M A G N U M 6 K S W I T C H E S , M N S - 6 K U S E R G U I D Ethe packet into one of the two queues, and depending on the precedence levels thequeue could be rearranged to meet the QoS requirements.QoS refers to the level of preferential treatment a packet receives when it is beingsent through a network. QoS allows time sensitive packets such as voice and video,to be given priority over time insensitive packets such as data. Differentiated Services(DiffServ or DS) are a set of technologies defined by the IETF (Internet EngineeringTask Force) to provide quality of service for traffic on IP networks.DiffServ and QoSDiffServ is designed for use at the edge of an Enterprise where corporate traffic enters theservice provider environment. DiffServ is a layer-3 protocol and requires no specific layer-2capability, allowing it to be used in the LAN, MAN, and WAN. DiffServ works by taggingeach packet (at the originating device or an intermediate switch) for the requested level ofservice it requires across the network.FIGURE 114 – ToS and DSCPDMAC SMAC ProtocolType ToS Data FCSDiffserv Code Points (DSCP) UnusedIP HeaderDMAC SMAC ProtocolType ToS Data FCSDiffserv Code Points (DSCP) UnusedIP HeaderDiffServ inserts a 6-bit DiffServ code point (DSCP) in the Type of Service (ToS) field of theIP header, as shown in the picture above. Information in the DSCP allows nodes todetermine the Per Hop Behavior (PHB), which is an observable forwarding behavior foreach packet. PHBs are defined according to:• Resources required (e.g., bandwidth, buffer size)• Priority (based on application or business requirements)• Traffic characteristics (e.g., delay, jitter, packet loss)Nodes implement PHBs through buffer management and packet scheduling mechanisms.This hop-by-hop allocation of resources is the basis by which DiffServ provides quality ofservice for different types of communications traffic.206
