15 213 Recitation Proxy Lab Elie Krevat with wisdom from past terms Outline Intro to Proxy Lab This week Sequential Proxy HTTP over TCP IP What to parse from HTTP headers What headers to suppress Handling broken pipes and using RIO Testing and Debugging Next week Concurrency and caching What is a proxy Middle man between browser client and web server Acts as client to web server Also acts as server to browser Useful as firewall logger cache Proxy Lab What we give you Tiny Web Server Example of web server code Debug Change code to control behavior csapp c h RIO package wrapper helper functions port for user pl Script to generate port for your proxy proxy c Empty Proxy Lab What you ll do Part 1 Sequential Web Proxy Accept conn read req parse it forward req to server get reply forward to client Part 2 Thread based Concurrent Proxy Spawn threads for each request in parallel Part 3 Adding a Cache Apply LRU eviction policy Make cache efficient and thread safe Start early please Proxy Lab is less intricate than Malloc BUT you ll be writing a full proxy with code basically from scratch and you ll still need to understand some conceptual hurdles AND IT TAKES ABOUT AS LONG IF NOT LONGER THAN MALLOC LAB So please start early What Proxy Lab covers Software engineering skills Writing projects from scratch in groups Reading formal specifications Testing and extending functionality Unix socket programming Internet communication Threading and concurrency Caching and data structure design Socket programming briefly Socket is a file descriptor special init Identifies endpoint of communication Imp functions connect bind accept Sockets opened with sockaddr For Internet use sockaddr in HTTP over TCP IP Everything is handled in layers IP handles addressing unreliable comm Which computer basic message passing TCP over IP handles multiplexing and reliable comm Which process moving bytes in order through congestion and packet loss HTTP over TCP handles semantics Bytes become ordered text and pictures on page HTTP Request HTTP defines protocol between web servers and clients Headers hold meta data of connection Type of request GET PUT POST Proxy Lab only covers GET requests URL destination http www cmu edu Body holds actual data HTTP Request cont Request Type Host Path Version GET http csapp cs cmu edu simple html HTTP 1 1 Host csapp cs cmu edu User Agent Mozilla 5 0 Accept text xml application xml Accept Language en us en q 0 5 Accept Encoding gzip deflate An empty line r n terminates a request Parsing the headers GET http www cmu edu 80 index html HTTP 1 1 other information Complete URL Extract the path URI for HTTP request Version Change to HTTP 1 0 for server request Hostname Needed for the Host field in server request Port Proxy needs dest port of server default 80 Forwarding requests GET http www cmu edu 80 index html HTTP 1 1 other information Web Browser process Web proxy process Web Server process Connects to target web server sends request GET index html HTTP 1 0 other information in the original request Proxy parses HTTP request Port not always specified default 80 Proxy suppresses modifies headers for server req Headers to suppress Connection Proxy Connection Change the field value to close Keep Alive Remove this don t want persistent connections with HTTP 1 0 Keep the rest HTTP Response Status HTTP 1 1 200 OK Date Mon 20 Nov 2006 03 34 17 GMT Server Apache 1 3 19 Unix Last Modified Mon 28 Nov 2005 23 31 35 GMT Content Length 129 Connection Keep Alive Content Type text html Status indicates success Send complete response back to client Broken pipe errors Occurs when writing to socket and connection closed prematurely at other end E g click stop on browser Kernel returns normally on first write But on subsequent writes kernel sends SIGPIPE Terminates process by default can be blocked or caught Returns 1 with errno set to EPIPE When reading from socket with closed connection Returns 1 with errno set to ECONNRESET Using Robust I O RIO Avoid upper case wrapper functions terminate all Instead close the offending connection Optionally print error message Handle client request Use rio readlineb to read client req r n signals end of the request rio writen to send request to server Handle server response Use rio readnb to read server response Binary data so difference is memcpy vs strcpy rio writen to send response to client browser Debug Is this my problem Web server issued HTTP redirect to the client DNS lookup for requested hostname failed Hostname ok but rest of URL bogus server ret 404 Web server crashed while it was replying Server sent me mp3 but firefox won t play it Client crashed while I was sending server s reply Webpage contains images that I haven t requested Server sent me something too big for me to cache Client is sending lots of indecipherable headers Debug Is this my problem Web server issued HTTP redirect to the client DNS lookup for requested hostname failed Hostname ok but rest of URL bogus server ret 404 Web server crashed while it was replying Server sent me mp3 but firefox won t play it Client crashed while I was sending server s reply Webpage contains images that I haven t requested Server sent me something too big for me to cache Client is sending lots of indecipherable headers Many things can go wrong but most are out of scope Testing Your Proxy Try a variety of web pages Test for both static dynamic content Test binary files e g images See proxylab writeup for more tips Next Week Concurrency Shell lab handled asynchronous signals Proxy lab enables concurrent threads Similar ideas Both handle race conditions when running code at the same time But threads are constantly switching and allow more memory sharing Summary Proxy Lab covers many concepts with lots of code to write from scratch Proxy parses and forwards HTTP reqs Also clean error handling broken pipes Start early Next week Multi threaded goodness
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