The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)                 9  ICMP Message Types and Formats                      9  ICMP Version 4 (ICMPv4) Informational Message Types and Formats

ICMP Version 4 (ICMPv4) Informational Message Types and Formats 1 2 3 ICMPv4 Timestamp (Request) and Timestamp Reply Messages

One of the main purposes of ICMP informational messages is to enable testing and diagnostics, to help identify and correct problems on an internetwork. The most basic test that can be conducted between two devices is simply checking if they are capable of sending datagrams to each other. The usual way that this is done is to have one device send a test message to a second device, which receives the message and replies back to tell the first device it received the message.

ICMPv4 includes a pair of messages specifically for connection testing. Suppose Device A wants to see if it can reach Device B. Device A begins the test process by sending an ICMPv4 Echo message to B. Device B, when it receives the Echo, responds back to Device A with an Echo Reply message. When Device A receives this message, it knows that it is able to communicate (both send and receive) successfully with Device B.

ICMP Version 4 (ICMPv4) Informational Message Types and Formats 1 2 3 ICMPv4 Timestamp (Request) and Timestamp Reply Messages