Traffic shaping (also known as "packet shaping") is an attempt to control computer
network traffic in order to optimize or guarantee performance, low latency, and/or bandwidth by delaying packets.[1]
Traffic shaping deals with concepts of classification, queue disciplines, enforcing policies, congestion management, quality of
service (QoS), and fairness.
Summary
Traffic shaping provides a mechanism to control the volume of traffic being sent into a network (bandwidth throttling), and the rate at which the
traffic is being sent (rate limiting). For this reason, traffic shaping schemes are
commonly implemented at the network edges to control traffic entering the network. This control can be accomplished in many ways
and for many reasons but traffic shaping always simply consists in delaying packets. Traffic
policing is the related practice of packet dropping and packet marking. Traffic shaping can be applied by the traffic
source (for example, computer or network card) or by an element in the network.
Implementation
A traffic shaper works by delaying metered traffic such that each packet complies with the relevant traffic contract. Metering may be implemented with for example the leaky
bucket or token bucket algorithms (the former typically in ATM and the latter in IP networks). Metered
packets or cells are then stored in a buffer until they can be transmitted in compliance with the
prevailing traffic contract. This may occur immediately (if the traffic arriving at the shaper is already compliant), after some
delay (waiting in the buffer until its scheduled release time) or never (in case of buffer overflow).
Overflow Condition
All traffic shaper implementations have a finite buffer, and must cope with the case where the buffer is full . A simple and
common approach is to drop traffic arriving while the buffer is full (tail drop), thus resulting in traffic policing as
well as shaping. A more sophisticated implementation could apply a dropping algorithm such as Random Early Discard; a crude alternative would be to allow overflowing traffic through
unshaped.
Traffic Classification
Simple traffic shaping schemes shape all traffic uniformly by rate. More sophisticated shapers first classify traffic.
'Traffic classification categorises traffic (for example, based on port
number or protocol number); each resulting traffic class can be treated differently to
differentiate service. For example, each traffic class could be subject to a different rate limit, shaped separately and/or
prioritised relative to other traffic classes. This differentiation can be used by a network operator to treat different types of
application traffic differently (for example, prioritise voice over file sharing), and to offer premium services at a higher
price point than basic ones[2].
Classification is achieved by various means. Matching bit patterns of data to those of known protocols is a simple, yet
widely-used technique. An example to match the BitTorrent Protocol Handshaking phase would be a simple check to see if a packet began with character 19 which was then followed
by the 19-byte string 'BitTorrent protocol'.[3] Upon
classifying a traffic flow using a particular protocol, a policy can be applied to it and other flows to either guarantee a
certain quality (as with VoIP or media streaming service[4]) or to provide best-effort delivery. This may be applied at the ingress point (the point at which traffic enters the network) with a granularity that allows the
traffic-shaping control mechanism to separate traffic into individual flows and shape them differently [5].
Relationship to Traffic Management
Traffic shaping is a specific technique and one of several which combined comprise Traffic
Management[6]. Current common usage, particularly in
discussion of domestic Internet service provision, frequently confuses traffic shaping with traffic management and traffic
policing, with classification policies and in general with any measure deliberately taken by an ISP which is detrimental to the
user's IP traffic performance.
ISPs
Traffic shaping is of interest especially to Internet Service Providers
(ISPs). Their high-cost, high-traffic networks are their major assets, and as such, are the focus of all their attentions. They
often use traffic shaping as a method to optimize the use of their network, sometimes by intelligently shaping traffic according
to importance, other times by discouraging uses of applications by harsh means.[citation needed]
Benefits
To ISPs, mere protocol identification (a large part of modern traffic shaping mechanisms) gives the intangible yet significant
benefit of seeing what internet traffic is flowing through the network. From this they can see which subscribers are doing what
on their network and can target services to the subscriber base they have attracted.
In addition, intelligent shaping schemes can guarantee a particular Quality of Service (often measured in jitter, packet loss, and latency) for an application or a user while still allowing other traffic to use all remaining
bandwidth. This allows ISPs to offer Differentiated services and to upsell
existing services to subscribers (such as offering minimum-latency computer gaming for an additional fee on top of basic
internet).
Three Types of Traffic
Internet traffic as viewed by ISPs can be thought of as placed into three categories: Sensitive, Best-Effort, and
Undesired.
Sensitive Traffic
Sensitive traffic is traffic whose Quality of Service ISPs care about. This usually includes VoIP, online gaming, video
streaming, and web surfing, but basically any application or protocol could fall under this umbrella. Shaping schemes are
generally tailored in such a way that the Quality of Service of these selected uses is guaranteed, or at least prioritized over
other classes of traffic. This can be accomplished by the absence of shaping schemes on these, or by positive shaping
(prioritization over others).
Best-Effort Traffic
Best effort traffic is all other kinds of non-detrimental traffic. This is traffic that is either not sensitive to Quality of
Service metrics (jitter, packet loss, latency) or traffic that is, and the ISP is not concerned about its Quality of Service. A
typical example of the former would be peer-to-peer traffic, the latter: online gaming
(though there are exceptions such as [1]).
Shaping schemes are generally tailored in such a way that this traffic gets 'what is left' of the bandwidth after sensitive
traffic has 'taken its share'.
Undesired Traffic
This category is generally limited to the delivery of spam and traffic created by worms, botnets, and other malicious attacks.
In some countries (for example, China), this definition can (and does) expand to such traffic as
non-local VoIP (Skype) or video streaming services, which are squelched to create a market for the
'in-house' services of the same type. Shaping schemes usually involve identifying and blocking this traffic entirely, or just by
severely hampering its operation. Rogers Communications in Canada has been accused of applying this type of shaping to peer-to-peer traffic in violation of Canadian Radio-television and Telecommunications Commission
(CRTC) regulations.[7]
Peer-To-Peer
Peer-to-peer (P2P) traffic is particularly troublesome for traffic shaping efforts for ISPs because it is designed to use any
and all available bandwidth which impacts QoS-sensitive applications (like online gaming)
that use comparatively small amounts of bandwidth. Most, if not all, popular P2P applications are location-agnostic[citation needed] (they care not/little if they
download from a user halfway around the world or one next door) makes them even more problematic, as traffic outside of a
network, which is the more expensive kind, will increase.
This has given P2P a bad reputation with internet service providers trying to roll out quality-dependent services (again, like
VoIP). Some may even view P2P as an 'attack' on their networks.[citation needed]
However, P2P is often listed as the reason subscribers choose broadband internet.[citation needed] Recent figures show that the usage
of one-fifth of the highest-usage subscribers must be added together to make up only close to 80% of P2P traffic on ISP networks.
[2]
Sandvine Incorporated has determined through traffic analysis that P2P traffic accounts for up
to 60% of traffic on most networks.[8] This shows, in contrast to previous studies and forecasts, that P2P has become more
mainstream than a handful of P2P users on the network.
These figures have influenced service providers to consider subscriber experience when implementing traffic
shaping.[citation needed] If P2P is being used by more
than the top fifth of an ISP's subscriber base, a harsh anti-P2P policy may have disastrous consequences on subscriber numbers,
increasing the subscriber Churn rate. In some extreme cases (like that of Rogers
Communications and ihaterogers.ca) this may damage
the ISPs reputation permanently.
P2P protocols are designed specifically to avoid being identified and with enough robustness that it is agnostic to standard
QoS metrics (out-of-order packets (jitter) just increase buffering, packet loss and latency just increase the download time)
means that it is best classified as Best-Effort traffic. At peak times when sensitive traffic is at its height, download speeds
will decrease. However, since P2P downloads are often background activities, it affects the subscriber experience little, so long
as the download speeds increase to their full potential when all other subscribers hang up their VoIP phones.
Enterprises
Traffic Shaping and Prioritization is becoming more and more common in the corporate market. Most companies with remote
offices are now connected via a WAN (Wide Area Network). Applications tend to become
centrally hosted at the head office and remote offices are expected to pull data from central databases and server farms. As
applications become more hungry in terms of bandwidth and prices of dedicated circuits being relatively high in most areas of the
world, instead of increasing the size of their WAN circuits, companies feel the need to properly manage their circuits to make
sure business-oriented traffic gets priority over best-effort traffic. Traffic shaping is thus a good means for companies to
avoid purchasing additional bandwidth while properly managing these resources.
Other upcoming technologies to this regards are Application Acceleration and WAN Optimization and Compression which are
fundamentally different from Traffic Shaping. Traffic Shaping defines bandwidth rules (or partitions as some vendors call them) whereas Application Acceleration using multiple
techniques like TCP Performance Enhancing Proxy. WAN Optimization and
Compression (WOC) on the other hand would use compression and differential algorithms and techniques to compress data streams or
send only differences in file updates. The latter is quite effective for chatty protocols like CIFS.
Uses
Traffic shaping is often used in combination with:
See also
Companies With Products Employing Traffic Shaping
Major Internet Service Providers Using Traffic Shaping
References
- ^ IETF RFC 2475 "An Architecture for Differentiated Services" section 2.3.3.3 - definition of
"Shaper"
- ^ PlusNet's Traffic Classes show classification and prioritisation policies used to differentiate between more and less
expensive Internet service
- ^ BitTorrent Protocol
- ^ SIN 450 Issue 1.2 May 2007 Suppliers' Information Note For The BT Network BT Wholesale - BT
IPstream Advanced Services - End User Speed Control and Downstream Quality of Service - Service Description
- ^ 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.
- ^ ATM Forum Traffic Management Specification, Version 4.0 Approved Specification 0056.00,
Section 5.5, Traffic Shaping
- ^ Geist, Michael. The Unintended Consequences of Rogers'
Packet Shaping.
- ^ Leydon, John. P2P swamps
broadband networks.
- "Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice" by John Evans, Clarence Filsfils (Morgan Kaufmann,
2007, ISBN 0-12-370549-5)
- "Peer-to-Peer File Sharing: The Impact of File Sharing on Service Provider Networks", Sandvine Incorporated, copyright
2002
External links
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
