Winter 2005

CMPE 151: Network Administration

Clients

Winter 2005

Client-Server Model

Winter 2005

Client-Server Model

Client

Kernel

File Server

Kernel

Printer Server

Kernel

Winter 2005

File transfer

Winter 2005

File transfer

Sharing remote files: “on-line” access versus “file transfer”.

“On-line” access transparent access to shared files, e.g., distributed file system.

Sharing through file transfer: user copies file then operates on it.

Winter 2005

FTP File transfer accounted for most of

the Internet traffic until the Web exploded!

Also uses TCP. Allows interactive access; format

specification (e.g., binary); authentication (clients required to authenticate themselves).

Winter 2005

FTP Operation Client Server

OS

Data Control

OS

Data Control

TCP connection

Winter 2005

Anonymous FTP Allows access to public files. No need to authenticate user. Access restricted to publicly

available files (e.g., in /usr/ftp).

Winter 2005

The Web and HTTP

Winter 2005

Some History Started in 1989 at CERN, European

center for nuclear research, in Switzerland.

Original motivation: need for scientists around the world to collaborate and share multi-media information.

Tim Berners-Lee came up with initial proposal of a web of linked documents

Winter 2005

More History… First text-based prototype demo in

12.91. Release of first graphical interface,

Mosaic, in 02.93 at NCSA by M. Andreessen.

In 1994, Andreessen creates Netscape. In 1994, CERM and MIT set up the

WWW Consortium to further develop the Web. www.w3.org for more information.

Winter 2005

The Web WWW, or the world-wide web is a

resource discovery service. Resource space is organized

hierarchically, and resources are linked to one another according to some relation.

Hypertext organization: link “granularity”; allows links within documents.

Graphical user interface.

Winter 2005

The client side Users perceive the Web as a vast

collection of information. Page is the Web’s information transfer unit. Each page may contain links to other

pages. Users follow links by clicking on them which

takes them to the corresponding page. This process can go on indefinetly,

traversing several pages located in different places.

Winter 2005

The browser Program running on client that retrieves

and displays pages. Interacts with server of page. Interprets commands and displays page.

Examples: Mosaic, Netscape’s Navigator and Communicator, Microsoft Internet Explorer.

Other features: back, forward, bookmark, caching, handle multimedia objects.

Winter 2005

DNS

Winter 2005

Domain Name System (DNS)

• IP addresses are not easy to remember.• The Domain Name System (DNS) maps

IP addresses to host names.• Host name is formed by machine name

followed by domain name.– Host_name.domain_name

• RFCs 1034 and 1035.

Winter 2005

DNS Basic function: translation of

names (ASCII strings) to network (IP) addresses and vice-versa.

Example: zephyr.isi.edu <-> 128.9.160.160

Winter 2005

DNS

• The domain_name is formed by the institutional site name and the Top-Level Domain name (TLD).– So the host name is of the

form:machine_name.Ist_site_name.TLD_name

• Examples: – sundance.ucsc.edu– soe.ucsc.edu (alias for sundance.ucsc.edu)– italia.cse.ucsc.edu– helios.jpl.nasa.gov– www.cnn.com

Winter 2005

Top-Level Domains (TLDs)

• TLD names identify organization types or country codes.

• Examples:.comCommercial org. .au Australia

.edu Educational site in US .ca Canada

.gov Government site in US .fr France

.mil Military organization in US .de Germany

.net Network site .uk Great Britain

.org Nonprofit organization .it Italy

.es Spain

Countries define their own internal hierarchy (e.g., .ac.uk, .edu.au)

Winter 2005

DNS hierarchy

• Organizations can create any internal DNS hierarchy.

• Authority for creating new subdomains within a domain name is delegated to each domain.– Administration of ucsc.edu has authority to

create cse.ucsc.edu and need not contact any central naming authority.

Winter 2005

Example DNS Hierarchy

Winter 2005

DNS Name Space

• DNS names are managed by a hierarchy of DNS servers.– Hierarchy is related to DNS domain hierarchy

• Root server at top of tree knows about next level servers.

• Next level servers, in turn, know about lower level servers.

Winter 2005

Example of DNS Hierarchy

Winter 2005

Example of DSN Hierarchy

Winter 2005

Choosing DNS Server Architecture

• Small organizations can use a single server.– Easy to administer.– Inexpensive.

• Large organizations often use multiple servers.– Reliability through redundancy.– Improved response time through load sharing.

Winter 2005

Name Resolution

• “Resolving a name” means mapping the host name to the IP address.• Reverse mapping is also possible.

• A client computer calls a DNS server for name resolution– DNS request contains name to be resolved.– DNS reply contains IP address for name in

request.

Winter 2005

Name resolution (cont’d) Client DNS (running on client

hosts), or resolver. Application calls resolver with

name. Resolver contacts local DNS server

(using UDP) passing the name. Server returns corresponding IP

address.

Winter 2005

Name resolution (cont’d) Application wants to resolve name. Resolver sends query to local name server.

Resolver configured with list of local name servers.

Select servers in round-robin fashion. If name is local, local name server returns

matching authoritative RRs. Authoritative RR comes from authority

managing the RR and is always correct. Cached RRs may be out of date.

Winter 2005

Name resolution (cont’d) If information not available locally

(not even cached), local NS will have to ask someone else. It asks the server of the top-level

domain of the name requested.

Winter 2005

Electronic mail Non-interactive.

Deferred mail (e.g., destination temporarily unavailable).

Spooling: Message delivery as background

activity. Mail spool: temporary storage area

for outgoing mail.

Winter 2005

Mail system

Userinterface

Usersends mail

Userreads mail

Outgoingmailspool

Mailboxes incomingmail

Client(send)

Server(receive)

TCPconnection(outgoing)

TCPconnection(incoming)

Winter 2005

Observations When user sends mail, message

stored is system spool area. Client transfer runs on background. Initiates transfer to remote

machine. If transfer succeeds, local copy of

message removed; otherwise, tries again later (30 min) for a maximum interval (3 days).

Winter 2005

Remote access

Winter 2005

Telnet

User’smachine

Telnetclient

OSTCP connectionover Internet

Telnetserver

OS

Winter 2005

Telnet basic operation When user invokes telnet, telnet client

on user machine establishes TCP connection to specified server.

TCP connection established; user’s keystrokes sent to remote machine.

Telnet server sends back response, echoed on user’s terminal.

Telnet server can accept multiple concurrent connections.