Chapter 16 BOOTP: Bootstrap Protocol
An example 16.3
Let's look at an example of an X terminal boot with BOOTP. FIG 16--3 shows the output of the tcpdump (named Proteus embodiment the client, the server name mercury tcpdump output is obtained on a different network, are other examples of this application has been used.).
In line 1, we see a request from the client 0.0. 0.0. 68, the transmission destination station is 255. 255. 255. 255. 67. The customer has to fill in the field is the number of seconds and its own Ethernet address. Our customers typically see the number of seconds will be set to 100.
tcpdump does not show the number of hops and transaction ID, because they are zero (0 transaction identifier indicates that the client ignores this field, because if you intend to return a response for authentication, it will set this field to a random value). Line 2 is the response returned by the server. Fill in the fields by the server that the client's IP address (tcpdump to display the name of proteus), IP address of the server (the name is displayed as mercury), IP address of the gateway (shown as the name mercury) and boot file name.
After receiving the BOOTP response, the client immediately sends an ARP request to see if there are other hosts on the network IP address. Followed who - Destination IP address corresponding to the name of proteus has (FIGS. 4 - 3), IP address of the sender is set to 0 0 0 0.... It then sends a ARP request in the same 0.5 seconds, followed by another 0.5 seconds and a fat. In the third ARP request (line 5), IP address, it changes the sender IP address of its own. This is a pointless ARP request (see section 4.7). Line 6 shows the customer wait another 0.5 seconds, another broadcast BOOTP request. The only difference between this request and the first line of customers at this time is to write it in the IP header of the IP address. It received the same response from the same server (line 7).
The client after waiting 2 seconds, and a broadcast BOOTP request (line 8), also received the same response from the same server.
After the client waits two seconds, mercury sends an ARP request (on line 10) to its server. Upon receipt of this ARP reply, it immediately sends a TFTP read request, a request to read its boot file (first 12 lines). File transfer process comprising 2,464 th TFTP packets and acknowledgment, the amount of data transferred to 5 1 2 × 2463 + 224 = 1 261 280 bytes. This operating system will be transferred to the X terminal. We have 16 chart - deleted most TFTP line 3.
As Figure 1 and 5 - 2 Comparative TFTP data exchange process is to be noted here that the TFTP client using the well-known port of the transmission process (69). Since one of the communicating parties are using the port 6 9, tcpdump know that these groups are TFTP packets, so it can use the TFTP protocol to explain each packet. This is why FIG 16--3 can specify which contains data, which includes a confirmation, and a block number of each packet. In Figure 15-- 2, we can not get this additional information because the two sides did not communicate using well-known port of TFTP data transfer. Because TFTP server as a multi-user system, and uses well-known port TFTP, and it is often TFTP client can not use that port. But here the system is in the process of being directed, not to provide a TFTP server, thus allowing customers to use the well-known port TFTP during transmission. This also implies that the TFTP server on mercury does not care what the client port number is - it only transfers the data to the client port, regardless of what happens.
From FIG 16--3 can be seen in 9 seconds were transferred 1,261,280 bytes. A data rate of about 140 000 bps. This is slower than most Ethernet access to a FTP file transfer form, but for a simple stop and wait protocol such as TFTP is already very good.
After the X terminal guidance system, the need to use TFTP transfer terminal font files, some DNS name server queries, then initialize the X protocol. FIG 16 - 3 all steps take approximately 5 seconds, the remaining steps 6 seconds, so the total time the system boot diskless X terminal is 21 seconds.
