Internet layer
Encyclopedia
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
. It is the group of methods, protocols, and specifications that 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 define communication between local (on-link) network nodes which fulfill 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.
) and transmit the packet to this host by passing it to the appropriate link layer
implementation; for incoming packets, capture packets and pass the packet payload up to the appropriate transport-layer protocol, if appropriate. 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 maximum 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.
(IP). It is implemented in two versions, IPv4
and IPv6
. The Internet Control Message Protocol
(ICMP) is primarily used for error and diagnostic functions. Different implementations exist for IPv4 and IPv6. The Internet Group Management Protocol
(IGMP) is used by IPv4 hosts and adjacent multicast routers to establish multicast group memberships.
(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 updated. The protocol was adapted for use in IPv4 and found widespread adoption in securing virtual private network
s.
(layer 3) in the Open Systems Interconnection
(OSI) protocol
stack. Although they have some overlap, these layering models represent different classification methods. In particular, the allowed characteristics of protocols (e.g., whether they are connection-oriented or connection-less) placed in these layers are different between the 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 network layer is often described to include protocols such as the Address Resolution Protocol
(ARP) which was placed in link layer
by the original TCP/IP architects (RFC 1122, RFC 1123).
Strict comparison between the TCP/IP model and the OSI model should be avoided. Layering in TCP/IP is not a principal design criterion and is in general considered to be harmful (RFC 3439, section 3: "Layering Considered Harmful").
Despite clear primary references (see References below) and normative standards documents the internet layer is still sometimes improperly called network layer, in analogy to the OSI model.
Internetworking
Internetworking is the practice of connecting a computer network with other networks through the use of gateways that provide a common method of routing information packets between the networks...
methods in the Internet protocol suite
Internet 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...
, commonly also called TCP/IP, which is the foundation 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...
. It is the group of methods, protocols, and specifications that 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
IP address
An 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...
) which is defined for this purpose by the Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP). The internet layer derives its name from its function of forming an internet (uncapitalized), or facilitating internetworking
Internetworking
Internetworking is the practice of connecting a computer network with other networks through the use of gateways that provide a common method of routing information packets between the networks...
, which is the concept of connecting multiple networks with each other through gateways
Gateway (telecommunications)
In telecommunications, the term gateway has the following meaning:*In a communications network, a network node equipped for interfacing with another network that uses different protocols....
.
Internet-layer protocols use IP-based packets. The internet layer does not include the protocols that define communication between local (on-link) network nodes which fulfill 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
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...
.
A particularly crucial aspect in the internet layer is the robustness principle
Robustness Principle
In computing, the robustness principle is a general design guideline for software:The principle is also known as Postel's law, after Internet pioneer Jon Postel, who wrote in an early specification of the Transmission Control Protocol that:...
: "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.
Purpose
The internet layer has three basic functions: For outgoing packets, select the next-hop host (gatewayGateway (telecommunications)
In telecommunications, the term gateway has the following meaning:*In a communications network, a network node equipped for interfacing with another network that uses different protocols....
) and transmit the packet to this host by passing it to the appropriate 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...
implementation; for incoming packets, capture packets and pass the packet payload up to the appropriate transport-layer protocol, if appropriate. In addition it provides error detection and diagnostic capability.
In Version 4 of the Internet Protocol (IPv4
IPv4
Internet 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...
), during both transmit and receive operations, IP is capable of automatic or intentional fragmentation or defragmentation
IP fragmentation
The Internet Protocol implements datagram fragmentation, so that packets may be formed that can pass through a link with a smaller maximum transmission unit than the original datagram size....
of packets, based, for example, on the maximum transmission unit
Maximum transmission unit
In computer networking, the maximum transmission unit of a communications protocol of a layer is the size of the largest protocol data unit that the layer can pass onwards. MTU parameters usually appear in association with a communications interface...
(MTU) of link elements. However, this feature has been dropped in IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
, as the communications end points, the hosts, now have to perform path MTU discovery
Path MTU discovery
Path MTU Discovery is a standardized technique in computer networking for determining the maximum transmission unit size on the network path between two Internet Protocol hosts, usually with the goal of avoiding IP fragmentation...
and assure that end-to-end transmissions don't exceed the maximum 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
Scalability
In electronics scalability is the ability of a system, network, or process, to handle growing amount of work in a graceful manner or its ability to be enlarged to accommodate that growth...
.
The function of providing reliability of service is the duty of higher level protocols, such as the Transmission Control Protocol
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
(TCP) in 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...
.
Integrity of packets is guaranteed only in IPv4 (not in IPv6) through checksums computed for IP packets.
Core protocols
The primary protocols in the internet layer are the Internet protocolInternet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP). It is implemented in two versions, IPv4
IPv4
Internet 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...
and IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
. The Internet Control Message Protocol
Internet Control Message Protocol
The 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...
(ICMP) is primarily used for error and diagnostic functions. Different implementations exist for IPv4 and IPv6. The Internet Group Management Protocol
Internet Group Management Protocol
The Internet Group Management Protocol is a communications protocol used by hosts and adjacent routers on IP networks to establish multicast group memberships....
(IGMP) is used by IPv4 hosts and adjacent multicast routers to establish multicast group memberships.
Security
Internet Protocol SecurityIPsec
Internet Protocol Security is a protocol suite for securing Internet Protocol communications by authenticating and encrypting each IP packet of a communication session...
(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
Key (cryptography)
In cryptography, a key is a piece of information that determines the functional output of a cryptographic algorithm or cipher. Without a key, the algorithm would produce no useful result. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa...
establishment. IPsec was originally designed as a base specification in IPv6 (RFC 1825, RFC 1829) in 1995, but later updated. The protocol was adapted for use in IPv4 and found widespread adoption in securing virtual private network
Virtual private network
A virtual private network is a network that uses primarily public telecommunication infrastructure, such as the Internet, to provide remote offices or traveling users access to a central organizational network....
s.
Relation to OSI model
The internet layer of the TCP/IP model is often compared directly with the network layerNetwork layer
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 3) in the Open Systems Interconnection
Open Systems Interconnection
Open Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
(OSI) protocol
OSI protocols
The Open Systems Interconnection protocols are a family of information exchange standards developed jointly by the ISO and the ITU-T starting in 1977....
stack. Although they have some overlap, these layering models represent different classification methods. In particular, the allowed characteristics of protocols (e.g., whether they are connection-oriented or connection-less) placed in these layers are different between the 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
Internetworking
Internetworking is the practice of connecting a computer network with other networks through the use of gateways that provide a common method of routing information packets between the networks...
using the Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
.
Because of this, the OSI network layer is often described to include protocols such as the Address Resolution Protocol
Address Resolution Protocol
Address Resolution Protocol is a telecommunications protocol used for resolution of network layer addresses into link layer addresses, a critical function in multiple-access networks. ARP was defined by RFC 826 in 1982. It is Internet Standard STD 37...
(ARP) which was placed in 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...
by the original TCP/IP architects (RFC 1122, RFC 1123).
Strict comparison between the TCP/IP model and the OSI model should be avoided. Layering in TCP/IP is not a principal design criterion and is in general considered to be harmful (RFC 3439, section 3: "Layering Considered Harmful").
Despite clear primary references (see References below) and normative standards documents the internet layer is still sometimes improperly called network layer, in analogy to the OSI model.
Further reading
- RFC 3439, Some Internet Architectural Guidelines and Philosophy, R. Bush, D. Meyer, December 2002