TCP/IP Model

The TCP/IP model is a specification for computer network protocols created in the 1970s by DARPA, an agency of the United States Department of Defense. It laid the foundations for ARPANET, which was the world's first wide area network and a predecessor of the Internet. The TCP/IP Model is sometimes called the Internet Reference Model, the DoD Model or the ARPANET Reference Model.
The TCP/IP Suite defines a set of rules to enable computers to communicate over a network. TCP/IP provides end-to-end connectivity specifying how data should be formatted, addressed, shipped, routed and delivered to the right destination. The specification defines protocols for different types of communication between computers and provides a framework for more detailed standards.

TCP/IP is generally described as having four abstraction layers (RFC 1122). This layer view is often compared with the seven-layer OSI Reference Model formalized after the TCP/IP specifications.
The TCP/IP model and related protocols are maintained by the Internet Engineering Task Force (IETF).

Key architectural principles
An early architectural document, RFC 1122, emphasizes architectural principles over layering.
1. End-to-End Principle: This principle has evolved over time. Its original expression put the maintenance of state and overall intelligence at the edges, and assumed the Internet that connected the edges retained no state and concentrated on speed and simplicity. Real-world needs for firewalls, network address translators, web content caches and the like have forced changes in this principle.
2. Robustness Principle: "In general, an implementation must be conservative in its sending behavior, and liberal in its receiving behavior. That is, it must be careful to send well-formed datagrams, but must accept any datagram that it can interpret (e.g., not object to technical errors where the meaning is still clear) RFC 791.The second part of the principle is almost as important: software on other hosts may contain deficiencies that make it unwise to exploit legal but obscure protocol features. RFC 1122"
Even when the layers are examined, the assorted architectural documents—there is no single architectural model such as ISO 7498, the OSI reference model—have fewer and less rigidly-defined layers than the OSI model, and thus provide an easier fit for real-world protocols. In point of fact, one frequently referenced document, RFC 1958, does not contain a stack of layers. The lack of emphasis on layering is a strong difference between the IETF and OSI approaches. It only refers to the existence of the "internetworking layer" and generally to "upper layers"; this document was intended as a 1996 "snapshot" of the architecture: "The Internet and its architecture have grown in evolutionary fashion from modest beginnings, rather than from a Grand Plan. While this process of evolution is one of the main reasons for the technology's success, it nevertheless seems useful to record a snapshot of the current principles of the Internet architecture."

RFC 1122 on Host Requirements is structured in paragraphs referring to layers, but refers to many other architectural principles not emphasizing layering. It loosely defines a four-layer model, with the layers having names, not numbers, as follows:
* Application (process-to-process) Layer: This is the scope within which applications create user data and communicate this data to other processes or applications on another or the same host. The communications partners are often called peers. This is where the "higher level" protocols such as SMTP, FTP, SSH, HTTP, etc. operate.
* Transport (host-to-host) Layer: The Transport Layer constitutes the networking regime between two network hosts, either on the local network or on remote networks separated by routers. The Transport Layer provides a uniform networking interface that hides the actual topology (layout) of the underlying network connections. This is where flow-control, error-correction, and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections between Internet hosts.
* Internet (internetworking) Layer: The Internet Layer has the task of exchanging datagrams across network boundaries. It is therefore also referred to as the layer that establishes internetworking, indeed, it defines and establishes the Internet. This layer defines the addressing and routing structures used for the TCP/IP protocol suite. The primary protocol in this scope is the Internet Protocol, which defines IP addresses. Its function in routing is to transport datagrams to the next IP router that has the connectivity to a network closer to the final data destination.
* Link Layer: This layer defines the networking methods with the scope of the local network link on which hosts communicate without intervening routers. This layer describes the protocols used to describe the local network topology and the interfaces needed to affect transmission of Internet Layer datagrams to next-neighbor hosts. (cf. the OSI Data Link Layer).

The Internet Protocol Suite and the layered protocol stack design were in use before the OSI model was established. Since then, the TCP/IP model has been compared with the OSI model in books and classrooms, which often results in confusion because the two models use different assumptions, including about the relative importance of strict layering.

Layers in the TCP/IP model
The layers near the top are logically closer to the user application, 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, for example, Ethernet and collision detection, while the lower layers avoid having to know the details of each and every application and its protocol.
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).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 Model, often do not consider ISO's later extensions to that model.
1. 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).
2. 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) [8]
3. 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.
4. 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.

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

Link Layer
The Link Layer is the networking scope of the local network connection to which a host is attached. This regime is called the link in Internet literature. This is the lowest component layer of the Internet protocols, as TCP/IP is designed to be hardware independent. As a result TCP/IP has been implemented on top of virtually any hardware networking technology in existence.
The Link Layer is used to move packets between the Internet Layer interfaces of two different hosts on the same link. The processes of transmitting packets on a given link and receiving packets from a link can be controlled both in the software device driver for the network card, as well as on firmware or specialist chipsets. These will perform data link functions such as adding a packet header to prepare it for transmission, then actually transmit the frame over a physical medium. The TCP/IP model includes specifications of translating the network addressing methods used in the Internet Protocol to data link addressing, such as Media Access Control (MAC), however all other aspects below that level are implicitly assumed to exist in the Link Layer, but are not explicitely defined.

The Link Layer can also be the layer where packets are intercepted to be sent over a virtual private network or other networking tunnel. When this is done, the Link Layer data is considered as application data and proceeds back down the IP stack for actual transmission. On the receiving end, the data goes up through the IP stack twice (once for routing and the second time for the tunnelling function). In these cases a transport protocol or even an application scope protocol constitutes a virtual link placing the tunnelling protocol in the Link Layer of the protocol stack. Thus, the TCP/IP model does not dictate a strict hierarchical encapsulation sequence and the description is dependent upon actual use and implementation.

Internet Layer
As originally defined, the Internet layer (or Network Layer) solves the problem of getting packets across a single network. Examples of such protocols are X.25, and the ARPANET's Host/IMP Protocol.
With the advent of the concept of internetworking, additional functionality was added to this layer, namely getting data from the source network to the destination network. This generally involves routing the packet across a network of networks, known as an internetwork or internet (lower case).
In the Internet Protocol Suite, IP performs the basic task of getting packets of data from source to destination. IP can carry data for a number of different upper layer protocols. These protocols are each identified by a unique protocol number: ICMP and IGMP are protocols 1 and 2, respectively.
Some of the protocols carried by IP, such as ICMP (used to transmit diagnostic information about IP transmission) and IGMP (used to manage IP Multicast data) are layered on top of IP but perform internetwork layer functions. This illustrates an incompatibility between the Internet and the IP stack and OSI model. Some routing protocols, such as OSPF, are also part of the network layer.

Transport Layer
The Transport Layer's responsibilities include end-to-end message transfer capabilities independent of the underlying network, along with error control, fragmentation and flow control. End to end message transmission or connecting applications at the transport layer can be categorized as either:
1. connection-oriented e.g. TCP
2. connectionless e.g. UDP
The Transport Layer can be thought of literally as a transport mechanism e.g. a vehicle whose responsibility is to make sure that its contents (passengers/goods) reach its destination safely and soundly, unless a higher or lower layer is responsible for safe delivery.
The Transport Layer provides this service of connecting applications together through the use of ports. Since IP provides only a best effort delivery, the Transport Layer is the first layer of the TCP/IP stack to offer reliability. Note that IP can run over a reliable data link protocol such as the High-Level Data Link Control (HDLC). Protocols above transport, such as RPC, also can provide reliability.
For example, TCP is a connection-oriented protocol that addresses numerous reliability issues to provide a reliable byte stream:
* data arrives in-order
* data has minimal error (i.e. correctness)
* duplicate data is discarded
* lost/discarded packets are resent
* includes traffic congestion control

The newer SCTP is also a "reliable", connection-oriented, transport mechanism. It is Message-stream-oriented — not byte-stream-oriented like TCP — and provides multiple streams multiplexed over a single connection. It also provides multi-homing support, in which a connection end can be represented by multiple IP addresses (representing multiple physical interfaces), such that if one fails, the connection is not interrupted. It was developed initially for telephony applications (to transport SS7 over IP), but can also be used for other applications.
UDP is a connectionless datagram protocol. Like IP, it is a best effort or "unreliable" protocol. Reliability is addressed through error detection using a weak checksum algorithm. UDP is typically used for applications such as streaming media (audio, video, Voice over IP etc) where on-time arrival is more important than reliability, or for simple query/response applications like DNS lookups, where the overhead of setting up a reliable connection is disproportionately large. RTP is a datagram protocol that is designed for real-time data such as streaming audio and video.

TCP and UDP are used to carry an assortment of higher-level applications. The appropriate transport protocol is chosen based on the higher-layer protocol application. For example, the File Transfer Protocol expects a reliable connection, but the Network File System assumes that the subordinate Remote Procedure Call protocol, not transport, will guarantee reliable transfer. Other applications, such as VoIP, can tolerate some loss of packets, but not the reordering or delay that could be caused by retransmission.
The applications at any given network address are distinguished by their TCP or UDP port. By convention certain well known ports are associated with specific applications. (See List of TCP and UDP port numbers.)

Application Layer
The Application Layer refers to the higher-level protocols used by most applications for network communication. Examples of application layer protocols include the File Transfer Protocol (FTP) and the Simple Mail Transfer Protocol (SMTP). Data coded according to application layer protocols are then encapsulated into one or (occasionally) more transport layer protocols (such as the Transmission Control Protocol (TCP) or User Datagram Protocol (UDP)), which in turn use lower layer protocols to effect actual data transfer.
Since the IP stack defines no layers between the application and transport layers, the application layer must include any protocols that act like the OSI's presentation and session layer protocols. This is usually done through libraries.

Application Layer protocols generally treat the transport layer (and lower) protocols as "black boxes" which provide a stable network connection across which to communicate, although the applications are usually aware of key qualities of the transport layer connection such as the end point IP addresses and port numbers. As noted above, layers are not necessarily clearly defined in the Internet protocol suite. Application layer protocols are most often associated with client-server applications, and the commoner servers have specific ports assigned to them by the IANA: HTTP has port 80; Telnet has port 23; etc. Clients, on the other hand, tend to use ephemeral ports, i.e. port numbers assigned at random from a range set aside for the purpose.
Transport and lower level layers are largely unconcerned with the specifics of application layer protocols. Routers and switches do not typically "look inside" the encapsulated traffic to see what kind of application protocol it represents, rather they just provide a conduit for it. However, some firewall and bandwidth throttling applications do try to determine what's inside, as with the Resource Reservation Protocol (RSVP). It's also sometimes necessary for Network Address Translation (NAT) facilities to take account of the needs of particular application layer protocols. (NAT allows hosts on private networks to communicate with the outside world via a single visible IP address using port forwarding, and is an almost ubiquitous feature of modern domestic broadband routers).

OSI and TCP/IP layering differences

The three top layers in the OSI model—the Application Layer, the Presentation Layer and the Session Layer—are not distinguished separately in the TCP/IP model where it is just the Application Layer. While some pure OSI protocol applications, such as X.400, also combined them, there is no requirement that a TCP/IP protocol stack needs to impose monolithic architecture above the Transport Layer. For example, the Network File System (NFS) application protocol runs over the eXternal Data Representation (XDR) presentation protocol, which, in turn, runs over a protocol with Session Layer functionality, Remote Procedure Call (RPC). RPC provides reliable record transmission, so it can run safely over the best-effort User Datagram Protocol (UDP) transport.
The Session Layer roughly corresponds to the Telnet virtual terminal functionality, which is part of text based protocols such as the HTTP and SMTP TCP/IP model Application Layer protocols. It also corresponds to TCP and UDP port numbering, which is considered as part of the transport layer in the TCP/IP model. The presentation layer has similarities to the MIME standard, which also is used in HTTP and SMTP.

Since the IETF protocol development effort is not concerned with strict layering, some of its protocols may not appear to fit cleanly into the OSI model. These conflicts, however, are more frequent when one only looks at the original OSI model, ISO 7498, without looking at the annexes to this model (e.g., ISO 7498/4 Management Framework), or the ISO 8648 Internal Organization of the Network Layer (IONL). When the IONL and Management Framework documents are considered, the ICMP and IGMP are neatly defined as layer management protocols for the network layer. In like manner, the IONL provides a structure for "subnetwork dependent convergence facilities" such as ARP and RARP.
IETF protocols can be applied recursively, as demonstrated by tunneling protocols such as Generic Routing Encapsulation (GRE). While basic OSI documents do not consider tunneling, there is some concept of tunneling in yet another extension to the OSI architecture, specifically the transport layer gateways within the International Standardized Profile framework [11]. The associated OSI development effort, however, has been abandoned given the overwhelming adoption of TCP/IP protocols.


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