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Re: Fwd: Re: [tcpm] FW: Call for Adoption: TCP Tuning for HTTP

From: Patrick McManus <mcmanus@ducksong.com>
Date: Fri, 4 Mar 2016 16:16:22 -0500
Message-ID: <CAOdDvNohs9TpLxMg-Vn-B=Ux2VumPL92hgqubARa7cgVinUmDg@mail.gmail.com>
To: Willy Tarreau <w@1wt.eu>
Cc: Patrick McManus <mcmanus@ducksong.com>, Joe Touch <touch@isi.edu>, HTTP Working Group <ietf-http-wg@w3.org>
To the extent this document can help promote efficient interop, rather than
just application tuning (and I acknowledge that is a blurry line), I think
it has promise as an IETF work and support adoption. I was skeptical of it
being able to play that role, but I think Wily has raised an interesting
point along those lines below. The document is clearly an -01 which needs
more exploration but it was nice of Daniel to put something forward.
(Daniel is afk for a bit - I'm sure we'll hear from him in the coming days.)

On Thu, Mar 3, 2016 at 5:01 PM, Willy Tarreau <w@1wt.eu> wrote:

> monitors). Many firewalls are tuned with aggressive FIN_WAIT/TIME_WAIT
> timeouts which cause their session to expire before the other side dares
> to retransmit. The server then remains for a long time in LAST_ACK state,
> resending this last ACK packet for some time before giving up.
>

Wily, thanks for the giant wall of text :). I have tried to distill it to
this, which I think is the essence of the discussion. I think the firewall
issue is one worth documenting for interop purposes - it can't know the end
state of receivers on either side so at least from a forwarding perspective
it should be configured permissively in an environment that anticipates low
TW. That seems like a valuable point to capture.


> But by this time, our nice client has already used all other ports and
> needs to reuse this port. Since its TIME_WAIT timeout was reduced to
> something lower than the server's LAST_ACK, it believes the port is
> free and reuses it. It sends a SYN which passes through the firewall,
> this SYN reaches the server which disagrees and sends an RST back
> (when the client picked a new SYN above the end of previous window)
> or an ACK which will generally be blocked by the firewall, or if the
> firewall accepts it, will be transmitted to the client which will then
> send an RST, wait one second and send the SYN again. As you can imagine,
> this dance is quite counter-productive for the performance since you
> convert hundreds of microseconds to multiples of seconds to establish
> certain connections.
>

This is interesting, and a bit different than the integrity issues an
established frame could inject normally associated with TW (which https can
protect against - at least in a deterministic fast fail sense).

on the one hand, we've gone from the client having a high rate of  fast
0RTT fails (blocked from initiating by TW).. to a situation where 3 things
are going on

1] an unquantified "large" fraction of quick successes. (no packet loss
impact, no state machine out of sync and no blockage by a timer)
2] a number of cases of fast 1RTT fail where a RST is received by the client
3] a fraction that may succeed or fail, but much more slowly due to retry
behavior with its set of lovely constants and backoffs

If I've got that (at least vaguely) right, there seem to be situational
tradeoffs as to whether that is 'better' or not. Sounds like good
discussion material in a tuning doc :)

Within the scope of "TCP for HTTP" would you say something different? (and
sure, I agree legacy TCP might not be the fun thing here.. but its the
topic at hand.)
Received on Friday, 4 March 2016 21:16:49 UTC

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