layer4 cheat sheet

Question 1

OSI Layer 4 (The Transport Layer)
🔹 Main Functions of the Transport Layer:
Segmentation
Reassembly
Flow Control
Error Checking
Multiplexing

Answer 1

Segmentation:
Breaks large data into smaller pieces (segments).
Reassembly:
Puts segments back together in the correct order.
Flow Control:
Prevents sender from overwhelming receiver (e.g., TCP window size).
Error Checking:
Ensures data is intact and in the right order (via checksums).
Multiplexing:
Uses port numbers to allow multiple conversations (apps) on one device.

Question 2

TCPTCP (Transmission Control Protocol)

Answer 2

Connection oriented (3-way handshake)
Reliability Reliable – resends lost packets
Ordering Guarantees correct order
Speed Slower (more overhead)
Examples HTTP, HTTPS, FTP, SSH, Telnet

Question 3

UDP (User Datagram Protocol)

Answer 3

Connectionless
Reliability Unreliable – no retransmission
Ordering No ordering guarantee
Speed Faster (less overhead)
Examples DNS, DHCP, TFTP, VoIP, streaming

Question 4

TCP Header Fields:

Answer 4

Source Port

Destination Port

Sequence Number

Acknowledgment Number

Flags (SYN, ACK, FIN, RST, etc.)

Window Size

Checksum

Question 5

UDP Header Fields:

Answer 5

Source Port

Destination Port

Length

Checksum