Please, I need an answer for problem 3: congestion control and throughput. In this problem, we make the following assumptions: • We always have cwnd ≤ rwnd, i.e., we can ignore TCP flow control. • All losses are detected through the 3 duplicate ACKs (2024)

FAQs

Why doesn't TCP Reno employ the slow start phase when a triple duplicate ACK is received? ›

In congestion avoidance phase, TCP's send rate increases linearly. Why does TCP Reno get rid of slow start phase when a triple duplicate ACK is received? The receipt of triple duplicate ACKs indicates that some segments are getting through, and so congestion may be less severe.

The problem of congestion control arises whenever multiple distributed agents try to make use of shared resources (Figure 1.1). It arises widely in different scenarios, including traffic control on highways or air traffic control.

TCP's general policy for handling congestion is based on three phases: slow start, congestion avoidance, and congestion detection. In the slow-start phase, the sender starts with a very slow rate of transmission, but increases the rate rapidly to reach a threshold.

The Receiver Window (rwnd) is a variable that advertises the amount of data that the destination side can receive. Together, the two variables are used to regulate data flow in TCP connections, minimize congestion, and improve network performance.

The reason that the sending side has to wait until the third duplicate ACK is described in RFC2001 as follows: " Since TCP does not know whether a duplicate ACK is caused by a lost segment or just a reordering of segments, it waits for a small number of duplicate ACKs to be received.

That means, if no ACK is seen after 20ms, it retransmits the packet. The Client itself is receiving the packet and the TCP Stack waits the Delayed ACK timer for the ACK (max 200ms), because there may be an answer from the application and the ACK + answer can be send in the same frame.

TCP uses a congestion window in the sender side to do congestion avoidance. The congestion window indicates the maximum amount of data that can be sent out on a connection without being acknowledged. TCP detects congestion when it fails to receive an acknowledgement for a packet within the estimated timeout.

Congestion Control Algorithms

They monitor network performance and adapt data transmission rates accordingly. Examples include TCP Tahoe, Reno, and BBR, which balance efficiency and fairness to prevent network congestion while maximizing data transfer speed.

Bandwidth: The most common cause of network congestion is bandwidth. Bandwidth refers to the ideal capacity of the network to transfer a certain amount of data from a source to a destination in a specific amount of time. A lack of bandwidth can lead to network outages.

What is standard TCP congestion control? ›

Transmission Control Protocol (TCP) uses a congestion control algorithm that includes various aspects of an additive increase/multiplicative decrease (AIMD) scheme, along with other schemes including slow start and a congestion window (CWND), to achieve congestion avoidance.

So during the congestion avoidance period, TCP is testing the tolerance of the network: after it has successfully transferring CWND amount of data, it adds one more packet to the congestion window: CWND + MSS and retest the network.

TCP flow control is a mechanism that regulates the amount of data that a sender can transmit to a receiver in a network. It ensures that the sender does not overwhelm the receiver with more data than it can process or store, which could lead to congestion, packet loss, and wasted resources.

The TCP Reno answer is known as slow start. If you are trying to guess a number in a fixed range, you are likely to use binary search. Not knowing the range for the “network ceiling”, a good strategy is to guess cwnd =1 (or cwnd =2) at first and keep doubling until you have gone too far.

Fast recovery means that when the sender receives three duplicate ACKs in succession, it executes a multiplication reduction algorithm and halves the slow start threshold to prevent network congestion. The CWND increases in an accumulative manner, causing the CWND to increase slowly and linearly.

Limitation of TCP Reno:

It can't detect multiple packet loss in a single cwnd window size. It reduces the cwnd multiple times (equal to packet lost) in single window size. Thus it reduces the cwnd significantly which is unnecessary cause the network can come out of the congestion state by reducing the cwnd once.

Hence tcp implemented duplicate acks and the sole purpose of it is to intimate the sender before timeout occurs . If the sender receivers duplicate packets greater than 3 then it will retransmit the packet. Duplicate packets are send immediately by receiver if out of order segments are arrived.