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 [RFC2002] 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 [RFC1661] 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) [RFC1332]. Using this
option, two peers can communicate their support for Mobile IP during
the IPCP phase of PPP. Familiarity with Mobile IP [RFC2002], IPCP
[RFC1332], and PPP [RFC1661] 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 [RFC2002] 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 [RFC1332], 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 [RFC1332].

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 RFC2119.

1.2. Terminology

This document uses the following terms as defined in [RFC2002]:

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 [RFC2002] currently specifies only the
following with respect to PPP:

"The Point-to-Point-Protocol (PPP) [RFC1661] and its Internet
Protocol Control Protocol (IPCP) [RFC1332], 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 [RFC1332], 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 [RFC2002]
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 [RFC2002] 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 [RFC1332]. 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 [RFC1144], then the mobile node
MUST NOT set the 'V' bit in its ensuing Mobile IP Registration
Request [RFC2002]. 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 [RFC1661], 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
[RFC2002]) 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, [RFC2002]). 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,
[RFC2002]). 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 [RFC1994])
to prevent theft-of-service. Additionally, users requiring
confidentiality should use PPP link encryption [RFC1968], IP-layer
encryption [RFC1827], or application-layer encryption, depending
upon their individual requirements. Finally, Mobile IP
authentication [RFC2002] protects against trivial denial-of-service
attacks that could otherwise be waged against a mobile node and its
home agent.

5. References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC1144, January 1990.

[RFC1332] McGregor, G., "The PPP Internet Protocol Control Protocol
(IPCP)," RFC1332, May 1992.

[RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)
for the Transmission of Multi-protocol Datagrams over Point-to-
Point Links", STD 51, RFC1661, July 1994.

[RFC1827] Atkinson, R., "IP Encapsulating Security Payload (ESP)",
RFC1827, August 1995.

[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC1994, August 1996.

[RFC1968] Meyer, G., "The PPP Encryption Control Protocol (ECP)",
RFC1968, June 1996.

[RFC2002] Perkins, C., Editor, "IP Mobility Support", RFC2002,
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 [RFC1661] in order to ensure consistency of
terminology and specification. The same goes for some of Charlie
Perkins' definitions, and other relavent text, from [RFC2002].

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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