Session layer

Session layer

The services provided by the first four layers (physical, data link, network and transport) are not sufficient for some processes. The session layer is the network dialogue controller. It establishes, and synchronizes the interaction between communicating system. Specific responsibilities of the session layer include the following:
Ø  Dialogue control. The session layer allows two system to enter into a dialogue. It allows the communication between two processes to take place in either half-duplex (one way at a time) or full-duplex (two way at a time) mode.
Ø  Synchronization. The session layer allows a process to add check points (synchronization points) into a stream of data. For example, if a system is sending a file of 2000 pages, it is advisable to insert check point after every 100 pages to ensure that each 100-page unit is received and acknowledged independently In this case, if a crash happens during the transmission of page 515, the only pages that need be resent after system recovery are pages 475 to. Pages previous to 475 need not be resent.

Transport layer

Transport layer

The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on the host. Whereas the network layer oversees source-to-destination delivery of individual packets, it does not recognize any relationship between those packets. It treats each one independently, as though each piece belong to a separate message, whether or not it does. The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level. Other responsibilities of the transport layer include the following:
Ø  Service-point addressing. Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on the other. The transport layer header must therefore include a type of address called a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.
Ø  Segmentation and reassembling. A message is divided into transmittable segments, with each segment containing sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.
Ø  Connection control. The transport layer can be either connectionless or connection oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination computer. A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets after all the data is transferred, the connection is terminated.
Ø  Flow control. Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link.
Ø  Error control. Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to-process rather than to across a single link. The sending transport layer make sure that the entire message arrives at the receiving transport layer without error (damage, loss or duplication). Error correction is usually achieved through transmission.

Network layer

Network Layer

The network layer is responsible for the source-to destination delivery of a packet, possibly across multiple networks (links). Whereas the data link layer oversees the delivery of the packet between two system on the same network (link), the network layer ensures that each packet gets from its point if origin to its final destination.
        If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached two different networks (link) with connecting devices between the networks (link), there is often a need for the network layer to accomplish source-to-destination delivery. Other responsibilities of the network layer include the following:
Ø  Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system distinguish the source and destination system. The network layer adds a header to the packet coming from the upper layer that, among other thing, includes the logical addresses of the sender and receiver.
Ø  Routing. When independent networks or links are connected together to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the function of the network layer is to provides this mechanism.

Data link layer

Data link layer

The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer(network layer). Other responsibilities of the data link layer include the following:
Ø  Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frame.
Ø  Physical addressing. If frames are to be distributed to different system on the network, the data link layers add a header to the frame to define the sender and /or receiver of the frame. If the frame is intended for a system outside the sender’s network, the receiver address is the address of the connecting device that connects the network to the next one.
Ø  Flow control. If the rate at which the data is absorbed by the receiver is less than the rate produced at the sender, the data link layer imposes a flow control mechanism to prevent overwhelming the receiver.
Ø  Error control. The data link layer adds reliability to the physical layer by adding mechanism to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize a duplicate frame. Error control is normally achieved through a trailer added to the end of the frame.
Ø  Access control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

Physical Lyer

Layers in the OSI model

Physical layer

The physical layer coordinates the function required to carry a bit stream over a physical medium. Physical Layer deals with the mechanical and electrical specifications of the interface and transmission media Physical Layer also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur.
The physical layer is responsible for moving individual bits from one
(NODE) to the next.
Physical layer is concerned with the following
Ø  Physical characteristics of interfaces and media.                The physical layer defines the characteristics of the interface between the devices and the transmission media. It also defines the type of transmission media.
Ø  Representation of bits. The physical layer data consists of a stream of bits (sequence of 0s and 1s ) with no interpretation. To be transmitted bits must be encoded into signals (electrical or optical). The physical layer defines the type of encoding (how 0s and 1s are changed to signals).
Ø  Data rate.            The transmission rate – the number of bits sent each second –is also defined by the physical layer. In other words, the physical layer defines the duration of a bit, which is how long it lasts.
Ø  Synchronization                                The sender and receiver must not only use the same bit rate but must also be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized.
Ø  Line configuration.          The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected together through a dedicated link. In a multipoint configuration a link is shared between several devices .
Ø  Physical topology.            The physical topology define how devices are connected to make a network. Devices can be connected using a mesh topology (every device connected to every other device), a star topology (devices are connected through a central device), a ring topology (each device is connected to the next. Forming a ring), or a bus topology (every device ion a common link)
Ø  Transmission mode         The physical layer also defines the direction of transmission between two devices: simplex, half duplex. In the simplex mode, only one device can send; the other can only receive. The simplex mode is a one way communication. In the half duplex mode, two devices can send and receive, but not at the same time, in a full-duplex (or simple duplex) mode, two devices can send and receive at the same time.

SIP (Session Initiation Protocol)

SIP (Session Initiation Protocol)

SIP is an application layer protocol designed to be independent of the underlying transport layer; it can run on transmission control protocol (TCP), UDP, or SCTP. It is a text based protocol, incorporating many elements of the HTTP and the SMTP.

The session Initiation Protocol is a signaling communication protocol, widely used for controlling multimedia communication sessions such as voice and video calls over IP.
The protocol defines the messages that are sent between peers which govern establishment, terminations and other essential element of call. SIP can be used for creating, modifying, and terminating two party (unicast) or multiparty (multicast) sessions consisting one or several media streams. Other SIP applications include video conferencing, streaming multimedia distribution, instant messaging, presence information, file transfer and online games.
SIP work in conjunction with several other application layer protocols that identify and carry the session media identification and negotiation is achieved with session description protocol (SDP). For the transmission of media stream a(voice, video) SIP typically employs the Real-Time transport protocol (RTP), which many be secured with the secure Real-Time transport protocol (RTP). For secure transmissions of SIP message the protocol may be encrypted with transport layer security TLS.

NNTP (Network News Transport Protocol)


NNTP (Network News Transfer Protocol)

The News Transport Protocol (NNTP) is an application protocol used for transporting Usenet news articles (net news) between news server and for reading and posting articles by end client application.
Usenet was originally designed based on the UUCP network, with most article transfer taking place over direct point to point telephone link between news sever and which were powerful time sharing systems. Readers and posters logged into this computer reading the article directly from the local disc.
As local area network and internet participation it became describe to allow news readers to be run on personal computers connected to local networks. Because distributed files system were not yet widely available, a news protocol was developed based on the client server model. It resembled the simple Mail Transfer Protocol (SMTP), but was tailored for exchanging news group article.
A news reader, also known as a news client, is a software application that reads article on Usenet, either directly from the news server's disks or via the NNTP. The well-known TCP port 119 is reserved for NNTP. When client connect to a news server with transport layer security (TLS), TCP port 563 is used. This is sometimes referred to as NNTPS.
In October 2006, the IETF released RFC 3977 which updates the NNTP protocol and codifies many of the additions over the years since RFC977.
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