Question 251 - 312-38 discussion

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 roundtrip 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.