Traffic policing


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


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Traffic policing — Le Traffic policing, ou limitation du flux, consiste à vérifier que les flux réseau se conforment à un accord de service et à prendre les mesures pour faire respecter un tel contrat. Les sources de données qui sont informées de l existence d un… …   Wikipédia en Français

  • Traffic shaping — (also known as packet shaping ) is the control of computer network traffic in order to optimize or guarantee performance, lower latency, and/or increase usable bandwidth by delaying packets that meet certain criteria. [… …   Wikipedia

  • Traffic shaping — Le Traffic shaping ou régulation de flux est le contrôle du volume des échanges sur un réseau informatique dans le but d’optimiser ou de garantir les performances, une latence plus basse ou d’augmenter la bande passante utilisable en retardant… …   Wikipédia en Français

  • Traffic flow (computer networking) — For other uses, see Flow (disambiguation). In packet switching networks, traffic flow, packet flow or network flow is a sequence of packets from a source computer to a destination, which may be another host, a multicast group, or a broadcast… …   Wikipedia

  • Policing in the United States — is one of three major components of the criminal justice system, along with courts and corrections. Although there exists an inherent interrelatedness between the different groups that make up the criminal justice system based on their crime… …   Wikipedia

  • Traffic Cops — For the policing function see Road traffic control Traffic Cops The current Traffic Cops and Motorway Cops title cards. Genre Motoring/ …   Wikipedia

  • Policing in Belgium — The Belgian police is the government agency charged with upholding the law and public order in Belgium. It is an integrated police service structured on the federal and local levels. Both forces are autonomous and subordinate to different… …   Wikipedia

  • policing —    It seemed at the time that reform of the police system after the death of Franco in 1975 was agonizingly slow. It had to be transformed from the repressive instrument of an authoritarian regime into a system that would protect the… …   Encyclopedia of contemporary Spanish culture

  • Traffic Operational Command Unit — The Traffic Operational Command Unit (CO15) is a Central Operations branch of the London Metropolitan Police Service. The unit is responsible for policing of the roads, such as the London road network. It has been in existence since 1930, when… …   Wikipedia

  • Traffic police — *Police controlling traffic, see rules of the road *Highway patrol *Road Policing Unit …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.