Communication Networks Recitation 4 Scheduling Drop Policies Comnet

Communication Networks Recitation 4 Scheduling & Drop Policies Comnet 2006 1

Queueing Basics • A queue consists of a scheduling discipline and a drop policy input drop policy: what is dropped upon overflow queued packets Comnet 2006 scheduling discipline: what packet gets sent next 2

Generalized Processor Sharing • The 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 Comnet 2006 3

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. Comnet 2006 4

WFQ Round -- Virtual Time • Round 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: – 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 Comnet 2006 5

Virtual time Example 1 1/3 1/2 1/3 A F 1=1 B F 1=2 L=2 C F 1=2 L=1 F 2=3. 5 L=2 Comnet 2006 6

Understanding bit by bit WFQ 4 queues, sharing 4 bits/sec of bandwidth 6 2 5 1 4 0 3 Time A 1 = 4 1 B 1 = 3 C 2 = 1 C 1 = 1 D 2 = 2 D 1 = 1 1 Weights : 1: 1 6 2 5 1 A 2 = 2 4 0 3 A 2, C 3 arrive A 1 = 4 1 B 1 = 3 C 3 = 2 C 2 = 1 C 1 = 1 D 2 = 2 6 2 Weights : 1: 1 A 2 = 2 5 1 D 1, C 1 Depart at R=1 Time D 1 = 1 4 0 A 1 = 4 B 1 = 3 3 C 3 = 2 C 2 = 1 C 1 = 1 D 2 = 2 D 1 = 1 D 1 1 1 C 1 B 1 A 1 Round 1 1 Time C 2 Departs at R=2 1 1 1 D 2 C 2 B 1 Round 2 1 A 1 D 1 C 1 B 1 A 1 Round 1 Weights : 1: 1 Comnet 2006 7

Understanding bit by bit WFQ 6 2 5 1 A 2 = 2 4 queues, sharing 4 bits/sec of bandwidth 4 0 3 A 1 = 4 B 1 = 3 C 3 = 2 C 2 = 1 C 1 = 1 D 2 = 2 6 2 Weights : 1: 1 5 4 1 0 A 2 = 2 D 1 = 1 B 1 = 3 C 2 = 1 C 1 = 1 D 2 = 2 D 1 = 1 D 2, 1 D 2 1 B 1 C 3 Depart at R=3 B 1 A 1 5 1 A 2 = 2 4 0 A 1 D 1 C 1 B 1 A 1 Round 1 1 Time 1 1 1 A 2 C 3, A 1 Depart at R=4 Departs at R=6 A 2 6 5 C 3 A 1 D 2 Round 4 C 3 B 1 A 1 D 2 Round 3 C 2 B 1 A 1 D 1 Round 2 C 1 B 1 A 1 Round 1 1 3 Time 1 B 1 = 3 C 2 = 1 C 1 = 1 D 2 = 2 B 1 Sort packets A 1 = 4 C 3 = 2 C 2 Round 2 Weights : 1: 1 6 2 D 2 Round 3 1 3 A 1 = 4 C 3 = 2 Time D 1 = 1 1 Departure order for packet by packet WFQ: Sort by finish round of packets A 2 C 3 A 1 A 1 D 2 B 1 B 1 C 2 D 1 C 1 1 1 Weights : 1: 1 Comnet 2006 8

WF²Q • 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. • Almost identical to that of GPS, differing by no more than one maximum size packet. Comnet 2006 9

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Drop-tail • Signals congestion only when the queue is already saturated • Likely to drop more packets from the same flow • Unfair with bursty flows Comnet 2006 11

Random Early Detection (RED) Comnet 2006 12

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The Advantages of RED • No bias against bursty traffic • No global synchronization • Packet marking probability proportional to connection’s share of bandwidth • Scalable: no per-connection state Comnet 2006 15

Explicit Congestion Notification (ECN) Packet Drops 7 6 5 4 3 2 1 Sender Receiver 1 ECN 2 7 4 6 5 5 4 6 3 7 2 1 Sender Receiver 1 2 3 Comnet 4 2006 5 6 7 16

ECN Details • Packets have a special Early Congestion Notification (ECN) bit • The ECN bit is set to 1 • Receivers forward ECN bit state back to sender in acknowledgments • Sender can adjust rate accordingly Comnet 2006 17