Telecom and Network Security
 Intro to OSI model
 LAN topologies
 OSI revisited
• hardware
• bridging,routing
• routed protocols, WANs
 IP addressing, subnet masks
 Routing Protocols

OSI/ISO ??
 OSI model developed by ISO, International
Standards Organization
 IEEE - Institute of Electrical and Electronics
Engineers
 NSA - National Security Agency
 NIST - National Institute for Standards and
Technology
 ANSI - American National Standards Institute
 CCITT - International Telegraph and Telephone
Consultative Committee

OSI Reference Model
 Open Systems Interconnection Reference Model
 Standard model for network communications
 Allows dissimilar networks to communicate

 Defines 7 protocol layers (a.k.a. protocol stack)

 Each layer on one workstation communicates with its
respective layer on another workstation using protocols
(i.e. agreed-upon communication formats)
 “Mapping” each protocol to the model is useful for
comparing protocols.

The OSI Layers
7 Application Provides specific services for applications such as
file transfer

6 Presentation Provides data representation between systems
5 Session Establishes, maintains, manages sessions
example - synchronization of data flow

4 Transport Provides end-to-end data transmission integrity
3 Network Switches and routes information units
2 Data Link Provides transfer of units of information to other
end of physical link
1 Physical Transmits bit stream on physical medium

Mnemonic: Please Do Not Take Sales Person Advice

Data Flow in
OSI Reference Model
Host 1 Host 2

Data travels down the stack
7 Applicatio

Then up the receiving stack
7 Applicatio
n n
6 Presentation 6 Presentation
5 Session 5 Sessio
4 Transport n
4 Transport
3 Network 3 Network
2 Data Link 2 Data Link
1 Physical 1 Physical

Through the network
As the data passes through each layer on the client information about that
layer is added to the data.. This information is stripped off by the
corresponding layer on the server.

OSI Model
 Protocols required for Networking are covered
in OSI model
 Keep model in mind for rest of course
 All layers to be explored in more detail

LAN Topologies
 Star Topology

 Bus Topology

LAN Topologies Cont…
 Ring Topology

Star Topology
 Telephone wiring is one common example
 Center of star is the wire closet
 Star Topology easily maintainable

Bus Topology
 Basically a cable that attaches many devices
 Can be a “daisy chain” configuration
 Computer I/O bus is example

Tree Topology
 Can be extension of bus and star topologies

 Tree has no closed loops

Ring Topology
 Continuous closed path between devices
 A logical ring is usually a physical star
 Don’t confuse logical and physical topology

Network topologies

Topology Advantages Disadvantages
Bus • Passive transmission medium • Channel access technique
• Localized failure impact (contention)
• Adaptive Utilization
Star • Simplicity • Reliability of central node
• Central routing • Loading of central node
• No routing decisions
Ring • Simplicity • Failure modes with global effect
• Predictable delay
• No routing decisions

LAN Access Methods
 Carrier Sense Multiple Access with Collision
Detection (CSMA/CD)
 Talk when no one else is talking
 Token
 Talk when you have the token
 Slotted
 Similar to token, talk in free “slots”

LAN Signaling Types
 Baseband
 Digital signal, serial bit stream
 Broadband
 Analog signal
 Cable TV technology

Ethernet
 Bus topology
 CSMA/CD
 Baseband
 Most common network type
 IEEE 802.3
 Broadcast technology - transmission stops at
terminators

Token Bus
 IEEE 802.4
 Very large scale, expensive
 Usually seen in factory automation
 Used when one needs:
 Multichannel capabilities of a broadband LAN
 resistance to electrical interference

Token Ring
 IEEE 802.5
 Flow is unidirectional
 Each node regenerates signal (acts as repeater)
 Control passed from interface to interface by
“token”
 Only one node at a time can have token
 4 or 16 Mbps

Fiber Distributed Data Interface (FDDI)
 Dual counter rotating rings
 Devices can attach to one or both rings
 Single attachment station (SAS), dual (DAS)
 Uses token passing
 Logically and physically a ring
 ANSI governed

WAN
 WANs connect LANs
 Generally a single data link
 Links most often come from Regional Bell Operating
Companies (RBOCs) or Post, Telephone, and
Telegraph (PTT) agencies
 Wan link contains Data Terminal Equipment (DTE)
on user side and Data Circuit-Terminating Equipment
(DCE) at WAN provider’s end
 MAN - Metropolitan Area Network

ISDN
 Integrated services digital network (ISDN) is a
worldwide public network service that can provide
end-to-end digital communications and fully integrate
technologies
 The basic rate interface (BRI) - 2B+D
 The primary rate interface (PRI) - 23B+D
 B channel - 64-Kbps bandwidth and are appropriate
for either voice or data transmission
 D channel - 16-Kbps signaling channel, is designed to
control transmission of the B channel

Typical Point-to –Point WAN

The Connections
T1 – 1.544 Mbps of electronic information
T2 - a T-carrier that can handle 6.312 Mbps or 96 voice
channels.
channels.
channels

WAN Cont…
 Cable Modem and DSL
 ADSL - Asymmetric Digital Subscriber Line - 144
Kbps to 1.5 Mbps
 SDSL - Single Line Digital Subscriber Line -
1.544 Mbps to 2.048 Mbps
 HDSL - High data rate Digital Subscriber Line -
1.544 Mbps to 42.048 Mbps
 VDSL - Very high data rate Digital Subscriber
Line - 13 to 52 Mbps 1.5 to 2.3 Mbps

WAN Cont…
 Frame Relay and X.25 - Packet-switched technologies

 Evolved from standardization work on ISDN
 Designed to eliminate much of the overhead in X.25
 DTE - Data Terminal Equipment
 DCE - Data Circuit-terminating Equipment
 CIR - Committed Information Rate

OSI Model -Layers
 Physical
 Data Link
 Network
 Transport
 Session
 Presentation
 Application

Physical Layer
 Specifies the electrical, mechanical,
procedural, and functional requirements for
activating, maintaining, and deactivating the
physical link between end systems
 Examples of physical link characteristics
include voltage levels, data rates, maximum
transmission distances, and physical
connectors

Physical Layer Hardware
 Cabling
 twisted pair
 10baseT
 10base2
 10base5
 fiber
 transceivers
 hubs
 topology

Twisted Pair
 10BaseT (10 Mbps, 100 meters w/o repeater)
 Unshielded and shielded twisted pair (UTP most
common)
 two wires per pair, twisted in spiral
 Typically 1 to 10 Mbps, up to 100Mbps possible
 Noise immunity and emanations improved by
shielding

Coaxial Cable
 10Base2 (10 Mbps, repeater every 200 m)
 ThinEthernet or Thinnet or Coax
 2-50 Mbps
 Needs repeaters every 200-500 meters
 Terminator: 50 ohms for ethernet, 75 for TV
 Flexible and rigid available, flexible most common
 Noise immunity and emanations very good

Coaxial Cables, cont
 Ethernet uses “T” connectors and 50 ohm
terminators
 Every segment must have exactly 2
terminators
 Segments may be linked using repeaters, hubs

Standard Ethernet
 10Base5
 Max of 100 taps per segment
 Nonintrusive taps available (vampire tap)
 Uses AUI (Attachment Unit Interface)

Fiber-Optic Cable
 Consists of Outer jacket, cladding of glass, and
core of glass
 Fast

Transceivers
 Physical devices to allow you to connect
different transmission media
 May include Signal Quality Error (SQE) or
“heartbeat” to test collision detection
mechanism on each transmission
 May include “link light”, lit when connection
exists

Hubs
 A device which connects several other devices
 Also called concentrator, repeater, or multi-
station access unit (MAU)

OSI Model - Layers

 Physical
 Data Link
 Network
 Transport
 Session
 Presentation
 Application

Data Link Layer
 Provides data transport across a physical link
 Data Link layer handles physical addressing,
network topology, line discipline, error
notification, orderly delivery of frames, and
optional flow control
 Bridges operate at this layer

Data Link Sub-layers
 Media Access Control (MAC)
 refers downward to lower layer hardware functions
 Logical Link Control (LLC)
 refers upward to higher layer software functions

Medium Access Control
 MAC address is “physical address”, unique for LAN
interface card
 Also called hardware or link-layer address
 The MAC address is burned into the Read Only
Memory (ROM)
 MAC address is 48 bit address in 12 hexadecimal
digits
 1st six identify vendor, provided by IEEE
 2nd six unique, provided by vendor

Logical Link Control
 Presents a uniform interface to upper layers
 Enables upper layers to gain independence
over LAN media access
 upper layers use network addresses rather than
MAC addresses
 Provide optional connection, flow control, and
sequencing services

Bridges
 Device which forwards frames between data link
layers associated with two separate cables
 Stores source and destination addresses in table
 When bridge receives a frame it attempts to find the
destination address in its table
 If found, frame is forwarded out appropriate port

 If not found, frame is flooded on all other ports

Bridges
 Can be used for filtering
 Make decisions based on source and destination address,
type, or combination thereof
 Filtering done for security or network management
reasons
 Limit bandwidth hogs
 Prevent sensitive data from leaving
 Bridges can be for local or remote networks
 Remote has “half” at each end of WAN link

Network Layer
 Which path should traffic take through
networks?
 How do the packets know where to go?
 What are protocols?
 What is the difference between routed and
routing protocols?

Network Layer
 Only two devices which are directly connected by
the same “wire” can exchange data directly
 Devices not on the same network must
communicate via intermediate system
 Router is an intermediate system
 The network layer determines the best way to
transfer data. It manages device addressing and
tracks the location of devices. The router operates
at this layer.

Network Layer
Bridge vs. Router
 Bridges can only extend a single network
 All devices appear to be on same “wire”
 Network has finite size, dependent on topology,
protocols used
 Routers can connect bridged subnetworks
 Routed network has no limit on size
 Internet, SIPRNET

Network Layer
 Provides routing and relaying
 Routing: determining the path between two end systems
 Relaying: moving data along that path
 Addressing mechanism is required
 Flow control may be required
 Must handle specific features of subnetwork
 Mapping between data link layer and network layer
addresses

Connection-Oriented vs. Connectionless
Network Layer
 Connection-Oriented
 provides a Virtual Circuit (VC) between two end
systems (like a telephone)
 3 phases - call setup, data exchange, call close
 Examples include X.25, OSI CONP, IBM SNA
 Ideal for traditional terminal-host networks of finite
size

Network Layer
 Connectionless (CL)
 Each piece of data independently routed
 Sometimes called “datagram” networking
 Each piece of data must carry all addressing and routing
info
 Basis of many current LAN/WAN operations
 TCP/IP, OSI CLNP, IPX/SPX
 Well suited to client/server and other distributed system
networks

Network Layer
 Arguments can be made Connection Oriented is best
for many applications
 Market has decided on CL networking
 All mainstream developments on CL
 Majority of networks now built CL
 Easier to extend LAN based networks using CL WANs
 We will focus on CL

Network switching
 Circuit-switched
 Transparent path between devices
 Dedicated circuit
 Phone call
 Packet-switched
 Data is segmented, buffered, & recombined

Network Layer Addressing
 Impossible to use MAC addresses
 Hierarchical scheme makes much more sense
(Think postal - city, state, country)
 This means routers only need to know regions
(domains), not individual computers
 The network address identifies the network and
the host

 Network Address - path part used by router
 Host Address - specific port or device

1.1
1.2 2.1 2.2
Router

1.3 Network Host
1 1,2,3
2.3
2 1,2,3

IP example
 IP addresses are like street addresses for
computers
 Networks are hierarchically divided into subnets
called domains
 Domains are assigned IP addresses and names
 Domains are represented by the network

portion of the address
 IP addresses and Domains are issued by InterNIC
(cooperative activity between the National Science
Foundation, Network Solutions, Inc. and AT&T)

Network Layer Addressing - IP
 IP uses a 4 octet (32 bit) network address
 The network and host portions of the address can
vary in size
 Normally, the network is assigned a class according
to the size of the network
 Class A uses 1 octet for the network
 Class B uses 2 octets for the network
 Class C uses 3 octets for the network
 Class D is used for multicast addresses

Class A Address
 Used in an inter-network that has a few
networks and a large number of hosts
 First octet assigned, users designate the other
3 octets (24 bits)
 Up to 128 Class A Domains
 Up to 16,777,216 hosts per domain

This Field is 24 Bits of
Fixed by IAB Variable Address

0-127 0-255 0-255 0-255

Class B Address
 Used for a number of networks having a
number of hosts
 First 2 octets assigned, user designates the
other 2 octets (16 bits)
 16384 Class B Domains
 Up to 65536 hosts per domain
These Fields are 16 Bits of
Fixed by IAB Variable Address

128-191 0-255 0-25 0-25
5 5

Class C Address
 Used for networks having a small amount of
hosts
 First 3 octets assigned, user designates last
octet (8 bits)
 Up to 2,097,152 Class C Domains
 Up to 256 hosts per domain
These Fields are 8 Bits of
Fixed by IAB Variable
Address

191-223 0-255 0-255 0-255

IP Addresses
 A host address of all ones is a broadcast
 A host address of zero means the wire itself
 These host addresses are always reserved and
can never be used

Subnets & Subnet Masks
 Every host on a network (i.e. same cable segment)
must be configured with the same subnet ID.
 First octet on class A addresses
 First & second octet on class B addresses
 First, second, & third octet on class C addresses
 A Subnet Mask (Netmask) is a bit pattern that
defines which portion of the 32 bits represents a
subnet address.
 Network devices use subnet masks to identify which
part of the address is network and which part is host

Network Layer
Routed vs. Routing Protocols
 Routed Protocol - any protocol which provides
enough information in its network layer
address to allow the packet to reach its
destination
 Routing Protocol - any protocol used by
routers to share routing information

Routed Protocols
 IP
 IPX

 SMB

 Appletalk

 DEC/LAT

OSI Reference Model
Protocol Mapping

TCP/IP UDP/IP SPX/IPX
Application using Application using Application using
7 Applicatio TCP/IP UDP/IP SPX/IPX
n
6 Presentation

5 Session SPX
4 Transport TCP UDP
3 Network IP IP IPX
2 Data Link
1 Physical

Network-level Protocols
 IPX (Internet Packet Exchange protocol)
 Novell Netware & others
 Works with the Session-layer protocol SPX (Sequential Packet
Exchange Protocol)
 NETBEUI (NetBIOS Extended User Interface)
 Windows for Workgroups & Windows NT
 IP (Internet Protocol)
 Win NT, Win 95, Unix, etc…
 Works with the Transport-layer protocols TCP (Transmission Control
Protocol) and UDP (User Datagram Protocol)
 SLIP (Serial-line Input Protocol) & PPP (Point-to-Point
Protocol)

TCP/IP
Consists of a suite of protocols (TCP & IP)
Handles data in the form of packets
Keeps track of packets which can be
Out of order
Damaged
Lost
Provides universal connectivity
reliable full duplex stream delivery (as opposed to
the unreliable UDP/IP protocol suite used by such
applications as PING and DNS)

TCP/IP Cont…
 Primary Services (applications) using TCP/IP
 FileTransfer (FTP)
 Remote Login (Telnet)

 Electronic Mail (SMTP)

 Currently the most widely used protocol
(especially on the Internet)
 Uses the IP address scheme

Routing Protocols
 Distance -Vector
 List of destination networks with direction and distance
in hops
 Link-state routing
 Topology map of network identifies all routers and
subnetworks
 Route is determined from shortest path to destination
 Routes can be manually loaded (static) or
dynamically maintained

Routing Internet
Management Domains
 Core of Internet uses Gateway-Gateway Protocol
(GGP) to exchange data between routers
 Exterior Gateway Protocol (EGP) is used to
exchange routing data with core and other
autonomous systems
 Interior Gateway Protocol (IGP) is used within
autonomous systems

Routing
Internet Management Domains
Internet Core
GGP

EGP
EGP

IGP IGP
Autonomous systems

Routing Protocols
 Static routes
 not a protocol
 entered by hand
 define a path to a network or subnet
 Most secure

Routing Protocols
RIP
 Distance Vector
 Interior Gateway Protocol
 Noisy, not the most efficient
 Broadcast routes every 30 seconds
 Lowest cost route always best
 A cost of 16 is unreachable
 No security, anyone can pretend to be a router

Routing Protocols
OSPF
 Link-state
 Interior Gateway Protocol
 Routers elect “Designated Router”
 All routers establish a topology database using
DR as gateway between areas
 Along with IGRP, a replacement for outdated
RIP

Routing Protocols
BGP
 Border Gateway Protocol is an EGP
 Can support multiple paths between
autonomous systems
 Can detect and suppress routing loops
 Lacks security
 Internet recently down because of incorrectly
configured BGP on ISP router

Source Routing
 Source (packet sender) can specify route a
packet will traverse the network
 Two types, strict and loose
 Allows IP spoofing attacks
 Rarely allowed across Internet

Transport Layer
 TCP
 UDP
 IPX Service Advertising Protocol
 Are UDP and TCP connectionless or
connection oriented?
 What is IP?
 Explain the difference

Session Layer
 Establishes, manages and terminates sessions
between applications
 coordinates service requests and responses that
occur when applications communicate between
different hosts
 Examples include: NFS, RPC, X Window
System, AppleTalk Session Protocol

Presentation Layer
 Provides code formatting and conversion
 For example, translates between differing text and
data character representations such as EBCDIC and
ASCII
 Also includes data encryption
 Layer 6 standards include JPEG, GIF, MPEG, MIDI

Application-layer Protocols
 FTP (File Transfer Protocol)
 TFTP (Trivial File Transfer Protocol)
 Used by some X-Terminal systems
 HTTP (HyperText Transfer Protocol)
 SNMP (Simple Network Management Protocol
 Helps network managers locate and correct problems in a
TCP/IP network
 Used to gain information from network devices such as count
of packets received and routing tables
 SMTP (Simple Mail Transfer Protocol)
 Used by many email applications

Identification & Authentication
 Identify who is connecting - userid
 Authenticate who is connecting
 password (static) - something you know
 token (SecureID) - something you have
 biometric - something you are
 RADIUS, TACACS, PAP, CHAP
 DIAMETER

Firewall Terms
 Network address translation (NAT)
 Internal addresses unreachable from external
network
 DMZ - De-Militarized Zone
 Hosts that are directly reachable from untrusted
networks
 ACL - Access Control List
 can be router or firewall term

Firewall Terms
 Choke, Choke router
 A router with packet filtering rules (ACLs)
enabled
 Gate, Bastion host, Dual Homed Host
 A server that provides packet filtering and/or
proxy services
 proxy server
 A server that provides application proxies

Firewall types
 Packet-filtering router
 Most common
 Uses Access Control Lists (ACL)
 Port
 Source/destination address

Screened host
 Packet-filtering and Bastion host
 Application layer proxies
 Screened subnet (DMZ)
 2 packet filtering routers and bastion host(s)
 Most secure

Firewall Models
 Proxy servers
 Intermediary
 Think of bank teller
 Stateful Inspection
 State and context analyzed on every packet in
connection

VPN – Virtual Private Network
 PPTP
 L2TP

 IPSec

 Tunnel Mode

 Transport Mode

 Site-to-Site VPN

 Client-to-Site VPN

 SSL

 SSH

Intrusion Detection (IDS)
 Host or network based
 Context and content monitoring
 Positioned at network boundaries
 Basically a sniffer with the capability to detect
traffic patterns known as attack signatures

Web Security
 Secure sockets Layer (SSL)
 Transport layer security (TCP based)
 Widely used for web based applications
 by convention, https:
 Secure Hypertext Transfer Protocol (S-HTTP)
 Less popular than SSL
 Used for individual messages rather than sessions
 Secure Electronic Transactions (SET)
 PKI
 Financial data
 Supported by VISA, MasterCard, Microsoft, Netscape

IPSEC
 IP Security
 Set of protocols developed by IETF
 Standard used to implement VPNs
 Two modes
 Transport Mode
 encrypted payload (data), clear text header
 Tunnel Mode
 encrypted payload and header
 IPSEC requires shared public key

Spoofing
 TCP Sequence number prediction
 UDP - trivial to spoof (CL)
 DNS - spoof/manipulate IP/hostname pairings
 Source Routing

Sniffing
 Passive attack
 Monitor the “wire” for all traffic - most
effective in shared media networks
 Sniffers used to be “hardware”, now are a
standard software tool

Session Hijacking
 Uses sniffer to detect sessions, get pertinent session
info (sequence numbers, IP addresses)
 Actively injects packets, spoofing the client side of
the connection, taking over session with server
 Bypasses I&A controls
 Encryption is a countermeasure, stateful inspection
can be a countermeasure

IP Fragmentation
 Use fragmentation options in the IP header to
force data in the packet to be overwritten upon
reassembly
 Used to circumvent packet filters
 Leads to Denial of Service Attack

IDS Attacks
 Insertion Attacks
 Insert information to confuse pattern matching
 Evasion Attacks
 Trick the IDS into not detecting traffic
 Example - Send a TCP RST with a TTL setting
such that the packet expires prior to reaching its
destination

Syn Floods
 Remember the TCP handshake?
 Syn, Syn-Ack, Ack
 Send a lot of Syns
 Don’t send Acks
 Victim has a lot of open connections, can’t
accept any more incoming connections
 Denial of Service

Telecom/Remote Access Security
 Dial up lines are favorite hacker target
 War dialing
 social engineering
 PBX is a favorite phreaker target
 blue box, gold box, etc.
 Voice mail

Remote Access Security
 SLIP - Serial Line Internet Protocol
 PPP - Point to Point Protocol
 SLIP/PPP about the same, PPP adds error checking, SLIP
obsolete
 PAP - Password authentication protocol
 clear text password
 CHAP - Challenge Handshake Auth. Prot.
 Encrypted password

Remote Access Security
 TACACS, TACACS+
 Terminal Access Controller Access Control
System
 Network devices query TACACS server to
verify passwords
 “+” adds ability for two-factor (dynamic)
passwords
 Radius
 Remote Auth. Dial-In User Service

RAID
 Redundant Array of Inexpensive(or
Independent) Disks - 7 levels
 Level 0 - Data striping (spreads blocks of each file
across multiple disks)
 Level 1 - Provides disk mirroring
 Level 3 - Same as 0, but adds a disk for error
correction
 Level 5 - Data striping at byte level, error
correction too

5. telecomm & network security

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