IP has the task of delivering packets from the source host to the destination host solely based on the IP addresses in the packet headers. For this purpose, IP defines packet structures that encapsulate the data to be delivered. It also defines addressing methods that are used to label the datagram with source and destination information.
Encapsulation of application data carried by UDP to a link protocol frame
The Internet Protocol is responsible for addressing host interfaces, encapsulating data into datagrams (including fragmentation and reassembly) and routing datagrams from a source host interface to a destination host interface across one or more IP networks.[2] For these purposes, the Internet Protocol defines the format of packets and provides an addressing system.
Each datagram has two components: a header and a payload. The IP header includes a source IP address, a destination IP address, and other metadata needed to route and deliver the datagram. The payload is the data that is transported. This method of nesting the data payload in a packet with a header is called encapsulation.
IP addressing entails the assignment of IP addresses and associated parameters to host interfaces. The address space is divided into subnets, involving the designation of network prefixes. IP routing is performed by all hosts, as well as routers, whose main function is to transport packets across network boundaries. Routers communicate with one another via specially designed routing protocols, either interior gateway protocols or exterior gateway protocols, as needed for the topology of the network.[3]
Version history
A timeline for the development of the transmission control Protocol TCP and Internet Protocol IPFirst Internet demonstration, linking the ARPANET, PRNET, and SATNET on November 22, 1977
IP versions 1 to 3 were experimental versions, designed between 1973 and 1978.[5] The following Internet Experiment Note (IEN) documents describe version 3 of the Internet Protocol, prior to the modern version of IPv4:
IEN 2 (Comments on Internet Protocol and TCP), dated August 1977 describes the need to separate the TCP and Internet Protocol functionalities (which were previously combined). It proposes the first version of the IP header, using 0 for the version field.
IEN 26 (A Proposed New Internet Header Format), dated February 1978 describes a version of the IP header that uses a 1-bit version field.
IEN 28 (Draft Internetwork Protocol Description Version 2), dated February 1978 describes IPv2.
IEN 41 (Internetwork Protocol Specification Version 4), dated June 1978 describes the first protocol to be called IPv4. The IP header is different from the modern IPv4 header.
IEN 44 (Latest Header Formats), dated June 1978 describes another version of IPv4, also with a header different from the modern IPv4 header.
IEN 54 (Internetwork Protocol Specification Version 4), dated September 1978 is the first description of IPv4 using the header that would be standardized in RFC760.
The dominant internetworking protocol in the Internet Layer in use is IPv4; the number 4 identifies the protocol version, carried in every IP datagram. IPv4 is described in RFC791 (1981).
Versions 2 and 3, and a draft of version 4, allowed an address length of up to 128 bits,[6] but this was reduced to 32 bits in the final version of IPv4.
Version number 5 was used by the Internet Stream Protocol, an experimental streaming protocol that was not adopted.[5]
The successor to IPv4 is IPv6. IPv6 was a result of several years of experimentation and dialog during which various protocol models were proposed, such as TP/IX (RFC1475), PIP (RFC1621) and TUBA (TCP and UDP with Bigger Addresses, RFC1347). Its most prominent difference from version 4 is the size of the addresses. While IPv4 uses 32 bits for addressing, yielding c. 4.3 billion (4.3×109) addresses, IPv6 uses 128-bit addresses providing c. 3.4×1038 addresses. Although adoption of IPv6 has been slow, as of January 2023[update], most countries in the world show significant adoption of IPv6,[7] with over 41% of Google's traffic being carried over IPv6 connections.[8]
The assignment of the new protocol as IPv6 was uncertain until due diligence assured that IPv6 had not been used previously.[9] Other Internet Layer protocols have been assigned version numbers,[10] such as 7 (IP/TX), 8 and 9 (historic). Notably, on April 1, 1994, the IETF published an April Fools' Day joke about IPv9.[11] IPv9 was also used in an alternate proposed address space expansion called TUBA.[12] A 2004 Chinese proposal for an "IPv9" protocol appears to be unrelated to all of these, and is not endorsed by the IETF.
Reliability
The design of the Internet protocol suite adheres to the end-to-end principle, a concept adapted from the CYCLADES project. Under the end-to-end principle, the network infrastructure is considered inherently unreliable at any single network element or transmission medium and is dynamic in terms of the availability of links and nodes. No central monitoring or performance measurement facility exists that tracks or maintains the state of the network. For the benefit of reducing network complexity, the intelligence in the network is located in the end nodes.
As a consequence of this design, the Internet Protocol only provides best-effort delivery and its service is characterized as unreliable. In network architectural parlance, it is a connectionless protocol, in contrast to connection-oriented communication. Various fault conditions may occur, such as data corruption, packet loss and duplication. Because routing is dynamic, meaning every packet is treated independently, and because the network maintains no state based on the path of prior packets, different packets may be routed to the same destination via different paths, resulting in out-of-order delivery to the receiver.
All fault conditions in the network must be detected and compensated by the participating end nodes. The upper layer protocols of the Internet protocol suite are responsible for resolving reliability issues. For example, a host may buffer network data to ensure correct ordering before the data is delivered to an application.
IPv4 provides safeguards to ensure that the header of an IP packet is error-free. A routing node discards packets that fail a header checksum test. Although the Internet Control Message Protocol (ICMP) provides notification of errors, a routing node is not required to notify either end node of errors. IPv6, by contrast, operates without header checksums, since current link layer technology is assumed to provide sufficient error detection.[13][14]
Link capacity and capability
The dynamic nature of the Internet and the diversity of its components provide no guarantee that any particular path is actually capable of, or suitable for, performing the data transmission requested. One of the technical constraints is the size of data packets possible on a given link. Facilities exist to examine the maximum transmission unit (MTU) size of the local link and Path MTU Discovery can be used for the entire intended path to the destination.[15]
The IPv4 internetworking layer automatically fragments a datagram into smaller units for transmission when the link MTU is exceeded. IP provides re-ordering of fragments received out of order.[16] An IPv6 network does not perform fragmentation in network elements, but requires end hosts and higher-layer protocols to avoid exceeding the path MTU.[17]
The Transmission Control Protocol (TCP) is an example of a protocol that adjusts its segment size to be smaller than the MTU. The User Datagram Protocol (UDP) and ICMP disregard MTU size, thereby forcing IP to fragment oversized datagrams.[18]
Security
During the design phase of the ARPANET and the early Internet, the security aspects and needs of a public, international network could not be adequately anticipated. Consequently, many Internet protocols exhibited vulnerabilities highlighted by network attacks and later security assessments. In 2008, a thorough security assessment and proposed mitigation of problems was published.[19] The IETF has been pursuing further studies.[20]
^Cerf, V.; Kahn, R. (1974). "A Protocol for Packet Network Intercommunication"(PDF). IEEE Transactions on Communications. 22 (5): 637–648. doi:10.1109/TCOM.1974.1092259. ISSN1558-0857. The authors wish to thank a number of colleagues for helpful comments during early discussions of international network protocols, especially R. Metcalfe, R. Scantlebury, D. Walden, and H. Zimmerman; D. Davies and L. Pouzin who constructively commented on the fragmentation and accounting issues; and S. Crocker who commented on the creation and destruction of associations.
