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Edit: I think based on the below answer here, it seems the answer is "client and server basically only communicate on one port, 3478 (or equivalent")
rfc 5766 : Issue when Both devices support TURN

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I have been reading several sources on TURN, including RFC.

I get the whole premise:

  1. Client creates allocation on TURN server
  2. Client sends data to Peer through TURN that relays via the relayed transport address
  3. Same way around from peer --> Server --> client

Most resources focus on setting up the server and what ports need to be configured.

The point that I am unclear is on the client side:
After the allocation is done and the client can start sending data, do they send that data to the relayed transport address that the Server allocated? Or do they send it to the standard TURN port e.g. 3478, and then the server takes care of looking up the allocation for this client and send it through the relayed address to the peer?

Example:
Client address 192.6.12.123:45677 (let's assume it's the NAT)
TURN server listens on 34.45.34.123:3478
TURN server has done an allocation for client on 34.45.34.123:50678

So when the client wants to send to a peer application data, do they send on port 3478 or port 50678?

My assumption (based also on some wireshark captures I tried) is that the client always send everything on port 3478 and the server takes care to send via the relayed address.

Vaggouras
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1 Answers1

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My assumption (based also on some wireshark captures I tried) is that the client always send everything on port 3478

The client will pick a random local port (e.g 45677), but traffic sent from this port goes to the server's port 3478 (or 5349 if using TLS) on the server. The server will forward it through its allocated port (50678) to whatever remote port the other client established during ICE negotiation.

selbie
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