Library /sys$common/syshlp/nclhelp.hlb set, routing, phaseiv_buffer_size  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Support: All
    Default: 576           Value: 1-65535

    Size, in octets, of buffers used for routing to adjacent Phase IV
    nodes. This value is actually six octets greater than the maximum
    buffer size, and does not include the headers for the Routing and
    Data Link layers.