Subject: Re: [Tsvwg] Working Group last call for RTO draft

From: Mark Allman <[email-masked]>
Date: Fri, 05 May 2000 08:59:31 -0400

> One question about the draft.  In section 2,
> 
>     (2.2) When the first RTT measurement R is made, the host MUST set
> 
>               SRTT <- R
>               RTTVAR <- R/2
>               RTO <- SRTT + max (G, K*RTTVAR)
> 
>           where K = 4.
> 
> Suppose it is a long thin network, and the first R is measured
> using a small packet.  The next segment is in full MSS size.  But
> the network is so "thin" that the segment takes longer than 2*R to
> transmit to the other end, the above MUST will probably cause a
> timeout because RTO is 3*R (assuming 2*R > G) and another R is
> needed for the ACK to get back to sender.  Should the requirement
> be changed to something like "RTO MUST be set to a minimum of" the
> above calculation?

Two answers, I think...

(1) By "small packet" above do you mean the 3WHS packets?  I believe
    the I-D leaves out (i.e., as an implmentation detail) which set
    of segments you should use to take the first RTT measurement.
    So, rather than using the SYN-SYN/ACK exchange you could use the
    first data segment and the corresponding ACK, I think.  

    Of course, the first data segment could be small, as well, so...

(2) The intro of the I-D says...

      In some situations it may be beneficial for a TCP sender to be more
      conservative than the algorithms detailed in this document allow.
      However, a TCP MUST NOT be more aggressive than the following
      algorithms allow.

    I.e., you're always allowed to be more conservative than any
    given point in the remainder of the I-D allows.  (We use this
    statement in the intro rather than restating this in each point
    in the rest of the ID).

allman

---
http://roland.grc.nasa.gov/~mallman/

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