/mcpVoice Traffic Priority There are two important components in prioritizing voice. The first is classifying and marking interesting voice traffic. The second is prioritizing the marked interesting voice traffic. The two subsections here discuss various approaches to classifying, marking, and prioritizing voice. Classification and Marking In order to guarantee bandwidth for VoIP packets, a network device must be able to identify the packets in all the IP traffic that flows through it. Network devices use the source and destination IP address in the IP header, or the source and destination UDP port numbers in the UDP header, to identify VoIP packets. This identification and grouping process is called classification. It is the basis for providing any QoS. Packet classification can be processor intensive. Therefore, classification needs to be done as far out towards 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 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. Refer to the Request for Comments (RFC). Differentiated Services (DiffServ) is a new model in which traffic is treated by intermediate systems with relative priorities based on the ToS field. Defined in RFC 2474 and RFC 2475 , the DiffServ standard supersedes the original specification for defining packet priority described in RFC 791 . DiffServ increases the number of definable priority levels by reallocating bits of an IP packet for priority marking. The DiffServ architecture defines the DiffServ field. It supersedes the ToS byte in IP V4 to make Per−Hop Behavior (PHB) decisions about packet classification and traffic conditioning functions such as metering, marking, shaping, and policing. In addition to the previously mentioned RFCs, RFC 2597 defines the Assured Forwarding (AF) classes. This is a breakdown of the DSCP fields. For more information on DSCP, refer to Implementing Quality of Service Policies with DSCP. ToS Byte − P2 P1 P0 T3 T2 T1 T0 CU IP precedence: three bits (P2−P0), ToS: four bits (T3−T0), CU: one bit DiffServ Field − DS5 DS4 DS3 DS2 DS1 DS0 ECN ECN DSCP: six bits (DS5−DS0), ECN: two bits XXX00000 Bits 0, 1, 2 (DS5, DS4, DS3) are Precedence bits, where: 111 = Network Control = Precedence 7 • 110 = Internetwork Control = Precedence 6 • 101 = CRITIC/ECP = Precedence 5 • 100 = Flash Override = Precedence 4 • 011 = Flash = Precedence 3 • 010 = Immediate = Precedence 2 • 001 = Priority = Precedence 1 • 000 = Routine = Precedence 0 • 000XXX00 Bits 3, 4, 5 (DS2, DS1, DS0) are Delay, Throughput, and Reliability bits. Bit 3 = Delay [D] (0 = Normal; 1 = Low) •