Subject: Re: SYN cookie ?
To: Ignatios Souvatzis <>
From: Bill Sommerfeld <>
List: tech-kern
Date: 04/18/2001 16:11:58
> On Wed, Apr 18, 2001 at 10:57:51AM -0400, Bill Sommerfeld wrote:
> > Others (notably SSH, FTP, SMTP) involve the server speaking first and
> > the client waiting for a application-layer message before sending
> > anything -- and, for those protocols, if the ACK of the SYN-ACK is
> > lost, the clients will wait forever (or until the user or client loses
> > patience) for the server to retransmit.
> Ah... I think I understand, but... 
> this is something the end node would do, right? 

The end node doing the tcp passive open (aka, the server) would do

SYN cookies are misnamed, as they appear in the SYN-ACK and ACK of
SYN-ACK (packets 2 and 3 of the 3-way handshake).

They're implemented by the "server" (system doing the passive tcp
open), allegedly to allow the "server" to avoid keeping state.

The tcp protocol requires that the "server" retransmit the SYN-ACK,
and does not require the "client" to retransmit the ack-of-SYN-ACK.

					- Bill