Protocol: MSDP (Multicast Source Discovery Protocol)
Transport: TCP
Defined: RFC 3618 (October 2003)
Status: Experimental (never made a proposed standard)
Port 639 is where MSDP peers connect. MSDP—Multicast Source Discovery Protocol—solves a problem that existed in the late 1990s and still exists today: how do you make multicast traffic work across different network domains without flooding the entire Internet?
The Problem MSDP Solved
Multicast is efficient within a single domain. One source sends packets once, routers duplicate them only where needed, and interested receivers get the stream. But what happens when a source is in one ISP's network and receivers are in another?
Before MSDP, there wasn't a good answer. You either flooded multicast everywhere (wasteful), or you manually configured everything (doesn't scale).
MSDP created a way for different PIM-SM (Protocol Independent Multicast Sparse Mode) domains to tell each other "hey, we have a multicast source at this address sending to this group." Routers could then decide whether they cared—whether anyone in their domain wanted that traffic.
How It Works
MSDP peers establish TCP connections on port 639. In a peering relationship, one peer listens on port 639 while the other makes the connection. The peer with the higher IP address listens. The peer with the lower IP address connects.1 This prevents both sides from trying to connect simultaneously.
Once connected, peers exchange Source-Active (SA) messages. These messages say: "I have a source at address X sending to multicast group Y." If the receiving peer has interested receivers in its domain, it can join the multicast tree and start receiving the traffic.
The genius is what MSDP doesn't do. It doesn't forward the actual multicast packets. It doesn't build a full multicast routing tree across domains. It just shares knowledge about active sources. The actual multicast traffic stays within the domain boundaries unless someone explicitly wants it.
The BGP Connection
MSDP has a peculiar relationship with BGP. The protocol spec requires that MSDP topology follow BGP topology—your MSDP peers should generally match your BGP peers.2 MSDP uses BGP routing data to perform RPF (Reverse Path Forwarding) checks, verifying that SA messages are coming from the expected peer.
This means MSDP lives in the same world as BGP: the world of ISP peering, AS numbers, and inter-domain routing. It's infrastructure-level protocol, not something end users interact with.
The Strange Status
RFC 3618 is marked "Experimental." MSDP became the IPv4 de facto standard for inter-domain multicast—it's running on routers across the Internet right now. But development stopped in 2006, and the authors decided not to pursue making it a proposed standard.3
Why? Partly because IPv6 multicast takes a different approach (embedded-RP). Partly because inter-domain multicast never achieved the widespread deployment that seemed inevitable in the 1990s. Video streaming went a different direction—unicast CDNs won, multicast lost.
But MSDP is still here. Still carrying SA messages on port 639. Still experimental. Still working.
Security Considerations
MSDP peers trust each other. A malicious peer can announce fake sources, potentially causing routers to join multicast groups unnecessarily or directing traffic incorrectly.
The protocol has no built-in authentication in its original spec, though implementations often use TCP MD5 signatures (the same mechanism BGP uses) to verify peer identity.
Most importantly: MSDP is not for everyone. If you're not running a PIM-SM domain that needs to peer with other domains, you don't need MSDP. This is ISP and large enterprise territory.
How to Check What's Listening
On Linux or macOS:
On Windows:
If you see something listening on port 639 and you're not running MSDP on a router, that's worth investigating.
Related Ports
- Port 520: RIP (Routing Information Protocol) - another routing protocol, much simpler
- Port 179: BGP (Border Gateway Protocol) - MSDP's routing cousin and topology reference
- Port 89: OSPF - interior gateway protocol that works within domains
Frequently Asked Questions About Port 639
এই পৃষ্ঠাটি কি সহায়ক ছিল?