TCP & UDP

What are the three Transport Layer protocols in TCP/IP?

TCP, UDP, and SCTP.

Which protocol is connection-oriented and reliable?

TCP (Transmission Control Protocol).

Which protocol is connectionless and unreliable?

UDP (User Datagram Protocol).

What is SCTP and its purpose?

Stream Control Transmission Protocol — designed for reliable transmission over a connectionless network.

What does TCP provide between hosts?

A reliable host-to-host connection.

What are the main TCP controls?

Initiation, prioritization, flow control, congestion avoidance, and termination.

What does the TCP header define?

The structure of control fields for managing connections and data reliability.

Which organization assigns TCP port numbers?

IANA (Internet Assigned Numbers Authority).

What are “well-known” ports used for?

Standard services like HTTP (80), HTTPS (443), and FTP (21).

What does the Sequence Number field represent?

The identifier of the segment being transmistted.

What does the Acknowledgement Number field represent?

The next sequence number expected from the other device.

What does the Data Offset field indicate?

Where the data begins in the segment (measured in 32-bit words).

What are the six main TCP control flags?

URG, ACK, PSH, RST, SYN, FIN.

What does the URG flag indicate?

The Urgent Pointer field is valid (priority data transfer).

What does the ACK flag indicate?

The Acknowledgement Number field is valid.

What does PSH do?

Pushes data immediately to the receiving application.

What does RST do?

Resets the TCP connection.

What does SYN do?

Synchronizes sequence numbers to start a connection.

What does FIN do?

Indicates the end of data transmission.W

What does the Window field show?

The number of bytes the device can accept.

What does the Checksum field ensure?

Data integrity (16-bit one’s complement sum).

What is the process used to establish a TCP connection?

The three-way handshake.

Describe the three steps of the TCP handshake.

SYN — Client sends a synchronize request.
SYN + ACK — Server acknowledges and sends its own SYN.
ACK — Client confirms, and the connection is established.

What sequence is used to close a TCP session?

FIN —> ACK —> FIN —> ACK

What is the TCP termination sequence commonly called?

TCP termination handshake.

Why does TCP use a four-step termination instead of three?

To allow both sides to close independently and ensure all data is transmitted.

What does the TCP state machine describe?

The transitions between states during connection setup, data transfer, and termination.

What command shows active TCP and UDP connections on a system?

netstat -a

How does UDP differ from TCP?

UDP is connectionless and provides no reliability, flow control, or acknowledement.

What is the benefit of UDP’s simplicity?

Lower overhead and faster transmission.

What is a typical use case for UDP?

Streaming media, DNS queries, VoIP.

How many fields are in the UDP header?

Four (Source Port, Destination Port, Length, Checksum).

What does the Length field represent?

The entire length of the UDP segment (header + data).

What does the Checksum field represent?

A 16-bit one’s complement sum over the entire segment.

Based on the UDP checksum design, what can UDP not guarantee?

Reliable delivery or retransmission of lost/corrupt packets.