cwnd and sstresh monitor

[Alin-Adrian Anton]

Robert Watson wrote:
> 
> On Thu, 1 Dec 2005, Vlad GALU wrote:
> 
>> On 12/1/05, Alin-Adrian Anton <aanton at spintech.ro> 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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