Networking on z/OS
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Class of service (CoS) and type of service (ToS)

Networking on z/OS

One of the biggest issues facing those who want to transport SNA over an IP network is the question of maintaining SNA's class of service (CoS). In SNA, the class of service specified for a particular session is used to determine both the route taken by the session and the transmission priority allotted to it.

With an IP backbone, the route is essentially unpredictable because of IP's connectionless property. However, IP provides for a transmission priority using the precedence bits in the IP header. Many routers now support the use of these bits. However, in the past, they tended to use the TCP or UDP port number as a means of assigning priorities to packets.

Enterprise Extender supports the use of both precedence bits and port numbers to inform the IP network of the transmission priority. You should use precedence bits because they are in the IP header. The UDP or TCP port numbers are carried inside the IP datagram, so encrypted packets have unreadable port numbers, and fragmented packets have no port numbers after the first fragment. For such encrypted or fragmented packets, intermediate routers cannot determine the appropriate priority.

IP type of service (ToS)

The IP header ToS field assigns the priority of the IP packet. Routers along the data path forward packets with higher type of service (ToS) values prior to forwarding lower priority ToS packets.

IP type of service (ToS) is related to the APPN class of service (CoS) and the UDP port numbers used for Enterprise Extender traffic. The APPN CoS specifies a transmission priority (with the keyword PRIORITY), which can be one of the data values listed in Table 1, with their corresponding UDP ports.

Table 1. ToS values and corresponding priorities
SNA priority IP ToS UDP port
LLC2 commands B'110' 12000
Network B'110' 12001
High (TP=2) B'100' 12002
Medium (TP=1) B'010' 12003
Low (TP=0) B'001' 12004

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