Implementing QoS At Layer 3

At Layer 3, Differentiated Services (DiffServ) provides a scalable mechanism for prioritizing packets and providing QoS on a VoIP network. There are three main levels identified by DiffServ. The highest priority level is Expedited Forwarding (EF). Expedited Forwarding packets get priority over all other packets. Assured Forwarding (AF) is the mid-level prioritization. Anything that is not EF or AF is classified as Default, and receives the lowest priority. The DiffServ identifier is found within the DiffServ Code Point (DSCP) found in the header of the IP packet.

DiffServ identifies Per-Hop Behaviors (PHB) for each packet that is sent through. A packet classified as EF will receive the highest priority, and is ideal for low-latency applications such as VoIP. An EF package will almost never be discarded from a DiffServ-capable device.

Assured Forwarding packets are subdivided into three behavior groups – Low Drop, Medium Drop, and High Drop. Each of these groups is further divided into four classes, Class 1, Class 2, Class 3, and Class 4. This gives us a total of twelve AF classifications. The drop precedence of these classifications is often defined by the queuing methods of QoS defined in the previous post. The most common methods are Fair Queuing or Weighted Fair Queuing. The level of forwarding assurance is determined by the load of the AF class and the drop precedence of the packet (in the case of congestion within a class).

A Default (DE) packet is sent using a best-effort behavior. If no DiffServ argument is included in a packet it is assumed to be part of the default category.

There are multiple points where a package can be marked for DiffServ. It can be marked using the end point within the device or application from which it originates, it can be marked using intelligent switches where the device is connected to a network based on its TCP/UDP Port, IP Address, or VLAN. The packet can be marked at a core or edge router where the packet encounters links. Traffic entering a network is classified, and possibly conditioned, at the boundaries of the network and is assigned to different behavior aggregates identified by a single DSCP. DiffServ edge router and switch interfaces can be configured to trust or distrust previously marked DSCP or 802.1p tagged packets. If the packet is trusted, the DSCP tag will remain the same. If it is distrusted, the DiffServ router will assign a new DSCP tag.
There are three types of nodes on a DiffServ domain: Edge nodes, Boundary Nodes, and Interior Nodes. The Edge node is the first IP Aware device that applies QoS policies. Edge Nodes and Boundary Nodes are configured to trust or distrust the DSCP identification and forward the packets from one node to another.

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