Editing Quality of service (section)

==Mechanisms==
Circuit switched networks, especially those intended for voice transmission, such as ATM or [[GSM]], have QoS in the core protocol, resources are reserved at each step on the network for the call as it is set up, there is no need for additional procedures to achieve required performance. Shorter data units and built-in QoS were some of the [[unique selling point]]s of ATM for applications such as [[video on demand]].

When the expense of mechanisms to provide QoS is justified, network customers and providers can enter into a contractual agreement termed a [[service-level agreement]] (SLA) which specifies guarantees for the ability of a connection to give guaranteed performance in terms of throughput or latency based on mutually agreed measures.

===Over-provisioning===
An alternative to complex QoS control mechanisms is to provide high quality communication by generously over-provisioning a network so that capacity is based on peak traffic load estimates. This approach is simple for networks with predictable peak loads. This calculation may need to appreciate demanding applications that can compensate for variations in bandwidth and delay with large receive buffers, which is often possible for example in video streaming.

Over-provisioning can be of limited use in the face of transport protocols (such as [[Transmission Control Protocol|TCP]]) that over time increase the amount of data placed on the network until all available bandwidth is consumed and packets are dropped. Such greedy protocols tend to increase latency and packet loss for all users.

The amount of over-provisioning in interior links required to replace QoS depends on the number of users and their traffic demands. This limits usability of over-provisioning. Newer more bandwidth intensive applications and the addition of more users results in the loss of over-provisioned networks. This then requires a physical update of the relevant network links which is an expensive process. Thus over-provisioning cannot be blindly assumed on the Internet.

Commercial VoIP services are often competitive with traditional telephone service in terms of call quality even without QoS mechanisms in use on the user's connection to their ISP and the VoIP provider's connection to a different ISP. Under high load conditions, however, VoIP may degrade to cell-phone quality or worse. The mathematics of packet traffic indicate that network requires just 60% more raw capacity under conservative assumptions.<ref>{{Cite book| first1 = M.| last1 = Yuksel| first2 = K. K.| last2 = Ramakrishnan| first3 = S.| last3 = Kalyanaraman| first4 = J. D.| last4 = Houle| first5 = R.| last5 = Sadhvani| date = 2007| title = 2007 Fifteenth IEEE International Workshop on Quality of Service| chapter = Value of Supporting Class-of-Service in IP Backbones| url = http://www.cse.unr.edu/~yuksem/my-papers/iwqos07.pdf| pages = 109–112| location = Evanston, IL, USA| doi = 10.1109/IWQOS.2007.376555| isbn = 978-1-4244-1185-6| citeseerx = 10.1.1.108.3494| s2cid = 10365270| access-date = 2009-01-24| archive-date = 2012-04-30| archive-url = https://web.archive.org/web/20120430131119/http://www.cse.unr.edu/~yuksem/my-papers/iwqos07.pdf| url-status = dead}}</ref>

===IP and Ethernet efforts===
Unlike single-owner networks, the [[Internet]] is a series of exchange points interconnecting private networks.<ref name="KAHNVID">{{cite web |url=http://archive.computerhistory.org/lectures/an_eveninig_with_robert_kahn.lecture.2007.01.09.wmv |title=An Evening With Robert Kahn |archive-url=https://web.archive.org/web/20081219124325/http://archive.computerhistory.org/lectures/an_eveninig_with_robert_kahn.lecture.2007.01.09.wmv |archive-date=December 19, 2008 |publisher=[[Computer History Museum]] |date=9 Jan 2007}}</ref> Hence the Internet's core is owned and managed by a number of different [[network service provider]]s, not a single entity. Its behavior is much more [[predictability|unpredictable]].

There are two principal approaches to QoS in modern packet-switched IP networks, a parameterized system based on an exchange of application requirements with the network, and a prioritized system where each packet identifies a desired service level to the network.
*[[Integrated services]] ("IntServ") implements the parameterized approach. In this model, applications use the [[Resource Reservation Protocol]] (RSVP) to request and reserve resources through a network.
*[[Differentiated services]] ("DiffServ") implements the prioritized model. DiffServ marks packets according to the type of service they desire. In response to these markings, routers and switches use various [[network scheduler|scheduling]] strategies to tailor performance to expectations. Differentiated services code point (DSCP) markings use the first 6 bits in the [[Type of service|ToS]] field (now renamed as the DS field) of the [[IPv4 header|IP(v4) packet header]].

Early work used the integrated services (IntServ) philosophy of reserving network resources. In this model, applications used RSVP to request and reserve resources through a network. While IntServ mechanisms do work, it was realized that in a broadband network typical of a larger service provider, Core routers would be required to accept, maintain, and tear down thousands or possibly tens of thousands of reservations. It was believed that this approach would not scale with the growth of the Internet,<ref>{{Cite book |title=Handbook of Image and Video Processing |date=2005 |edition=2nd |isbn=978-0-12-119792-6 |section=4.9 |quote=However, the effort required in setting flow-based resource reservations along the route is enormous. Further, the control signaling required and state maintenance at routers limit the scalability of this approach.}}</ref> and in any event was antithetical to the [[end-to-end principle]], the notion of designing networks so that core routers do little more than simply switch packets at the highest possible rates.

Under DiffServ, packets are marked either by the traffic sources themselves or by the [[edge device]]s where the traffic enters the network. In response to these markings, routers and switches use various queuing strategies to tailor performance to requirements. At the IP layer, DSCP markings use the 6 bit DS field in the IP packet header. At the MAC layer, [[VLAN]] [[IEEE 802.1Q]] can be used to carry 3 bit of essentially the same information. Routers and switches supporting DiffServ configure their network scheduler to use multiple queues for packets awaiting transmission from bandwidth constrained (e.g., wide area) interfaces. Router vendors provide different capabilities for configuring this behavior, to include the number of queues supported, the relative priorities of queues, and bandwidth reserved for each queue.

In practice, when a packet must be forwarded from an interface with queuing, packets requiring low jitter (e.g., [[VoIP]] or [[videoconferencing]]) are given priority over packets in other queues. Typically, some bandwidth is allocated by default to network control packets (such as [[Internet Control Message Protocol]] and routing protocols), while best-effort traffic might simply be given whatever bandwidth is left over.

At the [[medium access control]] (MAC) layer, [[VLAN]] [[IEEE 802.1Q]] and [[IEEE 802.1p]] can be used to distinguish between Ethernet frames and classify them. Queueing theory models have been developed on performance analysis and QoS for MAC layer protocols.<ref>{{cite journal|title=Performance analysis of the IEEE 802.11 distributed coordination function|last1=Bianchi|first1=Giuseppe|year=2000|doi=10.1109/49.840210|journal=IEEE Journal on Selected Areas in Communications|volume=18|issue=3|pages=535–547|citeseerx=10.1.1.464.2640}}</ref><ref>{{cite journal|title=Analytical Models for Understanding Misbehavior and MAC Friendliness in CSMA Networks|last1=Shi|first1=Zhefu|last2=Beard|first2=Cory|last3=Mitchell|first3=Ken|year=2009|doi=10.1016/j.peva.2009.02.002|journal=Performance Evaluation|volume=66|issue=9–10|pages=469|citeseerx=10.1.1.333.3990}}</ref>

[[Cisco IOS]] NetFlow and the Cisco Class Based QoS (CBQoS) Management Information Base (MIB) are marketed by [[Cisco Systems]].
<ref>{{cite web |author= Ben Erwin |publisher= [[NetQoS]] |title= How To Manage QoS In Your Environment, Part 1 of 3 |work= Network Performance Daily video |date= December 16, 2008 |url= http://www.networkperformancedaily.com/2008/12/whiteboard_series_how_to_manag.html |access-date= October 15, 2011 |archive-date= September 29, 2011 |archive-url= https://web.archive.org/web/20110929212249/http://www.networkperformancedaily.com/2008/12/whiteboard_series_how_to_manag.html |url-status= dead }}</ref>

One compelling example of the need for QoS on the Internet relates to [[congestive collapse]]. The Internet relies on congestion avoidance protocols, primarily as built into [[Transmission Control Protocol]] (TCP), to reduce traffic under conditions that would otherwise lead to congestive collapse. QoS applications, such as [[VoIP]] and [[IPTV]], require largely constant bitrates and low latency, therefore they cannot use TCP and cannot otherwise reduce their traffic rate to help prevent congestion. [[Service-level agreement]]s limit traffic that can be offered to the Internet and thereby enforce [[traffic shaping]] that can prevent it from becoming overloaded, and are hence an indispensable part of the Internet's ability to handle a mix of real-time and non-real-time traffic without collapse.

===Protocols===
Several QoS mechanisms and schemes exist for IP networking.
*The [[type of service]] (ToS) field in the [[IPv4 header]] (now superseded by [[DiffServ]])
*[[Differentiated services]] (DiffServ)
*[[Integrated services]] (IntServ)
*[[Resource Reservation Protocol]] (RSVP)
*[[RSVP-TE]]

QoS capabilities are available in the following network technologies.
*[[Multiprotocol Label Switching]] (MPLS) provides eight QoS classes<ref>{{cite web|title=VoIP on MPLS|url=http://searchunifiedcommunications.techtarget.com/tutorial/VoIP-on-MPLS|publisher=Search Unified Communications|access-date=12 March 2012}}</ref>
*[[Frame Relay]]
*[[X.25]]
*Some [[DSL modem]]s
*ATM
*[[Ethernet]] supporting [[IEEE 802.1Q]] with [[Audio Video Bridging]] and [[Time-Sensitive Networking]]
*[[Wi-Fi]] supporting [[IEEE 802.11e]]
*[[HomePNA]] home networking over coax and phone wires
*The [[G.hn]] home networking standard provides QoS by means of ''contention-free transmission opportunities'' (CFTXOPs) which are allocated to flows which require QoS and which have negotiated a contract with the network controller. G.hn also supports non-QoS operation by means of contention-based time slots.