draft-ietf-tewg-interas-mpls-te-req-09.txt   draft-ietf-tewg-interas-mpls-te-req-10.txt 
IETF Internet Draft Raymond Zhang, Editor This Internet-Draft, draft-ietf-tewg-interas-mpls-te-req-09.txt, was published as an Informational RFC, RFC 4216
Internet Engineering Task Force Infonet Services Corporation (http://www.ietf.org/rfc/rfc4216.txt), on 2005-11-21.
draft-ietf-tewg-interas-mpls-te-req-09.txt JP Vasseur, Co-Editor
Expires: March. 2005 CISCO Systems,Inc
September 2004
MPLS Inter-AS Traffic Engineering requirements
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004).
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.
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].
MPLS Inter-AS TE requirements............... September 2004
Table of Contents
1. Introduction.......................................................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......6
3.2.1. Inter-AS Bandwidth Guarantees..................................6
3.2.2. Inter-AS Resource Optimization.................................7
3.2.3. Fast Recovery across ASes......................................8
3.3. Inter-AS Traffic Engineering Requirements Statement..............8
4. Application Scenarios..............................................8
4.1. Application Scenarios Requiring Inter-AS Bandwidth Guarantees....9
4.1.1. Scenario I - Extended or Virtual PoP...........................9
4.1.2. Scenario II - Extended or Virtual Trunk.......................10
4.1.3. Scenario III - End-to-end Inter-AS MPLS TE From CE to CE......11
4.2. Application Scenarios Requiring Inter-AS Resource Optimization..12
4.2.1. Scenario IV - TE across multi-AS within a Single SP
Administrative Domain.........................................12
4.2.2. Scenario V - Transit ASes as Primary and Redundant Transport..13
5. Detailed Requirements for Inter-AS MPLS Traffic Engnineering......14
5.1. Requirements within one SP Administrative Domain................14
5.1.1. Inter-AS MPLS TE Operations and Interoperability..............14
5.1.2. Protocol Signaling and Path Computations......................15
5.1.3. Optimality....................................................15
5.1.4. Support of diversely routed inter-AS TE LSP...................15
5.1.5. Re-optimization...............................................16
5.1.6. Fast Recovery support using MPLS TE Fast Reroute..............16
5.1.7. DS-TE Support.................................................17
5.1.8. Scalability and Hierarchical LSP Support......................17
5.1.9. Mapping of traffic onto Inter-AS MPLS TE Tunnels..............17
5.1.10. Inter-AS MPLS TE Management..................................18
5.1.10.1. Inter-AS MPLS TE MIB Requirements..........................18
5.1.10.2. Inter-AS MPLS TE Fault Management Requirements.............18
5.1.11. Extensibility................................................19
5.1.12. Complexity and Risks.........................................19
5.1.13. Backward Compatibility.......................................19
5.1.14. Performance..................................................19
5.2. Requirements for Inter-AS MPLS TE across Multiple SP
Administrative Domains..........................................20
5.2.1. Confidentiality...............................................20
5.2.2. Policy Control................................................20
5.2.2.1. Inter-AS TE Agreement Enforcement Polices...................21
5.2.2.2. Inter-AS TE Rewrite Policies................................21
5.2.2.3 Inter-AS Traffic Policing....................................22
6. Security Considerations...........................................22
7. Acknowledgements..................................................22
8. Editor's Addresses................................................22
9. Normative References............................................. 23
10. Informative References...........................................23
MPLS Inter-AS TE requirements September 2004
11. Full Copyright Statement.........................................25
12. Intellectual Property............................................25
13. Acknowledgement..................................................25
Appendix A. Brief Description of BGP based Inter-AS Traffic
Engineering..............................................26
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 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 a SP's
aministrative 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.
MPLS Inter-AS TE requirements.............. September 2004
2. Contributing Authors
The text and content of this document was contributed to by the
co-authors listed below (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
E-mail : 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
E-mail: jeanlouis.leroux@francetelecom.com
Yonghwan Kim
SBC Laboratories, Inc.
4698 Willow Road
Pleasanton, CA 94588, USA
Email: Yonghwan_Kim@labs.sbc.com
MPLS Inter-AS TE requirements.............. September 2004
3. Definitions and Requirements Statement
3.1. Definitions
The following provides a list of abbreviations or 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
ASBR Routers: Border routers used to connect to another AS of a
different or the same Service Provider via one or more
links inter-connecting between ASes.
Inter-AS TE Path: An TE path traversing multiple ASes and ASBRs,
e.g. AS1-ASBR1-inter-AS link(s)-ASBR2-AS2...
ASBRn-ASn.
MPLS Inter-AS TE requirements.............. September 2004
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].
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 drafts
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]
that can be activated at the edge or over a DiffServ domain to
contribute to the enforcement of a (set of) QoS policy(ies), 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 Quality of Service (QOS) bounds are
required, admission control inside the backbone of a network is in
some cases required in addition to current Diffserv mechanisms.
MPLS Inter-AS TE requirements September 2004
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 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.
MPLS Inter-AS TE requirements September 2004
3.2.3. Fast Recovery across ASes
When extending services such as VoIP across ASes, customers often
reqiure 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 a 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 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.
MPLS Inter-AS TE requirements June 2004
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 achieve
some degrees of transparency to SP2's network changes in terms of
capacity and network conditions, one or more Inter-AS MPLS TE
trunk(s) can be built between SP1's ASBR or PE router inside AS1 and
SP1's PE routers co-locating 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 SP's networks may need to provision Diffserv PHB at each hop
supporting a set of traffic classes with compatible performance
targets. The subsequent issues regarding Service Level Agreement
(SLA) boundaries, reporting and measuring system inter-operability
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 paths.
MPLS Inter-AS TE requirements September 2004
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 a customer CEs are directly
connected) and SP1's ASBR or PE routers inside SP2'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-----+
MPLS Inter-AS TE requirements September 2004
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 SPsĘ 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 extenal 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.
Scenario III however, 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.
MPLS Inter-AS TE requirements Septmber 2004
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 searching 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, operating more likely 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 since 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 diagragm below:
MPLS Inter-AS TE requirements September 2004
<===================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.
MPLS Inter-AS TE requirements September 2004
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 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.
MPLS Inter-AS TE requirements September 2004
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 to either
explicitly select or 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] for instance.
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 life-time of the LSP
5.1.4 Support of diversely routed inter-AS TE LSP
In some cases it might be desirable to set up multiple inter-AS TE
LSPs between a pair of LSRs when:
MPLS Inter-AS TE requirements September 2004
(1) a single TE LSP satisfying the required set of constraints
cannot be found, in which case it may require load splitting;
(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
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 path, 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 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 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.
MPLS Inter-AS TE requirements September 2004
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 (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.
MPLS Inter-AS TE requirements September 2004
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 headend, 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, a 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.
MPLS Inter-AS TE requirements September 2004
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 a 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.
MPLS Inter-AS TE requirements September 2004
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-PCS signaling [PATH-COMP], such that A (PCC)
could use this information to compute an optimal path, even if the
computed path is not provided by AS2.
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 modify some
requested parameters conveyed by incoming inter-AS MPLS TE signaling
requests.
MPLS Inter-AS TE requirements September 2004
It is worth noting that such a policy control mechanism may also be
used between ASes within a SP.
This section only discusses 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 are 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 SHOULD 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 upon mutual
agreements. The following lists 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
MPLS Inter-AS TE requirements September 2004
Similarly, the re-writing 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 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), allow and possibly denying inter-AS signaling requests
and 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 Nadeauor, Yakov Rekhter and Bert Wijnen
for their suggestions and helpful comments during the discussions of
this draft.
8. Editor's Addresses
Raymond Zhang
Infonet Services Corporation
2160 E. Grand Ave.
El Segundo, CA 90025
USA
Email: raymond_zhang@infonet.com
JP Vasseur
CISCO Systems, Inc.
300 Beaver Brook Road
Boxborough , MA - 01719
USA
Email: jpv@cisco.com
MPLS Inter-AS TE requirements September 2004
9. Normative References
[TE-REQ], Awduche et. al., "Requirements for Traffic Engineering
over MPLS", RFC2702, September 1999.
[TE-RSVP], Awduche et. al., "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001
[RFC-2119], S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997
10. Informative References
[MPLS-ARCH], Rosen, et. al., "Multiprotocol Label Switching
Architecture", RFC 3031, January 2001
[BGP-MPLSVPN], Rosen, et. al., "BGP/MPLSVPN", draft-ietf-l3vpn
-rfc2547bis-01.txt, September 2003 (work in progress).
[DIFF_ARCH], Blake, et. al., "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[DIFF_AF], Heinanen,et. al., "Assured Forwarding PHB Group", RFC
2597, June 1999.
[DIFF_EF], Davie, et. al., "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, March 2002.
[MPLS-Diff], Le Faucheur, et. al., "MPLS Support of Differentiated
Services", RFC 3270, May 2002
[TE-OVW], Awduche, et. al., "Overview and Principles of Internet
Traffic Engineering", RFC 3272,May 2002
[PSTE], Li, et. al., "A Provider Architecture for Differentiated
Services and Traffic Engineering", RFC 2430, October 1998
[TE-APP], Boyle, et. al., "Applicability Statement of Traffic
Engineering", RFC 3346, August 2002.
[TE-SURVIV], Lai, et. al., "Network Hierachy and Multilayer
Suvivability", RFC 3386, November, 2002.
[GMPLS-ROUT], Kompella, et. al., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions, RFC 3473, January 2003.
[BGP], Rekhter, et. al., "A Border Gateway Protocol 4 (BGP-4)",
RFC 1771, March 1995
[LSPPING], Kompella, et.. al.," Detecting Data Plane Liveliness in
MPLS", Internet Draft <draft-ietf-mpls-lsp-ping-05.txt>,
August 2004, (Work in Progress)
MPLS Inter-AS TE requirements September 2004
[MPLS-TTL], Agarwal, et. al., "Time to Live (TTL) Processing in MPLS
Networks", RFC 3443, January, 2003
[DS-TE], Le Faucheur, et. al., ''Requirements for support of
DiffServ-aware MPLS Traffic Engineering'', RFC 3564, July, 2003
[TE-FRR], Pan, et. al., "Fast Reroute Techniques in RSVP-TE",
draft-ietf-mpls-rsvp-lsp-fastreroute-05.txt, November 2003
(Work in Progress).
[ISIS-TE], Smit, Li, "IS-IS extensions for Traffic Engineering",
draft-ietf-isis-traffic-05.txt, August, 2003 (Work in Progress).
[OSPF-TE] Katz, Yeung, "Traffic Engineering (TE) Extensions to
OSPF Version 2", RFC 2370, September 2003.
[PATH-COMP], Vasseur, et. al., ''RSVP Path computation request and
reply messages'', draft-vasseur-mpls-computation-rsvp-03.txt, June
2002. (Work in Progress)
[OSPF-TE-CAP], Vasseur, Psenak. "OSPF TE TLV capabilities",
draft-vasseur-mpls-ospf-te-cap-00.txt, October 2002.
(Work in Progress)
[MPLS-LSPHIE] Kompella, Rekhter, "LSP Hierarchy with Generalized
MPLS TE", draft-ietf-mpls-lsp-hierarchy-08.txt , September 2002.
(work in progress)
[MPLS-Recov], Sharma V., et al, "Framework for Multi-Protocol Label
Switching (MPLS)-based Recovery", RFC 3469, Feb, 2003
[BGP-Label], Rekhter and Rosen, "Carrying Label Information in
BGP-4", RFC 3107, May 2001
[INTER-AS-TE], Vasseur and Ayyangar, "Inter-AS MPLS Traffic
Engineering", draft-vasseur-ccamp-inter-area-as-te-00.txt,
August, 2004 (work in progress).
[EXCLUDE-ROUTE], Farrel, et. al., "draft-ietf-ccamp-rsvp-te-exclude
-route-01.txt", June 2004 (work in progress).
MPLS Inter-AS TE requirements September 2004
11. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
12. Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
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attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
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The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
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13. Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
MPLS Inter-AS TE requirements September 2004
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 (ASs) 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 AS 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 un-deterministic factors in the
practices of inter-AS traffic engineering employing the methods
mentioned above:
- Sub-optimum traffic distribution across inter-AS links
- Un-deterministic 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 that are specific to one or a set
of inter-AS links for ingress traffic destined to certain PoPs or
regions within SP's network from another AS by tagging certain sets
of routes with a specific attribute when announcing to another AS.
This of course goes under the assumption that the other AS permits
automated egress policy by matching the predefined attribute from
incoming routes.
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