Re: cwnd and sstresh monitor

Alin-Adrian Anton <aanton@xxxxxxxxxxx> · Sat, 03 Dec 2005 03:25:57 +0200

Robert Watson wrote:

On Thu, 1 Dec 2005, Vlad GALU wrote:
On 12/1/05, Alin-Adrian Anton <aanton@xxxxxxxxxxx> wrote:
Dear Hackers,
        I would like to monitor the changes of cwnd and sstresh 
values during

TCP traffic, in order to plot graphs and interpret congestion.
        So I need (cwnd, timestamp) and (sstresh, timestamp) records 
to be

taken everytime one of the two variables is modified.
        I'd like to ask you for suggestions, which would be the best 
aproach

(kernel patch, kernel module, etc?), and how would this be done best?

(the interception of values, the storage of snapshots, etc)?

  Does exporting them via sysctl, and graph them using anything
(rrdtool) sound reasonable ?

I thought about this too, however, this loses precision and provides 
constant units of time. Knowing the timestamps for each packet may be 
interesting to underline timeouts on the graphic.

I've not used it, but there is a TCPDEBUG kernel option that gathers TCP 
state information for debugging and tracing purposes.  I know this has 
been used quite effectively in the past for this sort of work, but 
unfortunately I know very little about it.  With all the TCP changes in 
the last few years (SACK, etc), it could be that it needs some 
enhancements, cleanups, fixes, etc.

I used it now, and with a small patch it shows exactly what I need (seq, 
ack, timestamp, cwnd and ssthresh). I just added my knob to trpt.c .
I also modified the iptime() function to provide microsecond resolution 
instead of miliseconds, because most of the packets have the same 
timestamp attached. Still, a decent number of packets have the same 
timestamp. I'm looking at them only on one side of the connection (the 
transmitter), I wonder if there is any better solution then timestamping 
them on both sides - and mixing the values.
Thanks guys for the precious information, it helped a lot!
Yours Sincerely,
--
Alin-Adrian Anton
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"It is dangerous to be right when the government is wrong." - Voltaire
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Re: cwnd and sstresh monitor

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