QoS for Virtual Private Networks

By implementing QoS, you can grant the appropriate service levels to your mission-critical applications. Because remote-access users do not usually care about the network topology or the high level of security/encryption or firewalls that handle their traffic, your solution must be able to give them what they do care about: an acceptable response time for their applications.

Your users' acceptance levels for delays will vary, depending on the application they are using at the time. What is an acceptable level of delay for FTP might not meet with the same acceptance when accessing a database or running voice over IP.

QoS gives you the mechanisms necessary to give your users this level of performance. QoS is a vital tool designed to ensure that all applications coexist and function at acceptable levels of performance. The primary QoS features you will be concerned with, especially when dealing with VPNs, are as follows:

Packet classification using committed access rate (CAR)
Bandwidth management by policing with CAR, shaping with Generic Traffic Shaping/Frame Relay Traffic Shaping (GTS/FRTS), and bandwidth allocation with WFQ
Congestion avoidance using WRED
Continuity of packet priority over Layer 2 and Layer 3 VPNs with tag switching/Multiprotocol Label Switching (MPLS)

Each of these features is discussed in the following sections.