Which of the following is a congestion control mechanism that is designed for unicast flows
operating in an Internet environment and competing with TCP traffic?

A.
Sliding Window

B.
TCP Friendly Rate Control

C.
Selective Acknowledgment

D.
Additive increase/multiplicative-decrease

Explanation:

TCP-Friendly Rate Control (TFRC) is a congestion control mechanism that is designed for unicast
flows operating in an Internet environment and competing with TCP traffic. Its goal is to compete
fairly with TCP traffic on medium timescales, but to be much less variable than TCP on short
timescales.
TCP congestion control works by maintaining a window of packets that have not yet been
acknowledged. This window is increased by one packet every round-trip time if no packets have
been lost, and is decreased by half if a packet loss is detected. Thus, TCP’s window is a function
of the losses observed in the network and the round trip time experienced by the flow.
The idea behind TFRC is to measure the loss probability and round trip time and to use these as
the parameters to a model of TCP throughput. The expected throughput from this model is then
used to directly drive the transmit rate of a TFRC flow.

Answer option D is incorrect. The additive increase/multiplicative-decrease (AIMD) algorithm is a
feedback control algorithm used in TCP Congestion Avoidance. Its major goal is to achieve
fairness and efficiency in allocating resources. AIMD combines linear growth of the congestion
window with an exponential reduction when congestion takes place.
The approach taken is to increase the transmission rate (window size), probing for usable
bandwidth, until loss occurs. The policy of additive increase may, for instance, increase the
congestion window by 1 MSS (Maximum segment size) every RTT (Round Trip Time) until a loss
is detected. When loss is detected, the policy is changed to be one of multiplicative decrease,
which may, for instance, cut the congestion window in half after the loss. A loss event is generally
described to be either a timeout or the event of receiving 3 duplicate ACKs.
Answer option C is incorrect. Selective Acknowledgment (SACK) is one of the forms of
acknowledgment. With selective acknowledgments, the sender can be informed by a data receiver
about all segments that have arrived successfully, so the sender retransmits only those segments
that have actually been lost. The selective acknowledgment extension uses two TCP options:
The first is an enabling option, “SACK-permitted”, which may be sent in a SYN segment to indicate
that the SACK option can be used once the connection is established.
The other is the SACK option itself, which can be sent over an established connection once
permission has been given by “SACK-permitted”.
Answer option A is incorrect. Sliding Window Protocols are a feature of packet-based data
transmission protocols. They are used where reliable in-order delivery of packets is required, such
as in the data link layer (OSI model) as well as in TCP.
Conceptually, each portion of the transmission (packets in most data link layers, but bytes in TCP)
is assigned a unique consecutive sequence number, and the receiver uses the numbers to place
received packets in the correct order, discarding duplicate packets and identifying missing ones.
The problem with this is that there is no limit of the size of the sequence numbers that can be
required.