Terry L Anderson wrote:
See comments below.
Rich Bowen wrote:
Robustness folks,
A couple of comments on TD-42/Osaka -
- While adding the ASN.1 from TD-42 to the draft H.225.0 v4, I noticed
the following paragraph about using StatusInquiry as a keepalive mechanism:
"The element closer to the called party shall send StatusInquiry periodically (this is the direction of least traffic during established calls). The period should be configurable. Two seconds is the recommended default, in order to allow detection of failure before other messages timeout. The selected value must be added to StatusInquiry so that the recipient can also monitor failure without an additional StatusInquiry/Status exchange in the opposite direction. The recipient system needs only to maintain a timer using the indicated value as a timeout."
This clause indicates that the period at which StatusInquiry is sent is also the amount of time that the receiving system should use as a timeToLive timer. This arrangement would result in a race condition since the next StatusInquiry would be expected at the exact moment that the timeToLive timer was due to expire.
IMHO, the StatusInquiry period and the timeToLive should be separately configurable, both having default values defined in Annex R, with the obvious restriction that timeToLive shall be greater than the StatusInquiry period.
What we had in mind is essentially a duplication of the RAS lightweight registration method. There timeToLive is indicated by either GW in RRQ or GK in RCF (RCF is the definitive) and is the only parameter. It is up to the GW to register enough earlier or the GK to allow registration to live enough longer to avoid the race. I would guess in practice that most systems do both. I see no reason the same cannot work here. I would assume that the recipient would add some suitable padding to the time before concluding failure. I see no reason to carry yet one more field in the message.
Actually the way the timeToLive has been handled in the LW RRQ has created some confusion in deployment, because different vendors use different algorithms to determine when to send the next RRQ. I think a standard approach makes network administration simpler.
A more typical approach in other protocols I'm familiar with is to define the amount of padding, and optionally allow it to be configurable (e.g., via a MIB object) to allow network tuning. Note that this doesn't add another field to the message, because there's no need to communicate the padding value between endpoints.
For example, one approach might be for the called endpoint to set the timeToLive sent in the StatusInquiry to be the sending period plus a padding value. The padding value could be defined in the standard to have some range, with a default value of say 1000ms. No padding would be necessary at the calling endpoint.
- Rich
- Has consideration been given to adding the ability for the endpoint
to indicate to the GK via the ARQ that a particular call requires robustness, or to allowing the GK to indicate to the EP that the EP must use the robustness procedure for a particular call? Several other features added in v4 use this model and it seems to be a useful framework.
I think that the group has assumed that robustness is a property of network elements - either it has it for all calls or none. But we could certainly consider it on a per call basis if this seems useful. I suppose since there is some processing cost associated with the mechanism, one might choose to NOT use it for some calls even when it is available.
Do others think a per call choice is useful?
- Rich
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