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Network WG                                                   James Polk
Internet-Draft                                          Toerless Eckert
Intended status: PS                                       Cisco Systems
Expires: January 15, 2014                                 July 15, 2013

      Differentiated Services Delay-and-Loss vs. Loss-Rate-Adaptive
                            Service Classes


   This document discusses how RTCWeb/RMCAT applications could best
   leverage existing and new DiffServ assignments and why we think it
   is necessary to differentiate the assignments of delay-and-loss-
   based vs. just-loss-based rate-adaptive video applications.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on January 15, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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1.  Introduction

   The current guidelines for DSCP assignments of traffic in the IETF
   is the informational RFC 4594 [RFC4594]. The TSV Working Group is
   currently reviewing how to update these assignments with one
   proposal being [ID-4594bis].

   This document discusses how RTCWeb/RMCAT applications could best
   leverage [RFC4594] & [ID-4594bis] assignments and why we think it is
   necessary to differentiate the assignments of video for these type
   of applications from non-rate adaptive video flows (i.e., MPEG2).

   The current text of [ID-4594bis] has audio flows within an
   interactive communication assigned to EF. The current text of
   [RFC4594] has interactive video flows would be assigned to AF4x.

   The definition of AF4x directly matches the requirements for delay
   and loss-based rate-adaptive, self-friendly video traffic as
   targeted by RTCWeb applications and RMCAT congestion mechanisms.
   Splitting audio and video this way into separate traffic classes
   would specifically provide the benefit of guaranteeing no
   congestion/loss for the audio part while allowing congestion in
   video to cause rate adaptation where there is contention in the
   network. Unfortunately, the reality of deployments of AF4x in
   network in the last decade or longer does not match this original
   target of [RFC4594], instead relying only on loss in the network to
   indicate congestion.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119] when they
   appear in ALL CAPS. These words may also appear in this document in
   lower case as plain English words, absent their normative meanings.

3.  Rate-Adaptive Traffic is Inelastic (and often badly provisioned)

   Because delay-based rate-adaptive video for conferencing has for the
   most part been a fairly recent development, the majority of video
   traffic deployed in many networks into AF41 or AF4x in general has
   been inelastic, highly loss-sensitive video traffic at fixed
   bitrates. This type of video deployment is accompanied by some form
   of either

   o a vendor specific "Locations CAC" (LCAC) mechanism,

   o an on-path RSVP based CAC or

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   o a much more commonly what is perceived to be sufficient

   This is because LCAC is often considered to be too provisioning
   intense and RSVP CAC is not supported on a critical mass of
   application/devices to be deployable in most networks.

   In addition to Video, AF4x often also carries in practice, the
   (non-rate adaptive) voice part of collaborative sessions, primarily
   because creating lip-sync between EF audio and AF4x video traffic
   when they experiences differing jitter and latency in the network
   was seen as a complexity that especially lower end hardware based
   video endpoints wanted to avoid implementing. Additional reasons for

   o Endpoints are too lazy to mark, so network devices mark and cannot
     tell what flow is video vs. audio;

   o Implementers simply did not give the option to mark voice or video

   o People want the same per-hop behavior for audio and video. After
     all, getting one type of media there faster is of little value
     since it just has to be put in a buffer and queued, waiting for
     the other flow.

   Alas, what might have been sufficient overprovisioning often turns
   out to be "pray and suffer" in the face of often badly controllable
   addition of new applications, devices, users or increased in
   utilization. For example, the busy hour in large enterprises often
   shrinks and becomes even more busy when expanding operations across
   multiple time zones. All these problems are the primary reason to
   propose the move to rate adaptive video, as seen in many recent
   products and IETF RTCweb/RMCAT work, but with the often long-term
   investments in a lot of this equipment in enterprises and cost of
   migrating deployment designs, the issues with this current use of
   AF4x is not going to go away for a long time. The recent interest in
   more application or network based circuit-breaker methods
   [ID-AVT-CB] for this type of inelastic traffic is also a good proof
   for this pain point (including the demands within the IETF to
   support them on any inelastic solution (i.e,. RTCWeb & RMCAT

   Because of these fairly widespread deployments of AF4x and their
   issues, these flows make for a fairly bad PHB to put RMCAT/RTCweb
   traffic into. Rate adaptive traffic will always come up short
   against non-rate adaptive, incongestible traffic when being put into
   the same queue. This is especially true when provisioning for AF4x
   is leveraging the above "pray and suffer + circuit breaker"

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4.  Jitter from Non-Rate Adaptive Video Traffic is Bad

   Even if the existing non-rate-adaptive traffic was correctly
   provisioned such that there is no congestion, and even if all that
   traffic is known in a particular deployment to only use AF41, it
   would still not be a good idea to put rate-adaptive RTCweb/RMCAT
   traffic into AF42/AF43.

   The reason is that according to the definition of these traffic
   classes, all AF4x traffic should be in a single queue because there
   must be no reordering between AF4x packets. The permissible
   differentiation between AF41/AF42/AF43 therefore are differential
   drop profiles via WRED or other methods.  This means that even if
   there is never ongoing congestion caused by badly behaving legacy
   non-rate adaptive traffic, the new rate-adaptive video traffic would
   still suffer from the jitter introduced by that old video traffic
   because it has to share a queue. And it would suffer proportional to
   the amount of traffic in the queue, aka: it would suffer most during
   the busy hours.

   This jitter from unknown/legacy/bad video traffic is especially
   troublesome because delay variation schemes considered for
   rate-control in RMCAT will be conscious of that jitter and likely
   work less well when exposed to it.

   It is hard enough - even unavoidable - for RMCAT to figure out how
   deal with competing TCP traffic on a single BE "Internet Queue". It
   is a total waste of effort and easily avoidable for RMCAT having to
   figure out how to deal with the jitter and loss introduced by legacy
   non-rate-adaptive video traffic in controlled environments where
   traffic can be separated by DSCP.

5.  Suggested Behavior in the Network

5.1 CS4 and CS4-Discardable for rate-adaptive video

   This document is therefore asking to assign a Service Class to delay
   and loss-based rate-adaptive, self-friendly conversational video
   traffic that is separate from AF4x.

   We think that CS4 has been a traffic class that so far has seen
   little use and most likely is the easiest traffic class to use for
   this traffic.

   In addition, it would be very helpful for future improvements in
   delay and loss-based rate-adaptive video traffic in the network if
   there is a second traffic class, e.g., "CS4-Discardable", for
   lower-priority packets within video flows that can easily be

   Given how rate-adaptive video will not always be able to avoid all

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   packet drops, video codecs can improve the visual quality in
   conjunction with the network by creating discardable P-frames (e.g.,
   P-frames not referenced by other P-frames) and having the network
   preferentially drop those frames. This can easily achieved with two
   DSCP assigned values, where one has a higher drop priority that the
   other. This, of course, is exactly the logic also designated for
   AF41/AF42/AF43, in other words we are simply asking for a new PHB be
   assigned to the existing AF41 (suggest: CS4) and a new AF42
   (suggested: CS4-Discardable).

5.2 AF4x for Non-Rate-Adaptive Video

   To represent the reality of deployments in the IETF guidelines, AF41
   should be re-designated for non-rate adaptive conversational video
   with explicit admission control (off-path or non-path). AF42 should
   be re-designated for non-rate-adaptive conversational video without
   explicit admission control (e.g., relying solely on circuit

   This proposal, in effect, is suggesting the text in RFC 4594
   regarding AF4X, as written, be ported/moved over to replace the text
   in that same RFC that was for CS4 PHB. New text will need to be
   written in the RFC4594bis document addressing AF4X taking over the
   more legacy traffic behaviors from non-adaptive (based on loss)
   video traffic.

6.  Additional Recommendations for Other PHBs Affected

   Specifying that delay and loss-based rate-adaptive video use CS4
   (and 'CS4+' or 'CS4-Discardable' or 'CS4-D') means the current RFC
   4594 assignment of CS4 to the Realtime-Interactive (RTI) service
   class needs modification. Rather than create new definitions for
   CS4, currently occupied with the Realtime Interactive (RTI) PHB
   traffic according to RFC 4594, our proposal is to move the RTI
   service class definitions to CS5 (as is the case in [ID-4594bis] -
   where the RTI will have a minimal effect on the existing installed
   base of implementations because it has few.

   The CS5 PHB, which is assigned for H.323 and SIP signaling, would be
   then moved to another DSCP. Perhaps near Low-Latency Data (CS2)
   according to [ID-4594bis].

7.  Acknowledgements

   The authors would like to thank Paul Jones, Charles Eckel, Charles
   Ganzhorn, Fred Baker, Mo Zanaty, Michael Ramalho for their active
   participation in this effort.

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8.  IANA Considerations

   If the WG agrees with this effort, then there will need to be a
   reassignment of AF4X and CS4, as well as the new assignment of an
   additional CS4+ or CS4-discardable PHB.

9.  Security Considerations


10. References

10.1 Normative References

 [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
           Requirement Levels", RFC 2119, March 1997

 [RFC4594] J. Babiarz, K. Chan, F Baker, "Configuration Guidelines for
           Diffserv Service Classes", RFC 4594, August 2006

 [ID-4594bis] J. Polk, "Standard Configuration of DiffServ Service
           Classes", "work in progress", March 2012

 [ID-AVT-CB] C. Perkins, V. Singh, "Multimedia Congestion Control:
           Circuit Breakers for Unicast RTP Sessions", work in
           progress, July 2013

10.2 Informative References

   none at this time

Author's Addresses

   James Polk
   Cisco Systems, Inc.
   8017 Hallmark Dr.
   North Richland Hills, TX 76182

   Phone: +1 817 271 3552
   Email: jmpolk@cisco.com

   Toerless Eckert
   Cisco Systems, Inc.
   San Jose

   Email: eckert@cisco.com

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