tailieunhanh - Internetworking with TCP/IP- P14

Internetworking with TCP/IP- P14: 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 | 98 Internet Protocol Connectionless Datagram Delivery Chap. 7 an IP datagram or merely a datagram. Like a typical physical network frame a datagram is divided into header and data areas. Also like a frame the datagram header contains the source and destination addresses and a type field that identifies the contents of the datagram. The difference of course is that the datagram header contains IP addresses whereas the frame header contains physical addresses. Figure shows the general form of a datagram DATAGRAM HEADER DATAGRAM DATA AREA Figure General form of an IP datagram the TCP IP analogy to a network frame. IP specifies the header format including the source and destination IP addresses. IP does not specify the format of the data area it can be used to transport arbitrary data. Datagram Format Now that we have described the general layout of an IP datagram we can look at the contents in more detail. Figure shows the arrangement of fields in a datagram 0 4 8 16 19 24 31 VERS HLEN SERVICE TYPE TOTAL LENGTH IDENTIFICATION FLAGS FRAGMENT OFFSET TIME TO LIVE PROTOCOL HEADER CHECKSUM SOURCE IP ADDRESS DESTINATION IP ADDRESS IP OPTIONS IF ANY PADDING DATA . . . Figure Format of an Internet datagram the basic unit of transfer in a TCP IP internet. Because datagram processing occurs in software the contents and format are not constrained by any hardware. For example the first 4-bit field in a datagram VERS contains the version of the IP protocol that was used to create the datagram. It is used to verify that the sender receiver and any routers in between them agree on the format Sec. The Internet Datagram 99 of the datagram. All IP software is required to check the version field before processing a datagram to ensure it matches the format the software expects. If standards change machines will reject datagrams with protocol versions that differ from theirs preventing them from misinterpreting datagram contents according to an outdated format. .