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

On 2007/08/17  , at 18:25, Ted Faber wrote:

> On Fri, Aug 17, 2007 at 06:06:55PM +0200, Daniel Schaffrath wrote:
>> 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?
>
> Sure.  If the code in the destination unreachable message is 2-4 abort
> the connection.  Otherwise, don't.  RFC 1122.
Ok... however, aborting a connection seems somewhat a bit too harsh  
congestion control to me :)

> 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.
Yes. And that exactly what I am after. Don't you think that this a  
bit to conservative? Especially if you consider exponential back-off.  
I mean, if TCP retransmits after RTO and gets no reply, this  
indicates severe congestion and back-off is fine. But if it gets an  
ICMP unreachable message again, this might allow a less conservative  
behavior?

Another thing is that RTO needs to expire to reach slow-start  
recovery. If my understanding of RFC 1122 is correct, it's still  
standard compliant to continue sending after having received the  
first appropriate ICMP message. Maybe that's a bit too aggressive (as  
opposed to slow-start recovery after an "ICMP induced" RTO).

Thank you,
Daniel

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