msdp

Overview ¶

Package msdp decodes MSDP (Multicast Source Discovery Protocol) packets per RFC 3618. MSDP is the inter-domain multicast protocol that completes the multicast trio alongside IGMP (host↔router, covered by `igmp_decode`) and PIM (router↔router intra-domain, covered by `pim_decode`). Each PIM-SM domain has its own Rendezvous Points (RPs); MSDP connects RPs across domains so that a receiver in one domain can join a multicast group whose source is in another.

Operationally, every major Internet exchange + carrier core that carries multicast traffic (financial-market data feeds, IPTV peering, content distribution) runs MSDP between RPs over TCP port 639.

Wrap-vs-native judgement

Native. RFC 3618 is fully public; MSDP messages are
plain TLVs: 1-byte Type + 2-byte Length (including the
3-byte header) + per-type body. No crypto, no
compression. Operators paste MSDP bytes (TCP port 639)
from a `tcpdump -X tcp port 639` line or a Wireshark
Follow-TCP-Stream view and get the documented type +
body breakdown.

What this package covers

• **3-byte TLV header** (RFC 3618 §3):

• byte 0: **Type** with **6-entry name table**: 1 IPv4 Source-Active, 2 IPv4 SA Request, 3 IPv4 SA Response, 4 Keepalive, 6 Notification, 7 Traceroute in Progress (deprecated), 8 Traceroute Reply (deprecated).

• bytes 1-2: Length (uint16 BE; total including this 3-byte header).

• **IPv4 Source-Active body** (Type 1; RFC 3618 §4.1):

• byte 0: Entry Count (uint8; number of (S, G) entries).

• bytes 1-4: RP Address (IPv4 — the originating Rendezvous Point).

• **Entry × Entry Count** (each 12 bytes):

• 3-byte Reserved.

• 1-byte Sprefix Len (typically 32; the source prefix length).

• 4-byte Group Address (IPv4 multicast).

• 4-byte Source Address (IPv4 unicast).

• Optional encapsulated multicast datagram (raw hex — typically the first packet from a new source, sent to bootstrap MSDP peers that haven't yet built (S, G) state).

• **IPv4 SA Request body** (Type 2; RFC 3618 §4.2):

• byte 0: Reserved.

• bytes 1-4: Group Address (IPv4 multicast).

• **IPv4 SA Response body** (Type 3; RFC 3618 §4.3) — same layout as Source-Active.

• **Keepalive body** (Type 4) — empty (header only; length = 3).

• **Notification body** (Type 6; RFC 3618 §6.1):

• byte 0: **O (Open) bit** (high bit) + 7-bit **Error Code** with **7-entry name table** (RFC 3618 §6.1): 1 Message Header Error, 2 SA-Request Error, 3 SA-Message/SA-Response Error, 4 Hold Timer Expired, 5 Finite State Machine Error, 6 Notification, 7 Cease.

• byte 1: Error Subcode.

• bytes 2+: Data (variable; surfaced as hex).

What this package does NOT cover (deliberately out of scope)

• TCP framing — feed MSDP bytes after the TCP payload extraction. MSDP runs on TCP port 639.

• MSDP state-machine reasoning (peer setup, SA cache, hold-timer expiry, mesh-group RPF check) — higher- level analysis.

• Encapsulated multicast datagram dissection — when SA carries a bootstrap data packet, it's surfaced as opaque hex; operators can feed it into `ip_packet_decode` to walk the inner IP frame.