Introduction to Internet Resources and Services

Spring '99
#43405 (Tue)
#43400 (Thu)

Class Notes: Names and Numbers and Protocols

Clients and Servers

• The Internet is based on a client/server architecture.

• A client is a program that performs tasks for a user.

• A server is a program that manages resources.

• Example: client = Netscape Navigator, server = Apache

• Two key elements typify a client/server system:
  • a resource managed by the server

  • separation across a communication pathway, such as a network

• Result: application is location insensitive and platform independent.

• The the terms client and server sometimes are used to refer to the machinery that runs such programs.

The TCP/IP Protocol Suite

• A communications protocol is a procedure for formatting and handling data.

• The "DARPA Internet Protocol Suite" is the collection of protocols used on the Internet.

• The two most important protocols in this suite are TCP and IP.

The TCP/IP Protocol Stack

• Communication protocols often are implemented as a set of layers.

• TCP/IP uses a four-layer protocol stack.

        (application program)
                 |
      +----------------------+
      |  Application Layer   |
      +----------------------+
      |   Transport Layer    |
      +----------------------+
      |    Network Layer     |
      +----------------------+
      |   Datalink Layer     |
      +----------------------+
                 |
             (hardware)
  

• Application Layer
  • Concerned with the semantics of work, e.g. formatting electronic mail messages.

  • Example Protocols:
    • Simple Mail Transfer Protocol (SMTP)
    • Hypertext Transfer Protocol (HTTP)

• Transport Layer
  • Provides high-level data transmission services.

  • Two most common transport protocols: TCP and UDP.

    Transmission Control Protocol	User Datagram Protocol
    stream oriented	datagram oriented
    connection oriented	connectionless
    reliable	not reliable

  • Whoever designed the application protocol chose which transport they want to use.

  • Most (but not all) common Internet services use TCP.

• Network Layer
  • The sole function of the network layer is to send data packets to their destination.

  • It is implemented by the Internet Protocol (IP).

  • IP is a best-effort, packet switching protocol.

  • IP is not reliable. Packets can be lost, corrupted, or arrive out of order. If desired, this must be handled at a higher layer.

• Datalink Layer
  • Provides a uniform interface to communications media.

  • Example Protocols:
    • Point-to-Point Protocol (PPP)
    • Ethernet

IP Addresses

• Every device connected to a TCP/IP network has a unique 32-bit IP address. Sometimes called network address or (if device is connected to the Internet) Internet address.

• 2³² = 4,294,967,296

• The IP layer stamps every packet with the source and destination IP addresses.

• IP addresses typically are written as a dotted quad of four decimal numbers (0 through 255), such as 128.83.248.28.

• Typically, we'll specify a mnemonic host name instead of an IP address: lexus.gslis.utexas.edu == 128.83.248.28.

• Connectivity may be tested with ping.
• The Internet address space is coordinated by the Internet Assigned Numbers Authority (IANA).

• The Internet address space is allocated by one of a number of regional registries:
  • American Registry for Internet Numbers (ARIN)
  • Asia-Pacific Network Information Center (APNIC)
  • Reseau IP Europeens (RIPE)

• This is all for the current IPv4 protocol. IPv6 (Internet II) uses 128-bit addresses.

Port Numbers

• The transport layer uses a port number to associate data with client and server processes.

• Port numbers range from 0 through 65,535.

• A server listens at a particular port number for incoming requests. The client sends its requests to that port number.

• Port numbers 0 through 1023 are reserved as well-known port numbers. The IANA allocates these ports for various applications.

  Some Well-Known TCP Ports
  Protocol	Port	Application
  echo	7	Character Echo (for testing)
  daytime	13	Current Day and Time
  chargen	19	Character Generator (for testing)
  ftp	21	File Transfer Protocol
  telnet	23	Network Terminal Emulation
  smtp	25	Simple Mail Transfer Protocol
  http	80	Hypertext Transfer Protocol

URLs and TCP/IP Communications

• Network objects, particularly on the World Wide Web, are specified by Uniform Resource Locators (URLs).

• URLs encode the destination address and port number at which to contact a server.

      http://www.acme.com/      = host www.acme.com + port 80
      http://www.acme.com:1080/ = host www.acme.com + port 1080
  

• The scheme portion of the URL implies how to setup the protocol stack.

      http://www.acme.com/           => HTTP + TCP + IP + (datalink)
      ftp://ftp.acme.com/            => FTP  + TCP + IP + (datalink)
      mailto:chipr@gslis.utexas.edu  => SMTP + TCP + IP + (datalink)
  

The Domain Name System (DNS)

• Name-to-Number Translation
  • Although IP uses numeric addresses, people prefer names.

  • The most important function of the DNS is to convert host names to numbers (IP addresses).

  • It also performs several other functions, including reverse lookups and email exchange lookups.

  • The DNS is a distributed database.

  • It may be accessed with the nslookup utility.

• Hierarchical Naming
  • Top-Level Domain (TLD), edu

  • Organization Domain, utexas.edu

  • Sub-Domain, gslis.utexas.edu

  • Fully Qualified Domain Name (FQDN), lexus.gslis.utexas.edu. Lehnert calls this a DNS name.

  • FQDN = (unqualified) host name + domain name

  • Term host name may be used to mean unqualified or qualified, depending on context.

• Generic Top-Level Domains (GTLDs) [ftp://ftp.isi.edu/in-notes/rfc1591.txt]

  COM	benjerry.com	Commercial Entities
  EDU	utexas.edu	Four-Year Colleges & Universities
  NET	internic.net	Network Providers Infrastructure
  ORG	eff.org	Miscellaneous Organizations, e.g. Non-Profits
  INT	nato.int	International Organizations
  GOV	whitehouse.gov	United States Federal Government
  MIL	arpa.mil	United States Military

  • Historical discrepancies: State of Texas should not be texas.gov.

  • Not enforced: austinfree.net probably belongs in ORG.

• Two-Letter Country-Code Domains (ISO-3166) [http://www.isi.edu/in-notes/iana/assignments/country-codes]

  US	United States of America
  MX	Mexico
  CA	Canada
  UK	United Kingdom
  AU	Australia

  • In theory the GTLDs (except gov and mil) are for everybody.

  • In practice, organizations outside the US frequently prefer to appear in their country domain.

• us Domain Structure [http://www.isi.edu/in-notes/usdnr/]

  Locality	austin.tx.us	organizations and individuals in Austin
  State	state.tx.us	State of Texas [http://www.state.tx.us/]
  City	ci.austin.tx.us	City of Austin [http://www.ci.austin.tx.us/]
  County	co.travis.tx.us	Travis County [http://www.co.travis.tx.us/]
  Community Colleges	austin.cc.tx.us	Austin Community College [http://www.austin.cc.tx.us/]
  Public School Districts	arlington.k12.tx.us	Arlington Independent School District [http://www.arlington.k12.tx.us/]
  Libraries	westbank.lib.tx.us	Westbank Community Library [http://www.westbank.lib.tx.us/]

• The Future of the DNS
  • The DNS is the final piece of the Internet still funded by the U.S. Government. They want out.

  • There is pressure for additional GTLDs. An earlier proposal called for the addition of several new TLDs: FIRM, SHOP, WEB, ARTS, REC, INFO, and NOM

  • ICANN (Internet Corporation for Assigned Names and Numbers) was formed in October 1998.

• Other Concepts
  • Synchronous versus Asynchronous Communications

  • Packet Switching versus Circuit Switching

  • Host versus Router

  • Router versus Gateway

  • Bandwidth

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