| Rfc | 1479 |
|---|
| Title | Inter-Domain Policy Routing Protocol Specification: Version 1 |
|---|
| Author | M.
Steenstrup |
|---|
| Date | July 1993 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | HISTORIC |
|---|
|
Network Working Group M. Steenstrup
Request for Comments: 1479 BBN Systems and Technologies
July 1993
Inter-Domain Policy Routing Protocol Specification: Version 1
Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
Abstract
We present the set of protocols and procedures that constitute
Inter-Domain Policy Routing (IDPR). IDPR includes the virtual
gateway protocol, the flooding protocol, the route server query
protocol, the route generation procedure, the path control protocol,
and the data message forwarding procedure.
Contributors
The following people have contributed to the protocols and procedures
described in this document: Helen Bowns, Lee Breslau, Ken Carlberg,
Isidro Castineyra, Deborah Estrin, Tony Li, Mike Little, Katia
Obraczka, Sam Resheff, Martha Steenstrup, Gene Tsudik, and Robert
Woodburn.
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Domain Elements . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Policy. . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. IDPR Functions. . . . . . . . . . . . . . . . . . . . . . . . 5
1.3.1. IDPR Entities . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. Policy Semantics. . . . . . . . . . . . . . . . . . . . . . . 7
1.4.1. Source Policies . . . . . . . . . . . . . . . . . . . . . . 7
1.4.2. Transit Policies. . . . . . . . . . . . . . . . . . . . . . 8
1.5. IDPR Message Encapsulation. . . . . . . . . . . . . . . . . . 9
1.5.1. IDPR Data Message Format. . . . . . . . . . . . . . . . . .11
1.6. Security. . . . . . . . . . . . . . . . . . . . . . . . . . .12
1.7. Timestamps and Clock Synchronization. . . . . . . . . . . . .13
1.8. Network Management. . . . . . . . . . . . . . . . . . . . . .14
1.8.1. Policy Gateway Configuration. . . . . . . . . . . . . . . .17
1.8.2. Route Server Configuration. . . . . . . . . . . . . . . . .18
2. Control Message Transport Protocol. . . . . . . . . . . . . . .18
2.1. Message Transmission. . . . . . . . . . . . . . . . . . . . .20
2.2. Message Reception . . . . . . . . . . . . . . . . . . . . . .22
2.3. Message Validation. . . . . . . . . . . . . . . . . . . . . .23
2.4. CMTP Message Formats. . . . . . . . . . . . . . . . . . . . .24
3. Virtual Gateway Protocol. . . . . . . . . . . . . . . . . . . .27
3.1. Message Scope . . . . . . . . . . . . . . . . . . . . . . . .28
3.1.1. Pair-PG Messages. . . . . . . . . . . . . . . . . . . . . .28
3.1.2. Intra-VG Messages . . . . . . . . . . . . . . . . . . . . .29
3.1.3. Inter-VG Messages . . . . . . . . . . . . . . . . . . . . .29
3.1.4. VG Representatives. . . . . . . . . . . . . . . . . . . . .31
3.2. Up/Down Protocol. . . . . . . . . . . . . . . . . . . . . . .31
3.3. Implementation. . . . . . . . . . . . . . . . . . . . . . . .33
3.4. Policy Gateway Connectivity . . . . . . . . . . . . . . . . .35
3.4.1. Within a Virtual Gateway. . . . . . . . . . . . . . . . . .35
3.4.2. Between Virtual Gateways. . . . . . . . . . . . . . . . . .37
3.4.3. Communication Complexity. . . . . . . . . . . . . . . . . .40
3.5. VGP Message Formats . . . . . . . . . . . . . . . . . . . . .41
3.5.1. UP/DOWN . . . . . . . . . . . . . . . . . . . . . . . . . .41
3.5.2. PG CONNECT. . . . . . . . . . . . . . . . . . . . . . . . .42
3.5.3. PG POLICY . . . . . . . . . . . . . . . . . . . . . . . . .43
3.5.4. VG CONNECT. . . . . . . . . . . . . . . . . . . . . . . . .44
3.5.5. VG POLICY . . . . . . . . . . . . . . . . . . . . . . . . .45
3.5.6. Negative Acknowledgements . . . . . . . . . . . . . . . . .46
4. Routing Information Distribution. . . . . . . . . . . . . . . .47
4.1. AD Representatives. . . . . . . . . . . . . . . . . . . . . .48
4.2. Flooding Protocol . . . . . . . . . . . . . . . . . . . . . .48
4.2.1. Message Generation. . . . . . . . . . . . . . . . . . . . .50
4.2.2. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . .52
4.2.3. Message Acceptance. . . . . . . . . . . . . . . . . . . . .52
4.2.4. Message Incorporation . . . . . . . . . . . . . . . . . . .54
4.2.5. Routing Information Database. . . . . . . . . . . . . . . .56
4.3. Routing Information Message Formats . . . . . . . . . . . . .57
4.3.1. CONFIGURATION . . . . . . . . . . . . . . . . . . . . . . .57
4.3.2. DYNAMIC . . . . . . . . . . . . . . . . . . . . . . . . . .62
4.3.3. Negative Acknowledgements . . . . . . . . . . . . . . . . .63
5. Route Server Query Protocol . . . . . . . . . . . . . . . . . .64
5.1. Message Exchange. . . . . . . . . . . . . . . . . . . . . . .64
5.2. Remote Route Server Communication . . . . . . . . . . . . . .65
5.3. Routing Information . . . . . . . . . . . . . . . . . . . . .66
5.4. Routes. . . . . . . . . . . . . . . . . . . . . . . . . . . .67
5.5. Route Server Message Formats. . . . . . . . . . . . . . . . .67
5.5.1. ROUTING INFORMATION REQUEST . . . . . . . . . . . . . . . .67
5.5.2. ROUTE REQUEST . . . . . . . . . . . . . . . . . . . . . . .68
5.5.3. ROUTE RESPONSE. . . . . . . . . . . . . . . . . . . . . . .71
5.5.4. Negative Acknowledgements . . . . . . . . . . . . . . . . .72
6. Route Generation. . . . . . . . . . . . . . . . . . . . . . . .73
6.1. Searching . . . . . . . . . . . . . . . . . . . . . . . . . .74
6.1.1. Implementation. . . . . . . . . . . . . . . . . . . . . . .75
6.2. Route Directionality. . . . . . . . . . . . . . . . . . . . .78
6.3. Route Database. . . . . . . . . . . . . . . . . . . . . . . .79
6.3.1. Cache Maintenance . . . . . . . . . . . . . . . . . . . . .80
7. Path Control Protocol and Data Message Forwarding Procedure . .80
7.1. An Example of Path Setup. . . . . . . . . . . . . . . . . . .81
7.2. Path Identifiers. . . . . . . . . . . . . . . . . . . . . . .84
7.3. Path Control Messages . . . . . . . . . . . . . . . . . . . .85
7.4. Setting Up and Tearing Down a Path. . . . . . . . . . . . . .87
7.4.1. Validating Path Identifiers . . . . . . . . . . . . . . . .89
7.4.2. Path Consistency with Configured Transit Policies . . . . .89
7.4.3. Path Consistency with Virtual Gateway Reachability. . . . .91
7.4.4. Obtaining Resources . . . . . . . . . . . . . . . . . . . .92
7.4.5. Target Response . . . . . . . . . . . . . . . . . . . . . .93
7.4.6. Originator Response . . . . . . . . . . . . . . . . . . . .93
7.4.7. Path Life . . . . . . . . . . . . . . . . . . . . . . . . 94
7.5. Path Failure and Recovery . . . . . . . . . . . . . . . . . 95
7.5.1. Handling Implicit Path Failures . . . . . . . . . . . . . 96
7.5.2. Local Path Repair . . . . . . . . . . . . . . . . . . . . 97
7.5.3. Repairing a Path. . . . . . . . . . . . . . . . . . . . . 98
7.6. Path Control Message Formats. . . . . . . . . . . . . . . . 100
7.6.1. SETUP . . . . . . . . . . . . . . . . . . . . . . . . . . 101
7.6.2. ACCEPT. . . . . . . . . . . . . . . . . . . . . . . . . . 103
7.6.3. REFUSE. . . . . . . . . . . . . . . . . . . . . . . . . . 103
7.6.4. TEARDOWN. . . . . . . . . . . . . . . . . . . . . . . . . 104
7.6.5. ERROR . . . . . . . . . . . . . . . . . . . . . . . . . . 105
7.6.6. REPAIR. . . . . . . . . . . . . . . . . . . . . . . . . . 106
7.6.7. Negative Acknowledgements . . . . . . . . . . . . . . . . 106
8. Security Considerations . . . . . . . . . . . . . . . . . . . 106
9. Authors's Address . . . . . . . . . . . . . . . . . . . . . . 107
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
1. Introduction
In this document, we specify the protocols and procedures that
compose Inter-Domain Policy Routing (IDPR). The objective of IDPR is
to construct and maintain routes between source and destination
administrative domains, that provide user traffic with the services
requested within the constraints stipulated for the domains
transited. IDPR supports link state routing information distribution
and route generation in conjunction with source specified message
forwarding. Refer to [5] for a detailed justification of our
approach to inter-domain policy routing.
1.1. Domain Elements
The IDPR architecture has been designed to accommodate an
internetwork with tens of thousands of administrative domains
collectively containing hundreds of thousands of local networks.
Inter-domain policy routes are constructed using information about
the services offered by, and the connectivity between, administrative
domains. The intra-domain details - gateways, networks, and links
traversed - of an inter-domain policy route are the responsibility of
intra-domain routing and are thus outside the scope of IDPR.
An "administrative domain" (AD) is a collection of contiguous hosts,
gateways, networks, and links managed by a single administrative
authority. The domain administrator defines service restrictions for
transit traffic and service requirements for locally-generated
traffic, and selects the addressing schemes and routing procedures
that apply within the domain. Within the Internet, each domain has a
unique numeric identifier assigned by the Internet Assigned Numbers
Authority (IANA).
"Virtual gateways" (VGs) are the only IDPR-recognized connecting
points between adjacent domains. Each virtual gateway is a
collection of directly-connected "policy gateways" (see below) in two
adjoining domains, whose existence has been sanctioned by the
administrators of both domains. The domain administrators may agree
to establish more than one virtual gateway between the two domains.
For each such virtual gateway, the two administrators together assign
a local numeric identifier, unique within the set of virtual gateways
connecting the two domains. To produce a virtual gateway identifier
unique within its domain, a domain administrator concatenates the
mutually assigned local virtual gateway identifier together with the
adjacent domain's identifier.
Policy gateways (PGs) are the physical gateways within a virtual
gateway. Each policy gateway enforces service restrictions on IDPR
transit traffic, as stipulated by the domain administrator, and
forwards the traffic accordingly. Within a domain, two policy
gateways are "neighbors" if they are in different virtual gateways.
A single policy gateway may belong to multiple virtual gateways.
Within a virtual gateway, two policy gateways are "peers" if they are
in the same domain and are "adjacent" if they are in different
domains. Adjacent policy gateways are "directly connected" if the
only Internet-addressable entities attached to the connecting medium
are policy gateways in the virtual gateways. Note that this
definition implies that not only point-to-point links but also
networks may serve as direct connections between adjacent policy
gateways. The domain administrator assigns to each of its policy
gateways a numeric identifier, unique within that domain.
A "domain component" is a subset of a domain's entities such that all
entities within the subset are mutually reachable via intra-domain
routes, but no entities outside the subset are reachable via intra-
domain routes from entities within the subset. Normally, a domain
consists of a single component, namely itself; however, when
partitioned, a domain consists of multiple components. Each domain
component has an identifier, unique within the Internet, composed of
the domain identifier together with the identifier of the lowest-
numbered operational policy gateway within the component. All
operational policy gateways within a domain component can discover
mutual reachability through intra-domain routing information. Hence,
all such policy gateways can consistently determine, without explicit
negotiation, which of them has the lowest number.
1.2. Policy
With IDPR, each domain administrator sets "transit policies" that
dictate how and by whom the resources in its domain should be used.
Transit policies are usually public, and they specify offered
services comprising:
- Access restrictions: e.g., applied to traffic to or from certain
domains or classes of users.
- Quality: e.g., delay, throughput, or error characteristics.
- Monetary cost: e.g., charge per byte, message, or unit time.
Each domain administrator also sets "source policies" for traffic
originating in its domain. Source policies are usually private, and
they specify requested services comprising:
- Access restrictions: e.g., domains to favor or avoid in routes.
- Quality: e.g., acceptable delay, throughput, and reliability.
- Monetary cost: e.g., acceptable session cost.
1.3. IDPR Functions
IDPR comprises the following functions:
- Collecting and distributing routing information including domain
transit policies and inter-domain connectivity.
- Generating and selecting policy routes based on the routing
information distributed and on the source policies configured or
requested.
- Setting up paths across the Internet using the policy routes
generated.
- Forwarding messages across and between domains along the
established paths.
- Maintaining databases of routing information, inter-domain policy
routes, forwarding information, and configuration information.
1.3.1. IDPR Entities
Several different entities are responsible for performing the IDPR
functions.
Policy gateways, the only IDPR-recognized connecting points between
adjacent domains, collect and distribute routing information,
participate in path setup, forward data messages along established
paths, and maintain forwarding information databases.
"Path agents", resident within policy gateways and within "route
servers" (see below), act on behalf of hosts to select policy routes,
to set up and manage paths, and to maintain forwarding information
databases. Any Internet host can reap the benefits of IDPR, as long
as there exists a path agent configured to act on its behalf and a
means by which the host's messages can reach the path agent.
Specifically, a path agent in one domain may be configured to act on
behalf of hosts in another domain. In this case, the path agent's
domain is an IDPR "proxy" for the hosts' domain.
Route servers maintain both the routing information database and the
route database, and they generate policy routes using the routing
information collected and the source policies requested by the path
agents. A route server may reside within a policy gateway, or it may
exist as an autonomous entity. Separating the route server functions
from the policy gateways frees the policy gateways from both the
memory intensive task of database (routing information and route)
maintenance and the computationally intensive task of route
generation. Route servers, like policy gateways, each have a unique
numeric identifier within their domain, assigned by the domain
administrator.
Given the size of the current Internet, each policy gateway can
perform the route server functions, in addition to its message
forwarding functions, with little or no degradation in message
forwarding performance. Aggregating the routing functions into
policy gateways simplifies implementation; one need only install IDPR
protocols in policy gateways. Moreover, it simplifies communication
between routing functions, as all functions reside within each policy
gateway. As the Internet grows, the memory and processing required
to perform the route server functions may become a burden for the
policy gateways. When this happens, each domain administrator should
separate the route server functions from the policy gateways in its
domain.
"Mapping servers" maintain the database of mappings that resolve
Internet names and addresses to domain identifiers. Each host is
contained within a domain and is associated with a proxy domain which
may be identical with the host's domain. The mapping server function
will be integrated into the existing DNS name service (see [6]) and
will provide mappings between a host and its local and proxy domains.
"Configuration servers" maintain the databases of configured
information that apply to IDPR entities within their domains.
Configuration information for a given domain includes transit
policies (i.e., service offerings and restrictions), source policies
(i.e., service requirements), and mappings between local IDPR
entities and their names and addresses. The configuration server
function will be integrated into a domain's existing network
management system (see [7]-[8]).
1.4. Policy Semantics
The source and transit policies supported by IDPR are intended to
accommodate a wide range of services available throughout the
Internet. We describe the semantics of these policies, concentrating
on the access restriction aspects. To express these policies in this
document, we have chosen to use a syntactic variant of Clark's policy
term notation [1]. However, we provide a more succinct syntax (see
[7]) for actually configuring source and transit policies.
1.4.1. Source Policies
Each source policy takes the form of a collection of sets as follows:
Applicable Sources and Destinations:
{((H(1,1),s(1,1)),...,(H(1,f1),s(1,f1))),...,((H(n,1),s(n,1)),...,
(H(n,fn),s(n,fn)))}: The set of groups of source/destination
traffic flows to which the source policy applies. Each traffic
flow group ((H(i,1),s(i,1)),...,(H(i,fi),s(i,fi))) contains a set
of source hosts and corresponding destination hosts. Here, H(i,j)
represents a host, and s(i,j), an element of {SOURCE,
DESTINATION}, represents an indicator of whether H(i,j) is to be
considered as a source or as a destination.
Domain Preferences: {(AD(1),x(1)),...,(AD(m),x(m))}: The set of
transit domains that the traffic flows should favor, avoid, or
exclude. Here, AD(i) represents a domain, and x(i), an element of
{FAVOR, AVOID, EXCLUDE}, represents an indicator of whether routes
including AD(i) are to be favored, avoided if possible, or
unconditionally excluded.
UCI: The source user class for the traffic flows listed.
RequestedServices: The set of requested services not related to
access restrictions, i.e., service quality and monetary cost.
When selecting a route for a traffic flow from a source host H(i,j)
to a destination host H(i,k), where 1 < or = i < or = n and 1 < or =
j, k < or = fi, the path agent (see section 1.3.1) must honor the
source policy such that:
- For each domain, AD(p), contained in the route, AD(p) is not equal
to any AD(k), such that 1 < or = k < or = m and x(k) = EXCLUDE.
- The route provides the services listed in the set Requested
Services.
1.4.2. Transit Policies
Each transit policy takes the form of a collection of sets as
follows:
Source/Destination Access Restrictions:
{((H(1,1),AD(1,1),s(1,1)),...,(H(1,f1),AD(1,f1),s(1,f1))),...,
((H(n,1),AD(n,1),s(n,1)),...,(H(n,fn),AD(n,fn),s(n,fn)))}: The set
of groups of source and destination hosts and domains to which the
transit policy applies. Each domain group
((H(i,1),AD(i,1),s(i,1)),...,(H(i,fi),AD(i,fi),s(i,fi))) contains
a set of source and destination hosts and domains such that this
transit domain will carry traffic from each source listed to each
destination listed. Here, H(i,j) represents a set of hosts,
AD(i,j) represents a domain containing H(i,j), and s(i,j), a
subset of {SOURCE, DESTINATION}, represents an indicator of
whether (H(i,j),AD(i,j)) is to be considered as a set of sources,
destinations, or both.
Temporal Access Restrictions: The set of time intervals during which
the transit policy applies.
User Class Access Restrictions: The set of user classes to which the
transit policy applies.
Offered Services: The set of offered services not related to access
restrictions, i.e., service quality and monetary cost.
Virtual Gateway Access Restrictions:
{((VG(1,1),e(1,1)),...,(VG(1,g1),e(1,g1))),...,((VG(m,1),e(m,1)),
gateways to which the transit policy applies. Each virtual
gateway group ((VG(i,1),e(i,1)),...,(VG(i,gi),e(i,gi))) contains a
set of domain entry and exit points such that each entry virtual
gateway can reach (barring an intra-domain routing failure) each
exit virtual gateway via an intra-domain route supporting the
transit policy. Here, VG(i,j) represents a virtual gateway, and
e(i,j), a subset of {ENTRY, EXIT}, represents an indicator of
whether VG(i,j) is to be considered as a domain entry point, exit
point, or both.
The domain advertising such a transit policy will carry traffic from
any host in the set H(i,j) in AD(i,j) to any host in the set H(i,k)
in AD(i,k), where 1 < or = i < or = n and 1 < or = j, k < or = fi,
provided that:
- SOURCE is an element of s(i,j).
- DESTINATION is an element of s(i,k).
- Traffic from H(i,j) enters the domain during one of the intervals
in the set Temporal Access Restrictions.
- Traffic from H(i,j) carries one of the user class identifiers in
the set User Class Access Restrictions.
- Traffic from H(i,j) enters via any VG(u,v) such that ENTRY is an
element of e(u,v), where 1 < or = u < or = m and 1 < or = v < or =
gu.
- Traffic to H(i,k) leaves via any VG(u,w) such that EXIT is an
element of e(u,w), where 1 < or = w < or = gu.
1.5. IDPR Message Encapsulation
There are two kinds of IDPR messages:
- "Data messages" containing user data generated by hosts.
- "Control messages" containing IDPR protocol-related control
information generated by policy gateways and route servers.
Within an internetwork, only policy gateways and route servers are
able to generate, recognize, and process IDPR messages. The
existence of IDPR is invisible to all other gateways and hosts,
including mapping servers and configuration servers. Mapping servers
and configuration servers perform necessary but ancillary functions
for IDPR, and thus they are not required to handle IDPR messages.
An IDPR entity places IDPR-specific information in each IDPR control
message it originates; this information is significant only to
recipient IDPR entities. Using "encapsulation" across each domain,
an IDPR message tunnels from source to destination across an
internetwork through domains that may employ disparate intra-domain
addressing schemes and routing procedures.
As an alternative to encapsulation, we had considered embedding IDPR
in IP, as a set of IP options. However, this approach has the
following disadvantages:
- Only domains that support IP would be able to participate in IDPR;
domains that do not support IP would be excluded.
- Each gateway, policy or other, in a participating domain would at
least have to recognize the IDPR option, even if it did not execute
the IDPR protocols. However, most commercial routers are not
optimized for IP options processing, and so IDPR message handling
might require significant processing at each gateway.
- For some IDPR protocols, in particular path control, the size
restrictions on IP options would preclude inclusion of all of the
necessary protocol-related information.
For these reasons, we decided against the IP option approach and in
favor of encapsulation.
An IDPR message travels from source to destination between
consecutive policy gateways. Each policy gateway encapsulates the
IDPR message with information, for example an IP header, that will
enable the message to reach the next policy gateway. Note that the
encapsulating header and the IDPR-specific information may increase
the message size beyond the MTU of the given domain. However,
message fragmentation and reassembly is the responsibility of the
protocol, for example IP, that encapsulates IDPR messages for
transport between successive policy gateways; it is not currently the
responsibility of IDPR itself.
A policy gateway, when forwarding an IDPR message to a peer or a
neighbor policy gateway, encapsulates the message in accordance with
the addressing scheme and routing procedure of the given domain and
indicates in the protocol field of the encapsulating header that the
message is indeed an IDPR message. Intermediate gateways between the
two policy gateways forward the IDPR message as they would any other
message, using the information in the encapsulating header. Only the
recipient policy gateway interprets the protocol field, strips off
the encapsulating header, and processes the IDPR message.
A policy gateway, when forwarding an IDPR message to a directly-
connected adjacent policy gateway, encapsulates the message in
accordance with the addressing scheme of the entities within the
virtual gateway and indicates in the protocol field of the
encapsulating header that the message is indeed an IDPR message. The
recipient policy gateway strips off the encapsulating header and
processes the IDPR message. We recommend that the recipient policy
gateway perform the following validation check of the encapsulating
header, prior to stripping it off. Specifically, the recipient
policy gateway should verify that the source address and the
destination address in the encapsulating header match the adjacent
policy gateway's address and its own address, respectively.
Moreover, the recipient policy gateway should verify that the message
arrived on the interface designated for the direct connection to the
adjacent policy gateway. These checks help to ensure that IDPR
traffic that crosses domain boundaries does so only over direct
connections between adjacent policy gateways.
Policy gateways forward IDPR data messages according to a forwarding
information database which maps "path identifiers", carried in the
data messages, into next policy gateways. Policy gateways forward
IDPR control messages according to next policy gateways selected by
the particular IDPR control protocols associated with the messages.
Distinguishing IDPR data messages and IDPR control messages at the
encapsulating protocol level, instead of at the IDPR protocol level,
eliminates an extra level of dispatching and hence makes IDPR message
forwarding more efficient. When encapsulated within IP messages,
IDPR data messages and IDPR control messages carry the IP protocol
numbers 35 and 38, respectively.
1.5.1. IDPR Data Message Format
The path agents at a source domain determine which data messages
generated by local hosts are to be handled by IDPR. To each data
message selected for IDPR handling, a source path agent prepends the
following header:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VERSION | PROTO | LENGTH |
+---------------+---------------+-------------------------------+
| PATH ID |
| |
+---------------------------------------------------------------+
| TIMESTAMP |
+---------------------------------------------------------------+
| INT/AUTH |
| |
+---------------------------------------------------------------+
VERSION (8 bits) Version number for IDPR data messages, currently
equal to 1.
PROTO (8 bits) Numeric identifier for the protocol with which to
process the contents of the IDPR data message. Only the path agent
at the destination interprets and acts upon the contents of the PROTO
field.
LENGTH (16 bits) Length of the entire IDPR data message in bytes.
PATH ID (64 bits) Path identifier assigned by the source's path agent
and consisting of the numeric identifier for the path agent's domain
(16 bits), the numeric identifier for the path agent's policy gateway
(16 bits), and the path agent's local path identifier (32 bits) (see
section 7.2).
TIMESTAMP (32 bits) Number of seconds elapsed since 1 January 1970
0:00 GMT.
INT/AUTH (variable) Computed integrity/authentication value,
dependent on the type of integrity/authentication requested during
path setup.
We describe the IDPR control message header in section 2.4.
1.6. Security
IDPR contains mechanisms for verifying message integrity and source
authenticity and for protecting against certain types of denial of
service attacks. It is particularly important to keep IDPR control
messages intact, because they carry control information critical to
the construction and use of viable policy routes between domains.
All IDPR messages carry a single piece of information, referred to as
the "integrity/authentication value", which may be used not only to
detect message corruption but also to verify the authenticity of the
message source. In the Internet, the IANA will sanction the set of
valid algorithms which may be used to compute the
integrity/authentication values. This set may include algorithms
that perform only message integrity checks such as n-bit cyclic
redundancy checksums (CRCs), as well as algorithms that perform both
message integrity and source authentication checks such as signed
hash functions of message contents.
Each domain administrator is free to select any
integrity/authentication algorithm, from the set specified by the
IANA, for computing the integrity/authentication values contained in
its domain's messages. However, we recommend that IDPR entities in
each domain be capable of executing all of the valid algorithms so
that an IDPR control message originating at an entity in one domain
can be properly checked by an entity in another domain.
Each IDPR control message must carry a non-null
integrity/authentication value. We recommend that control message
integrity/authentication be based on a digital signature algorithm
applied to a one-way hash function, such as RSA applied to MD5 [17],
which simultaneously verifies message integrity and source
authenticity. The digital signature may be based on either public-
key or private-key cryptography. Our approach to digital signature
use in IDPR is based on the privacy-enhanced Internet electronic mail
service [13]-[15], already available in the Internet.
We do not require that IDPR data messages carry a non-null
integrity/authentication value. In fact, we recommend that a higher
layer (end-to-end) procedure, and not IDPR, assume responsibility for
checking the integrity and authenticity of data messages, because of
the amount of computation involved.
1.7. Timestamps and Clock Synchronization
Each IDPR message carries a timestamp (expressed in seconds elapsed
since 1 January 1970 0:00 GMT, following the UNIX precedent) supplied
by the source IDPR entity, which serves to indicate the age of the
message. IDPR entities use the absolute value of the timestamp to
confirm that a message is current and use the relative difference
between timestamps to determine which message contains the more
recent information.
All IDPR entities must possess internal clocks that are synchronized
to some degree, in order for the absolute value of a message
timestamp to be meaningful. The synchronization granularity required
by IDPR is on the order of minutes and can be achieved manually.
Thus, a clock synchronization protocol operating among all IDPR
entities in all domains, while useful, is not necessary.
An IDPR entity can determine whether to accept or reject a message
based on the discrepancy between the message's timestamp and the
entity's own internal clock time. Any IDPR message whose timestamp
lies outside of the acceptable range may contain stale or corrupted
information or may have been issued by a source whose internal clock
has lost synchronization with the message recipient's internal clock.
Timestamp checks are required for control messages because of the
consequences of propagating and acting upon incorrect control
information. However, timestamp checks are discretionary for data
messages but may be invoked during problem diagnosis, for example,
when checking for suspected message replays.
We note that none of the IDPR protocols contain explicit provisions
for dealing with an exhausted timestamp space. As timestamp space
exhaustion will not occur until well into the next century, we expect
timestamp space viability to outlast the IDPR protocols.
1.8. Network Management
In this document, we do not describe how to configure and manage
IDPR. However, in this section, we do provide a list of the types of
IDPR configuration information required. Also, in later sections
describing the IDPR protocols, we briefly note the types of
exceptional events that must be logged for network management.
Complete descriptions of IDPR entity configuration and IDPR managed
objects appear in [7] and [8] respectively.
To participate in inter-domain policy routing, policy gateways and
route servers within a domain each require configuration information.
Some of the configuration information is specifically defined within
the given domain, while some of the configuration information is
universally defined throughout an internetwork. A domain
administrator determines domain-specific information, and in the
Internet, the IANA determines globally significant information.
To produce valid domain configurations, the domain administrators
must receive the following global information from the IANA:
- For each integrity/authentication type, the numeric
identifier, syntax, and semantics. Available integrity and
authentication types include but are not limited to:
o public-key based signatures;
o private-key based signatures;
o cyclic redundancy checksums;
o no integrity/authentication.
- For each user class, the numeric identifier, syntax, and
semantics. Available user classes include but are not limited to:
o federal (and if necessary, agency-specific such as NSF, DOD,
DOE, etc.);
o research;
o commercial;
o support.
- For each offered service that may be advertised in transit
policies, the numeric identifier, syntax, and semantics. Available
offered services include but are not limited to:
o average message delay;
o message delay variation;
o average bandwidth available;
o available bandwidth variation;
o maximum transfer unit (MTU);
o charge per byte;
o charge per message;
o charge per unit time.
- For each access restriction that may be advertised in transit
policies, the numeric identifier, syntax, and semantics. Available
access restrictions include but are not limited to:
o Source and destination domains and host sets.
o User classes.
o Entry and exit virtual gateways.
o Time of day.
- For each requested service that may appear within a path setup
message, the numeric identifier, syntax, and semantics. Available
requested services include but are not limited to:
o maximum path life in minutes, messages, or bytes;
o integrity/authentication algorithms to be used on data
messages sent over the path;
o upper bound on path delay;
o minimum delay path;
o upper bound on path delay variation;
o minimum delay variation path;
o lower bound on path bandwidth;
o maximum bandwidth path;
o upper bound on monetary cost;
o minimum monetary cost path.
In an internetwork-wide implementation of IDPR, the set of global
configuration parameters and their syntax and semantics must be
consistent across all participating domains. The IANA, responsible
for establishing the full set of global configuration parameters in
the Internet, relies on the cooperation of the administrators of all
participating domains to ensure that the global parameters are
consistent with the desired transit policies and user service
requirements of each domain. Moreover, as the syntax and semantics
of the global parameters affects the syntax and semantics of the
corresponding IDPR software, the IANA must carefully define each
global parameter so that it is unlikely to require future
modification.
The IANA provides configured global information to configuration
servers in all domains participating in IDPR. Each domain
administrator uses the configured global information maintained by
its configuration servers to develop configurations for each IDPR
entity within its domain. Each configuration server retains a copy
of the configuration for each local IDPR entity and also distributes
the configuration to that entity using, for example, SNMP.
1.8.1. Policy Gateway Configuration
Each policy gateway must contain sufficient configuration information
to perform its IDPR functions, which subsume those of the path agent.
These include: validating IDPR control messages; generating and
distributing virtual gateway connectivity and routing information
messages to peer, neighbor, and adjacent policy gateways;
distributing routing information messages to route servers in its
domain; resolving destination addresses; requesting policy routes
from route servers; selecting policy routes and initiating path
setup; ensuring consistency of a path with its domain's transit
policies; establishing path forwarding information; and forwarding
IDPR data messages along existing paths. The necessary configuration
information includes the following:
- For each integrity/authentication type, the numeric identifier,
syntax, and semantics.
- For each policy gateway and route server in the given domain, the
numeric identifier and set of addresses or names.
- For each virtual gateway connected to the given domain, the numeric
identifier, the numeric identifiers for the constituent peer policy
gateways, and the numeric identifier for the adjacent domain.
- For each virtual gateway of which the given policy gateway is a
member, the numeric identifiers and set of addresses for the
constituent adjacent policy gateways.
- For each policy gateway directly-connected and adjacent to the
given policy gateway, the local connecting interface.
- For each local route server to which the given policy gateway
distributes routing information, the numeric identifier.
- For each source policy applicable to hosts within the given domain,
the syntax and semantics.
- For each transit policy applicable to the domain, the numeric
identifier, syntax, and semantics.
- For each requested service that may appear within a path setup
message, the numeric identifier, syntax, and semantics.
- For each source user class, the numeric identifier, syntax, and
semantics.
1.8.2. Route Server Configuration
Each route server must contain sufficient configuration information
to perform its IDPR functions, which subsume those of the path agent.
These include: validating IDPR control messages; deciphering and
storing the contents of routing information messages; exchanging
routing information with other route servers and policy gateways;
generating policy routes that respect transit policy restrictions and
source service requirements; distributing policy routes to path
agents in policy gateways; resolving destination addresses; selecting
policy routes and initiating path setup; establishing path forwarding
information; and forwarding IDPR data messages along existing paths.
The necessary configuration information includes the following:
- For each integrity/authentication type, the numeric identifier,
syntax, and semantics.
- For each policy gateway and route server in the given domain, the
numeric identifier and set of addresses or names.
- For each source policy applicable to hosts within the given domain,
the syntax and semantics.
- For access restriction that may be advertised in transit
policies, the numeric identifier, syntax, and semantics.
- For each offered service that may be advertised in transit policies,
the numeric identifier, syntax, and semantics.
- For each requested service that may appear within a path setup
message, the numeric identifier, syntax, and semantics.
- For each source user class, the numeric identifier, syntax, and
semantics.
2. Control Message Transport Protocol
IDPR control messages convey routing-related information that
directly affects the policy routes generated and the paths set up
across the Internet. Errors in IDPR control messages can have
widespread, deleterious effects on inter-domain policy routing, and
so the IDPR protocols have been designed to minimize loss and
corruption of control messages. For every control message it
transmits, each IDPR protocol expects to receive notification as to
whether the control message successfully reached the intended IDPR
recipient. Moreover, the IDPR recipient of a control message first
verifies that the message appears to be well-formed, before acting on
its contents.
All IDPR protocols use the Control Message Transport Protocol (CMTP),
a connectionless, transaction-based transport layer protocol, for
communication with intended recipients of control messages. CMTP
retransmits unacknowledged control messages and applies integrity and
authenticity checks to received control messages.
There are three types of CMTP messages:
DATAGRAM:
Contains IDPR control messages.
ACK: Positive acknowledgement in response to a DATAGRAM message.
NAK: Negative acknowledgement in response to a DATAGRAM message.
Each CMTP message contains several pieces of information supplied by
the sender that allow the recipient to test the integrity and
authenticity of the message. The set of integrity and authenticity
checks performed after CMTP message reception are collectively
referred to as "validation checks" and are described in section 2.3.
When we first designed the IDPR protocols, CMTP as a distinct
protocol did not exist. Instead, CMTP-equivalent functionality was
embedded in each IDPR protocol. To provide a cleaner implementation,
we later decided to provide a single transport protocol that could be
used by all IDPR protocols. We originally considered using an
existing transport protocol, but rejected this approach for the
following reasons:
- The existing reliable transport protocols do not provide all of the
validation checks, in particular the timestamp and authenticity
checks, required by the IDPR protocols. Hence, if we were to use
one of these protocols, we would still have to provide a separate
protocol on top of the transport protocol to force retransmission of
IDPR messages that failed to pass the required validation checks.
- Many of the existing reliable transport protocols are window-based
and hence can result in increased message delay and resource use
when, as is the case with IDPR, multiple independent messages use
the same transport connection. A single message experiencing
transmission problems and requiring retransmission can prevent the
window from advancing, forcing all subsequent messages to queue
behind it. Moreover, many of the window-based protocols do not
support selective retransmission of failed messages but instead
require retransmission of not only the failed message but also all
preceding messages within the window.
For these reasons, we decided against using an existing transport
protocol and in favor of developing CMTP.
2.1. Message Transmission
At the transmitting entity, when an IDPR protocol is ready to issue a
control message, it passes a copy of the message to CMTP; it also
passes a set of parameters to CMTP for inclusion in the CMTP header
and for proper CMTP message handling. In turn, CMTP converts the
control message and associated parameters into a DATAGRAM by
prepending the appropriate header to the control message. The CMTP
header contains several pieces of information to aid the message
recipient in detecting errors (see section 2.4). Each IDPR protocol
can specify all of the following CMTP parameters applicable to its
control message:
- IDPR protocol and message type.
- Destination.
- Integrity/authentication scheme.
- Timestamp.
- Maximum number of transmissions allotted.
- Retransmission interval in microseconds.
One of these parameters, the timestamp, can be specified directly by
CMTP as the internal clock time at which the message is transmitted.
However, two of the IDPR protocols, namely flooding and path control,
themselves require message generation timestamps for proper protocol
operation. Thus, instead of requiring CMTP to pass back a timestamp
to an IDPR protocol, we simplify the service interface between CMTP
and the IDPR protocols by allowing an IDPR protocol to specify the
timestamp in the first place.
Using the control message and accompanying parameters supplied by the
IDPR protocol, CMTP constructs a DATAGRAM, adding to the header
CMTP-specific parameters. In particular, CMTP assigns a "transaction
identifier" to each DATAGRAM generated, which it uses to associate
acknowledgements with DATAGRAM messages. Each DATAGRAM recipient
includes the received transaction identifier in its returned ACK or
NAK, and each DATAGRAM sender uses the transaction identifier to
match the received ACK or NAK with the original DATAGRAM.
A single DATAGRAM, for example a routing information message or a
path control message, may be handled by CMTP at many different policy
gateways. Within a pair of consecutive IDPR entities, the DATAGRAM
sender expects to receive an acknowledgement from the DATAGRAM
recipient. However, only the IDPR entity that actually generated the
original CMTP DATAGRAM has control over the transaction identifier,
because that entity may supply a digital signature that covers the
entire DATAGRAM. The intermediate policy gateways that transmit the
DATAGRAM do not change the transaction identifier. Nevertheless, at
each DATAGRAM recipient, the transaction identifier must uniquely
distinguish the DATAGRAM so that only one acknowledgement from the
next DATAGRAM recipient matches the original DATAGRAM. Therefore,
the transaction identifier must be globally unique.
The transaction identifier consists of the numeric identifiers for
the domain and IDPR entity (policy gateway or route server) issuing
the original DATAGRAM, together with a 32-bit local identifier
assigned by CMTP operating within that IDPR entity. We recommend
implementing the 32-bit local identifier either as a simple counter
incremented for each DATAGRAM generated or as a fine granularity
clock. The former always guarantees uniqueness of transaction
identifiers; the latter guarantees uniqueness of transaction
identifiers, provided the clock granularity is finer than the minimum
possible interval between DATAGRAM generations and the clock wrapping
period is longer than the maximum round-trip delay to and from any
internetwork destination.
Before transmitting a DATAGRAM, CMTP computes the length of the
entire message, taking into account the prescribed
integrity/authentication scheme, and then computes the
integrity/authentication value over the whole message. CMTP includes
both of these quantities, which are crucial for checking message
integrity and authenticity at the recipient, in the DATAGRAM header.
After sending a DATAGRAM, CMTP saves a copy and sets an associated
retransmission timer, as directed by the IDPR protocol parameters.
If the retransmission timer fires and CMTP has received neither an
ACK nor a NAK for the DATAGRAM, CMTP then retransmits the DATAGRAM,
provided this retransmission does not exceed the transmission
allotment. Whenever a DATAGRAM exhausts its transmission allotment,
CMTP discards the DATAGRAM, informs the IDPR protocol that the
control message transmission was not successful, and logs the event
for network management. In this case, the IDPR protocol may either
resubmit its control message to CMTP, specifying an alternate
destination, or discard the control message altogether.
2.2. Message Reception
At the receiving entity, when CMTP obtains a DATAGRAM, it takes one
of the following actions, depending upon the outcome of the message
validation checks:
- The DATAGRAM passes the CMTP validation checks. CMTP then delivers
the DATAGRAM with enclosed IDPR control message, to the appropriate
IDPR protocol, which in turn applies its own integrity checks to
the control message before acting on the contents. The recipient
IDPR protocol, except in one case, directs CMTP to generate an ACK
and return the ACK to the sender. That exception is the up/down
protocol (see section 3.2) which determines reachability of
adjacent policy gateways and does not use CMTP ACK messages to
notify the sender of message reception. Instead, the up/down
protocol messages themselves carry implicit information about
message reception at the adjacent policy gateway. In the cases
where the recipient IDPR protocol directs CMTP to generate an ACK,
it may pass control information to CMTP for inclusion in the ACK,
depending on the contents of the original IDPR control message.
For example, a route server unable to fill a request for routing
information may inform the requesting IDPR entity, through an ACK
for the initial request, to place its request elsewhere.
- The DATAGRAM fails at least one of the CMTP validation checks.
CMTP then generates a NAK, returns the NAK to the sender, and
discards the DATAGRAM, regardless of the type of IDPR control
message contained in the DATAGRAM. The NAK indicates the nature of
the validation failure and serves to help the sender establish
communication with the recipient. In particular, the CMTP NAK
provides a mechanism for negotiation of IDPR version and
integrity/authentication scheme, two parameters crucial for
establishing communication between IDPR entities.
Upon receiving an ACK or a NAK, CMTP immediately discards the message
if at least one of the validation checks fails or if it is unable to
locate the associated DATAGRAM. CMTP logs the latter event for
network management. Otherwise, if all of the validation checks pass
and if it is able to locate the associated DATAGRAM, CMTP clears the
associated retransmission timer and then takes one of the following
actions, depending upon the message type:
- The message is an ACK. CMTP discards the associated DATAGRAM and
delivers the ACK, which may contain IDPR control information, to
the appropriate IDPR protocol.
- The message is a NAK. If the associated DATAGRAM has exhausted its
transmission allotment, CMTP discards the DATAGRAM, informs the
appropriate IDPR protocol that the control message transmission was
not successful, and logs the event for network management.
Otherwise, if the associated DATAGRAM has not yet exhausted its
transmission allotment, CMTP first checks its copy of the DATAGRAM
against the failure indication contained in the NAK. If its
DATAGRAM copy appears to be intact, CMTP retransmits the DATAGRAM
and sets the associated retransmission timer. However, if its
DATAGRAM copy appears to be corrupted, CMTP discards the DATAGRAM,
informs the IDPR protocol that the control message transmission was
not successful, and logs the event for network management.
2.3. Message Validation
On every CMTP message received, CMTP performs a set of validation
checks to test message integrity and authenticity. The order in
which these tests are executed is important. CMTP must first
determine if it can parse enough of the message to compute the
integrity/authentication value. (Refer to section 2.4 for a
description of CMTP message formats.) Then, CMTP must immediately
compute the integrity/authentication value before checking other
header information. An incorrect integrity/authentication value
means that the message is corrupted, and so it is likely that CMTP
header information is incorrect. Checking specific header fields
before computing the integrity/authentication value not only may
waste time and resources, but also may lead to incorrect diagnoses of
a validation failure.
The CMTP validation checks are as follows:
- CMTP verifies that it can recognize both the control message
version type contained in the header. Failure to recognize either
one of these values means that CMTP cannot continue to parse the
message.
- CMTP verifies that it can recognize and accept the
integrity/authentication type contained in the header; no
integrity/authentication is not an acceptable type for CMTP.
- CMTP computes the integrity/authentication value and verifies that
it equals the integrity/authentication value contained in the
header. For key-based integrity/authentication schemes, CMTP may
use the source domain identifier contained in the CMTP header to
index the correct key. Failure to index a key means that CMTP
cannot compute the integrity/authentication value.
- CMTP computes the message length in bytes and verifies that it
equals the length value contained in the header.
- CMTP verifies that the message timestamp is in the acceptable
range. The message should be no more recent than cmtp_new (300)
seconds ahead of the entity's current internal clock time. In this
document, when we present an IDPR system configuration parameter,
such as cmtp_new, we usually follow it with a recommended value in
parentheses. The cmtp_new value allows some clock drift between
IDPR entities. Moreover, each IDPR protocol has its own limit on
the maximum age of its control messages. The message should be no
less recent than a prescribed number of seconds behind the
recipient entity's current internal clock time. Hence, each IDPR
protocol performs its own message timestamp check in addition to
that performed by CMTP.
- CMTP verifies that it can recognize the IDPR protocol designated
for the enclosed control message.
Whenever CMTP encounters a failure while performing any of these
validation checks, it logs the event for network management. If the
failure occurs on a DATAGRAM, CMTP immediately generates a NAK
containing the reason for the failure, returns the NAK to the sender,
and discards the DATAGRAM message. If the failure occurs on an ACK
or a NAK, CMTP discards the ACK or NAK message.
2.4. CMTP Message Formats
In designing the format of IDPR control messages, we have attempted
to strike a balance between efficiency of link bandwidth usage and
efficiency of message processing. In general, we have chosen compact
representations for IDPR information in order to minimize the link
bandwidth consumed by IDPR-specific information. However, we have
also organized IDPR information in order to speed message processing,
which does not always result in minimum link bandwidth usage.
To limit link bandwidth usage, we currently use fixed-length
identifier fields in IDPR messages; domains, virtual gateways, policy
gateways, and route servers are all represented by fixed-length
identifiers. To simplify message processing, we currently align
fields containing an even number of bytes on even-byte boundaries
within a message. In the future, if the Internet adopts the use of
super domains, we will offer hierarchical, variable-length identifier
fields in an updated version of IDPR.
The header of each CMTP message contains the following information:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VERSION | PRT | MSG | DPR | DMS | I/A TYP |
+---------------+-------+-------+-------+-------+---------------+
| SOURCE AD | SOURCE ENT |
+-------------------------------+-------------------------------+
| TRANS ID |
+---------------------------------------------------------------+
| TIMESTAMP |
+-------------------------------+-------------------------------+
| LENGTH | message specific |
+-------------------------------+-------------------------------+
| DATAGRAM AD | DATAGRAM ENT |
+-------------------------------+-------------------------------+
| INFORM |
+---------------------------------------------------------------+
| INT/AUTH |
| |
+---------------------------------------------------------------+
VERSION
(8 bits) Version number for IDPR control messages, currently
equal to 1.
PRT (4 bits) Numeric identifier for the control message transport
protocol, equal to 0 for CMTP.
MSG (4 bits) Numeric identifier for the CMTP message type,equal to 0
for a DATAGRAM, 1 for an ACK, and 2 for a NAK.
DPR (4 bits) Numeric identifier for the original DATAGRAM's IDPR
protocol type.
DMS (4 bits) Numeric identifier for the original DATAGRAM's IDPR
message type.
I/A TYP (8 bits) Numeric identifier for the integrity/authentication
scheme used. CMTP requires the use of an
integrity/authentication scheme; this value must not be set
equal to 0, indicating no integrity/authentication in use.
SOURCE AD (16 bits) Numeric identifier for the domain containing the
IDPR entity that generated the message.
SOURCE ENT (16 bits) Numeric identifier for the IDPR entity that
generated the message.
TRANSACTION ID (32 bits) Local transaction identifier assigned by the
IDPR entity that generated the original DATAGRAM.
TIMESTAMP (32 bits) Number of seconds elapsed since 1 January 1970
0:00 GMT.
LENGTH (16 bits) Length of the entire IDPR control message, including
the CMTP header, in bytes.
message specific (16 bits) Dependent upon CMTP message type.
For DATAGRAM and ACK messages:
RESERVED
(16 bits) Reserved for future use and currently set
equal to 0.
For NAK messages:
ERR TYP (8 bits) Numeric identifier for the type of CMTP
validation failure encountered. Validation failures
include the following types:
1. Unrecognized IDPR control message version number.
2. Unrecognized CMTP message type.
3. Unrecognized integrity/authentication scheme.
4. Unacceptable integrity/authentication scheme.
5. Unable to locate key using source domain.
6. Incorrect integrity/authentication value.
7. Incorrect message length.
8. Message timestamp out of range.
9. Unrecognized IDPR protocol designated for the
enclosed control message.
ERR INFO (8 bits) CMTP supplies the following additional
information for the designated types of validation
failures:
Type 1:
Acceptable IDPR control message version number.
Types 3 and 4: Acceptable integrity/authentication
type.
DATAGRAM AD
(16 bits) Numeric identifier for the domain containing the IDPR
entity that generated the original DATAGRAM. Present only in
ACK and NAK messages.
DATAGRAM ENT (16 bits) Numeric identifier for the IDPR entity that
generated the original DATAGRAM. Present only in ACK and NAK
messages.
INFORM (optional,variable) Information to be interpreted by the IDPR
protocol that issued the original DATAGRAM. Present only in ACK
messages and dependent on the original DATAGRAM's IDPR protocol
type.
INT/AUTH (variable) Computed integrity/authentication value,
dependent on the type of integrity/authentication scheme used.
3. Virtual Gateway Protocol
Every policy gateway within a domain participates in gathering
information about connectivity within and between virtual gateways of
which it is a member and in distributing this information to other
virtual gateways in its domain. We refer to these functions
collectively as the Virtual Gateway Protocol (VGP).
The information collected through VGP has both local and global
significance for IDPR. Virtual gateway connectivity information,
distributed to policy gateways within a single domain, aids those
policy gateways in selecting routes across and between virtual
gateways connecting their domain to adjacent domains. Inter-domain
connectivity information, distributed throughout an internetwork in
routing information messages, aids route servers in constructing
feasible policy routes.
Provided that a domain contains simple virtual gateway and transit
policy configurations, one need only implement a small subset of the
VGP functions. The connectivity among policy gateways within a
virtual gateway and the heterogeneity of transit policies within a
domain determine which VGP functions must be implemented, as we
explain toward the end of this section.
3.1. Message Scope
Policy gateways generate VGP messages containing information about
perceived changes in virtual gateway connectivity and distribute
these messages to other policy gateways within the same domain and
within the same virtual gateway. We classify VGP messages into three
distinct categories: "pair-PG", "intra-VG", and "inter-VG", depending
upon the scope of message distribution.
Policy gateways use CMTP for reliable transport of VGP messages. The
issuing policy gateway must communicate to CMTP the maximum number of
transmissions per VGP message, vgp_ret, and the interval between VGP
message retransmissions, vgp_int microseconds. The recipient policy
gateway must determine VGP message acceptability; conditions of
acceptability depend on the type of VGP message, as we describe
below.
Policy gateways store, act upon, and in the case of inter-VG
messages, forward the information contained in acceptable VGP
messages. VGP messages that pass the CMTP validation checks but fail
a specific VGP message acceptability check are considered to be
unacceptable and are hence discarded by recipient policy gateways. A
policy gateway that receives an unacceptable VGP message also logs
the event for network management.
3.1.1. Pair-PG Messages
Pair-PG message communication occurs between the two members of a
pair of adjacent, peer, or neighbor policy gateways. With IDPR, the
only pair-PG messages are those periodically generated by the up/down
protocol and used to monitor mutual reachability between policy
gateways.
A pair-PG message is "acceptable" if:
- It passes the CMTP validation checks.
- Its timestamp is less than vgp_old (300) seconds behind the
recipient's internal clock time.
- Its destination policy gateway identifier coincides with the
identifier of the recipient policy gateway.
- Its source policy gateway identifier coincides with the identifier
of a policy gateway configured for the recipient's domain or
associated virtual gateway.
3.1.2. Intra-VG Messages
Intra-VG message communication occurs between one policy gateway and
all of its peers. Whenever a policy gateway discovers that its
connectivity to an adjacent or neighbor policy gateway has changed,
it issues an intra-VG message indicating the connectivity change to
all of its reachable peers. Whenever a policy gateway detects that a
previously unreachable peer is now reachable, it issues, to that
peer, intra-VG messages indicating connectivity to adjacent and
neighbor policy gateways. If the issuing policy gateway fails to
receive an analogous intra-VG message from the newly reachable peer
within twice the configured VGP retransmission interval, vgp_int
microseconds, it actively requests the intra-VG message from that
peer. These message exchanges ensure that peers maintain a
consistent view of each others' connectivity to adjacent and neighbor
policy gateways.
An intra-VG message is "acceptable" if:
- It passes the CMTP validation checks.
- Its timestamp is less than vgp_old (300) seconds behind the
recipient's internal clock time.
- Its virtual gateway identifier coincides with that of a virtual
gateway configured for the recipient's domain.
3.1.3. Inter-VG Messages
Inter-VG message communication occurs between one policy gateway and
all of its neighbors. Whenever the lowest-numbered operational
policy gateway in a set of mutually reachable peers discovers that
its virtual gateway's connectivity to the adjacent domain or to
another virtual gateway has changed, it issues an inter-VG message
indicating the connectivity change to all of its neighbors.
Specifically, the policy gateway distributes an inter-VG message to a
"VG representative" policy gateway (see section 3.1.4 below) in each
virtual gateway in the domain. Each VG representative in turn
propagates the inter-VG message to each of its peers.
Whenever the lowest-numbered operational policy gateway in a set of
mutually peers detects that one or more previously unreachable peers
are now reachable, it issues, to the lowest-numbered operational
policy gateway in all other virtual gateways, requests for inter-VG
information indicating connectivity to adjacent domains and to other
virtual gateways. The recipient policy gateways return the requested
inter-VG messages to the issuing policy gateway, which in turn
distributes the messages to the newly reachable peers. These message
exchanges ensure that virtual gateways maintain a consistent view of
each others' connectivity, while consuming minimal domain resources
in distributing connectivity information.
An inter-VG message contains information about the entire virtual
gateway, not just about the issuing policy gateway. Thus, when
virtual gateway connectivity changes happen in rapid succession,
recipients of the resultant inter-VG messages should be able to
determine the most recent message and that message must contain the
current virtual gateway connectivity information. To ensure that the
connectivity information distributed is consistent and unambiguous,
we designate a single policy gateway, namely the lowest-numbered
operational peer, for generating and distributing inter-VG messages.
It is a simple procedure for a set of mutually reachable peers to
determine the lowest-numbered member, as we describe in section 3.2
below.
To understand why a single member of a virtual gateway must issue
inter-VG messages, consider the following example. Suppose that two
peers in a virtual gateway each detect a different connectivity
change and generate separate inter-VG messages. Recipients of these
messages may not be able to determine which message is more recent if
policy gateway internal clocks are not perfectly synchronized.
Moreover, even if the clocks were perfectly synchronized, and hence
message recency could be consistently determined, it is possible for
each peer to issue its inter-VG message before receiving current
information from the other. As a result, neither inter-VG message
contains the correct connectivity from the perspective of the virtual
gateway. However, these problems are eliminated if all inter-VG
messages are generated by a single peer within a virtual gateway, in
particular the lowest-numbered operational policy gateway.
An inter-VG message is "acceptable" if:
- It passes the CMTP validation checks.
- Its timestamp is less than vgp_old (300) seconds behind the
recipient's internal clock time.
- Its virtual gateway identifier coincides with that of a virtual
gateway configured for the recipient's domain.
- Its source policy gateway identifier represents the lowest numbered
operational member of the issuing virtual gateway, reachable from
the recipient.
Distribution of intra-VG messages among peers often triggers
generation and distribution of inter-VG messages among virtual
gateways. Usually, the lowest-numbered operational policy gateway in
a virtual gateway generates and distributes an inter-VG message
immediately after detecting a change in virtual gateway connectivity,
through receipt or generation of an intra-VG message. However, if
this policy gateway is also waiting for an intra-VG message from a
newly reachable peer, it does not immediately generate and distribute
the inter-VG message.
Waiting for intra-VG messages enables the lowest-numbered operational
policy gateway in a virtual gateway to gather the most recent
connectivity information for inclusion in the inter-VG message.
However, under unusual circumstances, the policy gateway may fail to
receive an intra-VG message from a newly reachable peer, even after
actively requesting such a message. To accommodate this case, VGP
uses an upper bound of four times the configured retransmission
interval, vgp_int microseconds, on the amount of time to wait before
generating and distributing an inter-VG message, when receipt of an
intra-VG message is pending.
3.1.4. VG Representatives
When distributing an inter-VG message, the issuing policy gateway
selects as recipients one neighbor, the VG Representative, from each
virtual gateway in the domain. To be selected as a VG
representative, a policy gateway must be reachable from the issuing
policy gateway via intra-domain routing. The issuing policy gateway
gives preference to neighbors that are members of more than one
virtual gateway. Such a neighbor acts as a VG representative for all
virtual gateways of which it is a member and restricts inter-VG
message distribution as follows: any policy gateway that is a peer in
more than one of the represented virtual gateways receives at most
one copy of the inter-VG message. This message distribution strategy
minimizes the number of message copies required for disseminating
inter-VG information.
3.2. Up/Down Protocol
Directly-connected adjacent policy gateways execute the Up/Down
Protocol to determine mutual reachability. Pairs of peer or neighbor
policy gateways can determine mutual reachability through information
provided by the intra-domain routing procedure or through execution
of the up/down protocol. In general, we do not recommend
implementing the up/down protocol between each pair of policy
gateways in a domain, as it results in O(n**2) (where n is the number
of policy gateways within the domain) communications complexity.
However, if the intra-domain routing procedure is slow to detect
connectivity changes or is unable to report reachability at the IDPR
entity level, the reachability information obtained through the
up/down protocol may well be worth the extra communications cost. In
the remainder of this section, we decribe the up/down protocol from
the perspective of adjacent policy gateways, but we note that the
identical protocol can be applied to peer and neighbor policy
gateways as well.
The up/down protocol determines whether the direct connection between
adjacent policy gateways is acceptable for data traffic transport. A
direct connection is presumed to be "down" (unacceptable for data
traffic transport) until the up/down protocol declares it to be "up"
(acceptable for data traffic transport). We say that a virtual
gateway is "up" if there exists at least one pair of adjacent policy
gateways whose direct connection is acceptable for data traffic
transport, and that a virtual gateway is "down" if there exists no
such pair of adjacent policy gateways.
When executing the up/down protocol, policy gateways exchange UP/DOWN
messages every ud_per (1) second. All policy gateways use the same
default period of ud_per initially and then negotiate a preferred
period through exchange of UP/DOWN messages. A policy gateway
reports its desired value for ud_per within its UP/DOWN messages. It
then chooses the larger of its desired value and that of the adjacent
policy gateway as the period for exchanging subsequent UP/DOWN
messages. Policy gateways also exchange, in UP/DOWN messages,
information about the identity of their respective domain components.
This information assists the policy gateways in selecting routes
across virtual gateways to partitioned domains.
Each UP/DOWN message is transported using CMTP and hence is covered
by the CMTP validation checks. However, unlike other IDPR control
messages, UP/DOWN messages do not require reliable transport.
Specifically, the up/down protocol requires only a single
transmission per UP/DOWN message and never directs CMTP to return an
ACK. As pair-PG messages, UP/DOWN messages are acceptable under the
conditions described in section 3.1.1.
Each policy gateway assesses the state of its direct connection, to
the adjacent policy gateway, by counting the number of acceptable
UP/DOWN messages received within a set of consecutive periods. A
policy gateway communicates its perception of the state of the direct
connection through its UP/DOWN messages. Initially, a policy gateway
indicates the down state in each of its UP/DOWN messages. Only when
the direct connection appears to be up from its perspective does a
policy gateway indicate the up state in its UP/DOWN messages.
A policy gateway can begin to transport data traffic over a direct
connection only if both of the following conditions are true:
- The policy gateway receives from the adjacent policy gateway at
least j acceptable UP/DOWN messages within the last m consecutive
periods. From the recipient policy gateway's perspective, this
event up. Hence, the recipient policy gateway indicates the up
state in its subsequent UP/DOWN messages.
- The UP/DOWN message most recently received from the adjacent policy
gateway indicates the up state, signifying that the adjacent policy
gateway considers the direct connection to be up.
A policy gateway must cease to transport data traffic over a direct
connection whenever either of the following conditions is true:
- The policy gateway receives from the adjacent policy gateway at
most acceptable UP/DOWN messages within the last n consecutive
periods.
- The UP/DOWN message most recently received from the adjacent policy
gateway indicates the down state, signifying that the adjacent
policy gateway considers the direct connection to be down.
From the recipient policy gateway's perspective, either of these
events constitutes a state transition of the direct connection from
up to down. Hence, the policy gateway indicates the down state in
its subsequent UP/DOWN messages.
3.3. Implementation
We recommend implementing the up/down protocol using a sliding
window. Each window slot indicates the UP/DOWN message activity
during a given period, containing either a "hit" for receipt of an
acceptable UP/DOWN message or a "miss" for failure to receive an
acceptable UP/DOWN message. In addition to the sliding window, the
implementation should include a tally of hits recorded during the
current period and a tally of misses recorded over the current
window.
When the direct connection moves to the down state, the initial
values of the up/down protocol parameters must be set as follows:
- The sliding window size is equal to m.
- Each window slot contains a miss.
- The current period hit tally is equal to 0.
- The current window miss tally is equal to m.
When the direct connection moves to the up state, the initial values
of the up/down protocol parameters must be set as follows:
- The sliding window size is equal to n.
- Each window slot contains a hit.
- The current period hit tally is equal to 0.
- The current window miss tally is equal to 0.
At the conclusion of each period, a policy gateway computes the miss
tally and determines whether there has been a state transition of the
direct connection to the adjacent policy gateway. In the down state,
a miss tally of no more than m - j signals a transition to the up
state. In the up state, a miss tally of no less than n - k signals a
transition to the down state.
Computing the correct miss tally involves several steps. First, the
policy gateway prepares to slide the window by one slot so that the
oldest slot disappears, making room for the newest slot. However,
before sliding the window, the policy gateway checks the contents of
the oldest window slot. If this slot contains a miss, the policy
gateway decrements the miss tally by 1, as this slot is no longer
part of the current window.
After sliding the window, the policy gateway determines the proper
contents. If the hit tally for the current period equals 0, the
policy gateway records a miss for the newest slot and increments the
miss tally by 1. Otherwise, if the hit tally for the current period
is greater than 0, the policy gateway records a hit for the newest
slot and decrements the hit tally by 1. Moreover, the policy gateway
applies any remaining hits to slots containing misses, beginning with
the newest and progressing to the oldest such slot. For each such
slot containing a miss, the policy gateway records a hit in that slot
and decrements both the hit and miss tallies by 1, as the hit cancels
out a miss. The policy gateway continues to apply each remaining hit
tallied to any slot containing a miss, until either all such hits are
exhausted or all such slots are accounted for. Before beginning the
next up/down period, the policy gateway resets the hit tally to 0.
Although we expect the hit tally, within any given period, to be no
greater than 1, we do anticipate the occasional period in which a
policy gateway receives more than one UP/DOWN message from an
adjacent policy gateway. The most common reasons for this occurrence
are message delay and clock drift. When an UP/DOWN message is
delayed, the receiving policy gateway observes a miss in one period
followed by two hits in the next period, one of which cancels the
previous miss. However, excess hits remaining in the tally after
miss cancellation indicate a problem, such as clock drift. Thus,
whenever a policy gateway accumulates excess hits, it logs the event
for network management.
When clock drift occurs between two adjacent policy gateways, it
causes the period of one policy gateway to grow with respect to the
period of the other policy gateway. Let p(X) be the period for PG X,
let p(Y) be the period for PG Y, and let g and h be the smallest
positive integers such that g * p(X) = h * p(Y). Suppose that p(Y) >
p(X) because of clock drift. In this case, PG X observes g - h
misses in g consecutive periods, while PG Y observes g - h surplus
hits in h consecutive periods. As long as (g - h)/g < (n - k)/n and
(g - h)/g < or = (m - j)/m, the clock drift itself will not cause the
direct connection to enter or remain in the down state.
3.4. Policy Gateway Connectivity
Policy gateways collect connectivity information through the intra-
domain routing procedure and through VGP, and they distribute
connectivity changes through VGP in both intra-VG messages to peers
and inter-VG messages to neighbors. Locally, this connectivity
information assists policy gateways in selecting routes, not only
across a virtual gateway to an adjacent domain but also across a
domain between two virtual gateways. Moreover, changes in
connectivity between domains are distributed, in routing information
messages, to route servers throughout an internetwork.
3.4.1. Within a Virtual Gateway
Each policy gateway within a virtual gateway constantly monitors its
connectivity to all adjacent and to all peer policy gateways. To
determine the state of its direct connection to an adjacent policy
gateway, a policy gateway uses reachability information supplied by
the up/down protocol. To determine the state of its intra-domain
routes to a peer policy gateway, a policy gateway uses reachability
information supplied by either the intra-domain routing procedure or
the up/down protocol.
A policy gateway generates a PG CONNECT message whenever either of
the following conditions is true:
- The policy gateway detects a change, in state or in adjacent
domain component, associated with its direct connection to an
adjacent policy gateway. In this case, the policy gateway
distributes a copy of the message to each peer reachable via
intra-domain routing.
- The policy gateway detects that a previously unreachable peer is
now reachable. In this case, the policy gateway distributes a
copy of the message to the newly reachable peer.
A PG CONNECT message is an intra-VG message that includes information
about each adjacent policy gateway directly connected to the issuing
policy gateway. Specifically, the PG CONNECT message contains the
adjacent policy gateway's identifier, status (reachable or
unreachable), and domain component identifier. If a PG CONNECT
message contains a "request", each peer that receives the message
responds to the sender with its own PG CONNECT message.
All mutually reachable peers monitor policy gateway connectivity
within their virtual gateway, through the up/down protocol, the
intra-domain routing procedure, and the exchange of PG CONNECT
messages. Within a given virtual gateway, each constituent policy
gateway maintains the following information about each configured
adjacent policy gateway:
- The identifier for the adjacent policy gateway.
- The status of the adjacent policy gateway: reachable/unreachable,
directly connected/not directly connected.
- The local exit interfaces used to reach the adjacent policy
gateway, provided it is reachable.
- The identifier for the adjacent policy gateway's domain component.
- The set of peers to which the adjacent policy gateway is
directly-connected.
Hence, all mutually reachable peers can detect changes in
connectivity across the virtual gateway to adjacent domain
components.
When the lowest-numbered operational peer policy gateway within a
virtual gateway detects a change in the set of adjacent domain
components reachable through direct connections across the given
virtual gateway, it generates a VGCONNECT message and distributes a
copy to a VG representative in all other virtual gateways connected
to its domain. A VG CONNECT message is an inter-VG message that
includes information about each peer's connectivity across the given
virtual gateway. Specifically, the VG CONNECT message contains, for
each peer, its identifier and the identifiers of the domain
components reachable through its direct connections to adjacent
policy gateways. Moreover, the VG CONNECT message gives each
recipient enough information to determine the state, up or down, of
the issuing virtual gateway.
The issuing policy gateway, namely the lowest-numbered operational
peer, may have to wait up to four times vgp_int microseconds after
detecting the connectivity change, before generating and distributing
the VGCONNECT message, as described in section 3.1.3. Each recipient
VG representative in turn distributes a copy of the VG CONNECT
message to each of its peers reachable via intra-domain routing. If
a VG CONNECT message contains a "request", then in each recipient
virtual gateway, the lowest-numbered operational peer that receives
the message responds to the original sender with its own VGCONNECT
message.
3.4.2. Between Virtual Gateways
At present, we expect transit policies to be uniform over all intra-
domain routes between any pair of policy gateways within a domain.
However, when tariffed qualities of service become prevalent
offerings for intra-domain routing, we can no longer expect
uniformity of transit policies throughout a domain. To monitor the
transit policies supported on intra-domain routes between virtual
gateways requires both a policy-sensitive intra-domain routing
procedure and a VGP exchange of policy information between neighbor
policy gateways.
Each policy gateway within a domain constantly monitors its
connectivity to all peer and neighbor policy gateways, including the
transit policies supported on intra-domain routes to these policy
gateways. To determine the state of its intra-domain connection to a
peer or neighbor policy gateway, a policy gateway uses reachability
information supplied by either the intra-domain routing procedure or
the up/down protocol. To determine the transit policies supported on
intra-domain routes to a peer or neighbor policy gateway, a policy
gateway uses policy-sensitive reachability information supplied by
the intra-domain routing procedure. We note that when transit
policies are uniform over a domain, reachability and policy-sensitive
reachability are equivalent.
Within a virtual gateway, each constituent policy gateway maintains
the following information about each configured peer and neighbor
policy gateway:
- The identifier for the peer or neighbor policy gateway.
- The identifiers corresponding to the transit policies configured to
be supported by intra-domain routes to the peer or neighbor policy
gateway.
- According to each transit policy, the status of the peer or
neighbor policy gateway: reachable/unreachable.
- For each transit policy, the local exit interfaces used to reach
the peer or neighbor policy gateway, provided it is reachable.
- The identifiers for the adjacent domain components reachable
through direct connections from the peer or neighbor policy
gateway, obtained through VG CONNECT messages.
Using this information, a policy gateway can detect changes in its
connectivity to an adjoining domain component, with respect to a
given transit policy and through a given neighbor. Moreover,
combining the information obtained for all neighbors within a given
virtual gateway, the policy gateway can detect changes in its
connectivity, with respect to a given transit policy, to that virtual
gateway and to adjoining domain components reachable through that
virtual gateway.
All policy gateways mutually reachable via intra-domain routes
supporting a configured transit policy need not exchange information
about perceived changes in connectivity, with respect to the given
transit policy. In this case, each policy gateway can infer
another's policy-sensitive reachability to a third, through mutual
intra-domain reachability information provided by the intra-domain
routing procedure. However, whenever two or more policy gateways are
no longer mutually reachable with respect to a given transit policy,
these policy gateways can no longer infer each other's reachability
to other policy gateways, with respect to that transit policy. In
this case, these policy gateways must exchange explicit information
about changes in connectivity to other policy gateways, with respect
to that transit policy.
A policy gateway generates a PG POLICY message whenever either of the
following conditions is true:
- The policy gateway detects a change in its connectivity to another
virtual gateway, with respect to a configured transit policy, or to
an adjoining domain component reachable through that virtual
gateway. In this case, the policy gateway distributes a copy of
the message to each peer reachable via intra-domain routing but not
currently reachable via any intra-domain routes of the given
transit policy.
- The policy gateway detects that a previously unreachable peer is
reachable. In this case, the policy gateway distributes a copy of
the message to the newly reachable peer.
A PG POLICY message is an intra-VG message that includes information
about each configured transit policy and each virtual gateway
configured to be reachable from the issuing policy gateway via
intra-domain routes of the given transit policy. Specifically, the
PGPOLICY message contains, for each configured transit policy:
- The identifier for the transit policy.
- The identifiers for the virtual gateways associated with the given
transit policy and currently reachable, with respect to that
transit policy, from the issuing policy gateway.
- The identifiers for the domain components reachable from and
adjacent to the members of the given virtual gateways.
If a PG POLICY message contains a "request", each peer that receives
the message responds to the original sender with its own PG POLICY
message.
In addition to connectivity between itself and its neighbors, each
policy gateway also monitors the connectivity, between domain
components adjacent to its virtual gateway and domain components
adjacent to other virtual gateways, through its domain and with
respect to the configured transit policies. For each member of each
of its virtual gateways, a policy gateway monitors:
- The set of adjacent domain components currently reachable
through direct connections across the given virtual gateway. The
policy gateway obtains this information through PG CONNECT messages
from reachable peers and through UP/DOWN messages from adjacent
policy gateways.
- For each configured transit policy, the set of virtual gateways
currently reachable from the given virtual gateway with respect to
that transit policy and the set of adjoining domain components
currently reachable through direct connections across those virtual
gateways. The policy gateway obtains this information through PG
POLICY messages from peers, VG CONNECT messages from neighbors, and
the intra-domain routing procedure. Using this information, a
policy gateway can detect connectivity changes, through its domain
and with respect to a given transit policy, between adjoining
domain components.
When the lowest-numbered operational policy gateway within a virtual
gateway detects a change in the connectivity between a domain
component adjacent to its virtual gateway and a domain component
adjacent to another virtual gateway in its domain, with respect to a
configured transit policy, it generates a VG POLICY message and
distributes a copy to a VG representative in selected virtual
gateways connected to its domain. In particular, the lowest-numbered
operational policy gateway distributes a VG POLICY message to a VG
representative in every other virtual gateway containing a member
reachable via intra-domain routing but not currently reachable via
any routes of the given transit policy. A VG POLICY message is an
inter-VG message that includes information about the connectivity
between domain components adjacent to the issuing virtual gateway and
domain components adjacent to the other virtual gateways in the
domain, with respect to configured transit policies. Specifically,
the VG POLICY message contains, for each transit policy:
- The identifier for the transit policy.
- The identifiers for the virtual gateways associated with the given
transit policy and currently reachable, with respect to that
transit policy, from the issuing virtual gateway.
- The identifiers for the domain components reachable from and
adjacent to the members of the given virtual gateways.
The issuing policy gateway, namely the lowest-numbered operational
peer, may have to wait up to four times vgp_int microseconds after
detecting the connectivity change, before generating and distributing
the VG POLICY message, as described in section 3.1.3. Each recipient
VG representative in turn distributes a copy of the VG POLICY message
to each of its peers reachable via intra-domain routing. If a VG
POLICY message contains a "request", then in each recipient virtual
gateway, the lowest-numbered operational peer that receives the
message responds to the original sender with its own VG POLICY
message.
3.4.3. Communication Complexity
We offer an example, to provide an estimate of the number of VGP
messages exchanged within a domain, AD X, after a detected change in
policy gateway connectivity. Suppose that an adjacent domain, AD Y,
partitions such that the partition is detectable through the exchange
of UP/DOWN messages across a virtual gateway connecting AD X and AD
Y. Let V be the number of virtual gateways in AD X. Suppose each
virtual gateway contains P peer policy gateways, and no policy
gateway is a member of multiple virtual gateways. Then, within AD X,
the detected partition will result in the following VGP message
exchanges:
- P policy gateways each receive at most P-1 PG CONNECT messages.
Each policy gateway detecting the adjacent domain partition
generates a PG CONNECT message and distributes it to each reachable
peer in the virtual gateway.
- P * (V-1) policy gateways each receive at most one VG CONNECT
message. The lowest-numbered operational policy gateway in the
virtual gateway detecting the partition of the adjacent domain
generates a VG CONNECT message and distributes it to a VG
representative in all other virtual gateways connected to the
domain. In turn, each VG representative distributes the VG CONNECT
message to each reachable peer within its virtual gateway.
- P * (V-1) policy gateways each receive at most P-1 PG POLICY
messages, and only if the domain has more than a single uniform
transit policy. Each policy gateway in each virtual gateway
generates a PG POLICY message and distributes it to all reachable
peers not currently reachable with respect to the given transit
policy.
- P * V policy gateways each receive at most V-1 VG POLICY messages,
only if the domain has more than a single uniform transit policy.
The lowest-numbered operational policy gateway in each virtual
gateway generates a VG POLICY message and distributes it to a VG
representative in all other virtual gateways containing at least
one reachable member not currently reachable with respect to the
given transit policy. In turn, each VG representative distributes
a VG POLICY message to each peer within its virtual gateway.
3.5. VGP Message Formats
The virtual gateway protocol number is equal to 0. We describe the
contents of each type of VGP message below.
3.5.1. UP/DOWN
The UP/DOWN message type is equal to 0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRC CMP | DST AD |
+-------------------------------+---------------+---------------+
| DST PG | PERIOD | STATE |
+-------------------------------+---------------+---------------+
SRC CMP
(16 bits) Numeric identifier for the domain component containing
the issuing policy gateway.
DST AD (16 bits) Numeric identifier for the destination domain.
DST PG (16 bits) Numeric identifier for the destination policy
gateway.
PERIOD (8 bits) Length of the UP/DOWN message generation period, in
seconds.
STATE (8 bits) Perceived state (1 up, 0 down) of the direct
connection from the perspective of the issuing policy gateway,
contained in the right-most bit.
3.5.2. PG CONNECT
The PG CONNECT message type is equal to 1. PG CONNECT messages are
not required for any virtual gateway containing exactly two policy
gateways.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ADJ AD | VG | RQST |
+-------------------------------+---------------+---------------+
| NUM RCH | NUM UNRCH |
+-------------------------------+-------------------------------+
For each reachable adjacent policy gateway:
+-------------------------------+-------------------------------+
| ADJ PG | ADJ CMP |
+-------------------------------+-------------------------------+
For each unreachable adjacent policy gateway:
+-------------------------------+
| ADJ PG |
+-------------------------------+
ADJ AD
(16 bits) Numeric identifier for the adjacent domain.
VG (8 bits) Numeric identifier for the virtual gateway.
RQST (8 bits) Request for a PG CONNECT message (1 request, 0 no
request) from each recipient peer, contained in the right-most
bit.
NUM RCH (16 bits) Number of adjacent policy gateways within the
virtual gateway, which are directly-connected to and currently
reachable from the issuing policy gateway.
NUM UNRCH (16 bits) Number of adjacent policy gateways within the
virtual gateway, which are directly-connected to but not
currently reachable from the issuing policy gateway.
ADJ PG (16 bits) Numeric identifier for a directly-connected adjacent
policy gateway.
ADJ CMP (16 bits) Numeric identifier for the domain component
containing the reachable, directly-connected adjacent policy
gateway.
3.5.3. PG POLICY
The PG POLICY message type is equal to 2. PG POLICY messages are not
required for any virtual gateway containing exactly two policy
gateways or for any domain with a single uniform transit policy.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ADJ AD | VG | RQST |
+-------------------------------+---------------+---------------+
| NUM TP |
+-------------------------------+
For each transit policy associated with the virtual gateway:
+-------------------------------+-------------------------------+
| TP | NUM VG |
+-------------------------------+-------------------------------+
For each virtual gateway reachable via the transit policy:
+-------------------------------+---------------+---------------+
| ADJ AD | VG | UNUSED |
+-------------------------------+---------------+---------------+
| NUM CMP | ADJ CMP |
+-------------------------------+-------------------------------+
ADJ AD
(16 bits) Numeric identifier for the adjacent domain.
VG (8 bits) Numeric identifier for the virtual gateway.
RQST (8 bits) Request for a PG POLICY message (1 request, 0 no
request) from each recipient peer, contained in the right-most
bit.
NUM TP (8 bits) Number of transit policies configured to include the
virtual gateway.
TP (16 bits) Numeric identifier for a transit policy associated with
the virtual gateway.
NUM VG (16 bits) Number of virtual gateways reachable from the
issuing policy gateway, via intra-domain routes supporting the
transit policy.
UNUSED (8 bits) Not currently used; must be set equal to 0.
NUM CMP (16 bits) Number of adjacent domain components reachable via
direct connections through the virtual gateway.
ADJ CMP (16 bits) Numeric identifier for a reachable adjacent domain
component.
3.5.4. VG CONNECT
The VG CONNECT message type is equal to 3.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ADJ AD | VG | RQST |
+-------------------------------+---------------+---------------+
| NUM PG |
+-------------------------------+
For each reach policy gateway in the virtual gateway:
+-------------------------------+-------------------------------+
| PG | NUM CMP |
+-------------------------------+-------------------------------+
| ADJ CMP |
+-------------------------------+
ADJ AD
(16 bits) Numeric identifier for the adjacent domain.
VG (8 bits) Numeric identifier for the virtual gateway.
RQST (8 bits) Request for a VG CONNECT message (1 request, 0 no
request) from a recipient in each virtual gateway, contained in
the right-most bit.
NUM PG (16 bits) Number of mutually-reachable peer policy gateways in
the virtual gateway.
PG (16 bits) Numeric identifier for a peer policy gateway.
NUM CMP (16 bits) Number of components of the adjacent domain
reachable via direct connections from the policy gateway.
ADJ CMP (16 bits) Numeric identifier for a reachable adjacent domain
component.
3.5.5. VG POLICY
The VG POLICY message type is equal to 4. VG POLICY messages are not
required for any domain with a single uniform transit policy.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ADJ AD | VG | RQST |
+-------------------------------+---------------+---------------+
| NUM TP |
+-------------------------------+
For each transit policy associated with the virtual gateway:
+-------------------------------+-------------------------------+
| TP | NUM GRP |
+-------------------------------+-------------------------------+
For each virtual gateway group reachable via the transit policy:
+-------------------------------+-------------------------------+
| NUM VG | ADJ AD |
+---------------+---------------+-------------------------------+
| VG | UNUSED | NUM CMP |
+---------------+---------------+-------------------------------+
| ADJ CMP |
+-------------------------------+
ADJ AD
(16 bits) Numeric identifier for the adjacent domain.
VG (8 bits) Numeric identifier for the virtual gateway.
RQST (8 bits) Request for a VG POLICY message (1 request, 0 no
request) from a recipient in each virtual gateway, contained in
the right-most bit.
NUM TP (16 bits) Number of transit policies configured to include the
virtual gateway.
TP (16 bits) Numeric identifier for a transit policy associated with
the virtual gateway.
NUM GRP (16 bits) Number of groups of virtual gateways, such that all
members in a group are reachable from the issuing virtual
gateway via intra-domain routes supporting the given transit
policy.
NUM VG (16 bits) Number of virtual gateways in a virtual gateway
group.
UNUSED (8 bits) Not currently used; must be set equal to 0.
NUM CMP (16 bits) Number of adjacent domain components reachable via
direct connections through the virtual gateway.
ADJ CMP (16 bits) Numeric identifier for a reachable adjacent domain
component.
Normally, each VG POLICY message will contain a single virtual
gateway group. However, if the issuing virtual gateway becomes
partitioned such that peers are mutually reachable with respect to
some transit policies but not others, virtual gateway groups may be
necessary. For example, let PG X and PG Y be two peers in VG A which
configured to support transit policies 1 and 2. Suppose that PG X
and PG Y are reachable with respect to transit policy 1 but not with
respect to transit policy 2. Furthermore, suppose that PG X can
reach members of VG B via intra-domain routes of transit policy 2 and
that PG Y can reach members of VG C via intra-domain routes of
transit policy 2. Then the entry in the VG POLICY message issued by
VG A will include, for transit policy 2, two groups of virtual
gateways, one containing VG B and one containing VG C.
3.5.6. Negative Acknowledgements
When a policy gateway receives an unacceptable VGP message that
passes the CMTP validation checks, it includes, in its CMTP ACK, an
appropriate VGP negative acknowledgement. This information is placed
in the INFORM field of the CMTP ACK (described previously in section
2.4); the numeric identifier for each type of VGP negative
acknowledgement is contained in the left-most 8 bits of the INFORM
field. Negative acknowledgements associated with VGP include the
following types:
1. Unrecognized VGP message type. Numeric identifier for the
unrecognized message type (8 bits).
2. Out-of-date VGP message.
3. Unrecognized virtual gateway source. Numeric identifier for the
unrecognized virtual gateway including the adjacent domain
identifier (16 bits) and the local identifier (8 bits).
4. Routing Information Distribution
Each domain participating in IDPR generates and distributes its
routing information messages to route servers throughout an
internetwork. IDPR routing information messages contain information
about the transit policies in effect across the given domain and the
virtual gateway connectivity to adjacent domains. Route servers in
turn use IDPR routing information to generate policy routes between
source and destination domains.
There are three different procedures for distributing IDPR routing
information:
- The flooding protocol. In this case, a representative policy
gateway in each domain floods its routing information messages to
route servers in all other domains.
- Remote route server communication. In this case, a route server
distributes its domain's routing information messages to route
servers in specific destination domains, by encapsulating these
messages within IDPR data messages. Thus, a domain administrator
may control the distribution of the domain's routing information by
restricting routing information exchange with remote route servers.
Currently, routing information distribution restrictions are not
included in IDPR configuration information.
- The route server query protocol. In this case, a policy gateway or
route server requests routing information from another route
server, which in turn responds with routing information from its
database. The route server query protocol may be used for quickly
updating the routing information maintained by a policy gateway
or route server that has just been connected or reconnected to an
internetwork. It may also be used to retrieve routing information
from domains that restrict distribution of their routing
information.
In this section, we describe the flooding protocol only. In section
5, we describe the route server query protocol, and in section 5.2,
we describe communication between route servers in separate domains.
Policy gateways and route servers use CMTP for reliable transport of
IDPR routing information messages flooded between peer, neighbor, and
adjacent policy gateways and between those policy gateways and route
servers. The issuing policy gateway must communicate to CMTP the
maximum number of transmissions per routing information message,
flood_ret, and the interval between routing information message
retransmissions, flood_int microseconds. The recipient policy
gateway or route server must determine routing information message
acceptability, as we describe in section 4.2.3 below.
4.1. AD Representatives
We designate a single policy gateway, the "AD representative", for
generating and distributing IDPR routing information about its
domain, to ensure that the routing information distributed is
consistent and unambiguous and to minimize the communication required
for routing information distribution. There is usually only a single
AD representative per domain, namely the lowest-numbered operational
policy gateway in the domain. Within a domain, policy gateways need
no explicit election procedure to determine the AD representative.
Instead, all members of a set of policy gateways mutually reachable
via intra-domain routes can agree on set membership and therefore on
which member has the lowest number.
A partitioned domain has as many AD representatives as it does domain
components. In fact, the numeric identifier for an AD representative
is identical to the numeric identifier for a domain component. One
cannot normally predict when and where a domain partition will occur,
and thus any policy gateway within a domain may become an AD
representative at any time. To prepare for the role of AD
representative in the event of a domain partition, every policy
gateway must continually monitor its domain's IDPR routing
information, through VGP and through the intra-domain routing
procedure.
4.2. Flooding Protocol
An AD representative policy gateway uses unrestricted flooding among
all domains to distribute its domain's IDPR routing information
messages to route servers in an internetwork. There are two kinds of
IDPR routing information messages issued by each AD representative:
CONFIGURATION and DYNAMIC messages. Each CONFIGURATION message
contains the transit policy information configured by the domain
administrator, including for each transit policy, its identifier, its
specification, and the sets of virtual gateways configured as
mutually reachable via intra-domain routes supporting the given
transit policy. Each DYNAMIC message contains information about
current virtual gateway connectivity to adjacent domains and about
the sets of virtual gateways currently mutually reachable via intra-
domain routes supporting the configured transit policies.
The IDPR Flooding Protocol is similar to the flooding procedures
described in [9]-[11]. Through flooding, the AD representative
distributes its routing information messages to route servers in its
own domain and in adjacent domains. After generating a routing
information message, the AD representative distributes a copy to each
of its peers and to a selected VG representative (see section 3.1.4)
in all other virtual gateways connected to the domain. Each VG
representative in turn distributes a copy of the routing information
message to each of its peers. We note that distribution of routing
information messages among virtual gateways and among peers within a
virtual gateway is identical to distribution of inter-VG messages in
VGP, as described in section 3.1.3.
Within a virtual gateway, each policy gateway distributes a copy of
the routing information message:
- To each route server in its configured set of route servers. A
domain administrator should ensure that each route server not
contained within a policy gateway appears in the set of configured
route servers for at least two distinct policy gateways. Hence,
such a route server will continue to receive routing information
messages, even when one of the policy gateways becomes unreachable.
However, the route server will normally receive duplicate copies of
a routing information message.
- To certain directly-connected adjacent policy gateways. A policy
gateway distributes a routing information message to a
directly-connected adjacent policy gateway in an adjacent domain
component, only when it is the lowest-numbered operational peer
with a direct connection to the given domain component. We note
that each policy gateway knows, through information provided by
VGP, which peers have direct connections to which components of
the adjacent domain. If the policy gateway has direct connections
to more than one adjacent policy gateway in that domain component,
it selects the routing information message recipient according to
the order in which the adjacent policy gateways appear in its
database, choosing the first one encountered. This selection
procedure ensures that a copy of the routing information message
reaches each component of the adjacent domain, while limiting the
number of copies distributed.
Once a routing information message reaches an adjacent policy
gateway, that policy gateway distributes copies of the message
throughout its domain. The adjacent policy gateway, acting as the
first recipient of the routing information message in its domain,
follows the same message distribution procedure as the AD
representative in the source domain, as described above. The
flooding procedure terminates when all reachable route servers in an
internetwork receive a copy of the routing information message.
Neighbor policy gateways may receive copies of the same routing
information message from different adjoining domains. If two
neighbor policy gateways receive the message copies simultaneously,
they will distribute them to VG representatives in other virtual
gateways within their domain, resulting in duplicate message
distribution. However, each policy gateway stops the spread of
duplicate routing information messages as soon as it detects them, as
described in section 4.2.3 below. In the Internet, we expect
simultaneous message receptions to be the exception rather than the
rule, given the hierarchical structure of the current topology.
4.2.1. Message Generation
An AD representative generates and distributes a CONFIGURATION
message whenever there is a configuration change in a transit policy
or virtual gateway associated with its domain. This ensures that
route servers maintain an up-to-date view of a domain's configured
transit policies and adjacencies. An AD representative may also
distribute a CONFIGURATION message at a configurable period of
conf_per (500) hours. A CONFIGURATION message contains, for each
configured transit policy, the identifier assigned by the domain
administrator, the specification, and the set of associated "virtual
gateway groups". Each virtual gateway group comprises virtual
gateways configured to be mutually reachable via intra-domain routes
of the given transit policy. Accompanying each virtual gateway
listed is an indication of whether the virtual gateway is configured
to be a domain entry point, a domain exit point, or both according to
the given transit policy. The CONFIGURATION message also contains
the set of local route servers that the domain administrator has
configured to be available to IDPR clients in other domains.
An AD representative generates and distributes a DYNAMIC message
whenever there is a change in currently supported transit policies or
in current virtual gateway connectivity associated with its domain.
This ensures that route servers maintain an up-to-date view of a
domain's supported transit policies and existing adjacencies and how
they differ from those configured for the domain. Specifically, an
AD representative generates a DYNAMIC message whenever there is a
change in the connectivity, through the given domain and with respect
to a configured transit policy, between two components of adjoining
domains. An AD representative may also distribute a DYNAMIC message
at a configurable period of dyn_per (24) hours. A DYNAMIC message
contains, for each configured transit policy, its identifier,
associated virtual gateway groups, and domain components reachable
through virtual gateways in each group. Each DYNAMIC message also
contains the set of currently "unavailable", either down or
unreachable, virtual gateways in the domain.
We note that each virtual gateway group expressed in a DYNAMIC
message may be a proper subset of one of the corresponding virtual
gateway groups expressed in a CONFIGURATION message. For example,
suppose that, for a given domain, the virtual gateway group (VG
A,...,VG E) were configured for a transit policy such that each
virtual gateway was both a domain entry and exit point. Thus, all
virtual gateways in this group are configured to be mutually
reachable via intra-domain routes of the given transit policy. Now
suppose that VG E becomes unreachable because of a power failure and
furthermore that the remaining virtual gateways form two distinct
groups, (VG A,VG B) and (VG C,VG D), such that although virtual
gateways in both groups are still mutually reachable via some intra-
domain routes they are no longer mutually reachable via any intra-
domain routes of the given transit policy. In this case, the virtual
gateway groups for the given transit policy now become (VG A,VG B)
and (VG C,VG D); VG E is listed as an unavailable virtual gateway.
A route server uses information about the set of unavailable virtual
gateways to determine which of its routes are no longer viable, and
it subsequently removes such routes from its route database.
Although route servers could determine the set of unavailable virtual
gateways using information about configured virtual gateways and
currently reachable virtual gateways, the associated processing cost
is high. In particular, a route server would have to examine all
virtual gateway groups listed in a DYNAMIC message to determine
whether there are any unavailable virtual gateways in the given
domain. To reduce the message processing at each route server, we
have chosen to include the set of unavailable virtual gateways in
each DYNAMIC message.
In order to construct a DYNAMIC message, an AD representative
assembles information gathered from intra-domain routing and from
VGP. Specifically, the AD representative uses the following
information:
- VG CONNECT and UP/DOWN messages to determine the state, up or down,
of each of its domain's virtual gateways and the adjacent domain
components reachable through a given virtual gateway.
- Intra-domain routing information to determine, for each of its
domain's transit policies, whether a given virtual gateway in the
domain is reachable with respect to that transit policy.
- VG POLICY messages to determine the connectivity of adjoining
domain components, across the given domain and with respect to a
configured transit policy, such that these components are adjacent
to virtual gateways not currently reachable from the AD
representative's virtual gateway according to the given transit
policy.
4.2.2. Sequence Numbers
Each IDPR routing information message carries a sequence number
which, when used in conjunction with the timestamp carried in the
CMTP message header, determines the recency of the message. An AD
representative assigns a sequence number to each routing information
message it generates, depending upon its internal clock time:
- The AD representative sets the sequence number to 0, if its
internal clock time is greater than the timestamp in its previously
generated routing information message.
- The AD representative sets the sequence number to 1 greater than
the sequence number in its previously generated routing information
message, if its internal clock time equals the timestamp for its
previously generated routing information message and if the
previous sequence number is less than the maximum value
(currently 2**16 - 1). If the previous sequence number equals the
maximum value, the AD representative waits until its internal clock
time exceeds the timestamp in its previously generated routing
information message and then sets the sequence number to 0.
In general, we do not expect generation of multiple distinct IDPR
routing information messages carrying identical timestamps, and so
the sequence number may seem superfluous. However, the sequence
number may become necessary during synchronization of an AD
representative's internal clock. In particular, the AD
representative may need to freeze the clock value during
synchronization, and thus distinct sequence numbers serve to
distinguish routing information messages generated during the clock
synchronization interval.
4.2.3. Message Acceptance
Prior to a policy gateway forwarding a routing information message or
a route server incorporating routing information into its routing
information database, the policy gateway or route server assesses
routing information message acceptability. An IDPR routing
information message is "acceptable" if:
- It passes the CMTP validation checks.
- Its timestamp is less than conf_old (530) hours behind the
recipient's internal clock time for CONFIGURATION messages and less
than dyn_old (25) hours behind the recipient's internal clock time
for DYNAMIC messages.
- Its timestamp and sequence number indicate that it is more recent
than the currently-stored routing information from the given
domain. If there is no routing information currently stored from
the given domain, then the routing information message contains, by
default, the more recent information.
Policy gateways acknowledge and forward acceptable IDPR routing
information messages, according to the flooding protocol described in
section 4.2 above. Moreover, each policy gateway retains the
timestamp and sequence number for the most recently accepted routing
information message from each domain and uses these values to
determine acceptability of routing information messages received in
the future. Route servers acknowledge the receipt of acceptable
routing information messages and incorporate the contents of these
messages into their routing information databases, contingent upon
criteria discussed in section 4.2.4 below; however, they do not
participate in the flooding protocol. We note that when a policy
gateway or route server first returns to service, it immediately
updates its routing information database with routing information
obtained from another route server, using the route server query
protocol described in section 5.
An AD representative takes special action upon receiving an
acceptable routing information message, supposedly generated by
itself but originally obtained from a policy gateway or route server
other than itself. There are at least three possible reasons for the
occurrence of this event:
- The routing information message has been corrupted in a way that is
not detectable by the integrity/authentication value.
- The AD representative has experienced a memory error.
- Some other entity is generating routing information messages on
behalf of the AD representative.
In any case, the AD representative logs the event for network
management. Moreover, the AD representative must reestablish its own
routing information messages as the most recent for its domain. To
do so, the AD representative waits until its internal clock time
exceeds the value of the timestamp in the received routing
information message and then generates a new routing information
message using the currently-stored domain routing information
supplied by VGP and by the intra-domain routing procedure. Note that
the length of time the AD representative must wait to generate the
new message is at most cmtp_new (300) seconds, the maximum CMTP-
tolerated difference between the received message's timestamp and the
AD representative's internal clock time.
IDPR routing information messages that pass the CMTP validity checks
but appear less recent than stored routing information are
unacceptable. Policy gateways do not forward unacceptable routing
information messages, and route servers do not incorporate the
contents of unacceptable routing information messages into their
routing information databases. Instead, the recipient of an
unacceptable routing information message acknowledges the message in
one of two ways:
- If the routing information message timestamp and sequence number
equal to the timestamp and sequence number associated with the
stored routing information for the given domain, the recipient
assumes that the routing information message is a duplicate and
acknowledges the message.
- If the routing information message timestamp and sequence number
indicate that the message is less recent than the stored routing
information for the domain, the recipient acknowledges the message
with an indication that the routing information it contains is
out-of-date. Such a negative acknowledgement is a signal to the
sender of the routing information message to request more up-to-
date routing information from a route server, using the route
server query protocol. Furthermore, if the recipient of the out-
of-date routing information message is a route server, it
regenerates a routing information message from its own routing
information database and forwards the message to the sender. The
sender may in turn propagate this more recent routing information
message to other policy gateways and route servers.
4.2.4. Message Incorporation
A route server usually stores the entire contents of an acceptable
IDPR routing information message in its routing information database,
so that it has access to all advertised transit policies when
generating a route and so that it can regenerate routing information
messages at a later point in time if requested to do so by another
route server or policy gateway. However, a route server may elect
not to store all routing information message contents. In
particular, the route server need not store any transit policy that
excludes the route server's domain as a source or any routing
information from a domain that the route server's domain's source
policies exclude for transit. Selective storing of routing
information message contents simplifies the route generation
procedure since it reduces the search space of possible routes, and
it limits the amount of route server memory devoted to routing
information. However, selective storing of routing information also
means that the route server cannot always regenerate the original
routing information message, if requested to do so by another route
server or policy gateway.
An acceptable IDPR routing information message may contain transit
policy information that is not well-defined according to the route
server's perception. A CONFIGURATION message may contain an
unrecognized domain, virtual gateway, or transit policy attribute,
such as user class access restrictions or offered service. In this
case, "unrecognized" means that the value in the routing information
message is not listed in the route server's configuration database,
as described previously in section 1.8.2. A DYNAMIC message may
contain an unrecognized transit policy or virtual gateway. In this
case, "unrecognized" means that the transit policy or virtual gateway
was not listed in the most recent CONFIGURATION message for the given
domain.
Each route server can always parse an acceptable routing information
messsage, even if some of the information is not well-defined, and
thus can always use the information that it does recognize.
Therefore, a route server can store the contents of acceptable
routing information messages from domains in which it is interested,
regardless of whether all contents appear to be well-defined at
present. If a routing message contains unrecognized information, the
route server may attempt to obtain the additional information it
needs to decipher the unrecognized information. For a CONFIGURATION
message, the route server logs the event for network management; for
a DYNAMIC message, the route server requests, from another route
server, the most recent CONFIGURATION message for the domain in
question.
When a domain is partitioned, each domain component has its own AD
representative, which generates routing information messages on
behalf of that component. Discovery of a domain partition prompts
the AD representative for each domain component to generate and
distribute a DYNAMIC message. In this case, a route server receives
and stores more than one routing information message at a time for
the given domain, namely one for each domain component.
When the partition heals, the AD representative for the entire domain
generates and distributes a DYNAMIC message. In each route server's
routing information database, the new DYNAMIC message does not
automatically replace all of the currently-stored DYNAMIC messages
for the given domain. Instead, the new message only replaces that
message whose AD representative matches the AD representative for the
new message. The other DYNAMIC messages, generated during the period
over which the partition occurred, remain in the routing information
database until they attain their maximum lifetime, as described in
section 4.2.5 below. Such stale information may lead to the
generation of routes that result in path setup failures and hence the
selection of alternative routes. To reduce the chances of path setup
failures, we will investigate, for a future version of IDPR,
mechanisms for removing partition-related DYNAMIC messages
immediately after a partition disappears.
4.2.5. Routing Information Database
We expect that most of the IDPR routing information stored in a
routing information database will remain viable for long periods of
time, perhaps until a domain reconfiguration occurs. By "viable", we
mean that the information reflects the current state of the domain
and hence may be used successfully for generating policy routes. To
reduce the probability of retaining stale routing information, a
route server imposes a maximum lifetime on each database entry,
initialized when it incorporates an accepted entry into its routing
information database. The maximum lifetime should be longer than the
corresponding message generation period, so that the database entry
is likely to be refreshed before it attains its maximum lifetime.
Each CONFIGURATION message stored in the routing information database
has a maximum lifetime of conf_old (530) hours; each DYNAMIC message
stored in the routing information database has a maximum lifetime of
dyn_old (25) hours. Periodic generation of routing information
messages makes it unlikely that any routing information message will
remain in a routing information database for its full lifetime.
However, a routing information message may attain its maximum
lifetime in a route server that is separated from a internetwork for
a long period of time.
When an IDPR routing information message attains its maximum lifetime
in a routing information database, the route server removes the
message contents from its database, so that it will not generate new
routes with the outdated routing information nor distribute old
routing information in response to requests from other route servers
or policy gateways. Nevertheless, the route server continues to
dispense routes previously generated with the old routing
information, as long as path setup (see section 7) for these routes
succeeds.
The route server treats routing information message lifetime
expiration differently, depending on the type of routing information
message. When a CONFIGURATION message expires, the route server
requests, from another route server, the most recent CONFIGURATION
message issued for the given domain. When a DYNAMIC message expires,
the route server does not initially attempt to obtain more recent
routing information. Instead, if route generation is necessary, the
route server uses the routing information contained in the
corresponding CONFIGURATION message for the given domain. Only if
there is a path setup failure (see section 7.4) involving the given
domain does the route server request, from another route server, the
most recent DYNAMIC message issued for the given domain.
4.3. Routing Information Message Formats
The flooding protocol number is equal to 1. We describe the contents
of each type of routing information message below.
4.3.1. CONFIGURATION
The CONFIGURATION message type is equal to 0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AD CMP | SEQ |
+-------------------------------+-------------------------------+
| NUM TP | NUM RS |
+-------------------------------+-------------------------------+
| RS |
+-------------------------------+
For each transit policy configured for the domain:
+-------------------------------+-------------------------------+
| TP | NUM ATR |
+-------------------------------+-------------------------------+
For each attribute of the transit policy:
+-------------------------------+-------------------------------+
| ATR TYP | ATR LEN |
+-------------------------------+-------------------------------+
For the source/destination access restrictions attribute:
+-------------------------------+
| NUM AD GRP |
+-------------------------------+
For each domain group in the source/destination access restrictions:
+-------------------------------+-------------------------------+
| NUM AD | AD |
+---------------+---------------+-------------------------------+
| AD FLGS | NUM HST | HST SET |
+---------------+---------------+-------------------------------+
For the temporal access restrictions attribute:
+-------------------------------+
| NUM TIM |
+-------------------------------+
For each set of times in the temporal access restrictions:
+---------------+-----------------------------------------------+
| TIM FLGS | DURATION |
+---------------+-----------------------------------------------+
| START |
+-------------------------------+-------------------------------+
| PERIOD | ACTIVE |
+-------------------------------+-------------------------------+
For the user class access restrictions attribute:
+-------------------------------+
| NUM UCI |
+-------------------------------+
For each UCI in the user class access restrictions:
+---------------+
| UCI |
+---------------+
For each offered service attribute:
+---------------------------------------------------------------+
| OFR SRV |
+---------------------------------------------------------------+
For the virtual gateway access restrictions attribute:
+-------------------------------+
| NUM VG GRP |
+-------------------------------+
For each virtual gateway group in the virtual gateway access
restrictions:
+-------------------------------+-------------------------------+
| NUM VG | ADJ AD |
+---------------+---------------+-------------------------------+
| VG | VG FLGS |
+---------------+---------------+
AD CMP
(16 bits) Numeric identifier for the domain component containing
the AD representative policy gateway.
SEQ (16 bits) Routing information message sequence number.
NUM TP (16 bits) Number of transit policy specifications contained in
the routing information message.
NUM RS (16 bits) Number of route servers advertised to serve clients
outside of the domain.
RS (16 bits) Numeric identifier for a route server.
TP (16 bits) Numeric identifier for a transit policy specification.
NUM ATR (16 bits) Number of attributes associated with the transit
policy.
ATR TYP (16 bits) Numeric identifier for a type of attribute. Valid
attributes include the following types:
- Set of virtual gateway access restrictions (see section 1.4.2)
associated with the transit policy (variable). This attribute must
be included.
- Set of source/destination access restrictions (see section 1.4.2)
associated with the transit policy (variable). This attribute may
be omitted. Absence of this attribute implies that traffic from
any source to any destination is acceptable.
- Set of temporal access restrictions (see section 1.4.2) associated
with the transit policy (variable). This attribute may be omitted.
Absence of this attribute implies that the transit policy applies
at all times.
- Set of user class access restrictions (see section 1.4.2)
associated with the transit policy (variable). This attribute may
be omitted. Absence of this attribute implies that traffic from
any user class is acceptable.
- Average delay in milliseconds (16 bits). This attribute may be
omitted.
- Delay variation in milliseconds (16 bits). This attribute may be
omitted.
- Average available bandwidth in bits per second (48 bits). This
attribute may be omitted.
- Available bandwidth variation in bits per second (48 bits). This
attribute may be omitted.
- MTU in bytes (16 bits). This attribute may be omitted.
- Charge per byte in thousandths of a cent (16 bits). This attribute
may be omitted.
- Charge per message in thousandths of a cent (16 bits). This
attribute may be omitted.
- Charge for session time in thousandths of a cent per second (16
bits). This attribute may be omitted. Absence of any charge
attribute implies that the domain provides free transit service.
ATR LEN (16 bits) Length of an attribute in bytes, beginning with the
subsequent field.
NUM AD GRP (16 bits) Number of source/destination domain groups (see
section 1.4.2) associated with the source/destination access
restrictions.
NUM AD (16 bits) Number of domains or sets of domains in a domain
group.
AD (16 bits) Numeric identifier for a domain or domain set.
AD FLGS (8 bits) Set of five flags indicating how to interpret the AD
field, contained in the right-most bits. Proceeding left to right,
the first flag indicates whether the transit policy applies to all
domains or to specific domains (1 all, 0 specific), and when set to
1, causes the second and third flags to be ignored. The second flag
indicates whether the domain identifier signifies a single domain or
a domain set (1 single, 0 set). The third flag indicates whether the
transit policy applies to the given domain or domain set (1 applies,
0 does not apply) and is used for representing complements of sets of
domains. The fourth flag indicates whether the domain is a source (1
source, 0 not source). The fifth flag indicates whether the domain
is a destination (1 destination, 0 not destination). At least one of
the fourth and fifth flags must be set to 1.
NUM HST (8 bits) Number of "host sets" (see section 1.4.2) associated
with a particular domain or domain set. The value 0 indicates that
all hosts in the given domain or domain set are acceptable sources or
destinations, as specified by the fourth and fifth AD flags.
HST SET (16 bits) Numeric identifier for a host set.
NUM TIM (16 bits) Number of time specifications associated with the
temporal access restrictions. Each time specification is split into
a set of continguous identical periods, as we describe below.
TIM FLGS (8 bits) Set of two flags indicating how to combine the time
specifications, contained in the right-most bits. Proceeding left to
right, the first flag indicates whether the transit policy applies
during the periods specified in the time specification (1 applies, 0
does not apply) and is used for representing complements of policy
applicability intervals. The second flag indicates whether the time
specification takes precedence over the previous time specifications
listed (1 precedence, 0 no precedence). Precedence is equivalent to
the boolean OR and AND operators, in the following sense. At any
given instant, a transit policy either applies or does not apply,
according to a given time specification, and we can assign a boolean
value to the state of policy applicability according to a given time
specification. If the second flag assumes the value 1 for a given
time specification, that indicates the boolean operator OR should be
applied to the values of policy applicability, according to the given
time specification and to all previously listed time specifications.
If the second flag assumes the value 0 for a given time
specification, that indicates the boolean operator AND should be
applied to the values of policy applicability, according to the given
time specification and to all previously listed time specifications.
DURATION (24 bits) Length of the time specification duration, in
minutes. A value of 0 indicates an infinite duration.
START (32 bits) Time at which the time specification first takes
effect, in seconds elapsed since 1 January 1970 0:00 GMT.
PERIOD (16 bits) Length of each time period within the time
specification, in minutes.
ACTIVE (16 bits) Length of the policy applicable interval during each
time period, in minutes from the beginning of the time period.
NUM UCI (16 bits) Number of user classes associated with the user
class access restrictions.
UCI (8 bits) Numeric identifier for a user class.
NUM VG GRP (16 bits) Number of virtual gateway groups (see section
1.4.2) associated with the virtual gateway access restrictions.
NUM VG (16 bits) Number of virtual gateways in a virtual gateway
group.
ADJ AD (16 bits) Numeric identifier for the adjacent domain to which
a virtual gateway connects.
VG (8 bits) Numeric identifier for a virtual gateway.
VG FLGS (8 bits) Set of two flags indicating how to interpret the VG
field, contained in the right-most bits. Proceeding left to right,
the first flag indicates whether the virtual gateway is a domain
entry point (1 entry, 0 not entry). The second flag indicates
whether the virtual gateway is a domain exit point (1 exit, 0 not
exit). At least one of the first and second flags must be set to 1.
4.3.2. DYNAMIC
The DYNAMIC message type is equal to 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AD CMP | SEQ |
+-------------------------------+-------------------------------+
| UNAVL VG | NUM PS |
+-------------------------------+-------------------------------+
For each unavailable virtual gateway in the domain:
+-------------------------------+---------------+---------------+
| ADJ AD | VG | UNUSED |
+-------------------------------+---------------+---------------+
For each set of transit policies for the domain:
+-------------------------------+-------------------------------+
| NUM TP | NUM VG GRP |
+-------------------------------+-------------------------------+
| TP |
+-------------------------------+
For each virtual gateway group associated with the transit policy
set:
+-------------------------------+-------------------------------+
| NUM VG | ADJ AD |
+---------------+---------------+-------------------------------+
| VG | VG FLGS | NUM CMP |
+---------------+---------------+-------------------------------+
| ADJ CMP |
+-------------------------------+
AD CMP
(16 bits) Numeric identifier for the domain component containing
the AD representative policy gateway.
SEQ (16 bits) Routing information message sequence number.
UNAVL VG (16 bits) Number of virtual gateways in the domain component
that are currently unavailable via any intra-domain routes.
NUM PS (16 bits) Number of sets of transit policies listed. Transit
policy sets provide a mechanism for reducing the size of DYNAMIC
messages. A single set of virtual gateway groups applies to all
transit policies in a given set.
ADJ AD (16 bits) Numeric identifier for the adjacent domain to which
a virtual gateway connects.
VG (8 bits) Numeric identifier for a virtual gateway.
UNUSED (8 bits) Not currently used; must be set equal to 0.
NUM TP (16 bits) Number of transit policies in a set.
NUM VGGRP (16 bits) Number of virtual gateway groups currently
associated with the transit policy set.
TP (16 bits) Numeric identifier for a transit policy.
NUM VG (16 bits) Number of virtual gateways in a virtual gateway
group.
VG FLGS (8 bits) Set of two flags indicating how to interpret the VG
field, contained in the right-most bits. Proceeding left to
right, the first flag indicates whether the virtual gateway is a
domain entry point (1 entry, 0 not entry). The second flag
indicates whether the virtual gateway is a domain exit point (1
exit, 0 not exit). At least one of the first and second flags
must be set to 1.
NUM CMP (16 bits) Number of adjacent domain components reachable via
direct connections through the virtual gateway.
ADJ CMP (16 bits) Numeric identifier for a reachable adjacent domain
component.
4.3.3. Negative Acknowledgements
When a policy gateway or route server receives an unacceptable IDPR
routing information message that passes the CMTP validation checks,
it includes, in its CMTP ACK, an appropriate negative
acknowledgement. This information is placed in the INFORM field of
the CMTP ACK (described previously in section 2.4); the numeric
identifier for each type of routing information message negative
acknowledgement is contained in the left-most 8 bits of the INFORM
field. Negative acknowledgements associated with routing information
messages include the following types:
1. Unrecognized IDPR routing information message type. Numeric
identifier for the unrecognized message type (8 bits).
2. Out-of-date IDPR routing information message. This is a signal
to the sender that it may not have the most recent routing
information for the given domain.
5. Route Server Query Protocol
Each route server is responsible for maintaining both the routing
information database and the route database and for responding to
database information requests from policy gateways and other route
servers. These requests and their responses are the messages
exchanged via the Route Server Query Protocol (RSQP).
Policy gateways and route servers normally invoke RSQP to replace
absent, outdated, or corrupted information in their own routing
information or route databases. In section 4, we discussed some of
the situations in which RSQP may be invoked; in this section and in
section 7, we discuss other such situations.
5.1. Message Exchange
Policy gateways and route servers use CMTP for reliable transport of
route server requests and responses. RSQP must communicate to CMTP
the maximum number of transmissions per request/response message,
rsqp_ret, and the interval between request/response message
retransmissions, rsqp_int microseconds. A route server
request/response message is "acceptable" if:
- It passes the CMTP validation checks.
- Its timestamp is less than rsqp_old (300) seconds behind the
recipient's internal clock time.
With RSQP, a requesting entity expects to receive an acknowledgement
from the queried route server indicating whether the route server can
accommodate the request. The route server may fail to fill a given
request for either of the following reasons:
- Its corresponding database contains no entry or only a partial
entry for the requested information.
- It is governed by special message distribution rules, imposed by
the domain administrator, that preclude it from releasing the
requested information. Currently, such distribution rules are not
included in IDPR configuration information.
For all requests that it cannot fill, the route server responds with
a negative acknowledgement message carried in a CMTP acknowledgement,
indicating the set of unfulfilled requests (see section 5.5.4).
If the requesting entity either receives a negative acknowledgement
or does not receive any acknowledgement after rsqp_ret attempts
directed at the same route server, it queries a different route
server, as long as the number of attempted requests to different
route servers does not exceed rsqp_try (3). Specifically, the
requesting entity proceeds in round-robin order through its list of
addressable route servers. However, if the requesting entity is
unsuccessful after rsqp_try attempts, it abandons the request
altogether and logs the event for network management.
A policy gateway or a route server can request information from any
route server that it can address. Addresses for local route servers
within a domain are part of the configuration for each IDPR entity
within a domain; addresses for remote route servers in other domains
are obtained through flooded CONFIGURATION messages, as described
previously in section 4.2.1. However, requesting entities always
query local route servers before remote route servers, in order to
contain the costs associated with the query and response. If the
requesting entity and the queried route server are in the same
domain, they can communicate over intra-domain routes, whereas if the
requesting entity and the queried route server are in different
domains, they must obtain a policy route and establish a path before
they can communicate, as we describe below.
5.2. Remote Route Server Communication
RSQP communication involving a remote route server requires a policy
route and accompanying path setup (see section 7) between the
requesting and queried entities, as these entities reside in
different domains. After generating a request message, the
requesting entity hands to CMTP its request message along with the
remote route server's entity and domain identifiers. CMTP encloses
the request in a DATAGRAM and hands the DATAGRAM and remote route
server information to the path agent. Using the remote route server
information, the path agent obtains, and if necessary sets up, a path
to the remote route server. Once the path to the remote route server
has been successfully established, the path agent encapsulates the
DATAGRAM within an IDPR data message and forwards the data message
along the designated path.
When the path agent in the remote route server receives the IDPR data
message, it extracts the DATAGRAM and hands it to CMTP. In addition,
the path agent, using the requesting entity and domain identifiers
contained in the path identifier, obtains, and if necessary sets up,
a path back to the requesting entity.
If the DATAGRAM fails any of the CMTP validation checks, CMTP returns
a NAK to the requesting entity. If the DATAGRAM passes all of the
CMTP validation checks, the remote route server assesses the
acceptability of the request message. Provided the request message
is acceptable, the remote route server determines whether it can
fulfill the request and directs CMTP to return an ACK to the
requesting entity. The ACK may contain a negative acknowledgement if
the entire request cannot be fulfilled.
The remote route server generates responses for all requests that it
can fulfill and returns the responses to the requesting entity.
Specifically, the remote route server hands to CMTP its response and
the requesting entity information. CMTP in turn encloses the
response in a DATAGRAM.
When returning an ACK, a NAK, or a response to the requesting entity,
the remote route server hands the corresponding CMTP message and
requesting entity information to the path agent. Using the
requesting entity information, the path agent retrieves the path to
the requesting entity, encapsulates the CMTP message within an IDPR
data message, and forwards the data message along the designated
path.
When the path agent in the requesting entity receives the IDPR data
message, it extracts the ACK, NAK, or response to its request and
performs the CMTP validation checks for that message. In the case of
a response messsage, the requesting entity also assesses message
acceptability before incorporating the contents into the appropriate
database.
5.3 Routing Information
Policy gateways and route servers request routing information from
route servers, in order to update their routing information
databases. To obtain routing information from a route server, the
requesting entity issues a ROUTING INFORMATION REQUEST message
containing the type of routing information requested - CONFIGURATION
messages, DYNAMIC messages, or both - and the set of domains from
which the routing information is requested.
Upon receiving a ROUTING INFORMATION REQUEST message, a route server
first assesses message acceptability before proceeding to act on the
contents. If the ROUTING INFORMATION REQUEST message is deemed
acceptable, the route server determines how much of the request it
can fulfill and then instructs CMTP to generate an acknowledgement,
indicating its ability to fulfill the request. The route server
proceeds to fulfill as much of the request as possible by
reconstructing individual routing information messages, one per
requested message type and domain, from its routing information
database. We note that only a regenerated routing information
message whose entire contents match that of the original routing
information message may pass the CMTP integrity/authentication
checks.
5.4. Routes
Path agents request routes from route servers when they require
policy routes for path setup. To obtain routes from a route server,
the requesting path agent issues a ROUTE REQUEST message containing
the destination domain and applicable service requirements, the
maximum number of routes requested, a directive indicating whether to
generate the routes or retrieve them from the route database, and a
directive indicating whether to refresh the routing information
database with the most recent CONFIGURATION or DYNAMIC message from a
given domain, before generating the routes. To refresh its routing
information database, a route server must obtain routing information
from another route server. The path agent usually issues routing
information database refresh directives in response to a failed path
setup. We discuss the application of these directives in more detail
in section 7.4.
Upon receiving a ROUTE REQUEST message, a route server first assesses
message acceptability before proceeding to act on the contents. If
the ROUTE REQUEST message is deemed acceptable, the route server
determines whether it can fulfill the request and then instructs CMTP
to generate an acknowledgement, indicating its ability to fulfill the
request. The route server proceeds to fulfill the request with
policy routes, either retrieved from its route database or generated
from its routing information database if necessary, and returns these
routes in a ROUTE RESPONSE message.
5.5. Route Server Message Formats
The route server query protocol number is equal to 2. We describe
the contents of each type of RSQP message below.
5.5.1. ROUTING INFORMATION REQUEST
The ROUTING INFORMATION REQUEST message type is equal to 0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QRY AD | QRY RS |
+-------------------------------+-------------------------------+
| NUM AD | AD |
+---------------+---------------+-------------------------------+
| RIM FLGS | UNUSED |
+---------------+---------------+
QRY AD
(16 bits) Numeric identifier for the domain containing the
queried route server.
QRY RS (16 bits) Numeric identifier for the queried route server.
NUM AD (16 bits) Number of domains about which routing information is
requested. The value 0 indicates a request for routing
information from all domains.
AD (16 bits) Numeric identifier for a domain. This field is absent
when NUM AD equals 0.
RIM FLGS (8 bits) Set of two flags indicating the type of routing
information messages requested, contained in the right-most
bits. Proceeding left to right, the first flag indicates
whether the request is for a CONFIGURATION message (1
CONFIGURATION, 0 no CONFIGURATION). The second flag indicates
whether the request is for a DYNAMIC message (1 DYNAMIC, 0 no
DYNAMIC). At least one of the first and second flags must be
set to 1.
UNUSED (8 bits) Not currently used; must be set equal to 0.
5.5.2. ROUTE REQUEST
The ROUTE REQUEST message type is equal to 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QRY AD | QRY RS |
+-------------------------------+-------------------------------+
| SRC AD | HST SET |
+---------------+---------------+-------------------------------+
| UCI | UNUSED | NUM RQS |
+---------------+---------------+-------------------------------+
| DST AD | PRX AD |
+---------------+---------------+-------------------------------+
| NUM RTS | GEN FLGS | RFS AD |
+---------------+---------------+-------------------------------+
| NUM AD |
+-------------------------------+
For each domain to be favored, avoided, or excluded:
+-------------------------------+---------------+---------------+
| AD | AD FLGS | UNUSED |
+-------------------------------+---------------+---------------+
For each requested service:
+-------------------------------+-------------------------------+
| RQS TYP | RQS LEN |
+-------------------------------+-------------------------------+
| RQS SRV |
+---------------------------------------------------------------+
QRY AD
(16 bits) Numeric identifier for the domain containing the
queried route server.
QRY RS (16 bits) Numeric identifier for the queried route server.
SRC AD (16 bits) Numeric identifier for the route's source domain.
HST SET (16 bits) Numeric identifier for the source's host set.
UCI (8 bits) Numeric identifier for the source user class. The value
0 indicates that there is no particular source user class.
UNUSED (8 bits) Not currently used; must be set equal to 0.
NUM RQS (16 bits) Number of requested services. The value 0
indicates that the source requests no special services.
DST AD (16 bits) Numeric identifier for the route's destination
domain.
PRX AD (16 bits) Numeric identifier for the destination domain's
proxy (see section 1.3.1). If the destination domain provides
the path agent function for its hosts, then the destination and
proxy domains are identical. A route server constructs routes
between the source domain's proxy and the destination domain's
proxy. We note that the source domain's proxy is identical to
the domain issuing the CMTP message containing the ROUTE REQUEST
message, and hence available in the CMTP header.
NUM RTS (8 bits) Number of policy routes requested.
GEN FLGS (8 bits) Set of three flags indicating how to obtain the
requested routes, contained in the right-most bits. Proceeding
left to right, the first flag indicates whether the route server
should retrieve existing routes from its route database or
generate new routes (1 retrieve, 0 generate). The second flag
indicates whether the route server should refresh its routing
information database before generating the requested routes (1
refresh, 0 no refresh) and when set to 1, causes the third flag
and the RFS AD field to become significant. The third flag
indicates whether the routing information database refresh
should include CONFIGURATION messages or DYNAMIC messages (1
configuration, 0 dynamic).
RFS AD (16 bits) Numeric identifier for the domain for which routing
information should be refreshed. This field is meaningful only
if the second flag in the GEN FLGS field is set to 1.
NUM AD (16 bits) Number of transit domains that are to be favored,
avoided, or excluded during route selection (see section 1.4.1).
AD (16 bits) Numeric identifier for a transit domain to be favored,
avoided, or excluded.
AD FLGS (8 bits) Three flags indicating how to interpret the AD
field, contained in the right-most bits. Proceeding left to
right, the first flag indicates whether the domain should be
favored (1 favored, 0 not favored). The second flag indicates
whether the domain should be avoided (1 avoided, 0 not avoided).
The third flag indicates whether the domain should be excluded
(1 excluded, 0 not excluded). No more than one of the first,
second, and third flags must set to 1.
RQS TYP (16 bits) Numeric identifier for a type of requested service.
Valid requested services include the following types:
1. Upper bound on delay, in milliseconds (16 bits). This attribute
may be omitted.
2. Minimum delay route. This attribute may be omitted.
3. Upper bound on delay variation, in milliseconds (16 bits). This
attribute may be omitted.
4. Minimum delay variation route. This attribute may be omitted.
5. Lower bound on bandwidth, in bits per second (48 bits). This
attribute may be omitted.
6. Maximum bandwidth route. This attribute may be omitted.
7. Upper bound on monetary cost, in cents (32 bits). This attribute
may be omitted.
8. Minimum monetary cost route. This attribute may be omitted.
9. Path lifetime in minutes (16 bits). This attribute may be omitted
but must be present if types 7 or 8 are present. Route servers
use path lifetime information together with domain charging
method to compute expected session monetary cost over a given
domain.
10. Path lifetime in messages (16 bits). This attribute may be
omitted but must be present if types 7 or 8 are present.
11. Path lifetime in bytes (48 bits). This attribute may be omitted
but must be present if types 7 or 8 are present.
RQS LEN
(16 bits) Length of the requested service, in bytes, beginning
with the next field.
RQS SRV
(variable) Description of the requested service.
5.5.3. ROUTE RESPONSE
The ROUTE RESPONSE message type is equal to 2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NUM RTS |
+---------------+
For each route provided:
+---------------+---------------+
| NUM AD | RTE FLGS |
+---------------+---------------+
For each domain in the route:
+---------------+---------------+-------------------------------+
| AD LEN | VG | ADJ AD |
+---------------+---------------+-------------------------------+
| ADJ CMP | NUM TP |
+-------------------------------+-------------------------------+
| TP |
+-------------------------------+
NUM RTS
(16 bits) Number of policy routes provided.
RTE FLGS (8 bits) Set of two flags indicating the directions in which
a route can be used, contained in the right-most bits. Refer to
sections 6.2, 7, and 7.2 for detailed discussions of path
directionality. Proceeding left to right, the first flag
indicates whether the route can be used from source to
destination (1 from source, 0 not from source). The second flag
indicates whether the route can be used from destination to
source (1 from destination, 0 not from destination). At least
one of the first and second flags must be set to 1, if NUM RTS
is greater than 0.
NUM AD (8 bits) Number of domains in the policy route, not including
the first domain on the route.
AD LEN (8 bits) Length of the information associated with a
particular domain, in bytes, beginning with the next field.
VG (8 bits) Numeric identifier for an exit virtual gateway.
ADJ AD (16 bits) Numeric identifier for the adjacent domain connected
to the virtual gateway.
ADJ CMP (16 bits) Numeric identifier for the adjacent domain
component. Used by policy gateways to select a route across a
virtual gateway connecting to a partitioned domain.
NUM TP (16 bits) Number of transit policies that apply to the section
of the route traversing the domain component.
TP (16 bits) Numeric identifier for a transit policy.
5.5.4. Negative Acknowledgements
When a policy gateway receives an unacceptable RSQP message that
passes the CMTP validation checks, it includes, in its CMTP ACK, an
appropriate negative acknowledgement. This information is placed in
the INFORM field of the CMTP ACK (described previously in section
2.4); the numeric identifier for each type of RSQP negative
acknowledgement is contained in the left-most 8 bits of the INFORM
field. Negative acknowledgements associated with RSQP include the
following types:
1. Unrecognized RSQP message type. Numeric identifier for the
unrecognized message type (8 bits).
2. Out-of-date RSQP message.
3. Unable to fill requests for routing information from the
following domains. Number of domains for which requests cannot
be filled (16 bits); a value of 0 indicates that the route
server cannot fill any of the requests. Numeric identifier for
each domain for which a request cannot be filled (16 bits).
4. Unable to fill requests for routes to the following destination
domain. Numeric identifier for the destination domain (16 bits).
6. Route Generation
Route generation is the most computationally complex part of IDPR,
because of the number of domains and the number and heterogeneity of
policies that it must accommodate. Route servers must generate
policy routes that satisfy the requested services of the source
domains and respect the offered services of the transit domains.
We distinguish requested qualities of service and route generation
with respect to them as follows:
- Requested service limits include upper bounds on route delay, route
delay variation, and session monetary cost and lower bounds on
available route bandwidth. Generating a route that must satisfy
more than one quality of service constraint, for example route delay
of no more than X seconds and available route bandwidth of no less
than Y bits per second, is an NP-complete problem.
- Optimal requested services include minimum route delay, minimum
route delay variation, minimum session monetary cost, and maximum
available route bandwidth. In the worst case, the computational
complexity of generating a route that is optimal with respect to a
given requested service is O((N + L) log N) for Dijkstra's shortest
path first (SPF) search and O(N + (L * L)) for breadth-first (BF)
search, where N is the number of nodes and L is the number of links
in the search graph. Multi-criteria optimization, for example
finding a route with minimal delay variation and minimal session
monetary cost, may be defined in several ways. One approach to
multi-criteria optimization is to assign each link a single value
equal to a weighted sum of the values of the individual offered
qualities of service and generate a route that is optimal with
respect to this new criterion. However, selecting the weights that
yield the desired route generation behavior is itself an
optimization procedure and hence not trivial.
To help contain the combinatorial explosion of processing and memory
costs associated with route generation, we supply the following
guidelines for generation of suitable policy routes:
- Each route server should only generate policy routes from the
perspective of its own domain as source; it need not generate policy
routes for arbitrary source/destination domain pairs. Thus, we can
distribute the computational burden over all route servers.
- Route servers should precompute routes for which they anticipate
requests and should generate routes on demand only in order to
satisfy unanticipated route requests. Hence, a single route server
can distribute its computational burden over time.
- Route servers should cache the results of route generation, in order
to minimize the computation associated with responding to future
route requests.
- To handle requested service limits, a route server should always
select the first route generated that satisfies all of the requested
service limits.
- To handle multi-criteria optimization in route selection, a route
server should generate routes that are optimal with respect to the
first optimal requested service listed in the ROUTE REQUEST message.
The route server should resolve ties between otherwise equivalent
routes by evaluating these routes according to the other optimal
requested services contained in the ROUTE REQUEST message, in the
order in which they are listed. With respect to the route server's
routing information database, the selected route is optimal
according to the first optimal requested service listed in the ROUTE
REQUEST message but is not necessarily optimal according to any
other optimal requested service listed in the ROUTE REQUEST message.
ti 2 - To handle a mixture of requested service limits and optimal
requested services, a route server should generate routes that
satisfy all of the requested service limits. The route server
should resolve ties between otherwise equivalent routes by
evaluating these routes as described in the multi-criteria
optimization case above.
ti 2 - All else being equal, a route server should always prefer
minimum-hop routes, because they minimize the amount of network
resources consumed by the routes.
ti 2 - A route server should generate at least one route to each
component of a partitioned destination domain, because it may not
know in which domain component the destination host resides. Hence,
a route server can maximize the chances of providing a feasible
route to a destination within a partitioned domain.
6.1 Searching
All domains need not execute the identical route generation
procedure. Each domain administrator is free to specify the IDPR
route generation procedure for route servers in its own domain,
making the procedure as simple or as complex as desired.
We offer an IDPR route generation procedure as a model. With slight
modification, this procedure can be made to search in either BF or
SPF order. The procedure can be used either to generate a single
policy route from the source to a specified destination domain or to
generate a set of policy routes from the source domain to all
destination domains. If the source or destination domain has a
proxy, then the source or destination endpoint of the policy route
is a proxy domain and not the actual source or destination domain.
For high-bandwidth traffic flows, BF search is the recommended
search technique, because it produces minimum-hop routes. For low-
bandwidth traffic flows, the route server may use either BF search
or SPF search. The computational complexity of BF search is O(N +
L) and hence it is the search procedure of choice, except when
generating routes with optimal requested services. We recommend
using SPF search only for optimal requested services and never in
response to a request for a maximum bandwidth route.
6.1.1. Implementation
Data Structures:
The routing information database contains the graph of an
internetwork, in which virtual gateways are the nodes and intra-
domain routes between virtual gateways are the links. During route
generation, each route is represented as a sequence of virtual
gateways, domains, and relevant transit policies, together with a
list of route characteristics, stored in a temporary array and
indexed by destination domain.
- Execute the Policy Consistency routine, first with the source
domain the given domain and second with the destination domain as
the given domain. If any policy inconsistency precludes the
requested traffic flow, go to Exit.
- For each domain, initialize a null route, set the route bandwidth
to and set the following route characteristics to infinity: route
delay, route delay variation, session monetary cost, and route
length in hops.
- With each operational virtual gateway in the source or source proxy
domain, associate the initial route characteristics.
- Initialize a next-node data structure which will contain, for each
route in progress, the virtual gateway at the current endpoint of
the route together with the associated route characteristics. The
next-node data structure determines the order in which routes get
expanded.
BF: A fifo queue.
SPF: A heap, ordered according to the first optimal requested
service listed in the ROUTE REQUEST message.
Remove Next Node: These steps are performed for each virtual gateway
in the next-node data structure.
- If there are no more virtual gateways in the next-node data
structure, go to Exit.
- Extract a virtual gateway and its associated route
characteristics from the next-node data structure, obtain the
adjacent domain, and:
SPF: Remake the heap.
- If there is a specific destination domain and if for the primary
optimal service:
BF: Route length in hops.
SPF: First optimal requested service listed in the ROUTE
REQUEST message.
the extracted virtual gateway's associated route characteristic
is no better than that of the destination domain, go to Remove
Next Node.
- Execute the Policy Consistency routine with the adjacent domain
as given domain. If any policy inconsistency precludes the
requested traffic flow, go to Remove Next Node.
- Check that the source domain's transit policies do not preclude
traffic generated by members of the source host set with the
specified user class and requested services, from flowing to the
adjacent domain as destination. This check is necessary because
the route server caches what it considers to be all feasible
routes, to intermediate destination domains, generated during
the computation of the requested route. If there are no policy
inconsistencies, associate the route and its characteristics
with the adjacent domain as destination.
- If there is a specific destination domain and if the adjacent
domain is the destination or destination proxy domain, go to
Remove Next Node.
- Record the set of all exit virtual gateways in the adjacent
domain which the adjacent domain's transit policies permit the
requested traffic flow and which are currently reachable from
the entry virtual gateway.
Next Node:
These steps are performed for all exit virtual gateways in the
above set.
- If there are no exit virtual gateways in the set, go to Remove
Next Node.
- Compute the characteristics for the route to the exit virtual
gateway, and check that all of the route characteristics are
within the requested service limits. If any of the route
characteristics are outside of these limits, go to Next Node.
- Compare these route characteristics with those already
associated with the exit virtual gateway (there may be none, if
this is the first time the exit virtual gateway has been visited
in the search), according to the primary optimal service.
- Select the route with the optimal value of the primary optimal
service, resolve ties by considering optimality according to any
other optimal requested services in the order in which they are
listed in the ROUTE REQUEST message, and associate the selected
route and its characteristics with the exit virtual gateway.
- Add the virtual gateway to the next-node structure:
BF: Add to the end of the fifo queue.
SPF: Add to the heap.
and go to Next Node.
Exit:
Return a response to the route request, consisting of either a
set of candidate policy routes or an indication that the route
request cannot be fulfilled.
Policy Consistency: Check policy consistency for the given domain.
- Check that the given domain is not specified as an excluded
domain in the route request.
- Check that the given domain's transit policies do not preclude
traffic generated by members of the source host set with the
specified user class and requested services, from flowing to the
destination domain.
During the computation of the requested routes, a route server also
caches what it considers to be all feasible routes to intermediate
destination domains, thus increasing the chances of being able to
respond to a future route request without having to generate a new
route. The route server does perform some policy consistency checks
on the routes, as they are generated, to intermediate destinations.
However, these routes may not in fact be feasible; the transit
domains contained on the routes may not permit traffic between the
source and the given intermediate destinations. Hence, before
dispensing such a route in response to a route request, a route
server must check that the transit policies of the constituent
domains are consistent with the source and destination of the traffic
flow.
6.2. Route Directionality
A path agent may wish to set up a bidirectional path using a route
supplied by a route server. (Refer to sections 7.2 and 7.4 for
detailed discussions of path directionality.) However, a route
server can only guarantee that the routes it supplies are feasible if
used in the direction from source to destination. The reason is that
the route server, which resides in the source or source proxy domain,
does not have access to, and thus cannot account for, the source
policies of the destination domain. Nevertheless, the route server
can provide the path agent with an indication of its assessment of
route feasibility in the direction from destination to source.
A necessary but insufficient condition for a route to be feasible in
the direction from destination to source is as follows. The route
must be consistent, in the direction from destination to source, with
the transit policies of the domains that compose the route. The
transit policy consistency checks performed by the route server
during route generation account for the direction from source to
destination but not for the direction from destination to source.
Only after a route server generates a feasible route from source to
destination does it perform the transit policy consistency checks for
the route in the direction from destination to source. Following
these checks, the route server includes in its ROUTE RESPONSE message
to the path agent an indication of its assessment of route
feasibility in each direction.
6.3. Route Database
A policy route, as originally specified by a route server, is an
ordered list of virtual gateways, domains, and transit policies: VG 1
- AD 1 - TP 1 - ... - VG n - AD n - TP n. where VG i is the virtual
gateway that serves as exit from AD i-1 and entry to AD i, and TP i
is the set of transit policies associated with AD i and relevant to
the particular route. Each route is indexed by source and
destination domain. Route servers and paths agents store policy
routes in route databases maintained as caches whose entries must be
periodically flushed to avoid retention of stale policy routes. A
route server's route database is the set of all routes it has
generated on behalf of its domain as source or source proxy;
associated with each route in the database are its route
characteristics. A path agent's route database is the set of all
routes it has requested and received from route servers on behalf of
hosts for which it is configured to act.
When attempting to locate a feasible route for a traffic flow, a path
agent first consults its own route database before querying a route
server. If the path agent's route database contains one or more
routes between the given source and destination domains and
accommodating the given host set and UCI, then the path agent checks
each such route against the set of excluded domains listed in the
source policy. The path agent either selects the first route
encountered that does not include the excluded domains, or, if no
such route exists in its route database, requests a route from a
route server.
A path agent must query a route server for routes when it is unable
to fulfill a route request from its own route database. Moreover, we
recommend that a path agent automatically forward to a route server,
all route requests with non-null requested services. The reason is
that the path agent retains no route characteristics in its route
database. Hence, the path agent cannot determine whether an entry in
its route database satisfies the requested services.
When responding to a path agent's request for a policy route, a route
server first consults its route database, unless the ROUTE REQUEST
message contains an explicit directive to generate a new route. If
its route database contains one or more routes between the given
source and destination domains and accommodating the given host set
and UCI, the route server checks each such route against the set of
excluded domains listed in the ROUTE REQUEST message. The route
server either selects all routes encountered that do not include the
excluded domains, or, if no such route exists in its route database,
attempts to generate such a route. Once the route server selects a
set of routes, it then checks each such route against the services
requested by the path agent and the services offered by the domains
composing the route. To obtain the offered services information, the
route server consults its routing information database. The route
server either selects the first route encountered that is consistent
with both the requested and offered services, or, if no such route
exists in its route database, attempts to generate such a route.
6.3.1. Cache Maintenance
Each route stored in a route database has a maximum cache lifetime
equal to rdb_rs minutes for a route server and rdb_ps minutes for a
path agent. Route servers and path agents reclaim cache space by
flushing entries that have attained their maximum lifetimes.
Moreover, paths agents reclaim cache space for routes whose paths
have failed to be set up successfully or have been torn down (see
section 7.4).
Nevertheless, cache space may become scarce, even with reclamation of
entries. If a cache fills, the route server or path agent logs the
event for network management. To obtain space in the cache when the
cache is full, the route server or path agent deletes from the cache
the oldest entry.
7. Path Control Protocol and Data Message Forwarding Procedure
Two entities in different domains may exchange IDPR data messages,
only if there exists an IDPR path set up between the two domains.
Path setup requires cooperation among path agents and intermediate
policy gateways. Path agents locate policy routes, initiate the Path
Control Protocol (PCP), and manage existing paths between
administrative domains. Intermediate policy gateways verify that a
given policy route is consistent with their domains' transit
policies, establish the forwarding information, and forward messages
along existing paths.
Each policy gateway and each route server contains a path agent. The
path agent that initiates path setup in the source or source proxy
domain is the "originator", and the path agent that handles the
originator's path setup message in the destination or destination
proxy domain is the "target". Every path has two possible directions
of traffic flow: from originator to target and from target to
originator. Path control messages are free to travel in either
direction, but data messages may be restricted to only one direction.
Once a path for a policy route is set up, its physical realization is
a set of consecutive policy gateways, with policy gateways or route
servers forming the endpoints. Two successive entities in this set
belong to either the same domain or the same virtual gateway. A
policy gateway or route server may, at any time, recover the
resources dedicated to a path that goes through it by tearing down
that path. For example, a policy gateway may decide to tear down a
path that has not been used for some period of time.
PCP may build multiple paths between source and destination domains,
but it is not responsible for managing such paths as a group or for
eliminating redundant paths.
7.1. An Example of Path Setup
We illustrate how path setup works by stepping through an example.
Suppose host Hx in domain AD X wants to communicate with host Hy in
domain AD Y and that both AD X and AD Y support IDPR. Hx need not
know the identity of its own domain or of Hy's domain in order to
send messages to Hy. Instead, Hx simply forwards a message bound for
Hy to one of the gateways on its local network, according to its
local forwarding information only. If the recipient gateway is a
policy gateway, the resident path agent determines how to forward the
message outside of the domain. Otherwise, the recipient gateway
forwards the message to another gateway in AD X, according to its
local forwading information. Eventually, the message will arrive at
a policy gateway in AD X, as policy gateways are the only egress
points to other domains, in domains that support IDPR.
The path agent resident in the recipient policy gateway uses the
message header, including source and destination addresses and any
requested service information (for example, type of service), in
order to determine whether it is an intra-domain or inter-domain
message, and if inter-domain, whether it requires an IDPR policy
route. Specifically, the path agent attempts to locate a forwarding
information database entry for the given traffic flow, from the
information contained in the message header. In the future, for IP
messages, the relevant header information may also include special
service-specific IP options or even information from higher layer
protocols.
Forwarding database entries exist for all of the following:
- All intra-domain traffic flows. Intra-domain forwarding
information is integrated into the forwarding information database
as soon as it is received.
- Inter-domain traffic flows that do not require IDPR policy routes.
Non-IDPR forwarding information is integrated into the forwarding
database as soon as it is received.
- IDPR inter-domain traffic flows for which a path has already been
set up. IDPR forwarding information is integrated into the
forwarding database only during path setup.
The path agent uses the message header contents to guide the search
for a forwarding information database entry for a given traffic flow.
We recommend a radix search to locate such an entry. When the search
terminates, it produces either an entry, or, in the case of a new
IDPR traffic flow, a directive to generate an entry. If the search
terminates in an existing forwarding information database entry, the
path agent forwards the message according to that entry.
Suppose that the search terminates indicating that the traffic flow
from Hx to Hy requires an IDPR policy route and that no entry in the
forwarding information database yet exists for that traffic flow. In
this case, the path agent first determines the source and destination
domains associated with the message's source and destination
addresses, before attempting to obtain a policy route. The path
agent relies on the mapping servers to supply the domain information,
but it caches all mapping server responses locally to limit the
number of future queries. When attempting to resolve an address to a
domain, the path agent always checks its local cache before
contacting a mapping server.
After obtaining the domain information, the path agent attempts to
obtain a policy route to carry the traffic from Hx to Hy. The path
agent relies on route servers to supply policy routes, but it caches
all route server responses locally to limit the number of future
queries. When attempting to locate a suitable policy route, the path
agent usually consults its local cache before contacting a route
server, as described previously in section 6.3.
If no suitable cache entry exists, the path agent queries the route
server, providing it with the source and destination domains together
with source policy information carried in the host message or
specified through configuration. Upon receiving a policy route
query, a route server consults its route database. If it cannot
locate a suitable route in its route database, the route server
attempts to generate at least one route to AD Y, consistent with the
requested services for Hx.
The route server always returns a response to the path agent,
regardless of whether it is successful in locating a suitable policy
route. The response to a successful route query consists of a set of
candidate routes, from which the path agent makes its selection. We
expect that a path agent will normally choose a single route from a
candidate set. Nevertheless, IDPR does not preclude a path agent
from selecting multiple routes from the candidate set. A path agent
may desire multiple routes to support features such as fault
tolerance or load balancing; however, IDPR does not currently specify
how the path agent should use multiple routes.
If the policy route is a new route provided by the route server,
there will be no existing path for the route, and thus the path agent
must set up such a path. However, if the policy route is an existing
route extracted from the path agent's cache, there may well be an
existing path for the route, set up to accommodate a host traffic
flow. IDPR permits multiple traffic flows to use the same path,
provided that all traffic flows sharing the path travel between the
same endpoint domains and have the same service requirements.
Nevertheless, IDPR does not preclude a path agent from setting up
distinct paths along the same policy route to preserve the
distinction between host traffic flows.
The path agent associates an identifier with the path, which is
included in each message that travels down the path and is used by
the policy gateways along the path in order to determine how to
forward the message. If the path already exists, the path agent uses
the preexisting identifier. However, for new paths, the path agent
chooses a path identifier that is different from those of all other
paths that it manages. The path agent also updates its forwarding
information database to reference the path identifier and modifies
its search procedure to yield the correct entry in the forwarding
information database given the data message header.
For new paths, the path agent initiates path setup, communicating the
policy route, in terms of requested services, constituent domains,
relevant transit policies, and the connecting virtual gateways, to
policy gateways in intermediate domains. Using this information, an
intermediate policy gateway determines whether to accept or refuse
the path and to which next policy gateway to forward the path setup
information. The path setup procedure allows policy gateways to set
up a path in both directions simultaneously. Each intermediate
policy gateway, after path acceptance, updates its forwarding
information database to include an entry that associates the path
identifier with the appropriate previous and next hop policy
gateways.
When a policy gateway in AD Y accepts a path, it notifies the source
path agent in AD X. We expect that the source path agent will
normally wait until a path has been successfully established before
using it to transport data traffic. However, PCP does not preclude a
path agent from forwarding messages along a path prior to
confirmation of successful path establishment. Paths remain in place
until they are torn down because of failure, expiration, or when
resources are scarce, preemption in favor of other paths.
We note that data communication between Hx and Hy may occur over two
separate IDPR paths: one from AD X to AD Y and one from AD Y to AD X.
The reasons are that within a domain, hosts know nothing about path
agents nor IDPR paths, and path agents know nothing about other path
agents' existing IDPR paths. Thus, in AD Y, the path agent that
terminates the path from AD X may not be the same as the path agent
that receives traffic from Hy destined for Hx. In this case, receipt
of traffic from Hy forces the second path agent to set up an
independent path from AD Y to AD X.
7.2. Path Identifiers
Each path has an associated path identifier, unique throughout an
internetwork. Every IDPR data message travelling along that path
includes the path identifier, used for message forwarding. The path
identifier is the concatenation of three items: the identifier of the
originator's domain, the identifier of the originator's policy
gateway or route server, and a 32-bit local path identifier specified
by the originator. The path identifier and the CMTP transaction
identifier have analogous syntax and play analogous roles in their
respective protocols.
When issuing a new path identifier, the originator always assigns a
local path identifier that is different from that of any other active
or recently torn-down path originally set up by that path agent.
This helps to distinguish new paths from replays. Hence, the
originator must keep a record of each extinct path for long enough
that all policy gateways on the path will have eliminated any
reference to it from their memories. The right-most 30 bits of the
local identifier are the same for each path direction, as they are
assigned by the originator. The left-most 2 bits of the local
identifier indicate the path direction.
At path setup time, the originator specifies which of the path
directions to enable contingent upon the information received from
the route server in the ROUTE RESPONSE message. By "enable", we mean
that each path agent and each intermediate policy gateway establishes
an association between the path identifier and the previous and next
policy gateways on the path, which it uses for forwarding data
messages along that path. IDPR data messages may travel in the
enabled path directions only, but path control messages are always
free to travel in either path direction. The originator may enable
neither path direction, if the entire data transaction can be carried
in the path setup message itself. In this case, the path agents and
the intermediate policy gateways do not establish forwarding
associations for the path, but they do verify consistency of the
policy information contained in the path setup message, with their
own transit policies, before forwarding the setup message on to the
next policy gateway.
The path direction portion of the local path identifier has different
interpretations, depending upon message type. In an IDPR path setup
message, the path direction indicates the directions in which the
path should be enabled: the value 01 denotes originator to target,
the value 10 denotes target to originator, the value 11 denotes both
directions, and the value 00 denotes neither direction. Each policy
gateway along the path interprets the path direction in the setup
message and sets up the forwarding information as directed. In an
IDPR data message, the path direction indicates the current direction
of traffic flow: either 01 for originator to target or 10 for target
to originator. Thus, if for example, an originator sets up a path
enabling only the direction from target to originator, the target
sends data messages containing the path identifier selected by the
originator together with the path direction set equal to 10.
Instead of using path identifiers that are unique throughout an
internetwork, we could have used path identifiers that are unique
only between a pair of consecutive policy gateways and that change
from one policy gateway pair to the next. The advantage of locally
unique path identifiers is that they may be much shorter than
globally unique identifiers and hence consume less transmission
bandwidth. However, the disadvantage is that the path identifier
carried in each IDPR data message must be modified at each policy
gateway, and hence if the integrity/authentication information covers
the path identifier, it must be recomputed at each policy gateway.
For security reasons, we have chosen to include the path identifier
in the set of information covered by the integrity/authentication
value, and moreover, we advocate public-key based signatures for
authentication. Thus, it is not possible for intermediate policy
gateways to modify the path identifier and then recompute the correct
integrity/authentication value. Therefore, we have decided in favor
of path identifiers that do not change from hop to hop and hence must
be globally unique. To speed forwarding of IDPR data messages with
long path identifiers, policy gateways should hash the path
identifiers in order to index IDPR forwarding information.
7.3. Path Control Messages
Messages exchanged by the path control protocol are classified into
"requests": SETUP, TEARDOWN, REPAIR; and "responses": ACCEPT, REFUSE,
ERROR. These messages have significance for intermediate policy
gateways as well as for path agents.
SETUP:
Establishes a path by linking together pairs of policy gateways.
The SETUP message is generated by the originator and propagates
to the target. In response to a SETUP message, the originator
expects to receive an ACCEPT, REFUSE, or ERROR message. The
SETUP message carries all information necessary to set up the
path including path identifier, requested services, transit
policy information relating to each domain traversed, and
optionally, expedited data.
ACCEPT: Signals successful path establishment. The ACCEPT message is
generated by the target, in response to a SETUP message, and
propagates back to the originator. Reception of an ACCEPT
message by the originator indicates that the originator can now
safely proceed to send data along the path. The ACCEPT message
contains the path identifier and an optional reason for
conditional acceptance.
REFUSE: Signals that the path could not be successfully established,
either because resources were not available or because there was
an inconsistency between the services requested by the source
and the services offered by a transit domain along the path.
The REFUSE message is generated by the target or by any
intermediate policy gateway, in response to a SETUP message, and
propagates back to the originator. All recipients of a REFUSE
message recover the resources dedicated to the given path. The
REFUSE message contains the path identifier and the reason for
path refusal.
TEARDOWN: Tears down a path, typically when a non-recoverable failure
is detected. The TEARDOWN message may be generated by any path
agent or policy gateway in the path and usually propagates in
both path directions. All recipients of a TEARDOWN message
recover the resources dedicated to the given path. The TEARDOWN
message contains the path identifier and the reason for path
teardown.
REPAIR: Establishes a repaired path by linking together pairs of
policy gateways. The REPAIR message is generated by a policy
gateway after detecting that the next policy gateway on one of
its existing paths is unreachable. A policy gateway that
generates a REPAIR message propagates the message forward at
most one virtual gateway. In response to a REPAIR message, the
policy gateway expects to receive an ACCEPT, REFUSE, TEARDOWN,
or ERROR message. The REPAIR message carries the original SETUP
message.
ERROR: Transports information about a path error back to the
originator, when a PCP message contains unrecognized
information. The ERROR message may be generated by the target
or by any intermediate policy gateway and propagates back to the
originator. Most, but not all, ERROR messages are generated in
response to errors encountered during path setup. The ERROR
message includes the path identifier and an explanation of the
error detected.
Policy gateways use CMTP for reliable transport of PCP messages,
between path agents and policy gateways and between consecutive
policy gateways on a path. PCP must communicate to CMTP the maximum
number of transmissions per path control message, pcp_ret, and the
interval between path contol message retransmissions, pcp_int
microseconds. All path control messages, except ERROR messages, may
be transmitted up to pcp_ret times; ERROR messages are never
retransmitted. A path control message is "acceptable" if:
- It passes the CMTP validation checks.
- Its timestamp is less than pcp_old (300) seconds behind the
recipient's internal clock time.
- It carries a recognized path identifier, provided it is not a SETUP
message.
An intermediate policy gateway on a path forwards acceptable PCP
messages. As we describe in section 7.4 below, SETUP messages must
undergo additional tests at each intermediate policy gateway prior to
forwarding. Moreover, receipt of an acceptable ACCEPT, REFUSE,
TEARDOWN, or ERROR message at either path agent or at any
intermediate policy gateway indirectly cancels any active local CMTP
retransmissions of the original SETUP message. When a path agent or
intermediate policy gateway receives an unacceptable path control
message, it discards the message and logs the event for network
management. The path control message age limit reduces the
likelihood of denial of service attacks based on message replay.
7.4. Setting Up and Tearing Down a Path
Path setup begins when the originator generates a SETUP message
containing:
- The path identifier, including path directions to enable.
- An indication of whether the message includes expedited data.
- The source user class identifier.
- The requested services (see section 5.5.2) and source-specific
information (see section 7.6.1) for the path.
- For each domain on the path, the domain component, applicable
transit policies, and entry and exit virtual gateways.
The only mandatory requested service is the maximum path lifetime,
pth_lif, and the only mandatory source-specific information is the
data message integrity/authentication type. If these are not
specified in the path setup message, each recipient policy gateway
assigns them default values, (60) minutes for pth_lif and no
authentication for integrity/authentication type. Each path agent
and intermediate policy gateway tears down a path when the path
lifetime is exceeded. Hence, no single source can indefinitely
monopolize policy gateway resources or still functioning parts of
partially broken paths.
After generating the SETUP message and establishing the proper local
forwarding information, the originator selects the next policy
gateway on the path and forwards the SETUP message to the selected
policy gateway. The next policy gateway selection procedure,
described below, applies when either the originator or an
intermediate policy gateway is making the selection. We have elected
to describe the procedure from the perspective of a selecting
intermediate policy gateway.
The policy gateway selects the next policy gateway on a path, in
round-robin order from its list of policy gateways contained in the
present or next virtual gateway, as explained below. In selecting
the next policy gateway, the policy gateway uses information
contained in the SETUP message and information provided by VGP and by
the intra-domain routing procedure.
If the selecting policy gateway is a domain entry point, the next
policy gateway must be:
- A member of the next virtual gateway listed in the SETUP message.
- Reachable according to intra-domain routes supporting the transit
policies listed in the SETUP message.
- Able to reach, according to VGP, the next domain component listed
in the SETUP message.
In addition, the selecting policy gateway may use quality of service
information supplied by intra-domain routing to resolve ties between
otherwise equivalent next policy gateways in the same domain. In
particular, the selecting policy gateway may select the next policy
gateway whose connecting intra-domain route is optimal according to
the requested services listed in the SETUP message.
If the selecting policy gateway is a domain exit point, the next
policy gateway must be:
- A member of the current virtual gateway listed in the SETUP message
(which is also the selecting policy gateway's virtual gateway).
- Reachable according to VGP.
- A member of the next domain component listed in the SETUP message.
Once the originator or intermediate policy gateway selects a next
policy gateway, it forwards the SETUP message to the selected policy
gateway. Each recipient (policy gateway or target) of an acceptable
SETUP message performs several checks on the contents of the message,
in order to determine whether to establish or reject the path. We
describe these checks in detail below from the perspective of a
policy gateway as SETUP message recipient.
7.4.1. Validating Path Identifiers
The recipient of a SETUP message first checks the path identifier, to
make sure that it does not correspond to that of an already existing
or recently extinct path. To detect replays, malicious or otherwise,
path agents and policy gateways maintain a record of each path that
they establish, for max{pth_lif, pcp_old} seconds. If the path
identifier and timestamp carried in the SETUP message match a stored
path identifier and timestamp, the policy gateway considers the
message to be a retransmission and does not forward the message. If
the path identifier carried in the SETUP message matches a stored
path identifier but the two timestamps do not agree, the policy
gateway abandons path setup, logs the event for network management,
and returns an ERROR message to the originator via the previous
policy gateway.
7.4.2. Path Consistency with Configured Transit Policies
Provided the path identifier in the SETUP message appears to be new,
the policy gateway proceeds to determine whether the information
contained within the SETUP message is consistent with the transit
policies configured for its domain. The policy gateway must locate
the source and destination domains, the source host set and user
class identifier, and the domain-specific information for its own
domain, within the SETUP message, in order to evaluate path
consistency. If the policy gateway fails to recognize the source
user class (or one or more of the requested services), it logs the
event for network management but continues with path setup. If the
policy gateway fails to locate its domain within the SETUP message,
it abandons path setup, logs the event for network management, and
returns an ERROR message to the originator via the previous policy
gateway. The originator responds by tearing down the path and
subsequently removing the route from its cache.
Once the policy gateway locates its domain-specific portion of the
SETUP message, it may encounter the following problems with the
contents:
- The domain-specific portion lists a transit policy not configured
for the domain.
- The domain-specific portion lists a virtual gateway not configured
for the domain.
In each case, the policy gateway abandons path setup, logs the event
for network management, and returns an ERROR message to the
originator via the previous policy gateway. These types of ERROR
messages indicate to the originator that the route may have been
generated using information from an out-of-date CONFIGURATION
message.
The originator reacts to the receipt of such an ERROR message as
follows. First, it tears down the path and removes the route from
its cache. Then, it issues to a route server a ROUTE REQUEST message
containing a directive to refresh the routing information database,
with the most recent CONFIGURATION message from the domain that
issued the ERROR message, before generating a new route.
Once it verifies that its domain-specific information in the SETUP
message is recognizable, the policy gateway then checks that the
information contained within the SETUP message is consistent with the
transit policies configured for its domain. A policy gateway at the
entry to a domain checks path consistency in the direction from
originator to target, if the enabled path directions include
originator to target. A policy gateway at the exit to a domain
checks path consistency in the direction from target to originator,
if the enabled path directions include target to originator.
When evaluating the consistency of the path with the transit policies
configured for the domain, the policy gateway may encounter any of
the following problems with SETUP message contents:
- A transit policy does not apply in the given direction between the
virtual gateways listed in the SETUP message.
- A transit policy denies access to traffic from the given host set
between the source and destination domains listed in the SETUP
message.
- A transit policy denies access to traffic from the source user
class listed in the SETUP message.
- A transit policy denies access to traffic at the current time.
In each case, the policy gateway abandons path setup, logs the event
for network management, and returns a REFUSE message to the
originator via the previous policy gateway. These types of REFUSE
messages indicate to the originator that the route may have been
generated using information from an out-of-date CONFIGURATION
message. The REFUSE message also serves to teardown the path.
The originator reacts to the receipt of such a REFUSE message as
follows. First, it removes the route from its cache. Then, it issues
to a route server a ROUTE REQUEST message containing a directive to
refresh the routing information database, with the most recent
CONFIGURATION message from the domain that issued the REFUSE message,
before generating a new route.
7.4.3. Path Consistency with Virtual Gateway Reachability
Provided the information contained in the SETUP message is consistent
with the transit policies configured for its domain, the policy
gateway proceeds to determine whether the path is consistent with the
reachability of the virtual gateway containing the potential next
hop. To determine virtual gateway reachability, the policy gateway
uses information provided by VGP and by the intra-domain routing
procedure.
When evaluating the consistency of the path with virtual gateway
reachability, the policy gateway may encounter any of the following
problems:
- The virtual gateway containing the potential next hop is down.
- The virtual gateway containing the potential next hop is not
reachable via any intra-domain routes supporting the transit
policies listed in the SETUP message.
- The next domain component listed in the SETUP message is not
reachable.
Each of these determinations is made from the perspective of a single
policy gateway and may not reflect actual reachability. In each
case, the policy gateway encountering such a problem returns a REFUSE
message to the previous policy gateway which then selects a different
next policy gateway, in round-robin order, as described in
previously. If the policy gateway receives the same response from
all next policy gateways selected, it abandons path setup, logs the
event for network management, and returns the REFUSE message to the
originator via the previous policy gateway. These types of REFUSE
messages indicate to the originator that the route may have been
generated using information from an out-of-date DYNAMIC message. The
REFUSE message also serves to teardown the path.
The originator reacts to the receipt of such a REFUSE message as
follows. First, it removes the route from its cache. Then, it
issues to a route server a ROUTE REQUEST message containing a
directive to refresh the routing information database, with the most
recent DYNAMIC message from the domain that issued the REFUSE
message, before generating a new route.
7.4.4. Obtaining Resources
Once the policy gateway determines that the SETUP message contents
are consistent with the transit policies and virtual gateway
reachability of its domain, it attempts to gain resources for the new
path. For this version of IDPR, path resources consist of memory in
the local forwarding information database. However, in the future,
path resources may also include reserved link bandwidth.
If the policy gateway does not have sufficient resources to establish
the new path, it uses the following algorithm to determine whether to
generate a REFUSE message for the new path or a TEARDOWN message for
an existing path in favor of the new path. There are two cases:
- No paths have been idle for more than pcp_idle (300) seconds. In
this case, the policy gateway returns a REFUSE message to the
previous policy gateway. This policy gateway then tries to select
a different next policy gateway, as described previously, provided
the policy gateway that issued the REFUSE message was not the
target. If the REFUSE message was issued by the target or if there
is no available next policy gateway, the policy gateway returns
the REFUSE message to the originator via the previous policy
gateway and logs the event for network management. The REFUSE
message serves to tear down the path.
- At least one path has been idle for more than pcp_idle seconds. In
this case, the policy gateway tears down an older path in order to
accommodate the newer path and logs the event for network
management. Specifically, the policy gateway tears down the least
recently used path among those that have been idle for longer than
pcp_idle seconds, resolving ties by choosing the oldest such path.
If the policy gateway has sufficient resources to establish the path,
it attempts to update its local forwarding information database with
information about the path identifier, previous and next policy
gateways on the path, and directions in which the path should be
enabled for data traffic transport.
7.4.5 Target Response
When an acceptable SETUP message successfully reaches an entry policy
gateway in the destination or destination proxy domain, this policy
gateway performs all of the SETUP message checks described in the
above sections. The policy gateway's path agent then becomes the
target, provided no checks fail, unless there is an explicit target
specified in the SETUP message. For example, remote route servers
act as originator and target during RSQP message exchanges (see
section 5.2). If the recipient policy gateway is not the target, it
attempts to forward the SETUP message to the target along an intra-
domain route. However, if the target is not reachable via intra-
domain routing, the policy gateway abandons path setup, logs the
event for network management, and returns a REFUSE message to the
originator via the previous policy gateway. The REFUSE message
serves to tear down the path.
Once the SETUP message reaches the target, the target determines
whether it has sufficient path resources. The target generates an
ACCEPT message, provided it has sufficient resources to establish the
path. Otherwise, it generates a REFUSE message.
The target may choose to use the reverse path to transport data
traffic to the source domain, if the enabled path directions include
10 or 11. However, the target must first verify the consistency of
the reverse path with its own domain's configured transit policies,
before sending data traffic over that path.
7.4.6. Originator Response
The originator expects to receive an ACCEPT, REFUSE, or ERROR message
in response to a SETUP message and reacts as follows:
- The originator receives an ACCEPT message, confirming successful
path establishment. To expedite data delivery, the originator may
forward data messages along the path prior to receiving an ACCEPT
message, with the understanding that there is no guarantee that the
path actually exists.
- The originator receives a REFUSE message or an ERROR message,
implying that the path could not be successfully established. In
response, the originator attempts to set up a different path to the
same destination, as long as the number of selected different paths
does not exceed setup_try (3). If the originator is unsuccessful
after setup_try attempts, it abandons path setup and logs the event
for network management.
- The originator fails to receive any response to the SETUP message
within setup_int microseconds after transmission. In this case,
the originator attempts path setup using the same policy route and
a new path identifier, as long as the number of path setup attempts
using the same route does not exceed setup_ret (2). If the
originator fails to receive a response to a SETUP message after
setup_ret attempts, it logs the event for network management and
then proceeds as though it received a negative response, namely a
REFUSE or an ERROR, to the SETUP message. Specifically, it
attempts to set up a different path to the same destination, or it
abandons path setup altogether, depending on the value of
setup_try.
7.4.7. Path Life
Once set up, a path does not live forever. A path agent or policy
gateway may tear down an existing path, provided any of the following
conditions are true:
- The maximum path lifetime (in minutes, bytes, or messages) has been
exceeded at the originator, the target, or an intermediate policy
gateway. In each case, the IDPR entity detecting path expiration
logs the event for network management and generates a TEARDOWN
message as follows:
o The originator path agent generates a TEARDOWN message for
propagation toward the target.
o The target path agent generates a TEARDOWN message for
propagation toward the originator.
o An intermediate policy gateway generates two TEARDOWN messages,
one for propagation toward the originator and one for
propagation toward the target.
- The previous or next policy gateway becomes unreachable, across a
virtual gateway or across a domain according to a given transit
policy, and the path is not reparable. In either case, the policy
gateway detecting the reachability problem logs the event for
network management and generates a TEARDOWN message as follows:
o If the previous policy gateway is unreachable, an intermediate
policy gateway generates a TEARDOWN message for propagation to
the target.
o If the next policy gateway is unreachable, an intermediate
policy gateway generates a TEARDOWN message for propagation to
the originator.
- All of the policy gateway's path resources are in use at the
originator, the target, or an intermediate policy gateway, a new
path requires resources, and the given existing path is expendable,
according to the least recently used criterion discussed in section
7.4.4 above. In each case, the IDPR entity initiating path
preemption logs the event for network management and generates a
TEARDOWN message as follows:
o The originator path agent generates a TEARDOWN message for
propagation toward the originator.
o The target path agent generates a TEARDOWN message for
propagation toward the originator.
o An intermediate policy gateway generates two TEARDOWN messages,
one for propagation toward the originator and one for
propagation toward the target.
Path teardown at a path agent or policy gateway, whether initiated by
one of the above events, by receipt of a TEARDOWN message, or by
receipt of a REFUSE message during path setup (as discussed in the
previous sections), results in the path agent or policy gateway
releasing all resources devoted to both directions of the path.
7.5. Path Failure and Recovery
When a policy gateway fails, it may not be able to save information
pertaining to its established paths. Thus, when the policy gateway
returns to service, it may have no recollection of the paths set up
through it and hence may no longer be able to forward data messages
along these paths. We expect that when a policy gateway fails, it
will usually be out of service for long enough that the up/down
protocol and the intra-domain routing procedure can detect that the
particular policy gateway is no longer reachable. In this case,
adjacent or neighbor policy gateways that have set up paths through
the failed policy gateway and that have detected the failure, attempt
local path repair (see section 7.5.2 below), and if unsuccessful,
issue TEARDOWN messages for all affected paths.
7.5.1. Handling Implicit Path Failures
Nevertheless, policy gateways along a path must be able to handle the
case in which a policy gateway fails and subsequently returns to
service without either the up/down protocol or the intra-domain
routing procedure detecting the failure; we do not expect this event
to occur often. If the policy gateway, prior to failure, contained
forwarding information for several established paths, it may now
receive many IDPR data messages containing unrecognized path
identifiers. The policy gateway should alert the data sources that
their paths through it are no longer viable.
Policy gateways that receive IDPR data messages with unrecognized
path identifiers take one of the following two actions, depending
upon their past failure record:
- The policy gateway has not failed in the past pg_up (24) hour
period. In this case, there are at least four possible reasons for
the unrecognized path identifier in the data message:
o The data message path identifier has been corrupted in a way
that is not detectable by the integrity/authentication value, if
one is present.
o The policy gateway has experienced a memory error.
o The policy gateway failed sometime during the life of the path
and source sent no data on the path for a period of pg_up hours
following the failure. Although paths may persist for more than
pg_up hours, we expect that they will also be used more
frequently than once every pg_up hours.
o The path was not successfully established, and the originator
sent data messages down the path prior to receiving a response
to its SETUP message.
In all cases, the policy gateway discards the data message and
logs the event for network management.
- The policy gateway has failed at least once in the past pg_up hour
period. Thus, the policy gateway assumes that the unrecognized
path identifier in the data message may be attributed to its
failure. In response to the data message, the policy gateway
generates an ERROR message containing the unrecognized path
identifier. The policy gateway then sends the ERROR message back
to the entity from which it received the data message, which should
be equivalent to the previous policy gateway on the path.
When the previous policy gateway receives such an ERROR message, it
decides whether the message is acceptable. If the policy gateway
does not recognize the path identifier contained in the ERROR
message, it does not find the ERROR message acceptable and
subsequently discards the message. However, if the policy gateway
does find the ERROR message acceptable, it then determines whether it
has already received an ACCEPT message for the given path. If the
policy gateway has not received an ACCEPT message for that path, it
discards the ERROR message and takes no further action.
If the policy gateway has received an ACCEPT message for that path,
it then attempts path repair, as described in section 7.5.2 below.
Only if path repair is unsuccessful does the previous policy gateway
generate a TEARDOWN message for the path and return it to the
originator. The TEARDOWN message includes the domain and virtual
gateway containing the policy gateway that failed, which aids the
originator in selecting a new path that does not include the domain
containing the failed policy gateway. This mechanism ensures that
path agents quickly discover and recover from disrupted paths, while
guarding against unwarranted path teardown.
7.5.2. Local Path Repair
Failure of one of more entities on a given path may render the path
unusable. If the failure is within a domain, IDPR relies on the
intra-domain routing procedure to find an alternate route across the
domain, which leaves the path unaffected. If the failure is in a
virtual gateway, policy gateways must bear the responsibility of
repairing the path. Policy gateways nearest to the failure are the
first to recognize its existence and hence can react most quickly to
repair the path.
Relinquishing control over path repair to policy gateways in other
domains may be unacceptable to some domain administrators. The
reason is that these policy gateways cannot guarantee construction of
a path that satisfies the source policies of the source domain, as
they have no knowledge of other domains' source policies.
Nevertheless, limited local path repair is feasible, without
distributing either source policy information throughout an
internetwork or detailed path information among policy gateways in
the same domain or in the same virtual gateway. We say that a path
is "locally reparable" if there exists an alternate route between two
policy gateways, separated by at most one virtual gateway, on the
path. This definition covers path repair in the presence of failed
routes between consecutive policy gateways as well as failed policy
gateways themselves.
An IDPR entity attempts local repair of an established path, in the
direction from originator to target, immediately after detecting that
the next policy gateway on the path is no longer reachable. To
prevent multiple path repairs in response to the same failure, we
have stipulated that path repair can only be initiated in the
direction from originator to target. The IDPR entity initiating
local path repair attempts to find an alternate path to the policy
gateway immediately following the unreachable policy gateway on the
path.
Local path repair minimizes the disruption of data traffic flow
caused by certain types of failures along an established path.
Specifically, local path repair can accommodate an individual failed
policy gateway or failed direct connection between two adjacent
policy gateways. However, it can only be attempted through virtual
gateways containing multiple peer policy gateways. Local path repair
is not designed to repair paths traversing failed virtual gateways or
domain partitions. Whenever local path repair is impossible, the
failing path must be torn down.
7.5.3. Repairing a Path
When an IDPR entity detects through an ERROR message that the next
policy gateway has no knowledge of a given path, it generates a
REPAIR message and forwards it to the next policy gateway. This
REPAIR message will reestablish the path through the next policy
gateway.
When an entity detects that the next policy gateway on a path is no
longer reachable, it takes one of the following actions, depending
upon whether the entity is a member of the next policy gateway's
virtual gateway.
- If the entity is not a member of the next policy gateway's virtual
gateway, then one of the following two conditions must be true:
o The next policy gateway has a peer that is reachable via an
intra-domain route consistent with the requested services. In
this case, the entity generates a REPAIR message containing the
original SETUP message and forwards it to the next policy
gateway's peer.
o The next policy gateway has no peers that are reachable via
intra-domain routes consistent with the requested services. In
this case, the entity tears down the path back to the
originator.
- If the entity is a member of the next policy gateway's virtual
gateway, then one of the following four conditions must be true:
o The next policy gateway has a peer that belongs to the same
domain component and is directly-connected to and reachable from
the entity. In this case, the entity generates a REPAIR message
and forwards it to the next policy gateway's peer.
o The next policy gateway has a peer that belongs to the same
domain component, is not directly-connected to the entity, but
is directly-connected to and reachable from one of the entity's
peers, which in turn is reachable from the entity via an intra-
domain route consistent with the requested services. In this
case, the entity generates a REPAIR message and forwards it to
its peer.
o The next policy gateway has no operational peers within its
domain component, but is directly-connected to and reachable
from one of the entity's peers, which in turn is reachable from
the entity via an intra-domain route consistent with the
requested services. In this case, the entity generates a REPAIR
message and forwards it to its peer.
o The next policy gateway has no operational peers within its
domain component, and the entity has no operational peers which
are both reachable via intra-domain routes consistent with the
requested services and directly-connected to and reachable from
the next policy gateway. In this case, the entity tears down
the path back to the originator.
A recipient of a REPAIR message takes the following steps, depending
upon its relationship to the entity that issued the REPAIR message.
- The recipient and the issuing entity are in the same domain or in
same virtual gateway. In this case, the recipient extracts the
SETUP message contained within the REPAIR message and treats the
message as it would any other SETUP message. Specifically, the
recipient checks consistency of the path with its domain's transit
policies and virtual gateway reachability. If there are
unrecognized portions of the SETUP message, the recipient generates
an ERROR message, and if there are path inconsistencies, the
recipient generates a REFUSE message. In either case, the
recipient returns the corresponding message to the policy gateway
from which it received the REPAIR message. Otherwise, if the
recipient accepts the REPAIR message, it updates its local
forwarding information database accordingly and forwards the REPAIR
message to a potential next policy gateway, according to the
information contained in the enclosed SETUP message.
- The recipient and the issuing entity are in different domains and
different virtual gateways. In this case, the recipient extracts
the SETUP message from the REPAIR message and determines whether
the associated path matches any of its established paths. If the
path does not match an established path, the recipient generates a
REFUSE message and returns it to the policy gateway from which it
received the REPAIR message. In response to the receipt of a
REFUSE message, the policy gateway tries a different next policy
gateway.
The path is reparable, if a path match is discovered. In this case,
the recipient updates the path entry in the local forwarding
information database and issues an ACCEPT message to the policy
gateway from which it received the REPAIR message, which in turn
returns the message to the entity that issued the REPAIR message.
The path is irreparable if all potential next policy gateways have
been exhausted and a path match has yet to be discovered. In this
case, the policy gateway that fails to locate a next policy gateway
issues a TEARDOWN message to return to the originator.
An IDPR entity expects to receive an ACCEPT, TEARDOWN, REFUSE, or
ERROR message in response to a REPAIR message and reacts to these
responses differently as follows:
- The entity always returns a TEARDOWN message to the originator via
previous policy gateway.
- The entity does not return an ACCEPT message to the originator, but
receipt of such a message indicates that the path has been
successfully repaired.
- The entity infers that the path is irreparable and subsequently
tears down the path and logs the event for network management, upon
receipt of a REFUSE or ERROR message or when no response to the
REPAIR message arrives within setup_int microseconds.
When an entity detects that the previous policy gateway on a path
becomes unreachable, it expects to receive a REPAIR message within
setup_wait microseconds. If the entity does not receive a REPAIR
message for the path within that time, it infers that the path is
irreparable and subsequently tears down the path and logs the event
for network management.
7.6. Path Control Message Formats
The path control protocol number is equal to 3. We describe the
contents of each type of PCP message below.
7.6.1. SETUP
The SETUP message type is equal to 0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PATH ID |
| |
+-------------------------------+-------------------------------+
| SRC AD | HST SET |
+---------------+---------------+-------------------------------+
| UCI | UNUSED | NUM RQS |
+---------------+---------------+-------------------------------+
| DST AD | TGT ENT |
+-------------------------------+-------------------------------+
| AD PTR |
+-------------------------------+
For each requested service for the path:
+-------------------------------+-------------------------------+
| RQS TYP | RQS LEN |
+-------------------------------+-------------------------------+
| RQS SRV |
+---------------------------------------------------------------+
For each domain contained in the path:
+---------------+---------------+-------------------------------+
| AD LEN | VG | ADJ AD |
+---------------+---------------+-------------------------------+
| ADJ CMP | NUM TP |
+-------------------------------+-------------------------------+
| TP |
+-------------------------------+
PATH ID
(64 bits) Path identifier consisting of the numeric identifier
for the originator's domain (16 bits), the numeric identifier
for the originator policy gateway or route server (16 bits), the
path direction (2 bits), and the local path identifier (30
bits).
SRC AD (16 bits) Numeric identifier for the source domain, which may
be different from the originator domain if the originator domain
is a proxy for the source.
HST SET (16 bits) Numeric identifier for the source's host set.
UCI (8 bits) Numeric identifier for the source user class. The value
0 indicates that there is no particular source user class.
UNUSED (8 bits) Not currently used; must be set equal to 0.
NUM RQS (16 bits) Number of requested services.
DST AD (16 bits) Numeric identifier for the destination domain, which
may be different from the target domain if the target domain is
a proxy for the destination.
TGT ENT (16 bits) Numeric identifier for the target entity. A value
of 0 indicates that there is no specific target entity.
AD PTR (16 bits) Byte offset from the beginning of the message
indicating the location of the beginning of the domain-specific
information, contained in the right-most 15 bits. The left-most
bit indicates whether the message includes expedited data (1
expedited data, 0 no expedited data).
RQS TYP (16 bits) Numeric identifier for a type of requested service
or source-specific information. Valid requested services are
described in section 5.5.2. Valid source source-specific
information includes the following types:
12. MD4/RSA data message authentication (see [16]).
13. MD5/RSA data message authentication (see [17]).
14. Billing address (variable).
15. Charge number (variable).
RQS LEN (16 bits) Length of the requested service or source-specific
information, in bytes, beginning with the next field.
RQS SRV (variable) Description of the requested service or source-
specific information.
AD LEN (8 bits) Length of the information associated with a
particular domain on the route, in bytes, beginning with the
next field.
VG (8 bits) Numeric identifier for an exit virtual gateway.
ADJ AD (16 bits) Numeric identifier for an adjacent domain.
ADJ CMP (16 bits) Numeric identifier for a component of the adjacent
domain. Used to aid a policy gateway in routing across a
virtual gateway connected to a partitioned domain.
NUM TP (16 bits) Number of transit policies that apply to the section
of the path traversing the given domain component.
TP (16 bits) Numeric identifier for a transit policy.
7.6.2. ACCEPT
The ACCEPT message type is equal to 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PATH ID |
| |
+---------------+-----------------------------------------------+
| RSN TYP | REASON |
+---------------+-----------------------------------------------+
PATH ID
(64 bits) Path identifier contained in the original SETUP
message.
RSN TYP (optional, 8 bits) Numeric identifier for the reason for
conditional path acceptance.
REASON (optional, variable) Description of the reason for conditional
path acceptance. Currently, no reasons have been defined.
7.6.3 REFUSE
The REFUSE message type is equal to 2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PATH ID |
| |
+---------------+-----------------------------------------------+
| RSN TYP | REASON |
+---------------+-----------------------------------------------+
PATH ID
(64 bits) Path identifier contained in the original SETUP
message.
RSN TYP (8 bits) Numeric identifier for the reason for path refusal.
REASON (variable) Description of the reason for path refusal. Valid
reasons include the following types:
1. Transit policy does not apply between the virtual gateways in a
given direction. Numeric identifier for the transit policy (16
bits).
2. Transit policy denies access to traffic from the host set between
the source and destination domains. Numeric identifier for the
transit policy (16 bits).
3. Transit policy denies access to traffic from the source user
class. Numeric identifier for the transit policy (16 bits).
4. Transit policy denies access to traffic at the current time.
Numeric identifier for the transit policy (16 bits).
5. Virtual gateway is down. Numeric identifier for the virtual
gateway (8 bits) and associated adjacent domain (16 bits).
6. Virtual gateway is not reachable according to the given transit
policy. Numeric identifier for the virtual gateway (8 bits),
associated adjacent domain (16 bits), and transit policy (16
bits).
7. Domain component is not reachable. Numeric identifier for the
domain (16 bits) and the component (16 bits).
8. Insufficient resources to establish the path.
9. Target is not reachable via intra-domain routing.
10. No existing path with the given path identifier, in response to
a REPAIR message only.
7.6.4. TEARDOWN
The TEARDOWN message type is equal to 3.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PATH ID |
| |
+---------------+-----------------------------------------------+
| RSN TYP | REASON |
+---------------+-----------------------------------------------+
PATH ID
(64 bits) Path identifier contained in the original SETUP
message.
RSN TYP (8 bits) Numeric identifier for the reason for path teardown.
REASON (variable) Description of the reason for path teardown. Valid
reasons include the following types:
1. Virtual gateway is down. Numeric identifier for the virtual
gateway (8 bits) and associated adjacent domain (16 bits).
2. Virtual gateway is not reachable according to the given transit
policy. Numeric identifier for the virtual gateway (8 bits),
associated adjacent domain (16 bits), and transit policy (16
bits).
3. Domain component is not reachable. Numeric identifier for the
domain (16 bits) and the component (16 bits).
4. Maximum path lifetime exceeded.
5. Preempted path.
6. Unable to repair path.
7.6.5. ERROR
The ERROR message type is equal to 4.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PATH ID |
| |
+---------------+---------------+-------------------------------+
| MSG | RSN TYP | REASON |
+---------------+---------------+-------------------------------+
PATH ID
(64 bits) Path identifier contained in the path control or data
message in error.
MSG (8 bits) Numeric identifier for the type of path control message
in error. This field is ignored for error type 5.
RSN TYP (8 bits) Numeric identifier for the reason for the PCP
message error.
REASON (variable) Description of the reason for the PCP message
error. Valid reasons include the following types:
1. Path identifier is already currently active.
2. Domain does not appear in the SETUP message.
3. Transit policy is not configured for the domain. Numeric
identifer for
the transit policy (16 bits).
4. Virtual gateway not configured for the domain. Numeric
identifier
for the virtual gateway (8 bits) and associated adjacent domain
(16
bits).
5. Unrecognized path identifier in an IDPR data message.
7.6.6. REPAIR
The REPAIR message type is equal to 5. A REPAIR message contains the
original SETUP message only.
7.6.7. Negative Acknowledgements
When a policy gateway receives an unacceptable PCP message that
passes the CMTP validation checks, it includes, in its CMTP ACK, an
appropriate negative acknowledgement. This information is placed in
the INFORM field of the CMTP ACK (described previously in section
2.4); the numeric identifier for each type of PCP negative
acknowledgement is contained in the left-most 8 bits of the INFORM
field. Negative acknowledgements associated with PCP include the
following types:
1. Unrecognized PCP message type. Numeric identifier for the
unrecognized message type (8 bits).
2. Out-of-date PCP message.
3. Unrecognized path identifier (for all PCP messages except SETUP).
Numeric identifier for the unrecognized path (64 bits).
8. Security Considerations
Refer to sections 1.6, 1.7, and 2.3 for details on security in IDPR.
9. Author's Address
Martha Steenstrup
BBN Systems and Technologies
10 Moulton Street
Cambridge, MA 02138
Phone: (617) 873-3192
Email: msteenst@bbn.com
References
[1] Clark, D., "Policy Routing in Internet Protocols", RFC 1102, May
1989.
[2] Estrin, D., "Requirements for Policy Based Routing in the
Research Internet", RFC 1125, November 1989.
[3] Little, M., "Goals and Functional Requirements for Inter-
Autonomous System Routing", RFC 1126, July 1989.
[4] Breslau, L. and Estrin, D., "Design of Inter-Administrative
Domain Routing Protocols", Proceedings of the ACM SIGCOMM '90
Symposium, September 1990.
[5] Steenstrup, M., "An Architecture for Inter-Domain Policy Rout-
ing", RFC 1478, July 1993.
[6] Austein, R., "DNS Support for IDPR", Work in Progress, March
1993.
[7] Bowns, H. and Steenstrup, M., "Inter-Domain Policy Routing Con-
figuration and Usage", Work in Progress, July 1991.
[8] Woodburn, R., "Definitions of Managed Objects for Inter-Domain
Policy Routing (Version 1)", Work in Progress, March 1993.
[9] McQuillan, J., Richer, I., Rosen, E., and Bertsekas, D.,
"ARPANET Routing Algorithm Improvements: Second Semiannual
Technical Report", BBN Report No. 3940, October 1978.
[10] Moy, J., "The OSPF Specification", RFC 1131, October 1989.
[11] Oran, D. (editor), "Intermediate System to Intermediate System
Routeing Exchange Protocol for Use in Conjunction with the Pro-
tocol for Providing the Connectionless-mode Network Service (ISO
8473)", ISO/IEC JTC1/SC6/WG2, October 1989.
[12] Estrin, D., and Tsudik, G., "Secure Control of Transit Internet-
work Traffic, TR-89-15, Computer Science Department, University
of Southern California.
[13] Linn, J., "Privacy Enhancement for Internet Electronic Mail:
Part I - Message Encipherment and Authentication Procedures",
RFC 1113, August 1989.
[14] Kent, S., and Linn, J., "Privacy Enhancement for Internet Elec-
tronic Mail: Part II - Certificate-based Key Management", RFC
1114, August 1989.
[15] Linn, J., "Privacy Enhancement for Internet Electronic Mail:
Part III - Algorithms, Modes, and Identifiers", RFC 1115, August
1989.
[16] Rivest, R., "The MD4 Message-Digest Algorithm", RFC 1320, April
1992.
[17] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
1992.