TCP/IP Tutorial and Technical Overview - IBM Redbooks

9.3 Internet Control Message Protocol Version 6
(ICMPv6)
IP concerns itself with moving data from one node to another. However, in order
for IP to perform this task successfully, there are many other functions that need
to be carried out: error reporting, route discovery, and diagnostics, to name a
few. All these tasks are carried out by the Internet Control Message Protocol
(see 3.2, “Internet Control Message Protocol (ICMP)” on page 109). In addition,
ICMPv6 carries out the tasks of conveying multicast group membership
information, a function that was previously performed by the IGMP protocol in
IPv4 (see 3.3, “Internet Group Management Protocol (IGMP)” on page 119) and
address resolution, previously performed by ARP (see 3.4, “Address Resolution
Protocol (ARP)” on page 119).
ICMPv6 messages and their use are specified in RFC 4443 – Internet Control
Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification and RFC 2461 – Neighbor Discovery for IP Version 6 (IPv6). Both
RFCs are draft standards with a status of elective.
Every ICMPv6 message is preceded by an IPv6 header (and possibly some IP
extension headers). The ICMPv6 header is identified by a Next Header value of
58 in the immediately preceding header.
ICMPv6 messages all have a similar format, shown in Figure 9-14.
Figure 9-14 ICMPv6 general message format
Where:
Type There are two classes of ICMPv6 messages. Error
messages have a Type from 0 to 127. Informational
messages have a Type from 128 to 255.
352 TCP/IP Tutorial and Technical Overview
0 8 16 31
Type Checksum
Code
Body of ICMP Message
1 Destination Unreachable
2 Packet Too Big