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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 5969

Internet Engineering Task Force                              W. Townsley
Internet-Draft                                                  O. Troan
Intended status: Standards Track                                   Cisco
Expires: August 26, 2010                               February 22, 2010

             IPv6 via IPv4 Service Provider Networks "6rd"


   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 which are
   served by the mechanism.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on August 26, 2010.

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

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   (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
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   described in the BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  6rd Prefix Delegation  . . . . . . . . . . . . . . . . . . . .  6
   5.  Troubleshooting and Traceability . . . . . . . . . . . . . . .  7
   6.  Address Selection  . . . . . . . . . . . . . . . . . . . . . .  8
   7.  6rd Configuration  . . . . . . . . . . . . . . . . . . . . . .  8
     7.1.  Customer Edge Configuration  . . . . . . . . . . . . . . .  8
       7.1.1.  6rd DHCPv4 Option  . . . . . . . . . . . . . . . . . .  9
     7.2.  Border Relay Configuration . . . . . . . . . . . . . . . . 11
   8.  Neighbor Unreachability Detection  . . . . . . . . . . . . . . 11
   9.  IPv6 in IPv4 Encapsulation . . . . . . . . . . . . . . . . . . 13
     9.1.  Maximum Transmission Unit  . . . . . . . . . . . . . . . . 13
     9.2.  Receiving Rules  . . . . . . . . . . . . . . . . . . . . . 14
   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
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18

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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, extended for use
   in more general environments, and intended for advancement on the
   IETF Standards Track.  Throughout this document, the term 6to4 is
   used to refer to the mechanism described in [RFC3056] and 6rd 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 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.
   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) 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.

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   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 aside of 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 in fact be viewed as a superset of 6to4. 6to4 service can be
   made with 6rd by setting the 6rd prefix to 2002::/16.  Unlike 6to4,
   6rd is for use only in an environment where a service provider
   cooperates closely to deliver the 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 is considered out of scope for this

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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     A 6rd CE is a device functioning as a Customer
                         Edge 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 CE router serving a residential site
                         has one CE WAN side interface, one or more CE
                         LAN side interfaces, and a virtual 6rd

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                         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.

   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

   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

   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

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                         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

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].

   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 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
   sum of the number of bits used by each determines the size of the
   prefix that is delegated to the CE.

   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

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   address is used (e.g all CE IPv4 addresses can be aggregated by a, 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.

   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 (whether set via DHCP, PPP, or otherwise),
   thus the 6rd service is also tied to this in terms of authorization,
   accounting, etc.  For example, the 6rd delegated prefix has the same
   DHCP lease time 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.

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 which 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

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   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

   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 is being
                       used), IPv4MaskLen is equal to 8.

   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 which are the same across all
   CEs within a 6rd domain.  The values may be configured in a variety
   of manners, including automatic provisioning methods such as the
   Broadband Forum's "TR-69" Residential Gateway management interface,
   an XML-based object retrieved after IPv4 connectivity is established,
   a DNS record, an SNMP MIB, PPP IPCP, or manual configuration by an
   end-user or operator.  This document describes how to configure the
   necessary parameters via a single DHCP option.  In order to guarantee

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   interoperability, a CE SHOULD implement this DHCP option.  For
   consistency and convenience, this option format may be used by other
   automatic configuration methods by normative reference to this

   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 PD or other means.  Each 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(TBD)

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   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.

   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.

   No more than one instance of the OPTION_6RD DHCP option MUST occur in
   a given DHCP packet.  Provisioning of a CE router connected to
   multiple 6rd domains is outside the scope of this protocol

   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

   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

   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, a BR IPv4 address
   of, an IPv4MaskLen of 8, 2001:ABC0::/32 as the 6rdPrefix,
   and one /64 prefix assigned to a LAN side Interface, a typical CE
   Routing Information Base (RIB) will look like:

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     ::/0 -> 6rd-virtual-int0 via 2001:ABC0:0:0100:: (default route)
     2001:ABC0::/32 -> 6rd-virtual-int0 (direct connect to 6rd)
     2001:ABC0:6464:0100::/56 -> Null0 (delegated prefix sink route)
     2001:ABC0:6464:0100::/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
   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

   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

   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 which allows
   reachability detection packets to follow a typical data forwarding
   path without special processing by the BR.  One such method is
   described below.

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   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 for which the CE itself will process
          (e.g., a CE "loopback" or other type of local interface

       *  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 for any
          packet destined for the BR.  That is, the IPv4 destination
          address is that of the BR, and source address is the CE IPv4

   3.  The CE sends the constructed packet out the proper interface it
       is monitoring BR reachability on.  On successful receipt at the
       BR, the BR MUST decapsulate and forward the packet normally.
       This 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.

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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 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.

   The 6rd link is modeled as an NBMA [RFC2491] link similar to other
   automatic IPv6 in IPv4 tunneling mechanisms like [RFC5214] with all
   6rd CEs and BRs defined 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

   MTU and fragmentation issues for IPv6 in IPv4 tunneling are discussed
   in detail in section 3.2 of RFC4213 [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
   RFC4213 [RFC4213] or the 6rd Tunnel MTU may be explicitly configured.

   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 may be set to 1480 bytes.  Absent of more specific information
   the 6rd Tunnel MTU SHOULD default to 1280 bytes.

   A 6rd CE SHOULD advertise the 6rd Tunnel MTU, whether determined
   automatically or configured directly, on the LAN side by setting the
   MTU option in Router Advertisements [RFC4861] messages to the 6rd
   Tunnel MTU.

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9.2.  Receiving Rules

   In order to prevent spoofing of IPv6 addresses, the 6rd BR and CE
   MUST validate the 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 packets sourced by the configured BR
   IPv4 Address.

   The CE router SHOULD 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 may migrate to IPv6 at its own pace with little or no
   effect on customers being provided IPv6 via 6rd.  In the event native
   IPv6 connectivity is also available, the CE MAY 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

11.  IPv6 Address Space Usage

   As 6rd uses service provider address space, 6rd uses the normal
   address delegation a service provider gets from its 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 AS number holders (32K today)
   then the total address usage of 6rd would be equivalent to a /9.  If

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   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, none
   used any of 6rd's address compression techniques, and that none have
   moved to native IPv6 and reclaimed the 6rd space which 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
   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 an 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

   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

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   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

   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 may be reflected off a
   single BR, but the looping condition will not occur.  Tunnelled
   packets with the BR IPv4 address as the source address may also be
   filtered to prohibit 6rd tunnels from exiting the 6rd domain.

   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.

   The BR MUST install a sink route 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 is requested to assign a new DHCP Option code point for

14.  Acknowledgements

   This draft 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, contributions and 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, Jerry Huang, 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.

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

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

   [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.

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

   [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

   [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.

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

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

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              Nakibly and Arov, "Routing Loop Attacks using IPv6
              Tunnels", August 2009, <http://www.usenix.org/event/

Authors' Addresses

   Mark Townsley

   Email: mark@townsley.net

   Ole Troan

   Email: ot@cisco.com

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