Internet Engineering Task Force H. Levkowetz
Internet-Draft ipUnplugged
Expires: April 24, 2003 October 24, 2002
DHCP Option for Mobile IP Mobility Agents
<draft-ietf-dhc-mipadvert-opt-00.txt>
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines a new Dynamic Host Configuration Protocol
(DHCP) option with suboptions. One suboption is passed from the DHCP
Server to the DHCP Client to announce the presence of one or more
Mobile IP Mobility Agents. For each announced Mobility Agent,
information is provided which is the same as that of the Mobile IP
Agent Advertisement extension to ICMP Router Advertisements. There
is also one suboption which may be used by a DHCP client to provide
identity information to the DHCP server.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements terminology . . . . . . . . . . . . . . . . . . . 3
3. Mobility Agent Information Option . . . . . . . . . . . . . . 3
3.1 Mobility Agent Information Option Definition . . . . . . . . . 3
3.2 Network Access Identifier Sub-Option . . . . . . . . . . . . . 4
3.3 Mobility Agent Announcement Sub-Option . . . . . . . . . . . . 4
4. Mobility Agent Option Usage . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
Normative References . . . . . . . . . . . . . . . . . . . . . 7
Informative References . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 8
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
There already exists a DHCP option to announce Mobile IP Home Agent
addresses, described in RFC 2132 [2]. There is, however, no option
available to announce Mobile IP Foreign Agents.
Announcement of available Mobile IP Mobility Agents by means of DHCP
provides possibilities for selective and individual assignment of
Mobility Agents to Mobile Nodes. This in turn makes load-sharing and
selective service offerings easier. This draft describes a DHCP
option for announcing Mobility Agents to DHCP Clients.
2. Requirements terminology
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 [8].
The Mobile IP related terminology used in this document is described
in RFC 3220 [5].
3. Mobility Agent Information Option
3.1 Mobility Agent Information Option Definition
This document defines a new DHCP Option called the Mobility Agent
Option. It is a "container" option for specific agent- supplied sub-
options. The format of the Mobility Agent option is:
Code Len Mobility Agent Information Field
+-------+------+------+------+------+------+--...-+------+
| TBD | N | a1 | a2 | a3 | a4 | | aN |
+-------+------+------+------+------+------+--...-+------+
The length N gives the total number of octets in the Mobility Agent
Field. The Mobility Agent Information field consists of a sequence
of SubOpt/Length/Value tuples for each sub-option, encoded in the
following manner:
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SubOpt Len Sub-option Value
+------+------+------+------+------+------+--...-+------+
| 1 | N | s1 | s2 | s3 | s4 | | sN |
+------+------+------+------+------+------+--...-+------+
SubOpt Len Sub-option Value
+------+------+------+------+------+------+--...-+------+
| 2 | N | t1 | t2 | t3 | t4 | | tN |
+------+------+------+------+------+------+--...-+------+
The Mobility Agent Information field shall NOT be terminated with a
255 sub-option. The length N of the DHCP Mobility Agent Information
Option shall include all bytes of the sub-option code/length/value
tuples. Since at least one sub-option must be defined, the minimum
Mobility Agent Information length is two (2). The length N of the
sub-options shall be the number of octets in only that sub-option's
value field. A sub-option length may be zero. The sub-options need
not appear in sub-option code order.
3.2 Network Access Identifier Sub-Option
The Network Access Identifier (NAI) defined in RFC 2486 [3] is
already used in Mobile IP as an alternative to the home address as an
identifier of a mobile node [9].
The Network Access Identifier sub-option of the Mobility Agent
Information Option MAY be used by the DHCP client to provide
identifying information to the DHCP server, as part of the
DHCPDISCOVER message. The server may then use this information in
selecting mobility agent announcement parameters for the client.
The format of the Network Access Identifier sub-option is as follows:
SubOpt Len Sub-option Value
+------+-----+-----+-----+-----+-----+--...-+-----+
| 1 | N | Network Access Identifier |
+------+-----+-----+-----+-----+-----+--...-+-----+
3.3 Mobility Agent Announcement Sub-Option
The Mobility Agent Announcement sub-option announces the address of
one or more mobility agents, together with all the information about
the mobility agent which is normally found in a Mobile IP Agent
Advertisement extension to ICMP Router Advertisements as described in
RFC 3220 [5].
All fields are defined so as to correspond to fields of the same name
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in a Mobility Agent Advertisement Extension as described in RFC 3220
[5], and if in the future additional bits are allocated from the
'reserved' field for the Mobility Agent Advertisement Extension, they
should be equally valid in a DHCP Mobility Agent option.
This option may contain announcements of one or more Mobility Agents,
in sequence. The length of each individual Mobility Agent
Announcement is determined from the Adv-Length field of the
announcement.
SubOpt Len Sub-option Value (Announcements)
+------+-----+-----+-----+--...-+-----+-----+-----+--...-+-----+...
| 2 | N | Announcement 1 | Announcement 2 |
+------+-----+-----+-----+--...-+-----+-----+-----+--...-+-----+...
The format of one Mobility Agent Announcement is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobility Agent IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Adv-Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Registration Lifetime |R|B|H|F|M|G|r|T| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| zero or more care-of addresses |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option code
DHCP_MIP_OPTION (to be assigned by IANA)
Length
Length in bytes of this option, not including the Option code
and Length bytes.
Agent IP Address
The address trough which the Mobile Node may reach the
announced Mobility Agent in order to do a Mobile IP
registration.
Type
16. This is the same value as for the type field in a Mobility
Agent Advertisement Extension as described in RFC 3220 [5]. If
other Mobility Agent Advertisement Extensions are defined in
the future, this field will make it possible to differentiate
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between them without using new DHCP option numbers.
Reserved-0
Sent as zero; required to be zero on reception for the option
described in this document.
Adv-Length
(6 + 4*N), where 6 accounts for the number of bytes in the
Sequence Number, Registration Lifetime, flags, and reserved
fields, and N is the number of care-of addresses advertised for
the Mobility Agent.
Sequence Number
The count of Mobility Agent DHCP announcements made since the
DHCP server was initialized (RFC 3220, Section 2.3.2 [5]).
Registration Lifetime
The longest lifetime (measured in seconds) that this agent is
willing to accept in any Registration Request. A value of
0xffff indicates infinity.
R
Registration required. Registration with this foreign agent
(or another foreign agent listed in this DHCP option) is
required even when using a co-located care-of address.
B
Busy. The foreign agent will not accept registrations from
additional mobile nodes.
H
Home agent. This agent offers service as a home agent on the
link on which this mobility agent announcement is sent.
F
Foreign agent. This agent offers service as a foreign agent on
the link on which this mobility agent announcement is sent.
M
Minimal encapsulation. This agent implements receiving
tunneled datagrams that use minimal encapsulation [7].
G
GRE encapsulation. This agent implements receiving tunneled
datagrams that use GRE encapsulation [6].
r
Sent as zero; ignored on reception. SHOULD NOT be allocated
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for any other uses.
T
Foreign agent supports reverse tunneling [11].
reserved
Sent as zero; ignored on reception.
Care-of Address(es)
The foreign agent care-of address(es) provided by this foreign
agent. An DHCP Mobility Agent Announcement MUST include at
least one care-of address if the 'F' bit is set. The number of
care-of addresses present is determined by the Length field in
the Extension.
4. Mobility Agent Option Usage
The requesting and sending of this option follows the rules for DHCP
options in RFC 2131 [1]
5. Security Considerations
DHCP provides an authentication mechanism, as described in RFC 3118
[4], which may be used if authentication is required before offering
the Mobility Agent option described here. On the other hand, Mobile
IP Agent Advertisements as described in RFC 3220 [5] requires no
authentication for Agent Advertisement and Agent Solicitation
messages.
By providing Agent Advertisements by means of DHCP as an alternative
to extended ICMP Router Advertisement messages it is possible to do
so more selectively, and it does not offer any new threat to the
internet.
6. IANA Considerations
The value for the DHCP_MIP_OPTION code must be assigned from the
numbering space defined for public DHCP Options in RFC 2939 [10].
7. Acknowledgements
Normative References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[2] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
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Extensions", RFC 2132, March 1997.
[3] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
2486, January 1999.
[4] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[5] Perkins, C., "IP Mobility Support for IPv4", RFC 3220, January
2002.
Informative References
[6] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701, October 1994.
[7] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
October 1996.
[8] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[9] Calhoun, P. and C. Perkins, "Mobile IP Network Access
Identifier Extension for IPv4", RFC 2794, March 2000.
[10] Droms, R., "Procedures and IANA Guidelines for Definition of
New DHCP Options and Message Types", BCP 43, RFC 2939,
September 2000.
[11] Montenegro, G., "Reverse Tunneling for Mobile IP, revised", RFC
3024, January 2001.
Author's Address
Henrik Levkowetz
ipUnplugged AB
Arenavagen 33
Stockholm S-121 28
SWEDEN
Phone: +46 8 725 9513
EMail: henrik@levkowetz.com
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