Internetwork Chapter 3: Network Infrastructure

Characterizing the Existing Internetwork

• Examining a customer's existing network is an important step in top-down network design
• This helps to meet expectations for network scalability, performance, and availability
• Examining the network includes learning about the topology and physical structure
• Examination includes assessing the network performance

Characterizing the Network Infrastructure

• Important aspects of characterizing the network infrastructure:
  • Develop a set of network maps
  • Learn the location of major internetworking devices and network segments
  • Document the names and addresses of major devices and segments
  • Identify standard methods for addressing and naming
  • Document the types and lengths of physical cabling
  • Investigate architectural and environmental constraints

Developing a Network Map

• A map or set of maps can characterize the existing network
• Mapping is done by learning the location of major hosts, interconnection devices, and network segments

Characterizing Large Internetworks

• Developing a single network map might not be possible for large internetworks
• Approaches to solving this problem:
  • Developing many maps, one for each location
  • Apply a top down method, starting with a map of sets of maps that shows high-level information
• High level information includes:
  • Geographical information such as countries, states or provinces, cities, and campuses
  • WAN connections between countries, states, and cities
  • WAN and LAN connections between buildings and between campuses
• For each campus network, develop more precise maps showing more detailed information
• Detailed information includes:
  • Building and floors, and possibly rooms or cubicles
  • The location of major servers or server farms
  • The location of routers and switches
  • The location of firewall, Network Address Translation (NAT) devices, Intrusion Detection System (IDS), and Intrusion Prevention System (IPS)
  • The location of mainframe
  • The location of major network management stations
  • The location and reach of virtual LAN (VLAN)
  • Some indication of where workstations reside (not necessarily explicit location)
• Another method of characterizing large, complex networks is to use the top down approach influenced by the OSI reference model
  • Develop a logical map that shows applications and services used by the network user
  • Develop a map that shows network services and also develop a map that depicts layer 3 topology of the internetwork
  • Develop a map or a set of maps that shows detailed information about data link layer links and devices

Characterizing the Logical Architecture

• The logical topology illustrates the architecture of the network
• Architecture can be hierarchical, flat, structured, unstructured, layered, or not
• The logical topology also describes methods for connecting devices in geometric shape (star, ring, bus, hub and spoke, or mesh)
• The logical topology can affect your ability to upgrade a network
• A flat topology doesn't scale as well as a hierarchical topology

Developing a Modular Block Diagram

• In addition to developing a set of detailed maps, it is helpful to draw a simplified block diagram of the network or parts of it
• The diagram can depict the major functions of the network in a modular fashion
• Block, modularized network topology map example: the Cisco Enterprise Composite Network Model

Characterizing Network Addressing and Naming

• Characterizing the logical infrastructure of a network involves documenting any strategies your customer has for network addressing and naming
• When drawing detailed network maps, include the names of major sites, routers, network segments, and servers
• Investigate the network layer addresses the customer uses
• The customer's addressing scheme can influence your ability to adapt the network to new design goals
• Understanding your customer's addressing scheme might help you know:
  • Your customer might use unregistered IP addresses that will need to be changed or translated before connecting to the internet
  • Your customer might have a goal of using route summarization
  • Your customer's existing addressing scheme might affect the routing protocols you can select

Characterizing Wiring and Media

• While exploring the cabling design assess how well equipment and cables are labeled in the current network
• Your network diagram should document:
  • The connection between buildings
  • Information on the number of pairs of wires and the type of wiring (or wireless technology) in use
  • How far buildings are from one another
• Wiring (or wireless technology) between buildings is one of the following:
  • Single-mode fiber
  • Multi-mode fiber
  • Shielded twisted-pair (STP) copper
  • Unshielded twisted-pair (UTP) copper
  • Coaxial cable
  • Microwave
  • Laser
  • Radio
  • Infrared

Checking Architectural and Environmental Constraints

• When investigating cabling, pay attention to environmental issues
• Examples: Cabling near creeks that could flood or railroads or highways where traffic could jostle cables
• Pay attention to architectural issues that could affect the feasibility of implementing your network design

Checking Site for a Wireless Installation

• A common goal for modern campus network designs is to install a wireless LAN (WLAN) based on IEEE 802.11 standards
• An important aspect of inspecting architectural and environmental constraints of a site is determining the feasibility of using wireless transmission
• Wireless site survey is often used to describe the process of analyzing a site to see if it will be appropriate for wireless transmission
• A site survey starts with a draft WLAN design
• Using a floor plan or blueprint for the site, the designer decides on the initial placement of the wireless access points
• An access point is a station that transmits and receives data for users of the WLAN
• It usually serves as the point of interconnection between the WLAN and the wired Ethernet network
• A network designer can decide where to place access points for initial testing
• Decisions are based on knowledge of where the users will be located, characteristics of the access points' antennas, and the location of major obstructions
• The initial placement of an access point is based on an estimate of the signal loss that will occur between the access point and the users
• The starting point for an estimate depends on how much loss in power a signal would experience in the vacuum of space, without any obstructions or other interference
• This is called the free space path loss and is specified in decibels (dB)
• The estimate is tuned with an understanding that the actual expected signal loss depends on the medium through which the signal will travel, which is undoubtedly not a vacuum
• An RF signal traveling through objects of various sorts can be affected by many different problems, includes reflection, absorption, refraction, and diffraction

Checking the Health of the Existing Internetwork

• Studying the performance of the existing internetwork gives you a baseline measurement from which to measure new network performance
• With measurement of the present internetwork, you can demonstrate to your customer how much better the new internetwork performs once your design is implemented

Developing a Baseline of Network Performance

• Developing an accurate baseline of a network's performance is not an easy task
• One challenging aspect is selecting a time to do the analysis
• Allocate a lot of time (multiple days) if you want the baseline to be accurate
• If measurements are made over too short a timeframe, temporary errors appear more significant than they are

Analyzing Network Availability

• To document availability characteristics of the existing network, gather any statistics that the customer has on the mean time between failure (MTBF) and mean time to repair (MTTR) for the internetwork as a whole and major network segments

Analyzing Network Utilization

• Network utilization is a measurement of the amount of bandwidth that is in use during a specific time interval
• Utilization is commonly specified as a percentage of capacity
• If a network-monitoring tool says that network utilization on a Fast Ethernet segment is 70 percent, that means that 70 percent of the 100-Mbps capacity is in use, averaged over a specified timeframe or window

Measuring Bandwidth Utilization by Protocol

• Developing a baseline of network performance should also include measuring utilization from broadcast traffic versus unicast traffic, and by each major protocol
• To measure bandwidth utilization by protocol, place a protocol analyzer or remote monitoring (RMON) probe on each major network segment and fill out a chart
• If the analyzer supports relative and absolute percentages, specify the bandwidth used by protocols as relative and absolute
• Relative usage specifies how much bandwidth is used by the protocol in comparison to the total bandwidth currently in use on the segment
• Absolute usage specifies how much bandwidth is used by the protocol in comparison to the total capacity of the segment (for example, compared to 100 Mbps on Fast Ethernet)

Analyzing Network Accuracy

• With packet-switched networks, it makes more sense to measure frame (packet) errors because a whole frame is considered bad if a single bit is changed or dropped
• In packet switched networks, a sending station calculates a CRC based on the bits in a frame
• The sending station places the value of the CRC in the frame
• A receiving station determines if a bit has been changed or dropped by calculating the CRC again and comparing the result to the CRC in the frame
• A frame with a bad CRC is dropped and must be retransmitted by the sender
• Usually an upper-layer protocol has the job of retransmitting frames that do not get acknowledged

Analyzing Delay and Response Time

• To verify that performance of a new network design meets a customer's requirements, measure response time between significant network devices before and after a new network design is implemented
• Response time can be measured many ways
• Using a protocol analyzer, you can look at the amount of time between frames and get a rough estimate of response time at the data link layer, transport layer, and application layer
• A more common way to measure response time is to send ping packets and measure the round-trip time (RTT) to send a request and receive a response
• While measuring RTT, you can also measure an RTT variance
• Variance measurements are important for applications that cannot tolerate much jitter (for example, voice and video applications)
• You can also document any loss of packets

Checking the Status of Major Routers, Switches, and Firewalls

• The final step in characterizing the existing internetwork is to check the behavior of the internetworking devices in the internetwork
• This includes routers and switches that connect layers of a hierarchical topology, and devices that will have the most significant roles in your new network design
• It's not necessary to check every LAN switch, just the major switches, routers, and firewalls
• Checking the behavior and health of an internetworking device includes determining how busy the device is (CPU utilization), how many packets it has processed, how many packets it has dropped, and the status of buffers and queues
• Your method for assessing the health of an internetworking device depends on the vendor and architecture of the device
• Methods for checking status:
  • show buffers
  • show cdp neighbors detail
  • show environment
  • show interfaces
  • show memory
  • show processes
  • show running-config
  • show startup-config
  • show version

Network Health Checklist

• Here is a checklist to assist you in verifying the health of an existing internetwork
• The Network Health checklist is generic in nature and documents a best-case scenario
• Thresholds might not apply to all networks
• Examples of items on the checklist:
  • The network topology and physical infrastructure are well documented
  • Network addresses and names are assigned in a structured manner and are well documented
  • Network wiring is installed in a structured manner and is well labeled
  • Network wiring has been tested and certified
  • Network wiring between telecommunications closets and end stations is no more than 100 meters
  • Network availability meets current customer goals
  • Network security meets current customer goals
  • No LAN or WAN segments are becoming saturated (70 percent average network utilization in a 10-minute window)
  • There are no collisions on Ethernet full-duplex links
  • Broadcast traffic is less than 20 percent of all traffic on each network segment (some networks are more sensitive to broadcast traffic and should use a 10 percent threshold)
  • Wherever possible and appropriate, frame sizes have been optimized to be as large as possible for the data link layer in use
  • No routers are overused (5-minute CPU utilization is under 75 percent)
  • On average, routers are not dropping more than 1 percent of packets (for networks that are intentionally oversubscribed to keep costs low, a higher threshold can be used)
  • Up-to-date router, switch, and other device configurations have been collected, archived, and analyzed as part of the design study
  • The response time between clients and hosts is generally less than 100 ms (1/10th of a second)

Characterizing Traffic Flow

• Characterizing traffic flow involves identifying sources and destinations of network traffic and analyzing the direction and symmetry of data traveling between sources and destinations
• In some applications, the flow is bidirectional and symmetric (both ends of the flow send traffic at about the same rate)
• In other applications, the flow is bidirectional and asymmetric (clients send small queries and servers send large streams of data)
• In a broadcast application, the flow is unidirectional and asymmetric

Identifying Major Traffic Sources and Stores

• To understand network traffic flow, you should first identify user communities and data stores for existing and new applications
• A user community is a set of workers who use a particular application or set of applications
• A user community can be a corporate department or set of departments
• In many environments, application usage crosses departmental boundaries because more corporations use matrix management and form virtual teams to complete ad hoc projects
• It becomes increasingly necessary to characterize user communities by application and protocol usage rather than by departmental boundary
• To document user communities, ask your customer to help you fill out a chart
• In addition to documenting user communities, characterizing traffic flow also requires that you document major data stores
• A data store (sometimes called a data sink) is an area in a network where application layer data resides
• A data store can be a server, a server farm, a storage-area network (SAN), a mainframe, a tape backup unit, a digital video library, or any device or component of an internetwork where large quantities of data are stored
• To document major data stores use a chart

Documenting Traffic Flow on the Existing Network

• Documenting traffic flow involves identifying and characterizing individual traffic flows between traffic sources and stores
• Measuring the traffic flow behavior can also help network designers do the following
  • Characterize the behavior of existing networks
  • Plan for network development and expansion
  • Quantify network performance
  • Verify the quality of network service
  • Ascribe network usage to users and applications
• An individual network traffic flow can be defined as protocol and application information transmitted between communicating entities during a single session
• The simplest method for characterizing the size of a flow is to measure the number of megabytes per second (MBps) between communicating entities
• To characterize the size of a flow, use a protocol analyzer or network management system to record load between important sources and destinations
• You can also use Cisco NetFlow, which collects and measures data as it enters router and switch interfaces, including source and destination IP addresses, source and destination TCP or UDP port numbers, packet and byte counts, and so on
• Chart to document information about the direction and volume of traffic flows

Characterizing Types of Traffic Flow for New Network Applications

• A network flow can be characterized by its direction and symmetry
• Direction specifies whether data travels in both directions or in just one direction
• Direction also specifies the path that a flow takes as it travels from source to destination through an internetwork
• Symmetry describes whether the flow tends to have higher performance or QoS requirements in one direction than the other direction
• A good technique for characterizing network traffic flow is to classify applications as supporting one of a few well-known flow types:
  • Terminal/host traffic flow
  • Client/server traffic flow
  • Peer-to-peer traffic flow
  • Server/server traffic flow
  • Distributed computing traffic flow

Documenting Traffic Flow for New and Existing Network Applications

• To document traffic flow for new (and existing) network applications, characterize the flow type for each application and list the user communities and data stores that are associated with applications

Characterizing Traffic Load

• To select appropriate topologies and technologies to meet a customer's goals, it is important to characterize traffic load with traffic flow
• Characterizing traffic load can help you design networks with sufficient capacity for local usage and internetwork flows
• Because of the many factors involved in characterizing network traffic, traffic load estimates are unlikely to be precise
• The goal is simply to avoid a design that has any critical bottlenecks
• To avoid bottlenecks, you can research application-usage patterns, idle times between packets and sessions, frame sizes, and other traffic behavioral patterns for application and system protocols
• For customers with numerous applications, this level of analysis might not be practical
• Limit the analysis to the top five or ten applications
• Another approach to avoiding bottlenecks is simply to throw large amounts of bandwidth at the problem (also known as over provisioning)
• LAN bandwidth is extremely cheap
• No excuse for not using Fast Ethernet (or better) on all new workstations and switches, and most organizations can also afford to use Gigabit Ethernet on switch-to-switch and switch-to-server links
• Characterize QoS requirements for applications if bandwidth will not be a constraint

Documenting Application-Usage Patterns

• The first step in documenting application-usage patterns is to identify user communities, the number of users in the communities, and the applications the users employ
• Document the following information:
  • The frequency of application sessions (number of sessions per day, week, month, or whatever time period is appropriate)
  • The length of an average application session
  • The number of simultaneous users of an application

Characterizing Quality of Service Requirements

• Analyze network traffic requirements
• Characterize traffic behavior such as broadcast and error recovery behavior
• Characterize the QoS requirements for applications

IETF Integrated Services Working Group QoS Specifications

• RSVP implements QoS for a particular data flow using mechanisms collectively called traffic control
• These mechanisms include:
  • A packet classifier that determines the QoS class (and perhaps the route) for each packet
  • An admission control function that determines whether the node has sufficient available resources to supply the requested QoS
  • A packet scheduler that determines when particular packets are forwarded to meet QoS requirements of a flow

Network Traffic Checklist

• Use this checklist to determine if you have completed all the steps for characterizing network traffic:
  • Have identified major traffic sources and stores and documented traffic flow between them
  • Have categorized the traffic flow for each application as being terminal/host, client/server, peer-to-peer, server/server, or distributed computing
  • Have estimated the bandwidth requirements for each application
  • Have estimated the bandwidth requirements for routing protocols
  • Have characterized network traffic in terms of broadcast/multicast rates, efficiency, frame sizes, windowing and flow control, and error-recovery mechanisms
  • Have categorized the QoS requirements of each application
  • Have discussed the challenges associated with implementing end-to-end QoS and the need for devices across the network to do their part in implementing QoS strategies