Well here's what we know so far. We want to use this diff serve QOS model and
because diff server lies on all of these different routers and switches working
together we're going to need a way to identify and classify our traffic and so
we are going to use marking or tagging of our packets. We have COS for that we
have DSCP for that as our primary mechanisms with IP precedence being a
legacy marking method but now what what are we going to do at this point WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
legacy marking method but now what what are we going to do at this point
because
what we have is we have these devices that are receiving tagged traffic so
again
maybe we have a packet here a voice over IP packet tagged with DSCP EF
expedited forwarding decimal value 46 and so that's coming into this router and
we know that we've got a link upstream to the next router this is the next
top interface and maybe this is a hundred meg connection and this is marked as WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
top interface and maybe this is a hundred meg connection and this is marked as
a
very high priority type of packet so what do we do what kinds of policies can
we
be deploying against this traffic well the one we often cite is the concept of
reservation and specifically we're talking about bandwidth reservation at
this point so we just mentioned we have a hundred meg there we might want to
reserve let's say one megabit per second for voice over IP so that'd be one WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
reserve let's say one megabit per second for voice over IP so that'd be one
percent of that link now that doesn't sound like much at first one percent one
megabit per second however keep in mind that voice over IP streams are usually
64 kilobits per second it depends on the codec the way that we are encoding our
analog voice communications into the digital world but usually we can think of
them as being about 64 kilobits per second so we could get about 10 to 15 WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
them as being about 64 kilobits per second so we could get about 10 to 15
calls on one megabit per second if this is a small branch office we don't have
a
whole lot of phones over here and so 10 to 15 simultaneous calls is probably
sufficient now if this is a major headquarters connection that goes to
another major headquarters then 10 to 15 might not be enough but naturally that
's
going to be a conversation depending on the scenario so either way we
understand
that we want to reserve some bandwidth we might also reserve bandwidth for WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that we want to reserve some bandwidth we might also reserve bandwidth for
other applications maybe we want to take 10 megabits per second and reserve
that
for video maybe that's not quite enough maybe it's plenty again depends on the
situation we could always go with a percentage maybe we say let's just
reserve 20% of the link that would give us 20 megabits per second on that 100
meg connection something along those lines so bandwidth reservation is great
but it's not enough we also need to consider the concept of traffic
prioritization so we need to be able to prioritize traffic this is the second WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
prioritization so we need to be able to prioritize traffic this is the second
part of our Qs conversations and the prioritization of traffic is going to
come down to a couple of different factors first of all the order in which
we send packets so order for transmitting packets here and we also have to
consider the order of magnitude or the likelihood you know the probability WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
consider the order of magnitude or the likelihood you know the probability
we want to say it we'll say the likelihood here of dropping packets so
let's look at these in a little bit more detail so we just said that we want to
reserve traffic and we maybe have reserved one megabit per second and
we're making the claim that that's not enough well that's not enough and here's
why what happens if we have this router and we have this interface and yeah we WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
why what happens if we have this router and we have this interface and yeah we
are sending out on a hundred megabit per second connection we want to reserve
that but what if we're just sitting here at about 25 out of the hundred megab
its
per second well we're not full at all and if we're not full that means that the
bandwidth reservation doesn't actually mean anything I mean voice over IP could
take 75 megabits per second if needed on this link because that's how much is
available so that's all well and good but at this point we have bulk traffic WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
available so that's all well and good but at this point we have bulk traffic
coming through then we have voice over IP and video traffic and this router is
going to view all of these traffic types as the same yeah if we were full on
the
bandwidth then we would make sure we keep some of that set aside so that
video traffic could get through and voice traffic could get through but at
this point we're saying that well if everything is fine we're just going to
send traffic in the order we received it well what if we have a lot of bulk WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
send traffic in the order we received it well what if we have a lot of bulk
traffic that's stacked up when I say bulk traffic I mean transferring data
downloading from the internet and internet stream these types of traffic
flows they are not based on real-time applications so this is where we
said before that if we had you know an increase in 500 milliseconds for one of
these types of applications I mean nobody's going to notice if there's a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
these types of applications I mean nobody's going to notice if there's a
half a second delay on the amount of time it takes me to download a file from
the internet but if we have an increase in 500 of 500 milliseconds rather for
voice and video calls that lag is going to be very noticeable and so these are
considered to be real-time applications and there are other real-time
applications
that exist as well so this is where we care about the order of transmission if
I've got a bunch of this bulk packets stacked up and a video packet shows up or WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
I've got a bunch of this bulk packets stacked up and a video packet shows up or
a voice packet shows up I would want to push that to the front of the line and
have it get sent first so the order of the transmitted packets matters
furthermore if this router gets so overwhelmed that it now needs to start
dropping traffic well that can actually happen as well so we might just be
maxed out on the ingress we might be maxed out an internal buffer there's a lot
of ways we can get overwhelmed as a network device and so we have to choose WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
of ways we can get overwhelmed as a network device and so we have to choose
which traffic to drop in fact so often when it comes to talking about quality
of service sometimes it makes it feel like what we're saying is that we
can make sure that we don't ever drop any traffic with QOS and that's
absolutely not the case the better way to describe QOS is to say that we're
going to choose which traffic to drop we're going to choose the order of our
packets I can't magically make this hundred megabits per second into 200 WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
packets I can't magically make this hundred megabits per second into 200
megabits per second it just doesn't work that way sure we could use some
compression techniques and things like that to maximize our efficiency but we
don't really use those anymore because we've got a lot of bandwidth we just
have
to make sure we're making the most of it I'm not talking about T1s and ISDN
lines
anymore which are you know way way smaller and as far as bandwidth is
concerned so if we're going to choose what type of traffic to drop well now
that comes back to our ability to prioritize different traffic flows so WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that comes back to our ability to prioritize different traffic flows so
all of this is going to come down to a QOS technique that's known as congestion
management congestion management is ultimately taking the concept of Qs and
making it so that we sort our traffic into different Qs and then we schedule
traffic to be sent out of those respective Qs so we're going to be
diving into the details of that but just like we had with classification and
marking we do have a little bit more of a formal way of talking about queuing WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
marking we do have a little bit more of a formal way of talking about queuing
and
scheduling and that in the QOS world is congestion management so let's go ahead
and dive into that whole new concept.