Subject: Re: Problem with tcp (2.6.31) as first, http://bugzilla.kernel.org/show_bug.cgi?id=14580

From: Eric Dumazet <[email-masked]>
Date: Fri, 27 Nov 2009 12:48:05 +0100

Ilpo Järvinen a écrit :

> What would you expect to happen? If out-of-window stuff arrives we send 
> dupacks. If we would send resets, that would introduce blind rst attacks.
> In theory we might be able to quench the loop by using pingpong thing but 
> that needs very careful thought in order to not introduce other problems,
> and even then your connections will not be re-usable until either end 
> times out so the gain is rather limited. We simply cannot rst the 
> connection, that's not an option.
> 
> I find this problem simply stem from the introduced loss of end-to-end 
> connectivity. Would you really "lose" that server so that its TCP state is 
> not maintained, you'd get resets etc (crash, scheduled reboot or 
> whatever). Only real solution would be a kill switch for TCP connection 
> when you break e-2-e connectivity (ie., switch servers so that the same IP 
> is reacquired by somebody else). In theory you can "simulate" the kill 
> switch by setting tcp_retries sysctls to small values to make the 
> connections to timeout much faster, but still that might not be enough for 
> you (and has other implications you might not like). 
>

RST is not an option, sure, but ACK storms are unlikely good things too.

Could'nt we do something smart in presence of tcp timestamps ?

11:23:27.669910 IP 192.168.20.110.3434 > 192.168.200.200.333: . ack 2457299512 win 92 <nop,nop,timestamp 42408589 1506086404>
11:23:27.669991 IP 192.168.200.200.333 > 192.168.20.110.3434: . ack 11687 win 91 <nop,nop,timestamp 1704614538 42406583>
11:23:27.670000 IP 192.168.20.110.3434 > 192.168.200.200.333: . ack 2457299512 win 92 <nop,nop,timestamp 42408589 1506086404>
11:23:27.670093 IP 192.168.200.200.333 > 192.168.20.110.3434: . ack 11687 win 91 <nop,nop,timestamp 1704614538 42406583>
11:23:27.670099 IP 192.168.20.110.3434 > 192.168.200.200.333: . ack 2457299512 win 92 <nop,nop,timestamp 42408589 1506086404>
11:23:27.670175 IP 192.168.200.200.333 > 192.168.20.110.3434: . ack 11687 win 91 <nop,nop,timestamp 1704614538 42406583>
11:23:27.670183 IP 192.168.20.110.3434 > 192.168.200.200.333: . ack 2457299512 win 92 <nop,nop,timestamp 42408589 1506086404>
11:23:27.670268 IP 192.168.200.200.333 > 192.168.20.110.3434: . ack 11687 win 91 <nop,nop,timestamp 1704614538 42406583>
11:23:27.670276 IP 192.168.20.110.3434 > 192.168.200.200.333: . ack 2457299512 win 92 <nop,nop,timestamp 42408589 1506086404>
11:23:27.670359 IP 192.168.200.200.333 > 192.168.20.110.3434: . ack 11687 win 91 <nop,nop,timestamp 1704614538 42406583>
11:23:27.670368 IP 192.168.20.110.3434 > 192.168.200.200.333: . ack 2457299512 win 92 <nop,nop,timestamp 42408589 1506086404>

Or we could 

Count number N of strange/bad acks we received from peer.

- At first one, send our ACK immediately

- For following, delay our ACK answer by N*100 ms, to reduce the flood.
(or if we have data in flight, only rely on retransmit timer and not sending acks)