Subject: RE: [Simple] Re: Draft-campbell-simple-im-session-01: Head ofLin e Blocking

From: Adam Roach <[email-masked]>
Date: Mon, 28 Apr 2003 15:01:10 -0500

> -----Original Message-----
> From: Henning Schulzrinne [mailto:[email-masked]]
>
> I haven't followed the 64K messages on this topic, but I 
> would be very 
> concerned about conclusions that SCTP actually helps with HOL 
> blocking. 
> SCTP cannot throttle one stream, so if you have 'forking', as in

[diagram removed]

> the left-most link, SCTP or not, will get blocked waiting for the 
> slowest outbound link.

My understanding was that each SCTP stream is individually
flow controlled. This seems to be held up by RFC 3286:

   Another example of possible use of multi-streaming is the delivery of
   multimedia documents, such as a web page, when done over a single
   session.  Since multimedia documents consist of objects of different
   sizes and types, multi-streaming allows transport of these components
   to be partially ordered rather than strictly ordered, and may result
   in improved user perception of transport.

/a
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From: Paul Kyzivat <[email-masked]>
Date: Mon, 28 Apr 2003 16:22:21 -0400

>    in improved user perception of transport.

I think Henning is right. As I recall (and its been a long time since I 
have looked at the SCTP spec), flow control is over the entire 
collection of streams. Messages are chunked to prevent HOL blocking, but 
if the receiver doesn't pull the received chunks then eventually flow 
control will stop all the streams.

If you have a relay receiving messages for both slow and fast outbound 
links, and input for the slow link is arriving faster than it can be 
sent, then the relay has to keep pulling and buffering messages for the 
slow link, or else the fast link will eventually be blocked.

	Paul

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From: "James Undery" <[email-masked]>
Date: Mon, 28 Apr 2003 21:40:18 +0100

Yes Henning is right, and the only fixes I can think of from the top of my
head are too ugly to commit to email. When I said in a previous email that
chunking is likely to be a difficult problem I think I seriously
underestimated it in light of this, and have to withdraw SCTP a suggested
fix.

James

----- Original Message ----- 
From: "Paul Kyzivat" <[email-masked]>