10] as its primary mechanism for Agent Discovery. An Agent Advertisement is formed by including a Mobility Agent Advertisement Extension in an ICMP Router Advertisement message (Section 2.1). An Agent Solicitation message is identical to an ICMP Router Solicitation, except that its IP TTL MUST be set to 1 (Section 2.2). This section describes the message formats and procedures by which mobile nodes, foreign agents, and home agents cooperate to realize Agent Discovery. Agent Advertisement and Agent Solicitation may not be necessary for link layers that already provide this functionality. The method by which mobile nodes establish link-layer connections with prospective agents is outside the scope of this document (but see Appendix B). The procedures described below assume that such link-layer connectivity has already been established. No authentication is required for Agent Advertisement and Agent Solicitation messages. They MAY be authenticated using the IP Authentication Header , which is unrelated to the messages described in this document. Further specification of the way in which Advertisement and Solicitation messages may be authenticated is outside of the scope of this document.
10], the IP destination address of an multicast Agent Advertisement MUST be either the "all systems on this link" multicast address (184.108.40.206)  or the "limited broadcast" address (255.255.255.255). The subnet-directed broadcast address of the form <prefix>.<-1> cannot be used since mobile nodes will not generally know the prefix of the foreign network. When the Agent Advertisement is unicast to a mobile node, the IP home address of the mobile node SHOULD be used as the Destination Address.
- ICMP Fields Code The Code field of the agent advertisement is interpreted as follows: 0 The mobility agent handles common traffic -- that is, it acts as a router for IP datagrams not necessarily related to mobile nodes. 16 The mobility agent does not route common traffic. However, all foreign agents MUST (minimally) forward to a default router any datagrams received from a registered mobile node (Section 4.2.2). Lifetime The maximum length of time that the Advertisement is considered valid in the absence of further Advertisements. Router Address(es) See Section 2.3.1 for a discussion of the addresses that may appear in this portion of the Agent Advertisement. Num Addrs The number of Router Addresses advertised in this message. Note that in an Agent Advertisement message, the number of router addresses specified in the ICMP Router Advertisement portion of the message MAY be set to 0. See Section 2.3.1 for details. If sent periodically, the nominal interval at which Agent Advertisements are sent SHOULD be no longer than 1/3 of the advertisement Lifetime given in the ICMP header. This interval MAY be shorter than 1/3 the advertised Lifetime. This allows a mobile node to miss three successive advertisements before deleting the agent from its list of valid agents. The actual transmission time for each advertisement SHOULD be slightly randomized  in order to avoid synchronization and subsequent collisions with other Agent Advertisements that may be sent by other agents (or with other Router Advertisements sent by other routers). Note that this field has no relation to the "Registration Lifetime" field within the Mobility Agent Advertisement Extension defined below.
F Foreign agent. This agent offers service as a foreign agent on the link on which this Agent Advertisement message is sent. M Minimal encapsulation. This agent implements receiving tunneled datagrams that use minimal encapsulation . G GRE encapsulation. This agent implements receiving tunneled datagrams that use GRE encapsulation . r Sent as zero; ignored on reception. SHOULD NOT be allocated for any other uses. T Foreign agent supports reverse tunneling . reserved Sent as zero; ignored on reception. Care-of Address(es) The advertised foreign agent care-of address(es) provided by this foreign agent. An Agent Advertisement MUST include at least one care-of address if the 'F' bit is set. The number of care-of addresses present is determined by the Length field in the Extension. A home agent MUST always be prepared to serve the mobile nodes for which it is the home agent. A foreign agent may at times be too busy to serve additional mobile nodes; even so, it must continue to send Agent Advertisements, so that any mobile nodes already registered with it will know that they have not moved out of range of the foreign agent and that the foreign agent has not failed. A foreign agent may indicate that it is "too busy" to allow new mobile nodes to register with it, by setting the 'B' bit in its Agent Advertisements. An Agent Advertisement message MUST NOT have the 'B' bit set if the 'F' bit is not also set. Furthermore, at least one of the 'F' bit and the 'H' bit MUST be set in any Agent Advertisement message sent. When a foreign agent wishes to require registration even from those mobile nodes which have acquired a co-located care-of address, it sets the 'R' bit to one. Because this bit applies only to foreign agents, an agent MUST NOT set the 'R' bit to one unless the 'F' bit is also set to one.
Note that unlike other Extensions used in Mobile IP, the One-byte Padding Extension is encoded as a single byte, with no "Length" nor "Data" field present. The One-byte Padding Extension is defined as follows: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | Type | +-+-+-+-+-+-+-+-+ Type 0 (One-byte Padding Extension) 10], except that: - a mobility agent MUST limit the rate at which it sends broadcast or multicast Agent Advertisements; the maximum rate SHOULD be chosen so that the Advertisements do not consume a significant amount of network bandwidth, AND - a mobility agent that receives a Router Solicitation MUST NOT require that the IP Source Address is the address of a neighbor (i.e., an address that matches one of the router's own addresses on the arrival interface, under the subnet mask associated with that address of the router). - a mobility agent MAY be configured to send Agent Advertisements only in response to an Agent Solicitation message.
If the home network is not a virtual network, then the home agent for any mobile node SHOULD be located on the link identified by the mobile node's home address, and Agent Advertisement messages sent by the home agent on this link MUST have the 'H' bit set. In this way, mobile nodes on their own home network will be able to determine that they are indeed at home. Any Agent Advertisement messages sent by the home agent on another link to which it may be attached (if it is a mobility agent serving more than one link), MUST NOT have the 'H' bit set, unless the home agent also serves as a home agent (to other mobile nodes) on that other link. A mobility agent MAY use different settings for each of the 'R', 'H', and 'F' bits on different network interfaces. If the home network is a virtual network, the home network has no physical realization external to the home agent itself. In this case, there is no physical network link on which to send Agent Advertisement messages advertising the home agent. Mobile nodes for which this is the home network are always treated as being away from home. On a particular subnet, either all mobility agents MUST include the Prefix-Lengths Extension or all of them MUST NOT include this Extension. Equivalently, it is prohibited for some agents on a given subnet to include the Extension but for others not to include it. Otherwise, one of the move detection algorithms designed for mobile nodes will not function properly (Section 2.4.2).
10], except that the mobile node MAY solicit more often than once every three seconds, and that a mobile node that is currently not connected to any foreign agent MAY solicit more times than MAX_SOLICITATIONS. The rate at which a mobile node sends Solicitations MUST be limited by the mobile node. The mobile node MAY send three initial Solicitations at a maximum rate of one per second while searching for an agent. After this, the rate at which Solicitations are sent MUST be reduced so as to limit the overhead on the local link. Subsequent Solicitations MUST be sent using a binary exponential backoff mechanism, doubling the interval between consecutive Solicitations, up to a maximum interval. The maximum interval SHOULD be chosen appropriately based upon the characteristics of the media over which the mobile node is soliciting. This maximum interval SHOULD be at least one minute between Solicitations. While still searching for an agent, the mobile node MUST NOT increase the rate at which it sends Solicitations unless it has received a positive indication that it has moved to a new link. After successfully registering with an agent, the mobile node SHOULD also increase the rate at which it will send Solicitations when it next begins searching for a new agent with which to register. The increased solicitation rate MAY revert to the maximum rate, but then MUST be limited in the manner described above. In all cases, the recommended solicitation intervals are nominal values. Mobile nodes MUST randomize their solicitation times around these nominal values as specified for ICMP Router Discovery . Mobile nodes MUST process received Agent Advertisements. A mobile node can distinguish an Agent Advertisement message from other uses of the ICMP Router Advertisement message by examining the number of advertised addresses and the IP Total Length field. When the IP total length indicates that the ICMP message is longer than needed for the number of advertised addresses, the remaining data is interpreted as one or more Extensions. The presence of a Mobility Agent Advertisement Extension identifies the advertisement as an Agent Advertisement.
If there is more than one advertised address, the mobile node SHOULD pick the first address for its initial registration attempt. If the registration attempt fails with a status Code indicating rejection by the foreign agent, the mobile node MAY retry the attempt with each subsequent advertised address in turn. When multiple methods of agent discovery are in use, the mobile node SHOULD first attempt registration with agents including Mobility Agent Advertisement Extensions in their advertisements, in preference to those discovered by other means. This preference maximizes the likelihood that the registration will be recognized, thereby minimizing the number of registration attempts. A mobile node MUST ignore reserved bits in Agent Advertisements, as opposed to discarding such advertisements. In this way, new bits can be defined later, without affecting the ability for mobile nodes to use the advertisements even when the newly defined bits are not understood.
addition, if the home network is using ARP , the mobile node MUST follow the procedures described in Section 4.6 with regard to ARP, proxy ARP, and gratuitous ARP.
- discover the address of a home agent if the mobile node is not configured with this information.
When the mobile node instead registers directly with its home agent, the registration procedure requires only the following two messages: a) The mobile node sends a Registration Request to the home agent. b) The home agent sends a Registration Reply to the mobile node, granting or denying the Request. The registration messages defined in Sections 3.3 and 3.4 use the User Datagram Protocol (UDP) . A nonzero UDP checksum SHOULD be included in the header, and MUST be checked by the recipient. A zero UDP checksum SHOULD be accepted by the recipient. The behavior of the mobile node and the home agent with respect to their mutual acceptance of packets with zero UDP checksums SHOULD be defined as part of the mobility security association which exists between them.
See Sections 220.127.116.11 and 18.104.22.168 for details. UDP fields: Source Port variable Destination Port 434 The UDP header is followed by the Mobile IP fields shown below: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type |S|B|D|M|G|r|T|x| Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Agent | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Care-of Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Identification + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extensions ... +-+-+-+-+-+-+-+- Type 1 (Registration Request) S Simultaneous bindings. If the 'S' bit is set, the mobile node is requesting that the home agent retain its prior mobility bindings, as described in Section 22.214.171.124. B Broadcast datagrams. If the 'B' bit is set, the mobile node requests that the home agent tunnel to it any broadcast datagrams that it receives on the home network, as described in Section 4.3. D Decapsulation by mobile node. If the 'D' bit is set, the mobile node will itself decapsulate datagrams which are sent to the care-of address. That is, the mobile node is using a co-located care-of address. M Minimal encapsulation. If the 'M' bit is set, the mobile node requests that its home agent use minimal encapsulation  for datagrams tunneled to the mobile node.
G GRE encapsulation. If the 'G' bit is set, the mobile node requests that its home agent use GRE encapsulation  for datagrams tunneled to the mobile node. r Sent as zero; ignored on reception. SHOULD NOT be allocated for any other uses. T Reverse Tunneling requested; see . x Sent as zero; ignored on reception. Lifetime The number of seconds remaining before the registration is considered expired. A value of zero indicates a request for deregistration. A value of 0xffff indicates infinity. Home Address The IP address of the mobile node. Home Agent The IP address of the mobile node's home agent. Care-of Address The IP address for the end of the tunnel. Identification A 64-bit number, constructed by the mobile node, used for matching Registration Requests with Registration Replies, and for protecting against replay attacks of registration messages. See Sections 5.4 and 5.7. Extensions The fixed portion of the Registration Request is followed by one or more of the Extensions listed in Section 3.5. An authorization-enabling extension MUST be included in all Registration Requests. See Sections 126.96.36.199 and 188.8.131.52 for information on the relative order in which different extensions, when present, MUST be placed in a Registration Request message.
The UDP header is followed by the Mobile IP fields shown below: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Agent | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Identification + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extensions ... +-+-+-+-+-+-+-+- Type 3 (Registration Reply) Code A value indicating the result of the Registration Request. See below for a list of currently defined Code values. Lifetime If the Code field indicates that the registration was accepted, the Lifetime field is set to the number of seconds remaining before the registration is considered expired. A value of zero indicates that the mobile node has been deregistered. A value of 0xffff indicates infinity. If the Code field indicates that the registration was denied, the contents of the Lifetime field are unspecified and MUST be ignored on reception. Home Address The IP address of the mobile node. Home Agent The IP address of the mobile node's home agent. Identification A 64-bit number used for matching Registration Requests with Registration Replies, and for protecting against replay attacks of registration messages. The value is
based on the Identification field from the Registration Request message from the mobile node, and on the style of replay protection used in the security context between the mobile node and its home agent (defined by the mobility security association between them, and SPI value in the authorization-enabling extension). See Sections 5.4 and 5.7. Extensions The fixed portion of the Registration Reply is followed by one or more of the Extensions listed in Section 3.5. An authorization-enabling extension MUST be included in all Registration Replies returned by the home agent. See Sections 184.108.40.206 and 220.127.116.11 for rules on placement of extensions to Reply messages. The following values are defined for use within the Code field. Registration successful: 0 registration accepted 1 registration accepted, but simultaneous mobility bindings unsupported Registration denied by the foreign agent: 64 reason unspecified 65 administratively prohibited 66 insufficient resources 67 mobile node failed authentication 68 home agent failed authentication 69 requested Lifetime too long 70 poorly formed Request 71 poorly formed Reply 72 requested encapsulation unavailable 73 reserved and unavailable 77 invalid care-of address 78 registration timeout 80 home network unreachable (ICMP error received) 81 home agent host unreachable (ICMP error received) 82 home agent port unreachable (ICMP error received) 88 home agent unreachable (other ICMP error received)
Registration denied by the home agent: 128 reason unspecified 129 administratively prohibited 130 insufficient resources 131 mobile node failed authentication 132 foreign agent failed authentication 133 registration Identification mismatch 134 poorly formed Request 135 too many simultaneous mobility bindings 136 unknown home agent address Up-to-date values of the Code field are specified in the most recent "Assigned Numbers" . 23] to compute a 128-bit "message digest" of the registration message. The data over which the HMAC is computed is defined as: - the UDP payload (that is, the Registration Request or Registration Reply data), - all prior Extensions in their entirety, and - the Type, Length, and SPI of this Extension. Note that the Authenticator field itself and the UDP header are NOT included in the computation of the default Authenticator value. See Section 5.1 for information about support requirements for message authentication codes, which are to be used with the various authentication Extensions.
The Security Parameter Index (SPI) within any of the authentication Extensions defines the security context which is used to compute the Authenticator value and which MUST be used by the receiver to check that value. In particular, the SPI selects the authentication algorithm and mode (Section 5.1) and secret (a shared key, or appropriate public/private key pair) used in computing the Authenticator. In order to ensure interoperability between different implementations of the Mobile IP protocol, an implementation MUST be able to associate any SPI value with any authentication algorithm and mode which it implements. In addition, all implementations of Mobile IP MUST implement the default authentication algorithm (HMAC-MD5) specified above. 2] which result from the uncontrolled propagation of remote redirects in the Internet. The location of the extension marks the end of the authenticated data. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | SPI .... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... SPI (cont.) | Authenticator ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 32 Length 4 plus the number of bytes in the Authenticator. SPI Security Parameter Index (4 bytes). An opaque identifier (see Section 1.6). Authenticator (variable length) (See Section 3.5.1.)
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | SPI .... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... SPI (cont.) | Authenticator ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 33 Length 4 plus the number of bytes in the Authenticator. SPI Security Parameter Index (4 bytes). An opaque identifier (see Section 1.6). Authenticator (variable length) (See Section 3.5.1.)
address of one or more of its home agents; otherwise, the mobile node MAY discover a home agent using the procedures described in Section 18.104.22.168. If the mobile node is not configured with a home address, it MAY use the Mobile Node NAI extension  to identify itself, and set the Home Address field of the Registration Request to 0.0.0.0. In this case, the mobile node MUST be able to assign its home address after extracting this information from the Registration Reply from the home agent. For each pending registration, the mobile node maintains the following information: - the link-layer address of the foreign agent to which the Registration Request was sent, if applicable, - the IP destination address of the Registration Request, - the care-of address used in the registration, - the Identification value sent in the registration, - the originally requested Lifetime, and - the remaining Lifetime of the pending registration. A mobile node SHOULD initiate a registration whenever it detects a change in its network connectivity. See Section 2.4.2 for methods by which mobile nodes MAY make such a determination. When it is away from home, the mobile node's Registration Request allows its home agent to create or modify a mobility binding for it. When it is at home, the mobile node's (de)Registration Request allows its home agent to delete any previous mobility binding(s) for it. A mobile node operates without the support of mobility functions when it is at home. There are other conditions under which the mobile node SHOULD (re)register with its foreign agent, such as when the mobile node detects that the foreign agent has rebooted (as specified in Section 2.4.4) and when the current registration's Lifetime is near expiration. In the absence of link-layer indications of changes in point of attachment, Agent Advertisements from new agents SHOULD NOT cause a mobile node to attempt a new registration, if its current registration has not expired and it is still also receiving Agent Advertisements from the foreign agent with which it is currently registered. In the absence of link-layer indications, a mobile node MUST NOT attempt to register more often than once per second.
A mobile node MAY register with a different agent when transport- layer protocols indicate excessive retransmissions. A mobile node MUST NOT consider reception of an ICMP Redirect from a foreign agent that is currently providing service to it as reason to register with a new foreign agent. Within these constraints, the mobile node MAY register again at any time. Appendix D shows some examples of how the fields in registration messages would be set up in some typical registration scenarios.
layer destination address copied from the link-layer source address of the Agent Advertisement message in which it learned this foreign agent's IP address. - When the mobile node is registering directly with its home agent and knows the (unicast) IP address of its home agent, the destination address MUST be set to this address. - If the mobile node is registering directly with its home agent, but does not know the IP address of its home agent, the mobile node may use dynamic home agent address resolution to automatically determine the IP address of its home agent (Section 22.214.171.124). In this case, the IP destination address is set to the subnet-directed broadcast address of the mobile node's home network. This address MUST NOT be used as the destination IP address if the mobile node is registering via a foreign agent, although it MAY be used as the Home Agent address in the body of the Registration Request when registering via a foreign agent. IP Time to Live: - The IP TTL field MUST be set to 1 if the IP destination address is set to the "All Mobility Agents" multicast address as described above. Otherwise a suitable value should be chosen in accordance with standard IP practice .
A mobile node MAY set the 'B' bit to request its home agent to forward to it, a copy of broadcast datagrams received by its home agent from the home network. The method used by the home agent to forward broadcast datagrams depends on the type of care-of address registered by the mobile node, as determined by the 'D' bit in the mobile node's Registration Request: - If the 'D' bit is set, then the mobile node has indicated that it will decapsulate any datagrams tunneled to this care-of address itself (the mobile node is using a co-located care-of address). In this case, to forward such a received broadcast datagram to the mobile node, the home agent MUST tunnel it to this care-of address. The mobile node de-tunnels the received datagram in the same way as any other datagram tunneled directly to it. - If the 'D' bit is NOT set, then the mobile node has indicated that it is using a foreign agent care-of address, and that the foreign agent will thus decapsulate arriving datagrams before forwarding them to the mobile node. In this case, to forward such a received broadcast datagram to the mobile node, the home agent MUST first encapsulate the broadcast datagram in a unicast datagram addressed to the mobile node's home address, and then MUST tunnel this resulting datagram to the mobile node's care-of address. When decapsulated by the foreign agent, the inner datagram will thus be a unicast IP datagram addressed to the mobile node, identifying to the foreign agent the intended destination of the encapsulated broadcast datagram, and will be delivered to the mobile node in the same way as any tunneled datagram arriving for the mobile node. The foreign agent MUST NOT decapsulate the encapsulated broadcast datagram and MUST NOT use a local network broadcast to transmit it to the mobile node. The mobile node thus MUST decapsulate the encapsulated broadcast datagram itself, and thus MUST NOT set the 'B' bit in its Registration Request in this case unless it is capable of decapsulating datagrams. The mobile node MAY request alternative forms of encapsulation by setting the 'M' bit and/or the 'G' bit, but only if the mobile node is decapsulating its own datagrams (the mobile node is using a co- located care-of address) or if its foreign agent has indicated support for these forms of encapsulation by setting the corresponding bits in the Mobility Agent Advertisement Extension of an Agent Advertisement received by the mobile node. Otherwise, the mobile node MUST NOT set these bits.
The Lifetime field is chosen as follows: - If the mobile node is registering with a foreign agent, the Lifetime SHOULD NOT exceed the value in the Registration Lifetime field of the Agent Advertisement message received from the foreign agent. When the method by which the care-of address is learned does not include a Lifetime, the default ICMP Router Advertisement Lifetime (1800 seconds) MAY be used. - The mobile node MAY ask a home agent to delete a particular mobility binding, by sending a Registration Request with the care-of address for this binding, with the Lifetime field set to zero (Section 3.8.2). - Similarly, a Lifetime of zero is used when the mobile node deregisters all care-of addresses, such as upon returning home. The Home Address field MUST be set to the mobile node's home address, if this information is known. Otherwise, the Home Address MUST be set to zeroes. The Home Agent field MUST be set to the address of the mobile node's home agent, if the mobile node knows this address. Otherwise, the mobile node MAY use dynamic home agent address resolution to learn the address of its home agent. In this case, the mobile node MUST set the Home Agent field to the subnet-directed broadcast address of the mobile node's home network. Each home agent receiving such a Registration Request with a broadcast destination address MUST reject the mobile node's registration and SHOULD return a rejection Registration Reply indicating its unicast IP address for use by the mobile node in a future registration attempt. The Care-of Address field MUST be set to the value of the particular care-of address that the mobile node wishes to (de)register. In the special case in which a mobile node wishes to deregister all care-of addresses, it MUST set this field to its home address. The mobile node chooses the Identification field in accordance with the style of replay protection it uses with its home agent. This is part of the mobility security association the mobile node shares with its home agent. See Section 5.7 for the method by which the mobile node computes the Identification field.
Also, the Registration Reply MUST be checked for presence of an authorization-enabling extension. For all Registration Reply messages containing a Status Code indicating status from the Home Agent, the mobile node MUST check for the presence of an authorization-enabling extension, acting in accordance with the Code field in the Reply. The rules are as follows: a) If the mobile node and the foreign agent share a mobility security association, exactly one Mobile-Foreign Authentication Extension MUST be present in the Registration Reply, and the mobile node MUST check the Authenticator value in the Extension. If no Mobile-Foreign Authentication Extension is found, or if more than one Mobile-Foreign Authentication Extension is found, or if the Authenticator is invalid, the mobile node MUST silently discard the Reply and SHOULD log the event as a security exception. b) If the Code field indicates that service is denied by the home agent, or if the Code field indicates that the registration was accepted by the home agent, exactly one Mobile-Home Authentication Extension MUST be present in the Registration Reply, and the mobile node MUST check the Authenticator value in the Extension. If the Registration Reply was generated by the home agent but no Mobile-Home Authentication Extension is found, or if more than one Mobile-Home Authentication Extension is found, or if the Authenticator is invalid, the mobile node MUST silently discard the Reply and SHOULD log the event as a security exception. If the Code field indicates an authentication failure, either at the foreign agent or the home agent, then it is quite possible that any authenticators in the Registration Reply will also be in error. This could happen, for example, if the shared secret between the mobile node and home agent was erroneously configured. The mobile node SHOULD log such errors as security exceptions.
If the mobile node has registered on a foreign network, it SHOULD re-register before the expiration of the Lifetime of its registration. As described in Section 3.6, for each pending Registration Request, the mobile node MUST maintain the remaining lifetime of this pending registration, as well as the original Lifetime from the Registration Request. When the mobile node receives a valid Registration Reply, the mobile node MUST decrease its view of the remaining lifetime of the registration by the amount by which the home agent decreased the originally requested Lifetime. This procedure is equivalent to the mobile node starting a timer for the granted Lifetime at the time it sent the Registration Request, even though the granted Lifetime is not known to the mobile node until the Registration Reply is received. Since the Registration Request is certainly sent before the home agent begins timing the registration Lifetime (also based on the granted Lifetime), this procedure ensures that the mobile node will re-register before the home agent expires and deletes the registration, in spite of possibly non-negligible transmission delays for the original Registration Request and Reply that started the timing of the Lifetime at the mobile node and its home agent.
Code 136: (Denied by home agent, Unknown home agent address) This code is returned by a home agent when the mobile node is performing dynamic home agent address resolution as described in Sections 126.96.36.199 and 188.8.131.52. In this case, the Home Agent field within the Reply will contain the unicast IP address of the home agent returning the Reply. The mobile node MAY then attempt to register with this home agent in future Registration Requests. In addition, the mobile node SHOULD adjust the parameters it uses to compute the Identification field based upon the corresponding field in the Registration Reply, before issuing any future Registration Requests.