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Active Queue (Buffer) Management

Active Queue (Buffer) Management. Sonia Fahmy Department of Computer Sciences Purdue University fahmy@cs.purdue.edu http://www.cs.purdue.edu/homes/fahmy/. Overview. Drop Tail PPD/EPD RED RED Variants ECN. Routers. Drop tail: drop when buffer is full Problem: Partial packets

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Active Queue (Buffer) Management

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  1. Active Queue (Buffer)Management Sonia Fahmy Department of Computer Sciences Purdue University fahmy@cs.purdue.edu http://www.cs.purdue.edu/homes/fahmy/

  2. Overview • Drop Tail • PPD/EPD • RED • RED Variants • ECN

  3. Routers • Drop tail: drop when buffer is full • Problem: Partial packets • Partial packet discard (PPD) • Early packet discard (EPD): do not admit new packets beyond threshold to leave space for partial packets • Random Early Detection (RED): between maxth and minth, drop with p = f(avg Q) • Potential advantages: early detection (TCP), avoid synchronization, punish misbehaving flows, avoid unfairness against bursty sources Output port

  4. Random Early Detection (RED) Pdrop/mark No dropping or marking Drop with p=1 1 Qlen 0 gentle Pmax Mark/drop with P Linearly increasing From 0 to Pmax Qavg 0 Thmin Thmax

  5. RED Variants • Gentle RED (Rosolen, Bonaventure 1998) • Flow RED (Lin,Kung) compare Qi to Qave/N • Per flow state required. Also BRED • SRED: estimate number of active flows • ARED: variable pmax, BLUE: variable pdrop • WRED, RIO (parameters): diffserv • ECN (RFC 2481 (sec7), CCR 10/94): • 4 bits: ECN capable and CE bit to signal congestion, ECN-echo, CWR • In addition to timeout and 3 dupacks

  6. Explicit Congestion Notification (ECN) ECN marked Router Source Dest ACKs With ECN • React to ECN only once per RTT (like loss event in TFRC) • Advantage: Packet drops avoided, no need to wait for a Timeout • React to ECN similar to packet drop to preserve TCP-friendliness, but not drastically

  7. Key Points • RFC 2309 • See www.aciri.org/floyd/red.html for links to papers discussing RED, RED vairants, RED modeling, simulations and measurements

  8. Thank You! Questions?

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