Network Working Group C. Perkins, Editor
Request for Comments: 2002 IBM
Category: Standards Track October 1996
IP Mobility Support
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.
Abstract
This document specifies protocol enhancements that allow transparent
routing of IP datagrams to mobile nodes in the Internet. Each mobile
node is always identified by its home address, regardless of its
current point of attachment to the Internet. While situated away
from its home, a mobile node is also associated with a care-of
address, which provides information about its current point of
attachment to the Internet. The protocol provides for registering
the care-of address with a home agent. The home agent sends
datagrams destined for the mobile node through a tunnel to the care-
of address. After arriving at the end of the tunnel, each datagram
is then delivered to the mobile node.
Table of Contents
1. Introduction 3
1.1. Protocol Requirements . . . . . . . . . . . . . . . . . . 3
1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
1.5. New Architectural Entities . . . . . . . . . . . . . . . 5
1.6. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.7. Protocol Overview . . . . . . . . . . . . . . . . . . . . 8
1.8. Specification Language . . . . . . . . . . . . . . . . . 11
1.9. Message Format and Protocol Extensibility . . . . . . . . 12
2. Agent Discovery 14
2.1. Agent Advertisement . . . . . . . . . . . . . . . . . . . 14
2.1.1. Mobility Agent Advertisement Extension . . . . . 16
2.1.2. Prefix-Lengths Extension . . . . . . . . . . . . 18
2.1.3. One-byte Padding Extension . . . . . . . . . . . 19
2.2. Agent Solicitation . . . . . . . . . . . . . . . . . . . 19
2.3. Foreign Agent and Home Agent Considerations . . . . . . . 19
2.3.1. Advertised Router Addresses . . . . . . . . . . . 20
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2.3.2. Sequence Numbers and Rollover Handling . . . . . 21
2.4. Mobile Node Considerations . . . . . . . . . . . . . . . 21
2.4.1. Registration Required . . . . . . . . . . . . . . 22
2.4.2. Move Detection . . . . . . . . . . . . . . . . . 22
2.4.3. Returning Home . . . . . . . . . . . . . . . . . 24
2.4.4. Sequence Numbers and Rollover Handling . . . . . 24
3. Registration 24
3.1. Registration Overview . . . . . . . . . . . . . . . . . . 25
3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 26
3.3. Registration Request . . . . . . . . . . . . . . . . . . 26
3.4. Registration Reply . . . . . . . . . . . . . . . . . . . 29
3.5. Registration Extensions . . . . . . . . . . . . . . . . . 32
3.5.1. Computing Authentication Extension Values . . . . 32
3.5.2. Mobile-Home Authentication Extension . . . . . . 33
3.5.3. Mobile-Foreign Authentication Extension . . . . . 33
3.5.4. Foreign-Home Authentication Extension . . . . . . 34
3.6. Mobile Node Considerations . . . . . . . . . . . . . . . 34
3.6.1. Sending Registration Requests . . . . . . . . . . 36
3.6.2. Receiving Registration Replies . . . . . . . . . 40
3.6.3. Registration Retransmission . . . . . . . . . . . 42
3.7. Foreign Agent Considerations . . . . . . . . . . . . . . 43
3.7.1. Configuration and Registration Tables . . . . . . 44
3.7.2. Receiving Registration Requests . . . . . . . . . 44
3.7.3. Receiving Registration Replies . . . . . . . . . 47
3.8. Home Agent Considerations . . . . . . . . . . . . . . . . 49
3.8.1. Configuration and Registration Tables . . . . . . 49
3.8.2. Receiving Registration Requests . . . . . . . . . 49
3.8.3. Sending Registration Replies . . . . . . . . . . 53
4. Routing Considerations 55
4.1. Encapsulation Types . . . . . . . . . . . . . . . . . . . 56
4.2. Unicast Datagram Routing . . . . . . . . . . . . . . . . 56
4.2.1. Mobile Node Considerations . . . . . . . . . . . 56
4.2.2. Foreign Agent Considerations . . . . . . . . . . 57
4.2.3. Home Agent Considerations . . . . . . . . . . . . 58
4.3. Broadcast Datagrams . . . . . . . . . . . . . . . . . . . 59
4.4. Multicast Datagram Routing . . . . . . . . . . . . . . . 60
4.5. Mobile Routers . . . . . . . . . . . . . . . . . . . . . 61
4.6. ARP, Proxy ARP, and Gratuitous ARP . . . . . . . . . . . 62
5. Security Considerations 66
5.1. Message Authentication Codes . . . . . . . . . . . . . . 66
5.2. Areas of Security Concern in this Protocol . . . . . . . 66
5.3. Key Management . . . . . . . . . . . . . . . . . . . . . 67
5.4. Picking Good Random Numbers . . . . . . . . . . . . . . . 67
5.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.6. Replay Protection for Registration Requests . . . . . . . 68
5.6.1. Replay Protection using Timestamps . . . . . . . 68
5.6.2. Replay Protection using Nonces . . . . . . . . . 69
6. Acknowledgments 71
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A. Patent Issues 72
A.1. IBM Patent #5,159,592 . . . . . . . . . . . . . . . . . . 72
A.2. IBM Patent #5,148,479 . . . . . . . . . . . . . . . . . . 72
B. Link-Layer Considerations 73
C. TCP Considerations 73
C.1. TCP Timers . . . . . . . . . . . . . . . . . . . . . . . 73
C.2. TCP Congestion Management . . . . . . . . . . . . . . . . 73
D. Example Scenarios 74
D.1. Registering with a Foreign Agent Care-of Address . . . . 74
D.2. Registering with a Co-Located Care-of Address . . . . . . 75
D.3. Deregistration . . . . . . . . . . . . . . . . . . . . . 76
E. Applicability of Prefix Lengths Extension 76
Editor's Address 79
1. Introduction
IP version 4 assumes that a node's IP address uniquely identifies the
node's point of attachment to the Internet. Therefore, a node must
be located on the network indicated by its IP address in order to
receive datagrams destined to it; otherwise, datagrams destined to
the node would be undeliverable. For a node to change its point of
attachment without losing its ability to communicate, currently one
of the two following mechanisms must typically be employed:
a) the node must change its IP address whenever it changes its
point of attachment, or
b) host-specific routes must be propagated throughout much of
the Internet routing fabric.
Both of these alternatives are often unacceptable. The first makes
it impossible for a node to maintain transport and higher-layer
connections when the node changes location. The second has obvious
and severe scaling problems, especially relevant considering the
explosive growth in sales of notebook (mobile) computers.
A new, scalable, mechanism is required for accommodating node
mobility within the Internet. This document defines such a
mechanism, which enables nodes to change their point of attachment to
the Internet without changing their IP address.
1.1. Protocol Requirements
A mobile node must be able to communicate with other nodes after
changing its link-layer point of attachment to the Internet, yet
without changing its IP address.
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A mobile node must be able to communicate with other nodes that do
not implement these mobility functions. No protocol enhancements are
required in hosts or routers that are not acting as any of the new
architectural entities introduced in Section 1.5.
All messages used to update another node as to the location of a
mobile node must be authenticated in order to protect against remote
redirection attacks.
1.2. Goals
The link by which a mobile node is directly attached to the Internet
may often be a wireless link. This link may thus have a
substantially lower bandwidth and higher error rate than traditional
wired networks. Moreover, mobile nodes are likely to be battery
powered, and minimizing power consumption is important. Therefore,
the number of administrative messages sent over the link by which a
mobile node is directly attached to the Internet should be minimized,
and the size of these messages should be kept as small as is
reasonably possible.
1.3. Assumptions
The protocols defined in this document place no additional
constraints on the assignment of IP addresses. That is, a mobile
node can be assigned an IP address by the organization that owns the
machine.
This protocol assumes that mobile nodes will generally not change
their point of attachment to the Internet more frequently than once
per second.
This protocol assumes that IP unicast datagrams are routed based on
the destination address in the datagram header (and not, for example,
by source address).
1.4. Applicability
Mobile IP is intended to enable nodes to move from one IP subnet to
another. It is just as suitable for mobility across homogeneous
media as it is for mobility across heterogeneous media. That is,
Mobile IP facilitates node movement from one Ethernet segment to
another as well as it accommodates node movement from an Ethernet
segment to a wireless LAN, as long as the mobile node's IP address
remains the same after such a movement.
One can think of Mobile IP as solving the "macro" mobility management
problem. It is less well suited for more "micro" mobility management
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RFC 2002 IP Mobility Support October 1996
applications -- for example, handoff amongst wireless transceivers,
each of which covers only a very small geographic area. As long as
node movement does not occur between points of attachment on
different IP subnets, link-layer mechanisms for mobility (i.e.,
link-layer handoff) may offer faster convergence and far less
overhead than Mobile IP.
1.5. New Architectural Entities
Mobile IP introduces the following new functional entities:
Mobile Node
A host or router that changes its point of attachment from one
network or subnetwork to another. A mobile node may change its
location without changing its IP address; it may continue to
communicate with other Internet nodes at any location using its
(constant) IP address, assuming link-layer connectivity to a
point of attachment is available.
Home Agent
A router on a mobile node's home network which tunnels
datagrams for delivery to the mobile node when it is away from
home, and maintains current location information for the mobile
node.
Foreign Agent
A router on a mobile node's visited network which provides
routing services to the mobile node while registered. The
foreign agent detunnels and delivers datagrams to the mobile
node that were tunneled by the mobile node's home agent. For
datagrams sent by a mobile node, the foreign agent may serve as
a default router for registered mobile nodes.
A mobile node is given a long-term IP address on a home network.
This home address is administered in the same way as a "permanent" IP
address is provided to a stationary host. When away from its home
network, a "care-of address" is associated with the mobile node and
reflects the mobile node's current point of attachment. The mobile
node uses its home address as the source address of all IP datagrams
that it sends, except where otherwise described in this document for
datagrams sent for certain mobility management functions (e.g., as in
Section 3.6.1.1).
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1.6. Terminology
This document frequently uses the following terms:
Agent Advertisement
An advertisement message constructed by attaching a
special Extension to a router advertisement [4] message.
Care-of Address
The termination point of a tunnel toward a mobile node,
for datagrams forwarded to the mobile node while it is
away from home. The protocol can use two different types
of care-of address: a "foreign agent care-of address" is
an address of a foreign agent with which the mobile node
is registered, and a "co-located care-of address" is an
externally obtained local address which the mobile node
has associated with one of its own network interfaces.
Correspondent Node
A peer with which a mobile node is communicating. A
correspondent node may be either mobile or stationary.
Foreign Network
Any network other than the mobile node's Home Network.
Home Address
An IP address that is assigned for an extended period of
time to a mobile node. It remains unchanged regardless
of where the node is attached to the Internet.
Home Network
A network, possibly virtual, having a network prefix
matching that of a mobile node's home address. Note that
standard IP routing mechanisms will deliver datagrams
destined to a mobile node's Home Address to the mobile
node's Home Network.
Link A facility or medium over which nodes can communicate at
the link layer. A link underlies the network layer.
Link-Layer Address
The address used to identify an endpoint of some
communication over a physical link. Typically, the
Link-Layer address is an interface's Media Access Control
(MAC) address.
Mobility Agent
Either a home agent or a foreign agent.
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Mobility Binding
The association of a home address with a care-of address,
along with the remaining lifetime of that association.
Mobility Security Association
A collection of security contexts, between a pair
of nodes, which may be applied to Mobile IP protocol
messages exchanged between them. Each context indicates
an authentication algorithm and mode (Section 5.1), a
secret (a shared key, or appropriate public/private
key pair), and a style of replay protection in use
(Section 5.6).
Node A host or a router.
Nonce A randomly chosen value, different from previous choices,
inserted in a message to protect against replays.
Security Parameter Index (SPI)
An index identifying a security context between a pair
of nodes among the contexts available in the Mobility
Security Association. SPI values 0 through 255 are
reserved and MUST NOT be used in any Mobility Security
Association.
Tunnel The path followed by a datagram while it is encapsulated.
The model is that, while it is encapsulated, a datagram
is routed to a knowledgeable decapsulating agent, which
decapsulates the datagram and then correctly delivers it
to its ultimate destination.
Virtual Network
A network with no physical instantiation beyond a router
(with a physical network interface on another network).
The router (e.g., a home agent) generally advertises
reachability to the virtual network using conventional
routing protocols.
Visited Network
A network other than a mobile node's Home Network, to
which the mobile node is currently connected.
Visitor List
The list of mobile nodes visiting a foreign agent.
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1.7. Protocol Overview
The following support services are defined for Mobile IP:
Agent Discovery
Home agents and foreign agents may advertise their
availability on each link for which they provide service.
A newly arrived mobile node can send a solicitation on
the link to learn if any prospective agents are present.
Registration
When the mobile node is away from home, it registers
its care-of address with its home agent. Depending on
its method of attachment, the mobile node will register
either directly with its home agent, or through a foreign
agent which forwards the registration to the home agent.
The following steps provide a rough outline of operation of the
Mobile IP protocol:
- Mobility agents (i.e., foreign agents and home agents) advertise
their presence via Agent Advertisement messages (Section 2). A
mobile node may optionally solicit an Agent Advertisement message
from any locally attached mobility agents through an Agent
Solicitation message.
- A mobile node receives these Agent Advertisements and determines
whether it is on its home network or a foreign network.
- When the mobile node detects that it is located on its home
network, it operates without mobility services. If returning
to its home network from being registered elsewhere, the mobile
node deregisters with its home agent, through exchange of a
Registration Request and Registration Reply message with it.
- When a mobile node detects that it has moved to a foreign
network, it obtains a care-of address on the foreign network.
The care-of address can either be determined from a foreign
agent's advertisements (a foreign agent care-of address), or by
some external assignment mechanism such as DHCP [6] (a co-located
care-of address).
- The mobile node operating away from home then registers its
new care-of address with its home agent through exchange of a
Registration Request and Registration Reply message with it,
possibly via a foreign agent (Section 3).
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- Datagrams sent to the mobile node's home address are intercepted
by its home agent, tunneled by the home agent to the mobile
node's care-of address, received at the tunnel endpoint (either
at a foreign agent or at the mobile node itself), and finally
delivered to the mobile node (Section 4.2.3).
- In the reverse direction, datagrams sent by the mobile node
are generally delivered to their destination using standard IP
routing mechanisms, not necessarily passing through the home
agent.
When away from home, Mobile IP uses protocol tunneling to hide a
mobile node's home address from intervening routers between its home
network and its current location. The tunnel terminates at the
mobile node's care-of address. The care-of address must be an
address to which datagrams can be delivered via conventional IP
routing. At the care-of address, the original datagram is removed
from the tunnel and delivered to the mobile node.
Mobile IP provides two alternative modes for the acquisition of a
care-of address:
- A "foreign agent care-of address" is a care-of address provided
by a foreign agent through its Agent Advertisement messages. In
this case, the care-of address is an IP address of the foreign
agent. In this mode, the foreign agent is the endpoint of the
tunnel and, upon receiving tunneled datagrams, decapsulates them
and delivers the inner datagram to the mobile node. This mode
of acquisition is preferred because it allows many mobile nodes
to share the same care-of address and therefore does not place
unnecessary demands on the already limited IPv4 address space.
- A "co-located care-of address" is a care-of address acquired
by the mobile node as a local IP address through some external
means, which the mobile node then associates with one of its own
network interfaces. The address may be dynamically acquired as
a temporary address by the mobile node such as through DHCP [6],
or may be owned by the mobile node as a long-term address for its
use only while visiting some foreign network. Specific external
methods of acquiring a local IP address for use as a co-located
care-of address are beyond the scope of this document. When
using a co-located care-of address, the mobile node serves as the
endpoint of the tunnel and itself performs decapsulation of the
datagrams tunneled to it.
The mode of using a co-located care-of address has the advantage that
it allows a mobile node to function without a foreign agent, for
example, in networks that have not yet deployed a foreign agent.
Perkins Standards Track [Page 9]RFC 2002 IP Mobility Support October 1996
It does, however, place additional burden on the IPv4 address space
because it requires a pool of addresses within the foreign network to
be made available to visiting mobile nodes. It is difficult to
efficiently maintain pools of addresses for each subnet that may
permit mobile nodes to visit.
It is important to understand the distinction between the care-of
address and the foreign agent functions. The care-of address is
simply the endpoint of the tunnel. It might indeed be an address of
a foreign agent (a foreign agent care-of address), but it might
instead be an address temporarily acquired by the mobile node (a co-
located care-of address). A foreign agent, on the other hand, is a
mobility agent that provides services to mobile nodes. See Sections
3.7 and 4.2.2 for additional details.
A home agent MUST be able to attract and intercept datagrams that are
destined to the home address of any of its registered mobile nodes.
Using the proxy and gratuitous ARP mechanisms described in Section
4.6, this requirement can be satisfied if the home agent has a
network interface on the link indicated by the mobile node's home
address. Other placements of the home agent relative to the mobile
node's home location MAY also be possible using other mechanisms for
intercepting datagrams destined to the mobile node's home address.
Such placements are beyond the scope of this document.
Similarly, a mobile node and a prospective or current foreign agent
MUST be able to exchange datagrams without relying on standard IP
routing mechanisms; that is, those mechanisms which make forwarding
decisions based upon the network-prefix of the destination address in
the IP header. This requirement can be satisfied if the foreign
agent and the visiting mobile node have an interface on the same
link. In this case, the mobile node and foreign agent simply bypass
their normal IP routing mechanism when sending datagrams to each
other, addressing the underlying link-layer packets to their
respective link-layer addresses. Other placements of the foreign
agent relative to the mobile node MAY also be possible using other
mechanisms to exchange datagrams between these nodes, but such
placements are beyond the scope of this document.
If a mobile node is using a co-located care-of address (as described
in (b) above), the mobile node MUST be located on the link identified
by the network prefix of this care-of address. Otherwise, datagrams
destined to the care-of address would be undeliverable.
For example, the figure below illustrates the routing of datagrams to
and from a mobile node away from home, once the mobile node has
registered with its home agent. In the figure below, the mobile node
is using a foreign agent care-of address:
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RFC 2002 IP Mobility Support October 1996
2) Datagram is intercepted 3) Datagram is
by home agent and detunneled and
is tunneled to the delivered to the
care-of address. mobile node.
+-----+ +-------+ +------+
|home | =======> |foreign| ------> |mobile|
|agent| | agent | <------ | node |
+-----+ +-------+ +------+
1) Datagram to /|\ /
mobile node | / 4) For datagrams sent by the
arrives on | / mobile node, standard IP
home network | / routing delivers each to its
via standard | |_ destination. In this figure,
IP routing. +----+ the foreign agent is the
|host| mobile node's default router.
+----+
1.8. Specification Language
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that
the definition is an absolute requirement of the
specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means
that, in some circumstances, valid reasons may exist
to ignore this item, but the full implications must
be understood and carefully weighed before choosing
a different course. Unexpected results may result
otherwise.
MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST
be prepared to interoperate with another implementation
which does include the option.
Perkins Standards Track [Page 11]RFC 2002 IP Mobility Support October 1996
silently discard
The implementation discards the datagram without
further processing, and without indicating an error
to the sender. The implementation SHOULD provide the
capability of logging the error, including the contents
of the discarded datagram, and SHOULD record the event
in a statistics counter.
1.9. Message Format and Protocol Extensibility
Mobile IP defines a set of new control messages, sent with UDP [17]
using well-known port number 434. Currently, the following two
message types are defined:
1 Registration Request
3 Registration Reply
Up-to-date values for the message types for Mobile IP control
messages are specified in the most recent "Assigned Numbers" [20].
In addition, for Agent Discovery, Mobile IP makes use of the existing
Router Advertisement and Router Solicitation messages defined for
ICMP Router Discovery [4].
Mobile IP defines a general Extension mechanism to allow optional
information to be carried by Mobile IP control messages or by ICMP
Router Discovery messages. Each of these Extensions (with one
exception) is encoded in the following Type-Length-Value format:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Type | Length | Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Type Indicates the particular type of Extension.
Length Indicates the length (in bytes) of the data field within
this Extension. The length does NOT include the Type and
Length bytes.
Data The particular data associated with this Extension. This
field may be zero or more bytes in length. The format
and length of the data field is determined by the type
and length fields.
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Extensions allow variable amounts of information to be carried within
each datagram. The end of the list of Extensions is indicated by the
total length of the IP datagram.
Two separately maintained sets of numbering spaces, from which
Extension Type values are allocated, are used in Mobile IP:
- The first set consists of those Extensions which may appear only
in Mobile IP control messages (those sent to and from UDP port
number 434). Currently, the following Types are defined for
Extensions appearing in Mobile IP control messages:
32 Mobile-Home Authentication
33 Mobile-Foreign Authentication
34 Foreign-Home Authentication
- The second set consists of those extensions which may appear only
in ICMP Router Discovery messages [4]. Currently, Mobile IP
defines the following Types for Extensions appearing in ICMP
Router Discovery messages:
0 One-byte Padding (encoded with no Length nor Data field)
16 Mobility Agent Advertisement
19 Prefix-Lengths
Each individual Extension is described in detail in a separate
section later in this document. Up-to-date values for these
Extension Type numbers are specified in the most recent "Assigned
Numbers" [20].
Due to the separation (orthogonality) of these sets, it is
conceivable that two Extensions that are defined at a later date
could have identical Type values, so long as one of the Extensions
may be used only in Mobile IP control messages and the other may be
used only in ICMP Router Discovery messages.
When an Extension numbered in either of these sets within the range 0
through 127 is encountered but not recognized, the message containing
that Extension MUST be silently discarded. When an Extension
numbered in the range 128 through 255 is encountered which is not
recognized, that particular Extension is ignored, but the rest of the
Extensions and message data MUST still be processed. The Length
field of the Extension is used to skip the Data field in searching
for the next Extension.
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2. Agent Discovery
Agent Discovery is the method by which a mobile node determines
whether it is currently connected to its home network or to a foreign
network, and by which a mobile node can detect when it has moved from
one network to another. When connected to a foreign network, the
methods specified in this section also allow the mobile node to
determine the foreign agent care-of address being offered by each
foreign agent on that network.
Mobile IP extends ICMP Router Discovery [4] as its primary mechanism
for Agent Discovery. An Agent Advertisement is formed by including a
Mobility Agent Advertisement Extension in an ICMP Router
Advertisement message (Section 2.1). An Agent Solicitation message
is identical to an ICMP Router Solicitation, except that its IP TTL
MUST be set to 1 (Section 2.2). This section describes the message
formats and procedures by which mobile nodes, foreign agents, and
home agents cooperate to realize Agent Discovery.
Agent Advertisement and Agent Solicitation may not be necessary for
link layers that already provide this functionality. The method by
which mobile nodes establish link-layer connections with prospective
agents is outside the scope of this document (but see Appendix B).
The procedures described below assume that such link-layer
connectivity has already been established.
No authentication is required for Agent Advertisement and Agent
Solicitation messages. They MAY be authenticated using the IP
Authentication Header [1], which is unrelated to the messages
described in this document. Further specification of the way in
which Advertisement and Solicitation messages may be authenticated is
outside of the scope of this document.
2.1. Agent Advertisement
Agent Advertisements are transmitted by a mobility agent to advertise
its services on a link. Mobile nodes use these advertisements to
determine their current point of attachment to the Internet. An
Agent Advertisement is an ICMP Router Advertisement that has been
extended to also carry an Mobility Agent Advertisement Extension
(Section 2.1.1) and, optionally, a Prefix-Lengths Extension (Section
2.1.2), One-byte Padding Extension (Section 2.1.3), or other
Extensions that might be defined in the future.
Within an Agent Advertisement message, ICMP Router Advertisement
fields of the message are required to conform to the following
additional specifications:
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RFC 2002 IP Mobility Support October 1996
- Link-Layer Fields
Destination Address
The link-layer destination address of a unicast
Agent Advertisement MUST be the same as the source
link-layer address of the Agent Solicitation which
prompted the Advertisement.
- IP Fields
TTL The TTL for all Agent Advertisements MUST be set
to 1.
Destination Address
As specified for ICMP Router Discovery [4], the IP
destination address of an Agent Advertisement MUST
be either the "all systems on this link" multicast
address (224.0.0.1) [5] or the "limited broadcast"
address (255.255.255.255). The subnet-directed
broadcast address of the form <prefix>.<-1> cannot be
used since mobile nodes will not generally know the
prefix of the foreign network.
- ICMP Fields
Code The Code field of the agent advertisement is
interpreted as follows:
0 The mobility agent handles common traffic -- that
is, it acts as a router for IP datagrams not
necessarily related to mobile nodes.
16 The mobility agent does not route common traffic.
However, all foreign agents MUST (minimally)
forward to a default router any datagrams received
from a registered mobile node (Section 4.2.2).
Lifetime
The maximum length of time that the Advertisement
is considered valid in the absence of further
Advertisements.
Router Address(es)
See Section 2.3.1 for a discussion of the addresses
that may appear in this portion of the Agent
Advertisement.
Perkins Standards Track [Page 15]RFC 2002 IP Mobility Support October 1996
Num Addrs
The number of Router Addresses advertised in this
message. Note that in an Agent Advertisement
message, the number of router addresses specified in
the ICMP Router Advertisement portion of the message
MAY be set to 0. See Section 2.3.1 for details.
If sent periodically, the nominal interval at which Agent
Advertisements are sent SHOULD be 1/3 of the advertisement Lifetime
given in the ICMP header. This allows a mobile node to miss three
successive advertisements before deleting the agent from its list of
valid agents. The actual transmission time for each advertisement
SHOULD be slightly randomized [4] in order to avoid synchronization
and subsequent collisions with other Agent Advertisements that may be
sent by other agents (or with other Router Advertisements sent by
other routers). Note that this field has no relation to the
"Registration Lifetime" field within the Mobility Agent Advertisement
Extension defined below.
2.1.1. Mobility Agent Advertisement Extension
The Mobility Agent Advertisement Extension follows the ICMP Router
Advertisement fields. It is used to indicate that an ICMP Router
Advertisement message is also an Agent Advertisement being sent by a
mobility agent. The Mobility Agent Advertisement Extension 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 | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Registration Lifetime |R|B|H|F|M|G|V| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| zero or more Care-of Addresses |
| ... |
Type 16
Length (6 + 4*N), where N is the number of care-of addresses
advertised.
Sequence Number
The count of Agent Advertisement messages sent since the
agent was initialized (Section 2.3.2).
Perkins Standards Track [Page 16]RFC 2002 IP Mobility Support October 1996
Registration Lifetime
The longest lifetime (measured in seconds) that this
agent is willing to accept in any Registration Request.
A value of 0xffff indicates infinity. This field has no
relation to the "Lifetime" field within the ICMP Router
Advertisement portion of the Agent Advertisement.
R Registration required. Registration with this foreign
agent (or another foreign agent on this link) is required
rather than using a co-located care-of address.
B Busy. The foreign agent will not accept registrations
from additional mobile nodes.
H Home agent. This agent offers service as a home agent
on the link on which this Agent Advertisement message is
sent.
F Foreign agent. This agent offers service as a foreign
agent on the link on which this Agent Advertisement
message is sent.
M Minimal encapsulation. This agent implements receiving
tunneled datagrams that use minimal encapsulation [15].
G GRE encapsulation. This agent implements receiving
tunneled datagrams that use GRE encapsulation [8].
V Van Jacobson header compression. This agent supports use
of Van Jacobson header compression [10] over the link
with any registered mobile node.
reserved
Sent as zero; ignored on reception.
Care-of Address(es)
The advertised foreign agent care-of address(es) provided
by this foreign agent. An Agent Advertisement MUST
include at least one care-of address if the 'F' bit
is set. The number of care-of addresses present is
determined by the Length field in the Extension.
A home agent MUST always be prepared to serve the mobile nodes for
which it is the home agent. A foreign agent may at times be too busy
to serve additional mobile nodes; even so, it must continue to send
Agent Advertisements, so that any mobile nodes already registered
with it will know that they have not moved out of range of the
foreign agent and that the foreign agent has not failed. A foreign
Perkins Standards Track [Page 17]RFC 2002 IP Mobility Support October 1996
agent may indicate that it is "too busy" to allow new mobile nodes to
register with it, by setting the 'B' bit in its Agent Advertisements.
An Agent Advertisement message MUST NOT have the 'B' bit set if the
'F' bit is not also set, and at least one of the 'F' bit and the 'H'
bit MUST be set in any Agent Advertisement message sent.
When a foreign agent wishes to require registration even from those
mobile nodes which have acquired a co-located care-of address, it
sets the 'R' bit to one. Because this bit applies only to foreign
agents, an agent MUST NOT set the 'R' bit to one unless the 'F' bit
is also set to one.
2.1.2. Prefix-Lengths Extension
The Prefix-Lengths Extension MAY follow the Mobility Agent
Advertisement Extension. It is used to indicate the number of bits
of network prefix that applies to each Router Address listed in the
ICMP Router Advertisement portion of the Agent Advertisement. Note
that the prefix lengths given DO NOT apply to care-of address(es)
listed in the Mobility Agent Advertisement Extension. The Prefix-
Lengths Extension 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 | Prefix Length | ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 19 (Prefix-Lengths Extension)
Length N, where N is the value of the Num Addrs field in
the ICMP Router Advertisement portion of the Agent
Advertisement.
Prefix Length(s)
The number of leading bits that define the network number
of the corresponding Router Address listed in the ICMP
Router Advertisement portion of the message. The prefix
length for each Router Address is encoded as a separate
byte, in the order that the Router Addresses are listed
in the ICMP Router Advertisement portion of the message.
See Section 2.4.2 for information about how the Prefix Lengths
Extension MAY be used by a mobile node when determining whether it
has moved. See Appendix E for implementation details about the use
of this Extension.
Perkins Standards Track [Page 18]RFC 2002 IP Mobility Support October 1996
2.1.3. One-byte Padding Extension
Some IP protocol implementations insist upon padding ICMP messages to
an even number of bytes. If the ICMP length of an Agent
Advertisement is odd, this Extension MAY be included in order to make
the ICMP length even. Note that this Extension is NOT intended to be
a general-purpose Extension to be included in order to word- or
long-align the various fields of the Agent Advertisement. An Agent
Advertisement SHOULD NOT include more than one One-byte Padding
Extension and if present, this Extension SHOULD be the last Extension
in the Agent Advertisement.
Note that unlike other Extensions used in Mobile IP, the One-byte
Padding Extension is encoded as a single byte, with no "Length" nor
"Data" field present. The One-byte Padding Extension is defined as
follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type |
+-+-+-+-+-+-+-+-+
Type 0 (One-byte Padding Extension)
2.2. Agent Solicitation
An Agent Solicitation is identical to an ICMP Router Solicitation
with the further restriction that the IP TTL Field MUST be set to 1.
2.3. Foreign Agent and Home Agent Considerations
Any mobility agent which cannot be discovered by a link-layer
protocol MUST send Agent Advertisements. An agent which can be
discovered by a link-layer protocol SHOULD also implement Agent
Advertisements. However, the Advertisements need not be sent, except
when the site policy requires registration with the agent (i.e., when
the 'R' bit is set), or as a response to a specific Agent
Solicitation. All mobility agents SHOULD respond to Agent
Solicitations.
The same procedures, defaults, and constants are used in Agent
Advertisement messages and Agent Solicitation messages as specified
for ICMP Router Discovery [4], except that:
- a mobility agent MUST limit the rate at which it sends broadcast
or multicast Agent Advertisements; a recommended maximum rate is
once per second, AND
Perkins Standards Track [Page 19]RFC 2002 IP Mobility Support October 1996
- a mobility agent that receives a Router Solicitation MUST NOT
require that the IP Source Address is the address of a neighbor
(i.e., an address that matches one of the router's own addresses
on the arrival interface, under the subnet mask associated with
that address of the router).
- a mobility agent MAY be configured to send Agent Advertisements
only in response to an Agent Solicitation message.
If the home network is not a virtual network, then the home agent for
any mobile node SHOULD be located on the link identified by the
mobile node's home address, and Agent Advertisement messages sent by
the home agent on this link MUST have the 'H' bit set. In this way,
mobile nodes on their own home network will be able to determine that
they are indeed at home. Any Agent Advertisement messages sent by
the home agent on another link to which it may be attached (if it is
a mobility agent serving more than one link), MUST NOT have the 'H'
bit set, unless the home agent also serves as a home agent (to other
mobile nodes) on that other link.
If the home network is a virtual network, the home network has no
physical realization external to the home agent itself. In this
case, there is no physical network link on which to send Agent
Advertisement messages advertising the home agent. Mobile nodes for
which this is the home network are always treated as being away from
home.
On a particular subnet, either all mobility agents MUST include the
Prefix-Lengths Extension or all of them MUST NOT include this
Extension. Equivalently, it is prohibited for some agents on a given
subnet to include the Extension but for others not to include it.
Otherwise, one of the move detection algorithms designed for mobile
nodes will not function properly (Section 2.4.2).
2.3.1. Advertised Router Addresses
The ICMP Router Advertisement portion of the Agent Advertisement MAY
contain one or more router addresses. Thus, an agent MAY include one
of its own addresses in the advertisement. A foreign agent MAY
discourage use of this address as a default router by setting the
preference to a low value and by including the address of another
router in the advertisement (with a correspondingly higher
preference). Nevertheless, a foreign agent MUST route datagrams it
receives from registered mobile nodes (Section 4.2.2).
Perkins Standards Track [Page 20]RFC 2002 IP Mobility Support October 1996
2.3.2. Sequence Numbers and Rollover Handling
The sequence number in Agent Advertisements ranges from 0 to 0xffff.
After booting, an agent MUST use the number 0 for its first
advertisement. Each subsequent advertisement MUST use the sequence
number one greater, with the exception that the sequence number
0xffff MUST be followed by sequence number 256. In this way, mobile
nodes can distinguish reductions in sequence numbers that result from
reboots, from reductions that result in rollover of the sequence
number after it attains the value 0xffff.
2.4. Mobile Node Considerations
Every mobile node MUST implement Agent Solicitation. Solicitations
SHOULD only be sent in the absence of Agent Advertisements and when a
care-of address has not been determined through a link-layer protocol
or other means. The mobile node uses the same procedures, defaults,
and constants for Agent Solicitation as specified for ICMP Router
Solicitation messages [4], except that the mobile node MAY solicit
more often than once every three seconds, and that a mobile node that
is currently not connected to any foreign agent MAY solicit more
times than MAX_SOLICITATIONS.
The rate at which a mobile node sends Solicitations MUST be limited
by the mobile node. The mobile node MAY send three initial
Solicitations at a maximum rate of one per second while searching for
an agent. After this, the rate at which Solicitations are sent MUST
be reduced so as to limit the overhead on the local link. Subsequent
Solicitations MUST be sent using a binary exponential backoff
mechanism, doubling the interval between consecutive Solicitations,
up to a maximum interval. The maximum interval SHOULD be chosen
appropriately based upon the characteristics of the media over which
the mobile node is soliciting. This maximum interval SHOULD be at
least one minute between Solicitations.
While still searching for an agent, the mobile node MUST NOT increase
the rate at which it sends Solicitations unless it has received a
positive indication that it has moved to a new link. After
successfully registering with an agent, the mobile node SHOULD also
increase the rate at which it will send Solicitations when it next
begins searching for a new agent with which to register. The
increased solicitation rate MAY revert to the maximum rate, but then
MUST be limited in the manner described above. In all cases, the
recommended solicitation intervals are nominal values. Mobile nodes
MUST randomize their solicitation times around these nominal values
as specified for ICMP Router Discovery [4].
Perkins Standards Track [Page 21]
RFC 2002 IP Mobility Support October 1996
Mobile nodes MUST process received Agent Advertisements. A mobile
node can distinguish an Agent Advertisement message from other uses
of the ICMP Router Advertisement message by examining the number of
advertised addresses and the IP Total Length field. When the IP
total length indicates that the ICMP message is longer than needed
for the number of advertised addresses, the remaining data is
interpreted as one or more Extensions. The presence of a Mobility
Agent Advertisement Extension identifies the advertisement as an
Agent Advertisement.
When multiple methods of agent discovery are in use, the mobile node
SHOULD first attempt registration with agents including Mobility
Agent Advertisement Extensions in their advertisements, in preference
to those discovered by other means. This preference maximizes the
likelihood that the registration will be recognized, thereby
minimizing the number of registration attempts.
2.4.1. Registration Required
When the mobile node receives an Agent Advertisement with the 'R' bit
set, the mobile node SHOULD register through the foreign agent, even
when the mobile node might be able to acquire its own co-located
care-of address. This feature is intended to allow sites to enforce
visiting policies (such as accounting) which require exchanges of
authorization.
2.4.2. Move Detection
Two primary mechanisms are provided for mobile nodes to detect when
they have moved from one subnet to another. Other mechanisms MAY
also be used. When the mobile node detects that it has moved, it
SHOULD register (Section 3) with a suitable care-of address on the
new foreign network. However, the mobile node MUST NOT register more
frequently than once per second on average, as specified in Section
3.6.3.
Perkins Standards Track [Page 22]
RFC 2002 IP Mobility Support October 1996
2.4.2.1. Algorithm 1
The first method of move detection is based upon the Lifetime field
within the main body of the ICMP Router Advertisement portion of the
Agent Advertisement. A mobile node SHOULD record the Lifetime
received in any Agent Advertisements, until that Lifetime expires.
If the mobile node fails to receive another advertisement from the
same agent within the specified Lifetime, it SHOULD assume that it
has lost contact with that agent. If the mobile node has previously
received an Agent Advertisement from another agent for which the
Lifetime field has not yet expired, the mobile node MAY immediately
attempt registration with that other agent. Otherwise, the mobile
node SHOULD attempt to discover a new agent with which to register.
2.4.2.2. Algorithm 2
The second method uses network prefixes. The Prefix-Lengths
Extension MAY be used in some cases by a mobile node to determine
whether or not a newly received Agent Advertisement was received on
the same subnet as the mobile node's current care-of address. If the
prefixes differ, the mobile node MAY assume that it has moved. If a
mobile node is currently using a foreign agent care-of address, the
mobile node SHOULD NOT use this method of move detection unless both
the current agent and the new agent include the Prefix-Lengths
Extension in their respective Agent Advertisements; if this Extension
is missing from one or both of the advertisements, this method of
move detection SHOULD NOT be used. Similarly, if a mobile node is
using a co-located care-of address, it SHOULD not use this method of
move detection unless the new agent includes the Prefix-Lengths
Extension in its Advertisement and the mobile node knows the network
prefix of its current co-located care-of address. On the expiration
of its current registration, if this method indicates that the mobile
node has moved, rather than re-registering with its current care-of
address, a mobile node MAY choose instead to register with a the
foreign agent sending the new Advertisement with the different
network prefix. The Agent Advertisement on which the new
registration is based MUST NOT have expired according to its Lifetime
field.
Perkins Standards Track [Page 23]
RFC 2002 IP Mobility Support October 1996
2.4.3. Returning Home
A mobile node can detect that it has returned to its home network
when it receives an Agent Advertisement from its own home agent. If
so, it SHOULD deregister with its home agent (Section 3). Before
attempting to deregister, the mobile node SHOULD configure its
routing table appropriately for its home network (Section 4.2.1). In
addition, if the home network is using ARP [16], the mobile node MUST
follow the procedures described in Section 4.6 with regard to ARP,
proxy ARP, and gratuitous ARP.
2.4.4. Sequence Numbers and Rollover Handling
If a mobile node detects two successive values of the sequence number
in the Agent Advertisements from the foreign agent with which it is
registered, the second of which is less than the first and inside the
range 0 to 255, the mobile node SHOULD register again. If the second
value is less than the first but is greater than or equal to 256, the
mobile node SHOULD assume that the sequence number has rolled over
past its maximum value (0xffff), and that reregistration is not
necessary (Section 2.3).
3. Registration
Mobile IP registration provides a flexible mechanism for mobile nodes
to communicate their current reachability information to their home
agent. It is the method by which mobile nodes:
- request forwarding services when visiting a foreign network,
- inform their home agent of their current care-of address,
- renew a registration which is due to expire, and/or
- deregister when they return home.
Registration messages exchange information between a mobile node,
(optionally) a foreign agent, and the home agent. Registration
creates or modifies a mobility binding at the home agent, associating
the mobile node's home address with its care-of address for the
specified Lifetime.
Perkins Standards Track [Page 24]RFC 2002 IP Mobility Support October 1996
Several other (optional) capabilities are available through the
registration procedure, which enable a mobile node to:
- maintain multiple simultaneous registrations, so that a copy of
each datagram will be tunneled to each active care-of address
- deregister specific care-of addresses while retaining other
mobility bindings, and
- discover the address of a home agent if the mobile node is not
configured with this information.
3.1. Registration Overview
Mobile IP defines two different registration procedures, one via a
foreign agent that relays the registration to the mobile node's home
agent, and one directly with the mobile node's home agent. The
following rules determine which of these two registration procedures
to use in any particular circumstance:
- If a mobile node is registering a foreign agent care-of address,
the mobile node MUST register via that foreign agent.
- If a mobile node is using a co-located care-of address, and
receives an Agent Advertisement from a foreign agent on the
link on which it is using this care-of address, the mobile node
SHOULD register via that foreign agent (or via another foreign
agent on this link) if the 'R' bit is set in the received Agent
Advertisement message.
- If a mobile node is otherwise using a co-located care-of address,
the mobile node MUST register directly with its home agent.
- If a mobile node has returned to its home network and is
(de)registering with its home agent, the mobile node MUST
register directly with its home agent.
Both registration procedures involve the exchange of Registration
Request and Registration Reply messages (Sections 3.3 and 3.4). When
registering via a foreign agent, the registration procedure requires
the following four messages:
a) The mobile node sends a Registration Request to the
prospective foreign agent to begin the registration process.
b) The foreign agent processes the Registration Request and then
relays it to the home agent.
Perkins Standards Track [Page 25]RFC 2002 IP Mobility Support October 1996
c) The home agent sends a Registration Reply to the foreign
agent to grant or deny the Request.
d) The foreign agent processes the Registration Reply and then
relays it to the mobile node to inform it of the disposition
of its Request.
When the mobile node instead registers directly with its home agent,
the registration procedure requires only the following two messages:
a) The mobile node sends a Registration Request to the home
agent.
b) The home agent sends a Registration Reply to the mobile
node, granting or denying the Request.
The registration messages defined in Sections 3.3 and 3.4 use the
User Datagram Protocol (UDP) [17]. A nonzero UDP checksum SHOULD be
included in the header, and MUST be checked by the recipient.
3.2. Authentication
Each mobile node, foreign agent, and home agent MUST be able to
support a mobility security association for mobile entities, indexed
by their SPI and IP address. In the case of the mobile node, this
must be its Home Address. See Section 5.1 for requirements for
support of authentication algorithms. Registration messages between
a mobile node and its home agent MUST be authenticated with the
Mobile-Home Authentication Extension (Section 3.5.2). This Extension
immediately follows all non-authentication Extensions, except those
foreign agent-specific Extensions which may be added to the message
after the mobile node computes the authentication.
3.3. Registration Request
A mobile node registers with its home agent using a Registration
Request message so that its home agent can create or modify a
mobility binding for that mobile node (e.g., with a new lifetime).
The Request may be relayed to the home agent by the foreign agent
through which the mobile node is registering, or it may be sent
directly to the home agent in the case in which the mobile node is
registering a co-located care-of address.
IP fields:
Source Address Typically the interface address from which the
message is sent.
Perkins Standards Track [Page 26]RFC 2002 IP Mobility Support October 1996
Destination Address Typically that of the foreign agent or the
home agent.
See Sections 3.6.1.1 and 3.7.2.2 for details.
UDP fields:
Source Port variable
Destination Port 434
The UDP header is followed by the Mobile IP fields shown below:
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 |S|B|D|M|G|V|rsv| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Agent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Care-of Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
Type 1 (Registration Request)
S Simultaneous bindings. If the 'S' bit is set, the mobile
node is requesting that the home agent retain its prior
mobility bindings, as described in Section 3.6.1.2.
B Broadcast datagrams. If the 'B' bit is set, the mobile
node requests that the home agent tunnel to it any
broadcast datagrams that it receives on the home network,
as described in Section 4.3.
D Decapsulation by mobile node. If the 'D' bit is set, the
mobile node will itself decapsulate datagrams which are
sent to the care-of address. That is, the mobile node is
using a co-located care-of address.
Perkins Standards Track [Page 27]RFC 2002 IP Mobility Support October 1996
M Minimal encapsulation. If the 'M' bit is set, the
mobile node requests that its home agent use minimal
encapsulation [15] for datagrams tunneled to the mobile
node.
G GRE encapsulation. If the 'G' bit is set, the
mobile node requests that its home agent use GRE
encapsulation [8] for datagrams tunneled to the mobile
node.
V The mobile node requests that its mobility agent use Van
Jacobson header compression [10] over its link with the
mobile node.
rsv Reserved bits; sent as zero
Lifetime
The number of seconds remaining before the registration
is considered expired. A value of zero indicates a
request for deregistration. A value of 0xffff indicates
infinity.
Home Address
The IP address of the mobile node.
Home Agent
The IP address of the mobile node's home agent.
Care-of Address
The IP address for the end of the tunnel.
Identification
A 64-bit number, constructed by the mobile node, used for
matching Registration Requests with Registration Replies,
and for protecting against replay attacks of registration
messages. See Sections 5.4 and 5.6.
Extensions
The fixed portion of the Registration Request is followed
by one or more of the Extensions listed in Section 3.5.
The Mobile-Home Authentication Extension MUST be included
in all Registration Requests. See Sections 3.6.1.3
and 3.7.2.2 for information on the relative order in
which different extensions, when present, MUST be placed
in a Registration Request message.
Perkins Standards Track [Page 28]RFC 2002 IP Mobility Support October 1996
3.4. Registration Reply
A mobility agent returns a Registration Reply message to a mobile
node which has sent a Registration Request (Section 3.3) message. If
the mobile node is requesting service from a foreign agent, that
foreign agent will receive the Reply from the home agent and
subsequently relay it to the mobile node. The Reply message contains
the necessary codes to inform the mobile node about the status of its
Request, along with the lifetime granted by the home agent, which MAY
be smaller than the original Request.
The foreign agent MUST NOT increase the Lifetime selected by the
mobile node in the Registration Request, since the Lifetime is
covered by the Mobile-Home Authentication Extension, which cannot be
correctly (re)computed by the foreign agent. The home agent MUST NOT
increase the Lifetime selected by the mobile node in the Registration
Request, since doing so could increase it beyond the maximum
Registration Lifetime allowed by the foreign agent. If the Lifetime
received in the Registration Reply is greater than that in the
Registration Request, the Lifetime in the Request MUST be used. When
the Lifetime received in the Registration Reply is less than that in
the Registration Request, the Lifetime in the Reply MUST be used.
IP fields:
Source Address Typically copied from the destination
address of the Registration Request to which
the agent is replying. See Sections 3.7.2.3
and 3.8.3.1 for complete details.
Destination Address Copied from the source address of the
Registration Request to which the agent is
replying
UDP fields:
Source Port <variable>
Destination Port Copied from the source port of the
corresponding Registration Request
(Section 3.7.1).
Perkins Standards Track [Page 29]RFC 2002 IP Mobility Support October 1996
The UDP header is followed by the Mobile IP fields shown below:
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 | Code | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Agent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
Type 3 (Registration Reply)
Code A value indicating the result of the Registration
Request. See below for a list of currently defined Code
values.
Lifetime
If the Code field indicates that the registration was
accepted, the Lifetime field is set to the number of
seconds remaining before the registration is considered
expired. A value of zero indicates that the mobile node
has been deregistered. A value of 0xffff indicates
infinity. If the Code field indicates that the
registration was denied, the contents of the Lifetime
field are unspecified and MUST be ignored on reception.
Home Address
The IP address of the mobile node.
Home Agent
The IP address of the mobile node's home agent.
Perkins Standards Track [Page 30]RFC 2002 IP Mobility Support October 1996
Identification
A 64-bit number used for matching Registration Requests
with Registration Replies, and for protecting against
replay attacks of registration messages. The value is
based on the Identification field from the Registration
Request message from the mobile node, and on the style of
replay protection used in the security context between
the mobile node and its home agent (defined by the
mobility security association between them, and SPI
value in the Mobile-Home Authentication Extension). See
Sections 5.4 and 5.6.
Extensions
The fixed portion of the Registration Reply is followed
by one or more of the Extensions listed in Section 3.5.
The Mobile-Home Authentication Extension MUST be included
in all Registration Replies returned by the home agent.
See Sections 3.7.2.2 and 3.8.3.3 for rules on placement
of extensions to Reply messages.
The following values are defined for use within the Code field.
Registration successful:
0 registration accepted
1 registration accepted, but simultaneous mobility
bindings unsupported
Registration denied by the foreign agent:
64 reason unspecified
65 administratively prohibited
66 insufficient resources
67 mobile node failed authentication
68 home agent failed authentication
69 requested Lifetime too long
70 poorly formed Request
71 poorly formed Reply
72 requested encapsulation unavailable
73 requested Van Jacobson compression unavailable
80 home network unreachable (ICMP error received)
81 home agent host unreachable (ICMP error received)
82 home agent port unreachable (ICMP error received)
88 home agent unreachable (other ICMP error received)
Perkins Standards Track [Page 31]RFC 2002 IP Mobility Support October 1996
Registration denied by the home agent:
128 reason unspecified
129 administratively prohibited
130 insufficient resources
131 mobile node failed authentication
132 foreign agent failed authentication
133 registration Identification mismatch
134 poorly formed Request
135 too many simultaneous mobility bindings
136 unknown home agent address
Up-to-date values of the Code field are specified in the most recent
"Assigned Numbers" [20].
3.5. Registration Extensions
3.5.1. Computing Authentication Extension Values
The Authenticator value computed for each authentication Extension
MUST protect the following fields from the registration message:
- the UDP payload (that is, the Registration Request or
Registration Reply data),
- all prior Extensions in their entirety, and
- the Type and Length of this Extension.
The default authentication algorithm uses keyed-MD5 [21] in
"prefix+suffix" mode to compute a 128-bit "message digest" of the
registration message. The default authenticator is a 128-bit value
computed as the MD5 checksum over the the following stream of bytes:
- the shared secret defined by the mobility security association
between the nodes and by SPI value specified in the
authentication Extension, followed by
- the protected fields from the registration message, in the order
specified above, followed by
- the shared secret again.
Note that the Authenticator field itself and the UDP header are NOT
included in the computation of the default Authenticator value. See
Section 5.1 for information about support requirements for message
authentication codes, which are to be used with the various
authentication Extensions.
Perkins Standards Track [Page 32]RFC 2002 IP Mobility Support October 1996
The Security Parameter Index (SPI) within any of the authentication
Extensions defines the security context which is used to compute the
Authenticator value and which MUST be used by the receiver to check
that value. In particular, the SPI selects the authentication
algorithm and mode (Section 5.1) and secret (a shared key, or
appropriate public/private key pair) used in computing the
Authenticator. In order to ensure interoperability between different
implementations of the Mobile IP protocol, an implementation MUST be
able to associate any SPI value with any authentication algorithm and
mode which it implements. In addition, all implementations of Mobile
IP MUST implement the default authentication algorithm (keyed-MD5)
and mode ("prefix+suffix") defined above.
3.5.2. Mobile-Home Authentication Extension
Exactly one Mobile-Home Authentication Extension MUST be present in
all Registration Requests and Registration Replies, and is intended
to eliminate problems [2] which result from the uncontrolled
propagation of remote redirects in the Internet. The location of the
extension marks the end of the authenticated data.
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 | SPI ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... SPI (cont.) | Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 32
Length 4 plus the number of bytes in the Authenticator.
SPI Security Parameter Index (4 bytes). An opaque
identifier (see Section 1.6).
Authenticator (variable length) (See Section 3.5.1.)
3.5.3. Mobile-Foreign Authentication Extension
This Extension MAY be included in Registration Requests and Replies
in cases in which a mobility security association exists between the
mobile node and the foreign agent. See Section 5.1 for information
about support requirements for message authentication codes.
Perkins Standards Track [Page 33]RFC 2002 IP Mobility Support October 1996
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 | SPI ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... SPI (cont.) | Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 33
Length 4 plus the number of bytes in the Authenticator.
SPI Security Parameter Index (4 bytes). An opaque
identifier (see Section 1.6).
Authenticator (variable length) (See Section 3.5.1.)
3.5.4. Foreign-Home Authentication Extension
This Extension MAY be included in Registration Requests and Replies
in cases in which a mobility security association exists between the
foreign agent and the home agent. See Section 5.1 for information
about support requirements for message authentication codes.
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 | SPI ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... SPI (cont.) | Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 34
Length 4 plus the number of bytes in the Authenticator.
SPI Security Parameter Index (4 bytes). An opaque
identifier (see Section 1.6).
Authenticator (variable length) (See Section 3.5.1.)
3.6. Mobile Node Considerations
A mobile node MUST be configured with its home address, a netmask,
and a mobility security association for each home agent. In
addition, a mobile node MAY be configured with the IP address of one
or more of its home agents; otherwise, the mobile node MAY discover a
home agent using the procedures described in Section 3.6.1.2.
Perkins Standards Track [Page 34]RFC 2002 IP Mobility Support October 1996
For each pending registration, the mobile node maintains the
following information:
- the link-layer address of the foreign agent to which the
Registration Request was sent, if applicable,
- the IP destination address of the Registration Request,
- the care-of address used in the registration,
- the Identification value sent in the registration,
- the originally requested Lifetime, and
- the remaining Lifetime of the pending registration.
A mobile node SHOULD initiate a registration whenever it detects a
change in its network connectivity. See Section 2.4.2 for methods by
which mobile nodes MAY make such a determination. When it is away
from home, the mobile node's Registration Request allows its home
agent to create or modify a mobility binding for it. When it is at
home, the mobile node's (de)Registration Request allows its home
agent to delete any previous mobility binding(s) for it. A mobile
node operates without the support of mobility functions when it is at
home.
There are other conditions under which the mobile node SHOULD
(re)register with its foreign agent, such as when the mobile node
detects that the foreign agent has rebooted (as specified in Section
2.4.4) and when the current registration's Lifetime is near
expiration.
In the absence of link-layer indications of changes in point of
attachment, Agent Advertisements from new agents SHOULD NOT cause a
mobile node to attempt a new registration, if its current
registration has not expired and it is still also receiving Agent
Advertisements from the foreign agent with which it is currently
registered. In the absence of link-layer indications, a mobile node
MUST NOT attempt to register more often than once per second.
A mobile node MAY register with a different agent when transport-
layer protocols indicate excessive retransmissions. A mobile node
MUST NOT consider reception of an ICMP Redirect from a foreign agent
that is currently providing service to it as reason to register with
a new foreign agent. Within these constraints, the mobile node MAY
register again at any time.
Appendix D shows some examples of how the fields in registration
messages would be set up in some typical registration scenarios.
Perkins Standards Track [Page 35]RFC 2002 IP Mobility Support October 1996
3.6.1. Sending Registration Requests
The following sections specify details for the values the mobile node
MUST supply in the fields of Registration Request messages.
3.6.1.1. IP Fields
This section provides the specific rules by which mobile nodes pick
values for the IP header fields of a Registration Request.
IP Source Address:
- When registering on a foreign network with a co-located care-of
address, the IP source address MUST be the care-of address.
- In all other circumstances, the IP source address MUST be the
mobile node's home address.
IP Destination Address:
- When the mobile node has discovered the agent with which it is
registering, through some means (e.g., link-layer) that does not
provide the IP address of the agent (the IP address of the agent
is unknown to the mobile node), then the "All Mobility Agents"
multicast address (224.0.0.11) MUST be used. In this case, the
mobile node MUST use the agent's link-layer unicast address in
order to deliver the datagram to the correct agent.
- When registering with a foreign agent, the address of the agent
as learned from the IP source address of the corresponding Agent
Advertisement MUST be used. In addition, when transmitting
this Registration Request message, the mobile node MUST use a
link-layer destination address copied from the link-layer source
address of the Agent Advertisement message in which it learned
this foreign agent's IP address.
- When the mobile node is registering directly with its home
agent and knows the (unicast) IP address of its home agent, the
destination address MUST be set to this address.
Perkins Standards Track [Page 36]RFC 2002 IP Mobility Support October 1996
- If the mobile node is registering directly with its home
agent, but does not know the IP address of its home agent,
the mobile node may use dynamic home agent address resolution
to automatically determine the IP address of its home agent
(Section 3.6.1.2). In this case, the IP destination address is
set to the subnet-directed broadcast address of the mobile node's
home network. This address MUST NOT be used as the destination
IP address if the mobile node is registering via a foreign agent,
although it MAY be used as the Home Agent address in the body of
the Registration Request when registering via a foreign agent.
IP Time to Live:
- The IP TTL field MUST be set to 1 if the IP destination address
is set to the "All Mobility Agents" multicast address as
described above. Otherwise a suitable value should be chosen in
accordance with standard IP practice [19].
3.6.1.2. Registration Request Fields
This section provides specific rules by which mobile nodes pick
values for the fields within the fixed portion of a Registration
Request.
A mobile node MAY set the 'S' bit in order to request that the home
agent maintain prior mobility binding(s). Otherwise, the home agent
deletes any previous binding(s) and replaces them with the new
binding specified in the Registration Request. Multiple simultaneous
mobility bindings are likely to be useful when a mobile node using at
least one wireless network interface moves within wireless
transmission range of more than one foreign agent. IP explicitly
allows duplication of datagrams. When the home agent allows
simultaneous bindings, it will tunnel a separate copy of each
arriving datagram to each care-of address, and the mobile node will
receive multiple copies of datagrams destined to it.
The mobile node SHOULD set the 'D' bit if it is registering with a
co-located care-of address. Otherwise, the 'D' bit MUST NOT be set.
A mobile node MAY set the 'B' bit to request its home agent to
forward to it, a copy of broadcast datagrams received by its home
agent from the home network. The method used by the home agent to
forward broadcast datagrams depends on the type of care-of address
registered by the mobile node, as determined by the 'D' bit in the
mobile node's Registration Request:
Perkins Standards Track [Page 37]RFC 2002 IP Mobility Support October 1996
- If the 'D' bit is set, then the mobile node has indicated that it
will decapsulate any datagrams tunneled to this care-of address
itself (the mobile node is using a co-located care-of address).
In this case, to forward such a received broadcast datagram to
the mobile node, the home agent MUST tunnel it to this care-of
address. The mobile node de-tunnels the received datagram in the
same way as any other datagram tunneled directly to it.
- If the 'D' bit is NOT set, then the mobile node has indicated
that it is using a foreign agent care-of address, and that the
foreign agent will thus decapsulate arriving datagrams before
forwarding them to the mobile node. In this case, to forward
such a received broadcast datagram to the mobile node, the home
agent MUST first encapsulate the broadcast datagram in a unicast
datagram addressed to the mobile node's home address, and then
MUST tunnel this resulting datagram to the mobile node's care-of
address.
When decapsulated by the foreign agent, the inner datagram will
thus be a unicast IP datagram addressed to the mobile node,
identifying to the foreign agent the intended destination of the
encapsulated broadcast datagram, and will be delivered to the
mobile node in the same way as any tunneled datagram arriving for
the mobile node. The foreign agent MUST NOT decapsulate the
encapsulated broadcast datagram and MUST NOT use a local network
broadcast to transmit it to the mobile node. The mobile node thus
MUST decapsulate the encapsulated broadcast datagram itself, and
thus MUST NOT set the 'B' bit in its Registration Request in this
case unless it is capable of decapsulating datagrams.
The mobile node MAY request alternative forms of encapsulation by
setting the 'M' bit and/or the 'G' bit, but only if the mobile node
is decapsulating its own datagrams (the mobile node is using a co-
located care-of address) or if its foreign agent has indicated
support for these forms of encapsulation by setting the corresponding
bits in the Mobility Agent Advertisement Extension of an Agent
Advertisement received by the mobile node. Otherwise, the mobile
node MUST NOT set these bits.
The Lifetime field is chosen as follows:
- If the mobile node is registering with a foreign agent, the
Lifetime SHOULD NOT exceed the value in the Registration Lifetime
field of the Agent Advertisement message received from the
foreign agent. When the method by which the care-of address is
learned does not include a Lifetime, the default ICMP Router
Advertisement Lifetime (1800 seconds) MAY be used.
Perkins Standards Track [Page 38]RFC 2002 IP Mobility Support October 1996
- The mobile node MAY ask a home agent to delete a particular
mobility binding, by sending a Registration Request with the
care-of address for this binding, with the Lifetime field set to
zero (Section 3.8.2).
- Similarly, a Lifetime of zero is used when the mobile node
deregisters all care-of addresses, such as upon returning home.
The Home Agent field MUST be set to the address of the mobile node's
home agent, if the mobile node knows this address. Otherwise, the
mobile node MAY use dynamic home agent address resolution to learn
the address of its home agent. In this case, the mobile node MUST
set the Home Agent field to the subnet-directed broadcast address of
the mobile node's home network. Each home agent receiving such a
Registration Request with a broadcast destination address MUST reject
the mobile node's registration and SHOULD return a rejection
Registration Reply indicating its unicast IP address for use by the
mobile node in a future registration attempt.
The Care-of Address field MUST be set to the value of the particular
care-of address that the mobile node wishes to (de)register. In the
special case in which a mobile node wishes to deregister all care-of
addresses, it MUST set this field to its home address.
The mobile node chooses the Identification field in accordance with
the style of replay protection it uses with its home agent. This is
part of the mobility security association the mobile node shares with
its home agent. See Section 5.6 for the method by which the mobile
node computes the Identification field.
3.6.1.3. Extensions
This section describes the ordering of any mandatory and any optional
Extensions that a mobile node appends to a Registration Request.
This following ordering MUST be followed:
a) The IP header, followed by the UDP header, followed by the
fixed-length portion of the Registration Request, followed by
b) If present, any non-authentication Extensions expected to be
used by the home agent (which may or may not also be used by
the foreign agent), followed by
c) The Mobile-Home Authentication Extension, followed by
d) If present, any non-authentication Extensions used only by
the foreign agent, followed by
Perkins Standards Track [Page 39]RFC 2002 IP Mobility Support October 1996
e) The Mobile-Foreign Authentication Extension, if present.
Note that items (a) and (c) MUST appear in every Registration Request
sent by the mobile node. Items (b), (d), and (e) are optional.
However, item (e) MUST be included when the mobile node and the
foreign agent share a mobility security association.
3.6.2. Receiving Registration Replies
Registration Replies will be received by the mobile node in response
to its Registration Requests. Registration Replies generally fall
into three categories:
- the registration was accepted,
- the registration was denied by the foreign agent, or
- the registration was denied by the home agent.
The remainder of this section describes the Registration Reply
handling by a mobile node in each of these three categories.
3.6.2.1. Validity Checks
Registration Replies with an invalid, non-zero UDP checksum MUST be
silently discarded.
In addition, the low-order 32 bits of the Identification field in the
Registration Reply MUST be compared to the low-order 32 bits of the
Identification field in the most recent Registration Request sent to
the replying agent. If they do not match, the Reply MUST be silently
discarded.
Also, the authentication in the Registration Reply MUST be checked.
That is, the mobile node MUST check for the presence of a valid
authentication Extension, acting in accordance with the Code field in
the Reply. The rules are as follows:
a) If the mobile node and the foreign agent share a
mobility security association, exactly one Mobile-Foreign
Authentication Extension MUST be present in the Registration
Reply, and the mobile node MUST check the Authenticator
value in the Extension. If no Mobile-Foreign Authentication
Extension is found, or if more than one Mobile-Foreign
Authentication Extension is found, or if the Authenticator is
invalid, the mobile node MUST silently discard the Reply and
SHOULD log the event as a security exception.
Perkins Standards Track [Page 40]RFC 2002 IP Mobility Support October 1996
b) If the Code field indicates that service is denied by
the home agent, or if the Code field indicates that the
registration was accepted by the home agent, exactly one
Mobile-Home Authentication Extension MUST be present in
the Registration Reply, and the mobile node MUST check the
Authenticator value in the Extension. If no Mobile-Home
Authentication Extension is found, or if more than one
Mobile-Home Authentication Extension is found, or if the
Authenticator is invalid, the mobile node MUST silently
discard the Reply and SHOULD log the event as a security
exception.
If the Code field indicates an authentication failure, either at the
foreign agent or the home agent, then it is quite possible that any
authenticators in the Registration Reply will also be in error. This
could happen, for example, if the shared secret between the mobile
node and home agent was erroneously configured. The mobile node
SHOULD log such errors as security exceptions.
3.6.2.2. Registration Request Accepted
If the Code field indicates that the request has been accepted, the
mobile node SHOULD configure its routing table appropriately for its
current point of attachment (Section 4.2.1).
If the mobile node is returning to its home network and that network
is one which implements ARP, the mobile node MUST follow the
procedures described in Section 4.6 with regard to ARP, proxy ARP,
and gratuitous ARP.
If the mobile node has registered on a foreign network, it SHOULD
re-register before the expiration of the Lifetime of its
registration. As described in Section 3.6, for each pending
Registration Request, the mobile node MUST maintain the remaining
lifetime of this pending registration, as well as the original
Lifetime from the Registration Request. When the mobile node
receives a valid Registration Reply, the mobile node MUST decrease
its view of the remaining lifetime of the registration by the amount
by which the home agent decreased the originally requested Lifetime.
This procedure is equivalent to the mobile node starting a timer for
the granted Lifetime at the time it sent the Registration Request,
even though the granted Lifetime is not known to the mobile node
until the Registration Reply is received. Since the Registration
Request is certainly sent before the home agent begins timing the
registration Lifetime (also based on the granted Lifetime), this
procedure ensures that the mobile node will re-register before the
home agent expires and deletes the registration, in spite of possibly
non-negligible transmission delays for the original Registration
Perkins Standards Track [Page 41]RFC 2002 IP Mobility Support October 1996
Request and Reply that started the timing of the Lifetime at the
mobile node and its home agent.
3.6.2.3. Registration Request Denied
If the Code field indicates that service is being denied, the mobile
node SHOULD log the error. In certain cases the mobile node may be
able to "repair" the error. These include:
Code 69: (Denied by foreign agent, Lifetime too long)
In this case, the Lifetime field in the Registration Reply will
contain the maximum Lifetime value which that foreign agent is
willing to accept in any Registration Request. The mobile node
MAY attempt to register with this same agent, using a Lifetime
in the Registration Request that MUST be less than or equal to
the value specified in the Reply.
Code 133: (Denied by home agent, Identification mismatch)
In this case, the Identification field in the Registration
Reply will contain a value that allows the mobile node to
synchronize with the home agent, based upon the style of replay
protection in effect (Section 5.6). The mobile node MUST
adjust the parameters it uses to compute the Identification
field based upon the information in the Registration Reply,
before issuing any future Registration Requests.
Code 136: (Denied by home agent, Unknown home agent address)
This code is returned by a home agent when the mobile node is
performing dynamic home agent address resolution as described
in Sections 3.6.1.1 and 3.6.1.2. In this case, the Home Agent
field within the Reply will contain the unicast IP address of
the home agent returning the Reply. The mobile node MAY then
attempt to register with this home agent in future Registration
Requests. In addition, the mobile node SHOULD adjust the
parameters it uses to compute the Identification field based
upon the corresponding field in the Registration Reply, before
issuing any future Registration Requests.
3.6.3. Registration Retransmission
When no Registration Reply has been received within a reasonable
time, another Registration Request MAY be transmitted. When
timestamps are used, a new registration Identification is chosen for
each retransmission; thus it counts as a new registration. When
nonces are used, the unanswered Request is retransmitted unchanged;
Perkins Standards Track [Page 42]RFC 2002 IP Mobility Support October 1996
thus the retransmission does not count as a new registration (Section
5.6). In this way a retransmission will not require the home agent
to resynchronize with the mobile node by issuing another nonce in the
case in which the original Registration Request (rather than its
Registration Reply) was lost by the network.
The maximum time until a new Registration Request is sent SHOULD be
no greater than the requested Lifetime of the Registration Request.
The minimum value SHOULD be large enough to account for the size of
the messages, twice the round trip time for transmission to the home
agent, and at least an additional 100 milliseconds to allow for
processing the messages before responding. The round trip time for
transmission to the home agent will be at least as large as the time
required to transmit the messages at the link speed of the mobile
node's current point of attachment. Some circuits add another 200
milliseconds of satellite delay in the total round trip time to the
home agent. The minimum time between Registration Requests MUST NOT
be less than 1 second. Each successive retransmission timeout period
SHOULD be at least twice the previous period, as long as that is less
than the maximum as specified above.
3.7. Foreign Agent Considerations
The foreign agent plays a mostly passive role in Mobile IP
registration. It relays Registration Requests between mobile nodes
and home agents, and, when it provides the care-of address,
decapsulates datagrams for delivery to the mobile node. It SHOULD
also send periodic Agent Advertisement messages to advertise its
presence as described in Section 2.3, if not detectable by link-layer
means.
A foreign agent MUST NOT transmit a Registration Request except when
relaying a Registration Request received from a mobile node, to the
mobile node's home agent. A foreign agent MUST NOT transmit a
Registration Reply except when relaying a Registration Reply received
from a mobile node's home agent, or when replying to a Registration
Request received from a mobile node in the case in which the foreign
agent is denying service to the mobile node. In particular, a
foreign agent MUST NOT generate a Registration Request or Reply
because a mobile node's registration Lifetime has expired. A foreign
agent also MUST NOT originate a Registration Request message that
asks for deregistration of a mobile node; however, it MUST relay
valid (de)Registration Requests originated by a mobile node.
Perkins Standards Track [Page 43]
RFC 2002 IP Mobility Support October 1996
3.7.1. Configuration and Registration Tables
Each foreign agent MUST be configured with a care-of address. In
addition, for each pending or current registration, the foreign agent
MUST maintain a visitor list entry containing the following
information obtained from the mobile node's Registration Request:
- the link-layer source address of the mobile node
- the IP Source Address (the mobile node's Home Address)
- the IP Destination Address (as specified in 3.6.2.3)
- the UDP Source Port
- the Home Agent address
- the Identification field
- the requested registration Lifetime, and
- the remaining Lifetime of the pending or current registration.
As with any node on the Internet, a foreign agent MAY also share
mobility security associations with any other nodes. When relaying a
Registration Request from a mobile node to its home agent, if the
foreign agent shares a mobility security association with the home
agent, it MUST add a Foreign-Home Authentication Extension to the
Request and MUST check the required Foreign-Home Authentication
Extension in the Registration Reply from the home agent (Sections 3.3
and 3.4). Similarly, when receiving a Registration Request from a
mobile node, if the foreign agent shares a mobility security
association with the mobile node, it MUST check the required Mobile-
Foreign Authentication Extension in the Request and MUST add a
Mobile-Foreign Authentication Extension to the Registration Reply to
the mobile node.
3.7.2. Receiving Registration Requests
If the foreign agent accepts a Registration Request from a mobile
node, it then MUST relay the Request to the indicated home agent.
Otherwise, if the foreign agent denies the Request, it MUST send a
Registration Reply to the mobile node with an appropriate denial
Code, except in cases where the foreign agent would be required to
send out more than one such denial per second to the same mobile
node. The following sections describe this behavior in more detail.
If a foreign agent receives a Registration Request from a mobile node
in its visitor list, the existing visitor list entry for the mobile
node SHOULD NOT be deleted or modified until the foreign agent
receives a valid Registration Reply from the home agent with a Code
indicating success. The foreign agent MUST record the new pending
Perkins Standards Track [Page 44]RFC 2002 IP Mobility Support October 1996
Request separately from the existing visitor list entry for the
mobile node. If the Registration Request requests deregistration,
the existing visitor list entry for the mobile node SHOULD NOT be
deleted until the foreign agent has received a successful
Registration Reply. If the Registration Reply indicates that the
Request (for registration or deregistration) was denied by the home
agent, the existing visitor list entry for the mobile node MUST NOT
be modified as a result of receiving the Registration Reply.
3.7.2.1. Validity Checks
Registration Requests with an invalid, non-zero UDP checksum MUST be
silently discarded.
Also, the authentication in the Registration Request MUST be checked.
If the foreign agent and the mobile node share a mobility security
association, exactly one Mobile-Foreign Authentication Extension MUST
be present in the Registration Request, and the foreign agent MUST
check the Authenticator value in the Extension. If no Mobile-Foreign
Authentication Extension is found, or if more than one Mobile-Foreign
Authentication Extension is found, or if the Authenticator is
invalid, the foreign agent MUST silently discard the Request and
SHOULD log the event as a security exception. The foreign agent also
SHOULD send a Registration Reply to the mobile node with Code 67.
3.7.2.2. Forwarding a Valid Request to the Home Agent
If the foreign agent accepts the mobile node's Registration Request,
it MUST relay the Request to the mobile node's home agent as
specified in the Home Agent field of the Registration Request. The
foreign agent MUST NOT modify any of the fields beginning with the
fixed portion of the Registration Request up through and including
the Mobile-Home Authentication Extension. Otherwise, an
authentication failure is very likely to occur at the home agent. In
addition, the foreign agent proceeds as follows:
- It MUST process and remove any Extensions following the
Mobile-Home Authentication Extension,
- It MAY append any of its own non-authentication Extensions of
relevance to the home agent, if applicable, and
- It MUST append the Foreign-Home Authentication Extension, if the
foreign agent shares a mobility security association with the home
agent.
Perkins Standards Track [Page 45]RFC 2002 IP Mobility Support October 1996
Specific fields within the IP header and the UDP header of the
relayed Registration Request MUST be set as follows:
IP Source Address
The foreign agent's address on the interface from which
the message will be sent.
IP Destination Address
Copied from the Home Agent field within the Registration
Request.
UDP Source Port
<variable>
UDP Destination Port
434
After forwarding a valid Registration Request to the home agent, the
foreign agent MUST begin timing the remaining lifetime of the pending
registration based on the Lifetime in the Registration Request. If
this lifetime expires before receiving a valid Registration Reply,
the foreign agent MUST delete its visitor list entry for this pending
registration.
3.7.2.3. Denying Invalid Requests
If the foreign agent denies the mobile node's Registration Request
for any reason, it SHOULD send the mobile node a Registration Reply
with a suitable denial Code. In such a case, the Home Address, Home
Agent, and Identification fields within the Registration Reply are
copied from the corresponding fields of the Registration Request.
If the Reserved field is nonzero, the foreign agent MUST deny the
Request and SHOULD return a Registration Reply with status code 70 to
the mobile node. If the Request is being denied because the
requested Lifetime is too long, the foreign agent sets the Lifetime
in the Reply to the maximum Lifetime value it is willing to accept in
any Registration Request, and sets the Code field to 69. Otherwise,
the Lifetime SHOULD be copied from the Lifetime field in the Request.
Specific fields within the IP header and the UDP header of the
Registration Reply MUST be set as follows:
IP Source Address
Copied from the IP Destination Address of Registration
Request, unless the "All Agents Multicast" address was
used. In this case, the foreign agent's address (on the
interface from which the message will be sent) MUST be
Perkins Standards Track [Page 46]RFC 2002 IP Mobility Support October 1996
used.
IP Destination Address
Copied from the IP Source Address of the Registration
Request.
UDP Source Port
434
UDP Destination Port
Copied from the UDP Source Port of the Registration
Request.
3.7.3. Receiving Registration Replies
The foreign agent updates its visitor list when it receives a valid
Registration Reply from a home agent. It then relays the
Registration Reply to the mobile node. The following sections
describe this behavior in more detail.
If upon relaying a Registration Request to a home agent, the foreign
agent receives an ICMP error message instead of a Registration Reply,
then the foreign agent SHOULD send to the mobile node a Registration
Reply with an appropriate "Home Agent Unreachable" failure Code
(within the range 80-95, inclusive). See Section 3.7.2.3 for details
on building the Registration Reply.
3.7.3.1. Validity Checks
Registration Replies with an invalid, non-zero UDP checksum MUST be
silently discarded.
When a foreign agent receives a Registration Reply message, it MUST
search its visitor list for a pending Registration Request with the
same mobile node home address as indicated in the Reply. If no
pending Request is found, the foreign agent MUST silently discard the
Reply. The foreign agent MUST also silently discard the Reply if the
low-order 32 bits of the Identification field in the Reply do not
match those in the Request.
Also, the authentication in the Registration Reply MUST be checked.
If the foreign agent and the home agent share a mobility security
association, exactly one Foreign-Home Authentication Extension MUST
be present in the Registration Reply, and the foreign agent MUST
check the Authenticator value in the Extension. If no Foreign-Home
Authentication Extension is found, or if more than one Foreign-Home
Authentication Extension is found, or if the Authenticator is
invalid, the foreign agent MUST silently discard the Reply and SHOULD
Perkins Standards Track [Page 47]RFC 2002 IP Mobility Support October 1996
log the event as a security exception. The foreign agent also MUST
reject the mobile node's registration and SHOULD send a Registration
Reply to the mobile node with Code 68.
3.7.3.2. Forwarding Replies to the Mobile Node
A Registration Reply which satisfies the validity checks of Section
3.8.2.1 is relayed to the mobile node. The foreign agent MUST also
update its visitor list entry for the mobile node to reflect the
results of the Registration Request, as indicated by the Code field
in the Reply. If the Code indicates that the mobile node has
accepted the registration and the Lifetime field is nonzero, the
foreign agent MUST set the Lifetime in the visitor list entry to the
value specified in the Lifetime field of the Registration Reply. If,
instead, the Code indicates that the Lifetime field is zero, the
foreign agent MUST delete its visitor list entry for the mobile node.
Finally, if the Code indicates that the registration was denied by
the home agent, the foreign agent MUST delete its pending
registration list entry, but not its visitor list entry, for the
mobile node.
The foreign agent MUST NOT modify any of the fields beginning with
the fixed portion of the Registration Reply up through and including
the Mobile-Home Authentication Extension. Otherwise, an
authentication failure is very likely to occur at the mobile node.
In addition, the foreign agent SHOULD perform the following
additional procedures:
- It MUST process and remove any Extensions following the
Mobile-Home Authentication Extension,
- It MAY append its own non-authentication Extensions of relevance
to the mobile node, if applicable, and
- It MUST append the Mobile-Foreign Authentication Extension, if
the foreign agent shares a mobility security association with the
mobile node.
Specific fields within the IP header and the UDP header of the
relayed Registration Reply are set according to the same rules
specified in Section 3.7.2.3.
After forwarding a valid Registration Reply to the mobile node, the
foreign agent MUST update its visitor list entry for this
registration as follows. If the Registration Reply indicates that
the registration was accepted by the home agent, the foreign agent
resets its timer of the lifetime of the registration to the Lifetime
granted in the Registration Reply; unlike the mobile node's timing of
the registration lifetime as described in Section 3.6.2.2, the
foreign agent considers this lifetime to begin when it forwards the
Perkins Standards Track [Page 48]RFC 2002 IP Mobility Support October 1996
Registration Reply message, ensuring that the foreign agent will not
expire the registration before the mobile node does. On the other
hand, if the Registration Reply indicates that the registration was
rejected by the home agent, the foreign agent deletes its visitor
list entry for this attempted registration.
3.8. Home Agent Considerations
Home agents play a reactive role in the registration process. The
home agent receives Registration Requests from the mobile node
(perhaps relayed by a foreign agent), updates its record of the
mobility bindings for this mobile node, and issues a suitable
Registration Reply in response to each.
A home agent MUST NOT transmit a Registration Reply except when
replying to a Registration Request received from a mobile node. In
particular, the home agent MUST NOT generate a Registration Reply to
indicate that the Lifetime has expired.
3.8.1. Configuration and Registration Tables
Each home agent MUST be configured with an IP address and with the
prefix size for the home network. The home agent MUST be configured
with the home address and mobility security association of each
authorized mobile node that it is serving as a home agent. When the
home agent accepts a valid Registration Request from a mobile node
that it serves as a home agent, the home agent MUST create or modify
the entry for this mobile node in its mobility binding list
containing:
- the mobile node's care-of address
- the Identification field from the Registration Reply
- the remaining Lifetime of the registration
The home agent MAY also maintain mobility security associations with
various foreign agents. When receiving a Registration Request from a
foreign agent, if the home agent shares a mobility security
association with the foreign agent, the home agent MUST check the
Authenticator in the required Foreign-Home Authentication Extension
in the message, based on this mobility security association.
Similarly, when sending a Registration Reply to a foreign agent, if
the home agent shares a mobility security association with the
foreign agent, the home agent MUST include a Foreign-Home
Authentication Extension in the message, based on this mobility
security association.
3.8.2. Receiving Registration Requests
Perkins Standards Track [Page 49]RFC 2002 IP Mobility Support October 1996
If the home agent accepts an incoming Registration Request, it MUST
update its record of the the mobile node's mobility binding(s) and
SHOULD send a Registration Reply with a suitable Code. Otherwise
(the home agent denies the Request), it SHOULD send a Registration
Reply with an appropriate Code specifying the reason the Request was
denied. The following sections describe this behavior in more
detail.
3.8.2.1. Validity Checks
Registration Requests with an invalid, non-zero UDP checksum MUST be
silently discarded by the home agent.
The authentication in the Registration Request MUST be checked. This
involves the following operations:
a) The home agent MUST check for the presence of a valid
Mobile-Home Authentication Extension, and perform the
indicated authentication. Exactly one Mobile-Home
Authentication Extension MUST be present in the Registration
Request, and the home agent MUST check the Authenticator
value in the Extension. If no Mobile-Home Authentication
Extension is found, or if more than one Mobile-Home
Authentication Extension is found, or if the Authenticator
is invalid, the home agent MUST reject the mobile node's
registration and SHOULD send a Registration Reply to the
mobile node with Code 131. The home agent MUST then discard
the Request and SHOULD log the error as a security exception.
b) The home agent MUST check that the registration
Identification field is correct using the context selected by
the SPI within the Mobile-Home Authentication Extension. See
Section 5.6 for a description of how this is performed. If
incorrect, the home agent MUST reject the Request and SHOULD
send a Registration Reply to the mobile node with Code 133,
including an Identification field computed in accordance with
the rules specified in Section 5.6. The home agent MUST do
no further processing with such a Request, though it SHOULD
log the error as a security exception.
c) If the home agent shares a mobility security association with
the foreign agent, the home agent MUST check for the presence
of a valid Foreign-Home Authentication Extension. Exactly
one Foreign-Home Authentication Extension MUST be present in
the Registration Request in this case, and the home agent
MUST check the Authenticator value in the Extension. If no
Foreign-Home Authentication Extension is found, or if more
than one Foreign-Home Authentication Extension is found, or
Perkins Standards Track [Page 50]RFC 2002 IP Mobility Support October 1996
if the Authenticator is invalid, the home agent MUST reject
the mobile node's registration and SHOULD send a Registration
Reply to the mobile node with Code 132. The home agent
MUST then discard the Request and SHOULD log the error as a
security exception.
In addition to checking the authentication in the Registration
Request, home agents MUST deny Registration Requests that are sent to
the subnet-directed broadcast address of the home network (as opposed
to being unicast to the home agent). The home agent MUST discard the
Request and SHOULD returning a Registration Reply with a Code of 136.
In this case, the Registration Reply will contain the home agent's
unicast address, so that the mobile node can re-issue the
Registration Request with the correct home agent address.
3.8.2.2. Accepting a Valid Request
If the Registration Request satisfies the validity checks in Section
3.8.2.1, and the home agent is able to accommodate the Request, the
home agent MUST update its mobility binding list for the requesting
mobile node and MUST return a Registration Reply to the mobile node.
In this case, the Reply Code will be either 0 if the home agent
supports simultaneous mobility bindings, or 1 if it does not. See
Section 3.8.3 for details on building the Registration Reply message.
The home agent updates its record of the mobile node's mobility
bindings as follows, based on the fields in the Registration Request:
- If the Lifetime is zero and the Care-of Address equals the mobile
node's home address, the home agent deletes all of the entries in
the mobility binding list for the requesting mobile node. This
is how a mobile node requests that its home agent cease providing
mobility services.
- If the Lifetime is zero and the Care-of Address does not equal
the mobile node's home address, the home agent deletes only the
entry containing the specified Care-of Address from the mobility
binding list for the requesting mobile node. Any other active
entries containing other care-of addresses will remain active.
- If the Lifetime is nonzero, the home agent adds an entry
containing the requested Care-of Address to the mobility binding
list for the mobile node. If the 'S' bit is set and the home
agent supports simultaneous mobility bindings, the previous
mobility binding entries are retained. Otherwise, the home agent
removes all previous entries in the mobility binding list for the
mobile node.
Perkins Standards Track [Page 51]RFC 2002 IP Mobility Support October 1996
In all cases, the home agent MUST send a Registration Reply to the
source of the Registration Request, which might indeed be a different
foreign agent than that whose care-of address is being
(de)registered. If the home agent shares a mobility security
association with the foreign agent whose care-of address is being
deregistered, and that foreign agent is different from the one which
relayed the Registration Request, the home agent MAY additionally
send a Registration Reply to the foreign agent whose care-of address
is being deregistered. The home agent MUST NOT send such a Reply if
it does not share a mobility security association with the foreign
agent. If no Reply is sent, the foreign agent's visitor list will
expire naturally when the original Lifetime expires.
The home agent MUST NOT increase the Lifetime above that specified by
the mobile node in the Registration Request. However, it is not an
error for the mobile node to request a Lifetime longer than the home
agent is willing to accept. In this case, the home agent simply
reduces the Lifetime to a permissible value and returns this value in
the Registration Reply. The Lifetime value in the Registration Reply
informs the mobile node of the granted lifetime of the registration,
indicating when it SHOULD re-register in order to maintain continued
service. After the expiration of this registration lifetime, the
home agent MUST delete its entry for this registration in its
mobility binding list.
If the Registration Request duplicates an accepted current
Registration Request, the new Lifetime MUST NOT extend beyond the
Lifetime originally granted. A Registration Request is a duplicate
if the home address, care-of address, and Identification fields all
equal those of an accepted current registration.
In addition, if the home network implements ARP [16], and the
Registration Request asks the home agent to create a mobility binding
for a mobile node which previously had no binding (the mobile node
was previously assumed to be at home), then the home agent MUST
follow the procedures described in Section 4.6 with regard to ARP,
proxy ARP, and gratuitous ARP. If the mobile node already had a
previous mobility binding, the home agent MUST continue to follow the
rules for proxy ARP described in Section 4.6.
3.8.2.3. Denying an Invalid Request
If the Registration Reply does not satisfy all of the validity checks
in Section 3.8.2.1, or the home agent is unable to accommodate the
Request, the home agent SHOULD return a Registration Reply to the
mobile node with a Code that indicates the reason for the error. If
a foreign agent was involved in relaying the Request, this allows the
foreign agent to delete its pending visitor list entry. Also, this
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RFC 2002 IP Mobility Support October 1996
informs the mobile node of the reason for the error such that it may
attempt to fix the error and issue another Request.
This section lists a number of reasons the home agent might reject a
Request, and provides the Code value it should use in each instance.
See Section 3.8.3 for additional details on building the Registration
Reply message.
Many reasons for rejecting a registration are administrative in
nature. For example, a home agent can limit the number of
simultaneous registrations for a mobile node, by rejecting any
registrations that would cause its limit to be exceeded, and
returning a Registration Reply with error code 135. Similarly, a
home agent may refuse to grant service to mobile nodes which have
entered unauthorized service areas by returning a Registration Reply
with a Code of 129.
If the Reserved field is nonzero, it MUST deny the Request with a
Code of 134.
3.8.3. Sending Registration Replies
If the home agent accepts a Registration Request, it then MUST update
its record of the mobile node's mobility binding(s) and SHOULD send a
Registration Reply with a suitable Code. Otherwise (the home agent
has denied the Request), it SHOULD send a Registration Reply with an
appropriate Code specifying the reason the Request was denied. The
following sections provide additional detail for the values the home
agent MUST supply in the fields of Registration Reply messages.
3.8.3.1. IP/UDP Fields
This section provides the specific rules by which mobile nodes pick
values for the IP and UDP header fields of a Registration Reply.
IP Source Address
Copied from the IP Destination Address of Registration
Request, unless a multicast or broadcast address was
used. If the IP Destination Address of the Registration
Request was a broadcast or multicast address, the IP
Source Address of the Registration Reply MUST be set to
the home agent's (unicast) IP address.
IP Destination Address
Copied from the IP Source Address of the Registration
Request.
Perkins Standards Track [Page 53]RFC 2002 IP Mobility Support October 1996
UDP Source Port
Copied from the UDP Destination Port of the Registration
Request.
UDP Destination Port
Copied from the UDP Source Port of the Registration
Request.
When sending a Registration Reply in response to a Registration
Request that requested deregistration of the mobile node (the
Lifetime is zero and the Care-of Address equals the mobile node's
home address) and in which the IP Source Address was also set to the
mobile node's home address (this is the normal method used by a
mobile node to deregister when it returns to its home network), the
IP Destination Address in the Registration Reply will be set to the
mobile node's home address, as copied from the IP Source Address of
the Request.
In this case, when transmitting the Registration Reply, the home
agent MUST transmit the Reply directly onto the home network as if
the mobile node were at home, bypassing any mobility binding list
entry that may still exist at the home agent for the destination
mobile node. In particular, for a mobile node returning home after
being registered with a care-of address, if the mobile node's new
Registration Request is not accepted by the home agent, the mobility
binding list entry for the mobile node will still indicate that
datagrams addressed to the mobile node should be tunneled to the
mobile node's registered care-of address; when sending the
Registration Reply indicating the rejection of this Request, this
existing binding list entry MUST be ignored, and the home agent MUST
transmit this Reply as if the mobile node were at home.
3.8.3.2. Registration Reply Fields
This section provides specific rules by which home agents pick values
for the fields within the fixed portion of a Registration Reply. The
Code field of the Registration Reply is chosen in accordance with the
rules specified in the previous sections. When replying to an
accepted registration, a home agent SHOULD respond with Code 1 if it
does not support simultaneous registrations.
The Lifetime field MUST be copied from the corresponding field in the
Registration Request, unless the requested value is greater than the
maximum length of time the home agent is willing to provide the
requested service. In such a case, the Lifetime MUST be set to the
length of time that service will actually be provided by the home
agent. This reduced Lifetime SHOULD be the maximum Lifetime allowed
by the home agent (for this mobile node and care-of address).
Perkins Standards Track [Page 54]RFC 2002 IP Mobility Support October 1996
The Home Address field MUST be copied from the corresponding field in
the Registration Request.
If the Home Agent field in the Registration Request contains a
unicast address of this home agent, then that field MUST be copied
into the Home Agent field of the Registration Reply. Otherwise, the
home agent MUST set the Home Agent field in the Registration Reply to
its unicast address. In this latter case, the home agent MUST reject
the registration with a suitable code (e.g., Code 136) to prevent the
mobile node from possibly being simultaneously registered with two or
more home agents.
3.8.3.3. Extensions
This section describes the ordering of any required and any optional
Mobile IP Extensions that a home agent appends to a Registration
Reply. The following ordering MUST be followed:
a) The IP header, followed by the UDP header, followed by the
fixed-length portion of the Registration Reply,
b) If present, any non-authentication Extensions used by the
mobile node (which may or may not also be used by the foreign
agent),
c) The Mobile-Home Authentication Extension,
d) If present, any non-authentication Extensions used only by
the foreign agent, and
e) The Foreign-Home Authentication Extension, if present.
Note that items (a) and (c) MUST appear in every Registration Reply
sent by the home agent. Items (b), (d), and (e) are optional.
However, item (e) MUST be included when the home agent and the
foreign agent share a mobility security association.
4. Routing Considerations
This section describes how mobile nodes, home agents, and (possibly)
foreign agents cooperate to route datagrams to/from mobile nodes that
are connected to a foreign network. The mobile node informs its home
agent of its current location using the registration procedure
described in Section 3. See the protocol overview in Section 1.7 for
the relative locations of the mobile node's home address with respect
to its home agent, and the mobile node itself with respect to any
foreign agent with which it might attempt to register.
Perkins Standards Track [Page 55]RFC 2002 IP Mobility Support October 1996
4.1. Encapsulation Types
Home agents and foreign agents MUST support tunneling datagrams using
IP in IP encapsulation [14]. Any mobile node that uses a co-located
care-of address MUST support receiving datagrams tunneled using IP in
IP encapsulation. Minimal encapsulation [15] and GRE encapsulation
[8] are alternate encapsulation methods which MAY optionally be
supported by mobility agents and mobile nodes. The use of these
alternative forms of encapsulation, when requested by the mobile
node, is otherwise at the discretion of the home agent.
4.2. Unicast Datagram Routing
4.2.1. Mobile Node Considerations
When connected to its home network, a mobile node operates without
the support of mobility services. That is, it operates in the same
way as any other (fixed) host or router. The method by which a
mobile node selects a default router when connected to its home
network, or when away from home and using a co-located care-of
address, is outside the scope of this document. ICMP Router
Advertisement [4] is one such method.
When registered on a foreign network, the mobile node chooses a
default router by the following rules:
- If the mobile node is registered using a foreign agent care-of
address, then the mobile node MUST choose its default router
from among the Router Addresses advertised in the ICMP Router
Advertisement portion of that Agent Advertisement message. The
mobile node MAY also consider the IP source address of the Agent
Advertisement as another possible choice for the IP address of a
default router, along with the (possibly empty) list of Router
Addresses from the ICMP Router Advertisement portion of the
message. In such cases, the IP source address MUST be considered
to be the worst choice (lowest preference) for a default router.
- If the mobile node is registered directly with its home agent
using a co-located care-of address, then the mobile node SHOULD
choose its default router from among those advertised in any
ICMP Router Advertisement message that it receives for which
its externally obtained care-of address and the Router Address
match under the network prefix. If the mobile node's externally
obtained care-of address matches the IP source address of the
Agent Advertisement under the network prefix, the mobile node
MAY also consider that IP source address as another possible
choice for the IP address of a default router, along with the
(possibly empty) list of Router Addresses from the ICMP Router
Perkins Standards Track [Page 56]RFC 2002 IP Mobility Support October 1996
Advertisement portion of the message. If so, the IP source
address MUST be considered to be the worst choice (lowest
preference) for a default router. The network prefix MAY
be obtained from the Prefix-Lengths Extension in the Router
Advertisement, if present. The prefix MAY also be obtained
through other mechanisms beyond the scope of this document.
Beyond these rules, the actual selection of the default router is
made by the selection method specified for ICMP Router Discovery [4],
among the Router Addresses specified above. In any case, a mobile
node registered via a foreign agent MAY choose its foreign agent as a
default router.
Note that Van Jacobson header compression [10] will not function
properly unless all TCP IP datagrams to and from the mobile node
pass, respectively, through the same first and last-hop router. The
mobile node, therefore, MUST select its foreign agent as its default
router if it performs Van Jacobson header compression with its
foreign agent.
4.2.2. Foreign Agent Considerations
Upon receipt of an encapsulated datagram sent to its advertised
care-of address, a foreign agent MUST compare the inner destination
address to those entries in its visitor list. When the destination
does not match the address of any mobile node currently in the
visitor list, the foreign agent MUST NOT forward the datagram without
modifications to the original IP header, because otherwise a routing
loop is likely to result. The datagram SHOULD be silently discarded.
ICMP Destination Unreachable MUST NOT be sent when a foreign agent is
unable to forward an incoming tunneled datagram. Otherwise, the
foreign agent forwards the decapsulated datagram to the mobile node.
The foreign agent MUST NOT advertise to other routers in its routing
domain, nor to any other mobile node, the presence of a mobile router
(Section 4.5).
The foreign agent MUST route datagrams it receives from registered
mobile nodes. At a minimum, this means that the foreign agent must
verify the IP Header Checksum, decrement the IP Time To Live,
recompute the IP Header Checksum, and forward such datagrams to a
default router. In addition, the foreign agent SHOULD send an
appropriate ICMP Redirect message to the mobile node.
Perkins Standards Track [Page 57]RFC 2002 IP Mobility Support October 1996
4.2.3. Home Agent Considerations
The home agent MUST be able to intercept any datagrams on the home
network addressed to the mobile node while the mobile node is
registered away from home. Proxy and gratuitous ARP MAY be used in
enabling this interception, as specified in Section 4.6.
The home agent must examine the IP Destination Address of all
arriving datagrams to see if it is equal to the home address of any
of its mobile nodes registered away from home. If so, the home agent
tunnels the datagram to the mobile node's currently registered care-
of address or addresses. If the home agent supports the optional
capability of multiple simultaneous mobility bindings, it tunnels a
copy to each care-of address in the mobile node's mobility binding
list. If the mobile node has no current mobility bindings, the home
agent MUST NOT attempt to intercept datagrams destined for the mobile
node, and thus will not in general receive such datagrams. However,
if the home agent is also a router handling common IP traffic, it is
possible that it will receive such datagrams for forwarding onto the
home network. In this case, the home agent MUST assume the mobile
node is at home and simply forward the datagram directly onto the
home network.
See Section 4.1 regarding methods of encapsulation that may be used
for tunneling. Nodes implementing tunneling SHOULD also implement
the "tunnel soft state" mechanism [14], which allows ICMP error
messages returned from the tunnel to correctly be reflected back to
the original senders of the tunneled datagrams.
Home agents SHOULD be able to decapsulate and further deliver packets
addressed to themselves, sent by a mobile node for the purpose of
maintaining location privacy, as described in Section 5.5.
If the Lifetime for a given mobility binding expires before the home
agent has received another valid Registration Request for that mobile
node, then that binding is deleted from the mobility binding list.
The home agent MUST NOT send any Registration Reply message simply
because the mobile node's binding has expired. The entry in the
visitor list of the mobile node's current foreign agent will expire
naturally, probably at the same time as the binding expired at the
home agent. When a mobility binding's lifetime expires, the home
agent MUST delete the binding, but it MUST retain any other (non-
expired) simultaneous mobility bindings that it holds for the mobile
node.
When a home agent receives a datagram, intercepted for one of its
mobile nodes registered away from home, the home agent MUST examine
the datagram to check if it is already encapsulated. If so, special
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RFC 2002 IP Mobility Support October 1996
rules apply in the forwarding of that datagram to the mobile node:
- If the inner (encapsulated) Destination Address is the same
as the outer Destination Address (the mobile node), then the
home agent MUST also examine the outer Source Address of the
encapsulated datagram (the source address of the tunnel). If
this outer Source Address is the same as the mobile node's
current care-of address, the home agent MUST silently discard
that datagram in order to prevent a likely routing loop. If,
instead, the outer Source Address is NOT the same as the mobile
node's current care-of address, then the home agent SHOULD
forward the datagram to the mobile node. In order to forward
the datagram in this case, the home agent MAY simply alter the
outer Destination Address to the care-of address, rather than
re-encapsulating the datagram.
- Otherwise (the inner Destination Address is NOT the same as the
outer Destination Address), the home agent SHOULD encapsulate
the datagram again (recursive encapsulation), with the new outer
Destination Address set equal to the mobile node's care-of
address. That is, the home agent forwards the entire datagram
to the mobile node in the same way as any other datagram
(encapsulated already or not).
4.3. Broadcast Datagrams
When a home agent receives a broadcast datagram, it MUST NOT forward
the datagram to any mobile nodes in its mobility binding list other
than those that have requested forwarding of broadcast datagrams. A
mobile node MAY request forwarding of broadcast datagrams by setting
the 'B' bit in its Registration Request message (Section 3.3). For
each such registered mobile node, the home agent SHOULD forward
received broadcast datagrams to the mobile node, although it is a
matter of configuration at the home agent as to which specific
categories of broadcast datagrams will be forwarded to such mobile
nodes.
If the 'D' bit was set in the mobile node's Registration Request
message, indicating that the mobile node is using a co-located care-
of address, the home agent simply tunnels appropriate broadcast IP
datagrams to the mobile node's care-of address. Otherwise (the 'D'
bit was NOT set), the home agent first encapsulates the broadcast
datagram in a unicast datagram addressed to the mobile node's home
address, and then tunnels this encapsulated datagram to the foreign
agent. This extra level of encapsulation is required so that the
foreign agent can determine which mobile node should receive the
datagram after it is decapsulated. When received by the foreign
agent, the unicast encapsulated datagram is detunneled and delivered
Perkins Standards Track [Page 59]RFC 2002 IP Mobility Support October 1996
to the mobile node in the same way as any other datagram. In either
case, the mobile node must decapsulate the datagram it receives in
order to recover the original broadcast datagram.
4.4. Multicast Datagram Routing
As mentioned previously, a mobile node that is connected to its home
network functions in the same way as any other (fixed) host or
router. Thus, when it is at home, a mobile node functions
identically to other multicast senders and receivers. This section
therefore describes the behavior of a mobile node that is visiting a
foreign network.
In order receive multicasts, a mobile node MUST join the multicast
group in one of two ways. First, a mobile node MAY join the group
via a (local) multicast router on the visited subnet. This option
assumes that there is a multicast router present on the visited
subnet. If the mobile node is using a co-located care-of address, it
SHOULD use this address as the source IP address of its IGMP [5]
messages. Otherwise, it MUST use its home address.
Alternatively, a mobile node which wishes to receive multicasts MAY
join groups via a bi-directional tunnel to its home agent, assuming
that its home agent is a multicast router. The mobile node tunnels
IGMP messages to its home agent and the home agent forwards multicast
datagrams down the tunnel to the mobile node. The rules for
multicast datagram delivery to mobile nodes in this case are
identical to those for broadcast datagrams (Section 4.3). Namely, if
the mobile node is using a co-located care-of address (the 'D' bit
was set in the mobile node's Registration Request), then the home
agent SHOULD tunnel the datagram to this care-of address; otherwise,
the home agent MUST first encapsulate the datagram in a unicast
datagram addressed to the mobile node's home address and then MUST
tunnel the resulting datagram (recursive tunneling) to the mobile
node's care-of address.
A mobile node that wishes to send datagrams to a multicast group also
has two options: (1) send directly on the visited network; or (2)
send via a tunnel to its home agent. Because multicast routing in
general depends upon the IP source address, a mobile node which sends
multicast datagrams directly on the visited network MUST use a co-
located care-of address as the IP source address. Similarly, a
mobile node which tunnels a multicast datagram to its home agent MUST
use its home address as the IP source address of both the (inner)
multicast datagram and the (outer) encapsulating datagram. This
second option assumes that the home agent is a multicast router.
Perkins Standards Track [Page 60]
RFC 2002 IP Mobility Support October 1996
4.5. Mobile Routers
A mobile node can be a router, which is responsible for the mobility
of one or more entire networks moving together, perhaps on an
airplane, a ship, a train, an automobile, a bicycle, or a kayak. The
nodes connected to a network served by the mobile router may
themselves be fixed nodes or mobile nodes or routers. In this
document, such networks are called "mobile networks".
A mobile router MAY act as a foreign agent and provide a foreign
agent care-of address to mobile nodes connected to the mobile
network. Typical routing to a mobile node via a mobile router in
this case is illustrated by the following example:
a) A laptop computer is disconnected from its home network and
later attached to a network port in the seat back of an
aircraft. The laptop computer uses Mobile IP to register on
this foreign network, using a foreign agent care-of address
discovered through an Agent Advertisement from the aircraft's
foreign agent.
b) The aircraft network is itself mobile. Suppose the node
serving as the foreign agent on the aircraft also serves as
the default router that connects the aircraft network to the
rest of the Internet. When the aircraft is at home, this
router is attached to some fixed network at the airline's
headquarters, which is the router's home network. While
the aircraft is in flight, this router registers from time
to time over its radio link with a series of foreign agents
below it on the ground. This router's home agent is a node
on the fixed network at the airline's headquarters.
c) Some correspondent node sends a datagram to the laptop
computer, addressing the datagram to the laptop's home
address. This datagram is initially routed to the laptop's
home network.
d) The laptop's home agent intercepts the datagram on the home
network and tunnels it to the laptop's care-of address, which
in this example is an address of the node serving as router
and foreign agent on the aircraft. Normal IP routing will
route the datagram to the fixed network at the airline's
headquarters.
Perkins Standards Track [Page 61]RFC 2002 IP Mobility Support October 1996
e) The aircraft router and foreign agent's home agent there
intercepts the datagram and tunnels it to its current care-of
address, which in this example is some foreign agent on the
ground below the aircraft. The original datagram from the
correspondent node has now been encapsulated twice: once
by the laptop's home agent and again by the aircraft's home
agent.
f) The foreign agent on the ground decapsulates the datagram,
yielding a datagram still encapsulated by the laptop's home
agent, with a destination address of the laptop's care-of
address. The ground foreign agent sends the resulting
datagram over its radio link to the aircraft.
g) The foreign agent on the aircraft decapsulates the datagram,
yielding the original datagram from the correspondent node,
with a destination address of the laptop's home address.
The aircraft foreign agent delivers the datagram over the
aircraft network to the laptop's link-layer address.
This example illustrated the case in which a mobile node is attached
to a mobile network. That is, the mobile node is mobile with respect
to the network, which itself is also mobile (here with respect to the
ground). If, instead, the node is fixed with respect to the mobile
network (the mobile network is the fixed node's home network), then
either of two methods may be used to cause datagrams from
correspondent nodes to be routed to the fixed node.
A home agent MAY be configured to have a permanent registration for
the fixed node, that indicates the mobile router's address as the
fixed host's care-of address. The mobile router's home agent will
usually be used for this purpose. The home agent is then responsible
for advertising connectivity using normal routing protocols to the
fixed node. Any datagrams sent to the fixed node will thus use
recursive tunneling as described above.
Alternatively, the mobile router MAY advertise connectivity to the
entire mobile network using normal IP routing protocols through a
bi-directional tunnel to its own home agent. This method avoids the
need for recursive tunneling of datagrams.
4.6. ARP, Proxy ARP, and Gratuitous ARP
The use of ARP [16] requires special rules for correct operation when
wireless or mobile nodes are involved. The requirements specified in
this section apply to all home networks in which ARP is used for
address resolution.
Perkins Standards Track [Page 62]RFC 2002 IP Mobility Support October 1996
In addition to the normal use of ARP for resolving a target node's
link-layer address from its IP address, this document distinguishes
two special uses of ARP:
- A Proxy ARP [18] is an ARP Reply sent by one node on behalf
of another node which is either unable or unwilling to answer
its own ARP Requests. The sender of a Proxy ARP reverses the
Sender and Target Protocol Address fields as described in [16],
but supplies some configured link-layer address (generally, its
own) in the Sender Hardware Address field. The node receiving
the Reply will then associate this link-layer address with the
IP address of the original target node, causing it to transmit
future datagrams for this target node to the node with that
link-layer address.
- A Gratuitous ARP [23] is an ARP packet sent by a node in order to
spontaneously cause other nodes to update an entry in their ARP
cache. A gratuitous ARP MAY use either an ARP Request or an ARP
Reply packet. In either case, the ARP Sender Protocol Address
and ARP Target Protocol Address are both set to the IP address
of the cache entry to be updated, and the ARP Sender Hardware
Address is set to the link-layer address to which this cache
entry should be updated. When using an ARP Reply packet, the
Target Hardware Address is also set to the link-layer address to
which this cache entry should be updated (this field is not used
in an ARP Request packet).
In either case, for a gratuitous ARP, the ARP packet MUST be
transmitted as a local broadcast packet on the local link. As
specified in [16], any node receiving any ARP packet (Request or
Reply) MUST update its local ARP cache with the Sender Protocol
and Hardware Addresses in the ARP packet, if the receiving node
has an entry for that IP address already in its ARP cache. This
requirement in the ARP protocol applies even for ARP Request
packets, and for ARP Reply packets that do not match any ARP
Request transmitted by the receiving node [16].
While a mobile node is registered on a foreign network, its home
agent uses proxy ARP [18] to reply to ARP Requests it receives that
seek the mobile node's link-layer address. When receiving an ARP
Request, the home agent MUST examine the target IP address of the
Request, and if this IP address matches the home address of any
mobile node for which it has a registered mobility binding, the home
agent MUST transmit an ARP Reply on behalf of the mobile node. After
exchanging the sender and target addresses in the packet [18], the
home agent MUST set the sender link-layer address in the packet to
the link-layer address of its own interface over which the Reply will
be sent.
Perkins Standards Track [Page 63]RFC 2002 IP Mobility Support October 1996
When a mobile node leaves its home network and registers a binding on
a foreign network, its home agent uses gratuitous ARP to update the
ARP caches of nodes on the home network. This causes such nodes to
associate the link-layer address of the home agent with the mobile
node's home (IP) address. When registering a binding for a mobile
node for which the home agent previously had no binding (the mobile
node was assumed to be at home), the home agent MUST transmit a
gratuitous ARP on behalf of the mobile node. This gratuitous ARP
packet MUST be transmitted as a broadcast packet on the link on which
the mobile node's home address is located. Since broadcasts on the
local link (such as Ethernet) are typically not guaranteed to be
reliable, the gratuitous ARP packet SHOULD be retransmitted a small
number of times to increase its reliability.
When a mobile node returns to its home network, the mobile node
and its home agent use gratuitous ARP to cause all nodes on the
mobile node's home network to update their ARP caches to once again
associate the mobile node's own link-layer address with the mobile
node's home (IP) address. Before transmitting the (de)Registration
Request message to its home agent, the mobile node MUST transmit this
gratuitous ARP on its home network as a local broadcast on this link.
The gratuitous ARP packet SHOULD be retransmitted a small number of
times to increase its reliability, but these retransmissions SHOULD
proceed in parallel with the transmission and processing of its
(de)Registration Request.
When the mobile node's home agent receives and accepts this
(de)Registration Request, the home agent MUST also transmit a
gratuitous ARP on the mobile node's home network. This gratuitous
ARP also is used to associate the mobile node's home address with
the mobile node's own link-layer address. A gratuitous ARP is
transmitted by both the mobile node and its home agent, since in the
case of wireless network interfaces, the area within transmission
range of the mobile node will likely differ from that within range
of its its home agent. Th ARP packet from the home agent MUST be
transmitted as a local broadcast on the mobile node's home link,
and SHOULD be retransmitted a small number of times to increase
its reliability; these retransmissions, however, SHOULD proceed in
parallel with the transmission and processing of its (de)Registration
Reply.
While the mobile node is away from home, it MUST NOT transmit any
broadcast ARP Request or ARP Reply messages. Finally, while the
mobile node is away from home, it MUST NOT reply to ARP Requests
in which the target IP address is its own home address, unless the
ARP Request is sent by a foreign agent with which the mobile node
is attempting to register or a foreign agent with which the mobile
node has an unexpired registration. In the latter case, the mobile
Perkins Standards Track [Page 64]
RFC 2002 IP Mobility Support October 1996
node MUST use a unicast ARP Reply to respond to the foreign agent.
Note that if the mobile node is using a co-located care-of address
and receives an ARP Request in which the target IP address is this
care-of address, then the mobile node SHOULD reply to this ARP
Request. Note also that, when transmitting a Registration Request on
a foreign network, a mobile node may discover the link-layer address
of a foreign agent by storing the address as it is received from the
Agent Advertisement from that foreign agent, but not by transmitting
a broadcast ARP Request message.
The specific order in which each of the above requirements for the
use of ARP, proxy ARP, and gratuitous ARP are applied, relative to
the transmission and processing of the mobile node's Registration
Request and Registration Reply messages when leaving home or
returning home, are important to the correct operation of the
protocol.
To summarize the above requirements, when a mobile node leaves its
home network, the following steps, in this order, MUST be performed:
- The mobile node decides to register away from home, perhaps
because it has received an Agent Advertisement from a foreign
agent and has not recently received one from its home agent.
- Before transmitting the Registration Request, the mobile node
disables its own future processing of any ARP Requests it
may subsequently receive requesting the link-layer address
corresponding to its home address, except insofar as necessary to
communicate with foreign agents on visited networks.
- The mobile node transmits its Registration Request.
- When the mobile node's home agent receives and accepts the
Registration Request, it performs a gratuitous ARP on behalf
of the mobile node, and begins using proxy ARP to reply to ARP
Requests that it receives requesting the mobile node's link-layer
address. If, instead, the home agent rejects the Registration
Request, no ARP processing (gratuitous nor proxy) is performed by
the home agent.
When a mobile node later returns to its home network, the following
steps, in this order, MUST be performed:
- The mobile node decides to register at home, perhaps because it
has received an Agent Advertisement from its home agent.
Perkins Standards Track [Page 65]RFC 2002 IP Mobility Support October 1996
- Before transmitting the Registration Request, the mobile node
re-enables its own future processing of any ARP Requests it may
subsequently receive requesting its link-layer address.
- The mobile node performs a gratuitous ARP for itself.
- The mobile node transmits its Registration Request.
- When the mobile node's home agent receives and accepts the
Registration Request, it stops using proxy ARP to reply to
ARP Requests that it receives requesting the mobile node's
link-layer address, and then performs a gratuitous ARP on behalf
of the mobile node. If, instead, the home agent rejects the
Registration Request, no ARP processing (gratuitous nor proxy) is
performed by the home agent.
5. Security Considerations
The mobile computing environment is potentially very different from
the ordinary computing environment. In many cases, mobile computers
will be connected to the network via wireless links. Such links are
particularly vulnerable to passive eavesdropping, active replay
attacks, and other active attacks.
5.1. Message Authentication Codes
Home agents and mobile nodes MUST be able to perform authentication.
The default algorithm is keyed MD5 [21], with a key size of 128 bits.
The default mode of operation is to both precede and follow the data
to be hashed, by the 128-bit key; that is, MD5 is to be used in
"prefix+suffix" mode. The foreign agent MUST also support
authentication using keyed MD5 and key sizes of 128 bits or greater,
with manual key distribution. More authentication algorithms,
algorithm modes, key distribution methods, and key sizes MAY also be
supported.
5.2. Areas of Security Concern in this Protocol
The registration protocol described in this document will result in a
mobile node's traffic being tunneled to its care-of address. This
tunneling feature could be a significant vulnerability if the
registration were not authenticated. Such remote redirection, for
instance as performed by the mobile registration protocol, is widely
understood to be a security problem in the current Internet if not
authenticated [2]. Moreover, the Address Resolution Protocol (ARP)
is not authenticated, and can potentially be used to steal another
host's traffic. The use of "Gratuitous ARP" (Section 4.6) brings
with it all of the risks associated with the use of ARP.
Perkins Standards Track [Page 66]RFC 2002 IP Mobility Support October 1996
5.3. Key Management
This specification requires a strong authentication mechanism (keyed
MD5) which precludes many potential attacks based on the Mobile IP
registration protocol. However, because key distribution is
difficult in the absence of a network key management protocol,
messages with the foreign agent are not all required to be
authenticated. In a commercial environment it might be important to
authenticate all messages between the foreign agent and the home
agent, so that billing is possible, and service providers do not
provide service to users that are not legitimate customers of that
service provider.
5.4. Picking Good Random Numbers
The strength of any authentication mechanism depends on several
factors, including the innate strength of the authentication
algorithm, the secrecy of the key used, the strength of the key used,
and the quality of the particular implementation. This specification
requires implementation of keyed MD5 for authentication, but does not
preclude the use of other authentication algorithms and modes. For
keyed MD5 authentication to be useful, the 128-bit key must be both
secret (that is, known only to authorized parties) and pseudo-random.
If nonces are used in connection with replay protection, they must
also be selected carefully. Eastlake, et al. [7] provides more
information on generating pseudo-random numbers.
5.5. Privacy
Users who have sensitive data that they do not wish others to see
should use mechanisms outside the scope of this document (such as
encryption) to provide appropriate protection. Users concerned about
traffic analysis should consider appropriate use of link encryption.
If absolute location privacy is desired, the mobile node can create a
tunnel to its home agent. Then, datagrams destined for correspondent
nodes will appear to emanate from the home network, and it may be
more difficult to pinpoint the location of the mobile node. Such
mechanisms are all beyond the scope of this document.
Perkins Standards Track [Page 67]
RFC 2002 IP Mobility Support October 1996
5.6. Replay Protection for Registration Requests
The Identification field is used to let the home agent verify that a
registration message has been freshly generated by the mobile node,
not replayed by an attacker from some previous registration. Two
methods are described in this section: timestamps (mandatory) and
"nonces" (optional). All mobile nodes and home agents MUST implement
timestamp-based replay protection. These nodes MAY also implement
nonce-based replay protection (but see Appendix A.2 for a patent that
may apply to nonce-based replay protection).
The style of replay protection in effect between a mobile node and
its home agent is part of the mobile security association. A mobile
node and its home agent MUST agree on which method of replay
protection will be used. The interpretation of the Identification
field depends on the method of replay protection as described in the
subsequent subsections.
Whatever method is used, the low-order 32 bits of the Identification
MUST be copied unchanged from the Registration Request to the Reply.
The foreign agent uses those bits (and the mobile node's home
address) to match Registration Requests with corresponding replies.
The mobile node MUST verify that the low-order 32 bits of any
Registration Reply are identical to the bits it sent in the
Registration Request.
The Identification in a new Registration Request MUST NOT be the same
as in an immediately preceding Request, and SHOULD NOT repeat while
the same security context is being used between the mobile node and
the home agent. Retransmission as in Section 3.6.3 is allowed.
5.6.1. Replay Protection using Timestamps
The basic principle of timestamp replay protection is that the node
generating a message inserts the current time of day, and the node
receiving the message checks that this timestamp is sufficiently
close to its own time of day. Obviously the two nodes must have
adequately synchronized time-of-day clocks. As with any messages,
time synchronization messages may be protected against tampering by
an authentication mechanism determined by the security context
between the two nodes.
If timestamps are used, the mobile node MUST set the Identification
field to a 64-bit value formatted as specified by the Network Time
Protocol [13]. The low-order 32 bits of the NTP format represent
fractional seconds, and those bits which are not available from a
time source SHOULD be generated from a good source of randomness.
Note, however, that when using timestamps, the 64-bit Identification
Perkins Standards Track [Page 68]
RFC 2002 IP Mobility Support October 1996
used in a Registration Request from the mobile node MUST be greater
than that used in any previous Registration Request, as the home
agent uses this field also as a sequence number. Without such a
sequence number, it would be possible for a delayed duplicate of an
earlier Registration Request to arrive at the home agent (within the
clock synchronization required by the home agent), and thus be
applied out of order, mistakenly altering the mobile node's current
registered care-of address.
Upon receipt of a Registration Request with a valid Mobile-Home
Authentication Extension, the home agent MUST check the
Identification field for validity. In order to be valid, the
timestamp contained in the Identification field MUST be close enough
to the home agent's time of day clock and the timestamp MUST be
greater than all previously accepted timestamps for the requesting
mobile node. Time tolerances and resynchronization details are
specific to a particular mobility security association.
If the timestamp is valid, the home agent copies the entire
Identification field into the Registration Reply it returns the Reply
to the mobile node. If the timestamp is not valid, the home agent
copies only the low-order 32 bits into the Registration Reply, and
supplies the high-order 32 bits from its own time of day. In this
latter case, the home agent MUST reject the registration by returning
Code 133 (identification mismatch) in the Registration Reply.
As described in Section 3.6.2.1, the mobile node MUST verify that the
low-order 32 bits of the Identification in the Registration Reply are
identical to those in the rejected registration attempt, before using
the high-order bits for clock resynchronization.
5.6.2. Replay Protection using Nonces
Implementors of this optional mechanism should examine Appendix A.2
for a patent that may be applicable to nonce-based replay protection.
The basic principle of nonce replay protection is that node A
includes a new random number in every message to node B, and checks
that node B returns that same number in its next message to node A.
Both messages use an authentication code to protect against
alteration by an attacker. At the same time node B can send its own
nonces in all messages to node A (to be echoed by node A), so that it
too can verify that it is receiving fresh messages.
The home agent may be expected to have resources for computing
pseudo-random numbers useful as nonces [7]. It inserts a new nonce
as the high-order 32 bits of the identification field of every
Registration Reply. The home agent copies the low-order 32 bits of
Perkins Standards Track [Page 69]
RFC 2002 IP Mobility Support October 1996
the Identification from the Registration Request message into the
low-order 32 bits of the Identification in the Registration Reply.
When the mobile node receives an authenticated Registration Reply
from the home agent, it saves the high-order 32 bits of the
identification for use as the high-order 32 bits of its next
Registration Request.
The mobile node is responsible for generating the low-order 32 bits
of the Identification in each Registration Request. Ideally it
should generate its own random nonces. However it may use any
expedient method, including duplication of the random value sent by
the home agent. The method chosen is of concern only to the mobile
node, because it is the node that checks for valid values in the
Registration Reply. The high-order and low-order 32 bits of the
identification chosen SHOULD both differ from their previous values.
The home agent uses a new high-order value and the mobile node uses a
new low-order value for each registration message. The foreign agent
uses the low-order value (and the mobile host's home address) to
correctly match registration replies with pending Requests (Section
3.7.1).
If a registration message is rejected because of an invalid nonce,
the Reply always provides the mobile node with a new nonce to be used
in the next registration. Thus the nonce protocol is self-
synchronizing.
Perkins Standards Track [Page 70]
RFC 2002 IP Mobility Support October 1996
6. Acknowledgments
Special thanks to Steve Deering (Xerox PARC), along with Dan Duchamp
and John Ioannidis (JI) (Columbia), for forming the working group,
chairing it, and putting so much effort into its early development.
Thanks also to Kannan Alaggapan, Greg Minshall, and Tony Li for their
contributions to the group while performing the duties of
chairperson, as well as for their many useful comments.
Thanks to the active members of the Mobile IP Working Group,
particularly those who contributed text, including (in alphabetical
order)
- Ran Atkinson (Naval Research Lab),
- Dave Johnson (Carnegie Mellon University),
- Frank Kastenholz (FTP Software),
- Anders Klemets (KTH),
- Chip Maguire (KTH),
- Andrew Myles (Macquarie University),
- Al Quirt (Bell Northern Research),
- Yakov Rekhter (IBM), and
- Fumio Teraoka (Sony).
Thanks to Charlie Kunzinger and to Bill Simpson, the editors who
produced the first drafts for of this document, reflecting the
discussions of the Working Group. Much of the new text of this memo
is due to Jim Solomon and Dave Johnson.
Thanks to Greg Minshall (Novell), Phil Karn (Qualcomm), and Frank
Kastenholz (FTP Software) for their generous support in hosting
interim Working Group meetings.
Perkins Standards Track [Page 71]RFC 2002 IP Mobility Support October 1996
A. Patent Issues
As of the time of publication, the IETF had been made aware of two
patents that may be relevant to implementors of the protocol
described in this technical specification.
A.1. IBM Patent #5,159,592
Charles Perkins, editor of this memo, is sole inventor of U.S. Patent
No. 5,159,592, assigned to IBM. In a letter dated May 30, 1995, IBM
brought this patent to the attention of the IETF, stating that this
patent "relates to the Mobile IP." We understand that IBM did not
intend to assert that any particular implementation of Mobile IP
would or would not infringe the patent, but rather that IBM was
meeting what it viewed as a duty to disclose information that could
be relevant to the process of adopting a standard.
Based on a review of the claims of the patent, IETF believes that a
system of registering an address obtained from a foreign agent, as
described in the document, would not necessarily infringe any of the
claims of the patent; and that a system in which an address is
obtained elsewhere and then registered can be implemented without
necessarily infringing any claims of the patent. Accordingly, our
view is that the proposed protocol can be implemented without
necessarily infringing the Perkins Patent.
Parties considering adopting this protocol must be aware that some
specific implementations, or features added to otherwise non-
infringing implementations, may raise an issue of infringement with
respect to this patent or to some other patent.
This statement is for the IETF's assistance in its standard-setting
procedure, and should not be relied upon by any party as an opinion
or guarantee that any implementation it might make or use would not
be covered by the Perkins Patent and any other patents. In
particular, IBM might disagree with the interpretation of this patent
described herein.
A.2. IBM Patent #5,148,479
This patent, also assigned to IBM, may be relevant to those who
implement nonce-based replay protection as described in Section
5.6.2. Note that nonce-based replay protection is an optional
feature of this specification. Timestamp-based replay protection, on
the other hand, (Section 5.6.1) is a requirement of this
specification.
Perkins Standards Track [Page 72]
RFC 2002 IP Mobility Support October 1996
B. Link-Layer Considerations
The mobile node MAY use link-layer mechanisms to decide that its
point of attachment has changed. Such indications include the
Down/Testing/Up interface status [11], and changes in cell or
administration. The mechanisms will be specific to the particular
link-layer technology, and are outside the scope of this document.
The Point-to-Point-Protocol (PPP) [22] and its Internet Protocol
Control Protocol (IPCP) [12], negotiates the use of IP addresses.
The mobile node SHOULD first attempt to specify its home address, so
that if the mobile node is attaching to its home network, the
unrouted link will function correctly. When the home address is not
accepted by the peer, but a transient IP address is dynamically
assigned to the mobile node, and the mobile node is capable of
supporting a co-located care-of address, the mobile node MAY register
that address as a co-located care-of address. When the peer
specifies its own IP address, that address MUST NOT be assumed to be
a foreign agent care-of address or the IP address of a home agent.
C. TCP Considerations
C.1. TCP Timers
Most hosts and routers which implement TCP/IP do not permit easy
configuration of the TCP timer values. When high-delay (e.g.,
SATCOM) or low-bandwidth (e.g., High-Frequency Radio) links are in
use, the default TCP timer values in many systems may cause
retransmissions or timeouts, even when the link and network are
actually operating properly with greater than usual delays because of
the medium in use. This can cause an inability to create or maintain
TCP connections over such links, and can also cause unneeded
retransmissions which consume already scarce bandwidth. Vendors are
encouraged to make TCP timers more configurable. Vendors of systems
designed for the mobile computing markets should pick default timer
values more suited to low-bandwidth, high-delay links. Users of
mobile nodes should be sensitive to the possibility of timer-related
difficulties.
C.2. TCP Congestion Management
Mobile nodes often use media which are more likely to introduce
errors, effectively causing more packets to be dropped. This
introduces a conflict with the mechanisms for congestion management
found in modern versions of TCP [9]. Now, when a packet is dropped,
the correspondent node's TCP implementation is likely to react as if
there were a source of network congestion, and initiate the slow-
Perkins Standards Track [Page 73]
RFC 2002 IP Mobility Support October 1996
start mechanisms [9] designed for controlling that problem. However,
those mechanisms are inappropriate for overcoming errors introduced
by the links themselves, and have the effect of magnifying the
discontinuity introduced by the dropped packet. This problem has
been analyzed by Caceres, et al. [3]; there is no easy solution
available, and certainly no solution likely to be installed soon on
all correspondent nodes. While this problem is beyond the scope of
this document, it does illustrate that providing performance
transparency to mobile nodes involves understanding mechanisms
outside the network layer. It also indicates the need to avoid
designs which systematically drop packets; such designs might
otherwise be considered favorably when making engineering tradeoffs.
D. Example Scenarios
This section shows example Registration Requests for several common
scenarios.
D.1. Registering with a Foreign Agent Care-of Address
The mobile node receives an Agent Advertisement from a foreign agent
and wishes to register with that agent using the advertised foreign
agent care-of address. The mobile node wishes only IP-in-IP
encapsulation, does not want broadcasts, and does not want
simultaneous mobility bindings:
IP fields:
Source Address = mobile node's home address
Destination Address = copied from the IP source address of the
Agent Advertisement
Time to Live = 1
UDP fields:
Source Port = <any>
Destination Port = 434
Registration Request fields:
Type = 1
S=0,B=0,D=0,M=0,G=0
Lifetime = the Registration Lifetime copied from the
Mobility Agent Advertisement Extension of the
Router Advertisement message
Home Address = the mobile node's home address
Home Agent = IP address of mobile node's home agent
Care-of Address = the Care-of Address copied from the
Mobility Agent Advertisement Extension of the
Router Advertisement message
Identification = Network Time Protocol timestamp or Nonce
Extensions:
The Mobile-Home Authentication Extension
Perkins Standards Track [Page 74]RFC 2002 IP Mobility Support October 1996
D.2. Registering with a Co-Located Care-of Address
The mobile node enters a foreign network that contains no foreign
agents. The mobile node obtains an address from a DHCP server [6]
for use as a co-located care-of address. The mobile node supports
all forms of encapsulation (IP-in-IP, minimal encapsulation, and
GRE), desires a copy of broadcast datagrams on the home network, and
does not want simultaneous mobility bindings:
IP fields:
Source Address = care-of address obtained from DHCP server
Destination Address = IP address of home agent
Time to Live = 64
UDP fields:
Source Port = <any>
Destination Port = 434
Registration Request fields:
Type = 1
S=0,B=1,D=1,M=1,G=1
Lifetime = 1800 (seconds)
Home Address = the mobile node's home address
Home Agent = IP address of mobile node's home agent
Care-of Address = care-of address obtained from DHCP server
Identification = Network Time Protocol timestamp or Nonce
Extensions:
The Mobile-Home Authentication Extension
Perkins Standards Track [Page 75]RFC 2002 IP Mobility Support October 1996
D.3. Deregistration
The mobile node returns home and wishes to deregister all care-of
addresses with its home agent.
IP fields:
Source Address = mobile node's home address
Destination Address = IP address of home agent
Time to Live = 1
UDP fields:
Source Port = <any>
Destination Port = 434
Registration Request fields:
Type = 1
S=0,B=0,D=0,M=0,G=0
Lifetime = 0
Home Address = the mobile node's home address
Home Agent = IP address of mobile node's home agent
Care-of Address = the mobile node's home address
Identification = Network Time Protocol timestamp or Nonce
Extensions:
The Mobile-Home Authentication Extension
E. Applicability of Prefix Lengths Extension
Caution is indicated with the use of the Prefix Lengths Extension
over wireless links, due to the irregular coverage areas provided by
wireless transmitters. As a result, it is possible that two foreign
agents advertising the same prefix might indeed provide different
connectivity to prospective mobile nodes. The Prefix-Lengths
Extension SHOULD NOT be included in the advertisements sent by agents
in such a configuration.
Perkins Standards Track [Page 76]RFC 2002 IP Mobility Support October 1996
Foreign agents using different wireless interfaces would have to
cooperate using special protocols to provide identical coverage in
space, and thus be able to claim to have wireless interfaces situated
on the same subnetwork. In the case of wired interfaces, a mobile
node disconnecting and subsequently connecting to a new point of
attachment, may well send in a new Registration Request no matter
whether the new advertisement is on the same medium as the last
recorded advertisement. And, finally, in areas with dense
populations of foreign agents it would seem unwise to require the
propagation via routing protocols of the subnet prefixes associated
with each individual wireless foreign agent; such a strategy could
lead to quick depletion of available space for routing tables,
unwarranted increases in the time required for processing routing
updates, and longer decision times for route selection if routes
(which are almost always unnecessary) are stored for wireless
"subnets".
References
[1] Atkinson, R., "IP Authentication Header", RFC 1826, August 1995.
[2] S. M. Bellovin. Security Problems in the TCP/IP Protocol Suite.
ACM Computer Communications Review, 19(2), March 1989.
[3] Ramon Caceres and Liviu Iftode. Improving the Performance
of Reliable Transport Protocols in Mobile Computing
Environments. IEEE Journal on Selected Areas in Communications,
13(5):850--857, June 1995.
[4] Deering, S., Editor, "ICMP Router Discovery Messages",
RFC 1256, September 1991.
[5] Deering, S., "Host Extensions for IP Multicasting", STD 5,
RFC 1112, August 1989.
[6] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
October 1993.
[7] Eastlake, D., Crocker, S., and J. Schiller, "Randomness
Requirements for Security", RFC 1750, December 1994.
[8] Hanks, S., Li, R., Farinacci, D., and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701, October 1994.
[9] Van Jacobson. Congestion Avoidance and Control. In Proceedings
of the SIGCOMM '88 Symposium: Communications Architectures &
Protocols, pages 314--329, August 1988.
Perkins Standards Track [Page 77]RFC 2002 IP Mobility Support October 1996
[10] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed Serial
Links", RFC 1144, February 1990.
[11] McCloghrie, K., and F. Kastenholz, "Evolution of the
Interfaces Group of MIB-II", RFC 1573, January 1994.
[12] McGregor, G., "The PPP Internet Protocol Control Protocol
(IPCP)", RFC 1332, May 1992.
[13] Mills, D., "Network Time Protocol (Version 3):
Specification, Implementation and Analysis", RFC 1305, March
1992.
[14] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[15] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
October 1996.
[16] Plummer, D., "An Ethernet Address Resolution Protocol:
Or Converting Network Protocol Addresses to 48.bit Ethernet
Addresses for Transmission on Ethernet Hardware", STD 37,
RFC 826, November 1982.
[17] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
1980.
[18] Postel, J., "Multi-LAN Address Resolution", RFC 925, October
1984.
[19] Postel, J., Editor, "Internet Protocol", STD 5, RFC 791,
September 1981.
[20] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
RFC 1700, October 1994.
[21] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[22] Simpson, W., Editor, "The Point-to-Point Protocol
(PPP)", STD 51, RFC 1661, July 1994.
[23] W. Richard Stevens. TCP/IP Illustrated, Volume 1: The
Protocols. Addison-Wesley, Reading, Massachusetts, 1994.
Perkins Standards Track [Page 78]RFC 2002 IP Mobility Support October 1996
Editor's Address
Questions about this memo can also be directed to the editor:
Charles Perkins
Room H3-D34
T. J. Watson Research Center
IBM Corporation
30 Saw Mill River Rd.
Hawthorne, NY 10532
Work: +1-914-784-7350
Fax: +1-914-784-6205
EMail: perk@watson.ibm.com
The working group can be contacted via the current chair:
Jim Solomon
Motorola, Inc.
1301 E. Algonquin Rd.
Schaumburg, IL 60196
Work: +1-847-576-2753
EMail: solomon@comm.mot.com
Perkins Standards Track [Page 79]
