Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 176
ICMPv6 and ICMPv4

ICMPv6 cleaned up ICMPv4, mainly any control messages that were not used (gone from being specifically identified as ICMP messages are timestamp, timestamp reply, source quench, information request and reply). Most of these procedures are incorporated into other protocols. For example, source quench is not used anymore, for other mechanisms, such as TCP “slow start and congestion control,” were found to be more useful. Why put the onus on the routers when a mechanism is better defined and more useful elsewhere? Also, certain codes for a specific type have been moved or eliminated (for example, source route failed code for the Destination Unreachable Type). The address fields, obviously, had to be extended to handle ICMPv6 128-bit addressing. The multicast control functions of IGMP were also included in ICMPv6 to allow for the group membership Query, Report, and Termination. With all of the changes just discussed, ICMPv6 is not backwards compatible with ICMPv4. It uses the next-header function of IP and the next-header type of 56. One last change is that ICMPv4 messages copied the original IP header and 64 bits of data in the returned message. The exception to this is Echo Request or Echo Reply. ICMPv6 allows for a maximum of 576 bytes of data to be copied from the offending datagram.

The format of the ICMPv6 header is shown in the slide. It is the same format as ICMPv4. The type field indicates the type of the message. Its value determines the format of the remaining data. Error messages are identified as such by 0 a 0 in the high-order bit of their message Type field values. Thus, error messages have message Types from 0 to 127; informational messages have message Types from 128 to 255. The code field depends on the message type and further identifies the ICMP message. The checksum field is used to detect data corruption in the ICMPv6 message and parts of the IPv6 header. For the most part, the destination address is set to the source address of the previously received offending packet.