Force Client-SSL Profile to X25519, Instead of Post-Quantum Cryptography

Hi Kazeem_Yusuf​ ,

Enabling PQC ciphers can increase the size of the Client_Hello,  reference https://datatracker.ietf.org/doc/draft-ietf-uta-pqc-app/  See Section 4.1

" Challenges: - The size of the hybrid key exchange algorithm key share may exceed the Maximum Transmission Unit (MTU), potentially causing the ClientHello message to be fragmented across multiple packets. In TLS, this results in multiple TCP segments. In DTLS, handshake messages are explicitly fragmented at the record layer as specified in [RFC9147], with each fragment sent in its own UDP datagram. In both cases, larger ClientHello messages increase latency and the risk of handshake delay, especially in lossy networks."

Have you tried any custom cipher groups?

Here is something to test out in a lower / non-prod environment:

This locks down DH groups to only X25519, it will disable the Kyber Drafts.  *** Older SSL Clients might have issues with these settings ***

tmsh create ltm cipher rule rule_X25519_only cipher DEFAULT dh-groups X25519 signature-algorithms DEFAULT

tmsh create ltm cipher group group_X25519_only allow add { rule_X25519_only }

tmsh modify ltm profile client-ssl <your_client_ssl_profile_name> ciphers group_X25519_only

If you need to add support for secp256r1(P-256) for older SSL clients then add this string to the DH groups "X25519:secp256r1:secp384r1"  1st tmsh line above.

Give that a go hopefully it helps