2. Introduction to Internet Applications

1. Representation and Transfer
2. Web Protocols
3. Some Other Application Layer Protocols
4. Uniform Resource Identifiers (URIs)
5. Uniform Resource Locators (URLs)
6. URL example
7. URL schemes
8. Escaping Special URI characters
9. Domain Name System (DNS)
10. Name Resolution
11. DNS Design
12. Top-Level Domains
13. DNS Server Hierarchy
14. DNS Server Model (1)
15. DNS Server Model (2)
16. DNS Caching
17. Full Name Resolution
18. Internet e-mail
19. Sending e-mail
20. Example SMTP Session
21. Email Representation Standards
22. Multi-purpose Internet Mail Extensions (MIME)
23. MIME Headers
24. Base64 Encoding
25. Links to more information

2.1. Representation and Transfer

• application-layer protocols specify two aspects of interaction
  • representation
  • transfer
• representation:
  • syntax of data items exchanged
  • specific form during transfer
  • translation of integers, characters and files between computers
• transfer:
  • interaction between client and server
  • message syntax and semantics
  • valid and invalid exchange
  • error handling
  • 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
  • HyperText Markup Language (HTML): representation standard specifying contents and layout of a web page
  • Uniform Resource Identifier (URI): representation standard specifying format and meaning of web page identifiers
  • HyperText Transfer Protocol (HTTP): transfer protocol specifying how a browser interacts with a web server
• standardised by the World Wide Web Consortium (W3C)

2.3. Some Other Application Layer Protocols

• (usually standardised in a Request for Comments (RFC) by the Internet Engineering Task Force (IETF))
• telnet (for remote login)
  • defined in RFC 318 (1972)
• ftp (file transfer protocol)
  • defined in RFC 454 (1973)
• email protocols
  • SMTP (Simple Mail Transfer Protocol)
  • POP3 (Post Office Protocol version 3)
  • IMAP4 (Internet Mail Access Protocol)
• DNS (Domain Name System)
  • defined in RFC 1034 and RFC 1035 (1987)
• RTP (Real-time Transfer Protocol) for audio and video
  • 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
  • scheme identifies a naming scheme, e.g., http
  • scheme-specific-part identifies resource in some way specific to the scheme
  • most commonly used URIs are Uniform Resource Locators (URLs)
  • URIs also include Uniform Resource Names (URNs) which we won't discuss further
• URIs have been generalised to Internationalized Resource Identifiers (IRIs) in RFC3987

2.5. Uniform Resource Locators (URLs)

• scheme examples include
  • 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
  • user and password are not often used
  • host is a fully qualified domain name or IP address
  • port is optional (usually a default)
  • url-path is the path to the resource, specific to scheme
  • query-string includes parameters associated with the request (usually form fields)
  • anchor is a reference to a part of a resource (a fragment identifier)

2.6. URL example

In https://www.bbc.co.uk/search?q=brexit

• https is the scheme
• www.bbc.co.uk is the host
• search is the url-path
• q=brexit is the query string

2.7. URL schemes

• http
  • user name and password usually not applicable
  • default port number is 80
• https
  • HTTP encrypted by Transport Layer Security (TLS) (or previously Secure Sockets Layer (SSL))
  • default port number is 443
• ftp
  • user name and password can be given
  • if not, anonymous ftp used
  • default port number is 21
• telnet
  • host is mandatory
  • default port number is 23
• mailto
  • no need for url-path to be specified
  • program should prompt user for message, then send using SMTP

2.8. Escaping Special URI characters

• the space character is not allowed in URIs
• the characters /, #, ?, e.g., have special meaning in URIs
• also & is used to separate parameters in a query string
• so if we need any of these as an ordinary character in a URI, we use the escaped version
• the escaped version is the character % followed by the ASCII hexadecimal value of the character
• now % has a special meaning too
• the escaped versions of the above special characters are as follows:

  symbol	escaped version
  %	%25
  /	%2F
  #	%23
  ?	%3F
  space	%20
  &	%26

2.9. Domain Name System (DNS)

• provides a service mapping (human-readable) DNS names to IP addresses
• browsers, mail software and most other Internet applications use DNS
• although the TCP/IP protocols themselves use only IP addresses
• DNS has two advantages:
  • easier to remember www.w3.org than 128.30.52.37
  • higher level of abstraction allows simpler reorganisation
• names are organised hierarchically:
  • most significant part of the name on the right (specified by DNS)
  • left-most segment of a name is the name of an individual computer
• DNS is essentially
  • a distributed database implemented as a hierarchy of DNS servers
  • an application-layer protocol allowing hosts to query the database

2.10. 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)
• this software is usually built in to the application
• a resolver uses the DNS protocol to contact a DNS server on port 53
• e.g., browser uses a DNS server to map DNS name to IP address as follows:

  

2.11. 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:
  • it is a single point of failure
  • the need to handle huge volumes of queries
  • a single server cannot be "close" to all clients
  • it would also have to handle all updates for new hosts

2.12. Top-Level Domains

• right-most domains of the hierarchy are top-level domains:
  • either country-code top-level domain (ccTLD)
  • 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 RFC 1591; some examples:
  • edu: educational institutions
  • com: commercial entities, i.e., companies
  • net: network providers
  • org: organisations, e.g. NGOs
  • gov: government agencies
  • mil: US military
  • int: organisations established by international treaties

2.13. DNS Server Hierarchy

• the above figure shows a portion of the hierarchy of DNS servers
• there are 13 root DNS servers (each is actually a cluster of replicated servers)
  • these return IP addresses of top-level domain servers
• top-level domain servers are responsible for top-level domains
  • they return IP addresses of authoritative servers for organisations
• each organisation must provide an authoritative DNS server for its publically accessible hosts

2.14. DNS Server Model (1)

• each organisation is free to choose how to organise its servers
  • a small organisation might use an ISP to run a DNS server
  • a larger organisation might place all names on a single server
  • a large organisation might divide its names among several servers

2.15. DNS Server Model (2)

• DNS allows each organisation to
  • assign names to computers, or
  • 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

2.16. DNS Caching

• DNS servers employ caching in order to improve performance and reduce load
• mappings between names and addresses can be cached
• the length of time a mapping stays in the cache is given by its time to live (TTL)
• a mapping coming from the authoritative DNS server for a name is called an authoritative answer
• a mapping coming from the cache of some DNS server is called a non-authoritative answer
• e.g., one can use nslookup on Windows/Unix-based systems

  nslookup www.dcs.bbk.ac.uk
  ...
  Non-authoritative answer:
  Name:	 www.dcs.bbk.ac.uk
  Address: 193.61.29.21
  

2.17. Full Name Resolution

• when nothing is cached, the local name server might have to perform full name resolution

2.18. Internet e-mail

• e-mail client responsible for
  • retrieving mail from server (POP3, IMAP4)
  • sending mail to server (SMTP)
• e-mail server responsible for
  • collecting mail from client (SMTP)
  • distributing mail to client (POP3, IMAP4)
  • relaying mail between e-mail servers (SMTP)

2.19. Sending e-mail

• SMTP (Simple Mail Transfer Protocol)
• defined in RFC 821 and 822 (1982), superseded 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

2.20. Example SMTP Session

• mail message is transferred from user John_Q_Smith on computer example.edu to two users on computer somewhere.com

2.21. Email Representation Standards

• two important standards exist
  • RFC (Request For Comments) 2822 mail message format
  • Multi-purpose Internet Mail Extensions (MIME)
• RFC 2822 format comprises
  • a header section
  • a blank line
  • 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

2.22. Multi-purpose Internet Mail Extensions (MIME)

• SMTP originally only used the 7-bit ASCII format
• inadequate for non-English and non-textual data
• MIME was defined in RFCs 2045, 2046, 2047, 2048, 2049; allows
  • non-ASCII message bodies
  • extensible set of different formats for non-textual bodies
  • multi-part message bodies
  • non-ASCII textual header information

2.23. MIME Headers

• MIME headers include:
  • MIME-Version
  • Content-Type: specifies a type and subtype
  • Content-Transfer-Encoding: specifies auxiliary encoding for transfer
• contents of the Content-Type header is the MIME type
• examples of MIME types are text/html, image/gif and multipart/mixed
• example of Content-Transfer-Encoding is base64:
  • preferred encoding for 8-bit binary data
  • each group of 3 bytes (24 bits) is encoded as 4 ASCII characters

2.24. Base64 Encoding

2.25. Links to more information

• The RFC homepage
• FAQs on DNS From Internic, a registry of domain name registrars
• Internet Corporation for Assigned Names and Numbers (ICANN): organisation in overall charge of the DNS
• Wikipedia articles on DNS, SMTP, IMAP, POP3, MIME