Prof. Shervin Shirmohammadi SITE, University of Ottawa. Network Analysis: Process Part 1. Lecture 6: Prof. Shervin Shirmohammadi CEG

Transcription

1 Lecture 6: Network Analysis: Process Part 1 Prof. Shervin Shirmohammadi SITE, University of Ottawa Prof. Shervin Shirmohammadi CEG

2 Overview The principles, guidelines, and procedures for requirements analysis are part of the process model. How do you gather and mange user, application, device, and network requirements? Determine which service metrics to measure. Introduce modeling and simulation. Develop performance requirements for delay, capacity, and RMA. General and environment specific thresholds. The importance of predictable and guaranteed performance. Lecture 7 Traffic flow analysis by mapping requirements to geographic locations. Prof. Shervin Shirmohammadi CEG

3 Requirements Analysis Process Gather and List Requirements Develop Service Metrics Characterize Behavior Develop Requirements Map Requirements Prof. Shervin Shirmohammadi CEG

4 Gathering and Listing Requirements Determining initial conditions Type of Project: New network, a modification, an analysis of network problems, outsourcing, consolidation, or upgrade. The scope of the Project: size, distance, number of sites Example of initial design goals: Upgrade technology, improve performance, support new users, You might or might not be aware of outside forces: political, administrative, financial Prof. Shervin Shirmohammadi CEG

5 Existing Components This particular initial condition is a common and crucial one in that it will often act as constraints to your design. Is the performance target a multi-tier or single tier network. Multi-tier network: where one or more user, application, and/or devices requirements are significantly greater than those of other performance requirements for that network. Single-tier network: has no distinctive set of applications, users or hosts that have significantly greater performance requirements. Exercise 1 Prof. Shervin Shirmohammadi CEG

6 Working with Users Customer Expectations: Estimating resources and schedule, from a rapid initial evaluation of the problem, to quickly evaluate feasibility of expectations Most of the time, customer expectations need to be adjusted This is a very soft skill Survey/questionnaire, ( , fax, mail) Follow up surveys with telephone calls Follow up with face to face meetings. Conduct whiteboard sessions. Spend time with some high-performance users This is an interactive and continuous process: should not be approached in a hit-and-run fashion. Prof. Shervin Shirmohammadi CEG

7 Taking Performance Measurements Can be done in a controlled environment (left figure) or actual network (right figure). Pros and cons of each? Application Session Traffic Application Traffic across Network Performance and Session Data Collected Existing Network Performance and Session Data Collected Prof. Shervin Shirmohammadi CEG

8 Requirements Management and Tracking Since the requirements gathering phase is interactive and continuous, tracking and management of changes becomes important. Similar to software requirements management, although not as complicated as that. Tools should be used to capture requirements: Word processor Spread sheet Proprietary tools Different versions must be kept, for historical reasons and for accountability, and managed. Can use a configuration management tool for this Prof. Shervin Shirmohammadi CEG

9 Mapping Location Information Application G North Campus LAN Servers Applications E,F Servers 88 Central Campus LAN 51 2 Storage Server Application C South Campus LAN Applications A,D Prof. Shervin Shirmohammadi CEG

10 Developing Service Metrics For performance characteristics to be useful, they must be configurable, measurable, and verifiable. Availability, in terms of percent uptime or downtime Recoverability or stability, in terms of MTBF, MTTR and MTBSO (Service Outage) Error and loss rates, BER, CLR (Cell Loss Rate), CMR (Cell misinsertion ratio), frame and packet loss rates Service metrics for capacity Data rates: Peak data Rate (PDR), sustained data rate (SDR), minimum data rate Data size: burst size and duration Service metrics for delay End-to-end, round-trip or system delay Latency Delay variation Timeliness Prof. Shervin Shirmohammadi CEG

11 Measurement Tools Configuring and measuring service levels: use your basic Network Management Tools Simple Network Management Protocol (SNMP) Common Management Information Protocol (CMIP) Bytes in/out, IP packets in/out, dropped ICMP packets, capacity limits, burst tolerance, delay, downtime. Ping, traceroot, pathchar ( round-trip delay, per-link capacity, path traces TCPdump Exercise 2 Prof. Shervin Shirmohammadi CEG

12 Characterizing Behaviour How do users and applications use the network? Helps us understand their requirements. Usage patterns Frequency and duration of application sessions Number of simultaneous sessions Application behaviour Data sizes Frequency and time duration data passes the network Traffic flow characteristics Degree of multicasting (broadcasting) Prof. Shervin Shirmohammadi CEG

13 Modeling and Simulation This is often useful in predicting, determining, or estimating requirements and data flows. Good in the architecture and design process for understanding temporal and spatial behaviors of traffic flows, equipment type, placement, configuration, and behavior under stress. The advantage of this is that they can be used over and over again. We are using the OPNET IT Guru tool in this course for simulation. Disadvantage? Difference with analytical modeling? Where do traffic sources in simulation tools come from? Prof. Shervin Shirmohammadi CEG

14 An example of a Queue Web server: handles requests in 1 msec If requests arrive at a constant rate of 1000 req/sec or less, everything works fine. In reality, arrival rate is not constant but varies. Suppose arrival rate is irregular with an average of 500 req/sec. Prof. Shervin Shirmohammadi CEG

15 Normalized Rate: Ratio of arrival rate to processing rate. Prof. Shervin Shirmohammadi CEG

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18 Queue Behavior The behavior of a system with a queue may not be according to our intuition. When arrival rate is 50%, after 50 seconds, an average buffer size of 43 requests, with a peak of over 600 requests, is needed. When arrival rate is 95% (~2 times 50%), the average buffer size rises to 1859 (~40 times as before). When arrival rate is 99% compared to 95% (a tiny increase), the average buffer size rises to 2583 (~40% increase). Prof. Shervin Shirmohammadi CEG

19 RMA Requirements This is done through defining thresholds on the values. General thresholds are those that apply to most or all networks Environment thresholds are determined from the environment of the current network. These measures are generally defined as: MTBF/MTBCF/MTTR These are generally expressed in hours. Reliability is estimated by measuring the failure rate per hours of operation. MTBF = 1/failureRate The failure rate of a system is computed by adding the failure rates (total hours) of the components and then inverting this value. MTTR: Time to restore the system to fully operational status once it has experienced a fault. Availability is a Relationship of Reliability to Maintainability: (MTBF) / (MTBF + MTTR) OR MTBCF / (MTBCF + MTTR) Reflects only failure time, not scheduled maintenance downtime. Can be improved by: Replacing faulty components at a quicker scheduled rate. Design redundant paths for critical components. (expensive) Is it possible to have MTBF and MTBCF on the same network? Prof. Shervin Shirmohammadi CEG

20 Availability Measures A common measure for availability is a percentage of uptime or downtime. For example % ( five nines reliability) Most systems operate at 99.99% uptime (general threshold) % Uptime Yearly Monthly Weekly Daily 99% 87.6h 7.3h 1.68h 14.4m 99.9% 8.76h 44m 10m 1.4m 99.99% 53m 4.4m 1m 8.6s % 5.3m 26.3s 6s 0.86s Is it enough to just specify a percentage as a requirement? Prof. Shervin Shirmohammadi CEG

21 Availability Examples Pacific Bell s switched multimegabit data services (SMDS) Mean time between service outages of 3500 hours or more, with an MTTR of 3.5 hrs or less. Bellcore specifications for SMDS Availability of 99.9%. Similar measure to Pacific but less specific. Testbed Low Performance High Performance 99.5 %Uptime Prof. Shervin Shirmohammadi CEG

22 Delay Requirements Several delay measures are used: End to end Round trip Delay variation. Delay requirements Interaction Delay (INTD) waiting time seconds Human Response Time (HRT) human perceptive delay 100 ms Network propagation delay Depends on technology and distance (< 10 msec) Prof. Shervin Shirmohammadi CEG

23 Interactive Applications Delays Interactive-Burst Interactive-Burst or Interactive-Bulk Interactive-Bulk Human Response Time Delay (Seconds) Interaction Delay Prof. Shervin Shirmohammadi CEG

24 Delay Thresholds A guideline is to determine the limiting factor from the delay values (the ultimate delay bottleneck within the system). These can be reduced one at a time until the delay is at an acceptable level. We must keep in mind real physical limitations early in the design, for example: Real physical limitations determine that the physical delay of a network from Los Angeles and Tokyo is ~120 ms. No matter what you do you cannot improve this. Where can one look in order to improve on the delay of a network that theoretically can support say 10 ms delay but actually has a 100 ms delay? Improve or tune the OSs and/or network protocols. Difficult to do and requires a great deal of experience Get better device peripherals. Quick research can determine performance problems with devices. Prof. Shervin Shirmohammadi CEG

25 System Responsiveness System responsiveness = HRT/TCT, when HRT>=RTT System responsiveness = HRT/(RTT*TCT), when HRT<RTT HRT: Human response time RTT: Round-trip time TCT: Task Completion time SR < 3, a low degree of responsiveness (interactive bulk) SR > 3, a high degree of responsiveness (interactive burst) Prof. Shervin Shirmohammadi CEG

26 Delay Variation (Jitter) Requirements Delay variation is often coupled with end-to end or round trip delay to give an overall delay performance value. A rule of thumb is use a 1% to 2% of end-to-end delay as the delay variation. For example: An estimate for delay variation when the end-to-end delay is 40 ms is approximately 400 to 800 µs. What type of applications are impacted more by jitter than by the delay itself? Prof. Shervin Shirmohammadi CEG

27 Capacity Requirements Capacity requirements often used are: Peak data rate, sustained data rate, minimum data rate, or combinations of all of these. We only have a intuitive feeling of particular application limits: Telnet will not have a data rate of 100 Mb/s and that FTP should not run at 10 Kb/s if greater capacity is available. Prof. Shervin Shirmohammadi CEG

28 Capacity for Some Typical Apps Application Average Completion Time (Seconds) Average Data Size (Bytes) Distributed Computing Web Transactions Database Entries/Queries Payroll Entries Teleconference What about audio? Video? Prof. Shervin Shirmohammadi CEG