[c-nsp] Hierarchical QoS Policies

[mejllistor at carlson.homeunix.net]

On Thu, Apr 07, 2005 at 05:50:21PM +0200, Andre Beck wrote:

> Looking closer at them, I realized
> that at 100kbps, a full size packet of 1500 bytes aka 12000 bits will
> have a serialization delay of 120ms, so if the shaper emulates the
> queueing behavior of a real interface up to and including the blocking
> of packets due to serialization delay of other packets that have just
> seized the interface, well, it would explain the RTT values I see.

that very much highlights the issues with qos and low bandwidths. if for
example 10 ms delay of priority traffic due to head of line blocking is
acceptable, the bandwidth must be at least
1500 * 8 (bits) / 10e-3 (s) = 1.2 Mbps!

a way to circumvent this on lines with low bandwidth is lfi
(link fragmentation and interleaving), which is available on fr and
ppp-lines.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_c/fqcprt6/qcflem.htm

with ethernet, it is not much you can do except trying to reduce the
max packet length. for tcp traffic, 'ip tcp adjust-mss' will do that.
for udp, there is no buttons...

> I have not found detailed documentation about the shaper and how it
> interacts with LLQ so far, any pointers are welcome.

there were a similar thread last december on this mailing list. rodney dunn
posted some explanations how shaping works.

https://puck.nether.net/pipermail/cisco-nsp/2004-December/015462.html

-- 

Regards, Per Carlson

According to all the latest reports, there was no truth in any 
of the earlier reports.