9.4. Terminology
To properly understand more complicated configurations it is necessary to
explain a few concepts first. Because of the complexity and he relative
youth of the subject, a lot of different words are used when people in fact
mean the same thing.
The following is loosely based on
draft-ietf-diffserv-model-06.txt,
An Informal Management Model for Diffserv Routers.
It can currently be found at
http://www.ietf.org/internet-drafts/draft-ietf-diffserv-model-06.txt.
Read it for the strict definitions of the terms used.
- Queueing Discipline
An algorithm that manages the queue of a device, either incoming (ingress)
or outgoing (egress).
- Classless qdisc
A qdisc with no configurable internal subdivisions.
- Classful qdisc
A classful qdisc contains multiple classes. Each of these classes contains a
further qdisc, which may again be classful, but need not be. According to
the strict definition, pfifo_fast *is* classful, because it contains three
bands which are, in fact, classes. However, from the user's configuration
perspective, it is classless as the classes can't be touched with the tc
tool.
- Classes
A classful qdisc may have many classes, which each are internal to the
qdisc. Each of these classes may contain a real qdisc.
- Classifier
Each classful qdisc needs to determine to which class it needs to send a
packet. This is done using the classifier.
- Filter
Classification can be performed using filters. A filter contains a number of
conditions which if matched, make the filter match.
- Scheduling
A qdisc may, with the help of a classifier, decide that some packets need to
go out earlier than others. This process is called Scheduling, and is
performed for example by the pfifo_fast qdisc mentioned earlier. Scheduling
is also called 'reordering', but this is confusing.
- Shaping
The process of delaying packets before they go out to make traffic confirm
to a configured maximum rate. Shaping is performed on egress. Colloquially,
dropping packets to slow traffic down is also often called Shaping.
- Policing
Delaying or dropping packets in order to make traffic stay below a
configured bandwidth. In Linux, policing can only drop a packet and not
delay it - there is no 'ingress queue'.
- Work-Conserving
A work-conserving qdisc always delivers a packet if one is available. In
other words, it never delays a packet if the network adaptor is ready to
send one (in the case of an egress qdisc).
- non-Work-Conserving
Some queues, like for example the Token Bucket Filter, may need to hold on
to a packet for a certain time in order to limit the bandwidth. This means
that they sometimes refuse to give up a packet, even though they have one
available.
Now that we have our terminology straight, let's see where all these things
are.
Userspace programs
^
|
+---------------+-----------------------------------------+
| Y |
| -------> IP Stack |
| | | |
| | Y |
| | Y |
| ^ | |
| | / ----------> Forwarding -> |
| ^ / | |
| |/ Y |
| | | |
| ^ Y /-qdisc1-\ |
| | Egress /--qdisc2--\ |
--->->Ingress Classifier ---qdisc3---- | ->
| Qdisc \__qdisc4__/ |
| \-qdiscN_/ |
| |
+----------------------------------------------------------+ |
Thanks to Jamal Hadi Salim for this ASCII representation.
The big block represents the kernel. The leftmost arrow represents traffic
entering your machine from the network. It is then fed to the Ingress
Qdisc which may apply Filters to a packet, and decide to drop it. This
is called 'Policing'.
This happens at a very early stage, before it has seen a lot of the kernel.
It is therefore a very good place to drop traffic very early, without
consuming a lot of CPU power.
If the packet is allowed to continue, it may be destined for a local
application, in which case it enters the IP stack in order to be processed,
and handed over to a userspace program. The packet may also be forwarded
without entering an application, in which case it is destined for egress.
Userspace programs may also deliver data, which is then examined and
forwarded to the Egress Classifier.
There it is investigated and enqueued to any of a number of qdiscs. In the
unconfigured default case, there is only one egress qdisc installed, the
pfifo_fast, which always receives the packet. This is called 'enqueueing'.
The packet now sits in the qdisc, waiting for the kernel to ask for
it for transmission over the network interface. This is called 'dequeueing'.
This picture also holds in case there is only one network adaptor - the
arrows entering and leaving the kernel should not be taken too literally.
Each network adaptor has both ingress and egress hooks.
