30 lines
2.1 KiB
Plaintext
30 lines
2.1 KiB
Plaintext
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Glare is a PITA.
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Although configuration of ISDN links can be done to minimize glare, to be pedantic we must have a clear policy
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on how the FreeTDM API is supposed to behave on glare across signaling modules.
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There is a well-known race in the FreeTDM API since the beginning. When a user wants to place a call there is 2 APIs that
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must be used:
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1. ftdm_channel_open_xx (to hunt the channel by group, span or select a channel individually)
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2. ftdm_channel_call_place() to place the actual call.
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Since the user has no access to channel locking, between opening a channel and placing a call, an incoming call could be
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received. Therefore the user needs to be aware of the following:
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1. Between ftdm_channel_open_xx and ftdm_channel_call_place() a SIGEVENT_START can be received, if the user application
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is smart enough, upon receive of SIGEVENT_START it can avoid doing anything else with the channel (from an outgoing call perspective)
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since that channel is now a channel owned by the incoming call. It can for example hunt another channel using
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ftdm_channel_open_xx again and avoid calling ftdm_channel_call_place. However, if the app is not smart enough and still calls
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ftdm_channel_call_place even though already received FTDM_SIGEVENT_START on that channel, ftdm_channel_call_place will return
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FTDM_BREAK to inform the user the outgoing call could not be placed and that there is already an incoming call on that channel.
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2. If SIGEVENT_START was not received before calling ftdm_channel_call_place, it could still come while ftdm_channel_call_place()
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is being executed, in such situation ftdm_channel_place_call() will return FTDM_BREAK to inform the user the call could
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not be placed due to glare and the incoming call won the channel, he user should back off since the channel is
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now owned by the incoming call (it can touch the channel having in mind there is now an incoming call on it)
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3. After ftdm_channel_call_place returns, if glare is detected and the signaling stack decides to drop the local call, a regular
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SIGEVENT_STOP will be sent with the hangup cause FTDM_CAUSE_REQUESTED_CHAN_UNAVAIL.
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