Operation Manual – QoSH3C S7500 Series Ethernet Switches Chapter 1 QoS Configuration1-21.1.1 TrafficTraffic means service traffic, that is, all the packets passing the switch.1.1.2 Traffic ClassificationTraffic classification means to identify packets conforming to certain charactersaccording to certain rules.A classification rule is a filter rule configured to meet your management requirements. Itcan be very simple. For example, you can use a classification rule to identify traffic withdifferent priorities according to the ToS field in the IP packet header. It can be verycomplicated too. For example, you can use a classification rule to identify the packetsaccording to the combination of link layer (Layer 2), network layer (Layer 3) andtransport layer (Layer 4) information including MAC addresses, IP protocols, sourceaddresses, destination addresses, the port numbers of applications and so on.Classification is generally based on the information in the packet header and rarelybased on the packet content.1.1.3 Precedence1) IP precedence, ToS precedence and DSCP precedence0 76DSCPClass SelectorcodepointsDifferentiated ServicesCodepoint (DSCP)RFC 2474CurrentlyUnused RFC 1122IP Type ofRFMBZService (ToS)C 791MustBeZeroRFC 1349DS-Field(for IPv4,ToSoctet,and forIPv6,TrafficClass octet )IPv4 ToSbyte1 5432Bits: 0 761 5432Bits:PrecedenceCU Type ofServicesFigure 1-1 DS fields and TOS byteThe ToS field in an IP header contains 8 bits:z The high-order 3 bits represent IP precedence in the range of 0 to 7.z Bit 3 to bit 6 represent ToS precedence in the range of 0 to 15.z RFC2474 re-defines the ToS field in the IP packet header, which is called the DSfield. The high-order 6 (bit 0 to bit 5) bits of the DS field represent DSCPprecedence in the range of 0 to 63.The high-order 3 bits in DSCP precedence areclass selector codepoints, bit 4 and bit 5 represent drop precedence, and bit 6being zero indicates that the device sets the service class by using the DS model.z The lower-order 2 bits (bit 6 and bit 7) are reserved bits.
