Internet Layer

The Internet Layer is a group of internetworking methods in the TCP/IP protocol suite which is the foundation of the Internet (RFC 1122). It is the group of methods, protocols, and specifications which are used to transport datagrams (packets) from the originating host across network boundaries, if necessary, to the destination host specified by a network address (IP address) which is defined for this purpose by the Internet Protocol (IP). The Internet Layer derives its name from its function of forming an "internet" (uncapitalized), or facilitating "internetworking", which is the concept of connecting multiple networks with each other through gateways.

Internet Layer protocols use IP-based packets. The Internet Layer does not include the protocols that have the limited scope of communicating with other local ("on-link") network nodes for the purpose of maintaining link states between the local nodes, such as the local network topology, and that usually use protocols that are based on the framing of packets specific to the link types. Such protocols belong to the Link Layer.

A particularly crucial aspect in the Internet layer is the "Robustness Principle": "Be liberal in what you accept, and conservative in what you send" (RFC 1122), as a misbehaving host can deny Internet service to many other users.

Internet Layer functions

The Internet Layer has three basic functions: For outgoing packets, select the "next hop" host (gateway) and transmit the packet to this host by passing it to the appropriate Link Layer drivers; for incoming packets, capture packets and pass the packet payload up to the appropriate Transport Layer module. In addition it provides error detection and diagnostic capability.

In Version 4 of the Internet Protocol (IPv4), during both transmit and receive operations, IP is capable of automatic or intentional fragmentation or defragmentation of packets, based, for example, on the maximum transmission unit (MTU) of link elements. However, this feature has been dropped in IPv6, as the communications end points, the hosts, now have to perform path MTU discovery and assure that end-to-end transmissions don't exceed the minumum discovered.

In its operation, the Internet layer is not responsible for reliable transmission. It provides only an "unreliable" service, and "best effort" delivery. This means that the network makes no guarantees about packets' proper arrival (see also Internet Protocol#Reliability). This was an important design principle and change from the previous protocols used on the early ARPANET. Since packet delivery across diverse networks is inherently an unreliable and failure-prone operation, the burden of providing reliability was placed with the end points of a communication path, i.e., the hosts, rather than on the network. This is one of the reasons of the resiliency of the Internet against individual link failures and its proven scalability.

The function of providing reliability of service is the duty of higher level protocols, such as the Transmission Control Protocol (TCP) in the Transport Layer.

Integrity of packets is guaranteed only in IPv4 (not in IPv6) through checksums computed for IP packets.

Internet Layer protocols

The core protocols in the Internet Layer are:
* Internet protocol (IP), it is implemented in two versions, for IPv4 and IPv6.
* Internet Control Message Protocol (ICMP), primariy used for error and diagnostic functions, different implementations exist for IPv4 and IPv6.
* Internet Group Management Protocol (IGMP), used by IPv4 hosts and adjacent multicast routers to establish multicast group memberships.

Security

Internet Protocol Security (IPsec) is a suite of protocols for securing Internet Protocol (IP) communications by authenticating and/or encrypting each IP packet in a data stream. IPsec also includes protocols for cryptographic key establishment. IPsec was originally designed as a base specification in IPv6 (RFC 1825, RFC 1829) in 1995, but later superseded, see IPsec. The protocol was quickly back-ported for use in IPv4 as well.

Relation to OSI model

The Internet Layer of the TCP/IP model is often compared directly with the Network Layer (Layer 3) in the Open Systems Interconnection (OSI) protocol stack. Although both have some overlap, they are actually quite different. In particular, the allowed characteristics of protocols (e.g., whether they are connection-oriented or connection-less) placed in this layer are different in the two models. OSI's Network Layer is a "catch-all" layer for all protocols that facilitate network functionality. The Internet Layer, on the other hand, is specifically a suite of protocols that facilitate internetworking using the Internet Protocol.

Because of this, the OSI layer is often described to include protocols such as the Address Resolution Protocol (ARP), which clearly belongs in the Link Layer in the TCP/IP model.

In general, direct or strict comparisons between the TCP/IP model and the OSI model should be avoided. Layering in TCP/IP is not a principal design criterion and in general considered to be "harmful" (RFC 3439).

Unfortunately, despite clear primary references (cf. References) and normative standards documents the Internet Layer is still sometimes improperly called network layer, in analogy with the OSI model.

See also

* Network Layer
* Link Layer

References

* RFC 1122, Requirements for Internet Hosts -- Communication Layers, IETF, R. Braden (Editor), October 1989
* RFC 1123, Requirements for Internet Hosts -- Application and Support, IETF, R. Braden (Editor), October 1989
* RFC 791, Internet Protocol (IP), J. Postel, September 1981
* RFC 792, Internet Control Message Protocol (ICMP), J. Postel, September 1981
* RFC 950, Internet Standard Subnetting Procedure, J. Mogul and J. Postel, August 1985
* RFC 1112, Host Extensions for IP Multicasting, S. Deering, August 1989
* RFC 879, The TCP Maximum Segment Size and Related Topics, J. Postel, November 1983
* RFC 1108: Internet Protocol Security Options, B. Schofield, October 1989
* RFC 815: IP Datagram Reassembly Algorithms, D. Clark, July 1982
* RFC 816: Fault Isolation and Recovery, D. Clark, July 1982

Further reading

* RFC 3439, Some Internet Architectural Guidelines and Philosophy, R. Bush, D. Meyer, December 2002