TCP/IP model: Difference between revisions

Revision as of 07:48, 27 March 2012

Revision as of 07:51, 27 March 2012

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This abstraction also allows upper layers to provide services that the lower layers cannot, or choose not to, provide. Again, the original OSI model was extended to include connectionless services (OSIRM CL).<ref>[ OSI: Reference Model Addendum 1: Connectionless-mode Transmission,ISO7498/AD1],ISO7498/AD1, May 1986</ref> For example, IP is not designed to be reliable and is a [[best effort delivery]] protocol. This means that all transport layer implementations 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 acknowledges the reception of 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 reject: kings, presidents and voting. We believe in: rough consensus and running code." Criticisms of this model, which have been made with respect to the OSI Reference Modelmodel, 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).<ref>[http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=16011 Internal Organization of the Network layer], ISO 8648</ref>

# 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) <ref>[http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=14258&ICS1=35&ICS2=100&ICS3=70 Open Systems Interconnection -- Basic Reference Model -- Part 4: Management framework], ISO 7498/4</ref>

# The SSL/TLS library operates above the transport layer (uses 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 Modelmodel.

The following is a description of each layer in the TCP/IP networking model starting from the lowest level.