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# ABC: A Simple Explicit Congestion Controller for Wireless Networks

* 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 
  * ![](/files/3j3h96TwiCJXW2ikWnDC)
  * 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 
  * ![](/files/d6oFlnxZkIzKE84d3Wp3)
  * 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 
      * ![](/files/DCrvy6helLDDmqaYXqxs)
      * ![](/files/hADwZz4aRCQKxDmwdpw3)
      * ABC predicts the future enqueue rate for improved control 
      * ![](/files/2ZHbaQ4amlEThutijGVi)
      * 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