How the WebRTC API Works

The API is implemented as a gRPC service exchanging a specific set of Protocol Buffer messages with your Terminal.

To register your Terminal with our core, you can use one of the two methods:

  • Pipe()
  • MultiPipe()

Both methods serve the same purpose, but there is also a difference.

  • If you want to handle multiple ongoing calls simultaneously, use MultiPipe()
  • For a single call at a time, use Pipe()

Both methods receive and/or return a stream of WebTerminalMessages (two-way streaming).

A WebTerminalMessage is a structure used to exchange messages between a Terminal and our core (see proto).

For example, if you:

  1. register your Terminal (by calling either Pipe or MultiPipe)
  2. initiate a call by pushing a WebTerminalMessage that contains an Offer to our core
  3. receive a WebTerminalMessage from our core containing Ringing followed by an Answer

then

  1. your Terminal will know that the callee's phone¹ was first ringing, and then the callee answered the call.

¹ or another Terminal

In this manner, you are able to build quite a sophisticated application with the help of this API.

Common scenarios

Depending on the method used to register a WebRTC terminal, call_id in the scenarios below is either ignored (Pipe) or mandatory (MultiPipe).

call_id is basically a random UUID either created by Terminal or by the core.

Initiating a call from a WebRTC terminal

# ↔ Note
1 T → {Offer{sdp, msisdn}, call_id} Terminal sends an Offer with an SDP, a new UUID as call_id and the msisdn of the callee
2 T ← {Answer{sdp}, call_id} OR {Bye{}, call_id} Terminal receives either an Answer with an SDP enclosed, or a Bye rejecting the call
3 T ↔︎ {Bye{}, call_id} If established, the call eventually gets terminated with a Bye from either party

Receiving an incoming call from the core

# ↔ Note
1 T ← {Offer{sdp, msisdn}, call_id} Terminal receives an Offer with an SDP, a new UUID as call_id and the msisdn of the caller
2 T → {Ringing{}, call_id} (optional) Terminal tells the core that it is alerting the user
3 T → {Answer{sdp}, call_id} OR {Bye{}, call_id} Terminal sends either an Answer with an SDP enclosed, or a Bye rejecting the call
4 T ↔︎ {Bye{}, call_id} If established, the call eventually gets terminated with a Bye from either party

Observing status

# ↔ Note
1 T ← {InCall{msisdn}, call_id} Terminal receives an InCall notification whenever its msisdn becomes a participant of a call on another terminal
2 T ← {Idle{msisdn}, call_id} Terminal receives Idle whenever its msisdn becomes a non-participant of any call

Pipe(): call_id in Idle is empty as these messages are about the associated msisdn.

MultiPipe(): call_id indicates the call in question, for your Terminal to know which particular call has started/ended

Transferring call to Terminal

Upon reception of an InCall notification, Terminal can transfer the call to itself by sending a new Offer to the core

# ↔ Note
1 T → {Offer{sdp, msisdn}, call_id} call_id must be the one from the InCall received
2 T ← {Answer{sdp}, call_id} OR {Bye{}, call_id} Terminal receives either an Answer with an SDP enclosed, or a Bye rejecting the call
3 T ↔︎ {Bye{}, call_id} If established, the call eventually gets terminated with a Bye from either party

Transferring call to the mobile terminal

Whenever in a call, Terminal can transfer it to the associated mobile phone by sending {Action{ToPhone{}} message to the core.

# ↔ Note
1 T → {Action{ToPhone{}}, call_id} Requesting transfer to the phone (call_id must be the id of the call ongoing)
2 T ← {InCall{msisdn}, call_id} Indicates that the call has been transfered to the phone
3 T ↔︎ {Idle{msisdn}, call_id} Indicates that the call has ended

ICE

Terminal may send its candidates to the core within an SDP of Offer or Answer. WG2 media servers are on public Internet and will report its candidates in SDP.

Renegotiating SDP

During an ongoing call, Terminal must be prepared to receive (and handle) Offer with a new SDP even though the Offer/Answer exchange has completed.

# ↔ Note
1 T ← {Offer{sdp, msisdn}, call_id} The core uses Offer to update SDP