i'm sure you've heard of quality of service or class of service before
you've probably heard that it makes your network faster that it drops some
traffic and allows others that it removes network congestion or
that you need it for voice traffic qos can do a lot of good things but
there's also a lot of misunderstandings about it
to address this we're going to have a high level vendor neutral look
at qos and how it works and while we're at it
we'll clear up some of those misunderstandings
so let's start with the basics qos or quality of service
is not one single technology instead it's a group of technologies and
techniques that work together it becomes important when you have
limited bandwidth and one of your links is full
imagine that you have a one gigabit link and then you try to send 1.1 gigabit of
data down it that's more data than the link is
capable of transporting so while most traffic will make it some
will have to be dropped in a case like this quality of service
can be used to decide which traffic will be allowed to pass
and which traffic is selected to be dropped
a simple definition then is that qos prioritizes
more important traffic over less important traffic
in times of congestion more important traffic will be allowed to pass while
less important traffic will drop this is not
a perfect definition but it is a good place to start from
now let's clear up two misconceptions around qos that you might have heard
first that it makes the network faster this simply isn't true the only way to
make the network faster is to add more links or faster links or better
equipment the second is that qos prevents network
congestion this is also not true the only way to
truly prevent congestion is to have so much network capacity that
you will not fill it. as you know that's a difficult goal to
reach so why do these misconceptions exist then?
when your links are full of traffic qos helps us to manage that traffic
by managing the traffic better qos helps certain applications
to perform better however the link is still full
so while qos helps one application to perform better
it may be at the expense of another application
this traffic management that qos performs may give the illusion of a
faster network or less congestion let's look at an example to
show what I mean by this here we have a main office and a branch
office they are connected by a 100 meg wan link
which is reaching capacity this link carries all types of traffic
including web browsing and phone calls when the link is over capacity some
traffic needs to drop there's no way around that there is a
limit to how much data this link can carry
so without qos it's first in first served
and there's no more logic to that we will probably find then that
voice traffic and web browsing traffic are both affected
and it might even make phone calls drop out
phone calls are usually more important so we can configure qos to prioritize
voice traffic in this way the voice traffic will
always pass through while the less important traffic will be
left to drop so notice that this still drops some
traffic qos does not magically make your links
better faster or anything like that neither
does it make the congestion go away but it does allow the more important or
more sensitive traffic to go through first
as we dig deeper into qos you will find that it's important to understand the
characteristics of different traffic types
the way these are managed on the network will affect each application
of course we all know about bandwidth some traffic types like video and file
transfers use a lot of bandwidth while other
traffic types use very little but aside from bandwidth there are three
other characteristics to consider these are loss latency and jitter
loss is the easy one this is simply where traffic does not reach its
destination there may be several places along the
network where traffic can be lost sometimes this is intentional such as
when we have a firewall or acl to drop traffic for security reasons
but we're more concerned with how an application will work if traffic is lost
accidentally this includes a link that is full or is
down or where something has a hardware fault
think about how traffic loss is handled tcp traffic needs to be acknowledged
so missing traffic will need to be resent
udp traffic however will just move on with life
however some udp-based applications may ask for missing data to be reset anyway
if traffic needs to be resent sometimes this results in more traffic being
generated
next we have latency this is the amount of time it takes a packet to travel from
one endpoint to another these endpoints may
be directly connected or there may be devices in between
a similar term round trip time or rtt describes the time taken for the packet
to reach the other end and for the response to come back
latency and bandwidth are often confused think of it like a water pipe
the bandwidth is how wide your pipe is which controls how much water you can
fit into the pipe at one time the latency is the length of the pipe
and that affects how long it takes for the water to travel from one end to the
other so more bandwidth does not always mean a
lower latency many applications are very tolerant to
latency such as ones that use http others particularly real-time traffic
like voice and video care quite a lot about latency think of
streaming video which needs a steady stream of traffic
to deliver frames on time to make the video smooth if the packets
come in late a video may have to pause while it's
buffering bi-directional real-time traffic is the
worst hit by bad latency a video call for example which is a
two-way stream of video as it's real time we don't even have the
luxury of buffering now the interesting one is jitter this
is closely related to latency while latency measures the time for a
packet to travel between two end points jitter measures the variance
in this time for example imagine that we send five
packets and they each take 10 milliseconds
in this case there is no variance between the packets they all take the
same time this means that the jitter is 0
milliseconds however if the first packet were to take
10 milliseconds the second takes 20, the third is 37, the fourth
is 55, and the fifth is 37 milliseconds we now have a lot of variance between
the smallest and the largest delivery times
this is jitter jitter is particularly destructive to real-time applications
like voice and video as they require a steady stream of data
we want packets to arrive smoothly and consistently
the more jitter the more erratic it becomes
now it is normal to see some jitter but if you're seeing a lot then you might
have a problem somewhere on the network perhaps a link or device is saturated
which slows down delivery on some packets
so let's consider how quality of service may help us here
the first job of qos is to identify certain traffic
and categorize it for example we might have a real-time category which contains
voice and video traffic we might have another category for
network management which contains routing updates
there might be a category for business critical applications and business
critical websites and finally we may have a category for
non-essential traffic like games or in many cases social media
of course this is all customizable and it has to be social media may be
non-essential for one business but it may be absolutely critical for
say a marketing company now that traffic has been categorized we
can handle each category differently for example we may reserve a portion of
our bandwidth for real-time traffic and knowing it's sensitive to jitter and
loss we might make sure that packets aren't dropped or delayed
for another type of traffic we might use a strategy called shaping
which delays the forwarding of packets until there is available bandwidth
for other traffic types we might consider policing this is where we allow
a certain amount of bandwidth and no more
anything above that limit is dropped we'll talk about shaping and policing in
an upcoming video it is critical to understand that each
device makes its own qos decisions this means that each device along a path
must have qos configuration to be completely effective of course
there may be times that you only want to apply some config in one place
this might include a firewall at the edge of the network and perhaps it
limits the amount of bandwidth that's allocated to traffic from a particular
source
even though each device is independent they can still share some information
this is done by marking packets after we classify traffic as part of a category
the router can put a marking in the header of the packet
this is like adding a label that tells other devices which category that packet
belongs to this means that other routers in the
path can just take a look at the marking and make a qos decision based on that
marking is a very important process and we're going to look at that in detail in
the next video
one key point that you should remember is that qos is especially helpful when
our links are congested or to prevent them from becoming
congested if you're regularly experiencing
congestion you may need to consider upgrading your links
or reviewing the traffic that you allow on them
qos should be considered a part of the solution not the entire solution
do not avoid upgrading your capacity simply because you're using qos
something else that we need to clear up is class of service and what that means
it sounds very similar to quality of service so the two are often confused
quality of service is the umbrella term for the traffic management technologies
that we've been talking about so far this includes marking packets policing
shaping and other things these technologies generally work at
layer 3 of the osi model which means they focus on ip packets
class of service on the other hand is a value in the ethernet header
which means it operates at layer 2. specifically it's used over 802.1q
trunk links which generally means it's used on traffic between switches
the switches set priority on these frames and decide which frames to
prioritize over others class of service is still a
traffic management technology so it can be considered one of the qos
technologies
the last thing i'd like to point out on this to avoid confusion
is if you use juniper routers and switches you will find they use a term
class of service when they are talking about configuring
quality of service so if you use their devices just be aware that they say
class of service and they mean the software process for
configuring quality of service i'm sure they have their reasons for
this but personally i find this confusing
we've covered the overview of qos and what it does for your network
and what it does not do in the next video we're going to look at the process
of classifying traffic this is identifying traffic and marking
it the video after that we'll talk about taking action on this traffic
so i hope you will join me for both of those videos