| Internet-Draft | SID as source address | June 2026 |
| Yang & Lin | Expires 20 December 2026 | [Page] |
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.¶
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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.¶
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.¶
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.¶
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.¶
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.¶
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].¶
Control and Management Traffic will not be terminated by VPN, thus will not be impacted.¶
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.¶
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 * ********************** **********************
+---+ +---+ +--------+ +---+ +---+ |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 * ********************** **********************
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 <source, destination> tuple of the packet from PE1 to PE2 is <PE1-IP-ADDR, PE2-VPN-SID>, and the other direction is <PE2-IP-ADDR, PE1-VPN-SID>. 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.¶
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.¶
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 * ********************** **********************
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.¶
This document has no IANA actions.¶