TCP/IP model

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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.

TCP/IP 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 'layers', or five if you include the bottom physical layer. The layer view of TCP/IP is based on the seven-layer OSI Reference Model written long after the original TCP/IP specifications, and is not officially recognized. Regardless, it makes a good analogy for how TCP/IP works, and comparison of the models is common.

The TCP/IP Model and related protocols are currently maintained by the Internet Engineering Task Force (IETF).

[edit] Key Architectural Principles

An early architectural document, RFC 1122, emphasizes architectural principles over layering^[1].

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]
2. Robustness Principle: "Be liberal in what you accept, and conservative in what you send. Software on other hosts may contain deficiencies that make it unwise to exploit legal but obscure protocol features".^{[citation needed]}

Even when layer is examined, the assorted architectural documents -- there is no single architectural model such as ISO 7498, the OSI Reference Model -- have fewer, less rigidly defined layers than the commonly referenced OSI model, and thus provides an easier fit for real-world protocols. In point of fact, one frequently referenced document 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."

No document officially specifies the model, another reason to deemphasize the emphasis on layering. Different names are given to the layers by different documents, and different numbers of layers are shown by different documents.

There are versions of this model with four layers and with five^{[citation needed]} layers. RFC 1122 on Host Requirements makes general reference to layering, but refers to many other architectural principles not emphasizing layering. It loosely defines a four-layer version, with the layers having names, not numbers, as

• Process Layer or Application Layer: this is where the "higher level" protocols such as SMTP, FTP, SSH, HTTP, etc. operate.
• Host-To-Host (Transport) Layer: this is where flow-control and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections, ensuring that packets are in fact received.
• Internet or Internetworking Layer: this layer defines IP addresses, with many routing schemes for navigating packets from one IP address to another.
• Network Access Layer: this layer describes both the protocols (i.e., the OSI Data Link Layer) used to mediate access to shared media, and the physical protocols and technologies necessary for communications from individual hosts to a medium.

The Internet protocol suite (and corresponding protocol stack), and its layering model, were in use before the OSI model was established. Since then, the TCP/IP model has been compared with the OSI model numerous times in books and classrooms, which often results in confusion because the two models use different assumptions, including about the relative importance of strict layering.

[edit] Layers in the TCP/IP model

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

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.

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).^[3] 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 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) ^[4].
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) ^[5]
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.

[edit] OSI and TCP/IP Layering Differences

The three top layers in the OSI model - the application layer, the presentation layer and the session layer - usually are lumped into one layer in the TCP/IP model. While some pure OSI protocol applications, such as X.400, also lumped them together, there is no requirement that a TCP/IP protocol stack needs to be monolithic 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 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 ^[6]. The associated OSI development effort, however, has been abandoned given the real-world adoption of TCP/IP protocols.

[edit] The layers

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The following is a description of each layer in the IP suite stack.

[edit] 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)^[7]. 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).

[edit] 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.)

[edit] Network layer

As originally defined, the 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 (lower-case) internet.^[8]

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, illustrating an incompatibility between the Internet and the IP stack and OSI model. All routing protocols, such as OSPF, and RIP are also part of the network layer. What makes them part of the network layer is that their payload is totally concerned with management of the network layer. The particular encapsulation of that payload is irrelevant for layering purposes.

[edit] Data link layer

The link layer, which is the method used to move packets from the network layer on two different hosts, is not really part of the Internet protocol suite, because IP can run over a variety of different link layers. The processes of transmitting packets on a given link layer and receiving packets from a given link layer 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.

For Internet access over a dial-up modem, IP packets are usually transmitted using PPP. For broadband Internet access such as ADSL or cable modems, PPPoE is often used. On a local wired network, Ethernet is usually used, and on local wireless networks, IEEE 802.11 is usually used. For wide-area networks, either PPP over T-carrier or E-carrier lines, Frame relay, ATM, or packet over SONET/SDH (POS) are often used.

The link layer can also be the layer where packets are intercepted to be sent over a virtual private network. When this is done, the link layer data is considered the application data and proceeds back down the IP stack for actual transmission. On the receiving end, the data goes up the IP stack twice (once for routing and the second time for the VPN).

The link layer can also be considered to include the physical layer, which is made up of the actual physical network components (hubs, repeaters, fiber optic cable, coaxial cable, network cards, Host Bus Adapter cards and the associated network connectors: RJ-45, BNC, etc), and the low level specifications for the signals (voltage levels, frequencies, etc).

[edit] Physical layer

The Physical layer is responsible for encoding and transmission of data over network communications media. It operates with data in the form of bits that are sent from the Physical layer of the sending (source) device and received at the Physical layer of the destination device.

Ethernet, Token Ring, SCSI, hubs, repeaters, cables and connectors are standard network devices that function at the Physical layer. The Physical layer is also considered the domain of many hardware-related network design issues, such as LAN and WAN topology and wireless technology.

[edit] Hardware and software implementation

Normally the application programmers are in charge of layer 5 protocols (the application layer), while the layer 3 and 4 protocols are services provided by the TCP/IP stack in the operating system. Microcontroller firmware in the network adapter typically handle layer 2 issues, supported by a driver software in the operational system. Non-programmable analog and digital electronics are normally in charge of the physical layer, typically using an application-specific integrated circuit (ASIC) chipset for each radio interface or other physical standard.

However, hardware or software implementation is not stated in the protocols or the layered reference model. High-performance routers are to a large extent based on fast non-programmable digital electronics, carrying out layer 3 switching. In modern modems and wireless equipment, the physicapt was an example of CPU software implementation of the physical layer, making it possible to emulate some modem standards.

[edit] See also

• OSI model

[edit] References

1. ^ Architectural Principles of the Internet, RFC 1958, B. Carpenter, June 1996
2. ^ Rethinking the design of the Internet: The end to end arguments vs. the brave new world, Marjory S. Blumenthal, David D. Clark, August 2001
3. ^ [ OSI: Reference Model Addendum 1: Connectionless-mode Transmission,ISO7498/AD1],ISO7498/AD1, May 1986
4. ^ Internal Organization of the Network Layer, ISO 8648
5. ^ Open Systems Interconnection -- Basic Reference Model -- Part 4: Management framework,ISO 7498/4
6. ^ Framework and taxonomy of International Standardized Profiles, ISO 10000, October 1998
7. ^ TCP/IP Illustrated: the protocols, ISBN 0-201-63346-9, W. Richard Stevens, February 1994
8. ^ IP Packet Structure

[edit] External links

• The TCP/IP Stack and the OSI Model
• Understanding TCP/IP