DHCPv6 PD Exclude: Prefix Exclude Option for DHCPv6-based Prefix Delegation
Lets a DHCPv6 server carve one prefix out of a delegated block, such as the one already used on the link.
Korhonen, J., Ed., Savolainen, T., Krishnan, S., and O. Troan, "Prefix Exclude Option for DHCPv6-based Prefix Delegation", RFC 6603, DOI 10.17487/RFC6603, May 2012, <https://www.rfc-editor.org/info/rfc6603>.
@misc{rfc6603,
series = {Request for Comments},
number = 6603,
howpublished = {RFC 6603},
publisher = {RFC Editor},
doi = {10.17487/RFC6603},
url = {https://www.rfc-editor.org/info/rfc6603},
author = {J. Korhonen and T. Savolainen and S. Krishnan and O. Troan},
title = {{Prefix Exclude Option for DHCPv6-based Prefix Delegation}},
pagetotal = 10,
year = 2012,
month = may,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. Korhonen, Ed.
Request for Comments: 6603 Nokia Siemens Networks
Updates: 3633 T. Savolainen
Category: Standards Track Nokia
ISSN: 2070-1721 S. Krishnan
Ericsson
O. Troan
Cisco Systems, Inc
May 2012
Prefix Exclude Option for DHCPv6-based Prefix Delegation
Abstract
This specification defines an optional mechanism to allow exclusion
of one specific prefix from a delegated prefix set when using
DHCPv6-based prefix delegation. The new mechanism updates RFC 3633.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6603.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Requirements and Terminology ....................................2
3. Problem Background ..............................................3
4. Solution ........................................................3
4.1. Prefix Delegation with Excluded Prefixes ...................3
4.2. Prefix Exclude Option ......................................4
5. Delegating Router Solicitation ..................................6
5.1. Requesting Router ..........................................6
5.2. Delegating Router ..........................................6
6. Requesting Router Initiated Prefix Delegation ...................7
6.1. Requesting Router ..........................................7
6.2. Delegating Router ..........................................8
7. Security Considerations .........................................8
8. IANA Considerations .............................................8
9. Acknowledgements ................................................8
10. References .....................................................9
10.1. Normative References ......................................9
10.2. Informative References ....................................9
1. Introduction
This specification defines an optional mechanism and the related
DHCPv6 option to allow exclusion of one specific prefix from a
delegated prefix set when using DHCPv6-based prefix delegation.
The prefix exclusion mechanism is targeted at deployments where
DHCPv6-based prefix delegation is used, but a single aggregated
route/prefix has to represent one customer, instead of using one
prefix for the link between the delegating router and the requesting
router and another prefix for the customer network. The mechanism
defined in this specification allows a delegating router to use a
prefix out of the delegated prefix set on the link through which it
exchanges DHCPv6 messages with the requesting router, and is intended
for use in networks where each requesting router is on its own
layer-2 domain.
2. Requirements and 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].
3. Problem Background
DHCPv6 Prefix Delegation (DHCPv6-PD) [RFC3633] has an explicit
limitation described in Section 12.1 of [RFC3633] that a prefix
delegated to a requesting router cannot be used by the delegating
router. This restriction implies that the delegating router will
have two (non-aggregatable) routes towards a customer: one for the
link between the requesting router and the delegating router, and one
for the customer site behind the requesting router.
There are architectures and link models where a host (e.g., a mobile
router, also acting as a requesting router) always has a single (/64)
prefix configured on its uplink interface and the delegating router
is also the requesting router's first-hop router. Furthermore, it
may be required that the prefix configured on the uplink interface
has to be aggregatable with the delegated prefixes. This introduces
a problem in how to use DHCPv6-PD together with stateless [RFC4862]
or stateful [RFC3315] address autoconfiguration on a link where the
/64 advertised is also part of the prefix delegated (e.g., /56) to
the requesting router.
4. Solution
4.1. Prefix Delegation with Excluded Prefixes
This specification defines a new DHCPv6 option, OPTION_PD_EXCLUDE
(67), that is used to exclude exactly one prefix from a delegated
prefix. The OPTION_PD_EXCLUDE is included in the OPTION_IAPREFIX
IAprefix-options field. There can be at most one OPTION_PD_EXCLUDE
option in one OPTION_IAPREFIX option. The OPTION_PD_EXCLUDE option
allows prefix delegation where a requesting router is delegated a
prefix (e.g., /56) and the delegating router uses one prefix (e.g.,
/64) on the link through which it exchanges DHCPv6 messages with the
requesting router with a prefix out of the same delegated prefix set.
A requesting router includes an OPTION_ORO option with the
OPTION_PD_EXCLUDE option code in a Solicit, Request, Renew, or Rebind
message to inform the delegating router about the support for the
prefix delegation functionality defined in this specification. A
delegating router may include the OPTION_PD_EXCLUDE option code in an
OPTION_ORO option in a Reconfigure message to indicate that the
requesting router should request OPTION_PD_EXCLUDE from the
delegating router.
The delegating router includes the prefix in the OPTION_PD_EXCLUDE
option that is excluded from the delegated prefix set. The
requesting router MUST NOT assign the excluded prefix to any of its
downstream interfaces.
4.2. Prefix Exclude 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_PD_EXCLUDE | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| prefix-len | IPv6 subnet ID (1 to 16 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prefix Exclude Option
o option-code: OPTION_PD_EXCLUDE (67).
o option-len: 1 + length of IPv6 subnet ID in octets. A valid
option-len is between 2 and 17.
o prefix-len: The length of the excluded prefix in bits. The
prefix-len MUST be between 'OPTION_IAPREFIX prefix-length'+1 and
128.
o IPv6 subnet ID: A variable-length IPv6 subnet ID up to 128 bits.
The IPv6 subnet ID contains prefix-len minus 'OPTION_IAPREFIX prefix-
length' bits extracted from the excluded prefix starting from the bit
position 'OPTION_IAPREFIX prefix-length'. The extracted subnet ID
MUST be left-shifted to start from a full octet boundary, i.e., left-
shift of 'OPTION_IAPREFIX prefix-length' mod 8 bits. The subnet ID
MUST be zero-padded to the next full octet boundary.
The encoding of the IPv6 subnet ID can be expressed in a C-like
pseudocode as shown below:
uint128_t p1; // the delegated IPv6 prefix
uint128_t p2; // the excluded IPv6 prefix
uint16_t a; // the OPTION_IAPREFIX prefix-length
uint8_t b; // the excluded IPv6 prefix length
uint8_t s;
// sanity checks
s = 128-a; // size of non-prefix bits
assert(b>a); // b must be at least a+1
assert(p1>>s == p2>>s); // p1 and p2 must share a common
// prefix of 'a' bits
// calculate the option content
uint16_t c = b-a-1; // the IPv6_subnet_ID_length-1 in bits
uint16_t d = (c/8)+1; // the IPv6_subnet_ID_length in octets
uint128_t p = p2<<a; // p is the IPv6 subnet ID that has the
// common p1 prefix left-shifted out to
// a full octet boundary (trailing bits
// are zeroed)
// populate the option
uint8_t* id = &OPTION_PD_EXCLUDE.IPv6_subnet_ID;
OPTION_PD_EXCLUDE.option_len = d+1;
OPTION_PD_EXCLUDE.prefix_len = b;
while (d-- > 0) {
*id++ = p>>120;
p <<= 8;
}
The OPTION_PD_EXCLUDE option MUST only be included in the
OPTION_IAPREFIX IAprefix-options [RFC3633] field.
Any prefix excluded from the delegated prefix MUST be contained in
OPTION_PD_EXCLUDE options within the corresponding OPTION_IAPREFIX.
The prefix included in the OPTION_PD_EXCLUDE option shares the same
preferred-lifetime and valid-lifetime as the delegated prefix in the
encapsulating OPTION_IAPREFIX option.
The prefix in the OPTION_PD_EXCLUDE option MUST be part of the
delegated prefix in the OPTION_IAPREFIX. For example, the requesting
router has earlier been assigned a 2001:db8:dead:beef::/64 prefix by
the delegating router, and the delegated prefix in the
OPTION_IAPREFIX is 2001:db8:dead:bee0::/59. In this case, 2001:db8:
dead:beef::/64 is a valid prefix to be used in the OPTION_PD_EXCLUDE
option. The OPTION_PD_EXCLUDE option would be encoded 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_PD_EXCLUDE | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 64 |0|1|1|1|1|0|0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^ ^
| |
| +- 3 zero-padded bits follow
|
+- using C syntax: 0xef << (59 % 8)
Note: 59 mod 8 = 3
5. Delegating Router Solicitation
The requesting router locates and selects a delegating router in the
same way as described in Section 11 [RFC3633]. This specification
only describes the additional steps required by the use of the
OPTION_PD_EXCLUDE option.
5.1. Requesting Router
If the requesting router implements the solution described in Section
4.1, then the requesting router SHOULD include the OPTION_PD_EXCLUDE
option code in the OPTION_ORO option in Solicit messages.
Once receiving Advertise message(s), the requesting router uses the
prefix(es) received in OPTION_PD_EXCLUDE, in addition to the
advertised prefixes, to choose the delegating router. The requesting
router MUST proceed to the Prefix Delegation procedure described in
Section 6.1. If the Advertise message did not include the
OPTION_PD_EXCLUDE option, then the requesting router MUST fall back
to normal behavior, as described in Section 11.1 of [RFC3633].
5.2. Delegating Router
If the OPTION_ORO option in the Solicit message includes the
OPTION_PD_EXCLUDE option code, then the delegating router knows that
the requesting router supports the solution defined in this
specification. If the Solicit message also contains an IA_PD option,
the delegating router can delegate to the requesting router a prefix
that includes the prefix already assigned to the requesting router's
uplink interface. The delegating router includes the prefix
originally, or to be, assigned to the requesting router in the
OPTION_PD_EXCLUDE option within the OPTION_IAPREFIX IAprefix-option
in the Advertise message.
If the OPTION_ORO option in the Solicit message does not include the
OPTION_PD_EXCLUDE option code, then the delegating router MUST fall
back to normal behavior, as described in Section 11.2 of [RFC3633].
If the OPTION_ORO option in the Solicit message includes the
OPTION_PD_EXCLUDE option code but the delegating router does not
support the solution described in this specification, then the
delegating router acts as specified in [RFC3633].
6. Requesting Router-Initiated Prefix Delegation
The procedures described in the following sections are aligned with
Section 12 of [RFC3633]. In this specification, we only describe the
additional steps required by the use of the OPTION_PD_EXCLUDE option.
6.1. Requesting Router
The requesting router behavior regarding the use of the
OPTION_PD_EXCLUDE option is mostly identical to the steps described
in Section 5.1, with the difference being the use of a DHCPv6 Request
instead of an Solicit message. The requesting router SHOULD include
the OPTION_PD_EXCLUDE option code in the OPTION_ORO option for DHCPv6
messages as described in Section 22.7 of [RFC3315]. The requesting
router SHOULD include the OPTION_PD_EXCLUDE option code in the
OPTION_ORO option for DHCPv6 messages as described in Section 22.7 of
[RFC3315].
The requesting router uses a Release message to return the delegated
prefix(es) to a delegating router. The prefix(es) to be released
MUST be included in the IA_PDs along with the excluded prefix
included in the OPTION_PD_EXCLUDE option. The requesting router MUST
NOT use the OPTION_PD_EXCLUDE option to introduce an additional
excluded prefix in the Release message for which it originally got a
valid binding.
The requesting router must create sink routes for the delegated
prefixes, minus the excluded prefixes. This may be done by creating
sink routes for delegated prefixes and more specific routes for the
excluded prefixes.
6.2. Delegating Router
The delegating router behavior regarding the use of the
OPTION_PD_EXCLUDE option is more or less identical to the step
described in Section 5.2. The only difference is the DHCPv6 messages
used to carry the OPTION_PD_EXCLUDE option.
The delegating router may mark any prefix(es) in the IA_PD Prefix
options in a Release message from a requesting router as 'available',
excluding the prefix included in the OPTION_PD_EXCLUDE options. If
the Release message contains a 'new' excluded prefix in any
OPTION_PD_EXCLUDE option, the delegating router MUST send a Reply
message with the Status Code set to NoBinding for that IA_PD option.
7. Security Considerations
Security considerations for DHCPv6 are described in Section 23 of
[RFC3315]. For DHCPv6 Prefix Delegation, they are described in
Section 15 of [RFC3633]. In particular, RFC 3633 provides
recommendations for protection against prefix delegation attacks.
This specification does not add any new security considerations
beyond those provided by RFC 3633.
8. IANA Considerations
A new DHCPv6 Option Code has been reserved from the "Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)" registry for DHCP Option
Codes.
OPTION_PD_EXCLUDE (67)
9. Acknowledgements
The authors would like to thank Ralph Droms, Frank Brockners, Ted
Lemon, Julien Laganier, Fredrik Garneij, Sri Gundavelli, Mikael
Abrahamsson, Behcet Sarikaya, Jyrki Soini, Deng Hui, Stephen Jacob,
Hemant Singh, Gaurav Halwasia, Lorenzo Colitti, and Tomasz Mrugalski
for their valuable comments and discussions.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
10.2. Informative References
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
Authors' Addresses
Jouni Korhonen (editor)
Nokia Siemens Networks
Linnoitustie 6
FI-02600 Espoo
Finland
EMail: jouni.nospam@gmail.com
Teemu Savolainen
Nokia
Hermiankatu 12 D
FI-33720 Tampere
Finland
EMail: teemu.savolainen@nokia.com
Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada
EMail: suresh.krishnan@ericsson.com
Ole Troan
Cisco Systems, Inc
Oslo
Norway
EMail: ot@cisco.com
AI ELI5
On this page
§1Introduction
RFC 6603 defines an optional mechanism and a related DHCPv6 option for excluding one specific prefix from a delegated prefix set during DHCPv6-based prefix delegation. It updates RFC 3633.
It targets deployments where a single aggregated route or prefix has to represent one customer, rather than one prefix for the link between the delegating and requesting routers and another for the customer network. It is intended for networks where each requesting router is on its own layer-2 domain.
Normally a router that gives away a block of addresses cannot use any of them itself. This note says it can keep one piece back for itself.
§2The Problem With Plain Delegation
Section 12.1 of RFC 3633 limits DHCPv6 prefix delegation: a prefix delegated to a requesting router cannot be used by the delegating router. The delegating router therefore ends up with two non-aggregatable routes toward a customer, one for the link to the requesting router and one for the customer site behind it.
Some architectures make this awkward. A host, for example a mobile router that is also a requesting router, may always have a single /64 prefix on its uplink, with the delegating router as its first-hop router. That /64 may need to be aggregatable with the delegated prefixes, for example a /56. The difficulty is combining prefix delegation with stateless (RFC 4862) or stateful (RFC 3315) address autoconfiguration when the advertised /64 is also part of the delegated prefix.
It is like a landlord who gives you a whole floor but also needs the hallway on it. Without a way to say so, the landlord needs a separate address for the hallway and the floor.
§3The Exclude Option Idea
The new option, OPTION_PD_EXCLUDE (option code 67), excludes exactly one prefix from a delegated prefix. It is carried in the IAprefix-options field of OPTION_IAPREFIX, and there can be at most one such option in each OPTION_IAPREFIX.
With it, a requesting router can be delegated a prefix such as a /56, while the delegating router uses one prefix such as a /64 from that same set on the link where they exchange DHCPv6 messages. The delegating router puts the excluded prefix in the option, and the requesting router MUST NOT assign it to any of its downstream interfaces.
To signal support, a requesting router includes the option code in an OPTION_ORO option in a Solicit, Request, Renew or Rebind message. A delegating router may include the code in an OPTION_ORO in a Reconfigure message to indicate that the requesting router should request the option.
The router that gives out the block writes a note saying which piece is off limits. The router that receives the block promises not to hand that piece out to anyone behind it.
§4The Prefix Exclude Option Format
The option has four parts: the option code (OPTION_PD_EXCLUDE, 67), option-len, prefix-len and a variable-length IPv6 subnet ID of 1 to 16 octets. The option-len is 1 plus the subnet ID length in octets, so a valid value is between 2 and 17.
prefix-len is the length of the excluded prefix in bits. It MUST be between the delegated prefix's OPTION_IAPREFIX prefix-length plus 1 and 128.
The subnet ID holds the bits of the excluded prefix that come after the delegated prefix length, that is, prefix-len minus the OPTION_IAPREFIX prefix-length bits, starting at the bit position equal to that prefix-length. These bits MUST be left-shifted to start at a full octet boundary (a shift of the prefix-length mod 8 bits) and MUST be zero-padded to the next full octet boundary. The RFC gives C-like pseudocode for this encoding.
The excluded prefix MUST be part of the delegated prefix, and it shares the preferred-lifetime and valid-lifetime of the encapsulating OPTION_IAPREFIX. The option MUST only appear in the IAprefix-options field, and any prefix excluded from the delegated prefix MUST be carried in OPTION_PD_EXCLUDE options within that OPTION_IAPREFIX.
The worked example excludes 2001:db8:dead:beef::/64 from the delegated prefix 2001:db8:dead:bee0::/59. The option-len is 2, the prefix-len is 64, and the subnet ID octet carries the bits 01111000, which is 0xef shifted left by 59 mod 8 = 3 bits, with 3 zero-padded bits.
The note says how long the reserved piece is and then spells out only the part of its address that differs from the big block, squeezed into whole bytes.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
OPTION-CODE | OPTION_PD_EXCLUDE | 2 bytes | 67 | Option code 67. |
OPTION-LEN | option-len | 2 bytes | 1 plus the subnet ID length in octets; valid values are 2 to 17. | |
PREFIX-LEN | prefix-len | 1 byte | Length of the excluded prefix in bits. | |
SUBNET-ID | IPv6 subnet ID | 3 bytes | 1 to 16 octets; varies in length. |
§5How Routers Use the Option
The requesting router finds a delegating router as in Section 11 of RFC 3633. If it implements this specification it SHOULD include the option code in the OPTION_ORO of its Solicit messages. When Advertise messages arrive, it uses the prefixes received in OPTION_PD_EXCLUDE, in addition to the advertised prefixes, to choose a delegating router. If an Advertise has no such option, the requesting router MUST fall back to normal behavior from Section 11.1 of RFC 3633.
On the delegating side, an OPTION_ORO with the option code tells the server that the client supports this feature. If the Solicit also holds an IA_PD option, the delegating router can delegate a prefix that includes the prefix already assigned to the requesting router's uplink, and it puts that prefix in OPTION_PD_EXCLUDE in the Advertise. If the code is absent, the delegating router MUST fall back to normal behavior (Section 11.2 of RFC 3633). A server that does not support the feature acts as specified in RFC 3633.
The requesting-router-initiated exchanges work the same way, using a Request instead of a Solicit. To return prefixes in a Release message, the requesting router MUST include the delegated prefixes in the IA_PDs along with the excluded prefix, and MUST NOT use the option to introduce a new excluded prefix for which it did not originally get a valid binding. If a Release contains such a new excluded prefix, the delegating router MUST reply with the Status Code NoBinding for that IA_PD option.
The requesting router must also create sink routes for the delegated prefixes minus the excluded prefixes, for example a sink route for each delegated prefix plus more specific routes for the excluded ones.
The receiving router says it understands the trick, and the giving router then marks the reserved piece. If either side does not know the trick, they just do things the old way.
§6Security and IANA Notes
The RFC says the option adds no new security considerations beyond those in RFC 3633. DHCPv6 security is covered in Section 23 of RFC 3315, and RFC 3633 gives recommendations for protection against prefix delegation attacks.
For IANA, the DHCPv6 option code OPTION_PD_EXCLUDE (67) was reserved from the DHCPv6 option codes registry.
Adding the reserved piece does not open any new doors for troublemakers. The same safety advice as before still applies.