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Versions: (draft-westerlund-avtext-sdes-hdr-ext) 00 01 02 03 04 05 06 07 RFC 7941

Network Working Group                                      M. Westerlund
Internet-Draft                                                 B. Burman
Intended status: Standards Track                                Ericsson
Expires: December 12, 2016                                       R. Even
                                                     Huawei Technologies
                                                               M. Zanaty
                                                           Cisco Systems
                                                           June 10, 2016


         RTP Header Extension for RTCP Source Description Items
                   draft-ietf-avtext-sdes-hdr-ext-07

Abstract

   Source Description (SDES) items are normally transported in RTP
   control protocol (RTCP).  In some cases it can be beneficial to speed
   up the delivery of these items.  Mainly when a new source (SSRC)
   joins an RTP session and the receivers need this source's identity,
   relation to other sources, or its synchronization context, all of
   which may be fully or partially identified using SDES items.  To
   enable this optimization, this document specifies a new RTP header
   extension that can carry SDES items.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 12, 2016.

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
   Provisions Relating to IETF Documents



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   (http://trustee.ietf.org/license-info) in effect on the date of
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     2.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  SDES Item Header Extension  . . . . . . . . . . . . . . .   5
       4.1.1.  One-Byte Format . . . . . . . . . . . . . . . . . . .   5
       4.1.2.  Two-Byte Format . . . . . . . . . . . . . . . . . . .   6
     4.2.  Usage of the SDES Item Header Extension . . . . . . . . .   6
       4.2.1.  One or Two Byte Headers . . . . . . . . . . . . . . .   6
       4.2.2.  MTU and Packet Expansion  . . . . . . . . . . . . . .   7
       4.2.3.  Transmission Considerations . . . . . . . . . . . . .   7
       4.2.4.  Different Usages  . . . . . . . . . . . . . . . . . .   9
       4.2.5.  SDES Items in RTCP  . . . . . . . . . . . . . . . . .   9
       4.2.6.  Update Flaps  . . . . . . . . . . . . . . . . . . . .  10
       4.2.7.  RTP Header Compression  . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Registration of an SDES Base URN  . . . . . . . . . . . .  11
     5.2.  Creation of an SDES Sub-Registry  . . . . . . . . . . . .  11
     5.3.  Registration of SDES Item . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   This specification defines an RTP header extension [RFC3550][RFC5285]
   that can carry RTCP source description (SDES) items.  Normally the
   SDES items are carried in their own RTCP packet type [RFC3550].  By
   including selected SDES items in a header extension the determination
   of relationship and synchronization context for new RTP streams
   (SSRCs) in an RTP session can be optimized.  Which relationship and
   what information depends on the SDES items carried.  This becomes a
   complement to using only RTCP for SDES Item delivery.



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   It is important to note that not all SDES items are appropriate to
   transmit using RTP header extensions.  Some SDES items performs
   binding or identifies synchronization context with strict timeliness
   requirements, while many other SDES items do not have such
   requirements.  In addition, security and privacy concerns for the
   SDES item information need to be considered.  For example, the Name
   and Location SDES items are highly sensitive from a privacy
   perspective and should not be transported over the network without
   strong security.  No use case has identified where this information
   is required at the same time as the first RTP packets arrive.  A few
   seconds delay before such information is available to the receiver
   appears acceptable.  Therefore only appropriate SDES items will be
   registered for use with this header extension, such as CNAME.

   First, some requirements language and terminology are defined.  The
   following section motivates why this header extension is sometimes
   required or at least provides a significant improvement compared to
   waiting for regular RTCP packet transmissions of the information.
   This is followed by a specification of the header extension and usage
   recommendations.  Next, a sub-space of the header-extension URN is
   defined to be used for existing and future SDES items, and then the
   appropriate existing SDES items are registered.

2.  Definitions

2.1.  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].

2.2.  Terminology

   This document uses terminology defined in "A Taxonomy of Semantics
   and Mechanisms for Real-Time Transport Protocol (RTP) Sources"
   [RFC7656].  In particular the following definitions:

      Media Source

      RTP Stream

      Media Encoder

      Participant







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3.  Motivation

   Source Description (SDES) items are associated with a particular SSRC
   and thus RTP stream.  The source description items provide various
   meta data associated with the SSRC.  How important it is to have this
   data no later than when receiving the first RTP packets depends on
   the item itself.  The CNAME item is one item that is commonly needed
   either at reception of the first RTP packet for this SSRC, or at
   least by the time the first media can be played out.  If it is not
   available, the synchronization context cannot be determined and thus
   any related streams cannot be correctly synchronized.  Thus, this is
   a valuable example for having this information early when a new RTP
   stream is received.

   The main reason for new SSRCs in an RTP session is when media sources
   are added.  This can be either because an end-point is adding a new
   actual media source, or additional participants in a multi-party
   session are added to the session.  Another reason for a new SSRC can
   be an SSRC collision that forces both colliding parties to select new
   SSRCs.

   For the case of rapid media synchronization, one may use the RTP
   header extension for Rapid Synchronization of RTP Flows [RFC6051].
   This header extension carries the clock information present in the
   RTCP sender report (SR) packets.  It however assumes that the CNAME
   binding is known, which can be provided via signaling [RFC5576] in
   some cases, but not all.  Thus an RTP header extension for carrying
   SDES items like CNAME is a powerful combination to enable rapid
   synchronization in all cases.

   The Rapid Synchronization of RTP Flows specification does provide an
   analysis of the initial synchronization delay for different sessions
   depending on number of receivers as well as on session bandwidth
   (Section 2.1 of [RFC6051]).  These results are applicable also for
   other SDES items that have a similar time dependency until the
   information can be sent using RTCP.  These figures can be used to
   determine the benefit of reducing the initial delay before
   information is available for some use cases.

   Rapid Synchronization of RTP Flows [RFC6051] also discusses the case
   of late joiners, and defines an RTCP Feedback format to request
   synchronization information, which is another potential use case for
   SDES items in RTP header extension.  It would for example be natural
   to include CNAME SDES item with the header extension containing the
   NTP formatted reference clock to ensure synchronization.

   The ongoing work on bundling SDP media descriptions
   [I-D.ietf-mmusic-sdp-bundle-negotiation] has identified a new SDES



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   item that can benefit from timely delivery.  A corresponding RTP SDES
   compact header extension is therefore also defined and registered in
   that document:

   MID:  This is a media description identifier that matches the value
      of the Session Description Protocol (SDP) [RFC4566] a=mid
      attribute [RFC5888], to associate RTP streams multiplexed on the
      same transport with their respective SDP media description.

4.  Specification

   This section first specifies the SDES item RTP header extension
   format, followed by some usage considerations.

4.1.  SDES Item Header Extension

   An RTP header extension scheme allowing for multiple extensions is
   defined in "A General Mechanism for RTP Header Extensions" [RFC5285].
   That specification defines both short and long item headers.  The
   short headers (One-byte) are restricted to 1 to 16 bytes of data,
   while the long format (Two-byte) supports a data length of 0 to 255
   bytes.  Thus the RTP header extension formats are capable of
   supporting any SDES item from a data length perspective.

   The ID field, independent of short or long format, identifies both
   the type of RTP header extension and, in the case of the SDES item
   header extension, the type of SDES item.  The mapping is done in
   signaling by identifying the header extension and SDES item type
   using a URN, which is defined in the IANA consideration (Section 5)
   for the known SDES items appropriate to use.

4.1.1.  One-Byte Format

   The one-byte header format for an SDES item extension element
   consists of the one-byte header (defined in Section 4.2 of
   [RFC5285]), which consists of a 4-bit ID followed by a 4-bit length
   field (len) that identifies the number of data bytes (len value +1)
   following the header.  The data part consists of len+1 bytes of UTF-8
   [RFC3629] text.  The type of text and its mapping to the SDES item
   type is determined by the ID field value.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  ID   |  len  | SDES Item text value ...                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 1



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4.1.2.  Two-Byte Format

   The two-byte header format for an SDES item extension element
   consists of the two-byte header (defined in Section 4.3 of
   [RFC5285]), which consists of an 8-bit ID followed by an 8-bit length
   field (len) that identifies the number of data bytes following the
   header.  The data part consists of len bytes of UTF-8 text.  The type
   of text and its mapping to the SDES item type is determined by the ID
   field value.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      ID       |      len      |  SDES Item text value ...     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 2

4.2.  Usage of the SDES Item Header Extension

   This section discusses various usage considerations; which form of
   header extension to use, the packet expansion, and when to send SDES
   items in header extension.

4.2.1.  One or Two Byte Headers

   The RTP header extensions for SDES items MAY use either the one-byte
   or two-byte header formats, depending on the text value size for the
   used SDES items and the requirement from any other header extensions
   used.  The one-byte header SHOULD be used when all non SDES item
   header extensions supports the one-byte format and all SDES item text
   values contain at most 16 bytes.  Note that the RTP header extension
   specification does not allow mixing one-byte and two-byte headers for
   the same RTP stream (SSRC), so if the value size of any of the SDES
   items value requires the two-byte header, then all other header
   extensions MUST also use the two-byte header format.

   For example using CNAMEs that are generated according to "Guidelines
   for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)"
   [RFC7022], using short term persistent values, and if 96-bit random
   values prior to base64 encoding are sufficient, then they will fit
   into the one-byte header format.

   An RTP middlebox needs to take care choosing between one-byte headers
   and two-byte headers when creating the first packets for an outgoing
   stream (SSRC) with header extensions.  First of all it needs to
   consider all the header extensions that may potentially be used.
   Secondly, it needs to know the size of the SDES items that are going



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   to be included, and use two bytes headers if any are longer than 16
   bytes.  An RTP middlebox that forwards a stream, i.e., not mixing it
   or combing it with other streams, may be able to base its choice on
   the header size in incoming streams.  This is assuming that the
   middlebox does not modify the stream or add additional header
   extensions to the stream it sends, in which case it needs to make its
   own decision.

4.2.2.  MTU and Packet Expansion

   The RTP packet size will clearly increase when a header extension is
   included.  How much depends on the type of header extensions and
   their data content.  The SDES items can vary in size.  There are also
   some use-cases that require transmitting multiple SDES items in the
   same packet to ensure that all relevant data reaches the receiver.
   An example of that is when both CNAME, a MID, and the rapid time
   synchronization extension from RFC 6051 are needed.  Such a
   combination is quite likely to result in at least 16+3+8 bytes of
   data plus the headers, which will be another 7 bytes for one-byte
   headers, plus two bytes of header padding to make the complete header
   extension 32-bit word aligned, thus in total 36 bytes.

   If the packet expansion cannot be taken into account when producing
   the RTP payload, it can cause an issue.  An RTP payload that is
   created to meet a particular IP level Maximum Transmission Unit
   (MTU), taking the addition of IP/UDP/RTP headers but not RTP header
   extensions into account, could exceed the MTU when the header
   extensions are present, thus resulting in IP fragmentation.  IP
   fragmentation is known to negatively impact the loss rate due to
   middleboxes unwilling or not capable of dealing with IP fragments, as
   well as increasing the target surface for other types of packet
   losses.

   As this is a real issue, the media encoder and payload packetizer
   should be flexible and be capable of handling dynamically varying
   payload size restrictions to counter the packet expansion caused by
   header extensions.  If that is not possible, some reasonable worst
   case packet expansion should be calculated and used to reduce the RTP
   payload size of all RTP packets the sender transmits.

4.2.3.  Transmission Considerations

   The general recommendation is to only send header extensions when
   needed.  This is especially true for SDES items that can be sent in
   periodic repetitions of RTCP throughout the whole session.  Thus, the
   different usages (Section 4.2.4) have different recommendations.
   First some general considerations for getting the header extensions
   delivered to the receiver:



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   1.  The probability for packet loss and burst loss determine how many
       repetitions of the header extensions will be required to reach a
       targeted delivery probability, and if burst loss is likely, what
       distribution would be needed to avoid getting all repetitions of
       the header extensions lost in a single burst.

   2.  If a set of packets are all needed to enable decoding, there is
       commonly no reason for including the header extension in all of
       these packets, as they share fate.  Instead, at most one instance
       of the header extension per independently decodable set of media
       data would be a more efficient use of the bandwidth.

   3.  How early the SDES item information is needed, from the first
       received RTP data or only after some set of packets are received,
       can guide if the header extension(s) should be in all of the
       first N packets or be included only once per set of packets, for
       example once per video frame.

   4.  The use of RTP level robustness mechanisms, such as RTP
       retransmission [RFC4588], or Forward Error Correction, e.g.,
       [RFC5109] may treat packets differently from a robustness
       perspective, and SDES header extensions should be added to
       packets that get a treatment corresponding to the relative
       importance of receiving the information.

   As a summary, the number of header extension transmissions should be
   tailored to a desired probability of delivery taking the receiver
   population size into account.  For the very basic case, N repetitions
   of the header extensions should be sufficient, but may not be
   optimal.  N is selected so that the header extension target delivery
   probability reaches 1-P^N, where P is the probability of packet loss.
   For point to point or small receiver populations, it might also be
   possible to use feedback, such as RTCP, to determine when the
   information in the header extensions has reached all receivers and
   stop further repetitions.  Feedback that can be used includes the
   RTCP XR Loss RLE report block [RFC3611], which will indicate
   successful delivery of particular packets.  If the RTP/AVPF Transport
   Layer Feedback Messages for generic NACK [RFC4585] is used, it can
   indicate the failure to deliver an RTP packet with the header
   extension, thus indicating the need for further repetitions.  The
   normal RTCP report blocks can also provide an indicator of successful
   delivery, if no losses are indicated for a reporting interval
   covering the RTP packets with the header extension.  Note that loss
   of an RTCP packet reporting on an interval where RTP header extension
   packets were sent, does not necessarily mean that the RTP header
   extension packets themselves were lost.





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4.2.4.  Different Usages

4.2.4.1.  New SSRC

   A new SSRC joins an RTP session.  As this SSRC is completely new for
   everyone, the goal is to ensure, with high probability, that all RTP
   session participants receives the information in the header
   extension.  Thus, header extension transmission strategies that allow
   some margins in the delivery probability should be considered.

4.2.4.2.  Late Joiner

   In a multi-party RTP session where one or a small number of receivers
   join a session where the majority of receivers already have all
   necessary information, the use of header extensions to deliver
   relevant information should be tailored to reach the new receivers.
   The trigger to send header extensions can for example either be RTCP
   from new receiver(s) or an explicit request like the Rapid
   Resynchronization Request defined in [RFC6051].  In centralized
   topologies where an RTP middlebox is present, it can be responsible
   for transmitting the known information, possibly stored, to the new
   session participant only, and not repeat it to all the session
   participants.

4.2.4.3.  Information Change

   If the SDES information is tightly coupled with the RTP data, and the
   SDES information needs to be updated, then the use of the RTP header
   extension is superior to RTCP.  Using the RTP header extension
   ensures that the information is updated on reception of the related
   RTP media, ensuring synchronization between the two.  Continued use
   of the old SDES information can lead to undesired effects in the
   application.  Thus, header extension transmission strategies with
   high probability of delivery should be chosen.

4.2.5.  SDES Items in RTCP

   The RTP header extension information, i.e., SDES items, can and will
   be sent also in RTCP.  Therefore, it is worth making some reflections
   on this interaction.  As an alternative to the header extension, it
   is possible to schedule a non-regular RTCP packet transmission
   containing important SDES items, if one uses an RTP/AVPF-based RTP
   profile.  Depending on which mode one's RTCP feedback transmitter is
   working on, extra RTCP packets may be sent as immediate or early
   packets, enabling more timely SDES information delivery.

   There are however two aspects that differ between using RTP header
   extensions and any non-regular transmission of RTCP packets.  First,



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   as the RTCP packet is a separate packet, there is no direct relation
   and also no fate sharing between the relevant media data and the SDES
   information.  The order of arrival for the packets will matter.  With
   a header-extension, the SDES items can be ensured to arrive if the
   media data to play out arrives.  Secondly, it is difficult to
   determine if an RTCP packet is actually delivered.  This, as the RTCP
   packets lack both sequence number and a mechanism providing feedback
   on the RTCP packets themselves.

4.2.6.  Update Flaps

   The SDES item may arrive both in RTCP and in RTP header extensions,
   potentially causing the value to flap back and forth at the time of
   updating.  There are at least two reasons for these flaps.  The first
   one is packet reordering, where a pre-update RTP or RTCP packet with
   an SDES item is delivered to the receiver after the first RTP/RTCP
   packet with the updated value.  The second reason is the different
   code-paths for RTP and RTCP in implementations.  An update to the
   sender's SDES item parameter can take a different time to propagate
   to the receiver than the corresponding media data.  For example, an
   RTCP packet with the SDES item included that may have been generated
   prior to the update can still reside in a buffer and be sent
   unmodified.  The update of the item's value can at the same time
   cause RTP packets to be sent including the header extension, prior to
   the RTCP packet being sent.

   However, most of these issues can be avoided by the receiver
   performing some checks before updating the receiver's stored value.
   To handle flaps caused by reordering, SDES items received in RTP
   packets with the same or a lower extended sequence number than the
   last change MUST NOT be applied, i.e., discard items that can be
   determined to be older than the current one.  For compound RTCP
   packets, which will contain a Sender Report (SR) packet (assuming an
   active RTP sender), the receiver can use the RTCP SR Timestamp field
   to determine at what approximate time it was transmitted.  If the
   timestamp is earlier than the last received RTP packet with a header
   extension carrying an SDES item, and especially if carrying a
   previously used value, the SDES item in the RTCP SDES packet can be
   ignored.  Note that media processing and transmission pacing can
   easily cause the RTP header timestamp field as well as the RTCP SR
   timestamp field to not match with the actual transmission time.

4.2.7.  RTP Header Compression

   When robust header compression (ROHC) [RFC5225] is used with RTP, the
   RTP header extension [RFC5285] data itself is not part of what is
   being compressed and thus does not impact header compression
   performance.  The extension indicator (X) bit in the RTP header is



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   however compressed.  It is classified as rarely changing, which may
   no longer be true for all RTP header extension usage, in turn leading
   to lower header compression efficiency.

5.  IANA Considerations

   This section makes the following requests to IANA:

   o  Create a new sub-registry reserved for RTCP SDES items with the
      URN sub-space "urn:ietf:params:rtp-hdrext:sdes:" in the RTP
      Compact Header Extensions registry.

   o  Register the SDES items appropriate for use with the RTP header
      extension defined in this document.

   RFC-editor note: Please replace all occurrences of RFCXXXX with the
   RFC number this specification receives when published.

5.1.  Registration of an SDES Base URN

   IANA is requested to register the below entry in the RTP Compact
   Header Extensions registry:

   Extension URI: urn:ietf:params:rtp-hdrext:sdes
   Description:   Reserved as base URN for RTCP SDES items that are also
                  defined as RTP Compact header extensions.
   Contact:       Authors of [RFCXXXX]
   Reference:     [RFCXXXX]

   The reason to register a base URN for an SDES sub-space is that the
   name represents an RTCP Source Description item, where a
   specification is strongly recommended [RFC3550].

5.2.  Creation of an SDES Sub-Registry

   IANA is requested to create a sub-registry to the RTP Compact Header
   Extensions registry, with the same basic requirements, structure and
   layout as the RTP Compact Header Extensions registry.

   o  Registry name: RTP SDES Compact Header Extensions

   o  Specification: RFCXXXX and RFCs updating RFCXXXX

   o  Information required: Same as for RTP Header Extensions [RFC5285]
      registry






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   o  Review process: Same as for RTP Header Extensions [RFC5285]
      registry, with the following requirements added to the expert
      review:

      1.  Any registration using an Extension URI that starts with
          "urn:ietf:params:rtp-hdrext:sdes:" (Section 5.1) MUST also
          have a registered Source Description item in the "RTP SDES
          item types" registry.

      2.  A security and privacy consideration for the SDES item MUST be
          provided with the registration.

      3.  Information MUST be provided on why this SDES item requires
          timely delivery, motivating it to be transported in a header
          extension rather than as RTCP only.

   o  Size and format of entries: Same as for RTP Header Extensions
      [RFC5285] registry.

   o  Initial assignments: See Section 5.3 below.

5.3.  Registration of SDES Item

   It is requested that the following SDES item is registered in the
   newly formed RTP SDES Compact Header Extensions registry:

   Extension URI: urn:ietf:params:rtp-hdrext:sdes:cname
   Description:   Source Description: Canonical End-Point Identifier
                  (SDES CNAME)
   Contact:       Authors of [RFCXXXX]
   Reference:     [RFCXXXX]

6.  Security Considerations

   Source Description items may contain data that are sensitive from a
   security perspective.  There are SDES items that are or may be
   sensitive from a user privacy perspective, like CNAME, NAME, EMAIL,
   PHONE, LOC and H323-CADDR.  Some may contain sensitive information,
   like NOTE and PRIV, while others may be sensitive from profiling
   implementations for vulnerability or other reasons, like TOOL.  The
   CNAME sensitivity can vary depending on how it is generated and what
   persistence it has.  A short term CNAME identifier generated using a
   random number generator [RFC7022] may have minimal security
   implications, while a CNAME of the form user@host has privacy
   concerns, and a CNAME generated from a MAC address has long term
   tracking potentials.





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   In RTP sessions where any type of confidentiality protection is
   enabled for RTCP, the SDES item header extensions MUST also be
   protected.  This implies that to provide confidentiality, users of
   SRTP need to implement and use encrypted header extensions per
   [RFC6904].  SDES items carried as RTP header extensions MUST then use
   commensurate strength algorithms and SHOULD use the same
   cryptographic primitives (algorithms, modes) as applied to RTCP
   packets carrying corresponding SDES items.  If the security level is
   chosen to be different for an SDES item in RTCP and RTP header
   extension, it is important to motivate the exception, and to consider
   the security properties as the worst in each aspect for the different
   configurations.  It is worth noting that the current Secure RTP
   (SRTP) [RFC3711] only provides protection for the next trusted RTP/
   RTCP hop, which is not necessarily end-to-end.

   The general RTP header extension mechanism [RFC5285] does not itself
   contain any functionality that is a significant risk for a denial of
   service attack, neither from processing nor storage requirements.
   The extension for SDES items defined in this document, can
   potentially be a risk.  The risk depends on the received SDES item
   and its content.  If the SDES item causes the receiver to perform a
   large amount of processing, create significant storage structures, or
   emit network traffic, such a risk does exist.  The CNAME SDES item in
   the RTP header extension is only a minor risk, as reception of a
   CNAME item will create an association between the stream carrying the
   SDES item and other RTP streams with the same SDES item.  This
   usually results in time synchronizing the media streams, thus some
   additional processing is performed.  However, the application's media
   quality is likely more affected by an erroneous or changing
   association and media synchronization, than the application quality
   impact caused by the additional processing.

   As the SDES items are used by the RTP based application to establish
   relationships between RTP streams or between an RTP stream and
   information about the originating participant, there SHOULD be strong
   integrity protection and source authentication of the header
   extensions.  If not, an attacker can modify the SDES item value to
   create erroneous relationship bindings in the receiving application.
   For information regarding options for securing RTP, see [RFC7201].

7.  Acknowledgments

   The authors likes to thank the following individuals for feedback and
   suggestions: Colin Perkins, Ben Campbell, and Samuel Weiler.







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8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <http://www.rfc-editor.org/info/rfc3550>.

   [RFC5285]  Singer, D. and H. Desineni, "A General Mechanism for RTP
              Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
              2008, <http://www.rfc-editor.org/info/rfc5285>.

   [RFC6904]  Lennox, J., "Encryption of Header Extensions in the Secure
              Real-time Transport Protocol (SRTP)", RFC 6904,
              DOI 10.17487/RFC6904, April 2013,
              <http://www.rfc-editor.org/info/rfc6904>.

8.2.  Informative References

   [I-D.ietf-mmusic-sdp-bundle-negotiation]
              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
              negotiation-30 (work in progress), June 2016.

   [RFC3611]  Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
              "RTP Control Protocol Extended Reports (RTCP XR)",
              RFC 3611, DOI 10.17487/RFC3611, November 2003,
              <http://www.rfc-editor.org/info/rfc3611>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <http://www.rfc-editor.org/info/rfc3629>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <http://www.rfc-editor.org/info/rfc3711>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <http://www.rfc-editor.org/info/rfc4566>.



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   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <http://www.rfc-editor.org/info/rfc4585>.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              DOI 10.17487/RFC4588, July 2006,
              <http://www.rfc-editor.org/info/rfc4588>.

   [RFC5109]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
              Correction", RFC 5109, DOI 10.17487/RFC5109, December
              2007, <http://www.rfc-editor.org/info/rfc5109>.

   [RFC5225]  Pelletier, G. and K. Sandlund, "RObust Header Compression
              Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and
              UDP-Lite", RFC 5225, DOI 10.17487/RFC5225, April 2008,
              <http://www.rfc-editor.org/info/rfc5225>.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
              <http://www.rfc-editor.org/info/rfc5576>.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888,
              DOI 10.17487/RFC5888, June 2010,
              <http://www.rfc-editor.org/info/rfc5888>.

   [RFC6051]  Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
              Flows", RFC 6051, DOI 10.17487/RFC6051, November 2010,
              <http://www.rfc-editor.org/info/rfc6051>.

   [RFC7022]  Begen, A., Perkins, C., Wing, D., and E. Rescorla,
              "Guidelines for Choosing RTP Control Protocol (RTCP)
              Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022,
              September 2013, <http://www.rfc-editor.org/info/rfc7022>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
              <http://www.rfc-editor.org/info/rfc7201>.

   [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>.



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Authors' Addresses

   Magnus Westerlund
   Ericsson
   Farogatan 6
   SE-164 80 Stockholm
   Sweden

   Phone: +46 10 714 82 87
   Email: magnus.westerlund@ericsson.com


   Bo Burman
   Ericsson
   Gronlandsgatan 31
   Stockholm  16480
   Sweden

   Email: bo.burman@ericsson.com


   Roni Even
   Huawei Technologies
   Tel Aviv
   Israel

   Email: roni.even@mail01.huawei.com


   Mo Zanaty
   Cisco Systems
   7100 Kit Creek
   RTP, NC  27709
   USA

   Email: mzanaty@cisco.com















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