community.roxen.com
Not logged in Date: September 23, 2014
 DEMO  DOCS  PIKE
 COMMUNITY  DOWNLOAD
Home Developer tools Internet Documents RFCs www.roxen.com
Newest Categories... 1..499 500..999 1000..1499 1500..1999 2000..2499 2500..2999 3000..3499 3500..3999 4000..4499 4500..4999 5000..5499 5500..5999 6000..6499 6500..6999 7000..7372
[Text version]

Network Working Group
Request for Comments: 5316
Category: Standards Track
M. Chen
R. Zhang
Huawei Technologies Co., Ltd
X. Duan
China Mobile
December 2008

ISIS Extensions in Support of Inter-Autonomous System (AS)
  MPLS and GMPLS Traffic Engineering

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © 2008 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.

Abstract

This document describes extensions to the ISIS (ISIS) protocol 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.

No support for flooding information from within one AS to another AS is proposed or defined in this document.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................3
   2. Problem Statement ...............................................3
      2.1. A Note on Non-Objectives ...................................4
      2.2. Per-Domain Path Determination ..............................4
      2.3. Backward Recursive Path Computation ........................6
   3. Extensions to ISIS-TE ...........................................7
      3.1. Inter-AS Reachability TLV ..................................7
      3.2. TE Router ID ...............................................9
      3.3. Sub-TLV Detail .............................................9
           3.3.1. Remote AS Number Sub-TLV ............................9
           3.3.2. IPv4 Remote ASBR ID Sub-TLV ........................10
           3.3.3. IPv6 Remote ASBR ID Sub-TLV ........................11
           3.3.4. IPv4 TE Router ID sub-TLV ..........................11
           3.3.5. IPv6 TE Router ID sub-TLV ..........................12
   4. Procedure for Inter-AS TE Links ................................12
      4.1. Origin of Proxied TE Information ..........................14
   5. Security Considerations ........................................14
   6. IANA Considerations ............................................15
      6.1. Inter-AS Reachability TLV .................................15
      6.2. Sub-TLVs for the Inter-AS Reachability TLV ................15
      6.3. Sub-TLVs for the IS-IS Router Capability TLV ..............17
   7. Acknowledgments ................................................17
   8. References .....................................................17
      8.1. Normative References ......................................17
      8.2. Informative References ....................................17

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 that describe the TE attributes for a TE link.

[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, a new TLV, which is referred to as the inter-AS reachability TLV, is defined to advertise inter-AS TE information, and three new sub-TLVs are defined for inclusion in the inter-AS reachability TLV to carry the information about the remote AS number and remote ASBR ID. The sub-TLVs defined in [ISIS-TE], [ISIS-TE-V3], and other documents for inclusion in the extended IS reachability TLV for describing the TE properties of a TE link are applicable to be included in the inter-AS reachability TLV for describing the TE properties of an inter-AS TE link as well. Also, two more new sub- TLVs are defined for inclusion in the IS-IS router capability TLV to carry the TE Router ID when the TE Router ID needs to reach all routers within an entire ISIS routing domain. The extensions are equally applicable to IPv4 and IPv6 as identical extensions to [ISIS-TE] and [ISIS-TE-V3]. Detailed definitions and procedures are discussed in the following sections.

This document does not propose or define any mechanisms to advertise any other extra-AS TE information within ISIS. See Section 2.1 for a full list of non-objectives for this work.

1.1. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119 [RFC2119].

2. Problem Statement

As described in [INTER-AS-TE-REQ], in the case of establishing an inter-AS TE LSP that traverses 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 being 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].

The following features are explicitly excluded:

   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.

o No ISIS adjacencies are formed on the inter-AS link.

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 (ASBRs) within the local AS are connected to the downstream AS. That way, it can compute a TE LSP segment across the local AS to one of those LSRs and forward the Path message to that LSR and hence into the next AS. See Figure 1 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 will be: 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 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 the entry ASBR that computed the segment.

More details can be found in Section 4 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). However, this information is also needed throughout the local AS if path computation functionality 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 (ASes) are assumed to be selected before computation starts. Each downstream PCE in domain(i) returns to its upstream neighbor PCE in domain(i-1) a multipoint-to-point tree of potential paths. 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 for the tree of paths provided by one PCE to its neighbor to be correlated, the identities of the ASBRs for each path need to be referenced. Thus, the PCE must know the identities of the ASBRs in the remote AS that are reached by any inter-AS TE link, and, in order to provide only suitable paths in the tree, the PCE must know the TE properties of the inter-AS TE links. See the following figure as an example.

                   PCE1<------>PCE2<-------->PCE3
                   /       :             :
                  /        :             :
                R1------R3----R5-----R7------R9-----R11
                        |     | \    |      / |
                        |     |  \   |  ----  |
                        |     |   \  | /      |
                R2------R4----R6   --R8------R10----R12
                           :              :
                <-- AS1 -->:<---- AS2 --->:<--- AS3 --->
               Figure 2: BRPC for Inter-AS Reference Model

The figure shows three ASes (AS1, AS2, and AS3), three PCEs (PCE1, PCE2, and PCE3), 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. PCE1, PCE2, and PCE3 cooperate to perform inter-AS path computation and are responsible for path segment computation within their own domain(s).

If an inter-AS TE LSP is planned to be established from R1 to R12, the traversed domains are assumed to be selected: AS1->AS2->AS3, and the PCE chain is: PCE1->PCE2->PCE3. First, the path computation request originated from the PCC (R1) is relayed by PCE1 and PCE2 along the PCE chain to PCE3. Then, PCE3 begins to compute the path segments from the entry boundary nodes that provide connection from AS2 to the destination (R12). But, to provide suitable path segments, PCE3 must determine which entry boundary nodes provide connectivity to its upstream neighbor AS (identified by its AS number), and must know the TE properties of the inter-AS TE links. In the same way, PCE2 also needs to determine the entry boundary nodes according to its upstream neighbor AS and the inter-AS TE link capabilities.

Thus, to support Backward Recursive Path Computation, the same information listed in Section 2.2 is required. The AS number of the neighboring AS connected to by each inter-AS TE link is particularly important.

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, for the advertisement of inter-AS TE links, a new TLV, which is referred to as the inter-AS reachability TLV, is defined. Three new sub-TLVs are also defined for inclusion in the inter-AS reachability TLV to carry the information about the neighboring AS number and the remote ASBR ID of an inter-AS link. The sub-TLVs defined in [ISIS-TE], [ISIS-TE-V3], and other documents for inclusion in the extended IS reachability TLV are applicable to be included in the inter-AS reachability TLV for inter-AS TE links advertisement. Also, two other new sub-TLVs are defined for inclusion in the IS-IS router capability TLV to carry the TE Router ID when the TE Router ID is needed to reach all routers within an entire ISIS routing domain.

While some of the TE information of an inter-AS TE link may be available within the AS from other protocols, in order to avoid any dependency on where such protocols are processed, this mechanism carries all the information needed for the required TE operations.

3.1. Inter-AS Reachability TLV

The inter-AS reachability TLV has type 141 (see Section 6.1) and contains a data structure consisting of:
      o  4 octets of Router ID
      o  3 octets of default metric
      o  1 octet of control information, consisting of:
         -  1 bit of flooding-scope information (S bit)
         -  1 bit of up/down information (D bit)
         -  6 bits reserved
      o  1 octet of length of sub-TLVs
      o  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

Compared to the extended reachability TLV, which is defined in [ISIS-TE], the inter-AS reachability TLV replaces the "7 octets of System ID and Pseudonode Number" field with a "4 octets of Router ID" field and introduces an extra "control information" field, which consists of a flooding-scope bit (S bit), an up/down bit (D bit), and 6 reserved bits.

The Router ID field of the inter-AS reachability TLV is 4 octets in length, which contains the Router ID of the router who generates the inter-AS reachability TLV. The Router ID MUST be unique within the ISIS area. If the router generates inter-AS reachability TLV with entire ISIS routing domain flooding scope, then the Router ID MUST also be unique within the entire ISIS routing domain. The Router ID could be used to indicate the source of the inter-AS reachability TLV.

The flooding procedures for inter-AS reachability TLV are identical to the flooding procedures for the GENINFO TLV, which are defined in Section 4 of [GENINFO]. These procedures have been previously discussed in [ISIS-CAP]. The flooding-scope bit (S bit) SHOULD be set to 0 if the flooding scope is to be limited to within the single IGP area to which the ASBR belongs. It MAY be set to 1 if the information is intended to reach all routers (including area border routers, ASBRs, and PCEs) in the entire ISIS routing domain. The choice between the use of 0 or 1 is an AS-wide policy choice, and configuration control SHOULD be provided in ASBR implementations that support the advertisement of inter-AS TE links.

The sub-TLVs defined in [ISIS-TE], [ISIS-TE-V3], and other documents for describing the TE properties of a TE link are also applicable to the inter-AS reachability TLV for describing the TE properties of an inter-AS TE link. Apart from these sub-TLVs, three new sub-TLVs are defined for inclusion in the inter-AS reachability TLV defined in this document:

   Sub-TLV type    Length  Name
   ------------    ------  ---------------------------
             24        4   remote AS number
             25        4   IPv4 remote ASBR identifier
             26       16   IPv6 remote ASBR identifier

The detailed definitions of the three new sub-TLVs are described in Section 3.3.

3.2. TE Router ID

The IPv4 TE Router ID TLV and IPv6 TE Router ID TLV, which are defined in [ISIS-TE] and [ISIS-TE-V3] respectively, only have area flooding-scope. When performing inter-AS TE, the TE Router ID MAY be needed to reach all routers within an entire ISIS routing domain and it MUST have the same flooding scope as the inter-AS reachability TLV does.

[ISIS-CAP] defines a generic advertisement mechanism for ISIS, which allows a router to advertise its capabilities within an ISIS area or an entire ISIS routing domain. [ISIS-CAP] also points out that the TE Router ID is a candidate to be carried in the IS-IS router capability TLV when performing inter-area TE.

This document uses such mechanism for TE Router ID advertisement when the TE Router ID is needed to reach all routers within an entire ISIS Routing domain. Two new sub-TLVs are defined for inclusion in the IS-IS router capability TLV to carry the IPv4 and IPv6 TE Router IDs, respectively:

   Sub-TLV type   Length  Name
   ------------    ------  -----------------
             11        4   IPv4 TE Router ID
             12       16   IPv6 TE Router ID

Detailed definitions of the two new sub-TLVs are described in Section 3.3.

3.3. Sub-TLV Detail

3.3.1. Remote AS Number Sub-TLV

A new sub-TLV, the remote AS number sub-TLV, is defined for inclusion in the inter-AS 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 24 (see Section 6.2) and is 4 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 only 2 octets are used for the AS number, as in current deployments, the left (high-order) 2 octets MUST be set to 0. The remote AS number sub-TLV MUST be included when a router advertises an inter-AS TE link.

3.3.2. IPv4 Remote ASBR ID Sub-TLV

A new sub-TLV, which is referred to as the IPv4 remote ASBR ID sub- TLV, is defined for inclusion in the inter-AS reachability TLV when advertising inter-AS links. The IPv4 remote ASBR ID sub-TLV specifies the IPv4 identifier of the remote ASBR to which the advertised inter-AS link connects. This could be any stable and routable IPv4 address of the remote ASBR. Use of the TE Router ID as specified in the Traffic Engineering router ID TLV [ISIS-TE] is RECOMMENDED.

The IPv4 remote ASBR ID sub-TLV is TLV type 25 (see Section 6.2) and is 4 octets in length. The format of the IPv4 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                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The IPv4 remote ASBR ID sub-TLV MUST be included if the neighboring ASBR has an IPv4 address. If the neighboring ASBR does not have an IPv4 address (not even an IPv4 TE Router ID), the IPv6 remote ASBR ID sub-TLV MUST be included instead. An IPv4 remote ASBR ID sub-TLV and IPv6 remote ASBR ID sub-TLV MAY both be present in an extended IS reachability TLV.

3.3.3. IPv6 Remote ASBR ID Sub-TLV

A new sub-TLV, which is referred to as the IPv6 remote ASBR ID sub- TLV, is defined for inclusion in the inter-AS reachability TLV when advertising inter-AS links. The IPv6 remote ASBR ID sub-TLV specifies the IPv6 identifier of the remote ASBR to which the advertised inter-AS link connects. This could be any stable and routable IPv6 address of the remote ASBR. Use of the TE Router ID as specified in the IPv6 Traffic Engineering router ID TLV [ISIS-TE-V3] is RECOMMENDED.

The IPv6 remote ASBR ID sub-TLV is TLV type 26 (see Section 6.2) and is 16 octets in length. The format of the IPv6 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                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Remote ASBR ID (continued)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Remote ASBR ID (continued)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Remote ASBR ID (continued)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The IPv6 remote ASBR ID sub-TLV MUST be included if the neighboring ASBR has an IPv6 address. If the neighboring ASBR does not have an IPv6 address, the IPv4 remote ASBR ID sub-TLV MUST be included instead. An IPv4 remote ASBR ID sub-TLV and IPv6 remote ASBR ID sub-TLV MAY both be present in an extended IS reachability TLV.

3.3.4. IPv4 TE Router ID sub-TLV

The IPv4 TE Router ID sub-TLV is TLV type 11 (see Section 6.3) and is 4 octets in length. The format of the IPv4 TE Router 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       TE Router ID                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
When the TE Router ID is needed to reach all routers within an entire ISIS routing domain, the IS-IS Router capability TLV MUST be included in its LSP. If an ASBR supports Traffic Engineering for IPv4 and if the ASBR has an IPv4 TE Router ID, the IPv4 TE Router ID sub-TLV MUST be included. If the ASBR does not have an IPv4 TE Router ID, the IPv6 TE Router sub-TLV MUST be included instead. An IPv4 TE Router ID sub-TLV and IPv6 TE Router ID sub-TLV MAY both be present in an IS-IS router capability TLV.

3.3.5. IPv6 TE Router ID sub-TLV

The IPv6 TE Router ID sub-TLV is TLV type 12 (see Section 6.3) and is 4 octets in length. The format of the IPv6 TE Router 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       TE Router ID                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       TE Router ID   (continued)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       TE Router ID   (continued)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       TE Router ID   (continued)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

When the TE Router ID is needed to reach all routers within an entire ISIS routing domain, the IS-IS router capability TLV MUST be included in its LSP. If an ASBR supports Traffic Engineering for IPv6 and if the ASBR has an IPv6 TE Router ID, the IPv6 TE Router ID sub-TLV MUST be included. If the ASBR does not have an IPv6 TE Router ID, the IPv4 TE Router sub-TLV MUST be included instead. An IPv4 TE Router ID sub-TLV and IPv6 TE Router ID sub-TLV MAY both be present in an IS-IS router capability TLV.

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 MUST take precautions against excessive re-advertisements.
Hellos MUST NOT be exchanged over the inter-AS link, and consequently, an ISIS adjacency MUST NOT be formed.

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.

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 of this document. They will continue to flood the LSP, but will not attempt to use the information received.

In the current operation of ISIS TE, the LSRs at each end of a TE link emit LSAs describing the link. The databases in the LSRs then have two entries (one locally generated, the other from the peer) that describe the different 'directions' of the link. This enables Constrained Shortest Path First (CSPF) to do a two-way check on the link when performing path computation and eliminate it from consideration unless both directions of the link satisfy the required constraints.

In the case we are considering here (i.e., of a TE link to another AS), there is, by definition, no IGP peering and hence no bidirectional TE link information. In order for the CSPF route computation entity to include the link as a candidate path, we have to find a way to get LSAs describing its (bidirectional) TE properties into the TE database.

This is achieved by the ASBR advertising, internally to its AS, information about both directions of the TE link to the next AS. The ASBR will normally generate an LSA describing its own side of a link; here we have it 'proxy' for the ASBR at the edge of the other AS and generate an additional LSA that describes that device's 'view' of the link.

Only some essential TE information for the link needs to be advertised; i.e., the Interface Address, the remote AS number, and the remote ASBR ID of an inter-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.

4.1. Origin of Proxied TE Information

Section 4 describes how an ASBR advertises TE link information as a proxy for its neighbor ASBR, but does not describe where this information comes from.

Although the source of this information is outside the scope of this document, it is possible that it will be a configuration requirement at the ASBR, as are other local properties of the TE link. Further, where BGP is used to exchange IP routing information between the ASBRs, a certain amount of additional local configuration about the link and the remote ASBR is likely to be available.

We note further that it is possible, and may be operationally advantageous, to obtain some of the required configuration information from BGP. Whether and how to utilize these possibilities is an implementation matter.

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 (e.g., using the cleartext passwords or Hashed Message Authentication Codes - Message Digest 5 (HMAC-MD5) algorithm, which are defined in [ISIS] and [RFC5304], respectively).

There is no exchange of information between ASes, and no change to the ISIS security relationship between the ASes. In particular, since no ISIS adjacency is formed on the inter-AS links, there is no requirement for ISIS security between the ASes.

Some of the information included in these new advertisements (e.g., the remote AS number and the remote ASBR ID) is obtained manually from a neighboring administration as part of a commercial relationship. The source and content of this information should be carefully checked before it is entered as configuration information at the ASBR responsible for advertising the inter-AS TE links.

It is worth noting that in the scenario we are considering, a Border Gateway Protocol (BGP) peering may exist between the two ASBRs and that this could be used to detect inconsistencies in configuration (e.g., the administration that originally supplied the information may be lying, or some manual mis-configurations or mistakes may be made by the operators). For example, if a different remote AS number is received in a BGP OPEN [BGP] from that locally configured to ISIS-TE, as we describe here, then local policy SHOULD be applied to determine whether to alert the operator to a potential mis- configuration or to suppress the ISIS advertisement of the inter-AS TE link. Note further that if BGP is used to exchange TE information as described in Section 4.1, the inter-AS BGP session SHOULD be secured using mechanisms as described in [BGP] to provide authentication and integrity checks.

For a discussion of general security considerations for IS-IS, see [RFC5304].

6. IANA Considerations

IANA has made the following allocations from registries under its control.

6.1. Inter-AS Reachability TLV

This document defines the following new ISIS TLV type, described in Section 3.1, which has been registered in the ISIS TLV codepoint registry:
              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 (described in Sections 3.3.1, 3.3.2, and 3.3.3) of top-level TLV 141 (see Section 6.1 above), which have been registered in the ISIS sub-TLV registry for TLV 141. Note that these three new sub-TLVs SHOULD NOT appear in TLV 22 (or TLV 222) and MUST be ignored in TLV 22 (or TLV 222).
     Type        Description
     ----        ------------------------------
       24        remote AS number
       25        IPv4 remote ASBR Identifier
       26        IPv6 remote ASBR Identifier

As described above in Section 3.1, the sub-TLVs defined in [ISIS-TE], [ISIS-TE-V3], and other documents for describing the TE properties of a TE link are applicable to describe an inter-AS TE link and MAY be included in the inter-AS reachability TLV when adverting inter-AS TE links.

IANA has updated the registry that was specified as "Sub-TLVs for TLV 22" to be named "Sub-TLVs for TLVs 22, 141, and 222". Three new columns have been added to the registry to show in which TLVs the sub-TLVs may be present. All sub-TLVs currently defined may be present in all three TLVs, hence the registry (with the definition of the new sub-TLVs defined here) should read as follows.

                                               TLV TLV TLV
   Type    Description                          22  141 222 Reference
   ------- ------------------------------------ --- --- --- ---------
      0    Unassigned                            y   y   y
      1    Unassigned                            y   y   y
      2    Unassigned                            y   y   y
      3    Administrative group (color)          y   y   y  [RFC5305]
      4    Link Local/Remote Identifiers         y   y   y
                                                   [RFC4205][RFC5307]
      5    Unassigned                            y   y   y
      6    IPv4 interface address                y   y   y  [RFC5305]
      7    Unassigned                            y   y   y
      8    IPv4 neighbor address                 y   y   y  [RFC5305]
      9    Maximum link bandwidth                y   y   y  [RFC5305]
     10    Maximum reservable link bandwidth     y   y   y  [RFC5305]
     11    Unreserved bandwidth                  y   y   y  [RFC5305]
     12    Unassigned                            y   y   y
     13    Unassigned                            y   y   y
     14    Unassigned                            y   y   y
     15    Unassigned                            y   y   y
     16    Unassigned                            y   y   y
     17    Unassigned                            y   y   y
     18    TE Default metric                     y   y   y  [RFC5305]
     19    Link-attributes                       y   y   y  [RFC5029]
     20    Link Protection Type                  y   y   y
                                                      [RFC4205][RFC5307]
     21    Interface Switching Capability Desc   y   y   y
                                                      [RFC4205][RFC5307]
     22    Bandwidth Constraints                 y   y   y  [RFC4124]
     23    Unconstrained TE LSP Count (sub-)TLV  y   y   y  [RFC5330]
     24    remote AS number                      n   y   n  [RFC5316]
     25    IPv4 remote ASBR identifier           n   y   n  [RFC5316]
     26    IPv6 remote ASBR identifier           n   y   n  [RFC5316]
   27-249  Unassigned
   250-254 Reserved for Cisco-specific exts
   255     Reserved for future expansion

Further sub-TLVs may be defined in the future for inclusion in any of the TLVs 22, 141, or 222. The re-naming of the registry as above ensures that there is no accidental overlap of sub-TLV codepoints. The introduction of the columns within the registry clarify the use of the sub-TLVs.

6.3. Sub-TLVs for the IS-IS Router Capability TLV

This document defines the following new sub-TLV types, described in Sections 3.3.4 and 3.3.5, of top-level TLV 242 (which is defined in [ISIS-CAP]) that have been registered in the ISIS sub-TLV registry for TLV 242:
      Type        Description                        Length
      ----        ------------------------------   --------
        11        IPv4 TE Router ID                       4
        12        IPv6 TE Router ID                      16

7. Acknowledgments

The authors would like to thank Adrian Farrel, Jean-Louis Le Roux, Christian Hopps, Les Ginsberg, and Hannes Gredler for their review and comments on this document.

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.
   [RFC5304]         Li, T. and R. Atkinson, "IS-IS Cryptographic
                     Authentication", RFC 5304, October 2008.
   [ISIS]            Callon, R., "Use of OSI IS-IS for routing in TCP/IP
                     and dual environments", RFC 1195, December 1990.
   [ISIS-CAP]        Vasseur, JP., Ed., Shen, N., Ed., and R. Aggarwal,
                     Ed., "Intermediate System to Intermediate System
                     (IS-IS) Extensions for Advertising Router
                     Information", RFC 4971, July 2007.

8.2. Informative References

   [INTER-AS-TE-REQ] Zhang, R., Ed., and J.-P. Vasseur, Ed., "MPLS
                     Inter-Autonomous System (AS) Traffic Engineering
                     (TE) Requirements", RFC 4216, November 2005.
   [PD-PATH]         Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang,
                     "A Per-Domain Path Computation Method for
                     Establishing Inter-Domain Traffic Engineering (TE)
                     Label Switched Paths (LSPs)", RFC 5152, February
                     2008.
   [BRPC]            Vasseur, JP., Ed., Zhang, R., Bitar, N., JL. Le
                     Roux, "A Backward Recursive PCE-Based Computation
                     (BRPC) Procedure to Compute Shortest Inter-Domain
                     Traffic Engineering Label Switched Paths", Work in
                     Progress, April 2008.
   [PCE]             Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
                     Computation Element (PCE)-Based Architecture", RFC
                     4655, August 2006.
   [ISIS-TE]         Li, T. and H. Smit, "IS-IS Extensions for Traffic
                     Engineering", RFC 5305, October 2008.
   [ISIS-TE-V3]      Harrison, J., Berger, J., and Bartlett, M., "IPv6
                     Traffic Engineering in IS-IS", Work in Progress,
                     June 2008.
   [GMPLS-TE]        Kompella, K., Ed., and Y. Rekhter, Ed., "IS-IS
                     Extensions in Support of Generalized Multi-Protocol
                     Label Switching (GMPLS)", RFC 5307, October 2008.
   [BGP]             Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed.,
                     "A Border Gateway Protocol 4 (BGP-4)", RFC 4271,
                     January 2006.
   [GENINFO]         L. Ginsberg., Previdi, S., and M. Shand,
                     "Advertising Generic Information in IS-IS", Work in
                     Progress, June 2008.

Authors' Addresses

   Mach (Guoyi) 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
   Xiaodong Duan
   China Mobile
   53A, Xibianmennei Ave.
   Xunwu District
   Beijing, China
   EMail: duanxiaodong@chinamobile.com