Network Layer


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OSI Model
7 Application Layer
6 Presentation Layer
5 Session Layer
4 Transport Layer
3 Network Layer
2 Data Link Layer
1 Physical Layer

The Network Layer is Layer 3 of the seven-layer OSI model of computer networking.

The Network Layer is responsible for end-to-end (source to destination) packet delivery including routing through intermediate hosts, whereas the Data Link Layer is responsible for node-to-node (hop-to-hop) frame delivery on the same link.

The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination host via one or more networks while maintaining the quality of service and error control functions.

Functions of the Network Layer include:

For example, snail mail is connectionless, in that a letter can travel from a sender to a recipient without the recipient having to do anything. On the other hand, the telephone system is connection-oriented, because the other party is required to pick up the phone before communication can be established. The OSI Network Layer protocol can be either connection-oriented, or connectionless. In contrast, the TCP/IP Internet Layer supports only the connectionless Internet Protocol (IP); but connection-oriented protocols exist higher at other layers of that model.
  • Host addressing
Every host in the network needs to have a unique address which determines where it is. This address will normally be assigned from a hierarchical system, so you can be "Fred Murphy" to people in your house, "Fred Murphy, Main Street 1" to Dubliners, or "Fred Murphy, Main Street 1, Dublin" to people in Ireland, or "Fred Murphy, Main Street 1, Dublin, Ireland" to people anywhere in the world. On the Internet, addresses are known as Internet Protocol (IP) addresses.
  • Message forwarding
Since many networks are partitioned into subnetworks and connect to other networks for wide-area communications, networks use specialized hosts, called gateways or routers to forward packets between networks. This is also of interest to mobile applications, where a user may move from one location to another, and it must be arranged that his messages follow him. Version 4 of the Internet Protocol (IPv4) was not designed with this feature in mind, although mobility extensions exist. IPv6 has a better designed solution.

Within the service layering semantics of the OSI network architecture the Network Layer responds to service requests from the Transport Layer and issues service requests to the Data Link Layer.

Contents

Relation to TCP/IP model

The TCP/IP model describes the protocol suite of the Internet (RFC 1122). This model has a layer called the Internet Layer, located above the Link Layer. In many text books and other secondary references the Internet Layer is often equated with OSI's Network Layer. However, this is misleading as the allowed characteristics of protocols (e.g., whether they are connection-oriented or connection-less) placed into these layer are different in the two models. The Internet Layer of TCP/IP is in fact only a subset of functionality of the Network Layer. It only describes one type of network architecture, the Internet.

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

See also

References

  • RFC 1122
  • RFC 3439
  • Computer Networks, Fourth Edition, Andrew S.Tanenbaum, Prentice Hall, ISBN: 0130661023.

External links







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