126.96.36.199.1 Abstract Data Model
The shared state necessary to support the transmission and reception of RDP6.0-BC compressed data between a client and server requires a history buffer and a current offset into the history buffer (HistoryOffset). The size of the history buffer is 65,536 bytes. Any data that is being compressed MUST be smaller in size than the history buffer. The HistoryOffset MUST start initialized to zero, while the history buffer MUST be filled with zeros. After it has been initialized, the entire history buffer is immediately regarded as valid.
In addition to the history buffer and HistoryOffset, a small cache MUST also be managed by the client and server endpoints. This cache is referred to as the OffsetCache and is used to store the last four unique copy-offsets encountered during data compression (copy-offsets are described in [MS-RDPBCGR] section 188.8.131.52). This saves on bandwidth in cases where there are many repeated copy-offsets. Whenever the history buffer is initialized or reinitialized, the OffsetCache MUST be emptied.
When compressing data, the sender MUST first check that the uncompressed data can be inserted into the history buffer at the position in the history buffer given by the HistoryOffset. If the data will not fit into the history buffer (the sum of the HistoryOffset and the size of the uncompressed data exceeds the size of the history buffer), then the history buffer MUST be slid back by half its size and the HistoryOffset MUST be reset to the middle of the history buffer (offset 32768). If the data will fit into the history buffer, the sender endpoint inserts the uncompressed data at the position in the history buffer given by the HistoryOffset, and then advances the HistoryOffset by the amount of data added.
As the receiver endpoint decompresses the data, it inserts the decompressed data at the position in the history buffer given by its local copy HistoryOffset. If a slide-back occurs, the sender endpoint MUST notify the target receiver so it can reset its local state. In this way, the sender and receiver endpoints maintain an exact replica of the history buffer and HistoryOffset.