Preparation for Data Center TCP (DCTCP) recitation

• Read Data Center TCP (DCTCP)
• Skim section 4 (Results)
• Closely observe figures 15 and 19

DCTCP customizes the TCP congestion control algorithm for data centers. It leverages the Explicit Congestion Notification (ECN) to obtain an early congestion feedback from routers/switches, before the queue drops packets. Further, DCTCP provides a smooth reaction to congestion, i.e., when congestion is limited, it reduces its congestion window by a small amount. In contrast, when congestion is severe, it reduces its congestion window by a large amount.

Read the first 3 sections of the paper and try to understand how DCTCP compares to TCP. Skim the Results section (i.e., section 4) to check that the empirical results match your expectations. In particular check Figures 15 and 19, which show the queue occupancy as a function of time, and number of sources.

The following slides summarize the paper and could be helpful in understanding the content: PPTx Version | PDF Version

Please answer the following questions for recitation:

• What is the incast problem and why does it arise?
• What particular features in the design of DCTCP cause the queues to be smaller than the case where the network uses TCP?

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