Transport Layer
- Responsible for Source-to-Destination delivery of Entire Message
- Network Layer oversees source-to-destination delivery of the entire packets but it does not recognize any relationship b/w those packets
- Network layer treats each packet independently
- Transport Layer ensures that whole message arrives at the destination intact
Functions of Transport Layer
- Service Point Addressing
- Computers run several programs at the same time
- Source-to-Destination delivery means delivery not only from only from one computer to the other but also from a specific process on one computer to a specific process on the other
- Transport layer header includes a type of address called Service Point Address or PORT Address
- Network layer each packet to the correct computer while Transport layer gets entire message to the correct process on that comuter
- Segmentation and Reassembly
- Message is divided into transmittable segments
- Each segment contains a sequence number
- These sequence no.s enable Transport layer at the receiving m/c to reassemble message correctly at the destination and to identify and replace lost packets
- Connection Control
- Transport layer can be either connection-less or connection-oriented
- Connectionless
- Treats each segment as an independent packet and delivers it to the transport layer of the destination m/c
- Connection-Oriented
A connection is established first with Transport layer before delivering the packet. - After all data is Tx. , the connection is disconnected
- Flow Control
- Like Data link layer, Transport layer is also responsible for Flow control
- Flow control is performed end-to-end rather than across a single link
- Error Control
- Like data link layer, Transport layer is responsible for the Error Control
- Error control is performed end-to-end
- This layer makes sure that entire message reaches Rx Transport layer w/o error
- Error can be a result of Lost, damaged or duplicated data and usually Re Tx is done
Example Figure
Session Layer
- Session layer is the Network Dialog Controller
- Establishes, Maintains, and Synchronizes the interaction between communicating systems
Function of Session Layer
- Dialog Control
- Session layer allows two systems to enter into a dialog.
- It allows communication between two processes to take place either in half duplex or full duplex mode
- Synchronization
- Session layer allows a process to add check points (synchronization points) in a stream of data
- If a system is sending a file of 2000 pages, it is advisable to insert check points after every 100 page to ensure that each 100 page unit is received and acknowledged independently
- In this case, if a crash happens during the transmission of page 523 , retransmission at page 501
- Page 1-500 need not be retransmitted
Presentation Layer
- This layer is concerned with Syntax and Semantics of info exchange between two systems
Summary
- The OSI Model
- Functions of Layers
Reading Sections
- Section 3.2 “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan