CN Unit-4

Transport layer:

In computer networking, the transport layer or layer 4 provides end-to-end communication

services for applications within a layered architecture of network components and protocols. The

transport layer provides convenient services such as connection-oriented data stream support,

reliability, flow control, and multiplexing.

Different protocols used in Transport layer

TCP

UDP

DCCP

SCTP 

RSVP

The most well-known transport protocol is the Transmission Control Protocol (TCP). It lent its name to the title of the entire Internet Protocol Suite, TCP/IP. It is used for connection-oriented transmissions, whereas the connectionless User Datagram Protocol (UDP) is used for simpler messaging transmissions. TCP is the more complex protocol, due to its stateful design incorporating reliable transmission and data stream services. Other prominent protocols in this group are the Datagram Congestion Control Protocol (DCCP) and the Stream Control Transmission Protocol (SCTP).

There are many services that can be optionally provided by a transport-layer protocol, and different protocols may or may not implement them.

• Connection-oriented communication: It is normally easier for an application to interpret a connection as a data stream rather than having to deal with the underlying connection-less models, such as the datagram model of the User Datagram Protocol (UDP) and of the Internet Protocol (IP).
•  Byte orientation: Rather than processing the messages in the underlying communication syste  format, it is often easier for an application to process the data stream as a sequence of bytes. This simplification helps applications work with various underlying message formats.
• Same order delivery: The network layer doesn't generally guarantee that packets of data will arrive in the same order that they were sent, but often this is a desirable feature. This is usually done through the use of segment numbering, with the reciever passing them to the application in order. This can cause head-of-line blocking.
• Reliability: Packets may be lost during transport due to network congestion and errors. By means of an error detection code, such as a checksum, the transport protocol may check that the data is not corrupted, and verify correct receipt by sending an ACK or NACK message to the sender. Automatic repeat request schemes may be used to retransmit lost or corrupted data.
• Flow control: The rate of data transmission between two nodes must sometimes be
  managed to prevent a fast sender from transmitting more data than can be supported by
  the receiving data buffer, causing a buffer overrun. This can also be used to improve
  efficiency by reducing buffer under run.
• Congestion avoidance: Congestion control can control traffic entry into a telecommunications network, so as to avoid congestive collapse by attempting to avoid oversubscription of any of the processing or link capabilities of the intermediate nodes and networks and taking resource reducing steps, such as reducing the rate of sending packets. For example, automatic repeat requests may keep the network in a congested state; this situation can be avoided by adding congestion avoidance to the flow control, including slow-start. This keeps the bandwidth consumption at a low level in the beginning of the transmission, or after packet retransmission.
• Multiplexing: Ports can provide multiple endpoints on a single node. For example, the name on a postal address is a kind of multiplexing, and distinguishes between different recipients of the same location.
  Computer applications will each listen for information on their own ports, which enables the use of more than one network service at the same time.
  It is part of the transport layer in the TCP/IP model, but of the session layer in the OSI
  model. 

Connection management:

A symmetric connection management service between two service access points is specified, using a state transition system and safety and progress requirements. At each access point. the user can request connection establishment, request connection termination, and signal whether or not they are willing to accept connection requests from the remote user. The protocol can indicate connection establishment, connection termination, and rejection of a connection establishment request. The authors then specify a protocol and verify that it offers the service, given communication channels between the access points that can lose, reorder, and duplicate messages, but which guarantee delivery of a message that is repeatedly sent. The protocol achieves the service using 2-way and 3-way handshakes, and can be directly combined with any existing single-connection data transfer protocols to provide a transport layer protocol

that offers both connection management and data transfer services.

Three way hand shaking:

Before the sending device and the receiving device start the exchange of data, both devices need to be synchronized. During the TCP initialization process, the sending device and the receiving device exchange a few control packets for synchronization purposes. This exchange is known as a three-way handshake.

The three-way handshake begins with the initiator sending a TCP segment with the SYN control bit flag set.

TCP allows one side to establish a connection. The other side may either accept the connection or refuse it. If we consider this from application layer point of view, the side that is establishing the connection is the client and the side waiting for a connection is the server.

TCP identifies two types of OPEN calls:

Active Open: In an Active Open call a device (client process) using TCP takes the active role and initiates the connection by sending a TCP SYN message to start the connection.

Passive Open: A passive OPEN can specify that the device (server process) is waiting for an active OPEN from a specific client. It does not generate any TCP message segment. The server processes listening for the clients are in Passive Open mode.

 

Session layers:

The session protocol defines the format of the data sent over the connections. Session layer establish and manages the session between the two users at different ends in a network. Session layer also manages who can transfer the data in a certain amount of time and for how long. The examples of session layers and the interactive logins and file transfer sessions. Session layer reconnect the session if it disconnects. It also reports and logs and upper layer errors. 

The session layer allows session establishment between processes running on different stations.

Functions of Session layer:

Session eatablishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.

Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging and so on.

 Protocols: The protocols that work on the session layer are NetBIOS, Mail Slots, Names Pipes, RPC.

  

Presentation layer:

Presentation layer is also called translation layer. The presentation layer presents the data into a uniform format and masks the difference of data format between two dissimilar systems.

The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, and then translate the common format to a format known to the application layer at the receiving station.

Functions of Presentation layer:

 Character code translation: for example, ASCII to EBCDIC.

 Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.

 Data compression: reduces the number of bits that need to be transmitted on the network.

 Data compression: reduces the number of bits that need to be transmitted on the network.

Data encryption: encrypt data for security purposes. For example, password encryption.

 

Application layer:

The application layer serves as the window for users and application processes to access network services. The application layer makes the interface between the program that is sending or is receiving data and the protocol stack. When you download or send e-mails, your e-mail program contacts this layer. This layer provides network services to the end-users like Mail, ftp, telnet, DNS.

Function of Application Layer:

• Resource sharing and device redirection.
• Remote file access.
• Remote printer access.
• Inter-process communication.
• Network management.
• Directory services.
• Electronic messaging (such as mail)
• Network virtual terminals.
• Protocols used at application layer are FTP, DNS, SNMP, SMTP, FINGER, and TELNET.