LANS, performance and Client/Server design issues

CP3397 Network design and security Lecture 3

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

Throughput v Capacity

Optimum 100% Max throughput Actual 0% Load

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)

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 10 6 on LANs

Key performance relationships

Payload (TCP/IP over Ethernet)   Payload = MTU – ( TCP Overhead + IP Overhead + MAC Overhead ) MTU is maximum transmission unit  Overheads are: TCP 20 bytes; IP 20 Bytes; MAC 18 bytes Maximum packet rate  PPS max = Channel Speed (8 bits x PDU size ) For example at 64 kbps with 128 byte PDUs PPS max =64000/(8 x 128) = 62.5 pps

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

Key performance relationships

Packet rate and link speed  Ensure links do not exceed PPS max Error probability and frame size  Larger packets are more likely to contain an error   Protocol efficiency E E= S data _ [R(S data +S prot +S ack )]    S data = data size; S prot =protocol overhead; S ack = ack size R = expected number of transmissions per packet Or R=1+packet error rate e.g 1.001

if 1 in 1000 errors

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

Main types of server

File Servers Database Servers Transaction Servers GroupWare Servers Web Servers

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

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

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

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

Client/Server bottlenecks

Client and servers are subject to constraints from     Memory CPU cycles Network and disc input/output System bus throughput

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)

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

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

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

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

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

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

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

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

VLANs

Logical hierarchy imposed on a flat switched network allowing     Scalability Formation of workgroups Simplified admin Better security

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

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