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2.2.- FIFO queuing discipline
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| FIFO is absolutely explained by Chuck Sumeria
[11] in a few sentences and a figure; let's see how: |
| First-in, first-out (FIFO) queuing is the
most basic queue scheduling discipline. In FIFO queuing, all packets are
treated equally by placing them into a single queue, and then servicing them
in the same order that they were placed into the queue. FIFO queuing is also
referred to as First-come, first-served (FCFS) queuing. (See Figure 2.2.1)
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| Easy. Isn't it? By the way, FIFO is the default
queuing discipline used by Linux and most of the routers around the world.
In case you don't specifiy any specific qdisc Linux assembles its
interfaces with this type of queue. Let's see now how we can setup a FIFO
queue on ethernet interface eth0 using tc; at the Linux command prompt just
write: |
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| The command is self-explanatory. 'tc' is the
utility; 'qdisc' for telling tc we are configuring a queue discipline (it
could be 'class' or 'filter' if we are configuring a class or a filter
respectively); 'add' because we are adding a new qdisc; 'dev eth0' for
adding the qdisc to the device or interface Ethernet eth0; 'root' because it
is a root qdisc (it doesn't apply to pfifo that is classless - only a root
qdisc exists - but required to normalize the command utilization); 'pfifo'
because our queue is a pfifo (packet-fifo) queue. Finally pfifo requires
only one parameter: just 'limit' to indicate the length of the queue (number
of packets that the queue can hold). Our pfifo queue is a 10 packets queue. |
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| After creating our queueing discipline we can
ask tc what we have configured; the following command: |
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| The command just says: show us what qdisc we
have on device eth0. tc responds by telling us that we have a pfifo qdisc
numbered as 8001: (this really means 8001:0) with a limit capacity of 10
packets. Qdiscs and components are numbered, or better yet identified by a
32 bits handler formed by a 16 bits major number and a 16 bits minor number.
The minor number is always zero for queues. When we added our pfifo queue we
didn't assign it any handler and then tc created one for it (8001:0). |
| FIFO has spread as the grass but it has
benefits and limitations. Chuck [11] exposes about this in a magisterial
approach: |
| FIFO queuing offers the following benefits:
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- For software-based routers, FIFO queuing places an extremely low
computational load on the system when compared with more elaborate queue
scheduling disciplines.
- The behavior of a FIFO queue is very predictable: packets
are not reordered and the maximum delay is determined by the maximum depth
of the queue.
- As long as the queue depth remains short, FIFO queuing provides
simple contention resolution for network resources without adding
significantly to the queuing delay experienced at each hop.
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| FIFO queuing also poses the following
limitations: |
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- A single FIFO queue does not allow routers to organize buffered
packets, and then service one class of traffic differently from other
classes of traffic.
- A single FIFO queue impacts all flows equally, because the mean
queuing delay for all flows increases as congestion increases. As a
result, FIFO queuing can result in increased delay, jitter, and loss for
realtime applications traversing a FIFO queue.
- During periods of congestion, FIFO queuing benefits UDP flows over
TCP flows. When experiencing packet loss due to congestion, TCPbased
applications reduce their transmission rate, but UDPbased applications
remain oblivious to packet loss and continue transmitting packets at their
usual rate. Because TCPbased applications slow their transmission rate to
adapt to changing network conditions, FIFO queuing can result in increased
delay, jitter, and a reduction in the amount of output bandwidth consumed
byTCP applications traversing the queue.
- A bursty flow can consume the entire buffer space of a FIFO queue,
and that causes all other flows to be denied service until after the burst
is serviced. This can result in increased delay, jitter, and loss for the
other wellbehaved TCP and UDP flows traversing the queue.
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| More clearly, impossible. Well, with all these
problems let's delete the pfifo queue from our device eth0: |
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| That brought to the english just means: delete
from device eth0 the root queue discipline. Okay. We are over with FIFO. Now
let's try with PRIO. |
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