tailieunhanh - Internetworking with TCP/IP- P15

Internetworking with TCP/IP- P15: 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 | 108 Internet Protocol Connectionless Datagram Delivery Chap. 7 0 1 2 3 4 5 6 7 COPY OPTION CLASS OPTION NUMBER Figure The division of the option code octet into three fields of length 1 2 and 5 bits. The fields of the OPTION CODE consist of a 1 -bit COPY flag a 2-bit OPTION CLASS and the 5-bit OPTION NUMBER. The COPY flag controls how routers treat options during fragmentation. When the COPT bit is set to 1 it specifies that the option should be copied into all fragments. When set to 0 the COPY bit means that the option should only be copied into the first fragment and not into all fragments. The OPTION CLASS and OPTION NUMBER bits specify the general class of the option and a specific option in that class. The table in Figure shows how option classes are assigned. Option Class Meaning 0 1 2 3 Datagram or network control Reserved for future use Debugging and measurement Reserved for future use Figure Classes of IP options as encoded in the OPTION CLASS bits of an option code octet. The table in Figure lists examples of options that can accompany an IP datagram and gives their OPTION CLASS and OPTION NUMBER values. As the list shows most options are used for control purposes. Sec. Internet Datagram Options 109 Option Class Option Number Length Description 0 0 - End of option list. Used if options do not end at end of header see header padding field for explanation . 0 1 - No operation. Used to align octets in a list of options. 0 2 11 Security and handling restrictions for military applications . 0 3 var Loose source route. Used to request routing that includes the specified routers. 0 7 var Record route. Used to trace a route. 0 8 4 Stream identifier. Used to carry a SATNET stream identifier obsolete . 0 9 var Strict source route. Used to specify a exact path through the internet. 0 11 4 MTU Probe. Used for path MTU discovery. 0 12 4 MTU Reply. Used for path MTU discovery. 0 20 4 Router Alert. Router should examine this datagram even if not