agent is denying service to the mobile node. In particular, a
foreign agent MUST NOT generate a Registration Request or Reply
because a mobile node's registration Lifetime has expired. A foreign
agent also MUST NOT originate a Registration Request message that
asks for deregistration of a mobile node; however, it MUST relay
valid (de)Registration Requests originated by a mobile node.
3.7.1. Configuration and Registration Tables
Each foreign agent MUST be configured with a care-of address. In
addition, for each pending or current registration the foreign agent
MUST maintain a visitor list entry containing the following
information obtained from the mobile node's Registration Request:
- the link-layer source address of the mobile node
- the IP Source Address (the mobile node's Home Address) or its
co-located care-of address (see description of the 'R' bit in
section 2.1.1)
- the IP Destination Address (as specified in 3.6.1.1)
- the UDP Source Port
- the Home Agent address
- the Identification field
- the requested registration Lifetime, and
- the remaining Lifetime of the pending or current registration.
If the mobile node's Home Address is zero in the Registration Request
message, then the foreign agent MUST follow the procedures specified
in RFC2794 [6]. In particular, if the foreign agent cannot manage
pending registration request records with such a zero Home Address
for the mobile node, the foreign agent MUST return a Registration
Reply with Code indicating NONZERO_HOMEADDR_REQD (see [6]).
The foreign agent MAY configure a maximum number of pending
registrations that it is willing to maintain (typically 5).
Additional registrations SHOULD then be rejected by the foreign agent
with code 66. The foreign agent MAY delete any pending Registration
Request after the request has been pending for more than 7 seconds;
in this case, the foreign agent SHOULD reject the Request with code
78 (registration timeout).
As with any node on the Internet, a foreign agent MAY also share
mobility security associations with any other nodes. When relaying a
Registration Request from a mobile node to its home agent, if the
foreign agent shares a mobility security association with the home
agent, it MUST add a Foreign-Home Authentication Extension to the
Request and MUST check the required Foreign-Home Authentication
Extension in the Registration Reply from the home agent (Sections 3.3
and 3.4). Similarly, when receiving a Registration Request from a
mobile node, if the foreign agent shares a mobility security
association with the mobile node, it MUST check the required Mobile-
Foreign Authentication Extension in the Request and MUST add a
Mobile-Foreign Authentication Extension to the Registration Reply to
the mobile node.
3.7.2. Receiving Registration Requests
If the foreign agent accepts a Registration Request from a mobile
node, it checks to make sure that the indicated home agent address
does not belong to any network interface of the foreign agent. If
not, the foreign agent then MUST relay the Request to the indicated
home agent. Otherwise, if the foreign agent denies the Request, it
MUST send a Registration Reply to the mobile node with an appropriate
denial Code, except in cases where the foreign agent would be
required to send out more than one such denial per second to the same
mobile node. The following sections describe this behavior in more
detail.
If the foreign agent has configured one of its network interfaces
with the IP address specified by the mobile node as its home agent
address, the foreign agent MUST NOT forward the request again. If
the foreign agent serves the mobile node as a home agent, the foreign
agent follows the procedures specified in section 3.8.2. Otherwise,
if the foreign agent does not serve the mobile node as a home agent,
the foreign agent rejects the Registration Request with code 136
(unknown home agent address).
If a foreign agent receives a Registration Request from a mobile node
in its visitor list, the existing visitor list entry for the mobile
node SHOULD NOT be deleted or modified until the foreign agent
receives a valid Registration Reply from the home agent with a Code
indicating success. The foreign agent MUST record the new pending
Request as a separate part of the existing visitor list entry for the
mobile node. If the Registration Request requests deregistration,
the existing visitor list entry for the mobile node SHOULD NOT be
deleted until the foreign agent has received a successful
Registration Reply. If the Registration Reply indicates that the
Request (for registration or deregistration) was denied by the home
agent, the existing visitor list entry for the mobile node MUST NOT
be modified as a result of receiving the Registration Reply.
3.7.2.1. Validity Checks
Registration Requests with an invalid, non-zero UDP checksum MUST be
silently discarded. Requests with non-zero bits in reserved fields
MUST be rejected with code 70 (poorly formed request). Requests with
the 'D' bit set to 0, and specifying a care-of address not offered by
the foreign agent, MUST be rejected with code 77 (invalid care-of
address).
Also, the authentication in the Registration Request MUST be checked.
If the foreign agent and the mobile node share a mobility security
association, exactly one Mobile-Foreign Authentication Extension MUST
be present in the Registration Request, and the foreign agent MUST
check the Authenticator value in the Extension. If no Mobile-Foreign
Authentication Extension is found, or if more than one Mobile-Foreign
Authentication Extension is found, or if the Authenticator is
invalid, the foreign agent MUST silently discard the Request and
SHOULD log the event as a security exception. The foreign agent also
SHOULD send a Registration Reply to the mobile node with Code 67.
3.7.2.2. Forwarding a Valid Request to the Home Agent
If the foreign agent accepts the mobile node's Registration Request,
it MUST relay the Request to the mobile node's home agent as
specified in the Home Agent field of the Registration Request. The
foreign agent MUST NOT modify any of the fields beginning with the
fixed portion of the Registration Request up through and including
the Mobile-Home Authentication Extension or other authentication
extension supplied by the mobile node as an authorization-enabling
extension for the home agent. Otherwise, an authentication failure
is very likely to occur at the home agent. In addition, the foreign
agent proceeds as follows:
- It MUST process and remove any Extensions following the
Mobile-Home Authentication Extension,
- It MAY append any of its own non-authentication Extensions of
relevance to the home agent, if applicable, and
- It MUST append the Foreign-Home Authentication Extension, if
the foreign agent shares a mobility security association with
the home agent.
Specific fields within the IP header and the UDP header of the
relayed Registration Request MUST be set as follows:
IP Source Address
The foreign agent's address on the interface from which
the message will be sent.
IP Destination Address
Copied from the Home Agent field within the Registration
Request.
UDP Source Port
<variable>
UDP Destination Port
434
After forwarding a valid Registration Request to the home agent, the
foreign agent MUST begin timing the remaining lifetime of the pending
registration based on the Lifetime in the Registration Request. If
this lifetime expires before receiving a valid Registration Reply,
the foreign agent MUST delete its visitor list entry for this pending
registration.
3.7.2.3. Denying Invalid Requests
If the foreign agent denies the mobile node's Registration Request
for any reason, it SHOULD send the mobile node a Registration Reply
with a suitable denial Code. In such a case, the Home Address, Home
Agent, and Identification fields within the Registration Reply are
copied from the corresponding fields of the Registration Request.
If the Reserved field is nonzero, the foreign agent MUST deny the
Request and SHOULD return a Registration Reply with status code 70 to
the mobile node. If the Request is being denied because the
requested Lifetime is too long, the foreign agent sets the Lifetime
in the Reply to the maximum Lifetime value it is willing to accept in
any Registration Request, and sets the Code field to 69. Otherwise,
the Lifetime SHOULD be copied from the Lifetime field in the Request.
Specific fields within the IP header and the UDP header of the
Registration Reply MUST be set as follows:
IP Source Address
Copied from the IP Destination Address of Registration
Request, unless the "All Agents Multicast" address was
used. In this case, the foreign agent's address (on the
interface from which the message will be sent) MUST be
used.
IP Destination Address
If the Registration Reply is generated by the Foreign
Agent in order to reject a mobile node's Registration
Request, and the Registration Request contains a Home
Address which is not 0.0.0.0, then the IP Destination
Address is copied from the Home Address field of the
Registration Request. Otherwise, if the Registration
Reply is received from the Home Agent, and contains a
Home Address which is not 0.0.0.0, then the IP
Destination Address is copied from the Home Address field
of the Registration Reply. Otherwise, the IP Destination
Address of the Registration Reply is set to be
255.255.255.255.
UDP Source Port
434
UDP Destination Port
Copied from the UDP Source Port of the Registration
Request.
3.7.3. Receiving Registration Replies
The foreign agent updates its visitor list when it receives a valid
Registration Reply from a home agent. It then relays the
Registration Reply to the mobile node. The following sections
describe this behavior in more detail.
If upon relaying a Registration Request to a home agent, the foreign
agent receives an ICMP error message instead of a Registration Reply,
then the foreign agent SHOULD send to the mobile node a Registration
Reply with an appropriate "Home Agent Unreachable" failure Code
(within the range 80-95, inclusive). See Section 3.7.2.3 for details
on building the Registration Reply.
3.7.3.1. Validity Checks
Registration Replies with an invalid, non-zero UDP checksum MUST be
silently discarded.
When a foreign agent receives a Registration Reply message, it MUST
search its visitor list for a pending Registration Request with the
same mobile node home address as indicated in the Reply. If no such
pending Request is found, and if the Registration Reply does not
correspond with any pending Registration Request with a zero mobile
node home address (see section 3.7.1), the foreign agent MUST
silently discard the Reply. The foreign agent MUST also silently
discard the Reply if the low-order 32 bits of the Identification
field in the Reply do not match those in the Request.
Also, the authentication in the Registration Reply MUST be checked.
If the foreign agent and the home agent share a mobility security
association, exactly one Foreign-Home Authentication Extension MUST
be present in the Registration Reply, and the foreign agent MUST
check the Authenticator value in the Extension. If no Foreign-Home
Authentication Extension is found, or if more than one Foreign-Home
Authentication Extension is found, or if the Authenticator is
invalid, the foreign agent MUST silently discard the Reply and SHOULD
log the event as a security exception. The foreign agent also MUST
reject the mobile node's registration and SHOULD send a Registration
Reply to the mobile node with Code 68.
3.7.3.2. Forwarding Replies to the Mobile Node
A Registration Reply which satisfies the validity checks of Section
3.8.2.1 is relayed to the mobile node. The foreign agent MUST also
update its visitor list entry for the mobile node to reflect the
results of the Registration Request, as indicated by the Code field
in the Reply. If the Code indicates that the home agent has accepted
the registration and the Lifetime field is nonzero, the foreign agent
SHOULD set the Lifetime in the visitor list entry to the minimum of
the following two values:
- the value specified in the Lifetime field of the Registration
Reply, and
- the foreign agent's own maximum value for allowable
registration lifetime.
If, instead, the Code indicates that the Lifetime field is zero, the
foreign agent MUST delete its visitor list entry for the mobile node.
Finally, if the Code indicates that the registration was denied by
the home agent, the foreign agent MUST delete its pending
registration list entry, but not its visitor list entry, for the
mobile node.
The foreign agent MUST NOT modify any of the fields beginning with
the fixed portion of the Registration Reply up through and including
the Mobile-Home Authentication Extension. Otherwise, an
authentication failure is very likely to occur at the mobile node.
In addition, the foreign agent SHOULD perform the following
additional procedures:
- It MUST process and remove any Extensions following the
Mobile-Home Authentication Extension,
- It MAY append its own non-authentication Extensions of
relevance to the mobile node, if applicable, and
- It MUST append the Mobile-Foreign Authentication Extension, if
the foreign agent shares a mobility security association with
the mobile node.
Specific fields within the IP header and the UDP header of the
relayed Registration Reply are set according to the same rules
specified in Section 3.7.2.3.
After forwarding a valid Registration Reply to the mobile node, the
foreign agent MUST update its visitor list entry for this
registration as follows. If the Registration Reply indicates that
the registration was accepted by the home agent, the foreign agent
resets its timer of the lifetime of the registration to the Lifetime
granted in the Registration Reply; unlike the mobile node's timing of
the registration lifetime as described in Section 3.6.2.2, the
foreign agent considers this lifetime to begin when it forwards the
Registration Reply message, ensuring that the foreign agent will not
expire the registration before the mobile node does. On the other
hand, if the Registration Reply indicates that the registration was
rejected by the home agent, the foreign agent deletes its visitor
list entry for this attempted registration.
3.8. Home Agent Considerations
Home agents play a reactive role in the registration process. The
home agent receives Registration Requests from the mobile node
(perhaps relayed by a foreign agent), updates its record of the
mobility bindings for this mobile node, and issues a suitable
Registration Reply in response to each.
A home agent MUST NOT transmit a Registration Reply except when
replying to a Registration Request received from a mobile node. In
particular, the home agent MUST NOT generate a Registration Reply to
indicate that the Lifetime has expired.
3.8.1. Configuration and Registration Tables
Each home agent MUST be configured with an IP address and with the
prefix size for the home network. The home agent MUST be configured
with the mobility security association of each authorized mobile node
that it is serving as a home agent.
When the home agent accepts a valid Registration Request from a
mobile node that it serves as a home agent, the home agent MUST
create or modify the entry for this mobile node in its mobility
binding list containing:
- the mobile node's home address
- the mobile node's care-of address
- the Identification field from the Registration Reply
- the remaining Lifetime of the registration
The home agent MAY optionally offer the capability to dynamically
associate a home address to a mobile node upon receiving a
Registration Request from that mobile node. The method by which a
home address is allocated to the mobile node is beyond the scope of
this document, but see [6]. After the home agent makes the
association of the home address to the mobile node, the home agent
MUST put that home address into the Home Address field of the
Registration Reply.
The home agent MAY also maintain mobility security associations with
various foreign agents. When receiving a Registration Request from a
foreign agent, if the home agent shares a mobility security
association with the foreign agent, the home agent MUST check the
Authenticator in the required Foreign-Home Authentication Extension
in the message, based on this mobility security association.
Similarly, when sending a Registration Reply to a foreign agent, if
the home agent shares a mobility security association with the
foreign agent, the home agent MUST include a Foreign-Home
Authentication Extension in the message, based on this mobility
security association.
3.8.2. Receiving Registration Requests
If the home agent accepts an incoming Registration Request, it MUST
update its record of the the mobile node's mobility binding(s) and
SHOULD send a Registration Reply with a suitable Code. Otherwise
(the home agent denies the Request), it SHOULD send a Registration
Reply with an appropriate Code specifying the reason the Request was
denied. The following sections describe this behavior in more
detail. If the home agent does not support broadcasts (see section
4.3), it MUST ignore the 'B' bit (as opposed to rejecting the
Registration Request).
3.8.2.1. Validity Checks
Registration Requests with an invalid, non-zero UDP checksum MUST be
silently discarded by the home agent.
The authentication in the Registration Request MUST be checked. This
involves the following operations:
a) The home agent MUST check for the presence of an
authorization-enabling extension, and perform the indicated
authentication. Exactly one authorization-enabling extension
MUST be present in the Registration Request; and the home agent
MUST either check the Authenticator value in the extension or
verify that the authenticator value has been checked by another
agent with which it has a security association. If no
authorization-enabling extension is found, or if more than one
authorization-enabling extension is found, or if the
Authenticator is invalid, the home agent MUST reject the mobile
node's registration and SHOULD send a Registration Reply to the
mobile node with Code 131. The home agent MUST then discard
the Request and SHOULD log the error as a security exception.
b) The home agent MUST check that the registration Identification
field is correct using the context selected by the SPI within
the authorization-enabling extension. See Section 5.7 for a
description of how this is performed. If incorrect, the home
agent MUST reject the Request and SHOULD send a Registration
Reply to the mobile node with Code 133, including an
Identification field computed in accordance with the rules
specified in Section 5.7. The home agent MUST do no further
processing with such a Request, though it SHOULD log the error
as a security exception.
c) If the home agent shares a mobility security association with
the foreign agent, the home agent MUST check for the presence
of a valid Foreign-Home Authentication Extension. Exactly one
Foreign-Home Authentication Extension MUST be present in the
Registration Request in this case, and the home agent MUST
check the Authenticator value in the Extension. If no
Foreign-Home Authentication Extension is found, or if more than
one Foreign-Home Authentication Extension is found, or if the
Authenticator is invalid, the home agent MUST reject the mobile
node's registration and SHOULD send a Registration Reply to the
mobile node with Code 132. The home agent MUST then discard
the Request and SHOULD log the error as a security exception.
In addition to checking the authentication in the Registration
Request, home agents MUST deny Registration Requests that are sent to
the subnet-directed broadcast address of the home network (as opposed
to being unicast to the home agent). The home agent MUST discard the
Request and SHOULD returning a Registration Reply with a Code of 136.
In this case, the Registration Reply will contain the home agent's
unicast address, so that the mobile node can re-issue the
Registration Request with the correct home agent address.
Note that some routers change the IP destination address of a
datagram from a subnet-directed broadcast address to 255.255.255.255
before injecting it into the destination subnet. In this case, home
agents that attempt to pick up dynamic home agent discovery requests
by binding a socket explicitly to the subnet-directed broadcast
address will not see such packets. Home agent implementors should be
prepared for both the subnet-directed broadcast address and
255.255.255.255 if they wish to support dynamic home agent discovery.
3.8.2.2. Accepting a Valid Request
If the Registration Request satisfies the validity checks in Section
3.8.2.1, and the home agent is able to accommodate the Request, the
home agent MUST update its mobility binding list for the requesting
mobile node and MUST return a Registration Reply to the mobile node.
In this case, the Reply Code will be either 0 if the home agent
supports simultaneous mobility bindings, or 1 if it does not. See
Section 3.8.3 for details on building the Registration Reply message.
The home agent updates its record of the mobile node's mobility
bindings as follows, based on the fields in the Registration Request:
- If the Lifetime is zero and the Care-of Address equals the
mobile node's home address, the home agent deletes all of the
entries in the mobility binding list for the requesting mobile
node. This is how a mobile node requests that its home agent
cease providing mobility services.
- If the Lifetime is zero and the Care-of Address does not equal
the mobile node's home address, the home agent deletes only the
entry containing the specified Care-of Address from the
mobility binding list for the requesting mobile node. Any
other active entries containing other care-of addresses will
remain active.
- If the Lifetime is nonzero, the home agent adds an entry
containing the requested Care-of Address to the mobility
binding list for the mobile node. If the 'S' bit is set and
the home agent supports simultaneous mobility bindings, the
previous mobility binding entries are retained. Otherwise, the
home agent removes all previous entries in the mobility binding
list for the mobile node.
In all cases, the home agent MUST send a Registration Reply to the
source of the Registration Request, which might indeed be a different
foreign agent than that whose care-of address is being
(de)registered. If the home agent shares a mobility security
association with the foreign agent whose care-of address is being
deregistered, and that foreign agent is different from the one which
relayed the Registration Request, the home agent MAY additionally
send a Registration Reply to the foreign agent whose care-of address
is being deregistered. The home agent MUST NOT send such a Reply if
it does not share a mobility security association with the foreign
agent. If no Reply is sent, the foreign agent's visitor list will
expire naturally when the original Lifetime expires.
The home agent MUST NOT increase the Lifetime above that specified by
the mobile node in the Registration Request. However, it is not an
error for the mobile node to request a Lifetime longer than the home
agent is willing to accept. In this case, the home agent simply
reduces the Lifetime to a permissible value and returns this value in
the Registration Reply. The Lifetime value in the Registration Reply
informs the mobile node of the granted lifetime of the registration,
indicating when it SHOULD re-register in order to maintain continued
service. After the expiration of this registration lifetime, the
home agent MUST delete its entry for this registration in its
mobility binding list.
If the Registration Request duplicates an accepted current
Registration Request, the new Lifetime MUST NOT extend beyond the
Lifetime originally granted. A Registration Request is a duplicate
if the home address, care-of address, and Identification fields all
equal those of an accepted current registration.
In addition, if the home network implements ARP [36], and the
Registration Request asks the home agent to create a mobility binding
for a mobile node which previously had no binding (the mobile node
was previously assumed to be at home), then the home agent MUST
follow the procedures described in Section 4.6 with regard to ARP,
proxy ARP, and gratuitous ARP. If the mobile node already had a
previous mobility binding, the home agent MUST continue to follow the
rules for proxy ARP described in Section 4.6.
3.8.2.3. Denying an Invalid Request
If the Registration Reply does not satisfy all of the validity checks
in Section 3.8.2.1, or the home agent is unable to accommodate the
Request, the home agent SHOULD return a Registration Reply to the
mobile node with a Code that indicates the reason for the error. If
a foreign agent was involved in relaying the Request, this allows the
foreign agent to delete its pending visitor list entry. Also, this
informs the mobile node of the reason for the error such that it may
attempt to fix the error and issue another Request.
This section lists a number of reasons the home agent might reject a
Request, and provides the Code value it should use in each instance.
See Section 3.8.3 for additional details on building the Registration
Reply message.
Many reasons for rejecting a registration are administrative in
nature. For example, a home agent can limit the number of
simultaneous registrations for a mobile node, by rejecting any
registrations that would cause its limit to be exceeded, and
returning a Registration Reply with error code 135. Similarly, a
home agent may refuse to grant service to mobile nodes which have
entered unauthorized service areas by returning a Registration Reply
with a Code of 129.
Requests with non-zero bits in reserved fields MUST be rejected with
code 134 (poorly formed request).
3.8.3. Sending Registration Replies
If the home agent accepts a Registration Request, it then MUST update
its record of the mobile node's mobility binding(s) and SHOULD send a
Registration Reply with a suitable Code. Otherwise (the home agent
has denied the Request), it SHOULD send a Registration Reply with an
appropriate Code specifying the reason the Request was denied. The
following sections provide additional detail for the values the home
agent MUST supply in the fields of Registration Reply messages.
3.8.3.1. IP/UDP Fields
This section provides the specific rules by which home agents pick
values for the IP and UDP header fields of a Registration Reply.
IP Source Address
Copied from the IP Destination Address of Registration
Request, unless a multicast or broadcast address was
used. If the IP Destination Address of the Registration
Request was a broadcast or multicast address, the IP
Source Address of the Registration Reply MUST be set to
the home agent's (unicast) IP address.
IP Destination Address
Copied from the IP Source Address of the Registration
Request.
UDP Source Port
Copied from the UDP Destination Port of the Registration
Request.
UDP Destination Port
Copied from the UDP Source Port of the Registration
Request.
When sending a Registration Reply in response to a Registration
Request that requested deregistration of the mobile node (the
Lifetime is zero and the Care-of Address equals the mobile node's
home address) and in which the IP Source Address was also set to the
mobile node's home address (this is the normal method used by a
mobile node to deregister when it returns to its home network), the
IP Destination Address in the Registration Reply will be set to the
mobile node's home address, as copied from the IP Source Address of
the Request.
In this case, when transmitting the Registration Reply, the home
agent MUST transmit the Reply directly onto the home network as if
the mobile node were at home, bypassing any mobility binding list
entry that may still exist at the home agent for the destination
mobile node. In particular, for a mobile node returning home after
being registered with a care-of address, if the mobile node's new
Registration Request is not accepted by the home agent, the mobility
binding list entry for the mobile node will still indicate that
datagrams addressed to the mobile node should be tunneled to the
mobile node's registered care-of address; when sending the
Registration Reply indicating the rejection of this Request, this
existing binding list entry MUST be ignored, and the home agent MUST
transmit this Reply as if the mobile node were at home.
3.8.3.2. Registration Reply Fields
This section provides the specific rules by which home agents pick
values for the fields within the fixed portion of a Registration
Reply.
The Code field of the Registration Reply is chosen in accordance with
the rules specified in the previous sections. When replying to an
accepted registration, a home agent SHOULD respond with Code 1 if it
does not support simultaneous registrations.
The Lifetime field MUST be copied from the corresponding field in the
Registration Request, unless the requested value is greater than the
maximum length of time the home agent is willing to provide the
requested service. In such a case, the Lifetime MUST be set to the
length of time that service will actually be provided by the home
agent. This reduced Lifetime SHOULD be the maximum Lifetime allowed
by the home agent (for this mobile node and care-of address).
If the Home Address field of the Registration Request is nonzero, it
MUST be copied into the Home Address field of the Registration Reply
message. Otherwise, if the Home Address field of the Registration
Request is zero as specified in section 3.6, the home agent SHOULD
arrange for the selection of a home address for the mobile node, and
insert the selected address into the Home Address field of the
Registration Reply message. See [6] for further relevant details in
the case where mobile nodes identify themselves using an NAI instead
of their IP home address.
If the Home Agent field in the Registration Request contains a
unicast address of this home agent, then that field MUST be copied
into the Home Agent field of the Registration Reply. Otherwise, the
home agent MUST set the Home Agent field in the Registration Reply to
its unicast address. In this latter case, the home agent MUST reject
the registration with a suitable code (e.g., Code 136) to prevent the
mobile node from possibly being simultaneously registered with two or
more home agents.
3.8.3.3. Extensions
This section describes the ordering of any required and any optional
Mobile IP Extensions that a home agent appends to a Registration
Reply. The following ordering MUST be followed:
a) The IP header, followed by the UDP header, followed by the
fixed-length portion of the Registration Reply,
b) If present, any non-authentication Extensions used by the
mobile node (which may or may not also be used by the foreign
agent),
c) The Mobile-Home Authentication Extension,
d) If present, any non-authentication Extensions used only by the
foreign agent, and
e) The Foreign-Home Authentication Extension, if present.
Note that items (a) and (c) MUST appear in every Registration Reply
sent by the home agent. Items (b), (d), and (e) are optional.
However, item (e) MUST be included when the home agent and the
foreign agent share a mobility security association.
4. Routing Considerations
This section describes how mobile nodes, home agents, and (possibly)
foreign agents cooperate to route datagrams to/from mobile nodes that
are connected to a foreign network. The mobile node informs its home
agent of its current location using the registration procedure
described in Section 3. See the protocol overview in Section 1.7 for
the relative locations of the mobile node's home address with respect
to its home agent, and the mobile node itself with respect to any
foreign agent with which it might attempt to register.
4.1. Encapsulation Types
Home agents and foreign agents MUST support tunneling datagrams using
IP in IP encapsulation [32]. Any mobile node that uses a co-located
care-of address MUST support receiving datagrams tunneled using IP in
IP encapsulation. Minimal encapsulation [34] and GRE encapsulation
[16] are alternate encapsulation methods which MAY optionally be
supported by mobility agents and mobile nodes. The use of these
alternative forms of encapsulation, when requested by the mobile
node, is otherwise at the discretion of the home agent.
4.2. Unicast Datagram Routing
4.2.1. Mobile Node Considerations
When connected to its home network, a mobile node operates without
the support of mobility services. That is, it operates in the same
way as any other (fixed) host or router. The method by which a
mobile node selects a default router when connected to its home
network, or when away from home and using a co-located care-of
address, is outside the scope of this document. ICMP Router
Advertisement [10] is one such method.
When registered on a foreign network, the mobile node chooses a
default router by the following rules:
- If the mobile node is registered using a foreign agent care-of
address, it MAY use its foreign agent as a first-hop router.
The foreign agent's MAC address can be learned from Agent
Advertisement. Otherwise, the mobile node MUST choose its
default router from among the Router Addresses advertised in
the ICMP Router Advertisement portion of that Agent
Advertisement message.
- If the mobile node is registered directly with its home agent
using a co-located care-of address, then the mobile node SHOULD
choose its default router from among those advertised in any
ICMP Router Advertisement message that it receives for which
its externally obtained care-of address and the Router Address
match under the network prefix. If the mobile node's
externally obtained care-of address matches the IP source
address of the Agent Advertisement under the network prefix,
the mobile node MAY also consider that IP source address as
another possible choice for the IP address of a default router.
The network prefix MAY be obtained from the Prefix-Lengths
Extension in the Router Advertisement, if present. The prefix
MAY also be obtained through other mechanisms beyond the scope
of this document.
While they are away from the home network, mobile nodes MUST NOT
broadcast ARP packets to find the MAC address of another Internet
node. Thus, the (possibly empty) list of Router Addresses from the
ICMP Router Advertisement portion of the message is not useful for
selecting a default router, unless the mobile node has some means not
involving broadcast ARP and not specified within this document for
obtaining the MAC address of one of the routers in the list.
Similarly, in the absence of unspecified mechanisms for obtaining MAC
addresses on foreign networks, the mobile node MUST ignore redirects
to other routers on foreign networks.
4.2.2. Foreign Agent Considerations
Upon receipt of an encapsulated datagram sent to its advertised
care-of address, a foreign agent MUST compare the inner destination
address to those entries in its visitor list. When the destination
does not match the address of any mobile node currently in the
visitor list, the foreign agent MUST NOT forward the datagram without
modifications to the original IP header, because otherwise a routing
loop is likely to result. The datagram SHOULD be silently discarded.
ICMP Destination Unreachable MUST NOT be sent when a foreign agent is
unable to forward an incoming tunneled datagram. Otherwise, the
foreign agent forwards the decapsulated datagram to the mobile node.
The foreign agent MUST NOT advertise to other routers in its routing
domain, nor to any other mobile node, the presence of a mobile router
(Section 4.5) or mobile node in its visitor list.
The foreign agent MUST route datagrams it receives from registered
mobile nodes. At a minimum, this means that the foreign agent must
verify the IP Header Checksum, decrement the IP Time To Live,
recompute the IP Header Checksum, and forward such datagrams to a
default router.
A foreign agent MUST NOT use broadcast ARP for a mobile node's MAC
address on a foreign network. It may obtain the MAC address by
copying the information from an Agent Solicitation or a Registration
Request transmitted from a mobile node. A foreign agent's ARP cache
for the mobile node's IP address MUST NOT be allowed to expire before
the mobile node's visitor list entry expires, unless the foreign
agent has some way other than broadcast ARP to refresh its MAC
address associated with the mobile node's IP address.
Each foreign agent SHOULD support the mandatory features for reverse
tunneling [27].
4.2.3. Home Agent Considerations
The home agent MUST be able to intercept any datagrams on the home
network addressed to the mobile node while the mobile node is
registered away from home. Proxy and gratuitous ARP MAY be used in
enabling this interception, as specified in Section 4.6.
The home agent must examine the IP Destination Address of all
arriving datagrams to see if it is equal to the home address of any
of its mobile nodes registered away from home. If so, the home agent
tunnels the datagram to the mobile node's currently registered care-
of address or addresses. If the home agent supports the optional
capability of multiple simultaneous mobility bindings, it tunnels a
copy to each care-of address in the mobile node's mobility binding
list. If the mobile node has no current mobility bindings, the home
agent MUST NOT attempt to intercept datagrams destined for the mobile
node, and thus will not in general receive such datagrams. However,
if the home agent is also a router handling common IP traffic, it is
possible that it will receive such datagrams for forwarding onto the
home network. In this case, the home agent MUST assume the mobile
node is at home and simply forward the datagram directly onto the
home network.
For multihomed home agents, the source address in the outer IP header
of the encapsulated datagram MUST be the address sent to the mobile
node in the home agent field of the registration reply. That is, the
home agent cannot use the the address of some other network interface
as the source address.
See Section 4.1 regarding methods of encapsulation that may be used
for tunneling. Nodes implementing tunneling SHOULD also implement
the "tunnel soft state" mechanism [32], which allows ICMP error
messages returned from the tunnel to correctly be reflected back to
the original senders of the tunneled datagrams.
Home agents MUST decapsulate packets addressed to themselves, sent by
a mobile node for the purpose of maintaining location privacy, as
described in Section 5.5. This feature is also required for support
of reverse tunneling [27].
If the Lifetime for a given mobility binding expires before the home
agent has received another valid Registration Request for that mobile
node, then that binding is deleted from the mobility binding list.
The home agent MUST NOT send any Registration Reply message simply
because the mobile node's binding has expired. The entry in the
visitor list of the mobile node's current foreign agent will expire
naturally, probably at the same time as the binding expired at the
home agent. When a mobility binding's lifetime expires, the home
agent MUST delete the binding, but it MUST retain any other (non-
expired) simultaneous mobility bindings that it holds for the mobile
node.
When a home agent receives a datagram, intercepted for one of its
mobile nodes registered away from home, the home agent MUST examine
the datagram to check if it is already encapsulated. If so, special
rules apply in the forwarding of that datagram to the mobile node:
- If the inner (encapsulated) Destination Address is the same as
the outer Destination Address (the mobile node), then the home
agent MUST also examine the outer Source Address of the
encapsulated datagram (the source address of the tunnel). If
this outer Source Address is the same as the mobile node's
current care-of address, the home agent MUST silently discard
that datagram in order to prevent a likely routing loop. If,
instead, the outer Source Address is NOT the same as the mobile
node's current care-of address, then the home agent SHOULD
forward the datagram to the mobile node. In order to forward
the datagram in this case, the home agent MAY simply alter the
outer Destination Address to the care-of address, rather than
re-encapsulating the datagram.
- Otherwise (the inner Destination Address is NOT the same as the
outer Destination Address), the home agent SHOULD encapsulate
the datagram again (nested encapsulation), with the new outer
Destination Address set equal to the mobile node's care-of
address. That is, the home agent forwards the entire datagram
to the mobile node in the same way as any other datagram
(encapsulated already or not).
4.3. Broadcast Datagrams
When a home agent receives a broadcast datagram, it MUST NOT forward
the datagram to any mobile nodes in its mobility binding list other
than those that have requested forwarding of broadcast datagrams. A
mobile node MAY request forwarding of broadcast datagrams by setting
the 'B' bit in its Registration Request message (Section 3.3). For
each such registered mobile node, the home agent SHOULD forward
received broadcast datagrams to the mobile node, although it is a
matter of configuration at the home agent as to which specific
categories of broadcast datagrams will be forwarded to such mobile
nodes.
If the 'D' bit was set in the mobile node's Registration Request
message, indicating that the mobile node is using a co-located care-
of address, the home agent simply tunnels appropriate broadcast IP
datagrams to the mobile node's care-of address. Otherwise (the 'D'
bit was NOT set), the home agent first encapsulates the broadcast
datagram in a unicast datagram addressed to the mobile node's home
address, and then tunnels this encapsulated datagram to the foreign
agent. This extra level of encapsulation is required so that the
foreign agent can determine which mobile node should receive the
datagram after it is decapsulated. When received by the foreign
agent, the unicast encapsulated datagram is detunneled and delivered
to the mobile node in the same way as any other datagram. In either
case, the mobile node must decapsulate the datagram it receives in
order to recover the original broadcast datagram.
4.4. Multicast Datagram Routing
As mentioned previously, a mobile node that is connected to its home
network functions in the same way as any other (fixed) host or
router. Thus, when it is at home, a mobile node functions
identically to other multicast senders and receivers. This section
therefore describes the behavior of a mobile node that is visiting a
foreign network.
In order to receive multicasts, a mobile node MUST join the multicast
group in one of two ways. First, a mobile node MAY join the group
via a (local) multicast router on the visited subnet. This option
assumes that there is a multicast router present on the visited
subnet. If the mobile node is using a co-located care-of address, it
SHOULD use this address as the source IP address of its IGMP [11]
messages. Otherwise, it MAY use its home address.
Alternatively, a mobile node which wishes to receive multicasts MAY
join groups via a bi-directional tunnel to its home agent, assuming
that its home agent is a multicast router. The mobile node tunnels
IGMP messages to its home agent and the home agent forwards multicast
datagrams down the tunnel to the mobile node. For packets tunneled
to the home agent, the source address in the IP header SHOULD be the
mobile node's home address.
The rules for multicast datagram delivery to mobile nodes in this
case are identical to those for broadcast datagrams (Section 4.3).
Namely, if the mobile node is using a co-located care-of address (the
'D' bit was set in the mobile node's Registration Request), then the
home agent SHOULD tunnel the datagram to this care-of address;
otherwise, the home agent MUST first encapsulate the datagram in a
unicast datagram addressed to the mobile node's home address and then
MUST tunnel the resulting datagram (nested tunneling) to the mobile
node's care-of address. For this reason, the mobile node MUST be
capable of decapsulating packets sent to its home address in order to
receive multicast datagrams using this method.
A mobile node that wishes to send datagrams to a multicast group also
has two options: (1) send directly on the visited network; or (2)
send via a tunnel to its home agent. Because multicast routing in
general depends upon the IP source address, a mobile node which sends
multicast datagrams directly on the visited network MUST use a co-
located care-of address as the IP source address. Similarly, a
mobile node which tunnels a multicast datagram to its home agent MUST
use its home address as the IP source address of both the (inner)
multicast datagram and the (outer) encapsulating datagram. This
second option assumes that the home agent is a multicast router.
4.5. Mobile Routers
A mobile node can be a router that is responsible for the mobility of
one or more entire networks moving together, perhaps on an airplane,
a ship, a train, an automobile, a bicycle, or a kayak. The nodes
connected to a network served by the mobile router may themselves be
fixed nodes or mobile nodes or routers. In this document, such
networks are called "mobile networks".
A mobile router MAY act as a foreign agent and provide a foreign
agent care-of address to mobile nodes connected to the mobile
network. Typical routing to a mobile node via a mobile router in
this case is illustrated by the following example:
a) A laptop computer is disconnected from its home network and
later attached to a network port in the seat back of an
aircraft. The laptop computer uses Mobile IP to register on
this foreign network, using a foreign agent care-of address
discovered through an Agent Advertisement from the aircraft's
foreign agent.
b) The aircraft network is itself mobile. Suppose the node
serving as the foreign agent on the aircraft also serves as the
default router that connects the aircraft network to the rest
of the Internet. When the aircraft is at home, this router is
attached to some fixed network at the airline's headquarters,
which is the router's home network. While the aircraft is in
flight, this router registers from time to time over its radio
link with a series of foreign agents below it on the ground.
This router's home agent is a node on the fixed network at the
airline's headquarters.
c) Some correspondent node sends a datagram to the laptop
computer, addressing the datagram to the laptop's home address.
This datagram is initially routed to the laptop's home network.
d) The laptop's home agent intercepts the datagram on the home
network and tunnels it to the laptop's care-of address, which
in this example is an address of the node serving as router and
foreign agent on the aircraft. Normal IP routing will route
the datagram to the fixed network at the airline's
headquarters.
e) The aircraft router and foreign agent's home agent there
intercepts the datagram and tunnels it to its current care-of
address, which in this example is some foreign agent on the