All things related to multicast

[Bill Nickless <>]

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insensitivity to destination MAC addresses is no longer valid.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