HTB(8) Linux HTB(8)
NAME
HTB - Hierarchy Token Bucket
SYNOPSIS
tc qdisc ... dev dev ( parent classid | root) [ handle major: ] htb [
default minor-id ] [ r2q divisor ] [ offload ]
tc class ... dev dev parent major:[minor] [ classid major:minor ] htb
rate rate [ ceil rate ] burst bytes [ cburst bytes ] [ prio priority ]
[ quantum bytes ]
DESCRIPTION
HTB is meant as a more understandable and intuitive replacement for the
CBQ qdisc in Linux. Both CBQ and HTB help you to control the use of the
outbound bandwidth on a given link. Both allow you to use one physical
link to simulate several slower links and to send different kinds of
traffic on different simulated links. In both cases, you have to spec-
ify how to divide the physical link into simulated links and how to de-
cide which simulated link to use for a given packet to be sent.
Unlike CBQ, HTB shapes traffic based on the Token Bucket Filter algo-
rithm which does not depend on interface characteristics and so does
not need to know the underlying bandwidth of the outgoing interface.
SHAPING ALGORITHM
Shaping works as documented in tc-tbf (8).
CLASSIFICATION
Within the one HTB instance many classes may exist. Each of these
classes contains another qdisc, by default tc-pfifo(8).
When enqueueing a packet, HTB starts at the root and uses various meth-
ods to determine which class should receive the data.
In the absence of uncommon configuration options, the process is rather
easy. At each node we look for an instruction, and then go to the
class the instruction refers us to. If the class found is a barren
leaf-node (without children), we enqueue the packet there. If it is not
yet a leaf node, we do the whole thing over again starting from that
node.
The following actions are performed, in order at each node we visit,
until one sends us to another node, or terminates the process.
(i) Consult filters attached to the class. If sent to a leafnode, we
are done. Otherwise, restart.
(ii) If none of the above returned with an instruction, enqueue at
this node.
This algorithm makes sure that a packet always ends up somewhere, even
while you are busy building your configuration.
LINK SHARING ALGORITHM
FIXME
QDISC
The root of a HTB qdisc class tree has the following parameters:
parent major:minor | root
This mandatory parameter determines the place of the HTB in-
stance, either at the root of an interface or within an existing
class.
handle major:
Like all other qdiscs, the HTB can be assigned a handle. Should
consist only of a major number, followed by a colon. Optional,
but very useful if classes will be generated within this qdisc.
default minor-id
Unclassified traffic gets sent to the class with this minor-id.
r2q divisor
Divisor used to calculate quantum values for classes. Classes
divide rate by this number. Default value is 10.
offload
Offload the HTB algorithm to hardware (requires driver and de-
vice support).
CLASSES
Classes have a host of parameters to configure their operation.
parent major:minor
Place of this class within the hierarchy. If attached directly
to a qdisc and not to another class, minor can be omitted.
Mandatory.
classid major:minor
Like qdiscs, classes can be named. The major number must be
equal to the major number of the qdisc to which it belongs. Op-
tional, but needed if this class is going to have children.
prio priority
In the round-robin process, classes with the lowest priority
field are tried for packets first.
rate rate
Maximum rate this class and all its children are guaranteed.
Mandatory.
ceil rate
Maximum rate at which a class can send, if its parent has band-
width to spare. Defaults to the configured rate, which implies
no borrowing
burst bytes
Amount of bytes that can be burst at ceil speed, in excess of
the configured rate. Should be at least as high as the highest
burst of all children.
cburst bytes
Amount of bytes that can be burst at 'infinite' speed, in other
words, as fast as the interface can transmit them. For perfect
evening out, should be equal to at most one average packet.
Should be at least as high as the highest cburst of all chil-
dren.
quantum bytes
Number of bytes to serve from this class before the scheduler
moves to the next class. Default value is rate divided by the
qdisc r2q parameter. If specified, r2q is ignored.
NOTES
Due to Unix timing constraints, the maximum ceil rate is not infinite
and may in fact be quite low. On Intel, there are 100 timer events per
second, the maximum rate is that rate at which 'burst' bytes are sent
each timer tick. From this, the minimum burst size for a specified
rate can be calculated. For i386, a 10mbit rate requires a 12 kilobyte
burst as 100*12kb*8 equals 10mbit.
SEE ALSO
tc(8)
HTB website: http://luxik.cdi.cz/~devik/qos/htb/
AUTHOR
Martin Devera <devik@cdi.cz>. This manpage maintained by bert hubert
<ahu@ds9a.nl>
iproute2 10 January 2002 HTB(8)
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