1. Proxy Lab Recitation I
Monday Nov 20, 2006

2. Outline What is a HTTP proxy? HTTP Tutorial
HTTP Request
HTTP Response
Sequential vs. concurrent proxies
Caching

3. What is a proxy? Proxy Client Server Why a proxy?
Browser
Why a proxy?
Access control (allowed websites)
Filtering (viruses, for example)
Caching (multiple people request CNN)

4. Brief HTTP Tutorial Hyper-Text Transfer Protocol
Protocol spoken between a browser and a web-server
From browser  web-server: REQUEST
GET HTTP/1.0
From web-server  browser: RESPONSE
HTTP 200 OK
Other stuff…

5. HTTP Request GET http://csapp.cs.cmu.edu/simple.html HTTP/1.1
Request Type
Host
Path
Version
GET HTTP/1.1
Host: csapp.cs.cmu.edu
User-Agent: Mozilla/
Accept: text/xml,application/xml ...
Accept-Language: en-us,en;q=
Accept-Encoding: gzip,deflate ...
An empty line terminates a HTTP request

6. HTTP Request GET http://csapp.cs.cmu.edu/simple.html HTTP/1.1
Host: csapp.cs.cmu.edu
User-Agent: Mozilla/
Accept: text/xml,application/xml ...
Accept-Language: en-us,en;q=
Accept-Encoding: gzip,deflate ...
The Host header is optional in HTTP/1.0 but we
recommend that it be always included

7. HTTP Request GET http://csapp.cs.cmu.edu/simple.html HTTP/1.1
Host: csapp.cs.cmu.edu
User-Agent: Mozilla/
Accept: text/xml,application/xml ...
Accept-Language: en-us,en;q=
Accept-Encoding: gzip,deflate ...
The User agent identifies the browser type. Some websites
use it to determine what to send. And reject you if you say
you use MyWeirdBrowser  Proxy must send this and all other
headers through…

8. HTTP Response HTTP/1.1 200 OK Date: Mon, 20 Nov 2006 03:34:17 GMT
Status
HTTP/ OK
Date: Mon, 20 Nov :34:17 GMT
Server: Apache/ (Unix) …
Last-Modified: Mon, 28 Nov :31:35 GMT
Content-Length: 129
Connection: Keep-Alive
Content-Type: text/html
Status indicates whether it was successful or not, if it is a “redirect”, etc.
The complete response should be transparently sent back to the
client by the proxy.

9. HTTP Response HTTP/1.1 200 OK Date: Mon, 20 Nov 2006 03:34:17 GMT
Server: Apache/ (Unix) …
Last-Modified: Mon, 28 Nov :31:35 GMT
Content-Length: 129
Connection: Keep-Alive
Content-Type: text/html
This field identifies how many bytes are there in the response.
Not sent by all web-servers. DO NOT RELY ON IT !

10. Concurrent Proxy Need to handle multiple requests simultaneously
From different clients
From the same client
E.g., each individual image in a HTML document needs to be requested separately
Serving requests sequentially decreases throughput
Server is waiting for I/O most of the time
This time can be used to start serving other clients
Multiple outstanding requests

11. Concurrent Proxy Use threads for making proxy concurrent
Create one thread for each new client request
The thread finishes and exists after serving the client request
Use pthread library
pthread_create(), pthread_detach(), etc.
Can use select() as well for adding concurrency
Much more difficult to get right

12. Caching Proxy Most geeks visit http://slashdot.org/ every 2 minutes
Why fetch the same content again and again?
(If it doesn’t change frequently)
The proxy can cache responses
Serve directly out of its cache
Reduces latency, network-load

13. Caching: Implementation Issues
Use the GET URL (host/path) to locate the appropriate cache entry
THREAD SAFETY
A single cache is accessed by multiple threads
Easy to create bugs: thread 1 is reading an entry, while thread 2 is deleting the same entry

14. General advice Use RIO routines When to use strcpy() vs. memcpy()
rio_readnb, rio_readlineb
Be very careful when you are reading line-by-line (HTTP request), versus just a stream of bytes (HTTP response)
When to use strcpy() vs. memcpy()
gethostbyname(), inet_ntoa() are not thread-safe!
Path: sequential  + concurrency  + caching