Cursor-based Pagination of SCIM Resources

Introduction The two common patterns for result pagination are index-based pagination and cursor-based pagination. Rather than attempt to compare and contrast the advantages and disadvantages of competing pagination patterns, this document simply recognizes that SCIM service providers are commonly implemented as an interoperability layer on top of already existing application codebases, databases, and/or APIs that already have a well established pagination pattern. Translating from an underlying cursor-based pagination pattern to the index-based pagination defined in Section 3.4.2.4 of ultimately requires the SCIM service provider to fully iterate the underlying cursor, store the results, and then serve indexed pages from the stored results. This task of "pagination translation" dramatically increases complexity and memory requirements for implementing a SCIM service provider, and may be an impediment to SCIM adoption for some applications and identity systems. This document defines a simple addition to the SCIM protocol that allows SCIM service providers to reuse underlying cursors without expensive translation. Support for cursor-based pagination in SCIM encourages broader cross-application identity management interoperability by encouraging SCIM service provider implementations for applications and identity systems where cursor-based pagination is already well-established.

Notational Conventions 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.

Query Parameters and Response Attributes The following table describes the URL pagination parameters for requesting cursor-based pagination: Query Parameters

Parameter	Description
`cursor`	The string value of the nextCursor attribute from a previous result page. The cursor value MUST be empty or omitted for the first request of a cursor-paginated query. This value may only contained characters from the unreserved characters set defined in section 2.2 of
`count`	A positive integer. Specifies the desired maximum number of query results per page, e.g., `count=10`. When specified, the service provider MUST NOT return more results than specified, although it MAY return fewer results. If count is not specified in the query, the maximum number of results is set by the service provider.

The following table describes cursor-based pagination attributes returned in a paged query response: Response Attributes

Element	Description
`nextCursor`	A cursor value string that MAY be used in a subsequent request to obtain the next page of results. Service providers supporting cursor-based pagination MUST include `nextCursor` in all paged query responses except when returning the last page. `nextCursor` is omitted from a response only to indicate that there are no more result pages.
`previousCursor`	A cursor value string that MAY be used in a subsequent request to obtain the previous page of results. Returning `previousCursor` is OPTIONAL.

Cursor values are opaque; clients MUST not make assumptions about their structure. When the client wants to retrieve another result page for a query, it MUST query the same service provider endpoint with all query parameters and values being identical to the initial query with the exception of the cursor value which SHOULD be set to a nextCursor (or previousCursor) value that was returned by service provider in a previous response. For example, to retrieve the first 10 Users with userName starting with J, use an empty cursor and set the count to 10: The SCIM provider in response to the query above returns metadata regarding pagination similar to the following example (actual resources removed for brevity): Given the example above, to request the next page or results, use the same query parameters and values except set the cursor to the value of nextCursor (VZUTiyhEQJ94IR): In the example above, the response includes the OPTIONAL previousCursor indicating that the service provider supports forward and reverse traversal of result pages. As described in Section 3.4.1 of service providers SHOULD return an accurate value for totalResults which is the total number of resources for all pages. Service providers implementing cursor pagination that are unable to estimate totalResults MAY choose to omit the totalResults attribute.

Pagination errors If a service provider encounters invalid pagination query parameters (invalid cursor value, count value, etc), or other error conditions, the service provider SHOULD return the appropriate HTTP response status code and detailed JSON error response as defined in Section 3.12 of . Most pagination error conditions would generate an HTTP response with status code 400. Since many pagination error conditions are not user recoverable, error messages SHOULD focus on communicating error details to the SCIM client developer. For HTTP status code 400 (Bad Request) responses, the following detail error types are defined. These error types extend the list of error types defined in Section 3.12, Table 9: SCIM Detail Error Keyword Values. Pagination Errors

scimType	Description	Applicability
`invalidCursor`	Cursor value is invalid. Cursor value SHOULD be empty to request the first page and set to the `nextCursor` or `previousCursor` value for subsequent queries.	`GET` (Section 3.4.2 of )
`expiredCursor`	Cursor has expired. Do not wait longer than `cursorTimeout` (600 sec) to request additional pages.	`GET` (Section 3.4.2 of )
`invalidCount`	Count value is invalid. Count value must be between 1 - and maxPageSize (500)	`GET` (Section 3.4.2 of )

Sorting If sorting is implemented as described Section 3.4.2.3 of , then cursor-paged results SHOULD be sorted.

Default Pagination Method When a service provider supports both index- and cursor-based pagination, clients can use the 'startIndex' and 'cursor' query parameters to request a specific method. Service providers supporting both pagination methods MUST choose a pagination method to use when responding to requests that have not specified a pagination query parameter. Service providers MUST NOT return an error due to the pagination method being unspecified when pagination is required to complete the response. If the default pagination method is not advertised in the Service Provider Configuration data, service provider implementers MAY dynamically determine which pagination method is used for each response based on criteria of their choosing.

Cursors as the Only Pagination Method A service provider MAY require cursor-based pagination to retrieve all results for a query by including a nextCursor value in the response even when the query does not include the cursor parameter. For example: The service provider may respond to the above query with a page containing defaultPageSize results and a nextCursor value as shown in the below example (Resources omitted for brevity):

Backwards Compatibility Considerations Implementers of SCIM service providers that previously supported index-based pagination and are adding support for cursor-based pagination SHOULD carefully consider the impact to existing SCIM clients before changing the default pagination method in a return set. SCIM clients that previously expected index-based pagination may not be compatible with cursor-based pagination without making changes to the SCIM client. Adding cursor-based pagination support but leaving the default return set pagination method as-is SHOULD not impact existing SCIM clients. SCIM clients can query the provider configuration endpoint to determine if index-based, cursor-based or both types of pagination are supported.

Querying Resources using HTTP POST Section 3.4.2.4 of defines how clients MAY execute the HTTP POST method combined with the /.search path extension to issue execute queries without passing parameters on the URL. When using /.search, the client would pass the parameters defined in Section 2 Which would return a result containing a nextCursor value which may be used by the client in a subsequent call to return the next page of resources

Service Provider Configuration The /ServiceProviderConfig resource defined in Section 4 of facilitates discovery of SCIM service provider features. A SCIM service provider implementing cursor-based pagination SHOULD include the following additional attribute in JSON document returned by the /ServiceProviderConfig endpoint:

pagination: A complex type that indicates pagination configuration options. OPTIONAL.
cursor: A Boolean value specifying support of cursor-based pagination. REQUIRED.
index: A Boolean value specifying support of index-based pagination. REQUIRED.
defaultPaginationMethod: A string value specifying the type of pagination that the service provider defaults to when the client has not specified which method it wishes to use. Possible values are "cursor" and "index". OPTIONAL.
defaultPageSize: Positive integer value specifying the default number of results returned in a page when a count is not specified in the query. OPTIONAL.
maxPageSize: Positive integer specifying the maximum number of results returned in a page regardless of what is specified for the count in a query. The maximum number of results returned may be further restricted by other criteria. OPTIONAL.
cursorTimeout: Positive integer specifying the maximum number seconds that a cursor is guaranteed to be valid between page requests. Clients waiting too long between cursor pagination requests may receive an invalid cursor error response. OPTIONAL.

Service providers may choose not to advertise Service Provider Configuration information regarding default pagination method, page size or cursor validity. Clients MUST NOT interpret the lack of published Service Provider Configuration values to mean that no defaults or limits on page sizes or cursor lifetimes exist, or that there is no default pagination method. Service providers may choose not to publish values for the pagination sub-attributes for many reasons. Examples include:

Service providers containing multiple resource types may have different values set for each resource type.
Default and maximum page size may be determined by factors besides or in addition to the number of resources returned, such as the size of each resource on the page.

Before using cursor-based pagination, a SCIM client MAY fetch the Service Provider Configuration document from the SCIM service provider and verify that cursor-based pagination is supported. For example: A service provider supporting both cursor-based pagination and index- based pagination would return a document similar to the following (full ServiceProviderConfig schema defined in Section 5 of has been omitted for brevity): Service provider implementors SHOULD ensure that misuse of pagination by a SCIM client does not deplete service provider resources or prevent valid requests from other clients being handled. Defenses for a SCIM service provider are similar those used to protect other Web API services -- including the use of a "Web API gateway" layer, to provide authentication, rate limiting, IP allow/block lists, logging and monitoring, response caching, etc. For example, an obvious protection against abuse is for the service provider to require client authentication in order to retrieve large result sets and enforce an overriding totalResults limit for non- authenticated clients. Another example would be for a service provider that implements cursor pagination to restrict the number of cursors that can be allocated by a client or enforce cursor lifetimes.

Security Considerations This section elaborates on the security considerations associated with the implementation of cursor pagination in SCIM. This draft is under the same security and privacy considerations of those described in RFC 7644. It is imperative that implementers consider the following security aspects to safeguard against both deliberate attacks and inadvertent misuse that may compromise the system's security posture.

Threat Model and Security Environment The threat landscape is characterized by two primary types of actors:

Unauthenticated and Authenticated Malicious Actors: These individuals or entities represent a malevolent threat. Their objectives include unauthorized access to data, alteration, or deletion through cursor-enabled queries. They may also seek to deplete service provider resources deliberately, aiming to cause a denial-of-service state, thereby reducing service availability.
Authenticated Benign Users: This category includes legitimate users who, due to confusion or a lack of understanding, inadvertently engage in actions that consume service provider resources excessively. Such actions, while not ill-intended, can lead to unintended denial of service by overwhelming the system's capacity.

Confidentiality To ensure that confidential data remains appropriately secured:

Implementors MUST ensure that pagination through results sets is strictly confined to the data that the actor's current identity has been authorized to access. This holds true even in cases where the actor has obtained a cursor pertaining to a result set that was generated by a different actor.
Authorization checks MUST BE continuously applied as an actor navigates through the result set associated with a cursor. Under no circumstances should possession of a cursor be interpreted as granting any supplementary access privileges to the actor.
In alignment with Section 2, cursor values SHOULD be treated as opaque entities. Clients should avoid making any inferences or assumptions about their internal structure.
The system SHOULD handle error scenarios gracefully, while not exposing sensitive data. For instance, if an actor attempts to access a page of results outside their authorized scope, or if a request is made for a non-existent page, the system should respond with identical error messages, so as not to disclose any details of the underlying data or the nature of the authorization failure. It is acceptable, however, for the system to log different messages to a log accessible by administrators or other authorized personnel.

Integrity The extension discussed herein is query-only and does not inherently pose a substantial risk to data integrity. However, the focus is placed on safeguarding the integrity of the applications and clients that depend on this extension, rather than the integrity of the service provider. Specific considerations include: It is not required to tie a cursor to specific actor. However, if a cursor is tied to an actor and if the actor's permissions change, and the actor is still using the cursor, the actor may miss records OR there may be unauthorized access to data.

When possible, service providers SHOULD invalidate all tokens/watermarks corresponding to an actor immediately following a change in permissions. This ensures that any queries executed post-permission change, utilizing old tokens/watermarks, will be denied.
As an alternative approach, service provider may opt to retain the existing tokens/watermarks but must ensure that any metadata tied to the result set, such as record counts, is updated to reflect the new permissions accurately.

Availability The concern for availability primarily stems from the potential for Denial of Service (DoS) attacks. If the service provider elects to retain substantial data or metadata for each cursor, numerous concurrent queries with &cursor could strain and eventually exhaust service provider resources. This could be orchestrated by an attacker with malicious intent or could occur innocuously as a result of actions taken by a benign but confused actor. To mitigate such risks, the following strategies are recommended:

Implementation of rate limiting to control the volume and cadence of cursor requests. This approach should adhere to established standards for rate limiting, details of which can be found in .
Cursor mechanisms must be designed in a manner that avoids any additional consumption of service provider resources with the initiation of new &cursor requests.
It is advisable to establish a ceiling on the number of cursors permissible at any given time. Alternatively, the adoption of an opaque identifier system that conservatively utilizes resources may be used.
Token invalidation mechanisms (including mechanisms triggered by permissions changes) must be designed to be resource-efficient to prevent them from being exploited for DoS attacks.
Actors may face challenges in maintaining a seamless pagination experience if their permissions are in a state of flux. Proactive measures should be taken to ensure that permission changes do not disrupt the user experience.

Other Security References Using URIs to describe and locate resources has its own set of security considerations discussed in Section 7 of . IANA Considerations

IANA Considerations This specification requests IANA to amends the registry "SCIM Schema URIs for Data Resources" established by , for the urn:ietf:params:scim:api:messages:2.0:SearchRequest message URI and adds the following new fields: SCIM cursor attribute

Field Name: cursor.
Status: permanent.
Specification Document: this specification, Section 2
Comments: see section 3.4.3 of System for Cross-domain Identity Management: Protocol

SCIM count attribute

Field Name: count
Status: permanent
Specification Document: this specification, Section 2
Comments: see section 3.4.3 of System for Cross-domain Identity Management: Protocol

This specification amends the entry for urn:ietf:params:scim:api:messages:2.0:ListResponse message URI, and adds the following fields: SCIM nextCursor attribute

Field Name: nextCursor
Status: permanent
Specification Document: this specification, Section 2
Comments: see section 3.4.2 of System for Cross-domain Identity Management: Protocol

SCIM previousCursor attribute

Field Name: previousCursor
Status: permanent
Specification Document: this specification, Section 2
Comments: see section 3.4.2 of System for Cross-domain Identity Management: Protocol

This specification amends the entry for urn:ietf:params:scim:schemas:core:2.0:ServiceProviderConfig schema URI, and adds the following field: SCIM pagination attribute

Field Name: pagination
Status: permanent
Specification Document: this specification, Section 4
Comments: see section 5 of System for Cross-domain Identity Management: Protocol

Change Log -05

Various updates in response to WG/IETF Last Call feedback

-04

Added IANA Considerations section
Added Security Considerations section
Added Backwards Compatibility Considerations section

-03

Minor grammatical/typo fixes, rename + changes to maxPageSize SCP definition

-02

Typos/semantics, acknowledgements, expansion of cursorTimeout SCP definition

-01

Updated after Httpdir review.

-00

Adopted by SCIM WG.

Acknowledgments and Contributions The authors would like to acknowledge the contribution of Paul Lanzi (IDenovate) in leading the writing of security considerations section. The authors would also like to acknowledge the following individuals who provided valuable feedback while reviewing the draft:

Aaron Parecki - Okta
David Brossard - Axiomatics
Dean H. Saxe - Amazon Web Services
Pamela Dingle - Microsoft