Effects of Global Synchronization and TCP Slow Start
Global synchronization combined with TCP slow starts can lead to some very undesirable results. When many TCP flows have one or more packets dropped in a tail drop, they all go through TCP slow start at the same time. This means these flows all slow down at the same time, and traffic on the network as a whole drops abruptly. The network gets significantly quieter than it was before the tail drop (again, this assumes most of the traffic is TCP). Next, because the flows are synchronized, they all start to speed up at the same time. This continues until the aggregate rate of the flows once again climbs to a maximum rate, at which time congestion causes another tail drop and the cycle repeats.
From a link-utilization standpoint, this cycle of traffic rising to saturation and crashing down looks like a sawtooth-shaped graph. This effect is depicted in Figure 5-10.
Figure 5-10 Synchronized TCP Slow Starts Cause a Sawtooth Pattern of Network Utilization
Tail drop
Synchronized TCP slow starts
Link utilization
Figure 5-10 Synchronized TCP Slow Starts Cause a Sawtooth Pattern of Network Utilization
Tail drop
Synchronized TCP slow starts
- Time
sessions
sessions
Utilization of the link climbs to 100 percent, drops sharply when the TCP flows go into slow start, and starts climbing back up to 100 percent only to crash down again. Although there is nothing fundamentally wrong with TCP (slow start is a useful flow-control mechanism that throttles endstations for the good of the network), you can observe from the sawtooth pattern the following problems:
• Link utilization is not 100 percent the entire time, and overall throughput is less than the optimal rate you can get out of the link.
• Transmission rates start and stop; throughput is inconsistent. This could cause problems for applications and users.
• Recognizing bandwidth oversubscription and planning for network upgrades is difficult because average link utilization measures short of full capacity.
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