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 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8759.¶

Copyright Notice

Copyright (c) 2020 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 (https://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

TTML (Timed Text Markup Language)[TTML2] is a media type fordescribing timed text, such as closed captions and subtitles in televisionworkflows or broadcasts, as XML. This document specifies how TTML should bemapped into an RTP stream in streaming workflows, including (but not restrictedto) those described in the television-broadcast-oriented European BroadcastingUnion Timed Text (EBU-TT) Part 3[TECH3370] specification. This document does not define a media typefor TTML but makes use of the existing application/ttml+xml media type[TTML-MTPR].¶

2.Conventions and Definitions

Unless otherwise stated, the term "document" refers to the TTML documentbeing transmitted in the payload of the RTP packet(s).¶

The term "word" refers to a data word aligned to a specified number of bitsin a computing sense and not to linguistic words that might appear inthe transported text.¶

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14[RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3.Media Format Description

4.Payload Format

In addition to the required RTP headers, the payload contains a section forthe TTML document being transmitted (User Data Words) and a field for thelength of that data. Each RTP payload contains one or part of one TTMLdocument.¶

A representation of the payload format for TTML isFigure 1.¶

 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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|V=2|P|X| CC    |M|    PT       |        Sequence Number        |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|                           Timestamp                           |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|           Synchronization Source (SSRC) Identifier            |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|           Reserved            |             Length            |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|                       User Data Words...+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.Payload Content Restrictions

This section defines constraints on the content of TTML documents carriedover RTP.¶

Multiple TTML subtitle streamsMUST NOT be interleaved in asingle RTP stream.¶

The TTML document instance's roottt element in thehttp://www.w3.org/ns/ttml namespaceMUST include atimeBase attribute in thehttp://www.w3.org/ns/ttml#parameter namespace containing the valuemedia.¶

This is equivalent to the TTML2 content profile definition document inFigure 2.¶

<?xml version="1.0" encoding="UTF-8"?><profile xmlns="http://www.w3.org/ns/ttml#parameter"    xmlns:ttm="http://www.w3.org/ns/ttml#metadata"    xmlns:tt="http://www.w3.org/ns/ttml"    type="content"    designator="urn:ietf:rfc:8759#content"    combine="mostRestrictive">    <features xml:base="http://www.w3.org/ns/ttml/feature/">        <tt:metadata>            <ttm:desc>                This document is a minimal TTML2 content profile                definition document intended to express the                minimal requirements to apply when carrying TTML                over RTP.            </ttm:desc>        </tt:metadata>        <feature value="required">#timeBase-media</feature>        <feature value="prohibited">#timeBase-smpte</feature>        <feature value="prohibited">#timeBase-clock</feature>    </features></profile>

6.Payload Processing Requirements

This section defines constraints on the processing of the TTML documents carried over RTP.¶

If a TTML document is assessed to be invalid, then itMUST bediscarded. This includes empty documents, i.e., those of zero length. Whenprocessing a valid document, the following requirements apply.¶

Each TTML document becomes active at its epoch E. EMUST beset to the RTP Timestamp in the header of the RTP packet carrying the TTMLdocument. Computed TTML media times are offset relative to E, in accordancewith Section I.2 of[TTML2].¶

When processing a sequence of TTML documents, where each is delivered inthe same RTP stream, exactly zero or one documentSHALL beconsidered active at each moment in the RTP time line.In the event that a documentD_n-1 with E_n-1 is active, and document D_n isdelivered with E_n where E_n-1 < E_n,processing of D_n-1MUST be stopped at E_nand processing of D_nMUST begin.¶

When all defined content within a document has ended, then processing of thedocumentMAY be stopped. This can be tested by constructing theintermediate synchronic document sequence from the document, as defined by[TTML2]. If the last intermediate synchronic document in thesequence is both active and contains no region elements, then all definedcontent within the document has ended.¶

As described above, the RTP Timestamp does not specify the exact timing ofthe media in this payload format. Additionally, documents may be fragmentedacross multiple packets. This renders the RTCP jitter calculationunusable.¶

<?xml version="1.0" encoding="UTF-8"?><profile xmlns="http://www.w3.org/ns/ttml#parameter"    xmlns:ttm="http://www.w3.org/ns/ttml#metadata"    xmlns:tt="http://www.w3.org/ns/ttml"    type="processor"    designator="urn:ietf:rfc:8759#processor"    combine="mostRestrictive">    <features xml:base="http://www.w3.org/ns/ttml/feature/">        <tt:metadata>            <ttm:desc>                This document is a minimal TTML2 processor profile                definition document intended to express the                minimal requirements of a TTML processor able to                process TTML delivered over RTP according to                RFC 8759.            </ttm:desc>        </tt:metadata>        <feature value="required">#timeBase-media</feature>        <feature value="optional">            #profile-full-version-2        </feature>    </features>    <extensions xml:base="urn:ietf:rfc:8759">        <extension restricts="#timeBase-media" value="required">            #rtp-relative-media-time        </extension>    </extensions></profile>

7.Payload Examples

Figure 4 is an example of a valid TTML document that maybe carried using the payload format described in this document.¶

<?xml version="1.0" encoding="UTF-8"?><tt xml:lang="en" xmlns="http://www.w3.org/ns/ttml" xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:ttp="http://www.w3.org/ns/ttml#parameter" xmlns:tts="http://www.w3.org/ns/ttml#styling" ttp:timeBase="media" >  <head>    <metadata>      <ttm:title>Timed Text TTML Example</ttm:title>      <ttm:copyright>The Authors (c) 2006</ttm:copyright>    </metadata>    <styling>      <!--        s1 specifies default color, font, and text alignment      -->      <style xml:id="s1"        tts:color="white"        tts:fontFamily="proportionalSansSerif"        tts:fontSize="100%"        tts:textAlign="center"      />    </styling>    <layout>      <region xml:id="subtitleArea"               tts:extent="78% 11%"        tts:padding="1% 5%"        tts:backgroundColor="black"        tts:displayAlign="after"      />    </layout>  </head>  <body region="subtitleArea">    <div>      <p xml:id="subtitle1" dur="5.0s">        How truly delightful!      </p>    </div>  </body></tt>

8.Fragmentation of TTML Documents

Many of the use cases for TTML are low bit-rate with RTP packets expectedto fit within the Path MTU. However, some documents may exceed the PathMTU. In these cases, they may be split between multiple packets. Wherefragmentation is used, the following guidelinesMUST befollowed:¶

• It isRECOMMENDED that documents be fragmented as seldomas possible, i.e., the least possible number of fragments is created out of adocument.¶

• Text stringsMUST split at character boundaries. Thisenables decoding of partial documents. As a consequence, documentfragmentation requires knowledge of the UTF-8/UTF-16 encoding formats todetermine character boundaries.¶

• Document fragmentsSHOULD be protected against packetlosses. More information can be found inSection 9.¶

When a document spans more than one RTP packet, the entire document isobtained by concatenating User Data Words from each consecutive contributingpacket in ascending order of Sequence Number.¶

As described inSection 6, only zero or one TTMLdocument may be active at any point in time. As such, thereMUST only be one document transmitted for a given RTPTimestamp. Furthermore, as stated inSection 4.1, themarker bitMUST be set for a packet containing the lastfragment of a document. A packet following one where the marker bit is setcontains the first fragment of a new document. The first fragment might alsobe the last.¶

9.Protection against Loss of Data

Consideration must be devoted to keeping loss of documents due to packetloss within acceptable limits. What is deemed acceptable limits is dependenton the TTML profile(s) used and use case, among other things. As such, specificlimits are outside the scope of this document.¶

DocumentsMAY be sent without additional protection ifend-to-end network conditions guarantee that document loss will be withinacceptable limits under all anticipated load conditions. Where such guaranteescannot be provided, implementationsMUST use a mechanism toprotect against packet loss. Potential mechanisms include Forward ErrorCorrection (FEC)[RFC5109], retransmission[RFC4588], duplication[ST2022-7], or an equivalenttechnique.¶

10.Congestion Control Considerations

Congestion control for RTPSHALL be used in accordance with[RFC3550] and with any applicable RTP profile, e.g.,[RFC3551]. "Multimedia Congestion Control: Circuit Breakers forUnicast RTP Sessions"[RFC8083] is an update to"RTP: A Transport Protocol for Real-timeApplications"[RFC3550], which defines criteria for when one is required tostop sending RTP packet streams. Applications implementing this standardMUST comply with[RFC8083], with particularattention paid to Section4.4on Media Usability.[RFC8085] provides additional informationon the best practices for applying congestion control to UDP streams.¶

11.Payload Format Parameters

This RTP payload format is identified using the existingapplication/ttml+xml media type as registered with IANA[IANA]and defined in[TTML-MTPR].¶

m=application 30000 RTP/AVP 112a=rtpmap:112 ttml+xml/90000a=fmtp:112 charset=utf-8;codecs=im2t

12.IANA Considerations

This document has no IANA actions.¶

13.Security Considerations

RTP packets using the payload format defined in this specification aresubject to the security considerations discussed in the RTP specification[RFC3550] and in any applicable RTP profile, such as RTP/AVP[RFC3551], RTP/AVPF[RFC4585], RTP/SAVP[RFC3711], or RTP/SAVPF[RFC5124].However, as"Securing the RTP Protocol Framework: Why RTP Does Not Mandate a Single MediaSecurity Solution"[RFC7202] discusses, it is not an RTPpayload format's responsibility to discuss or mandate what solutions are usedto meet the basic security goals (like confidentiality, integrity, and sourceauthenticity) for RTP in general. This responsibility lays on anyone using RTPin an application. They can find guidance on available security mechanismsand important considerations in "Options for Securing RTP Sessions"[RFC7201]. ApplicationsSHOULD use one or moreappropriate strong security mechanisms. The rest of this SecurityConsiderations section discusses the security impacting properties of thepayload format itself.¶

To avoid potential buffer overflow attacks, receivers should take care tovalidate that the User Data Words in the RTP payload are of the appropriatelength (using the Length field).¶

This payload format places no specific restrictions on the size of TTMLdocuments that may be transmitted. As such, malicious implementations could beused to perform denial-of-service (DoS) attacks.[RFC4732] provides more information on DoS attacks and describes somemitigation strategies. Implementers should take into consideration that thesize and frequency of documents transmitted using this format may vary overtime. As such, sender implementations should avoid producing streams thatexhibit DoS-like behaviour, and receivers should avoid false identification ofa legitimate stream as malicious.¶

As with other XML types and as noted inSection 10 of "XML Media Types" [RFC7303],repeated expansion of maliciously constructed XMLentities can be used to consume large amounts of memory, which may cause XMLprocessors in constrained environments to fail.¶

In addition, because of the extensibility features for TTML and of XML ingeneral, it is possible that "application/ttml+xml" may describe content thathas security implications beyond those described here. However, TTML does notprovide for any sort of active or executable content, and if the processorfollows only the normative semantics of the published specification, thiscontent will be outside TTML namespaces and may be ignored. Only in the casewhere the processor recognizes and processes the additional content or wherefurther processing of that content is dispatched to other processors wouldsecurity issues potentially arise. And in that case, they would fall outsidethe domain of this RTP payload format and the application/ttml+xmlregistration document.¶

Although not prohibited, there are no expectations that XML signatures orencryption would normally be employed.¶

Further information related to privacy and security at a document level canbe found in Appendix P of[TTML2].¶

Acknowledgements

Thanks toNigel Megitt,James Gruessing,Robert Wadge,Andrew Bonney,James Weaver,John Fletcher,Frans de Jong, andWillem Vermost for their valuablefeedback throughout thedevelopment of this document. Thanks to the W3C Timed Text Working Group andEBU Timed Text Working Group for their substantial efforts in developing thetimed text format this payload format is intended to carry.¶

Author's Address