Lectures Top-Down Network Design - Chapter 2: Analyzing Technical Goals and Tradeoffs

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1. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Top-Down Network Design Chapter Two Analyzing Technical Goals and Tradeoffs Copyright 2010 Cisco Press & Priscilla Oppenheimer Technical Goals • Scalability • Ava ila bility • Performance • Security • Manageability • Usability • Adaptability • Affordability Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 1
2. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Scalability • Scalability refers to the ability to grow • SthliSome technologies are more sca lbllable – Flat network designs, for example, don’t scale well • Try to learn – Number of sites to be added – What will be needed at each of these sites – How many users will be added – How many more servers will be added Availability • Availability can be expressed as a percent uptime per year, month, week, day, or hour, compared to the total time in that period – For example: • 24/7 operation • Network is up for 165 hours in the 168-hour week • Availability is 98.21% • Different applications may require different levels • Some enterprises may want 99.999% or “Five Nines” availability Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 2
3. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Availability Downtime in Minutes Per Hour Per Day Per Week Per Year 99.999% .0006 .01 .10 5 99.98% .012 .29 2 105 99.95% .03 .72 5 263 99.90% .06 1.44 10 526 99.70% .18 4.32 30 1577 99.999% Availability May Require Triple Redundancy ISP 1 ISP 2 ISP 3 Enterprise • Can the customer afford this? Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 3
4. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Availability • Availability can also be expressed as a mean time between failure (MTBF) and mean time to repair (MTTR) • Availability = MTBF/(MTBF + MTTR) – For example: • The network should not fail more than once everyy, 4,000 hours ( 166 day y)s) and it should be fixed within one hour • 4,000/4,001 = 99.98% availability Network Performance • Common performance factors include – Bandwidth – Throughput – Bandwidth utilization – Offered load – Accuracy – Efficiency – Delay (latency) and delay variation – Response time Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 4
5. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Bandwidth Vs. Throughput • Bandwidth and throughput are not the same thing • Bandwidth is the data carrying capacity of a circuit • Usually specified in bits per second • Throughput is the quantity of error free data transmitted per u nit of time • Measured in bps, Bps, or packets per second (pps) Bandwidth, Throughput, Load 100 % of Capacity T h r Actual o u g h p u t 100 % of Capacity Offered Load Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 5
6. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Other Factors that Affect Throughput • The size of packets •Inter-frame gaps between packets • Packets-per-second ratings of devices that forward packets • Client speed (CPU, memory, and HD access speeds) • Server speed (CPU, memory, and HD access speeds) • Network design • Protocols • Distance • Errors • Time of day, etc., etc., etc. Throughput Vs. Goodput • You need to decide what you mean by throughput • Are you referring to bytes per second, regardless of whether the bytes are user data bytes or packet header bytes – Or are you concerned with application-layer througgphput of user b ytes, sometimes called “goodput” • In that case, you have to consider that bandwidth is being “wasted” by the headers in every packet Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 6
7. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Performance (continued) • Efficiency – How much overhead is required to deliver an amount of data? – How large can packets be? • Larger better for efficiency (and goodput) • But too large means too much data is lost if a packet is damaged • How many packets can be sent in one bunch without an acknowledgment? Efficiency Small Frames (Less Efficient) Large Frames (More Efficient) Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 7
8. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Delay from the User’s Point of View • Response Time – A function of the application and the equipment the application is running on, not just the network – Most users expect to see something on the screen in 100 to 200 milliseconds Delay from the Engineer’s Point of View • Propagation delay – A signal travels in a cable at about 2/3 the speed of light in a vacuum • Transmission delay (also known as serialization delay) – Time to put digital data onto a transmission line • For example, it takes about 5 ms to output a 1,024 byte packet on a 1.544 Mbps T1 line • Packet-switching delay • Queuing delay Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 8
9. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Queuing Delay and Bandwidth Utilization 15 12 9 6 ge Queue Depth a 3 0 Aver 0.5 0.6 0.7 0.8 0.9 1 Average Utilization Number of packets in a queue increases exponentially as utilization increases Example • A packet switch has 5 users, each offering packets at a rate of 10 packets per second • The average length of the packets is 1,024 bits • The packet switch needs to transmit this data over a 56-Kbps WAN circuit – Load = 5 x 10 x 1,024 = 51,200 bps – Utilization = 51,200/56,000 = 91.4% – Average number of packets in queue = (0.914)/(1-0.914) = 10.63 packets Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 9
10. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Delay Variation • The amount of time average delay varies – Also known as jitter • Voice, video, and audio are intolerant of delay variation • So forget everything we said about m aximizin g pack et siz es – There are always tradeoffs – Efficiency for high-volume applications versus low and non-varying delay for multimedia Security • Focus on requirements first • Detailed security planning later (Chapter 8) • Identify network assets – Including their value and the expected cost associated with losing them due to a security problem • Analyze security risks Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 10
11. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Network Assets • Hardware • Software • Applications •Data • Intellectual property • Trade secrets • Company’s reputation Security Risks • Hacked network devices – Data can be intercepted, analyzed, altered, or deleted – User passwords can be compromised – Device configurations can be changed • Reconnaissance attacks • Denial-of-service attacks Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 11
12. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Manageability • Fault management • Configuration management • Accounting management • Performance management • Security management Usability • Usability: the ease of use with which netktwork users can access th thtkde network and services • Networks should make users’ jobs easier • Some design decisions will have a negative affect on usability: – Strict security, for example Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 12
13. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Adaptability • Avoid incorporating any design elements that would make it hard to implement new technologies in the future • Change can come in the form of new protocols, new business practices, new fiscal goals, new legislation • A flexible desiggpggn can adapt to changing traffic patterns and Quality of Service (QoS) requirements Affordability • A network should carry the maximum amount of traffic possible for a given financial cost • Affordability is especially important in campus network designs • WANs are expected to cost more, but costs can be reduced with the ppproper use of technology – Quiet routing protocols, for example Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 13
14. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Network Applications Technical Requirements Name of Cost of Acceptable Acceptable Throughput Delay Must be Delay Application Downtime MTBF MTTR Goal Less Than: Variation Must be Less Than: Making Tradeoffs Scalability 20 Availability 30 Network performance 15 Security 5 Manageability 5 Usa bility 5 Adaptability 5 Affordability 15 Total (must add up to 100) 100 Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 14
15. Top-Down Network Design, Ch. 2: Analyzing Technical Goals and Tradeoffs Summary • Continue to use a systematic, top-down approach • Don’t select products until you understand goals for scalability, availability, performance, security, manageability, usability, adaptability, and affordability • Tradeoffs are almost always necessary Review Questions • What are some typical technical goals for organizations today? • How do bandwidth and throughput differ? • How can one improve network efficiency? • What tradeoffs may be necessary in order to imppyrove network efficiency? Copyright 2004 Cisco Press & Priscilla Oppenheimer Page 15