Along with the transmission rate in the transmitting direction, the same consideration also applies to the receiving direction. In this case, the device's WAN interface receives significantly less data from the DSL modem than the Ethernet interface can theoretically handle. All data packets received on the WAN interface are transferred to the LAN with equal priority.
In order to be able to prioritize incoming data, an artificial “brake” needs to be applied to the received data. As with the transmitting direction, the transmission rate of the interface in the receive direction is adapted to match the speed of the provider, e.g. a downstream rate of 16 Mbps. Again, as with the upstream rate, the gross data rate may also be entered, if known.
Reducing the reception bandwidth now makes it possible to handle the received data packets in a suitable manner. The preferred data packets are passed directly to the LAN up to the guaranteed minimum bandwidth, the remaining data packets run into congestion. This congestion usually leads to the acknowledgment of the packets being delayed. On a TCP connection, the sending server will respond to these delays, decreasing its transmit frequency and adapting to the available bandwidth.
The following queues are used when receiving data:
- Deferred acknowledge queue Each WAN interface additionally has a QoS receive queue, which receives the packets to be slowed down. The storage period of each individual packet depends on the length of the packet and the currently permitted reception bandwidth. Packets that are granted a minimum receive bandwidth by a QoS rule are allowed to pass without delay, as long as the minimum bandwidth is not exceeded.
- Standard reception queue All received packets not given priority treatment by a QoS rule end up in this queue. Packets in this queue are forwarded or acknowledged directly, without consideration of maximum bandwidths.
