The TCP/IP Guide - ICMPv6 Echo Request and Echo Reply Messages

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The format for ICMPv6 Echo Request and Echo Reply messages is very similar to that of the ICMPv4 version, as shown in Table 108 and Figure 157.

**Table 108: ICMPv6 *Echo Request* and *Echo Reply* Message Format**
Field Name	Size (bytes)	Description
*Type*	1	*Type:* Identifies the ICMPv6 message type; for Echo Request messages the value is 128, and for Echo Reply messages 129. (Recall that in ICMPv6, informational messages always have a Type value of 128 or higher.)
*Code*	1	*Code:* Not used; 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 optional identification field that can be used to help in matching Echo Request and Echo Reply messages.
*Sequence Number*	2	*Sequence Number:* A sequence number to help in matching Echo Request and Echo Reply messages.
*Optional Data*	Variable	*Optional Data:* Additional optional data to be sent along with the message. If this is sent in the Echo Request it is copied into the Echo Reply to be sent back to the source.

As with ICMPv4, it is often necessary to match an Echo Reply message with the Echo Request message that led to it being generated. Two special fields are used within the format of these messages to allow Echo Request and Echo Reply messages to be matched together, and to allow a sequence of messages to be exchanged. The Identifier field is provided so a particular test session can be identified, and the Sequence Number field allows a series of tests in a session to be numbered. The use of both fields is optional.

ICMPv6 Echo Request and Echo Reply messages are used via the IPv6 version of the IP ping utility, commonly called ping6. Like its IPv4 predecessor this utility allows an administrator to configure a number of test options to perform either a simple or rigorous test of the connection between a pair of devices. See the topic describing ping for a full explanation.