tailieunhanh - Practical TCP/IP and Ethernet Networking- P26

Practical TCP/IP and Ethernet Networking- P26: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | Internet layer protocols 107 The RARP server changes the opcode to 4 RARP reply . It then inserts the missing address in the target IP address field and sends the reply directly back to the requesting machine. The requesting machine then stores it in memory until next time it reboots. All RARP servers on a network will reply to a RARP request even though only one reply is required. The RARP software on the requesting machine sets a timer when sending a request and retransmits the request if the timer expires before a reply has been received. On a best-effort local area network such as Ethernet the provision of more than one RARP server reduces the likelihood of RARP replies being lost or dropped because the server is down or overloaded. This is important because a diskless workstation often requires its own IP address before it can complete its bootstrap procedure. To avoid multiple and unnecessary RARP responses on a broadcast-type network such as Ethernet each machine on the network is assigned a particular server called its primary RARP server. When a machine broadcasts a RARP request all servers will receive it and record its time of arrival but only the primary server for that machine will reply. If the primary server is unable to reply for any reason the sender s timer will expire it will rebroadcast its request and all non-primary servers receiving the rebroadcast so soon after the initial broadcast will respond. Alternatively all RARP servers can be programmed to respond to the initial broadcast with the primary server set to reply immediately and all other servers set to respond after a random time delay. The retransmission of a request should be delayed for long enough for these delayed RARP replies to arrive. RARP has several drawbacks. It has to be implemented as a server process. It is also prudent to have more than one server since no diskless workstation can boot up if the single RARP server goes down. In addition to this very little information only