[Jack Jackson <jack@xxxxxxxxxxxxxxx>]

At 05:43 PM 10/24/2005, ronnie sahlberg wrote:

On 10/25/05, Jack Jackson <jack@xxxxxxxxxxxxxxx> wrote:

Likewise I have never seen data sent

with SYN, which is also legal.

No this is not legal.   TTCP tried to experiment long time ago with this in order to  reduce the cost of latency for small shortlived transactions but issues were found with this approach (data inside the syn segment) which essentially killed TTCP off.

In TCP it is not legal to transfer data in the SYN segment.

Perhaps it has changed, but RFC 793 clearly allows data in SYN
segments.

 From Section 3.3

"For sequence number purposes, the SYN is considered to occur before
the first actual data octet of the segment in which it occurs, while the
FIN is considered  to occur after the last actual data octet in a
segment in which it occurs.  The segment length (SEG.LEN) includes
both data and sequence space occupying controls."

 From Section 3.4

"Several examples of connection initiation follow.  Although
these examples do not show connection synchronization using data-carrying
segments, this is perfectly legitimate, so long as the receiving TCP
doesn't deliver the data to the user until it is clear the data is valid
(i.e., the data must be buffered at the receiver until the connection
reaches the ESTABLISHED state)."

It is also discussed as legal in Douglas E. Comer's "Internetworking
with TCP/IP Vol 1" (13.24)