Re: [PATCH 1/1] tcp: Wrong timeout for SYN segments

From: "H.K. Jerry Chu" <hkjerry.chu@gmail.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Alex Bergmann <alex@linlab.net>,
	davem@davemloft.net, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org, Jerry Chu <hkchu@google.com>,
	Neal Cardwell <ncardwell@google.com>,
	Nandita Dukkipati <nanditad@google.com>
Subject: Re: [PATCH 1/1] tcp: Wrong timeout for SYN segments
Date: Wed, 22 Aug 2012 10:29:26 -0700	[thread overview]
Message-ID: <CAFbMe2N-Ag_tUnHCr1=cJJmC4FTfXVJumn2Bk-oVfxuBoSCGrQ@mail.gmail.com> (raw)
In-Reply-To: <1345629806.5158.933.camel@edumazet-glaptop>

On Wed, Aug 22, 2012 at 3:03 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> On Wed, 2012-08-22 at 12:00 +0200, Eric Dumazet wrote:
>> On Wed, 2012-08-22 at 11:29 +0200, Alex Bergmann wrote:
>>
>> > Actual 6 SYN frames are sent. The initial one and 5 retries.
>> >
>>
>> first one had a t0 + 0 delay. How can it count ???
>>
>> > The kernel is waiting another 32 seconds for a SYN+ACK and then gives
>> > the ETIMEDOUT back to userspace.
>> >
>> > Do you mean that we have to send another SYN packet after the 3 minutes?
>> >
>>
>> First SYN is not a retransmit
>>
>> R2 = time_of_last_SYN - time_of_initial_SYN (t0) = 31
>>
>> If you read RFC it states :
>>
>> "In particular, R2 for a SYN segment MUST
>>  be set large enough to provide retransmission of the segment
>>  for at least 3 minutes."
>>
>>
>> That means that the last _retransmit_ MUST happen after 180 seconds.
>>
>> And not :
>>
>> Send all the restransmits at t0 + 1, then wait 180 seconds before giving
>> connect() a timeout indication.
>>
>>
>
> Therefore, the minimal connect() timeout should be : 180 + 100 seconds
>
> (allowing 100 seconds for the SYNACKs sent in answer of the very last
> retransmit to come back)
>
> (100 seconds is the R2 for non SYN frames)
>
> RFC quote : The value of R2 SHOULD
>             correspond to at least 100 seconds.

I agree if you take RFC1122 literally the last retransmission must
happen no less than 3 minutes from the 1st SYN... Oh actually it'd be
3 minutes plus initRTO because the 3 minutes applies only to
"retransmission" as in

"R2 for a SYN segment MUST be set large enough to provide retransmission
of the segment for at least 3 minutes.:

But IMHO 6 retries providing 1+2+4+8+16+32 = 63 secs retransmission plus
64 secs wait time totaling 127 secs is really plenty enough.

You have a good point on SYN-ACK.

Jerry

>
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