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NISCS_MAX_PKTSZ is a system parameter that 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 packetsize used for cluster communications on network communicationpaths. PEDRIVER automatically allocates memory to supportthe largest packet size that is usable by any virtual circuitconnected to the system up to the limit set by this parameter. OnAlpha and Integrity servers, to optimize performance, the defaultvalue is the largest packet size currently supported by OpenVMS. PEDRIVER uses NISCS_MAX_PKTSZ to compute the maximum amount ofdata 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 besmaller than the NISCS_MAX_PKTSZ limit for any of the followingreasons:

  • On a per-LAN path basis, if PEdriver determines that the LANpath between two nodes, including the local and remote LANadapters and intervening LAN equipment, can only convey alesser size. In other words, only nodes with large-packet LAN adaptersconnected end-to-end by large-packet LAN equipment can uselarge packets. Nodes connected to large-packet LANs but havingan end-to-end path that involves an Ethernet segment restrictpacket size to that of an Ethernet packet (1498 bytes).
  • For performance reasons, PEDRIVER might further limitthe upper bound on packet size so that the packets can beallocated from a lookaside list in the nonpaged pool.

The actual memory allocation includes the required data structureoverhead 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 Minimum Value for NISCS_MAX_PKTSZ
Type of LAN Minimum Value for NISCS_MAX_PKTSZ
Ethernet 1498
FDDI 4382 (before Version 7.3)
4396 (Version 7.3 and later)
Gigabit Ethernet 8192
ATM 7606

Note that the maximum packet size for some Gigabit Ethernetadapters is larger than the maximum value of NISCS_MAX_PKTSZ(8192 bytes). See the LAN_FLAGS parameter for a description ofhow to enable jumbo frames on Gigabit Ethernet-that is, packetsizes larger than those noted for Ethernet. On Alpha and Integrity servers, NISCS_MAX_PKTSZ is an AUTOGEN parameter.