1. WP3 Budapest SEP-2002 EDG Timestamping John Ryan, Brian Coghlan Trinity College Dublin

2. Contents EDG timestamping: overview Decisions made for TB1.2 Issues Future

3. EDG Timestamping Universal Time (UT/UT0/UT1/UT2): based on rotation of earth on its axis TAI (aka atomic time): based on caesium resonance Coordinated Universal Time is derived: UTC = TAI – leapseconds UTC kept within 0.9s of UT1 IERS (Paris) decides introduces +/- leapseconds, very irregularly

4. EDG Timestamping c consumes information Ia(ta) The problem: c a produces information Ia with timestamp ta Ia(ta) Sites a & c are corrected for leap seconds, b is not ba b consumes information Ia(ta) When b utilises Ia(ta) it must first convert to binary, this value is now incorrect, by as much as 32 seconds, but at least 1

5. EDG Timestamping Solution: all sites get leapsecond info, from same information source Uses R-GMA Obtains data from USNO ftp site. Publishes info via DBProducerServlet Timestamp class uses this info to correctly convert between formats

6. Timestamp obj Application Consumer Servlet (Local Cache) Leap Second data EDG Timestamping SQL-DB DB Producer Leapsecond sensor DB Producer Servlet U.S.N.O.

7. Decisions Made for TB1.2 Epoch: ASCII: UTC [date string] == 1-JAN-1972 binary: TAI [second count] == 1-JAN-1958 Binary fraction-of-second format Count of pS Accuracy/resolution info Preserved in ASCII if defined in binary Preserved in binary if defined in ASCII

8. Issues: CRON Scripts None in CVS tree yet Need scripts to: obtain leapsecond info from USNO NOT DONE populate cache files NOT DONE randomize the timing of these actions NOT DONE Must be able to be set up by LCFG

9. Future: Timestamp Formats TB1.2 ASCII format: does not preserve record of use of leapseconds CAN use lots of space on disk 2002-01-16T12:46:48.7567Z.00000001r.00001a 32 + 16 + 16 + 16 + 16 + 16 bits +.756 … 112 bits before fractional secs excluding T, Z, r and a: can be implied excluding accuracy and resolution excluding record of use of leapseconds

10. Future: Timestamp Formats TB1.2 binary format: Count of pS long[2] if accuracy & resolution NOT used long[4] if accuracy & resolution are used Fractional parts are fixed-format Issues : 4 longs use lots of space on disk does not preserve record of use of leapseconds cannot distinguish between EDG & UNIX time

11. Future: Timestamp Formats We propose to : add optional record of use of leapseconds last_leapsecond_update (L) field add to EDG ASCII and binary timestamps allows missed updates to be recognized add EDG packed binary timestamp format(s) particularly for secondary storage packed types use less space

12. Future: Timestamp Formats EDG packed binary timestamp formats: distinct EDG formats extensible typing using format type field automatic recognition of EDG formats include resolution & accuracy fields include last_leapsecond_update field preserves record of use of leapseconds default packed format = Long[2] counting pS

13. Future: Timestamp Formats Things to think about: Leapseconds may be dispensed with OGSA may define their own timestamp

14. EDG Timestamping THE END