On most networks you will see a wide range of applications, each application is unique and has its own requirements when it comes to bandwidth, delay, jitter, etc. For example, an FTP application used for backups of large files might require a lot of bandwidth but delay and jitter won’t matter since it’s not an interactive application.
Voice over IP on the other hand doesn’t require much bandwidth but delay and jitter are very important. When your delay is too high your calls will become walkie-talkie conversations and jitter screws up the sound quality.
To make sure each application gets the treatment that it requires we have to implement QoS (Quality of Service).
The first step when implementing QoS is classification, that’s what this lesson is all about.
By default your router doesn’t care what kind of IP packets it is forwarding…the only important thing is looking at the destination IP address, doing a routing table lookup and whoosh…the IP packet has been forwarded.
Before we can configure any QoS methods like queuing, policing or shaping we have to look at the traffic that is running through our router and identify (classify) it so we know to which application it belongs. That’s what classification is about.
Once the traffic has been classified, we will mark it and apply a QoS policy to it. Marking and configuring QoS policies are a whole different story so in this lesson, we’ll just stick to classification.
On IOS routers there are a couple of methods we can use for classification:
- Header inspection
- Payload inspection
There are quite some fields in our headers that we can use to classify applications. For example, telnet uses TCP port 23 and HTTP uses TCP port 80. Using header inspection you can look for:
- Layer 2: MAC addresses
- Layer 3: source and destination IP addresses
- Layer 4: source and destination port numbers and protocol
This is a really simple method of classification that works well but has some downsides. For example, you can configure your router that everything that uses TCP and destination port number 80 is “HTTP” but it’s possible that some other applications (instant messaging for example) are also using TCP port 80. Your router will perform the same action for IM and HTTP traffic.
Payload inspection is more reliable as it will do deep packet inspection. Instead of just looking at layer 2/3/4 information the router will look at the contents of the payload and will recognize the application. On Cisco IOS routers this is done with NBAR (Network-Based Application Recognition).
When you enable NBAR on an interface, the router will inspect all incoming IP packets and tries to match them with signatures and attributes in the PDLM (Packet Description Language Module). For example, NBAR can detect HTTP traffic no matter what ports you are using and it can also match on things like:
- MIME type (zip file, image, etc)
- User-agent (Mozilla, Opera, etc)
Since NBAR can see the URL, it is also commonly used to block websites and a popular choice for classification.
You should now have an idea what classification is about, let’s look at some routers and configure classification.
We’ll start with a simple example where I use an access-list to classify some telnet traffic. Here’s the topology that I will use:
R1 will be our telnet client and R2 the telnet server. We will classify the packets when they arrive at R2. Let’s look at the configuration!
Classification with access-list
First I have to create an access-list that matches on telnet traffic:
R2(config)#ip access-list extended TELNET R2(config-ext-nacl)#permit tcp any any eq 23
This will match on all IP packets that use TCP as the transport protocol and destination port 23. Normally when you configure an access-list for filtering, we apply it to the interface. When configuring QoS we have to use the MQC (Modular Quality of Service Command-Line Interface). The name is pretty spectacular but it’s a really simple method to configure QoS.
We use something called a policy-map where we configure the QoS actions we want to perform…marking, queueing, policing, shaping, etc. These actions are performed on a class-map, and that’s where we specify the traffic. Let me show you how this is done:
R2(config)class-map TELNET R2(config-cmap)#match ? access-group Access group any Any packets class-map Class map cos IEEE 802.1Q/ISL class of service/user priority values destination-address Destination address discard-class Discard behavior identifier dscp Match DSCP in IP(v4) and IPv6 packets flow Flow based QoS parameters fr-de Match on Frame-relay DE bit fr-dlci Match on fr-dlci input-interface Select an input interface to match ip IP specific values mpls Multi Protocol Label Switching specific values not Negate this match result packet Layer 3 Packet length precedence Match Precedence in IP(v4) and IPv6 packets protocol Protocol qos-group Qos-group source-address Source address vlan VLANs to match
I created a class-map called “TELNET” and when you create a class-map you have a lot of options. On top you see access-group which uses an access-list to classify the traffic, that’s what I will use. Some other nice methods are the input-interface, frame-relay DLCI values, packet length, etc. The most simple option is probably the access-list:
R2(config-cmap)#match access-group name TELNET
My class-map called “TELNET” now matches traffic that is specified in the access-list called “TELNET”.
Now we can create a policy-map and refer to our class-map:
R2(config)#policy-map CLASSIFY R2(config-pmap)#class TELNET
The policy-map is called “CLASSIFY” and the class-map called “TELNET” belongs to it. Normally this is where I also specify the QoS action like marking, queueing, etc. I’m not configuring any action right since this lesson is only about classification.
Before the policy-map does anything, we have to attach it to an interface:
R2(config)#interface GigabitEthernet 0/1 R2(config-if)#service-policy input CLASSIFY
That’s it, our router can now classify telnet traffic. Let’s try it by telnetting from R1 to R2:
R1#telnet 192.168.12.2 Trying 192.168.12.2 ... Open
Let’s see what R2 thinks of this:
R2#show policy-map interface GigabitEthernet 0/1 GigabitEthernet0/1 Service-policy input: CLASSIFY Class-map: TELNET (match-all) 11 packets, 669 bytes 5 minute offered rate 0 bps Match: access-group name TELNET Class-map: class-default (match-any) 3 packets, 206 bytes 5 minute offered rate 0 bps, drop rate 0 bps Match: any
Great! Our router sees the telnet traffic that arrives on the GigabitEthernet 0/1 interface. You can see the name of the policy-map, the class-map and the access-list that we used. Something that you should remember is that all traffic that is not specified in a class-map will hit the class-default class-map. Not too bad right? Let’s see if we can also make this work with NBAR…
Classification with NBAR
The configuration of NBAR is quite easy. First let me show you a simple example of NBAR where it shows us all traffic that is flowing through an interface:
Thank you very much. This is really very good topic and it is very clear to me.
Thanks Renee! I’m gaining some traction on QOS
Thanks for your excellent introduction!
Here I have one concern, which “Tool” is better to identify the specific traffic? For example, If want to perform QoS for one of applications named ABC, how does router know which traffic is for Application - ABC?
If it’s a well known application like HTTP, HTTPS, SMTP, POP3, IMAP, SQL, etc. then NBAR can recognize them. Otherwise, it’s best to use an access-list to match the port numbers of your application.
Thanks for your feedback, and then, what’s the best way to get the port numbers of some particular applications?