[Ns-bugs] [Bug 1039] TCP Nagle algorithm and RTO calculation

[code@nsnam.ece.gatech.edu]

http://www.nsnam.org/bugzilla/show_bug.cgi?id=1039

Josh Pelkey <jpelkey at gatech.edu> changed:

           What    |Removed                     |Added
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                 CC|                            |jpelkey at gatech.edu

--- Comment #1 from Josh Pelkey <jpelkey at gatech.edu> 2011-03-16 11:42:43 EDT ---
(In reply to comment #0)
> I'm filing a reminder to look at two changes to TCP behavior when the new TCP
> socket architecture was defined.  These can be seen in the packet traces for
> routing-aodv-regression (tcp-chain) but need some separate tests once ns-3.10
> is released.
> 
> In looking at the tcp-chain traces in routing-aodv-regression, there are a few
> substantive differences in the model.
> 
>   a) this application is writing data periodically into the buffer (1200 bytes
> every 150 ms).  The old TCP would not send a partial segment but the new TCP
> will try to empty its buffer even if it is not a full segment.  In other words,
> it does not use Nagle's algorithm by default.

I can't find the logic for this in the old TCP implementation either, though
things are a bit harder to find in that one. I think this logic would fit in
TcpSocketBase::Send fairly easily.


>  b) in this experiment, there comes a time (after 1.9 seconds) where acks are
> lost.  Both the old and new TCP will take a coarse timeout, but the
> retransmission timeout is quite different.  In the old TCP, the retransmission
> of the segment sent  at 1.905 seconds comes at 2.791 seconds; about 800 ms.  In
> the new TCP, it comes at time 2.116 seconds; about 200 ms.  I think that this
> should also be looked at; 200ms seems too small for an RTO.  According to RFC
> 2988, 1 seconds is the recommended min RTO.

The minimum RTO (200ms) is set in rtt-estimator.cc. I'm betting this was set in
ns-3 to follow what Linux does, but I'm guessing Linux also does some special
rtt estimation to support a lower min RTO.  +1 to bump it up to 1 second.

I hope to get a patch up here soon.

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