UUCP `G' Protocol
The `G' protocol is used by SVR4 UUCP. It is identical to the `g' protocol, except that it is possible to modify the window and packet sizes. The SVR4 implementation of the `g' protocol reportedly is fixed at a packet size of 64 and a window size of 7. Supposedly SVR4 chose to implement a new protocol using a new letter to avoid any potential incompatibilities when using different packet or window sizes.
Most implementations of the `g' protocol that accept packets larger than 64 bytes will also accept packets smaller than whatever they requested in the `INITB' packet. The SVR4 `G' implementation is an exception; it will only accept packets of precisely the size it requests in the INITB packet.