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

DNS Structure (partial). educom gov utexastamuibm cs austin mac1 solar

DNS -- How it Works DNS Servers in a logical tree DNS clients on every host Iterative Queries Recursive Queries

ARP Address Resolution Protocol {translate network layer address to physical address} Part of general resolution procedure: name {e.g., neuron.cs.tamu.edu} IP Address {e.g., } Ethernet address {e.g., 08:00:20:08:58:78} DNS ARP

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 dominates the Internet Virtual Terminals –representing the abstract state of the real terminal Remote Job Execution –JTM: Job Transfer and Manipulation

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)

ISO Terminology SASE CASE “applications” [Specific|Common] Application Service Elements “think of them as programming libraries or APIs”

ISO Service Elements (examples) Association Control Service Element (primitives) Commitment, Concurrency and Recovery –atomic actions –two phase commit

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

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

Electronic Mail Motis and X.400 CCITT has aligned them for compatibility May replace SMTP The user agent user interface Message transfer agent post office

Virtual Terminals Scroll mode terminals no local resources; dumb display and transmission Page mode terminals 25 x 80 character display screen editing via termcap Form mode terminals local processing enabled Bitmapped terminals like X stations

Case Study: Internet File Transfer Electronic Mail Virtual Terminals

Getting Started RARP BOOTP TFTP DHCP

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

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

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

USENET--HOW TO AVOID GRADUATION Internet compatible (now) Variety of newsgroups Moderated newsgroups NNTP, Network News Transfer Protocol, allows selective downloading of messages to multiple sites

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 (!)

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., HTML –HyperText Markup Language

HTML Document Willis Marti’s Homepage Sample HTML Document To demonstrate HTML....

Web Future Directions HTML Enhancements Secure Transactions Uniform Naming Librarians & Brokers Information “push” or “pull”

Layer 7 Summary Service Element Model Not all Apps belong here... Common Network Services