Deployment Considerations for CUBIC TCP

CUBIC is a variant of TCP that was presented in ACM's 2008 SIGOPS conference. CUBIC extends and improves on the BIC variant of TCP. Like BIC, CUBIC is a variant of TCP useful in long, wide bandwidth links where the bandwidth-delay product is high. The TCP that most computers run today is not capable of reaching speeds higher than 100Mbps because transmission rate is increased linearly, with a low coefficient.

BIC solves this problem by using a high coefficient for growing the window, and then the coefficient smooths out over time. But a problem with BIC is that it uses more than its fair share of bandwidth when standard (NewReno, SACK) TCP must compete with it. CUBIC has the advantages of BIC, but also improves TCP-friendliness when in connections with low delay (~5ms). So why don't we set CUBIC as the default variant of TCP in our Operating Systems? There are several points to consider.

Incentives:

• Better RTT-fairness. Because the rate of window increase (in congestion avoidance) is dependent on the RTT in standard TCP, flows with a large delay perform worse than flows with low delay. CUBIC (and BIC) improve RTT-fairness because window increase is done differently.
  
• Better link utilization. As explained above, CUBIC will occupy a link's bandwidth much faster.
• Friendliness with standard TCP in low speed (~10Mbps), high delay (100ms) links.
  

Drawbacks:

• Unfriendliness in large-delay (~40ms), high-bandwidth paths. In one case, CUBIC takes 80% of the available bandwidth, and leaves standard TCP with 20%.
  
• Large bandwidth-convergence time among competing CUBIC flows. With a 400Mbps link, it can require several hundred seconds for an existing connection to lower its rate while the new connection increases its rate. However, convergence time is much lower when there is less available bandwidth, e.g. 100Mbps.
  

These considerations are perhaps a good start on considering whether to deploy CUBIC on the Internet. While much has gone unconsidered, one can get a general picture of how CUBIC performs.