Internet Protocol
The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork.
IP is a network layer protocol in the internet protocol suite and is encapsulated in a data link layer protocol (e.g., Ethernet). As a lower layer protocol, IP provides the service of communicable unique global addressing amongst computers.
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Packetization
Data from an upper layer protocol is encapsulated inside one or more packets/datagrams (the terms are basically synonymous in IP). No circuit setup is needed before a host tries to send packets to a host it has previously not communicated with (this is the point of a packet-switched network), thus IP (Internet protocol) is a connectionless protocol. This is quite unlike Public Switched Telephone Networks that require the setup of a circuit before a phone call may go through (a connection-oriented protocol).
Reliability
IP provides an unreliable service (i.e., best effort delivery). This means that the network makes no guarantees about the packet and none, some, or all of the following may apply:
- data corruption
- out of order (packet A may be sent before packet B, but B can arrive before A)
- duplicate arrival
- lost or dropped/discarded
In terms of reliability the only thing IP does is ensure the IP packet's header is error-free through the use of a checksum. This has the side-effect of discarding packets with bad headers on the spot, and with no required notification to either end (though an ICMP message may be sent).
To address any of these reliability issues, an upper layer protocol must handle it. For example, to ensure in-order delivery the upper layer may have to cache data until it can be passed up in order.
If the upper layer protocol does not self-police its own size by first looking at the Layer 2 Maximum Transmission Unit (MTU) size, and sends the IP layer too much data, IP is forced to fragment the original datagram into smaller fragments for transmission. IP does provide re-ordering of any fragments that arrive out of order by using the fragmentation flags and offset.<ref>Siyan, Karanjit. Inside TCP/IP, New Riders Publishing, 1997. ISBN 1-56205-714-6</ref> TCP is a good example of a protocol that will adjust its segment size to be smaller than the MTU. User Datagram Protocol (UDP) and Internet Control Message Protocol (ICMP) are examples of protocols that disregard MTU size thereby forcing IP to fragment oversized datagrams.<ref>http://www.securityfocus.com/infocus/1870</ref>
The primary reason for the lack of reliability is to reduce the complexity of routers. While this does give routers carte blanche to do as they please with packets, anything less than best effort yields a poorer experience for the user. So, even though no guarantees are made, the better the effort made by the network, the better the experience for the user. Also, most "reliable" networks end up being less than completely reliable so an upper layer usually has to do reliability checking anyway. See End-to-end principle.
IP addressing and routing
Perhaps the most complex aspects of IP are IP addressing and routing. Addressing refers to how end hosts become assigned IP addresses and how subnetworks of IP host addresses are divided and grouped together. IP routing is performed by all hosts, but most importantly by internetwork routers, which typically use either interior gateway protocols (IGPs) or external gateway protocols (EGPs) to help make IP datagram forwarding decisions across IP connected networks.
See Also
Further Reading
For further related information please refer to Wikipedia Internet protocol
