ooni-probe-cli/internal/tutorial/measurex/chapter13

Chapter XIII: Rewriting Web Connectivity

This chapter contains an exercise. We are going to use the measurex API to rewrite part of the Web Connectivity network experiment. (This is probably the right place to prod you to go to the ooni/spec repository, locate the ts-017-web-connectivity.md spec, and read it.)

Read the spec? Good, so what we are more precisely going to do here is implement the network measurement part of Web Connectivity where we:

1. enumerate all the IP addresses of the target URL using the system resolver;

2. build endpoints with such IPs with a suitable port, thus obtaining a list of HTTP endpoints;

3. TCP connect each of the endpoints and save the results into a measurement object compatible with Web Connectivity's data format;

4. TLS handshake each endpoint (only if this makes sense, of course);

5. HTTP GET the URL and follow redirects until we reach a webpage, fetch the body, and store it for later analysis (which we'll not implement as part of this exercise).

Let us now provide extra context that should help you figure out how to solve this exercise.

Regarding points 3-4

You already know all the primitives.

Regarding point 5

Historically this point has always been performed by a separate HTTP client. This means that any implementation:

• will not include any TCP or TLS event generated during point 5 in the measurement;

• most likely will resolve the URL's domain again (even though the probe-cli implementation uses a fake Resolver to avoid that);

• tries every available IP address and stops at the first one to which it can connect to (which is what a naive HTTP client does, whereas a more advanced one likely tries a couple of addrs in parallel, especially when both IPv4 and IPv6 are supported - this is also known as happy eyeballs).

In terms of measurex, the best API to do what you're required to do in point 5 is probably NewTracingHTTPTransportWithDefaultSettings, which allows you to trace only the HTTP round trip and ignores any other event.

Once you have such a transport, the best Measurer API for the task is probably HTTPClientGET.

Other remarks

You also need to learn about how to measure events at low level, which entails creating an instance of MeasurementDB, passing it to the relevant networking code, and then calling its AsMeasurement method to get back a measurement. (You can probably get an idea of how this is done in general by checking the implementation of Measurer.TCPConnect.)

Hopefully, this should be enough information to help you tackle this task. As you see below, the main function is there empty waiting for your implementation. We will provide our own solution to this problem in the next chapter.

(This file is auto-generated. Do not edit it directly! To apply changes you need to modify ./internal/tutorial/measurex/chapter13/main.go.)

The main.go file

package main

func main() {
}