Network Working Group                                      H. Alvestrand
Internet-Draft                                                    Google
Intended status: Standards Track                        October 26, 2016
Expires: April 29, 2017


                         Transports for WebRTC
                    draft-ietf-rtcweb-transports-17

Abstract

   This document describes the data transport protocols used by WebRTC,
   including the protocols used for interaction with intermediate boxes
   such as firewalls, relays and NAT boxes.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on April 29, 2017.

Copyright Notice

   Copyright (c) 2016 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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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements language . . . . . . . . . . . . . . . . . . . .   3
   3.  Transport and Middlebox specification . . . . . . . . . . . .   3
     3.1.  System-provided interfaces  . . . . . . . . . . . . . . .   3
     3.2.  Ability to use IPv4 and IPv6  . . . . . . . . . . . . . .   4
     3.3.  Usage of temporary IPv6 addresses . . . . . . . . . . . .   4
     3.4.  Middle box related functions  . . . . . . . . . . . . . .   5
     3.5.  Transport protocols implemented . . . . . . . . . . . . .   6
   4.  Media Prioritization  . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Local prioritization  . . . . . . . . . . . . . . . . . .   8
     4.2.  Usage of Quality of Service - DSCP and Multiplexing . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Change log . . . . . . . . . . . . . . . . . . . . .  16
     A.1.  Changes from -00 to -01 . . . . . . . . . . . . . . . . .  16
     A.2.  Changes from -01 to -02 . . . . . . . . . . . . . . . . .  16
     A.3.  Changes from -02 to -03 . . . . . . . . . . . . . . . . .  17
     A.4.  Changes from -03 to -04 . . . . . . . . . . . . . . . . .  17
     A.5.  Changes from -04 to -05 . . . . . . . . . . . . . . . . .  17
     A.6.  Changes from -05 to -06 . . . . . . . . . . . . . . . . .  17
     A.7.  Changes from -06 to -07 . . . . . . . . . . . . . . . . .  18
     A.8.  Changes from -07 to -08 . . . . . . . . . . . . . . . . .  18
     A.9.  Changes from -08 to -09 . . . . . . . . . . . . . . . . .  18
     A.10. Changes from -09 to -10 . . . . . . . . . . . . . . . . .  18
     A.11. Changes from -10 to -11 . . . . . . . . . . . . . . . . .  18
     A.12. Changes from -11 to -12 . . . . . . . . . . . . . . . . .  19
     A.13. Changes from -12 to -13 . . . . . . . . . . . . . . . . .  19
     A.14. Changes from -13 to -14 . . . . . . . . . . . . . . . . .  19
     A.15. Changes from -14 to -15 . . . . . . . . . . . . . . . . .  19
     A.16. Changes from -15 to -16 . . . . . . . . . . . . . . . . .  19
     A.17. Changes from -16 to -17 . . . . . . . . . . . . . . . . .  20
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   WebRTC is a protocol suite aimed at real time multimedia exchange
   between browsers, and between browsers and other entities.

   WebRTC is described in the WebRTC overview document,
   [I-D.ietf-rtcweb-overview], which also defines terminology used in
   this document, including the terms "WebRTC endpoint" and "WebRTC
   browser".



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   Terminology for RTP sources is taken from[RFC7656] .

   This document focuses on the data transport protocols that are used
   by conforming implementations, including the protocols used for
   interaction with intermediate boxes such as firewalls, relays and NAT
   boxes.

   This protocol suite intends to satisfy the security considerations
   described in the WebRTC security documents,
   [I-D.ietf-rtcweb-security] and [I-D.ietf-rtcweb-security-arch].

   This document describes requirements that apply to all WebRTC
   endpoints.  When there are requirements that apply only to WebRTC
   browsers, this is called out explicitly.

2.  Requirements language

   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.  Transport and Middlebox specification

3.1.  System-provided interfaces

   The protocol specifications used here assume that the following
   protocols are available to the implementations of the WebRTC
   protocols:

   o  UDP [RFC0768].  This is the protocol assumed by most protocol
      elements described.

   o  TCP [RFC0793].  This is used for HTTP/WebSockets, as well as for
      TURN/TLS and ICE-TCP.

   For both protocols, IPv4 and IPv6 support is assumed.

   For UDP, this specification assumes the ability to set the DSCP code
   point of the sockets opened on a per-packet basis, in order to
   achieve the prioritizations described in [I-D.ietf-tsvwg-rtcweb-qos]
   (see Section 4.2) when multiple media types are multiplexed.  It does
   not assume that the DSCP codepoints will be honored, and does assume
   that they may be zeroed or changed, since this is a local
   configuration issue.

   Platforms that do not give access to these interfaces will not be
   able to support a conforming WebRTC endpoint.




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   This specification does not assume that the implementation will have
   access to ICMP or raw IP.

   The following protocols may be used, but can be implemented by a
   WebRTC endpoint, and are therefore not defined as "system-provided
   interfaces":

   o  TURN - Traversal Using Relays Around NAT, [RFC5766]

   o  STUN - Session Traversal Utilities for NAT, [RFC5389]

   o  ICE - Interactive Connectivity Establishment,
      [I-D.ietf-ice-rfc5245bis]

   o  TLS - Transport Layer Security, [RFC5246]

   o  DTLS - Datagram Transport Layer Security, [RFC6347].

3.2.  Ability to use IPv4 and IPv6

   Web applications running in a WebRTC browser MUST be able to utilize
   both IPv4 and IPv6 where available - that is, when two peers have
   only IPv4 connectivity to each other, or they have only IPv6
   connectivity to each other, applications running in the WebRTC
   browser MUST be able to communicate.

   When TURN is used, and the TURN server has IPv4 or IPv6 connectivity
   to the peer or the peer's TURN server, candidates of the appropriate
   types MUST be supported.  The "Happy Eyeballs" specification for ICE
   [I-D.ietf-mmusic-ice-dualstack-fairness] SHOULD be supported.

3.3.  Usage of temporary IPv6 addresses

   The IPv6 default address selection specification [RFC6724] specifies
   that temporary addresses [RFC4941] are to be preferred over permanent
   addresses.  This is a change from the rules specified by [RFC3484].
   For applications that select a single address, this is usually done
   by the IPV6_PREFER_SRC_TMP preference flag specified in [RFC5014].
   However, this rule, which is intended to ensure that privacy-enhanced
   addresses are used in preference to static addresses, doesn't have
   the right effect in ICE, where all addresses are gathered and
   therefore revealed to the application.  Therefore, the following rule
   is applied instead:

   When a WebRTC endpoint gathers all IPv6 addresses on its host, and
   both non-deprecated temporary addresses and permanent addresses of
   the same scope are present, the WebRTC endpoint SHOULD discard the
   permanent addresses before exposing addresses to the application or



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   using them in ICE.  This is consistent with the default policy
   described in [RFC6724].

   If some of the temporary IPv6 addresses, but not all, are marked
   deprecated, the WebRTC endpoint SHOULD discard the deprecated
   addresses, unless they are used by an ongoing connection.  In an ICE
   restart, deprecated addresses that are currently in use MAY be
   retained.

3.4.  Middle box related functions

   The primary mechanism to deal with middle boxes is ICE, which is an
   appropriate way to deal with NAT boxes and firewalls that accept
   traffic from the inside, but only from the outside if it is in
   response to inside traffic (simple stateful firewalls).

   ICE [I-D.ietf-ice-rfc5245bis] MUST be supported.  The implementation
   MUST be a full ICE implementation, not ICE-Lite.  A full ICE
   implementation allows interworking with both ICE and ICE-Lite
   implementations when they are deployed appropriately.

   In order to deal with situations where both parties are behind NATs
   of the type that perform endpoint-dependent mapping (as defined in
   [RFC5128] section 2.4), TURN [RFC5766] MUST be supported.

   WebRTC browsers MUST support configuration of STUN and TURN servers,
   both from browser configuration and from an application.

   Note that there is other work around STUN and TURN sever discovery
   and management, including [I-D.ietf-tram-turn-server-discovery] for
   server discovery, as well as [I-D.ietf-rtcweb-return].

   In order to deal with firewalls that block all UDP traffic, the mode
   of TURN that uses TCP between the WebRTC endpoint and the TURN server
   MUST be supported, and the mode of TURN that uses TLS over TCP
   between the WebRTC endpoint and the TURN server MUST be supported.
   See [RFC5766] section 2.1 for details.

   In order to deal with situations where one party is on an IPv4
   network and the other party is on an IPv6 network, TURN extensions
   for IPv6 [RFC6156] MUST be supported.

   TURN TCP candidates, where the connection from the WebRTC endpoint's
   TURN server to the peer is a TCP connection, [RFC6062] MAY be
   supported.

   However, such candidates are not seen as providing any significant
   benefit, for the following reasons.



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   First, use of TURN TCP candidates would only be relevant in cases
   which both peers are required to use TCP to establish a
   PeerConnection.

   Second, that use case is supported in a different way by both sides
   establishing UDP relay candidates using TURN over TCP to connect to
   their respective relay servers.

   Third, using TCP between the WebRTC endpoint's TURN server and the
   peer may result in more performance problems than using UDP, e.g. due
   to head of line blocking.

   ICE-TCP candidates [RFC6544] MUST be supported; this may allow
   applications to communicate to peers with public IP addresses across
   UDP-blocking firewalls without using a TURN server.

   If TCP connections are used, RTP framing according to [RFC4571] MUST
   be used for all packets.  This includes the RTP packets, DTLS packets
   used to carry data channels, and STUN connectivity check packets.

   The ALTERNATE-SERVER mechanism specified in [RFC5389] (STUN) section
   11 (300 Try Alternate) MUST be supported.

   The WebRTC endpoint MAY support accessing the Internet through an
   HTTP proxy.  If it does so, it MUST include the "ALPN" header as
   specified in [RFC7639], and proxy authentication as described in
   Section 4.3.6 of [RFC7231] and [RFC7235] MUST also be supported.

3.5.  Transport protocols implemented

   For transport of media, secure RTP is used.  The details of the
   profile of RTP used are described in "RTP Usage"
   [I-D.ietf-rtcweb-rtp-usage], which mandates the use of a circuit
   breaker [I-D.ietf-avtcore-rtp-circuit-breakers] and congstion control
   (see [I-D.ietf-rmcat-cc-requirements] for further guidance).

   Key exchange MUST be done using DTLS-SRTP, as described in
   [I-D.ietf-rtcweb-security-arch].

   For data transport over the WebRTC data channel
   [I-D.ietf-rtcweb-data-channel], WebRTC endpoints MUST support SCTP
   over DTLS over ICE.  This encapsulation is specified in
   [I-D.ietf-tsvwg-sctp-dtls-encaps].  Negotiation of this transport in
   SDP is defined in [I-D.ietf-mmusic-sctp-sdp].  The SCTP extension for
   NDATA, [I-D.ietf-tsvwg-sctp-ndata], MUST be supported.

   The setup protocol for WebRTC data channels described in
   [I-D.ietf-rtcweb-data-protocol] MUST be supported.



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   Note: DTLS-SRTP as defined in [RFC5764] section 6.7.1 defines the
   interaction between DTLS and ICE ( [I-D.ietf-ice-rfc5245bis]).  The
   effect of this specification is that all ICE candidate pairs
   associated with a single component are part of the same DTLS
   association.  Thus, there will only be one DTLS handshake even if
   there are multiple valid candidate pairs.

   WebRTC endpoints MUST support multiplexing of DTLS and RTP over the
   same port pair, as described in the DTLS-SRTP specification
   [RFC5764], section 5.1.2, with clarifications in
   [I-D.ietf-avtcore-rfc5764-mux-fixes].  All application layer protocol
   payloads over this DTLS connection are SCTP packets.

   Protocol identification MUST be supplied as part of the DTLS
   handshake, as specified in [I-D.ietf-rtcweb-alpn].

4.  Media Prioritization

   The WebRTC prioritization model is that the application tells the
   WebRTC endpoint about the priority of media and data that is
   controlled from the API.

   In this context, a "flow" is used for the units that are given a
   specific priority through the WebRTC API.

   For media, a "media flow", which can be an "audio flow" or a "video
   flow", is what [RFC7656] calls a "media source", which results in a
   "source RTP stream" and one or more "redundancy RTP streams".  This
   specification does not describe prioritization between the RTP
   streams that come from a single "media source".

   All media flows in WebRTC are assumed to be interactive, as defined
   in [RFC4594]; there is no browser API support for indicating whether
   media is interactive or non-interactive.

   A "data flow" is the outgoing data on a single WebRTC data channel.

   The priority associated with a media flow or data flow is classified
   as "very-low", "low", "medium or "high".  There are only four
   priority levels at the API.

   The priority settings affect two pieces of behavior: Packet send
   sequence decisions and packet markings.  Each is described in its own
   section below.







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4.1.  Local prioritization

   Local prioritization is applied at the local node, before the packet
   is sent.  This means that the prioritization has full access to the
   data about the individual packets, and can choose differing treatment
   based on the stream a packet belongs to.

   When an WebRTC endpoint has packets to send on multiple streams that
   are congestion-controlled under the same congestion control regime,
   the WebRTC endpoint SHOULD cause data to be emitted in such a way
   that each stream at each level of priority is being given
   approximately twice the transmission capacity (measured in payload
   bytes) of the level below.

   Thus, when congestion occurs, a "high" priority flow will have the
   ability to send 8 times as much data as a "very-low" priority flow if
   both have data to send.  This prioritization is independent of the
   media type.  The details of which packet to send first are
   implementation defined.

   For example: If there is a high priority audio flow sending 100 byte
   packets, and a low priority video flow sending 1000 byte packets, and
   outgoing capacity exists for sending >5000 payload bytes, it would be
   appropriate to send 4000 bytes (40 packets) of audio and 1000 bytes
   (one packet) of video as the result of a single pass of sending
   decisions.

   Conversely, if the audio flow is marked low priority and the video
   flow is marked high priority, the scheduler may decide to send 2
   video packets (2000 bytes) and 5 audio packets (500 bytes) when
   outgoing capacity exists for sending > 2500 payload bytes.

   If there are two high priority audio flows, each will be able to send
   4000 bytes in the same period where a low priority video flow is able
   to send 1000 bytes.

   Two example implementation strategies are:

   o  When the available bandwidth is known from the congestion control
      algorithm, configure each codec and each data channel with a
      target send rate that is appropriate to its share of the available
      bandwidth.

   o  When congestion control indicates that a specified number of
      packets can be sent, send packets that are available to send using
      a weighted round robin scheme across the connections.





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   Any combination of these, or other schemes that have the same effect,
   is valid, as long as the distribution of transmission capacity is
   approximately correct.

   For media, it is usually inappropriate to use deep queues for
   sending; it is more useful to, for instance, skip intermediate frames
   that have no dependencies on them in order to achieve a lower
   bitrate.  For reliable data, queues are useful.

   Note that this specification doesn't dictate when disparate streams
   are to be "congestion controlled under the same congestion control
   regime".  The issue of coupling congestion controllers is explored
   further in [I-D.ietf-rmcat-coupled-cc].

4.2.  Usage of Quality of Service - DSCP and Multiplexing

   When the packet is sent, the network will make decisions about
   queueing and/or discarding the packet that can affect the quality of
   the communication.  The sender can attempt to set the DSCP field of
   the packet to influence these decisions.

   Implementations SHOULD attempt to set QoS on the packets sent,
   according to the guidelines in [I-D.ietf-tsvwg-rtcweb-qos].  It is
   appropriate to depart from this recommendation when running on
   platforms where QoS marking is not implemented.

   The implementation MAY turn off use of DSCP markings if it detects
   symptoms of unexpected behaviour like priority inversion or blocking
   of packets with certain DSCP markings.  Some examples of such
   behaviors are described in [ANRW16].  The detection of these
   conditions is implementation dependent.

   A particularly hard problem is when one media transport uses multiple
   DSCP code points, where one may be blocked and another may be
   allowed.  This is allowed even within a single media flow for video
   in [I-D.ietf-tsvwg-rtcweb-qos].  Implementations need to diagnose
   this scenario; one possible implementation is to send initial ICE
   probes with DSCP 0, and send ICE probes on all the DSCP code points
   that are intended to be used once a candidate pair has been selected.
   If one or more of the DSCP-marked probes fail, the sender will switch
   the media type to using DSCP 0.  This can be carried out
   simultaneously with the initial media traffic; on failure, the
   initial data may need to be resent.  This switch will of course
   invalidate any congestion information gathered up to that point.

   Failures can also start happening during the lifetime of the call;
   this case is expected to be rarer, and can be handled by the normal
   mechanisms for transport failure, which may involve an ICE restart.



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   Note that when a DSCP code point causes non-delivery, one has to
   switch the whole media flow to DSCP 0, since all traffic for a single
   media flow needs to be on the same queue for congestion control
   purposes.  Other flows on the same transport, using different DSCP
   code points, don't need to change.

   All packets carrying data from the SCTP association supporting the
   data channels MUST use a single DSCP code point.  The code point used
   SHOULD be that recommended by [I-D.ietf-tsvwg-rtcweb-qos] for the
   highest priority data channel carried.  Note that this means that all
   data packets, no matter what their relative priority is, will be
   treated the same by the network.

   All packets on one TCP connection, no matter what it carries, MUST
   use a single DSCP code point.

   More advice on the use of DSCP code points with RTP and on the
   relationship between DSCP and congestion control is given in
   [RFC7657].

   There exist a number of schemes for achieving quality of service that
   do not depend solely on DSCP code points.  Some of these schemes
   depend on classifying the traffic into flows based on 5-tuple (source
   address, source port, protocol, destination address, destination
   port) or 6-tuple (5-tuple + DSCP code point).  Under differing
   conditions, it may therefore make sense for a sending application to
   choose any of the configurations:

   o  Each media stream carried on its own 5-tuple

   o  Media streams grouped by media type into 5-tuples (such as
      carrying all audio on one 5-tuple)

   o  All media sent over a single 5-tuple, with or without
      differentiation into 6-tuples based on DSCP code points

   In each of the configurations mentioned, data channels may be carried
   in its own 5-tuple, or multiplexed together with one of the media
   flows.

   More complex configurations, such as sending a high priority video
   stream on one 5-tuple and sending all other video streams multiplexed
   together over another 5-tuple, can also be envisioned.  More
   information on mapping media flows to 5-tuples can be found in
   [I-D.ietf-rtcweb-rtp-usage].

   A sending implementation MUST be able to support the following
   configurations:



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   o  Multiplex all media and data on a single 5-tuple (fully bundled)

   o  Send each media stream on its own 5-tuple and data on its own
      5-tuple (fully unbundled)

   It MAY choose to support other configurations, such as bundling each
   media type (audio, video or data) into its own 5-tuple (bundling by
   media type).

   Sending data channel data over multiple 5-tuples is not supported.

   A receiving implementation MUST be able to receive media and data in
   all these configurations.

5.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.

6.  Security Considerations

   RTCWEB security considerations are enumerated in
   [I-D.ietf-rtcweb-security].

   Security considerations pertaining to the use of DSCP are enumerated
   in [I-D.ietf-tsvwg-rtcweb-qos].

7.  Acknowledgements

   This document is based on earlier versions embedded in
   [I-D.ietf-rtcweb-overview], which were the results of contributions
   from many RTCWEB WG members.

   Special thanks for reviews of earlier versions of this draft go to
   Eduardo Gueiros, Magnus Westerlund, Markus Isomaki and Dan Wing; the
   contributions from Andrew Hutton also deserve special mention.

8.  References

8.1.  Normative References









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   [I-D.ietf-avtcore-rfc5764-mux-fixes]
              Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              draft-ietf-avtcore-rfc5764-mux-fixes-11 (work in
              progress), September 2016.

   [I-D.ietf-avtcore-rtp-circuit-breakers]
              Perkins, C. and V. Singh, "Multimedia Congestion Control:
              Circuit Breakers for Unicast RTP Sessions", draft-ietf-
              avtcore-rtp-circuit-breakers-06 (work in progress), July
              2014.

   [I-D.ietf-ice-rfc5245bis]
              Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", draft-ietf-ice-
              rfc5245bis-04 (work in progress), June 2016.

   [I-D.ietf-mmusic-ice-dualstack-fairness]
              Martinsen, P., Reddy, T., and P. Patil, "ICE Multihomed
              and IPv4/IPv6 Dual Stack Fairness", draft-ietf-mmusic-ice-
              dualstack-fairness-02 (work in progress), September 2015.

   [I-D.ietf-mmusic-sctp-sdp]
              Loreto, S. and G. Camarillo, "Stream Control Transmission
              Protocol (SCTP)-Based Media Transport in the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sctp-sdp-07
              (work in progress), July 2014.

   [I-D.ietf-rmcat-cc-requirements]
              Jesup, R., "Congestion Control Requirements For RMCAT",
              draft-ietf-rmcat-cc-requirements-06 (work in progress),
              October 2014.

   [I-D.ietf-rtcweb-alpn]
              Thomson, M., "Application Layer Protocol Negotiation for
              Web Real-Time Communications (WebRTC)", draft-ietf-rtcweb-
              alpn-00 (work in progress), July 2014.

   [I-D.ietf-rtcweb-data-channel]
              Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
              Channels", draft-ietf-rtcweb-data-channel-12 (work in
              progress), September 2014.







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   [I-D.ietf-rtcweb-data-protocol]
              Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channel
              Establishment Protocol", draft-ietf-rtcweb-data-
              protocol-08 (work in progress), September 2014.

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

   [I-D.ietf-rtcweb-rtp-usage]
              Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
              Communication (WebRTC): Media Transport and Use of RTP",
              draft-ietf-rtcweb-rtp-usage-17 (work in progress), August
              2014.

   [I-D.ietf-rtcweb-security]
              Rescorla, E., "Security Considerations for WebRTC", draft-
              ietf-rtcweb-security-07 (work in progress), July 2014.

   [I-D.ietf-rtcweb-security-arch]
              Rescorla, E., "WebRTC Security Architecture", draft-ietf-
              rtcweb-security-arch-10 (work in progress), July 2014.

   [I-D.ietf-tsvwg-rtcweb-qos]
              Dhesikan, S., Jennings, C., Druta, D., Jones, P., and J.
              Polk, "DSCP and other packet markings for RTCWeb QoS",
              draft-ietf-tsvwg-rtcweb-qos-02 (work in progress), June
              2014.

   [I-D.ietf-tsvwg-sctp-dtls-encaps]
              Tuexen, M., Stewart, R., Jesup, R., and S. Loreto, "DTLS
              Encapsulation of SCTP Packets", draft-ietf-tsvwg-sctp-
              dtls-encaps-05 (work in progress), July 2014.

   [I-D.ietf-tsvwg-sctp-ndata]
              Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
              "Stream Schedulers and a New Data Chunk for the Stream
              Control Transmission Protocol", draft-ietf-tsvwg-sctp-
              ndata-01 (work in progress), July 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.





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

   [RFC4571]  Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
              and RTP Control Protocol (RTCP) Packets over Connection-
              Oriented Transport", RFC 4571, July 2006.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594, August
              2006.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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

   [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.

   [RFC6156]  Camarillo, G., Novo, O., and S. Perreault, "Traversal
              Using Relays around NAT (TURN) Extension for IPv6", RFC
              6156, April 2011.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6544]  Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
              "TCP Candidates with Interactive Connectivity
              Establishment (ICE)", RFC 6544, March 2012.

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012.



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   [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.

   [RFC7235]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Authentication", RFC 7235, June 2014.

   [RFC7639]  Hutton, A., Uberti, J., and M. Thomson, "The ALPN HTTP
              Header Field", RFC 7639, DOI 10.17487/RFC7639, August
              2015, <http://www.rfc-editor.org/info/rfc7639>.

   [RFC7656]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
              DOI 10.17487/RFC7656, November 2015,
              <http://www.rfc-editor.org/info/rfc7656>.

8.2.  Informative References

   [ANRW16]   Barik, R., Welzl, M., and A. Elmokashfi, "How to say that
              you're special: Can we use bits in the IPv4 header?", ACM,
              IRTF, ISOC Applied Networking Research Workshop (ANRW
              2016), Berlin , July 2016.

   [I-D.ietf-rmcat-coupled-cc]
              Islam, S., Welzl, M., and S. Gjessing, "Coupled congestion
              control for RTP media", draft-ietf-rmcat-coupled-cc-03
              (work in progress), July 2016.

   [I-D.ietf-rtcweb-return]
              Schwartz, B. and J. Uberti, "Recursively Encapsulated TURN
              (RETURN) for Connectivity and Privacy in WebRTC", draft-
              ietf-rtcweb-return-01 (work in progress), January 2016.

   [I-D.ietf-tram-turn-server-discovery]
              Patil, P., Reddy, T., and D. Wing, "TURN Server Auto
              Discovery", draft-ietf-tram-turn-server-discovery-00 (work
              in progress), July 2014.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC5014]  Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
              Socket API for Source Address Selection", RFC 5014,
              September 2007.

   [RFC5128]  Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to-
              Peer (P2P) Communication across Network Address
              Translators (NATs)", RFC 5128, March 2008.



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   [RFC7657]  Black, D., Ed. and P. Jones, "Differentiated Services
              (Diffserv) and Real-Time Communication", RFC 7657, DOI 10
              .17487/RFC7657, November 2015,
              <http://www.rfc-editor.org/info/rfc7657>.

Appendix A.  Change log

   This section should be removed before publication as an RFC.

A.1.  Changes from -00 to -01

   o  Clarified DSCP requirements, with reference to -qos-

   o  Clarified "symmetric NAT" -> "NATs which perform endpoint-
      dependent mapping"

   o  Made support of TURN over TCP mandatory

   o  Made support of TURN over TLS a MAY, and added open question

   o  Added an informative reference to -firewalls-

   o  Called out that we don't make requirements on HTTP proxy
      interaction (yet

A.2.  Changes from -01 to -02

   o  Required support for 300 Alternate Server from STUN.

   o  Separated the ICE-TCP candidate requirement from the TURN-TCP
      requirement.

   o  Added new sections on using QoS functions, and on multiplexing
      considerations.

   o  Removed all mention of RTP profiles.  Those are the business of
      the RTP usage draft, not this one.

   o  Required support for TURN IPv6 extensions.

   o  Removed reference to the TURN URI scheme, as it was unnecessary.

   o  Made an explicit statement that multiplexing (or not) is an
      application matter.

   .





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A.3.  Changes from -02 to -03

   o  Added required support for draft-ietf-tsvwg-sctp-ndata

   o  Removed discussion of multiplexing, since this is present in rtp-
      usage.

   o  Added RFC 4571 reference for framing RTP packets over TCP.

   o  Downgraded TURN TCP candidates from SHOULD to MAY, and added more
      language discussing TCP usage.

   o  Added language on IPv6 temporary addresses.

   o  Added language describing multiplexing choices.

   o  Added a separate section detailing what it means when we say that
      an WebRTC implementation MUST support both IPv4 and IPv6.

A.4.  Changes from -03 to -04

   o  Added a section on prioritization, moved the DSCP section into it,
      and added a section on local prioritization, giving a specific
      algorithm for interpreting "priority" in local prioritization.

   o  ICE-TCP candidates was changed from MAY to MUST, in recognition of
      the sense of the room at the London IETF.

A.5.  Changes from -04 to -05

   o  Reworded introduction

   o  Removed all references to "WebRTC".  It now uses only the term
      RTCWEB.

   o  Addressed a number of clarity / language comments

   o  Rewrote the prioritization to cover data channels and to describe
      multiple ways of prioritizing flows

   o  Made explicit reference to "MUST do DTLS-SRTP", and referred to
      security-arch for details

A.6.  Changes from -05 to -06

   o  Changed all references to "RTCWEB" to "WebRTC", except one
      reference to the working group




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   o  Added reference to the httpbis "connect" protocol (being adopted
      by HTTPBIS)

   o  Added reference to the ALPN header (being adopted by RTCWEB)

   o  Added reference to the DART RTP document

   o  Said explicitly that SCTP for data channels has a single DSCP
      codepoint

A.7.  Changes from -06 to -07

   o  Updated references

   o  Removed reference to draft-hutton-rtcweb-nat-firewall-
      considerations

A.8.  Changes from -07 to -08

   o  Updated references

   o  Deleted "bundle each media type (audio, video or data) into its
      own 5-tuple (bundling by media type)" from MUST support
      configuration, since JSEP does not have a means to negotiate this
      configuration

A.9.  Changes from -08 to -09

   o  Added a clarifying note about DTLS-SRTP and ICE interaction.

A.10.  Changes from -09 to -10

   o  Re-added references to proxy authentication lost in 07-08
      transition (Bug #5)

   o  Rearranged and rephrased text in section 4 about prioritization to
      reflect discussions in TSVWG.

   o  Changed the "Connect" header to "ALPN", and updated reference.
      (Bug #6)

A.11.  Changes from -10 to -11

   o  Added a definition of the term "flow" used in the prioritization
      chapter

   o  Changed the names of the four priority levels to conform to other
      specs.



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A.12.  Changes from -11 to -12

   o  Added a SHOULD NOT about using deprecated temporary IPv6
      addresses.

   o  Updated draft-ietf-dart-dscp-rtp reference to RFC 7657

A.13.  Changes from -12 to -13

   o  Clarify that the ALPN header needs to be sent.

   o  Mentioned that RFC 7657 also talks about congestion control

A.14.  Changes from -13 to -14

   o  Add note about non-support for marking flows as interactive or
      non-interactive.

A.15.  Changes from -14 to -15

   o  Various text clarifications based on comments in Last Call and
      IESG review

   o  Clarified that only non-deprecated IPv6 addresses are used

   o  Described handling of downgrading of DSCP markings when blackholes
      are detected

   o  Expanded acronyms in a new protocol list

A.16.  Changes from -15 to -16

   These changes are done post IESG approval, and address IESG comments
   and other late comments.  Issue numbers refer to https://github.com/
   rtcweb-wg/rtcweb-transport/issues.

   o  Moved RFC 4594, 7656 and -overview to normative (issue #28)

   o  Changed the terms "client", "WebRTC implementation" and "WebRTC
      device" to consistently be "WebRTC endpoint", as defined in
      -overview. (issue #40)

   o  Added a note mentioning TURN service discovery and RETURN (issue
      #42)

   o  Added a note mentioning that rtp-usage requires circut breaker and
      congestion control (issue #43)




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   o  Added mention of the "don't discard temporary IPv6 addresses that
      are in use" (issue #44)

   o  Added a reference to draft-ietf-rmcat-coupled-cc (issue #46)

A.17.  Changes from -16 to -17

   o  Added an informative reference to the "DSCP blackholing" paper

   o  Changed the reference for ICE from RFC 5245 to draft-ietf-ice-
      rfc5245bis

Author's Address

   Harald Alvestrand
   Google

   Email: harald@alvestrand.no

































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