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RFC2793 - RTP Payload for Text Conversation

王朝other·作者佚名  2008-05-31
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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 RFC2198.

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.

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 RFC2119 [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 RFC1889 [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

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 RFC2198 [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 RFC2198, 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 RFC2198 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").

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.

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 RFC2198.

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.

3.4 Transmission during "silent periods" when redundancy is used.

When using the redundancy transmission scheme, and there is nothing

more to transmit from T.140, the latest T140block has a risk of

getting old before it is transmitted as redundant data. The result is

less useful protection against packet loss at the end of a text input

sequence. For cases where this should be avoided, a zero-length

primary T140block MAY be transmitted with the redundant data.

Any zero-length T140blocks that are sent as primary data MUST be

included as redundant T140blocks on subsequent packets just as normal

text T140blocks would be so that sequence number inference for the

redundant T140blocks will be correct, as explained in section 2.3.

Redundancy for the last T140block SHOULD NOT be implemented by

repeatedly transmitting the same packet (with the same sequence

number) because this will cause the packet loss count, as reported in

RTCP, to decrement.

4. Examples

This is an example of a T140 RTP packet without redundancy.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

V=2PX CC=0 M T140 PT sequence number

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

timestamp (1000Hz)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

synchronization source (SSRC) identifier

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ T.140 encoded data +

+ +---------------+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This is an example of an RTP packet with one redundant T140block.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

V=2PX CC=0 M "RED" PT sequence number of primary

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

timestamp of primary encoding "P"

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

synchronization source (SSRC) identifier

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

1 T140 PT timestamp offset of "R" "R" block length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

0 T140 PT

+-+-+-+-+-+-+-+-+ +

+ "R" T.140 encoded redundant data +

+ +---------------+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +

"P" T.140 encoded primary data

+ +

+ +---------------+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure: Examples of RTP text packets.

5. Security Considerations

Since the intention of the described payload format is to carry text

in a text conversation, security measures in the form of encryption

are of importance. The amount of data in a text conversation session

is low and therefore any encryption method MAY be selected and

applied to T.140 session contents or to the whole RTP packets. When

redundant data is included, the same security considerations as for

RFC2198 apply.

6. MIME Media Type Registrations

This document defines a new RTP payload name and associated MIME

type, T140 (text/t140).

6.1 Registration of MIME media type text/t140

MIME media type name: text

MIME suBType name: t140

Required parameters: None

Optional parameters: None

Encoding considerations: T140 text can be transmitted with RTP as

specified in RFC2793.

Security considerations: None

Interoperability considerations: None

Published specification: ITU-T T.140 Recommendation.

RFC2793.

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 c/o Steve Casner casner@cisco.com

7. Author's Address

Gunnar Hellstrom

Omnitor AB

Alsnogatan 7, 4 tr

SE-116 41 Stockholm

Sweden

Phone: +46 708 204 288 / +46 8 556 002 03

Fax: +46 8 556 002 06

EMail: gunnar.hellstrom@omnitor.se

8. Acknowledgements

The author wants to thank Stephen Casner and Colin Perkins for

valuable support with reviews and advice on creation of this

document, to Mickey Nasiri at EriCsson Mobile Communication for

providing the development environment, and Michele Mizarro for

verification of the usability of the payload format for its intended

purpose.

9. References

[1] ITU-T Recommendation T.140 (1998) - Text conversation protocol

for multimedia application, with amendment 1, (2000).

[2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,

"RTP: A Transport Protocol for Real-Time Applications", RFC

1889, January 1996.

[3] Perkins, C., Kouvelas, I., Hardman, V., Handley, M. and J.

Bolot, "RTP Payload for Redundant Audio Data", RFC2198,

September 1997.

[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement

Levels", BCP 14, RFC2119, 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.

10. Full Copyright Statement

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

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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