Network Working Group Renhai. Zhang
Mach. Chen
Internet Draft Huawei Technologies Co.,Ltd
Expires: March 2007 October 12, 2006
Locate ASBR in PCE
draft-zhang-pce-locate-asbr-00.txt
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Abstract
The ability to compute constrained shortest Traffic Engineering (TE)
Label Switched Paths (LSPs) in Multiprotocol Label Switching (MPLS)
and Generalized MPLS (GMPLS) networks across multiple domains has
been identified as a key requirement. This document specifies a
procedure for an inter-AS PCE to locate boundary node within its
domain dynamically, which is used for path computation.
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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 [1].
Table of Contents
1. Introduction................................................2
2. BGP extension...............................................3
2.1. EBGP Session Summary NLRI..............................3
2.2. ESSNP capability.......................................4
3. Procedure...................................................4
4. Example.....................................................5
5. Security Considerations.....................................6
6. IANA Considerations.........................................6
7. References..................................................6
7.1. Normative References...................................6
7.2. Informative References.................................6
Author's Addresses.............................................7
Intellectual Property Statement................................7
Disclaimer of Validity.........................................7
Copyright Statement............................................8
Acknowledgment.................................................8
1. Introduction
The framework for inter-domain MPLS Traffic Engineering has been
provided in [I-D.ietf-ccamp-inter-domain-framework].
An inter-AS PCE must have reachability information to the LSP tail-
end and head-end. At minimum, this reachability information must
include the AS path to the LSP tail-end, and the AS in which the
tail-end and head-end of the LSP reside. Using this information, an
inter-AS PCE can determine whether it can fully handle the path
computation request by itself. PCE may obtain the reachability
information using PCECP protocol extension, which is running between
the ASBR and PCE. But this may increase the complexity of the ASBR.
At the same time, PCE need make a selection among many ASBRs. So an
IBGP session between an inter-AS PCE and each ASBR is a better choice.
[BRPC] defines a PCE-based TE LSP computation method to compute
optimal inter-domain constrained (G)MPLS TE LSPs. In this path
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computation method, an inter-AS PCE needs to locate ASBRs that
provide connectivity from a specified AS.
Network administrator could configure ASBRs information on the inter-
AS PCE. But it may not be the best way. When the inter-AS link of an
ASBR is down, the PCE may proceed the path computation without being
aware of the situation.. Of course the result is that it can not get
a successful path through this ASBR. In order to solve the problem,
this document provides a method by which the PCE can dynamically
obtain information on the ASBR and its interconnected AS.
According to the above statement, MP-BGP is a good choice for an
inter-AS PCE to achieve necessary information used in BRPC method.
Thus, some extension to BGP is needed.
2. BGP extension
EBGP speaker should report its connections to the inter-AS PCE. A new
NLRI named EBGP Session Summary NLRI is introduced with extension to
MP-BGP.
2.1. EBGP Session Summary NLRI
The EBGP Session Summary NLRI is described below. A new AFI and SAFI
need to be defined.
+------------------------------------+
| Length (2 octets) |
+------------------------------------+
| Router ID (4 octets) |
+------------------------------------+
| AS Number (2 octets) |
+------------------------------------+
/ /
/ /
/ /
+------------------------------------+
| Router ID (4 octets) |
+------------------------------------+
| AS Number (2 octets) |
+------------------------------------+
The use and the meaning of these fields are as follows:
a) Length:
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The Length field indicates the length in bytes of the whole
NLRI.
b) Router ID:
This field indicates the Router ID of the peer BGP speaker in
the neighboring AS.
c) AS Number:
This field indicates the AS number of the BGP peer.
The tuple of <Router ID, AS Number> means an EBGP peer in a
neighboring AS. It could appear more than once in the NLRI. With this
information received from all EBGP speakers within its AS, an inter-
AS PCE can have the knowledge of ASBRs in its AS and neighboring AS
needed in path computation.
2.2. ESSNP capability
New AFI and SAFI need to be defined and negotiated during BGP session
establishment. It represents a capability to process EBGP Session
Summary NLRI. We named this capability ESSNP (EBGP Session Summary
NLRI Processing) here.
As inter-AS PCE establish IBGP connections with all BGP speakers
within its scope, with the ESSNP capability, it can have knowledge of
the ASBRs within its domain and their corresponding interconnection
of all neighboring ASes. Thus, when it computes using BRPC method, it
can find appropriate ASBR.
3. Procedure
When an ASBR establishes a new IBGP session with an inter-AS PCE,
they SHOULD negotiate ESSNP capability firstly. Upon succeeding, the
ASBR SHUOLD encapsulate each EBGP connection information in EBGP
Session Summary NLRI, and send them to the PCE.
When an EBGP speaker establishes a new EBGP connection, it SHUOLD
encapsulate this connection information in EBGP Session Summary NLRI
and send an update message to all the inter-AS PCEs that support
ESSNP capability within the AS it resides in.
When an EBGP connection of a speaker is down, it SHUOLD encapsulate
this connection information in EBGP Session Summary NLRI and send a
withdraw message to all the inter-AS PCEs that support ESSNP
capability within the AS it resides in.
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4. Example
An inter-AS PCE SHOULD build an EBGP Session Summary table to
efficiently maintenance those information received from ASBR within
its AS scope, using AS number or AS number combined with the Router
ID of a neighboring ASBR as index to look up the local ASBR needed in
computation.
Let's see an example as follows:
Inter-AS Inter-AS Inter-AS
PCE1<---------->PCE2<--------------> PCE3
:: :: ::
R1---ASBR1====ASBR3---R3---ASBR5====ASBR7---R5---R6
| | | | / |
| | | | / |
| | | | / |
R2---ASBR2====ASBR4---R4---ASBR6 / R7
<==AS1=> <====AS2======> <=====AS3===>
Figure 1 Inter-AS PCE Reference Model
Assume that PCE1 received a computation request from R1 to R6. Then,
this request is relayed to PCE3 eventually. The EBGP Session Summary
table of PCE3 may be like this:
AS number Router ID Local ASBR
--------------------------------------------
AS2 ASBR5 ASBR7
AS2 ASBR6 ASBR7
Table 1: The EBGP Session Summary table for PCE3
PCE3 knows the request arriving from AS2. So it looks up Table 1 and
finds the only ASBR connecting to AS2 is ASBR7. PCE3 computes and
passes the result ASBR7-R5-R6-R7 to PCE2.
The EBGP Session Summary table of PCE2 may be like this:
AS number Router ID Local ASBR
--------------------------------------------
AS1 ASBR1 ASBR3
AS1 ASBR2 ASBR4
AS3 ASBR7 ASBR5, ASBR6
Table 2: The EBGP Session Summary table for PCE2
Using AS3 and ASBR7 as index to look up Table 2, PCE2 will locate the
ASBR in AS2 connecting to ASBR7 is ASBR5 and ASBR6. At the same time,
PCE2 should locate ASBR connecting to AS1 because it should compute
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the path from those ASBRes to R7 and pass the partial result to PCE1.
Using AS1 as index it will find ASBR3 and ASBR4. The computation
continues as described in [BRPC].
5. Security Considerations
No new security issues are raised in this document.
6. IANA Considerations
IANA is asked to allocate an AFI and SAFI for EBGP Statistic NLRI.
7. References
7.1. Normative References
[BRPC] JP. Vasseur, Ed., R. Zhang, N. Bitar, JL. Le Roux, " A
Backward Recursive PCE-based Computation (BRPC) procedure
to compute shortest inter-domain Traffic Engineering Label
Switched Paths ", draft-ietf-pce-brpc-00.txt, Work in
Progress, August 2006.
[RFC4655] A. Farrel, J.-P. Vasseur, J. Ash, " A Path Computation
Element (PCE)-Based Architecture ", RFC 4655, August 2006.
[BGP-MP] T. Bates, Y. Rekhter, R. Chandra, D. Katz, " Multiprotocol
Extensions for BGP-4 ", RFC 2858, June 2000.
7.2. Informative References
[2] Nabil Bitar, Jean-Louis Le Roux, Raymond Zhang, " Framework for
PCE based inter-domain path computation ", draft-bitar-pce-
inter-domain-frwk-00.txt, Work in Progress, June 2006.
[BGP-CAP] R. Chandra, J. Scudder, " Capabilities Advertisement with
BGP-4 ", RFC 3392, November 2002.
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Author's Addresses
Renhai Zhang
Huawei Technologies Co.,Ltd
Email: zhangrenhai@huawei.com
Mach chen
Huawei Technologies Co.,Ltd
Email: mach@huawei.com
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