RFC 2793 (rfc2793) - Page 3 of 10
RTP Payload for Text Conversation
Alternative Format: Original Text Document
RFC 2793 RTP Payload for Text Conversation May 2000
characters regardless of the number of octets per character. It also
implies that any composite character sequence (CCS) SHOULD be placed
within one block.
The T140blocks MAY be transmitted redundantly according to the
payload format defined in RFC 2198 [3]. In that case, the RTP header
is followed by one or more redundant data block headers, the same
number of redundant data fields carrying T140blocks from previous
packets, and finally the new (primary) T140block for this packet.
2.1 RTP packet header
Each RTP packet starts with a fixed RTP header. The following fields
of the RTP fixed header are used for T.140 text streams:
Payload Type (PT): The assignment of an RTP payload type is specific
to the RTP profile under which this payload format is used. For
profiles which use dynamic payload type number assignment, this
payload format is identified by the name "T140" (see section 6).
If redundancy is used per RFC 2198, the Payload Type MUST indicate
that payload format ("RED").
Sequence number: The Sequence Number MUST be increased by one for
each new transmitted packet. It is used for detection of packet
loss and packets out of order, and can be used in the process of
retrieval of redundant text, reordering of text and marking missing
text.
Timestamp: The RTP Timestamp encodes the approximate instance of
entry of the primary text in the packet. A clock frequency of 1000
Hz MUST be used. Sequential packets MUST NOT use the same
timestamp. Since packets do not represent any constant duration,
the timestamp cannot be used to directly infer packet losses.
2.2 Additional headers
There are no additional headers defined specific to this payload
format.
When redundant transmission of the data according to RFC 2198 is
desired, the RTP header is followed by one or more redundant data
block headers, one for each redundant data block to be included.
Each of these headers provides the timestamp offset and length of the
corresponding data block plus a payload type number indicating this
payload format ("T140").
Hellstrom Standards Track