[Nsi-wg] time issue
Guy Roberts
Guy.Roberts at dante.net
Thu Sep 30 11:06:49 CDT 2010
I support Radek’s approach on this one – in the absence of deterministic setup times, let’s take a heuristic approach and rely on the operating experience gained from systems such as AutoBAHN. As we gain more operating experience we may be able to find a better solution for future NSI versions.
Guy
From: Radek Krzywania [mailto:radek.krzywania at man.poznan.pl]
Sent: 30 September 2010 16:52
To: 'Artur Barczyk'
Cc: nsi-wg at ogf.org
Subject: Re: [Nsi-wg] time issue
Hi,
There is no, and will be no such mechanism to define static, constant or even predictable connection activation time in distributed environment. What we do is estimates. You have a VC at 2pm, ask for circuit available at 1:40, as you were warned connection setup time may take up to 20 minutes. Then you have a guarantee. Please mind, v1 can’t solve everything. Let’s just create something and improve it step by step.
Yes, in above case you extend reservation time, which means you pay more (in theory, depending on pay model). But think in reverse direction – how do you know how long do you need a connection? You usually guess and adds something just in case. So you don’t use it in optimal way anyway. For v1 – I would not care much. If we try to restrict it in details, we will stack in discussion and complexity of the protocol and its mechanisms. I would rather keep it simple in contrary.
Best regards
Radek
________________________________________________________________________
Radoslaw Krzywania Network Research and Development
Poznan Supercomputing and
radek.krzywania at man.poznan.pl<mailto:radek.krzywania at man.poznan.pl> Networking Center
+48 61 850 25 26 http://www.man.poznan.pl
________________________________________________________________________
From: Artur Barczyk [mailto:Artur.Barczyk at cern.ch]
Sent: Thursday, September 30, 2010 5:25 PM
To: radek.krzywania at man.poznan.pl
Cc: 'Jerry Sobieski'; 'Jeff W.Boote'; nsi-wg at ogf.org
Subject: Re: [Nsi-wg] time issue
Hi Radek, All,
hmmmm, I for my part would be quite annoyed (to put it mildly), if I miss the first
15 minutes of todays HD conf call just because I reserved the resources a week
in advance. "Around" has no place in a well defined protocol. No fuzzy logic, please :-)
Consider also the "bored child in a car" scenario:
RA: are we there yet? PA: no... RA: are we there yet? PA: nooo.... RA: are we there yet? PA: NO! etc.
Be aware that users complaining are users quite quickly lost. You don't want that.
So let's consider two example users:
- high volume data transfers through a managed system: a data movement scheduler has
reserved some bandwidth at a given time. This time comes, the application will just
throw data on the network, it might use connection-less protocol, or not, but it will
result in an error. It cannot wait "around" 15 minutes, as it will bring the transfer schedule
in complete disorder. Such a "service" is just useless.
- video conferencing/streaming. You reserve the network resource for 3pm because your
meeting starts then. How do you explain to the video conference participant that the
network prevented the conference to start for "around" 15 minutes? (Well, you can, but
this will be the last time you'll see the user using your network :-) )
In short, the only reasonable thing to do is to put the right mechanism in place to
guarantee the service is up when the user requested it (and you confirmed it).
The only acceptable reason for failing this is an error condition like network down (and we'll
talk about protection in v2 :-) )
I also think it is very dangerous to use "providing a service" as argument while the underlying
protocols are not yet correctly specified. This is not theoretical, the service needs to be useful
to the end-user, if you want some uptake. Fuzzy statements make it useless. The very reason people
are interested in this is that it's deterministic - you know what you get and when. Otherwise use the
routed network. :-)
Cheers,
Artur
On 09/30/2010 03:37 PM, Radek Krzywania wrote:
Hi,
It’s getting hard to solve everything here, so let’s don’t try to solve everything here at once. So how about defining a start time as a best effort for v1? So we promise to deliver the service, yet we are unable to guarantee the exact start time in precision of seconds. If user want connection to be available at 2pm, it will be around that time, but we can’t guarantee when exactly (1:50, 2:01, 2:15). Let’s take a quite long time as a timeout (e.g. 20 minutes), and start booking the circuit in 5 or 10 minutes in advance (no discussion for v1, just best feeling guess) . The result will be that in most cases we will deliver the service at AROUND specified time. For v1 is enough, as we will be able to deliver a service, while in v2 we can discuss possible upgrades (unless our engineering approach discovers it’s fine enough :) ).
For #1 – it may a problem for instant reservations. Here user want a circuit ASAP. We define ASAP as (see above approach) less than 20 minutes (typically 5-10 minutes probably, but that’s my guess), or not at all. Users may or may not complain on that. In the first case we are good. For the second case we will need to design an upgrade for v2.
Synchronization IMHO is important, and out of scope at the same time. We can make an assumption that agents times are synchronized with precision of let say 10 seconds, which should be far enough. The agents will use system clocks, so they need to be synchronized at the end (NTP or whatever), but that not even implementation but deployment issue. So let put into specification: “NSI protocol requires time synchronization with precision not less than 10seconds”. If we discover it’s insufficient, let’s upgrade it for v2.
We already have some features to implement, just to see if it works fine (works at all, actually). If user is booking a circuit a week in advance, I guess he will not mind if we set it up 15 minutes after start time (user IS aware of that as we specify this in the protocol description). We can’t however deliver the service shorter than user defined time. So we can agree (by voting, not discussing) the fixed time values. My proposal is as above:
20 minutes for reservation as set up time
Service availability time (e.g. 13 h)
Service tear down time (it’s not important from user perspective, as since any segment of connection is removed, the service is not available any more, but let’s say 15 minutes)
In that way, calendar booking needs to have reserve resources for 13h 35 minutes. IMHO we can agree on that by simply vote for v1 (doodle maybe), and collect more detailed requirements for v2 later on. I get the feeling we started quite theoretical discussion based on assumptions and guessing “what if”, instead of focusing on delivering any service (event with limited guarantee).
Best regards
Radek
________________________________________________________________________
Radoslaw Krzywania Network Research and Development
Poznan Supercomputing and
radek.krzywania at man.poznan.pl<mailto:radek.krzywania at man.poznan.pl> Networking Center
+48 61 850 25 26 http://www.man.poznan.pl
________________________________________________________________________
From: nsi-wg-bounces at ogf.org<mailto:nsi-wg-bounces at ogf.org> [mailto:nsi-wg-bounces at ogf.org] On Behalf Of Jerry Sobieski
Sent: Wednesday, September 29, 2010 9:33 PM
To: Jeff W.Boote
Cc: nsi-wg at ogf.org<mailto:nsi-wg at ogf.org>
Subject: Re: [Nsi-wg] time issue
Ok. I can buy this approach of #1. The Requested Start Time is immutable as the request goes down the tree (which disallows #2) - it is still a Requested Start Time, but NSAs are not allowed to change requested start time as the request goes down the tree. But you can't prevent #3 if thats what an NSA somewhere down the tree decides to do. The result would be a promise he may not be able to keep - but thats acceptable because the Estimated Start Time is just an estimate, its not binding.
The point is, the local NSA cannot tell whether a remote NSA is using #1 or #3 since its totally up to the remote NSA to select the guard time appropriate for that request. Likewise, even if the remote NSA misses the Estimated Start Time, the requesting RA has no recourse other than to a) just wait until the provisioning completes or b) give up and release the connection. An SLA might influence the bad NSA to not low ball his provisioning guard time in the future, or it may provide a rebate for the jilted user, but these are not a protocol or a standards issue.
This goes to John's comment on the call today about what happens inside the NSA between the PA role and the RA role... These actions are captured in "state routines" that are invoked when protocol events occur. These actions are generalized in the standard, but any heuristics like these approaches to guard time cannot always be mandated. In a protocol standard, what ever components are "required" or "must" items, must be verifiable in a conformance test. I.e. if someone comes up with an NSI imlementation, we should be able to put the reference implementation against the test implementation and we should be able to tell via protocol operation if the implementation under test is doing all the "must" items. If we say an NSA must use #1 above, there is no way to test it and confirm that it is doing so. If the test implementation uses #3, the only outward sign is that it may miss the start time on some connection(s), but it could have as easily just been a poor judgment call on the provisioning time - which is ok.
So, in the standard, we can only recommend #1 be used. Or we can say the NSA "should" use #1. But we cannot require it.
my $.02
Jerry
Jeff W.Boote wrote:
On Sep 29, 2010, at 7:31 AM, Gigi Karmous-Edwards wrote:
Jerry,
For your question : " While the guard times may be network specific, we do need to at least consider what we would like an NSA to do if for instance a provisioning guard time pushes a reservation forward into a previous reservation: Do we 1) reject the request since we can't prepend our guard time and still make the Requested Start Time? OR 2) Do we retard the Estimated Start Time to allow for the guard time? OR 3) do we reduce the guard time to fit the available lead time?"
In my opinion, I think the answer here has to be # 1) each NSA must reject the request if their process to establish the connection requested can not meet the Start time. In my opinion an NSA should NOT be allowed to change the requested start time (this will cause all types of problems for other NSAs), so # 2) is not an option. The guard time for each NSA will most likely be vastly different and very dependent on the tools used by that network domain to configure the network elements for the requested path, so an individual guard time of an NSA is also nonnegotiable, so option # 3) is not an option.
I agree #1 seems the most deterministic.
I agree with Radek, ONLY Start times and End times should be used in the protocol and that guard times are only private functions of each individual NSA.
I agree with this. The guard times are not additive across each NSA. The guard time from the perspective of the user will effectively be the maximum of each NSAa guard time in the chain. But, the user doesn't care as long as provisioning is accomplished by the users requested start time. That time would be in the protocol and would remain unchanged through each step of the chain. And, it shouldn't matter how long it takes to tear down the circuit either as long as the circuit is available until their requested end time.
As to how to manage this time synchronization... I think it is totally reasonable to depend upon existing protocols. There are other protocols that already depend upon time synchronization, and many of them use NTP. We are not talking about needing very tight synchronization anyway. 1 second or even 10 seconds is plenty close enough. It is more about bounding that error.
jeff
Kind regards,
Gigi
On 9/29/10 8:45 AM, Jerry Sobieski wrote:
Hi Inder- I am not sure I agree with all of this...
Inder Monga wrote:
Radek
I agree with your statements;
User is not interested in partial results, as he/she is not even aware/interested in which NSAs/domains are involved. User doesn’t care (if everything works fine ;) ).
The protocol should be designed with the user in mind. The user does not care about guard time values, differences in setup times for MPLS vs optical lambdas, and concern itself with choices an NSA/NRM will make in path-finding.
The protocol designers can keep the user in mind, but the protocol is between the RA and the PA and and has a specific purpose: to reserve and instantiate a connection across the globe. We need to keep in mind that the RA is not always the end user - it is by definition another NSA and could be an NSA in the tree/chain somewhere. If we want to differentiate between the user and the network, then we can create a simplified User to Network API, and a different Network to Network API...but I don't think thats what we want to do (:-) We need to IMO *not* think about the user, but to think about the Requesting Agent - regardless of who it represents.
Perhaps once the RA-PA protocol is tightly defined in all its nuances, we can develop/recommend an end user API that simplifies the the application's required interactions ?? This would allow an application to embed an RA in a runtime library/module and the application itself would only have to deal with the basic connection requirements.... just a thought.
In my opinion,
a. the user should specify "Expected Start Time, Expected End Time". The NSAs/domains along the path determine resource availability and booking in their schedules based on their own configured guard time (guard times are not specified by NSI protocol. NSI connection service architecture should discuss them as a suggested concept).
While the guard times may be network specific, we do need to at least consider what we would like an NSA to do if for instance a provisioning guard time pushes a reservation forward into a previous reservation: Do we 1) reject the request since we can't prepend our guard time and still make the Requested Start Time? OR 2) Do we retard the Estimated Start Time to allow for the guard time? OR 3) do we reduce the guard time to fit the available lead time?
I think we now agree that the Start Time is just an estimate, due primarily to the guard time itself being just an estimate. So none of these times are etched in stone...So which option do we recommend or require? The protocol is sensitive to these various times - they cause timers to go off, messages to be sent, error handling to kick in... If they are adjusted during scheduling or provisioning, we MUST understand what impact they will have to the protocol and how that will be carried through the service tree.
b. Within reasonable limits, the connection should be up as close to the start time as possible. The user can set his own policy/configuration on how long to wait after the start time to accept a connection. Since the resources are guaranteed, this is a connection of setup/provisioning only. Hence, there is no protocol state transition when start time is passed other than the messages that indicate the circuit is established end to end or teardown message initiated by the client.
Ah, but the rub here is that the "user" is an RA...but not all RAs are the end user. We are defining the actions of an RA, regardless of whether it is a user NSA or an network NSA. So we must insure that if the RA gets tired of waiting for provisioning to complete, that whatever actions it is allowed to take will be consistent and predictable through out the service tree for all the RA/PA interactions. So the "user" actions are not irrelevant to the protocol.
c. We should not design a protocol that depends on time synchronization to work. In my opinion, the start time, expected time to provision aka guard time is best handled/shared as a SLA/Service definition issue.
I agree: We cannot expect perfectly/exactly synchronized clocks anywhere in the network. And therefore we cannot depend upon clock synchronization for any part of the protocol to work. Which implies that the protocol must work when the clocks are NOT synchronized. How do we insure this? --> rigorous protocol analysis.
While the values of certain timers may be left to the Service Definition/SLA, as I state before, we must make sure that the protocol can function predictably and consistently in the face of all possible timing permutations that are possible among NSAs. This rapidly gets very complex if we allow too many variables for the SD/SLA to define. Sometimes, its ok to identify constants that the protocol must use so that we can validate the protocol and simplify implementation and deployment. Indeed, often times when clocks are only slightly skewed they introduce race conditions that become more likely to occur requiring more careful consideration.
d. Similar semantics apply to the end-time as well.
Pretty much. Across the board, things like clock events, estimates, and service specific choices will create situations where we need to insure the protocol and state machines will function properly across the full range of possible permuted values. This is in general why protocol designers say "make it only as complex as it needs to be, and no more" - options breed complexity.
br
Jerry
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Dr Artur Barczyk
California Institute of Technology
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