9. Internet Protocols

1. Internet Architecture
2. IP (Internet Protocol)
3. TCP (Transmission Control Protocol)
4. TCP Features
5. User Datagram Protocol (UDP)
6. Domain Name System (DNS)
7. Top-Level Domains
8. DNS Lookup
9. Application Layer Protocols
10. Internet Electronic mail
11. Sending e-mail
12. Multipurpose Internet Mail Extensions (MIME)
13. MIME Headers
14. Universal Resource Identifiers (URIs)
15. URIs, URNs and URLs
16. URI Syntax
17. Escaping Special URI characters
18. Uniform Resource Locators (URLs)
19. URL schemes
20. Uniform Resource Names (URNs)
21. Some Namespace Identifiers
22. Resolving URNs
23. Links to more information

9.1. Internet Architecture

• network layer concerned with physical connection between nodes
• internet layer deals with addressing and fragmentation of packets (includes ICMP)
• transport layer provides reliable connections (for TCP) between processes on hosts
• application layer contains protocols used by applications

9.2. IP (Internet Protocol)

• defined in RFC 791 (1981)
• for transmitting blocks of data (packets) from sources to destinations
• each packet has 32-bit destination address (IP address)
• 4 bytes written as n1.n2.n3.n4 where each ni is a decimal number, e.g., 18.23.0.22
• IP address standard is RFC 1166
• IP fragments packets if necessary (maximum size 65535 bytes)
• each packet routed independently, based on destination and network load and availability
• no mechanism for reliable delivery

9.3. TCP (Transmission Control Protocol)

• defined in Internet standard RFC 793 (1981)
• provides connection-oriented, reliable service
• supports addressing of individual processes via ports

9.4. TCP Features

• acknowledges safe receipt of packets
• detects missing/corrupted/duplicated packets
• provides method to resend missing/corrupted packets
• uses sequence numbers to reassemble packets in same order as sent
• so looks like stream from application's viewpoint
• used for ftp, telnet, http, smtp

9.5. User Datagram Protocol (UDP)

• provides connectionless, unreliable service
• so UDP faster than TCP
• adds only checksum and process-to-process addressing to IP
• used for DNS and NFS
• used when socket is opened in datagram mode

9.6. Domain Name System (DNS)

• DNS defined in RFC 1034 and 1035
• global service mapping DNS names to IP addresses
• names are organised hierarchically:
• 2 advantages:
  • easier to remember www.w3.org than 18.23.0.22
  • higher level of abstraction allows simpler reorganisation

9.7. Top-Level Domains

• domains on first level of 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 Internet informational RFC 1591:
  • 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

9.8. DNS Lookup

• DNS server is used by browser to map DNS name to IP address:

9.9. Application Layer Protocols

• HTTP (HyperText Transfer Protocol)
  • used by WWW to deliver documents
  • each document has a unique URI (Universal Resource Identifier)
• telnet (for remote login)
  • defined in RFC 318 (1972)
• ftp (file transfer protocol)
  • defined in RFC 454 (1973)
• NNTP and NNRP (network news protocols)
  • Network News Transfer Protocol (NNTP) used between servers
  • Network News Reading Protocol (NNRP), subset of NNTP, used between client and server
• SMTP, POP3 and IMAP4 (email protocols)

9.10. Internet Electronic 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)

9.11. Sending e-mail

• SMTP (Simple Mail Transfer Protocol)
• defined in RFC 821 and 822 (1982)
• use mailto: prefix in URI in browser
• uses socket port 25
• routing using DNS (Domain Name System)
• address of recipient is of form
  octopus@garden.under.the.sea
• also used for mailing lists and list servers

9.12. Multipurpose 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
  • non-US-ASCII message bodies
  • extensible set of different formats for non-textual bodies
  • multi-part message bodies
  • non-US-ASCII textual header information

9.13. MIME Headers

• MIME headers include:
  • MIME-Version
  • Content-Type: specifies type and subtype
  • Content-Transfer-Encoding: specifies auxiliary encoding
• contents of Content-Type header is 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

9.14. Universal Resource Identifiers (URIs)

• a Universal 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 maintained by IANA
  • IANA: Internet Assigned Numbers Authority
  • scheme-specific-part identifies resource in some way specific to the scheme

9.15. URIs, URNs and URLs

• a URI is either
  • a Uniform Resource Name (URN), or
  • a Uniform Resource Locator (URL)
• URN names a resource, while URL gives its address
• URN vs URL analogous to DNS name vs IP address

9.16. URI Syntax

• general rules for scheme specific part:
  • percent sign: % used as escape character for encoding non-printable or reserved characters (completeness)
  • hierarchical forms: / reserved to delimit resources which are arranged hierarchically
  • hash sign: # used to delimit a resource reference from a fragment identifier
  • query strings: ? used to delimit a resource reference from a query to the resource, + used for space in query string

9.17. Escaping Special URI characters

• if need %, /, #, ? or space in URI or + in query string, use escaped versions based on ASCII hexadecimal values:

  symbol	escaped version
  %	%25
  /	%2F
  #	%23
  ?	%3F
  space	%20
  +	%2B

9.18. Uniform Resource Locators (URLs)

• scheme is one of
  • ftp, http, https, mailto, news, nntp, telnet
• scheme specific part has syntax
  "//" [ user [ ":" password ] "@" ] host [ ":" port ] [ "/" url-path ] [ "?" query-string ] [ "#" anchor ]
  where
  • user and password are optional
  • host is fully qualified domain name or IP address
  • port is optional (usually a default)
  • url-path is path to resource, specific to scheme
  • query-string include parameters associated with request (usually form fields)
  • anchor is a reference to a part of a resource

9.19. URL schemes

• http
  • user name and password not applicable
  • default port number is 80
• https
  • HTTP over 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

9.20. Uniform Resource Names (URNs)

• overcome disadvantages of using URLs, namely:
  • dependence on host names
  • dependence on file structure on host
  • ease with which URL can be invalidated
• no syntactic difference between URN and URL
• URNs not yet supported by browsers
• syntax for URNs:
  "urn:" <NID> ":" <NSS>
  where
  • scheme is urn
  • scheme specific part is <NID> ":" <NSS>
  • <NID> is Namespace IDentifier
  • <NSS> is Namespace Specific String

9.21. Some Namespace Identifiers

• registered with IANA
• ISBN: International Standard Book Number
• ISSN: International Standard Serial Number
• PIN: Personal Internet Name
• IETF: Internet Engineering Task Force (for their documents)
• XMLORG: xml.org

9.22. Resolving URNs

• need Resolver Discovery Service (RDS) to find resolver for particular <NID>
• send <NSS> to resolver which returns URL of resource

9.23. Links to more information

• www.isoc.org/internet/history/
  History of the Internet
• www.rfc-editor.org/
  The RFC homepage