tailieunhanh - Internetworking with TCP/IP- P18

Internetworking with TCP/IP- P18: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | 138 Internet Protocol Error And Control Messages ICMP Chap. 9 a longer path than necessary . When router R5 receives the datagram it cannot send an ICMP redirect message to R because it does not know R s address. Later chapters explore the problem of how to propagate routes across multiple networks. Figure ICMP redirect messages do not provide routing changes among routers. In this example router R5 cannot redirect R to use the shorter path for datagrams from S to D. In addition to the requisite TYPE CODE and CHECKSUM fields each redirect message contains a 32-bit ROUTER INTERNET ADDRESS field and an INTERNET HEADER field as Figure shows. 0 8 16 31 TYPE 5 CODE 0 to 3 CHECKSUM ROUTER INTERNET ADDRESS INTERNET HEADER FIRST 64 BITS OF DATAGRAM . . . Figure ICMP redirect message format. The ROUTER INTERNET ADDRESS field contains the address of a router that the host is to use to reach the destination mentioned in the datagram header. The INTERNET HEADER field contains the IP header plus the next 64 bits of the datagram that triggered the message. Thus a host receiving an ICMP redirect examines the datagram prefix to determine the datagram s destination address. The CODE field of an ICMP redirect message further specifies how to interpret the destination address based on values assigned as follows Sec. Route Change Requests From Routers 139 Code Value _Meaning__ 0 Redirect datagrams for the Net now obsolete 1 Redirect datagrams for the Host 2 Redirect datagrams for the Type of Service and Net 3 Redirect datagrams for the Type of Service and Host As a general rule routers only send ICMP redirect requests to hosts and not to other routers. We will see in later chapters that routers use other protocols to exchange routing information. Detecting Circular Or Excessively Long Routes Because internet routers compute a next hop using local tables errors in routing tables can produce a routing cycle for some .