Smart QoS

QoS Background

Diverse
services result in a sharp increase in network traffic, which may cause
network congestion, increase forwarding delay, or even cause packet
loss. Any of these situations will cause service quality deterioration
or even service interruption. Therefore, real-time services require a
solution to prevent network congestion. The best solution is to increase
network bandwidth, but increasing network bandwidth is costly. The most
cost-effective way is to use a "guarantee" policy to manage traffic
congestion.

QoS
guarantees end-to-end service quality based on the requirements of
different services. It helps improve utilization of network resources
and allows different types of traffic to preempt network resources based
on their priorities; for example, voice, video, and important data
applications can be processed preferentially on network devices.

QoS Indicators

The
factors that affect the network service quality need to be learned to
improve network quality. Traditionally, factors that affect network
quality include link bandwidth, packet transmission delay, jitter, and
packet loss rate. To improve the network service quality, ensure the
bandwidth of transmission links, and reduce packet transmission delay,
jitter, and packet loss rate. These factors that affect the network
service quality become QoS indicators.

• Bandwidth

  The
  bandwidth, also called throughput, refers to the maximum number of
  transmitted data bits between two ends within a specified period (1
  second) or the average rate at which specified data flows are
  transmitted between two network nodes. Bandwidth is expressed in bit/s.

  Generally,
  data transmission capability and network service quality are
  accompanied by the bandwidth. In other words, a lane is positive to the
  traffic flow capacity with low traffic jam in a highway. Network users
  all expect higher bandwidth; however, the O&M costs are higher.
  Therefore, bandwidth becomes a serious bottleneck as the Internet
  develops rapidly and services become increasingly diversified.

• Delay

  The
  delay refers to the time required to transmit a packet or a group of
  packets from the transmit end to the receive end. It consists of the
  transmission delay and processing delay.

  Voice
  transmission is used as an example. A delay refers to the period during
  which words are spoken and then heard. Generally, people are
  insensitive to a delay of less than 100 ms. If a delay ranging from 100
  ms to 300 ms occurs, a speaker can sense slight pauses in the
  responder's reply, which can seem annoying to both. If a delay longer
  than 300 ms occurs, both the speaker and responder obviously sense the
  delay and have to wait for responses. If the speaker cannot wait but
  repeats what has been said, voices overlap and the quality of the
  conversation deteriorates severely.

• Jitter

  If
  network congestion occurs, the delays of packets over the same
  connection are different. The jitter is used to describe the degree of
  delay change, that is, the time difference between the maximum delay and
  the minimum delay.

  Jitter
  is an important parameter for real-time transmission, especially for
  real-time services, such as voice and video, which are zero-tolerant of
  jitters because jitters will cause voice or video interruptions.

  Jitters
  also affect protocol packet transmission. Specific protocol packets are
  transmitted at a fixed interval. High jitters may cause flapping of the
  protocols.

  Jitters
  exist on networks but the service quality will not be affected if
  jitters do not exceed a specific tolerance. The buffer can alleviate
  excess jitters but prolongs delays.

• Packet Loss Rate

  The
  packet loss rate refers to the ratio of lost packets to total packets.
  Slight packet loss does not affect services. For example, users are
  unaware of the loss of a bit or a packet in voice transmission. The loss
  of a bit or a packet in video transmission may cause the image on the
  screen to become garbled instantly, but the image can be restored
  quickly.

  TCP
  is used to transmit data to handle slight packet loss because TCP
  instantly retransmits the packets that have been lost. If severe packet
  loss does occur, the packet transmission efficiency is affected. QoS
  focuses on the packet loss rate. The network packet loss rate must be
  controlled within a certain range during transmission.

QoS Service Models

How
are QoS indicators defined within proper ranges to improve network
service quality? The QoS model is involved. The QoS model is not a
specific function, but an E2E QoS scheme. For example, intermediate
devices may be deployed between two connected hosts. E2E service quality
guarantee can be implemented only when all devices on a network use the
same QoS service model. International organizations such as the IETF
and ITU-T designed QoS models for their concerned services. The
following describes three main QoS service models.

• Best-Effort

  Best-Effort
  is the default service model for the Internet and applies to various
  network applications, such as the File Transfer Protocol (FTP) and
  email. It is the simplest service model, in which an application can
  send any number of packets at any time without notifying the network.
  The network then tries its best to transmit the packets but provides no
  guarantee of performance in terms of delay and reliability.

  The Best-Effort model is suitable for services that have low requirements for delay and packet loss rate.

• Integrated Service (IntServ)

  In
  the IntServ model, an application uses a signaling protocol to notify
  the network of its traffic parameters and apply for a specific level of
  QoS before sending packets. The network reserves resources for the
  application based on the traffic parameters. After the application
  receives an acknowledgement message and confirms that sufficient
  resources have been reserved, it starts to send packets within the range
  specified by the traffic parameters. The network maintains a state for
  each packet flow and performs QoS behaviors based on this state to
  guarantee application performance.

  The
  IntServ model uses the Resource Reservation Protocol (RSVP) for
  signaling. The RSVP protocol reserves resources such as bandwidth and
  priority on a known path, and each network element along the path must
  reserve required resources for data flows requiring QoS guarantee. That
  is, each network element maintains a soft state for each data flow. A
  soft state is a temporary state that is periodically updated through
  RSVP messages. Each network element checks whether sufficient resources
  can be reserved based on these RSVP messages. The path is available only
  if all involved network elements can provide sufficient resources.

• Differentiated Service (DiffServ)

  The
  DiffServ model classifies packets on a network into multiple classes
  and takes different actions for each class. When network congestion
  occurs, packets of different classes are processed based on their
  priorities, resulting in different packet loss rates, delay, and jitter.
  Packets of the same class are aggregated and sent as a whole to ensure
  consistent delay, jitter, and packet loss rate.

  Unlike
  the IntServ model, the DiffServ model does not require a signaling
  protocol. In this model, an application does not need to apply for
  network resources before sending packets. Instead, the application sets
  QoS parameters in the packets, through which the network can learn the
  QoS requirements of the application. The network provides differentiated
  services based on the QoS parameters of each data flow and does not
  need to maintain a state for each data flow. DiffServ takes full
  advantage of IP networks' flexibility and extensibility and transforms
  information in packets into per-hop behaviors (PHBs), greatly reducing
  signaling operations. DiffServ is the most commonly used QoS model on
  current networks. QoS implementation described in the subsequent

  sections is based on this model.

Mechanisms in the DiffServ Model

QoS
services based on the DiffServ model are supported on Huawei data
communications products, including switches, routers, WLAN products, and
firewalls. The DiffServ model involves the following QoS mechanisms:

• Traffic classification and marking

  Traffic
  classification and marking are prerequisites for differentiated
  services. Traffic classification divides packets into different classes
  or sets different priorities, and can be implemented using traffic
  classifiers configured on the Modular QoS Command-Line Interface (MQC).
  Traffic marking sets different priorities for packets and can be
  implemented through priority mapping and re-marking.

• Traffic policing, traffic shaping, and interface-based rate limiting

  Traffic
  policing and traffic shaping control the traffic rate within a
  bandwidth limit. Traffic policing drops excess traffic when the traffic
  rate exceeds the limit, whereas traffic shaping buffers excess traffic.
  Traffic policing and traffic shaping can be performed on an interface to
  implement interface-based rate limiting.

• Congestion management and congestion avoidance

  Congestion
  management buffers packets in queues upon network congestion and uses a
  scheduling algorithm to determine the forwarding order. Congestion
  avoidance monitors network resource usage and drops packets to mitigate
  network overload if congestion worsens.

Traffic
classification and marking are the basis of differentiated services.
Traffic policing, traffic shaping, interface-based rate limiting,
congestion management, and congestion avoidance control network traffic
and resource allocation to implement differentiated services.