Client/Server design issues

cursefarmNetworking and Communications

Oct 24, 2013 (3 years and 9 months ago)

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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

Max capacity

Throughput

Load

Optimum

Actual

100%

0%

Max throughput

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