fix(srv6): Add missing SRv6 SID Structure sub-sub-TLV - #299
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The BGP Prefix-SID attribute Galactic advertises for EVPN Type 5 paths carried the destination SID but never described its internal layout. RFC 9252 requires an SRv6 SID Structure Sub-Sub-TLV alongside the SID so receivers know how to interpret it — Galactic's uFMT 48+16 layout (48-bit Block, 16-bit Node-ID, 4-bit Function, 12-bit Argument, no transposition) was never signaled on the wire. prefixSIDAttr now builds that sub-sub-TLV from uformat's existing bit-width constants rather than hardcoding new values, keeping the wire encoding and the eBPF datapath's own bit layout from drifting apart. A new round-trip test serializes the attribute and decodes it back through GoBGP's own DecodeFromBytes to assert on the sub-sub-TLV fields. L2 (uEnd.DT2) and the receive side (nodes still parse SIDs via uformat's fixed offsets, not this TLV) are unaffected — out of scope until L2/EVI support lands. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
ecv
approved these changes
Aug 7, 2026
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Summary
The BGP Prefix-SID attribute Galactic advertises for EVPN Type 5 paths carries the destination SID but never described its internal layout. RFC 9252 requires an SRv6 SID Structure Sub-Sub-TLV alongside the SID so receivers know how to interpret it — Galactic's uFMT 48+16 layout (48-bit Block, 16-bit Node-ID, 4-bit Function, 12-bit Argument, no transposition) was never signaled on the wire. Impact was low, since every Galactic node parses SIDs via fixed offsets rather than this TLV, but a spec-compliant third-party EVPN/SRv6 receiver would have been missing the structure description entirely.
prefixSIDAttrnow builds the sub-sub-TLV fromuformat's existing bit-width constants instead of hardcoding new values, so the wire encoding and the eBPF datapath's own bit layout can't silently drift apart.L2 (
uEnd.DT2) and the receive-side path are unaffected — out of scope until L2/EVI support lands.Found via a compliance check against
datum-cloud/enhancements'sarchitecture/design/network/addressing/srv6.md, §"RFC 9252 SRv6 Service Encoding" — there's no tracked issue for this gap.Test plan
task linttask test:unit— covers a new round-trip test that serializes the attribute and decodes it back via GoBGP's ownDecodeFromBytes, asserting on the sub-sub-TLV fieldstask test:e2e— not run; the e2e suite doesn't exercise real BGP path exchange between two GoBGP instances, so it wouldn't observe this attribute on the wire