Re: [Dime] Conclusion for Sequence Numbers - was Re: OVLI: comments to 4.3

[Steve Donovan <[e-mail]>]

Jouni,

We need the sequence number to be strictly increasing.  I don't see the
need for it to increase in uniform amounts.  Using time does fit these
requirements.  I'm ok with using time as long as we don't call the AVP
timestamp.

Ulrich does bring up an interesting use case, where a client is
receiving realm reports for the same realm from different agents.  We
need to define the clients behavior in this case. 

Presumably the client needs to be able to determine who generated the
realm report.  This cannot be determine based on the content of the
message or the connection on which the message arrived.  It seems like
we might need "Report Generator Diameter ID" in the overload report
specifically for Realm reports. 

Once the client is able to differentiate between realm reports sent by
different agents (or servers) we need logic defining how the client
deals with a new overload report. 

I see a couple of options (others will probably see options I am missing):

- Use the last received realm report - This introduces the possibility
of thrashing between two different reduction values and different
durations.  Note that this approach does not require the source of the
report to be included in the report.

- Only listen to one source of realm overload - The approach would be to
remember who sent the first overload report from the realm and ignore
realm overload reports from other sources.  This behavior would likely
be constrained to a single occurrence of realm overload.  Meaning that
the "memory" of the report source would only last as long as that
overload event persists.  Once the overload event goes away, the report
source would be forgotten and a new source could be used for the next
occurrence.

On the surface, the second approach looks better to me.

Steve

On 12/11/13 2:15 AM, Jouni wrote:
> Ulrich,
>
> I might be slow but.. Section 4.4 says
>
>    control endpoints.  The sequence number is only required to be unique
>    between two overload control endpoints and does not need to be
>
> Unique between two endpoints..
>
> Section 5.1 talks about endpoints:
>
>    of an arbitrary Diameter network.  The overload control information
>    is exchanged over on a "DOIC association" between two communication
>    endpoints.  The endpoints, namely the "reacting node" and the
>    "reporting node" do not need to be adjacent Diameter peer nodes, nor
>
> So if your agents inject realm reports, they need to be endpoints to the
> client. Similar to Figure 5. Therefore the sequence number spaces between
> C-A1 and C-A2 are separate.
>
> Now it is not clear to me, whether in your reasoning the C would see
> the server identity (as the endpoint) when there is an active "DEP
> agent" on the path. That would not clearly work and not be align with
> the endpoint assumption.
>
> Note that at some point of time we had (at least on a discussion level
> in f2f meeting) report originator identity in the OLR. That would make
> endpoint identification trivial. Now a "DEP agent" needs to act as a 
> "server" for its clients in order to appear as an endpoint.
>
> - Jouni
>
> ps: still think the use of Time is simpler..
>
>
> On Dec 11, 2013, at 9:43 AM, Wiehe, Ulrich (NSN - DE/Munich) wrote:
>
>> That's not predictable. It may be the same server in some cases, and different servers in other cases.
>>
>> -----Original Message-----
>> From: ext Jouni [mailto:[e-mail]] 
>> Sent: Wednesday, December 11, 2013 8:38 AM
>> To: Wiehe, Ulrich (NSN - DE/Munich)
>> Cc: Ben Campbell; [e-mail] list; Steve Donovan
>> Subject: Re: [Dime] Conclusion for Sequence Numbers - was Re: OVLI: comments to 4.3
>>
>>
>> Ulrich,
>>
>> On Dec 11, 2013, at 9:21 AM, Wiehe, Ulrich (NSN - DE/Munich) wrote:
>>
>>> Jouni,
>>>
>>> ad 1. "monotonically" does not express your intention. What we are looking for may be "stepwise with fixed step".
>>>
>>> Ad 2. Is not necessarily a mistake that could result in out-of-sequence sequence numbers. When a client C sends a realm-type requests towards any server in the realm, an agent A1 that selects the server would send back the realm-type OLR with sequence number s1. The next realm-type request sent by C (that survived the throttling) may take a path that does not include A1 but A2. A2 then selects the server and sends back a sequence number s2. Nothing ensures that s1 and s2 are in sequence.
>> Would the server in both cases (via A1 and A2) be the same?
>>
>> - Jouni
>>
>>
>>> Ulrich
>>>
>>>
>>> -----Original Message-----
>>> From: ext Jouni Korhonen [mailto:[e-mail]] 
>>> Sent: Tuesday, December 10, 2013 10:31 PM
>>> To: Wiehe, Ulrich (NSN - DE/Munich)
>>> Cc: Ben Campbell; [e-mail] list; Steve Donovan
>>> Subject: Re: [Dime] Conclusion for Sequence Numbers - was Re: OVLI: comments to 4.3
>>>
>>> Ulrich,
>>>
>>> On Dec 10, 2013, at 4:31 PM, "Wiehe, Ulrich (NSN - DE/Munich)" <[e-mail]> wrote:
>>>
>>>> Jouni,
>>>>
>>>> 1. I find the texts
>>>> a) "The sequence number ... does not need to be monotonically increasing"
>>>> and 
>>> Means the delta from old-seqno to new-seqno can be any non-negative integer
>>> (within the given limits) not something fixed step/delta (like +1). As long as
>>> "new-seqno >= old-seqno" holds we are fine.
>>>
>>>> b) "...the new sequence number MUST be greater or equal than the old sequence number..."
>>>> contradicting.
>>>> Can you please clarify.
>>> See above. (mind the overflow case)
>>>
>>>> 2. The expected behaviour when receiving an out-of-sequence sequence number within OC-OLR is described in 4.3:
>>>> "The receiver SHOULD discard an OC-OLR AVP with a sequence number that is less than previously received one."
>>>> I don't find this very robust. Once a higher sequence number (received erroneously by mistake) is accepted you cannot (easily) recover.
>>> I find it more robust in a sense that I should not care about stale old information.
>>> However, since we are piggybacking (by popular demand) we have little room for seqno
>>> re-sync negotiation.
>>>
>>> What is the mistake you refer here? A misbehaving implementation? In that case, it 
>>> deserves to get a manual intervention once figured out by admins checking alarms and
>>> logs. If the mistake is due other things, like endpoints being out of sync, we currently
>>> have no written down mechanism to survive that.
>>>
>>>> 3. The expected behaviour when receiving an out-of-sequence sequence number within the OC-Supported-Features AVP is not described. What is the intention here?
>>> No intention. Just a sloppy specification. You are right that something needs to be
>>> done & clarified here. (again the semantics of Time would nice..)
>>>
>>> I'll propose something. Others should too ;)
>>>
>>> - Jouni
>>>
>>>> Ulrich
>>>>
>>>> -----Original Message-----
>>>> From: DiME [mailto:[e-mail]] On Behalf Of ext Jouni Korhonen
>>>> Sent: Tuesday, December 10, 2013 8:28 AM
>>>> To: Ben Campbell; [e-mail] list; Steve Donovan
>>>> Subject: Re: [Dime] Conclusion for Sequence Numbers - was Re: OVLI: comments to 4.3
>>>>
>>>>
>>>> Fine.. lets define then the sequence number semantics. Basic
>>>> unsigned integer math. The text proposal is the following:
>>>>
>>>> 4.4.  OC-Sequence-Number AVP
>>>>
>>>> The OC-Sequence-Number AVP (AVP code TBD3) is type of Unsigned64.
>>>> Its usage in the context of the overload control is described in
>>>> Sections 4.1 and 4.3.
>>>>
>>>> From the functionality point of view, the OC-Sequence-Number AVP
>>>> MUST be used as a non-volatile increasing counter between two
>>>> overload control endpoints.  The sequence number is only required
>>>> to be unique between two overload control endpoints and does not
>>>> need to be monotonically increasing.
>>>>
>>>> When comparing two sequence numbers, the new sequence number MUST
>>>> be greater or equal than the old sequence number within a window
>>>> that is half of the size of the maximum sequence number. This
>>>> allows a simple handling of the sequence number overflow using
>>>> unsigned integer arithmeticf:
>>>>
>>>>   #define WINDOW 0x8000000000000000ULL
>>>>
>>>>   bool verify_seqnum( uint64_t newsn, uint64_t oldsn ) {
>>>>       if (newsn - oldsn <= WINDOW)
>>>>           // newsn >= oldsn
>>>>           return true;   
>>>>       } else
>>>>           // outside window or newsn < oldsn
>>>>           return false;  
>>>>       }
>>>>   }
>>>>
>>>>
>>>>
>>>> The above should even work is someone shovels NTP times into
>>>> sequence numbers with a blind typecasting.
>>>>
>>>> - Jouni
>>>>
>>>> On Dec 10, 2013, at 12:34 AM, Ben Campbell <[e-mail]> wrote:
>>>>
>>>>> On Dec 9, 2013, at 10:00 AM, Steve Donovan <[e-mail]> wrote:
>>>>>
>>>>>> Jouni,
>>>>>>
>>>>>> I propose that we keep the name OC-Sequence-Number but that we use the Time type for OC-Sequence-Number.  It is misleading and potentially confusing to call it OC-Time-Stamp.  
>>>>>>
>>>>> I could live with that, although I would rather just define the expected properties of the sequence number, and leave the implementation up to the implementor. I assume your reasoning for not calling it a timestamp is that you do not want people to try to use it as a time base reference. If so, then we don't require any connection to a clock. We just need it to be monotonically increasing.
>>>>>
>>>>>> We might consider expanding on the format of the AVP to make it something like Session-ID, where it is a concatenation of the Diameter-ID of the generating node and a timestamp.  This might help the reacting node keep track of which sequence number it has received.
>>>>>>
>>>>> Do we need a uniqueness across multiple nodes property? If so, why?
>>>>>
>>>>>> Steve
>>>>>>
>>>>>> On 12/9/13 5:37 AM, Jouni Korhonen wrote:
>>>>>>> Folks
>>>>>>>
>>>>>>> Could we conclude on the sequence number vs. time stamp vs. something else?
>>>>>>> We got more important places to spend our energy than this ;)
>>>>>>>
>>>>>>> My proposal is the following (based on the original pre-00 design):
>>>>>>>
>>>>>>> o We change the OC-Sequence-Number to OC-Time-Stamp in all occurrences
>>>>>>> in the -01.
>>>>>>> o We use RFC6733 Time type for the OC-Time-Stamp. RFC6733 gives us
>>>>>>> already exact definition how to handle the AVP.
>>>>>>> o Define that the OC-Time-Stamp is the time of the creation of the 
>>>>>>> "original" AVP within whose context the time stamp is present.
>>>>>>> o The OC-Time-Stamp AVP uniqueness is still considered to be in scope
>>>>>>> of the communicating endpoints.
>>>>>>> o The time stamp can be used to quickly determine if the content of
>>>>>>> the encapsulating AVP context has changed (among other properties).
>>>>>>> This would be useful specifically in the future when the encapsulating
>>>>>>> grouped AVPs  grow in size and functionality.
>>>>>>>
>>>>>>>
>>>>>>> - Jouni
>>>>>>>
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>>>>>>>
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