bgp-ls: accept prefix-length 0 in LsTLVIPReachability (default route)

[Vadims06]

Problem

GoBGP rejects a BGP-LS IPv4/IPv6 Topology Prefix NLRI whose IP Reachability Information TLV (type 265) carries prefix-length 0 (the default route 0.0.0.0/0 or ::/0), sending a NOTIFICATION and resetting the session in a loop.

Observed with a Huawei NE40E advertising default-route-advertise always into BGP-LS. The session reached ESTABLISHED but immediately dropped:

Session will be reset due to malformed BGP message
Error="Incorrect IP reachability Info length"
sent notification Code=3(update) Subcode=10(invalid network field)

Root cause

LsTLVIPReachability.DecodeFromBytes in pkg/packet/bgp/bgp.go had two overly strict guards:

1. if len(value) < 2 — rejects a 1-byte value, but for prefix-length 0 the TLV value is exactly 1 byte (the 0x00 prefix-length octet with zero prefix bytes following).
2. if value[0] > 128 || value[0] == 0 — explicitly rejects prefix-length 0.

A default route is a valid BGP-LS Prefix NLRI per RFC 7752 §3.2.3.2: prefix-length 0 → 0 prefix bytes → TLV value length 1.

toPrefix() had the same latent bug: (int(0)-1)/8+1 evaluates to 1 (not 0) due to Go integer truncation, producing a wrong bytes count masked only by the if i >= len(l.Prefix) { break } guard inside the copy loop.

Fix

• Relax the length guard to < 1.
• Remove the value[0] == 0 rejection.
• Compute ll as 0 when prefix-length is 0.
• Apply the same guard to toPrefix() for correctness.

Test

Added a table-driven case for the exact captured TLV bytes {0x01, 0x09, 0x00, 0x01, 0x00} (type 265, len 1, value 0x00), verifying decode to prefix_length=0, prefix=[].

[fujita]

Pushed, thanks!