[queuing] RED: random early detection tcp

[ref: https://en.wikipedia.org/?title=Random_early_detection]

Random early detection (RED), also known as random early discard or random early drop is a queueing discipline for a network schedulersuited for congestion avoidance.[1]

In the conventional tail drop algorithm, a router or other network component buffers as many packets as it can, and simply drops the ones it cannot buffer. If buffers are constantly full, the network is congested. Tail drop distributes buffer space unfairly among traffic flows. Tail drop can also lead to TCP global synchronization as all TCP connections "hold back" simultaneously, and then step forward simultaneously. Networks become under-utilized and flooded by turns. RED addresses these issues.


RED monitors the average queue size and drops (or marks when used in conjunction with ECN) packets based on statistical probabilities. If the buffer is almost empty, all incoming packets are accepted. As the queue grows, the probability for dropping an incoming packet grows too. When the buffer is full, the probability has reached 1 and all incoming packets are dropped.

RED is more fair than tail drop, in the sense that it does not possess a bias against bursty traffic that uses only a small portion of the bandwidth. The more a host transmits, the more likely it is that its packets are dropped as the probability of a host's packet being dropped is proportional to the amount of data it has in a queue. Early detection helps avoid TCP global synchronization.



[network] dummynet tcp

[ref: http://info.iet.unipi.it/~luigi/dummynet/]

Description

datafiles/dummynet.gifdummynet is a live network emulation tool, originally designed for testing networking protocols, and since then used for a variety of applications including bandwidth management. It simulates/enforces queue and bandwidth limitations, delays, packet losses, and multipath effects. It also implements various scheduling algorithms. dummynet can be used on the machine running the user's application, or on external boxes acting as routers or bridges.
dummynet runs within your operating system (FreeBSD, OSX, Linux, Windows) and works by intercepting selected traffic on its way through the network stack, as in the figure above, and passing packets to objects called pipes which implement a set of queues, a scheduler, and a link, all with configurable features (bandwidth, delay, loss rate, queue size, scheduling policy...).
Traffic selection is done using the ipfw firewall, which is the main user interface for dummynet. ipfw lets you select precisely the traffic and direction you want to work on, making configuration and use incredibly simple. You can create multiple pipes, send traffic to different pipes, even build cascades of pipes.

[gnuplot]png resolution, xtics linux

set terminal png size 1280,900 truecolor enhanced

으로 png size 결과 resolution 조절

set xtics 50

xtics에 50 으로 set

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