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Versions: 00 01 02                                                      
               AVT Working Group
               Internet Draft                                             G. Hellstrom
               <draft-hellstrom-avt-rfc2793bis-02.txt>                      Omnitor AB
               Expires: April 2004
                                                                              P. Jones
                                                                   Cisco Systems, Inc.
                                                                          October 2003
               
               
               
                                    RTP Payload for Text Conversation
               
               
               Status of this Memo
               
                  This document is an Internet-Draft and is in full conformance with
                  all provisions of Section 10 of RFC 2026.
               
                  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 working documents as Internet-
                  Drafts.
               
                  Internet-Drafts are draft documents valid for a maximum of six months
                  and may be updated, replaced, or obsoleted by other documents at any
                  time.  It is inappropriate to use Internet-Drafts as reference
                  material or to cite them other than as "work in progress."
               
                  The list of current Internet-Drafts can be accessed at
                       http://www.ietf.org/ietf/1id-abstracts.txt
                  The list of Internet-Draft Shadow Directories can be accessed at
                       http://www.ietf.org/shadow.html.
               
                  [Note to RFC Editor: All references to RFC XXXX are to be replaced by
                  references to the RFC number of this memo, when published.]
               
               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.
               
               
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                                 RTP Payload for Text Conversation      October 2003
               
               
               
               Table of Contents
                    1. Introduction..................................................2
                    2. Conventions used in this document.............................3
                    3. Usage of RTP..................................................3
                        3.1 Payload Format for Transmission of text/t140 Data........4
                        3.2 Payload Format for Transmission of audio/t140 Data.......4
                        3.3 The "T140block"..........................................4
                        3.4 Use of Redundancy........................................5
                        3.5 Synchronization of Text with Other Media.................5
                        3.6 RTP packet header........................................5
                        3.7 Additional Headers.......................................6
                        3.8 T.140 Text Structure.....................................6
                    4. Recommended Procedure.........................................7
                        4.1 Recommended Basic Procedure..............................7
                        4.2 Recommended Procedure for Compensation for Lost Packets..8
                        4.3 Recommended Procedure for Compensation for Packets Out of
                            Order....................................................8
                        4.4 Transmission During "Silent Periods" when Redundancy is
                            Used.....................................................8
                    5. SDP Attribute for Flow Control................................9
                    6. Examples......................................................9
                        6.1 RTP Packetization Examples for the text/t140 format......9
                        6.2 RTP Packetization Examples for the audio/t140 format....11
                        6.3 SDP Examples............................................13
                    7. Security Considerations......................................14
                    8. MIME Media Type Registrations................................14
                        8.1 Registration of MIME Media Type text/t140...............14
                        8.2 Registration of MIME Media Type audio/t140..............15
                        8.3 Registration of MIME Media Type text/RED................16
                    9. Authors' Addresses...........................................17
                    10. Acknowledgements............................................18
                    11. Normative References........................................18
                    12. Informative References......................................18
                    13. Full Copyright Statement....................................18
               
               1. Introduction
               
                  This document defines two payload types 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
               
               
               
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                                 RTP Payload for Text Conversation      October 2003
               
               
                  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, though text may be transmitted at a much higher
                  rate (e.g., automated systems or "copy and paste" operations may
                  produce a lot of text very rapidly). Therefore, it is generally
                  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.
               
                  This document updates and extends RFC 2793.  The text is intended to
                  to clarify any ambiguities in RFC 2793, improve on the specific
                  implementation requirements learned through development experience,
                  give explicit usage examples and to introduce a means of transporting
                  text interleaved with voice within the same RTP session.
               
               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.
               
               3. Usage of RTP
               
                  When transport of T.140 text session data in RTP is desired, the
                  payloads as described in this specification SHOULD be used.
               
               
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                                 RTP Payload for Text Conversation      October 2003
               
               
               
               3.1 Payload Format for Transmission of text/t140 Data
               
                  A text conversation RTP packet as specified by this payload format
                  consists of an RTP header as defined in RFC 3550 [2] followed
                  immediately by a block of T.140 data, referred to as a "T140block"
                  (see section 3.3).  There is no additional header specific to this
                  payload format.
               
                  This format is primarily used when text is transmitted on a separate
                  RTP session dedicated for the transmission of text and not shared
                  with other media, such as audio, DTMF, etc.  IP textphone devices
                  most commonly use this format.
               
               3.2 Payload Format for Transmission of audio/t140 Data
               
                  A text conversation RTP packet as specified by this payload format
                  consists of an RTP header as defined in RFC 3550 followed immediately
                  by a 16-bit "t140block counter" followed by a "T140block" (see
                  section 3.3).  There is no additional header specific to this payload
                  format.
               
                  The T140block counter MUST be initialized to zero the first time that
                  a packet containing a T140block is transmitted and MUST be
                  incremented by 1 each time that a new block is transmitted.  Once the
                  counter reaches the value 0xFFFF, the counter is reset to 0 the next
                  time the counter is incremented.  This T140block counter may be
                  utilized to detect lost characters.
               
                  For the purposes of readability, the remainder of this document only
                  refers to the T140block without making explicit reference to the
                  T140block counter.  Readers should understand that when using the
                  audio/t140 format, the T140block counter MUST always precede the
                  actual T140block, including redundant data transmissions.
               
                  The primary purpose for this payload specification is to allow
                  gateways that are interconnecting two PSTN networks to interleave,
                  through a single RTP session, audio and text data received on the
                  PSTN circuit.  This is comparable to the way in which DTMF is
                  extracted and transmitted within an RTP session [8].
               
               3.3 The "T140block"
               
                  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
                  characters regardless of the number of octets per character. It also
               
               
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                                 RTP Payload for Text Conversation      October 2003
               
               
                  implies that any composite character sequence (CCS) SHOULD be placed
                  within one block.
               
               3.4 Use of Redundancy
               
                  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.
               
               3.5 Synchronization of Text with Other Media
               
                  Usually, each medium in a session utilizes a separate RTP stream. In
                  that case, 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 reference clock (refer to
                  the description of the timestamp field as it relates to
                  synchronization in section 5.1 of RFC 3550).  Association of RTP
                  streams is dependent on the particular session application and is
                  outside the scope of this document.
               
                  When audio/t140 is used, it is generally transmitted as interleaved
                  packets between voice packets or other kinds of audio packets.  One
                  should observe the RTP timestamps of the voice, text, or other audio
                  packets in order to reproduce the stream correctly when playing out
                  the audio.  Note, also, that incoming text from a PSTN circuit might
                  be at a higher bit-rate than can be played out on an egress PSTN
                  circuit.  As such, it is possible that, on the egress side, a gateway
                  may not complete the play out of the text packets before it is time
                  to play the next voice packet.  Given that this application is
                  primarily for the benefit of deaf users utilizing PSTN textphone
                  devices, it is strongly RECOMMENDED that all text packets be properly
                  reproduced on the egress gateway before considering any subsequent
                  voice or other audio packets.  If necessary, voice and other audio
                  packets should be discarded in order to properly reproduce the text
                  signals on the PSTN circuit.
               
               3.6 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 that use dynamic payload type number assignment, this
                    payload format is identified by the name "T140" (see section 8).
                    If redundancy is used per RFC 2198, the Payload Type MUST indicate
                    that payload format ("RED").
               
               
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                  Sequence number: The Sequence Number MUST be increased by one for
                    each new transmitted packet. 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.  (Character loss is detected through the T140block counter
                    when using the audio/t140 payload format.)
               
                  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 for text/t140.  The clock frequency may be
                    specified for audio/t140 and is generally set to be 8000 Hz, as
                    that is most common for audio.  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.
               
               
               3.7 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").  Redundant data older than 16383 divided by
                  the clock frequency MUST not be transmitted.
               
               3.8 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 and since the
                  audio/t140 format allows for other media to be interleaved, 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
                  when using the text/t140 payload format.  While the audio/t140
               
               
               
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                  payload format does not rely on the sequence numbers of packets to
                  identify missing data, the same rules apply.  They are:
               
                    - 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.
               
                  For the text/t140 payload format, 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.
               
               4. 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.
               
               4.1 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.
               
                  T.140 specifies that T.140 data MAY be buffered before transmission
                  for a short moment. A maximum buffering time of 500 ms is specified.
                  In order to keep the maximum bit rate usage for text at a reasonable
                  level, it is RECOMMENDED to buffer T.140 data for transmission in 300
                  ms intervals. This time is selected so that text users will still
                  perceive a real time text flow.
               
                  On reception of text/t140 data, the RTP sequence number is compared
                  with the sequence number of the last correctly received packet.  On
                  receipt of audio/t140 data, the T140block counter is compared with
                  the T140block counter of the last correctly received packet.  If they
                  are consecutive, the (only or primary) T140block is retrieved from
                  the packet.
               
               
               
               
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                                 RTP Payload for Text Conversation      October 2003
               
               
               4.2 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 three
                  redundant T140blocks in each packet. If there is a gap in the RTP
                  sequence numbers (for text/t140) or T140block counters (audio/t140),
                  and redundant T140blocks are available in a subsequent packet, the
                  sequence numbers or T140block counters for the redundant T140blocks
                  should be inferred by counting backwards from the sequence number or
                  T140block counter 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].
               
               4.3 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 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 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.
               
               4.4 Transmission During "Silent Periods" when Redundancy is Used
               
                  When using the redundancy transmission scheme, and there is redundant
                  data, but no new T.140 data to transmit after the transmit buffering
                  interval described in section 4.1 has passed, a packet MUST be
                  transmitted containing a zero-length primary T140block and the
                  properly positioned redundant data.  When using the audio/t140payload
                  format with an empty T140block, the T140block counter MUST also be
                  absent (as there is no actual T140block).
               
                  When using the text/t140 payload format, 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
               
               
               
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                                 RTP Payload for Text Conversation      October 2003
               
               
                  be so that sequence number inference for the redundant T140blocks
                  will be correct, as explained in section 3.8.
               
                  When using the audio/t140 payload format, zero-length T140blocks sent
                  as primary data MUST NOT be included as redundant T140blocks, as it
                  would simply be a waste of bandwidth to send them.
               
                  Redundancy for the last T140block MUST 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.
               
               5. SDP Attribute for Character Transmission Rate
               
                  In some cases, it is necessary to limit the rate at which characters
                  are transmitted.  While the "b=" SDP attribute could be used to limit
                  the rate of the RTP session, it may be that only the text stream in
                  an interleaved audio/text session needs special handling.  For
                  example, when a PSTN gateway is interworking between an IP device
                  (not necessarily a textphone) and a PSTN textphone, it may be
                  necessary to limit the character rate from the IP device in order to
                  avoid throwing away characters at the PSTN gateway.  At the same
                  time, no explicit bit rate restriction is necessarily applied to the
                  audio stream.
               
                  To control the character transmission rate, the "fmtp" attribute [7]
                  is used 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.
               
                  Devices in receipt of this parameter MUST adhere to the request to
                  apply by transmitting characters at a rate at or below the specified
                  <integer> value.
               
               6. Examples
               
               6.1 RTP Packetization Examples for the text/t140 format.
               
                     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)                       |
               
               
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                                 RTP Payload for Text Conversation      October 2003
               
               
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |           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                 |
                     +                                                               +
                     +                                               +---------------+
                     |                                               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               
                     This 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            |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               
               
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                                 RTP Payload for Text Conversation      October 2003
               
               
                     |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, this example 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 in order to
                     represent the empty 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 "R1" | "R1" block length |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |1|   T140 PT   |  timestamp offset of "R2" | "R2" block length |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |0|   T140 PT   |                                               |
                     +-+-+-+-+-+-+-+-+                                               +
                     |                                                               |
                     +               "R1" T.140 encoded redundant data               +
                     |                                                               |
                     +                                               +---------------+
                     |                                               |               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
                     |                "P" T.140 encoded primary data                 |
                     +                                                               +
                     +                                               +---------------+
                     |                                               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               
               
               6.2 RTP Packetization Examples for the audio/t140 format
               
               
               
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                     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 (8000Hz)                       |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |           synchronization source (SSRC) identifier            |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |     T140block Counter         |                               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
                     +                      T.140 encoded data                       +
                     |                                                               |
                     +                                               +---------------+
                     |                                               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               
                     This is an example of an RTP packet with one redundant T140block
                     using audio/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.  Note that since this is the
                     audio/t140 payload format, the redundant block of T.140 data
                     is immediately preceded with a T140block Counter.
                      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   |  T140block Counter            |               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
                     |                                                               |
                     +               "R" T.140 encoded redundant data                +
                     |                                                               |
                     +                                               +---------------+
                     |                                               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               
                     As a follow-on to the previous example, this example shows the
                     next RTP packet in the sequence which does contain a new real
                     T140block when using the audio/t140 payload format.  This
                     example has 2 levels of redundancy and one primary data block.
               
               
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                     Since the previous primary block was empty, no redundant data
                     is included for that block.  This is because when using the
                     audio/t140 payload format, any previously transmitted "empty"
                     T140blocks are NOT included as redundant data in subsequent
                     packets.
                      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 "R1" | "R1" block length |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |1|   T140 PT   |  T140block Counter            |               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
                     |                                                               |
                     +               "R1" T.140 encoded redundant data               +
                     |                                                               |
                     +                                               +---------------+
                     |                                               |    T140block_ |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | Counter       |     "P" T.140 encoded primary data            |
                     +-+-+-+-+-+-+-+-+                                               +
                     |                                                               |
                     +                                               +---------------+
                     |                                               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               
               
               
               6.3 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 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
               
               
               
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                  Below is an example of SDP describing RTP text interleaved with G.711
                  audio packets within the same RTP session from port 7200 and at a
                  maximum text rate of 6 characters per second:
               
                      m=audio 7200 RTP/AVP 0 98
                      a=rtpmap:98 t140/8000
                      a=fmtp:98 cps=6
               
                  Below is an example using RFC 2198 to provide redundancy to just the
                  text packets in an RTP session with interleaving text and G.711 at a
                  text rate no faster than 6 characters per second:
               
                      m=audio 7200 RTP/AVP 0 98 100
                      a=rtpmap:98 t140/8000
                      a=fmtp:98 cps=6
                      a=rtpmap:100 red/8000
                      a=fmtp:100 98/98
               
               7. 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.  Additionally, all of the security considerations
                  from section 14 or RFC 3550 apply.
               
               8. MIME Media Type Registrations
               
                  This document defines an RTP payload named "t140" and two associated
                  MIME types, "text/t140" and "audio/t140".  Additionally, the MIME
                  type "text/RED" is defined to allow RFC 2198 to be used to carry
                  redundant text payloads.
               
               8.1 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 character that may be received
                       per second.
               
               
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                     Encoding considerations: T.140 text can be transmitted with RTP as
                       specified in RFC <TDB>.
               
                     Security considerations: None
               
                     Interoperability considerations: None
               
                     Published specification: ITU-T T.140 Recommendation.
                                              RFC XXXX.
               
                     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            | IETF avt WG
                       gunnar.hellstrom@omnitor.se |
               
               8.2 Registration of MIME Media Type audio/t140
               
                     MIME media type name: audio
               
                     MIME subtype name: t140
               
                     Required parameters:
                       rate: The RTP timestamp clock rate, which is equal to the
                       sampling rate.
               
                     Optional parameters:
                       cps: The maximum number of character that may be received
                       per second.
               
                     Encoding considerations: T.140 text can be transmitted with RTP as
                       specified in RFC XXXX.
               
                     Security considerations: None
               
               
               
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                     Interoperability considerations: None
               
                     Published specification: ITU-T T.140 Recommendation.
                                              RFC <TDB>.
               
                     Applications which use this media type:
                       Text communication systems and text conferencing tools that
                       transmit text associated with audio and within the same RTP
                       session as the audio, such as PSTN gateways that transmit
                       audio and text signals between two PSTN textphone users
                       over an IP network.
               
                     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:
                       Paul E. Jones
                       E-mail: paulej@packetizer.com
               
                     Intended usage: COMMON
               
                     Author                        / Change controller:
                       Paul E. Jones               | IETF avt WG
                       paulej@packetizer.com       |
               
               8.3 Registration of MIME Media Type text/RED
               
                     MIME media type name: text
               
                     MIME subtype name: RED
               
                     Required parameters:
                       pt: a comma-separated list of RTP payload types.  Because
                       comma is a special character, the list must be a quoted-string
                       (enclosed in double quotes).  For static payload types, each
                       list element is simply the type number.  For dynamic payload
                       types, each list element is a mapping of the dynamic payload
                       type number to an embedded MIME content-type specification for
                       the payload format corresponding to the dynamic payload type.
                       The format of the mapping is:
               
                          dynamic-payload-type "=" content-type
               
                       If the content-type string includes a comma, then the
                       content-type string MUST be a quoted-string.  If the content-
                       type string does not include a comma, it MAY still be quoted.
               
               
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                       Since it is part of the list which must itself be a quoted-
                       string, that means the quotation marks MUST be quoted with
                       backslash quoting as specified in RFC 2045.  If the content-
                       type string itself contains a quoted-string, then the
                       requirement for backslash quoting is recursively applied.  To
                       specify the text/RED payload format in SDP, the pt parameter
                       is mapped to an a=fmtp attribute by eliminating the parameter
                       name (pt) and changing the commas to slashes.  For example,
                       'pt="101,102"' maps to 'a=fmtp:99 101/102'.
               
                     Optional parameters: ptime, maxptime
               
                     Encoding considerations:
                       This type is only defined for transfer via RTP [2].
               
                     Security considerations: None
               
                     Interoperability considerations: none
               
                     Published specification: RFC 2198
               
                     Applications which use this media type:
                       Text streaming and conferencing tools.
               
                     Additional information: none
               
                     Person & email address to contact for further information:
                       Paul E. Jones
                       E-mail: paulej@packetizer.com
               
                     Intended usage: COMMON
               
                     Author                        / Change controller:
                       Paul E. Jones               | IETF avt WG
                       paulej@packetizer.com       |
               
               9. 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.
               
               
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                  7025 Kit Creek Rd.
                  Research Triangle Park, NC 27709
                  Phone: +1 919 392 6948
                  E-mail: paulej@packetizer.com
               
               10. Acknowledgements
               
                  The authors want 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.
               
               11. 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
                       2279, January 1998.
               
                  [7] Handley, M., Jacobson, V., "SDP: Session Description Protocol",
                       RFC 2327, April 1998.
               
               12. Informative References
               
                  [8] Schulzrinne, H., Petrack, S., "RTP Payload for DTMF Digits,
                       Telephony Tones and Telephony Signals", May 2000.
               
               13. Intellectual Property Right Considerations
               
                  The IETF takes no position regarding the validity or scope of any
                  intellectual property or other rights that might be claimed to
                  pertain to the implementation or use of the technology described in
               
               
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                  this document or the extent to which any license under such rights
                  might or might not be available; neither does it represent that it
                  has made any effort to identify any such rights.  Information on the
                  IETF's procedures with respect to rights in standards-track and
                  standards-related documentation can be found in BCP-11.  Copies of
                  claims of rights made available for publication and any assurances of
                  licenses to be made available, or the result of an attempt made to
                  obtain a general license or permission for the use of such
                  proprietary rights by implementors or users of this specification can
                  be obtained from the IETF Secretariat.
               
                  The IETF invites any interested party to bring to its attention any
                  copyrights, patents or patent applications, or other proprietary
                  rights which may cover technology that may be required to practice
                  this standard.  Please address the information to the IETF Executive
                  Director.
               
               14. Full Copyright Statement
               
                  Copyright (C) The Internet Society (2003).  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
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                  copyrights defined in the Internet Standards process must be
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                  English.
               
                  The limited permissions granted above are perpetual and will not be
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                  This document and the information contained herein is provided on an
                  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
                  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
                  BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
                  HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
                  MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
               
                  Acknowledgement
               
                  Funding for the RFC Editor function is currently provided by the
                  Internet Society.
               
               
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