How can I know the PDU number to pass to the PER decoder if I don't know what encoded PDU I received?

Although PER encodings do not carry a tag as in BER, there is always a mechanism available for determining what the type (PDU number) is. With lower-level protocols it might be the channel on which the message is received. Other protocols might use the state of the protocol machine to determine the message type (e.g., originating a message, responding to a message, awaiting a connection, in the process of terminating, etc.). Yet other protocols use the value of some other type that is carried typically right in front of the message, such as that of an INTEGER or OBJECT IDENTIFIER type.

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