Re: detection of the third tcp packet in a tcp connection setup

[Ramin Alidousti <ramin@cannon.eng.us.uu.net>]

On Thu, Jun 27, 2002 at 08:44:03PM +0100, Antony Stone wrote:

> On Thursday 27 June 2002 8:44 pm, Patrick Schaaf wrote:
> 
> > > > There are some distinguishing characteristics... it is the first packet
> > > > sent by the client that is in state ESTABLISHED.  it should have ACK
> > > > set and no other flags.  the tcp data length should be zero.
> > >
> > > Isn't that in itself a bit of a giveaway ?   I can't think of a reason
> > > why a zero-length packet should ever occur in the remainder of the data
> > > stream... ?
> >
> > How to TCP keepalive packets look like?
> 
> Hmmm.   Don't know.   Hadn't thought about those...
> 
> > Also, isn't it possible that the third packet already carries data, in the
> > general (read TCP protocol as it is written) case?
> 
> Well, I'd always thought that this was allowed, yes, but I've also been told 
> by several different people (who play with real-world networks all the time) 
> that it never happens in practice - you get:
> 

If I'm not wrong you can always send data even in SYN and SYN/ACK.
Like Patrick said the default options in the socket interface might
not do it but I thought I saw some tweaking in one of Richard Steven's
(god bless his soul) books to push traffic in SYN packets.

Ramin

> SYN (no data)
> SYN/ACK (no data)
> ACK (no data)
> ACK (data)
> ACK (data)
> etc.....