Re: [quicwg/base-drafts] Rework Retry packet (#1498)

[Nick Banks <[e-mail]>]

nibanks commented on this pull request.



> +A Retry packet does not include a packet number and cannot be explictly
+acknowledged by a client.
+
+A server MUST only send a Retry in response to a client Initial packet.
+
+If the Original Destination Connection ID field does not match the Destination
+Connection ID from the most recent Initial packet it sent, clients MUST discard
+the packet.  This prevents an off-path attacker from injecting a Retry packet.
+
+The client responds to a Retry packet with an Initial packet that includes the
+provided Retry Token to continue connection establishment.
+
+A server that might send another Retry packet in response to a subsequent
+Initial packet MUST set the Source Connection ID to a new value of at least 8
+octets in length.  This allows clients to distinguish between Retry packets when
+the server sends multiple rounds of Retry packets.  Consequently, a valid Retry

Instead of requiring the server to use a new CID, why not just make the client use a new random CID for every new token it sends in the Initial packet? That way the client has direct control to be able to differentiate between the possibly Retry responses? And no additional complexity on the server side.

Going further, I was thinking during the talks today, if we are going to recommend that initial packet routing not take the client Initial's DCID into account (because it would be an attack surface) then I don't see any reason to support the Retry packet changing the CID at all. The primary reason for adding that support initially was for routing/load balancing; but that seems to be ill advised now. It just adds unnecessary complexity in the client code, IMO.

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