New Designs for the Internet

1. New Designs for the Internet • Why can’t I get higher throughput? • Why is my online video jerky? • How is capacity shared in the Internet?

2. Some Internet History • 1974: First draft of TCP/IP“A protocol for packet network interconnection”, Vint Cerf and Robert Kahn • 1983: ARPANET switches on TCP/IP • 1986: Congestion collapse • 1988: Congestion control for TCP“Congestion avoidance and control”, Van Jacobson “A Brief History of the Internet”, the Internet Society

3. TCP if (seqno > _last_acked) { if (!_in_fast_recovery) { _last_acked = seqno; _dupacks = 0; inflate_window(); send_packets(now); _last_sent_time = now; return; } if (seqno < _recover) { uint32_t new_data = seqno - _last_acked; _last_acked = seqno; if (new_data < _cwnd) _cwnd -= new_data; else _cwnd=0; _cwnd += _mss; retransmit_packet(now); send_packets(now); return; } uint32_t flightsize = _highest_sent - seqno; _cwnd = min(_ssthresh, flightsize + _mss); _last_acked = seqno; _dupacks = 0; _in_fast_recovery = false; send_packets(now); return; } if (_in_fast_recovery) { _cwnd += _mss; send_packets(now); return; } _dupacks++; if (_dupacks!=3) { send_packets(now); return; } _ssthresh = max(_cwnd/2, (uint32_t)(2 * _mss)); retransmit_packet(now); _cwnd = _ssthresh + 3 * _mss; _in_fast_recovery = true; _recover = _highest_sent; } bandwidth [0-100 kB/sec] time [0-8 sec]

4. How TCP shares capacity individualflowbandwidths availablebandwidth sum of flowbandwidths time

5. Random Walks, Electrical Networks • Kakutani (1945) • Consider a particle performing a random walk on a network

6. Random Walks, Electrical Networks • Kakutani (1945) • Consider a particle performing a random walk on a network • From node i it jumps to neighbouring node j at rate 1/rij j rij i

7. Random Walks, Electrical Networks • Kakutani (1945) • Consider a particle performing a random walk on a network • From node i it jumps to neighbouring node j at rate 1/rij

8. Random Walks, Electrical Networks • Kakutani (1945) • Consider a particle performing a random walk on a network • From node i it jumps to neighbouring node j at rate 1/rij

9. Random Walks, Electrical Networks • Kakutani (1945) • Consider a particle performing a random walk on a network • From node i it jumps to neighbouring node j at rate 1/rij

10. Random Walks, Electrical Networks • Kakutani (1945) • Consider a particle performing a random walk on a network • From node i it jumps to neighbouring node j at rate 1/rij

11. Random Walks, Electrical Networks • Kakutani (1945) • Consider a particle performing a random walk on a network • From node i it jumps to neighbouring node j at rate 1/rij

12. Random Walks, Electrical Networks • Let Vi be the probability that, starting at i, the particle hits node 1 before it hits node 0 • Let Iij=(Vj-Vi)/rij node 1 Vi node 0

13. Random Walks, Electrical Networks • Let Vi be the probability that, starting at i, the particle hits node 1 before it hits node 0 • Let Iij=(Vj-Vi)/rij • Then Vi are the potentials and Iij the currents in this electrical circuit node 1 resistance rij node 0

14. Three Levels of Description • Microscopic:particle-level rules of motion • Macroscopic:Ohm’s law and Kirchhoff’s laws • Teleological:Iij are such as to minimize heat dissipation minimize ½åi,jIij2rij over Iij

15. Macroscopic description • Consider several TCP flows sharing a single link • Let RTT be the round-trip time [sec]Let x be the mean bandwidth of a flow [pkts/sec]Let y be the total bandwidth of all flows [pkts/sec]Let C be the total available capacity [pkts/sec] • The total fraction of packets that are lost isp = (y-C)+/y • The TCP algorithm achievesaverage increase in rate = average decrease in rate1/RTT2 = (p x) x/2

16. Teleological description • Consider several TCP flows sharing a single link • Let RTT be the round-trip time [sec]Let xr be the mean bandwidth of flow r[pkts/sec]Let y be the total bandwidth of all flows [pkts/sec]Let C be the total available capacity [pkts/sec] • TCP and the network act so as to solvemaximise årU(xr) - (y - C log y/C-1)+ over xr where y=år xr U(x) x

17. Teleological description • Consider several TCP flows sharing a single link • Let RTT be the round-trip time [sec]Let xr be the mean bandwidth of flow r[pkts/sec]Let y be the total bandwidth of all flows [pkts/sec]Let C be the total available capacity [pkts/sec] • TCP and the network act so as to solvemaximise årU(xr) - (y - C log y/C-1)+ over xr where y=år xr U(x) little extra valued attached to high-bandwidth flows severe penalty for allocating too little bandwidth x

18. Teleological description • Consider several TCP flows sharing a single link • Let RTT be the round-trip time [sec]Let xr be the mean bandwidth of flow r[pkts/sec]Let y be the total bandwidth of all flows [pkts/sec]Let C be the total available capacity [pkts/sec] • TCP and the network act so as to solvemaximise årU(xr) - (y - C log y/C-1)+ over xr where y=år xr U(x) small RTT large RTT x

19. Teleological description • Consider several TCP flows sharing a single link • Let RTT be the round-trip time [sec]Let xr be the mean bandwidth of flow r[pkts/sec]Let y be the total bandwidth of all flows [pkts/sec]Let C be the total available capacity [pkts/sec] • TCP and the network act so as to solvemaximise årU(xr) - (y - C log y/C-1)+ over xr where y=år xr no penalty until y>Ci.e. the link is overloaded U(x) x

20. Teleological description • Is this what we want the Internet to optimize? • Does it make good use of the network? • Can it deliver high bandwidth? • Is it a fair allocation? • Can we design a better allocation? U(x) x

21. Stability of TCP • TCP was designed to prevent congestion collapse. How well does it work? • A more detailed macroscopic model for TCP is • Is this dynamical system stable?How fast does it converge?

22. Ongoing work • Kelly (Cambridge statslab) • Vinnicombe (Cambridge engineering)proposed+analysed a good macro model • Tom Kelly (CERN)implemented it in Linux: ScalableTCP • Raina (Cambridge management science)analysed stability & instability • Wischik (UCL computer science)micro/macro models for the link:how big should buffers be?

23. Ongoing work • Low, Doyle (Caltech electrical engineering)Proposed further micro/macro models, FAST TCP • Cottrell (SLAC)Testbed for TCP variants • Srikant (UIUC electrical/computer engineering)Towsley (UMass computer science)Fluid model analysis • Baccelli (ENS mathematics)Marsan (Turin electrical engineering)Mean field limit theory for TCP