Chapter 10 Policy and Static Routes
NXC Series User’s Guide
178
DiffServ
QoS is used to prioritize source-to-destination traffic flows. All packets in the same flow are given the
same priority. CoS (class of service) is a way of managing traffic in a network by grouping similar types of
traffic together and treating each type as a class. You can use CoS to give different priorities to different
packet types.
DiffServ (Differentiated Services) is a class of service (CoS) model that marks packets so that they
receive specific per-hop treatment at DiffServ-compliant network devices along the route based on the
application types and traffic flow. Packets are marked with DiffServ Code Points (DSCPs) indicating the
level of service desired. This allows the intermediary DiffServ-compliant network devices to handle the
packets differently depending on the code points without the need to negotiate paths or remember
state information for every flow. In addition, applications do not have to request a particular service or
give advanced notice of where the traffic is going.
DSCP Marking and Per-Hop Behavior
DiffServ defines a new DS (Differentiated Services) field to replace the Type of Service (TOS) field in the IP
header. The DS field contains a 2-bit unused field and a 6-bit DSCP field which can define up to 64
service levels. The following figure illustrates the DS field.
DSCP is backward compatible with the three precedence bits in the ToS octet so that non-DiffServ
compliant, ToS-enabled network device will not conflict with the DSCP mapping.
The DSCP value determines the forwarding behavior, the PHB (Per-Hop Behavior), that each packet
gets across the DiffServ network. Based on the marking rule, different kinds of traffic can be marked for
different kinds of forwarding. Resources can then be allocated according to the DSCP values and the
configured policies.
10.2 Policy Route
Click Configuration > Network > Routing to open this screen. Use this screen to see the configured policy
routes and turn policy routing based bandwidth management on or off.
A policy route defines the matching criteria and the action to take when a packet meets the criteria.
The action is taken only when all the criteria are met. The criteria can include the user name, source
address and incoming interface, destination address, schedule, IP protocol (ICMP, UDP, TCP, etc.) and
port.
The actions that can be taken include:
• Routing the packet to a different gateway or outgoing interface.
• Limiting the amount of bandwidth available and setting a priority for traffic.
IPPR follows the existing packet filtering facility of RAS in style and in implementation.
DSCP (6 bits) Unused (2 bits)
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