3 OSI Model and Protocols
The TCP/IP Protocol Suite (3/3)
-If the host-to-host (or transport) protocol is TCP, the matching functionality of the TCP/IP transport layer is found in the OSI transport and session layers combined -When using UDP, the TCP/IP transport layer is equivalent to the functions of only the transport layer of the OSI model -The TCP/IP application layer, when used with TCP, provides the functions of the OSI model's presentation and application layers; when used with UDP, its functions are equivalent to OSI session, presentation, and application layers combined
Can you name an example of enterprise extended wireless network?
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Bus Topology
§A bus topology connects each system to a trunk or backbone cable -All systems on the bus can transmit data simultaneously •To avoid collision, the systems employ a mechanism that listens for any other currently occurring traffic -When data is transmitted on a bus topology, all systems on the network see the data -The benefit of a bus topology is that if a single segment fails, communications on all other segments can continue •However, the central trunk line remains a single point of failure §Two types of bus topologies -Linear: single trunk line with all systems directly connected to it -Tree: single trunk line with branches that can support multiple systems
Communication Protocol
§A communication protocol is a set of rules that define a common language two computers use to exchange messages §The term comes from diplomacy, in which a protocol specifies the rules under which a diplomatic exchange occurs §A computer communication protocol defines: -The exact format and meaning of each message that a computer can send -The conditions under which a computer should send a given message -How a computer should respond when a message arrives §
Mesh Topology
§A mesh topology connects systems to other systems using numerous paths -A full mesh topology connects each system to all other systems on the network -A partial mesh topology connects many systems to many other systems §Mesh topologies provide redundant connections to systems -They allow multiple segment failures without seriously affecting connectivity
Ring Topology
§A ring topology connects each system as points on a unidirectional transmission loop -Only one system can transmit data at a time -Traffic management is performed by a token •A token is a special packet that travels around the ring until a system grabs it •A system in possession of the token can transmit §As the data travels around the loop, each system checks to see whether it is the intended recipient -Once the data is received, the token is released and returns in the loop §If any one segment of the loop is broken, all communication around the loop stops §Dual loops can be used as a fault tolerance mechanism
Star Topology
§A star topology employs a centralized connection device -This device can be a simple hub or switch §Each system is connected to the central hub by a dedicated segment -If any one segment fails, the other segments can continue to function -However, the central hub is a single point of failure §The star topology uses less cabling than other topologies and makes the identification of damaged cables easier §A logical bus and a logical ring can be implemented as a physical star -Ethernet is a bus-based technology •It can be deployed as a physical star
Layer 1 - Physical Layer
§Accepts frames from the Data Link layer and converts them into bits for transmission over the physical connection medium §Receives bits from the physical connection medium and converts them into frames to be used by the Data Link layer §Located within the Physical Layer are electrical specifications, protocols, and interface standards such as those defined by -IEEE, EIA/TIA, ANSI, and other similar standardization bodies §Network hardware devices that function at Layer 1 are -Network interface cards (NICs), hubs, repeaters, and concentrators •These devices perform hardware-based signal operations, such as sending a signal from one connection port out on all other ports (a hub) or amplifying the signal to increase the transmission distance (a repeater)
Layer 2 - Data Link Layer (1/2)
§Accepts packets from the Network layer and converts them into the proper format for transmission (frames) -The proper format is determined by the hardware and the technology of the network •Ethernet (IEEE 802.3) •Token Ring (IEEE 802.5) •Asynchronous Transfer Mode (ATM) §Once the frame is formatted, it is sent to the Physical layer for transmission §Protocols found within the Data Link layer include -Address Resolution Protocol (ARP) •Used to resolve IP addresses into MAC addresses -Reverse Address Resolution Protocol (RARP)
Network Cabling (2/2)
§Cabling types -Coaxial cable •ThinNet 10base2 •ThickNet 10base5 -Twisted Pair •Unshielded Twisted Pair (UTP) •Shielded Twisted Pair (STP) -Fiber Optic •Pulses of lights §Cabling issues -Cross-talk -Attenuation
Securing 802.11 Networks
§Change the default password §Disable the SSID broadcast §Change the SSID to something unique §Enable MAC filtering - If the pool of wireless clients is relatively small and static §Use the highest form of authentication encryption supported -WEP, WPA, or WPA-2 (802.11i) §Manage access using 802.1X §Isolate using a firewall §Monitor WAP-to-wired-network communications with an IDS §Require all transmissions to be encrypted
History of the OSI Reference Model
§Developed in late 1970s / early 1980s -Developed by the International Organization for Standardization (ISO) -TCP/IP was developed in early 1970s §Establishes a common communication structure or standard for computer systems -The OSI model was never widely adopted, but the theory behind it was readily accepted §Serves as an abstract framework or theoretical model for how protocols should function §Used as a common reference point amongst protocols, as well as software and hardware
OSI Functionality
§Divided into 7 layers each with specific tasks -Layers are numbered bottom to top and referred to by layer name or number §Layers communicate only with layers directly above or below -Each layer performs specific functions to support the layers above it and uses the services offered by the layers below it §The three lowest layers focus on passing traffic through the network to an end system (hardware/firmware) §The top four layers focus on establishing connections and supporting apps (software) §
Encapsulation / Deencapsulation (1/2)
§Encapsulation -The addition of a header, and possibly a footer, to the data received by each layer from the layer above (the payload), before it's handed off to the layer below •A footer is added by the Data Link layer -Occurs as data moves from the top layer to the bottom layer •As the message is encapsulated at each layer, the previous layer's header and payload become the payload of the current layer §Deencapsulation -Occurs as received data moves from the bottom layer to the top layer -Each layer's header (and footer) is processed and removed, before sending the payload up to the layer above •This information creates the logical channel that enables peer layers on different computers to communicate
Layer 4 - Transport Layer
§Establishes a logical connection between two devices and provides end-to-end transport services to ensure data delivery §Accepts PDUs (protocol data unit) from the Session layer and converts them into segments §Includes mechanisms for segmentation, sequencing, error checking, controlling the flow of data, error correction -Rules specify how much data each segment can contain, how to verify the integrity of data transmitted, and whether data has been lost §Protocols found within the Transport layer include -Transmission Control Protocol (TCP) -User Datagram Protocol (UDP) -Secure Sockets Layer (SSL) -Transport Layer Security (TLS)
LAN Technologies
§Ethernet - 802.3 -Shared-media LAN technology using collision and broadcast domains •Numerous devices communicate over the same medium -They take turns communicating and perform collision detection and avoidance •A broadcast domain is a physical grouping of systems in which all the systems in the group receive a broadcast sent by a single system in the group -Logical bus topology, often deployed as physical star -Fast Ethernet supports 100 Mbps throughput -Gigabit Ethernet supports 1 Gbps throughput §Token Ring - 802.5 -Logical ring topology, deployable as a physical star §Fiber Distributed Data Interface (FDDI) -Dual token ring: two rings with traffic flowing in opposite directions
Wireless Security
§IEEE 802.11 defines two methods that wireless clients can use to authenticate to WAPs -Open System Authentication (OSA) •No real authentication required -As long as a radio signal can be transmitted between client and WAP, communications are allowed -Networks using OSA typically transmit everything in clear text -Shared Key authentication (SKA) •Some form of authentication must take place •802 .11 defines a technique known as Wired Equivalent Privacy (WEP) -Encrypted authentication that employs RC4 -Supports only one-way authentication from client to WAP -Considered insufficient for security because of several deficiencies in its design and implementation: can be cracked in minutes •Two newer and more secure methods are WPA-2 and 802.1.X
Network Layer Protocols (2/2)
§Internet Control Messaging Protocol (ICMP) -Used to determine the health of a network or a specific link -ICMP is utilized by ping, trace route, and other network management tools -The type field in the ICMP header defines the type or purpose of the message contained within the ICMP payload -There are more than 40 defined types, but only 7 are commonly used •Complete list of type values available at www.iana.org/assignments/icmp-parameters -
Network Layer Protocols (1/2)
§Internet Protocol (IP) -Provides route addressing for data packets •This route addressing is the foundation of global Internet communications because it provides a means of identifying transmission paths -Unreliable connectionless datagram protocol •Does not offer guarantees that packets will be delivered or that packets will be delivered in the correct order •Does not guarantee that packets will not be delivered more than once -IPv4 vs. IPv6 •32-bit vs. 128-bit addressing scheme: 4.2 x 109 vs. 3.4 x 1038 addresses §Internet Group Management Protocol (IGMP) -Allows systems to support multicasting •Multicasting is the transmission of data to multiple specific recipients
Layer 7 - Application Layer (2/2)
§Numerous application-specific protocols are found the application layer, such as the following -Hypertext Transfer Protocol (HTTP) -File Transfer Protocol (FTP) -Simple Mail Transfer Protocol (SMTP) -Telnet -Electronic Data Interchange (EDI) -Post Office Protocol version 3 (POP3) -Internet Message Access Protocol (IMAP) -Simple Network Management Protocol (SNMP) -Network News Transport Protocol (NNTP) -Secure Remote Procedure Call (S-RPC) -Secure Electronic Transaction (SET)
Communicating Using Channels (1/2)
§One solution to sharing a communication medium is the use of channels §Cable television systems send multiple signals using multiple channels -Each channel is assigned a unique frequency -A carrier at that frequency is modulated to encode the information -All the signals are mixed together and sent over the cable -The television receiver extracts the signal for the selected channel and ignores the other channels
Layer 2 - Data Link Layer (2/2)
§Processing performed within the Data Link layer includes adding hardware source and destination addresses to the frame -The hardware address is the Media Access Control (MAC) address -A MAC address is a 6-byte (48-bit) address written in hexadecimal notation (for example, 00-13-02-lF-58-F5) •The first 3 bytes denote the manufacturer of the network interface •The last 3 bytes represent a unique number assigned to that interface by the manufacturer §Network devices that function at layer 2 are switches and bridges -These devices support MAC-based traffic routing •Bridges normally works in software •Switches use Application-Specific Integrated Circuits (ASICs) to perform the task on dedicated hardware
Layer 3 - Network Layer (1/5)
§Responsible for adding routing and addressing information to the data §Accepts segments from the Transport layer and adds information to it to create a packet -The packet includes the source and destination IP addresses §Protocols found within the Network layer include -Internet Control Message Protocol (ICMP) -Routing Information Protocol (RIP) -Open Shortest Path First (OSPF) -Border Gateway Protocol (BGP) -Internet Protocol (IP), IP Security (IPSec) -Network Address Translation (NAT)
Layer 5 - Session Layer
§Responsible for establishing, maintaining, and terminating communication sessions between two computers §Establishes checkpoints for recovery, and retransmits PDUs that have failed or been lost since the last verified checkpoint §Protocols operating at the Session layer include -Network File System (NFS) -Remote Procedure Call (RPC) §Communication sessions can operate in different control modes -Simplex: One-way communication -Half-duplex: Two-way communication, but only one direction can send data at a time -Full-duplex: Two-way communication, in which data can be sent in both directions simultaneously
Layer 7 - Application Layer (1/2)
§Responsible for interfacing user applications, network services, or the operating system with the protocol stack -Allows applications to communicate with their peers through the protocol stack §The Application layer determines whether a remote communication partner is available and accessible, and resources are available to support the requested communications -For instance, an email client can determine whether the mail server is available §Applications are not located within this layer -The protocols and services required to transmit files, exchange messages, and connect to remote terminals are found here
Layer 6 - Presentation Layer
§Responsible for transforming data received from the Application layer into a standard format that any system following the OSI model can understand §Responsible for encryption and compression §Acts as an interface between the network and the applications §Most file or data formats for images, video, sound, documents, email, and web pages operate within this layer, including -American Standard Code for Information Interchange (ASCII) -Tagged Image File Format (TIFF) -Joint Photographic Experts Group (JPEG) -Moving Picture Experts Group (MPEG)
Application Layer Protocols
§Telnet (23) §SSH (22) §FTP (21, 20) §TFTP (UDP 69) §SMTP (25, SSL 465) §POP3 (110, SSL 995) §IMAP4 (143) §DHCP/BootP (67, 68) §§HTTP (80, SSL 443) §LPD (515) §X Window (6000-6063) §NFS (2049) §SNMP (UDP 161, 162) §LDAP (389, SSL 636) §Kerberos (88)
Layer 3 - Network Layer (2/5)
§The Network layer is responsible for providing routing or delivery information, but it is not responsible for verifying guaranteed delivery -That is the responsibility of the Transport layer §The Network layer also manages error detection and traffic control §Routers are among the network hardware devices that function at layer 3 -Routers determine the best logical path for the transmission of packets based on speed, hops, preference, and so on -Routers use the destination IP address to guide the transmission of packets
TCP/IP Model (1/3)
§The TCP/IP model (aka the DARPA or the DOD model) consists of only four layers
The TCP/IP Protocol Suite (2/3)
§The TCP/IP model uses four layers to perform the functions of the seven-layer OSI model -The TCP/IP network access (or network interface) layer is functionally equivalent to the combination of OSI physical and data link layers -The TCP/IP Internet layer performs the same functions as the OSI network layer
Network Topologies
§The physical layout and organization of computers and networking devices is known as the network topology §The logical topology is the grouping of networked systems into trusted groups §The physical topology is not always the same as the logical topology §There are four basic topologies for the physical layout of a network -Ring -Bus -Star -Mesh •Full and partial
Network Cabling (1/2)
§The type of physical connectivity media employed in a network is important to the network's design, layout, and capabilities -Without the right cabling or transmission media, a network may not be able to support the necessary traffic volume -Common causes of network failures (in other words, violations of availability) are cable failures or misconfigurations §Different types of network devices and technologies are used with different types of cabling -Each cable type has unique useful lengths, throughput rates, and connectivity requirements §Nomenclature -XXyyyyZZ: speed, baseband/broadband, length or technology §
Layer 3 - The Internet Layer (3/5)
§To make it easier to manage the network and control the flow of packets, many organizations separate their network layer into smaller parts known as subnets -Routers use the network or subnet portion of the IP address to route traffic between different networks •Each router must be configured specifically for the networks or subnets that will be connected to its interfaces -Routers communicate with one another using routing protocols, such as Routing Information Protocol (RIP) and Open version of Shortest Path First (OSPF), to learn of other networks that are present and to calculate the best way to reach each network
Transport Layer Protocols
§Transmission Control Protocol (TCP) -Connection-oriented -Virtual circuit established via three-way handshake -Header composition - Flag field §User Datagram Protocol (UDP) -Connectionless -Simple header composition §When a connection is established between two systems, it is done using ports (16-bit numbers) -Ports allow a single IP address to be able to support multiple simultaneous communications, each using a different port number •The first 1,024 ports are called the well-known ports or the service ports -For example, port 80 is the standard port for web (HTTP) traffic -
Layer 3 - The Internet Layer (5/5)
§When passing packets between different networks, it may become necessary to adjust their outbound size to one that is compatible with the layer 2 protocol that is being used -Routers at the network layer accomplish this via a process known as fragmentation -All reassembly of fragmented packets happens at the network layer of the final destination system §Some basic security functionality can also be set up by filtering traffic using layer 3 addressing on routers or other similar devices
Wireless Communications
§Wireless communications employ radio waves to transmit signals over a distance §There is a finite amount of radio wave spectrum -Its use must be managed properly to allow multiple simultaneous communications with little to no interference -The radio spectrum is measured using frequency •Number of wave oscillations per time, measured in Hertz (Hz) -Radio waves have a frequency between 3 Hz and 300 GHz -Different ranges of frequencies have been designated for specific uses §To manage the simultaneous use of the limited radio frequencies, several spectrum-use techniques were developed -Spread Spectrum, FHSS, DSSS, OFDM •With spread spectrum communication occurs over multiple frequencies
Wireless Networking (802.11) (1/2)
§Wireless networking is based on the IEEE 802.11 standard §Two primary components -Host interfaces: wireless network interface cards (wireless NICs) -Wireless access point: the radio signal hub for the wireless network •Supports associations with host devices with wireless NICs •Converts radio signal transmissions into cable-based transmissions in order to support communications between wireless clients and the wired network §Wireless networks are assigned a Service Set Identifier (SSID) to differentiate one wireless network from another -If multiple base stations (wireless access points) are used in the same wireless network, an Extended Service Set Identifier (ESSID) is defined -The SSID is broadcast by the WAP via a special transmission called a beacon frame -
Wireless Networking (802.11) (2/2)
§Wireless networks can be deployed in two primary methods -Ad hoc or peer-to-peer •Wireless clients connect directly to one another without the use of a wireless access point -Infrastructure •Any network configuration using a wireless access point to connect wireless clients •A stand-alone mode infrastructure occurs when there is a wireless access point connecting wireless clients to each other, but not to any wired resources •A wired extension mode infrastructure occurs when the wireless access point acts as a connection point to link the wireless clients to the wired network •An enterprise extended mode infrastructure occurs when multiple wireless access points (WAPs) are used to connect a large physical area to a wired network -Each wireless access point will use the same Extended Service Set Identifier (ESSID) §
Domain Name Resolution
§Without addressing schemes, networked computers would not be able to communicate §Likewise, without naming schemes, humans would have to remember numeric addresses to identify computers -It is much easier to remember google.com than 64.233.187.99 §In TCP/IP-based networks there are three different layers -MAC address: a "permanent" physical address -IP address: a "temporary" logical address assigned over the MAC address -Domain name: a "temporary" human-friendly name assigned over the IP address §The Domain Name System (DNS) resolves a human-friendly domain name into its IP address equivalent -Then, ARP resolves the IP address into its MAC address equivalent
