Network Working Group
Request for Comments: 3056
Category: Standards Track

B. Carpenter
K. Moore
February 2001

Connection of IPv6 Domains via IPv4 Clouds

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Abstract

This memo specifies an optional interim mechanism for IPv6 sites to communicate with each other over the IPv4 network without explicit tunnel setup, and for them to communicate with native IPv6 domains via relay routers. Effectively it treats the wide area IPv4 network as a unicast point-to-point link layer. The mechanism is intended as a start-up transition tool used during the period of co-existence of IPv4 and IPv6. It is not intended as a permanent solution.

The motivation for this method is to allow isolated IPv6 domains or hosts, attached to an IPv4 network which has no native IPv6 support, to communicate with other such IPv6 domains or hosts with minimal manual configuration, before they can obtain natuve IPv6 connectivity. It incidentally provides an interim globally unique IPv6 address prefix to any site with at least one globally unique IPv4 address, even if combined with an IPv4 Network Address Translator (NAT).

   1. Introduction................................................. 2
   1.1. Terminology................................................ 4
   2. IPv6 Prefix Allocation....................................... 5
   2.1 Address Selection........................................... 6
   3. Encapsulation in IPv4........................................ 6
   3.1. Link-Local Address and NUD................................. 7
   4. Maximum Transmission Unit.................................... 7
   5. Unicast scenarios, scaling, and transition to normal prefixes 8
   5.1 Simple scenario - all sites work the same................... 8
   5.2 Mixed scenario with relay to native IPv6...................  9
   5.2.1 Variant scenario with ISP relay.......................... 12
   5.2.2 Summary of relay router configuration.................... 12
   5.2.2.1. BGP4+ not used........................................ 12
   5.2.2.2. BGP4+ used............................................ 12
   5.2.2.3. Relay router scaling.................................. 13
   5.2.3 Unwilling to relay....................................... 13
   5.3 Sending and decapsulation rules............................ 13
   5.4 Variant scenario with tunnel to IPv6 space................. 14
   5.5 Fragmented Scenarios....................................... 14
   5.6 Multihoming................................................ 16
   5.7 Transition Considerations.................................. 16
   5.8 Coexistence with firewall, NAT or RSIP..................... 16
   5.9 Usage within Intranets..................................... 17
   5.10 Summary of impact on routing.............................. 18
   5.11. Routing loop prevention.................................. 18
   6. Multicast and Anycast....................................... 19
   7. ICMP messages............................................... 19
   8. IANA Considerations......................................... 19
   9. Security Considerations..................................... 19
   Acknowledgements............................................... 20
   References..................................................... 20
   Authors' Addresses............................................. 22
   Intellectual Property.......................................... 22
   Full Copyright Statement....................................... 23

1. Introduction

The document defines a method for assigning an interim unique IPv6 address prefix to any site that currently has at least one globally unique IPv4 address, and specifies an encapsulation mechanism for transmitting IPv6 packets using such a prefix over the global IPv4 network. It also describes scenarios for using such prefixes during the co-existence phase of IPv4 to IPv6 transition. Note that these scenarios are only part of the total picture of transition to IPv6. Also note that this is considered to be an interim solution and that sites should migrate when possible to native IPv6 prefixes and native IPv6 connectivity. This will be possible as soon as the site's ISP offers native IPv6 connectivity.

The basic mechanism described in the present document, which applies to sites rather than individual hosts, will scale indefinitely by limiting the number of sites served by a given relay router (see Section 5.2). It will introduce no new entries in the IPv4 routing table, and exactly one new entry in the native IPv6 routing table (see Section 5.10).

Although the mechanism is specified for an IPv6 site, it can equally be applied to an individual IPv6 host or very small site, as long as it has at least one globally unique IPv4 address. However, the latter case raises serious scaling issues which are the subject of further study [SCALE].

The motivation for this method is to allow isolated IPv6 sites or hosts, attached to a wide area network which has no native IPv6 support, to communicate with other such IPv6 domains or hosts with minimal manual configuration.

IPv6 sites or hosts connected using this method do not require IPv4- compatible IPv6 addresses [MECH] or configured tunnels. In this way IPv6 gains considerable independence of the underlying wide area network and can step over many hops of IPv4 subnets. The abbreviated name of this mechanism is 6to4 (not to be confused with [6OVER4]). The 6to4 mechanism is typically implemented almost entirely in border routers, without specific host modifications except a suggested address selection default. Only a modest amount of router configuration is required.

Sections 2 to 4 of this document specify the 6to4 scheme technically. Section 5 discusses some, but not all, usage scenarios, including routing aspects, for 6to4 sites. Scenarios for isolated 6to4 hosts are not discussed in this document. Sections 6 to 9 discuss other general considerations.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

1.1. Terminology

The terminology of [IPV6] applies to this document.

6to4 pseudo-interface:

6to4 encapsulation of IPv6 packets inside IPv4 packets occurs at a point that is logically equivalent to an IPv6 interface, with the link layer being the IPv4 unicast network. This point is referred to as a pseudo-interface. Some implementors may treat it exactly like any other interface and others may treat it like a tunnel end-point.

6to4 prefix:

an IPv6 prefix constructed according to the rule in Section 2 below.

   6to4 address:  an IPv6 address constructed using a 6to4 prefix.
   
   Native IPv6 address:  an IPv6 address constructed using another type
         of prefix than 6to4.

6to4 router (or 6to4 border router):

an IPv6 router supporting a 6to4 pseudo-interface. It is normally the border router between an IPv6 site and a wide-area IPv4 network.

6to4 host:

an IPv6 host which happens to have at least one 6to4 address. In all other respects it is a standard IPv6 host.

Note: an IPv6 node may in some cases use a 6to4 address for a configured tunnel. Such a node may function as an IPv6 host using a 6to4 address on its configured tunnel interface, and it may also serve as a IPv6 router for other hosts via a 6to4 pseudo-interface, but these are distinct functions.

6to4 site:

a site running IPv6 internally using 6to4 addresses, therefore containing at least one 6to4 host and at least one 6to4 router.

Relay router:

a 6to4 router configured to support transit routing between 6to4 addresses and native IPv6 addresses.

6to4 exterior routing domain:

a routing domain interconnecting a set of 6to4 routers and relay routers. It is distinct from an IPv6 site's interior routing domain, and distinct from all native IPv6 exterior routing domains.

2. IPv6 Prefix Allocation

Suppose that a subscriber site has at least one valid, globally unique 32-bit IPv4 address, referred to in this document as V4ADDR. This address MUST be duly allocated to the site by an address registry (possibly via a service provider) and it MUST NOT be a private address [RFC 1918].

The IANA has permanently assigned one 13-bit IPv6 Top Level Aggregator (TLA) identifier under the IPv6 Format Prefix 001 [AARCH, AGGR] for the 6to4 scheme.Its numeric value is 0x0002, i.e., it is 2002::/16 when expressed as an IPv6 address prefix.

The subscriber site is then deemed to have the following IPv6 address prefix, without any further assignment procedures being necessary:

Prefix length: 48 bits
Format prefix: 001
TLA value: 0x0002
NLA value: V4ADDR

This is illustrated as follows:

     | 3 |  13  |    32     |   16   |          64 bits               |
     +---+------+-----------+--------+--------------------------------+
     |FP | TLA  | V4ADDR    | SLA ID |         Interface ID           |
     |001|0x0002|           |        |                                |
     +---+------+-----------+--------+--------------------------------+

Thus, this prefix has exactly the same format as normal /48 prefixes assigned according to [AGGR]. It can be abbreviated as 2002:V4ADDR::/48. Within the subscriber site it can be used exactly like any other valid IPv6 prefix, e.g., for automated address assignment and discovery according to the normal mechanisms such as [CONF, DISC], for native IPv6 routing, or for the "6over4" mechanism [6OVER4].

Note that if the IPv4 address is assigned dynamically, the corresponding IPv6 prefix will also be dynamic in nature, with the same lifetime.

2.1 Address Selection

To ensure the correct operation of 6to4 in complex topologies, source and destination address selection must be appropriately implemented. If the source IPv6 host sending a packet has at least one 2002:: address assigned to it, and if the set of IPv6 addresses returned by the DNS for the destination host contains at least one 2002:: address, then the source host must make an appropriate choice of the source and destination addresses to be used. The mechanisms for address selection in general are under study at the time of writing [SELECT]. Subject to those general mechanisms, the principle that will normally allow correct operation of 6to4 is this:

If one host has only a 6to4 address, and the other one has both a 6to4 and a native IPv6 address, then the 6to4 address should be used for both.

If both hosts have a 6to4 address and a native IPv6 address, then either the 6to4 address should be used for both, or the native IPv6 address should be used for both. The choice should be configurable. The default configuration should be native IPv6 for both.

3. Encapsulation in IPv4

IPv6 packets from a 6to4 site are encapsulated in IPv4 packets when they leave the site via its external IPv4 connection. Note that the IPv4 interface that is carrying the 6to4 traffic is notionally equivalent to an IPv6 interface, and is referred to below as a pseudo-interface, although this phrase is not intended to define an implementation technique. V4ADDR MUST be configured on the IPv4 interface.

IPv6 packets are transmitted in IPv4 packets [RFC 791] with an IPv4 protocol type of 41, the same as has been assigned [MECH] for IPv6 packets that are tunneled inside of IPv4 frames. The IPv4 header contains the Destination and Source IPv4 addresses. One or both of these will be identical to the V4ADDR field of an IPv6 prefix formed as specified above (see section 5 for more details). The IPv4 packet body contains the IPv6 header and payload.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Version|  IHL  |Type of Service|          Total Length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Identification        |Flags|      Fragment Offset    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Time to Live | Protocol 41   |         Header Checksum       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Source Address                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Destination Address                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Options                    |    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            IPv6 header and payload ...              /
     +-------+-------+-------+-------+-------+------+------+

The IPv4 Time to Live will be set as normal [RFC 791], as will the encapsulated IPv6 hop limit [IPv6]. Other considerations are as described in Section 4.1.2 of [MECH].

3.1. Link-Local Address and NUD

The link-local address of a 6to4 pseudo-interface performing 6to4 encapsulation would, if needed, be formed as described in Section 3.7 of [MECH]. However, no scenario is known in which such an address would be useful, since a peer 6to4 gateway cannot determine the appropriate link-layer (IPv4) address to send to.

Neighbor Unreachability Detection (NUD) is handled as described in Section 3.8 of [MECH].

4. Maximum Transmission Unit

MTU size considerations are as described for tunnels in [MECH].

If the IPv6 MTU size proves to be too large for some intermediate IPv4 subnet, IPv4 fragmentation will ensue. While undesirable, this is not necessarily disastrous, unless the fragments are delivered to different IPv4 destinations due to some form of IPv4 anycast. The IPv4 "do not fragment" bit SHOULD NOT be set in the encapsulating IPv4 header.

5. Unicast scenarios, scaling, and transition to normal prefixes

5.1 Simple scenario - all sites work the same

The simplest deployment scenario for 6to4 is to use it between a number of sites, each of which has at least one connection to a shared IPv4 Internet. This could be the global Internet, or it could be a corporate IP network. In the case of the global Internet, there is no requirement that the sites all connect to the same Internet service provider. The only requirement is that any of the sites is able to send IPv4 packets with protocol type 41 to any of the others. By definition, each site has an IPv6 prefix in the format defined in Section 2. It will therefore create DNS records for these addresses. For example, site A which owns IPv4 address 192.1.2.3 will create DNS records with the IPv6 prefix {FP=001,TLA=0x0002,NLA=192.1.2.3}/48 (i.e., 2002:c001:0203::/48). Site B which owns address 9.254.253.252 will create DNS records with the IPv6 prefix {FP=001,TLA=0x0002,NLA=9.254.253.252}/48 (i.e., 2002:09fe:fdfc::/48).

When an IPv6 host on site B queries the DNS entry for a host on site A, or otherwise obtains its address, it obtains an address with the prefix {FP=001,TLA=0x0002,NLA=192.1.2.3}/48 and whatever SLA and Interface ID applies. The converse applies when a host on site A queries the DNS for a host on site B. IPv6 packets are formed and transmitted in the normal way within both sites.

                            _______________________________
                           |                               |
                           |  Wide Area IPv4 Network       |
                           |_______________________________|
                                  /                    \
                        192.1.2.3/         9.254.253.252\
 _______________________________/_   ____________________\____________
|                              /  | |                     \           |
|IPv4 Site A          ##########  | |IPv4 Site B          ##########  |
| ____________________# 6to4   #_ | | ____________________# 6to4   #_ |
||                    # router # || ||                    # router # ||
||IPv6 Site A         ########## || ||IPv6 Site B         ########## ||
||2002:c001:0203::/48            || ||2002:09fe:fdfc::/48            ||
||_______________________________|| ||_______________________________||
|                                 | |                                 |
|_________________________________| |_________________________________|

Within a 6to4 site, addresses with the 2002::/16 prefix, apart from those with the local 2002:V4ADDR::/48 prefix, will be handled like any other non-local IPv6 address, i.e., by a default or explicit route towards the 6to4 border router.

When an outgoing packet reaches the 6to4 router, it is encapsulated as defined in Section 3, according to the additional sending rule defined in Section 5.3. Incoming packets are decapsulated according to the additional decapsulation rule defined in Section 5.3. The additional sending and decapsulation rules are the only changes to IPv6 forwarding, and they occur only at border routers. No IPv4 routing information is imported into IPv6 routing (nor vice versa).

In this scenario, any number of 6to4 sites can interoperate with no tunnel configuration, and no special requirements from the IPv4 service. All that is required is the appropriate DNS entries and the additional sending and decapsulation rules configured in the 6to4 router. This router SHOULD also generate the appropriate IPv6 prefix announcements [CONF, DISC].

Although site A and site B will each need to run IPv6 routing internally, they do not need to run an IPv6 exterior routing protocol in this simple scenario; IPv4 exterior routing does the job for them.

It is RECOMMENDED that in any case each site should use only one IPv4 address per 6to4 router, and that should be the address assigned to the external interface of the 6to4 router. Single-homed sites therefore SHOULD use only one IPv4 address for 6to4 routing. Multi- homed sites are discussed briefly in section 5.6.

Because of the lack of configuration, and the distributed deployment model, there are believed to be no particular scaling issues with the basic 6to4 mechanism apart from encapsulation overhead. Specifically, it introduces no new entries in IPv4 routing tables.

5.2 Mixed scenario with relay to native IPv6

During the transition to IPv6 we can expect some sites to fit the model just described (isolated sites whose only connectivity is the IPv4 Internet), whereas others will be part of larger islands of native or tunneled IPv6 using normal IPv6 TLA address space. The 6to4 sites will need connectivity to these native IPv6 islands and vice versa. In the 6to4 model, this connectivity is accomplished by IPv6 routers which possess both 6to4 and native IPv6 addresses. Although they behave essentially as standard IPv6 routers, for the purposes of this document they are referred to as relay routers to distinguish them from routers supporting only 6to4, or only native IPv6.

There must be at least one router acting as a relay between the 6to4 domain and a given native IPv6 domain. There is nothing special about it; it is simply a normal router which happens to have at least one logical 6to4 pseudo-interface and at least one other IPv6 interface. Since it is a 6to4 router, it implements the additional sending and decapsulation rules defined in Section 5.3.

We now have three distinct classes of routing domain to consider:

      1.  the internal IPv6 routing domain of each 6to4 site;
      2.  an exterior IPv6 routing domain interconnecting
          a given set of 6to4 border routers, including relay routers,
          among themselves, i.e., a 6to4 exterior routing domain;
      3.  the exterior IPv6 routing domain of each native IPv6 island.

1. The internal routing domain of a 6to4 site behaves as described in section 5.1.

2. There are two deployment options for a 6to4 exterior routing domain:

   2.1 No IPv6 exterior routing protocol is used.  The 6to4 routers
   using a given relay router each have a default IPv6 route pointing to
   the relay router.  The relay router MAY apply source address based
   filters to accept traffic only from  specific 6to4 routers.

2.2 An IPv6 exterior routing protocol is used. The set of 6to4 routers using a given relay router obtain native IPv6 routes from the relay router using a routing protocol such as BGP4+ [RFC 2283, BGP4+]. The relay router will advertise whatever native IPv6 routing prefixes are appropriate on its 6to4 pseudo-interface. These prefixes will indicate the regions of native IPv6 topology that the relay router is willing to relay to. Their choice is a matter of routing policy. It is necessary for network operators to carefully consider desirable traffic patterns and topology when choosing the scope of such routing advertisements. The relay router will establish BGP peering only with specific 6to4 routers whose traffic it is willing to accept.

Although this solution is more complex, it provides effective policy control, i.e., BGP4+ policy determines which 6to4 routers are able to use which relay router.

3. A relay router MUST advertise a route to 2002::/16 into the native IPv6 exterior routing domain. It is a matter of routing policy how far this routing advertisement of 2002::/16 is propagated in the native IPv6 routing system. Since there will in general be multiple relay routers advertising it, network operators will require to filter it in a managed way. Incorrect policy in this area will lead to potential unreachability or to perverse traffic patterns.

6to4 prefixes more specific than 2002::/16 must not be propagated in native IPv6 routing, to prevent pollution of the IPv6 routing table by elements of the IPv4 routing table. Therefore, a 6to4 site which also has a native IPv6 connection MUST NOT advertise its 2002::/48 routing prefix on that connection, and all native IPv6 network operators MUST filter out and discard any 2002:: routing prefix advertisements longer than /16.

Sites which have at least one native IPv6 connection, in addition to a 6to4 connection, will therefore have at least one IPv6 prefix which is not a 2002:: prefix. Such sites' DNS entries will reflect this and DNS lookups will return multiple addresses. If two such sites need to interoperate, whether the 6to4 route or the native route will be used depends on IPv6 address selection by the individual hosts (or even applications).

Now consider again the example of the previous section. Suppose an IPv6 host on site B queries the DNS entry for a host on site A, and the DNS returns multiple IPv6 addresses with different prefixes.

            ____________________________         ______________________
           |                            |       |                      |
           |  Wide Area IPv4 Network    |       |   Native IPv6        |
           |                            |       |   Wide Area Network  |
           |____________________________|       |______________________|
                /                    \             //
      192.1.2.3/         9.254.253.252\           // 2001:0600::/48
  ____________/_   ____________________\_________//_
             /  | |                     \       //  |
    ##########  | |IPv4 Site B          ##########  |
  __# 6to4   #_ | | ____________________# 6to4   #_ |
    # router # || ||                    # router # ||
    ########## || ||IPv6 Site B         ########## ||
               || ||2002:09fe:fdfc::/48            ||
  __Site A_____|| ||2001:0600::/48_________________||
    as before   | |                                 |
  ______________| |_________________________________|

If the host picks the 6to4 prefix according to some rule for multiple prefixes, it will simply send packets to an IPv6 address formed with the prefix {FP=001,TLA=0x0002,NLA=192.1.2.3}/48. It is essential that they are sourced from the prefix {FP=001,TLA=0x0002,NLA=9.254.253.252}/48 for two-way connectivity to be possible. The address selection mechanism of Section 2.1 will ensure this.

5.2.1 Variant scenario with ISP relay

The previous scenario assumes that the relay router is provided by a cooperative 6to4 user site. A variant of this is for an Internet Service Provider, that already offers native IPv6 connectivity, to operate a relay router. Technically this is no different from the previous scenario; site B is simply an internal 6to4 site of the ISP, possibly containing only one system, i.e., the relay router itself.

5.2.2 Summary of relay router configuration

A relay router participates in IPv6 unicast routing protocols on its native IPv6 interface and may do so on its 6to4 pseudo-interface, but these are independent routing domains with separate policies, even if the same protocol, probably BGP4+, is used in both cases.

A relay router also participates in IPv4 unicast routing protocols on its IPv4 interface used to support 6to4, but this is not further discussed here.

On its native IPv6 interface, the relay router MUST advertise a route to 2002::/16. It MUST NOT advertise a longer 2002:: routing prefix on that interface. Routing policy within the native IPv6 routing domain determines the scope of that advertisement, thereby limiting the visibility of the relay router in that domain.

IPv6 packets received by the relay router whose next hop IPv6 address matches 2002::/16 will be routed to its 6to4 pseudo-interface and treated according to the sending rule of Section 5.1.

5.2.2.1. BGP4+ not used

If BGP4+ is not deployed in the 6to4 exterior routing domain (option 2.1 of Section 5.2), the relay router will be configured to accept and relay all IPv6 traffic only from its client 6to4 sites. Each 6to4 router served by the relay router will be configured with a default IPv6 route to the relay router (for example, Site A's default IPv6 route ::/0 would point to the relay router's address under prefix 2002:09fe:fdfc::/48).

5.2.2.2. BGP4+ used

If BGP4+ is deployed in the 6to4 exterior routing domain (option 2.2 of Section 5.2), the relay router advertises IPv6 native routing prefixes on its 6to4 pseudo-interface, peering only with the 6to4 routers that it serves. (An alternative is that these routes could be advertised along with IPv4 routes using BGP4 over IPv4, rather than by running a separate BGP4+ session.) The specific routes advertised depend on applicable routing policy, but they must be chosen from among those reachable through the relay router's native IPv6 interface. In the simplest case, a default route to the whole IPv6 address space could be advertised. When multiple relay routers are in use, more specific routing prefixes would be advertised according to the desired routing policy. The usage of BGP4+ is completely standard so is not discussed further in this document.

5.2.2.3. Relay router scaling

Relay routers introduce the potential for scaling issues. In general a relay router should not attempt to serve more sites than any other transit router, allowing for the encapsulation overhead.

5.2.3 Unwilling to relay

It may arise that a site has a router with both 6to4 pseudo- interfaces and native IPv6 interfaces, but is unwilling to act as a relay router. Such a site MUST NOT advertise any 2002:: routing prefix into the native IPv6 domain and MUST NOT advertise any native IPv6 routing prefixes or a default IPv6 route into the 6to4 domain. Within the 6to4 domain it will behave exactly as in the basic 6to4 scenario of Section 5.1.

5.3 Sending and decapsulation rules

The only change to standard IPv6 forwarding is that every 6to4 router (and only 6to4 routers) MUST implement the following additional sending and decapsulation rules.

In the sending rule, "next hop" refers to the next IPv6 node that the packet will be sent to, which is not necessarily the final destination, but rather the next IPv6 neighbor indicated by normal IPv6 routing mechanisms. If the final destination is a 6to4 address, it will be considered as the next hop for the purpose of this rule. If the final destination is not a 6to4 address, and is not local, the next hop indicated by routing will be the 6to4 address of a relay router.

ADDITIONAL SENDING RULE for 6to4 routers

if the next hop IPv6 address for an IPv6 packet

does match the prefix 2002::/16, and
does not match any prefix of the local site

then
apply any security checks (see Section 8); encapsulate the packet in IPv4 as in Section 3,

with IPv4 destination address = the NLA value V4ADDR

extracted from the next hop IPv6 address;

queue the packet for IPv4 forwarding.

A simple decapsulation rule for incoming IPv4 packets with protocol type 41 MUST be implemented:

ADDITIONAL DECAPSULATION RULE for 6to4 routers

apply any security checks (see Section 8);
remove the IPv4 header;
submit the packet to local IPv6 routing.

5.4 Variant scenario with tunnel to IPv6 space

A 6to4 site which has no IPv6 connections to the "native" IPv6 Internet can acquire effective connectivity to the v6 Internet via a "configured tunnel" (using the terminology in [MECH]) to a cooperating router which does have IPv6 access, but which does not need to be a 6to4 router. Such tunnels could be autoconfigured using an IPv4 anycast address, but this is outside of the scope of this document. Alternatively a tunnel broker can be used. This scenario would be suitable for a small user-managed site.

These mechanisms are not described in detail in this document.

5.5 Fragmented Scenarios

If there are multiple relay routers between native IPv6 and the 6to4 world, different parts of the 6to4 world will be served by different relays. The only complexity that this introduces is in the scoping of 2002::/16 routing advertisements within the native IPv6 world. Like any BGP4+ advertisements, their scope must be correctly defined by routing policy to ensure that traffic to 2002::/16 follows the intended paths.

If there are multiple IPv6 stubs all interconnected by 6to4 through the global IPv4 Internet, this is a simple generalization of the basic scenarios of sections 5.1. and 5.2 and no new issues arise. This is shown in the following figure. Subject to consistent configuration of routing advertisements, there are no known issues with this scenario.

                    ______________
                   |     AS3      |
                   |_IPv6 Network_| Both AS1 and AS2 advertise
                   | AS1  | AS2   | 2002::/16, but only one of
                   |______|_______| them reaches AS3.
                    //          \\
         __________//_          _\\__________         ______________
        | 6to4 Relay1 |        | 6to4 Relay2 |       | IPv6 Network |
        |_____________|        |_____________|       |    AS4       |
               |                      |              |______________|
       ________|______________________|________             |
      |                                        |      ______|______
      |       Global IPv4 Network              |-----| 6to4 Relay3 |
      |________________________________________|     |_____________|
         |          |            |          |
     ____|___    ___|____    ____|___    ___|____
    |  6to4  |  |  6to4  |  |  6to4  |  |  6to4  |
    | Site A |  | Site B |  | Site C |  | Site D |
    |________|  |________|  |________|  |________|

If multiple IPv6 stubs are interconnected through multiple, disjoint IPv4 networks (i.e., a fragmented IPv4 world) then the 6to4 world is also fragmented; this is the one scenario that must be avoided. It is illustrated below to show why it does not work, since the 2002::/16 advertisement from Relay1 will be invisible to Relay2, and vice versa. Sites A and B therefore have no connectivity to sites C and D.

                    ______________
                   |     AS3      |
                   |_IPv6 Network_| Both AS1 and AS2 advertise
                   | AS1  | AS2   | 2002::/16, but sites A and B
                   |______|_______| cannot reach C and D.
                    //          \\
         __________//_          _\\__________
        | 6to4 Relay1 |        | 6to4 Relay2 |
        |_____________|        |_____________|
               |                      |
       ________|_______        _______|________
      | IPv4 Network   |      | IPv4 Network   |
      | Segment 1      |      | Segment 2      |
      |________________|      |________________|
         |          |            |          |
     ____|___    ___|____    ____|___    ___|____
    |  6to4  |  |  6to4  |  |  6to4  |  |  6to4  |
    | Site A |  | Site B |  | Site C |  | Site D |
    |________|  |________|  |________|  |________|

5.6 Multihoming

Sites which are multihomed on IPv4 MAY extend the 6to4 scenario by using a 2002:: prefix for each IPv4 border router, thereby obtaining a simple form of IPv6 multihoming by using multiple simultaneous IPv6 prefixes and multiple simultaneous relay routers.

5.7 Transition Considerations

If the above rules for routing advertisements and address selection are followed, then a site can migrate from using 6to4 to using native IPv6 connections over a long period of co-existence, with no need to stop 6to4 until it has ceased to be used. The stages involved are

1. Run IPv6 on site using any suitable implementation. True native IPv6, [6OVER4], or tunnels are all acceptable.

2. Configure a border router (or router plus IPv4 NAT) connected to the external IPv4 network to support 6to4, including advertising the appropriate 2002:: routing prefix locally. Configure IPv6 DNS entries using this prefix. At this point the 6to4 mechanism is automatically available, and the site has obtained a "free" IPv6 prefix.

3. Identify a 6to4 relay router willing to relay the site's traffic to the native IPv6 world. This could either be at another cooperative 6to4 site, or an ISP service. If no exterior routing protocol is in use in the 6to4 exterior routing domain, the site's 6to4 router will be configured with a default IPv6 route pointing to that relay router's 6to4 address. If an exterior routing protocol such as BGP4+ is in use, the site's 6to4 router will be configured to establish appropriate BGP peerings.

4. When native external IPv6 connectivity becomes available, add a second (native) IPv6 prefix to both the border router configuration and the DNS configuration. At this point, an address selection rule will determine when 6to4 and when native IPv6 will be used.

5. When 6to4 usage is determined to have ceased (which may be several years later), remove the 6to4 configuration.

5.8 Coexistence with firewall, NAT or RSIP

The 6to4 mechanisms appear to be unaffected by the presence of a firewall at the border router.

If the site concerned has very limited global IPv4 address space, and is running an IPv4 network address translator (NAT), all of the above mechanisms remain valid. The NAT box must also contain a fully functional IPv6 router including the 6to4 mechanism. The address used for V4ADDR will simply be a globally unique IPv4 address allocated to the NAT. In the example of Section 5.1 above, the 6to4 routers would also be the sites' IPv4 NATs, which would own the globally unique IPv4 addresses 192.1.2.3 and 9.254.253.252.

   Combining a 6to4 router with an IPv4 NAT in this way offers  the site
   concerned a globally unique IPv6 /48 prefix, automatically, behind
   the IPv4 address of the NAT.  Thus every host behind the NAT can
   become an IPv6 host with no need for additional address space
   allocation, and no intervention by the Internet service provider.  No
   address translation is needed by these IPv6 hosts.

A more complex situation arises if a host is more than one NAT hop away from the globally unique IPv4 address space, since only the outermost NAT has a unique IPv4 address. All IPv6 hosts in this situation must use addresses derived from the 2002: prefix constructed from the global IPv4 address of the outermost NAT. The IPv4 addresses of the inner NATs are not globally unique and play no part in the 6to4 mechanism, and 6to4 encapsulation and decapsulation can only take place at the outermost NAT.

   The Realm-Specific IP (RSIP) mechanism [RSIP] can also co-exist with
   6to4.  If a 6to4 border router is combined with an RSIP border
   router, it can support IPv6 hosts using 6to4 addresses, IPv4 hosts
   using RSIP, or dual stack hosts using both.  The RSIP function
   provides fine-grained management of dynamic global IPv4 address
   allocation and the 6to4 function provides a stable IPv6 global
   address to each host.  As with NAT, the IPv4 address used to
   construct the site's 2002:  prefix will be one of the global
   addresses of the RSIP border router.

5.9 Usage within Intranets

There is nothing to stop the above scenario being deployed within a private corporate network as part of its internal transition to IPv6; the corporate IPv4 backbone would serve as the virtual link layer for individual corporate sites using 2002:: prefixes. The V4ADDR MUST be a duly allocated global IPv4 address, which MUST be unique within the private network. The Intranet thereby obtains globally unique IPv6 addresses even if it is internally using private IPv4 addresses [RFC 1918].

5.10 Summary of impact on routing

   IGP (site) routing will treat the local site's 2002::/48  prefix
   exactly like a native IPv6 site prefix assigned to the local site.
   There will also be an IGP route to the generic 2002::/16 prefix,
   which will be a route to the site's 6to4 router, unless this is
   handled as a default route.

EGP (i.e., BGP) routing will include advertisements for the 2002::/16 prefix from relay routers into the native IPv6 domain, whose scope is limited by routing policy. This is the only non-native IPv6 prefix advertised by BGP.

It will be necessary for 6to4 routers to obtain routes to relay routers in order to access the native IPv6 domain. In the simplest case there will be a manually configured default IPv6 route to a relay router's address under the prefix {FP=001,TLA=0x0002,NLA=V4ADDR}/48, where V4ADDR is the IPv4 address of the relay router. Such a route could be used to establish a BGP session for the exchange of additional IPv6 routes.

By construction, unicast IPv6 traffic within a 6to4 domain will follow exactly the same path as unicast IPv4 traffic.

5.11. Routing loop prevention

Since 6to4 has no impact on IPv4 routing, it cannot induce routing loops in IPv4. Since 2002: prefixes behave exactly like standard IPv6 prefixes, they will not create any new mechanisms for routing loops in IPv6 unless misconfigured. One very dangerous misconfiguration would be an announcement of the 2002::/16 prefix into a 6to4 exterior routing domain, since this would attract all 6to4 traffic into the site making the announcement. Its 6to4 router would then resend non-local 6to4 traffic back out, forming a loop.

The 2002::/16 routing prefix may be legitimately advertised into the native IPv6 routing domain by a relay router, and into an IPv6 site's local IPv6 routing domain; hence there is a risk of misconfiguration causing it to be advertised into a 6to4 exterior routing domain.

To summarize, the 2002::/16 prefix MUST NOT be advertised to a 6to4 exterior routing domain.

6. Multicast and Anycast

It is not possible to assume the general availability of wide-area IPv4 multicast, so (unlike [6OVER4]) the 6to4 mechanism must assume only unicast capability in its underlying IPv4 carrier network. An IPv6 multicast routing protocol is needed [MULTI].

The allocated anycast address space [ANYCAST] is compatible with 2002:: prefixes, i.e., anycast addresses formed with such prefixes may be used inside a 6to4 site.

7. ICMP messages

ICMP "unreachable" and other messages returned by the IPv4 routing system will be returned to the 6to4 router that generated a encapsulated 2002:: packet. However, this router will often be unable to return an ICMPv6 message to the originating IPv6 node, due to the lack of sufficient information in the "unreachable" message. This means that the IPv4 network will appear as an undiagnosable link layer for IPv6 operational purposes. Other considerations are as described in Section 4.1.3 of [MECH].

8. IANA Considerations

No assignments by the IANA are required beyond the special TLA value 0x0002 already assigned.

9. Security Considerations

Implementors should be aware that, in addition to possible attacks against IPv6, security attacks against IPv4 must also be considered. Use of IP security at both IPv4 and IPv6 levels should nevertheless be avoided, for efficiency reasons. For example, if IPv6 is running encrypted, encryption of IPv4 would be redundant except if traffic analysis is felt to be a threat. If IPv6 is running authenticated, then authentication of IPv4 will add little. Conversely, IPv4 security will not protect IPv6 traffic once it leaves the 6to4 domain. Therefore, implementing IPv6 security is required even if IPv4 security is available.

By default, 6to4 traffic will be accepted and decapsulated from any source from which regular IPv4 traffic is accepted. If this is for any reason felt to be a security risk (for example, if IPv6 spoofing is felt to be more likely than IPv4 spoofing), then additional source address based packet filtering could be applied. A possible plausibility check is whether the encapsulating IPv4 address is consistent with the encapsulated 2002:: address. If this check is applied, exceptions to it must be configured to admit traffic from relay routers (Section 5). 2002:: traffic must also be excepted from checks applied to prevent spoofing of "6 over 4" traffic [6OVER4].

In any case, any 6to4 traffic whose source or destination address embeds a V4ADDR which is not in the format of a global unicast address MUST be silently discarded by both encapsulators and decapsulators. Specifically, this means that IPv4 addresses defined in [RFC 1918], broadcast, subnet broadcast, multicast and loopback addresses are unacceptable.

Acknowledgements

The basic idea presented above is probably not original, and we have had invaluable comments from Magnus Ahltorp, Harald Alvestrand, Jim Bound, Scott Bradner, Randy Bush, Matt Crawford, Richard Draves, Jun-ichiro itojun Hagino, Joel Halpern, Tony Hain, Andy Hazeltine, Bob Hinden, Geoff Huston, Perry Metzger, Thomas Narten, Erik Nordmark, Markku Savela, Ole Troan, Sowmini Varadhan, members of the Compaq IPv6 engineering team, and other members of the NGTRANS working group. Some text has been copied from [6OVER4]. George Tsirtsis kindly drafted two of the diagrams.

References

   [AARCH]    Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.
   
   [AGGR]     Hinden., R, O'Dell, M. and S. Deering, "An IPv6
              Aggregatable Global Unicast Address Format", RFC 2374,
              July 1998.
   
   [API]      Gilligan, R., Thomson, S., Bound, J. and W. Stevens,
              "Basic Socket Interface Extensions for IPv6", RFC 2553,
              March 1999.
   
   [BGP4+]    Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
              Extensions for IPv6 Inter-Domain Routing", RFC 2545, March
              1999.
   
   [CONF]     Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.
   
   [DISC]     Narten, T., Nordmark, E. and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461, December
              1998.
   
   [IPV6]     Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.
   
   [6OVER4]   Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529, March 1999.
   
   [ANYCAST]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", Work in Progress.
   
   [MULTI]    Thaler, D., "Support for Multicast over 6to4 Networks",
              Work in Progress.
   
   [SCALE]    Hain, T., "6to4-relay discovery and scaling", Work in
              Progress.
   
   [SELECT]   Draves, R., "Default Address Selection for IPv6", Work in
              Progress.
   
   [RFC 791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

[RFC 1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., de Groot, G.

and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996.

   [MECH]     Gilligan, R. and E. Nordmark, "Transition Mechanisms for
              IPv6 Hosts and Routers", RFC 2893, August 2000.
   
   [RSIP]     Borella, M., Grabelsky, D., Lo, J. and K. Tuniguchi,
              "Realm Specific IP: Protocol Specification", Work in
              Progress.
   
   [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC 2283] Bates, T., Chandra, R., Katz, D. and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 2283, February
              1998.

Authors' Addresses

Brian E. Carpenter
IBM
iCAIR, Suite 150
1890 Maple Avenue
Evanston IL 60201, USA

EMail:

          brian@icair.org
   
   Keith Moore
   UT Computer Science Department
   1122 Volunteer Blvd, Ste 203
   Knoxville, TN  37996-3450
   USA
   
   EMail: moore@cs.utk.edu

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