Normalization Marker for AF PHB Group in DiffServ
draft-lai-tsvwg-normalizer-01
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| Authors | Cheng-Jia Lai, Wenyi Wang, Stan Yang, Toerless Eckert , Fred Yip | ||
| Last updated | 2013-07-05 | ||
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draft-lai-tsvwg-normalizer-01
Network Working Group C. Lai
Internet-Draft W. Wang
Intended status: Informational S. Yang
Expires: January 6, 2014 T. Eckert
F. Yip
Cisco Systems
July 5, 2013
Normalization Marker for AF PHB Group in DiffServ
draft-lai-tsvwg-normalizer-01
Abstract
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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This Internet-Draft will expire on January 6, 2014.
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
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(http://trustee.ietf.org/license-info) in effect on the date of
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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 [RFC2998]. 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
distribution.
+------------+
| 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
provide.
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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
implementation.
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
errors.
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
precedence.
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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
symantics.
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
follows.
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
Thus,
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
follows:
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
flow.
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.
[RFC2998] Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L.,
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Speer, M., Braden, R., Davie, B., Wroclawski, J., and E.
Felstaine, "A Framework for Integrated Services Operation
over Diffserv Networks", RFC 2998, 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
US
Email: chelai AT cisco.com
Wenyi Wang
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
US
Email: wenywang AT cisco.com
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Stan Yang
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
US
Email: stanyang AT cisco.com
Toerless Eckert
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
US
Email: eckert AT cisco.com
Fred Yip
Cisco Systems
San Diego, CA
US
Email: fyip AT cisco.com
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