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Versions: 00 01 02 rfc2290                                              
PPP Extensions Working Group                        J. Solomon, Motorola
Internet Draft                                    S. Glass, FTP Software
expires September 24, 1997                                March 24, 1997

             Mobile IPv4 Configuration Option for PPP IPCP
                  <draft-ietf-pppext-ipcp-mip-00.txt>

Status of this Memo

   This document is a submission to the PPPEXT working group of the
   IETF, having already reached consensus within the MOBILEIP working
   group.  Questions can be directed to the respective mailing lists:
   ietf-ppp@merit.edu and mobile-ip@smallworks.com.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
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   ftp.isi.edu (US West Coast).

   Distribution of this memo is unlimited.

Abstract

   Mobile IP [RFC 2002] defines media-independent procedures by which a
   Mobile Node can maintain existing transport and application-layer
   connections despite changing its point-of-attachment to the Internet
   and without changing its IP address.  PPP [RFC 1661] provides a
   standard method for transporting multi-protocol packets over point-
   to-point links.  As currently specified, Mobile IP Foreign Agents
   which support Mobile Node connections via PPP can do so only by first
   assigning unique addresses to those Mobile Nodes, defeating one of
   the primary advantages of Foreign Agents.  This documents corrects
   this problem by defining the Mobile IPv4 Configuration Option to the
   Internet Protocol Control Protocol (IPCP) [RFC 1332].  Using this
   option, two peers can communicate their support for Mobile IP during
   the IPCP phase of PPP.  Familiarity with Mobile IP [RFC 2002], IPCP
   [RFC 1332], and PPP [RFC 1661] is assumed.



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                                 Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   2
       1.1. Terminology  . . . . . . . . . . . . . . . . . . . . . .   2
       1.2. Problem Statement  . . . . . . . . . . . . . . . . . . .   3
       1.3. Requirements . . . . . . . . . . . . . . . . . . . . . .   5
   2. Mobile IPv4 Configuration Option . . . . . . . . . . . . . . .   6
   3. Additional Requirements  . . . . . . . . . . . . . . . . . . .   9
       3.1. Other IPCP Options . . . . . . . . . . . . . . . . . . .   9
       3.2. Move Detection . . . . . . . . . . . . . . . . . . . . .   9
   4. Supported Scenarios  . . . . . . . . . . . . . . . . . . . . .  10
   5. Security Considerations  . . . . . . . . . . . . . . . . . . .  13
   6. References . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   7. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  14
   8. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  15

1. Introduction

   Mobile IP [RFC 2002] defines protocols and procedures by which
   packets can be routed to a mobile node, regardless of its current
   point-of-attachment to the Internet, and without changing its IP
   address.  Mobile IP is designed to run over any type of media and any
   type of data link-layer.  However, the interaction between Mobile IP
   and PPP is currently underspecified and generally results in an
   inappropriate application of Mobile IP when mobile nodes connect to
   the Internet via PPP.

   This document defines proper interaction between a mobile node [RFC
   2002] and a dialup server through which the mobile node connects to
   the Internet using PPP.  This requires the definition of a new option
   for IPCP [RFC 1332], named the "Mobile IPv4 Configuration Option",
   which is defined in this document.  The mobile node and the dialup
   server use this option to negotiate the appropriate use of Mobile IP
   over the PPP link.


1.1. Terminology

   This document uses the following terms as defined in [RFC 2002]:

      Mobile Node

         A host or router that changes its point of attachment from one
         network or subnetwork to another.  A mobile node may change its
         location without changing its IP address; it may continue to
         communicate with other Internet nodes at any location using its
         (constant) IP address, assuming link-layer connectivity to a
         point of attachment is available.



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      Home Agent

         A router on a mobile node's home network which tunnels
         datagrams for delivery to the mobile node when it is away from
         home, and maintains current location information for the mobile
         node.

      Foreign Agent

         A router on a mobile node's visited network which provides
         routing services to the mobile node while registered.  The
         foreign agent detunnels and delivers datagrams to the mobile
         node that were tunneled by the mobile node's home agent.  For
         datagrams sent by a mobile node, the foreign agent may serve as
         a default router for registered mobile nodes.

      Dialup Server

         The PPP peer of a mobile node.  A dialup server might support
         home agent functionality, foreign agent functionality, both, or
         neither.


1.2. Problem Statement

   In Mobile IP, packets sent to a mobile node's home address are routed
   first to the mobile node's home agent, a router on the mobile node's
   home link which intercepts packets sent to the home address.  The
   home agent then tunnels such packets to the mobile node's care-of
   address, where the packets are extracted from the tunnel and
   delivered to the mobile node.  There are two types of care-of
   addresses:

      Co-located Care-of Address

         An address temporarily assigned to a mobile node itself.  In
         this case, the mobile node is the exit-point of the tunnel and
         decapsulates packets encapsulated for delivery by its home
         agent.

      Foreign Agent Care-of Address

         An address of a foreign agent that has at least one interface
         on a mobile node's visited link.  In this case, the foreign
         agent decapsulates packets that have been tunneled by the home
         agent and delivers them to the mobile node over the visited
         link.




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   In Appendix B, Mobile IP [RFC 2002] currently specifies only the
   following with respect to PPP:

      "The Point-to-Point-Protocol (PPP) [RFC 1661] and its Internet
      Protocol Control Protocol (IPCP) [RFC 1332], negotiates the use of
      IP addresses.

      "The mobile node SHOULD first attempt to specify its home address,
      so that if the mobile node is attaching to its home network, the
      unrouted link will function correctly.  When the home address is
      not accepted by the peer, but a transient IP address is
      dynamically assigned to the mobile node, and the mobile node is
      capable of supporting a co-located care-of address, the mobile
      node MAY register that address as a co-located care-of address.
      When the peer specifies its own IP address, that address MUST NOT
      be assumed to be a foreign agent care-of address or the IP address
      of a home agent."

   Inspection of this text reveals that there is currently no way for
   the mobile node to use a foreign agent care-of address, without first
   being assigned a unique IP address, even if the dialup server also
   supports foreign agent functionality.  The reason for this can be
   seen by walking through the IPCP negotiation:

   1. A mobile node calls a dialup server and proposes its home address
      in IPCP's "IP Address" option.

   2. The dialup server has no way of knowing whether this is: (a) a
      mobile node proposing its home address; or (b) a conventional node
      proposing a non-routable address.  In this case, the dialup server
      must (conservatively) Nak the IP address option.

   3. The mobile node, in turn, has no way of knowing whether its home
      address was Nak'ed because the peer is a foreign agent being
      conservative, or because the peer does not implement Mobile IP at
      all.  Therefore, the mobile node must (conservatively) assume that
      the peer does not implement Mobile IP and continue the negotiation
      of an IP address in IPCP, after which point the mobile node can
      use the assigned address as a co-located care-of address.

   Here we observe that, even if the dialup server is a foreign agent
   and sends an Agent Advertisement to the mobile node after IPCP
   completes, the mobile node will still have negotiated a routable
   address in step 3 which it is likely already using as a co-located
   care-of address.  This defeats the purpose of foreign agent care-of
   addresses, which are designed to be shared by multiple mobile nodes
   and to eliminate the need to assign a unique address to each mobile
   node.



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1.3. Requirements

   The purpose of this document is to specify the behavior of both ends
   of the PPP link when one or more of the PPP peers supports Mobile IP.
   Specifically, the design of the option and protocol defined in this
   document is based upon the following requirements:

   1. The option and protocol described in this document must be
      backwards compatible with conventional nodes and dialup servers
      which do not implement this option nor any Mobile IP
      functionality.

   2. The option and protocol described in this document must
      accommodate a variety of scenarios, minimally those identified in
      Section 4.

   3. A unique address must not be assigned to a mobile node unless
      absolutely necessary.  Specifically, no such address is assigned
      to a mobile node that connects via PPP to its home link or a
      mobile node that connects via PPP to a foreign agent (and uses
      that foreign agent's care-of address).






























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2. Mobile IPv4 Configuration Option

   The Mobile IPv4 Configuration Option for IPCP 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     |D| reserved1   |C|H| reserved2 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Mobile Node's Home Address                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Assigned Co-Located Care-Of Address               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type         137  (Mobile IPv4 Configuration Option)

      Length       12  (The length of this entire extension in bytes)

      D            1, if the Mobile Node wants a Dynamically assigned,
                   co-located care-of address, 0 otherwise.

      reserved1    sent as 0, ignored on reception

      C            1, if the dialup server Cannot fulfill the mobile
                   node's request to be dynamically assigned a
                   co-located care-of address, 0 otherwise.

      H            1, if the PPP interface of the dialup server is a
                   neighbor of the mobile node's home address on the
                   home link, 0 otherwise.

      reserved2    sent as 0, ignored on reception

      Mobile Node's Home Address
                   The IP home address of the mobile node sending the
                   Mobile IPv4 Configuration Option.

      Assigned Co-Located Care-of Address
                   A place-holder for the dynamically assigned
                   co-located care-of address.  Ignored if the 'D' bit
                   is 0.

   The option works as follows.  A mobile node includes the Mobile IPv4
   Configuration Option in a Configure-Request of IPCP.  A mobile node
   MUST NOT include an IP Address Option (nor the deprecated IP-
   Addresses Option) along with the Mobile IPv4 Configuration Option but
   MAY include other options that do not involve negotiation of an IP
   address (see section 3.1).



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   The mobile node MUST set the 'D' bit to 1 if it wants to be assigned
   a co-located care-of address, otherwise it MUST set the 'D' bit to 0.
   The mobile node SHOULD set all other fields and bits to 0, except for
   the "Mobile Node's Home Address" field in which it MUST place its IP
   home address.

   The response generated at the other end of the PPP link depends upon
   the identity and the capabilities of the PPP peer (for simplicity, we
   call this peer a "dialup server").  Several cases are described as
   follows, which assume that the dialup server has received a
   Configure-Request from the mobile node containing a Mobile IPv4
   Configuration Option:

   1. If the Mobile IPv4 Configuration Option is not recognizable or is
      not acceptable for negotiation (as configured by a network
      administrator), then the dialup server MUST respond by sending a
      Configure-Reject, as described in [RFC 1661].  A mobile node that
      receives such a Configure-Reject MAY proceed with IPCP negotiation
      using the IP Address Option [RFC 1332] instead of the Mobile IPv4
      Configuration Option.  The address so negotiated MAY be used by
      the mobile node as a co-located care-of address.

      If, instead, the Mobile IPv4 Configuration Option is recognizable
      and is acceptable for negotiation, then the dialup server MUST
      respond with either a Configure-Ack or a Configure-Nak, depending
      upon the acceptability of the specific values contained within the
      Mobile IPv4 Configuration Option.  Each such case is described
      below where, due to the recognizability of the Mobile IPv4
      Configuration Option, we assume that the dialup server is either a
      home agent, a foreign agent, or both.

   2. If the dialup server is a bridge with one interface on the mobile
      node's home network or if the dialup server is a router whose
      address on its PPP interface is a neighbor to the mobile node's
      home address (i.e., the mobile node is connecting to its home
      link), then the dialup server sends a Configure-Nak in which it
      sets the 'H' bit to 1 and leaves all other fields unmodified.  A
      mobile node receiving this Configure-Nak MUST respond by sending a
      new Configure-Request (containing the Mobile IPv4 Configuration
      Option) with D=0, C=0, H=1, Mobile Node's Home Address field set
      to the mobile node's IP home address, and the Assigned Co-Located
      Care-of Address field set to 0.  The dialup server will
      subsequently send a Configure-Ack in response to this new
      Configure-Request and IPCP will complete.  The mobile node is not
      required to wait for an Agent Advertisement before de-registering
      with its home agent.





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      All the remaining cases assume that the dialup server is a foreign
      agent to the mobile node and specifically not connected to the
      mobile node's home link (as determined by the Mobile Node's Home
      Address field).

   3. If the mobile node set the 'D' bit to 1, if the Assigned Co-
      Located Care-of Address is set to a value never previously
      assigned to this mobile node (e.g., 0.0.0.0), and if the foreign
      agent is capable of assigning an address, then the foreign agent
      MUST respond with a Configure-Nak in which a newly assigned co-
      located care-of address is placed in the Assigned Co-Located
      Care-of Address field and all other fields are left unmodified.  A
      mobile node receiving such a Configure-Nak MUST respond by sending
      a new Configure-Request containing a Mobile IPv4 Configuration
      Option copied without modification from this received Configure-
      Nak.  The foreign agent MUST respond to this new Configure-Request
      with a Configure-Ack.

      As specified in [RFC 1661] and [RFC 1332], a mobile node MUST
      receive such a Configure-Ack before it can consider IPCP to be
      completed and therefore that it may use the assigned address as a
      co-located care-of address.  In addition, the mobile node MUST (!)
      wait for an Agent Advertisement before registering this co-located
      care-of address.  This is because the foreign agent might set the
      'R' Bit in its Agent Advertisements (see [RFC 2002]) which forces
      the mobile node to register via the foreign agent, even when using
      a co-located care-of address.  Accordingly, the foreign agent MUST
      send an Agent Advertisement over a PPP link immediately after IPCP
      for that link enters the Opened state.

   4. If the mobile node set the 'D' bit to 1, and if the Assigned Co-
      Located Care-of Address is set to a value previously assigned to
      this mobile node by this foreign agent (i.e., as assigned in case
      #3 above), then the foreign agent MUST respond with a Configure-
      Ack and IPCP completes.  The mobile node MUST (!) wait for an
      Agent Advertisement from the foreign agent before registering.
      This is because the foreign agent might set the 'R' Bit in its
      Agent Advertisements (see RFC 2002) which forces the mobile node
      to register via the foreign agent, even when using a co-located
      care-of address.  Accordingly, the foreign agent MUST send an
      Agent Advertisement over a PPP link immediately after IPCP for
      that link enters the Opened state.









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   5. If the mobile node set the 'D' bit to 1, and if the foreign agent
      is *not* capable of assigning an address, then the foreign agent
      MUST respond with a Configure-Nak in which the 'C' bit is set to 1
      and all other fields are left unmodified.  The mobile node must
      now either give up and try to find a dialup server that can assign
      an address, or proceed to use a foreign agent care-of address.  In
      the latter case, the mobile node sends a new Configure-Request
      containing the Mobile IPv4 Configuration Option with the 'D' bit
      set to 0, as described in item #6 below.

   6. If the mobile node set the 'D' bit to 0, then the foreign agent
      MUST return the option unmodified in a Configure-Ack and IPCP
      completes.  The mobile node MUST wait for an Agent Advertisement
      from the foreign agent before registering.  Accordingly, the
      foreign agent MUST send an Agent Advertisement over a PPP link
      immediately after IPCP for that link enters the Opened state.

   The design and semantics of the Mobile IPv4 Configuration Option are
   therefore optimized for the case of a mobile node making use of a
   foreign agent's care-of address.  The negotiation takes only one
   round-trip in this case.

3. Additional Requirements

3.1. Other IPCP Options

   Although a mobile node MUST NOT include an IP Address Option (nor the
   deprecated IP-Addresses Option) in any Configure-Request that
   contains a Mobile IPv4 Configuration Option, the mobile node MAY
   include an IP-Compression-Protocol Option or any other option that
   does not involve the negotiation of an IP address.  If a mobile node
   and a foreign agent or home agent agree in IPCP to use Van Jacobson
   Header Compression [RFC 1144], then the mobile node MUST NOT set the
   'V' bit in its ensuing, Mobile IP Registration Request [RFC 2002].

3.2. Move Detection

   Mobile nodes that connect via PPP MUST correctly implement PPP's
   IPCP, since movement by the mobile node will likely change its PPP
   peer.  Specifically, mobile nodes MUST be prepared to re-negotiate
   IPCP at any time, including, the re-negotiation of the Mobile IPv4
   Configuration Option described in this document.

   Also note that certain wireless links can employ handoff and proxying
   mechanisms that would not necessarily require bringing down a PPP
   link but would indeed require a mobile node to register with a new
   foreign agent.  Therefore, mobile nodes which connect to an agent via
   PPP MUST employ their move detection algorithms (see section 2.4.2 in



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   [RFC 2002]) and register whenever they detect a change in
   connectivity.

   Specifically, a mobile node that fails to receive an Agent
   Advertisement within the Lifetime advertised by its current foreign
   agent, MUST assume that it has lost contact with that foreign agent
   (see Section 2.4.2.1, [RFC 2002]).  If, in the mean time, the mobile
   node has received Agent Advertisements from another foreign agent,
   the mobile node SHOULD immediately register with that foreign agent
   upon timing out with its current foreign agent.

   Likewise, a mobile node that implements move detection based upon the
   Prefix-Length Extension MUST compare the prefix of any advertising
   agents with that of its current foreign agent (see Section 2.4.2.2,
   [RFC 2002]).  If such a mobile node receives an Agent Advertisement
   from a foreign agent specifying a different prefix than that of its
   current foreign agent, then the mobile node that employs this method
   of move detection MUST register with that new foreign agent.

   A mobile node MAY treat PPP link-establishment as a sufficient reason
   to proceed with a new Mobile IP registration.  Section 2 defines the
   circumstances under which mobile nodes MUST wait for an Agent
   Advertisement before registering.  Accordingly, foreign agents and
   home agents MUST send an Agent Advertisement over a PPP link
   immediately after IPCP for that link enters the Opened state.

4. Supported Scenarios

   The Mobile IPv4 Configuration Option is designed to accommodate the
   following scenarios.  This section also helps to illustrate the use
   of the option and the protocol specified above.

   In the scenarios which follow, the direction of message flow is
   indicated along with the type of IPCP message and the contents of the
   appropriate option.  "MN" refers to the mobile node and "dialup"
   refers to the PPP peer to which the mobile node connects.

   For sake of brevity, the Type, Length, Reserved, and Mobile Node's
   Home Address fields have been omitted from the Mobile IPv4
   Configuration Option below because these fields are always set to
   their obvious values.  Finally, the Assigned Co-Located Care-Of
   Address field is designated by "IPcol-coa".









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   A. A mobile node wants to use a co-located care-of address and the
      dialup server is a foreign agent that is capable of assigning such
      an address:

      [ From -> To ] PPP Message Type  Mobile IPv4 Configuration Option
      ============== ================= ================================
      [MN -> dialup] Configure-Request {D=1,C=0,H=0,IPcol-coa=0.0.0.0}
      [dialup -> MN] Configure-Nak     {D=1,C=0,H=0,IPcol-coa=new-coa}
      [MN -> dialup] Configure-Request {D=1,C=0,H=0,IPcol-coa=new-coa}
      [dialup -> MN] Configure-Ack     {D=1,C=0,H=0,IPcol-coa=new-coa}

      - Mobile node waits to receive an Agent Advertisement.
      - If (Advertisement has R-bit set) then
          Mobile node registers with co-located care-of address via the
          foreign agent;
        else
          Mobile node registers with co-located care-of address directly
          with its home agent.

   B. A mobile node wants to use a co-located care-of address and the
      dialup server is a foreign agent.  The foreign agent cannot assign
      a co-located care-of address (e.g., it has no pool of addresses
      from which to allocate for the purposes of assignment):

      [ From -> To ] PPP Message Type  Mobile IPv4 Configuration Option
      ============== ================= ================================
      [MN -> dialup] Configure-Request {D=1,C=0,H=0,IPcol-coa=0.0.0.0}
      [dialup -> MN] Configure-Nak     {D=1,C=1,H=0,IPcol-coa=0.0.0.0}

      The mobile node has two options: either proceed to use this
      foreign agent's care-of address or disconnect and try to find a
      different dialup server which can fulfill the request for a co-
      located care-of address.  In the former case, IPCP continues as
      follows:

      [ From -> To ] PPP Message Type  Mobile IPv4 Configuration Option
      ============== ================= ================================
      [MN -> dialup] Configure-Request {D=0,C=0,H=0,IPcol-coa=0.0.0.0}
      [dialup -> MN] Configure-Ack     {D=0,C=0,H=0,IPcol-coa=0.0.0.0}

      - IPCP completes.
      - Mobile node waits to receive an Agent Advertisement.
      - Mobile node registers with its home agent via the foreign agent.








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   C. A mobile node wants to use a foreign agent care-of address and the
      dialup server is a foreign agent which finds this state of affairs
      satisfactory:

      [ From -> To ] PPP Message Type  Mobile IPv4 Configuration Option
      ============== ================= ================================
      [MN -> dialup] Configure-Request {D=0,C=0,H=0,IPcol-coa=0.0.0.0}
      [dialup -> MN] Configure-Ack     {D=0,C=0,H=0,IPcol-coa=0.0.0.0}

      - IPCP completes.
      - Mobile node waits to receive an Agent Advertisement.
      - Mobile node registers with its home agent via the foreign agent.

   D. A mobile node connects to a dialup server which is connected to
      the mobile node's home link (for this scenario, it does not matter
      whether the mobile node wishes to use a co-located care-of address
      or a foreign agent care-of address).  The dialup server informs
      the mobile node that it is connected to its home link as follows
      (the notation "D=x" implies a "don't care" condition):

      [ From -> To ] PPP Message Type  Mobile IPv4 Configuration Option
      ============== ================= ================================
      [MN -> dialup] Configure-Request {D=x,C=0,H=0,IPcol-coa=0.0.0.0}
      [dialup -> MN] Configure-Nak     {D=x,C=0,H=1,IPcol-coa=0.0.0.0}
      [MN -> dialup] Configure-Request {D=0,C=0,H=1,IPcol-coa=0.0.0.0}
      [dialup -> MN] Configure-Ack     {D=0,C=0,H=1,IPcol-coa=0.0.0.0}

      - IPCP completes.
      - Mobile node de-registers with its home agent.






















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   E. A mobile node wants to use either type of care-of address and the
      dialup server does not implement the Mobile IPv4 Configuration
      Option.  The server rejects the option as follows (the notation
      "D=x" implies a "don't care" condition):

      [ From -> To ] PPP Message Type  Mobile IPv4 Configuration Option
      ============== ================= ================================
      [MN -> dialup] Configure-Request {D=x,C=0,H=0,IPcol-coa=0.0.0.0}
      [dialup -> MN] Configure-Reject  {D=x,C=0,H=0,IPcol-coa=0.0.0.0}

      At this point, the mobile node can use the IP Address Option [RFC
      1332] to negotiate an address which it can subsequently use as a
      co-located care-of address:

      [ From -> To ] PPP Message Type  ***IP Address Option***
      ============== ================= ===============================
      [MN -> dialup] Configure-Request {IPaddress = 0.0.0.0}
      [dialup -> MN] Configure-Nak     {IPaddress = assigned-address}
      [MN -> dialup] Configure-Request {IPaddress = assigned-address}
      [dialup -> MN] Configure-Ack     {IPaddress = assigned-address}

      - IPCP completes.
      - Mobile node registers "IPaddress" as a co-located care-of address
        with its home agent.

5. Security Considerations

   This document introduces no known security threats over and above
   those facing any node on the Internet that either connects via PPP or
   implements Mobile IP or both.  Specifically, service providers should
   use cryptographically strong authentication (e.g., CHAP [RFC 1994])
   to prevent theft-of-service.  Additionally, users requiring
   confidentiality should use PPP link encryption [RFC 1968], IP-layer
   encryption [RFC 1827], or application-layer encryption, depending
   upon their individual requirements.  Finally, Mobile IP
   authentication [RFC 2002] protects against trivial denial-of-service
   attacks that could otherwise be waged against a mobile node and its
   home agent.













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6. References

   [RFC 1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed
      Serial Links", RFC 1144, January 1990.

   [RFC 1332] McGregor, G., "The PPP Internet Protocol Control Protocol
      (IPCP)," RFC 1332, May 1992.

   [RFC 1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)
      for the Transmission of Multi-protocol Datagrams over Point-to-
      Point Links," RFC 1661, July 1994.

   [RFC 1827] Atkinson, R., "IP Encapsulating Security Payload (ESP)",
      RFC 1827, August 1995.

   [RFC 1994] Simpson, W., "PPP Challenge Handshake Authentication
      Protocol (CHAP)", RFC 1994, August 1996.

   [RFC 1968] Meyer, G., "The PPP Encryption Control Protocol (ECP)",
      RFC 1968, June 1996.

   [RFC 2002] Perkins, C., Editor, "IP Mobility Support", RFC 2002,
      October 1996.

7. Acknowledgments

   The design of this protocol and option were inspired by an earlier
   submission by B. Patel and C. Perkins, then of IBM, in draft-patel-
   mobileip-pppext-00.txt, which has since expired.  Tim Wilson and
   Chris Stanaway of Motorola contributed significantly to the design of
   this configuration option and protocol specification.

   Also, some of William Simpson's text was copied verbatim from [RFC
   1661] in order to ensure consistency of terminology and
   specification.  The same goes for Charlie Perkins' text, including
   definitions, from [RFC 2002].















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8. Authors' Addresses

   Questions about this memo can be directed to:

   Jim Solomon
   Motorola, Inc.
   1301 E. Algonquin Rd. - Rm 2240
   Schaumburg, IL  60196

   Voice:  +1-847-576-2753
   Fax:    +1-847-576-3240
   E-Mail: solomon@comm.mot.com


   Steven Glass
   FTP Software, Inc.
   2 High Street
   North Andover, MA  01845

   Voice:  +1-508-685-4000
   Fax:    +1-508-684-6105
   E-mail: glass@ftp.com





























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