Re: [quicwg/base-drafts] Authenticate connection IDs (#3499)

[ianswett <[e-mail]>]

ianswett approved this pull request.

Some small nits, but this still LG.

I'd feel more confident in the text's completeness and correctness if a few implementations implemented it and achieved interop.  I don't anticipate any issues, but when we change the handshake, unanticipated new edge cases can be introduced.

> @@ -1475,6 +1475,48 @@ lifetime of a connection, especially in response to connection migration
 ({{migration}}); see {{issue-cid}} for details.
 
 
+## Authenticating Connection IDs {#cid-auth}
+
+The choice each endpoint makes about connection IDs during the handshake is
+authenticated by including all values in transport parameters; see
+{{transport-parameters}}. This ensures that all connection IDs are authenticated
+by the cryptographic handshake.
+
+Each endpoint includes the value of the Source Connection ID field from Initial
+packets it sends in the handshake_connection_id transport parameter; see

One could read that this Connection ID is only used on Initial packets.  If so, I think initial_connection_id would be a better TP name.  But I think this CID can be used used on Initial and 0-RTT from the client and all Initial, Handshake and 1-RTT from the server?

> +* absence of the handshake_connection_id transport parameter from either
+  endpoint,
+
+* absence of the original_connection_id transport parameter from the server,
+
+* absence of the retry_connection_id transport parameter from the server after
+  receiving a Retry packet,
+
+* presence of the retry_connection_id transport parameter when no Retry packet
+  was received, or
+
+* a mismatch between values received from a peer in these transport parameters
+  and the value sent in the corresponding Destination Connection ID fields of
+  Initial packets.
+
+If a zero-length connection IDs is selected, the corresponding transport

```suggestion
If a zero-length connection ID is selected, the corresponding transport
```

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