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DeltaSync is a proprietary Microsoft communications protocol for synchronizing web services with offline clients.[1] Windows Live Hotmail is exposed to offline clients using DeltaSync; Outlook Connector and the Windows Live Mail client use it for offline access. Microsoft is pushing for this protocol instead of POP3 or IMAP4 for supporting various advanced features, like contacts, calendar, and notes synchronization. Using the DeltaSync protocol, Microsoft intends to provide a common platform for any compatible device to implement a client which can two-way synchronize mail, contacts, calendar, and notes.


In January 2007, Microsoft announced DeltaSync as the replacement for WebDAV. In April 2008, Microsoft sent an email warning that WebDAV would no longer be supported after 30 June 2008.[2] However, in May 2008, Microsoft stated that it would be "postponing the transition deadline previously announced" to allow customers more time to "evaluate alternative solutions".[3] In June 2009, Microsoft announced that WebDAV support would finally end on 1 September 2009.[4]

Format details[edit]

The DeltaSync compression code produces a stream with a four byte magic number of "HU01", followed by the stream header.

HU01 header[edit]

Offset Description
0 "HU01" magic number
4 Header size (should be greater than or equal to 28)
8..31 Unknown
32 Decompressed file size

Following this header is a series of compressed data blocks with the following structure.

SCBH header[edit]

Offset Description
0 "SCBH" magic number
4 Header size
8 Decompressed block size
12 Block CRC?
16 Compressed block size

This is followed by 256 bytes of decompression table and finally the actual compressed data.

See also[edit]


External links[edit]