The "preferred_address" extension in TLS 1.3 allows a server to provide an alternative IP address and port number that clients can use to establish a connection in case the original connection fails. This can be useful in scenarios where a server has multiple IP addresses or is load-balanced across multiple machines.

When a client connects to a server, the server can include a preferred_address extension in its initial TLS handshake message. This extension includes the IP address and port number that the server prefers the client to use in future connections. The client can then use this information to establish a connection to the preferred address instead of the original address.

One of the benefits of using the preferred_address extension is that it can improve the resilience of the connection. For example, if the server's original IP address becomes unavailable due to a network outage, the client can still connect to the server using the preferred address.

However, it's worth noting that the preferred_address extension is optional, and not all servers support it. Additionally, some network configurations may prevent clients from connecting to the preferred address. As with any TLS extension, it's important to carefully evaluate whether the preferred_address extension is appropriate for your specific use case.