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 09:44:07PM +0200, 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? Also, isn't it possible that the
> third packet already carries data, in the general (read TCP protocol as
> it is written) case? You probably won't get that with the normal socket
> interface from userlevel, but does TCP forbid it? I don't think so.

Correct. And besides when you receive data and have nothing to send
you'll ack with a zero-length packet.

Ramin

> 
> best regards
>   Patrick