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.