netdev - Re: TCP_MAXSEG option with TSO enabled

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Message-ID: <CAEnQRZB4sGP=6-Z_8G3P-BVo+wE_tbCdFUnTEgADd11XXBa6xw@mail.gmail.com>
Date:	Fri, 30 Mar 2012 21:07:57 +0300
From:	Daniel Baluta <daniel.baluta@...il.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	netdev@...r.kernel.org
Subject: Re: TCP_MAXSEG option with TSO enabled

On Fri, Mar 30, 2012 at 8:38 PM, Rick Jones <rick.jones2@...com> wrote:
> On 03/30/2012 04:33 AM, Daniel Baluta wrote:
>>
>> Hello,
>>
>> I am using TCP_MAXSEG option on an interface which has TSO enabled.
>>
>> strace output for connection looks as follows:
>>
>> socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 26
>> setsockopt(26, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
>> setsockopt(26, SOL_TCP, TCP_NODELAY, [1], 4) = 0
>> setsockopt(26, SOL_TCP, TCP_MAXSEG, [300], 4) = 0
>> connect(26, {sa_family=AF_INET, sin_port=htons(1935),
>> sin_addr=inet_addr("10.10.0.1")}, 16) = -1 EINPROGRESS (Operation now
>> in progress)
>>
>> I can see that MSS (300) is advertised in SYN packet, anyhow capturing
>> the traffic
>> I notice that are some TCP segments with length greater than MSS.
>>
>> Is this normal taking in consideration that TSO is enabled?
>
>
> I would think so.  What do the netstat stats suggest for segments and bytes
> per segment when you are running your test?  You could always look for an
> on-the-wire packet trace.
>
> Not sure that a trace on a receiver would give you that though - the inbound
> promiscuous tap may be above GRO (and almost certainly will be above LRO)

Thanks Rick. Indeed on-the-wire capture looks fine. I have TSO enabled
at sender and LRO at receiver, everything makes sense now.

thanks,
Daniel.
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