SPRING F. Yang Internet-Draft China Mobile Intended status: Informational C. Lin Expires: 20 December 2026 New H3C Technologies 18 June 2026 SID as source address in SRv6 draft-yang-spring-sid-as-source-address-11 Abstract SRv6 is being rapidly deployed and is currently primarily used in trusted-domain backbone networks. Both the carrier market and the enterprise market are adopting SRv6 for end-to-end service delivery. However, if a firewall exists along an SRv6 path, not only legitimate SRv6 traffic but also OAM response packets to head-end will be dropped. This proposal addresses this issue by using SID as source address in SRv6 packets. 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 20 December 2026. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. Yang & Lin Expires 20 December 2026 [Page 1] Internet-Draft SID as source address June 2026 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Using SRv6 SID as Source Address . . . . . . . . . . . . . . 3 2.1. User Traffic . . . . . . . . . . . . . . . . . . . . . . 4 2.2. ICMP Traffic . . . . . . . . . . . . . . . . . . . . . . 4 2.3. Control and Management Traffic . . . . . . . . . . . . . 4 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. SRv6 Network with SR-aware Stateful Firewall . . . . . . 4 3.1.1. Problem Statement . . . . . . . . . . . . . . . . . . 4 3.1.2. Solution for SRv6 Traffic Pass Thru SR-aware Stateful Firewall . . . . . . . . . . . . . . . . . . . . . . 6 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.1. Normative References . . . . . . . . . . . . . . . . . . 8 6.2. Informative References . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction SRv6 is being rapidly deployed and is currently primarily used in trusted-domain backbone networks. Both the carrier market and the enterprise market are adopting SRv6 for end-to-end service delivery. However, if a firewall exists along an SRv6 path, not only legitimate SRv6 traffic but also feedback packets to head-end will be dropped. Yang & Lin Expires 20 December 2026 [Page 2] Internet-Draft SID as source address June 2026 The reason has been elaborated in Section 8.1 of [I-D.draft-ietf-spring-srv6-security]. SRv6 implementations use the ingress PE’s loopback IPv6 address as the outer IPv6 source address for encapsulated traffic. This design leads to asymmetric bidirectional flow tuples. When stateful firewalls exist along the SRv6 forwarding path, legitimate bidirectional traffic and all upstream response packets, e.g. ICMP response, are dropped by firewalls, as they fail to match stateful session rules. Operators are forced to deploy additional multi-layer tunneling such as IPsec to bypass firewall filtering, which introduces significant header overhead and weakens the native programmability advantages of SRv6. 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. 1.2. Terminology AC: attachment circuit. PE: Provider Edge. SID: Segment Identifier, defined in [RFC8402]. SRv6: SR over IPv6, defined in [RFC8402]. VPLS: Virtual Private LAN Service. VPWS: Virtual Private Wire Service. VPN: Virtual Private Network. 2. Using SRv6 SID as Source Address Only unicast traffic are eligible for using SID as source address. There are several cases SHOULD be considered for using SRv6 SID as source address when there is firewall in middle. * User traffic. This includes SRv6 VPN-based services and VPN-less IP over SRv6 tunnels. * ICMP traffic. This is mainly for SRv6 Ping in SRv6 OAM[RFC9259] scenario. Yang & Lin Expires 20 December 2026 [Page 3] Internet-Draft SID as source address June 2026 2.1. User Traffic For SRv6 VPN-based services, the user traffic SHOULD use the SRv6 service SID as source address. The service SIDs are locally allocated by the PE per VPN/AC, packets received from an AC can always be unambiguously associated with a SRv6 service SID. All those End.DX* and End.DT* SIDs except End.DT2M SHOULD be used for source address. For VPN-less IP forwarding over SRv6 tunnels, the tunnels SHOULD use End SID as source address. 2.2. ICMP Traffic There are 2 cases, tunnel ends and transit node generated ICMP, respectively. For ping or the ICMP response generated by head or tail end of SRv6 tunnel, it SHOULD use the SRv6 SID as source address, such as ping, trace. For SRv6 VPN-based and VPN-less case, it SHOULD use the SID specified in Section 2.1 when there is a firewall in middle of SRv6 tunnel. Refer to RFC 8986 4.1.1, Allowing the processing of specific Upper-Layer header types is useful for Operations, Administration, and Maintenance (OAM). For the transit node generated ICMP error response message, the destination address would be a SID, when processing the Upper-Layer header of a packet matching a FIB entry locally instantiated as an VPN SID (End.DT4\End.DT6\End.DT46\End.DX4\End.DX6\End.DX2\End.DX2V\End.DT2U), process the packet as per Section 4.1.1 of [RFC8986]. 2.3. Control and Management Traffic Control and Management Traffic will not be terminated by VPN, thus will not be impacted. 3. Use Cases 3.1. SRv6 Network with SR-aware Stateful Firewall 3.1.1. Problem Statement To provide VPN service in an SRv6 network [RFC9252], the ingress PE encapsulates the payload in an outer IPv6 header with the Segment Routing Header (SRH) [RFC8754] carrying the SR Policy segment list along with the VPN Service SID. If the VPN service is with best- effort connectivity, the destination address of the outer IPv6 header carries the VPN service SID and the SRH is omitted. Yang & Lin Expires 20 December 2026 [Page 4] Internet-Draft SID as source address June 2026 Along the forwarding path in the SRv6 network, firewalls may be deployed to filter the traffics. If a firewall is SR-aware, it will retrieve the final destination of an SRv6 packet from the last entry in the SRH rather than the destination address field of the IPv6 header [I-D.draft-ietf-spring-sr-service-programming]. A stateful firewall keeps a track of the state of the network connections traveling across it. Whenever a packet arrives to seek permission to pass through it, the firewall checks from its state table if there is an active connection between identified by 3 tuple or 5 tuple. Thus only legitimate packets are allowed to be transmitted across it. Figure 1 and Figure 2 show the bidirectional VPN traffic packets passing through a firewall in an SRv6 network. The source address of the outer IPv6 header is the IPv6 address of ingress PE. The final destination address of the outer IPv6 header is the VPN Service SID of egress PE. In the SR-Policy-based way, the final destination address is encapsulated in the last entry in the SRH, Segment[0]. In the best-effort way, the SRH is omitted. +---+ +---+ +--------+ +---+ +---+ |CE1|---|PE1|--...--|Firewall|--...--|PE2|---|CE2| +---+ +---+ +--------+ +---+ +---+ Packet (PE1 ---> PE2): Packet (PE1 <--- PE2): ********************** ********************** * IPv6 * * IPv6 * * SA=PE1-IP-ADDR * * SA=PE2-IP-ADDR * * DA=NextSegment * * DA=NextSegment * ********************** ********************** * SRH * * SRH * * Seg[0]=PE2-VPN-SID * * Seg[0]=PE1-VPN-SID * * Seg[...] * * Seg[...] * ********************** ********************** * Eth/IPv4/IPv6 * * Eth/IPv4/IPv6 * * Source=CE1 * * Source=CE2 * * Destination=CE2 * * Destination=CE1 * ********************** ********************** * Payload * * Payload * ********************** ********************** Figure 1: SR-Policy-based VPN Traffic across Firewall Yang & Lin Expires 20 December 2026 [Page 5] Internet-Draft SID as source address June 2026 +---+ +---+ +--------+ +---+ +---+ |CE1|---|PE1|--...--|Firewall|--...--|PE2|---|CE2| +---+ +---+ +--------+ +---+ +---+ Packet (PE1 ---> PE2): Packet (PE1 <--- PE2): ********************** ********************** * IPv6 * * IPv6 * * SA=PE1-IP-ADDR * * SA=PE2-IP-ADDR * * DA=PE2-VPN-SID * * DA=PE1-VPN-SID * ********************** ********************** * Eth/IPv4/IPv6 * * Eth/IPv4/IPv6 * * Source=CE1 * * Source=CE2 * * Destination=CE2 * * Destination=CE1 * ********************** ********************** * Payload * * Payload * ********************** ********************** Figure 2: Best-Effort VPN Traffic across Firewall The stateful firewall will check the association relationships of the bidirectional VPN traffic packets. A common implementation may record the key information of the packets on forward way(internal to external), such as source address and destination address. When receiving a packet on backward way(external to internal), it checks the state table if there is an existing forward packet flow. For example, the firewall may require that the source address of packet on backward way matches the destination address of packet on forward way, and destination address will be checked in the similar way. If not matched, the packet on the backward path will be regarded as illegal and thus dropped. An SR-aware firewall is able to retrieve the final destination of an SRv6 packet from the last entry in the SRH. The tuple of the packet from PE1 to PE2 is , and the other direction is . However, the source address of the outer IPv6 packet is usually a loopback interface of the ingress PE. Eventually, the source address and destination address of the forward and backward VPN traffic are regarded as different flow, and they may be blocked by the firewall. 3.1.2. Solution for SRv6 Traffic Pass Thru SR-aware Stateful Firewall In the SRv6-based VPN service, the final destination of the outer IPv6 header is the VPN-SID of the egress PE, which is associated with that VPN service. But the source address of the outer IPv6 header is usually unrelated to the VPN service. So, it can be difficult for a stateful firewall to establish the association relationship between the bidirectional traffic flows. Yang & Lin Expires 20 December 2026 [Page 6] Internet-Draft SID as source address June 2026 The proposed solution is to unify the semantic of the source and destination address thus ensure the symmetry of the bidirectional flow. When an ingress PE receives the client packet from CE, it checks which L3 VPN service it belongs to, and uses the VPN-SID associated with that L3 VPN service as the source address when encapsulating the outer IPv6 header with the optional SRH. Outer IPv6 Header of SR-Policy-based VPN Traffic: ********************** ********************** * IPv6 * * IPv6 * * SA=PE1-VPN-SID * * SA=PE2-VPN-SID * * DA=NextSegment * * DA=NextSegment * ********************** ********************** * SRH * * SRH * * Seg[0]=PE2-VPN-SID * * Seg[0]=PE1-VPN-SID * * Seg[...] * * Seg[...] * ********************** ********************** Outer IPv6 Header of Best-effort VPN Traffic: ********************** ********************** * IPv6 * * IPv6 * * SA=PE1-VPN-SID * * SA=PE2-VPN-SID * * DA=PE2-VPN-SID * * DA=PE1-VPN-SID * ********************** ********************** Figure 3: Outer IPv6 Header in the Proposed Solution According to [RFC8402] and [RFC8986], an SRv6 VPN Service SID is an IPv6 address, and it is routable by its Locator prefix in the SRv6 network. In the proposed solution, when an SRv6 VPN Service SID is used as the source address of the outer IPv6 header in the SRv6 network, it is treated as a normal IPv6 address and does not perform any special behavior. 4. IANA Considerations This document has no IANA actions. 5. Security Considerations TBD. 6. References Yang & Lin Expires 20 December 2026 [Page 7] Internet-Draft SID as source address June 2026 6.1. Normative References [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006, . [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, . [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, . [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, . [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, . [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, . 6.2. Informative References Yang & Lin Expires 20 December 2026 [Page 8] Internet-Draft SID as source address June 2026 [I-D.draft-ietf-spring-sr-service-programming] Abdelsalam, A., Xu, X., Filsfils, C., Bernier, D., Li, C., Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W., and S. Salsano, "Service Programming with Segment Routing", Work in Progress, Internet-Draft, draft-ietf-spring-sr- service-programming-12, 3 November 2025, . [I-D.draft-ietf-spring-srv6-security] Buraglio, N., Mizrahi, T., tongtian124, Contreras, L. M., and F. Gont, "Segment Routing IPv6 Security Considerations", Work in Progress, Internet-Draft, draft- ietf-spring-srv6-security-14, 13 April 2026, . Authors' Addresses Feng Yang China Mobile China Email: yangfeng@chinamobile.com Changwang Lin New H3C Technologies China Email: linchangwang.04414@h3c.com Yang & Lin Expires 20 December 2026 [Page 9]