“Internet layer”的意思、由来-开放百科全书

The internet layer is a group of internetworking methods, protocols, and specifications in the Internet protocol suite that are used to transport network packets from the originating host across network boundaries, if necessary, to the destination host specified by an IP address. The internet layer derives its name from its function facilitating internetworking, which is the concept of connecting multiple networks with each other through gateways.

The internet layer does not include the protocols that fulfill the purpose of maintaining link states between the local nodes 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. Internet-layer protocols use IP-based packets.

A common design aspect in the internet layer is the robustness principle: "Be liberal in what you accept, and conservative in what you send"^[1] as a misbehaving host can deny Internet service to many other users.

Purpose

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 the proper arrival of packets. This in accordance with the end-to-end principle and a change from the previous protocols used on the early ARPANET. Since packet delivery across diverse networks is an inherently 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.

In IPv4, a checksum is used to protect the header of each datagram. The checksum ensures that the information in a received header is accurate, however, IPv4 does not attempt to detect errors that may have occurred to the data in each packet. IPv6 does not include this header checksum, instead relying on the link layer to assure data integrity for the entire packet including the checksum.

Core protocols

The primary protocols in the internet layer are the Internet Protocol (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 IP multicast routers to establish multicast group memberships.

Security

Relation to OSI model

Despite clear primary references and normative standards documents, the internet layer is often improperly called network layer.^[1]^[4] This is done because the internet layer of the TCP/IP model is easily compared directly with the network layer (layer 3) in the Open Systems Interconnection (OSI) protocol stack.^[5]^[6]^[7]^[8]

Although they have some overlap, these two 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.{{citation needed|date=December 2016}}

IETF standards

See also

References

1. ^¹{{cite IETF |title=Requirements for Internet Hosts -- Communication Layers |rfc=1122 |editor=R. Braden |date=October 1989 |publisher=IETF}}
2. ^{{citation |rfc=1825 |title=Security Architecture for the Internet Protocol |author=R. Atkinson |date=August 1995 |publisher=IETF}}
3. ^{{citation |rfc=1829 |title=Security Architecture for the Internet Protocol |author1=P. Karn |author2=P. Metzger |author3=W. Simpson |date=August 1995 |publisher=IETF}}
4. ^RFC 1123
5. ^{{cite web|url=https://www.electronicdesign.com/what-s-difference-between/what-s-difference-between-osi-seven-layer-network-model-and-tcpip|title=What’s The Difference Between The OSI Seven-Layer Network Model And TCP/IP?|date=2 October 2013|website=Electronic Design}}
6. ^{{cite web|url=https://www.studytonight.com/computer-networks/comparison-osi-tcp-model.php|title=Difference between OSI Reference Model and TCP/IP Reference Model - Studytonight|website=www.studytonight.com}}
7. ^{{cite web|url=http://www.omnisecu.com/tcpip/tcpip-model.php|title=Four Layers of TCP/IP model, Comparison and Difference between TCP/IP and OSI models|website=www.omnisecu.com}}
8. ^{{cite web|url=https://www.dummies.com/programming/networking/cisco/network-basics-tcpip-and-osi-network-model-comparisons/|title=Network Basics: TCP/IP and OSI Network Model Comparisons|publisher=}}