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.This implies that the data link layer need not provide this service.Ethernet provides globally unique addresses except it is not globally communicable (i.e., two arbitrarily chosen Ethernet devices will only be able to communicate if they are on the same bus).
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Encapsulation of user data in a UDP datagram inside an IP packet. |
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An example IP header as captured by ethereal. |
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 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).
Because of the abstraction provided by encapsulation, IP can be used over a
heterogenous network (i.e., a network connecting two computers can be any mix of
Ethernet,
ATM,
FDDI,
Wi-fi,
Token ring, etc.) and it makes no difference to the
upper layer protocols.
All the data link layers can (and do) have their own set of addressing (or possibly the complete lack of it) and the need to resolve IP addresses to data link addresses is needed.This resolving is addressed by the
Address Resolution Protocol (ARP).
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.
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.
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.
IP is the common element found in today's public
Internet. The current and most popular network layer protocol in use today is
IPv4; this version of the protocol is assigned version 4. IPv4 was adopted by the
United States Department of Defense as
MIL-STD-1778.
IPv6 is the proposed successor to IPv4 whose most prominent change is the addressing.IPv4 uses
32-bit addresses (~4 billion addresses) while IPv6 uses
128-bit addresses (~3.4×10
38 addresses). Although adoption of IPv6 has been slow, as of
2008, all
United States government systems must support IPv6.
[http://www.gcn.com/print/25_16/41051-1.html]Versions 0 through 3 were either reserved or unused; version 5 was used for an experimental stream protocol. Other version numbers have been assigned, usually for experimental protocols, but have not been widely used.
*
Connectionless protocol*
IANA*
Internet protocol suite*
IPv4*
IPv6*
IP address*
IP packet*
TCP and UDP port numbers*
Transmission Control Protocol* RFC 791
*
Your IP Address*
The Law of Leaky Abstractions by Joel Spolsky
