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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

Issues Using Timestamp and Timestamp Reply Messages

In practice, even with three timestamp fields, it is difficult to coordinate system clocks over an internetwork—especially a large one like the Internet. The main problem is that the amount of time it takes to send a datagram between any pair of devices varies from one datagram to the next. And again, since IP is unreliable, it's possible the time for a datagram to be received could be infinite—it might be lost or dropped by a router.

This means that a simple exchange of Timestamp and Timestamp Reply messages is simply not a reliable enough method for ensuring that two devices are synchronized on a typical IP internetwork. For this reason, modern devices often use a more sophisticated method for time synchronization, such as the Network Time Protocol (NTP).

Note that unlike many of the other ICMP message types, support for Timestamp and Timestamp Reply messages is optional, for both hosts and routers.

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