Network Working Group A.K. Vijayabhaskar
Internet-Draft Hewlett-Packard
Expires: Dec 19, 2002 19 Jun 2002
Client Preferred Prefix option for DHCPv6
draft-ietf-dhc-dhcpv6-opt-cliprefprefix-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes the Client Preferred Prefix option by which
the client can specify its preferred prefixes on which the addresses
need to be allocated by the server.
1. Introduction
Scenario 1: The client's link has multiple prefixes of different
scopes and the administrator policy on the server insists that the
addresses need to be allocated on site-local prefixes only. The
client will not be able to communicate with a node that belongs to a
different site, as the server allocates only site-local addresses in
IAs.
Scenario 2: The client's link has two prefixes: site-local and global.
The administrator policy insists that addresses need to be allocated
on both the prefixes. All the nodes on a link will not communicate
with external sites and thus all of them do not require global
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addresses. However, the server allocates addresses on both the
prefixes and hence most of the global addresses are wasted.
To overcome the problems described in Scenario 1 and 2, the client
can specify its preferred prefixes to the server using Client
Preferred Prefix option.
2. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [1]
3. Terminology
This document uses terminology specific to IPv6 and DHCPv6 as defined
in section "Terminology" of the DHCP specification.
4. Client Preferred Prefix option
Client Preferred Prefix option is used by the client to specify its
preferred prefixes to the server.
The format of the Client Preferred Prefix option is as shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_CLIENT_PREF_PREFIX | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| prefix-len | |
+-+-+-+-+-+-+-+-+-+ |
| |
| subnet prefix (n bytes) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| prefix-len | |
+-+-+-+-+-+-+-+-+-+ |
| |
| subnet prefix (n bytes) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. client-pref-prefix-options .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_CLIENT_PREF_PREFIX (tbd)
option-len: total length of the prefix-len and subnet prefix lists
and its encapsulated options.
prefix-len: prefix length of the subnet address.
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subnet prefix: 'n' bytes of subnet prefix, where 'n' is minimum
number of bytes required to refer 'prefix-len' bits
of the prefix.
client-pref-prefix-options: options associated with Client
Preferred Prefix option.
5. Server Behavior
If the server policy doesn't support client preferred prefix option,
then it can either send reply with OptionUnsupported in the
encapsulated error code option in client preferred prefix option or
allocate addresses based on its original policy. The server behavior
SHOULD be configurable by the administrator.
If the server policy supports client preferred prefix option and if
this option contains one or more prefixes which are not valid for the
client's link, then, the server MUST send the reply with error code
NotOnLink.
If the server policy supports client preferred prefix option and all
the prefixes in this option are valid for the client's link, then the
server MUST allocate addresses only on the prefixes specified in
client preferred prefix option encapsulated in the IAs.
6. Client Behavior
If the client has received OptionUnsupported error, it can either
choose the next server to send request, till the server list gets
exhausted or it can start the configuration exchange as specified in
Section 18.1.1 of [2] without the client preferred prefix option.
If the server list has exhausted then, it MUST start the configuration
exchange as specified in Section 18.1.1 of [2] without the client
preferred prefix option.
If the client has received the addresses with the prefixes that were
not specified in client preferred prefix option, it can release the
unwanted addresses.
7. Appearance of these options
Client Preferred Prefix option MUST occur only in Request and Reply
messages. This option MUST occur in Reply messages only if it
encapsulates the Error code option.
Client Preferred Prefix option MUST occur only as an encapsulated
option in the IA or IA_TA option.
Client Preferred Prefix option MUST only have Error code option as the
encapsulated option.
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8. Security Considerations
Since, this option can occur only in IA or IA_TA option, all the
IA-relevant security considerations are applicable to this option too.
To avoid attacks through this option, the DHCP client SHOULD use
authenticated DHCP (see section "Authentication of DHCP messages"
in the DHCPv6 specification [2]).
9. IANA Considerations
IANA is requested to assign an option code to this option from the
option-code space defined in section "DHCPv6 Options" of the DHCPv6
specification [2].
References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R.
Droms (ed.), "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-26 (work in progress), June
2002.
Author's Addresses
Vijayabhaskar A K
Hewlett-Packard ESD-I
29, Cunningham Road
Bangalore - 560052
India
Phone: +91-80-2051424
E-Mail: vijayak@india.hp.com
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society. Thanks to Jim Bound for his thorough review
of the document.
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