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rfc:rfc2290

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

Solomon & Glass Standards Track [Page 1] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

     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]:

Solomon & Glass Standards Track [Page 2] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

    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:

Solomon & Glass Standards Track [Page 3] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

 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.

Solomon & Glass Standards Track [Page 4] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

  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.

Solomon & Glass Standards Track [Page 5] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

  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:

Solomon & Glass Standards Track [Page 6] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

    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.

Solomon & Glass Standards Track [Page 7] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

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

Solomon & Glass Standards Track [Page 8] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

 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.

Solomon & Glass Standards Track [Page 9] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

      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:

Solomon & Glass Standards Track [Page 10] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

      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.

Solomon & Glass Standards Track [Page 11] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

         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)
  1. Mobile node waits to receive an Agent Advertisement.
  2. 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)
  1. IPCP completes.
  2. Mobile node waits to receive an Agent Advertisement.
  3. Mobile node registers using the peer's foreign agent care-of

address with its home agent.

Solomon & Glass Standards Track [Page 12] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

  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)
  1. IPCP completes.
  2. 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)
  1. IPCP completes.
  2. Mobile node waits to receive an Agent Advertisement.
  3. 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)
  1. IPCP completes.
  2. 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()
  1. IPCP completes.
  2. 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.

Solomon & Glass Standards Track [Page 13] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

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.

Solomon & Glass Standards Track [Page 14] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

 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.

Solomon & Glass Standards Track [Page 15] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

 [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

Solomon & Glass Standards Track [Page 16] RFC 2290 Mobile-IPv4 Option for PPP IPCP February 1998

8. Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Solomon & Glass Standards Track [Page 17]

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