BESS Yisong Liu Internet Draft China Mobile Intended status: Standards Track C. Lin Expires: January 07, 2026 M. Chen New H3C Technologies Y. Liu ZTE July 05, 2025 No Further Fast Reroute for L3 SRv6 Service SID draft-liu-bess-srv6-service-sid-nffrr-flag-03 Abstract In some multihoming SRv6 L3VPN and EVPN scenarios, once fast reroute has taken place, a second fast reroute is undesirable and may cause looping. This document proposes a mechanism to prevent further fast reroutes by advertising No-Further-FRR behaviors for L3 SRv6 Service SIDs in BGP messages. 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). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on January 07, 2026. Copyright Notice Copyright (c) 2025 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 Liu, et al. Expires January 07, 2026 [Page 1] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 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...................................................2 1.1. Requirements Language.....................................2 2. Use Case.......................................................3 2.1. SRv6 L3VPN Multihoming....................................3 3. Solution.......................................................5 4. Extensions for L3 SRv6 Endpoint Behaviors......................7 4.1. End.DT4.Reroute : End.DT4 with Fast Reroute...............7 4.2. End.DT6.Reroute : End.DT6 with Fast Reroute...............7 4.3. End.DT46.Reroute : End.DT46 with Fast Reroute.............8 4.4. End.DX4.Reroute : End.DX4 with Fast Reroute...............9 4.5. End.DX6.Reroute : End.DX6 with Fast Reroute..............10 5. Backward Compatibility........................................10 6. SID Allocation Optimization Considerations....................11 7. Security Considerations.......................................11 8. IANA Considerations...........................................12 9. References....................................................12 9.1. Normative References.....................................12 Authors' Addresses...............................................12 1. Introduction [RFC9252] defines procedures and messages for SRv6-based BGP services, including Layer 3 Virtual Private Network (L3VPN), Ethernet VPN (EVPN), and Internet services. In some multihoming scenarios, two egress PEs may establish a backup path between them and use it as the protection of PE-CE link failure. Once fast reroute (FRR) has taken place, a second fast reroute is undesirable and may cause looping. This document defines the No-Further-FRR behavior for L3 SRv6 Service SIDs carried in BGP messages and proposes a mechanism using the No-Further-FRR flavor to prevent further fast reroutes. 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. liu, et al. Expires January 07, 2026 [Page 2] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 2. Use Case 2.1. SRv6 L3VPN Multihoming In the multihoming SRv6 L3VPN scenarios, two egress PEs may establish a backup path between them and use it as the protection of PE-CE link failure. Take the network in Figure 1 as an example. When traffic goes from CE1 to CE2, it may be load-balanced between PE2 and PE3 or only forwarded to the main egress PE. If the link PE2-CE2 fails, PE2 can still forward the traffic for CE2 by sending it over the backup path to PE3 (and similarly for PE3 if link2 fails). +-----+ | CE1 | +-----+ | | +-----+ ------------------- | PE1 |*************** ^ +-----+ * | / \ * | / \ * | P1 P2 * | . . +------+ SRv6 VPN . *************.*******|BGP-RR| | . * . +------+ | P3 * P4 * | | * | * | | * | * v +-----+ Backup +-----+ * --------- | PE2 |#############| PE3 |***** +-----+ Path +-----+ \ / \ / +-----+ | CE2 | +-----+ Figure 1 Examples of BGP routes advertised by PE2 and PE3 are as following: liu, et al. Expires January 07, 2026 [Page 3] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 BGP Route by PE2: VPN Prefix of CE2: BGP Prefix SID Attr: SRv6 L3 Service TLV: SRv6 SID Information sub-TLV: SID: SID-2 Behavior: End.DT46 BGP Route by PE3: VPN Prefix of CE2: BGP Prefix SID Attr: SRv6 L3 Service TLV: SRv6 SID Information sub-TLV: SID: SID-3 Behavior: End.DT46 Examples of FIB entries for L3VPN service SID on PE2 and PE3 are as following: FIB on PE2: SID-2: Primary Next-hop: CE2 Backup Next-hop: Service SRv6 SID-3 FIB on PE3: SID-3: Primary Next-hop: CE2 Backup Next-hop: Service SRv6 SID-2 However, suppose CE2 is down. PE2 will think PE2-CE2 link is down and send traffic to PE3 over the backup path. PE3 will also think PE3-CE3 link is down and send the traffic back to PE2 over the backup path. So, traffic will loop between PE2 and PE3 until BGP convergence. The traffic forwarding when CE2 fails is as following: liu, et al. Expires January 07, 2026 [Page 4] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 +======+=============+=======+==============+ | Node | Packet | Next | Comment | +======+=============+=======+==============+ | PE1 | pkt | PE2 | | +------+-------------+-------+--------------+ | PE2 | pkt | CE2 | PE2-CE2 down | +------+-------------+-------+--------------+ | PE2 | pkt | PE3 | FRR | +------+-------------+-------+--------------+ | PE3 | pkt | CE2 | PE3-CE2 down | +------+-------------+-------+--------------+ | PE3 | pkt | PE2 | FRR | +------+-------------+-------+--------------+ | PE2 | -- | CE2 | PE2-CE2 down | +------+-------------+-------+--------------+ | PE2 | pkt | PE3 | FRR | +------+-------------+-------+--------------+ | ... | | | Loop! | +------+-------------+-------+--------------+ 3. Solution Each egress PE advertises an additional SRv6 Service SID in BGP routes which is called No-Further-FRR SID. The owner of No-Further-FRR SID will not provide local FRR for it. When the next-hop of No-Further-FRR SID is down, like PE-CE link failure or CE node failure, the PE will drop packets rather than apply FRR. The No-Further-FRR SID can used by other PE as the protection of local PE-CE link failure, without worrying about the looping problem. To support backwards compatibility and BGP RR deployment, both the normal SRv6 Service SID and the No-Further-FRR SID MAY be advertised together. A No-Further-FRR behavior is used to indicate the No- Further-FRR SID. Detailed BGP extensions will be described in Section 4. Still taking the network in Figure 1 as an example, the BGP routes advertised by PE2 and PE3 are as following: liu, et al. Expires January 07, 2026 [Page 5] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 BGP Route by PE2: VPN Prefix of CE2: BGP Prefix SID Attr: SRv6 L3 Service TLV: SRv6 SID Information sub-TLV: SID: SID-21 Behavior: End.DT46 SRv6 SID Information sub-TLV: SID: SID-22 Behavior: End.DT46.Reroute BGP Route by PE3: VPN Prefix of CE2: BGP Prefix SID Attr: SRv6 L3 Service TLV: SRv6 SID Information sub-TLV: SID: SID-31 Behavior: End.DT46 SRv6 SID Information sub-TLV: SID: SID-32 Behavior: End.DT46.Reroute The FIB entries for L3VPN service SID on PE2 and PE3 are as following: FIB on PE2: SID-21,Behavior: End.DT46: Primary Next-hop: CE2 Backup Next-hop: Service SRv6 SID-32 SID-22 (No-Further-FRR),Behavior: End.DT46.Reroute: Only Forwarding on Primary Next-hop: CE2 FIB on PE3: SID-31, Behavior: End.DT46: Primary Next-hop: CE2 Backup Next-hop: Service SRv6 SID-22 SID-32 (No-Further-FRR), Behavior: End.DT46.Reroute: Only Forwarding on Primary Next-hop: CE2 After adopting the proposed solution, if CE fails, PE2 will think PE2-CE2 link is down and send traffic to PE3 by using the No- Further-FRR SID-32. PE3 will also think PE3-CE3 link is down, but PE3 will drop the packets rather than apply FRR. The traffic forwarding when CE2 fails is as following: liu, et al. Expires January 07, 2026 [Page 6] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 +======+==============+=======+==============+ | Node | Packet | Next | Comment | +======+==============+=======+==============+ | PE1 | pkt | PE2 | | +------+--------------+-------+--------------+ | PE2 | pkt | CE2 | PE2-CE2 down | +------+--------------+-------+--------------+ | PE2 | pkt | PE3 | FRR | +------+--------------+-------+--------------+ | PE3 | pkt | CE2 | PE3-CE2 down | +------+--------------+-------+--------------+ | PE3 | - | - | Drop | +------+--------------+-------+--------------+ 4. Extensions for L3 SRv6 Endpoint Behaviors 4.1. End.DT4.Reroute : End.DT4 with Fast Reroute The "End.DT4 with Fast Reroute" behavior ("End.DT4.Reroute" for short) is a variant of the End.DT4 behavior. The End.DT4.Reroute behavior is defined for the fast-reroute application between two multi-homing peers, and extends the base End.DT4 behavior. When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.DT4 SID, N does the following: S01. If (Upper-Layer header type == 4(IPv4) ) { S02. Remove the outer IPv6 header with all its extension headers S03. Set the packet's associated FIB table to T S04. Submit the packet to the egress IPv4 FIB lookup for transmission to the new destination S05. if (The forwarding path for the new destination is the backup path generated by Fast Reroute) { S06. Drop the packet S07. } Else { S08. Transmission to the new destination S09. } S10. } Else { S11. Process as per Section 4.1.1 S12. } 4.2. End.DT6.Reroute : End.DT6 with Fast Reroute The "End.DT6 with Fast Reroute" behavior ("End.DT6.Reroute" for short) is a variant of the End.DT6 behavior. liu, et al. Expires January 07, 2026 [Page 7] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 The End.DT6.Reroute behavior is defined for the fast-reroute application between two multi-homing peers, and extends the base End.DT6 behavior. When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.DT6 SID, N does the following: S01. If (Upper-Layer header type == 41(IPv6) ) { S02. Remove the outer IPv6 header with all its extension headers S03. Set the packet's associated FIB table to T S04. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination S05. if (The forwarding path for the new destination is the backup path generated by Fast Reroute) { S06. Drop the packet S07. } Else { S08. Transmission to the new destination S09. } S10. } Else { S11. Process as per Section 4.1.1 S12. } 4.3. End.DT46.Reroute : End.DT46 with Fast Reroute The "End.DT46 with Fast Reroute" behavior ("End.DT46.Reroute" for short) is a variant of the End.DT46 behavior. The End.DT46.Reroute behavior is defined for the fast-reroute application between two multi-homing peers, and extends the base End.DT46 behavior. When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.DT46 SID, N does the following: liu, et al. Expires January 07, 2026 [Page 8] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 S01. If (Upper-Layer header type == 4(IPv4) ) { S02. Remove the outer IPv header with all its extension headers S03. Set the packet's associated FIB table to T4 S04. Submit the packet to the egress IPv4 FIB lookup for transmission to the new destination S05. if (The forwarding path for the new destination is the backup path generated by Fast Reroute) { S06. Drop the packet S07. } Else { S08. Transmission to the new destination S09. } S10. } Else If (Upper-Layer header type == 41(IPv6) ) { S11. Remove the outer IPv6 header with all its extension headers S12. Set the packet's associated FIB table to T6 S13. Submit the packet to the egress IPv6 FIB lookup for transmission to the new destination S14. if (The forwarding path for the new destination is the backup path generated by Fast Reroute) { S15. Drop the packet S16. } Else { S17. Transmission to the new destination S18. } S19. } Else { S20. Process as per Section 4.1.1 S21. } 4.4. End.DX4.Reroute : End.DX4 with Fast Reroute The "End.DX4 with Fast Reroute" behavior ("End.DX4.Reroute" for short) is a variant of the End.DX4 behavior. The End.DX4.Reroute behavior is defined for the fast-reroute application between two multi-homing peers, and extends the base End.DX4 behavior. When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.DX4 SID, N does the following: liu, et al. Expires January 07, 2026 [Page 9] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 S01. If (Upper-Layer header type == 4(IPv4) ) { S02. Remove the outer IPv6 header with all its extension headers S03. If (L3 adjacency J is the backup path generated by Fast Reroute) { S04. Drop the packet S05. } Else { S06. Forward the exposed IPv4 packet to the L3 adjacency J S07. } S08. } Else { S09. Process as per Section 4.1.1 S10. } 4.5. End.DX6.Reroute : End.DX6 with Fast Reroute The "End.DT6 with Fast Reroute" behavior ("End.DX6.Reroute" for short) is a variant of the End.DX6 behavior. The End.DX6.Reroute behavior is defined for the fast-reroute application between two multi-homing peers, and extends the base End.DX6 behavior. When processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an End.DX6 SID, N does the following: S01. If (Upper-Layer header type == 41(IPv6) ) { S02. Remove the outer IPv6 header with all its extension headers S03. If (L3 adjacency J is the backup path generated by Fast Reroute) { S04. Drop the packet S05. } Else { S06. Forward the exposed IPv6 packet to the L3 adjacency J S07. } S08. } Else { S09. Process as per Section 4.1.1 S10. } 5. Backward Compatibility To maintain backwards-compatibility, both End.DT4.Reroute and End.DT4 Behavior SIDs MAY be advertised together. Receiving PEs SHOULD use the SRv6 SID from the first instance of the Sub-TLV only (Section 3.1 of [RFC9252]), and ignore the SRv6 SID of unknown behavior End.DT4.Reroute (Section 3.2.1 of [RFC9252]). The same compatibility handling applies to other behaviors such as End.DT6.Reroute and End.DT6, End.DT46.Reroute and End.DT46, End.DX4.Reroute and End.DX4, and End.DX6.Reroute and End.DX6. liu, et al. Expires January 07, 2026 [Page 10] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 6. SID Allocation Optimization Considerations To reduce the allocation of SRv6 Service SIDs, only one SRv6 Service SID (with Behaviors End.DT4, End.DT6, End.DT46, End.DX4, End.DX6) can be allocated. Then, by setting the optional Fast Reroute argument "Arg.FR2," the SID can be distinguished as either for Fast Reroute or not. Specifically, "Arg.FR2" differentiates between Behaviors with Fast Reroute (End.DT4.Reroute, End.DT6.Reroute, End.DT46.Reroute, End.DX4.Reroute, End.DX6.Reroute) and those without Fast Reroute (End.DT4, End.DT6, End.DT46, End.DX4, End.DX6). For example, using End.DT4, the implementation process is as follows: The SRv6 L3 Service TLV in this case will carry two SRv6 SID Information sub-TLVs: * the first one with the base End.DT4 behavior and * the second one with the End.DT4.Reroute behavior variant. The second one will have a non-zero Arg length (AL) and convey Arg.FR2 embedded in the advertised SID. Following is an example representation of the BGP Prefix-SID Attribute encoding in this case for a 16-bit argument Arg.FR2 (0x0001): BGP Prefix SID Attr: SRv6 L3 Service TLV: SRv6 SID Information sub-TLV: SID: 2001:123:a:1:1234:: Behavior: End.DT4 SRv6 SID Structure sub-sub-TLV: LBL: 48, LNL: 16, FL: 16, AL: 0, TPOS-L: 0, TPOS-O: 0 SRv6 SID Information sub-TLV: SID: 2001:123:a:1:1234:0001:: Behavior: End.DT4.Reroute SRv6 SID Structure sub-sub-TLV: LBL: 48, LNL: 16, FL: 16, AL: 16, TPOS-L: 0, TPOS-O: 0 The processing of other types End.DT6, End.DT46, End.DX4, and End.DX6 is similar to that of DT4. 7. Security Considerations TBD. liu, et al. Expires January 07, 2026 [Page 11] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 8. IANA Considerations This document introduces two new Endpoint behaviors. This document requests IANA assign a two new values and update the "SRv6 Endpoint Behaviors" subregistry under the top-level "Segment Routing" registry as follows: +-------+-----+-------------------+---------------+ | Value | Hex | Endpoint Behavior | Reference | +-------+-----+-------------------+---------------+ | TBD | TBD | End.DT4.Reroute | This document | +-------+-----+-------------------+---------------+ | TBD | TBD | End.DT6.Reroute | This document | +-------+-----+-------------------+---------------+ | TBD | TBD | End.DT46.Reroute | This document | +-------+-----+-------------------+---------------+ | TBD | TBD | End.DX4.Reroute | This document | +-------+-----+-------------------+---------------+ | TBD | TBD | End.DX6.Reroute | This document | +-------+-----+-------------------+---------------+ Table 1: SRv6 Endpoint Behaviors Subregistry 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, May 2017 [RFC9252] Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene, B., Zhuang, S., and J. Rabadan, "BGP Overlay Services Based on Segment Routing over IPv6 (SRv6)", RFC 9252, DOI 10.17487/RFC9252, July 2022, . Authors' Addresses Yisong Liu China Mobile China Email: liuyisong@chinamobile.com liu, et al. Expires January 07, 2026 [Page 12] Internet-Draft SRv6 Service SID No-Further-FRR Flag July 2025 Changwang Lin New H3C Technologies China Email: linchangwang.04414@h3c.com Mengxiao Chen New H3C Technologies China Email: chen.mengxiao@h3c.com Yao Liu ZTE China Email: liu.yao71@zte.com.cn liu, et al. Expires January 07, 2026 [Page 13]