Question

Explain both TCP/IP and ISO/OSI communication protocol stacks, identifying each of the protocol layers. How does encapsulation work in this layered design? In terms of ratio header/payload, estimate the optimistic accumulate overhead these protocols incur for each application “packet” that is transmitted over the Internet.

Answer 1

TCP/IP protocol stacks :

Transmission Control Protocol/Internet Protocol (TCP/IP) is the language a computer uses to access the internet. It consists of a suite of protocols designed to establish a network of networks to provide a host with access to the internet.

TCP/IP is responsible for full-fledged data connectivity and transmitting the data end to end by providing other functions, including addressing, mapping and acknowledgment. TCP/IP contains four layers, which differ slightly from the OSI model.

The technology is so common that one would rarely use the full name. In other words, in common usage the acronym is now the term itself.

TCP/IP is the world's most widely-used non-proprietary protocol suite because it enables computers using diverse hardware and software platforms, on different types of networks, to communicate. The protocols work equally well in both LANs and WANs. TCP/IP is a collection of protocols named after its two best-known and most important protocols, the Transmission Control Protocol (TCP) and the Internet Protocol (IP). As well as these relatively low-level protocols, TCP/IP includes several higher level protocols that facilitate common applications such as electronic mail, terminal emulation, and file transfer. As we have seen, the Internet protocols used today were originally developed as part of the ARPANET research project which started in the 1960s and led to the emergence of the global network of networks we call the Internet.

Layers Of The TCP/IP Protocol Stack

The Internet architecture is made up of four layers that work together. These four layers are, from high to low:

The Application Layer

The Transport Layer

The Network Layer

The Physical Layer

The Application Layer

The application layer defines how certain services operate and how they can be used. Examples are the FTP service for transferring files, HTTP for serving Web pages and SMTP for e-mail.

These services are defined in a rather abstract manner. Two parties, called the client and the server, set up a connection over which they exchange messages in accordance with a specific protocol. The client starts the protocol by requesting the service. Often the next step is for the server to authenticate the client, for example by asking for a password or by executing a public-key based protocol.

Taking e-mail as an example, the protocol in question is called the Simple Mail Transfer Protocol (SMTP). The client and the server set up an SMTP connection over which they exchange identifying information. The client then tells who the message is from and who the intended recipient is. The server then indicates whether it accepts or refuses the message (for example if it’s spam or the intended recipient is unknown). If the message is accepted, the client sends the actual content of the message and the server stores it in the right mailbox.

Transport layer:

The transport layer oversees the end-to-end transfer of data, and can handle a number of data streams simultaneously. The main transport layer protocol is Transmission Control Protocol (TCP), which provides a reliable, connection-oriented service. User Datagram Protocol (UDP) provides an unreliable, connectionless service (delivery is not guaranteed, but UDP is useful for applications for which speed is more important than reliability). The transport layer roughly corresponds to its namesake in the OSI model.

The Network Layer

The network layer is responsible for transmitting and routing data packets over the network. The Internet uses the Internet Protocol or IP as its network layer. Each node on the network has an address, which of course is called the IP address. Data is sent as IP packets.

A transport layer connection is made up up of a large number of IP packets exchanged by the client and server. The Internet Protocol (IP) is very simple: a packet has a source, a destination and a payload, and it’s passed from one node in the network to another until it gets to the destination. The IP does not notice that a packet gets lost. It just never gets to the destination. If a particular node cannot pass the packet to the next node along the normal route, it will do its best to find an alternative path. That’s why IP is sometimes called a “best-effort” protocol.

When the client sends its TCP connection request, the network layer puts the request in a number of packets and transmits each of them to the server. Each packet can take a different route, and some of the packets may get lost along the way. If they all make it, the transport layer at the server is able to reconstruct the request, and it will prepare a response confirming that a TCP connection has been set up. This response is sent back again in a number of IP packets that will hopefully make it to the client.

The Physical Layer

The lowest layer is the physical layer, which defines how the cables, network cards, wireless transmitters and other hardware connect computers to networks and networks to the rest of the Internet. Examples of physical layer networks are Ethernet, WiFi, Token Ring and Fiber Data Distributed Interface (FDDI). Note that many of these technologies also have their own link layer protocol.

The physical layer provides the means to transfer the actual bits from one computer to another. In an Ethernet network, a computer is connected by plugging a network cable into its Ethernet card, and then plugging the other end of that cable into a router or switch. The physical layer specifies how bits of data are sent over that cable: how do the electrical currents or the pulses the card sends get turned back into the data for the higher level layers. For wireless networks, this works exactly the same, except of course there is no cable.

OSI model:

The Open Systems Interconnection (OSI) Model is a conceptual and logical layout that defines network communication used by systems open to interconnection and communication with other systems.

The model is broken into seven subcomponents, or layers, each of which represents a conceptual collection of services provided to the layers above and below it. The OSI Model also defines a logical network and effectively describes computer packet transfer by using different layer protocols.

The OSI Model may also be referred to as the seven-layer OSI Model or the seven-layer model.

The seven Open Systems Interconnection layers are:

Layer 7: The application layer. This is the layer at which communication partners are identified (Is there someone to talk to?), network capacity is assessed (Will the network let me talk to them right now?), and that creates a thing to send or opens the thing received. (This layer is not the application itself, it is the set of services an application should be able to make use of directly, although some applications may perform application layer functions.)

Layer 6: The presentation layer. This layer is usually part of an operating system (OS) and converts incoming and outgoing data from one presentation format to another (for example, from clear text to encrypted text at one end and back to clear text at the other).

Layer 5: The session layer. This layer sets up, coordinates and terminates conversations. Services include authentication and reconnection after an interruption. On the Internet, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) provide these services for most applications.

Layer 4: The transport layer. This layer manages packetization of data, then the delivery of the packets, including checking for errors in the data once it arrives. On the Internet, TCP and UDP provide these services for most applications as well.

Layer 3: The network layer. This layer handles the addressing and routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). IP is the network layer for the Internet.

Layer 2: The data-link layer. This layer sets up links across the physical network, putting packets into network frames. This layer has two sub-layers, the Logical Link Control Layer and the Media Access Control Layer. Ethernet is the main data link layer in use.

Layer 1: The physical layer. This layer conveys the bit stream through the network at the electrical, optical or radio level. It provides the hardware means of sending and receiving data on a carrier network.

encapsulation:

encapsulation is a method of designing modular communication protocols in which logically separate functions in the network are abstracted from their underlying structures by inclusion or information hiding within higher level objects.

The physical layer is responsible for physical transmission of the data. Link encapsulation allows local area networking and Internet Protocol (IP) provides global addressing of individual computers; Transmission Control Protocol (TCP) adds application or process selection, i.e., the port specifies the service such as a Web or TFTP server.

During encapsulation, each layer builds a protocol data unit (PDU) by adding a header (and sometimes trailer) containing control information to the PDU from the layer above. For example, in the Internet protocol suite, the contents of a web page are encapsulated with an HTTP header, then by a TCP header, an IP header, and, finally, by a frame header and trailer. The frame is forwarded to the destination node as a stream of bits, where it is decapsulated (or de-encapsulated) into the respective PDUs and interpreted at each layer by the receiving node.

The result of encapsulation is that each lower layer provides a service to the layer or layers above it, while at the same time each layer communicates with its corresponding layer on the receiving node. These are known as adjacent-layer interaction and same-layer interaction, respectively.

In discussions of encapsulation, the more abstract layer is often called the upper layer protocol while the more specific layer is called the lower layer protocol. Sometimes, however, the terms upper layer protocols and lower layer protocols are used to describe the layers above and below IP, respectively.

Encapsulation is a characteristic feature of most networking models, including both the OSI model and TCP/IP suite of protocols.

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