The HyperText Transfer Protocol

An important difference between HTTP/1.0, HTTP/1.1 and HTTP/2.0 is their utilization of the underlying transport connections. Answer the three questions below to confirm that you understand the difference between these versions of the HTTP protocol.

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  1. System administrators who are responsible for web servers often want to monitor these servers and check that they are running correctly. As a HTTP server uses TCP on port 80, the simplest solution is to open a TCP connection on port 80 and check that the TCP connection is accepted by the remote host. However, as HTTP/1.x is an ASCII-based protocol, it is also very easy to write a small script that downloads a web page on the server and compares its content with the expected one. Use telnet or ncat to verify that a web server is running on host www.computer-networking.info 1

  2. Instead of using telnet on port 80, it is also possible to use a command-line tool such as curl Use curl with the –trace-ascii tracefile option to store in tracefile all the information exchanged by curl when accessing the server.

    • what is the version of HTTP used by your version of curl ?

    • can you explain the different headers placed by curl in the request ?

    • can you explain the different headers found in the response ?

  3. HTTP 1.1, specified in RFC 2616, forces the client to include the Host: header in all its requests. HTTP 1.0 does not define the Host: header, by most implementations support it. By using telnet and curl retrieve the first page of the http://www.computer-networking.info web server by sending http requests with and without the Host: header. Explain the difference between the two.

  4. The headers sent in a HTTP request allow the client to provide additional information to the server. One of these headers is the Accept-Language header that allows to indicate the preferred language of the client 2. For example, curl -HAccept-Language:en http://www.google.be will send to http://www.google.be a HTTP request indicating English (en) as the preferred language. Does google provides a different page in French (fr) and Walloon (wa) ? Same question for http://www.uclouvain.be (given the size of the homepage, use diff to compare the different pages retrieved from www.uclouvain.be)

  5. Compare the size of the http://www.yahoo.com and http://www.google.com web pages by downloading them with curl

  6. The ipvfoo extension on google chrome allows to visually detect whether a website is using IPv6 and IPv4, but also to see which web sites have been contacted when rendering a given web page. Some websites are distributed over several dozens of different servers. Can you find one ?

  7. Some websites are reachable over both IPv4 and IPv6 while others are only reachable over IPv4 3. You can use the -6 (resp. -4) option to force curl to only use IPv6 (resp. IPv4). Verify that www.computer-networking.info is reachable over IPv6 and IPv4 and then check whether your university website already supports IPv6.

  8. curl supports a huge number of options and parameters that are described in its man page Some of them allow you to force the utilization of a specific version of HTTP. These include –http0.9, –http1.0, –http1.1 and http2. Using the latter, verify whether your favorite website supports HTTP/2.0.

As for the DNS, besides using software tools that implement the HTTP protocols, it can also be useful to analyze packet traces with wireshark . The exercises below contain simple packet traces collected with different versions of the HTTP protocol.

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Footnotes

1

The minimum command sent to a HTTP server is GET / HTTP/1.0 followed by CRLF and a blank line.

2

The list of available language tags can be found at http://www.iana.org/assignments/language-subtag-registry Versions in other formats are available at http://www.langtag.net/registries.html Additional information about the support of multiple languages in Internet protocols may be found in rfc5646

3

There are probably very few websites that only support IPv6 and not IPv4. If you find one, let us know by submitting a pull-request to change this exercise.