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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 Classes, Types and Codes 1 2 3 ICMP Common Message Format and Data Encapsulation

Limitations on ICMP Message Responses

Most of the issues related to message generation have to do with error messages, not informational messages. The latter class of messages usually don't cause problems because they are generated based on specific rules already established in the protocols that use them. For example, Router Advertisement messages are sent by routers on a regular basis, and the routers make sure this is infrequent. They are also sent in response to Router Solicitation messages sent on occasion by hosts, and as long as a host doesn't go haywire and start sending tons of Solicitations, there won't be a problem. Even then, a router can be given enough smarts not to send Router Advertisements too often.

The problem comes up with error messages specifically because they are sent in response to so many situations. Potentially, they may even be sent in response to each other. Without special care, loops or cascading message generation might occur. For example, consider a situation where device A encounters an error and sends an error report to device B. Device B finds an error in device A's message and sends an error report back to device A. This could result in billions of messages being sent back and forth clogging the network, until a human figures out what is wrong and fixes it.

To prevent such problems, an ICMP error message must not be generated in response to any of the following:

• An ICMP Error Message: This prevents loops of the type just mentioned. Note, however, that an ICMP error message can be generated in response to an ICMP informational message.
  
  
• A Broadcast or Multicast Datagram: What would happen if a datagram were broadcast to 5,000 hosts and each of them found an error in it and tried to send a report back to the source? Something unpleasant.
  
  
• IP Datagram Fragments Except The First: When a datagram is fragmented, errors may only be sent in response to the first fragment. Often, errors that are generated due to a problem with one fragment would also be generated by each successive one, causing unnecessary ICMP traffic.
  
  
• Datagrams With Non-Unicast Source Address: If a datagram's source address doesn't define a unique, unicast device address, an error message cannot be sent back to that source. This prevents ICMP messages from being broadcast, unicast, or sent to non-routable special addresses such as the loopback address.

These rules apply to both ICMPv4 and ICMPv6, but in ICMPv6 there are a couple of special cases. In certain circumstances an ICMPv6 Packet Too Big message may be sent to a multicast address, as this is required for Path MTU Discovery to work. Certain Parameter Problem messages may also be sent to multicast or broadcast addresses. Finally, in addition to the rules above, IPv6 implementations are specifically directed to limit the rate at which they send ICMPv6 messages overall.

ICMP Message Classes, Types and Codes 1 2 3 ICMP Common Message Format and Data Encapsulation