/mcpQuality of Service for Voice over IP Packet Classification 3 QoSVoIP.mif Packet Classification The basis for providing any QoS lies in the ability of a network device to identify and group specific packets. This identification process is called packet classification. After a packet has been classified, the packet needs to be marked by setting designated bits in the IP header. The following sections describe classification and marking: • Packet Classification Overview • Voice Dial Peers Classification and Marking Example • Committed Access Rate Classification and Marking Example • Policy-Based Routing Classification and Marking Example • Modular QoS Command-Line Interface Classification and Marking Example Packet Classification Overview To guarantee bandwidth for VoIP packets, a network device must be able to identify VoIP packets in all the IP traffic flowing through it. Network devices use the source and destination IP address in the IP header or the source and destination User Datagram Protocol (UDP) port numbers in the UDP header to identify VoIP packets. This identification and grouping process is called classification and it is the basis for providing any QoS. Besides the static classification methods involving Layer 3 or Layer 4 header information matching, you can use a mechanism such as Resource Reservation Protocol (RSVP) for dynamic classification. RSVP uses H.245 signaling packets to determine which UDP port the voice conversation will use. It then sets up dynamic access lists to identify VoIP traffic and places the traffic into a reserved queue. RSVP is discussed later in this document. Packet classification can be processor-intensive, so it should occur as far out toward the edge of the network as possible. Because every hop still needs to make a determination on the treatment a packet should receive, you need to have a simpler, more efficient classification method in the network core. This simpler classification is achieved through marking or setting the type of service (ToS) byte in the IP header. The three most significant bits of the ToS byte are called the IP Precedence bits. Most applications and vendors currently support setting and recognizing these three bits. Marking is evolving so that the six most significant bits of the ToS byte, called the Differentiated Services Code Point (DSCP), can be used to define differentiated services (DS) classes. DSCP is discussed later is this document. After every hop in the network is able to classify and identify the VoIP packets (either through port address information or through the ToS byte), those hops can then provide each VoIP packet with the required QoS. At that point, you can configure special techniques to provide priority queueing to make sure that large data packets do not interfere with voice data transmission, and to reduce bandwidth requirements by compressing the 40-byte IP plus UDP plus RTP header to 2 to 4 bytes. Classification and Marking Classification is the process of identifying the class or group to which a packet belongs. Network devices use various match criteria to place traffic into a certain number of classes. Matches are based on the following criteria: • The dial-peer voice voip global configuration command • Access list (standard and extended)