| Rfc | 6159 |
|---|
| Title | Session-Specific Explicit Diameter Request Routing |
|---|
| Author | T. Tsou, G.
Zorn, T. Taylor, Ed. |
|---|
| Date | April 2011 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Independent Submission T. Tsou
Request for Comments: 6159 Huawei Technologies (USA)
Category: Informational G. Zorn
ISSN: 2070-1721 Network Zen
T. Taylor, Ed.
Huawei Technologies
April 2011
Session-Specific Explicit Diameter Request Routing
Abstract
This document describes a mechanism to enable specific Diameter
proxies to remain in the path of all message exchanges constituting a
Diameter session.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6159.
IESG Note
Techniques similar to those discussed in this document were discussed
in the IETF Diameter Maintenance and Extensions (DIME) Working Group.
The group had no consensus that the problems addressed by such work
are a real concern in Diameter deployments. Furthermore, there was
no consensus that the proposed solutions are in line with the
architectural principles of the Diameter protocol. As a result, the
working group decided not to undertake the work. There has also not
been a formal request for this functionality from any standards body.
This RFC represents a continuation of the abandoned work. Readers of
this specification should be aware that the IETF has not reviewed
this specification and cannot say anything about suitability for a
particular purpose or compatibility with the Diameter architecture
and other extensions.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. The 3GPP Wireless LAN (WLAN) Access Architecture ................4
3.1. Maintaining the Routing Path ...............................5
4. Diameter Explicit Routing (ER) ..................................6
4.1. Originating a Request (ER-Originator) ......................6
4.2. Relaying and Proxying Requests (ER-Proxy) ..................8
4.3. Receiving Requests (ER-Destination) .......................10
4.4. Diameter Answer Processing ................................11
4.5. Failover and Failback Considerations ......................12
4.6. Attribute-Value Pairs .....................................12
4.6.1. Explicit-Path-Record AVP ...........................12
4.6.1.1. Proxy-Host AVP ............................13
4.6.1.2. Proxy-Realm AVP ...........................13
4.6.2. Explicit-Path AVP ..................................13
4.7. Error Handling ............................................13
5. Example Message Flow ...........................................14
6. RADIUS/Diameter Protocol Interactions ..........................16
7. Security Considerations ........................................17
8. Acknowledgements ...............................................17
9. References .....................................................18
9.1. Normative References ......................................18
9.2. Informative References ....................................18
1. Introduction
In the Diameter base protocol [RFC3588], the routing of request
messages is based solely on the routing decisions made separately by
each node along the path. [RFC5729] has added the ability to force
messages to pass through a specified set of realms through the use of
Network Access Identifier (NAI) decoration. However, no other
specification provides the ability to force routing through a
specific set of agents. Therefore, in a topology where multiple
paths exist from source to destination, there is no guarantee that
all messages relating to a given session will take the same path. In
general, this has not caused problems, but some architectures (e.g.,
WLAN Third Generation Partnership Project (3GPP) IP access
[TS23.234]) require that once certain agents become engaged in a
session, they be able to process all subsequent messages for that
session.
While the solution presented in this document is valid, it violates
one of the basic premises of Diameter -- the robustness of its
architecture. With normal Diameter routing, sessions will survive
failures of agents along the routing path. With the proposals in
this document, routing becomes pinned to specific agents whose
failure will terminate the session.
The authors see no interaction between explicit routing and the
specific applications with which it is employed. Hence, in principle
it can be added to existing applications if they support the
necessary extensibility, and equally can be used with new
applications.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The following terms are used to define the functionality and
participants in the routing extensions described in this document.
ER
Explicit routing -- the mechanism provided by this specification
to allow proxies traversed by the initial message of a session to
ensure that they remain on the messaging path for all subsequent
request messages of a session.
ER-Proxy
A proxy that implements the ER mechanism and can therefore use it
to remain in the path for subsequent messages of a session.
ER-Destination
A Diameter node that is capable of participating in ER and that
will ultimately consume the request sent by an ER-Originator.
ER-Originator
A Diameter node initiating a session and sending the requests.
The ER-Originator can be any Diameter node sending a request,
i.e., a client, server or proxy capable of initiating sessions and
participating in ER.
Authentication, Authorization, and Accounting (AAA) Relays
Other Diameter nodes interspersed between the ER-Originator,
ER-Proxies, and the ER-Destination. These nodes represent
existing Diameter agents and proxies that do not participate in ER
and do not recognize Explicit-Path Attribute Value Pairs (AVPs).
3. The 3GPP Wireless LAN (WLAN) Access Architecture
The 3GPP WLAN IP access architecture [TS23.234] is one example of a
system requiring that certain agents (stateful proxies, in this case)
remain in the forwarding path of all session messages. The 3GPP WLAN
interworking architecture extends 3GPP services to the WLAN access
side, enabling a 3GPP subscriber to use a WLAN to access 3GPP
services.
WLAN AAA provides access to the WLAN to be authenticated and
authorized through the 3GPP system. This access control can permit
or deny a subscriber access to the WLAN system and/or the 3GPP
system.
There are two 3GPP WLAN interworking reference models:
1. In the non-roaming case, the model includes the WLAN access
network and the 3GPP AAA server in the home network. The 3GPP
AAA server is responsible for access control as well as charging.
2. In the roaming case, the model includes the WLAN access network,
the 3GPP AAA proxy in the visited network, and the 3GPP AAA
server in the home network. The 3GPP AAA server is responsible
for access control. Charging records may be generated by the AAA
proxy and/or the AAA server. The AAA proxy relays access control
and charging messages to the AAA server. The AAA proxy will also
do offline charging, if required.
The roaming case presents two problems for which the Diameter routing
mechanism described in [RFC3588] does not offer any unambiguous and
standard solution.
Network Selection
Selecting an initial message path for the Diameter session through
(possibly many) alternative visited network(s) to the home
network.
Explicit Routing (ER)
Maintaining the selected message path for all messages in the
Diameter session.
Selecting an initial message path is outside the scope of this
document. A mechanism for maintaining the selected message path is
described in detail below.
3.1. Maintaining the Routing Path
After a successful authentication, a Diameter session is established
involving (at least) the following stateful entities:
o the Diameter client in the WLAN access node (e.g., the 3GPP AAA
client in the terminal visited network),
o a Diameter proxy in the visited mobile network (e.g., the 3GPP AAA
proxy in the terminal visited network), and
o a Diameter server in the user's home realm (e.g., the destination
3GPP AAA server in the terminal home network).
Message routing for the initial session request uses the normal
Diameter routing tables (Section 2.7 of [RFC3588]) in the 3GPP AAA
client, the 3GPP AAA proxy in the visited network, and any
intermediate proxies after that. The 3GPP AAA client sends the
initial session request to the 3GPP AAA proxy in the visited network.
The 3GPP AAA proxy processes the request, then forwards it towards
the destination 3GPP AAA server, through an intermediate proxy if
necessary. The request may be forwarded through other intermediate
proxies in the same way, until it reaches the destination 3GPP AAA
server in the terminal home network.
The functions assigned to the 3GPP AAA proxy include:
o Reporting charging information to the offline charging system in
the visited network,
o Policy enforcement based on roaming agreements, and
o Service termination initiated by the visited network's operator.
These functions all require that state be maintained within the
visited network. The 3GPP's choice is to maintain that state at the
3GPP AAA proxy. This means that the latter must remain in the
messaging path for all subsequent messages relating to the same
session.
4. Diameter Explicit Routing (ER)
This section outlines a Diameter ER mechanism by which Diameter nodes
participating in ER can remain in the path of all request messages
for a specific session. A new Explicit-Path AVP is defined to enable
ER participants to manipulate the Destination-Host and/or
Destination-Realm AVPs of request messages in order to ensure the
correct routing behavior. The following sections describe the
extensions to the request routing in [RFC3588] to implement the ER
mechanism. The proposed extensions utilize existing routing
strategies in [RFC3588] and do not mandate modifications to it. The
mechanism imposes loose rather than strict source routing, in that
subsequent messages of a session are forced through the participating
nodes, but not through any individual non-participating nodes. In
summary, only Diameter nodes interested in participating in the ER
scheme will be involved in it.
4.1. Originating a Request (ER-Originator)
A Diameter node acting as an ER-Originator for a particular session
MUST maintain a local cache that enumerates all the Diameter
identities of the ER-Proxies that the request messages must traverse
along the path to the ER-Destination. The identity of a Diameter
node is defined in [RFC3588]. The local cache MAY also include the
node's realm. The data structure of the cache is left up to the
implementation and SHOULD persist as part of the session attributes
or properties.
An ER-Originator sending request messages MUST add an Explicit-Path
AVP to these requests. The contents of the cache SHOULD be used to
populate the Explicit-Path AVP, with each cached entry represented by
a corresponding instance of the Explicit-Path-Record AVP. ER-Proxies
along the path of the request message MUST examine the contents of
the Explicit-Path AVP and make routing adjustments based on records
it contains. An example of the message flow is shown in Section 5.
Note that the ER-Originator can be any Diameter node, i.e., a client,
server, or proxy.
The ER-Originator can populate the cache either by pre-configuring
its contents or by using the first request message of the session to
gather identities of participating ER-Proxies along the routing path.
The latter scheme is known as Explicit-Path discovery. The contents
of the cache can be pre-configured if the ER-Originator has explicit
knowledge of the ER-Proxies the request messages must traverse;
otherwise, the ER-Originator can use Explicit-Path discovery. It is
RECOMMENDED that Explicit-Path discovery be used whenever possible
since pre-configuration is less flexible by nature.
Explicit-Path discovery is useful if the identities of the ER-Proxies
are not known or if there are several ER-capable proxies (a cluster
of proxies) that can be dynamically chosen based on other routing
policies. In Explicit-Path discovery, the cache of the ER-Originator
is initially empty. To initiate discovery, when the ER-Originator
sends the first request message of a session, it MUST include the
Explicit-Path AVP containing a single Explicit-Path-Record AVP with
the identity and/or the realm of the ER-Originator. The
ER-Originator MUST set the Destination-Host and/or Destination-Realm
AVP of the request message to the identity and/or the realm of the
ER-Destination, respectively, as specified in [RFC3588].
Note that ER-Originator initial request message routing procedures
and the process of population of the Destination-Realm may be
affected by the User-Name AVP NAI decoration [RFC5729]. NAI
decoration is a form of request message source routing and defines
realms that the request message must traverse through before
routing towards the ER-Destination. Diameter nodes participating
in request message routing must examine and process the User-Name
AVP, and modify the Destination-Realm AVP accordingly as long as
there are realms left in the decorated NAI. Source routing based
upon NAI decoration does not affect Explicit-Path discovery as
defined in this document.
If the path taken by the initial request encounters one or more
participating ER-Proxies and a participating ER-Destination, the
procedures described in Section 4.2 and Section 4.3 ensure that a
successful response to that request will contain an Explicit-Path AVP
that includes one or more Explicit-Path-Records containing the
ER-Originator's identity, the identities of all participating
ER-Proxies, and the identity of the ER-Destination. The
ER-Originator SHOULD populate its local cache with the contents of
the Explicit-Path AVP received in this initial answer message.
If the answer message does not contain an Explicit-Path AVP or the
Result-Code AVP is set to DIAMETER_ER_NOT_AVAILABLE (Section 4.7), it
is an indication to the ER-Originator that the destination of the
request does not support ER and that the ER-Originator SHOULD avoid
sending an Explicit-Path AVP in subsequent request messages.
If the initial request message initiated Explicit-Path discovery, but
the Explicit-Path AVP in the answer message contains Explicit-Path-
Records for the ER-Originator and ER-Destination only, it is an
indication to the ER-Originator that there are no Diameter proxies
capable of participating in ER along the path and that the
ER-Originator SHOULD NOT send an Explicit-Path AVP in subsequent
request messages of this session. See Section 4.5 for more
discussion. In such cases, the situation may be transient, and
Explicit-Path discovery may find participating proxies in succeeding
sessions. It is left up to the ER-Originator to decide if Explicit-
Path discovery should be attempted in succeeding sessions.
Once the ER-Originator's local cache has been populated, whether by
pre-configuration or through Explicit-Path discovery, all request
messages for the session MUST include the Explicit-Path AVP using the
contents of the local cache. The Explicit-Path AVP MUST contain the
Explicit-Path-Records of all the nodes enumerated in the cache except
that of the ER-Originator itself. The identities enumerated in the
Explicit-Path AVP MUST appear in the order they will be traversed in
the routing path. The last entry in the Explicit-Path AVP MUST be
the Explicit-Path-Record of the ER-Destination. In addition, the
value of the Destination-Host and possibly the Destination-Realm in
the request message MUST be copied from the values of the Proxy-Host
AVP and, if present, the Proxy-Realm AVP of the first Explicit-Path-
Record AVP present in the Explicit-Path AVP.
This ensures that the ER-Originator as well as any AAA relays
between the ER-Originator and the first ER-Proxy will route the
message towards the first ER-Proxy as specified in RFC 3588
[RFC3588].
Subsequent actions taken by the first ER-Proxy upon receipt of the
message are described in Section 4.2 and will mimic those of the
ER-Originator.
Answer messages received by the ER-Originator to subsequent request
messages after the Explicit-Path has been established SHOULD NOT have
an Explicit-Path AVP. If they do, this SHOULD be considered a
suspect condition that may be caused by a misbehaving ER participant.
It is left up to the ER-Originator whether to continue using the ER
scheme when such a condition arises or to attempt another Explicit-
Path discovery for subsequent sessions.
4.2. Relaying and Proxying Requests (ER-Proxy)
The basic action taken by an ER-Proxy upon receiving a request is to
check whether explicit routing is supported in the request and if so,
check whether it is already a participant in explicit routing for the
said request. If it is not an existing participant, if Explicit-Path
discovery is in progress, and if it wishes to participate, it appends
an Explicit-Path-Record AVP identifying itself to the end of the
Explicit-Path AVP. If it is an existing participant, the ER-Proxy
pops/removes the Explicit-Path-Record AVP pertaining to itself from
the Explicit-Path AVP and then uses the next Explicit-Path-Record AVP
for subsequent routing. Details of this operation follow.
An ER-Proxy is not required to keep local state or cache state
regarding the explicit routing procedure. However, it MUST check
whether an incoming request contains an Explicit-Path AVP. The
following cases can occur.
1. If an incoming request does not contain an Explicit-Path AVP,
then the ER-Proxy takes no action beyond processing and
forwarding the request as specified in [RFC3588].
2. If the incoming request contains an Explicit-Path AVP, the
ER-Proxy MUST check whether its identity is present in the
Explicit-Path AVP. Determining whether its identity is present
can be done by matching its identity to the Proxy-Host AVP
contained in each Explicit-Path-Record. If its identity is not
present, then:
A. If it wishes to participate in explicit routing, the ER-Proxy
MUST verify that Explicit-Path discovery is in progress by
verifying that the Proxy-Host AVP in the first Explicit-Path-
Record AVP in the Explicit-Path AVP does not match the
Destination-Host AVP (if present). If this verification
succeeds or the Destination-Host AVP is absent, the ER-Proxy
MAY append a new Explicit-Path-Record as the last AVP in the
Explicit-Path AVP prior to forwarding the request. The new
Explicit-Path-Record MUST contain a Proxy-Host AVP set to the
proxy's identity, and MAY contain a Proxy-Realm AVP giving
the proxy's realm. If, however, the Destination-Host AVP is
present and matches the Proxy-Host AVP of the first Explicit-
Path-Record AVP, then the Explicit-Path contains an already-
defined source route that does not include the ER-Proxy. The
ER-Proxy SHOULD process the request as if the ER-Path AVP
were absent.
B. If the ER-Proxy does not wish to participate in the ER, it
SHOULD NOT modify the Explicit-Path AVP and SHOULD simply
process and forward the request as specified in [RFC3588]
using the existing values of the Destination-Host and/or
Destination-Realm AVPs. Non-ER-Proxies and relays that do
not support ER and do not recognize Explicit-Path AVP will
take the same action.
3. If the identity of the ER-Proxy is present in the Explicit-Path
AVP, then:
A. If it is not the first Explicit-Path-Record in the AVP, this
MUST be considered an error, and an answer message with the
'E' bit set and the Result-Code set to
DIAMETER_INVALID_PROXY_PATH_STACK MUST be sent back to the
ER-Originator (Section 4.7).
B. If the identity of the ER-Proxy matches the first Explicit-
Path-Record, the ER-Proxy MUST remove this record from the
Explicit-Path AVP and repopulate the Destination-Host and
possibly the Destination-Realm AVP from the next Explicit-
Path-Record present in the Explicit-Path AVP. Setting the
Destination-Host and possibly the Destination-Realm AVP will
ensure that the ER-Proxy as well as all AAA relays between
the current ER-Proxy and the next ER-Proxy enumerated in the
Explicit-Path AVP will route the message towards the next
ER-Proxy. The process of removing the ER-Proxy's record is
analogous to popping an entry from a stack represented by the
Explicit-Path AVP.
The behavior specified above also applies to a Diameter node that
acts as a relay agent and participates in the ER scheme.
4.3. Receiving Requests (ER-Destination)
A Diameter node that locally processes requests sent by the
ER-Originator (Section 4.1) and is able to support ER (an
ER-Destination) MUST check for the presence of an Explicit-Path AVP
in the request message.
1. If an incoming request does not contain an Explicit-Path AVP,
then it is an indication that messages belonging to this session
will not use ER. The ER-Destination MUST simply process the
request for local consumption and formulate an answer message as
specified in [RFC3588].
2. If the incoming request contains an Explicit-Path AVP, the
ER-Destination MUST check whether its identity is present in the
Explicit-Path AVP. If its identity is not present, indicating
that Explicit-Path discovery is in progress, then:
A. If it wishes to participate in the ER, and subject to
paragraph B below, the ER-Destination MUST append a new
Explicit-Path-Record to the Explicit-Path AVP in the received
message. The new Explicit-Path-Record MUST contain at the
least a Proxy-Host AVP set to the ER-Destination's identity.
The ER-Destination MUST then copy the resulting Explicit-Path
AVP to the subsequent answer message.
B. If there is only one Explicit-Path-Record in the incoming
Explicit-Path AVP, then this is an indication of a successful
Explicit-Path discovery, but with no participating
ER-Proxies. The ER-Destination SHOULD NOT copy the Explicit-
Path AVP into the subsequent answer message.
C. If the ER-Destination supports ER but does not wish to or
cannot participate, it MAY send a Result-Code AVP set to
DIAMETER_ER_NOT_AVAILABLE as defined in Section 4.7. The
ER-Destination MUST NOT include any Explicit-Path AVP in the
subsequent answer. Diameter servers that do not support ER
and do not recognize the Explicit-Path AVP will also omit the
Explicit-Path AVP from the answer message.
3. If the identity of the ER-Destination matches a record in the
Explicit-Path AVP, then it MUST be the only Explicit-Path-Record
present in the Explicit-Path AVP. Otherwise, this MUST be
considered an error, and an answer message with the 'E' bit set
and containing an Experimental-Result-Code AVP set to
DIAMETER_INVALID_PROXY_PATH_STACK MUST be sent back to the
ER-Originator (Section 4.7). If the identity of the
ER-Destination does match the only existing Explicit-Path-Record,
then this is an indication that the request reached the
ER-Destination by way of a successfully executed explicit route.
The ER-Destination MUST NOT include the Explicit-Path AVP in the
subsequent answer message.
4.4. Diameter Answer Processing
There is no requirement on Diameter nodes participating in ER to
provide special handling or routing of answer messages. Answer
messages SHOULD be processed normally as specified in [RFC3588].
However, a Diameter node acting as an ER-Destination MUST formulate a
proper Explicit-Path AVP in answer messages as described in
Section 4.3.
4.5. Failover and Failback Considerations
If there is no ER-Proxy along the selected path, the answer message
MAY contain an Explicit-Path AVP that contains only the Explicit-
Route-Records of the ER-Originator and the ER-Destination, indicating
that there is no ER support found in Diameter nodes along the path.
It is left to the ER-Originator to continue with processing of the
request without ER support or terminate the session. The
ER-Originator SHOULD NOT attempt to perform Explicit-Path discovery
in subsequent request messages of this session in such cases, to
protect against failback conditions where an ER-Proxy suddenly
appears in the path and attempts to add a new Explicit-Path-Record
for request messages other than the initial request.
Allowing an ER-Proxy to join the session after the initial request
makes sense only if the application requirements do not mandate
that every participating ER-Proxy receive all of the messages of a
session.
However, depending on local policy, the ER-Originator MAY attempt ER
path discovery in subsequent sessions despite the lack of proxy
participants in the earlier attempt.
If a failover occurs in a Diameter node preceding an ER-Proxy when
the Explicit-Path is already established, it is possible that a
DIAMETER_UNABLE_TO_DELIVER error will be received by the
ER-Originator if there are no alternative paths towards the ER-Proxy.
In such a case, it is left to the ER-Originator to handle the error
as specified in the Diameter application or in [RFC3588].
4.6. Attribute-Value Pairs
The following sections define the AVPs used in the ER process. All
of these AVPs MUST have the 'V' bit set and the 'M' bit cleared, with
the Vendor-ID field set to 2011 (as assigned by IANA in "Private
Enterprise Numbers" registry; see http://www.iana.org/).
4.6.1. Explicit-Path-Record AVP
The Explicit-Path-Record AVP (AVP Code 35001) is of type Group. The
identity added in the Proxy-Host [RFC3588] element of this AVP MUST
be the same as the one advertised by the Diameter node in the Origin-
Host AVP during the Capabilities Exchange messages.
Explicit-Path-Record ::= < AVP Header: 35001 >
{ Proxy-Host }
[ Proxy-Realm ]
4.6.1.1. Proxy-Host AVP
The Proxy-Host AVP (AVP Code 35004) is of type DiameterIdentity. It
identifies the ER node that is inserting the record. The Proxy-Host
AVP MUST be present.
4.6.1.2. Proxy-Realm AVP
The Proxy-Realm AVP (AVP Code 35002) is of type DiameterIdentity, and
contains the realm of the ER node inserting the record. The Proxy-
Realm AVP MAY be present in the Explicit-Path-Record. If it is
present, the realm name included in the value of the Proxy-Host AVP
MUST match the value of the Proxy-Realm AVP.
4.6.2. Explicit-Path AVP
The Explicit-Path AVP (AVP Code 35003) is of type Grouped. This AVP
MUST be present in all request messages performing ER. It MAY be
present in the answer to the initial session request message if
Explicit-Path discovery was successfully executed for the request.
Explicit-Path ::= < AVP Header: 35003 >
1* [ Explicit-Path-Record ]
* [ AVP ]
4.7. Error Handling
The following error conditions may occur during ER processing. All
error indications MUST be encapsulated in an instance of the
Experimental-Result AVP [RFC3588] with the Vendor-ID AVP set to 2011
and the Experimental-Result-Code set as specified below.
DIAMETER_INVALID_PROXY_PATH_STACK 3501
A request message received by an ER-Proxy or ER-Destination after
an Explicit-Path has been established has the first or only
Explicit-Path-Record AVP not matching the ER-Proxy's or the
ER-Destination's identity. The same error applies to
ER-Destinations receiving an Explicit-Path-AVP containing more
than one Explicit-Path-Record or an Explicit-Path-AVP with only
one Explicit-Path-Record not matching its own identity.
This error SHOULD be considered a protocol failure and SHOULD be
treated on a per-hop basis; Diameter proxies may attempt to
correct the error, if possible. Diameter answer messages
containing this error indication MUST have the 'E' bit set and
MUST conform to Section 7.2 of [RFC3588].
DIAMETER_ER_NOT_AVAILABLE 4501
An ER-Destination that supports ER routing but is unable to comply
for unknown reasons MAY send an answer message with the Result-
Code AVP set to this error code. This error value SHOULD be
considered a transient failure indicating that subsequent ER
attempts may succeed.
5. Example Message Flow
The example presented here illustrates the flow of Diameter messages
with the typical attributes present in the ER scenario.
The ER-Originator in the example below shows the use of Explicit-Path
discovery with the first request. However, the ER-Originator could
also use a pre-configured cache. The ER-Originator can be any
Diameter node sending a request, i.e., a client, server, or proxy.
In this scenario, the local cache of the ER-Originator is initially
empty.
The AAA relays between the ER-Proxies, ER-Originator, and
ER-Destination may or may not be present and are shown here to depict
routing paths that the requests may take prior to being processed by
nodes participating in the ER scheme. The AAA relays also depict
existing Diameter relays or proxies that do not recognize Explicit-
Path AVPs and therefore do not participate in ER.
ER- ER- ER- ER-
Originator AAA relays Proxy1 AAA relays Proxy2 Destination
(o.r1 (p.r1 (p.r2 (d.r2
.example) .example) .example) .example)
| | | | |
cache=(empty) | | | | |
------------->|--------->| | | |
(1st request of the session)| | | |
Explicit-Path= | | | |
o.r1.example,r1.example | | |
dest-host=d.r2.example | | | |
dest-realm=r2.example | | | |
| | | | |
| |--------->|--------->| |
| | (forwarded request)| |
| | Explicit-Path= | |
| | record1=o.r1.example,r1.example
| | record2=p.r1.example,r1.example
| | dest-host=d.r2.example |
| | dest-realm=r2.example |
| | | | |
| | | |--------->|
| | | (forwarded request)
| | | Explicit-Path=
| | | record1=o.r1.example,
| | | r1.example
| | | record2=p.r1.example,
| | | r1.example
| | | record3=p.r2.example,
| | | r2.example
| | | dest-host=d.r2.example
| | | dest-realm=r2.example
| | | | |
cache= |<---------|<---------|<---------|<---------|
record1=o.r1.example,r1.example (answer) |
record2=p.r1.example,r1.example Explicit-Path=
record3=p.r2.example,r2.example record1=o.r1.example,r1.example
record4=d.r2.example,r2.example record2=p.r1.example,r1.example
| | record3=p.r2.example,r2.example
| | record4=d.r2.example,r2.example
Note: An originator pre-configuring | | |
its local cache can skip the | | |
exchange above and send the | | |
initial request as shown below. | | |
| | | | |
------------->|--------->| | | |
(subsequent request of the session) | | |
Explicit-Path= | | | |
record1=p.r1.example,r1.example | | |
record2=p.r2.example,r2.example | | |
record3=d.r2.example,r2.example | | |
dest-host=p.r1.example | | | |
dest-realm=r1.example | | | |
| |--------->|--------->| |
| | (forwarded request)| |
| | Explicit-Path= | |
| | record1=p.r2.example,r2.example
| | record2=d.r2.example,r2.example
| | dest-host=p.r2.example |
| | dest-realm=r2.example |
| | | | |
| | | |--------->|
| | | (forwarded request)
| | | Explicit-Path
| | | record1=d.r2.example,
| | | r2.example
| | | dest-host=d.r2.example
| | | dest-realm=r2.example
| | | | |
cache= |<---------|<---------|<---------|<---------|
record1=o.r1.example,r1.example (answer) | |
record2=p.r1.example,r1.example * no Explicit-Path-AVP present
record3=p.r2.example,r2.example | | |
record4=d.r2.example,r2.example | | |
| | | | |
| | | | |
(subsequent request of the session will repeat the process above)
| | | | |
| | | | |
Figure 1: Example ER Message Flow
6. RADIUS/Diameter Protocol Interactions
No actions need to be taken with regards to RADIUS/Diameter
interaction. The routing extension described in this document is
transparent to any translation gateway and relevant only to Diameter
routing. The assumption is that if there is a RADIUS proxy chain
between Diameter translation agents, the route between translation
agents remains stable during the session and does not cause an
invalidation of the proxy path stack.
7. Security Considerations
The security considerations in [RFC3588] apply to this extension. In
addition, this extension raises questions of authorization and can
potentially allow a new denial-of-service attack.
The authorization issue comes about because the proxies that
participate in ER are self-selected. An ER-Proxy is able, through
the operation of ER, to guarantee that it can monitor every message
of a session. This is in contrast to ordinary Diameter routing,
where some messages may pass by an alternate route. The question is
whether the originating party is prepared to extend this additional
degree of trust to arbitrary parties along the path. If not, the
ER-Originator requires a mechanism to determine whether an ER-Proxy
listed in the returned Explicit-Path AVP can be trusted. If it has
such a mechanism, then an unwanted ER-Proxy can be deleted from its
cache and thus not appear in the ER-Path AVP in subsequent requests.
This specification assumes that either the originating party is
prepared to allow arbitrary Diameter nodes along the path to attach
themselves to the session as ER-Proxies, or the ER-Originator
maintains a pre-configured list of ER-Proxies in its cache.
The potential denial-of-service attack is not a serious one because
the same result can be obtained more directly. An attacker with
control of a Diameter node along the path of the original request
could insert an Explicit-Path-Record containing the identity of
another node or a non-existent node, rather than its own identity.
Routing subsequent messages of the session through another node could
result in violation of the trust assumptions made upstream. Routing
subsequent messages to a non-existent node causes them to be lost and
terminates the session. It would seem simpler to perpetrate whatever
harm the attacker intends at the subverted Diameter node itself. The
advantage of using ER to accomplish either of the attacks is that it
makes it more difficult to determine which node misbehaved, but the
extra effort involved to implement the attack does not seem to be
worth the potential gain.
8. Acknowledgements
The authors gratefully acknowledge the contributions of Tony Zhang,
Fortune Huang, Rajith R., Victor Fajardo, Jouni Korhonen, Tolga
Asveren, Mark Jones, Avi Lior, Steve Norreys, Lionel Morand, Dave
Frascone, and Hannes Tschofenig.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588,
September 2003.
[RFC5729] Korhonen, J., Ed., Jones, M., Morand, L., and T. Tsou,
"Clarifications on the Routing of Diameter Requests Based
on the Username and the Realm", RFC 5729, December 2009.
9.2. Informative References
[TS23.234] 3GPP, "3GPP system to Wireless Local Area Network (WLAN)
interworking; System description", TS 23.234
Version 7.4.0, 2006.
Authors' Addresses
Tina Tsou
Huawei Technologies (USA)
2330 Central Expressway
Santa Clara, CA 95050
USA
Phone: +1 408 330 4424
EMail: tena@huawei.com
URI: http://tinatsou.weebly.com/contact.html
Glen Zorn
Network Zen
227/358 Thanon Sanphawut
Bang Na, Bangkok 10260
Thailand
Phone: +66 (0) 87-040-4617
EMail: gwz@net-zen.net
Tom Taylor (editor)
Huawei Technologies
1852 Lorraine Ave.
Ottawa
Canada
EMail: tom111.taylor@bell.net