What is rate shaping?

Rate shaping is used to add a bound or constraint on the rate of a network traffic flowing through a router to a particular bit rate or a certain rate of packets per second. Rate shaping requires queues, queue management, and scheduling.

How is it implemented?

Rate shaping is often achieved by limiting the frequency of how often a queue is emptied, even if the scheduler that is servicing the queue has nothing better to do. If packets arrive with a shorter interpacket gap than is allowed by the scheduler, the packet is queued: smoothing out the burstiness of the flow.

Often, this is done by implementing a leaky bucket (which is different than a token bucket!) which allowes a packet to be de-queued every so often (every T seconds). This type of queue will always report it is empty, even if it contains data, unless T seconds has elapsed: thus ensuring that when packets arrive, they depart with at least T seconds between them.

Why would one want to rate shape traffic?

There are a couple of reasons, one technical and one emotional:

• Rate smoothing reduces the burstiness of a traffic stream, thus smoothing out traffic patterns for downstream routers.
• Shaping can also balance customer expectations. If the customer experiences faster than lightning performance when your network is not loaded and then the "payed-for" performance later on: they may perceive a degredation in service quality. Limiting their rate early on can act as an investment against this problem from occuring.

See also: Quality of Service, CQS, Token Bucket, & Traffic Engineering