Slow Start Phase (Exponential Increase)

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This lecture introduces TCP congestion policy, focusing on the slow start phase. It begins by listing the three main components: Slow Start (Exponential Increase), Congestion Avoidance (Additive Increase), and Congestion Detection. The core of the lesson explains how the congestion window size (cwnd) starts at 1 MSS and doubles with each round trip time, following the formula cwnd = 2^n. A client-server timing diagram is used to illustrate this exponential growth, showing segment and ACK exchanges alongside a cwnd bar. The concept of the slow start threshold (Ssthresh) is defined as half the maximum number of TCP segments the receiver window can accommodate. A graph plotting MSS against Time visually distinguishes the curved slow start phase from the linear congestion avoidance phase. The lecture concludes by returning to the overview slide, summarizing the three phases of TCP congestion control.

Chapters

1. 0:00 – 2:00 00:00-02:00

   The lecture opens with a slide titled 'TCP Congestion Policy,' listing three key phases: Slow Start (Exponential Increase), Congestion Avoidance (Additive Increase), and Congestion Detection. The instructor then transitions to a detailed slide for the 'Slow Start Phase,' introducing a client-server diagram. On-screen text defines the initial state: 'Initially, sender sets congestion window size = Maximum Segment Size (1 MSS).' The instructor begins explaining the exponential growth, showing that after 1 round trip time, the congestion window size becomes (2)^1 = 2 MSS.

2. 2:00 – 5:00 02:00-05:00

   The explanation of the slow start phase continues with a step-by-step breakdown of the exponential increase. The slide displays formulas for subsequent round trips: after 2 RTTs, cwnd = (2)^2 = 4 MSS; after 3 RTTs, cwnd = (2)^3 = 8 MSS. A timing diagram below the text illustrates this with segment and ACK arrows, a 'cwnd' bar at the bottom, and red handwritten annotations. The instructor introduces the slow start threshold (Ssthresh), defined on-screen as 'Maximum number of TCP segments that receiver window can accommodate / 2.' Around the 225-second mark, the slide changes to a graph with 'Time' and 'MSS' axes, showing a curved line for the 'Slow start phase' transitioning into a straight line for the 'Congestion Avoidance phase.'

3. 5:00 – 6:03 05:00-06:03

   The lecture concludes the slow start explanation by returning to a client-server diagram. The instructor adds handwritten annotations, such as '64 MSS' and '1KB = 1MSS,' to clarify the threshold calculation. The final transition returns to the 'TCP Congestion Policy' overview slide, which lists the three phases again: 1. Slow Start (Exponential Increase), Congestion Avoidance, and Congestion Detection. This serves as a summary of the lesson's main topic, reinforcing the relationship between the exponential growth of the congestion window and the broader TCP congestion control mechanism.

The lecture provides a structured introduction to TCP's slow start phase, emphasizing its exponential nature. The teaching flow moves from a high-level overview of the three congestion policy phases to a detailed, formula-driven explanation of slow start. Key concepts include the doubling of the congestion window (cwnd) per round trip time, starting from 1 MSS. The use of a client-server timing diagram and an MSS-vs-Time graph helps visualize the transition from exponential growth to linear congestion avoidance. The definition of Ssthresh as half the receiver window size is a critical detail linking slow start to the next phase. The lesson effectively combines textual formulas, visual diagrams, and handwritten annotations to clarify the mechanics of TCP congestion control.