ADSL-Bandwidth-Management-HOWTO.pdf

ADSL Bandwidth Management HOWTO

Dan Singletary

dvsing@sonicspike.net

Revision History

Revision 1.2 2002−09−26 Revised by: ds

Added link to new Email Discussion List. Added small teaser to caveat section regarding new and improved

QoS for Linux designed specifically for ADSL to be released soon.

Revision 1.1 2002−08−26 Revised by: ds

A few corrections (Thanks to the many that pointed them out!). Added informational caveat to

implementation section.

Revision 1.0

2002−08−21

Revised by: ds

Better control over bandwidth, more theory, updated for 2.4 kernels

Revision 0.1

2001−08−06

Revised by: ds

Initial publication

This document describes how to configure a Linux router to more effectively manage outbound traffic on an

ADSL modem or other device with similar bandwidth properties (cable modem, ISDN, etc). Emphasis is

placed on lowering the latency for interactive traffic even when the upstream and/or downstream bandwidth is

fully saturated.

ADSL Bandwidth Management HOWTO

Table of Contents

1. Introduction. ....................................................................................................................................................1

1.1. New Versions of This Documen. .....................................................................................................1

1.2. Email Discussion List. ......................................................................................................................1

1.3. Disclaimer. ........................................................................................................................................1

1.4. Copyright and License. .....................................................................................................................1

1.5. Feedback and corrections. .................................................................................................................1

2. Background .....................................................................................................................................................2

2.1. Prerequisites. .....................................................................................................................................2

2.2. Layout. ..............................................................................................................................................2

2.3. Packet Queues. ..................................................................................................................................3

2.3.1. The Upstream. .........................................................................................................................3

2.3.2. The Downstream. ....................................................................................................................3

3. How it Works. ..................................................................................................................................................5

3.1. Throttling Outbound Traffic with Linux HTB. .................................................................................5

3.2. Priority Queuing with HTB. .............................................................................................................5

3.3. Classifying Outbound Packets with iptables. ...................................................................................6

3.4. A few more tweaks.... .......................................................................................................................6

3.5. Attempting to Throttle Inbound Traffic. ...........................................................................................7

3.5.1. Why Inbound Traffic Limiting isn't all That Good. ................................................................7

4. Implementation. ..............................................................................................................................................8

4.1. Caveats. .............................................................................................................................................8

4.2. Script: myshaper. ..............................................................................................................................8

5. Testing the New Queue. ................................................................................................................................14

6. OK It Works!! Now What?. .........................................................................................................................15

1. Introduction

The purpose of this document is to suggest a way to manage outbound traffic on an ADSL (or cable modem)

connection to the Internet. The problem is that many ADSL lines are limited in the neighborhood of 128kbps

for upstream data transfer. Aggravating this problem is the packet queue in the ADSL modem which can take

2 to 3 seconds to empty when full. Together this means that when the upstream bandwidth is fully saturated it

can take up to 3 seconds for any other packets to get out to the Internet. This can cripple interactive

applications such as telnet and multi−player games.

1.1. New Versions of This Document

You can always view the latest version of this document on the World Wide Web at the URL:

http://www.tldp.org .

New versions of this document will also be uploaded to various Linux WWW and FTP sites, including the

LDP home page a t http://www.tldp.org .

1.2. Email Discussion List

For questions and update information regarding ADSL Bandwidth Management please subscribe to the ADSL

Bandwidth Management email list at http://jared.sonicspike.net/mailman/listinfo/adsl−qos .

1.3. Disclaimer

Neither the author nor the distributors, or any other contributor of this HOWTO are in any way responsible for

physical, financial, moral or any other type of damage incurred by following the suggestions in this text.

1.4. Copyright and License

Documentation License, which is hereby incorporated by reference.

1.5. Feedback and corrections

If you have questions or comments about this document, please feel free to contact the author at

dvsing@sonicspike.net .

1. Introduction

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:

· 2.4.18−5 Kernel with QoS Support fully enabled (modules OK) and including the following kernel

patches (which may eventually be included in later kernels):

Red Hat Linux 7.3

HTB queue − http://luxik.cdi.cz/~devik/qos/htb/

Note: it has been reported that kernels since version 2.4.18−3 shipped with Mandrake (8.1,

8.2) have already been patched for HTB.

IMQ device − http://luxik.cdi.cz/~patrick/imq/

iptables v1.2.6a or later (version of iptables distributed with Red Hat 7.3 is missing the length

module)

Note: Previous versions of this document specified a method of bandwidth control that involved patching

the existing sch_prio queue. It was found later that this patch was entirely unnecessary. Regardless, the

newer methods outlined in this document will give you better results (although at the writing of this

document 2 kernel patches are now necessary. :) Happy patching.)

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

−−−−−−−−−−−−−−−−−

| |

| Linux Router |

| |

−−−−−−−−−−−−−−−−−

| .. | eth1..ethN

| |

V V

Local Network

2. Background

ADSL Bandwidth Management HOWTO

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 a t 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

2. Background

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