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

From: Sally Floyd <[email-masked]>
Date: Tue, 30 Oct 2001 16:03:39 -0800

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

That is explicitly allowed in the ECN standard.  And I would
recommend it.  This was motivated by the unfortunate presence of
firewalls and load-balancers in the network that either drop ECN
SYN packets or respond with a Reset.

Section 6.1.1.1 of RFC 3168.  Ah, the joys of dealing with the
real world...

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

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From: Joe Touch <[email-masked]>
Date: Tue, 30 Oct 2001 16:46:34 -0800

> Section 6.1.1.1 of RFC 3168.  Ah, the joys of dealing with the

Is there anything preventing sending both SYNs at the same time?
(that way the timeout could be avoided, and the unused connection could 
be RST).

Joe

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From: Alan Cox <[email-masked]>
Date: Wed, 31 Oct 2001 01:08:50 +0000 (GMT)

> (that way the timeout could be avoided, and the unused connection could 

Servers that create sessions and piggy back data onto the syn|ack might not
be too happy. Also a lot of firewalls will drop the RST.

Another question to ask perhaps the important one - for a typical connection
does the overhead of building two connections and shooting one exceed the
gain from using ECN in the first place. 

Alan

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From: David Nicol <[email-masked]>
Date: Tue, 30 Oct 2001 19:36:31 -0600

Sally Floyd wrote, from the real world :

> recommend it.

Yet it does not appear to happen with linux ECN :(  perhaps by
continuing to suggest this someone else will write the patch :)

Alan Cox wrote, from a position of omniscience:

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

Yet another purpose for the (proposed)per-peer IP data cache, along
with preferred window size

> (that way the timeout could be avoided, and the unused connection could

how do you differentiate this from opening two channels at once?
Oh, obviously, they have the same local port number, so it would look
like a duplicated SYN to the non-initiated

> gain from using ECN in the first place. 

we're not building a second connection, we're sending a duplicate SYN
packet with different option bits.

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