IGP Encoding for SRv6 Mirror SID in Egress Protection

Internet-Draft	Mirror SID IGP Encoding	July 2026
He, et al.	Expires 20 January 2027	[Page]

Abstract

This document specifies the IGP protocol extensions required to support SRv6 path egress protection using the Mirror SID (End.M) mechanism. It reuses the existing SRv6 End SID sub-TLV defined in RFC 9352 (IS-IS, Section 7.2) and RFC 9513 (OSPFv3, Section 8) with the End.M endpoint behavior (74) to advertise the Mirror SID and the set of protected locators, and defines a new Protected Locators sub-(sub-)TLV. enabling a backup egress node (protector) to signal its capability to protect a primary egress node within a single link-state IGP area.¶

This document is a companion to [I-D.ietf-rtgwg-srv6-egress-protection], which specifies the overall SRv6 path egress protection mechanism and the End.M behavior. The IGP encoding defined herein provides the signaling substrate for that mechanism.¶

1. Introduction

[I-D.ietf-rtgwg-srv6-egress-protection] specifies a mechanism for fast protecting the egress node and link of a Segment Routing for IPv6 (SRv6) path. The mechanism introduces a Mirror SID (End.M) behavior and requires the backup egress node (protector) to advertise the tuple <PEB, PEA, Mirror SID> together with the protected locators through the IGP. This enables the Point of Local Repair (PLR) to pre-compute backup paths toward the protector for use upon failure of the primary egress.¶

This document provides the IGP protocol encoding for that mechanism. It reuses the existing SRv6 End SID sub-TLV (RFC 9352 Section 7.2 for IS-IS and RFC 9513 Section 8 for OSPFv3), setting the endpoint behavior to 74 (End.M), to carry the Mirror SID and protected locator information within link-state advertisements, and defines a new Protected Locators sub-(sub-)TLV carried inside that sub-TLV. The overall egress protection mechanism, including the End.M behavior definition, protection procedures, and operational guidelines, is described in [I-D.ietf-rtgwg-srv6-egress-protection].¶

The IGP extensions defined in this document operate within a single link-state IGP area/level. They carry locator-level protection information only; no per-service (e.g., per-VPN or per-Service SID) signaling is introduced in the IGP. The encoding is designed to minimize IGP flooding overhead while enabling the PLR to compute Loop-Free Alternates (LFAs) for egress protection.¶

Note: The applicability and scaling considerations for the overall mechanism are discussed in [I-D.ietf-rtgwg-srv6-egress-protection]. This document focuses solely on the encoding of the IGP extensions.¶

3. SRv6 Mirror SID Overview

This section provides a brief overview of the SRv6 Mirror SID (End.M) mechanism. For a complete description, including the protection procedures, packet walks, and operational guidelines, refer to [I-D.ietf-rtgwg-srv6-egress-protection].¶

In SRv6 path egress protection, a backup egress node PEB is provisioned to protect a primary egress node PEA. A Mirror SID (End.M, SRv6 Endpoint Behavior value 74) is configured on PEB. The Mirror SID is associated with an IPv6 FIB table that contains the forwarding entries for the services anchored on PEA. When PEA fails, the PLR (typically the upstream neighbor of PEA) re-routes the traffic to PEB by encapsulating the packet with the Mirror SID as the destination. PEB decapsulates the packet and forwards the inner packet using the FIB table identified by the Mirror SID, thus reproducing the egress behavior of the failed PEA.¶

To enable this protection, PEB must advertise through the IGP the information <PEB, PEA, Mirror SID>, together with the locators of PEA that are protected. This advertisement allows the PLR to learn that PEB protects PEA and to compute a backup path toward PEB. The following sections specify how the existing SRv6 End SID sub-TLV (RFC 9352 Section 7.2 for IS-IS and RFC 9513 Section 8 for OSPFv3) is reused with endpoint behavior 74 to carry this information, and define the Protected Locators sub-(sub-)TLV for carrying the protected locators.¶

4. IS-IS Extensions for Mirror SID

This section defines the IS-IS extensions for advertising the Mirror SID and associated protected locators.¶

4.1. IS-IS SRv6 Mirror SID sub-TLV

This document reuses the SRv6 End SID sub-TLV defined in [RFC9352] (Section 7.2, Type 5) to advertise the Mirror SID; no new sub-TLV is defined. When the endpoint behavior is 74 (End.M), the sub-TLV carries the Mirror SID in the SID field and the protected locators in the Protected Locators sub-sub-TLV defined below. The SRv6 Mirror SID inherits the topology and algorithm from the parent locator TLV. The format of the SRv6 End SID sub-TLV (annotated for Mirror SID usage) is illustrated below.¶

  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 (5)    |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Flags      |     Endpoint Behavior         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SID (16 octets)                       |
 :                                                               :
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         sub-sub-TLVs                          |
 :                                                               :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: IS-IS SRv6 End SID sub-TLV (reused for Mirror SID)

Type:: 5, 1 octet, as defined in [RFC9352] (Section 7.2). No new type is assigned.¶
Length:: 1 octet, as defined in [RFC9352].¶
Flags:: 1 octet. This field MUST be set to zero on transmit and ignored on receipt, as defined in [RFC9352].¶
SRv6 Endpoint Function:: 2 octets. It MUST contain the endpoint behavior 74 (End.M) when advertising a Mirror SID. The SRv6 End SID sub-TLV MUST be ignored if the endpoint behavior is not 74.¶
SID:: 16 octets. This field contains the SRv6 Mirror SID to be advertised. It MUST NOT be zero (0).¶

4.2. IS-IS Protected Locators sub-sub-TLV

A Protected Locators sub-sub-TLV is defined and used to carry the locators of the egress node to be protected by the SRv6 Mirror SID. When advertising a Mirror SID, the SRv6 End SID sub-TLV (RFC 9352 Section 7.2) MUST include exactly one IS-IS Protected Locators sub-sub-TLV. 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 (TBD1)  |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Locator-Size  |        Locator (variable)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                                                               :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Locator-Size  |        Locator (variable)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: IS-IS Protected Locators sub-sub-TLV

Type:: TBD1 (suggested value 1) is to be assigned by IANA.¶
Length:: 1 octet. Its value MUST NOT be less than 2. The entire IS-IS SRv6 Mirror SID sub-TLV MUST be ignored if the length is less than 2.¶
Locator-Size:: 1 octet. Number of bits in the Locator field, which MUST be from the range (1-128). The entire IS-IS SRv6 Mirror SID sub-TLV MUST be ignored if the Locator-Size is outside this range.¶
Locator:: 1-16 octets. This field encodes an SRv6 Locator of an egress node to be protected by the SRv6 Mirror SID. The 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.¶

4.3. IS-IS Advertisement Procedures

When a backup egress node PEB advertises that it protects a primary egress node PEA with a Mirror SID through an LSP, the LSP MUST contain an SRv6 Locator TLV [RFC9352] carrying an SRv6 End SID sub-TLV (Type 5) with the endpoint behavior set to 74 (End.M). This sub-TLV includes the Mirror SID in the SID field and PEA's locators in the IS-IS Protected Locators sub-sub-TLV.¶

The SRv6 End SID sub-TLV used for a Mirror SID MUST include exactly one Protected Locators sub-sub-TLV and MUST NOT carry per-service (e.g., VPN or Service-SID) enumerations. Attempting to list individual services at large scale is not suitable due to IGP flooding and convergence considerations, as discussed in [I-D.ietf-rtgwg-srv6-egress-protection].¶

5. OSPFv3 Extensions for Mirror SID

This section defines the OSPFv3 extensions for advertising the Mirror SID and associated protected locators.¶

5.1. OSPFv3 SRv6 Mirror SID sub-TLV

This document reuses the SRv6 End SID sub-TLV defined in [RFC9513] (Section 8, Type 1) to advertise the Mirror SID; no new sub-TLV is defined. When the endpoint behavior is 74 (End.M), the sub-TLV carries the Mirror SID in the SID field and the protected locators in the Protected Locators sub-TLV defined below. Note that OSPFv3 sub-TLVs use 2-octet Type and Length fields, unlike IS-IS which uses 1-octet fields. The format of the SRv6 End SID sub-TLV (annotated for Mirror SID usage) is illustrated below.¶

  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 (1)         |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Flags     |    Reserved   |        Endpoint Behavior      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         SID (16 octets)                       |
 :                                                               :
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                            sub-TLVs                           |
 :                                                               :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: OSPFv3 SRv6 End SID sub-TLV (reused for Mirror SID)

Type:: 1, 2 octets, as defined in [RFC9513] (Section 8). No new type is assigned.¶
Length:: 2 octets, as defined in [RFC9513].¶
Flags:: 1 octet. This field MUST be set to zero on transmit and ignored on receipt, as defined in [RFC9513].¶
Reserved:: 1 octet. This field MUST be set to zero on transmit and ignored on receipt, as defined in [RFC9513].¶
SRv6 Endpoint Function:: 2 octets. It MUST contain the endpoint behavior 74 (End.M) when advertising a Mirror SID. The SRv6 End SID sub-TLV MUST be ignored if the endpoint behavior is not 74.¶
SID:: 16 octets. This field contains the SRv6 Mirror SID to be advertised. It MUST NOT be zero (0).¶

5.2. OSPFv3 Protected Locators sub-TLV

A Protected Locators sub-TLV is defined and used to carry the locators of the node to be protected by the SRv6 Mirror SID. When advertising a Mirror SID, the SRv6 End SID sub-TLV (RFC 9513 Section 8) MUST include exactly one OSPFv3 Protected Locators sub-TLV. 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 (TBD2)           |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Locator-Size  |           Locator (variable)                  ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                                                               :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Locator-Size  |           Locator (variable)                  ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: OSPFv3 Protected Locators sub-TLV

Type:: TBD2 (suggested value 1) is to be assigned by IANA.¶
Length:: 2 octets. Its value MUST NOT be less than 2. The entire OSPFv3 SRv6 Mirror SID sub-TLV MUST be ignored if the Length is less than 2.¶
Locator-Size:: 1 octet. Number of bits in the Locator field, which MUST be from the range (1-128). The entire OSPFv3 SRv6 Mirror SID sub-TLV MUST be ignored if the Locator-Size is outside this range.¶
Locator:: 1-16 octets. This field encodes an SRv6 Locator of an egress node to be protected by the SRv6 Mirror SID. The 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.¶

5.3. OSPFv3 Advertisement Procedures

When a backup egress node PEB advertises that it protects a primary egress node PEA with a Mirror SID through an LSA, the LSA MUST contain an SRv6 Locator TLV [RFC9513] carrying an SRv6 End SID sub-TLV (Type 1) with the endpoint behavior set to 74 (End.M). This sub-TLV includes the Mirror SID in the SID field and PEA's locators in the OSPFv3 Protected Locators sub-TLV.¶

The SRv6 End SID sub-TLV used for a Mirror SID MUST include exactly one Protected Locators sub-TLV and MUST NOT carry per-service (e.g., VPN or Service-SID) enumerations, for the same scaling reasons discussed in [I-D.ietf-rtgwg-srv6-egress-protection].¶

5.4. Comparison of IS-IS and OSPFv3 Encodings

The IS-IS and OSPFv3 encodings for the Mirror SID reuse the SRv6 End SID sub-TLV (RFC 9352 Section 7.2 for IS-IS, Type 5; RFC 9513 Section 8 for OSPFv3, Type 1). They are semantically equivalent but differ in the following structural aspects of that sub-TLV:¶

o: Type field: 1 octet in the IS-IS SRv6 End SID sub-TLV vs. 2 octets in the OSPFv3 SRv6 End SID sub-TLV.¶
o: Length field: 1 octet in the IS-IS SRv6 End SID sub-TLV vs. 2 octets in the OSPFv3 SRv6 End SID sub-TLV.¶
o: Reserved field: 1 octet (Flags) in the IS-IS SRv6 End SID sub-TLV vs. 2 octets (Flags + Reserved) in the OSPFv3 SRv6 End SID sub-TLV.¶
o: Nested TLV naming: sub-sub-TLVs in IS-IS vs. sub-TLVs in OSPFv3, reflecting the terminology conventions of each protocol.¶

6. IANA Considerations

This document requests IANA to make the following assignments in the specified registries.¶

6.1. IS-IS Sub-Sub-TLV Registrations

This document does not define a new IS-IS sub-TLV; it reuses the SRv6 End SID sub-TLV (Type 5) defined in [RFC9352] (Section 7.2). Under the existing registry "Sub-Sub-TLVs for SRv6 End SID sub-TLV" (RFC 9352 Section 11.6), IANA is requested to assign the following new sub-sub-TLV:¶

  +==============+=========================+===============+
  |    Value     | Sub-Sub-TLV Name       | Reference     |
  +==============+=========================+===============+
  |     TBD1     | Protected Locators      | This document |
  +--------------+-------------------------+---------------+

6.2. OSPFv3 Sub-TLV Registrations

This document does not define a new OSPFv3 sub-TLV; it reuses the SRv6 End SID sub-TLV (Type 1) defined in [RFC9513] (Section 8). Under the existing sub-TLV registry of the SRv6 End SID sub-TLV (RFC 9513), IANA is requested to assign the following new sub-TLV:¶

  +==============+============================+===============+
  | Sub-TLV Type | Sub-TLV Name               | Reference     |
  +==============+============================+===============+
  |     TBD2     | Protected Locators Sub-TLV | This document |
  +--------------+----------------------------+---------------+

7. Security Considerations

The IGP extensions defined in this document are used within a single link-state IGP area/level under a single administrative domain. They reuse the SRv6 End SID sub-TLV in IS-IS and OSPFv3 to advertise the Mirror SID (End.M) and define a new Protected Locators sub-(sub-)TLV for the protected locator information.¶

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.¶

Security concerns for OSPFv3 are described in [RFC5340] and [RFC8362]. While OSPFv3 is under a single administrative domain, there can be deployments where potential attackers have access to one or more networks in the OSPFv3 routing domain. In these deployments, stronger authentication mechanisms such as those specified in [RFC4552] and [RFC7166] SHOULD be used.¶

The following additional security considerations apply specifically to the Mirror SID IGP extensions:¶

Mirror SID Authentication: Since the Mirror SID is advertised through IGP, it is essential that IGP advertisements are authenticated to prevent malicious nodes from advertising counterfeit Mirror SIDs. Implementations SHOULD support the cryptographic authentication mechanisms specified in [RFC5304] for IS-IS and [RFC4552] for OSPFv3.¶

Unauthorized Rerouting: An attacker could attempt to trigger unnecessary rerouting by advertising false protection relationships. This is mitigated by authenticating IGP advertisements and limiting protector selection to trusted nodes within the same administrative domain.¶

Traffic Interception: An attacker could attempt to become a protector to intercept traffic destined to a primary egress node. This is mitigated by authenticating IGP advertisements of Mirror SIDs and monitoring for unexpected protector advertisements.¶

Control Plane Overload: An attacker could attempt to flood the IGP with excessive protection advertisements, causing control plane overload and convergence issues. This is mitigated by:¶

o: Limiting the number of protection relationships per node.¶
o: Implementing IGP flooding rate limiting.¶
o: Following the operational guidelines in [I-D.ietf-rtgwg-srv6-egress-protection] to limit the scale of deployments.¶
o: Monitoring IGP database size and convergence times.¶

8. References

8.1. Normative References

[I-D.ietf-rtgwg-srv6-egress-protection]: He, T., Hu, Z., Chen, H., Toy, M., and C. Cao, "SRv6 Path Egress Protection", Work in Progress, Internet-Draft, draft-ietf-rtgwg-srv6-egress-protection, 2026, <https://datatracker.ietf.org/doc/draft-ietf-rtgwg-srv6-egress-protection/>.
[ISO10589]: ISO, "Intermediate System to Intermediate System Intra-Domain Routing Exchange Protocol for use in Conjunction with the Protocol for Providing the Connectionless-mode Network Service (ISO 8473)", 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>.
[RFC4552]: Gupta, M. and N. Melam, "Authentication/Confidentiality for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006, <https://www.rfc-editor.org/info/rfc4552>.
[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>.
[RFC5340]: Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, <https://www.rfc-editor.org/info/rfc5340>.
[RFC7166]: Bhatia, M., Manral, V., and A. Lindem, "Supporting Authentication Trailer for OSPFv3", RFC 7166, DOI 10.17487/RFC7166, March 2014, <https://www.rfc-editor.org/info/rfc7166>.
[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>.
[RFC8362]: Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and F. Baker, "OSPFv3 Link State Advertisement (LSA) Extensibility", RFC 8362, DOI 10.17487/RFC8362, April 2018, <https://www.rfc-editor.org/info/rfc8362>.
[RFC9352]: 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>.
[RFC9513]: Li, Z., Hu, Z., Talaulikar, K., Ed., and P. Psenak, "OSPFv3 Extensions for Segment Routing over IPv6 (SRv6)", RFC 9513, DOI 10.17487/RFC9513, December 2023, <https://www.rfc-editor.org/info/rfc9513>.