I GETTING TO EQUILIBRIUM: SLOW-START Since that time, we have put seven new algorithms into the 4BSD TCP round-trip-time variance estimation (ii) exponential retransmit timer backoff (iii) slow-start (iv) more aggressive receiver ack policy (v) dynamic window sizing on congestion (vi Karn's clamped retransmit backoff (vii) fast retransmit Our measurements and the reports of beta testers suggest that the final product is fairly good at dealing with congested conditions on the Internet tr This paper is a brief description of (i)-(v)and the rationale behind them. (vi) is an algo- thm recently developed by Phil Karn of Bell Communications Research, described in [16] (vii) is described in a soon-to-be-published RFC(ARPANET Request for Comments) Algorithms()-(v) spring from one observation: The fow on a TCP connection(or ISO TP-4 or Xerox NS SPP connection) should obey a conservation of packets' principle And, if this principle were obeyed, congestion collapse would become the exception rather than the rule. Thus congestion control involves finding places that violate conservation and fixing them By 'conservation of packets' we mean that for a connection in equilibrium,1.e,run- ning stably with a full window of data in transit, the packet fow is what a physicist would call"conservative: A new packet isn t put into the network until an old packet leaves. The physics of flow predicts that systems with this property should be robust in the face of congestion. Observation of the Internet suggests that it was not particularly robust. Why the di There are only three ways for packet conservation to fail 1. The connection doesnt get to equilibrium,or 2. A sender injects a new packet before an old packet has exited, or 3. The equilibrium can t be reached because of resource limits along the path In the following sections we treat each of these in turn 1 Getting to Equilibrium: Slow-start Failure (1)has to be from a connection that is either starting or restarting after a packet loss. Another way to look at the conservation property is to say that the sender uses acks as a clock to strobe new packets into the network. Since the receiver can generate acks ne 'A conservative flow means that for any given time the integral of the packet density around the sender- receiver-sender loop is a constant. Since packets have to'diffuse'around this loop, the integral is suficiently ontinuous to be a Lyapunov function for the system. A constant function trivially meets the conditions for Lyapunov stability so the system is stable and any superposition of such systems is stable. (See [3], chap. 111 GETTING TO EQUILIBRIUM: SLOW-START 2 Since that time, we have put seven new algorithms into the 4BSD TCP: (i) round-trip-time variance estimation (ii) exponential retransmit timer backoff (iii) slow-start (iv) more aggressive receiver ack policy (v) dynamic window sizing on congestion (vi) Karn’s clamped retransmit backoff (vii) fast retransmit Our measurements and the reports of beta testers suggest that the final product is fairly good at dealing with congested conditions on the Internet. This paper is a brief description of (i) – (v) and the rationale behind them. (vi) is an algo￾rithm recently developed by Phil Karn of Bell Communications Research, described in [16]. (vii) is described in a soon-to-be-published RFC (ARPANET “Request for Comments”). Algorithms (i) – (v) spring from one observation: The flow on a TCP connection (or ISO TP-4 or Xerox NS SPP connection) should obey a ‘conservation of packets’ principle. And, if this principle were obeyed, congestion collapse would become the exception rather than the rule. Thus congestion control involves finding places that violate conservation and fixing them. By ‘conservation of packets’ we mean that for a connection ‘in equilibrium’, i.e., run￾ning stably with a full window of data in transit, the packet flow is what a physicist would call ‘conservative’: A new packet isn’t put into the network until an old packet leaves. The physics of flow predicts that systems with this property should be robust in the face of congestion.1 Observation of the Internet suggests that it was not particularly robust. Why the discrepancy? There are only three ways for packet conservation to fail: 1. The connection doesn’t get to equilibrium, or 2. A sender injects a new packet before an old packet has exited, or 3. The equilibrium can’t be reached because of resource limits along the path. In the following sections, we treat each of these in turn. 1 Getting to Equilibrium: Slow-start Failure (1) has to be from a connection that is either starting or restarting after a packet loss. Another way to look at the conservation property is to say that the sender uses acks as a ‘clock’ to strobe new packets into the network. Since the receiver can generate acks no 1A conservative flow means that for any given time, the integral of the packet density around the sender– receiver–sender loop is a constant. Since packets have to ‘diffuse’ around this loop, the integral is sufficiently continuous to be a Lyapunov function for the system. A constant function trivially meets the conditions for Lyapunov stability so the system is stable and any superposition of such systems is stable. (See [3], chap. 11– 12 or [21], chap. 9 for excellent introductions to system stability theory.)