[LARTC] simple TOS based setup vs more complex ones

[Martin A. Brown]

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Hello Gustavo,

 : After reading section 9 of LARTC it seemed to me that a pure TOS 
 : based QoS setup with be sufficient for a small newtork. 
 : Interactive packets could have the highest priority, second 
 : highest for DNS and small HTTP packets and lowest prio for all 
 : others.
 : 
 : The advantage is that, the setup would be simply a couple of 
 : iptables lines, because the default pfifo_fast qdisc already 
 : implements priorities.

In your proposed system, is still possible for a flood of DNS 
queries to cause queue depths upstream (and queue depths translate 
directly to queue backups and delays).

 : For this case, what is the recommended way to limit the outgoing 
 : rate to ensure that nothing is queued on the modem?

The answer depends on what you are trying to do.  Consider HTB 
and/or HFSC.  Although you might find that TBF is sufficient, you 
are already talking about ToS, so TBF probably won't cut the 
mustard in your situation.

 : Can this be done with pfifo_fast?

Not really.  Although, the actual qdisc proposed is different, 
please see this recent exchange [0] about prio qdisc.

If you are using a work-conserving qdisc (i.e., a qdisc that 
performs no shaping), you'll not really be able to guarantee 
anything about the quality of traffic from one point to another.  
In order to offer some sort of guarantees on any link, your device 
must be the bottleneck.  This requires shaping or, at least, some 
sort of non-work-conserving qdisc.

Good luck,

- -Martin

 [0] http://mailman.ds9a.nl/pipermail/lartc/2006q2/019130.html
     http://mailman.ds9a.nl/pipermail/lartc/2006q2/019138.html
     http://mailman.ds9a.nl/pipermail/lartc/2006q2/019143.html
     http://mailman.ds9a.nl/pipermail/lartc/2006q2/019158.html

- -- 
Martin A. Brown
http://linux-ip.net/
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