Re: [tcpm] Extending RFC 1323 window scaling?

>   (1) The semantics of the SEG.WND field in the TCP header could be
>       changed, i. e., the receive window is scaled in <SYN,ACK>
>       segments under some constraints. Whether or not the receive
>       window is scaled could be announced explicitly by a flag, or
>       implicitly, e. g., due to the presence of Quick-Start options or
>       other options. However, changing the semantics of the receive
>       window header field creates interoperation issues with hosts
>       that do not implement such a modification.
> 
>   (2) The true amount of free buffer space is announced somehow, and
>       overwrites the (unscaled) value in the TCP header, which remains
>       unscaled in order to be compatible with hosts that do not
>       implement this mechanism. This requires some new mechanism to
>       signal a scaled receive window in <SYN,ACK> segments.
> 
> The proposed new TCP option is a straightforward realization of
> approach (2).

I am not sure if we need a (2).  As you note, Quick-Start is currently
the only thing that really runs into a problem and maybe something else
could.  To be precise, I think we can say that the only place where this
is a problem is when the initiator of a connection wants to (and can)
send more than 64KB in the first RTT of the connection.  Unless we
pretty drastically change the rules it seems that this is going to have
to be done only with some explicit communication and so using that as an
implicit signal seems fine to me.  (We clearly botched this in terms of
Quick-Start because we were too dumb to think of it and you found it
after publication.)  Further, it is not clear that we need to add new
functionality like this for something experimental (Quick-Start).  But,
maybe ...

If I were going to take approach (2) I would suggest a two-byte option
that indicates the value in the standard TCP header is to be scaled.
Why add another 2 byte window value when one of them is going to be
ignored?  This fails safe because if a host doesn't understand the new
("the window is scaled") option it will think the advertised window is
unscaled and so less than it really is and so there is no danger in
running into problems.

allman