Domain Name System

1
Domain Name System

• A heirarchial, distributed database
• A service primarily aimed at mapping names to IP
  addresses
• Partitioned for ease of administration

2
DNS Structure (partial)
3
DNS -- How it Works

• DNS Servers in a logical tree
• DNS clients on every host
• Iterative Queries
• Recursive Queries

4
ARP

• Address Resolution Protocol translate network
  layer address to physical address
• Part of general resolution procedure

name e.g., neuron.cs.tamu.edu
DNS
IP Address e.g., 128.194.133.1
ARP
Ethernet address e.g., 080020085878
5
Application Layer

• File Transfer, Access and Management
• virtual file store
• file servers and requesting clients
• Electronic Mail
• process to process traffic expected to dominate
  Internet
• actually email dominates the Internet
• Virtual Terminals
• representing the abstract state of the real
  terminal
• Remote Job Execution
• JTM Job Transfer and Manipulation

6
ISO Applications

• FTAM FTP File Transfer, Access Manipulation
• VTS TELNET Virtual Terminal Service
• JTM ??? Job Transfer Manipulation maybe rsh,
  RPC?
• MHS SMTP Message Handling System (X.400)

7
ISO Terminology
SpecificCommon Application Service
Elements think of them as programming libraries
or APIs
8
ISO Service Elements(examples)

• Association Control Service Element (primitives)
• Commitment, Concurrency and Recovery
• atomic actions
• two phase commit

9
File Servers

• 1. File Structure
• 2. File Attributes
• e.g. identifier, size of storage, access control
• 3. File Operations
• create, delete move
• OSI has defined virtual filestore operations

10
Replicated Files

• Why multiple file servers?
• 1. To split the workload over multiple servers.
• 2. To allow file access to occur even if one
  file server is down
• 3. To increase reliability by having independent
  backups of each file
• Updates
• Voting

11
Electronic Mail

• Motis and X.400CCITT has aligned them for
  compatibilityMay replace SMTP
• The user agentuser interface
• Message transfer agentpost office

12
Virtual Terminals

• Scroll mode terminalsno local resources dumb
  display and transmission
• Page mode terminals25 x 80 character
  displayscreen editing via termcap
• Form mode terminalslocal processing enabled
• Bitmapped terminals
• like X stations

13
Case Study Internet

• File Transfer
• Electronic Mail
• Virtual Terminals

14
Getting Started

• RARP
• BOOTP
• TFTP
• DHCP

15
File Transfer Protocol

• FTP recognizes four file types
• 1. Image
• bit by bit transfer
• 2. ASCII
• 3. EBCDIC
• 4. Logical Byte files
• binary files which use byte size other than 8 bits

16
Electronic Mail

• Pioneered by ARPANET
• RFC 822 (widely used)
• Simple Mail Transfer Protocol (SMTP)
• Supports only ASCII text
• name_at_domain addressing

17
Virtual Terminals - TELNET

• Designed for scroll mode terminals
• Hit a key , 8-bit bytes are sent
• 95 ASCII and 7 control characters legal

18
USENET--HOW TO AVOID GRADUATION

• Internet compatible (now)
• Variety of newsgroups
• Moderated newsgroups
• NNTP, Network News Transfer Protocol, allows
  selective downloading of messagesto multiple
  sites

19
The Web

• the killer application for the Internet
• Two components for popularity
• http combines multiple access (gopher, ftp, etc)
  methods
• hypertext interface supports point-and-click
  interface
• Who will organize the information?
• No one...
• Database experts
• Librarians (!)

20
Web Terminology

• Web Browsers
• Netscape
• Mosaic
• Web Servers
• http daemon
• httd.conf - main server config file
• srm.con - server resource config file
• access.conf - global access control file
• Home Page
• Eg., www.cs.tamu.edu
• HTML
• HyperText Markup Language

21
HTML Document

• ltHTMLgt
• ltHEADgt
• ltTITLEgtWillis Martis Homepagelt/TITLEgt
• ltLINK REVOWNER HREFmailtowillis_at_cs.tamu.edu
  gt
• lt/HEADgt
• ltBODYgt
• ltIMG SRCmy-logo.gif ALTlogogt
• ltH1gt Sample HTML Documentlt/H1gt
• ltEMgt To demonstrate HTML lt/EMgt
• ltHRgt
• ....

22
Web Future Directions

• HTML Enhancements
• Secure Transactions
• Uniform Naming
• Librarians Brokers
• Information push or pull

23
Layer 7 Summary

• Service Element Model
• Not all Apps belong here...
• Common Network Services