DiffServ Applied to Real-time Transports                   D. Black, Ed.
Internet-Draft                                                       EMC
Intended status: Informational                                  P. Jones
Expires: February 26, 2015                                         Cisco
                                                         August 25, 2014

     Differentiated Services (DiffServ) and Real-time CommunicHtion


   This document describes the interaction between Differentiated
   Services (DiffServ) network quality of service (QoS) functionality
   and real-time network communication, including communication based on
   the Real-time Transport Protocol (RTP).  DiffServ is based on network
   nodes applying different forwarding treatments to packets whose IP
   headers are marked with different DiffServ Code Points (DSCPs).
   WebRTC applications, as well as some conferencing applications, have
   begun using the Session Description Protocol (SDP) bundle negotiation
   mechanism to send multiple traffic streams with different QoS
   requirments using the same network 5-tuple.  Use of different DSCPs
   to obtain different QoS treatments within a single network 5-tuple,
   the results (e.g., reordering) may cause transport protocol
   interactions, particularly with congestion control functionality.  In
   addition, DSCP markings may be changed or removed between the traffic
   source and destination.  This document covers the implications of
   these DiffServ aspects for real-time network communication, including

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on February 26, 2015.

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Copyright Notice

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Real Time Communications  . . . . . . . . . . . . . . . . . .   3
     2.1.  RTP Background  . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  RTP Multiplexing  . . . . . . . . . . . . . . . . . . . .   6
   3.   Differentiated Services (DiffServ) . . . . . . . . . . . . .   7
     3.1.  Diffserv PHBs (Per-Hop Behaviors) . . . . . . . . . . . .   9
     3.2.  Traffic Classifiers and DSCP Remarking  . . . . . . . . .  10
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  DiffServ Interactions . . . . . . . . . . . . . . . . . . . .  12
     5.1.  DiffServ, Reordering and Transport Protocols  . . . . . .  12
     5.2.  DiffServ, Reordering and Real-Time Communication  . . . .  14
     5.3.  Drop Precedence and Transport Protocols . . . . . . . . .  15
   6.  Guidelines  . . . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     10.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   This document describes the interactions between Differentiated
   Services (DiffServ) network quality of service (QoS) functionality
   [RFC2475] and real-time network communication, including
   communication based on the Real-time Transport Protocol (RTP)
   [RFC3550].  DiffServ is based on network nodes applying different
   forwarding treatments to packets whose IP headers are marked with
   different DiffServ Code Points (DSCPs)[RFC2474].  In the past,
   distinct RTP streams have been sent over different transport level

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   flows, sometimes multiplexed with RTCP.  WebRTC applications, as well
   as some conferencing applications, are now using the Session
   Description Protocol (SDP) [RFC4566] bundle negotiation mechanism
   [I-D.ietf-mmusic-sdp-bundle-negotiation] to send col (SDP) bundle
   negotiation mechanism to send multiple traffic streams with different
   QoS requirments using the same network 5-tuple.  Use of different
   DSCPs to obtain different QoS treatments within a single network
   5-tuple, the results (e.g., reordering) may cause transport protocol
   interactions, particularly with congestion control functionality.  In
   addition, DSCP markings may be changed or removed between the traffic
   source and destination.  This document covers the implications of
   these DiffServ aspects for real-time network communication, including
   WebRTC traffic [I-D.ietf-rtcweb-overview].

   The document is organized as follows.  Background is provided in
   Section 2 on real time communications and Section 3 on Differentiated
   Services.  Section 4 describes some examples of DiffServ usage with
   real time communications.  Section 5 explains how use of DiffServ
   features interacts with both transport and real time communications
   protocols and Section 6 provides guidance on DiffServ feature usage
   to control undesired interactions.  Security considerations are
   discussed in Section 8 (Section 7 is an empty IANA Considerations

2.  Real Time Communications

   Real-time communications enables communication in real time over an
   IP network using voice, video, text, content sharing, etc.  It is
   possible to use one or more of these modalities in parallel to
   provide a rich communication experience.

   A simple example of real-time communications is a voice call placed
   over the Internet where an audio stream is transmitted in each
   direction between two users.  A more complex example is an immersive
   videoconferencing system that has multiple video screens, multiple
   cameras, multiple microphones, and some means of sharing content.
   For such complex systems, there may be multiple media and non-media
   streams transmitted via a single IP address and port or via multiple
   IP addresses and ports.

2.1.  RTP Background

   The most common protocol used for real time media is the Real-Time
   Transport Protocol (RTP) [RFC3550].  RTP defines a common
   encapsulation format and handling rules for real-time data
   transmitted over the Internet.  Unfortunately, RTP terminology usage
   has been inconsistent.  For example, the document on RTP grouping
   terminology [I-D.ietf-avtext-rtp-grouping-taxonomy] observes that:

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      RFC 3550 [RFC3550] uses the terms media stream, audio stream,
      video stream and streams of (RTP) packets interchangeably.

   Terminology in this document is based on that RTP grouping
   terminology document with the following terms being of particular
   importance (see that terminology document for full definitions):

   Source Stream:  A reference clock synchronized, time progressing,
      digital media stream.

   RTP Stream:  A stream of RTP packets containing media data, which may
      be source data or redundant data.  The RTP Stream is identified by
      an RTP synchronization source (SSRC) belonging to a particular RTP

   Media encoding and packetization of a source stream results in a
   source RTP stream plus zero or more redundancy RTP streams that
   provide resilience against loss of packets from the source RTP stream
   [I-D.ietf-avtext-rtp-grouping-taxonomy].  Redundancy information may
   also be carried in the same RTP stream as the encoded source stream,
   e.g., see Section 7.2 of [RFC5109].  With most applications, a single
   media type (e.g., audio) is transmitted within a single RTP session.
   However, it is possible to transmit multiple, distinct source streams
   over the same RTP session as one or more individual RTP streams.
   This is referred to as RTP multiplexing.  In addition, an RTP stream
   may contain multiple source streams, e.g., components or programs in
   an MPEG Transport Stream [H.222.0].

   The number of source streams and RTP streams in an overall real-time
   interaction can be surprisingly large.  In addition to a voice source
   stream and a video source stream, there could be separate source
   streams for each of the cameras or microphones on a videoconferencing
   system.  As noted above, there might also be separate redundancy RTP
   streams that provide protection to a source RTP stream, using
   techniques such as Forward Error Correction.  Another example is
   simulcast transmission, where a video source stream can be
   transmitted as high resolution and low resolution RTP streams at the
   same time.  In this case, a media processing function might choose to
   send one or both RTP streams onward to a receiver based on bandwidth
   availability or who the active speaker is in a multipoint conference.
   Lastly, a transmitter might send the same media content concurrently
   as two RTP streams using different encodings (e.g., video encoded as
   VP8 in parallel with H.264) to allow a media processing function to
   select a media encoding that best matches the capabilities of the

   For the WebRTC protocol suite [I-D.ietf-rtcweb-transports], an
   individual source stream is a MediaStreamTrack, and a MediaStream

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   contains one or more MediaStreamTracks
   [W3C.WD-mediacapture-streams-20130903].  A MediaStreamTrack is
   transmitted as a source RTP stream plus zero or more redundant RTP
   streams, so a MediaStream that consists of one MediaStreamTrack is
   transmitted as a single source RTP stream plus zero or more redundant
   RTP streams.  For more information on use of RTP in WebRTC, see

   RTP is usually carried over a datagram protocol, such as
   UDP[RFC0768], UDP-Lite [RFC3828] or DCCP [RFC4340]; UDP is most
   commonly used, but a non-datagram protocol (e.g., TCP) may also be
   used.  Transport protocols other than UDP or UDP-Lite may also be
   used to transmit real-time data or near-real-time data.  For example,
   SCTP [RFC4960] can be utilized to carry application sharing or
   whiteboarding information as part of an overall interaction that
   includes real-time media.  These additional transport protocols can
   be multiplexed with an RTP session via UDP or TCP encapsulation,
   thereby using a single pair of UDP ports.

   The WebRTC protocol suite encompasses a number of forms of

   1.  Individual source streams are carried in one or more individual
       RTP streams.  These RTP streams can be multiplexed onto a single
       transport-layer flow or sent as separate transport-layer flows.
       This document only considers the case where the RTP streams are
       to be multiplexed onto a single transport-layer flow, forming a
       single RTP session as described in [RFC3550];

   2.  The RTP Control Protocol (RTCP) (see [RFC3550]) may be
       multiplexed onto the same transport-layer flow as the RTP stream
       with which it is associated, as described in [RFC5761] or it may
       be sent on a separate transport-layer flow;

   3.  An RTP session could be multiplexed with a single SCTP over DTLS
       association plus STUN [RFC5389] and TURN [RFC5766] traffic into a
       single transport-layer flow as described in [RFC5764] with the
       updates in [I-D.petithuguenin-avtcore-rfc5764-mux-fixes].  The
       STUN [RFC5389] and TURN [RFC5766] protocols provide NAT/FW
       (Network Address Translator / Firewall) traversal and port

   The resulting transport layer flow is identified by a network
   5-tuple, i.e., a combination of two IP addresses (source and
   destination), two ports (source and destination), and the transport
   protocol used (e.g., UDP).  SDP bundle negotiation restrictions
   [I-D.ietf-mmusic-sdp-bundle-negotiation] limit WebRTC to using at
   most a single DTLS session per network 5-tuple.  In contrast,

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   multiple SCTP associations can be directly multiplexed over a single
   UDP 5-tuple [RFC6951] when DTLS is not used to encapsulate SCTP.

   The STUN and TURN protocols were originally designed for use of UDP,
   however, TURN has been extended to use TCP as a transport for
   situations in which UDP does not work [RFC6062].  When TURN selects
   use of TCP, the entire real-time communications session is carried
   over a single TCP 5-tuple.

   For IPv6, addition of the flow label [RFC6437] to network 5-tuples
   results in network 6-tuples, but in practice, use of a flow label is
   unlikely to result in a finer-grain traffic subset than the
   corresponding network 5-tuple (e.g., the flow label is likely to
   represent the combination of two ports with use of the UDP protocol).
   For that reason, discussion in this draft focuses on UDP 5-tuples.

2.2.  RTP Multiplexing

   Section 2.1 explains how source streams can be multiplexed in a
   single RTP session, which can in turn be multiplexed over UDP with
   packets generated by other transport protocols.  This section
   provides background on why this level of multiplexing is desirable.
   The rationale in this section applies both to multiplexing of source
   streams in a single RTP session and multiplexing of an RTP session
   with traffic from other transport protocols via UDP encapsulation.

   Multiplexing reduces the number of ports utilized for real-time and
   related communication in an overall interaction.  While a single
   endpoint might have plenty of ports available for communication, this
   traffic often traverses points in the network that are constrained on
   the number of available ports or whose performance degrades as the
   number of ports in use increases.  A good example is a Network
   Address Translator and Firewall (NAT/FW) device sitting at the
   network edge.  As the number of simultaneous protocol sessions
   increases, so does the burden placed on these devices to provide port

   Another reason for multiplexing is to help reduce the time required
   to establish bi-directional communication.  Since any two
   communicating users might be situated behind different NAT/FW
   devices, it is necessary to employ techniques like STUN and TURN
   along with ICE [RFC5245] to get traffic to flow between the two
   devices [I-D.ietf-rtcweb-transports].  Performing the tasks required
   by these protocols takes time, especially when multiple protocol
   sessions are involved.  While tasks for different sessions can be
   performed in parallel, it is nonetheless necessary for applications
   to wait for all sessions to be opened before communication between
   two users can begin.  Reducing the number of STUN/ICE/TURN steps

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   reduces the likelihood of loss of a packet for one of these
   protocols; any such loss adds delay to setting up a communication
   session.  Further, reducing the number of STUN/ICE/TURN tasks places
   a lower burden on the STUN and TURN servers.

   Multiplexing may reduce the complexity and resulting load on an
   endpoint.  A single instance of STUN/ICE/TURN is simpler to execute
   and manage than multiple instances STUN/ICE/TURN operations happening
   in parallel, as the latter require synchronization and create more
   complex failure situations that have to be cleaned up by additional

3.  Differentiated Services (DiffServ)

   The DiffServ architecture [RFC2475][RFC4594] is intended to enable
   scalable service discrimination in the Internet without requiring
   each node in the network to store per-flow state and participate in
   per-flow signaling.  The services may be end-to-end or within a
   network; they include both those that can satisfy quantitative
   performance requirements (e.g., peak bandwidth) and those based on
   relative performance (e.g., "class" differentiation).  Services can
   be constructed by a combination of well-defined building blocks
   deployed in network nodes that:

   o  classify traffic and set bits in an IP header field at network
      boundaries or hosts,

   o  use those bits to determine how packets are forwarded by the nodes
      inside the network, and

   o  condition the marked packets at network boundaries in accordance
      with the requirements or rules of each service.

   Traffic conditioning may include changing the DSCP in a packet
   (remarking it), delaying the packet (as a consequence of traffic
   shaping) or dropping the packet (as a consequence of traffic

   A network node that supports DiffServ includes a classifier that
   selects packets based on the value of the DS field in IP headers (the
   DiffServ codepoint or DSCP), along with buffer management and packet
   scheduling mechanisms capable of delivering the specific packet
   forwarding treatment indicated by the DS field value.  Setting of the
   DS field and fine-grain conditioning of marked packets need only be
   performed at network boundaries; internal network nodes operate on
   traffic aggregates that share a DS field value, or in some cases, a
   small set of related values.

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   The DiffServ architecture[RFC2475] maintains distinctions among:

   o  the QoS service provided to a traffic aggregate,

   o  the conditioning functions and per-hop behaviors (PHBs) used to
      realize services,

   o  the DSCP in the IP header used to mark packets to select a per-hop
      behavior, and

   o  the particular implementation mechanisms that realize a per-hop

   This document focuses on PHBs and the usage of DSCPs to obtain those
   behaviors.  In a network node's forwarding path, the DSCP is used to
   map a packet to a particular forwarding treatment, or per-hop
   behavior (PHB) that specifies the forwarding treatment.

   The specification of a PHB describes the externally observable
   forwarding behavior of a network node for network traffic marked with
   a DSCP that selects that PHB.  In this context, "forwarding behavior"
   is a general concept - for example, if only one DSCP is used for all
   traffic on a link, the observable forwarding behavior (e.g., loss,
   delay, jitter) will often depend only on the loading of the link.  To
   obtain useful behavioral differentiation, multiple traffic subsets
   are marked with different DSCPs for different PHBs for which node
   resources such as buffer space and bandwidth are allocated.  PHBs
   provide the framework for a DiffServ network node to allocate
   resources to traffic subsets, with network-scope differentiated
   services constructed on top of this basic hop-by-hop resource
   allocation mechanism.

   The codepoints (DSCPs) may be chosen from a small set of fixed values
   (the class selector codepoints), or from a set of recommended values
   defined in PHB specifications, or from values that have purely local
   meanings to a specific network that supports DiffServ; in general,
   packets may be forwarded across multiple such networks between source
   and destination.

   The mandatory DSCPs are the class selector code points as specified
   in [RFC2474].  The class selector codepoints (CS0-CS7) extend the
   deprecated concept of IP Precedence in the IPv4 header; three bits
   are added, so that the class selector DSCPs are of the form 'xxx000'.
   The all-zero DSCP ('000000' or CS0) is always assigned to a Default
   PHB that provides best-effort forwarding behavior and the remaining
   class selector code points are intended to provide relatively better
   per-hop-forwarding behavior in increasing numerical order, but:

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   o  There is no requirement that any two adjacent class selector
      codepoints be assigned to different PHBs; adjacent class selector
      codepoints may use the same pool of resources on each network node
      in some networks.  This generalizes to ranges of class selector
      codepoints, but with limits - for example CS6 and CS7 are often
      used for network control (e.g., routing) traffic [RFC4594] and
      hence are likely to provide better forwarding behavior under
      network load to prioritize network recovery from disruptions.

   o  CS1 ('001000') was subsequently designated as the recommended
      codepoint for the Lower Effort (LE) PHB [RFC3662].  An LE service
      forwards traffic with "lower" priority than best effort and can be
      "starved" by best effort and other "higher" priority traffic.  Not
      all networks offer an LE service, hence traffic marked with the
      CS1 DSCP may not receive lower effort forwarding; such traffic may
      be forwarded with a different PHB (e.g., the Default PHB),
      remarked to another DSCP (e.g., CS0) and forwarded accordingly, or
      dropped.  See [RFC3662] for further discussion of the LE PHB and

   One cannot rely upon different class selector codepoints providing
   differentiated services or upon the presence of an LE service that is
   selected by the CS1 DSCP.  There is no effective way for a network
   endpoint to determine which PHBs are selected by the class selector
   codepoints or whether the CS1 DSCP selects an LE service on a
   specific network, let alone end-to-end.  Packets marked with the CS1
   DSCP may be forwarded with best effort service or another "higher"
   priority service, see [RFC2474].

3.1.  Diffserv PHBs (Per-Hop Behaviors)

   Although Differentiated Services is a general architecture that may
   be used to implement a variety of services, three fundamental
   forwarding behaviors (PHBs) have been defined and characterized for
   general use.  These are:

   1.  Default Forwarding (DF) for elastic traffic [RFC2474].  The
       Default PHB is always selected by the all-zero DSCP and provides
       best-effort forwarding.

   2.  Assured Forwarding (AF) [RFC2597] to provide differentiated
       service to elastic traffic.  Each instance of the AF behavior
       consists of three PHBs that differ only in drop precedence, e.g.,
       AF11, AF12 and AF13; such a set of three AF PHBs is referred to
       as an AF class, e.g., AF1x.  There are four defined AF classes,
       AF1x through AF4x, with higher numbered classes intended to
       receive better forwarding treatment than lower numbered classes.

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   3.  Expedited Forwarding (EF) [RFC3246] intended for inelastic
       traffic.  Beyond the basic EF PHB, the VOICE-ADMIT PHB [RFC5865]
       is an admission controlled variant of the EF PHB.  Both of these
       PHBs are based on pre-configured limited forwarding capacity;
       traffic in excess of that capacity is expected to be dropped.

3.2.  Traffic Classifiers and DSCP Remarking

   DSCP markings are not end-to-end in general.  Each network can make
   its own decisions about what PHBs to use and which DSCP maps to each
   PHB.  While every PHB specification includes a recommended DSCP, and
   RFC 4594 [RFC4594] recommends their end-to-end usage, there is no
   requirement that every network support any PHBs or use any specific
   DSCPs, with the exception of the support requirements for the class
   selector codepoints (see RFC 2474 [RFC2474]).  When DiffServ is used,
   the edge or boundary nodes of a network are responsible for ensuring
   that all traffic entering that network conforms to that network's
   policies for DSCP and PHB usage, and such nodes may change DSCP
   markings on traffic to achieve that result.  As a result, DSCP
   remarking is possible at any network boundary, including the first
   network node that traffic sent by a host encounters.  Remarking is
   also possible within a network, e.g., for traffic shaping.

   DSCP remarking is part of traffic conditioning; the traffic
   conditioning functionality applied to packets at a network node is
   determined by a traffic classifier [RFC2475].  Edge nodes of a
   DiffServ network classify traffic based on selected packet header
   fields; typical implementations do not look beyond the traffic's
   network 5-tuple in the IP and transport protocol headers (e.g., for
   SCTP or RTP enapsulated in UDP, header-based classification is
   unlikely to look beyond the outer UDP header).  As a result, when
   multiple DSCPs are used for traffic that shares a network 5-tuple,
   remarking at a network boundary may result in all of the traffic
   being forwarded with a single DSCP, thereby removing any
   differentiation within the network 5-tuple downstream of the
   remarking location.  Network nodes within a DiffServ network
   generally classify traffic based solely on DSCPs, but may perform
   finer grain traffic conditioning similar to that performed by edge

   So, for two arbitrary network endpoints, there can be no assurance
   that the DSCP set at the source endpoint will be preserved and
   presented at the destination endpoint.  Rather, it is quite likely
   that the DSCP will be set to zero (e.g., at the boundary of a network
   operator that distrusts or does not use the DSCP field) or to a value
   deemed suitable by an ingress classifier for whatever network 5-tuple
   it carries.

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   In addition, remarking may remove application-level distinctions in
   forwarding behavior - e.g., if multiple PHBs within an AF class are
   used to distinguish different types of frames within a video RTP
   stream, token-bucket-based remarkers operating in Color-Blind mode
   (see [RFC2697] and [RFC2698] for examples) may remark solely based on
   flow rate and burst behavior, removing the drop precedence
   distinctions specified by the source.

   Backbone and other carrier networks may employ a small number of
   DSCPs (e.g., less than half a dozen) to manage a small number of
   traffic aggregates; hosts that use a larger number of DSCPs can
   expect to find that much of their intended differentiation is removed
   by such networks.  Better results may be achieved when DSCPs are used
   to spread traffic among a smaller number of DiffServ-based traffic
   subsets or aggregates, see [I-D.geib-tsvwg-diffserv-intercon] for one
   proposal.  This is of particular importance for MPLS-based networks
   due to the limited size of the Traffic Class (TC) field in an MPLS
   label [RFC5462] that is used to carry DiffServ information and the
   use of that TC field for other purposes, e.g., ECN [RFC5129].  For
   further discussion on use of DiffServ with MPLS, see [RFC3270] and

4.  Examples

   For real-time communications, one might want to mark the audio
   packets using EF and the video packets as AF41.  However, in a video
   conference receiving the audio packets significantly ahead of the
   video is not useful because lip sync is necessary between audio and
   video.  It may still be desirable to send audio with a PHB that
   provides better service, because more reliable arrival of audio helps
   assure smooth audio rendering, which is often more important than
   fully faithful video rendering.  There are also limits, as some
   devices have difficulties in synchronizing voice and video when
   packets that need to be rendered together arrive at significantly
   different times.  It makes more sense to use different PHBs when the
   audio and video source streams do not share a strict timing
   relationship.  For example, video content may be shared within a
   video conference via playback, perhaps of an unedited video clip that
   is intended to become part of a television advertisement.  Such
   content sharing video does not need precise synchronization with
   video conference audio, and could use a different PHB, as content
   sharing video is more tolerant to jitter, loss, and delay.

   Within a layered video RTP stream, ordering of frame communication is
   preferred, but importance of frame types varies, making use of PHBs
   with different drop precedences appropriate.  For example, I-frames
   that contain an entire image are usually more important than P-frames
   that contain only changes from the previous image because loss of a

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   P-frame (or part thereof) can be recovered (at the latest) via the
   next I-frame, whereas loss of an I-frame (or part thereof) may cause
   rendering problems for all of the P-frames that depend on the missing
   I-frame.  For this reason, it is appropriate to mark I-frame packets
   with a PHB that has lower drop precedence than the PHB used for
   P-frames, as long as the PHBs preserve ordering among frames (e.g.,
   are in a single AF class) - AF41 for I-frames and AF43 for P-frames
   is one possibility.  Additional spatial and temporal layers beyond
   the base video layer could also be marked with higher drop precedence
   than the base video layer, as their loss reduces video quality, but
   does not disrupt video rendering.

   Additional RTP streams in a real-time communication interaction could
   be marked with CS0 and carried as best effort traffic.  One example
   is real-time text transmitted as specified in RFC 4103 [RFC4103].
   Best effort forwarding suffices because such real-time text has loose
   timing requirements; RFC 4103 recommends sending text in chunks every
   300ms.  Such text is technically real-time, but does not need a PHB
   promising better service than best effort, in contrast to audio or

   A WebRTC application involves one or more RTP streams, as discussed
   above.  In addition, it may include an SCTP-based data channel
   [I-D.ietf-rtcweb-data-channel] whose QoS treatment depends on the
   nature of the application.  For example, best effort treatment of
   data channels is likely to suffice for messaging, shared white board,
   and guided browsing applications, whereas latency-sensitive games
   could require better QoS for their data channels.

5.  DiffServ Interactions

5.1.  DiffServ, Reordering and Transport Protocols

   Transport protocols provide data communication behaviors beyond those
   possible at the IP layer.  An important example is that TCP [RFC0793]
   provides reliable in-order delivery of data with congestion control.
   SCTP [RFC4960] provides additional properties such as preservation of
   message boundaries, and the ability to avoid head-of-line blocking
   that may occur with TCP.

   In contrast, UDP [RFC0768] is a basic unreliable datagram protocol
   that provides port-based multiplexing and demultiplexing on top of
   IP.  Two other unreliable datagram protocols are UDP-Lite [RFC3828],
   a variant of UDP that may deliver partially corrupt payloads when
   errors occur, and DCCP [RFC4340], which provides a range of
   congestion control modes for its unreliable datagram service.

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   Transport protocols that provide reliable delivery (e.g., TCP, SCTP)
   are sensitive to network reordering of traffic.  When a protocol that
   provides reliable delivery receives a packet other than the next
   expected packet, the protocol usually assumes that the expected
   packet has been lost and respond with a retransmission request for
   that packet.  In addition, congestion control functionality in
   transport protocols (including DCCP) usually infers congestion when
   packets are lost.  This creates additional sensitivity to significant
   network packet reordering, as such reordering may be
   (mis-)interpreted as loss of the out-of-order packets, causing a
   congestion control response.

   This sensitivity to reordering remains even when ECN [RFC3168] is in
   use, as ECN receivers are required to treat missing packets as
   potential indications of congestion, because:

   o  Severe congestion may cause ECN-capable network nodes to drop
      packets, and

   o  ECN traffic may be forwarded by network nodes that do not support
      ECN and hence drop packets to indicate congestion.

   Congestion control is an important aspect of the Internet
   architecture, see [RFC2914] for further discussion.

   In general, marking packets with different DSCPs results in different
   PHBs being applied at nodes in the network, making reordering
   possible due to use of different pools of forwarding resources for
   each PHB.  This should not be done for current transport protocols
   within a single network 5-tuple, with the exception of UDP and UDP-

   When PHBs that allow reordering are mixed within a single
   network5-tuple, the effect is to mix QoS-based traffic classes within
   the scope of a single transport connection or association.  Such QoS-
   based traffic classes receive different network QoS treatments and
   hence use different pools of network resources.  Transport protocol
   support for multiple QoS-based traffic classes within a single
   network 5-tuple adds complexity to congestion-controlled transport
   protocols by comparison to current designs because it requires that
   network congestion information for each QoS-based traffic class be
   disambiguated so that congestion control is managed separately for
   each such traffic class.  Traffic in different QoS-based traffic
   classes may use different paths through the network; this complicates
   path integrity checking in connection- or association-based
   protocols, as those paths may fail independently.

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   The primary example where usage of multiple PHBs does not allow
   reordering within a single network 5-tuple is use of PHBs from a
   single AF class (e.g., AF1x).  Traffic reordering within the scope of
   a network 5-tuple that uses a single PHB or AF class may occur for
   other transient reasons (e.g., routing changes or ECMP rebalancing).

   Reordering also affects other forms of congestion control, such as
   techniques for RTP congestion control that were under development
   when this document was published, see
   [I-D.ietf-rmcat-cc-requirements] for requirements.  These techniques
   prefer use of a common (coupled) congestion controller for RTP
   streams between the same endpoints to reduce packet loss and delay by
   reducing competition for resources at any shared bottleneck.

   Shared bottlenecks can be detected via techniques such as correlation
   of one-way delay measurements across RTP streams.  An alternate
   approach is to assume that the set of packets on a single network
   5-tuple marked with DSCPs that do not allow reordering will utilize a
   common network path and common forwarding resources at each network
   node.  Under that assumption, any bottleneck encountered by such
   packets is shared among all of them, making it safe to use a common
   (coupled) congestion controller, see [I-D.welzl-rmcat-coupled-cc].
   This is not a safe assumption when the packets involved are marked
   with DSCP values that allow reordering because a bottleneck may not
   be shared among all such packets (e.g., if the DSCPs result in use of
   different queues at a network node, only one of which is a

   UDP and UDP-Lite are not sensitive to reordering in the network,
   because they do not provide reliable delivery or congestion control.
   On the other hand, when used to encapsulate other protocols (e.g., as
   UDP is used by WebRTC, see Section 2.1), the reordering
   considerations for the encapsulated protocols apply.  For the
   specific usage of UDP by WebRTC, every encapsulated protocol (i.e.,
   RTP, SCTP and TCP) is sensitive to reordering as further discussed in
   this document.  In addition, [RFC5405] provides general guidelines
   for use of UDP (and UDP-Lite); the congestion control guidelines in
   that document apply to protocols encapsulated in UDP (or UDP-Lite).

5.2.  DiffServ, Reordering and Real-Time Communication

   Real-time communications are also sensitive to network reordering of
   packets.  Such reordering may lead to spurious NACK generation and
   unneeded retransmission, as is the case for reliable delivery
   protocols (see Section 5.1).  The degree of sensitivity depends on
   protocol or stream timers, in contrast to reliable delivery protocols
   that usually react to all reordering.

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   Receiver jitter buffers have important roles in the effect of
   reordering on real time communications:

   o  Minor packet reordering that is contained within a jitter buffer
      usually has no effect on rendering of the received RTP stream
      because packets that arrive out of order are retrieved in order
      from the jitter buffer for rendering.

   o  Packet reordering that exceeds the capacity of a jitter buffer can
      cause user-perceptible quality problems (e.g., glitches, noise)
      for delay sensitive communication, such as interactive
      conversations for which small jitter buffers are necessary to
      preserve human perceptions of real-time interaction.  Interactive
      real-time communication implementations often discard data that is
      sufficiently late that it cannot be rendered in source stream
      order, making retransmission counterproductive.  For this reason,
      implementations of interactive real-time communication often do
      not use retransmission.

   o  In contrast, replay of recorded media can tolerate significantly
      longer delays than interactive conversations, so replay is likely
      to use larger jitter buffers than interactive conversations.
      These larger jitter buffers increase the tolerance of replay to
      reordering by comparison to interactive conversations.  The size
      of the jitter buffer imposes an upper bound on replay tolerance to
      reordering, but does enable retransmission to be used when the
      jitter buffer is significantly larger than the amount of data that
      can be expected to arrive during the round-trip latency for

   Network packet reordering has no effective upper bound, and can
   exceed the size of any reasonable jitter buffer.  In practice, the
   size of jitter buffers for replay is limited by external factors such
   as the amount of time that a human is willing to wait for replay to
   start.  Use of different DSCPs that allow reordering is more likely
   to cause reordering problems than use of a single DSCP or set of
   DSCPs that does not allow reordering.

5.3.  Drop Precedence and Transport Protocols

   Packets within the same network 5-tuple that use PHBs within a single
   AF class can be expected to draw upon the same forwarding resources
   on network nodes (e.g., use the same router queue) and hence use of
   multiple drop precedences within an AF class is not expected to cause
   latency variation.  When PHBs within a single AF class are mixed
   within a flow, the resulting overall likelihood that packets will be
   dropped from that flow is a mix of the drop likelihoods of the PHBs

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   There are situations in which drop precedences should not be mixed.
   A simple example is that there is little value in mixing drop
   precedences within a TCP connection, because TCP's ordered delivery
   behavior results in any drop requiring the receiver to wait for the
   dropped packet to be retransmitted.  Any resulting delay depends on
   the RTT and not the packet that was dropped.  Hence a single DSCP
   should be used for all packets in a TCP connection.

   As a consequence, when TCP is selected for NAT/FW traversal (e.g., by
   TURN), a single DSCP should be used for all traffic on that TCP
   connection.  An additional reason for this recommendation is that
   packetization for STUN/ICE/TURN occurs before passing the resulting
   packets to TCP; TCP resegmentation may result in a different
   packetization on the wire, breaking any association between DSCPs and
   specific data to which they are intended to apply.

   SCTP [RFC4960] differs from TCP in a number of ways, including the
   ability to deliver messages in an order that differs from the order
   in which they were sent and support for unreliable streams.  However,
   SCTP performs congestion control and retransmission across the entire
   association, and not on a per-stream basis.  Although there may be
   advantages to using multiple drop precedence across SCTP streams or
   within an SCTP stream that does not use reliable ordered delivery,
   there is no practical operational experience in doing so (e.g., the
   SCTP sockets API [RFC6458] does not support use of more than one DSCP
   for an SCTP association).  As a consequence, the impacts on SCTP
   protocol and implementation behavior are unknown and difficult to
   predict.  Hence a single DSCP should be used for all packets in an
   SCTP association, independent of the number or nature of streams in
   that association.  Similar reasoning applies to a DCCP connection; a
   single DSCP should be used because the scope of congestion control is
   the connection and there is no operational experience with using more
   than one DSCP.

   Guidance on transport protocol design and implementation to provide
   support for use of multiple PHBs and DSCPs in a transport protocol
   connection (e.g., DCCP) or transport protocol association (e.g.,
   SCTP) is out of scope for this document.

   RTCP multi-stream reporting optimizations for an RTP session
   [I-D.ietf-avtcore-rtp-multi-stream-optimisation] assume that the RTP
   streams involved experience the same packet loss behavior.  This
   mechanism is highly inappropriate when the RTP streams involved use
   different PHBs, even if those PHBs differ solely in drop precedence.

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6.  Guidelines

   The only use of multiple standardized PHBs and DSCPs that prevents
   network reordering among packets marked with different DSCPs is use
   of PHBs within a single AF class.  All other uses of multiple PHBs
   and/or the class selector DSCPs allow network reordering of packets
   that are marked with different DSCPs.  Based on this and the
   foregoing discussion, the guidelines in this section apply to use of
   DiffServ with real-time communications.

   Applications and other traffic sources:

   o  Should limit use of DSCPs within a single RTP stream to those
      whose corresponding PHBs do not allow packet reordering.  If this
      is not done, significant network reordering may overwhelm
      implementation assumptions about reordering limits, e.g., jitter
      buffer size, causing poor user experiences, see Section 5.2 above.
      This guideline applies to all of the RTP streams that are within
      the scope of a common (coupled) congestion controller that does
      not use per-RTP-stream measurements for bottleneck detection.

   o  Should use a single DSCP for an RTCP session, primarily to avoid
      RTCP reordering (and because there is no compelling reason for use
      of different drop precedences).  One of the PHBs and associated
      DSCP used for the associated RTP traffic would be an appropriate
      choice.  [Editor's note: This bullet is an open technical issue.]

   o  Should use a single DSCP for all packets within a reliable
      transport protocol session (e.g., TCP connection, SCTP
      association) or DCCP connection, see Section 5.1 and Section 5.3.
      For SCTP, this requirement applies across the entire SCTP
      association, and not just to individual streams within an
      association.  When TURN selects TCP for NAT/FW traversal, this
      guideline applies to all traffic multiplexed onto that TCP
      connection, in contrast to use of UDP for NAT/FW traversal.

   o  May use different DSCPs whose corresponding PHBs allow reordering
      within a single UDP or UDP-Lite 5-tuple, subject to the above
      constraints.  The service differentiation provided by such usage
      is unreliable, as it may be removed or changed by DSCP remarking
      at network boundaries as described in Section 3.2 above.

   o  Cannot rely on end-to-end preservation of DSCPs as network node
      remarking can change DSCPs and remove drop precedence distinctions
      (see Section 3.2 above).  For example, if a source uses drop
      precedence distinctions within an AF class to identify different
      types of video frames, using those DSCP values at the receiver to
      identify frame type is inherently unreliable.

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   o  Should limit use of the CS1 codepoint to traffic for which best
      effort forwarding is acceptable, as network support for use of CS1
      to select a "less than best effort" PHB is inconsistent.  Further,
      some networks may treat CS1 as providing "better than best effort"
      forwarding behavior.

   There is no guidance in this document on how network operators should
   differentiate traffic.  Networks may support all of the PHBs
   discussed herein, classify EF and AFxx traffic identically, or even
   remark all traffic to best effort at some ingress points.
   Nonetheless, it is useful for applications and other traffic sources
   to provide finer granularity DSCP marking on packets for the benefit
   of networks that offer QoS service differentiation.  A specific
   example is that traffic originating from a browser may benefit from
   QoS service differentiation in within-building and residential access
   networks, even if the DSCP marking is subsequently removed or
   simplified.  This is because such networks and the boundaries between
   them are likely traffic bottleneck locations (e.g., due to customer
   aggregation onto common links and/or speed differences among links
   used by the same traffic).

7.  IANA Considerations

   This document includes no request to IANA.

8.  Security Considerations

   The security considerations for all of the technologies discussed in
   this document apply; in particular see the security considerations
   for RTP in [RFC3550] and DiffServ in [RFC2474] and[RFC2475].

   Multiplexing of multiple protocols onto a single UDP 5-tuple via
   encapsulation has implications for network functionality that
   monitors or inspects individual protocol flows, e.g., firewalls and
   traffic monitoring systems.  When implementations of such
   functionality lack visibility into encapsulated traffic (likely for
   many current implementations), it may be difficult or impossible to
   apply network security policy and associated controls at a finer
   granularity than the overall UDP 5-tuple.

   Use of multiple DSCPs that allow reordering within an overall real-
   time communication interaction enlarges the set of network forwarding
   resources used by that interaction, thereby increasing exposure to
   resource depletion or failure, independent of whether the underlying
   cause is benign or malicious.  This represents an increase in the
   effective attack surface of the interaction, and is a consideration
   in selecting an appropriate degree of QoS differentiation among the
   components of the real-time communication interaction.

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   Use of multiple DSCPs to provide differentiated QoS service may
   reveal information about the encrypted traffic to which different
   service levels are provided.  For example, DSCP-based identification
   of RTP streams combined with packet frequency and packet size could
   reveal the type or nature of the encrypted source streams.  The IP
   header used for forwarding has to be unencrypted for obvious reasons,
   and the DSCP likewise has to be unencrypted to enable different IP
   forwarding behaviors to be applied to different packets.  The nature
   of encrypted traffic components can be disguised via encrypted dummy
   data padding and encrypted dummy packets, e.g., see the discussion of
   traffic flow confidentiality in [RFC4303].  Encrypted dummy packets
   could even be added in a fashion that an observer of the overall
   encrypted traffic might mistake for another encrypted RTP stream.

9.  Acknowledgements

   This document is the result of many conversations that have occurred
   within the dart working group and multiple other working groups in
   the RAI and Transport areas.  Many thanks to Aamer Akhter, Harald
   Alvestrand, Erin Bournival, Ben Campbell, Brian Carpenter, Keith
   Drage, Gorry Fairhurst, Ruediger Geib, Cullen Jennings, Jonathan
   Lennox, Karen Nielsen, Colin Perkins, James Polk, Michael Welzl, Dan
   York and the dart WG participants for their reviews and comments.

10.  References

10.1.  Normative References

              Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro,
              "A Taxonomy of Grouping Semantics and Mechanisms for Real-
              Time Transport Protocol (RTP) Sources", draft-ietf-avtext-
              rtp-grouping-taxonomy-02 (work in progress), June 2014.

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

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474, December

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

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

   [RFC3246]  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
              J., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, March 2002.

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

   [RFC3662]  Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
              Per-Domain Behavior (PDB) for Differentiated Services",
              RFC 3662, December 2003.

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
              G. Fairhurst, "The Lightweight User Datagram Protocol
              (UDP-Lite)", RFC 3828, July 2004.

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

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
              4960, September 2007.

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405, November

   [RFC5865]  Baker, F., Polk, J., and M. Dolly, "A Differentiated
              Services Code Point (DSCP) for Capacity-Admitted Traffic",
              RFC 5865, May 2010.

   [RFC6951]  Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
              Control Transmission Protocol (SCTP) Packets for End-Host
              to End-Host Communication", RFC 6951, May 2013.

10.2.  Informative References

   [H.222.0]  ITU-T, "H.222.0 : Information technology - Generic coding
              of moving pictures and associated audio information", June

              Geib, R., "DiffServ interconnection classes and practice",
              draft-geib-tsvwg-diffserv-intercon-06 (work in progress),
              July 2014.

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              Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
              "Sending Multiple Media Streams in a Single RTP Session:
              Grouping RTCP Reception Statistics and Other Feedback",
              draft-ietf-avtcore-rtp-multi-stream-optimisation-03 (work
              in progress), July 2014.

              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
              negotiation-08 (work in progress), August 2014.

              Jesup, R., "Congestion Control Requirements For RMCAT",
              draft-ietf-rmcat-cc-requirements-05 (work in progress),
              July 2014.

              Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
              Channels", draft-ietf-rtcweb-data-channel-11 (work in
              progress), July 2014.

              Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", draft-ietf-rtcweb-overview-11
              (work in progress), August 2014.

              Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
              Communication (WebRTC): Media Transport and Use of RTP",
              draft-ietf-rtcweb-rtp-usage-16 (work in progress), July

              Alvestrand, H., "Transports for WebRTC", draft-ietf-
              rtcweb-transports-06 (work in progress), August 2014.

              Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              draft-petithuguenin-avtcore-rfc5764-mux-fixes-00 (work in
              progress), July 2014.

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

   [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.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41, RFC
              2914, September 2000.

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

   [RFC3270]  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
              P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
              Protocol Label Switching (MPLS) Support of Differentiated
              Services", RFC 3270, May 2002.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, June 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
              4303, December 2005.

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

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

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

   [RFC5127]  Chan, K., Babiarz, J., and F. Baker, "Aggregation of
              Diffserv Service Classes", RFC 5127, February 2008.

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, January 2008.

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   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, February 2009.

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

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.

   [RFC6062]  Perreault, S. and J. Rosenberg, "Traversal Using Relays
              around NAT (TURN) Extensions for TCP Allocations", RFC
              6062, November 2010.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437, November 2011.

   [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
              Yasevich, "Sockets API Extensions for the Stream Control
              Transmission Protocol (SCTP)", RFC 6458, December 2011.

              Burnett, D., Bergkvist, A., Jennings, C., and A.
              Narayanan, "Media Capture and Streams", World Wide Web
              Consortium WD WD-mediacapture-streams-20130903, September
              2013, <http://www.w3.org/TR/2013/

Appendix A.  Change History

   [To be removed before RFC publication.]

   Changes from draft-york-dart-dscp-rtp-00 to -01:

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   o  Added examples (Section 5)

   o  Reworked text on RTP session multiplexing, at most one RTP session
      can be used per UDP 5-tuple.

   o  Initial terminology alignment with RTP grouping taxonomy draft.

   o  Added Section 2.5 on real-time communication interaction w/
      reordering based on text from Harald Alvestrand.

   o  Strengthened warnings on loss of differentiation, but indicate
      that differentiation may still be useful from source to point of

   o  Added a few sentences on DiffServ and MPLS.

   o  Added discussion of UDP-encapsulated protocols that are reordering

   o  Added initial security considerations.

   o  Many editorial changes

   Changes from draft-york-dart-dscp-rtp-01 to -02:

   o  More terminology alignment with RTP grouping taxonomy draft: "RTP
      packet stream" -> "RTP stream"

   o  Aligned terminology for less-than-best-effort with RFC 3662 - LE
      (Lower Effort) PHB and service

   o  Minor reference updates

   Changes from draft-york-dart-dscp-rtp-02 to draft-ietf-dart-dscp-rtp-

   o  Reduce author list and convert to Informational - remove RFC 2119
      reference and keywords

   o  Strengthen TCP and SCTP text.

   o  Add section 2.6 on drop precedence.

   o  Remove discussion of multiplexing multiple RTP sessions on a
      single UDP 5-tuple

   o  Add discussions of RTCP, STUN/ICE/TURN and coupled congestion

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   o  Many editorial changes.

   o  Lots of additional references

   Changes from draft-ietf-dart-dscp-rtp-00 to -01:

   o  Merge the two TCP/SCTP guideline bullets.

   o  Add DCCP and UDP-Lite material, plus reference to RFC 5405 for UDP
      (and UDP-Lite) usage guidelines.

   o  Add "attack surface" security consideration.

   o  Many editorial changes.

   o  More references, and moved some references to normative.

   Changes from draft-ietf-dart-dscp-rtp-01 to -02:

   o  Reorganize text for better topic flow and make related edits.

   Changes from draft-ietf-dart-dscp-rtp-02 to -03:

   o  Correct text on treatment of excess EF traffic to indicate that
      excess traffic is dropped.

   o  Say that transport protocol design and implementation guidance is
      not provided on use of multiple DSCPs and PHBs in a single
      transport protocol connection or association.

   o  RTCWEB -> WebRTC, and correct problems in descriptions of how it
      uses multiplexing.

   o  Fix DCCP congestion control discussion and text on coupled
      congestion controllers.

   o  Strengthen text on what happens when TURN selects TCP for NAT

   o  Note open issue on how to mark RTCP traffic.

   o  Many editorial changes.

   Changes from draft-ietf-dart-dscp-rtp-03 to -04:

   o  Add abstract/intro text on SDP bundle usage, e.g., by WebRTC

   o  Remove erroneous use of SSRC w/source stream in Section 2.1

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   o  Add text on WebRTC data channel examples

   o  Add text on transport protocol complexities that would be
      necessary to deal with multiple QoS levels in same protocol
      connection or association

   o  Additional minor edits.

Authors' Addresses

   David Black (editor)
   176 South Street
   Hopkinton, MA  01748

   Phone: +1 508 293-7953
   Email: david.black@emc.com

   Paul Jones
   7025 Kit Creek Road
   Research Triangle Park, MA  27502

   Phone: +1 919 476 2048
   Email: paulej@packetizer.com

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