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

2. 2 Sonia FahmyPurdue University q Drop Tail q PPD/EPD q RED q RED Variants q ECN Overview

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

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

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

6. 6 Sonia FahmyPurdue University Explicit Congestion Notification (ECN) Router SourceDest ACKs With ECN ECN marked  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. 7 Sonia FahmyPurdue University Key Points q RFC 2309 q See www.aciri.org/floyd/red.html for links to papers discussing RED, RED vairants, RED modeling, simulations and measurementswww.aciri.org/floyd/red.html

8. 8 Sonia FahmyPurdue University Thank You! Questions?