RFC 5944
IP Mobility Support for IPv4, Revised
3.7. Foreign Agent Considerations
The foreign agent plays a mostly passive role in Mobile IP registration. It relays Registration Requests between mobile nodes and home agents, and, when it provides the care-of address,
decapsulates datagrams for delivery to the mobile node. It SHOULD also send periodic Agent Advertisement messages to advertise its presence as described in Section 2.3, if not detectable by link-layer means. A foreign agent MUST NOT transmit a Registration Request, unless the request is being relayed from a mobile node to that mobile node's home agent. A foreign agent MUST NOT transmit a Registration Reply except when relaying a Registration Reply received from a mobile node's home agent, or when replying to a Registration Request received from a mobile node in the case in which the foreign agent is denying service to the mobile node. In particular, a foreign agent MUST NOT generate a Registration Request or Reply because a mobile node's registration Lifetime has expired. A foreign agent also MUST NOT originate a Registration Request message that asks for deregistration of a mobile node; however, it MUST relay well-formed (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: o the link-layer source address of the mobile node o 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) o the IP Destination Address (as specified in Section 3.6.1.1) o the UDP Source Port o the home agent address o the Identification field o the requested registration Lifetime, and o the remaining Lifetime of the pending or current registration If there is an NAI extension in the Registration Request message (often, for example, when the mobile node's Home Address is zero), then the foreign agent MUST follow the procedures specified in RFC
2794 [2]. 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 a code indicating NONZERO_HOMEADDR_REQD (see [2]). 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. In this case, when the Registration Reply has nonzero Lifetime, the foreign agent 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 194 (Invalid 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 asks for 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, nonzero Lifetime, 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:
o It MUST process and remove any extensions that do not precede any
authorization-enabling extension,
o It MAY append any of its own non-authentication Extensions of
relevance to the home agent, if applicable, and
o If the foreign agent shares a Mobility Security Association with
the home agent, and the Request has Lifetime != 0, then it MUST
append the Foreign-Home Authentication Extension.
Specific fields within the IP header and the UDP header of the
relayed Registration Request MUST be set as follows:
IP Source Address
The care-of address offered by the foreign agent for the mobile
node sending the Registration Request.
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 the 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 that 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 that 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 there are multiple entries with the same home address, and if the Registration Reply has the Mobile Node NAI extension [2], the foreign agent MUST use the NAI to disambiguate the pending Registration Requests with the same home address. If no matching 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 that 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: o the value specified in the Lifetime field of the Registration Reply, and o 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: o It MUST process and remove any Extensions that are not covered by any authorization-enabling extension, o It MAY append its own non-authentication Extensions that supply information to the mobile node, if applicable, and o 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: o the mobile node's home address o the mobile node's care-of address o the Identification field from the Registration Reply o 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 [2]. 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, unless the Lifetime field equals 0. When processing a Registration Request with Lifetime = 0, the HA MAY skip checking for the presence and validity of a Foreign-Home Authentication Extension. 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 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 in most cases 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 at least one authorization-enabling extension, and ensure that all indicated authentications are carried out. At least 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 the home agent receives a Registration Request from a mobile
node with which it does not have any security association, the
home agent MUST silently discard the Registration Request.
If the home agent receives a Registration Request without any
authorization-enabling extension, the home agent MUST silently
discard the Registration Request.
If the Authenticator is invalid, the home agent MUST reject the
mobile node's registration. Further action to be taken in this
case depends upon whether the Request has a valid Foreign-Home
authentication extension (as follows):
* If there is a valid Foreign-Home authentication extension, the
home agent MUST send a Registration Reply with Code 131.
* Otherwise, if there is no Foreign-Home Security Association,
the home agent MAY send a Registration Reply with Code 131.
If the home agent sends a Registration Reply, it MUST contain
a valid Mobile-Home Authentication Extension. In constructing
the Reply, the home agent SHOULD choose a security association
that is likely to exist in the mobile node; for example, this
may be an older security association or one with a longer
lifetime than the one that the mobile node attempted to use in
its Request. Deployments should take care when updating
security associations to ensure that there is at least one
common security association shared between the mobile node and
home agent. In any case of a failed Authenticator, the home
agent MUST then discard the Request without further processing
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 that the home agent used to
authenticate the mobile node's Registration Request. 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, and this is a Registration Request (has non-zero
Lifetime), 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.
d. If the home agent and the foreign agent do not share a Mobility
Security Association, and the Registration contains a Foreign-
Home Authentication Extension, the home agent MUST 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 return 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 code in the Registration Reply 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:
o 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.
o 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.
o 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 that
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.
When a foreign agent relays a deregistration message containing a
care-of address that it does not own, it MUST NOT add a Foreign-Home
Authentication Extension to that deregistration. See Section 3.5.4
for more details.
The home agent MUST NOT increase the Lifetime above that specified by
the mobile node in the Registration Request. However, it is not an
error for the mobile node to request a Lifetime longer than the home
agent is willing to accept. In this case, the home agent simply
reduces the Lifetime to a permissible value and returns this value in
the Registration Reply. The Lifetime value in the Registration Reply
informs the mobile node of the granted Lifetime of the registration,
indicating when it SHOULD re-register in order to maintain continued
service. After the expiration of this registration Lifetime, the home agent MUST delete its entry for this registration in its mobility binding list. If the Registration Request duplicates an accepted current Registration Request, the new Lifetime MUST NOT extend beyond the Lifetime originally granted. A Registration Request is a duplicate if the home address, care-of address, and Identification fields all equal those of an accepted current registration. In addition, if the home network implements ARP [16], and the Registration Request asks the home agent to create a mobility binding for a mobile node that 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 Request 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 a Code of 135. Similarly, a home agent may refuse to grant service to mobile nodes that 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 in most cases 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 the 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 non-zero, it MUST be copied into the Home Address field of the Registration Reply message. If the home agent cannot support the specified nonzero unicast address in the Home Address field of the Registration Request, then the home agent MUST reject the Registration Request with a Code of 129. 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 [2] 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 [14]. Any mobile node that uses a co-located care-of address MUST support receiving datagrams tunneled using IP in IP encapsulation. Minimal encapsulation [15] and GRE encapsulation [13] are alternate encapsulation methods that 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 [5] is one such method. When registered on a foreign network, the mobile node chooses a default router by the following rules: o 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 the foreign agent's Agent Advertisement message. 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. o 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 [12].
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 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 [14], which allows ICMP error messages returned from the tunnel to correctly be reflected back to the original senders of the tunneled datagrams. Home agents 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 [12]. 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:
o 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.
o 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 [6] messages. Otherwise, it MAY use its home address. Alternatively, a mobile node that wishes to receive multicasts MAY join groups via a bidirectional 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 that 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 that 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 the 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
intercept the datagram and tunnel it to its current care-of
address, which in this example is some foreign agent on the
ground below the aircraft. The original datagram from the
correspondent node has now been encapsulated twice: once by the
laptop's home agent and again by the aircraft's home agent.
f. The foreign agent on the ground decapsulates the datagram,
yielding a datagram still encapsulated by the laptop's home
agent, with a Destination Address of the laptop's care-of
address. The ground foreign agent sends the resulting datagram
over its radio link to the aircraft.
g. The foreign agent on the aircraft decapsulates the datagram,
yielding the original datagram from the correspondent node, with
a Destination Address of the laptop's home address. The aircraft
foreign agent delivers the datagram over the aircraft network to
the laptop's link-layer address.
This example illustrates the case in which a mobile node is attached
to a mobile network. That is, the mobile node is mobile with respect
to the network, which itself is also mobile (here with respect to the
ground). If, instead, the node is fixed with respect to the mobile
network (the mobile network is the fixed node's home network), then
either of two methods may be used to cause datagrams from
correspondent nodes to be routed to the fixed node.
For the fixed node, a home agent MAY be configured to have a
permanent registration that indicates the mobile router's address as
the fixed host's care-of address. The mobile router's home agent
will normally be used for this purpose. The home agent is then
responsible for advertising connectivity using normal routing
protocols to the fixed node. Any datagrams sent to the fixed node
will thus use nested tunneling as described above.
Alternatively, the mobile router MAY advertise connectivity to the
entire mobile network using normal IP routing protocols through a
bidirectional tunnel to its own home agent. This method avoids the
need for nested tunneling of datagrams.
4.6. ARP, Proxy ARP, and Gratuitous ARP
The use of ARP [16] requires special rules for correct operation when wireless or mobile nodes are involved. The requirements specified in this section apply to all home networks in which ARP is used for address resolution. 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: o A Proxy ARP [49] is an ARP Reply sent by one node on behalf of another node that is either unable or unwilling to answer its own ARP Requests. The sender of a Proxy ARP reverses the Sender and Target Protocol Address fields as described in [16], but supplies some configured link-layer address (generally, its own) in the Sender Hardware Address field. The node receiving the Reply will then associate this link-layer address with the IP address of the original target node, causing it to transmit future datagrams for this target node to the node with that link-layer address. o A Gratuitous ARP [45] is an ARP packet sent by a node in order to spontaneously cause other nodes to update an entry in their ARP cache. A gratuitous ARP MAY use either an ARP Request or an ARP Reply packet. In either case, the ARP Sender Protocol Address and ARP Target Protocol Address are both set to the IP address of the cache entry to be updated, and the ARP Sender Hardware Address is set to the link-layer address to which this cache entry should be updated. When using an ARP Reply packet, the Target Hardware Address is also set to the link-layer address to which this cache entry should be updated (this field is not used in an ARP Request packet). In either case, for a gratuitous ARP, the ARP packet MUST be transmitted as a local broadcast packet on the local link. As specified in [16], any node receiving any ARP packet (Request or Reply) MUST update its local ARP cache with the Sender Protocol and Hardware Addresses in the ARP packet, if the receiving node has an entry for that IP address already in its ARP cache. This requirement in the ARP protocol applies even for ARP Request packets, and for ARP Reply packets that do not match any ARP Request transmitted by the receiving node [16]. While a mobile node is registered on a foreign network, its home agent uses proxy ARP [49] to reply to ARP Requests it receives that seek the mobile node's link-layer address. When receiving an ARP Request, the home agent MUST examine the target IP address of the Request, and if this IP address matches the home address of any
mobile node for which it has a registered mobility binding, the home agent MUST transmit an ARP Reply on behalf of the mobile node. After exchanging the sender and target addresses in the packet [49], 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. This gratuitous ARP packet MUST be transmitted as a broadcast packet on the link on which the mobile node's home address is located. Since broadcasts on the local link (such as Ethernet) are typically not guaranteed to be reliable, the gratuitous ARP packet SHOULD be retransmitted a small number of times to increase its reliability. When a mobile node returns to its home network, the mobile node and its home agent use gratuitous ARP to cause all nodes on the mobile node's home network to update their ARP caches to once again associate the mobile node's own link-layer address with the mobile node's home (IP) address. Before transmitting the (de)Registration Request message to its home agent, the mobile node MUST transmit this gratuitous ARP on its home network as a local broadcast on this link. The gratuitous ARP packet SHOULD be retransmitted a small number of times to increase its reliability, but these retransmissions SHOULD proceed in parallel with the transmission and processing of the mobile node's (de)Registration Request. When the mobile node's home agent receives and accepts this (de)Registration Request, the home agent MUST also transmit a gratuitous ARP on the mobile node's home network. This gratuitous ARP also is used to associate the mobile node's home address with the mobile node's own link-layer address. A gratuitous ARP is transmitted by both the mobile node and its home agent, since in the case of wireless network interfaces, the area within transmission range of the mobile node will likely differ from that within range of its home agent. The ARP packet from the home agent MUST be transmitted as a local broadcast on the mobile node's home link, and SHOULD be retransmitted a small number of times to increase its reliability; these retransmissions, however, SHOULD proceed in parallel with the transmission and processing of the mobile node's (de)Registration Reply.
While the mobile node is away from home, it MUST NOT transmit any
broadcast ARP Request or ARP Reply messages. Finally, while the
mobile node is away from home, it MUST NOT reply to ARP Requests in
which the target IP address is its own home address unless the ARP
Request is unicast by a foreign agent with which the mobile node is
attempting to register or a foreign agent with which the mobile node
has an unexpired registration. In the latter case, the mobile node
MUST use a unicast ARP Reply to respond to the foreign agent. Note
that if the mobile node is using a co-located care-of address and
receives an ARP Request in which the target IP address is this care-
of address, then the mobile node SHOULD reply to this ARP Request.
Note also that, when transmitting a Registration Request on a foreign
network, a mobile node may discover the link-layer address of a
foreign agent by storing the address as it is received from the Agent
Advertisement from that foreign agent, but not by transmitting a
broadcast ARP Request message.
The specific order in which each of the above requirements for the
use of ARP, proxy ARP, and gratuitous ARP are applied, relative to
the transmission and processing of the mobile node's Registration
Request and Registration Reply messages when leaving home or
returning home, are important to the correct operation of the
protocol.
To summarize the above requirements, when a mobile node leaves its
home network, the following steps, in this order, MUST be performed:
o 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.
o Before transmitting the Registration Request, the mobile node
disables its own future processing of any ARP Requests it may
subsequently receive requesting the link-layer address
corresponding to its home address, except insofar as necessary to
communicate with foreign agents on visited networks.
o The mobile node transmits its Registration Request.
o 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. In the gratuitous ARP, the ARP Sender Hardware Address
is set to the link-layer address of the home agent. If, instead,
the home agent rejects the Registration Request, no ARP processing
(neither 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:
o The mobile node decides to register at home, perhaps because it
has received an Agent Advertisement from its home agent.
o 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.
o The mobile node performs a gratuitous ARP for itself. In this
gratuitous ARP, the ARP Sender Hardware Address is set to the
link-layer address of the mobile node.
o The mobile node transmits its Registration Request.
o 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. In this gratuitous ARP, the ARP Sender Hardware
Address is set to the link-layer address of the mobile node. If,
instead, the home agent rejects the Registration Request, the home
agent MUST NOT make any change to the way it performs ARP
processing (neither gratuitous nor proxy) for the mobile node. In
this latter case, the home agent should operate as if the mobile
node has not returned home, and continue to perform proxy ARP on
behalf of the mobile node.
(page 77 continued on part 4)
