cs 453 computer networks

CS 453CS 453Computer NetworksComputer Networks

Lecture 23Lecture 23

Network Management and Network Management and PerformancePerformance

Performance Issues

Clearly, users of networks expect them to perform well…

…and usually notice when they do not…

… and expect the network management to fix the problems.

Performance Issues

Down routes – network routes fail or shutdown for a variety of reasons, network management software must be able to detect route or link failures, …

…and provide a means to resolve or work around the problem….

For example, rerouting traffic, until preferred routes are restored

Performance Issues

Congestion – Networks (particularly WANs) have multiple routes to provide end-to-end connectivity

There are usually one or more preferred routes… But these preferred routes often aggregate traffic from

multiple sources… Which can overload links and create congestion

(compounding problem) Network management systems should be able to—

Recognize congestion

…and shed traffic to other routes, if possible

Performance Issues

Low end processing hardware/software High performance networks connected to

old/slow hardware (routers, PCs) can cause performance problems

Messages arrive faster than the hardware can process them…

Messages are lost, and retransmitted,… And this problem can compound if the

overload is not transient

Performance Issues

Synchronous overloads – Suppose a bad message is broadcast to a

network of 10,000 machines…. …and each of these machines replies with an

error message… Result can be massive overload of the

network Broadcast Storm UDP protocol changes to inhibit error reports

for broadcast messages

Performance Issues

Synchronous Overloads- Imagine the simultaneous restart of all

machines/routers in a network… … like a regional power failure Every machine on the network will start and

may issue a DHCP Discovery request, simultaneously…

Routers will update their routes and configurations,

Network hosted software will reload And the network will come to it’s knees

Performance Issues

Synchronous Overloads- This is not far fetched Happens all of the time on a modest scale Recently, happened on a global scale… Due to a MS Windows automatic update Update required a restart Something like 6 million computers worldwide

restarted at the same time

Performance Issues

Tuning Insufficient buffer space in routers,

source/destination computers Scheduling algorithm giving too little priority to

processing incoming data, drops messages, causes retransmits

Timer settingsToo short – messages in-flight reported as lost

Too long – waste time waiting for lost messages

Performance Issues

Inefficient use of resources

For example, high performance network with low feed

Performance Issues

Jitter We want a short mean arrival time Also want small standard deviation for arrival

time Large standard deviation in arrival time is

called Jitter Jitter big problem with new services like

VoIPStreaming audioStreaming video

Can mitigate some

Network Management

More things to see http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/

snmp.htm http://en.wikipedia.org/wiki/Network_performance_management http://en.wikipedia.org/wiki/Snmp http://www.cotse.com/tools/netman.htm

TCP/IP http://www.redbooks.ibm.com/Redbooks.nsf/RedbookAbstracts/gg243376.html?Open