Network design
Topic 3
Analysing network traffic
Agenda
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Traffic flows
Traffic load
Traffic behaviour
Quality of Service (QoS)
User communities and data stores
• A user community is a set of workers who use a particular
application or set of applications
– May be located within a department
– May be a virtual team that crosses department boundaries
• Survey users to identify which applications they use and
where the user is located
– Document user community name, number of users, location and
applications used
• Locate data stores
– Server, server farm, SAN, mainframe, tape back-up, digital video
library
– Document the data store name, the location, the application that
uses the data store and the user community that uses the
application
Traffic flows
• An individual traffic flow is protocol and application
information transmitted between hosts during a single
session
• Attributes include:
– Direction
• both directions or just one direction
– Symmetry
• is higher performance (QoS) required in one direction
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Routing path
Number of packets
Number of bytes
End point addresses
• Measured by protocols analysers or network management
systems NMS
– Cisco FlowCollector and data analyzer
Well known flow types
• Terminal/host traffic flow
– Usually asymmetric, telnet
• Client/server traffic flow
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Clients send queries and requests to servers, servers respond with data,
Flow is bidirectional and asymmetric, SMB, NFS, HTTP
Caching can change the flow
Thin clients, Citrix and MS Terminal services, large volumes of data often at the
same time of day
• Peer-to-peer traffic flow
– Bidirectional and symmetric, small LANs access to data
– Peer-to-peer applications for downloading music, software, videos
– Videoconferencing
• Server/server traffic flow
– Directory services, caching, data backup, management applications
– Generally bidirectional, symmetry depends on application
• Distributed computing traffic flow
– Multiple nodes share the processing load
– Flows are individual
Voice over IP flows
• Multiple flows
– Call setup and teardown
• Client-server flow, phone to gatekeeper or gateway, H.323,
skinny, SGCP and MGCP, SIP
• Server or phone switch handles call control, call setup and
teardown, addressing and routing, rules and capabilities,
information and supplementary services
• Call switching, moving calls through infrastructure
– Audio voice flow and video flow
• Peer-to-peer between phones or software such as Cisco
Softphone
• Distinct traffic flow which may follows a different path than
call setup packets and requires QoS and bandwidth
Documenting traffic flows
• Create a table in your documentation to identify:
– Name of application
– Type of traffic flow
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Terminal/host
Client/server
Peer-to-peer
Server/server
Protocols used
User community using application
Data stores
Approximate bandwidth requirements
QoS requirements
Traffic load
• Traffic load is the sum of all the data that network hosts
have ready to send at a particular time
• Network capacity should be adequate to handle the
traffic load to avoid bottlenecks
• Consider:
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Number of stations
Average time that a station is idle between sending frames
Time required to transmit a message – frame size
Number of stations * bits per second sent
• Estimate the load per application flow
• Investigate infrequent flows
– Such as printing monthly statements
Application usage patterns
• Identify:
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User communities
Number of users in the community
Applications used by users
Frequency, number of sessions per day, week, month
Length of average session
Number of simultaneous users of an application
• Use information to predict total bandwidth
requirement for all users of the application
Estimating traffic load of applications
• What size are the data objects sent by
applications?
• What is the size of overhead caused by
protocols?
– 802.3 frame header and trailer = 46 bytes
– IP header = 20 bytes
– TCP header = 20 bytes
• What is the size of any additional load caused
by the application flow (initialisation)?
Estimating traffic load of routing
protocols
• Large distance vector (RIP) routing tables can
be sent every 30 secs
– Significant load on slow WAN links
• OSPF and EIGRP use very little bandwidth
– However OSPF database synchronisation packets
every 30 mins could be a concern
– Hello packets (OSPF 10 secs, EIGRP 5 secs) are very
small and effect is negligible
Traffic behaviour
• Broadcast traffic
– Broadcast radiation, the effect of broadcasting by a
host can degrade performance
– NICs pass broadcasts and some multicasts to
processor
• If more than 20% of network traffic is broadcast or
multicast traffic segment the network with routers or
VLANs
• Misconfiguration of subnet masks can cause intermittent
broadcast storms
• IP network, limit the number of stations in a single
broadcast domain to 500
Network efficiency
• Whether applications and protocols use
bandwidth efficiently
– Frame size
• Use the largest MTU possible for large data transfers
• Configure on routers
– Protocols used
• Tune protocol timers
• Investigate read/write speeds of storage
– Windowing and flow control
• By increasing memory and CPU power on receiving hosts a
larger receive window can be supported
– Error-recovery
• Selective ACKS, only missing segments are retransmitted
Quality of Service
• Is the bandwidth requirement flexible or non-flexible?
– Voice is inflexible to delay
– Sensitive to packet loss – clipped speech
– Packet loss occurs on congested links
• Protocols (RSVP) to allow hosts to reserve network
bandwidth in advance and receive a guarantee of a
negotiated level of service
– Packet classifier that determines the QoS class
– Admission control – are sufficient resources available on the
intermediate nodes
– Packet scheduler – determines when packets are forwarded
to meet the QoS requirements
Agenda
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Traffic flows
Traffic load
Traffic behaviour
Quality of Service