fix: rv-live should honor the BMP peer A flag

[pforemski]

Problem

rv-live dropped UPDATEs from any peer that did not negotiate 4-byte ASNs (RFC 6793) with the collector:

ASPATH: invalid ASPATH segment length: 22 < 2+4*10

Reported by a RouteViews operator against route-views7 peer AS40864, and reproducible on RIPE RIS too.

processRecord() parsed the BGP message without ever setting Meta.ParseAS4, so attribute parsing fell back to the pipe-wide caps — which default to CAP_AS4 on and CAP_AS_GUESS off. Every peer was therefore parsed as 4-byte, and legacy peers failed hard and were dropped.

Fix

The per-peer answer was already in hand and being discarded. Bmp.FromBytes() parses the Per-Peer header, whose A flag (RFC 7854 §4.2) announces the legacy 2-byte AS_PATH format. bmp.Reader.setMeta() honors it — which is why read --format=obmp has always handled these peers correctly. Only the Kafka path ignored it.

Eight lines, no parser swap: just read the flag that bm.Peer already carries, before fixPath() triggers attribute parsing.

Why the flag is trusted only when set

The strict reading of RFC 7854 is that a clear A flag means 4-byte. That turns out to be actively harmful in the field, because senders that never set the flag then pin every peer to 4-byte — and ParseAS4 = 1 also short-circuits the retry in attrs.Aspath.Unmarshal, silently disabling --guess-asn.

RouteViews is such a sender. Measured on their live feed, route-views7, ~92k route-monitoring messages across 15 peers:

PEER_AS    Aflag_set Aflag_clr   only2B   only4B   MISMATCH
40864              0       199      147        0        147  <-- 2-byte on the wire, flag says otherwise
16276              0     68421        0    63700          0
48112              0      1457        0     1353          0
...
total route-monitoring msgs: 91628, distinct peers: 15

The flag is clear on 100% of messages. For the 14 modern peers that is accidentally correct; for AS40864 it is wrong.

Validation

Against live routeviews route-views7, 75–90s windows:

	ASPATH errors	with --guess-asn
before	233	0
after	143	0

The strict variant was implemented first and measured at 720 errors with --guess-asn (vs 0), which is what motivated the narrower rule.

Since RouteViews never sets the flag, this change is a no-op against their feed today — the before/after difference without --guess-asn is window-to-window variance in AS40864's announcement rate, not an effect. It is correct for any conformant sender, brings rv-live in line with read --format=obmp, and leaves --guess-asn working as the escape hatch for non-conformant ones.

Also verified with synthetic OpenBMP records carrying a real 2-byte UPDATE: flag set parses [61292,24482,16509], flag clear reproduces the reported error byte-for-byte.

[Copilot]

🟢 Approval recommended

The change is minimal, well-scoped to the Kafka rv-live path, and addresses the described failure mode without introducing risky behavioral changes for non-legacy peers.

Pull request overview

This PR fixes rv-live (Kafka/OpenBMP ingestion path) so it honors the BMP per-peer A flag when deciding whether to parse AS_PATH as legacy 2-byte ASNs, aligning its behavior with the existing read --format=obmp BMP reader and preventing UPDATE drops from legacy peers.

Changes:

• Set m.ParseAS4 = -1 when the BMP peer indicates 2-byte AS_PATH (bm.Peer.Is2ByteAS()), before any UPDATE attribute parsing is triggered (e.g., by fixPath).
• Document why the flag is only trusted when set (to avoid breaking --guess-asn in the presence of non-conformant senders).

File summaries

File	Description
stages/rv-live/openbmp.go	Apply per-peer BMP A-flag-derived AS_PATH parsing mode to the message meta before UPDATE attribute parsing occurs.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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