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rfc:rfc2793

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.

Hellstrom Standards Track [Page 1] 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

Hellstrom Standards Track [Page 2] 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 [Page 3] RFC 2793 RTP Payload for Text Conversation May 2000

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:
  1. 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:
  1. reorder text received out of order.
  2. mark where text is missing because of packet loss.
  3. compensate for lost packets by using redundant data.

Hellstrom Standards Track [Page 4] RFC 2793 RTP Payload for Text Conversation May 2000

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.

Hellstrom Standards Track [Page 5] RFC 2793 RTP Payload for Text Conversation May 2000

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.

Hellstrom Standards Track [Page 6] RFC 2793 RTP Payload for Text Conversation May 2000

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 Standards Track [Page 7] RFC 2793 RTP Payload for Text Conversation May 2000

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

Hellstrom Standards Track [Page 8] RFC 2793 RTP Payload for Text Conversation May 2000

 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.

Hellstrom Standards Track [Page 9] RFC 2793 RTP Payload for Text Conversation May 2000

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.

Hellstrom Standards Track [Page 10]

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