Request block headers from a particular block hash.
        Will receive up to 2000 blocks, starting with the block *after*
        the given block hash (prev_block_hash)

def send_getheaders( self, prev_block_hash ):
        """
        Request block headers from a particular block hash.
        Will receive up to 2000 blocks, starting with the block *after*
        the given block hash (prev_block_hash)
        """

        getheaders = GetHeaders()

        getheaders.add_block_hash( prev_block_hash )

        log.debug("send getheaders")
        self.send_message( getheaders )