1. HTTP request message: general format

2. Uploading form input Post method: Web page often includes form input
Input is uploaded to server in entity body
URL method:
Uses GET method
Input is uploaded in URL field of request line:

3. Method types HTTP/1.0 GET POST HEAD HTTP/1.1 GET, POST, HEAD PUT
asks server to leave requested object out of response
HTTP/1.1
GET, POST, HEAD
PUT
uploads file in entity body to path specified in URL field
DELETE
deletes file specified in the URL field

4. HTTP response message status line (protocol status code status phrase)
header
lines
data, e.g.,
requested
HTML file
Data………………….

5. HTTP response status codes
In first line in server->client response message.
A few sample codes:
200 OK
request succeeded, requested object later in this message
301 Moved Permanently
requested object moved, new location specified later in this message (Location:)
400 Bad Request
request message not understood by server
404 Not Found
requested document not found on this server
505 HTTP Version Not Supported

6. User-server state: cookies
Many major Web sites use cookies
Four components:
1) cookie header line in the HTTP response message
2) cookie header line in HTTP request message
3) cookie file kept on user’s host and managed by user’s browser
4) back-end database at Web site
Example:
Susan access Internet always from same PC
She visits a specific e- commerce site for first time
When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID

7. Cookies: keeping “state” (cont.)
client
server
usual http request msg
usual http response +
Set-cookie: 1678
cookie: 1678
usual http response msg
cookie-
specific
action
spectific
Cookie file
ebay: 8734
server
creates ID
1678 for user
entry in backend
database
Cookie file
amazon: 1678
ebay: 8734
access
one week later:
access
Cookie file
amazon: 1678
ebay: 8734

8. Cookies (continued) Cookies and privacy:
aside
Cookies and privacy:
cookies permit sites to learn a lot about you
you may supply name and to sites
search engines use redirection & cookies to learn yet more
advertising companies obtain info across sites
What cookies can bring:
authorization
shopping carts
recommendations
user session state (Web )
Do cookies compromise security?
Can it be used for authentication?

9. Web caches (proxy server)
Goal: satisfy client request without involving origin server
user sets browser: Web accesses via cache
browser sends all HTTP requests to cache
object in cache: cache returns object
else cache requests object from origin server, then returns object to client
origin
server
Proxy
server
HTTP request
HTTP request
client
HTTP response
HTTP response
HTTP request
HTTP response
client
origin
server
Your Web request may be intercepted
using a transparent TCP proxy!

10. More about Web caching Cache acts as both client and server
Typically cache is installed by ISP (university, company, residential ISP)
Why Web caching?
Reduce response time for client request.
Reduce traffic on an institution’s access link.
Internet dense with caches enables “poor” content providers to effectively deliver content (but so does P2P file sharing)

11. Types of Web Caches User Agent Cache (Browser Cache)
Single user
Shared Cache (Proxy Cache)
Multi-user
Gateway Cache

12. Caching example Assumptions origin average object size = 100,000 bits
avg. request rate from institution’s browsers to origin servers = 15/sec
delay from institutional router to any origin server and back to router = 2 sec
Consequences
utilization on LAN =
1.5Mbps/10Mbps = 15%
utilization on access link = 100%
total delay = Internet delay + access delay + LAN delay
= 2 sec + minutes + milliseconds
origin
servers
public
Internet
institutional
network
10 Mbps LAN
1.5 Mbps
access link
cache

13. Caching example (cont)
origin
servers
Possible solution
increase bandwidth of access link to, say, 10 Mbps
Consequences
utilization on LAN = 15%
utilization on access link = 15%
Total delay = Internet delay + access delay + LAN delay
= 2 sec + msecs + msecs
often a costly upgrade
public
Internet
10 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache

14. Caching example (cont)
origin
servers
Install cache
suppose hit rate is 0.4
Consequence
40% requests will be satisfied almost immediately
60% requests satisfied by origin server
utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec)
total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + milliseconds < 1.4 secs
public
Internet
1.5 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache

15. Conditional GET server cache
Goal: don’t send object if cache has up-to-date cached version
cache: specify date of cached copy in HTTP request
If-modified-since: <date>
server: response contains no object if cached copy is up-to- date:
HTTP/ Not Modified
HTTP request msg
If-modified-since: <date>
object
not
modified
HTTP response
HTTP/1.0
304 Not Modified
HTTP request msg
If-modified-since: <date>
object
modified
HTTP response
HTTP/ OK
<data>

16. Controlling Cache HTML Meta Tags HTTP Headers
Written in <head> section of an HTML page.
Can mark expiration date or as uncacheable.
Only used by some browser caches and not seen by proxy caches.
HTTP Headers
Automatically created by Web server.
Sent before HTML.
Seen by browser and proxy caches.

17. HTTP Defining Mechanisms
Freshness – Content is able to be loaded from cache without having to check with the original server.
Validation – The process of confirming with the original server whether or not cached content is still valid to load.

18. HTTP Response Headers Example
HTTP/ OK
Date: Fri, 30 Oct :19:41 GMT
Server: Apache/1.3.3 (Unix)
Cache-Control: max-age=3600, must-revalidate
Expires: Fri, 30 Oct :19:41 GMT
Last-Modified: Mon, 29 Jun :28:12 GMT
ETag: "3e fbbc"
Content-Length: 1040
Content-Type: text/html
Validation
Freshness

19. Expires Header and Freshness
Expires: Fri, 30 Oct :19:41 GMT
Indicates how long the representation is fresh. After this time passes, the cache will communicate with the original server to see if there have been any changes.
Beneficial for static page images as well as continually changing content.
Web server and cache must be synchronized.

20. Cache-Control Header and Freshness
Cache-Control: max-age=3600, must-revalidate
Gives control beyond Expires header.
Cache control response-headers:
max-age=[seconds]
s-maxage=[seconds]
public
no-cache
no-store
must-revalidate
proxy-revalidate

21. Last-Modified and Validation
Last-Modified: Mon, 29 Jun :28:12 GMT
Validate cache by looking at the last time the document was altered (Last-Modified).
If-Modified-Since request is sent to the original server. If changes have been made since the date given, the entire document is returned. Otherwise, the cached document can be loaded.

22. ETag and Validation Unique identifiers created by server.
ETag: "3e fbbc"
Unique identifiers created by server.
Changed each time the representation is altered on the original server.
If-None-Match, request is sent to server and a simple comparison is used to validate the content.

23. Content Delivery Network (CDN)
4a. Get embedded documents from local cache or server (if not already cached)
Cache
CDN
server
4b. Embedded documents
5. Embedded documents
3. Get embedded documents
1. Get base documents
Client
Original
server
2. Document with refs to embedded documents
Cache producer within Akamai CDN

24. References
Nottingham, Mark. “Caching Tutorial: for Web Authors and Webmasters.” July <
Tanenbaum, Andrew S. Distributed Systems: Principles and Paradigms. New Jersey: Prentice- Hall, 2002.
“Web cache.” Wikipedia. <