4.2 An Example

4.2 An Example

Whenever we type a command of the form

 %  ftp bsdi  

the following steps take place. These numbered steps are shown in Figure 4.2.

Figure 4.2. Operation of ARP when user types " ftp hostname".

1. The application, the FTP client, calls the function gethostbyname (3) to convert the hostname ( bsdi ) into its 32-bit IP address. This function is called a resolver in the DNS (Domain Name System), which we describe in Chapter 14. This conversion is done using the DNS, or on smaller networks, a static hosts file ( /etc/hosts ).

2. The FTP client asks its TCP to establish a connection with that IP address.

3. TCP sends a connection request segment to the remote host by sending an IP datagram to its IP address. (We'll see the details of how this is done in Chapter 18.)

4. If the destination host is on a locally attached network (e.g., Ethernet, token ring, or the other end of a point-to-point link), the IP datagram can be sent directly to that host. If the destination host is on a remote network, the IP routing function determines the Internet address of a locally attached next -hop router to send the IP datagram to. In either case the IP datagram is sent to a host or router on a locally attached network.

5. Assuming an Ethernet, the sending host must convert the 32-bit IP address into a 48-bit Ethernet address. A translation is required from the logical Internet address to its corresponding physical hardware address. This is the function of ARP.

   ARP is intended for broadcast networks where many hosts or routers are connected to a single network.

6. ARP sends an Ethernet frame called an ARP request to every host on the network. This is called a broadcast. We show the broadcast in Figure 4.2 with dashed lines. The ARP request contains the IP address of the destination host (whose name is bsdi ) and is the request "if you are the owner of this IP address, please respond to me with your hardware address."

7. The destination host's ARP layer receives this broadcast, recognizes that the sender is asking for its hardware address, and replies with an ARP reply. This reply contains the IP address and the corresponding hardware address.

8. The ARP reply is received and the IP datagram that forced the ARP request-reply to be exchanged can now be sent.

9. The IP datagram is sent to the destination host.

The fundamental concept behind ARP is that the network interface has a hardware address (a 48-bit value for an Ethernet or token ring interface). Frames exchanged at the hardware level must be addressed to the correct interface. But TCP/IP works with its own addresses: 32-bit IP addresses. Knowing a host's IP address doesn't let the kernel send a frame to that host. The kernel (i.e., the Ethernet driver) must know the destination's hardware address to send it data. The function of ARP is to provide a dynamic mapping between 32-bit IP addresses and the hardware addresses used by various network technologies.

Point-to-point links don't use ARP. When these links are configured (normally at bootstrap time) the kernel must be told of the IP address at each end of the link. Hardware addresses such as Ethernet addresses are not involved.