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Communication Networks

This document provides a comprehensive overview of queueing fundamentals in communication networks, focusing on scheduling disciplines and drop policies. It covers the principles of Generalized Processor Sharing (GPS) and Weighted Fair Queueing (WFQ), examining both ideal models and practical implementations. Key topics include the mechanics of virtual time, the implications of packet service order, and various drop strategies like Random Early Detection (RED) and Explicit Congestion Notification (ECN). This resource is essential for understanding traffic management and congestion control in network communications.

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Communication Networks

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  1. Communication Networks Recitation 4 Netcomm 2005

  2. Scheduling & Drop Policies Netcomm 2005

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

  4. 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 Netcomm 2005

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

  6. 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 Netcomm 2005

  7. fA A r= 1 fB B fC C fA = fB = fC = 1 A F1=1 L=1 F2=3.5 L=2 B F1=2 L=2 C F1=2 L=2 Virtual time Example 1 1/3 1/2 1/3 Netcomm 2005

  8. 6 5 4 3 2 1 0 Time A1 = 4 1 1 1 1 1 1 1 1 1 1 1 1 B1 = 3 C2 = 1 C1 = 1 D1 = 1 D2 = 2 Weights : 1:1:1:1 6 5 4 3 2 1 0 D1, C1 Depart at R=1 Time A2, C3 arrive A2 = 2 A1 = 4 D1 C1 B1 A1 B1 = 3 C3 = 2 C2 = 1 C1 = 1 Round 1 D1 = 1 D2 = 2 Weights : 1:1:1:1 6 5 4 3 2 1 0 Time C2 Departs at R=2 A2 = 2 A1 = 4 D2 C2 B1 A1 D1 C1 B1 A1 B1 = 3 C3 = 2 C2 = 1 C1 = 1 Round 2 Round 1 D1 = 1 D2 = 2 Weights : 1:1:1:1 Understanding bit by bit WFQ 4 queues, sharing 4 bits/sec of bandwidth Netcomm 2005

  9. Time 6 5 4 3 2 1 0 D2, B1 Depart at R=3 A2 = 2 A1 = 4 1 1 1 1 1 1 1 1 1 1 1 1 D2 C2 B1 A1 D2 C3 B1 A1 D1 C1 B1 A1 B1 = 3 C3 = 2 C2 = 1 C1 = 1 D1 = 1 D2 = 2 6 5 4 3 2 1 0 Weights : 1:1:1:1 A2 Departs at R=6 C3, A1 Depart at R=4 Time A2 = 2 A1 = 4 D2 C2 B1 A1 A2 A2 C3 A1 D2 C3 B1 A1 D1 C1 B1 A1 B1 = 3 C3 = 2 C2 = 1 C1 = 1 6 5 Round 4 Round 3 D1 = 1 D2 = 2 Sort packets Weights : 1:1:1:1 Time 6 5 4 3 2 1 0 Departure order for packet by packet WFQ: Sort by finish round of packets A2 = 2 A1 = 4 A2 A2 C3 C3 A 1 A 1 A1 A 1 D2 D2 B1 B1 B1 C2 D1 C1 B1 = 3 Round 2 Round 1 Round 2 Round 1 C3 = 2 C2 = 1 C1 = 1 D1 = 1 D2 = 2 Weights : 1:1:1:1 Understanding bit by bit WFQ 4 queues, sharing 4 bits/sec of bandwidth Round 3 Netcomm 2005

  10. 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. Netcomm 2005

  11. Netcomm 2005

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

  13. Random Early Detection (RED) Netcomm 2005

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

  15. 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 Netcomm 2005

  16. Explicit Congestion Notification (ECN) Netcomm 2005

  17. 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 Netcomm 2005

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