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Re: (MSDP problems in the internet)


Chronological Thread 
  • From: Bill Nickless <>
  • To: John Zwiebel <>
  • Cc: Marshall Eubanks <>, Bill Owens <>, <>, <>
  • Subject: Re: (MSDP problems in the internet)
  • Date: Mon, 13 May 2002 18:37:38 -0500


On Mon, 13 May 2002, John Zwiebel wrote:

> Let me take a moment to bug everyone with some -obvious- points.
>
> 1) MSDP is and always has been a kludge.

No, John,

The service model and wire protocol in RFC-1112 are the kludges.

RFC-1112 is built around the idea that a host can simply put an IPv4 multicast group-addressed packet on a wire and it will get to all the interested receivers with the same reliability as unicast UDP. That's true in a single collision domain. But collision domains don't scale.

Regarding internetwork routing, RFC-1112 takes the easy way out. It simply says that "multicast router(s) attached to the local network take responsibility for forwarding...towards all other networks that have members of the destination group". It says nothing about how the "multicast router(s) attached to the local network" know where the interested group members are. That's left as an exercise to the reader.

SSM comes close to giving the "multicast router(s)" the information they need to fulfill their RFC-1112 duties. How? In SSM, a receiver must give the network the knowledge of a source that will (may someday?) put a packet on its local wire. Given that knowledge, the internetwork has at least a small prayer to create the forwarding state necessary for the future packets to be delivered from source to destination.

Your complaint with MSDP is really a complaint about the ASM (RFC-1112) service model. That is, the network can't know about the existence of active sources until the source goes active in the first place. Spreading that knowledge around is the purpose of MSDP.

Also:

IGMP is a broken kludge, again designed for networks that are composed of single 10Base5 thick Etherhoses. That is: a single collision domain, where the underlying wire is insensitive to the actual MAC destination addresses of a packet.

But what happens when the MAC address is used to make forwarding decisions, like in a bridged Ethernet? That RFC-1112 assumption of media insensitivity to destination MAC addresses is no longer valid.

Somehow we've bludgeoned the Ethernet (802.x) switch builders into looking far deeper into Ethernet multicast frames than they ever should have had to; that's how IGMPv2 got deployed. Now we're expecting them to redo all that work, look even FURTHER into the Ethernet multicast frames, to deploy IGMPv3? For what payback? Especially from their perspective? Oh, and by the way, SSM/IMGPv3 doesn't solve the exchange point problem (PIM Asserts) either. Anyone want to bet how long it will take for 802.x vendors to handle IPv6 multicast in their devices?

I've thought about writing an IGMPv4 draft. IGMPv4 speakers would negotiate media-specific multicast addresses for (*,G) or (S,G) or even (S,G,upstream) sources. The interested listeners would then do media-specific multicast joins (like 802.1p GARP/GMRP or ATM point-to-multipoint SVCs or point-to-multipoint MPLS TEs or Optical Control Plane Splitters or whatever.) This would separate the Internet Protocol Group management from the media-specific inter-collision-domain forwarding.

In the meantime, we have a set of protocols for ASM that are designed, implemented, deployed, and operational for IPv4. Those protocols are IGMPv2, MSDP, PIM-SM, and M-BGP. They're close enough that we can write books about them, teach classes on how to use them, and put them into production. There's plenty of data that they're working fine today on the Internet.

MSDP is eminently completable. I agree with Hans Kuhn about putting SDP files on the "small research project" World Wide Web. We should quit being afraid of breaking SDR and therefore stop encapsulating data in MSDP. Separating the control plane and the data plane has been a good design principle for a long time. MSDP, used purely to DISCOVER SOURCES, can and should be done right.

===
Bill Nickless http://www.mcs.anl.gov/people/nickless +1 630 252 7390
PGP:0E 0F 16 80 C5 B1 69 52 E1 44 1A A5 0E 1B 74 F7





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