After looking at webtorrent a while ago I really wanted to dig deep down into WebRTC to see how it actually worked. WebRTC is a web standard that allows two browsers to set up a peer to peer connection (usually browsers only talk through intermediary servers). It also brings unreliable and unordered networking to the browser, which is great for some kinds of applications.
Searching for WebRTC and Java on Github mostly gave me some very simple applications that use a Java server to set up a peer to peer application between two browsers. There also seemed to be some unmaintained JNI wrappers for the WebRTC libs. What I wanted was a pure Java WebRTC implementation so that I could talk directly to the browser more on my terms.
Since this was lacking I decided to try to write my own Java WebRTC implementation. To limit the scope, I decided that I would only try to talk to Firefox, and only support an unordered unreliable DataChannel. Which is WebRTC’s greatest selling point for me. What I ended up with can be found on my profile on GitHub. It is not pretty or solid, but it sort of works.
WebRTC is made up of bunch of protocols: ICE (to establish communication), STUN (binding and NAT traversal), DTLS (encryption), SCPT (over UDP) (message flow control), TURN (fallback for difficult network situations, which I have chosen to ignore).
The initial part of starting a WebRTC connection uses the ICE protocol, the ICE protocol is typically carried out over an encrypted WebSocket connection, though you can use encrypted pigeons if you like.
To be honest I have not looked a lot at ICE, but the parts most relevant for WebRTC works a bit like this:
- A goes: This is my password and certificate signature, this is referred to as an offer.
- B goes: This is my password and certificate signature, this is referred to as an answer.
- Then A and B send each other different options of connectivity as candidates. I am currently blissfully ignorant of how this is supposed to work, in my current implementation I just send the first candidate from the browser and replace IP, port and auth data in the response.
The message format used in the browsers WebRTC implementation for offers and answers is JSON containing an SDP. The SDP contains tons of information that is not used (afaik). The values I have touched are marked in green below, for more information about the fields, this is a great overview.
Important SDP values:
- fingerprint:sha-256 – DTLS certificate sha-256 fingerprint.
- ice-pwd – Your chosen password for STUN.
- ice-ufrag – Your chosen user for STUN.
- 192.168.1.100 58713 – Your offered address and port.
Once candidates have been exchanged, a STUN binding request is sent to establish connectivity, this request must be responded to with a binding response with a xor mapped address. The received message integrity hash is a HMAC-SHA1 which can be checked against the message hashed with the ice-pwd as key.
After the STUN the next step is a DTLS handshake. Thank you Legion of the Bouncy Castle, without you I would have given up here.
Over the same UDP socket where DTLS is running, STUN must be multiplexed. STUN is here used to send “consent” requests every X seconds. If you do not respond to the STUN the browser reports ICE error and shuts down the connection. Bouncy Castle DTLS will silently drop these STUN “consent” packets, so this was a pain to figure out and debug.
Once the DTLS is set up an SCTP connection is initiated. SCTP can be configured to work in all constellations of ordered/unordered and reliable/unreliable.
SCTP over UDP challenges
With SCTP over UDP several issues crop up. My main languages are all JVM based (Kotlin, Java). There is no SCTP implementation in Java, but there is an API for the systems SCTP. The API is very hard to integrate with an UDP socket though (I tried a bit, but it seemed very hard), so the simpler solution is probably to use a userland C lib like the one used by libjitsi.
Sadly i did not know libjitsi existed so I started hacking my own SCTP implementation in Java land. Way more fun…
Writing my own SCTP implementation
The SCTP spec is mostly ok, if a bit unclear in some parts. Once I found the SCTP parameter list it got better. It should also be mentioned that SCTP uses a CRC32C for checksums, this differs from the one used by STUN. This took me quite a while to figure out.
I currently have a very minimal “working” SCTP implementation. Meaning that it can talk to a browser, but does not shut down nicely, and does not do congestion control, and does not handle resends properly. These are very tricky areas to do well, and I have no clue, so I will probably change to use the lib used by libjitsi.
Getting the all important firefox logs
To not go in blind when debugging, logging from the connected browser is invaluable. This is my script to run firefox with WebRTC logging. The different modules can be turned on and off by not including them in the NSPR_LOG_MODULES. It took me a while to figure out the correct modules, so hopefully this helps someone else.
Where is the code?
If this was interesting and you want to take a look at the source code. It is available on my GitHub profile.