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6. 3 A D DI T IOnA L I P-R e L AT eD S e RV IC e S
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Figure 6.10 Host A is attempting to send packets to Host B, but Host B is no longer available. Without an ICMP message, it will be up to Host A s upper-layer protocols to time out the connection. As an alternative, if the network link to Host B is down, a router can send an ICMP destination unreachable message to Host A s IP process so that it can inform the upper-layer protocols.
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Host B
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Host A Normal IP packet flow from Host A to Host B Destination link is broken ICMP destination unreachable message is sent to source
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Recall from 3 that an ethernet device has a hardware or MAC address associated with its ethernet interface. In order for a device on an ethernet (or other LAn) network to communicate with another device on the same ethernet network, it must know the device s MAC address. However, in many cases, a device may just know another device s IP address, not its MAC address. In order to get the IP packet to its destination, a host must have some mechanism to match a destination IP address to a destination MAC address. This is the purpose of the Address Resolution Protocol. ARP is defined in RFC 826. However, RFC 826 contained some ambiguities that were clarified in RFC 1122 (Host network Requirements). As such, ARP implementations need to incorporate both RFC 826 and RFC 1122 in order to work reliably and consistently with other implementations. RFC 826 introduced the concept of an Address Resolution Protocol as a useful way for devices to locate the ethernet hardware address of another IP host on the same local network. All Local Area network (LAn) media and many WAn media now use ARP to locate the hardware addresses of other IP devices on the local network. When a device needs to send an IP packet to another device on the local network, the IP software will first check to see if it knows the hardware address associated with
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the destination IP address (typically by consulting a cache of saved information from previous requests). If so, the sender just transmits the data to the destination system, using the protocols and addressing appropriate for whatever network medium is in use by the two devices. However, if the destination system s hardware address is not known, the IP software has to locate it before any data can be sent. At this point, IP will call on ARP to locate the hardware address of the destination system. ARP performs this task by issuing an IP broadcast onto the network, requesting (ARP request) that the system that is using the specified IP address respond with its hardware address. If the destination system is powered up and on the network, it will see this broadcast (as will all of the other devices on the local network), and it will send an ARP response back to the original system. note that the response is not broadcast back over the network, but is instead sent directly to the requesting system. All the local devices should monitor the network for ARP broadcasts, and whenever they see a request for themselves (as indicated in the destination IP address field of the ARP request), they must generate a response packet and send it back to the requesting system. The response packet will consist of the local device s IP and hardware addresses. The response will also be marked as such, with the message-type field indicating that the current packet is an ARP response. The new ARP packet is then unicast directly to the original requester, where it is received and processed. In Figure 6.11, Host 1 tries to ping Host 2. Host 1 looks in its cache of MAC addresses for the destination MAC address of Host 2. If it is not there, Host 1 queues the ICMP packet and sends an ARP request message. The ARP request is a broadcast message, and it is sent to all hosts in the broadcast domain. each host opens the frame and checks the destination IP address. If it is not their address, the host ignores the packet. However, when Host 2 receives the request, it sees that it is the destination and sends an ARP reply. This ARP reply is transmitted in a frame that has the MAC address of Host 1 as its destination, and the MAC address of Host 2 as the source. Upon receiving the reply, Host 1 now learns the MAC address of Host 2 and is able to frame the ICMP message and send it to Host 2. When the requesting system gets an ARP response, it stores the hardware and IP address pair of the requested device in a local cache, as illustrated in Figure 6.12. The next time that system needs to send data, it checks the local cache. If an entry is found, it uses that, eliminating the need to broadcast another request.
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