Re: [quicwg/base-drafts] Restore CID sequence numbers (#1465)

[Martin Thomson <[e-mail]>]

martinthomson commented on this pull request.

Not 100% sure about the organization here, but the text is definitely worth having.  We definitely need to do some reorganizing of the document real soon now, so I'm not going to stress about where text goes.

> +selected during the handshake coming first.  Additional connection IDs are
+provided using the NEW_CONNECTION_ID frame ({{frame-new-connection-id}}), each
+with a specified sequence number.  The series of connection IDs issued SHOULD be
+contiguous, but might not appear to be upon receipt due to reordering or loss.
+
+Each connection ID MUST be used on only one local address. When packets are sent
+for the first time on a new local address, a new connection ID MUST be used with
+a higher sequence number than any connection ID previously used on any local
+address.  At any time, an endpoint MAY change to a new connection ID on a local
+address already in use.
+
+An endpoint MUST NOT send packets with a connection ID which has a lower
+sequence number than the highest sequence number of any connection ID ever sent
+or received on that local address.
+
+Implementations SHOULD ensure that peers have a connection ID with a matching

If this isn't MUST, then the above MUST NOT will just result in broken connections.

> +series of connection IDs over the lifetime of a connection. The series of
+connection IDs issued by an endpoint is ordered, with the final connection ID
+selected during the handshake coming first.  Additional connection IDs are
+provided using the NEW_CONNECTION_ID frame ({{frame-new-connection-id}}), each
+with a specified sequence number.  The series of connection IDs issued SHOULD be
+contiguous, but might not appear to be upon receipt due to reordering or loss.
+
+Each connection ID MUST be used on only one local address. When packets are sent
+for the first time on a new local address, a new connection ID MUST be used with
+a higher sequence number than any connection ID previously used on any local
+address.  At any time, an endpoint MAY change to a new connection ID on a local
+address already in use.
+
+An endpoint MUST NOT send packets with a connection ID which has a lower
+sequence number than the highest sequence number of any connection ID ever sent
+or received on that local address.

This is going to be hard to understand.  It's precise, but I think that we might provide a little more help.  Something like "This guarantees that when an endpoint migrates to a new path the packets sent on that path use new connection IDs in both directions." 

I don't know how to say this, but it seems like it might be worth adding: "Note: A connection ID MUST NOT include an unprotected encoding of the associated sequence number.  Endpoints need to be able to recover the sequence number associated with each connection ID they provide without relying on information available to the recipient of the connection ID.  A connection ID that encodes an unencrypted sequence number could be used to correlate connection IDs across network paths."

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