ntp_time.r2py

"""
   Author: Justin Cappos
     Edited by Eric Kimbrel, this was originally time.r2py 

   Start Date: 8 August 2008

   Description:

   This is an implementation of time_interface.r2py

   This module handles getting the time from an external source, via UDP.
   It gets the remote time once and then uses the offset from the local 
   clock from then on
   to return the current time.

   To use this module, first make a call to time_updatetime(localport) with a
   local UDP port that you have permission to send/recv on. This will
   contact some random subset of NTP servers to get and store the local time.

   Then, to get the actual time, call time_gettime() which will return
   the current time (in seconds). time_gettime() can be called at any point
   after having called time_updatetime(localport) since time_gettime() simply
   calculates how much time has elapsed since the local time was originally 
   acquired from one of the NTP servers.

   Note that time_gettime() will raise TimeError if no NTP server responded or
   if time_updatetime(localport) was never previously called.  If time_gettime()
   fails, then time_updatetime(localport) can be called again to sample time
   from another random set of NTP servers.
"""


time_interface = dy_import_module('time_interface.r2py')

# Use for random sampling...
random = dy_import_module('random.r2py')



timeservers = ["time-a.nist.gov", "time-b.nist.gov", 
    "time-a.timefreq.bldrdoc.gov", "time-b.timefreq.bldrdoc.gov", 
    "time-c.timefreq.bldrdoc.gov", "utcnist.colorado.edu", "time.nist.gov", 
    "nist.netservicesgroup.com"]




#BUG: Do I need to compensate for the time taken to contact the time server?    (#353)
def ntp_time_updatetime(localport):
  """
   <Purpose>
    Obtains and stores the local time from a subset of NTP servers.

   <Arguments>
    localport:
             The local port to be used when contacting the NTP server(s).

   <Exceptions>
    TimeError when getmyip() fails or one of the subset of NTP servers will not
    respond.

   <Side Effects>
    time_interface.time_settime(currenttime) is called as the subprocess of a subprocess, which
    adjusts the current time.

   <Returns>
    None.
  """

  ip = getmyip()

  socketobj = listenformessage(ip, localport)

  # always close the handle before returning...
  try: 
    # try five random servers, and send requests to all of the servers and 
    # then wait for a response from any server for atmost 5 seconds. This way
    # the total wait time is reduced to 5 seconds, since any one of the server's
    # is guaranteed to give a response.
    for servername in random.random_sample(timeservers,5):

      # this sends a request, version 3 in "client mode"
      ntp_request_string = chr(27)+chr(0)*47
      try:
        sendmessage(gethostbyname(servername), 123, ntp_request_string,\
          ip, localport) # 123 is the NTP port
      except NetworkAddressError:
        # gethostbyname couldn't resolve servername, try with next name
        pass

    # Wait for 5 seconds to get a response from any of the 5 servers that was
    # requested above.
    starttime = getruntime()
    responsetime = 5.0

    while (getruntime() - starttime) < responsetime:
      try:
        (remoteip, remoteport, mess) = socketobj.getmessage()
      except SocketWouldBlockError:
        sleep(0.05) # Retry amount time...
      else:
        # XXX Check for matching address/port, #1268
        time_interface.time_settime(_time_convert_timestamp_to_float(mess[40:48]))
        return
    
  finally:
    socketobj.close()

  # Failure, tried servers without luck...
  raise time_interface.TimeError, "Time Server update failed.  Perhaps retry later..."



### Do the conversion / decoding for NTP.   More details about the 
### format of NTP are at RFC 2030 (http://www.ietf.org/rfc/rfc2030.txt)

# this unpacks the data from the packet and changes it to a float
def _time_convert_timestamp_to_float(timestamp):
  integerpart = (ord(timestamp[0])<<24) + (ord(timestamp[1])<<16) + (ord(timestamp[2])<<8) + (ord(timestamp[3]))
  floatpart = (ord(timestamp[4])<<24) + (ord(timestamp[5])<<16) + (ord(timestamp[6])<<8) + (ord(timestamp[7]))
  return integerpart + floatpart / float(2**32)


#register the update method
time_interface.time_register_method('ntp',ntp_time_updatetime)