Subject: Re: [Tsvwg] Question on Limited Transmit (RFC3042)

From: Sally Floyd <[email-masked]>
Date: Tue, 19 Nov 2002 13:47:40 -0800

>I remain of the opinion that Early Retransmit is a useful idea and
>it could certainly be useful.

I agree.  I think it needs a document making the worst-case behavior
quite clear, and arguing why this worst-case example is sufficiently
unrealistic that it is (or is not) an acceptable risk.

Something like the following:

* Worst-case scenario:  
- All transfers on this link are from two-packet TCP connections
(because the web server only has this one two-packet page:).
- All pairs of packets are reordered in the network.
- As a result, all connections get the (S)ACK for the second packet before 
the ACK for the first packet, invoke Early Retransmit, and
unnecessarily retransmit the first packet.
- The result is that one-third of the bandwidth on the
link in question is wasted on unnecessary retransmissions.

* Argument why this worst-case scenario is an OK risk:
- There are very few (if any) links where most of the packets
are from a data-limited connection in where the sender does not
have a new packet that it wants to send.
- Thus, even with reordering of all packet pairs, there are very
few links where this would lead to anywhere close to one-third
of the link bandwidth being wasted on unnecessary retransmissions.
- There are transport-level mechanisms to limit the use of Early
Retransmit within a single data-limited connection, so that a single
connection does not repeatedly use Early Retransmit to unnecessarily
retransmit a packet.

* Unacceptable arguments:
- My own view would be that it is unacceptable to argue that networks
that reorder packet pairs are rare, and that therefore it is ok to
have bad behavior in this case.  I think that the goal should be a
robust transport protocol even in a network that reorders packet pairs.

- Sally
http://www.icir.org/floyd/

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From: Marco Mellia <[email-masked]>
Date: Wed, 27 Nov 2002 12:03:06 +0100

Dear Mark, Sally,
    as you might remember, a few months ago we submitted a draft that
addressed, trying to alleviate it, the RTO penalty for TCP
connections in the "small window" regime.

It was available as "draft-mellia-tsvwg-tcp-smartframing-00.txt"

The idea is to exploit a smart segmentation (TCP-Smart Framing is indeed 
the name of the proposal) algorithm, so that even if the cwnd is smaller 
than 4, you get the chance to receive enough dup-acks to trigger a fast 
retransmit.
This avoids RTO expiration, while remaining robust to packet reordering.

There was some discussion in this mailing lists, but was never presented 
at a  IETF meeting, and we didn't update the draft.

We were just wondering if this idea might take part in the ongoing
discussion.

Ciao, Marco, Michela and Claudio

P.S. A paper about TCP-Smart Framing was also presented at IEEE GLOBECOM 
2001 in San Antonio -