Could you please help me to understand whether a client's certificate will be accepted by a server as valid during mutual TLS challenge based on diagram below? The client side knows nothing about server's Intermediate CA B, and vice versa a server side knows nothing about client's Intermediate CA A. Will validation going to happen on Level 1, if Level 2 missing information about Intermediate CAs?
When the client connects via SSL/TLS, the server will present its certificate chain, i.e. it will present its own certificate but can also offer the certificate for CA B.
If the server only presents its own certificate, then the client cannot validate the certificate and must close the connection or stop to ask the user.
If the server presents both certificates (and the client trusts the root CA, and certificates are valid for the usages and the server name) the client will accept the certificate as trusted.
In the case of PEM encoded certificates, the certificate chain is created by concatenating the certificates:
cat CA_B.pem >>myserver.pem
With pkcs12, its slightly more involved.