RFC6597: RTP Payload Format for Society of Motion Picture and Television Engineers (SMPTE) ST 336 Encoded Data

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Internet Engineering Task Force (IETF)                     J. Downs, Ed.
Request for Comments: 6597                  PAR Government Systems Corp.
Category: Standards Track                               J. Arbeiter, Ed.
ISSN: 2070-1721                                               April 2012

                         RTP Payload Format for
       Society of Motion Picture and Television Engineers (SMPTE)
                          ST 336 Encoded Data


   This document specifies the payload format for packetization of KLV
   (Key-Length-Value) Encoded Data, as defined by the Society of Motion
   Picture and Television Engineers (SMPTE) in SMPTE ST 336, into the
   Real-time Transport Protocol (RTP).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012

Table of Contents

   1. Introduction ....................................................2
   2. Conventions, Definitions, and Acronyms ..........................3
   3. Media Format Background .........................................3
   4. Payload Format ..................................................4
      4.1. RTP Header Usage ...........................................5
      4.2. Payload Data ...............................................5
           4.2.1. The KLVunit .........................................5
           4.2.2. KLVunit Mapping to RTP Packet Payload ...............6
      4.3. Implementation Considerations ..............................6
           4.3.1. Loss of Data ........................................6
         Damaged KLVunits ...........................7
         Treatment of Damaged KLVunits ..............9
   5. Congestion Control ..............................................9
   6. Payload Format Parameters .......................................9
      6.1. Media Type Definition ......................................9
      6.2. Mapping to SDP ............................................10
           6.2.1. Offer/Answer Model and Declarative Considerations ..10
   7. IANA Considerations ............................................11
   8. Security Considerations ........................................11
   9. References .....................................................12
      9.1. Normative References ......................................12
      9.2. Informative References ....................................12

1.  Introduction

   This document specifies the payload format for packetization of KLV
   (Key-Length-Value) Encoded Data, as defined by the Society of Motion
   Picture and Television Engineers (SMPTE) in [SMPTE-ST336], into the
   Real-time Transport Protocol (RTP) [RFC3550].

   The payload format is defined in such a way that arbitrary KLV data
   can be carried.  No restrictions are placed on which KLV data keys
   can be used.

   A brief description of SMPTE ST 336, "Data Encoding Protocol Using
   Key-Length-Value", is given.  The payload format itself, including
   use of the RTP header fields, is specified in Section 4.  The media
   type and IANA considerations are also described.  This document
   concludes with security considerations relevant to this payload

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012

2.  Conventions, Definitions, and Acronyms

   The term "Universal Label Key" is used in this document to refer to a
   fixed-length, 16-byte SMPTE-administered Universal Label (see
   [SMPTE-ST298]) that is used as an identifying key in a KLV item.

   The term "KLV item" is used in this document to refer to one single
   Universal Label Key, length, and value triplet encoded as described
   in [SMPTE-ST336].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Media Format Background

   [SMPTE-ST336], "Data Encoding Protocol Using Key-Length-Value",
   defines a byte-level data encoding protocol for representing data
   items and data groups.  This encoding protocol definition is
   independent of the application or transportation method used.

   SMPTE ST 336 data encoding can be applied to a wide variety of binary
   data.  This encoding has been used to provide diverse and rich
   metadata sets that describe or enhance associated video
   presentations.  Use of SMPTE ST 336 encoded metadata in conjunction
   with video has enabled improvements in multimedia presentations,
   content management and distribution, archival and retrieval, and
   production workflow.

   The SMPTE ST 336 standard defines a KLV triplet as a data interchange
   protocol for data items or data groups where the Key identifies the
   data, the Length specifies the length of the data, and the Value is
   the data itself.  The KLV protocol provides a common interchange
   point for all compliant applications irrespective of the method of
   implementation or transport.

   The Key of a KLV triplet (a Universal Label Key) is coded using a
   fixed-length 16-byte SMPTE-administered Universal Label.
   [SMPTE-ST298] further details the structure of 16-byte SMPTE-
   administered Universal Labels.  Universal Label Keys are maintained
   in registries published by SMPTE (see, for example, [SMPTE-ST335] and

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012

   The standard also provides methods for combining associated KLV
   triplets in data sets where the set of KLV triplets is itself coded
   with the KLV data coding protocol.  Such sets can be coded in either
   full form (Universal Sets) or one of four increasingly bit-efficient
   forms (Global Sets, Local Sets, Variable Length Packs, and Defined
   Length Packs).  The standard provides a definition of each of these
   data constructs.

   Additionally, the standard defines the use of KLV coding to provide a
   means to carry information that is registered with a non-SMPTE
   external agency.

4.  Payload Format

   The main goal of the payload format design for SMPTE ST 336 data is
   to provide carriage of SMPTE ST 336 data over RTP in a simple, yet
   robust manner.  All forms of SMPTE ST 336 data can be carried by the
   payload format.  The payload format maintains simplicity by using
   only the standard RTP headers and not defining any payload headers.

   SMPTE ST 336 KLV data is broken into KLVunits.  A KLVunit is simply a
   logical grouping of otherwise unframed KLV data, grouped based on
   source data timing (see Section 4.2.1).  Each KLVunit is then placed
   into one or more RTP packet payloads.  The RTP header marker bit is
   used to assist receivers in locating the boundaries of KLVunits.

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012

4.1.  RTP Header Usage

   This payload format uses the RTP packet header fields as described in
   the table below:

   | Field     | Usage                                                 |
   | Timestamp | The RTP Timestamp encodes the instant along a         |
   |           | presentation timeline that the entire KLVunit encoded |
   |           | in the packet payload is to be presented.  When one   |
   |           | KLVunit is placed in multiple RTP packets, the RTP    |
   |           | timestamp of all packets comprising that KLVunit MUST |
   |           | be the same.  The timestamp clock frequency is        |
   |           | defined as a parameter to the payload format          |
   |           | (Section 6).                                          |
   |           |                                                       |
   | M-bit     | The RTP header marker bit (M) is used to demarcate    |
   |           | KLVunits.  Senders MUST set the marker bit to '1' for |
   |           | any RTP packet that contains the final byte of a      |
   |           | KLVunit.  For all other packets, senders MUST set the |
   |           | RTP header marker bit to '0'.  This allows receivers  |
   |           | to pass a KLVunit for parsing/decoding immediately    |
   |           | upon receipt of the last RTP packet comprising the    |
   |           | KLVunit.  Without this, a receiver would need to wait |
   |           | for the next RTP packet with a different timestamp to |
   |           | arrive, thus signaling the end of one KLVunit and the |
   |           | start of another.                                     |

   The remaining RTP header fields are used as specified in [RFC3550].

4.2.  Payload Data

4.2.1.  The KLVunit

   A KLVunit is a logical collection of all KLV items that are to be
   presented at a specific time.  A KLVunit is comprised of one or more
   KLV items.  Compound items (sets, packs) are allowed as per
   [SMPTE-ST336], but the contents of a compound item MUST NOT be split
   across two KLVunits.  Multiple KLV items in a KLVunit occur one after
   another with no padding or stuffing between items.

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012

4.2.2.  KLVunit Mapping to RTP Packet Payload

   An RTP packet payload SHALL contain one, and only one, KLVunit or a
   fragment thereof.  KLVunits small enough to fit into a single RTP
   packet (RTP packet size is up to the implementation but should
   consider underlying transport/network factors such as MTU
   limitations) are placed directly into the payload of the RTP packet,
   with the first byte of the KLVunit (which is the first byte of a KLV
   Universal Label Key) being the first byte of the RTP packet payload.

   KLVunits too large to fit into a single RTP packet payload MAY span
   multiple RTP packet payloads.  When this is done, the KLVunit data
   MUST be sent in sequential byte order, such that when all RTP packets
   comprising the KLVunit are arranged in sequence number order,
   concatenating the payload data together exactly reproduces the
   original KLVunit.

   Additionally, when a KLVunit is fragmented across multiple RTP
   packets, all RTP packets transporting the fragments of a KLVunit MUST
   have the same timestamp.

   KLVunits are bounded with changes in RTP packet timestamps.  The
   marker (M) bit in the RTP packet headers marks the last RTP packet
   comprising a KLVunit (see Section 4.1).

4.3.  Implementation Considerations

4.3.1.  Loss of Data

   RTP is generally deployed in network environments where packet loss
   might occur.  RTP header fields enable detection of lost packets, as
   described in [RFC3550].  When transmitting payload data described by
   this payload format, packet loss can cause the loss of whole KLVunits
   or portions thereof.

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012  Damaged KLVunits

   A damaged KLVunit is any KLVunit that was carried in one or more RTP
   packets that have been lost.  When a lost packet is detected (through
   use of the sequence number header field), the receiver

   o  MUST consider the KLVunit partially received before a lost packet
      as damaged.  This damaged KLVunit includes all packets prior to
      the lost one (in sequence number order) back to, but not
      including, the most recent packet in which the M-bit in the RTP
      header was set to '1'.

   o  MUST consider the first KLVunit received after a lost packet as
      damaged.  This damaged KLVunit includes the first packet after the
      lost one (in sequence number order) and, if the first packet has
      its M-bit in the RTP header set to '0', all subsequent packets up
      to and including the next one with the M-bit in the RTP header set
      to '1'.

   The above applies, regardless of the M-bit value in the RTP header of
   the lost packet itself.  This enables very basic receivers to look
   solely at the M-bit to determine the outer boundaries of damaged
   KLVunits.  For example, when a packet with the M-bit set to '1' is
   lost, the KLVunit that the lost packet would have terminated is
   considered damaged, as is the KLVunit comprised of packets received
   subsequent to the lost packet (up to and including the next received
   packet with the M-bit set to '1').

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012

   The example below illustrates how a receiver would handle a lost
   packet in another possible packet sequence:

          +---------+-------------+    +--------------+
          | RTP Hdr | Data        |    |              |
          +---------+-------------+    +--------------+
     .... | ts = 30 | KLV KLV ... |    |              |  >---+
          | M = 1   |             |    |              |      |
          | seq = 5 | ... KLV KLV |    |              |      |
          +---------+-------------+    +--------------+      |
           Last RTP pkt for time 30      Lost RTP Pkt        |
                                           (seq = 6)         |
    |     +---------+-------------+    +---------+-------------+
    |     | RTP Hdr |     Data    |    | RTP Hdr |     Data    |
    |     +---------+-------------+    +---------+-------------+
    +-->  | ts = 45 | KLV KLV ... |    | ts = 45 | ... KLV ... | >---+
          | M = 0   |             |    | M = 1   |             |     |
          | seq = 7 | ... KLV ... |    | seq = 8 | ... KLV KLV |     |
          +---------+-------------+    +---------+-------------+     |
             RTP pkt for time 45        Last RTP pkt for time 45     |
              KLVunit carried in these two packets is "damaged"      |
    |     +---------+-------------+
    |     | RTP Hdr |     Data    |
    |     +---------+-------------+
    +-->  | ts = 55 | KLV KLV ... |   ....
          | M = 1   |             |
          | seq = 9 | ... KLV ... |
           Last and only RTP pkt
               for time 55

   In this example, the packets with sequence numbers 7 and 8 contain
   portions of a KLVunit with a timestamp of 45.  This KLVunit is
   considered "damaged" due to the missing RTP packet with sequence
   number 6, which might have been part of this KLVunit.  The KLVunit
   for timestamp 30 (ended in packet with sequence number 5) is
   unaffected by the missing packet.  The KLVunit for timestamp 55,
   carried in the packet with sequence number 9, is also unaffected by
   the missing packet and is considered complete and intact.

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012  Treatment of Damaged KLVunits

   SMPTE ST 336 KLV data streams are built in such a way that it is
   possible to partially recover from errors or missing data in a
   stream.  Exact specifics of how damaged KLVunits are handled are left
   to each implementation, as different implementations can have
   differing capabilities and robustness in their downstream KLV payload
   processing.  Because some implementations can be particularly limited
   in their capacity to handle damaged KLVunits, receivers MAY drop
   damaged KLVunits entirely.

5.  Congestion Control

   The general congestion control considerations for transporting RTP
   data apply; see RTP [RFC3550] and any applicable RTP profile, like
   AVP [RFC3551].

   Further, SMPTE ST 336 data can be encoded in different schemes that
   reduce the overhead associated with individual data items within the
   overall stream.  SMPTE ST 336 grouping constructs, such as local sets
   and data packs, provide a mechanism to reduce bandwidth requirements.

6.  Payload Format Parameters

   This RTP payload format is identified using the application/smpte336m
   media type, which is registered in accordance with [RFC4855], and
   using the template of [RFC4288].

6.1.  Media Type Definition

   Type name: application

   Subtype name: smpte336m

   Required parameters:

      rate: RTP timestamp clock rate.  Typically chosen based on
         sampling rate of metadata being transmitted, but other rates
         can be specified.

   Optional parameters: None

   Encoding considerations: This media type is framed and binary; see
      Section 4.8 of [RFC4288].

   Security considerations: See Section 8 of RFC 6597.

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012

   Interoperability considerations: Data items in smpte336m can be very
      diverse.  Receivers might only be capable of interpreting a subset
      of the possible data items; unrecognized items are skipped.
      Agreement on data items to be used out of band, via application
      profile or similar, is typical.

   Published specification: RFC 6597

   Applications that use this media type: Streaming of metadata
      associated with simultaneously streamed video and transmission of
      [SMPTE-ST336]-based media formats (e.g., Material Exchange Format
      (MXF) [SMPTE-ST377]).

   Additional Information: none

   Person & email address to contact for further information: J. Downs
      <jeff_downs@partech.com>; IETF Payload Working Group

   Intended usage: COMMON

   Restrictions on usage: This media type depends on RTP framing, and
      hence is only defined for transfer via RTP ([RFC3550]).  Transport
      within other framing protocols is not defined at this time.


      J. Downs <jeff_downs@partech.com>

      J. Arbeiter <jimsgti@gmail.com>

   Change controller: IETF Payload working group delegated from the

6.2.  Mapping to SDP

   The mapping of the above defined payload format media type and its
   parameters SHALL be done according to Section 3 of [RFC4855].

6.2.1.  Offer/Answer Model and Declarative Considerations

   This payload format has no configuration or optional format
   parameters.  Thus, when offering SMPTE ST 336 Encoded Data over RTP
   using the Session Description Protocol (SDP) in an Offer/Answer model
   [RFC3264] or in a declarative manner (e.g., SDP in the Real-Time
   Streaming Protocol (RTSP) [RFC2326] or the Session Announcement
   Protocol (SAP) [RFC2974]), there are no specific considerations.

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012

7.  IANA Considerations

   IANA has registered application/smpte336m as specified in
   Section 6.1.  The media type has been added to the IANA registry for
   "RTP Payload Format media types"

8.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550], and in any applicable RTP profile.  The main
   security considerations for the RTP packet carrying the RTP payload
   format defined within this memo are confidentiality, integrity, and
   source authenticity.  Confidentiality is achieved by encryption of
   the RTP payload.  Integrity of the RTP packets is achieved through a
   suitable cryptographic integrity protection mechanism.  Cryptographic
   systems may also allow the authentication of the source of the
   payload.  A suitable security mechanism for this RTP payload format
   should provide confidentiality, integrity protection, and at least
   source authentication capable of determining whether or not an RTP
   packet is from a member of the RTP session.

   Note that the appropriate mechanism to provide security to RTP and
   payloads following this memo may vary.  It is dependent on the
   application, the transport, and the signaling protocol employed.
   Therefore, a single mechanism is not sufficient, although if suitable
   the usage of the Secure Real-time Transport Protocol (SRTP) [RFC3711]
   is recommended.  Other mechanisms that may be used are IPsec
   [RFC4301] and Transport Layer Security (TLS) [RFC5246] (RTP over
   TCP), but other alternatives may exist as well.

   This RTP payload format presents the possibility for significant
   non-uniformity in the receiver-side computational complexity during
   processing of SMPTE ST 336 payload data.  Because the length of SMPTE
   ST 336 encoded data items is essentially unbounded, receivers must
   take care when allocating resources used in processing.  It is easy
   to construct pathological data that would cause a naive decoder to
   allocate large amounts of resources, resulting in denial-of-service
   threats.  Receivers SHOULD place limits on resource allocation that
   are within the bounds set forth by any application profile in use.

   This RTP payload format does not contain any inherently active
   content.  However, individual SMPTE ST 336 KLV items could be defined
   to convey active content in a particular application.  Therefore,
   receivers capable of decoding and interpreting such data items should
   use appropriate caution and security practices.  In particular,
   accepting active content from streams that lack authenticity or

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012

   integrity protection mechanisms places a receiver at risk of attacks
   using spoofed packets.  Receivers not capable of decoding such data
   items are not at risk; unknown data items are skipped over and
   discarded according to SMPTE ST 336 processing rules.

9.  References

9.1.  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.

   [RFC3551]      Schulzrinne, H. and S. Casner, "RTP Profile for Audio
                  and Video Conferences with Minimal Control", STD 65,
                  RFC 3551, July 2003.

   [RFC4288]      Freed, N. and J. Klensin, "Media Type Specifications
                  and Registration Procedures", BCP 13, RFC 4288,
                  December 2005.

   [RFC4855]      Casner, S., "Media Type Registration of RTP Payload
                  Formats", RFC 4855, February 2007.

9.2.  Informative References

   [RFC2326]      Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
                  Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [RFC2974]      Handley, M., Perkins, C., and E. Whelan, "Session
                  Announcement Protocol", RFC 2974, October 2000.

   [RFC3264]      Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
                  Model with Session Description Protocol (SDP)",
                  RFC 3264, June 2002.

   [RFC3711]      Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
                  K. Norrman, "The Secure Real-time Transport Protocol
                  (SRTP)", RFC 3711, March 2004.

   [RFC4301]      Kent, S. and K. Seo, "Security Architecture for the
                  Internet Protocol", RFC 4301, December 2005.

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RFC 6597            RTP Format for SMPTE ST 336 Data          April 2012

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

   [SMPTE-RP210]  Society of Motion Picture and Television Engineers,
                  "SMPTE RP 210v12:2010 Data Element Dictionary", 2010,

   [SMPTE-ST298]  Society of Motion Picture and Television Engineers,
                  "SMPTE ST 298:2009 Universal Labels for Unique
                  Identification of Digital Data", 2009,

   [SMPTE-ST335]  Society of Motion Picture and Television Engineers,
                  "SMPTE ST 335:2012 Metadata Element Dictionary
                  Structure", 2012, <http://www.smpte.org>.

   [SMPTE-ST336]  Society of Motion Picture and Television Engineers,
                  "SMPTE ST 336:2007 Data Encoding Protocol Using Key-
                  Length-Value", 2007, <http://www.smpte.org>.

   [SMPTE-ST377]  Society of Motion Picture and Television Engineers,
                  "SMPTE ST 377-1:2011 Material Exchange Format (MXF) -
                  File Format Specification", 2011,

Authors' Addresses

   J. Downs (editor)
   PAR Government Systems Corp.

   EMail: jeff_downs@partech.com

   J. Arbeiter (editor)

   EMail: jimsgti@gmail.com

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