Traffic shaping
Traffic shaping (including traffic shaping ) refers to a type of queue management in packet-switched data networks, in which data packets are delayed or discarded according to certain criteria, in order to satisfy certain requirements profiles. This function is carried out by a network scheduler and is basically a form of data rate limitation . Traffic shaping is unidirectional , which means that, in contrast to data flow control, it workswithout control information from the other side.
With traffic shaping, the network traffic is divided into different areas. These classifications are, for example, applications and protocols , transmission directions between client and host , time and type of connection.
Application example
With the Transmission Control Protocol (TCP), the receipt of a certain amount of data must be confirmed by the other side before the sender can send new data. If these confirmations of receipt do not come quickly enough, the sender has to wait and there are delays and drops in the transfer rate. Especially with ADSL it can happen that a download is slowed down by overloading the return channel (which has the smaller bandwidth anyway) with an upload, so that there is no longer enough transmission capacity for the immediate sending of the confirmation of receipt of the download. The confirmation packets are buffered (within the computer and also within the DSL modem) and, although they are very small, have to wait in the worst case until all the packets generated before them have been sent. The larger the buffers, the greater the delay they cause. If the performance capacity is continuously overloaded, packets are also discarded, which increases the delay even further.
To compensate for this effect, some DSL drivers or tuning programs increase the TCP window size so that the sender can send more data before it has to wait for an acknowledgment of receipt. This advantage is paid for by a significant increase in latency (ping times), so surfing during a data transfer is unusually slow. (If the TCP window size value is set to over 65,535 bytes, TCP window scaling must be activated at the same time , otherwise no throughput can be increased corresponding to the TCP window size. Without TCP window scaling, the maximum is Throughput at around 5 Mbit / s with 100 ms packet cycle time .)
The traffic shaping now analyzes the incoming data traffic in both directions (with a stronger effect on uploads) and rearranges the waiting data packets so that the data transfer in the opposite direction is less hindered. If the packages to be sent are those that are only supposed to confirm receipt of data, then these are given priority, i.e. sent first. This shortens the phases in which the remote station is not sending any data and the transmission rate increases. As a side effect, this largely prevents these packets from being discarded due to overload. In order to maximize this effect, the entire data stream is also slowed down so that the computer sends its data a little slower than the DSL modem can transmit it. This means that the buffers in the DSL modem are always empty and data that the computer sends to the modem are transferred immediately. So there are no delays caused by the buffers in the modem. This has the following advantages:
- Higher download rate despite uploads
- Higher download rate with multiple downloads and uploads
- Shorter response times for z. B. Surfing (better ping times, important for online games, for example)
Algorithms
The most common algorithms for queue management are the leaky bucket algorithm and the token bucket algorithm .
Hardware / provider used
The router firmware offered can be based on Linux, BSD or VxWorks . Generally available algorithms with their own settings or algorithms developed in-house are used for the network scheduler. With the help of the Linux distribution OpenWrt , the latest technologies can even be used on inexpensive routers, such as data packet sniffers .
The firmware of the Fritz! Box from AVM is based on Linux, but the core, the dsld , is a proprietary in-house development.
With the product cFosSpeed, cFos Software GmbH offers a software traffic shaper for Microsoft Windows, which can be installed cooperatively on all Windows clients in a homogeneous local Windows network.