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2. The Internet Protocol

IP provides the functionality for interconnecting end systems across multiple networks. For this purpose, IP is implemented in each end system and in routers, which are devices that provide connection between networks.

    2.2. First generation: IPv4

        2.2.1. Aim of IPv4

IP stands for Internet protocol. It is a network layer protocol (according to the Open Systems Interconnection (OSI) model designed by ISO) that contains addressing information and some control information that enables packets to be routed. IP was devised for use in interconnected systems of packet switched computer communication networks.
IP has two main functions: supplying connectionless (no call set up), best effort delivery of datagrams through an internetwork and providing fragmentation and reassembly of datagrams to support data links with different maximum transmission unit (MTU) sizes.

Each communicating device is assigned a fixed length IP address. The address identifies the network (which may be divided into sub-networks) and the host. The term "host" refers to any communicating device in an IP network. It includes any PC, printer, gateway, file server, or other device that has an IP address and talks on an IP network. Each datagram contains enough information to carry it to its destination. The network, however, does not make any guarantees if the datagram is delivered to destination. If the packet is lost, corrupted or for some other reason not delivered to its intended destination, the network does nothing to recover from the failure. This service model is commonly called as best effort or unreliable service. IP does not either guarantee anything about order in which packets arrive to destination nor that the packets are delivered at most once (datagrams may duplicate). However, there is a limited lifetime for each datagram.

        2.2.2. IP wide usage

The best effort connectionless service is the simplest service an internetwork can provide: this makes it possible to transfer datagrams over any link layer technology. 
Moreover IP delivers a few basic services that everyone needs (file transfer, electronic mail, remote logon) across a very large number of client and server systems. Several computers in a small department can use TCP/IP (along with other protocols) on a single LAN. The IP component provides routing from the department to the enterprise network, then to regional networks, and finally to the global Internet. On the battlefield a communications network will sustain damage, so the DOD designed TCP/IP to be robust and automatically recover from any node or phone line failure. This design allows the construction of very large networks with less central management. That is why IP was initially successful.
However, because of the automatic recovery, network problems can go undiagnosed and uncorrected for long periods of time.
Unfortunately, multimedia traffic most notably, telephony and multimedia conferencing, which constitutes a significant portion of a potential multicast traffic, possesses different characteristics. The Internet Protocol and mainly TCP which were designed for providing a reliable data transmission with minimal or no delay constraints require consequently some improvements or new protocols to provide the necessary services. Quality of Service should provide predictable and measurable network performance in terms of throughput and latency. This is a challenge for unicast where there's only a single sender and receiver but the challenge is much larger for multicast where a single sender can have any (changing) number of receivers. Thanks to the improvements concerning the physical way of telecommunications, IP has been able until now to provide with quite good service

        2.2.3. Shortcomings related to IPv4 protocol

The most important issue concerning IP deals with the address space of IPv4 that is filling rapidly. IPv4 addresses are implemented on 32 bits that allows more than 4 billions of possibilities. At the beginning, there were enough addresses but nowadays because of the amazing boom of the Internet, more addresses are needed. 
By using the hierarchic mechanism in order to allocate the addresses to hosts, most of them are wasted. Moreover, this issue could worsen because of the paradigm shift.
Another problem deals with the size of the routing tables. Indeed, the routing tables include information about the routes between different addresses. As the Internet has grown, the size of the routing tables has exploded. The largest problem is the subnet division to classes. The number of very large nets is very limited. The organization will be similar to CIDR (classless interdomain routing), which is the current patch for IPv4 to help in routing table problems. 
Besides, IPv4 do not supply any security mechanism as the authentication of the packet sender, the encryption of the packets before delivery. 
Finally, IPv4 do not enable to know the kind of data transmitted. It does really matter for applications that require guaranteed throughput, end-to-end delay, and/or jitter, such as multimedia or real-time communications. 
The fact that the Internet was really fast evolving as well as the appearance of new market stresses the IP flaws. That is why, the authorities have tried to improve the last IP version by creating IPv6.