Buffer Bloat – Bufferbloat

[UK Sentinel]

Bufferbloat! is a phenomenon in packet-switched networks, in which excess buffering of packets causes high latency and packet delay variation (also known as jitter), as well as reducing the overall network throughput. When a router or switch is configured to use excessively large buffers, even very high-speed networks can become practically unusable for many interactive applications like Voice over IP (VoIP), online gaming, and even web surfing.

The bufferbloat phenomenon was initially described as far back as in 1985, and gradually became more recognized as an issue. It gained more widespread attention starting in 2009.

Some equipment manufactures placed overly large buffers in some of their models. In such equipment, bufferbloat occurs when a network link becomes congested, causing packets to become queued in buffers for too long. In a first-in first-out queuing system, overly large buffers result in longer queues and higher latency.

Buffering!
The rule of thumb for the network equipment manufacturers was to provide buffers large enough to accommodate 250 ms (or more) worth of traffic passing through a device. For example, a router’s 1 Gbit/s Ethernet interface would require a relatively large 32 MB buffer[3] Such sizing of the buffers can lead to TCP’s congestion-avoidance algorithms breaking, causing problems such as high and variable latency, and choking network bottlenecks for all other flows as the buffer becomes full of the packets of one TCP stream and other packets are then dropped. The buffers then take some time to drain, before the TCP connection ramps back up to speed and then floods the buffers again.

A bloated buffer has an effect only when this buffer is actually used. In other words, oversized buffers have a damaging effect only when the link they buffer for becomes a bottleneck. When the current bottleneck on the route from or to another host is not contended, it is easy to check whether it is bloated or not using the ping utility provided by most operating systems. First, the other host should be pinged continuously; then, a several-seconds-long download from it should be started and stopped a few times. By design, the TCP congestion avoidance algorithm rapidly fills up tk on the ro. If downloading (and uploading, respectively) correlates with a direct and important increase of the round trip time reported by ping, then it proves that the buffer of the current bottleneck in the download (and upload, respectively) direction is bloated. Since the increase of the round trip time is caused by the buffer on the bottleneck, the maximum increase gives a rough estimation of its size in milliseconds.

Ref: https://en.wikipedia.org/wiki/Bufferbloat

You can read more about buffer bloat, and the various solutions available to you, at http://www.dslreports.com/faq/17883

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