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

ICMPv4 Echo and Echo Reply Message Format

Table 96 and Figure 146 show the format for both ICMPv4 Echo and Echo Reply messages.

It is possible that a source device may want to send more than one Echo message to either a single destination or multiple destinations. Conversely, a single destination might receive Echo messages from more than one source. It is essential that a device receiving an Echo Reply knows which Echo prompted it to be sent.

Two special fields are used within the format of these messages to allow Echo and Echo Reply messages to be matched together, and to allow a sequence of messages to be exchanged. The Identifier field was envisioned as being used as a higher-level label, like a session identifier, while the Sequence Number was seen as something to identify individual test messages within a series. However, the use of these fields is up to the particular implementation. In some cases, the Identifier field is filled in with the process number of the application that is using the Echo or Echo Reply message, to allow several users to use utilities like ping without interference.

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