- From: Drummond Reed <drummond.reed@cordance.net>
- Date: Thu, 21 Aug 2008 12:24:09 -0700
- To: "'Williams, Stuart (HP Labs, Bristol)'" <skw@hp.com>, "'John Bradley'" <john.bradley@wingaa.com>
- Cc: "'Mark Baker'" <distobj@acm.org>, "'Henry S. Thompson'" <ht@inf.ed.ac.uk>, <www-tag@w3.org>
- Message-ID: <080201c903c3$7cc52250$0500a8c0@ELROND>
This thread has grown long and somewhat difficult to follow. However because it’s important to the XRI discussion, I offer the following as a summary of some of the key points (and then will start a separate thread on a related topic): 1) A persistent XRI (an XRI consisting entirely of persistent subsegments, often called an i-number) is defined in XRI Syntax 2.0 [1] as meeting the functional requirements of a URN as specified in RFC 1737 [2]. 2) That means that a persistent XRI is a claim by the identifier authority assigning it that the identification relationship (graph arc) between the persistent XRI and the resource it identifies (graph node) will never be reassigned, i.e., the arc will never be changed to point at a different node. For example, if the XRI is assigned to identify a specific person, the claim is that it will always identify that specific person (even after they die). If it is assigned to identify a specific organization, the claim is that it will always represent that specific organization (even after it is merged into another). If it is assigned to represent a specific document, the claim will always represent that specific document (even if it is subsequently renamed, moved, or becomes unavailable on the network). 3) In URN architecture, a resolvable URN is resolved to a descriptor of the target resource called a URC (Uniform Resource Characteristics) (see section 1 of [1]). The URC contains metadata describing the target resource, which typically includes (but is not required to include) concrete identifiers or “locators” of the resource. 4) In XRI architecture, the URC is an XRDS document. XRDS documents enable mapping of an XRI (either a persistent XRI or a reassignable XRI) to concrete identifiers or “locators” called service endpoints. A service endpoint (very similar to an endpoint reference or EPR in web services architecture) can be either typed or untyped, and can contain zero or more concrete URIs for the target resource. In addition to service endpoints, XRDS documents also support synonym assertions (such as CanonicalID). These are assertions by the identifier authority of other identifiers that identify the same target resource as the original XRI. 5) While the XRDS document may itself be considered a first-class resource on the Web, by definition it is NOT the resource being identified by the XRI, just as a URC is not the resource identified by the URN. (More about this below.) 6) The last and most subtle point about the relationship between XRIs, XRDS documents, and XRI authorities (which again is directly analogous to the relationship between URNs, URCs, and URN assignment authorities) is that in the authority component of an XRI, delegation between authorities is supported just like it is in DNS. This uses XRI subsegment architecture, which operates from left-to-right (vs. right-to-left delegation in DNS names). For example, in the theoretical XRI xri://@!1!2!3, the identifier authority for XRI @ registry assigns the persistent subsegment !1 to identifier authority #1; identifier authority #1 assigns the persistent subsegment !2 to identifier authority #2, and so on. In such delegations, the target resource (node) being identified _is by definition the identifier authority for any additional delegations_. For persistent XRIs, _it is this delegation relationship (arc) that is persistent_. For example, in the theoretical XRI xri://@!1!2!3, the XRI @ identifier authority (which is a registry) is asserting that the subsegment !1 has been assigned once to a registrant and will not be reassigned to another registrant. That registrant, who is now the adminstrative authority for xri://@!1, is in turn asserting that the subsegment !2 has been assigned once to whatever type of resource may be identified in its scope, and will not be reassigned to another resource. If that resource is a registry for other identifier authorites, the identifier authority chain continues. If however any point in the chain you can reach an identifier for a resource that is NOT an identifier authority, such as a document, you’ve reached the end of the delegation chain. For example, if @!1!2 represented a document management system for persistent documents, then @!1!2!3 identifies a specific document in this registry. This XRI represents an assertion by the @!1!2 identifier authority that !3 has been assigned to one specific document and will not be reassigned to another document. In all cases, however, the XRDS documents resolved in the process of traversing that resolution chain are _not_ the resources being identified. I can understand the source of the confusion, however, because XRDS documents are a defined media type retreived from an http: or https: URI, so by AWWW guidelines they are first-class resources on the Web. I don’t believe that is a problem, however; indeed, one might say that the whole premise of abstract identifier architecture is that an abstract identifier can be resolved to a resource that describes another resource, and this set of relationships is known a priori from the abstract identifier architecture. This, in a nutshell, is why it is essential to know, a priori, that an identifier is abstract, just as any application can know, a priori, that a URN is abstract just by looking at the urn: scheme prefix. Which leads me to my next point, for which I’ll start a new thread. =Drummond [1] http://docs.oasis-open.org/xri/xri-syntax/2.0/specs/cs01/xri-syntax-V2.0-cs. html [2] http://www.w3.org/Addressing/rfc1737.txt _____ From: www-tag-request@w3.org [mailto:www-tag-request@w3.org] On Behalf Of Williams, Stuart (HP Labs, Bristol) Sent: Thursday, August 21, 2008 6:30 AM To: John Bradley Cc: Mark Baker; Henry S. Thompson; www-tag@w3.org Subject: RE: Perisistence (was RE: [XRI] Private naming conventions and hypermedia) Hello John, _____ From: John Bradley [mailto:john.bradley@wingaa.com] Sent: 08 August 2008 00:48 To: Williams, Stuart (HP Labs, Bristol) Cc: Mark Baker; Henry S. Thompson; www-tag@w3.org Subject: Re: Perisistence (was RE: [XRI] Private naming conventions and hypermedia) Hi Stuart, Reply inline. On 7-Aug-08, at 2:57 AM, Williams, Stuart (HP Labs, Bristol) wrote: <snip/> This must be an identifier for which the parent authority is the final authority. For any XRI other than a community root authority (DNS) the CID must consist of the parent authorities CanonicalID plus one additional subsegment. Once assigned, a parent authority SHOULD NEVER a) change or reassign a CanonicalID value Never change => only ever assign one CID to a given thing? <snip/> Assuming that that's out of the way, back to what it is that is persistent. Your response to my question on that was: B) is the closest. It is the mapping assigned by the parent authority to the relationship between and administrative authority and the identifier. where B) was b) The association between the identifier and whatever it refers to (a resource?)? I think this in part gets close to the heart of the matter. What I believe you are saying is that what it is that is persistent is, as you say, the relationship between an (persistent) identifier and the authority that administers it. Yes it is an administrative relationship. ! segments are used to specify persistent identifiers―identifiers that are permanently assigned to a resource and will not be reassigned at a future date. In this context the resource is the XRD document. The administrative control of that XRD document must not be reassigned. Ah... we're back into the data v metadata thing. In what I wrote I was meaning the resource to be whatever the XRI was intended to be used to refer to (denote), rather than metadata (an XRD document) about that thing. eg. in the example I tried to construct earlier of an XRI for the "XRI 2.0 Resolution Spec" the resource would be just that, the spec, and not the XRD document which (one presumes) would describe the spec. (or at least how to obtain a copy of it). >From your answer above, what is persistent is the relationship between and XRD 'resolved' by the resolution protocol and the administrative authority that controls that document. As the content of the XRD document itself can change, and could presumably over time described different things (maybe) then the relationship between a persistent XRI and the thing it denotes could change by fiat of a change in an XRD... though that may be a violation of XRI architecture which might demand a fresh persistent XRI CID in such a circumstance - which then breaks the premise (because we have a new CID and a new persistent relationship). If =skw is assigned to you along with =!BF81.FD97.C81B.B4E5 and at some point you stop paying for =skw and TBL wants to purchase it he can get =skw but not =!BF81.FD97.C81B.B4E5, a new CID would be created if he needed it. As the openid.claimed_id is the CID and not the iName TBL would not be able to get access to your resources. It is always possible to reassign the administrative control of a XRD to some other entity if the owner requests it. That might be necessary if a company is sold or something like that. This is largely a social matter wrt to administrative policy rather than a technical one. The underlying social issue that this seeks to address is a lack of stabilty in this relationship induced by the administrative policies assoicated with the DNS - specifically, that the authority for a given domain name can change over time. If DNS domains were not 'rented' on a 1-2 year renewal basis where an authority can 'accidentally' loose administrative control over the name through inattention, or simply relinquish control which is subsequently picked up by another party. Is that the primary issues that motivates persistent (top-level) XRI path names? Yes DNS has no way to detect identifier recycling. That's a pity. <snip/> Anyway, the persistence XRI speaks of for persistent identifiers is the persistent in the relationship between and identifier and its administrative authority. Yes, the identifier is assigned to a XRD and the administrative authority for that XRD will not be reassigned. There's that data/metadata thing again... we (or I) need to have a consistent understanding of what an XRI is assigned to, AIUI, an XRD(S) document arising as part of the resolution process is a stepping stone in making that determination, but in general is *not* what is denoted or referred to by the identifier being resolved. I think that there may be a further obligation on the administrative authority not to change *what* is being described in an XRD(S) document without assigning a new CID. By contrast a reassignable XRI can be assigned by an identifier authority to represent a different resource at some future time. I hope that helps. Best Regards John Bradley OASIS IDTRUST-SC http://xri.net/=jbradley 五里霧中 <snip/> Regards Stuart -- Hewlett-Packard Limited registered Office: Cain Road, Bracknell, Berks RG12 1HN Registered No: 690597 England
Received on Thursday, 21 August 2008 19:25:47 UTC