[c-nsp] understanding the IP SLA "icmp-jitter" calculations

[Martin T]

Hi,

I configured an icmp-jitter type of IP SLA entry in Cisco CSR1000V
router and enabled IP SLA debug. Following two sequential debug
messages show the sending of the ICMP "Timestamp Request" and
receiving of the ICMP "Timestamp Reply" message:

*Apr  4 09:55:25.095: IPSLA-OPER_TRACE:OPER:300 Sending ID: 38

*Apr  4 09:55:25.396: IPSLA-OPER_TRACE:OPER:300 Rev Seq: 1, Id: 38
        STS: 2183208743  RTD: 35723058 STD: 35723058  RTS: 35725395


This 2183208743 decimal timestamp(10000010001000010001111100100111 in
binary) is a non-standard
value(https://tools.ietf.org/html/rfc792#page-17) as the most
significant bit is set. 10001000010001111100100111 converts to
35725095 in decimal, i.e debug output above can be translated into:

*Apr  4 09:55:25.396: IPSLA-OPER_TRACE:OPER:300 Rev Seq: 1, Id: 38
        STS: 35725095  RTD: 35723058 STD: 35723058  RTS: 35725395

This means that ICMP "Timestamp Request" reached the destination
-2037(35723058-35725095) milliseconds later. This makes sense because
the system time in destination is set to be bit more than two seconds
behind. Processing in destination took less than a
millisecond(35723058-35723058). ICMP "Timestamp Reply" was sent at
35723058 and reached the router at 35725395, i.e time difference is
2337ms. 2037ms is the clock difference and 300ms added by server
egress queue discipline. This 300ms difference can also be seen in
debug messages timestamps, i.e "Apr  4 09:55:25.095" vs "Apr  4
09:55:25.396". In short, the numbers make perfect sense.

However, when I check the IP SLA statistics, then it shows that RTT
was 1ms which is clearly wrong:

IPSLAs Latest Operation Statistics

IPSLA operation id: 300
Type of operation: icmp-jitter
        Latest RTT: 1 milliseconds
Latest operation start time: 09:55:25 UTC Thu Apr 4 2019
Latest operation return code: OK
RTT Values:
        Number Of RTT: 1                RTT Min/Avg/Max: 1/1/1 milliseconds
Latency one-way time:
        Number of Latency one-way Samples: 0
        Source to Destination Latency one way Min/Avg/Max: 0/0/0 milliseconds
        Destination to Source Latency one way Min/Avg/Max: 0/0/0 milliseconds
Jitter Time:
        Number of SD Jitter Samples: 0
        Number of DS Jitter Samples: 0
        Source to Destination Jitter Min/Avg/Max: 0/0/0 milliseconds
        Destination to Source Jitter Min/Avg/Max: 0/0/0 milliseconds
Over Threshold:
        Number Of RTT Over Threshold: 0 (0%)
Packet Late Arrival: 0
Out Of Sequence: 0
        Source to Destination: 0        Destination to Source 0
        In both Directions: 0
Packet Skipped: 0       Packet Unprocessed: 0
Packet Loss: 0
        Loss Periods Number: 0
        Loss Period Length Min/Max: 0/0
        Inter Loss Period Length Min/Max: 0/0
Number of successes: 1
Number of failures: 0
Operation time to live: Forever


1) Why does "sh ip sla statistics" report 1ms RTT?
2) Does "icmp-jitter" expect the timestamps to be STS < RTD < STD <
RTS and if not, then no calculations are made?
3) How practical such measurements are as even a small, few
millisecond time offset in remote host, can screw up the latency
measurement results?


thanks,
Martin