rfc2290.txt

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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



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       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]:







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      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:








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   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.






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    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.






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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