Re: [tcpm] SYN in SYN_RCVD state

"Vishal Study" <vishal.study@gmail.com> Mon, 22 October 2007 19:39 UTC

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Date: Mon, 22 Oct 2007 12:37:23 -0700
From: "Vishal Study" <vishal.study@gmail.com>
To: "Ted Faber" <faber@isi.edu>
Subject: Re: [tcpm] SYN in SYN_RCVD state
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Hi Ted:

The server would retransmit SYN+ACK when server times out in SYN-RECEIVED state.
I agree and have seen this.

But, when the server hasn't timed out in SYN-RECEIVED state and it
receives retransmitted SYN from client (due to client's timeout), what
is the expected behavior on server side? Is server supposed to send
SYN+ACK?

Thanks!

On 10/22/07, Ted Faber <faber@isi.edu> wrote:
> On Sun, Oct 21, 2007 at 04:25:25AM -0700, Vishal Study wrote:
> > > Vishal Study ha scritto:
> > > > Consider the following scenario:
> > > >
> > > > 1. TCP client sends SYN to server; server goes to SYN_SENT state
> > > >
> > > > 2. server responds with SYN+ACK;  server goes to SYN_RCVD state
> > > > but SYN+ACK sent to client is lost somewhere in the network.
> > > >
> > > > 3. client re-sends SYN on its timeout (couple of seconds later) to server
> > > >
> > > > What should be the server behavior? Should it retransmit SYN+ACK or
> > > > should it send a RST?
> > > >
> > But RFC 793, pg 71 seem to say that the server should send out a RST
> > if SYN is rx in SYN-RCVD state (assuming SYN seq number is within the
> > window, which is true in the example I had mentioned).
> >
> > Any thoughts on this?
>
> I think that because RCV.NEXT was set to SEG.SEQ+1 when the "server"
> entered SYN-RECEIVED state (as documented on page 66 of 793) the resent
> SYN is outside the window and will be acked.  The "server's" SYN+ACK
> will be resent if it times out.
>
> --
> Ted Faber
> http://www.isi.edu/~faber           PGP: http://www.isi.edu/~faber/pubkeys.asc
> Unexpected attachment on this mail? See http://www.isi.edu/~faber/FAQ.html#SIG
>
>


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