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Network Working Group                                       G. HellstromRequest for Comments: 2793                                    Omnitor ABCategory: Standards Track                                       May 2000RTP Payload for Text ConversationStatus 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 followsRFC 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 inRFC 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 inRFC 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 encodedHellstrom                   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 inRFC 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" (seesection 6).     If redundancy is used perRFC 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 toRFC 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 20002.3 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.Hellstrom                   Standards Track                     [Page 4]

RFC 2793           RTP Payload for Text Conversation            May 20003.1 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 inRFC 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 20003.4 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 insection 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 20004. 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 20005.  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 forRFC 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 inRFC 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.seHellstrom                   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.com7. 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.se8. 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",RFC1889, 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",RFC2279, January 1998.Hellstrom                   Standards Track                     [Page 9]

RFC 2793           RTP Payload for Text Conversation            May 200010. 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]