Network Layer
Encyclopedia
The network layer is layer 3 of the seven-layer OSI model
of computer networking.
The network layer is responsible for packet forwarding including routing
through intermediate routers, whereas the data link layer
is responsible for media access control, flow control and error checking.
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
functions.
Functions of the network layer include:
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
.
of the Internet
(RFC 1122). This model has a layer called the Internet layer
, located above the link layer
. In many textbooks and other secondary references the Internet layer is equated with OSI's network layer. However, this comparison is misleading as the allowed characteristics of protocols (e.g., whether they are connection-oriented or connection-less) placed into these layers 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 the Internet Engineering Task Force
(IETF) considers it to be "harmful" (RFC 3439).
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
of computer networking.
The network layer is responsible for packet forwarding including routing
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
through intermediate routers, whereas the data link layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
is responsible for media access control, flow control and error checking.
The network layer provides the functional and procedural means of transferring variable length data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
sequences from a source to a destination host via one or more networks while maintaining the quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
functions.
Functions of the network layer include:
- Connection model: connectionless communication
- For example, IPInternet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
is connectionless, in that a frame can travel from a sender to a recipient without the recipient having to send an acknowledgement. Connection-oriented protocols exist at other, higher 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, 1 Main Street" to Dubliners, or "Fred Murphy, 1 Main Street, Dublin" to people in Ireland, or "Fred Murphy, 1 Main Street, Dublin, Ireland" to people anywhere in the world. On the Internet, addresses are known as Internet Protocol (IP) addressesIP addressAn Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
.- 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 ProtocolInternet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IPv4IPv4Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
) was not designed with this feature in mind, although mobility extensions exist. IPv6IPv6Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
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
Transport layer
In computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols...
and issues service requests to the data link layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
.
Protocols
- IPv4/IPv6, Internet ProtocolInternet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
- DVMRP, Distance Vector Multicast Routing Protocol
- ICMP, Internet Control Message ProtocolInternet Control Message ProtocolThe Internet Control Message Protocol is one of the core protocols of the Internet Protocol Suite. It is chiefly used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be...
- IGMP, Internet Group Multicast Protocol
- PIM-SM, Protocol Independent Multicast Sparse ModeProtocol Independent MulticastProtocol-Independent Multicast is a family of multicast routing protocols for Internet Protocol networks that provide one-to-many and many-to-many distribution of data over a LAN, WAN or the Internet...
- PIM-DM, Protocol Independent Multicast Dense ModeProtocol Independent MulticastProtocol-Independent Multicast is a family of multicast routing protocols for Internet Protocol networks that provide one-to-many and many-to-many distribution of data over a LAN, WAN or the Internet...
- IPsec, Internet Protocol SecurityIPsecInternet Protocol Security is a protocol suite for securing Internet Protocol communications by authenticating and encrypting each IP packet of a communication session...
- IPX, Internetwork Packet ExchangeIPXInternetwork Packet Exchange is the OSI-model Network layer protocol in the IPX/SPX protocol stack.The IPX/SPXM protocol stack is supported by Novell's NetWare network operating system. Because of Netware's popularity through the late 1980s into the mid 1990s, IPX became a popular internetworking...
- RIP, Routing Information ProtocolRouting Information ProtocolThe Routing Information Protocol is a distance-vector routing protocol, which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops allowed for RIP is 15....
- DDP, Datagram Delivery ProtocolDatagram Delivery ProtocolDatagram Delivery Protocol is a member of the AppleTalk networking protocol suite. Its main responsibility is for socket-to-socket delivery of datagrams over an AppleTalk network....
- BGP, Border Gateway ProtocolBorder Gateway ProtocolThe Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector protocol...
Relation to TCP/IP model
The TCP/IP model describes the protocol suiteInternet protocol suite
The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
of the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
(RFC 1122). This model has a layer called the Internet layer
Internet layer
The internet layer or IP layer is a group of internetworking methods in the Internet protocol suite, commonly also called TCP/IP, which is the foundation of the Internet...
, located above the link layer
Link Layer
In computer networking, the link layer is the lowest layer in the Internet Protocol Suite , the networking architecture of the Internet . It is the group of methods or protocols that only operate on a host's link...
. In many textbooks and other secondary references the Internet layer is equated with OSI's network layer. However, this comparison is misleading as the allowed characteristics of protocols (e.g., whether they are connection-oriented or connection-less) placed into these layers 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 the Internet Engineering Task Force
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
(IETF) considers it to be "harmful" (RFC 3439).