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Network Working Group                                             X. Zhu
Internet Draft                                                    R. Pan
Intended Status: Informational                             Cisco Systems
Expires: April 17, 2013                                 October 14, 2012


     NADA: A Unified Congestion Control Scheme for Real-Time Media
                        draft-zhu-rmcat-nada-00


Abstract

   This document describes a scheme named network-assisted dynamic
   adaptation (NADA), a novel congestion control approach for
   interactive real-time media applications, such as video conferencing.
   In the proposed scheme, the sender regulates its sending rate based
   on either implicit or explicit congestion signaling, in a unified
   approach. The scheme can reap the benefits of explicit congestion
   notification markings from network nodes. It also maintains
   consistent sender behavior in the absence of such markings, by
   reacting to queuing delays instead.

   We present here the overall system architecture, recommended
   behaviors at the sender and the receiver, as well as expected network
   nodes operations. Results from extensive simulation studies of the
   proposed scheme are available upon request.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   http://www.ietf.org/shadow.html




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Copyright and License Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.



Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3. System Model  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4. Network Node Operations . . . . . . . . . . . . . . . . . . . .  4
     4.1 Default behavior of drop tail  . . . . . . . . . . . . . . .  4
     4.2 ECN marking  . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.3 PCN marking  . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.4 Comments and Discussions . . . . . . . . . . . . . . . . . .  6
   5. Sender Behavior . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.1 Encoder rate control . . . . . . . . . . . . . . . . . . . .  6
     5.2 Rate shaping buffer  . . . . . . . . . . . . . . . . . . . .  7
     5.3 Encoder target rate calculator . . . . . . . . . . . . . . .  7
       5.3.1 Slow-start behavior  . . . . . . . . . . . . . . . . . .  7
       5.3.2 ECN-enabled mode . . . . . . . . . . . . . . . . . . . .  8
     5.4 Sending rate calculator  . . . . . . . . . . . . . . . . . .  8
   6. Receiver Behavior . . . . . . . . . . . . . . . . . . . . . . .  8
   7. Incremental Deployment  . . . . . . . . . . . . . . . . . . . .  9
   8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  9
   9. References  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     9.1  Normative References  . . . . . . . . . . . . . . . . . . .  9
     9.2  Informative References  . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10










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1. Introduction

   Interactive real-time media applications bring about a unique set of
   challenges for congestion control. Unlike TCP, the mechanism used for
   real-time media needs to adapt fast to instantaneous bandwidth
   changes, accommodate fluctuations in the output of video encoder rate
   control, and cause low queuing delay over the network. An ideal
   scheme should also make effective use of all types of congestion
   signals, including packet losses, queuing delay, and explicit
   congestion notification (ECN) markings.

   Based on the above considerations, we present a scheme named network-
   assisted dynamic adaptation (NADA). The proposed design benefits from
   explicit congestion control signals (e.g., ECN markings) from the
   network, and remains compatible in the presence of implicit signals
   (delay or loss) only. In addition, it supports weighted bandwidth
   sharing among competing video flows.

   This documentation describes the overall system architecture,
   recommended designs at the sender and receiver, as well as expected
   network nodes operations. The signaling mechanism consists of
   standard RTP timestamp [RFC3550] and standard RTCP feedback reports.

2. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].


3. System Model

   The system consists of the following elements:

        * Incoming media stream, in the form of consecutive raw video
        frames and audio samples;

        * Media encoder with rate control capabilities. It takes the
        incoming media stream and encodes it to an RTP stream at a
        target bit rate R_o. Note that the actual output rate from the
        encoder R_v may fluctuate randomly around R_o. Also, the encoder
        can only change its rate at rather coarse time intervals, on the
        order of seconds.

        * RTP sender, responsible for calculating the target bit rate
        R_o based on network congestion signals (delay or ECN marking
        reports from the receiver), and for regulating the actual
        sending rate R_s accordingly. The difference between the video



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        encoder output R_v and sending rate R_s are absorbed in a rate
        shaping buffer. The buffer size L_s, together with R_v,
        influences the calculation of R_s. The RTP sender also generates
        RTP timestamp.

        * RTP receiver, responsible for measuring and estimating end-to-
        end queuing delay d based on sender RTP timestamp. In the
        presence of ECN markings, it also maintains the statistics of
        the marking ratio p. The receiver feeds these statistics back to
        the sender via periodic RTCP reports.

        * Network node, with several modes of operation. The system can
        work with the default behavior of a simple drop tail queue.  It
        can also benefit from advanced AQM features such as RED-based
        ECN marking, and PCN marking using a token bucket algorithm.

In the following, we will elaborate on the respective operations at the
network node, the sender, and the receiver.

4. Network Node Operations

We consider three variations of queue management behavior at the network
node, leading to either implicit or explicit congestion signals.

4.1 Default behavior of drop tail

In conventional network with drop tail or RED queues, congestion is
inferred from the estimation of end-to-end queuing delay. No special
action is required at network node.

This leads to the default operation of delay-based congestion control at
the sender.

4.2 ECN marking

In this mode, the network node randomly marks the ECN field in the IP
packet header following the Random Early Detection (RED) algorithm
[RFC2309]. Calculation of the marking probability involves the following
steps:

    * upon packet arrival, update smoothed queue size q_avg as:

                  q_avg = alpha*q + (1-alpha)*q_avg.

    The smoothing parameter alpha is a value between 0 and 1. A value of
    alpha=0 corresponds to performing no smoothing at all, and
    calculating the marking probability based on instantaneous queue
    size.



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    * calculate marking probability p as:

        p = 0, if q<q_lo;

                   q_avg - q_lo
        p = p_max*--------------, if q_lo<=q<q_hi;
                   q_hi - q_lo

        p = 1, if q>=q_hi.

Here, q_lo and q_hi corresponds to the low and high thresholds of queue
occupancy. The maximum parking probability is p_max.

The ECN markings will trigger the ECN-enabled mode of sender behavior.

4.3 PCN marking

As a more advanced feature, we also envision network nodes which support
PCN marking based on virtual queues. In such a case, the marking
probability of the ECN bit in the IP packet header is calculated as
follows:

    * upon packet arrival, meter packet against token bucket (r,b);

    * update token level b_tk;

    * calculate the marking probability as:

        p = 0, if b-b_tk < b_lo;

                    b-b_tk-b_lo
        p = p_max* --------------, if b_lo<= b-b_tk <b_hi;
                     b_hi-b_lo

        p = 1, if b-b_tk>=b_hi.

Here, the token bucket lower and upper limits are denoted by b_lo and
b_hi, respectively. The parameter b indicates the size of the token
bucket. The parameter r is chosen as r=gamma*C, where gamma<1 is the
target utilization ratio and C designates link capacity. The maximum
marking probability is p_max.

Note that the sender will respond to the observation of markings with
exactly the same ECN-enabled mode as in last section. However, the
virtual queuing mechanism at the network from the PCN marking algorithm
will lead to additional benefits such as zero standing queues and
smoother streaming rate.




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4.4 Comments and Discussions

In all three flavors described above, the network queue operates with
the simple first-in-first-out (FIFO) principle. There is no need to
maintain per-flow state. Such a simple design ensures that the system
can scale easily with large number of video flows and high link
capacity.

5. Sender Behavior

As illustrated in Fig. 1, the sender comprises four modules: a) encoder
rate control; b) rate shaping buffer; c) encoder target rate calculator,
and d) sending rate calculator.

The following sections describe these modules in further details, and
explain how they interact with each other.

------------
|          |        Rv       --------------
|          |---------------->     | | | | | -------------->
|  Encoder |                      | | | | |        / \
|          |                 --------------         |
------------              Rate Shaping Buffer       |
     |                              |               |
     |                              |               |
     |                              |               | Rs
     | Ro                           |               |
     |                          Ls  |               |
-----------------                   |      -----------------
|               |                   |      |               |
|  Target Rate  |                   |----->| Sending Rate  |
|  Calculation  |------------------------> | Calculation   |
|               |                          -----------------
----------------
     /\
      |
      --------------- RTCP report (loss/delay/ECN/PC)

                       Figure 1 Sender Structure



5.1 Encoder rate control

The encoder rate control procedure has the following characteristics:

    * Rate changes can happen only at large intervals, on the order of
    seconds.



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    * Given a target rate R_o, the encoder output rate may randomly
    fluctuate around it.

    * The encoder output rate is further constrained by video content
    complexity. The range of the final rate output is [R_min, R_max].
    Note that it's content-dependent, and may change over time.


5.2 Rate shaping buffer

A rate shaping buffer is employed at the sender, to absorb any
instantaneous mismatch between encoder rate output R_v and regulated
sending rate R_s. The size of the buffer evolves over time, as:

             L_s(t) = max [0, L_s(t-tau)+R_v*tau-R_s*tau].


A large rate shaping buffer contributes to higher end-to-end delay,
which may harm the performance of real-time media communications.
Therefore, the sender has a strong incentive to constrain the size of
the shaping buffer. It can either deplete it faster by choosing a larger
sending rate R_s, or limit its growth by reducing the video encoder
target rate R_o.

5.3 Encoder target rate calculator

The sender calculates the encoder target rate based on network
congestion information from receiver RTCP reports. When only delay
information is available, the target rate is calculated as


                           R_max-R_min
    R_o = R_min + kappa*w*-------------      (1)
                                d

Here, R_min and R_max denote the content-dependent rate range the
encoder can produce. The weight of priority level is w. The scaling
factor kappa can be tuned to determine how sensitive the rate adaptation
scheme is in reaction to fluctuations in observed delay d. The final
target rate Ro is clipped within the range of [Rmin, Rmax].

5.3.1 Slow-start behavior

In addition, the initial sending rate of a stream is regulated to grow
linearly, no more than R_ss at time t:

                           t-t_0
        R_ss(t) = R_min + -------(R_max-R_min).



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                             T

The start time of the stream is t_0, and T represents the time horizon
over which the slow-start mechanism is effective. The encoder target
rate is chosen to be the minimum of R_o and R_ss in the first T
seconds.

5.3.2 ECN-enabled mode

If the receiver reports on observed ECN marking probability p, the
target rate calculation of Eq. (1) is replaced by the following, with a
scaling factor eta:

                             R_max - R_min
        R_o = R_min + eta*w*----------------.        (2)
                                 p

All other procedures remain the same.

Note that the sender does not need to differentiate whether the network
node operates with RED-based ECN marking, or token-bucket-level-based
PCN marking. It reacts to observed ECN marking probabilities in exactly
the same manner.

5.4 Sending rate calculator

Finally, the actual outgoing rate over the network is R_s. Its value is
calculated based on both the encoder target rate and rate shaping buffer
size, as follows:

                             L_s
        R_s = R_o + beta * -------.
                            tau_v

The first term indicates the rate calculated from network congestion
feedback alone.  The second term exerts additional pressure to send out
more packets, if the rate shaping buffer is building up. The scaling
factor beta can be tuned to balance between these two competing goals.

6. Receiver Behavior

The role of the receiver is fairly straightforward. It observes and
estimates end-to-end queuing delay d and ECN marking ratio p of the
stream. The former can be obtained from the RTP timestamp provided by
the sender. The latter can be obtained by keeping a running average of
the number of marked and un-marked packets. The detailed mechanisms for
obtaining such estimates can be varied, and is out of the scope of this
paper.



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Periodically, the receiver sends back the updated values of d and p in
RTCP messages, to aid the sender in its calculation of target rate.
Note that the size of acknowledgement packets are typically on the order
of tens of bytes, and are significantly smaller than average video
packet sizes. Therefore, the bandwidth overhead of the receiver
acknowledgement stream is sufficiently low.

7. Incremental Deployment

One nice property of proposed design is the consistent behavior of video
end points regardless of variations in network node operations. This
facilitates gradual, incremental adoption of the scheme.

To start off with, the scheme operating in delay-assisted congestion
control mode can be implemented without any explicit support from the
network.

When ECN is enabled at the network nodes, together with RED-based
marking, the sender can react to these explicit congestion signals
instead. Ultimately, networks equipped with proactive marking based on
token bucket level metering can reap the additional benefits, including
zero standing queues and more smooth streaming rates.

8. IANA Considerations

There are no actions for IANA.

9. References

9.1  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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


9.2  Informative References

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.

   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,



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              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, April 1998.




Authors' Addresses


   Xiaoqing Zhu
   Cisco Systems,
   510 McCarthy Blvd,
   Milpitas, CA 95134, USA
   EMail: xiaoqzhu@cisco.com

   Rong Pan
   Cisco Systems
   510 McCarthy Blvd,
   Milpitas, CA 95134, USA
   Email: ropan@cisco.com






























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