nx-x11proto-core-dev 2:3.5.99.16-1 (amd64 binary) in ubuntu cosmic

 NX is a software suite which implements very efficient
 compression of the X11 protocol. This increases performance when
 using X applications over a network, especially a slow one.
 .
 This package provides development headers describing the wire protocol
 for the core nx-X11 protocol, and also provides a number of utility headers,
 used to abstract OS-specific functions.