CUBIC CONGESTION CONTROL

Question 1

its unique window growth function

Answer 1

The main feature of BIC-TCP

Question 2

BIC-TCP: If the window grows past the maximum,

Answer 2

the equilibrium window size must be larger than the current maximum and a new maximum must be found.

Question 3

BIC-TCP enters this new phase that uses a window growth function exactly symmetric to those used in additive increase and binary search (which is logarithmic; its reciprocal will be exponential) and then additive increase.

Answer 3

“max probing”

Question 4

During max

probing,

Answer 4

the window grows slowly initially to find the new
maximum nearby, and after some time of slow growth, if it does not find the new maximum (i.e., packet losses), then it guesses the new maximum is further away so it switches to a faster increase by switching to additive increase where the window size is incremented by a large fixed increment.

Question 5

The good performance of BIC-TCP comes from

Answer 5

the slow increase around Wmax and linear increase during additive increase and max probing.

Question 6

achieves good scalability in high speed networks,

fairness among competing flows of its own and stability with low window oscillations.

Answer 6

BIC-TCP

Question 7

can still be too aggressive for TCP, especially under

short RTT or low speed networks.

Answer 7

BIC-TCP’s growth func-

tion

Question 8

a new high-speed TCP variant:

Answer 8

CUBIC

Question 9

a cubic function whose shape is very

similar to the growth function of BIC-TCP.

Answer 9

the window growth
function of CUBIC

Question 10

uses both the concave and convex profiles of a cubic function for window increase.

Answer 10

CUBIC

Question 11

The window growth function of CUBIC

Answer 11

W(t) = C(t - K)3 +Wmax

Question 12

if cwnd is less
than the window size that (standard) TCP would reach at
time t after the last loss event,

Answer 12

then CUBIC is in the TCP
mode (we describe below how to determine this window size
of standard TCP in term of time t).

Question 13

if cwnd is less than Wmax, then

Answer 13

CUBIC is in the concave region,

Question 14

if cwnd is larger than Wmax,

Answer 14

CUBIC is in the convex region.

Question 15

When receiving an ACK in congestion avoidance, if the protocol is not in the TCP mode and cwnd is less than Wmax, then the protocol is in the concave region. In this region,cwnd is incremented by W(t+RTT)-cwnd
cwnd ,

Answer 15

Concave region

Question 16

When the window size of CUBIC is larger than Wmax, it

passes the plateau of the cubic function after which CUBIC follows the convex profile of the cubic function.

Answer 16

Convex region