2. Background

2.3.1. The Upstream

With an ADSL modem, bandwidth is asymmetric with 1.5Mbit/s typical downstream and 128kbit/sec typical upstream. Although this is the line speed, the interface between the Linux Router PC and the ADSL modem is typically at or above 10Mbit/s. If the interface to the Local Network is also 10Mbit/s, there will typically be NO QUEUING at the router when packets are sent from the Local Network to the Internet. Packets are sent out eth0 as fast as they are received from the Local Network. Instead, packets are queued at the ADSL modem since they are arriving at 10Mbit/s and only being sent at 128kbit/s. Eventually the packet queue at the ADSL modem will become full and any more packets sent to it will be silently dropped. TCP is designed to handle this and will adjust it's transmit window size accordingly to take full advantage of the available bandwidth.

While packet queues combined with TCP result in the most effective use of bandwidth, large FIFO queues can increase the latency for interactive traffic.

Another type of queue that is somewhat like FIFO is an n-band priority queue. However, instead of having just one queue that packets line up in, the n-band priority queue has n FIFO queues which packets are placed in by their classification. Each queue has a priority and packets are always dequeued from the highest priority queue that contains packets. Using this discipline FTP packets can be placed in a lower priority queue than telnet packets so that even during an FTP upload, a single telnet packet will jump the queue and be sent immediately.

This document has been revised to use a new queue in linux called the Hierarchical Token Bucket (HTB). The HTB queue is much like the n-band queue described above, but it has the capability to limit the rate of traffic in each class. In addition to this, it has the ability to set up classes of traffic beneath other classes creating a hierarchy of classes. Fully describing HTB is beyond the scope of this document, but more information can be found at http://www.lartc.org

2.3.2. The Downstream

Traffic coming inbound on your ADSL modem is queued in much the same way as outbound traffic, however the queue resides at your ISP. Because of this, you probably don't have direct control of how packets are queued or which types of traffic get preferential treatment. The only way to keep your latency low here is to make sure that people don't send you data too fast. Unfortunately, there's no way to directly control the speed at which packets arrive, but since a majority of your traffic is most likely TCP, there are some ways to slow down the senders:

• Intentionally drop inbound packets - TCP is designed to take full advantage of the available bandwidth while also avoiding congestion of the link. This means that during a bulk data transfer TCP will send more and more data until eventually a packet is dropped. TCP detects this and reduces it's transmission window. This cycle continues throughout the transfer and assures data is moved as quickly as possible.

• Manipulate the advertised receive window - During a TCP transfer, the receiver sends back a continuous stream of acknowledgment (ACK) packets. Included in the ACK packets is a window size advertisement which states the maximum amount of unacknowledged data the receiver should send. By manipulating the window size of outbound ACK packets we can intentionally slow down the sender. At the moment there is no (free) implementation for this type of flow-control on Linux (however I may be working on one!).