ANNEX C Part 2: Address Resolution Protocol
What are the types of Ethernet broadcast messages and Ethernet unicast messages?
ARP requests and ARP replies
What does a node do when it needs to create a frame and the ARP cache does not contain a map of an IP address to a destination MAC address?
It generates an ARP request
What two things must a node use in order to send data?
MAC address and an IP address
What are the two functions of the ARP protocol?
Resolving IPv4 addresses to MAC addresses and maintaining a table of mappings
Resolving IPv4 Addresses to MAC addresses
o For a frame to be placed on the LAN media, it must have a destination MAC address. When a packet is sent to the data link layer to be encapsulated into a frame, the node refers to a table in its memory to find the data link layer address that is mapped to the destination IPv4 address. This table is called the ARP table or the ARP cache. The ARP table is stored in the RAM of the device. o Each entry, or row, of the ARP table binds an IP address with a MAC address. We call the relationship between the two values a map - it simply means that you can locate an IP address in the table and discover the corresponding MAC address. The ARP table temporarily saves (caches) the mapping for the devices on the local LAN. o To begin the process, a transmitting node attempts to locate the MAC address mapped to an IPv4 destination. If this map is found in the table, the node uses the MAC address as the destination MAC in the frame that encapsulates the IPv4 packet. The frame is then encoded onto the networking media.
Maintaining the ARP Table
· The ARP table is maintained dynamically. There are two ways that a device can gather MAC addresses. One way is to monitor the traffic that occurs on the local network segment. As a node receives frames from the media, it can record the source IP and MAC address as a mapping in the ARP table. As frames are transmitted on the network, the device populates the ARP table with address pairs. · Another way a device can get an address pair is to send an ARP request. An ARP request is a Layer 2 broadcast to all devices on the Ethernet LAN. The ARP request contains the IP address of the destination host and the broadcast MAC address, FFFF.FFFF.FFFF. Since this is a broadcast, all nodes on the Ethernet LAN will receive it and look at the contents. The node with the IP address that matches the IP address in the ARP request will reply. The reply will be a unicast frame that includes the MAC address that corresponds to the IP address in the request. This response is then used to make a new entry in the ARP table of the sending node. · Entries in the ARP table are time stamped in much the same way that MAC table entries are time stamped in switches. If a device does not receive a frame from a particular device by the time the time stamp expires, the entry for this device is removed from the ARP table. · Additionally, static map entries can be entered in an ARP table, but this is rarely done. Static ARP table entries do not expire over time and must be manually removed.
ARP Request Process
· When ARP receives a request to map an IPv4 address to a MAC address, it looks for the cached map in its ARP table. If an entry is not found, the encapsulation of the IPv4 packet fails and the Layer 2 processes notify ARP that it needs a map. The ARP processes then send out an ARP request packet to discover the MAC address of the destination device on the local network. If a device receiving the request has the destination IP address, it responds with an ARP reply. A map is created in the ARP table. Packets for that IPv4 address can now be encapsulated in frames. · If no device responds to the ARP request, the packet is dropped because a frame cannot be created. This encapsulation failure is reported to the upper layers of the device. If the device is an intermediary device, like a router, the upper layers may choose to respond to the source host with an error in an ICMPv4 packet. · All frames must be delivered to a node on the local network segment. If the destination IPv4 host is on the local network, the frame will use the MAC address of this device as the destination MAC address. · If the destination IPv4 host is not on the local network, the source node needs to deliver the frame to the router interface that is the gateway or next hop used to reach that destination. The source node will use the MAC address of the gateway as the destination address for frames containing an IPv4 packet addressed to hosts on other networks. · The gateway address of the router interface is stored in the IPv4 configuration of the hosts. When a host creates a packet for a destination, it compares the destination IP address and its own IP address to determine if the two IP addresses are located on the same Layer 3 network. If the receiving host is not on the same network, the source uses the ARP process to determine a MAC address for the router interface serving as the gateway. · In the event that the gateway entry is not in the table, the normal ARP process will send an ARP request to retrieve the MAC address associated with the IP address of the router interface. · For each device, an ARP cache timer removes ARP entries that have not been used for a specified period of time. The times differ depending on the device and its operating system. For example, some Windows operating systems store ARP cache entries for 2 minutes. If the entry is used again during that time, the ARP timer for that entry is extended to 10 minutes. · Commands may also be used to manually remove all or some of the entries in the ARP table. After an entry has been removed, the process for sending an ARP request and receiving an ARP reply must occur again to enter the map in the ARP table. Each device has an operating system-specific command to delete the contents of the ARP cache. These commands do not invoke the execution of ARP in any way. They merely remove the entries of the ARP table. ARP service is integrated within the IPv4 protocol and implemented by the device. Its operation is transparent to both upper layer applications and users.
