Optimizing Converged Cisco Networks (ONT) Module 3: Introduction to IP QoS
Introducing QoS
Objectives Explain why converged networks require QoS. Identify the major quality issues with converged networks. Calculate available bandwidth given multiple flows. Describe mechanisms designed to use bandwidth more efficiently. Describe types of delay. Identify ways to reduce the impact of delay on quality. Describe packet loss and ways to prevent or reduce packet loss in the network.
Traditional Nonconverged Network Traditional data traffic characteristics: Bursty data flow FIFO access Not overly time-sensitive; delays OK Brief outages are survivable
Converged Network Realities Converged network realities: Constant small-packet voice flow competes with bursty data flow. Critical traffic must have priority. Voice and video are time-sensitive. Brief outages are not acceptable. Converged networks must provide secure, predictable, measurable, and, sometimes, guaranteed services.
Converged Network Quality Issues Lack of bandwidth: Multiple flows compete for a limited amount of bandwidth. End-to-end delay (fixed and variable): Packets have to traverse many network devices and links; this travel adds up to the overall delay. (Fixed: serialization + propagation. Variable: Processing + queuing) Variation of delay (jitter): Sometimes there is a lot of other traffic, which results in varied and increased delay. Packet loss: Packets may have to be dropped when a link is congested.
Measuring Available Bandwidth The maximum available bandwidth is the bandwidth of the slowest link. Multiple flows are competing for the same bandwidth, resulting in much less bandwidth being available to one single application. A lack in bandwidth can have performance impacts on network applications.
Increasing Available Bandwidth Upgrade the link (the best but also the most expensive solution). Improve QoS with advanced queuing mechanisms to forward the important packets first. Compress the payload of Layer 2 frames (end to end, takes time, may increase delay). Compress IP packet headers (TCP or RTP, hop by hop).
Using Available Bandwidth Efficiently Voice (Highest) Data (High) 1 1 2 2 Voice LLQ RTP header compression 4 3 2 1 1 Data (Medium) Data (Low) 3 3 3 4 4 4 4 Data CBWFQ TCP header compression Using advanced queuing and header compression mechanisms, the available bandwidth can be used more efficiently: Voice: LLQ and RTP header compression Interactive traffic: CBWFQ and TCP (Van Jacobson) header compression
Types of Delay Processing delay: The time it takes for a router to take the packet from an input interface, examine the packet, and put the packet into the output queue of the output interface. Queuing delay: The time a packet resides in the output queue of a router. Serialization delay: The time it takes to place the bits on the wire. Propagation delay: The time it takes for the packet to cross the link from one end to the other.
The Impact of Delay and Jitter on Quality For private networks, 200 ms of delay is a reasonable goal and 250 ms a limit. End-to-end delay: The sum of all propagation, processing, serialization, and queuing delays in the path Jitter: The variation in the delay. In best-effort networks, propagation and serialization delays are fixed, while processing and queuing delays are unpredictable.
Ways to Reduce Delay Upgrade the link (the best solution but also the most expensive). Forward the important packets first. Enable reprioritization of important packets. Compress the payload of Layer 2 frames (takes time). Compress IP packet headers.
Reducing Delay in a Network Customer routers perform: TCP/RTP header compression LLQ Prioritization ISP routers perform: Reprioritization according to the QoS policy
The Impacts of Packet Loss Telephone call: I cannot understand you. Your voice is breaking up. Teleconferencing: The picture is very jerky. Voice is not synchronized. Publishing company: This file is corrupted. Call center: Please hold while my screen refreshes.
Types of Packet Drops Tail drops occur when the output queue is full. Tail drops are common and happen when a link is congested. Other types of drops, usually resulting from router congestion, include input drop, ignore, overrun, and frame errors. These errors can often be solved with hardware upgrades.
Ways to Prevent Packet Loss Upgrade the link (the best solution but also the most expensive). Guarantee enough bandwidth for sensitive packets. Prevent congestion by randomly dropping less important packets before congestion occurs.
Traffic Traffic Traffic Traffic Traffic Policing and Traffic Shaping Traffic Rate Policing Traffic Rate Time Time Traffic Rate Traffic Rate Shaping Time Time Only policing can be applied to inbound traffic on an interface. Shaping requires sufficient memory and a scheduling function for later transmission of any delayed packets i.e. CBWFQ and LLQ.
Reducing Packet Loss in a Network Problem: Interface congestion causes TCP and voice packet drops, resulting in slowing FTP traffic and jerky speech quality. Conclusion: Congestion avoidance and queuing can help. Solution: Use WRED (Weighted Random Early Detection and LLQ (Low Latency Queuing).
Summary Converged networks carry different types of traffic over a shared infrastructure. This creates the need to differentiate traffic and give priority to time-sensitive traffic. Various mechanisms exist that help to maximize the use of the available bandwidth, including queuing techniques and compression mechanisms. All networks experience delay. Delay can effect time sensitive traffic such as voice and video. Without proper provisioning and management, networks can experience packet loss. Packet loss is especially important with voice and video, as no resending of lost packets can occur.
Resources Quality of Service Networking http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/qos.ht m QoS Congestion Avoidance http://www.cisco.com/en/us/tech/tk543/tk760/tsd_technology_s upport_protocol_home.html QoS Congestion Management (queuing) http://www.cisco.com/en/us/tech/tk543/tk544/tsd_technology_s upport_protocol_home.html
Implementing Cisco IOS QoS
Objectives Describe the need for QoS as it relates to various types of network traffic. Identify QoS mechanisms. Describe the steps used to implement QoS.
What Is Quality of Service? Two Perspectives The user perspective Users perceive that their applications are performing properly Voice, video, and data The network manager perspective Need to manage bandwidth allocations to deliver the desired application performance Control delay, jitter, and packet loss
Different Types of Traffic Have Different Needs Real-time applications especially sensitive to QoS Interactive voice Videoconferencing Causes of degraded performance Congestion losses Variable queuing delays The QoS challenge Manage bandwidth allocations to deliver the desired application performance Control delay, jitter, and packet loss Application Examples Interactive Voice and Video Streaming Video Transactional/ Interactive Bulk Data Email File Transfer Delay Sensitivity to QoS Metrics Jitter Packet Loss Y Y Y N Y Y Y N N N N N Need to manage bandwidth allocations
Cisco IOS QoS Tools Congestion management: Priority Queuing (PQ) Custom Queuing (CQ) Weighted Fair Queuing (WFQ) Class Based WFQ (CBWFQ) Queue management Weighted Random Early Detection (WRED) Link efficiency Link fragmentation and interleave RTP and Compressed RTP (crtp) Traffic shaping and traffic policing QoS Toolbox
Priority Queuing PQ puts data into four levels of queues: high, medium, normal, and low.
Custom Queuing CQ handles traffic by assigning a specified amount of queue space to each class of packet and then servicing up to 17 queues in a round-robin fashion.
Weighted Fair Queuing WFQ makes the transfer rates and interarrival periods of active high-volume conversations much more predictable.
Weighted Random Early Detection WRED provides a method that stochastically (random) discards packets if congestion begins to increase. WRED combines the capabilities of the RED algorithm with IP precedence and is RSVP aware.
Implementing QoS Step 1: Identify types of traffic and their requirements. Step 2: Divide traffic into classes. Step 3: Define QoS policies for each class.
Step 1: Identify Types of Traffic and Their Requirements Network audit: Identify traffic on the network (CPU use). Tools: NetFlow Accounting, Network-based Application Recognition (NBAR), and QoS Device Manager (QDM). Business audit: Determine how important each type of traffic is for business. Service levels required: Determine required response time.
Understanding the characteristics of applications
Step 2: Define Traffic Classes Priorities and QoS policy 5, use LLQ 4, use CBWFQ 3, use CBWFQ 2, use CBWFQ Scavenger Class Less than Best Effort 1, use WRED
Step 3: Define QoS Policy A QoS policy is a networkwide definition of the specific levels of QoS that are assigned to different classes of network traffic. Activities: Setting a minimum bandwidth guarantee Setting a maximum bandwidth limit Assigning priorities to each class Using QoS technologies, such as advanced queuing, to manage congestion
Quality of Service Operations How Do QoS Tools Work? Classification and Marking Queuing and (Selective) Dropping Post-Queuing Operations
Self Check 1. What types of applications are particularly sensitive to QoS issues? 2. What is WFQ? How is it different than FIFO? 3. What are the 3 basic steps involved in implementing QoS? 4. What is Scavenger Class?
Summary QoS is important to both the end user and the network administrator. End users experience lack of QoS as poor voice quality, dropped calls or outages. Network traffic differs in its ability to handle delay, jitter and packet loss. Traffic sensitive to these issues requires priority treatment. QoS measures can provide priority to sensitive traffic, while still providing services to more resilient traffic. Implementing QoS involves 3 basic steps: identify the types of traffic on your network, divide the traffic into classes, and define a QoS policy for each traffic class.
Q and A
Resources QoS Best Practices At-A-Glance http://www.cisco.com/application/pdf/en/us/guest/tech/tk759/c14 82/cdccont_0900aecd80295aa1.pdf QoS Tools At-A-Glance http://www.cisco.com/application/pdf/en/us/guest/tech/tk759/c14 82/cdccont_0900aecd80295abf.pdf