Chapter 6

Question 1

What is the primary goal of TCP congestion control?

Answer 1

To determine the available capacity in the network to avoid congestion

Question 2

Which mechanism does TCP use to adjust the size of the congestion window? A) Fast Retransmit

Answer 2

• Additive Increase/Multiplicative Decrease (AIMD)
• Slow Start

Question 3

How does TCP infer the occurrence of network congestion?

Answer 3

By detecting timeouts and duplicate ACKs

Question 4

What happens during TCP’s slow start phase?

Answer 4

The congestion window increases exponentially

Question 5

What triggers the ‘Fast Retransmit’ mechanism in TCP?

Answer 5

The receipt of multiple duplicate ACKs

Question 6

Which TCP mechanism adjusts the congestion window based on perceived network congestion without waiting for a packet loss?

Answer 6

Fast Recovery

Question 7

What does the TCP ‘Additive Increase, Multiplicative Decrease’ strategy help prevent?

Answer 7

• Buffer overflow at the routers
• Congestion collapse
• Uncontrolled bandwidth usage

Question 8

During TCP congestion control, when is the ‘Slow Start’ algorithm re-invoked?

Answer 8

After a timeout indicating packet loss

Question 9

Which of the following best describes TCP’s Fast Retransmit?

Answer 9

It triggers retransmission of a packet if multiple duplicate ACKs are received.

Question 10

In TCP congestion control, what is the purpose of the ‘Congestion Window’?

Answer 10

To control the number of bytes the sender is allowed to transmit without an ACK

Question 11

Congestion Window (CWND)

Answer 11

• The congestion window controls the amount of data TCP can send into the network before requiring an acknowledgment.
• It starts small and adjusts dynamically based on network responses to find an optimal data flow rate without inducing congestion.

Question 12

Additive Increase/Multiplicative Decrease (AIMD):

Answer 12

• This is the primary mechanism of TCP congestion control
• Where the congestion window increases gradually (additively) to probe for usable bandwidth and decreases sharply (multiplicatively) when congestion is detected (usually through packet loss indicated by timeouts or duplicate acknowledgments).

Question 13

Slow Start Phase

Answer 13

• When a TCP connection begins, or after a timeout, TCP uses the slow start algorithm, where the congestion window size increases exponentially each round-trip time (RTT) until it detects packet loss or reaches a threshold.
• This rapid increase helps quickly utilize available bandwidth.

Question 14

Fast Retransmit

Answer 14

Fast Retransmit triggers a retransmission of packets when multiple duplicate ACKs are received, suggesting that a packet has been lost but subsequent packets have been received.

Question 15

Fast Recovery

Answer 15

Fast Recovery adjusts the congestion window in response to congestion detected via Fast Retransmit, allowing the congestion window to shrink less drastically than it would with multiplicative decrease alone, thus speeding recovery.

Question 16

Congestion Avoidance:

Answer 16

After the slow start phase, TCP enters congestion avoidance where it increases the
congestion window more slowly to avoid causing congestion. This phase continues until a packet loss is detected.

Question 17

Thresholds and Adjustments:

Answer 17

The threshold, or ssthresh, is a boundary between slow start and congestion avoidance. TCP uses this threshold to switch from the exponential growth of the slow start to the linear increase of congestion avoidance.

Question 18

TCP Self-Clocking

Answer 18

• TCP uses the principle of self-clocking, where the receipt of ACKs paces the sending of new data.
• This mechanism ensures that data transmission is naturally regulated by the rate at which the network can handle the traffic.

Question 19

Interaction with Network Conditions:

Answer 19

• TCP congestion control adapts not just to packet loss but also variations in round-trip time and other network conditions, adjusting its behavior to maintain efficient and stable data transfer.

Question 20

Protocols and Algorithms:

Answer 20

TCP employs several protocols and algorithms to handle various scenarios and network conditions, such as selective acknowledgments (SACK) to improve performance in networks with high packet reordering.

Question 21

TCP Timers and Their Role in Congestion Control:

Answer 21

• TCP uses various timers, such as the Retransmission Timeout (RTO), to detect lost packets.
• The behavior of these timers directly influences congestion control dynamics, as timeouts often trigger reductions in the congestion window.

Question 22

Retransmission Strategies

Answer 22

TCP’s retransmission strategies, including Fast Retransmit, are crucial for maintaining throughput in the face of packet loss without waiting for the expiration of timers, which can significantly slow down transmission.

Question 23

Network Feedback Utilization

Answer 23

• TCP interprets duplicate ACKs and timeouts as signals of potential network congestion.
• This feedback mechanism allows TCP to adjust the sending rate preemptively before more severe network congestion occurs.

Question 24

Slow Start Restart (SSR)

Answer 24

• If a connection goes idle for a period, TCP may restart the slow start process, assuming that the state of the network may have changed during the idle period.
• This cautious approach helps avoid sudden bursts of data that might lead to packet loss.

Question 25

Interaction with Advertised Window

Answer 25

* The congestion window operates in conjunction with the receiver’s advertised window, which is based on the available buffer space at the receiver. * This ensures that the sender does not overwhelm the receiver.

Question 26

TCP's Reaction to Packet Loss:

Answer 26

Upon detecting packet loss, TCP not only reduces the congestion window but also enters into a recovery phase where it tries to retransmit lost packets and regain the lost throughput efficiently.

Question 27

Role of ACKs in Data Pacing

Answer 27

* TCP uses the arrival of ACKs to pace out the sending of new packets. * This is known as ACK-clocking, where each incoming ACK allows the sender to send one or more new packets * Helping to smooth the flow of data and adjust to the available network capacity.

Question 28

Congestion Window Validation (CWV)

Answer 28

* TCP implementations may include mechanisms like Congestion Window Validation to reduce the size of the congestion window after a connection has been idle for an extended period, reflecting the assumption that network conditions may have changed.

Question 29

Sensitivity to Round Trip Time (RTT)

Answer 29

TCP's performance is sensitive to the accurate measurement of RTT, as it influences the calculation of RTO and the pace at which the congestion window grows during the slow start and congestion avoidance phases

Question 30

Fine-Tuning Congestion Control Algorithms

Answer 30

* Over the years, TCP’s congestion control algorithms have been fine-tuned to handle various network conditions * Incorporating enhancements like selective acknowledgments and explicit congestion notification, which help in environments with high packet loss or varying latency

Question 31

Network Model - Packet-Switched Network

Answer 31

* Definition: Resource allocation in a packet-switched network with multiple links and switches. * Key Point: A source may have enough capacity on the outgoing link but encounter congestion in the middle of the network. * Example: Two high-speed links feeding a low-speed link, creating a bottleneck. * Important Note: Congestion control is different from routing. Routing around a congested link does not always solve congestion.

Question 32

Network Model - Connectionless Flows

Answer 32

* Definition: Network is assumed to be connectionless; connection-oriented service is in the transport protocol (e.g., TCP/IP). * Key Point: IP provides connectionless datagram delivery, TCP implements end-to-end connection abstraction. * Flow: Sequence of packets sent between a source and destination following the same route. * Pro of Flow Abstraction: Flows can be defined at different granularities (e.g., host-to-host, process-to-process).

Question 33

Connectionless Flows - Implicit vs Explicit Flows

Answer 33

* Implicit Flow: Routers observe packets traveling between the same source/destination and treat them as part of the same flow. * Explicit Flow: Source sends a flow setup message declaring a flow of packets, helping in resource allocation without end-to-end semantics. * Benefit: Explicit flows allow resource reservation at each router, potentially avoiding congestion.

Question 34

Service Model - Best-Effort Service

Answer 34

- Definition: All packets are treated equally, with no guarantees for preferential service. - Key Point: No opportunity for end hosts to ask for specific guarantees. - Contrast: A service model with multiple qualities of service (QoS) offers guarantees for specific flows, e.g., video streaming bandwidth. - QoS Example: Guaranteeing the bandwidth needed for a video stream.

Question 35

Taxonomy of Resource Allocation Mechanisms

Answer 35

- Router-Centric: Routers decide packet forwarding, dropping, and inform hosts about allowable packets to send. - Host-Centric: End hosts observe network conditions and adjust their behavior accordingly. - Note: These two groups are not mutually exclusive.

Question 36

Taxonomy - Reservation-Based vs. Feedback-Based Mechanisms

Answer 36

- **Reservation-Based**: End host requests capacity for a flow; routers allocate resources if possible. - **Key Point**: Ensures enough resources before data transmission. - **Example**: Reserving buffers at each router. - **Feedback-Based**: Hosts send data without reservations and adjust based on network feedback (explicit or implicit). - **Explicit Feedback**: Congested router sends a "please slow down" message to the host. - **Implicit Feedback**: End host adjusts based on observable network behavior, such as packet losses.

Question 37

Taxonomy - Window-based vs Rate-based Control

Answer 37

- **Window-Based Control**: Limits data transmission based on buffer space at the receiver. - **Flow Control**: Ensures sender does not overwhelm receiver’s buffer. - **Rate-Based Control**: Controls data transmission rate (bits per second) that the network can handle. - **Application**: Useful for multimedia applications that generate data at a steady rate.

Question 38

Evaluation Criteria - Effective Resource Allocation

Answer 38

- **Goal**: Maximize power (Throughput/Delay). - **Key Point**: Balance between conservative packet sending and avoiding excessive queuing delays. - **Optimal Load**: Achieved when throughput and delay are balanced for maximum efficiency. - **Illustration**: Too few packets lead to underutilization, too many packets increase delays.

Question 39

Evaluation Criteria - Fair Resource Allocation

Answer 39

- **Goal**: Ensure each flow receives an equal share of bandwidth. - **Challenge**: Fairness calculation when path lengths differ, e.g., one four-hop flow vs. three one-hop flows. - **Fairness Index**: Used to measure the fairness of resource allocation among flows.

Question 40

Effective Resource Allocation - Power Formula

Answer 40

- **Formula**: Power = Throughput / Delay - **Optimal Load**: Achieved when throughput and delay are balanced. - **Important Concept**: Conservative sending leads to underutilization, aggressive sending increases delays due to queuing.

Question 41

Fair Resource Allocation - Jain's Fairness Index

Answer 41

- **Definition**: Measures fairness among flows. - **Formula**: ((i = 1 to n)Σxi)^2/(n(i = 1 to n)Σxi^2) - **Range**: 0 to 1, where 1 represents perfect fairness. - **Example Calculation**: Given flow throughputs (x1, x2, ..., xn), calculate the fairness index.

Question 42

**Queuing Disciplines - Introduction**

Answer 42

- **Definition**: Mechanisms that control how packets are buffered and transmitted by routers. - **Key Point**: Each router must implement some queuing discipline to manage bandwidth and buffer space, affecting latency.

Question 43

**FIFO (First In, First Out)**

Answer 43

- **Definition**: Packets are processed in the order they arrive. - **Key Point**: Simple and straightforward, but can lead to inefficiencies. - **Tail Drop**: If the buffer is full, incoming packets are dropped.

Question 44

**Priority Queuing**

Answer 44

- **Definition**: Packets are marked with priority levels and processed based on these levels. - **Key Point**: High-priority packets are transmitted first, potentially starving lower-priority packets. - **Example**: Marking packets in the IP header to indicate priority.

Question 45

**Fair Queuing (FQ)**

Answer 45

- **Definition**: Each flow gets a separate queue, ensuring fair bandwidth distribution. - **Key Point**: Prevents ill-behaved traffic sources from affecting well-behaved ones. - **Challenge**: Packets of different lengths require consideration for fair bandwidth allocation.

Question 46

**Work-Conserving vs. Non-Work-Conserving**

Answer 46

- **Work-Conserving**: Link is never left idle as long as there is a packet in the queue. - **Non-Work-Conserving**: Allows for controlled idle periods to manage queue lengths and delays.

Question 47

**Weighted Fair Queuing (WFQ)**

Answer 47

- **Definition**: Each flow is assigned a weight, dictating how many bits are transmitted each time the queue is serviced. - **Key Point**: Provides more bandwidth to flows with higher weights. - **Example**: A flow with weight 2 gets twice the bandwidth of a flow with weight 1.