Registration of the text/red MIME Sub-Type
The registration that gives redundant real-time text its official MIME name, text/red.
Jones, P., "Registration of the text/red MIME Sub-Type", RFC 4102, DOI 10.17487/RFC4102, June 2005, <https://www.rfc-editor.org/info/rfc4102>.
@misc{rfc4102,
series = {Request for Comments},
number = 4102,
howpublished = {RFC 4102},
publisher = {RFC Editor},
doi = {10.17487/RFC4102},
url = {https://www.rfc-editor.org/info/rfc4102},
author = {P. Jones},
title = {{Registration of the text/red MIME Sub-Type}},
pagetotal = 6,
year = 2005,
month = jun,
} RFC text
Contents
Network Working Group P. Jones
Request for Comments: 4102 Cisco Systems, Inc.
Category: Standards Track June 2005
Registration of the text/red MIME Sub-Type
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines the text/red MIME sub-type. "Red" is short for
redundant. The actual RTP packetization for this MIME type is
specified in RFC 2198.
1. Introduction
Text is an important component of any multimedia communication
system. Like audio, the transport of text can benefit from the use
of redundancy in order to improve reliability and end-user
experience.
RFC 2198 [1] defines an RTP [2] payload format for redundant audio
data. The format defined in that document is quite suitable for
providing redundancy for text, as well as audio.
RFC 4103 [8] specifies one usage of RFC 2198 and the text/red MIME
type for the transport of redundant text data.
This memo provides the MIME sub-type registration information for
text/red. While this document focuses on the use of this MIME sub-
type in SDP [5], the application of this MIME sub-type is not
restricted to SDP.
2. Conventions Used in This Document
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 [3].
3. IANA Considerations
One new MIME sub-type has been registered by the IANA, as described
below:
MIME media type name: text
MIME subtype name: RED
Required parameters:
rate: the RTP clock rate of the payload carried within the RTP
packet. Typically, this rate is 1000, but other rates MAY be
specified. This parameter MUST be set equal to the clock rate of
the text payload format carried as the primary encoding.
pt: a comma-separated ordered list of RTP payload types
enumerating the primary, secondary, etc., in accordance with RFC
2198. Because comma is a special character, the list MUST be a
quoted-string (enclosed in double quotes). For static payload
types, each list element is simply the type number. For dynamic
payload types, each list element is a mapping of the dynamic
payload type number to an embedded MIME content-type specification
for the payload format corresponding to the dynamic payload type.
The format of the mapping is:
dynamic-payload-type "=" content-type
If the content-type string includes a comma, then the content-
type string MUST be a quoted-string. If the content-type string
does not include a comma, it MAY still be quoted. Because it is
part of the list, which must itself be a quoted-string, the
quotation marks MUST be quoted with backslash quoting as specified
in RFC 2045 [4]. If the content-type string itself contains a
quoted-string, then the requirement for backslash quoting is
recursively applied.
Optional parameters: ptime, maxptime (these attributes are originally
defined in RFC 2327 [5] and RFC 3267 [6], respectively)
Restrictions on Usage:
This type is defined only for transfer via RTP.
It shall not be defined for a storage format.
Encoding considerations:
See restrictions on Usage above; this section is included per
the requirements in RFC 3555 [7].
Security considerations: Refer to section 5 of RFC 4102.
Interoperability considerations: none
Published specification: RFC 2198
Applications which use this media type:
Text streaming and conferencing tools.
Additional information: none
Person & email address to contact for further information:
Paul E. Jones
E-mail: paulej@packetizer.com
Intended usage: COMMON
Author:
Paul E. Jones
paulej@packetizer.com
Change Controller:
AVT Working Group delegated from the IESG
4. Mapping to SDP Parameters
The information carried in the MIME media type specification has a
specific mapping to fields in the Session Description Protocol (SDP)
[5], which is commonly used to describe RTP sessions. When SDP is
used to specify sessions employing the RFC 2198 in a text session,
the mapping is as follows:
- The MIME type ("text") goes in SDP "m=" as the media name.
- The value of the parameter "rate" goes in SDP "a=rtpmap".
- The MIME subtype (RED) goes in SDP "a=rtpmap" as the encoding
name.
- The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and
"a=maxptime" attributes, respectively.
- The pt parameter is mapped to an a=fmtp attribute by eliminating
the parameter name (pt) and changing the commas to slashes. For
example, 'pt="101,102"' maps to 'a=fmtp:99 101/102', where = '99'
is the payload type of the redundancy frames. Note that the
single quote marks (') used in this example are not present in the
actual message encoding, but are present here only for
readability. The level of redundancy is shown by the number of
elements in the payload type list.
Any dynamic payload type in the list MUST be represented by its
payload type number and not by its content-type. The mapping of
payload types to the content-type is done using the normal SDP
procedures with "a=rtpmap".
An example of SDP is:
m=text 11000 RTP/AVP 98 100
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98
For each redundancy payload type defined, the ordering of the primary
and redundancy encoding(s) is fixed. If more than one combination of
primary and redundancy encoding(s) is desired, multiple redundancy
payload types needs to be defined.
5. Security Considerations
The security considerations listed in RFC 2198 apply. Further, it
should be understood that text data, perhaps even more so than audio
data, is susceptible to unwanted modification that may lead to
undesired results. To prevent modification of the primary,
secondary, or header information, payload integrity protection over
at least the complete RTP packet is RECOMMENDED, for example using
SRTP [9].
6. Normative References
[1] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, M.,
Bolot, J., Vega-Garcia, A., and S. Fosse-Parisis, "RTP Payload
for Redundant Audio Data", RFC 2198, September 1997.
[2] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[5] Handley, M., Jackson, V., "SDP: Session Description Protocol",
RFC 2327, April 1998.
[6] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, "Real-
Time Transport Protocol (RTP) Payload Format and File Storage
Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate
Wideband (AMR-WB) Audio Codecs", RFC 3267, June 2002.
[7] Casner, S. and P. Hoschka, "MIME Type Registration of RTP Payload
Formats", RFC 3555, July 2003.
7. Informative References
[8] Hellstrom, G. and P. Jones, "RTP Payload for Text Conversation",
RFC 4103, June 2005.
[9] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004.
Author's Address
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709, USA
Phone: +1 919 392 6948
EMail: paulej@packetizer.com
Full Copyright Statement
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AI ELI5
On this page
§1Why Text Needs a Redundancy Type
Text is an important part of any multimedia communication system. Like audio, text transport can use redundancy to improve reliability and the end-user experience. In the name red, the word is short for redundant.
RFC 2198 already defines an RTP payload format for redundant audio data, and the memo notes that the same format suits text too. RFC 4103 specifies one use of that format and of the text/red MIME type for redundant text.
This memo only supplies the MIME sub-type registration. It focuses on use in SDP, but says the type is not restricted to SDP.
If you say something twice, a friend who missed it the first time can still catch it. This memo just writes down the official name for text sent that way.
§2The text/red Registration and Its Parameters
The registration is for media type name text and subtype name RED. It has two required parameters.
rate: the RTP clock rate of the payload. Typically this is 1000, but other rates MAY be specified. It MUST equal the clock rate of the text payload format carried as the primary encoding.pt: a comma-separated, ordered list of RTP payload types naming the primary, secondary and later encodings, as in RFC 2198. Because a comma is a special character, the list MUST be a quoted-string.
For static payload types each list element is just the type number. For dynamic payload types each element maps the payload type number to an embedded MIME content-type, written as dynamic-payload-type "=" content-type. Quotation marks inside the quoted list MUST be escaped with backslash quoting as in RFC 2045, applied recursively if the content-type itself contains a quoted-string.
The optional parameters are ptime and maxptime, defined originally in RFC 2327 and RFC 3267.
The name tag has two blanks to fill in: how fast the clock ticks, and a list saying which kinds of text are the main copy and which are the spare copies.
§3Restrictions and Registration Details
The type is defined only for transfer via RTP. It shall not be defined for a storage format, so the encoding considerations simply point back to this restriction, as RFC 3555 requires.
The registration lists the published specification as RFC 2198, the intended usage as COMMON, and the applications as text streaming and conferencing tools. It reports no interoperability considerations. The change controller is the AVT Working Group, delegated from the IESG.
This kind of text is meant to be sent live over the network, not saved in a file.
§4Mapping to SDP Parameters
The Session Description Protocol (SDP) is commonly used to describe RTP sessions, and the memo spells out where each piece of the MIME registration goes.
- The MIME type
textgoes in the SDPm=line as the media name. rategoes ina=rtpmap, and the subtypeREDgoes there as the encoding name.ptimeandmaxptimego in thea=ptimeanda=maxptimeattributes.ptbecomes ana=fmtpattribute: drop the parameter name and change the commas to slashes. The number of elements in the list shows the level of redundancy.
Any dynamic payload type in the list MUST be written as its number, not its content-type; the number-to-type mapping is done with normal SDP a=rtpmap lines. For each redundancy payload type the ordering of primary and redundancy encodings is fixed, so wanting more than one combination means defining more than one redundancy payload type.
When two programs set up a call, they write a little note describing it. This part says which line of the note holds which piece of the text/red name tag.
m=text 11000 RTP/AVP 98 100
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98§5Security Considerations
The security considerations of RFC 2198 apply. The memo adds that text data, perhaps even more than audio, is susceptible to unwanted modification that may lead to undesired results.
To prevent modification of the primary data, the secondary data or the header information, payload integrity protection over at least the complete RTP packet is RECOMMENDED, for example using SRTP.
Someone could sneak in and change the words on the way. Putting a tamper-proof seal on the whole package stops that.