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.
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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
184.108.40.206. Damaged KLVunits ...........................7
220.127.116.11. 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
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
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
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 [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
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
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
Additionally, the standard defines the use of KLV coding to provide a
means to carry information that is registered with a non-SMPTE
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.
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.
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
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.
18.104.22.168. 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
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').
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.
22.214.171.124. 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
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
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.
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
Additional Information: none
Person & email address to contact for further information: J. Downs
<firstname.lastname@example.org>; 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 <email@example.com>
J. Arbeiter <firstname.lastname@example.org>
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.
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
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.
9.1. 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,
[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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
[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,
J. Downs (editor)
PAR Government Systems Corp.
J. Arbeiter (editor)