Subject: [tcpm] Congestion control in face of ICMP unreachable messages

From: Daniel Schaffrath <[email-masked]>
Date: Fri, 17 Aug 2007 18:06:55 +0200

Dear Community,

I was wondering if there are any recommendations regarding congestion  
control in face of ICMP (host/network) unreachable messages. If I am  
not mistaken there is nothing in the various TCPM documents... nor in  
any RFC.

Maybe you have any pointers?

Thank you,
Daniel Schaffrath

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From: Ted Faber <[email-masked]>
Date: Fri, 17 Aug 2007 09:25:42 -0700

> Maybe you have any pointers?

Sure.  If the code in the destination unreachable message is 2-4 abort
the connection.  Otherwise, don't.  RFC 1122.

I wouldn't use ICMP reachability information in congestion control,
and I don't believe that any standard requires an implementer to do so.
If packets aren't being delivered, the sending endpoint(s) will slow
down pretty dramatically, pretty quickly without intervention.  A
retransmisson timeout puts the endpoint into slow start.

You're still technically required to respond to an ICMP source quench,
but I personally wouldn't.

-- 
Ted Faber
http://www.isi.edu/~faber           PGP: http://www.isi.edu/~faber/pubkeys.asc
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From: Joe Touch <[email-masked]>
Date: Fri, 17 Aug 2007 09:41:05 -0700

Daniel,

AFAIK, congestion control response to in-band messages (ACKs) and
timeouts (implied loss, as TCP interprets it), as well as to some new
fields (ECN, etc.) which are carried in the same packets.

Reactions to external congestion signals - ICMP source quench (which has
been informally deprecated a while ago, but has not been documented as
such yet) or otherwise - would constitute another opportunity for a DOS
attack, and seem like a bad idea to encourage.

Joe

Daniel Schaffrath wrote:
> control in face of ICMP (host/network) unreachable messages. If I am not
> mistaken there is nothing in the various TCPM documents... nor in any RFC.
>