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rfc:rfc3543

Network Working Group S. Glass Request for Comments: 3543 Sun Microsystems Category: Standards Track M. Chandra

                                                         Cisco Systems
                                                           August 2003
               Registration Revocation in Mobile IPv4

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 This document defines a Mobile IPv4 Registration Revocation mechanism
 whereby a mobility agent involved in providing Mobile IP services to
 a mobile node can notify the other mobility agent providing Mobile IP
 services to the same mobile node of the termination of this
 registration.  The mechanism is also usable by a home agent to notify
 a co-located mobile node of the termination of its binding as well.
 Moreover, the mechanism provides for this notification to be
 acknowledged.  A signaling mechanism already defined by the Mobile
 IPv4 protocol is leveraged as a way to inform a mobile node of the
 revocation of its binding.

Table of Contents

 1.  Introduction and Applicability . . . . . . . . . . . . . . . .  2
 2.  Terminology. . . . . . . . . . . . . . . . . . . . . . . . . .  4
 3.  Registration Revocation Extensions and Messages. . . . . . . .  4
     3.1.  Advertising Registration Revocation Support. . . . . . .  5
     3.2.  Revocation Support Extension . . . . . . . . . . . . . .  6
     3.3.  Registration Revocation Message. . . . . . . . . . . . .  8
     3.4.  Registration Revocation Acknowledgment Message . . . . . 11
     3.5.  Replay Protection. . . . . . . . . . . . . . . . . . . . 14
 4.  Registration Revocation Overview . . . . . . . . . . . . . . . 15
     4.1.  Mobile Node Notification . . . . . . . . . . . . . . . . 15
     4.2.  Registration Revocation Mechanism - Agent Notification . 17
           4.2.1.  Negotiating Revocation Support . . . . . . . . . 17

Glass & Chandra Standards Track [Page 1] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

           4.2.2.  Home Domain Revoking a Registration. . . . . . . 19
                   4.2.2.1.  Home Agent Responsibilities. . . . . . 19
                   4.2.2.2.  Foreign Agent Responsibilities . . . . 20
                   4.2.2.3.  'Direct' Co-located Mobile Node
                             Responsibilities . . . . . . . . . . . 20
           4.2.3.  Foreign Domain Revoking a Registration . . . . . 21
                   4.2.3.1.  Foreign Agent Responsibilities . . . . 21
                   4.2.3.2.  Home Agent Responsibilities. . . . . . 22
           4.2.4.  Mobile Node Deregistering a Registration . . . . 23
     4.3.  Mobile IP Registration Bits in the Revocation Process. . 23
           4.3.1.  The 'R' Bit in Use . . . . . . . . . . . . . . . 23
           4.3.2.  The 'D' Bit in Use (co-located mobile nodes) . . 23
 5.  Error Codes. . . . . . . . . . . . . . . . . . . . . . . . . . 24
 6.  Security Considerations. . . . . . . . . . . . . . . . . . . . 24
     6.1.  Agent Advertisements . . . . . . . . . . . . . . . . . . 24
     6.2.  Revocation Messages. . . . . . . . . . . . . . . . . . . 25
 7.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 27
     7.1.  New Message Types. . . . . . . . . . . . . . . . . . . . 27
     7.2.  New Extension Values . . . . . . . . . . . . . . . . . . 27
     7.3.  New Error Codes. . . . . . . . . . . . . . . . . . . . . 27
 8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     8.1.  Normative (Numerical References) . . . . . . . . . . . . 27
     8.2.  Informational (Alphabetical References). . . . . . . . . 28
 Appendix A  An Example of the New Messages in Use. . . . . . . . . 29
             A.1.  The Registration Phase . . . . . . . . . . . . . 29
             A.2.  The Revocation Phase . . . . . . . . . . . . . . 29
 Appendix B  Disparate Address, and Receiver Considerations . . . . 30
 Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . 32
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32
 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 33

1. Introduction and Applicability

 Mobile IP [1] defines registration of a mobile node's location to
 provide connectivity between the mobile node and its home domain,
 facilitating communication between mobile nodes and any correspondent
 node.  At any time, either the home or foreign agent may wish to
 cease servicing a mobile node, or for administrative reasons may no
 longer be required to service a mobile node.
 This document defines a general registration revocation mechanism for
 Mobile IPv4, whereby a mobility agent can notify another mobility
 agent (or a 'direct' co-located mobile node) of the termination of
 mobility bindings.  A mobility agent that receives a revocation
 notification no longer has to provide services to the mobile node
 whose registration has been revoked.  A signaling mechanism already
 defined by the Mobile IPv4 protocol [1] is leveraged as a way to
 inform a mobile node of the revocation of its binding.

Glass & Chandra Standards Track [Page 2] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 The registration revocation protocol provides the following
 advantages:
 1. Timely release of Mobile IP resources.  Resources being consumed
    to provide Mobile IP services for a mobile node that has stopped
    receiving Mobile IP services by one agent, can be reclaimed by the
    other agent in a more timely fashion than if it had to wait for
    the binding to expire.  This also applies to the case in which a
    mobile node roams away from a foreign agent to another foreign
    agent.  Notification to the previous foreign agent would allow it
    to reclaim resources.
 2. Accurate accounting.  This has a favorable impact on resolving
    accounting issues with respect to the length of mobility bindings
    in both domains, as the actual end of the registration is relayed.
 3. Earlier adoption of domain policy changes with regards to services
    offered/required of a Mobile IP binding.  For example, the home
    domain may now require reverse tunnels [C], yet there are existing
    bindings that do not use them.  Without a revocation mechanism,
    new services can only be put in place or removed as bindings are
    re-registered.
 4. Timely notification to a mobile node that it is no longer
    receiving mobility services, thereby significantly shortening any
    'black-hole' periods to facilitate a more robust recovery.
 The revocation protocol is an active, yet unobtrusive mechanism
 allowing more timely communication between the three Mobile IP
 entities in the various administrative domains.  Since many mobile
 nodes may not understand the concept of revocation, care has been
 taken to ensure backwards compatibility with [1].
 The registration revocation protocol does not replace the methods
 described in [1] for Mobile IP deregistration, as the purpose of
 these mechanisms is fundamentally different.  Deregistration messages
 are used by a mobile node to inform its home agent that it has e.g.,
 roamed back to its home subnet, whereas revocation messages are used
 between mobility agents to signal the termination of mobility
 bindings.  More specifically, the revocation message defined here is
 NOT for use by 'direct' co-located mobile nodes that are terminating
 their registration as deregistration messages are already sufficient
 for this purpose.  A 'direct' co-located mobile node, however, may
 wish to process revocation messages as it is a useful mechanism to
 trigger the re-negotiation of required services from the home domain.

Glass & Chandra Standards Track [Page 3] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

2. Terminology

 It is assumed that the reader is familiar with the terminology used
 in [1].  In addition, the following terms are defined:
 'Direct' Co-located Mobile Node
    A mobile node registering directly with its home agent, with the
    'D' bit set in its registration request, and NOT registering
    through a foreign agent.
 Mobile IP Resources
    Various functional elements allocated by a mobility agent to
    support a Mobile IP binding, e.g., memory.
 Mobile IP Services
    Various responsibilities of a mobility agent in supporting a
    mobile node as defined in [1], e.g., encapsulation of packets
    addressed to a mobile node by a home agent, decapsulation of these
    packets by a foreign agent for delivery to a mobile node, etc.
 Mobility Agent
    The home agent or foreign agent as specified in [1].
 Revocation
    Premature termination of a mobility binding.
 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in BCP 14, RFC 2119 [3].

3. Registration Revocation Extensions and Messages

 Registration revocation in Mobile IPv4 is accomplished via the
 following:
  1. Advertising Registration Revocation Support (Section 3.1.):
    o  A flag in the Agent Advertisement extension has been reserved
       for agents to advertise their support of revocation messages.

Glass & Chandra Standards Track [Page 4] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

  1. Revocation Support Extension (Section 3.2.):
    o  This extension is appended to a registration request or
       registration reply by a mobility agent to indicate its support
       of registration revocation.
    o  This extension is appended to a registration request by a
       'direct' co-located mobile node to indicate its understanding
       of revocation messages.
  1. Registration Revocation Message (Section 3.3.):
    o  A message sent by a mobility agent to inform another mobility
       agent, or a 'direct' co-located mobile node, that it has
       revoked the binding of a mobile node.
  1. Registration Revocation Acknowledgment Message (Section 3.4.):
    o  A message sent by mobility agents or 'direct' co-located mobile
       nodes to indicate the receipt of a revocation message.
 Security considerations related to the above messages and extensions
 are covered in Section 6.

3.1. Advertising Registration Revocation Support

 Mobility agents can advertise their support of registration
 revocation with a modification to the Mobility Agent Advertisement
 extension described in [1].  An 'X' bit is introduced to indicate an
 agent's support for Registration Revocation.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |        Sequence Number        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Registration Lifetime      |R|B|H|F|M|G|r|T|U|X| reserved  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  zero or more Care-of Addresses               |
 |                              ...                              |
    X   The mobility agent supports Registration Revocation
 A foreign agent that sets the 'X' bit in an agent advertisement
 extension MUST support registration revocation messages on that link,
 specifically the Revocation Support Extension (section 3.2.),
 Revocation Messages (section 3.3.), and Revocation Acknowledgment

Glass & Chandra Standards Track [Page 5] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 (section 3.4.).  It is not required that all agents advertising on
 the same link support registration revocation, nor is it required
 that an agent advertise this support on all of its links.
 Note that using this information, a mobile node can select a foreign
 agent that supports Registration Revocation.  Should a mobile node
 not understand this bit, it simply ignores it as per [1].
 As a bit in the agent advertisement, use of the 'X' bit has no impact
 on other messages, such as e.g., Challenge-Response [2].

3.2. Revocation Support Extension

 The Mobile IP revocation support extension indicates support of
 registration revocation, and so MUST be attached to a registration
 request or registration reply by any entity that wants to receive
 revocation messages.  Normally, this is either a foreign agent, or a
 home agent.  However a 'direct' co-located mobile node MAY also
 include a revocation support extension in its registration request.
 A mobile node which is not co-located MUST NOT include a Revocation
 Support Extension in its registration.
 A foreign agent advertising the 'X' bit on the link on which the
 registration request was received, and that has a security
 relationship with the home agent identified in the same registration
 request, MUST attach a revocation support extension to the forwarded
 registration request.  A home agent that receives a registration
 request that does not contain a revocation extension SHOULD NOT
 include a revocation support extension in the associated registration
 reply.
 The format of the revocation support extension is based on the Type-
 Length-Value Extension Format given in [1] and is defined as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |     Length    |I|        Reserved            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |                            Timestamp                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
    Type     137

Glass & Chandra Standards Track [Page 6] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

    Length
             Length (in bytes, currently 6).  Does NOT include Type
             and Length fields (in accordance with section 1.9. of
             [1]).  This allows for a longer extension length should
             more bits be required in the future.
    Timestamp
             Current 4-byte timestamp of the mobility agent or
             'direct' co-located mobile node.  This is used to
             identify the ordering of registrations as they are
             forwarded, how they relate to the sending of any
             revocation messages, and to identify the approximate
             offset between the clocks of the mobility agents
             providing support for this binding, or between a 'direct'
             co-located mobile node and its home agent.
    'I' Bit
             This bit is set to '1' by a mobility agent to indicate it
             supports the use of the 'I' bit in revocation messages
             (section 3.3.)
             When sent by a foreign agent in a registration request:
             If set to 1, the FA is willing to have the home agent use
             the 'I' bit in the revocation process to determine
             whether the mobile node should be informed of the
             revocation or not.
             If set to 0, indicates to the home agent that the foreign
             agent will follow its own policy with regards to
             informing the mobile node in the event of a revocation.
             When sent by a home agent in response to a revocation
             extension in which the 'I' bit was set to '1':
             If set to 1, the home agent agrees to use the 'I' bit in
             the revocation process to indicate to the foreign agent
             whether or not the mobile node should be informed.
             If set to 0, the home agent will not use the 'I' bit in
             the revocation process, thereby yielding to the foreign
             agent's default behavior with regard to informing the
             mobile node.
             To preserve the robustness of the protocol, the
             recommended default behavior for a foreign agent is to
             inform the mobile node of its revocation as described in
             Section 4.1.

Glass & Chandra Standards Track [Page 7] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

    Reserved
             Reserved for future use.  MUST be set to 0 on sending,
             MUST be ignored on receiving.
 When appearing in a registration request, or registration reply, the
 Mobile IP revocation support extension MUST be protected either by a
 foreign-home authentication extension, a mobile-home authentication
 extension, or any other equivalent mechanism [1], e.g., via AAA [A],
 [B], or perhaps IPsec.  If the extension appearing in either of these
 registration messages is NOT protected, the appropriate action as
 described by [1] (Sections 3.8.2.1. and Sections 3.7.3.1.) MUST be
 taken.
 Support of the 'I' bit is OPTIONAL.  If a mobility agent does not
 support the specified functionality, it MUST set the 'I' bit to zero.
 Note that the home agent setting the 'I' bit to '1' in response to a
 revocation extension from the foreign agent in which the 'I' bit was
 set to '0' is undefined, and SHOULD NOT be done.
 'I' bit support has been negotiated when both agents have set the 'I'
 bit to '1' in their revocation support extensions.
 It is important to note that this extension is skippable (i.e., if
 the receiving mobility agent does not understand this extension, it
 MUST skip it, and continue processing the remainder of the
 registration request).

3.3. Registration Revocation Messages

 A revocation message is sent by a mobility agent to inform another
 mobility agent, or a 'direct' co-located mobile node, that it is
 revoking the binding of a mobile node.
 IP Fields:
    Source Address       In the case of the home agent issuing the
                         registration revocation, the address
                         registered with the care-of address as that
                         of the home agent (that is the address
                         identified as the home address of this
                         binding).
                         In the case of the foreign agent issuing the
                         registration revocation, the address
                         registered with the home agent as the care-of
                         address.

Glass & Chandra Standards Track [Page 8] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

    Destination Address  In the case of the home agent issuing the
                         registration revocation, the source address
                         of the last approved registration request for
                         this binding, i.e., the destination address
                         of the last registration reply indicating
                         success for this binding.
                         In the case of the foreign agent issuing the
                         registration revocation, the address
                         registered as that of the home agent by the
                         mobile node whose registration is being
                         revoked.
 UDP Fields:
    Source Port          variable
    Destination Port     434
 The UDP header is followed by the Mobile IP fields shown below:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |   Reserved    |A|I|          Reserved         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Home Address                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Home Domain Address                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Foreign Domain Address                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Revocation Identifier                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Extensions...
 +-+-+-+-+-+-+-+-+-+-+-+-+-
 |   Authenticator...
 +-+-+-+-+-+-+-+-+-+-+-+-+-
    Type     7
    Reserved MUST be sent as 0, ignored when received.
    A        Agent bit ('direction' bit).
             This bit identifies the role of the agent sending the
             revocation, that is the 'direction' of the revocation
             message.  This is useful for detecting reflection

Glass & Chandra Standards Track [Page 9] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

             attacks, particularly when symmetric keying is being
             used.
             Set to '0' if the revoking agent is servicing this
             binding as a foreign agent.
             Set to '1' if the revoking agent is servicing this
             binding as a home agent.
    I        Inform bit.
             This bit MUST NOT be set to '1' unless 'I' bit support
             was negotiated in the revocation extension messages
             passed in the registration process, otherwise the results
             can be unpredictable.
             When sent by the home agent to a foreign agent:
             Set to '0' to request that the mobile node SHOULD NOT be
             informed of the revocation, or because the use of the 'I'
             bit was not agreed upon.
             Set to '1' to request that the mobile node be informed of
             the revocation.
             When sending a revocation message to a 'direct' co-
             located mobile node, this bit is essentially irrelevant,
             but SHOULD be set to '1'.
             When sent by the foreign agent:
             Set to '0' to indicate that the foreign agent is using
             foreign domain policy as to whether or not the mobile
             node should be informed of the revocation, or because 'I'
             bit support was not agreed upon.
             Set to '1' to ask the home agent if the mobile node
             should be informed of the revocation.
    Reserved
             MUST be sent as 0, ignored when received.
    Home Address
             The home IP address of the mobile node whose registration
             is being revoked.

Glass & Chandra Standards Track [Page 10] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

    Foreign Domain Address
             The relevant IP address in the foreign domain to identify
             which binding is being revoked.  This is one of the
             following: (i) the foreign agent's IP address, or (ii)
             the co-located care-of address.
    Home Domain Address
             The IP address of the home agent to identify which
             binding is being revoked.
    Revocation Identifier
             Protects against replay attacks.  The revoking agent MUST
             insert its current 4-byte timestamp running off the same
             clock as it is using to fill in the timestamp in its
             revocation extensions.  See section 3.5.
 A registration revocation message MUST be protected by either a valid
 authenticator as specified in [1], namely a home-foreign
 authenticator, if the communication is between home and foreign
 agents, or a mobile-home authenticator if the communication is being
 sent from a home agent to a 'direct' co-located mobile node, or
 another security mechanism at least as secure, and agreed upon by the
 home and foreign domains, e.g., IPsec.  If any agent, or 'direct'
 co-located mobile node, receives a registration revocation message
 that does not contain a valid authenticator, and is not adequately
 protected, the revocation message MUST be ignored, and silently
 discarded.
 A revocation message MUST NOT be sent for any registration that has
 expired, and MAY only be sent prior to the expiration of a mobile
 node's registration.  Note, however, due to the nature of datagram
 delivery, this does not guarantee these messages will arrive before
 the natural expiration of any binding.
 An agent MUST NOT send more than one revocation message or
 registration message per second for the same binding.  Note that this
 updates [1] by including revocation messages in the rate limit
 specified in [1], i.e., that an agent MUST NOT send more than one
 registration message per second for the same binding.
 An example of the use of revocation messages is given in Appendix A.

3.4. Registration Revocation Acknowledgment Message

 A revocation acknowledgment message is sent by mobility agents or
 'direct' co-located mobile nodes to indicate the successful receipt
 of a revocation message.

Glass & Chandra Standards Track [Page 11] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 IP fields:
    Source Address       Copied from the destination address of the
                         received registration revocation message for
                         which this registration revocation
                         acknowledgment message is being generated.
    Destination Address  Copied from the source address of the
                         received registration revocation message for
                         which this registration revocation
                         acknowledgment message is being generated.
 UDP fields:
    Source Port          434 (copied from the destination port of the
                         revocation message).
    Destination Port     Copied from the source port of the revocation
                         message.
 The UDP header is followed by the Mobile IP fields shown below:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |     Reserved  |I|         Reserved            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Home Address                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Revocation Identifier                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Extensions...
 +-+-+-+-+-+-+-+-+-+-+-+-+-
 | Authenticator...
 +-+-+-+-+-+-+-+-+-+-+-+-+-
    Type     15
    Reserved
             MUST be sent as 0, ignored when received.
    I        Inform bit.
             The 'I' bit MUST NOT be set to '1' in the revocation
             acknowledgment messages unless it was set to '1' in the
             revocation message.  If an agent receives a revocation
             acknowledgment message in which the 'I' bit is set to
             '1', but for which the revocation message being

Glass & Chandra Standards Track [Page 12] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

             acknowledged had the 'I' bit set to '0', the 'I' bit in
             the revocation acknowledgment message MUST be ignored.
             When sent by the home agent:
             Set to '1' by the home agent to request the foreign agent
             inform the mobile node of the revocation.
             Set to '0' by the home agent to request the foreign agent
             not inform the mobile node of the revocation.
             When sent by a foreign agent:
             Set to '1' to indicate to the home agent that the mobile
             node was informed.
             Set to '0' to indicate to the home agent that the foreign
             agent used local policy to determine whether or not the
             mobile node should be informed.  For purposes of protocol
             robustness, it is highly recommended that such a default
             be set for the foreign agent to inform the mobile node of
             the revocation.
    Reserved
             MUST be sent as 0, ignored when received.
    Home Address
             The home address copied from the revocation message for
             which this acknowledgment is being sent.
    Revocation Identifier
             Copied from the Revocation Identifier of the revocation
             message for which this acknowledgment is being sent.  See
             Section 3.5.
 A registration revocation acknowledgment message MUST be sent in
 response to a valid and authenticated registration revocation
 message.
 A registration acknowledgment message MUST be protected by either a
 valid authenticator as specified in [1], namely a home-foreign
 authenticator if the communication is between home and foreign
 agents, or a mobile-home authenticator if the communication is
 between home agent and 'direct' co-located mobile node, or another
 security mechanism at least as secure and agreed upon by the home and
 foreign domains, e.g., IPsec.

Glass & Chandra Standards Track [Page 13] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 An example of the use of Revocation Acknowledgment Messages is given
 in Appendix A.

3.5. Replay Protection

 As registration revocation messages are designed to terminate service
 for a mobile node, or multiple mobile nodes simultaneously, replay
 protection is crucial to prevent denial of service attacks by
 "malicious repeaters" - those who store datagrams with the intent of
 replaying them at a later time, or by "malicious reflectors" - those
 who reflect packets back at their original source (both a form of
 "active" attack).  See Section 6. for a discussion of these security
 considerations.
 All Revocation Messages and Revocation Acknowledgment Messages MUST
 be authenticated as well be replay-protected.  The order in which
 they are done, however, is up to implementation.
 Replay protection is handled with a simple timestamp mechanism, using
 a single 32-bit identifier field in the registration revocation
 message, in conjunction with the home address field, to associate any
 revocation acknowledgment messages with its revocation messages.  To
 do this:
  1. The revoking agent sets the 'A' bit to its agent-type, and the

Revocation Identifier field in the registration revocation message

    to a valid 32-bit timestamp from the same clock it is using to set
    the timestamp field of its revocation extensions included in
    registration messages.
  1. Upon receipt of an authenticated revocation message, the receiving

agent (or 'direct' co-located mobile node) MUST check the value of

    the 'A' bit, and Revocation Identifier to make sure this
    revocation message is not a replay of an old revocation message
    received from the same agent.  The receiving agent MUST also check
    that the message is not a reflection of a revocation message it
    sent in relation to the identified binding.  If the 'A' bit and
    Identifier field imply this packet is a replay, the revocation
    message MUST be silently discarded.
  1. When building a revocation acknowledgment message, the

acknowledging agent (or 'direct' co-located mobile node) copies

    the values of the Home Address and Revocation Identifier fields
    from the revocation message into the  Home Address and the
    Revocation Identifier of the revocation acknowledgment message.
    This is so the revoking agent can match this revocation
    acknowledgment to its corresponding revocation message.

Glass & Chandra Standards Track [Page 14] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

  1. Upon receiving a valid revocation acknowledgment, the revoking

agent MUST check the Home Address and Identifier fields to make

    sure they match those fields from a corresponding revocation
    message it sent to the acknowledging agent.  If not, this
    revocation acknowledgment message MUST be silently discarded.
 Note that since the Identifier in an incoming revocation message is a
 32-bit timestamp, it is possible for an agent to check the validity
 of the Identifier fields without having to remember all identifiers
 sent by that corresponding agent.
 Note: as it is possible for a mobile node to register at different
 times with different home agents, and at different times with
 different foreign agents, it is crucial that it not be required that
 the Identifier fields be unique in messages from different agents as
 there is no guarantee that clocks on different agents will be
 synchronized.  For example, if a mobile node has simultaneous
 bindings with multiple foreign agents, and if revocation messages are
 received by more than one such foreign agent "simultaneously", it is
 possible the revocation message from one of these foreign agents may
 contain Identifier fields that happen to match those of any or all
 the other foreign agents.  This MUST NOT result in any of these
 revocation messages being ignored.

4. Registration Revocation Overview

 Registration Revocation consists of two distinct pieces: a signaling
 mechanism between tunnel endpoints, and a signaling mechanism between
 foreign agent and mobile node.  A 'direct' co-located mobile node MAY
 implement revocation extensions and revocation acknowledgment in
 order to receive and respond to revocation messages from its home
 agent, however, a 'direct' co-located mobile node MUST NOT send a
 revocation message as de-registration messages defined in [1] are
 sufficient for this purpose.
 For further discussion on security issues related to registration
 revocation, refer to Section 6.

4.1. Mobile Node Notification

 A mechanism which provides a foreign agent a way to actively notify a
 mobile node that its binding has been reset already exists in [1],
 though it has been overlooked for this purpose.
 A brief overview of the mechanics of the sequence number in agent
 advertisement from [1] is given so that the mechanism by which the
 foreign agent 'implies' to the mobile node that its binding is no
 longer active is clearly understood.

Glass & Chandra Standards Track [Page 15] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 When a foreign agent begins sending agent advertisements, it starts
 with a sequence number of 0, and [monotonically] increments the
 sequence number with each subsequent agent advertisement.  In order
 for a mobile node to be able to distinguish between a foreign agent
 that has simply exhausted the sequence number space from one which
 has been reset, when the agent increments the sequence number counter
 past its maximum value, it sets the sequence number to 256 instead of
 rolling to 0 [1].  In this way, a mobile node would have to miss, at
 that time, 256 advertisements in a row to mistake a reset as a roll-
 over.  Moreover, the lifetimes contained within an agent
 advertisement should be set in such a way that when a mobile node
 believes it has missed 3 beacons, the entry for this foreign agent
 should time out, and if the mobile node is registered there, it
 should send an agent solicitation [1].  If, however, an agent is
 somehow reset, it will begin advertising with a sequence number of 0,
 and the mobile node can presume this foreign agent has lost its
 binding, and the mobile node SHOULD re-register to make sure it is
 still obtaining Mobile IP services through this foreign agent.
 Leveraging this mechanism, a foreign agent may consciously notify all
 mobile nodes currently bound to it that it has "reset" all of their
 bindings, even if the agent itself has not been reset, by simply
 [re]setting the sequence number of the next agent advertisement to 0.
 Moreover, a foreign agent may inform all mobile nodes currently bound
 to it that they should re-register with a different foreign agent by
 simultaneously setting the 'B' bit in the advertisement to 1,
 indicating this foreign agent is busy and is not accepting new
 registrations [1].  In these situations, any mobile node in
 compliance with [1] will presume this foreign agent has lost its
 binding, and must re-register if they wish to re-establish Mobile IP
 functionality with their home subnet.
 To indicate to any registered mobile node that its binding no longer
 exists, the foreign agent with which the mobile node is registered
 may unicast an agent advertisement with the sequence number set to 0
 to the mobile node [1], [D].  Moreover, if such a foreign agent
 wishes to indicate to the mobile node that its binding has been
 revoked, and that the mobile node should not attempt to renew its
 registration with it, the foreign agent MAY also set the 'B' bit to 1
 in these agent advertisements, indicating it is busy, and is not
 accepting new registrations [1].  All mobile nodes compliant with [1]
 will understand that this means the agent is busy, and MAY either
 immediately attempt to re-register with another agent in their
 foreign agent cache, or MAY solicit for additional agents.  In the
 latter case, a foreign agent can optionally remember the mobile
 node's binding was revoked, and respond to the solicit in the same
 way, namely with the 'B' bit set to 1.  It should be noted, though,
 that since the foreign agent is likely to not be setting the 'B' bit

Glass & Chandra Standards Track [Page 16] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 to 1 in its broadcasted agent advertisements (sent to the entire
 link), the revoked mobile node, upon hearing this agent's multicast
 agent advertisement without the 'B' bit set, may attempt to
 [re]register with it.  If this happens, depending on foreign domain
 policy, the foreign agent can simply deny the mobile node with an
 appropriate error code (e.g., "administratively prohibited").  At
 this time, a mobile node can use foreign agent fallback to attempt to
 register with a different foreign agent as described in [1].
 Mobile nodes which understand the revocation mechanism described by
 this document may understand that a unicast agent advertisement with
 the sequence number reset to 0 could indicate a revocation, and may
 attempt to re-register with the same foreign agent, or register with
 a different foreign agent, or co-locate.
 Agent Advertisements unicast to a mobile node MUST be sent as
 described in [1] in addition to any methods currently in use on the
 link to make them secure or authenticatable to protect from denial-
 of-service attacks.

4.2. Registration Revocation Mechanism - Agent Notification

 A foreign agent that is currently supporting registration revocation
 on a link MUST set the 'X' bit in its Agent Advertisement Extensions
 being sent on that link.  This allows mobile nodes requiring
 Registration Revocation services to register with those foreign
 agents advertising its support.

4.2.1. Negotiation of Revocation Support

 During the registration process, if the foreign agent wishes to
 participate in revocation messages with the home domain, it MUST have
 an existing security association with the home agent identified in
 the registration request, and append a revocation support extension
 (defined in Section 3.2.) to it.  If the corresponding registration
 reply from this home agent does not contain a revocation support
 extension, the foreign agent SHOULD assume the home agent does not
 understand registration revocation, or is unwilling to participate.
 If this is unacceptable to the foreign agent, it MAY deny the
 registration with e.g., "Administratively Prohibited".  Note that in
 this case, where a security association exists, as specified in [1],
 both registration request and registration reply MUST still contain
 home-foreign authenticators.
 If a home agent wishes to be able to exchange revocation messages
 with the foreign domain, it MUST have an existing security
 association with the foreign agent who relayed the registration
 request, and it MUST append a revocation support extension to the

Glass & Chandra Standards Track [Page 17] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 registration reply.  If the registration request from a foreign agent
 did not contain a revocation support extension, the home agent SHOULD
 assume the foreign agent does not understand registration revocation,
 or is unwilling to participate specifically for this binding.  If
 this is unacceptable to the home agent, it MAY deny the registration
 with e.g., "Administratively Prohibited".  The home agent MAY include
 a revocation support extension in the registration reply.
 If a 'direct' co-located mobile node wishes to be informed of a
 released binding by its home agent, it MUST insert a revocation
 support extension into the registration request.  If this is
 acceptable to the home agent, it MUST include a revocation support
 extension in its registration reply.  Note that if this is not
 acceptable, the home agent MAY deny the registration, or it MAY
 simply not include a revocation support extension in its registration
 reply indicating to the mobile node that it will not participate in
 revocation for this binding.  A home agent which receives a
 registration request from a 'direct' co-located mobile node which
 does not contain a revocation support extension MAY deny the
 registration with e.g., "Administratively Prohibited" and also MAY or
 MAY NOT include a revocation support extension in the registration
 reply.
 Note that a non-colocated mobile node MUST NOT insert a revocation
 support extension into its registration request.  If a foreign agent
 receives such a registration request, it MUST silently discard it,
 and MAY log it as a protocol error.
 The 'I' bit in the revocation extension is used to indicate whether
 or not the decision to inform the mobile node that its binding is
 terminated will be left to the home agent.  This functionality is
 offered by the foreign agent, and accepted by the home agent.  More
 precisely, by sending a revocation extension attached to a
 registration request in which the 'I' bit is set to 1, the foreign
 agent is indicating to the home agent that it MAY leave the decision
 to inform this mobile node that its registration is terminated up to
 the home agent.  (The term "MAY" is used here because it is
 recognized that domain policy may change during the lifetime of any
 registration).  The home agent can acknowledge that it wishes to do
 this by setting the 'I' bit to 1, or it can indicate it will not do
 so by setting the 'I' bit to 0, in the revocation extension appearing
 in the registration reply.
 Revocation support is considered to be negotiated for a binding when
 both sides have included a revocation support extension during a
 successful registration exchange.

Glass & Chandra Standards Track [Page 18] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

4.2.2. Home Domain Revoking/Releasing a Registration

 The following section details the responsibilities of each party
 depending on the functionality negotiated in the revocation support
 extensions when the home domain is revoking a registration.

4.2.2.1. Home Agent Responsibilities

 In the case where a home agent is revoking a mobile node's binding,
 and revocation support has been negotiated, the home agent MUST
 notify the foreign domain address it is terminating the tunnel entry
 point by sending a revocation message.  Note that the foreign domain
 address can either be the foreign agent care-of address, or the co-
 located care-of address of a 'direct' co-located mobile node.
 As a home agent, it MUST set the 'A' bit to '1', indicating this
 packet is coming from the home agent servicing this binding.
 When a revocation message is being sent to a foreign agent, and the
 use of the 'I' bit was negotiated in the registration process, the
 home agent MUST set the 'I' bit to 1 if the home agent would like the
 foreign agent to inform the mobile node of the revocation.
 Conversely, if the home agent does not want the mobile node notified,
 it MUST set the 'I' bit to 0.  Note that the home agent could also
 set the 'I' bit to '0' because it knows the mobile node has
 registered with a different foreign agent, and so there is no need
 for the foreign agent to attempt a notification.
 The home agent MUST set the Identifier field as defined in Section
 3.5., and MUST include a valid authenticator as specified in Section
 3.3.
 If the home agent does not receive a revocation acknowledgment
 message within a reasonable amount of time, it MUST retransmit the
 revocation message.  How long the home agent waits to retransmit, and
 how many times the message is retransmitted is limited by the
 requirement that:
  1. every time the home agent is about to retransmit the revocation

message, it MUST update the value of the timestamp in the

    revocation identifier with a current value from the same clock
    used to generate the timestamps in the revocation extensions sent
    to this foreign agent.  Note that this also necessarily means
    updating any fields derived using the revocation identifier (e.g.,
    a home-foreign authenticator).
  1. the home agent MUST NOT send more than one revocation per second

for a particular binding,

Glass & Chandra Standards Track [Page 19] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

  1. the time between retransmissions SHOULD fall-back in analogy with

the registration guidelines in [1], namely exponential backoff,

    and
  1. the home agent MUST NOT retransmit revocation messages beyond the

normal life of the binding identified by the revocation message.

4.2.2.2. Foreign Agent Responsibilities

 Upon receiving a registration revocation message, the foreign agent
 MUST check that the validity of the authenticator, the 'A' bit, and
 the identifier field against replay as defined by Section 3.5.  The
 foreign agent MUST also identify the binding described by the home
 agent as being released using the information in the revocation
 message, namely the addresses identified by the mobile node address,
 the foreign domain address, the home domain address, as well as the
 timestamp in the revocation message, and also the timestamp in the
 last accepted registration message; revocations are only valid for
 existing registrations, and so the timestamp of a registration MUST
 precede the revocation message (note that both of those timestamps
 were set by the same home agent).  Upon locating the binding, the
 foreign agent MUST revoke it, and MUST respond with a revocation
 acknowledgment sent to the source address of the revocation message.
 If the 'I' bit was negotiated, the foreign agent MUST check the value
 of the 'I' bit in the revocation message and act accordingly.
 If notifying the mobile node by the methods described in Section
 4.1., the foreign agent MUST set the 'I' bit to '1' in the revocation
 acknowledgment to be sent to the home agent, or if not notifying the
 mobile node, the foreign agent MUST set the 'I' bit to '0'.
 The foreign agent may discontinue all Mobile IP services by the
 former binding at this time, and free up any resources that were
 being used by it.
 The foreign agent MUST then generate a revocation acknowledgment,
 setting the Home Address and Identifier field in the revocation
 acknowledgment message as described by Section 3.5., and protect it
 with a valid authenticator as specified in Section 3.3.

4.2.2.3. 'Direct' co-located mobile node Responsibilities

 Upon receiving a revocation message, the 'direct' co-located mobile
 node MUST validate the authenticator, and check the home address and
 identifier specified in the revocation message for replay.  If the
 packet passes authentication, and the identifier reveals this
 revocation to be new, the mobile node MUST verify that the
 information contained in the revocation messages identifies the home

Glass & Chandra Standards Track [Page 20] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 agent with which it has a current binding, that this binding
 identifies correctly this mobile node and any foreign domain address
 it is currently using.  If the mobile node is able to identify such a
 binding, the mobile node SHOULD first generate a revocation
 acknowledgment message which MUST be sent to the IP source address of
 the revocation message.  The mobile node may then terminate any
 reverse tunnel encapsulation [C] it is using to this home agent, and
 consider its binding revoked, and free up any other resources
 associated with the former binding.

4.2.3. Foreign Domain Revoking/Releasing a Registration

 The following section details the responsibilities of each party
 depending on the functionality negotiated in the revocation support
 extensions when the foreign domain is revoking a registration.  Note
 that revocation support for a co-located mobile node registering via
 a foreign agent (because the 'R' bit was set in the agent's
 advertisement) is not supported.  See Section 4.3.1. for details.

4.2.3.1. Foreign Agent Responsibilities

 If the use of the 'I' bit was negotiated, and the foreign domain
 policy of informing the mobile node has not changed since the last
 successful registration exchange, the foreign agent MUST NOT inform
 any mobile node of its revocation at this time.  Instead, the foreign
 agent MUST set the 'I' bit to '1' in the revocation message, thereby
 asking the home agent to use the 'I' bit in the revocation
 acknowledgment to indicate if it should notify the effected mobile
 nodes.  If the policy on the foreign domain was to not notify the
 mobile node, or if it has changed since the most recent successful
 registration, and the foreign agent is no longer able to use the 'I'
 bit, the foreign agent MUST set the 'I' bit to '0', and follow the
 policies of the foreign domain with regard to notifying the mobile
 node.
 Note that the 'A' bit MUST be set to '0' to indicate that the
 revocation message is coming from the foreign agent servicing this
 binding.
 Before transmitting the revocation message, the foreign agent MUST
 set the revocation identifier as described by section 3.5., and MUST
 include an authenticator as described by section 3.3.
 If the foreign agent does not receive a revocation acknowledgment
 message within a reasonable amount of time, it MUST retransmit the
 revocation message.  How long the foreign agent waits to retransmit,
 and how many times the message is retransmitted is only limited by
 the following specifications:

Glass & Chandra Standards Track [Page 21] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

  1. every time the foreign agent is about to retransmit the revocation

message, it MUST update the value of the timestamp in the

    revocation identifier with a current value from the same clock
    used to generate the timestamps in the revocation extensions sent
    to this home agent.  Note that this also necessarily means
    updating any fields derived using the revocation identifier (e.g.,
    a home-foreign authenticator).
  1. MUST NOT send more than one revocation per second for a particular

binding,

  1. SHOULD set its retransmissions to fall-back in analogy with the

registration guidelines in [1], namely exponential backoff, and

  1. MUST NOT retransmit revocation messages beyond the normal life of

the binding identified by the revocation message.

4.2.3.2. Home Agent Responsibilities

 Upon receiving a registration revocation message, the home agent MUST
 check the 'A' bit, and identifier field, as well as the
 authenticator.  If the packet is acceptable, the home agent MUST
 locate the binding identified by the foreign agent as being released
 using the information in the revocation message, namely the addresses
 identified by the home address, the foreign domain address and the
 home domain address fields.  As revocations are only valid for
 existing registrations, the timestamp of a registration MUST precede
 the revocation message (note that both of those timestamps were set
 by the same foreign agent).  Since this binding is no longer active,
 the home agent can free up any resources associated with the former
 binding and discontinue all Mobile IP services for it.
 Upon processing a valid registration revocation message, the home
 agent MUST send a revocation acknowledgment to the IP source address
 of the registration revocation message.
 If use of the 'I' bit was negotiated, and the 'I' bit is set to '1'
 in the revocation message, the home agent should decide if it wants
 the mobile node informed of the revocation of this binding.  If it
 does want the mobile node informed, it MUST set the 'I' bit in the
 revocation acknowledgment message to '1'.  If it does not want the
 mobile node informed, it MUST set the 'I' bit to '0'.
 The home agent MUST set the Home Address, and Revocation Identifier
 fields as described by Section 3.5., and protect the revocation
 acknowledgment message with a valid authenticator as specified in
 Section 3.3.

Glass & Chandra Standards Track [Page 22] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

4.2.4. Mobile Node Deregistering a Registration

 The cases where a mobile node is registered with its home agent,
 whether it is registered directly with its home agent ('direct' co-
 located mobile node), or registered via a foreign agent, and wishes
 to terminate its own binding, the mobile node MUST NOT send a
 revocation message, but SHOULD simply deregister the appropriate
 care-of address with its home agent as described by [1].

4.3. Mobile IP Registration Bits in the Revocation Process

 Several of the bits used in the registration process need special
 consideration when using the revocation mechanism.

4.3.1. The 'R' Bit in Use

 If the foreign agent wishes to be able to revoke a mobile node's
 registration, it MUST set the 'R' bit in its agent advertisements.
 (A foreign agent advertising the 'R' bit requests every mobile node,
 even one that is co-located (and whose registration would otherwise
 by-pass the foreign agent), to register with the foreign agent.)
 However, in this case, the foreign agent SHOULD deny a registration
 request as "Administratively Prohibited" from a mobile node that is
 registering in a co-located fashion.  The reason being that the
 foreign agent will not be able to revoke the binding of a co-located
 mobile node due to reasons outlined in Section 4.3.2.
 How the foreign agent and/or foreign domain enforce the 'R' bit is
 beyond the scope of this document.

4.3.2. The 'D' bit in Use

 A mobile node registering directly with its home agent in a co-
 located fashion with the 'D' bit set in its registration request is
 supported in registration revocation.  However, support for a co-
 located mobile node (with the 'D' bit set in its registration
 request) registering via a foreign agent is not supported for the
 following reasons.
 Registration requests where the 'D' bit is set, and which are relayed
 through a foreign agent (e.g., due to the advertising of the 'R' bit)
 should theoretically contain the foreign agent address as the source
 address of the registration request when received by the home agent.
 A home agent may conclude that the source address of this
 registration request is not the same as the co-located care-of
 address contained in the registration request, and is therefore
 likely to be the address of the foreign agent.  However, since there
 is no way to guarantee that this IP source address is in fact an

Glass & Chandra Standards Track [Page 23] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 address of the foreign agent servicing the mobile node, accepting a
 revocation message from this IP source address may lead to a denial-
 of-service attack by a man-in-the-middle on the mobile node.
 Moreover, there is currently no method for the foreign agent
 servicing the mobile node to identify itself to the home agent during
 the Mobile IP registration phase.  Even if a foreign agent could
 identify itself, the co-located mobile node would also need to
 authorize that this foreign agent is indeed the agent that is
 providing it the Mobile IP services.  This is to thwart a denial-of-
 service attack on the mobile node by a foreign agent that has a
 security association with the home agent, and is on the path between
 the co-located mobile node and the home agent.

5. Error Codes

 As the intent of a registration revocation message is not a request
 to discontinue services, but is a notification that Mobile IP
 services are discontinued, there are no new error codes.

6. Security Considerations

 There are two potential vulnerabilities, one in the agent
 advertisement mechanism, and one related to unauthorized revocation
 messages.

6.1. Agent Advertisements

 Although the mechanisms defined by this document do not introduce
 this problem, it has been recognized that agent advertisements as
 defined in [1] subject mobile nodes to a denial-of-service potential.
 This is because the agent advertisement as defined in [1] may be
 spoofed by other machines residing on the link.  This makes it
 possible for such nodes to trick the mobile node into believing its
 registration has been revoked either by unicasting an advertisement
 with a reset sequence number to the link-local address of the mobile
 node, or by broadcasting it to the subnet, thereby tricking all
 mobile nodes registered with a particular foreign agent into
 believing all their registrations have been lost.
 There has been some work in this working group and others (e.g.,
 IPsec) to secure such router advertisements, though at the time of
 this publication, no solutions have become common practice.  To help
 circumvent possible denial of service issues here, bringing their
 potential for disruption to a minimum, mobile node implementors
 should ensure that any agent advertisement which doesn't conform to a
 strict adherence to [1], specifically those whose TTL is not 1, or
 which do not emanate from the same link-address (when present) as

Glass & Chandra Standards Track [Page 24] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 other agent advertisements supposedly from the same agent, or even
 that of the last successful registration reply, be silently
 discarded.

6.2. Revocation Messages

 As registration revocation, when performed, terminates Mobile IP
 services being provided to the mobile node, it is crucial that all
 security and replay protection mechanisms be verified before a
 mobility agent believes that the other agent has revoked a binding.
 Messages which are sent link-local (e.g., between mobile node and
 foreign agent) MAY also be secured by methods outlined in [1], namely
 the use of mobile-foreign authenticators, but these have no direct
 relation to registration revocation.
 RFC 3344 [1] defines a security mechanism that MUST be used between
 home agents and mobile nodes, and MAY used between home agents and
 foreign agents, namely the use of authenticators.  All foreign and
 home agents MUST support protection of revocation messages via the
 foreign-home authenticators defined in [1].  They MAY implement other
 mechanisms of equal or greater strength; if such mechanisms are known
 to be available to both parties, they MAY be used instead.
 Revocation messages are at least as secure as registration messages
 passed between home and foreign agents and containing home-foreign
 authenticators as defined in [1].  Thus, there are no new security
 threats introduced by the revocation mechanism other than those
 present in [1] with respect to the compromise of the shared secret
 which is used to generate the home-foreign authenticators.
 That said, there are two types of active attacks which use messages
 captured "in flight" by a man-in-the-middle between the home and
 foreign agents - "malicious repeaters" and "malicious reflectors".
 In the case of a "malicious repeater", a man-in-the-middle captures a
 revocation message, then replays it to the same IP destination
 address at a later time.  Presuming the authenticator of the original
 packet was deemed valid, without replay protection, the home-foreign
 authenticator of the replayed packet will (again) pass
 authentication.  Note that since datagrams are not guaranteed to
 arrive unduplicated, a replay may occur by "design".
 In the case of a "malicious reflector," a man-in-the-middle captures
 a revocation message, then returns it to its originator at a later
 time.  If the security association between home and foreign domains
 uses a security association involving a (single) shared secret which
 only protects the contents of the UDP portion of the packet (such as
 home-foreign authenticators as defined by [1]), without replay

Glass & Chandra Standards Track [Page 25] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 protection, the sender of the packet will also believe the revocation
 message to be authentic.
 The replay protection mechanism used by the revocation messages
 defined by this document is designed to protect against both of these
 active attacks.  As a benefit, by using a 32-bit timestamp it can be
 more quickly determined if revocation messages are replays, though
 the reader is advised to use caution in this approach.  An agent
 which receives an authenticated revocation message can compare the
 Identifier field to that of a previously received revocation message,
 and if the timestamp in the new message is found to have been
 generated after that of the time-stamp in the last revocation message
 received, it can immediately be determined as not being a replay.
 Note however that since datagrams are not guaranteed to arrive in
 order, it should not be presumed that because the values contained in
 an Identifier field are timestamps that they will necessarily be
 increasing with each successive revocation message received.  Should
 an implementor decide to base his replay detection mechanism on
 increasing timestamps, and therefore increasing Identifier values, a
 suitable time window should be defined in which revocation messages
 can be received.  At worst, ignoring any revocation message should
 result in the retransmission of another revocation message, this time
 with timestamp later than the last one received.
 Note that any registration request or reply can be replayed.  With
 the exchanging of time-stamps by agents in revocation extensions, an
 agent should have a belief that such messages have been delivered in
 a timely manner.  For purposes of registration revocation, the
 timeliness of a registration packet is simply based on the
 granularity of each registration.  Since [1] provides a replay
 mechanism for the home agent to use, it has a way to tell if the
 registration request being presented to it is new.  The foreign
 agent, however, has no such mechanism in place with the mobile node.
 Foreign agents are advised to continue to consider registrations
 'outstanding' until the associated registration reply is returned
 from the home agent before using the information in any of its
 visitor entries.  Even so, this leaves the foreign agent open to a
 potential denial of service attack in which registration requests and
 replies are replayed by multiple nodes.  When this happens, the
 foreign agent could be lead to believe such registrations are active,
 but with old information, which can have adverse effects on them, as
 well as to the ability of that agent to successfully use the
 procedures outlined in this document.  Sufficient protection against
 this scenario is offered by the challenge-response mechanism [2] by
 which a foreign agent generates a live challenge to a mobile node for
 the purposes of making sure, among other things, that the
 registration request is not a replay.

Glass & Chandra Standards Track [Page 26] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

7. IANA Considerations

 This document defines an additional set of messages between the home
 and foreign agent specific to the services being provided to the same
 mobile node, or sub-set of mobile nodes.  To ensure correct
 interoperation based on this specification, IANA has reserved values
 in the Mobile IP number space for two new message types, and a single
 new extension.

7.1. New Message Types

 The following message types are introduced by this specification:
 Registration Revocation: A new Mobile IP control message, using UDP
 port 434, type 7.  This value has been taken from the same number
 space as Mobile IP Registration Request (Type = 1), and Mobile IP
 Registration Reply (Type = 3).
 Registration Revocation Acknowledgment: A new Mobile IP control
 message, using UDP port 434, type 15.  This value has been taken from
 the same number space as Mobile IP Registration Request (Type = 1),
 and Mobile IP Registration Reply (Type = 3).

7.2. New Extension Values

 The following extensions are introduced by this specification:
 Revocation Support Extension: A new Mobile IP Extension, appended to
 a Registration Request, or Registration Reply.  The value assigned is
 137.  This extension is derived from the Extension number space.  It
 MUST be in the 'skippable' (128 - 255) range as defined in RFC 3344.

7.3. New Error Codes

 There are no new Mobile IP error codes introduced by this document.

8. References

8.1. Normative References (Numerical)

 [1] Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3344,
     August 2002.
 [2] Perkins, C. and P. Calhoun, "Mobile IPv4 Challenge/Response
     Extensions", RFC 3012, November 2000.
 [3] Bradner, S., "Key Words for us in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.

Glass & Chandra Standards Track [Page 27] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

8.2. Informational References (Alphabetical)

 [A] Glass, S., Hiller, T., Jacobs, S. and C. Perkins, "Mobile IP
     Authentication, Authorization, and Accounting Requirements", RFC
     2977, October 2000.
 [B] Aboba, B., Calhoun, P., Glass, S., Hiller, T., McCann, P.,
     Shiino, H., Walsh, P., Zorn, G., Dommety, G., Perkins, C., Patil,
     B., Mitton, D., Manning, S., Beadles, M., Chen, X., Sivalingham,
     S., Hameed, A., Munson, M., Jacobs, S., Lim, B., Hirschman, B.,
     Hsu, R., Koo, H., Lipford, M., Campbell, E., Xu, Y., Baba, S. and
     E. Jaques, "Criteria for Evaluating AAA Protocols for Network
     Access", RFC 2989, November 2000.
 [C] Montenegro, G., Ed., "Reverse Tunneling for Mobile IP, revised",
     RFC 3024, January 2001.
 [D] Deering, S., Ed., "ICMP Router Discovery Messages", RFC 1256,
     September 1991.
 [E] Calhoun, P. and C. Perkins, "Mobile IP Network Access Identifier
     Extension for IPv4", RFC 2794, March 2000.

Glass & Chandra Standards Track [Page 28] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

Appendix A: An Example of the Revocation Messages in Use

 For clarity, the following example is meant to illustrate the use of
 the new messages in the registration phase, and the revocation phase.
 In this example, a foreign agent and home agent will negotiate
 revocation during the registration phase.  During the revocation
 phase, the foreign agent will revoke the binding of a mobile node.

A.1. The Registration Phase

 Consider a foreign agent that supports registration revocation, and
 has a security association with a home agent to which it is
 forwarding a registration request.  The foreign agent will include
 the revocation support extension after the mobile-home authenticator.
 Assume that the foreign agent supports the use of the 'I' bit, and is
 willing to let the home agent decide if the mobile node should be
 informed of the revocation of its registration. Thus, the foreign
 agent will set the 'I' bit to '1'.  The foreign agent will append a
 foreign-home authenticator to the registration request.
 Upon receiving the registration request containing a revocation
 extension, the home agent will include a revocation support extension
 in the registration reply.  Since the foreign agent set the 'I' bit
 to '1' in its revocation extension, and the home agent supports the
 use of the 'I' bit, the home agent will set the 'I' bit in its
 registration extension to '1'.  Additionally, the home agent will
 append a home-foreign authenticator to the registration request.
 Upon receiving the authenticated registration reply, the foreign
 agent will check the revocation support extension and note that the
 home agent wants to decide if the mobile node should be notified in
 the event this registration is revoked, i.e., since the home agent
 set the 'I' bit in the return revocation extension.

A.2. The Revocation Phase

 The foreign agent revokes a mobile node's binding, and generates a
 revocation message to be sent to the mobile node's home agent.  Since
 the 'I' bit was negotiated in the revocation extensions, and the
 foreign agent is still willing to let the home agent indicate whether
 this mobile node should be informed about the revocation, it will set
 the 'I' bit to '1' in the revocation message.  The foreign agent also
 makes sure the 'A' bit is set to '0'.
 The foreign agent will also place the address of the mobile node
 whose registration it wishes to revoke in the home address field, the
 address that the mobile node registered as the care-of address in the
 foreign domain field, and the address registered as the home agent in

Glass & Chandra Standards Track [Page 29] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 the home domain address field.  The foreign agent will set the
 Revocation Identifier to the current 32-bit timestamp, and append the
 foreign-home authenticator.
 Upon receiving the above revocation message, the home agent uses the
 address identified as the foreign domain address to identify the
 security association, and authenticate the revocation message.  After
 authenticating the message, the home agent will check to make sure
 the 'A' bit and Identifier indicate that this revocation is not a
 replay.  The home agent then uses the mobile node home address,
 foreign domain address, and home domain address to locate the mobile
 node whose registration is being revoked.
 Upon processing a valid registration revocation message, the home
 agent generates a revocation acknowledgment message.  Since the 'I'
 bit was set to '1' in the revocation message and the home agent
 wishes for the identified mobile node to be informed of the
 revocation, it will set the 'I' bit in the revocation acknowledgment
 to '1'.  The home agent then copies the home address and the
 Revocation Identifier field into the revocation acknowledgement.  The
 home agent protects the revocation acknowledgment with a home-foreign
 authenticator.
 Upon receiving a valid revocation acknowledgment (in which the
 authenticator and Identifier fields are acceptable), the foreign
 agent checks the state of the 'I' bit.  Since the 'I' bit is set to
 '1', the foreign agent will notify the mobile node of the revocation.

Appendix B: Disparate Address, and Receiver Considerations

 Since the registration revocation message comes from a source address
 that is topologically routable from the interface receiving the
 datagram, the agents, by definition, are topologically connected (if
 this were not the case, the initial registration mechanism would have
 failed).  If either are the ultimate hop from this topologically
 connected region to one or more disparate address spaces, no problems
 are foreseen.  In order for the mobile node to have successfully
 registered with its home agent, it MUST have provided to the network
 (foreign agent) to which it is currently attached a routable address
 of its home agent.  Conversely, the care-of address being used by the
 mobile node must also be topologically significant to the home agent
 in order for the registration reply to have been received, and the
 tunnel initiated.  By definition, then, the home agent address and
 the care-of address must each be significant, and either address must
 form a unique pair in the context of this mobile node to both agents.

Glass & Chandra Standards Track [Page 30] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

 Another way of understanding this is that the tunnel endpoints are in
 some way connected, and hence each are unique as far as the other end
 is concerned.  The address at the other end of the tunnel, in
 combination with the address of the mobile node, must therefore form
 a unique pair that can be identified by the agent receiving the
 registration revocation message.
 As an example, consider a mobile node who's home address lies in
 disparate address space A behind its home agent.  In the following
 diagram, [*] indicates an interface of the entity in which it
 appears.
    MN[a]-----[c]FA[b]=====((()))=====[b]HA[a]-----[a]CN
        Address      Some topologically      Address
        Space C      connected network       Space A
 We presume a binding for MN exists, and hence a tunnel between FA[b]
 and HA[b] exists.  Then, since the address assigned to MN[a] MUST be
 unique in address space A, the pair {FA[b],MN[a]} is guaranteed to be
 unique in the binding table of HA, and the pair {HA[b],MN[a]} is
 guaranteed to be unique in the foreign agent's visitor list.
 As a result, a home agent receiving a registration revocation message
 and foreign-home authenticator for MN[a] from FA[b] is able to
 determine the unique mobile node address being deregistered.
 Conversely a foreign agent receiving a registration revocation
 message and home-foreign authenticator for MN[a] from HA[b] is able
 to determine the exact mobile node address being deregistered.  For
 this reason, if a foreign agent receives a registration revocation
 message with the home domain field set to the zero address it MUST be
 silently discarded.  This is to prevent confusion in the case of
 overlapping private addresses; when multiple mobile nodes are
 registered via the same care-of address and coincidentally using the
 same (disparate/private) home address, the home agent address
 appearing in the home domain field is the only way a foreign agent
 can discern the difference between these mobile nodes.

Glass & Chandra Standards Track [Page 31] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

Acknowledgments

 The authors would like to thank Rajesh Bhalla, Kent Leung, and Alpesh
 Patel for their contributions to the concepts detailed in
 draft-subbarao-mobileip-resource-00.txt, "Releasing Resources in
 Mobile IP," from which the revocation support extension, and the
 acknowledgment mechanism contained in this document were derived.
 The authors would also like to thank Pete McCann for his discussions
 on replay mechanisms, and security concerns, and Ahmad Muhanna for
 pointing out a problem with the initial replay mechanism, which
 eventually lead to the addition of a time stamp to the Revocation
 Extension.
 The authors would also like to acknowledge Henrik Levkowetz for his
 detailed review of the document, and Michael Thomas for his review of
 the replay mechanism described herein.

Authors' Addresses

 Steven M. Glass
 Solaris Network Technologies
 Sun Microsystems
 1 Network Drive
 Burlington, MA.  01801
 Phone: +1.781.442.0000
 Fax:   +1.781.442.1706
 EMail: steven.glass@sun.com
 Madhavi W. Chandra
 IOS Technologies Division
 Cisco Systems
 7025 Kit Creek Road
 Research Triangle Park, NC 27709
 Phone: +1.919.392.8387
 EMail: mchandra@cisco.com

Glass & Chandra Standards Track [Page 32] RFC 3543 Registration Revocation in Mobile IPv4 August 2003

Full Copyright Statement

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Acknowledgement

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 Internet Society.

Glass & Chandra Standards Track [Page 33]

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