Definition

A device implemented for use in QoS applications. A token bucket is a technique for deciding how to mark packets to be considered out of profile. The token bucket allows for a small degree of burstiness within a particular traffic class while generally enforcing a lower long-term (average) rate limit.

Tokens are added to a "bucket" at some fixed rate X (tokens per second) and are removed from the bucket whenever a packet arrives. A bucket also has a finite depth--it never will contain more than Y tokens.

Now, how this applies to traffic is thus: when a packet arrives and at least one token is avaiable, the token is removed and the packet is considered to be in profile. If no tokens are in the bucket when a packet arrives, that packet is declared to be out of profile. The token replensihment rate X represents the long-term average rate limit if packets are to remain in profile. By tuning X and Y one can tune Quality of Service attributes in respect to burst rates with ease.

Illustration


                 Token Bucket                       Token Bucket                       Token Bucket

             [-------------------]              [-------------------]              [-------------------]
   Packet    |                   |    Packet    |                   |    Packet    |                   |
  Arrives    | [---------------] |   Arrives    | [---------------] |   Arrives    | [---------------] |   ^
             | |               | |              | |               | |              | |               | |   |
     |       | [---------------] |      |       | [---------------] |      |       | [---------------] |   | B
     |       |                   |      |       |                   |      |       |                   |   | u
     |       | [---------------] |      |       | [---------------] |      |       | [---------------] |   | f
     |       | |               | |      |       | |               | |      |       | |               | |   | f
     |       | [---------------] |      |       | [---------------] |      |       | [---------------] |   | e
     |       |                   |      |       |                   |      |       |                   |   | r
     |       | [---------------] |      |       | [---------------] |      |       | [---------------] |   |
     |       | |               | |      |       | |               | |      |       | |               | |   |
     |       | [---------------] |      |       | [---------------] |      |       | [---------------] |   | 
     |       |                   |      |       |                   |      |       |                   |   | D
     |       | [---------------] |      |       | [---------------] |      |       | [---------------] |   | e
     |       | |###############| |      |       | |               | |      |       | |               | |   | p
     V       | [---------------] |      V       | [---------------] |      V       | [---------------] |   | t
             |                   |              |                   |              |                   |   | h
 Takes       | [---------------] |  Takes       | [---------------] |  No          | [---------------] |   |
 Token ----> | |###############| |  Token ----> | |###############| |  Token ----> | |               | |   |
             | [---------------] |              | [---------------] |              | [---------------] |   V
             |                   |              |                   |              |                   |
             [-------------------]              [-------------------]              [-------------------]

               Result: In Profile                 Result: In Profile               Result: Out of Profile

See also: CQS, MPLS, & Traffic Engineering

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