Description

Two-Way Active Measurement Protocol (TWAMP) defines an open protocol for measuring two-way or round-trip metrics with greater accuracy than other methods by using timestamps (processing delays can be factored as well).  This article explains why certain TWAMP OIDs might not be displayed. 
 

Symptoms

Over each collection of probes, TWAMP calculates statistics for several sets of measurements.  These sets include the following:

  1. roundTripTime -- the set of round trip delays
  2. rttJitter (posRttJitter) -- the set of positive round trip jitter measurements
  3. rttInterarrivalJitter (negRttJitter) -- the set of negative round trip jitter measurements
  4. egress -- the set of outgoing (source to destination) one-way delays
  5. egressJitter (posEgressJitter) -- the set of positive egress jitter measurements
  6. egressInterarrivalJitter (negEgressJitter) -- the set of negative egress jitter measurements
  7. ingress -- the set of incoming (destination to source) one-way delays
  8. ingressJitter (posIngressJitter) -- the set of positive ingress jitter measurements
  9. ingressInterarrivalJitter (negIngressJitter) -- the set of negative ingress jitter measurements

In the described scenario, it was noticed that ingress and egress measures are not collected:

root@> request services rpm twamp start client                       

root@> show snmp mib walk 1.3.6.1.4.1.2636.3.77.1.1.1.1.2.2.99.49.2.116.49   
jnxTwampResSampleValue.2.99.49.2.116.49.1 = 262
jnxTwampResSampleValue.2.99.49.2.116.49.2 = 16
jnxTwampResSampleValue.2.99.49.2.116.49.3 = 20
jnxTwampResSampleValue.2.99.49.2.116.49.5 = 13
jnxTwampResSampleValue.2.99.49.2.116.49.6 = 18
jnxTwampResSampleValue.2.99.49.2.116.49.8 = 2
jnxTwampResSampleValue.2.99.49.2.116.49.9 = 1

Solution

There are 4 timestamps used in TWAMP: 

  • T1: Session-Sender sending time
  • T2: Session-Reflector receiving time
  • T3: Session-Reflector sending time
  • T4: Session-Sender receiving time.
For most TWAMP OIDs, the calculation is based on the measurements from the same side. For example, RTT calculation will be (T4 - T1) - both values are from the sender side. 

However, the "egress" OID is calculated as (T2 - T1) and "ingress" OID is calculated as (T4 - T3). These are so-called "one-way OIDs", meaning that the values are taken from different sides.

This makes these OIDs dependent on the clock synchronization between the TWAMP client and the TWAMP server. Additionally, the values of "egress" and "ingress" must satisfy the following conditions, according to TWAMP specification:
  • They must be greater than 0
  • The sum of both must be less than the calculated RTT
When the clock is not synchronized, it might happen that the values will not meet the above requirements (for example, they might be negative).  In this case, they will be omitted. 

In scenarios where the RTT is smaller than the offset difference between TWAMP client and server, it is expected to lose the one way OIDs as they will probably be negative and not match the prerequisite to exist.  This behavior is expected. Even when external NTP server is used; as there might be a certain offset between the TWAMP client and the TWAMP server.  

Workaround:

Manually synchronize the clock with the NTP server right before running the TWAMP test using the command: ' set ntp update '
​This will make the values of "egress" and "ingress" OIDs accurate, bring the sync time between TWAMP client and server really close, so it meets the requirements and makes it possible for the values of OIDs 4 and 7 to be displayed.


 

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