[PATCH] SRv6 support for BIRD

[Sébastien PARISOT]

Hello BIRD team,

This patch (for master branch / 2.18) adds SRv6 L3VPN support to BIRD. It applies on top of the Multiple Labels capability (RFC 8277) patch I sent earlier.

- Patch 0001 is preparatory work that adds a new internal attribute BA_RAW_MPLS_LABEL_STACK (0xfd) that stores the full 24-bit wire values from MPLS label fields in BGP NLRI, alongside the existing BA_MPLS_LABEL_STACK (0xfe) which stores decoded 20-bit label values.

The existing BA_MPLS_LABEL_STACK only preserves the 20-bit label value from each 24-bit NLRI label field, discarding the traffic class and BOS fields. This is fine for normal MPLS but is lossy for SRv6: RFC 9252 uses the NLRI label field to carry part of an SRv6 SID via transposition, where the full 24 bits are significant and BOS may not be set.

- Patch 0002 adds SRv6 to the nest, it includes SID stack in next hop, adds the parser, display in CLI outputs.

- Patch 0003 adds Netlink support for SEG6 routes in Linux kernel (but not SEG6LOCAL routes).

- Patch 0004 adds SRv6 support to static protocol.

- Patch 0005 updates l3vpn protocol to operate without MPLS.

- Patch 0006 adds Prefix SID BGP attribute support for L3VPN.

With this patch set BIRD can act as a SRv6 PE/headend, but there are some limitations:
- Dynamic SID allocator is not implemented (SID can be read routing table with netlink or configured on static routes)
- The code supports a SID stack but SRV6_MAX_SID_STACK is currently #defined to 1 because L3VPN uses only one (no SRv6 TE support)
- The SID structure sub-sub-TLV is parsed on import (for transposition) but not generated on export (it is not mandatory per RFC 9252)
- Endpoint behavior is set the opaque value (0xffff) in generated SID information sub-TLV (it is not mandatory to specify the codepoint of actual SID behavior per RFC 9252)

A pair of BIRD configuration file are also included in attachment as examples. Example "show route" output:

65001:1 10.10.1.0/24
    unicast [static_vpn 17:37:31.789] * (200)
        via 10.0.0.100 on veth1 srv6 <fc00:a1::1>
        Type: static univ
65002:1 10.20.1.0/24
    unicast [peer2 17:37:36.192 from 10.0.0.2] * (100/) [AS65002i]
        via 10.0.0.200 on veth1 srv6 <fc00:a2::1>
        Type: BGP univ
        BGP.origin: IGP
        BGP.as_path: 65002
        BGP.next_hop: 10.0.0.200
        BGP.local_pref: 100
        BGP.prefix_sid: SRv6 L3 Service SID=fc00:a2::1, Flags=0x00, Behavior=0xffff
        BGP.raw_mpls_label_stack: 0x000000
        BGP.mpls_label_stack: 0

Looking forward to your feedback.

Thanks!
--
Sébastien
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