Traffic Conditioning Exposed

Traffic conditioning is a somewhat abstract concept that each network will instantiate in a different form, depending on the network's needs. Traffic conditioning has four essential components: markers, which place or change the Differentiated Services (DiffServ) Codepoint mark in packet header; meters, which measure traffic and check it against traffic profiles; shapers, which delay traffic to make it conform to a certain traffic profile; and droppers, which simply drop packets. These four components work together.

Let's assume every client has a service level agreement (SLA) with an ISP. The SLA specifies how much traffic the client can send for a certain fee and what will happen to excessive traffic. The essential element of the SLA is the Traffic Conditioning Specification (TCS), which is a technical specification for the treatment of packets. The IETF Internet Draft "A Framework for Differentiated Services" ( draft-ietf-diffserv-framework-02.htm) outlines the DiffServ Codepoint mark, the profile, the scope, and the disposition of nonconforming traffic as the minimum TCS requirements for a network ingress point.

The traffic conditioning mechanism at the network ingress examines each packet and pulls a DiffServ Codepoint mark from the header. Then, the mechanism compares the packet with the specified profile for packets carrying that DiffServ Codepoint mark. The profile might consist of data such as packet size, packet origin, or some encrypted code or key. If the packet conforms to the profile for its particular DiffServ Codepoint mark, the ingress lets it pass through. The scope specifies egress points that the routers can carry the packet to. If the packet doesn't conform to the profile, the ingress can re-mark the packet for a lower service level, delay the packet, or drop it. The ingress can also forward the packet at the requested priority level with the violation noted and the packet sender subject to a surcharge.

The current DiffServ framework expects customers to mark and shape their traffic. Providers police traffic that customers submit at the edge. Providers can also mark and shape customers' traffic as a value-added service.

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