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1. Introduction to the Internet 1. Computer Networks 2. The Internet 3. Part of the Internet 4. Packet Switching 5. History of the Internet 6. Growth of the Internet 7. Communication Protocols 8. Protocols and Layering 9. TCP/IP 5-layer Reference Model 10. TCP/IP layers with some protocols 11. Data Passing Through Layers 12. Headers and Layers 13. Internet Communication Paradigms 14. Connection-Oriented Communication 15. Client-Server Model 16. Client Software 17. Server Software 18. Server Identification 19. Service Identification 20. Client-Server Interaction 21. A specific example
1.1. Computer Networks
computer networks are everywhere they form an essential part of our infrastructure used o at home o at work o by governments o on the move o ... there are many different types of networks and standards we will concentrate on the Internet
1.2. The Internet
(the above is a very old figure showing an internet) each ellipse represents a network connecting a number of computers directly an internet is a federation of computer networks, connected by routers the Internet is the world-wide federation of packet-switched networks running TCP/IP important applications include email and the World Wide Web (WWW)
1.4. Packet Switching
early communication networks evolved from telephone systems used physical pair of wires between two parties to form a dedicated circuit circuit switching was the task of deciding which circuit to use when two parties wanted to communicate the circuit is reserved for the two parties during communication so it is not available to other parties the Internet uses packet switching which is considered more efficient packet switching o divides data into small blocks, called packets o allows multiple users to share a network o includes identification of the intended recipient in each packet o devices throughout the network each have information about how to reach each possible destination
1.5. History of the Internet
(1957) Advanced Research Projects Agency (ARPA) established by US Department of Defense (1968-9) first packet-switching networks (1972) Telnet (1973) File Transfer Protocol (FTP); ARPANET goes international: o University College, London (UK) o Royal Radar Establishment (Norway) (1974) design of TCP (Transmission Control Protocol) (1977) email (1982) TCP and IP (Internet Protocol) used for ARPANET (1984) DNS (Domain Name Service) introduced (1991) WWW released
1.6. Growth of the Internet
1.7. Communication Protocols
communication always involves at least two entities o one that sends information and another that receives it all entities in a network must agree on how information will be represented and communicated o the way that electrical signals are used to represent data o procedures used to initiate and conduct communication o the format of messages all communicating parties follow the same set of rules , a set of specifications a specification for network communication is called a communication protocol
1.10. TCP/IP layers with some protocols
1.11. Data Passing Through Layers
1.12. Headers and Layers
1.13. Internet Communication Paradigms
Internet supports two basic communication paradigms: o stream paradigm o message paradigm
stream paradigm message paradigm connection-oriented connectionless one-to-one communication many-to-many communication sequence of individual bytes sequence of individual messages arbitrary length transfer each message limited to 64 Kbytes used by most applications used for multimedia applications built on TCP protocol built on UDP protocol
we will focus on the stream paradigm
1.14. Connection-Oriented Communication
Internet stream service is connection-oriented two applications must request that a connection be created once it has been established, the connection allows the applications to send data in either direction finally, when they finish communicating, the applications request that the connection be terminated
o an identifier for a service on the computer identifying a computer o each computer on the Internet is assigned a unique 32- bit identifier known as an Internet Protocol address ( IP address ) o 4 bytes written as n 1 .n 2 .n 3 .n 4 where each ni is a decimal number, e.g., 18.23.0. o a client must specify the server's IP address o to make server identification easy for humans, each computer is also assigned a name, and the Domain Name System (DNS) is used to translate a name into an address o thus, a user specifies a name such as www.dcs.bbk.ac.uk rather than an integer address
1.19. Service Identification
each service available in the Internet is assigned a unique 16- bit identifier known as a protocol port number (or port number ) o examples: email -> port number 25, and the web -> port number 80 when a server begins execution o it registers with its local OS by specifying the port number for its service when a client contacts a remote server to request service o the request contains a port number when a request arrives at a server computer o software on the server uses the port number in the request to determine which application on the server computer should handle the request
1.20. Client-Server Interaction
following diagram illustrates client-server interaction o client on the left o server on the right
1.21. A specific example
suppose I want to retrieve a web page from www.w3.org my browser will use DNS to find the IP address 128.30.52. my browser will compose a message based on HTTP asking to get the page HTTP will ask TCP to connect to port 80 on 128.30.52. TCP will ask IP to send the message to 128.30.52. IP will send the message to a router on the local network this router will send the message to another router ... the router on the local network for 128.30.52.37 will receive the message it will send the message to 128.30.52. IP will receive it and pass it up to TCP TCP will see that it is for port 80 and will pass it to the web server process the web server will interpret the HTTP and send the page to my browser
o error handling o termination of interaction
2.2. Web Protocols
World Wide Web (WWW) is one of the most widely used services on the Internet major WWW standards are o HyperText Markup Language (HTML): representation standard specifying contents and layout of a web page o Uniform Resource Identifier (URI): representation standard specifying format and meaning of web page identifiers o HyperText Transfer Protocol (HTTP): transfer protocol specifying how a browser interacts with a web server
2.3. Some Other Application Layer Protocols
telnet (for remote login) o defined in RFC 318 (1972) ftp (file transfer protocol) o defined in RFC 454 (1973) email protocols o SMTP (Simple Mail Transfer Protocol) o POP3 (Post Office Protocol version 3) o IMAP4 (Internet Mail Access Protocol) DNS (Domain Name System) o defined in RFC 1034 and RFC 1035 (1987) RTP (Real-time Transfer Protocol) for audio and video o defined in RFC 3550 (2003)
2.4. Uniform Resource Identifiers (URIs)
a Uniform Resource Identifier (URI) is a unique identifier for identifying a resource on the internet basic syntax is: scheme ":" scheme-specific-part where o scheme identifies a naming scheme, e.g., http o scheme-specific-part identifies resource in some way specific to the scheme
o most commonly used URIs are Uniform Resource Locators (URLs)
2.5. Uniform Resource Locators (URLs)
scheme examples include o ftp, http, https, mailto, telnet in the following syntax [ ... ] denotes optional everything else not in quotes denotes a string to be supplied scheme specific part has syntax "//" [ user [ ":" password ] "@" ] host [ ":" port ] [ "/" url- path ] [ "?" query-string ] [ "#" anchor ] where o user and password are not often used o host is a fully qualified domain name or IP address o port is optional (usually a default) o url-path is the path to the resource, specific to scheme o query-string includes parameters associated with the request (usually form fields) o anchor is a reference to a part of a resource (a fragment identifier) e.g. in http://vili.dcs.bbk.ac.uk/dept/staffperson05.asp?name=ptw o http is the scheme o vili.dcs.bbk.ac.uk is the host o dept/staffperson05.asp is the url-path o name=ptw is the query string
2.6. URL schemes
http o user name and password not applicable o default port number is 80 https o HTTP over Secure Sockets Layer (SSL) o default port number is 443 ftp o user name and password can be given o if not, anonymous ftp used o default port number is 21 telnet o host is mandatory o default port number is 23 mailto
2.9. DNS Design
why is DNS distributed? a simpler design would have been to have one DNS server storing all the mappings problems with this centralised design include: o it is a single point of failure o the need to handle huge volumes of queries o a single server cannot be "close" to all clients o it would also have to handle all updates for new hosts
2.10. Top-Level Domains
right-most domains of the hierarchy are top-level domains : o either country-code top-level domain (ccTLD) o or generic top-level domain (gTLD) ccTLD represented by two-letter country-codes from ISO 3166, e.g., uk, fr, de, ch gTLD given in Internet informational RFC 1591: o edu: educational institutions o com: commercial entities, i.e., companies o net: network providers o org: organisations, e.g. NGOs o gov: government agencies o mil: US military o int: organisations established by international treaties
2.11. DNS Server Hierarchy
the following shows a portion of the hierarchy of DNS servers
there are 13 root DNS servers (each is actually a cluster of replicated servers) o these return IP addresses of top-level domain servers top-level domain servers are responsible for top-level domains o they return IP addresses of authoritative servers for organisations each organisation must provide an authoritative DNS server for its publically accessible hosts
2.12. DNS Server Model
each organization is free to choose how to organise its servers o a small organisation might use an ISP to run a DNS server o a larger organisation might place all names on a single server o a large organisation might divide its names among several servers DNS allows each organization to assign names to computers or to change those names without informing a central authority each DNS server contains information linking it to other DNS servers up and down the hierarchy a given server can be replicated replication is useful for heavily used servers, such as root servers that provide information about top-level domains DNS servers employ caching in order to improve performance and reduce load
2.13. Name Resolution
translation of a domain name into an address is called name resolution the name is said to be resolved to an address software to perform the translation is known as a name resolver (or simply resolver ) DNS server is used by a browser to map DNS name to IP address:
e-mail server responsible for o collecting mail from client (SMTP) o distributing mail to client (POP3, IMAP4) o relaying mail between e-mail servers (SMTP)
2.17. Sending e-mail
SMTP (Simple Mail Transfer Protocol) defined in RFC 821 and 822 (1982), superceded by RFC 2822 (2001) use mailto: prefix in URI in browser uses TCP port 25 address of recipient is of the form name@dept.inst.ac.uk uses DNS (Domain Name System) to map domain name to IP address
2.18. Example SMTP Session
mail message is transferred from user John_Q_Smith on computer example.edu to two users on computer somewhere.com
2.19. Email Representation Standards
two important standards exist o RFC (Request For Comments) 2822 mail message format o Multi-purpose Internet Mail Extensions (MIME) RFC 2822 format comprises o a header section o a blank line o and a body header lines each have the form keyword: information
where keywords include From, To, Subject, Cc
the mail message (including headers) makes up the DATA as sent by SMTP
2.20. Multi-purpose Internet Mail Extensions (MIME)
SMTP uses 7-bit ASCII format inadequate for non-English and non-textual data MIME defined in RFCs 2045, 2046 , 2047 , 2048 , 2049 ; allows
