Network Working Group G. Hellstrom
Request for Comments: 4103 Omnitor AB
Obsoletes: 2793 P. Jones
Category: Standards Track Cisco Systems, Inc.
June 2005
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 (2005).
Abstract
This memo obsoletes RFC 2793; it 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.
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" ...........................................5
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 ................7
5. Recommended Procedure ..........................................8
5.1. Recommended Basic Procedure ...............................8
5.2. Transmission before and after "Idle Periods" ..............8
5.3. Detection of Lost Text Packets ............................9
5.4. Compensation for Packets Out of Order ....................10
6. Parameter for Character Transmission Rate .....................10
7. Examples ......................................................11
7.1. RTP Packetization Examples for the text/t140 Format ......11
7.2. SDP Examples .............................................13
8. Security Considerations .......................................14
8.1. Confidentiality ..........................................14
8.2. Integrity ................................................14
8.3. Source Authentication ....................................14
9. Congestion Considerations .....................................14
10. IANA Considerations ...........................................16
10.1. Registration of MIME Media Type text/t140 ...............16
10.2. SDP Mapping of MIME Parameters ..........................17
10.3. Offer/Answer Consideration ..............................17
11. Acknowledgements ..............................................18
12. Normative References ..........................................18
13. Informative References ........................................19
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 with 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.
This makes it easy to implement internationally useful applications
and to handle the text in modern information technology environments.
The payload of an RTP packet that follows 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 in order to lower the risk of loss.
Because 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, for example, 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
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 is 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 RFC 2793. It has been 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 depend 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, carried in network byte order (see RFC 791 [12]).
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
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 the 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 4102 [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 out-of-order packets,
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. Because 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.
4. Protection against Loss of Data
Consideration must be devoted to keeping loss of text due to 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, in addition to a payload type number
(indicating the payload format text/t140).
The redundant data block headers are followed by the redundant data
fields carrying T140blocks from previous packets. Finally, the new
(primary) T140block for this packet follows.
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
Because 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, because sequence numbers are not provided in the redundant
header, some additional rules must be followed to allow redundant
data that corresponds to missing primary data to be properly merged
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 the 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 the next idle period. This is done in order to
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 as
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 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].
Because empty T140blocks are transmitted in the beginning of an idle
period, there is a slight risk of falsely marking loss of text, when
only an empty T140block was lost. Procedures based on detection of
the packet with the M-bit set to one MAY be used to reduce the risk
of 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, therefore, is not suitable
for applications 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. Therefore, text/t140 is 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 that does not arrive before the time limit
expires should be treated as lost and a missing text marker should be
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 Public Switched Telephone
Network (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).
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:
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