VMS Help Sys Parameters, NISCS_MAX_PKTSZ  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    This parameter specifies an upper limit on the size, in bytes,
    of the user data area in the largest packet sent by NISCA on any
    local area network (LAN).

    NISCS_MAX_PKTSZ allows the system manager to change the packet
    size used for cluster communications on network communication
    paths. PEDRIVER automatically allocates memory to support
    the largest packet size that is usable by any virtual circuit
    connected to the system up to the limit set by this parameter.
    Its default values are different for OpenVMS Alpha and OpenVMS
    VAX:

    o  On Alpha, to optimize performance, the default value is the
       largest packet size currently supported by OpenVMS.

    o  On VAX, to conserve memory, the default value is the Ethernet
       packet size.

    PEDRIVER uses NISCS_MAX_PKTSZ to compute the maximum amount of
    data to transmit in any LAN packet:

    LAN packet size <= LAN header (padded Ethernet format)
                       + NISCS_MAX_PKTSZ
                       + NISCS checksum (only if data checking is enabled)
                       + LAN CRC or FCS

    The actual packet size automatically used by PEDRIVER might be
    smaller than the NISCS_MAX_PKTSZ limit for any of the following
    reasons:

    o  On a per-LAN path basis, if PEdriver determines that the LAN
       path between two nodes, including the local and remote LAN
       adapters and intervening LAN equipment, can only convey a
       lesser size.

       In other words, only nodes with large-packet LAN adapters
       connected end-to-end by large-packet LAN equipment can use
       large packets. Nodes connected to large-packet LANs but having
       an end-to-end path that involves an Ethernet segment restrict
       packet size to that of an Ethernet packet (1498 bytes).

    o  For performance reasons, PEDRIVER might further limit
       the upper bound on packet size so that the packets can be
       allocated from a lookaside list in the nonpaged pool.

    The actual memory allocation includes the required data structure
    overhead used by PEDRIVER and the LAN drivers, in addition to the
    actual LAN packet size.

    The following table shows the minimum NISCS_MAX_PKTSZ value
    required to use the maximum packet size supported by specified
    LAN types.

    Type of
    LAN         Minimum Value for NISCS_MAX_PKTSZ

    Ethernet    1498
    FDDI        4468
    Gigabit     7532
    Ethernet
    ATM         7606

    On Alpha systems, NISCS_MAX_PKTSZ is an AUTOGEN parameter.