deegree WMS 1.3 does not return exception when mandatory version parameter is missing

[dstenger]

This occurs when a GetMap request is send:

• https://inspire.brandenburg.de/services/ef_sem_wms?BBOX=228152.00000000%2C5690412.00000000%2C493382.00000000%2C5939023.00000000&CRS=EPSG%3A25833&FORMAT=image%2Fpng&HEIGHT=500&LAYERS=EF.EnvironmentalMonitoringProgrammes&REQUEST=GetMap&SERVICE=WMS&STYLES=EF.EnvironmentalMonitoringProgrammes.Default.Point&WIDTH=500

A map is returned instead of an exception.
This is backed up by the specification (OGC® 06-042) as version is a mandatory parameter.

[dstenger]

However, it is not 100% clear what exception type shall be thrown as deegree does not know what WMS version is requested.
Please also see: #1182 (comment)

[dstenger]

We should be aware that some clients may omit the version parameter in a GetMap request.
So, a fix might break the compatibility of deegree with some clients.

Geoserver, for example, ignores a missing version.
https://maps1.geosolutionsgroup.com/geoserver/osm/wms?SERVICE=WMS&REQUEST=GetMap&FORMAT=image%2Fpng8&TRANSPARENT=true&LAYERS=osm&tiled=true&STYLES&WIDTH=256&HEIGHT=256&SRS=EPSG%3A3857&BBOX=1487158.8223163895%2C5165920.119625352%2C1526294.5807983999%2C5205055.878107362 is working as well

[dstenger]

From TMC meeting 2021-09-24: We propose to add a configuration flag such as "strict-validation" that can enable the required behavior.

[dstenger]

I posted this problem to the OSGeo-Standards mailing list and received an answer how GeoServer is handling this problem: https://lists.osgeo.org/pipermail/standards/2021-October/001283.html

GeoServer uses the same solution as proposed in #1183 (comment)

Also, the version number negotiation of GeoServer follows a logical pattern. A similar mechanism might be introduced to deegree as well.