Registration of the text/red MIME Sub-Type
updated by
- rfc-6354 — Forward-Shifted RTP Redundancy Payload Support
Extracted elements (14)
The ordering of primary and redundancy encodings is fixed for each redundancy payload type. If more than one combination of primary and redundancy encoding(s) is desired, multiple redundancy payload types must be defined rather than reordering within a single type.
Optional parameters ptime and maxptime (originally defined in RFC 2327 and RFC 3267, respectively) are supported by the text/red MIME type registration.
The text/red MIME type is based on RFC 2198 (RTP Payload for Redundant Audio Data), which is also the published specification for this MIME type. The format defined in RFC 2198 for audio redundancy is equally suitable for text redundancy.
Any dynamic payload type in the SDP 'pt' list MUST be represented by its payload type number and not by its content-type. The mapping of payload types to content-type is done using normal SDP procedures with 'a=rtpmap'.
For dynamic payload types in the 'pt' list, if the content-type string includes a comma, then the content-type string MUST be a quoted-string. Quotation marks within the list MUST be quoted with backslash quoting as specified in RFC 2045.
The 'pt' parameter list MUST be a quoted-string (enclosed in double quotes) because comma is a special character in MIME parameter syntax.
The text/red type is defined only for transfer via RTP and shall not be defined for a storage format.
Dynamic payload type mapping format within the 'pt' parameter: 'dynamic-payload-type "=" content-type'. This allows dynamic payload types to reference their corresponding MIME content-type specification.
SDP mapping: the MIME type 'text' goes in 'm=' as media name; 'rate' goes in 'a=rtpmap'; subtype 'RED' goes in 'a=rtpmap' as encoding name; 'ptime' and 'maxptime' map to 'a=ptime' and 'a=maxptime' attributes respectively.
The 'pt' parameter maps to an 'a=fmtp' attribute by eliminating the parameter name and replacing commas with slashes. For example, 'pt="101,102"' maps to 'a=fmtp:99 101/102' where 99 is the payload type of the redundancy frames.
The 'pt' required parameter is a comma-separated ordered list of RTP payload types enumerating the primary, secondary, etc., encodings in accordance with RFC 2198. Because comma is a special character, the list MUST be a quoted-string enclosed in double quotes.
The 'rate' required parameter specifies the RTP clock rate of the payload carried within the RTP packet. Typically 1000, but other rates MAY be specified; it MUST be set equal to the clock rate of the text payload format carried as the primary encoding.
Registers the text/red MIME sub-type with IANA. The media type is 'text' and the subtype is 'RED', intended for RTP transport of redundant text data as specified in RFC 2198.
Text data is susceptible to unwanted modification that may lead to undesired results, perhaps even more so than audio data. Payload integrity protection over at least the complete RTP packet is RECOMMENDED, for example using SRTP (RFC 3711), to prevent modification of primary, secondary, or header information.