Quality of service is important for real-time streaming multimedia applications such as voice over IP, multiplayer online games and IPTV, since these often require fixed bit rate and are delay sensitive.
During the session it may monitor the achieved level of performance, for example the data rate and delay, and dynamically control scheduling priorities in the network nodes.
An alternative to complex QoS control mechanisms is to provide high quality communication over a best-effort network by over-provisioning the capacity so that it is sufficient for the expected peak traffic load.
QoS is sometimes used in application layer services such as telephony and streaming video to describe a metric that reflects or predicts the subjectively experienced quality.
Examples are Frame Relay, Asynchronous Transfer Mode (ATM) and Multiprotocol Label Switching (MPLS) (a technique between layer 2 and 3).
[4] Many things can happen to packets as they travel from origin to destination, resulting in the following problems as seen from the point of view of the sender and receiver: A defined quality of service may be desired or required for certain types of network traffic, for example: These types of service are called inelastic, meaning that they require a certain minimum bit rate and a certain maximum latency to function.
Shorter data units and built-in QoS were some of the unique selling points 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.
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 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 TCP) that over time increase the amount of data placed on the network until all available bandwidth is consumed and packets are dropped.
The amount of over-provisioning in interior links required to replace QoS depends on the number of users and their traffic demands.
It was believed that this approach would not scale with the growth of the Internet,[7] 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.
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.
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.
[13] The more scalable traffic engineering version, RSVP-TE, is used in many networks to establish traffic-engineered Multiprotocol Label Switching (MPLS) label-switched paths.
Research consortia such as "end-to-end quality of service support over heterogeneous networks" (EuQoS, from 2004 through 2007)[16] and fora such as the IPsphere Forum[17] developed more mechanisms for handshaking QoS invocation from one domain to the next.
EuQoS conducted experiments to integrate Session Initiation Protocol, Next Steps in Signaling and IPsphere's SSS with an estimated cost of about 15.6 million Euro and published a book.
[23] A broader European project called "Architecture and design for the future Internet" known as 4WARD had a budget estimated at 23.4 million Euro and was funded from January 2008 through June 2010.
As all electronic commerce on the Internet requires the use of such strong cryptography protocols, unilaterally downgrading the performance of encrypted traffic creates an unacceptable hazard for customers.
Protocols like ICA and RDP may encapsulate other traffic (e.g. printing, video streaming) with varying requirements that can make optimization difficult.
[29][a] The group predicted that "logistical, financial, and organizational barriers will block the way toward any bandwidth guarantees" by protocol modifications aimed at QoS.
[30] They believed that the economics would encourage network providers to deliberately erode the quality of best effort traffic as a way to push customers to higher priced QoS services.
[31] Bachula's testimony has been cited by proponents of a law banning quality of service as proof that no legitimate purpose is served by such an offering.
[32] Some QoS-related IETF Request for Comments (RFC)s are Baker, Fred; Black, David L.; Nichols, Kathleen; Blake, Steven L. (December 1998), Definition of the Differentiated services Field (DS Field) in the IPv4 and IPv6 Headers, doi:10.17487/RFC2474, RFC 2474, and Braden, Robert T.; Zhang, Lixia; Berson, Steven; Herzog, Shai; Jamin, Sugih (September 1997), Braden, R.