BEHAVE WG M. Bagnulo
Internet-Draft UC3M
Intended status: Standards Track P. Matthews
Expires: September 8, 2009 Alcatel-Lucent
I. van Beijnum
IMDEA Networks
March 7, 2009
NAT64: Network Address and Protocol Translation from IPv6 Clients to
IPv4 Servers
draft-bagnulo-behave-nat64-03
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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 . . . . . . . . . . . . . . . . . . . . 4
1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1. NAT64 solution elements . . . . . . . . . . . . . . . 6
1.2.2. Walkthough . . . . . . . . . . . . . . . . . . . . . . 7
1.2.3. Filtering . . . . . . . . . . . . . . . . . . . . . . 9
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. NAT64 Normative Specification . . . . . . . . . . . . . . . . 11
3.1. Determining the Incoming 5-tuple . . . . . . . . . . . . . 13
3.2. Filtering and Updating Session Information . . . . . . . . 14
3.2.1. UDP Session Handling . . . . . . . . . . . . . . . . . 14
3.2.2. TCP Session Handling . . . . . . . . . . . . . . . . . 15
3.2.3. Computing the Outgoing 5-Tuple . . . . . . . . . . . . 15
3.2.4. Translating the Packet . . . . . . . . . . . . . . . . 16
3.2.5. Handling Hairpinning . . . . . . . . . . . . . . . . . 17
3.3. FTP ALG . . . . . . . . . . . . . . . . . . . . . . . . . 17
4. Application scenarios . . . . . . . . . . . . . . . . . . . . 17
4.1. Enterprise IPv6 only network . . . . . . . . . . . . . . . 18
4.2. Reaching servers in private IPv4 space . . . . . . . . . . 18
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.1. About the Prefix used to map the IPv4 address space
into IPv6 . . . . . . . . . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Changes from Previous Draft Versions . . . . . . . . . . . . . 23
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 23
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. Normative References . . . . . . . . . . . . . . . . . . . 24
11.2. Informative References . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
This document specifies NAT64, a mechanism for IPv6-IPv4 transition
and co-existence. Together with DNS64 [I-D.bagnulo-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.
The translation is done by translating the packet headers according
to SIIT [RFC2765], translating the IPv4 server address by adding or
removing a /96 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 /96 prefix to the IPv4
address to create an IPv6 address, where the /96 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-enabled devices connecting an
IPv6-only network to the IPv4-only Internet, along with the
deployment of a few DNS64-enabled name servers in the IPv6-only
network. However, some advanced features 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].
1.1. Features of NAT64
The features of NAT64 and DNS64 are:
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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 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
communcation 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 [I-D.ietf-behave-tcp], and ICMP
[I-D.ietf-behave-nat-icmp].
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
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
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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 SIIT [RFC2765].
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
a /96 IPv6 prefix (hereafter called Pref64::/96). This allows each
IPv4 address to be mapped into a different IPv6 address by simply
concatenating the /96 prefix assigned as the IPv6 address pool of the
NAT64, with the IPv4 address being mapped (i.e. an IPv4 address X is
mapped into the IPv6 address Pref64:X). The NAT64 prefix Pref64::/96
is assigned by the administrator of the NAT64 box from the global
unicast IPv6 address block assigned to the site. It should be noted
that the the prefix used as the IPv6 address pool is assigned to a
specific NAT64 box and if there are multiple NAT64 boxes, each box is
allocated a different prefix. Assigning the same prefix to multiple
boxes may lead to communication failures due to internal routing
fluctuations.
The IPv4 address pool, however, is a set of IPv4 addresses, normally
a small prefix assigned by the local administrator to the NAT64's
external (IPv4) interface. 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.bagnulo-behave-dns64] is used to
synthesize an AAAA record from the A record (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 by adding the
Pref64::/96 of a NAT64 to the responder's IPv4 address (i.e. if the
IPv4 node has IPv4 address X, then the synthetic AAAA RR will contain
the IPv6 address formed as Pref64:X). 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. Walkthough
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 IPv6
Internet.
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)".
The scenario for this case is depicted in the following figure:
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+---------------------------------------+ +-----------+
|IPv6 site +-------------+ | | |
| +----+ | Name server | +-------+ | IPv4 |
| | H1 | | with DNS64 | | NAT64 |----| Internet |
| +----+ +-------------+ +-------+ +-----------+
| |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 Internet. This NAT64
has a /96 prefix (called Pref64::/96) associated to its IPv6
interface and an IPv4 address T assigned to its IPv4 interface.
Also shown is a local name server with DNS64 functionality. For the
purpose of this example, we assume that the name server is a dual-
stack node, so that H1 can contact it via IPv6, while it can contact
IPv4-only name servers via IPv4.
The local name server needs to know the /96 prefix assigned to the
local NAT64 (Pref64::/96). 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 PRefix64::/96 associated to the NAT64 box and the IPv4
address of H2 in the lower 32 bits.
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,x), where y and x are ports chosen by H1.
3. The packet is routed to the IPv6 interface of the NAT64 (since
Pref64::/96 has been associated to this interface).
4. The NAT64 receives the packet and performs the following actions:
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* 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
SIIT.
* 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 lower 32
bits of the destination IPv6 address of the received IPv6
packet that is being translated.
The NAT64 sends the translated packet out its IPv4 interface and
the packet arrives at H2.
5. H2 node responds by sending a packet with destination transport
address (T,t) and source transport address (X,x).
6. 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
SIIT.
* The NAT64 includes (Y',y) as destination transport address in
the packet and (Pref64:X,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.
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 IPv4 address 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::/96
in this document) to the original IPv4 address.
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.
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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
destination IP address equal to the source IP address of the incoming
packet.
NAT64 filtering is consistent with the recommendations of RFC 4787
[RFC4787].
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:
DNS64: A logical function that synthesizes AAAA records (containing
IPv6 addresses) from A records (containing IPv4 addresses).
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.
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 or UDP 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.
5-Tuple: The tuple (source IP address, source port, destination IP
address, destination port, transport protocol). A 5-tuple
uniquely identifies a session. When a session flows through a
NAT64, each session has two different 5-tuples: one with IPv4
addresses and one with IPv6 addresses.
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Session table: A table of sessions kept by a NAT64. Each NAT64 has
two session tables, one for TCP and one for UDP.
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).
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.
BIB: Binding Information Base. A table of mappings kept by a NAT64.
Each NAT64 has two BIBs, one for TCP and one for UDP.
Endpoint-Independent Mapping: In NAT64, using the same mapping for
all sessions between an IPv6 that have the same IPv6 transport
address endpoint. 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.
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 one IPv6 interface and one IPv4 interface.
The IPv6 interface MUST have a unicast /96 IPv6 prefix assigned to
it, denoted Pref64::/96. The IPv4 interface MUST have one or more
unicast IPv4 addresses assigned to it.
A NAT64 uses the following dynamic data structures:
o UDP BIB
o UDP Session Table
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o TCP BIB
o TCP Session Table
A NAT64 has two Binding Information Bases: one for TCP and one for
UDP. 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
IPv4 interface of 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.
A NAT64 also has two session tables: one for TCP sessions and one for
UDP sessions. Each entry keeps information on the state of the
corresponding session: see Section 3.2. 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 5-tuple or an outgoing
5-tuple.
For each 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). However, a single BIB entry can have
multiple corresponding sessions. When the last corresponding session
is deleted, the BIB entry is deleted.
The processing of an incoming IP packet takes the following steps:
1. Determining the incoming 5-tuple
2. Filtering and updating session information
3. Computing the outgoing 5-tuple
4. Translating the packet
5. Handling hairpinning
The details of these steps are specified in the following
subsections.
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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 in the same.
TBD: Add support for ICMP Query packets. (ICMP Error packets are
handled).
3.1. Determining the Incoming 5-tuple
This step associates a incoming 5-tuple (source IP address, source
port, destination IP address, destination port, transport protocol)
with every incoming IP packet for use in subsequent steps.
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.
If the incoming IP packet contains a complete (un-fragmented) ICMP
message, then the 5-tuple is computed by extracting the appropriate
fields from the IP packet embedded inside the ICMP 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 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
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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 Session Information
This step updates the per-session information stored in the
appropriate session table. This affects the lifetime of the session,
which in turn affects the lifetime of the corresponding BIB entry.
This step may also filter incoming packets, if desired.
The details of this step depend on the transport protocol (UDP or
TCP).
3.2.1. UDP Session Handling
The state information stored for a UDP session 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 UDP session
is deleted.
The incoming packet is processed as follows:
1. If the packet arrived on the IPv4 interface and 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 destination IPv4 address 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 SHOULD 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).
2. The NAT64 searches for the session table entry corresponding to
the incoming 5-tuple. If no such entry if found, a new entry is
created.
3. 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.
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3.2.2. TCP Session Handling
TBD: Describe the state machine required to track the state of the
TCP session. This is a simplified version of the state machine used
by the endpoints.
3.2.3. Computing the Outgoing 5-Tuple
This step computes the outgoing 5-tuple by translating the addresses
and ports in the incoming 5-tuple. The transport protocol in the
outgoing 5-tuple is always the same as that in the incoming 5-tuple.
In the text below, a reference to the "the BIB" means either the TCP
BIB or the UDP BIB as appropriate, as determined by the transport
protocol in the 5-tuple.
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 adding or removing the /96 prefix.
This distinction is important; without it, hairpinning doesn't
work correctly.
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:
If the BIB contains a entry (S',s) <--> (T,t), then the outgoing
source transport address is (T,t).
Otherwise, create a new BIB entry (S',s) <--> (T,t) as described
below. The outgoing source transport address is (T,t).
The outgoing destination address is computed as follows:
If D' is composed of the NAT64's prefix followed by an IPv4
address D, then the outgoing destination transport address is
(D,d).
Otherwise, discard the packet.
When translating in the IPv4 --> IPv6 direction, let the incoming
source and destination transport addresses in the 5-tuple be (S,s)
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and (D,d) respectively. The outgoing source transport address is
computed as follows:
The outgoing source transport address is (Pref64::S,s).
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 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.
TBD: Do we delete the session entry if we cannot create a BIB entry?
If the rules specify that the packet is discarded, then the NAT64
SHOULD 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).
3.2.4. Translating the Packet
This step translates the packet from IPv6 to IPv4 or vica-versa.
The translation of the packet is as specified in section 3 and
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section 4 of SIIT [RFC2765], 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 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 which contains the source and destination IP addresses. An
algorithm for efficently updating these checksums is described in
[RFC3022].
o When the protocol following the IP header is ICMP (sections 3.4
and 4.4) 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).
3.2.5. 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.
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.
3.3. FTP ALG
TBD: Describe the FTP ALG, a mechanism for translating the embedded
IP addresses inside FTP commands, that enables FTP sessions to pass
through NAT64.
4. Application scenarios
In this section, we describe how to apply NAT64/DNS64 to the suitable
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scenarios described in draft-arkko-townsley-coexistence.
4.1. Enterprise IPv6 only network
The Enterprise 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
NAT64 function should be located in the GW device that connects the
IPv6 site to the IPv4 Internet. The DNS64 functionality can be
placed in different places. Probably the best trade-off between
architectural cleanness deployment simplicity would be to place it in
the local recursive DNS server of the enterprise site. The option
that is easier to deploy would be to co-locate it with the NAT64 box.
The cleanest option would be included in the local resolver of the
IPv6 hosts, but this option seems the harder to deploy cause it
implies changes to the hosts.
The proposed NAT64/DNS64 approach satisfies the requirements of this
scenario, in particular cause it doesn't require any changes to
current IPv6 hosts in the site to obtain basic functionality.
4.2. Reaching servers in private IPv4 space
The scenario of servers using IPv4 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 don't
have public VIPv4 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.
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+----+
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 needed. Since the server's site is
running the NAT64 and the servers, it can publish in its own DNS
server the AAAA RR corresponding to the servers i.e. AAAA RR
associating the FQDN of the server and the Pref64:ServerIPv4Addr. In
this case, there is no need to synthesize AAAA RR cause the site can
configure them in the DNS itself.
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.
5. Discussion
5.1. About the Prefix used to map the IPv4 address space into IPv6
In the NAT64 approach, we need to represent the IPv4 addresses in the
IPv6 Internet. Since there is enough address space in IPv6, we can
easily embed the IPv4 address into an IPv6 address, so that the IPv4
address information can be extracted from the IPv6 address without
requiring additional state. One way to that is to use an IPv6 prefix
Pref64::/96 and juxtapose the IPv4 address at the end (there are
other ways of doing it, but we are not discussing the different
formats here). In this document the Pref64::/96 prefix is extracted
from the address block assigned to the site running the NAT64 box.
However, one could envision the usage of other prefixes for that
function. In particular, it would be possible to define a well-known
prefix that can be used by the NAT64 devices to map IPv4 (public)
addresses into IPv6 addresses, irrespectively of the address space of
the site where the NAT64 is located. In this section, we discuss the
pro and cons of the different options.
the different options for Pref64::/96 are the following
Local: A locally assigned prefix out of the address block of the
site running the NAT64 box
Well-known: A well know prefix that is reserved for this purpose.
We have the following different options:
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IPv4 mapped prefix
IPv4 compatible prefix
A new prefix assigned by IANA for this purpose
The reasons why using a well-known prefix is attractive are the
following: Having a global well-know prefix would allow to identify
which addresses are "real" IPv6 addresses with native connectivity
and which addresses are IPv6 addresses that represent an IPv4
address. From an architectural perspective, it seems the right thing
to do to make this visible since hosts an applications could react
accordingly and avoid or prefer such type of connectivity if needed.
From the DNS64 perspective, using the well-know prefix would imply
that the same synthetic AAAA RR will be created throughout the IPv6
Internet, which would result in consistent view of the RR
irrespectively of the location in the topology. From a more
practical perspective, having a well-know prefix would allow to
completely decouple the DNS64 from the NAT64, since the DNS64 would
always use the well-know prefix to create the synthetic AAAA RR and
there is no need to configure the same Pref64::/96 both in the DNS64
and the NAT64 that work together.
Among the different options available for the well-know prefix, the
option of using a pre-existing prefix such as the IPv4-mapped or
IPv4-compatible prefix has the advantage that would potentially allow
the default selection of native connectivity over translated
connectivity for legacy hosts in communications involving dual-stack
hosts. This is because current RFC3484 default policy table include
entries for the IPv4-mapped prefix and the IPv4-compatible prefix,
implying that native IPv6 prefixes will be preferred over these.
However, current implementations do not use the IPv4-mapped prefix on
the wire, beating the purpose of support unmodified hosts. The IPv4-
compatible prefix is used by hosts on the wire, but has a higher
priority than the IPv4-mapped prefix, which implies that current
hosts would prefer translated connectivity over native IPv4
connectivity (represented by the IPv4-mapped prefix in the default
policy table). So neither of the prefixes that are present in the
default policy table would result in the legacy hosts preferring
native connectivity over translated connectivity, so it doesn't seem
to be a compelling reason to re-use neither the IPv4-mapped not the
IPv4-compatible prefix for this. So, we conclude that among the the
well know prefix options, the preferred option would be to ask for a
new prefix from IANA to be allocated for this.
However, there are several issues when considering using the well-
know prefix option, namely:
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The well-know prefix is suitable only for mapping IPv4 public
addresses into IPv6. IPv4 public addresses can be mapped using
the same prefix cause they are globally unique. However, the
well-known prefix is not suitable for mapping IPv4 private
addresses. This is so because we cannot leverage on the
uniqueness of the IPv4 address to achieve uniqueness of the IPv6
address, so we need to use a different IPv6 prefix to disambiguate
the different private IPv4 address realms. As we describe above,
there is a clear use case for mapping IPv4 private addresses, so
there is a pressing need to map IPv4 private addresses. In order
to do so we will need to use at least for IPv4 private addresses,
IPv6 local prefixes. In that case, the architectural goal of
distinguishing the "real" IPv6 addresses from the IPv6 addresses
that represent IPv4 addresses can no longer be achieved in a
general manner, making this option less attractive.
The usage of a single well-known prefix to map IPv4 addresses
irrespectively of the NAT64 used, may results in failure modes in
sites that have more than one NAT64 device. The main problem is
that intra-site routing fluctuations that result in packets of an
ongoing communication flow through a different NAT64 box that the
one they were initially using (e.g. a change in an ECMP load
balancer), would break ongoing communications. This is so because
the different NAT64 boxes will use a different IPv4 address, so
the IPv4 peer of the communications will receive packets coming
from a different IPv4 address. This is avoided using a local
address, since each NAT64 box can have a different Pref64::/06
associated, to routing fluctuations would not result in using a
different NAT64 box.
The usage of a well-known prefix is also problematic in the case
that different routing domains want to exchange routing
information involving these routes. Consider the case of an IPv6
site that has multiple providers and that each of these providers
provides access to the IPv4 Internet using the well know prefix.
Consider the hypothetical case that different parts of the IPv4
Internet are reachable through different IPv6 ISPs (yes, this
means that in a futuristic scenario, the IPv4 Internet is
partitioned). In order to reach the different parts through the
different ISPs, more specific routes representing the different
IPv4 destinations reachable need to be injected in the IPv6 sites.
This basically means that such configuration would imply to import
the IPv4 routing entropy into the IPv6 routing system. If
different local prefixes are used, then each ISP only announces
its own local prefix, and then the burden of defining which IPv4
destination is reachable through which ISP is placed somewhere
else (e.g. in the DNS64).
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6. Security Considerations
Implications on end-to-end security, IPSec and TLS.
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 [RFC2765].
TBD: TLS implications
Filtering.
NAT64 creates binding state using packets flowing from the IPv6 side
to the IPv4 side. So, NAT64 implements by definition, at least,
endpoint independent filtering, meaning that in order to enable any
packet to flow from the IPv4 side to the IPv6 side, there must have
been a packet flowing from the IPv6 side to the IPv4 side the created
the binding information to be used for packets in the other
direction. Endpoint independent filtering allows that once a binding
is created, it can be used by any node on the IPv4 side to send
packets to the IPv6 transport address that created the binding. This
basically means that as long a the IPv6 node does not open a hole in
the NAT64, incoming communications are blocked and that once that the
IPv6 node has sent the first packet, this packet opens the door for
any node on the IPv4 side to send packets to that IPv6 transport
address. It is possible to configure the NAT64 to implement more
stringent security policy, if endpoint independent filtering is
considered not secure enough. In particular, if the security policy
of the NAT64 requires it, is it possible to configure the NAT64 to
perform address dependent filtering. This basically means that the
binding state created can only be used by to send packets from the
IPv4 address to which the original packet that created the binding
was sent to. This basically means that the door is open only for
that IPv4 address to send packet to the IPv6 transport address.
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 address that is 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 address. It should be
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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 resource available in the NAT64 such as
memory or link capacity. For instance, if the NAT64 implements
reassembly of fragmented packets, 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 if 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.
7. IANA Considerations
The IANA is requested to assign an EDNS Option Code value for the SAS
option.
TBD: Set up an IANA registry for SAS flags??
8. Changes from Previous Draft Versions
Note to RFC Editor: Please remove this section prior to publication
of this document as an RFC.
[[This section lists the changes between the various versions of this
draft.]]
9. Contributors
George Tsirtsis
Qualcomm
tsirtsis@googlemail.com
10. 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.
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Marcelo Bagnulo and Iljitsch van Beijnum are partly funded by
Trilogy, a research project supported by the European Commission
under its Seventh Framework Program.
11. References
11.1. 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.
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[I-D.ietf-behave-tcp]
Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP",
draft-ietf-behave-tcp-08 (work in progress),
September 2008.
[I-D.ietf-behave-nat-icmp]
Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP protocol",
draft-ietf-behave-nat-icmp-12 (work in progress),
January 2009.
[I-D.bagnulo-behave-dns64]
Bagnulo, M., Sullivan, A., Matthews, P., Beijnum, I., and
M. Endo, "DNS64: DNS extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers",
draft-bagnulo-behave-dns64-02 (work in progress),
March 2009.
11.2. Informative References
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766,
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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.
[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.
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
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Philip Matthews
Unaffiliated
600 March Road
Ottawa, Ontario
Canada
Phone: +1 613-592-4343 x224
Fax:
Email: philip_matthews@magma.ca
URI:
Iljitsch van Beijnum
IMDEA Networks
Av. Universidad 30
Leganes, Madrid 28911
Spain
Phone: +34-91-6246245
Email: iljitsch@muada.com
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