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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Below is an example of an RTP packet with one redundant T140block
using text/t140 payload format. The primary data block is empty,
which is the case when transmitting a packet for the sole purpose of
forcing the redundant data to be transmitted in the absence of any
new data.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC=0 |M| "RED" PT | sequence number of primary |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of primary encoding "P" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R" | "R" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T140 PT | "R" T.140 encoded redundant data |
+-+-+-+-+-+-+-+-+ +---------------+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
As a follow-on to the previous example, the example below shows the
next RTP packet in the sequence, which does contain a real T140block
when using the text/t140 payload format. Note that the empty block
is present in the redundant transmissions of the text/t140 payload
format. This example shows two levels of redundancy and one primary
data block. The value of the "R2 block length" would be set to zero
in order to represent the empty T140block.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC=0 |M| "RED" PT | sequence number of primary |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of primary encoding "P" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R2" | "R2" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R1" | "R1" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T140 PT | "R1" T.140 encoded redundant data |
+-+-+-+-+-+-+-+-+ +---------------+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+
| "P" T.140 encoded primary data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.2. SDP Examples
Below is an example of SDP, which describes RTP text transport on
port 11000:
m=text 11000 RTP/AVP 98
a=rtpmap:98 t140/1000
Below is an example of SDP that is similar to the above example, but
also utilizes RFC 2198 to provide the recommended two levels of
redundancy for the text packets:
m=text 11000 RTP/AVP 98 100
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98/98
Note: Although these examples utilize the RTP/AVP profile, it is not
intended to limit the scope of this memo. Any appropriate profile
may be used in conjunction with this memo.
8. Security Considerations
All of the security considerations from Section 14 of RFC 3550 [2]
apply.
8.1. Confidentiality
Because 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. Therefore, any encryption method MAY be
selected and applied to T.140 session contents or to whole RTP
packets. Secure Real-time Transport Protocol (SRTP) [14] provides a
suitable method for ensuring confidentiality.
8.2. Integrity
It may be desirable to protect the text contents of an RTP stream
against manipulation. SRTP [14] provides methods for providing
integrity that MAY be applied.
8.3. Source Authentication
There are several methods of making sure the source of the text is
the intended one.
Text streams are usually used in a multimedia control environment.
Security measures for authentication are available and SHOULD be
applied in the registration and session establishment procedures, so
that the identity of the sender of the text stream is reliably
associated with the person or device setting up the session. Once
established, SRTP [14] mechanisms MAY be applied to ascertain that
the source is maintained the same during the session.
9. Congestion Considerations
The congestion considerations from Section 10 of RFC 3550 [2],
Section 6 of RFC 2198 [3], and any used profile (e.g., the section
about congestion in chapter 2 of RFC 3551 [11]) apply with the
following application-specific considerations.
Automated systems MUST NOT use this format to send large amounts of
text at rates significantly above those a human user could enter.
Even if the network load from users of text conversation is usually
very low, for best-effort networks an application MUST monitor the
packet loss rate and take appropriate actions to reduce its sending
rate (if this application sends at higher rate than what TCP would
achieve over the same path). The reason for this is that this
application, due to its recommended usage of two or more redundancy
levels, is very robust against packet loss. At the same time, due to
the low bit-rate of text conversations, if one considers the
discussion in RFC 3714 [13], this application will experience very
high packet loss rates before it needs to perform any reduction in
the sending rate.
If the application needs to reduce its sending rate, it SHOULD NOT
reduce the number of redundancy levels below the default amount
specified in Section 4. Instead, the following actions are
RECOMMENDED in order of priority:
- Increase the shortest time between transmissions (described in
Section 5.1) from the recommended 300 ms to 500 ms, which is the
highest value allowed according to T.140.
- Limit the maximum rate of characters transmitted.
- Increase the shortest time between transmissions to a higher value,
not higher than 5 seconds. This will cause unpleasant delays in
transmission, beyond what is allowed according to T.140, but text
will still be conveyed in the session with some usability.
- Exclude participants from the session.
Please note that if the reduction in bit-rate achieved through the
above measures is not sufficient, the only remaining action is to
terminate the session.
As guidance, some load figures are provided here as examples based on
use of IPv4, including the load from IP, UDP, and RTP headers without
compression .
- Experience tells that a common mean character transmission rate,
during a complete PSTN text telephony session, is around two
characters per second.
- A maximum performance of 20 characters per second is enough even
for voice-to-text applications.
- With the (unusually high) load of 20 characters per second, in a
language that makes use of three octets per UTF-8 character, two
redundant levels, and 300 ms between transmissions, the maximum
load of this application is 3300 bits/s.
- When the restrictions mentioned above are applied, limiting
transmission to 10 characters per second, using 5 s between
transmissions, the maximum load of this application, in a language
that uses one octet per UTF-8 character, is 300 bits/s.
Note that this payload can be used in a congested situation as a last
resort to maintain some contact when audio and video media need to be
stopped. The availability of one low bit-rate stream for text in
such adverse situations may be crucial for maintaining some
communication in a critical situation.
10. IANA Considerations
This document updates the RTP payload format named "t140" and the
associated MIME type "text/t140", in the IANA RTP and Media Type
registries.
10.1. Registration of MIME Media Type text/t140
MIME media type name: text
MIME subtype name: t140
Required parameters: rate: The RTP timestamp clock rate, which is
equal to the sampling rate. The only valid value is 1000.
Optional parameters: cps: The maximum number of characters that may
be received per second. The default value is 30.
Encoding considerations: T.140 text can be transmitted with RTP as
specified in RFC 4103.
Security considerations: See Section 8 of RFC 4103.
Interoperability considerations: This format is the same as specified
in RFC2793. For RFC2793 the "cps=" parameter was not defined.
Therefore, there may be implementations that do not consider this
parameter. Receivers need to take that into account.
Published specification: ITU-T T.140 Recommendation. RFC 4103.
Applications which use this media type: Text communication terminals
and text conferencing tools.
Additional information: This type is only defined for transfer via
RTP.
Magic number(s): None
File extension(s): None
Macintosh File Type Code(s): None
Person & email address to contact for further information:
Gunnar Hellstrom
E-mail: gunnar.hellstrom@omnitor.se
Intended usage: COMMON
Author / Change controller:
Gunnar Hellstrom | IETF avt WG
gunnar.hellstrom@omnitor.se |
10.2. SDP Mapping of MIME Parameters
The information carried in the MIME media type specification has a
specific mapping to fields in the Session Description Protocol (SDP)
[7], which is commonly used to describe RTP sessions. When SDP is
used to specify sessions employing the text/t140 format, the mapping
is as follows:
- The MIME type ("text") goes in SDP "m=" as the media name.
- The MIME subtype (payload format name) goes in SDP "a=rtpmap" as
the encoding name. The RTP clock rate in "a=rtpmap" MUST be 1000
for text/t140.
- The parameter "cps" goes in SDP "a=fmtp" attribute.
- When the payload type is used with redundancy according to RFC
2198, the level of redundancy is shown by the number of elements in
the slash-separated payload type list in the "fmtp" parameter of
the redundancy declaration as defined in RFC 4102 [9] and RFC 2198
[3].
10.3. Offer/Answer Consideration
In order to achieve interoperability within the framework of the
offer/answer model [10], the following consideration should be made:
- The "cps" parameter is declarative. Both sides may provide a
value, which is independent of the other side.
11. Acknowledgements
The authors want to thank Stephen Casner, Magnus Westerlund, and
Colin Perkins for valuable support with reviews and advice on
creation of this document, to Mickey Nasiri at Ericsson Mobile
Communication for providing the development environment, Michele
Mizarro for verification of the usability of the payload format for
its intended purpose, and Andreas Piirimets for editing support and
validation.
12. 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., Hodson, O., Hardman, V., Handley, M.,
Bolot, J., Vega-Garcia, A., and S. Fosse-Parisis, "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", STD
63, RFC 3629, November 2003.
[7] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[8] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for
Generic Forward Error Correction", RFC 2733, December 1999.
[9] Jones, P., "Registration of the text/red MIME Sub-Type", RFC
4102, June 2005.
[10] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
the Session Description Protocol (SDP)", RFC 3264, June 2002.
[11] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conference with Minimal Control", STD 65, RFC 3551, July 2003.
[12] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
13. Informative References
[13] Floyd, S. and J. Kempf, "IAB Concerns Regarding Congestion
Control for Voice Traffic in the Internet", RFC 3714, March
2004.
[14] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004.
[15] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits,
Telephony Tones and Telephony Signals", RFC 2833, May 2000.
[16] Hellstrom, G., "RTP Payload for Text Conversation", RFC 2793,
May 2000.
[17] ITU-T Recommendation F.703, Multimedia Conversational Services,
November 2000.
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
EMail: gunnar.hellstrom@omnitor.se
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
Phone: +1 919 392 6948
EMail: paulej@packetizer.com
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