ABC: A Simple Explicit Congestion Controller for Wireless Networks

https://www.usenix.org/conference/nsdi20/presentation/goyal

• Wireless Links have Variable Rates

• Drop-based E2E Schemes: NewReno, Cubic, etc.

  • Rely on drops to infer congestion

  • Drops are infrequent --> under-utilization or bufferbloat

  • 
  • Cubic: deploy deep buffer

• AQM Schemes: RED, PIE, CoDel, etc.

  • Drops and ECN signal only congestion

  • Do not signal that higher rates are possible

• Other E2E Schemes: Verus, Copa, Sprout, etc.

  • Use RTT, send/receive rate, etc.

  • Require queue build-ups to estimate available rate

    • Queues --> trade-off between throughput and delay

  • Need more feedback for increasing rates

• Explicit Schemes: XCP, RCP, etc.

  • Explicit feedback using the packet header

  • Compare enqueue rate to link rate for computing feedback

  • 
  • Challenges:

    • Control loop designed for fixed capacity links

      • Less accurate in tracking wireless link rates

    • Multi-bit feedback --> hard to deploy

      • IP header has no space

      • IP options are not an option

      • TCP options: IPSec problems

  • Accel-Brake Control (ABC)

    • Single bit feedback - signal both increase and decrease

      • Repurpose ECN for deployment

    • Control algorithm for fast-varying links

      • Use dequeue rate for better feedback

    • Coexistence

      • With legacy bottleneck routers

      • With legacy transport protocol

  • ABC Overview

    • Single bit feedback

      • Sender adjusts window using these accelerate and brake signals

        • Accelerate: W += 1

        • Brake: W -= 1

      • Router controls rate by setting f(t)

        • f(t) = 0, throttles the transmission

        • f(t) = 1/2, maintain the current rate

        • f(t) = 1, doubles the congestion windows every RTT

      • Within 1 RTT,

        • W --> 2 * f(t) * W

      • Sender's rate can be varied from 0 to 2 * current_rate

    • Control loop for time-varying links

      • ABC Router: target rate

      • 
      • 
      • ABC predicts the future enqueue rate for improved control

      • 
      • Varying link capacity --> enqueue and dequeue rate can differ significantly

      • Building block for congestion control loops!

    • ABC: high utilization, low delay

    • Evaluation

      • ABC endpoints

        • Linux - pluggabel TCP API

      • ABC router

        • Cellular

          • Trace driven emulation with Mahimahi

        • Wi-Fi

          • Commodity router - Netgear WNDR 3800

          • Linux implementation using OpenWrt

  • Summary

    • Single bit feedback - signal both increase and decrease

      • Control sender's rate from 0 to 2*Current Rate in 1 RTT

    • Control loop for fast-varying wireless links

      • ABC quickly and accurately adapts to varying link rates