# Trying to understand HTB subclasses rate

I'm currently studying the opportunity of using an Ubuntu server to provide QoS instead of my consumer-class router. I've read a lot of resources about tc and HTB queuing disciplines - which look to be the one I need for my QoS needs, and even if it mostly seems clear now, there is still something that bugs me with subclasses rate.

Let's take a look at this sample configuration found as an answer to this question :

tc class add dev eth0 parent 1: classid 1:1 htb rate 90kbps ceil 90kbps
tc class add dev eth0 parent 1:1 classid 1:10 htb rate 30kbps ceil 60kbps
tc class add dev eth0 parent 1:1 classid 1:11 htb rate 30kbps ceil 60kbps
tc class add dev eth0 parent 1:1 classid 1:12 htb rate 30kbps ceil 60kbps

This is very straightforward when you know how tc works : the three subclasses are all guaranteed a bandwidth of 30kbps but are allowed to borrow 30 more kbps of available bandwidth from their parent class (30 + 30 = 60kbps ceil).

This example makes sense to me. The total bandwidth of the parent class is 90kbps, and each of the three subclasses are guaranteed 30kbps - 3 x 30kbps = 90kbps.

Now, let's take a look at this other example from this tutorial :

# tc class add dev eth0 parent 1: classid 1:1 htb rate 6mbit burst 15k
# tc class add dev eth0 parent 1:1 classid 1:10 htb rate 5mbit burst 15k
# tc class add dev eth0 parent 1:1 classid 1:20 htb rate 3mbit ceil 6mbit burst 15k
# tc class add dev eth0 parent 1:1 classid 1:30 htb rate 1kbit ceil 6mbit burst 15k

That's where things are getting confusing to me. The first subclass is guaranteed a bandwidth of 5mbit while the second one is guaranteed of bandwidth of 3mbit. But the parent class only have a bandwidth of 6mbit !

What is the purpose of such a rule ? None of the subclasses will ever be able to obtain its guaranteed bandwidth.

Even more confusing is the conclusion of the tutorial :

HTB certainly looks wonderful – if 10: and 20: both have their guaranteed bandwidth, and more is left to divide, they borrow in a 5:3 ratio, just as you would expect.

How could both classes have their guaranteed bandwidth and their parent still have bandwidth to borrow ?

Without a doubt, there is something that I'm missing here. It could be the tutorial that is flawed, but I found a lot of other samples with the same kind of confusing setup - here for instance :

/sbin/tc class add dev eth3 parent 1: classid 1:1 htb rate 2000kbit
/sbin/tc class add dev eth3 parent 1:1 classid 1:10 htb prio 1 rate 1500kbit ceil 1950kbit
/sbin/tc class add dev eth3 parent 1:1 classid 1:20 htb prio 2 rate 500kbit ceil 1600kbit

With this configuration, in which case could the 1:20 class be able to borrow bandwidth to the parent class (it is allowed to borrow up to 1600kbit) since the guaranteed bandwidth of its sibling is 1500kbit - and 1500kbit of its sibling plus 500kbit of its own guaranteed bandwidth already match the parent 2000kbit bandwidth.

Can anyone clarify the situation ?