Allow per-user TURN relay allocation

[enobufs]

Summary

Current TURN server API allows us to customize how to allocate and create a net.PacketConn via ManagerConfig.AllocatePacketConn. This callback method however does not provide information of the TURN users for which the allocation was requested.

Motivation

TURN server is a middlebox where we could enforce policies such as how WebRTC traffic should be relayed. It could also be great place to run various diagnostic services. Allowing these services to be customizable per-user basis creates many business opportunities.

Describe alternatives you've considered

Per-process customization is possible but it is clunky and less flexible.

Additional context

classDiagram
    TURNServer --> UserManager: AuthHandler(username, realm)  
    TURNClient --> TURNServer: Allocate()  
    TURNServer --> RelayAddressGenerator: AllocatePacketConn(username)
    RelayAddressGenerator --> UserManager: LookupUser(username)

    namespace user_code {
        class UserManager {
        }
        class RelayAddressGenerator {
        }
    }
end

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[rg0now]

Interesting issue, and thanks for the PR too. Can you please elaborate a bit? What is the simplest use case imaginable that would rely on this feature?

I'm not nitpicking, just trying to find the justification for introducing a breaking change.

[enobufs]

I use this feature in my project to use TURN server (pion/turn fork) as a network conditioner which is implemented inside the net.PacketConn AllocatePacketConn() returns. The associating a user with a specific allocation allows it to have different network settings (works as presets).

You can think of many applications:

• Rate limiter per user
• WebRTC traffic diagnostic service (the traffic is encrypted, but you can still tap into RTP/RTCP headers, STUN, etc)
• Selective QA/intergarion tests/troubleshooting system

[enobufs]

Once this feature lands, I'd be happy to add the TURN-based network conditioner to pion's offerings. I talked about this with @Sean-Der a while ago. FYI.