Vegas A (Modified TCP-Vegas)

TCP Vegas is a modified TCP Vegas. It depends on Round Trip Time values on the accurate calculation of Base RTT. The TCP detects congestion that can be avoided at different Stages based on Round Trip Time(RTT). Here A stands for Adaptive. TCP Vegas emphasizes packet delay rather than loss of the packet. The TCP Vegas was first coined by Larry L Peterson and Lawrence Brakmo in 1994. The TCP Vegas has better throughput with less loss of packets by keeping packets in the congestion window to be smaller during the transmission.


TCP Vegas is an algorithm that avoids congestion at a different incipient stage i.e TCP congestion avoidance algorithm. The TCP detects congestion that can be avoided at different Stages based on Round Trip Time(RTT)

TCP Vegas detect network congestion and also recognizes the problem of end-to-end measurements of the round-trip time(RTT).

The Congestion avoidance mechanism

The network to be more efficient it needs to estimate the available network. The TCP controls the network flow rate using the feedback of the network.

TCP is responsible for the check of the availability of Bandwidth and its maximum usage using the Bandwidth Estimation Scheme.

The TCP Congestion avoidance algorithm is TCP vegas which sees packet delay more than the loss of the packet. It determines the rates for sending the packets.

In today's world, in modern technology more efficient congestion control technologies, are emerging for instance 5G, IOT, etc which use TCP Vegas that suits the performance in accord. Such manner mostly TCP Vegas can be seen in the latest internet but not in routers or any operating system. TCP Vegas is more stable and fair than another available networks.

TCP Vegas Algorithm

The TCP Vegas contains two terms α and β. There are three cases based on the value of the difference between the data in the queues.

Terms that are used in the algorithm as follows

Actual Rate of throughput at time t = When the data rate is achieved without congestion.

Expected Rate of throughput at time t = The data with congestion.

RTT = Round Trip Time.

Difference(Diff) = Expected Rate – Actual Rate

The value of Diff is constant i.e lies between α and β then two cases arise

  • Case i) − If Diff is smaller than α then the window size(cw) increases by 1.

  • Case ii) − If Diff is greater than β then the window size(cw) decreases by 1.

Here α and β are thresholds.

Also, RTT value is significant then the window size(cw) decreases which is triggered as β-threshold results in a decrease in window size. We can also see congestion resulting due to heavy loads of packets in the queue.

If the RTT value is small then the window size(cw) avoids risk which is triggered as α-threshold. We can also see the queue is null. However, there is a need to avoid the risk associated with not utilizing the full available bandwidth. Hence increase the window size (cw).

As TCP Vegas is dependent on the precise value of BaseRTT. The BaseRTT value is so small then the protocol throughput will stay below the available Bandwidth

In simple words, the data is queued more than there the congestion happens in the network.

Whereas no data is queued will make the risk of not utilizing bandwidth. The threshold trigger change according to actual and expected values to avoid congestion.

Issues related to TCP Vegas are Rerouting

The rerouting can result in a substantial decrease in throughput. The route is changed by the switch then without an explicit signal it cannot detect the end host. If the new route is a longer propagation delay then TCP vegas cannot detect it is due to a change in route or congestion in the network causing the round trip delay. However if the most likely packets experiences short round delay the Base RTT is updated.

There is shortness in delay due to backlogs but regain once retain stability.

Advantages of TCP Vegas

  • In TCP Vegas, 40% to 70% throughput would be possible with a one-half to one-fifth loss.

  • It also helps in maintaining high throughput and avoiding unnecessary packet loss. But we care as have to maintain parameter optimization performance and modifications in some conditions.

Disadvantages of TCP Vegas

  • As the load increases or the number of router buffers decreases the Vegas congestion avoidance mechanism is not effective.

  • Vegas limits its usage of router buffer as specified by threshold hence TCP Vegas behavior is less aggressive in using router buffer.


TCP Vegas helps improve network performances. It decreases the delay and increases the throughput. By using available bandwidth effectively rather than retransmission aggressively for achieving the throughput. The throughput is minimized whereas transmission is maximized.

If a duplicate ACK is received then the process of retransmission occurs. This helps in reducing the time of window size to one-half. When we compare TCP Vegas to TCP Reno then TCP Vegas is the far better performer.

Updated on: 12-Apr-2023


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