On Mon, Apr 13, 2015 at 1:34 AM, peter van der Stok <[email protected]> wrote: > Dear all, > > Trying to work out all your recommendations, I fail to understand the need > for the solution. > > Large clients and small servers is not tru any more. > The assumption is that clients and servers are small, they are not > dynamically augmented with new modules. They only know about hash values and > have no knowledge what so ever of the corresponding names. >
The term "dynamically loaded" is misleading, because a new revision of the device, or a device that supports firmware upgrade, can have new YANG modules added. > Let's concentrate on the one module clash first. > Apparently, it is envisaged to load modules that already contain hash > clashes inside. > For me that is an unsolvable problem: There is one hash value that points to > different names (memory stores) in the server. > Servers only know about hash values, they have no name tables. Suppose one > of the names has been rehashed (where? certainly not within the server). > We might add a statement in the server that the old hash value goes to the > new rehashed value. > It is not clear to me how the server can decide which memory location goes > with the old hash value and which with the new value. > If a value gets rehashed, it is because there is a collision. I am not sure if module numbers can be added to YANG, but it should be possible to either avoid 1-module collisions or add a YANG extension to the module to resolve the collision so that every implementation uses the same rehash (known in advance). > Apparently, something needs to be done before the module is loaded into the > server. For me that means that only modules can be loaded without hash > clashes. > For modules intended for CoMI, we can certainly make sure no names used in the module produce any collisions. > Suppose in a given server two names from two modules clash. The solution is > then that the module ID distinguishes between the two clashing values. > It is then proposed that module names are hashed. That solution is the same > as saying that you need more than 32 bits for a hash value to reduce clash > probability. > > The only valid remaining approach is a registry that maps module name to a > 32 bit? ID. Transporting the module ID means additional transport costs. > It should be possible to come up with a short module-id format, and only use the long form (full module name) if no short-form exists. > I can imagine though that in a given installation the servers and their > module combinations are known. > Any clashes can be solved before load time. The appropriate code can be > loaded in the small clients and servers. > This does not allow for the old client/new server scenario to keep working, if the new server adds a module with a hash collision. > The problem arrives when a new server with a set of modules (including a new > module) finds a clash. > This clash does not affect the old clients and servers (they are not aware > of the new module) But what if the module they know about is the one that gets rehashed? There is no saved or canonical order for processing YANG modules. > Consequently, the clash can be solved by renaming the clashing name in the > newly added module. > Only if the collision is detected and corrected before the new module is published. > All clients, and servers which have to use the clashing name of the new > module will use the hash value of the renamed name, and do not need to be > aware of the rehashing. > > At the operational level, there are only unique hashes within an > installation. > At the organization level, the rehash value of the new module needs to be > registered and used when code for new clients or servers are generated. > > Consequently, I do not see a need for module IDs. YANG is modular, and different naming authorities (SDOs and vendors) work at different speeds and publish independently of each other. Applications can use different modules on the same server without impacting each other. Vendors can add their own modules before or after standard modules are written and added. If module A and module B both have objects that hash to the same value then they cannot be used together without re-hashing 1 of the objects. > > Any mistakes in the above? I think the combinations of modules that will be available on a given implementation cannot be controlled in advance. We cannot assume all client devices and server devices can be upgraded together (flag day upgrade). > > Peter > Andy > Michel Veillette schreef op 2015-04-02 20:37: >> >> If I summarize: >> >> Reducing the scope of uniqueness of YAND hash values to each module >> allows detection of hash collision(s) at design time instead of at run >> time. >> >> To reduce this scope, we need to: >> - Create a unique module identifier (e.g. 20 bits organization ID >> registered at IANA, 10 bits module ID) >> - Add this module identifier to the URI (e.g. one to five base64 >> characters) >> - Add a map at the root of the CBOR objects carry within the CoAP >> payload to associate module IDs with modules data nodes (e.g. one to >> five byes per module) >> >> The problem that still need to be resolved is how we document data >> nodes rehash offline. >> - One option is to use the description statement of the YANG module or >> a new YANG statement specifically added for this purpose. >> - Another option is create a IANA registry for these rehash values. >> >> Hash collisions within YANG modules are very unlikely to happen and >> the solution don't need to be highly scalable. In that respect, using >> a IANA registry might be a good solution since it can be used for >> already published yang modules and can be use after the fact with any >> future YANG modules. >> >> Michel Veillette >> System Architecture Director >> Trilliant Inc. >> Tel: 450-375-0556 ext. 237 >> [email protected] >> www.trilliantinc.com >> >> >> -----Original Message----- >> From: Pascal Thubert (pthubert) [mailto:[email protected]] >> Sent: 2 avril 2015 13:09 >> To: Carsten Bormann; Andy Bierman >> Cc: Michel Veillette; [email protected]; [email protected]; >> [email protected] >> Subject: RE: [6tisch] [core] COMI hash values globally unique vs. >> unique within a module >> >> Yes, we asked about that as well. >> >> It help to have a registry that guarantees the uniqueness of the first >> element in the path, so as to make it easier to check for collision >> within one path only. >> >> Cheers, >> >> Pascal >> >> >>> -----Original Message----- >>> From: 6tisch [mailto:[email protected]] On Behalf Of Carsten >>> Bormann >>> Sent: jeudi 2 avril 2015 18:41 >>> To: Andy Bierman >>> Cc: Michel Veillette; [email protected]; [email protected]; >>> [email protected] >>> Subject: Re: [6tisch] [core] COMI hash values globally unique vs. >>> unique within a module >>> >>> I think I'm losing track of what "this" is. >>> Assigning numbers when doing a (version of a) module should be doable, >>> even if it requires some additional tooling. >>> Assigning numbers to modules would require a registry. >>> No rehashing (or hashing at all) required. >>> (Now, that registry could be filled using a hash...) >>> >>> BTW, I'm not sure that the "rehashing" in the current spec is the >>> easiest way to handle collisions -- I also don't understand how >>> multiple paths that arrive at the same hash value are handled. >>> >>> Grüße, Carsten >>> >>> Andy Bierman wrote: >>> > On Thu, Apr 2, 2015 at 8:25 AM, Andy Bierman <[email protected]> >>> wrote: >>> >> Hi, >>> >> >>> >> I should make it clear that I was trying to use a module-id so the >>> >> objects in the module could be given a simple integer OID. >>> >> The goal was to support identifiers that could fit in 1 or 2 bytes. >>> >> But this was dropped when we changed to using hashes. >>> >> >>> >> It might be a huge effort to create and use numeric module IDs, but >>> >> YANG module names tend to be long strings. It might be worth it, if >>> >> a per-module hash is used. >>> >> >>> >> If the collisions were coupled to specific modules, then they might >>> >> be avoided before publication or the client can be hard-wired to >>> >> use the rehashed values from the start. >>> >> >>> >> A client would not need to know the XPath strings for any objects. >>> >> It could be hard-coded to work with module X, Y, Z, and it would >>> >> always work, no matter what additional modules were added to the >>> >> server. >>> >> This does seem very useful for constrained clients. >>> >> >>> > >>> > A standard, deterministic renaming algorithm would be needed to make >>> > this viable. If each server is free to choose its own rehash, then >>> > the client still needs to store all the strings, and resolve the >>> > collisions at run-time instead of compile-time. >>> > (So therefore a module-name or module-id would not solve anything). >>> > >>> >> Andy >>> > >>> > Andy >>> > >>> >> >>> >> On Thu, Apr 2, 2015 at 5:55 AM, Carsten Bormann <[email protected]> wrote: >>> >>> Michel Veillette wrote: >>> >>>> strings represents */5323 bytes/* >>> >>> If we want to free the implementations completely of having to use >>> >>> these strings, we'll need to assign numbers. This can be done at >>> >>> the module level, either manually or algorithmically. Using a >>> >>> per-module hash to do this is suboptimal; it is much better to >>> >>> fill a small >>> linear space. >>> >>> >>> >>> We then need a way to compose these numbers with numbers assigned >>> >>> to module IDs. SMIv2 had this way to name modules and their >>> >>> components... >>> >>> >>> >>> Grüße, Carsten >>> > >>> > _______________________________________________ >>> > 6tisch mailing list >>> > [email protected] >>> > https://www.ietf.org/mailman/listinfo/6tisch >>> > >>> > >>> >>> _______________________________________________ >>> 6tisch mailing list >>> [email protected] >>> https://www.ietf.org/mailman/listinfo/6tisch _______________________________________________ 6tisch mailing list [email protected] https://www.ietf.org/mailman/listinfo/6tisch
