Code change:
connection if servers make reasonable selections when generating new IDs for that connection. <ins> 10.3. A simple deployment of QUIC-LB in a cloud provider might use the same global QUIC-LB configuration across all its load balancers that route to customer servers.  An attacker could then simply become a customer, obtain the configuration, and then extract server IDs of other customers' connections at will. To avoid this, the configuration agent SHOULD issue QUIC-LB configurations to mutually distrustful servers that have different keys (for the block cipher or stream cipher algorithms) or routing masks and divisors (for the obfuscated algorithm).  The load balancers can distinguish these configurations by external IP address, or by assigning different values to the config rotation bits (config-rotation).  Note that either of these techniques exposes information to outside observers, as traffic destined for each server set can be easily distinguished. These techniques are not necessary for the plaintext algorithm, as it does not attempt to conceal the server ID. 10.4. Section 21.9 of QUIC-TRANSPORT discusses the Stateless Reset Oracle attack.  For a server deployment to be vulnerable, an attacking client must be able to cause two packets with the same Destination CID to arrive at two different servers that share the same cryptographic context for Stateless Reset tokens.  As QUIC-LB requires deterministic routing of DCIDs over the life of a connection, it is a sufficient means of avoiding an Oracle without additional measures. </ins> 11. There are no IANA requirements.

Related discussion:
text: Added two sections to Security Considerations (Stateless Reset Oracle and Local Configurations Only). and .
Can you speak a little about your reasoning for the recent changes? If this attack is limited to extracting the identity of server instances for other co-hosted entities, that's probably OK, but I'm having trouble connecting the SHOULD here with the preceding text.
Sure. If an attacker has the same QUIC-LB config as the victim, then it can extract the server mapping, which defeats the whole point of the spec. Obviously, an LB must have the QUIC-LB config for servers it routes to. If I'm an attacker within that group of servers I may already see the packet headers that give away the server mapping, so there's little added linkability here (though there is a little). The moment I share configuration among multiple server pools, I'm expanding the number of entities with access to the config, for no operational benefit except to make the administrator's life easier. In the absurd limit, all of AWS has the same QUIC-LB config, and essentially everyone can extract the server mapping from any CID that goes to AWS.
Add a security consideration to avoid the following scenario: MyCloudProvider has a single QUIC-LB config for all its load balancers. It rotates keys periodically, etc, but everyone gets the same config. Obviously, all the attacker has to do is open an account with MyCloudProvider and it is able to recover all the server IDs. Configs ought to be restricted to load balancers serving a finite set of servers. It is possible another MyCloudProvider customer is in the pool behind that load balancer, but that's already a privileged position as already described in the draft. Obviously, this will require some wordsmithing, as the statement above isn't very precise.
As server clusters increase in size, the need to reallocate server identifiers becomes more acute. In one model, the configuration ID is used to indicate a stable routing configuration. Server identifiers for a given configuration ID are routed to the same server, no matter how many other instances are added or removed. In order to allow for changes in the cluster, the configuration ID is used so that old servers can be removed from consideration and new ones added. If these changes happen frequently enough, the number of bits allocated to identifying a configuration might be insufficient. Why not make the length of the identifier flexible?