RSPRO is the Remote SIM PROtocol, which is used between the osmo-remsim components (bankd, client, server).
It is a binary protocol running inside TCP, with an IPA ("GSM over IP") multiplex layer in between.
The actual protocol is described in ASN.1 and uses BER encoding.
The ASN.1 syntax can be found inside the osmo-remsim.git repository, see https://gitea.osmocom.org/sim-card/osmo-remsim/src/branch/master/asn1/RSPRO.asn
There is a (currently experimental) wireshark dissector available at https://gitea.osmocom.org/osmocom/wireshark/src/branch/laforge/rspro which we aim to eventually merge to master.
You have to manually select "Decode as..." "GSM over IP" for TCP port 9999 and 9998 in order to get the decode.