This REQUIRED attribute is a counter to maintain the integrity of real-time text. (The bounds of seq is 31-bits, the range of positive values of a signed integer.)

Senders MUST increment the seq attribute by 1 in each subsequent <rtt/> transmitted without an event attribute. When an <rtt/> element has an event attribute, senders MAY instead use any value as the new starting value for seq. A random starting seq value is RECOMMENDED for best integrity during Usage with Multi-User Chat and Simultaneous Logins. Senders SHOULD limit the size of the new starting seq value, to keep <rtt/> compact while allowing plenty of incrementing room, and prevent wraparound from ever happening.

Recipients MUST monitor the seq value to verify the integrity of real-time text. See Keeping Real-Time Text Synchronized.

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 event values. The use of <rtt/> elements without an event attribute is for transmitting changes to an existing real-time message. The event attribute is used in the situations specified below:

event='new'
Senders MUST use this value when transmitting the first <rtt/> element containing Action Elements (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 event attribute.

event='reset'
For recipients, both 'new' and 'reset' 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 event attribute. Recipients MUST be able to process 'reset' without first receiving 'new'. See Message Reset, used for Keeping Real-Time Text Synchronized and Basic Real-Time Text.

event='init'
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 Activating and Deactivating Real-Time Text.

event='cancel'
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 'init' is used again). See Activating and Deactivating Real-Time Text.

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.

This id attribute refers to the <message/> stanza containing the <body/> that is being edited (See 'Business Rules' in XEP-0308). When used, id 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'/> (Message Reset) 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).

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.

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 Preserving Key Press Intervals). For detailed information, see List of Action Elements.

• For Element <e/> – Erase Text:
  The n attribute is a length value, in number of characters. If n is omitted, the default value of n MUST be 1.

• For Element <t/> – Insert Text and Element <e/> – Erase Text:
  The p 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 p is omitted, the default value of p MUST point to the end of the message (i.e. p is set to the current length of the real-time message).

• For the purpose of this specification, the word "character" represents a single Unicode code point. See Unicode Character Counting.

• Senders MUST NOT use negative values for any attribute, nor use p values beyond the current message length. However, recipients receiving such values MUST clip negative values to 0, and clip excessively high p values to the current length of the real-time message. Modifications only occur within the boundaries of the current real-time message.

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. Basic Real-Time Text). Support for <w/> is RECOMMENDED for both senders and recipients, in order to accommodate Preserving Key Press Intervals. Recipients MUST ignore unexpected or unsupported elements within <rtt/>, while continuing to process subsequent action elements (Compatibility is ensured via Namespace Versioning). Action elements are immediate child elements of the <rtt/> element, and are never nested. See examples in Use Cases.

For <rtt/> elements that do not contain an event attribute:

1. Senders MUST increment the seq attribute in steps of 1, for consecutively transmitted <rtt/> elements.
2. Recipients MUST monitor the seq attribute value of received <rtt/> elements, to verify that it is incrementing by 1.

Loss of sync occurs if the seq attribute do not increment by 1 as expected when Staying In Sync. In this case:

• Recipients MUST keep the pre-existing real-time message unchanged; and
• Recipients MUST ignore action elements within the current and subsequent <rtt/> elements; and
• An indication can be used to show the loss of sync (e.g. color coding, modified chat state message).

Recovery occurs when the recipient receives the following:

• A <body/> element. The Body Element supersedes the real-time message.
• An <rtt/> element containing an event attribute (e.g. new message, or Message Reset).

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.

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: 

• When the recipient starts sending messages from a different full JID (e.g. switched systems);
• When the recipient sends a presence update (e.g. from offline to online);
• When the sender resumes composing after an extended pause (e.g. recipient may have cleared Stale Messages);
• When the conversation is unlocked (e.g. section 5.1 of &xmppim;);

A message reset is done using the <rtt/> attribute event value of 'reset' (see RTT Attributes).

This is a retransmission of the entire real-time message. ]]>

Note: There are no restrictions on using multiple Action Elements during a message reset (e.g. typing or backspacing occurring at the end of a retransmitted message).

For this specification, a "character" represents a single Unicode code point. This is the same definition used in section 1.1 of IETF RFC 5198 RFC 5198: Unicode Format for Network Interchange. <http://tools.ietf.org/html/rfc5198>.. For platform-independent interoperability of Action Elements, calculations on Attribute Values (p and n) MUST be based on counts of Unicode code points.

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:

• Multiple Unicode code points (e.g. combining marks, accents) can form a combining character sequence.
  Action elements operate on Unicode code points individually.

• Unicode code points U+10000 through U+10FFFF are represented as a surrogate pair in some Unicode encodings (e.g. UTF-16).
  Action elements operate on Unicode code points as a whole, not on separate components of a surrogate pair.

• XMPP transmission uses UTF-8, which use a variable number of bytes per Unicode code point.
  Action elements operate on Unicode code points as a whole, not on separate bytes.

Lengths and positions in Attribute Values 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 Attribute Values as array indexes.

Sender clients MUST generate real-time text (Action Elements and Attribute Values) 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;).

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.

If Unicode combining character sequences (e.g. letter with multiple accents) are used for Element <t/> – Insert Text, 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 Element <e/> – Erase Text 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 Guidelines for Recipients).

For the purpose of calculating Attribute Values, 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 XML XML: Extensible Markup Language 1.0 (Fifth Edition). <http://www.w3.org/TR/xml/>.. In addition, XML character entities are counted as a single character.

For Element <t/> – Insert Text, 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 Action Elements and Attribute Values (The recipient client can separately process/modify a copy of the same real-time message text, if necessary for the purpose of display presentation).

It is possible for Element <t/> – Insert Text 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.

If a long Transmission Interval is used without Preserving Key Press Intervals, then incoming text will appear in intermittent bursts if the display of text is not smoothed. This hurts user experience of real-time text.

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 Element <w/> – Wait Interval between other Action Elements. 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 Examples of Key Press Intervals.

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.

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 Transmission Interval. It is not necessary to transmit Element <w/> – Wait Interval for real-time transcription.

Some software platforms (e.g. JavaScript, BOSH, mobile devices) may have low-precision timers that impact Transmission Interval and/or Preserving Key Press Intervals. Clients can optimize for bandwidth, performance and/or screen repaints by eliminating, merging, or ignoring Element <w/> – Wait Interval 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 Transmission Interval. Compression can also be used to reduce bandwidth (e.g. TLS compression or &xep0138;) .

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.

Sender clients can simply begin transmitting real-time text (i.e. send <rtt/> elements), either immediately or upon user-initiated activation.

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:

• Activate immediately (begin transmitting <rtt/> elements too); or
• Activate after user confirmation prompt (for Privacy considerations); or
• Deny (transmit <rtt event='cancel'/>); or
• Ignore (discard incoming <rtt/> elements); or
• Display only incoming real-time text (e.g. Multi-User Chat participants control their own outgoing real-time text).

For any client, the preferred first <rtt/> element to send is <rtt event='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.

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 Determining Support). 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 Usage with Chat States.

Real-time text can be deactivated by transmitting <rtt event='cancel'/>, or simply by ending the chat session. Recipient clients can respond to deactivation with appropriate response(s), including:

• Stop transmitting <rtt/> elements as well (not applicable to Multi-User Chat); and
• Handle the sender's unfinished incoming real-time message (such as clearing it and/or saving it); and
• Inform the recipient user that sender ended real-time text (or denied/cancelled, if no real-time text was received).

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'/>.

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 p attribute, according to Attribute Values. The recipient can calculate the cursor position as follows:

• After Element <t/> – Insert Text, the cursor position is the p attribute plus the length of the text being inserted. The cursor position is put at the end of the inserted text.
  This is the normal forward cursor movement during text insertion.

• After Element <e/> – Erase Text, the cursor position is the p attribute minus the n attribute.
  This is the normal backwards cursor movement to a backspace key.

• After an empty Element <t/> – Insert Text (in the format of <t p='#'/> with no text to insert), the cursor position is the p attribute, and no text modification is done.
  This allows cursor response to arrow keys and/or mouse repositioning the cursor.

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 '|'.

It is acceptable for the sender to transmit Element <t/> – Insert Text as empty elements (with the cursor position in the p 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.

Experience has found that the most reliable method for generating real-time text, is to monitor for text changes to the sender’s message entry field, instead of key press events. Text change events have the following advantages:

• It captures all typing, including edits and deletes.
• It captures copy & paste operations, as well as edits made via a pointing device.
• It captures all automatic text changes (e.g. spell checker, auto-correct, macros, transcription, assistive devices).
• It captures characters requiring multiple key presses to compose (e.g. accents, combining marks).
• It makes no assumptions about different keyboards or input method editors (e.g. Chinese).
• Text change events are more portable across platforms, including on mobile phones.

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 Action Elements for a single text block deletion and/or insertion. In addition, if Preserving Key Press Intervals is supported, then Element <w/> – Wait Interval records the time elapsed between text change events.

Sender software can do the following:

1. 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.

2. During every Transmission Interval, 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.

3. If there are no message changes occurring, no unnecessary transmission takes place.

Real-time text can be generated via monitoring key presses. However, this does not have the advantages of Monitoring Message Changes Instead Of Key Presses. 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.

The use of Element <t/> – Insert Text without any attributes, simply appends text to the end of a message, while the use of Element <e/> – Erase Text 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).

It is possible for sender clients to use Message Reset to retransmit the whole real-time message, whenever there are text changes. The advantage is very simple implementation. Disadvantages can include the lack of Preserving Key Press Intervals, and extra bandwidth consumption that can occur with longer messages, unless stream compression is used. The below illustrates transmission of the real-time message “Hello there!” at a regular Transmission Interval while the sender is typing.

Hel Hello th Hello there! ]]>

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 Preserving Key Press Intervals 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 Message Reset can instead be used, in order to save bandwidth. (Stream compression is another approach.)

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 Message Reset. 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/>.

For specialized clients that send continuous real-time text (e.g. news ticker, captioning, transcription, TTY gateway), a Body Element 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.

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:

• For <rtt/> transmitted without an accompanying <body/>, include <composing/> chat state.
• For <rtt/> transmitted with an accompanying <body/>, include the <active/> chat state.
• Other chat states are handled as specified by XEP-0085 Chat States.

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.

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 Multi-User Chat. In addition, it may be a resource-consumption concern, as part of Congestion Considerations.

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 Multi-User Chat, 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).

Senders that resume composing a message (i.e. continues a partially-composed message hours later) can transmit a Message Reset, which allows recipients to redisplay the real-time message.

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.

Battery life considerations are closely related to performance, as the addition of real-time text may impact battery life. If Preserving Key Press Intervals are supported, then the implementation of Element <w/> – Wait Interval needs to be implemented in a battery-efficient manner. The Transmission Interval may vary dynamically to optimize for battery life and wireless reception. For devices where screen updates are an unavoidable inefficient bottleneck, see Low-Bandwidth and Low-Precision Text Smoothing to reduce the number of screen updates per second.

Hello Bob, this is Alice! ]]>

Final result of real-time message: "Hello, this is Alice!"
This example outputs "Hello Bob, this is Alice!" then <e n='4' p='9'/> erases 4 characters before character position index 9. The Element <e/> – Erase Text removes the text " Bob" including the preceding space character.

Hello, this is Alice! Bob ]]>

Final result of real-time message: "Hello Bob, this is Alice!"
This is because this example outputs "Hello, this is Alice!" then the <t p='5'> inserts the specified text " Bob" at position 5, using Element <t/> – Insert Text.

Hello Bob, tihsd is Alice! this ]]>

Final result of real-time message: "Hello Bob, this is Alice!"
This example outputs "Hello Bob, tihsd is Alice!", then <e p='16' n='5'/> erases 5 characters at position 16 in the string of text (which erases the mistyped word "tihsd"). Finally, <t p='11'>this</t> inserts the text "this" place of the original misspelled word.

This is an example message containing multiple consecutive real-time message edits. This illustrates valid use of the <rtt/> element.

Helo lo...planet World there, ]]>

Resulting real-time message: "Hello there, World", completed in the following series of action elements:

*The Cursor Position column is only relevant if the Optional Remote Cursor is implemented.

This example does not illustrate Preserving Key Press Intervals. Also, it is noted that most situations, during normal typing speeds at a normal Transmission Interval, the above series of Action Elements will normally be spread over multiple separate <rtt/> elements.

All examples shown below, result in the same real-time message “HELLO”. Only the last example follows Preserving Key Press Intervals.

HELLO ]]>

The above example outputs “HELLO” in a single action element (Element <t/> – Insert Text).

H E L L O ]]>

The above example outputs “HELLO” in separate action elements for each key press.

H E L L O ]]>

The above example outputs “HELLO” in separate action elements for each key press, while also Preserving Key Press Intervals. The Element <w/> – Wait Interval specifies the number of milliseconds between key presses, to allow smooth presentation in recipient clients that support <w/> action elements.

This example is a transmission of “Hello there!” while Preserving Key Press Intervals. It illustrates a four-second typing sequence:

• The misspelled phrase “Hello tehre!” is typed;
• Optional transmission of cursor movements towards the typing mistake;
• Two backspaces to delete the typing mistake;
• Two correct key presses to correctly spell the word “there”.

The use Element <w/> – Wait Interval, 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.

H e l l o t e h r e ! h e Hello there! ]]>

This example also illustrates the following:

• Typing is done via Element <t/> – Insert Text.
• Backspaces are done via Element <e/> – Erase Text.
• There is a final transmission with a Body Element, when the message is finished.
• Intervals between key presses are done via Element <w/> – Wait Interval.
• Each <message/> is delivered at a regular Transmission Interval, typically 0.7 seconds.
• Cursor movements via empty <t/> elements. Sender transmission is not essential, but can be desirable for recipient clients supporting an Optional Remote Cursor.
• Recipient clients that do not support Preserving Key Press Intervals and/or Optional Remote Cursor, will still display this message normally.
• The total sum of all values in Element <w/> – Wait Interval in one <message/> equal the Transmission Interval 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 Receiving Real-Time Text.
• See Monitoring Message Changes Instead Of Key Presses for the best method of implementation.