Network Working Group                                             C. Lai
Internet-Draft                                                   W. Wang
Intended status: Standards Track                                 S. Yang
Expires: August 12, 2013                                       T. Eckert
                                                                  F. Yip
                                                           Cisco Systems
                                                        February 8, 2013

           Normalization Marker for AF PHB Group in DiffServ


   This memo describes a Normalization Marker (NM) at DiffServ (DS)
   nodes to normalize the distribution of DS codepoint (DSCP) markings
   for an Assured Forwarding (AF) Per-Hop Behavior (PHB) group.  NM is
   useful for traffic conditioning with Active Queue Management (AQM)
   when the AF PHB group is deployed for multimedia service classes that
   carry video packets with advanced codec technologies.  Using NM can
   offer an incentive that is however missing in today's ecosystem of
   practical deployment, i.e., when congestion occurs in the AF PHB
   group, the network should allow a video codec to benefit from its
   effort of generating finer layers of intra-flow precedence (IFP) with
   discardable packets in a more balanced distribution.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 12, 2013.

Copyright Notice

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

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Video Packets in Structure . . . . . . . . . . . . . . . .  6
     2.2.  Intra-Flow Precedence (IFP)  . . . . . . . . . . . . . . .  8
     2.3.  Mapping IFP to AF Markings . . . . . . . . . . . . . . . .  9
   3.  Normalization Marker (NM)  . . . . . . . . . . . . . . . . . . 11
     3.1.  Color-Aware vs. Color-Blind Mode . . . . . . . . . . . . . 12
     3.2.  Distribution Meter . . . . . . . . . . . . . . . . . . . . 12
     3.3.  Normalizer . . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

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1.  Introduction

   Assured Forwarding (AF) Per-Hop Behavior (PHB) groups are described
   in [RFC2597] (with terminology clarified in [RFC3260]) for DiffServ
   (DS) multimedia service classes such as realtime video conferencing
   and on-demand streaming.  Four AF PHB groups have been defined in
   [RFC4594] with DS codepoint (DSCP): AF1x, AF2x, AF3x and AF4x where
   x=1, 2 or 3 for drop precedence in each independent AF PHB group.
   The DS nodes that support an AF PHB group must set configuration of
   Active Queue Management (AQM) properly w.r.t. those DSCP markings.
   For example, for AF4x PHB group which includes AF41, AF42 and AF43
   markings, an AQM implementation by Weighted Random Early Detection
   (WRED) should be configured with some drop probabilities and queue
   thresholds such that the packet loss rate of AF41 <= AF42 <= AF43 on
   congestion of the queue.

   For an AF PHB group, a DS boundary node or host in the DS domain
   should use a marking algorithm that properly assigns AF markings of
   drop precedence to all packets w.r.t. the traffic profiles and
   Service Level Agreements (SLA).  For example, [RFC2697] and [RFC2698]
   use a token-bucket mechanism for metering each stream of packets and
   respectively define "srTCM" and "trTCM" markers, to mark packets by
   the data rate and burst size limit in traffic profiles.  Those rate-
   control markers can be useful at DS boundary nodes for traffic
   conditioning [RFC2475] and to support IntServ/RSVP traffic over DS
   regions [RFC2988].  Multiple markers may be applied to the same
   stream, either on the same or multiple DS nodes along the path.  For
   example, srTCM and trTCM can operate in a so-called "color-aware"
   mode such that for each incoming packet that already carries an AF
   marking, the local srTCM/trTCM either keeps the same or lowers the
   drop precedence of that packet by metering.

   However, modern video codec technologies are being advanced not only
   in coding efficiency (i.e. better compression ratio) but also in two
   key areas for transport on IP networks: (1) encoder rate-control and
   dynamic adaptation; (2) ability to generate discardable packets in
   multiple layers to tolerate packet losses in the network without
   significant degradation of video quality observed at the decoder.
   For (1), the encoder dynamically limits its output rate of packets
   into the AF PHB group, i.e., the encoder's host is the first DS node
   equiped with srTCM/trTCM if it marks packets in that behavior.  The
   next DS node is the first-hop router which may add extra srTCM/trTCM
   to enforce the traffic conditioning or policing from the network's
   perspective.  Thus, we consider this an incentive for (1) because an
   encoder using a self rate-control is less likely to see packet losses
   by the network.  Unfortunately, an incentive for (2) is arguably
   missing today.

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   To see the missing incentive for (2), consider the following example
   where 2 video flows A and B with rate control are sent in AF4x PHB
   group.  Each sends 5Mbps on average with some burstiness, but still
   complies with the rate and burst limit in its traffic profile.
   However, A and B generate packets with AF4x markings in different
   distributions of precentage:

      Flow A

         80% or 4Mbps in AF41

         20% or 1Mbps in AF42

          0% or 0Mbps in AF43

      Flow B

         40% or 2Mbps in AF41

         40% or 2Mbps in AF42

         20% or 1Mbps in AF43

   Flow B at above is likely using a more advanced video technology to
   generate multiple layers of discardable video packets, and thus, its
   distribution of AF4x markings looks finer and more balanced.  That
   is, flow B acts more friendly to other flows in this AF4x PHB group.

   Thus, we argue that the ecosystem in practical deployment should
   offer an incentive for flows to behave similarly to what flow B is
   doing above, i.e., on congestion, the AF4x PHB group should try to
   drop packets in the same amount from each flow, while a flow with
   finer layers of discardable packets and/or in a more balanced
   distribution should be able to benefit from its own efforts and see
   good results in video quality preservation.

   Unfortunately, this incentive is still missing today.  Suppose that
   congestion occurs in the AF4x WRED queue where A and B compete for
   bandwidth and there is no other flow, for simplicity.  B's packet
   loss rate is very likely to become higher than A's, despite B's
   effort of acting friendly:

   o  If the queue drops 1Mbps in total,

         A sees  0% or 0Mbps loss;

         B sees 20% or 1Mbps loss (all its AF43 are lost).

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   o  If the queue drops 4Mbps in total,

         A sees 20% or 1Mbps loss (all its AF42 are lost);

         B sees 60% or 3Mbps loss (all its AF42 and AF43 are lost).

   Thus, to create the missing incentive at above, we propose a new
   "Normalization Marker" (NM) and describe it in this memo.  NM can be
   deployed on DS boundary nodes for traffic conditioning in practical
   deployment with AF PHB groups for multimedia service classes.  In
   summary, if NM is applied to a DS boundary node for an AF PHB group,
   it re-assigns the AF markings of all packets per flow such that the
   distributions of the AF markings are similar in all flows, i.e., it
   "normalizes" the distributions of AF markings in all flows.  It also
   attempts to maintain the original orders of the intra-flow drop
   precedence carried by the input AF markings, as linearly as possible.
   After the AF-marking distributions are normalized, all those flows
   should see very similar packet loss rates at AQM for this AF PHB
   group on congestion of the queue.  Then, a codec implementation may
   have better video quality preservation on network congestion if it
   employs a more advanced video technology to generate discardable
   packets with finer markings of drop precedence in a more balanced

                               |  Runtime   |
                               | Statistics |
                               |            V
                           +-------+    +--------+
                           |       |    |        |
           AF-Marked       | Distr.|    |  Norm  |       AF-Marked
         Packet Stream ===>| Meter |===>| Marker |===> Packet Stream
                           |       |    |        |
                           +-------+    +--------+

                Normalization Marker (NM) with AF PHB Group

                                 Figure 1

   Note that the use of NM is not necessarily limited to video service
   classes, but could be extended to wherever AF PHB groups can be used,
   or to any other PHB groups that require a similar incentive NM can

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2.  Background

2.1.  Video Packets in Structure

   Modern video codec technologies such as ITU-T H.264/MPEG-4 AVC [H264]
   typically generate a stream of encoded video packets with internal
   structure of data dependency for decoding.  This has been designed
   for at least 3 fundamental reasons:

   o  Coding Efficiency: An encoder improves its coding efficiency
      typically by reducing spatial and temporal redundancy of the
      input.  For video, spatial redundancy is reduced by intra-frame
      motion prediction and compensation, while temporal redundancy
      refers to inter-frame since a video stream is composed of a
      sequence of frames or pictures in the temporal order.  With motion
      prediction, a frame can be encoded by referencing some pixels of
      the picture data that will be decoded earlier either in the same
      (intra) or another (inter) frame so that it can use significantly
      fewer bits to encode this frame.  The frame where the pixels are
      referenced by any other frame is thus called a referenced frame in
      the video stream; for example, Instantaneous Decoding Refresh
      (IDR) in H.264 or Intra (I) frames are typically referenced by
      subsequential frames, while Predictive (P) frames may be
      referenced at the encoder's choice, by the Group of Picture (GOP)
      profile, and/or by some proprietary algorithm in the codec

   o  Lossy Network: To use network transport that may lose packets, the
      encoder may choose to generate a stream with two or more layers
      each of which the packets are marked with some layer identifier
      (ID).  The network can simply use the layer ID to determine the
      drop precedence of each packet in the video stream.

      *  Layers in Hierarchy of Dependency: If these layers are coded in
         hierarchy of dependency, the packets in an "enhancement" layer
         will depend on 1 or more "base" layers to get decoded without
         errors, while packets in a base layer without dependency can be
         independently decoded without errors.

         +  If some enhancement layer packets are lost, the decoding
            errors in that picture frame will not stay or cascade to
            other frames given that no others depend on those lost data.
            This nice property allows the network to safely drop packets
            in some enhancement layers, if needed, without badly
            impacting the video quality at decoder.

         +  If some base layer packets are lost, the impact can be
            severe since these decoding errors will stay in buffer and

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            cascade to all other picture pixels that depend on the lost
            data to decode in the current and/or a later frame.  This
            impact can last tens of seconds as the video quality
            continues getting worse, resulting in unpleasant user
            experiences, until the decoder receives the next IDR or I
            frame, either on-demand or periodically, to remove those

         For example, H.264 Annex G defines Scalable Video Coding (SVC)
         using a 3-dimensional (i.e. spatical, temporal and quality)
         hierarchy of layer dependency at the encoder's choice, but for
         simplicity, it also defines a scalar number called Priority ID
         (PID) in its header so the network could instead use PID, if
         set by the encoder, to determine drop precedence in the stream.

      *  Layers NOT in Hierarchy of Dependency: Sometimes the encoder
         will generate multiple layers without any dependency between
         those layers.  These mechanisms usually enlarge the amount of
         encoded video data for vairous purposes.  For example,

         +  Forward Error Correction (FEC) may be used at the encoder to
            generate extra FEC packets, so that the decoder can tolerate
            certain amounts of packet losses.

         +  Simulcast (i.e. simultaneous multicast) by an encoder will
            actually generate multiple layers each of which can be
            transmitted and decoded independently, in parallel by IP or
            application multicast.  Each layer carries video in a
            different resolution and/or quality.  The decoder can choose
            1 or more of those layers to receive according to the
            required, available or detected bandwidth, packet losses,
            delays, jitter etc. in its network service.

         With FEC and/or Simulcast, the encoder can still mark the
         packets with different drop precedence in those layers to
         better protect the more important data for video quality at
         decoding when congestion occurs.

   o  In-Band Signaling: An encoded video stream usually carries in-band
      control messages that are most critical for adequate encoder and
      decoder behaviors.  For example,

      *  H.264 Annex D defines Supplemental Enhancement Information
         (SEI), which could also carry proprietary codec parameters.
         These in-band control signals should be given the highest drop

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      *  Real Time Control Protocol (RTCP) carries in-band control
         messages for Real Time Protocol (RTP) [RFC3550], which is
         mostly used for realtime multimedia transmission on IP
         networks.  RTCP messages are defined as RTP packets with
         special payload types in the RTP stream.  RTCP packets should
         be given the highest drop precedence but should receive the
         same delay/jitter as regular RTP packets in the same stream.

2.2.  Intra-Flow Precedence (IFP)

   For abstraction, we define "Intra-Flow Precedence" (IFP) to represent
   the drop precedence in one individual flow that may carry a video
   stream of IP packets in multimedia networks.  Here is a summary of
   IFP characteristics:

   o  IFPs are drop precedence levels that are only significant within
      each individual flow.

   o  IFPs are integer numbers that can be numerically compared if
      needed. 0 represents the highest precedence.  The larger numerical
      value an IFP is, the lower precedence it represents.

   o  The number of IFP levels in each flow is not necessarily the same.

   o  IFPs between any 2 flows should NOT be compared to determine drop
      precedence between their packets in a queue.

   o  IFPs may be assigned by the original encoder of the stream and
      carried in some bits field of all packets in the stream.

   o  IFPs may be assigned or re-assigned by a middle box or router if
      it is capable of understanding the stream packet format and codec

   For example, an H.264 AVC flow may have the following IFP assignments
   at the video encoder's choice.

      IFP = 0 for in-band signals

      IFP = 1 for IDR frames

      IFP = 2 for referenced P (rP) frames

      IFP = 3 for non-referenced P (nrP) frames and others

   IFP assignements as well as their distribution can vary a lot among
   different encoder implementations and codec profiles.  For example,
   some encoders may generate both long-term and short-term referenced P

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   frames, where a long-term referenced P frame should have higher drop
   precedence.  In case of H.264 SVC, the IFP assignments could simply
   be the same as the PID assignments if set by the encoder properly, or
   be calculated based on the SVC layer ID that has 3 tuples for the
   spatial, temporal and quality dimensions, respectively.

2.3.  Mapping IFP to AF Markings

   When a flow is sent in an AF PHB group, the number of its IFP levels
   is not necessarily equal to the number of the AF markings.  In fact,
   since each of the currently defined AF PHB groups has only 3 AF
   markings, it is likely that an encoder or DS node needs to apply an
   n-to-1 mapping from IFPs to AF markings in practice.

   The mapping decision is made usually by the encoder, but can also be
   made by another DS node if necessary and if the DS node is able to
   understand the encoded video packets, which may require Deep Packet
   Inspection (DPI), e.g. to read in RTP payload and parse the H.264
   headers [RFC6184], or in a proprietary bits field in the IP payload,
   to retrieve or calculate the IFP of each packet in a flow before
   locally mapping the IFP to an AF marking.

   This n-to-1 mapping can be arbitrary but should be appropriate.
   Consider 2 IFPs, say x and y, where x and y are mapped to AF markings
   AF(x) and AF(y), respectively.  Then, the mapping should ideally obey
   the following criteria to keep linearity from IFPs to AF markings.

      If x < y, AF(x) <= AF(y);

      If x > y, AF(x) >= AF(y).

   Although the above two do NOT imply that if x = y, AF(x) = AF(y), it
   is usually so in practical implementation as it is straightforward.
   Then, if the encoder algorithm generates a lot of packets with the
   same IFP, all those packets will be assigned the same AF marking,
   possibly resulting in an unbalanced distribution of AF markings in
   the AF PHB group.  Thus, an encoder with advanced technologies should
   make good efforts to generate packets with a finer and more balanced
   IFP distribution in the first place.

   For example, if AF4x PHB group is used to send an H.264 AVC flow with
   the IFP assignments in the example of Section 2.2, one possible IFP-
   to-AF4x mapping is:

      AF(0) = AF41

      AF(1) = AF41

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      AF(2) = AF42

      AF(3) = AF43

   This mapping actually results in the following AF markings:

      AF41 for in-band signals and IDR frames

      AF42 for referenced P (rP) frames

      AF43 for non-referenced P (nrP) frames and others

   Now, consider two encoders that generate flow A and B, respectively,
   both using this mapping, but with different IFP distributions as

      Flow A

          5% in IFP=0 for in-band signals

         75% in IFP=1 for IDR frames

         20% in IFP=2 for rP frames

      Flow B

          5% in IFP=0 for in-band signals

         35% in IFP=1 for IDR frames

         40% in IFP=2 for rP frames

         20% in IFP=3 for nrP frames


      Flow A

         80% in AF41

         20% in AF42

          0% in AF43

      Flow B

         40% in AF41

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         40% in AF42

         20% in AF43

   This results in exactly the two AF marking distribtions that we have
   previously used in Section 1.

   Note that in terms of encoded data size, an IDR frame is typically 10
   times larger than a P frame on average.  Assume that flow B's coding
   efficiency has rP twice as large as nrP.  Then, flow A and B might be
   sending frames periodically in patterns by Group of Picture (GOP) as

      Flow A: IDR, rP, rP, rP

      Flow B: IDR, rP, nrP, rP, nrP, rP, nrP, rP, nrP

   If so, it shows that flow B's encoder is making efforts to generate
   discardable packets with more layers in a more balanced distribution,
   which is desirable.

3.  Normalization Marker (NM)

   Referring to Figure 2, NM has 3 major components: IFP reconstructor,
   IFP distribution meter, and normalizer.  NM may operate in either
   "color-aware" (CA) or "color-blind" (CB) mode.

        +---------------+    +--------------+    +------------+
        |               |    |              |    |            |
        |      IFP      |    |     IFP      |    |            |
    ===>| Reconstructor |===>| Distribution |===>| Normalizer |===>
        |  in CA or CB  |    |    Meter     |    |            |
        |               |    |              |    |            |
        +---------------+    +--------------+    +------------+

                  Normalization Marker (NM) Architecture

                                 Figure 2

   The packets arrive at the IFP reconstructor which determines the IFP
   of each packet depending on whether NM is in CA or CB mode.  This is
   fed into the IFP distribution meter that keeps a runtime statistics.
   Then, by the runtime statistics and the IFP of the very packet, the
   normalizer writes a proper AF-marking in that packet.

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3.1.  Color-Aware vs. Color-Blind Mode

   When NM operates in "color-aware" (CA) mode, it reads the incoming
   AF-markings that are carried in the packets as the drop precedence.
   This CA mode should be supported in all NM implementations.

   When NM operates in "color-blind" (CB) mode, which is optionally
   supported, it reads certain bits field(s) other than the AF-markings
   in the packets to determine the actual drop precedence of that
   packet.  This implies that NM may need DPI in the packets, e.g.
   parsing into H.264 AVC header in each RTP packets, or alternatively
   use some method where the drop precedence is carried from the encoder
   in a customized bits field other than the AF-marking in each packet.

   In comparison, CB is more complex than CA in implementation.
   However, CB could probabily produce better normalization results
   because the AF-markings are actually outcomes of an n-to-1 mapping
   from IFPs, as previoulsy mentioned in Section 2.3, which can reduce
   granularity, e.g. for IFPs x and y, if x > y at encoder, it is
   possible that AF(x) = AF(y) when NM sees those packets in CA mode.
   On the contrary, NM in CB mode may reconstruct IFPs x > y for those
   packets by local DPI.

   Note that NM in CB mode may fail to determine the IFP of a packet for
   various reasons at runtime.  If so, NM should randomly assign an IFP
   to each of those packets with an even distribution over the IFPs.
   The failure could be due to payload encryption that prevents DPI.
   Another reason may be that the NM does not support the codec used for
   encoding those packets in the flow.  For example, an NM might only
   support H.264 AVC but is unable to parse packets in H.264 Annex G
   (SVC), so it fails to determine the IFPs of packets in an H.264 SVC

3.2.  Distribution Meter

   The IFP distribution meter keeps a runtime statistics of the IFPs per
   flow so that the normalizer will be able to assign a proper AF-
   marking for each packet.  The types of statistics to collect at
   runtime depend on the NM algorithm in the implementation.

   For example, an NM implementation may keep a counter of packets per
   IFP in a flow since the beginning of the flow's lifetime.  Another
   implementation may choose to keep only the running average of the
   packet counter per IFP.  An even simpler implementation may choose to
   keep only the running average of IFPs of all packets per flow.

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3.3.  Normalizer

   The normalizer should reference the runtime statistics kept by the
   IFP distribution meter, and adaptively map the IFP of the very packet
   to an AF marking, such that the resulting AF-marking distributions
   for all flows are similar or even identical to a target distribution.

   The target distribution of an NM can be simply an even distribution
   over all possible AF-markings in the AF PHB group.  However, in a
   more complex NM implementation, it may allow configuration for other
   target distributions as appropriate with the AQM configuration.

4.  Acknowledgements

   The authors would like to thank many colleagues for comments and
   support of this work, with a special thank to Shuai Dai, who helped
   in the NM prototyping and testing with actual video encoders.

5.  IANA Considerations

   This memo includes no request to IANA.

6.  Security Considerations

   This memo has no security consideration at the time of writing.

7.  References

7.1.  Normative References

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597, June 1999.

   [RFC2697]  Heinanen, J. and R. Guerin, "A Single Rate Three Color
              Marker", RFC 2697, September 1999.

   [RFC2698]  Heinanen, J. and R. Guerin, "A Two Rate Three Color
              Marker", RFC 2698, September 1999.

   [RFC2988]  Paxson, V. and M. Allman, "Computing TCP's Retransmission

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              Timer", RFC 2988, November 2000.

   [RFC3260]  Grossman, D., "New Terminology and Clarifications for
              Diffserv", RFC 3260, April 2002.

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

7.2.  Informative References

   [H264]     ITU-T Recommendation H.264, "Advanced video coding for
              generic audiovisual services", March 2010.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC6184]  Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
              Payload Format for H.264 Video", RFC 6184, May 2011.

Authors' Addresses

   Cheng-Jia Lai
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134

   Phone: +1 408 853 6076
   Email: chelai@cisco.com

   Wenyi Wang
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134

   Phone: +1 408 853 6178
   Email: wenywang@cisco.com

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Internet-Draft    Normalization Marker for AF PHB Group    February 2013

   Stan Yang
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134

   Phone: +1 408 527 5338
   Email: stanyang@cisco.com

   Toerless Eckert
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134

   Phone: +1 408 902 2043
   Email: eckert@cisco.com

   Fred Yip
   Cisco Systems
   San Diego, CA

   Phone: +1 408 527 6521
   Email: fyip@cisco.com

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