The sfq queuing discipline
--------------------------

Parameters.

  quantum <QUANTUM>
    This is how many bytes a class gets to send before the
    scheduler starts sending from the next class with packets
    waiting.  Must be no smaller than the devices maximum
    transmission unit, as printed by the mtu option in "ip 
    link list".  It defaults to the devices maximum
    transmission unit.

  perturb <PERTURB>
    If present this extends the time between randomisations
    of the classifier.  If not specified the time between
    randomisations is 10msec.  This is the number of 10msec
    units to extend that time by.


Classes.  The sfq queuing discipline allocates each different
tcp, udp and esp connection to a class.  Other literature calls
these classes flows.  Sfq does not allow you to configure its
classes manually.

Scheduling.  The scheduler sends packets from each class in a
round robin fashion.  The maximum number of bytes each class
can send in a single round is set by the quantum parameter.

Policing.  Attempting to enqueuing a packet when the queue already
contains 128 packets will cause the packet to be dropped.

Rate limiting.  The sfq queuing discipline does not rate limit
traffic.

Classifier.  A hashing function is used to determine the class
for a connection, so each class may contain several connections.
This hashing function changes in a random way at regular intervals.
This interval configured by the perturb parameter.  Sfq does not
allow you to alter its classification system.

Comments.

- The sqf queuing discipline allocates bandwidth fairly to each
  of its classes.  As the classes roughly correspond to a single
  tcp or udp connection this means sfq ensures no single
  connection hogs the bandwidth of a device.  It does this very
  well, with (uncharacteristically for the traffic control engine)
  almost no setup and very little CPU overhead.

- As a side comment, sfq would be much more useful if it allowed
  you to configure what a class is.  Currently a class corresponds
  to a connection.  But imagine you are trying to allocate bandwidth
  to a class room of PC's.  The students would rapidly discover
  that they could get around this by creating many TCP connections.
  If you could configure sfq to use the senders IP, or perhaps their
  MAC address, the creative little darlings would be thwarted.  If
  you are trying to allocate bandwidth among departments a senders
  network might be a better way to classify a packet.