idnits 2.17.1 draft-ietf-krb-wg-kdc-model-15.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document seems to contain a disclaimer for pre-RFC5378 work, and may have content which was first submitted before 10 November 2008. The disclaimer is necessary when there are original authors that you have been unable to contact, or if some do not wish to grant the BCP78 rights to the IETF Trust. If you are able to get all authors (current and original) to grant those rights, you can and should remove the disclaimer; otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (January 14, 2013) is 4119 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 KERBEROS WORKING GROUP Johansson 3 Internet-Draft SUNET 4 Intended status: Standards Track January 14, 2013 5 Expires: July 18, 2013 7 An information model for Kerberos version 5 8 draft-ietf-krb-wg-kdc-model-15 10 Abstract 12 This document describes an information model for Kerberos version 5 13 from the point of view of an administrative service. There is no 14 standard for administrating a kerberos 5 KDC. This document 15 describes the services exposed by an administrative interface to a 16 KDC. 18 Status of this Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on July 18, 2013. 35 Copyright Notice 37 Copyright (c) 2013 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 This document may contain material from IETF Documents or IETF 51 Contributions published or made publicly available before November 52 10, 2008. The person(s) controlling the copyright in some of this 53 material may not have granted the IETF Trust the right to allow 54 modifications of such material outside the IETF Standards Process. 55 Without obtaining an adequate license from the person(s) controlling 56 the copyright in such materials, this document may not be modified 57 outside the IETF Standards Process, and derivative works of it may 58 not be created outside the IETF Standards Process, except to format 59 it for publication as an RFC or to translate it into languages other 60 than English. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 65 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4 66 3. Information model demarcation . . . . . . . . . . . . . . . . 6 67 4. Information model specification . . . . . . . . . . . . . . . 7 68 4.1. Principal . . . . . . . . . . . . . . . . . . . . . . . . 7 69 4.1.1. Principal: Attributes . . . . . . . . . . . . . . . . 7 70 4.1.2. Principal: Associations . . . . . . . . . . . . . . . 8 71 4.2. KeySet . . . . . . . . . . . . . . . . . . . . . . . . . . 9 72 4.2.1. KeySet: Attributes . . . . . . . . . . . . . . . . . . 9 73 4.2.2. KeySet: Associations . . . . . . . . . . . . . . . . . 9 74 4.3. Key . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 75 4.3.1. Key: Attributes . . . . . . . . . . . . . . . . . . . 10 76 4.3.2. Key: Associations . . . . . . . . . . . . . . . . . . 10 77 4.3.3. Key: Remarks . . . . . . . . . . . . . . . . . . . . . 11 78 4.4. Policy . . . . . . . . . . . . . . . . . . . . . . . . . . 11 79 4.4.1. Policy: Attributes . . . . . . . . . . . . . . . . . . 11 80 4.4.2. Mandatory-to-implement Policy . . . . . . . . . . . . 12 81 5. Implementation Scenarios . . . . . . . . . . . . . . . . . . . 13 82 5.1. LDAP backend to KDC . . . . . . . . . . . . . . . . . . . 13 83 5.2. LDAP frontend to KDC . . . . . . . . . . . . . . . . . . . 13 84 5.3. SOAP . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 85 5.4. Netconf . . . . . . . . . . . . . . . . . . . . . . . . . 13 86 6. Security Considerations . . . . . . . . . . . . . . . . . . . 14 87 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 88 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16 89 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 90 9.1. Normative References . . . . . . . . . . . . . . . . . . . 17 91 9.2. Informative References . . . . . . . . . . . . . . . . . . 17 92 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18 94 1. Introduction 96 The Kerberos version 5 authentication service described in [RFC4120] 97 describes how a Key Distribution Center (KDC) provides authentication 98 to clients. The standard does not stipulate how a KDC is managed and 99 several "kadmin" servers have evolved. This document describes the 100 services required to administer a KDC and the underlying information 101 model assumed by a kadmin-type service. 103 The information model is written in terms of "attributes" and 104 "services" or "interfaces" but the use of these particular words must 105 not be taken to imply any particular modeling paradigm. Neither an 106 object oriented model nor an LDAP [RFC4510] schema is intended. The 107 author has attempted to describe in natural language the intended 108 semantics and syntax of the components of the model. An LDAP schema 109 (for instance) based on this model will be more precise in the 110 expression of the syntax while preserving the semantics of this 111 model. 113 Implementations of this document MAY decide to change the names used 114 (e.g. principalName). If so an implementation MUST provide a name to 115 name mapping to this document. In particular schema languages may 116 have different conventions for caseing, eg camelCase vs use of '_' 117 and '-' to separate 'words' in a name. Implementations MUST call out 118 such conventions explicitly. 120 Implementations of this document MUST be able to support default 121 values for attributes as well as the ability to specify syntax for 122 attribute values. 124 2. Requirements notation 126 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 127 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 128 document are to be interpreted as described in [RFC2119]. 130 This document uses the standard normative key words ("MUST", "MUST 131 NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", 132 "RECOMMENDED", "MAY", and "OPTIONAL") but does not reference 133 [RFC2119]. The reason for this (which was discussed extensively in 134 the kerberos WG) is as follows: 136 This document describes an information model for kerberos 5 but does 137 not directly describe any mapping onto a particular schema- or 138 modelling language. Hence an implementation of this model consists 139 of a mapping to such a language - e.g. an LDAP or SQL schema. The 140 standard normative key word therefore require precise definition: 142 The terms MUST or REQUIRED means that schema implementing this model 143 must have a way to represent a feature (i.e that it is mandatory to 144 implement in schema) but that unless otherwise specified the feature 145 may represent an optional element in the chosen schema definition 146 language. 148 However MUST also means that a KDC or administrative interface 149 implementing this information model MUST provide the feature and 150 associated behavior consistent with schema. 152 For instance, principalLastFailedAuthentication (cf below) represents 153 the last time an authentication failed for a principal. In an LDAP 154 schema (for instance) this may be represented as an optional 155 attribute even though all KDCs implementing this specification must 156 support this attribute. 158 The terms MAY or OPTIONAL means that the feature is optional to 159 implement by a KDC or administrative interface implementing this 160 information model. It also means that the feature is optional to 161 implement in schema. 163 Implementors of schema should be aware that unless there is a way to 164 represent critical but optional elements in the schema definition 165 language confusion may arise when optional elements are used but not 166 understood by all implementations in a particular deployment. 168 The expression "MUST NOT be OPTIONAL" means that a feature is 169 mandatory to implement ("MUST" cf above) and that additionally it 170 must not be marked optional in the schema language. In particular 171 this means that the feature is both mandatory to implement and must 172 be present in all representations of the object to which it applies. 174 The term SHOULD or RECOMMENDED means that the consequences of not 175 implementing the feature as if it was described with the "MUST" 176 keyword must be carefully weighed before choosing a different course. 177 In particular this implies that interoperability concerns may arise 178 from not following the recommended practice in schema that implements 179 this model. 181 The context will determine if the "SHOULD" key word applies to 182 schema, or to underlying behaviour of the KDC or both. For instance, 183 principalIsDisabled (cf below) SHOULD default to FALSE implies both a 184 recommendation for the behaviour of KDCs aswell as a rekommendation 185 for the representation of that behaviour in schema. 187 3. Information model demarcation 189 The inforsmation model specified in the next chapter describes 190 objects, properties of those objects and relations between those 191 objects. These elements comprise an abstract view of the data 192 represented in a KDC. It is important to understand that the 193 information model is not a schema. In particular the way objects are 194 compared for equality beyond that which is implied by the 195 specification of a syntax is not part of this specification. Nor is 196 ordering specified between elements of a particular syntax. 198 Further work on Kerberos will undoubtedly prompt updates to this 199 information model to reflect changes in the functions performed by 200 the KDC. Such extensions to the information model should always use 201 a normative reference to the relevant RFCs detailing the change in 202 KDC function. 204 This model describes a number of elements related to password policy 205 management. Not all of the elements in this model are unique to 206 Kerberos; an LDAP implementation of this model should incorporate 207 existing LDAP schema where functional overlap exists, rather than 208 defining additional Kerberos-specific elements. 210 4. Information model specification 212 4.1. Principal 214 The fundamental entity stored in a KDC is the principal. The 215 Principal is associated to keys and generalizes the "user" concept. 216 The Principal MUST be implemented in full and MUST NOT be OPTIONAL in 217 an implementation 219 4.1.1. Principal: Attributes 221 4.1.1.1. principalName 223 The principalName MUST uniquely identify the Principal within the 224 administrative context of the KDC. The principalName MUST be 225 equivalent to the string representation of the Principal name 226 (section 2.1.1 of [RFC1964]) including, if applicable for the name 227 type, the realm. 229 The attribute MAY be multi-valued if the implementation supports 230 aliases and/or enterprise names. In that case exactly one of the 231 principalName values MAY be designated the canonical principalName 232 and if the implementation supports enctypes which require salt then 233 exactly one of the values of principalName MAY be designated as the 234 canonical salting principalName. 236 Implementations (i.e. schema) that support enterprise names and/or 237 aliases SHOULD provide for efficient lookup of Principal objects 238 based on alias/enterprise name. 240 4.1.1.2. principalNotUsedBefore 242 The Principal MUST NOT be used before this date. The syntax of the 243 attribute MUST be Internet Date/Time Format from [RFC3339]. The 244 attribute MUST be single-valued. 246 4.1.1.3. principalNotUsedAfter 248 The Principal MUST NOT be used after this date. The syntax of the 249 attribute MUST be Internet Date/Time Format from [RFC3339]. The 250 attribute MUST be single-valued. 252 4.1.1.4. principalIsDisabled 254 A boolean attribute used to disable a Principal. The attribute 255 SHOULD default to boolean FALSE. 257 4.1.1.5. principalLastCredentialChangeTime 259 This single-valued attribute contains the time of the last successful 260 change of credential (e.g. password or private key) associated with 261 this Principal. The syntax of the attribute MUST be Internet Date/ 262 Time Format from [RFC3339]. 264 4.1.1.6. principalCreateTime 266 This single-valued attribute contains the time and date when this 267 Principal was created. The syntax of the attribute MUST be Internet 268 Date/Time Format from [RFC3339]. 270 4.1.1.7. principalModifyTime 272 This single-valued attribute contains the time and date when this 273 Principal was last modified excluding credentials change. The syntax 274 of the attribute MUST be Internet Date/Time Format from [RFC3339]. 276 4.1.1.8. principalMaximumTicketLifetime 278 This single-valued attribute contains the time in seconds 279 representing the maximum lifetime for tickets issued for this 280 Principal. 282 4.1.1.9. principalMaximumRenewableTicketLifetime 284 This single-valued attribute contains the delta time in seconds 285 representing the maximum amount of time a ticket may be renewed for 286 this Principal. 288 4.1.1.10. principalAllowedEnctype 290 This OPTIONAL multi-valued attribute lists the enctypes allowed for 291 this principal. If empty or absent any enctype supported by the 292 implementation is allowed for this Principal. 294 This attribute is intended as a policy attribute and restricts all 295 uses of enctypes including server, client, and session keys. Data 296 models MAY choose to use policy objects in order to represent more 297 complex decision mechanisms. 299 4.1.2. Principal: Associations 301 Each Principal MAY be associated with 0 or more KeySet and MAY be 302 associated with 0 or more Policies. The KeySet is represented as an 303 object in this model since it has attributes associated with it (the 304 key version number). In typical situations the Principal is 305 associated with exactly 1 KeySet but implementations MUST NOT assume 306 this case, i.e. an implementation of this standard MUST be able to 307 handle the general case of multiple KeySet associated with each 308 principal. Multiple KeySets may for instance be useful when 309 performing a key rollover for a principal. 311 4.2. KeySet 313 In Kerberos principals are associated with zero or more symmetric 314 secret keys, and each key has a key version number (kvno) and 315 enctype. In this model we group keys by kvno into KeySet objects. A 316 Principal can have zero or more KeySet objects associated with it, 317 each of which MUST have one or more keys. Each KeySet is associated 318 with exactly one principal. Schemas derived from this model MAY lack 319 a direct analogue of KeySet as described in this document. 321 It is expected that most Kerberos implementations will use a special- 322 purpose interface for setting and changing Principal passwords and 323 keys. 325 If a server supports an enctype for a Principal that enctype must be 326 present in at least one key for the Principal in question. For any 327 given enctype a KeySet MUST NOT contain more than one Key with that 328 enctype. 330 The security of Kerberos 5 depends absolutely on the confidentiality 331 and integrity of the Key objects stored in the KDC. Implementations 332 of this standard MUST facilitate, to the extent possible, an 333 administrator's ability to place more restrictive access controls on 334 KeySets than on other Principal data, and to arrange for more secure 335 backup for KeySets. 337 4.2.1. KeySet: Attributes 339 4.2.1.1. kvno 341 Also knowns as the key version number. This is a single-valued 342 attribute containing a non-negative integer. This number is 343 incremembed by one each time a key in the KeySet is changed. 345 4.2.2. KeySet: Associations 347 To each KeySet MUST be associated a set of 1 or more Keys. 349 4.3. Key 351 Implementations of this model MUST NOT REQUIRE keys to be 352 represented. 354 4.3.1. Key: Attributes 356 4.3.1.1. keyEncryptionType 358 The enctype SHOULD be represented as an enumeration of the enctypes 359 supported by the KDC using the string name ("encryption type") of the 360 enctype from the IANA registry of Kerberos Encryption Type Numbers. 361 One example is 'aes128-cts-hmac-sha1-96'. 363 4.3.1.2. keyValue 365 The binary representation of the key data. This MUST be a single- 366 valued octet string. 368 4.3.1.3. keySaltValue 370 The binary representation of the key salt. This MUST be a single- 371 valued octet string. 373 4.3.1.4. keyStringToKeyParameter 375 This MUST be a single-valued octet string representing an opaque 376 parameter associated with the enctype. This parameter is specified 377 in the "string-to-key" method in section 3 of [RFC3961]. 379 4.3.1.5. keyNotUsedBefore 381 This key MUST NOT be used before this date. The syntax of the 382 attribute MUST be semantically equivalent with the standard ISO date 383 format. This MUST be a single-valued attribute. 385 4.3.1.6. keyNotUsedAfter 387 This key MUST NOT be used after this date. The syntax of the 388 attribute MUST be semantically equivalent with the standard ISO date 389 format. This MUST be a single-valued attribute. 391 4.3.1.7. keyIsDisabled 393 This is a boolean attribute which SHOULD be set to false by default. 394 If this attribute is true the key MUST NOT be used. This is used to 395 temporarily disable a key. 397 4.3.2. Key: Associations 399 None 401 4.3.3. Key: Remarks 403 The security of the keys is an absolute requirement for the operation 404 of Kerberos 5. If keys are implemented adequate protection from 405 unauthorized modification and disclosure MUST be available and 406 REQUIRED by the implementation. 408 4.4. Policy 410 Implementations SHOULD implement policy but MAY allow them to be 411 OPTIONAL. The Policy should be thought of as a 'typed hole'. i.e. an 412 opaque binary value paired with an identifier of type of data 413 contained in the binary value. Both attributes (type and value) must 414 be present. 416 4.4.1. Policy: Attributes 418 4.4.1.1. policyIdentifier 420 The policyIdentifier MUST be globally unique. Possible types of 421 identifiers include: 423 An Object Identifier (OID) [RFC4517] 425 A URI [RFC3986] 427 A UUID [RFC4122] 429 Implementations of this specification are expected to assign globally 430 unique identifiers to the list of standard policy below in accordance 431 with best-practice for identifier-management for the schema-language 432 used. 434 4.4.1.2. policyIsCritical 436 This boolean attribute indicates that the KDC MUST be able to 437 correctly interpret and apply this policy for the Principal to be 438 used. 440 4.4.1.3. policyContent 442 This is an optional single opaque binary value used to store a 443 representation of the policy. In general a policy cannot be fully 444 expressed using attribute-value pairs. The policyContent is OPTIONAL 445 in the sense that an implementation MAY use it to store an opaque 446 value for those policy-types which are not directly representable in 447 that implementation. 449 4.4.1.4. policyUse 451 This is an optional single enumerated string value used to describe 452 the use of the policy. Implementations SHOULD provide this attribute 453 and MUST (if the attribute is implemented) describe the enumerated 454 set of possible values. The intent is that this attribute be useful 455 in providing an initial context-based filtering. 457 4.4.2. Mandatory-to-implement Policy 459 All implementations that represent Policy objects MUST be able to 460 represent the policies listed in this section. Implementations are 461 not required to use the same underlying data-representation for the 462 policyContent binary value but SHOULD use the same OIDs as the 463 policyIdentifier. In general the expression of policy may require a 464 Turing-complete language. This specification does not attempt to 465 model policy expression language. 467 4.4.2.1. Password Quality Policy 469 Password quality policy controls the requirements placed by the KDC 470 on new passwords. 472 4.4.2.2. Password Management Policy 474 Password management policy controls how passwords are changed. 476 4.4.2.3. Keying Policy 478 A keying policy specifies the association of enctypes with new 479 principals, e.g. when a Principal is created one of the applicable 480 keying policies is used to determine the set of keys to associate 481 with the principal. 483 4.4.2.4. Ticket Flag Policy 485 A ticket flag policy specifies the ticket flags allowed for tickets 486 issued for a principal. 488 5. Implementation Scenarios 490 There are several ways to implement an administrative service for 491 Kerberos 5 based on this information model. In this section we list 492 a few of them. 494 5.1. LDAP backend to KDC 496 Given an LDAP schema implementation of this information model it 497 would be possible to build an administrative service by back-ending 498 the KDC to a directory server where principals and keys are stored. 499 Using the security mechanisms available on the directory server keys 500 are protected from access by anyone apart from the KDC. 501 Administration of the principals, policy, and other non-key data is 502 done through the directory server while the keys are modified using 503 the set/change password protocol 504 [I-D.ietf-krb-wg-kerberos-set-passwd]. 506 5.2. LDAP frontend to KDC 508 An alternative way to provide a directory interface to the KDC is to 509 implement an LDAP-frontend to the KDC which exposes all non-key 510 objects as entries and attributes. As in the example above all keys 511 are modified using the set/change password protocol 512 [I-D.ietf-krb-wg-kerberos-set-passwd]. In this scenario the 513 implementation would typically not use a traditional LDAP 514 implementation but treat LDAP as an access protocol to data in the 515 native KDC database. 517 5.3. SOAP 519 Given an XML schema implementation of this information model it would 520 be possible to build a SOAP interface to the KDC. This demonstrates 521 the value of creating an abstract information model which is mappable 522 to multiple schema representations. 524 5.4. Netconf 526 Given a YAML implementation of this information model it would be 527 possible to create a Netconf-based interface to the KDC, enabling 528 management of the KDC from standard network management applications. 530 6. Security Considerations 532 This document describes an abstract information model for Kerberos 5. 533 The Kerberos 5 protocol depends on the security of the keys stored in 534 the KDC. The model described here assumes that keys MUST NOT be 535 transported in the clear over the network and furthermore that keys 536 are treated as write-only attributes that SHALL only be modified 537 (using the administrative interface) by the change-password protocol 538 [I-D.ietf-krb-wg-kerberos-set-passwd]. 540 Exposing the object model of a KDC typically implies that objects can 541 be modified and/or deleted. In a KDC not all principals are created 542 equal, so that for instance deleting krbtgt/EXAMPLE.COM@EXAMPLE.COM 543 effectively disables the EXAMPLE.COM realm. Hence access control is 544 paramount to the security of any implementation. This document does 545 not mandate access control. This only implies that access control is 546 beyond the scope of the standard information model, i.e. that access 547 control may not be accessible via any protocol based on this model. 548 If access control objects are exposed via an extension to this model 549 the presence of access control may in itself provide points of attack 550 by giving away information about principals with elevated rights etc. 552 7. IANA Considerations 554 This document has no IANA actions. 556 8. Acknowledgments 558 The author wishes to extend his thanks to Love Hoernquist-Aestrand 559 and Sam Hartman for their important contributions to this document. 561 9. References 563 9.1. Normative References 565 [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", 566 RFC 1964, June 1996. 568 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 569 Requirement Levels", BCP 14, RFC 2119, March 1997. 571 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the 572 Internet: Timestamps", RFC 3339, July 2002. 574 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for 575 Kerberos 5", RFC 3961, February 2005. 577 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 578 Resource Identifier (URI): Generic Syntax", STD 66, 579 RFC 3986, January 2005. 581 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 582 Kerberos Network Authentication Service (V5)", RFC 4120, 583 July 2005. 585 [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally 586 Unique IDentifier (UUID) URN Namespace", RFC 4122, 587 July 2005. 589 [RFC4517] Legg, S., "Lightweight Directory Access Protocol (LDAP): 590 Syntaxes and Matching Rules", RFC 4517, June 2006. 592 9.2. Informative References 594 [I-D.ietf-krb-wg-kerberos-set-passwd] 595 Williams, N., "Kerberos Set/Change Key/Password Protocol 596 Version 2", draft-ietf-krb-wg-kerberos-set-passwd-08 (work 597 in progress), November 2008. 599 [RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol 600 (LDAP): Technical Specification Road Map", RFC 4510, 601 June 2006. 603 Author's Address 605 Leif Johansson 606 Swedish University Network 607 Thulegatan 11 608 Stockholm 610 Email: leifj@sunet.se 611 URI: http://www.sunet.se