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

From: Jamshid Mahdavi <[email-masked]>
Date: 05 May 2000 09:53:27 -0700

The message below suggests a useful way of defining the maximum MSS
for an LTN.

Since the minimum RTO is defined in this I-D to be 1 second, it makes
sense that the packet transmission time on an LTN should be no more
than one second.  So the maximum MSS which should be used is BW*1sec.
For 9.6k, this means MSS no more than 1200 bytes.

By and large these are probably larger MSS than people would choose
anyway, but for very slow paths (esp. in the 1 kb/s range), a suitably
low MSS would help avoid the problem which Kacheong describes below.

--Jamshid

PS. There might be a refinement of this calculation which would yield
a higher MSS for very slow paths (based on the min pkt size of 40
bytes and the variance part of the computation) but my brain isn't
quite in high enough gear to sort this all out yet.

------- Start of forwarded message -------
Date: Thu, 4 May 2000 16:41:29 -0700 (PDT)
From: Kacheong Poon <[email-masked]>
Reply-To: Kacheong Poon <[email-masked]>
Subject: Re: [Tsvwg] Working Group last call for RTO draft
To: [email-masked]
Message-ID: <Roam.SIMC.2.0.6.957483689.5747.kcpoon@jurassic>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; CHARSET=US-ASCII

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?

							K. Poon.
							[email-masked]

_______________________________________________
tsvwg mailing list
[email-masked]
http://www1.ietf.org/mailman/listinfo/tsvwg
------- End of forwarded message -------