EECS 1520 Final: Exam Notes #3

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York University
Electrical Engineering and Computer Science
EECS 1520
John Hofbauer

Computer Networking Why network? • Share data (e.g., documents, photos) • Share resources (e.g., storage, printer) • Authenticate credentials (e.g., e-payment, security access) • Lookup information (e.g., sports scores, stock prices) • Disseminate information (e.g., event invites, job postings) • Backup data (to facilitate recovery of lost/corrupted data) • Manage resources (e.g., configure company computers) • Remote operation (e.g., home monitoring, Nest thermostat) Client-Server Model • Describes the interaction of networked computers o Clients request information from servers o Servers respond with the information (if applicable) • Misleading, as any computer can act as a client or server o Computer typically described as “client” or “server” based on its role the majority of the time • E.g.: File Server 1 requests a file from File Server 2 o FS1 is the client, FS2 is the server in this interaction Networking Terms • Bandwidth, the amount of data transmitted along a communication path in a given unit of time; typically expressed in bits per second (bps) • Latency, the amount of time it takes for a piece of data to travel between two points in a network Wired Networking Ethernet • Developed by Xerox PARC in mid 1970s • Commercialized and standardized (called IEEE 802.3) in early 1980s • Used to implement local area networks(LANs) in home, schools, and businesses • Sockets and cables resemble those for telephones, but are wider and thicker • Ethernet cables contain four pairs of wires that are twisted together to mitigate electro-magnetic interference o Cable categories (Cat3, Cat5, Cat5e, Cat6) describe the cable’s construction and its maximum speed More than just wires… • Network devices join computers to the rest of the network o They analyze the data and direct it towards its destination • Switch, directs data between computers within the same network • Router or gateway, passes data from one network to another; joins two networks • Firewall, monitors network traffic and blocks communication that violates access rules (pre-defined and user-specified) • These network entities can exist in stand-alone devices or can be combined into a single multi-function device Message Switching • Data is sent in its entirety and routed to its destination • Each intermediate point in the network (called a node) must wait to receive the entire message before forwarding it Advantages: data arrives at its destination intact Disadvantages: 1. Network devices must have sufficient memory to store entire messages (possibly of unlimited length) 2. Latency is higher, as devices must receive entire message before forwarding it (i.e., not good for real- time communication) 3. Any corruption to the data means resending the entire message. Packet Switching • Data is divided into packets and routed to its destination • Each packet could reach its destination with a different path • Disadvantages: Packets could arrive out of order and need to be re-arranged • Advantages: 1. If a packet is lost/corrupted, only it needs to be resent 2. With large data, the first packet can be sent right away (i.e., lower latency, which is good for real- time applications) 3. Network devices only need enough memory to store a sufficient number of packets • Computer networks typically use packet switching • Packet are typically only 1500 bytes in size, but network devices can be configured to use sizes up to 9000 bytes (called “jumbo frames”) • Included in the packet is information about the data, its source, and its destination (like a parcel in the mail) • But how does the network know where the packet has to go? How does the network differentiate computers? Network Addressing • Internet Protocol (IP) address uniquely identifies each client on a network (like a phone number represent a phone line) o 4 bytes long, with each byte representing a value 0-255 o Takes the form x.y.a.b where x, y, a, and bare numbers 0-255 ▪ Example using York’s IP: • Each IP address also has virtual “ports” (like phone ext.) o Specifying a port is optional or handled automatically by software o A port is specified using a colon at the end of the IP address: ▪ Example using port 80: • Some IP addresses are reserved for private use (e.g., in a home, small business, or portions of a larger network): –, –, – Network Address Translation (NAT) • Used in routers to map a single IP address to a series of network clients, each with their own private IP address • Allows the reuse of private IP addresses in subsections(“subnets”) of a LAN • Also reduces the need for non-private IP addresses Wireless Networking Wi-Fi • Developed by NCR Corp. (subsidiary of AT&T Corp.) in 1991 • Standardized in 1997 (IEEE 802.11) and popularized in 1999 • Implement wireless local area networks (WLANs): • Numerous IEEE 802.11 (a, b, g, n, ac) standards o Provides speeds from 11 Mb/s to 1.67 Gb/s o Uses the 2.4 GHz and/or 5 GHz radio frequency band ▪ 2.4 GHz communication vulnerable to fluorescent lighting and microwaves • Devices called access points (APs) connect the wireless clients of a WLAN to the rest of the LAN Sharing the Air • When using wireless technology, all other clients within range can capture (“sniff”) your data packets o Packet sniffing can also occur on wired networks, but the network clients don’t often change and are relatively trustworthy • Your data transmissions can influence theirs and visa versa o Wireless clients take turns transmitting to decrease such interference (called “collisions”) o Modern Wi-Fi standards (n and ac) can now direct signals (“beamforming”) and use multiple input and multiple output antennas (“MIMO”) to reduce the negative effects of interference Bluetooth • Often used to create a personal area network (PAN), connecting devices within your immediate vicinity o Game controllers, keyboards, mice o Mobile devices (e.g., smartphones, smart watches) o Fitness trackers o Hands-free devices (e.g., headset, car speakerphone) • Communication profiles for different types of data o Advanced Audio Distribution Profile (A2DP) –audio streaming o Hands-Free Profile (HFP) –headsets o Human Interface Device Profile –computer input devices • Data rates up to 25 Mb/s • Device range listed by “class” o Class 1 (100 m), Class 2 (10 m), Class 3 (1 m) Near Field Communication (NFC) • Transfer data between devices in close proximity ~4 cm • Data rates up to 424 kb/s (not meant for large files) • Can read codes/data on radio-frequency ID (RFID) tags: typically found in smart cards (e.g. debit cards), shipping containers (for tracking) • NFC in smartphones can read and emulate RFID tags • RFID tags in smart cards can be read at up to 3 feet, but other kinds can be read up to 30 or 300 feet Wireless Security Vulnerabilities Wi-Fi: • Data transmitted exposed to all other user in vicinity • Older security measure (WEP encryption) easily broken • Use Wi-Fi Protected Access (WPA or WPA2) instead NFC used to “steal” info from RFID debit/credit cards: • CBC news story • GlobalTV news story [Use at own risk] • Android app to read your RFID cards • Can be blocked using special wallets, cases, or aluminum foil Bluetooth devices sometimes lack sufficient security: • Outline of threats, consequences, and preventative measures Motor vehicles are increasingly connected: • Car vulnerable to hacking • Public service announcement from the FBI The Internet What is the Internet? • Global network of networks • Considered a wide area network (WAN) of diverse devices • Communication is facilitated using agreed-upon protocols (i.e., rules for transmitting and receiving data) • Data travels over the Internet using packet switching • Computers on the Internet are identified using IP addresses Who controls the Internet? • No central body controls the Internet • There are organizations that help standardize protocols and carry out administrative tasks o Internet Engineering Task Force (IETF) expertise to recommend future Internet development o Internet Society (ISOC)Internet for global benefit o Internet Assigned Numbers Authority (IANA) assignment of IP address on the Internet o Internet Corporation for Assigned Names and Numbers (ICANN) administrates assignment of domain names on the Internet • No one owns the entire Internet either, as it is comprised of many separate networks o These networks are each owned and operated by various entities o People access the Internet via Internet service providers Internet Backbone • Sometimes also called the “Internet Trunk” • Built upon long distance networks of telephone companies • Primary connections between large networks on Internet • Networks connect through Internet exchange points (IXPs) Internet Service Providers • Provide Internet access to customers and to each other • Large telecommunication companies (e.g., Bell, Telus, Rogers) own the network (e.g., lines to your home) o Sell access to business and residential customers o Sell wholesale access to “smaller” ISPs (e.g., Teksavvy, Acanac, Primus, and others) • Digital Subscriber Line (DSL) and Fibre-To-The-Node (FTTN) o Provided by Bell, Telus, and smaller ISPs o Require a “modem” (often integrated in a Wi-Fi router) to access network • Cable o Provided by Rogers and smaller ISPs o Require a “modem” (often integrated in a Wi-Fi router) to access network • Mobile o Provided by your mobile phone provider (e.g., Bell, Rogers, Wind) o Smartphone can serve as a Wi-Fi access point (“hotspot”) High-Level Protocols • Rules for Internet services and application to exchange data • Initial communication uses a specific network port Protocol Port Description File Transfer Protocol (FTP) 21 Exchanges files between computers Secure Shell Host (SSH) 22 Remote login to a computer (like Telnet) Simple Mail Transfer Protocol (SMTP) 25 Used by email servers to send and receive messages Hypertext Transfer Protocol (HTTP) 80 Requests web documents, typically using a web browser Secure HTTP 443 Encrypted version of HTTP for security Transport Protocols • Data is divided into data packets (a.k.a. segments) and sent • Segments are transported using one of two protocols: o TCP ▪ Emphasize reliability ▪ “If a segment is lost/damaged, send it again, and I’ll put it in order” ▪ For each segment received, the recipient send an acknowledgement (if the segment is intact) or a negative acknowledgement (if the segment is damaged) ▪ If the sender does not receive an acknowledgement within a set amount of time (or receives a neg ack), it resends the data o UDP ▪ Emphasizes timeliness ▪ “If a segment is lost, don’t worry, it’s too late to be of use” Network Addressing on the Internet • Similar to addressing in the section “Computer Networking” • Two versions of addressing in use: IPv4: • 4 bytes long • Takes the form x.x.x.x where x is any number in the range 0-255 • Number of available IPv4 addresses ran out in Sept. 2015 IPv6: • 16 bytes long • Takes the form hhhh:hhhh:hhhh:hhhh:hhhh:hhhh:hhhh:hhhh, where his a hexadecimal digit • Devised to take over from IPv4; transition ongoing and growing, but difficult, as IPv6 is not compatible with IPv4 Domain Names • Remembering an IP address is difficult (especially IPv6) • Domain names are strings of characters used to identify computers (typically servers) on the Internet • Multiple domains can be combined (with dots) to form a fully qualified domain name (e.g., • Domains are more general (higher level) moving L  Rwww: the web server in the EECS Department o eecs: the EECS Department at York University o yorku: York University in Canada o ca: the Canadian “top level domain” • Top level domains determined by ICANN • Can be associated with a specific economic sector (.com, .edu, .org) or a country (.ca, .uk, .ru, .nl, .cn) (not strictly enforced) Domain Name Servers (DNS) • Keep track of registered domain names and their associated IP addresses • Updates are shared between DNSs • Every request for an internet resource using a domain name involves at least one access to a DNS • ISPs typically host their own DNS to improve service quality o Slow access to a DNS results in slow Internet communication Ping • Network utility to test if a network computer is accessible o Like poking your friend to get a response
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