The TCP/IP Guide by Charles M. Kozierok, Chapter 5: General OSI Reference Model Issues and Concepts

Notes on Chapter 13, Sections Address Resolution Concepts and Issues and TCP/IP Address Resolution Protocol.

Link to Champlain College Book Library

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Address Resolution Concepts and Issues

• MAC addresses are used for local transmissions between hardware devices that can communicate directly, assigned by the NIC
• ARP is a part of the TCP/IP Protocol
• ARP belongs in layer2/3 (data link and network) of the OSI model
• "The layer 2 addresses (such as IEEE 802 MAC addresses) are used for local transmissions between hardware devices that can communicate directly. They are used to implement basic local area network (LAN), wireless LAN (WLAN), and wide area network (WAN) technologies. In contrast, layer 3 addresses (most commonly, IP addresses) are used in internetworking to create the equivalent of a massive virtual network at the network layer."

"Address resolution is required because internetworked devices communicate logically using layer 3 addresses, but the actual transmissions between devices take place using layer 2 (hardware) addresses."

• "Address resolution can be accomplished in two basic ways: direct [static] mapping and dynamic resolution."
• "When the layer 2 address is smaller than the layer 3 address, it is possible to define a direct mapping between them so that the hardware address can be determined directly from the network layer address. This makes address resolution extremely simple, but reduces flexibility in how addresses are assigned."

"In the direct mapping technique, a formula is used to map the higher-layer address into the lower-layer address." Direct mapping is not efficient when the hardware address size exceeds the network layer address size.

Dynamic Addressing Limo Analogy

The ARP cache table is both static and dynamic -- it can be manually updated, but the protocol itself is a dynamic address resolution. The Limo Driver analogy helps to explain how dynamic addressing works. A limo driver knows who they are picking up and they hold up a sign with the riders name, however they do not know the rider's face until they meet and great each other. This is similar to dynamic addressing -- ARP will send a broadcast to the network (like the limo sign) with an IP address (the rider's name). The broadcast will only be received by the device who currently owns the IP address (only the rider will approach the sign), and the device will respond with it's MAC address (the rider will great the driver, and the driver will see their face). Vroom vroom.

Enchance Dynamic Addressing with caching mechanism (otherwise, broadcasts would clog up bandwidth).

TCP/IP Address Resolution Protocol (ARP)

• ARP is a dynamic resolution protocol that matches IP addresses to the data link layer MAC address.

• First was used on the Ethernet as IEEE 802.3, but has transferred over to use in TCP/IP protocol suite.

• "ARP was developed to facilitate dynamic address resolution between IP and Ethernet and can now be used on other layer 2 technologies as well. It works by allowing an IP device to send a broadcast on the local network, and it requests a response with a hardware address from another device on the same local network."

• Source vs. Destination

• Two messages: to the destination and from the destination --> Sender vs. Target Device

Steps of ARP: 0.5. Before beginning the ARP transaction, ARP must decide if the destination device's IP addresses is a part of the local network or distant network. If local, the ARP broadcast occurs asking for destination MAC using the IP address. If distant, the ARP broadcast occurs asking for MAC of default gateway's/router's IP.

1. Source Device Checks Cache
2. Source Device Generate ARP Request Messege
3. Source Device Broadcasts ARP Request Message
4. Local Devices Process ARP Request Message
5. Destination Device Generate ARP Reply Message
6. Destination Device Updates ARP Cache
7. Destination Device Sends ARP Reply Message
8. Source Device Processes ARP Reply Message
9. Source Device Updates ARP Cache

ARP Message Format

• ARP Caching can be done statically (permanent) or dynamically (automatic and temporary)
• Dynamic Caching helps relieve these issues: Device Hardware Changes, Device IP Address Changes, Device Removal.

ARP Proxy:

"Since ARP relies on broadcasts for address resolution, and broadcasts are not propagated beyond a physical network, ARP cannot function between devices on different physical networks. When such operation is required, a device, such as a router, can be configured as an ARP proxy to respond to ARP requests on the behalf of a device on a different network."