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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)

PPP Multilink Protocol (MP) Frame Format 1 Address Resolution and the TCP/IP Address Resolution Protocol (ARP)

The second layer of the OSI Reference Model is the data link layer; it corresponds to the TCP/IP network interface layer. It is there that most LAN, WAN and WLAN technologies are defined, such as Ethernet and IEEE 802.11. The third layer is the network layer, also called the internet layer in the TCP/IP model, where internetworking protocols are defined, the most notable being the Internet Protocol. These two layers are intimately related, because messages sent at the network layer must be carried over individual physical networks at the data link layer. They perform different tasks but as neighbors in the protocol stack, must cooperate with each other.

There is a set of protocols that serves the important task of linking together these two layers and allowing them to work together. The problem with them is deciding where exactly they should live! They are sort of the “black sheep” of the networking world—nobody denies their importance, but they always think they belong in “the other guy's” layer. For example, since these protocols pass data on layer two networks, the folks who deal with layer two technologies say they belong at layer three. But those who work with layer three protocols consider these “low level” protocols that provide services to layer three, and hence put them as part of layer two.

So where do they go? Well, to some extent it doesn't really matter. Even if they are “black sheep” I consider them somewhat special, so I gave them their own home. Welcome to “networking layer limbo”, also known as “OSI layer two-and-a-half”. J This is where a couple of protocols are described that serve as “glue” between the data link and network layers. The main job performed here is address resolution, or providing mappings between layer two and layer three addresses. This resolution can be done in either direction, and is represented by the two TCP/IP protocols ARP and RARP (which, despite their similarities, are used for rather different purposes in practice.)

PPP Multilink Protocol (MP) Frame Format 1 Address Resolution and the TCP/IP Address Resolution Protocol (ARP)