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

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.¶

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.¶

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.¶

 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) . . .         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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) . . .            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.¶

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.¶

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]).¶

Contributors

The following people gave a substantial contribution to the content of this document and should be considered coauthors:¶

Authors' Addresses