Network work group Mach Chen
Renhai Zhang
Internet Draft Huawei Technologies Co.,Ltd
Expires: August 2007 February 1, 2007
OSPF Extensions in Support of Inter-AS (G)MPLS TE
draft-chen-ccamp-ospf-interas-te-extension-01.txt
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Abstract
This document describes extensions to the OSPF protocol to support
Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS)
Traffic Engineering (TE) for multiple Autonomous Systems (ASes). It
defines OSPF extensions for the flooding of inter-AS links which can
be used to perform inter-AS path computation.
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.
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Table of Contents
1. Introduction.................................................2
2. Problem statement............................................3
2.1. Per-Domain Path Determination...........................3
2.2. Backward Recursive Path Computation.....................4
3. Extensions to OSPF...........................................5
3.1. Remote AS Number Sub-TLV................................5
3.2. Inter-AS Link Type......................................6
3.3. Link ID.................................................6
4. Inter-AS links procedure.....................................6
5. Security Considerations......................................7
6. IANA Considerations..........................................7
6.1. OSPF LSA Sub-TLVs type..................................7
6.2. OSPF TE Link Type.......................................7
7. References...................................................9
7.1. Normative References....................................9
7.2. Informative References..................................9
Author's Addresses.............................................10
Intellectual Property Statement................................10
Disclaimer of Validity.........................................11
Copyright Statement............................................11
Acknowledgment.................................................11
1. Introduction
[OSPF-TE] defines extensions to the OSPF protocol [OSPF] to support
intra-area Traffic Engineering (TE). The extensions provide a way of
encoding the TE information for TE-enabled links within the network
(TE links) and flooding this information within an area. Type 10
opaque LSAs [RFC2370] are used to carry such TE information. Two top-
level TLVs are defined in [OSPF-TE]: Router Address TLV and Link TLV.
The Link TLV has several nested sub-TLVs which describe the TE
attributes for a TE link.
Requirements for establishing Multiprotocol Label Switching (MPLS) TE
Label Switched Paths (LSPs) that cross multiple Autonomous Systems
(ASes) are described in [INTERAS-TE-REQ]. As described in [INTERAS-
TE-REQ], a method SHOULD provide the ability to compute a path
spanning multiple ASes. So a path computation entity that may be the
Head-end Label Switching Router (LSR), an AS Border Router (ASBR), or
a Path Computation Element (PCE [PCE]) needs to know not only the TE
information of the links within an AS, but also of the links that
connect to other ASes.
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In this document, some extensions to [OSPF-TE] are defined in support
of carrying inter-AS TE link information for inter-AS Traffic
Engineering. A new sub-TLV is added to the Link TLV and a new link
type is introduced. The detailed definitions and procedures are
discussed in the following sections.
2. Problem statement
As described in [INTERAS-TE-REQ], in the case of establishing an
inter-AS TE LSP traversing multiple ASes, the Path message [RFC3209]
may include the following elements in the Explicit Route Object(ERO)
in order to describe the path of the LSP:
- a set of AS numbers as loose hops; and/or
- a set of LSRs including ASBRs as loose hops.
Two methods for determining inter-AS paths are currently discussed.
The per-domain method [PD-PATH] determines the path one domain at a
time. The backward recursive method [BRPC] uses cooperation between
PCEs to determine an optimum inter-domain path. The sections that
follow examine how inter-AS TE link information could be useful in
both cases.
2.1. Per-Domain Path Determination
In the per-domain method of determining an inter-AS path for an MPLS-
TE LSP, when an LSR that is an entry-point to an AS receives a PATH
message from an upstream AS with an ERO containing a next hop that is
an AS number, it needs to find which LSRs within the local AS are
connected to the downstream AS so that it can compute a TE LSP
segment across the AS to that LSR and forward the PATH message to the
LSR and hence into the next AS. See the below figure for example:
R1------R3----R5-----R7------R9-----R11
| | \ | / |
| | \ | ---- |
| | \ | / |
R2------R4----R6 --R8------R10----R12
: :
<-- AS1 -->:<---- AS2 --->:<--- AS3 --->
Figure 1: Inter-AS Reference Model
The figure shows three ASes (AS1, AS2, and AS3) and twelve LSRs (R1
through R12). R3 and R4 are ASBRs in AS1. R5, R6, R7, and R8 are
ASBRs in AS2. R9 and R10 are ASBRs in AS3.
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If an inter-AS TE LSP is planned to be established from R1 to R12,
the AS sequence is limited as: AS1, AS2, AS3.
Suppose that the Path message enters AS2 from R3. The next hop in the
ERO shows AS3, and R5 must determine a path segment across AS2 to
reach AS3. It has a choice of three exit points from AS2 (R6, R7, and
R8) and it needs to know which of these provide TE connectivity to
AS3, and whether the TE connectivity (for example, available
bandwidth) is adequate for the requested LSP.
Alternatively, if the next hop in the ERO is the entry ASBR for AS3
(say R9), R5 needs to know which of its exit ASBRs has a TE link that
connects to R9. Since there may be multiple exist ASBRs that are
connected to R9(both R7 and R8 in this example), R5 also needs to
know the TE properties of the inter-AS TE links so that it can select
the correct exit ASBR.
Once the path message reaches the exit ASBR, any choice of inter-AS TE
link can be made by the ASBR if not already made by entry ASBR that
computed the segment.
To enable R5 to make the correct choice of exit ASBR the following
information is needed:
- List of all inter-AS TE links for the local AS.
- TE properties of each inter-AS TE link.
- AS number of the neighboring AS connected to by each inter-AS TE
link.
- Identity (TE Router ID) of the neighboring ASBR connected to by
each inter-AS TE link.
This information is needed throughout the local AS if path
computation function is fully distributed among LSRs in the local AS.
2.2. Backward Recursive Path Computation
Another scenario using PCE techniques has the same problem. [BRPC]
defines a PCE-based TE LSP computation method (called Backward
Recursive Path Computation) to compute optimal inter-domain
constrained MPLS-TE or GMPLS LSPs. In this path computation method, a
specific set of traversed domains are assumed to be selected before
computation starts. Each downstream PCE in domain(i) returns a
multipoint-to-point tree of potential paths to its upstream neighbor
PCE in domain(i-1). Each tree consists of the set of paths from all
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Boundary Nodes located in domain(i) to the destination where each
path satisfies the set of required constraints for the TE LSP
(bandwidth, affinities, etc.).
So a PCE needs to select Boundary Nodes (that is, ASBRs) that provide
connectivity from the upstream AS. In order that the tree of paths
provided by one PCE to its neighbor can be correlated, the identities
of the ASBRs for each path need to be referenced, so the PCE must
know the identities of the ASBRs in the remote AS reached by any
inter-AS TE link, and, in order that it provides only suitable paths
in the tree, the PCE must know the TE properties of the inter-AS TE
links.
Thus, to support Backward Recursive Path Computation the same
information as listed in Section 2.1 is required.
3. Extensions to OSPF
The extensions defined in this document allow a TE link advertisement
to be easily identified as such by the use of a new link type. A new
sub-TLV to the Link TLV is defined to carry the information about the
neighboring AS.
3.1. Remote AS Number Sub-TLV
As described in [OSPF-TE], the Link TLV describes a single link and
consists of a set of sub-TLVs. A new sub-TLV, the Remote AS Number
sub-TLV is added to the Link TLV when advertising inter-AS links. The
Remote AS Number sub-TLV specifies the AS number of the neighboring
AS to which the advertised link connects.
The Remote AS number sub-TLV is TLV type 21 (which needs to be
confirmed by IANA), and is four octets in length. The format is as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote AS Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Remote AS number field has 4 octets. When two octets are used for
the AS number, as in current deployments, the left two octets MUST be
set to zero.
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3.2. Inter-AS Link Type
To identify a link to be an inter-AS link and allow easy
identification of these new advertisements, a new Link Type value is
defined for use in the Link Type sub-TLV. The value of the Link Type
for an inter-AS point-to-point link is 3 (which needs to be confirmed
by IANA).
3.3. Link ID
For an inter-AS link, the Link ID carried in the Link ID sub-TLV is
the TE Router ID of the remote ASBR reached through this inter-AS
link.
4. Inter-AS links procedure
When TE is enabled on an inter-AS link and the link is up, the ASBR
SHOULD advertise this link using the normal procedures for OSPF-TE
[OSPF-TE]. When either the link is down or TE is disabled on the link,
the ASBR SHOULD withdraw the advertisement. When there are changes to
the TE parameters for the link (for example, when the available
bandwidth changes) the ASBR SHOULD re-advertise the link, but the
ASBR MUST take precautions against excessive re-advertisements as
described in [OSPF-TE].
The information advertised comes from the ASBR's knowledge of the TE
capabilities of the link, the ASBR's knowledge of the current status
and usage of the link, and configuration at the ASBR of the remote AS
number and remote ASBR TE Router ID.
The TE link advertisement SHOULD be carried in a Type 10 Opaque LSA
if the flooding scope is to be limited to within the single IGP area
to which the ASBR belongs, or MAY be carried in a Type 11 Opaque LSA
if the information should reach all routers (including area border
routers, ASBRs, and PCEs) in the AS.
Legacy routers receiving an advertisement for an inter-AS TE link are
able to ignore it because the Link Type carries an unknown value.
They will continue to flood the LSA, but will not attempt to use the
information received as if the link were an intra-AS TE link.
Routers or PCEs that are capable of processing advertisements of
inter-AS TE links SHOULD NOT use such links to compute paths that
exit an AS to a remote ASBR and then immediately re-enter the AS.
Such paths would constitute extremely rare occurrences and MUST only
be allowed as the result of specific policy configuration at the
router or PCE computing the path.
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5. Security Considerations
The protocol extensions defined in this document are relatively minor
and can be secured within the AS in which they are used by the
existing OSPF security mechanisms.
It should be noted, however, that some of the information included in
these new advertisements (the remote AS number and the remote ASBR ID)
are obtained from a neighboring administration and cannot be verified
in anyway. Since the means of delivery of this information is likely
to be part of a commercial relationship, the source of the
information should be carefully checked before it is entered as
configuration information at the ASBR
6. IANA Considerations
IANA is requested to make the following allocations from registries
under its control.
6.1. OSPF LSA Sub-TLVs type
IANA maintains the "Open Shortest Path First (OSPF) Traffic
Engineering TLVs" registry with sub-registry "Types for sub-TLVs in a
TE Link TLV". IANA is requested to assign a new sub-TLV as follows.
The number 21 is suggested.
Value Meaning
21 Remote AS Number sub-TLV.
6.2. OSPF TE Link Type
IANA is requested to create a new sub-registry "TE Link Types" of the
registry "Open Shortest Path First (OSPF) Traffic Engineering TLVs"
to track TE Link Types.
The sub-registry should read as follows:
[RFC3630] defines the Link Type sub-TLV of the Link TLV. The
following values are defined.
Value Meaning Reference
1 Point-to-point link [RFC3630]
2 Multi-access link [RFC3630]
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3 Inter-AS link [this document]
New allocations from this registry are by IETF Standards Action.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3209] 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.
[RFC2370] R. Coltun, "The OSPF Opaque LSA Option", RFC2370, July
1998.
[OSPF] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[OSPF-TE] Katz, D., Kompella, K., and Yeung, D., "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630, September
2003.
7.2. Informative References
[INTERAS-TE-REQ] Zhang and Vasseur, "MPLS Inter-AS Traffic
Engineering Requirements", RFC4216, November 2005.
[PER-DOMAIN] Ayyangar, A., Vasseur, JP., and Zhang, R., "A Per-domain
path computation method for establishing Inter-domain",
draft-ietf-ccamp-inter-domain-pd-path-comp, (work in
progress).
[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, (work in progress)
[PCE] Farrel, A., Vasseur, JP., and Ash, J., "A Path Computation
Element (PCE)-Based Architecture", RFC4655, August 2006.
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Author's Addresses
Mach Chen
Huawei Technologies Co.,Ltd
KuiKe Building, No.9 Xinxi Rd.,
Hai-Dian District
Beijing, 100085
P.R. China
Email: mach@huawei.com
Renhai Zhang
Huawei Technologies Co.,Ltd
KuiKe Building, No.9 Xinxi Rd.,
Hai-Dian District
Beijing, 100085
P.R. China
Email: zhangrenhai@huawei.com
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Acknowledgment
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