Layer 4

The protocols that use a three-way handshake to transfer information can be found within which layer of the OSI model?

A.

Layer 2

B.

Layer 3

C.

Layer 4

D.

Layer 5

Layer 4: Transport Layer

Just above the Network Layer is the Transport Layer (Layer 4). The Transport Layer provides a valuable service in network communication: the ability to ensure that data is sent completely and correctly through the use of error recovery and flow control techniques. On the surface, the Transport Layer and its function might seem similar to the Data Link Layer because it also ensures the reliability of communication. However, the Transport Layer not only guarantees the link between stations; it also guarantees the actual delivery of data.

Connection Versus Connectionless

At the Transport Layer are the two protocols known as TCP and UDP; these protocols are known as connection and connectionless, respectively. Connection-oriented protocols operate by acknowledging or confirming every connection request or transmission, much like getting a return receipt for a letter. Connectionless protocols are those that do not require an acknowledgment and in fact, do not ask facknowledgmentone. The difference between these two is the overhead involved. Due to connection-oriented protocols’ need for acknowledgments, the overhead is more and the performance is less, while connectionless is faster due to its lack of this requirement.

From a high-level perspective, the Transport Layer is responsible for communication between host computers and verifying that both the sender and receiver are ready to initiate the data transfer. The two most widely known protocols found at the Transport Layer are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). TCP is connection-oriented, whereas UDP is connectionless. TCP provides reliable communication through the use of handshaking, acknowledgments, error detection, and session teardown. UDP is a connectionless protocol that offers speed and low overhead as its primary advantage.

OSI layers and common protocols.
OSI REFERENCE MODEL LAYER	COMON PROTOCOLS AND APLICATIONS
Application	FTP, TFTP, SNMP, Telnet, HTTP, DNS, and POP3
Presentation	ASCII, EBCDIC, TIFF, JPEG, MPEG, and MIDI
Session	NetBIOS, SQL, RPC, and NFS
Transport	TCP, UDP, SSL, and SPX
Network	IP, ICMP, IGMP, BGP, OSPF, and IPX
Data Link	ARP, RARP, PPP, SLIP, TLS, L2TP, and LTTP
Physical	HSSI, X.21, and EIA/TIA-232