Wireshark: How to know if a sample was filtered on the writer side when using ContentFilteredTopics

ContentFilteredTopics allow a DataReader receive only a subset of the samples written by the Writer. The application of the filter may be done on the DataWriter side or on the DataReader side. To minimize network overhead it is desirable that the filter evaluation is applied on the writer side. Wireshark may be used to verify where the filtering is actually happening. 

When the DataReader filters:

• The DataReader will have to spend some extra CPU cycles in order to apply the filter.

• Some data was transferred through the network, although it was of no interest to the DataReader. The processing of this data needlessly wastes network resources and could result in a loss of network performance.

The DataWriter applies the filter if certain conditions are met, as described in the User’s Manual (here for the current release or here for release 6.01).

So, how can you tell if the filter is being applied by the DataReader or the DataWriter?

The DataWriter adds a signature to the RTPS packet to tell the DataReader that the sample was already filtered on the DataWriter side, and there is no need to apply it on the DataReader side. This signature is contained within the RTPS packet, under DATA/inlineQoS/PID_CONTENT_FILTER_INFO/filterSignature.

There is no public API for accessing this information (although DDS_DataReaderProtocolStatus provides statistics on filtered samples on the DataReader side), so you need to rely on Wireshark for the information. If filterSignature appears in a sample, the sample was filtered on the DataWriter side. If filterSignature doesn’t appear in a sample, the sample was not filtered on the DataWriter side (either there was no ContentFilteredTopic or it was filtered on the DataReader side). Additionally, the filterSignature changes when the ContentFilteredTopic changes.