SyncML works by exchanging commands, which can be requests and responses. As an example:
the mobile sends an Alert command for signaling the wish to begin a refresh-only synchronization
the computer responds with a Status command for accepting the request
the mobile sends one or more Sync command containing an Add sub-command for each item ; if the number of entries is large, it does not include the tag;
in the latter case, the computer requests to continue with an appropriate Alert message, and the mobile sends another chunk of items; otherwise, the computer confirms it received all data with a Status command
Commands are grouped into messages. Each message and each of its commands has an identifier, so that the pair MsgID,CmdID uniquely determine a command. Responses like Status commands include the pair identifying the command they are responding to. Before commands, messages contain a header specifying various data regarding the transaction. An example message containing the Alert command for begin a refresh synchronization, like in the previous example, is:
1.1 SyncML/1.1 1 1 PC Suite IMEI:3405623856456 8000
1 203 Events /telecom/cal.vcs 4242
The response from the computer could be an xml document like :
1.1 SyncML/1.1 1 1 IMEI:3405623856456 PC Suite
1 1 0 SyncHdr PC Suite IMEI:3405623856456 200
2 1 1 Alert Events /telecom/cal.vcs 00 200
The transaction then proceeds with a message from the mobile containing the Sync 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 initial Alert command can be used to initiate other kinds of synchronizations. For example, in a "two-way sync", only the changes from the last synchronization are sent to the computer, which does the same. The Last and Next 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, a mobile may use progressive numbers 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 with Last 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 sync case, in a normal sync, each device sends all changes since the last synchronization.
