TCP Protocol: Features, Handshake, Flow Control, and Reliability

https://moodle.maynoothuniversity.ie/pluginfile.php/2001435/mod_resource/content/0/Lab5%20-%20TCP%20notes%20from%20Hannah%20.pdf

What do negative acknowledgements indicate in TCP?

They inform the sender that a packet was not received correctly.

What is the purpose of sequence numbers in TCP?

They allow the sender to keep track of the order of packets and identify lost packets.

What is window/pipelining in TCP?

It allows the sender to transmit multiple packets without waiting for acknowledgements, as long as they are within a specified sequence number range.

What are the main features of TCP's connection-oriented transport?

TCP relies on error detection, retransmission, cumulative acknowledgements, and timers for reliable data transfer.

What must occur before data can be sent in a TCP connection?

A handshake process must take place to establish connection parameters.

What is the maximum segment size (MSS) in TCP?

It is the maximum amount of application data that can be sent in a TCP segment, determined by the largest link-layer frame.

What is the role of the TCP header?

It contains important fields such as source and destination port numbers, sequence and acknowledgment numbers, and flags for controlling the connection.

What does the ACK bit in the TCP flag field signify?

It indicates that the acknowledgment number in the segment is valid.

What is the purpose of the SYN bit in TCP?

It is used for connection setup and teardown.

What does the FIN bit in TCP indicate?

It signals the termination of a connection.

How does TCP ensure reliable data transfer?

By using sequence numbers, acknowledgment numbers, and retransmission of lost packets.

What is the function of the receive window in TCP?

It indicates the number of bytes the receiver can accept at a given time.

What does the options field in a TCP header allow?

It allows the sender and receiver to negotiate parameters such as the maximum segment size.

How does TCP handle urgent data?

It uses the URG bit and the Urgent Data Pointer field to indicate the location of urgent data.

What happens to data in TCP after it is received?

It is placed in the TCP connection's receive buffer for the application to read.

What is the significance of the acknowledgment number in TCP?

It indicates the sequence number of the next byte that the sender expects to receive.

What is the structure of a TCP segment?

It includes a header with source and destination ports, sequence and acknowledgment numbers, and a payload.

What is a third-way handshake in TCP?

It is the process where the client and server exchange special TCP segments to establish a connection.

How does TCP manage data flow between two processes?

It allows full-duplex communication, enabling data to flow simultaneously in both directions.

What is the role of timers in TCP?

They are used to retransmit packets if they are lost during transmission.

What is the significance of the 32-bit sequence number field in TCP?

It numbers the data flowing from sender to receiver, helping to identify lost packets.

What type of acknowledgements does TCP provide?

Cumulative acknowledgements.

How does TCP handle out of order data?

TCP can either discard out of order segments or keep them and wait for missing bytes.

What is the purpose of the timeout/retransmit mechanism in TCP?

To recover lost segments.

What should the length of the timeout be in relation to the round-trip time (RTT)?

It should be larger than the connection's round-trip time.

How is SampleRTT estimated in TCP?

SampleRTT is estimated for transmitted but currently unacknowledged segments.

What is the formula for calculating EstimatedRTT?

EstimatedRTT = (1 - a) EstimatedRTT + a SampleRTT, with a recommended value of 0.125.

What does DevRTT estimate in TCP?

It estimates how much SampleRTT deviates from EstimatedRTT.

What is the recommended value for B in the DevRTT calculation?

0.25.

What is the relationship between the timeout interval and EstimatedRTT?

The timeout interval should be greater than or equal to EstimatedRTT.

What happens to the timeout interval after a timeout occurs?

The timeout interval is doubled.

What is the purpose of TCP's cumulative acknowledgements?

To ensure reliable data transfer by acknowledging one or more previous unacknowledged segments.

What does TCP do when it receives three duplicate ACKs?

It performs a fast retransmit of the missing segment before the timer expires.

What is the difference between Go-Back-N (GBN) and TCP's selective acknowledgment?

GBN retransmits all subsequent packets after a lost packet, while TCP only retransmits the lost packet.

What is flow control in TCP?

A service that prevents the sender from overflowing the receiver's buffer by matching the sending rate with the reading rate of the application.

What variables does TCP maintain for flow control?

Receive window (rwnd), LastByteRead, and LastByteRcvd.

What happens when the receive window (rwnd) is full?

The sender will continue to send 1-byte segments and will receive ACKs with new rwnd values once space is made.

What are the steps for establishing a TCP connection?

1. Client sends a SYN segment to the server. 2. Server allocates resources and sends a connection-granted segment back.

What does the SYN bit indicate in a TCP segment?

It indicates that the segment is a request to establish a connection.

What information is contained in the connection-granted segment from the server?

SYN = 1, acknowledgment field set to client_isn + 1, and the server's initial sequence number.

What is the purpose of the SYNACK segment in TCP?

It indicates that the server has granted the connection and is part of the three-way handshake process.

What happens when a client receives a SYNACK segment?

The client allocates buffers and variables for the connection and sends an acknowledgment segment back to the server.

What does the FIN bit signify in a TCP segment?

It indicates that the sender wants to terminate the connection.

What are the states a client TCP transitions through during a connection?

CLOSED, SYN_SENT, ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, TIME_WAIT.

What occurs in the FIN_WAIT_1 state?

The client waits for an acknowledgment segment from the server after sending a FIN segment.

What is the purpose of the TIME_WAIT state in TCP?

It allows the client to resend the final acknowledgment in case it was lost.

What triggers a TCP sender to reduce its send rate?

Perceived congestion on the path or a lost segment.

What is a SYN Flood Attack?

A DoS attack that sends numerous TCP SYN segments without completing the handshake, exhausting server resources.

What is the role of TCP congestion control?

To manage the rate of data transmission to avoid overwhelming the network.

What does cwnd stand for in TCP?

Congestion window, which limits the amount of unacknowledged data that can be sent.

How does TCP use acknowledgments to manage congestion?

TCP increases the congestion window size when acknowledgments are received, indicating successful data delivery.

What is the slow start mechanism in TCP?

A method where the congestion window starts at 1 MSS and doubles each time a segment is acknowledged.

What happens when a loss event occurs in TCP?

The congestion window is reset to 1 MSS, and the slow start process begins again.

What is the purpose of bandwidth probing in TCP?

To adjust the transmission rate in response to received ACKs until a loss event occurs.

What is the significance of the ssthresh variable in TCP?

It represents the slow start threshold, which is set to half the congestion window value when congestion is detected.

What are the three major components of TCP congestion control?

Slow start, congestion avoidance, and fast recovery.

What does the term 'self-clocking' refer to in TCP?

The mechanism where TCP uses acknowledgments to trigger increases in the congestion window size.

What happens if a host receives a TCP segment with mismatched port numbers?

The host sends a special reset segment with the RST flag set to 1.

What does the term 'half-open connections' refer to?

Connections that are allocated resources but not fully established due to incomplete handshakes.

What is the effect of a high delay or low-bandwidth link on TCP's congestion window?

The congestion window will increase at a slow rate.

What is the initial sending rate in TCP when the connection begins?

It is roughly MSS/RTT, where MSS is the maximum segment size and RTT is the round-trip time.

What happens when the congestion window (cwnd) equals the slow start threshold (ssthresh)?

Slow start ends, and TCP transitions into congestion avoidance mode.

How does TCP increase the congestion window in congestion avoidance mode?

TCP increases cwnd cautiously by 1 Maximum Segment Size (MSS) for each round-trip time (RTT).

What triggers TCP to perform a fast retransmit?

The detection of three duplicate acknowledgments (ACKs).

What is the effect of TCP splitting on networking delay?

It reduces networking delay from approximately 4 RTT to RTT.

What is the behavior of TCP when entering the congestion avoidance state?

Cwnd is set to approximately half the value it was when congestion was last encountered.

How does TCP adjust the congestion window when a timeout occurs?

Cwnd is reset to 1 MSS, and ssthresh is updated to half of cwnd when a loss occurs.

What happens to cwnd during fast recovery after detecting three duplicate ACKs?

Cwnd is increased by 1 MSS for each duplicate ACK received.

What is the difference between TCP Tahoe and TCP Reno in terms of congestion control?

TCP Tahoe unconditionally cuts cwnd to 1 MSS after a timeout, while TCP Reno incorporates fast recovery.

What is the additive-increase, multiplicative-decrease (AIMD) approach in TCP congestion control?

It refers to the linear increase of cwnd by 1 MSS per RTT and halving cwnd on triple duplicate ACK events.

What is the purpose of the TCP Vegas algorithm?

To avoid congestion while maintaining good throughput by detecting congestion before packet loss occurs.

How does the average throughput of a long-lived TCP connection relate to cwnd and RTT?

TCP's transmission rate is roughly equal to cwnd divided by RTT.

What is the average throughput of a TCP connection when a loss event occurs?

The average throughput ranges from W/(2 RTT) to W/RTT, where W is the value of cwnd at the time of loss.

What is the formula for average throughput of a TCP connection as a function of loss rate?

Average throughput = (1.22 MSS) / (RTT √L), where L is the loss rate.

What is meant by fairness in TCP congestion control?

Fairness is achieved when the average transmission rate of each connection is approximately R/K, where R is the transmission rate and K is the number of TCP connections.

How do sessions with smaller RTTs affect bandwidth usage in TCP?

Sessions with smaller RTTs can grab available bandwidth more quickly, resulting in higher throughput compared to connections with larger RTTs.

What is the impact of UDP traffic on TCP congestion control?

UDP sources do not decrease their transmission rate in response to congestion, which can crowd out TCP traffic.

What happens when an application uses multiple parallel TCP connections?

It can obtain a larger fraction of bandwidth in a congested link.