Classification and Marking

Almost every QoS tool uses classification to some degree. To put one packet into a different queue than another packet, the IOS must somehow differentiate between the two packets. To perform header compression on RTP packets, but not on other packets, the IOS must determine which packets have Real Time Protocol (RTP) headers. To shape data traffic going into a Frame Relay network, so that the voice traffic gets enough bandwidth, the IOS must differentiate between Voice over IP (VoIP) and data packets. If an IOS QoS feature needs to treat two packets differently, you must use classification.

Classification involves differentiating one packet from another, typically by examining fields inside the headers. After classification, a QoS tool can treat packets in one class differently than others. To just give all VoIP traffic preference over all other traffic, the queuing tool would need to classify traffic into one of two categories: VoIP or not-VoIP.

Because most QoS tools need to differentiate between packets, most QoS tools have classification features. In fact, you may already know something about several of the QoS tools described in this book. You may realize that you already know how to perform classification using some of those tools. For instance, many QoS tools enable you to classify using access-control lists (ACLs)—for instance, if ACL 101 "permits" a packet, the packet falls into one queue; if ACL 102 permits a packet, it is placed in a second queue; and so on. In one way of thinking, queuing could instead be called "classification and queuing," because the queuing feature must somehow decide which packets end up in which queue. Similarly, traffic shaping could be called "classification and traffic shaping," policing could be called "classification and policing," and so on. Because most QoS tools classify traffic, however, the names of most QoS tools never evolved to mention the classification function of the tool.

Only one category of QoS tool, called classification and marking, highlights the classification feature in the name of the tool. For other tools, the classification function is just part of the story; with classification and marking tools, classification is the whole point of the tool. To appreciate the need for classification and marking tools, consider Figure 2-1.

The figure shows the QoS policies for traffic flowing right to left. R3 performs queuing and shaping, and R2 performs queuing only. However, for both sets of queues, and for the shaping function, classification must occur. The classification part of the effort seems to be a simple task, but it may cause many comparisons to be made. For instance, each packet exiting R3's S0 and R2's S0 interfaces might be compared for the following:

• From source address 10.1.1.1, TCP source port 80 (Server1 web traffic)

• Using User Datagram Protocol (UDP), port number range 16384 to 32767 (voice pay-load)—may also want to check IP address ranges to match IP Phones' voice subnets, or voice gateway IP addresses

• Using TCP port 1720 (H.323 voice signaling)

• Using TCP port range 11000 to 11999 (Voice signaling)

• Using TCP port 1719 (Voice signaling)

• Using TCP port 2000 to 2002 (Skinny voice signaling)

• Using UDP port 2427 and 2428 (MGCP voice signaling)

Figure 2-1 Sample Network, with Queuing and Shaping Tools Enabled

Figure Shows QoS for Packets Flows Right-to-Left

Hannal

Queuing

Classify

Shaping Queuing

Classify

Queuing

Classify

Shaping Queuing

Classify

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Classification and marking tools simplify the classification process of the other QoS tools. Even with seemingly simple requirements, the classification functions can require many comparisons to every packet. Rather than have each tool do extensive packet matching, classification and marking tools do the extensive classification once, and mark a field in a packet header. The remaining QoS tools just need to look for this marked field, simplifying the repetitive classification work.

The two most commonly used marking fields in the IP header are the IP Precedence field, and the Differentiated Services Code Point (DSCP) field. You will see the details of these two fields, along with the other fields that can be used for marking, later in this chapter. Consider Figure 2-2, where classification and marking is performed on input of R3.

The queuing and shaping features can now classify more efficiently. Queuing is still performed on R3 and R2, and shaping is still performed on R3. However, the extensive matching logic for each packet done for all incoming traffic can be performed once on R3's FA0/0 interface, or once on one of the LAN switches, such as SW3. Once marked, the other QoS tools can react to the marked value, which each QoS tool can efficiently match in the end-to-end path through the network.

Figure 2-2 Sample Network, with Simplified Classification as a Result of Classification and Marking

Figure Shows QoS for Packets Flows Right-to-Left

Figure 2-2 Sample Network, with Simplified Classification as a Result of Classification and Marking

Figure Shows QoS for Packets Flows Right-to-Left

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