Network Working Group C. Perkins, Editor
Request for Comments: 2002 IBM
Category: Standards Track October 1996 IP Mobility Support
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
This document specifies protocol enhancements that allow transparent
routing of IP datagrams to mobile nodes in the Internet. Each mobile
node is always identified by its home address, regardless of its
current point of attachment to the Internet. While situated away
from its home, a mobile node is also associated with a care-of
address, which provides information about its current point of
attachment to the Internet. The protocol provides for registering
the care-of address with a home agent. The home agent sends
datagrams destined for the mobile node through a tunnel to the care-
of address. After arriving at the end of the tunnel, each datagram
is then delivered to the mobile node.
Table of Contents
1. Introduction 31.1. Protocol Requirements . . . . . . . . . . . . . . . . . . 31.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 41.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 41.5. New Architectural Entities . . . . . . . . . . . . . . . 51.6. Terminology . . . . . . . . . . . . . . . . . . . . . . . 61.7. Protocol Overview . . . . . . . . . . . . . . . . . . . . 81.8. Specification Language . . . . . . . . . . . . . . . . . 111.9. Message Format and Protocol Extensibility . . . . . . . . 122. Agent Discovery 142.1. Agent Advertisement . . . . . . . . . . . . . . . . . . . 142.1.1. Mobility Agent Advertisement Extension . . . . . 162.1.2. Prefix-Lengths Extension . . . . . . . . . . . . 182.1.3. One-byte Padding Extension . . . . . . . . . . . 192.2. Agent Solicitation . . . . . . . . . . . . . . . . . . . 192.3. Foreign Agent and Home Agent Considerations . . . . . . . 192.3.1. Advertised Router Addresses . . . . . . . . . . . 20
A. Patent Issues 72A.1. IBM Patent #5,159,592 . . . . . . . . . . . . . . . . . . 72A.2. IBM Patent #5,148,479 . . . . . . . . . . . . . . . . . . 72B. Link-Layer Considerations 73C. TCP Considerations 73C.1. TCP Timers . . . . . . . . . . . . . . . . . . . . . . . 73C.2. TCP Congestion Management . . . . . . . . . . . . . . . . 73D. Example Scenarios 74D.1. Registering with a Foreign Agent Care-of Address . . . . 74D.2. Registering with a Co-Located Care-of Address . . . . . . 75D.3. Deregistration . . . . . . . . . . . . . . . . . . . . . 76E. Applicability of Prefix Lengths Extension 76
Editor's Address 79
IP version 4 assumes that a node's IP address uniquely identifies the
node's point of attachment to the Internet. Therefore, a node must
be located on the network indicated by its IP address in order to
receive datagrams destined to it; otherwise, datagrams destined to
the node would be undeliverable. For a node to change its point of
attachment without losing its ability to communicate, currently one
of the two following mechanisms must typically be employed:
a) the node must change its IP address whenever it changes its
point of attachment, or
b) host-specific routes must be propagated throughout much of
the Internet routing fabric.
Both of these alternatives are often unacceptable. The first makes
it impossible for a node to maintain transport and higher-layer
connections when the node changes location. The second has obvious
and severe scaling problems, especially relevant considering the
explosive growth in sales of notebook (mobile) computers.
A new, scalable, mechanism is required for accommodating node
mobility within the Internet. This document defines such a
mechanism, which enables nodes to change their point of attachment to
the Internet without changing their IP address.
1.1. Protocol Requirements
A mobile node must be able to communicate with other nodes after
changing its link-layer point of attachment to the Internet, yet
without changing its IP address.
A mobile node must be able to communicate with other nodes that do
not implement these mobility functions. No protocol enhancements are
required in hosts or routers that are not acting as any of the new
architectural entities introduced in Section 1.5.
All messages used to update another node as to the location of a
mobile node must be authenticated in order to protect against remote
The link by which a mobile node is directly attached to the Internet
may often be a wireless link. This link may thus have a
substantially lower bandwidth and higher error rate than traditional
wired networks. Moreover, mobile nodes are likely to be battery
powered, and minimizing power consumption is important. Therefore,
the number of administrative messages sent over the link by which a
mobile node is directly attached to the Internet should be minimized,
and the size of these messages should be kept as small as is
The protocols defined in this document place no additional
constraints on the assignment of IP addresses. That is, a mobile
node can be assigned an IP address by the organization that owns the
This protocol assumes that mobile nodes will generally not change
their point of attachment to the Internet more frequently than once
This protocol assumes that IP unicast datagrams are routed based on
the destination address in the datagram header (and not, for example,
by source address).
Mobile IP is intended to enable nodes to move from one IP subnet to
another. It is just as suitable for mobility across homogeneous
media as it is for mobility across heterogeneous media. That is,
Mobile IP facilitates node movement from one Ethernet segment to
another as well as it accommodates node movement from an Ethernet
segment to a wireless LAN, as long as the mobile node's IP address
remains the same after such a movement.
One can think of Mobile IP as solving the "macro" mobility management
problem. It is less well suited for more "micro" mobility management
applications -- for example, handoff amongst wireless transceivers,
each of which covers only a very small geographic area. As long as
node movement does not occur between points of attachment on
different IP subnets, link-layer mechanisms for mobility (i.e.,
link-layer handoff) may offer faster convergence and far less
overhead than Mobile IP.
1.5. New Architectural Entities
Mobile IP introduces the following new functional entities:
A host or router that changes its point of attachment from one
network or subnetwork to another. A mobile node may change its
location without changing its IP address; it may continue to
communicate with other Internet nodes at any location using its
(constant) IP address, assuming link-layer connectivity to a
point of attachment is available.
A router on a mobile node's home network which tunnels
datagrams for delivery to the mobile node when it is away from
home, and maintains current location information for the mobile
A router on a mobile node's visited network which provides
routing services to the mobile node while registered. The
foreign agent detunnels and delivers datagrams to the mobile
node that were tunneled by the mobile node's home agent. For
datagrams sent by a mobile node, the foreign agent may serve as
a default router for registered mobile nodes.
A mobile node is given a long-term IP address on a home network.
This home address is administered in the same way as a "permanent" IP
address is provided to a stationary host. When away from its home
network, a "care-of address" is associated with the mobile node and
reflects the mobile node's current point of attachment. The mobile
node uses its home address as the source address of all IP datagrams
that it sends, except where otherwise described in this document for
datagrams sent for certain mobility management functions (e.g., as in
This document frequently uses the following terms:
An advertisement message constructed by attaching a
special Extension to a router advertisement  message.
The termination point of a tunnel toward a mobile node,
for datagrams forwarded to the mobile node while it is
away from home. The protocol can use two different types
of care-of address: a "foreign agent care-of address" is
an address of a foreign agent with which the mobile node
is registered, and a "co-located care-of address" is an
externally obtained local address which the mobile node
has associated with one of its own network interfaces.
A peer with which a mobile node is communicating. A
correspondent node may be either mobile or stationary.
Any network other than the mobile node's Home Network.
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.
A network, possibly virtual, having a network prefix
matching that of a mobile node's home address. Note that
standard IP routing mechanisms will deliver datagrams
destined to a mobile node's Home Address to the mobile
node's Home Network.
Link A facility or medium over which nodes can communicate at
the link layer. A link underlies the network layer.
The address used to identify an endpoint of some
communication over a physical link. Typically, the
Link-Layer address is an interface's Media Access Control
Either a home agent or a foreign agent.
The association of a home address with a care-of address,
along with the remaining lifetime of that association.
Mobility Security Association
A collection of security contexts, between a pair
of nodes, which may be applied to Mobile IP protocol
messages exchanged between them. Each context indicates
an authentication algorithm and mode (Section 5.1), a
secret (a shared key, or appropriate public/private
key pair), and a style of replay protection in use
Node A host or a router.
Nonce A randomly chosen value, different from previous choices,
inserted in a message to protect against replays.
Security Parameter Index (SPI)
An index identifying a security context between a pair
of nodes among the contexts available in the Mobility
Security Association. SPI values 0 through 255 are
reserved and MUST NOT be used in any Mobility Security
Tunnel The path followed by a datagram while it is encapsulated.
The model is that, while it is encapsulated, a datagram
is routed to a knowledgeable decapsulating agent, which
decapsulates the datagram and then correctly delivers it
to its ultimate destination.
A network with no physical instantiation beyond a router
(with a physical network interface on another network).
The router (e.g., a home agent) generally advertises
reachability to the virtual network using conventional
A network other than a mobile node's Home Network, to
which the mobile node is currently connected.
The list of mobile nodes visiting a foreign agent.
1.7. Protocol Overview
The following support services are defined for Mobile IP:
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.
When the mobile node is away from home, it registers
its care-of address with its home agent. Depending on
its method of attachment, the mobile node will register
either directly with its home agent, or through a foreign
agent which forwards the registration to the home agent.
The following steps provide a rough outline of operation of the
Mobile IP protocol:
- Mobility agents (i.e., foreign agents and home agents) advertise
their presence via Agent Advertisement messages (Section 2). A
mobile node may optionally solicit an Agent Advertisement message
from any locally attached mobility agents through an Agent
- A mobile node receives these Agent Advertisements and determines
whether it is on its home network or a foreign network.
- When the mobile node detects that it is located on its home
network, it operates without mobility services. If returning
to its home network from being registered elsewhere, the mobile
node deregisters with its home agent, through exchange of a
Registration Request and Registration Reply message with it.
- When a mobile node detects that it has moved to a foreign
network, it obtains a care-of address on the foreign network.
The care-of address can either be determined from a foreign
agent's advertisements (a foreign agent care-of address), or by
some external assignment mechanism such as DHCP  (a co-located
- The mobile node operating away from home then registers its
new care-of address with its home agent through exchange of a
Registration Request and Registration Reply message with it,
possibly via a foreign agent (Section 3).
- Datagrams sent to the mobile node's home address are intercepted
by its home agent, tunneled by the home agent to the mobile
node's care-of address, received at the tunnel endpoint (either
at a foreign agent or at the mobile node itself), and finally
delivered to the mobile node (Section 4.2.3).
- In the reverse direction, datagrams sent by the mobile node
are generally delivered to their destination using standard IP
routing mechanisms, not necessarily passing through the home
When away from home, Mobile IP uses protocol tunneling to hide a
mobile node's home address from intervening routers between its home
network and its current location. The tunnel terminates at the
mobile node's care-of address. The care-of address must be an
address to which datagrams can be delivered via conventional IP
routing. At the care-of address, the original datagram is removed
from the tunnel and delivered to the mobile node.
Mobile IP provides two alternative modes for the acquisition of a
- A "foreign agent care-of address" is a care-of address provided
by a foreign agent through its Agent Advertisement messages. In
this case, the care-of address is an IP address of the foreign
agent. In this mode, the foreign agent is the endpoint of the
tunnel and, upon receiving tunneled datagrams, decapsulates them
and delivers the inner datagram to the mobile node. This mode
of acquisition is preferred because it allows many mobile nodes
to share the same care-of address and therefore does not place
unnecessary demands on the already limited IPv4 address space.
- A "co-located care-of address" is a care-of address acquired
by the mobile node as a local IP address through some external
means, which the mobile node then associates with one of its own
network interfaces. The address may be dynamically acquired as
a temporary address by the mobile node such as through DHCP ,
or may be owned by the mobile node as a long-term address for its
use only while visiting some foreign network. Specific external
methods of acquiring a local IP address for use as a co-located
care-of address are beyond the scope of this document. When
using a co-located care-of address, the mobile node serves as the
endpoint of the tunnel and itself performs decapsulation of the
datagrams tunneled to it.
The mode of using a co-located care-of address has the advantage that
it allows a mobile node to function without a foreign agent, for
example, in networks that have not yet deployed a foreign agent.
It does, however, place additional burden on the IPv4 address space
because it requires a pool of addresses within the foreign network to
be made available to visiting mobile nodes. It is difficult to
efficiently maintain pools of addresses for each subnet that may
permit mobile nodes to visit.
It is important to understand the distinction between the care-of
address and the foreign agent functions. The care-of address is
simply the endpoint of the tunnel. It might indeed be an address of
a foreign agent (a foreign agent care-of address), but it might
instead be an address temporarily acquired by the mobile node (a co-
located care-of address). A foreign agent, on the other hand, is a
mobility agent that provides services to mobile nodes. See Sections
3.7 and 4.2.2 for additional details.
A home agent MUST be able to attract and intercept datagrams that are
destined to the home address of any of its registered mobile nodes.
Using the proxy and gratuitous ARP mechanisms described in Section
4.6, this requirement can be satisfied if the home agent has a
network interface on the link indicated by the mobile node's home
address. Other placements of the home agent relative to the mobile
node's home location MAY also be possible using other mechanisms for
intercepting datagrams destined to the mobile node's home address.
Such placements are beyond the scope of this document.
Similarly, a mobile node and a prospective or current foreign agent
MUST be able to exchange datagrams without relying on standard IP
routing mechanisms; that is, those mechanisms which make forwarding
decisions based upon the network-prefix of the destination address in
the IP header. This requirement can be satisfied if the foreign
agent and the visiting mobile node have an interface on the same
link. In this case, the mobile node and foreign agent simply bypass
their normal IP routing mechanism when sending datagrams to each
other, addressing the underlying link-layer packets to their
respective link-layer addresses. Other placements of the foreign
agent relative to the mobile node MAY also be possible using other
mechanisms to exchange datagrams between these nodes, but such
placements are beyond the scope of this document.
If a mobile node is using a co-located care-of address (as described
in (b) above), the mobile node MUST be located on the link identified
by the network prefix of this care-of address. Otherwise, datagrams
destined to the care-of address would be undeliverable.
For example, the figure below illustrates the routing of datagrams to
and from a mobile node away from home, once the mobile node has
registered with its home agent. In the figure below, the mobile node
is using a foreign agent care-of address:
2) Datagram is intercepted 3) Datagram is
by home agent and detunneled and
is tunneled to the delivered to the
care-of address. mobile node.
+-----+ +-------+ +------+
|home | =======> |foreign| ------> |mobile|
|agent| | agent | <------ | node |
+-----+ +-------+ +------+
1) Datagram to /|\ /
mobile node | / 4) For datagrams sent by the
arrives on | / mobile node, standard IP
home network | / routing delivers each to its
via standard | |_ destination. In this figure,
IP routing. +----+ the foreign agent is the
|host| mobile node's default router.
1.8. Specification Language
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that
the definition is an absolute requirement of the
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means
that, in some circumstances, valid reasons may exist
to ignore this item, but the full implications must
be understood and carefully weighed before choosing
a different course. Unexpected results may result
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.
The implementation discards the datagram without
further processing, and without indicating an error
to the sender. The implementation SHOULD provide the
capability of logging the error, including the contents
of the discarded datagram, and SHOULD record the event
in a statistics counter.
1.9. Message Format and Protocol Extensibility
Mobile IP defines a set of new control messages, sent with UDP 
using well-known port number 434. Currently, the following two
message types are defined:
1 Registration Request
3 Registration Reply
Up-to-date values for the message types for Mobile IP control
messages are specified in the most recent "Assigned Numbers" .
In addition, for Agent Discovery, Mobile IP makes use of the existing
Router Advertisement and Router Solicitation messages defined for
ICMP Router Discovery .
Mobile IP defines a general Extension mechanism to allow optional
information to be carried by Mobile IP control messages or by ICMP
Router Discovery messages. Each of these Extensions (with one
exception) is encoded in the following Type-Length-Value format:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
| Type | Length | Data ...
Type Indicates the particular type of Extension.
Length Indicates the length (in bytes) of the data field within
this Extension. The length does NOT include the Type and
Data The particular data associated with this Extension. This
field may be zero or more bytes in length. The format
and length of the data field is determined by the type
and length fields.
Extensions allow variable amounts of information to be carried within
each datagram. The end of the list of Extensions is indicated by the
total length of the IP datagram.
Two separately maintained sets of numbering spaces, from which
Extension Type values are allocated, are used in Mobile IP:
- The first set consists of those Extensions which may appear only
in Mobile IP control messages (those sent to and from UDP port
number 434). Currently, the following Types are defined for
Extensions appearing in Mobile IP control messages:
32 Mobile-Home Authentication
33 Mobile-Foreign Authentication
34 Foreign-Home Authentication
- The second set consists of those extensions which may appear only
in ICMP Router Discovery messages . Currently, Mobile IP
defines the following Types for Extensions appearing in ICMP
Router Discovery messages:
0 One-byte Padding (encoded with no Length nor Data field)
16 Mobility Agent Advertisement
Each individual Extension is described in detail in a separate
section later in this document. Up-to-date values for these
Extension Type numbers are specified in the most recent "Assigned
Due to the separation (orthogonality) of these sets, it is
conceivable that two Extensions that are defined at a later date
could have identical Type values, so long as one of the Extensions
may be used only in Mobile IP control messages and the other may be
used only in ICMP Router Discovery messages.
When an Extension numbered in either of these sets within the range 0
through 127 is encountered but not recognized, the message containing
that Extension MUST be silently discarded. When an Extension
numbered in the range 128 through 255 is encountered which is not
recognized, that particular Extension is ignored, but the rest of the
Extensions and message data MUST still be processed. The Length
field of the Extension is used to skip the Data field in searching
for the next Extension.
2. Agent Discovery
Agent Discovery is the method by which a mobile node determines
whether it is currently connected to its home network or to a foreign
network, and by which a mobile node can detect when it has moved from
one network to another. When connected to a foreign network, the
methods specified in this section also allow the mobile node to
determine the foreign agent care-of address being offered by each
foreign agent on that network.
Mobile IP extends ICMP Router Discovery  as its primary mechanism
for Agent Discovery. An Agent Advertisement is formed by including a
Mobility Agent Advertisement Extension in an ICMP Router
Advertisement message (Section 2.1). An Agent Solicitation message
is identical to an ICMP Router Solicitation, except that its IP TTL
MUST be set to 1 (Section 2.2). This section describes the message
formats and procedures by which mobile nodes, foreign agents, and
home agents cooperate to realize Agent Discovery.
Agent Advertisement and Agent Solicitation may not be necessary for
link layers that already provide this functionality. The method by
which mobile nodes establish link-layer connections with prospective
agents is outside the scope of this document (but see Appendix B).
The procedures described below assume that such link-layer
connectivity has already been established.
No authentication is required for Agent Advertisement and Agent
Solicitation messages. They MAY be authenticated using the IP
Authentication Header , which is unrelated to the messages
described in this document. Further specification of the way in
which Advertisement and Solicitation messages may be authenticated is
outside of the scope of this document.
2.1. Agent Advertisement
Agent Advertisements are transmitted by a mobility agent to advertise
its services on a link. Mobile nodes use these advertisements to
determine their current point of attachment to the Internet. An
Agent Advertisement is an ICMP Router Advertisement that has been
extended to also carry an Mobility Agent Advertisement Extension
(Section 2.1.1) and, optionally, a Prefix-Lengths Extension (Section
2.1.2), One-byte Padding Extension (Section 2.1.3), or other
Extensions that might be defined in the future.
Within an Agent Advertisement message, ICMP Router Advertisement
fields of the message are required to conform to the following
- Link-Layer Fields
The link-layer destination address of a unicast
Agent Advertisement MUST be the same as the source
link-layer address of the Agent Solicitation which
prompted the Advertisement.
- IP Fields
TTL The TTL for all Agent Advertisements MUST be set
As specified for ICMP Router Discovery , the IP
destination address of an Agent Advertisement MUST
be either the "all systems on this link" multicast
address (22.214.171.124)  or the "limited broadcast"
address (255.255.255.255). The subnet-directed
broadcast address of the form <prefix>.<-1> cannot be
used since mobile nodes will not generally know the
prefix of the foreign network.
- ICMP Fields
Code The Code field of the agent advertisement is
interpreted as follows:
0 The mobility agent handles common traffic -- that
is, it acts as a router for IP datagrams not
necessarily related to mobile nodes.
16 The mobility agent does not route common traffic.
However, all foreign agents MUST (minimally)
forward to a default router any datagrams received
from a registered mobile node (Section 4.2.2).
The maximum length of time that the Advertisement
is considered valid in the absence of further
See Section 2.3.1 for a discussion of the addresses
that may appear in this portion of the Agent
The number of Router Addresses advertised in this
message. Note that in an Agent Advertisement
message, the number of router addresses specified in
the ICMP Router Advertisement portion of the message
MAY be set to 0. See Section 2.3.1 for details.
If sent periodically, the nominal interval at which Agent
Advertisements are sent SHOULD be 1/3 of the advertisement Lifetime
given in the ICMP header. This allows a mobile node to miss three
successive advertisements before deleting the agent from its list of
valid agents. The actual transmission time for each advertisement
SHOULD be slightly randomized  in order to avoid synchronization
and subsequent collisions with other Agent Advertisements that may be
sent by other agents (or with other Router Advertisements sent by
other routers). Note that this field has no relation to the
"Registration Lifetime" field within the Mobility Agent Advertisement
Extension defined below.
2.1.1. Mobility Agent Advertisement Extension
The Mobility Agent Advertisement Extension follows the ICMP Router
Advertisement fields. It is used to indicate that an ICMP Router
Advertisement message is also an Agent Advertisement being sent by a
mobility agent. The Mobility Agent Advertisement Extension is
defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Type | Length | Sequence Number |
| Registration Lifetime |R|B|H|F|M|G|V| reserved |
| zero or more Care-of Addresses |
| ... |
Length (6 + 4*N), where N is the number of care-of addresses
The count of Agent Advertisement messages sent since the
agent was initialized (Section 2.3.2).
The longest lifetime (measured in seconds) that this
agent is willing to accept in any Registration Request.
A value of 0xffff indicates infinity. This field has no
relation to the "Lifetime" field within the ICMP Router
Advertisement portion of the Agent Advertisement.
R Registration required. Registration with this foreign
agent (or another foreign agent on this link) is required
rather than using a co-located care-of address.
B Busy. The foreign agent will not accept registrations
from additional mobile nodes.
H Home agent. This agent offers service as a home agent
on the link on which this Agent Advertisement message is
F Foreign agent. This agent offers service as a foreign
agent on the link on which this Agent Advertisement
message is sent.
M Minimal encapsulation. This agent implements receiving
tunneled datagrams that use minimal encapsulation .
G GRE encapsulation. This agent implements receiving
tunneled datagrams that use GRE encapsulation .
V Van Jacobson header compression. This agent supports use
of Van Jacobson header compression  over the link
with any registered mobile node.
Sent as zero; ignored on reception.
The advertised foreign agent care-of address(es) provided
by this foreign agent. An Agent Advertisement MUST
include at least one care-of address if the 'F' bit
is set. The number of care-of addresses present is
determined by the Length field in the Extension.
A home agent MUST always be prepared to serve the mobile nodes for
which it is the home agent. A foreign agent may at times be too busy
to serve additional mobile nodes; even so, it must continue to send
Agent Advertisements, so that any mobile nodes already registered
with it will know that they have not moved out of range of the
foreign agent and that the foreign agent has not failed. A foreign
agent may indicate that it is "too busy" to allow new mobile nodes to
register with it, by setting the 'B' bit in its Agent Advertisements.
An Agent Advertisement message MUST NOT have the 'B' bit set if the
'F' bit is not also set, and at least one of the 'F' bit and the 'H'
bit MUST be set in any Agent Advertisement message sent.
When a foreign agent wishes to require registration even from those
mobile nodes which have acquired a co-located care-of address, it
sets the 'R' bit to one. Because this bit applies only to foreign
agents, an agent MUST NOT set the 'R' bit to one unless the 'F' bit
is also set to one.
2.1.2. Prefix-Lengths Extension
The Prefix-Lengths Extension MAY follow the Mobility Agent
Advertisement Extension. It is used to indicate the number of bits
of network prefix that applies to each Router Address listed in the
ICMP Router Advertisement portion of the Agent Advertisement. Note
that the prefix lengths given DO NOT apply to care-of address(es)
listed in the Mobility Agent Advertisement Extension. The Prefix-
Lengths Extension is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Type | Length | Prefix Length | ....
Type 19 (Prefix-Lengths Extension)
Length N, where N is the value of the Num Addrs field in
the ICMP Router Advertisement portion of the Agent
The number of leading bits that define the network number
of the corresponding Router Address listed in the ICMP
Router Advertisement portion of the message. The prefix
length for each Router Address is encoded as a separate
byte, in the order that the Router Addresses are listed
in the ICMP Router Advertisement portion of the message.
See Section 2.4.2 for information about how the Prefix Lengths
Extension MAY be used by a mobile node when determining whether it
has moved. See Appendix E for implementation details about the use
of this Extension.
2.1.3. One-byte Padding Extension
Some IP protocol implementations insist upon padding ICMP messages to
an even number of bytes. If the ICMP length of an Agent
Advertisement is odd, this Extension MAY be included in order to make
the ICMP length even. Note that this Extension is NOT intended to be
a general-purpose Extension to be included in order to word- or
long-align the various fields of the Agent Advertisement. An Agent
Advertisement SHOULD NOT include more than one One-byte Padding
Extension and if present, this Extension SHOULD be the last Extension
in the Agent Advertisement.
Note that unlike other Extensions used in Mobile IP, the One-byte
Padding Extension is encoded as a single byte, with no "Length" nor
"Data" field present. The One-byte Padding Extension is defined as
0 1 2 3 4 5 6 7
| Type |
Type 0 (One-byte Padding Extension)
2.2. Agent Solicitation
An Agent Solicitation is identical to an ICMP Router Solicitation
with the further restriction that the IP TTL Field MUST be set to 1.
2.3. Foreign Agent and Home Agent Considerations
Any mobility agent which cannot be discovered by a link-layer
protocol MUST send Agent Advertisements. An agent which can be
discovered by a link-layer protocol SHOULD also implement Agent
Advertisements. However, the Advertisements need not be sent, except
when the site policy requires registration with the agent (i.e., when
the 'R' bit is set), or as a response to a specific Agent
Solicitation. All mobility agents SHOULD respond to Agent
The same procedures, defaults, and constants are used in Agent
Advertisement messages and Agent Solicitation messages as specified
for ICMP Router Discovery , 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 MUST NOT
require that the IP Source Address is the address of a neighbor
(i.e., an address that matches one of the router's own addresses
on the arrival interface, under the subnet mask associated with
that address of the router).
- a mobility agent MAY be configured to send Agent Advertisements
only in response to an Agent Solicitation message.
If the home network is not a virtual network, then the home agent for
any mobile node SHOULD be located on the link identified by the
mobile node's home address, and Agent Advertisement messages sent by
the home agent on this link MUST have the 'H' bit set. In this way,
mobile nodes on their own home network will be able to determine that
they are indeed at home. Any Agent Advertisement messages sent by
the home agent on another link to which it may be attached (if it is
a mobility agent serving more than one link), MUST NOT have the 'H'
bit set, unless the home agent also serves as a home agent (to other
mobile nodes) on that other link.
If the home network is a virtual network, the home network has no
physical realization external to the home agent itself. In this
case, there is no physical network link on which to send Agent
Advertisement messages advertising the home agent. Mobile nodes for
which this is the home network are always treated as being away from
On a particular subnet, either all mobility agents MUST include the
Prefix-Lengths Extension or all of them MUST NOT include this
Extension. Equivalently, it is prohibited for some agents on a given
subnet to include the Extension but for others not to include it.
Otherwise, one of the move detection algorithms designed for mobile
nodes will not function properly (Section 2.4.2).
2.3.1. Advertised Router Addresses
The ICMP Router Advertisement portion of the Agent Advertisement MAY
contain one or more router addresses. Thus, an agent MAY include one
of its own addresses in the advertisement. A foreign agent MAY
discourage use of this address as a default router by setting the
preference to a low value and by including the address of another
router in the advertisement (with a correspondingly higher
preference). Nevertheless, a foreign agent MUST route datagrams it
receives from registered mobile nodes (Section 4.2.2).
2.3.2. Sequence Numbers and Rollover Handling
The sequence number in Agent Advertisements ranges from 0 to 0xffff.
After booting, an agent MUST use the number 0 for its first
advertisement. Each subsequent advertisement MUST use the sequence
number one greater, with the exception that the sequence number
0xffff MUST be followed by sequence number 256. In this way, mobile
nodes can distinguish reductions in sequence numbers that result from
reboots, from reductions that result in rollover of the sequence
number after it attains the value 0xffff.
2.4. Mobile Node Considerations
Every mobile node MUST implement Agent Solicitation. Solicitations
SHOULD only be sent in the absence of Agent Advertisements and when a
care-of address has not been determined through a link-layer protocol
or other means. The mobile node uses the same procedures, defaults,
and constants for Agent Solicitation as specified for ICMP Router
Solicitation messages , except that the mobile node MAY solicit
more often than once every three seconds, and that a mobile node that
is currently not connected to any foreign agent MAY solicit more
times than MAX_SOLICITATIONS.
The rate at which a mobile node sends Solicitations MUST be limited
by the mobile node. The mobile node MAY send three initial
Solicitations at a maximum rate of one per second while searching for
an agent. After this, the rate at which Solicitations are sent MUST
be reduced so as to limit the overhead on the local link. Subsequent
Solicitations MUST be sent using a binary exponential backoff
mechanism, doubling the interval between consecutive Solicitations,
up to a maximum interval. The maximum interval SHOULD be chosen
appropriately based upon the characteristics of the media over which
the mobile node is soliciting. This maximum interval SHOULD be at
least one minute between Solicitations.
While still searching for an agent, the mobile node MUST NOT increase
the rate at which it sends Solicitations unless it has received a
positive indication that it has moved to a new link. After
successfully registering with an agent, the mobile node SHOULD also
increase the rate at which it will send Solicitations when it next
begins searching for a new agent with which to register. The
increased solicitation rate MAY revert to the maximum rate, but then
MUST be limited in the manner described above. In all cases, the
recommended solicitation intervals are nominal values. Mobile nodes
MUST randomize their solicitation times around these nominal values
as specified for ICMP Router Discovery .
Mobile nodes MUST process received Agent Advertisements. A mobile
node can distinguish an Agent Advertisement message from other uses
of the ICMP Router Advertisement message by examining the number of
advertised addresses and the IP Total Length field. When the IP
total length indicates that the ICMP message is longer than needed
for the number of advertised addresses, the remaining data is
interpreted as one or more Extensions. The presence of a Mobility
Agent Advertisement Extension identifies the advertisement as an
When multiple methods of agent discovery are in use, the mobile node
SHOULD first attempt registration with agents including Mobility
Agent Advertisement Extensions in their advertisements, in preference
to those discovered by other means. This preference maximizes the
likelihood that the registration will be recognized, thereby
minimizing the number of registration attempts.
2.4.1. Registration Required
When the mobile node receives an Agent Advertisement with the 'R' bit
set, the mobile node SHOULD register through the foreign agent, even
when the mobile node might be able to acquire its own co-located
care-of address. This feature is intended to allow sites to enforce
visiting policies (such as accounting) which require exchanges of
2.4.2. Move Detection
Two primary mechanisms are provided for mobile nodes to detect when
they have moved from one subnet to another. Other mechanisms MAY
also be used. When the mobile node detects that it has moved, it
SHOULD register (Section 3) with a suitable care-of address on the
new foreign network. However, the mobile node MUST NOT register more
frequently than once per second on average, as specified in Section
126.96.36.199. Algorithm 1
The first method of move detection is based upon the Lifetime field
within the main body of the ICMP Router Advertisement portion of the
Agent Advertisement. A mobile node SHOULD record the Lifetime
received in any Agent Advertisements, until that Lifetime expires.
If the mobile node fails to receive another advertisement from the
same agent within the specified Lifetime, it SHOULD assume that it
has lost contact with that agent. If the mobile node has previously
received an Agent Advertisement from another agent for which the
Lifetime field has not yet expired, the mobile node MAY immediately
attempt registration with that other agent. Otherwise, the mobile
node SHOULD attempt to discover a new agent with which to register.
188.8.131.52. Algorithm 2
The second method uses network prefixes. The Prefix-Lengths
Extension MAY be used in some cases by a mobile node to determine
whether or not a newly received Agent Advertisement was received on
the same subnet as the mobile node's current care-of address. If the
prefixes differ, the mobile node MAY assume that it has moved. If a
mobile node is currently using a foreign agent care-of address, the
mobile node SHOULD NOT use this method of move detection unless both
the current agent and the new agent include the Prefix-Lengths
Extension in their respective Agent Advertisements; if this Extension
is missing from one or both of the advertisements, this method of
move detection SHOULD NOT be used. Similarly, if a mobile node is
using a co-located care-of address, it SHOULD not use this method of
move detection unless the new agent includes the Prefix-Lengths
Extension in its Advertisement and the mobile node knows the network
prefix of its current co-located care-of address. On the expiration
of its current registration, if this method indicates that the mobile
node has moved, rather than re-registering with its current care-of
address, a mobile node MAY choose instead to register with a the
foreign agent sending the new Advertisement with the different
network prefix. The Agent Advertisement on which the new
registration is based MUST NOT have expired according to its Lifetime
2.4.3. Returning Home
A mobile node can detect that it has returned to its home network
when it receives an Agent Advertisement from its own home agent. If
so, it SHOULD deregister with its home agent (Section 3). Before
attempting to deregister, the mobile node SHOULD configure its
routing table appropriately for its home network (Section 4.2.1). In
addition, if the home network is using ARP , the mobile node MUST
follow the procedures described in Section 4.6 with regard to ARP,
proxy ARP, and gratuitous ARP.
2.4.4. Sequence Numbers and Rollover Handling
If a mobile node detects two successive values of the sequence number
in the Agent Advertisements from the foreign agent with which it is
registered, the second of which is less than the first and inside the
range 0 to 255, the mobile node SHOULD register again. If the second
value is less than the first but is greater than or equal to 256, the
mobile node SHOULD assume that the sequence number has rolled over
past its maximum value (0xffff), and that reregistration is not
necessary (Section 2.3).