Issue #2928 explains that in order to deal with "post quantum" crypto, we should be able to carry Client Hello messages that are too large to fit in a single packet. To test support to that feature, we need to create client hello packets that do not fit in one packet. This can be achieved by sending a large Transport Parameter. We've reserved 0x173E for this purpose. The transport parameter is defined as such:

• Parameter name: discard
• Parameter code: 0x173E
• Parameter value: anything, it's discarded!

The client who wants to test Quantum readiness inserts the parameter in the transport parameter list. Endpoints that receive this parameter MUST ignore it.

As an alternative, clients can send a large TP with an ID reserved for greasing, or split small ClientHello into multiple QUIC packets.