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IETF RFC 6751
Last modified on Monday, October 15th, 2012
 Permanent link to RFC 6751
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Independent Submission R. Despres, Ed.
Request for Comments: 6751 RD-IPtech
Category: Experimental B. Carpenter
ISSN: 2070-1721 Univ. of Auckland
D. Wing
Cisco
S. Jiang
Huawei Technologies Co., Ltd.
October 2012
Native IPv6 behind IPv4-to-IPv4 NAT Customer Premises Equipment (6a44)
 Abstract
In customer sites having IPv4-only Customer Premises Equipment (CPE),
Teredo (RFC 4380, RFC 5991, RFC 6081) provides last-resort IPv6
connectivity. However, because it is designed to work without the
involvement of Internet Service Providers, it has significant
limitations (connectivity between IPv6 native addresses and Teredo
addresses is uncertain; connectivity between Teredo addresses fails
for some combinations of NAT types). 6a44 is a complementary
solution that, being based on ISP cooperation, avoids these
limitations. At the beginning of 6a44 IPv6 addresses, it replaces
the Teredo well-known prefix, present at the beginning of Teredo IPv6
addresses, with network-specific /48 prefixes assigned by local ISPs
(an evolution similar to that from 6to4 to 6rd (IPv6 Rapid Deployment
on IPv4 Infrastructures)). The specification is expected to be
complete enough for running code to be independently written and the
solution to be incrementally deployed and used.
Despres, et al. Experimental PAGE 1 
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see 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/RFC 6751.
Copyright Notice
Copyright (c) 2012 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.
Despres, et al. Experimental PAGE 2
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
Table of Contents
1. Introduction ....................................................3
2. Requirements Language ...........................................5
3. Definitions .....................................................5
4. Design Goals, Requirements, and Model of Operation ..............7
4.1. Hypotheses about NAT Behavior ..............................7
4.2. Native IPv6 Connectivity for Unmanaged Hosts behind
NAT44s .....................................................7
4.3. Operational Requirements ...................................8
4.4. Model of Operation .........................................9
5. 6a44 Addresses .................................................12
6. Specification of Clients and Relays ............................14
6.1. Packet Formats ............................................14
6.2. IPv6 Packet Encapsulations ................................14
6.3. 6a44 Bubbles ..............................................14
6.4. MTU Considerations ........................................16
6.5. 6a44 Client Specification .................................16
6.5.1. Tunnel Maintenance .................................16
6.5.2. Client Transmission ................................19
6.5.3. Client Reception ...................................20
6.6. 6a44 Relay Specification ..................................23
6.6.1. Relay Reception in IPv6 ............................23
6.6.2. Relay Reception in IPv4 ............................24
6.7. Implementation of Automatic Sunset ........................26
7. Security Considerations ........................................26
8. IANA Considerations ............................................30
9. Acknowledgments ................................................30
10. References ....................................................30
10.1. Normative References .....................................30
10.2. Informative References ...................................31
1. Introduction
Although most Customer Premises Equipment (CPE) should soon be dual-
stack capable, a large installed base of IPv4-only CPEs is likely to
remain for several years. Their operation is based on IPv4-to-IPv4
NATs (NAT44s). Also, due to the IPv4 address shortage, more and more
Internet Service Providers (ISPs), and more and more mobile
operators, will assign private IPv4 addresses ([RFC 1918]) to their
customers (the [NAT444] model). For rapid and extensive use of IPv6
[RFC 2460], there is therefore a need for IPv6 connectivity behind
NAT44s, including those of the [NAT444] model.
Despres, et al. Experimental PAGE 3
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
At the moment, there are two tunneling techniques specified for IPv6
connectivity behind NAT44s:
o Configured tunnels. These involve tunnel brokers with which users
must register [RFC 3053]. Well-known examples include deployments
of the Hexago tool, and the SixXS collaboration, which are
suitable for IPv6 early trials. However, this approach is not
adequate for mass deployment: it imposes the restriction that even
if two hosts are in the same customer site, IPv6 packets between
them must transit via tunnel servers, which may be far away.
o Automatic Teredo tunnels [RFC 4380] [RFC 5991]. Teredo is specified
as a last-resort solution that, due to its objective to work
without local ISP involvement, has the following limitations:
* Connectivity between IPv6 native addresses and Teredo addresses
is uncertain. (As explained in [RFC 4380] Section 8.3, this
connectivity depends on paths being available from all IPv6
native addresses to some Teredo relays. ISPs lack sufficient
motivations to ensure it.)
* Between two Teredo addresses, IPv6 connectivity fails for some
combinations of NAT44 types ([RFC 6081] Section 3).
* According to [RFC 4380] Section 5.2, each Teredo host has to be
configured with the IPv4 address of a Teredo server (a
constraint that can, however, be avoided in some
implementations).
6a44 is designed to avoid Teredo limitations: with 6a44, ISPs can
participate in the solution. The approach for this is similar to the
approach that permitted 6rd [RFC 5569] [RFC 5969] to avoid the
limitations of 6to4 [RFC 3056] [RFC 3068]: at the beginning of IPv6
addresses, the Teredo well-known prefix is replaced by network-
specific prefixes assigned by local ISPs.
This document is organized as follows: terms used in the document are
defined in Section 3; design goals and model of operation are
presented in Section 4; Section 5 describes the format of 6a44 IPv6
addresses; Section 6 specifies in detail the behaviors of 6a44
clients and 6a44 relays; security and IANA considerations are covered
in Sections 7 and 8, respectively.
This specification is expected to be complete enough for running code
to be independently written and the solution to be incrementally
deployed and used. Its status is Experimental rather than Standards
Track, to reflect uncertainty as to which major Internet players may
be willing to support it.
Despres, et al. Experimental PAGE 4
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
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].
3. Definitions
The following definitions are used in this document:
MAJOR NEW DEFINITIONS
6a44 ISP network: An IPv4-capable ISP network that supports at least
one 6a44 relay. Additional conditions are that it assigns
individual IPv4 addresses to its customer sites (global or
private), that it supports ingress filtering [RFC 2827], and that
its path MTUs are at least 1308 octets.
6a44 relay: A node that supports the 6a44 relay function defined in
this document and that has interfaces to an IPv6-capable upstream
network and to an IPv4-capable downstream network.
6a44 client: A host that supports the 6a44 client function defined
in this document and has no means other than 6a44 to have an IPv6
native address.
6a44 tunnel: A tunnel established and maintained between a 6a44
client and 6a44 relays of its ISP network.
6a44 bubble: A UDP/IPv4 packet sent from a 6a44 client to the
6a44-relay address, or vice versa, and having a UDP payload that
cannot be confused with an IPv6 packet. In the client-to-relay
direction, it is a request for a response bubble. In the relay-
to-client direction, it conveys the up-to-date IPv6 prefix of the
client.
SECONDARY NEW DEFINITIONS
(This list is for reference and can be skipped by readers familiar
with the usual terminology.)
6a44 service: The service offered by a 6a44 ISP network to its 6a44
clients.
6a44-client IPv6 address: The IPv6 address of a 6a44 client. It is
composed of the client IPv6 prefix, received from a 6a44 relay,
followed by the client local IPv4 address.
Despres, et al. Experimental PAGE 5
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
6a44-client IPv6 prefix: For a 6a44 client, the IPv6 prefix (/96)
composed of the IPv6 prefix of the local 6a44 network (/48)
followed by the UDP/IPv4 mapped address of the client (32 +
16 bits).
6a44-client UDP/IPv4 mapped address: For a 6a44 client, the external
UDP/IPv4 address that, in the CPE NAT44 of the site, is that of
its 6a44 tunnel.
6a44-client UDP/IPv4 local address: For a 6a44 client, the
combination of its local IPv4 address and the 6a44 port.
6a44 port: UDP port 1027, reserved by IANA for 6a44 (see Section 8).
6a44-relay UDP/IPv4 address: The UDP/IPv4 address composed of the
6a44-relay anycast address and the 6a44 port.
6a44-relay anycast address: IPv4 anycast address 192.88.99.2,
reserved by IANA for 6a44 (see Section 8).
6a44-network IPv6 prefix: An IPv6 /48 prefix assigned by an ISP to a
6a44 network.
USUAL DEFINITIONS
(This list is for reference and can be skipped by readers familiar
with the usual terminology.)
Upstream direction: For a network border node, the direction toward
the Internet core.
Downstream direction: For a network border node, the direction
toward end-user nodes (opposite to the upstream direction).
IPv4 private address: An address that starts with one of the three
[RFC 1918] prefixes (10/8, 172.16/12, or 192.168/16).
IPv6 native address: An IPv6 global unicast address that starts with
an aggregatable prefix assigned to an ISP.
UDP/IPv4 address: The combination of an IPv4 address and a UDP port.
UDP/IPv4 packet: A UDP datagram contained in an IPv4 packet.
IPv6/UDP/IPv4 packet: An IPv6 packet contained in a UDP/IPv4 packet.
Despres, et al. Experimental PAGE 6
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
4. Design Goals, Requirements, and Model of Operation
4.1. Hypotheses about NAT Behavior
6a44 is designed to work with NAT44 behaviors identified in Section 3
of [RFC 6081]. In particular, it has to work with endpoint-dependent
mappings as well as with endpoint-independent mappings, including
cases where there are dynamic changes from one mode to the other.
The only assumption is that, after a mapping has been established in
the NAT44, it is maintained as long as it is reused at least once, in
each direction, every 30 seconds.
NOTE: 30 seconds is the value used for the same mapping-maintenance
purpose in Teredo [RFC 4380] and in SIP [RFC 5626].
4.2. Native IPv6 Connectivity for Unmanaged Hosts behind NAT44s
The objective remains that, as soon as possible, CPEs and ISPs
support IPv6 native prefixes. 6a44 is therefore designed only as a
temporary solution for hosts to obtain IPv6 native addresses in sites
whose CPEs are not IPv6 capable yet.
As noted in Section 1, IPv6 native addresses obtainable with
configured tunnels have important limitations. However, compared to
6a44 addresses, they have the advantage of remaining unchanged in the
case of NAT44 reset. 6a44 therefore remains the last-resort solution
for IPv6 native addresses in unmanaged hosts of IPv4-only-CPE sites,
while configured tunnels may still be preferred for some managed
hosts if reported limitations of configured tunnels are judged to be
acceptable. As their scopes are different, the two solutions can
usefully coexist.
Note that Teredo remains a last-resort solution for hosts to have
IPv6 addresses where IPv6 native addresses cannot be made available
(and where Teredo limitations are judged to be acceptable).
Despres, et al. Experimental PAGE 7
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
4.3. Operational Requirements
Operational requirements of 6a44 include the following:
Robust IPv6 connectivity: A node having a 6a44 address must have
paths across the Internet to and from all IPv6 native addresses
that are not subject to voluntary firewall filtering.
Intra-site path efficiency: Packets exchanged between 6a44 clients
that are behind the same CPE NAT44 must not have to traverse it.
If these clients have IPv4 connectivity using their private IPv4
addresses, they must also have IPv6 connectivity using their 6a44
addresses.
Plug-and-play operation of 6a44 clients: In order to obtain a 6a44
address from its local ISP, a 6a44 client must need no parameter
configuration.
Scalability of ISP functions: For the solution to be easily
scalable, ISP-supported functions have to be completely stateless.
Anti-spoofing protection: Where address anti-spoofing is ensured in
IPv4 with ingress filtering [RFC 2827] [RFC 3704], IPv6 addresses
must benefit from the same degree of anti-spoofing protection.
Overall operational simplicity: To paraphrase what Antoine de Saint-
Exupery said in [TheTool], "it seems that perfection is attained
not when there is nothing more to add, but when there is nothing
more to remove".
Incremental deployability: Hosts and ISP networks must be able to
become 6a44 capable independently of each other. IPv6 must be
operational where both are available, and there must be no
perceptible effect where they are not both available.
Despres, et al. Experimental PAGE 8
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
4.4. Model of Operation
Operation of 6a44 involves two types of nodes: 6a44 clients and 6a44
relays. Figure 1 shows the two applicability scenarios:
o In the first one, IPv4 addresses assigned to customer sites are
global IPv4.
o In the second one, they are private IPv4 addresses (the [NAT444]
model, where ISPs operate one or several NAT44s, also called
Carrier-Grade NATs (CGNs)).
(A) GLOBAL IPv4 ISP NETWORK
+------------------+
6a44 customer network(s) |GLOBAL IPv4 | Upstream
+-----------+ ---| MTU >= 1308 +--- IPv4 network
---| Private | | ingress filtering| (<== no route
+----+ | IPv4 +-----+ | IPv6 optional | to 6a44 relays)
| |-----| |NAT44|----+ |
+----+ | +-----+ | +-------------+
6a44 ---|MTU >= 1308| | --+6a44 relay(s)|--- Upstream
client(s) | no | ---| +-------------+ IPv6 network
|native IPv6| | |
+-----------+ +------------------+
(B) PRIVATE IPv4 ISP NETWORK
+------------------+
|PRIVATE IPv4 |
| as above |
---| |
| +--------------+
| --+ ISP NAT44(s) |--- Upstream
as above ----+ +--------------+ IPv4 network
| |
| +--------------+
---| --+6a44 relay(s) |--- Upstream
| +--------------+ IPv6 network
| |
+------------------+
Figure 1: 6a44 Applicability Scenarios
Despres, et al. Experimental PAGE 9
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
In both configurations, the ISP network may also assign IPv6 prefixes
to customer sites:
o If customer sites are only assigned IPv4 addresses (IPv6 prefix
available neither natively nor with any tunnel), 6a44 applies not
only to sites whose CPEs are IPv4-only capable but also to those
whose CPEs are dual-stack capable.
o If customer sites are assigned both IPv4 addresses and IPv6
prefixes, 6a44 only applies to sites whose CPEs are IPv4-only
capable.
Figure 2 illustrates paths of IPv6 packets between a 6a44 client, A,
and various possible locations of remote hosts (E in the same site, F
in another 6a44 site of the same ISP, G in a non-6a44 IPv6 site of
the same ISP, D in an IPv6 site of another ISP). Between 6a44
clients of a same site, IPv6 packets are encapsulated in IPv4
packets. Those between 6a44 clients and 6a44 relays are encapsulated
in UDP/IPv4 packets.
6a44 operates as follows (details in Section 6):
1. A 6a44 client starts operation by sending a 6a44 bubble to the
6a44-relay UDP/IPv4 address.
2. When a 6a44 relay receives a bubble from one of its 6a44
clients, it returns to this client a bubble containing the IPv6
prefix of this client.
3. When a 6a44 client receives a bubble from a 6a44 relay, it
updates (or confirms) its 6a44 address. It is an update if the
client has no IPv6 address yet or if, due to a CPE reset, this
address has changed. After receiving a bubble, a client is
ready to start, or to continue, IPv6 operation.
4. When a 6a44 client having a 6a44 address has an IPv6 packet to
send whose destination IS in the same customer site, it
encapsulates it in an IPv4 packet whose destination is found in
the IPv6 destination address. It then sends the resulting IPv6/
IPv4 packet.
5. When a 6a44 client receives a valid IPv6/IPv4 packet from a 6a44
client of the same site, it decapsulates the IPv6 packet and
submits it to further IPv6 processing.
Despres, et al. Experimental PAGE 10
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
6. When a 6a44 client having a 6a44 address has an IPv6 packet to
send whose destination IS NOT in the same customer site, it
encapsulates the packet in a UDP/IPv4 packet whose destination
is the 6a44-relay UDP/IPv4 address. It then sends the IPv6/UDP/
IPv4 packet.
7. When a 6a44 relay receives via its IPv4 interface a valid IPv6/
UDP/IPv4 packet whose destination IS one of its 6a44 clients, it
forwards the contained IPv6 packet in a modified IPv6/UDP/IPv4
packet. The UDP/IPv4 destination of this packet is found in the
IPv6 destination address.
8. When a 6a44 client receives a valid IPv6/UDP/IPv4 packet from a
6a44 relay, it decapsulates the IPv6 packet and submits it to
further IPv6 processing.
9. When a 6a44 relay receives via its IPv4 interface a valid IPv6/
UDP/IPv4 packet whose IPv6 destination IS NOT one of its 6a44
clients, it decapsulates the IPv6 packet and sends it via its
IPv6 interface.
10. When a 6a44 relay receives via its IPv6 interface a valid IPv6
packet whose destination is one of its 6a44 clients, it
encapsulates the packet in a UDP/IPv4 packet whose destination
is the UDP/IPv4 address found in the IPv6 destination address.
It then sends the resulting IPv6/UDP/IPv4 packet via its IPv4
interface.
11. To maintain the NAT44 mapping of its 6a44 tunnel, and to quickly
detect the need to change its 6a44 address in case of NAT44
reset, a 6a44 client from time to time sends a bubble to the
6a44-relay address (see Section 6.5.1).
12. When a 6a44 relay receives via its IPv4 interface an IPv6/UDP/
IPv4 packet whose IPv6 and UDP/IPv4 source addresses are not
consistent, it discards the invalid packet and returns a bubble
to the UDP/IPv4 source address. (This permits the 6a44 client
at this address to update its IPv6 address.)
Despres, et al. Experimental PAGE 11
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
CUSTOMER +-------------------------+
SITES | ISP NETWORK |
+---------+ +----------------+ |
| | |6a44 ISP NETWORK| | GLOBAL
| | | | | INTERNET
HOSTS | IPv6/UDP/IPv4 +---------+ | HOST
+-+ | +-----+ | B| 6a44 |C/48| IPv6 +-+
|A|---|--.---|NAT44|----|----------.---------.----|--- - - - ---|D|
+-+ | \ +-----+ | /| relay(s)|\ | +-+
+-+ | / | | ' +---------+ ' |
|E|---|--' | | | | | |
+-+ IPv6/IPv4 | | | | | |
+---------+ | | | | |
| | | | |
+---------+ | | | | |
| IPv6/UDP/IPv4 . | | |
+-+ | +-----+ | / | | |
|F|---|------|NAT44|----|------' | | |
+-+ | +-----+ | | | |
| | +----------------+ | |
+---------+ | . |
+-+ | / |
|G|---- - - - - - - ----|--------------------' |
+-+ IPv6 | |
+-------------------------+
IPv6 PATHS A-D: D is IPv6 of another ISP
A-E: E is a 6a44 client in the same site
A-F: F is a 6a44 client in another site of the same ISP
A-G: G is IPv6 of the same ISP, other than 6a44
Figure 2: IPv6 Paths between 6a44 Hosts and Remote Hosts
5. 6a44 Addresses
The 6a44 IPv6 address an ISP assigns to a host must contain all
pieces of information needed to reach it from other IPv6 addresses.
These pieces are described below and illustrated in Figure 3:
o the 6a44-network IPv6 prefix C (a /48 the ISP has assigned to its
6a44 relays);
o the customer-site IPv4 address N (either global IPv4 or, if the
ISP uses a [NAT444] model, private IPv4);
Despres, et al. Experimental PAGE 12
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
o the mapped port Z of the 6a44 tunnel (i.e., the external port
assigned by the NAT44 to the tunnel that the client maintains
between its UDP/IPv4 local address A:W and the 6a44-relay UDP/IPv4
address B:W);
o the client local IPv4 address A (i.e., the private IPv4 address
assigned to the client in its customer site; it is needed for
intra-site IPv6 connectivity).
Customer network ISP network
+--------------+ +------------------+
Client |IPv4 CPE |IPv4 |
+----+ | +-----+ | +----------+
| ^ |-----| |NAT44|----+ |6a44 relay|---- IPv6
+-|-^+ | +-----+ | +----------+^
| | | ^ | ^ | ^ | |
| | +----------|---+ | +---------|--------+ |
| | | ^ | | |
| | >0/0| | |N/32< | |
| | | | |
| | Mapping | |
| | <a:w>-<N:Z> (*) | |
| | | |
| |A:W< >B:W| |
| |
IPv6 |C.N.Z.A/128< |C/48<
(*) With NAT44(s) between client and CPE, a:w may differ from A:W
|0 47|48 79|80 95|96 127|
+-------+-------+-------+-------+-------+-------+-------+-------+
| 6a44-network | Customer-site |Tunnel | 6a44-client |
| IPv6 prefix | IPv4 address |mapped | local IPv4 |
| (C) | (N) |port(Z)| address (A) |
+-------+-------+-------+-------+-------+-------+-------+-------+
6a44-client
<-- UDP/IPv4 address -->
<------------ 6a44-client IPv6 prefix --------->
<---------------- 6a44-client IPv6 address --------------------->
Figure 3: Host-Address Construction
NOTE: 6a44 addresses are not guaranteed to comply with the rule
listed in [RFC 4291], according to which bits 64-127 of aggregatable
unicast addresses have to be in Modified-EUI-64 Interface Identifier
(IID) format. However, these bits within the 6a44 addresses are
interpreted only where 6a44 addresses are processed, i.e., in 6a44
Despres, et al. Experimental PAGE 13
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
relays and clients. No operational problem is therefore foreseen.
Besides, because it is a purely transitional tool, it shouldn't
prevent any "development of future technology that can take advantage
of interface identifiers with universal scope" (the purpose of this
format, as expressed in [RFC 4291].
6. Specification of Clients and Relays
6.1. Packet Formats
6.2. IPv6 Packet Encapsulations
For NAT44 traversal, an IPv6 packet transmitted from a 6a44 client to
a 6a44 relay, or vice versa, is encapsulated in a UDP/IP packet whose
source and destination addresses are those of the two endpoints (A:W
and B:W in the notations of Figure 3). The IPv4 packet is that of a
complete datagram (its more-fragment bit is set to 0, its offset is
set to 0, and its datagram identification may be set to 0). The UDP
checksum is set to 0 (there is no need for an additional layer of
checksum protection). The length of the IPv6 packet SHOULD NOT
exceed 1280 octets (see Section 6.4).
Octets: |0 |20 |28 |68 |
+----------+---+-------------------+-------//-----+
| IPv4 |UDP| IPv6 header | IPv6 payload |
+----------+---+-------------------+-------//-----+
An IPv6 packet transmitted from a 6a44 client to another 6a44 client
of the same site is encapsulated in an IPv4 packet whose source and
destination addresses are the private IPv4 addresses of the two
hosts. The IPv4 packet is that of a complete datagram (its
more-fragment bit is set to 0, its offset is set to 0, and its
datagram identification may be set to 0). The size of the IPv6
packet SHOULD NOT exceed 1280 octets (see Section 6.4).
Octets: |0 |20 |60 |
+----------+-------------------+-------//-----+
| IPv4 | IPv6 header | IPv6 payload |
+----------+-------------------+-------//-----+
6.3. 6a44 Bubbles
A "bubble" is a UDP/IPv4 packet whose UDP payload is comprised of a
"6a44-client IPv6 prefix" field and a "Bubble ID" field and whose UDP
checksum is set to 0. Having no UDP checksum protection in bubbles
is a simplification that is acceptable because bubble contents are
Despres, et al. Experimental PAGE 14
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
regularly updated and non-critical (a client accepting a corrupted
IPv6 prefix never leads to any IPv6 packet being accepted by any
wrong destination).
"6a44-client IPv6 prefix" field
. from a 6a44 client = 0 (also denoted by ::/96)
. from a 6a44 relay = 6a44-client IPv6 prefix
|
Octets: |0 |20 |28| |40 |48
+----------+---+--|-+---+
| IPv4 |UDP| . | . |
+----------+---+----+-|-+
|
"Bubble ID" field
. from a 6a44 client: a client-selected value
. from a 6a44 relay:
- in a response bubble, copy of the received Bubble ID
- in an error-signaling bubble, 0
Figure 4: 6a44 Bubble Format
In a bubble from a 6a44 client to a 6a44 relay, the "6a44-client
IPv6 prefix" field is only reserved space for the response and is set
to 0. In a bubble from a 6a44 relay to a 6a44 client, this field
contains the IPv6 prefix of the client, left-justified.
In a bubble from a 6a44 client to a 6a44 relay, the "Bubble ID" field
contains a randomly chosen value, renewed under the circumstances
defined in Section 6.5.1. In a bubble from a 6a44 relay to a 6a44
client, if the bubble is a response to a bubble received from the
client, the field contains the value found in the received bubble; if
the bubble is a reaction to a received IPv6/UDP/IPv4 packet whose
IPv6 and UDP/IPv4 sources are inconsistent (i.e., not conforming to
R44-2 condition (3) in Section 6.6.2), the field is set to 0. The
purpose of this field is to protect against 6a44-relay spoofing
attacks (see Section 7).
In order to preserve forward compatibility with any extension of
bubble formats -- should one prove useful in the future -- 6a44
clients and 6a44 relays MUST be configured to receive bubbles whose
UDP payload lengths are longer than 20 octets (up to that of an IPv6-
packet header since, as detailed in Sections 6.5.3 and 6.6.2, bubbles
are recognized by the fact that their lengths are shorter than that
of tunneled IPv6 packets).
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
6.4. MTU Considerations
Reassembly of a fragmented IPv4 datagram necessitates that its
identifier be remembered from reception of the first fragment to
reception of the last one, and necessitates a timeout protection
against packet losses. If such stateful IP-layer processing would be
necessary for 6a44, it would make it more complex than needed, would
introduce a vulnerability to denial-of-service attacks, and would
impose the restriction that all fragments of a fragmented IPv4
datagram go to the same relay. This last point would be a constraint
on how load balancing may be performed between multiple 6a44 relays,
and would therefore be detrimental to scalability.
For 6a44 processing to remain completely stateless, IPv4 packets
containing encapsulated IPv6 packets must never be fragmented (DF
always set to 1). For this requirement to be met, the following
apply:
o In customer sites, 6a44 clients MUST have IPv4 link MTUs that
support encapsulated IPv6 packets of lengths up to 1280 octets,
i.e., for IPv6/UDP/IPv4 packets that traverse the CPE, link MTUs
of at least 1280+20+8=1308 octets. (This condition is in general
satisfied.)
o For the same reason, 6a44 ISP networks must have IPv4 path MTUs of
at least 1308 octets. (This condition is in general satisfied.)
o 6a44 clients SHOULD limit the size of IPv6 packets they transmit
to 1280 octets.
o 6a44 relays SHOULD set their IPv6 MTU to 1280. (If a relay
receives an IPv6 packet longer than this MTU via its IPv6 upstream
interface, it MUST return an ICMPv6 Packet Too Big error message.)
Typical ISP networks have path MTUs that would permit IPv6 MTUs of
6a44 devices to be longer than 1280 octets, but accepting 1280
octets is a precaution that guarantees against problems with
customer sites that may have internal path MTUs smaller than those
supported by their ISP networks.
6.5. 6a44 Client Specification
6.5.1. Tunnel Maintenance
For a 6a44-client IPv6 address to remain valid, the port mapping of
the 6a44 tunnel MUST be maintained in the CPE NAT44.
For this, the 6a44 client SHOULD apply the equivalent of the
following TM-x rules, as illustrated in Figure 5.
Despres, et al. Experimental PAGE 16
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
TM-1 At initialization, a timer value T1 is randomly chosen in the
recommended range of 1 to 1.5 seconds, and the "6a44 disabled"
state is entered. (Randomness of this value is a precaution to
avoid the following scenario: if many hosts happened to be
re-initialized at the same time, the bubble traffic resulting
from the following rules would be synchronized.)
TM-2 In the "6a44-disabled" state, if it appears that the interface
has no IPv6 native address BUT has a private IPv4 address, then
(1) the Attempt count (a local variable) is set to 1; (2) a new
Bubble ID (another local variable) is randomly chosen (it is
not critical how random this new value is, as explained in
Section 7); (3) a bubble is sent with this Bubble ID; (4) the
"Bubble sent" state is entered with the timer set to T1.
TM-3 In the "Bubble sent" state, if the timer expires AND the
Attempt count is less than 4, then (1) the Attempt count is
increased by 1; (2) a new bubble is sent with the current
Bubble ID; (3) the "Bubble sent" state is re-entered with the
timer reset to T1.
TM-4 In the "Bubble sent" state, if a bubble is received, then
(1) the 6a44-client IPv6 address is set to the received
6a44-client IPv6 prefix followed by the host local IPv4
address; (2) the "Bubble received" state is entered with the
timer set to T2, whose recommended value is 30 seconds minus 4
times T1.
TM-5 In the "Bubble sent" state, if timer T1 expires AND the Attempt
count is equal to 4, then the "No 6a44 relay" state is entered
with the timer set to T3, whose recommended value is 30
minutes.
TM-6 In the "Bubble sent" state, OR the "Bubble received" state, OR
the "No 6a44 relay" state, if an IPv6 native address is
obtained by some other means, OR if the private IPv4 address of
the host is no longer valid, then (1) the timer is disarmed;
(2) the "6a44 disabled" state is entered.
TM-7 In the "Bubble received" state, if timer T2 expires, then
(1) the Attempt count is reset to 1; (2) a new Bubble ID is
randomly chosen; (3) a bubble is sent with this Bubble ID;
(4) the "Bubble sent" state is entered with the timer set
to T1.
TM-8 In the "Bubble received" state, if a bubble is received, then
the timer is reset to T2. (NOTE: Since a bubble is received by
a 6a44 client either in response to a bubble it has sent or in
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
reaction to a packet it has sent with inconsistent IPv6 and
UDP/IPv4 source addresses, receiving a bubble is a sign that
the tunnel mapping reported in the received bubble prefix has
recently been used in BOTH directions, a condition required by
some NAT44s to maintain their mappings.)
TM-9 In the "No 6a44 relay" state, if the timer expires, then
(1) the Attempt count is reset to 1; (2) a new Bubble ID is
randomly chosen; (3) a bubble is sent with this Bubble ID;
(4) the "Bubble sent" state is entered with the timer set
to T1.
Initialization
________v________
/ \
| "6a44 disabled" |------------<-----------------+
\_________________/ ^
v no v6-add AND v4-add ^
+--------->--------------v ^
^ +--------------v--------------+ ^
^ | Reset the Attempt count | ^
^ | Renew the Bubble ID | ^
^ +--------------+--------------+ ^
^ +----->-------------v ^
^ ^ +--------------v--------------+ ^
^ ^ | Send a bubble | ^
^ ^ +--------------v--------------+ ^
^ ^ ________v________ ^
^ ^ Timer T1 / \ 4 attempts without answer ^
^ +----<-----| "Bubble sent" |-------->----------------+ ^
^ (1 to 1.5 s)\_________________/ v ^
^ v \ v6-add OR no v4-add v ^
^ Bubble received v +-----------------------------+
^ v-----------------<-----------+ v ^
^ _________v_________ ^ v ^
^ Timer T2 / \Bubble received ^ v ^
+----------<---| "Bubble received" |-------->----------+ v ^
^ (30 s - 4*T1)\___________________/ v ^
^ \ v6-add OR no v4-add v ^
^ +------->--------------------+
^ v ^
^ +----------------------------------+ ^
^ _______v________ ^
^ Timer T3 / \ v6-add OR no v4-add ^
+-----------<----| "No 6a44 relay" |----->-----------------------+
(30 min) \_________________/
Figure 5: Tunnel Maintenance Algorithm
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
6.5.2. Client Transmission
A 6a44 client transmits packets according to the following CT-x
rules. In figures that illustrate these rules, symbols used in
Section 5 are reused; packets are represented as a succession of
significant fields separated by commas, with sources preceding
destinations as usual; != means "different from".
CT-1 BUBBLE SENT BY A 6a44 CLIENT
(IPv4, A, B, UDP[W, W, ::/96, <current Bubble ID>])
|
+-------+--------+ |
| | 6a44 | |
| | client +------>---------- >B:W
| |function|A:W< UDP/IPv4
+-------+--------+
Host
Bubbles are transmitted from time to time. Conditions of their
transmission are specified in Section 6.5.1, and their format is
specified in Section 6.3.
CT-2 IPv6/IPv4 PACKET SENT TO A HOST OF THE SAME SITE
[IPv6, <C.N.Z.A>, <C.N..E>,...]
|
| (IPv4, A, A2, IP-in-IP[encapsulated packet])
| |
+----|--+--------+ |
| | | 6a44 | |
| -->--+ client +------>------ >A2
| IPv6 |function|<A IPv4
+-------+--------+
Host
If an IPv6 packet is submitted for transmission with ALL the
following conditions satisfied, the 6a44 client MUST encapsulate the
IPv6 packet in an IPv4 packet whose protocol is set to IP in IP
(protocol = 41) and whose IPv4 destination is copied from the last 32
bits of the IPv6 destination: (1) the IPv6 source address is the
6a44-client IPv6 address; (2) the IPv6 destination is a 6a44 address
of the same site (it has the same 80 bits as the 6a44-client IPv6
address); (3) either the IPv6 packet does not exceed 1280 octets, or
it is longer but it does not exceed the IPv4 link MTU minus 20 octets
and the IPv4 destination address starts with the IPv4 link prefix.
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
CT-3 IPv6/UDP/IPv4 PACKET TO A HOST OF ANOTHER SITE
[IPv6, <C.N.Z.A>, X != <C.N...>, ...]
|
| (IPv4, B, A, UDP(W, W, [encapsulated packet])
| |
+----|--+--------+ |
| | | 6a44 | |
| -->--+ client +------>---------- >B:W
| IPv6 |function|A:W< UDP/IPv4
+-------+--------+
Host
If an IPv6 packet is submitted for transmission and ALL the following
conditions are satisfied, the IPv6 packet MUST be encapsulated in a
UDP/IPv4 packet whose destination is the 6a44-relay anycast address
and whose source and destination ports are both the 6a44 port:
(1) the source address is the local 6a44-client IPv6 address; (2) the
destination is not a 6a44 address of the same site (its first 80 bits
differ from those of the 6a44-client IPv6 address); (3) the IPv6
packet does not exceed 1280 octets.
CT-4 IPv6 PACKET THAT DOESN'T CONCERN 6a44
If an IPv6 packet is submitted to the 6a44 client function for
transmission with an IPv6 source address that is not the
6a44-client IPv6 address, the packet does not concern 6a44. It
MUST be left for any other IPv6 transmission function that may
apply (the source address can be a link-local address or a
Unique Local Address (ULA) [RFC 4193]).
6.5.3. Client Reception
Upon reception of an IPv4 packet, a 6a44 client applies the following
CR-x rules:
CR-1 BUBBLE RECEIVED FROM A 6a44 RELAY
(IPv4, B, A, UDP(W, W, [<C.N.Z>, <current Bubble ID>])
|
+-------+--------+ |
| | 6a44 | |
| | client +------<---------- <B:W
| | |A:W< UDP/IPv4
+-------+--------+
Host
(updates C.N.Z)
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
If ALL the following conditions are satisfied (i.e., the packet is a
6a44 bubble from a 6a44 relay), the 6a44-client IPv6 address MUST be
updated using the received IPv6 prefix C.N.Z: (1) the IPv4 packet
contains a complete UDP datagram (protocol = 17, offset = 0,
more-fragment bit = 0); (2) both ports of the UDP datagram are the
6a44 port, and the payload length is enough to contain a 6a44-client
IPv6 prefix and a Bubble ID but shorter than an IPv6-packet header
(protocol = 17, UDP payload length = at least 20 octets and less than
40 octets); (3) the received Bubble ID matches the current value of
the Bubble-ID local variable.
CR-2 IPv6/IPv4 PACKET FROM A HOST OF THE SAME SITE
(IPv4, E, A, IP-in-IP, [IPv6, <C.N..A2>, <C.N.Z.A>, ...])
|
[decapsulated packet] |
| |
+----|--+--------+ |
| | | 6a44 | |
| --<--+ client +------<------ <A2
| IPv6 | |A< IPv4
+-------+--------+
Host
If ALL the following conditions are satisfied (i.e., the packet comes
from a 6a44 client of the same site), the 6a44 client MUST
decapsulate the inner packet and treat it as a received IPv6 packet:
(1) the IPv4 packet contains a complete UDP datagram (protocol = 17,
offset = 0, more-fragment bit = 0); (2) both ports of the UDP
datagram are the 6a44 port, and the UDP payload is an IPv6 packet
(UDP length of at least 40 octets, version = 6); (3) the IPv6 source
address is one of the same site (the first 80 bits match those of the
6a44-client IPv6 address; (4) its last 32 bits are equal to the IPv4
source address; (5) the IPv6 destination address is the 6a44-client
IPv6 address.
CR-3 IPv6/UDP/IPv4 PACKET FROM A HOST OF ANOTHER SITE
(IPv4, B, A, UDP(W, W, [IPv6, X, <C.N.Z.A>,...])
|
[decapsulated packet] |
| |
+----|--+--------+ |
| | | 6a44 | |
| --<--+ client +------<---------- <B:W
| IPv6 | |A:W< UDP/IPv4
+-------+--------+
Host
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
If ALL the following conditions are satisfied (i.e., the packet has
been relayed by a 6a44 relay), the 6a44 client MUST decapsulate the
inner packet and treat it as a received IPv6 packet: (1) the IPv4
packet contains a complete UDP datagram (protocol = 17, offset = 0,
more-fragment bit = 0); (2) the UDP payload is an IPv6 packet (length
of at least 40 octets, version = 6); (3) the UDP/IPv4 source address
is the 6a44-relay UDP/IPv4 address; (4) the IPv6 destination address
is the 6a44-client IPv6 address.
CR-4 RECEIVED ICMPv4 ERROR MESSAGE CONCERNING A 6a44 PACKET
If the 6a44 client receives an IPv4 error message [RFC 792]
that concerns a discarded 6a44 packet (i.e., if the copied
header of the discarded packet is that of a transmitted packet
according to CT-2 or CT-3), it SHOULD translate it into an
ICMPv6 error message [RFC 4443] and then treat it as a received
IPv6 packet. Translation of Type and Code conversions between
IPv4 and IPv6 is described in Section 4.2 of [RFC 6145], under
"ICMPv4 error messages".
CR-5 RECEIVED IPv4 PACKET OTHER THAN 6a44
If ANY one or more of the following conditions are verified,
the received IPv4 packet does not concern 6a44 and MUST
therefore be left for any other IPv4 reception function that
may apply: (1) the IPv4 payload is neither UDP nor IPv6
(protocol = neither 17 nor 41, or protocol = 41 and IP version
in the payload is not = 6); (2) the IPv4 packet is an
IP-datagram fragment other than the first one (offset > 0);
(3) the IPv4 packet contains the first or unique fragment of a
UDP datagram (protocol = 17, offset = 0), with neither port
equal to the 6a44 port.
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
6.6. 6a44 Relay Specification
6.6.1. Relay Reception in IPv6
Upon reception of a packet via its IPv6 interface with a destination
address starting with the 6a44-network IPv6 prefix, a 6a44 relay MUST
apply the following RR6-x rules:
RR6-1 VALID IPv6 PACKET FROM OUTSIDE THE 6a44 ISP NETWORK
[IPv6, (X != <C...> AND != <Teredo(IPv4=B)>), <C.<N != B>.Z...>,...]
|
(IPv4, B, N, UDP(W, Z, |
[encapsulated packet])) |
| |
| +--------+ |
| >B:W | 6a44 |C/48< |
N:Z< ---<--------| relay |-------<---- C.N.Z...<
IPv4 | | IPv6
+--------+
If ALL the following conditions are satisfied, the IPv6 packet MUST
be encapsulated in a UDP/IPv4 packet whose UDP/IPv4 destination is
copied from bits 48 to 95 of the IPv6 destination address: (1) the
IPv6 source address is not that of a 6a44 client of the ISP (it does
not start with the 6a44-network IPv6 prefix); (2) the IPv6 source
address is not a Teredo address whose embedded UDP/IPv4 address is
the 6a44-relay anycast address; (3) the customer-site IPv4 address
embedded in the 6a44 destination address is not the 6a44-relay
anycast address; (4) the packet has at most 1280 octets.
RR6-2 INVALID IPv6 PACKET FROM OUTSIDE THE 6a44 ISP NETWORK
If ANY one or more of the following conditions are satisfied,
the IPv6 packet MUST be discarded: (1) the packet has more
than 1280 octets (in this case, an ICMPv6 Packet Too Big error
message MUST be returned to the source); (2) the customer-site
IPv4 address embedded in the IPv6 destination address is the
6a44-relay anycast address; (3) the IPv6 source address is a
Teredo address whose embedded IPv4 address is the 6a44-relay
anycast address.
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
6.6.2. Relay Reception in IPv4
Upon reception via its IPv4 downstream interface of an IPv4 packet
that contains a complete IP datagram (fragment offset = 0 and
more-fragment bit = 0) and that contains a UDP datagram whose UDP/
IPv4 destination is the 6a44-relay UDP/IPv4 address, a 6a44 relay
MUST apply the following rules:
RR4-1 BUBBLE FROM 6a44 CLIENT
(IPv4, N, B, UDP(Z, W, [::/96, Bubble ID]))
|
IPv4 | +--------+
------->----| |
>B:W| 6a44 |
| relay |
N:Z< -------<----| |
IPv4 | +--------+
|
|
(IPv4, B, N, UDP(W, Z, [<C.N.Z>, Bubble ID]))
If the following condition is satisfied, the 6a44 relay MUST return
to the source a bubble derived from the bubble it just received by
permuting its UDP/IPv4 source and destination, and by putting in its
6a44-client-IPv6-prefix field the received UDP/IPv4 source address:
the UDP payload is a bubble, i.e., has at least 20 octets and less
than 40 octets.
RR4-2 IPv6 PACKET FROM A 6a44 CLIENT TO ANOTHER 6a44 CLIENT
(IPv4, N1, B, UDP(Z1, W, [IPv6, <C.N1.Z1...>, <C.N2.Z2...>, ...]))
|
IPv4 | +--------+
------->----| |
>B:W| 6a44 |
| relay |
| |
N2.Z2< -------<----| |
IPv4 | +--------+
| 6a44 relay
|
(IPv4, B, N2, UDP(W, Z2, [encapsulated packet]))
If ALL the following conditions are satisfied, the 6a44 relay MUST
return back via its downstream IPv4 interface an IPv6/ UDP/IPv4
packet containing the same encapsulated packet, having its UDP/IPv4
destination set to the UDP/IPv4 address found in the 6a44 destination
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
address, and having its UDP/IPv4 source set to the 6a44-relay
UDP/IPv4 address: (1) the IPv4 packet contains a complete UDP
datagram (protocol = 17, offset = 0, more-fragment bit = 0); (2) the
UDP payload is an IPv6 packet (length of at least 40 octets, version
= 6); (3) the IPv6 source address starts with the 6a44-network IPv6
prefix followed by the UDP/IPv4 source address of the received
packet; (4) the IPv6 destination address starts with the 6a44-network
IPv6 prefix.
RR4-3 IPv6 PACKET FROM A 6a44 CLIENT TO A NON-6a44 CLIENT
(IPv4, N, B, UDP(Z, W, [IPv6, <C.N.Z...>,
| (X != <C...> AND != <Teredo(IPv4=B)), ...]))
|
| [decapsulated packet]
| |
| +--------+ |
| B:W>| 6a44 | |
>B:W --->----------| relay |------->---- >
IPv4 | | IPv6
+--------+
If ALL the following conditions are satisfied, the 6a44 relay MUST
decapsulate the IPv6 packet and forward it via the IPv6 interface:
(1) the IPv4 packet contains a complete UDP datagram (protocol = 17,
offset = 0, more-fragment bit = 0); (2) the UDP payload is an IPv6
packet (length of at least 40 octets, version = 6); (3) the IPv6
source address starts with the 6a44-network IPv6 prefix followed by
the UDP/IPv4 source address of the received packet; (4) the IPv6
destination address does not start with the 6a44-network IPv6 prefix
and is not a Teredo address whose embedded IPv4 address is the
6a44-relay anycast address.
RR4-4 RECEIVED ICMPv4 ERROR MESSAGE CONCERNING A 6a44 PACKET
If the 6a44 relay receives an IPv4 error message [RFC 792]
that concerns a discarded 6a44 packet (i.e., if the copied
header of the discarded packet is that of a transmitted packet
according to RR6-1 or RR4-2), it SHOULD translate it into an
ICMPv6 error message [RFC 4443] and then treat it as a received
IPv6 packet. Translation of Type and Code conversions between
IPv4 and IPv6 is described in Section 4.2 of [RFC 6145], under
"ICMPv4 error messages".
RR4-5 INVALID IPv6/UDP/IPv4 PACKET
For ANY other case, the 6a44 relay MUST discard the packet.
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
6.7. Implementation of Automatic Sunset
6a44 is designed as an interim transition mechanism, not to be used
any longer than strictly necessary. Its sole purpose is to
accelerate availability of IPv6 native addresses where, for any
reason, CPEs cannot quickly be replaced, or where, for any reason,
ISP networks cannot quickly support dual-stack routing or 6rd.
A 6a44-capable ISP can first have an increase in its 6a44 traffic as
more and more hosts behind IPv4-only CPEs support the 6a44 client
function, but it should later have a decrease in this traffic as more
and more CPEs operate in dual stack.
When this traffic becomes sufficiently negligible, the ISP may, after
due prior notice, discontinue 6a44-relay operation. This terminates
its sunset procedure.
In a host that obtains an IPv6 native address by some means other
than 6a44, the effect of having the 6a44 function in its protocol
stack is inexistent. OS providers may therefore keep this function
in their code for many years. When it becomes clear that the number
of users of this function has become negligible, they can delete it
from later releases. This terminates their sunset procedure.
7. Security Considerations
Incoming reachability:
Hosts that acquire 6a44 addresses become reachable from the
Internet in IPv6 while they remain unreachable in IPv4 at their
private IPv4 addresses.
For ordinary use, this should not introduce a perceptible new
security risk for two reasons: (1) hosts can, without IPv6, use
NAT44 hole-punching techniques such as Interactive Connectivity
Establishment (ICE) [RFC 5245] to receive incoming connections;
(2) by default, modern operating systems that support IPv6 have
their own protections against incoming connections.
If 6a44 reachability across an ordinary NAT44 nevertheless has to
be barred, this can be done by configuring its port-forwarding
function with the 6a44 port bound to any internal address that is
not assigned to any host. Thus, no bubble from a 6a44 relay can
reach any 6a44-capable host, and this is sufficient to prevent
hosts from using 6a44.
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
For more sophisticated uses with managed firewalls, default
configurations generally specify that packets that are not
explicitly authorized are discarded. Thus, 6a44 can be used only
if the 6a44 port is deliberately opened to incoming traffic.
Subscriber authentication:
Any authentication that applies to an IPv4 address extends its
effect to 6a44 addresses that are derived from it.
Host-address spoofing:
With ingress filtering required in 6a44 ISP networks, and with the
address checks specified in Section 6, no new IPv6 address-
spoofing vulnerability is introduced by 6a44.
Address-and-port scanning:
To mitigate the (limited) risk of a malicious user trying to scan
IPv4 address/port pairs to reach a host, Teredo addresses contain
12 random bits [RFC 5991]. 6a44 addresses have no random bits but
contain local IPv4 addresses of clients. Since possible values of
these addresses are not deterministically known from outside
customer sites and are in ranges that can be configured in typical
NAT44s, some protection against address and port scanning is thus
achieved. This protection may be less effective than that
achieved with random bits but is in any case better for 6a44 IPv6
addresses than for IPv4 addresses alone.
Denial of service:
Provided 6a44 relays are provisioned with enough processing power,
which is facilitated by their being completely stateless, 6a44
introduces no denial-of-service vulnerabilities of its own.
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
Routing loops:
A risk of routing-loop attacks has been identified in [RFC 6324].
Without taking precautions, it applies to some combinations of
automatic-tunnel mechanisms such as 6to4, the Intra-Site Automatic
Tunnel Addressing Protocol (ISATAP), 6rd, and Teredo. This risk
does not exist with 6a44 for the following reasons:
1. When a packet enters a 6a44 relay via its IPv6 interface, the
following apply:
+ An IPv6/UDP/IPv4 packet cannot be sent to another 6a44
relay because its IPv4 destination would have to be a
6a44-relay IPv4 address. This is prevented by rule RR6-1
of Section 6.6.1.
+ If an IPv6/UDP/IPv4 packet is sent to the address of a 6to4
relay, 6rd relay, or ISATAP relay, it will be discarded
there because these relays don't accept UDP/IPv4 packets.
+ If an IPv6/UDP/IPv4 packet is sent to a Teredo relay, it
will be discarded there because (1) Teredo relays check
that the IPv4 address that is embedded in the IPv6 source
address of a received IPv6/IPv4 packet matches the IPv4
source address of the encapsulating packet (Section 5.4.2
of [RFC 4380]); (2) encapsulating packets sent by 6a44
relays have the 6a44-relay anycast address as the IPv4
source address; (3) a 6a44 relay forwards a received IPv6
packet as an IPv6/UDP/IPv4 packet only if its IPv6 source
address is not a Teredo address whose embedded IPv4 address
is the 6a44-relay IPv4 address.
2. When a packet enters a 6a44 relay via its IPv4 interface, the
following apply:
+ The received packet cannot come from another 6a44 relay (as
just explained, 6rd relays do not send IPv6/UDP/IPv4
packets to other 6a44 relays).
+ If the IPv4 packet comes from a 6to4 relay, a 6rd relay, or
an ISATAP relay, its IPv6 encapsulated packet cannot be
forwarded (the received packet is IPv6/IPv4 instead of
being IPv6/UDP/IPv4, as required by rules RR4-2 and RR4-3
of Section 6.6.2).
+ If the received packet is an IPv6/UDP/IPv4 packet coming
from a Teredo relay, this packet cannot have been sent to
the Teredo relay by a 6a44 relay: (1) in order to reach the
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
6a44 relay, the IPv6 destination of the IPv6 encapsulated
packet must be a Teredo address whose embedded IPv4 address
is the 6a44-relay anycast address (Section 5.4.1 of
[RFC 4380]); (2) a 6a44 relay does not forward via its IPv6
interface an IPv6 packet whose destination is a Teredo
address whose embedded IPv4 address is the 6a44-relay
anycast address (rule RR4-3 of Section 6.6.2).
6a44-relay spoofing:
In a 6a44 network, no node can spoof a 6a44 relay because ingress
filtering prevents any 6a44-relay anycast address from being
spoofed.
In a network that does not support ingress filtering (and
therefore is not a 6a44 network), the following apply:
* 6a44 packets sent by 6a44-capable hosts are discarded in the
IPv4 backbone because their IPv4 destination, the 6a44-relay
anycast address, does not start with any ISP-assigned prefix.
* If an attacker tries to send to a 6a44-capable host a fake
relay-to-client bubble, the probability that it would be
accepted by its destination is negligible. It would require
that all the following conditions be simultaneously satisfied:
+ The UDP/IPv4 destination set by the attacker must reach a
NAT44 node in which it is the external mapping of a 6a44
tunnel established by a 6a44-capable host.
+ This host must be in the "Bubble sent" state -- the only one
in which it listens to bubbles when its ISP is not 6a44
capable. This state is taken only for a few seconds every
30 minutes (rule TM-5 of Section 6.5.1).
+ This host accepts the bubble only if its Bubble ID has the
right value -- an extremely unlikely possibility with a
64-bit randomly chosen Bubble ID (see Section 6.5.1).
* If a 6a44-capable host -- despite this scenario being very
unlikely -- accepts a fake bubble, the effect is that it
wrongly believes, for about 30 seconds, that it has an assigned
public IPv6 address. All IPv6 packets it then sends with this
address as the source cannot be accepted by any destination (no
relay will forward them, and no host of the same site will
accept them). The consequences of this scenario would
therefore not impair security.
Despres, et al. Experimental PAGE 29
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
8. IANA Considerations
IANA has assigned the following:
1. IPv4 address 192.88.99.2 as the 6a44-relay anycast address (B in
this document).
2. UDP port 1027 as the 6a44 port (W in this document).
The choice of 192.88.99.2 as the 6a44 IPv4 anycast address doesn't
conflict with any existing IETF specification because
o it starts with the 6to4 prefix 192.88.99.0/24 [RFC 3068].
o it differs from the only currently assigned address that starts
with this prefix (the anycast address of 6to4 relays --
192.88.99.1 [RFC 3068].
This choice is made to permit implementations of 6a44 relays in
physical nodes that are independent from any 6to4 relay or, if found
to be more optimum, in nodes in which 6to4 relays and 6a44 relays are
collocated.
9. Acknowledgments
This specification, whose origin is a convergence effort based on two
independent proposals -- [6rd+] and [SAMPLE] -- has benefited from
various suggestions. Comments have been received during this
process, in particular from Dave Thaler, Fred Templin, Ole Troan,
Olivier Vautrin, Pascal Thubert, Washam Fan, and Yu Lee. The authors
wish to thank them, and all others, for their useful contributions.
Special recognition is due to Dave Thaler and John Mann. Their
detailed reviews led to a few useful modifications and editorial
improvements.
10. References
10.1. Normative References
[RFC 792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Despres, et al. Experimental PAGE 30
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
10.2. Informative References
[6rd+] Despres, R., "Rapid Deployment of Native IPv6 Behind IPv4
NATs (6rd+)", Work in Progress, July 2010.
[NAT444] Yamaguchi, J., Shirasaki, Y., Miyakawa, S., Nakagawa, A.,
and H. Ashida, "NAT444 addressing models", Work
in Progress, July 2012.
[RFC 1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC 2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC 3053] Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6
Tunnel Broker", RFC 3053, January 2001.
[RFC 3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC 3068] Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
RFC 3068, June 2001.
[RFC 3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC 4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC 4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC 4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443,
March 2006.
Despres, et al. Experimental PAGE 31
RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
[RFC 5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC 5569] Despres, R., "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd)", RFC 5569, January 2010.
[RFC 5626] Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed.,
"Managing Client-Initiated Connections in the Session
Initiation Protocol (SIP)", RFC 5626, October 2009.
[RFC 5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) -- Protocol Specification",
RFC 5969, August 2010.
[RFC 5991] Thaler, D., Krishnan, S., and J. Hoagland, "Teredo
Security Updates", RFC 5991, September 2010.
[RFC 6081] Thaler, D., "Teredo Extensions", RFC 6081, January 2011.
[RFC 6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, April 2011.
[RFC 6324] Nakibly, G. and F. Templin, "Routing Loop Attack Using
IPv6 Automatic Tunnels: Problem Statement and Proposed
Mitigations", RFC 6324, August 2011.
[SAMPLE] Carpenter, B. and S. Jiang, "Legacy NAT Traversal for
IPv6: Simple Address Mapping for Premises Legacy Equipment
(SAMPLE)", Work in Progress, June 2010.
[TheTool] de Saint-Exupery, A., "Wind, Sand and Stars", Chapter III
(The Tool), 1939.
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RFC 6751 Native IPv6 behind NAT44 CPEs (6a44) October 2012
Authors' Addresses
Remi Despres (editor)
RD-IPtech
3 rue du President Wilson
Levallois
France
EMail: despres.remi@laposte.net
Brian Carpenter
University of Auckland
Department of Computer Science
PB 92019
Auckland 1142
New Zealand
EMail: brian.e.carpenter@gmail.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
EMail: dwing@cisco.com
Sheng Jiang
Huawei Technologies Co., Ltd.
Q14, Huawei Campus - No. 156 Beiqing Road
Hai-Dian District, Beijing 100095
P.R. China
EMail: jiangsheng@huawei.com
Despres, et al. Experimental PAGE 33
RFC TOTAL SIZE: 73468 bytes
PUBLICATION DATE: Monday, October 15th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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