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List: dccp
Subject: Re: RFC 4342 Erratum: use Request/Response RTT
From: Eddie Kohler <kohler () cs ! ucla ! edu>
Date: 2007-02-08 1:20:54
Message-ID: 45CA7AF6.7050702 () cs ! ucla ! edu
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Hi Gerrit,
I don't actually completely understand where you're coming from. The initial
send rate is not 1 packet per second, it is 2-4 packets per RTT, as per
RFC4342. So the initial t_ipi is going to be s/X, where X = 2-4 packets per RTT.
If you follow the logic through RFC3448 4.2-4.4, you will see that due to this
line,
If (t_now - tld >= R)
X = max(min(2*X, 2*X_recv), s/R);
tld = t_now;
(where tld is initialized to -1 as regards section 4.2), the sending rate X
becomes 1 packet/RTT (= s/R) as soon as a round-trip time sample is available.
The erratum allows the implementer to sidestep all of this silliness by
simply starting at X = 2-4 packets/RTT, since the RTT sample is available
immediately.
Eddie
Gerrit Renker wrote:
> Hi Eddie,
>
> following a discussion with Ian, it became clear that the best way to fix
> some of the outstanding scheduling problems is to implement the Erratum to
> RFC 4342 and use the initial Request/Response handshake as basis for calculating t_ipi.
>
> I am referring to this posting (sorry the original was no longer in the inbox):
> http://www.mail-archive.com/dccp@vger.kernel.org/msg01319.html
>
> and was using the RFC 4340/2 erratum page:
> http://www.read.cs.ucla.edu/dccp/rfc4340errata
>
> Now, having spent almost half a day to come up with a solution for initial RTT
> estimates, I found that there are still several things open/unclear:
>
> * RFC 3448, 4.2 specifies an initial sending rate of 1 packet per second
> * RFC 3448, 4.6 specifies t_ipi = s/X => still gives 1 second
> * I can't find an override-clause in RFC 4342 for this
>
> Question: Is the aim to use section 4.2 of RFC 3448 and if yes, to what extent
> (this section has quite a few changes to other parts of the algorithm, too).
>
> Can you please clarify, maybe I have got the wrong erratum.
>
> It would be good to have this clear - so as to implement it.
>
>
> Gerrit
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