Layers in the TCP/IP model

From Wikipedia
Alan Isherwoo


Layers in the TCP/IP model

The layers near the top are logically closer to the user application (as opposed to the human user), while those near the bottom are logically closer to the physical transmission of the data. Viewing layers as providing or consuming a service is a method of abstraction to isolate upper layer protocols from the nitty gritty detail of transmitting bits over, say, Ethernet and collision detection, while the lower layers avoid having to know the details of each and every application and its protocol.
IP suite stack showing the physical network connection of two hosts via two routers and the corresponding layers used at each hop. The dotted line represents a virtual connection
Sample encapsulation of data within a UDP datagram within an IP packet

This abstraction also allows upper layers to provide services that the lower layers cannot, or choose not to, provide. Again, the original OSI Reference Model was extended to include connectionless services (OSIRM CL).[5] For example, IP is not designed to be reliable and is a best effort delivery protocol. This means that all transport layers must choose whether or not to provide reliability and to what degree. UDP provides data integrity (via a checksum) but does not guarantee delivery; TCP provides both data integrity and delivery guarantee (by retransmitting until the receiver receives the packet).

This model lacks the formalism of the OSI Reference Model and associated documents, but the IETF does not use a formal model and does not consider this a limitation, as in the comment by David D. Clark, "We don't believe in kings, presidents, or voting. We believe in rough consensus and running code." Criticisms of this model, which have been made with respect to the OSI Reference Model, often do not consider ISO's later extensions to that model.

For multiaccess links with their own addressing systems (e.g. Ethernet) an address mapping protocol is needed. Such protocols can be considered to be below IP but above the existing link system. While the IETF does not use the terminology, this is a subnetwork dependent convergence facility according to an extension to the OSI model, the Internal Organization of the Network Layer (IONL) [6].
ICMP & IGMP operate on top of IP but do not transport data like UDP or TCP. Again, this functionality exists as layer management extensions to the OSI model, in its Management Framework (OSIRM MF) [7]
The SSL/TLS library operates above the transport layer (utilizes TCP) but below application protocols. Again, there was no intention, on the part of the designers of these protocols, to comply with OSI architecture.
The link is treated like a black box here. This is fine for discussing IP (since the whole point of IP is it will run over virtually anything). The IETF explicitly does not intend to discuss transmission systems, which is a less academic but practical alternative to the OSI Reference Model.