| Rfc | 4216 |
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| Title | MPLS Inter-Autonomous System (AS) Traffic Engineering (TE)
Requirements |
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| Author | R. Zhang, Ed., J.-P. Vasseur, Ed. |
|---|
| Date | November 2005 |
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| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Network Working Group R. Zhang, Ed.
Request for Comments: 4216 Infonet Services Corporation
Category: Informational J.-P. Vasseur, Ed.
Cisco Systems, Inc.
November 2005
MPLS Inter-Autonomous System (AS)
Traffic Engineering (TE) Requirements
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document discusses requirements for the support of inter-AS MPLS
Traffic Engineering (MPLS TE). Its main objective is to present a
set of requirements and scenarios which would result in general
guidelines for the definition, selection, and specification
development for any technical solution(s) meeting these requirements
and supporting the scenarios.
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
2. Contributing Authors ............................................4
3. Definitions and Requirements Statement ..........................5
3.1. Definitions ................................................5
3.2. Objectives and Requirements of Inter-AS Traffic
Engineering ................................................7
3.2.1. Inter-AS Bandwidth Guarantees .......................7
3.2.2. Inter-AS Resource Optimization ......................8
3.2.3. Fast Recovery across ASes ...........................8
3.3. Inter-AS Traffic Engineering Requirements Statement ........9
4. Application Scenarios ...........................................9
4.1. Application Scenarios Requiring Inter-AS Bandwidth
Guarantees .................................................9
4.1.1. Scenario I - Extended or Virtual PoP (VPoP) .........9
4.1.2. Scenario II - Extended or Virtual Trunk ............11
4.1.3. Scenario III - End-to-End Inter-AS MPLS TE
from CE to CE ......................................12
4.2. Application Scenarios Requiring Inter-AS Resource
Optimization ..............................................13
4.2.1. Scenario IV - TE across multi-AS within a
Single SP ..........................................13
4.2.2. Scenario V - Transit ASes as Primary and
Redundant Transport ................................14
5. Detailed Requirements for Inter-AS MPLS Traffic Engineering ....16
5.1. Requirements within One SP Administrative Domain ..........16
5.1.1. Inter-AS MPLS TE Operations and Interoperability ...16
5.1.2. Protocol Signaling and Path Computations ...........16
5.1.3. Optimality .........................................17
5.1.4. Support of Diversely Routed Inter-AS TE LSP ........17
5.1.5. Re-Optimization ....................................18
5.1.6. Fast Recovery Support Using MPLS TE Fast Reroute ...18
5.1.7. DS-TE Support ......................................18
5.1.8. Scalability and Hierarchical LSP Support ...........19
5.1.9. Mapping of Traffic onto Inter-AS MPLS TE Tunnels ...19
5.1.10. Inter-AS MPLS TE Management .......................19
5.1.10.1. Inter-AS MPLS TE MIB Requirements ........19
5.1.10.2. Inter-AS MPLS TE Fault Management
Requirements .............................20
5.1.11. Extensibility .....................................21
5.1.12. Complexity and Risks ..............................21
5.1.13. Backward Compatibility ............................21
5.1.14. Performance .......................................21
5.2. Requirements for Inter-AS MPLS TE across Multiple SP ......22
5.2.1. Confidentiality ....................................22
5.2.2. Policy Control .....................................23
5.2.2.1. Inter-AS TE Agreement Enforcement
Polices ...................................23
5.2.2.2. Inter-AS TE Rewrite Policies ..............24
5.2.2.3. Inter-AS Traffic Policing .................24
6. Security Considerations ........................................24
7. Acknowledgements ...............................................24
8. Normative References ...........................................25
9. Informative References .........................................25
Appendix A. Brief Description of BGP-based Inter-AS Traffic
Engineering ...........................................27
1. Introduction
The MPLS Traffic Engineering (TE) mechanism documented in [TE-RSVP]
may be deployed by Service Providers (SPs) to achieve some of the
most important objectives of network traffic engineering as described
in [TE-OVW]. These objectives are summarized as:
- Supporting end-to-end services requiring Quality of Service (QoS)
guarantees
- Performing network resource optimization
- Providing fast recovery
However, this traffic engineering mechanism can only be used within
an Autonomous System (AS).
This document discusses requirements for an inter-AS MPLS Traffic
Engineering mechanism that may be used to achieve the same set of
objectives across AS boundaries within or beyond an SP's
administrative domains.
The document will also present a set of application scenarios where
the inter-AS traffic engineering mechanism may be required. This
mechanism could be implemented based upon the requirements presented
in this document.
These application scenarios will also facilitate discussions for a
detailed requirements list for this inter-AS Traffic Engineering
mechanism.
Please note that there are other means of traffic engineering
including Interior Gateway Protocol (IGP); metrics-based (for use
within an AS); and Border Gateway Protocol (BGP) attribute-based (for
use across ASes, as described in Appendix A), which provide coarser
control of traffic paths. However, this document addresses
requirements for a MPLS-based, fine-grained approach for inter-AS TE.
This document doesn't make any claims with respect to whether it is
possible to have a practical solution that meets all the requirements
listed in this document.
1.1. Conventions Used in This Document
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 [RFC-2119].
2. Contributing Authors
The co-authors listed below contributed to the text and content of
this document. (The contact information for the editors appears in
section 9, and is not repeated below.)
Kenji Kumaki
KDDI Corporation
Garden Air Tower
Iidabashi, Chiyoda-ku,
Tokyo 102-8460, JAPAN
EMail : ke-kumaki@kddi.com
Paul Mabey
Qwest Communications
950 17th Street,
Denver, CO 80202, USA
EMail: pmabey@qwest.com
Nadim Constantine
Infonet Services Corporation
2160 E. Grand Ave.
El Segundo, CA 90025. USA
EMail: nadim_constantine@infonet.com
Pierre Merckx
EQUANT
1041 route des Dolines - BP 347
06906 SOPHIA ANTIPOLIS Cedex, FRANCE
EMail: pierre.merckx@equant.com
Ting Wo Chung
Bell Canada
181 Bay Street, Suite 350
Toronto, Ontario, Canada, M5J 2T3
EMail: ting_wo.chung@bell.ca
Jean-Louis Le Roux
France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex, France
EMail: jeanlouis.leroux@francetelecom.com
Yonghwan Kim
SBC Laboratories, Inc.
4698 Willow Road
Pleasanton, CA 94588, USA
EMail: Yonghwan_Kim@labs.sbc.com
3. Definitions and Requirements Statement
3.1. Definitions
The following provides a list of abbreviations and acronyms
specifically pertaining to this document:
SP: Service Providers including regional or global
providers.
SP Administrative
Domain: a single SP administration over a network or
networks that may consist of one AS or multiple
ASes.
IP-only networks: SP's network where IP routing protocols such as
IGP/BGP are activated.
IP/MPLS networks: SP's network where MPLS switching capabilities and
signaling controls (e.g., ones described in
[MPLS-ARCH]) are activated in addition to IP
routing protocols.
Intra-AS TE: A generic definition for traffic engineering
mechanisms operating over IP-only and/or IP/MPLS
network within an AS.
Inter-AS TE: A generic definition for traffic engineering
mechanisms operating over IP-only and/or IP/MPLS
network across one or multiple ASes. Since this
document only addresses IP/MPLS networks, any
reference to Inter-AS TE in this document refers
only to IP/MPLS networks and is not intended to
address IP-only TE requirements.
TE LSP: MPLS Traffic Engineering Label Switched Path.
Intra-AS MPLS TE: An MPLS Traffic Engineering mechanism where its TE
Label Switched Path (LSP), Head-end Label Switching
Router (LSR), and Tail-end LSR reside in the same
AS for traffic engineering purposes.
Inter-AS MPLS TE: An MPLS Traffic Engineering mechanism where its TE
LSPs, Head-end LSR, and Tail-end LSR do not reside
within the same AS or both Head-end LSR and Tail-
end LSR are in the same AS, but the TE LSP
transiting path may be across different ASes.
ASBRs: Autonomous System Border Routers used to connect to
another AS of a different or the same Service
Provider via one or more links that interconnect
ASes.
Inter-AS TE Path: A TE path traversing multiple ASes and ASBRs, e.g.,
AS1-ASBR1-inter-AS link(s)-ASBR2-AS2... ASBRn-ASn.
Inter-AS TE
Segment: A portion of the Inter-AS TE path.
Inter-AS DS-TE: Diffserv-aware Inter-AS TE.
CE: Customer Edge Equipment
PE: Provider Edge Equipment that has direct connections
to CEs.
P: Provider Equipment that has backbone trunk
connections only.
VRF: Virtual Private Network (VPN) Routing and
Forwarding Instance.
PoP: Point of presence or a node in SP's network.
SRLG: A set of links may constitute a 'shared risk link
group' (SRLG) if they share a resource whose
failure may affect all links in the set as defined
in [GMPLS-ROUT].
PCC: Path Computation Client; any client application
requesting a path computation to be performed by
the Path Computation Element.
PCE: Path Computation Element; an entity (component,
application or network node) that is capable of
computing a network path or route based on a
network graph and applying computational
constraints.
Please note that the terms of CE, PE, and P used throughout this
document are generic in their definitions. In particular, whenever
such acronyms are used, it does not necessarily mean that CE is
connected to a PE in a VRF environment described in such IETF
documents as [BGP-MPLSVPN].
3.2. Objectives and Requirements of Inter-AS Traffic Engineering
As mentioned in section 1 above, some SPs have requirements for
achieving the same set of traffic engineering objectives as presented
in [TE-OVW] across AS boundaries.
This section examines these requirements in each of the key
corresponding areas: 1) Inter-AS bandwidth guarantees; 2) Inter-AS
Resource Optimization and 3) Fast Recovery across ASes, i.e.,
Recovery of Inter-AS Links/SRLG and ASBR Nodes.
3.2.1. Inter-AS Bandwidth Guarantees
The Diffserv IETF working group has defined a set of mechanisms
described in [DIFF_ARCH], [DIFF_AF], and [DIFF_EF] or [MPLS-Diff].
These mechanisms can be activated at the edge of or over a Diffserv
domain to contribute to the enforcement of a QoS policy (or a set of
QoS policies), which can be expressed in terms of maximum one-way
transit delay, inter-packet delay variation, loss rate, etc.
Many SPs have partial or full deployment of Diffserv implementations
in their networks today, either across the entire network or
minimally on the edge of the network across CE-PE links.
In situations where strict QoS bounds are required, admission control
inside the backbone of a network is in some cases required in
addition to current Diffserv mechanisms.
When the propagation delay can be bounded, the performance targets,
such as maximum one-way transit delay, may be guaranteed by providing
bandwidth guarantees along the Diffserv-enabled path.
One typical example of this requirement is to provide bandwidth
guarantees over an end-to-end path for VoIP traffic classified as EF
(Expedited Forwarding [DIFF_EF]) class in a Diffserv-enabled network.
When the EF path is extended across multiple ASes, inter-AS bandwidth
guarantee is then required.
Another case for inter-AS bandwidth guarantee is the requirement for
guaranteeing a certain amount of transit bandwidth across one or
multiple ASes.
Several application scenarios are presented to further illustrate
this requirement in section 4 below.
3.2.2. Inter-AS Resource Optimization
In Service Provider (SP) networks, the BGP protocol [BGP] is deployed
to exchange routing information between ASes. The inter-AS
capabilities of BGP may also be employed for traffic engineering
purposes across the AS boundaries. Appendix A provides a brief
description of the current BGP-based inter-AS traffic engineering
practices.
SPs have managed to survive with this coarse set of BGP-based traffic
engineering facilities across inter-AS links in a largely best-effort
environment. Certainly, in many cases, ample bandwidth within an
SP's network and across inter-AS links reduces the need for more
elaborate inter-AS TE policies.
However, in the case where a SP network is deployed over multiple
ASes (for example, as the number of inter-AS links grows), the
complexity of the inter-AS policies and the difficulty in inter-AS TE
path optimization increase to a level such that it may soon become
unmanageable.
Another example is where inter-AS links are established between
different SP administrative domains. Nondeterministic factors such
as uncoordinated routing and network changes, as well as sub-optimum
traffic conditions, would potentially lead to a complex set of
inter-AS traffic engineering policies where current traffic
engineering mechanisms would probably not scale well.
In these situations where resource optimization is required and/or
specific routing requirements arise, the BGP-based inter-AS
facilities will need to be complemented by a more granular inter-AS
traffic engineering mechanism.
3.2.3. Fast Recovery across ASes
When extending services such as VoIP across ASes, customers often
require SPs to maintain the same level of performance targets, such
as packet loss and service availability, as achieved within an AS.
As a consequence, fast convergence in a stable fashion upon
link/SRLG/node failures becomes a strong requirement. This is
clearly difficult to achieve with current inter-domain techniques,
especially in cases of link/SRLG failures between ASBRs or ASBR node
failures.
3.3. Inter-AS Traffic Engineering Requirements Statement
Just as in the applicable case of deploying MPLS TE in an SP's
network, an inter-AS TE method in addition to BGP-based traffic
engineering capabilities needs to be deployed across inter-AS links
where resource optimization, bandwidth guarantees and fast recovery
are required.
This is especially critical in a Diffserv-enabled, multi-class
environment described in [PSTE] where statistical performance targets
must be maintained consistently over the entire path across different
ASes.
The approach of extending current intra-AS MPLS TE capabilities
[TE-RSVP] across inter-AS links for IP/MPLS networks is considered
here because of already available implementations and operational
experiences.
Please note that the inter-AS traffic engineering over an IP-only
network is for future consideration since there is not sufficient
interest for similar requirements to those of IP/MPLS networks at
this time. More specifically, this document only covers the inter-AS
TE requirements for packet-based IP/MPLS networks.
4. Application Scenarios
The following sections present a few application scenarios over
IP/MPLS networks where requirements cannot be addressed with the
current intra-AS MPLS TE mechanism and give rise to considerations
for inter-AS MPLS traffic engineering requirements.
Although not explicitly noted in the following discussions, fast
recovery of traffic path(s) crossing multiple ASes in a stable
fashion is particularly important in the case of link/SRLG/node
failures at AS boundaries for all application scenarios presented
here.
4.1. Application Scenarios Requiring Inter-AS Bandwidth Guarantees
4.1.1. Scenario I - Extended or Virtual PoP (VPoP)
A global service provider (SP1) would like to expand its reach into a
region where a regional service provider's (SP2) network has already
established a denser network presence.
In this scenario, the SP1 may establish interconnections with SP2 in
one or multiple points in that region. In their customer-dense
regions, SP1 may utilize SP2's network as an extended transport by
co-locating aggregation routers in SP2's PoPs.
In order to ensure bandwidth capacity provided by SP2 and to achieve
some degrees of transparency to SP2's network changes in terms of
capacity and network conditions, one or more inter-AS MPLS TE LSPs
can be built between SP1's ASBR or PE router inside AS1 and SP1's PE
routers co-located in SP2's PoPs, as illustrated in the diagram
below:
<===========Inter-AS MPLS TE Tunnel===========>
----- -----
________|ASBR |___Inter-AS___|ASBR |________
| | RTR | Link | RTR | |
---- ----- ----- ----- -----
|SP1 |_Inter-AS_| SP2 | | SP1 |
|VPoP| Link |P/PE | |P/PE |
---- ----- ----- ----- -----
|________|ASBR |___Inter-AS___|ASBR |________|
| RTR | Link | RTR |
----- -----
<=================Inter-AS MPLS TE Tunnel======================>
+-SP1 AS1-+ +---SP2 AS2-----+ +------SP1 AS1------+
In situations where end-to-end Diffserv paths must be maintained,
both SPs' networks may need to provision Diffserv PHB at each hop in
order to support a set of traffic classes with compatible performance
targets. The subsequent issues regarding Service Level Agreement
(SLA) boundaries, reporting and measuring system interoperability and
support demarcations are beyond the scope of this document and are
not discussed further.
If either SP1's or SP2's network is not a Diffserv-aware network, the
scenario would still apply to provide bandwidth guarantees.
The SP2, on the other hand, can similarly choose to expand its reach
beyond its servicing region over SP1's network via inter-AS MPLS TE
tunnels.
It is worth mentioning that these remote aggregation routers co-
located in another SP's network are unlikely to host SP1's IGP and
BGP routing planes and will more likely maintain their own AS or be
part of the SP1's AS. In this case, such TE tunnels may cross
several ASes, but the Head-end and Tail-end LSRs of TE tunnel may
have the same AS number, as shown in the diagram above.
4.1.2. Scenario II - Extended or Virtual Trunk
Instead of co-locating a PE router in SP2's PoP, SP1 may also choose
to aggregate customer VPN sites onto a SP2's PE router where inter-AS
TE tunnels can be built and signaled through SP2's MPLS network
between the SP2 PoP (to which SP1 and customer CEs are directly
connected) and SP1's ASBR or PE routers inside SP1's network. This
allows SP1's customers connected to SP2 PE router to receive a
guaranteed bandwidth service up to the TE LSP tail-end router located
in SP1's network.
In this scenario, there could be two applicable cases:
Case 1 - the inter-AS MPLS TE tunnel functions as an extended or
virtual trunk aggregating SP1's CE's local-loop access circuits on
SP2's MPLS network over which the bandwidth can be guaranteed to the
TE LSP tail-end router located in SP1's network, as shown in the
diagram below:
<====Inter-AS MPLS TE Tunnel====>
or
< ===Inter-AS MPLS TE Tunnel===============>
---- ----- ----- ----- -----
| CE |_____Local___| SP2 |___|ASBR |___Inter-AS___|ASBR |___|SP1 |
| | Loop | PE | | RTR | Link | RTR | |PE |
---- ----- ----- ----- -----
+SP1 Customer ASx+ +-----SP2 AS2---+ +-SP1 AS1-------+
Case 2 - the inter-AS MPLS TE tunnel in this case functions as an
extended or virtual local access link from SP1's CE on SP2's network
to the SP1's ASBR or PE:
<==============Inter-AS MPLS TE Tunnel==============>
or
<==============Inter-AS MPLS TE Tunnel========================>
---- ----- ----- ----- -----
| CE |____Local_____| SP2 |___|ASBR |___Inter-AS___|ASBR |___|SP1 |
| | Loop | PE | | RTR | Link | RTR | |PE |
---- ----- ----- ----- -----
+SP1 Customer ASx+ +------SP2 AS2---+ +--SP1 AS1-----+
In Case 2 above, SP2 may elect to establish an aggregating or
hierarchical intra-AS MPLS TE tunnel between the transiting P or PE
router and SP2's ASBR router just to reduce the number of tunnel
states signaled from the SP2 PE to where SP1's CEs are connected.
4.1.3. Scenario III - End-to-End Inter-AS MPLS TE from CE to CE
In this scenario as illustrated below, customers require the
establishment of MPLS TE tunnel from CE1 to CE2 end-to-end across
several SPs' networks.
<======================Inter-AS MPLS TE Tunnel==================>
--- ----- ----- ----- ----- ---
|CE1|_____| SP2 |___|ASBR |__Inter-AS__|ASBR |____| SP1 |_____|CE2|
| | | PE | | RTR | Link | RTR | | PE | | |
--- ----- ----- ----- ----- ---
+Cust ASx+ +---SP2 AS-----+ +-------SP1 AS-------+ +Cust ASy+
The diagram below illustrates another example where CE1 and CE2 are
customers of SP1 with external BGP (eBGP) peering relationships
established across the CE-PE links. An inter-AS MPLS TE tunnel may
then be established from CE1 in ASx to CE2, which may belong to the
same AS or a different AS than that of CE1 across SP1's network in
AS2.
<===============Inter-AS MPLS TE Tunnel=====================>
--- ----- ---- ---- ----- ---
|CE1|______| SP1 |_____|SP1 |____|SP1 |____| SP1 |_________|CE2|
| | | PE1 | |P1 | |P2 | | PE2 | | |
--- ----- ---- ---- ----- ---
+-Cust ASx-+ +-------------SP1 AS2----------------+ +-Cust ASy-+
The above example shows that SP1's network has a single AS.
Obviously, there may be multiple ASes between CE1 and CE2, as well as
in the SP1's network.
In addition, where both CE1 and CE2 reside in the same AS, they will
likely share the same private AS number.
However, Scenario III will not scale well if there is a greater
number of inter-AS TE MPLS tunnels in some degrees of partial mesh or
full mesh. Therefore, it is expected that this scenario will have
few deployments, unless some mechanisms such as hierarchical intra-AS
TE-LSPs are used to reduce the number of signaling states.
4.2. Application Scenarios Requiring Inter-AS Resource Optimization
The scenarios presented in this section mainly deal with inter-AS
resource optimization.
4.2.1. Scenario IV - TE across multi-AS within a Single SP
Administrative Domain
As mentioned in [TE-APP], SPs have generally admitted that the
current MPLS TE mechanism provides a great deal of tactical and
strategic value in areas of traffic path optimization [TE-RSVP] and
rapid local repair capabilities [TE-FRR] via a set of on-line or
off-line constraint-based path computation algorithms.
From a service provider's perspective, another way of stating the
objectives of traffic engineering is to utilize available capacity in
the network for delivering customer traffic without violating
performance targets, and/or to provide better QoS services via an
improved network utilization, more likely operating below congestion
thresholds.
It is worth noting that situations where resource provisioning is not
an issue (e.g., low density in inter-AS connectivity or ample inter-
AS capacity), it may not require more scalable and granular TE
facilities beyond BGP routing policies. This is because such
policies can be rather simple and because inter-AS resource
optimization is not an absolute requirement.
However many SPs, especially those with networks across multiple
continents, as well as those with sparsely connected networks, have
designed their multi-AS routing policies along or within the
continental or sub-continental boundaries where the number of ASes
can range from a very few to dozens. Generally, inter-continent or
sub-continent capacity is very expensive. Some Service Providers
have multiple ASes in the same country and would like to optimize
resources over their inter-region links. This would demand a more
scalable degree of resource optimization, which warrants the
consideration of extending current intra-AS MPLS TE capabilities
across inter-AS links.
In addition, one may only realize higher efficiency in conducting
traffic optimization and path protection/restoration planning when
coordinating all network resources as a whole, rather than partially.
For a network which may consist of many ASes, this could be realized
via the establishment of inter-AS TE LSPs, as shown in the diagram
below:
<===================Inter-AS MPLS Tunnel=============>
-------- -------- --------
| |_______________| |____________| |
| SP1 |_______________| SP1 |____________| SP1 |
| AS1 |_______________| AS2 |____________| AS3 |
| | | | | |
-------- -------- --------
|| ||
|| --------- ||
||___________________| SP1 |________________||
|____________________| AS4 |_________________|
| |
---------
The motivation for inter-AS MPLS TE is even more prominent in a
Diffserv-enabled network over which statistical performance targets
are to be maintained from any point to any point of the network as
illustrated in the diagram below with an inter-AS DS-TE LSP:
<===================Inter-AS MPLS DS-TE Tunnel=============>
---- ----- ----- ----- ----- ----
| PE |__| P |___|ASBR |___Inter-AS___|ASBR |___|P |___|PE |
| RTR| | RTR | | RTR | Link | RTR | |RTR | |RTR |
---- ----- ----- ----- ----- ----
+------------SP1 AS1---------+ +------------SP1 AS2------+
For example, the inter-AS MPLS DS-TE LSP shown in the diagram above
could be used to transport a set of L2 Pseudo Wires or VoIP traffic
with corresponding bandwidth requirement.
Furthermore, fast recovery in case of ASBR-ASBR link failure or ASBR
node failure is a strong requirement for such services.
4.2.2. Scenario V - Transit ASes as Primary and Redundant Transport
Scenario V presents another possible deployment case. SP1 with AS1
wants to link a regional network to its core backbone by building an
inter-AS MPLS TE tunnel over one or multiple transit ASes belonging
to SP2, SP3, etc., as shown in the following diagram:
<===========Inter-AS MPLS TE Tunnel=======>
[ ] [ ] [ ]
[ ---- ---- ] [ ---- ---- ] [ ---- ---- ]
[ |P/PE|__|ASBR|]_Inter-AS_[|ASBR|.|ASBR|]_Inter-AS_[|ASBR| |P/PE|]
[ |RTR | |RTR |] Link [|RTR | |RTR |] Link [|RTR | |RTR |]
[ ---- ---- ] [ ---- ---- ] [ ---- ---- ]
[ ] [ ] [ ]
<================Inter-AS MPLS TE Tunnel=====================>
+SP1 Regional ASx+ +Transit SP2 AS2,etc...SPi ASi+ +------SP1 AS1-+
This scenario can be viewed as a broader case of Scenario I shown in
section 4.1.1 where the "VPoP" could be expanded into a regional
network of SP1. By the same token, the AS number for SP1's regional
network ASx may be the same as or different from AS1.
The inter-AS MPLS TE LSP in this case may also be used to backup an
internal path, as depicted in the diagram below, although this could
introduce routing complexities:
<===========Inter-AS MPLS TE Tunnel=======>
+----------------------------SP1 AS1-----------------------------+
[ ]
[ ---- ---- ---- ---- ]
[ |P/PE|__|ASBR|__________Primary Intera-AS________|P | |PE |]
[ |RTR | |RTR | Link |RTR | |RTR |]
[ ---- ---- ---- ---- ]
[ | | ]
[ ---- ---- ]
[ |ASBR| |ASBR| ]
[ |RTR | |RTR | ]
[ ---- ---- ]
^ | | ^
| | | |
| | [ ] | |
| | [ ---- ---- ] | |
| |__ Inter-AS_[|ASBR|..|ASBR|]_Inter-AS_| |
| Link [|RTR | |RTR |] Link |
| [ ---- ---- ] |
| [ ] |
| |
+======Backup Inter-AS MPLS TE Tunnel======+
+Transit SP2 AS2,SP3 AS3,etc....SPi ASi+
5. Detailed Requirements for Inter-AS MPLS Traffic Engineering
This section discusses detailed requirements for inter-AS MPLS TE in
two principal areas: 1) requirements for inter-AS MPLS TE in the same
SP administrative domain and 2) requirements for inter-AS MPLS TE
across different SP administrative domains.
5.1. Requirements within One SP Administrative Domain
This section presents detailed requirements for inter-AS MPLS TE
within the same SP administrative domain.
5.1.1. Inter-AS MPLS TE Operations and Interoperability
The inter-AS MPLS TE solution SHOULD be consistent with requirements
discussed in [TE-REQ] and the derived solution MUST be such that it
will interoperate seamlessly with the current intra-AS MPLS TE
mechanism and inherit its capability sets from [TE-RSVP].
The proposed solution SHOULD allow the provisioning of a TE LSP at
the Head/Tail-end with end-to-end Resource Reservation Protocol
(RSVP) signaling (eventually with loose paths) traversing across the
interconnected ASBRs, without further provisioning required along the
transit path.
5.1.2. Protocol Signaling and Path Computations
One can conceive that an inter-AS MPLS TE tunnel path signaled across
inter-AS links consists of a sequence of ASes, ASBRs, and inter-AS
links.
The proposed solution SHOULD provide the ability either to select
explicitly or to auto-discover the following elements when signaling
the inter-AS TE LSP path:
- a set of AS numbers as loose hops and/or
- a set of LSRs including ASBRs
It should also specify the above elements in the Explicit Route
Object (ERO) and record them in the Record Route Object (RRO) of the
Resv message just to keep track of the set of ASes or ASBRs traversed
by the inter-AS TE LSP.
In the case of establishing inter-AS TE LSP traversing multiple ASes
within the same SP networks, the solution SHOULD also allow the
Head-end LSR to explicitly specify the hops across any one of the
transiting ASes and the TE tunnel Head-end SHOULD also check the
explicit segment to make sure that the constraints are met.
In addition, the proposed solution SHOULD provide the ability to
specify and signal that certain loose or explicit nodes (e.g., AS
numbers, etc.) and resources are to be explicitly excluded in the
inter-AS TE LSP path establishment, such as one defined in
[EXCLUDE-ROUTE].
5.1.3. Optimality
The solution SHOULD allow the set-up of an inter-AS TE LSP that
complies with a set of TE constraints defined in [TE-REQ]) and
follows an optimal path.
An optimal path is defined as a path whose end-to-end cost is
minimal, based upon either an IGP or a TE metric. Note that in the
case of an inter-AS path across several ASes having completely
different IGP metric policies, the notion of minimal path might
require IGP metric normalization.
The solution SHOULD provide mechanism(s) to compute and establish an
optimal end-to-end path for the inter-AS TE LSP and SHOULD also allow
for reduced optimality (or sub-optimality) since the path may not
remain optimal for the lifetime of the LSP.
5.1.4. Support of Diversely Routed Inter-AS TE LSP
Setting up multiple inter-AS TE LSPs between a pair of LSRs might be
desirable when:
(1) a single TE LSP satisfying the required set of constraints
cannot be found, in which case it may require load sharing;
(2) multiple TE paths may be required to limit the impact of a
network element failure to a portion of the traffic (as an
example, two VoIP gateways may load balance the traffic among a
set of inter-AS TE LSPs);
(3) path protection (e.g., 1:1 or 1:N) as discussed in
[MPLS-Recov].
In the examples above, being able to set up diversely routed TE LSPs
becomes a requirement for inter-AS TE.
The solution SHOULD be able to set up a set of link/SRLG/Node
diversely routed inter-AS TE LSPs.
5.1.5. Re-Optimization
Once an inter-AS TE LSP has been established, and should there be any
resource or other changes inside anyone of the ASes, the solution
MUST be able to re-optimize the LSP accordingly and non-disruptively,
either upon expiration of a configurable timer or upon being
triggered by a network event or a manual request at the TE tunnel
Head-End.
The solution SHOULD provide an option for the Head-End LSRs to
control if re-optimizing or not should there exist a more optimal
path in one of the ASes.
In the case of an identical set of traversed paths, the solution
SHOULD provide an option for the Head-End LSRs to control whether
re-optimization will occur because there could exist a more optimal
path in one of the transit ASes along the inter-AS TE LSP path.
Furthermore, the solution MUST provide the ability to reject re-
optimization at AS boundaries.
5.1.6. Fast Recovery Support Using MPLS TE Fast Reroute
There are, in general, two or more inter-AS links between multiple
pairs of ASBRs for redundancy. The topological density between ASes
in a SP network with multi-ASes is generally much higher. In the
event of an inter-AS link failure, rapid local protection SHOULD also
be made available and SHOULD interoperate with the current intra-AS
MPLS TE fast re-route mechanism from [TE-FRR].
The traffic routed onto an inter-AS TE tunnel SHOULD also be fast
protected against any node failure where the node could be internal
to an AS or at the AS boundary.
5.1.7. DS-TE Support
The proposed inter-AS MPLS TE solution SHOULD satisfy core
requirements documented in [DS-TE].
It is worth pointing out that the compatibility clause in section 4.1
of [DS-TE] SHOULD also be faithfully applied to the solution
development.
5.1.8. Scalability and Hierarchical LSP Support
The proposed solution(s) MUST have a minimum impact on network
scalability from both intra- and inter-AS perspectives.
This requirement applies to all of the following:
- IGP (impact in terms of IGP flooding, path computation, etc.)
- BGP (impact in terms of additional information carried within
BGP, number of routes, flaps, overload events, etc.)
- RSVP TE (impact in terms of message rate, number of retained
states, etc.)
It is also conceivable that there would potentially be scalability
issues as the number of required inter-AS MPLS TE tunnels increases.
In order to reduce the number of tunnel states to be maintained by
each transiting PoP, the proposed solution SHOULD allow TE LSP
aggregation such that individual tunnels can be carried onto one or
more aggregating LSP(s). One such mechanism, for example, is
described in [MPLS-LSPHIE].
5.1.9. Mapping of Traffic onto Inter-AS MPLS TE Tunnels
There SHOULD be several possibilities to map particular traffic to a
particular destination onto a specific inter-AS TE LSP.
For example, static routing could be used if IP destination addresses
are known. Another example is to utilize static routing using
recursive BGP route resolution.
The proposed solution SHOULD also provide the ability to "announce"
the inter-AS MPLS TE tunnels as a link into the IGPs (ISIS or OSPF)
with the link's cost associated with it. By doing so, PE routers
that do not participate in the inter-AS TE path computation can take
into account such links in its IGP-based SPF computation.
5.1.10. Inter-AS MPLS TE Management
5.1.10.1. Inter-AS MPLS TE MIB Requirements
An inter-AS TE Management Information Base (MIB) is required for use
with network management protocols by SPs to manage and configure
inter-AS traffic engineering tunnels. This new MIB SHOULD extend
(and not reinvent) the existing MIBs to accommodate this new
functionality.
An inter-AS TE MIB should have features that include:
- The setup of inter-AS TE tunnels with associated constraints
(e.g., resources).
- The collection of traffic and performance statistics not only at
the tunnel head-end, but any other points of the TE tunnel.
- The inclusion of both IPv4/v6 + AS# or AS# subobjects in the ERO
in the path message, e.g.:
EXPLICIT_ROUTE class object:
address1 (loose IPv4 Prefix, /AS1)
address2 (loose IPv4 Prefix, /AS1)
AS2 (AS number)
address3 (loose IPv4 prefix, /AS3)
address4 (loose IPv4 prefix, /AS3) - destination
or
address1 (loose IPv4 Prefix, /AS1)
address2 (loose IPv4 Prefix, /AS1)
address3 (loose IPv4 Prefix, /AS2)
address4 (loose IPv4 Prefix, /AS2)
address5 (loose IPv4 prefix, /AS3)
address6 (loose IPv4 prefix, /AS3) - destination
- Similarly, the inclusion of the RRO object in the Resv message
recording sub-objects such as interface IPv4/v6 address (if not
hidden), AS number, a label, a node-id (when required), etc.
- Inter-AS specific attributes as discussed in section 5 of this
document including, for example, inter-AS MPLS TE tunnel
accounting records across each AS segment.
5.1.10.2. Inter-AS MPLS TE Fault Management Requirements
In a MPLS network, an SP wants to detect both control plane and data
plane failures. But tools for fault detection over LSPs haven't been
widely developed so far. SPs today manually troubleshoot such
failures in a hop-by-hop fashion across the data path. If they
detect an error on the data plane, they have to check the control
plane in order to determine where the faults come from.
The proposed solution SHOULD be able to interoperate with fault
detection mechanisms of intra-AS TE and MAY or MAY NOT require the
inter-AS TE tunnel ending addresses to be known or routable across
IGP areas (OSPF) or levels (IS-IS) within the transiting ASes with
working return paths.
For example, [LSPPING] is being considered as a failure detection
mechanism over the data plane against the control plane and could be
used to troubleshoot intra-AS TE LSPs. Such facilities, if adopted,
SHOULD then be extended to inter-AS TE paths.
However, the above example depicts one such mechanism that does
require a working return path such that diagnostic test packets can
return via an alternate data plane, such as a global IPv4 path in the
event that the LSP is broken.
[MPLS-TTL] presents how TTL may be processed across hierarchical MPLS
networks, and such a facility as this SHOULD also be extended to
inter-AS TE links.
5.1.11. Extensibility
The solution(s) MUST allow extensions as both inter-AS MPLS TE and
current intra-AS MPLS TE specifications evolve.
5.1.12. Complexity and Risks
The proposed solution(s) SHOULD NOT introduce unnecessary complexity
to the current operating network to such a degree that it would
affect the stability and diminish the benefits of deploying such a
solution over SP networks.
5.1.13. Backward Compatibility
The deployment of inter-AS MPLS TE SHOULD NOT impact existing BGP-
based traffic engineering or MPLS TE mechanisms, but allow for a
smooth migration or co-existence.
5.1.14. Performance
The solution SHOULD be evaluated taking into account various
performance criteria:
- Degree of path optimality of the inter-AS TE LSP path
- TE LSP setup time
- Failure and restoration time
- Impact and scalability of the control plane due to added
overheads, etc.
- Impact and scalability of the data/forwarding plane due to added
overheads, etc.
5.2. Requirements for Inter-AS MPLS TE across Multiple SP
Administrative Domains
The requirements for inter-AS MPLS TE across multiple SP admin
domains SHOULD include all requirements discussed in section 5.1
above in addition to those that are presented in this section here.
Please note that the SP with multi-AS networks may choose not to turn
on the features discussed in the following two sections when building
TE tunnels across ASes in its own domain.
5.2.1. Confidentiality
Since an inter-AS TE LSP may span multiple ASes belonging to
different SPs, the solution MIGHT allow hiding the set of hops used
by the TE LSP within an AS, as illustrated in the following example:
[ ASBR1-----ASBR2 ]
[ ] [ ]
[ A ] [ B ]
[ AS1 ] [ AS2 ]
[ SP1 ]-----[ SP2 ]
[ ] [ ]
Suppose there is an inter-AS TE LSP from A (within AS1 of SP1) to B
(within AS2 of SP2). When computing an inter-AS TE LSP path, the set
of hops within AS2 might be hidden to AS1. In this case, the
solution will allow A to learn that the more optimal TE LSP path to B
(that complies with the set of constraints) traverses ASBR2, without
a detailed knowledge of the lists of hops used within AS2.
Optionally, the TE LSP path cost within AS2 could be provided to A
via, for example, PCC-PCE communication, such that A (PCC) could use
this information to compute an optimal path, even if the computed
path is not provided by AS2. (See [PCE-COM] for PCC-PCE
communication and [PCE] for a description of the PCE-based path
computation architecture.)
In addition, the management requirements discussed in section 5.1.10
above, when used across different SP admin domains, SHOULD include
similar confidentiality requirements discussed here in terms of
"hiding" intermediate hops or interface address and/or labels in the
transiting or peering SPs.
5.2.2. Policy Control
In some cases, policy control might be necessary at the AS
boundaries, namely ingress policy controls enabling SPs to enforce
the inter-AS policies per interconnect agreements or to modify some
requested parameters conveyed by incoming inter-AS MPLS TE signaling
requests.
It is worth noting that such a policy control mechanism may also be
used between ASes within a SP.
This section discusses only the elements that may be used to form a
set of ingress control policies, but exactly how SPs establish
bilateral or multilateral agreements upon which the control policies
can be built is beyond the scope of this document.
5.2.2.1. Inter-AS TE Agreement Enforcement Polices
The following provides a set of TE-LSP parameters in the inter-AS TE
Requests (RSVP Path Message) that could be enforced at the AS
boundaries:
- RSVP-TE session attributes: affinities and preemption priorities
- Per AS or SP bandwidth admission control to ensure that RSVP-TE
messages do not request for bandwidth resources over their
allocation
- Request origins which can be represented by Head-End tunnel
ending IP address, originating AS#, neighbor AS#, neighbor ASBR
interface IP address, etc.
- DS-TE TE-Class <Class-Type, Preemption>
- FRR attribute: local protection desired bit, node protection
desired bit, and bandwidth protection desired bit carried in the
- SESSION ATTRIBUTE or the FAST-REROUTE objects in the RSVP Path
message as defined in [TE-FRR]
- Optimization allowed or not allowed
In some cases, a TE policy server could also be used for the
enforcement of inter-AS TE policies. Implementations SHOULD allow
the use of a policy enforcement server. This requirement could allow
SPs to make the inter-AS TE policies scale better.
The signaling of a non-policy-compliant request SHOULD trigger the
generation of a RSVP Path Error message by the policy enforcing node
towards the Head-end LSR, indicating the cause. The Head-end LSR
SHOULD take appropriate actions, such as re-route, upon receipt of
such a message.
5.2.2.2. Inter-AS TE Rewrite Policies
In some situations, SPs may need to rewrite some attributes of the
incoming inter-AS TE signaling requests due to a lack of resources
for a particular TE-Class, non-compliant preemption, or mutual
agreements. The following provides a non-exhaustive list of the
parameters that can potentially be rewritten at the AS boundaries:
- RSVP-TE session attributes: affinities and preemption priorities
- DS-TE TE-Class <Class-Type, Preemption>
- ERO expansion requests
Similarly, the rewriting node SHOULD generate a RSVP Path Error
Message towards the Head-end LSR indicating the cause in terms of
types of changes made so as to maintain the end-to-end integrity of
the inter-AS TE LSP.
5.2.2.3. Inter-AS Traffic Policing
The proposed solution SHOULD also provide a set of policing
mechanisms which could be configured on the inter-AS links to ensure
that traffic routed through the tunnel does not exceed the bandwidth
negotiated during LSP signaling.
For example, an ingress policer could be configured to enforce the
traffic contract on the mutually agreed resource requirements of the
established inter-AS TE LSP (i.e., RSVP bandwidth) on the interface
to which the inter-AS link is connected.
6. Security Considerations
The proposed solution(s) MUST address security issues across multiple
SP administrative domains. Although inter-AS MPLS TE is not expected
to add specific security extensions beyond those of current intra-AS
TE, greater considerations MUST be given in terms of how to establish
a trusted model across AS boundaries. SPs SHOULD have a means to
authenticate (such as using RSVP INTEGRITY Object), to allow, and to
possibly deny inter-AS signaling requests. Also, SPs SHOULD be
protected from DoS attacks.
7. Acknowledgements
We would like to thank Yuichi Ikejiri, David Allan, Kurt Erik
Lindqvist, Dave McDysan, Christian Jacquenet, Kireeti Kompella, Ed
Kern, Jim Boyle, Thomas Nadeau, Yakov Rekhter, and Bert Wijnen for
their suggestions and helpful comments during the discussions of this
document.
8. Normative References
[TE-REQ] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M.,
and J. McManus, "Requirements for Traffic Engineering
Over MPLS", RFC 2702, September 1999.
[TE-RSVP] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
V., and G. Swallow, "RSVP-TE: Extensions to RSVP for
LSP Tunnels", RFC 3209, December 2001.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9. Informative References
[MPLS-ARCH] Rosen, E., Viswanathan, A., and R. Callon,
"Multiprotocol Label Switching Architecture", RFC
3031, January 2001.
[BGP-MPLSVPN] Rosen, E. and Y. Rekhter, "BGP/MPLS IP VPNs", Work in
Progress, October 2004.
[DIFF_ARCH] Blake, S., Black, D., Carlson, M., Davies, E., Wang,
Z., and W. Weiss, "An Architecture for Differentiated
Service", RFC 2475, December 1998.
[DIFF_AF] Heinanen, J., Baker, F., Weiss, W., and J.
Wroclawski, "Assured Forwarding PHB Group", RFC 2597,
June 1999.
[DIFF_EF] Davie, B., Charny, A., Bennet, J.C., Benson, K., Le
Boudec, J., Courtney, W., Davari, S., Firoiu, V., and
D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, March 2002.
[MPLS-Diff] Le Faucheur, F., Wu, L., Davie, B., Davari, S.,
Vaananen, P., Krishnan, R., Cheval, P., and J.
Heinanen, "Multi-Protocol Label Switching (MPLS)
Support of Differentiated Services", RFC 3270, May
2002.
[TE-OVW] Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., and
X. Xiao, "Overview and Principles of Internet Traffic
Engineering", RFC 3272, May 2002.
[PSTE] Li, T. and Y. Rekhter, "A Provider Architecture for
Differentiated Services and Traffic Engineering
(PASTE)", RFC 2430, October 1998.
[TE-APP] Boyle, J., Gill, V., Hannan, A., Cooper, D., Awduche,
D., Christian, B., and W. Lai, "Applicability
Statement for Traffic Engineering with MPLS", RFC
3346, August 2002.
[GMPLS-ROUT] Berger, L., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions",
RFC 3473, January 2003.
[BGP] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[LSPPING] Kompella, K. and G. Swallow, "Detecting MPLS Data
Plane Failures", Work in Progress, May 2005.
[MPLS-TTL] Agarwal, P. and B. Akyol, "Time To Live (TTL)
Processing in Multi-Protocol Label Switching (MPLS)
Networks", RFC 3443, January 2003.
[DS-TE] Le Faucheur, F. and W. Lai, "Requirements for Support
of Differentiated Services-aware MPLS Traffic
Engineering", RFC 3564, July 2003.
[TE-FRR] Pan, P., Swallow, G. and A. Atlas, "Fast Reroute
Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May
2005.
[MPLS-LSPHIE] Kompella, K. and Y. Rekhter, "Label Switched Paths
(LSP) Hierarchy with Generalized Multi-Protocol Label
Switching (GMPLS) Traffic Engineering (TE)", RFC
4206, September 2005.
[MPLS-Recov] Sharma, V. and F. Hellstrand, "Framework for Multi-
Protocol Label Switching (MPLS)-based Recovery", RFC
3469, February 2003.
[EXCLUDE-ROUTE] Lee, CY., Farrel, A., and S. De Cnodder, "Exclude
Routes - Extension to RSVP-TE", Work in Progress,
August 2005.
[PCE] Farrel, A., Vasseur, J.-P., and J. Ash, "Path
Computation Element (PCE) Architecture", Work in
Progress, September 2005.
[PCE-COM] Vasseur, J.-P., et al., "Path Computation Element
(PCE) communication Protocol (PCEP) - Version 1",
Work in Progress, September 2005.
Appendix A. Brief Description of BGP-based Inter-AS Traffic
Engineering
In today's Service Provider (SP) network, BGP is deployed to meet two
different sets of requirements:
- Establishing a scalable exterior routing plane separate from the
data forwarding plane within SP's administrative domain
- Exchanging network reachability information with different BGP
autonomous systems (ASes) that could belong to a different SP or
simply, a different AS within a SP network
Over connections across the AS boundaries, traffic engineering may
also be accomplished via a set of BGP capabilities by appropriately
enforcing BGP-based inter-AS routing policies. The current BGP-based
inter-AS traffic engineering practices may be summarized as follows:
- "Closest exit" routing where egress traffic from one SP to
another follows the path defined by the lowest IGP or intra-AS
MPLS TE tunnel metrics of the BGP next-HOP of exterior routes
learned from other ASes over the inter-AS links
- "BGP path attribute"-based routing selection mechanism where the
egress traffic path is determined by interconnect (peering or
transit) policies based upon one or a combination of BGP path
attributes, like AS_PATH, MULTI_EXIT_DISC (MED), and Local_Pref.
SPs have often faced a number of nondeterministic factors in the
practices of inter-AS traffic engineering employing the methods
mentioned above:
- Sub-optimum traffic distribution across inter-AS links
- Nondeterministic traffic condition changes due to uncoordinated
IGP routing policies or topology changes within other AS and
uncoordinated BGP routing policy changes (MED or as-prepend,
etc.)
In addition, to achieve some degrees of granularity, SPs may choose
to enforce BGP inter-AS policies. These policies are specific to one
inter-AS link or to a set of inter-AS links for ingress traffic. By
tagging certain sets of routes with a specific attribute when
announcing to another AS, the ingress traffic is destined to certain
PoPs or to regions within SP's network from another AS. Of course,
this operates on the assumption that the other AS permits automated
egress policy by matching the predefined attribute from incoming
routes.
Editors' Addresses
Raymond Zhang
Infonet Services Corporation
2160 E. Grand Ave.
El Segundo, CA 90025
USA
EMail: raymond_zhang@infonet.com
J.-P. Vasseur
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough, MA 01719
USA
EMail: jpv@cisco.com
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