[Iccrg] Re: Why don't we stop treating ECN and loss similarly

[Ingemar Johansson S]

Hi

Late with the response so I don't include any earlier conversation in this email
Answer is, Yes. It is doable and sometime beneficial to treat ECN and implicit signals of congestion (delay, packet loss) differently. We explored the possibilities in 
http://www.tschofenig.priv.at/cc-workshop/irtf_iab-ccirtcpaper4.pdf 
The trick we did was to include the channel conditions into the ECN decision meaning that the cost per transmitted bit is included, this and the combination of only a limited decrease of the bitrate (~20%) gives a dramatically improved performance compared to delay based rate adaptation. ECN with cost per transmitted bit only makes sense with wireless access as it is only then the cost may differ between the terminals.

Problems ?
The studies that was done was targeted at operator controlled Video telephony (VoLTE), in which case the operator has more control of the whole transmission path including backhaul. It can become a bit more tricky with OTT services but there are probably solutions to this.
One can tune AQMs to trigger ECN earlier than drops in backhaul and thus open up for a differentiated behavior between ECN and loss response. The effect of this will be that the ECN enabled flows will have a higher congestion bitrate, which may give the false impression that ECN enabled flows are more aggressive. How does an operator deal with this ?
ConEx is one solution, the ConEx option use an L bit and a C bit which makes it possible for a ConEx based policer at the network ingress to police ECN based and loss based congestion differently, the policer has possibly some knowledge about the ECN marking behavior in the domain which the policer is set to protect.

A tough question is .. How is the end user made aware of the differentiated ECN and drop behavior?. 

/Ingemar
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Ingemar Johansson  M.Sc. 
Senior Researcher

Ericsson AB
Wireless Access Networks
Labratoriegränd 11
971 28, Luleå, Sweden
Phone +46-1071 43042
SMS/MMS +46-73 078 3289
ingemar.s.johansson at ericsson.com
www.ericsson.com

"However beautiful the strategy, 
 you should occasionally look at the results" 
        Sir Winston Churchill
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