On Dec 10, 2007, at 11:35 PM, rihad wrote:
Pipes and queues are two different things; a pipe simulates a
network link, and a queue is used to hold packets which are
backlogged because they are arriving faster than the outbound link
(ie, a pipe) can transmit them.
So you mean queues are only used on the sending side? (the box is
acting as a router) and that their purpose is to diminish packet
loss rate due to network congestion? Now I get it, thanks.
Queues are used to buffer packets before sending. For a router, this
affects traffic in both directions, but it's normally the case that
that Internet<->router link is slower than the router<->LAN link.
What if I set it to 10000 slots, why should it wait to fill the
queue before starting to send stuff out the network interface?
Well, it *doesn't* wait for the queue to be filled before starting
to send stuff out the network.
This ipfw manpage section was the reason I asked (sorry for the
formatting). What's with the "queuing delay" part? I'm totally
confused.
queue {slots | sizeKbytes}
Queue size, in slots or KBytes. Default value is 50
slots, which
is the typical queue size for Ethernet devices. Note that
for slow
speed links you should keep the queue size short or your
traffic
might be affected by a significant queueing delay. E.g.,
50 max-
sized ethernet packets (1500 bytes) mean 600Kbit or 20s of
queue on
a 30Kbit/s pipe. Even worse effects can result if you get
packets
from an interface with a much larger MTU, e.g. the
loopback inter-
face with its 16KB packets.
The issue is that if you have a really slow upstream link, you can end
up queuing many seconds worth of traffic using the default queue
size-- depending on the priorities, you might have traffic being
buffers so long that it starts breaking connections or causing
needless TCP retries...
--
-Chuck
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