[LARTC] Re: HFSC

[Nuutti Kotivuori <naked@iki.fi>]

Patrick McHardy wrote:
> This is currently all there is. If you have some specific questions,
> just ask (but please CC lartc). If anyone wants to write some
> documentation I'd be happy to help, but I don't have time for it
> myself.

I am not sure if the original poster has specific questions, but I
sure do.

I just recently got into this HFSC mess myself, so I'm a bit fuzzy in
all the terms and differences in implementation. I read the paper
(SIGCOM97) on HFSC and I think I understood most of it. But there are
some things in the implementation that I couldn't really realize.

I'll quote the usage here for reference:

,----
| Usage: ... hfsc [ rt SC ] [ ls SC ] [ ul SC ]
|  
| SC := [ [ m1 BPS ] [ d SEC ] m2 BPS
|  
|  m1 : slope of first segment
|  d  : x-coordinate of intersection
|  m2 : slope of second segment
`----

Okay, the SC parameters I think I understand rather well - they are
there to define the service curve itself. But the way hfsc takes three
optional parameters of service curves puzzles me.

I believe 'rt' referes to 'Real-Time Service Curve', 'ls' to 'Link
Sharing Service Curve' and 'ul' to 'Upper Limit Service Curve'. If I
understand correctly, the SIGCOM97 paper mentioned that the link
sharing selection need not be the same as the real time selection, but
in examples assumed for simplicity that they were. Also, from the
source I conclude that 'Upper Limit Service Curve' cannot be specified
without 'Link Sharing Service Curve'.

So, this, all in all, baffles me :-)

The possible combinations I can make here are:

  Real-Time
  Link-Sharing
  Real-Time, Link-Sharing
  Link-Sharing, Upper-Limit
  Real-Time, Link-Sharing, Upper-Limit

How do these behave? If I specify *only* the real-time curve, what is
used for the link-sharing part? Or does that mean that there is no
sharing? Or if I only specify the link-sharing curve, does that mean
that no specific deadlines are for packets, just that they are sent
based on the link-sharing model? And what actually is the upper-limit
service curve? I take it that it is some kind of a packet drop curve,
but I don't know how it would behave - nor why it would require the
link-sharing curve.

So, any pointers on these would be helpful, or if you manage to get
the time to explain it specifically.

I will probably cook up atleast an example script using HFSC for
normal QoS if I manage to understand how it works, perhaps even some
documentation.

-- Naked
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