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2. Background

2.1. Prerequisites

The method outlined in this document should work in other Linux configurations however it remains untested in any configuration but the following:

2.2. Layout

In order to keep things simple, all references to network devices and configuration in this document will be with respect to the following network layout diagram:

               <-- 128kbit/s      --------------     <-- 10Mbit -->
  Internet <--------------------> | ADSL Modem | <--------------------
                1.5Mbit/s -->     --------------                     |
                                                                     | eth0
                                                                     V
                                                         -----------------
                                                         |               |
                                                         | Linux Router  |
                                                         |               |
                                                         -----------------
                                                          | .. | eth1..ethN
                                                          |    |
                                                          V    V
                   
                                                       Local Network
      

2.3. Packet Queues

Packet queues are buckets that hold data for a network device when it can't be immediately sent. Most packet queues use a FIFO (first in, first out) discipline unless they've been specially configured to do otherwise. What this means is that when the packet queue for a device is completely full, the packet most recently placed in the queue will be sent over the device only after all the other packets in the queue at that time have been sent.

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




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