    Network Working Group                                       G. Hellstrom
    Request for Comments: 2793                                    Omnitor AB
    Category: Standards Track                                       May 2000

                       RTP Payload for Text Conversation

    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 (2000).  All Rights Reserved.

    Abstract

       This memo describes how to carry text conversation session contents
       in RTP packets. Text conversation session contents are specified in
       ITU-T Recommendation T.140 [1].

       Text conversation is used alone or in connection to other
       conversational facilities such as video and voice, to form multimedia
       conversation services.

       This RTP payload description contains an optional possibility to
       include redundant text from already transmitted packets in order to
       reduce the risk of text loss caused by packet loss. The redundancy
       coding follows RFC 2198.

    1. Introduction

       This memo defines a payload type for carrying text conversation
       session contents in RTP packets. Text conversation session contents
       are specified in ITU-T Recommendation T.140 [1]. Text conversation is
       used alone or in connection to other conversational facilities such
       as video and voice, to form multimedia conversation services. Text in
       text conversation sessions is sent as soon as it is available, or
       with a small delay for buffering.

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    RFC 2793           RTP Payload for Text Conversation            May 2000

       The text is supposed to be entered by human users from a keyboard,
       handwriting recognition, voice recognition or any other input method.
       The rate of character entry is usually at a level of a few characters
       per second or less. Therefore, the expected number of characters to
       transmit is low. Only one or a few new characters are expected to be
       transmitted with each packet.

       T.140 specifies that text and other T.140 elements MUST be
       transmitted in ISO 10 646-1 code with UTF-8 transformation. That
       makes it easy to implement internationally useful applications, and
       to handle the text in modern information technology environments.
       The payload of an RTP packet following this specification consists of
       text encoded according to T.140 without any additional framing.  A
       common case will be a single ISO 10646 character, UTF-8 encoded.

       T.140 requires the transport channel to provide characters without
       duplication and in original order.  Text conversation users expect
       that text will be delivered with no or a low level of lost
       information. If lost information can be indicated, the willingness to
       accept loss is expected to be higher.

       Therefore a mechanism based on RTP is specified here. It gives text
       arrival in correct order, without duplications, and with detection
       and indication of losses.  It also includes an optional possibility
       to repeat data for redundancy to lower the risk of loss. Since packet
       overhead is usually much larger than the T.140 contents, the increase
       in channel load by the redundancy scheme is minimal.

    1.1 Terminology

       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 [4]

    2. Usage of RTP

       When transport of T.140 text session data in RTP is desired, the
       payload as described in this specification SHOULD be used.

       A text conversation RTP packet as specified by this payload format
       consists of an RTP header as defined in RFC 1889 [2] followed
       immediately by a block of T.140 data, defined here to be a
       "T140block".  There is no additional header specific to this payload
       format.  The T140block contains one or more T.140 code elements as
       specified in [1].  Most T.140 code elements are single ISO 10646 [5]
       characters, but some are multiple character sequences.  Each
       character is UTF-8 encoded [6] into one or more octets. This implies
       that each block MUST contain an integral number of UTF-8 encoded

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       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").

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    2.3 T.140 Text structure

       T.140 text is UTF-8 coded as specified in T.140 with no extra
       framing. When using the format with redundant data, the transmitter
       MAY select a number of T140block generations to retransmit in each
       packet. A higher number introduces better protection against loss of
       text but increases the data rate.

       Since packets are not generated at regular intervals, the timestamp
       is not sufficient to identify a packet in the presence of loss unless
       extra information is provided. Since sequence numbers are not
       provided in the redundant header, some additional rules must be
       followed to allow the redundant data corresponding to missing primary
       data to be merged properly into the stream of primary data
       T140blocks:

          - Each redundant data block MUST contain the same data as a
            T140block previously transmitted as primary data, and be
            identified with a timestamp offset equating to the original
            timestamp for that T140block.
          - The redundant data MUST be placed in age order with most
            recent redundant T140block last in the redundancy area.
          - All T140blocks from the oldest desired generation up through
            the generation immediately preceding the new (primary)
            T140block MUST be included.

       These rules allow the sequence numbers for the redundant T140blocks
       to be inferred by counting backwards from the sequence number in the
       RTP header.  The result will be that all the text in the payload will
       be contiguous and in order.

    3. Recommended procedures

       This section contains RECOMMENDED procedures for usage of the payload
       format.  Based on the information in the received packets, the
       receiver can:

          - reorder text received out of order.
          - mark where text is missing because of packet loss.
          - compensate for lost packets by using redundant data.

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    3.1 Recommended basic procedure

       Packets are transmitted only when there is valid T.140 data to
       transmit. The sequence number is used for sequencing of T.140 data.

       On reception, the RTP sequence number is compared with the sequence
       number of the last correctly received packet. If they are
       consecutive, the (only or primary) T140block is retrieved from the
       packet.

    3.2 Recommended procedure for compensation for lost packets.

       For reduction of data loss in case of packet loss, redundant data MAY
       be included in the packets following to the procedures in RFC 2198.
       If network conditions are not known, it is RECOMMENDED to use one
       redundant T140block in each packet. If there is a gap in the RTP
       sequence numbers, and redundant T140blocks are available in a
       subsequent packet, the sequence numbers for the redundant T140blocks
       should be inferred by counting backwards from the sequence number in
       the RTP header for that packet.  If there are redundant T140blocks
       with sequence numbers matching those that are missing, the redundant
       T140blocks may be substituted for the missing T140blocks.

       Both for the case when redundancy is used and not used, missing data
       SHOULD be marked by insertion of a missing text marker in the
       received stream for each missing T140block, as specified in ITU-T
       T.140. Addendum 1 [1].

    3.3 Recommended procedure for compensation for packets out of order.

       For protection against packets arriving out of order, the following
       procedure MAY be implemented in the receiver.  If analysis of a
       received packet reveals a gap in the sequence and no redundant data
       is available to fill that gap, the received packet can be kept in a
       buffer to allow time for the missing packet(s) to arrive.  It is
       suggested that the waiting time be limited to 0.5 seconds. For the
       case when redundancy is used the waiting time SHOULD be extended to
       the number of redundancy generations times the T.140 buffering timer
       if this product is known to be greater than 0.5 seconds.

       If a packet with a T140block belonging to the gap arrives before the
       waiting time expires, this T140block is inserted into the gap and
       then consecutive T140blocks from the leading edge of the gap may be
       consumed.  Any T140block which does not arrive before the time limit
       expires should be treated as lost.

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    3.4 Transmission during "silent periods" when redundancy is used.

       When using the redundancy transmission scheme, and there is nothing
       more to transmit from T.140, the latest T140block has a risk of
       getting old before it is transmitted as redundant data. The result is
       less useful protection against packet loss at the end of a text input
       sequence. For cases where this should be avoided, a zero-length
       primary T140block MAY be transmitted with the redundant data.

       Any zero-length T140blocks that are sent as primary data MUST be
       included as redundant T140blocks on subsequent packets just as normal
       text T140blocks would be so that sequence number inference for the
       redundant T140blocks will be correct, as explained in section 2.3.

       Redundancy for the last T140block SHOULD NOT be implemented by
       repeatedly transmitting the same packet (with the same sequence
       number) because this will cause the packet loss count, as reported in
       RTCP, to decrement.

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    4. Examples

       This is an example of a T140 RTP packet without redundancy.
        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=0  |M|   T140 PT   |       sequence number         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      timestamp (1000Hz)                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           synchronization source (SSRC) identifier            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +                      T.140 encoded data                       +
       |                                                               |
       +                                               +---------------+
       |                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       This is an example of an RTP packet with one redundant T140block.
        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=0  |M|  "RED" PT   |   sequence number of primary  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              timestamp  of primary encoding "P"               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           synchronization source (SSRC) identifier            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1|   T140 PT   |  timestamp offset of "R"  | "R" block length  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0|   T140 PT   |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                                                               |
       +               "R" T.140 encoded redundant data                +
       |                                                               |
       +                                               +---------------+
       |                                               |               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
       |                "P" T.140 encoded primary data                 |
       +                                                               +
       +                                               +---------------+
       |                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure: Examples of RTP text packets.

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    5.  Security Considerations

       Since the intention of the described payload format is to carry text
       in a text conversation, security measures in the form of encryption
       are of importance. The amount of data in a text conversation session
       is low and therefore any encryption method MAY be selected and
       applied to T.140 session contents or to the whole RTP packets. When
       redundant data is included, the same security considerations as for
       RFC 2198 apply.

    6.  MIME Media Type Registrations

       This document defines a new RTP payload name and associated MIME
       type, T140 (text/t140).

    6.1  Registration of MIME media type text/t140

       MIME media type name: text

       MIME subtype name: t140

       Required parameters: None

       Optional parameters: None

       Encoding considerations: T140 text can be transmitted with RTP as
       specified in RFC 2793.

       Security considerations: None

       Interoperability considerations: None

       Published specification: ITU-T T.140 Recommendation.
                                RFC 2793.

       Applications which use this media type:
         Text communication terminals and text conferencing tools.

       Additional information: None

         Magic number(s): None
         File extension(s): None
         Macintosh File Type Code(s): None

       Person & email address to contact for further information:
         Gunnar Hellstrom
         e-mail: gunnar.hellstrom@omnitor.se

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       Intended usage: COMMON
         Author                      / Change controller:
         Gunnar Hellstrom            | IETF avt WG
         gunnar.hellstrom@omnitor.se | c/o Steve Casner casner@cisco.com

    7. Author's Address

       Gunnar Hellstrom
       Omnitor AB
       Alsnogatan 7, 4 tr
       SE-116 41 Stockholm
       Sweden

       Phone: +46 708 204 288 / +46 8 556 002 03
       Fax:   +46 8 556 002 06
       EMail: gunnar.hellstrom@omnitor.se

    8. Acknowledgements

       The author wants to thank Stephen Casner and Colin Perkins for
       valuable support with reviews and advice on creation of this
       document, to Mickey Nasiri at Ericsson Mobile Communication for
       providing the development environment, and Michele Mizarro for
       verification of the usability of the payload format for its intended
       purpose.

    9. References

       [1]  ITU-T Recommendation T.140 (1998) - Text conversation protocol
            for multimedia application, with amendment 1, (2000).

       [2]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
            "RTP: A Transport Protocol for Real-Time Applications", RFC
            1889, January 1996.

       [3]  Perkins, C., Kouvelas, I., Hardman, V., Handley, M. and J.
            Bolot, "RTP Payload for Redundant Audio Data", RFC 2198,
            September 1997.

       [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
            Levels", BCP 14, RFC 2119, March 1997.

       [5]  ISO/IEC 10646-1: (1993), Universal Multiple Octet Coded
            Character Set.

       [6]  Yergeau, F., "UTF-8, a transformation format of ISO 10646",  RFC
            2279, January 1998.

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    10. Full Copyright Statement

       Copyright (C) The Internet Society (2000).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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