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Network Working Group B. Aboba Request for Comments: 2607 Microsoft Corporation Category: Informational J. Vollbrecht

                                                  Merit Networks, Inc.
                                                             June 1999
        Proxy Chaining and Policy Implementation in Roaming

Status of this Memo

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

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

1. Abstract

 This document describes how proxy chaining and policy implementation
 can be supported in roaming systems. The mechanisms described in this
 document are in current use.
 However, as noted in the security considerations section, the
 techniques outlined in this document are vulnerable to attack from
 external parties as well as susceptible to fraud perpetrated by the
 roaming partners themselves. As a result, such methods are not
 suitable for wide-scale deployment on the Internet.

2. Terminology

 This document frequently uses the following terms:
 Network Access Server
    The Network Access Server (NAS) is the device that clients contact
    in order to get access to the network.
 RADIUS server
    This is a server which provides for authentication/authorization
    via the protocol described in [3], and for accounting as described
    in [4].

Aboba & Vollbrecht Informational [Page 1] RFC 2607 Proxy Chaining and Policy in Roaming June 1999

 RADIUS proxy
    In order to provide for the routing of RADIUS authentication and
    accounting requests, a RADIUS proxy can be employed. To the NAS,
    the RADIUS proxy appears to act as a RADIUS server, and to the
    RADIUS server, the proxy appears to act as a RADIUS client.
 Network Access Identifier
    In order to provide for the routing of RADIUS authentication and
    accounting requests, the userID field used in PPP (known as the
    Network Access Identifier or NAI) and in the subsequent RADIUS
    authentication and accounting requests, can contain structure.
    This structure provides a means by which the RADIUS proxy will
    locate the RADIUS server that is to receive the request. The NAI
    is defined in [6].
 Roaming relationships
    Roaming relationships include relationships between companies and
    ISPs, relationships among peer ISPs within a roaming association,
    and relationships between an ISP and a roaming consortia.
    Together, the set of relationships forming a path between a local
    ISP's authentication proxy and the home authentication server is
    known as the roaming relationship path.

3. Requirements language

 In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
 "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
 described in [5].

4. Introduction

 Today, as described in [1], proxy chaining is widely deployed for the
 purposes of providing roaming services. In such systems,
 authentication/authorization and accounting packets are routed
 between a NAS device and a home server through a series of proxies.
 Consultation of the home server is required for password-based
 authentication, since the home server maintains the password database
 and thus it is necessary for the NAS to communicate with the home
 authentication server in order to verify the user's identity.

Aboba & Vollbrecht Informational [Page 2] RFC 2607 Proxy Chaining and Policy in Roaming June 1999

4.1. Advantages of proxy chaining

 Proxies serve a number of functions in roaming, including:
 Scalability improvement
 Authentication forwarding
 Capabilities adjustment
 Policy implementation
 Accounting reliability improvement
 Atomic operation
 Scalability improvement
    In large scale roaming systems, it is necessary to provide for
    scalable management of keys used for integrity protection and
    Proxy chaining enables implementation of hierarchical
    forwarding within roaming systems, which improves scalability
    in roaming consortia based on authentication protocols without
    automated key management.  Since RADIUS as described in [3]
    requires a shared secret for each client-server pair, a
    consortium of 100 roaming partners would require 4950 shared
    secrets if each partner were to contact each other directly,
    one for each partner pair.  However, were the partners to
    route authentication requests through a central proxy, only
    100 shared secrets would be needed, one for each partner. The
    reduction in the number of partner pairs also brings with it
    other benefits, such as a reduction in the number of bilateral
    agreements and accounting and auditing overhead.  Thus,
    hierarchical routing might be desirable even if an
    authentiation protocol supporting automated key exchange were
 Capabilities adjustment
    As part of the authentication exchange with the home server,
    the NAS receives authorization parameters describing the
    service to be provided to the roaming user.  Since RADIUS,
    described in [3], does not support capabilities negotiation,
    it is possible that the authorization parameters sent by the
    home server will not match those required by the NAS. For
    example, a static IP address could be specified that would not
    be routable by the NAS. As a result, capbilities adjustment is
    performed by proxies in order to enable communication between
    NASes and home servers with very different feature sets.

Aboba & Vollbrecht Informational [Page 3] RFC 2607 Proxy Chaining and Policy in Roaming June 1999

    As part of capabilities adjustment, proxies can edit
    attributes within the Access-Accept in order to ensure
    compatibility with the NAS.  Such editing may include
    addition, deletion, or modification of attributes. In
    addition, in some cases it may be desirable for a proxy to
    edit attributes within an Access-Request in order to clean up
    or even hide information destined for the home server.  Note
    that if the proxy edits attributes within the Access-Accept,
    then it is possible that the service provided to the user may
    not be the same as that requested by the home server. This
    creates the possibility of disputes arising from inappropriate
    capabilities adjustment.
    Note that were roaming to be implemented based on an
    authentication/authorization protocol with built-in capability
    negotiation, proxy-based capabilities adjustment would
    probably not be necessary.
 Authentication forwarding
    Since roaming associations frequently implement hierarchical
    forwarding in order to improve scalability, in order for a NAS
    and home server to communicate, authentication and accounting
    packets are forwarded by one or more proxies. The path
    travelled by these packets, known as the roaming relationship
    path, is determined from the Network Access Identifier (NAI),
    described in [6]. Since most NAS devices do not implement
    forwarding logic, a proxy is needed to enable forwarding of
    authentication and accounting packets. For reasons that are
    described in the security section, in proxy systems it is
    desirable for accounting and authentication packets to follow
    the same path.
    Note: The way a proxy learns the mapping between NAI and the
    home server is  beyond  the  scope  of this document. This
    mapping can be accomplished by static configuration in the
    proxy, or by some currently undefined protocol that provides
    for dynamic mapping. For the purposes of this document, it is
    assumed that such a mapping capability exists in the proxy.
 Policy implementation
    In roaming systems it is often desirable to be able to
    implement policy. For example, a given partner may only be
    entitled to use of a given NAS during certain times of the
    day. In order to implement such policies, proxies may be
    implemented at the interface between administrative domains
    and programmed to modify authentication/authorization packets
    forwarded between the NAS and the home server. As a result,
    from a security point of view, a proxy implementing policy

Aboba & Vollbrecht Informational [Page 4] RFC 2607 Proxy Chaining and Policy in Roaming June 1999

    operates as a "man in the middle."
 Accounting reliability improvement
    In roaming systems based on proxy chaining, it is necessary
    for accounting information to be forwarded between the NAS and
    the home server. Thus roaming is inherently an interdomain
    This represents a problem since the RADIUS accounting
    protocol, described in [4] is not designed for use on an
    Internet scale.  Given that in roaming accounting packets
    travel between administrative domains, packets will often pass
    through network access points (NAPs) where packet loss may be
    substantial. This can result in unacceptable rates of
    accounting data loss.
    For example, in a proxy chaining system involving four
    systems, a one percent failure rate on each hop can result in
    loss of 3.9 percent of all accounting transactions. Placement
    of an accounting proxy near the NAS may improve reliability by
    enabling enabling persistent storage of accounting records and
    long duration retry.
 Atomic operation
    In order to ensure consistency among all parties required to
    process accounting data, it can be desirable to assure that
    transmission of accounting data is handled as an atomic
    operation. This implies that all parties on the roaming
    relationship path will receive and acknowledge the receipt of
    the accounting data for the operation to complete. Proxies can
    be used to ensure atomic delivery of accounting data by
    arranging for delivery of the accounting data in a serial
    fashion, as discussed in section 5.2.

5. Proxy chaining

 An example of a proxy chaining system is shown below.
       (request)          (request)          (request)
   NAS ----------> Proxy1 ----------> Proxy2 ----------> Home
       (reply)            (reply)            (reply)     Server
       <---------         <---------         <---------
 In the above diagram, the NAS generates a request and sends it to
 Proxy1.  Proxy1 forwards the request to Proxy2 and Proxy2 forwards
 the request to the Home Server.  The Home Server generates a reply
 and sends it to Proxy2.  Proxy2 receives the reply, matches it with
 the request it had sent, and forwards a reply to Proxy1. Proxy1

Aboba & Vollbrecht Informational [Page 5] RFC 2607 Proxy Chaining and Policy in Roaming June 1999

 matches the reply with the request it sent earlier and forwards a
 reply to the NAS.  This model applies to all requests, including
 Access Requests and Accounting Requests.
 Except for the two cases described below, a proxy server such as
 Proxy2 in the diagram above SHOULD NOT send a Reply packet to Proxy1
 without first having received a Reply packet initiated by the Home
 Server.  The two exceptions are when the proxy is enforcing policy as
 described in section 5.1 and when the proxy is acting as an
 accounting store (as in store and forward), as described in section
 The RADIUS protocol described in [3] does not provide for end-to-end
 security services, including integrity or replay protection,
 authentication or confidentiality. As noted in the security
 considerations section, this omission results in several security
 problems within proxy chaining systems.

5.1. Policy implementation

 Proxies are frequently used to implement policy in roaming
 situations.  Proxies implementing policy MAY reply directly to
 Access-Requests without forwarding the request. When replying
 directly to an Access-Request, the proxy MUST reply either with an
 Access-Reject or an Access-Challenge packet. A proxy MUST NOT reply
 directly with an Access-Accept.  An example of this would be when the
 proxy refuses all connections from a particular realm during prime
 time. In this case the home server will never receive th Access-
 Request.  This situation is shown below:
       (request)          (request)
   NAS ----------> Proxy1 ----------> Proxy2             Home
       (reply)            (reply)                        Server
       <---------         <---------
 A proxy MAY also decide to Reject a Request that has been accepted by
 the home server.  This could be based on the set of attributes
 returned by the home server.  In this case the Proxy SHOULD send an
 Access-Reject to the NAS and an Accounting-Request with Acct-Status-
 Type=Proxy-Stop (6) to the home server.  This lets the home server
 know that the session it approved has been denied downstream by the
 proxy.  However, a proxy MUST NOT send an Access-Accept after
 receiving an Access-Reject from a proxy or from the home server.

Aboba & Vollbrecht Informational [Page 6] RFC 2607 Proxy Chaining and Policy in Roaming June 1999

       (Access-Req)       (Access-Req)       (Access-Req)
   NAS ----------> Proxy1 ----------> Proxy2 ---------->     Home
       (Access-Reject)    (Access-Accept)    (Access-Accept) Server
       <---------         <---------         <---------
                          (AcctPxStop)       (AcctPxStop)
                          ---------->        ---------->

5.2. Accounting behavior

 As described above, a proxy MUST NOT reply directly with an Access-
 Accept, and MUST NOT reply with an Access-Accept when it has received
 an Access-Reject from another proxy or Home Server. As a result, in
 all cases where an accounting record is to be generated (accepted
 sessions), no direct replies have occurred, and the Access-Request
 and Access-Accept have passed through the same set of systems.
 In order to allow proxies to match incoming Accounting-Requests with
 previously handled Access-Requests and Access-Accepts, a proxy SHOULD
 route the Accounting-Request along the same realm path travelled in
 authentication/authorization.  Note that this does not imply that
 accounting packets will necessarily travel the identical path,
 machine by machine, as did authentication/authorization packets.
 This is because it is conceivable that a proxy may have gone down,
 and as a result the Accounting-request may need to be forwarded to an
 alternate server. It is also conceivable that
 authentication/authorization and accounting may be handled by
 different servers within a realm.
 The Class attribute can be used to match Accounting Requests with
 prior Access Requests.  It can also be used to match session log
 records between the home Server, proxies, and NAS. This matching can
 be accomplished either in real-time (in the case that authentication
 and accounting packets follow the same path, machine by machine), or
 after the fact.
 Home servers SHOULD insert a unique session identifier in the Class
 attribute in an Access-Accept and Access-Challenge.  Proxies and
 NASes MUST forward the unmodified Class attribute.  The NAS MUST
 include the Class attribute in subsequent requests, in particular for
 Accounting-Requests. The sequence of events is shown below:

Aboba & Vollbrecht Informational [Page 7] RFC 2607 Proxy Chaining and Policy in Roaming June 1999

  1. ——→ ——–> ———>

NAS Proxy1 Proxy2 Home (add class)

   <-class--          <-class-           <-class--
   (Accounting-req)   (Accounting-req)  (Accounting-req)
       w/class           w/class            w/class
NAS ----------> Proxy1 ----------> Proxy2 ---------->       Home
    (Accounting-reply) (Accounting-reply)(Accounting-reply) Server
    <---------         <---------         <---------
 Since there is no need to implement policy in accounting, a proxy
 MUST forward all Accounting Requests to the next server on the path.
 The proxy MUST guarantee that the Accounting Request is received by
 the End Server and all intermediate servers.  The proxy may do this
 either by: 1) forwarding the Accounting Request and not sending a
 Reply until it receives the matching Reply from the upstream server,
 or 2) acting as a store point which takes responsibility for
 reforwarding the Accounting Request until it receives a Reply.
 Note that when the proxy does not send a reply until it receives a
 matching reply, this ensures that Accounting Start and Stop messages
 are received and can be logged by all servers along the roaming
 relationship path. If one of the servers is not available, then the
 operation will fail. As a result the entire accounting transaction
 will either succeed or fail as a unit, and thus can be said to be
 Where store and forward is implemented, it is possible that one or
 more servers along the roaming relationship path will not receive the
 accounting data while others will. The accounting operation will not
 succeed or fail as a unit, and is therefore not atomic.  As a result,
 it may not be possible for the roaming partners to reconcile their
 audit logs, opening new opportunities for fraud.  Where store and
 forward is implemented, forwarding of Accounting Requests SHOULD be
 done as they are received so the downstream servers will receive them
 in a timely way.
 Note that there are cases where a proxy will need to forward an
 Accounting packet to more than one system. For example, in order to
 allow for proper accounting in the case of a NAS that is shutting
 down, the proxy can send an Accounting-Request with Acct-Status-
 Type=Accounting-Off (8) to all realms that it forwards to.  In turn,
 these proxies will also flood the packet to their connected realms.

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6. References

 [1]  Aboba, B., Lu J., Alsop J., Ding J. and W. Wang, "Review of
      Roaming Implementations", RFC 2194, September 1997.
 [2]  Aboba, B. and G. Zorn, "Criteria for Evaluating Roaming
      Protocols", RFC 2477, January 1999.
 [3]  Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote
      Authentication Dial In User Service (RADIUS)", RFC 2138, April
 [4]  Rigney, C., "RADIUS  Accounting", RFC 2139, April 1997.
 [5]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [6]  Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
      2486, January 1999.

7. Security Considerations

 The RADIUS protocol described in [3] was designed for intra-domain
 use, where the NAS, proxy, and home server exist within a single
 administrative domain, and proxies may be considered a trusted
 component. However, in roaming the NAS, proxies, and home server will
 typically be managed by different administrative entities. As a
 result, roaming is inherently an inter-domain application, and
 proxies cannot necessarily be trusted.  This results in a number of
 security threats, including:
    Message editing
    Attribute editing
    Theft of passwords
    Theft and modification of accounting data
    Replay attacks
    Connection hijacking
    Fraudulent accounting

7.1. Message editing

 Through the use of shared secrets it is possible for proxies
 operating in different domains to establish a trust relationship.
 However, if only hop-by-hop security is available then untrusted
 proxies are capable of perpetrating a number of man-in-the-middle
 attacks.  These include modification of messages.

Aboba & Vollbrecht Informational [Page 9] RFC 2607 Proxy Chaining and Policy in Roaming June 1999

 For example, an Access-Accept could be substituted for an Access-
 Reject, and without end-to-end integrity protection, there is no way
 for the NAS to detect this. On the home server, this will result in
 an accounting log entry for a session that was not authorized.
 However, if the proxy does not forward accounting packets or session
 records to the home server, then the home server will not be able to
 detect the discrepancy until a bill is received and audited.
 Note that a proxy can also send an Access-Reject to the NAS after
 receiving an Access-Accept from the home server. This will result in
 an authentication log entry without a corresponding accounting log
 entry.  Without the proxy sending an Accounting-Request with Acct-
 Status-Type=Proxy-Stop (6) to the home server, then there will be no
 way for the home server to determine whether the discrepancy is due
 to policy implementation or loss of accounting packets.  Thus the use
 of Acct-Status-Type=Proxy-Stop can be of value in debugging roaming
 It should be noted that even if end-to-end security were to be
 available, a number of sticky questions would remain. While the end-
 points would be able to detect that the message from the home server
 had been modified by an intermediary, the question arises as to what
 action should be taken. While the modified packet could be silently
 discarded, this could affect the ability of the home server to .
 accept an Acct-Status-Type=Proxy-Stop message from an intermediate
 proxy. Since this message would not be signed by the NAS, it may need
 to be dropped by the home server.
 This is similar to the problem that IPSEC-capable systems face in
 making use of ICMP messages from systems with whom they do not have a
 security association. The problem is more difficult here, since in
 RADIUS retransmission is driven by the NAS.  Therefore the home
 server does not receive acknowledgement for Access-Accepts and thus
 would have no way of knowing that its response has not been honored.

7.2. Attribute editing

 RADIUS as defined in [3] does not provide for end-to-end security or
 capabilities negotiation. As a result there is no way for a home
 server to securely negotiate a mutually acceptable configuration with
 the NAS or proxies. As a result, a number of attribute editing
 attacks are possible.
 For example, EAP attributes might be removed or modified so as to
 cause a client to authenticate with EAP MD5 or PAP, instead of a
 stronger authentication method. Alternatively, tunnel attributes
 might be removed or modified so as to remove encryption, redirect the
 tunnel to a rogue tunnel server, or otherwise lessen the security

Aboba & Vollbrecht Informational [Page 10] RFC 2607 Proxy Chaining and Policy in Roaming June 1999

 provided to the client.  The mismatch between requested and received
 services may only be detectable after the fact by comparing the
 Access-Accept attributes against the attributes included in the
 Accounting-Request. However, without end-to-end security services, it
 is possible for a rogue proxy to cover its tracks.
 Due to the complexity of proxy configuration, such attacks need not
 involve malice, but can occur due to mis-configuration or
 implementation deficiencies.  Today several proxy implementations
 remove attributes that they do not understand, or can be set up to
 replace attribute sets sent in the Access-Accept with sets of
 attributes appropriate for a particular NAS.
 In practice, it is not possible to define a set of guidelines for
 attribute editing, since the requirements are very often
 implementation-specific. At the same time, protection against
 inappropriate attribute editing is necessary to guard against attacks
 and provide assurance that users are provisioned as directed by the
 home server.
 Since it is not possible to determine beforehand whether a given
 attribute is editable or not, a mechanism needs to be provided to
 allow senders to indicate which attributes are editable and which are
 not, and for the receivers to detect modifications of "non-editable"
 attributes.  Through implementation of end-to-end security it may be
 possible to detect unauthorized addition, deletion, or modification
 of integrity-protected attributes. However, it would still possible
 for a rogue proxy to add, delete or modify attributes that are not
 integrity-protected. If such attributes influence subsequent charges,
 then the possibility of fraud would remain.

7.3. Theft of passwords

 RADIUS as defined in [3] does not provide for end-to-end
 confidentiality. As a result, where clients authenticate using PAP,
 each proxy along the path between the local NAS and the home server
 will have access to the cleartext password. In many circumstances,
 this represents an unacceptable security risk.

7.4. Theft and modification of accounting data

 Typically in roaming systems, accounting packets are provided to all
 the participants along the roaming relationship path, in order to
 allow them to audit subsequent invoices. RADIUS as described in [3]
 does not provide for end-to-end security services, including
 integrity protection or confidentiality. Without end-to-end integrity
 protection, it is possible for proxies to modify accounting packets
 or session records.  Without end-to-end confidentiality, accounting

Aboba & Vollbrecht Informational [Page 11] RFC 2607 Proxy Chaining and Policy in Roaming June 1999

 data will be accessible to proxies.  However, if the objective is
 merely to prevent snooping of accounting data on the wire, then IPSEC
 ESP can be used.

7.5. Replay attacks

 In this attack, a man in the middle or rogue proxy collects CHAP-
 Challenge and CHAP-Response attributes, and later replays them. If
 this attack is performed in collaboration with an unscrupulous ISP,
 it can be used to subsequently submit fraudulent accounting records
 for payment.  The system performing the replay need not necessarily
 be the one that initially captured the CHAP Challenge/Response pair.
 While RADIUS as described in [3] is vulnerable to replay attacks,
 without roaming the threat is restricted to proxies operating in the
 home server's domain. With roaming, such an attack can be mounted by
 any proxy capable of reaching the home server.

7.6. Connection hijacking

 In this form of attack, the attacker attempts to inject packets into
 the conversation between the NAS and the home server. RADIUS as
 described in [3] is vulnerable to such attacks since only Access-
 Reply and Access-Challenge packets are authenticated.

7.7. Fraudulent accounting

 In this form of attack, a local proxy transmits fraudulent accounting
 packets or session records in an effort to collect fees to which they
 are not entitled. This includes submission of packets or session
 records for non-existent sessions. Since in RADIUS as described in
 [3], there is no end-to-end security, a rogue proxy may insert or
 edit packets without fear of detection.
 In order to detect submissions of accounting packets or session
 records for non-existent sessions, parties receiving accounting
 packets or session records would be prudent to reconcile them with
 the authentication logs. Such reconciliation is only typically
 possible when the party acts as an authentication proxy for all
 sessions for which an accounting record will subsequently be
 In order to make reconciliation easier, home servers involved in
 roaming include a Class attribute in the Access-Accept.  The Class
 attribute uniquely identifies a session, so as to allow an
 authentication log entry to be matched with a corresponding
 accounting packet or session record.

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 If reconciliation is put in place and all accounting log entries
 without a corresponding authentication are rejected, then the
 attacker will need to have obtained a valid user password prior to
 submitting accounting packets or session records on non-existent
 sessions. While use of end-to-end security can defeat unauthorized
 injection or editing of accounting or authentication packets by
 intermediate proxies, other attacks remain feasible. For example,
 unless replay protection is put in place, it is still feasible for an
 intermediate proxy to resubmit authentication or accounting packets
 or session records. In addition, end-to-end security does not provide
 protection against attacks by the local proxy, since this is
 typically where end-to-end security will be initiated. To detect such
 attacks, other measures need to be put in place, such as systems for
 detecting unusual activity of ISP or user accounts, or for
 determining whether a user or ISP account is within their credit
 Note that implementation of the store and forward approach to proxy
 accounting makes it possible for some systems in the roaming
 relationship path to receive accounting records that other systems do
 not get. This can result in audit discrepancies. About the best that
 is achievable in such cases is to verify that the accounting data is
 missing by checking against the authentication logs.

8. Acknowledgments

 Thanks to Pat Calhoun of Sun Microsystems, Mark Beadles of
 CompuServe, Aydin Edguer of Morningstar, Bill Bulley of Merit, and
 Steven P. Crain of Shore.Net for useful discussions of this problem

Aboba & Vollbrecht Informational [Page 13] RFC 2607 Proxy Chaining and Policy in Roaming June 1999

9. Authors' Addresses

 Bernard Aboba
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052
 Phone: 425-936-6605
 John R. Vollbrecht
 Merit Network, Inc.
 4251 Plymouth Rd.
 Ann Arbor, MI 48105-2785
 Phone: 313-763-1206

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10. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Aboba & Vollbrecht Informational [Page 15]

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