ICMPv6 Informational Messages

The ICMPv6 informational messages defined in RFC 2463 provide a simple diagnostic capability to aid in troubleshooting and consist of the following messages:

  • Echo Request (ICMPv6 Type 128)
  • Echo Reply (ICMPv6 Type 129)

Additional ICMPv6 informational messages are used for ND and MLD. For more information, see Chapter 6, "Neighbor Discovery" and Chapter 7, "Multicast Listener Discovery."

Echo Request

An ICMPv6 Echo Request message is sent to a destination to solicit an immediate Echo Reply message. The Echo Request/Echo Reply message facility provides a simple diagnostic function to aid in the troubleshooting of a variety of reachability and routing problems. Figure 5-6 shows the structure of the Echo Request message.

Figure 5-6. The structure of the Echo Request message

In the Echo Request message, the Type field is set to 128 and the Code field is set to 0. Following the Checksum field are the 16-bit Identifier and the 16-bit Sequence Number fields. The Identifier and Sequence Number fields are set by the sending host so that they can be used to match an incoming Echo Reply message with a sent Echo Request message. The Data field is zero or more bytes of optional data that is also set by the sending host.

Network Monitor Capture

Here is an example of an Echo Request message as displayed by Network Monitor (frame 1 of capture 05_03 in the \NetworkMonitorCaptures folder on the companion CD-ROM):

 + Frame: Base frame properties + ETHERNET:  EType = IPv6  + IP6: Proto = ICMP6; Len = 40   ICMP6: Echo Request; ID = 0, Seq = 24       ICMP6: Type = 128 (Echo Request)       ICMP6: Code = 0 (0x0)       ICMP6: Checksum = 0xCB4D       ICMP6: Identifier = 0 (0x0)       ICMP6: Sequence Number = 24 (0x18)       ICMP6: Data: Number of data bytes remaining = 32 (0x0020) 

Echo Reply

An ICMPv6 Echo Reply message is sent in response to the receipt of an ICMPv6 Echo Request message. Figure 5-7 shows the structure of the Echo Reply message.

Figure 5-7. The structure of the Echo Reply message

In the Echo Reply message, the Type field is set to 129 and the Code field is set to 0. Following the Checksum field are the 16-bit Identifier and the 16-bit Sequence Number fields. The Identifier, Sequence Number, and Data fields are set with the same values as those in the Echo Request message that prompted the Echo Reply.

Echo Request messages can be sent to a multicast address. As specified in RFC 2463, an Echo Request message sent to a multicast address should be answered with an Echo Reply message, sent from a unicast address assigned to the interface on which the Echo Request was received.

Network Monitor Capture

Here is an example of an Echo Reply message as displayed by Network Monitor (frame 2 of capture 05_03 in the \NetworkMonitorCaptures folder on the companion CD-ROM):

 + Frame: Base frame properties + ETHERNET:  EType = IPv6  + IP6: Proto = ICMP6; Len = 40   ICMP6: Echo Reply; ID = 0, Seq = 24       ICMP6: Type = 129 (Echo Reply)       ICMP6: Code = 0 (0x0)       ICMP6: Checksum = 0xCA4D       ICMP6: Identifier = 0 (0x0)       ICMP6: Sequence Number = 24 (0x18)       ICMP6: Data: Number of data bytes remaining = 32 (0x0020) 

This Echo Reply message was sent in response to the previously displayed Echo Request message. Notice how the Identifier, Sequence Number, and Data fields (actual contents of the Data field are not shown) match the original Echo Request message.



Understanding IPv6
Understanding Ipv6
ISBN: 0735612455
EAN: 2147483647
Year: 2005
Pages: 124
Authors: Joseph Davies

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