The basic AIMD algorithm then goes as follows, assuming that packet loss is used as an indicator of congestion: On modern Linux kernels, the initial CWND is 10 MSS. When a connection is set up, the CWND is set to a small multiple of the maximum segment size (MSS) allowed on that connection. You can not find the CWND by inspecting packet headers. The CWND is not advertised or exchanged between the sender and receiver - it is a private value maintained locally by each end host. In other words: if the number of unacknowledged segments is equal to the CWND, the sender stops sending data until more acknowledgements are received. For each connection, TCP maintains a CWND that limits the total number of unacknowledged packets that may be in transit end-to-end ("bytes in flight"). How does the sender reduce its sending rate? Most congestion control algorithm use a congestion window (CWND) to control sending rate. The sender will realize that packets are lost when no ACK is received for them, and it will reduce its sending rate. When the buffer is full, the router has no choice but to drop new packets as they arrive. As the sender continues to transmit faster than packets can leave the buffer, the buffer will fill with packets until eventually there is no room for more packets. When a sender transmits TCP packets at rate faster than the capacity of the bottleneck (the link with the smallest capacity in the path), the bottleneck router puts packets in a buffer. There are various indicators of congestion that can signal that the network is over-utilized. If it believes the network is over-utilized, then it should decrease its sending rate if it believes the network is under-utilized, then it should increase its sending rate.
The basic idea of congestion control is that the sender transmits TCP packets on the network, then reacts to observable events to either increase or decrease its sending rate. answering the question "How fast should I send?" AIMD: additive increase, multiplicative decreaseĪdditive-increase/multiplicative-decrease (AIMD) is a feedback control algorithm that is the primary mechanism for adjusting the rate of a TCP flow, i.e. However, in this experiment, we'll examine an early variant of TCP congestion control that uses a basic additive increase, multiplicative decrease rule. As the Internet (and the characteristics of Internet traffic) changes, TCP congestion control must evolve along with it. The effective throughput (the volume of useful data transferred over the link) dropped by a factor of ~1000!īecause TCP congestion control is so essential, it is also continuously undergoing improvement and refinement.
#Tcp packet sender in c series#
In October of '86, the Internet had the first of what became a series of 'congestion collapses.' During this period, the data throughput from LBL to UC Berkeley (sites separated by 400 yards and three IMP hops) dropped from 32 Kbps to 40 bps. In fact, an Internet collapse has occurred in the past, due to insufficient congestion control. This is a state known as congestion collapse, and occurs when the link is "busy" but not getting useful work done - for example, if the network is clogged with unnecessary retransmissions of lost packets. TCP is arguably one of the most important Internet protocols, as it carries a much higher volume of traffic on the Internet than any other transport-layer protocol.īecause TCP carries so much traffic, its congestion control algorithm is the main technique which prevents the Internet from slowing to a crawl due to over-utilization. To reproduce this experiment on Cloudlab, you will need an account on Cloudlab, you will need to have joined a project, and you will need to have set up SSH access. Cloudlab-specific instructions: Prerequisites
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