Understanding Linux Network Internals

Part VI. Neighboring Subsystem

Packets use a Layer three protocol such as IP to reach a LAN, and then a Layer two protocol such as Ethernet to go from the router on the local network to the system where the endpoint application is running. But a step is missing in this scenario. How do the router and the application host know who each other are? In more technical terms, how can a host find the L2 address (such as a MAC address) that corresponds to a given IP address? The action of finding the L2 address associated with a given L3 address is referred to as “resolving the L3 address."The missing piece is filled in by a neighboring protocol.

The most familiar neighboring protocol is Address Resolution Protocol (ARP), and Chapter 28 describes it in general terms. The corresponding protocol used in IPv6 is Neighbor Discovery (ND). But the key principles and tasks of a neighboring protocol, and a neighboring subsystem within an operating system, can be generalized.

Here is what each chapter discusses:

Chapter 26 Neighboring Subsystem: Concepts

Describes why and when a neighboring protocol is used and lays out its major tasks.

Chapter 27 Neighboring Subsystem: Infrastructure

Discusses the infrastructure that is common to all neighboring protocols.

Chapter 28 Neighboring Subsystem: Address Resolution Protocol (ARP)

Describes how ARP, the most common neighboring protocol and the one readers are most likely to have interacted with, uses the infrastructure.

Chapter 29 Neighboring Subsystem: ...