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RE: [bacnet-mstpwg] Re: Should a node be allowed to do more than one token retry?

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  • Hartman, John
    I don t see how that differs much from just doing another token retry. Clause 9 specifies the same timeout (Tusage_delay, 15 msec) for a node to begin using
    Message 1 of 12 , Oct 8, 2012
      I don't see how that differs much from just doing another token retry. Clause 9 specifies the same timeout (Tusage_delay, 15 msec) for a node to begin using the token or respond to a PFM; and the same timeout (Tusage_timeout, 20 to 100 msec) for the sender of the token or PFM to time out.

      So the funk-ridden device would have the same amount of time to respond to either frame.

      Actually, since maintenance PFMs are common and token retries are (hopefully) rare, some Trane devices use a longer timeout (near 100 msec) for token retries and a shorter timeout (30 to 50 msec) on PFMs. That way, if the device is just SLOW in using the token (as opposed to having not seen the token at all), we give it more time in order to avoid the pain of dropping a device from the rotation. Using a 100 msec timeout on PFM could adversely affect the link bandwidth unless max-masters is kept tight, which isn't always possible or desirable.

      -----Original Message-----
      From: bacnet-mstpwg@yahoogroups.com [mailto:bacnet-mstpwg@yahoogroups.com] On Behalf Of kerlyn2001
      Sent: Thursday, October 04, 2012 12:36 PM
      To: bacnet-mstpwg@yahoogroups.com
      Subject: [bacnet-mstpwg] Re: Should a node be allowed to do more than one token retry?

      Rather than increasing the number of token retries, would it make sense (in the
      case of a lost token) for ThisStation to begin the PFM cycle at NS rather than
      NS+1? This would allow just a little extra time for NS to recover from its funk
      w/o adding to the overall token rotation time. I'll appeal to the institutional
      memory to see if this has been suggested before.

      -K-

      --- In bacnet-mstpwg@yahoogroups.com, "Hartman, John" <jhartman@...> wrote:
      >
      > Thanks for all the responses.
      >
      > I think I am spending too much time trying to figure out how to operate on links with lame devices. I started to write out the scenario where increasing the token retry helps (based on several third party devices that have caused us problems in the field), but it became clear that it helped because of the specific flaws in their implementation. And as several people pointed out, increasing the retry count could CAUSE problems in some other scenarios.
      >
      > So I think that the best approach may be for me to quietly allow this as a non-compliant setting that we can configure in the field as necessary, just as we can stretch Tusage_timeout beyond 100 msec to deal with various implementations that can't meet that (much less the 15 msec Tusage_delay they are SUPPOSED to meet).
      >
      > From: bacnet-mstpwg@yahoogroups.com [mailto:bacnet-mstpwg@yahoogroups.com] On Behalf Of Carl Neilson
      > Sent: Wednesday, July 18, 2012 4:30 PM
      > To: bacnet-mstpwg@yahoogroups.com
      > Subject: RE: [bacnet-mstpwg] Should a node be allowed to do more than one token retry?
      >
      >
      >
      >
      > My 2 cents:
      >
      > My concern with adding in retries for the token is that it makes it easier for lame implementations to slow the whole network down. I would hate to see implementation that always take 3 token passes before they pick it up.
      >
      > I expect that any device that does a reply-postponed does not go and build the response when it receives the token. The packet is built while it doesn't have the token. When the token is received, the processing is interrupted and any queued packet is sent out. So it really should not matter that the device is busy building some other response; it is either has the packet ready to go or it doesn't and the receipt of a token has to interrupt the normally app layer processing.
      >
      > We had no problem meeting the timing requirements back in 96, why would we have a problem now?
      >
      > Carl
      >
      > From: bacnet-mstpwg@yahoogroups.com [mailto:bacnet-mstpwg@yahoogroups.com] On Behalf Of Coleman Brumley
      > Sent: Wednesday, July 18, 2012 11:06 AM
      > To: bacnet-mstpwg@yahoogroups.com
      > Subject: RE: [bacnet-mstpwg] Should a node be allowed to do more than one token retry?
      >
      >
      > What John is proposing was discussed during a meeting recently (I forget where it was, but I believe it was within context of the new baud rates or making max-master configurable). The idea was, why drop the token and start a (potentially lengthy) PFM cycle when the NS "was just there a few milliseconds ago". I agree with John that a second (or even third) token retry isn't that much more overhead, and it's actually way faster than waiting for a complete PFM cycle.
      >
      > I don't think it was discarded, but it was tabled as outside of the scope of that particular proposal. I'm all for opening the discussion on this topic again as I think the concept is beneficial to MS/TP.
      >
      > From: bacnet-mstpwg@yahoogroups.com<mailto:bacnet-mstpwg@yahoogroups.com> [mailto:bacnet-mstpwg@yahoogroups.com]<mailto:[mailto:bacnet-mstpwg@yahoogroups.com]> On Behalf Of Clifford.H.Copass@...<mailto:Clifford.H.Copass@...>
      > Sent: Wednesday, July 18, 2012 1:58 PM
      > To: bacnet-mstpwg@yahoogroups.com<mailto:bacnet-mstpwg@yahoogroups.com>
      > Subject: Re: [bacnet-mstpwg] Should a node be allowed to do more than one token retry?
      >
      >
      >
      > My understanding is that one retry is part of the concept of minimizing the chance of multiple tokens being active at once. Since a sender not seeing activity could be either a sender fault or a receiver fault, more reties would increase the chance of multiple tokens being active (a good receiver could pass the token while a flaky sender is still retrying its previous token pass).
      >
      > If a token pass had a positive acknowledgement back to the sender (and all the extra performance robbing overhead that adds), I would agree with offering more retries. Lacking that, I am not so sure.
      >
      > In my experience, there is a greater chance of being dropped from the token ring than would be considered ideal, but it is seldom a real problem unless the communication reliability is bad enough that there are a lot of data messages being lost also. Thus when things start to "fall apart", there is a lot that falls apart, not just token passing.
      >
      > Cliff Copass
      > Johnson Controls, Inc.
      >
      >
      > From:
      >
      > "nomeekgeek" <jhartman@...<mailto:jhartman@...>>
      >
      > To:
      >
      > bacnet-mstpwg@yahoogroups.com<mailto:bacnet-mstpwg@yahoogroups.com>
      >
      > Date:
      >
      > 07/17/2012 02:09 PM
      >
      > Subject:
      >
      > [bacnet-mstpwg] Should a node be allowed to do more than one token retry?
      >
      >
      >
      >
      > ________________________________
      >
      >
      >
      > Clause 9 currently says that when a node passes the token, it waits for NS to begin using it. If no such usage is seen, the node may retry ONCE, after which it uses PFM to find a new NS.
      >
      > This seems a bit severe: most nodes will do 3 or more retries at the APDU level - not just one.
      >
      > Once a node has been dropped from the rotation, it is locked out until it is offered a PFM. The PFM poll may not come for 50 rotations. I have seen links with large max-info-frames where rotation time is 5 seconds or more, so it could be a looong time.
      >
      > If the dropped node uses postponed replies, either always or as part of transferring a segmented reply, it can easily become overwhelmed - other nodes continue to make requests of it, but responses must be queued until it can send previously queued responses.
      >
      > It would be trivial in both the standard and most implementations to increase the token retry count.
      >
      > Is this worth a formal proposal, or have the Olympians already considered this and discarded the idea?
      >
      >
      >
      > ------------------------------------
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