Network work group Mach Chen
Internet Draft Renhai Zhang
Expires: December 2007 Huawei Technologies Co.,Ltd
Category: Standards Track June 28, 2007
ISIS Traffic Engineering (ISIS-TE) Extensions in Support of Inter-AS
Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS)
Traffic Engineering
draft-chen-ccamp-isis-interas-te-extension-00.txt
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Abstract
This document describes extensions to the ISIS Traffic Engineering
(ISIS-TE) mechanisms to support Multiprotocol Label Switching (MPLS)
and Generalized MPLS (GMPLS) Traffic Engineering (TE) for multiple
Autonomous Systems (ASes). It defines ISIS-TE extensions for the
flooding of TE information about inter-AS links which can be used to
perform inter-AS TE 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 [RFC2119].
Table of Contents
1. Introduction.................................................2
2. Problem statement............................................3
2.1. A Note on Non-Objectives................................3
2.2. Per-Domain Path Determination...........................4
2.3. Backward Recursive Path Computation.....................5
3. Extensions to ISIS-TE........................................6
3.1. Link Type sub-TLV.......................................6
3.2. Remote AS Number Sub-TLV................................7
3.3. Remote ASBR ID sub-TLV..................................7
3.4. Inter-AS reachability TLV...............................8
4. Procedure for Inter-AS TE Links..............................9
5. Security Considerations.....................................10
6. IANA Considerations.........................................10
6.1. Inter-AS Reachability TLV..............................10
6.2. Sub-TLVs for the Inter-AS Reachability TLV.............11
6.3. Sub-TLVs for the Extended IS Reachability TLV..........11
6.4. ISIS TE Link Type......................................11
7. Acknowledgments.............................................12
8. References..................................................12
8.1. Normative References...................................12
8.2. Informative References.................................12
Author's Addresses.............................................13
Intellectual Property Statement................................13
Disclaimer of Validity.........................................14
Copyright Statement............................................14
Acknowledgment.................................................14
1. Introduction
[ISIS-TE] defines extensions to the ISIS protocol [ISIS] 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. The Extended
IS reachability TLV and Traffic Engineering Router ID TLV, which are
defined in [ISIS-TE], are used to carry such TE information. The
Extended IS reachability TLV has several nested sub-TLVs which
describe the TE attributes for a TE link.
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[ISIS-TE-V3] and [GMPLS-TE] define similar extensions to ISIS [ISIS]
in support of IPv6 and GMPLS traffic engineering respectively.
Requirements for establishing Multiprotocol Label Switching (MPLS) TE
Label Switched Paths (LSPs) that cross multiple Autonomous Systems
(ASes) are described in [INTER-AS-TE-REQ]. As described in [INTER-AS-
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 the TE
information not only of the links within an AS, but also of the links
that connect to other ASes.
In this document, some extensions to ISIS-TE are defined in support
of carrying inter-AS TE link information for inter-AS Traffic
Engineering. Three new sub-TLVs are added to the Extended IS
reachability TLV, and a new TLV, which is referred to as inter-AS
reachability TLV, is defined. The extensions are equally applicable
to IPv4 and IPv6 as identical extensions to [ISIS-TE] and [ISIS-TE-
V3]. The detailed definitions and procedures are discussed in the
following sections.
2. Problem statement
As described in [INTER-AS-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. A Note on Non-Objectives
It is important to note that this document does not make any change
to the confidentiality and scaling assumptions surrounding the use of
ASes in the Internet. In particular, this document is conformant to
the requirements set out in [INTER-AS-TE-REQ].
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The following lists of features are explicit exclusions.
o There is no attempt to distribute TE information from within one
AS to another AS.
o There is no mechanism proposed to distribute any form of TE
reachability information for destinations outside the AS.
o There is no proposed change to the PCE architecture or usage.
o TE aggregation is not supported or recommended.
o There is no exchange of private information between ASes.
2.2. 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 figure below for an 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.
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
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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.
More details can be found in the Section 4.0 of [PD-PATH], which
clearly points out why advertising of inter-AS links is desired.
To enable R5 to make the correct choice of exit ASBR the following
information is needed:
o List of all inter-AS TE links for the local AS.
o TE properties of each inter-AS TE link.
o AS number of the neighboring AS connected to by each inter-AS TE
link.
o Identity (TE Router ID) of the neighboring ASBR connected to by
each inter-AS TE link.
In GMPLS networks further information may also be required to select
the correct TE links as defined in [GMPLS-TE].
The example above shows how this information is needed at the entry
point ASBRs for each AS (or the PCEs that provide computation
services for the ASBRs), but this information is also needed
throughout the local AS if path computation function is fully
distributed among LSRs in the local AS, for example to support LSPs
that have start points (ingress nodes) within the AS.
2.3. 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
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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
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 2.2 is required.
3. Extensions to ISIS-TE
Note that this document does not define mechanisms for distribution
of TE information from one AS to another, does not distribute any
form of TE reachability information for destinations outside the AS,
does not change the PCE architecture or usage, does not suggest or
recommend any form of TE aggregation, and does not feed private
information between ASes. See section 2.1.
In this document, three new sub-TLVs are added to the extended IS
reachability TLV to carry the information about the neighboring AS,
the remote ASBR ID and the Link Type of an inter-AS link. An new TLV,
which is referred to as inter-AS reachability TLV, is defined to
flood the information about the neighboring AS and the remote ASBR ID
within a whole AS.
3.1. Link Type sub-TLV
To identify a link as an inter-AS link and allow easy identification
of these new advertisements, a new Link Type sub-TLV is added to the
extended IS reachability TLV to identify the type of the links.
The Link Type sub-TLV is TLV type 22 (which needs to be confirmed by
IANA), and is one octet in length. The format of the link type sub-
TLV is as follows:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Type |
+-+-+-+-+-+-+-+-+
The value of the Link Type for an inter-AS point-to-point link is 3
(which needs to be confirmed by IANA). The use of multi-access inter-
AS TE links and intra-area TE links is for future study.
3.2. Remote AS Number Sub-TLV
As described in [ISIS-TE], the Extended IS reachability 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 extended IS reachability 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 23 (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 (high-order) two
octets MUST be set to zero.
3.3. Remote ASBR ID sub-TLV
A new sub-TLV, which is referred to as the Remote ASBR ID sub-TLV, is
added to the extended IS reachability TLV when advertising inter-AS
links. The remote ASBR ID sub-TLV specifies the Router ID or TE
Router ID of the remote ASBR to which the advertised inter-AS link
connects, which provides a stable, routable identifier of the remote
ASBR.
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The Remote ASBR ID sub-TLV is TLV type 24 (which needs to be
confirmed by IANA), and is four or sixteen octets in length. The
format of the remote ASBR ID sub-TLV 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 ASBR ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
or
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 ASBR ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If a router implements traffic engineering for IPv4, the length of
the remote ASBR ID is four. If a router implements traffic
engineering for IPv6, the length of the remote ASBR ID is sixteen.
3.4. Inter-AS reachability TLV
The inter-AS reachability TLV has type 141 (which needs to be
confirmed by IANA). This is an optional TLV, when needed, it is used
to flood the reachability information of the inter-AS links within a
whole AS. And such reachability information SHOULD include the
neighboring AS number and the remote ASBR ID to which an inter-AS
link connects. The inter-AS reachability TLV contains a data
structure consisting of:
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6 octets of System ID
1 octet of Pseudonode Number
1 octet flags
1 bit of up/down information
1 bit indicating the presence of sub-TLVs
6 bits reserved
1 octet of length of sub-TLVs
0-246 octets of sub-TLVs
where each sub-TLV consists of a sequence of:
1 octet of sub-type
1 octet of length of the value field of the sub-TLV
0-244 octets of value
In this document, two sub-TLVs are defined for the inter-AS
Reachability TLV, they are:
Sub-TLV type length Name
23 4 Remote AS number
24 4or16 Remote ASBR Identifier
These two sub-TLVs have the same format and semantics as defined in
Section 3.1 and section 3.2 of this memo.
4. Procedure for Inter-AS TE Links
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 ISIS-TE
[ISIS-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.
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.
When the inter-AS reachability information needs to reach all
routers(including area border routers, ASBRs, and PCEs) in the AS,
the ASBR SHOULD carry the Remote AS sub-TLV and Remote ASBR ID sub-
TLV in the inter-AS reachability TLV. As defined in Section 4.1 of
[ISIS-TE], the inter-AS reachability TLV also defines an up/down bit
to facilitate the redistribution of inter-AS reachability information
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freely between level 1 and level 2. The semantics of the up/down bit
in the new inter-AS reachability TLV are identical to the semantics
of the up/down bit defined in [ISIS-TE]. That is, the up/down bit
SHALL be set to 0 when the inter-AS reachability information first
injected into ISIS [ISIS], and the up/dawn bit SHALL be set to 1 if
the inter-AS reachability information needs to be advertised from
high level to low level.
Legacy routers receiving an advertisement for an inter-AS TE link are
able to ignore it because they do not know the new TLV and sub-TLVs
that are defined in Section 3 in this document. They will continue to
flood the LSP, 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
through another TE link. Such paths would constitute extremely rare
occurrences and SHOULD NOT be allowed except as the result of
specific policy configurations at the router or PCE computing the
path.
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 ISIS 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 responsible for advertising the
inter-AS TE links.
6. IANA Considerations
IANA is requested to make the following allocations from registries
under its control.
6.1. Inter-AS Reachability TLV
This document defines the following new ISIS TLV type that needs to
be reflected in the ISIS TLV code-point registry:
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Type Description IIH LSP SNP
---- ---------------------- --- --- ---
141 Inter-AS reachability n y n
information
6.2. Sub-TLVs for the Inter-AS Reachability TLV
This document defines the following new sub-TLV types of top-
level TLV 141 that need to be reflected in the ISIS sub-TLV registry
for TLV 141:
Type Description Length
---- ------------------------------ --------
23 Remote AS number 4
24 Remote ASBR Identifier 4 or 16
6.3. Sub-TLVs for the Extended IS Reachability TLV
This document also defines the following new sub-TLV types of top-
level TLV 22 that need to be reflected in the ISIS sub-TLV registry
for TLV 22:
Type Description Length
---- ------------------------------ --------
22 Link Type 1
23 Remote AS number 4
24 Remote ASBR Identifier 4 or 16
6.4. ISIS TE Link Type
IANA is requested to create a new sub-registry "TE Link Types" of the
registry "Intermediate System to Intermediate System (ISIS) Traffic
Engineering TLVs" to track TE Link Types.
The sub-registry should read as follows:
This document defines the Link Type sub-TLV of the extended IS
reachability TLV. The following values are defined.
Value Meaning Reference
---- -------------------- ----------
3 Inter-AS link [this document]
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New allocations from this registry are by IETF Standards Action.
7. Acknowledgments
8. References
8.1. 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.
[ISIS] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[ISIS-TE] Smit, H. and T. Li, "Intermediate System to Intermediate
System (IS-IS) Extensions for Traffic Engineering (TE)",
RFC 3784, June 2004.
[GMPLS-TE] K.Kompella and Y.Rekhter, "IS-IS Extensions in Support of
Generalized Multi-Protocol Label Switching", RFC 4205,
October 2005.
8.2. Informative References
[INTER-AS-TE-REQ] Zhang and Vasseur, "MPLS Inter-AS Traffic
Engineering Requirements", RFC4216, November 2005.
[PD-PATH] 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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[ISIS-TE-V3] Harrison, J., Berger, J., and Bartlett, M., "IPv6
Traffic Engineering in IS-IS", draft-ietf-isis-ipv6-te-
03.txt, {work in progress}.
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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