dhc Working Group R. Droms
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track September 8, 2008
Expires: March 12, 2009
Container Option for Server Configuration
draft-ietf-dhc-container-opt-01.txt
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Abstract
In some DHCP service deployments, it is desirable for a DHCP server
in one administrative domain to pass configuration options to a DHCP
server in a different administrative domain. This DHCP option
carries a set of DHCP options that can be used by another DHCP
server.
1. Introduction
In some DHCP service deployments, it is desirable to pass
configuration options from a DHCP server in one administrative domain
to another DHCP server in a different administrative domain. In one
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example of such a deployment, an IPTV service provider (SP) may need
to provide certain SP domain-specific information to IPTV device(s)
located in the consumer domain. This information is sent from the
IPTV SP DHCP server to the consumer DHCP server located in the
Residential Gateway (RG), which can then be passed along to IPTV
device(s) in the subscriber network.
Existing RGs may pass some configuration information received by the
RG DHCP client to the RG server for configuration of devices attached
to the consumer network. There are several motivations for this
option:
o The devices attached to the consumer network may require different
configuration information than the DHCP options provided to the RG
o Existing RG DHCP clients are typically not be coded to process new
DHCP options and, therefore, will be unable to pass those new
options to the RG DHCP server
o Existing RG DHCP clients are typically coded to pass only a fixed
list of DHCP options to the RG DHCP server and, therefore, will be
unable to pass newly defined options to the RG DHCP server.
The DHCP Container option defined in this document provides a
mechanism through which the RG DHCP client can pass DHCP options to
the RG DHCP server without explicit knowledge of the semantics of
those options. With this option, the SP DHCP server can pass both
current and future DHCP options to the RG DHCP server.
The DHCP Container option does not carry IP addresses, IPv6 prefixes
or other information about leases. It carries other configuration
information.
2. 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 RFC2119 [RFC2119].
The following terms and acronyms are used in this document:
DHCPv4 "Dynamic Host Configuration Protocol" [RFC2131]
DHCPv6 "Dynamic Host Configuration Protocol for IPv6"
[RFC3315]
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DHCP DHCPv4 and/or DHCPv6
RG "home gateway"; the device through which the
consumer network connects to the broadband WAN;
typically a layer 3 forwarding device
RG DHCP client (or "RG client") the DHCP client in the RG
RG DHCP server (or "RG server") the DHCP server in the RG
SP DHCP server (or "SP server") the DHCP server managed by the
service provider (SP)
This document uses terminology for DHCPv4 and DHCPv6 as defined in
RFCs 2131 and 2132, respectively.
3. Problem statement and requirements for RG DHCP server configuration
The following diagram shows the components in a network deployment
using the DHCP Container option:
Client STB/CPE -+ +---------+ +------+
| | RG | | SP |
Client STB/CPE -+ | Client+--- ... ---+ DHCP |
+--+Server | |server|
Client STB/CPE -+ +---------+ +------+
In this diagram, the RG client engages in DHCP message exchanges with
the SP server to obtain its IP address and other configuration
information.
The problem under consideration in this document is to transmit
configuration information from the SP DHCP server to devices attached
to the consumer network. The problem solution has the following
requirements:
o The SP server MUST be able to transmit different configuration
information to the consumer devices than the DHCP options provided
to the RG
o The SP server MUST be able to control which DHCP options are
transmitted to the consumer device
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o There MUST be a way for the SP server to pass DHCP options to be
defined in the future to consumer devices
4. Design alternatives
The following three designs meet the solution requirements:
o SP server passes container option to RG client, which forwards
contents to RG server; this alternative is the preferred solution
o RG server does direct DHCP info request to SP server; this
alternative is not preferred:
* requires that the RG server include a DHCP client
* requires that the SP server be able to differentiate between RG
client and server requests
* does not scale well, as it at least doubles the load on the SP
server
o RG server passes device requests to SP DHCP server; this
alternative is not preferred:
* requires that the RG also function as a DHCP relay
* requires that the RG relay function be configured with the IP
addresses of the SP DHCP server(s)
* requires that the RG relay function differentiate between DHCP
messages that are processed by the RG server and DHCP messages
that are processes by the SP server
A variant on the preferred design would allow the inclusion of
multiple sets of DHCP options intended for different classes of
devices in the consumer network; e.g., the design would allow for one
set of options for video set-top boxes and a second set of options
for VoIP MTAs. The variant would require the specification of rules
to be provided by the SP server through which the RG server would
differentiate its clients and send the appropriate set of options to
each device. At present, there is no requirement for differential
configuration of consumer devices and this alternative is not defined
in this document.
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5. Semantics and syntax of the Container option
Along with configuration information intended for the RG, the SP
server can include the DHCP Container option. When the RG client
receives the DHCP Container option, it passes the contents of the
option to the RG server. The means through which the information is
passed between the RG client and the RG server is out of the scope of
this document and left unspecified.
The DHCP options in this container are carried in DHCP message format
(option-code/length/value). In this format, the contained options
can be passed through a DHCP client to a co-located DHCP server
without specific knowledge on the part of the client or the server of
the semantics of the options.
5.1. DHCPv4 Container option
The DHCPv4 Container option has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | len | DHCP Options for RG server |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code OPTION_CONTAINER_V4 (TBD)
len Length of options for RG server, in octets
5.2. DHCPv6 Container option
The DHCPv6 Container option has the following format:
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_CONTAINER_V6 | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DHCP Options for RG server |
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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option-code OPTION_CONTAINER_V6 (TBD).
option-len Length of options for RG server, in octets
5.3. SP server behavior
The SP server MAY include the Container option in any DHCP message
sent to an RG client.
The policy through which the SP server is instructed to include a
Container option for an RG client, and the policy determining the
contents of the Container object are out of scope of this document
and left unspecified.
5.4. RG client behavior
The RG client MUST pass the contents of the received Container option
to the RG server without alteration. The details of the
implementation through which the RG client parses the content of the
Container option and passes the options to the RG server are out of
scope for this document and left unspecified.
5.5. RG server behavior
The RG server MUST discard any options related to IP address
assignment, IPv6 prefix delegation or operation of the DHCP protocol
itself. Appendices TBD give a list of DHCPv4 and DHCPv6 options that
the RG server MUST discard.
The Container option provides a mechanism through which the SP might
be able to unilaterally control the configuration settings passed
from a CPE DHCP server to a CPE device. This configuration channel
must be handled with some care if the subscriber is to retain desired
control over the CPE configurations. The following behaviors limit
the degree to which the SP con control CPE configuration:
o The RG server MAY discard any undesired options, as determined by
policy in the RG.
o The RG server MUST return to any DHCP client only those options
requested by the DHCP client in a Parameter Request List option
(DHCPv4 option code 55) or an Option Request option (DHCPv6 option
code 6).
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6. Security Considerations
A rogue server can use this option to pass invalid information to the
RG client, which would then be passed to the Client STB/CPEs. This
invalid information could be used to mount a denial of service attack
or a man-in-the-middle attack against some applications.
Authentication of DHCP messages(RFC 3118 [RFC3118] for DHCPv4 or
section 20 of RFC 3315 [RFC3315]) can be used to ensure that the
contents of this option are not altered in transit between the DHCP
server and client.
7. IANA Considerations
When this document is published, IANA is asked to assign an option
tag from the "BOOTP Vendor Extensions and DHCP Options" registry for
OPTION_CONTAINER_V4.
When this document is published, IANA is asked to assign an option
code from the "DHCPv6 Option Codes" registry for OPTION_CONTAINER_V6.
8. Change Log
If this document is accepted for publication as an RFC, this change
log is to be removed before publication.
o Corrected a cut-and-paste error in section "DHCPv6 Container
option": The Time Protocol Servers option -> The DHCPv4 Container
option
o Added text to section "RG Server Behavior" to address policy
management concerns
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
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and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC 3118, June 2001.
Author's Address
Ralph Droms
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
USA
Phone: +1 978.936.1674
Email: rdroms@cisco.com
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