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 E
the packet into one of the two queues, and depending on the precedence levels the
queue could be rearranged to meet the QoS requirements.
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 and QoS
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.
F
IGURE
109
–
ToS and DSCP
DMAC
SMAC
Protocol
Type
ToS
Data
FCS
Diffserv Code Points (DSCP)
Unused
IP Header
DMAC
SMAC
Protocol
Type
ToS
Data
FCS
Diffserv Code Points (DSCP)
Unused
IP Header
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. 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 of
service for different types of communications traffic.
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