Transport Layer Part 3

Question 1

Principles of Congestion Control

Congestion

Answer 1

• Too many sources sending too much data too fast for network to handle.
• Causes long delays (queuing)
• Causes packet loss (buffer overflow)

Question 2

Principles of Congestion Control

Approaches to Congestion Control

Answer 2

1. End-end congestion control
2. Network-assisted congestion control

Question 3

Principles of Congestion Control

End-end Congestion Control

Answer 3

• No explicit feedback from network
• Congestion inferred from observed loss, delay
• Approach taken by TCP

Question 4

Principles of Congestion Control

Network-assisted Congestion Control

Answer 4

• Routers provide direct feedback to sending/receiving hosts about flows passing through congested router
• May indicate congestion level or explicitly set sending rate
• ECN protocol

Question 5

Principles of Congestion Control

ECN Protocol

Answer 5

• Network-assisted Congestion Control
• 2 bits in IP header marked by network router to indicate congestion
• Congestion indication carried to destination
• Destination sets ECN bit on ACK seg to notify sender on congestion
• Involves both IP (IP header bit) and TCP (TCP header)

Question 6

TCP Congestion Control

cwnd

Answer 6

• Congestion window
• Sender in-flight packets cannot exceed this number

Question 7

TCP Congestion Control

rwnd

Answer 7

• Receiver window
• Sender in-flight packets cannot exceed this number

Question 8

TCP Congestion Control

cwnd & rwnd

Answer 8

LastByteSent - LastByteAcked <= min{cwnd, rwnd}

Amount of unACKed data at sender cannot exceed minimum of cwnd and rwnd.

Question 9

TCP Congestion Control

TCP Congestion Phases

Answer 9

1. Slow Start Phase
2. Congestion Avoidance Phase
3. Congestion Detection Phase

Question 10

TCP Congestion Control

Threshold

Answer 10

Point where congestion began in the past.

Question 11

TCP Congestion Control

Slow Start Phase

Answer 11

Exponential increment: After every RTT the congestion window size increments exponentially

Question 12

TCP Congestion Control

Congestion Avoidance Phase

Answer 12

Additive Increment: Starts after threshold val is reached. After each RTT, cwnd is incremented by 1.

Question 13

TCP Congestion Control

Congestion Detection Phase

Answer 13

Multiplicative Decrement: If congestion occurs, cwnd is cut in half. Only way for sender to guess that congestion occured is if it needs to retransmits a segment.

Retransmission can occur if timer times out or when 3 duplicate ACKs are received.

Question 14

TCP Congestion Control

AIMD

Answer 14

• Additive Increase, Multipicative Decrease
• Senders can increase sending rate until packet loss occurs, then decrease sending rate on loss event
• “Sawtooth” behavior, probing for bandwidth

Question 15

TCP Congestion Control

Congestion Detection Phase Case 1

Answer 15

TCP Tahoe: Retransmission due to timeout.
* ssthresh is reduced to half of current window size
* set cwnd = 1
* start with Slow Start Phase again

Question 16

TCP Congestion Control

Congestion Detection Phase Case 2

Answer 16

TCP Reno: Retransmission due to 3 ACKs
* ssthresh reduced to half of current window size
* set cwnd = ssthresh
* start with Congestion Avoidance Phase