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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 Concepts and General Operation

ICMP Message Creation and Processing Conventions and Rules 1 2 ICMP Message Types and Formats

Original Datagram Inclusion In ICMP Error Messages

The inclusion of original IP datagram information is done differently for the two ICMP versions.

Each error message includes the full IP header and the first 8 bytes of the payload. Since the beginning of the payload will contain the encapsulated higher-layer header, this means the ICMP message also carries either the full UDP header, or the first 8 bytes of the TCP header. In both cases, the source and destination port numbers are part of what is included.

If the original header was a standard IP header with no options, the Message Body will therefore have a length of 28 bytes; if options are present, it will be larger.

Each error message includes as much of the IPv6 datagram as will fit without causing the size of the ICMPv6 error message (including its IP header encapsulation) to exceed the minimum IPv6 maximum transmission unit size, which is 1280 bytes. This provides additional information for diagnostic purposes compared to ICMPv4, while ensuring that no ICMPv6 error messages will be too large for any physical network segment. The larger size of the included data allows the IPv6 extension headers to be included in the error message, since the error could be in one of those extension headers.

Remember that in IPv6, routers cannot fragment IP datagrams; any datagram that is “oversized” for an underlying physical network is dropped. ICMPv6 is thus designed to ensure that this does not happen by not creating ICMPv6 datagrams over the universal IPv6 MTU size of 1280.

After an ICMP message is formatted, it is encapsulated in an IP datagram like any other IP message. This is why some people believe ICMP is architecturally a higher layer than IP, though as we discussed earlier it is really more of a special case. You can also see then, that when an ICMP error message is generated, we end up with the original IP header and part or all of the payload, encapsulated in the ICMP message, in turn encapsulated within a new IP header to be sent back as an error report usually to the device that sent the original IP message.

ICMP Message Creation and Processing Conventions and Rules 1 2 ICMP Message Types and Formats