Lecture 5: Congestion Control TCP Reno

Question 1

Wie unterscheidet man bei TCP Reno?

Answer 1

..ob wir eine schwere Stausituation oder leichte Stausituation haben.

Question 2

Cwnd: congestion window

Answer 2

wie viel darf ich schicken ohne ein Stau im Netz zu erzeugen.

Question 3

What is congestion?

Answer 3

the traffic load exceed the capacity of a linke –> congestion

Question 4

Why is congestion bad?

Answer 4

• router buffers fill up => queueing delays
• router buffers are finite => packet drops
• need retransmissions => wasted bandwidth, low goodput, congestion collapse

Question 5

Congestion control

Answer 5

distributed algorithm that shares network resources among competing users in a fair and efficient way.

Question 6

TCP congestion control

Answer 6

• is end 2 end an window based.
• TCP senders have at most cwnd packets in the pipe (cwnd transmitted but not acknowledged packets)
• > a small cwnd = fewer packets are out at any one time and less traffic = less congrestion
• > a large cwnd = better throughput, up to the point of congestion
• the challenge is to adapt cwnd based on the stream of returning acknowledgments ACK

Question 7

TCP self-clocking

Answer 7

• packets arrive at the receiver with a spacing of packet size / bottleneck capacity
• acknowledgments are generated with the same spacing
• they trigger new packets with bottleneck spacing

Question 8

TCP congestion window adaptation

Answer 8

• it starts cautiously with a small cwnd
• whenever packets are acknowledged, it cautiously increases cwnd (to surpass the knee)
• if it detects loss (surpassed the cliff) it drastically reduces cwnd to resolve congestion quickly

Question 9

BAndwidth delay product

Answer 9

Given

• a window size of cwnd packets
• a max segment size MSS
• a pipe with capacity C
• round trip time RTT

TCP can transmit: cwnd*MSS bytes per RTT

the achievable Throughput = cwnd*MSS/RTT

In order to fill the pipe with capacity C (throughput = C) a congestion window of C*RTT = cwnd *MSS is needed

C*RTT is referred to as the bandwidth delay product of the pipe.

Question 10

Max-min Fairness

Answer 10

Maximize the rate of the flow that has the minimal rate

• max-min fairness is achieved, of no rate can be increased without decreasing an already smaller rate
• algorithm: water filling
• –increase the rates of all flows evenly until a link is saturated
• –repeat the previous for all flows that do not use saturated links