SSL and TLS renegotiation are vulnerable to an attack in which the
attacker forms a TLS connection with the target server, injects
content of his choice, and then splices in a new TLS connection from
a client. The server treats the client's initial TLS handshake as a
renegotiation and thus believes that the initial data transmitted by
the attacker is from the same entity as the subsequent client data.
This draft defines a TLS extension to cryptographically tie
renegotiations to the TLS connections they are being performed over,
thus preventing this attack.