Networking Models

On networking models, for theory and practice.

• OSI Model (1984)
• TCP/IP Model (1989)
• Models Compared
• Details on Layers

OSI Model

From wikipedia: OSI model:

"The Open Systems Interconnection model (OSI model) is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard protocols. The model partitions a communication system into abstraction layers. The original version of the model defined seven layers."

...

"In the late 1970s, one project was administered by the International Organization for Standardization (ISO), while another was undertaken by the International Telegraph and Telephone Consultative Committee, or CCITT (the abbreviation is from the French version of the name). These two international standards bodies each developed a document that defined similar networking models.

In 1983, these two documents were merged to form a standard called The Basic Reference Model for Open Systems Interconnection. The standard is usually referred to as the Open Systems Interconnection Reference Model, the OSI Reference Model, or simply the OSI model. It was published in 1984 by both the ISO, as standard ISO 7498, and the renamed CCITT (now called the Telecommunications Standardization Sector of the International Telecommunication Union or ITU-T) as standard X.200.

OSI had two major components, an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols."

OSI Model
Layer		Protocol data unit (PDU)	Function[3]
Host layers	7. Application	Data	High-level APIs, including resource sharing, remote file access
	6. Presentation		Translation of data between a networking service and an application; including character encoding, data compression and encryption/decryption
	5. Session		Managing communication sessions, i.e. continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes
	4. Transport	Segment (TCP) / Datagram (UDP)	Reliable transmission of data segments between points on a network, including segmentation, acknowledgement and multiplexing
Media layers	3. Network	Packet	Structuring and managing a multi-node network, including addressing, routing and traffic control
	2. Data link	Frame	Reliable transmission of data frames between two nodes connected by a physical layer
	1. Physical	Bit	Transmission and reception of raw bit streams over a physical medium

• wikipedia: OSI model

 

TCP/IP Model and Family

Model - the theory, framework, model with four layers

Family - the set of actual protocols enabling communications designed within the framework (e.g. TCP, IP, DNS, FTP, and HTTP).

"The Internet protocol suite is the computer networking model and set of communications protocols used on the Internet and similar computer networks. It is commonly known as TCP/IP, because its most commonly used protocols, the Transmission Control Protocol (TCP) and the Internet Protocol (IP) were the first networking protocols defined during its development. It is occasionally known as the Department of Defense (DoD) model, because the development of the networking model was funded by DARPA, an agency of the United States Department of Defense." [wikipedia]

1982: the US Department of Defense declared TCP/IP as the standard for all military computer networking.

1985: In 1985, the Internet Advisory Board (later renamed theInternet Architecture Board) held a three-day workshop on TCP/IP for the computer industry, attended by 250 vendor representatives, promoting the protocol and leading to its increasing commercial use.

In 1985, the first Interop conference focused on network interoperability by broader adoption of TCP/IP. 

Two RFCs in 1989:

1. RFC1122 - Requirements for Internet Hosts -- Communication Layers (Oct 1989), covers the communications protocol layers: link layer, IP layer, and transport layer;
2. RFC1123 - Requirements for Internet Hosts -- Application and Support (Oct 1989), covers the application and support protocols.

The TCP/IP Model uses four layers, as exemplified in figure below (w UDP transport protocol):

• wikipedia TCP/IP model (redirects to wikipedia Internet protocol suite)

From wikipedia: OSI model:

"Despite using a different concept for layering than the OSI model, these layers are often compared with the OSI layering scheme in the following way:

• The Internet application layer includes the OSI application layer, presentation layer, and most of the session layer.
• Its end-to-end transport layer includes the graceful close function of the OSI session layer as well as the OSI transport layer.
• The internetworking layer (Internet layer) is a subset of the OSI network layer.
• The link layer includes the OSI data link layer and sometimes the physical layers, as well as some protocols of the OSI's network layer.

These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the network layer document.[citation needed]

The presumably strict layering of the OSI model as it is usually described does not present contradictions in TCP/IP, as it is permissible that protocol usage does not follow the hierarchy implied in a layered model. Such examples exist in some routing protocols (e.g., OSPF), or in the description of tunneling protocols, which provide a link layer for an application, although the tunnel host protocol might well be a transport or even an application-layer protocol in its own right.[citation needed]"

 

Models Compared

General note: the layers in the different models does NOT correlate and should NOT be viewed as strict to any degree. The side-by-side below is only to provide some indications on how the models compare.

	OSI Model		TCP/IP Model
Briefly	"a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology." [wikipedia]		The Internet protocol suite is the computer networking model and set of communications protocols used on the Internet and similar computer networks. [wikipedia]
I.e.	More theoretical and useful for learning and discussions		More practical and useful for using data networks.
History	First official publication in 1984, as ISO 7498 and CCITT - now ITU-T - X.200.		Two RFCs in 1989: RFC1122 and RFC1123.
	OSI Model Layers (7)		TCP/IP Model Layers (4)
Host layers	7. Application - High-level APIs, including resource sharing, remote file access		(4) The Internet application layer includes the OSI application layer, presentation layer, and most of the session layer.
	6. Presentation - Translation of data between a networking service and an application; including character encoding, data compression and encryption/decryption
	5. Session - Managing communication sessions, i.e. continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes
			(3) Its end-to-end transport layer includes the graceful close function of the OSI session layer as well as the OSI transport layer.
	4. Transport - Reliable transmission of data segments between points on a network, including segmentation, acknowledgement and multiplexing
Media layers	3. Network - Structuring and managing a multi-node network, including addressing, routing and traffic control		(2) The internetworking layer (Internet layer) is a subset of the OSI network layer.
			(1) The link layer includes the OSI data link layer and sometimes the physical layers, as well as some protocols of the OSI's network layer.
	2. Data link - Reliable transmission of data frames between two nodes connected by a physical layer
	1. Physical - Transmission and reception of raw bit streams over a physical medium
			(Sometimes outside TCP/IP Model)

 

 

Details on Layers

¹ I.e., the level on which address format is defined. Address in new format is then available for layer(s) above.
² Actually transmitted pages, parts of big files, etc
³ First layer actually concerned with any form of deciding which interface/similar is Data link, layer 2.