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 DataControl OS DataControl 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 Release of first graphical interface, Mosaic, in 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. 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

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 –

Winter 2005 Top-Level Domains (TLDs) TLD names identify organization types or country codes. Examples:. comCommercial org..auAustralia.eduEducational site in US.caCanada.govGovernment site in US.frFrance.milMilitary organization in US.deGermany.netNetwork site.ukGreat Britain.orgNonprofit organization.itItaly.esSpain 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 User interface User sends mail User reads mail Outgoing mail spool Mailboxes incoming mail Client (send) Server (receive) TCP connection (outgoing) TCP connection (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’s machine Telnet client OS TCP connection over Internet Telnet server 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.