target = "https://www.rfc-editor.org/rfc/rfc8312#section-4.2"

# 4.2.  TCP-Friendly Region
#
# Standard TCP performs well in certain types of networks, for example,
# under short RTT and small bandwidth (or small BDP) networks.  In
# these networks, we use the TCP-friendly region to ensure that CUBIC
# achieves at least the same throughput as Standard TCP.
# 
# The TCP-friendly region is designed according to the analysis
# described in [FHP00].  The analysis studies the performance of an
# Additive Increase and Multiplicative Decrease (AIMD) algorithm with
# an additive factor of alpha_aimd (segments per RTT) and a
# multiplicative factor of beta_aimd, denoted by AIMD(alpha_aimd,
# beta_aimd).  Specifically, the average congestion window size of
# AIMD(alpha_aimd, beta_aimd) can be calculated using Eq. 3.  The
# analysis shows that AIMD(alpha_aimd, beta_aimd) with
# alpha_aimd=3*(1-beta_aimd)/(1+beta_aimd) achieves the same average
# window size as Standard TCP that uses AIMD(1, 0.5).
# 
#     AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) /
#                    (2*(1-beta_aimd)*p) ]^0.5 (Eq. 3)
# 
# Based on the above analysis, CUBIC uses Eq. 4 to estimate the window
# size W_est of AIMD(alpha_aimd, beta_aimd) with
# alpha_aimd=3*(1-beta_cubic)/(1+beta_cubic) and beta_aimd=beta_cubic,
# which achieves the same average window size as Standard TCP.  When
# receiving an ACK in congestion avoidance (cwnd could be greater than
# 
# or less than W_max), CUBIC checks whether W_cubic(t) is less than
# W_est(t).  If so, CUBIC is in the TCP-friendly region and cwnd SHOULD
# be set to W_est(t) at each reception of an ACK.
# 
#     W_est(t) = W_max*beta_cubic +
#                 [3*(1-beta_cubic)/(1+beta_cubic)] * (t/RTT) (Eq. 4)

[[spec]]
level = "SHOULD"
quote = '''
If so, CUBIC is in the TCP-friendly region and cwnd SHOULD
be set to W_est(t) at each reception of an ACK.
'''