Explicitly require transports to validate inbound messages

The L1 specification has been updated to explicitly require transports
to validate inbound messages and discard invalid ones. This change
enhances the robustness and security of the protocol by ensuring that
only well-formed messages are processed by higher levels.

Note that the validation of outbound messages is already required by
the "send" operation.

Author: sophokles73

up-l1/README.adoc

@@ -220,9 +220,9 @@ UTransport implementations
 * *MUST* fail invocations of <<receive>> with a `UCode.UNIMPLEMENTED`, if the transport does not support the _pull_ <<delivery-method, delivery method>>.
 --
 
-[.specitem,oft-sid="dsn~utransport-receive-error-notfound~1",oft-needs="impl,utest",oft-tags="TransportLayerImplPull"]
+[.specitem,oft-sid="dsn~utransport-receive-error-notfound~2",oft-needs="impl,utest",oft-tags="TransportLayerImplPull"]
 --
-* *MUST* fail invocations of <<receive>> with a `UCode.NOT_FOUND`, if there are no matching messages available.
+* *MUST* fail invocations of <<receive>> with a `UCode.NOT_FOUND`, if there are no matching messages available. This is also the case if the a package data unit cannot be deserialized into a valid xref:../basics/umessage.adoc[uProtocol message]. Transport implementations *SHOULD* use the validation functionality provided by language libraries for this purpose, if available.
 --
 
 [mermaid]
@@ -335,6 +335,11 @@ UTransport implementations
 * *MUST* deliver matching messages to a successfully registered listener. This means that for each message that the transport receives _after_ <<register-listener>> has completed successfully, and which matches the listener's source and sink filter criteria according to the xref:../basics/uri.adoc#pattern-matching[UUri pattern matching rules], the transport *MUST* invoke the listener's <<on-receive>> method _at least once_.
 --
 
+[.specitem,oft-sid="dsn~utransport-registerlistener-discard-invalid-messages~1",oft-needs="impl,utest",oft-tags="TransportLayerImplPush"]
+--
+* *MUST* discard any inbound transport specific package data unit that cannot be deserialized into a valid xref:../basics/umessage.adoc[uProtocol message]. Transport implementations *SHOULD* use the validation functionality provided by language libraries for this purpose, if available.
+--
+
 [.specitem,oft-sid="req~utransport-registerlistener-prevent-unauthorized-access~1",oft-needs="dsn,uman",oft-tags="TransportLayerImpl"]
 --
 * *MUST* provide means to prevent a uEntity using this transport from consuming messages that it is not authorized to process.