WHAT CAN WE DO WITH QOS?

QoS is essential for managing traffic in today’s packet-based networks and includes these capabilities:

● Prioritizing traffic over other traffic based on protocol, address and port number.

● Filtering traffic upon ingress or egress.

● Controlling the allowed bandwidth transmitted or received on the device.

● Reading and writing QoS behavior requirements in the packet header.

● Controlling congestion so that the device sends the highest priority traffic based on scheduler priorities.

● Controlling packet loss using random early detection (RED) algorithms, so that the device knows the packets to drop or process.

IMPLEMENTING QOS

When implementing QoS tools and strategies, organizations should follow these steps:

PLANNING

The organization should gain an understanding of each department's service needs and requirements, choose a fitting model, and cultivate buy-in from stakeholders.

DESIGN

The organization should then take note of all significant software and hardware changes, and apply the chosen QoS model to the specifics of its network architecture.

TESTING

The organization should test QoS settings and policies in a safe, controlled testing environment where bugs can be worked out.

DEPLOYMENT

Policies should be rolled-out iteratively, in phases. An organization may choose to roll out policies by network segment or by separate QoS function (what each policy does).

MONITORING AND ANALYSIS

Policies should be adjusted to improve performance according to performance data.

Three models exist to implement QoS: Best Effort, Integrated Services and Differentiated Services.

Best Effort A QoS model where all the packets receive the same priority, and there is no guaranteed delivery of packets. Best Effort is applied when networks have not configured QoS policies or when the infrastructure does not support QoS.

Integrated Services (IntServ) A QoS model that reserves bandwidth along a specific path on the network. Applications ask the network for resource reservation, and network devices monitor the flow of packets to make sure network resources can accept the packets.

IntServ Functions

● Admission control

● Classification

● Policing

● Queuing

● Scheduling

Implementing IntServ requires IntServ-capable routers and uses the Resource Reservation Protocol (RSVP) for network resource reservation. IntServ has limited scalability and high consumption of network resources.

Pros ● Explicit resource admission control (end to end) ● Per-request policy admission control ● Signaling of dynamic port numbers

Cons ● Continuous signaling because of stateful architecture ● Flow-based approach not scalable to large implementations, such as public Internet

Differentiated Services (DiffServ) A QoS model where network elements, such as routers and switches, are configured to service multiple classes of traffic with different priorities. Network traffic must be divided into classes based on a company's configuration.

For example, voice traffic can be assigned a higher priority than other types of traffic. Packets are assigned priorities using Differentiated Services Code Point (DSCP) for classification. DiffServ also uses per-hop behavior to apply QoS techniques, such as queuing and prioritization, to packets.

Network architecture also affects how an organization implements QoS. A Multiprotocol Label Switching (MPLS) network includes a private link that offers end-to-end QoS along a single path. SLAs for MPLS specify bandwidth, QoS, latency and uptime. However, an MPLS can be expensive for organizations.

Software-defined WAN (SD-WAN) uses multiple connectivity types, including MPLS and broadband. SD-WAN monitors the state of current network connections for performance issues and uses its multiple connectivity types to fail over based on state. For example, if packet loss exceeds a certain level on one connection, SD-WAN capabilities will look for an alternative connection.

Pros ● High scalable ● Many levels of quality possible

Cons ● No absolute guarantee ● Complex mechanisms