Re: [PATCH 0/3][v2] tcp: fix ICMP-RTO war

Damian Lukowski
2010-02-08T21:58:44Z

Damian Lukowski schrieb:
> Am 01.02.2010, 08:33 Uhr, schrieb David Miller <davem@davemloft.net>:
>
>> From: Damian Lukowski <damian@tvk.rwth-aachen.de>
>> Date: Fri, 29 Jan 2010 23:15:51 +0100
>>
>>> This patches fix the current RTO calculation routine, when
>>> srtt and rttvar are zero, yielding an RTO of zero
>>> Under some circumstances, TCPs srtt and rttvar are zero,
>>> yielding a calculated RTO of zero.
>>> This is particularly unfortunate for ICMP based RTO recalculation
>>> as introduced in f1ecd5d9e736660 (Revert Backoff [v3]: Revert RTO
>>> on ICMP destination unreachable), as it results in RTO retransmission
>>> flooding.
>>>
>>> Thanks to Ilpo Jarvinen for providing debug patches and to
>>> Denys Fedoryshchenko for reporting and testing.
>>>
>>> Signed-off-by: Damian Lukowski <damian@tvk.rwth-aachen.de>
>>
>> I still haven't seen a detailed enough analysis of why these
>> tiny RTOs can come to exist in the first place.
>>
>> Please show me a list of events, function by function, the value of
>> relevant variables and per-socket TCP state, in the TCP stack, that
>> show how this ends up happening.
>>
>> Thanks for all of your work on this so far.
>
> I might have figured it out, but could not verify it, so maybe you can
> comment my thought.
>
> When a listening TCP receives a SYN, it will send a SYN+ACK
> and wait for an ACK to complete the handshake.
> Look at tcp_rcv_state_process::step 5::case SYN_RECV::acceptable
> and the code after the comment "tcp_ack considers this ACK as duplicate
> and does not calculate rtt".
>
> If the connecting client has disabled timestamps, the rtt statistics
> won't be updated here, while the state is changed above.
> I printk'ed at the very end of TCP_SYN_RECV and got the following:
> state 1 (ESTABLISHED), srtt 0, rttvar 0.
>
> So my suspicion is: If connectivity breaks right after a listening TCP
> has completed the handshake without timestamps, and the listening TCP
> sends data after establishing the connection, we will get the observed
> behaviour.

Just verified that by dropping pure ACKs coming from the originally
listening TCP using iptables.

Regards
Damian