Chapter 9
More on HTTP and DNS
Professor Rick Han
University of Colorado at Boulder
rhan@cs.colorado.edu
Announcements
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HW #4 due April 16
Programming Assignment #3 Thursday
Midterm solutions on Web
Lecture slides from last week on the Web
Next, more on HTTP and DNS, also SMTP
Prof. Rick Han, University of
Colorado at Boulder
Recap of Previous Lecture
• HTTP
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A Simple Stateless Request-Response Protocol
Simplicity leads to poor interaction with TCP
Persistent HTTP 1.1 multiplexes multiple HTTP
requests onto a single TCP connection
Caching Proxies
• Reduce client response time
• Reduce wide-area network traffic
• Reduce server processing
• Combining Caching and Prefetching
Prof. Rick Han, University of
Colorado at Boulder
More on HTTP
• Configure the Web browser to access the Web
via the HTTP proxy:
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Internet Explorer: Tools  Internet Options 
Connections  LAN Settings
Netscape: Edit  Preferences  Advanced  Proxies
 Manual
• DNS lookup by HTTP Proxy
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Given URL http://www.cs.colorado.edu/index.html,
then proxy must call DNS to translate
www.cs.colorado.edu to 128.138.242.195
Then, proxy establishes HTTP over TCP connection to
128.138.242.195 to retrieve URL’s page
Prof. Rick Han, University of
Colorado at Boulder
More on HTTP (2)
• Most browsers now support HTTP 1.1
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Compatibility with HTTP 1.0 is “expected” but not
mandated
• Eased via HTTP proxies
• Load balancing via HTTP Redirect
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In response to a GET request, a server can return an
HTTP Redirect Response
• Server selects another server that is less loaded
• Client is redirected to again send GET request to less
loaded server
Redirect (2)
HTTP
HTTP
Server 2
Server 1
GET (1) Client GET (3)
Prof. Rick Han, University of
Colorado at Boulder
• GET:
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More on HTTP (3)
Retrieve document, No payload
One-step roundtrip process
Incorporate parameters via long URL
The server returns a response file with a MIME
header identifying the type of file.
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MIME was developed for email, but is reused by HTTP
• POST: sent from Client to Server
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typically used by HTML <form> to send data to a
back-end CGI script
Two-roundtrip process: contact form-processing
server, then send data
Give information to a server, has payload
Expect a response
Prof. Rick Han, University of
Colorado at Boulder
More on HTTP (4)
• POST vs. GET
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Use POST instead of GET if you want to send
complex long text fields/parameters to server
• PUT: sent from Client to Server
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Store document at server under specified URL,
May be disabled at server to avoid modifying
files
Receive a response: Created, Modified, …
• POST vs. PUT
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POST URL specifies the CGI process that will
handle the enclosed form
PUT URL specifies the enclosed document to be
created/storedProf. Rick Han, University of
Colorado at Boulder
DNS Lookup Example
root & edu
DNS server
www.cs.cmu.edu
Client
Local
DNS server
Prof. Rick Han, University of
Colorado at Boulder
cmu.edu
DNS server
cs.cmu.edu
Authoritative
DNS
server
Courtesy: Srini Seshan
More on DNS
• In addition to name translation, DNS helps with
• Host aliasing
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DNS supports multiple host names for a single IP
address, e.g. yahoo.com and www.yahoo.com
• Load distribution
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Instead of HTTP Redirect, use DNS!
A busy site like cnn.com will have multiple
replicated Web servers, each with a different IP
address
• A set of IP addresses associated with cnn.com
DNS can return multiple records that match a
single name
• Order of replicated server addresses is
rotated Prof. Rick Han, University of
Colorado at Boulder
DNS Message Format
12 bytes
Name, type fields
for a query
RRs in response to
query
Records for
authoritative
servers
Additional “helpful
info that may be
used
Identification
Flags
No. of Questions
No. of Answer RRs
No. of Authority RRs
No. of Additional RRs
Questions (variable number of answers)
Answers (variable number of resource records)
Authority (variable number of resource records)
Additional Info (variable number of resource records
Prof. Rick Han, University of
Colorado at Boulder
More on DNS (2)
Server Farm
DNS
Client
• DNS helps with
• Load distribution (cont.)
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DNS round robin to N servers
Akamai’zing – Smarter than DNS round robin:
choose the server closest to you – better
response time!
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Akamai serves a subset of cnn.com
Each URL in Akamai subset has a name for which the
Akamai DNS server is authoritative
www.cnn.com/foo.gif  a128.g.akamai.net/foo.gif
Prof. Rick Han, University of
Colorado at Boulder
More on DNS (3)
• RFC 1794, DNS Support for Load Balancing
• DNS also helps with :
• Mail server aliasing
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Given hotmail.com, return the specific host
name
• BIND is a popular name server for Unix
Prof. Rick Han, University of
Colorado at Boulder
More on DNS (4)
• Dynamic DNS:
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Mapping your well-known Web name to a dynamic IP
address (from DHCP)
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Solution: your PC includes code snippet to update
DNS each time your PC gets a new IP address via
DHCP
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Suppose you’ve reserved a hostname www.myweb.org to
serve Web pages from your home PC
Each time your PC connects via cable/DSL, your ISP assigns
your PC a different dynamic IP address via DHCP
Users won’t know your dynamic IP address but may
remember your Web address. How can they reach you?
Your PC must have authorization at a DNS server to update
its DNS record
Dynamic DNS services are being offered on the
Web, some for free,
others
you
pay
Prof. Rick Han,
University
of
Colorado at Boulder
Load Balancing Techniques
• HTTP Redirection
• DNS Load Balancing
• Router-based
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Zany idea #1: N servers each advertise the same IP
address. Let IP shortest-hop routing determine
the nearest server.
• Hopefully no loops.
Prof. Rick Han, University of
Colorado at Boulder
Load Balancing Techniques (2)
• Router-based
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Better idea: Place an IP router in front of N
servers – the router balances the load
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Example: each server has different IP address, and router
substitutes IP address of lightest loaded server
If a TCP connection is established to a specific server X,
router must remember to route packets for this TCP
connection to server X only
• router can’t just choose most lightly loaded server L,
because L might not be X, so server L would not be
expecting server X’s TCP packets
• NAT-Based (see NAT section)
Prof. Rick Han, University of
Colorado at Boulder
Email: SMTP, POP, IMAP
• SMTP – Simple Mail Transfer Protocol
• Defines email header and email body
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ASCII text for header and body
To:, Subject:, Date:, From:, Received:
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Supplements original email RFC 822
Allows email to carry more than just text:
images, audio, video, MS Word, …
• MIME = Multipurpose Internet Mail
Extensions
Prof. Rick Han, University of
Colorado at Boulder
Email: SMTP, POP, IMAP (2)
• MIME = Multipurpose Internet Mail Extensions
• Augments email header with fields
• MIME-version:,
• Content-Description:,
• Content-Transfer-Encoding:
• Content-Type: namely, the MIME type
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text/html
application/postscript
image/gif
multipart – allows multiple types in an email,
e.g. multipart/mixed; each part has its own
Rick Han, University of
header Prof.Colorado
at Boulder
Email: SMTP, POP, IMAP (3)
• Mail client uses SMTP over TCP to send to
mail server
• Mail reader uses POP3 or IMAP4 to read
email from mail server
• SMTP
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Sendmail is UNIX SMTP implementation
Mail may traverse several mail forwarding
gateways en route to destination
• Each gateway will have sendmail or an SMTP
equivalent to set up a new SMTP connection
SMTP client sends a command (HELO, MAIL,
RCPT, …), server responds with a code
Prof. Rick Han, University of
Colorado at Boulder