Re: [rtcweb] SDP_PRANSWER followed by SDP_OFFER scenario in JSEP

Roman Shpount <roman@telurix.com> Wed, 02 May 2012 16:23 UTC

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Date: Wed, 2 May 2012 12:23:27 -0400
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From: Roman Shpount <roman@telurix.com>
To: Harald Alvestrand <harald@alvestrand.no>
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Subject: Re: [rtcweb] SDP_PRANSWER followed by SDP_OFFER scenario in JSEP
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On Wed, May 2, 2012 at 11:54 AM, Harald Alvestrand <harald@alvestrand.no>wrote:

>  It does mean that the implementation will have to do reference counting
> to know when it can close the port - if one clones the socket and binds the
> clones to different remote ports, I think the OS will take care of it on
> Unix, I'm not sure how it goes for other OSes.
>
> Does someone know what the semantic of bind() is here - whether one needs
> to have an unbound port to fork from in order to bind to different remote
> addresses? I've not tried this myself.
>

I actually do not think this cloning the socket will work properly even on
Unix. You can connect a cloned UDP socket to a different remote IP/port but
both sockets will continue to receive packets from ALL the remote IP/ports.

Even if it does work, this does not prevent the implementation from
reference counting, since the same TURN allocation would need to be reused
across multiple peerconnections, with a new set of permissions/channels
created for each connection.

In the case of hardware codecs that must be allocated to a specific stream
> .... if one supports forking, which connection (if any) gets the codec? The
> first one to start using it? What happens to the second one?
>

My understanding was that you do not really need to allocated hardware
resources until the answer arrives and ICE handshake completes. So, my
assumption was that in case of forking, original connection does not
allocate any codec related resources. If the connection is cloned, then it
does not allocation any codec related resources either and stores the
reference for the sockets and TURN allocations from the original
connection. When either connection receives an answer, it conducts the ICE
handshake and allocates resources as needed.


>  I kind of think it's less disruptive to the people who don't want to fork
> stuff if you must instantiate a different object in order to support
> forking. Then implementations that don't support forking can simply not
> offer a constructor for that object.
>

Alternatively clone can return and error/null if cloning is not supported.
_____________
Roman Shpount