[Sois-tcoa] Time Distribution - again

[Abhijit Sengupta]

Continuing on time distribution services, here are some thoughts on other 
services, parameters, implementation etc etc

    * The time value might be used as the internationally accepted standard 
as year-month-date-T-time etc – for example 20050310T145959.45 means 550 
msec before 3pm of 10th March of 2005 at GMT.
    * Some implementation aspect: I presume the following:
        * when a user application A1 running on processor Pi invokes 
Time.Request on processor Pj, TDS running on Pi checks whether i = j,
                          i.      if true, Pi issues OS-dependent “time” 
command or accesses “time counter” in hardware to determine the current 
time and responds to A1 (through Time.indication)
                         ii.      if false, sends an “inquiry” message to 
TDS on Pj using MTS or some form of AMS (basically using some messaging 
service) and responds with received data (through Time.indication)
    * when TDS running on processor Pi receives an “inquiry” message from 
the messaging system,
                          i.      Pi extracts request token, sender id from 
message
                         ii.      Pi issues OS-dependent “time” command or 
accesses “time counter” in hardware to determine the current time
    * If using the messaging system, the timeout is used to define latency 
bound in QoS.
    * If lower layer returns QoS cannot be satisfied, Time.indication is 
issued with error condition – basically I am assuming if the timeout cannot 
be satisfied, then the inquiry does not go out.
    * A more realistic alternative to 3(d):
        * presuming timeout interval is not “critical” and it identifies 
after what interval the time value will be known, instead of timeout an 
interval driven mechanism
    * Data structure:
        * The messaging system on requesting side puts a time stamp of its 
own clock on the message and also the messaging system on recipient side 
puts a time stamp of its own clock when message is received.
        * Similar time stamp while sending responses
        * These provide some estimate of propagation delay and delay 
because of processing overhead on each side. Such estimate might be useful 
for time critical cases.



Abhijit


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