Demo/sgi/video/vtime.py

   1 #
   2 # Module vtime - Keep virtual time between two nodes.
   3 #
   4 # We try for synchronised clocks by sending a packet of the for
   5 # (1,mytime,0) to the other side, and waiting (at most) a second for
   6 # a reply. This reply has the form (2,mytime,histime), and we can
   7 # estimate the time difference by defining histime to be exactly half-way
   8 # between the time we sent our message and got our reply. We send a
   9 # final (3,mynewtime,histime) message to allow the other side to do the
  10 # same computations.
  11 #
  12 # Note that the protocol suffers heavily from the 2-army problem.
  13 # It'll have to do until I can read up on time-sync protocols, though.
  14 #
  15 from socket import *
  16 import time
  17
  18 MSGSIZE = 100
  19 MSGTIMEOUT = 1000
  20
  21 recv_timeout = 'receive timeout'
  22 bad_connect = 'Bad connection'
  23
  24 def timeavg(a,b):
  25     return int((long(a)+b)/2L)
  26 def tryrecv(s):
  27     cnt = 0
  28     while 1:
  29         if s.avail():
  30             return s.recvfrom(MSGSIZE)
  31         time.millisleep(100)
  32         cnt = cnt + 100
  33         if cnt > MSGTIMEOUT:
  34             raise recv_timeout
  35
  36 class VTime():
  37     def init(self,(client,host,port)):
  38         s = socket(AF_INET, SOCK_DGRAM)
  39         host = gethostbyname(host)
  40         localhost = gethostbyname(gethostname())
  41         raddr = (host,port)
  42         s.bind((localhost,port))
  43         if client:
  44             #
  45             # We loop here because we want the *second* measurement
  46             # for accuracy
  47             for loopct in (0,2):
  48                 curtijd = time.millitimer()
  49                 check = `(loopct,curtijd,0)`
  50                 s.sendto(check,raddr)
  51                 while 1:
  52                     try:
  53                         if loopct:
  54                             data, other = s.recvfrom(MSGSIZE)
  55                         else:
  56                             data, other = tryrecv(s)
  57                         newtijd = time.millitimer()
  58                         if other <> raddr:
  59                             print 'Someone else syncing to us: ', other
  60                             raise bad_connect
  61                         data = eval(data)
  62                         if data[:2] == (loopct+1,curtijd):
  63                             break
  64                         if data[0] <> 2:
  65                             print 'Illegal sync reply: ', data
  66                             raise bad_connect
  67                     except recv_timeout:
  68                         curtijd = time.millitimer()
  69                         check = `(loopct,curtijd,0)`
  70                         s.sendto(check,raddr)
  71             histime = data[2]
  72             s.sendto(`(4,newtijd,histime)`,raddr)
  73             mytime = timeavg(curtijd,newtijd)
  74             #mytime = curtijd
  75             self.timediff = histime - mytime
  76         else:
  77             while 1:
  78                 data,other = s.recvfrom(MSGSIZE)
  79                 if other <> raddr:
  80                     print 'Someone else syncing to us: ', other, ' Wanted ', raddr
  81                     raise bad_connect
  82                 data = eval(data)
  83                 if data[0] in (0,2):
  84                     curtijd = time.millitimer()
  85                     s.sendto(`(data[0]+1,data[1],curtijd)`,raddr)
  86                 elif data[0] == 4:
  87                     newtijd = time.millitimer()
  88                     histime = data[1]
  89                     mytime = timeavg(curtijd,newtijd)
  90                     #mytime = curtijd
  91                     self.timediff = histime-mytime
  92                     break
  93                 else:
  94                     print 'Funny data: ', data
  95                     raise bad_connect
  96         return self
  97         #
  98     def his2mine(self,tijd):
  99         return tijd - self.timediff
 100     #
 101     def mine2his(self, tijd):
 102         return tijd + self.timediff
 103
 104 def test(clt, host, port):
 105     xx = VTime().init(clt,host,port)
 106     print 'Time diff: ', xx.his2mine(0)