Winter 2005 CMPE 151: Network Administration Clients
Jan 20, 2016
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.