Whats the need for TURN

HI,
I come across the statement when STUN does not work then we resort to TURN.
whats the reason for this why wont STUN work. I have read up two reasons for this.
1) When UDP is blocked by firewall
2) client is behind symmetric NAT

Please help in explaining why wont STUN work in both of these cases.

Thanks
Rohit BajajAsked:
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nociConnect With a Mentor Software EngineerCommented:
Stun uses UDP packets port 1493 so if those are blocked there is no traffic for STUN and no answer as well.
NAT may cause a different address for STUN as for your VOIP equipment.

There is also a 3rd method for STUN to fail, if a Firewall tries to help voip using an ALG  (Applic. Level. Gateway).
where the ALG wrecks packets in stead of translating them correctly for this mode of operation.
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nociSoftware EngineerCommented:
answer is imho complete
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