Subject: [Tsvwg] Max Burst again

From: "Randall R. Stewart (home)" <[email-masked]>
Date: Thu, 19 Jun 2003 07:10:42 -0500

Mark/Kacheong/All:

I have been doing a bit of poking around on the
max.burst issue. I have not adjusted the KAME-SCTP stack
so that you can apply max-burst in either the "Kacheong" method i.e.
counting packets... or the "Mark" method of adjusting the cwnd
(default is the kacheong method :>).

I have done a few preliminary studies...I do need more work yet to
prove anything conclusive though.. my findings are so far:

1) It does not seem to make a difference on a LAN 100Meg link
    as to if we use either options. I tranfer in roughly the same time a
    large elephant type transfer. And it seems to run at about line-rate.

2) It is true that the cwnd seems to get to be a much larger value on
    the Kacheong method... close to the rwnd... but I think the max.burst
    kicking in aka 4 packets sent.. shuts down further sending...

3) Without a maxburst (of any type) it appears that I over-send to the 
network card and
     actually drop packets... need to test this again ... but this 
causes a decrease
    in performance.

4) There is one issue that I think may be a bad thing for the kacheong 
method. Consider
    a sender doing:
    while(1){
         send(100-bytes);
    }
    with nagle off... What I have seen in the above case is oversending 
by the sender. The reason
    being the cwnd is never blocking us. What seems to occur is the sack 
comes in and we burst
    out 4 packets.. then right behind that a send comes in from the user 
and we burst out 4 packets.
    Now this assumes a very high send-side socket buffer (I have both 
the receive/send sockbuf's at
    512k).... and it may be an extreme example.. but it does show a 
weakness to the counting packets
    method I think... aka an opportunity to send is not only on a SACK 
but also possibly on a user
   send... which means you do have the potential to burst out packets in 
larger than maxburst. With the
   "Mark" method.. the cwnd is adjusted down and gates you at maxburst 
packets... so when the user
    sends presto... no packets are sent since the full cwnd is used up 
already...

    I need to do a bit more study on the two issues .. and I would like 
to get on a Gigabity e-net.. which will
    have to wait until I get another large server (sometime in 
August)... I am curious to see if higher speed
    will also have an effect on use of the bandwidth..

R

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