[c-nsp] Performance issue on link

CiscoNSP List cisconsp_list at hotmail.com
Wed Apr 3 18:52:30 EDT 2013


If I run 1000 pings (1500 byte, df-bit set) in both directions, I am not seeing any packet loss?

#tcp window size in the "bad" direction:

starts at win=6912 and grows to 165504 (This is where I start to see a heap of TCP Dup ACK and TCP segment of a reassembled PDU)), then increases to 353408 (Again, more TCP Dup ACK's), and looks to max out at 669056.

#tcp window size in the "good" direction:

starts at win=6912 and looks to max out at 1995392 and has very few TCP Dup ACK and TCP segment of a reassembled PDU

Thanks again for your help.

> Date: Wed, 3 Apr 2013 09:39:54 +0300
> From: saku at ytti.fi
> To: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] Performance issue on link
> On (2013-04-03 16:54 +1100), CiscoNSP List wrote:
> > Thanks Mikael - I have captured a wget with tcpdump, but Im not exactly sure what I should be looking for in wireshark....I see a LOT of "TCP segment of a reassembled PDU", a LOT of "Dup ACK's" and graph shows peaks of 25Mb...
> For typical TCP stack, if you've received TCP segment 42 previously then
> you'll receive TCP segment 43, you'll duplicate ACK 42, in order to signal
> to sender that you believe you're experiencing packet loss (which might not
> be packet loss, it might be just reordering, but modern TCP stack does not
> cope with reordering, because it copes aggressively well with packet loss)
> So you're experiencing packet loss, which shouldn't be surprising I think
> Mikael was also interested in hearing your TCP window size, preferably when
> it was highest.
> (You need to capture session from the start to know TCP window scale
> option)
> -- 
>   ++ytti
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