Subject: Re: [aqm] Question re draft-baker-aqm-recommendations recomendation #2

From: Wesley Eddy <[email-masked]>
Date: Wed, 01 May 2013 23:09:22 -0400

On 4/30/2013 6:17 PM, Linda Dunbar wrote:
> Wes, 
> 
> Question inserted below: 
> 
>> -----Original Message-----
>> """
>>    Hence, network communication to the host regarding the moderation of
>>    its traffic flow SHOULD use an AQM algorithm to determine which
>>    packets it should affect, and then implement that effect by marking
>>    ECN-capable traffic "Congestion Experienced (CE)" or dropping non-
>>    ECN-capable traffic.
> 
> 
> [Linda] Do you mean that downstream nodes treat packets with ECN bits marked differently than that packets whose ECN bits are not marked? 
> In another words, is "congestion experienced (CE)" encoded in the packets? 
> 

Hi Linda, that was actually a direct quote from Fred's document, so I
can't take credit for it ... however, all that it's saying is that
the congested queue set the CE bit (when ECT is set), per normal ECN
protocol.  It isn't saying anything new about the downstream nodes that
see CE has already been set.

-- 
Wes Eddy
MTI Systems

---

From: John Leslie <[email-masked]>
Date: Thu, 2 May 2013 12:43:58 -0400

Wesley Eddy <[email-masked]> wrote:
>> Do you mean that downstream nodes treat packets with ECN bits marked
>> differently than that packets whose ECN bits are not marked? 
>... all that it's saying is that the congested queue set the CE bit
> (when ECT is set), per normal ECN protocol. It isn't saying anything
> new about the downstream nodes that see CE has already been set.

   A downstream node _could_ treat the packet differently: it wouldn't
matter much for what we're interested in.

   Note that a downstream node could reasonably drop a packet already
CE-marked if that node _also_ is congested. It wouldn't actually help
IMHO if the flow containing the marked packet is well-behaved; but
there's really no way to tell whether the flow is well-behaved.

   With room for doubt, it's arguably right to drop it at an even
lighter level of congestion: in all reasonable cases of two or more
nodes congested on the path, we should expect another packet to come
along _not_ already CE-marked.

   (I'm not actually proposing this -- least of all proposing to
_standardize_ this; but perhaps it will help folks to think through
the issue.)

   In some future "perfect world", where CE can mark packets _before_
there is any reason to drop any, I might talk about standardizing
such a practice. ;^)

--
John Leslie <[email-masked]>