# Prioritize network traffic by IP in Linux

## December 28, 2020

I have a server on a residental internet connection with relatively limited upload bandwidth. That server hosts this website along with several services for personal use including:

If I’m listening to an audio book or streaming some other media to myself, I certainly don’t want to buffer if someone else wants to download a large file or its time to ship regularly scheduled backups off to the Google Panopticon (encrypted, of course). Fortunately, there is built-in functionality in Linux that allows upload packets to be prioritized and reserve some fraction of the available bandwidth. This functionality can also limit the total upload rate from a particular linux machine such that there is always bandwidth left over for other machines on the same network. Much more complicated behavior is possible, but I decided to stick with something simple for now.

## Traffic shaping

A machine only has control over the packets it transmits. I.E. there is no shaping of incomming traffic, but outgoing traffic can be controlled as finely as you’re willing to specify rules. The TrafficControl (tc) utility is the standard tool for modifying these rules, and it has absolutely bonkers syntax. Each network adapter on a Linux machine can have a “queuing discipline” assigned to it that orders outgoing packets for transmission at the kernel level. By default this is always set to a first-in-first-out queue (pfifo_fast) that takes no configuration, and simple options like a stochastic fairness queuing (SFQ) exist to equally share bandwidth between all outgoing connections.

For more fine-grained control, queuing disciplines like hierarchy token bucket (HTB) exist to assign outgoing packets to several different classes, with each class having rules for minimum guaranteed bandwidth, maximum potential bandwidth, and in what order spare bandwidth is allocated. This is precisely what I want to do. The one caveat that I couldn’t find a well documented solution for is how to prioritize packets to a particular outbound IP (the IP I stream to), so this post documents the solution I ended up with in case it is useful to others. In short, after setting up an HTB queuing discipline with appropriate rules, the most straightforward solution I arrived at was to mark packets with iptables rules and have those packets be assigned to a particular HTB class, while all other packets go to a default HTB class.

## HTB classes

In this section assume I have one network adapter named eth1 for which I want a maximum of 11mbit(/s) upload bandwidth. The following will create a HTB queue discipline on eth1 named 1::

tc qdisc add dev eth1 root handle 1: htb default 20


It is unclear to me why the name 1: is used here, but perhaps someone willing to read more of the (complicated) tc documentation will understand. Conceptually, this is the “root” of a tree-like structure below which HTB classes will be defined. Importantly, this specifies that some leaf of this tree structure with the name 1:20 (1: + 20 evidently) will be the default class for unclassified packets.

The next step is to define the “child class” of this HTB “root” that allocates a certain amount of bandwidth (11mbit) on the interface:

tc class add dev eth1 parent 1: classid 1:1 htb rate 11mbit burst 15k


Note this class is named 1:1 with the parent 1: created in the previous command. Here, burst refers to the amount of data transmitted before potentially switching to a different class. If it is too small, there will be a lot of overhead in the traffic shaping logic, and bandwidth will suffer. If it is too large, traffic shaping will be discontinuous, and latency may be erratic. It should be as large as the largest child burst.

I’ll allocate minimum 8mbit bandwidth to my prioritized traffic (plenty for a 720p HD video stream), which can potentially use up to 11mbit (all) available bandwidth.

tc class add dev eth1 parent 1:1 classid 1:10 htb rate 8mbit ceil 11mbit burst 15k prio 0


The prio 0 part signifies this (lower is first) rule will receive additional bandwidth, up to its ceiling, before other rules (with larger prio).

I’ll allocate the other 3mbit bandwidth as the minimum for all other traffic, potentially using up to 10mbit to never max out the available upload bandwidth.

tc class add dev eth1 parent 1:1 classid 1:20 htb rate 3mbit ceil 10mbit burst 15k prio 1


Note the class is named 1:20, which was the default given in the queue discipline.

The final step is a quality-of-life addition and likely not strictly required, but after packets are assigned to HTB classes, the default is simply another first-in-first-out queue. To make this a bit more balanced, a simple SFQ fairshare queue is added as a leaf to each class.

tc qdisc add dev eth1 parent 1:10 handle 10: sfq perturb 10
tc qdisc add dev eth1 parent 1:20 handle 20: sfq perturb 10


The perturb option relates to how frequently the fairshare parameters are recalculated. Like the burst arguments, this should be big enough but not too big.

Now, by default, all outgoing packets on the eth1 interface will be assigned to 1:20 with the bandwidth rules it implies. To assign packets to other rules, tc includes an overly-complex filter syntax that I simply can’t be bothered to learn for a one-off solution like this. Fortunately, the filter syntax can detect packets marked by iptables rules we’ll setup in the next section. This command will assign packets marked 0x1 (handle 1) to the higher-bandwidth 1:10 class:

tc filter add dev eth1 parent 1:0 protocol ip prio 1 handle 1 fw classid 1:10


I will take no questions on this filter syntax!

## Marking packets with iptables

The last step in this endeavor is to mark outgoing packets on eth1 with 0x1 if they should go to the high bandwidth class. This can be done with mangle table rules, and should be applied to any packets traversing the FORWARD chain (if you are running a router or have packets coming from a sandboxed Docker instance) or OUTPUT chain (for packets originating from a local process). The most straightforward way to accomplish this seems to be to create a new chain QOS that all outbound packets on eth1 going through FORWARD and OUTPUT are sent to.

iptables -t mangle -N QOS
iptables -t mangle -A FORWARD -o eth1 -j QOS
iptables -t mangle -A OUTPUT -o eth1 -j QOS


Then packets matching any standard iptables rule can be marked with 0x1. Here, I mark packets going to ${DEST_IP} such that these packets get guaranteed higher bandwidth. iptables -t mangle -A QOS -d${DEST_IP} -j MARK --set-mark 0x1


No more buffering for me!

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