Remote Replication Agent (or RRA) is as the name suggests a protocol used for synchronization. It is used for all synchronization on Windows Mobile devices predating Windows Mobile 2005, but is still used partly on the latter platform for all the non-PIM items (files, favorites, media and notes), whilst AirSync is used for the PIM ones (appointments, contacts, e-mails and tasks).
RemSync, or Remote Sync is a protocol used for status notifications. The device connects to the host on port 999, and the host sends notification messages in the various stages of the synchronization process.
Desktop pass through (or DTPT) is a protocol used for sharing the host's internet connection with the device, ie. a proprietary proxy protocol (yes, another one). The host listens on TCP port 5721 and the device connects to it with one or more clients. The MessageType of the first request initializes the connection to either a NSPSession or a ConnectionSession. A NSPSession is used to perform DNS lookups, whilst a ConnectionSession is used to connect to a given host (a new session is created for every external host that the device wishes to have a session with).
