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Why It is Called “Karn’s Algorithm”

Why It is Called “Karn’s Algorithm”. Craig Partridge BBN Technologies. Fall 1986. After doing DNS work for most of the year, I start graduate work at Harvard, while working full time for BBN Parking in Harvard Square for courses (mid-day) is a pain I decide to do a Master’s thesis

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Why It is Called “Karn’s Algorithm”

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  1. Why It is Called “Karn’s Algorithm” Craig Partridge BBN Technologies

  2. Fall 1986 • After doing DNS work for most of the year, I start graduate work at Harvard, while working full time for BBN • Parking in Harvard Square for courses (mid-day) is a pain • I decide to do a Master’s thesis • I can get out of 1/4 of my course requirements • BBN friends suggest I implement RDP (RFC 908) • “You won’t really understand networking until you implement a reliable transport protocol”

  3. Late Fall 1986 • I view the fall foliage from my office window while • Waiting for my kernels to compile • Pinging Goonhilly Echo • IP echo server in UK • 2 second satellite delay means I can observe protocol state in real-time • Learning I don’t know how to handle round-trip timers

  4. Zhang’s SIGCOMM ‘86 Paper • Retransmission ambiguity precludes proper RTT estimation • Send a packet and then retransmit it • Now an ack comes back • Do you measure the RTT? • Yes • From first transmission? (Estimate goes to infinity) • From last transmission? (Estimate goes to 1/2 RTT) • No • You fail to adapt to changes in RTT

  5. Solving Retransmission Ambiguity • On gut instinct, I think the problem is solvable • Two complete nobodies on TCP-IP list agree with me • Phil Karn, a Bellcore techie fascinated by packet radio • Van Jacobson, an LBL staff programmer who talks control theory that I don’t understand • About New Years 1987 I decide to figure out if the problem is really solvable • I spend the weekend with the key papers, reading and re-reading • And I find an approach (based on restarting the timer, ala connection initialization)

  6. Why The Solution is Karn’s Algorithm • Having struggled all weekend I now knew solutions existed • The challenge was to find a good one • I figured I’d wait until the next weekend • On Monday, I came into the office and found email from Phil Karn asking my opinion on an algorithm he’d thought of and tried that weekend...

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