LANS, performance and Client/Server design issues

1. LANS, performance and Client/Server design issues CP3397 Network design and security Lecture 3

2. Basic performance definitions • Bandwidth • Raw data rate of links • Capacity • Theoretical limit of data transfer • Measured over the network, sub-net or link • Throughput • Actual data transmitted (e.g. packets per second) • Limited by protocol overhead, delays, latency etc

3. Throughput v Capacity Optimum 100% Max throughput Throughput Actual Max capacity 0% Load

4. Basic performance definitions • Latency • End-to-end delay, comprising • propagation delay (near speed of light), • transmission delay (media speed), • store-and-forward delay (bridge/switch/router buffering), • processing delay (action on protocol elements) • Sensitivity to delay is application dependent • video is very sensitive and • virtual terminal (Telnet) is medium sensitive (user-dependent)

5. Basic performance definitions • Jitter • The variability of latency • Buffering can smooth the delay • Media access delay • LAN access delay depends on • Access scheme used • No. of contending devices • Accuracy • Data corruption • Bit error rate on WAN links • < 1 in 106 on LANs

6. Key performance relationships • Payload (TCP/IP over Ethernet) • Payload = MTU – (TCPOverhead + IPOverhead+ MACOverhead) • MTU is maximum transmission unit • Overheads are: TCP 20 bytes; IP 20 Bytes; MAC 18 bytes • Maximum packet rate • PPSmax =Channel Speed (8 bits x PDUsize ) For example at 64 kbps with 128 byte PDUs PPSmax =64000/(8 x 128) = 62.5 pps

7. Performance issues • Different network types have different maximum packet/frame sizes • Overlarge packets need fragmentation and re-assembly to be transmitted • limits throughput • reduces performance • Compression can be used to improve performance on slower speed links

8. Key performance relationships • Packet rate and link speed • Ensure links do not exceed PPSmax • Error probability and frame size • Larger packets are more likely to contain an error • Protocol efficiency E • E= Sdata _ [R(Sdata+Sprot+Sack)] • Sdata= data size; Sprot=protocol overhead;Sack = ack size • R = expected number of transmissions per packet • Or R=1+packet error rate e.g 1.001 if 1 in 1000 errors

9. Typical bottlenecks • Shared services (centralised servers etc) • Multi-user applications and databases • Low-speed NICs • Shared LAN segments • Low-bandwidth WAN links • Core routing and switching components • Firewalls (particularly public-facing) • Inappropriate compression usage

10. Main types of server • File Servers • Database Servers • Transaction Servers • GroupWare Servers • Web Servers

11. Middleware • Resides between the client and server • Gives the single system image • Typically a major component in a NOS • Provides: directory services, network security etc • Contains proprietary elements where required

12. Scalable Client Server • For the single User • Client, middleware and most of the business services on a single machine • For the SME • Use of small LAN • Often involves multiple clients talking to a local server • For the Enterprise • Connection of multiple servers across a network • To utilise fully requires low cost, high speed bandwidth

13. Features of Server S/W • Wait for client initiated requests • Execute many requests at the same time • Are able to prioritise requests • Can run activities in background • Are resilient and keep running • Main contenders; • Netware • Windows (and NT) Server • Unix/Linux

14. Features of Client S/W • Communicate service requests to a server • Needs to be robust • Provide protection from programs that crash • Provide a mechanism for file transfer • Provide multi tasking • Allow background processes to take place

15. Client/Server bottlenecks • Client and servers are subject to constraints from • Memory • CPU cycles • Network and disc input/output • System bus throughput

16. Client/Server Design Issues • User requirements (applications, response rate, latency etc) • NOS (free choice or pre-determined) • Topology (technology determined) • Server placement (on the network) • Thick/thin client (balance of services) • Groupware (CSCW) use • Maintenance (ability/cost)

17. Protocol Issues • TCP/IP protocol performance depends on • The implementation/stack used • The OS and platform • Packet size distribution of the application • Background traffic characteristics of the contended paths • LAN, MAN, WAN media properties , overheads and BERs • Intermediate device-forwarding characteristics • TCPs sliding window behaviour

18. Typical bottlenecks • The LAN/WAN interface • WANs are typically an order of magnitude slower • Routers need to buffer WAN traffic • Buffers require sufficient memory • Insufficient buffer space leads to more re-transmissions – lowering efficiency • Queuing/buffering also increases end-to-end latency • Some applications may not tolerate high latency, timeout and re-transmissions will occur increasing the problem

19. Data modelling • Gather information of the users to derive • Application maps • Which are used and where • Data flow • How much data flows from machine to machine • Traffic types • Terminal/host, Client/Server, Peer-to-peer, Server-to server, Distributed entity traffic • Local:Remote 80:20 50:50 in modern intranets • Build user-type and server profiles • Traffic matrices • Characterise data in and data out of each site

20. Hierarchical network design • Three-layer architecture • Backbone layer • High-speed switching layer • Mesh design for resilience/minimise outages • Distribution layer • Link points between campus LANs and core backbone • Access layer • End user interface • Typically LAN environment

21. Advantages of hierarchical network design • Scalability • Easier to add to the network • Manageability • Easier to identify location of problems • Broadcast traffic segmentation • Traffic confined to smaller broadcast domains • Less traffic over expensive links

22. Ethernets • Generic Ethernet design rules • Max. stations in a collision domain =1024 • (collision domain is where the time taken to transmit a min. frame is shorter than the time to detect a collision) • Only use repeaters at link-ends • Avoid exceeding standard specs • No more than 4 repeaters in a collision domain • No more than 3 coax segments in a collision domain • Inter-repeater links are best implemented by fibre (10baseFL, 10baseFB) or 10baseT • 10base5, 10base2 and 10baseT can be mixed if wanted

23. LAN performance considerations • Fixed parameters • Bit rate, slot time etc • Variable factors • Packet length distribution • No.of hosts in a collision domain • Arrival rate of frames • Average length of cable • Distance between nodes • Average medium acquisition time

24. Ethernet design rules • To optimise performance • Use shorter cables - Long cables increase collision detection time • Do not attach too many nodes to a segment • Use largest possible packet size – this reduces collisions • Try not to mix real-time and heavy bulk data traffic in the same collision domain

25. VLANs • Logical hierarchy imposed on a flat switched network allowing • Scalability • Formation of workgroups • Simplified admin • Better security

26. Wireless LANs • Use Wireless LAN access points(WLAP) • Simplest LAN use single WLAP • Effectively a wireless star topology • Multiple WLAPs can be used • Can incorporate wired and wireless segments • WLAPS can support • 10-50 clients • Over a 30-60m radius (depends on radio transmission environment) • Wireless LANs can simplify installation and reduce costs – especially in smaller and older buildings

27. Summary • Good design should optimise performance • Many factors affect performance • Technology • Software tuning • Physical environment • The interaction of all network components needs to be considered