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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 Message Types and Formats                      9  ICMP Version 4 (ICMPv4) Informational Message Types and Formats

ICMPv4 Timestamp (Request) and Timestamp Reply Messages 1 2 3 4 ICMPv4 Address Mask Request and Reply Messages

ICMPv4 Router Advertisement Message Format

The ICMPv4 Router Advertisement message format is shown in Table 98 and Figure 148.

Table 98: ICMPv4 Router Advertisement Message Format

Field Name	Size (bytes)	Description
Type	1	Type: Identifies the ICMP message type. For Router Advertisement messages the value is 9.
Code	1	Code: Normally set to 0. When a Mobile IP agent is sending a Router Advertisement with an Agent Advertisement extension, it may set the value to 16 if the device is a mobile agent only and doesn’t intend to handle normal traffic. See the discussion of Mobile IP agent discovery for details.
Checksum	2	Checksum: 16-bit checksum field for the ICMP header, as described in the topic on the ICMP common message format.
Num Addrs	1	Number Of Addresses: The number of addresses associated with this router that are included in this advertisement.
Addr Entry Size	1	Address Entry Size: The number of 32-bit words of information included with each address. Since in this message format each router address has a 32-bit address and a 32-bit preference level, this value is fixed at 2.
Lifetime	2	Lifetime: The number of seconds that a host should consider the information in this message valid.
Router Address Entries	Value of Num Addrs field * 8

Figure 148: ICMPv4 Router Advertisement Message Format

ICMPv4 Timestamp (Request) and Timestamp Reply Messages 1 2 3 4 ICMPv4 Address Mask Request and Reply Messages

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