Common PDU rules for IM2000 protocols

The IM2000 protocols are described using Abstract Syntax Notation 1 (ISO/IEC 8824-1:2003), and are transferred using the Distinguished subset of the ASN.1 Basic Encoding Rules (ISO/IEC 8825-1:2003). In order to support future extensions to the protocols, codecs in clients and servers must be capable of handling, and ignoring, elements of SEQUENCE encodings whose tags they do not recognize.

If a server receives a command PDU from the client which it cannot parse (because, for example, the encoding structures or the lengths of the data are incorrect), or receives a PDU from the client which is not a command, or receives a command PDU from the client that is not appropriate for the current position with a transaction, it:

must abort the transaction in progress (if any),
must send a response PDU to the client signifying a protocol error and signifying that the transaction has been aborted, and
should continue the session by awaiting further transactions.

If a client receives a response PDU from the server which it cannot parse, or that is not a response PDU, it should terminate the session. Whether the transaction that was in progress has been completed or aborted cannot be determined by the client in such a case.

If a client receives a response PDU from the server with the abort flag set, or if it receives a response PDU that does not have the abort flag set but that is not an expected response to the command, it

must abort the transaction in progress, and should continue the session by initiating any further transactions.

Servers may implement limits on the length of time that they will wait idly for command PDUs to arrive from clients. If a server times out without receiving a command PDU it:

must abort the transaction in progress (if any),
must send a response PDU to the client signifying a timeout error and signifying that the transaction has been aborted, and
must terminate the session after sending the response.

Note that older tools pre-dating the 1998 standards may not support the UTF8String type used in the ASN.1 definitions.

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