The Illustrated Network- P24:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 7 Internet Control Message Protocol 199 1 byte_1 byte_1 byte__ _1 byte Type Code Checksum Content Depends on Type Code IP Header 20 bytes and First 8 bytes of Original Packet Data usually TCP UDP header a Usually all 0 unused except for Type 3 Code 4 Destination unreachable fragmentation needed fields are 2 bytes unused and 2-byte link MTU size Type 3 Code 5 Destination unreachable redirect field is router IP address Type 12 Code 0 Parameter problem field is 4-bit pointer to parameter rest all 0 1 byte 1 byte 1 byte 1 byte Type 3 Code Checksum Identifier for Request Response pairs usually PID in Unix Sequence Number set to 0 initially and incremented Content depends on Query Type b FIGURE ICMP error and query messages. Note that error messages include the IP header that generated the error. a ICMP error message. b ICMP query message. Let s take a look at what the specifications say about ICMP messages. First we ll look at error messages and then query messages and then all the rest. ICMP Error Messages ICMP Error messages report semipermanent network conditions. The five ICMP error messages are displayed in Table which shows how routers and hosts should handle each type. Time-exceeded errors result from TTL expiration Code 0 or when fragments cannot be completed quickly enough at a receiver Code 1 . Parameter problems are usually sent in regard to IP options. The codes are for a bad IP header 0 missing a required option field 1 or a bad length 2 . Which of these message types are essential to device operation and should not be blocked Generally the Destination Unreachable is essential it is used by traceroute and used in MTU path calculations. Of the others the Redirect message is most often 200 PART II Core Protocols Table ICMP Error Messages Type Meaning Codes Data Router Sends Router Receives Host Sends Host Receives 3 Destination Unreachable 0-15 IP hdr 8 bytes M M M M 4 Source Quench 0 IP hdr 8 .
