1 Netcomm 2005 Communication Networks Recitation 4

2 Netcomm 2005 Scheduling & Drop Policies

3 Netcomm 2005 Queueing Basics A queue consists of a scheduling discipline and a drop policyA queue consists of a scheduling discipline and a drop policy queued packets scheduling discipline: what packet gets sent next drop policy: what is dropped upon overflow input

4 Netcomm 2005 Generalized Processor Sharing The ideal max-min fair scheduling schemeThe ideal max-min fair scheduling scheme –Visit each non-empty queue in turn –Serve infinitesimal from each –GPS is not implementable; we can serve only packets

5 Netcomm 2005 Weighted Fair Queueing Problem: We need to serve a whole packet at a time. Solution: 1.Determine what time a packet, p, would complete if we served flows by GPS. Call this the packet’s finish time, F( p ). 2.Serve packets in the order of increasing finish time.

6 Netcomm 2005 WFQ Round -- Virtual Time Round number is a real-valued variable that increases at a rate inversely proportional to the number/weight of active connectionsRound number is a real-valued variable that increases at a rate inversely proportional to the number/weight of active connections Updating the number of connections:Updating the number of connections: –A connection becomes active when a packet arrives to an empty queue –A connection becomes inactive when R(t) > F( p ), where p is the last packet served

7 Netcomm 2005 Virtual time Example A L=1L=2 B C 1/3 1/2 1/3 1 F 1 =1 F 1 =2 F 2 =3.5  A r = 1  A =  B =  C = 1 B C  CC

8 Netcomm 2005 Understanding bit by bit WFQ 4 queues, sharing 4 bits/sec of bandwidth Weights : 1:1:1: B1 = 3 A1 = 4 D2 = 2 D1 = 1 C2 = 1C1 = 1 Time B1 = 3 A1 = 4 D2 = 2 D1 = 1 C2 = 1C1 = 1 A1B1C1D1 A2 = 2 C3 = 2 Weights : 1:1:1:1 D1, C1 Depart at R=1 A2, C3 arrive Time Round 1 Weights : 1:1:1: B1 = 3 A1 = 4 D2 = 2 D1 = 1 C2 = 1C1 = 1 A1B1C1D1 A2 = 2 C3 = 2 A1 B1C2D2 C2 Departs at R=2 Time Round 1Round 2

9 Netcomm 2005 Understanding bit by bit WFQ 4 queues, sharing 4 bits/sec of bandwidth Weights : 1:1:1: B1 = 3 A1 = 4 D2 = 2 D1 = 1 C2 = 1C1 = 1 A1B1C1D1 A2 = 2 C3 = 2 A1B1C2D2 D2, B1 Depart at R=3 A1B1C3D2 Time Round 1Round 2Round 3 Weights : 1:1:1: B1 = 3 A1 = 4 D2 = 2 D1 = 1 C2 = 1C3 = 2C1 = 1 C1D1C2B1 D2 A 1 A2 = 2 C3 A2 Departure order for packet by packet WFQ: Sort by finish round of packets Time Sort packets B1 = 3 A1 = 4 D2 = 2 D1 = 1 C2 = 1C1 = 1 A1B1C1D1 A2 = 2 C3 = 2 A1B1C2D2 A1 Depart at R=4 A1B1C3D2A1C3A2 Time Round 1Round 2Round 3Round 4 C3,A2 Departs at R=6 56

10 Netcomm 2005 WF²Q Packet approximation algorithm of GPS.Packet approximation algorithm of GPS. Choosing the packet with the smallest finish time among all the packets that have already started service in the corresponding GPS emulation.Choosing the packet with the smallest finish time among all the packets that have already started service in the corresponding GPS emulation. Almost identical to that of GPS, differing by no more than one maximum size packet.Almost identical to that of GPS, differing by no more than one maximum size packet.

11 Netcomm 2005

12 Netcomm 2005 Drop-tail Signals congestion only when the queue is already saturatedSignals congestion only when the queue is already saturated Likely to drop more packets from the same flowLikely to drop more packets from the same flow Unfair with bursty flowsUnfair with bursty flows

13 Netcomm 2005 Random Early Detection (RED)

14 Netcomm 2005 RED Details Two queue length thresholds:Two queue length thresholds: if AvgLen <= MinThreshold then enqueue the packet enqueue the packet if MinThreshold < AvgLen < MaxThreshold then calculate probability P calculate probability P drop arriving packet with probability P drop arriving packet with probability P if Maxhreshold <= AvgLen then drop arriving packet drop arriving packet

15 Netcomm 2005 The Advantages of RED No bias against bursty trafficNo bias against bursty traffic No global synchronizationNo global synchronization Packet marking probability proportional to connection’s share of bandwidthPacket marking probability proportional to connection’s share of bandwidth Scalable: no per-connection stateScalable: no per-connection state

16 Netcomm 2005 Explicit Congestion Notification (ECN)

17 Netcomm 2005 ECN Details Packets have a special Early Congestion Notification (ECN) bitPackets have a special Early Congestion Notification (ECN) bit The ECN bit is set to 1The ECN bit is set to 1 Receivers forward ECN bit state back to sender in acknowledgmentsReceivers forward ECN bit state back to sender in acknowledgments Sender can adjust rate accordinglySender can adjust rate accordingly