Common request parameters |
The VERSION parameter specifies the protocol version number. The format of the version number, and version negotiation, see Version numbering and negotiation.
The REQUEST parameter indicates which service operation is being invoked. The value shall be the name of one of the operations offered by the server.
The FORMAT parameter specifies the output format of the response to an operation.
A WMS may offer only a subset of the formats known for that type of operation. The server must detailedly list its supported formats in its service metadata, and must accept operation requests of any kind of format it lists. The server can choose to provide a new format, which cannot be provided by any other instances before. Moreover, it does not require the client to accept or handle this unknown format. When the request contains a kind of format that is not provided by a certain server, this server will use the default format to respond to this operation if the default format has been predefined; or the server will throw a service exception (the code is "InvalidFormat") if the default format has not been defined.
The client only accepts subsets of formats corresponding to operation types. If the client and the server do not support any kind of consistent format, the client will determine, to interrupt the communication with the server, or to search for other service providers that can convert formats, or to permit users to select other deployment methods.
Formats are represented detailedly in the service metadata and operation requests using the MIME type. Each operation has an individual supported format list. Some formats can be provided by several operations, and copied in required lists.
EXCEPTION request parameters represent formats of reporting errors. (Please see service exception)
WMS permits to extend capabilities and operations selectively. When needing to use additional functionalities or specialization, it can define extend contents in information entities. It does not require or expect the general client to use extend modules. If necessary, extended capabilities or operations are defined through the abstract <_ExtendedCapabilities> element or <ExtendedCapabiltites> element of instances provided in service metadata modes of entities. Extended capabilities provide additional metadata about services, so that optional new parameters are contained or not contained in operation requests. Extended operations permit to use predefined additional operations.
Even if extended capabilities use parameters that do not exist or are not correct, or provide server-unknown parameters, the server would have to respond to operations having been defined by OGC standards validly (for example, the server must respond to extended capabilities using default values if default values have been defined).
Service providers must select extend names cautiously, so as to avoid conflicts with standard metadata fields, parameters and operations.