Per Flow Metering

The main components of the Meter Table are table features, such as the max number of entries and supported band types, and a Hash Map of meter entries. Other members include a reference pointer to the Datapath, allowing a Meter Table to call the function to send OpenFlow messages; and two counters: one for the number of meter entries and another one for the quantity of bands.

The Meter Table has a set of functions for: initialization and destruction of the Meter Table; meter mod and meter features messages handling; finding and applying a meter entry.

Structures of meter entries are composed of a configuration - which contains information about the meter id and meter bands - and a struct for recording statistics. In addition, the meter entry has pointers to the Datapath and the Meter Table, similar to what is done in the Meter Table struct. Finally, it has a list of flow references. If the meter entry is deleted, all flows sending packets to the meter entry are deleted.

Meter entry bands are chosen accordingly to a configured rate - in Kilo packets (Kpps) per second or Kilobits per second (Kbps). Thus, it is necessary to measure the flow matched packets in function of one of the specified unities. The first idea to implement rate measuring scheme considered the use of matched flow counters, divided by the number of matched bytes by some time interval. Although easy to implement, this approach proved itself inaccurate after some attempts to limit the bandwidth between two hosts connected to the switch.

After a better analysis of the task and some literature research, we found and implemented a simple and efficient algorithm used for rate policy: the Token Bucket. The Figure below illustrates how the Token Bucket works within a meter band.

Simply put, each meter band has a bucket attached to it. At every second the bucket is refilled with a number of tokens equal to the meter rate. When a packet is sent to the Meter Table, it goes through each band's bucket belonging to the meter entry. Inside the bucket, packets consume a number of tokens equal to their size. If there are enough tokens, the OpenFlow pipeline continues processing the packet, otherwise, the meter band is chosen and executed.