The TCP/IP Guide - ICMPv4 Timestamp (Request) and Timestamp Reply Messages

The ICMPv4 Timestamp and Timestamp Reply messages have the same format. The originating device fills in some of the fields, and the replying device others. The format is as shown in Table 97 and Figure 147.

**Table 97: ICMPv4 *Timestamp* and *Timestamp Reply* Message Format**
Field Name	Size (bytes)	Description
*Type*	1	*Type:* Identifies the ICMP message type. For Timestamp messages the value is 13; for Timestamp Reply messages the value is 14.
*Code*	1	*Code:* Not used for Timestamp and Timestamp Reply messages; set to 0.
*Checksum*	2	*Checksum:* 16-bit checksum field for the ICMP header, as described in the topic on the ICMP common message format.
*Identifier*	2	*Identifier:* An identification field that can be used to help in matching Timestamp and Timestamp Reply messages.
*Sequence Number*	2	*Sequence Number:* A sequence number to help in matching Timestamp and Timestamp Reply messages.
*Originate Timestamp*	4	*Originate Timestamp:* A time value filled in by the originating device just before sending the Timestamp message.
*Receive Timestamp*	4	*Receive Timestamp:* A time value filled in by the responding device just as it receives the Timestamp message.
*Transmit Timestamp*	4	*Transmit Timestamp:* A time value filled in by the responding device just before sending back the Timestamp Reply message.

The Identifier and Sequence Number fields are used to match Timestamp and Timestamp Reply messages, exactly as they were used for Echo and Echo Reply messages. The Identifier field is intended as a higher-level label, like a session identifier, while the Sequence Number is often used to identify individual messages within a series. However, the use of these fields is up to the particular implementation.

All three timestamps are represented as the number of milliseconds since midnight, Universal Time (UT, also called Greenwich Mean Time or GMT). The reason there are three timestamps instead of the two you might ordinarily expect, is that a separate timestamp is recorded by the responding device both when it receives the Timestamp message and when it generates the Timestamp Reply. When the Reply is received back by the originating device, it then has the times that both the Timestamp and the Timestamp Reply were sent. This allows the originating device to differentiate between the time required for transmitting datagrams over the network, and the time for the other device to process the Timestamp and turn it into a Timestamp Reply.