[e2e] how far it queues?

Alexandre L. Grojsgold algold at rnp.br
Wed Oct 2 08:13:52 PDT 2002


As a curious net admin, and having read a lot about QoS, I got engaged in
searching more detailed information on what really goes on inside the
boxes, trying to figure out how far can QoS mechanisms really improve the
way packets are forwarded. Or how much QoS features actually and
noticeably make life better to privileged traffic.

Up to my undestanding, give that interface speeds are mostly fixed and
routers can't do much to push bits faster down the cables, and given that
most of present days routers easily operate at wire speed, the only thing
a router can do to increase the quality of service figures of a given flow
is to change the way its individual packets are queued at the output
interface they are supposed to go.

It means that the influence of QoS processing over a flow will be more
noticed for long queues than for short ones. It means also that if queue
length is zero most of the time (meaning that the interface is idle or
just starting to transmit a packet), QoS will do little - most of the
time. And that for hi speed interfaces, where a typical 1500 byte packet
will be completly sent in a sub-milisecond time frame, passing around a 3
or 4 packet queue is not of much help.

Again, trying to figure out how things happen in real life, with real
traffic, to my bigest surprise I failed miserably in finding papers and
articles that showed queue behavior and mean queue lengths within internet

Maybe I used the wrong search engines, maybe I am asking myself the wrong
question :(

Any hints? What I am trying to find out is the mean length of an output
queue in a typical router, within a typical (backbone) mesh, in normal
non-congested operation.


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