Zero-lenght Client Connection IDs about quic-go HOT 2 CLOSED

Hi @backkem, thank you for opening this well-researched issue!

I assume that you're referring to this check in handleInitialImpl:

This code implements the check required by section 7.2 of RFC 9000, which says:

When an Initial packet is sent by a client that has not previously received an Initial or Retry packet from the server, the client populates the Destination Connection ID field with an unpredictable value. This Destination Connection ID MUST be at least 8 bytes in length. Until a packet is received from the server, the client MUST use the same Destination Connection ID value on all packets in this connection.

This only applies to the initial packet, for packets later in the connection a zero-length CID can be negotiated. In fact, that's what quic-go does when you use quic.Dial.

To further debug this, could you record a tcpdump of the packets that are being sent here?

from quic-go.

Ah, thanks for the pointers! In this case the C++ client may be the one misbehaving. Let me check on that first.

from quic-go.