OSI Model & TCP/IP Model

OSI Model 

Application Layer: 

• This layer includes the Protocols which support an Application.  It does not contain the application itself. 
• When the application needs to send the data across, then it passes the data to the protocols that support it. 
• Eg: A browser generates an HTTP Request on form submission.
• HTTP, SMTP, FTP

Presentation Layer:

• Formats the information into a form that everyone will understand.
• This layer is concerned with the Syntax and format of Data.
• If a browser is used to upload a picture, it will be converted into an HTTP Request and the picture will be represented in a standard format like JPEG, TIFF, GIF, which the receiving side will understand the type of Information contained in the Data.
• This layer is responsible for Translation ( System dependent to System Independent format)
• This layer is responsible for Encryption or Compression of Information 

Session Layer:

• Is responsible for establishing/ maintaining/ terminating a session with an Application.
• Allows communication between applications in Simplex, Half-Duplex and Full-Duplex forms.
• It is also called "port layer"
• It manages Application-to-Application Sessions
• Example: Images, HTML, CSS files are all stored as separate files on a web server. To download each of them a separate download session is started. The session layer keeps track of which packets and data belong to which file and keeps track of each download session.
• The session layer is responsible for session checkpointing and recovery. 
• Session layer allows information of different streams, to be properly combined or synchronized (Eg - different files being downloaded for loading a single webpage)
• In TCP/IP this functionality is handled by application software(browser) addressing a connection to a remote machine and using a different local port number for each connection
• AppleTalk Session Protocol, NetBios are Session Layer Protocols.

Transport Layer

• Transport Layer ensures delivery from one (Source IP, Port) to Another (Destination IP, Port)
• The transport layer ensures that messages/data are delivered without errors, in sequence, and with no data content losses or data content duplications.
• Session Layer acts as a manager and manages the multiple files delivered by Transport Layer to be properly combined or synchronized.
• TCP & UDP are transport layer protocols.

Network Layer

• Responsible for Source IP address to Destination IP address delivery
• IP, ICMP, OSPF, BGP, IGMP protocols work at this layer.
• Ping operates by sending Internet Control Message Protocol (ICMP) echo request packets to the target host and waiting for an ICMP response. In the process it measures the time from transmission to reception (round-trip time) and records any packet loss. Since PING uses ICMP protocol, it is not associated with any PORT number.
• If two systems were connected to the same Link, there would be no need for a Network Layer.
• Network layer is needed when, two devices are connected to two different Networks
• Network layer provides the routing mechanism for packets.

Data Link Layer:

• Responsible for moving a FRAME from one node to the next.
• Physical Addressing - Assigns the sender and receiver of the frame.
• Flow Control - Rate at which data is absorbed by the receiver
• Error Control - Detects and re-transmits damaged or lost frames. Error control is achieved through check-sum added at the end of frame.
• Access Control - Determines which node has control of the link at any given time.
• ARP, RARP, PPP ( Point to Point Protocol)

Physical Layer:

• Moves bits from one hop to the next.
• This layer converts bits into voltage for transmission.
• This layer controls synchronization, data rate, line noise and medium access.