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TRAM                                                          M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                                B. Aboba
Expires: January 5, 2015                                       Microsoft
                                                             A. Johnston
                                                           O. Moskalenko
                                                          public project
                                                            July 4, 2014

                     A Bandwidth Attribute for TURN


   An attribute is defined for Session Traversal Utilities for NAT
   (STUN) that allows for declarations of bandwidth limits on the
   negotiated flow.  The application of this attribute is the
   negotiation of bandwidth between a Traversal Using Relays around NAT
   (TURN) client and a TURN server.

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
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

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

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   3.  The BANDWIDTH Attribute . . . . . . . . . . . . . . . . . . . . 4
   4.  Applications  . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  STUN Usage  . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.2.  TURN Usage  . . . . . . . . . . . . . . . . . . . . . . . . 5
     4.3.  ICE Usage . . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Bandwidth Measurement Considerations  . . . . . . . . . . . . . 5
     5.1.  Rate Enforcement  . . . . . . . . . . . . . . . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . . . 6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   This document defines a BANDWIDTH attribute that can be used to
   request and allocate bandwidth at a Traversal Using Relays around NAT
   (TURN) relay [RFC5766].

   The operator of a TURN server will likely wish to provide fairness
   between relayed sessions.  A TURN server might also wish to limit the
   use of service to audio-only sessions, or low bandwidth video and
   audio sessions.  In addition, the server may apply rate-limiting
   policy depending on the credential used for authentication, or the
   origin of the client.  Without the BANDWIDTH attribute, there is no
   way for a client to indicate the expected bandwidth utilization, or
   for the server to indicate the maximum bandwidth utilization allowed
   before rate limiting could be applied.

   This attribute is used for indicating a bandwidth limit that is set
   in policy.  The sender is not advised or required to utilize
   bandwidth up to this limit; limits are usually set well in excess of
   application needs.  Senders also limit their use of bandwidth in
   reaction to path congestion and "circuit breakers".

   Note that the BANDWIDTH attribute was originally in the TURN draft up
   to version draft-ietf-behave-turn-07 where it was removed as "the
   requirements for this feature were not clear and it was felt the
   feature could be easily added later."  This draft proposes adding
   this attribute back into TURN.  A related error code 507
   "Insufficient Bandwidth Capacity" was also defined in the TURN
   Internet-Draft, but is not proposed in this draft.  This attribute
   has also been proposed to be used by ICE to provide communication
   consent [I-D.thomson-mmusic-rtcweb-bw-consent].  No use cases have
   been identified where bandwidth information is useful for a STUN
   server which is responding to STUN binding requests.

   There have been discussions about what other media-related
   information could be usefully exchanged between a TURN client and a
   TURN server.  One proposal was for the actual media type (voice,
   video, data) to be exchanged.  Other proposals include more
   granularity over the bandwidth, including max, min, average, etc.
   While these could be added, the authors do not feel the use cases for
   these data have been sufficiently developed yet.  Also, this
   information is known in signaling through the SDP attributes and
   parameters.  In a particular implementation, it could be possible for
   a signaling-aware entity to share this information with a TURN server
   in order to apply policy for the media relay.

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2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [RFC2119] and indicate requirement
   levels for compliant implementations.

   The terms client, server, and peer are those used for TURN, as
   defined in [RFC5766].

3.  The BANDWIDTH Attribute

   The BANDWIDTH attribute (identifier TBD) identifies the rate of
   packet transmission in kilobits per second that is permitted for a
   given transport flow.  The BANDWIDTH attribute is a comprehension-
   optional attribute (see Section 15 from [RFC5389]).  Figure 1 shows
   the format of this attribute.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |     Attribute Type (TBD)      |          Length (4)           |
      |                          Bandwidth                            |

                   Figure 1: Bandwidth Attribute Format

   The value of this attribute is an unsigned integer that represents
   the maximum bandwidth for the flow in kilobits per second (1 kilobit
   = 1024 bits).  This is the original format of the Bandwidth
   attribute.  This format could include a maximum and average
   bandwidth, as the BANDWIDTH-USAGE attribute proposed in

4.  Applications

   This section discusses the application of the BANDWIDTH attribute for
   STUN, TURN, and ICE.

4.1.  STUN Usage

   Since the bandwidth of a communications session has no bearing on a
   STUN server that simply responds to binding requests, this attribute
   MUST NOT be used for client-server STUN requests or responses.

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4.2.  TURN Usage

   This attribute can be useful for communication between a TURN client
   and a TURN server.

   The BANDWIDTH attribute indicates a limit to available bandwidth for
   TURN [RFC5766] allocation.  The bandwidth limit is symmetric; the
   value covers the bandwidth of data sent from a peer toward the TURN
   server and the bandwidth of data sent from client to the TURN server.

   A BANDWIDTH attribute MAY be present in an Allocate request.  This
   attribute indicates that the given bandwidth is requested.  A
   BANDWIDTH attribute MAY be present in an Allocate response.  This
   attribute in a response indicates the limit that will be applied by
   the TURN server.  The value a TURN server provides could be
   influenced by the value that a TURN client requests at the discretion
   of server policy.  A client could use this bandwidth limitation of
   the TURN server in choosing media types or in choosing codecs for a
   media session.

4.3.  ICE Usage

   While [I-D.thomson-mmusic-rtcweb-bw-consent] proposed the use of the
   BANDWIDTH attribute to provide bandwidth consent for ICE, this draft
   does not do so.  This attribute MUST NOT be used with ICE.

5.  Bandwidth Measurement Considerations

   Allocation messages (Binding and Allocate) sent to and from the TURN
   server are exempt from any bandwidth measurement accounting.

   In calculating bandwidth, the entire IP packet - including the header
   - is measured.  This is identical to the measurement performed by the
   Real-time Transport Protocol (RTP) [RFC3550].  At a TURN server,
   bandwidth measurement is performed on the packets arriving at or
   leaving from the TURN server, prior to the encapsulation that occurs
   between TURN server and TURN client.

   Determining the rate requires that the bits be allocated to specific
   intervals of time.  How bits are allocated MAY vary between

   Measurement of bandwidth is imperfect and inconsistent.  Packet
   jitter can result in fluctuations in received packet rate so that a
   receiver might see an instantaneous bandwidth that is different to
   what the sender might have transmitted.  Jitter can cause the
   observed bandwidth of incoming packets to temporarily increase above

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   the permitted rate.  At a minimum, implementations SHOULD allow for
   short periods of excessive bandwidth to allow for these temporary

5.1.  Rate Enforcement

   Enforcement of limits by the TURN server SHOULD provide an allowance
   for application usages that temporarily exceed the limit.  For
   example, assessing observed bandwidth usage as an average over 10
   seconds ensures that real-time video does not clip unnecessarily;
   shorter durations could result in the enforcement affecting valuable

6.  Security Considerations

   For STUN requests or responses that are not sent using TLS or DTLS
   transport, the bandwidth information contained in the BANDWIDTH
   attribute will be available to an eavesdropper who could use it to
   learn about the nature of a session to be established.  For example,
   they might be able to deduce from the bandwidth requested that the
   session is likely to be audio only, or audio and video.  However, an
   on-path attacker can likely learn this same information from either
   the signaling channel or by inspecting the RTP packet headers, which
   are in the clear for SRTP, or simply by measuring the media bandwidth

   If a STUN request or response is transported using TCP or UDP, the
   BANDWIDTH attribute will have integrity protection from the MESSAGE-
   INTEGRITY attribute if the request is authenticated using the STUN
   short-term or long-term authentication method.  Unauthenticated TCP
   or UDP requests will not have integrity protection and could be
   modified by a MitM attacker.  The use of DTLS transport
   [I-D.ietf-tram-stun-dtls] provides integrity protection for the
   BANDWIDTH attribute regardless of the STUN authentication method

7.  IANA Considerations

   The STUN BANDWIDTH attribute uses the TBD value in the comprehension-
   optional range.  This attribute is registered in the "STUN Attribute"
   Registry following the procedures of Section 18.2 of [RFC5389].

8.  Implementation Status

   Note to RFC Editor: Please remove this entire section prior to

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   publication, including the reference to RFC 6982.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC6982].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may

   According to [RFC6982], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   A multiple realms capable advanced open source TURN server (named
   'Coturn') has been created by Oleg Moskalenko and is freely licensed
   under the New BSD license.  This reference implementation and proof-
   of-concept provides a clone (a spin-off) of the rfc5766-turn-server
   project adding STUN BANDWIDTH attribute support, among other TRAM
   Working Group STUN and TURN extensions.

   'Coturn' is backward-compatible with rfc5766-turn-server project but
   the code is more complex and it uses a different (also more complex)
   database structure.  It is the intent to add all IETF TRAM TURN
   server related capabilities to this project as they mature.  'Coturn'
   is publicly available and can be found at:

9.  References

9.1.  Normative References

              Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
              Layer Security (DTLS) as Transport for Session Traversal
              Utilities for NAT (STUN)", draft-ietf-tram-stun-dtls-05
              (work in progress), June 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate

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              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

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

   [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.

9.2.  Informative References

              Martinsen, P. and H. Wildfeuer, "Differentiated prIorities
              and Status Code-points Using Stun Signalling (DISCUSS)",
              draft-martinsen-tram-discuss-00 (work in progress),
              February 2014.

              Thomson, M. and B. Aboba, "Bandwidth Constraints for
              Session Traversal Utilities for NAT (STUN)",
              draft-thomson-mmusic-rtcweb-bw-consent-00 (work in
              progress), October 2012.

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

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982,
              July 2013.

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Authors' Addresses

   Martin Thomson
   331 E Evelyn Street
   Mountain View, CA  94041

   Phone: +1 650-353-1925
   Email: martin.thomson@gmail.com

   Bernard Aboba
   One Microsoft Way
   Redmond, WA  98052

   Email: bernard_aboba@outlook.com

   Alan Johnston
   St. Louis, MO

   Email: alan.b.johnston@gmail.com

   Oleg Moskalenko
   public project rfc5766-turn-server
   Walnut Creek, CA

   Email: mom040267@gmail.com
   URI:   https://code.google.com/p/rfc5766-turn-server/

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