Tell me how to filter by protocol
Observer’s Protocol Data Field filter rule lets you search for specific values in selected protocol header fields. For example, you can filter for ICMP destination unreachable packets and wireless control, data, and management packets. You can also define your own custom protocol filter, either by port or search pattern.
 
Figure 19: Protocol Filters
 
Click Add and give the protocol filter a descriptive name and choose whether you want to define the protocol by a pattern filter or a port filter. After you click OK, the appropriate filter dialog is displayed allowing you to enter the pattern or port that defines the protocol.