Winter 2002 1

CMPE 155

Week 5

Winter 2002 2

Uniform Resource Locator (URL)

Way to identify objects (pages).– What is page called?– Where is it located? – How to access page?

URL has 3 parts:– Protocol (or scheme).– Machine’s name/address.– Local name (file name).

Winter 2002 3

URL

Ability to handle other protocols.– HTTP, FTP, news, gopher, mail, telnet.

Universal Resource Identifier (URI).– Location transparency.– Replication.

Winter 2002 4

HyperText Markup Language (HTML)

Allows users to produce Web pages including text, graphics, pointers, etc.

Application of the ISO SGML (Standard Generalized Markup Language) standard.

Markup language: describe how objects are to be formatted.– Contains explicit commands for formatting.– Example: <B> and </B>.– Advantages: easy to parse.

Winter 2002 5

Forms

HTML 1.0: one-way exchange.– Enable users to get information from

servers.– Users could not send information back.– Example: on-line order forms.

HTML 2.0 allows forms.– INPUT, SUBMIT.– <FORM> and </FORM>.

Winter 2002 6

CGI Common Gateway Interface. CGI scripts: programs that read input,

process it, and generate output.– Example: after completing a form, user clicks

on SUBMIT; this causes browser to invoke associated CGI script (given by URL). CGI script can generate output in the form of a HTML page that is returned to browser, which interprets and displays it.

– Commercial use of the Web.

Winter 2002 7

Making the Web more interactive

HTML used to write static Web pages. With CGI, some “activation” possible. Java makes the Web more interactive.

– Java program is executed directly on the client’s machine.

– Allows to add animation, sound, etc. to Web pages efficiently

– Remote execution model.• Instead of buying software, installing it, etc., just click on the

appropriate applet from vendor’s Web page and get applet downloaded, executed, etc.

Winter 2002 8

Java

Developed at Sun Microsystems. Originally targeted embedded devices

(e.g., “smart” appliances). Object-oriented language a la C++.

Winter 2002 9

Java and the Web Web page can point to a Java program, or

applet.– Browser gets to the applet pointer, it downloads the

applet to the client’s machine where it is executed.• Example: Income tax filing.

– Go to IRS Web page, get appropriate tax applet downloaded; applet contacts employer, banks, etc, fills tax form and returns to user for verification.

– Security issues:• Applet cannot perform unauthorized access.

Winter 2002 10

More servers: DNS and Mail

Winter 2002 11

Domain Name System (DNS)

Basic function: translation of names (ASCII strings) to network (IP) addresses and vice-versa.

Example: – zephyr.isi.edu <-> 128.9.160.160

Winter 2002 12

History Original approach (ARPANET, 1970’s):

– File hosts.txt listed all hosts and their IP addresses.

– Every night every host fetches file from central repository.

– OK for a few hundred hosts.– Scalability?

• File size.• Centrally managed.

Winter 2002 13

DNS

Hierarchical name space. Distributed database. RFCs 1034 and 1035.

Winter 2002 14

How is it used?

Client-server model.– 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 2002 15

Namespace Flat versus hierarchical. Flat:

– Sequence of characters with no structure.– Short, convenient names.– But, doesn’t scale! Why?

• Unique names.• Hard to decentralize.

Hierarchical:– Name space partitioned and decentralized.– Portions delegated to different authorities.

Winter 2002 16

DNS Name Space

Tree-based hierarchy.

int com edu gov mil org net us ca …

usc

cs ee

ibm

eng sales

Winter 2002 17

Name Space Structure Top-level domains:

– Generic.– Countries.

Leaf domains: no sub-domains. In practice all US organizations are

under a generic domain, while everything outside the US is under the corresponding country domain.

Winter 2002 18

DNS Names Domain names:

– Concatenation of all domain names starting from its own all the way to the root separated by “.”.

– Refers to a tree node and all names under it.– Case insensitive.– Components up to 63 characters.– Full name less than 255 characters.

Winter 2002 19

Name Space Management Domains are autonomous.

– Organizational boundaries.– Each domain manages its own name space

independently of other domains. Delegation:

– When creating new domain: register with parent domain.

• For name uniqueness.• For name resolution.

Winter 2002 20

Resource Records Entry in the DNS database. Several types of entries or RRs. Example: RR “A” contains IP address. Name <-> several resource records. RR format: five-tuple.

– Name.– TTL (in seconds).– Class (usually “IN” for Internet info).– Type: type of RR.– Value.

Winter 2002 21

RR Types 1 SOA: start of authority.

– Marks beginning of zone’s database.– Provides general info about the zone: e-mail

address of admin, default TTL, etc.

A: address.– Contains 32-bit IP address.– Single name <-> several A RRs.

MX: mail exchange.– Name of mail server for this domain.

Winter 2002 22

RR Types 2 NS: name server.

– Name of name server for this domain. CNAME: canonical name.

– Alias. HINFO: host description.

– Provides information about host, e.g., CPU type, OS, etc.

TXT: arbitrary string of characters.– Generic description of the domain, where it is

located, etc.

Winter 2002 23

Name Servers Entire database in a single name server.

– Practical?– Why?

DNS database is partitioned into zones. Each zone contains part of the DNS tree. Zone <-> name server.

– Each zone may be served by more than 1 server.– A server may serve multiple zones.

Primary and secondary name servers.

Winter 2002 24

Name Resolution 1 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 2002 25

Name Resolution 2

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 2002 26

Recursive Resolution Recursive query:

– Each server that doesn’t have info forwards it to someone else.

– Response finds its way back. Alternative: iterative resolution

– Name server not able to resolve query, sends back the name of the next server to try.

– Some servers use this method.– More control for clients.

Winter 2002 27

Example Suppose resolver on flits.cs.vu.nl wants to

resolve linda.cs.yale.edu.– Local NS, cs.vu.nl, gets queried but cannot resolve it.– It then contacts .edu server.– .edu server forwards query to yale.edu server.– yale.edu contacts cs.yale.edu, which has the

authoritative RR.– Response finds its way back to originator.– cs.vu.nl caches this info.

• Not authoritative (since may be out-of-date).• RR TTL determines how long RR should be cached.

Winter 2002 28

Caching

Name servers cache recent requests and corresponding mappings.

Also, cache server that provided mapping.

Cached information is non-authoritative. Clients may choose to use them or go

to authoritative server for fresh copy.

Winter 2002 29

Cache consistency

TTL: how long cached copy is valid. Specified by original server. May be different for each object. Long versus short TTLs.

Winter 2002 30

More details…

RFC 1034 and 1035.

Winter 2002 31

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 2002 32

Mail system

Userinterface

Usersends mail

Userreads mail

Outgoingmailspool

Mailboxes incomingmail

Client(send)

Server(receive)

TCPconnection(outgoing)

TCPconnection(incoming)

Winter 2002 33

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 2002 34

Mail alias expansion

Mapping of e-mail identifiers to mail addresses.– Mail interface consults local alias database and

performs mapping before passing message to outgoing mail spool.

– One-to-many (e.g., mailing lists) and many-to-one (e.g., multiple ways to refer to a single user) mapping.

– Incoming mail also goes through alias expansion before delivery.

Winter 2002 35

SMTP

Simple Mail Transfer Protocol How messages are transferred over a

TCP/IP internet. Defines commands used to exchange

mail between mail clients and servers. Problems reported to user by e-mail.

Winter 2002 36

Example SMTP exchangeUser smith@alpha.edu sends message to jones, green, and brown@beta.gov.

S: 220 beta.gov readyC: Helo alpha.eduS: 250 beta.govC: MAIL FROM smith@alpha.eduS: 250 OKC: RCPT TO: gren@beta.govS: 250 OKC: RCPT TO: brow@beta.govS: 550 no such userC: DATA …

Winter 2002 37

Mail retrieval

SMTP implies server is always listening. What about machines with intermittent

Internet access?– 2-stage delivery: message delivered to

user permanent mailbox; then user connects to retrieve messages.

– User needs protocol to retrieve messages from “permanent” mailboxes.

Winter 2002 38

Post Office Protocol version 3

POP3. User invokes POP3 client; connects to POP3

server through TCP. Requires authentication (user id and passwd). Commands to retrieve and delete messages

from permanent mailbox. Mail server needs to run SMTP and POPP3.

– Mailbox a shared data.

Winter 2002 39

More details…

RFC 821 and 822 specify SMTP and its message formats.