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ICMP

The Internet Control Message Protocol, or ICMP, described in RFC 792, specifies a variety of messages whose common purpose is to manage the internetwork. ICMP messages may be classified as either error messages or queries and responses. Figure 2.28 shows the general ICMP packet format. The packets are identified by type; many of the packet types have more specific types, and these are identified by the code field. Table 2.5 lists the various ICMP packet types and their codes, as described in RFC 1700.

Figure 2.28. The ICMP packet header includes a type field, a code field that further identifies some types, and a checksum. The rest of the fields depend on the type and code.

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Table 2.5. ICMP packet types and code fields.

Type	Code	Name
		ECHO REPLY
3		DESTINATION UNREACHABLE
		Network Unreachable
	1	Host Unreachable
	2	Protocol Unreachable
	3	Port Unreachable
	4	Fragmentation Needed and Don't Fragment Flag Set
	5	Source Route Failed
	6	Destination Network Unknown
	7	Destination Host Unknown
	8	Source Host Isolated
	9	Destination Network Administratively Prohibited
	10	Destination Host Administratively Prohibited
	11	Destination Network Unreachable for Type of Service
	12	Destination Host Unreachable for Type of Service
4		SOURCE QUENCH
5		REDIRECT
		Redirect Datagram for the Network (or Subnet)
	1	Redirect Datagram for the Host
	2	Redirect Datagram for the Network and Type of Service
	3	Redirect Datagram for the Host and Type of Service
6		ALTERNATE HOST ADDRESS
8		ECHO
9		ROUTER ADVERTISEMENT
10		ROUTER SELECTION
11		TIME EXCEEDED
		Time to Live Exceeded in Transit
	1	Fragment Reassembly Time Exceeded
12		PARAMETER PROBLEM
		Pointer Indicates the Error
	1	Missing a Required Option
	2	Bad Length
13		TIMESTAMP
14		TIMESTAMP REPLY
15		INFORMATION REQUEST (Obsolete)
16		INFORMATION REPLY (Obsolete)
17		ADDRESS MASK REQUEST
18		ADDRESS MASK REPLY
30		TRACEROUTE
31		DATAGRAM CONVERSION ERROR
32		MOBILE HOST REDIRECT
33		IPv6 WHERE-ARE-YOU
34		IPv6 I-AM-HERE
35		MOBILE REGISTRATION REQUEST
36		MOBILE REGISTRATION REPLY

Figures 2.29 and 2.30 show analyzer captures of two of the most well-known ICMP messages ”Echo Request and Echo Reply, which are used by the ping function.

Figure 2.29. An ICMP Echo message, shown with its IP header.

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Figure 2.30. An ICMP Echo Reply without the IP header displayed. The packet summary window in the background shows four Echo/Echo Reply pairs, which make up four Pings.

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Although most ICMP types have some bearing on routing functionality, three types are of particular importance.

Router Advertisement and Router Selection , types 9 and 10, respectively, are used by the ICMP Router Discovery Protocol (IRDP).

Redirect , ICMP type 5, is used by routers to notify hosts of another router on the data link that should be used for a particular destination. Suppose two routers, router A and router B, are connected to the same Ethernet. Host X, also on the Ethernet, is configured to use router A as its default gateway; the host sends a packet to router A, and A sees that the destination address of the packet is reachable via router B (that is, router A must forward the packet out the same interface on which it was received). Router A forwards the packet to B but also sends an ICMP redirect to host X informing it that in the future, to reach that particular destination, X should forward the packet to router B. Figure 2.31 shows a router sending a redirect.

Figure 2.31. Using the debugging function debug ip icmp, this router can be seen sending a redirect to host 10.158.43.25, informing it that the correct router for reaching destination 10.158.40.1 is reachable via gateway (gw) 10.158.43.10.

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A frequently used trick to avoid redirects on data links with multiple attached gateways is to set each host's default gateway as its own IP address. The hosts will then ARP for any address, and if the address is not on the data link, the correct router should respond via proxy ARP. The benefits of using this tactic merely to avoid redirects are debatable; redirects are decreased or eliminated but at the expense of increased ARP traffic.

Redirects are enabled by default on Cisco routers and may be disabled on a per interface basis with the command no ip redirects .

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