Re: [aqm] Updated draft-ietf-aqm-ecn-benefits - comments still welcome

gorry@erg.abdn.ac.uk Thu, 19 March 2015 07:55 UTC

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Date: Thu, 19 Mar 2015 07:54:32 -0000
From: gorry@erg.abdn.ac.uk
To: "John Leslie" <john@jlc.net>
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Cc: Gorry Fairhurst <gorry@erg.abdn.ac.uk>, Dave Taht <dave.taht@gmail.com>, "aqm@ietf.org" <aqm@ietf.org>
Subject: Re: [aqm] Updated draft-ietf-aqm-ecn-benefits - comments still welcome
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Thanks Dave for reading this ID and providing your comments. It's really
good to explore what may be missing.

> Dave Taht <dave.taht@gmail.com> wrote:
>>
>> section 6 addition. (could use more verbiage)
>>
>> 6.3 "An AQM that is ECN aware MUST have overload protection.
>
>    I fear I cannot discern what you mean this to say. :^(
>
>> It is trivial for a malbehaved application/worm/bot to mark all
>> its packets with ECN and thus gain priority over other traffic
>> not ecn marked.
>
>    This somewhat-paranoid claim rests on several assumptions that I
> hope we will recommend against.
>
> - the most obvious is an assumption that a tail-drop node will mark
>   _instead_ of dropping ECN-capable packets. This is not actually
>   possible, and I hope we will strongly deprecate it. Tail-drop should
>   drop packets regardless of ECN bits.
>
> - there is also an assumption that an ECN-capable transport can mark
>   its packets as ECN-capable and then never reduce its sending rate.
>   I suppose it could; but not-ECN-capable transports can also never
>   reduce the sending rate. :^( And the not-ECN-capable transports
>   could accomplish the same reduction in "lost" packets by FEC.
>
>    I believe we are going to "suggest" a lower marking threshhold for
> ECN-capable packets than the dropping threshhold for not-ECN-capable
> packets at AQM-capable nodes. This should reduce the paranoia level,
> I hope, since the ECN-capable flows will get congestion signals when
> not-ECN-capable packets are _not_ being dropped.
>
>    We should concentrate our efforts on providing useful signals:
> that some transports might make poor use of these signals is beyond
> our scope.
>
I understand that router overload needs to be considered in the design of
an  AQM algorithm, but I inclined to think there is not much say to
application designers, and that this need may have been said said in the
AQM Recommendations document. Agreeing with John, I don't see this as the
place to start putting detail on how routers implement AQM.

>> 6.4 Enabling ECN at the application layer requires access to the IP
>>     header fields, which are usually abstracted out completely at the
>>     tcp layer, and hard to access from udp with multiple non-portable
>>     methods to do so.
>
>    Yes, there are TCP stacks which are ECN-unfriendly; but there are
> enough _today_ which are friendly to ECN.
>
I also agree with what you say - although, again I'm not sure we need to
add this here, I think the design of transports is really the topic of
RFC5405.bis,

>>     ECN over UDP in new applications such as webrtc and Quic has
>>     great potential for many other applications, however the same
>>     care of design that went into ECN on TCP needs to go into
>>     future UDP based protocols.
>
>    I wouldn't disagree; but those issues are essentially-solved
> problems today.
>
>> Some other section that may end up here?
>>
>> ECN marking other sorts of flows (example routing packets) that have a
>> higher priority than other flows on link-local packets may be of benefit
>> with wider availability of aqm technologies that are ecn aware...
>
 I'm not sure I understand what you are suggesting with respect to ECN.

>    I suppose there might be _some_ use for ECN on routing packets; but
> I doubt this is desirable today. ECN is not-at-all about getting a
> higher priority -- it's about getting congestion signals without
> packet loss.
>
I think the IETF would normally recommend diffserv priority marking for
network control traffic.

> --
> John Leslie <john@jlc.net>
>

Gorry