ADSL Bandwidth Management HOWTO, Dan Singletary [free ebook reader for ipad .txt] 📗

| | |

| ----------------- |

| | .. | 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] 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!). How it Works

There are two basic steps to optimize upstream bandwidth. First we have to

find a way to prevent the ADSL modem from queuing packets since we have no

control over how it handles the queue. In order to do this we will throttle

the amount of data the router sends out eth0 to be slightly less than the

total upstream bandwidth of the ADSL modem. This will result in the router

having to queue packets that arrive from the Local Network faster than it is

allowed to send them.

The second step is to set up priority queuing discipline on the router. We'll

investigate a queue that can be configured to give priority to interactive

traffic such as telnet and multi-player games.

+---------------------------------------------------------------------------+

| |

| By using the HTB queue we can accomplish bandwidth shaping and priority |

| queuing at the same time while also assuring that no priority class is |

| starved by another. Avoiding starvation wasn't possible using the method |

| outlined in the 0.1 revision of this document. |

| |

+---------------------------------------------------------------------------+

The final step is to configure the firewall to prioritize packets by using

fwmark.

3.1. Throttling Outbound Traffic with Linux HTB

Although the connection between the router and the modem is at 10Mbit/s, the

modem is only able to send data at 128kbit/s. Any data sent in excess of that

rate will be queued at the modem. Thus, a ping packet sent from the router

may go to the modem immediately, but may take a few seconds to actually get

sent out to the Internet if the queue in the modem has any packets in it.

Unfortunately most ADSL modems provide no mechanism to specify how packets

are dequeued or how large the queue is, so our first objective is to move the

place where the outbound packets are queued to somewhere where we have more

control over the queue.

We'll do this by using the HTB queue to limit the rate at which we send

packets to the ADSL modem. Even though our upstream bandwidth may be 128kbit/

s we'll have to limit the rate at which we send packets to be slightly below

that. If we want to lower the latency we have to be SURE that not a single

packet is ever queued at the modem. Through experimentation I have found that

limiting the outbound traffic to about 90kbit/s gives me almost 95% of the

bandwidth I could achieve without HTB rate control. With HTB enabled at this

rate, we've prevented the ADSL modem from queuing packets.

3.2. Priority Queuing with HTB

+---------------------------------------------------------------------------+

| |

| Note: previous claims in this section (originally named N-band priority |

| queuing) were later found to be incorrect. It actually WAS possible to |

| classify packets into the individual bands of the priority queue by only |

| using the fwmark field, however it was poorly documented at the writing |

| of version 0.1 of this document