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SyncML, or Synchronization Markup Language, was originally developed as aplatform-independent standard forinformation synchronization. Established by theSyncML Initiative, this project has evolved to become a key component indata synchronization and device management. The project is currently referred to asOpen Mobile Alliance Data Synchronization and Device Management.^[1] The purpose ofSyncML is to offer anopen standard as a replacement for existingdata synchronization solutions; which have mostly been somewhatvendor,application, oroperating system specific. SyncML 1.0specification was released on December 17, 2000,^[2] and 1.1 on February 26, 2002.^[3]

ASyncML message is a well-formedXML document that adheres to thedocument type definition (DTD), but which does not require validation.

SyncML works by exchangingcommands, which can be requests and responses. As an example:

• themobile phone sends anAlert command for signaling the wish to begin a refresh-onlysynchronization
• thecomputer responds with aStatus command for accepting the request
• the phone sends one or moreSync command containing an Add sub-command for each item (e.g.,phonebook entry); if the number of entries is large, it does not include the <Final/> tag;
• in the latter case, the computer requests to continue with an appropriateAlert message, and the mobile sends another chunk of items; otherwise, the computer confirms it received all data with aStatus command

Commands (Alert,Sync,Status, etc.) are grouped into messages. Each message and each of its commands has an identifier, so that the pair (MsgID, CmdID) uniquely determines a command. Responses likeStatus commands include the pair identifying the command they are responding to.

Before commands, messages contain a header specifying various data regarding thetransaction. An example message containing theAlert command for begin a refresh synchronization, like in the previous example, is:

<?xml version="1.0"?><!DOCTYPE SyncML PUBLIC "-//SYNCML//DTD SyncML 1.2//EN" "http://www.openmobilealliance.org/tech/DTD/OMA-TS-SyncML_RepPro_DTD-V1_2.dtd"><SyncMLxmlns="SYNCML:SYNCML1.2"><SyncHdr><VerDTD>1.1</VerDTD><VerProto>SyncML/1.1</VerProto><SessionID>1</SessionID><MsgID>1</MsgID><Target><LocURI>PCSuite</LocURI></Target><Source><LocURI>IMEI:3405623856456</LocURI></Source><Meta><MaxMsgSizexmlns="syncml:metinf">8000</MaxMsgSize></Meta></SyncHdr><SyncBody><Alert><CmdID>1</CmdID><Data>203</Data><!-- 203 = mobile signals a refresh from it to computer --><Item><Target><LocURI>Events</LocURI></Target><Source><LocURI>/telecom/cal.vcs</LocURI></Source><Meta><Anchorxmlns="syncml:metinf"><Last>42</Last><Next>42</Next></Anchor></Meta></Item></Alert><Final/></SyncBody></SyncML>

The response from thecomputer could be anXML document like (comments addedfor the sake of explanation):

<?xml version="1.0"?><!DOCTYPE SyncML PUBLIC "-//SYNCML//DTD SyncML 1.2//EN" "http://www.openmobilealliance.org/tech/DTD/OMA-TS-SyncML_RepPro_DTD-V1_2.dtd"><SyncML><SyncHdr><VerDTD>1.1</VerDTD><VerProto>SyncML/1.1</VerProto><SessionID>1</SessionID><MsgID>1</MsgID><Target><LocURI>IMEI:3405623856456</LocURI></Target><Source><LocURI>PCSuite</LocURI></Source></SyncHdr><SyncBody><!-- accept the header of the last message from the client --><Status><CmdID>1</CmdID><MsgRef>1</MsgRef><CmdRef>0</CmdRef><!-- 0 = header of the message --><Cmd>SyncHdr</Cmd><TargetRef>PCSuite</TargetRef><SourceRef>IMEI:3405623856456</SourceRef><Data>200</Data><!-- 200 = ok, accepted --></Status><!-- accept the request of the mobile for a sync --><Status><CmdID>2</CmdID><!-- this is command #2 --><MsgRef>1</MsgRef><CmdRef>1</CmdRef><!-- it respond to command msg=1,cmd=1 --><Cmd>Alert</Cmd><TargetRef>Events</TargetRef><SourceRef>/telecom/cal.vcs</SourceRef><Meta><Anchorxmlns="syncml:metinf"><Next>0</Next><Last>0</Last></Anchor></Meta><Data>200</Data><!-- 200 = ok, accepted --></Status><Final/></SyncBody></SyncML>

The transaction then proceeds with a message from the mobile containing theSync command, and so on.

This example is a refresh where the mobile sends all its data to the computer and nothing in the other way around. Different codes in the initialAlert command can be used to initiate other kinds of synchronizations. For example, in a "two-way sync", only the changes from the lastsynchronization are sent to thecomputer, which does the same.

TheLast andNext tags are used to keep track of a possible loss of sync.Last represents the time of the last operation of synchronization, as measured by each device. For example, amobile may use progressive numbers (1,2,3, ...) to represent time, while the computer uses strings like "20140112T213401Z".Next is the current time in the same representation. This latter data is stored and then compared withLast in the next synchronization. Any difference indicates a loss of sync. Appropriate actions involving sending all data can be then taken to put the devices back in sync.

Anchors are only used to detect a loss of sync; they do not indicate which data is to be sent. Apart from the loss-of-synchronization situation, in a normal (non-refresh) synchronization, each device sends alog of changes since the last synchronization.

¹SAN = Server Alert Notification. This SyncML Push technology is based on definitions by theOpen Mobile Alliance and extends the existing SyncML protocol specification by offering a method of server initiated synchronization.

• CalDAV
• CardDAV
• Critical Path SyncML Server
• iCalendar
• The SyncML Initiative
• Yahoo! Mobile andYahoo! Calendar -Yahoo services offered in some countries that uses SyncML technology.

• SyncML
• Open formats
• Computer standards
• Web syndication formats
• XML-based standards

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