<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def _request_eip(interface, vm_):
    '''
    Request and return Elastic IP
    '''
    params = {'Action': 'AllocateAddress'}
    params['Domain'] = interface.setdefault('domain', 'vpc')
    eips = aws.query(params,
                     return_root=True,
                     location=get_location(vm_),
                     provider=get_provider(),
                     opts=__opts__,
                     sigver='4')
    for eip in eips:
        if 'allocationId' in eip:
            return eip['allocationId']
    return None