TCP performance

Question 1

diff between flow and congestion control?

Answer 1

Flow control: do not overload the receiver. –Congestion control: do not overload the network.

Question 2

why is tcp no longer as good?

Answer 2

it was designed to work on a low speed network relative to today, performance is impacted by improved speeds

Question 3

how do we prevent wrapped sequence numbers in tcp?

Answer 3

PAWS
protection against wrapped sequence nums

uses tcp extension header with timestamps, compares timestamps as well as sequence number

Question 4

how can single-packet trans. be sped up?

Answer 4

pipelining - tx continues sending data before the first ack

Question 5

what is the bandwidtch delay product?

Answer 5

capacity of a “pipe” - the path between tx and rx

bdp - data-rate x RTT

Question 6

what is end-to-end flow control?

Answer 6

Receiver may wish to control transmission from sender, e.g. due to load at receiver • End-to-end flow control: –Receiver signals sender to control sending rate.

Question 7

how does bdp relate to tcp rwnd?

Answer 7

rwnd size dictactes the utilisation of the pipe. if rwnd > bdp then 100% util,

otherwise its a fraction

e.g. Example: 1Gbps Ethernet, TRTT = 1ms: – BDP: 109 ´0.001 = 125,000 bytes. – 65,536 / 125,000 = ~52% utilisation. • Example: 1Gbps, TRTT = 200ms: – BDP: 109 ´0.2 = 25MB. – 65,536 / 25MB = ~0.003% utilisation.

so you wanna increase your rwnd using a window scale option, recommeneded that this is used with paws

Question 8

what is congestion collapse?

Answer 8

Congestive collapse is the condition in which congestion prevents or limits useful communication. Congestion collapse generally occurs at choke points in the network, where incoming traffic exceeds outgoing bandwidth

Question 9

what causes congestion? what are the effects of congestion?

Answer 9

Congestion effects: – Higher end-to-end delay. – Lost packets. – Network instability, – Loss of service,

• Causes of congestion: – Buffer overflow in routers. – Unpredictable traffic patterns. – Route changes. – Time-of-day traffic. – etc…

Question 10

how does tcp reno do congestion control?

Answer 10

slow start (exp) into sawtooth (linear)

restarts at N/2 and increases linearly per RTT

Question 11

what is jfi? flaw?

Answer 11

Jain’s Fairness Index (JFI): – Widely used. – A “power” measure.
JFI assumes that resources can be consumed equally by systems/flows being measured

Different TCP variants behave differently: –unequal shares of available resources