RFC 3220
IP Mobility Support for IPv4
2. Agent Discovery
Agent Discovery is the method by which a mobile node determines whether it is currently connected to its home network or to a foreign network, and by which a mobile node can detect when it has moved from one network to another. When connected to a foreign network, the methods specified in this section also allow the mobile node to determine the foreign agent care-of address being offered by each foreign agent on that network. Mobile IP extends ICMP Router Discovery [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 [22], which is unrelated to the messages described in this document. Further specification of the way in which Advertisement and Solicitation messages may be authenticated is outside of the scope of this document.
2.1. Agent Advertisement
Agent Advertisements are transmitted by a mobility agent to advertise its services on a link. Mobile nodes use these advertisements to determine their current point of attachment to the Internet. An Agent Advertisement is an ICMP Router Advertisement that has been extended to also carry an Mobility Agent Advertisement Extension (Section 2.1.1) and, optionally, a Prefix-Lengths Extension (Section 2.1.2), One-byte Padding Extension (Section 2.1.3), or other Extensions that might be defined in the future. Within an Agent Advertisement message, ICMP Router Advertisement fields of the message are required to conform to the following additional specifications: - Link-Layer Fields Destination Address The link-layer destination address of a unicast Agent Advertisement MUST be the same as the source link-layer address of the Agent Solicitation which prompted the Advertisement. - IP Fields TTL The TTL for all Agent Advertisements MUST be set to 1. Destination Address As specified for ICMP Router Discovery [10], the IP destination address of an multicast Agent Advertisement MUST be either the "all systems on this link" multicast address (224.0.0.1) [11] 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 [10] in order to
avoid synchronization and subsequent collisions with other Agent
Advertisements that may be sent by other agents (or with other Router
Advertisements sent by other routers). Note that this field has no
relation to the "Registration Lifetime" field within the Mobility
Agent Advertisement Extension defined below.
2.1.1. Mobility Agent Advertisement Extension
The Mobility Agent Advertisement Extension follows the ICMP Router Advertisement fields. It is used to indicate that an ICMP Router Advertisement message is also an Agent Advertisement being sent by a mobility agent. The Mobility Agent Advertisement Extension is defined as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Registration Lifetime |R|B|H|F|M|G|r|T| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | zero or more Care-of Addresses | | ... | Type 16 Length (6 + 4*N), where 6 accounts for the number of bytes in the Sequence Number, Registration Lifetime, flags, and reserved fields, and N is the number of care-of addresses advertised. Sequence Number The count of Agent Advertisement messages sent since the agent was initialized (Section 2.3.2). Registration Lifetime The longest lifetime (measured in seconds) that this agent is willing to accept in any Registration Request. A value of 0xffff indicates infinity. This field has no relation to the "Lifetime" field within the ICMP Router Advertisement portion of the Agent Advertisement. R Registration required. Registration with this foreign agent (or another foreign agent on this link) is required even when using a co-located care-of address. B Busy. The foreign agent will not accept registrations from additional mobile nodes. H Home agent. This agent offers service as a home agent on the link on which this Agent Advertisement message is sent.
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 [34].
G GRE encapsulation. This agent implements receiving
tunneled datagrams that use GRE encapsulation [16].
r Sent as zero; ignored on reception. SHOULD NOT be
allocated for any other uses.
T Foreign agent supports reverse tunneling [27].
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.
2.1.2. Prefix-Lengths Extension
The Prefix-Lengths Extension MAY follow the Mobility Agent Advertisement Extension. It is used to indicate the number of bits of network prefix that applies to each Router Address listed in the ICMP Router Advertisement portion of the Agent Advertisement. Note that the prefix lengths given DO NOT apply to care-of address(es) listed in the Mobility Agent Advertisement Extension. The Prefix- Lengths Extension is defined as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Prefix Length | .... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 19 (Prefix-Lengths Extension) Length N, where N is the value (possibly zero) of the Num Addrs field in the ICMP Router Advertisement portion of the Agent Advertisement. Prefix Length(s) The number of leading bits that define the network number of the corresponding Router Address listed in the ICMP Router Advertisement portion of the message. The prefix length for each Router Address is encoded as a separate byte, in the order that the Router Addresses are listed in the ICMP Router Advertisement portion of the message. See Section 2.4.2 for information about how the Prefix-Lengths Extension MAY be used by a mobile node when determining whether it has moved. See Appendix E for implementation details about the use of this Extension.2.1.3. One-byte Padding Extension
Some IP protocol implementations insist upon padding ICMP messages to an even number of bytes. If the ICMP length of an Agent Advertisement is odd, this Extension MAY be included in order to make the ICMP length even. Note that this Extension is NOT intended to be a general-purpose Extension to be included in order to word- or long-align the various fields of the Agent Advertisement. An Agent Advertisement SHOULD NOT include more than one One-byte Padding Extension and if present, this Extension SHOULD be the last Extension in the Agent Advertisement.
Note that unlike other Extensions used in Mobile IP, the One-byte
Padding Extension is encoded as a single byte, with no "Length" nor
"Data" field present. The One-byte Padding Extension is defined as
follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type |
+-+-+-+-+-+-+-+-+
Type 0 (One-byte Padding Extension)
2.2. Agent Solicitation
An Agent Solicitation is identical to an ICMP Router Solicitation
with the further restriction that the IP TTL Field MUST be set to 1.
2.3. Foreign Agent and Home Agent Considerations
Any mobility agent which cannot be discovered by a link-layer
protocol MUST send Agent Advertisements. An agent which can be
discovered by a link-layer protocol SHOULD also implement Agent
Advertisements. However, the Advertisements need not be sent, except
when the site policy requires registration with the agent (i.e., when
the 'R' bit is set), or as a response to a specific Agent
Solicitation. All mobility agents MUST process packets that they
receive addressed to the Mobile-Agents multicast group, at address
224.0.0.11. A mobile node MAY send an Agent Solicitation to
224.0.0.11. All mobility agents SHOULD respond to Agent
Solicitations.
The same procedures, defaults, and constants are used in Agent
Advertisement messages and Agent Solicitation messages as specified
for ICMP Router Discovery [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).2.3.1. Advertised Router Addresses
The ICMP Router Advertisement portion of the Agent Advertisement MAY contain one or more router addresses. An agent SHOULD only put its own addresses, if any, in the advertisement. Whether or not its own address appears in the Router Addresses, a foreign agent MUST route datagrams it receives from registered mobile nodes (Section 4.2.2).2.3.2. Sequence Numbers and Rollover Handling
The sequence number in Agent Advertisements ranges from 0 to 0xffff. After booting, an agent MUST use the number 0 for its first advertisement. Each subsequent advertisement MUST use the sequence number one greater, with the exception that the sequence number 0xffff MUST be followed by sequence number 256. In this way, mobile nodes can distinguish a reduction in the sequence number that occurs after a reboot from a reduction that results in rollover of the sequence number after it attains the value 0xffff.
2.4. Mobile Node Considerations
Every mobile node MUST implement Agent Solicitation. Solicitations SHOULD only be sent in the absence of Agent Advertisements and when a care-of address has not been determined through a link-layer protocol or other means. The mobile node uses the same procedures, defaults, and constants for Agent Solicitation as specified for ICMP Router Solicitation messages [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 [10]. 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.2.4.1. Registration Required
When the mobile node receives an Agent Advertisement with the 'R' bit set, the mobile node SHOULD register through the foreign agent, even when the mobile node might be able to acquire its own co-located care-of address. This feature is intended to allow sites to enforce visiting policies (such as accounting) which require exchanges of authorization. If formerly reserved bits require some kind of monitoring/enforcement at the foreign link, foreign agents implementing the new specification for the formerly reserved bits can set the 'R' bit. This has the effect of forcing the mobile node to register through the foreign agent, so the foreign agent could then monitor/enforce the policy.2.4.2. Move Detection
Two primary mechanisms are provided for mobile nodes to detect when they have moved from one subnet to another. Other mechanisms MAY also be used. When the mobile node detects that it has moved, it SHOULD register (Section 3) with a suitable care-of address on the new foreign network. However, the mobile node MUST NOT register more frequently than once per second on average, as specified in Section 3.6.3.
2.4.2.1. Algorithm 1
The first method of move detection is based upon the Lifetime field within the main body of the ICMP Router Advertisement portion of the Agent Advertisement. A mobile node SHOULD record the Lifetime received in any Agent Advertisements, until that Lifetime expires. If the mobile node fails to receive another advertisement from the same agent within the specified Lifetime, it SHOULD assume that it has lost contact with that agent. If the mobile node has previously received an Agent Advertisement from another agent for which the Lifetime field has not yet expired, the mobile node MAY immediately attempt registration with that other agent. Otherwise, the mobile node SHOULD attempt to discover a new agent with which to register.2.4.2.2. Algorithm 2
The second method uses network prefixes. The Prefix-Lengths Extension MAY be used in some cases by a mobile node to determine whether or not a newly received Agent Advertisement was received on the same subnet as the mobile node's current care-of address. If the prefixes differ, the mobile node MAY assume that it has moved. If a mobile node is currently using a foreign agent care-of address, the mobile node SHOULD NOT use this method of move detection unless both the current agent and the new agent include the Prefix-Lengths Extension in their respective Agent Advertisements; if this Extension is missing from one or both of the advertisements, this method of move detection SHOULD NOT be used. Similarly, if a mobile node is using a co-located care-of address, it SHOULD not use this method of move detection unless the new agent includes the Prefix-Lengths Extension in its Advertisement and the mobile node knows the network prefix of its current co-located care-of address. On the expiration of its current registration, if this method indicates that the mobile node has moved, rather than re-registering with its current care-of address, a mobile node MAY choose instead to register with a the foreign agent sending the new Advertisement with the different network prefix. The Agent Advertisement on which the new registration is based MUST NOT have expired according to its Lifetime field.2.4.3. Returning Home
A mobile node can detect that it has returned to its home network when it receives an Agent Advertisement from its own home agent. If so, it SHOULD deregister with its home agent (Section 3). Before attempting to deregister, the mobile node SHOULD configure its routing table appropriately for its home network (Section 4.2.1). In
addition, if the home network is using ARP [36], the mobile node MUST follow the procedures described in Section 4.6 with regard to ARP, proxy ARP, and gratuitous ARP.2.4.4. Sequence Numbers and Rollover Handling
If a mobile node detects two successive values of the sequence number in the Agent Advertisements from the foreign agent with which it is registered, the second of which is less than the first and inside the range 0 to 255, the mobile node SHOULD register again. If the second value is less than the first but is greater than or equal to 256, the mobile node SHOULD assume that the sequence number has rolled over past its maximum value (0xffff), and that reregistration is not necessary (Section 2.3).3. Registration
Mobile IP registration provides a flexible mechanism for mobile nodes to communicate their current reachability information to their home agent. It is the method by which mobile nodes: - request forwarding services when visiting a foreign network, - inform their home agent of their current care-of address, - renew a registration which is due to expire, and/or - deregister when they return home. Registration messages exchange information between a mobile node, (optionally) a foreign agent, and the home agent. Registration creates or modifies a mobility binding at the home agent, associating the mobile node's home address with its care-of address for the specified Lifetime. Several other (optional) capabilities are available through the registration procedure, which enable a mobile node to: - discover its home address, if the mobile node is not configured with this information. - maintain multiple simultaneous registrations, so that a copy of each datagram will be tunneled to each active care-of address - deregister specific care-of addresses while retaining other mobility bindings, and
- discover the address of a home agent if the mobile node is not
configured with this information.
3.1. Registration Overview
Mobile IP defines two different registration procedures, one via a
foreign agent that relays the registration to the mobile node's home
agent, and one directly with the mobile node's home agent. The
following rules determine which of these two registration procedures
to use in any particular circumstance:
- If a mobile node is registering a foreign agent care-of
address, the mobile node MUST register via that foreign agent.
- If a mobile node is using a co-located care-of address, and
receives an Agent Advertisement from a foreign agent on the
link on which it is using this care-of address, the mobile node
SHOULD register via that foreign agent (or via another foreign
agent on this link) if the 'R' bit is set in the received Agent
Advertisement message.
- If a mobile node is otherwise using a co-located care-of
address, the mobile node MUST register directly with its home
agent.
- If a mobile node has returned to its home network and is
(de)registering with its home agent, the mobile node MUST
register directly with its home agent.
Both registration procedures involve the exchange of Registration
Request and Registration Reply messages (Sections 3.3 and 3.4). When
registering via a foreign agent, the registration procedure requires
the following four messages:
a) The mobile node sends a Registration Request to the prospective
foreign agent to begin the registration process.
b) The foreign agent processes the Registration Request and then
relays it to the home agent.
c) The home agent sends a Registration Reply to the foreign agent
to grant or deny the Request.
d) The foreign agent processes the Registration Reply and then
relays it to the mobile node to inform it of the disposition of
its Request.
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) [37]. 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.
3.2. Authentication
Each mobile node, foreign agent, and home agent MUST be able to
support a mobility security association for mobile entities, indexed
by their SPI and IP address. In the case of the mobile node, this
must be its Home Address. See Section 5.1 for requirements for
support of authentication algorithms. Registration messages between
a mobile node and its home agent MUST be authenticated with an
authorization-enabling extension, e.g. the Mobile-Home Authentication
Extension (Section 3.5.2). This extension MUST be the first
authentication extension; other foreign agent-specific extensions MAY
be added to the message after the mobile node computes the
authentication.
3.3. Registration Request
A mobile node registers with its home agent using a Registration
Request message so that its home agent can create or modify a
mobility binding for that mobile node (e.g., with a new lifetime).
The Request may be relayed to the home agent by the foreign agent
through which the mobile node is registering, or it may be sent
directly to the home agent in the case in which the mobile node is
registering a co-located care-of address.
IP fields:
Source Address Typically the interface address from which the
message is sent.
Destination Address Typically that of the foreign agent or the
home agent.
See Sections 3.6.1.1 and 3.7.2.2 for details. UDP fields:
Source Port variable
Destination Port 434
The UDP header is followed by the Mobile IP fields shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |S|B|D|M|G|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
3.6.1.2.
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 [34] 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
[16] 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 [27].
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 3.6.1.3 and
3.7.2.2 for information on the relative order in which
different extensions, when present, MUST be placed in a
Registration Request message.
3.4. Registration Reply
A mobility agent returns a Registration Reply message to a mobile node which has sent a Registration Request (Section 3.3) message. If the mobile node is requesting service from a foreign agent, that foreign agent will receive the Reply from the home agent and subsequently relay it to the mobile node. The Reply message contains the necessary codes to inform the mobile node about the status of its Request, along with the lifetime granted by the home agent, which MAY be smaller than the original Request. The foreign agent MUST NOT increase the Lifetime selected by the mobile node in the Registration Request, since the Lifetime is covered by an authentication extension which enables authorization by the home agent. Such an extension contains authentication data which cannot be correctly (re)computed by the foreign agent. The home agent MUST NOT increase the Lifetime selected by the mobile node in the Registration Request, since doing so could increase it beyond the maximum Registration Lifetime allowed by the foreign agent. If the Lifetime received in the Registration Reply is greater than that in the Registration Request, the Lifetime in the Request MUST be used. When the Lifetime received in the Registration Reply is less than that in the Registration Request, the Lifetime in the Reply MUST be used. IP fields: Source Address Typically copied from the destination address of the Registration Request to which the agent is replying. See Sections 3.7.2.3 and 3.8.3.1 for complete details. Destination Address Copied from the source address of the Registration Request to which the agent is replying UDP fields: Source Port <variable> Destination Port Copied from the source port of the corresponding Registration Request (Section 3.7.1).
The UDP header is followed by the Mobile IP fields shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Agent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
Type 3 (Registration Reply)
Code 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 3.7.2.2 and 3.8.3.3 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" [40].
3.5. Registration Extensions
3.5.1. Computing Authentication Extension Values
The Authenticator value computed for each authentication Extension
MUST protect the following fields from the registration message:
- 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.
The default authentication algorithm uses HMAC-MD5 [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.3.5.2. Mobile-Home Authentication Extension
Exactly one authorization-enabling extension MUST be present in all Registration Requests, and also in all Registration Replies generated by the Home Agent. The Mobile-Home Authentication Extension is always an authorization-enabling for registration messages specified in this document. This requirement is intended to eliminate problems [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.)3.5.3. Mobile-Foreign Authentication Extension
This Extension MAY be included in Registration Requests and Replies in cases in which a mobility security association exists between the mobile node and the foreign agent. See Section 5.1 for information about support requirements for message authentication codes.
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.)
3.5.4. Foreign-Home Authentication Extension
This Extension MAY be included in Registration Requests and Replies
in cases in which a mobility security association exists between the
foreign agent and the home agent. See Section 5.1 for information
about support requirements for message authentication codes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | SPI ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... SPI (cont.) | Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 34
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.)
3.6. Mobile Node Considerations
A mobile node MUST be configured with a netmask and a mobility
security association for each of its home agents. In addition, a
mobile node MAY be configured with its home address, and the IP
address of one or more of its home agents; otherwise, the mobile node MAY discover a home agent using the procedures described in Section 3.6.1.2. If the mobile node is not configured with a home address, it MAY use the Mobile Node NAI extension [6] 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.3.6.1. Sending Registration Requests
The following sections specify details for the values the mobile node MUST supply in the fields of Registration Request messages.3.6.1.1. IP Fields
This section provides the specific rules by which mobile nodes pick values for the IP header fields of a Registration Request. IP Source Address: - When registering on a foreign network with a co-located care-of address, the IP source address MUST be the care-of address. - Otherwise, if the mobile node does not have a home address, the IP source address MUST be 0.0.0.0. - In all other circumstances, the IP source address MUST be the mobile node's home address. IP Destination Address: - When the mobile node has discovered the agent with which it is registering, through some means (e.g., link-layer) that does not provide the IP address of the agent (the IP address of the agent is unknown to the mobile node), then the "All Mobility Agents" multicast address (224.0.0.11) MUST be used. In this case, the mobile node MUST use the agent's link-layer unicast address in order to deliver the datagram to the correct agent. - When registering with a foreign agent, the address of the agent as learned from the IP source address of the corresponding Agent Advertisement MUST be used. This MAY be an address which does not appear as an advertised care-of address in the Agent Advertisement. In addition, when transmitting this Registration Request message, the mobile node MUST use a link-
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 3.6.1.2). 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 [38].
3.6.1.2. Registration Request Fields
This section provides specific rules by which mobile nodes pick
values for the fields within the fixed portion of a Registration
Request.
A mobile node MAY set the 'S' bit in order to request that the home
agent maintain prior mobility binding(s). Otherwise, the home agent
deletes any previous binding(s) and replaces them with the new
binding specified in the Registration Request. Multiple simultaneous
mobility bindings are likely to be useful when a mobile node using at
least one wireless network interface moves within wireless
transmission range of more than one foreign agent. IP explicitly
allows duplication of datagrams. When the home agent allows
simultaneous bindings, it will tunnel a separate copy of each
arriving datagram to each care-of address, and the mobile node will
receive multiple copies of datagrams destined to it.
The mobile node SHOULD set the 'D' bit if it is registering with a
co-located care-of address. Otherwise, the 'D' bit MUST NOT be set.
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.
a) The IP header, followed by the UDP header, followed by the
fixed-length portion of the Registration Request, followed by
b) If present, any non-authentication Extensions expected to be
used by the home agent (which may or may not also be useful to
the foreign agent), followed by
c) An authorization-enabling extension, followed by
d) If present, any non-authentication Extensions used only by the
foreign agent, followed by
e) The Mobile-Foreign Authentication Extension, if present.
Note that items (a) and (c) MUST appear in every Registration Request
sent by the mobile node. Items (b), (d), and (e) are optional.
However, item (e) MUST be included when the mobile node and the
foreign agent share a mobility security association.
3.6.2. Receiving Registration Replies
Registration Replies will be received by the mobile node in response
to its Registration Requests. Registration Replies generally fall
into three categories:
- the registration was accepted,
- the registration was denied by the foreign agent, or
- the registration was denied by the home agent.
The remainder of this section describes the Registration Reply
handling by a mobile node in each of these three categories.
3.6.2.1. Validity Checks
Registration Replies with an invalid, non-zero UDP checksum MUST be
silently discarded.
In addition, the low-order 32 bits of the Identification field in the
Registration Reply MUST be compared to the low-order 32 bits of the
Identification field in the most recent Registration Request sent to
the replying agent. If they do not match, the Reply MUST be silently
discarded.
Also, the Registration Reply MUST be checked for 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.
3.6.2.2. Registration Request Accepted
If the Code field indicates that the request has been accepted, the
mobile node SHOULD configure its routing table appropriately for its
current point of attachment (Section 4.2.1).
If the mobile node is returning to its home network and that network
is one which implements ARP, the mobile node MUST follow the
procedures described in Section 4.6 with regard to ARP, proxy ARP,
and gratuitous ARP.
If the mobile node has registered on a foreign network, it SHOULD
re-register before the 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.3.6.2.3. Registration Request Denied
If the Code field indicates that service is being denied, the mobile node SHOULD log the error. In certain cases the mobile node may be able to "repair" the error. These include: Code 69: (Denied by foreign agent, Lifetime too long) In this case, the Lifetime field in the Registration Reply will contain the maximum Lifetime value which that foreign agent is willing to accept in any Registration Request. The mobile node MAY attempt to register with this same agent, using a Lifetime in the Registration Request that MUST be less than or equal to the value specified in the Reply. Code 133: (Denied by home agent, Identification mismatch) In this case, the Identification field in the Registration Reply will contain a value that allows the mobile node to synchronize with the home agent, based upon the style of replay protection in effect (Section 5.7). The mobile node MUST adjust the parameters it uses to compute the Identification field based upon the information in the Registration Reply, before issuing any future Registration Requests. Code 136: (Denied by home agent, Unknown home agent address) This code is returned by a home agent when the mobile node is performing dynamic home agent address resolution as described in Sections 3.6.1.1 and 3.6.1.2. In this case, the Home Agent field within the Reply will contain the unicast IP address of
the home agent returning the Reply. The mobile node MAY then
attempt to register with this home agent in future Registration
Requests. In addition, the mobile node SHOULD adjust the
parameters it uses to compute the Identification field based
upon the corresponding field in the Registration Reply, before
issuing any future Registration Requests.
3.6.3. Registration Retransmission
When no Registration Reply has been received within a reasonable
time, another Registration Request MAY be transmitted. When
timestamps are used, a new registration Identification is chosen for
each retransmission; thus it counts as a new registration. When
nonces are used, the unanswered Request is retransmitted unchanged;
thus the retransmission does not count as a new registration (Section
5.7). In this way a retransmission will not require the home agent
to resynchronize with the mobile node by issuing another nonce in the
case in which the original Registration Request (rather than its
Registration Reply) was lost by the network.
The maximum time until a new Registration Request is sent SHOULD be
no greater than the requested Lifetime of the Registration Request.
The minimum value SHOULD be large enough to account for the size of
the messages, twice the round trip time for transmission to the home
agent, and at least an additional 100 milliseconds to allow for
processing the messages before responding. The round trip time for
transmission to the home agent will be at least as large as the time
required to transmit the messages at the link speed of the mobile
node's current point of attachment. Some circuits add another 200
milliseconds of satellite delay in the total round trip time to the
home agent. The minimum time between Registration Requests MUST NOT
be less than 1 second. Each successive retransmission timeout period
SHOULD be at least twice the previous period, as long as that is less
than the maximum as specified above.
(page 46 continued on part 3)
