Mapping Logical to Physical Address: ARP

Mapping Logical to Physical Address: ARP

Anytime a host or a router has an IP datagram to send to another host or router, it has the logical (IP) address of the receiver. The logical (IP) address is obtained from the DNS if the sender is the host or it is found in a routing table if the sender is a router. But the IP datagram must be encapsulated in a frame to be able to pass through the physical network. This means that the sender needs the physical address of the receiver. The host or the router sends an ARP query packet. The packet includes the physical and IP addresses of the sender and the IP address of the receiver. Because the sender does not know the physical address of the receiver, the query is broadcast over the network

1. Cache Memory

Using ARP is inefficient if system A needs to broadcast an ARP request for each IP packet it needs to send to system B. It could have broadcast the IP packet itself. ARP can be useful if the ARP reply is cached because a system normally sends several packets to the same destination. A system that receives an ARP reply stores the mapping in the cache memory and

keeps it for 20 to 30 minutes unless the space in the cache is exhausted. Before sending an ARP request, the system first checks its cache to see if it can find the mapping.

2. Packet Format

The fields are as follows:

· Hardware type: This is a 16-bit field defining the type of the network on which ARP isrunning. Each LAN has been assigned an integer based on its type. For example, Ethernet is given type 1. ARP can be used on any physical network

Protocol type: This is a 16-bit field defining the protocol. For example, the value of thisfield for the IPv4 protocol is 080016, ARP can be used with any higher-level protocol.

Hardware length: This is an 8-bit field defining the length of the physical address inbytes. For example, for Ethernet the value is 6.

Protocol length: This is an 8-bit field defining the length of the logical address in bytes.For example, for the IPv4 protocol the value is 4.

Operation: This is a 16-bit field defining the type of packet. Two packet types aredefined: ARP request (1) and ARP reply (2).

Sender hardware address: This is a variable-length field defining the physical addressof the sender. For example, for Ethernet this field is 6 bytes long.

Sender protocol address: This is a variable-length field defining the logical (forexample, IP) address of the sender. For the IP protocol, this field is 4 bytes long.

Target hardware address: This is a variable-length field defining the physical addressof the target. For example, for Ethernet this field is 6 bytes long. For an ARP request message, this field is alIOs because the sender does not know the physical address of the target.

Target protocol address: This is a variable-length field defining the logical (forexample, IP) address of the target. For the IPv4 protocol, this field is 4 bytes long.

3. Encapsulation

An ARP packet is encapsulated directly into a data link frame. For example, an ARP packet is encapsulated in an Ethernet frame. Note that the type field indicates that the data carried by the frame are an ARP packet

Operation

The steps involved in an ARP process:

1. The sender knows the IP address of the target.

2.  IP asks ARP to create an ARP request message, filling in the sender physical address, the sender IP address, and the target IP address. The target physical address field is filled with 0s.

3. The message is passed to the data link layer where it is encapsulated in a frame by using the physical address of the sender as the source address and the physical broadcast address as the destination address.

4.  Every host or router receives the frame. Because the frame contains a broadcast destination address, all stations remove the message and pass it to ARP. All machines except the one targeted drop the packet. The target machine recognizes its IP address.

5. The target machine replies with an ARP reply message that contains its physical address. The message is unicast.

6.  The sender receives the reply message. It now knows the physical address of the target machine.

7.  The IP datagram, which carries data for the target machine, is now encapsulated in a frame and is unicast to the destination.

4. Four Different Cases:

The following are four different cases in which the services of ARP can be used

1. The sender is a host and wants to send a packet to another host on the same network. In this case, the logical address that must be mapped to a physical address is the destination IP address in the datagram header.

2.  The sender is a host and wants to send a packet to another host on another network. In this case, the host looks at its routing table and finds the IP address of the next hop (router) for this destination. If it does not have a routing table, it looks for the IP address of the default router. The IP address of the router becomes the logical address that must be mapped to a physical address.

3. The sender is a router that has received a datagram destined for a host on another network. It checks its routing table and finds the IP address of the next router. The IP address of the next router becomes the logical address that must be mapped to a physical address.

4. The sender is a router that has received a datagram destined for a host on the same network. The destination IP address of the datagram becomes the logical address that must be mapped to a physical address.

Example 3.17

A host with IP addresses 130.23.43.20 and physical address B2:34:55: 10:22: 10 has a packet to send to another host with IP address 130.23.43.25 and physical address A4:6E:F4:59:83:AB. The two hosts are on the same Ethernet network. Show the ARP request and reply packets encapsulated in Ethernet frames.

Solution

The ARP request and reply packets. Note that the ARP data field in this case is 28 bytes, and that the individual addresses do not fit in the 4-byte boundary. That is why we do not show the regular 4-byte boundaries for these addresses.

5. Proxy ARP

A technique called proxy ARP is used to create a subnetting effect. A proxy ARP is an ARP that acts on behalf of a set of hosts. Whenever a router running a proxy ARP receives an ARP request looking for the IP address of one of these hosts, the router sends an ARP reply announcing its own hardware (physical) address. After the router receives the actual IP packet, it sends the packet to the appropriate host or router. Let us give an example.

However, the administrator may need to create a subnet without changing the whole system to recognize subnetted addresses. One solution is to add a router running a proxy ARP. In this case, the router acts on behalf of all the hosts installed on the subnet. When it receives an ARP request with a target IP address that matches the address of one of its proteges (141.23.56.21, 141.23.56.22, or 141.23.56.23), it sends an ARP reply and announces its hardware address as the target hardware address. When the router receives the IP packet, it sends the packet to the appropriate host. This may happen in two cases:

1.        A diskless station is just booted. The station can find its physical address by checking its interface, but it does not know its IP address.

2.        An organization does not have enough IP addresses to assign to each station; it needs to assign IP addresses on demand. The station can send its physical address and ask for a short time lease.