Internet Protocol Concepts and OverviewIP is a very important protocol in internetworking; it wouldnt be anexaggeration to say that you cant really comprehend modern networkingwithout a good understanding of IP. Unfortunately, IP can be somewhatdifficult to understand. This is probably because due to its importance, a largeamount of complexity has become associated with the protocol over theyears, 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 ahigh-level look at IP and what it does. In this section I provide a briefexamination of basic concepts related to the Internet Protocol and how itworks. I begin with an overview of IP, how it operates in basic terms and themost important characteristics of how it does its job. I then expand on thisdiscussion by describing the main functions of the Internet Protocol, which canbe used as an introduction to the remainder of the sections that explain IP inconsiderably more detail. I conclude with a brief look at the history ofdevelopment of IP, its versions, and how it has spawned the development ofseveral 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 thatconstitute it; several dozen are covered in this Guide. Most of the criticalprotocols 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 andtransport 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 lowerlayers, this is the largest chapter of The TCP/IP Guide. It contains foursubsections. The first describes the two TCP/IP protocols that reside at thenetwork interface layer, layer 2 of the OSI model: PPP and SLIP. The seconddescribes a couple of “special” protocols that reside architecturally betweenlayers 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 andsupport protocol. The fourth describes the TCP/IP transport layer protocolsTCP and UDP.
TCP/IP Internet Layer (OSI Network Layer) ProtocolsThe first two layers of the OSI Reference Model, the physical layer and datalink layer, deal primarily with physical network details. The various LAN,WLAN and WAN protocols function primarily at these two layers to connectdevices to create networks, and perform functions such as physicalconnection and signaling, media access control and local delivery of databetween devices on the same network. Above these layers, we move beyondthe hardware aspects of networking and closer to the more abstract realm ofsoftware-related network functions.The third OSI layer is the network layer. We are of course talking aboutnetworks in this Guide, and it is no coincidence that the layer bearing thatname is one of the most important in comprehending how networks function. Itis here that we find protocols that tie networks together to createinternetworks, and also where cross-network addressing and routing areperformed. The network layer is also called the internet layer in the TCP/IPmodel.In this section I provide details for the various TCP/IP protocols that residearchitecturally at the TCP/IP internet layer / OSI network layer. Much of thefocus here is on the all-important Internet Protocol; the section covering IPincludes extensive coverage of IP version 4, IP version 6, and IP-relatedprotocols such as IPSec, Mobile IP and IP Network Address Translation(NAT). The following three subsections cover IP support protocols such as theInternet 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-RelatedProtocols (IP NAT, IPSec, Mobile IP)The idea of singling out any one protocol as being more important than theothers in a network is kind of pointless, if you think about it. The protocols andtechnologies work as a team to accomplish the goal of communication acrossthe network. Like any team, no single member can get the job done alone, nomatter how good they are. Still, if we were to try to pick a “most valuableplayer” in the world of networking, a good case could be made that we have ithere 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
functions including addressing, datagram handling and routing, and is thefoundation upon which other TCP/IP protocols are built. Even the ones lowerin the TCP/IP architecture such as ARP and PPP are easier to understandwhen you know how IP works. In addition to the main functions implementedby the IP protocol itself, there are also several protocols that have beendeveloped over the years that I call “IP-Related” because they are based onIP but add new functionality or capabilities for special purposes.In this section I provide considerable coverage of the TCP/IP Internet Protocoland to several protocols that are closely related to IP. I begin with a sectionthat takes a general look at IP concepts and provides an overview of how IPworks. I then have a large section that looks at IP version 4, the currentversion 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 providesections covering three IP-related protocols: the IP Network AddressTranslation protocol (IP NAT), the IP Security protocol set (IPSec), and theadaptation of IP for mobile devices (Mobile IP).IP Security (IPSec) ProtocolsOne of the weaknesses of the original Internet Protocol is that it lacks any sortof general purpose mechanism for ensuring the authenticity and privacy ofdata as it is passed over the internetwork. Since IP datagrams must usuallybe routed between two devices over unknown networks, any information inthem is subject to being intercepted and even possibly changed. With theincreased use of the Internet for critical applications, security enhancementswere needed for IP. To this end, a set of protocols called IP Security orIPSec was developed.In this section I provide a brief description of IPSec concepts and protocols. Ibegin with an overview of IPSec, including a discussion of the history of thetechnology and defining standards. I describe the main components andprotocols of the IPSec suite, and its different architectures and methods forimplementation. 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 constructssuch as the Security Parameter Index (SPI). The last three topics cover thethree main IPSec protocols: IPSec Authentication Header (AH), IPSecEncapsulating Security Payload (ESP) and the IPSec Internet Key Exchange(IKE).