Experiment with a WebRTC client

[tomaka]

cc #1712

Opening this PR as a draft, mostly to keep the overview that I wrote down.

The code in this PR makes the browser open a TCP connection to 127.0.0.1:41000.
I haven't experimented with a DTLS+SCTP server yet.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

---

 Delta Bytes │ Item
─────────────┼──────────────────
          +0 ┊ Σ [0 Total Rows]

[tomaka]

The code in this PR causes Chrome to send UDP packets to the target IP address. Here's what the UDP packets look like (sampled):

[0, 1, 0, 80, 33, 18, 164, 66, 85, 57, 89, 118, 119, 54, 107, 122, 68, 104, 47, 73, 0, 6, 0, 11, 101, 56, 54, 101, 51, 49, 58, 101, 67, 120, 50, 0, 192, 87, 0, 4, 0, 0, 3, 231, 128, 42, 0, 8, 132, 114, 208, 65, 199, 193, 81, 224, 0, 37, 0, 0, 0, 36, 0, 4, 110, 0, 30, 255, 0, 8, 0, 20, 182, 225, 121, 168, 185, 100, 50, 30, 148, 179, 186, 186, 102, 170, 30, 185, 198, 35, 151, 165, 128, 40, 0, 4, 240, 64, 154, 82]
[0, 1, 0, 80, 33, 18, 164, 66, 76, 89, 69, 99, 114, 98, 80, 118, 86, 49, 115, 120, 0, 6, 0, 11, 101, 56, 54, 101, 51, 49, 58, 101, 67, 120, 50, 0, 192, 87, 0, 4, 0, 0, 3, 231, 128, 42, 0, 8, 132, 114, 208, 65, 199, 193, 81, 224, 0, 37, 0, 0, 0, 36, 0, 4, 110, 0, 30, 255, 0, 8, 0, 20, 227, 94, 105, 131, 50, 107, 7, 153, 218, 151, 138, 68, 235, 237, 11, 213, 104, 253, 200, 230, 128, 40, 0, 4, 7, 71, 25, 93]
[0, 1, 0, 80, 33, 18, 164, 66, 90, 106, 85, 43, 111, 102, 81, 55, 81, 70, 89, 90, 0, 6, 0, 11, 101, 56, 54, 101, 51, 49, 58, 101, 67, 120, 50, 0, 192, 87, 0, 4, 0, 0, 3, 231, 128, 42, 0, 8, 132, 114, 208, 65, 199, 193, 81, 224, 0, 37, 0, 0, 0, 36, 0, 4, 110, 0, 30, 255, 0, 8, 0, 20, 5, 250, 68, 136, 190, 252, 253, 66, 9, 117, 185, 73, 34, 87, 159, 210, 0, 161, 121, 180, 128, 40, 0, 4, 160, 32, 77, 51]
[0, 1, 0, 80, 33, 18, 164, 66, 87, 79, 111, 82, 112, 110, 109, 113, 70, 112, 69, 118, 0, 6, 0, 11, 101, 56, 54, 101, 51, 49, 58, 101, 67, 120, 50, 0, 192, 87, 0, 4, 0, 0, 3, 231, 128, 42, 0, 8, 132, 114, 208, 65, 199, 193, 81, 224, 0, 37, 0, 0, 0, 36, 0, 4, 110, 0, 30, 255, 0, 8, 0, 20, 210, 87, 139, 5, 61, 124, 209, 84, 61, 220, 141, 170, 92, 23, 46, 53, 5, 37, 246, 121, 128, 40, 0, 4, 36, 55, 70, 123]
[0, 1, 0, 80, 33, 18, 164, 66, 82, 80, 71, 87, 97, 66, 119, 51, 90, 100, 88, 43, 0, 6, 0, 11, 101, 56, 54, 101, 51, 49, 58, 101, 67, 120, 50, 0, 192, 87, 0, 4, 0, 0, 3, 231, 128, 42, 0, 8, 132, 114, 208, 65, 199, 193, 81, 224, 0, 37, 0, 0, 0, 36, 0, 4, 110, 0, 30, 255, 0, 8, 0, 20, 110, 19, 133, 234, 218, 112, 177, 122, 9, 171, 33, 66, 60, 222, 99, 71, 172, 110, 71, 81, 128, 40, 0, 4, 47, 18, 50, 245]
[0, 1, 0, 80, 33, 18, 164, 66, 105, 80, 122, 71, 114, 103, 115, 121, 50, 57, 51, 100, 0, 6, 0, 11, 101, 56, 54, 101, 51, 49, 58, 101, 67, 120, 50, 0, 192, 87, 0, 4, 0, 0, 3, 231, 128, 42, 0, 8, 132, 114, 208, 65, 199, 193, 81, 224, 0, 37, 0, 0, 0, 36, 0, 4, 110, 0, 30, 255, 0, 8, 0, 20, 98, 12, 46, 36, 28, 177, 141, 67, 167, 131, 7, 120, 53, 118, 207, 104, 1, 73, 30, 36, 128, 40, 0, 4, 251, 211, 160, 136]

Unfortunately, this isn't a valid DTLS record, as bytes 2 and 3 of the packet are supposed to be a version number equal to 254 and 253 (see https://datatracker.ietf.org/doc/html/rfc6347#section-4.1).

[tomaka]

This is because it's STUN traffic. Need to understand ICE better.

[tomaka]

RFC 8445, section 7:

It is important to note that STUN requests are sent to and from the
exact same IP addresses and ports that will be used for data (e.g.,
RTP, RTCP, or other protocols). Consequently, agents demultiplex
STUN and data using the contents of the packets rather than the port
on which they are received.

[melekes]

Closing in favor of #2135.