Subject: Re: [tcpm] small bit on ecnsyn-00 and syn cookies

From: Sally Floyd <[email-masked]>
Date: Mon, 20 Mar 2006 09:20:55 -0800

>Just a slight bit more discussion of syn cookies is required in the
>ecnsyn draft as it stands.

Many thanks for the email.  Clearly the draft needs to be clarified.

I don't think it is necessary to incorporate the ECN-capable bit into the
cookie, at least for using ECN for the SYN/ACK packet.

* When host B sees an incoming SYN trying to negotiate ECN, it can send
  the syn-ack as ECN-capable.

* If host B gets back an ACK for the SYN/ACK reporting ECN-Echo, then it 
  has to respond appropriately (i.e., set cwnd to one packet, maybe wait
  for a timer, and set CWR on the first outgoing data packet.)

Neither of these require much by way of maintaining state.

I will add a discussion of this to the syn-ack draft.  If you wanted
to add something to the draft-eddy-syn-flood-01.txt, that would be
good also.

In general, here are the places where state is required for using ECN:

* How long to send ECN-Echo:
If host B receives packets marked as CE (Congestion Experienced),
then it has to respond with ECN-Echo, and continuing sending ECN-Echo
until it receives CWR (Congestion Window Reduced).

* Whether to send data packets as ECN-Capable?
Host B also has to know that it is ok to send data packets as
ECN-Capable, if it is going to do so.  So it has to remember that
the other side has agreed to do ECN.

- Sally

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