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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

ICMPv4 Echo (Request) and Echo Reply Messages 1 2 3 ICMPv4 Router Advertisement and Router Solicitation Messages

All of the hosts and routers on an internetwork operate independently of each other. One aspect of this autonomy is that each device maintains a separate system clock. Since even highly-accurate clocks have slight differences in how accurately they keep time, as well as the time they are initialized with, this means that under normal circumstances, no two devices on an internetwork are guaranteed to have exactly the same time.

The creators of TCP/IP recognized that certain applications might not work properly if there was too much differential between the system clocks of a pair of devices. To support this requirement, they created a pair of ICMP messages that allow devices to exchange system time information. The initiating device creates a Timestamp message and sends it to the device with which it wishes to synchronize. That device responds with a Timestamp Reply message. Timestamp fields in these messages are used to mark the times that these messages are sent and received to allow the devices' clocks to be synchronized.

ICMPv4 Echo (Request) and Echo Reply Messages 1 2 3 ICMPv4 Router Advertisement and Router Solicitation Messages