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According to KeyCDN, TLSv1.3 connections are established in a single round trip.

If that is the case, then the client's first message (after the TCP handshake) must include everything that the server will need for symmetric encryption, including the key and cipher suite. That means that the client would have to choose a suite without any input from the server.

How can the client be sure to pick a cipher suite that the server supports?

According to KeyCDN, TLSv1.3 connections are established in a single round trip:

While TLS 1.2 requires two round-trips to complete the TLS handshake, 1.3 gets the job done in just one trip

If that is the case, then the client's first message (after the TCP handshake) must include everything that the server will need for symmetric encryption, including the key and cipher suite. That means that the client would have to choose a suite without any input from the server.

How can the client be sure to pick a cipher suite that the server supports?

According to KeyCDN, TLSv1.3 connections are established in a single round trip.

If that is the case, then in the first packet to the server (after the TCP handshake), the client must include everything the server will need for symmetric encryption, including the key and cipher suite. That means that the client would have to choose a suite without any input from the server.

How can the client be sure to pick a cipher suite that the server supports?

According to KeyCDN, TLSv1.3 connections are established in a single round trip.

If that is the case, then the client's first message (after the TCP handshake) must include everything that the server will need for symmetric encryption, including the key and cipher suite. That means that the client would have to choose a suite without any input from the server.

How can the client be sure to pick a cipher suite that the server supports?