Outbound priorities

For QDIO devices, z/OS® Communications Server and OSA-Express provide a function that assigns a priority value to each outbound datagram and that attempts to provide preferential service to the higher-priority data.

z/OS Communications Server supports four priority values in the range 1–4 for outbound QDIO traffic (with 1 being the highest priority).

TCP/IP uses the first three bits of the Type of Service (ToS) byte in the IP header to determine the outbound priority value for a given datagram. The default mapping of ToS values to priorities is:

ToS	Priority
000	4
001	4
010	3
011	2
100	1
101	1
110	1
111	1

You can use the z/OS UNIX Service Policy Agent to override the default mapping of ToS values to priorities. (See the description of the SetSubnetPrioTosMask statement in z/OS Communications Server: IP Configuration Guide for details on how to override the default priority for a given ToS setting.)

Note: You cannot use a virtual IP address (VIPA) as the SubnetAddr value on the SetSubnetPrioTosMask statement.