5.1. Old Client, New Server

5.1. Old Client, New Server

Server implementations MAY support a configurable compatibility flag that enables compatibility with old versions. When this flag is on, the server SHOULD identify its protoversion as "1.99". Clients using protocol 2.0 MUST be able to identify this as identical to "2.0". In this mode, the server SHOULD NOT send the Carriage Return character (ASCII 13) after the identification string.

In the compatibility mode, the server SHOULD NOT send any further data after sending its identification string until it has received an identification string from the client. The server can then determine whether the client is using an old protocol, and can revert to the old protocol if required. In the compatibility mode, the server MUST NOT send additional data before the identification string.

When compatibility with old clients is not needed, the server MAY send its initial key exchange data immediately after the identification string.