1. One-way Delay Measurement Vladimir Smotlacha vs@cesnet.cz TF-NGN, October 2002

2. One-way delay sender and receiver evaluate timestamps of packet OWD = Ts - Tr –Tr........packet receiving timestamp –Ts........ packet sending timestamp problem: time in both sites of measurement has to be synchronized

3. Box Specification Pentium III (450MHz and 860MHz) Linux kernel 2.4.16 with nanokernel patch NTP package (4.1.71) Rude/Crude 0.50 PPS connected to serial port

4. Box Configuration two workstation: traffic generator and receiver RUDE sends UDP stream, CRUDE captures it NTP configuration –setup I and II: server maxpoll 6 –setup IIa:server PPS processing (to evaluate exact OWD)

5. What was measured Ts - timestamp of packet sending (from CRUDE log) Tr - timestamp of packet receiving (from CRUDE log) Os - offset of sender clock (reported by NTP) Or - offset of receiver clock (reported by NTP) Ps - exact offset of sender clock (from PPS capture log) Pr - exact offset of receiver clock (from PPS capture log)

6. What was calculated Raw one-way delay obtained from CRUDE log OWD_r = Tr - Ts One-way delay corrected by estimated NTP offsets OWD_n = Tr - Or - (Ts - Os) Exact one-way delay calculated from GPS time OWD_e = Tr - Pr - (Ts - Ps)

7. OWD Measurement Setup I + II NTP_ A NTP_B | | PPS --> Box_A ----- network ----- Box_B <-- PPS NTP | PPS --> Box_A ----- network ----- Box_B <-- PPS

8. Results (setup I)

9. Error of measurement (setup I)

10. Results (setup II)

11. Error of measurement (setup II)

12. Results (setup IIa)

13. Error of measurement (setup IIa)

14. OWD Measurement Setup III NTP ---------- ----------- | \ / | PPS --> Box_A ----- network ----- Box_B <-- PPS GEANT ------> TELIA <------

15. Results (setup III) Cesnet -> Heanet (TELIA) red: measured OWD (about 28ms) green:exact OWD (about 37 ms) Heanet -> Cesnet (GEANT) red: measured OWD (about 28ms) green:exact OWD (about 20 ms)

16. Results (setup III) Cesnet -> Heanet (TELIA) red: measured OWD (about 28ms) green:exact OWD (about 36 ms) red: corrected OWD (about 28ms)

17. Results (setup III) Heanet -> Cesnet (GEANT) red: measured OWD (about 28ms) green:exact OWD (about 20 ms) red: corrected OWD (about 28ms)

18. Errors (setup III) Cesnet: error of NTP reported offset Heanet: error of NTP reported offset

19. Conclusions Max. error is less than round-trip delay between NTP server and client (site of measurement), estimated error is the asymmetry Setup I (local NTP server in each site of measurement) robust, estimated error in the order of 100 us assumed low offset between both NTP servers Setup II (one common NTP server) accuracy depends on NTP server position estimated error less than 1 ms in case of symmetric routing careful setup of ntpd necessary (differs from default ! )

20. Conclusions (cont.) Setup III (one NTP server, asymmetric routing ) stable asymmetry in OWD can not be detected mean value of measured OWD in both directions is the same estimated error of measurement is one half of the asymmetry

21. OWD Measurement Setup IV NTP_A ----- network ----- NTP_B | | institute_A network institute_B network | | NTP_A_sec NTP_B_sec | | (PPS --> ) Box_A Box_B ( <-- PPS)