Editing Quality of service (section)

===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.