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Versions: 00 01 02 03 04 rfc2793                         Standards Track
Internet Engineering Task Force                                   AVT WG
Internet Draft                                          Gunnar Hellstrom
draft-ietf-avt-rtp-text-04.txt                              LM  Ericsson
February,  6, 2000
Expires: August 6, 2000

                      RTP Payload for Text Conversation


   This document is an Internet-Draft and it is in full conformance with
   all provisions of section 10 of RFC2026. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
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   and may be updated, replaced, or obsoleted by other documents at any
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   Distribution of this document is unlimited.


   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.

Hellstrom                                                       [Page 1]

Internet Draft

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.
   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",
   document are to be interpreted as described in RFC 2119 [4]

Hellstrom                                                       [Page 2]

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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

   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

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.

Hellstrom                                                       [Page 3]

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2.2 Additional headers

   There are no additional headers defined specific to this payload

   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").

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
   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.

Hellstrom                                                       [Page 4]

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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

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. T.140 defines a missing data marker (Unicode
   character 2607 "Lightning") which SHOULD be used as a marker to
   indicate each missing T140block.

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.

Hellstrom                                                       [Page 5]

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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

   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.

Hellstrom                                                       [Page 6]

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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.

Hellstrom                                                       [Page 7]

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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 "draft-ietf-avt-rtp-text-04".

     Security considerations: None

     Interoperability considerations: None

     Published specification: ITU-T T.140 Recommendation.

     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

     Intended usage: COMMON
     Author/Change controller:
       Gunnar Hellstrom
       e-mail: gunnar.hellstrom@omnitor.se
Hellstrom                                                       [Page 8]

Internet Draft                                   Expires: August, 7, 2000

7.  Author's Address

   Gunnar Hellstrom
   Ericsson Mobile Communication AB
   Home Communications
   SE-164 80 Stockholm

   e-mail: gunnar.hellstrom@omnitor.se
   Tel:    +46 708 204 288
   Fax:    +46 8 556 002 06

8. Acknowledgements

   The author wants to thank Stephen Casner and Colin Perkins for
   valuable support with reviews and advice on creation of this
   document, 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 1999.

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

[3] C. Perkins, I. Kouvelas, V. Hardman, M. Handley, and J. Bolot,
   "RTP payload for redundant audio data," RFC 2198, Internet
   Engineering Task Force, Sept. 1997.

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

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

[6] F. Yergeau, "UTF-8, a transformation format of ISO 10646,"  RFC
    2279, Internet Engineering Task Force, Jan. 1998.

10.   Revision history.

    Oct 6 1999. draft-01. Explanations in recommended procedures
    Oct 22 1999. draft-02. Nomenclature, ordering of redundant packets,
                           send empty info to expire redundancy.
    Jan 13 2000. draft-03. Clarification after working group last call,
                           Clarify that timestamp is not used for
                           Sequencing. Improvements in clarity.
    Feb 6  2000. draft-04. Four "SHALL" changed to "MUST" because they
                           was the intention. Two typos corrected.

Hellstrom                                                       [Page 9]