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Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers
draft-ietf-v6ops-rfc6146-bis-10

Versions:

Document	Type	Active Internet-Draft (v6ops WG)
	Authors	Marcelo Bagnulo , Philip Matthews , Jordi Palet Martinez
	Last updated	2026-07-21 (Latest revision 2026-07-02)
	Replaces	draft-palet-v6ops-rfc6146-bis, draft-v6ops-rfc6146-bis
	RFC stream	Internet Engineering Task Force (IETF)
	Intended RFC status	Internet Standard
	Formats	txt html xml htmlized bibtex bibxml
	Reviews	ARTART Telechat review by John Levine Ready w/issues DNSDIR Telechat review by Jim Reid Ready w/nits ARTART IETF Last Call review (of -07) by John Levine Ready w/issues INTDIR IETF Last Call review (of -07) by Satoru Matsushima Ready w/nits OPSDIR IETF Last Call review (of -07) by Tony Li Not ready TSVART IETF Last Call review (of -07) by Joerg Ott Ready w/nits DNSDIR IETF Last Call review (of -07) by Jim Reid Ready w/nits SECDIR Telechat Review due 2026-08-04 Incomplete GENART IETF Last Call Review due 2026-06-30 Incomplete
	Additional resources	Mailing list discussion
Stream	WG state	Submitted to IESG for Publication
	Document shepherd	XiPeng Xiao
	Shepherd write-up	Show Last changed 2026-06-16
IESG	IESG state	IESG Evaluation
	Action Holder	Mohamed Boucadair 25
	Consensus boilerplate	Yes
	Telechat date	On agenda of 2026-08-06 IESG telechat Has a DISCUSS. Needs 6 more YES or NO OBJECTION positions to pass.
	Responsible AD	Mohamed Boucadair
	Send notices to	xipengxiao@gmail.com
IANA	IANA review state	Version Changed - Review Needed
	IANA expert review state	Expert Reviews OK
	IANA expert review comments	The IPFIX Information Elements updates have been approved.

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draft-ietf-v6ops-rfc6146-bis-10

v6ops M. Bagnulo
Internet-Draft UC3M
Obsoletes: 6146 (if approved) P. Matthews
Intended status: Standards Track
Expires: 3 January 2027 J. Palet Martinez, Ed.
The IPv6 Company
2 July 2026

Stateful NAT64: Network Address and Protocol Translation
from IPv6 Clients to IPv4 Servers
draft-ietf-v6ops-rfc6146-bis-10

Abstract

This document describes stateful NAT64 translation, which allows
IPv6-only clients to contact IPv4 servers using unicast UDP, TCP, or
ICMP. One or more public IPv4 addresses assigned to a stateful NAT64
translator are shared among several IPv6-only clients. When stateful
NAT64 is used in conjunction with DNS64, no changes are required in
either the IPv6 client or the IPv4 server.

This document obsoletes RFC 6146.

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on 3 January 2027.

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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Features of Stateful NAT64 . . . . . . . . . . . . . . . 5
1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.1. Stateful NAT64 Solution Elements . . . . . . . . . . 7
1.2.2. Stateful NAT64 Behavior Walk-Through . . . . . . . . 9
1.2.3. Filtering . . . . . . . . . . . . . . . . . . . . . . 11
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 12
3. Stateful NAT64 Normative Specification . . . . . . . . . . . 14
3.1. Binding Information Bases . . . . . . . . . . . . . . . . 15
3.2. Session Tables . . . . . . . . . . . . . . . . . . . . . 16
3.3. Packet Processing Overview . . . . . . . . . . . . . . . 17
3.4. Determining the Incoming Tuple . . . . . . . . . . . . . 19
3.5. Filtering and Updating Binding and Session Information . 21
3.5.1. UDP Session Handling . . . . . . . . . . . . . . . . 21
3.5.1.1. Rules for Allocation of IPv4 Transport Addresses
for UDP . . . . . . . . . . . . . . . . . . . . . . 24
3.5.2. TCP Session Handling . . . . . . . . . . . . . . . . 25
3.5.2.1. State Definition . . . . . . . . . . . . . . . . 25
3.5.2.2. State Machine for TCP Processing in the Stateful
NAT64 . . . . . . . . . . . . . . . . . . . . . . . 27
3.5.2.3. Rules for Allocation of IPv4 Transport Addresses
for TCP . . . . . . . . . . . . . . . . . . . . . . 35
3.5.3. ICMP Query Session Handling . . . . . . . . . . . . . 35
3.5.4. Generation of the IPv6 Representations of IPv4
Addresses . . . . . . . . . . . . . . . . . . . . . . 38
3.6. Computing the Outgoing Tuple . . . . . . . . . . . . . . 38
3.6.1. Computing the Outgoing 5-Tuple for TCP, UDP, and for
ICMP Error Messages Containing TCP or UDP Packets . . 39
3.6.2. Computing the Outgoing 3-Tuple for ICMP Query Messages
and for ICMP Error Messages Containing an ICMP Query 40
3.7. Translating the Packet . . . . . . . . . . . . . . . . . 40
3.8. Handling Hairpinning . . . . . . . . . . . . . . . . . . 41
4. Protocol Constants . . . . . . . . . . . . . . . . . . . . . 41
5. Operational Considerations . . . . . . . . . . . . . . . . . 42
5.1. Stateful NAT64 Prefix . . . . . . . . . . . . . . . . . . 42
5.2. Stateful NAT64 Prefix Discovery . . . . . . . . . . . . . 42
5.3. Stateful NAT64 in Other Protocols . . . . . . . . . . . . 43

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5.4. Port Control Protocol . . . . . . . . . . . . . . . . . . 43
5.5. QUIC . . . . . . . . . . . . . . . . . . . . . . . . . . 43
5.6. Issues with IP Address Sharing . . . . . . . . . . . . . 43
5.7. Previous Operational Experience . . . . . . . . . . . . . 43
5.8. Benchmarking and Scalability . . . . . . . . . . . . . . 44
5.9. Port Allocation Schemes . . . . . . . . . . . . . . . . . 44
5.10. Logging, Alarms and Event Reporting . . . . . . . . . . . 44
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 44
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
8. Security Considerations . . . . . . . . . . . . . . . . . . . 47
8.1. Implications on End-to-End Security . . . . . . . . . . . 47
8.2. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 47
8.3. Attacks on Stateful NAT64 . . . . . . . . . . . . . . . . 49
8.4. Avoiding Hairpinning Loops . . . . . . . . . . . . . . . 50
8.5. DNS64 and DNSSEC . . . . . . . . . . . . . . . . . . . . 50
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 50
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 52
10.1. Normative References . . . . . . . . . . . . . . . . . . 52
10.2. Informative References . . . . . . . . . . . . . . . . . 53
Appendix A. Changes from RFC 6146 . . . . . . . . . . . . . . . 57
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 58

1. Introduction

This document specifies stateful NAT64, a mechanism for IPv4-IPv6
transition and coexistence. Together with DNS64 [RFC6147], these two
mechanisms enable an IPv6-only client to initiate communications to
an IPv4-only server. They also enable peer-to-peer communication
between an IPv4 and an IPv6 node, where the communication can be
initiated when either end uses existing, NAT-traversal, peer-to-peer
communication techniques, such as Interactive Connectivity
Establishment (ICE) [RFC8445] [RFC8839]. Stateful NAT64 also
supports IPv4-initiated communications to a subset of the IPv6 hosts
through statically configured bindings in the stateful NAT64, which
is usually implemented, following the same approach as for Explicit
Address Mappings for Stateless IP/ICMP Translation [RFC7757].

Note that the server may be actually a peer "exposing a server
function", so across the document, references to IPv4-only servers
also mean IPv4-only peers even if not specifically stated.

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Stateful NAT64 is a mechanism for translating IPv6 packets to IPv4
packets and vice versa. The translation is done by translating the
packet headers according to the IP/ICMP Translation Algorithm defined
in [RFC7915]. The IPv4 addresses of IPv4 hosts are algorithmically
translated to and from IPv6 addresses by using the algorithm defined
in [RFC6052] and an IPv6 prefix assigned to the stateful NAT64 for
this specific purpose. The IPv6 addresses of IPv6 hosts are
translated to and from IPv4 addresses by installing mappings in the
normal Network Address Port Translation (NAPT) manner [RFC3022].

The current specification only defines how stateful NAT64 translates
unicast packets carrying TCP, UDP, and ICMP traffic. Multicast
packets and other protocols, including the Stream Control
Transmission Protocol (SCTP), the Datagram Congestion Control
Protocol (DCCP), and IPsec, are out of the scope of this
specification.

"Stateful NAT64 function" is the logical translation function,
independent whether implemented in a device, Virtual Network Function
(VNF) or Service Function (SF). This document defines no SFC-
specific behavior.

DNS64 [RFC6147] is a mechanism for synthesizing AAAA Resource Records
(RRs) from existing A RRs. The IPv6 address contained in the
synthetic AAAA RR is algorithmically generated from the IPv4 address
and the IPv6 prefix assigned to a stateful NAT64 function by using
the same algorithm defined in [RFC6052]. This synthesis can also be
done in the IPv6 clients (DNS64 in stub-resolver mode or "self-
synthesis").

Together, these two mechanisms allow any of the following:

* an IPv6-only-strict client (i.e., a host with a networking stack
that only implements or uses IPv6)

* a dual-stack client connected to an IPv6-only network

* a dual-stack client willing to use only IPv6 connectivity
(IPv6-Mostly [RFC8925])

* a host running an IPv6-only application

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to initiate communications to an IPv4-only server. Note that in some
cases, when using stateful NAT64 together with other mechanisms (such
as a 464XLAT [RFC6877] customer side translator - CLAT), this may be
possible just using stateful NAT64, without the need of DNS64.
However, as described in [RFC8683], it has the impact of forcing a
double translation and may induce an extra delay for the connection
establishment.

For the remainder of this document, an IPv6-only client or node
refers to one of the cases enumerated in the preceding paragraph,
unless explicitly stated otherwise.

These mechanisms play a critical role in IPv4-IPv6 transition and
coexistence. Due to public IPv4 address depletion and the limited
size of [RFC1918] addressing space (in hyperscale data centers or
mobile networks, for example), new clients are IPv6-only and still
need to connect to the existing IPv4-only servers. The stateful
NAT64 and DNS64 mechanisms are easily deployable, since they do not
require changes to either the IPv6 client or the IPv4 server. For
basic functionality, the approach only requires the deployment of the
stateful NAT64 function somewhere in the path between the devices
connecting an IPv6-only network to the IPv4-only network, along with
the deployment of a DNS64-enabled name server accessible to the
IPv6-only hosts. If a host can be updated, then the DNS64
functionality can be built-in, as well as supporting new features
which improve the functionality, such as the support of IPv6-Mostly.
An analysis of the application scenarios can be found in [RFC6144].

This document obsoletes [RFC6146]. The primary changes are listed in
Appendix A.

1.1. Features of Stateful NAT64

The features of NAT64 are:

* Stateful NAT64 is compliant with the recommendations for how NATs
should handle UDP [RFC4787], TCP [RFC5382], and ICMP [RFC5508].
As such, stateful NAT64 only supports Endpoint-Independent
Mappings and supports both Endpoint-Independent and Address-
Dependent Filtering. Because of the compliance with the
aforementioned requirements, stateful NAT64 is compatible with
current NAT traversal techniques, such as ICE [RFC8445] [RFC8839],
and with other NAT traversal techniques.

* In the absence of preexisting state in a stateful NAT64 function,
only IPv6 nodes can initiate sessions to IPv4 nodes. This works
for roughly the same class of applications that work through IPv4-
to-IPv4 NATs (NAT44).

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* Depending on the filtering policy used (Endpoint-Independent or
Address-Dependent), IPv4 nodes might be able to initiate sessions
to a given IPv6 node, if the stateful NAT64 has an appropriate
mapping (i.e., state) for an IPv6 node, via one of the following
mechanisms:

- The IPv6 node has recently initiated a session to the same or
another IPv4 node. This is also the case if the IPv6 node has
used a NAT-traversal technique (such as ICE).

- A statically configured mapping exists for the IPv6 node, e.g.
by means of [RFC7757].

- A dynamic mapping configured by a protocol such as Port Control
Protocol (PCP) [RFC6887].

* IPv4 address sharing: Stateful NAT64 allows multiple IPv6-only
nodes to share a single IPv4 address to access the IPv4 Internet.
This help address some of the problems of IPv4 address exhaustion.
The stateful NAT64 can even be operated as a service by other
parties, not necessarily the operator providing the Internet
connectivity.

* Only TCP, UDP, and ICMP are supported. Support for other
protocols (such as other transport protocols and IPsec without UDP
encapsulation [RFC3948]) is not defined in this document.

1.2. Overview

A non-normative introduction to stateful NAT64 is provided. This is
achieved by describing the stateful NAT64 behavior involving a simple
setup that involves a single stateful NAT64 function, a single DNS64,
and a simple network topology. The goal of this description is to
provide the reader with a general view of stateful NAT64. It is not
the goal of this section to describe all possible configurations nor
to provide a normative specification of the stateful NAT64 behavior.
A more complete set of possible deployment scenarios is described in
[RFC8683]. For the sake of clarity, only TCP and UDP are described
in this overview; the details of ICMP, fragmentation, and other
aspects of translation are purposefully avoided in this overview.
The normative specification of stateful NAT64 is provided in
Section 3.

The stateful NAT64 function is implemented in a device that has (at
least) two interfaces, an IPv4 interface connected to the IPv4
network, and an IPv6 interface connected to the IPv6 network.
Packets generated in the IPv6 network for a receiver located in the
IPv4 network will be forwarded within the IPv6 network towards the

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stateful NAT64 function. The stateful NAT64 will translate them and
forward them as IPv4 packets through the IPv4 network to the IPv4
receiver. The reverse takes place for packets generated by hosts
connected to the IPv4 network for an IPv6 receiver. Stateful NAT64,
however, is not symmetric. In order to be able to perform IPv6-IPv4
translation, stateful NAT64 requires state. The state contains the
binding of an IPv6 address and TCP/UDP port (hereafter called an IPv6
transport address) to an IPv4 address and TCP/UDP port number
(hereafter called an IPv4 transport address).

Such binding state is either statically configured in the stateful
NAT64 or it is created when the first packet flowing from the IPv6
network to the IPv4 network is translated. After the binding state
has been created, packets flowing in both directions on that
particular flow are translated. The result is that, in the general
case, stateful NAT64 only supports communications initiated by the
IPv6-only node towards an IPv4-only node. Some additional mechanisms
(like ICE) or static binding configuration can be used to provide
support for communications initiated by an IPv4-only node to an
IPv6-only node.

1.2.1. Stateful NAT64 Solution Elements

The different elements involved in the stateful NAT64 approach are
described below.

The main component of the solution is the translator itself. The
translator has essentially two main parts, the address translation
mechanism and the protocol translation mechanism.

Protocol translation from an IPv4 packet header to an IPv6 packet
header and vice versa is performed according to the IP/ICMP
Translation Algorithm [RFC7915].

Address translation maps IPv6 transport addresses to IPv4 transport
addresses and vice versa. In order to create these mappings, the
stateful NAT64 has two pools of addresses: an IPv6 address pool (to
represent IPv4 addresses in the IPv6 network) and an IPv4 address
pool (to represent IPv6 addresses in the IPv4 network).

The IPv6 address pool is one or more IPv6 prefixes assigned to the
translator itself. Hereafter, the IPv6 address pool is referenced as
Pref64::/n; in the case there is more than one prefix assigned to the
stateful NAT64, the comments made about Pref64::/n apply to each of
them. Pref64::/n will be used by the stateful NAT64 to construct
IPv4-Converted IPv6 addresses as defined in [RFC6052]. Due to the
abundance of IPv6 address space, it is possible to assign one or more
Pref64::/n, each of them being equal to or even bigger than the size

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of the whole IPv4 address space. This allows each IPv4 address to be
mapped into a different IPv6 address by simply concatenating a
Pref64::/n with the IPv4 address being mapped and a suffix. The
provisioning of the Pref64::/n as well as the address format are
defined in [RFC6052].

The IPv4 address pool is a set of IPv4 addresses, normally a prefix
assigned by the local administrator. Since IPv4 address space is a
scarce resource, the IPv4 address pool is small and typically not
sufficient to establish permanent one-to-one mappings with IPv6
addresses. So, except for the static/manually created ones, mappings
using the IPv4 address pool will be created and released dynamically.
Moreover, because of the IPv4 address scarcity, the usual practice
for stateful NAT64 is likely to be the binding of IPv6 transport
addresses into IPv4 transport addresses, instead of IPv6 addresses
into IPv4 addresses directly, enabling a higher utilization of the
limited IPv4 address pool. This implies that stateful NAT64 performs
both address and port translation.

Because of the dynamic nature of the IPv6-to-IPv4 address mapping and
the static nature of the IPv4-to-IPv6 address mapping, it is far
simpler to allow communications initiated from the IPv6 side toward
an IPv4 node, whose address is algorithmically mapped into an IPv6
address, than communications initiated from IPv4-only nodes to an
IPv6 node. In that case, an IPv4 address needs to be associated with
the IPv6 node's address dynamically.

Using a mechanism such as DNS64, an IPv6 client obtains an IPv6
address that embeds the IPv4 address of the IPv4 server and sends a
packet to that IPv6 address. The packets are intercepted by the
stateful NAT64 function, which associates an IPv4 transport address
out of its IPv4 pool to the IPv6 transport address of the initiator,
creating binding state, so that reply packets can be translated and
forwarded back to the initiator. The binding state is kept while
packets are flowing. Once the flow stops, and based on a timer, the
IPv4 transport address is returned to the IPv4 address pool so that
it can be reused for other communications.

To allow an IPv6 initiator to do a DNS lookup to learn the address of
the responder, DNS64 [RFC6147] is used to synthesize AAAA RRs from
the A RRs. The IPv6 addresses contained in the synthetic AAAA RRs
contain a Pref64::/n assigned to the stateful NAT64 and the IPv4
address of the responder. The synthetic AAAA RRs are passed back to
the IPv6 initiator, which will initiate an IPv6 communication with an
IPv6 address associated to the IPv4 receiver. The packet will be
forwarded to the stateful NAT64 function, which will create the IPv6-
to-IPv4 address mapping as described before.

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1.2.2. Stateful NAT64 Behavior Walk-Through

This section describes a simple example of the stateful NAT64
behavior. For that, an IPv6 node located in an IPv6-only network
initiates a TCP connection to an IPv4-only node located in the
IPv4-only network. The role of the stateful NAT64 function is to
interconnect both networks and enable the communication between the
nodes, by translating the packets in both directions.

The scenario for this case is depicted in the following figure:

+---------------------+ +---------------+
|IPv6 network | | IPv4 |
| +-------------+ | network |
| |--| Name server |--| |
| | | with DNS64 | | +----+ |
| +----+ | +-------------+ | | H2 | |
| | H1 |---| | | +----+ |
| +----+ | +-------+ | 192.0.2.1 |
|2001:db8::1|------| NAT64 |----| |
| +-------+ | |
| | | |
+---------------------+ +---------------+

Figure 1: Sample stateful NAT64 Topology

Figure 1 shows an IPv6 node H1 with an IPv6 address 2001:db8::1 and
an IPv4 node H2 with IPv4 address 192.0.2.1. H2 has h2.example.com
as its Fully Qualified Domain Name (FQDN).

A stateful NAT64 connects the IPv6 network to the IPv4 network. This
stateful NAT64 uses the Well-Known Prefix (WKP) 64:ff9b::/96 defined
in [RFC6052] to represent IPv4 addresses in the IPv6 address space
and a single IPv4 address 203.0.113.1 assigned to its IPv4 interface.
The routing is configured in such a way that the IPv6 packets
addressed to a destination address in 64:ff9b::/96 are forwarded to
the IPv6 interface of the stateful NAT64 function.

Also shown is a local resolving DNS server with DNS64 functionality.
The local DNS server uses the Well-Known Prefix 64:ff9b::/96 to
create the IPv6 addresses in the synthetic RRs.

For this example, assume the typical DNS situation where IPv6 hosts
have only stub resolvers, and the local resolving DNS server does the
recursive lookups and the DNS64 synthesis.

The steps by which H1 establishes communication with H2 are:

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1. H1 performs a DNS query for h2.example.com and receives the
synthetic AAAA RR from the local resolving DNS server that
implements the DNS64 functionality. The AAAA record contains an
IPv6 address formed by the Well-Known Prefix and the IPv4 address
of H2 (i.e., 64:ff9b::192.0.2.1).

2. H1 sends a TCP SYN packet to H2. The packet is sent from a
source transport address of (2001:db8::1,1500) to a destination
transport address of (64:ff9b::192.0.2.1,80), where the ports are
set by H1.

3. The packet is forwarded to the IPv6 interface of the stateful
NAT64 (since IPv6 routing is configured that way).

4. The stateful NAT64 receives the packet and performs the following
actions:

* The stateful NAT64 selects an unused port (e.g., 2000) on its
IPv4 address 203.0.113.1 and creates the mapping entry
(2001:db8::1,1500) <--> (203.0.113.1,2000)

* The stateful NAT64 translates the IPv6 header into an IPv4
header using the IP/ICMP Translation Algorithm [RFC7915].

* The stateful NAT64 includes (203.0.113.1,2000) as the source
transport address in the packet and (192.0.2.1,80) as the
destination transport address in the packet. Note that
192.0.2.1 is extracted directly from the destination IPv6
address of the received IPv6 packet that is being translated.
The destination port number 80 of the translated packet is the
same as the destination port of the received IPv6 packet.

5. The stateful NAT64 sends the translated packet out of its IPv4
interface and the packet arrives at H2.

6. H2 node responds by sending a TCP SYN+ACK packet with the
destination transport address (203.0.113.1,2000) and source
transport address (192.0.2.1,80).

7. Since the IPv4 address 203.0.113.1 is assigned to the IPv4
interface of the stateful NAT64 device, the packet is forwarded
to the stateful NAT64 function, which will look for an existing
mapping containing (203.0.113.1,2000). Since the mapping
(2001:db8::1,1500) <--> (203.0.113.1,2000) exists, the stateful
NAT64 performs the following operations:

* The stateful NAT64 translates the IPv4 header into an IPv6
header using the IP/ICMP Translation Algorithm [RFC7915].

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* The stateful NAT64 includes (2001:db8::1,1500) as the
destination transport address in the packet and
(64:ff9b::192.0.2.1,80) as the source transport address in the
packet. Note that 192.0.2.1 is extracted directly from the
source IPv4 address of the received IPv4 packet that is being
translated. The source port number 80 of the translated
packet is the same as the source port of the received IPv4
packet.

8. The translated packet is sent out of the IPv6 interface to H1.

The packet exchange between H1 and H2 continues, and packets are
translated in the different directions as previously described.

It is important to note that the translation still works if the IPv6
initiator H1 learns the IPv6 representation of H2's IPv4 address
(i.e., 64:ff9b::192.0.2.1) through some scheme other than a DNS
lookup. This is because the DNS64 processing does not result in any
state being installed in the stateful NAT64 and because the mapping
of the IPv4 address into an IPv6 address is the result of
concatenating the Well-Known Prefix to the original IPv4 address.

1.2.3. Filtering

Stateful NAT64 may do filtering, which means that it only allows a
packet in through an interface under certain circumstances. The
stateful NAT64 can filter IPv6 packets based on the administrative
rules to create entries in the binding and session tables. The
filtering can be flexible and general, but the idea of the filtering
is to provide the administrators necessary control to avoid denial-
of-service (DoS) attacks that would result in exhaustion of the
stateful NAT64's IPv4 address, port, memory, and CPU resources.
Filtering techniques of incoming IPv6 packets are not specific to the
stateful NAT64 and therefore are not described in this specification.

Filtering of IPv4 packets, on the other hand, is tightly coupled to
the stateful NAT64 state and therefore is described in this
specification. This document considers that the stateful NAT64 may
do no filtering, or it may filter incoming IPv4 packets.

Stateful NAT64 filtering of incoming IPv4 packets is consistent with
the recommendations of [RFC4787] and [RFC5382]. Because of that, the
stateful NAT64 supports both Endpoint-Independent Filtering and
Address-Dependent Filtering, both for TCP and UDP as well as
filtering of ICMP packets.

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If a stateful NAT64 performs Endpoint-Independent Filtering of
incoming IPv4 packets, then an incoming IPv4 packet is dropped unless
the stateful NAT64 has state for the destination transport address of
the incoming IPv4 packet.

If a stateful NAT64 performs Address-Dependent Filtering of incoming
IPv4 packets, then an incoming IPv4 packet is dropped unless the
stateful NAT64 has state involving the destination transport address
of the IPv4 incoming packet and the particular source IP address of
the incoming IPv4 packet.

2. Terminology

This section provides a definitive reference for all the terms used
in the document.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

The following additional terms are used in this document:

3-Tuple: The tuple (source IP address, destination IP address, ICMP
Identifier). A 3-tuple uniquely identifies an ICMP Query session.
When an ICMP Query session flows through a stateful NAT64, each
session has two different 3-tuples: one with IPv4 addresses and
one with IPv6 addresses.

5-Tuple: The tuple (source IP address, source port, destination IP
address, destination port, transport protocol). A 5-tuple
uniquely identifies a UDP/TCP session. When a UDP/TCP session
flows through a stateful NAT64, each session has two different
5-tuples: one with IPv4 addresses and one with IPv6 addresses.

BIB: Binding Information Base. A table of bindings kept by a
stateful NAT64. Each stateful NAT64 has a BIB for each translated
protocol. An implementation compliant to this document would have
a BIB for TCP, one for UDP, and one for ICMP Queries. Additional
BIBs would be added to support other protocols, such as SCTP.

Endpoint-Independent Mapping: In stateful NAT64, using the same

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mapping for all the sessions involving a given IPv6 transport
address of an IPv6 host (irrespectively of the transport address
of the IPv4 host involved in the communication). Endpoint-
Independent Mapping is important for peer-to-peer communication.
See [RFC4787] for the definition of the different types of
mappings in IPv4-to-IPv4 NATs.

Filtering, Endpoint-Independent: The stateful NAT64 only filters
incoming IPv4 packets destined to a transport address for which
there is no state in the stateful NAT64, regardless of the source
IPv4 transport address. The NAT forwards any packets destined to
any transport address for which it has state. In other words,
having state for a given transport address is sufficient to allow
any packets back to the internal endpoint. See [RFC4787] for the
definition of the different types of filtering in IPv4-to-IPv4
NATs.

Filtering, Address-Dependent: The stateful NAT64 filters incoming
IPv4 packets destined to a transport address for which there is no
state (similar to the Endpoint-Independent Filtering).
Additionally, the stateful NAT64 will filter out incoming IPv4
packets coming from a given IPv4 address X and destined for a
transport address for which it has state if the stateful NAT64 has
not sent packets to X previously (independently of the port used
by X). In other words, for receiving packets from a specific IPv4
endpoint, it is necessary for the IPv6 endpoint to send packets
first to that specific IPv4 endpoint's IP address.

Hairpinning: Having a packet do a "U-turn" inside a NAT and come
back out the same side as it arrived on. If the destination IPv6
address and its embedded IPv4 address are both assigned to the
stateful NAT64 itself, then the packet is being sent to another
IPv6 host connected to the same stateful NAT64. Such a packet is
called a 'hairpin packet'. A stateful NAT64 that forwards hairpin
packets back to the IPv6 host is defined as supporting
"hairpinning". Hairpinning support is important for peer-to-peer
applications, as there are cases when two different hosts on the
same side of a NAT can only communicate using sessions that
hairpin through the NAT. Hairpin packets can be either TCP or
UDP. More detailed explanation of hairpinning and examples for
the UDP case can be found in [RFC4787].

ICMP Query packet: ICMP packets that are not ICMP error messages.
For ICMPv6, ICMPv6 Query Messages are the ICMPv6 Informational
messages as defined in [RFC4443]. For ICMPv4, ICMPv4 Query
messages are all ICMPv4 messages that are not ICMPv4 error
messages.

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Mapping or Binding: A mapping between an IPv6 transport address and
a IPv4 transport address or a mapping between an (IPv6 address,
ICMPv6 Identifier) pair and an (IPv4 address, ICMPv4 Identifier)
pair. Used to translate the addresses and ports / ICMP
Identifiers of packets flowing between the IPv6 host and the IPv4
host. In stateful NAT64, the IPv4 address and port / ICMPv4
Identifier is always one assigned to the stateful NAT64 itself,
while the IPv6 address and port / ICMPv6 Identifier belongs to
some IPv6 host.

Session: The flow of packets between two different hosts. This may
be TCP, UDP, or ICMP Queries. In stateful NAT64, typically one
host is an IPv4 host, and the other one is an IPv6 host. However,
due to hairpinning, both hosts might be IPv6 hosts.

Session table: A table of sessions kept by a stateful NAT64. Each
stateful NAT64 has three session tables, one for TCP, one for UDP,
and one for ICMP Queries.

Stateful NAT64: A function that has per-flow state that translates
IPv6 packets to IPv4 packets and vice versa, for TCP, UDP, and
ICMP. The stateful NAT64 uses binding state to perform the
translation between IPv6 and IPv4 addresses.

Stateful NAT64 device: The device where the stateful NAT64 function
is executed.

Transport Address: The combination of an IPv6 or IPv4 address and a
port. Typically written as (IP address,port), e.g.,
(192.0.2.15,8001).

Tuple: Refers to either a 3-tuple or a 5-tuple as defined above.

For a detailed understanding of this document, the reader should also
be familiar with NAT terminology [RFC4787].

3. Stateful NAT64 Normative Specification

A stateful NAT64 is a device with at least one IPv6 interface and at
least one IPv4 interface. Each stateful NAT64 device MUST have at
least one unicast /n IPv6 prefix assigned to it, denoted Pref64::/n.
Additional considerations about the Pref64::/n are presented in
Section 3.5.4. A stateful NAT64 MUST have one or more unicast IPv4
addresses assigned to it.

A stateful NAT64 uses the following conceptual dynamic data
structures:

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* UDP Binding Information Base

* UDP Session Table

* TCP Binding Information Base

* TCP Session Table

* ICMP Query Binding Information Base

* ICMP Query Session Table

These tables contain information needed for the stateful NAT64
processing. The actual division of the information into six tables
is done in order to ease the description of the stateful NAT64
behavior. Stateful NAT64 implementations are free to use different
data structures but they MUST store all the required information, and
the externally visible outcome MUST be the same as the one described
in this document.

The notation used is the following: uppercase letters are IPv4
addresses; uppercase letters with a prime(') are IPv6 addresses;
lowercase letters are ports; IPv6 prefixes of length n are indicated
by "P::/n"; mappings are indicated as "(X,x) <--> (Y',y)".

3.1. Binding Information Bases

A stateful NAT64 has three Binding Information Bases (BIBs): one for
TCP, one for UDP, and one for ICMP Queries. In the case of UDP and
TCP BIBs, each BIB entry specifies a mapping between an IPv6
transport address and an IPv4 transport address:

(X',x) <--> (T,t)

where X' is some IPv6 address, T is an IPv4 address, and x and t are
ports. T will always be one of the IPv4 addresses assigned to the
stateful NAT64. The BIB has then two columns: the BIB IPv6 transport
address and the BIB IPv4 transport address. A given IPv6 or IPv4
transport address can appear in at most one entry in a BIB: for
example, (2001:db8::17, 49832) can appear in at most one TCP and at
most one UDP BIB entry. TCP and UDP have separate BIBs because the
port number space for TCP and UDP are distinct. If the BIBs are
implemented as specified in this document, it results in Endpoint-
Independent Mappings in the stateful NAT64. The information in the
BIBs is also used to implement Endpoint-Independent Filtering.
(Address-Dependent Filtering is implemented using the session tables
described below.)

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In the case of the ICMP Query BIB, each ICMP Query BIB entry
specifies a mapping between an (IPv6 address, ICMPv6 Identifier) pair
and an (IPv4 address, ICMPv4 Identifier) pair.

(X',i1) <--> (T,i2)

where X' is some IPv6 address, T is an IPv4 address, i1 is an ICMPv6
Identifier, and i2 is an ICMPv4 Identifier. T will always be one of
the IPv4 addresses assigned to the stateful NAT64. A given (IPv6 or
IPv4 address, ICMPv6 or ICMPv4 Identifier) pair can appear in at most
one entry in the ICMP Query BIB.

Entries in any of the three BIBs can be created dynamically as the
result of the flow of packets as described in Section 3.5, but they
can also be created manually by an administrator. Stateful NAT64
implementations SHOULD support manually configured BIB entries for
any of the three BIBs. Dynamically created entries are deleted from
the corresponding BIB when the last session associated with the BIB
entry is removed from the session table. Manually configured BIB
entries are not deleted when there is no corresponding Session
Table Entry and can only be deleted by the administrator.

3.2. Session Tables

A stateful NAT64 also has three session tables: one for TCP sessions,
one for UDP sessions, and one for ICMP Query sessions. Each entry
keeps information on the state of the corresponding session. In the
TCP and UDP session tables, each entry specifies a mapping between a
pair of IPv6 transport addresses and a pair of IPv4 transport
addresses:

(X',x),(Y',y) <--> (T,t),(Z,z)

where X' and Y' are IPv6 addresses, T and Z are IPv4 addresses, and
x, y, z, and t are ports. T will always be one of the IPv4 addresses
assigned to the stateful NAT64. Y' is always the IPv6 representation
of the IPv4 address Z, so Y' is obtained from Z using the algorithm
applied by the stateful NAT64 to create IPv6 representations of IPv4
addresses. y will always be equal to z.

For each TCP or UDP Session Table Entry (STE), there are then five
columns. The terminology used for the STE columns is from the
perspective of an incoming IPv6 packet being translated into an
outgoing IPv4 packet. The columns are:

The STE source IPv6 transport address; (X',x) in the example
above.

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The STE destination IPv6 transport address; (Y',y) in the example
above.

The STE source IPv4 transport address; (T,t) in the example above.

The STE destination IPv4 transport address; (Z,z) in the example
above.

The STE lifetime.

In the ICMP Query session table, each entry specifies a mapping
between a 3-tuple of IPv6 source address, IPv6 destination address,
and ICMPv6 Identifier and a 3-tuple of IPv4 source address, IPv4
destination address, and ICMPv4 Identifier:

(X',Y',i1) <--> (T,Z,i2)

where X' and Y' are IPv6 addresses, T and Z are IPv4 addresses, i1 is
an ICMPv6 Identifier, and i2 is an ICMPv4 Identifier. T will always
be one of the IPv4 addresses assigned to the stateful NAT64. Y' is
always the IPv6 representation of the IPv4 address Z, so Y' is
obtained from Z using the algorithm applied by the stateful NAT64 to
create IPv6 representations of IPv4 addresses.

For each ICMP Query Session Table Entry (STE), there are then seven
columns:

The STE source IPv6 address; X' in the example above.

The STE destination IPv6 address; Y' in the example above.

The STE ICMPv6 Identifier; i1 in the example above.

The STE source IPv4 address; T in the example above.

The STE destination IPv4 address; Z in the example above.

The STE ICMPv4 Identifier; i2 in the example above.

The STE lifetime.

3.3. Packet Processing Overview

The stateful NAT64 uses the session state information to determine
when the session is completed, and also uses session information for
Address-Dependent Filtering. A session can be uniquely identified by
either an incoming tuple or an outgoing tuple.

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For each TCP or UDP session, there is a corresponding BIB entry,
uniquely specified by either the source IPv6 transport address (in
the IPv6 --> IPv4 direction) or the destination IPv4 transport
address (in the IPv4 --> IPv6 direction). For each ICMP Query
session, there is a corresponding BIB entry, uniquely specified by
either the source IPv6 address and ICMPv6 Identifier (in the IPv6 -->
IPv4 direction) or the destination IPv4 address and the ICMPv4
Identifier (in the IPv4 --> IPv6 direction). However, for all the
BIBs, a single BIB entry can have multiple corresponding sessions.
When the last corresponding session is deleted, if the BIB entry was
dynamically created, the BIB entry is deleted.

The stateful NAT64 will receive packets through its interfaces.
These packets can be either IPv6 packets or IPv4 packets, and they
may carry TCP traffic, UDP traffic, or ICMP traffic. The processing
of the packets will be described next. In the case that the
processing is common to all the aforementioned types of packets, will
refer to the packet as the incoming IP packet in general. In the
case that the processing is specific to IPv6 packets, will explicitly
refer to the incoming packet as an incoming IPv6 packet; analogous
terminology will apply in the case of processing that is specific to
IPv4 packets.

The processing of an incoming IP packet takes the following steps:

1. Determining the incoming tuple

2. Filtering and updating binding and session information

3. Computing the outgoing tuple

4. Translating the packet

5. Handling hairpinning

The details of these steps are specified in the following
subsections.

This breakdown of the stateful NAT64 behavior into processing steps
is done for ease of presentation. A stateful NAT64 MAY perform the
steps in a different order or MAY perform different steps, but the
externally visible outcome MUST be the same as described.

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3.4. Determining the Incoming Tuple

This step associates an incoming tuple with every incoming IP packet
for use in subsequent steps. In the case of TCP, UDP, and ICMP error
packets, the tuple is a 5-tuple consisting of the source IP address,
source port, destination IP address, destination port, and transport
protocol. In case of ICMP Queries, the tuple is a 3-tuple consisting
of the source IP address, destination IP address, and ICMP
Identifier.

If the incoming IP packet contains a complete (un-fragmented) UDP or
TCP protocol packet, then the 5-tuple is computed by extracting the
appropriate fields from the received packet.

If the incoming packet is a complete (un-fragmented) ICMP Query
message (i.e., an ICMPv4 Query message or an ICMPv6 Informational
message), the 3-tuple is the source IP address, the destination IP
address, and the ICMP Identifier.

If the incoming IP packet contains a complete (un-fragmented) ICMP
error message containing a UDP or a TCP packet, then the incoming
5-tuple is computed by extracting the appropriate fields from the IP
packet embedded inside the ICMP error message. However, the role of
source and destination is swapped when doing this: the embedded
source IP address becomes the destination IP address in the incoming
5-tuple, the embedded source port becomes the destination port in the
incoming 5-tuple, etc. If it is not possible to determine the
incoming 5-tuple (perhaps because not enough of the embedded packet
is reproduced inside the ICMP message), then the incoming IP packet
MUST be silently discarded.

If the incoming IP packet contains a complete (un-fragmented) ICMP
error message containing an ICMP error message, then the packet is
silently discarded.

If the incoming IP packet contains a complete (un-fragmented) ICMP
error message containing an ICMP Query message, then the incoming
3-tuple is computed by extracting the appropriate fields from the IP
packet embedded inside the ICMP error message. However, the role of
source and destination is swapped when doing this: the embedded
source IP address becomes the destination IP address in the incoming
3-tuple, the embedded destination IP address becomes the source
address in the incoming 3-tuple, and the embedded ICMP Identifier is
used as the ICMP Identifier of the incoming 3-tuple. If it is not
possible to determine the incoming 3-tuple (perhaps because not
enough of the embedded packet is reproduced inside the ICMP message),
then the incoming IP packet MUST be silently discarded.

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If the incoming IP packet contains a fragment, then more processing
may be needed. This specification leaves open the exact details of
how a stateful NAT64 handles incoming IP packets containing
fragments, and simply requires that the external behavior of the
stateful NAT64 be compliant with the following conditions:

The stateful NAT64 MUST handle fragments. In particular, stateful
NAT64 MUST handle fragments arriving out of order, conditional on
the following:

- The stateful NAT64 MUST limit the amount of resources devoted
to the storage of fragmented packets in order to protect from
DoS attacks.

- As long as the stateful NAT64 has available resources, the
stateful NAT64 MUST allow the fragments to arrive over a time
interval. The time interval SHOULD be configurable and the
default value MUST be of at least FRAGMENT_MIN.

- The stateful NAT64 MAY require that the UDP, TCP, or ICMP
header be completely contained within the fragment that
contains fragment offset equal to zero.

For incoming packets carrying TCP or UDP fragments with a non-zero
checksum, stateful NAT64 MAY elect to queue the fragments as they
arrive and translate all fragments at the same time. In this
case, the incoming tuple is determined as documented above to the
un-fragmented packets. Alternatively, a stateful NAT64 MAY
translate the fragments as they arrive, by storing information
that allows it to compute the 5-tuple for fragments other than the
first. In the latter case, subsequent fragments may arrive before
the first, and the rules (in the bulleted list above) about how
the stateful NAT64 handles (out-of-order) fragments apply.

For incoming IPv4 packets carrying UDP packets with a zero
checksum, if the stateful NAT64 has enough resources, the stateful
NAT64 MUST reassemble the packets and MUST calculate the checksum.
If the stateful NAT64 does not have enough resources, then it MUST
silently discard the packets. The handling of fragmented and un-
fragmented UDP packets with a zero checksum as specified above
deviates from that specified in [RFC7915].

Implementers of stateful NAT64 should be aware that there are a
number of well-known attacks against IP fragmentation; see
[RFC1858] and [RFC3128]. Implementers should also be aware of
additional issues with reassembling packets at high rates,
described in [RFC4963], as well as fragment translation processing
in sections 1.4 and 5.1.1 of [RFC7915].

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If the incoming packet is an IPv6 packet that contains a protocol
other than TCP, UDP, or ICMPv6 in the last Next Header, then the
packet SHOULD be discarded and, if the security policy permits, the
stateful NAT64 SHOULD send an ICMPv6 Destination Unreachable error
message with Code 4 (Port Unreachable) to the source address of the
received packet. NOTE: This behavior may be updated by future
documents that define how other protocols such as SCTP or DCCP are
processed by stateful NAT64.

If the incoming packet is an IPv4 packet that contains a protocol
other than TCP, UDP, or ICMPv4, then the packet SHOULD be discarded
and, if the security policy permits, the stateful NAT64 SHOULD send
an ICMPv4 Destination Unreachable error message with Code 2 (Protocol
Unreachable) to the source address of the received packet. NOTE:
This behavior may be updated by future documents that define how
other protocols such as SCTP or DCCP are processed by stateful NAT64.

3.5. Filtering and Updating Binding and Session Information

This step updates binding and session information stored in the
appropriate tables. May also filter incoming packets, if desired.

The details depend on the protocol, i.e., UDP, TCP, or ICMP. The
behaviour for UDP, TCP, and ICMP Queries is described in
Section 3.5.1, Section 3.5.2, and Section 3.5.3, respectively. For
the case of ICMP error messages, they do not affect in any way either
the BIBs or the session tables, so there is no processing resulting
from these messages in this section. ICMP error message processing
continues in Section 3.6.

Irrespective of the transport protocol used, the stateful NAT64 MUST
silently discard all incoming IPv6 packets containing a source
address that contains the Pref64::/n. This is required in order to
prevent hairpinning loops as described in Section 8. In addition,
the stateful NAT64 MUST only process incoming IPv6 packets that
contain a destination address that contains Pref64::/n. Likewise,
the stateful NAT64 MUST only process incoming IPv4 packets that
contain a destination address that belongs to the IPv4 pool assigned
to the stateful NAT64.

3.5.1. UDP Session Handling

The following state information is stored for a UDP session:

Binding:(X',x),(Y',y) <--> (T,t),(Z,z)

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Lifetime: a timer that tracks the remaining lifetime of the UDP
session. When the timer expires, the UDP session is deleted. If
all the UDP sessions corresponding to a dynamically created UDP
BIB entry are deleted, then the UDP BIB entry is also deleted.

An IPv6 incoming packet with an incoming tuple with source transport
address (X',x) and destination transport address (Y',y) is processed
as follows:

The NAT64 searches for a UDP BIB entry that contains the BIB IPv6
transport address that matches the IPv6 source transport address
(X',x). If such an entry does not exist, the stateful NAT64 tries
to create a new entry (if resources and policy permit). The
source IPv6 transport address of the packet (X',x) is used as the
BIB IPv6 transport address, and the BIB IPv4 transport address is
set to (T,t), which is allocated using the rules defined in
Section 3.5.1.1. The result is a BIB entry as follows: (X',x)
<--> (T,t).

The stateful NAT64 searches for the Session Table Entry
corresponding to the incoming 5-tuple. If no such entry is found,
the stateful NAT64 tries to create a new entry (if resources and
policy permit). The information included in the session table is
as follows:

- The STE source IPv6 transport address is set to (X',x), i.e.,
the source IPv6 transport address contained in the received
IPv6 packet.

- The STE destination IPv6 transport address is set to (Y',y),
i.e., the destination IPv6 transport address contained in the
received IPv6 packet.

- The STE source IPv4 transport address is extracted from the
corresponding UDP BIB entry, i.e., it is set to (T,t).

- The STE destination IPv4 transport is set to (Z(Y'),y), y being
the same port as the STE destination IPv6 transport address and
Z(Y') being algorithmically generated from the IPv6 destination
address (i.e., Y') using the reverse algorithm (see
Section 3.5.4).

The result is a Session Table Entry as follows:

(X',x),(Y',y) <--> (T,t),(Z(Y'),y)

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The stateful NAT64 sets (or resets) the timer in the Session
Table Entry to the maximum session lifetime. The maximum session
lifetime MAY be configurable, and the default SHOULD be at least
UDP_DEFAULT. The maximum session lifetime MUST NOT be less than
UDP_MIN. The packet is translated and forwarded as described in
the following sections.

An IPv4 incoming packet, with an incoming tuple with source IPv4
transport address (W,w) and destination IPv4 transport address (T,t)
is processed as follows:

The stateful NAT64 searches for a UDP BIB entry that contains the
BIB IPv4 transport address matching (T,t), i.e., the IPv4
destination transport address in the incoming IPv4 packet. If
such an entry does not exist, the packet MUST be dropped. An ICMP
error message with Type 3 (Destination Unreachable) MAY be sent to
the original sender of the packet.

If the stateful NAT64 applies Address-Dependent Filters on its
IPv4 interface, then the stateful NAT64 checks to see if the
incoming packet is allowed according to the Address-Dependent
Filtering rule. To do this, it searches for a Session Table Entry
with an STE source IPv4 transport address equal to (T,t), i.e.,
the destination IPv4 transport address in the incoming packet, and
STE destination IPv4 address equal to W, i.e., the source IPv4
address in the incoming packet. If such an entry is found (there
may be more than one), packet processing continues. Otherwise,
the packet is discarded. If the packet is discarded, then an ICMP
error message MAY be sent to the original sender of the packet.
The ICMP error message, if sent, has Type 3 (Destination
Unreachable) and Code 13 (Communication Administratively
Prohibited).

In case the packet is not discarded in the previous processing
(either because the stateful NAT64 is not filtering or because the
packet is compliant with the Address-Dependent Filtering rule),
then the stateful NAT64 searches for the Session Table Entry
containing the STE source IPv4 transport address equal to (T,t)
and the STE destination IPv4 transport address equal to (W,w). If
no such entry is found, the stateful NAT64 tries to create a new
entry (if resources and policy permit). In case a new UDP Session
Table Entry is created, it contains the following information:

- The STE source IPv6 transport address is extracted from the
corresponding UDP BIB entry.

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- The STE destination IPv6 transport address is set to (Y'(W),w),
w being the same port w as the source IPv4 transport address
and Y'(W) being the IPv6 representation of W, generated using
the algorithm described in Section 3.5.4.

- The STE source IPv4 transport address is set to (T,t), i.e.,
the destination IPv4 transport addresses contained in the
received IPv4 packet.

- The STE destination IPv4 transport is set to (W,w), i.e., the
source IPv4 transport addresses contained in the received IPv4
packet.

3.5.1.1. Rules for Allocation of IPv4 Transport Addresses for UDP

When a new UDP BIB entry is created for a source transport address of
(S',s), the stateful NAT64 allocates an IPv4 transport address for
this BIB entry as follows:

If there exists some other BIB entry containing S' as the IPv6
address and mapping it to some IPv4 address T, then the stateful
NAT64 SHOULD use T as the IPv4 address. Otherwise, use any IPv4
address of the IPv4 pool assigned to the stateful NAT64 to be used
for translation.

If the port s is in the Well-Known port range 0-1023, and the
stateful NAT64 has an available port t in the same port range,
then the stateful NAT64 SHOULD allocate the port t. If the
stateful NAT64 does not have a port available in the same range,
the stateful NAT64 MAY assign a port t from another range where it
has an available port. (This behavior is recommended in REQ 3-a
of [RFC4787].)

If the port s is in the range 1024-65535, and the stateful NAT64
has an available port t in the same port range, then the stateful
NAT64 SHOULD allocate the port t. If the stateful NAT64 does not
have a port available in the same range, the stateful NAT64 MAY
assign a port t from another range where it has an available port.
(This behavior is recommended in REQ 3-a of [RFC4787].)

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The stateful NAT64 SHOULD preserve the port parity (odd/even), as
per Section 4.2.2 of [RFC4787]. The stateful NAT64 MAY disable
port parity preservation for all dynamic Mappings, per Section 8
of [RFC7857].

In all cases, the allocated IPv4 transport address (T,t) MUST NOT
be in use in another entry in the same BIB, but can be in use in
other BIBs (e.g., the TCP and ICMP BIBs).

If it is not possible to allocate an appropriate IPv4 transport
address or create a BIB entry, then the packet is discarded. The
stateful NAT64 SHOULD send an ICMPv6 Destination Unreachable error
message with Code 3 (Address Unreachable).

3.5.2. TCP Session Handling

A description about how the TCP BIB and session tables are populated
is provided by defining the state machine that the stateful NAT64
uses for TCP. First the states and the information contained in them
are described, followed by the actual state machine and state
transitions.

3.5.2.1. State Definition

The following state information is stored for a TCP session:

Binding:(X',x),(Y',y) <--> (T,t),(Z,z)

Lifetime: a timer that tracks the remaining lifetime of the TCP
session. When the timer expires, the TCP session is deleted. If
all the TCP sessions corresponding to a TCP BIB entry are deleted,
then the dynamically created TCP BIB entry is also deleted.

Because the TCP session inactivity lifetime is set to at least 2
hours and 4 minutes (as per [RFC5382]), it is important that each TCP
Session Table Entry corresponds to an existing TCP session. In order
to do that, for each TCP session established, the TCP connection
state is tracked using the following state machine.

The states are as follows:

CLOSED: Analogous to [RFC9293], CLOSED is a fictional state
because it represents the state when there is no state for this
particular 5-tuple, and therefore no connection.

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V4 INIT: An IPv4 packet containing a TCP SYN was received by the
stateful NAT64, implying that a TCP connection is being initiated
from the IPv4 side. The stateful NAT64 is now waiting for a
matching IPv6 packet containing the TCP SYN in the opposite
direction.

V6 INIT: An IPv6 packet containing a TCP SYN was received,
translated, and forwarded by the stateful NAT64, implying that a
TCP connection is being initiated from the IPv6 side. The
stateful NAT64 is now waiting for a matching IPv4 packet
containing the TCP SYN in the opposite direction.

ESTABLISHED: Represents an open connection, with data able to flow
in both directions.

V4 FIN RCV: An IPv4 packet containing a TCP FIN was received by
the stateful NAT64, data can still flow in the connection, and the
stateful NAT64 is waiting for a matching TCP FIN in the opposite
direction.

V6 FIN RCV: An IPv6 packet containing a TCP FIN was received by
the stateful NAT64, data can still flow in the connection, and the
stateful NAT64 is waiting for a matching TCP FIN in the opposite
direction.

V6 FIN + V4 FIN RCV: Both an IPv4 packet containing a TCP FIN and
an IPv6 packet containing a TCP FIN for this connection were
received by the stateful NAT64. The stateful NAT64 keeps the
connection state alive and forwards packets in both directions for
a short period of time to allow remaining packets (in particular,
the ACKs) to be delivered.

TRANS: The lifetime of the state for the connection is set to
TCP_TRANS minutes either because a packet containing a TCP RST was
received by the stateful NAT64 for this connection or simply
because the lifetime of the connection has decreased and there are
only TCP_TRANS minutes left. The stateful NAT64 will keep the
state for the connection for TCP_TRANS minutes, and if no other
data packets for that connection are received, the state for this
connection is then terminated.

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3.5.2.2. State Machine for TCP Processing in the Stateful NAT64

The state machine used by the stateful NAT64 for the TCP session
processing is depicted next. The described state machine handles all
TCP segments received through the IPv6 and IPv4 interface. There is
one state machine per TCP connection that is potentially established
through the stateful NAT64. After bootstrapping of the stateful
NAT64 function, all TCP sessions are in CLOSED state. As mentioned
above, the CLOSED state is a fictional state when there is no state
for that particular connection in the stateful NAT64. It should be
noted that there is one state machine per connection, so only packets
belonging to a given connection are inputs to the state machine
associated to that connection. In other words, when in the state
machine below is stated that a packet is received, it is implicit
that the incoming 5-tuple of the data packet matches to the one of
the state machine.

A TCP segment with the SYN flag set that is received through the IPv6
interface is called a V6 SYN, similarly, V4 SYN, V4 FIN, V6 FIN, V6
FIN + V4 FIN, V6 RST, and V4 RST.

Figure 2 presents a simplified version of the state machine; refer to
the text for the full specification of the state machine.

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+-----------------------------+
| |
V |
V6 +------+ V4 |
+----SYN------|CLOSED|-----SYN------+ |
| +------+ | |
| ^ | |
| |TCP_TRANS T.O. | |
V | V |
+-------+ +-------+ +-------+ |
|V6 INIT| | TRANS | |V4 INIT| |
+-------+ +-------+ +-------+ |
| | ^ | |
| data pkt | | |
| | V4 or V6 RST | |
| | TCP_EST T.O. | |
V4 SYN V | V6 SYN |
| +--------------+ | |
+--------->| ESTABLISHED |<---------+ |
+--------------+ |
| | |
V4 FIN V6 FIN |
| | |
V V |
+---------+ +----------+ |
| V4 FIN | | V6 FIN | |
| RCV | | RCV | |
+---------+ +----------+ |
| | |
V6 FIN V4 FIN TCP_TRANS
| | T.O.
V V |
+---------------------+ |
| V4 FIN + V6 FIN RCV |--------------------+
+---------------------+

Figure 2: Stateful NAT64 State Machine

Next, the state information and the transitions are described.

*** CLOSED ***

If a V6 SYN is received with an incoming tuple with source transport
address (X',x) and destination transport address (Y',y) (this is the
case of a TCP connection initiated from the IPv6 side), the
processing is as follows:

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1. The stateful NAT64 searches for a TCP BIB entry that matches the
IPv6 source transport address (X',x).

If such an entry does not exist, the stateful NAT64 tries to
create a new BIB entry (if resources and policy permit). The
BIB IPv6 transport address is set to (X',x), i.e., the source
IPv6 transport address of the packet. The BIB IPv4 transport
address is set to an IPv4 transport address allocated using
the rules defined in Section 3.5.2.3. The processing of the
packet continues as described in bullet 2.

If the entry already exists, then the processing continues as
described in bullet 2.

2. Then the stateful NAT64 tries to create a new TCP session entry
in the TCP session table (if resources and policy permit). The
information included in the session table is as follows:

The STE source IPv6 transport address is set to (X',x), i.e.,
the source transport address contained in the received V6 SYN
packet.

The STE destination IPv6 transport address is set to (Y',y),
i.e., the destination transport address contained in the
received V6 SYN packet.

The STE source IPv4 transport address is set to the BIB IPv4
transport address of the corresponding TCP BIB entry.

The STE destination IPv4 transport address contains the port y
(i.e., the same port as the IPv6 destination transport
address) and the IPv4 address that is algorithmically
generated from the IPv6 destination address (i.e., Y') using
the reverse algorithm as specified in Section 3.5.4.

The lifetime of the TCP Session Table Entry is set to at least
TCP_TRANS (the transitory connection idle timeout as defined
in [RFC5382]).

3. The state of the session is moved to V6 INIT.

4. The stateful NAT64 translates and forwards the packet as
described in the following sections.

If a V4 SYN packet is received with an incoming tuple with source
IPv4 transport address (Y,y) and destination IPv4 transport address
(X,x) (this is the case of a TCP connection initiated from the IPv4
side), the processing is as follows:

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If the security policy requires silently dropping externally
initiated TCP connections, then the packet is silently discarded.

Else, if the destination transport address contained in the
incoming V4 SYN (i.e., X,x) is not in use in the TCP BIB, then:

- The stateful NAT64 tries to create a new Session Table Entry in
the TCP session table (if resources and policy permit),
containing the following information:

o The STE source IPv4 transport address is set to (X,x), i.e.,
the destination transport address contained in the V4 SYN.

o The STE destination IPv4 transport address is set to (Y,y),
i.e., the source transport address contained in the V4 SYN.

o The STE source IPv6 transport address is left unspecified
and may be populated by other protocols that are out of the
scope of this specification.

o The STE destination IPv6 transport address contains the port
y (i.e., the same port as the STE destination IPv4 transport
address) and the IPv6 representation of Y (i.e., the IPv4
address of the STE destination IPv4 transport address),
generated using the algorithm described in Section 3.5.4.

- The state is moved to V4 INIT.

- The lifetime of the STE entry is set to TCP_INCOMING_SYN as per
[RFC5382], and the packet is stored. The result is that the
stateful NAT64 will not drop the packet based on the filtering,
nor create a BIB entry. Instead, the stateful NAT64 will only
create the Session Table Entry and store the packet. The
motivation for this is to support simultaneous open of TCP
connections.

If the destination transport address contained in the incoming V4
SYN (i.e., X,x) is in use in the TCP BIB, then:

- The stateful NAT64 tries to create a new Session Table Entry in
the TCP session table (if resources and policy permit),
containing the following information:

o The STE source IPv4 transport address is set to (X,x), i.e.,
the destination transport address contained in the V4 SYN.

o The STE destination IPv4 transport address is set to (Y,y),
i.e., the source transport address contained in the V4 SYN.

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o The STE source IPv6 transport address is set to the IPv6
transport address contained in the corresponding TCP BIB
entry.

- The state is moved to V4 INIT.

- If the stateful NAT64 is performing Address-Dependent
Filtering, the lifetime of the STE entry is set to
TCP_INCOMING_SYN as per [RFC5382], and the packet is stored.
The motivation for creating the Session Table Entry and storing
the packet (instead of simply dropping the packet based on the
filtering) is to support simultaneous open of TCP connections.

- If the stateful NAT64 is not performing Address-Dependent
Filtering, the lifetime of the STE is set to at least TCP_TRANS
(the transitory connection idle timeout as defined in
[RFC5382]), and it translates and forwards the packet as
described in the following sections.

For any other packet belonging to this connection:

If there is a corresponding entry in the TCP BIB, the packet
SHOULD be translated and forwarded if the security policy allows
doing so. The state remains unchanged.

If there is no corresponding entry in the TCP BIB, the packet is
silently discarded.

*** V4 INIT ***

If a V6 SYN is received with incoming tuple with source transport
address (X',x) and destination transport address (Y',y), then the
lifetime of the TCP Session Table Entry is set to at least the
maximum session lifetime. The value for the maximum session lifetime
MAY be configurable, but it MUST NOT be less than TCP_EST (the
established connection idle timeout as defined in [RFC5382]). The
default value for the maximum session lifetime SHOULD be set to
TCP_EST. The packet is translated and forwarded. The state is moved
to ESTABLISHED.

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If the lifetime expires, an ICMP Port Unreachable error (Type 3, Code
3) containing the IPv4 SYN packet stored is sent back to the source
of the v4 SYN, the Session Table Entry is deleted, and the state is
moved to CLOSED.

For any other packet, the packet SHOULD be translated and forwarded
if the security policy allows doing so. The state remains unchanged.

*** V6 INIT ***

If a V4 SYN is received (with or without the ACK flag set), with an
incoming tuple with source IPv4 transport address (Y,y) and
destination IPv4 transport address (X,x), then the state is moved to
ESTABLISHED. The lifetime of the TCP Session Table Entry is set to
at least the maximum session lifetime. The value for the maximum
session lifetime MAY be configurable, but it MUST NOT be less than
TCP_EST (the established connection idle timeout as defined in
[RFC5382]). The default value for the maximum session lifetime
SHOULD be set to TCP_EST. The packet is translated and forwarded.

If the lifetime expires, the Session Table Entry is deleted, and the
state is moved to CLOSED.

If a V6 SYN packet is received, the packet is translated and
forwarded. The lifetime of the TCP Session Table Entry is set to at
least TCP_TRANS. The state remains unchanged.

For any other packet, the packet SHOULD be translated and forwarded
if the security policy allows doing so. The state remains unchanged.

*** ESTABLISHED ***

If a V4 FIN packet is received, the packet is translated and
forwarded. The state is moved to V4 FIN RCV.

If a V6 FIN packet is received, the packet is translated and
forwarded. The state is moved to V6 FIN RCV.

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If a V4 RST or a V6 RST packet is received, the packet is translated
and forwarded. The lifetime is set to TCP_TRANS and the state is
moved to TRANS. (Since the stateful NAT64 is uncertain whether the
peer will accept the RST packet, instead of moving the state to
CLOSED, it moves to TRANS, which has a shorter lifetime. If no other
packets are received for this connection during the short timer, the
stateful NAT64 assumes that the peer has accepted the RST packet and
moves to CLOSED. If packets keep flowing, the stateful NAT64 assumes
that the peer has not accepted the RST packet and moves back to the
ESTABLISHED state. This is described below in the TRANS state
processing description.)

If any other packet is received, the packet is translated and
forwarded. The lifetime of the TCP Session Table Entry is set to at
least the maximum session lifetime. The value for the maximum
session lifetime MAY be configurable, but it MUST NOT be less than
TCP_EST (the established connection idle timeout as defined in
[RFC5382]). The default value for the maximum session lifetime
SHOULD be set to TCP_EST. The state remains unchanged as
ESTABLISHED.

If the lifetime expires, then the stateful NAT64 SHOULD send a probe
packet (as defined next) to at least one of the endpoints of the TCP
connection. The probe packet is a TCP segment for the connection
with no data. The sequence number and the acknowledgment number are
set to zero. All flags but the ACK flag are set to zero. The state
is moved to TRANS.

Upon the reception of this probe packet, the endpoint will reply
with an ACK containing the expected sequence number for that
connection. It should be noted that, for an active connection,
each of these probe packets will generate one packet from each end
involved in the connection, since the reply of the first point to
the probe packet will generate a reply from the other endpoint.

*** V4 FIN RCV ***

If a V6 FIN packet is received, the packet is translated and
forwarded. The lifetime is set to TCP_TRANS. The state is moved to
V6 FIN + V4 FIN RCV.

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If any packet other than the V6 FIN is received, the packet is
translated and forwarded. The lifetime of the TCP Session
Table Entry is set to at least the maximum session lifetime. The
value for the maximum session lifetime MAY be configurable, but it
MUST NOT be less than TCP_EST (the established connection idle
timeout as defined in [RFC5382]). The default value for the maximum
session lifetime SHOULD be set to TCP_EST. The state remains
unchanged as V4 FIN RCV.

If the lifetime expires, the Session Table Entry is deleted, and the
state is moved to CLOSED.

*** V6 FIN RCV ***

If a V4 FIN packet is received, the packet is translated and
forwarded. The lifetime is set to TCP_TRANS. The state is moved to
V6 FIN + V4 FIN RCV.

If any packet other than the V4 FIN is received, the packet is
translated and forwarded. The lifetime of the TCP Session
Table Entry is set to at least the maximum session lifetime. The
value for the maximum session lifetime MAY be configurable, but it
MUST NOT be less than TCP_EST (the established connection idle
timeout as defined in [RFC5382]). The default value for the maximum
session lifetime SHOULD be set to TCP_EST. The state remains
unchanged as V6 FIN RCV.

If the lifetime expires, the Session Table Entry is deleted and the
state is moved to CLOSED.

*** V6 FIN + V4 FIN RCV ***

All packets are translated and forwarded.

If the lifetime expires, the Session Table Entry is deleted and the
state is moved to CLOSED.

*** TRANS ***

If a packet other than a RST packet is received, the lifetime of the
TCP Session Table Entry is set to at least the maximum session
lifetime. The value for the maximum session lifetime MAY be
configurable, but it MUST NOT be less than TCP_EST (the established
connection idle timeout as defined in [RFC5382]). The default value
for the maximum session lifetime SHOULD be set to TCP_EST. The state
is moved to ESTABLISHED.

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If the lifetime expires, the Session Table Entry is deleted and the
state is moved to CLOSED.

3.5.2.3. Rules for Allocation of IPv4 Transport Addresses for TCP

When a new TCP BIB entry is created for a source transport address of
(S',s), the stateful NAT64 allocates an IPv4 transport address for
this BIB entry as follows:

If there exists some other BIB entry in any of the BIBs that
contains S' as the IPv6 address and maps it to some IPv4 address
T, then T SHOULD be used as the IPv4 address. Otherwise, use any
IPv4 address of the IPv4 pool assigned to the stateful NAT64 to be
used for translation.

In all cases, the allocated IPv4 transport address (T,t) MUST NOT
be in use in another entry in the same BIB, but can be in use in
other BIBs (e.g., the UDP and ICMP BIBs).

3.5.3. ICMP Query Session Handling

The following state information is stored for an ICMP Query session
in the ICMP Query session table:

Binding:(X',Y',i1) <--> (T,Z,i2)

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Lifetime: a timer that tracks the remaining lifetime of the ICMP
Query session. When the timer expires, the session is deleted.
If all the ICMP Query sessions corresponding to a dynamically
created ICMP Query BIB entry are deleted, then the ICMP Query BIB
entry is also deleted.

An incoming ICMPv6 Informational packet with IPv6 source address X',
IPv6 destination address Y', and ICMPv6 Identifier i1 is processed as
follows:

If the local security policy determines that ICMPv6 Informational
packets are to be filtered, the packet is silently discarded.
Else, the stateful NAT64 searches for an ICMP Query BIB entry that
matches the (X',i1) pair. If such an entry does not exist, the
stateful NAT64 tries to create a new entry (if resources and
policy permit) with the following data:

- The BIB IPv6 address is set to X' (i.e., the source IPv6
address of the IPv6 packet).

- The BIB ICMPv6 Identifier is set to i1 (i.e., the ICMPv6
Identifier).

- If there exists another BIB entry in any of the BIBs that
contains the same IPv6 address X' and maps it to an IPv4
address T, then use T as the BIB IPv4 address for this new
entry. Otherwise, use any IPv4 address assigned to the IPv4
interface.

- Any available value is used as the BIB ICMPv4 Identifier, i.e.,
any identifier value for which no other entry exists with the
same (IPv4 address, ICMPv4 Identifier) pair.

The stateful NAT64 searches for an ICMP Query Session Table Entry
corresponding to the incoming 3-tuple (X',Y',i1). If no such
entry is found, the stateful NAT64 tries to create a new entry (if
resources and policy permit). The information included in the new
Session Table Entry is as follows:

- The STE IPv6 source address is set to X' (i.e., the address
contained in the received IPv6 packet).

- The STE IPv6 destination address is set to Y' (i.e., the
address contained in the received IPv6 packet).

- The STE ICMPv6 Identifier is set to i1 (i.e., the identifier
contained in the received IPv6 packet).

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- The STE IPv4 source address is set to the IPv4 address
contained in the corresponding BIB entry.

- The STE ICMPv4 Identifier is set to the IPv4 identifier
contained in the corresponding BIB entry.

- The STE IPv4 destination address is algorithmically generated
from Y' using the reverse algorithm as specified in
Section 3.5.4.

The stateful NAT64 sets (or resets) the timer in the session table
entry to the maximum session lifetime. By default, the maximum
session lifetime is ICMP_DEFAULT. The maximum lifetime value
SHOULD be configurable. The packet is translated and forwarded as
described in the following sections.

An incoming ICMPv4 Query packet with source IPv4 address Y,
destination IPv4 address X, and ICMPv4 Identifier i2 is processed as
follows:

The stateful NAT64 searches for an ICMP Query BIB entry that
contains X as the IPv4 address and i2 as the ICMPv4 Identifier.
If such an entry does not exist, the packet is dropped. An ICMP
error message MAY be sent to the original sender of the packet.
The ICMP error message, if sent, has Type 3, Code 1 (Host
Unreachable).

In contrast to the TCP and UDP specifications in previous
sections, the definition of the ICMP Query session do not have
transport ports and therefore do not have an analogous Address-
Dependent Filtering case at that stage.

Consequently, in case the packet is not discarded in the previous
processing steps, the stateful NAT64 then searches for a Session
Table Entry with an STE source IPv4 address equal to X, an STE
ICMPv4 Identifier equal to i2, and a STE destination IPv4 address
equal to Y. If no such entry is found, the stateful NAT64 tries
to create a new entry (if resources and policy permit) with the
following information:

- The STE source IPv4 address is set to X.

- The STE ICMPv4 Identifier is set to i2.

- The STE destination IPv4 address is set to Y.

- The STE source IPv6 address is set to the IPv6 address of the
corresponding BIB entry.

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- The STE ICMPv6 Identifier is set to the ICMPv6 Identifier of
the corresponding BIB entry.

- The STE destination IPv6 address is set to the IPv6
representation of the IPv4 address of Y, generated using the
algorithm described in Section 3.5.4.

- The stateful NAT64 sets (or resets) the timer in the session
table entry to the maximum session lifetime. By default, the
maximum session lifetime is ICMP_DEFAULT. The maximum lifetime
value SHOULD be configurable. The packet is translated and
forwarded as described in the following sections.

3.5.4. Generation of the IPv6 Representations of IPv4 Addresses

Stateful NAT64 supports multiple algorithms for the generation of the
IPv6 representation of an IPv4 address and vice versa. The
constraints imposed on the generation algorithms are the following:

The algorithm MUST be reversible, i.e., it MUST be possible to
derive the original IPv4 address from the IPv6 representation.

The input for the algorithm MUST be limited to the IPv4 address,
the IPv6 prefix (denoted Pref64::/n) used in the IPv6
representations, and optionally a set of stable parameters that
are configured in the stateful NAT64 (such as a fixed string to be
used as a suffix).

- "n" is the length of the prefix Pref64::/n, and MUST be less
than or equal to 96. If a Pref64::/n is configured through any
means in the stateful NAT64 (such as manually configured, or
other automatic means not specified in this document), the
default algorithm MUST use this prefix. If no prefix is
configured, the algorithm SHOULD use the Well-Known Prefix
(64:ff9b::/96) defined in [RFC6052].

Stateful NAT64 MUST support the algorithm for generating IPv6
representations of IPv4 addresses defined in Section 2.3 of
[RFC6052]. The aforementioned algorithm SHOULD be used as default
algorithm.

3.6. Computing the Outgoing Tuple

This step computes the outgoing tuple by translating the IP addresses
and port numbers or ICMP Identifier in the incoming tuple.

In the text below, a reference to a BIB means the TCP BIB, the UDP
BIB, or the ICMP Query BIB, as appropriate.

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NOTE: Not all addresses are translated using the BIB. BIB entries
are used to translate IPv6 source transport addresses to IPv4
source transport addresses, and IPv4 destination transport
addresses to IPv6 destination transport addresses. They are NOT
used to translate IPv6 destination transport addresses to IPv4
destination transport addresses, nor to translate IPv4 source
transport addresses to IPv6 source transport addresses. The
latter cases are handled by applying the algorithmic
transformation described in Section 3.5.4. This distinction is
important; without it, hairpinning doesn't work correctly.

3.6.1. Computing the Outgoing 5-Tuple for TCP, UDP, and for ICMP Error
Messages Containing TCP or UDP Packets

The transport protocol in the outgoing 5-tuple is always the same as
that in the incoming 5-tuple. When translating from IPv4 ICMP to
IPv6 ICMP, the protocol number in the last next header field in the
protocol chain is set to 58 (IPv6-ICMP). When translating from IPv6
ICMP to IPv4 ICMP, the protocol number in the protocol field of the
IP header is set to 1 (ICMP).

When translating in the IPv6 --> IPv4 direction, let the source and
destination transport addresses in the incoming 5-tuple be (S',s) and
(D',d), respectively. The outgoing source transport address is
computed as follows: if the BIB contains an entry (S',s) <--> (T,t),
then the outgoing source transport address is (T,t).

The outgoing destination address is computed algorithmically from D'
using the address transformation described in Section 3.5.4.

When translating in the IPv4 --> IPv6 direction, let the source and
destination transport addresses in the incoming 5-tuple be (S,s) and
(D,d), respectively. The outgoing source transport address is
computed as follows:

The outgoing source transport address is generated from S using
the address transformation algorithm described in Section 3.5.4.

The BIB table is searched for an entry (X',x) <--> (D,d), and if
one is found, the outgoing destination transport address is set to
(X',x).

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3.6.2. Computing the Outgoing 3-Tuple for ICMP Query Messages and for
ICMP Error Messages Containing an ICMP Query

When translating in the IPv6 --> IPv4 direction, let the source and
destination addresses in the incoming 3-tuple be S' and D',
respectively, and the ICMPv6 Identifier be i1. The outgoing source
address is computed as follows: if the BIB contains an entry (S',i1)
<--> (T,i2), then the outgoing source address is T and the ICMPv4
Identifier is i2.

The outgoing IPv4 destination address is computed algorithmically
from D' using the address transformation described in Section 3.5.4.

When translating in the IPv4 --> IPv6 direction, let the source and
destination addresses in the incoming 3-tuple be S and D,
respectively, and the ICMPv4 Identifier is i2. The outgoing source
address is generated from S using the address transformation
algorithm described in Section 3.5.4. The BIB is searched for an
entry containing (X',i1) <--> (D,i2), and, if found, the outgoing
destination address is X' and the outgoing ICMPv6 Identifier is i1.

3.7. Translating the Packet

This step translates the packet from IPv6 to IPv4 or vice versa.

The translation of the packet is as specified in Sections 4 and 5 of
the IP/ICMP Translation Algorithm [RFC7915], with the following
modifications:

* When translating an IP header (Sections 4.1 and 5.1 of [RFC7915]),
the source and destination IP address fields are set to the source
and destination IP addresses from the outgoing tuple as determined
in Section 3.6.

* When the protocol following the IP header is TCP or UDP, then the
source and destination ports are modified to the source and
destination ports from the outgoing 5-tuple. In addition, the TCP
or UDP checksum must also be updated to reflect the translated
addresses and ports; note that the TCP and UDP checksum covers the
pseudo-header that contains the source and destination IP
addresses. An algorithm for efficiently updating these checksums
is described in [RFC3022].

* When the protocol following the IP header is ICMP and it is an
ICMP Query message, the ICMP Identifier is set to the one from the
outgoing 3-tuple as determined in Section 3.6.2.

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* When the protocol following the IP header is ICMP error containing
an ICMP Query message, the source and destination transport
addresses and ICMP identifier in the embedded packet are set to
the destination and source transport addresses from the outgoing
3-tuple (note the swap of source and destination).

* When the protocol following the IP header is ICMP and it is an
ICMP error message, the source and destination transport addresses
in the embedded packet are set to the destination and source
transport addresses from the outgoing 5-tuple (note the swap of
source and destination).

The size of outgoing packets as well and the potential need for
fragmentation is done according to the behavior defined in the IP/
ICMP Translation Algorithm [RFC7915].

3.8. Handling Hairpinning

If the destination IP address of the translated packet is an IPv4
address assigned to the stateful NAT64 itself, then the packet is a
hairpin packet. Hairpin packets are processed as follows:

* The outgoing 5-tuple becomes the incoming 5-tuple.

* The packet is treated as if it was received on the outgoing
interface.

* Processing of the packet continues at step 2 -- "Filtering and
Updating Binding and Session Information" (Section 3.5).

4. Protocol Constants

UDP_MIN: 2 minutes (as defined in [RFC4787])

UDP_DEFAULT: 5 minutes (as defined in [RFC4787])

TCP_TRANS: 4 minutes (as defined in [RFC5382])

TCP_EST: 2 hours (The minimum lifetime for an established TCP session
defined in [RFC5382] is 2 hours and 4 minutes, which is achieved by
adding the 2 hours with this timer and the 4 minutes with the
TCP_TRANS timer.)

TCP_INCOMING_SYN: 6 seconds (as defined in [RFC5382])

FRAGMENT_MIN: 2 seconds

ICMP_DEFAULT: 60 seconds (as defined in [RFC5508])

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5. Operational Considerations

Since [RFC6146] was published, there have been a notable number of
specifications that, in conjunction with stateful NAT64, have made
significant improvements in the deployment of IPv6, greatly
facilitating the transition.

This non-normative section briefly summarizes those specifications
and their relevance to the stateful NAT64 updated specification, as
well as relevant operational considerations.

5.1. Stateful NAT64 Prefix

Stateful NAT64 does not make an assumption about whether the WKP or a
Network Specific Prefix (NSP) is used. Such decision is deployment-
specific. However, [RFC6052] used to have a deployment constraint
for the use of WKP and includes a restriction against the use of non-
global IPv4 addresses. This restriction is relaxed in
[I-D.ietf-v6ops-nat64-wkp-1918].

Further, [RFC8215] specifies a Local-Use IPv4/IPv6 Translation
Prefix, adjacent to the WKP, facilitating the coexistence of multiple
IPv4/IPv6 translation mechanisms in a single network domain.

5.2. Stateful NAT64 Prefix Discovery

[RFC7050], updated by [RFC8880], defines a best effort method for
clients to discover the Pref64::/n being used by the stateful NAT64.

In order to improve the discovery of the Pref64::/n, [RFC8781]
specifies a ND option to be used in RAs. [RFC9872] further provides
a recommendation for using [RFC8781] instead of [RFC7050].

One more alternative is specified by [RFC7225] "Discovering NAT64
IPv6 Prefixes Using the Port Control Protocol (PCP)".

Section 3 of [RFC7051] already exposed the issues of the stateful
NAT64 prefix discovery, and most of them are resolved in [RFC8781],
but it is important for operators to review that in order to ensure a
proper deployment.

It should be noticed that the DHCPv6 OPTION_V6_PREFIX64, defined in
[RFC8115], is not a stateful NAT64 prefix discovery mechanism and
should not be used for that purpose. This is remarked to ensure
avoiding confusion for implementers who notice the option name or the
uPrefix64 field.

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5.3. Stateful NAT64 in Other Protocols

464XLAT [RFC6877] resolves some of the issues of stateful NAT64, such
as the reachability of IPv4-only destinations when DNS is not being
used (literal IPv4 addresses, code-embedded IPv4 addresses, etc.).
Concretely, [RFC6877] allows stateful NAT64 to be used without DNS64
if a CLAT function is present in the host or other elements of the
network. Note that not using DNS64 has some implications, as already
described in [RFC8683]. [RFC8585] added information about the steps
needed to configure CLAT in a Customer Edge Router (CE) in order to
facilitate the deployment of stateful NAT64 in broadband networks.

Taking advantage of 464XLAT, and the IPv6-Only Preferred Option for
DHCPv4 [RFC8925], [I-D.ietf-v6ops-claton] further specifies the CLAT
and the usage of CLAT in hosts and routers.

5.4. Port Control Protocol

The Port Control Protocol (PCP) [RFC6887], provides a mechanism to
control how incoming packets are forwarded by upstream devices, such
as the stateful NAT64, avoiding the need for keepalive traffic.
IPv6-only hosts discover NAT64 prefixes [RFC7225], program their
mappings, discover the external IP address and implement local
address synthesis.

5.5. QUIC

QUIC [RFC9000] is carried over UDP, so it works when QUIC packets are
translated by a stateful NAT64. However, QUIC/HTTP/3 traffic depends
on reasonable UDP session lifetimes, and stateful NAT64
implementations should not use QUIC Connection IDs as NAT state keys.
Refer to [RFC9312], which provides relevant guidance.

5.6. Issues with IP Address Sharing

Similarly to NAT44, stateful NAT64 shares many of the issues
described in [RFC6269]. This needs to be carefully evaluated in any
stateful NAT64 deployment.

5.7. Previous Operational Experience

Many operators have deployed stateful NAT64 in different
environments, and there are extensive recommendations based on that
experience. Two complementary documents provide advice, from
slightly different perspectives: [RFC7269] and [RFC8683].

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5.8. Benchmarking and Scalability

For dimensioning of stateful NAT64 deployments, [RFC9693] provides
useful considerations. In addition, some benchmarking results for
stateful NAT64 implementations are provided by [Len2023] and
[Len2024].

5.9. Port Allocation Schemes

Stateful NAT64 translators are often configured by default to
maximize the use of port numbers per IPv4 public address, not pre-
allocating a port-range per subscriber. This is one of the
advantages of stateful NAT64 compared with other transition
mechanisms, as described in [RFC9313] (sections 3.4 and 4.7). This
means that a much smaller IPv4 address pool can be used to serve a
larger number of subscribers. The trade-off is that logging files
are bigger. However, this should be balanced with local logging
regulations requirements.

5.10. Logging, Alarms and Event Reporting

[RFC6888] analyzes common requirements for translators and is also
applicable to stateful NAT64. Section 4 of [RFC6888] is devoted to
logging requirements.

Operators also may need to configure alarms and event reporting,
which can be done by using [RFC8158] to monitor address consumption,
in particular.

6. Implementation Status

Note to RFC Editor: Please remove this section before publication, as
it is only intended for the IESG evaluation.

The known status of existing and interoperable implementations, as
well as closely related protocols is summarized. This is following
([RFC7942]) and intended to assist the relevant WGs, IESG and IETF as
a whole, in the evaluation of the document for the document progress
through the standardization process.

The description of the implementations does not imply any IETF
endorsement and is solely based on publicly available information,
which has not been formally confirmed by specific interoperability
testing for this document publication; however, it is known to be
confirmed by existing commercial working deployments worldwide and
without known interoperability issues.

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Stateful NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers ([RFC6146]) was originally published in April
2011.

[RFC6146] is implemented together with other related protocols (just
to name a few of the most relevant ones) such as:

* IPv6 Addressing of IPv4/IPv6 Translators ([RFC6052]).

* IP/ICMP Translation Algorithm ([RFC7915]).

Follows a list of known implementations by different products/
vendors, known to be mature and in production products/networks/
services worldwide:

* 6Wind. Implemented in multiple products.
https://www.6wind.com/6wind-cg-nat-vrouter-with-nat64/.

* A10. Implemented in multiple products.

* AlliedTelesis. Implemented in multiple products.
https://www.alliedtelesis.com/sites/default/files/documents/
configuration-guides/
transitioning_ipv4_to_ipv6_feature_overview_guide.pdf.

* Amazon. Virtual Private Cloud.
https://docs.aws.amazon.com/vpc/latest/userguide/nat-gateway-
nat64-dns64.html.

* Apple. Implemented since 2016.
https://developer.apple.com/library/archive/documentation/NetworkingInternetWeb/Conceptual/
NetworkingOverview/UnderstandingandPreparingfortheIPv6Transition/
UnderstandingandPreparingfortheIPv6Transition.html.

* Arista. Implemented in multiple products.
https://www.arista.com/en/support/toi/eos-4-24-0f/14495-map-t-
border-relay.

* Broadcom. Implemented in VMWare.
https://techdocs.broadcom.com/us/en/vmware-cis/nsx/nsxt-dc/3-1/
administration-guide/network-address-translation/configure-an-nsx-
nat64.html.

* Cisco. Implemented in multiple series of products since 2010.
https://www.cisco.com/c/en/us/td/docs/routers/ios/config/17-x/ip-
addressing/b-ip-addressing/m_iadnat-stateless-nat64.html.

* Ecdysis. http://ecdysis.viagenie.ca/.

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* F5. Implemented in multiple products. https://techdocs.f5.com/kb/
en-us/products/big-ip_ltm/manuals/product/cgn-implementations-
11-6-0/2.html.

* Fortinet. Implemented in multiple products.
https://docs.fortinet.com/document/fortigate/7.4.6/fortinet-
carrier-grade-nat-field-reference-architecture-guide/891965/nat64.

* Huawei. Implemented in multiple series of products.
https://support.huawei.com/enterprise/en/doc/EDOC1100278545/
fe351de4/nat64-configuration.

* Infoblox. Implemented as part of the DNS64 support.
https://www.a10networks.com/products/thunder-cgn/.

* Jool. Implemented since 2014. https://nicmx.github.io/Jool/en/
index.html.

* Juniper. Implemented in multiple series of products.
https://www.juniper.net/documentation/us/en/software/nce/nce-
nat64-ipv6-ipv4-depletion/topics/concept/ipv6-nat64-ipv4-
depletion-overview.html.

* Nokia. Implemented in multiple products.
https://documentation.nokia.com/html/0_add-h-f/93-0262-
HTML/7750_SR_OS_MSISA_Guide/Application-Assurance-NAT.pdf.

* OpenWrt. https://github.com/openwrt and https://openwrt.org.

* Palo Alto. Implemented in multiple products.
https://docs.paloaltonetworks.com/ngfw/networking/nat64.

* Sophos. Implemented in multiple products.
https://news.sophos.com/en-us/2025/04/08/sophos-firewall-v21-5-
early-access-is-now-available/.

* Tayga. https://github.com/openthread/tayga and
https://github.com/apalrd/tayga.

* VPP. https://docs.fd.io/vpp/17.07/nat64_doc.html.

* ZTE. Implemented in multiple products.
https://www.zte.com.cn/global/product_index/ip_network_en/68e_e/
zxr10-6800e/zxr10-6800e.html.

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Note that even an effort has been done to compile an extensive list
(including a relevant URL), there may be many more implementations
not publicly known, so this list does not pretend to be exclusive,
just an indication of a sufficient number of implementations, as
required for the evaluation of the current implementation status.

7. IANA Considerations

This document requests IANA to replace references to [RFC6146] in the
following registry groups with references to this document as
follows:

* Service Function Chaining Service Function Types available at
https://www.iana.org/assignments/service-function-chaining-
service-function-types/service-function-chaining-service-function-
types.xhtml. Reference "[RFC6146]" should be replaced with "[rfc-
to-be-this-document]" for Value 42 (NAT64).

* IP Flow Information Export (IPFIX) Entities available at
https://www.iana.org/assignments/ipfix/ipfix.xhtml. "See
[RFC6146] for nat64 specification" should be replaced with "See
[rfc-to-be-this-document] for NAT64 specification" in ElementID
281 (postNATSourceIPv6Address) and 282
(postNATDestinationIPv6Address). "See [RFC6146] for the
definition of NAT64" should be replaced with "See [rfc-to-be-this-
document] for the definition of NAT64" in ElementID 297 (natType).

8. Security Considerations

8.1. Implications on End-to-End Security

Any protocols that protect IP header information are essentially
incompatible with stateful NAT64. This implies that end-to-end IPsec
verification will fail when the Authentication Header (AH) is used
(both transport and tunnel mode) and when ESP is used in transport
mode. This is inherent in any network-layer translation mechanism.
End-to-end IPsec protection can be restored, using UDP encapsulation
as described in [RFC3948]. The actual extensions to support IPsec
are out of the scope of this specification.

8.2. Filtering

Stateful NAT64 creates binding state using packets flowing from the
IPv6 side to the IPv4 side. In accordance with the procedures
defined in this document following the guidelines defined in
[RFC4787], a stateful NAT64 MUST offer "Endpoint-Independent
Mapping". This means:

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For any IPv6 packet with source (S'1,s1) and destination
(Pref64::D1,d1) that creates an external mapping to (S1,s1v4),
(D1,d1), for any subsequent packet from (S'1,s1) to
(Pref64::D2,d2) that creates an external mapping to (S2,s2v4),
(D2,d2), within a given binding timer window,

(S1,s1v4) = (S2,s2v4) for all values of D2,d2

Implementations MAY also provide support for "Address-Dependent
Mapping" as also defined in this specification and following the
guidelines defined in [RFC4787].

The security properties, however, are determined by which packets the
stateful NAT64 filter allows in and which it does not. The security
properties are determined by the filtering behavior and filtering
configuration in the filtering portions of the stateful NAT64, not by
the address mapping behavior. For example:

Without filtering - When "Endpoint-Independent Mapping" is used in
stateful NAT64, once a binding is created in the IPv6 ---> IPv4
direction, packets from any node on the IPv4 side destined to the
IPv6 transport address will traverse the stateful NAT64 gateway
and be forwarded to the IPv6 transport address that created the
binding. However,

With filtering - When "Endpoint-Independent Mapping" is used in
stateful NAT64, once a binding is created in the IPv6 ---> IPv4
direction, packets from any node on the IPv4 side destined to the
IPv6 transport address will first be processed against the
filtering rules. If the source IPv4 address is permitted, the
packets will be forwarded to the IPv6 transport address. If the
source IPv4 address is explicitly denied -- or the default policy
is to deny all addresses not explicitly permitted -- then the
packet will be discarded. A dynamic filter may be employed
whereby the filter will only allow packets from the IPv4 address
to which the original packet that created the binding was sent.
This means that only the IPv4 addresses to which the IPv6 host has
initiated connections will be able to reach the IPv6 transport
address, and no others. This essentially narrows the effective
operation of the stateful NAT64 function to an "Address-Dependent
Mapping" behavior, though not by its mapping behavior, but instead
by its filtering behavior.

As currently specified, the stateful NAT64 only requires filtering
traffic based on the 5-tuple. In some cases (e.g., statically
configured mappings), this may make it easy for an attacker to guess.
An attacker need not be able to guess other fields, e.g., the TCP
sequence number, to get a packet through the stateful NAT64. While

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such traffic might be dropped by the final destination, it does not
provide additional mitigations against bandwidth/CPU attacks
targeting the internal network. To avoid this type of abuse, a
stateful NAT64 MAY keep track of the sequence number of TCP packets
in order to verify the proper sequencing of exchanged segments, in
particular, those of the SYNs and the FINs.

8.3. Attacks on Stateful NAT64

The stateful NAT64 function itself is a potential victim of different
types of attacks. In particular, the stateful NAT64 can be a victim
of DoS attacks. The stateful NAT64 device has a limited number of
resources that can be consumed by attackers creating a DoS attack.
The stateful NAT64 has a limited number of IPv4 addresses that it
uses to create the bindings. Even though the stateful NAT64 performs
address and port translation, it is possible for an attacker to
consume all the IPv4 transport addresses by sending IPv6 packets with
different source IPv6 transport addresses. This attack can only be
launched from the IPv6 side, since IPv4 packets are not used to
create binding state. DoS attacks can also affect other limited
resources available in the stateful NAT64 such as memory or link
capacity. For instance, it is possible for an attacker to launch a
DoS attack on the memory of the stateful NAT64 device by sending
fragments that the stateful NAT64 will store for a given period. If
the number of fragments is high enough, the memory of the stateful
NAT64 could be exhausted. Similarly, a DoS attack against the
stateful NAT64 can be crafted by sending either V4 or V6 SYN packets
that consume memory in the form of session and/or binding table
entries. In the case of IPv4 SYNs the situation is aggravated by the
requirement to also store the data packets for a given amount of
time, requiring more memory from the stateful NAT64 device. Stateful
NAT64 devices MUST implement proper protection against such attacks,
for instance, allocating a limited amount of memory for fragmented
packet storage as specified in Section 3.4.

Another consideration related to stateful NAT64 resource depletion
refers to the preservation of binding state. Attackers may try to
keep a binding state alive forever by sending periodic packets that
refresh the state. In order to allow the stateful NAT64 to defend
against such attacks, the stateful NAT64 MAY choose not to extend the
session entry lifetime for a specific entry upon the reception of
packets for that entry through the external interface. As described
in the framework document [RFC6144], the stateful NAT64 can be
deployed in multiple scenarios, in some of which the Internet side is
the IPv6 one, and in others of which the Internet side is the IPv4
one. It is then important to properly set which is the Internet side
of the stateful NAT64 in each specific configuration.

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8.4. Avoiding Hairpinning Loops

If an IPv6-only client can guess the IPv4 binding address that will
be created, it can use the IPv6 representation of that address as the
source address for creating this binding. Then, any packet sent to
the binding's IPv4 address could loop in the stateful NAT64. This is
prevented in the current specification by filtering incoming packets
containing Pref64::/n in the source address, as described below.

Consider the following example:

Suppose that the IPv4 pool is 192.0.2.0/24

Then, the IPv6-only client sends this to stateful NAT64:

Source: [Pref64::192.0.2.1]:500

Destination: any

The stateful NAT64 allocates 192.0.2.1:500 as the IPv4 binding
address. Now anything sent to 192.0.2.1:500, be it a hairpinned IPv6
packet or an IPv4 packet, could loop.

It is not hard to guess the IPv4 address that will be allocated.
First, the attacker creates a binding and uses (for example) Simple
Traversal of the UDP Protocol through NAT (STUN) [RFC8489] to learn
its external IPv4 address. New bindings will always have this
address. Then, it uses a source port in the range 1-1023. This will
increase the chances to 1/512 (since range and parity are preserved
by stateful NAT64 in UDP).

In order to address this vulnerability, the stateful NAT64 MUST drop
IPv6 packets whose source address is in Pref64::/n, as defined in
Section 3.5.

8.5. DNS64 and DNSSEC

If DNS64 is used, also see the Security Considerations of [RFC6147].

9. Acknowledgements

Thanks to Mohamed Boucadair, Michael Richardson, Tom Petch, Ted
Lemon, Daryll Sweer, Brian E. Carpenter, Goetz Goerisch, Gabor
Lencse, XiPeng Xiao, Tony Li, John Levine, Jim Reid, Joerg Ott,
Satoru Matsushima and Paul Aitken for the inputs provided.

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Special thanks to Alberto Leiva Popper, who reported erratum 4756,
engaged in private discussions and provided a very detailed
explanation, as experienced stateful NAT64 implementor (Jool),
facilitating the drafting of the text for resolving it. Also to Marc
Lepage, who reported erratum 8416.

Original authors of RFC 6146 (2011)

Marcelo Bagnulo
UC3M
Av. Universidad 30
28911 Leganes Madrid
Spain
Phone: +34-91-6249500
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es/marcelo

Philip Matthews
Alcatel-Lucent
600 March Road
Ottawa Ontario
Canada
Phone: +1 613-592-4343 x224
Email: philip_matthews@magma.ca

Iljitsch van Beijnum
IMDEA Networks
Avda. del Mar Mediterraneo, 22
28918 Leganes Madrid
Spain
Email: iljitsch@muada.com

Contributors listed in RFC6146:

George Tsirtsis

Qualcomm

tsirtsis@googlemail.com

Greg Lebovitz

Juniper

gregory.ietf@gmail.com

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Simon Perreault

Viagenie

simon.perreault@viagenie.ca

Acknowledgements from RFC 6146:

Lorenzo Colitti, Dave Thaler, Dan Wing, Alberto Garcia-Martinez,
Reinaldo Penno, Ranjana Rao, Lars Eggert, Senthil Sivakumar, Zhen
Cao, Xiangsong Cui, Mohamed Boucadair, Dong Zhang, Bryan Ford,
Kentaro Ebisawa, Charles Perkins, Magnus Westerlund, Ed Jankiewicz,
David Harrington, Peter McCann, Julien Laganier, Pekka Savola, and
Joao Damas reviewed the document and provided useful comments to
improve it.

The content of the document was improved thanks to discussions with
Christian Huitema, Fred Baker, and Jari Arkko.

Marcelo Bagnulo and Iljitsch van Beijnum are partly funded by
Trilogy, a research project supported by the European Commission
under its Seventh Framework Program.

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.

[RFC4787] Audet, F., Ed. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
2007, <https://www.rfc-editor.org/info/rfc4787>.

[RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, DOI 10.17487/RFC5382, October 2008,
<https://www.rfc-editor.org/info/rfc5382>.

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[RFC5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP", BCP 148, RFC 5508,
DOI 10.17487/RFC5508, April 2009,
<https://www.rfc-editor.org/info/rfc5508>.

[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
DOI 10.17487/RFC6052, October 2010,
<https://www.rfc-editor.org/info/rfc6052>.

[RFC7857] Penno, R., Perreault, S., Boucadair, M., Ed., Sivakumar,
S., and K. Naito, "Updates to Network Address Translation
(NAT) Behavioral Requirements", BCP 127, RFC 7857,
DOI 10.17487/RFC7857, April 2016,
<https://www.rfc-editor.org/info/rfc7857>.

[RFC7915] Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
"IP/ICMP Translation Algorithm", RFC 7915,
DOI 10.17487/RFC7915, June 2016,
<https://www.rfc-editor.org/info/rfc7915>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2. Informative References

[I-D.ietf-v6ops-claton]
Colitti, L., Linkova, J., and T. Jensen, "464XLAT
Customer-side Translator (CLAT): Node Behavior and
Recommendations", Work in Progress, Internet-Draft, draft-
ietf-v6ops-claton-16, 5 March 2026,
<https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-
claton-16>.

[I-D.ietf-v6ops-nat64-wkp-1918]
Kumari, W. and J. Linkova, "NAT64 WKP", Work in Progress,
Internet-Draft, draft-ietf-v6ops-nat64-wkp-1918-02, 16 May
2026, <https://datatracker.ietf.org/doc/html/draft-ietf-
v6ops-nat64-wkp-1918-02>.

[Len2023] Lencse, G., Shima, K., and K. Cho, "Benchmarking
methodology for stateful NAT64 gateways", Computer
Communications, vol. 210, no. 1, pp. 256-272,
DOI 10.1016/j.comcom.2023.08.009, 1 October 2023,
<https://www.sciencedirect.com/science/article/pii/
S0140366423002931>.

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[Len2024] Lencse, G., "Making Stateless and Stateful Network
Performance Measurements Unbiased", Computer
Communications, vol. 225, no. 1, pp. 141-155,
DOI 10.1016/j.comcom.2024.05.018, 1 September 2024,
<https://www.sciencedirect.com/science/article/abs/pii/
S0140366424001993>.

[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
DOI 10.17487/RFC1858, October 1995,
<https://www.rfc-editor.org/info/rfc1858>.

[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <https://www.rfc-editor.org/info/rfc1918>.

[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
DOI 10.17487/RFC3022, January 2001,
<https://www.rfc-editor.org/info/rfc3022>.

[RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128,
DOI 10.17487/RFC3128, June 2001,
<https://www.rfc-editor.org/info/rfc3128>.

[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, DOI 10.17487/RFC3948, January 2005,
<https://www.rfc-editor.org/info/rfc3948>.

[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963,
DOI 10.17487/RFC4963, July 2007,
<https://www.rfc-editor.org/info/rfc4963>.

[RFC6144] Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation", RFC 6144, DOI 10.17487/RFC6144,
April 2011, <https://www.rfc-editor.org/info/rfc6144>.

[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <https://www.rfc-editor.org/info/rfc6146>.

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[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
DOI 10.17487/RFC6147, April 2011,
<https://www.rfc-editor.org/info/rfc6147>.

[RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269,
DOI 10.17487/RFC6269, June 2011,
<https://www.rfc-editor.org/info/rfc6269>.

[RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
Combination of Stateful and Stateless Translation",
RFC 6877, DOI 10.17487/RFC6877, April 2013,
<https://www.rfc-editor.org/info/rfc6877>.

[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
DOI 10.17487/RFC6887, April 2013,
<https://www.rfc-editor.org/info/rfc6887>.

[RFC6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
A., and H. Ashida, "Common Requirements for Carrier-Grade
NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,
April 2013, <https://www.rfc-editor.org/info/rfc6888>.

[RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
the IPv6 Prefix Used for IPv6 Address Synthesis",
RFC 7050, DOI 10.17487/RFC7050, November 2013,
<https://www.rfc-editor.org/info/rfc7050>.

[RFC7051] Korhonen, J., Ed. and T. Savolainen, Ed., "Analysis of
Solution Proposals for Hosts to Learn NAT64 Prefix",
RFC 7051, DOI 10.17487/RFC7051, November 2013,
<https://www.rfc-editor.org/info/rfc7051>.

[RFC7225] Boucadair, M., "Discovering NAT64 IPv6 Prefixes Using the
Port Control Protocol (PCP)", RFC 7225,
DOI 10.17487/RFC7225, May 2014,
<https://www.rfc-editor.org/info/rfc7225>.

[RFC7269] Chen, G., Cao, Z., Xie, C., and D. Binet, "NAT64
Deployment Options and Experience", RFC 7269,
DOI 10.17487/RFC7269, June 2014,
<https://www.rfc-editor.org/info/rfc7269>.

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[RFC7757] Anderson, T. and A. Leiva Popper, "Explicit Address
Mappings for Stateless IP/ICMP Translation", STD 103,
RFC 7757, DOI 10.17487/RFC7757, February 2016,
<https://www.rfc-editor.org/info/rfc7757>.

[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.

[RFC8115] Boucadair, M., Qin, J., Tsou, T., and X. Deng, "DHCPv6
Option for IPv4-Embedded Multicast and Unicast IPv6
Prefixes", RFC 8115, DOI 10.17487/RFC8115, March 2017,
<https://www.rfc-editor.org/info/rfc8115>.

[RFC8158] Sivakumar, S. and R. Penno, "IP Flow Information Export
(IPFIX) Information Elements for Logging NAT Events",
RFC 8158, DOI 10.17487/RFC8158, December 2017,
<https://www.rfc-editor.org/info/rfc8158>.

[RFC8215] Anderson, T., "Local-Use IPv4/IPv6 Translation Prefix",
RFC 8215, DOI 10.17487/RFC8215, August 2017,
<https://www.rfc-editor.org/info/rfc8215>.

[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.

[RFC8489] Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
D., Mahy, R., and P. Matthews, "Session Traversal
Utilities for NAT (STUN)", RFC 8489, DOI 10.17487/RFC8489,
February 2020, <https://www.rfc-editor.org/info/rfc8489>.

[RFC8585] Palet Martinez, J., Liu, H. M.-H., and M. Kawashima,
"Requirements for IPv6 Customer Edge Routers to Support
IPv4-as-a-Service", RFC 8585, DOI 10.17487/RFC8585, May
2019, <https://www.rfc-editor.org/info/rfc8585>.

[RFC8683] Palet Martinez, J., "Additional Deployment Guidelines for
NAT64/464XLAT in Operator and Enterprise Networks",
RFC 8683, DOI 10.17487/RFC8683, November 2019,
<https://www.rfc-editor.org/info/rfc8683>.

[RFC8781] Colitti, L. and J. Linkova, "Discovering PREF64 in Router
Advertisements", RFC 8781, DOI 10.17487/RFC8781, April
2020, <https://www.rfc-editor.org/info/rfc8781>.

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[RFC8839] Petit-Huguenin, M., Nandakumar, S., Holmberg, C., Keränen,
A., and R. Shpount, "Session Description Protocol (SDP)
Offer/Answer Procedures for Interactive Connectivity
Establishment (ICE)", RFC 8839, DOI 10.17487/RFC8839,
January 2021, <https://www.rfc-editor.org/info/rfc8839>.

[RFC8880] Cheshire, S. and D. Schinazi, "Special Use Domain Name
'ipv4only.arpa'", RFC 8880, DOI 10.17487/RFC8880, August
2020, <https://www.rfc-editor.org/info/rfc8880>.

[RFC8925] Colitti, L., Linkova, J., Richardson, M., and T.
Mrugalski, "IPv6-Only Preferred Option for DHCPv4",
RFC 8925, DOI 10.17487/RFC8925, October 2020,
<https://www.rfc-editor.org/info/rfc8925>.

[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.

[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/rfc9293>.

[RFC9312] Kühlewind, M. and B. Trammell, "Manageability of the QUIC
Transport Protocol", RFC 9312, DOI 10.17487/RFC9312,
September 2022, <https://www.rfc-editor.org/info/rfc9312>.

[RFC9313] Lencse, G., Palet Martinez, J., Howard, L., Patterson, R.,
and I. Farrer, "Pros and Cons of IPv6 Transition
Technologies for IPv4-as-a-Service (IPv4aaS)", RFC 9313,
DOI 10.17487/RFC9313, October 2022,
<https://www.rfc-editor.org/info/rfc9313>.

[RFC9693] Lencse, G. and K. Shima, "Benchmarking Methodology for
Stateful NATxy Gateways", RFC 9693, DOI 10.17487/RFC9693,
January 2025, <https://www.rfc-editor.org/info/rfc9693>.

[RFC9872] Buraglio, N., Jensen, T., and J. Linkova, "Recommendations
for Discovering IPv6 Prefix Used for IPv6 Address
Synthesis", RFC 9872, DOI 10.17487/RFC9872, September
2025, <https://www.rfc-editor.org/info/rfc9872>.

Appendix A. Changes from RFC 6146

* Resolved 2 errata (4756 and 8416). None of the errata have any
implications in the protocol itself.

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- Erratum 4756: ICMP does not have an Address-Dependent Filtering
rule.

- Erratum 8416: Simple typo carried from copy an paste from a
previous section.

* Updated references.

* Clarified the behavior when port parity preservation is not
followed.

* Improved/updated text in intro, related to EAM, usage of stateful
NAT64 for 464XLAT and IPv6-Mostly, Pref64 discovery and self-
synthesis.

* Added new section with Operational Considerations.

* Fixed a few grammar corrections and shortened sentences.

Authors' Addresses

Marcelo Bagnulo
UC3M
Av. Universidad 30
28911 Leganes Madrid
Spain
Phone: +34-91-6249500
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es/marcelo

Philip Matthews
Canada
Email: philip_matthews@magma.ca

Jordi Palet Martinez (editor)
The IPv6 Company
Molino de la Navata, 75
28420 La Navata - Galapagar (Madrid)
Spain
Email: jordi.palet@theipv6company.com
URI: http://www.theipv6company.com/

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Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers
draft-ietf-v6ops-rfc6146-bis-10