Mobile IPv4 for PPP: Mobile-IPv4 Configuration Option for PPP IPCP

Network Working Group                                         J. Solomon
Request for Comments: 2290                                      Motorola
Updates: 2002                                                   S. Glass
Category: Standards Track                                   FTP Software
                                                           February 1998


             Mobile-IPv4 Configuration Option for PPP IPCP

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 (1998).  All Rights Reserved.

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.

Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   2
       1.1. Specification Language . . . . . . . . . . . . . . . . .   2
       1.2. Terminology  . . . . . . . . . . . . . . . . . . . . . .   2
       1.3. Problem Statement  . . . . . . . . . . . . . . . . . . .   3
       1.4. Requirements . . . . . . . . . . . . . . . . . . . . . .   5
   2. Mobile-IPv4 Configuration Option . . . . . . . . . . . . . . .   6
       2.1. Option Format  . . . . . . . . . . . . . . . . . . . . .   6
       2.2. Overview . . . . . . . . . . . . . . . . . . . . . . . .   7
       2.3. High-Level Requirements for Non-Mobile-Nodes . . . . . .   7
       2.4. High-Level Requirements for Mobile Nodes . . . . . . . .   8
       2.5. Detailed Description . . . . . . . . . . . . . . . . . .   8
       2.6. Example Scenarios  . . . . . . . . . . . . . . . . . . .  12
   3. Additional Requirements  . . . . . . . . . . . . . . . . . . .  14
       3.1. Other IPCP Options . . . . . . . . . . . . . . . . . . .  14
       3.2. Move Detection . . . . . . . . . . . . . . . . . . . . .  14
   4. Security Considerations  . . . . . . . . . . . . . . . . . . .  15
   5. References . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   6. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  16
   7. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  16
   8. Full Copyright Statement . . . . . . . . . . . . . . . . . . .  17

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 peer 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 peer use this
   option to negotiate the appropriate use of Mobile IP over the PPP
   link.

   The Mobile-IPv4 option defined in this document is intended to work
   in conjunction with the existing IP-Address option [RFC 1332].

1.1. Specification Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

1.2. 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
         link 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 (permanent)
         home, IP address, assuming link-layer connectivity is available
         at its current location.

      Home Agent

         A router with at least one interface on a mobile node's home
         link.  A home agent intercepts packets destined to a mobile
         node's home address and tunnels them to the mobile node's
         care-of address when the mobile node is connected to a foreign
         link.  A mobile node informs its home agent of its current
         care-of address through an authenticated registration protocol
         defined by Mobile IP.

      Foreign Agent

         A router with at least one interface on a mobile node's
         (current) foreign link.  When a mobile node uses a foreign
         agent's care-of address, the foreign agent detunnels and
         delivers packets to the mobile node that were tunneled by the
         mobile node's home agent.  A foreign agent might also serve as
         a default router for packets sent by a registered mobile node.

      Peer

         The PPP peer of a mobile node.  The mobile node's peer might
         support home agent functionality, foreign agent functionality,
         both, or neither.

1.3. 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.
      A Co-located Care-of Address may be used by exactly one mobile
      node at any point in time.

   Foreign Agent Care-of Address

      An address of a foreign agent that has at least one interface on a
      mobile node's visited, foreign 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.
      A Foreign Agent Care-of Address may be used simultaneously by many
      mobile nodes at any point in time.

   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 [sic] 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 [link], 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 peer also supports
   foreign agent functionality.  The reason for this can be seen by
   walking through the IPCP negotiation:

    1. A mobile node connects to a peer via PPP and proposes its home
       address in an IPCP Configure-Request containing the IP-Address
       option.  In this scenario, we assume that the mobile node is
       connecting to some foreign link.

    2. The peer has no way of knowing whether this Configure-Request was
       received from: (a) a mobile node proposing its home address; or
       (b) a conventional node proposing some topologically non-routable
       address.  In this case, the peer must (conservatively) send a
       Configure-Nak of the IP-Address option supplying a topologically
       appropriate address for use by the node at the other end of the
       PPP link.

    3. The mobile node, in turn, has no way of knowing whether this
       Configure-Nak was received 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 mobile node's peer is a foreign
   agent and sends an Agent Advertisement to the mobile node after IPCP
   reaches the Opened state, 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.

1.4. 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 their potential
       peers 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 provided in
       the examples of Section 2.6.

    3. The option and protocol described in this document must not
       duplicate any functionality already defined in other IPCP
       options; specifically, the IP-Address option.

    4. 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).

2. Mobile-IPv4 Configuration Option

   This section defines the Mobile-IPv4 Configuration Option and
   provides several examples of its use.

2.1. Option Format

   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     |         Mobile Node's ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         ...  Home Address         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Type

      4 (Mobile-IPv4)

   Length

      6 (The length of this entire extension in bytes)

   Mobile Node's Home Address

      In a Configure-Request, the IP home address of the mobile node
      sending this Configuration Option, otherwise the (unmodified) IP
      home address of the mobile node when sent in a Configure-Ack or
      Configure-Reject. Configure-Nak'ing this option is undefined and
      MUST NOT be sent by implementations complying with this version of
      the specification.  This field MUST NOT be zero.

   Default Value

      The Mobile-IPv4 Configuration Option defaults to the sending
      mobile node's home address.

   In describing the operation of the Mobile-IPv4 Configuration Option
   (in conjunction with the IP-Address Configuration Option), we use the
   following abbreviations:

      PPP Message Types:
          Request = Configure-Request
           Reject = Configure-Reject
              Ack = Configure-Ack
              Nak = Configure-Nak

      IPCP Configuration Options:
            MIPv4 = Mobile-IPv4
               IP = IP-Address

      IP addresses:
          a.b.c.d = some non-zero IP address
          w.x.y.z = some non-zero IP address other than a.b.c.d
             home = a mobile node's IP Home address
              coa = an IP Care-Of Address
                0 = the all-zeroes IP address (0.0.0.0)

2.2. Overview

   The Mobile-IPv4 Configuration Option is designed to be used in
   conjunction with the IP-Address Configuration Option.  For the
   convenience of implementors, the detailed description in section 2.5
   includes all possible combinations of these two options that might be
   sent by a PPP peer during IPCP.  Along with each possibility is a
   description of how the receiver should interpret the contents as well
   as a suggested course of action.

2.3. High-Level Requirements for Non-Mobile-Nodes

   A node that is not performing mobile node functionality (such as
   non-Mobile-IP-aware nodes as well as nodes performing only home agent
   functionality, foreign agent functionality, or both) MUST NOT include
   a Mobile-IPv4 Configuration Option within any Configure-Request
   message.  As per [RFC 1332], such a node SHOULD send a Configure-
   Request containing an IP-Address Configuration Option in which the
   IP-Address field is set to a non-zero IP address that the node has
   assigned to one of its interfaces.  If an explicit IP address has
   been assigned to the node's PPP interface then this address SHOULD be
   sent in preference to any of the node's other addresses.

   A node MUST NOT send a Configure-Nak containing a Mobile-IPv4
   Configuration Option.  Doing so is currently "undefined" and might
   cause interoperability problems when a useful meaning for Configure-
   Nak is ultimately defined for the Mobile-IPv4 Configuration Option.
   A node that sends a Configure-Ack containing a Mobile-IPv4
   Configuration Option SHOULD send an Agent Advertisement [RFC 2002]
   immediately upon IPCP for that link entering the Opened state.

2.4. High-Level Requirements for Mobile Nodes

   A mobile node SHOULD begin its IPCP negotiation by sending the
   Configure-Request described in either item #1 or item #4 in Section
   2.5.  The mobile node MAY begin its negotiation with one of the other
   numbered items in Section 2.5 under extenuating circumstances.

   A mobile node that receives a Configure-Ack containing a Mobile-IPv4
   Configuration Option MUST receive an Agent Advertisement, possibly in
   response to an Agent Solicitation, before sending a Registration
   Request [RFC 2002] if that mobile node is connecting to a foreign
   link.  This is because the peer might be a foreign agent that
   enforces a policy which requires a mobile node to register with that
   foreign agent even if the mobile node is using a co-located care-of
   address.  A mobile node need not wait for such an advertisement if it
   connects to its home link.  See item 7a in section 2.5 for one way in
   which a mobile node can determine if it has connected to its home
   link.  Another way is by receiving an explicit notification of this
   fact from its peer, such as receipt of the messages in items 1b, 2c,
   and 3a in section 2.5.

   A mobile node that receives a Configure-Reject containing a Mobile-
   IPv4 Configuration Option SHOULD fall back to IPCP negotiation using
   the IP-Address option [RFC 1332].  A mobile node SHOULD begin this
   negotiation with Request(IP=home) or Request(IP=0), depending on
   whether or not the mobile node is connecting to its home link,
   respectively.  A mobile node MAY make this determination by
   inspection of an IP-Address option contained within a Configure-
   Request sent by its peer.  If the prefix of the peer's stated IP-
   address is equal to the prefix of the mobile node's home address,
   then the mobile node MAY conclude that it is connecting to its home
   link.  Otherwise, if the mobile node is connecting to a foreign link,
   then the mobile node SHOULD send Request(IP=0) since its peer might
   have no means for assigning addresses other than IPCP.  This
   specification therefore updates this behavior as described in [RFC
   2002], the latter of which recommends that a mobile node begin IP-
   Address negotiation with Request(IP=Home) under all circumstances.

   A peer that is performing neither home agent nor foreign agent
   functionality SHOULD send a Reject in response to any Request
   received from its peer that contains a Mobile-IPv4 Configuration
   Option.

2.5. Detailed Description

   The numbered items below show all possible combinations of Mobile-
   IPv4 and IP-Address Configuration Options that a mobile node (or a
   conventional node) might send to its peer.  Mobile nodes SHOULD begin
   their IPCP negotiation with item #1 or item #4 depending on whether
   they prefer a co-located or a foreign agent care-of address
   respectively.  The lettered items list the possible legal responses
   that a peer might send to the mobile node (or conventional node) in
   response to the numbered Request.

   In each case, an interpretation is defined and a suggested course of
   action is provided.  Finally, it is believed that the presentation
   below has the advantages of conciseness and precision in comparison
   to an equivalent presentation in "prose form."

    1. Request(IP=0,MIPv4=home) means "I prefer a co-located care-of
       address to a foreign agent care-of address."  Peer MUST respond
       with one of the following:

        a. Nak(IP=coa) means "use coa as your co-located care-of
           address".  Goto 2.
        b. Nak(IP=home) means "you're at home and don't need a care-of
           address".  Goto 3.
        c. Reject(IP=0) means "I cannot assign a co-located care-of
           address but you're welcome to use me as a foreign agent".
           Goto 4.
        d. Reject(MIPv4=home) means "I do not implement the Mobile-IPv4
           option".  If the peer also sent Request(IP=address) and the
           prefix of the peer's assigned address is equal to that of the
           mobile node's home address, then goto 6 with a.b.c.d=home;
           otherwise, goto 5.
        e. Reject(IP=0,MIPv4=home) means "use the default".  Goto 7.

        => Ack(IP=0, ...), Nak(MIPv4=any, ...) MUST NOT be sent.

    2. Request(IP=coa,MIPv4=home) means "I want to use coa as my co-
       located care-of address."  Peer MUST respond with one of the
       following:

        a. Ack(IP=coa,MIPv4=home) means "ok, use coa as your co-located
           care-of address; be sure to wait for an advertisement."
           Opened.
        b. Nak(IP=alternate-coa) means "no, use alternate-coa as your
           co-located care-of address".  Goto 2.
        c. Nak(IP=home) means "you're at home and don't need a co-
           located care-of address".  Goto 3.
        d. Reject(IP=coa) means "coa is not a useful value for a co-
           located care-of address on this link and I cannot assign a
           useful one (or I will not negotiate the IP-Address option) --
           you may use me as a foreign agent".  Goto 4.

        e. Reject(MIPv4=home) means "I do not implement the Mobile-IPv4
           option".  If the peer also sent Request(IP=address) and the
           prefix of the peer's address is equal to that of the mobile
           node's home address, then goto 6 with a.b.c.d=home;
           otherwise, goto 5.
        f. Reject(IP=coa,MIPv4=home) means "use the default".  Goto 7.

        => Nak(MIPv4=any, ...) MUST NOT be sent.

    3. Request(IP=home,MIPv4=home) means "I think I'm at home but if I'm
       wrong then I prefer a co-located care-of address to a foreign
       agent care-of address."  Peer MUST respond with one of the
       following:

        a. Ack(IP=home,MIPv4=home) means "yes, you're at home".  Opened.
        b. Nak(IP=coa) means "you're not at home, use coa as your co-
           located care-of address".  Goto 2.
        c. Reject(IP=home) means "you're not at home and I cannot assign
           a co-located care-of address (or I will not negotiate the
           IP-Address option) -- you may use me as a foreign agent".
           Goto 4.
        d. Reject(MIPv4=home) means "I do not implement the Mobile-IPv4
           option".  If the peer also sent Request(IP=address) and the
           prefix of the peer's address is equal to that of the mobile
           node's home address, then goto 6 with a.b.c.d=home;
           otherwise, goto 5.
        e. Reject(IP=home,MIPv4=home) means "use the default".  Goto 7.

        => Nak(MIPv4=any, ...) MUST NOT be sent.

    4. Request(MIPv4=home) means "I want to run Mobile IP over this link
       and I don't want a co-located care-of address." Peer MUST respond
       with one of the following:

        a. Ack(MIPv4=home) means "ok, wait for an advertisement to
           figure out where you are."  Opened.
        b. Reject(MIPv4=home) means "I do not implement the Mobile-IPv4
           option".  If the peer also sent Request(IP=address) and the
           prefix of the peer's address is equal to that of the mobile
           node's home address, then goto 6 with a.b.c.d=home;
           otherwise, goto 5.

        => Nak(MIPv4=any, ...) MUST NOT be sent.

    5. Request(IP=0) means "Please assign an address/co-located-care-
       of-address".  Peer MUST respond with one of the following:

        a. Nak(IP=a.b.c.d) means "use a.b.c.d as your address/co-
           located-care-of-address".  Goto 6.
        b. Reject(IP=0) means "I cannot assign an address (for the
           Mobile Node to use as a co-located-care-of-address), or I do
           not implement the IP-Address option".  Goto 7.

        => Ack(IP=0) MUST NOT be sent and historically means "I don't
           know your address either".  Opened.  An implementation MUST
           NOT use 0 as its IP address upon receiving Ack(IP=0) but MAY
           use some other, non-zero, interface address for packets sent
           on its PPP interface.

    6. Request(IP=a.b.c.d) means "I want to use a.b.c.d as my
       address/home-address/co-located-care-of-address".  Peer MUST
       respond with one of the following:

        a. Ack(IP=a.b.c.d) means "ok, a.b.c.d is your address/home-
           address/co-located-care-of-address".  Opened.
        b. Nak(IP=w.x.y.z) means "no, use w.x.y.z as your address/home-
           address/co-located-care-of-address".  Goto 6.
        c. Reject(IP=a.b.c.d) means "a.b.c.d is a bad address to use,
           but I cannot give you a good one" or "I do not implement the
           IP-Address option".  Goto 7.

    7. Request() means "I want to use the default".  Peer MUST respond
       with one of the following:

        a. Ack() means "ok, use the default".  Opened.

           In this case the mobile node will use the "default" values of
           the IP-Address option (no address configured by IPCP) and the
           Mobile-IPv4 option (the mobile node's IP home address).  The
           mobile node SHOULD send Agent Solicitations to see if there
           are any agents present on the current link. (Note that the
           current "link" might also include a shared medium if the
           mobile node's PPP peer is a bridge.)  If an agent is present
           and the mobile node receives an Agent Advertisement, then the
           mobile node employs its move-detection algorithm(s) and
           registers accordingly.

           In any case, if the mobile node's peer supplied an IP-Address
           option containing a non-zero value within an IPCP Configure-
           Request, the mobile node MAY use this address to determine
           whether or not it is connected to its home link.  This can be
           accomplished by comparing the stated IP address with the
           mobile node's home address under the prefix-length associated
           with the home link.  If the mobile node is connected to its
           home link then it SHOULD de-register with its home agent.

           Otherwise, the mobile node MAY attempt to obtain a
           topologically routable address through any of its supported
           means (e.g., DHCP, manual configuration, etc.)  for use as a
           co-located care-of address.  If the mobile node is successful
           in obtaining such an address then it SHOULD register this
           address with its home agent.

        => Nak(IP=0) MUST NOT be sent.  Goto 6.

        => Nak() MUST NOT be sent.

        => Reject() MUST NOT be sent.

2.6. Example Scenarios

   This section illustrates the use of the option and protocol as
   defined in the previous sections.  In the examples which follow, a
   Configure-Request sent by a mobile node and the response generated by
   the peer are shown on the same line.  The number and letter to the
   left of each request/response refer to the numbered and lettered
   items in Section 2.5.

    A. A mobile node prefers a co-located care-of address and the peer
       is a foreign agent that is capable of assigning such an address:

       (1)(a) Request(IP=0,MIPv4=Home) / Nak(IP=coa)
       (2)(a) Request(IP=coa,MIPv4=Home) / Ack(IP=coa,MIPv4=Home)

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

    B. A mobile node prefers a co-located care-of address and the peer
       is a foreign agent that cannot assign a co-located care-of
       address (e.g., it has no pool of addresses from which to allocate
       for the purpose of assignment):

       (1)(c) Request(IP=0,MIPv4=Home) / Reject(IP=0)
       (4)(a) Request(MIPv4=Home) / Ack(MIPv4=Home)

         - IPCP completes.
         - Mobile node waits to receive an Agent Advertisement.
         - Mobile node registers using the peer's foreign agent care-of
           address with its home agent.

    C. A mobile node prefers a co-located care-of address and the peer
       determines that the mobile node's home address is such that the
       mobile node is connecting to its home link:

       (1)(b) Request(IP=0,MIPv4=Home) / Nak(IP=Home)
       (3)(a) Request(IP=Home,MIPv4=Home) / Ack(IP=Home,MIPv4=Home)

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

    D. A mobile node prefers a foreign agent care-of address and the
       peer is a foreign agent which finds this state of affairs
       satisfactory:

       (4)(a) Request(MIPv4=Home) / Ack(MIPv4=Home)

         - IPCP completes.
         - Mobile node waits to receive an Agent Advertisement.
         - Mobile node registers using the peer's foreign agent care-of
           or de-registers at home, depending on the values in the Agent
           Advertisement.

    E. A mobile node prefers a co-located care-of address and the peer
       does not implement the Mobile-IPv4 Configuration Option.  The
       peer is, however, capable of assigning dynamic addresses:

       (1)(d) Request(IP=0,MIPv4=Home) / Reject(MIPv4=Home)
       (5)(a) Request(IP=0) / Nak(IP=a.b.c.d)
       (6)(a) Request(IP=a.b.c.d) / Ack(IP=a.b.c.d)

         - IPCP completes.
         - Mobile node registers using a.b.c.d as a co-located care-of
           address with its home agent.

    F. A mobile node prefers a co-located care-of address and the peer
       does not implement the Mobile-IPv4 Configuration Option. The peer
       is not capable of assigning dynamic addresses:

       (1)(e) Request(IP=0,MIPv4=Home) / Reject(IP=0,MIPv4=Home)
       (7)(a) Request() / Ack()

         - IPCP completes.
         - Mobile node sends an Agent Solicitation and/or attempts to
           obtain a co-located care-of address via means outside IPCP
           (e.g., DHCP or manual configuration), or it gives up.

3. Additional Requirements

3.1. Other IPCP Options

   A mobile node MUST NOT include the deprecated IP-Addresses option in
   any Configure-Request that contains a Mobile-IPv4 option, an IP-
   Address option, or both.

   Conversely, the mobile node MAY include an IP-Compression-Protocol
   option and any other options that do not involve the negotiation of
   IP addresses.

   If a mobile node and a foreign agent or a 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].  If the PPP peer entities are utilizing VJ header
   compression there is no gain for the mobile ip entities to do so, and
   requesting this option is likely to cause confusion.

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 renegotiate
   IPCP at any time, including, the renegotiation of the IP-Address
   Configuration Option and the Mobile-IPv4 Configuration Option
   described in this document.  As per [RFC 1661], a mobile node in the
   Opened state MUST renegotiate IPCP upon receiving an IPCP Configure-
   Request from its peer.

   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
   [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 SHOULD send an Agent Advertisement over a PPP link
   immediately after IPCP for that link enters the Opened state.

4. 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.

5. References

   [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
      Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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", STD 51, 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.

6. Acknowledgments

   The design of this protocol and option were inspired by an earlier
   submission by B. Patel and C. Perkins, then of IBM, in a now expired
   internet draft.  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 some of Charlie
   Perkins' definitions, and other relavent text, from [RFC 2002].

   Tim Wilson and Chris Stanaway (Motorola) contributed significantly to
   the design of this Configuration Option and protocol specification.
   Special thanks to Vernon Schryver (SGI), Craig Fox (Cisco), Karl Fox
   (Ascend), and John Bray (FTP) for their helpful suggestions,
   comments, and patience.

7. Authors' Addresses

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

   Phone:  +1-847-576-2753
   Fax:    +1-847-576-3240
   EMail:  solomon@comm.mot.com


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

   Phone:  +1-508-685-4000
   Fax:    +1-508-684-6105
   EMail:  glass@ftp.com

8.  Full Copyright Statement

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