Question

nowadays computer network communication depends on 5 layers. if we would like to improve the QOS quality of service infrastructure what would be your suggestion for each layer?
explain in detail

Answer 1

To improve the QOS quality of service infrastructure these changes can be done:

1. General model for traffic prioritization :

Traffic should always follow a prioritization scheme in order to guarantee specific bandwidth requirements from real time communications. This scheme can be represented in the form of a general model which applies to all applications with special conditions (such as maximum delay) to be met. This model is represented in Figure 1. After receiving the traffic through the incoming interface, the first step would be to mark the incoming frames/packets according to our needs. This should be done using a different traffic class for every kind of traffic with different needs. A common practice is to classify voice and video in its own class, away from any other type. Next, the traffic should be ready to be classified according to our own requirements. Delay sensitive data should receive a special treatment to avoid delay at all costs. Any other data should be considered as delay tolerant and further processed in order to provide the bandwidth only to those applications that do really need it. The most important traffic class should receive a strict priority using Low Latency Queuing (LLQ). LLQ allows traffic to skip directly to the output interface, reducing its processing time. By specifying a reasonable amount of the total bandwidth, we will be guaranteeing the required resources for this traffic type. The rest of the classes must go through WRED congestion avoidance mechanisms and queuing (see figure 1). This process would divide the remaining bandwidth according to the policies configured for each data class. Once all conditions are met and all policies are applied, the now marked and prioritized traffic is sent through the router’s outgoing interface to its destination.

Now that we have shown the general model, we will describe in detail what we propose to improve the performance in each layer of our model.

2, Improving data link layer :

To improve the performance of the network at layer 2, we need to configure the operation mode and traffic prioritization of the switches at the data link layer.

i)Switch mode Operation. How a frame is switched from the source port to its destination is a trade off between latency and reliability. A switch can start to transfer the frame as soon as the destination MAC address is received. This is called cutthrough and results in the lowest latency through the switch. No error checking is available, but considering the application, it is more important to transfer frames faster than to lose some frames.

ii) Traffic Prioritization with 802.1p. If VLANs are used inside our network and traffic is sent among users of the same VLAN, the traffic will never go past layer 2. For this reason, we need to add layer 2 priorities to our designed infrastructure. The IEEE 802.1p is an extension of the IEEE 802.1Q (VLANs tagging) standard. The VLAN tag has two parts: VLAN ID (12-bit) and Prioritization (3-bit). The prioritization field was not defined in the 802.1Q, but is defined in 802.1p. VLAN frame tagging is an approach that has been specifically developed for switched communications and gives the possibility of using the prioritization field. The 802.1p standard also offers provisions to filter multicast traffic to ensure it does not proliferate over layer 2-switched networks. The 802.1p header includes a three-bit field for prioritization, which allows packets to be grouped into various traffic classes. It can also be defined as best-effort QoS (Quality of Service) or CoS (Class of Service) at Layer 2 and can be implemented in network adapters and switches without involving any reservation setup. 802.1p traffic is simply classified and sent to the destination; no bandwidth reservations are established. IEEE 802.1p establishes eight levels of priority. The highest priority is seven, which might go to network-critical traffic like Open Shortest Path First (OSPF) table updates. Values five and six may be used for delay-sensitive applications such as interactive video and voice. Data classes four through one range from controlled-load applications such as streaming multimedia and business-critical traffic - carrying SAP data, for instance - down to "loss eligible" traffic. The zero value is used as a besteffort default, invoked automatically when no other value has been set. Using the described datagram fields will create a faster infrastructure in the data link layer. This is sometimes described as "layer 2 quality of service".  

Improving network layer :

When willing to provide QoS for traffic that will flow outside our LAN, we need to specify layer 3 priorities to obtain the desired latency and bandwidth. QoS refers to both class of service (CoS) and type of service (ToS). The basic goal of these is to guarantee specific bandwidth and latency for a particular application. To achieve this, we use the Differentiated Services Codepoint (DSCP) or the IP Precedence field in the packet header. These values provide the necessary marking as suggested by the first step of our general model (Figure 1) for layer 3 traffic. DSCP is composed by the first six bits in the ToS byte, while the IP Precedence is created with the first three bits in the ToS value. The IP Precedence value is actually part of the IP DSCP value, so both values can not be set simultaneously. If both values are set simultaneously, the DSCP value overwrites the IP precedence one. The marking of traffic at layers 2 or 3 is crucial to providing QoS within a network. We suggest deciding at which layer to mark after considering the following: • Layer 2 marking can be performed for non IP traffic. This is the only option available for non IP aware switches. • Layer 3 marking will carry the QoS information end-to-end. We propose to use both DSCP to mark packets and use CoS to mark frames to allow layer 2 devices to provide the QoS requirements of frames at the data link layer. A mapping between layer two CoS and layer three QoS (DSCP) is possible. However, since we are just trying to improve QoS inside our Autonomous System, we will only propose tools associated with the network edge. After completing the marking stage, classification will be needed to create different classes of traffic with different priorities.