RTP Payload for Text Conversation
draft-ietf-avt-rfc2793bis-09

Network Working Group                                      G. Hellstrom 
Internet Draft                                               Omnitor AB 
<draft-ietf-avt-rfc2793bis-09.txt>                             P. Jones 
Expires: February 2005                              Cisco Systems, Inc. 
                                                            August 2004 
    
    
                    RTP Payload for Text Conversation 
    
    
Status of this Memo 
 
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   accordance with RFC 3668 (BCP 79). 
    
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Abstract 
    
   This memo describes how to carry real time text conversation 
   session contents in RTP packets. Text conversation session contents 
   are specified in ITU-T Recommendation T.140. 
    
   One payload format is described for transmitting text on a separate 
   RTP session dedicated for the transmission of text.  

   This RTP payload description recommends a method to include 
   redundant text from already transmitted packets in order to reduce 
   the risk of text loss caused by packet loss. 
    

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Table of Contents 
 
   1. Introduction...................................................3 
   2. Conventions used in this document..............................4 
   3. Usage of RTP...................................................4 
      3.1 Motivations and rationale..................................4 
      3.2 Payload Format for Transmission of text/t140 Data..........4 
      3.3 The "T140block"............................................4 
      3.4 Synchronization of Text with Other Media...................5 
      3.5 RTP packet header..........................................5 
   4. Protection against loss of data................................6 
      4.1 Payload Format when using Redundancy.......................6 
      4.2 Using redundancy with the text/t140 format.................6 
   5. Recommended Procedure..........................................7 
      5.1 Recommended Basic Procedure................................7 
      5.2 Transmission before and after "Idle Periods"...............8 
      5.3 Detection of Lost Text Packets.............................8 
      5.4 Compensation for Packets Out of Order......................9 
   6. Parameter for Character Transmission Rate......................9 
   7. Examples......................................................10 
      7.1 RTP Packetization Examples for the text/t140 format.......10 
      7.2 SDP Examples..............................................12 
   8. Security Considerations.......................................12 
      8.1 Confidentiality...........................................13 
      8.2 Integrity.................................................13 
      8.3 Source authentication.....................................13 
   9. Congestion Considerations.....................................13 
   10. IANA considerations..........................................15 
      10.1 Registration of MIME Media Type text/t140................15 
      10.2 SDP mapping of MIME parameters...........................16 
      10.3 Offer/Answer Consideration...............................16 
   11. Authors' Addresses...........................................16 
   12. Acknowledgements.............................................17 
   13. Normative References.........................................17 
   14. Informative References.......................................18 
   15. Intellectual Property Statement..............................18 
   16. Copyright Statement..........................................18 
    
   [Notes to RFC Editor:  
   1. All references to RFC XXXX are to be replaced by references to 
      the RFC number of this memo, when published.  
   2. All references to RFC YYYY [9] are to be replaced by references 
      to the document that registers the text/red MIME type.] 

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1. Introduction 
    
   This document defines a payload type for carrying text conversation 
   session contents in RTP [2] 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 multimedia conversation sessions is sent 
   character-by-character as soon as it is available, or with a small 
   delay for buffering. 
    
   The text is intended 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. In general, only one or a few new 
   characters are expected to be transmitted with each packet. Small 
   blocks of text may be prepared by the user and pasted into the user 
   interface for transmission during the conversation, occasionally 
   causing packets to carry more payload. 
    
   T.140 specifies that text and other T.140 elements must be 
   transmitted in ISO 10646-1[5] code with UTF-8 [6] 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. 
    
   Therefore a mechanism based on RTP is specified here. It gives text 
   arrival in correct order, without duplication, and with detection 
   and indication of loss. 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 bandwidth with the use of redundancy is minimal. 
    
   By using RTP for text transmission in a multimedia conversation 
   application, uniform handling of text and other media can be 
   achieved in, as examples, conferencing systems, firewalls, and 
   network translation devices.  This, in turn, eases the design and 
   increases the possibility for prompt and proper media delivery. 
    
   This document obsoletes RFC 2793 [16].  The text clarifies 
   ambiguities in RFC 2793, improves on the specific implementation 

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   requirements learned through development experience and gives 
   explicit usage examples. 
    
2. Conventions used in this document 
    
   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]. 
    
3. Usage of RTP  
    
   The payload format for real-time text transmission with RTP [2] 
   described in this memo is intended for general text conversation 
   use and called text/t140 after its MIME registration. 
    
3.1 Motivations and rationale 
 
   The text/t140 format is intended to be used for text transmitted on 
   a separate RTP session dedicated for the transmission of text and 
   not shared with other media. 
    
   The text/t140 format MAY be used for any non-gateway application as 
   well as in gateways. It MAY be used simultaneously with other media 
   streams, transmitted as a separate RTP session, as required in real 
   time multimedia applications. 
    
   The text/t140 format specified in this memo is compatible with its 
   earlier definition in RFC2793. It is just refined, with the main 
   intention to minimize interoperability problems and encourage good 
   reliability and functionality. 
    
   By specifying text transmission as a text medium, many good effects 
   are gained. Routing, device selection, invocation of transcoding, 
   selection of quality of service parameters and other high and low 
   level functions are depending on each medium being explicitly 
   specified. 
    
3.2 Payload Format for Transmission of text/t140 Data 
    
   A text/t140 conversation RTP payload format consists of one and 
   only one block of T.140 data, referred to as a "T140block" (see 
   section 3.3).  There are no additional headers specific to this 
   payload format. The fields in the RTP header are set as defined in 
   section 3.5. 
    
3.3 The "T140block" 
    
   T.140 text is UTF-8 coded as specified in T.140 with no extra 
   framing. 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 

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   character is UTF-8 encoded [6] into one or more octets. Each block 
   MUST contain an integral number of UTF-8 encoded characters 
   regardless of the number of octets per character. Any composite 
   character sequence (CCS) SHOULD be placed within one block. 
    
3.4 Synchronization of Text with Other Media 
    
   Usually, each medium in a session utilizes a separate RTP stream. 
   As such, if synchronization of the text and other media packets is 
   important, the streams MUST be associated when the sessions are 
   established and the streams MUST share the same reference clock 
   (refer to the description of the timestamp field as it relates to 
   synchronization in section 5.1 of RFC 3550 [2]).  Association of 
   RTP streams can be done through the CNAME field of RTCP SDES 
   function. It is dependent on the particular application and is 
   outside the scope of this document. 
    
    
3.5 RTP packet header 
    
   Each RTP packet starts with a fixed RTP header. The following 
   fields of the RTP fixed header are specified 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 that use dynamic payload type number 
     assignment, this payload format can be identified by the MIME 
     type "text/t140" (see section 10).  If redundancy is used per RFC 
     2198, another payload type number needs to be provided for the 
     redundancy format. The MIME type for identifying RFC 2198 is 
     available in RFC YYYY [9]. 
    
   Sequence number: The definition of sequence numbers is available in 
     RFC 3550 [2]. When transmitting text using the payload format for 
     text/t140, 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 loss. 
    
   M-bit: The M-bit MUST be included. The first packet in a session, 
     and the first packet after an idle period, SHOULD be 
     distinguished by setting the marker bit in the RTP data header to 
     one.  The marker bit in all other packets MUST be set to zero.  
     The reception of the marker bit MAY be used for refined methods 
     for detection of loss.  

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4. Protection against loss of data 
    
   Consideration must be devoted to keeping loss of text caused by 
   packet loss within acceptable limits. (See ITU-T F.703 [17]) 
    
   The default method that MUST be used when no other method is 
   explicitly selected is redundancy in accordance with RFC 2198 [3]. 
   When this method is used, the original text and two redundant 
   generations SHOULD be transmitted if the application or end-to-end 
   conditions do not call for other levels of redundancy to be used. 
    
   Forward Error Correction mechanisms as per RFC 2733 [8] or any 
   other mechanism with the purpose of increasing the reliability of 
   text transmission MAY be used as an alternative or complement to 
   redundancy. Text data MAY be sent without additional protection if 
   end-to-end network conditions allow the text quality requirements 
   specified in ITU-T F.703 [17] to be met in all anticipated load 
   conditions. 
    
    
4.1 Payload Format when using Redundancy 
    
   When using the payload 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 marginally increases the data rate. 
    
   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 the 
   payload format text/t140.   
    
   After the redundant data block headers follows the redundant data 
   fields carrying T140blocks from previous packets, and finally the 
   new (primary) T140block for this packet. 
    
   Redundant data that would need a timestamp offset higher than 16383 
   due to its age at transmission MUST NOT be included in transmitted 
   packets. 
    
4.2 Using redundancy with the text/t140 format. 
    
   Since text is transmitted only when there is text to transmit, the 
   timestamp is not used to identify a lost packet. Rather, missing 
   sequence numbers are used to detect lost text packets at reception. 
   Also, 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 

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   properly into the stream of primary data T140blocks. 
    They are: 
    
     - Each redundant data block MUST contain the same data as a 
       T140block previously transmitted as primary data.  
     - 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. 
    
   If there is a gap in the received 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. 
    
    
5. Recommended Procedure 
 
   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. 
    
5.1 Recommended Basic Procedure 
    
   Packets are transmitted when there is valid T.140 data to transmit.  
    
   T.140 specifies that T.140 data MAY be buffered for transmission 
   with a maximum buffering time of 500 ms. A buffering time of 300 ms 
   is RECOMMENDED, when the application or end-to-end network 
   conditions are not known to require another value.  
    
   If no new data is available for a longer period than the buffering 
   time, the transmission process is in an idle period. 
    
   When new text is available for transmission after an idle period, 
   it is RECOMMENDED to send it as soon as possible. After this 
   transmission, it is RECOMMENDED to buffer T.140 data in buffering 
   time intervals, until next idle period. This is done in order to 

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   keep the maximum bit rate usage for text at a reasonable level. The 
   buffering time MUST be selected so that text users will perceive a 
   real time text flow. 
    
5.2 Transmission before and after "Idle Periods". 
    
   When valid T.140 data has been sent and no new T.140 data is 
   available for transmission after the selected buffering time, an 
   empty T140block SHOULD be transmitted. This situation is regarded 
   to be the beginning of an idle period. The procedure is recommended 
   in order to more rapidly detect potentially missing text before an 
   idle period. 
    
   An empty T140block contains no data.  
    
   When redundancy is used, transmission continues with a packet at 
   every transmission timer expiration and insertion of an empty 
   T.140block as primary, until the last non-empty T140block has been 
   transmitted as primary and as redundant data with all intended 
   generations of redundancy. The last packet before an idle period 
   will contain only one non-empty T140block as redundant data, while 
   the remainder of the redundancy packet will contain empty 
   T140blocks. 
    
   Any empty T140block that is sent as primary data MUST be included 
   as redundant T140blocks in subsequent packets just as normal text 
   T140blocks would be, unless the empty T140block is too old to be 
   transmitted. This is done so that sequence number inference for the 
   redundant T140blocks will be correct, as explained in section 4.2.  
    
   After an idle period, the transmitter SHOULD set the M-bit to one 
   in the first packet with new text. 
     
5.3 Detection of Lost Text Packets 
    
   Packet loss for text/t140 packets MAY be detected by observing gaps 
   in the sequence numbers of RTP packets received by the receiver. 
    
   With text/t140 the loss of packets is usually detected by 
   comparison of the sequence of RTP packets as they arrive. Any 
   discrepancy MAY be used to indicate loss. The highest RTP sequence 
   number received may also be compared with that in RTCP reports, as 
   an additional check for loss of the last packet before an idle 
   period. 
    
   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]. 
    
   Since empty T140blocks are transmitted in the beginning of an idle 
   period, there is a slight risk of falsely marking loss of text, 

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   when only an empty T140block was lost and when using text/t140. 
   Procedures based on detection of the packet with the M-bit set to 
   one MAY be used to reduce the risk for introducing false markers of 
   loss.  
    
   If redundancy is used with the text/t140 format, and a packet is 
   received with fewer redundancy levels than normally in the session, 
   it SHOULD be treated as if one empty T140block has been received 
   for each excluded level in the received packet. This is because the 
   only occasion when a T140block is excluded from transmission is 
   when it is an empty T140block that has become too old to be 
   transmitted. 
    
   If two successive packets have the same number of redundant 
   generations, it SHOULD be treated as the general redundancy level 
   for the session. Change of the general redundancy level SHOULD only 
   be done after an idle period. 
    
   The text/t140 format relies on use of the sequence number in the 
   RTP packet header for detection of loss and is therefore not 
   suitable for an application where it needs to be alternating with 
   other payloads in the same RTP stream. It would be complicated and 
   unreliable to try to detect loss of data at the edges of the shifts 
   between t140 text and other stream contents. It is therefore 
   RECOMMENDED to be the only payload type in the RTP stream. 
     
5.4 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 SHOULD be kept 
   in a buffer to allow time for the missing packet(s) to arrive.  It 
   is RECOMMENDED that the waiting time be limited to 1 second.  
    
   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 and a missing text 
   marker inserted ( see section 5.3 ). 
    
6. Parameter for Character Transmission Rate 
    
   In some cases, it is necessary to limit the rate at which 
   characters are transmitted.  For example, when a PSTN gateway is 
   interworking between an IP device and a PSTN textphone, it may be 
   necessary to limit the character rate from the IP device in order 
   to avoid throwing away characters in case of buffer overflow at the 
   PSTN gateway.  
    

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   To control the character transmission rate, the MIME parameter 
   "cps" in the "fmtp" attribute [7] is defined (see section 10 ). It 
   is used in SDP with the following syntax: 
    
       a=fmtp:<format> cps=<integer> 
    
   The <format> field is populated with the payload type that is used 
   for text.  The <integer> field contains an integer representing the 
   maximum number of characters that may be received per second. The 
   value shall be used as a mean value over any 10 second  interval. 
   The default value is 30. 
    
   Examples of use in SDP are found in section 7.2. 
    
   In receipt of this parameter, devices MUST adhere to the request by 
   transmitting characters at a rate at or below the specified 
   <integer> value. Note that this parameter was not defined in 
   RFC 2793 [16]. Therefore implementations of the text/t140 format 
   may be in use that do not recognize and act according to this 
   parameter. Receivers of text/t140 SHALL therefore be designed so 
   that they can handle temporary reception of characters at a higher 
   rate than this parameter specifies, so that malfunction because of 
   buffer overflow is avoided for text conversation with human input.  
    
7. Examples 
    
7.1 RTP Packetization Examples for the text/t140 format. 
    
   Below is an example of a text/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                       | 
   +                                               +---------------+ 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    
   Below is an example of a text/t140 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"               | 

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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |           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       | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                            
    
    
   Below is an example of an RTP packet with one redundant T140block 
   using text/t140 payload format.  The primary data block is 
   empty, which is the case when transmitting a packet for the 
   sole purpose of forcing the redundant data to be transmitted 
   in the absence of any new data. 
    
    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              | 
   +-+-+-+-+-+-+-+-+                               +---------------+ 
   |                                               | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    
   As a follow-on to the previous example, the example below shows 
   the next RTP packet in the sequence which does contain a real 
   T140block when using the text/t140 payload format.  Note that the 
   empty block is present in the redundant transmissions of the 
   text/t140 payload format.  This example shows 2 levels of 
   redundancy and one primary data block.  The value of the "R2 
   block length" would be set to zero in order to 
   represent the empty T140block. 

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    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 "R2" | "R2" block length | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |1|   T140 PT   |  timestamp offset of "R1" | "R1" block length | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
   |0|   T140 PT   | "R1" T.140 encoded redundant data             | 
   +-+-+-+-+-+-+-+-+                               +---------------+ 
   |                                               |               | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+         +-+-+-+ 
   |              "P" T.140 encoded primary data             | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    
    
    
    
7.2 SDP Examples 
    
   Below is an example of SDP describing RTP text transport on port 
   11000: 
    
       m=text 11000 RTP/AVP 98 
       a=rtpmap:98 t140/1000 
    
   Below is an example of SDP similar to the above example, but also 
   utilizing RFC 2198 to provide the recommended two levels of 
   redundancy for the text packets: 
    
       m=text 11000 RTP/AVP 98 100 
       a=rtpmap:98 t140/1000 
       a=rtpmap:100 red/1000 
       a=fmtp:100 98/98/98 
    
    
   Note - While these examples utilize the RTP/AVP profile, it is not 
   intended to limit the scope of this memo to use with only that 
   profile.  Rather, any appropriate profile may be used in 
   conjunction with this memo. 
    
8. Security Considerations 
    
   All of the security considerations from section 14 of RFC 3550 [2] 
   apply. 
    

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8.1 Confidentiality 
    
   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. SRTP [14] provides a suitable method for ensuring 
   confidentiality.  
    
8.2 Integrity 
    
   It may be desirable to protect the text contents of an RTP stream 
   against manipulation.  SRTP [14] provides methods for providing 
   integrity that MAY be applied.  
    
8.3 Source authentication 
    
   Measures to make sure that the source of text is the intended one 
   can be accomplished by a combination of methods. 
    
   Text streams are usually used in a multimedia control environment. 
   Security measures for authentication are available and SHOULD be 
   applied in the registration and session establishment procedures, 
   so that the identity of the sender of the text stream is reliably 
   associated with the person or device setting up the session. Once 
   established, SRTP [14] mechanisms MAY be applied to ascertain that 
   the source is maintained the same during the session.  
    
9. Congestion Considerations 
    
   The congestion considerations from section 10 of RFC 3550 [2], 
   section 6 of RFC 2198 [3] and any used profile, e.g. the section 
   about congestion in chapter 2 of RFC 3551 [11] apply with the 
   following application specific considerations. 
    
   Automated systems MUST NOT use this format to send large amounts of 
   text at a rate significantly above that which a human user could 
   enter. 
    
   Even if the network load from users of text conversation is usually 
   very low, for best-effort networks an application MUST monitor the 
   packet loss rate and take appropriate actions to reduce its sending 
   rate if this application sends at higher rate than what TCP would 
   achieve over the same path. The reason is that this application, 
   due to its recommended usage of two or more redundancy levels, is 
   very robust against packet loss. At the same time, due to the low 
   bit-rate of text conversations, if one considers the discussion in 
   RFC 3714 [13], this application will experience very high packet 
   loss rates before it needs to perform any reduction in the sending 
   rate. 

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   If the application needs to reduce its sending rate, it SHOULD NOT  
   reduce the number of redundancy levels below the default amount 
   specified in section 4. Instead, the following actions are 
   RECOMMENDED in order of priority: 
    
   - Increase the shortest time between transmissions described in 
    section 5.1 from the recommended 300 ms to 500 ms that is the 
    highest value allowable according to T.140. 
    
   - Limit the maximum rate of characters transmitted. 
    
   - Increase the shortest time between transmissions to a higher 
    value, not higher than 5 seconds. This will cause unpleasant 
    delays in transmission, beyond what is allowed according to 
    T.140, but text will still be conveyed in the session with some 
    usability. 
    
   - Exclude participants from the session. 
    
   Please note that if the reduction in bit-rate achieved through the 
   above measures are not sufficient, the only remaining action is to 
   terminate the session. 
    
   As guidance, some load figures are provided here as examples based 
   on use of IPv4, including the load from IP, UDP and RTP headers 
   without compression. 
     
   -Experience tells that a common mean character transmission rate 
   during a complete PSTN text telephony session in reality is around 
   2 characters per second. 
    
   -A maximum performance of 20 characters per second is enough even 
   for voice to text applications. 
    
   -With the (unusually high) load of 20 characters per second, in a 
   language that make use of three octets UTF-8 characters, two 
   redundant levels and 300 ms between transmissions, the maximum load 
   of this application is 3300 bits/s. 
    
   -When the restrictions mentioned above are applied, limiting 
   transmission to 10 characters per second, using 5 s between 
   transmissions, the maximum load of this application in a language 
   that uses one octet per UTF-8 character is 300 bits/s. 
    
   Note also, that this payload can be used in a congested situation 
   as a last resort to maintain some contact when audio and video 
   media need to be stopped. The availability of one low bit-rate 
   stream for text in such adverse situations may be crucial for 
   maintaining some communication in a critical situation. 
    

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10. IANA considerations 
    
   This document defines one RTP payload format named "t140" and an 
   associated MIME type "text/t140", to be registered by IANA. 
    
10.1 Registration of MIME Media Type text/t140 
    
      MIME media type name: text 
    
      MIME subtype name: t140 
    
      Required parameters: 
        rate: The RTP timestamp clock rate, which is equal to the 
        sampling rate.  The only valid value is 1000. 
    
      Optional parameters: 
        cps: The maximum number of characters that may be received 
        per second. The deafult value is 30. 
    
      Encoding considerations: T.140 text can be transmitted with RTP 
      as specified in RFC XXXX. 
    
      Security considerations: See section 8 of RFC XXXX. 
    
      Interoperability considerations: This format is the same as 
      specified in RFC2793. For RFC2793 the "cps=" parameter was not 
      defined. Therefore there may be implementations that do not 
      consider this parameter. Receivers need to take that into 
      account. 
    
      Published specification: ITU-T T.140 Recommendation. 
                               RFC XXXX. 
    
      Applications which use this media type: 
        Text communication terminals and text conferencing tools. 
    
      Additional information: This type is only defined for transfer 
      via RTP. 
    
        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            | IETF avt WG 

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        gunnar.hellstrom@omnitor.se | 
     
    
10.2 SDP mapping of MIME parameters 
    
   The information carried in the MIME media type specification has a 
   specific mapping to fields in the Session Description Protocol 
   (SDP) [7], which is commonly used to describe RTP sessions. When 
   SDP is used to specify sessions employing the text/t140 format, the 
   mapping is as follows: 
    
     - The MIME type ("text" ) goes in SDP "m=" as the media name. 
    
     - The MIME subtype (payload format name) goes in SDP "a=rtpmap" 
       as the encoding name. The RTP clock rate in "a=rtpmap" MUST be 
       1000 for text/t140. 
    
     - The parameter "cps" goes in SDP "a=fmtp" attribute. 
    
    -  When the payload type is used with redundancy according to 
       RFC 2198, the level of redundancy is shown by the number of 
       elements in the slash-separated payload type list in the 
       "fmtp" parameter of the redundancy declaration as defined in 
       RFC YYYY [9] and RFC 2198 [3].  
    
10.3 Offer/Answer Consideration 
    
   In order to achieve interoperability within the framework of the 
   offer/answer model [10], the following consideration should be 
   made: 
    
    -   The "cps" parameter is declarative. Both sides may provide a 
       value, which is independent of the other side. 
    
11. Authors' Addresses 
    
   Gunnar Hellstrom 
   Omnitor AB 
   Renathvagen 2 
   SE-121 37 Johanneshov 
   Sweden 
   Phone: +46 708 204 288 / +46 8 556 002 03 
   Fax:   +46 8 556 002 06 
   E-mail: gunnar.hellstrom@omnitor.se 
    
   Paul E. Jones 
   Cisco Systems, Inc. 
   7025 Kit Creek Rd. 
   Research Triangle Park, NC 27709 
   USA 
   Phone: +1 919 392 6948 

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   E-mail: paulej@packetizer.com 
    
12. Acknowledgements 
    
   The authors want to thank Stephen Casner, Magnus Westerlund 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, Michele 
   Mizarro for verification of the usability of the payload format for 
   its intended purpose, and Andreas Piirimets for editing support and 
   validation. 
    
13. Normative 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 
       3550, July 2003. 
    
   [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 3629, December 2003. 
    
   [7] Handley, M., Jacobson, V., "SDP: Session Description 
       Protocol", RFC 2327, April 1998. 
    
   [8] Rosenberg, J., Schulzrinne, H., "An RTP Payload Format for 
       Generic Forward Error Correction", RFC 2733, December 1999. 
    
   [9] Jones, P. , "Registration of the text/red MIME Sub-Type", 
       draft-ietf-avt-text-red, RFC YYYY, 2004.  
    
   [10] Rosenberg, J., Schulzrinne, H., "An Offer/Answer Model with 
       the Session Description Protocol (SDP)", RFC 3264, June 2002. 
    
   [11] Schultzrinne, J., Perkins, C., "RTP Profile for Audio and 
       Video Conference with Minimal Control", RFC 3551, July 2003.  
    
   [12] Postel, J.,"Internet Protocol", RFC 791, 1981.  
    

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14. Informative References 
    
   [13] Floyd, S., Kempf, J., IAB Concerns Regarding Congestion 
       Control for Voice Traffic in the Internet, RFC 3714,March 2004 
    
   [14] Baugher, McGrew, Carrara, Naslund, Norrman, The Secure Real-
       Time Transport Protocol (SRTP), RFC 3711, March 2004. 
    
   [15] Schulzrinne, H., Petrack, S., "RTP Payload for DTMF Digits, 
       Telephony Tones and Telephony Signals", RFC 2833, May 2000. 
    
   [16] Hellstrom, G., "RTP Payload for text conversation.", RFC2793, 
       2000 
    
   [17] ITU-T Recommendation F.703, Multimedia Conversational 
       Services, Nov 2000. 
    
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   Information on the IETF's procedures with respect to rights in IETF 
   Documents can be found in RFC 3667 (BCP 78) and RFC 3668 (BCP 79). 
    
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   at http://www.ietf.org/ipr. 
    
   The IETF invites any interested party to bring to its attention any 
   copyrights, patents or patent applications, or other proprietary 
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16. Copyright Statement 
    
   Copyright (C) The Internet Society (2004).  
    
   This document is subject to the rights, licenses and restrictions 
   contained in BCP 78, and except as set forth therein, the authors 
   retain all their rights. 
    
Disclaimer of Validity 
    

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   This document and the information contained herein are provided on 
   an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE 
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND 
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. 
    

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