netdev - Re: Detecting TCP loss on the receiving side?

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Message-ID: <1e41a3230807101434s48dc0aaic4760f64f2e78956@mail.gmail.com>
Date:	Thu, 10 Jul 2008 14:34:30 -0700
From:	"John Heffner" <johnwheffner@...il.com>
To:	"Rick Jones" <rick.jones2@...com>
Cc:	"Dan Noé" <dnoe@...ebrokerage.com>,
	netdev@...r.kernel.org
Subject: Re: Detecting TCP loss on the receiving side?

On Thu, Jul 10, 2008 at 2:20 PM, Rick Jones <rick.jones2@...com> wrote:
> John Heffner wrote:
>>
>> Looking for loss at the receiver is a bit tricky.  It doesn't look
>> like struct tcp_info has enough information to do this easily.  If you
>> are able to install a custom kernel on this machine, the Web100 patch
>> would be able to gather enough information to figure it out.  The
>> basic idea would be to look for a difference between RcvNxt and
>> RcvMax.
>
> And even then it depends on the connections having multiple segments in
> flight at one time.  Although I suppose that cuts both ways and affects the
> tracing too, but perhaps not to the same extent.
>
> Dan - seeing "brokerage" in your email and worries about latency makes me
> think that your app(s) are pushing around lots of small messages - are those
> spread-out across lots of connections, or are they consolidated into a
> rather smaller number of connections?  Also, what is the magnitude of the
> latency in these latency events?


Yes, a very slow rate makes things trickier, but transmitting less
than one segment per minRTO (200 ms) seems pretty unlikely.
Otherwise, the receiver should observe reordering.

  -John
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