tailieunhanh - Internetworking with TCP/IP- P13

Internetworking with TCP/IP- P13: 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 | 6 Determining An Internet Address At startup RARP Introduction We now know that physical network addresses are both low-level and hardware dependent and we understand that each machine using TCP IP is assigned one or more 32-bit IP addresses that are independent of the machine s hardware addresses. Application programs always use the IP address when specifying a destination. Because hosts and routers must use a physical address to transmit a datagram across an underlying hardware network they rely on address resolution schemes like ARP to map between an IP address and an equivalent hardware address. Usually a computer s IP address is kept on its secondary storage where the operating system finds it at startup. The question arises How does a machine without a permanently attached disk determine its IP address The problem is critical for workstations that store files on a remote server or for small embedded systems because such machines need an IP address before they can use standard TCP IP file transfer protocols to obtain their initial boot image. This chapter explores the question of how to obtain an IP address and describes a low-level protocol that such machines can use before they boot from a remote file server. Chapter 23 extends the discussion of bootstrapping and considers popular alternatives to the protocol presented here. Because an operating system image that has a specific IP address bound into the code cannot be used on multiple computers designers usually try to avoid compiling a machine s IP address in the operating system code or support software. In particular the bootstrap code often found in Read Only Memory ROM is usually built so the same image can run on many machines. When such code starts execution it uses the network to contact a server and obtain the computer s IP address. 89 90 Determining An Internet Address At Startup RARP Chap. 6 The bootstrap procedure sounds paradoxical a machine communicates with a remote server to obtain an .