Udacity Lesson 6 Flashcards
Quiz Content Covered in Udacity Lesson 6
What is congestion control? why do we need it?
Fill the internets pipes without overflowing them
Throughput less than the bottleneck length
Congestion Collapse
Increase in load -> decrease in useful work
Congestion Collapse
cause of congestion collapse
spurious retransmission
undelivered packets
spurious retransmission
“better timers
tcp congestion control”
undelivered packets
congestion control of all packets
congestion control main goals
“use network resources efficiently
preserve fair allocatino of resources
avoid congestion collapse”
congestion control approaches
“end-to-end congestion control
network-assisted”
end-to-end congestion control
“no feedback from network
congestion inferred by loss and delay”
network-assisted congestion control
“routers provide feedback:
single bit for congestion
explicit rate specification”
tcp congestion control
senders increase rate until packet drop occurs
T/F: packet loss is always a sign of congestion
False.
Wireless networks can experience packet loss due to interference
packet loss occurs when
a router has no room in its buffer for incoming packets
window-based algorithm
“sender can only have certain number of packets ““in-flight”” referred to as ““window size””.
sender cannot send additional packets until ACK is returned from receiver.”
AIMD additive increase
increment window size based on number of returned ACK packets
AIMD multiplicative decrease
reduce window size by half
AIMD
“Window-based congestion control: additive increase, multiplicative decrease.
Distributed, Fair, Efficient”
Sending Rate: RTT, Packet size, Window_size
( Window_Size / RTT ) * ( Bits / Packet )
Fairness
every sender gets fair share of network resources
Efficiency
The network resources are used well.
Phase Plot
see plot
Multiplicative Decrease improves
fairness
Additive Increase improves
efficiency
Loss Rate
“1/2 * (w_m/2)*(w_m/2 + 1)
~w_m^2 / 8”
Throughput
3/4 * w_m/(RTT)
Solution to TCP Incast
“Finer granularity retransmission. (timers need to operate on a granularity close to that of the datacenter)
Fewer ACKs. (ACKs for every-other packet)”
Jitter
Variation in transmission delay
Which pathologies can streaming audio/video tolerate?
“Initial Delay
Minor Packet Loss
CANNOT tolerate Jitter”
T/F: TCP is good for video/audio transmission
FALSE
UDP
User Datagram Protocol
Does UDP retransmit?
No
Does UDP adapt sending rate?
No
QoS
Quality of Service: Marking and Policing
QoS problem with Fixed Bandwidth
leads to inefficiency
Common QoS strategies
“Marking Packets by Priority
Scheduling High-Priority Queues”
