RADIUS for 6rd: RADIUS Attribute for IPv6 Rapid Deployment on IPv4 Infrastructures (6rd)
A RADIUS attribute that lets an AAA server hand 6rd settings to the gateway that configures home routers.
Guo, D., Jiang, S., Ed., Despres, R., and R. Maglione, "RADIUS Attribute for IPv6 Rapid Deployment on IPv4 Infrastructures (6rd)", RFC 6930, DOI 10.17487/RFC6930, April 2013, <https://www.rfc-editor.org/info/rfc6930>.
@misc{rfc6930,
series = {Request for Comments},
number = 6930,
howpublished = {RFC 6930},
publisher = {RFC Editor},
doi = {10.17487/RFC6930},
url = {https://www.rfc-editor.org/info/rfc6930},
author = {D. Guo and S. Jiang and R. Despres and R. Maglione},
title = {{RADIUS Attribute for IPv6 Rapid Deployment on IPv4 Infrastructures (6rd)}},
pagetotal = 12,
year = 2013,
month = apr,
} RFC text
Contents
Internet Engineering Task Force (IETF) D. Guo
Request for Comments: 6930 S. Jiang, Ed.
Category: Standards Track Huawei Technologies Co., Ltd
ISSN: 2070-1721 R. Despres
RD-IPtech
R. Maglione
Cisco Systems
April 2013
RADIUS Attribute for IPv6 Rapid Deployment
on IPv4 Infrastructures (6rd)
Abstract
The IPv6 Rapid Deployment on IPv4 Infrastructures (6rd) provides both
IPv4 and IPv6 connectivity services simultaneously during the
IPv4/IPv6 coexistence period. The Dynamic Host Configuration
Protocol (DHCP) 6rd option has been defined to configure the 6rd
Customer Edge (CE). However, in many networks, the configuration
information may be stored in the Authentication Authorization and
Accounting (AAA) servers, while user configuration is mainly acquired
from a Broadband Network Gateway (BNG) through the DHCP protocol.
This document defines a Remote Authentication Dial-In User Service
(RADIUS) attribute that carries 6rd configuration information from
the AAA server to BNGs.
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/rfc6930.
Copyright Notice
Copyright (c) 2013 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 ....................................................3
2. Terminology .....................................................3
3. IPv6 6rd Configuration with RADIUS ..............................4
4. Attributes ......................................................6
4.1. IPv6-6rd-Configuration Attribute ...........................6
4.2. Table of Attributes ........................................9
5. Diameter Considerations .........................................9
6. Security Considerations .........................................9
7. IANA Considerations ............................................10
8. Acknowledgments ................................................10
9. References .....................................................10
9.1. Normative References ......................................10
9.2. Informative References ....................................11
1. Introduction
Recently, providers have started to deploy IPv6 and to consider
transition to IPv6. The IPv6 Rapid Deployment (6rd) [RFC5969]
provides both IPv4 and IPv6 connectivity services simultaneously
during the IPv4/IPv6 coexistence period. 6rd is used to provide IPv6
connectivity service through legacy IPv4-only infrastructure. 6rd
uses the Dynamic Host Configuration Protocol (DHCP) [RFC2131], and
the 6rd Customer Edge (CE) uses the DHCP 6rd option [RFC5969] to
discover a 6rd Border Relay and to configure an IPv6 prefix and
address.
In many networks, user-configuration information is managed by
Authentication, Authorization, and Accounting (AAA) servers. The
Remote Authentication Dial-In User Service (RADIUS) protocol
[RFC2865] is usually used by AAA servers to communicate with network
elements. In a fixed-line broadband network, the Broadband Network
Gateways (BNGs) act as the access gateway for users. The BNGs are
assumed to embed a DHCP server function that allows them to handle
locally any DHCP requests issued by hosts.
Since the 6rd configuration information is stored in AAA servers, and
user configuration is mainly through DHCP between BNGs and hosts/CEs,
new RADIUS attributes are needed to propagate the information from
AAA servers to BNGs.
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].
The terms 6rd Customer Edge (6rd CE) and 6rd Border Relay (BR) are
defined in [RFC5969]. "MAC" stands for Media Access Control.
3. IPv6 6rd Configuration with RADIUS
Figure 1 illustrates how DHCP and the RADIUS protocol cooperate to
provide 6rd CE with 6rd configuration information.
6rd CE BNG AAA Server
| | |
|-------DHCPDISCOVER------>| |
|(Parameter Request w/ 6rd option) |
| |--Access-Request(6rd Attr)-->|
| | |
| |<--Access-Accept(6rd Attr)---|
|<-------DHCPOFFER---------| |
| (6rd option) | |
| | |
DHCP RADIUS
Figure 1: The Cooperation between DHCP and RADIUS
When Combined with RADIUS Authentication
The BNG acts as a client of RADIUS and as a DHCP server. First, the
6rd CE MAY initiate a DHCPDISCOVER message that includes a Parameter
Request option (55) [RFC2132] with the 6rd option [RFC5969]. When
the BNG receives the DHCPDISCOVER, it SHOULD initiate a RADIUS
Access- Request message to the RADIUS server. In that message,
- the User-Name attribute (1) SHOULD be filled by the 6rd CE MAC
address, and
- the User-Password attribute (2) SHOULD be filled by the shared 6rd
password that has been preconfigured on the DHCP server.
The BNG requests authentication, as defined in [RFC2865], with the
IPv6-6rd-Configuration attribute (Section 4.1) in the desired
attribute list. If the authentication request is approved by the AAA
server, an Access-Accept message MUST be acknowledged with the
IPv6-6rd-Configuration attribute. Then, the BNG SHOULD respond to
the 6rd CE with a DHCPOFFER message, which contains a DHCP 6rd
option. The recommended format of the MAC address is as defined in
Calling-Station-Id ([RFC3580], Section 3.20) without the SSID
(Service Set Identifier) portion.
Figure 2 describes another scenario -- later re-authorization -- in
which the authorization operation is not coupled with authentication.
Authorization relevant to 6rd is done independently after the
authentication process.
6rd CE BNG AAA Server
| | |
|--------DHCPREQUEST------>| |
|(Parameter Request w/ 6rd option) |
| |--Access-Request(6rd Attr)-->|
| | |
| |<--Access-Accept(6rd Attr)---|
| | |
|<---------DHCPACK---------| |
| (6rd option) | |
| | |
DHCP RADIUS
Figure 2: The Cooperation between DHCP and RADIUS
When Decoupled from RADIUS Authentication
In this scenario, the Access-Request packet SHOULD contain a Service-
Type attribute (6) with the value Authorize Only (17); thus,
according to [RFC5080], the Access-Request packet MUST contain a
State attribute that it obtains from the previous authentication
process.
In both above-mentioned scenarios, Message-Authenticator (type 80)
[RFC2865] SHOULD be used to protect both Access-Request and Access-
Accept messages.
After receiving the IPv6-6rd-Configuration attribute in the initial
Access-Accept, the BNG SHOULD store the received 6rd configuration
parameters locally. When the 6rd CE sends a DHCP Request message to
request an extension of the lifetime for the assigned address, the
BNG does not have to initiate a new Access-Request towards the AAA
server to request the 6rd configuration parameters. The BNG could
retrieve the previously stored 6rd configuration parameters and use
them in its reply.
If the BNG does not receive the IPv6-6rd-Configuration attribute in
the Access-Accept, it MAY fall back to a preconfigured default 6rd
configuration, if any. If the BNG does not have any preconfigured
default 6rd configuration or if the BNG receives an Access-Reject,
the tunnel cannot be established.
As specified in [RFC2131], Section 4.4.5 ("Reacquisition and
expiration"), if the DHCP server to which the DHCP Request message
was sent at time T1 has not responded by time T2 (typically
0.375*duration_of_lease after T1), the 6rd CE (the DHCP client)
SHOULD enter the REBINDING state and attempt to contact any server.
In this situation, the secondary BNG receiving the new DHCP message
MUST initiate a new Access-Request towards the AAA server. The
secondary BNG MAY include the IPv6-6rd-Configuration attribute in its
Access-Request.
4. Attributes
This section defines the IPv6-6rd-Configuration attribute that is
used in both above-mentioned scenarios. The attribute design follows
[RFC6158] and refers to [RFC6929].
4.1. IPv6-6rd-Configuration Attribute
The specification requires that multiple IPv4 addresses are
associated with one IPv6 prefix. Given that RADIUS currently has no
recommended way of grouping multiple attributes, the design below
appears to be a reasonable compromise. The IPv6-6rd-Configuration
attribute is structured 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | SubType1 | SubLen1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4MaskLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SubType2 | SubLen2 | Reserved | 6rdPrefixLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ 6rdPrefix +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SubType3 | SubLen3 | 6rdBRIPv4Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6rdBRIPv4Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
173
Length
28 + n*6 (the length of the entire attribute in octets, where n
is the number of BR IPv4 addresses and minimum n is 1)
SubType1
1 (SubType number, for the IPv4 Mask Length suboption)
SubLen1
6 (the length of the IPv4 Mask Length suboption)
IPv4MaskLen
The number of high-order bits that are identical across all CE
IPv4 addresses within a given 6rd domain. This may be any
value between 0 and 32. Any value greater than 32 is invalid.
Since [RFC6158], Appendix A.2.1, has forbidden 8-bit fields, a
32-bit field is used here.
SubType2
2 (SubType number for the 6rd prefix suboption)
SubLen2
20 (the length of the 6rd prefix suboption)
Reserved
Set to all 0 for now. Reserved for future use. To be
compatible with other IPv6 prefix attributes in the RADIUS
protocol, the bits MUST be set to zero by the sender and MUST
be ignored by the receiver.
6rdPrefixLen
The IPv6 Prefix length of the Service Provider's 6rd IPv6
prefix in number of bits. The 6rdPrefixLen MUST be less than
or equal to 128.
6rdPrefix
The Service Provider's 6rd IPv6 prefix represented as a
16-octet IPv6 address. The bits after the 6rdPrefixlen number
of bits in the prefix SHOULD be set to zero.
SubType3
3 (SubType number, for the 6rd Border Relay IPv4 address
suboption)
SubLen3
6 (the length of the 6rd Border Relay IPv4 address suboption)
6rdBRIPv4Address
One or more IPv4 addresses of the 6rd Border Relay(s) for a
given 6rd domain. The maximum RADIUS attribute length of 255
octets results in a limit of 37 IPv4 addresses.
Since the subtypes have values, they can appear in any order. If
multiple 6rdBRIPv4Address (subtype 3) appear, they are RECOMMENDED to
be placed together.
The IPv6-6rd-Configuration attribute is normally used in Access-
Accept messages. It MAY be used in Access-Request packets as a hint
to the RADIUS server; for example, if the BNG is preconfigured with a
default 6rd configuration, these parameters MAY be inserted in the
attribute. The RADIUS server MAY ignore the hint sent by the BNG,
and it MAY assign different 6rd parameters.
If the BNG includes the IPv6-6rd-Configuration attribute, but the AAA
server does not recognize it, this attribute MUST be ignored by the
AAA server.
If the BNG does not receive the IPv6-6rd-Configuration attribute in
the Access-Accept, it MAY fallback to a preconfigured default 6rd
configuration, if any. If the BNG does not have any preconfigured
default 6rd configuration, the 6rd tunnel cannot be established.
If the BNG is pre-provisioned with a default 6rd configuration and
the 6rd configuration received in Access-Accept is different from the
configured default, then the 6rd configuration received in the
Access-Accept message MUST be used for the session.
If the BNG cannot support the received 6rd configuration for any
reason, the tunnel SHOULD NOT be established.
4.2. Table of Attributes
The following table adds to the one in [RFC2865], Section 5.44,
providing a guide to the quantity of IPv6-6rd-Configuration
attributes that may be found in each kind of packet.
Request Accept Reject Challenge Accounting # Attribute
Request
0-1 0-1 0 0 0-1 173 IPv6-6rd-
Configuration
0-1 0-1 0 0 0-1 1 User-Name
0-1 0 0 0 0-1 2 User-Password
0-1 0-1 0 0 0-1 6 Service-Type
0-1 0-1 0-1 0-1 0-1 80 Message-Authenticator
The following key defines the meanings of the above table entries.
0 This attribute MUST NOT be present in packet.
0+ Zero or more instances of this attribute MAY be present in
packet.
0-1 Zero or one instance of this attribute MAY be present in
packet.
1 Exactly one instance of this attribute MUST be present in
packet.
5. Diameter Considerations
This attribute is usable within either RADIUS or Diameter [RFC6733].
Since the attribute defined in this document has been allocated from
the standard RADIUS type space, no special handling is required by
Diameter entities.
6. Security Considerations
In 6rd scenarios, both CE and BNG are within a provider network,
which can be considered as a closed network and a lower-threat
environment. A similar consideration can be applied to the RADIUS
message exchange between the BNG and the AAA server.
In 6rd scenarios, the RADIUS protocol is run over IPv4. Known
security vulnerabilities of the RADIUS protocol are discussed in
[RFC2607], [RFC2865], and [RFC2869]. Use of IPsec [RFC4301] for
providing security when RADIUS is carried in IPv6 is discussed in
[RFC3162].
To get unauthorized 6rd configuration information, a malicious user
may use MAC address spoofing and/or a dictionary attack on the shared
6rd password that has been preconfigured on the DHCP server. The
relevant security issues have been considered in Section 12 of
[RFC5969].
Security issues that may arise specifically between the 6rd CE and
BNG are discussed in [RFC5969]. Furthermore, generic DHCP security
mechanisms can be applied to DHCP intercommunication between 6rd CE
and BNG.
Security considerations for the Diameter protocol are discussed in
[RFC6733].
7. IANA Considerations
Per this document, IANA has assigned one new RADIUS Attribute Type in
the "Radius Types" registry (currently located at
http://www.iana.org/assignments/radius-types) for the following
attribute:
IPv6-6rd-Configuration (173)
8. Acknowledgments
The authors would like to thank Alan DeKok, Yong Cui, Leaf Yeh, Sean
Turner, Joseph Salowey, Glen Zorn, Dave Nelson, Bernard Aboba, Benoit
Claise, Barry Lieba, Stephen Farrell, Adrian Farrel, Ralph Droms, and
other members of the SOFTWIRE WG, RADEXT WG, AAA Doctors, and
Security Directorate for valuable comments.
9. References
9.1. 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.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC
2865, June 2000.
[RFC3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6", RFC
3162, August 2001.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5080] Nelson, D. and A. DeKok, "Common Remote Authentication
Dial In User Service (RADIUS) Implementation Issues and
Suggested Fixes", RFC 5080, December 2007.
[RFC5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) -- Protocol Specification", RFC
5969, August 2010.
[RFC6158] DeKok, A., Ed., and G. Weber, "RADIUS Design Guidelines",
BCP 158, RFC 6158, March 2011.
[RFC6733] Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
Ed., "Diameter Base Protocol", RFC 6733, October 2012.
9.2. Informative References
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[RFC2869] Rigney, C., Willats, W., and P. Calhoun, "RADIUS
Extensions", RFC 2869, June 2000.
[RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G., and J. Roese,
"IEEE 802.1X Remote Authentication Dial In User Service
(RADIUS) Usage Guidelines", RFC 3580, September 2003.
[RFC6929] DeKok, A. and A. Lior, "Remote Authentication Dial-In User
Service (RADIUS) Protocol Extensions", RFC 6929, April
2013.
Authors' Addresses
Dayong Guo
Huawei Technologies Co., Ltd
Q14 Huawei Campus, 156 BeiQi Road,
ZhongGuan Cun, Hai-Dian District, Beijing 100095
P.R. China
EMail: guoseu@huawei.com
Sheng Jiang (Editor)
Huawei Technologies Co., Ltd
Q14 Huawei Campus, 156 BeiQi Road,
ZhongGuan Cun, Hai-Dian District, Beijing 100095
P.R. China
EMail: jiangsheng@huawei.com
Remi Despres
RD-IPtech
3 rue du President Wilson
Levallois
France
EMail: despres.remi@laposte.net
Roberta Maglione
Cisco Systems
181 Bay Street
Toronto, ON
M5J 2T3
Canada
EMail: robmgl@cisco.com
AI ELI5
On this page
§1Why 6rd Needs a RADIUS Attribute
6rd (IPv6 Rapid Deployment, defined in RFC 5969) provides IPv6 service through legacy IPv4-only infrastructure, while IPv4 and IPv6 run side by side. A 6rd Customer Edge (CE) device uses the DHCP 6rd option to discover a 6rd Border Relay (BR) and to configure an IPv6 prefix and address.
In many networks, user configuration is kept on Authentication, Authorization, and Accounting (AAA) servers, which talk to network elements using RADIUS (RFC 2865). In fixed-line broadband, the Broadband Network Gateway (BNG) is the access gateway and is assumed to embed a DHCP server. The 6rd settings sit in the AAA server, while the customer asks over DHCP, so a new RADIUS attribute is needed to carry the information from the AAA server to the BNG.
The settings live in one place, but the router asks a different helper. This document gives the two helpers a way to pass the settings along.
§3How DHCP and RADIUS Cooperate
The BNG acts as a RADIUS client and as a DHCP server. The 6rd CE MAY send a DHCPDISCOVER whose Parameter Request option (55) asks for the 6rd option. On receiving it, the BNG SHOULD send a RADIUS Access-Request. Its User-Name attribute (1) SHOULD hold the CE's MAC address, and its User-Password attribute (2) SHOULD hold the shared 6rd password preconfigured on the DHCP server. The recommended MAC format is that of Calling-Station-Id in RFC 3580, without the SSID part.
The BNG asks for the IPv6-6rd-Configuration attribute. If the AAA server approves, the Access-Accept MUST carry the attribute, and the BNG SHOULD answer the CE with a DHCPOFFER containing a DHCP 6rd option.
A second scenario decouples authorization from authentication. The CE sends a DHCPREQUEST, and the BNG's Access-Request SHOULD contain Service-Type (6) with the value Authorize Only (17). Under RFC 5080, that request MUST then contain a State attribute obtained from the earlier authentication. In both scenarios, Message-Authenticator (80) SHOULD protect Access-Request and Access-Accept.
The router asks the front desk for its settings. The front desk phones the account office, gets the answer, and passes it back to the router.
- DHCPDISCOVERParameter Request with the 6rd option.
- Access-Request (6rd Attr)
- Access-Accept (6rd Attr)
- DHCPOFFERCarries the 6rd option.
§3.1Storing, Fallback and Failover
After the first Access-Accept, the BNG SHOULD store the received 6rd parameters locally. When the CE later asks to extend the lifetime of its address, the BNG does not have to send a new Access-Request, and could reuse the stored values in its reply.
If the Access-Accept has no IPv6-6rd-Configuration attribute, the BNG MAY fall back to a preconfigured default 6rd configuration. With no default, or after an Access-Reject, the tunnel cannot be established. If a default exists but the Access-Accept carries a different configuration, the received configuration MUST be used for the session. If the BNG cannot support the received configuration for any reason, the tunnel SHOULD NOT be established.
Following RFC 2131, if the DHCP server contacted at time T1 has not responded by T2, the CE SHOULD enter the REBINDING state and contact any server. A secondary BNG receiving that message MUST send a fresh Access-Request to the AAA server, and MAY include the IPv6-6rd-Configuration attribute in it.
The front desk writes the settings down so it need not call again. If the office has no answer, it can use a default card, but settings from the office win over the default card.
§4.1The IPv6-6rd-Configuration Attribute
The attribute has Type 173 and a Length of 28 + n*6 octets, where n is the number of BR IPv4 addresses (at least 1). It is built from three suboptions, because RADIUS has no recommended way to group several attributes and the 6rd configuration needs several IPv4 addresses tied to one IPv6 prefix. The design follows RFC 6158 and refers to RFC 6929.
- SubType 1 (SubLen 6):
IPv4MaskLen, the number of high-order bits identical across all CE IPv4 addresses in a 6rd domain. It may be 0 to 32, and a larger value is invalid. It is a 32-bit field because RFC 6158 forbids 8-bit fields. - SubType 2 (SubLen 20): a Reserved field (all zero, MUST be zero when sent and MUST be ignored on receipt),
6rdPrefixLen(MUST be 128 or less) and6rdPrefix, a 16-octet IPv6 address whose bits beyond the prefix length SHOULD be zero. - SubType 3 (SubLen 6): one
6rdBRIPv4Address, repeated for each Border Relay. The 255-octet RADIUS limit allows 37 addresses.
Subtypes can appear in any order. Several subtype 3 entries are RECOMMENDED to sit together.
The form has three boxes: how much of the street address is shared, the provider's special address prefix, and a list of relay stations. The boxes can come in any order.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | 173 | Attribute type. |
LENGTH | Length | 1 byte | Whole attribute in octets: 28 + n*6, n being the number of BR addresses. | |
SUBTYPE1 | SubType1 | 1 byte | 1 | IPv4 Mask Length suboption. |
SUBLEN1 | SubLen1 | 1 byte | 6 | Length of that suboption. |
IPV4-MASK-LEN | IPv4MaskLen | 4 bytes | Count of high-order bits shared by all CE IPv4 addresses; 0 to 32. | |
SUBTYPE2 | SubType2 | 1 byte | 2 | 6rd prefix suboption. |
SUBLEN2 | SubLen2 | 1 byte | 20 | Length of that suboption. |
RESERVED | Reserved | 1 byte | All zero; ignored by the receiver. | |
PREFIX-LEN | 6rdPrefixLen | 1 byte | Prefix length in bits; 128 or less. | |
PREFIX | 6rdPrefix | 16 bytes | The provider's 6rd IPv6 prefix as a 16-octet address. | |
SUBTYPE3 | SubType3 | 1 byte | 3 | 6rd Border Relay IPv4 address suboption. |
SUBLEN3 | SubLen3 | 1 byte | 6 | Length of that suboption. |
BR-ADDR | 6rdBRIPv4Address | 4 bytes | A Border Relay IPv4 address; repeated for each relay. |
§4.2Where the Attribute May Appear
The attribute is normally used in Access-Accept messages. It MAY appear in an Access-Request as a hint, for example when the BNG has a default 6rd configuration. The RADIUS server MAY ignore the hint and assign different parameters. If the AAA server does not recognize the attribute, it MUST ignore it.
The RFC adds a table to the one in RFC 2865: zero or one instance of the attribute may be present in an Access-Request, Access-Accept or Accounting-Request, and none in an Access-Reject or Access-Challenge.
The attribute is usable in RADIUS or Diameter. Since it was allocated from the standard RADIUS type space, Diameter entities need no special handling (RFC 6733).
The settings normally come back in the answer. The front desk can also suggest some in its question, but the office does not have to agree.
§6Security Considerations
The RFC argues that the CE and BNG sit inside a provider network, a closed and lower-threat environment, and the same reasoning applies to the BNG-to-AAA exchange. RADIUS runs over IPv4 in 6rd scenarios, and its known weaknesses are discussed in RFC 2607, RFC 2865 and RFC 2869. IPsec for RADIUS over IPv6 is covered in RFC 3162.
A malicious user could try to obtain unauthorized 6rd configuration through MAC address spoofing or a dictionary attack on the shared 6rd password. Those issues are treated in Section 12 of RFC 5969, and generic DHCP security mechanisms can protect the CE-to-BNG leg.
Someone could pretend to be another customer's router or guess the shared password. The network is private, which lowers the risk.
§7IANA Assignment
IANA assigned one new RADIUS Attribute Type in the Radius Types registry: IPv6-6rd-Configuration, value 173. That is the only registry action in the document.
The attribute got its own official number, 173, so everyone uses the same one.