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Network Working Group                            S.E. Hardcastle-Kille
Requests for Comments 1275                   University College London
                                                         November 1991

Replication Requirements to provide an Internet Directory using X.500

Status of this Memo
    This memo provides information for the Internet community.  It
    does not specify an Internet standard.  Distribution of this memo
    is unlimited.

    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 1991

1  Distributed Operation Extensions

The Internet Directory will operate DSAs over TCP/IP using RFC 1006
[RC87], and DSAs over the an ISO Network Service.  Distributed
operation procedures should not require full connectivity.

2  Knowledge Replication

Knowledge information is critical to resolution of names, and
performing searches.  Knowledge information high up the tree needs to
be widely available.  Consider resolving a name below ``Country=US''.
To do this, a DSA needs to have full knowledge at this point.  Many
DSAs need to be able to do this, in order to give reasonable response
and availability.  It would be an unacceptable bottleneck to force
such resolution to a single or small number of DSAs.  To replicate
this knowledge widely, a systematic approach to replication is needed.

3  Data Replication

Searches are often made at the root and country level, and this is a
vital service (e.g., an approximate match of an organisation name).
Data needs to be collected in such a way that this sort of searching
is reasonably efficient.  The usual X.500 approach of subordinate
references militates against this.  At a node in the DIT, subordinate
references to the entries below are held.  These entries will be in
many DSAs, each of which needs to be accessed in order to perform the
single level search.  It is suggested that replication of data is
necessary to achieve this.

The major requirement for this replication is high up the DIT, where
information must be replicated between different implementations.  At
lower levels of the DIT, it is reasonable for DSAs to be of the same
implementation and to use implementation specific techniques in order
to achieve performance and availability.

4  Alternate DSAs

When a DSA Referral is returned, only the master DSA is indicated.
This will lead to a single point of failure.  It seems important to
allow for additional references to slave copies, in order to get

Hardcastle-Kille                                                Page 1

RFC 1275            Replication Requirements             November 1991

better availability.  This needs to be solved in conjunction with the
problem described in the previous section.

5  Guidelines for use of Replication

To be effective, the replication specification needs to provide
guidelines for deployment in the pilot, in order to meet the desired
service criteria.

6  Some scaling targets

Most techniques for replication have scaling limits.  It is important
that mechanisms used do not stress the limits of the mechanism.  The
order of magnitude envisioned in the pilot is 100 000 non-leaf entries
and several million leaf entries.


[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 Considerations

Security 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

Hardcastle-Kille                                                Page 2

RFC 1275            Replication Requirements             November 1991

    Phone:  +44-71-380-7294

    EMail:  S.Kille@CS.UCL.AC.UK

Hardcastle-Kille                                                Page 3