VMS Help TCPIP Services, Programming Interfaces, Sockets API, $CANCEL, Arguments  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 chan
    OpenVMS usage:channel
    type:         word (unsigned)
    access:       read only
    mechanism:    by value
    I/O channel on which I/O is to be canceled. The chan argument is
    a word containing the channel number.