Traffic policing

Traffic policing is monitoring network traffic for conformity with a traffic contract and if required, dropping traffic to enforce compliance with that contract. Traffic sources which are aware of a traffic contract sometimes apply Traffic Shaping in order to ensure their output stays within the contract and is thus not dropped. Traffic exceeding a traffic contract may be tagged as non-compliant, dropped, or left as-is depending on circumstances.

Effect

The recipient of traffic that has been "policed" will observe packet loss distributed throughout periods which exceeded the contract. If the source does not respond to this (for example, through a feedback mechanism), this will continue, and may appear to the recipient as if link errors or some other disruption is causing random packet loss.

The received traffic will typically comply with the contract, give or take jitter introduced by elements in the network downstream of the policer.

Impact on Congestion-Controlled Sources

Sources with feedback-based congestion control mechanisms (for example TCP) typically adapt rapidly to static policing, converging on a rate just below the policed sustained rate. Co-operative policing mechanisms, such as packet-based discard [Design and applications of ATM LAN/WAN adapters. Bonjour, D.; De Hauteclocque, G.; Le Moal, J. ATM, 1998. ICATM-98., IEEE International Conference, 22-24 Jun 1998 Page(s):191 - 198 Digital Object Identifier 10.1109/ICATM.1998.688177] facilitate more rapid convergence, higher stability and more efficient resource sharing.

As a result, it may be hard for endpoints to distinguish TCP traffic that has been merely policed from TCP traffic that has been shaped.

Impact on Self-limiting Sources

Self-limiting sources without feedback may suffer severe degradation due to policing depending on the nature of the application.

Where cell-level dropping is enforced (as opposed to that achieved through packet-based policing) the impact is particularly severe on longer packets. Since cells are typically much shorter than the maximum packet size, conventional policers discard cells which do not respect packet boundaries, and hence the total amount of traffic dropped will typically be distributed throughout a number of packets. Almost all known packet reassembly mechanisms will respond to a missing cell by dropping the packet entirely, and consequently a very large number of packet losses can result from moderately exceeding the policed contract.

Implementation

Traffic policing elements comprise a "meter" and a "dropper". [ [http://tools.ietf.org/html/rfc2475#section-2.3.3.1 IETF RFC 2475] "An Architecture for Differentiated Services" section 2.3.3 - definitions of meter, dropper and marker] They may also optionally include a "marker". The meter measures the traffic and determines whether or not it exceeds the contract (for example by GCRA). Where it exceeds the contract, some policy determines if any given PDU is dropped, or if marking is implemented, if and how it is to be marked. Marking can comprise setting a congestion flag (such as ECN flag or CLP bit) or setting a traffic aggregate indication (such as Differentiated Services Code Point).

Traffic policing requires maintenance of numerical statistics and measures for each policed traffic flow, but it does not require implementation or management of significant volumes of packet buffer. Consequently it is significantly less complex to implement than "traffic shaping".

Connection-Oriented Networks and Connection Admission Control

Policing can be imposed at every node in an ATM network. Sources are required to ensure their traffic complies with the contract in force if they wish to avoid policing, and may implement traffic shaping to achieve this optimally.

Connection-oriented networks (for example ATM systems) perform connection admission control based on traffic contracts.An application that wishes to use a connection-oriented network to transport traffic must first request a connection (through signalling, for example Q.2931), which involves informing the network about the characteristics of the traffic and the quality of service (QoS) required by the application. [Ferguson P., Huston G., Quality of Service: Delivering QoS on the Internet and in Corporate Networks, John Wiley & Sons, Inc., 1998. ISBN 0-471-24358-2.] This information is stored in a traffic contract. If the connection request is accepted, the application is permitted to use the network to transport traffic.

This function protects the network resources from malicious connections and enforces the compliance of every connection to its negotiated traffic contract. The network can also discard non-conformant traffic in the network (using Priority Control). Traffic policing in ATM networks is known as Usage Parameter Control (UPC) and Network Parameter Control (NPC). [Hiroshi Saito, Teletraffic Technologies in ATM Networks, Artech House, 1993. ISBN 0-89006-622-1.]

See also

* Broadband Networks
* Network Parameter Control (NPC)
* Teletraffic engineering in broadband networks
* Usage Parameter Control (UPC)

References