Network Working Group M. Chen

Network Working Group M. Chen
Request for Comments: 5316 R. Zhang
Category: Standards Track 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 (c) 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.

Chen, et al. Standards Track [Page 1]

RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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

Chen, et al. Standards Track [Page 2]

RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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.

Chen, et al. Standards Track [Page 3]

RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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.

Chen, et al. Standards Track [Page 4]

RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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.

Chen, et al. Standards Track [Page 5]

RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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

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RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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:

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RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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:

Chen, et al. Standards Track [Page 8]

RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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.

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RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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.

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RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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.

Chen, et al. Standards Track [Page 12]

RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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.

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RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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-

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RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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

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RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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.

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RFC 5316 ISIS Extensions for Inter-AS TE December 2008

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.

Chen, et al. Standards Track [Page 17]

RFC 5316 ISIS Extensions for Inter-AS TE December 2008

[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.

Chen, et al. Standards Track [Page 18]

RFC 5316 ISIS Extensions for Inter-AS TE December 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: [email protected]

Renhai Zhang
Huawei Technologies Co., Ltd
KuiKe Building, No.9 Xinxi Rd.
Hai-Dian District
Beijing, 100085
P.R. China

EMail: [email protected]

Xiaodong Duan
China Mobile
53A, Xibianmennei Ave.
Xunwu District
Beijing, China

EMail: [email protected]

Chen, et al. Standards Track [Page 19]