Clarify handling of NEW_CONNECTION_ID and disable_active_migration

[huitema]

Asking the following questions in the QUICDEV chat:

We have three types of "migration": voluntary migration by changing IP address; involuntary migration due to NAT rebinding; and rotating the DCID for privacy reasons. I am concerned about the interaction between "rotating the CID" and the parameters disable_active_migration and active_connection_id_limit. Specifically:

1. If a peer has set disable_active_migration , is it still OK to send NEW_CONNECTION_ID frames to that peer within the limits of active_connection_id_limit ? I think the answer is yes, but I wanted to make sure.
2. If a peer has set disable_active_migration but has also sent NEW_CONNECTION_ID frames, is it OK to rotate the destination connection ID? Again I think the answer is yes.
3. If a peer has set disable_active_migration and preferred_address , is it OK to rotate the destination connection ID before acting on the preferred address parameter? I believe the answer to that one is NO, but I would like to be sure.

Based on answers, we may need to fix the code, which currently does not process NEW CONNECTION ID if active migration is disabled.

[huitema]

First answer from Marteen Seeman:

1. Yes. You can change CIDs without migrating. That also answers 2.
2. I don’t think we have any ordering requirement. As soon as you learn about a CID, you can use it for whatever purpose you like

[huitema]

OK, consensus seems to be, just send new connection ID. Fixing the code in PR #1150.