On Tue, Sep 15, 1998 at 10:23:10PM -0700, David S. Miller wrote:
> Setting the initial mdev to m<<1 is a bug in the BSD code which we
> just copied blindly in 2.0.x, the calculation in 2.1.x (m<<2) is the
> correct one, check the RFCs :-). I believe this is mentioned in
> Steven's books and the tcp-impl "known TCP implementation problems"
> draft, amongst other places.
The current code does
tp->srtt = m<<3; /* take the measured time to be rtt */
tp->mdev = m<<2; /* make sure rto = 3*rtt */
...
tp->rto = (tp->srtt >> 3) + tp->mdev;
tp->rto += (tp->rto >> 2) + cwnd_stuff
where m is a round trip time for a single packet.
So we initialize our RTO to (m + 4m) * (1+1/4+C)
i.e. at least to 25/4 m (about 6m).
The value seems to be fairly large and contradicts to the comments.
I haven't look though all RFCs thoroughly. The only point to
mdev initialization I remembered was a phrase in RFC1122
that mdev should be initialized so that RTO be 3 sec.
I have no arguments for a certain initial value of RTO.
If 25/4 of the initial RTT is a design decision rather than a misprint
I'm quite satisfied.
Best wishes
Andrey V.
Savochkin
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