Internet Engineering Task Force S. Glass
Mobile IP Working Group Sun Microsystems
Internet Draft M. Chandra
Cisco Systems
August 2002
Registration Revocation in Mobile IPv4
draft-ietf-mobileip-reg-revok-04.txt
Status of This Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
This document is a submission to the Mobile IP Working Group of the
Internet Engineering Task Force (IETF). Comments should be
submitted to the MOBILE-IP@SUNROOF.ENG.SUN.COM mailing list.
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Distribution of this memo is unlimited.
Abstract
This document defines a Mobile IPv4 Registration Revocation mechanism
whereby either mobility agent participating in providing Mobile IP
services to the same mobile node can notify the other mobility agent
(or co-located mobile node) of the termination of either a single, or
multiple mobility bindings, and for this notification to be
acknowledged. Furthermore, if desired, a signaling mechanism already
defined by the base Mobile IP protocol [1] is leveraged as a way to
inform the mobile node of the revocation of this binding.
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Table of Contents
1. Introduction..................................................... 1
2. Motivation....................................................... 2
3. Revocation Message Formats....................................... 3
3.1 Revocation Support Extension................................... 4
3.1.1 Use in Agent Advertisements................................. 4
3.1.2 Use in Registration Messages................................ 5
3.1.3 Revocation Message Extension Format......................... 5
3.2 Revocation Message............................................. 7
3.2.1 Revocation Mask.............................................11
3.3 Revocation Acknowledgment Message..............................13
3.4 Replay Protection..............................................15
4. Registration Revocation Overview.................................17
4.1 Mobile Node Notification.......................................17
4.2 Registration Revocation Mechanism - Notification...............19
4.2.1 Home Domain Revoking a Registration.........................20
4.2.1.1 Home Agent Responsibilities..............................20
4.2.1.2 Foreign Agent Responsibilities...........................21
4.2.1.3 Co-located Mobile Node Responsibilities..................22
4.2.2 Foreign Domain Revoking a Registration......................22
4.2.2.1 Foreign Agent Responsibilities...........................22
4.2.2.2 Home Agent Responsibilities..............................23
4.2.3 Mobile Node Deregistering a Registration....................24
4.3 The Use of Bits................................................25
4.3.1 The 'R' Bit in Use..........................................25
4.3.2 The 'D' Bit in Use..........................................26
5. Error Codes......................................................27
6. Multiple Binding Support.........................................28
7. Security Considerations..........................................28
7.1 Agent Advertisements...........................................28
7.2 Revocation Messages............................................29
Appendix A Registration Revocation in 'R' and 'D' Bit Worlds........31
Appendix B Disparate Address, and Receiver Considerations...........33
Appendix C Using Registration Revocation to Help Release Resources..35
Appendix D An Example of the New Messages in Use....................36
D.1 The Registration Phase.................................36
D.2 The Revocation Phase...................................37
Acknowledgments......................................................39
Where to Direct Comments.............................................39
References...........................................................40
Full Copyright Statement.............................................41
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1. Introduction
A general registration revocation mechanism is defined for Mobile
IPv4, whereby a mobility agent can notify another mobility agent (or
co-located mobile node) of termination of a mobility binding. A
mobility agent which receives a revocation notification no longer has
to provide services to the mobile node whose registration has been
revoked. This means resources being consumed to provide Mobile IP
services for a mobile node that has stopped receiving Mobile IP
services by one agent, can be reclaimed by the other agent in a more
timely fashion than if it had to wait for the binding to expire.
(Note that Mobile IP 'services' refers to the various responsibilities
of the mobility agents as defined in [1]. Mobile IP 'resources'
refers to the different functional elements allocated by the mobility
agents to support the mobility binding, e.g., memory.)
Should foreign domain policy allow for it, informing the mobile node
that it is no longer being provided Mobile IP services enables that
mobile node to react faster than if it had to discover this for itself
(e.g. by waiting to have its attempt to re-register denied). This is
crucial as revocation messages are also useful when it is desirable
to prompt a mobile node to reregister. For example, if reverse
tunnels are now required by home domain policy, revoking registrations
that don't use reverse tunnelings prompt reregistration, which can be
denied with error 138: "reverse tunnel is mandatory, and 'T' bit not
set" [2].
The registration revocation protocol is broken into two disjoint
pieces. First, a signaling mechanism between home and foreign agents
is described which allows either the home or foreign domain to inform
the other it has torn down the current tunnel, allowing both to free
up the resources associated with the identified Mobile IP binding, and
to acknowledge this understanding. Second, a mechanism to inform the
mobile node of the revocation of its current binding is described
using a mechanism which already exists in the base Mobile IP protocol
[1]. This means any mobile node which is designed to comply with [1]
will understand it's registration has been revoked.
The mechanism described in this document is intended for mobility
agents playing the active role in ending a mobile node's service, and
is relayed between them. The revocation messages defined here are a
logical addition to the messages defined in [1] for the generic Mobile
IP registration process, and apply to situations where mobility agents
need to play an active role in administering Mobile IP registrations.
This document therefore describes methods to be used only when
mobility agents are initiating, and/or coordinating the release of
Mobile IP services.
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The mechanism described in this document do not replace the methods
described in [1] for deregistration messages as those apply to
situations where the mobile node is playing the active role in
informing the home agent of its location when it has roamed e.g., back
to its home subnet. This is to say the revocation message defined by
this document is NOT for use by co-located mobile nodes that are
terminating their registration as deregistration requests are already
sufficient for this purpose. A co-located mobile node, however, may
wish to process the messages defined in this document as it is a
useful mechanism to trigger the renegotiation of required services
from the home domain.
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [6].
2. Motivation
Mobile IP [1] defines registration of a mobile node's location to
provide connectivity between the mobile node and its home domain,
facilitating communication between mobile nodes and any correspondent
node. During the original design of Mobile IP, a passive mechanism,
namely the denial of a subsequent registration from a mobile node, in
conjunction with the recommendation for short registration lifetimes,
was considered sufficient for this purpose. Investigations into the
requirements for a AAA protocol specific to Mobile IP have forced
reconsideration of a more pro-active Mobile IP registration revocation
feature whereby if either domain wishes to revoke a current
registration, this can be communicated to the other domain providing
Mobile IP services for the mobile node whose binding is being revoked,
and potentially also have the mobile node informed that it is no
longer receiving Mobile IP services.
At any time, either home or foreign agent may wish to stop servicing a
mobile node, or for administrative reasons may no longer be required
to service a mobile node.
If the home agent stops providing Mobile IP services (for a mobile
node), the foreign agent will simply stop seeing tunnel packets whose
inner-destination address is that of the mobile node, just as if
e.g. a route stopped functioning, or if there is simply no traffic for
the mobile node. If, before prematurely shutting down a binding, the
home agent is capable of sending a revocation message to the foreign
agent, both the foreign agent (and the mobile node) get earlier
indication.
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If the foreign agent shuts down the tunnel, the home agent will
continue encapsulating packets destined for the mobile node, which
will likely be silently discarded by the foreign agent. An aware
mobile node may notice the lack of response to packets it is
generating, eventually guessing its binding may have vanished, then
attempt to re-register. Only at that time will the mobile node get a
registration denied error with some hint as to why its service was
stopped, and by which agent (based on the error code).
Another, more common scenario occurs when a mobile node roams away
from a foreign agent, which now no longer needs to provide Mobile IP
services to it. Notification to the previous foreign agent of this
state-change by a home agent could be useful to it (see Appendix C).
Another scenario occurs when either the home or foreign domain change
their policy with regards to services offered/required of Mobile IP
binding. For example, the home domain now requires reverse tunnels,
yet there are existing bindings that do not use them. Without a
revocation mechanism, new services can only be put in place or removed
as bindings are re-registered.
A revocation protocol is also useful in scenarios where a timely
release of resources is desirable, regardless of whether the mobile
node's binding was administratively revoked or terminated for another
reason. This has a favorable impact on resolving accounting issues
with respect to mobility binding in both domains as the actual end of
the registration is relayed.
In any of these scenarios, a mobile node must be able to continue to
operate as described by [1], even if its registration has been revoked
since many mobile nodes may not understand the concept of revocation.
Clearly, a more active yet unobtrusive mechanism needs to be provided
allowing more timely communication between the three Mobile IP
entities in the various administrative domains. Until now, no such
mechanism has been defined.
3. Revocation Message Formats
This section details the format of the signaling protocol between home
and foreign agents, or from a home agent to a co-located mobile node.
The following message-types are introduced by this document for
relaying revocation information:
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- Revocation Support Extension (Section 3.1):
o An extension appended to an agent advertisement by a foreign
agent to indicate its support of registration revocation.
o The revocation support extension may also be included in a
registration request or registration reply by a mobility
agent indicating its support of registration revocation to
the receiving agent.
o This extension may also be appended to a registration
request by a co-located mobile node to indicate its
understanding of revocation messages.
- Revocation Message (Section 3.2):
o A message sent by a mobility agent to inform another
mobility agent, or co-located mobile node, that it is
revoking the binding of a[t least one] mobile node. These
messages are not sent by mobile nodes, but may be received
by co-located mobile nodes.
o This message MUST NOT be sent by a co-located mobile node.
- Revocation Acknowledgment Message (Section 3.3):
o A message to indicate the successful receipt of a revocation
message.
o These messages are sent by mobility agents, or co-located
mobile nodes.
3.1 Revocation Support Extension
This section describes the use of the Revocation Support Extension by
foreign agents in their agent advertisements, and by home and foreign
agents in the registration process.
3.1.1 Used in Agent Advertisements
A foreign agent MAY include a Revocation Support Extension in its
agent advertisements to advertise to any of its links that it supports
the registration revocation mechanism on that link. This information
may be useful to a mobile node when deciding which foreign agent to
register through e.g. should the home domain require a binding with
such support. It is NOT required that all foreign agents advertising
on any link support registration revocation (unlike support of the 'R'
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bit as described by [1]), nor that registration revocation is
supported by any particular foreign agent on all of its links.
Security issues the reader should be aware of with agent
advertisements are covered in Section 7 and [5].
3.1.2 Used in the Registration Process
The Mobile IP revocation support extension MUST be attached to a
registration message by any entity that wants to receive registration
revocation messages. Normally, this is either a foreign agent, or a
home agent, however a co-located mobile node MAY also include a
revocation support extension in its registration request.
When appearing in a registration request, or registration reply, the
Mobile IP registration support extension MUST be protected either by a
foreign-home Authentication Extension, a mobile-home authentication
extension, or any other equivalent mechanism [1], e.g. via AAA [A],
[B]. If the extension appearing in either of these registration
messages is NOT protected, the appropriate action as described by [1]
MUST be taken. This is due to the security risks as described by
Section 7.
3.1.3 Revocation Support Extension Format
The format of the revocation support extension is based on the
Type-Length-Value Extension Format given in [1] and 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 |A|I| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
Skippable. To be assigned by IANA.
Length
Length (in bytes). Does NOT include Type and Length
(in accordance with [1]).
'A' Bit
When this bit is set to '1', the mobility agent is
specifying that it will send a revocation
acknowledgment in receipt of a registration revocation
message for this registration (if the other side
agrees to do the same). See Sections 3.2 and 4.2.
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'I' Bit
This bit is set to '1' by a mobility agent to indicate
it supports the use of the 'I' bit in revocation
messages. See Sections 3.2 and 4.2. Note that in all
revocation scenarios the use of the 'I' bit requires
acknowledgment, so if the 'I' bit is set to 1 in a
revocation extension, the 'A' bit MUST also be set to
'1'.
When sent by a foreign agent in a registration request:
If set to 1, indicates to the home agent that the
foreign agent is willing to have the 'I' bit as set by
the home agent in the revocation process determine
whether the mobile node is informed of the revocation,
or not.
If set to 0, indicates to the home agent that the
foreign agent will follow its own policy with regards
to informing the mobile node in the event of a
revocation.
When sent by a home agent in response to a revocation
extension in which the 'I' bit was set to 1:
If set to 1, the home agent agrees to use the 'I' bit
in the revocation process to indicate to the foreign
agent if the mobile node should be informed.
If set to 0, the home agent will not use the 'I' bit in
the revocation process, thereby yielding to the foreign
agent's default behavior with regard to informing the
mobile node.
Reserved
Reserved for future use. MUST be set to 0 on sending,
MUST be ignored on receiving.
Support of the 'A' bit and 'I' bit are OPTIONAL. If a mobility agent
does not support the specified behavior, it MUST set the respective
bit(s) to zero.
It is important to note that this extension is of the skippable
variety, that is if the receiving mobility agent doesn't understand
this extension, it MUST skip it, and continue processing the remainder
of the registration request. This is so a home agent will continue to
process the registration request which has been forwarded to it, and
send a registration reply even though the home agent doesn't support
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registration revocation. In this way, if local policy in the foreign
domain requires registrations to be made only with home agents which
support this feature, the foreign agent may actively deny the
registration with this home agent, and indicate this to the mobile
node (e.g. via 65 "administratively prohibited"). In contrast, if
this extension were not skippable, a home agent which doesn't
understand the revocation extension would silently discard the
registration, and there would be no feedback to either the foreign
agent, or the mobile node as to why this was occuring. In this way,
use of registration revocation can be negotiated for each
registration, and each domain has a clear understanding of what is
expected.
3.2 Revocation Message
The format of revocation message is analogous to registration request
and registration reply messages from [1].
IP fields:
Source Address In the case of the home agent issuing the
registration revocation, the address
registered with the care-of address as
that of the home agent.
In the case of the foreign agent issuing
the registration revocation:
If the registration being revoked is NOT
for a co-located mobile node, the address
registered with the home agent as the
care-of address.
In the case of the revocation of a
co-located mobile node, any of the
addresses of the foreign agent associated
with the foreign agent NAI which MUST
have been included in the most recent
registration request to the home agent.
Destination Address In the case of the home agent issuing the
registration revocation:
In the case where the revocation is not
for a co-located mobile node, the address
identified as the care-of address of the
mobile node,
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In the case where the home agent is
notifying the foreign agent servicing a
co-located mobile node, any of the
addresses associated with the NAI of the
foreign agent included in the most recent
approved registration request.
In the case of the foreign agent issuing
the registration revocation, the address
registered as that of the home agent by
the mobile node whose registration is
being revoked.
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 | Mask |A|I| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Foreign Domain Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Domain Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Foreign Domain Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Domain Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type To be assigned by IANA. (Registration Revocation)
Mask A set of flags indicating the applicability of the
registration revocation. See Section 3.2.1 for
definitions.
A Acknowledge bit.
This bit MUST NOT be set to '1' unless acknowledgment
support was negotiated in the last successful
registration of at least one of the mobile nodes whose
registration is being revoked, otherwise the results
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can be unpredictable.
Set to '0' when the revoking agent does NOT want an
acknowledgment for this revocation message, or when
the 'A' bit was not negotiated.
Set to '1' when the revoking agent is expecting an
acknowledgment of this revocation message.
I Inform bit.
This bit MUST NOT be set to '1' unless 'I' bit support
was negotiated in the revocation extension messages
passed in the registration process, otherwise the
results can be unpredictable.
When sent by the home agent to a foreign agent:
Set to '0' when the mobile node should NOT be informed
of the revocation or because the use of the 'I' bit
was not agreed upon.
Set to '1' to request that the mobile node be informed
of the revocation.
In either case, if the home agent wants the foreign
agent to acknowlege whether the mobile node was
informed or not, it MUST also set the 'A' bit to '1'.
Note that when sending a revocation message to a
co-located mobile node, this bit is irrelevant.
When sent by the foreign agent:
Set to '0' when the home agent doesn't have say in
whether the mobile node is to be informed of the
revocation, or because 'I' bit support was not agreed
upon.
Set to '1' to ask the home agent if the mobile node
should be informed of the revocation. Note that in
this case the foreign agent MUST also set the 'A' bit
to indicate to the home agent that it needs to
acknowledge this revocation message.
reserved MUST be sent as 0, ignored when received.
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Home Address
The IP address of the mobile node whose registration
is being revoked, or the subnet address to indicate
all mobile node registrations belonging to this subnet
are being revoked, or the zero address to indicate all
mobile nodes registered using this home agent and this
care-of address are being revoked. See Section 3.2.1.
Foreign Domain Address
The relevant IP address in the foreign domain to
identify which bindings are being revoked. This is
one of the following: (i) the foreign agent's IP
address, (ii) the co-located care-of address, (iii) the
subnet address of either (i) or (ii) indicating all
mobile nodes using a care-of address on the identified
subnet are having their bindings revoked, or (iv) the
zero address if all mobile node bindings using the
identified Home Domain Address(es) are being revoked.
See Section 3.2.1.
Home Domain Address
The IP address in the home domain to identify which
bindings are being revoked. This can be one of the
following: (i) the home agent's IP address, (ii) the
subnet address indicating that all mobile nodes using
a home agent on the identified subnet are having their
bindings revoked. See Section 3.2.1. The zero
address MUST NOT be used, as discussed in Appendix B.
Foreign Domain Identifier
Protects against replay and reflection attacks. When
used by the foreign domain, a 32-bit timestamp. When
used by the home domain, extends the timestamp in the
Home Domain Identifier field to uniquely identify a
specific binding. See Section 3.4.
Home Domain Identifier
Protects against replay and reflection attacks. When
used by the home domain, a 32-bit timestamp. When
used by the foreign domain, extends the timestamp in
the Foreign Domain Identifier field to uniquely
identify a specific binding. See Section 3.4.
A registration revocation message MUST be protected by a valid
authenticator, namely a home-foreign authenticator if the
communication is between home and foreign agents, or a mobile-home
authenticator if the communication is being sent from a home agent to
a co-located mobile node.
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An example of the use of revocation messages is given in Appendix D.
3.2.1 Revocation Mask
The use of the revocation mask in the revocation message is OPTIONAL.
If it is not used, it MUST be set to all zeros. The registration
revocation message MAY use this to indicate the "extent" of the
revocation, and is used in combination with the address fields defined
above. The following bit-values are defined. Note the low-order bits
apply to addresses, and the high-order bits apply to NAIs.
Value Meaning
----- -----------------------------------------------------------
0x01 Applies to all bindings where the mobile node home address
belongs to the same subnet as the address appearing in the
home address field. The address appearing in the home
address field MUST either be that of a registered mobile
node, or a subnet address. The issuing agent MUST include
a prefix-length extension defined by [1] appearing after
the revocation message to indicate the bits of address
being effected
(e.g. revoke: 10.1.1.128, prefixLen: 25).
0x02 Applies to all bindings where the care-of address belongs
to the same subnet as the address appearing in the foreign
domain address field. The address appearing in the foreign
domain address field MUST either be the co-located address
belonging to a mobile node, or a subnet address. The
issuing agent MUST include a prefix-length extension as
defined by [1] appearing after the revocation message to
indicate the bits of address being effected
(e.g. revok: 10.1.1.192, prefixLen: 26).
0x04 Applies to all bindings where the registration is using the
same foreign address appearing in the foreign domain
address field. The address appearing in the foreign domain
address filed MUST either the address registered as the
foreign agent address, or a subnet address in which case
the issuing agent MUST include a prefix-length extension as
defined by [1] appearing after the revocation message to
indicate the bits of address being effected
(e.g. revoke: 10.1.1.224, prefixLen: 27).
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0x08 Applies to all bindings where the registration is using the
same home agent address appearing in the Home Domain
Address field. The address appearing in the Home Domain
Address field MUST either be the address registered as the
home agent address, or a subnet address. The issuing agent
MUST include a prefix-length extension as defined by [1]
appearing after the revocation message to indicate the bits
of address being effected
(e.g. revoke: 10.1.1.240, prefixLen: 28).
Note that this address MUST NOT be the zero address, see
Appendix B.
0x10 Applies to all bindings within the same administrative
domain as the mobile node. The issuing agent MUST include
the NAI of [one of] the mobile nodes whose registration is
being revoked in an NAI extension, appearing after the
revocation message.
0x20 Applies to all bindings with the same administrative domain
as the care-of address. The issuing agent MUST include the
NAI from the care-of domain in an NAI extension, appearing
after the revocation message.
0x40 Applies to all bindings with the same administrative domain
as the foreign agent. The issuing agent MUST include the
foreign agent NAI in an NAI extension appearing after the
revocation message.
0x80 Applies to all bindings with the same administrative domain
as the home agent. The issuing agent MUST include the home
agent NAI in an NAI extension appearing after the
revocation message.
When multiple flags are set, the corresponding prefix-length and NAI
extensions MUST appear in the same order as the flags appearing in the
mask field of the revocation message. e.g. if the 0x02 and 0x01 bits
are set, the first prefix length extension is for the subnet of the
care-of address, and the second prefix length extension is for the
subnet of the home address.
For example, if the home agent is revoking bindings for all mobile
nodes belonging to a particular home subnet, the home agent sets the
mask to 0x01, and includes the prefix length extension for that
subnet. When the foreign agent receives an authenticated revocation
message in which the mask field is set to 0x01, it understands that
all mobile nodes whose home address falls in the subnet range
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indicated by the home address field and the prefix length extension
(using the home agent whose address appears in the home domain address
field) have been revoked.
For authentication where domain bindings are being revoked, the
appropriate NAI extension MUST be appended to the revocation message.
For example, a foreign agent revoking all binding from the same NAI
domain as mn@home.domain would set the mask to 0x10, and include an
NAI extension after the revocation message including the NAI
mn@home.domain. When the foreign agent receives an authenticated
message in which the mask is set to 0x10, it understands that all
mobile nodes that used an NAI containing @home.domain have been
revoked.
Note that the scope of the revocation MUST only apply to the subset of
bindings between the revoking agent, and the agent receiving the
revocation message. For example, if foreign agent x.y.z.t sends a
revocation message to home agent a.b.c.d for mobile node subnet
a.b.c.0, and sets the revocation mask to 0x01, the home agent MUST NOT
revoke all of its bindings for mobile nodes on the same home link as
the revoked mobile node, but only those mobile nodes that are also
visiting the foreign agent sending the revocation message.
Note: the distinction between bindings in the same subnet as the CoA
(0x02), and bindings in the same subnet as the foreign agent (0x04) is
to be able to discern bindings which may be on different links but
being served by the same foreign agent, e.g., a foreign agent may be
servicing different classless subnets [7], each with their own NAI, or
it may be multihomed but using the same CoA for all its subnets
(e.g. disparate address support). See Appendix B in [2] on disparate
address support for a related discussion.
3.3 Revocation Acknowledgment Message
The format of revocation acknowledgment message is analogous to the
revocation message.
IP fields:
Source Address Copied from the destination address of
the received registration revocation
message for which this registration
revocation acknowledgment message is
being generated.
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Destination Address Copied from the source address of the
received registration revocation message
for which this registration revocation
acknowledgment message is being
generated.
UDP fields:
Source Port 434 (copied from the destination port of
the revocation message)
Destination Port Copied from the source port of the
revocation 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 | Length |I| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Foreign Domain Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Domain Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type To be assigned by IANA. (Revocation Acknowledgment)
Length 10. The length of the revocation acknowledgment
message on octets, not including Type and Length fields.
I Inform bit.
The 'I' bit MUST NOT be used in the revocation
acknowledgment messages unless it was set to '1' in the
revocation message.
When sent by the home agent:
Set to '1' by the home agent to request the foreign
agent inform the mobile node of the revocation.
Set to '0' by the home agent to request the foreign
agent not inform the mobile node of the revocation.
When sent by a foreign agent:
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Set to '1' to indicate to the home agent that the mobile
node was/will be informed of the revocation.
Set to '0' to indicate to the home agent that the mobile
node was/will not be informed of the revocation
Set to the OPPOSITE of what the home agent set the 'I'
bit to in the revocation message to indicate the foreign
agent is NOT revealing whether the mobile node was
informed or not. See Section 4.1.1.2 for an example.
reserved MUST be sent as 0, ignored when received.
Foreign Domain Identifier
Copied from the Foreign Domain Identifier field of the
revocation message for which this revocation
acknowledgment is being generated. See Section 3.4.
Home Domain Identifier
Copied from the Home Domain Identifier field of the
revocation message for which this revocation
acknowledgment is being generated. See Section 3.4.
A registration revocation acknowledgment message MUST only be sent in
response to a registration revocation messages in which the 'A' bit
was set to 1.
A revocation acknowledgment message MUST contain a valid authenticator,
namely a home-foreign authenticator if the communication is between
home and foreign agents, or a mobile-home authenticator if the
communication is between home agents and co-located mobile nodes.
An example of the use of the revocation messages is given in Appendix D.
3.4 Replay Protection
As registration revocation messages are designed to terminate service
for a mobile node, replay protection is crucial to prevent denial of
service attacks by "malicious repeaters" - those who store datagrams
with the intent of replaying them at a later time, or by "malicious
reflectors" - those who reflect packets back at their original source
(both a form of "man-in-the-middle" attack). See Section 7 for a
discussion of these security considerations.
Replay protection is handled in analogy with the mechanism defined by
[1], through two 32-bit identifier fields in the registration
revocation message and revocation acknowlegement messages.
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- The agent revoking a registration sets their domain Identifier
field in the registration revocation message to a valid 32-bit
timestamp, and sets the other Identifier field to a value which,
when combined with the 32-bit timestamp, uniquely identifies this
revocation. In order to protect against reflection attacks, the
value used in the other domain's Identifier field MUST NOT be the
same as the timestamp value appearing in that domain's own field.
- Upon receipt of an authenticated revocation message, the
receiving agent MUST check the values in both Identifier fields
to make sure this revocation message is neither a replay of an
old revocation message received from the same agent, nor a
reflection of a revocation message it sent. If the Identifier
fields imply this packet is a replay, the revocation message MUST
be silently discarded.
- When building a revocation acknowledgment message the
acknowledging agent copies the Foreign Domain Identifier and Home
Domain Identifier fields of the registration revocation
message into the Foreign Domain Identifier and Home Domain
Identifier fields of the revocation acknowledgment message
respectively.
- Upon receiving a valid revocation acknowledgment, the revoking
agent MUST check both Identifier fields to make sure they match
the Identifier fields from the corresponding revocation message
it sent. If not, the revocation acknowledgment message MUST be
silently discarded.
Note that since one of the identifiers in an incoming revocation
message is a 32-bit timestamp, it is possible for an agent, upon
identifying the binding(s) for which the revocation message applies,
to check the validity of the Identifier fields without having to
remember all identifiers sent by a corresponding agent. Implementors
are advised to see the information Section 7 when taking this
approach.
Note: as it is possible for a mobile node to register at different
times with different home agents, and at different times with
different foreign agents, it is crucial that it not be required that
the Identifier fields be unique in messages from different agents as
there is no method for this information to be communicated between
agents. For example, if a mobile node has simultaneous bindings with
multiple foreign agents, and two or more are being revoked
simultaneously, it is possible the revocation message from one of
these foreign agents may contain Identifier fields that happen to
match that of any or all the other foreign agents. This MUST NOT
result in any of these revocation message being ignored.
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4. Registration Revocation Overview
Registration Revocation consists of two disjoint pieces: a signaling
mechanism between tunnel endpoints, and a signaling mechanism between
foreign agent and mobile node. A co-located mobile node (a mobile
node which is decapsulating its own tunnel traffic) MAY implement the
revocation acknowlegement portion of this protocol in order to respond to
revocation messages from its home agent. However, a co-located mobile
node MUST NOT send a revocation message as deregistration messages
defined in [1] are sufficient for this purpose.
4.1 Mobile Node Notification
A mechanism which provides a foreign agent a way to actively notify a
mobile node that its binding has been reset already exists in [1],
though it has been overlooked for this purpose.
The following paragraph contains a brief overview of the mechanics of
the sequence number in agent advertisement from [1] so the mechanism
by which the foreign agent 'implies' to the mobile node that its
binding is no longer active is clearly understood.
When a foreign agent begins sending agent advertisements, it starts
with a sequence number of 0, and [monotonically] increments the
sequence number with each subsequent agent advertisement. In order
for a mobile node to be able to distinguish between a foreign agent
that has simply exhausted the sequence number space from one which has
been reset, and thereby lost the mobile node's binding information, a
foreign agent sets the sequence number to 256 instead of rolling to 0.
In this way, a mobile node would have to miss, at that time, 256
advertisements in a row to mistake a reset as a roll-over. Moreover,
the lifetimes contained within an agent advertisement should be set in
such a way that when a mobile node believes it has missed 3 beacons,
the entry for this foreign agent should time out, and if the mobile
node is registered there, it should send an agent solicitation [1].
If, however, an agent is somehow reset, it will begin advertising with
a sequence number of 0, and the mobile node can presume this foreign
agent has lost its binding, and the mobile node SHOULD re-register to
make sure it is still obtaining Mobile IP services through this
foreign agent.
Leveraging this mechanism, a foreign agent may consciously notify all
mobile nodes currently bound to it that it has "reset" all their
bindings, even if the agent itself has not been reset, by simply
[re]setting the sequence number of the next agent advertisement to 0.
Moreover, a foreign agent may inform all mobile nodes currently bound
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to it they should re-register with a different foreign agent by
simultaneously setting the sequence number to 0, and also setting the
'B' bit in the advertisement to 1, indicating this foreign agent is
busy and is not accepting new registrations [1]. In these situations,
any mobile node in compliance with [1] will presume this foreign agent
has lost its binding, and must re-register if they wish to
re-establish Mobile IP functionality with their home subnet.
To indicate to any registered mobile node that its binding no longer
exists, the foreign agent with which the mobile node is registered may
unicast an agent advertisement with the sequence number set to 0 to
the mobile node [1], [3].
Moreover, if such a foreign agent wishes to indicate to the mobile
node that its binding has been revoked, and that the mobile node
should not attempt to renew it's registration with it, the foreign
agent MAY also set the 'B' bit to 1 in these agent advertisements,
indicating it is busy, and is not accepting new registrations [1].
All mobile nodes compliant with [1] will understand this means the
agent is busy, and MAY either immediately attempt to re-register with
another agent in their foreign agent cache, or MAY solicit for
additional agents. In the latter case, a foreign agent can optionally
remember the mobile node's binding was revoked, and respond to the
solicit in the same way, namely with the 'B' bet set to 1. It should
be noted, though, that since the foreign agent is likely to not be
setting the 'B' bit to 1 in its broadcasted agent advertisements (sent
to the entire link), the revoked mobile node, upon hearing this
agent's multicast agent advertisement without the 'B' bit set may
attempt to [re]register with it. If this happens, depending of
foreign domain policy, the foreign agent can simply deny the mobile
node with an appropriate error code (e.g., "administratively
prohibited"). At this time, a mobile node MAY use foreign agent
fallback to attempt to register with a different foreign agent as
described in [1]. Mobile nodes which understand the revocation
mechanism described by this document may understand that a unicast
agent advertisement with the sequence number reset to 0 could indicate
a revocation, and may attempt to register, or co-locate.
Agent Advertisements unicast to a mobile node MUST be sent as
described in [1] in addition to any methods currently in use on the
link to make them secure or authenticatable. Such mechanisms are
highly desirable to protect from denial-of-service attacks [5].
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4.2 Registration Revocation Mechanism - Notification
Any active mechanism designed to be useful to both home and foreign
domains must contain a way to securely revoke a current binding. This
means the registration process must supply both home and foreign
domains with enough information to be able to inform the other side of
any revocation. Moreover, a revocation message MUST NOT be sent for a
registration that has expired, and MUST only be sent prior to the
expiration of a mobile node's registration.
During the registration process, if the foreign agent wishes to
participate in revocation messages with the home domain, it MUST
append a revocation support extension (defined in Section 3.1) to the
registration request. If the corresponding registration reply from
this home agent does not contain a revocation support extension, the
foreign agent MAY assume the home agent/domain does not understand
registration revocation, or is unwilling to participate. Note that in
this case, as specified in [1], both registration request and
registration reply MUST contain a home-foreign authenticator.
If a home agent wishes to participate in revocation messages with the
foreign domain, it MUST append a revocation support extension to the
registration reply. If the registration request from a foreign agent
does not contain a revocation support extension, the home agent MAY
assume the foreign agent/domain does not understand registration
revocation, or is unwilling to participate specifically for this
binding. The home agent MAY include a revocation support extension in
the registration reply anyway.
If a co-located mobile node wishes to be informed of a released
binding by the home agent, it MUST insert a revocation support
extension into the registration request.
The 'A' bit in the revocation extension is used to indicate the desire
to use acknowledgment messages in response to revocation messages for
the binding to which they correspond. The use of acknowledgment
messages for this registration is negotiated, and is offered by the
foreign agent, and agreed to by the home agent. More precisely, by
sending a revocation extension in which the 'A' bit is set, the
foreign agent is indicating it would like the home agent to resend any
revocation message for this registration for which it does not receive
a revocation acknowledgment message, and is offering to do the same
for the home agent. The home agent then can disagree, in which case
it simply does not set the 'A' bit, or agree by setting the 'A' bit to
1 in the revocation extension appearing in the registration reply.
The 'I' bit in the revocation extension is used to indicate the
decision to inform the mobile node its binding has gone away will be
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left to the home agent. As with the 'A' bit, this functionality is
offered by the foreign agent, and accepted by the home agent. More
precisely, by sending a revocation extension attached to a
registration request in which the 'I' bit is set to 1, the foreign
agent is indicating to the home agent that it MAY leave the decision
to inform this mobile node its registration has gone away to the home
agent (the term "MAY" is used here because it is recognized that
domain policy may change during the lifetime of any registration).
The home agent can acknowledge that it wishes to do this by setting
the 'I' bit to 1, or it can indicate it will not do so by setting the
'I' bit to 0 in the revocation extension appearing in the registration
reply.
If any agent, or co-located mobile node receives a registration
revocation message that does not contain a valid authenticator, namely
a home-foreign authenticator if the revocation message is between home
and foreign agents, or a mobile-home authenticator if the message is
from home agent to co-located mobile node, the revocation message MUST
be ignored, and silently discarded.
Revocation support is considered to be negotiated when both sides have
included a revocation support extension during a successful
registration exchange.
4.2.1 Home Domain Revoking/Releasing a Registration
The following section details the responsibilities of each party
depending on the functionality negotiated in the revocation support
extension when the home domain is revoking a registration.
4.2.1.1 Home Agent Responsibilities
In the case where a home agent is revoking/releasing a mobile node's
binding, and revocation support has been negotiated, the home agent
MUST notify the foreign domain address it is terminating the tunnel
entry point by sending a revocation message. Note that the foreign
domain address can either be the foreign agent's care-of address, or a
co-located care-of address.
Note that if a co-located mobile node included a revocation support
extension in its registration request, and passed it to a foreign
agent (because the 'R' bit was set) in which case the foreign agent
also included a revocation support extension, there may be a need to
send a revocation message to both mobile node and foreign agent. If
the 'A' bit was negotiated because it was set to '1' in both
revocation extensions passed in the registration process, the home
agent MUST set the 'A' bit to '1' in the revocation message and expect
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a revocation acknowledgment in return. If the home agent does not
receive a revocation acknowledgment message with in a reasonable
amount of time, it MUST retransmit the revocation message. How long
the home agent waits to retransmit, and how many times the message is
retransmitted is only limited by the requirement that the home agent
MUST NOT send more than one revocation per second for a particular
binding, that the time between retransmissions SHOULD fall-back in
analogy with the registration guidelines in [1], and that it MUST NOT
retransmit revocation messages beyond the normal life of the revoked
binding.
If the revocation message is being sent to a foreign agent, and the
use of the 'I' bit was negotiated in the registration process, the
home agent MUST set the 'I' bit to 1 if the home agent would like the
foreign agent to inform the mobile node of the revocation. Conversely,
if the home agent does not want the mobile node notified, it MUST set
the 'I' bit to 0. Use of the 'I' bit, by definition, requires the 'A'
bit to be set to '1'.
The home agent MUST set the Identifier fields as defined in Section
3.4, and include either a home-foreign authenticator, or a mobile-home
authenticator if the revocation message is being sent to a co-located
mobile node.
4.2.1.2 Foreign Agent Responsibilities
Upon receiving a registration revocation message, the foreign agent
MUST check the home-foreign authenticator as defined by [1], and the
Identifier fields as defined by Section 3.4. The foreign agent must
then identify the binding(s) described by the home agent as being
released using the information in the revocation message, namely the
addresses identified by the mobile node address, the foreign domain
address, and the home domain address. Upon locating such bindings, if
the 'A' bit was negotiated in the registration phase, and the 'A' bit
is now set in the revocation message, the foreign agent MUST respond
with a revocation acknowledgment sent to the source address of the
revocation message. If the 'I' bit was also negotiated, the foreign
agent MUST check the value of the 'I' bit in the revocation message
and act accordingly. If notifying the mobile node by the methods
described in Section 4.1, the foreign agent MUST set the 'I' bit to
'1' in the revocation acknowlegement to be sent to the home agent, or
if not notifying the mobile node, the foreign agent MUST set the 'I'
bit to '0'. If however, since the negotiation of the 'I' bit in the
most recent registration, the policy on the foreign domain has changed
so that use of the 'I' bit is no longer allowed, the foreign agent
MUST set the 'I' bit in the revocation acknowledgment to the OPPOSITE
of what was sent by the home agent. This indicates to the home agent
that its wishes were not necessarily followed, and the setting of the
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'I' bit does not indicate whether the mobile node was notified or
not. The foreign agent may also free up any resources that were being
used by the former binding(s), and discontinue all Mobile IP services
for the identified mobile nodes.
4.2.1.3 Co-located Mobile Node Responsibilities
Upon receiving a revocation message, the co-located mobile node MUST
check the mobile-home authenticator, and the identifiers. If the
packet passes authentication, the mobile node MUST verify that the
information contained in the revocation messages identifies the home
agent with which it has a current binding, that this binding
identifies correctly this mobile node and any foreign domain address
it is currently using. If the mobile node is able to identify such a
binding, the mobile node SHOULD immediately terminate any reverse
tunnel encapsulation it is using to this home agent. The mobile node
may also free up any other resources associated with the former
binding. In addition, if the 'A' bit was negotiated in the
registration process, and the 'A' bit was set in the revocation
message, the mobile node MUST immediately generate a revocation
acknowledgment message in response. The revocation acknowledgment
message MUST be sent to the IP source address of the revocation
message.
4.2.2 Foreign Domain Revoking/Releasing a Registration
The following section details the responsibilities of each party
depending on the functionality negotiated in the revocation support
extensions when the foreign domain is revoking a registration.
4.2.2.1 Foreign Agent Responsibilities
If the most recent registration request indicated the mobile node is
using a co-located care-of address (the 'D' bit was set), and hence
the registration request was unlikely to contain any information about
the foreign agent, the foreign agent MUST include an NAI extension for
identification by the home agent in the revocation message. (Note: it
is presumed the foreign agent had the 'R' bit set when such a
co-located mobile node last registered, but even if not, obviously the
foreign agent is aware of this binding. In this case, the foreign
agent is implying to the home agent this it and/or the foreign domain
in general have some way of controlling communications between the
mobile node and home agent, and therefore a revocation message in this
case implies to the home agent that such lines of communication are
going away. See Section 4.3.1 "Use of the 'R' bit").
If the 'A' bit was negotiated in the revocation extension, the foreign
agent MUST set the 'A' bit to 1 in the revocation message.
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If the use of the 'I' bit was negotiated, and the policy of informing
the mobile node has not changed since the last successful registration
exchange, the foreign agent MUST NOT inform any mobile node of its
revocation at this time. Instead, the foreign agent MUST set the 'I'
bit to '1' in the revocation message, thereby asking the home agent to
use the 'I' bit in the revocation acknowledgment to indicate if it
should notify the effected mobile nodes. If the policy on the foreign
domain has changed since the most recent successful registration, and
the foreign agent is no longer able to use the 'I' bit, the foreign
agent MUST set the 'I' bit to '0', and follow the policies of the
foreign domain with regard to notifying the mobile node.
Note, when setting the 'I' bit to 1, the 'A' bit MUST also be set to
'1'.
If the use of the 'I' bit was not negotiated, the foreign agent MAY
inform the mobile node of its revocation as described in Section 4.1,
before sending the revocation message to the home agent, just after
sending the revocation message to the home agent, or after it receives
a revocation acknowledgment message from the home agent, or MAY NOT
inform the mobile node at all.
If the 'A' bit was set to '1' in the revocation message, the foreign
agent SHOULD wait for a revocation acknowledgment message from the
home agent. How long the foreign agent waits to retransmit, and how
many times the message is retransmitted is only limited by the
specification that the foreign agent MUST NOT send more than one
revocation per second for a particular binding, SHOULD set its
retransmissions to fall-back in analogy with the registration
guidelines in [1], and MUST NOT retransmit revocation messages beyond
the normal life of the revoked binding.
4.2.2.2 Home Agent Responsibilities
Upon receiving a registration revocation message, the home agent MUST
check the foreign-home authenticator, and identifier fields. If the
packet is acceptable, the home agent MUST locate the binding(s)
identified by the foreign agent as being released using the
information in the revocation message, namely the addresses identified
by the home address, the foreign domain address, and the home domain
address fields. Since these bindings are no longer active, the home
agent can free up any resources associated with the former bindings
and discontinue all Mobile IP services for them.
If a home agent receives a registration revocation from a foreign
agent indicating the registration for a co-located mobile node has
been revoked, the home agent MUST verify this message is coming from
the same foreign agent that relayed the registration request for this
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binding. If this can not be verified, or it can be determined that
the foreign agent issuing this registration revocation is not the
foreign agent which relayed the registration request for the current
binding, the home agent MUST silently ignore this revocation message.
This means if a mobile node has one or more bindings with different
co-located care-of addresses, and in addition has one or more bindings
using foreign agent provided care-of addresses (as in the case of
multi-homed foreign agent), each set of bindings must be revoked
separately.
If use of the 'A' bit was negotiated, and the 'A' bit is set to '1' in
the revocation message, the home agent MUST send a revocation
acknowledgment to the IP source address of the registration revocation
message.
If use of the 'I' bit was negotiated, and the 'I' bit is set to '1' in
the revocation message, the home agent SHOULD decide if it wants the
mobile node informed of the revocation of this binding. If it does
want the mobile node informed, it MUST set the 'I' bit in the
revocation acknowledgment message to '1'. If it does not want the
mobile node informed, it MUST set the 'I' bit to '0'.
The home agent MUST set the Identifier field as described by
Section 3.4, and protect the revocation acknowledgment message with a
home-foreign authenticator or equivalent mechanism before sending the
revocation acknowledgment message.
4.2.3 Mobile Node Deregistering a Registration
The cases where a mobile node is registered with its home agent,
whether it is registered directly with its home agent, or registered
via a foreign agent (and in either case may be co-located), and wishes
to terminate it's own binding, the mobile node MUST NOT send a
revocation message, but SHOULD simply deregister the appropriate
care-of address with its home agent as described by [1].
Note that when a co-located mobile node includes a revocation
extension in its registration request, it will expect to see a
revocation extension in the registration reply from the home agent
acknowledging that the home agent is willing to send revocation
messages to such a mobile node. However, the home agent should
understand that it will not receive revocation messages from such a
mobile node, so the decision to include a revocation extension in the
registration reply MUST NOT be made based on the home agent's desire
to see such messages. A co-located mobile node, and home agent MAY
negotiate the use of the 'A' bit, and use it in the revocation
process.
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4.3 The Use of Bits
Several of the bits used in the registration process play an important
role in the topology of the binding, and therefore the revocation.
The 'R' bit in the agent advertisement is an effective way to assure
that the foreign domain is provided with the binding information
necessary to revoke it. This is especially important in the case of a
co-located mobile node whose registration request may otherwise
by-pass the foreign agent. How the foreign domain enforces this
policy is beyond the scope of this document, but a few examples are
through either restricted access to topologically correct addresses
with which to co-locate, some form of access control lists on the
routers, etc.
The 'D' bit in the registration request is a sign to a home domain
that a foreign agent address likely does NOT appear in the
registration request, and so the home agent may be missing the
necessary information to be able to notify the foreign domain in the
event of a registration revocation. A home agent that receives a
registration request in which the 'D' bit is set SHOULD look for a
foreign agent NAI extension appearing after the mobile-home
authenticator, indicating the foreign agent to which any revocation
messages are to be sent. Note that if the registration request came
from a foreign agent (e.g. the 'R' bit was set), then the source
address of the registration request could be that of the foreign
agent. The home agent MAY check to see if that address is the care-of
address identified in the care-of address field of the registration
request (if not, the source address of the registration request is
likely that of the foreign agent). Alternatively, as any such NAI
extension MUST be secured via a foreign-home authenticator [1], the
security association identified by the foreign agent's NAI MAY be used
to identify an IP address to which any revocation message is to be
sent.
More information, and an example, is provided in "Appendix A:
registration revocation in 'R' and 'D' Bit Worlds."
4.3.1 The 'R' Bit in Use
If the foreign agent wishes to be able to revoke a mobile node's
registration, it MUST set the 'R' bit in its agent advertisements. A
foreign agent advertising the 'R' bit requests every mobile node, even
one that is co-located, to register with the foreign agent, making the
foreign agent then capable of revoking the mobile node's registration
as it is now privy to the information in the registration request,
namely the mobile node's link address, home IP address, and the
address of the mobile node's home agent. The foreign agent will then
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be capable of unicasting an agent advertisement to the mobile node,
and sending a registration revocation message to the home agent, as
described in Sections 4.1 and 4.2.2. This makes advertising the 'R'
bit attractive in domains that wish to have control over registered
mobile nodes, and moreover, sending revocation notification to the
home domain may be a contractual necessity.
Enforcement of the 'R' bit is beyond the scope of this document, and
when the foreign domain wishes to be able to enforce the revocation,
and/or to notify the home domain of revoked registrations, such an
enforcement mechanism is crucial. Since the co-located care-of
address is by definition topologically correct, the reader must
understand that a co-located mobile node may send the registration
request directly to the home agent, and perhaps receive a registration
reply from the home agent even in the presence of agent advertisements
in which the 'R' bit is set (though this is considered "bad form", and
the 'R' bit is a "hint" to the mobile node that such direct contact is
policed in some way). In the event a foreign domain wishes to enforce
the revocation of a co-located mobile node's registration, the foreign
domain MUST be able to control the flow of datagrams to, and/or from,
the mobile node. The easiest way to do this is for local policy to
not provide a mechanism for co-location, or for the foreign agent to
simply not accept registration requests from co-located mobile nodes,
though there are enforcement mechanisms available which allow
co-location.
4.3.2 The 'D' bit in Use
If the mobile node has set the 'D' bit in the registration request,
and forwards the registration via a foreign agent (see Section 4.3.1),
and the foreign agent determines it will accept the registration from
the mobile node, then the foreign agent MUST append a revocation
extension if it wants to be notified of registration revocations for
this mobile node (in addition to any already included by the mobile
node and protected by the mobile-home authenticator). The foreign
agent MAY also append the prefix length extension [1] that appears in
the agent advertisements on the link from which the mobile node is
registering. The inclusion of the prefix length extension is so the
home agent understands the topology of the link the mobile node is
visiting in the event it wants to issue a single registration
revocation for multiple mobile nodes visiting the same link. In order
to authenticate these extensions, the foreign agent MUST append its
NAI extension so the home agent may know how to validate the
foreign-home authenticator.
Note that registration requests where the 'D' bit is set, and which
are relayed through a foreign agent due to the advertising of the 'R'
bit can contain the foreign agent address as the source address of the
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registration request as received by the home agent. A home agent MAY
verify that the source address of this registration request is not the
same as the care-of address contained in the registration request, and
is therefore likely to be the address of a foreign agent. The reader
should note that according to [1] the home agent is required to send
the registration reply to the source IP address of the registration
request.
If the home agent wishes to support registration revocation for this
binding, which can be conveyed to either the co-located mobile node,
and/or the foreign agent whose NAI appears in the registration
request, it MUST include a revocation support extension in the
mobile-home portion of the registration reply if it agrees to notify
the mobile node of a release of this registration, and/or MUST include
a revocation support extension in the foreign-home portion of the
registration reply if it agrees to notify the foreign agent of a
release of this registration. Note that, as determined by the
security association with the foreign agent, it may be necessary for
the home agent to include its NAI extension for use by the foreign
agent in authenticating the registration reply.
Upon release of this registration, if revocation support was
negotiated with BOTH the co-located mobile node, and the foreign
agent, revocation messages for this binding MUST be first sent to the
mobile node's co-located care-of address to ensure that they reach it.
This includes the time for any necessary retransmissions should a
revocation acknowledgment not be forthcoming. A revocation message
MUST then be sent to the foreign agent. If a revocation message was
sent to the foreign agent first, there may be no way for a revocation
message to then reach the mobile node. Also, in order for the home
agent to be able to send a revocation message to the foreign agent, it
is presumed that the source IP address of the most recently accepted
registration request is that of the foreign agent, or that the
security association between the home and foreign agents contains
(among other things) the NAI and IP addresses of the foreign agent.
5. Error Codes
As the intent of a registration revocation message is not a request to
discontinue services, but is a notification that Mobile IP services
are discontinued, there are no new error codes.
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6. Multiple Binding Support
RFC 3220 [1] allows a mobile node to register multiple care-of
addresses with its home agent. Each one of these bindings is it's own
discrete registration, and hence can be treated as a separate case for
registration revocation. Consider the situation where a mobile node
has multiple care-of addresses registered with a single foreign agent
(either because the mobile node has multiple co-located care-of
addresses and the foreign agent has the 'R' bit set, or because the
foreign agent is multi-homed). Either agent can indicate the
revocation of all these bindings by setting the care-of address field
to the zero address (and their addresses in the correct places),
indicating all care-of addresses being used by the indicated mobile
node that are bound to the agent identifying itself by this revocation
message.
If a foreign agent revokes a particular mobile node's binding(s) with
it, the home agent MAY or MAY NOT then revoke any of the mobile node's
other bindings (with other foreign agents). If a home agent is
revoking one of a mobile node's bindings, it MAY revoke none, or all
of the mobile node's other bindings as it sees fit. If a home agent
decides to revoke a single binding for a mobile node, the foreign
agent MAY decide to revoke other bindings for the same mobile node as
it sees fit.
The revocation mask is designed to make revoking multiple bindings
easier to specify, and, through the use of NAIs, even in situations
where network topologies make revoking multiple mobile node bindings
difficult to specify numerically.
7. Security Considerations
There are two basic susceptibilities, one in agent advertisement
mechanism, and one relating to unauthorized revocation messages.
7.1 Agent Advertisements
Although the mechanisms defined by this document do not introduce
this, it has been recognized that agent advertisements as defined in
[1] provide a denial-of-service potential. This is because the agent
advertisements as defined in [1] may be spoofed by other machines
residing on the link. This makes it possible for such nodes to trick
the mobile node into believing its registration has been revoked
either by unicasting an advertisement with a reset sequence number to
the link-local address of the mobile node, or by broadcasting it to
the subnet, thereby tricking all mobile nodes registered with a
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particular foreign agent into believing all their registrations have
been lost. For particulars regarding this issue, the reader is
referred to [5].
There has been some work in this working group and others (e.g. IPSec)
to secure such router advertisements, though at the time of this
publication, no solutions have become common practice. To help
circumvent possible denial of service issues here, bringing their
potential for disruption to a minimum, mobile node implementors should
ensure that any agent advertisement which doesn't conform to a strict
adherence to [1], specifically those whose TTL is not 1 or e.g. which
do not emanate from the same link-address (when present) as the last
successful registration reply, be silently discarded.
7.2 Revocation Messages
As registration revocation, when performed, terminates Mobile IP
services being provided to the mobile node, it is crucial that all
security and replay protection mechanisms be verified before a
mobility agent recognizes that the other agent has revoked a mobile
node's binding, and frees up the resources it has reserved for this
service. Messages which are sent link-local (e.g. between mobile node
and foreign agent) MAY also be secured by methods outlined in [1],
namely the use of challenge-response and mobile-foreign authenticators,
but these have no relation, and hence do not add any security to the
enhancements described by this document.
RFC 3220 [1] defines a security mechanism that MUST be used between
home agents and mobile nodes, and MAY used between home agents and
foreign agents, namely the use of authenticators. However, revocation
messages as defined in this document which are passed between home and
foreign agents in the revocation process MUST be protected by the same
foreign-home authenticators defined in [1]. As a result, these
messages are as secure as registration messages passed between home
and foreign agents and containing home-foreign authenticators as
defined in [1]. As a result, there are no new security threats
introduced by the revocation mechanism than those present in [1] with
respect to the compromise of the shared secret which is used to
generate the home-foreign authenticators.
That said, there are two types of replay attacks which use messages
captured "in flight" by a man-in-the-middle between the home and
foreign agents - "malicious repeaters" and "malicious reflectors".
In the case of a "malicious repeater", a man-in-the-middle captures a
revocation message then replays it to the same IP destination address
at a later time. Presuming the authenticator of the original packet
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was deemed valid, without replay protections the home-foreign
authenticator of the replayed packet will (again) pass authentication.
Note that since datagrams are not guaranteed to arrive unduplicated, a
replay may occur by "design".
In the case of a "malicious reflector" where a man-in-the-middle
captures a revocation message then returns it to its originator at a
later time, using a security association that involved a (single)
shared secret which only protects the contents of the UDP portion of
the packet (such as home-foreign authenticators as defined by [1]),
without replay protection the sender of the packet will also believe
the revocation message to be authenticated.
The replay protection mechanism used by the revocation messages
defined by this document is designed to protect against both of these
man-in-the-middle attacks. As a benefit, by using a 32-bit timestamp
it can be more quickly determined if revocation messages are replays,
though the reader is advised to use caution in this approach. An
agent which receives an authenticated revocation message can compare
the Identifier field to that of a previously received revocation
message, and if the timestamp in the Identifier field is found to
postdate that of the revocation message with the latest timestamp, it
can immediately be determined as not being a replay. Note however
that since datagrams are not guaranteed to arrive in order, it should
not be presumed that because the values contained in an Identifier
field are timestamps that they will necessarily be increasing with
each successive revocation message received. Should an implementor
decide to base his replay detection mechanism on increasing
timestamps, and therefore increasing Identifier values, a suitable
time window should be defined in which revocation messages can be
received. At worst, should the 'A' bit have been negotiated, ignoring
any revocation message should result in the retransmission of another
revocation message, this time with timestamp later than the last one
received.
RFC 2794 [4] describes the use of the Network Access Identifier in
Mobile IP. Its relevant use here is for agents to be able to identify
each other in a revocation message. In cases where foreign and home
domains are going to approve registrations from co-located mobile
nodes (registering with the 'D' bit [1]), and in topologies where the
foreign agent is setting the 'R' bit in its agent advertisements [1],
if the agents are going to use their NAIs to identify themselves the
security association they share MUST include the IP address the
registration revocation messages are to be sent to, and will be sent
from. For a more involved discussion, see Appendix A.
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Appendix A: Registration Revocation in 'R' and 'D' Bit Worlds.
Two issues influence the registration of a mobile node on a foreign
subnet. First is the mobile node's desire to co-locate (use of the
'D' bit). Second is whether the foreign domain is requiring
registration through foreign agents (the infamous 'R' bit). This
leads to several different registration topologies in Mobile IP (note:
cases where the mobile node is using a private address are considered
in Appendix B):
- The most generic is the mobile node that doesn't co-locate,
and registers using a foreign agent's care-of address
(regardless of the state of the 'R' bit in the foreign agent
advertisements) to its home agent. In this case, all the
necessary information to revoke a mobile node's registration
is contained in the registration. The security association
between the agents can be based on IP address, or NAI.
- Second is a mobile node that co-locates, then registers
directly to its home agent without registering through a
foreign agent (foreign agents ignored, 'R' bit irrelevant).
In this case, the home agent MAY notify the mobile node that
its registration is being revoked, but it isn't possible
through Mobile IP for the foreign domain to revoke a mobile
node's registration in this way, nor is it possible for a
foreign domain to suspend Mobile IP services being provided
to a co-located mobile node as the mobile node is doing its
own tunnel decapsulations.
- Lastly is a mobile node that co-locates setting the 'D' bit,
then due to the presence of the 'R' bit in the agent
advertisement(s), registers through a foreign agent with its
home agent.
In the last case, the mobile node has set MNaddr in the registration
request to it's home address, HAaddr to that of its home agent, and
COaddr to its co-located address. Note that nowhere in this
registration request appears the IP address of any foreign agent.
Upon sending the registration request to a foreign agent, that foreign
agent processes the registration, remembering the information it
contains. If the foreign agent shares a security association with the
identified home agent, it MUST append its NAI, and a foreign-home
authenticator. The foreign agent then sends the registration request
to the identified home agent using its IP address as the source
address of the datagram. Upon receiving such a registration request,
the home agent MUST check the foreign-home authenticator. It does so
by using the NAI if present, or perhaps the source address of the
request. The registration reply the home agent sends MAY contain the
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home agent's NAI, but it MUST contain a home-foreign authenticator if
a foreign-home authenticator was present in the registration request,
and the home agent MUST send the registration reply to the source
address of the registration request [1], in this case the IP address
of the foreign agent.
Note that in cases where a foreign agent is able to revoke the
registration of a co-located mobile node, it is expected that such a
foreign agent has the power to control the data-flow of a co-located
mobile node even though tunnel datagrams are being addressed directly
to the mobile node. This is akin to how such a foreign agent would
enforce the 'R' bit, preventing the mobile node from sending the
registration request directly to the home agent.
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Appendix B: Disparate Address, and Receiver Considerations
Since the registration revocation message comes from a source address
that is topologically routable from the interface receiving the
datagram, the agents, by definition, are topologically connected (if
this were not the case, the initial registration mechanism would have
failed). If either are the ultimate hop from this topologically
connected region to one or more disparate address spaces, no problems
are foreseen. In order for the mobile node to have successfully
registered with its home agent, it MUST have provided to the network
(foreign agent) to which it is currently attached a routable address
of its home agent. Conversely, the care-of address being used by the
mobile node must also be topologically significant to the home agent
in order for the registration reply to have been received, and the
tunnel initiated. By definition, then, the home agent address and the
care-of address must each be significant, and either address must form
a unique pair in the context of this mobile node to both agents.
Another way of understanding this is that the tunnel endpoint are in
some way connected, and hence each are unique as far as the other end
is concerned. The address at the other end of the tunnel, in
combination with the address of the mobile node, must therefore form a
unique pair that can be identified by the agent receiving the
registration revocation message.
As an example, consider a mobile node who's home address lies in
disparate address space A behind it's home agent. In the following
diagram, [*] indicates an interface of the entity in which it appears.
MN[a]-----[c]FA[b]=====((()))=====[b]HA[a]-----[a]CN
Address Some topologically Address
Space C connected network Space A
We presume a binding for MN exists, and hence a tunnel between FA[b]
and HA[b] exists. Then, since the address assigned to MN[a] MUST be
unique in address space A, the pair {FA[b],MN[a]} is guaranteed to be
unique in the binding table of HA, and the pair {HA[b],MN[a]} is
guaranteed to be unique in the foreign agent's visitor list.
As a result, a home agent receiving a registration revocation message
and foreign-home authenticator for MN[a] from FA[b] is able to
determine the unique mobile node address(es) being deregistered.
Conversely a foreign agent receiving a registration revocation message
and home-foreign authenticator for MN[a] from HA[b] is able to
determine the exact mobile node address being deregistered.
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For this reason, if a foreign agent receives a registration revocation
message with the home domain field set to the zero address it MUST be
silently discarded. This is to prevent confusion in the case of
overlapping private addresses; when multiple mobile nodes are
registered via the same care-of address and coincidentally using the
same (disparate/private) home address, the home agent address
appearing in the home domain field is the only way a foreign agent can
discern the difference between these mobile nodes.
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Appendix C: Using Registration Revocation to Help Release Resources
When a mobile node moves out/around a foreign domain from where it had
registered, relevant resources at a previous foreign agent may be
consumed until its visitor entry for the mobile node expires. For
example, consider a cdma2000 network deployment using Mobile IP. The
Wireless IP Network Standard [8] defines foreign agent/Packet Data
Servicing Node (PDSN) procedures for managing the PPP session for a
mobile node, i.e., when the Radio Network opens an R-P session for an
mobile node, the foreign agent/PDSN initiates establishment of a PPP
session. As a mobile node moves from one foreign domain serviced by a
foreign agent/PDSN (source PDSN) to another PDSN (target PDSN) during
an inter-PDSN handoff, a Mobile IP registration is sent to the home
agent and a new PPP session is established at the target PDSN. The
PPP session at the source PDSN remains exhausted until the PPP
inactivity timer expires. Maintaining these PPP sessions may consume
valuable resources at the source PDSN that could otherwise be used to
support additional mobile nodes.
Registration revocation messages can be used to help identify
resources that could be released (without actually having an
administrative revocation).
To accomplish this, the revocation support extension would need to be
appended to a registration request as it is forwarded by a foreign
agent that supports registration revocation. Note that the revocation
support extension is protected by the foreign-home authenticator. When a
home agent that supports registration revocation receives such a
registration request, as required by this specification it MUST record
the capability of this foreign agent. The home agent may then use a
registration revocation message to notify this foreign agent if the
mobile node's care-of address changes, e.g., when the mobile node
moves from one foreign agent to another, or when the mobile node
deregisters with the home agent. As a result, upon receipt of an
authenticated revocation message, the foreign agent can delete the
visitor entry for this mobile node, thereby releasing the resources
being used to support it.
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Appendix D: An Example of the New Messages in Use
For clarity, the following examples are meant to illustrate the use of
these messages in the registration phase, and the revocation phase.
In cases where there may be some question to the order of processing,
any order indicated by [1] MUST be followed.
D.1 The Registration Phase
A foreign agent that supports registration revocation, and has a
security association with a home agent to which it is forwarding a
registration request MAY include the revocation support extension
after the mobile-home authenticator. If the foreign agent wishes to
receive a revocation acknowledgment from the home agent in the event
that the registration for this mobile node is revoked, and/or if it is
willing to send a revocation acknowledgment message in response to a
revocation message from the home agent for this mobile node, it MUST
set the 'A' bit to '1'. Furthermore, if the foreign agent supports
the use of the 'I' bit, and is willing to let the home agent decide if
the mobile node should be informed of the revocation of its
registration, it MAY set the 'I' bit to '1'. Additionally, the
foreign agent MUST append a foreign-home authenticator to the
registration request, and include any other extension needed by the
foreign agent to verify the authenticator [1].
Upon receiving a registration request containing a revocation
extension, a home agent that supports registration revocation MAY
include a revocation support extension in the registration reply. If
the 'A' bit is set in the revocation extension sent by the foreign
agent, a home agent that wishes that foreign agent to retransmit
unacknowledged revocation messages, and agrees to do the same, MUST
set the 'A' bit in its revocation extension to '1'. Furthermore, if
the foreign agent set the 'A' bit to '1', and also set 'I' bit to '1'
in its revocation extension, and the home agent supports the use of
the 'I' bit, and wishes to determine whether this mobile node is to be
informed in the event that its registration has been revoked, the home
agent MUST set the 'A' bit, and the 'I' bit in its registration
extension to '1'. Additionally, the home agent MUST append a
home-foreign authenticator to the registration request, and include
any other extension needed by the foreign agent to verify the
authenticator [1].
Upon receiving the authenticated registration reply, the foreign agent
MUST check the revocation support extension. If none is present, the
foreign agent SHOULD assume that the home agent doesn't understand
revocation messages, or is unwilling to participate. If a revocation
support extension is present, and if the foreign agent included the
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revocation extension in the registration request with the 'A' and 'I'
bit were set to '1', the foreign agent MUST check the 'A' bit to see
if the home agent will provide revocation acknowledgment services (if
they were requested), and MUST also check the 'I' bit to see if the
home agent wants to decide if the mobile node should be notified in
the event this registration is revoked.
D.2 The Revocation Phase
If a foreign agent wishes inform a home agent that is is revoking a
mobile node's registration, it generates a revocation message. If the
'A' bit was negotiated in the revocation extension for this
registration, the foreign agent MUST set the 'A' bit in the revocation
message to '1'. If the 'A' bit and 'I' bits were negotiated in the
revocation extensions, and the foreign agent is willing to let the
home agent indicate whether this mobile node should be informed that
its registration has been revoked, it will set both the 'A' bit and
the 'I' bit to '1'. The foreign agent will also place the address of
the mobile node whose registration it wishes to revoke in the home
address field, the address that mobile node registered as the care-of
address in the foreign domain field, and the address the mobile node
registered in the home domain address field. The foreign agent MUST
set the Foreign Domain Identifier to the current 32-bit timestamp, and
set the Home Domain Identifier to a value that will, when combined
with the timestamp, uniquely identify this binding to the foreign
agent. The foreign agent MUST append a foreign-home authenticator,
and any additional information it needs to include for the home agent
to validate it (e.g. if the care-of address is not its own, it can use
its NAI extension).
Upon receiving the above revocation message, the home agent assures
the revocation is protected with a valid foreign-home authenticator.
The home agent uses either the foreign agent NAI if present, or the
address identified as the foreign domain address to identify the
security association, and authenticate the revocation message. If the
authenticator is bad, the home agent MUST ignore, and silently discard
the revocation message. If the authenticator is good, the home agent
MUST check to make sure the Identifier does NOT indicate this
revocation is a replay. The home agent then uses the mobile node home
address, foreign domain address, and home domain address to locate the
mobile node(s) whose registration(s) is/are being revoked. If the 'A'
bit was negotiated in the registration phase for the binding(s)
identified, and set to '1' in the revocation message, the home agent
MUST generate a revocation acknowledgment message. If the 'I' bit
was negotiated in the registration phase for the binding(s)
identified, the home agent will check to see if the 'A' bit and 'I'
bit are both set to '1' in the revocation message. If so, and the
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home agent wishes for the identified mobile node(s) to be informed of
the revocation, it MUST set the 'I' bit in the revocation
acknowledgment to '1', or if the home agent does not wish the
identified mobile node(s) to be informed of the revocation, it MUST
set the 'I' bit in the revocation acknowledgment to '0'. The home
agent then copies the addresses, and the Identifier field into the
reply. The home agent MUST protect the revocation acknowledgment with
a home-foreign authenticator (and any other extension necessary for
the foreign agent to authenticate the home-foreign authenticator,
e.g. its NAI extension).
Upon receiving a valid revocation acknowledgment (in which the
authenticator and Identifier fields are acceptable), if the foreign
agent set the 'I' bit in the revocation message to '1', it MUST check
the 'I' bit in the revocation acknowledgment, and if set to '1', MUST
notify the mobile node of the revocation, or if the 'I' bit is set to
'0' in the revocation acknowledgment, the foreign agent MUST NOT
notify the mobile node of the revocation.
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Acknowledgments
The authors would like to thank Rajesh Bhalla, Kent Leung, and Alpesh
Patel for their work on draft-subbarao-mobileip-resource-00.txt
"Releasing Resources in Mobile IP" of which the revocation support
extension, and the acknowledgment mechanism are contained in this
document.
Where to Direct Comments
Questions and comments about this draft should be directed at the
Mobile IP working group:
mobile-ip@sunroof.eng.sun.com
Questions and comments about this draft may also be directed to the
authors:
Steven M. Glass Madhavi W. Chandra
Solaris Network Technologies IOS Technologies Division
Sun Microsystems Cisco Systems
1 Network Drive 7025 Kit Creek Road
Burlington, MA. 01801 Research Triangle Park,
NC 27709
Phone: 1.781.442.0504 Phone: 1.919.392.8387
Fax: 1.781.442.1706
E-mail: steven.glass@sun.com E-mail: mchandra@cisco.com
The working group may also be contacted via the current chairs:
Basavaraj Patil Phil Roberts
Nokia Corporation Megisto Systems, Inc.
6000 Connection Drive 20251 Century Blvd
M/S M8-540 Suite 120
Irving, Texas 75039 Germantown, MD 20874
USA USA
Phone: +1 972-894-6709 Phone: +1 301-444-1700
Fax : +1 972-894-5349 Fax: +1 301-515-3675
E-Mail: Basavaraj.Patil@nokia.com E-Mail: proberts@megisto.com
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References
[A] Mobile IP Authentication, Authorization, and Accounting Requirements
RFC 2977, October 2000
S. Glass, T. Hiller, S. Jacobs, C. Perkins.
[B] Criteria for Evaluating AAA Protocols for Network Access
RFC2989, November 2000
B. Aboba, P. Calhoun, S. Glass, T. Hiller, P. McCann, H. Shiino,
P. Walsh, G. Zorn, G. Dommety, C. Perkins, B. Patil, D. Mitton,
S. Manning, M. Beadles, X. Chen, S. Sivalingham, A. Hameed,
M. Munson, S. Jacobs, B. Lim, B. Hirschman, R. Hsu, H. Koo,
M. Lipford, E. Campbell, Y. Xu, S. Baba, E. Jaques
[1] IP Mobility Support for IPv4
RFC3220, January 2002
C. Perkins, Editor.
[2] Reverse Tunneling for Mobile IP, revisited
RFC3024, January 2001
G. Montenegro, Editor.
[3] ICMP Router Discovery Messages
RFC 1256, September 1991
S. Deering, Editor.
[4] Mobile IP Network Access Identifier Extension for IPv4
RFC 2794, March 2000
P. Calhoun, C. Perkins.
[5] Security Issues in Mobile IP
draft-glass-mobileip-securities-issues-02.txt
S. Glass.
[6] Key Words for us in RFCs to Indicate Requirement Levels
RFC 2119, March 1997
S. Bradner
[7] CIDR and Classful Routing
RFC 1817, August 1995
Y. Rekhter
[8] TIA/EIA/IS-835, Wireless IP Network Standard, June 2000.
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S. Glass, M. Chandra Expires February 2003 [page 41]
