Thanks Kaleb for the WONDERFUL reply.
Thanks a ton for taking out the time for such exhaustively useful post !!!
In my particular case, I disabled client-certificate-authorization at the server.
That lowered the wolfSSL_connect bytes-exchanges from 17 KB to just 5 KB (as the server stopped sending the long list of allowed client-certificate-CA-names).
ajay@ajay-HP-15-Notebook-PC:~$ openssl s_client -connect device.instamsg.io:8883
CONNECTED(00000003)
depth=2 C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO RSA Certification Authority
verify error:num=20:unable to get local issuer certificate
verify return:0
---
Certificate chain
0 s:/OU=Domain Control Validated/OU=PositiveSSL Multi-Domain/CN=instamsg.io
i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
1 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Certification Authority
2 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Certification Authority
i:/C=SE/O=AddTrust AB/OU=AddTrust External TTP Network/CN=AddTrust External CA Root
---
Server certificate
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
subject=/OU=Domain Control Validated/OU=PositiveSSL Multi-Domain/CN=instamsg.io
issuer=/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
---
No client certificate CA names sent
---
SSL handshake has read 4934 bytes and written 509 bytes
---
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-DES-CBC3-SHA
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
Protocol : TLSv1.2
Cipher : ECDHE-RSA-DES-CBC3-SHA
Session-ID: 5A26271571D58DE8068741AD8E6B3DA949DEF9CCEB0B00906B884C5828F6B678
Session-ID-ctx:
Master-Key: 0EBD937ADC38D5795F2AADDBAADE67CB49C70AF6F78B4775E3EBAB0A816CAC01374094534664A4C32B4B8D73BE8212BE
Key-Arg : None
PSK identity: None
PSK identity hint: None
SRP username: None
Start Time: 1512449813
Timeout : 300 (sec)
Verify return code: 20 (unable to get local issuer certificate)
---
Curently, No client certificate CA names sent is sent.
Earlier (with client-cert-authorization optional at server), server used to send a LOONGG list of allowed-CAs.
Once again, thanks a ton for your time.
Please mark this thread as solved.
Thanks and Regards,
Ajay