Internet Engineering Task Force (IETF) M. Bagnulo
Request for Comments: 6146 UC3M
Category: Standards Track P. Matthews
ISSN: 2070-1721 Alcatel-Lucent
I. van Beijnum
IMDEA Networks
April 2011
Stateful NAT64: Network Address and Protocol Translation
from IPv6 Clients to IPv4 Servers
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 NAT64
translator are shared among several IPv6-only clients. When stateful
NAT64 is used in conjunction with DNS64, no changes are usually
required in the IPv6 client or the IPv4 server.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6146.
Bagnulo, et al. Standards Track [Page 1]
RFC 6146 Stateful NAT64 April 2011
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(
http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Bagnulo, et al. Standards Track [Page 2]
RFC 6146 Stateful NAT64 April 2011
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Features of Stateful NAT64 . . . . . . . . . . . . . . . . 5
1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.1. Stateful NAT64 Solution Elements . . . . . . . . . . . 6
1.2.2. Stateful NAT64 Behavior Walk-Through . . . . . . . . . 8
1.2.3. Filtering . . . . . . . . . . . . . . . . . . . . . . 10
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 11
3. Stateful NAT64 Normative Specification . . . . . . . . . . . . 14
3.1. Binding Information Bases . . . . . . . . . . . . . . . . 14
3.2. Session Tables . . . . . . . . . . . . . . . . . . . . . . 15
3.3. Packet Processing Overview . . . . . . . . . . . . . . . . 17
3.4. Determining the Incoming Tuple . . . . . . . . . . . . . . 18
3.5. Filtering and Updating Binding and Session Information . . 20
3.5.1. UDP Session Handling . . . . . . . . . . . . . . . . . 21
3.5.1.1. Rules for Allocation of IPv4 Transport
Addresses for UDP . . . . . . . . . . . . . . . . 23
3.5.2. TCP Session Handling . . . . . . . . . . . . . . . . . 24
3.5.2.1. State Definition . . . . . . . . . . . . . . . . . 24
3.5.2.2. State Machine for TCP Processing in the NAT64 . . 25
3.5.2.3. Rules for Allocation of IPv4 Transport
Addresses for TCP . . . . . . . . . . . . . . . . 33
3.5.3. ICMP Query Session Handling . . . . . . . . . . . . . 33
3.5.4. Generation of the IPv6 Representations of IPv4
Addresses . . . . . . . . . . . . . . . . . . . . . . 36
3.6. Computing the Outgoing Tuple . . . . . . . . . . . . . . . 36
3.6.1. Computing the Outgoing 5-Tuple for TCP, UDP, and
for ICMP Error Messages Containing a TCP or UDP
Packets . . . . . . . . . . . . . . . . . . . . . . . 37
3.6.2. Computing the Outgoing 3-Tuple for ICMP Query
Messages and for ICMP Error Messages Containing an
ICMP Query . . . . . . . . . . . . . . . . . . . . . . 38
3.7. Translating the Packet . . . . . . . . . . . . . . . . . . 38
3.8. Handling Hairpinning . . . . . . . . . . . . . . . . . . . 39
4. Protocol Constants . . . . . . . . . . . . . . . . . . . . . . 39
5. Security Considerations . . . . . . . . . . . . . . . . . . . 40
5.1. Implications on End-to-End Security . . . . . . . . . . . 40
5.2. Filtering . . . . . . . . . . . . . . . . . . . . . . . . 40
5.3. Attacks on NAT64 . . . . . . . . . . . . . . . . . . . . . 41
5.4. Avoiding Hairpinning Loops . . . . . . . . . . . . . . . . 42
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 43
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 43
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
8.1. Normative References . . . . . . . . . . . . . . . . . . . 43
8.2. Informative References . . . . . . . . . . . . . . . . . . 44
Bagnulo, et al. Standards Track [Page 3]
RFC 6146 Stateful NAT64 April 2011
1. Introduction
This document specifies stateful NAT64, a mechanism for IPv4-IPv6
transition and IPv4-IPv6 coexistence. Together with DNS64 [RFC6147],
these two mechanisms allow 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) [RFC5245]. Stateful NAT64 also
supports IPv4-initiated communications to a subset of the IPv6 hosts
through statically configured bindings in the stateful NAT64.
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 [RFC6145]. 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.
DNS64 is a mechanism for synthesizing AAAA resource records (RRs)
from 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 NAT64 device by using the same algorithm defined in
[RFC6052].
Together, these two mechanisms allow an IPv6-only client (i.e., a
host with a networking stack that only implements IPv6, a host with a
networking stack that implements both protocols but with only IPv6
connectivity, or a host running an IPv6-only application) to initiate
communications to an IPv4-only server (which is analogous to the
IPv6-only host above).
These mechanisms are expected to play a critical role in IPv4-IPv6
transition and IPv4-IPv6 coexistence. Due to IPv4 address depletion,
it is likely that in the future, the new clients will be IPv6-only
and they will want 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.
Bagnulo, et al. Standards Track [Page 4]
RFC 6146 Stateful NAT64 April 2011
For basic functionality, the approach only requires the deployment of
the stateful NAT64 function in the devices connecting an IPv6-only
network to the IPv4-only network, along with the deployment of a few
DNS64-enabled name servers accessible to the IPv6-only hosts. An
analysis of the application scenarios can be found in [RFC6144].
For brevity, in the rest of the document, we will refer to the
stateful NAT64 either as stateful NAT64 or simply as NAT64.
1.1. Features of Stateful NAT64
The features of NAT64 are:
o NAT64 is compliant with the recommendations for how NATs should
handle UDP [RFC4787], TCP [RFC5382], and ICMP [RFC5508]. As such,
NAT64 only supports Endpoint-Independent Mappings and supports
both Endpoint-Independent and Address-Dependent Filtering.
Because of the compliance with the aforementioned requirements,
NAT64 is compatible with current NAT traversal techniques, such as
ICE [RFC5245], and with other NAT traversal techniques.
o In the absence of preexisting state in a NAT64, 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.
o 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 NAT64 somehow 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.
o IPv4 address sharing: NAT64 allows multiple IPv6-only nodes to
share an IPv4 address to access the IPv4 Internet. This helps
with the forthcoming IPv4 exhaustion.
o As currently defined in this NAT64 specification, only TCP, UDP,
and ICMP are supported. Support for other protocols (such as
other transport protocols and IPsec) is to be defined in separate
documents.
Bagnulo, et al. Standards Track [Page 5]
RFC 6146 Stateful NAT64 April 2011
1.2. Overview
This section provides a non-normative introduction to NAT64. This is
achieved by describing the NAT64 behavior involving a simple setup
that involves a single NAT64 device, a single DNS64, and a simple
network topology. The goal of this description is to provide the
reader with a general view of NAT64. It is not the goal of this
section to describe all possible configurations nor to provide a
normative specification of the NAT64 behavior. So, 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 NAT64 is provided in Section 3.
The NAT64 mechanism 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 routed within the IPv6 network towards the NAT64 device. The
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. NAT64, however, is not symmetric. In order to be able to
perform IPv6-IPv4 translation, 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
(hereafter called an IPv4 transport address).
Such binding state is either statically configured in the 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, 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
In this section, we describe the different elements involved in the
NAT64 approach.
The main component of the proposed solution is the translator itself.
The translator has essentially two main parts, the address
translation mechanism and the protocol translation mechanism.
Bagnulo, et al. Standards Track [Page 6]
RFC 6146 Stateful NAT64 April 2011
Protocol translation from an IPv4 packet header to an IPv6 packet
header and vice versa is performed according to the IP/ICMP
Translation Algorithm [RFC6145].
Address translation maps IPv6 transport addresses to IPv4 transport
addresses and vice versa. In order to create these mappings, the
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, we will call the IPv6 address pool
Pref64::/n; in the case there is more than one prefix assigned to the
NAT64, the comments made about Pref64::/n apply to each of them.
Pref64::/n will be used by the 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 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 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 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
NAT64 device, which associates an IPv4 transport address out of its
Bagnulo, et al. Standards Track [Page 7]
RFC 6146 Stateful NAT64 April 2011
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 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 routed
to the NAT64 device, which will create the IPv6-to-IPv4 address
mapping as described before.
1.2.2. Stateful NAT64 Behavior Walk-Through
In this section, we provide a simple example of the NAT64 behavior.
We consider an IPv6 node that is located in an IPv6-only site and
that initiates a TCP connection to an IPv4-only node located in the
IPv4 network.
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 |----| |
| | +-------+ | |
| | | | |
+---------------------+ +---------------+
The figure above 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 NAT64 connects the IPv6 network to the IPv4 network. This NAT64
uses the Well-Known Prefix 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
Bagnulo, et al. Standards Track [Page 8]
RFC 6146 Stateful NAT64 April 2011
configured in such a way that the IPv6 packets addressed to a
destination address in 64:ff9b::/96 are routed to the IPv6 interface
of the NAT64 device.
Also shown is a local name server with DNS64 functionality. The
local name 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 name server does the
recursive lookups.
The steps by which H1 establishes communication with H2 are:
1. H1 performs a DNS query for h2.example.com and receives the
synthetic AAAA RR from the local name 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 routed to the IPv6 interface of the NAT64 (since
IPv6 routing is configured that way).
4. The NAT64 receives the packet and performs the following actions:
* The 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 NAT64 translates the IPv6 header into an IPv4 header using
the IP/ICMP Translation Algorithm [RFC6145].
* The 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 80 of the translated packet is the same as
the destination port of the received IPv6 packet.
5. The NAT64 sends the translated packet out of its IPv4 interface
and the packet arrives at H2.
Bagnulo, et al. Standards Track [Page 9]
RFC 6146 Stateful NAT64 April 2011
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 NAT64 device, the packet is routed to the NAT64
device, 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 NAT64 performs the following
operations:
* The NAT64 translates the IPv4 header into an IPv6 header using
the IP/ICMP Translation Algorithm [RFC6145].
* The 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 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 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
NAT64 may do filtering, which means that it only allows a packet in
through an interface under certain circumstances. The 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 NAT64's IPv4 address,
port, memory, and CPU resources. Filtering techniques of incoming
IPv6 packets are not specific to the NAT64 and therefore are not
described in this specification.
Bagnulo, et al. Standards Track [Page 10]
RFC 6146 Stateful NAT64 April 2011
Filtering of IPv4 packets, on the other hand, is tightly coupled to
the NAT64 state and therefore is described in this specification. In
this document, we consider that the NAT64 may do no filtering, or it
may filter incoming IPv4 packets.
NAT64 filtering of incoming IPv4 packets is consistent with the
recommendations of [RFC4787] and [RFC5382]. Because of that, the
NAT64 as specified in this document supports both Endpoint-
Independent Filtering and Address-Dependent Filtering, both for TCP
and UDP as well as filtering of ICMP packets.
If a NAT64 performs Endpoint-Independent Filtering of incoming IPv4
packets, then an incoming IPv4 packet is dropped unless the NAT64 has
state for the destination transport address of the incoming IPv4
packet.
If a NAT64 performs Address-Dependent Filtering of incoming IPv4
packets, then an incoming IPv4 packet is dropped unless the 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 this document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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 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 NAT64, each session has two different 5-tuples:
one with IPv4 addresses and one with IPv6 addresses.
Bagnulo, et al. Standards Track [Page 11]
RFC 6146 Stateful NAT64 April 2011
BIB: Binding Information Base. A table of bindings kept by a NAT64.
Each 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 NAT64, using the same 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 NAT64 only filters incoming
IPv4 packets destined to a transport address for which there is no
state in the 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 NAT64 filters incoming IPv4
packets destined to a transport address for which there is no
state (similar to the Endpoint-Independent Filtering).
Additionally, the 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 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
NAT64 itself, then the packet is being sent to another IPv6 host
connected to the same NAT64. Such a packet is called a 'hairpin
packet'. A 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].
Bagnulo, et al. Standards Track [Page 12]
RFC 6146 Stateful NAT64 April 2011
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.
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 NAT64, the IPv4 address and port / ICMPv4 Identifier is
always one assigned to the 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 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 NAT64. Each 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 NAT64 uses binding state to perform the translation
between IPv6 and IPv4 addresses. In this document, we also refer
to stateful NAT64 simply as NAT64.
Stateful NAT64 device: The device where the NAT64 function is
executed. In this document, we also refer to stateful NAT64
device simply as NAT64 device.
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].
Bagnulo, et al. Standards Track [Page 13]
RFC 6146 Stateful NAT64 April 2011
3. Stateful NAT64 Normative Specification
A NAT64 is a device with at least one IPv6 interface and at least one
IPv4 interface. Each 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 NAT64 MUST have one or more unicast IPv4 addresses assigned to it.
A NAT64 uses the following conceptual dynamic data structures:
o UDP Binding Information Base
o UDP Session Table
o TCP Binding Information Base
o TCP Session Table
o ICMP Query Binding Information Base
o ICMP Query Session Table
These tables contain information needed for the NAT64 processing.
The actual division of the information into six tables is done in
order to ease the description of the NAT64 behavior. 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 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
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,
Bagnulo, et al. Standards Track [Page 14]
RFC 6146 Stateful NAT64 April 2011
(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 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.)
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 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. 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 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 NAT64. Y' is always the IPv6 representation of the
IPv4 address Z, so Y' is obtained from Z using the algorithm applied
by the NAT64 to create IPv6 representations of IPv4 addresses. y will
always be equal to z.
Bagnulo, et al. Standards Track [Page 15]
RFC 6146 Stateful NAT64 April 2011
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.
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 NAT64. Y' is always the
IPv6 representation of the IPv4 address Z, so Y' is obtained from Z
using the algorithm applied by the 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.
Bagnulo, et al. Standards Track [Page 16]
RFC 6146 Stateful NAT64 April 2011
3.3. Packet Processing Overview
The 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.
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 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, we will refer to the packet
as the incoming IP packet in general. In the case that the
processing is specific to IPv6 packets, we 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.
Bagnulo, et al. Standards Track [Page 17]
RFC 6146 Stateful NAT64 April 2011
This breakdown of the NAT64 behavior into processing steps is done
for ease of presentation. A NAT64 MAY perform the steps in a
different order or MAY perform different steps, but the externally
visible outcome MUST be the same as the one described in this
document.
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
Bagnulo, et al. Standards Track [Page 18]
RFC 6146 Stateful NAT64 April 2011
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.
If the incoming IP packet contains a fragment, then more processing
may be needed. This specification leaves open the exact details of
how a NAT64 handles incoming IP packets containing fragments, and
simply requires that the external behavior of the NAT64 be compliant
with the following conditions:
The NAT64 MUST handle fragments. In particular, NAT64 MUST handle
fragments arriving out of order, conditional on the following:
* The 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 NAT64 has available resources, the 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 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, 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 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 NAT64 handles
(out-of-order) fragments apply.
For incoming IPv4 packets carrying UDP packets with a zero
checksum, if the NAT64 has enough resources, the NAT64 MUST
reassemble the packets and MUST calculate the checksum. If the
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 [RFC6145].
Bagnulo, et al. Standards Track [Page 19]
RFC 6146 Stateful NAT64 April 2011
Implementers of 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].
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
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
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 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 NAT64.
3.5. Filtering and Updating Binding and Session Information
This step updates binding and session information stored in the
appropriate tables. This step may also filter incoming packets, if
desired.
The details of this step depend on the protocol, i.e., UDP, TCP, or
ICMP. The behaviors for UDP, TCP, and ICMP Queries are 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 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 5. In addition, the NAT64
MUST only process incoming IPv6 packets that contain a destination
address that contains Pref64::/n. Likewise, the NAT64 MUST only
process incoming IPv4 packets that contain a destination address that
belongs to the IPv4 pool assigned to the NAT64.
Bagnulo, et al. Standards Track [Page 20]
RFC 6146 Stateful NAT64 April 2011
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)
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 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 NAT64 searches for the Session Table Entry corresponding to
the incoming 5-tuple. If no such entry is found, the 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).
Bagnulo, et al. Standards Track [Page 21]
RFC 6146 Stateful NAT64 April 2011
The result is a Session Table Entry as follows:
(X',x),(Y',y) <--> (T,t),(Z(Y'),y)
The 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 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 NAT64 applies Address-Dependent Filters on its IPv4
interface, then the 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 NAT64 is not filtering or because the packet
is compliant with the Address-Dependent Filtering rule), then the
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 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.
Bagnulo, et al. Standards Track [Page 22]
RFC 6146 Stateful NAT64 April 2011
* 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.
The 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.
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 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 NAT64
SHOULD use T as the IPv4 address. Otherwise, use any IPv4 address
of the IPv4 pool assigned to the NAT64 to be used for translation.
If the port s is in the Well-Known port range 0-1023, and the
NAT64 has an available port t in the same port range, then the
NAT64 SHOULD allocate the port t. If the NAT64 does not have a
port available in the same range, the 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 NAT64 has an
available port t in the same port range, then the NAT64 SHOULD
allocate the port t. If the NAT64 does not have a port available
in the same range, the 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].)
The NAT64 SHOULD preserve the port parity (odd/even), as per
Section 4.2.2 of [RFC4787]).
Bagnulo, et al. Standards Track [Page 23]
RFC 6146 Stateful NAT64 April 2011
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 TCP BIBs).
If it is not possible to allocate an appropriate IPv4 transport
address or create a BIB entry, then the packet is discarded. The
NAT64 SHOULD send an ICMPv6 Destination Unreachable error message
with Code 3 (Address Unreachable).
3.5.2. TCP Session Handling
In this section, we describe how the TCP BIB and session table are
populated. We do so by defining the state machine that the NAT64
uses for TCP. We first describe the states and the information
contained in them, and then we describe 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 [RFC0793], CLOSED is a fictional state
because it represents the state when there is no state for this
particular 5-tuple, and therefore no connection.
V4 INIT: An IPv4 packet containing a TCP SYN was received by the
NAT64, implying that a TCP connection is being initiated from the
IPv4 side. The NAT64 is now waiting for a matching IPv6 packet
containing the TCP SYN in the opposite direction.
Bagnulo, et al. Standards Track [Page 24]
RFC 6146 Stateful NAT64 April 2011
V6 INIT: An IPv6 packet containing a TCP SYN was received,
translated, and forwarded by the NAT64, implying that a TCP
connection is being initiated from the IPv6 side. The 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 NAT64, data can still flow in the connection, and the 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 NAT64, data can still flow in the connection, and the 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 an TCP FIN for this connection were
received by the NAT64. The 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 NAT64 for this connection or simply because the
lifetime of the connection has decreased and there are only
TCP_TRANS minutes left. The 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.
3.5.2.2. State Machine for TCP Processing in the NAT64
The state machine used by the 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 NAT64. After bootstrapping of the NAT64 device, all TCP sessions
are in CLOSED state. As we mention above, the CLOSED state is a
fictional state when there is no state for that particular connection
in the 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 we state 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.
Bagnulo, et al. Standards Track [Page 25]
RFC 6146 Stateful NAT64 April 2011
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.
The figure presents a simplified version of the state machine; refer
to the text for the full specification of the state machine.
+-----------------------------+
| |
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 |--------------------+
+---------------------+
We next describe the state information and the transitions.
Bagnulo, et al. Standards Track [Page 26]
RFC 6146 Stateful NAT64 April 2011
*** 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:
1. The NAT64 searches for a TCP BIB entry that matches the IPv6
source transport address (X',x).
If such an entry does not exist, the 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 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.
Bagnulo, et al. Standards Track [Page 27]
RFC 6146 Stateful NAT64 April 2011
4. The 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:
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 NAT64 tries to create a new Session Table Entry in the TCP
session table (if resources and policy permit), containing the
following information:
+ The STE source IPv4 transport address is set to (X,x), i.e.,
the destination transport address contained in the V4 SYN.
+ The STE destination IPv4 transport address is set to (Y,y),
i.e., the source transport address contained in the V4 SYN.
+ The STE transport IPv6 source address is left unspecified
and may be populated by other protocols that are out of the
scope of this specification.
+ 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
NAT64 will not drop the packet based on the filtering, nor
create a BIB entry. Instead, the 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:
Bagnulo, et al. Standards Track [Page 28]
RFC 6146 Stateful NAT64 April 2011
The NAT64 tries to create a new Session Table Entry in the TCP
session table (if resources and policy permit), containing the
following information:
+ The STE source IPv4 transport address is set to (X,x), i.e.,
the destination transport address contained in the V4 SYN.
+ The STE destination IPv4 transport address is set to (Y,y),
i.e., the source transport address contained in the V4 SYN.
+ The STE transport IPv6 source address is set to the IPv6
transport address contained in the corresponding TCP BIB
entry.
+ 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.
If the 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 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.
Bagnulo, et al. Standards Track [Page 29]
RFC 6146 Stateful NAT64 April 2011
*** 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.
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.
Bagnulo, et al. Standards Track [Page 30]
RFC 6146 Stateful NAT64 April 2011
If a V6 FIN packet is received, the packet is translated and
forwarded. The state is moved to V6 FIN RCV.
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 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 NAT64
assumes that the peer has accepted the RST packet and moves to
CLOSED. If packets keep flowing, the 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 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.
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
Bagnulo, et al. Standards Track [Page 31]
RFC 6146 Stateful NAT64 April 2011
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.
If the lifetime expires, the Session Table Entry is deleted and the
state is moved to CLOSED.
Bagnulo, et al. Standards Track [Page 32]
RFC 6146 Stateful NAT64 April 2011
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 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 NAT64 to be used for
translation.
If the port s is in the Well-Known port range 0-1023, and the
NAT64 has an available port t in the same port range, then the
NAT64 SHOULD allocate the port t. If the NAT64 does not have a
port available in the same range, the NAT64 MAY assign a port t
from another range where it has an available port.
If the port s is in the range 1024-65535, and the NAT64 has an
available port t in the same port range, then the NAT64 SHOULD
allocate the port t. If the NAT64 does not have a port available
in the same range, the NAT64 MAY assign a port t from another
range where it has an available port.
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 TCP BIBs).
If it is not possible to allocate an appropriate IPv4 transport
address or create a BIB entry, then the packet is discarded. The
NAT64 SHOULD send an ICMPv6 Destination Unreachable error message
with Code 3 (Address Unreachable).
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)
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.
Bagnulo, et al. Standards Track [Page 33]
RFC 6146 Stateful NAT64 April 2011
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 NAT64 searches for an ICMP Query BIB entry that matches
the (X',i1) pair. If such an entry does not exist, the 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 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 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).
* 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.
Bagnulo, et al. Standards Track [Page 34]
RFC 6146 Stateful NAT64 April 2011
* The STE IPv4 destination address is algorithmically generated
from Y' using the reverse algorithm as specified in
Section 3.5.4.
The 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 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).
If the NAT64 filters on its IPv4 interface, then the 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 address equal to X, an STE
ICMPv4 Identifier equal to i2, and a STE destination IPv4 address
equal to Y. 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
steps (either because the NAT64 is not filtering or because the
packet is compliant with the Address-Dependent Filtering rule),
then the NAT64 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 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.
Bagnulo, et al. Standards Track [Page 35]
RFC 6146 Stateful NAT64 April 2011
* The STE source IPv6 address is set to the IPv6 address of the
corresponding BIB entry.
* 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 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
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 NAT64 (such as a fixed string to be used as
a suffix).
If we note n the length of the prefix Pref64::/n, then n MUST
be less than or equal to 96. If a Pref64::/n is configured
through any means in the 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
available, the algorithm SHOULD use the Well-Known Prefix
(64:ff9b::/96) defined in [RFC6052].
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.
Bagnulo, et al. Standards Track [Page 36]
RFC 6146 Stateful NAT64 April 2011
In the text below, a reference to a BIB means the TCP BIB, the UDP
BIB, or the ICMP Query BIB, as appropriate.
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 a 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).
Bagnulo, et al. Standards Track [Page 37]
RFC 6146 Stateful NAT64 April 2011
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: 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 [RFC6145], with the following
modifications:
o When translating an IP header (Sections 4.1 and 5.1 of [RFC6145]),
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.
o 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].
o 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.
Bagnulo, et al. Standards Track [Page 38]
RFC 6146 Stateful NAT64 April 2011
o 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 [RFC6145].
3.8. Handling Hairpinning
If the destination IP address of the translated packet is an IPv4
address assigned to the NAT64 itself, then the packet is a hairpin
packet. Hairpin packets are processed as follows:
o The outgoing 5-tuple becomes the incoming 5-tuple.
o The packet is treated as if it was received on the outgoing
interface.
o 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])
Bagnulo, et al. Standards Track [Page 39]
RFC 6146 Stateful NAT64 April 2011
5. Security Considerations
5.1. Implications on End-to-End Security
Any protocols that protect IP header information are essentially
incompatible with 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 document.
5.2. Filtering
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 NAT64 MUST
offer "Endpoint-Independent Mapping". This means:
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 document and following the
guidelines defined in [RFC4787].
The security properties, however, are determined by which packets the
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 NAT64, not by the
address mapping behavior. For example:
Without filtering - When "Endpoint-Independent Mapping" is used in
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 NAT64 gateway and be forwarded
to the IPv6 transport address that created the binding. However,
With filtering - When "Endpoint-Independent Mapping" is used in
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
Bagnulo, et al. Standards Track [Page 40]
RFC 6146 Stateful NAT64 April 2011
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 NAT64 device to an "Address-Dependent Mapping" behavior,
though not by its mapping behavior, but instead by its filtering
behavior.
As currently specified, the 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 NAT64. While 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 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.
5.3. Attacks on NAT64
The NAT64 device itself is a potential victim of different types of
attacks. In particular, the NAT64 can be a victim of DoS attacks.
The NAT64 device has a limited number of resources that can be
consumed by attackers creating a DoS attack. The NAT64 has a limited
number of IPv4 addresses that it uses to create the bindings. Even
though the 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 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 NAT64 device by
sending fragments that the NAT64 will store for a given period. If
the number of fragments is high enough, the memory of the NAT64 could
be exhausted. Similarly, a DoS attack against the 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
Bagnulo, et al. Standards Track [Page 41]
RFC 6146 Stateful NAT64 April 2011
memory from the NAT64 device. 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 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 NAT64 to defend against such
attacks, the 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 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 NAT64 in each specific
configuration.
5.4. 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 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 NAT64:
Source: [Pref64::192.0.2.1]:500
Destination: any
The 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) [RFC5389] 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 NAT64 in UDP).
Bagnulo, et al. Standards Track [Page 42]
RFC 6146 Stateful NAT64 April 2011
In order to address this vulnerability, the NAT64 MUST drop IPv6
packets whose source address is in Pref64::/n, as defined in
Section 3.5.
6. Contributors
George Tsirtsis
Qualcomm
[email protected]
Greg Lebovitz
Juniper
[email protected]
Simon Perreault
Viagenie
[email protected]
7. Acknowledgements
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.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
Bagnulo, et al. Standards Track [Page 43]
RFC 6146 Stateful NAT64 April 2011
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.
[RFC5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP", BCP 148, RFC 5508,
April 2009.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
October 2010.
[RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, April 2011.
8.2. Informative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128, June 2001.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
Bagnulo, et al. Standards Track [Page 44]
RFC 6146 Stateful NAT64 April 2011
[RFC6144] Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation", RFC 6144, April 2011.
[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,
April 2011.
Authors' Addresses
Marcelo Bagnulo
UC3M
Av. Universidad 30
Leganes, Madrid 28911
Spain
Phone: +34-91-6249500
EMail:
[email protected]
URI:
http://www.it.uc3m.es/marcelo
Philip Matthews
Alcatel-Lucent
600 March Road
Ottawa, Ontario
Canada
Phone: +1 613-592-4343 x224
EMail:
[email protected]
Iljitsch van Beijnum
IMDEA Networks
Avda. del Mar Mediterraneo, 22
Leganes, Madrid 28918
Spain
EMail:
[email protected]
Bagnulo, et al. Standards Track [Page 45]
