netdev - Re: TCP socket receives strange packet

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Date:	Tue, 14 Oct 2014 11:40:44 -0400
From:	John Heffner <johnwheffner@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"Yurij M. Plotnikov" <Yurij.Plotnikov@...etlabs.ru>,
	Netdev <netdev@...r.kernel.org>,
	"Alexandra N. Kossovsky" <Alexandra.Kossovsky@...etlabs.ru>
Subject: Re: TCP socket receives strange packet

On Tue, Oct 14, 2014 at 11:00 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2014-10-14 at 18:09 +0400, Yurij M. Plotnikov wrote:
>> Connected TCP socket receives packet without timestamps option which
>> exists in SYN, SYNACK and ACK. It is packet 4 in attached tcpdump output.
>>
>> tcpdump output description: The host has address 10.208.10.1 (server)
>> and the peer host has address 10.208.10.2 (client).
>>
>> Establishing connection: Timestamps option exists in SYN, SYNACK and ACK
>> (packets 1, 2 and 3 in attached file), so accepted socket should receive
>> packets only with timestamps option.
>
> Can you point the RFC paragraph stating so ?
>
> I have wondering if this behavior was correct some time ago, and could
> not find a definitive answer.
>
> RFC 1323 4.2.1 seems to suggest it is valid to accept a segment without
> TS.
>
> R1) If there is a Timestamps option in the arriving segment...
>
>
>  There is no : Else drop the segment.


I can't think of a good reason to drop unless you're trying to use the
timestamp fields as extra security against off-path injection attacks.
  (It doesn't currently help much for that.)

  -John
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