Active Queue Management Design active router queue management to aid congestion control Why? Router has unified view of queuing behavior Routers see actual queue occupancy(distinguish queue delay and propagation delay) Routers can decide on transient congestion, based on workload
11 Active Queue Management • Design active router queue management to aid congestion control • Why? • Router has unified view of queuing behavior • Routers see actual queue occupancy (distinguish queue delay and propagation delay) • Routers can decide on transient congestion, based on workload
Design objectives Keep throughput high and delay low High power(throughput/delay) Accommodate bursts Queue size should reflect ability to accept bursts rather than steady-state queuing Improve TCP performance with minimal hardware changes
12 Design Objectives • Keep throughput high and delay low • High power (throughput/delay) • Accommodate bursts • Queue size should reflect ability to accept bursts rather than steady-state queuing • Improve TCP performance with minimal hardware changes
Lock-out Problem Random drop Packet arriving when queue is full causes some random packet to be dropped Drop front On full queue, drop packet at head of queue Random drop and drop front solve the lock-out problem but not the full-queues problem
13 Lock-out Problem • Random drop • Packet arriving when queue is full causes some random packet to be dropped • Drop front • On full queue, drop packet at head of queue • Random drop and drop front solve the lock-out problem but not the full-queues problem
Full Queues Problem Drop packets before queue becomes full (early drop) Intuition: notify senders of incipient congestion EXample: early random drop (ErD) If qlen drop level, drop each new packet with fixed probability p Does not control misbehaving users 14
14 Full Queues Problem • Drop packets before queue becomes full (early drop) • Intuition: notify senders of incipient congestion • Example: early random drop (ERD): • If qlen > drop level, drop each new packet with fixed probability p • Does not control misbehaving users
Random Early Detection (RED) Detect incipient congestion Assume hosts respond to lost packets Avoid window synchronization Randomly mark packets Avoid bias against bursty traffic 15
15 Random Early Detection (RED) • Detect incipient congestion • Assume hosts respond to lost packets • Avoid window synchronization • Randomly mark packets • Avoid bias against bursty traffic