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Committed access rate (CAR) is a feature from Cisco that is used in network optimization and security. It limits the input or output traffic rate on an interface or sub-interface based on criteria such as IP precedence, IP access list or incoming interface. When the traffic reaches the set limit, CAR specifies certain actions to be carried out. These actions can be configured using CAR commands that use the values of traffic rate limit, burst rate allowed and the action to be performed when the traffic reaches or exceeds the set limit.
Committed access rate is similar to traffic shaping, where the incoming or outgoing traffic rate is limited according to certain criteria. CAR differs from traffic shaping in the handling of excess data when there is a burst of data. While traffic shaping buffers the excess data, CAR performs an exceed action that has been specified by the user.
The two major functionalities of CAR are:
A CAR feature can be implemented with the help of specifications related to:
Rate limits define the conformance of a packet based upon the values of average traffic rate, normal burst size and excess burst size.
When a packet either conforms to or exceeds the rate limit set by the user, any of the exceed actions such as drop, set precedence, transmit or continue are performed on the packet.
The rate policies can be used to specify the matching criteria conditions on which to perform the rate limiting. They may be based upon all IP traffic, rate-limit access list or the standard IP access list. Each interface or sub-interface can have multiple CAR policies and each packet must undergo an examination under each of these policies. When none of the matching criteria is satisfied, the packet is transmitted. If any condition is met, the corresponding exceed action is performed on the packet and it is either dropped or compared with the next rate policy.
CAR can be effectively used to tune the network behavior during congestion situations. It allows the lowering of precedence of high-priority packets before dropping them, and thus helps in providing a useful intermediate step before dropping a packet.
Though using CAR may degrade the network performance, it protects the network from Internet-based denial of service (DoS) attacks like PING flood or SYN attacks.