The network layer is level three of the seven level OSI model. It responds to service requests from the transport layer and issues service requests to the data link layer.

The network layer addresses messages and translates logical addresses and names into physical addresses. It also determines the route from the source to the destination computer and manages traffic problems, such as switching, routing, and controlling the congestion of data packets.

In essence, the network layer is responsible for end to end (source to destination) packet delivery, whereas the data link layer is responsible for node to node (hop to hop) packet delivery.

The network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the transport layer. The Network layer performs network routing, flow control, network segmentation/desegmentation, and error control functions.

The network layer deals with transmitting information all the way from its source to its destination - and transmitting things from anywhere you like, to anywhere you like. If you can't contact a place at the network layer, then you can't contact that place at all. Here are some things that the network layer needs to address:

• Is the network connection-oriented or connectionless?

For example, snail mail is connectionless, because you can send a letter to someone without them doing anything and they will receive the letter. On the other hand, the telephone system is connection-oriented, because it requires the other party has to pick up the phone before communication can be established. The OSI Network Layer protocol can be either connection-oriented, or connectionless. The TCP/IP Internet Layer (equivalent to OSI's Network Layer) supports only the connectionless Internet Protocol (IP).

• What are the Global Addresses?

Everybody in the network needs to have a unique address which determines who they are. This address will normally be hierarchical, so you can be "Fred Murphy" to Dubliners, or "Fred Murphy, Dublin" to people in Ireland, or "Fred Murphy, Dublin, Ireland" to people anywhere in the world. On the internet, these addresses are known as IP Numbers.

• How do you forward a message?

This is of particular interest to mobile applications, where a user may rapidly move from place to place, and it must be arranged that his messages follow him. Version 4 of the Internet Protocol (IPv4) doesn't really allow for this, though it has been hacked somewhat since its inception. Fortunately, the forthcoming IPv6 has a much better designed solution, which should make this type of applications much smoother.

In the traditional postage system (commonly referred to as snail mail) this role is provided by the postman (to some extent).

Examples

• IP/IPv6, Internet Protocol
  • DVMRP, Distance Vector Multicast Routing Protocol
  • ICMP, Internet Control Message Protocol
  • IGMP, Internet Group Multicast Protocol
  • PIM-SM, Protocol Independent Multicast Sparse Mode
  • PIM-DM, Protocol Independent Multicast Dense Mode
  • SLIP, Serial Line IP
• IPSec, Internet Protocol Security
• IPX, Internetwork Packet Exchange
  • RIP, Routing Information Protocol
  • NLSP, NetWare Link State Protocol
• X.25, Packet Level Protocol
  • X.75, Packet Switched Signaling Between Public Networks
• DDP, Datagram Delivery Protocol

See also

• Router
• ISO protocol suite
• DECnet

OSI protocols

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