A common reason for RTP not being able to reach an endpoint would be because of the use of NATs and Firewalls between the endpoints that are trying to communicate. For SIP, an Application Layer Gateway (ALG) or NAT traversal schemes built into the endpoints can reduce the likelihood of problems.
A common reason for an endpoint not being able to process a received RTP stream would be due to an improper implementation. Devices from some manufacturers can only receive RTP packets of fixed packet sizes; if one endpoint sends 30ms RTP packets instead of 20ms RTP packets, for example, the other endpoint may not be able to process them.
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