Internet Engineering Task Force C. Perkins, editor
INTERNET DRAFT IBM
21 December 1995
IP Mobility Support
draft-ietf-mobileip-protocol-14.txt
Status of This Memo
This document is a submission by the Mobile-IP Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
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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 packets
destined for the mobile node through a tunnel to the care-of address.
After arriving at the end of the tunnel, the packets are then
delivered to the mobile node.
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Contents
Status of This Memo i
Abstract i
1. Introduction 1
1.1. Protocol Requirements . . . . . . . . . . . . . . . . . . 1
1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 2
1.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 2
1.5. New Architectural Entities . . . . . . . . . . . . . . . 3
1.6. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.7. Protocol Overview . . . . . . . . . . . . . . . . . . . . 5
1.8. Specification Language . . . . . . . . . . . . . . . . . 9
1.9. Message Format and Protocol Extensibility . . . . . . . . 9
2. Agent Discovery 12
2.1. Agent Advertisement . . . . . . . . . . . . . . . . . . . 12
2.1.1. Mobile Service Extension . . . . . . . . . . . . 14
2.1.2. Prefix-Lengths Extension . . . . . . . . . . . . 15
2.1.3. One-byte Padding Extension . . . . . . . . . . . 16
2.2. Agent Solicitation . . . . . . . . . . . . . . . . . . . 17
2.3. Foreign/Home Agent Considerations . . . . . . . . . . . . 17
2.3.1. Advertised Router Addresses . . . . . . . . . . . 18
2.3.2. Sequence Numbers, and Rollover Handling . . . . . 18
2.4. Mobile Node Considerations . . . . . . . . . . . . . . . 18
2.4.1. Registration Required . . . . . . . . . . . . . . 19
2.4.2. Move Detection . . . . . . . . . . . . . . . . . 19
2.4.3. Returning Home . . . . . . . . . . . . . . . . . 20
2.4.4. Sequence Numbers, and Rollover Handling . . . . . 20
3. Registration 21
3.1. Registration Overview . . . . . . . . . . . . . . . . . . 21
3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 22
3.3. Registration Request . . . . . . . . . . . . . . . . . . 22
3.4. Registration Reply . . . . . . . . . . . . . . . . . . . 24
3.5. Registration Extensions . . . . . . . . . . . . . . . . . 27
3.5.1. Mobile-Home Authentication Extension . . . . . . 27
3.5.2. Mobile-Foreign Authentication Extension . . . . . 28
3.5.3. Foreign-Home Authentication Extension . . . . . . 28
3.6. Mobile Node Considerations . . . . . . . . . . . . . . . 29
3.6.1. Sending Registration Requests . . . . . . . . . . 30
3.6.2. Receiving Registration Replies . . . . . . . . . 33
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3.6.3. Registration Retransmission . . . . . . . . . . . 35
3.7. Foreign Agent Considerations . . . . . . . . . . . . . . 36
3.7.1. Configuration and Registration Tables . . . . . . 36
3.7.2. Receiving Registration Requests . . . . . . . . . 37
3.7.3. Receiving Registration Replies . . . . . . . . . 39
3.8. Home Agent Considerations . . . . . . . . . . . . . . . . 40
3.8.1. Configuration and Registration Tables . . . . . . 40
3.8.2. Receiving Registration Requests . . . . . . . . . 41
3.8.3. Sending Registration Replies . . . . . . . . . . 44
4. Routing Considerations 46
4.1. Encapsulation Types . . . . . . . . . . . . . . . . . . . 46
4.2. Unicast Packet Routing . . . . . . . . . . . . . . . . . 46
4.2.1. Mobile Node Considerations . . . . . . . . . . . 46
4.2.2. Foreign Agent Considerations . . . . . . . . . . 47
4.2.3. Home Agent Considerations . . . . . . . . . . . . 48
4.3. Broadcast packets . . . . . . . . . . . . . . . . . . . . 48
4.4. Multicast Packet Routing . . . . . . . . . . . . . . . . 49
4.5. Mobile Routers . . . . . . . . . . . . . . . . . . . . . 50
4.6. ARP, Proxy ARP, and Gratuitous ARP . . . . . . . . . . . 51
5. Security Considerations 55
5.1. Message Authentication Codes . . . . . . . . . . . . . . 55
5.2. Areas of security concern in this protocol . . . . . . . 55
5.3. Key management . . . . . . . . . . . . . . . . . . . . . 55
5.4. Picking good random numbers . . . . . . . . . . . . . . . 56
5.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 56
5.6. Replay Protection for Registration Requests . . . . . . . 56
5.6.1. Replay Protection using Timestamps . . . . . . . 57
5.6.2. Replay Protection using Nonces . . . . . . . . . 58
6. Acknowledgments 58
A. Patent Issues 59
A.1. IBM Patent #5,159,592 . . . . . . . . . . . . . . . . . . 60
A.2. IBM Patent #5,148,479 . . . . . . . . . . . . . . . . . . 60
B. Link-Layer considerations 61
B.1. Point-to-Point Link-Layers . . . . . . . . . . . . . . . 61
B.2. Multi-Point Link-Layers . . . . . . . . . . . . . . . . . 61
C. TCP Considerations 61
C.1. TCP Timers . . . . . . . . . . . . . . . . . . . . . . . 61
C.2. TCP Congestion Management . . . . . . . . . . . . . . . . 62
D. Example Scenarios 63
D.1. Registering with a Foreign Agent's Care-of Address . . . 63
D.2. Registering with a Dynamic Care-of Address . . . . . . . 63
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D.3. Deregistration . . . . . . . . . . . . . . . . . . . . . 64
E. Applicability of Prefix Lengths Extension 65
Chair's Address 68
Editor's Address 68
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1. Introduction
IP version 4, like its predecessors, 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 packets destined to it; otherwise,
packets 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 computers.
A new, scalable, mechanism is evidently required to accommodate node
mobility within the Internet. This document defines such a mechanism
and enables nodes to change their point of attachment to the Internet
without changing their IP address.
1.1. Protocol Requirements
Implementation of the protocol described in this document shall allow
a mobile node to communicate with other nodes after changing its
point of physical attachment to the Internet, yet without changing
its IP address.
Implementation of the protocol described in this document shall allow
a mobile node to communicate with other nodes that do not implement
these mobility functions. No protocol enhancements are required
in hosts or routers that are not providing any of the mobility
functions.
Messages used by this protocol which exchange location information
must be authenticated in order to protect against remote redirection
attacks.
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1.2. Goals
The link by which the mobile node is directly attached to the
Internet is likely to be bandwidth limited, and experience a higher
rate of errors than traditional wired networks. Moreover, mobile
nodes are more likely to be battery powered, and minimizing power
consumption is important. Therefore, only a few administrative
messages should be sent between a mobile node and an agent, and the
size of these messages should be kept as short as is reasonably
possible.
1.3. Assumptions
The protocols defined in this document place no additional
constraints on assignment of IP addresses. That is, a mobile node
can be assigned an IP address by the organization that owns the
machine, and will be able to use that IP address regardless of the
current point of attachment.
It is assumed that mobile nodes will not change their point of
attachment to the Internet more frequently than once per second.
It is assumed that IP unicast datagrams are routed based on the
destination address in the datagram header (i.e., not by source
address).
1.4. Applicability
Mobile IP is intended to solve node mobility across changes in IP
subnet. 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
applications -- for example, handoff amongst wireless transceivers,
each of which covers only a very small geographic area. In this
later situation, link-layer mechanisms for link maintenance (i.e.
link-layer handoff) may offer faster convergence and far less
overhead than Mobile IP.
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1.5. New Architectural Entities
Mobile IP introduces these 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 losing connectivity and without changing its
IP address.
Home Agent
A router on a mobile node's home network which tunnels packets
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 when it registers. The
foreign agent detunnels and delivers packets to the mobile node
that were tunneled by the mobile node's home agent. In the
reverse direction, the foreign agent may serve as a default
router for registered mobile node.
A mobile node is given a long-term IP address on a home network.
This home address is administered in much the same way as "permanent"
IP addresses are provided to stationary hosts. When away from its
home network, the "care-of address" associated with the mobile node
reflects the mobile node's current point of attachment. The mobile
node creates new network connections with existing Internet hosts
using its home address.
1.6. Terminology
This document frequently uses the following terms:
Agent Advertisement
A periodic advertisement constructed by attaching a special
extension to a router advertisement [7] message.
Care-of Address
The care-of address is the termination point of a tunnel toward
a mobile node that is away from its home network. Depending
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on the configuration, the care-of address can be either an
address of a foreign agent or a temporary address acquired by
the mobile node.
Correspondent
A peer with which a mobile node is communicating. The
correspondent 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 packets 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 the endpoints of the 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.
Mobility Binding
The association of a home address with a care-of address, along
with the remaining lifetime of that association.
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Mobility Security Association
The mobility security association between a pair of nodes
is a collection of security contexts which may be applied
to Mobile IP protocol messages exchanged by them. Each
context indicates an authentication algorithm and mode
(subsection 5.1), a secret (a shared key, or appropriate
public/private key pair), and a style of replay protection in
use (subsection 5.6).
Node
A host or a router.
Nonce
A random value, different from previous choices, inserted in a
packet to protect against replays.
Security Parameter Index (SPI)
The SPI indicates the security context between a pair of nodes
among those available in the Mobility Security Association. A
value of zero is reserved and MUST NOT be used in any Mobility
Security Association.
Tunnel
The path followed by a packet while it is encapsulated. The
model is that, while it is encapsulated, a packet is routed
to a knowledgeable decapsulating agent, which decapsulates
the packet and then correctly delivers it to its ultimate
destination.
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.
1.7. Protocol Overview
The following support services are defined for Mobile IP:
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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 is a rough outline of the mobile-IP protocol:
- Foreign agents and home agents advertise their presence via Agent
Advertisements (see section 2).
- A mobile node receives these 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.
- 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, or by some assignment mechanism (for
example, DHCP [8]).
- The mobile node then registers its new care-of address with its
home agent, possibly via a foreign agent (see section 3).
- Packets sent to the mobile node's Home Address are received
by the home agent, tunneled by the home agent to the 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 (see subsection 4.2.3).
- In the reverse direction, packets originated by the mobile node
are typically 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
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network and its current location. The tunnel terminates at the
mobile node's care-of address -- an address to which packets can
be delivered via conventional IP routing. At the care-of address,
the original packet is removed from the tunnel and delivered to the
mobile node.
Mobile IP provides two distinct modes for the acquisition of a
care-of address:
a) A care-of address may be provided by a foreign agent. This
mode 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. In this mode, the foreign agent is the endpoint of
the tunnel and, upon receiving tunneled packets from the
mobile node's home agent, decapsulates them and delivers the
inner packet to the mobile node.
b) A care-of address may be acquired by, and/or owned by, a
mobile node when visiting a foreign network. The method
by which it obtains such an address is beyond the scope
of this document (but see, for example, DHCP [8]). With
its own care-of address, the mobile node itself performs
decapsulation of the packets tunneled by its home agent.
The latter mode has the advantage that it allows a mobile node to
function without a foreign agent, for example, in installations that
have not yet deployed Mobile IP. It does, however, place additional
burden on the IPv4 address space because it requires a pool of
addresses to be made available to visiting mobile nodes. It is
difficult to efficiently maintain pools of addresses for each subnet
which 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, but it also might be an address temporarily
acquired by the mobile node. On the other hand, a foreign agent
is a mobility agent that provides services to mobile nodes. See
subsections 3.7 and 4.2.2 for additional details.
A home agent MUST be able to attract and intercept packets 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
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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 packets 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 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 packets to each other,
addressing the underlying link-layer frames 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 packets between these nodes. Such placements are beyond the
scope of this document.
If a mobile node owns its own 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, packets destined
to the care-of address will be undeliverable.
.....................................................................
: :
: 2) packet is received 3) packet is :
: by home agent and detunneled :
: is tunneled to the and delivered :
: care-of address to mobile node :
: :
: +-----+ +-------+ +------+ :
: |home | =======> |foreign| ------> |mobile| :
: |agent| | agent | <------ | node | :
: +-----+ +-------+ +------+ :
: 1) packet to /|\ / :
: mobile node | / 4) In the opposite direction, :
: arrives on | / standard IP routing delivers :
: home network | / the packet to its destination. :
: via standard | |_ In this figure, the foreign :
: IP routing. +----+ agent is the mobile node's :
: |host| default router. :
: +----+ :
: :
: Figure 1: Packet Delivery for Mobile Nodes Away from Home :
:...................................................................:
Figure one illustrates packet routing to/from a mobile node away from
home, once the mobile node has registered with its home agent. Shown
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is the mode in which the care-of address is provided by a foreign
agent.
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 there may exist valid reasons in
particular circumstances 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.
silently discard The implementation discards the packet 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 packet,
and SHOULD record the event in a statistics
counter.
1.9. Message Format and Protocol Extensibility
Each message in Mobile IP begins with a short fixed part, followed
by one or more extensions in Type-Length-Value format, with one
exception. That exception is the One-Byte Padding Extension which
has only a Type but no Length and no Data fields.
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There are two separately maintained sets of numbering spaces from
which extension Type values are allocated. The first set defines
those extensions which may only appear in ICMP Router Advertisement
messages. The second set defines those extensions which may only
appear in Mobile IP messages (those sent to and from UDP port number
434). Each individual extension is described in detail within the
section of this document that is applicable.
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
applies to ICMP Router Advertisement and the other applies to Mobile
IP registration.
When an extension numbered in either of these sets within the range
0-127 is encountered but not recognized, the packet containing the
extension must be dropped. When an extension numbered in the range
128-255 is encountered which is not recognized, that particular
extension is ignored, but the rest of the extensions and packet data
can still be processed. The Length field of the extension is used to
skip the Data field in searching for the next extension.
Below is the list of extensions that are currently defined within
each respective set. Up-to-date values for extensions to ICMP Router
Advertisement and to Mobile IP messages are specified in the most
recent "Assigned Numbers" [21].
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 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
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Type
The following extension Type values are allocated from the set
of extensions to ICMP Router Advertisement:
0 One-byte Padding (has no Length nor Data field)
16 Mobile Service
19 Prefix-Lengths
The following extension Type values are allocated from the set
of extensions to Mobile IP messages (e.g. registration):
32 Mobile-Home Authentication
33 Mobile-Foreign Authentication
34 Foreign-Home Authentication
All other values in these respective sets are currently
undefined (see [21] for the most up-to-date lists).
Length
Indicates the length (in bytes) of the data field. The length
does not include the Type and Length bytes.
Data
This field is zero or more bytes in length and contains the
value(s) for this extension. The format and length of the data
field is determined by the type and length fields.
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.
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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 a foreign
network. When on a foreign network, the methods specified in this
section allow the mobile node to determine the care-of address being
offered by foreign agents on that network.
Mobile IP extends ICMP Router Discovery [7] as its primary mechanism
for Agent Discovery. An Agent Advertisement is formed by appending
one or more of the extensions defined in this section to an ICMP
Router Advertisement. An Agent Solicitation is identical to an ICMP
Router Solicitation. 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 which 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 [2], which is external to the messages
described here. Further specification of the way that advertisement
and solicitation messages are authenticated is outside of the scope
of this document.
2.1. Agent Advertisement
Agent Advertisements are periodic transmissions sent by a mobility
agent. Mobile nodes use these advertisements to determine their
current point of attachment to the Internet. Agent advertisements,
as specified in this document, are ICMP Router Advertisements
that have been modified to carry the Mobile Service Extension and,
optionally, the Prefix-Lengths Extension or future extensions that
might be defined.
The following fields within the ICMP Router Advertisement portion of
the Agent Advertisement are further refined as follows:
- Link Layer Fields
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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 Agent Advertisements MUST be set to 1.
Destination Address
As per RFC1256 [7], the IP destination address of an
Agent Advertisement MUST be either the "all systems
on this link" multicast address (224.0.0.1) [6] or the
"Limited broadcast" address (255.255.255.255). This is
because subnet-directed broadcast addresses of the form
<prefix>.<-1> are generally useless to mobile nodes that
are visiting foreign networks. Such mobile nodes will have
a different prefix than that of the advertising agents.
- ICMP Fields
Code
The Code field of the agent advertisement is interpreted as
follows:
0 The mobility agent handles common traffic -- that
is, IP data packets 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 those packets
received from a registered mobile node (see
subsection 4.2.2).
Lifetime
The maximum length of time that the Advertisement is
considered valid in the absence of further Advertisements.
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Router Addresses
See subsection 2.3.1 for a discussion of the addresses that
may appear in this portion of the Agent Advertisement.
The nominal advertisement period as shown in the Lifetime field
SHOULD be 1/3 of the advertisement Lifetime. This allows a mobile
node to miss three successive advertisements before deleting the
agent from its list of valid agents. The actual time of issuance of
advertisements should be slightly randomized to avoid synchronization
and subsequent collisions with other agent advertisements which
may be present on the same medium. Note that this field has no
relation whatsoever to the "Lifetime" field within the Mobile Service
Extension defined below.
The ICMP fields are immediately followed by the Mobile Service
Extension defined in subsection 2.1.1. The Mobile Service
Extension MAY optionally be followed by the Prefix-Lengths
Extension (subsection 2.1.2), the One-byte Padding Extension
(subsection 2.1.3), or future extensions which might be defined.
2.1.1. Mobile Service Extension
The Mobile Service Extension immediately follows the ICMP Router
Advertisement fields. It is used to indicate that an ICMP Router
Advertisement message is actually an Agent Advertisement being sent
by a mobility agent. The Mobile Service 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 advertisement messages sent since
the agent was initialized (see section 2.3.2).
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Lifetime The longest lifetime (measured in seconds)
that the agent is willing to accept in any
registration request. A value of all ones
indicates infinity. This field has nothing
whatsoever to do with the "Lifetime" field
within the ICMP Router Advertisement portion of
the Agent Advertisement described above.
R Foreign agent registration required bit.
B Busy bit. The foreign agent will not accept
more registrations. Only valid if F=1.
H Agent offers service as a home agent.
F Agent offers service as a foreign agent.
M Agent offers minimal encapsulation (see [16]).
G Agent offers GRE encapsulation (see [10]).
V Agent supports Van Jacobson header compression
[11]
reserved Sent as zero; ignored on reception.
Care of Address A foreign agent's care-of address(es). An Agent
Advertisement MUST include at least one care-of
address if F=1.
Any home agent MUST always be prepared to serve its mobile nodes.
Thus, it is an error to have the 'B' bit set without also having the
'F' bit set, since only foreign agents are permitted to be too busy
to service new requests. An agent MUST NOT send Agent Advertisements
with neither the 'F' bit nor the 'H' bit set to one.
When a foreign agent wishes to require registration even from those
mobile nodes which have acquired local, temporary care-of addresses,
it sets the 'R' bit to one. Because this 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 Mobile Service Extension.
It is used to indicate the number of bits of network prefix that
applies to each address listed in the ICMP Router Advertisement
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portion of the Agent Advertisement. Note that the prefix lengths
DO NOT apply to care-of address(es) listed in the Mobile Service
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 The length of this extension excluding the
Type field and the Length field. This field
MUST equal 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 which define the
network number of the corresponding router
address listed in the ICMP Router Advertisement
portion of the message.
See subsection 2.4.2 for information about how to use the Prefix
Lengths extension when determining whether movement has occurred.
See appendix E for implementation details about the use of this
extension.
2.1.3. One-byte Padding Extension
Some kernel implementations insist upon padding ICMP packets 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. In fact,
an Agent Advertisement SHOULD NOT include more than one One-byte
Padding Extension and this extension SHOULD be the last extension
present in an Agent Advertisement. NOTE the absence of both a
"Length" field and a "Data" field in the One-byte Pad Extension.
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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/Home Agent Considerations
Any mobility agent which is not indicated by a link-layer protocol
MUST send Agent Advertisements. An agent which is indicated 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 solicitation. All mobility
agents SHOULD respond to Agent Solicitations.
The same procedures, defaults, and constants are used in agent
advertisements as described in RFC 1256 [7], 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
- a mobility agent that receives a Router Solicitation does 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.)
The home agent for a given mobile node SHOULD be located on the link
identified by the home address, if the home network is not merely a
virtual network. In this case, the home agent MUST send out agent
advertisements with the 'H' bit set, so that mobile nodes on their
home network will be able to determine that they are indeed at 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
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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 (see subsection 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). A foreign agent MUST route packets it receives from
registered mobile nodes (see subsection 4.2.2).
2.3.2. Sequence Numbers, and Rollover Handling
The sequence number in Agent Advertisements ranges from 0 to
0xffff. After booting, an agent shall use the number 0 for its first
advertisement. Each subsequent advertisement shall use the sequence
number one greater, with the exception that the sequence number
0xffff shall 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 described in RFC 1256 for
router solicitation, except that the mobile node may solicit more
often than once every three seconds and MAX_SOLICITATIONS does not
apply for mobile nodes that are currently unconnected to any foreign
agent.
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,
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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 MAY also
increase the rate at which it will send solicitations when it next
begins searching for a new agent with which to register. In both
cases, 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 mandated by RFC 1256 [7].
Mobile nodes must process Agent Advertisements. Mobile nodes
can distinguish Agent Advertisements from "standard" ICMP Router
Advertisements 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 Mobile Service Extension
identifies the advertisement as an Agent Advertisement.
When multiple methods of agent identification are in use, the
mobile node SHOULD first attempt registration with agents including
Mobile Service Extensions in their advertisements in preference
to those sending link-layer advertisements. This order 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 to 1, the mobile node SHOULD register through the foreign
agent, even when the mobile node might be able to acquire its own
temporary care-of address. This feature allows 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
movement from one subnet to another. Other mechanisms MAY be used.
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When the mobile node detects that it has moved, it SHOULD register
(see section 3) with a suitable care-of address on the new foreign
network (but not too often -- see subsection 3.6.3).
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. 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
the agent and therefore it SHOULD attempt to discover and then
register with other agents.
The second method uses network prefixes. The Prefix-Lengths
Extension MAY be used by mobile nodes to determine whether or not a
newly received Agent Advertisement was received on the same subnet
as the mobile node's current agent. If the prefixes differ, the
mobile node assumes that it has moved. A 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. On the expiration of registration, a mobile node may choose to
register with the care-of addresses received in the most recent agent
advertisement.
2.4.3. Returning Home
If the mobile node detects that it has moved to its home network,
it MUST deregister with its home agent (see section 3). Before
attempting to deregister, the mobile node SHOULD configure
its routing table appropriately for its home network (see
subsection 4.2.1). In addition, if the home network is using
ARP [17], 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 advertisements from its foreign agent, the second of which is
less than the first and inside the range 0 to 255, the mobile node
MUST register again. If the second value is less than the first, but
greater than or equal to 256, the mobile node may assume that the
sequence number has rolled over past its maximum value (0xffff), and
that there is no need to re-register (see subsection 2.3).
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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.
Several other (optional) capabilities are available through the
registration procedure, which enable a mobile node to:
- maintain multiple simultaneous registrations, wherein 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
If a mobile node has associated a care-of address to an interface on
a link over which it has not received Agent Advertisements with the
'R' bit set, or if the mobile node is returning home, the mobile node
registers or deregisters directly with a home agent by the exchange
of only 2 messages:
a) The mobile node sends a registration request to a home agent,
asking it to provide service.
b) The home agent sends a registration reply to the mobile node,
granting or denying service.
If a mobile node is using a care-of address offered by a foreign
agent, the mobile node MUST register via that foreign agent. Even
if a mobile node has associated a care-of address with one of its
own interfaces on a link over which it receives Agent Advertisements
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with the 'R' bit set, the mobile node SHOULD register via one of the
foreign agents on that link. In these cases, the foreign agent acts
as a relay between the mobile node and the home agent. This extended
registration process requires 4 messages:
a) The mobile node sends a registration request to the
prospective foreign agent to begin the registration process.
b) The foreign agent relays the request to the home agent,
asking the home agent to register the mobile node at the
foreign agent's care-of address.
c) The home agent sends a registration reply to the foreign
agent to grant or deny service.
d) The foreign agent relays the registration reply to the mobile
node to inform it of the disposition of its request.
The registration messages defined in subsections 3.3 and 3.4 use the
User Datagram Protocol (UDP) header [18]. A nonzero UDP checksum
SHOULD be included in the header, and checked by each 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 IP address. See section 5.1 for requirements for support
of authentication algorithms. Registration messages between mobile
node and home agent MUST be authenticated with the Mobile-Home
Authentication Extension (subsection 3.5.1). This extension
immediately follows all non-authentication extensions, except those
foreign agent-specific extensions which may be added to the packet
after the mobile node computes the authentication.
3.3. Registration Request
A mobile node sends a registration request message so that its home
agent can create or modify a mobility binding for that mobile node
(with a new lifetime). The request may be relayed to the home agent
by the foreign agent from which the mobile node is accepting service,
or it may be sent directly in case the mobile node has received a
care-of address by some other means (e.g, DHCP [8]).
IP fields:
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Source Address Typically the interface address from which
the packet is sent.
Destination Address Typically that of the foreign agent or the
home agent.
See subsections 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|rsvd | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Agent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Care-of Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
Type 1 (Registration Request)
S If the 'S' bit is set, the mobile node is
requesting that the home agent retain its
prior mobility bindings, as described in
subsection 3.6.1.2.
B If the 'B' bit is set, the mobile node requests
that the home agent send to it broadcasts on the
home network, as described in subsection 4.3.
D 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
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has assocated a care-of address with one of its
own interfaces.
M If the 'M' bit is set, the mobile node asks its
home agent to use minimal encapsulation [16].
G If the 'G' bit is set, the mobile node asks its
home agent to use GRE encapsulation [10].
rsvd 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 all ones indicates infinity.
Home Address The IP address of the mobile node.
Home Agent The IP address of a home agent.
Care-of Address The IP address for the end of a tunnel.
Identification A 64-bit number, constructed by the mobile node,
useful for matching requests with replies, and
for protecting against replay attacks (see
subsections 5.4, 5.6).
Extensions The fixed portion of the Registration Request
is followed by one or more of the extensions
listed in subsection 3.5. The Mobile-Home
Authentication Extension MUST be included in
all Registration Requests. See the sections on
mobile node, and foreign agent considerations
(3.6.1.3 and 3.7.2.2) for ordering rules on
extensions.
3.4. Registration Reply
A mobility agent returns a registration reply message to a mobile
node which has sent a registration request (subsection 3.3) message.
If the mobile node is accepting 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.
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Mobility agents MUST NOT increase the lifetime selected by the mobile
node in the registration request. If the lifetime of the reply is
greater than the original request, the excess time MUST be ignored.
When the lifetime of the reply is smaller than the original request,
another registration SHOULD occur before the smaller lifetime
expires.
IP fields:
Source Typically copied from the destination address of the
Registration Request to which the agent is replying.
See subsections 3.7.2.3 and 3.8.3.1 for complete
details.
Destination 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
(subsection 3.7.1).
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 One of the following codes:
0 registration accepted
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1 registration accepted; simultaneous mobility
bindings unsupported
Service 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 VJ compression unavailable
80 home network unreachable (ICMP error)
81 home agent host unreachable (ICMP error)
82 home agent port unreachable (ICMP error)
88 home agent unreachable (other ICMP error)
Service 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 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" [21].
Lifetime The seconds remaining before the registration is
considered expired. A value of zero confirms a
request for deregistration. A value of all ones
indicates infinity.
Home Address The IP address of the mobile node.
Home Agent The IP address of a home agent.
Identification The registration identification is copied from
the request message, for use by the mobile
node in matching its reply with an outstanding
request.
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Extensions The fixed portion of the Registration Reply
is followed by one or more of the extensions
listed in subsection 3.5. The Mobile-Home
Authentication Extension MUST be included in all
Registration Replies returned by the home agent.
See the sections on foreign agent and home agent
considerations (3.7.2.2 and 3.8.3.3) for the
ordering rules on extensions.
3.5. Registration Extensions
Each authenticator required in the authentication extensions which
follow is defined to be a value computed from a stream of bytes
including:
- the shared secret,
- the UDP payload (that is, the registration request or reply data),
- all prior extensions in their entirety, and
- the type and length of this extension,
but not including the Authenticator field itself nor the UDP header.
See subsection 5.1 for information about support requirements for
message authentication codes, etc. which are to be used with the
various authentication extensions.
3.5.1. Mobile-Home Authentication Extension
This extension must be present in all registration requests and
replies, and is intended to eliminate problems [3] which result from
the uncontrolled propagation of remote redirects in the Internet.
The location of the authentication 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 The number of data bytes in the extension.
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SPI Security Parameter Index. (4 bytes) An opaque
identifier other than zero (reserved).
Authenticator (variable length) (see 3.5)
3.5.2. Mobile-Foreign Authentication Extension
This extension may be found in registration requests and replies
where a security association exists between the mobile node and a
foreign agent. See subsection 5.1 for information about support
requirements for message authentication codes, etc.
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
SPI Security Parameter Index. (4 bytes) An opaque
identifier other than zero (reserved).
Length The number of data bytes in the extension.
Authenticator (variable length) (see 3.5)
3.5.3. Foreign-Home Authentication Extension
This extension may be found in registration requests and replies
where a security association exists between the foreign agent and
a home agent. See subsection 5.1 for information about support
requirements for message authentication codes, etc.
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
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SPI Security Parameter Index. (4 bytes) An opaque
identifier other than zero (reserved).
Length The number of data bytes in the extension.
Authenticator (variable length) (see 3.5)
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.
For each pending registration, the mobile node maintains the
following:
- link-layer address of foreign agent, if applicable
- Care-of Address
- registration Identification
- registration Lifetime
A mobile node initiates registration whenever it detects a change in
its network connectivity. See section 2.4.2 for methods by which
mobile nodes can 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. When it is at home, the mobile
node's (de)Registration Request allows its home agent to erase any
previous mobility binding(s). 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 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 do not necessarily
affect a current registration. 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 register with a new foreign agent because
it has received an ICMP Redirect from the foreign agent that is
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currently providing service to it. Within these constraints, the
mobile node MAY register again at any time.
Please refer to the examples in appendix D to see how the fields in
registration messages would be set up in some typical registration
scenarios.
3.6.1. Sending Registration Requests
The following sections provide additional detail 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 mobility agents
pick values for the IP header fields of a Registration Request.
IP Source Address:
- When registering on a foreign network with a care-of address
associated to one of its own interfaces, 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 IP address is unknown (e.g., the agent was discovered
via a link-layer protocol), the "All Mobility Agents" multicast
address (224.0.0.11) MUST be used. In such a case, the mobile
node SHOULD 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.
- When registering directly with the home agent, the destination
address is set to the (unicast) address that the mobile node uses
for its home agent.
- If the mobile node is connected to its home network or if the
mobile node is registering without a foreign agent, and the mobile
node is attempting to perform home agent discovery, then the
IP destination address is set to the subnet-directed broadcast
address of the home network. This address MUST NOT be used as the
destination IP address if the mobile node is registering via a
foreign agent.
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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 moves
within range of multiple cellular systems. IP explicitly allows
duplication of datagrams. When the home agent allows simultaneous
bindings, it will encapsulate a separate copy of each arriving
datagram to each care-of address, and the mobile node will receive
multiple copies of its datagrams.
The mobile node SHOULD set the 'D' bit if it is registering with
a care-of address associated with one of its own interfaces.
Otherwise, this bit MUST NOT be set.
A mobile node MAY set the 'B' bit if the mobile node would like to
receive a copy of IP broadcasts on its home network. Note that in
order to "shield" local broadcast packets from nodes on the foreign
network (particularly the foreign agent), the home agent is required
to tunnel broadcasts either directly to a care-of address associated
with one of the mobile node's interfaces (hence the 'D' bit) -or- it
must recursively tunnel such packets first to the mobile node's home
address and then to the (foreign agent-provided) care-of address.
The mobile node must be capable of de-tunneling packets in order to
obtain the original broadcast datagram. For this reason, the 'B' bit
MUST NOT be set unless the mobile node is capable of de-tunneling
packets.
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 packets (with a care-of address associated
with one of its own interfaces) or if its foreign agent has indicated
support for these forms of encapsulation by setting the corresponding
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bits in the Mobile Service Extension of an Agent Advertisement.
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 learned in the Agent
Advertisement. 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.
- The mobile node MAY ask a home agent to terminate forwarding
service to a particular care-of address, by sending a registration
with a Lifetime of zero (see section 3.8.2).
- Similarly, a Lifetime of zero is used when the mobile node
deregisters all care-of addresses upon returning home.
The Home Agent field is set to the address of one of the mobile
node's home agents, if the mobile node possesses this information.
Otherwise, the mobile node MAY discover a home agent by setting this
field to the subnet-directed broadcast address of the home network.
(Each home agent will reject the mobile node's registration, but in
the reply they will provide their unicast address for use by the
mobile node in a future registration attempt).
The Care-of Address field is set to the value of the particular
care-of address that the mobile node wishes to (de)register. In the
special case when a mobile node wishes to deregister all care-of
addresses, it sets this field to the value of the its home address.
A mobile node on its home network need not register again with a home
agent when a change of sequence number occurs, or the advertisement
lifetime expires, or even when the home agent crashes, since it is
not seeking service from the home agent.
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 on replay protection 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
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b) Any non-authentication extensions relevant to the home agent
(which may or may not also be relevant to the foreign agent)
c) The Mobile-Home Authentication Extension
d) Any non-authentication extensions relevant only to the
foreign agent.
e) The Mobile-Foreign Authentication Extension
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 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:
- service request was accepted,
- service request was denied by foreign agent, and
- service request was denied by home agent.
The remainder of this section describes handling by mobile nodes
under these various categories, based upon the contents of the
Registration Reply.
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
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 Registration Reply MUST be checked for authenticity.
That is, the mobile node MUST check for the presence of a valid
authentication extension, based upon the Code field in the Reply.
The rules are as follows:
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a) If the mobile node and foreign agent share a security
association, the mobile node MUST check for a valid
Mobile-Foreign Authentication Extension. If no such
extension is found, the mobile node MUST silently discard the
Reply and the mobile node SHOULD log the event as a security
exception.
b) If the Code field indicates that service is denied by the
home agent, or if the Code field indicates that the service
request was accepted, the mobile node MUST check for a valid
Mobile-Home Authentication Extension. If no such extension
is found, the mobile node MUST silently discard the Reply and
the mobile node 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. Service Request Accepted
If the Code field indicates that service will be provided, the mobile
node SHOULD configure its routing table appropriately for its current
point of attachment (see subsection 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 granted Lifetime expires.
3.6.2.3. Service Request Denied
If the Code field indicates that service is being denied, the mobile
node SHOULD log the error. There are several scenarios under which
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 amount of time for which that foreign
agent is willing to accept registrations. The mobile node MAY
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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. (See section 5.6 for details). 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 home agent discovery as described in
subsections 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 is 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;
thus the retransmission does not count as a new registration (see
subsection 5.6). In this way a retransmission will not require the
home agent to resynchronize with the mobile node by issuing another
nonce.
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 packets, twice the round trip time for transmission at the
link speed, and at least an additional 100 milliseconds to allow
for processing the packets before responding. Some circuits add
another 200 milliseconds of satellite delay. The minimum time
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between Registration Requests MUST NOT be less than 1 second. Each
successive wait SHOULD be at least twice the previous wait, as long
as that is less than the maximum.
3.7. Foreign Agent Considerations
The foreign agent plays a 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 MAY also advertise its presence as
described in subsection 2.3.
The foreign agent MUST NOT originate a Registration Request or Reply
that has not been prompted by the mobile node. The foreign agent
MUST NOT generate a Registration Request or Reply to indicate that
the service Lifetime has expired. A foreign agent MUST NOT originate
a message that asks for deregistration of a mobile node; however, it
MUST relay valid deregistration requests originated by a mobile node.
3.7.1. Configuration and Registration Tables
Each foreign agent must offer 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:
- link-layer source address
- IP Source Address (mobile node's Home Address)
- UDP Source Port
- Home Agent
- Lifetime
- Identification
As with any host on the Internet, a foreign agent may also maintain
a security association for each pending or current registrant,
and use it to authenticate the Registration Requests and Replies
of the mobile node or its home agent (subsections 3.3, 3.4).
The foreign agent may use an available security association
with the home agent to compute the authentication data for the
Foreign-Home Authentication Extension. Even if a foreign agent
implements authentication, it might not use authentication with each
registration, because of the key management difficulties.
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3.7.2. Receiving Registration Requests
If the foreign agent is able to satisfy an incoming Registration
Request, it then relays the Request to the home agent. Otherwise, it
denies the request by sending a Registration Reply to the mobile node
with an appropriate rejection Code. The following sections describe
this behavior in more detail.
If a foreign agent receives a deregistration request from a mobile
node in its visitor list, the visitor list entry SHOULD NOT be purged
until the home agent sends back a Registration Reply with a Code
indicating success.
3.7.2.1. Validity Checks
Registration Requests with an invalid, non-zero UDP checksum MUST be
silently discarded.
Also, the Registration Request MUST be checked for authenticity.
That is, the foreign agent MUST check for the presence of a valid
Mobile-Foreign Authentication Extension if it shares a security
association with the mobile node. If, under these circumstances, no
such extension is found, the foreign agent MAY reject the Request by
sending a Registration Reply to the mobile node with Code 67. The
foreign agent SHOULD do no further processing with such a Request.
3.7.2.2. Forwarding a Valid Request to the Home Agent
If the foreign agent is able to satisfy the mobile node's
Registration Request, it relays the Request to the mobile node's home
agent. The foreign agent MUST NOT modify any of the fields beginning
within 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 MUST perform the following additional
procedures:
- It MUST consume any extensions following the Mobile-Home
Authentication Extension,
- It SHOULD 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 security association with the home agent.
Specific fields within the IP header and the UDP header of the
relayed Registration Request MUST be set as follows:
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IP Source Address
The foreign agent's address on the interface from which the
packet will be sent.
IP Destination Address
Copied from the Home Agent field within the Registration
Request.
UDP Source Port
<variable>
UDP Destination Port
434
3.7.2.3. Denying Invalid Requests
If the foreign agent is unable to satisfy the mobile node's
Registration Request, it SHOULD send the mobile node a Registration
Reply with a suitable rejection 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 a foreign agent detects unknown bits set in the Reserved Bits
field of the registration request, it MUST deny the request with
status code 134. 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 length of time for which it is willing to
accept a registration, 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 packet will be sent) is used.
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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 sends 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 of building
the Registration Reply.
3.7.3.1. Validity Checks
Registration Replies with an invalid, non-zero UDP checksum MUST be
silently discarded.
A Registration Reply MUST be silently discarded if the low-order
32 bits of the Identification field do not match that of a pending
Registration Request.
Also, the Registration Reply MUST be checked for authenticity.
That is, the foreign agent MUST check for the presence of a valid
Foreign-Home Authentication Extension if it shares a security
association with the home agent. If, under these circumstances, no
such extension is found, the foreign agent MAY reject the Request
by sending a Registration Reply to the mobile node with Code
68. The foreign agent MUST NOT perform further processing on the
Reply, though the foreign agent SHOULD log the error as a security
exception.
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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. If the reply contains
a status code indicating that service will be provided, then the
foreign agent updates its visitor list accordingly.
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 consume any extensions following the Mobile-Home
Authentication Extension,
- It SHOULD 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 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 set
forth in section 3.7.2.3.
3.8. Home Agent Considerations
Home agents also play a reactive role in the registration process.
They receive Registration Requests from mobile nodes (perhaps relayed
by a foreign agent), update their mobility bindings appropriately,
and issue suitable Registration Replies in response.
A home agent MUST NOT originate a Registration Reply that has not
been prompted by the mobile node. The home agent MUST NOT generate a
Registration Reply to indicate that the service Lifetime has expired.
3.8.1. Configuration and Registration Tables
Each home agent must have an IP address, and, if the home network is
not a virtual network, the prefix size for the home network. The
home agent must keep track of the home address and mobility security
association of each authorized mobile node. When an authorized
mobile node becomes registered, the home agent will create or modify
its mobility binding list entry containing:
- care-of address
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- registration Identification
- registration Lifetime
The home agent MAY also maintain security associations with
various foreign agents. The home agent may use these security
associations to compute the authentication data for the Foreign-Home
Authentication Extension.
3.8.2. Receiving Registration Requests
If the home agent is able to satisfy an incoming Registration
Request, it then updates the mobile node's mobility binding(s)
and issues a Registration Reply with a suitable Code. Otherwise,
it denies the request by sending a Registration Reply with an
appropriate Code specifying the reason the request was rejected. 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.
Also, the Registration Request MUST be checked for authenticity.
This minimally involves the following operations:
a) The home agent MUST check for the presence of a valid
Mobile-Home Authentication Extension. If no such extension
is found, the home agent MAY reject the Request by sending
a Registration Reply to the mobile node with Code 131. The
home agent MUST do no further processing with such a Request,
though it SHOULD log the error as a security exception.
b) The home agent MUST check that the registration
Identification field is correct under the context selected
by the security parameter index within the Mobile-Home
Authentication Extension. See section 5.6 for a description
of how this is performed. If incorrect, the home agent MAY
reject the Request by sending a Registration Reply to the
mobile node with Code 133, and including an Identification
field computed in accordance with the rules set forth in 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) In addition, the home agent MUST check for the presence of a
valid Foreign-Home Authentication Extension if it shares a
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security association with the foreign agent. If, under these
circumstances, no such extension is found, the home agent MAY
reject the Request by sending a Registration Reply to the
mobile node with Code 132. The home agent MUST do no further
processing with such a Request, but is SHOULD log the error
as a security exception.
In addition to validating the authenticity of a Registration Request,
home agents MUST NOT grant service for 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 MAY
reject such a request by returning status code 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 updates its mobility binding list for the
requesting mobile node and returns 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 of building the Registration Reply
message.
The home agent updates its mobility bindings as follows:
- 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 to one, and the
home agent supports simultaneous mobility bindings, the previous
mobility binding entries remain active. Otherwise, the home agent
removes all previous entries in the mobility binding list for the
mobile node.
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In all cases, the home agent sends 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 security association with
the foreign agent whose care-of address is being deregistered --
wherein said 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 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.
It is not an error for the mobile node to request a Lifetime longer
than the home agent is willing to accept. In such a case, the home
agent simply reduces the Lifetime to a permissible amount and returns
this amount in the Registration Reply. This informs the mobile node
when it should reregister. The home agent MUST NOT increase the
Lifetime above that specified by the mobile node in the Registration
Request.
If the Registration Request duplicates an accepted current
Registration Request, the new Lifetime MUST NOT extend beyond the
Lifetime originally granted.
In addition, if the home network implements ARP [17], 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 subsection 3.8.2.1, or the home agent is unable to accommodate the
request, the home agent returns 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
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.
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See subsection 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 error code 129.
If a home agent detects unknown bits set in the Reserved Bits field
of the registration request, it MUST deny the request with status
code 134.
3.8.3. Sending Registration Replies
If the home agent is able to satisfy an incoming Registration
Request, it then updates the mobile node's mobility binding(s)
and issues a Registration Reply with a suitable Code. Otherwise,
it denies the request by sending a Registration Reply with an
appropriate Code specifying the reason the request was rejected. 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. In such a
case, the home agent's address by which
it is known to the requesting mobile node
is used.
IP Destination Address Copied from the IP Source Address of the
Registration Request.
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.
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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 Type field in a Registration Reply is always set to 3. The Code
field is chosen according to the rules set forth in the previous
sections. When responding to accepted registrations, a home agent
SHOULD respond with Code 1 if it does not support simultaneous
registrations.
The Lifetime field is 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.
The Home Address field is 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 is copied into
the Home Agent field of the Registration Reply. Otherwise, the home
agent supplies its unicast address in the Home Agent field of the
Registration Reply. 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 being simultaneously registered with two or more
home agents.
3.8.3.3. Extensions
This section describes the ordering of any mandatory and any optional
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) Any non-authentication extensions relevant to the mobile node
(which may or may not also be relevant to the foreign agent),
and
c) The Mobile-Home Authentication Extension
d) Any non-authentication extensions relevant only to the
foreign agent.
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e) The Foreign-Home Authentication Extension
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 security association.
4. Routing Considerations
This section describes how mobile nodes, home agents, and (possibly)
foreign agents cooperate to route packets 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.
4.1. Encapsulation Types
Support for IP in IP encapsulation [15] is required in home agents
and foreign agents, and any mobile node which can associate a care-of
address to one of its own interfaces. Minimal encapsulation [16]
and GRE encapsulation [10] are alternate encapsulation methods which
MAY optionally be supported by mobility agents and mobile nodes.
Minimal encapsulation MUST NOT be used when the original datagram is
a fragment. 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 Packet 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 just like
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 care-of address associated to one of its
own interfaces, is outside the scope of this document. ICMP Router
Advertisement [7] is one such method.
When registered on a foreign network, the mobile node chooses a
default router by examining the Agent Advertisements sent by its
foreign agent. The default router is selected by the procedure
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described in RFC1256; that is, the mobile node SHOULD choose as its
default router the highest preference router address listed in the
ICMP Router Advertisement portion of the Agent Advertisement. The
mobile node MAY add the IP source address of the Agent Advertisement
to this (possibly empty) list of router addresses wherein the
IP source address is considered to be the worst choice (lowest
preference) for a default router. In any case, the mobile node MAY
choose its foreign agent as its default router.
Note that Van Jacobson header compression [11] will not function
properly unless all TCP packets to and from the mobile node pass,
respectively, through the same first and last-hop router. The mobile
node, therefore, SHOULD select its foreign agent as its default
router if it performs Van Jacobson header compression with its
foreign agent.
In order for a mobile node to associate a care-of address to one of
its own interfaces, it MUST be able to detunnel packets sent to this
address. The method by which the mobile node obtained its local
care-of address SHOULD also be capable of supplying the mobile node
with the address of a default router.
4.2.2. Foreign Agent Considerations
Upon receipt of an encapsulated packet 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 any 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 naturally
forwards the decapsulated packet 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
(see subsection 4.5).
The foreign agent MUST route packets 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 packets to
a default router. In addition, the foreign agent should send
appropriate ICMP error messages to the mobile node.
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4.2.3. Home Agent Considerations
Packets destined for a mobile node will arrive at a home agent that
advertises connectivity to the home network indicated by the address
of the mobile nodes. The home agent must examine the IP header of
all arriving traffic to see if it contains a destination address
equal to the home address of any of its mobile nodes.
If so, the home agent tunnels the datagram to the mobile node's most
recently registered care-of address. 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 assumes the mobile node is at home and simply forwards
the datagram directly to it. In this case, however, it is likely
that the datagram will never be received by the home agent.
See section 4.1 about methods of encapsulation that may be used for
tunneling. Maintenance of "soft tunnel state" (described in [15])
effectively reduces transmission errors in the tunnel.
If the lifetime for a given mobility binding expires before the home
agent has received another Registration Request, then that binding is
erased from the mobility binding list. No special Registration Reply
is sent to the foreign agent. The entry in its visitor list will
expire naturally, and probably at the same time. When a mobility
binding's lifetime expires, the home agent drops it regardless of
whether or not simultaneous bindings are supported.
Suppose an encapsulated datagram arrives at the home agent, that
is to be delivered to one of its mobile nodes. If the destination
of the inner header is not that same mobile node, the home agent
may recursively encapsulate it for delivery to the mobile node's
care-of address. Otherwise, the home agent may simply alter the
outer destination to the care-of address, unless the care-of address
is the same as the origination point of the encapsulated datagram.
In the latter case, if the home agent receives a datagram for one of
its mobile nodes, and the packet's IP source address is identical to
the care-of address contained in the mobility binding list, the home
agent MUST silently discard that packet. Otherwise, a routing loop
is likely to result.
4.3. Broadcast packets
When a home agent receives a broadcast packet, it may transmit the
packet to only those mobile nodes on its mobility binding list that
have requested broadcast service. Mobile nodes request encapsulated
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delivery of broadcast packets by setting the 'B' bit in their
Registration Request packets (subsection 3.3). It is a matter of
configuration at the home agent as to which specific categories of
broadcast packets will be sent to such mobile nodes.
If the mobile node is using a care-of address associated with one of
its own interfaces, as indicated by the 'D' bit in its Registration
Request packet, the home agent simply tunnels appropriate broadcast
IP datagrams to the mobile node's care-of address. Otherwise, when
the mobile node registered through a foreign agent, 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 foreign agent can determine which
mobile node should receive the packet after it is decapsulated. When
received by the foreign agent, the unicast encapsulated datagram is
detunneled and delivered 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 Packet Routing
As mentioned previously, a mobile node that is connected to its home
network functions just like any other (stationary) 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. Mobile nodes MAY join multicast groups in order to receive
transmissions in one of two ways. First, they 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. The mobile node SHOULD use a care-of address associated with
one of its own interfaces (if it has one) as the source IP address of
its IGMP [6] packets. Otherwise, it MAY use its home address.
Alternatively, a mobile node which wishes to receive multicasts can
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 packets to its home agent and the home agent forwards multicast
packets down the tunnel to the mobile node. The rules for multicast
packet delivery to mobile nodes in this case are identical to those
for broadcast packets (see section 4.3). Namely, the home agent
must tunnel the packet directly to the care-of address associated to
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one of the mobile node's network interfaces, or, the packet must be
tunneled first to the mobile node's home address and then recursively
tunneled to the foreign agent-provided care-of address.
A mobile node which wishes to send packets 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 packets directly on the visited network MUST use
a care-of address which is assigned to one of its own interfaces
as the IP source address. Similarly, a mobile node which tunnels
a multicast packet to its home agent MUST use its home address as
the IP source address of both the (inner) multicast packet and the
(outer) encapsulating packet. This second option assumes that the
home agent is a multicast router.
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
subsection, such networks are called "mobile networks".
A mobile router may provide a care-of address to mobile nodes
connected to the mobile network. In this case, when a correspondent
host sends a packet to the mobile node, the actions described in the
next paragraph should occur.
Normal IP procedures will route the packet addressed to the mobile
node from the correspondent host to the mobile node's home agent.
This home agent's binding for the mobile node causes it to tunnel the
packet to the mobile router. Normal IP procedures will then route
the packet from this home agent to the mobile router's home agent.
That home agent's binding for the mobile router causes the packet
to be doubly tunneled to the mobile router's care-of address. For
the sake of discussion, assume there is a foreign agent available at
that care-of address. The mobile router's foreign agent will then
detunnel the packet and use its visitor list entry to deliver the
packet to the mobile router. The mobile router will then detunnel
the packet and use its visitor list entry to deliver the packet
finally to the mobile node.
If a fixed node is connected to a mobile network then either of two
methods may be used to cause packets from correspondent hosts to be
routed to the fixed node.
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A home agent may be configured that has 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 packets 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 packets.
4.6. ARP, Proxy ARP, and Gratuitous ARP
The use of ARP [17] 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.
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 [20] 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 [17],
but supplies its own link-layer address in the Sender Hardware
Address field. The node receiving the Reply will then associate
the link-layer address of the replying node with the IP address
of the original target node, causing it to transmit future
datagrams for this target node to the node that sent the Proxy
ARP Reply.
- A Gratuitous ARP is an ARP Reply that is sent without having been
prompted by the receipt of any ARP Request. The ARP Reply is
transmitted as a local broadcast packet on the local link. Any
node receiving any ARP packet (Request or Reply) MUST update its
local ARP cache with the sender IP and link-layer address in
the ARP packet, if the receiving node has an entry for that IP
address already in its ARP cache [17]. This requirement in the
ARP protocol applies even if the ARP packet is an ARP Reply that
does not match any ARP Request transmitted by the receiving node.
While a mobile node is registered on a foreign network, its home
agent uses proxy ARP [20] to reply to ARP Requests it receives that
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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 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 [20], 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.
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. The gratuitous ARP
Reply packet is sent on the link on which the mobile node's home
address is located. In this ARP Reply, both the sender and target
IP addresses MUST be set to the home address of the mobile node, and
both the sender and target link-layer addresses MUST be set to the
home agent's link-layer address on its interface to the link on which
the mobile node's home address is located. This ARP Reply 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 link 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 a gratuitous
ARP Reply packet on its home network. This ARP Reply packet MUST be
transmitted as a local broadcast on this link. Both the sender and
target IP addresses MUST be set to the home address of the mobile
node, and both the sender and target link-layer addresses MUST be
set to the mobile node's link-layer address on its interface to
the home network. This ARP Reply packet SHOULD be retransmitted
a small number of times to increase its reliability, but these
retransmissions MAY 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 transmit a gratuitous
ARP Reply packet on the mobile node's home network. This ARP Reply
packet MUST be transmitted as a local broadcast on this link. As
with the gratuitous ARP Reply sent by the mobile node above, both
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the sender and target IP addresses in the home agent's gratuitous
ARP Reply MUST be set to the home address of the mobile node, and
both the sender and target link-layer addresses MUST be set to the
mobile node's link-layer address on its interface to this link. This
ARP Reply packet SHOULD be retransmitted a small number of times to
increase its reliability, but these retransmissions MAY proceed in
parallel with the transmission and processing of its (de)Registration
Reply.
Finally, while the mobile node is away from home, it MUST NOT reply
to ARP Requests that seek its own link-layer 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 already successfully registered.
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 its link-layer address.
- 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.
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- 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.
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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 [22], 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 suffix+prefix mode. The foreign agent SHOULD 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 authentic. Such remote redirection, for
instance as performed by the mobile registration protocol, is widely
understood to be a security problem in the current Internet [3].
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" (see subsection 4.6) brings with it all of the
risks associated with the use of ARP.
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 don't
provide service to users that are not legitimate customers of that
service provider.
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5.4. Picking good random numbers
The strength of any authentication mechanism is dependent 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. [9] 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, packets destined for correspondent
hosts will appear to emanate from the Home Network, and it may be
more difficult to pinpoint the location of the mobile node.
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 here: timestamps (mandatory) and "nonces"
(optional). All mobile nodes and home agents MUST implement
timestamp-based replay protection. These nodes MAY 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 to match Registration Requests
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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 during
the lifetime of the selected security context between the mobile
node and the home agent. Retransmission as in subsection 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 usual all messages
are protected against tampering by a cryptographic checksum.
If timestamps are used, the mobile node sets the Identification
field to a 64-bit value formatted as specified by the Network Time
Protocol [14]. 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.
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 mobile security association.
If the timestamp is valid, the home agent copies the entire
Identification field into the Registration Reply it returns 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.
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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 a cryptographic checksum 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 [9]. 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
the Identification from the registration request message. 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 replay protection. The foreign agent uses
the low-order value to correctly match registration replies with
pending requests (see subsection 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.
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.
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Thanks also to Kannan Alaggapan and Greg Minshall for their
contributions to the group while performing the duties of
chairperson.
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 - also, JI's advisor and early contributor)
- 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 in the latest
drafts is due to Jim Solomon.
Thanks to Greg Minshall (Novell), Phil Karn (Qualcomm), and Frank
Kastenholz (FTP Software) for their generous support in hosting
interim Working Group meetings.
Implementors may note that Anders Klemets has an implementation
of the protocol specified here for mobile nodes, foreign agents,
and home agents running under SunOS v4.1.3. He is willing to
provide it to people wishing to perform beta testing. Contact
him at <klemets@sics.se> if you would like a copy. There
is also a version of mobile-IP which was developed by Vipul
Gupta <vgupta@cs.binghamton.edu> at the State University of
New York Binghamton. The software (along with supporting
documentation) is available from the Linux Mobile-IP home page at
http://anchor.cs.binghamton.edu/~mobileip.
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.
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A.1. IBM Patent #5,159,592
Charles Perkins, editor of this draft, 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 draft, 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, or would not be asserted by IBM to be covered, by the
Perkins Patent or any other patent.
As the sole inventor of the patent, Charles Perkins also feels
honor-bound to submit his opinion (not informed by legal counsel on
the matter, nor by IBM corporate opinion) that the aforementioned
patent is relevant to a wide range of technology involving the action
of home agents assisting the routing of packets for mobile nodes
addressed via a home network.
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. Conversely, timestamp-based
replay protection (section 5.6.1) is a requirement of this
specification.
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B. Link-Layer considerations
The mobile node primarily uses link-layer mechanisms to decide that
its point of attachment has changed. Such indications include
the Down/Testing/Up interface status [12], 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.
B.1. Point-to-Point Link-Layers
The Point-to-Point-Protocol (PPP) [23] and its Internet Protocol
Control Protocol (IPCP) [13], negotiates the use of IP addresses.
The mobile node SHOULD first attempt to specify its home address.
This allows an unrouted link to function correctly.
When the home address is not accepted by the peer, but a transient
IP address is dynamically assigned, that address MAY be used as the
care-of address for registration. When the peer specifies its own IP
address, that address MUST NOT be assumed to be the care-of address
of a foreign agent or the IP address of a home agent.
When router advertisements are received which contain the Mobile
Service Extension, registration with the agent SHOULD take place as
usual. If the link is bandwidth limited, this method is preferred
over use of the transient care-of address. The encapsulation will
be removed by the peer, allowing header compression techniques to
function correctly [11].
B.2. Multi-Point Link-Layers
Another link establishment protocol, IEEE 802.11 [1], might yield the
link address of an agent. This link-layer address SHOULD be used to
attempt registration.
The receipt of an agent's address via a router advertisement
supersedes that obtained via IEEE 802.11.
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
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in use, the default TCP timer values in many systems may cause
retransmissions or timeouts, even when the link and network is
actually operating properly with greater than usual delays because
of the medium in use. This can cause an inability to create or
maintain 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
Mobility nodes are likely to use media which have low bandwidth and
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. Now, when
a packet is dropped, the correspondent's TCP implementation is
likely to react as if there were a source of network congestion,
and initiate the slow-start mechanisms [5] 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. [4];
there is no easy solution available, and certainly no solution likely
to be installed soon on all correspondents. While this problem has
nothing to do with any of the specifications in 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.
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D. Example Scenarios
This section shows example Registration Requests for several common
scenarios.
D.1. Registering with a Foreign Agent's Care-of Address
The mobile node receives an Agent Advertisement from a foreign agent
and wishes to register with that agent. The mobile node wishes only
standard 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 Lifetime copied from the Mobile Service
Extension of the Agent Advertisement
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 Mobile
Service Extension of the Agent Advertisement
Identification = Network Time Protocol timestamp or Nonce
Extensions:
The Mobile-Home Authentication Extension
D.2. Registering with a Dynamic 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 for
use as its care-of address. The mobile node supports all forms of
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encapsulation, desires a copy of all broadcasts 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
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
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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 many 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.
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 to another 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".
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References
[1] Draft Standard, Wireless LAN MAC and PHY Specifications, Rev.
D1. IEEE Document P802.11/D1-94/12, Dec 1994.
[2] R. Atkinson. IP Authentication Header. RFC 1826, August 1995.
[3] S.M. Bellovin. Security Problems in the TCP/IP Protocol Suite.
ACM Computer Communications Review, 19(2), March 1989.
[4] Ramon Caceres and Liviu Iftode. The Effects of Mobility on
Reliable Transport Protocols. In Proceedings of the 14th
International Conference on Distributed Computing Systems, June
1994.
[5] Douglas E. Comer. Principles, Protocols, and Architecture,
volume 1 of Internetworking with TCP/IP. Prentice Hall,
Englewood Cliffs, N.J., second edition, 1991.
[6] S. Deering. Host Extensions for IP Multicasting. RFC 1112,
August 1989.
[7] S. Deering. Router Discovery. RFC 1256, September 1991.
[8] R. Droms. Dynamic Host Configuration Protocol. RFC 1541,
October 1993.
[9] D.E. Eastlake, S.D. Crocker, and J.I. Schiller. Randomness
Requirements for Security. RFC 1750, December 1994.
[10] S. Hanks, T. Li, D. Farinacci, and P. Traina. Generic Routing
Encapsulation (GRE). RFC 1701, October 1994.
[11] V. Jacobson. Compressing TCP/IP Headers for Low-Speed Serial
Links. RFC 1144, February 1990.
[12] K. McCloghrie and F. Kastenholz. Evolution of the Interfaces
Group MIP-II. RFC 1573, January 1994.
[13] G. McGregor. The PPP Internet Procotol Control Protocol (IPCP).
RFC 1332, May 1992.
[14] D. Mills. Network Time Protocol (Version 3). RFC 1305, March
1992.
[15] C. Perkins. IP Encapsulation within IP. Internet Draft -- work
in progress, October 1995.
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[16] C. Perkins. Minimal Encapsulation within IP. Internet Draft --
work in progress, July 1995.
[17] D. Plummer. An Ethernet Address Resolution Protocol. RFC 826,
November 1982.
[18] J. Postel. User Datagram Protocol. RFC 768, August 1980.
[19] J. Postel. Internet Protocol. RFC 791, September 1981.
[20] J. Postel. Multi-LAN Address Resolution. RFC 925, October
1984.
[21] J. Reynolds and J. Postel. Assigned Numbers. RFC 1700, October
1994.
[22] R. Rivest. The MD5 Message-Digest Algorithm. RFC 1321, April
1992.
[23] W. Simpson (Editor). The Point-to-Point Protocol (PPP). RFC
1661, July 1994.
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Chair's Addresses
The working group can be contacted via the current chairs:
Jim Solomon Tony Li
Motorola, Inc. cisco systems
1301 E. Algonquin Rd. 170 W. Tasman Dr.
Schaumburg, IL 60196 San Jose, CA 95134
Work: +1-708-576-2753 Work: +1-408-526-8186
E-mail: solomon@comm.mot.com E-mail: tli@cisco.com
Editor's Address
Questions about this memo can also be directed to:
Charles Perkins
Room J1-A25
T. J. Watson Research Center
IBM Corporation
30 Saw Mill River Rd.
Hawthorne, NY 10532
Work: +1-914-784-7350
Fax: +1-914-784-7007
E-mail: perk@watson.ibm.com
Perkins, editor Expires 21 June 1996 [Page 68]
