All things related to multicast

[Hugh LaMaster <>]

On Sun, 3 Mar 2002, Bill Nickless wrote:

> I can't begin to count the number of times we find duplex mismatches, dirty 
> circuits (clock skew is my favorite), routers mis-configured to handle 
> multicast forwarding in software, IGMP brokenness, 5% of circuit speed rate 
> limits on multicast traffic (!!) and so on.
> 
> The challenge with many of these problems is that TCP hides them too 
> well.  Often the multimedia application is the first to find a problem 
> that's existed for a long time.

Bill Nickless is right on.  Very, very often, I have seen
underlying problems from duplex mismatch, or, autonegotiation
failure/reset, other switch problems, dirty fiber connectors,
bad fiber jumpers, other SONET-layer problems, marginal switch 
boards, and on and on, not show up with TCP other than the 
inevitable slowness, and be revealed by multimedia UDP applications, 
either multicast.  We've seen cases where error rates went way
up as traffic went way up, guessed that the behavior implied layer-3,
only to discover a bad fiber jumper was the problem.  Things like
this have happened so many times, but, it is a hard lesson to learn.

And, for some strange reason, the gut-level reaction that 
many of us have when we first see the problem is to blame
the multimedia application, rather than try to debug the
layer-1/2 problems.  

Multicast multimedia applications are actually great tools 
for verifying the integrity of the network paths end-end.


--
 Hugh LaMaster, M/S 233-21,    Email: 

 NASA Ames Research Center     Or:    

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 Phone: 650/604-1056           Disc:  Unofficial, personal *opinion*.