Softwire WG M. Boucadair
Internet-Draft Orange
Intended status: Standards Track I. Farrer
Expires: April 3, 2017 Deutsche Telekom
September 30, 2016
Unified IPv4-in-IPv6 Softwire CPE: A DHCPv6-based Prioritization
Mechanism
draft-ietf-softwire-unified-cpe-08
Abstract
In IPv6-only provider networks, transporting IPv4 packets
encapsulated in IPv6 is a common solution to the problem of IPv4
service continuity. A number of differing functional approaches have
been developed for this, each having their own specific
characteristics. As these approaches share a similar functional
architecture and use the same data plane mechanisms, this memo
specifies a DHCPv6 option whereby a single CPE can interwork with all
of the standardized and proposed approaches to providing encapsulated
IPv4 in IPv6 services by providing a prioritization mechanism.
Requirements Language
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 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 3, 2017.
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Copyright Notice
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document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. DHCPv6 S46 Priority Option . . . . . . . . . . . . . . . 4
1.4. DHCPv6 Client Behavior . . . . . . . . . . . . . . . . . 5
1.5. DHCPv6 Server Behavior . . . . . . . . . . . . . . . . . 6
2. Operator Deployment Considerations for Deploying Multiple
Sotfwire Mechanisms . . . . . . . . . . . . . . . . . . . . . 6
2.1. Client Address Planning . . . . . . . . . . . . . . . . . 6
2.2. Backwards Compatability with Existing Softwire Clients . 7
3. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4.1. S46 Mechanisms and their Identifying Option Codes . . . . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
IPv4 service continuity is one of the major technical challenges
which must be considered during IPv6 migration. Over the past few
years, a number of different approaches have been developed to assist
with this problem (e.g., [RFC6333], [RFC7596], or [RFC7597]). These
approaches, referred to as 'S46 mechanisms' in this document, exist
in order to meet the particular deployment, scaling, addressing and
other requirements of different service provider's networks.
A common feature shared between all of the differing modes is the
integration of softwire tunnel end-point functionality into the
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Customer Premise Equipment (CPE) router. Due to this inherent data
plane similarity, a single CPE may be capable of supporting several
different approaches. Users may also wish to configure a specific
mode of operation.
A service provider's network may also have more than one S46
mechanism enabled in order to support a diverse CPE population with
differing client functionality, such as during a migration between
mechanisms, or where services require specific supporting softwire
architectures.
For softwire based services to be successfully established, it is
essential that the customer end-node, the service provider end-node
and provisioning systems are able to indicate their capabilities and
preferred mode of operation.
A number of DHCPv6 options for the provisioning of softwires have
been standardized:
RFC6334 Defines DHCPv6 option 64 for configuring Basic Bridging
BroadBand (B4, [RFC6333]) elements with the IPv6 address of
the Address Family Transition Router (AFTR, [RFC6333]).
RFC7341 Defines DHCPv6 option 88 for configuring the address of a
DHCPv4 over DHCPv6 server, which can then be used by a
softwire client for obtaining further configuration.
RFC7598 Defines DHCPv6 options 94, 95 and 96 for provisioning Mapping
of Address and Port with Encapsulation (MAP-E, [RFC7597]),
Mapping of Address and Port using Translation (MAP-T,
[RFC7599]), and Lightweight 4over6 [RFC7596] respectively.
This document describes a DHCPv6 based prioritization method whereby
a CPE which supports several S46 mechanisms and receives
configuration for more than one can prioritise which mechanism to
use. The method requires no server side logic to be implemented and
only uses a simple S46 mechanism prioritization to be implemented in
the CPE.
The prioritization method as described here does not provide
redundancy between S46 mechanisms for the client. I.e. If the
highest priority S46 mechanism which has been provisioned to the
client is not available for any reason, the means for identifying
this and falling back to the S46 mechanism with the next highest
priority is not in the scope of this document.
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1.1. Terminology
This document makes use of the following terms:
o Address Family Transition Router (AFTR): is the IPv4-in-IPv6
tunnel termination point and the NAT44 function deployed in the
operator's network [RFC6333].
o Border Relay (BR): a MAP-enabled router managed by the service
provider at the edge of a MAP domain. A BR has at least an
IPv6-enabled interface and an IPv4 interface connected to the
native IPv4 network [RFC7597].
o Customer Premise Equipment (CPE): denotes the equipment at the
customer edge that terminates the customer end of an IPv6
transitional tunnel. In some documents (e.g., [RFC7597]), this
functional entity is called CE (Customer Edge).
1.2. Rationale
The following rationale has been adopted for this document:
(1) Simplify solution migration paths: Define unified CPE behavior,
allowing for smooth migration between the different s46
mechanisms.
(2) Deterministic CPE co-existence behavior: Specify the behavior
when several S46 mechanisms co-exist in the CPE.
(3) Deterministic service provider co-existence behavior: Specify
the behavior when several modes co-exist in the service
providers network.
(4) Re-usability: Maximize the re-use of existing functional blocks
including tunnel end-points, port restricted NAPT44, forwarding
behavior, etc.
(5) Solution agnostic: Adopt neutral terminology and avoid (as far
as possible) overloading the document with solution-specific
terms.
(6) Flexibility: Allow operators to compile CPE software only for
the mode(s) necessary for their chosen deployment context(s).
(7) Simplicity: Provide a model that allows operators to only
implement the specific mode(s) that they require without the
additional complexity of unneeded modes.
1.3. DHCPv6 S46 Priority Option
The S46 Priority Option is used to convey a priority order of IPv4
service continuity mechanisms. Figure 1 shows the format of the S46
Priority Option.
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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_S46_PRIORITY | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| s46-option-code | s46-option-code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | s46-option-code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: S46 Priority Option
o option-code: OPTION_S46_PRIORITY (TBD)
o option-length: >=2 and a multiple of 2, in octets.
o s46-option-code: 16-bits long IANA registered option code of the
DHCPv6 option which is used to identify the softwire mechanism.
S46 mechanism are prioritized in the appearance order in the S46
Priority Option.
Codes in OPTION_S46_PRIORITY are processed in order; in the event
that a client receives more than one s46-option-code with a
particular value, this should be considered as invalid. DHCP servers
MAY validate the list of s46-option-code values to detect invalid
values and duplicates. The option MUST contain at least one s46-
option-code.
1.4. DHCPv6 Client Behavior
Clients MAY request option OPTION_S46_PRIORITY, as defined in
[RFC3315], Sections 17.1.1, 18.1.1, 18.1.3, 18.1.4, 18.1.5, and 22.7.
As a convenience to the reader, we mention here that the client
includes requested option codes in the Option Request Option.
Upon receipt of a DHCPv6 Advertise message from the server containing
OPTION_S46_PRIORITY the client performs the following steps:
1. Check the contents of the DHCPv6 message for options containing
valid S46 mechanism configuration. A candidate list of possible
S46 mechanisms is created from these option codes.
2. Check the contents of OPTION_S46_PRIORITY for the DHCPv6 option
codes contained in the included s46-option-code fields. From
this, an S46 mechanism priority list is created, ordered from
highest to lowest following the appearance order.
3. Sequentially check the priority list against the candidate list
until a match is found.
4. When a match is found, the client MUST configure the resulting
S46 mechanism.
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In the event that no match is found between the priority list and the
candidate list, the client MAY proceed with configuring one or more
of the provisioned S46 softwire mechanism(s). In this case, which
mechanism(s) are chosen by the client is implementation-specific and
not defined here.
If an invalid OPTION_S46_PRIORITY option is received, the client MAY
proceed with configuring the provisioned S46 mechanisms as if
OPTION_S46_PRIORITY had not been received.
If an unknown option code is received in OPTION_S46_PRIORITY option,
the client MUST skip it and continue processing other listed option
codes if they exist. The initial option codes that are allowed to be
included in a OPTION_S46_PRIORITY option are listed in Section 4.1.
1.5. DHCPv6 Server Behavior
Sections 17.2.2 and 18.2 of [RFC3315] govern server operation in
regards to option assignment. As a convenience to the reader, we
mention here that the server will send a particular option code only
if configured with specific values for that option code and if the
client requested it.
Option OPTION_S46_PRIORITY is a singleton. Servers MUST NOT send
more than one instance of the OPTION_S46_PRIORITY option.
2. Operator Deployment Considerations for Deploying Multiple Sotfwire
Mechanisms
The following sub-sections describe some considerations for operators
who are planning on implementing multiple softwire mechanisms in
their network (e.g., during a migration between mechanisms).
2.1. Client Address Planning
As an operator's available IPv4 resources are likely to be limited,
it may be desirable to use a common range of IPv4 addresses across
all of the active Softwire mechanisms. However, this is likely to
result in difficulties in routing ingress IPv4 traffic to the correct
Border Relay (BR)/AFTR instance which is actively serving a given CE.
For example, a client which is configured to use MAP-E may send its
traffic to the MAP-E BR, but on the return path, the ingress IP
traffic gets routed to a MAP-T BR. The resulting translated packet
that gets forwarded to the MAP-E client will be dropped.
Therefore, operators are advised to use separate IPv4 pools for each
of the different mechanisms to simplify planning and IPv4 routing.
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For IPv6 planning there is less of a constraint as the BR/AFTR
elements for the different mechanisms can contain configuration for
overlapping client's IPv6 addresses, providing only one mechanism is
actively serving a given client at a time. However, the IPv6 address
that is used as the tunnel concentrator's endpoint (BR/AFTR address)
needs to be different for each mechanisms to ensure correct
operation.
2.2. Backwards Compatability with Existing Softwire Clients
Deployed clients which can support multiple softwire mechanisms, but
do not implement the prioritization mechanism described here may
require additional planning. In this scenario, the CPE would request
configuration for all of the supported softwire mechanisms in its
DHCPv6 Option Request Option (ORO), but would not request
OPTION_S46_PRIORITY. By default, the DHCPv6 server will respond with
configuration for all of the requested mechanisms which could result
in unpredictable and unwanted client configuration.
In this scenario, it may be necessary for the operator to implement
logic within the DHCPv6 server to identify such clients and only
provision them with configuration for a single softwire mechanism.
It should be noted that this can lead to complexity and reduced
scalability in the DHCPv6 server implementation due to the addition
DHCPv6 message processing overhead.
3. Security Considerations
Security considerations discussed in [RFC6334] and [RFC7598] apply
for this document.
Misbehaving intermediate nodes may alter the content of the S46
Priority Option. This may lead to setting a different IPv4 service
continuity mechanism than the one initially preferred by the network
side. Also, a misbehaving node may alter the content of the S46
Priority Option and other DHCPv6 options (e.g., DHCPv6 Option #64 or
#90) so that the traffic is intercepted by an illegitimate node.
Those attacks are not unique to the S46 Priority Option but are
applicable to any DHCPv6 option that can be altered by a misbehaving
intermediate node.
4. IANA Considerations
IANA is kindly requested to allocate the following DHCPv6 option
code:
TBD for OPTION_S46_PRIORITY
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All values should be added to the DHCPv6 option code space defined in
Section 24.3 of [RFC3315].
4.1. S46 Mechanisms and their Identifying Option Codes
This document requests that IANA create a new registry entitled
"Option Codes permitted in the S46 Priority Option". This registry
will enumerate the set of DHCPv6 Option Codes that can be included in
OPTION_S46_PRIORITY option. Options may be added to this list using
the IETF Review process described in Section 4.1 of [RFC5226].
The following table shows the option codes which are currently
defined and the S46 mechanisms which they represent. The contents of
this table shows the format and the initial values for the new
registry. Option codes that have not been requested to be added
according to the stated procedure should not be mentioned at all in
the table, and should not be listed as "reserved" or "unassigned".
The valid range of values for the registry is the range of DHCPv6
Option Codes (1-65535).
+-------------+--------------------+-----------+
| Option Code | S46 Mechanism | Reference |
+-------------+--------------------+-----------+
| 64 | DS-Lite | [RFC6334] |
| 88 | DHCPv4 over DHCPv6 | [RFC7341] |
| 94 | MAP-E | [RFC7598] |
| 95 | MAP-T | [RFC7598] |
| 96 | Lightweight 4over6 | [RFC7598] |
+-------------+--------------------+-----------+
Table 1: DHCPv6 Option to S46 Mechanism Mappings
5. Acknowledgements
Many thanks to O. Troan, S. Barth. A. Yourtchenko, B. Volz, T.
Mrugalski, J. Scudder, P. Kyzivat, F. Baker, and B. Campbell for
their input and suggestions.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.
[RFC6334] Hankins, D. and T. Mrugalski, "Dynamic Host Configuration
Protocol for IPv6 (DHCPv6) Option for Dual-Stack Lite",
RFC 6334, DOI 10.17487/RFC6334, August 2011,
<http://www.rfc-editor.org/info/rfc6334>.
[RFC7341] Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport",
RFC 7341, DOI 10.17487/RFC7341, August 2014,
<http://www.rfc-editor.org/info/rfc7341>.
[RFC7598] Mrugalski, T., Troan, O., Farrer, I., Perreault, S., Dec,
W., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
Configuration of Softwire Address and Port-Mapped
Clients", RFC 7598, DOI 10.17487/RFC7598, July 2015,
<http://www.rfc-editor.org/info/rfc7598>.
6.2. Informative References
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,
<http://www.rfc-editor.org/info/rfc6333>.
[RFC7596] Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
Farrer, "Lightweight 4over6: An Extension to the Dual-
Stack Lite Architecture", RFC 7596, DOI 10.17487/RFC7596,
July 2015, <http://www.rfc-editor.org/info/rfc7596>.
[RFC7597] Troan, O., Ed., Dec, W., Li, X., Bao, C., Matsushima, S.,
Murakami, T., and T. Taylor, Ed., "Mapping of Address and
Port with Encapsulation (MAP-E)", RFC 7597,
DOI 10.17487/RFC7597, July 2015,
<http://www.rfc-editor.org/info/rfc7597>.
[RFC7599] Li, X., Bao, C., Dec, W., Ed., Troan, O., Matsushima, S.,
and T. Murakami, "Mapping of Address and Port using
Translation (MAP-T)", RFC 7599, DOI 10.17487/RFC7599, July
2015, <http://www.rfc-editor.org/info/rfc7599>.
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Authors' Addresses
Mohamed Boucadair
Orange
Rennes
France
Email: mohamed.boucadair@orange.com
Ian Farrer
Deutsche Telekom
Germany
Email: ian.farrer@telekom.de
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