Extended Service Plan. Sold by Circuit City employees, almost never used because of unknowing customers. Best Buy sells a similar service plan called PSP or Performace Service Plan. Even if Best Buy isn't on commission, they still have to push PSP's and accessories. Circuit City employees are just trying to make some mad cash.

Electronic Stability Program (ESP), also called Vehicle Dynamics Control, Dynamic Stability Control, etc., is a passive safety feature for your car that constantly verifies whether your vehicle is headed in the direction the steering wheel is indicating. If the system detects that your car is veering off the desired course, it will instantly react by individually applying the brakes on up to two specific wheels in order to cause the car to swing back on course.

The detection portion of the system works by comparing data from a sensor embeded in the steering wheel and directional data obtained from a yaw sensor.

This type of course correction cannot be performed manually with today's cars because there is only one brake pedal that applies the brakes on all four wheels. To manually achieve the same effect as ESP, you would need to have 4 brake pedals, one for each wheel. And you would need to instinctively know which brake pedal would have which effect under any given circumstance, such as oversteer or understeer.

The ESP system can apply the brakes to the wheels of its choice thanks to your car's ABS system which is already designed to be able to individually control each wheel's braking.

To visualize how the system uses the brakes to correct your car's course, suppose that you are traveling straight and you hit "the brake for the left rear wheel". This will cause your car to tend towards rotating left. Simulate it with the help of a pointy knife and a piece of paper that represents a car. Move the piece of paper forward on your table and while you are doing so jab the knife in the area of the paper where the left rear wheel would be. Notice that your forward pushing of the paper now causes it to rotate left instead of move forward. The knife, of course, represents the friction of the wheel being braked (Note: this excercise will probably damage your table). In a real world situation, suppose you wanted to turn left but the car started to skid straight ahead. Hitting the left rear wheel brake pedal might allow your car to regain its full grip on the road and thus go where you want it to.

Encapsulating Security Payload (ESP) is a header format of IPsec. In particular, it is designed to provide confidentiality and limited traffic-flow confidentiality, as well as similar services to the authentication header. This is accomplished primarily through encryption.

ESP provides encrypted encapsulation. The TCP packet (or any other protocol running on IP) is encrypted and included as part of ESP. Alternatively, the entire IP packet can encapsulated in another IP packet. Encrypting only the subprotocol of IP is called transport mode, while encrypting the entire packet is called tunnel mode.

Tunnel mode is much more common in my experience, as generally you want to set up a virtual private network (VPN) that allows you to communicate with multiple hosts. In transport mode, you would have to negotiate keys with each host. In tunnel mode, you only have to nogotiate keys with a single server, making deployment and management much simpler.

ESP is designed for use with symmetric encryption algorithms. Moreover, because packet order is not guaranteed, each packet must be self-contained. Consistent with the rest of IPsec, ESP does not specify the encryption or authentication to use, leaving that to the security association part of ISAKMP. However, the specification does list required algorithms to support in a conforming implementation.

Note that ESP contains within it support for authentication data, providing similar functionality that AH in addition to encryption. However, ESP does not protect any header fields unless they are part of the encapsulation. Thus, the outside IP header is not authenticated by ESP.

ESP is defined in RFC 2406.

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