A command is addressed to a specific Modbus address, and while other devices may receive the message, only the addressed device will respond.
Up to 32 devices could be implemented on a single RS-485 serial link, requiring each device communicating via Modbus be assigned a unique address. Modbus can be implemented on either an RS-232C (point-to-point) or RS-485 (multidrop) physical layer. Modbus protocol transaction (error-free).
4.Īn (optional) message body: The message body may contain a document that is sent in response to the request.įigure 6.3. 3.Īn empty line (to indicate the end of the response header). Examples are Content-Language (indicating the language of the document in the message body, e.g., Content-Language: en), Content-Length, and Last-Modified (indicating the time the document was last modified, which is relevant for caching, e.g., Last-Modified: Sat, 18:43:00 GMT). HTTP headers: Similar to HTTP headers in the request, the response also uses them to provide more additional information. Examples of status codes (and their verbal description) are 200 (“OK”), 404 (“Not Found”), and 503 (“Service Unavailable”). The line consists of the HTTP version used by the server, the status code, and a verbal description of the status code.
Status code line: The status code line indicates the type of response the server provides. 11Īn HTTP response consists of the following. The reason the certificate was revoked can also be returned in the response. Responses can also contain a nextUpdate time, which indicates how long the client can consider the OCSP response definitive. OCSP responses contain the basic information as to the status of the certificate, in the set of “good,” “revoked,” or “unknown.” They also contain a thisUpdate time, similarly to a CRL, and are signed. It has the design tradeoff of requiring that clients validating certificates have network connectivity to the required OCSP server. This removes the need to download the entire list of revoked certificates and allows for essentially instantaneous revocation of invalid certificates.
The OCSP server checks the certificate status and returns an indication as to the current status of the certificate. A client validating a certificate transmits the certificate serial number, a hash of the issuer name, and a hash of the subject name, to that OCSP server.
A CA certificate contains a reference to an OCSP server. The Online Certificate Status Protocol (OCSP) was designed with the goal of reducing the costs of CRL transmission and eliminating the time lag between certificate invalidity and certificate revocation inherent in CRL-based designs. Terence Spies, in Computer and Information Security Handbook, 2009 Online Certificate Status Protocol