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Versions: 00 01 draft-ietf-rmcat-app-interaction

RMCAT WG                                                       M. Zanaty
Internet-Draft                                                     Cisco
Intended status: Informational                                  V. Singh
Expires: January 5, 2015                                Aalto University
                                                           S. Nandakumar
                                                               Z. Sarker
                                                             Ericsson AB
                                                            July 4, 2014

          RTP Application Interaction with Congestion Control


   Interactive real-time media applications that use the Real-time
   Transport Protocol (RTP) over the User Datagram Protocol (UDP) must
   use congestion control techniques above the UDP layer since it
   provides none.  This memo describes the interactions and conceptual
   interfaces necessary between the application components that relate
   to congestion control, including the RTP layer, the higher-level
   media codec control layer, and the lower-level transport interface,
   as well as components dedicated to congestion control functions.

Status of This Memo

   This Internet-Draft is submitted 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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   This Internet-Draft will expire on January 5, 2015.

Copyright Notice

   Copyright (c) 2014 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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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Key Words for Requirements  . . . . . . . . . . . . . . . . .   3
   3.  Conceptual Model  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Implementation Model  . . . . . . . . . . . . . . . . . . . .   5
   5.  Interfaces and Interactions . . . . . . . . . . . . . . . . .   6
     5.1.  Config - Codec Interactions . . . . . . . . . . . . . . .   6
     5.2.  Config - RTP/RTCP Interactions  . . . . . . . . . . . . .   6
     5.3.  Codec - RTP Interactions  . . . . . . . . . . . . . . . .   6
     5.4.  Codec - CC Interactions . . . . . . . . . . . . . . . . .   7
     5.5.  RTP - CC Interactions . . . . . . . . . . . . . . . . . .   9
     5.6.  CC - UDP Interactions . . . . . . . . . . . . . . . . . .   9
     5.7.  CC - Shared State Interactions  . . . . . . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Interactive real-time media applications most commonly use RTP
   [RFC3550] over UDP [RFC0768].  Since UDP provides no form of
   congestion control, which is essential for any application deployed
   on the Internet, these RTP applications have historically implemented
   one of the following options at the application layer to address
   their congestion control requirements.

   1.  For media with relatively low packet rates and bit rates, such as
       many speech codecs, some applications use a simple form of
       congestion control that stops transmission permanently or
       temporarily after observing significant packet loss over a
       significant period of time, similar to the RTP circuit breakers

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   2.  Some applications have no explicit congestion control, despite
       the clear requirements in RTP and its profiles AVP [RFC3551] and
       AVPF [RFC4585], under the expectation that users will terminate
       media flows that are significantly impaired by congestion (in
       essence, human circuit breakers).

   3.  For media with substantially higher packet rates and bit rates,
       such as many video codecs, various non-standard congestion
       control techniques are often used to adapt transmission rate
       based on receiver feedback.

   4.  Some experimental applications use standardized techniques such
       as TCP-Friendly Rate Control (TFRC) [RFC5348].  However, for
       various reasons, these have not been widely deployed.

   The RTP Media Congestion Avoidance Techniques (RMCAT) working group
   was chartered to standardize appropriate and effective congestion
   control for RTP applications.  It is expected such applications will
   migrate from the above historical solutions to the RMCAT solution(s).

   The RMCAT requirements [I-D.ietf-rmcat-cc-requirements] include low
   delay, reasonably high throughput, fast reaction to capacity changes
   including routing or interface changes, stability without over-
   reaction or oscillation, fair bandwidth sharing with other instances
   of itself and TCP flows, sharing information across multiple flows
   when possible [I-D.welzl-rmcat-coupled-cc], and performing as well or
   better in networks which support Active Queue Management (AQM),
   Explicit Congestion Notification (ECN), or Differentiated Services
   Code Points (DSCP).

   In order to meet these requirements, interactions are necessary
   between the application's congestion controller, the RTP layer, media
   codecs, other components, and the OS UDP stack.  This memo discusses
   these interactions, presents a conceptual model of the required
   interfaces based on a simplified application decomposition, and
   proposes specific information exchange across these interfaces along
   with corresponding component behavior.

   Note that RTP can also operate over other transports with integrated
   congestion control such as TCP [RFC5681] and DCCP [RFC4340], but that
   is beyond the scope of RMCAT and this memo.

2.  Key Words for Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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3.  Conceptual Model

   It is useful to decompose an RTP application into several components
   to facilitate understanding and discussion of where congestion
   control functions operate, and how they interface with the other
   components.  The conceptual model in Figure 1 consists of the
   following components.

                  |       +-----Config-----+   |
                  |       |       |        |   |
                  |       |     Codec      |   |
                  |       |     | | |      |   |
                  | APP   +---RTP | RTCP---+   |
                  |       |     | | |          |
                  |       |     | | |          |
                  |       +---Congestion-------|---Shared
                  |            Control         |   State
                  | OS           UDP           |

                                 Figure 1

   o  APP: Application containing one or more RTP streams and the
      corresponding media codecs and congestion controllers.  For
      example, a WebRTC browser.

   o  Config: Configuration specified by the application that provides
      the media and transport parameters, RTP and RTCP parameters and
      extensions, and congestion control parameters.  For example, a
      WebRTC Javascript application may use the 'constraints' API to
      affect the media configuration, and SDP applications may negotiate
      the media and transport parameters with the remote peer.  This
      determines the initial static configuration negotiated in session
      establishment.  The dynamic state may differ due to congestion or
      other factors, but still must conform to limits established in the

   o  Codec: Media encoder/decoder or other source/sink for the RTP
      payload.  The codec may be, for example, a simple monaural audio
      format, a complex scalable video codec with several dependent
      layers, or a source/sink with no live encoding/decoding such as a
      mixer which selectively switches and forwards streams rather than
      mixes media.

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   o  RTP: Standard RTP stack functions, including media packetization /
      depacketization and header processing, but excluding existing
      extensions and possible new extensions specific to congestion
      control (CC) such as absolute timestamps or relative transmission
      time offsets in RTP header extensions.  RTCP: Standard RTCP
      functions, including sender reports, receiver reports, extended
      reports, circuit breakers [I-D.ietf-avtcore-rtp-circuit-breakers],
      feedback messages such as NACK [RFC4585] and codec control
      messages such as TMMBR [RFC5104], but excluding existing
      extensions and possible new extensions specific to congestion
      control (CC) such as REMB [I-D.alvestrand-rmcat-remb] (for
      receiver-side CC), ACK (for sender-side CC), absolute and/or
      relative timestamps (for sender-side or receiver-side CC), etc.

   o  Congestion Control: All functions directly responsible for
      congestion control, including possible new RTP/RTCP extensions,
      send rate computation (for sender-side CC), receive rate
      computation (for receiver-side CC), other statistics, and control
      of the UDP sockets including packet scheduling for traffic

   o  Shared State: Storage and exchange of congestion control state for
      multiple flows within the application and beyond it.

   o  OS: Operating System containing the UDP socket interface and other
      network functions such as ECN, DSCP, physical interface events,
      interface-level traffic shaping and packet scheduling, etc.

4.  Implementation Model

   There are advantages and drawbacks to implementing congestion control
   in the application layer.  It avoids OS dependencies and allows for
   rapid experimentation, evolution and optimization for each
   application.  However, it also puts the burden on all applications,
   which raises the risks of improper or divergent implementations.  One
   motivation of this memo is to mitigate such risks by giving proper
   guidance on how the application components relating to congestion
   control should interact.

   Another drawback of congestion control in the application layer is
   that any decomposition, including the one presented in Figure 1, is
   purely conceptual and illustrative, since implementations have
   differing designs and decompositions.  Conversely, this can be viewed
   as an advantage to distribute congestion control functions wherever
   expedient without rigid interfaces.  For example, they may be
   distributed within the RTP/RTCP stack itself, so the separate
   components in Figure 1 are combined into a single RTP+RTCP+CC
   component as shown in Figure 2.

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                  |       +-----Config         |
                  |       |       |            |
                  |       |     Codec          |
                  | APP   |       |            |
                  |       +---RTP+RTCP+CC------|---Shared
                  +----------------------------+   State
                  | OS           UDP           |

                                 Figure 2

   The conceptual model in Figure 1 will be used throughout this memo to
   establish clearer boundaries between functions.  But actual
   implementations may be closer to the looser model in [Singh12].

5.  Interfaces and Interactions

5.1.  Config - Codec Interactions

   The primary interactions between the config and the codec that are
   relevant to congestion control are the multiplexing of media streams
   [I-D.ietf-mmusic-sdp-bundle-negotiation] and RTP/RTCP [RFC5761] on
   the same UDP port.

   The config also establishes limits for the codec such as maximum bit
   rate and other codec-specific parameters.  For example, a video codec
   config often sets limits on maximum resolution and frame rate as well
   as bit rate.

5.2.  Config - RTP/RTCP Interactions

   The config establishes the negotiated RTP and RTCP attributes and
   extensions such as Extended Reports (XR), reduced size [RFC5506],
   codec control [RFC5104], transmission time [RFC5450], etc.

5.3.  Codec - RTP Interactions

   Packetization of codec frames into RTP packets can be an important
   interaction.  Some network interfaces may benefit from small packet
   sizes well below the MTU, while others may benefit from large packets
   approaching the MTU.  Equalizing packet sizes of a frame may also be
   beneficial in some cases, rather than a combination of large and
   small packets.  For example, in some FEC schemes, the FEC bandwidth
   overhead depends on the largest source packet size.  Equalizing the

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   source packet sizes can yield lower overhead than a combination of
   large and small packets.

5.4.  Codec - CC Interactions

   Allowed Rate (from CC to Codec): The max transmit rate allowed over
   the next time interval.  The time interval may be specified or may
   use a default, for example, one second.  The rate may be specified in
   bytes or packets or both.  The rate must never exceed permanent
   limits established in session signaling such as the SDP bandwidth
   attribute [RFC4566] nor temporary limits in RTCP such as TMMBR
   [RFC5104] or REMB [I-D.alvestrand-rmcat-remb].  This is the most
   important interface among all components, and is always required in
   any RMCAT solution.  In the simplest possible solution, it may be the
   only CC interface required.

   Media Elasticity (from Codec to CC): Many live media encoders are
   highly elastic, often able to achieve any target bit rate within a
   wide range, by adapting the media quality.  For example, a video
   encoder may support any bit rate within a range of a few tens or
   hundreds of kbps up to several Mbps, with rate changes registering as
   fast as the next video frame, although there may be limitations in
   the frequency of changes.  Other encoders may be less elastic,
   supporting a narrower rate range, coarser granularity of rate steps,
   slower reaction to rate changes, etc.  Other media, particularly some
   audio codecs, may be fully inelastic with a single fixed rate.  CC
   can beneficially use codec elasticity, if provided, to plan Allowed
   Rate changes, especially when there are multiple flows sharing CC
   state and bandwidth.

   Startup Ramp (from Codec to CC, and from CC to Codec): Startup is an
   important moment in a conversation.  Rapid rate adaptation during
   startup is therefore important.  The codec should minimize its
   startup media rate as much as possible without adversely impacting
   the user experience, and support a strategy for rapid rate ramp.  The
   CC should allow the highest startup media rate as possible without
   adversely impacting network conditions, and also support rapid rate
   ramp until stabilizing on the available bandwidth.  Startup can be
   viewed as a negotiation between the codec and the CC.  The codec
   requests a startup rate and ramp, and the CC responds with the
   allowable parameters which may be lower/slower.  The RMCAT
   requirements also include the possibility of bandwidth history to
   further accelerate or even eliminate startup ramp time.  While this
   is highly desirable from an application viewpoint, it may be less
   acceptable to network operators, since it is in essence a gamble on
   current congestion state matching historical state, with the
   potential for significant congestion contribution if the gamble was

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   wrong.  Note that startup can often commence before user interaction
   or conversation to reduce the chance of clipped media.

   Delay Tolerance (from Codec to CC): An ideal CC will always minimize
   delay and target zero.  However, real solutions often need a real
   non-zero delay tolerance.  The codec should provide an absolute delay
   tolerance, perhaps expressed as an impairment factor to mix with
   other metrics.

   Loss Tolerance (from Codec to CC): An ideal CC will always minimize
   packet loss and target zero.  However, real solutions often need a
   real non-zero loss tolerance.  The codec should provide an absolute
   loss tolerance, perhaps expressed as an impairment factor to mix with
   other metrics.  Note this is unrecoverable post-repair loss after
   retransmission or forward error correction.

   Throughput Sensitivity (from Codec to CC): An ideal CC will always
   maximize throughput.  However, real solutions often need a trade-off
   between throughput and other metrics such as delay or loss.  The
   codec should provide throughput sensitivity, perhaps expressed as an
   impairment factor (for low throughputs) to mix with other metrics.

   Rate Stability (from Codec to CC): The CC algorithm must strike a
   balance between rate stability and fast reaction to changes in
   available bandwidth.  The codec should provide its preference for
   rate stability versus fast and frequent reaction to rate changes,
   perhaps expressed as an impairment factor (for high rate variance
   over short timescales) to mix with other metrics.

   Forward Error Correction (FEC): Simple FEC schemes like XOR Parity
   codes [RFC5109] may not handle consecutive or burst loss well.  More
   complex FEC schemes like Reed-Solomon [RFC6865] or Raptor [RFC6330]
   codes are more effective at handling bursty loss.  The sensitivity to
   packet loss therefore depends on the media (source) encoding as well
   as the FEC (channel) encoding, and this sensitivity may differ for
   different loss patterns like random, periodic, or consecutive loss.
   Expressing this sensitivity to the congestion controller may help it
   choose the right balance between optimizing for throughput versus low

   Probing for Available Bandwidth: FEC can also be used to probe for
   additional available bandwidth, if the application desires a higher
   target rate than the current rate.  FEC is preferable to synthetic
   probes since any contribution to congestion by the FEC probe will not
   impact the post-repair loss rate of the source media flow while
   synthetic probes may adversely affect the loss rate [Nagy14].  Note
   that any use of FEC or retransmission must ensure that the total flow

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   of all packets including FEC, retransmission and original media never
   exceeds the Allowed Rate.

5.5.  RTP - CC Interactions

   RTP Circuit Breakers: The intent behind RTP circuit breakers
   [I-D.ietf-avtcore-rtp-circuit-breakers] is to provide a kill switch
   of last resort, not true congestion control.  The breakers should
   never trip when an effective congestion control is operating.  This
   may impose some boundaries on RMCAT solutions to ensure the
   congestion control never approaches situations which may trigger the

   RTCP Feedback: The primary method of communicating CC information is

   RTP Header Extensions: While RTCP is likely to be the primary carrier
   of CC feedback, the RMCAT requirements also include the possibility
   of using RTP header extensions in bidirectional flows for CC
   feedback.  Transmission time [RFC5450], or possibly absolute time,
   also use header extensions, as would any per packet priority markings
   expected to survive across different networks and administrative

5.6.  CC - UDP Interactions

   Pacing / Shaping: Simple pacing / shaping strategies delay the
   transmission of packets to equalize inter-packet time intervals,
   assuming the bottleneck is most sensitive to packet rate.  More
   complex pacing strategies may go beyond simple even distribution of
   transmission times.  For example, Sprout [Winstein13] attempts to
   predict the optimal transmission time (and rate) with the highest
   probability of success for each packet based on channel statistics.
   Pacing may be always on, or adaptively enabled / disabled based on
   congestion state to minimize delay.  Pacing may be performed within
   the CC for a single flow or across multiple flows.  It may also be
   performed across all or selective traffic over the network interface
   if the OS supports interface-level traffic shaping.

   Detection of Transport Capabilities: The CC can query the OS for
   useful transport capabilities such as ECN, DSCP, traffic shaping,
   etc.  This may also aid upper layers in making better decisions such
   as whether or not to multiplex media streams.  For example, if audio
   can be given differentiated network treatment from video when using
   separate ports.

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   ECN: If the OS and transport path support ECN, the CC can react
   faster than a loss-based CC and more reliably to congestion onset and

   DSCP: If the OS and transport path support DSCP, the CC can map per-
   packet priority from RTP header extensions to DSCP (and layer 2 QoS
   if available) for better network handling under congestion.

   AQM: If AQM is present in the bottleneck, and working effectively,
   there should be little or no excess delay observed when varying the
   transmission rate.  The loss of such delay signals may hinder the
   performance of congestion control algorithms that are highly
   dependent on delay variation for adapting transmission rate.  If the
   application has knowledge of the presence of AQM, through any means
   which are beyond the scope of this memo, it should communicate this
   to the CC.  The CC may use this to alter its signal collection and
   rate adaptation strategies.  The CC must not rely solely on this as a
   reliable indicator.  It must continue to monitor statistics to
   validate this application hint, and react appropriately if the
   statistics suggest different network behavior.

5.7.  CC - Shared State Interactions

   Multiple Flows: Sharing state across multiple flows within the
   application can yield better CC decisions.  Sharing state across even
   more flows beyond the application can yield even better CC decisions.
   The actual benefits and mechanisms of state sharing and coupled CC
   are described in [I-D.welzl-rmcat-coupled-cc].

   Weighted Fairness: An important consideration in CC of multiple flows
   is their relative application-specified weights.  Within an
   application, it is likely the different flows have different rate
   requirements, so equal bandwidth sharing may not be fair nor
   desirable, and weighted fairness may be required.

6.  Acknowledgements

   The RMCAT design team discussions contributed to this memo.

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

   Amplification attacks often use UDP traffic to launch denial of
   service attacks.  Attackers may attempt to subvert congestion control
   protocols in UDP applications to launch amplification attacks by

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   signaling more bandwidth than is actually available.  For example,
   sending a victim a forged REMB or a few fast ACKs may result in the
   victim sending a high rate RTP stream.  Attacks on conference servers
   could lead to further amplification if it distributes the streams to
   many others.  One mitigation is to use SRTCP for congestion control
   messages where supported.  Even if SRTCP is only authenticated not
   encrypted, SRTCP packets should always pass authentication checks
   before any message contents are interpreted.  Non-secure RTCP should
   be avoided where possible.

9.  References

9.1.  Normative References

              Alvestrand, H., "RTCP message for Receiver Estimated
              Maximum Bitrate", draft-alvestrand-rmcat-remb-03 (work in
              progress), October 2013.

              Perkins, C. and V. Singh, "Multimedia Congestion Control:
              Circuit Breakers for Unicast RTP Sessions", draft-ietf-
              avtcore-rtp-circuit-breakers-05 (work in progress),
              February 2014.

              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
              negotiation-07 (work in progress), April 2014.

              Jesup, R., "Congestion Control Requirements For RMCAT",
              draft-ietf-rmcat-cc-requirements-04 (work in progress),
              April 2014.

              Singh, V. and J. Ott, "Evaluating Congestion Control for
              Interactive Real-time Media", draft-ietf-rmcat-eval-
              criteria-01 (work in progress), March 2014.

              Welzl, M., Islam, S., and S. Gjessing, "Coupled congestion
              control for RTP media", draft-welzl-rmcat-coupled-cc-03
              (work in progress), May 2014.

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

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

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              July 2003.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, February 2008.

   [RFC5450]  Singer, D. and H. Desineni, "Transmission Time Offsets in
              RTP Streams", RFC 5450, March 2009.

   [RFC5506]  Johansson, I. and M. Westerlund, "Support for Reduced-Size
              Real-Time Transport Control Protocol (RTCP): Opportunities
              and Consequences", RFC 5506, April 2009.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761, April 2010.

9.2.  Informative References

   [Nagy14]   Nagy, M., Singh, V., Ott, J., and L. Eggert, "Congestion
              Control using FEC for Conversational Multimedia
              Communication", Proc. of 5th ACM Internation Conference on
              Multimedia Systems , 3 2014.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC5109]  Li, A., "RTP Payload Format for Generic Forward Error
              Correction", RFC 5109, December 2007.

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   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification", RFC
              5348, September 2008.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.

   [RFC6330]  Luby, M., Shokrollahi, A., Watson, M., Stockhammer, T.,
              and L. Minder, "RaptorQ Forward Error Correction Scheme
              for Object Delivery", RFC 6330, August 2011.

   [RFC6865]  Roca, V., Cunche, M., Lacan, J., Bouabdallah, A., and K.
              Matsuzono, "Simple Reed-Solomon Forward Error Correction
              (FEC) Scheme for FECFRAME", RFC 6865, February 2013.

   [Singh12]  Singh, V., Ott, J., and C. Perkins, "Congestion Control
              for Interactive Media: Control Loops & APIs", Proc. of
              IAB/IRTF Workshop on Congestion Control for Interactive
              RTC , 7 2012.

              Winstein,, K., Sivaraman,, A., and H. Balakrishnan,
              "Stochastic Forecasts Achieve High Throughput and Low
              Delay over Cellular Networks", Proc. of the 10th USENIX
              Symposium on Networked Systems Design and Implementation ,
              4 2013.

Authors' Addresses

   Mo Zanaty
   Raleigh, NC

   Email: mzanaty@cisco.com

   Varun Singh
   Aalto University
   Espoo, FIN

   Email: varun@comnet.tkk.fi

Zanaty, et al.           Expires January 5, 2015               [Page 13]

Internet-Draft        RMCAT Application Interaction            July 2014

   Suhas Nandakumar
   San Jose, CA

   Email: snandaku@cisco.com

   Zaheduzzaman Sarker
   Ericsson AB

   Email: zaheduzzaman.sarker@ericsson.com

Zanaty, et al.           Expires January 5, 2015               [Page 14]

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