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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Concepts and Overview

IP Overview and Key Operational Characteristics 1 IP History, Standards, Versions and Closely-Related Protocols

In the preceding topic I described the general operation of IP and boiled down its primary job as internetwork datagram delivery. I also explained the most important characteristics of how IP does this job. With that as a foundation, let's now look a bit deeper, at how IP “gets the job done”. A good way to do this is to examine the various functions that the Internet Protocol includes.

The exact number of IP functions depends on where you “draw the line” between certain activities. For explanatory purposes, however, I view IP as having four basic functions (or more accurately, function sets):

• Addressing: In order to perform the job of delivering datagrams, IP must know where to deliver them to! For this reason, IP includes a mechanism for host addressing. Furthermore, since IP operates over internetworks, its system is designed to allow unique addressing of devices across arbitrarily large networks. It also contains a structure to facilitate the routing of datagrams to distant networks if that is required.
  
  Since most of the other TCP/IP protocols use IP, understanding the IP addressing scheme is of vital importance to comprehending much of what goes on in TCP/IP.
  
  
• Data Encapsulation and Formatting/Packaging: As the TCP/IP network layer protocol, IP accepts data from the transport layer protocols UDP and TCP. It then encapsulates this data into an IP datagram using a special format prior to transmission.
  
  
• Fragmentation and Reassembly: IP datagrams are passed down to the data link layer for transmission on the local network. However, the maximum frame size of each physical/data-link network using IP may be different. For this reason, IP includes the ability to fragment IP datagrams into pieces so they can each be carried on the local network. The receiving device uses the reassembly function to recreate the whole IP datagram again.

• Routing / Indirect Delivery: When an IP datagram must be sent to a destination on the same local network, this can be done easily using the network's underlying LAN/WLAN/WAN protocol using what is sometimes called direct delivery. However, in many (if not most cases) the final destination is on a distant network not directly attached to the source. In this situation the datagram must be delivered indirectly. This is accomplished by routing the datagram through intermediate devices (shockingly called routers). IP accomplishes this in concert with support from the other protocols including ICMP and the TCP/IP gateway/routing protocols such as RIP and BGP.

As you continue on in this section on IP will find that I have structured the sub-sections that provide more detail one the main IP version and IP-related protocols based on these general functions.

IP Overview and Key Operational Characteristics 1 IP History, Standards, Versions and Closely-Related Protocols