Transport Layer in OSI Model

Application Layer and Transport Layer

• The application layer sends and receives messages as a single block of data.
• The transport layer breaks the data into smaller segments for transmission and reassembles them at the receiver's end.

Segmenting

• Segmenting is the process of breaking down a large message from the application layer into smaller segments for transmission.
• The transport layer (TCP) breaks down the message into smaller packets and assigns a sequence number to each packet.
• At the receiver's end, the transport layer reassembles the packets into the original message.

Session Management

• A session is a conversation between two computers, where the sender establishes a connection with the receiver before transmitting data.
• Connection-oriented messaging (TCP) establishes a session before transmitting data, ensuring that the data is delivered reliably.
• Connectionless messaging (UDP) does not establish a session before transmission, and packets may arrive out of sequence at the receiver's end.

Connection-Oriented Messaging (TCP)

• TCP sets up a connection (session) between the sender and receiver before transmitting data.
• The connection is established by exchanging synchronize (SYN) and acknowledge (ACK) segments.
• If errors occur during transmission, the receiver requests retransmission of the affected segment.
• TCP is a full-duplex connection, where each side of the session can terminate the connection independently.

Connectionless Messaging (UDP)

• UDP does not establish a connection before transmission, and each packet is treated separately.
• Packets may take different routes and arrive out of sequence at the receiver's end.
• The maximum size of a TCP message field is 1444 bytes (Ethernet's max message size minus IP and TCP headers).

Transport Layer Functions

• The transport layer has three main functions: linking to the application layer, segmenting, and session management.
• Linking to the application layer involves linking software applications to the network.
• Segmenting involves breaking down large messages into smaller blocks for transmission.
• Session management ensures end-to-end delivery of messages.

Linking to the Application Layer

• The transport layer uses port numbers to identify applications running on a computer.
• Port numbers are located in the TCP header fields and indicate the source and destination applications.
• Standard port numbers are assigned to common applications (e.g., HTTP port 80, FTP port 21, SMTP port 25).
• Nonstandard port numbers can be used, but require configuration of TCP by the programmer.