Subject: Re: [Tsvwg] SCTP Max.burst and CWND (fwd)

From: Mark Allman <[email-masked]>
Date: Tue, 05 Mar 2002 23:43:43 -0500

> Why?  cwnd already limits things to the maximum burst that was
> successful in the past?

and, later:

> As such, cwnd is no greater than the latest and greatest
> successful burst.

Just not true.

Using a large cwnd a little at a time (like clocking out 2-3
segments per ACK) is quite a bit different than sending the whole
lot at once.  In the first case we end up spreading the transmission
over time (ideally the entire RTT) a bit.  In the second case we
attempt to send the entire cwnd at once.

I am not wading into this entire SCTP vs. TCP thing...  As I have
said before, I think they should both use the same algorithms in
this space to the extent possible.

What I will note is that having no burst limit may in fact *hurt*
your performance.  Significant line-rate bursts have a greater
chance of causing additional loss and therefore further reducing
cwnd (and causing more congestion in the network).  There is a bunch
of TCP data on this in one of my former officemate's thesis:

    Chris Hayes. Analyzing the Performance of New TCP
    Implementations over Satellite Links. Master's Thesis, Ohio
    University, August 1997. 
    http://irg.cs.ohiou.edu/papers/hayes-thesis.pdf

There is plenty of evidence in there that shows that having a rwnd
that does not allow for sending during recovery often causes a
secondary loss event upon leaving recovery -- due to the line rate
burst.

I.e., the point is that this anti-bursting stuff is not necessarily
a disadvantage.  Just because some scheme says "send less" doesn't
necessarily mean you'll lose in the end...

allman

--
Mark Allman -- NASA GRC/BBN -- http://roland.grc.nasa.gov/~mallman/

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From: Mark Allman <[email-masked]>
Date: Tue, 05 Mar 2002 23:14:22 -0500

> Are you proposing that SCTP be disadvantaged wrt the
> current TCP standard?

I am transport protocol neutral.  I think both protocols should use
the same CC schemes (to the extent possible).  So, I think both TCP
and SCTP should be trying to ameliorate bursts in a similar
fashion.  (I have sketched the scheme I think is reasonable at this
moment -- but, it is certainly up in the air whether others buy it
or not.)

allman