Autumn 2000John Kristoff1 Congestion Control Computer Networks

Autumn 2000John Kristoff2 Where are we?

Autumn 2000John Kristoff3 Recall ÑData Link Layer Ñ Link level specific transmission ÑNetwork Layer Ñ End-to-End host addressing and routing ÑTransport Layer Ñ End-to-End application multiplexing and message flow-control An expert: Sally Floyd An expert: Van Jacobson

Autumn 2000John Kristoff4 Note Flow control is a subset of congestion control. The former attempts to properly match the rate of the sender with that of the network and receiver. The later deals with the sustained overload of intermediate network elements such as internetwork routers.

Autumn 2000John Kristoff5 Congestion Collapse ÑAs the network load increases, packet drops and thus packet retransmissions increase ÑFragments dropped are especially annoying, the remaining fragments get sent, but cannot be used ÑAs retransmissions increase, less actual work gets done

Autumn 2000John Kristoff6 Some Congestion Fixes Ñ When congestion increases, slow down! Ñ Additive Increase, Multiplicative Decrease is used in TCP Ñ Setup reservations or service classes Ñ Packets failing to adhere to their class or reservation are simply discarded or put onto a low priority queue/link Ñ Discover end-to-end MTU if fragments are getting dropped

Autumn 2000John Kristoff7 Fairness ÑEqual share bandwidth to end stations ÑFair share based on application ÑFair share based on timeliness of data ÑFair share based on value of data ÑFair share based on price paid Ñ...and so on

Autumn 2000John Kristoff8 Active Congestion Control Mechanisms ÑEligible discard ÑQueue management ÑNetwork Signaling and Notification ÑEnd station avoidance ÑClass of service signaling ÑQuality of service reservations

Autumn 2000John Kristoff9 Eligible Discard ÑFrames, cells or packets are marked according to a drop priority ÑSource or edge intermediate device may mark based on some policy Ñ watermark/threshold reached Ñ data type Ñ source Ñ destination Ñ cost Ñ Usually implemented at data link or network layer

Autumn 2000John Kristoff10 Eligible Discard Illustrated

Autumn 2000John Kristoff11 Queue Management ÑFirst in, first dropped (FIFO) ÑTail drop (LIFO) ÑLeaky bucket ÑToken bucket ÑRandom early detection (RED) ÑWeighted Fair Queueing Ñ Usually implemented in intermediate devices such as routers and switches

Autumn 2000John Kristoff12 First In, First Out Illustrated ÑQueue pointers need to be updated ÑSender learns of drop sooner

Autumn 2000John Kristoff13 Last In, First Out Illustrated ÑSimple - no queue pointers to update ÑSource cannot react as quick

Autumn 2000John Kristoff14 Leaky Bucket Illustrated Ñ From Tanenbaum Figure 5-24, graphic will print to a Postscript printer

Autumn 2000John Kristoff15 Token Bucket Illustrated Ñ From Tanenbaum Figure 5-26, graphic will print to a Postscript printer

Autumn 2000John Kristoff16 RED Illustrated ÑProbability marking applied to each packet based on queue length, packet being dropped

Autumn 2000John Kristoff17 Weighted Fair Queueing

Autumn 2000John Kristoff18 Network Signaling and Notification ÑAlso called choke packets ÑIn Frame Relay Ñ Forward Explicit Congestion Notification (FECN) Ñ Backward Explicit Congestion Notification (BECN) Ñ Bit in frame set ÑExperimental Internet mechanism Ñ Explicit Congestion Notification (ECN) Ñ Bits set in packets to hosts

Autumn 2000John Kristoff19 End Station Avoidance ÑAlso called end-to-end control ÑTCP ÑSlow start ÑCongestion avoidance ÑFast Retransmit ÑFast Recovery

Autumn 2000John Kristoff20 Class of Service Signaling ÑPackets marked to a particular traffic class ÑIEEE 802.1p ÑDifferentiated Services (DiffServ) ÑRe-defines IP Type of Service (ToS) bit fields ÑAsynchronous Transfer Mode

Autumn 2000John Kristoff21 Quality of Service Reservations ÑResource ReSerVation Protocol ÑReserve resources in routers ÑRequires stateful path ÑAsynchronous Transfer Mode (ATM)