71 | | [inki@Client1 ~]$ wget -m -p http://pc484.emulab.net |
72 | | --2012-07-06 04:33:49-- http://pc484.emulab.net/ |
73 | | Resolving pc484.emulab.net... 155.98.38.84 |
74 | | Connecting to pc484.emulab.net|155.98.38.84|:80... connected. |
75 | | HTTP request sent, awaiting response... 200 OK |
76 | | Length: 548 [text/html] |
77 | | Saving to: “pc484.emulab.net/index.html” |
78 | | |
79 | | 100%[======================================>] 548 --.-K/s in 0s |
80 | | |
81 | | 2012-07-06 04:33:49 (118 MB/s) - “pc484.emulab.net/index.html” saved [548/548] |
82 | | |
83 | | Loading robots.txt; please ignore errors. |
84 | | --2012-07-06 04:33:49-- http://pc484.emulab.net/robots.txt |
85 | | Connecting to pc484.emulab.net|155.98.38.84|:80... connected. |
86 | | HTTP request sent, awaiting response... 404 Not Found |
87 | | 2012-07-06 04:33:49 ERROR 404: Not Found. |
88 | | |
89 | | --2012-07-06 04:33:49-- http://pc484.emulab.net/top.html |
90 | | Connecting to pc484.emulab.net|155.98.38.84|:80... connected. |
91 | | HTTP request sent, awaiting response... 200 OK |
92 | | Length: 917 [text/html] |
93 | | Saving to: “pc484.emulab.net/top.html” |
94 | | |
95 | | 100%[======================================>] 917 --.-K/s in 0s |
96 | | |
97 | | 2012-07-06 04:33:49 (172 MB/s) - “pc484.emulab.net/top.html” saved [917/917] |
98 | | |
99 | | --2012-07-06 04:33:49-- http://pc484.emulab.net/home.html |
100 | | Connecting to pc484.emulab.net|155.98.38.84|:80... connected. |
101 | | HTTP request sent, awaiting response... 200 OK |
102 | | Length: 822 [text/html] |
103 | | Saving to: “pc484.emulab.net/home.html” |
104 | | |
105 | | 100%[======================================>] 822 --.-K/s in 0s |
106 | | |
107 | | 2012-07-06 04:33:49 (215 MB/s) - “pc484.emulab.net/home.html” saved [822/822] |
108 | | |
109 | | --2012-07-06 04:33:49-- http://pc484.emulab.net/links.html |
110 | | Connecting to pc484.emulab.net|155.98.38.84|:80... connected. |
111 | | HTTP request sent, awaiting response... 200 OK |
112 | | Length: 958 [text/html] |
113 | | Saving to: “pc484.emulab.net/links.html” |
114 | | |
115 | | 100%[======================================>] 958 --.-K/s in 0s |
116 | | |
117 | | 2012-07-06 04:33:49 (267 MB/s) - “pc484.emulab.net/links.html” saved [958/958] |
118 | | |
119 | | --2012-07-06 04:33:49-- http://pc484.emulab.net/media/GENILogo.png |
120 | | Connecting to pc484.emulab.net|155.98.38.84|:80... connected. |
121 | | HTTP request sent, awaiting response... 200 OK |
122 | | Length: 22678 (22K) [image/png] |
123 | | Saving to: “pc484.emulab.net/media/GENILogo.png” |
124 | | |
125 | | 100%[======================================>] 22,678 --.-K/s in 0.001s |
126 | | |
127 | | 2012-07-06 04:33:49 (42.8 MB/s) - “pc484.emulab.net/media/GENILogo.png” saved [22678/22678] |
128 | | |
129 | | --2012-07-06 04:33:49-- http://pc484.emulab.net/media/topgeni.png |
130 | | Connecting to pc484.emulab.net|155.98.38.84|:80... connected. |
131 | | HTTP request sent, awaiting response... 200 OK |
132 | | Length: 116224 (114K) [image/png] |
133 | | Saving to: “pc484.emulab.net/media/topgeni.png” |
134 | | |
135 | | 100%[======================================>] 116,224 --.-K/s in 0.002s |
136 | | |
137 | | 2012-07-06 04:33:49 (66.7 MB/s) - “pc484.emulab.net/media/topgeni.png” saved [116224/116224] |
138 | | |
139 | | --2012-07-06 04:33:49-- http://pc484.emulab.net/media/hello.png |
140 | | Connecting to pc484.emulab.net|155.98.38.84|:80... connected. |
141 | | HTTP request sent, awaiting response... 200 OK |
142 | | Length: 11497 (11K) [image/png] |
143 | | Saving to: “pc484.emulab.net/media/hello.png” |
144 | | |
145 | | 100%[======================================>] 11,497 --.-K/s in 0s |
146 | | |
147 | | 2012-07-06 04:33:49 (45.8 MB/s) - “pc484.emulab.net/media/hello.png” saved [11497/11497] |
148 | | |
149 | | --2012-07-06 04:33:49-- http://pc484.emulab.net/media/experimenter.png |
150 | | Connecting to pc484.emulab.net|155.98.38.84|:80... connected. |
151 | | HTTP request sent, awaiting response... 200 OK |
152 | | Length: 14020 (14K) [image/png] |
153 | | Saving to: “pc484.emulab.net/media/experimenter.png” |
154 | | |
155 | | 100%[======================================>] 14,020 --.-K/s in 0s |
156 | | |
157 | | 2012-07-06 04:33:49 (47.5 MB/s) - “pc484.emulab.net/media/experimenter.png” saved [14020/14020] |
158 | | |
159 | | --2012-07-06 04:33:49-- http://pc484.emulab.net/media/keyboard.png |
160 | | Connecting to pc484.emulab.net|155.98.38.84|:80... connected. |
161 | | HTTP request sent, awaiting response... 200 OK |
162 | | Length: 9533 (9.3K) [image/png] |
163 | | Saving to: “pc484.emulab.net/media/keyboard.png” |
164 | | |
165 | | 100%[======================================>] 9,533 --.-K/s in 0s |
166 | | |
167 | | 2012-07-06 04:33:49 (73.4 MB/s) - “pc484.emulab.net/media/keyboard.png” saved [9533/9533] |
168 | | |
169 | | FINISHED --2012-07-06 04:33:49-- |
170 | | Downloaded: 9 files, 173K in 0.003s (59.7 MB/s) |
171 | | [inki@Client1 ~]$ |
| 73 | [inki@Client1 ~]$ wget -m -p http://server |
173 | | |
174 | | * Use the dataplane interface. Up to this point we have used the public interface of the webserver. As you are testing your environment, you should use the dataplane connections between the clients and the server. These are the private connections that are represented with a lines between the machines in Flack. To do that you first need to figure out the IP address of the server on each of these links. In the above example, the line that connects the Server with Client1 has a box that is called `lan0`. Press on the (i) button of that box and see the IP server that is assigned on the server. Use this IP to run the wget command : |
175 | | {{{ |
176 | | [inki@Client1 ~]$ wget -m -p http://10.10.1.1 |
177 | | }}} |
178 | | |
179 | | === Viewing and Adjusting link characteristics === |
| 75 | This will produce a directory with the followin data structure, run: |
| 76 | {{{ |
| 77 | [inki@Client1 ~]$ ls server/ |
| 78 | home.html index.html links.html media top.html |
| 79 | }}} |
| 80 | |
| 81 | == Build your own Server == |
| 82 | At a high level, a web server listens for connections on a socket (bound to a specific port on a host machine). Clients connect to this socket and use a simple text-based protocol to retrieve files from the server. For example, you might try the following command on `Client1`: |
| 83 | |
| 84 | {{{ |
| 85 | % telnet server 80 |
| 86 | GET /index.html HTTP/1.0 |
| 87 | }}} |
| 88 | (Type two carriage returns after the "GET" command). This will return to you (on the command line) the HTML representing the "front page" of the web server that is running on the `Server` host.) |
| 89 | |
| 90 | One of the key things to keep in mind in building your web server is that the server is translating relative filenames (such as index.html ) to absolute filenames in a local filesystem. For example, you might decide to keep all the files for your server in ~10abc/cs339/server/files/, which we call the document root. When your server gets a request for index.html (which is the default web page if no file is specified), it will prepend the document root to the specified file and determine if the file exists, and if the proper permissions are set on the file (typically the file has to be world readable). If the file does not exist, a file not found error is returned. If a file is present but the proper permissions are not set, a permission denied error is returned. Otherwise, an HTTP OK message is returned along with the contents of a file. |
| 91 | |
| 92 | In our setup we are using the [http://httpd.apache.org/ Apache web server]. The default document root for Apache on a host running Fedora 10 is under `/var/www/html`. |
| 93 | * Login to the `Server` host |
| 94 | * Run |
| 95 | {{{ |
| 96 | [inki@server ~]$ ls /var/www/html/* |
| 97 | }}} |
| 98 | |
| 99 | |
| 100 | You should also note that since index.html is the default file, web servers typically translate "GET /" to "GET /index.html". That way index.html is assumed to be the filename if no explicit filename is present. This is also why the two URLs http://www.cs.williams.edu and http://www.cs.williams.edu/index.html return equivalent results. |
| 101 | |
| 102 | When you type a URL into a web browser, the server retrieves the contents of the requested file. If the file is of type text/html and HTTP/1.0 is being used, the browser will parse the html for embedded links (such as images) and then make separate connections to the web server to retrieve the embedded files. If a web page contains 4 images, a total of five separate connections will be made to the web server to retrieve the html and the four image files. |
| 103 | |
| 104 | Using HTTP/1.0, a separate connection is used for each requested file. This implies that the TCP connections being used never get out of the slow start phase. HTTP/1.1 attempts to address this limitation. When using HTTP/1.1, the server keeps connections to clients open, allowing for "persistent" connections and pipelining of client requests. That is, after the results of a single request are returned (e.g., index.html), the server should by default leave the connection open for some period of time, allowing the client to reuse that connection to make subsequent requests. One key issue here is determining how long to keep the connection open. This timeout needs to be configured in the server and ideally should be dynamic based on the number of other active connections the server is currently supporting. Thus if the server is idle, it can afford to leave the connection open for a relatively long period of time. If the server is busy servicing several clients at once, it may not be able to afford to have an idle connection sitting around (consuming kernel/thread resources) for very long. You should develop a simple heuristic to determine this timeout in your server. |
| 105 | |
| 106 | For this assignment, you will need to support enough of the HTTP/1.0 and HTTP/1.1 protocols to allow an existing web browser (Firefox) to connect to your web server and retrieve the contents of the Willams CS front page from your server. (Of course, this will require that you copy the appropriate files to your server's document directory.) Note that you DO NOT have to support script parsing (php, javascript), and you do not have to support HTTP POST requests. You should support images, and you should return appropriate HTTP error messages as needed. |
| 107 | |
| 108 | At a high level, your web server will be structured something like the following: |
| 109 | |
| 110 | Forever loop: |
| 111 | Listen for connections |
| 112 | Accept new connection from incoming client |
| 113 | Parse HTTP request |
| 114 | Ensure well-formed request (return error otherwise) |
| 115 | Determine if target file exists and if permissions are set properly (return error otherwise) |
| 116 | Transmit contents of file to connect (by performing reads on the file and writes on the socket) |
| 117 | Close the connection (if HTTP/1.0) |
| 118 | |
| 119 | You will have three main choices in how you structure your web server in the context of the above simple structure: |
| 120 | |
| 121 | 1) A multi-threaded approach will spawn a new thread for each incoming connection. That is, once the server accepts a connection, it will spawn a thread to parse the request, transmit the file, etc. |
| 122 | |
| 123 | 2) A multi-process approach maintains a worker pool of active processes to hand requests off to from the main server. This approach is largely appropriate because of its portability (relative to assuming the presence of a given threads package across multiple hardware/software platform). It does face increased context-switch overhead relative to a multi-threaded approach. |
| 124 | |
| 125 | 3) An event-driven architecture will keep a list of active connections and loop over them, performing a little bit of work on behalf of each connection. For example, there might be a loop that first checks to see if any new connections are pending to the server (performing appropriate bookkeeping if so), and then it will loop overall all existing client connections and send a "block" of file data to each (e.g., 4096 bytes, or 8192 bytes, matching the granularity of disk block size). This event-driven architecture has the primary advantage of avoiding any synchronization issues associated with a multi-threaded model (though synchronization effects should be limited in your simple web server) and avoids the performance overhead of context switching among a number of threads. |
| 126 | |
| 127 | You may choose from C or C++ to build your web server but you must do it in Linux (although the code should run on any Unix system). In C/C++, you will want to become familiar with the interactions of the following system calls to build your system: socket(), select(), listen(), accept(), connect() . We outline a number of resources below with additional information on these system calls. A good book is also available on this topic (there is a reference copy of this in the lab). |