Header field introduced in HTTP/1.1; in the exchange, the client begins by making a cleartext request, which is later upgraded to a newer HTTP protocol version or switched to a different protocol; connection upgrade must be requested by the client
header field introduced in HTTP/1.1; in the exchange, the client begins by making a cleartext request, which is later upgraded to a newer HTTP protocol version or switched to a different protocol; connection upgrade must be requested by the client (en)