| Network Working Group | C.J. Lai |
| Internet-Draft | W.Y. Wang |
| Intended status: Informational | S. Yang |
| Expires: January 09, 2014 | T. Eckert |
| F. Yip | |
| Cisco Systems | |
| July 08, 2013 |
Normalization Marker for AF PHB Group in DiffServ
draft-lai-tsvwg-normalizer-02
In DiffServ, preferential dropping of packets in AF PHB groups has long been considered beneficial, typically for video flows with discardable packets. Unfortunately, the ecosystem of bandwidth contention at congestion is very likely to discourage those video endpoints from generating packets with lower precedence markings, i.e. they would lose more packets if doing so. Thus, to offer an incentive for more collaborative and mutually beneficial behaviors of video endpoints in AF PHB groups, we propose a Normalization Marker (NM) for traffic conditioning at network edges. Deployment of NM will encourage the video endpoints to generate finer layers of intra-flow precedence (IFP) with discardable packets in more balanced distributions.
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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.
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 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:
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.
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:
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.
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.
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:
For example, an H.264 AVC flow may have the following IFP assignments at the video encoder's choice.
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 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.
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.
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: Section 1.
This mapping actually results in the following AF markings:
Now, consider two encoders that generate flow A and B, respectively, both using this mapping, but with different IFP distributions as follows.
Thus,
This results in exactly the two AF marking distribtions that we have previously used in
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:
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.
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.
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.
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.
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.
The authors would like to thank many colleagues for comments and supports, and thank Shuai Dai for testing NM with actual H.264 video endpoints.
This memo includes no request to IANA.
This memo has no security consideration at the time of writing.
| [RFC2475] | Blake, S., Black, D.L., Carlson, M.A., 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., 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. |
| [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.-K., Even, R., Kristensen, T. and R. Jesup, "RTP Payload Format for H.264 Video", RFC 6184, May 2011. |
| [H264] | ITU-T Recommendation H.264, "Advanced video coding for generic audiovisual services", March 2010. |