Congestion

Terms

Incipient Congestion

• Happens from multiplexing
• Statistical burst of traffic
• Sometimes packets, buffer or drop
• Irrespective of a long-term load

Persistent Congestion

• Over-Subscription
• Packet loss

CC — Congestion Control

AQM — Active Queue Management

• Prevents the queue from filling even faster
• Drops packets selectively

RED — Random Early Detection

• Based on Queue Depth
  • Queue is 100 packets
    • 30 packets, drop rate of a new packet is 20%
    • 50 packets, drop rate is 50%
    • 100 packet, drop rate is 100%

Congestion Collapse

• Badly designed protocols react to congestion by trying to send faster or more often.

TCP’s implementation of CC is responsible for the Internet not suffering from Congestion Collapse.

TCP

• ECN is marked
• Dropped packets cause streams to slow down.

Global Synchronization

TCP on its own, will settle into this pattern, when the queues fill, multiple flows all speeding up and slowing down around the same period.

References

RFC 7567: IETF Recommendations Regarding Active Queue Management | RFC Editor

RFC 7141: Byte and Packet Congestion Notification | RFC Editor

Random Early Detection Gateways for Congestion Avoidance - 1993 IEEE