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Table Of Contents  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 6 (ICMPv6) Informational Message Types and Formats

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ICMP Version 6 (ICMPv6) Informational Message Types and Formats
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ICMPv6 Router Advertisement and Router Solicitation Messages
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ICMPv6 Echo Request and Echo Reply Messages
(Page 2 of 2)

ICMPv6 Echo and Echo Reply Message Format

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.



Figure 157: ICMPv6 Echo Request and Echo Reply Message Format

 


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.

Application of Echo and Echo Reply Messages

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.

Key Concept: ICMPv6 Echo Request and Echo Reply messages are used to facilitate network reachability testing. A device tests its ability to communicate with another by sending it an Echo Request message and waiting for an Echo Reply in response. The ping utility, a widely-used diagnostic tool in TCP/IP internetworks, makes use of these messages.



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