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dxpc is an X protocol compressor designed to improve the
speed of X11 applications run over low-bandwidth links
(such as dialup PPP connections).
dxpc consists of two processes:
1. a Client Proxy that runs on the "remote" machine (the machine where
the X clients are running)
2. a Server Proxy that runs on the "local" machine (the machine where
the X server is running)
(Starting in the dxpc-3.0, release, the Client Proxy and Server Proxy
are instances of the same program, called "dxpc"; command-line arguments
tell the program whether it is acting as a Client Proxy or a Server Proxy.)
The Client Proxy mimics an X server. X client applications connect
to the Client Proxy using display "unix:8" (or "<hostname>:8"; dxpc
supports both UNIX domain and TCP sockets). The Client Proxy receives
X requests from the application, compresses them, and sends them to
the Server Proxy. The Server Proxy uncompresses the requests and
sends them to the real X server. Similarly, the Server Proxy receives
X events, replies, and errors from the real X server. It compresses
these messages and sends them to the Client Proxy, which uncompresses
them and sends them to the client application.
dxpc attempts to exploit patterns in X protocol messages to limit
the amount of data sent between the Client Proxy and Server Proxy.
For many X message types, each field has a high probability of having
the same value as it had in some previous message of the the same type.
For such fields, dxpc maintains caches of the last 'n' values, with a
least-recently-used replacement policy. If a field value in a new
message is already present in the corresponding cache, dxpc transmits
the value's index within the cache rather than the value itself.
Because the number of bits needed to represent this index is typically
much smaller than the number of bits needed to represent the value
itself, transmission of cache indices typically results in a
significant reduction in the number of bytes transmitted over
the low-bandwidth link.
In other cases, the value of a field in an X message may differ from
that field's value in the last message of the same type by a small
value. Some X messages contain sequence numbers or timestamps that
have this property. X requests that create new objects also tend
to have this property; in a "Create Window" request, for example,
the value of the "Window ID" being created is typically equal to
"(Window ID of the last window created) + (some small positive integer)."
For fields like these, dxpc transmits the difference between the field
value in the new message and the value of the corresponding field in
the previous message of the same type. This value usually is a
small number that can be encoded in far fewer bits than the actual
field value.
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