Subject: Re: Questions on TCP backlog
To: B. James Phillippe <>
From: der Mouse <mouse@Rodents.Montreal.QC.CA>
List: tech-net
Date: 10/20/2001 06:08:04
> 2.) IPv4 TCP ~ general connection request handling
> 	In any case, I'm trying to understand the mechanism used to
> 	queue new connection requests.  Specifically, I am interested
> 	to know the design WRT SYN flood type attacks against local
> 	sockets.  Is state tracked for SYN's, or is there some kind of
> 	mechanism similar to SYN cookies which obviates state tracking
> 	until the last stage of the 3WHS?

As I understand it, NetBSD tracks state for SYNs; someone put a good
deal of effort into it and managed to get the per-nascent-connection
overhead down to some insanely low figure - what I remember is 4 bytes

No stateless mechanism (like SYN cookies) can possibly conform to the
TCP spec, because SYN+ACKs will never be retransmitted (which, it's
fairly easy to see, can lead to connections wedging, so this is a
practical as well as strict-conformance problem.)

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