1. Internet Protocol Concepts and Overview
IP is a very important protocol in internetworking; it wouldn't be an
exaggeration to say that you can't really comprehend modern networking
without a good understanding of IP. Unfortunately, IP can be somewhat
difficult to understand. This is probably because due to its importance, a large
amount of complexity has become associated with the protocol over the
years, to allow it to meet the many demands placed upon it.
Before diving into the details of how IP works, I feel it is worthwhile to take a
high-level look at IP and what it does. In this section I provide a brief
examination of basic concepts related to the Internet Protocol and how it
works. I begin with an overview of IP, how it operates in basic terms and the
most important characteristics of how it does its job. I then expand on this
discussion by describing the main functions of the Internet Protocol, which can
be used as an introduction to the remainder of the sections that explain IP in
considerably more detail. I conclude with a brief look at the history of
development of IP, its versions, and how it has spawned the development of
several IP-related protocols.
TCP/IP Lower-Layer (Interface, Internet and Transport)
Protocols (OSI Layers 2, 3 and 4)
The TCP/IP protocol suite is largely defined in terms of the protocols that
constitute it; several dozen are covered in this Guide. Most of the critical
protocols of the suite function at the lower layers of the OSI Reference Model:
layers 2, 3 and 4, which correspond to the network interface, internet and
transport layers in the TCP/IP model architecture. Included here are the all-
important Internet Protocol (IP) at layer 3 and Transmission Control Protocol
(TCP) at layer 4, which combine to give TCP/IP its name.
Due to the importance of these and other TCP/IP protocols at the lower
layers, this is the largest chapter of The TCP/IP Guide. It contains four
subsections. The first describes the two TCP/IP protocols that reside at the
network interface layer, layer 2 of the OSI model: PPP and SLIP. The second
describes a couple of “special” protocols that reside architecturally between
layers 2 and 3: ARP and RARP. The third covers the TCP/IP internet layer
(OSI network layer, layer 3), including IP and several other related and
support protocol. The fourth describes the TCP/IP transport layer protocols
TCP and UDP.

2. TCP/IP Internet Layer (OSI Network Layer) Protocols
The first two layers of the OSI Reference Model, the physical layer and data
link layer, deal primarily with physical network details. The various LAN,
WLAN and WAN protocols function primarily at these two layers to connect
devices to create networks, and perform functions such as physical
connection and signaling, media access control and local delivery of data
between devices on the same network. Above these layers, we move beyond
the hardware aspects of networking and closer to the more abstract realm of
software-related network functions.
The third OSI layer is the network layer. We are of course talking about
networks in this Guide, and it is no coincidence that the layer bearing that
name is one of the most important in comprehending how networks function. It
is here that we find protocols that tie networks together to create
internetworks, and also where cross-network addressing and routing are
performed. The network layer is also called the internet layer in the TCP/IP
model.
In this section I provide details for the various TCP/IP protocols that reside
architecturally at the TCP/IP internet layer / OSI network layer. Much of the
focus here is on the all-important Internet Protocol; the section covering IP
includes extensive coverage of IP version 4, IP version 6, and IP-related
protocols such as IPSec, Mobile IP and IP Network Address Translation
(NAT). The following three subsections cover IP support protocols such as the
Internet Control Message Protocol (ICMP) and IPv6 Neighbor Discovery (ND)
protocol, and the complete set of IP routing protocols.
Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related
Protocols (IP NAT, IPSec, Mobile IP)
The idea of singling out any one protocol as being more important than the
others in a network is kind of pointless, if you think about it. The protocols and
technologies work as a team to accomplish the goal of communication across
the network. Like any team, no single member can get the job done alone, no
matter how good they are. Still, if we were to try to pick a “most valuable
player” in the world of networking, a good case could be made that we have it
here in this section: the TCP/IP Internet Protocol (IP).
Even though it gets “second billing” in the name of the TCP/IP protocol suite,
IP is in fact the “workhorse” of TCP/IP. It implements key network-layer

3. functions including addressing, datagram handling and routing, and is the
foundation upon which other TCP/IP protocols are built. Even the ones lower
in the TCP/IP architecture such as ARP and PPP are easier to understand
when you know how IP works. In addition to the main functions implemented
by the IP protocol itself, there are also several protocols that have been
developed over the years that I call “IP-Related” because they are based on
IP but add new functionality or capabilities for special purposes.
In this section I provide considerable coverage of the TCP/IP Internet Protocol
and to several protocols that are closely related to IP. I begin with a section
that takes a general look at IP concepts and provides an overview of how IP
works. I then have a large section that looks at IP version 4, the current
version of the protocol that is in use on TCP/IP networks everywhere.
Following this I look at the “next generation” of IP, IP version 6. I then provide
sections covering three IP-related protocols: the IP Network Address
Translation protocol (IP NAT), the IP Security protocol set (IPSec), and the
adaptation of IP for mobile devices (Mobile IP).
IP Security (IPSec) Protocols
One of the weaknesses of the original Internet Protocol is that it lacks any sort
of general purpose mechanism for ensuring the authenticity and privacy of
data as it is passed over the internetwork. Since IP datagrams must usually
be routed between two devices over unknown networks, any information in
them is subject to being intercepted and even possibly changed. With the
increased use of the Internet for critical applications, security enhancements
were needed for IP. To this end, a set of protocols called IP Security or
IPSec was developed.
In this section I provide a brief description of IPSec concepts and protocols. I
begin with an overview of IPSec, including a discussion of the history of the
technology and defining standards. I describe the main components and
protocols of the IPSec suite, and its different architectures and methods for
implementation. I then move to actually discussing how IPSec works,
beginning with a description of the two IPSec modes (transport and tunnel)
and how they differ. I describe security associations and related constructs
such as the Security Parameter Index (SPI). The last three topics cover the
three main IPSec protocols: IPSec Authentication Header (AH), IPSec
Encapsulating Security Payload (ESP) and the IPSec Internet Key Exchange
(IKE).