suggesting to increase timeout_req default to 7 seconds

[Nils Goroll]

Hi,

some time has passed since my initial email regarding this suggestion and it
still holds.

Unless there is a strong argument against it, I think we really should increase
the default timeout_req to 7 seconds. I think the argumentation for this value
is sound and I haven't found any reasons against it.

Please keep this suggestion separate from the suggestion to re-introduce
SO_LINGER. I still need to do production system tests with it.

Nils

On 26/02/15 11:27, Nils Goroll wrote:
> This tcpdump output illustrates an issue we seem to have with default Linux tcp
> timeouts and the default timeout_req of 2 seconds:
> 
> 16:47:44.542049 IP client.49550 > varnish.80: Flags [S], seq 29295818, win 4380,
> options [mss 1460,sackOK,eol], length 0
> 16:47:44.542080 IP varnish.80 > client.49550: Flags [S.], seq 3652568857, ack
> 29295819, win 29200, options [mss 1460,nop,nop,sackOK], length 0
> 16:47:44.542250 IP client.49550 > varnish.80: Flags [.], ack 1, win 4380, length 0
> 16:47:46.080501 IP client.49550 > varnish.80: Flags [P.], seq 1:1453, ack 1, win
> 4380, length 1452
> 16:47:46.080528 IP varnish.80 > client.49550: Flags [.], ack 1453, win 31944,
> length 0
> 16:47:48.082783 IP varnish.80 > client.49550: Flags [F.], seq 1, ack 1453, win
> 31944, length 0
> 16:47:48.083070 IP client.49550 > varnish.80: Flags [.], ack 2, win 4380, length 0
> 16:47:48.350763 IP client.49550 > varnish.80: Flags [P.], seq 1453:2905, ack 2,
> win 4380, length 1452
> 16:47:48.350792 IP varnish.80 > client.49550: Flags [R], seq 3652568859, win 0,
> length 0
> 
> The packet at 16:47:46.080501 contains the first part of a request up to the
> start of a very long cookie line.
> 
> At 16:47:48 varnish closes after reaching timeout_req of 2s. Then, the client
> immediately acks.
> 
> My understanding is that the varnish->client ack 1453 got lost and the client
> did not get around to retransmit seq 1:1453 before we timed out.
> 
> 
> The most helpful online reference regarding recommended initial tcp
> retransmittion timeouts I have found so far is
> http://tools.ietf.org/html/rfc6298#ref-PA00
> 
> In summary, an initial timeout (RTO) of 1s is now recommended, but the former 3s
> RTO remains valid. So, for any client following the former 3s recommendation,
> current we don't even tolerate a single packet retransmission after 3way is
> complete. For those clients following the new 1s recommended RTO, timing is also
> really tight it seems unlikely that we tolerate retransmission of two packets.
> 
> Based on this, I'd suggest to raise the default timeout_req to 7 seconds to
> allow for two retransmissions at RTO=3.
> 
> This seems to be particularly relevant with the growing popularity of mobile
> clients.
> 
> The risk is increased resource usage for malicious requests. To address it, I'd
> suggest to document that lowering timeout_req can be an option to mitigate
> certain DoS (slowloris) attacks.
> 
> 
> Nils
> 
> 
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