Subject: Re: Copy of reserved bits from SYN into SYN-ACK

From: Sally Floyd <[email-masked]>
Date: Tue, 30 Oct 2001 13:57:29 -0800

Joe -

>Notably about ECN - ECN requires that a non-ECN receiver respond with 
>all-zeroes ECN bits. Copying the bits on the return path is a violation 
>of ECN -
>
>(Page 14):
>  * If a host has received an ECN-setup SYN packet, then it MAY send
>         an ECN-setup SYN-ACK packet.  Otherwise, it MUST NOT send an
>         ECN-setup SYN-ACK packet.

Actually, the ECN spec does not say anything one way or another
about copying the bits of the TCP Reserved field from the SYN into
the SYN-ACK.  It just makes sure that its negotiation of ECN-capability
is robust even in the presence of TCP receivers that do just copy
the bits.  ECN does *not* require that a non-ECN receiver respond with
all-zeroes ECN bits.

That is, ECN took some care to make sure that the ECN bits set from
the Reserved field in the ECN-setup SYN-ACK packet are not the same
as the ECN bits set in the ECN-setup SYN packet.  So, if the receiver
just copies the bits from the Reserved field from the SYN packet
into the SYN-ACK packet, this is not interpreted by the TCP sender
as negotiating ECN.

I would say that if you have only a single bit, then copying that
bit from the SYN to the SYN-ACK packet is not an acceptably robust
way to negotiate something.

- Sally
http://www.aciri.org/floyd/

---

From: Joe Touch <[email-masked]>
Date: Tue, 30 Oct 2001 15:03:00 -0800

> into the SYN-ACK packet, this is not interpreted by the TCP sender

Thanks - missed that. Got lost in the 3-letter acronyms.

> way to negotiate something.

Yes - however, TCP did require that the bits be 0 on send, so...
  :-)

Joe

---

From: David Nicol <[email-masked]>
Date: Tue, 30 Oct 2001 17:30:52 -0600

Would a host responding to a timeout while waiting for an ECN SYN-ACK
by sending a non-ECN SYN break anything?

-- 
                                                  David Nicol 816.235.1187
there are no troops outside the gates of eden http://wmbr.mit.edu/wmbr.ram

---

From: Alan Cox <[email-masked]>
Date: Tue, 30 Oct 2001 23:52:26 +0000 (GMT)

> by sending a non-ECN SYN break anything?

Your user has already pressed "reload" five times by then. 

Alan