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Clock synchronization and transit time estimation

Clock synchronization and transit time estimation 
8.2.3 This page explains how ICMP timestamps are used to solve clock synchronization issues.
The TCP/IP protocol suite allows systems to connect to one another over vast distances through multiple networks. Each network provides clock synchronization in its own way. As a result, hosts on different networks who attempt to communicate with software that requires time synchronization can encounter problems. The ICMP timestamp message type is designed to help alleviate this problem.
The ICMP timestamp request message allows a host to ask for the current time according to the remote host. The remote host uses an ICMP timestamp reply message to respond to the request.
The type field on an ICMP timestamp message can be either 13 for a timestamp request or 14 for a timestamp reply. The code field value is always set to 0 because there are no additional parameters available. The ICMP timestamp request contains an originate timestamp, which is the time on the requesting host just before the timestamp request is sent. The receive timestamp is the time that the destination host receives the ICMP timestamp request. The transmit timestamp is filled in just before the ICMP timestamp reply is returned. Originate, receive, and transmit timestamps are computed in milliseconds elapsed since midnight Universal Time (UT).
All ICMP timestamp reply messages contain the originate, receive, and transmit timestamps. Using these three timestamps, the host can determine transit time across the network by subtracting the originate time from the receive time. Or it could determine transit time in the return direction by subtracting the transmit time from the current time. The host that originated the timestamp request can also estimate the local time on the remote computer.
While ICMP timestamp messages provide a simple way to estimate time on a remote host and total network transmit time, this is not the best way to obtain this information. Instead, more robust protocols such as Network Time Protocol (NTP) at the upper layers of the TCP/IP protocol stack perform clock synchronization in a more reliable manner.
The next page will discuss ICMP information request and reply messages.

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