Principles of congestion control
-
Upload
whitney-moss -
Category
Documents
-
view
96 -
download
2
description
Transcript of Principles of congestion control
ContentsCauses and cost of congestion
Three examples How to handle congestion
End-to-end Network-assisted
TCP congestion controlATM ABR congestion controlSummary
What is congestion?Overflowing of routers buffersPacket retransmission
Delay the problem?How to treat problem?
Mechanisms to throttle senders
Causes and cost Example 1
2 senders Router with infinite buffer spaceNo retransmissionSending time infinteLink capacity (R)
•Large queuing delays
Causes and cost
Causes and costExample 2
2 sendersRouter with finite buffer spaceRetransmissionOffered loadLink capacity (R)
Causes and cost•Retransmission due to buffer overflow•Retrassmission of unneeded copies
Causes and costExample 3
4 senders4 routers with finite buffer spaceMultihop pathsLink capacity (R)Offered load
Causes and cost•Throughput goes to zero in the the limit of heavy traffic•Wasted use of upstream links capacity
How to handle congestionEnd-to-end congestion control
No explicit support from networkIP layer provides no feedbackObserved network behavior, ex. packet loss or
delayTCP use end-to-end control
TCP congestion controlAdditive-increase, multiple-decrease (AIMD)Slow start (exponential)Congestion avoidance (linear)Fast recovery
Timeout Tripple duplicat ACK’s
TCP Tahoe TCP Reno (Newer versions of TCP)
How to handle congestion Network-assisted
Explicit feedbackDirect feedbackFeedback via receiverLow impact on trafficATM (Asynchronous Transfer Mode)ABR (Available Bit-Rate)XCP protocol
ATM ABR congestion control ATM takes a VC approach
ATM is a kind of architectureTrack behavior of individual senders
ABR takes advantage of spare bandwidthResource management cells (RM cells)RM cells convey congestion-related
informationDirect feedback or feedback via receiver
ABRThree mechanisms
Explicit forward congestion indication (EFCI) bit
Congestion indication and No increase (CI and NI) bit
Expclicit rate (ER) field
SummaryWhy do we need to handle congestion?
Minimize delaysMake traffic more effectiveMaximize use of bandwidth
How is congestion problem solved?End-to-end congestion controlNetwork-assisted congestion control
Questions?