The comments below convey one or two valid points, but are marred by failure to recognize that different functions within H.323 have different message routing requirements. The valid points are: - efficient handling of LRQs is tricky (but getting around that is what a good bit of the Annex G work is all about) - if a call crosses several administrative domains, each domain has to have a chance to authorize the use of the resources to be consumed by the call.
My further comments are interspersed with those of Mr. Roy.
Tom Taylor
-----Original Message----- From: Roy, Radhika R, ALTEC [SMTP:rrroy@ATT.COM] Sent: Thursday, August 20, 1998 12:35 AM To: ITU-SG16@MAILBAG.INTEL.COM Subject: Re: Contribution (Inter-GK Communications)
Hi Everyone:
Here are my comments on APC-1422 Hierarchical Model Based VideoServer's Proposal:
It is appreciable that Mr. Santo Wiaryman, VideoServer, presented an addressing scheme that ranges from multiple zones to multiple domains. The basic idea of the addressing scheme related to the zone and the domain is the core of the proposal, and is applicable in other situations as well no matter whether the architectural model is hierarchical or non-hierarchical. The opportunities and problems that are presented are real. This contribution has increased our understanding related to the addressing scheme in the context of zones and domains.
However, the addressing scheme has been applied using the model specified in APC-1422 that uses hierarchical architecture using the border GKs. In this context, the following comments will reveal the fundamental aspects of the model proposed in APC-1422 considering the insights that have provided by this proposal:
It appears that a sort of routing scheme(s) needs to be used forsending the addressing information from a source zone GK through a series of hierarchical border GKs up to the root border GK, and from the root border GK through a series border GKs to the destination zone GK. 2. No mechanism is proposed in the signaling messages to protect against routing loops when the "abstraction" of routing is made between the source zone GK , a series of hierarchical border GKs, and the destination zone GK.
[Taylor, Tom [CAR:B318-I:EXCH]] First valid point: handling of LRQs or their functional equivalent is tricky. Your proposal to add time-to-live to LRQs seems to have been accepted.
Is there any inter-GK protocol messages (e.g., resourceavailability) needed between the zone GK and the border GK and between the border GKs other than the zone messages considering the (networking) configurations of the GKs? 4. It appears that a root border GK needs to be defined. Who will decide the root GK from which a hierarchy will be establsihed? Is it any international organization like IANA? 5. The path between the source and the destination GK is the pre-specified hierarchical logical path, and may not be optimal between the source-destination GK.
[Taylor, Tom [CAR:B318-I:EXCH]] Postulating such a path, it is used only to distribute addressing information. Call signaling and media packet routing do not need to follow the same path, and in fact most of the routing decisions are made at the transport rather than the application level.
The signaling message only passes through the source and thedestination zone GK, and other hierarchical GKs. As a result, if a call is established between the source-destination path, the call may have to pass through many "intermediate zones" in addition to the source and destination zone. Consequently, the intermediate zone GKs will be completely unable to play any roles.
[Taylor, Tom [CAR:B318-I:EXCH]] There is a kernel of truth here, the second valid point I noted above. However, the RAS messages you list below are designed for communication between the endpoint and its Gatekeeper. There are other solutions to the authorization problem (e.g. use of RADIUS/DIAMETER) besides propagation of these messages. Moreover, authorization will not typically be done by the Gatekeeper itself, but by an authorization server which it consults. It will be more efficient fto communicate with the authorization server for each administrative domain directly wherever possible rather than invoke a chain of Gatekeepers to do it indirectly.
For example, bandwidth/QOS resources that are supposed to be allocated in each zone by each GK between the source-destination path before placing the call cannot be done. The RAS messages such as ARQ/ACF/ARJ, BRQ/BCJ/BRJ, URQ/UCF/URJ, DRQ/DCF/DRJ, RAI/RAC, and others may not be able to play proper roles for all zones between the source-destination path through which a call is established. 7. Is there any solution provided by the model described in APC-1422 if the zone boundaries become logical instead of physical?
[Taylor, Tom [CAR:B318-I:EXCH]] Not clear how a zone as defined in H.323 is anything but logical.
This simple example presented by VideoServer using the proposed APC-1422 model can also lead to a very high-level comparison between the hierarchical and non-hierarchical model. The following table may provide a high-level summary of comparison between the two models:
Table 1: High Level Summary of Comparsion
Description Hierarchical Model: APC-1422/Example VideoServer Proposal Non-Hierarchical Model: AT&T's Proposal Remarks
Routing between the GKs Routing is needed:
Static routing through the pre-specified logical pathNo scope for path optimizationNo mechanism for avoiding loops Routing is needed:Dynamic routing between the source-destination GKsPath is optimizedMechanism is provided to avoid loops(Static routing can also be done if needed) Non-hierarchicalmodel appears to be much superior ARQ/ACF/ARJ Bandwidth/QOS allocation cannot be confirmed because the signaling message does not pass through all zones between the source-destination path. Bandwidth/QOS allocation can be confirmed because the signaling message passes through all zones between the source-destination path Non-hierarchical model appears to be much superior
BRQ/BCJ/BRJ Bandwidth/QOS change cannot be confirmed because the signaling message does not pass through all zones between the source-destination path. Bandwidth/QOS change can be confirmed because the signaling message passes through all zones between the source-destination path Non-hierarchical model appears to be much superior
All RAS signaling messages that may have implications for all zones between the source-destination path of the call Signaling messages cannot pass through the intermediate zones Signaling messages can pass through all zones between the source-destination path Non-hierarchical model appears to be much superior Root-GK A root-GK needs to be defined (does it mean to have an international authority like IANA?) No need to define a root-GK Non-hierarchical model appears to be much superior Logical zone boundary Probably cannot be defined (may be limited to physical zone boundaries only) Can be defined (in addition to physical one) Non-hierarchical model appears to be much superior
If you have any questions, please let me know.
Thanks and regards,
Radhika R. Roy AT&T, USA Tel: +1 732 949 8657 Email: rrroy@att.com