<remark><p>Spelling, grammar, and clarification edits, including section clarifications recommended from public discussion. Interop with XEP-0308 message correction.</p></remark>
<p><strong>Real-time text is text transmitted instantly while it is being typed or created.</strong> The recipient can immediately read the sender's text as it is written, without waiting. Text can be used conversationally, similar to a telephone conversation, where one listens while the other is speaking. It eliminates waiting times found in messaging. Real-time text has been around for decades in various implementations:</p>
<li>For SIP, real-time text is sent using <spanclass="ref"><strong><linkurl="http://tools.ietf.org/html/rfc4103">IETF RFC 4103</link></strong></span><note>RFC 4103: RTP Payload for Text Conversation. <<linkurl="http://tools.ietf.org/html/rfc4103">http://tools.ietf.org/html/rfc4103</link>>.</note> with <spanclass="ref"><strong><linkurl="http://www.itu.int/rec/T-REC-T.140">ITU-T T.140</link></strong></span><note>ITU-T T.140: Protocol for multimedia application text conversation. <<linkurl="http://www.itu.int/rec/T-REC-T.140">http://www.itu.int/rec/T-REC-T.140</link>>.</note> presentation coding.</li>
<li>The <spanclass="ref"><strong><linkurl="http://help.aol.com/help/microsites/microsite.do?cmd=displayKC&externalId=223568">Real-Time IM</link></strong></span><note>AOL AIM Real Time Text: <<linkurl="http://help.aol.com/help/microsites/microsite.do?cmd=displayKC&externalId=223568">http://help.aol.com/help/microsites/microsite.do?cmd=displayKC&externalId=223568</link>>.</note> feature found in AOL Instant Messenger.</li>
<li>A component within Total Conversation, used by <spanclass="ref"><strong><linkurl="http://www.reach112.eu">Reach112</link></strong></span><note>Reach112: European emergency service with real-time text. <<linkurl="http://www.reach112.eu">http://www.reach112.eu</link>>.</note> in Europe, an accessible emergency service with real-time text.</li>
<p>Real-time text allows continuous and rapid communication in text, to complement general-purpose instant messaging. It can also allow immediate conversation in situations where speech cannot be used (e.g. quiet environments, privacy, deaf and hard of hearing). Real-time text is also beneficial in emergency situations, due to its immediacy. For a visual animation of real-time text, see <spanclass="ref"><strong><linkurl="http://www.realtimetext.org">Real-Time Text Taskforce</link></strong></span>
<note>Real-Time Text Taskforce, a foundation for real-time text standardization <<linkurl="http://www.realtimetext.org">http://www.realtimetext.org</link>>.</note>.</p>
<li>Support transmission and reproduction of the original intervals between key presses, to preserve look-and-feel of typing independently of transmission intervals.</li>
<li>Allow XMPP to follow the <spanclass="ref"><strong><linkurl="http://www.itu.int/rec/T-REC-F.703">ITU-T Rec. F.703</link></strong></span><note>ITU-T Rec. F.703: Multimedia conversational services. <<linkurl="http://www.itu.int/rec/T-REC-F.703">http://www.itu.int/rec/T-REC-F.703</link>>.</note> Total Conversation standard for simultaneous voice, video, and real-time text.</li>
<li>Be suitable for transcription services and (when coupled with voice at user's choice) for TTY/text telephone alternatives, relay services, and captioned telephone systems.</li>
<p><strong>real-time</strong>– A conversational latency of less than 1 second, as defined by <spanclass="ref"><strong><linkurl="http://www.itu.int/rec/T-REC-F.700">ITU-T Rec. F.700</link></strong></span><note>ITU-T Rec. F.700: Framework Recommendation for multimedia services <<linkurl="http://www.itu.int/rec/T-REC-F.700">http://www.itu.int/rec/T-REC-F.700</link>>.</note>, section 2.1.2.1.</p>
<p>Real-time text is transmitted via an <rtt/> child element of a <message/> stanza. The <rtt/> element is transmitted at regular intervals by the sender client while a message is being composed, to allow the recipient to see the latest message text from the sender, without waiting for the full message sent in a <body/> element.</p>
<p>This is a basic example of a <em><strong>real-time message</strong></em> "Hello, my Juliet!” transmitted in real-time while it is being typed, before a final message delivery in a <body/> element (to remain <linkurl="#backwards_compatible">Backwards Compatible</link>):</p>
<p>The <rtt/> element contains a series of one or more child elements called <em><strong>action elements</strong></em> that represent <linkurl="#realtime_text_actions">Real-Time Text Actions</link> such as text being appended, inserted, or deleted. Example 1 illustrates only the <t/> action element, which appends text to the end of a message.</p>
<p>Transmission of the <rtt/> element occurs at a regular <linkurl="#transmission_interval">Transmission Interval</link> whenever the sender is actively composing a message. If there are no changes to the message since the last transmission, no transmission occurs.</p>
<p>There MUST NOT be more than one <rtt/> element per <message/> stanza.</p>
<p>The namespace of the <rtt/> element is “urn:xmpp:rtt:0”.</p>
<p>This REQUIRED attribute is a counter to maintain the integrity of real-time text. (The bounds of <strong>seq</strong> is 31-bits, the range of positive values of a signed integer.)</p>
<p>Senders MUST increment the <strong>seq</strong> attribute by 1 in each subsequent <rtt/> transmitted without an <linkurl="#event">event</link> attribute. When an <rtt/> element has an <linkurl="#event">event</link> attribute, senders MAY instead use any value as the new starting value for <strong>seq</strong>. A random starting <strong>seq</strong> value is RECOMMENDED for best integrity during <linkurl="#usage_with_multiuser_chat_and_simultaneous_logins">Usage with Multi-User Chat and Simultaneous Logins</link>. Senders SHOULD limit the size of the new starting <strong>seq</strong> value, to keep <rtt/> compact while allowing plenty of incrementing room, and prevent wraparound from ever happening.</p>
<p>Recipients MUST monitor the <strong>seq</strong> value to verify the integrity of real-time text. See <linkurl="#keeping_realtime_text_synchronized">Keeping Real-Time Text Synchronized</link>.</p>
<p>This attribute signals events for real-time text, such as the start of a new real-time message. Recipient clients MUST ignore <rtt/> containing unsupported <strong>event</strong> values. The use of <rtt/> elements without an <strong>event</strong> attribute is for transmitting changes to an existing real-time message. The <strong>event</strong> attribute is used in the situations specified below:</p>
Senders MUST use this value when transmitting the first <rtt/> element containing <linkurl="#action_elements">Action Elements</link> (i.e. when sending the first character(s) of a new message). Recipient clients MUST initialize a new real-time message for display, and then process action elements within the <rtt/> element. If a real-time message already exists, from the same sender in the same chat session, its content MUST be replaced (i.e. cleared prior to processing action elements). Senders MAY send subsequent <rtt/> elements that do not contain an <strong>event</strong> attribute.</p>
For recipients, both <strong>'new'</strong> and <strong>'reset'</strong> are logically identical, and differ only for implementation purposes (e.g. highlighting newly-started messages). Recipient clients MUST initialize a new real-time message for display, and then process action elements within the <rtt/> element. If a real-time message already exists, from the same sender in the same chat session, its content MUST be replaced (i.e. cleared prior to processing action elements). Senders MAY send subsequent <rtt/> elements that do not contain an <strong>event</strong> attribute. Recipients MUST be able to process <strong>'reset'</strong> without first receiving <strong>'new'</strong>. See <linkurl="#message_reset">Message Reset</link>, used for <linkurl="#keeping_realtime_text_synchronized">Keeping Real-Time Text Synchronized</link> and <linkurl="#basic_realtime_text">Basic Real-Time Text</link>.</p>
Clients MAY use this value to signal the other end that real-time text is being activated. It can be used to signal activation of real-time text before starting a new real-time message (since it may take a while before the sender starts composing). If used, this <rtt/> element MUST be empty with no action elements. See <linkurl="#activating_and_deactivating_realtime_text">Activating and Deactivating Real-Time Text</link>.</p>
Clients MAY use this value to signal the other end to stop transmitting real-time text. If used, this <rtt/> element MUST be empty with no action elements. Recipients SHOULD discontinue sending back <rtt/> elements for the remainder of the same chat session (or unless <strong>'init'</strong> is used again). See <linkurl="#activating_and_deactivating_realtime_text">Activating and Deactivating Real-Time Text</link>.</p>
<p>This attribute is used only if &xep0308; (XEP-0308) is implemented at the same time as this specification. If a client supports both XEP-0301 and XEP-0308, clients MUST use this attribute if real-time text is used during editing of the previously delivered message.</p>
<p>This <strong>id</strong> attribute refers to the <message/> stanza containing the <body/> that is being edited (See 'Business Rules' in XEP-0308). When used, <strong>id</strong> MUST be included in all <rtt/> elements transmitted during message correction of the previous message. The whole message MUST be retransmitted via <rtt event='reset'/> (<linkurl="#message_reset">Message Reset</link>) when beginning to edit the previous message, or when switching between messages (e.g. editing the new partially-composed message versus editing of the previously delivered message).</p>
<p>The real-time message is considered complete upon receipt of a <body/> element in a <message/> stanza. The delivered text in the <body/> element is considered the final message text, and supersedes the real-time message. In the ideal case, the text from the <body/> element is redundant since this delivered text is identical to the final contents of the real-time message.</p>
<p>Senders MAY transmit the <body/> element in the same or separate message stanza as the one containing the <rtt/> element. Senders MUST include an <linkurl="#event">event</link> attribute in the next <rtt/> element that is transmitted after a message stanza containing a <body/> element.</p>
<p>The real-time text standard simply provides early delivery of text before the <body/> element. The <body/> element continues to follow the &xmppcore; specification. In particular, XMPP implementations need to ignore XML elements they do not understand. Clients, that do not support real-time text, will continue to behave normally, displaying complete lines of messages as they are delivered.</p>
<p>For the best balance between interoperability and usability, the default transmission interval of <rtt/> elements for a continuously-changing message SHOULD be approximately <strong>0.7 seconds</strong>. This interval meets ITU-T Rec. F.700 Section A.3.2.1 for good quality real-time conversation. If a different transmission interval needs to be used, the interval SHOULD be <strong>between 0.3 seconds and 1 second</strong>.</p>
<p>A longer interval will lead to a less optimal user experience in most network conditions. Conversely, a much shorter interval can lead to <linkurl="#congestion_considerations">Congestion Considerations</link>. To provide fluid real-time text, one or more of the following methods can be used:</p>
<li><linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link> for natural typing display, independently of the transmission interval.</li>
<li>Use of <linkurl="#time_critical_and_low_latency_methods">Time Critical and Low Latency Methods</link>, for real-time captioning/transcription.</li>
<li>For other options or reduced-precision options, see <linkurl="#lowbandwidth_and_lowprecision_text_smoothing">Low-Bandwidth and Low-Precision Text Smoothing</link>.</li>
<p>The <rtt/> element MAY contain one or more <em><strong>action elements</strong></em> representing real-time text operations, including text being appended, inserted, or deleted.</p>
<p>Many chat clients allow a sender to edit their message before sending (via a Send button, or pressing Enter). The inclusion of real-time text functionality, in existing client software, needs to preserve the sender's existing expectation of being able to edit their messages. In a chat session with real-time text, the recipient can see the sender compose and edit their message before it is completed.</p>
<p>This is a short summary of action elements that operate on a real-time message. These elements are kept compact in order to save bandwidth, since a single <rtt/> element can contain a huge number of action elements (e.g. during <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link>). For detailed information, see <linkurl="#list_of_action_elements">List of Action Elements</link>.</p>
The <em><strong>n</strong></em> attribute is a length value, in number of characters. If <em><strong>n</strong></em> is omitted, the default value of <em><strong>n</strong></em> MUST be 1.</p></li>
<li><p>For <linkurl="#element_t_insert_text">Element <t/>– Insert Text</link> and <linkurl="#element_e_erase_text">Element <e/>– Erase Text</link>:<br/>
The <em><strong>p</strong></em> attribute is an absolute position value, as a character position index into the real-time message, where 0 represents the beginning of the message. If <em><strong>p</strong></em> is omitted, the default value of <em><strong>p</strong></em> MUST point to the end of the message (i.e. <em><strong>p</strong></em> is set to the current length of the real-time message).</p></li>
<li><p>For the purpose of this specification, the word "character" represents a single Unicode code point. See <linkurl="#unicode_character_counting">Unicode Character Counting</link>.</p></li>
<li><p>Senders MUST NOT use negative values for any attribute, nor use <em><strong>p</strong></em> values beyond the current message length. However, recipients receiving such values MUST clip negative values to 0, and clip excessively high <em><strong>p</strong></em> values to the current length of the real-time message. Modifications only occur within the boundaries of the current real-time message.</p></li>
<p>Recipients MUST be able to process all <t/> and <e/> action elements for incoming <rtt/> transmissions, even if senders do not use all of these for outgoing <rtt/> transmissions (e.g. <linkurl="#basic_realtime_text">Basic Real-Time Text</link>). Support for <w/> is RECOMMENDED for both senders and recipients, in order to accommodate <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link>. Recipients MUST ignore unexpected or unsupported elements within <rtt/>, while continuing to process subsequent action elements (Compatibility is ensured via <linkurl="#namespace_versioning">Namespace Versioning</link>). Action elements are immediate child elements of the <rtt/> element, and are never nested. See examples in <linkurl="#use_cases">Use Cases</link>.</p>
<em>Note: Text can be any</em><em>subset of text allowed in the <body/> element of a <message/>. If <t/> is empty, no text modification takes place.</em></p>
<p>Supports the behavior of backspace key presses. Text is removed towards beginning of the message. This element is also used for all delete operations, including the delete key, and text block deletes.<br/>
<em>Note: Excess backspaces MUST be ignored. Thus, text is backspaced only to the beginning of the message, in situations where the value of p is excessively large.</em></p>
<p>Allow for the transmission of intervals, between real-time text actions, to recreate the pauses between key presses. See <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link>.</p>
<p>Wait <em><strong>n</strong></em> milliseconds before processing the next action element. This pause MAY be approximate, and not necessarily be of millisecond precision. The <em><strong>n</strong></em> value SHOULD NOT exceed the <linkurl="#transmission_interval">Transmission Interval</link>. Also, if a <linkurl="#body_element">Body Element</link> arrives, pauses SHOULD be interrupted to prevent a delay in message delivery.</p>
<section2topic="Keeping Real-Time Text Synchronized"anchor="keeping_realtime_text_synchronized">
<p>In a chat session, it is important that real-time text stays identical on both the sender and recipient ends. The loss of a single <rtt/> transmission could represent missing text or missing edits. Also, recipients can connect after the sender has already started composing a message. Recovery of in-progress real-time message via <linkurl="#message_reset">Message Reset</link> is useful in several situations:</p>
<p>Recipient clients MUST keep track of separate real-time message per sender, including maintaining independent <linkurl="#seq">seq</link> values. For implementation simplicity, recipient clients MAY track incoming <rtt/> elements per bare JID. Conflicting <rtt/> elements, from separate <linkurl="#simultaneous_logins">Simultaneous Logins</link>, is handled via the remainder of this section. Alternatively, recipient clients MAY keep track of separate real-time messages per full JID and/or per <thread/>.</p>
<li>Senders MUST increment the <linkurl="#seq">seq</link> attribute in steps of 1, for consecutively transmitted <rtt/> elements.</li>
<li>Recipients MUST monitor the <linkurl="#seq">seq</link> attribute value of received <rtt/> elements, to verify that it is incrementing by 1.</li>
<p>Loss of sync occurs if the <linkurl="#seq">seq</link> attribute do not increment by 1 as expected when <linkurl="#staying_in_sync">Staying In Sync</link>. In this case:</p>
<li>Recipients MUST ignore action elements within the current and subsequent <rtt/> elements; and</li>
<li>An indication can be used to show the loss of sync (e.g. color coding, modified chat state message).</li>
</ul>
<p>Recovery occurs when the recipient receives the following:</p>
<ul>
<li>A <body/> element. The <linkurl="#body_element">Body Element</link> supersedes the real-time message.</li>
<li>An <rtt/> element containing an <linkurl="#event">event</link> attribute (e.g. new message, or <linkurl="#message_reset">Message Reset</link>).</li>
<p>A message reset is a retransmission of the sender's partially composed text. The recipient can redisplay the real-time message as a result. It allows real-time text conversation to resume quickly, without waiting for senders to start a new message.</p>
<p>Retransmission SHOULD be done at an average interval of 10 seconds during active typing or composing. This interval is frequent enough to minimize user waiting time, while being infrequent enough to reduce bandwidth overhead. This interval MAY vary to a longer interval, in order to reduce average bandwidth (e.g. long messages, infrequent or minor message changes). For quicker recipient recovery, senders MAY adjust the timing of the message retransmissions to occur right after any of the following additional events: </p>
<p>A message reset is done using the <rtt/> attribute <linkurl="#event">event</link> value of <strong>'reset'</strong> (see <linkurl="#rtt_attributes">RTT Attributes</link>).</p>
<p>Note: There are no restrictions on using multiple <linkurl="#action_elements">Action Elements</link> during a message reset (e.g. typing or backspacing occurring at the end of a retransmitted message).</p>
</section3>
</section2>
<section2topic="Accurate Processing of Action Elements"anchor="accurate_processing_of_action_elements">
<p>Real-time text is generated based on text normally allowed to be transmitted within the <body/> element.</p>
<p>Incorrectly generated <linkurl="#action_elements">Action Elements</link> and <linkurl="#attribute_values">Attribute Values</link> can lead to inconsistencies between the sender and recipient during real-time editing. The Unicode characters of the real-time text needs to make it transparently from the sender to the recipient, without unexpected modifications after sender pre-processing. This is the chain between the sender's creation of real-time text, to the recipient's processing of real-time text. Transparent transmission of Unicode characters is possible with sender pre-processing, as long as the transmission from the sender to the recipient remains standards-compliant, including compliant XML processors and compliant XMPP servers.</p>
<p>Any inconsistencies that occur during real-time message editing (e.g. non-compliant servers that modify messages, incorrect <linkurl="#unicode_character_counting">Unicode Character Counting</link>) will recover upon delivery of the <linkurl="#body_element">Body Element</link>, upon the next <linkurl="#message_reset">Message Reset</link>, and/or during <linkurl="#basic_realtime_text">Basic Real-Time Text</link>.</p>
<section3topic="Unicode Character Counting"anchor="unicode_character_counting">
<p>For this specification, a "character" represents a single Unicode code point. This is the same definition used in section 1.1 of <spanclass="ref"><strong><linkurl="http://tools.ietf.org/html/rfc5198">IETF RFC 5198</link></strong></span><note>RFC 5198: Unicode Format for Network Interchange. <<linkurl="http://tools.ietf.org/html/rfc5198">http://tools.ietf.org/html/rfc5198</link>>.</note>. For platform-independent interoperability of <linkurl="#action_elements">Action Elements</link>, calculations on <linkurl="#attribute_values">Attribute Values</link> (<em><strong>p</strong></em> and <em><strong>n</strong></em>) MUST be based on counts of Unicode code points.</p>
<p>Many platforms use different internal encodings (i.e. string formats) that are different from the transmission encoding (UTF-8). These factors need to be considered:</p>
<ul>
<li><p>Multiple Unicode code points (e.g. combining marks, accents) can form a combining character sequence.<br/>
<em>Action elements operate on Unicode code points individually.</em></p></li>
<li><p>Unicode code points U+10000 through U+10FFFF are represented as a surrogate pair in some Unicode encodings (e.g. UTF-16).<br/>
<em>Action elements operate on Unicode code points as a whole, not on separate components of a surrogate pair.</em></p></li>
<li><p>XMPP transmission uses UTF-8, which use a variable number of bytes per Unicode code point.<br/><em>Action elements operate on Unicode code points as a whole, not on separate bytes.</em></p></li>
</ul>
<p>Lengths and positions in <linkurl="#attribute_values">Attribute Values</link> are relative to the internal Unicode text of the real-time message, independently of the directionality of actual displayed text. As a result, any valid Unicode text direction can be used with real-time text (right-to-left, left-to-right, and bidirectional). One way for implementers to visualize this, is to simply visualize Unicode text as an array of individual code points, and treat <linkurl="#attribute_values">Attribute Values</link> as array indexes.</p>
</section3>
<section3topic="Guidelines for Senders"anchor="guidelines_for_senders">
<p>Sender clients MUST generate real-time text (<linkurl="#action_elements">Action Elements</link> and <linkurl="#attribute_values">Attribute Values</link>) based on the plain text version of the sender's message with pre-processing completed. This is separate and concurrent to any displayed presentation of the same message (e.g. formatting, emoticon graphics, &xep0071;).</p>
<p>Pre-processing before generating real-time text, include Unicode normalization, conversion of emoticons graphics to text, removal of illegal characters, line-break conversion, and any other necessary text modifications. For Unicode normalization, sender clients SHOULD ensure the message is in Unicode Normalization Form C ("NFC"), specified by section 3 of RFC 5198, and within many other standards such as Canonical XML 1.0.</p>
<p>If Unicode combining character sequences (e.g. letter with multiple accents) are used for <linkurl="#element_t_insert_text">Element <t/>– Insert Text</link>, then complete combining character sequences SHOULD be sent. In situations where modifications are required to an existing combining character sequence (e.g. adding an additional accent), an <linkurl="#element_e_erase_text">Element <e/>– Erase Text</link> SHOULD be used to delete the existing combining character sequence, before transmitting a complete replacement sequence via the <t/> element. (However, recipients SHOULD NOT assume this behavior from sending clients. See <linkurl="#guidelines_for_recipients">Guidelines for Recipients</link>).</p>
<p>For the purpose of calculating <linkurl="#attribute_values">Attribute Values</link>, any line breaks MUST be treated as a single character. Conversion of line breaks into a single LINE FEED U+000A is REQUIRED for XML processors, according to section 2.11 of <spanclass="ref"><strong><linkurl="http://www.w3.org/TR/xml/">XML</link></strong></span><note>XML: Extensible Markup Language 1.0 (Fifth Edition). <<linkurl="http://www.w3.org/TR/xml/">http://www.w3.org/TR/xml/</link>>.</note>. In addition, XML character entities are counted as a single character.</p>
<section3topic="Guidelines for Recipients"anchor="guidelines_for_recipients">
<p>For <linkurl="#element_t_insert_text">Element <t/>– Insert Text</link>, text MUST be obtained using compliant XML processing (including entities converted to characters). Recipient clients SHOULD ensure that the received text is in Unicode Normalization Form C ("NFC"). After this, recipient clients MUST NOT do any other modifications to resulting real-time messages. This is to allow accurate processing of subsequent <linkurl="#action_elements">Action Elements</link> and <linkurl="#attribute_values">Attribute Values</link> (The recipient client can separately process/modify a copy of the same real-time message text, if necessary for the purpose of display presentation).</p>
<p>It is possible for <linkurl="#element_t_insert_text">Element <t/>– Insert Text</link> to contain any subset sequence of Unicode code points from the sender’s message. This can result in situations where text transmitted in <t/> elements is an incomplete combining character sequence (e.g. Unicode combining mark(s) without a base character) which becomes a complete sequence when inserted within the recipient's real-time message (e.g. additional accent for an existing combining character sequence). These are still complete individual code points, even if the sequence is incomplete.</p>
<p>If a client supports this real-time text protocol, it MUST advertise that fact in its responses to &xep0030; information requests ("disco#info") by returning a feature of <strong>urn:xmpp:rtt:0</strong></p>
<p>In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in &xep0115;. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.</p>
<p>See <linkurl="#activating_and_deactivating_realtime_text">Activating and Deactivating Real-Time Text</link> for more information, including implicit discovery.</p>
<p>If a long <linkurl="#transmission_interval">Transmission Interval</link> is used without <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link>, then incoming text will appear in intermittent bursts if the display of text is not smoothed. This hurts user experience of real-time text.</p>
<p>For high quality presentation of real-time text, the original look-and-feel of typing can be preserved independently of the transmission interval. This is achieved using <linkurl="#element_w_wait_interval">Element <w/>– Wait Interval</link> between other <linkurl="#action_elements">Action Elements</link>. Sender clients can transmit the length of pauses between key presses, and send multiple key presses in a single <message/> stanza. Recipient clients that process <w/> elements are able to display the sender's typing smoothly, without sudden bursts of text. See <linkurl="#examples_of_key_press_intervals">Examples of Key Press Intervals</link>.</p>
<p>When key press intervals are preserved at high precision, all subtleties of typing are preserved, including the 'mood' (calm typing versus panicked or emphatic typing, etc.). Much as VoIP allows accurate packet transmission of sound, this spec allows accurate packet transmission of original typing look-and-feel. This enables the real-time feel of typing over virtually any network connection, without requiring frequent transmission intervals. Look and feel of typing is also preserved over variable latency connections including &xep0206;, mobile phone, satellite and long international connections with heavy packet-bursting tendencies.</p>
<section3topic="Time Critical and Low Latency Methods"anchor="time_critical_and_low_latency_methods">
<p>There are specialized situations such as live transcriptions and captioning (e.g. transcription service, closed captioning provider, captioned telephone, Communication Access Realtime Translation (CART), relay services) that demands low latency transmission. Such systems typically use voice recognition and/or stenotype machines, which output text in word bursts rather than a character at a time. It is acceptable for senders with bursty output to immediately transmit word bursts of text without buffering. This eliminates any lag caused by the <linkurl="#transmission_interval">Transmission Interval</link>. It is not necessary to transmit <linkurl="#element_w_wait_interval">Element <w/>– Wait Interval</link> for real-time transcription.</p>
<p>Some software platforms (e.g. JavaScript, BOSH, mobile devices) may have low-precision timers that impact <linkurl="#transmission_interval">Transmission Interval</link> and/or <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link>. Clients can optimize for bandwidth, performance and/or screen repaints by eliminating, merging, or ignoring <linkurl="#element_w_wait_interval">Element <w/>– Wait Interval</link> selectively, especially those containing shorter intervals. In addition, it is acceptable for the transmission interval of <rtt/> to vary, either intentionally for optimizations, or due to precision limitation, preferably within the range recommended by <linkurl="#transmission_interval">Transmission Interval</link>. Compression can also be used to reduce bandwidth (e.g. TLS compression or &xep0138;) .</p>
<p>Clients can choose to implement alternate text-smoothing methods, such as adaptive-rate character-at-a-time output, and/or word buffering for incoming real-time text. Word buffering prevents most typing mistakes from being displayed, which can be a useful mode of operation for certain recipients who may dislike watching the sender's typing mistakes.</p>
<p>There are many possible ways to implement turning on/off real-time text. Clients can send outgoing real-time text by default. Other clients might choose to do user-initiated activation (e.g. via a button).</p>
<p>Sender clients can <strong>simply begin transmitting real-time text</strong> (i.e. send <rtt/> elements), either immediately or upon user-initiated activation.</p>
<p>For one-on-one chats, it can be beneficial for clients to easily synchronize the enabling/disabling of real-time text. Upon receiving incoming real-time text, recipient clients can automatically do an appropriate response, such as:</p>
<li>Display only incoming real-time text (e.g. <linkurl="#multiuser_chat">Multi-User Chat</link> participants control their own outgoing real-time text).</li>
<p>For any client, the preferred first <rtt/> element to send is <rtt <linkurl="#event">event</link>='init'/> as it can quickly signal activation of real-time text, without waiting for the sender to begin composing a new message, and since it is usable regardless of discovery.</p>
<p>Before sending real-time text, it is preferable for a sender client to explicitly discover whether the recipient client supports the protocol defined herein (using <linkurl="#determining_support">Determining Support</link>). In the absence of explicit discovery or negotiation, the sender client can implicitly request and discover the use of real-time text, by sending <rtt event='init'/> upon activation. It is inappropriate to send any further <rtt/> elements, until support is confirmed either by incoming <rtt/> elements or via discovery. Implicit discovery allows usage of real-time text as an enhancement to &xep0085; (XEP-0085 Section 5.1), during all situations where it can be used. See <linkurl="#usage_with_chat_states">Usage with Chat States</link>.</p>
<p>Real-time text can be deactivated by transmitting <rtt <linkurl="#event">event</link>='cancel'/>, or simply by ending the chat session. Recipient clients can respond to deactivation with appropriate response(s), including:</p>
<p>Sending an <rtt event='cancel'/> is also useful in situations where the user closes a chat window, and ends the chat session. It is also useful when the user wants to deactivate or deny real-time text, while still continuing the chat session. After <rtt event='cancel'/>, any client can reactivate real-time text again using <rtt event='init'/>.</p>
<p>Recipient clients might choose to display a cursor (or caret) within incoming real-time messages. This enhances usability of real-time text further, since it becomes easier for a recipient to observe the sender's real-time message edits.</p>
<p>If a remote cursor is not used, then clients can simply ignore calculating a cursor position and skip this section. <linkurl="#action_elements">Action Elements</link> only use absolute positioning (relative positions are not used by this standard), so clients do not need to remember the position value from previous action elements.</p>
<p>When <t/> or <e/> action elements are processed in incoming real-time text, the beginning value for the cursor position calculation is the absolute position value of the <em><strong>p</strong></em> attribute, according to <linkurl="#attribute_values">Attribute Values</link>. The recipient can calculate the cursor position as follows:</p>
<li><p>After <linkurl="#element_t_insert_text">Element <t/>– Insert Text</link>, the cursor position is the <em><strong>p</strong></em> attribute plus the length of the text being inserted. The cursor position is put at the end of the inserted text.<br/>
<li><p>After <linkurl="#element_e_erase_text">Element <e/>– Erase Text</link>, the cursor position is the <em><strong>p</strong></em> attribute minus the <em><strong>n</strong></em> attribute.<br/>
<li><p>After an empty <linkurl="#element_t_insert_text">Element <t/>– Insert Text</link> (in the format of <strong><t p='#'/></strong> with no text to insert), the cursor position is the <strong>p</strong> attribute, and no text modification is done.<br/>
<em>This allows cursor response to arrow keys and/or mouse repositioning the cursor.</em></p></li>
<p>The remote cursor needs to be clearly distinguishable from the sender's real local cursor. One example is to use a non-blinking cursor, easily emulated with a Unicode character or the vertical bar character '|'.</p>
<p>It is acceptable for the sender to transmit <linkurl="#element_t_insert_text">Element <t/>– Insert Text</link> as empty elements (with the cursor position in the <em><strong>p</strong></em> attribute) whenever the cursor position is changing without any text modifications (i.e. via arrow keys or mouse). This allows recipients supporting a remote cursor, to show the cursor movements. These extra elements are ignored by recipients that do not support a remote cursor.</p>
<p>This section lists several possible methods of generating real-time text for transmission. For most situations, the preferred methodology is <linkurl="#monitoring_message_changes_instead_of_key_presses">Monitoring Message Changes Instead Of Key Presses</link>.</p>
<section3topic="Monitoring Message Changes Instead Of Key Presses"anchor="monitoring_message_changes_instead_of_key_presses">
<p>Experience has found that the most reliable method for generating real-time text, is to monitor for <strong>text changes</strong> to the sender’s message entry field, instead of key press events. Text change events have the following advantages:</p>
<ul>
<li>It captures all typing, including edits and deletes.</li>
<li>It captures copy & paste operations, as well as edits made via a pointing device.</li>
<li>It captures all automatic text changes (e.g. spell checker, auto-correct, macros, transcription, assistive devices).</li>
<p>During a text change event, the sender’s current message text can be compared to the old message text from the previous text change event. The difference in text, between consecutive text change events, is typically a one character difference (e.g. key press) or one text block difference (e.g. auto-correct, cut, paste). In order to calculate what text changes took place, the first changed character and the last changed character is determined. From this, it is simple to generate <linkurl="#action_elements">Action Elements</link> for a single text block deletion and/or insertion. In addition, if <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link> is supported, then <linkurl="#element_w_wait_interval">Element <w/>– Wait Interval</link> records the time elapsed between text change events.</p>
<li><p>Monitor for text changes in the sender’s message. Whenever a text change event occurs, compute action element(s) and append these action element(s) to a buffer. Repeating this step during every text change event, is equivalent to recording a small sequence of typing.</p></li>
<li><p>During every <linkurl="#transmission_interval">Transmission Interval</link>, all buffered action elements are transmitted in <rtt/> element in a <message/> stanza. This is equivalent to transmitting a small sequence of typing at a time.</p></li>
<li><p>If there are no message changes occurring, no unnecessary transmission takes place.</p></li>
<p>Real-time text can be generated via monitoring key presses. However, this does not have the advantages of <linkurl="#monitoring_message_changes_instead_of_key_presses">Monitoring Message Changes Instead Of Key Presses</link>. Care needs be taken with automatic changes to the message, generated by means other than key presses. This includes spell check auto-correct, copy and pastes, transcription, input method editors, and multiple key presses required to compose a character (i.e. accents). Key press events can miss these text changes, and this can potentially cause incorrect real-time text to be transmitted.</p>
<p>The use of <linkurl="#element_t_insert_text">Element <t/>– Insert Text</link> without any attributes, simply appends text to the end of a message, while the use of <linkurl="#element_e_erase_text">Element <e/>– Erase Text</link> without any attributes, simply erases text from the end of the message. This sending method can also be useful for special-purpose clients where mid-message editing capabilities are not used (e.g. simple transcription, news tickers, relay services, captioned telephone).</p>
<p>It is possible for sender clients to use <linkurl="#message_reset">Message Reset</link> to retransmit the whole real-time message, whenever there are text changes. The advantage is very simple implementation. Disadvantages can include the lack of <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link>, and extra bandwidth consumption that can occur with longer messages, unless stream compression is used. The below illustrates transmission of the real-time message “<strong>Hello there!</strong>” at a regular <linkurl="#transmission_interval">Transmission Interval</link> while the sender is typing.</p>
<p>In order to allow <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link> in incoming real-time text, recipient clients can do the following:</p>
<li><p>Upon receiving <linkurl="#action_elements">Action Elements</link> in incoming <rtt/> elements, they are added to a queue in the order they are received. This provides immunity to variable network conditions, since the queuing action will smooth out incoming transmission (e.g. receiving new <rtt/> while still processing a delayed <rtt/>).</p></li>
<li><p>The recipient client processes action elements in the queue in sequential order, including pauses from <linkurl="#element_w_wait_interval">Element <w/>– Wait Interval</link>. This is equivalent to playing back the sender's original typing.</p></li>
<li><p>Upon receiving a <linkurl="#body_element">Body Element</link> indicating a completed message, the full message text from <body/> can be displayed immediately in place of the real-time message, and unprocessed action elements can be cleared from the queue. This ensures final message delivery is not delayed by late processing of action elements.</p></li>
<p>In processing <linkurl="#element_w_wait_interval">Element <w/>– Wait Interval</link>, excess lag in incoming real-time text might occur if multiple delayed <rtt/> elements suddenly get delivered (e.g. congestion, intermittent wireless reception). Recipients can avoid excess lag by monitoring the queue for excess <w/> action elements (e.g. unprocessed <w/> elements from two <rtt/> elements ago) and then ignoring or shortening the intervals in <w/> elements. This allows lagged real-time text to "catch up" more quickly.</p>
<p>A large sequence of action elements can result in an <rtt/> larger than the size of a message <body/>. This can occur normally during fast typing when <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link> during small messages. However, if the <rtt/> element becomes unusually huge (e.g. macros, multiple copy and pastes, leading to an <rtt/> exceeding one kilobyte) a <linkurl="#message_reset">Message Reset</link> can instead be used, in order to save bandwidth. (Stream compression is another approach.)</p>
<p>Clients can limit the length of the text input for the sender's message, in order to keep the size of <message/> stanzas reasonable, including during <linkurl="#message_reset">Message Reset</link>. Also, large <rtt/> elements may occur in situations such as large copy and pastes. To keep message stanza sizes reasonable, <rtt/> can be transmitted in a separate <message/> than the one containing <body/>.</p>
<p>For specialized clients that send continuous real-time text (e.g. news ticker, captioning, transcription, TTY gateway), a <linkurl="#body_element">Body Element</link> can be sent and then a new real-time message started immediately after, every time a message reaches a reasonable size. This allows continuous real-time text without real-time messages becoming excessively large.</p>
<p>Real-time text can be used in conjunction with XEP-0085 Chat States. These are simple guidelines for <message/> stanzas that include an <rtt/> element:</p>
<p>The in-band nature of this real-time text standard allows one-to-many situations. Thus, real-time text is appropriate for use with &xep0045; (MUC), as well as concurrent simultaneous logins.</p>
<p>For simplicity, clients can implement real-time text only for one-on-one chat, and not for MUC. However, it can be appropriate to support <rtt/> elements in MUC, even if not all participants support real-time text. Participants that enable real-time text during group chat need to keep track of multiple concurrent real-time messages on a per-participant basis. Participants, with real-time text, will see real-time text coming from each participant that has real-time text enabled. Participant clients without real-time text (whether unsupported or turned off) will simply see group chat function normally on a line-by-line basis, since it is <linkurl="#backwards_compatible">Backwards Compatible</link>.</p>
<p>Participants that turn off real-time text for themselves, can simply ignore incoming <rtt/> and not transmit outgoing <rtt/>. Participant clients in MUC receiving an incoming <rtt <linkurl="#event">event</link>=’cancel’/> needs to keep outgoing transmission unaffected during <linkurl="#deactivation_guidelines">Deactivation Guidelines</link> (otherwise, one participant could deny real-time text between other willing participants).</p>
<p>To minimize on-screen clutter of multiple idle real-time messages, clients can hide idle messages, clear old <linkurl="#stale_messages">Stale Messages</link>, and/or prioritize the display of the most useful real-time messages. Prominent visibility of real-time text can be assigned to recent typists and/or moderators (e.g. classroom teacher, convention speaker). For the same participant logged in multiple times in the same room, see <linkurl="#simultaneous_logins">Simultaneous Logins</link>. In situations of simultaneous typing by a large number of participants, see <linkurl="#congestion_considerations">Congestion Considerations</link>.</p>
<p>In simultaneous login situations, transmitting of <rtt/> works in one-to-many situations without any special software support. For many-to-one situations where there is incoming <rtt/> from more than one simultaneous login, <linkurl="#keeping_realtime_text_synchronized">Keeping Real-Time Text Synchronized</link> will pause the real-time message upon conflicting <rtt/>, and resume during the next <linkurl="#message_reset">Message Reset</link>, presumably from the active login. This provides a seamless system-switching experience. A good implementation of <linkurl="#message_reset">Message Reset</link> will improve user experience, regardless of whether or not the client follows Best Practices For Resource Locking (XEP-0296). Clients can choose to distinguish the <rtt/> streams (via full JID and/or via <thread/>) and keep multiple concurrent real-time messages similar in manner to <linkurl="#multiuser_chat">Multi-User Chat</link>, with the <linkurl="#stale_messages">Stale Messages</link> being timed-out.</p>
<p>There are situations where senders pause typing indefinitely. This can result in recipients displaying a real-time message for an extended time period. It may also be a screen clutter concern during <linkurl="#multiuser_chat">Multi-User Chat</link>. In addition, it may be a resource-consumption concern, as part of <linkurl="#congestion_considerations">Congestion Considerations</link>.</p>
<p>It is acceptable for recipients to clear (and/or save) incoming real-time messages that have been idle for an extended time period. There is no specific time-out period defined by this specification. For <linkurl="#multiuser_chat">Multi-User Chat</link>, the time-out period might be shorter because of the need to reduce screen clutter. For normal chat sessions, the time-out period might need to be longer to allow reasonable interruptions (i.e. sender pausing during a long phone call).</p>
<p>Senders that resume composing a message (i.e. continues a partially-composed message hours later) can transmit a <linkurl="#message_reset">Message Reset</link>, which allows recipients to redisplay the real-time message.</p>
<p>With real-time text, frequent screen updates can occur. Screen updates are a potential performance bottleneck, since fast typists type many key presses per second. Optimizing screen updates becomes especially important for slower platforms. The real-time message might be implemented as a separate window or separate display element.</p>
<p>Battery life considerations are closely related to performance, as the addition of real-time text may impact battery life. If <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link> are supported, then the implementation of <linkurl="#element_w_wait_interval">Element <w/>– Wait Interval</link> needs to be implemented in a battery-efficient manner. The <linkurl="#transmission_interval">Transmission Interval</link> may vary dynamically to optimize for battery life and wireless reception. For devices where screen updates are an unavoidable inefficient bottleneck, see <linkurl="#lowbandwidth_and_lowprecision_text_smoothing">Low-Bandwidth and Low-Precision Text Smoothing</link> to reduce the number of screen updates per second.</p>
<p>Most of these examples are deliberately kept simple. In complete software implementations supporting key press intervals, transmissions will most resemble the last example, <linkurl="#full_message_including_key_press_intervals">Full Message Including Key Press Intervals</link>. For simplicity, these examples use a bare JID, even in situations where a full JID might be more appropriate.</p>
<section2topic="Example of Simple Real-Time Text"anchor="example_of_simple_realtime_text">
<p>All three examples shown below result in the same real-time message "<strong>HELLO</strong>" created by writing "<strong>HLL</strong>", backspacing two times, and then "<strong>ELLO</strong>". The action elements are <linkurl="#element_t_insert_text">Element <t/>– Insert Text</link> and <linkurl="#element_e_erase_text">Element <e/>– Erase Text</link>.</p>
<p>The example above sends the misspelled "<strong>HLL</strong>", then <strong><e/><e/></strong> backspaces 2 times, then sends "<strong>HELLO</strong>".</p>
<p>The example above splits the same real-time text over multiple <message/> stanzas, which would occur if the typing was occurring more slowly, over several <linkurl="#transmission_interval">Transmission Interval</link> cycles.</p>
<p>The example above represents moderate typing speed during a normal <linkurl="#transmission_interval">Transmission Interval</link>, such as 0.7 seconds between <message/> stanzas for continuous typing. It illustrates the following:</p>
Note: In most situations, during normal human typing speeds at a normal <linkurl="#transmission_interval">Transmission Interval</link>, smaller fragments of text will be spread over multiple <rtt/> elements, than these demonstration examples below. See <linkurl="#sending_realtime_text">Sending Real-Time Text</link>.</p>
This example outputs "<strong>Hello Bob, this is Alice!</strong>" then <strong><e n='4' p='9'/></strong> erases 4 characters before character position index 9. The <linkurl="#element_e_erase_text">Element <e/>– Erase Text</link> removes the text " <strong>Bob</strong>" including the preceding space character.</p>
This is because this example outputs "<strong>Hello, this is Alice!</strong>" then the <strong><t p='5'></strong> inserts the specified text " <strong>Bob</strong>" at position 5, using <linkurl="#element_t_insert_text">Element <t/>– Insert Text</link>.</p>
This example outputs "<strong>Hello Bob, tihsd is Alice!</strong>", then <strong><e p='16' n='5'/></strong> erases 5 characters at position 16 in the string of text (which erases the mistyped word "<strong>tihsd</strong>"). Finally, <strong><t p='11'>this</t></strong> inserts the text "<strong>this</strong>" place of the original misspelled word.</p>
<p>This example does not illustrate <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link>. Also, it is noted that most situations, during normal typing speeds at a normal <linkurl="#transmission_interval">Transmission Interval</link>, the above series of <linkurl="#action_elements">Action Elements</link> will normally be spread over multiple separate <rtt/> elements.</p>
<section2topic="Examples of Key Press Intervals"anchor="examples_of_key_press_intervals">
<section3topic="Comparison With and Without Intervals"anchor="comparison_with_and_without_intervals">
<p>All examples shown below, result in the same real-time message “<strong>HELLO</strong>”. Only the last example follows <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link>.</p>
<p>The above example outputs “<strong>HELLO</strong>” in a single action element (<linkurl="#element_t_insert_text">Element <t/>– Insert Text</link>).</p>
<p>The above example outputs “<strong>HELLO</strong>” in separate action elements for each key press, while also <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link>. The <linkurl="#element_w_wait_interval">Element <w/>– Wait Interval</link> specifies the number of milliseconds between key presses, to allow smooth presentation in recipient clients that support <w/> action elements.</p>
<p>This example is a transmission of “Hello there!” while <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link>. It illustrates a four-second typing sequence:</p>
<p>The use <linkurl="#element_w_wait_interval">Element <w/>– Wait Interval</link>, between key presses, allows the receiving client execute a small pause between action elements. This allows recipient clients to play back the sender's typing fluidly.</p>
<li>Intervals between key presses are done via <linkurl="#element_w_wait_interval">Element <w/>– Wait Interval</link>.</li>
<li>Each <message/> is delivered at a regular <linkurl="#transmission_interval">Transmission Interval</link>, typically 0.7 seconds.</li>
<li>Cursor movements via empty <t/> elements. Sender transmission is not essential, but can be desirable for recipient clients supporting an <linkurl="#optional_remote_cursor">Optional Remote Cursor</link>.</li>
<li>Recipient clients that do not support <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link> and/or <linkurl="#optional_remote_cursor">Optional Remote Cursor</link>, will still display this message normally.</li>
<li>The total sum of all values in <linkurl="#element_w_wait_interval">Element <w/>– Wait Interval</link> in one <message/> equal the <linkurl="#transmission_interval">Transmission Interval</link> during periods of continuous typing. This also results in some <w/> interval elements being split between consecutive messages. Although not critical, it can further improve the fluidity of <linkurl="#receiving_realtime_text">Receiving Real-Time Text</link>.</li>
<li>See <linkurl="#monitoring_message_changes_instead_of_key_presses">Monitoring Message Changes Instead Of Key Presses</link> for the best method of implementation.</li>
<p>There are other real-time text formats with interoperability considerations relating to the session setup level, the media transport level, and presentation level. Interoperability specifications between multiple real-time text formats can be found at Real-Time Text Taskforce (R3TF).</p>
<p>It is appropriate for implementers to choose the most appropriate real-time text standard for the session control standard in use during a particular session. For example, clients that use XMPP can utilize this XEP-0301 specification, and clients that use SIP might utilize IETF RFC 4103, <spanclass="ref"><strong><linkurl="http://tools.ietf.org/html/rfc5194">IETF RFC 5194</link></strong></span><note>RFC 5194: Framework for Real-Time Text over IP Using the Session Initiation Protocol (SIP). <<linkurl="http://tools.ietf.org/html/rfc5194">http://tools.ietf.org/html/rfc5194</link>>.</note> and ITU-T T.140). Clients that run on multiple networks, might need to utilize multiple real-time text standards. To interoperate between incompatible real-time text standards, gateway servers can transcode between different real-time text standards, along with other media such as audio and video. This can include TTY and textphones.</p>
<p>In the SIP environment, real-time text is specified in IETF RFC 4103 and ITU-T T.140. SIP is a popular real-time session control protocol, and there are many implementations of real-time text controlled by SIP. This includes emergency services in some regions.</p>
<p>Interoperability considerations include addressing translation, media negotiation and translation, and media transcoding. Transcoding is straightforward between this specification and T.140/RFC4103, except for editing in the middle of messages. Text insertions or deletions, occurring far back in the message, can cause a large number of erase operations in T.140 that consume time and bandwidth. T.140 specifies the use of ISO 6429 control codes for presentation characteristics, such as text color, that are not supported by this specification. During transcoding, these control codes needs to be filtered off in order to not disturb the presentation of text.</p>
<section2topic="Total Conversation – Combination with Audio and Video"anchor="total_conversation_combination_with_audio_and_video">
<p>According to ITU-T Rec. F.703, the “Total Conversation” accessibility standard defines the simultaneous use of audio, video, and real-time text. This is more widespread in certain regions (e.g. Reach 112 in Europe). For convenience, messaging applications can be designed to have automatic negotiation of as many as possible of the three media preferred by the users.</p>
<p>In the XMPP session environment, the Jingle protocol (&xep0166;) is available for negotiation and transport of the more time-critical, real-time audio and video media. Any combination of audio, video, and real-time text can be used together simultaneously.</p>
<p>The primary internationalization consideration involve real-time message editing using <linkurl="#action_elements">Action Elements</link>, where text is inserted and deleted using position and length values. For this, <linkurl="#accurate_processing_of_action_elements">Accurate Processing of Action Elements</link> including correct <linkurl="#unicode_character_counting">Unicode Character Counting</link> will ensure that all possible valid Unicode text can be used via this protocol. This includes text containing multiple scripts/languages, ideographic symbols (e.g. Chinese), right-to-left text (e.g. Arabic), and bidirectional text.</p>
<p>For accessibility considerations, there is an <spanclass="ref"><strong><linkurl="http://www.fasttext.org">International Symbol of Real-Time Text</link></strong></span>
<note>The International Symbol of Real-Time Text <<linkurl="http://www.fasttext.org">http://www.fasttext.org</link>>.</note> to alert users to the existence of this feature.</p>
<p>It is important for users to be made aware of real-time text (e.g. user consent, software notice, introductory explanation). Users of real-time text needs to be aware that their typing is now visible in real-time to everyone in the current chat conversation. This may have security implications if users copy & paste private information into their chat entry buffer (e.g. a shopping invoice) before editing out the private parts of the pasted text (e.g. a credit card number) before they send the message. With real-time message editing, recipients can watch all text changes that occur in the sender's text, before the sender finishes the message. Implementation behaviors and improved education can be added to reduce privacy issues. Examples include showing an introduction upon first activation of feature, special handling for copy and pastes (i.e. preventing them, or prompting for confirmation), recipient confirmation of real-time text via <linkurl="#activating_and_deactivating_realtime_text">Activating and Deactivating Real-Time Text</link>, etc.</p>
<p>Real-time text (<rtt/> elements) transmit the content contained within messages. Therefore, a client that encrypts <body/> also needs to encrypt <rtt/> as well:</p>
<li><p>Encryption at the stream level (e.g. TLS) can be used normally with this specification. Stream-level encryption is the most common form of encryption.</p></li>
<li><p>Encryption at the <message/> stanza level (e.g. XEP-0200) can be used for all stanzas containing either <rtt/> or <body/>. It is noted that real-time text can have a higher rate of message stanzas, contributing to additional overhead. See <linkurl="#congestion_considerations">Congestion Considerations</link>.</p></li>
<li><p>Encryption at the <body/> level (e.g. deprecated XEP-0027) do not encrypt <rtt/>. In this case, <rtt/> needs to be encrypted separately. It is preferable to use a broader level of encryption, where possible.</p></li>
<p>The nature of real-time text can result in more frequent transmission of <message/> stanzas than would otherwise happen in a non-real-time text conversation. This can lead to increased network and server loading of XMPP networks.</p>
<p>Transmission of real-time text can be throttled temporarily during poor network conditions. It is appropriate to use latency monitoring mechanisms (e.g. &xep0184; or &xep0198;) in order to temporarily adjust the <linkurl="#transmission_interval">Transmission Interval</link> of real-time text beyond the recommended range. This results in lagged text (less real-time) but is better than failure during poor network conditions. The use of <linkurl="#message_reset">Message Reset</link> can also retransmit real-time text lost by poor network conditions, including stanzas dropped during a network issue or server error. These techniques are useful for mission-critical applications such as next generation emergency services (e.g. text to 9-1-1).</p>
<p>Excess numbers of real-time messages (e.g. during DoS scenario in <linkurl="#multiuser_chat">Multi-User Chat</link>) might cause local resource-consumption issues, which can be mitigated by accelerated time-out of <linkurl="#stale_messages">Stale Messages</link>.</p>
<p>According to multiple university studies worldwide, the average length of instant messages is under 40 characters. The additional incremental bandwidth overhead of real-time text can be very low for an existing XMPP client, especially one already using many extensions. Bandwidth can also be further mitigated using stream compression, to benefit bandwidth-constrained networks (e.g. GPRS, 3G, satellite).</p>
<p>The XMPP Registrar should include "urn:xmpp:rtt:0" in its registry of protocol namespaces (see <<linkurl="http://xmpp.org/registrar/namespaces.html">http://xmpp.org/registrar/namespaces.html</link>>).</p>
<p>If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.</p>
<p>The members of the Real-Time Text Taskforce (R3TF), <linkclass="western"href="http://www.realtimetext.org/">www.realtimetext.org</link>, made significant contributions to this specification. Mark Rejhon leads the Jabber/XMPP Taskgroup at R3TF. Members of R3TF who have contributed to this specification include Gunnar Hellstrom, Paul E. Jones, Gregg Vanderheiden, Barry Dingle, and Arnoud van Wijk. Others contributors include Bernard Aboba, Mark Grady, Darren Sturman, Christian Vogler, Norm Williams, and several members from the XMPP Standards Mailing List, including Kevin Smith, Peter Saint Andre and many others.</p>
<p>The technique of <linkurl="#preserving_key_press_intervals">Preserving Key Press Intervals</link>, otherwise called "natural typing", was created by Mark Rejhon, who is deaf. It is incorporated into this specification in compliance of the XSF's Intellectual Property Rights Policy at <linkclass="western"href="http://xmpp.org/extensions/ipr-policy.shtml">http://xmpp.org/extensions/ipr-policy.shtml</link>.</p>
