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RFC4103 - RTP Payload for Text Conversation

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RFC4103 - RTP Payload for Text Conversation_电脑维修资料库

network working group g. hellstromrequest for comments: 4103 omnitor abobsoletes: 2793 p. jonescategory: standards track cisco systems, inc.june 2005rtp payload for text conversationstatus of this memothis document specifies an internet standards track protocol for theinternet community, and requests discussion and suggestions forimprovements. please refer to the current edition of the internetofficial protocol standards (std 1) for the standardization stateand status of this protocol. distribution of this memo is unlimited.copyright noticecopyright (c) the internet society (2005).abstractthis memo obsoletes rfc 2793; it describes how to carry real-timetext conversation session contents in rtp packets. text conversationsession contents are specified in itu-t recommendation t.140.one payload format is described for transmitting text on a separatertp session dedicated for the transmission of text.this rtp payload description recommends a method to include redundanttext from already transmitted packets in order to reduce the risk oftext loss caused by packet loss.table of contents1. introduction ...................................................32. conventions used in this document ..............................43. usage of rtp ...................................................43.1. motivations and rationale .................................43.2. payload format for transmission of text/t140 data .........43.3. the t140block ...........................................53.4. synchronization of text with other media ..................53.5. rtp packet header .........................................54. protection against loss of data ................................64.1. payload format when using redundancy ......................64.2. using redundancy with the text/t140 format ................75. recommended procedure ..........................................85.1. recommended basic procedure ...............................85.2. transmission before and after idle periods ..............85.3. detection of lost text packets ............................95.4. compensation for packets out of order ....................106. parameter for character transmission rate .....................107. examples ......................................................117.1. rtp packetization examples for the text/t140 format ......117.2. sdp examples .............................................138. security considerations .......................................148.1. confidentiality ..........................................148.2. integrity ................................................148.3. source authentication ....................................149. congestion considerations .....................................1410. iana considerations ...........................................1610.1. registration of mime media type text/t140 ...............1610.2. sdp mapping of mime parameters ..........................1710.3. offer/answer consideration ..............................1711. acknowledgements ..............................................1812. normative references ..........................................1813. informative references ........................................191. introductionthis document defines a payload type for carrying text conversationsession contents in rtp <2> packets. text conversation sessioncontents are specified in itu-t recommendation t.140 <1>. textconversation is used alone or in connection with other conversationalfacilities, such as video and voice, to form multimedia conversationservices. text in multimedia conversation sessions is sentcharacter-by-character as soon as it is available, or with a smalldelay 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 charactersper second or less. in general, only one or a few new characters areexpected to be transmitted with each packet. small blocks of textmay be prepared by the user and pasted into the user interface fortransmission during the conversation, occasionally causing packets tocarry more payload.t.140 specifies that text and other t.140 elements must betransmitted in iso 10646-1 <5> code with utf-8 <6> transformation.this makes it easy to implement internationally useful applicationsand to handle the text in modern information technology environments.the payload of an rtp packet that follows this specification consistsof 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 withoutduplication and in original order. text conversation users expectthat text will be delivered with no, or a low level, of lostinformation.therefore, a mechanism based on rtp is specified here. it gives textarrival in correct order, without duplication, and with detection andindication of loss. it also includes an optional possibility torepeat data for redundancy in order to lower the risk of loss.because packet overhead is usually much larger than the t.140contents, the increase in bandwidth, with the use of redundancy, isminimal.by using rtp for text transmission in a multimedia conversationapplication, uniform handling of text and other media can be achievedin, for example, conferencing systems, firewalls, and networktranslation devices. this, in turn, eases the design and increasesthe possibility for prompt and proper media delivery.this document obsoletes rfc 2793 <16>. the text clarifiesambiguities in rfc 2793, improves on the specific implementationrequirements learned through development experience and givesexplicit usage examples.2. conventions used in this documentthe key words must, must not, required, shall, shall not,should, should not, recommended, may, and optional in thisdocument are to be interpreted as described in rfc 2119 <4>.3. usage of rtpthe payload format for real-time text transmission with rtp <2>described in this memo is intended for general text conversation useand is called text/t140 after its mime registration.3.1. motivations and rationalethe text/t140 format is intended to be used for text transmitted on aseparate rtp session, dedicated for the transmission of text, and notshared with other media.the text/t140 format may be used for any non-gateway application, aswell as in gateways. it may be used simultaneously with other mediastreams, transmitted as a separate rtp session, as required in realtime multimedia applications.the text/t140 format specified in this memo is compatible with itsearlier definition in rfc 2793. it has been refined, with the mainintention to minimize interoperability problems and encourage goodreliability and functionality.by specifying text transmission as a text medium, many good effectsare gained. routing, device selection, invocation of transcoding,selection of quality of service parameters, and other high and lowlevel functions depend on each medium being explicitly specified.3.2. payload format for transmission of text/t140 dataa text/t140 conversation rtp payload format consists of one, and onlyone, block of t.140 data, referred to as a t140block (see section3.3). there are no additional headers specific to this payloadformat. the fields in the rtp header are set as defined in section3.5, carried in network byte order (see rfc 791 <12>).3.3. the t140blockt.140 text is utf-8 coded, as specified in t.140, with no extraframing. the t140block contains one or more t.140 code elements asspecified in <1>. most t.140 code elements are single iso 10646 <5>characters, but some are multiple character sequences. eachcharacter is utf-8 encoded <6> into one or more octets. each blockmust contain an integral number of utf-8 encoded charactersregardless of the number of octets per character. any compositecharacter sequence (ccs) should be placed within one block.3.4. synchronization of text with other mediausually, each medium in a session utilizes a separate rtp stream. assuch, if synchronization of the text and other media packets isimportant, the streams must be associated when the sessions areestablished and the streams must share the same reference clock(refer to the description of the timestamp field as it relates tosynchronization in section 5.1 of rfc 3550 <2>). association of rtpstreams can be done through the cname field of rtcp sdes function.it is dependent on the particular application and is outside thescope of this document.3.5. rtp packet headereach rtp packet starts with a fixed rtp header. the following fieldsof the rtp fixed header are specified for t.140 text streams:payload type (pt): the assignment of an rtp payload type is specificto the rtp profile under which the payload formatis used. for profiles that use dynamic payloadtype number assignment, this payload format can beidentified by the mime type text/t140 (seesection 10). if redundancy is used per rfc 2198,another payload type number needs to be providedfor the redundancy format. the mime type foridentifying rfc 2198 is available in rfc 4102 <9>.sequence number: the definition of sequence numbers is available inrfc 3550 <2>. when transmitting text using thepayload format for text/t140, it is used fordetection of packet loss and out-of-order packets,and can be used in the process of retrieval ofredundant text, reordering of text and markingmissing text.timestamp: the rtp timestamp encodes the approximate instanceof entry of the primary text in the packet. aclock frequency of 1000 hz must be used.sequential packets must not use the sametimestamp. because packets do not represent anyconstant duration, the timestamp cannot be used todirectly infer packet loss.m-bit: the m-bit must be included. the first packet in asession, and the first packet after an idleperiod, should be distinguished by setting themarker bit in the rtp data header to one. themarker bit in all other packets must be set tozero. the reception of the marker bit may be usedfor refined methods for detection of loss.4. protection against loss of dataconsideration must be devoted to keeping loss of text due to packetloss within acceptable limits. (see itu-t f.703 <17>)the default method that must be used, when no other method isexplicitly selected, is redundancy in accordance with rfc 2198 <3>.when this method is used, the original text and two redundantgenerations should be transmitted if the application or end-to-endconditions do not call for other levels of redundancy to be used.forward error correction mechanisms, as per rfc 2733 <8>, or anyother mechanism with the purpose of increasing the reliability oftext transmission, may be used as an alternative or complement toredundancy. text data may be sent without additional protection ifend-to-end network conditions allow the text quality requirements,specified in itu-t f.703 <17>, to be met in all anticipated loadconditions.4.1. payload format when using redundancywhen using the payload format with redundant data, the transmittermay select a number of t140block generations to retransmit in eachpacket. a higher number introduces better protection against loss oftext but marginally increases the data rate.the rtp header is followed by one or more redundant data blockheaders: one for each redundant data block to be included. each ofthese headers provides the timestamp offset and length of thecorresponding 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 datafields 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 transmittedpackets.4.2. using redundancy with the text/t140 formatbecause text is transmitted only when there is text to transmit, thetimestamp is not used to identify a lost packet. rather, missingsequence numbers are used to detect lost text packets at reception.also, because sequence numbers are not provided in the redundantheader, some additional rules must be followed to allow redundantdata that corresponds to missing primary data to be properly mergedinto the stream of primary data t140blocks. they are:- each redundant data block must contain the same data as a t140blockpreviously transmitted as primary data.- the redundant data must be placed in age order, with the mostrecent redundant t140block last in the redundancy area.- all t140blocks, from the oldest desired generation up through thegeneration immediately preceding the new (primary) t140block, mustbe included.these rules allow the sequence numbers for the redundant t140blocksto be inferred by counting backwards from the sequence number in thertp header. the result will be that all the text in the payload willbe contiguous and in order.if there is a gap in the received rtp sequence numbers, and redundantt140blocks are available in a subsequent packet, the sequence numbersfor the redundant t140blocks should be inferred by counting backwardsfrom the sequence number in the rtp header for that packet. if thereare redundant t140blocks with sequence numbers matching those thatare missing, the redundant t140blocks may be substituted for themissing t140blocks.5. recommended procedurethis section contains recommended procedures for usage of the payloadformat. based on the information in the received packets, thereceiver 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 procedurepackets are transmitted when there is valid t.140 data to transmit.t.140 specifies that t.140 data may be buffered for transmission witha maximum buffering time of 500 ms. a buffering time of 300 ms isrecommended when the application or end-to-end network conditions arenot known to require another value.if no new data is available for a longer period than the bufferingtime, the transmission process is in an idle period.when new text is available for transmission after an idle period, itis recommended to send it as soon as possible. after thistransmission, it is recommended to buffer t.140 data in bufferingtime intervals, until the next idle period. this is done in order tokeep the maximum bit rate usage for text at a reasonable level. thebuffering time must be selected so that text users will perceive areal-time text flow.5.2. transmission before and after idle periodswhen valid t.140 data has been sent and no new t.140 data isavailable for transmission after the selected buffering time, anempty t140block should be transmitted. this situation is regarded asthe beginning of an idle period. the procedure is recommended inorder to more rapidly detect potentially missing text before an idleperiod.an empty t140block contains no data.when redundancy is used, transmission continues with a packet atevery transmission timer expiration and insertion of an emptyt.140block as primary, until the last non-empty t140block has beentransmitted, as primary and as redundant data, with all intendedgenerations of redundancy. the last packet before an idle periodwill contain only one non-empty t140block as redundant data, whilethe remainder of the redundancy packet will contain empty t140blocks.any empty t140block sent as primary data must be included asredundant t140blocks in subsequent packets, just as normal textt140blocks would be, unless the empty t140block is too old to betransmitted. this is done so that sequence number inference for theredundant t140blocks will be correct, as explained in section 4.2.after an idle period, the transmitter should set the m-bit to one inthe first packet with new text.5.3. detection of lost text packetspacket loss for text/t140 packets may be detected by observing gapsin the sequence numbers of rtp packets received by the receiver.with text/t140, the loss of packets is usually detected by comparisonof the sequence of rtp packets as they arrive. any discrepancy maybe used to indicate loss. the highest rtp sequence number receivedmay also be compared with that in rtcp reports, as an additionalcheck for loss of the last packet before an idle period.missing data should be marked by insertion of a missing text markerin the received stream for each missing t140block, as specified initu-t t.140 addendum 1 <1>.because empty t140blocks are transmitted in the beginning of an idleperiod, there is a slight risk of falsely marking loss of text, whenonly an empty t140block was lost. procedures based on detection ofthe packet with the m-bit set to one may be used to reduce the riskof introducing false markers of loss.if redundancy is used with the text/t140 format, and a packet isreceived with fewer redundancy levels than normally in the session,it should be treated as if one empty t140block has been received foreach excluded level in the received packet. this is because the onlyoccasion when a t140block is excluded from transmission is when it isan empty t140block that has become too old to be transmitted.if two successive packets have the same number of redundantgenerations, it should be treated as the general redundancy level forthe session. change of the general redundancy level should only bedone after an idle period.the text/t140 format relies on use of the sequence number in the rtppacket header for detection of loss and, therefore, is not suitablefor applications where it needs to be alternating with other payloadsin the same rtp stream. it would be complicated and unreliable totry to detect loss of data at the edges of the shifts between t140text and other stream contents. therefore, text/t140 is recommendedto be the only payload type in the rtp stream.5.4. compensation for packets out of orderfor protection against packets arriving out of order, the followingprocedure may be implemented in the receiver. if analysis of areceived packet reveals a gap in the sequence and no redundant datais available to fill that gap, the received packet should be kept ina buffer to allow time for the missing packet(s) to arrive. it isrecommended that the waiting time be limited to 1 second.if a packet with a t140block belonging to the gap arrives before thewaiting time expires, this t140block is inserted into the gap andthen consecutive t140blocks from the leading edge of the gap may beconsumed. any t140block that does not arrive before the time limitexpires should be treated as lost and a missing text marker should beinserted (see section 5.3).6. parameter for character transmission ratein some cases, it is necessary to limit the rate at which charactersare transmitted. for example, when a public switched telephonenetwork (pstn) gateway is interworking between an ip device and apstn textphone, it may be necessary to limit the character rate fromthe ip device in order to avoid throwing away characters (in case ofbuffer overflow at the pstn gateway).to control the character transmission rate, the mime parameter cpsin the fmtp attribute <7> is defined (see section 10 ). it is usedin sdp with the following syntax:a=fmtp: cps=the field is populated with the payload type that is usedfor text. the field contains an integer representing themaximum number of characters that may be received per second. thevalue shall be used as a mean value over any 10-second interval. thedefault value is 30.examples of use in sdp are found in section 7.2.in receipt of this parameter, devices must adhere to the request bytransmitting characters at a rate at or below the specified value. note that this parameter was not defined in rfc 2793 <16>.therefore implementations of the text/t140 format may be in use thatdo not recognize and act according to this parameter. therefore,receivers of text/t140 must be designed so they can handle temporaryreception of characters at a higher rate than this parameterspecifies. as a result malfunction due to buffer overflow is avoidedfor text conversation with human input.7. examples7.1. rtp packetization examples for the text/t140 formatbelow is an example of a text/t140 rtp packet without redundancy.0 1 2 30 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) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| synchronization source (ssrc) identifier |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| t.140 encoded data |+ +---------------++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+below is an example of a text/t140 rtp packet with one redundantt140block.0 1 2 30 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 t140blockusing text/t140 payload format. the primary data block is empty,which is the case when transmitting a packet for the sole purpose offorcing the redundant data to be transmitted in the absence of anynew data.0 1 2 30 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 thenext rtp packet in the sequence, which does contain a real t140blockwhen using the text/t140 payload format. note that the empty blockis present in the redundant transmissions of the text/t140 payloadformat. this example shows two levels of redundancy and one primarydata block. the value of the r2 block length would be set to zeroin order to represent the empty t140block.0 1 2 30 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 examplesbelow is an example of sdp, which describes rtp text transport onport 11000:m=text 11000 rtp/avp 98a=rtpmap:98 t140/1000below is an example of sdp that is similar to the above example, butalso utilizes rfc 2198 to provide the recommended two levels ofredundancy for the text packets:m=text 11000 rtp/avp 98 100a=rtpmap:98 t140/1000a=rtpmap:100 red/1000a=fmtp:100 98/98/98note: although these examples utilize the rtp/avp profile, it is notintended to limit the scope of this memo. any appropriate profilemay be used in conjunction with this memo.8. security considerationsall of the security considerations from section 14 of rfc 3550 <2>apply.8.1. confidentialitybecause the intention of the described payload format is to carrytext in a text conversation, security measures in the form ofencryption are of importance. the amount of data in a textconversation session is low. therefore, any encryption method may beselected and applied to t.140 session contents or to whole rtppackets. secure real-time transport protocol (srtp) <14> provides asuitable method for ensuring confidentiality.8.2. integrityit may be desirable to protect the text contents of an rtp streamagainst manipulation. srtp <14> provides methods for providingintegrity that may be applied.8.3. source authenticationthere are several methods of making sure the source of the text isthe intended one.text streams are usually used in a multimedia control environment.security measures for authentication are available and should beapplied in the registration and session establishment procedures, sothat the identity of the sender of the text stream is reliablyassociated with the person or device setting up the session. onceestablished, srtp <14> mechanisms may be applied to ascertain thatthe source is maintained the same during the session.9. congestion considerationsthe congestion considerations from section 10 of rfc 3550 <2>,section 6 of rfc 2198 <3>, and any used profile (e.g., the sectionabout congestion in chapter 2 of rfc 3551 <11>) apply with thefollowing application-specific considerations.automated systems must not use this format to send large amounts oftext at rates significantly above those a human user could enter.even if the network load from users of text conversation is usuallyvery low, for best-effort networks an application must monitor thepacket loss rate and take appropriate actions to reduce its sendingrate (if this application sends at higher rate than what tcp wouldachieve over the same path). the reason for this is that thisapplication, due to its recommended usage of two or more redundancylevels, is very robust against packet loss. at the same time, due tothe low bit-rate of text conversations, if one considers thediscussion in rfc 3714 <13>, this application will experience veryhigh packet loss rates before it needs to perform any reduction inthe sending rate.if the application needs to reduce its sending rate, it should notreduce the number of redundancy levels below the default amountspecified in section 4. instead, the following actions arerecommended in order of priority:- increase the shortest time between transmissions (described insection 5.1) from the recommended 300 ms to 500 ms, which is thehighest 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 intransmission, beyond what is allowed according to t.140, but textwill 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 theabove measures is not sufficient, the only remaining action is toterminate the session.as guidance, some load figures are provided here as examples based onuse of ipv4, including the load from ip, udp, and rtp headers withoutcompression .- experience tells that a common mean character transmission rate,during a complete pstn text telephony session, is around twocharacters per second.- a maximum performance of 20 characters per second is enough evenfor voice-to-text applications.- with the (unusually high) load of 20 characters per second, in alanguage that makes use of three octets per utf-8 character, tworedundant levels, and 300 ms between transmissions, the maximumload of this application is 3300 bits/s.- when the restrictions mentioned above are applied, limitingtransmission to 10 characters per second, using 5 s betweentransmissions, the maximum load of this application, in a languagethat uses one octet per utf-8 character, is 300 bits/s.note that this payload can be used in a congested situation as a lastresort to maintain some contact when audio and video media need to bestopped. the availability of one low bit-rate stream for text insuch adverse situations may be crucial for maintaining somecommunication in a critical situation.10. iana considerationsthis document updates the rtp payload format named t140 and theassociated mime type text/t140, in the iana rtp and media typeregistries.10.1. registration of mime media type text/t140mime media type name: textmime subtype name: t140required parameters: rate: the rtp timestamp clock rate, which isequal to the sampling rate. the only valid value is 1000.optional parameters: cps: the maximum number of characters that maybe received per second. the default value is 30.encoding considerations: t.140 text can be transmitted with rtp asspecified in rfc 4103.security considerations: see section 8 of rfc 4103.interoperability considerations: this format is the same as specifiedin rfc2793. for rfc2793 the cps= parameter was not defined.therefore, there may be implementations that do not consider thisparameter. receivers need to take that into account.published specification: itu-t t.140 recommendation. rfc 4103.applications which use this media type: text communication terminalsand text conferencing tools.additional information: this type is only defined for transfer viartp.magic number(s): nonefile extension(s): nonemacintosh file type code(s): noneperson & email address to contact for further information:gunnar hellstrome-mail: gunnar.hellstrom@omnitor.seintended usage: commonauthor / change controller:gunnar hellstrom | ietf avt wggunnar.hellstrom@omnitor.se |10.2. sdp mapping of mime parametersthe information carried in the mime media type specification has aspecific mapping to fields in the session description protocol (sdp)<7>, which is commonly used to describe rtp sessions. when sdp isused to specify sessions employing the text/t140 format, the mappingis 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 asthe encoding name. the rtp clock rate in a=rtpmap must be 1000for text/t140.- the parameter cps goes in sdp a=fmtp attribute.- when the payload type is used with redundancy according to rfc2198, the level of redundancy is shown by the number of elements inthe slash-separated payload type list in the fmtp parameter ofthe redundancy declaration as defined in rfc 4102 <9> and rfc 2198<3>.10.3. offer/answer considerationin order to achieve interoperability within the framework of theoffer/answer model <10>, the following consideration should be made:- the cps parameter is declarative. both sides may provide avalue, which is independent of the other side.11. acknowledgementsthe authors want to thank stephen casner, magnus westerlund, andcolin perkins for valuable support with reviews and advice oncreation of this document, to mickey nasiri at ericsson mobilecommunication for providing the development environment, michelemizarro for verification of the usability of the payload format forits intended purpose, and andreas piirimets for editing support andvalidation.12. normative references<1> itu-t recommendation t.140 (1998) - text conversation protocolfor multimedia application, with amendment 1, (2000).<2> schulzrinne, h., casner, s., frederick, r. and v. jacobson,rtp: a transport protocol for real-time applications, rfc3550, july 2003.<3> perkins, c., kouvelas, i., hodson, o., hardman, v., handley, m.,bolot, j., vega-garcia, a., and s. fosse-parisis, rtp payloadfor redundant audio data, rfc 2198, september 1997.<4> bradner, s., key words for use in rfcs to indicate requirementlevels, bcp 14, rfc 2119, march 1997.<5> iso/iec 10646-1: (1993), universal multiple octet codedcharacter set.<6> yergeau, f., utf-8, a transformation format of iso 10646, std63, rfc 3629, november 2003.<7> handley, m. and v. jacobson, sdp: session descriptionprotocol, rfc 2327, april 1998.<8> rosenberg, j. and h. schulzrinne, an rtp payload format forgeneric forward error correction, rfc 2733, december 1999.<9> jones, p., registration of the text/red mime sub-type, rfc4102, june 2005.<10> rosenberg, j. and h. schulzrinne, an offer/answer model withthe session description protocol (sdp), rfc 3264, june 2002.<11> schulzrinne, h. and s. casner, rtp profile for audio and videoconference 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 congestioncontrol for voice traffic in the internet, rfc 3714, march2004.<14> baugher, m., mcgrew, d., naslund, m., carrara, e., and k.norrman, the secure real-time transport protocol (srtp), rfc3711, 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' addressesgunnar hellstromomnitor abrenathvagen 2se-121 37 johanneshovswedenphone: +46 708 204 288 / +46 8 556 002 03fax: +46 8 556 002 06email: gunnar.hellstrom@omnitor.sepaul e. jonescisco systems, inc.7025 kit creek rd.research triangle park, nc 27709usaphone: +1 919 392 6948email: paulej@packetizer.comfull copyright statementcopyright (c) the internet society (2005).this document is subject to the rights, licenses and restrictionscontained in bcp 78, and except as set forth therein, the authorsretain all their rights.this document and the information contained herein are provided on anas is basis and the contributor, the organization he/she representsor is sponsored by (if any), the internet society and the internetengineering task force disclaim all warranties, express or implied,including but not limited to any warranty that the use of theinformation herein will not infringe any rights or any impliedwarranties of merchantability or fitness for a particular purpose.intellectual propertythe ietf takes no position regarding the validity or scope of anyintellectual property rights or other rights that might be claimed topertain to the implementation or use of the technology described inthis document or the extent to which any license under such rightsmight or 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