rtgwg Working Group W. Cheng Internet-Draft W.Jiang Intended status: Experimental China Mobile Expires:January 26, 2027 C. Lin New H3C Technologies Z. Hu Huawei Technologies Y. Qiu New H3C Technologies July 23, 2026 SRv6 Egress Protection in Multi-homed scenario draft-cheng-rtgwg-srv6-multihome-egress-protection-12 Abstract This document describes a SRv6 egress node protection mechanism in multi-homed scenarios. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on January 26 2027. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. Cheng, et al. Expires January, 2027 [Page 1] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction...................................................2 2. Terminology....................................................3 3. Multi-homed SRv6 Egress Protection Mechanism...................3 3.1. B-flag in Segment Routing Header..........................4 3.2. Procedure on the Ingress Endpoint.........................4 3.3. Procedure on the Egress Endpoint..........................6 3.4. Procedure on the Penultimate Endpoint.....................7 4. Multi-homed SRv6 Egress Protection Example.....................8 5. Considerations for SRv6 Segment List Compression .......................9 6. Security Considerations.......................................10 7. IANA Considerations...........................................10 8. References....................................................11 8.1. Normative References.....................................11 8.2. Informative References...................................12 9. Acknowledgments...............................................13 Authors' Addresses...............................................13 1. Introduction The fast protection of a transit node of a Segment Routing (SR) path or tunnel is described in [I-D.ietf-rtgwg-segment-routing-ti-lfa] and [I-D.hu-spring-segment-routing-proxy-forwarding]. [RFC8400] specifies the fast protection of egress node(s) of an MPLS TE LSP tunnel including P2P TE LSP tunnel and P2MP TE LSP tunnel in details. However, these documents do not discuss the fast protection of the egress node of a Segment Routing for IPv6 (SRv6) path or tunnel. [I-D.ietf-rtgwg-srv6-egress-protection] proposes mirror protection mechanism and presents protocol extensions for the fast protection of the egress node of a SRv6 path or tunnel. However, the mechanism provided in this document is relatively complex. It is necessary to configure the Mirror SID for the protected egress node on the backup Cheng, et al. Expires January, 2027 [Page 2] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 egress node. The mirror relationship needs to be distributed through IGP and BGP protocols to automatically create mapping entries. This document introduces a simplified protection mechanism of the egress node of a SRv6 path. Only expanding the data plane can perform fast path switching in case of egress node failure. 2. Terminology The following terminologies are used in this document. SR: Segment Routing SRv6: SR for IPv6 SRH: Segment Routing Header SID: Segment Identifier CE: Customer Edge PE: Provider Edge VPN: Virtual Private Network PSD: Penultimate Segment Decapsulation 3. Multi-homed SRv6 Egress Protection Mechanism This section describes the mechanism of SRv6 path egress protection in multi-homed scenarios and the extension of SRH extension header. Figure 1 is used to explain the multi-homed egress node protection mechanism. Cheng, et al. Expires January, 2027 [Page 3] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 Locator: A3:1::/64 VPN SID: A3:1::B100 +---+ *** +---+ *** +---+ *** +---+ +---+ |PE1|-----| P1|-----| P2|-----|PE3|---|CE2| +---+ +---+ +---+ +---+ +---+ / | | | | \ / +---+/ | | | | \ / |CE1| | | | | X +---+\ | | | | / \ \ | | | | / \ +---+ +---+ +---+ +---+ +---+ |PE2|-----| P3|-----| P4|-----|PE4|---|CE3| +---+ +---+ +---+ +---+ +---+ Locator: A4:1::/64 VPN SID: A4:1::B100 PE3 Egress PE4 Backup Egress CEx Customer Edge Px Non-Provider Edge *** SR Path Figure 1 3.1. B-flag in Segment Routing Header [RFC8754] describes the Segment Routing Header (SRH) and how SR capable nodes use it. The SRH contains an 8-bit "Flags" field. This document defines the following bit in the SRH Flags field to carry the B-flag(TBD): Where: - B-flag(TBD): The marking bit of carrying backup SID in segment list. If the B-flag is set to 1, a backup SID is carried into the position of Segment List[0] and a primary SID is carried into the position of Segment List[1]. 3.2. Procedure on the Ingress Endpoint In the CE multi-homed or double-homed scenario, the ingress node learns the multi-homed or double-homed route of CE through the routing protocol, and then determines the optimal path and suboptimal path according to the routing optimization strategy. The egress node on the optimal path acts as the primary egress, and the SID of the primary egress node is used as the primary SID. The egress node on the suboptimal path acts as the backup egress, and the SID of the backup egress node is used as the backup SID. Cheng, et al. Expires January, 2027 [Page 4] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 On the path forwarded based on SRv6 TE policy, when the ingress node encapsulates the SRH extension header, judge whether the primary VPN SID of the egress node (PE1) has a backup SID. If yes, insert the backup SID into the position of Segment List[0], and set B-flag to 1 to identify that the backup SID has been carried in the first location of the segment list. The format of SRH extension header filling is shown in the following Figure 2. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len | Routing Type | SL = n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Last Entry=n |Flags(B-flag=1)| Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Backup SID (Segment List[0],128 bits IPv6 value) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Primary SID (Segment List[1], 128 bits IPv6 address) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // ... // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Segment List[n] (128 bits IPv6 address) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Optional Type Length Value objects (variable) // Cheng, et al. Expires January, 2027 [Page 5] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2 3.3. Procedure on the Egress Endpoint Normally, the traffic is forwarded along the path P1->P2->PE3->CE2. When the primary egress node PE3 receives a packet whose IPv6 DA is a local SID, PE3 removes the outer packet header with all its extension headers and submits the packet to the egress FIB lookup for transmission to the new destination. However, because Segment List[0] carries the backup SID, the SID of the primary egress node PE3 is encapsulated in the position of the penultimate SID. Therefore, according to [RFC8986], PE3 will not perform SRv6 decapsulation. In order to indicate the primary egress node with SID located in the penultimate position of the SRH Segment List array to decapsulate, this document define PSD(Penultimate Segment Decapsulation) Flavor. SR Segment Endpoint nodes receive the IPv6 packet with the Destination Address field of the IPv6 header equal to its SID address. When a node (PE3 in Figure 1) receives a packet whose IPv6 DA is a SID with PSD Flavor located in the penultimate position of the SRH Segment List array and that SID is a local SID, it indicates to remove the outer encapsulation of the packet, and forward the packet according to the exposed packet. PSD Flavor can apply to End.DT4, End.DT6, End.DT46, End.DX4, End.DX6, End.DX2, End.DX2V and End.DX2M. The SIDs can be advertised via routing protocol. The SRH processing of the End.DT4, End.DT6, End.DT46, End.DX4, End.DX6, End.DX2, End.DX2V and End.DX2M behaviors defined in [RFC8986] are modified; the instructions S02 are substituted by the following one: S02. If ((Segments Left != 0) && (Segments Left != 1)) { Due to the above pseudocode modification, the PSD operation only takes place at the egress node and does not happen at any transit node. When a SID of PSD flavor is processed at a transit node, the PSD behavior is not performed since Segments Left would not be 1 or 0. Cheng, et al. Expires January, 2027 [Page 6] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 Normally, the traffic is forwarded along the path P1->P2->PE3->CE2. When PE3 receives a packet whose IPv6 DA is S and S is a local PSD- flavored SID, PE3 removes the outer packet header with all its extension headers and submits the packet to the egress FIB lookup for transmission to the new destination. 3.4. Procedure on the Penultimate Endpoint After receiving the packet, if any of the following cases is met, the penultimate endpoint acting as the repair node can provide fast protection against the failure of the egress node by IGP fast convergence or BFD detection. Case 1: When the destination address of the received packet is the local END.X SID, the outgoing interface is down. Update the IPv6 DA with SRH[SL] and the SL is reduced. Case 2: When the destination address of the received packet is the local END SID, the FIB is looked up for the updated DA with Segment List[SL] and the SL is reduced. IF the primary outbound interface used to forward the packet failed or there is no FIB entry for forwarding the packet, the detailed processing to be performed by the penultimate node is as follows: IF B-flag = 1 THEN IF SL = 1 THEN SL decreases by 1 and becomes 0; Update the IPv6 DA with Segment List[0]; FIB lookup on the updated DA; Forward the packet according to the matched entry; When primary egress node (PE3) fails, the Penultimate Endpoint (P2) finds out that the PE3's SID is unreachable. For example, P2 quickly detects that the route to PE3 is unreachable through IGP convergence or BFD detection. Then, P2 sequentially looks up the Segment List after the current unreachable SID and finds the first reachable downstream nodes. The specific operations are as follows: For the packet with B-flag 1 and SL = 1, P2 decreases SL by 1 and then modifies the destination address of the packet to Segment List[SL]. Because Segment List[0] is the backup SID, if the backup egress node (PE4) is reachable, P2 sends the modified packet to PE4. In this way, P2 provides fast protection for the egress failure and greatly shortens the cut-off time. When the packet arrives at PE4, PE4 removes the outer IPv6 header, and forwards the original packet. Cheng, et al. Expires January, 2027 [Page 7] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 After the route convergence is completed, the ingress node (PE1) will reselect the forwarding path for the traffic to VPN, and switch the path (P1->P3->P4->PE4->CE2) to the CE to the egress node (PE4). After that, P2 no longer needs to forward the packet with the destination address of PE3's PSD-flavored SID. About penultimate node, it could be several hops away to egress node, according to [RFC8986], the transit node cannot process SRH. If there are transit node(s) between the penultimate endpoint node and the primary egress node, the multi-homed SRv6 egress protection mechanism does not take effect on the transit node. The multi-homed egress protection is only performed at the penultimate endpoint node. 4. Multi-homed SRv6 Egress Protection Example Figure 3 shows an example of protecting egress PE3 of a SRv6 TE path, which is from ingress PE1 to egress PE3. A0:1::1 A1:1::1 A2:1::A100 A3:1::B100 +---+ *** +---+ **** +---+ **** +---+ +---+ |PE1|-----| P1|------| P2|------|PE3|---|CE2| +---+ +---+ +---+ +---+ +---+ / | | |& | \ / +---+/ | | |& | \ / |CE1| | | |& | X +---+\ | | |& | / \ \ | | |& | / \ +---+ +---+ +---+ &&&& +---+ +---+ |PE2|-----| P3|------| P4|------|PE4|---|CE3| +---+ +---+ +---+ +---+ +---+ A4:1::B100 PE3 Egress PE4 Backup Egress CEx Customer Edge Px Non-Provider Edge *** SR Path &&& backup Path Figure 3 In this document, a SID list is represented as where S1 is the first SID to visit, S2 is the second SID to visit and S3 is the last SID to visit along the SRv6 path. In Figure 3, Both CE2 and CE3 are dual homed to PE3 and PE4. PE1 has a locator A0:1::/64. P1 has a locator A1:1::/64. P2 has a locator A2:1::/64 and END.X SID A2:1::A100. PE3 has a locator A3:1::/64 and a VPN SID A3:1::B100 with PSD-flavored behavior. PE4 has a locator Cheng, et al. Expires January, 2027 [Page 8] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 A4:1::/64 and VPN SID A4:1::B100. The traffic from CE1 to CE2 is forwarded along the path PE1->P1->P2->PE3. After the configuration, according to the BGP route selection principle, the ingress PE node selects the preferred route as the primary node and the second-best route as the backup node. PE1 determines that PE3 is the primary egress node and PE4 is the backup egress node. PE3's backup SID is PE4's VPN SID. After multi-homed egress protection is enabled, when PE1 receives the packet from CE1 to CE2, PE1 encapsulates the packet with IPv6 header. The segment list in SRH is designed as < A1:1::1, A2:1::A100, A3:1::B100, A4:1::B100>. The SL is set to 3 and B-flag is set to 1. In normal operations, When P2 receives a packet destined to END.X SID A2:1::A100, it decreases SL by 1 and forwards the packet with source A0:1::1 and destination PE3's VPN SID A3:1::B100 from the link between P2 and PE3 according to END.X SID. The SL of the packet sent to PE3 is 1. When the packet arrives at PE3, the destination address A3:1::B100 matches PE3's PSD-flavored SID. PE3 as the penultimate endpoint performs decapsulation and forwarding processing. The specific operations of PE3 are as follows: 1) Remove the outer packet header and all its extension headers. 2) Look up the FIB table according to the destination address of the original packet. 3) Send the packet to CE2 according to the FIB entry. After PE3 fails, P2 detects PE3 failure through IGP fast convergence or BFD detection. When P2 receives the packet to be sent to PE3's VPN SID A3:1::B100, after decreasing SL P2 finds that the outgoing interface is down or the route to PE3 is unreachable. If the B-flag is 1, P2 continues to decrease SL by 1 and obtains the next reachable SID of PE4's SID from the segment list. The SL changes to 0. P2 changes the destination address of the packet with the backup SID of Segment List[0] and sends the modified packet to A4:1::B100. When PE4 receives the modified packet, it decapsulates the packet and forwards the decapsulated packet by executing END.DT6 behavior for an END.DT6 SID instance. 5. Considerations for SRv6 Segment List Compression [RFC9800] enables a compressed encoding of the SRv6 Segment List in the SRH, which can reduce the SRv6 encapsulation size. The Cheng, et al. Expires January, 2027 [Page 9] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 protection proposed in this document can also be applied to the case of SRv6 Segment-List compression, ensuring that the Primary SID and Backup SID are not compressed, while other SIDs can be compressed according to the method defined in [RFC9800]. 6. Security Considerations [RFC8754] defines the notion of an SR domain and use of SRH within the SR domain. The use of egress protection mechanism described in this document is restricted to an SR domain. For example, similar to the SID manipulation, B-flag manipulation is not considered as a threat within the SR domain. Procedures for securing an SR domain are defined the section 5.1 and section 7 of [RFC8754]. This document does not impose any additional security challenges to be considered beyond security threats described in [RFC8754], [RFC8679] and [RFC8986]. 7. IANA Considerations This document requests IANA to allocate the following codepoints for PSD flavor behaviors within the "SRv6 Endpoint Behaviors" registry in the "Segment Routing registry group. +-------+--------+----------------------------+-----------+ | Value | Hex | Endpoint behavior | Reference | +-------+--------+----------------------------+-----------+ | 140 | 0x008C | End.DX6 with PSD | [This.ID] | | 141 | 0x008D | End.DX4 with PSD | [This.ID] | | 142 | 0x008E | End.DT6 with PSD | [This.ID] | | 143 | 0x008F | End.DT4 with PSD | [This.ID] | | 144 | 0x0090 | End.DT46 with PSD | [This.ID] | | 145 | 0x0091 | End.DX2 with PSD | [This.ID] | | 146 | 0x0092 | End.DX2V with PSD | [This.ID] | | 147 | 0x0093 | End.DT2U with PSD | [This.ID] | | 148 | 0x0094 | End.DT2M with PSD | [This.ID] | +-------+--------+----------------------------+-----------+ Table 1: IETF - SRv6 Endpoint Behaviors This document requests that IANA allocate the following registration in the "Segment Routing Header Flags" sub-registry for the "Internet Protocol Version 6 (IPv6) Parameters" registry maintained by IANA: Cheng, et al. Expires January, 2027 [Page 10] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 +-------+------------------------------+---------------+ | Bit | Description | Reference | +=======+==============================+===============+ | TBD | B-flag | This document | +-------+------------------------------+---------------+ 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8400] Chen, H., Liu, A., Saad, T., Xu, F., and L. Huang, "Extensions to RSVP-TE for Label Switched Path (LSP) Egress Protection", RFC 8400, DOI 10.17487/RFC8400, June 2018, . [RFC8679] Shen, Y., Jeganathan, M., Decraene, B., Gredler, H., Michel, C., and H. Chen, "MPLS Egress Protection Framework", RFC 8679, DOI 10.17487/RFC8679, December 2019, . [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, . [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, . [RFC9800] Cheng, W., Ed., Filsfils, C., Li, Z., Decraene, B., and F. Clad, Ed., "Compressed SRv6 Segment List Encoding", RFC 9800, DOI 10.17487/RFC9800, June 2025, . Cheng, et al. Expires January, 2027 [Page 11] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 [RFC9855] Bashandy, A., Litkowski, S., Filsfils, C., Francois, P., Decraene, B., and D. Voyer, "Topology Independent Fast Reroute using Segment Routing", RFC 9855, DOI 10.17487/RFC9855, October 2025, . 8.2. Informative References [I-D.hu-spring-segment-routing-proxy-forwarding] Hu, Z., Chen, H., Yao, J., Bowers, C., Yongqing, and Yisong, "SR-TE Path Midpoint Restoration", Work in Progress, Internet-Draft, draft-hu-spring-segment-routing-proxy-forwarding-24, 21 August 2023, . [I-D.ietf-rtgwg-srv6-egress-protection] Hu, Z., Chen, H., Chen, H., Wu, P., Toy, M., Cao, C., He T., Liu, L., Liu, X., "SRv6 Path Egress Protection", Work in Progress, Internet-Draft, draft-ietf-rtgwg-srv6-egress-protection-24, 19 June 2026, Cheng, et al. Expires January, 2027 [Page 12] Internet-Draft Multi-homed SRv6 Egress Protection July 2026 9. Acknowledgments The authors would like to thank the following for their valuable contributions of this document: Yisong Liu China Mobile Authors' Addresses Weiqiang Cheng China Mobile Email: chengweiqiang@chinamobile.com Wenying Jiang China Mobile Email: jiangwenying@chinamobile.com Changwang Lin New H3C Technologies Email: linchangwang.04414@h3c.com Zhibo Hu Huawei Technologies Email: huzhibo@huawei.com Yuanxiang Qiu New H3C Technologies Email: qiuyuanxiang@h3c.com Cheng, et al. Expires January, 2027 [Page 13]