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This overview of TCP's Slow Start and Congestion Avoidance mechanisms explains how the congestion window (cwnd) operates to manage data transmission. It highlights the exponential increase in cwnd during Slow Start, the impact of timeouts, and the transition to additive growth in Congestion Avoidance. Important concepts include the slow start threshold (ssthresh), loss detection through timeouts and Duplicate Acknowledgments, and mechanisms like Fast Retransmit. The goal is to optimize throughput while minimizing packet loss in congested networks.
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Recap • Slow start introduced cwnd • Can transmit up to min( cwnd, offered window ) • Flow control by the sender • cwnd increases by 1 (MSS) for every new ack
Recap • Exponential increase of cwnd • Eventually, packet drop • Detected by time-out • Let x = min ( cwnd, offered window ) at this point • After time-out cwnd = 1 again • Recover the lost packet
Recap • In the next cycle, cwnd allowed to increase exponentially till (1/2)*x • Motivation for multiplicative decrease • Over several RTTs, exponential decrease • Counteracting exponential increase in load
Congestion Avoidance • The parameter where (1/2)*x is stored ssthresh • Slow startthreshold: The value till which cwnd increases exponentially in the next cycle
cwnd 1 2 4 8 Time-out observed ssthresh = (1/2)*8 = 4 Next cycle: cwnd increases rapidly (by 1 for each new ack) till ssthresh After reaching ssthresh, cwnd increases more gently
Congestion Avoidance time Loss (timeout) cwnd reaches ssthresh; henceforth, different rule for incrementing cwnd Loss (timeout) SS cwnd=1 cwnd=1 SS SS Store related value in ssthresh; ssthresh=(1/2)*cwnd Store related value in ssthresh; ssthresh=(1/2)*cwnd Incrementing cwnd more conservatively, after it is = ssthresh; loss detection by timeout; next cycle begins with cwnd=1
Congestion Avoidance • Incrementing cwnd afterit has reached ssthresh • For each new ack, increment = 1/cwndsegment
Congestion Avoidance SS cwnd time RTT Congestion avoidance begun At this point, cwnd increases to (cwnd+1) Exponential increase with RTT in Slow Start but additive increase with RTT in Congestion Avoidance
SS and CA combined • SS and CA are found together in implementations • Upon reception of new ack if (cwnd <= ssthresh) /* slow start */ cwnd += 1 else /* congestion avoidance */ cwnd += 1/cwnd • SS and CA are distinct algorithms with different objectives, but both update the same parameter: cwnd
SS and CA • Slow start • Get the ack clock started • Congestion avoidance • Adapt to the congestion on the path • Congestion leads to packet loss • Loss indicated by timeout
n:n+s (s) n+s:n+2s (s) n+2s:n+3s (s) Congestion Avoidance • After Congestion Avoidance has been implemented, are there aspects that can yet be improved? • Loss detection still by timeout • A cycle always begins in Slow Start, with cwnd=1 Last ack carried no. n X Ack = n Ack = n
Duplicate acks • Early detection of congestion by watching for duplicate acks • Instead of detecting congestion via time-outs • Every time-out is followed by cwnd = 1, and this means the window has to be built up again • When bandwidth-delay product is large, this means considerable wastage
Fast Retransmit • Early retransmission, or “Fast Retransmit” • If K dup acks are observed, conclude that a packet was dropped and retransmit the packet • K: a small fraction of the bandwidth-delay product • Most implementations set K=3 • K is called the duplicate ack threshold
Bandwidth delay product (BDP) = 8 segments Also called “pipe capacity”
