[tcpm] rfc2581bis [2]: limiting cwnd inflation

Mark Allman <mallman@icir.org> Tue, 13 February 2007 15:06 UTC

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From: Mark Allman <mallman@icir.org>
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Subject: [tcpm] rfc2581bis [2]: limiting cwnd inflation
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Reminder, the draft should pop out of the usual places soon, but until
then you can slurp a copy from:


The second issue ...

We added a note to step 4 of the fast retransmit/recovery procedure:

    4.  For each additional duplicate ACK received (after the third),
        cwnd MUST be incremented by SMSS.  This artificially inflates
        the congestion window in order to reflect the additional segment
        that has left the network.
        Note: [SCWA99] discusses a receiver-based attack whereby many
        bogus duplicate ACKs are sent to the data sender in order to
        artificially inflate cwnd and cause a higher than appropriate
        sending rate to be used.  A TCP MAY therefore limit the number
        of times cwnd is artificially inflated during loss recovery
        to the number of outstanding segments (or, an approximation

That is, we explicitly allow a TCP to limit the number of dupacks that
are acceptable during fast recovery to prevent an attack such as
discussed in [SCWA99].

Note that (1) this is more conservative than the algorithm in 2581 and
so implicitly allowed already (that is, we are just calling attention to
this) and (2) we are not trying to mandate this behavior, just noting
that it might be useful.

Comments would be appreciated.