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Fair Internet bandwidth management on a network using OpenBSD

Written by Solène, on 30 August 2021.
Tags: #openbsd #bandwidth

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Introduction §

I have a simple DSL line with a 15 Mb/s download and 900 kb/s upload rates and there are many devices using the Internet and two people in remote work. Some poorly designed software (mostly on windows) will auto update without allowing to reduce the bandwidth or some huge bloated website will require lot of download and will impact workers using the network.

The point of this article is to explain how to use OpenBSD as a router on your network to allow the Internet access to be used fairly by devices on the network to guarantee everyone they will have at least a bit of Internet to continue working flawlessly.

I will use the queuing features from the OpenBSD firewall PF (Packet Filter) which relies on the CoDel network scheduler algorithm, which seems to bring all the features we need to do what we want.

pf.conf manual page: QUEUEING section

Wikipedia page about the CoDel network scheduler algorithm

Important §

I'm writing this in a separate section of the article because it is important to understand.

It is not possible to limit the download bandwidth, because once the data are already in the router, this mean they came from the modem and it's too late to try to do anything. But there is still hope, if the router receives data from the Internet it's that some devices on the network asked to receive it, we can work on the uploaded data to throttle what we receive. This is not obvious at first but it makes totally sense once you get the idea.

The biggest point to understand is that we can throttle download through the ACK packets. Think of two people on a phone, let's say Alice and Bob, Alice is your network and calls Bob who is very happy to tell his life to Alice. Bob speaking is data you download. In a normal conversation, Bob will talk and will hear some sounds from Alice who acknowledge what Bob is saying. If Alice stops or shut her microphone, Bob may ask if Alice is still listening and will wait for an answer. When Alice is making a sound (like "hmmhm or yes"), this is an acknowledgement for Bob to continue. Literally, Bob is sending a voice stream to Alice who is sending ACK (acknowledgement short name) packets to Bob so he can continue.

This is exactly where we can control bandwidth, if we reduce the bandwidth used by ACK packets for a download, we can reduce the given download. If we can allow multiple systems to fairly send their share of ACK, they should have a fair share of the downloaded data.

What's even more important is that you absolutely don't use all the upload bandwidth with ACK packets to reach your maximum download bandwidth. We will have to separate ACK from uploaded data so we don't limit file upload or similar flows.

Setup §

For the setup I used a laptop with two network cards, one was connected to the ISP box and the other was on the LAN side. I've enabled a DHCP server on the OpenBSD router to automatically give IP addresses and gateway and name servers addresses to devices on the network.

Basically, you can just plug an equivalent router on your current LAN, disable DHCP on your ISP router and enable DHCP on your OpenBSD system using a different subnet, both subnets will be available on the network but for tests it requires little changes, when you want to switch from a router to another by default, toggle the DHCP service on both and renew DHCP leases on your devices. This is extremely easy.


  +---------+
  |  ISP    |
  |  router |
  +---------+
       |
       |
       | re0
  +---------+
  | OpenBSD |
  | router  |
  +---------+
       | em0
       | 
       |
  +---------+
  | network |
  | switch  |
  +---------+

Configuration explained §

Line by line §

I'll explain first all the config lines from my /etc/pf.conf file, and later in this article you will find a block with the complete rules set.

The following lines are default and can be kept as-is except if you want to filter what's going in or out, but it's another topic as we only want to apply queues. Filtering would be as usual.

set skip on lo

block return	# block stateless traffic
pass		# establish keep-state

This is where it get interesting. The upstream router is accessed through the interface re0, so we create a queue of the speed of the link of that interface, which is 1 Gb/s. pf.conf syntax requires to use bits per second (b/s or bps) and not bytes per second (Bps or B/s) which can be misleading.

queue std on re0 bandwidth 1G

Then, we create a queue that inherits from the parent created before, this represent the whole upload bandwidth to reach the Internet. We will make all the traffic reaching the Internet to go through this queue.

I've set a bandwidth of 900K with a max of 900K, this mean, that this queue can't let pass more than 900 kilo bits per second (which represent 900/8 = 112.5 kB/s or kilo Bytes per second). This is the extreme maximum my Internet access allows me.

	queue internet parent std bandwidth 900K max 900K

The following lines are all sub queues to divide the upload usage, we want to have a separate queue for DNS request which must not be delayed to keep responsiveness, but also voip or VPN queues to guarantee a minimum available for the users.

The web queue is the one which is likely to pass the most data, if you upload a file through a website, it will pass through the web queue. The unknown queue is the outgoing traffic that is not known, it's up to you to put a maximum or not.

Finally, the ackp queue that is split into two other queues, it's the most important part of the setup.

The "bandwidth xxxK" values should sum up to something around the 900K defined as a maximum in the parent, this only mean we target to keep this amount for this queue, this doesn't enforce a minimum or a maximum which can be defined with min and max keywords.

As explained earlier, we can control the downloading speed by regulating the sent ACK packets, all ACK will go through the queues ack_web and ack.

ack_web is a queue dedicated for http/https downloads and the other ack queue is used for other protocol, I preferred to divide it in two so other protocol will have a bit more room for themselves to counterbalance a huge http download (Steam game platform like to make things hard on this topic by making downloads to simultaneous server for maximum bandwidth usage).

The two ack queues accumulated can't get over the parent queue set as 406K here. Finding the correct value is empirical, I'll explain later.

All these queues created will allow each queue to guarantee a minimum from the router point of view, roughly said per protocol here. Unfortunately, this won't guarantee computers on the network will have a fair share of the queues! This is a crucial understanding I lacked at first when trying to do this a few years ago. The solution is to use the "flow" scheduler by using the flow keyword in the queue, this will give some slot to every session on the network, guarantying (at least theoretically) every session have the same time passed to send data.

I used "flows" only for ACK, it proved to work perfectly fine for me as it's the most critical part but in fact, it could be applied to every leaf queues.

		queue web      parent internet bandwidth 220K qlimit 100
		queue dns      parent internet bandwidth   5K
		queue unknown  parent internet bandwidth 150K min 100K qlimit 150 default
                queue vpn      parent internet bandwidth 150K min 200K qlimit 100
                queue voip     parent internet bandwidth 150K min 150K
                queue ping     parent internet bandwidth  10K min  10K
                
		queue ackp     parent internet bandwidth 200K max 406K
			queue ack_web parent ackp bandwidth 200K flows 256
			queue ack     parent ackp bandwidth 200K flows 256

Because packets aren't magically assigned to queues, we need some match rules for the job. You may notice the notation with parenthesis, this mean the second member of the parenthesis is the queue dedicated for ACK packets.

The VOIP queuing is done a bit wide, it seems Microsoft Teams and Discord VOIP goes through these port ranges, it worked fine from my experience but may depend of protocols.

match proto tcp from em0:network to any queue (unknown,ack)
match proto tcp from em0:network to any port { 80 443 8008 8080 } queue (web,ack_web)
match proto tcp from em0:network to any port { 53 } queue (dns,ack)
match proto udp from em0:network to any port { 53 } queue dns

# VPN (wireguard, ssh, openvpn)
match proto udp from em0:network to any port { 4443 1194 } queue vpn
match proto tcp from em0:network to any port { 1194 22 } queue (vpn,ack)

# voip (teams)
match proto tcp from em0:network to any port { 3479 50000:50060 } queue voip
match proto udp from em0:network to any port { 3479 50000:50060 } queue voip

# keep some bandwidth for ping packets
match proto icmp from em0:network to any queue ping

Simple rule to enable NAT so devices from the LAN network can reach the Internet.

# NAT to the outside
pass out on egress from !(egress:network) nat-to (egress)

Default OpenBSD rules that can be kept here.

# By default, do not permit remote connections to X11
block return in on ! lo0 proto tcp to port 6000:6010

# Port build user does not need network
block return out log proto {tcp udp} user _pbuild

How to choose values §

In the previous section I used absolute values, like 900K or even 406K. A simple way to define them is to upload a big file to the Internet and check the upload rate, I use bwm-ng but vnstat or even netstat (with the correct combination of flags) could work, see your average bandwidth over 10 or 20 seconds while transferring, and use that value as a maximum in BITS as a maximum for the internet queue.

As for the ACK queue, it's a bit more tricky and you may tweak it a lot, this is a balance between full download mode or conservative download speed. I've lost a bit of download rate for the benefit of keeping room for more overall responsiveness. Like previously, monitor your upload rate when you download a big file (or even multiples files to be sure to fill your download link) and you will see how much will be used for ACK. It will certainly be a few try and guesses before you get the perfect value, too low and the maximum download rate will be reduced, and too high and your link will be filled entirely when downloading.

Full configuration §

set skip on lo

block return	# block stateless traffic
pass		# establish keep-state

queue std on re0 bandwidth 1G
	queue internet parent std bandwidth 900K min 900K max 900K
		queue web  parent internet bandwidth 220K qlimit 100
		queue dns  parent internet bandwidth   5K
		queue unknown  parent internet bandwidth 150K min 100K qlimit 120 default
                queue vpn  parent internet bandwidth 150K min 200K qlimit 100
                queue voip parent internet bandwidth 150K min 150K
                queue ping parent internet bandwidth 10K min 10K
		queue ackp parent internet bandwidth 200K max 406K
			queue ack_web parent ackp bandwidth 200K flows 256
			queue ack     parent ackp bandwidth 200K flows 256

match proto tcp from em0:network to any queue (unknown,ack)
match proto tcp from em0:network to any port { 80 443 8008 8080 } queue (web,ack_web)
match proto tcp from em0:network to any port { 53 } queue (dns,ack)
match proto udp from em0:network to any port { 53 } queue dns

# VPN (ssh, wireguard, openvpn)
match proto udp from em0:network to any port { 4443 1194 } queue vpn
match proto tcp from em0:network to any port { 1194 22 } queue (vpn,ack)

# voip (teams)
match proto tcp from em0:network to any port { 3479 50000:50060 } queue voip
match proto udp from em0:network to any port { 3479 50000:50060 } queue voip

# ICMP
match proto icmp from em0:network to any queue ping

# NAT
pass out on egress from !(egress:network) nat-to (egress)

# default OpenBSD rules
# By default, do not permit remote connections to X11
block return in on ! lo0 proto tcp to port 6000:6010

# Port build user does not need network
block return out log proto {tcp udp} user _pbuild

How to monitor §

There is an excellent tool to monitor the queues in OpenBSD which is systat in its queue view. Simply call it with "systat queue", you can define the refresh rate by pressing "s" and a number. If you see packets being dropped in a queue, you can try to increase the qlimit of the queue which is the amount of packets kept in the queue and delayed (it's a FIFO) before dropping them. The default qlimit is 50 and may be too low.

systat man page anchored to the queues parameter

Conclusion §

I've spent a week scrutinizing pf.conf manual and doing many tests with many hardware until I understand that ACK were the key and that the flow queuing mode was what I was looking for. As a result, my network is much more responsive and still usable even when someone/some device is using the network without any kind of limit.

The setup can appear a bit complicated but in the end it's only a few pf.conf lines and using the correct values for your internet access. I chose to make a lot of queues, but simply separating ack from the default queue may be enough.