]> Validation of Locations Around a Planned Change
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ART ecrit This document defines an extension to the Location to Service Translation (LoST) protocol (RFC5222) that allows a LoST server to notify a client of planned changes to location data. This extension is only useful with the validation function of LoST. It is beneficial for LoST validation clients to be aware of planned changes, since at a known future date, previously valid records may become invalid, and new records may become valid. This extension adds an element to the <findService> request to allow a LoST client to request validation as of a specified date. It adds an optional Time-To-Live element to the response, which informs clients of the current expected lifetime of a validation. It also adds a separate interface to a LoST server that allows a client to poll for planned changes. Additionally, this document provides a conventional XML schema for LoST, as a backwards compatible alternative to the RelaxNG schema in RFC5222.
Introduction Validation of civic locations involves dealing with data that may change over time. A typical example is when a portion of a county or district that was not part of a municipality is "annexed" to a municipality. Prior to the change, a Presence Information Data Format Location Object (PIDF-LO) specifying a civic location in the affected area would have a blank A3 element, or would contain some other value; after the change, a PIDF-LO specifying the same location would contain an A3 element set to the name of the municipality that annexed that part of the county/district. This kind of annexation has an effective date and time (typically 00:00 on the first or last day of a month), known in advance. Other kinds of changes may also occur, and these will almost always also have an effective date that is known in advance. Records in a LoST client such as a Location Information Server (LIS) must change around these kinds of events. Each affected old record must be discarded, and a new, validated record must be loaded into the client's database. It is often difficult for a LIS operator to know that records must be changed around such events. There are other circumstances where locations that were previously valid become invalid, such as a street renaming or renumbering event. Using validation, the only way for a LoST client such as a LIS to discover such changes is to periodically revalidate its entire database. Of course, this does not facilitate timely changes, is not coordinated with the actual change event, and also adds significant load to LoST servers as well as LoST clients such as LISs. Even when re-validation is used, a server has no mechanism to control, or suggest the time period for revalidation. This extension allows a LoST client to obtain from a LoST server sets of locations which may change. It makes use of "partial location information" which is a set of location elements and values that, when compared against client location records, identify which of the client records may change as a result of the planned change. A set of such partial locations is termed a "ChangeSet". ChangeSets have an ID, and a date when the change is effective. IDs are ordered so that changes can be completed in the correct order. The planned change interface is a REST/JSON interface that allows a client to poll a server using the last ID that it obtained from that server. The response to a poll is a list of all the newer planned changes the server knows about beyond the ChangeSet whose ID was included in the poll. The ID of the last ChangeSet in the returned list is used by the client in its next poll. When a LoST client such as a LIS receives a new ChangeSet, it may prepare one or more new records so that, at the precise planned event date and time, it may insert the new records into its active database and delete the old records. As part of preparing the new records in advance of the change, a client may use the "asOf" date component of this extension to perform a LoST validation of the new record as of the effective date. The "asOf" date component of this extension allow a LoST client such as a LIS to be prepared for and smoothly transition to planned changes. The polling mechanism allows a client to be informed of planned changes, while the "asOf" date allows it to verify the validity of locations before they become active. Unplanned changes will occur, and periodic revalidation can still be used to maintain the data in the client's records. However, LoST servers should be able to influence the rate of such revalidation. For this purpose, this extension adds an optional "expires" element to the <findServiceResponse> to provide advice from a server to a client as to when revalidation is suggested. In a <findServiceResponse>, a LoST server may include the "expires" element to expressly state when the location should be re-validated. This avoids clients blindly revalidating every address on an arbitrary schedule. There are quite a few implementations of LoST. Experience with these implementations indicates that the RelaxNG schema is very difficult to deal with, both because many commonly used development tools don't support it, and development staff is often unfamiliar with it. Informal alternative schemas have been circulated, which is undesirable as they may not be in conformance with the RelaxNG schema in . This document provides an XML schema that replaces the RelaxNG schema. It can be used by any implementation interchangeably with the RelaxNG schema.
Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. "Server" in this document refers to a LoST server and "Client" is a LoST client.
Planned Change Poll Interface This document defines a new interface to a LoST server. The interface has three endpoints:
  1. Versions, which returns the current version(s) the interface supports. This allows the interface to evolve over time.
  2. PlannedChangePoll, which supports a polling mechanism. The poll returns a list of changeSetIds which identify ChangeSet objects.
  3. GetChangeSet, which accepts a changeSetId and returns the identified ChangeSet object.
A ChangeSet object contains an identifier (changeSetId), a date (changeSetEffective) and an array of partial locations. A partial location is an array of location information element name and value pairs. A LoST client (such as a LIS) compares the location elements with its records. For each of the client's records where all of the location elements provided in the partial location have the same values as those in the partial location, that client record may be affected by the planned change. The client's records may have other location elements, but those are not considered in the comparison. So, for example, a partial location may have Country, A1, A2, A3 and A4 location elements, which means that any address with those Country, A1, A2, A3 and A4 values may be affected by the planned change, regardless of street name and number. As another example, a partial location with Country, A1, A2, A3, A4, RD and POD but not HN applies to any address number on the specified street. Servers supporting this extension maintain an ordered list of changeSetIds. A changeSetId is a string, which might not show the ordering. For example, the ID could be a hashed timestamp, or a hashed sequence number, among other things. Given a changeSetId returned in a prior poll, a server can identify which ChangeSets it has that come next, in order, after the specified changeSetId. A new client that does not know any IDs, or a client that lost the last changeSetId that it had, uses the PlannedChangePoll endpoint, omitting the changeSetId in the poll query. In the response, the server returns all the ChangeSets it knows about. The effective date of a ChangeSet returned by a server need not be in the future. Tardy clients might not keep up. On the other hand, a server is not obligated to keep ChangeSets whose changeSetEffective date has passed by more than some arbitrary time. A 12 month time period may be appropriate for a server whose service area doesn't have many changes, while a three month time period may be needed in a service area with frequent changes. A tardy client in a fast-changing environment might receive a large number of ChangeSetIds in response to a poll. Polls are expected every few minutes. A new client (or one that has lost its last changeSetID) performs a poll without specifying an ID, and the server responds with all the changeSetsIds that it knows about. Thereafter, the client retains the last changeSetId from its most recent poll and uses that in the next poll. If the response to that poll contains no changeSetIds, it means the changeSetId the client has is the latest change the server knows about; the client uses the same changeSetId in subsequent polls until the server returns a new changeSetId. The version mechanism returns a list of versions of the web service the server supports. This document specifies version 1.0. Versions are described as a major version and a minor version, where major and minor versions are integers, and a version description is a string consisting of the major version, a dot, and the minor version. A backwards-compatible change within a major version MAY increment only the minor version number. A non-backwards-compatible change MUST increment the major version number. To achieve backwards compatibility, implementations MUST ignore any object members they do not recognize. Minor version specifications SHALL only add objects, non-required members of existing objects, and non-mandatory-to-use functions and SHALL NOT delete any objects, members of objects or functions. This means an implementation of a specific major version and minor version is backwards compatible with all minor versions of the major version. The versions mechanism returns an array of supported versions, one for each major version supported, with the minor version listed being the highest supported minor version. When versions are written or used as a string, the major version is first and separated from the minor version with a period. For example major version 3, minor version 2 is written as "3.2". Discovering the Planned Change Poll interface uses the "lost+plannedchange" application tag with U-NAPTR using the name (application unique string) of the LoST server that provided validation as input along with this new service tag. The normative definition of the web interface for these endpoints is contained in
<asOf> Element This document defines a new element in the <findService> request called 'asOf', which contains a date and time in dateTime format with a required timezone. A server supporting this extension validates the location in the request as of the date and time specified, taking into account planned changes. This allows a client to verify in advance that it can make changes in its records in accordance with changes at the LoST server. A server responds with what it knows (as of the moment of the request) will be in effect on or before the 'asOf' date The LoST server is not expected to retain a history of changes. Therefore, an 'asOf' date in the past will return the same results as omitting it (meaning current data). There are no constraints on how far in advance planned changes may occur, and changes to the data may occur at any time. Therefore, two queries with the same future 'asOf' value placed at different times can return different results. The server responds with its current understanding of planned changes (including any planned or unplanned changes that have already occurred) and makes no representations about future queries of the same location.
"validUntil" in Response This extension adds an 'validUntil' attribute to the <locationValidation> element. The 'validUntil' attribute contains a date and time after which the client may wish to revalidate the location with the server. A server MAY add this attribute to a response when validation is requested. This attribute takes the form of the 'expires' attribute pattern of , which allows the values "NO-CACHE" or "NO-EXPIRATION" to be returned instead of a dateTime value. However, for the 'validUntil' attribute specified here, "NO-CACHE" has no meaning and MUST NOT be returned; "NO-EXPIRATION" means the server does not have a suggested revalidation period. Selecting a revalidation interval is a complex balancing of timeliness, server load, stability of the underlying data, and policy at the LoST server. Too short, and load on the server may be undesirably large. Too long, and invalid data may persist in clients for unacceptable lengths of time. The polling mechanism provides timely notice to synchronize changes, but even with it, it is advisable for clients to periodically revalidate their records. In areas that have little change in data, such as fully built out, stable communities already part of a municipality, it may be reasonable to set revalidation periods of six months or longer. In areas that are quickly growing, 20-30 day revalidation may be more appropriate, even though these revalidations might be a majority of the traffic at the LoST server. When a change is planned, typically the 'validUntil' value of the affected records is lowered approaching and after the change, so that clients with affected records revalide soon after the change is implemented. For example, one technique is to start lowering the 'validUntil' value of the affected records at the length of the previous 'validUntil' value in advance of the planned change. For example, if the old 'validUntil' value of the affected records is three months, then at least three months before the planned change, the 'validUntil' value of the affected records might be lowered to 20 days, and then 15-20 days before the planned change the value might be lowered again, to 10 days or so. It is not advisable to set the expiration precisely at the planned change time if a large number of records will be changed, since that would cause a large spike in traffic at the change time. Rather, the validUntil time should have a random additional time added to it to spread out the load.
Replacement XML schema This schema is an alternative to The Relax NG schema in . Future extensions to LoST are expected to use this schema as the base for the extensions, rather than the Relax NG schema. Existing extensions described using the Relax NG schema continue to be valid. Exception pattern. Any element not in the LoST namespace. A wildcard pattern for including any element from any other namespace. A wildcard pattern for including any element from any other namespace. A point where future extensions (elements from other namespaces) can be added. ]]>
Extension XML Schema This schema provides the extension to the prior section schema for planned changes: ]]>
plannedChangePoll Interface Description
Security Considerations As an extension to LoST, this document inherits the security issues raised in . Clients could poll at a rate which could affect other processing at the LoST server. This is not unlike any other type of denial of service attack at an HTTP server and similar mitigations are necessary.
IANA Considerations
Application Service Tag This document registers the following U-NAPTR application service tag:
Replacement XML Schema Registration The XML Schema is found in .
Planned Change Extension XML Schema Registration LoST Planned Change Namespace

Namespace for LoST

urn:ietf:params:xml:ns:lostPlannedChange1

See RFC????.

END ]]> The XML Schema is found in .
Normative References Revised Civic Location Format for Presence Information Data Format Location Object (PIDF-LO) This document defines an XML format for the representation of civic location. This format is designed for use with Presence Information Data Format Location Object (PIDF-LO) documents and replaces the civic location format in RFC 4119. The format is based on the civic address definition in PIDF-LO, but adds several new elements based on the civic types defined for Dynamic Host Configuration Protocol (DHCP), and adds a hierarchy to address complex road identity schemes. The format also includes support for the xml:lang language tag and restricts the types of elements where appropriate. [STANDARDS-TRACK] LoST: A Location-to-Service Translation Protocol This document describes an XML-based protocol for mapping service identifiers and geodetic or civic location information to service contact URIs. In particular, it can be used to determine the location-appropriate Public Safety Answering Point (PSAP) for emergency services. [STANDARDS-TRACK] Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.