GE ML1200 instruction manual DiffServ and QoS, IP Precedence

QUALITY OF SERVICE CHAPTER 14: QUALITY OF SERVICE

To make the preemptive queuing possible, most switches implement at least two queue buffers. The MultiLink ML1200 Managed Field Switch has two priority queues, 1 (low) and 0 (high).When tagged packets enter a switch port, the switch responds by placing the packet into one of the two queues, and depending on the precedence levels the queue could be rearranged to meet the QoS requirements.

14.1.3 DiffServ and QoS

QoS refers to the level of preferential treatment a packet receives when it is being sent 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 Engineering Task Force) to provide quality of service for traffic on IP networks.

DiffServ is designed for use at the edge of an Enterprise where corporate traffic enters the service provider environment. DiffServ is a layer-3 protocol and requires no specific layer-2 capability, allowing it to be used in the LAN, MAN, and WAN. DiffServ works by tagging each packet (at the originating device or an intermediate switch) for the requested level of service it requires across the network.

IP Header

DMAC SMAC Protocol

ToS

Data

FCS

DiffservCode Points (DSCP)

Unused

754725A1.CDR

FIGURE 14–1: ToS and DSCP

DiffServ inserts a 6-bit DiffServ code point (DSCP) in the Type of Service (ToS) field of the IP header, as shown in the picture above. Information in the DSCP allows nodes to determine the Per Hop Behavior (PHB), which is an observable forwarding behavior for each packet. Per hop behaviors 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 of service for different types of communications traffic.

14.1.4 IP Precedence

IP Precedence utilizes the three precedence bits in the IPv4 header's Type of Service (ToS) field to specify class of service for each packet. You can partition traffic in up to eight classes of service using IP precedence. The queuing technologies throughout the network can then use this signal to provide the appropriate expedited handling.

14–2 MULTILINK ML1200 MANAGED FIELD SWITCH – INSTRUCTION MANUAL