THE TRANSPORT LAYER - LAYER 4

The four main functions of the Transport layer are:
· Segment and assemble upper-layer applications
· Transport segments from one host to another host
· Establish and manage end-to-end operations
· Error recovery
In carrying out its duties, the Transport layer performs a range of support activities, including:
· Maintaining data integrity through flow control techniques
· Multiplexing the data from upper layer applications
· Setting up and tearing down any virtual circuits established to transport the data over the network
· Hiding any network-dependent information from the upper layers (which will only confuse them)
· Breaking down Session layer (layer 5) datagrams into segments
· Monitoring the error-free delivery of the data to its destination
· Providing for general connection management and data transfer services
· Providing for the reliable (but, not guaranteed) delivery of data
Transporting Protocols
· TCP (Transmission Control Protocol): The protocol primarily concerned with the reliable delivery of packets that requires an acknowledgement of a packet's arrival at its destination.
· UDP (User Datagram Protocol): The TCP/IP best-effort protocol that isn't concerned with the reliable delivery of packets and doesn't bother with overhead such as acknowledgments.
· SPX (Sequence Package Exchange): The Novell protocol most akin to TCP. It guarantees data delivery.
· NWLink (NetWare Link): Microsoft's version of Novell's IPX/SPX.
· ATP/NBP (AppleTalk Transaction Protocol/Name Binding Protocol): AppleTalk's data transport protocols.
· NetBIOS/NetBEUI (Network Basic Input/Output System/NetBIOS extended User Interface): Microsoft's network protocols that work together to manage communications and provide data transport services.
TCP, UDP and SPX are Transport layer protocols. Network layer protocols include IP, ICMP, and IPX.
SPX is connection-oriented and its packets are tracked through the use of a sequence number associated with each packet. A positive acknowledgment must be received from the destination device for each packet before another packet is sent. A print server is an example of an application that implements SPX.
A protocol is considered connection-oriented if it meets one of two criteria:
· Data is transmitted over a negotiated, established path, a virtual circuit, between two nodes.
· The protocol includes a process for error-recovery.
The following protocols and link types are connection-oriented:
· Frame Relay
· TCP
· SPX
· X.25
Connectionless protocols:
· IP
· IPX
· UDP Note: IP and IPX are Network layer protocols.
Layer 4 protocols that implement error recovery have the following characteristics:
· They are connection-oriented, which means they establish a connection prior to the transmission of data.
· Each PDU has header information used by the receiver to acknowledge the receipt of a packet and a system to check for errors in transmission.
· The sender requires notification of packets that have been successfully received.
You use three primary methods for error-checking. They are parity bit, check-sum, and CRC.
The three basic forms of flow control are:
· Buffering
· Congestion avoidance
· Windowing