The congestion control mechanisms described above (Slow Start, Congestion Avoidance, Fast Retransmit, Fast Recovery), found in TCP variants like Tahoe and Reno, are primarily loss-based congestion control algorithms.

• Loss-Based Congestion Control (e.g., TCP Tahoe, TCP Reno, TCP CUBIC - modern default for Linux):
• Principle: These algorithms implicitly assume that packet loss is the primary and direct signal of network congestion. When packets are lost (due to router buffer overflow), it's a clear indication that the network's capacity has been exceeded.
• Pros: Relatively simple to implement and robust across a wide variety of network conditions. They effectively prevent congestion collapse by backing off aggressively when loss is detected.
• Cons: They are reactive; they only detect congestion after it has already manifested as packet loss. This means network queues must be filled and overflow before the sender reduces its rate, which can lead to:
  • Bursty traffic: Cycles of rapid increase, then sharp drops in cwnd.
  • Underutilization: If network buffers are very shallow, losses can occur prematurely, preventing TCP from fully utilizing available bandwidth.
  • Poor performance over wireless: Wireless links can have non-congestion-related packet loss (e.g., due to interference), which a loss-based algorithm would misinterpret as congestion and unnecessarily reduce the sending rate.

Delay-Based Congestion Control (e.g., TCP Vegas, TCP BBR):

• Principle: These algorithms attempt to detect the onset of congestion proactively, before packet loss occurs. They achieve this primarily by monitoring changes in the Round-Trip Time (RTT). An increase in RTT generally indicates that packets are spending more time waiting in router queues, signaling incipient congestion.
• TCP Vegas:
• Approach: Vegas tries to keep a small, fixed amount of extra data in the network pipeline to fill potential idle spots without causing excessive queuing. It calculates the expected throughput (based on cwnd and the minimum observed RTT - the "base RTT" or "propagation RTT") and compares it to the actual observed throughput (based on current RTT).
• Reaction: If the actual throughput falls significantly below the expected throughput, it indicates packets are queuing up, and Vegas proactively reduces cwnd slightly to prevent a full buffer and subsequent packet loss.
• Pros: Can achieve lower packet loss and lower average queueing delays compared to loss-based TCP. Better for networks with shallow buffers.
• Cons: Can be overly conservative, sometimes underutilizing available bandwidth, especially on networks with high link capacity and deep buffers. Its performance can be sensitive to the accurate estimation of base RTT.
• TCP BBR (Bottleneck Bandwidth and Round-trip Propagation Time):
• Approach: A more modern and sophisticated "model-based" congestion control algorithm developed by Google. BBR attempts to explicitly estimate two key network parameters:
  • Bottleneck Bandwidth (BtlBW): The maximum achievable throughput of the bottleneck link on the path.
  • Round-Trip Propagation Time (RTprop): The minimum RTT observed, representing the true propagation delay across the path without queuing.
• Mechanism: BBR continuously measures these two parameters and then paces its sending rate to match the estimated BtlBW while keeping no more than BtlBW * RTprop (the Bandwidth-Delay Product or BDP) amount of data in flight, plus a small amount to fill any "pipe" fluctuations. This means it tries to keep the network pipe full but not overfilled, explicitly avoiding queue buildup.
• Pros: Can achieve significantly higher throughput and lower average latency compared to loss-based TCP, especially on high-bandwidth, high-latency (long-fat) networks, or networks with significant non-congestion loss (e.g., wireless). It aims to run the network at optimal efficiency by avoiding unnecessary packet loss.
• Cons: More complex to implement. Its performance can sometimes be affected by interactions with other non-BBR flows or specific network topologies.