RTP Header Extension for RTCP Source Description Items
draft-ietf-avtext-sdes-hdr-ext-01

Network Working Group                                      M. Westerlund
Internet-Draft                                                 B. Burman
Intended status: Standards Track                                Ericsson
Expires: November 12, 2015                                       R. Even
                                                     Huawei Technologies
                                                               M. Zanaty
                                                           Cisco Systems
                                                            May 11, 2015

         RTP Header Extension for RTCP Source Description Items
                   draft-ietf-avtext-sdes-hdr-ext-01

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 needs 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 November 12, 2015.

Copyright Notice

   Copyright (c) 2015 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

Westerlund, et al.      Expires November 12, 2015               [Page 1]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

   (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  . . . . . . . . . . . . . .   6
       4.2.3.  Transmission Considerations . . . . . . . . . . . . .   7
       4.2.4.  Different Usages  . . . . . . . . . . . . . . . . . .   8
       4.2.5.  SDES Items in RTCP  . . . . . . . . . . . . . . . . .   9
       4.2.6.  Update Flaps  . . . . . . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Reservation of the SDES URN sub-space . . . . . . . . . .  10
     5.2.  Registration of SDES Items  . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

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

   It is important to note that not all SDES items are appropriate to
   transmit using RTP header extensions.  Some SDES items performs

Westerlund, et al.      Expires November 12, 2015               [Page 2]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

   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 needs 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 is 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 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 Grouping
   Semantics and Mechanisms for Real-Time Transport Protocol (RTP)
   Sources" [I-D.ietf-avtext-rtp-grouping-taxonomy].  In particular the
   following definitions:

      Media Source

      RTP Stream

      Media Encoder

      Encoded Stream

      Participant

Westerlund, et al.      Expires November 12, 2015               [Page 3]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

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
   if not at reception of the first RTP packet for this SSRC, then at
   least by the time the first media can be played out.  If not, 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 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 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.  Thus the benefit of reducing the
   initial delay before information is available can be determined for
   some use cases from these figures.

   That document 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.

   There is an additional, newly defined SDES item that can benefit from
   timely delivery, and an RTP header extension SDES item is therefore
   defined for it:

Westerlund, et al.      Expires November 12, 2015               [Page 4]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

   MID:  This is a media description identifier that matches the value
      of the SDP a=mid attribute, to associate RTP streams multiplexed
      on the same transport with their respective SDP media description
      as described in [I-D.ietf-mmusic-sdp-bundle-negotiation].

4.  Specification

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

4.1.  SDES Item Header Extension

   The RTP header extension scheme that allows for multiple extensions
   to be included 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 how many bytes (len value +1) of data
   following the header.  The data part consists of len+1 bytes of UTF-8
   text.  The type of text is determined by the ID field value and its
   mapping to the type of SDES item.

    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

Westerlund, et al.      Expires November 12, 2015               [Page 5]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

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 how many bytes of data that follows the
   header.  The data part consists of len bytes of UTF-8 text.  The type
   of text is determined by the ID field value and its mapping to the
   type of SDES item.

    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, the 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.

4.2.2.  MTU and Packet Expansion

   The RTP packet size will clearly increase when it includes the header
   extension.  How much depends on which header extensions and their
   data parts.  The SDES items can vary in size.  There are also some

Westerlund, et al.      Expires November 12, 2015               [Page 6]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

   use-cases which require transmitting multiple SDES items in the same
   packet to ensure that all relevant data reaches the receiver.  An
   example of that is when you need both the CNAME, a MID, and the rapid
   time synchronization extension from RFC 6051.  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 padding headers to make the complete header extension word
   aligned, thus in total 36 bytes.

   The packet expansion can cause an issue when it cannot be taken into
   account when producing the RTP payload.  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:

   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
       dispersion would be needed to avoid getting multiple 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.

Westerlund, et al.      Expires November 12, 2015               [Page 7]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

   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.

   In 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
   succesful 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 succesful
   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.

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
   receivers receives the information in the header extension.  Thus,
   header extension transmission strategies that allow some margins in
   the delivery probability should be considered.

Westerlund, et al.      Expires November 12, 2015               [Page 8]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

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

   In cases when the SDES item text value is changed and the new SDES
   information is tightly coupled to and thus needs to be synchronized
   with a related change in the RTP stream, use of a header extension is
   far superior to RTCP SDES.  In this case it is equal or even more
   important with timely SDES information than in the case of new SSRCs
   (Section 4.2.4.1).  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 extensions information, i.e. SDES Items, can and will
   be sent also in RTCP.  Therefore, it is worth some reflections on
   this interaction.  An alternative to the header extension is the
   possibility to schedule a non-regular RTCP packet transmission
   containing important SDES items, if one uses a 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 delivery of SDES information.

   There is however two aspects that differ between using RTP header
   extensions and any non-regular transmission of RTCP packets.  First,
   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 or a mechanism providing feedback
   on the RTCP packets themselves.

Westerlund, et al.      Expires November 12, 2015               [Page 9]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

4.2.6.  Update Flaps

   The SDES item may arrive both in RTCP and in RTP header extensions,
   this can cause 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
   senders SDES item parameter, can take different time to propagate.
   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 performing some
   checks before updating the receiver's stored value.  To handle flaps
   caused by reordering, only SDES items received in RTP packets with a
   higher extended sequence number than the last change shall be
   applied, i.e. discard items that can be determined to be older than
   the current one.  For compound RTCP packets, which will contain an
   Sender Report (SR) packet (assuming an active RTP sender), the
   receiver can compare the RTCP Sender Report's Timestamp field, to
   determine at what approximate time it was transmitted.  If the
   timestamp is earlier than the last received RTP packet 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
   only lously couple with the actual transmission time.

5.  IANA Considerations

   This section makes the following requests to IANA:

   o  Register and reserve for SDES items 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.

5.1.  Reservation of the SDES URN sub-space

   The reason to require registering a URN within an SDES sub-space is
   that the name represents an RTCP Source Description item, where a
   specification is strongly recommended.  The formal policy is

Westerlund, et al.      Expires November 12, 2015              [Page 10]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

   maintained from the main space, i.e. Expert Review.  However, some
   additional considerations are provided here that needs to be
   considered when applying for a registration within this sub-space of
   the RTP Compact Header Extensions registry.

   Any registration using an Extension URI that starts with
   "urn:ietf:params:rtp-hdrext:sdes:" MUST also have a registered Source
   Description item in the "RTP SDES item types" registry.  Secondly, a
   security and privacy consideration for the SDES item must be provided
   with the registration, preferably in a publicly available reference.
   Thirdly, information must be provided on why this SDES item requires
   timely delivery, motivating it to be transported in an header
   extension rather than as RTCP only.

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

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

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

5.2.  Registration of SDES Items

   It is requested that the following SDES item is registered in the RTP
   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]

   We also note that the MID SDES item is already registered in the
   registry by [I-D.ietf-mmusic-sdp-bundle-negotiation].

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

Westerlund, et al.      Expires November 12, 2015              [Page 11]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

   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.

   The above security concerns may have to be put in relation to third
   party monitoring needs.  In RTP sessions where any type of
   confidentiality protection is enabled, the SDES item header
   extensions SHOULD also be protected per default.  This implies that
   to provide confidentiality, users of SRTP need to implement encrypted
   header extensions per [RFC6904].  Commonly, it is expected that the
   same security level is applied to RTCP packets carrying SDES items,
   and to an RTP header extension containing SDES items.  If the
   security level is different, it is important to consider the security
   properties as the worst in each aspect for the different
   configurations.

   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
   requirements on integrity 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.

7.  Acknowledgements

   The authors likes to thanks the following individuals for feedback
   and suggestions; Colin Perkins.

8.  References

8.1.  Normative References

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

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

   [RFC5285]  Singer, D. and H. Desineni, "A General Mechanism for RTP
              Header Extensions", RFC 5285, July 2008.

   [RFC6904]  Lennox, J., "Encryption of Header Extensions in the Secure
              Real-time Transport Protocol (SRTP)", RFC 6904, April
              2013.

Westerlund, et al.      Expires November 12, 2015              [Page 12]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

8.2.  Informative References

   [I-D.ietf-avtext-rtp-grouping-taxonomy]
              Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, "A Taxonomy of Grouping Semantics and
              Mechanisms for Real-Time Transport Protocol (RTP)
              Sources", draft-ietf-avtext-rtp-grouping-taxonomy-06 (work
              in progress), March 2015.

   [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-19 (work in progress), March 2015.

   [RFC3611]  Friedman, T., Caceres, R., and A. Clark, "RTP Control
              Protocol Extended Reports (RTCP XR)", RFC 3611, November
              2003.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
              2006.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              July 2006.

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

   [RFC6051]  Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
              Flows", RFC 6051, November 2010.

   [RFC7022]  Begen, A., Perkins, C., Wing, D., and E. Rescorla,
              "Guidelines for Choosing RTP Control Protocol (RTCP)
              Canonical Names (CNAMEs)", RFC 7022, September 2013.

Authors' Addresses

   Magnus Westerlund
   Ericsson
   Farogatan 6
   SE-164 80 Stockholm
   Sweden

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

Westerlund, et al.      Expires November 12, 2015              [Page 13]
Internet-Draft            RTP HE for RTCP SDES                  May 2015

   Bo Burman
   Ericsson
   Kistavagen 25
   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

Westerlund, et al.      Expires November 12, 2015              [Page 14]