TCP/IP-05-RARP Reverse Address Resolution Protocol

TCP/IP-05-RARP Reverse Address Resolution Protocol TCP/IP-04-ARP Address Resolution Protocol http://www.bkjia.com/net/201307/230435.html Chapter 2 RARP: Reverse Address Resolution Protocol 5th introduction when a system with a local disk is booted, the IP address is generally read from the configuration file on the disk. However, diskless machines, such as X terminals or diskless workstations, need to use other methods to obtain the IP address. Each system on the network has a unique hardware address, which is configured by the network interface manufacturer. The RARP implementation process of a diskless System reads the unique hardware address from the interface card, and then sends an RARP request (a frame of data broadcast on the network ), request a host to respond to the IP address of the diskless system (in RARP response ). This process is very simple in concept, but it is often more difficult to implement than ARP. The formal specification of RARP is RFC 903 [Finlayson et al. 1984]. 5.2 The format of the RARP group is basically the same as that of the ARP group. The main difference between them is that the frame type code of the RARP request or response is 0x8 0 3 5, the operation code of the RARP request is 3, and the response operation code is 4. Corresponding to ARP, RARP requests are transmitted in broadcast mode, while RARP responses are generally transmitted by unicast (u n I c a s t. 5.3 RARP in the Internet, we can force the sun host to boot from the network rather than from the local disk. If you run the RARP service program and the TCP d u m p command on host B s d I, you can get the output of 5-1. Use the-e parameter to print the hardware address using the TCP d u m p command: The RARP request is broadcast (1st rows), and The RARP response of 2nd rows is unicast. In the output of row 2nd, a tsun indicates that the RARP response contains the IP address of host sun (1 4 0. 2 5 2. 1 3. 3 3 ). In row 3rd, we can see that once sun receives the IP address, it sends a t f t p Read Request (r q) to the file 8 c f c 0 D 2 1. sun 4 C. The eight hexadecimal numbers in the file name are used to calculate the IP address of host sun 1 4 0. 2 5 2. 1 3. 3 3. This IP address is returned in the RARP response. The file name suffix sun 4 C indicates the type of the system to be guided. TCP d u m p indicates in row 3rd that the IP datagram length is 6 to 5 bytes, instead of a u d p datagram, because we run the TCP d u m p command with the-e parameter to view the address of the hardware layer. Another point to note is that the length of the Ethernet data frame in the 2nd rows is smaller than the minimum length. The reason is that we run the TCP d u m p command on the system that sends the Ethernet data frame (B s d I. The application RARP d writes 4 2 bytes to the B S D grouping filter device, which is the copy received by TCP d u m p. However, the Ethernet device driver needs to fill the short frame with white space characters to reach the minimum transmission length (6 0 ). If we run the TCP d u m p command on another system, the length will be 6 0. From this example, we can see that when the diskless system receives its IP address from the RARP response, it will send a t f t p request to read the boot image. At this point, we will not discuss in detail how diskless systems are guided. The destination address of each group is an Ethernet broadcast address. The ethernet address after who-is the destination hardware address, and the ethernet address after t e l is the hardware address of the sender. Pay attention to the frequency of retransmission. The first re-transmission is in 6. 5 5 seconds later, then increase to 4 2. 8 0 seconds, and then reduced to 5. 3 4 seconds and 6. 5 5 seconds, and then return to 4 2. 7 9 seconds. This uncertain situation continues. If we calculate the interval between two resends, we find that there is a double relationship: from 5. 3 4 to 6. 5 5 is 1. 2 1 second, from 6. 5 5 to 8. 9 7 is 2. 4 2 seconds, from 8. 9 7 to 1 3. 8 0 is 4. 8 or 3 seconds. Continue. When the time interval reaches a threshold value (greater than 4 2. 8 0 seconds), it is reset to 5. 3 4 seconds. The increasing time-out interval is better than the method with the same value every time. We will see an incorrect timeout and re-sending method. 5.4 RARP server design although RARP is very simple in concept, the design of an RARP server is related to the system and complicated. On the contrary, it is very easy to provide an ARP Server, which is usually part of TCP/IP implementation in the kernel. Because the kernel knows the IP address and hardware address, when it receives an ARP request asking the IP address, it only needs to provide a response using the corresponding hardware address. 5.4.1 the complexity of the RARP server, which acts as a user process, is that the server generally provides hardware address-to-IP address ing for multiple hosts (all diskless systems on the network. The ing is contained in a disk file (generally located in the/e t c/e t h e r s directory in the U n I x system ). Because the kernel generally does not read and analyze disk files, the functions of the RARP server are provided by the user process, rather than as part of the kernel's TCP/IP implementation. More complicated, RARP requests are transmitted as a special type of Ethernet data frame. This indicates that the RARP server must be able to send and receive such Ethernet data frames. In appendix A, we describe the B S D grouping filter, the network interface plug of sun, and the S V R 4 data link provider interface, which can be used to receive these data frames. Since the transmission and receipt of these data frames are related to the system, the implementation of the RARP server is bundled with the system. 5.4.2 a complex factor for the implementation of multiple RARP servers on each network is that RARP requests are broadcast on the hardware layer, which means they are not forwarded by routers. To enable the diskless system to boot when the RARP server is shut down, multiple RARP servers are usually provided on the same network. When the number of servers increases (to provide redundant backup), network traffic increases as each server sends an RARP response to each RARP request. The diskless system that sends RARP requests generally uses the first RARP response received. In addition, it is possible that each RARP server responds at the same time, which increases the probability of Ethernet conflict. 5.5 Summary The RARP protocol is used by many diskless systems to obtain IP addresses during boot. The format of the RARP group is basically the same as that of the ARP group. An RARP request is broadcast on the network. It indicates the hardware address of the sender in the group to request a response from the corresponding IP address. The response is generally sent by unicast. RARP issues include the use of link layer broadcast, which prevents most routers from forwarding RARP requests and returns only a few messages: only the system IP address. Although RARP is very simple in concept, the implementation of RARP server is related to the system. Therefore, not all TCP/IP implementations provide RARP servers.