TCP Cubic Flashcards
Why does the linear growth rate of TCP-RENO (1/RTT) perform poorly for short lived flows in networks with large bandwidth and delay products?
The time period required for the congestion window to reach its maximum value is very large (on the order of minutes and hours) for TCP-RENO in paths with large bandwidth delay products. Short lived flows may never reach a congestion event, meaning the flow unnecessarily transmitted slower than necessary over its entire lifetime to avoid congestion.
What is the purpose of the concave region in the CUBIC growth function?
The concave region of CUBIC’s growth function rapidly increases the congestion window to the previous value where a congestion event occurred, allowing for a quick recovery and high utilization of available bandwidth following a congestion event
What is the purpose of the plateau region in the CUBIC growth function?
The plateau is also known as the TCP friendly region. In this region of the growth curve, the congestion window is nearly constant as it approaches and potentially exceeds WMAX. This achieves stability, as WMAX represents the point where network utilization is at its highest under steady state conditions
What is the purpose of the convex region in the CUBIC growth function?
The convex region of CUBIC’s growth function exists to rapidly converge on a new value of WMAX following a change in available bandwidth.
This is considered a max probing phase, as the congestion window will grow exponentially in this region until another congestion event occurs and WMAX is reset.
How does CUBIC’s fast convergence mechanism detect a reduction in available bandwidth (i.e. a new flow competing for bandwidth)?
When new flows start competing for bandwidth, other flows must release some bandwidth to maintain fairness. CUBIC employs the fast convergence mechanism to accomplish this.
When two successive congestion events indicate a reduction in available bandwidth the new value of WMAX is further reduced (based on the multiplicative decrease factor used for resetting the congestion window) to free up additional bandwidth and reduce the number of congestion events required for all flows to converge on a fair distribution of bandwidth.
