BEHAVE WG M. Bagnulo
Internet-Draft UC3M
Intended status: Standards Track P. Matthews
Expires: April 13, 2010 Alcatel-Lucent
I. van Beijnum
IMDEA Networks
October 10, 2009
NAT64: Network Address and Protocol Translation from IPv6 Clients to
IPv4 Servers
draft-ietf-behave-v6v4-xlate-stateful-02
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Abstract
NAT64 is a mechanism for translating IPv6 packets to IPv4 packets and
vice-versa. DNS64 is a mechanism for synthesizing AAAA records from
A records. These two mechanisms together enable client-server
communication between an IPv6-only client and an IPv4-only server,
without requiring any changes to either the IPv6 or the IPv4 node,
for the class of applications that work through NATs. They also
enable peer-to-peer communication between an IPv4 and an IPv6 node,
where the communication can be initiated by either end using
existing, NAT-traversing, peer-to-peer communication techniques.
This document specifies NAT64, and gives suggestions on how they
should be deployed.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Features of NAT64 . . . . . . . . . . . . . . . . . . . . 5
1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1. NAT64 solution elements . . . . . . . . . . . . . . . 6
1.2.2. Walkthrough . . . . . . . . . . . . . . . . . . . . . 7
1.2.3. Filtering . . . . . . . . . . . . . . . . . . . . . . 10
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. NAT64 Normative Specification . . . . . . . . . . . . . . . . 12
3.1. Determining the Incoming tuple . . . . . . . . . . . . . . 14
3.2. Filtering and Updating Binding and Session Information . . 15
3.2.1. UDP Session Handling . . . . . . . . . . . . . . . . . 16
3.2.2. TCP Session Handling . . . . . . . . . . . . . . . . . 17
3.2.3. Rules for allocation of IPv4 transport addresses . . . 22
3.2.4. ICMP Query Session Handling . . . . . . . . . . . . . 22
3.2.5. Generation of the IPv6 representations of IPv4
addresses . . . . . . . . . . . . . . . . . . . . . . 24
4. Computing the Outgoing Tuple . . . . . . . . . . . . . . . . . 25
4.1. Computing the outgoing 5-tuple for TCP, UDP and ICMP
error messages . . . . . . . . . . . . . . . . . . . . . . 26
4.2. Computing the outgoing 3-tuple for ICMP Query messages . . 27
5. Translating the Packet . . . . . . . . . . . . . . . . . . . . 27
6. Handling Hairpinning . . . . . . . . . . . . . . . . . . . . . 28
7. Path MTU discovery and fragmentation . . . . . . . . . . . . . 28
7.1. Translating whole packets and PMTUD . . . . . . . . . . . 29
7.1.1. IPv6-to-IPv4 translation . . . . . . . . . . . . . . . 29
7.1.2. IPv4-to-IPv6 . . . . . . . . . . . . . . . . . . . . . 30
7.2. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 31
7.2.1. IPv4-to-IPv6 . . . . . . . . . . . . . . . . . . . . . 31
7.2.2. IPv6-to-IPv4 . . . . . . . . . . . . . . . . . . . . . 32
7.3. TCP MSS option . . . . . . . . . . . . . . . . . . . . . . 33
8. Application scenarios . . . . . . . . . . . . . . . . . . . . 33
8.1. Scenario 1: an IPv6 network to the IPv4 Internet . . . . . 33
8.2. Scenario 3: the IPv6 Internet to an IPv4 network . . . . . 34
9. Security Considerations . . . . . . . . . . . . . . . . . . . 34
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 36
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 37
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
13.1. Normative References . . . . . . . . . . . . . . . . . . . 37
13.2. Informative References . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 39
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1. Introduction
This document specifies NAT64, a mechanism for IPv6-IPv4 transition
and co-existence. Together with DNS64 [I-D.ietf-behave-dns64], these
two mechanisms allow a IPv6-only client to initiate communications to
an IPv4-only server, and also allow peer-to-peer communication
between IPv6-only and IPv4-only hosts.
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 IP/ICMP Translation Algorithm
[I-D.ietf-behave-v6v4-xlate], translating the IPv4 server address by
adding or removing an IPv6 prefix, and translating the IPv6 client
address by installing mappings in the normal NAT manner.
DNS64 is a mechanism for synthesizing AAAA resource records (RR) from
A RR. The synthesis is done by adding a IPv6 prefix to the IPv4
address to create an IPv6 address, where the IPv6 prefix is assigned
to a NAT64 device.
Together, these two mechanisms allow a IPv6-only client to initiate
communications to an IPv4-only server.
These mechanisms are expected to play a critical role in the IPv4-
IPv6 transition and co-existence. Due to IPv4 address depletion,
it's likely that in the future, a lot of IPv6-only clients will want
to connect to IPv4-only servers. The NAT64 and DNS64 mechanisms are
easily deployable, since they require no changes to either the IPv6
client nor the IPv4 server. For basic functionality, the approach
only requires the deployment of 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 in the IPv6-only
network. However, some advanced features such as support for DNSSEC
validating stub resolvers or support for some IPsec modes, require
software updates to the IPv6-only hosts.
The NAT64 and DNS64 mechanisms are related to the NAT-PT mechanism
defined in [RFC2766], but significant differences exist. First,
NAT64 does not define the NATPT mechanisms used to support IPv6 only
servers to be contacted by IPv4 only clients, but only defines the
mechanisms for IPv6 clients to contact IPv4 servers and its potential
reuse to support peer to peer communications through standard NAT
traversal techniques. Second, NAT64 includes a set of features that
overcomes many of the reasons the original NAT-PT specification was
moved to historic status [RFC4966].
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1.1. Features of NAT64
The features of NAT64 and DNS64 are:
o It enables IPv6-only nodes to initiate a client-server connection
with an IPv4-only server, without needing any changes on either
IPv4 or IPv6 nodes. This works for roughly the same class of
applications that work through IPv4-to-IPv4 NATs.
o It supports peer-to-peer communication between IPv4 and IPv6
nodes, including the ability for IPv4 nodes to initiate
communication with IPv6 nodes using peer-to-peer techniques (i.e.,
using a rendezvous server and ICE). To this end, NAT64 is
compliant with the recommendations for how NATs should handle UDP
[RFC4787], TCP [RFC4787], and ICMP [RFC5508].
o Compatible with ICE.
o Supports additional features with some changes on nodes. These
features include:
* Support for DNSSEC
* Some forms of IPsec support
1.2. Overview
This section provides a non-normative introduction to the mechanisms
of NAT64.
NAT64 mechanism is implemented in an NAT64 box which has two
interfaces, an IPv4 interface connected to the 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 box. The NAT64
box will translate them and forward them as IPv4 packets through the
IPv4 network to the IPv4 receiver. The reverse takes place for
packets generated in 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, binding an IPv6 address and
port (hereafter called an IPv6 transport address) to an IPv4 address
and port (hereafter called an IPv4 transport address).
Such binding state 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 either direction on that
particular flow are translated. The result is that NAT64 only
supports communications initiated by the IPv6-only node towards an
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IPv4-only node. Some additional mechanisms, like ICE, can be used in
combination with NAT64 to provide support for communications
initiated by the IPv4-only node to the IPv6-only node. The
specification of such mechanisms, however, is out of the scope of
this document.
1.2.1. 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.
Protocol translation from IPv4 packet header to IPv6 packet header
and vice-versa is performed according to IP/ICMP Translation
Algorithm [I-D.ietf-behave-v6v4-xlate].
Address translation maps IPv6 transport addresses to IPv4 transport
addresses and vice-versa. In order to create these mappings the
NAT64 box has two pools of addresses i.e. 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). Since there
is enough IPv6 address space, it is possible to map every IPv4
address into a different IPv6 address.
NAT64 creates the required mappings by using as the IPv6 address pool
an IPv6 IPv6 prefix (hereafter called Pref64::/n). This allows each
IPv4 address to be mapped into a different IPv6 address by simply
concatenating the Pref64::/n prefix assigned as the IPv6 address pool
of the NAT64, with the IPv4 address being mapped and a suffix (i.e.
an IPv4 address X is mapped into the IPv6 address Pref64:X:SUFFIX).
The NAT64 prefix Pref64::/n is assigned by the administrator of the
NAT64 box from the global unicast IPv6 address block assigned to the
site.
The IPv4 address pool is a set of IPv4 addresses, normally a small
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, 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 mapping of
IPv6 transport addresses into IPv4 transport addresses, instead of
IPv6 addresses into IPv4 addresses directly, which enable a higher
utilization of the limited IPv4 address pool.
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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 easy to
understand that it is far simpler to allow communication initiated
from the IPv6 side toward an IPv4 node, which address is permanently
mapped into an IPv6 address, than communications initiated from IPv4-
only nodes to an IPv6 node in which case IPv4 address needs to be
associated with it dynamically. For this reason NAT64 supports only
communications initiated from the IPv6 side.
An IPv6 initiator can know or derive in advance the IPv6 address
representing the IPv4 target and send packets to that address. The
packets are intercepted by the NAT64 device, which associates an IPv4
transport address of its IPv4 pool to the IPv6 transport address of
the initiator, creating binding state, so that reply packets can be
translated and forwarded back to the initiator. The binding state is
kept while packets are flowing. Once the flow stops, and based on a
timer, the IPv4 transport address is returned to the IPv4 address
pool so that it can be reused for other communications.
To allow an IPv6 initiator to do the standard DNS lookup to learn the
address of the responder, DNS64 [I-D.ietf-behave-dns64] is used to
synthesize an AAAA RR from the A RR (containing the real IPv4 address
of the responder). DNS64 receives the DNS queries generated by the
IPv6 initiator. If there is no AAAA record available for the target
node (which is the normal case when the target node is an IPv4-only
node), DNS64 performs a query for the A record. If an A record is
discovered, DNS64 creates a synthetic AAAA RR that includes the IPv6
representations of the IPv4 address created by concatenating the
Pref64::/n of a NAT64 to the responder's IPv4 address and a suffix
(i.e. if the IPv4 node has IPv4 address X, then the synthetic AAAA RR
will contain the IPv6 address formed as Pref64:X:SUFFIX). The
synthetic AAAA RR is passed back to the IPv6 initiator, which will
initiate an IPv6 communication with the 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. Walkthrough
In this example, we consider an IPv6 node located in a IPv6-only site
that initiates a communication to a IPv4 node located in the IPv4
network.
The notation used is the following: upper case letters are IPv4
addresses; upper case letters with a prime(') are IPv6 addresses;
lower case letters are ports; prefixes are indicated by "P::X", which
is a IPv6 address built from an IPv4 address X by adding the prefix
P, mappings are indicated as "(X,x) <--> (Y',y)".
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The scenario for this case is depicted in the following figure:
+---------------------------------------+ +---------------+
|IPv6 network +-------------+ | | |
| +----+ | Name server | +-------+ | IPv4 |
| | H1 | | with DNS64 | | NAT64 |----| Network |
| +----+ +-------------+ +-------+ | |
| |IP addr: Y' | | | | IP addr: X |
| --------------------------------- | | +----+ |
+---------------------------------------+ | | H2 | |
| +----+ |
+---------------+
The figure shows a IPv6 node H1 which has an IPv6 address Y' and an
IPv4 node H2 with IPv4 address X.
A NAT64 connects the IPv6 network to the IPv4 network. This NAT64
has a /n prefix (called Pref64::/n) that it uses to represent IPv4
addresses in the IPv6 address space and an IPv4 address T assigned to
its IPv4 interface. the routing is configured in such a way, that the
IPv6 packets addressed to a destination address containing Pref64::/n
are routed to the IPv6 interface of the NAT64 box.
Also shown is a local name server with DNS64 functionality. The
local name server needs to know the /n prefix assigned to the local
NAT64 (Pref64::/n). For the purpose of this example, we assume it
learns this through manual configuration.
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 FQDN(H2) 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 Pref64::/n associated to the NAT64 box and the IPv4
address of H2 and a suffix.
2. H1 sends a packet to H2. The packet is sent from a source
transport address of (Y',y) to a destination transport address of
(Pref64:X:SUFFIX,x), where y and x are ports set by H1.
3. The packet is routed to the IPv6 interface of the NAT64 (since
the IPv6 routing is configured that way).
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4. The NAT64 receives the packet and performs the following actions:
* The NAT64 selects an unused port t on its IPv4 address T and
creates the mapping entry (Y',y) <--> (T,t)
* The NAT64 translates the IPv6 header into an IPv4 header using
IP/ICMP Translation Algorithm [I-D.ietf-behave-v6v4-xlate].
* The NAT64 includes (T,t) as source transport address in the
packet and (X,x) as destination transport address in the
packet. Note that X is extracted directly from the
destination IPv6 address of the received IPv6 packet that is
being translated.
5. The NAT64 sends the translated packet out its IPv4 interface and
the packet arrives at H2.
6. H2 node responds by sending a packet with destination transport
address (T,t) and source transport address (X,x).
7. The packet is routed to the NAT64 box, which will look for an
existing mapping containing (T,t). Since the mapping (Y',y) <-->
(T,t) exists, the NAT64 performs the following operations:
* The NAT64 translates the IPv4 header into an IPv6 header using
IP/ICMP Translation Algorithm [I-D.ietf-behave-v6v4-xlate].
* The NAT64 includes (Y',y) as destination transport address in
the packet and (Pref64:X:SUFFIX,x) as source transport address
in the packet. Note that X is extracted directly from the
source IPv4 address of the received IPv4 packet that is being
translated.
8. The translated packet is sent out 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. Pref64:X:SUFFIX) through some scheme other than a DNS look-up.
This is because the DNS64 processing does NOT result in any state
installed in the NAT64 box and because the mapping of the IPv4
address into an IPv6 address is the result of concatenating the
prefix defined within the site for this purpose (called Pref64::/n in
this document) to the original IPv4 address and a suffix.
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1.2.3. Filtering
A NAT64 box may do filtering, which means that it only allows a
packet in through an interface if the appropriate permission exists.
A NAT64 may do no filtering, or it may filter on its IPv4 interface.
Filtering on the IPv6 interface is not supported, as mappings are
only created by packets traveling in the IPv6 --> IPv4 direction.
If a NAT64 performs address-dependent filtering according to RFC4787
[RFC4787] on its IPv4 interface, then an incoming packet is dropped
unless a packet has been recently sent out the interface with a
source transport address equal to the destination transport address
of the incoming packet and destination IP address equal to the source
IP address of the incoming packet.
NAT64 filtering is consistent with the recommendations of RFC 4787
[RFC4787], and the ones of RFC 5382 [RFC5382]
2. Terminology
This section provides a definitive reference for all the terms used
in 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 terms are used in this document:
3-Tuple: The tuple (source IP address, destination IP address, Query
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.
BIB: Binding Information Base. A table of mappings kept by a NAT64.
Each NAT64 has three BIBs, one for TCP, one for UDP and one for
ICMP Queries.
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DNS64: A logical function that synthesizes AAAA Resource Records
(containing IPv6 addresses) from A Resource Records (containing
IPv4 addresses).
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.
Hairpinning: Having a packet do a "U-turn" inside a NAT and come
back out the same interface as it arrived on. 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 though the NAT.
Mapping: A mapping between an IPv6 transport address and a IPv4
transport address. Used to translate the addresses and ports of
packets flowing between the IPv6 host and the IPv4 host. In
NAT64, the IPv4 transport address is always a transport address
assigned to the NAT64 itself, while the IPv6 transport address
belongs to some IPv6 host.
NAT64: A device that translates IPv6 packets to IPv4 packets and
vice-versa, with the provision that the communication must be
initiated from the IPv6 side. The translation involves not only
the IP header, but also the transport header (TCP or UDP).
Session: A TCP, UDP or ICMP Query session. In other words, the bi-
directional flow of packets between two ports on two different
hosts. In NAT64, typically one host is an IPv4 host, and the
other one is an IPv6 host.
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.
Synthetic RR: A DNS Resource Record (RR) that is not contained in
any zone data file, but has been synthesized from other RRs. An
example is a synthetic AAAA record created from an A record.
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).
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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 DNS terminology [RFC1035] and current NAT
terminology [RFC4787].
3. 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 one unicast /n IPv6
prefix assigned to it, denoted Pref64::/n. Additional consideration
about the Pref64::/n are presented in Section 3.2.5. Each NAT64 box
MUST have one or more unicast IPv4 addresses assigned to it.
A NAT64 uses the following 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
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. A given IPv6 or IPv4 transport address can appear in at most
one entry in a BIB: for example, (2001:db8::17, 4) 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.
In the case of the ICMP Query BIB, each ICMP Query BIB entry specify
a mapping between an (IPv6 address, Query Identifier) pair and an
(IPv4 address, Query Identifier pair).
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(X',I1) <--> (T,I2)
where X' is some IPv6 address, T is an IPv4 address, and I1 and I2
are Query Identifiers. T will always be one of the IPv4 addresses
assigned to the NAT64. A given (IPv6 or IPv4 address, Query Id) 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 the section Section 3.2
but the can also can be created manually by the system 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 to 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.
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 address and a pair of IPv4 transport address:
(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 is
always equal to z. In addition, each session table entry has a
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 Query Id and a 3-tuple of IPv4 source address, IPv4
destination address and ICMPv4 Query Id:
(X',Y',I1) <--> (T,Z,I2)
where X' and Y' are IPv6 addresses, T and Z are IPv4 addresses, and
I1 and I2 are ICMP query Ids. 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. In addition, each session table entry has a
lifetime.
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The NAT64 uses the session state information to determine when the
session is completed, and also uses session information for ingress
filtering. A session can be uniquely identified by either an
incoming tuple or an outgoing tuple.
For each session, there is a corresponding BIB entry, uniquely
specified by either the source IPv6 transport address or the source
IPv6 address and ICMPv6 Query Id (in the IPv6 --> IPv4 direction) or
the destination IPv4 transport address or the destination IPv4
address and the ICMPv4 Query Id (in the IPv4 --> IPv6 direction).
However, 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 processing of an incoming IP packet takes the following steps:
1. Determining the incoming tuple
2. Filtering and updating binding and session information
3. Computing the outgoing tuple
4. Translating the packet
5. Handling hairpinning
The details of these steps are specified in the following
subsections.
This breakdown of the 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, as long as the
externally visible outcome is the same.
3.1. Determining the Incoming tuple
This step associates a 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 source IP address,
source port, destination IP address, destination port, transport
protocol. In case of ICMP Queries, the tuple is a 3-tuple consisting
of the source IP address, destination IP address and Query
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 packet.
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If the incoming packet is an 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 Query
Identifier.
If the incoming IP packet contains a complete (un-fragmented) ICMP
error message, then the 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 5-tuple, the embedded source port becomes the
destination port in the 5-tuple, etc. If it is not possible to
determine the 5-tuple (perhaps because not enough of the embedded
packet is reproduced inside the ICMP message), then the incoming IP
packet is silently discarded.
NOTE: The transport protocol is always one of TCP or UDP, even if
the IP packet contains an ICMP Error message.
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 a NAT64 handle fragments arriving out-of-order.
A NAT64 MAY elect to queue the fragments as they arrive and translate
all fragments at the same time. 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, the NAT64 will still need to handle the situation where
subsequent fragments arrive before the first.
Implementors of NAT64 should be aware that there are a number of
well-known attacks against IP fragmentation; see [RFC1858] and
[RFC3128].
Assuming it otherwise has sufficient resources, a NAT64 MUST allow
the fragments to arrive over a time interval of at least 10 seconds.
A NAT64 MAY require that the UDP, TCP, or ICMP header be completely
contained within the first fragment.
3.2. 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 (UDP TCP or ICMP
Query).
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3.2.1. UDP Session Handling
The state information stored for a UDP session in the UDP session
table includes a timer that tracks the remaining lifetime of the UDP
session. The NAT64 decrements this timer at regular intervals. When
the timer expires, the UDP session is deleted. If all the UDP
sessions corresponding to a UDP BIB entry are deleted, then the UDP
BIB entry is also deleted (only applies to the case of dynamically
created entries).
An IPv6 incoming packet is processed as follows:
The NAT64 searches for a UDP BIB entry that matches the IPv6
source transport address. If such entry does not exists, a new
entry is created. As IPv6 address, the source IPv6 transport
address of the packet is included and an IPv4 transport address
allocated using the rules defined in Section 3.2.3 is included as
IPv4 address.
The NAT64 searches for the session table entry corresponding to
the incoming 5-tuple. If no such entry is found, a new entry is
created. The information included in the session table is as
follows: the IPv6 transport source and destination addresses
contained in the received IPv6 packet, the IPv4 transport source
address is extracted from the corresponding UDP BIB entry and the
IPv4 transport destination address contains the same port as the
IPv6 destination transport address and the IPv4 address that is
algorithmically generated from the IPv6 destination address using
the reverse algorithm as specified in Section 3.2.5.
The NAT64 sets or resets the timer in the session table entry to
maximum session lifetime. By default, the maximum session
lifetime is 5 minutes, but for specific destination ports in the
Well-Known port range (0..1023), the NAT64 MAY use a smaller
maximum lifetime. The packet is translated and forwarded as
described in the following sections.
An IPv4 incoming packet is processed as follows:
The NAT64 searches for a UDP BIB entry that matches the IPv4
destination transport address. If such entry does not exists, the
packet is dropped. An ICMP message MAY be sent to the original
sender of the packet, unless the discarded packet is itself an
ICMP message. The ICMP message, if sent, has a type of 3
(Destination 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-
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dependent filtering rule. To do this, it searches for a session
table entry with a source IPv4 transport address equal to the
destination IPv4 transport address in the incoming 5-tuple and
destination IPv4 address (in the session table entry) equal to the
source IPv4 address in the incoming 5-tuple. 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 message MAY be sent to the original sender of the
packet, unless the discarded packet is itself an ICMP message.
The ICMP message, if sent, has a type of 3 (Destination
Unreachable) and a code of 13 (Communication Administratively
Prohibited).
The NAT64 searches for the session table entry corresponding to
the incoming 5-tuple. If no such entry is found, a new entry is
created. The UDP session table entry contains the transport
source and destination address contained in the IPv4 packet and
the source IPv6 transport address (in the IPv6 --> IPv4 direction)
contained in the existing UDP BIB entry. The destination IPv6
transport address contains the same port than the destination IPv4
transport address and the IPv6 representation of the IPv4 address
of the destination IPv4 transport address, generated using the
algorithm described in Section 3.2.5.
The NAT64 sets or resets the timer in the session table entry to
maximum session lifetime. By default, the maximum session
lifetime is 5 minutes, but for specific destination ports in the
Well-Known port range (0..1023), the NAT64 MAY use a smaller
maximum lifetime.
3.2.2. TCP Session Handling
The state information stored for a TCP session:
Binding:(X',x),(Y',y) <--> (T,t),(Z,z)
Lifetime: is a timer that tracks the remaining lifetime of the UDP
session. The NAT64 decrements this timer at regular intervals.
When the timer expires, the TCP session is deleted. If all the
TCP sessions corresponding to a TCP BIB entry are deleted, then
the TCP BIB entry is also deleted (only applies to the case of
dynamically created entries).
TCP sessions are expensive, because their inactivity lifetime is set
to 2 hours and 4 min (as per [RFC5382]), so it is important that each
TCP session table entry corresponds to an existent TCP session. In
order to do that, the NAT64 tracks the TCP connection procedure. In
this section we describe how the NAT64 does that tracking by
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describing the state machine.
Temporarily the NAT64 TCP tracking state machine is depicted in
http:/www.it.uc3m.es/~marcelo/nat64_state_machine.pdf. Once it is
stable, we will include in the draft in ASCII art format.
The states are the following ones:
CLOSED
V4 SYN RCV
V6 SYN RCV
ESTABLISHED
V4 FIN RCV
V6 FIN RCV
V6 FIN + V4 FIN RCV
RST RCV
After bootstrapping of the NAT64 device, all TCP session are in
CLOSED state. We next describe the state information and the
transitions.
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.
*** CLOSED ***
If a V6 SYN is received, the processing is as follows:
The state of the session is moved to V6 SYN RCV.
The NAT64 searches for a TCP BIB entry that matches the IPv6
source transport address.
If such entry does not exists, a new entry is created. As IPv6
address, the source IPv6 transport address of the packet is
included and an IPv4 transport address allocated using the
rules defined in Section 3.2.3 is included as the IPv4
transport address.
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Then a new TCP session entry is created in the TCP session table.
The information included in the session table is as follows:
The IPv6 transport source and destination addresses contained
in the received V6 SYN packet,
The IPv4 transport source address is extracted from the
corresponding TCP BIB entry and,
the IPv4 transport destination address contains the same port
as the IPv6 destination transport address and the IPv4 address
that is algorithmically generated from the IPv6 destination
address using the reverse algorithm as specified in
Section 3.2.5.
The lifetime of the TCP session table entry is set to 4 min
(the transitory connection idle timeout as defined in
[RFC5382]).
The packet is translated and forwarded.
If a V4 SYN packet is received, 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 is not in use in the TCP BIB, then the packet is discarded and
an ICMP Port Unreachable error (Type 3, Code 3) is sent back to
the source of the v4 SYN. The state remains unchanged in CLOSED
If the destination transport address contained in the incoming V4
SYN is in use in the TCP BIB, then
The state is moved to V4 SYN RCV.
A new session table entry is created in the TCP session table,
containing the following information:
The transport source and destination address contained in
the V4 SYN and,
The source IPv6 transport address (in the IPv6 --> IPv4
direction) contained in the existing TCP BIB entry.
The destination IPv6 transport address contains the same
port than the destination IPv4 transport address and the
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IPv6 representation of the IPv4 address of the destination
IPv4 transport address, generated using the algorithm
described in Section 3.2.5.
The lifetime of the entry is set to 6 seconds as per
[RFC5382].
The packet is discarded.
For any other IPv6 packet, depending on the security policy other
packets MAY be forwarded or MAY be silently discarded. In any case,
the state remains unchanged.
For any other IPv4 packet,
If the destination transport address contained in the incoming
IPv4 packet is in use in the TCP BIB depending on the security
policy other packets MAY be forwarded or MAY be silently
discarded. In any case, the state remains unchanged.
If the destination transport address contained in the incoming
IPv4 packet is not in use in the TCP BIB the packet is silently
discarded.
*** V4 SYN RCV ***
If a V6 SYN is received, then the state is moved to ESTABLISHED. The
lifetime of the corresponding TCP session table entry is updated to 2
hours 4 min (the established connection idle timeout as defined in
[RFC5382]). The packet is translated and forwarded.
If the lifetime expires, an ICMP Port Unreachable error (Type 3, Code
3) 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, depending on the security policy other packets
MAY be forwarded or MAY be silently discarded. In any case, the
state remains unchanged.
*** V6 SYN RCV ***
If a V4 SYN is received (with or without the ACK flag set), then the
state is moved to ESTABLISHED. The timer is updated to 2 hours 4 min
(the established connection idle timeout as defined in [RFC5382]).
The packet is translated and forwarded.
If the lifetime expires, the session table entry is deleted and the
state is moved to CLOSED.
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For any other packet, depending on the security policy other packets
MAY be forwarded or MAY be silently discarded. In any case, the
state remains unchanged.
*** ESTABLISHED ***
If the lifetime expires, the session table entry is deleted and the
state is moved to CLOSED.
If a V4 FIN packet is received, the packet is translated and
forwarded. The state is moved to V4 FIN RCV.
If a V6 FIN packet is received, the packet is translated and
forwarded. The state is moved to V6 FIN RCV.
If a packet is received, the packet is translated and forwarded. The
lifetime is set to 2 hours and 4 min. The state remains unchanged as
ESTABLISHED.
If the lifetime expires, the session table entry is deleted and the
state is moved to CLOSED.
If a V4 RST or a V6 RST packet is received, the packet is translated
and forwarded. The lifetime is set to 4 min and state is moved to
RST RCV.
*** V4 FIN RCV ***
If a packet is received, the packet is translated and forwarded. The
lifetime is set to 2 hours and 4 min. The state remains unchanged as
V4 FIN RCV.
If a V6 FIN packet is received, the packet is translated and
forwarded. The lifetime is set to 4 min. The state is moved to V6
FIN + 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 packet is received, the packet is translated and forwarded. The
lifetime is set to 2 hours and 4 min. The state remains unchanged as
V6 FIN RCV.
If a V4 FIN packet is received, the packet is translated and
forwarded. The lifetime is set to 4 min. The state is moved to V6
FIN + V4 FIN RCV.
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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.
*** RST RCV ***
If a packet is received, the lifetime is set to 2 hours and 4 min and
the state is moved to ESTABLISHED.
If the lifetime expires, the session table entry is deleted and the
state is moved to CLOSED.
3.2.3. Rules for allocation of IPv4 transport addresses
If the rules specify that a new BIB entry is created for a source
transport address of (S',s), then 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 use T as the
IPv4 address. Otherwise, use any IPv4 address assigned to the
IPv4 interface.
If the port s is in the Well-Known port range 0..1023, then
allocate a port t from this same range. Otherwise, if the port s
is in the range 1024..65535, then allocate a port t from this
range. Furthermore, if port s is even, then t must be even, and
if port s is odd, then t must be odd.
In all cases, the allocated IPv4 transport address (T,t) MUST NOT
be in use in another entry in the same BIB, but MAY be in use in
the other BIB.
If it is not possible to allocate an appropriate IPv4 transport
address or create a BIB entry for some reason, then the packet is
discarded.
3.2.4. ICMP Query Session Handling
The state information stored for an ICMP Query session in the ICMP
Query session table includes a timer that tracks the remaining
lifetime of the session. The NAT64 decrements this timer at regular
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intervals. When the timer expires, the session is deleted. If all
the sessions corresponding to a ICMP Query BIB entry are deleted,
then the ICMP Query BIB entry is also deleted in the case of
dynamically created entries.
An incoming ICMPv6 Informational packet is processed as follows:
If the local security policy determines that ICMPv6 Informative
packets are to be filtered, the packet is silently discarded.
Else, the NAT64 searches for a ICMP Query BIB entry that matches
the (IPv6 source address, ICMPv6 Query Id) pair. If such entry
does not exist, a new entry is created. As (IPv6 address, ICMPv6
Query Id) pair, the source IPv6 address of the packet and the
ICMPv6 Query Identifier are included. The IPv4 address and ICMPv4
Query Identifier values are allocated as follows:
If there exists some other BIB entry containing the same IPv6
address and mapping it to some IPv4 address T, then use T as
the IPv4 address. Otherwise, use any IPv4 address assigned to
the IPv4 interface.
As ICMPv4 Identifier use any available value i.e. any
identifier value for which no other entry exists with the same
(IPv4 address, ICMPv4 Query Id) pair.
The NAT64 searches for the session table entry corresponding to
the incoming 3-tuple. If no such entry is found, a new entry is
created. The information included in the session table is as
follows: the IPv6 source and destination addresses contained in
the received IPv6 packet, the IPv4 source address, the ICMPv4
Query Id and the ICMPv6 Query Id are extracted from the
corresponding ICMP Query BIB entry and the IPv4 destination
address is algorithmically generated from the IPv6 destination
address using the reverse algorithm as specified in Section 3.2.5.
The NAT64 sets or resets the timer in the session table entry to
maximum session lifetime. By default, the maximum session
lifetime is 60 seconds. The maximum lifetime value SHOULD be
configurable. The packet is translated and forwarded as described
in the following sections.
An incoming ICMPv4 Query packet is processed as follows:
The NAT64 searches for a ICMP Query BIB entry that matches the
IPv4 destination address and ICMPv4 query Id pair. If such entry
does not exists, the packet is dropped. An ICMP message MAY be
sent to the original sender of the packet, unless the discarded
packet is itself an ICMP message. The ICMP message, if sent, has
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a type of 3 (Destination 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 a source IPv4 address and ICMP Query Id pair
equal to the destination IPv4 address and ICMP Query Id in the
incoming 3-tuple and destination IPv4 address (in the session
table entry) equal to the source IPv4 address in the incoming
3-tuple. 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 message MAY be sent to
the original sender of the packet, unless the discarded packet is
itself an ICMP message. The ICMP message, if sent, has a type of
3 (Destination Unreachable) and a code of 13 (Communication
Administratively Prohibited).
The NAT64 searches for the session table entry corresponding to
the incoming 3-tuple. If no such entry is found, a new entry is
created. The ICMP Query session table entry contains the ICMPv4
Query Identifier, source and destination address contained in the
IPv4 packet and the source IPv6 address and the ICMPv6 Query Id
(in the IPv6 --> IPv4 direction) contained in the existing ICMP
Query BIB entry. The destination IPv6 address contains is the
IPv6 representation of the IPv4 address of the destination IPv4
address, generated using the algorithm described in Section 3.2.5.
The NAT64 sets or resets the timer in the session table entry to
maximum session lifetime. By default, the maximum session
lifetime is 60 seconds. The maximum lifetime value SHOULD be
configurable. The packet is translated and forwarded as described
in the following sections.
3.2.5. Generation of the IPv6 representations of IPv4 addresses
NAT64 support multiple algorithms for the generation of the IPv6
representation of an IPv4 address. The constraints imposed to the
generation algorithms are the following:
The same algorithm to create an IPv6 address from an IPv4 address
MUST be used by:
The DNS64 to create the IPv6 address to be returned in the
synthetic AAAA RR from the IPv4 address contained in original A
RR, and,
The NAT64 to create the IPv6 address to be included in the
destination address field of the outgoing IPv6 packets from the
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IPv4 address included in the destination address field of the
incoming IPv4 packet.
The algorithm MUST be reversible, i.e. it MUST be possible to
extract 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 fixed string to be used as a
suffix).
If we note n the length of the prefix Pref64::/n, then n MUST
the less or equal than 96. If a Pref64::/n is configured
through any means in the DNS64 (such as manually configured, or
other automatic mean not specified in this document), the
default algorithm MUST use this prefix. If no prefix is
available, the algorithm MUST use the Well-Known prefix
(include here the prefix to be assigned by IANA) defined in
[I-D.ietf-behave-address-format]
NAT64 MUST support the following algorithms for generating IPv6
representations of IPv4 addresses defined in
[I-D.ietf-behave-address-format]:
Zero-Pad And Embed, defined in section 3.2.3 of
[I-D.ietf-behave-address-format]
Compensation-Pad And Embed, defined in section of 3.2.4 of
[I-D.ietf-behave-address-format]
Embed And Zero-Pad, defined in section of 3.2.5 of
[I-D.ietf-behave-address-format]
Preconfigured Mapping Table, defined in section of 3.2.6 of
[I-D.ietf-behave-address-format]
The default algorithm used by NAT64 must be Embed and Zero-Pad.
While the normative description of the algorithms is provided in
[I-D.ietf-behave-address-format].
4. Computing the Outgoing Tuple
This step computes the outgoing tuple by translating the addresses
and ports or ICMP Query Id in the incoming tuple.
In the text below, a reference to the the "BIB" means either the TCP
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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 applying the algorithmic transformation
described in Section 3.2.5. This distinction is important;
without it, hairpinning doesn't work correctly.
4.1. Computing the outgoing 5-tuple for TCP, UDP and ICMP error
messages
The transport protocol in the outgoing 5-tuple is always the same as
that in the incoming 5-tuple.
When translating in the IPv6 --> IPv4 direction, let the incoming
source and destination transport addresses in the 5-tuple be (S',s)
and (D',d) respectively. The outgoing source transport address is
computed as follows: the BIB contains a 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 3.2.5.
When translating in the IPv4 --> IPv6 direction, let the incoming
source and destination transport addresses in the 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 3.2.5.
The outgoing destination transport address is computed as follows:
If the BIB contains an entry (X',x) <--> (D,d), then the outgoing
destination transport address is (X',x).
Otherwise, discard the packet.
If the rules specify that the packet is discarded, then the NAT64 MAY
send an ICMP reply to the original sender, unless the packet being
translated contains an ICMP message. The type should be 3
(Destination Unreachable) and the code should be 0 (Network
Unreachable in IPv4, and No Route to Destination in IPv6).
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4.2. Computing the outgoing 3-tuple for ICMP Query messages
When translating in the IPv6 --> IPv4 direction, let the incoming
source and destination addresses in the 3-tuple be S' and D'
respectively and the ICMPv6 Query Identifier be I1. The outgoing
source address is computed as follows: the BIB contains a entry
(S',I1) <--> (T,I2), then the outgoing source address is T and the
ICMPv4 Query Id is I2.
The outgoing IPv4 destination address is computed algorithmically
from D' using the address transformation described in Section 3.2.5.
When translating in the IPv4 --> IPv6 direction, let the incoming
source and destination addresses in the 3-tuple be S and D
respectively and the ICMPv4 query Id is I2. The outgoing source
address is generated from S using the address transformation
algorithm described in Section 3.2.5. The outgoing destination
address and ICMPv6 Query Id are computed as follows:
If the BIB contains an entry (X',I1) <--> (D,I2), then the
outgoing destination address is X' and the outgoing ICMPv6 Query
Id is I1.
Otherwise, discard the packet.
NOTE: Not sure if this applies to ICMP query messages....If the rules
specify that the packet is discarded, then the NAT64 MAY send an ICMP
reply to the original sender, unless the packet being translated
contains an ICMP message. The type should be 3 (Destination
Unreachable) and the code should be 0 (Network Unreachable in IPv4,
and No Route to Destination in IPv6).
5. Translating the Packet
This step translates the packet from IPv6 to IPv4 or vice-versa.
The translation of the packet is as specified in section 3 and
section 4 of IP/ICMP Translation Algorithm
[I-D.ietf-behave-v6v4-xlate], with the following modifications:
o When translating an IP header (sections 3.1 and 4.1), the source
and destination IP address fields are set to the source and
destination IP addresses from the outgoing 5-tuple.
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
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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 which 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 query Identifier is set to the one of
the outgoing 3-tuple.
o When the protocol following the IP header is ICMP (sections 3.4
and 4.4) 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).
6. Handling Hairpinning
This step handles hairpinning if necessary.
If the destination IP address is an address assigned to the NAT64
itself (i.e., is one of the IPv4 addresses assigned to the IPv4
interface, or is covered by the /96 prefix assigned to the IPv6
interface), then the packet is a hairpin packet. The outgoing
5-tuple becomes the incoming 5-tuple, and the packet is treated as if
it was received on the outgoing interface. Processing of the packet
continues at step 2.
[R/T] The reference to step 2 here was a little confusing to us. Are
you referring to Filtering and Updating Session Information (Section
3.2)? MB> I am not sure about this anymore. I mean what if the
packet that is being hairpinned is an ICMP error msge, I mean don't
we still need step 1?
TBD: Is there such a thing as a hairpin loop (likely not naturally,
but perhaps through a special-crafted attack packet with a spoofed
source address)? If so, need to drop packets that hairpin more than
once.
7. Path MTU discovery and fragmentation
It's the job of the network layer to adapt to different maximum
packet sizes as packets move through the network. There are three
mechanisms that handle this: transport layer negotiations such as the
TCP MSS option, path MTU discovery and fragmentation. The difference
between the IPv4 and IPv6 header sizes requires some handling in a
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NAT64 translator, and there are complications because of the
differences between how IPv4 and IPv6 handle fragmentation, as well
as the issue of how to demultiplex fragmented IPv4 packets.
The vast majority of both IPv4 and IPv6 hosts use path MTU discovery
[RFC1191] [RFC1981]. With IPv4, PMTUD can be enabled on a per-packet
basis by setting the DF bit to 1. With IPv6, there is no need for
PMTUD for packets up to 1280 bytes because all IPv6 hosts are
required to be able to receive 1280-byte packets without
fragmentation. When sending larger packets, IPv6 hosts implicitly
use PMTUD.
The fragmentation behavior specified in [RFC2765] is that upon the
reception of an ICMPv6 "packet too big" message with an indicated
packet size of less than 1280 octets, IPv6 hosts will transmit 1280-
octet packets, but include a fragment header in those packets. In a
stateful translator, the identification value in this fragment header
can't be used, so the fragment header itself serves no purpose.
Additionally, the presence or absense of the fragment header isn't
enough to determine whether to set the DF bit in packets translated
to IPv4 to 0 (fragment header present) or 1 (no fragment header
present). The reason for this is that operators may decide to forego
path MTU discovery by configuring an MTU of 1280 and filtering
incoming "too big" messages. The behavior specified below is meant
to avoid PMTUD black holes in this situation
7.1. Translating whole packets and PMTUD
This section specifies the values in the fragmentation-related fields
in the IPv4 header when no fragmentation occurs, and how path MTU
discovery is handled.
7.1.1. IPv6-to-IPv4 translation
If the NAT64 has the same MTUs on its IPv6 and IPv4 interfaces, it
will never have to generate "packet too big" messages for incoming
IPv6 packets because the translation from IPv6 to IPv4 reduces the
packet size by 20 bytes, more if the IPv6 packet has extension
headers that are removed during the translation, such as the fragment
header. If the MTU on the IPv6 side is larger than 1280 bytes and
more than 20 bytes smaller than the MTU on the IPv4 side, the NAT64
MUST generate the appropriate "packet too big" messages on the IPv6
side.
To support PMTUD, for translated packets that are larger than 1260
bytes on the IPv4 side (1280 bytes IPv6 packets with 20 byte size
reduction through the translation), the DF bit is set to 1 in the
resulting IPv4 packet.
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IPv4 routers may generate "packet too big" messages indicating a
supported MTU size smaller than 1280 bytes. In those cases, the IPv6
hosts will continue to send packets larger than what the IPv4 path
MTU can support. To allow packets to be delivered successfully in
this case, the DF bit is set to 0 in all translated packets smaller
than or equal to 1260 bytes, to allow these packets to be fragmented
in the IPv4 network.
Note: it is highly recommended for IPv4 hosts running services that
may be used by IPv6 clients through a NAT64 translator to use an MTU
size of at least 1260 bytes and to properly generate "packet too big"
messages.
When a NAT64 translates "packet too big" messages from IPv6 to IPv4,
it adjusts the advertised MTU to the minimum of the original
advertised MTU + 20, the NAT64's MTU on the IPv6 side + 20 and the
NAT64's MTU on the IPv4 side.
The identification field in the IPv4 header MUST be filled with a
value generated by the NAT64 translator, similar to the way that
identification values are created for locally generated packets. It
is RECOMMENDED that a NAT64 translator keep an identification counter
for every combination of remote IPv4 destination and protocol.
In theory, IPv4 packets with DF set to 1 don't need a unique
identification value. However, it is not unheard of for operators to
configure equipment to clear the DF bit, at which time an
identification value with good uniqueness becomes necessary. As
such, it is recommended that translators include a unique
identification value in all packets, including those with DF set to
1. However, since more packets will be sent with DF set to 1, this
will use up identification values faster. Implementations may choose
to segment the identification space and assign values from non-
overlapping pools to packets with DF set to 0 and DF set to 1 to
provide a longer period of uniqueness to fragmentable packets.
7.1.2. IPv4-to-IPv6
Because it may be necessary to include a fragmentation header or
other extension header, the NAT64 MUST be prepared to generate
"packet too big" messages for packets with the DF bit set to 1
received from the IPv4 side, regardless of the MTU sizes on the IPv4
and IPv6 interfaces. If the packet with DF = 1 is larger than can be
transmitted on the IPv6 side after translation, the NAT64 returns a
"packet too big" message indicating the maximum IPv4 packet size that
would be supported using the same translation as the current packet.
This can be calculated as IPv4-packet-size - 20.
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When a NAT64 translates "packet too big" messages from IPv4 to IPv6,
it adjusts the advertised MTU to the minimum of the original
advertised MTU - 20, the NAT64's MTU on the IPv6 side and the NAT64's
MTU on the IPv4 side - 20. However, if the advertised MTU in "packet
too big" messages is smaller than 1260 bytes, the value put into the
translated "packet too big" message is 1280. This makes sure that
the IPv6 host will limit its packet sizes to 1280 bytes, so its
packets are subsequently translated into IPv4 packets with DF set to
0. (This deviates from [RFC2765].)
7.2. Fragmentation
Because NAT deviates from normal router behavior, the limitation that
IPv6 packets or IPv4 packets with DF set to 1 are not fragmented by
routers doesn't apply to a NAT64 translator. Where appropriate,
these packets are fragmented after translation as described below.
7.2.1. IPv4-to-IPv6
Because packets coming in on the IPv4 side may be larger than 1280
bytes after translation, a NAT64 MUST implement PMTUD on the IPv6
side. In other words, it must react to "packet too big" messages for
any IPv6 destination that it communicates with by limiting the size
of the packets that it sends to the advertised maximum.
In the case where, after translation from IPv4 to IPv6, a packet is
larger than a destination's PMTU, the NAT64 returns a "packet too
big" as outlined earlier in the case that the DF bit was set to 1 in
the IPv4 packet. If the DF bit was set to 0, the translator first
translates the IPv4 packet, and then fragments the resulting IPv6
packets using normal IPv6 fragmentation rules. The lower 16 bits of
the IPv6 identification field are copied from the IPv4 identification
field. The upper 16 bits of the IPv6 identification field are set to
0.
Because NAT64 provides a stateful many-to-one (perhaps even many-to-
many) translation, it is necessary to recognize which session a given
packet belongs to. In the IPv4-to-IPv6 direction, the TCP or UDP
port numbers must be known to accomplish this, but the port numbers
only occur in the first fragment of a fragmented packet. There are
two possible ways to deal with this:
1. Reassemble the packet before translating it.
2. Create translation state for the fragments belonging to the same
packet so each packet can be translated.
Strategy 2 is attractive in large installations because it requires
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less storage and processing. However, it may still be necessary to
buffer fragments for some time, as the fragment containing the first
part of the packet (and with that, the port numbers) may not be the
first one to arrive.
Note: based on the assumptions that hosts generate fragments in-order
and that reordering must happen through parallel network links and
that the path between these parallel links and a NAT64 supports
speeds of at least 10 Mbps, there is a very high probability that two
out-of-order fragments making up a packet will arrive at the NAT64
within 50 to 100 milliseconds. Further assuming that fragmented
traffic makes up less than 10% of all traffic, this only requires a
buffer of 6 to 12,500 fragments (50 ms at 10 Mbps to 100 ms at 10
Gbps).
In some cases, there may only be a single session matching the
fragment's source and destination addresses and protocol number. In
these cases, it would be possible to translate the fragments out-of-
order. A NAT64 translator MAY do this for TCP, however, it MUST NOT
translate UDP packets before the first fragment is available. The
reason for this is that the fragment could be part of a packet
setting up a new session. However, with TCP session establishment
packets don't carry data, so it's extremely unlikely that they are
fragmented. This is not the case with UDP, and in the IPv4-to-IPv6
direction, a UDP packet may have a zero checksum, which must be
recalculated when translating to IPv6, for which the entire packet
must be available.
7.2.2. IPv6-to-IPv4
For all IPv4 packets that the NAT64 creates through translation, the
translator generates an ID value. This applies to all packets,
regardless of their size or the value of the DF field. A NAT64
translator MAY employ strategies to avoid reusing an ID value for a
certain source, destination, protocol tuple as long as possible. If
the IPv4 packets are fragments of an IPv6 packet, then state is
created that makes it possible for all the fragments to have the same
ID value on the IPv4 side.
[RFC2765] specifies copying the lower bits from the IPv6 ID field in
a fragment header (if present) to the IPv4 ID field, but this runs
the risk of two IPv6 hosts talking to the same IPv4 destination
through the NAT64 using the same ID value.
Otherwise, when translating IPv6 packets with a fragmentation header,
the fragments are translated as per [RFC2765].
In the IPv6-to-IPv4 direction, there is no need to map a fragment to
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the session it belongs to in order to translate the fragment.
However, it is necessary that all the fragments have the same
identification value, so fragments may be translated individually,
but state must be kept to be able to translate subsequent fragments
of the same packet using the same identification value on the IPv4
side.
7.3. TCP MSS option
It is not recommended that NAT64 translators rewrite the TCP MSS
option [RFC0793]. As such, assuming the common case of all 1500-
octet MTUs, an IPv6 host will advertise a 1440-octet MSS, triggering
the IPv4 host to generate 1480-octet packets that are translated to
1500-octet IPv6 packets. IPv4 hosts will advertise a 1460-octet MSS,
which would be 1520-octet IPv6 packets. However, ethernet-connected
IPv6 hosts can only send 1500-octet packets, so in the all-ethernet
case, there is no dependency on path MTU discovery.
8. Application scenarios
In this section, we describe how to apply NAT64/DNS64 to the suitable
scenarios described in [I-D.ietf-behave-v6v4-framework] .
8.1. Scenario 1: an IPv6 network to the IPv4 Internet
An IPv6 only network basically has IPv6 hosts (those that are
currently available) and because of different reasons including
operational simplicity, wants to run those hosts in IPv6 only mode,
while still providing access to the IPv4 Internet. The scenario is
depicted in the picture below.
+----+ +-------------+
| +------------------+IPv6 Internet+
| | +-------------+
IPv6 host-----------------+ GW |
| | +-------------+
| +------------------+IPv4 Internet+
+----+ +-------------+
|-------------------------public v6-----------------------------|
|-------public v6---------|NAT|----------public v4--------------|
The proposed NAT64/DNS64 is perfectly suitable for this particular
scenario. The deployment of the NAT64/DNS64 would be as follows: The
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NAT64 function should be located in the GW device that connects the
IPv6 site to the IPv4 Internet. The DNS64 functionality can be
placed either in the local recursive DNS server or in the local
resolver in the hosts.
The proposed NAT64/DNS64 approach satisfies the requirements of this
scenario, in particular because it doesn't require any changes to
current IPv6 hosts in the site to obtain basic functionality.
8.2. Scenario 3: the IPv6 Internet to an IPv4 network
The scenario of servers using private addresses and being reached
from the IPv6 Internet basically includes the cases that for whatever
reason the servers cannot be upgraded to IPv6 and they even may not
have public IPv4 addresses and it would be useful to allow IPv6 nodes
in the IPv6 Internet to reach those servers. This scenario is
depicted in the figure below.
+----+
IPv6 Host(s)-------(Internet)-----+ GW +------Private IPv4 Servers
+----+
|---------public v6---------------|NAT|------private v4----------|
This scenario can again be perfectly served by the NAT64 approach.
In this case the NAT64 functionality is placed in the GW device
connecting the IPv6 Internet to the server's site. In this case, the
DNS64 functionality is not required in general since real (i.e. non
synthetic) AAAA RRs for the IPv4 servers containing the IPv6
representation of the IPv4 address of the servers can be created.
See more discussion about this in [I-D.ietf-behave-dns64]
Again, this scenario is satisfied by the NAT64 since it supports the
required functionality without requiring changes in the IPv4 servers
nor in the IPv6 clients.
9. Security Considerations
Implications on end-to-end security.
Any protocol that protect IP header information are essentially
incompatible with NAT64. So, this implies that end to end IPSec
verification will fail when AH is used (both transport and tunnel
mode) and when ESP is used in transport mode. This is inherent to
any network layer translation mechanism. End-to-end IPsec protection
can be restored, using UDP encapsulation as described in [RFC3948].
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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 RFC 4787 [RFC4787] a
NAT64 must offer "enpoint independent filtering". This means:
for any IPv6 side packet with source (S'1,s1) and destination
(Pref64::D1,d1) that creates an external mapping to (S1,s1),
(D1,d1),
for any subsequent external connection to from S'1 to (D2,d2)
within a given binding timer window,
(S1,s1) = (S2,s2) for all values of D2,d2
Implementations may also provide support for "Address-Dependent
Mapping" and "Address and Port-Dependent Mapping", as also defined in
this document and following the guidelines defined in RFC 4787
[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 filtering" 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 filtering" 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
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
discarded. A dynamic filter may be employed where by 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 D 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
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operation of the NAT64 device to a "Address Dependent" behavior,
though not by its mapping behavior, but instead by its filtering
behavior.
Attacks to NAT64.
The NAT64 device itself is a potential victim of different type of
attacks. In particular, the NAT64 can be a victim of DoS attacks.
The NAT64 box 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. It
should be noted that 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 to 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. NAT64 devices should implement proper
protection against such attacks, for instance allocating a limited
amount of memory for fragmented packet storage.
10. IANA Considerations
This document contains no IANA considerations.
11. Contributors
George Tsirtsis
Qualcomm
tsirtsis@googlemail.com
Greg Lebovitz
Juniper
gregory.ietf@gmail.com
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12. Acknowledgements
Dave Thaler, Dan Wing, Alberto Garcia-Martinez, Reinaldo Penno and
Joao Damas reviewed the document and provided useful comments to
improve it.
The content of the draft was improved thanks to discussions with 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.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.
[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.
[I-D.ietf-behave-dns64]
Bagnulo, M., Sullivan, A., Matthews, P., and I. Beijnum,
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"DNS64: DNS extensions for Network Address Translation
from IPv6 Clients to IPv4 Servers",
draft-ietf-behave-dns64-00 (work in progress), July 2009.
[I-D.ietf-behave-v6v4-xlate]
Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", draft-ietf-behave-v6v4-xlate-01 (work in
progress), September 2009.
[I-D.ietf-behave-address-format]
Huitema, C., Bao, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators",
draft-ietf-behave-address-format-00 (work in progress),
August 2009.
13.2. Informative References
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766,
February 2000.
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128, June 2001.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network
Address Translator - Protocol Translator (NAT-PT) to
Historic Status", RFC 4966, July 2007.
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[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[RFC3498] Kuhfeld, J., Johnson, J., and M. Thatcher, "Definitions of
Managed Objects for Synchronous Optical Network (SONET)
Linear Automatic Protection Switching (APS)
Architectures", RFC 3498, March 2003.
[I-D.ietf-behave-v6v4-framework]
Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation",
draft-ietf-behave-v6v4-framework-01 (work in progress),
September 2009.
Authors' Addresses
Marcelo Bagnulo
UC3M
Av. Universidad 30
Leganes, Madrid 28911
Spain
Phone: +34-91-6249500
Fax:
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es/marcelo
Philip Matthews
Alcatel-Lucent
600 March Road
Ottawa, Ontario
Canada
Phone: +1 613-592-4343 x224
Fax:
Email: philip_matthews@magma.ca
URI:
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Iljitsch van Beijnum
IMDEA Networks
Avda. del Mar Mediterraneo, 22
Leganes, Madrid 28918
Spain
Email: iljitsch@muada.com
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