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?