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3. Quality of Service

Communication Networks Quality of Service (QOS) 1

  • QoS:
    • It is a set of technologies that are used to manage network resources and provide better services to users.
    • It is an inter-networking issue.
    • It manages traffic to reduce packet loss and delays.
    • The primary goal is to provide priority for a specific type of data. E.g. voice and video data needs more priority over file transfer data.
  • QoS has 4 parameters:
    • Reliability:
      • The probability of a network to deliver packets without errors.
      • It involves re-transmitting of lost packets is an example of reliability.
      • It means the guarantee of the delivery of packets.
      • It is more important for file transfer, main, and Internet access than for voice and video packets.
    • Delay:
      • It is the time taken by a packet to travel from the source to the destination.
      • It is more important for voice, video, and internet access packets than for file transfer and email packets.
    • Jitter:
      • It is the variation in the delay of packets belonging to the same receiver.
      • It is more important for voice and video packets than for file transfer and email packets.
      • It may change the order of packets on the receiver side.
    • Bandwidth:
      • It is the maximum data rate of a network or internet connection.
  • Techniques to improve Qos:
    • Scheduling:
      • It is a technique to prioritize packets, for example, the router prioritizes which packets to send/forward first.
      • It uses different types of queues to prioritize packets:
        • FIFO (First In First Out) queues: If the packets are received at a higher rate than being sent, the queue will be full and the packets will be dropped.
        • Priority queues: It classifies packets into different queues; the low-priority queue is only processed if the high-priority queue is empty. It suffers from starvation if more packets are received in the higher priority queue, then packets in the lower priority queue will never be sent or get delayed.
        • Weighted Fair Queuing (WFQ) queues: It is a combination of FIFO and priority queues. It sends packets from different queues in a round-robin manner. After sending a packet; the algorithm decides the next queue that a packet will be picked from, thus no queue will starve.
    • Traffic Shaping.
    • Admission Control.
    • Resource Reservation.

Traffic Engineering: Shaping vs. Policing 2

  • Traffic engineering techniques:
    • Shaping: queueing or delaying certain packets.
    • Policing: dropping packets that exceed a certain rate or based on a certain policy.
    • Scheduling: prioritizing certain packets over others by enqueuing traffic in different queues according to heterogeneous policies.
    • Washing: dropping portions of packets that are not necessary to complete the service.
  • Traffic shaping is used to control the rate of traffic sent to the network to conform with policing policies.
  • Traffic policing is used to enforce the rate of traffic sent to the network for purposes like:
    • The user paid for a certain amount of bandwidth, and cannot exceed it.
    • Congestion control.
    • Etc.

Traffic Shaping

  • Traffic shaping is a bandwidth modeling technique to keep network traffic load at acceptable levels.
  • Traffic shaping can act on the network traffic in three ways:
    • Forwarding: the traffic load is under the acceptable level, so the traffic is forwarded.
    • Delaying: the traffic load is over the acceptable level, so the traffic is delayed as long as the queue is not full.
    • Dropping: the traffic load is over the acceptable level, and the queue is full, so the traffic is dropped.
  • Types of Traffic Shaping:
    • Leaky Bucket Shaper.
    • Token Bucket Shaper.
    • Generic Cell Rate Algorithm (GCRA) Shaper.

Traffic Policing 6

  • Traffic policing is a bandwidth management technique to control the rate of traffic sent to the network.
  • It immediately drops the traffic that exceeds the acceptable level.
  • It only executes Drop and Forward actions.
  • Types of Traffic Policing:
    • Single Rate Three Color Marker (srTCM). 6
    • Two Rate Three Color Marker (trTCM). 6
    • Token Bucket Policer.

What Is Quality of Service (QoS)? 3

  • QoS is a basic feature of Huawei data communications products, including switches, routers, WLAN products, and firewalls.
  • QoS guarantees end-to-end service quality based on the requirements of different services. It helps improve the 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.
  • 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.
  • Delay:
    • The delay refers to the time required to transmit a packet or a group of packets from the transmit end to the receiving end. It consists of the transmission delay and processing delay.
    • The delay is expressed in milliseconds (ms).
    • 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 expressed in milliseconds (ms).
    • 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 prolong delays.
  • Packet Loss Rate:
    • The packet loss rate refers to the ratio of the number of lost packets to the number of transmitted packets.
    • The packet loss rate is expressed in percentage (%).
    • 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 packet in voice transmission.
    • The loss of a bit or 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.

QoS Service Models

  • The QoS model is not a specific function, but an E2E QoS scheme. It defines the ranges for the QoS indicators to ensure a specific service quality.
  • 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.
  • Some models:
    • Best-Effort Model.
    • Integrated Service (IntServ) Model.
    • Differentiated Service (DiffServ) Model.

Best-Effort Model

  • This is the default service model for the Internet and applies to various applications such as 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) Model

  • 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 acknowledgment 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.
  • he 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) Model

  • 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 rates.
  • 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:
    • Traffic Classification and Marking:
      • Traffic classification and marking are prerequisites for differentiated services.
      • Traffic classification divides packets into different classes or sets **different prioritie**s, 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, 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.
      • 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.

Quality of Service in Computer Networks (QoS) 4

  • Examples of services that require QoS, mostly real-time resource-intensive services:
    • IPTV, Internet Protocol Television.
    • Online gaming.
    • Streaming media.
    • Video conferencing.
    • VOD, Video on Demand.
    • VoIP, Voice over Internet Protocol.
  • A flow can consist of:
    • Packets from a particular application or an incoming interface.
    • Source destination addresses,
    • Source destination socket numbers,
    • Session identifiers.
  • Techniques Involved in QoS:
    • Scheduling:
      • FIFO: First In First Out.
      • Priority Queuing: only processes the low-priority queue if the high-priority queue is empty.
      • Weighted Fair Queuing: all queues are processed, but with higher priority queues being processed more frequently.
    • Traffic Shaping:
      • Leaky Bucket.
      • Token Bucket:
        • A counter is incremented at a fixed rate and decremented when a packet is sent.
        • Before adding a packet to the queue, the counter is checked to see if the packet can be queued or not.
  • Implementations of Qos:
    • Best Effort: all packets are treated equally.
    • Integrated Services (IntServ): RSVP is used to reserve resources for a flow.
    • Differentiated Services (DiffServ): packets are marked with a DSCP value; routers treat packets differently based on this value.
  • Advantages of QoS:
    • Unlimited software prioritization.
    • Better network management.
    • Enhanced user experience.
    • Point-to-factor network management.
    • Packet loss prevention.
    • Latency reduction.
  • Disadvantages of QoS:
    • Complexity.
    • Cost.
    • Performance.
    • Security.
    • Scalability.

A Comparative Investigation on Different QoS Mechanisms in Multi-Homed Networks 5

  • QoS can be characterized as a set of precisely specified metrics such as data loss, latency, jitter, and network resource utilization that are linked to the sensation or notion of quality that a network user has.
  • The IntServ-DiffServ collaboration in the state-of-the-art involves a DiffServ area in the center of two IntServ-supported regions.
  • The results showed that:
    • IntServ and DiffServ were the best two mechanisms that outperform all other QoS mechanisms; they sufficiently showed the suitability for multi-homing, whereas these two mechanisms could not achieve a permanent end-to-end QoS along with all the connection time.
    • However, the IntServ-DiffServ takes full advantage of these two mechanisms.
    • Moreover, the IntServ mechanism might be adopted to deliver non-interrupted audio/video communications, accordingly with the DiffServ mechanism to deliver low latency and stable service to prioritized communication traffic.
    • The best-effort provides a decent service to non-prioritized traffic, e.g., web-based and file exchange.

References

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  1. SK Wish. (2020, February 2). Communication networks quality of service (QOS) [Video]. YouTube. https://www.youtube.com/watch?v=-cGMmSx9Ag0 ↩

  2. Fulber-Garcia, V. (2022, November 13). Traffic engineering: Shaping vs. policing. Baeldung. https://www.baeldung.com/cs/traffic-engineering-shaping-vs-policing ↩

  3. Huawei. (2022, September 13). What Is the quality of service (QoS)? https://support.huawei.com/enterprise/en/doc/EDOC1100086518 ↩

  4. Mishra, A. (2022, July 13). Quality of service in computer networks (QoS). Scaler Topics. https://www.scaler.com/topics/computer-network/qos/ ↩

  5. Oleiwi, H. W., Al-Taie, H. L., Saeed, N., & Mhawi, D. N. (2022). A comparative investigation of different QoS mechanisms in multi-homed networks. Iraqi Journal of Industrial Research, 9(1), 1-11. https://ijoir.gov.iq/ijoir/index.php/jou/article/view/141 ↩

  6. Extreme SLX-OS QoS and Traffic Management Configuration Guide, 20.4.2. (2022). Extremenetworks.com. https://documentation.extremenetworks.com/slxos/sw/20xx/20.4.2/traffic/GUID-AB18E6FD-09E4-4096-998C-76134F47F76E.shtml ↩↩↩