INTERNET-DRAFT Russel F. Weiser Informational Draft Digital Signature Trust Co. Expires 21 April 2000 Ellen Stokes IBM 21 October 1999 LDAP V3 Replication Requirements Status of this Memo This document is am Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Drafts Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document discusses the fundamental requirements for replication of data accessible via the LDAPv3 [RFC2251] protocol. It is intended to be a gathering place for general replication requirements needed to provide interoperability between informational directories. The key words MUST, MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Weiser & Stokes 21 April 2000 [PAGE 1] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 Table of Contents 1.Introduction.....................................................3 2. Terminology.....................................................3 3. Objective.......................................................5 4. Applicability Statement.........................................5 5. Replication Model..............................................10 6. Replication Protocol...........................................12 7. Schema.........................................................13 8. Administration and Management Considerations...................13 9. Acknowledgement................................................14 10. References....................................................15 11. Author's Address..............................................15 Weiser & Stokes 21 April 2000 [Page 2] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 1. Introduction The ability to distribute directory information throughout the network provides a two fold benefit to the network: (1) increasing the reliability of the directory through fault tolerance, and (2) brings the directory content closer to the clients using the data. LDAPÆs acceptance as an access protocol for directory information is driving the need to distribute LDAP directory content among servers within enterprise and Internet. Currently LDAP does not define a replication mechanism and only generally mentions LDAP shadow servers (see [RFC2251] and [Changelog]) in passing. The requirements for replication are critical to the successful deployment and acceptance of LDAP in the market place. 2. Terminology For the purposes of this document, the following terminology definitions are used: Area of replication - A whole or portion of a directory tree(DIT) making up a distinct unit of data to be replicated. This may also be known as "unit of replication". Atomic operation - The ability to treat and contain several updates or attribute changes as a single operation for replication purposes to guarantee that the several updates or attribute changes are propagated to a replica as a single unit. Authoritative Master Replica - The Primary updateable replica of the replicated information. Conflict resolution - Deterministic procedures within replication protocols, utilized to resolve change information conflicts that may arise due to conflicting changes affecting a directory entry. Fractional replication - The capability to replicate a subset of attributes of any given entry. Incremental Update - The process of updating a replica, or copy, of a naming context, by updating only those fields or objects which have changed. Master Slave, or Single Master Replication - Replication model that assumes only one server, the master, allows write access to the replicated data. Note that Master-Slave replication can be considered a proper subset of multi-master replication. Weiser & Stokes 21 April 2000 [Page 3] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 Multi-Master Replication - A replication model where entries can be written and updated on any of several updateable replica copies without requiring communication with other updateable replicas before the write or update is performed. Naming Context - Suffix of a Sub-tree. A sub-tree of entries held in a single server [X.500]. One-way Replication - The process of synchronization in a single direction where the authoritative source information is provided to a replica. Partial Replication - The capability to replicate some subset of entries in a naming context. Propagation behavior - The general behavior of the actual synchronization process between a consumer and a provider of replication information. Read-only Replica - A read-only copy of a replicated directory. A read-only replica is assumed to be a slave replica of master slave or single master replication definition. Replica - A single instance of a whole or portion of the Directory tree (DIT) as defined by area of replication. Replica Ring - A set of servers, which hold in common the same DIT information as, defined by ôArea of replicationö. These servers may be managed under a single replication agreement that handles all members of the set of servers as a group. Replica Cycle - When a change or groups of changes need to be propagated to the other member of a replica ring. The process of contacting a replica member would be considered the beginning of a replication cycle; the termination of communications with a replica is the end of the cycle whether its due to an error or successful exchange of update records. Replication - The process of copying portions of naming context information and content between multiple LDAP servers, such that certain predefined portions of the information are available from different servers. Replication can occur between either homogeneous implementations across heterogeneous platforms (operating systems) or heterogeneous implementations supporting identical replication across heterogeneous platforms (operating systems). Sparse Replica - A incomplete copy of a sub-tree which maybe inclusive with updateable, or Read-only. See Partial replication and Weiser & Stokes 21 April 2000 [Page 4] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 Fractional replication. Topology - Refers to the shape of the directed graph describing the relationships between replicas, as in the replicated directory topology. Two-way Replication - The process of synchronization where change information may flow bi-directionally between two replica. Update Propagation - Protocol-based process by which directory replicas are reconciled. Updateable Replica - A Non-authoritative read-writeable copy of the replicated information. Such that during conflict resolution a authoritative master takes precedents in resolving conflicts. 3. Objective The major objective is to provide an interoperable LDAP V3 directory synchronization protocol which is simple, highly efficient and flexible enough to support both multi-master and master-slave replication operations to meet the needs of both the internet and enterprise environments. 4. Applicability Statement Generally replication can be characterized by looking at data consistency models across existing technologies. This may provide insight to LDAP v3 replication requirements. The following is a brief examination of the following data models. Model 1: Tight Consistency -- Includes environments where all replicas must always contain exactly the same directory content. Two phase commit transaction models may be used to preserve transaction consistency. Model 2: Eventual Consistency or Transient Consistency -- Includes X.500 Directories, Bayou [XEROX], and NDS (Novell Directory Services) names service where definite knowledge of the global replica topology is provided through predetermined replication agreements. Such that every update propagates to every replica that it can reach via a path of stepwise eventual connectivity. Transaction consistency is preserved for transactions directed at the master server in X.500 implementations. NDS additionally provides deterministic consistency over time to all replicas due to its inherent replication policies. Weiser & Stokes 21 April 2000 [Page 5] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 Model 3: Limited Effort Eventual Consistency -- Includes Xerox Clearinghouse [XEROX] that provides a statistical probability of convergence with global knowledge of replica topology. Similar to "Eventual Consistency", except where replicas may purge updates therefore dropping propagation changes when some replica time boundary is exceeded, thus leaving some changes replicated to a portion of the replica topology. Transactional consistency is not preserved, though some weaker constraints on consistency are available. Model 4: Loosest Consistency -- Includes opportunistic or simple cache where information is provided from the cache until stale. Model 5: Ad hoc -- A copy of a date store where no follow up checks are made for the accuracy/freshness of the data. Consistency models 2, and 3 involve the use of prearranged replication agreements or "Predefined Replication Agreements" between cooperating servers. The complexity of Model 1's use of 2- phase commit adds additional overhead that should not considered at this time. Models 4 and 5 involve unregistered replicas which "pull" updates from another directory server without that server's knowledge. These models can be considered to violate a directory's security policies. Therefore models 1, 4, and 5 are declared to be out of scope of this working group. So through further review of these consistency models two application areas can then be derived with even further characterizations of the data types usages. Eventual Consistency or Transient Consistency (Model 2) - This model provides policy configuration through security management parameters; the data is more dynamic and utilizes dynamic address information. Limited Effort Eventual Consistency (Model 3) - This model matches a white-pages environment which contains fairly static data and address information. This model mainly replicates message attributes. Therefore it is believed an LDAP replication should be flexible enough to cover the above range of capabilities. The generalized use of LDUP replication environment is to provide for the distribution of LDAP directory information in order to improve accessibility and consistency of the information held by the directory. 4.1 Replication Scenarios Weiser & Stokes 21 April 2000 [Page 6] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 The following directory deployment examples are intended to substantiate and validate our replication requirements. It is assumed in all cases that directory implementations from different vendors are involved. 4.1.1 Extranet Example A company has a trading partner to whom it wishes to provide directory information. This information may be as simple as a corporate telephone directory, or as complex as an extranet work flow application. For performance reasons the company may wish to have a replica of its directory within the Partner Company, rather than simply exposed beyond its firewall. The requirements, which follow from this scenario, are: - One-way replication, single mastered. - Authentication of clients. - Common access control and access control identification. - Secure transmission of updates. - Selective attribute replication (Fractional Replication), so that only partial entries can be replicated. 4.1.2 Consolidation Example Company A acquires company B. In the transition period, whilst the organizations are merged, both directory services must coexist. Company A may wish to attach company B's directory to its own. The requirements, which follow from this scenario, are: - Multi-Master replication. - Common access control model. Access control model identification. - Secure transmission of updates. - Replication between DITs with potentially differing schema. 4.1.3 Replication Heterogeneous Deployment Example An organization may deliberately deploy multiple directory services within their enterprise to employ the differing benefits of each service. In this case multi-master replication will be required to ensure that the multiple updateable replicas of the DIT are synchronized. Some vendors may provide directory clients, which are tied to their own directory service. The requirements, which follow from this scenario, are: - Multi-Master replication Weiser & Stokes 21 April 2000 [Page 7] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 - Common access control model and Access control model identification. - Secure transmission of updates. - Replication between DITs with potentially differing schemas. 4.1.4 Shared Name Space Example Two organizations may choose to cooperate on some venture and need a shared name space to manage their operation. Both organizations will require administrative rights over the shared name space. The requirements, which follow from this scenario, are: - Multi-Master replication. - Common access control model and Access control model identification. - Secure transmission of updates. 4.1.5 Supplier Initiated Replication A single master environment, which maintains a number of replicas of the DIT by pushing changes, based on a defined schedule. The requirements, which follow from this scenario, are: - Single-master environment. - Supplier-initiated replication. - Secure transmission of updates. 4.1.6 Consumer Initiated Replication Again a single mastered replication topology, but the replica initiates the replication exchange rather than the master. An example of this is a replica that resides on a laptop computer that may run disconnected for a period of time. The requirements, which follow from this scenario, are: - Single-master environment. - Consumer initiated replication. - Open scheduling (anytime). 4.1.7 Prioritized attribute replication The password attribute can provide an example of the requirement for prioritized attribute replication. A user is working in Utah and the administrator resides in California. The user has forgotten his password. So the user calls or emails the administrator to request a new password. The administrator provides the updated password (a change). Policy states that this attribute is critical and must be Weiser & Stokes 21 April 2000 [Page 8] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 available to the user for login immediately (e.g. shortly) after the administrator changed it. Replication needs to occur immediately for critical attributes/objects. The requirements, which follow from this scenario, are: - Incremental replication of changes. - Automatic replication on change of certain attributes. - Replicate based on time/attribute semantics. 4.1.8 Bandwidth issues The replication of Server (A) R/W replica (a) in Katmandu is handled via a dial up phone link to Paris where server (B) R/W replica of (a) resides. Server (C) R/W replica of(a) is connected by a T1 connection to server (B). Each connection has a different performance characteristic. The requirements, which follow from this scenario, are: - Minimize repetitive updates when replicating from multiple replication paths. - Incremental replication of changes. - Provide replication cycles to delay and/or retry when connections can not be reached. - Allowances for consumer initiated or supplier initiated replication. 4.1.9 Interoperable Administration and Management The administrator with administrative authority of the corporate directory which is replicated by numerous geographically dispersed LDAP servers from different vendors notices that the replication process is not completing correctly as the change log is continuing to grow and/or error message informs him. The administrator uses his $19.95 RepCo LDAP directory replication diagnostics tools to look at Root DSE replica knowledge on server 17 and determines that server 42 made by LDAPÆRUS Inc. is not replicating properly due to an Object conflict. Using his Repco Remote repair tools he connects to server 42 and resolves the conflict on the remote server. The requirements, which follow from this scenario, are: - Provides replication audit history. - Provisions for managing conflict resolution. - Provide LDAP access to predetermined agreements, topology and policy attributes. - Provide operations for comparing replicaÆs content for validity. - Provide LDAP access to status and audit information. Weiser & Stokes 21 April 2000 [Page 9] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 4.1.10 Enterprise Directory Replication Mesh A Corporation builds a mesh of directory servers within the enterprise utilizing LDAP servers from various vendors. Five servers are holding the same area of replication. The predetermined replication agreement(s) for the enterprise mesh are under a single management, and the security domain allows a single predetermined replication agreement to manage the 5 servers replication. The requirements, which follow from this scenario, are: - Predefined replication agreements that manage more than a single area of replication that is held on numerous servers. - Common support of replication management knowledge across vendor implementation. - Rescheduling and continuation of a replication cycle when one server in a replica ring is busy and/or unavailable. 5. Replication Model 5.1 LDAP Replication MUST be allowed to span different vendors directory services in order to provide interoperability. 5.2 All replicas MUST eventually be updated with the changed information, if specified by the replication policy. 5.3 Replication schedules MUST be configurable to allow for periodic replication, with the replication period determined by administrator of the replicated system. 5.4 Replication Model MUST enable replication cycle to be initiated on change or based on the number of pending changes. 5.5 The replication model MUST allow for administrative initiation of replication cycle for any replica that may have just come back online or was unavailable during previous replication cycles. 5.6 The replication model MUST support both master-slave and authoritative multi-updateable replica relationships. 5.7 All replicated information between the master database and its replica databases MUST be identical including all non-user modify operational attributes such as time stamps. Note this does not imply that the entire database is identical from replica to replica, but that the subset of data, chosen to replicate is identical from replica to replica. Some operational attributes may be dynamically evaluated; these attributes will not necessarily appear to be identical. Weiser & Stokes 21 April 2000 [Page 10] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 5.8 In distributed multi-vendor environment, LDAP replication MUST NOT require all copies of the replicated information be complete copies of the replicated object. 5.9 LDAP replication MUST encompass common schema objects and attributes, access control, and name space information. 5.10 Sub-tree Replication MUST be defined to allow for greater flexibility in replication topologies of the DIT as defined by the area of replication called partial replication. 5.11 Replication of critical values MUST be synchronized and have priority over non-critical values. An example of a critical value might be a password or certificate value. 5.12 Replication activities MUST occur within the context of a predefined replication agreement that addresses proper knowledge of access requirements and credentials between the synchronizing directories. Currently X.525 DISP [X.525] discusses this as a shadowing agreement including such information as unit of replication, update mode, and access point defining many of the policies between the master and a replica. 5.13 The acceptance and usage of the Internet requires that LDAP replication be available across disparate vendor directory services. 5.14 LDAP replication MUST provide scalability to both enterprise and Internet environments, e.g. an LDAP server may provide replication services to replicas within an enterprise as well as across the Internet. 5.15 The replication model MUST define deterministic policy such that replication cycle startup time conflicts between two or more competing master replicas may be resolved programmatically. An example might be automatic submission and rescheduling by one of the masters. In such a case, these replication "conflicts" MUST be resolved by the replication policy. 5.16 Any replication capable LDAP server MUST allow replication where the 2 replicating servers agree they can replicate. This may be accomplished through administrative agreements assuming compatible access control model and common schema are provided. 5.17 The replication model MUST be able to handle convergence and resurrection of attributes and objects. This is a consequence of delete and move with respect to the replication process. Weiser & Stokes 21 April 2000 [Page 11] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 5.18 It is not realistic to assume that all vendors have cooperating schemas, but that replication may be allowed between diverse schema. The Model MAY allow for replication between divergent schema of objects. 6. Replication Protocol 6.1 The act of replication SHOULD have minimal impact on both the system and network performance. 6.2 The replica synchronization SHOULD be handled in such a manner as to not saturate network with repetitive entry replication from multiple synchronization providers points. 6.3 Replication MUST only be allowed after the authentication and verification of authorization of both the replica and the source directory. 6.4 The transport for LDAP synchronization MUST allow for the integrity and confidentiality of each replicated server. 6.5 Replicated data MUST be transferable in a secure manner. 6.6 Replication protocol MUST provide for recovery and rescheduling of a replication cycle due to a replication initiation conflicts (e.g. consumer busy replicating with other servers) and or loss of connection(e.g. supplier cannot reach a replica). The replication protocol MUST include restarting at the last acknowledged update prior to interruption rather than re-sending updates it had already sent to a consuming replica. 6.7 LDAP replication MUST allow for full update to facilitate replica initialization and reset loading utilizing a standardized format such as LDIF [LDIF] format. 6.8 The replication standard SHOULD NOT limit the size of a replica. The area of replication is defined to be a whole or portion of a DIT, also allowing a portion of a naming context to be replicated. Incremental replication SHOULD be allowed. 6.9 The replication agreements MUST accommodate multiple servers receiving the same replica under a single predefined agreement. 6.10 The replication protocol MUST allow either a master or replica to initiate the replication process. 6.11 Additionally the initiator MUST be allowed to determine whether it will become a consumer or supplier during the synchronization startup process. This would allow a replica to Weiser & Stokes 21 April 2000 [Page 12] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 be periodically connected and synchronized from remote sites at the local administrator's discretion. 6.12 Multiple LDAP changes to a single server: If transactional consistency is propagated during replication, then multiple LDAP changes submitted to a single server SHOULD BE treated as a single 'atomic unit of work'. 6.13 An LDAP Replication Standard SHOULD NOT limit the transaction rate of a replication session. 6.14 Entry change information MUST be purged or discarded in a timely manner when change information becomes outdated due to propagated to all replica members. 7. Schema 7.1 Replica knowledge MUST be provided as DSE attributes. 7.2 The Replication Protocol documents MUST define standard schema for representing replication agreements, and MUST define the semantics associated with modifying the attributes of replication agreements. The documents MUST also define a standard method for determining the location of these agreements accessible utilizing LDAP. 7.3 The Replication Protocol documents MUST define standard schema for publishing state information about a given replica, and MUST define a standard method for determining the location of this information. 7.4 A location independent management point MUST be defined to provide authorized administrators with well known access to the replication policies, regardless of network location. 7.5 Replication agreements of all servers containing replicated information MUST be accessible via LDAP. 7.6 All objects MUST be uniquely identifiable throughout the object lifetime . 8. Administration and Management Considerations 8.1 Replication policies MUST allow replication of changed information to be administratively postponed to a more Weiser & Stokes 21 April 2000 [Page 13] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 convenient period. 8.2 Allowance for non-scheduled replication of a replica MUST be provided upon request such that the replica server has been down or unconnected for a period of time. 8.3 Each copy of a replica MUST maintain audit history information of which servers it has replicated with and which servers have replicated with it. 8.4 A replica MUST store conflicted versions of the replicated object to allow optional human review and intervention. 8.5 Access to replication predetermined agreements, topologies, and policies attributes MUST be provided through LDAP access. 8.6 The capability to check the differences between two replicas for the same information SHOULD be provided for. This should entail a client invoking an operation at some server, which causes that server to extract the contents from some other server it has a replication agreement with and report the differences back to the client as the result. 8.7 Authenticated access SHOULD be provided so that Administrative LDAP clients may query a server for the current state and replication history for each replica that the server maintains replication agreements with. 8.8 The ability to view replication conflicts, and override the resolution derived by the replication policy MUST be provided. 8.9 The deletion of sensitive data MUST be handled in an orderly manner so that at no time will that data be available without proper access control. That is, access control information (ACI) associated with sensitive data must be deleted after or simultaneously with the delete of the sensitive data. Likewise, when adding sensitive data, ACI MUST be added first or simultaneously with the addition of that data. 9. Acknowledgement This document is based on input from IETF members interested in LDUP Replication. Weiser & Stokes 21 April 2000 [Page 14] INTERNET-DRAFT LDAP Replication Requirements 21 October 1999 10. References [RFC2251] M. Wahl, T. Howes, S. Kille "Lightweight Directory Access Protocal", RFC 2251. [RFC2119] S.Bradner, " Key words for use in RFCs to indicate Requirement Levels", RFC 2119. [LDIF] Gordon Good, "The LDAP Data Interchange Format (LDIF)", Internet draft, draft-ietf-asid-ldif-00.txt, November 1996. [Changelog] Gordon Good, "Definitions of an Object Class to Hold LDAP Change records", Internet Draft, draft-ietf-asid-changelog- 00.txt, November 1996. [X.501] ITU-T Recommendation X.501 (1993), | ISO/IEC 9594-2: 1993, Information Technology - Open Systems Interconnection - The Directory: Models [XEROX] Hauser, C. "Managing update conflicts in Bayou, a weakly connected replicated storage system". Palo Alto, CA: Xerox PARC, Computer Science Laboratory; 1995 August; CSL-95-4. [CSL-95-04] 11. Author's Address Russel F. Weiser Digital Signature Trust Co. One South Main Street Salt Lake City, Utah 84111 USA E-mail: rweiser@digsigtrust.com Telephone: +1-801-983-4415 Fax +1-801-983-4408 Ellen J. Stokes IBM 11400 Burnet Rd. Austin, Texas 78758 USA E-mail: stokes@austin.ibm.com Telephone: +1-512-838-3725 Fax: +1-512-838-0156 Weiser & Stokes 21 April 2000 [Page 15]