IETF Seamoby Working Group
Internet Draft Marco Liebsch
Category: Experimental Ajoy Singh
(Editors)
Hemant Chaskar
Daichi Funato
Eunsoo Shim
draft-ietf-seamoby-card-protocol-08.txt
Expires: March 2005 September 2004
Candidate Access Router Discovery
Status of this Memo
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Abstract
To enable seamless IP-layer handover of a mobile node (MN) from one
access router (AR) to another, the MN is required to discover the
identities of candidate ARs (CARs) for handover, along with their
capabilities, prior to the initiation of the IP-layer handover. The
act of discovery of CARs has two aspects to it: identifying the IP
addresses of the CARs and finding the capabilities of those CARs.
This process is called "candidate access router discovery" (CARD). At
the time of IP-layer handover, that CAR, whose capabilities is a good
match to the preferences of the MN, is chosen as the target AR for
handover. The protocol described in this document allows a mobile
node to perform CARD.
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TABLE OF CONTENTS
1. INTRODUCTION...................................................3
2. TERMINOLOGY....................................................3
3. CARD PROTOCOL FUNCTIONS........................................4
3.1 Reverse Address Translation................................4
3.2 Discovery of CAR Capabilities..............................4
4. CARD PROTOCOL OPERATION........................................5
4.1 Conceptual Data Structures.................................8
4.2 Mobile Node - Access Router Operation......................8
4.3 Current Access Router - Candidate Access Router Operation.11
4.4 CARD Protocol Message Piggybacking on the MN-AR Interface.13
5. PROTOCOL MESSAGES.............................................14
5.1 CARD Messages for the Mobile Node-Access Router Interface.14
5.2 CARD Inter-Access Router Messages.........................28
6. SECURITY CONSIDERATIONS.......................................30
6.1 Veracity of CARD Information..............................30
6.2 Security Association between AR and AR....................31
6.3 Security Association between AR and MN....................31
6.4 Router Certificate Exchange...............................32
6.5 DoS Attack................................................33
6.6 Replay Attacks............................................33
7. PROTOCOL CONSTANTS............................................34
8. IANA CONSIDERATIONS...........................................34
9. NORMATIVE REFERENCES..........................................34
10. INFORMATIVE REFERENCES.......................................35
11. AUTHORS' ADDRESSES...........................................36
12. IPR STATEMENTS...............................................36
13. DISCLAIMER OF VALIDITY.......................................37
14. COPYRIGHT NOTICE.............................................37
15. ACKNOWLEDGEMENTS.............................................37
Appendix A MAINTENANCE OF ADDRESS MAPPING TABLES IN
ACCESS ROUTERS. . . . . . . . . . . . . . . . . . . 39
Appendix A.1 Centralized Approach using a Server Functional
Entity. . . . . . . . . . . . . . . . . . . . . . . 39
Appendix A.2 Decentralized Approach using Mobile Terminals'
Handover. . . . . . . . . . . . . . . . . . . . . . 40
Appendix B APPLICATION SCENARIOS . . . . . . . . . . . . . . . 43
Appendix B.1 CARD Operation in a Mobile IPv6 Enabled Wireless
LAN Network . . . . . . . . . . . . . . . . . . . . 43
Appendix B.2 CARD Operation in a Fast Mobile IPv6 enabled
network . . . . . . . . . . . . . . . . . . . . . . 46
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Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [Brad97].
1. INTRODUCTION
IP mobility protocols, such as Mobile IP, enable mobile nodes to
execute IP-level handover among access routers. Additionally, work
is underway [Kood03][Malk03] to extend the mobility protocols to
allow seamless IP handover. The pre-requisite for the seamless IP
mobility protocols is the knowledge of candidate access routers
(CARs) to which a mobile node can be transferred. The CAR discovery
protocol enables the acquisition of information about the access
routers that are candidates for the mobile node's next handover.
CAR discovery involves identifying a CAR's IP address as well as the
capabilities that the mobile node might use for a handover decision.
There are cases when a mobile node has a choice of CARs. The mobile
node chooses one based on a match between the mobile node's
requirements for a handover candidate and the CAR's capabilities.
However, the decision algorithm itself is out of scope of this
document.
The problem statement for CAR discovery is documented in [TKCK02].
In this document, a protocol is described to perform CAR discovery.
Section 3 describes two main functions of the CAR discovery
protocol. Section 4 describes the core part of the CARD protocol
operation. The protocol message format is described in Section 5.
Section 6 discusses security considerations and Section 7 contains a
table of protocol parameters. Appendix A contains two alternative
techniques for dynamically constructing the CAR table mapping
between the access point L2 ID and Access Router IP address,
necessary for reverse address translation. The default method is
static configuration. Appendix B contains two sample scenarios for
using CARD.
2. TERMINOLOGY
This document uses terminology defined in [MaKo03].
In addition, the following terms are used:
Access Router (AR)
An IP router residing in an access network and connected to one or
more APs. An AR offers IP connectivity to MNs.
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Candidate AR (CAR)
An AR to which a MN has a choice when performing IP-level handover.
Capability of an AR
A characteristic of the service offered by an AR that may be of
interest to a MN when the AR is being considered as a handover
candidate.
L2 ID
Identifier of an AP that uniquely identifies that AP. For example, in
802.11, this could be a MAC address of an AP.
CARD Initiating Trigger
L2 trigger used to initiate the CARD process. For example, a MN can
initiate CARD as soon as it detects the L2 ID of a new AP during link
layer scan.
Access Point (AP)
A wireless access point, identified by a MAC address, providing
service to the wired network for wireless nodes.
3. CARD PROTOCOL FUNCTIONS
The CARD protocol accomplishes the following functions.
3.1 Reverse Address Translation
If a MN can listen to the L2 IDs of new APs prior to making a
decision about IP-level handover to CARs, a mechanism is needed for
reverse address translation. This function of the CARD protocol
enables the MN to map the received L2 ID of an AP to the IP address
of the associated CAR that connects to the AP. To get the CAR's IP
address, the MN sends the L2 ID of the AP to the current AR and the
current AR provides the associated CAR's IP address to the MN.
3.2 Discovery of CAR Capabilities
Information about capabilities of CARs can assist the MN in making
optimal handover decisions. This capability information serves as
input to the target AR selection algorithm. Some of the capability
parameters of CARs can be static, while others can change with time.
Definition of capabilities is out of scope of this document. Encoding
rules for capabilities and the format of a capability container for
capability transport are specified in Section 5.
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4. CARD PROTOCOL OPERATION
The CARD protocol allows MNs to resolve the L2 ID of one or more APs
to the IP address of the associated CARs. The L2 IDs are typically
discovered during operation by the MN and are potential handover
candidates. Additionally, CARD allows MNs to discover particular
capabilities associated with the CARs, such as available bandwidth,
that might influence the handover decision of the MN. Furthermore,
the protocol allows ARs to populate and maintain their local CAR
table (Section 4.1) with the capabilities of CARs. For this, the
CARD protocol makes use of CARD Request and CARD Reply messages
between a MN and its current AR (Section 5.1.2), and between a MN's
current AR and individual CARs respectively (Section 5.2.2).
To allow a MN to retrieve a CARs' address and capability
information, the CARD Request and CARD Reply messages used between a
MN and its current AR may contain one or more access points' L2 IDs
and the IP addresses of associated CARs respectively. Optionally,
the CARD Reply messages can also contain CARs' capability
information. A CAR's capabilities are specified as a list of
attribute-value pairs, which are conveyed in a Capability Container
message parameter.
Information about the CAR(s) and associated capabilities MAY be used
by the MN to perform target access router selection during its IP
handover. The current AR returns replies based on its CAR table (see
Section 4.1), and returns a RESOLVER ERROR (see Section 5.1.3.1) if
the request cannot be resolved.
The CARD protocol also enables a MN to optionally indicate its
preferences on capabilities of interest to its current AR by
including the Preferences message parameter in the CARD Request
message. The MN's current AR MAY use this information to perform
optional capability pre-filtering for optimization purposes and
returns only these capabilities of interest to the requesting MN.
The format of this optional Preferences message parameter is
described in Section 5.1.3.2.
Optionally, the MN can provide its current AR with a list of
capability attribute-value pairs, indicating not only the capability
parameters (attributes) as required for capability pre-filtering,
but also a specific value for a particular capability. This allows
the MN's current AR performing CAR pre-filtering and to send only
address and capability information of CARs, whose capability values
meet the requirements of the MN, back to the requesting MN. The
format of this optional Requirements message parameter is described
in Section 5.1.3.3.
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As an example, using the optional Preferences message parameter, a
MN may indicate to its current AR that it is interested only in
IEEE802.11a interface specific capability parameters, since this is
the only interface the MN has implemented. Hence, the MN's current
AR sends back only CARs' IEEE802.11a specific capabilities.
Similarly, using the optional Requirements message parameter, a MN
may indicate to its current AR that it is only interested in CARs
that can satisfy a given QoS constraint. Here, a MN sends the
respective QoS attribute with the QoS constraint value to its
current AR using the optional Requirements message parameter. The
QoS constraint is denoted as an attribute-value pair and
encapsulated with the Requirements message parameter, which is
appended to the MN-originated CARD Request message. The Requirements
message parameter may be used to indicate the cut off values of the
capabilities for the desired CAR(s). Based on the received optional
list of attributes in the Preferences parameter or a list of
attribute-value pairs in the Requirements message parameter, the
MN's current AR MAY use these parameters for deciding the content of
the solicited CARD Reply message, which is to be sent back to the
MN. Alternatively, in case no optimization with regard to capability
or CAR pre-filtering is performed by the MN's current AR, the
current AR MAY choose to silently ignore the optional Requirements
and Preferences message parameter as received in the CARD Request
message.
The MN can additionally request from the AR a certification path
that is anchored at a certificate from a shared, trusted anchor. The
MN includes in the CARD Request message a list of trusted anchors
for which the MN has a certificate and the AR replies with the
certification path. If no match is found, the AR returns the trusted
anchor names from the CARD Request. The MN can ask for a chain for
either the current AR or for a CAR. If the trusted anchor list is
accompanied by an AP L2 ID for the MN's current AP, the returned
chain is for the current AR. If the L2 ID is for an AP that the MN
has heard during scanning and is not connected to the current AR,
the returned chain is for a CAR. The chain is returned as a sequence
of CARD Reply messages, each message containing a single
certificate, the L2 identifier for the AP sent in the CARD Request
and a router address for the CAR (or for the AR itself if a request
was made for the AR). When the chain is complete, the MN can use it
to obtain the router's certified key, and thereby validate
signatures on CARD messages, and other messages between the MN and
AR. The MN only needs to send the trusted anchor option if it does
not have the certification path for the router already cached. If the
mobile node has the certification path cached, either through
preconfiguration, previous receipt of the chain from this router, or
through having received the chain through a previous router, then the
trusted anchor does not need to be sent. More information about
certificate exchange and its use in CARD security can be found in
Section 6.
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The CARD protocol operation, as described in this section,
distinguishes signaling messages exchanged between a MN and its
current AR and signaling messages exchanged between ARs. Hence,
description of signaling messages described in the following
sections has a preceding identifier, referring to the associated
interface. Messages that are exchanged between a MN and AR are
designated as "MN-AR", messages between ARs are designated as "AR-
AR" respectively.
+--------------+ (1a)AR-AR CARD Request +----------+
| Current |------------------------->| CAR |
| AR |<-------------------------| |
+--------------+ (2a)AR-AR CARD Reply +----------+
^ |
| | MN-AR
MN-AR | | CARD Reply(3m)
CARD Request(2m) V
+--------------+
| Mobile |
| Node |<-- CARD Init Trigger
+--------------+ (1m)
Figure 1: MN initiated CARD Protocol Overview
Figure 1 describes the operation of the MN-AR CARD Request/Reply
protocol and AR-AR CARD Request/Reply protocol. On reception of the
access points' L2 IDs or the appearance of a CARD initiation trigger
(1m), the MN may pass on one or more AP L2 ID(s) to its current AR
using the MN-AR CARD Request message (2m). In case the MN wants its
AR to perform capability discovery in addition to reverse address
translation, this must be indicated in the MN-AR CARD Request
message by setting the C-flag. If the C-flag is not set, the AR
receiving the CARD Request message will perform only reverse address
translation. The MN's current AR resolves the L2 ID to the IP
address of the associated CAR or, in case the MN has not attached
any L2 ID message parameters, it just reads out all CARs' IP address
information using the reverse address translation information (L2 ID
to IP address mapping) from its local CAR table. The current AR then
returns to the MN using the MN-AR CARD Reply message (3m) the IP
address of the CAR(s), each CAR's set of L2 IDs with CANDIDATE
indicated in the L2 ID sub-option status field, and, in case
capability information has been requested, associated capabilities.
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For the AR-AR CARD Request/Reply protocol, the requesting AR sends a
CARD Request message to its peer when the CAR table entries time out
(1a). The peer returns a CARD Reply message with the requested
information (2a).
4.1 Conceptual Data Structures
AR(s) SHALL maintain a L2-L3 address mapping table (CAR table) that
is used to resolve L2 IDs of candidate APs to the IP address of the
associated CAR. By default, this address-mapping table is configured
statically for the CARD protocol operation. Optionally, the CAR
table MAY be populated dynamically. Two possible approaches are
described in Appendices A.1 and A.2 respectively.
ARs SHOULD also keep and maintain individual CARs' capabilities in
the local CAR table, taking the associated capability lifetime into
account. If the lifetime of an individual capability entry has
expired, the respective capability information is updated.
An AR may also initiate capability exchange prior to expiration of
the capabilities associated with a CAR in the CAR table thereby
populating its CAR table. The ARs' CAR table may be implemented
differently, hence additional details are not provided here. ARs
MUST maintain their own AP to AR mappings and capability information
in their CAR tables, in order to provide newly booted MNs with this
information and so that a MN can obtain the AR's certification path.
MNs SHOULD maintain discovered address and capability information of
CARs in a local cache to avoid requesting the same information
repeatedly and to select an appropriate target AR from the list of
CARs as quickly as possible when a handover is imminent.
4.2 Mobile Node - Access Router Operation
4.2.1 Mobile Node Operation
To initiate CARD, a MN sends a CARD Request to its current AR,
requesting it to resolve the L2 ID of nearby access points to the IP
address of associated CARs, and also to obtain capability parameters
associated with these CARs. In case the requesting MN wants its
current AR to resolve specific L2 IDs, the MN-AR CARD Request MUST
contain the CARD protocol specific L2 ID message parameters. If the
MN wants its AR to perform only reverse address translation without
appending the CARs' capabilities, the MN refrains from setting the
C-flag in the CARD Request message. If the MN wants to perform
capability discovery, the CARD Request MUST have set the C-flag. The
CARD Request MAY also contain the Preferences or Requirements
message parameter, indicating the MN's preferences on capability
attributes of interest or its requirements on CARs' capability
attribute-value pairs.
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In case the MN appends multiple L2 ID sub-options to a CARD Request,
the AR MUST assume each L2 ID is associated with an AP that connects
to a different CAR. Since L2 IDs, address information and capability
information are transmitted with separate sub-options, each sub-
option carries a Context-ID, to allow matching parameters that
belong together. Hence, the MN MUST assign different Context-ID
values to the L2 ID sub-options it appends to the CARD Request
message. The Status-Code field of the L2 ID sub-option MUST always
be set to NONE (0x00) by a MN. The MN MUST set the sequence number
to a randomly generated value, and the AR MUST include the sequence
number in all messages of the reply. If the reply spans multiple
messages, each message contains the same sequence number.
Upon receipt of the corresponding MN-AR CARD Reply message, the MN
correlates the CARD Reply with the appropriate CARD Request message
and then processes all MN-AR CARD Reply message parameters to
retrieve its CARs' address and capability information. If the MN is
unable to correlate the CARD Reply with any previously sent CARD
Request messages, the MN SHOULD silently discard the reply. This may
happen when the MN reboots after sending a CARD Request Message to
the connected AR.
A MN uses exponential backoff to retransmit the CARD Request in the
event a CARD Reply is not received within CARD_REQUEST_RETRY
seconds. The retransmitted CARD Request MUST have the same sequence
number as the original. With the exception of certification paths,
which are large by nature, an AR SHOULD attempt to limit the
information in a CARD Reply to a single message. Should that not be
possible, the AR MAY send the reply in multiple messages. The last
message of a reply MUST always have the L-flag set in the CARD Reply
option to indicate that the message is the last for the associated
sequence number. An AR retransmitting replies to a CARD Request MUST
always send the full CARD Reply sequence. The Trusted Anchor sub-
option and the Router Certificate sub-option provide a means whereby
the MN can request specific certificates in a certification path, in
the event that the CARD Reply carrying a certification path spans
multiple messages and one of them is lost. However, a request for
specific certificates that were not received in the initial CARD
Reply MUST be treated as a new request by the MN and MUST use a
different sequence number.
Processing the Context-ID of Address sub-options allows the MN to
assign the resolved IP address of a specific CAR to a L2 ID.
In some cases a L2 ID parameter is present in a CARD Reply message.
The Status-Code field in the L2 ID parameter indicates one of the
following reasons for being sent towards the MN.
RESOLVER ERROR Status-Code indication:
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In case the MN's current AR could not resolve a particular L2 ID,
this status code is returned to the MN.
MATCH Status-Code indication:
If a L2 ID is encountered that shares a CAR with a previously
resolved L2 ID, the AR returns MATCH to the MN. This status code is
an indicator that the Context-ID of this particular L2 ID sub-option
has been set to the Context-ID of the associated CAR's Address and
Capability Container sub-option, which is sent with this CARD Reply
message. This approach avoids sending the same CAR's address and
capability information multiple times with the same CARD Reply
message in case two or more L2 IDs resolve to the same CAR. A MN
uses the Context-ID received in the L2 ID sub-option as the key to
find the serving CAR of the given AP from the content of the
received CARD Reply message.
CANDIDATE Status-Code indication:
In case the MN does not append any L2 ID to the CARD Request, the AR
sends back the L2 ID and address information of all CARs. Since the
received parameters' Context-IDs cannot be correlated with a L2 ID's
Context-ID of a previously sent request, the AR chooses values for
the Context-ID and marks these candidate L2 IDs with CANDIDATE in
the status code of the distributed L2 IDs. However, individual
values of L2 IDs' Context-ID allow the MN to assign a particular L2
ID to the associated Address and the possibly received Capability
Container sub-option.
As described in Section 4.5, a MN can use CARD when it boots up
initially to determine whether piggyback operation is possible. A MN
can also use CARD initially to determine the capabilities and
certificates for an AR on which it boots up, or if it cannot obtain
the certificates beforehand. To do this, the MN includes a L2
Identifier option with its current AP L2 ID, and the requested
information. The AR replies with its own information.
4.2.2 Current Access Router Operation
Upon receipt of a MN's MN-AR CARD Request, the connected AR SHALL
resolve the requested APs' L2 ID to the IP address of the associated
CAR(s). In case no L2 ID parameter has been sent with the MN-AR CARD
Request message, the receiving AR retrieves all CARs' IP addresses,
and, if the C-flag was set in the request, the capability
information.
In the first case, where the AR resolves only requested L2 IDs, the
AR does not send back the L2 ID to the requesting MN. If, however,
two or more L2 IDs match the same CAR information, the L2 ID sub-
option is sent back to the MN, indicating MATCH in the Status-Code
field of the L2 ID. Furthermore, the AR sets the Context-ID of the
returned L2 ID to the value of the resolved CAR's L2 ID, Address and
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Capability Container sub-option. In case an AR cannot resolve a
particular L2 ID, a L2 ID sub-option is sent back to the MN,
indicating RESOLVER ERROR in the L2 ID sub-option's Status-Code
field.
In the second case, where the AR did not receive any L2 ID with a
CARD Request, all candidate APs' L2 IDs are sent to a requesting MN
with the CARD Reply message. Here, the AR marks the Status-Code of
individual L2 IDs as CANDIDATE, indicating to the MN, that the
associated Context-ID cannot be matched with the ID of a previously
sent request.
In any case, the AR MUST set the Context-ID of the Address and the
Capability Container sub-option to the same value of the associated
L2 ID sub-option.
Optionally, when allowed by local policies and supported by
respective ARs for capability discovery, the AR MAY retrieve a
subset of capabilities or CARs, satisfying the optionally appended
Preferences and Requirement message parameter, from its local CAR
table. CARs' address information along with associated capabilities
are then delivered to the MN using the MN-AR CARD Reply message. The
CARs' IP address as well as the capabilities SHALL be encoded
according to the format for CARD protocol message parameters as
defined in Section 5.1.3 of this document. The capabilities are
encoded as attribute-value pairs, which are encapsulated in a
Capability Container message parameter according to the format
defined in Section 5.1.3.4. The responding current AR SHALL copy the
sequence number received in the MN-AR CARD Request to the MN-AR CARD
Reply.
4.3 Current Access Router - Candidate Access Router Operation
4.3.1 Current Access Router Operation
The MN's current AR MAY initiate capability exchange with CARs
either when it receives a MN-AR CARD Request or when it detects that
one or multiple of its local CAR table's capability entries'
lifetime is about to expire. An AR SHOULD preferentially utilize its
CAR table to fulfill requests rather than signaling the CAR
directly, and it SHOULD keep the CAR table up to date for this
purpose, in order to avoid injecting unnecessary delays into the MN
response.
The AR SHOULD issue an AR-AR CARD Request to the respective CAR(s)
if complete capability information of a CAR is not available in the
current AR's CAR table, or if such information is expired or about
to expire. The AR-AR CARD Request message format is defined in
Section 5.2.2. The sequence number on the AR-AR interface starts
with zero when the AR reboots. The sending AR MUST increment the
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sequence number in the CARD Request by one each time it sends a CARD
Request message.
The AR MAY append its own capabilities, which are encoded as
attribute-value pairs and encapsulated with the Capability Container
message parameter, to the released AR-AR CARD Request. In case the
AR-AR CARD Request conveys the current AR's capabilities to the CAR,
the associated Capability Container can have any value set for the
Context-ID, since there is no need for the receiving CAR to process
this field due to the absence of a L2 ID and an Address sub-option.
Furthermore, the current AR MAY set the P-flag in the Capability
Container sub-option to inform the CAR about its own capability to
perform CARD protocol message piggybacking.
Optionally, a current AR MAY append the Preferences sub-option to
the AR-AR CARD Request to obtain only capability parameters of
interest from a CAR.
Upon receipt of the AR-AR CARD Reply, which has been sent by the CAR
in response to the previously sent request, the MN's current AR
SHALL extract the capability information from the payload of the
received message and store the received capabilities in its local
CAR table. The lifetime of individual capabilities is to be set
according to the lifetime indicated for each capability received.
The value of the table entries' timeout shall depend upon the nature
of individual capabilities.
Optionally, CARs can send unsolicited CARD Reply messages to
globally adjacent ARs if the configuration of their APs or
capabilities changes dynamically. In case the current AR receives an
unsolicited CARD Reply message from a CAR for which there is an
entry in its local CAR table, the current AR checks that the
sequence number of the received CARD Reply has increased compared to
the previously received unsolicited CARD Reply message, which has
been sent from the same CAR. Then, the current AR can update its
local CAR table according to the received capabilities. If a new CAR
is added, an AR may receive a CARD Reply from a CAR that is not in
its CAR table, or from a CAR that has rebooted. In this case, the
sequence number is 0. The requirement that ARs share an IPsec
security association, detailed in Section 6, ensures that an AR
never accepts CARD information from an unauthenticated source.
4.3.2 Candidate Access Router Operation
Upon receipt of an AR-AR CARD Request, a CAR shall extract the
sending AR's capabilities, if the sending AR has included its
capabilities. The CAR SHALL store the received capabilities in its
CAR table and set the timer for individual capabilities
appropriately. The value of the table entries' timeout depends upon
the nature of capabilities in the AR-AR CARD Reply message. The CAR
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must include the same sequence number in the AR-AR CARD Reply
Message as received in the AR-AR CARD Request Message. The AR-AR
CARD Reply shall include the CAR's capabilities as list of
attribute-value pairs in the Capability Container message parameter.
In case the sending AR has appended an optional Preferences sub-
option, the CAR MAY perform capability filtering and send back only
these capabilities, which are of interest to the requesting AR,
identified according to the Preferences sub-option. Since the AR-AR
CARD Reply is based on a previously received AR-AR CARD Request, the
CAR MUST set the U-flag of the AR-AR CARD Reply to 0.
Optionally, the CAR MAY send an unsolicited CARD Reply message to
globally adjacent ARs in case one or more of its capability
parameters change. The unsolicited CARD Reply messages should have
as destination address the adjacent ARs' unicast address and must
have the U-flag set. Consecutive unsolicited CARD Reply messages
MUST have the sequence number incremented respectively, starting
with 0 when the AR boots.
4.4 CARD Protocol Message Piggybacking on the MN-AR Interface
CARD supports another mode of CAR information distribution, in which
the capabilities are distributed piggybacked on fast handover
protocol messages. To allow MNs and ARs appending the ICMP-option
type CARD Request and CARD Reply (Section 5.1.2) to the ICMP-type
Fast Mobile IPv6 [Kood03] signaling messages, the MN and AR should
know about the signaling peer's capability for CARD protocol message
piggybacking. This requires dynamic discovery of piggybacking
capability using the P-flag in the MN-AR CARD Request and the MN-AR
CARD Reply message, as well as in the Capability Container message
parameter. The format of these messages and parameters is described
in Section 5.1.
The MN sends the very first CARD Request to its current AR using the
ICMP-type CARD main header for transport, as described in Section
4.2.1. In case the MN supports CARD protocol message piggybacking,
the P-flag in this very first CARD Request message is set. On
reception of the CARD Request message, current AR learns about the
MN's piggybacking capability. To indicate its piggybacking
capability, the AR sets the P-flag in the CARD Reply message. In
case the AR does not support piggybacking, all subsequent CARD
protocol messages between the MN and the AR are sent stand-alone,
using the CARD main header. In case both nodes, the MN and its
current AR, support CARD protocol message piggybacking, subsequent
CARD protocol messages can be conveyed as an option via the Fast
Mobile IPv6 Router Solicitation for Proxy (RtSolPr) and Proxy Router
Advertisement (PrRtAdv) messages. During the CARD process, a MN
learns about CAR's piggybacking capability during the discovery
phase, since the Capability Container, as described in Section
5.1.3.4, also carries a P-flag. This allows the MN to immediately
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perform CARD protocol message piggybacking after a handover to a
selected CAR, assumed this CAR supports CARD protocol piggybacking.
If a MN prefers the reverse address translation function of the Fast
Mobile IPv6 protocol, it can use CARD protocol message piggybacking
to retrieve only the CARs' capability information. To indicate that
reverse address translation is not required, the piggybacked CARD
Request message MUST have the A-flag set. These causes the current
AR to append only Capability Container sub-options. To associate a
Capability Container, sent as a parameter of the CARD Reply message,
to the IP address for the appropriate CAR, the Context-ID of an
individual Capability Container MUST be used as an index, pointing
to the associated IP address in the PrRtAdv message options. The
Context-ID of individual Capability Containers is set appropriately
by the MN's current AR. Details about how individual Context-ID
values can be associated with a particular IP address option of the
PrRtAdv message is out of the scope of this document.
5. PROTOCOL MESSAGES
5.1 CARD Messages for the Mobile Node-Access Router Interface
5.1.1 MN-AR Transport
The MN-AR interface uses ICMP for transport. Because ICMP messages
are limited to a single packet, and because ICMP contains no
provisions for retransmitting packets if signaling is lost, the CARD
protocol incorporates provisions for improving transport performance
on the MN-AR interface. MNs SHOULD limit the amount of information
requested in a single ICMP packet, since ICMP has no provision for
fragmentation above the IP level.
Hosts and Access Routers use the Experimental ICMP type main header
[Ke04] when CARD protocol messages cannot be conveyed via ICMP-type
Fast Mobile IPv6 [Kood03]. The MN-AR interface MUST implement and
SHOULD use the CARD ICMP Type header for transport. If available,
the MN-AR interface MAY use the ICMP-type Fast Mobile IPv6 [Kood03]
for transport (Section 4.4).
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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+- - - -
IP Fields:
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Source Address:
An IP address assigned to the sending
interface.
Destination Address:
An IP address assigned to the receiving
interface.
Hop Limit: 255
ICMP Fields:
Type Experimental Mobility type (To be assigned by
IANA for IPv4 and IPv6, see [Ke04]).
Code 0
Checksum The ICMP checksum.
Subtype Experimental Mobility subtype for CARD, see
[Ke04].
Reserved This field is currently unused. It MUST be
initialized to zero by the sender and MUST be
ignored by the receiver.
Valid Options:
CARD Request: The CARD Request allows entities to request CARD
specific information from ARs. To support
processing the CARD Request message on the
receiver side, further sub-options may be
carried, serving as input to the reverse address
translation function and/or capability discovery
function.
CARD Reply: The CARD Reply carries parameters, previously
requested with a CARD Request, back to the
sender of the CARD Request.
Valid Sub-Options:
Support level indicated in parentheses:
Layer-2 ID (mandatory):
The Layer-2 ID sub-option [5.1.3.1] carries
information about the type of an access point
as well as the Layer-2 address of the access
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point associated with the CAR, whose IP address
and capability information is to be resolved.
Capability container (mandatory):
The Capability container sub-option carries
information about a single CAR's capabilities.
The format of this sub-option is described in
Section 5.1.3.4.
Address (mandatory):
The Address sub-option carries information on
an individual CAR's resolved IP address. The
format of the Address sub-option is described
in Section 5.1.3.5.
Trusted Anchor (mandatory):
The Trusted Anchor sub-option carries the name
of a trusted anchor for which the MN has
a certificate. The format of the Trusted Anchor
sub-option is described in Section 5.1.3.6.
Router Certificate (mandatory):
The Router Certificate sub-option carries one
certificate in the path for the AR or for a
CAR. The chain includes certificates starting
at a trusted anchor, which the router shares in
common with the mobile node, to the router
itself. The format of the Router Certification
sub-option is described in Section 5.1.3.7.
Preferences sub-option (optional):
The Preferences sub-option carries information
about attributes of interest to the requesting
entity. Attributes are encoded according to the
AVP encoding rule as described in Section
5.1.4. For proper settings of AVP Code and Data
field, see Section 5.1.3.2. This sub-option is
used only in case of performing optional
capability pre-filtering on ARs and provides
only capabilities of interest to a requesting
MN.
Requirements (optional):
The Requirements sub-option carries information
about attribute-value pairs required for pre-
filtering of CARs on a MN's current AR. This
parameter conveys MN specific attribute-value
pairs to allow a MN's current AR to send only
information about CARs of interest back to the
requesting MN. CARs are filtered on ARs
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according to CARs' capability parameters and
given policy or threshold, as encoded in the
Requirements sub-option. Attribute-value pairs
are encoded according to the AVP encoding rule
as described in Section 5.1.4. Rules for proper
setting of the AVP Code and Data field for the
Requirements sub-option are described in
Section 5.1.3.3.
CARD Requests which fail to elicit a response are retransmitted.
The initial retransmission occurs after a CARD_REQUEST_RETRY wait
period. Retransmissions MUST be made with exponentially increasing
wait intervals (doubling the wait each time). CARD Requests should
be retransmitted until either a response (which might be an error)
has been obtained, or for CARD_RETRY_MAX seconds occurs. ARs MUST
discard any CARD Requests having the same sequence number after
CARD_RETRY_MAX seconds. If a CARD Reply spans multiple ICMP
messages, the same sequence number MUST be used in each message.
MNs which retransmit a CARD Request use the same CARD sequence
number. This allows an AR to cache its reply to the original request
and then send it again, should a duplicate request arrive. This
cached information should only be held for a maximum of
CARD_RETRY_MAX seconds after receipt of the request. Sequence
numbers SHOULD be randomly chosen. Random sequence numbers avoid
duplicates if MNs restart frequently, and simplify sequence number
maintenance on both the MN and AR when MNs frequently appear and
disappear due to movement between CARs.
5.1.2 CARD Options Format
All options are of the form:
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 |Vers.| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: 8-bit identifier of the type of option,
assigned by IANA. See [Ke04] for CARD Request
and CARD Reply values.
Length: 8-bit unsigned integer. The length of
option including the type and length fields in
units of 8 octets. The value 0 is invalid.
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Vers.: 3-bit version code. For this specification,
Vers.=1.
5.1.2.1 CARD Request Option
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 |Vers.|P|C|A|T| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Options
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
Fields:
Type: To be assigned by IANA for IPv4 and IPv6, see [Ke04].
Length: The length of the option in units of 8 octets, including
the type and length fields as well as sub-options.
Vers.: 3-bit version code. For this specification,
Vers.=1.
Flags: P-flag: Indicates CARD protocol message piggybacking
capability of the CARD Request message sender.
A description for proper use of this flag can
be found in Section 4.4 of this document.
C-flag: Indicates that the requesting entity is
interested also in associated CARs'
capabilities. If the MN wants the AR to append
CARs' capability parameters to the CARD Reply
in addition to address information, the MN must
set this flag.
A-flag: Indicates that the requesting entity does NOT
want the receiver of this message to perform
reverse address translation. This flag is
set in case CARD protocol messages are
piggybacked with a protocol that performs
reverse address translation. For details refer
to Section 4.4 of this document.
T-flag: Indicates that the requesting entity is
interested in obtaining all certificates from
the responder. This flag is only valid on the
AR-AR interface.
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The flag combination A=1 and C=0 is invalid, and the
flag T=1 is invalid on the MN-AR interface. The AR MUST
discard an invalid message and log an appropriate error
message.
Reserved: Initialized to zero, ignored on reception.
Sequence Number:
Allows correlating requests with replies.
Valid Sub-Options:
- L2 ID sub-option
- Preferences sub-option
- Requirements sub-option
- Trusted Anchor sub-option
To ensure meeting requirements on boundary alignment, individual
sub-options MUST meet the 64-bit boundary alignment requirements
respectively. This will ensure that the entire CARD Request option
meets the 8n alignment constraint.
5.1.2.2 CARD Reply Option
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 |Vers.|P|U|L| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Options
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
Fields:
Type: To be assigned by IANA for IPv4 and IPv6 [Ke04].
Length: The length of the option in units of 8 octets, including
the type and length fields as well as sub-options.
Vers.: 3-bit version code. For this specification,
Vers.=1.
Flags: P-flag: Indicates CARD protocol message piggybacking
capability of the CARD Reply message sender.
A description for proper use of this flag can
be found in Section 4.5 of this document.
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U-flag: Indicates an unsolicited CARD Reply. This flag
is only valid on the AR-AR interface.
L-flag: Set if this message is the last message in a
a multiple ICMP message reply. This flag is only
valid on the MN-AR interface.
The flag U=1 on an AR-MN message is invalid. An invalid
message should be discarded and an appropriate error
message logged.
Reserved: Initialized to zero, ignored on reception.
Sequence Number:
Allows correlating requests with replies.
Valid Sub-Options:
- L2 ID sub-option
- Capability Container sub-option
- Address sub-option
- Router Certificate sub-option.
To ensure meeting requirements on boundary alignment, individual
sub-options MUST meet 64-bit boundary alignment requirements
respectively. This will ensure that the entire CARD Request option
meets the 8n alignment constraint.
5.1.3 Sub-Options Format
All sub-options are of the form:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-Option Type|Sub-Option Len | Sub-Option Data . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Option Type: 8-bit identifier of the type of option. The
sub-options defined in this document are listed
in the table below. The table also indicates
on which interfaces the sub-option is valid.
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Description Type Interface
| | / \
| | MN-AR AR-AR
---------------------------------------------------------------
L2 ID 0x01 x
Address 0x02 x
Capability Container 0x03 x x
Preferences 0x04 x x
Requirements 0x05 x
Trusted Anchor 0x06 x
Router Certificate 0x07 x x
Sub-Option-Length: 8-bit unsigned integer, indicating the length
of the sub-option, including sub-option type and
sub-option length fields. Sub-option lengths are
in units of 8 octets, aligned on a 64-bit
boundary. Sub-options that are shorter are
padded with null octets, the extent of the
padding is determined by the sub-option contents.
5.1.3.1 L2 ID Sub-Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-Option Type|Sub-Option Len | Context-ID | Status Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2-Type | L2 ID . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
Sub-Option Type:
0x01
Sub-Option Length:
Length of the sub-option.
Context-ID: Associates the L2 ID, IP address and other parameters
that belong to the same AR IP address but are encoded
in separate sub-options.
Status Code: This field allows ARs to inform a requesting entity
about processing results for a particular L2 ID. The
L2 ID sub-option MUST be sent back to the requesting
entity with a CARD Reply message.
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The following status codes are specified:
0x00: NONE - This value MUST be set when the
L2 ID is included in a CARD Request.
0x01: CANDIDATE - MUST be set in a CARD Reply when
a L2 ID sub-option is included with
information about candidate APs' L2 IDs.
Candidate L2 IDs are sent if the CARD Request
did not include a specific L2 ID for
resolution. If CANDIDATE is set, the AR MUST
set the Context-ID field of individual
parameters to a value that allows matching
associated L2 ID, address and capability
information on the receiver side.
0x02: MATCH - MUST be set in the CARD Reply to
identify that this L2 ID matches previously
resolved CAR information for a different L2
ID. If MATCH is set, the AR sets the Context-
ID in the L2-ID sub-option to identify the
matching previously resolved L2 ID.
0x03: RESOLVER ERROR - MUST be set in the CARD
Reply if the L2 ID cannot be resolved. The AR
sets this value for the Status Code in the
returned L2 ID sub-option.
L2 type: Indicates the interface type. Allocated by IANA
[Ke04].
L2 ID: The variable length Layer-2 identifier of an
individual CAR's access point. The length without
padding is determined by the L2 type.
5.1.3.2 Preferences Sub-Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-Option Type|Sub-Option Len | Preferences
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Option Type:
0x04
Sub-Option Length:
Length of the sub-option.
Preferences: List of capability attribute values (Section 5.1.4).
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Only ATTRIBUTE (AVP Code, see Section 5.1.4) fields MUST be present
and set for individual capabilities, which are of interest to the
requesting entity. The LIFETIME and VALUE (Data) indicator will not
be processed and can be omitted. The AVP LENGTH indicator is also
not present, since the preferences are indicated only with a list of
16-bit encoded ATTRIBUTE fields. In case 64-bit boundary alignment
requirements cannot be met with the list of ATTRIBUTE values,
padding the missing 16-bit MUST be done with an ATTRIBUTE value of
0x0000. An ATTRIBUTE code of 0x0 is reserved, so the end of the
ATTRIBUTE code list can be determined when an ATTRIBUTE value of 0x0
is read.
The use of the Preferences sub-option is optional and for
optimization purposes.
5.1.3.3 Requirements Sub-Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-Option Type|Sub-Option Len | Requirements
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Option Type:
0x05
Sub-Option Length:
Length of the sub-option.
Requirements: AVP encoded requirements (see Section 5.1.4)
AVPs MUST be encoded according to the rule described in Section
5.1.4. Both, the ATTRIBUTE (AVP Code) and VALUE (Data) field MUST be
present and set appropriately. The end of the Requirements list can
be determined when an ATTRIBUTE value of 0x0 is read.
The use of the Requirements sub-option is optional and for
optimization purpose.
5.1.3.4 Capability Container Sub-Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-Option Type|Sub-Option Len | Context-ID |P| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVPs
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
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Sub-Option Type:
0x03
Sub-Option Length:
Length of the sub-option.
Context-ID: Associates the L2 ID, IP address and other parameters
that belong to the same AR IP address but are encoded
in separate sub-options.
Flags: P-flag: Indicates piggybacking capability of the CAR
whose capabilities are conveyed in this
Capability Container. This flag allows a MN
to know after a CARD process whether a
selected new AR can perform piggybacking.
Reserved: Initialized to zero, ignored on reception.
AVPs: AVPs are a method of encapsulating capability
information relevant for the CARD protocol. See
Section 5.1.4 for the AVP encoding rule and list
parsing.
5.1.3.5 Address Sub-Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-Option Type|Sub-Option Len | Context-ID | Address Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - -
Sub-Option Type:
0x02
Sub-Option Length:
Length of the sub-option. For IPv4, the length is
1 (8 octets); for IPv6 the length is 3 (24 octets).
Context-ID: Associates the L2 ID, IP address and other parameters
that belong to the same AR IP address but are encoded
in separate sub-options.
Address Type: Indicates the type of the address.
0x01 IPv4
0x02 IPv6
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Address: The Candidate Access Router's IP address.
5.1.3.6 Trusted Anchor Sub-Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-Option Type|Sub-Option Len | Component |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Trusted Anchor Name
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - -
Sub-Option Type:
0x06
Sub-Option Length:
Length of the sub-option.
Reserved: Initialized to zero, ignored on reception.
Component:
A 2 octet unsigned integer field set to 65,535 if the
sender desires to retrieve all the certificates in
the certification path. Otherwise, it is set to the
component identifier corresponding to the certificate
that the receiver wants to retrieve.
Trusted Anchor Name: DER encoding for the X.501 name of
certification path component(see [Arkko04] for
more detail on certification path component
name encoding).
A CARD Request message containing Trusted Anchor sub-options MUST
NOT contain any other sub-options, except for a single L2 ID sub-
option identifying the AP of interest.
Trusted anchor sub-options SHOULD be retransmitted for individual
components not received within CARD_REQUEST_RETRY seconds, rather
than retransmitting a request for the whole list. Subsequent
retransmissions SHOULD take into account any received options and
only request those that have not been received.
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5.1.3.7 Router Certificate Sub-Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-Option Type|Sub-Option Len | Context-ID | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| All Components | Component |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Certificate... |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Option Type:
0x07
Sub-Option Length:
Length of the sub-option.
Context-ID: Associates the L2 ID, IP address and other parameters
that belong to the same AR IP address but are encoded
in separate sub-options.
Reserved: Initialized to zero, ignored on reception.
All Components: 2 octet unsigned integer giving the total number of
certificates in the certification path.
Component: 2 octet unsigned integer giving the location of this
certificate in the certification path.
Certificate: Variable length field containing the X.509v3 router
certificate encoded in ASN.1 (see [Arkko04] for more
detail on certificate profile including encoding)
Padding: A variable length field making the option length a
multiple of 8, beginning after the ASN.1 encoding of
the certificate continuing to the end of the option,
as specified by the Length field.
A CARD Reply containing a Router Certificate sub-option MUST NOT
include more than one such sub-option, and the CARD Reply MUST
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contain the matching L2 ID sub-option and router Address sub-option
for the router possessing the chain with the Context-ID field set to
a nonzero value, and no other sub-options. Any other sub-options
included in a CARD Reply SHOULD be ignored. If the reply spans
multiple ICMP messages, the L2 ID sub-option and router Address sub-
option MUST be included in the first message sent, and the Context-
ID field in the Router Certificate sub-options in all the messages
MUST be set to the same value as in the L2 ID and Address sub-
options. The replying AR SHOULD order the returned certification
path such that the certificate immediately after the trust anchor in
the path is the first certificate sent, in order to allow immediate
verification. The trust anchor certificate itself SHOULD NOT be
sent.
5.1.4 Capability AVP encoding rule
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Code | AVP Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Lifetime | Data . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - -
AVP Code: Identifies the attribute uniquely. The AVP Code
0x0000 is reserved and MUST NOT be assigned to a
capability.
AVP Length: The two octet AVP length field indicates the
number of octets in this AVP, including the AVP Code,
AVP Length, Reserved, Lifetime and Data field.
Reserved: Initialized to zero, ignored on reception.
Lifetime: Specifies the lifetime of the encoded capability
in seconds. In case of a static capability, the
Lifetime field MUST be set to the maximum value
(0xffff), which indicates that the lifetime of this
capability parameter never expires. A lifetime value
of 0x0000 deletes a capability entry.
Data: This variable length field has the Value of the
capability attribute encoded.
Since an AVP Code of 0x0 is reserved, it can be used by the sub-
option list parsing to determine when the end of a list of
Capabilities has been reached and where the sub-option padding
starts. AVPs themselves are not zero padded.
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Note: This document provides no detailed information on how to
encode the individual capability attribute values, which is to be
encoded in the Data field. Details on the interpretation of
individual capability parameters are out of scope of this document.
5.2 CARD Inter-Access Router Messages
5.2.1 AR-AR Transport
Since the types of access networks in which CARD might be useful are
not today deployed or, if they have been deployed, have not been
extensively measured, it is difficult to know whether congestion
will be a problem for inter-router CARD. Part of the research task
in preparing CARD for consideration as a candidate for possible
standardization is to quantify this issue. However, in order to
avoid potential interference with production applications should a
prototype CARD deployment involve running over the public Internet,
it seems prudent to recommend a default transport protocol that
accommodates congestion.
This suggests that implementations of CARD MUST support and
prototype deployments of CARD SHOULD use Stream Control Transport
Protocol (SCTP) [Stew00] for the transport protocol between routers,
especially if deployment over the public Internet is contemplated.
SCTP supports congestion control, fragmentation, and partial
retransmission based on a programmable retransmission timer. SCTP
also supports many advanced and complex features, such as multiple
streams and multiple IP addresses for failover, that are not
necessary for experimental implementation and prototype deployment
of CARD. The use of such SCTP features for CARD is not recommended
at this time.
The SCTP Payload Data Chunk carries the CARD messages. CARD messages
on the inter-router interface consist of just the CARD Request or
CARD Reply options. The User Data part of each SCTP message contains
the CARD option for the message type. For instance, a CARD Reply
message is constructed by including the CARD Reply option and all
the appropriate sub-options within the User Data part of an SCTP
message.
A single stream is used for CARD with in-sequence delivery of SCTP
messages. Each message, unless fragmented, corresponds to a single
CARD query or response. Unsolicited CARD Reply messages can also be
sent to peers to notify them of changes in network configuration or
capabilities. A single stream provides simplicity. Use of multiple
streams to prevent head-of-line blocking is for future study. Since
timeliness is not an issue with inter-router CARD and since there is
unlikely to be more than one CARD transaction between two routers
active at any one time, having ordered delivery simplifies the
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implementation. The Payload Protocol Identifier in the SCTP header
is 'CARD'. CARD uses the Seamoby SCTP port number [Ke04].
The format of Payload Data Chunk taken from [Stew00] is shown in the
following diagram.
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 = 0 | Reserved|U|B|E| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream Identifier S | Stream Sequence Number n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Protocol Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ User Data (seq n of Stream S) /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
'U' bit The Unordered bit. MUST be set to 0 (zero).
'B' bit The Beginning fragment bit. See [Stew00].
'E' bit The Ending fragment bit. See [Stew00].
TSN Transmission Sequence Number. See [Stew00].
Stream Identifier S
Identifies the CARD stream.
Stream Sequence Number n
Sequence number. See [Stew00].
Payload Protocol Identifier
Set to 'CARD'.
User Data Contains the CARD message.
In order to avoid generating congestion on startup, ARs MUST wait a
random amount of time between 0 and CARD_STARTUP_WAIT seconds upon
reboot before sending an AR-AR CARD Request to one of its CARs. An
AR that receives a CARD Request from an AR that is not in its CAR
table MUST NOT solicit the AR, but rather MUST wait until the AR
sends an unsolicited CARD Reply advertising the AR's information. An
AR that is starting up MUST sent unsolicited CARD Replies to all its
CARs to make sure their CAR tables are properly populated.
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The frequency of unsolicited CARD Reply messages MUST be strictly
limited to CARD_MIN_UPDATE_INTERVAL, in order to avoid overwhelming
CARs with traffic. ARs are free to discard messages that arrive more
frequently.
If a CARD deployment will never run over the public Internet, and it
is known that congestion is not a problem in the access network,
alternative transport protocols MAY be appropriate vehicles for
experimentation. Implementations of CARD MAY support UDP for such
purposes. In that case, the researcher MUST be careful to
accommodate good Internet transport protocol engineering practices,
including using retransmits with exponential backoff, etc. In
addition, it is an open research question whether SCTP is an
appropriate transport protocol for all inter-router CARD operations.
Investigation of this issue, for example to determine whether a
lighter weight protocol might be more appropriate than SCTP, may be
of interest to some researchers.
5.2.2 Protocol Payload Types
The AR-AR interface MUST insert the CARD Request option and CARD
Reply option directly in the body of the SCTP User Data field. The
sequence number for the CARD Request on the AR-AR interface MUST be
initialized to zero when the AR reboots, and MUST be incremented
every time a CARD Request message is sent. The replying AR MUST
include a sequence number from the CARD Request in the CARD Reply.
If an unsolicited CARD Reply is sent, the sending AR MUST increment
the sequence number. Sequentially increasing sequence numbers allows
the receiving AR to determine whether the information has already
been received.
On the AR-AR interface, the Capability Container parameter is used
to convey capabilities between ARs. Optionally, the Preferences
parameter can be used for capability pre-filtering during the inter-
AR capability discovery procedure. Payload types and encoding rules
are the same as described for the respective sub-option types in
Section 5.1 for the MN-AR interface. The same TLV-encoded format is
used to attach the options as payload to the protocol main header.
Additionally, an AR can set the T flag in the CARD Request header in
order to obtain the certificates for the CAR. The description of
sub-options in Section 5.1.3 includes information on what flag
settings are prohibited on the AR-AR interface.
6. SECURITY CONSIDERATIONS
6.1 Veracity of CARD Information
The veracity of the CARD protocol depends on the ability of an AR to
obtain accurate information about geographically neighboring ARs,
and to provide such accurate information about its own APs and
capabilities to other ARs. The CARD protocol described in the body
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of this document does not contain any support for determining the AR
to AP mapping or capabilities, either for a specific AR itself or
for a CAR. Therefore, methods for determining the accuracy of the
information exchanged between ARs are out of scope for the base CARD
protocol. The appendices of this draft describe procedures for
discovering the identities of the geographically adjacent ARs and
APs, including capabilities, and discuss relevant security
considerations. Alternatively, this information could be statically
configured into the AR.
6.2 Security Association between AR and AR
CARD does contain support allowing ARs to exchange capability
information. If this protocol is not protected from modification, a
malicious attacker can modify the information. Also if the
information is delivered in plain text, a third party can read it.
To prevent the information from being compromised, the CARD messages
between ARs MUST be authenticated. The messages also SHOULD be
encrypted for privacy of the information if required.
Confidentiality might be required if the traffic between two ARs in
an operator's network traversed the public Internet, for example.
Two ARs engaging in the CARD protocol MUST use IKE [HarCar98] to
negotiate an IPsec ESP security association for message
authentication. If confidentiality is desired, the two ARs MUST
additionally negotiate an ESP security association for encryption.
Replay protection SHOULD also be enabled with IKE. To protect CARD
protocol messages between ARs, IPsec ESP [AtKe98] MUST be used with
a non-null integrity protection and origin authentication algorithm
and SHOULD be used with a non-null encryption algorithm for
protecting the confidentiality of the CARD information.
An AR can provide the certificates for its CARs if the certificates
are available. The AR requests certificates from its CARs by setting
the T flag in the CARD Request message. All certificates are sent.
If CARD is used to exchange information between different
administrative domains, additional security policy issues may apply.
Such issues are out of scope of this document. Use of CARD between
administrative domains is not recommended at this time, until the
policy issues involved are more thoroughly understood.
6.3 Security Association between AR and MN
A malicious node can send bogus CARD Reply messages to MNs by
masquerading as the AR. The MN MUST authenticate the CARD Reply
messages from the AR. Since establishing an IPSec security
association between the MN and AR is likely to be a performance
issue, IKE is not an appropriate mechanism for setting up the
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security association. Instead, the SEND security association is used
[Arkko04]. ARs MUST include a SEND Signature Option on CARD Reply
messages. The format of the signature option is the same for both
IPv4 and IPv6 CARD, though SEND itself is only defined for IPv6. A
Mobile IPv4 ICMP Foreign Agent Advertisement option type code for
the SEND signature option [Ke04] has been allocated.
No authentication is required for CARD Requests since CARD
information is provided by the AR to optimize link access. In
contrast, CARD Reply authentication is required because a bogus AR
could provide the MN with CARD information that would lead the MN to
handover to a bogus router which could steal traffic or propagate a
denial of service attack on the MN. The asymmetry of the
authentication requirement is the same as that involving Router
Advertisements in IPv6 router discovery [SEND].
Since CARD is a discovery protocol, confidentiality is not necessary
in general on the MN-AR interface. In specific cases where different
network operators are sharing the same access network
infrastructure, network operators may want to hide information about
operator-specific capabilities for business reasons. The base CARD
protocol contains no support for such cases. However, should such a
case arise in the future, an AVP for an encrypted capability can be
defined at that time.
6.4 Router Certificate Exchange
Because SEND is only available in IPv6, the procedures for obtaining
certificates differ depend on whether CARD is used with IPv4 or
IPv6. In IPv6, when the MN receives a CARD reply with signature from
a router for which it does not have a certificate, it SHOULD use
SEND DCS/DCA to obtain the AR's certificate chain. ARs MUST be
configured with a certification path for this purpose, and MNs MUST
be configured with a set of certificates for shared trusted anchors
to allow verification of the AR certificates. A MN may not
necessarily need to use Cryptographically Generated Addresses (CGAs)
with CARD, CGA support is OPTIONAL for CARD. A certificate profile
for ARs is described in the SEND specification [Arkko04].
In IPv4, there is no DCS/DCA message for obtaining the certificate.
In that case, if the MN does not have a certificate for the router,
the MN sends a CARD Request message containing the L2 ID of its
current AP and one Trusted Anchor sub-option (Section 5.1.3.6) for
each shared trusted anchor for which the MN has a certificate, to
obtain the certification path for the current AR. The Component
field of the Trusted Anchor sub-option is set to 65535 to indicate
that the entire certification path is needed. No other options
should be included in the request. The AR replies by sending a CARD
Reply containing the L2 ID sub-option sent in the request, an
Address sub-option for itself and a Router Certificate sub-option
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(Section 5.1.3.7) containing one certificate in its certification
path, matching one of the requested trust anchors, and no other sub-
options, setting the Context-ID of all sub-options to match. The All
Components field is set to the path length and the Component field
is set to the number of this component in the path. If the path is
longer than one certificate, the router sends the LD ID sub-option
and the Address sub-option in the first certificate and the other
certificates in separate ICMP messages, due to the limitation on
ICMP message length, with the same Context-ID set on each Route
Certificate sub-option, and the Component field properly set. The
router SHOULD NOT send the trusted anchor's certificate and SHOULD
send certificates in order from the certificate after the trusted
anchor. If the trusted anchor option does not match any certificate,
the AR returns the Trusted Anchor sub-options in the reply. The MN
SHOULD immediately conduct a Certificate Revocation List (CRL) check
on any certificates obtained through CARD certificate exchange, to
make sure the certificates are still valid.
Certification paths for CARs may be fetched in advance of handover
by requesting them as part of the CARD protocol. In that case, the
MN includes Trusted Anchor sub-options in the CARD request along
with the L2 ID option for the AP for which the CAR certificate is
desired, and the AR replies as above, except the L2 ID, address and
certificates are for the CAR instead of for the AR itself. This
allows the MN to skip the DCS/DCA or CARD certificate exchange when
it moves to a new router.
Because the amount of space in an ICMP message is limited, the
router certification paths SHOULD be kept short.
6.5 DoS Attack
An AR can be overwhelmed with CARD Request messages. The AR SHOULD
implement a rate limiting policy so that it does not send or process
more than a certain number of messages per period. A suggested rate
limiting policy is the following. If the number of CARD messages
exceeds CARD_REQUEST_RATE, the AR SHOULD begin to randomly drop
messages until the rate is reduced. MNs SHOULD avoid sending
messages more frequently than CARD_REQUEST_RATE. ARs SHOULD also
avoid sending unsolicited CARD Replies or CARD Requests more
frequently than CARD_MIN_UPDATE_INTERVAL, but, in this case, the
existence of an IPsec security association assures that messages
from unknown entities will be discarded immediately during IPsec
processing.
MNs MUST discard CARD Replies for which there is no outstanding CARD
Request, indicated by the sequence number.
6.6 Replay Attacks
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To protect against replay attacks on the AR-AR interface, ARs SHOULD
enable replay protection when negotiating the IPsec security
association using IKE.
On the MN-AR interface, the MN MUST discard any CARD Replies for
which there is no outstanding request as determined by the sequence
number. For ARs, an attacker can replay a previous request from a
MN, but the attack is without serious consequence since the MN in
any case ignores the reply.
7. PROTOCOL CONSTANTS
Constant Section Default Value Meaning
--------------------------------------------------------------------
CARD_REQUEST_RETRY 5.1.1 2 seconds Wait interval before
initial retransmit
on MN-AR interface.
CARD_RETRY_MAX 5.1.1 15 seconds Give up on retry
on MN-AR interface.
CARD_STARTUP_WAIT 5.2.1 1-3 seconds Maximum startup wait
for an AR before
performing AR-AR
CARD.
CARD_MIN_UPDATE_INTERVAL 5.2.1 60 seconds Minimum AR-AR update
interval.
CARD_REQUEST_RATE 6.5 2 requests/ Maximum number of
sec. messages before
AR institutes rate
limiting.
8. IANA CONSIDERATIONS
See [Ke04] for instructions on IANA allocation.
9. NORMATIVE REFERENCES
[Brad97] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[Kemp02] Kempf, J., "Problem Description: Reasons For Performing
Context Transfers Between Nodes in an IP Access Network",
RFC 3374, September 2002.
[NaNS98] Narten, T., et al., "Neighbor Discovery for IP Version 6
(IPv6)", RFC 2461, December 1998.
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[Post80] Postel, J., "User Datagram Protocol", RFC 768, August 1980.
[Stew00] Stewert, R., et. al., "Stream Control Transmission
Protocol", RFC 2960, October, 2000.
[AtKe98] Atkinson, R., Kent, S.,"IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[NaAl98] Narten, T., Alvestrand, H., "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
[HarCar98] Harkins, D., and Carrel, D., "The Internet Key Exchange",
RFC 2409, November, 1998.
[Arkko04] Arkko, J., editor, Kempf, J., Sommerfelt, B., Zill, B., and
Nikander, P., "SEcure Neighbor Discovery(SEND)", Internet
draft, Work in progress.
[Ke04] Kempf, J., "Instructions for Seamoby and Experimental Mobility
Protocol IANA Allocations", Internet Draft, Work in progress.
10. INFORMATIVE REFERENCES
[TKCK02] Trossen, D., Krishanmurthi, G. Chaskar, H., Kempf, J.,
"Issues in candidate access router discovery for seamless
IP-level handoffs", Internet Draft, Work in progress.
[Kris02] Krishanmurti, G., "Requirements for CAR Discovery
Protocols", Internet Draft, Work in progress.
[Kenw02] Kenward, B., "General Requirements for Context
Transfer", Internet Draft, Work in progress.
[MaKo03] Manner, J., Kojo, M. (Ed), "Mobility Related Terminology",
Internet Draft, Work in progress.
[Kood03] Koodli, R, et al., "Fast handoffs for Mobile IPv6",
Internet Draft, Work in progress.
[Funa02] Funato, D. et al., "Geographically Adjacent Access Router
Discovery Protocol", Internet Draft, Work in progress.
[Tros03] Trossen, D. et al., "A Dynamic Protocol for Candidate
Access-Router Discovery", Internet Draft, Work in progress.
[ShGi00] Shim, E., Gitlin, R., "Fast Handoff Using Neighbor
Information", Internet Draft, Work in progress.
[Malk03] El Malki, K. et al., "Low Latency Handoffs in Mobile IPv4",
Internet Draft, Work in progress.
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11. AUTHORS' ADDRESSES
Hemant Chaskar
Nokia Research Center
5 Wayside Road
Burlington, MA 01803, USA
Phone: +1 781-993-3785
Email: Hemant.Chaskar@nokia.com
Daichi Funato
NTT DoCoMo USA Labs
181 Metro Drive, Suite 300
San Jose, CA 95110, USA
Phone: +1 408-451-4736
Email: funato@docomolabs-usa.com
Marco Liebsch
NEC Network Laboratories
Kurfuersten-Anlage 36 , 69115 Heidelberg
Germany
Phone: +49 6221-90511-46
Email: marco.liebsch@ccrle.nec.de
Eunsoo Shim
NEC Laboratories America, Inc.
4 Independence Way
Princeton, NJ 08540, USA
Phone: +1 609-951-2909
Email: eunsoo@nec-labs.com
Ajoy Singh
Motorola Inc
1501 West Shure Dr, USA
Phone: +1 847-632-6941
Email: asingh1@email.mot.com
12. IPR STATEMENTS
The IETF has been notified of intellectual property rights claimed
in regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
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made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
13. DISCLAIMER OF VALIDITY
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
14. COPYRIGHT NOTICE
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
15. CONTRIBUTORS
The authors would like to thank Vijay Devarapalli (Nokia) and Henrik
Petander (Helsinki University of Technology) for formally reviewing
the protocol specification draft and providing valuable comments and
input for technical discussions. The authors would also like to
thank James Kempf for reviewing and providing lots of valuable
comments and editing help.
15. ACKNOWLEDGEMENTS
The authors would like to thank (in alphabetical order) Dirk
Trossen, Govind Krishnamurthi, James Kempf, Madjid Nakhjiri, Pete
McCann, Rajeev Koodli, Robert C. Chalmers and other members of the
Seamoby WG for their valuable comments on the previous versions of
the document as well as for the general CARD related discussion and
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feedback. In addition, the authors would like to thank Erik Nordmark
for providing valuable insight about the piggybacking of CARD
options upon Fast Mobile IPv6 messages.
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APPENDIX A: MAINTENANCE OF ADDRESS MAPPING TABLES IN ACCESS ROUTERS
This appendix provides information on two optional CAR table
maintenance schemes for reverse address mapping in access routers.
These schemes replace static configuration of the AP L2 ID to CAR IP
address mapping in the CAR table. Details on these mechanisms are
out of the scope of this document and intention of this appendix is
to provide only a basic idea on flexible extensions to the CARD
protocol as described in this document.
Appendix A.1 Centralized Approach using a Server Functional Entity
The centralized approach performs CARD over the MN-AR interface as
described in Section 4 of this document. Additionally, the
centralized approach introduces a new entity, the CARD server, to
assist the current AR in performing reverse address translation. The
centralized approach requires neighboring AR(s) to register with the
CARD server to populate the reverse address translation table. The
registration of AR addresses with the CARD server is performed prior
to initiation of any reverse address translation request.
Figure A.1 illustrates a typical scenario of the centralized CARD
operation. In this example, ARs have registered their address
information with a CARD server in advance. When a MN discovers the
L2 ID of APs during L2 scanning, the MN passes one or more L2 ID(s)
to its current AR and the AR resolves it to the IP address of the
AR. For this, the AR first checks whether the mapping information is
locally available in its CAR table. If not, the MN's current AR
queries a CARD server with the L2 ID. In response, the CARD server
returns the IP address of the CAR to the current AR. Then, the
current AR directly contacts the respective CAR and performs
capability discovery with it. The current AR then passes the IP
address of the CAR and associated capabilities to the MN. The
current AR then stores the resolved IP address within its local CAR
table. The centralized CARD protocol operation introduces additional
signaling messages, which are exchanged between the MN's current AR
and the CARD server. The signaling messages between an AR and the
CARD server function are shown with the preceding identifier "AR-
Server", referring to the associated interface.
An initial idea of performing reverse address translation using a
centralized server has been described in [Funa02].
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+----------+
+------------>| CARD |<-------------+
|+------------| Server |-------------+|
|| +----------+ ||
|| ||
|| ~~~~~~~~~~~ ||
(3)AR-Server||(4)AR-Server{ } ||(0) CARD
CARD || CARD { } ||Reg Req/
Request || Reply { IP Cloud } | Reply
|| { } ||
|| { } ||
|V ~~~~~~~~~~~ V|
+---------+ (5)AR-AR CARD Request +-----+-----+
| Current |------------------------->| CAR | CAR |
| AR |<-------------------------| 1 | 2 |
+---------+ (6)AR-AR CARD Reply +-----+-----+
^ | | |
(2)MN-AR | |(7)MN-AR | |
CARD | | CARD | |
Request| V REPLY +---+ +---+
+--------------+ (1) AP1 L2 ID +--|AP1| |AP2|
| Mobile |<---------------------+ +---+ +---+
| Node |<--------------------------------+
+--------------+ (1) AP2 L2 ID
Figure A.1: Centralized Approach for L2-L3 mapping
Appendix A.2 Decentralized Approach using Mobile Terminals'
Handover
This approach performs CARD over the MN-AR interface as described in
Section 4. However, it employs one additional message, called the
Router Identity message, over the MN-AR interface to enable ARs to
learn about the reverse address translation tables of their
neighboring ARs, without being dependent on any centralized server.
In this approach, CAR identities in the CAR table of an AR are
maintained as soft state. The entries for CARs are removed from the
CAR table if not refreshed before the timeout period expires and are
created or refreshed according to the following mechanism.
The key idea behind the decentralized approach is to bootstrap and
maintain the association between two ARs as neighbors of each other
using the actual handover of MNs occurring between them as input.
The first handover between any two neighboring ARs serves as the
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bootstrap handover to invoke the discovery procedure and the
subsequent handover serves to refresh the association between the
neighboring ARs. After the bootstrap handover, the MNs can perform
CARD and thus seamless handover using the CAR information. This idea
was presented in [ShGi00] and [Tros03].
Maintenance of the CAR table is done using an additional option for
the CARD protocol operation performed between a MN and its current
AR. This message serves as Router Identity message.
Upon the completion of an inter-AR handover, the MN SHOULD send a
Router Identity message to its current AR. This message contains the
identity (IP address) of the previous AR (pAR), and can be sent as a
specific sub-option in the MN-AR CARD Request message. It SHOULD be
acknowledged with the MN-AR CARD Reply. The Router Identity message
enables the MN's current AR to learn that the pAR (still) has an AP
whose coverage overlaps with one of the APs of the current AR and
vice versa. With this information, the MN's current AR can create or
refresh an entry for the pAR as its neighbor. If handover is no
longer possible between two ARs, the associated entries eventually
timeout and are removed from each AR's CAR table.
Prior to trusting the MN's report, however, the current AR may
perform a number of checks to ensure the validity of the received
information. One simple method is to verify the accuracy of the
Router Identity message by sending an AR-AR CARD Request message to
the pAR. The AR-AR CARD Request includes the identity of the MN.
Upon receiving this message, the pAR verifies that the MN was indeed
attached to it during a reasonable past interval and responds to the
current AR. In this way, each handover of a MN results in a bi-
directional discovery process between the two participating ARs.
Upon receiving a positive verification response, the current AR
creates or refreshes as applicable the entry for the pAR in its
local CAR table. In the former case, the current AR and the pAR
exchange capabilities using the AR-AR CARD Request and AR-AR CARD
Reply protocol messages. When a new entry is created, the ARs MUST
exchange their reverse address translation tables. They may exchange
other capabilities at this time or may defer it to a later time when
some MN undergoing handover between them performs CARD as described
in Section 4. In the later (refresh) case, ARs may exchange
capabilities or defer it until a later time when another MN
undergoes handover.
Finally, note that in a handover-based protocol, a first handover
between a pAR and a MN's current AR cannot use CARD, as this
handover bootstraps the CAR table. However, in long term, such a
handover will only amount to a small fraction of total successful
handover between the two AR(s). Also, if the MN engaging in such a
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first handover is running a non-delay sensitive application at the
time of handover, the user may not even realize its impact.
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APPENDIX B: APPLICATION SCENARIOS
This section provides two examples of an application scenario for
CARD protocol operation. One scenario describes a CARD protocol
operation in a Mobile IPv6 (MIPv6) network, providing access to the
infrastructure via wireless LAN Access Points and associated Access
Routers. A second scenario describes CARD protocol operation in a
Mobile IPv6 enabled network, which has enhanced support for fast
handover integrated (Fast Mobile IPv6), also providing wireless LAN
access to the infrastructure.
Appendix B.1 CARD Operation in a Mobile IPv6 Enabled Wireless LAN
Network
This application scenario assumes a moving MN having access to the
infrastructure through wireless LAN (IEEE802.11) APs. Mobility
management is performed using the Mobile IPv6 protocol.
The following figure illustrates the assumed access network design.
-----------------------------
/ \ +----+
| NETWORK |---| HA |
\ / +----+
-----------------------------
| |
+-----+ +-----+
| AR1 |---------+ | AR2 |
+-----+ | +-----+
| subnet 1 | |subnet 2
+-----+ +-----+ +-----+
| AP1 | | AP2 | | AP3 |
+-----+ +-----+ +-----+
^ ^ ^
\
\
\
v
+-----+
| MN | - - ->>>- - - ->>>
+-----+
Figure B.1: Assumed network topology
A Mobile IPv6 Home Agent (HA), maintains location information for
the MN in its binding cache. From Figure B.1, the MN holds a care-of
address for the subnet 1, supported by AR1. As the MN moves, the
MN's current environment offers two further wireless LAN APs with
increasing link-quality as candidate APs for a handover. To
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facilitate decision making, parameters associated with ARs are taken
into account during the decision process. The AR-related parameters
can be, for example, available QoS resources or the type of access
technologies supported from an AR. To learn about these candidate
ARs' capabilities and associated IP address information, the MN
performs CARD. This requires retrieving information about candidate
APs' L2 IDs Furthermore, associated link-quality parameters are
retrieved to ascertain, whether or not approaching APs are eligible
candidates for a handover. Assume AP2 and AP3 are suitable candidate
APs. The MN encapsulates both L2 IDs (AP2 and AP3) into a CARD
Request message, using the L2 ID sub-option, and sends it to its
current AR (AR1).
AR1 resolves each L2 ID, listed in L2 ID options to the associated
IP address of the respective CAR, making use of its local CAR table.
According to the environment illustrated in Figure B.1, the
associated AR IP address of the candidate AP2 will be the same as
the MN is currently attached to, which is AR1. Respective IP address
of the candidate AR, to which AP3 is connected to, is the address of
AR2. Since IP addresses of the MN's CARs are now known to AR1, AR1
retrieves the CARs' capabilities from the CAR table, assumed it has
valid entries for respective capability parameters To refresh
dynamic capabilities, whose associated lifetime in AR1's CAR table
has expired, AR1 performs Inter-AR CARD for capability discovery.
Since capability information for AR1 is known to AR1, a respective
Inter-AR CARD Request is sent only to AR2. AR2 in response sends a
CARD Reply message back to AR1, encapsulating the requested
capability parameters with the signaling message, in a Capability
Container sub-option.
Now, AR1 sends its own capabilities and the dynamically discovered
ones of AR2 back to the MN via a CARD Reply message. Furthermore,
AR1 stores the capability parameters of AR2 with the associated
lifetimes in its local CAR table.
On reception of the CARD Reply message, the MN performs target AR
selection, taking AR1's and AR2's capability parameters as well as
associated APs' link-quality parameters into account. In case the
selected AP is AP2, no IP handover needs to be performed. In case
AP3 and the associated AR2 are selected, the MN needs to perform an
IP handover according to the Mobile IPv6 protocol operation.
Figure B.2 illustrates the signaling flow of the previously
described application scenario of CARD within a Mobile IPv6 enabled
network.
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MN AP1 AR1 AP2 AP3 AR2
| | | | | |
| connected | | | | |
0-------------0-------0 | | |
| | | | | |
| | | | | |
| | | |
| <~~~~~~~~~L2-SCAN (AP2)~~~~~| | |
| <~~~~~~~~~L2-SCAN (AP3)~~~~~~~~~~~~~~~~~| |
| | | |
| (MN-AR) CARD Req | | | |
|-------------------->| (AR-AR) CARD Req |
| | |---------------------------------------->|
| | | (AR-AR) CARD Repl |
| (MN-AR) CARD Repl |<----------------------------------------|
|<--------------------| | | |
| | | | | |
[target AR | | | | |
selection] | | | | |
| | | | | |
// // // // // //
[either...] | | | | |
| | | | | |
|-------- L2 attach --------->| | |
| | | | | |
| connected | | | |
0---------------------0-------0 | |
| | | | | |
// // // // // //
[... or] | | | | |
| | | | | |
|--------------- L2 attach -------------->| |
| | | | | |
| connected | | | |
0-----------------------------------------0---------------------0
| | | | | |
| | |
| MIPv6 Binding Update to the HA | |
|------------------------------------------------ - - - > |
| | | | | |
Figure B.2: CARD protocol operation within a Mobile IPv6 enabled
wireless LAN network.
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Appendix B.2 CARD Operation in a Fast Mobile IPv6 Network
This application scenario assumes ARs can perform the fast handover
protocol sequence for Mobile IPv6 [Kood03]. The MN scans for new APs
for handover similar to Figure B.1 To discover the ARs (CARs), the
MN attaches a MN-AR CARD Request option to the ICMP-type Fast Mobile
IPv6 RtSolPr message, which is sent to the MN's current AR (pAR,
previous AR).
Candidate APs' L2 IDs are encapsulated using the CARD protocol's L2
ID sub-options, which allow the MN to send multiple L2 IDs of
candidate APs to its current AR (potentially replaces the "New
Attachment Point Link-Layer Address" option of the Fast Mobile IPv6
protocol).
The pAR resolves the received list of candidate APs' L2 IDs to the
IP address of associated CARs. The pAR checks its local CAR table to
retrieve information about the CARs' capabilities. If any table
entries have expired, the pAR acquires this CAR's capabilities by
sending an AR-AR CARD Request to the respective CAR. The CAR replies
with an AR-AR CARD Reply message, encapsulating all capabilities in
a Capability Container sub-option and attaching them to the CARD
Reply option. On reception of the CARs' capability information, the
pAR updates its local CAR table and forwards the address and
capability information to the MN of attaching a MN-AR CARD Reply
option, to the Fast Mobile IPv6 PrRtAdv message. When the MN's
handover is imminent, the MN selects its new AR and the associated
new AP from the discovered list of CARs. According to the Fast
Mobile IPv6 protocol, the MN notifies the pAR of the selected new AR
with the Fast Binding Update (F-BU) message, allowing the pAR to
perform a fast handover according to the Fast Mobile IPv6 protocol.
Optionally, the pAR could perform selection of an appropriate new AR
on behalf of the MN after the pAR has the MN's CARs' addresses and
associated capabilities available. The MN must send its requirements
for the selection process to its pAR together with the MN-AR CARD
Request message After the pAR has selected the MN's new AR, the
address and associated capabilities of the chosen new AR are sent to
the MN with the CARD Reply option, in the Fast Mobile IPv6 PrRtAdv
message.
Figure B.3 illustrates how CARD protocol messages and functions work
together with the Fast Mobile IPv6 protocol.
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MN pAR NAR CAR2
| | as CAR1 |
| | | |
|-------RtSolPr------>| | |
| [MN-AR CARD Req] |-- AR-AR CARD Req*->| |
| |-- AR-AR CARD Req*------------>|
| |<--AR-AR CARD Repl*------------|
| |<--AR-AR CARD Repl*-| |
|<------PrRtAdv-------| | |
| [MN-AR CARD Repl] | | |
| | | |
NAR selection | | |
|------F-BU---------->|--------HI--------->| |
| |<------HACK---------| |
| <--F-BACK--|--F-BACK--> | |
| | | |
Disconnect | | |
| forward | |
| packets===============>| |
| | | |
| | | |
Connect | | |
| | | |
RS (with FNA option)======================>| |
|<-----------RA (with NAACK option)--------| |
|<=================================== deliver packets |
| | |
Figure B.3: Fast Handover protocol sequence with
CARD protocol options
*) In Figure B.3, the CARD protocol interaction between the pAR and
CARs is only required in case the lifetime of any capability entries
in the pAR's CAR table have expired. Otherwise, the pAR can respond
to the requesting MN immediately after retrieving the CARs' address
and capability information from its CAR table.
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