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https://en.wikipedia.org/wiki/Quality_of_service
Quality of service (QoS) is the overall performance of a telephony or computer network, particularly the performance seen by the users of the network. To quantitatively measure quality of service, several related aspects of the network service are often considered, such as error rates, bit rate, throughput, transmission delay, availability, jitter, etc.
In the field of computer networking and other packet-switched telecommunication networks, quality of service refers to traffic prioritization and resource reservation control mechanisms rather than the achieved service quality. Quality of service is the ability to provide different priority to different applications, users, or data flows, or to guarantee a certain level of performance to a data flow.
Quality of service is particularly important for the transport of traffic with special requirements. In particular, developers have introduced technology to allow computer networks to become as useful as telephone networks for audio conversations, as well as supporting new applications with even stricter service demands.
https://zh.wikipedia.org/wiki/服务质量
问题的提出
在因特网创建的初期并没有意识到QoS应用的需要，因此整个因特网运作如一个“竭尽全力”的系统。每段信息都有4个“服务类别”位和3个“优先级”位，但是他们完全没有派上用场。依发送和接收者看来，数据包从起点到终点的传输过程中会发生许多事情，并产生如下有问题的结果：
- 丢失数据包 - 当数据包到达一个缓冲器已满的路由器时，则代表此次的发送失败，路由器会依网络的状况决定要丢弃一部分、不丢弃或者是丢弃所有的数据包，而且这不可能在预先就知道，接收端的应用程序在这时必须请求重新发送，而这同时可能造成总体传输严重的延迟。
- 延迟 - 或许需要很长时间才能将数据包传送到终点，因为它会被漫长的队列迟滞，或需要运用间接路由以避免阻塞；也许能找到快速、直接的路由。总之，延迟非常难以预料。
- 传输顺序出错 - 当一群相关的数据包被路由经过因特网时，不同的数据包可能选择不同的路由器，这会导致每个数据包有不同的延迟时间。最后数据包到达目的地的顺序会和数据包从发送端发提交去的顺序不一致，这个问题必须要有特殊额外的协议负责刷新失序的数据包。
需要QoS的应用
流量约定（SLA, Service Level Agreement服务等级协议）给数据流设定优先级，以此在网络／协议层面上，根据相互商定的尺度，设定有保障的性能、通过量、延迟等界限。一些特定形式的网络数据流需要定义服务质量，例如：
这些类型的服务被称为非弹性，意思是它们需要固定的带宽才能运作--如果得到多余的带宽，它们也无法使用；如果得到较少的带宽，则根本无法工作。相形之下，弹性应用可以从多余的带宽中受益。
保持QoS
实质上有两种方式提供QoS保证。第一种，就是简单地提供大量的资源，用丰富、安全的余量设备应付预期中的“高峰”需求。这样既好又简单，然而有人认为这种方式代价昂贵，而且不能应对高峰需求超越预期的情形，部署额外的资源也很耗费时间。
第二种是要求用户预约带宽，并且仅在能够提供可靠服务的前提下接受预约。自然，可以为预约服务向用户收费。常用的实现方法有两类。
History
Conventional Internet routers and LAN switches operate on a best effort basis. This equipment is less expensive, less complex and faster and thus more popular than competing more complex technologies that provided QoS mechanisms. There were four “Type of service” bits and three “Precedence” bits provided in each IP packet header, but they were not generally respected. These bits were later re-defined as Differentiated services code points (DSCP).
With the advent of IPTV and IP telephony, QoS mechanisms are increasingly available to the end user.[citation needed]
A number of attempts for layer 2 technologies that add QoS tags to the data have gained popularity in the past. Examples are frame relay, asynchronous transfer mode (ATM) and multiprotocol label switching (MPLS) (a technique between layer 2 and 3). Despite these network technologies remaining in use today, this kind of network lost attention after the advent of Ethernet networks. Today Ethernet is, by far, the most popular layer 2 technology. Ethernet uses 802.1p to signal the priority of a frame.
Qualities of traffic
In packet-switched networks, quality of service is affected by various factors, which can be divided into “human” and “technical” factors. Human factors include: stability of service, availability of service, delays, user information. Technical factors include: reliability, scalability, effectiveness, maintainability, grade of service, etc.[5]
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:
- Low throughput
- Due to varying load from disparate users sharing the same network resources, the bit rate (the maximum throughput) that can be provided to a certain data stream may be too low for realtime multimedia services if all data streams get the same scheduling priority.
- Dropped packets
- The routers might fail to deliver (drop) some packets if their data loads are corrupted, or the packets arrive when the router buffers are already full. The receiving application may ask for this information to be retransmitted, possibly causing severe delays in the overall transmission.
- Errors
- Sometimes packets are corrupted due to bit errors caused by noise and interference, especially in wireless communications and long copper wires. The receiver has to detect this and, just as if the packet was dropped, may ask for this information to be retransmitted.
- Latency
- It might take a long time for each packet to reach its destination, because it gets held up in long queues, or it takes a less direct route to avoid congestion. This is different from throughput, as the delay can build up over time, even if the throughput is almost normal. In some cases, excessive latency can render an application such as VoIP or online gaming unusable.
- Jitter
- Packets from the source will reach the destination with different delays. A packet's delay varies with its position in the queues of the routers along the path between source and destination and this position can vary unpredictably. This variation in delay is known as jitter and can seriously affect the quality of streaming audio and/or video.
- Out-of-order delivery
- When a collection of related packets is routed through a network, different packets may take different routes, each resulting in a different delay. The result is that the packets arrive in a different order than they were sent. This problem requires special additional protocols responsible for rearranging out-of-order packets to an isochronous state once they reach their destination. This is especially important for video and VoIP streams where quality is dramatically affected by both latency and lack of sequence.