Internet Engineering Task Force (IETF) W. Townsley

Internet Engineering Task Force (IETF) W. Townsley
Request for Comments: 5969 O. Troan
Category: Standards Track Cisco
ISSN: 2070-1721 August 2010

IPv6 Rapid Deployment on IPv4 Infrastructures (6rd) --
Protocol Specification

Abstract

This document specifies an automatic tunneling mechanism tailored to
advance deployment of IPv6 to end users via a service provider's IPv4
network infrastructure. Key aspects include automatic IPv6 prefix
delegation to sites, stateless operation, simple provisioning, and
service, which is equivalent to native IPv6 at the sites that are
served by the mechanism.

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/rfc5969.

Copyright Notice

Copyright (c) 2010 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.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. 6rd Prefix Delegation . . . . . . . . . . . . . . . . . . . . 5
5. Troubleshooting and Traceability . . . . . . . . . . . . . . . 7
6. Address Selection . . . . . . . . . . . . . . . . . . . . . . 7
7. 6rd Configuration . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Customer Edge Configuration . . . . . . . . . . . . . . . 8
7.1.1. 6rd DHCPv4 Option . . . . . . . . . . . . . . . . . . 9
7.2. Border Relay Configuration . . . . . . . . . . . . . . . . 10
8. Neighbor Unreachability Detection . . . . . . . . . . . . . . 11
9. IPv6 in IPv4 Encapsulation . . . . . . . . . . . . . . . . . . 12
9.1. Maximum Transmission Unit . . . . . . . . . . . . . . . . 13
9.2. Receiving Rules . . . . . . . . . . . . . . . . . . . . . 13
10. Transition Considerations . . . . . . . . . . . . . . . . . . 14
11. IPv6 Address Space Usage . . . . . . . . . . . . . . . . . . . 14
12. Security Considerations . . . . . . . . . . . . . . . . . . . 15
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
15.1. Normative References . . . . . . . . . . . . . . . . . . . 16
15.2. Informative References . . . . . . . . . . . . . . . . . . 17

1. Introduction

The original idea and the name of the mechanism (6rd) described in
[RFC5569] details a successful commercial "rapid deployment" of the
6rd mechanism by a residential service provider and is recommended
reading. This document describes the 6rd mechanism, which has been
extended for use in more general environments. Throughout this
document, the term 6to4 is used to refer to the mechanism described
in [RFC3056] and 6rd is the mechanism defined herein.

6rd specifies a protocol mechanism to deploy IPv6 to sites via a
service provider's (SP's) IPv4 network. It builds on 6to4 [RFC3056],
with the key differentiator that it utilizes an SP's own IPv6 address
prefix rather than a well-known prefix (2002::/16). By using the
SP's IPv6 prefix, the operational domain of 6rd is limited to the SP
network and is under its direct control. From the perspective of
customer sites and the IPv6 Internet at large, the IPv6 service
provided is equivalent to native IPv6.

The 6rd mechanism relies upon an algorithmic mapping between the IPv6
and IPv4 addresses that are assigned for use within the SP network.
This mapping allows for automatic determination of IPv4 tunnel
endpoints from IPv6 prefixes, allowing stateless operation of 6rd.

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6rd views the IPv4 network as a link layer for IPv6 and supports an
automatic tunneling abstraction similar to the Non-Broadcast Multiple
Access (NBMA) [RFC2491] model.

A 6rd domain consists of 6rd Customer Edge (CE) routers and one or
more 6rd Border Relays (BRs). IPv6 packets encapsulated by 6rd
follow the IPv4 routing topology within the SP network among CEs and
BRs. 6rd BRs are traversed only for IPv6 packets that are destined to
or are arriving from outside the SP's 6rd domain. As 6rd is
stateless, BRs may be reached using anycast for failover and
resiliency (in a similar fashion to [RFC3068]).

On the "customer-facing" (i.e., "LAN") side of a CE, IPv6 is
implemented as it would be for any native IP service delivered by the
SP, and further considerations for IPv6 operation on the LAN side of
the CE is out of scope for this document. On the "SP-facing" (i.e.,
"WAN") side of the 6rd CE, the WAN interface itself, encapsulation
over Ethernet, ATM or PPP, as well as control protocols such as
PPPoE, IPCP, DHCP, etc. all remain unchanged from current IPv4
operation. Although 6rd was designed primarily to support IPv6
deployment to a customer site (such as a residential home network) by
an SP, it can equally be applied to an individual IPv6 host acting as
a CE.

6rd relies on IPv4 and is designed to deliver production-quality IPv6
alongside IPv4 with as little change to IPv4 networking and
operations as possible. Native IPv6 deployment within the SP network
itself may continue for the SP's own purposes while delivering IPv6
service to sites supported by 6rd. Once the SP network and
operations can support fully native IPv6 access and transport, 6rd
may be discontinued.

6rd utilizes the same encapsulation and base mechanism as 6to4 and
could be viewed as a superset of 6to4 (6to4 could be achieved by
setting the 6rd prefix to 2002::/16). Unlike 6to4, 6rd is for use
only in an environment where a service provider closely manages the
delivery of IPv6 service. 6to4 routes with the 2002::/16 prefix may
exist alongside 6rd in the 6rd CE router, and doing so may offer some
efficiencies when communicating directly with 6to4 routers.

The 6rd link model can be extended to support IPv6 multicast. IPv6
multicast support is left for future consideration.

How this mechanism should be used and other deployment and
operational considerations are considered out of scope for this
document.

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2. 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].

3. Terminology

6rd prefix An IPv6 prefix selected by the service provider
for use by a 6rd domain. There is exactly one
6rd prefix for a given 6rd domain. An SP may
deploy 6rd with a single 6rd domain or multiple
6rd domains.

6rd Customer Edge (6rd CE) A device functioning as a Customer Edge
router in a 6rd deployment. In a
residential broadband deployment, this
type of device is sometimes referred to
as a "Residential Gateway" (RG) or
"Customer Premises Equipment" (CPE). A
typical 6rd CE serving a residential site
has one WAN side interface, one or more
LAN side interfaces, and a 6rd virtual
interface. A 6rd CE may also be referred
to simply as a "CE" within the context of
6rd.

6rd delegated prefix The IPv6 prefix calculated by the CE for use
within the customer site by combining the 6rd
prefix and the CE IPv4 address obtained via
IPv4 configuration methods. This prefix can be
considered logically equivalent to a DHCPv6
IPv6 delegated prefix [RFC3633].

6rd domain A set of 6rd CEs and BRs connected to the same
virtual 6rd link. A service provider may
deploy 6rd with a single 6rd domain, or may
utilize multiple 6rd domains. Each domain
requires a separate 6rd prefix.

CE LAN side The functionality of a 6rd CE that serves the
"Local Area Network (LAN)" or "customer-facing"
side of the CE. The CE LAN side interface is
fully IPv6 enabled.

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CE WAN side The functionality of a 6rd CE that serves the
"Wide Area Network (WAN)" or "Service Provider-
facing" side of the CE. The CE WAN side is
IPv4-only.

6rd Border Relay (BR) A 6rd-enabled router managed by the service
provider at the edge of a 6rd domain. A Border
Relay router has at least one of each of the
following: an IPv4-enabled interface, a 6rd
virtual interface acting as an endpoint for the
6rd IPv6 in IPv4 tunnel, and an IPv6 interface
connected to the native IPv6 network. A 6rd BR
may also be referred to simply as a "BR" within
the context of 6rd.

BR IPv4 address The IPv4 address of the 6rd Border Relay for a
given 6rd domain. This IPv4 address is used by
the CE to send packets to a BR in order to
reach IPv6 destinations outside of the 6rd
domain.

6rd virtual interface Internal multi-point tunnel interface where 6rd
encapsulation and decapsulation of IPv6 packets
inside IPv4 occurs. A typical CE or BR
implementation requires only one 6rd virtual
interface. A BR operating in multiple 6rd
domains may require more than one 6rd virtual
interface, but no more than one per 6rd domain.

CE IPv4 address The IPv4 address given to the CE as part of
normal IPv4 Internet access (i.e., configured
via DHCP, PPP, or otherwise). This address may
be global or private [RFC1918] within the 6rd
domain. This address is used by a 6rd CE to
create the 6rd delegated prefix as well as to
send and receive IPv4-encapsulated IPv6
packets.

4. 6rd Prefix Delegation

The 6rd delegated prefix for use at a customer site is created by
combining the 6rd prefix and all or part of the CE IPv4 address.
From these elements, the 6rd delegated prefix is automatically
created by the CE for the customer site when IPv4 service is
obtained. This 6rd delegated prefix is used in the same manner as a
prefix obtained via DHCPv6 prefix delegation [RFC3633].

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In 6to4, a similar operation is performed by incorporating an entire
IPv4 address at a fixed location following a well-known /16 IPv6
prefix. In 6rd, the IPv6 prefix as well as the position and number
of bits of the IPv4 address incorporated varies from one 6rd domain
to the next. 6rd allows the SP to adjust the size of the 6rd prefix,
how many bits are used by the 6rd mechanism, and how many bits are
left to be delegated to customer sites. To allow for stateless
address auto-configuration on the CE LAN side, a 6rd delegated prefix
SHOULD be /64 or shorter.

The 6rd delegated prefix is created by concatenating the 6rd prefix
and a consecutive set of bits from the CE IPv4 address in order. The
length of the 6rd delegated prefix is equal to length of the 6rd
prefix (n) plus the number of bits from the CE IPv4 address (o).

The figure shows the format of an IPv6 address (Section 2.5.4 of
[RFC4291]) with a 6rd prefix and an embedded CE IPv4 address:

| n bits | o bits | m bits | 128-n-o-m bits |
+---------------+--------------+-----------+------------------------+
| 6rd prefix | IPv4 address | subnet ID | interface ID |
+---------------+--------------+-----------+------------------------+
|<--- 6rd delegated prefix --->|

Figure 1

For example, if the 6rd prefix is /32 and 24 bits of the CE IPv4
address is used (e.g., all CE IPv4 addresses can be aggregated by a
10.0.0.0/8), then the size of the 6rd delegated prefix for each CE is
automatically calculated to be /56 (32 + 24 = 56).

Embedding less than the full 32 bits of a CE IPv4 address is possible
only when an aggregated block of IPv4 addresses is available for a
given 6rd domain. This may not be practical with global IPv4
addresses, but is quite likely in a deployment where private
addresses are being assigned to CEs. If private addresses overlap
within a given 6rd deployment, the deployment may be divided into
separate 6rd domains, likely along the same topology lines the NAT-
based IPv4 deployment itself would require. In this case, each
domain is addressed with a different 6rd prefix.

Each 6rd domain may use a different encoding of the embedded IPv4
address, even within the same service provider. For example, if
multiple IPv4 address blocks with different levels of aggregation are
used at the same service provider, the number of IPv4 bits needed to
encode the 6rd delegated prefix may vary between each block. In this
case, different 6rd prefixes, and hence separate 6rd domains, may be
used to support the different encodings.

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Since 6rd delegated prefixes are selected algorithmically from an
IPv4 address, changing the IPv4 address will cause a change in the
IPv6 delegated prefix which would ripple through the site's network
and could be disruptive. As such, it is recommended that the service
provider assign CE IPv4 addresses with relatively long lifetimes.

6rd IPv6 address assignment, and hence the IPv6 service itself, is
tied to the IPv4 address lease; thus, the 6rd service is also tied to
this in terms of authorization, accounting, etc. For example, the
6rd delegated prefix has the same lifetime as its associated IPv4
address. The prefix lifetimes advertised in Router Advertisements or
used by DHCP on the CE LAN side MUST be equal to or shorter than the
IPv4 address lease time. If the IPv4 lease time is not known, the
lifetime of the 6rd delegated prefix SHOULD follow the defaults
specified in [RFC4861].

5. Troubleshooting and Traceability

A 6rd IPv6 address and associated IPv4 address for a given customer
can always be determined algorithmically by the service provider that
operates the given 6rd domain. This may be useful for referencing
logs and other data at a service provider that may have more robust
operational tools for IPv4 than IPv6. This also allows IPv4 data
path, node, and endpoint monitoring to be applicable to IPv6.

The 6rd CE and BR SHOULD support the IPv6 Subnet-Router anycast
address [RFC4291] for its own 6rd delegated prefix. This allows, for
example, IPv6 ICMP echo messages to be sent to the 6rd virtual
interface itself for additional troubleshooting of the internal
operation of 6rd at a given CE or BR. In the case of the BR, the
IPv4 address used to calculate the 6rd delegated prefix is the
configured BR IPv4 address.

6. Address Selection

All addresses assigned from 6rd delegated prefixes should be treated
as native IPv6. No changes to the source address selection or
destination address selection policy table [RFC3484] are necessary.

7. 6rd Configuration

For a given 6rd domain, the BR and CE MUST be configured with the
following four 6rd elements. The configured values for these four
6rd elements are identical for all CEs and BRs within a given 6rd
domain.

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IPv4MaskLen The number of high-order bits that are identical
across all CE IPv4 addresses within a given 6rd
domain. For example, if there are no identical
bits, IPv4MaskLen is 0 and the entire CE IPv4
address is used to create the 6rd delegated
prefix. If there are 8 identical bits (e.g., the
Private IPv4 address range 10.0.0.0/8 is being
used), IPv4MaskLen is equal to 8 and IPv4MaskLen
high-order bits are stripped from the IPv4
address before constructing the corresponding 6rd
delegated prefix.

6rdPrefix The 6rd IPv6 prefix for the given 6rd domain.

6rdPrefixLen The length of the 6rd IPv6 prefix for the given
6rd domain.

6rdBRIPv4Address The IPv4 address of the 6rd Border Relay for a
given 6rd domain.

7.1. Customer Edge Configuration

The four 6rd elements are set to values that are the same across all
CEs within a 6rd domain. The values may be configured in a variety
of manners, including provisioning methods such as the Broadband
Forum's "TR-69" [TR069] Residential Gateway management interface, an
XML-based object retrieved after IPv4 connectivity is established, a
DNS record, an SMIv2 MIB [RFC2578], PPP IPCP, or manual configuration
by an administrator. This document describes how to configure the
necessary parameters via a single DHCP option. A CE that allows IPv4
configuration by DHCP SHOULD implement this option. Other
configuration and management methods may use the format described by
this option for consistency and convenience of implementation on CEs
that support multiple configuration methods.

The only remaining provisioning information the CE requires in order
to calculate the 6rd delegated prefix and enable IPv6 connectivity is
an IPv4 address for the CE. This CE IPv4 address is configured as
part of obtaining IPv4 Internet access (i.e., configured via DHCP,
PPP, or otherwise). This address may be global or private [RFC1918]
within the 6rd domain.

A single 6rd CE MAY be connected to more than one 6rd domain, just as
any router may have more than one IPv6-enabled service provider
facing interface and more than one set of associated delegated
prefixes assigned by DHCPv6 prefix delegation or other means. Each

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domain a given CE operates within would require its own set of 6rd
configuration elements and would generate its own 6rd delegated
prefix.

7.1.1. 6rd DHCPv4 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_6RD | option-length | IPv4MaskLen | 6rdPrefixLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| 6rdPrefix |
| (16 octets) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6rdBRIPv4Address(es) |
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2

option-code OPTION_6RD (212)

option-length The length of the DHCP option in octets (22
octets with one BR IPv4 address).

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.

6rdPrefixLen The IPv6 prefix length of the SP's 6rd IPv6
prefix in number of bits. For the purpose of
bounds checking by DHCP option processing, the
sum of (32 - IPv4MaskLen) + 6rdPrefixLen MUST be
less than or equal to 128.

6rdBRIPv4Address One or more IPv4 addresses of the 6rd Border
Relay(s) for a given 6rd domain.

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6rdPrefix The service provider's 6rd IPv6 prefix
represented as a 16-octet IPv6 address. The bits
in the prefix after the 6rdPrefixlen number of
bits are reserved and MUST be initialized to zero
by the sender and ignored by the receiver.

The CE MUST include a Parameter Request List Option [RFC2132] for the
OPTION_6RD. Because the OPTION_6RD contains one IPv4MaskLen/
6rdPrefixLen/6rdPrefix block, and because DHCP cannot convey more
than one instance of an option, OPTION_6RD is limited to provision at
most a single 6rd domain. Provisioning of a CE router connected to
multiple 6rd domains is outside the scope of this protocol
specification.

The presence of the OPTION_6RD DHCP option is an indication of the
availability of the 6rd service. By default, receipt of a valid 6rd
DHCP option by a 6rd-capable CE results in configuration of the 6rd
virtual interface and associated delegated prefix for use on the CE's
LAN side. The CE MUST be able to configure the 6rd mechanism to be
disabled, in which case the 6rd DHCP option, if received, is silently
ignored.

A detailed description of CE behavior using multiple BR IPv4
addresses is left for future consideration. In such a case, a CE
MUST support at least one BR IPv4 address and MAY support more than
one.

When 6rd is enabled, a typical CE router will install a default route
to the BR, a black hole route for the 6rd delegated prefix, and
routes for any LAN side assigned and advertised prefixes. For
example, using a CE IPv4 address of 10.100.100.1, a BR IPv4 address
of 10.0.0.1, an IPv4MaskLen of 8, 2001:db8::/32 as the 6rdPrefix, and
one /64 prefix assigned to a LAN side interface, a typical CE routing
table will look like:

::/0 -> 6rd-virtual-int0 via 2001:db8:0:100:: (default route)
2001:db8::/32 -> 6rd-virtual-int0 (direct connect to 6rd)
2001:db8:6464:100::/56 -> Null0 (delegated prefix null route)
2001:db8:6464:100::/64 -> Ethernet0 (LAN interface)

7.2. Border Relay Configuration

The 6rd BR MUST be configured with the same 6rd elements as the 6rd
CEs operating within the same domain.

For increased reliability and load balancing, the BR IPv4 address may
be an anycast address shared across a given 6rd domain. As 6rd is
stateless, any BR may be used at any time. If the BR IPv4 address is

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anycast the relay MUST use this anycast IPv4 address as the source
address in packets relayed to CEs.

Since 6rd uses provider address space, no specific routes need to be
advertised externally for 6rd to operate, neither in IPv6 nor IPv4
BGP. However, if anycast is used for the 6rd IPv4 relays, the
anycast addresses must be advertised in the service provider's IGP.

8. Neighbor Unreachability Detection

Neighbor Unreachability Detection (NUD) for tunnels is described in
Section 3.8 of [RFC4213]. In 6rd, all CEs and BRs can be considered
as connected to the same virtual link and therefore neighbors to each
other. This section describes how to utilize neighbor unreachability
detection without negatively impacting the scalability of a 6rd
deployment.

A typical 6rd deployment may consist of a very large number of CEs
within the same domain. Reachability between CEs is based on IPv4
routing, and sending NUD or any periodic packets between 6rd CE
devices beyond isolated troubleshooting of the 6rd mechanism is NOT
RECOMMENDED.

While reachability detection between a given 6rd CE and BR is not
necessary for the proper operation of 6rd, in cases where a CE has
alternate paths for BR reachability to choose from, it could be
useful. Sending NUD messages to a BR, in particular periodic
messages from a very large number of CEs, could result in overloading
of the BR control message processing path, negatively affecting
scalability of the 6rd deployment. Instead, a CE that needs to
determine BR reachability MUST utilize a method that allows
reachability detection packets to follow a typical data forwarding
path without special processing by the BR. One such method is
described below.

1. The CE constructs a payload of any size and content to be sent to
the BR (e.g., a zero-length null payload, a padded payload
designed to test a certain MTU, a NUD message, etc.). The exact
format of the message payload is not important as the BR will not
be processing it directly.

2. The desired payload is encapsulated with the inner IPv6 and outer
IPv4 headers as follows:

* The IPv6 destination address is set to an address from the
CE's 6rd delegated prefix that is assigned to a virtual
interface on the CE.

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* The IPv6 source address is set to an address from the CE's 6rd
delegated prefix as well, including the same as used for the
IPv6 destination address.

* The IPv4 header is then added as it normally would be for any
packet destined for the BR. That is, the IPv4 destination
address is that of the BR, and the source address is the CE
IPv4 address.

3. The CE sends the constructed packet out the interface on which BR
reachability is being monitored. On successful receipt at the
BR, the BR MUST decapsulate and forward the packet normally.
That is, the IPv4 header is decapsulated normally, revealing the
IPv6 destination as the CE, which in turn results in the packet
being forwarded to that CE via the 6rd mechanism (i.e., the IPv4
destination is that of the CE that originated the packet, and the
IPv4 source is that of the BR).

4. Arrival of the constructed IPv6 packet at the CE's IPv6 address
completes one round trip to and from the BR, without causing the
BR to process the message outside of its normal data forwarding
path. The CE then processes the IPv6 packet accordingly
(updating keepalive timers, metrics, etc.).

The payload may be empty or could contain values that are meaningful
to the CE. Sending a proper NUD message could be convenient for some
implementations (note that the BR will decrement the IPv6 hop limit).
Since the BR forwards the packet as any other data packet without any
processing of the payload itself, the format of the payload is left
as a choice to the implementer.

9. IPv6 in IPv4 Encapsulation

IPv6 in IPv4 encapsulation and forwarding manipulations (e.g.,
handling packet markings, checksumming, etc.) is performed as
specified in Section 3.5 of "Basic Transition Mechanisms for IPv6
Hosts and Routers" [RFC4213], which is the same mechanism used by
6to4 [RFC3056]. ICMPv4 errors are handled as specified in Section
3.4 of [RFC4213]. By default, the IPv6 Traffic Class field MUST be
copied to the IPv4 ToS (Type of Service) field. This default
behavior MAY be overridden by configuration. See [RFC2983] and
[RFC3168] for further information related to IP Differentiated
Services and tunneling.

IPv6 packets from a CE are encapsulated in IPv4 packets when they
leave the site via its CE WAN side interface. The CE IPv4 address
MUST be configured to send and receive packets on this interface.

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The 6rd link is modeled as an NBMA link similar to other automatic
IPv6 in IPv4 tunneling mechanisms like [RFC5214], with all 6rd CEs
and BRs defined as off-link neighbors from one other. The link-local
address of a 6rd virtual interface performing the 6rd encapsulation
would, if needed, be formed as described in Section 3.7 of [RFC4213].
However, no communication using link-local addresses will occur.

9.1. Maximum Transmission Unit

Maximum transmission unit (MTU) and fragmentation issues for IPv6 in
IPv4 tunneling are discussed in detail in Section 3.2 of RFC 4213
[RFC4213]. 6rd's scope is limited to a service provider network.
IPv4 Path MTU discovery MAY be used to adjust the MTU of the tunnel
as described in Section 3.2.2 of RFC 4213 [RFC4213], or the 6rd
Tunnel MTU might be explicitly configured.

The use of an anycast source address could lead to any ICMP error
message generated on the path being sent to a different BR.
Therefore, using dynamic tunnel MTU Section 3.2.2 of [RFC4213] is
subject to IPv4 Path MTU blackholes.

Multiple BRs using the same anycast source address could send
fragmented packets to the same IPv6 CE at the same time. If the
fragmented packets from different BRs happen to use the same fragment
ID, incorrect reassembly might occur. For this reason, a BR using an
anycast source address MUST set the IPv4 Don't Fragment flag.

If the MTU is well-managed such that the IPv4 MTU on the CE WAN side
interface is set so that no fragmentation occurs within the boundary
of the SP, then the 6rd Tunnel MTU should be set to the known IPv4
MTU minus the size of the encapsulating IPv4 header (20 bytes). For
example, if the IPv4 MTU is known to be 1500 bytes, the 6rd Tunnel
MTU might be set to 1480 bytes. Absent more specific information,
the 6rd Tunnel MTU SHOULD default to 1280 bytes.

9.2. Receiving Rules

In order to prevent spoofing of IPv6 addresses, the 6rd BR and CE
MUST validate the embedded IPv4 source address of the encapsulated
IPv6 packet with the IPv4 source address it is encapsulated by
according to the configured parameters of the 6rd domain. If the two
source addresses do not match, the packet MUST be dropped and a
counter incremented to indicate that a potential spoofing attack may
be underway. Additionally, a CE MUST allow forwarding of packets
sourced by the configured BR IPv4 address.

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RFC 5969 6rd August 2010

By default, the CE router MUST drop packets received on the 6rd
virtual interface (i.e., after decapsulation of IPv4) for IPv6
destinations not within its own 6rd delegated prefix.

10. Transition Considerations

An SP network can migrate to IPv6 at its own pace with little or no
effect on customers being provided IPv6 via 6rd. When native IPv6
connectivity is available, an administrator can choose to disable
6rd.

The SP can choose to provision a separate IPv6 address block for
native service, or reuse the 6rd prefix block itself. If the SP uses
a separate address block, moving from 6rd to native IPv6 is seen as a
normal IPv6 renumbering event for the customer. Renumbering may also
be avoided by injecting the 6rd delegated prefix into the SP's IPv6
routing domain. Further considerations with regards to transitioning
from 6rd to native IPv6 are not covered in this protocol
specification.

11. IPv6 Address Space Usage

As 6rd uses service-provider address space, 6rd uses the normal
address delegation a service provider gets from its Regional Internet
Registry (RIR) and no global allocation of a single 6rd IANA-assigned
address block like the 6to4 2002::/16 is needed.

The service provider's prefix must be short enough to encode the
unique bits of all IPv4 addresses within a given 6rd domain and still
provide enough IPv6 address space to the residential site. Assuming
a worst case scenario using the full 32 bits for the IPv4 address,
assigning a /56 for customer sites would mean that each service
provider using 6rd would require a /24 for 6rd in addition to other
IPv6 addressing needs. Assuming that 6rd would be stunningly
successful and taken up by almost all Autonomous System (AS) number
holders (32K today), then the total address usage of 6rd would be
equivalent to a /9. If the SP instead delegated /60s to sites, the
service provider would require a /28, and the total global address
consumption by 6rd would be equivalent to a /13. Again, this assumes
that 6rd is used by all AS number holders in the IPv4 Internet today
at the same time, that none have used any of 6rd's address
compression techniques, and that none have moved to native IPv6 and
reclaimed the 6rd space that was being used for other purposes.

To alleviate concerns about address usage, 6rd allows for leaving out
redundant IPv4 prefix bits in the encoding of the IPv4 address inside
the 6rd IPv6 address. This is most useful where the IPv4 address
space is very well aggregated. For example, to provide each customer

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RFC 5969 6rd August 2010

with a /60, if a service provider has all its IPv4 customers under a
/12 then only 20 bits needs to be used to encode the IPv4 address and
the service provider would only need a /40 IPv6 allocation for 6rd.
If private address space is used, then a 10/8 would require a /36.
If multiple 10/8 domains are used, then up to 16 could be supported
within a /32.

If a service provider has a non-aggregatable IPv4 space and requiring
the use of the full 32-bit IPv4 address in the encoding of the 6rd
IPv6 address, the 6rd prefix MUST be no longer than /32 in order to
offer a 6rd delegated prefix of at least /64.

The 6rd address block can be reclaimed when all users of it have
transitioned to native IPv6 service. This may require renumbering of
customer sites and use of additional address space during the
transition period.

12. Security Considerations

A 6to4 relay router as specified in [RFC3056] can be used as an open
relay. It can be used to relay IPv6 traffic and as a traffic
anonymizer. By restricting the 6rd domain to within a provider
network, a CE only needs to accept packets from a single or small set
of known 6rd BR IPv4 addresses. As such, many of the threats against
6to4 as described in [RFC3964] do not apply.

When applying the receiving rules in Section 9.2, IPv6 packets are as
well protected against spoofing as IPv4 packets are within an SP
network.

A malicious user that is aware of a 6rd domain and the BR IPv4
address could use this information to construct a packet that would
cause a Border Relay to reflect tunneled packets outside of the
domain that it is serving. If the attacker constructs the packet
accordingly and can inject a packet with an IPv6 source address that
looks as if it originates from within another 6rd domain, forwarding
loops between 6rd domains may be created, allowing the malicious user
to launch a packet amplification attack between 6rd domains
[RoutingLoop].

One possible mitigation for this is to simply not allow the BR IPv4
address to be reachable from outside the SP's 6rd domain. In this
case, carefully constructed IPv6 packets still could be reflected off
a single BR, but the looping condition will not occur. Tunneled
packets with the BR IPv4 address as the source address might also be
filtered to prohibit 6rd tunnels from exiting the 6rd domain.

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RFC 5969 6rd August 2010

To avoid forwarding loops via other internal relays, the BR should
employ outgoing and incoming IPv4 packets filters, filtering out all
known relay addresses for internal 6rd BRs, ISATAP routers, or 6to4
relays, including the well-known anycast address space for 6to4.

Another possible mitigation to the routing loop issue is described in
[V6OPS-LOOPS].

The BR MUST install a null route [RFC4632] for its 6rd delegated
prefix created based on its BR IPv4 address, with the exception of
the IPv6 Subnet-Router anycast address.

13. IANA Considerations

IANA assigned a new DHCP Option code point for OPTION_6RD (212) with
a data length of 18 + N (OPTION_6RD with N/4 6rd BR addresses).

14. Acknowledgements

This RFC is based on Remi Despres' original idea described in
[RFC5569] and the work done by Rani Assaf, Alexandre Cassen, and
Maxime Bizon at Free Telecom. Brian Carpenter and Keith Moore
documented 6to4, which all of this work is based upon. We thank Fred
Templin for his review and contributions, and for sharing his
experience with ISATAP. Review and encouragement have been provided
by many others and in particular Chris Chase, Thomas Clausen, Wouter
Cloetens, Wojciech Dec, Bruno Decraene, Remi Despres, Alain Durand,
Washam Fan, Martin Gysi, David Harrington, Jerry Huang, Peter McCann,
Alexey Melnikov, Dave Thaler, Eric Voit, and David Ward.

15. References

15.1. Normative References

[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G.,
and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, February 1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP
Vendor Extensions", RFC 2132, March 1997.

[RFC2491] Armitage, G., Schulter, P., Jork, M., and G. Harter,
"IPv6 over Non-Broadcast Multiple Access (NBMA)
networks", RFC 2491, January 1999.

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RFC 5969 6rd August 2010

[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6
Domains via IPv4 Clouds", RFC 3056, February 2001.

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The
Addition of Explicit Congestion Notification (ECN) to
IP", RFC 3168, September 2001.

[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition
Mechanisms for IPv6 Hosts and Routers", RFC 4213,
October 2005.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)",
RFC 4861, September 2007.

15.2. Informative References

[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management
Information Version 2 (SMIv2)", STD 58, RFC 2578,
April 1999.

[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, October 2000.

[RFC3068] Huitema, C., "An Anycast Prefix for 6to4 Relay
Routers", RFC 3068, June 2001.

[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.

[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for
Dynamic Host Configuration Protocol (DHCP) version 6",
RFC 3633, December 2003.

[RFC3964] Savola, P. and C. Patel, "Security Considerations for
6to4", RFC 3964, December 2004.

[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and
Aggregation Plan", BCP 122, RFC 4632, August 2006.

[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)",
RFC 5214, March 2008.

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RFC 5969 6rd August 2010

[RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd)", RFC 5569, January 2010.

[RoutingLoop] Nakibly and Arov, "Routing Loop Attacks using IPv6
Tunnels", August 2009, <http://www.usenix.org/event/
woot09/tech/full_papers/nakibly.pdf>.

[TR069] "TR-069, CPE WAN Management Protocol v1.1, Version:
Issue 1 Amendment 2", December 2007.

[V6OPS-LOOPS] Nakibly, G. and F. Templin, "Routing Loop Attack using
IPv6 Automatic Tunnels: Problem Statement and Proposed
Mitigations", Work in Progress, May 2010.

Authors' Addresses

Mark Townsley
Cisco
Paris,
France

EMail: [email protected]

Ole Troan
Cisco
Bergen,
Norway

EMail: [email protected]

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