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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.
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Identifier
|
2
|
Identifier:
An optional identification field that can be used to help in matching
Echo Request and Echo Reply messages.
|
Sequence
Number
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2
|
Sequence Number:
A sequence number to help in matching Echo Request and Echo
Reply messages.
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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.
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Figure 157: ICMPv6 Echo Request and Echo Reply Message Format
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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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Version 3.0 - Version Date: September 20, 2005
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