RFC 9352 PROPOSED STANDARD ROUTING · IS-IS · SRV6
SRv6 for IS-IS: IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane
How IS-IS routers tell each other about SRv6 segments: locators, SIDs, behaviors and hardware limits.
RFC EDITOR
Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B., and Z. Hu, "IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352, February 2023, <https://www.rfc-editor.org/info/rfc9352>.
BIBTEX
@misc{rfc9352,
series = {Request for Comments},
number = 9352,
howpublished = {RFC 9352},
publisher = {RFC Editor},
doi = {10.17487/RFC9352},
url = {https://www.rfc-editor.org/info/rfc9352},
author = {P. Psenak and C. Filsfils and A. Bashandy and B. Decraene and Z. Hu},
title = {{IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane}},
pagetotal = 25,
year = 2023,
month = feb,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Internet Engineering Task Force (IETF) P. Psenak, Ed.
Request for Comments: 9352 C. Filsfils
Updates: 7370 A. Bashandy
Category: Standards Track Cisco Systems
ISSN: 2070-1721 B. Decraene
Orange
Z. Hu
Huawei Technologies
February 2023
IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane
Abstract
The Segment Routing (SR) architecture allows a flexible definition of
the end-to-end path by encoding it as a sequence of topological
elements called "segments". It can be implemented over the MPLS or
the IPv6 data plane. This document describes the IS-IS extensions
required to support SR over the IPv6 data plane.
This document updates RFC 7370 by modifying an existing registry.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9352.
Copyright Notice
Copyright (c) 2023 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
(https://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. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
2. SRv6 Capabilities Sub-TLV
3. Advertising Supported Algorithms
4. Advertising Maximum SRv6 SID Depths
4.1. Maximum Segments Left MSD Type
4.2. Maximum End Pop MSD Type
4.3. Maximum H.Encaps MSD Type
4.4. Maximum End D MSD Type
5. SRv6 SIDs and Reachability
6. Advertising Anycast Property
7. Advertising Locators and End SIDs
7.1. SRv6 Locator TLV Format
7.2. SRv6 End SID Sub-TLV
8. Advertising SRv6 Adjacency SIDs
8.1. SRv6 End.X SID Sub-TLV
8.2. SRv6 LAN End.X SID Sub-TLV
9. SRv6 SID Structure Sub-Sub-TLV
10. Advertising Endpoint Behaviors
11. IANA Considerations
11.1. SRv6 Locator TLV
11.1.1. SRv6 End SID Sub-TLV
11.1.2. IS-IS Sub-TLVs for TLVs Advertising Prefix
Reachability Registry
11.2. SRv6 Capabilities Sub-TLV
11.3. IS-IS Sub-Sub-TLVs for the SRv6 Capabilities Sub-TLV
Registry
11.4. SRv6 End.X SID and SRv6 LAN End.X SID Sub-TLVs
11.5. MSD Types
11.6. IS-IS Sub-Sub-TLVs for SRv6 SID Sub-TLVs Registry
11.7. Prefix Attribute Flags Sub-TLV
11.8. IS-IS SRv6 Capabilities Sub-TLV Flags Registry
11.9. IS-IS SRv6 Locator TLV Flags Registry
11.10. IS-IS SRv6 End SID Sub-TLV Flags Registry
11.11. IS-IS SRv6 Adjacency SID Sub-TLVs Flags Registry
12. Security Considerations
13. References
13.1. Normative References
13.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
With Segment Routing (SR) [RFC8402], a node steers a packet through
an ordered list of instructions, which are called segments.
Segments are identified through Segment Identifiers (SIDs).
SR can be directly instantiated on the IPv6 data plane through the
use of the Segment Routing Header (SRH) defined in [RFC8754]. SRv6
refers to this SR instantiation on the IPv6 data plane.
The network programming paradigm [RFC8986] is central to SRv6. It
describes how any behavior can be bound to a SID and how any network
program can be expressed as a combination of SIDs.
This document specifies IS-IS extensions that allow the IS-IS
protocol to encode some of these SIDs and their behaviors.
Familiarity with the network programming paradigm [RFC8986] is
necessary to understand the extensions specified in this document.
The new SRv6 Locator top-level TLV announces SRv6 Locators -- a form
of summary address for the set of topology-/algorithm-specific SIDs
instantiated at the node.
The SRv6 Capabilities sub-TLV announces the ability to support SRv6.
Several new sub-TLVs are defined to advertise various SRv6 Maximum
SID Depths (MSDs).
The SRv6 End SID sub-TLV, the SRv6 End.X SID sub-TLV, and the SRv6
LAN End.X SID sub-TLV are used to advertise which SIDs are
instantiated at a node and what Endpoint behavior is bound to each
instantiated SID.
This document updates [RFC7370] by modifying an existing registry
(Section 11.1.2).
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. SRv6 Capabilities Sub-TLV
A node indicates that it supports the SR Segment Endpoint Node
functionality as specified in [RFC8754] by advertising a new SRv6
Capabilities sub-TLV of the Router Capability TLV [RFC7981].
The SRv6 Capabilities sub-TLV may contain optional sub-sub-TLVs. No
sub-sub-TLVs are currently defined.
The SRv6 Capabilities sub-TLV has the following format:
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 | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional sub-sub-TLVs...
Type: 25. Single octet, as defined in Section 9 of [ISO10589].
Length: Single octet, as defined in Section 9 of [ISO10589]. The
length value is 2 + length of sub-sub-TLVs.
Flags: 2 octets. The following flags are defined:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |O| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
O-flag: If set, the router supports use of the O-bit in the
SRH, as defined in [RFC9259].
The remaining bits, including bit 0, are reserved for future
use. They MUST be set to zero on transmission and MUST be
ignored on receipt.
3. Advertising Supported Algorithms
An SRv6-capable router indicates one or more supported algorithms by
advertising the Segment Routing Algorithm sub-TLV, as defined in
[RFC8667].
4. Advertising Maximum SRv6 SID Depths
[RFC8491] defines the means to advertise node-/link-specific values
for MSDs of various types. Node MSDs are advertised in a sub-TLV of
the Router Capability TLV [RFC7981]. Link MSDs are advertised in a
sub-TLV of TLVs 22, 23, 25, 141, 222, and 223.
This document defines the relevant SRv6 MSDs and requests MSD type
assignments in the "IGP MSD-Types" registry created by [RFC8491].
4.1. Maximum Segments Left MSD Type
The Maximum Segments Left MSD Type signals the maximum value of the
"Segments Left" field [RFC8754] in the SRH of a received packet
before applying the Endpoint behavior associated with a SID.
SRH Max Segments Left Type: 41
If no value is advertised, the supported value is 0.
4.2. Maximum End Pop MSD Type
The Maximum End Pop MSD Type signals the maximum number of SIDs in
the SRH to which the router can apply "Penultimate Segment Pop (PSP)
of the SRH" or "Ultimate Segment Pop (USP) of the SRH" behavior, as
defined in "Flavors" (Section 4.16 of [RFC8986]).
SRH Max End Pop Type: 42
If the advertised value is zero or no value is advertised, then
the router cannot apply PSP or USP flavors.
4.3. Maximum H.Encaps MSD Type
The Maximum H.Encaps MSD Type signals the maximum number of SIDs that
can be added to the segment list of an SRH as part of the "H.Encaps"
behavior, as defined in [RFC8986].
SRH Max H.encaps Type: 44
If the advertised value is zero or no value is advertised, then
the headend can apply an SR Policy that only contains one segment
without inserting any SRH header.
A non-zero SRH Max H.encaps MSD indicates that the headend can
insert an SRH up to the advertised number of SIDs.
4.4. Maximum End D MSD Type
The Maximum End D MSD Type specifies the maximum number of SIDs
present in an SRH when performing decapsulation. As specified in
[RFC8986], the permitted SID types include, but are not limited to,
End.DX6, End.DT4, End.DT46, End with USD, and End.X with USD.
SRH Max End D Type: 45
If the advertised value is zero or no value is advertised, then
the router cannot apply any behavior that results in decapsulation
and forwarding of the inner packet if the outer IPv6 header
contains an SRH.
5. SRv6 SIDs and Reachability
As discussed in [RFC8986], an SRv6 Segment Identifier (SID) is 128
bits and consists of locator, function, and argument parts.
A node is provisioned with topology-/algorithm-specific locators for
each of the topology/algorithm pairs supported by that node. Each
locator is a covering prefix for all SIDs provisioned on that node
that have the matching topology/algorithm.
Locators MUST be advertised in the SRv6 Locator TLV (see
Section 7.1). Forwarding entries for the locators advertised in the
SRv6 Locator TLV MUST be installed in the forwarding plane of
receiving SRv6-capable routers when the associated topology/algorithm
is supported by the receiving node. The processing of the prefix
advertised in the SRv6 Locator TLV, the calculation of its
reachability, and the installation in the forwarding plane follows
the process defined for the Prefix Reachability TLV 236 [RFC5308] or
TLV 237 [RFC5120].
Locators associated with algorithms 0 and 1 (for all supported
topologies) SHOULD also be advertised in a Prefix Reachability TLV
(236 or 237) so that legacy routers (i.e., routers that do not
support SRv6) will install a forwarding entry for algorithms 0 and 1
SRv6 traffic.
In cases where the same prefix with the same prefix length, Multi-
Topology Identifier (MTID), and algorithm is received in both a
Prefix Reachability TLV and an SRv6 Locator TLV, the Prefix
Reachability advertisement MUST be preferred when installing entries
in the forwarding plane. This is to prevent inconsistent forwarding
entries between SRv6-capable and SRv6-incapable routers. Such
preference of Prefix Reachability advertisement does not have any
impact on the rest of the data advertised in the SRv6 Locator TLV.
Locators associated with Flexible Algorithms (see Section 4 of
[RFC9350]) SHOULD NOT be advertised in Prefix Reachability TLVs (236
or 237). Advertising the Flexible Algorithm locator in a regular
Prefix Reachability TLV (236 or 237) would make the forwarding for it
follow the algorithm 0 path.
SRv6 SIDs are advertised as sub-TLVs in the SRv6 Locator TLV, except
for SRv6 SIDs that are associated with a specific neighbor/link and
are therefore advertised as sub-TLVs in TLVs 22, 23, 25, 141, 222,
and 223.
SRv6 SIDs received from other nodes are not directly routable and
MUST NOT be installed in the forwarding plane. Reachability to SRv6
SIDs depends upon the existence of a covering locator.
Adherence to the rules defined in this section will ensure that SRv6
SIDs associated with a supported topology/algorithm pair will be
forwarded correctly, while SRv6 SIDs associated with an unsupported
topology/algorithm pair will be dropped. NOTE: The drop behavior
depends on the absence of a default/summary route covering a given
locator.
In order for forwarding to work correctly, the locator associated
with SRv6 SID advertisements must be the longest match prefix
installed in the forwarding plane for those SIDs. In order to ensure
correct forwarding, network operators should take steps to make sure
that this requirement is not compromised. For example, the following
situations should be avoided:
* Another locator associated with a different topology/algorithm is
the longest match.
* Another prefix advertisement (i.e., from TLV 236 or 237) is the
longest match.
6. Advertising Anycast Property
Both prefixes and SRv6 Locators may be configured as anycast; as
such, the same value can be advertised by multiple routers. It is
useful for other routers to know that the advertisement is for an
anycast identifier.
A new flag in the Prefix Attribute Flags sub-TLV [RFC7794] is defined
to advertise the anycast property:
Bit #: 4
Name: Anycast Flag (A-flag)
When the prefix/SRv6 Locator is configured as anycast, the A-flag
SHOULD be set. Otherwise, this flag MUST be clear.
The A-flag MUST be preserved when the advertisement is leaked between
levels.
The A-flag and the N-flag MUST NOT both be set. If both the N-flag
and the A-flag are set in the prefix/SRv6 Locator advertisement, the
receiving routers MUST ignore the N-flag.
The same prefix/SRv6 Locator can be advertised by multiple routers.
If at least one of them sets the A-flag in its advertisement, the
prefix/SRv6 Locator SHOULD be considered as anycast.
A prefix/SRv6 Locator that is advertised by a single node and without
an A-flag MUST be considered node specific.
All the nodes advertising the same anycast locator MUST instantiate
the exact same set of SIDs under that anycast locator. Failure to do
so may result in traffic being dropped or misrouted.
The Prefix Attribute Flags sub-TLV can be carried in the SRv6 Locator
TLV as well as the Prefix Reachability TLVs. When a router
originates both the Prefix Reachability TLV and the SRv6 Locator TLV
for a given prefix, it SHOULD advertise the Prefix Attribute Flags
sub-TLV, if used, in both TLVs and use the same flags. However,
unlike TLVs 236 [RFC5308] and 237 [RFC5120], the X-flag in the Prefix
Attributes Flags sub-TLV is valid when sent in the SRv6 Locator TLV.
When included in the Locator TLV, the state of the X-flag in the
Prefix Attributes Flags sub-TLV MUST match the setting of the
embedded "X-bit" in any advertisement for the same prefix in TLVs 236
[RFC5308] and 237 [RFC5120]. In case of any inconsistency between
the Prefix Attribute Flags advertised in the Locator TLV and in the
Prefix Reachability TLV, the ones advertised in the Prefix
Reachability TLV MUST be preferred.
7. Advertising Locators and End SIDs
The SRv6 Locator TLV is introduced to advertise SRv6 Locators and End
SIDs associated with each locator.
This new TLV shares the sub-TLV space defined for TLVs 135, 235, 236,
and 237.
7.1. SRv6 Locator TLV Format
The SRv6 Locator TLV has the following format:
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 |R|R|R|R| MTID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator Entries . . . |
Type: 27. Single octet, as defined in Section 9 of [ISO10589].
Length: Single octet, as defined in Section 9 of [ISO10589]. The
length value is variable.
R Bits: Reserved for future use. They MUST be set to zero on
transmission and MUST be ignored on receipt.
MTID: Multi-Topology Identifier, as defined in [RFC5120]. Note that
the value 0 is legal.
The SRv6 Locator TLV is followed by one or more locator entries of
the form:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Algorithm | Loc Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Locator (continued, variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLV-len | Sub-TLVs (variable) . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Metric: 4 octets, as described in Section 4 of [RFC5305].
Flags: 1 octet. The following flags are defined:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|D| Reserved |
+-+-+-+-+-+-+-+-+
D-flag: "up/down bit" as described in Section 4.1 of [RFC5305].
The remaining bits are reserved for future use. They MUST be
set to zero on transmission and MUST be ignored on receipt.
Algorithm: 1 octet, as defined in the "IGP Algorithm Types" registry
[RFC8665].
Loc-Size: 1 octet. Number of bits in the SRv6 Locator field, which
MUST be from the range (1-128). The entire TLV MUST be ignored if
the Loc-Size is outside this range.
Locator: 1-16 octets. This field encodes the advertised SRv6
Locator. The SRv6 Locator is encoded in the minimal number of
octets for the given number of bits. Trailing bits MUST be set to
zero and ignored when received.
Sub-TLV-length: 1 octet. Number of octets used by sub-TLVs.
Optional Sub-TLVs: Supported sub-TLVs are specified in
Section 11.1.2. Any sub-TLV that is not allowed in the SRv6
Locator TLV MUST be ignored.
The Prefix Attribute Flags sub-TLV [RFC7794] SHOULD be included in
the Locator TLV.
The Prefix Attribute Flags sub-TLV MUST be included in the Locator
TLV when it is leaked upwards in the hierarchy or originated as a
result of the redistribution from another protocol or another IS-IS
instance. If the Prefix Attribute Flags sub-TLV is not included in
these cases, receivers will be unable to determine the correct source
of the advertisement. The receivers will be unable to detect the
violation.
7.2. SRv6 End SID Sub-TLV
The SRv6 End SID sub-TLV is introduced to advertise SRv6 SIDs with
Endpoint behaviors that do not require a particular neighbor in order
to be correctly applied. SRv6 SIDs associated with a neighbor are
advertised using the sub-TLVs defined in Section 8.
Supported behavior values, together with parent TLVs in which they
are advertised, are specified in Section 10 of this document. Please
note that not all behaviors defined in [RFC8986] are defined in this
document, e.g., End.T is not.
This new sub-TLV is advertised in the SRv6 Locator TLV defined in the
previous section. SRv6 End SIDs inherit the topology/algorithm from
the parent locator.
The SRv6 End SID sub-TLV has the following format:
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 | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Endpoint Behavior |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (128 bits) . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (cont . . .) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (cont . . .) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (cont . . .) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-sub-TLV-len| Sub-sub-TLVs (variable) . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 5. Single octet, as defined in Section 9 of [ISO10589].
Length: Single octet, as defined in Section 9 of [ISO10589]. The
length value is variable.
Flags: 1 octet. No flags are currently defined. All bits are
reserved for future use. They MUST be set to zero on transmission
and MUST be ignored on receipt.
Endpoint Behavior: 2 octets, as defined in [RFC8986]. Supported
behavior values for this sub-TLV are defined in Section 10 of this
document. Unsupported or unrecognized behavior values are ignored
by the receiver.
SID: 16 octets. This field encodes the advertised SRv6 SID.
Sub-sub-TLV-length: 1 octet. Number of octets used by sub-sub-TLVs.
Optional Sub-sub-TLVs: Supported sub-sub-TLVs are specified in
Section 11.6. Any sub-sub-TLV that is not allowed in the SRv6 End
SID sub-TLV MUST be ignored.
The SRv6 End SID MUST be allocated from its associated locator. SRv6
End SIDs that are not allocated from the associated locator MUST be
ignored.
Multiple SRv6 End SIDs MAY be associated with the same locator. In
cases where the number of SRv6 End SID sub-TLVs exceeds the capacity
of a single TLV, multiple Locator TLVs for the same locator MAY be
advertised. For a given MTID/Locator, the algorithm MUST be the same
in all TLVs. If this restriction is not met, all TLVs for that MTID/
Locator MUST be ignored.
8. Advertising SRv6 Adjacency SIDs
Certain SRv6 Endpoint behaviors [RFC8986] are associated with a
particular adjacency.
This document defines two new sub-TLVs of TLVs 22, 23, 25, 141, 222,
and 223 -- namely "SRv6 End.X SID sub-TLVs" and "SRv6 LAN End.X SID
sub-TLVs".
IS-IS neighbor advertisements are topology specific but not algorithm
specific. SIDs advertised in SRv6 End.X SID and SRv6 LAN End.X SID
sub-TLVs therefore inherit the topology from the associated neighbor
advertisement, but the algorithm is specified in the individual SID.
All SIDs advertised in SRv6 End.X SID and SRv6 LAN End.X SID sub-TLVs
MUST be a subnet of a Locator with matching topology and algorithm
that are advertised by the same node in an SRv6 Locator TLV. SIDs
that do not meet this requirement MUST be ignored. This ensures that
the node advertising these SIDs is also advertising its corresponding
Locator with the algorithm that will be used for computing paths
destined to the SID.
8.1. SRv6 End.X SID Sub-TLV
This sub-TLV is used to advertise an SRv6 SID associated with a
point-to-point adjacency. Multiple SRv6 End.X SID sub-TLVs MAY be
associated with the same adjacency.
The SRv6 End.X SID sub-TLV has the following format:
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 | Flags | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Weight | Endpoint Behavior |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (128 bits) . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (cont . . .) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (cont . . .) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (cont . . .) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-sub-tlv-len| Sub-sub-TLVs (variable) . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 43. Single octet, as defined in Section 9 of [ISO10589].
Length: Single octet, as defined in Section 9 of [ISO10589]. The
length value is variable.
Flags: 1 octet.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|B|S|P|Reserved |
+-+-+-+-+-+-+-+-+
where:
B-Flag: Backup flag. If set, the SID is eligible for
protection, e.g., using IP Fast Reroute (IPFRR) [RFC5286],
as described in [RFC8355].
S-Flag: Set flag. When set, the S-flag indicates that the SID
refers to a set of adjacencies (and therefore MAY be
assigned to other adjacencies as well).
P-Flag: Persistent flag. When set, the P-flag indicates that
the SID is persistently allocated, i.e., the SID value
remains consistent across router restart and/or interface
flap.
Reserved bits: Reserved bits MUST be zero when originated and
MUST be ignored when received.
Algorithm: 1 octet, as defined in the "IGP Algorithm Types" registry
[RFC8665].
Weight: 1 octet. The value represents the weight of the SID for the
purpose of load balancing. The use of the weight is defined in
[RFC8402].
Endpoint Behavior: 2 octets, as defined in [RFC8986]. Supported
behavior values for this sub-TLV are defined in Section 10 of this
document. Unsupported or unrecognized behavior values are ignored
by the receiver.
SID: 16 octets. This field encodes the advertised SRv6 SID.
Sub-sub-TLV-length: 1 octet. Number of octets used by sub-sub-
TLVs.
Optional Sub-sub-TLVs: Supported sub-sub-TLVs are specified in
Section 11.6. Any sub-sub-TLV that is not allowed in SRv6 End.X
SID sub-TLV MUST be ignored.
Note that multiple TLVs for the same neighbor may be required in
order to advertise all the SRv6 SIDs associated with that neighbor.
8.2. SRv6 LAN End.X SID Sub-TLV
This sub-TLV is used to advertise an SRv6 SID associated with a LAN
adjacency. Since the parent TLV is advertising an adjacency to the
Designated Intermediate System (DIS) for the LAN, it is necessary to
include the System-ID of the physical neighbor on the LAN with which
the SRv6 SID is associated. Given that many neighbors may exist on a
given LAN, multiple SRv6 LAN END.X SID sub-TLVs may be associated
with the same LAN. Note that multiple TLVs for the same DIS neighbor
may be required in order to advertise all the SRv6 SIDs associated
with that neighbor.
The SRv6 LAN End.X SID sub-TLV has the following format:
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 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Neighbor System-ID (ID length octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Algorithm | Weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Endpoint Behavior |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (128 bits) . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (cont . . .) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (cont . . .) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID (cont . . .) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-sub-TLV-len| Sub-sub-TLVs (variable) . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 44. Single octet, as defined in Section 9 of [ISO10589].
Length: Single octet, as defined in Section 9 of [ISO10589]. The
length value is variable.
Neighbor System-ID: IS-IS System-ID of length "ID Length", as
defined in [ISO10589].
Flags: 1 octet.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|B|S|P|Reserved |
+-+-+-+-+-+-+-+-+
The B-, S-, and P-flags are as described in Section 8.1. Reserved
bits MUST be zero when originated and MUST be ignored when
received.
Algorithm: 1 octet, as defined in the "IGP Algorithm Types" registry
[RFC8665].
Weight: 1 octet. The value represents the weight of the SID for the
purpose of load balancing. The use of the weight is defined in
[RFC8402].
Endpoint Behavior: 2 octets, as defined in [RFC8986]. Supported
behavior values for this sub-TLV are defined in Section 10 of this
document. Unsupported or unrecognized behavior values are ignored
by the receiver.
SID: 16 octets. This field encodes the advertised SRv6 SID.
Sub-sub-TLV-length: 1 octet. Number of octets used by sub-sub-
TLVs.
Optional Sub-sub-TLVs: Supported sub-sub-TLVs are specified in
Section 11.6. Any sub-sub-TLV that is not allowed in SRv6 LAN
End.X SID sub-TLV MUST be ignored.
Note that multiple TLVs for the same neighbor, on the same LAN, may
be required in order to advertise all the SRv6 SIDs associated with
that neighbor.
9. SRv6 SID Structure Sub-Sub-TLV
The SRv6 SID Structure sub-sub-TLV is an optional sub-sub-TLV of:
* SRv6 End SID sub-TLV (Section 7.2)
* SRv6 End.X SID sub-TLV (Section 8.1)
* SRv6 LAN End.X SID sub-TLV (Section 8.2)
The SRv6 SID Structure sub-sub-TLV is used to advertise the structure
of the SRv6 SID, as defined in [RFC8986]. It has the following
format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LB Length | LN Length | Fun. Length | Arg. Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1. Single octet, as defined in Section 9 of [ISO10589].
Length: Single octet, as defined in Section 9 of [ISO10589]. The
length value is 4 octets.
LB Length: 1 octet. SRv6 SID Locator Block length in bits.
LN Length: 1 octet. SRv6 SID Locator Node length in bits.
Fun. Length: 1 octet. SRv6 SID Function length in bits.
Arg. Length: 1 octet. SRv6 SID Arguments length in bits.
The IS-IS SRv6 SID Structure sub-sub-TLV MUST NOT appear more than
once in its parent sub-TLV. If it appears more than once in its
parent sub-TLV, the parent sub-TLV MUST be ignored by the receiver.
The sum of all four sizes advertised in the IS-IS SRv6 SID Structure
sub-sub-TLV MUST be less than or equal to 128 bits. If the sum of
all four sizes advertised in the IS-IS SRv6 SID Structure sub-sub-TLV
is larger than 128 bits, the parent sub-TLV MUST be ignored by the
receiver.
The SRv6 SID Structure sub-sub-TLV is intended for informational use
by the control and management planes. It MUST NOT be used at a
transit node (as defined in [RFC8754]) for forwarding packets. As an
example, this information could be used for the following:
* validation of SRv6 SIDs being instantiated in the network and
advertised via IS-IS. These can be learned by controllers via
Border Gateway Protocol - Link State (BGP-LS) and then be
monitored for conformance to the SRv6 SID allocation scheme chosen
by the operator, as described in Section 3.2 of [RFC8986].
* verification and automation for securing the SRv6 domain by
provisioning filtering rules at SR domain boundaries, as described
in Section 5 of [RFC8754].
The details of these potential applications are outside the scope of
this document.
10. Advertising Endpoint Behaviors
Endpoint behaviors are defined in [RFC8986]. The codepoints for the
Endpoint behaviors are defined in the "SRv6 Endpoint Behaviors"
registry defined in [RFC8986]. If a behavior is advertised, it MUST
only be advertised in the TLV(s) marked with "Y" in the table below
and MUST NOT be advertised in the TLV(s) marked with "N" in the table
below.
+===================+===================+=====+=======+===========+
| Endpoint Behavior | Endpoint Behavior | End | End.X | Lan End.X |
| | Codepoint | SID | SID | SID |
+===================+===================+=====+=======+===========+
| End (PSP, USP, | 1-4, 28-31 | Y | N | N |
| USD) | | | | |
+-------------------+-------------------+-----+-------+-----------+
| End.X (PSP, USP, | 5-8, 32-35 | N | Y | Y |
| USD) | | | | |
+-------------------+-------------------+-----+-------+-----------+
| End.DX6 | 16 | N | Y | Y |
+-------------------+-------------------+-----+-------+-----------+
| End.DX4 | 17 | N | Y | Y |
+-------------------+-------------------+-----+-------+-----------+
| End.DT6 | 18 | Y | N | N |
+-------------------+-------------------+-----+-------+-----------+
| End.DT4 | 19 | Y | N | N |
+-------------------+-------------------+-----+-------+-----------+
| End.DT46 | 20 | Y | N | N |
+-------------------+-------------------+-----+-------+-----------+
Table 1: Endpoint Behaviors
11. IANA Considerations
This document requests allocation for the following TLVs, sub-TLVs,
and sub-sub-TLVs by updating the existing registries and defining new
registries under the "IS-IS TLV Codepoints" grouping.
11.1. SRv6 Locator TLV
The SRv6 Locator TLV shares sub-TLV space with TLVs advertising
prefix reachability. IANA has updated the "IS-IS Sub-TLVs for TLVs
Advertising Prefix Reachability" registry initially defined in
[RFC7370] by adding this document as a reference and updating the
description of that registry to include the SRv6 Locator TLV (27).
This document makes the following registration in the "IS-IS Top-
Level TLV Codepoints" registry:
+=======+==============+=====+=====+=====+=======+
| Value | Name | IIH | LSP | SNP | Purge |
+=======+==============+=====+=====+=====+=======+
| 27 | SRv6 Locator | n | y | n | n |
+-------+--------------+-----+-----+-----+-------+
Table 2: IS-IS Top-Level TLV Codepoints Registry
11.1.1. SRv6 End SID Sub-TLV
This document makes the following registration:
+======+==============+====+=====+=====+=====+=====+=============+
| Type | Description | 27 | 135 | 235 | 236 | 237 | Reference |
+======+==============+====+=====+=====+=====+=====+=============+
| 5 | SRv6 End SID | y | n | n | n | n | RFC 9352, |
| | | | | | | | Section 7.2 |
+------+--------------+----+-----+-----+-----+-----+-------------+
Table 3: IS-IS Sub-TLVs for TLVs Advertising Prefix
Reachability Registry
11.1.2. IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability
Registry
IANA has updated the "IS-IS Sub-TLVs for TLVs Advertising Prefix
Reachability" registry to include a column for the SRv6 Locator TLV
(27) as shown below:
+======+=======================+====+=====+=====+=====+=====+
| Type | Description | 27 | 135 | 235 | 236 | 237 |
+======+=======================+====+=====+=====+=====+=====+
| 1 | 32-bit Administrative | y | y | y | y | y |
| | Tag Sub-TLV | | | | | |
+------+-----------------------+----+-----+-----+-----+-----+
| 2 | 64-bit Administrative | y | y | y | y | y |
| | Tag Sub-TLV | | | | | |
+------+-----------------------+----+-----+-----+-----+-----+
| 3 | Prefix Segment | n | y | y | y | y |
| | Identifier | | | | | |
+------+-----------------------+----+-----+-----+-----+-----+
| 4 | Prefix Attribute | y | y | y | y | y |
| | Flags | | | | | |
+------+-----------------------+----+-----+-----+-----+-----+
| 6 | Flexible Algorithm | n | y | y | y | y |
| | Prefix Metric (FAPM) | | | | | |
+------+-----------------------+----+-----+-----+-----+-----+
| 11 | IPv4 Source Router ID | y | y | y | y | y |
+------+-----------------------+----+-----+-----+-----+-----+
| 12 | IPv6 Source Router ID | y | y | y | y | y |
+------+-----------------------+----+-----+-----+-----+-----+
| 32 | BIER Info | n | y | y | y | y |
+------+-----------------------+----+-----+-----+-----+-----+
Table 4: IS-IS Sub-TLVs for TLVs Advertising Prefix
Reachability Registry
11.2. SRv6 Capabilities Sub-TLV
This document makes the following registration in the "IS-IS Sub-TLVs
for IS-IS Router CAPABILITY TLV" registry:
+=======+===================+=====================+
| Value | Description | Reference |
+=======+===================+=====================+
| 25 | SRv6 Capabilities | RFC 9352, Section 2 |
+-------+-------------------+---------------------+
Table 5: IS-IS Sub-TLVs for IS-IS Router
CAPABILITY TLV Registry
11.3. IS-IS Sub-Sub-TLVs for the SRv6 Capabilities Sub-TLV Registry
IANA has created the "IS-IS Sub-Sub-TLVs for SRv6 Capabilities Sub-
TLV" registry under the "IS-IS TLV Codepoints" grouping for the
assignment of sub-TLV types for the SRv6 Capabilities sub-TLV
specified in this document (Section 2). This registry defines sub-
sub-TLVs for the SRv6 Capabilities sub-TLV (25) advertised in the IS-
IS Router CAPABILITY TLV (242).
The registration procedure is "Expert Review", as defined in
[RFC8126]. Guidance for the designated experts is provided in
[RFC7370]. No sub-sub-TLVs are defined by this document, except for
the reserved type 0.
+=======+=============+===========+
| Value | Description | Reference |
+=======+=============+===========+
| 0 | Reserved | RFC 9532 |
+-------+-------------+-----------+
| 1-255 | Unassigned | |
+-------+-------------+-----------+
Table 6: IS-IS Sub-Sub-TLVs for
SRv6 Capabilities Sub-TLV
Registry
11.4. SRv6 End.X SID and SRv6 LAN End.X SID Sub-TLVs
This document makes the following registrations in the "IS-IS Sub-
TLVs for TLVs Advertising Neighbor Information" registry:
+======+=============+====+====+====+=====+=====+=====+=============+
| Type | Description | 22 | 23 | 25 | 141 | 222 | 223 | Reference |
+======+=============+====+====+====+=====+=====+=====+=============+
| 43 | SRv6 End.X | y | y | y | y | y | y | RFC 9352, |
| | SID | | | | | | | Section |
| | | | | | | | | 8.1 |
+------+-------------+----+----+----+-----+-----+-----+-------------+
| 44 | SRv6 LAN | y | y | y | y | y | y | RFC 9352, |
| | End.X SID | | | | | | | Section |
| | | | | | | | | 8.2 |
+------+-------------+----+----+----+-----+-----+-----+-------------+
Table 7: IS-IS Sub-TLVs for TLVs Advertising Neighbor Information
Registry
11.5. MSD Types
This document makes the following registrations in the "IGP MSD-
Types" registry:
+=======+==================+===========+
| Value | Name | Reference |
+=======+==================+===========+
| 41 | SRH Max SL | RFC 9352 |
+-------+------------------+-----------+
| 42 | SRH Max End Pop | RFC 9352 |
+-------+------------------+-----------+
| 44 | SRH Max H.encaps | RFC 9352 |
+-------+------------------+-----------+
| 45 | SRH Max End D | RFC 9352 |
+-------+------------------+-----------+
Table 8: IGP MSD-Types
11.6. IS-IS Sub-Sub-TLVs for SRv6 SID Sub-TLVs Registry
IANA has created the "IS-IS Sub-Sub-TLVs for SRv6 SID Sub-TLVs"
registry under the "IS-IS TLV Codepoints" grouping to assign sub-TLV
types for the SID sub-TLVs specified in this document (Sections 7.2,
8.1, and 8.2).
This registry defines sub-sub-TLVs for SRv6 SID sub-TLVs. This
includes the following sub-TLVs:
* SRv6 End SID (5) (Advertised in SRv6 Locator TLV (27))
* SRv6 End.X SID (43) (Advertised in TLVs advertising neighbor
information)
* SRv6 LAN End.X SID (44) (Advertised in TLVs advertising neighbor
information)
The registration procedure is "Expert Review", as defined in
[RFC8126]. Guidance for the designated experts is provided in
[RFC7370]. The following assignments are made by this document:
+=======+====================+===+====+====+===========+
| Type | Description | 5 | 43 | 44 | Reference |
+=======+====================+===+====+====+===========+
| 0 | Reserved | | | | RFC 9352 |
+-------+--------------------+---+----+----+-----------+
| 1 | SRv6 SID Structure | y | y | y | RFC 9352 |
+-------+--------------------+---+----+----+-----------+
| 2-255 | Unassigned | | | | |
+-------+--------------------+---+----+----+-----------+
Table 9: IS-IS Sub-Sub-TLVs for SRv6 SID Sub-TLVs
Registry
11.7. Prefix Attribute Flags Sub-TLV
This document adds a new bit in the "IS-IS Bit Values for Prefix
Attribute Flags Sub-TLV" registry:
+=======+=======================+=====================+
| Bit # | Name | Reference |
+=======+=======================+=====================+
| 4 | Anycast Flag (A-flag) | RFC 9352, Section 6 |
+-------+-----------------------+---------------------+
Table 10: IS-IS Bit Values for Prefix Attribute
Flags Sub-TLV Registry
11.8. IS-IS SRv6 Capabilities Sub-TLV Flags Registry
IANA has created the "IS-IS SRv6 Capabilities Sub-TLV Flags" registry
under the "IS-IS TLV Codepoints" grouping to assign bits 0 to 15 in
the Flags field of the IS-IS SRv6 Capabilities sub-TLV specified in
this document (Section 2). This registry defines bit values
advertised in the Flags field of the SRv6 Capabilities sub-TLV (25).
This sub-TLV is advertised in the IS-IS Router CAPABILITY TLV (242).
The registration procedure is "Expert Review", as defined in
[RFC8126]. Guidance for the designated experts is provided in
[RFC7370]. The following assignments are made by this document:
+======+=============+=====================+
| Type | Description | Reference |
+======+=============+=====================+
| 0 | Unassigned | |
+------+-------------+---------------------+
| 1 | O-flag | RFC 9352, Section 2 |
+------+-------------+---------------------+
| 2-15 | Unassigned | |
+------+-------------+---------------------+
Table 11: IS-IS SRv6 Capabilities Sub-
TLV Flags Registry
11.9. IS-IS SRv6 Locator TLV Flags Registry
IANA has created the "IS-IS SRv6 Locator TLV Flags" registry under
the "IS-IS TLV Codepoints" grouping to assign bits 0 to 7 in the
Flags field of the SRv6 Locator TLV specified in this document
(Section 7.1). This registry defines bit values advertised in the
Flags field of the SRv6 Locator TLV (27).
The registration procedure is "Expert Review", as defined in
[RFC8126]. Guidance for the designated experts is provided in
[RFC7370]. The following assignments are made by this document:
+=======+=============+=======================+
| Value | Description | Reference |
+=======+=============+=======================+
| 0 | D-flag | RFC 9352, Section 7.1 |
+-------+-------------+-----------------------+
| 1-7 | Unassigned | |
+-------+-------------+-----------------------+
Table 12: IS-IS SRv6 Locator TLV Flags Registry
11.10. IS-IS SRv6 End SID Sub-TLV Flags Registry
IANA has created the "IS-IS SRv6 End SID Sub-TLV Flags" registry
under the "IS-IS TLV Codepoints" grouping to assign bits 0 to 7 in
the Flags field of the IS-IS SRv6 End SID sub-TLV specified in this
document (Section 7.2). This registry defines bit values advertised
in the Flags field of the SRv6 End SID sub-TLV (5), which is
advertised in the SRv6 Locator TLV (27).
The registration procedure is "Expert Review", as defined in
[RFC8126]. Guidance for the designated experts is provided in
[RFC7370]. No assignments are made by this document.
+=======+=============+
| Value | Description |
+=======+=============+
| 0-7 | Unassigned |
+-------+-------------+
Table 13: IS-IS SRv6
End SID Sub-TLV Flags
Registry
11.11. IS-IS SRv6 Adjacency SID Sub-TLVs Flags Registry
IANA has created the "IS-IS SRv6 Adjacency SID Sub-TLVs Flags"
registry under the "IS-IS TLV Codepoints" grouping to assign bits 0
to 7 in the Flags field of the IS-IS SRv6 End.X SID and LAN End.X SID
sub-TLVs (Sections 8.1 and 8.2).
This registry defines bit values advertised in the Flags field of
SRv6 SID sub-TLVs associated with adjacencies. These sub-TLVs are
advertised in TLVs advertising neighbor information. The list of
sub-TLVs includes:
* SRv6 End.X SID (43)
* SRv6 LAN End.X SID (44)
The registration procedure is "Expert Review", as defined in
[RFC8126]. Guidance for the designated experts is provided in
[RFC7370]. The following assignments are made by this document:
+=======+=============+=======================+
| Value | Description | Reference |
+=======+=============+=======================+
| 0 | B-flag | RFC 9352, Section 8.1 |
+-------+-------------+-----------------------+
| 1 | S-flag | RFC 9352, Section 8.1 |
+-------+-------------+-----------------------+
| 2 | P-flag | RFC 9352, Section 8.1 |
+-------+-------------+-----------------------+
| 3-7 | Unassigned | |
+-------+-------------+-----------------------+
Table 14: IS-IS SRv6 Adjacency SID Sub-TLVs
Flags Registry
12. Security Considerations
Security concerns for IS-IS are addressed in [ISO10589], [RFC5304],
and [RFC5310]. While IS-IS is deployed under a single administrative
domain, there can be deployments where potential attackers have
access to one or more networks in the IS-IS routing domain. In these
deployments, the stronger authentication mechanisms defined in the
aforementioned documents SHOULD be used.
This document describes the IS-IS extensions required to support SR
over an IPv6 data plane. The security considerations for SR are
discussed in [RFC8402]. [RFC8986] defines the SRv6 Network
Programming concept and specifies the main SR behaviors to enable the
creation of interoperable overlays; the security considerations from
that document apply too.
The advertisement for an incorrect MSD value may have negative
consequences; see [RFC8491] for additional considerations.
Security concerns associated with the setting of the O-flag are
described in [RFC9259].
Security concerns associated with the usage of Flexible Algorithms
are described in [RFC9350]).
13. References
13.1. Normative References
[ISO10589] ISO, "Information technology - Telecommunications and
information exchange between systems - Intermediate System
to Intermediate System intra-domain routeing information
exchange protocol for use in conjunction with the protocol
for providing the connectionless-mode network service (ISO
8473)", Second Edition, ISO/IEC 10589:2002, November 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<https://www.rfc-editor.org/info/rfc5308>.
[RFC7370] Ginsberg, L., "Updates to the IS-IS TLV Codepoints
Registry", RFC 7370, DOI 10.17487/RFC7370, September 2014,
<https://www.rfc-editor.org/info/rfc7370>.
[RFC7794] Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
March 2016, <https://www.rfc-editor.org/info/rfc7794>.
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
DOI 10.17487/RFC8491, November 2018,
<https://www.rfc-editor.org/info/rfc8491>.
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
[RFC9259] Ali, Z., Filsfils, C., Matsushima, S., Voyer, D., and M.
Chen, "Operations, Administration, and Maintenance (OAM)
in Segment Routing over IPv6 (SRv6)", RFC 9259,
DOI 10.17487/RFC9259, June 2022,
<https://www.rfc-editor.org/info/rfc9259>.
[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
DOI 10.17487/RFC9350, February 2023,
<https://www.rfc-editor.org/rfc/rfc9350>.
13.2. Informative References
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <https://www.rfc-editor.org/info/rfc5304>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <https://www.rfc-editor.org/info/rfc5310>.
[RFC8355] Filsfils, C., Ed., Previdi, S., Ed., Decraene, B., and R.
Shakir, "Resiliency Use Cases in Source Packet Routing in
Networking (SPRING) Networks", RFC 8355,
DOI 10.17487/RFC8355, March 2018,
<https://www.rfc-editor.org/info/rfc8355>.
Acknowledgements
Thanks to Christian Hopps for his review comments and shepherd work.
Thanks to Alvaro Retana and John Scudder for AD review and comments.
Contributors
The following people gave a substantial contribution to the content
of this document and should be considered coauthors:
Stefano Previdi
Huawei Technologies
Email: stefano@previdi.net
Paul Wells
Cisco Systems
Saint Paul, Minnesota
United States of America
Email: pauwells@cisco.com
Daniel Voyer
Email: daniel.voyer@bell.ca
Satoru Matsushima
Email: satoru.matsushima@g.softbank.co.jp
Bart Peirens
Email: bart.peirens@proximus.com
Hani Elmalky
Email: hani.elmalky@ericsson.com
Prem Jonnalagadda
Email: prem@barefootnetworks.com
Milad Sharif
Email: msharif@barefootnetworks.com
Robert Hanzl
Cisco Systems
Millenium Plaza Building, V Celnici 10, Prague 1
Prague
Czech Republic
Email: rhanzl@cisco.com
Ketan Talaulikar
Cisco Systems, Inc.
Email: ketant@cisco.com
Authors' Addresses
Peter Psenak (editor)
Cisco Systems
Pribinova Street 10
81109 Bratislava
Slovakia
Email: ppsenak@cisco.com
Clarence Filsfils
Cisco Systems
Brussels
Belgium
Email: cfilsfil@cisco.com
Ahmed Bashandy
Cisco Systems
Milpitas,
United States of America
Email: bashandy@cisco.com
Bruno Decraene
Orange
Chatillon
France
Email: bruno.decraene@orange.com
Zhibo Hu
Huawei Technologies
Email: huzhibo@huawei.com
AI ELI5
On this page
PROBLEMSegment Routing over IPv6 (SRv6) steers packets through a list of instructions called segments, but IS-IS had no way to advertise the locators, segment identifiers and behaviors that routers need to build such paths.
MECHANISMThe RFC adds a new SRv6 Locator TLV, an SRv6 Capabilities sub-TLV, End SID, End.X SID and LAN End.X SID sub-TLVs, an optional SID Structure sub-sub-TLV and new Maximum SID Depth types.
WHY IT MATTERSIS-IS can now carry everything a router needs to advertise SRv6 support and its SIDs. The RFC also updates RFC 7370 by modifying an existing IANA registry.
§1Why IS-IS Needs SRv6 Extensions
With Segment Routing (SR), a node steers a packet through an ordered list of instructions called segments. Segments are identified by Segment Identifiers (SIDs). SR can run directly on the IPv6 data plane using the Segment Routing Header, and that combination is called SRv6.
This document specifies the IS-IS extensions that let the protocol encode SIDs and the behaviors bound to them. The RFC assumes the reader knows the SRv6 network programming model from RFC 8986. In short, it adds a Locator TLV, a Capabilities sub-TLV, sub-TLVs for maximum SID depths, and three sub-TLVs that say which SIDs a node has and what Endpoint behavior each one has.
Routers already use IS-IS to share maps. This RFC teaches IS-IS a few new words so routers can also share the list of stops a trip may use.
§2Capabilities, Algorithms and Depth Limits
A node says it can act as an SR Segment Endpoint by advertising a new SRv6 Capabilities sub-TLV (type 25) inside the Router Capability TLV. Its only defined flag is the O-flag: when set, the router supports the O-bit in the Segment Routing Header. All other bits are reserved, MUST be zero when sent and MUST be ignored when received.
Supported algorithms are advertised with the Segment Routing Algorithm sub-TLV from RFC 8667.
The RFC also defines four Maximum SID Depth (MSD) types, building on RFC 8491. They tell other routers how many SIDs a node can handle. If no value is advertised for any of them, the router is assumed to lack the ability (for Max Segments Left the supported value is 0).
An incorrect MSD advertisement may have negative consequences, because other routers may build paths the node cannot process.
Each router raises a flag that says I understand these trips, and also says how long a list of stops it can handle at once.
- 41SRH Max Segments Left — Maximum Segments Left value in a received SRH before applying the Endpoint behavior.
- 42SRH Max End Pop — Maximum number of SIDs in the SRH for which PSP or USP can be applied.
- 44SRH Max H.encaps — Maximum number of SIDs that H.Encaps can add to the segment list.
- 45SRH Max End D — Maximum number of SIDs in an SRH when decapsulating.
§5Locators, SIDs and Reachability
An SRv6 SID is 128 bits made of locator, function and argument parts. A node has a locator for each topology/algorithm pair it supports, and the locator is a covering prefix for all SIDs of that pair on the node. Locators MUST be advertised in the new SRv6 Locator TLV (type 27). Receivers MUST install forwarding entries for them when they support the topology/algorithm, using the same process as Prefix Reachability TLVs 236 and 237.
Locators for algorithms 0 and 1 SHOULD also be advertised in TLV 236 or 237 so that legacy routers install a forwarding entry. If the same prefix shows up in both, the Prefix Reachability advertisement MUST be preferred. Flexible Algorithm locators SHOULD NOT be advertised in those TLVs, as that would make them follow the algorithm 0 path.
SIDs learned from other nodes are not directly routable and MUST NOT be installed in the forwarding plane. They are reachable only through a covering locator, which must be the longest match prefix for them.
Section 6 adds an anycast flag (bit 4, the A-flag) to the Prefix Attribute Flags sub-TLV. All nodes advertising the same anycast locator MUST instantiate the exact same set of SIDs under it, or traffic may be dropped or misrouted. The A-flag and the N-flag MUST NOT both be set.
A locator is like a street name that covers every house on it. Routers learn the street first, and only then can they find a particular house number (a SID).
§7The Locator TLV and SID Sub-TLVs
The SRv6 Locator TLV (type 27) holds a Multi-Topology Identifier and one or more locator entries. Each entry has a 4-octet metric, a flags octet (with the D up/down bit), a 1-octet algorithm, a 1-octet Loc-Size (1 to 128 bits; the whole TLV MUST be ignored if it is outside this range), the locator itself in 1 to 16 octets, and optional sub-TLVs. The Prefix Attribute Flags sub-TLV SHOULD be included, and MUST be included when the entry is leaked upwards or redistributed.
The SRv6 End SID sub-TLV (type 5) lives inside the Locator TLV and advertises SIDs whose behavior needs no particular neighbor. It carries a flags octet (none defined), a 2-octet Endpoint Behavior, the 16-octet SID and optional sub-sub-TLVs. The SID MUST be allocated from its locator, or it is ignored.
SIDs tied to a neighbor use the SRv6 End.X SID sub-TLV (type 43, point-to-point) and the SRv6 LAN End.X SID sub-TLV (type 44, which adds the neighbor System-ID). Their flags are B (eligible for protection), S (refers to a set of adjacencies) and P (persistently allocated), plus a 1-octet weight for load balancing. Such SIDs MUST be a subnet of a locator with matching topology and algorithm from the same node.
The optional SID Structure sub-sub-TLV (type 1) gives the lengths in bits of the locator block, locator node, function and arguments. The four lengths MUST add up to at most 128 bits, and it MUST NOT be used at a transit node for forwarding.
These are the forms routers fill in. One form lists a street and the houses on it. Other forms list doors that lead to one particular neighbor.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | 5 | Sub-TLV type. |
LENGTH | Length | 1 byte | Variable length of the sub-TLV. | |
FLAGS | Flags | 1 byte | No flags defined; all bits reserved. | |
BEHAVIOR | Endpoint Behavior | 2 bytes | Behavior code from the RFC 8986 registry. | |
SID | SID | 16 bytes | The advertised SRv6 SID (16 octets). | |
SUB-LEN | Sub-sub-TLV-len | 1 byte | Octets used by the optional sub-sub-TLVs. |
§10Which Behaviors Go Where
Endpoint behaviors and their codepoints come from the registry in RFC 8986. This RFC says which sub-TLV each may appear in. A behavior MUST only be advertised in the TLVs marked Y and MUST NOT be advertised in those marked N. Not every behavior from RFC 8986 is covered; End.T, for example, is not.
Behaviors that need no specific neighbor, such as End and the End.DT forms, go in the End SID sub-TLV. Behaviors that forward to a neighbor, such as End.X, End.DX6 and End.DX4, go in the End.X and LAN End.X sub-TLVs. A receiver ignores behavior values it does not support or recognize.
Some jobs are done at the router itself, and some jobs hand the packet to a neighbor. The RFC says which form to use for which kind of job.
| Behavior | Codepoint | End SID | End.X SID | LAN End.X SID |
|---|---|---|---|---|
| End (PSP, USP, USD) | 1-4, 28-31 | Y | N | N |
| End.X (PSP, USP, USD) | 5-8, 32-35 | N | Y | Y |
| End.DX6 | 16 | N | Y | Y |
| End.DX4 | 17 | N | Y | Y |
| End.DT6 | 18 | Y | N | N |
| End.DT4 | 19 | Y | N | N |
| End.DT46 | 20 | Y | N | N |
§11IANA Registrations
The RFC registers the SRv6 Locator TLV (27) in the IS-IS top-level TLV registry, the SRv6 End SID sub-TLV (5), the SRv6 Capabilities sub-TLV (25), the End.X and LAN End.X sub-TLVs (43 and 44), and the MSD types 41, 42, 44 and 45. It also adds the A-flag as bit 4 of the Prefix Attribute Flags registry.
New registries are created for sub-sub-TLVs of the Capabilities sub-TLV, sub-sub-TLVs of the SRv6 SID sub-TLVs, and flags of the Capabilities sub-TLV, the Locator TLV, the End SID sub-TLV and the adjacency SID sub-TLVs. Registration in them is by Expert Review. The existing registry from RFC 7370 for sub-TLVs of prefix reachability TLVs is updated with a column for the Locator TLV, which is why this RFC updates RFC 7370.
Every new word needs an official number in a shared dictionary, so two vendors never use the same number for different things.
§12Security Considerations
Security concerns for IS-IS are covered in ISO 10589, RFC 5304 and RFC 5310. IS-IS usually runs inside a single administrative domain, but where attackers may reach the routing domain, the stronger authentication mechanisms in those documents SHOULD be used.
The security considerations of SR (RFC 8402) and of SRv6 network programming (RFC 8986) apply too. The RFC also points to RFC 8491 for the harm an incorrect MSD value can cause, to RFC 9259 for the O-flag, and to RFC 9350 for Flexible Algorithm.
If a bad actor could whisper false directions to the routers, trips would go wrong. So routers should check who is talking to them.
§8References
[RFC8402]RFC 8402Filsfils, C., Ed., et al., "Segment Routing Architecture", RFC 8402, July 2018.July 2018[RFC8754]RFC 8754Filsfils, C., Ed., et al., "IPv6 Segment Routing Header (SRH)", RFC 8754, March 2020.March 2020[RFC8986]RFC 8986Filsfils, C., Ed., et al., "Segment Routing over IPv6 (SRv6) Network Programming", RFC 8986, February 2021.February 2021[RFC7370]RFC 7370Ginsberg, L., "Updates to the IS-IS TLV Codepoints Registry", RFC 7370, September 2014.September 2014[RFC8491]RFC 8491Tantsura, J., et al., "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, November 2018.November 2018[RFC9350]RFC 9350Psenak, P., Ed., et al., "IGP Flexible Algorithm", RFC 9350, February 2023.February 2023