Subject: [tcpm] simultaneous open with different options

From: Joe Touch <[email-masked]>
Date: Tue, 21 Oct 2014 18:30:41 -0700

Hi, all,

One of the questions that has kept coming up regarding SYN option
extension has to do with options that differ during simultaneous open.

IMO, it'd be useful to understand what happens in the current case
before assuming we can solve this case for extensions.

Here's the scenario

	host A			host B

	SYN w/X -> 		<- SYN w/Y

What happens next?

AFAICT, it depends on a few things:

- whether host A supports Y

- whether host B supports X

I.e., let's say that:

	host A supports Y

	host B does NOT support X

in that case, the next step would be:

	host A			host B

	SYN/ACK w/XY ->		<- SYN/ACK w/-

I.e., A supports both X and Y, and B supports neither.

At this point, the final ACK is critical to the completion of the
exchange, because it's where A finds out that neither option should be used:

	ACK w/- ->		<- ACK w/-

(these ACKs happen if both sides agree to skip the options they
requested. if not, then they drop the connection and send a RST)

Is there any agreement on this point?

Joe

---

From: Yuchung Cheng <[email-masked]>
Date: Fri, 24 Oct 2014 23:32:50 +0800

> - whether host B supports X
What existing option is like Y? i.e. not announce in SYN but
acknowledge its support if the remote peer announces it.

> _______________________________________________
> tcpm mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/tcpm

---

From: Joe Touch <[email-masked]>
Date: Fri, 24 Oct 2014 09:57:27 -0700

On 10/24/2014 8:32 AM, Yuchung Cheng wrote:
...
> acknowledge its support if the remote peer announces it.

AFAICT, TS and WS *can* work that way (at least).

Joe