Re: [ippm] Some though on draft-mirsky-ippm-twamp-light-yang-07

[Wei Luo S <[e-mail]>]

Please see my comments inline.

Thanks,
Wei Luo

From: Henrik Nydell [mailto:[e-mail]]
Sent: Tuesday, April 18, 2017 7:59 PM
To: Greg Mirsky <[e-mail]>
Cc: Wei Luo S <[e-mail]>; [e-mail]; [e-mail]
Subject: Re: [ippm] Some though on draft-mirsky-ippm-twamp-light-yang-07

I would ask you to consider adding some more valuable metrics
1) Loss burst size max


        leaf loss-burst-max {

                type int32;

                description

                "Highest number of lost packets back-to-back during interval.";

        }
[WEI] Agree, it’s valuable, I think we should add it. Besides, I suggest to add one more metrics: loss-burst-min, which is the minimum  number of lost packets back-to-back during interval.

2) Loss burst count (tells how many instances there was of packet loss during an interval, one "instance" means one or more consecutive packets lost.


        leaf loss-burst-count {

                type int32;

                description

                "Number of occasions with packet loss during interval.";

        }

To further explain the above metrics, consider 60-second interval below with 1 packet per second monitoring, where x means lost packet and o means recieved.

0                                                           60s
oooXooXooooXXXXooooooooooXXoooooXoooXooooooooXXXXXXXXXXXXooo

This 60s-interval has 22 lost packets ( 22/60 =36.67% loss)
Loss burst max is 12 packets, number of loss instances are 7

[WEI] Agree. It should be added. For stateful reflector, two more metrics can be added: loss-burst-count-far-end, loss-burst-count-near-end.

3) Percentiles are very useful and important. If they cannot be "configurable" in the Yang model I would suggest you to consider adding at least the 95th, 99th and 99.9th percentiles for all delay type metrics.
[WEI] Could you explain the meaning of “percentile” here?


On Thu, Apr 13, 2017 at 6:53 PM, Greg Mirsky <[e-mail]<mailto:[e-mail]>> wrote:
Dear All,
the new update of the TWAMP-Light YANG model<https://tools.ietf.org/html/draft-mirsky-ippm-twamp-light-yang-08> has been published. It includes the following:

  *   packet loss ratio as decimal64 type with fraction-size 5;
  *   session-reflector state parameter in session-sender container;
  *   defined continuous and periodic modes to execute a test session and how performance metrics are calculated in each of the modes;
  *   reporting of one-way packet loss metrics, both near-end and far-end, has dependency of reflector's mode.
Some questions still being discussed and we greatly appreciate suggestions, comments:

  *   include percentile in delay and delay-variation containers?
  *   default value for session-timeout in session-sender container is 900 seconds. Seems too big. What may be practical? Change units from seconds to centiseconds or milliseconds?
Regards,
Greg

On Tue, Mar 21, 2017 at 5:21 AM, Henrik Nydell <[e-mail]<mailto:[e-mail]>> wrote:
Some comments from the "field" as Accedian has several hundred thousand TWAMP sessions running (continously) at numerous Tier one mobile/fixed operators globally.

On Tue, Mar 21, 2017 at 11:52 AM, Wei Luo S <[e-mail]<mailto:[e-mail]>> wrote:
Hi Greg,

Thanks a lot for your response. Please see my reply inline tagged [WEI>>].

Regards,
Wei Luo

From: Greg Mirsky [mailto:[e-mail]<mailto:[e-mail]>]
Sent: Tuesday, March 21, 2017 1:16 AM
To: Wei Luo S <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: Re: Some though on draft-mirsky-ippm-twamp-light-yang-07

Hi Wei Luo,
many thanks for your thorough review and the most helpful comments to the TWAMP Light(Test) model. Please find my answers, notes in-line tagged GIM>>.

Regards,
Greg

On Sat, Mar 18, 2017 at 4:27 AM, Wei Luo S <[e-mail]<mailto:[e-mail]>> wrote:
Hi Greg & Adrian,

This is Wei Luo from Ericsson. I work on TWAMP light area in Ericsson. The current TWAMP Light YANG model is well defined. Thanks for your great job.
But by working closely with our customers, we got some new user cases on TWAMP light. I believe these user cases are valuable and popular enough to be modeled in TWAMP Light YANG. I hope I can be a contributor  and co-work with you move this draft forward.
I  drafted a new version of the TWAMP light YANG model based on version [e-mail]<mailto:[e-mail]>. Could you please comments on it? Any discussion is welcome.
The draft yang model and tree is attached. To make you find the updates quickly, I highlighted all the updates in file ietf-twamp-light-weiluo.pdf.

The following are the list of main updates:
1. Add a new typedef: percent. This is a new type defined for packet loss ratio.
Consideration:
1). From the customer perspective, packet loss ratio is a more meaningful data. In most of the time, the absolute number is meaningless to user, especially they do the TWAMP test continuously. They are more care about the ratio than the absolute number. So adding it makes this model more friendly to customer;
2). From the service layer assurance(SLA) perspective, the packet loss ratio is a major measures. So with adding packet loss ratio in model, the TWAMP can work in SLA framework more smoothly.
3). It seems some similar protocol’s YANG model has the same definition, e.g. ‘Service OAM Performance Monitoring YANG Module’, https://www.mef.net/Assets/Technical_Specifications/PDF/MEF_39.pdf.
Agreed packet loss is important, however another important loss metric is loss burst size (max/min) and number of loss bursts. A loss burst of 10 consecutive TWAMP-test packets can be deemed more serious than 10 lost packets spread evenly over the report interval.
GIM>> Indeed, packet loss more often expressed as packet loss ratio rather than as the absolute number. It would be most helpful to hear from network operators if they see introduction of Packet Loss Ratio into the TWAMP model helpful.
2. Add a new typedef: state-mode. It defines a common type for stateful/stateless reflector. This type will be used in both sender session and reflector session.
Consideration:
If the reflector is stateful, the TWAMP light can measure more items, e.g. one way packet loss. So for sender, the stats calculation and show is different. When the reflector is stateless, it doesn’t need to calculate the one way packet loss. The one way packet loss is invalid and shouldn’t be presented to customer. When the reflector is stateless, the sender needs to calculate the one way packet loss. And the data should be present to customer. So this is used as a ‘when’ condition in the model’s RO tree.
GIM>> Yes, if Session-Sender is aware of the mode corresponding Session-Reflector operates, the sender may avoid calculation of some performance metrics, e.g., one-way packet loss. On the other hand, the orchestrator is aware of the state-mode and should be capable to properly use metrics reported by the Session-Sender.
[WEI>>] Yes, the orchestrator could know that. But from the model side, this is not correct.  The model should represent the right behavior and shouldn’t do assumption on orchestrator.
I agree the model should describe both one-way loss metrics and roundtrip loss metrics, and the sender should be able to use either mode when calculating, potentially also populating the roundtrip delay values with proper t1-t0 + t3-t2 values, as well as reporting the t2-t1 values that would indicate buffer load/CPU load in the TWAMP responders processing time.
3. Add a new typedef: send-mode. This is a new type for sender session. It makes the sender session can send packet continuously and monitor the network all the time.
Consideration:
The user case is that: the user runs TWAMP light sessions to watch links quality continuously. The session number could be very big. These TWAMP sessions are managed by SLA framework or similar. SLA retrieves the stats from TWAMP periodically, e.g. 15mins. In other words, all the performance metrics are calculated based on the packets sent/received within 15mins. This makes the calculation become possible. With the periodical stats data, the Network Management software can do further actions if some abnormal stats observed.  This is a more general user case in customer site. While the non-continuous TWAMP sender session is generally used for debugging purpose on a link.
GIM>> I think that support of continuous measurement is in LMAP domain, not for TWAMP Test data model. To conduct continuous measurement he LMAP Controller, in my opinion, programs the Measurement Agent to perform TWAMP Test session with certain set of parameters and repeat it without any interval (interval = 0).

Many operators use TWAMP in continous mode, not only with Accedian test points and report at fixed intervals, typically ranging from 5s to 5 or 15 minutes, with 1-minute being the most popular granularity currently. The advantage is that the result calculation can be handled separately from the TWAMP-test sending/recieving, so that there is no parallelism required to monitor 24/7. If a start-stop-based methodology is used, the sender needs to start up the new test session even before the previous one has ended, since the previous session needs to wait X seconds (or at least Y 100s of milliseconds) before it stops waiting for packets to come back. And this new session needs to have a different signature in order for the sender to discern which packets belong to the previous interval and which belong to the current.

In a continous test-model, the sender can just simply record the sequence number of the last packet transmitted in the interval to be reported, wait for it to come back, or a MAXTIME, then report that result, while continuing to transmit for the next interval.

If the "interval==0" parameter is intended to be used for continous type tests, then what parameter should indicate to the sender at what intervals to produce results?
4. Add a new group: packet-loss-statistics. It grouping two packet loss statistics: loss-count and loss-ratio. This group will be used in RO stats tree.
GIM>> I'd like to continue discussion.
[WEI>>] OK.
5. Move leaf dscp out from grouping session-light-parameters. The leaf dscp is only valid when the dscp-handling-mode is use-configured-value. A when condition shall be added to it. So it can’t be in this group.
GIM>> I'm concerned that then the model will not be able to support concurrent TWAMP Test sessions between the same pair of Test Points (IP address+port number) at different CoS markings.
[WEI>>] Actually, I have concern on using five tuple(IP address+port number+dscp) to identify a TWAMP test session. The DSCP is not a constant value in packet. It could be modified by the routers in the path. For example, the sender has two sessions: session A’s five tuple is: Sip=1.1.1.1, Dip=2.2.2.2, Sport=50000, Dport=50001, DSCP=cs2. Session B’s five tuple is: Sip=1.1.1.1, Dip=2.2.2.2, Sport=50000, Dport=50001, DSCP=cs3. The only difference between session A and session B is DSCP. If the test packet’s DSCP of session B is modified to cs2 by a router in the path. The five tuples are exactly the same for reflector. It can’t differentiate which packet is from session A, which packet is from session B. It could mess the reflector’s session sequence number. And also, the sender will be messed because the received reply packet’s five tuple are exactly the same.
So I think it’s more reasonable to use four tuple to identify a session.

Yes, this would be appreciated by users. Changes in DSCP is a reasonably common network error that users can detect with continous TWAMP monitoring, thus it is good to not include the DSCP value as part of the "session identifiier" but instead use 4-tuple with UDP source port to identify several parallel flows between the same sender and responder.
6. Add leaf 'session-packet-send-mode' to /twamp-light/twamp-light-session-sender/test-session*. This leaf specifies the sender session's packet send mode: continuous or non-continuous.
GIM>> As discussed in #3, I think that it is already part of LMAP YANG model.
7. Add leaf 'reflector-light-mode-state' to /twamp-light/twamp-light-session-sender/test-session*. This leaf indicates the the reflector's mode: stateful or stateless. If the reflector's mode is stateful. Two one way packet loss statistics can be got: one-way-packet-loss-far-end, one-way-packet-loss-near-end.
Consideration:
Only valid data should be presented to user. Otherwise it could misleading user in some cases.
GIM>> A in response to #2.
8. Modify leaf /twamp-light/twamp-light-session-sender/test-session*/number-of-packets. Add a 'when' condition to this leaf. When send-mode is 'continuous', the leaf number-of-packets is meaningless. So add a 'when' condition to limit it.  Besides, added a default value ‘10’ to it. When the send-mode is 'non-continuous', the session can't work with an empty number-of-packets.
GIM>> As I've noted in #3. Will add default.
9. Add leaf time out to /twamp-light/twamp-light-session-sender/test-session*. A timeout mechanism is needed when the sender session can't get all the reply packets for a long time.
GIM>> Thank you, will add in the next update.
10. Modify leaf /twamp-light/twamp-light-session-sender/test-session*/interval. Change the units from ‘microseconds’ to ‘milliseconds’. Add a default value 1000.
Consideration:
    1). The aim of TWAMP is to measure network quality, but not fast failure detection. So a millisecond packet interval is enough.
    2). Interval is a necessary parameter for a session. A sender session can't work with an empty packet send interval. So added a default value to it.
GIM>> Thank you. We've made units of interval microseconds in the last update already. I think that changing to milliseconds may be too restrictive, limit use cases for TWAMP Test. Will add default value with the next update.
[WEI>>] Sorry, I do not see the reason. Are there any user cases to use microseconds?
11. Add leaf 'dscp' to /twamp-light/twamp-light-session-sender/test-session*. This is the leaf moved out from grouping session-light-parameters.
GIM>> As noted in response #5, the change may limit ability to run concurrent TWAMP Test sessions per CoS. I consider that to be valuable mode but would like to hear from network operators if that is indeed useful information.

See my comment above. I argue that it is useful to keep track of changing DSCP values, and treating DSCP as a metric of the TWAMP Session just like loss and delay
12. Move leaves 'ref-wait', 'reflector-light-mode-state' and 'dscp-handling-mode' from /twamp-light/twamp-light-session-reflector to /twamp-light/twamp-light-session-reflector/test-session*. These three attributes should be session specific. Different session could have different values. They are not common attributes.
GIM>> Agree, will make it in the next update.
13. Add leaf 'dscp' to /twamp-light/twamp-light-session-reflector/test-session*. This is the leaf moved out from grouping session-light-parameters. Besides the movement, added a 'when' condition to the leaf 'dscp'. This leaf is only valid when the dscp-handling-mode is 'use-configured-value'.
GIM>> As response to #5.
14. Modify leaf /twamp-light-state/twamp-light-session-sender-state/test-session-state*/current-stats/number-of-packets. Add a 'when' condition to this leaf. When send-mode is 'continuous', the leaf number-of-packets is meaningless.
GIM>> Similar to #3.
15. Modify leaf /twamp-light-state/twamp-light-session-sender-state/test-session-state*/current-stats/interval. Change the units from microseconds to milliseconds.
GIM>> I think that microseconds is reasonable.
16. Add leaves 'two-way-packet-loss', 'one-way-packet-loss-far-end' and 'one-way-packet-loss-near-end' to /twamp-light-state/twamp-light-session-sender-state/test-session-state*/current-stats/. These are the new statistics for stateful reflector.
GIM>> Thank you, will be coming in the next update.
17. Remove leaf loss-packet in /twamp-light-state/twamp-light-session-sender-state/test-session-state*/current-stats. The loss packeted is replaced with 'two-way-packet-loss' stated above.
GIM>> Agree.
18. Modify leaf to /twamp-light-state/twamp-light-session-sender-state/test-session-state*/history-stats*/interval. Change the units from microseconds to milliseconds.
GIM>> I think that will limit applicability of TWAMP Test.
19. Add leaves 'two-way-packet-loss', 'one-way-packet-loss-far-end' and 'one-way-packet-loss-near-end' to /twamp-light-state/twamp-light-session-sender-state/test-session-state*/history-stats*/. These are the new statistics for stateful reflector.
GIM>> Agree.

Thanks,
Wei Luo


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