Chapter 23 Materials Figure 23.1 A list of ICMP messages. Each message is identified by an 8-bit type field. Figure 23.2 Two levels of encapsulation that occur when an ICMP message is sent. The ICMP message is encapsulated in a datagram, which is encapsulated in a frame for transmission across a physical network. Data file 1 Trace of all IP traffic on Ethernet segment, including ICMP Data file 2 Anonymous FTP session with dir, get and put. Contains approximately 930Kbytes and 2300 packets.