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Network Working Group                            S.E. Hardcastle-KilleRequests for Comments 1275                   University College London                                                         November 1991Replication Requirements to provide an Internet Directory using X.500Status of this Memo    This memo provides information for the Internet community.  It    does not specify an Internet standard.  Distribution of this memo    is unlimited.Abstract    This RFCconsiders certain deficiencies of the 1988 X.500    standard, which need to be addressed before an effective open    Internet Directory can be established using these protocols and    services [CCI88].  The only areas considered are primary    problems, to which solutions must be found before a pilot can be    deployed.  This RFCconcerns itself with deficiencies which can    only be addressed by use of additional protocol or procedures for    distributed operation.

RFC 1275            Replication Requirements             November 19911  Distributed Operation ExtensionsThe Internet Directory will operate DSAs over TCP/IP usingRFC 1006[RC87], and DSAs over the an ISO Network Service.  Distributedoperation procedures should not require full connectivity.2  Knowledge ReplicationKnowledge information is critical to resolution of names, andperforming searches.  Knowledge information high up the tree needs tobe widely available.  Consider resolving a name below ``Country=US''.To do this, a DSA needs to have full knowledge at this point.  ManyDSAs need to be able to do this, in order to give reasonable responseand availability.  It would be an unacceptable bottleneck to forcesuch resolution to a single or small number of DSAs.  To replicatethis knowledge widely, a systematic approach to replication is needed.3  Data ReplicationSearches are often made at the root and country level, and this is avital service (e.g., an approximate match of an organisation name).Data needs to be collected in such a way that this sort of searchingis reasonably efficient.  The usual X.500 approach of subordinatereferences militates against this.  At a node in the DIT, subordinatereferences to the entries below are held.  These entries will be inmany DSAs, each of which needs to be accessed in order to perform thesingle level search.  It is suggested that replication of data isnecessary to achieve this.The major requirement for this replication is high up the DIT, whereinformation must be replicated between different implementations.  Atlower levels of the DIT, it is reasonable for DSAs to be of the sameimplementation and to use implementation specific techniques in orderto achieve performance and availability.4  Alternate DSAsWhen a DSA Referral is returned, only the master DSA is indicated.This will lead to a single point of failure.  It seems important toallow for additional references to slave copies, in order to getHardcastle-Kille                                                Page 1

RFC 1275            Replication Requirements             November 1991better availability.  This needs to be solved in conjunction with theproblem described in the previous section.5  Guidelines for use of ReplicationTo be effective, the replication specification needs to provideguidelines for deployment in the pilot, in order to meet the desiredservice criteria.6  Some scaling targetsMost techniques for replication have scaling limits.  It is importantthat mechanisms used do not stress the limits of the mechanism.  Theorder of magnitude envisioned in the pilot is 100 000 non-leaf entriesand several million leaf entries.References[CCI88] The Directory --- overview of concepts, models and services,        December 1988. CCITT X.500 Series Recommendations.[RC87]  Marshall T. Rose and Dwight E. Cass. ISO Transport Services        on top of the TCP. Request for Comments 1006, Northrop        Corporation Technology Center, May 1987.7  Security ConsiderationsSecurity considerations are not discussed in this memo.8  Author's Address    Steve Hardcastle-Kille    Department of Computer Science    University College London    Gower Street    WC1E 6BT    EnglandHardcastle-Kille                                                Page 2

RFC 1275            Replication Requirements             November 1991    Phone:  +44-71-380-7294    EMail:  S.Kille@CS.UCL.AC.UKHardcastle-Kille                                                Page 3