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The Illustrated Network- P25

The Illustrated Network- P25: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 209 Path MTU discovery is automatic and a new Packet Too Big message is sent to the source for over-large packets because IPv6 routers do not fragment. There is no Source Quench in ICMPv6 it is obsolete in ICMPv4 but still exists . IGMP for multicast is included in ICMPv6. ICMPv6 helps detect nonfunctioning routers and inactive partner hosts. ICMPv6 is so different that it now has its own IP protocol number. IPv6 uses the next header value of 58 for ICMPv6 messages. Basic ICMPv6 Messages The general ICMPv6 message format is similar to ICMPv4 but somewhat simpler. The structure of a generic ICMPv6 message and the common Destination Unreachable message are shown in Figure 7.6. ICMPv6 error messages are in the range 0 to 127. Some of the most common are shown in the figure as well. 1 byte 1 byte 1 byte 1 byte Type Code Checksum Message Body a Basic ICMPv6 Type field values 1 Destination Unreachable 2 Packet Too Big 3 Time Exceeded 4 Parameter Problem 5 Redirect 128 Echo Request 129 Echo Reply FIGURE 7.6 ICMPv6 message formats which can be compared to the IPv4 versions in Figure 7.4. a Generic ICMPv6 message format. b ICMPv6 Destination Unreachable message. 210 PART II Core Protocols Table 7.6 Destination Unreachable Codes for ICMPv6 Code Meaning 0 No route to destination 1 Communication with destination administratively prohibited 2 Next destination in the IPv6 Routing header is not a neighbor and this is a strict route routing headers are not currently supported 3 Address unreachable 4 Port unreachable Destination Unreachable In ICMPv6 the Destination Unreachable message type is Type 1. The codes that can be compared to Table 7.2 IPv4 codes number only five and are listed in Table 7.6. Packet Too Big A router sends an ICMPv6 Packet Too Big message to the source when the packet is bigger than the MTU for the next-hop link. The next-hop link s MTU size is reported in the message. In ICMPv4 this type of information was .