rfc2793.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                       G. Hellstrom
Request for Comments: 2793                                    Omnitor AB
Category: Standards Track                                       May 2000


                   RTP Payload for Text Conversation

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This memo describes how to carry text conversation session contents
   in RTP packets. Text conversation session contents are specified in
   ITU-T Recommendation T.140 [1].

   Text conversation is used alone or in connection to other
   conversational facilities such as video and voice, to form multimedia
   conversation services.

   This RTP payload description contains an optional possibility to
   include redundant text from already transmitted packets in order to
   reduce the risk of text loss caused by packet loss. The redundancy
   coding follows RFC 2198.

1. Introduction

   This memo defines a payload type for carrying text conversation
   session contents in RTP packets. Text conversation session contents
   are specified in ITU-T Recommendation T.140 [1]. Text conversation is
   used alone or in connection to other conversational facilities such
   as video and voice, to form multimedia conversation services. Text in
   text conversation sessions is sent as soon as it is available, or
   with a small delay for buffering.








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   The text is supposed to be entered by human users from a keyboard,
   handwriting recognition, voice recognition or any other input method.
   The rate of character entry is usually at a level of a few characters
   per second or less. Therefore, the expected number of characters to
   transmit is low. Only one or a few new characters are expected to be
   transmitted with each packet.

   T.140 specifies that text and other T.140 elements MUST be
   transmitted in ISO 10 646-1 code with UTF-8 transformation. That
   makes it easy to implement internationally useful applications, and
   to handle the text in modern information technology environments.
   The payload of an RTP packet following this specification consists of
   text encoded according to T.140 without any additional framing.  A
   common case will be a single ISO 10646 character, UTF-8 encoded.

   T.140 requires the transport channel to provide characters without
   duplication and in original order.  Text conversation users expect
   that text will be delivered with no or a low level of lost
   information. If lost information can be indicated, the willingness to
   accept loss is expected to be higher.

   Therefore a mechanism based on RTP is specified here. It gives text
   arrival in correct order, without duplications, and with detection
   and indication of losses.  It also includes an optional possibility
   to repeat data for redundancy to lower the risk of loss. Since packet
   overhead is usually much larger than the T.140 contents, the increase
   in channel load by the redundancy scheme is minimal.

1.1 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [4]

2. Usage of RTP

   When transport of T.140 text session data in RTP is desired, the
   payload as described in this specification SHOULD be used.

   A text conversation RTP packet as specified by this payload format
   consists of an RTP header as defined in RFC 1889 [2] followed
   immediately by a block of T.140 data, defined here to be a
   "T140block".  There is no additional header specific to this payload
   format.  The T140block contains one or more T.140 code elements as
   specified in [1].  Most T.140 code elements are single ISO 10646 [5]
   characters, but some are multiple character sequences.  Each
   character is UTF-8 encoded [6] into one or more octets. This implies
   that each block MUST contain an integral number of UTF-8 encoded



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   characters regardless of the number of octets per character. It also
   implies that any composite character sequence (CCS) SHOULD be placed
   within one block.

   The T140blocks MAY be transmitted redundantly according to the
   payload format defined in RFC 2198 [3].  In that case, the RTP header
   is followed by one or more redundant data block headers, the same
   number of redundant data fields carrying T140blocks from previous
   packets, and finally the new (primary) T140block for this packet.

2.1 RTP packet header

   Each RTP packet starts with a fixed RTP header. The following fields
   of the RTP fixed header are used for T.140 text streams:

   Payload Type (PT): The assignment of an RTP payload type is specific
     to the RTP profile under which this payload format is used.  For
     profiles which use dynamic payload type number assignment, this
     payload format is identified by the name "T140" (see section 6).
     If redundancy is used per RFC 2198, the Payload Type MUST indicate
     that payload format ("RED").

   Sequence number:  The Sequence Number MUST be increased by one for
     each new transmitted packet. It is used for detection of packet
     loss and packets out of order, and can be used in the process of
     retrieval of redundant text, reordering of text and marking missing
     text.

   Timestamp: The RTP Timestamp encodes the approximate instance of
     entry of the primary text in the packet. A clock frequency of 1000
     Hz MUST be used. Sequential packets MUST NOT use the same
     timestamp. Since packets do not represent any constant duration,
     the timestamp cannot be used to directly infer packet losses.

2.2 Additional headers

   There are no additional headers defined specific to this payload
   format.

   When redundant transmission of the data according to RFC 2198 is
   desired, the RTP header is followed by one or more redundant data
   block headers, one for each redundant data block to be included.
   Each of these headers provides the timestamp offset and length of the
   corresponding data block plus a payload type number indicating this
   payload format ("T140").






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2.3 T.140 Text structure

   T.140 text is UTF-8 coded as specified in T.140 with no extra
   framing. When using the format with redundant data, the transmitter
   MAY select a number of T140block generations to retransmit in each
   packet. A higher number introduces better protection against loss of
   text but increases the data rate.

   Since packets are not generated at regular intervals, the timestamp
   is not sufficient to identify a packet in the presence of loss unless
   extra information is provided. Since sequence numbers are not
   provided in the redundant header, some additional rules must be
   followed to allow the redundant data corresponding to missing primary
   data to be merged properly into the stream of primary data
   T140blocks:

      - Each redundant data block MUST contain the same data as a
        T140block previously transmitted as primary data, and be
        identified with a timestamp offset equating to the original
        timestamp for that T140block.
      - The redundant data MUST be placed in age order with most
        recent redundant T140block last in the redundancy area.
      - All T140blocks from the oldest desired generation up through
        the generation immediately preceding the new (primary)
        T140block MUST be included.

   These rules allow the sequence numbers for the redundant T140blocks
   to be inferred by counting backwards from the sequence number in the
   RTP header.  The result will be that all the text in the payload will
   be contiguous and in order.

3. Recommended procedures

   This section contains RECOMMENDED procedures for usage of the payload
   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.











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3.1 Recommended basic procedure

   Packets are transmitted only when there is valid T.140 data to
   transmit. The sequence number is used for sequencing of T.140 data.

   On reception, the RTP sequence number is compared with the sequence
   number of the last correctly received packet. If they are
   consecutive, the (only or primary) T140block is retrieved from the
   packet.

3.2 Recommended procedure for compensation for lost packets.

   For reduction of data loss in case of packet loss, redundant data MAY
   be included in the packets following to the procedures in RFC 2198.
   If network conditions are not known, it is RECOMMENDED to use one
   redundant T140block in each packet. If there is a gap in the RTP
   sequence numbers, and redundant T140blocks are available in a
   subsequent packet, the sequence numbers for the redundant T140blocks
   should be inferred by counting backwards from the sequence number in
   the RTP header for that packet.  If there are redundant T140blocks
   with sequence numbers matching those that are missing, the redundant
   T140blocks may be substituted for the missing T140blocks.

   Both for the case when redundancy is used and not used, missing data
   SHOULD be marked by insertion of a missing text marker in the
   received stream for each missing T140block, as specified in ITU-T
   T.140. Addendum 1 [1].

3.3 Recommended procedure for compensation for packets out of order.

   For protection against packets arriving out of order, the following
   procedure MAY be implemented in the receiver.  If analysis of a
   received packet reveals a gap in the sequence and no redundant data
   is available to fill that gap, the received packet can be kept in a
   buffer to allow time for the missing packet(s) to arrive.  It is
   suggested that the waiting time be limited to 0.5 seconds. For the
   case when redundancy is used the waiting time SHOULD be extended to
   the number of redundancy generations times the T.140 buffering timer
   if this product is known to be greater than 0.5 seconds.

   If a packet with a T140block belonging to the gap arrives before the
   waiting time expires, this T140block is inserted into the gap and
   then consecutive T140blocks from the leading edge of the gap may be
   consumed.  Any T140block which does not arrive before the time limit
   expires should be treated as lost.






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