Customizing IP type of service

EE maps the SNA transmission priority to the IP type of service so that routers can preserve the SNA transmission priority across the IP network.

The IP type of service (ToS) used for IPv4, and the IP traffic class used for IPv6, are related to the transmission priority of the chosen APPN class of service (COS) and the UDP port numbers used for EE traffic.

Tip: For IPv6 protocols, IP traffic class performs the same function as IP ToS does for IPv4 protocols. The term ToS is used in this document to see both functions.

The APPN COS specifies a transmission priority (with the keyword PRIORITY), which can be one of the data values shown in Table 1.

Restriction: Although the IPPORT operand enables you to change the port numbers used for EE, many platforms do not permit this. Therefore, do not change the IPPORT default value.

The UDP port value specified on the IPPORT keyword (default 12000), is reserved for signal data that is involved in establishing and keeping the EE connection active. The next four sequential UDP port numbers are reserved for the SNA traffic priorities (network, high, medium, and low).

IP routers can use the UDP port number to prioritize traffic. Likewise, by default, the ToS values in Table 5 tell IP routers that support this function how to prioritize data if they do not support prioritization by the port number. This also applies in cases where IPSec is being used.

Table 1. SNA priorities and corresponding port numbers and default ToS values
Priority	Port	ToS value
LDLC Signaling *	12000	C0
Network	12001	C0
High	12002	80
Medium	12003	40
Low	12004	20

Note: * Port 12000 carries EE LDLC signaling data and does not correspond to one of the four SNA transmission priorities.

Not all routers support prioritization of traffic based on port number or ToS value. Some might support none and others might support one or both. This is why two methods are needed (both ToS and port number) for the prioritization of HPR traffic in an IP network. In addition, if IPSec is being used, EE data can be encrypted in the IP network. This encryption includes the UDP port numbers, but not the ToS values.

Guideline: For optimum preservation of SNA transmission priority, use prioritization based on ToS value.

Rule: If HPR data prioritization in an IP network is needed while using IPSec, the IP routers in the network must support the ToS function. Also, all routers and EE endpoints must be consistently defined for ToS and the UDP port numbers.