BEHAVE WG M. Bagnulo
Internet-Draft UC3M
Intended status: Standards Track P. Matthews
Expires: June 17, 2010 Alcatel-Lucent
I. van Beijnum
IMDEA Networks
December 14, 2009
NAT64: Network Address and Protocol Translation from IPv6 Clients to
IPv4 Servers
draft-ietf-behave-v6v4-xlate-stateful-04
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.
NAT64 also support IPv4 initiated communications to a subset of the
IPv6 hosts through statically configured bindings in the NAT64. This
document specifies NAT64, and gives suggestions on how they should be
deployed.
Status of this Memo
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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. NAT64 Behaviour Walkthrough . . . . . . . . . . . . . 8
1.2.3. Filtering . . . . . . . . . . . . . . . . . . . . . . 10
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. NAT64 Normative Specification . . . . . . . . . . . . . . . . 12
3.1. Determining the Incoming tuple . . . . . . . . . . . . . . 16
3.2. Filtering and Updating Binding and Session Information . . 18
3.2.1. UDP Session Handling . . . . . . . . . . . . . . . . . 18
3.2.2. TCP Session Handling . . . . . . . . . . . . . . . . . 20
3.2.3. ICMP Query Session Handling . . . . . . . . . . . . . 27
3.2.4. Rules for allocation of IPv4 transport addresses . . . 29
3.2.5. Generation of the IPv6 representations of IPv4
addresses . . . . . . . . . . . . . . . . . . . . . . 30
3.3. Computing the Outgoing Tuple . . . . . . . . . . . . . . . 31
3.3.1. Computing the outgoing 5-tuple for TCP, UDP and
ICMP error messages . . . . . . . . . . . . . . . . . 31
3.3.2. Computing the outgoing 3-tuple for ICMP Query
messages . . . . . . . . . . . . . . . . . . . . . . . 32
3.4. Translating the Packet . . . . . . . . . . . . . . . . . . 32
3.5. Handling Hairpinning . . . . . . . . . . . . . . . . . . . 33
4. Security Considerations . . . . . . . . . . . . . . . . . . . 33
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 36
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
8.1. Normative References . . . . . . . . . . . . . . . . . . . 36
8.2. Informative References . . . . . . . . . . . . . . . . . . 37
Appendix A. Application scenarios . . . . . . . . . . . . . . . . 39
A.1. Scenario 1: an IPv6 network to the IPv4 Internet . . . . . 39
A.2. Scenario 3: the IPv6 Internet to an IPv4 network . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40
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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, also allow peer-to-peer communication between
IPv6-only and IPv4-only hosts. NAT64 also support IPv4 initiated
communications to a subset of the IPv6 hosts through statically
configured bindings in the NAT64.
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 the
general case of 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 are:
o NAT64 as specified in this document is compliant with the
recommendations for how NATs should handle UDP [RFC4787], TCP
[RFC5382], and ICMP [RFC5508]. As such, NAT64 only supports
Endpoint-Independent mappings and supports both Endpoint-
Independent and Address dependent filtering. Because of the
compliance with the aforementioned requirements, NAT64 is
compatible with ICE.
o In the absence of any state in NAT64 regarding a given IPv6 node,
only said IPv6 node can initiate sessions to IPv4 nodes. This
works for roughly the same class of applications that work through
IPv4-to-IPv4 NATs.
o Depending on the filtering policy used (Endpoint-Independent, or
Address-Dependent), IPv4-nodes MAY be able to initiate sessions to
a given IPv6 node, if the NAT64 somehow has an appropriate mapping
(i.e.,state) for said IPv6 node, via one of the following
mechanism.
* The IPv6 node has recently initiated a session to the same or
other external-IPv4 node.
* The IPv6 node has used a NAT-traversal technique (such as ICE)
which essentially results in the previous bullet point.
* If static configuration (i.e., mapping) exists regarding said
IPv6 node
1.2. Overview
This section provides a non-normative introduction to the mechanisms
of NAT64. This is achieved by describing the NAT64 behavior
involving a simple setup, that involves a single NAT64 box, a single
DNS64 box and a simple network topology. The goal of this
description is to provide the reader with a general view of NAT64.
It is not the goal of this section to describe all possible
configurations nor to provide a normative specification of the NAT64
behavior. The normative specification of NAT64 is provided in
Section 3.
NAT64 mechanism is implemented in an NAT64 box which has (at least)
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
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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 either statically configured in the NAT64 or it
is created when the first packet flowing from the IPv6 network to the
IPv4 network is translated. After the binding state has been
created, and if the filtering policy permits, packets flowing in
either direction on that particular flow are translated. The result
is that, in the general case, NAT64 only supports communications
initiated by the IPv6-only node towards an IPv4-only node. Some
additional mechanisms (like ICE) or static binding configuration, can
be used to provide support for communications initiated by the IPv4-
only node to the IPv6-only node.
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.
The IPv6 address pool is an IPv6 prefix assigned to the translator
itself (hereafter called Pref64::/n). Due to the abundance of IPv6
address space, it is possible to assign an Pref64::/n that is equal
or even bigger than the whole IPv4 address space. This allows each
IPv4 address to be mapped into a different IPv6 address by simply
concatenating the Pref64::/n with the IPv4 address being mapped and a
suffix (i.e. an IPv4 address X is mapped into the IPv6 address
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Pref64:X:SUFFIX). The provisioning of the Pref64::/n is discussed at
length in [I-D.ietf-behave-address-format]
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.
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
algorithmically 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.
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
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before.
1.2.2. NAT64 Behaviour 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)".
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:
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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).
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.
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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.
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 incoming IPv4 packets.
Filtering of incoming IPv6 packets is not described in this
specification.
NAT64 filtering is consistent with the recommendations of RFC 4787
[RFC4787], and the ones of RFC 5382 [RFC5382]. Because of that, the
NAT64 as specified in this document, supports both Endpoint-
Independent filtering and Address-Dependent filtering, both for TCP
and UDP.
If a NAT64 performs Endpoint-Independent filtering of incoming IPv4
packets, then an incoming IPv4 packet is dropped unless the NAT64 has
state for the destination transport address of the incoming IPv4
packet.
If a NAT64 performs Address-Dependent filtering of incoming IPv4
packets, then an incoming IPv4 packet is dropped unless the NAT64 has
state involving the destination transport address of the IPv4
incoming packet and the particular source IP address of the incoming
IPv4 packet.
2. Terminology
This section provides a definitive reference for all the terms used
in 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].
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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.
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.
Filtering, Endpoint-Independent: The NAT64 filters out only incoming
IPv4 packets not destined to a transport address for which there
is not state in the NAT64, regardless of the source IPv4 transport
address. The NAT forwards any packets destined to any transport
address for which it has state. In other words, having state for
a given transport address is sufficient to allow any packets back
to the internal endpoint.
Filtering, Address-Dependent: The NAT64 filters out incoming IPv4
packets not destined to a transport address for which there is no
state (similar to the Endpoint-Independent filtering).
Additionally, the NAT64 will filter out incoming IPv4 packets
coming from IPv4 address X and destined for a transport address
that it has state for if the NAT64 has not sent packets to X
previously (independently of the port used by X). In other words,
for receiving packets from a specific IPv4 endpoint, it is
necessary for the IPv6 endpoint to send packets first to that
specific IPv4 endpoint's IP address.
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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 through 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).
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
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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. The BIB has then two columns, the BIB IPv6 transport address
and the BIB IPv4 transport address. A given IPv6 or IPv4 transport
address can appear in at most one entry in a BIB: for example, (2001:
db8::17, 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. This implementation of the BIBs
ensures Endpoint-Independent mappings in the NAT64. The information
in the BIBs is also used to implement Endpoint-Independent filtering.
(Address-Dependent filtering is implemented using the Session tables
described below.)
In the case of the ICMP Query BIB, each ICMP Query BIB entry specify
a mapping between an (IPv6 address, IPv6 Identifier) pair and an
(IPv4 address, IPv6 Identifier) pair.
(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, IPv6 or IPv4
Identifier) pair can appear in at most one entry in the ICMP Query
BIB.
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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 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 will
always be equal to z.
For each Session Table Entry (STE), there are then five columns:
The STE source IPv6 transport address, (X',x) in the example
above,
The STE destination IPv6 transport address, (Y',y) in the example
above,
The STE source IPv4 transport address, (T,t) in the example above,
and,
The STE destination IPv4 transport address, (Z,z) in the example
above.
The STE lifetime.
The terminology used for the session table entry columns is from the
perspective of an incoming IPv6 packet being translated into an
outgoing IPv4 packet.
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
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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.
For each Session Table Entry (STE), there are then six columns:
The STE source IPv6 address, X' in the example above,
The STE destination IPv6 address, Y' in the example above,
The STE IPv6 Identifier, I1 in the example above,
The STE source IPv4 address, T in the example above,
The STE destination IPv4 address, Z in the example above, and,
The STE IPv4 Identifier, I2 in the example above.
The STE lifetime.
The NAT64 uses the session state information to determine when the
session is completed, and also uses session information for Address-
Dependent filtering. A session can be uniquely identified by either
an incoming tuple or an outgoing tuple.
For each TCP or UDP session, there is a corresponding BIB entry,
uniquely specified by either the source IPv6 transport address (in
the IPv6 --> IPv4 direction) or the destination IPv4 transport
address (in the IPv4 --> IPv6 direction). For each ICMP Query
session, there is a corresponding BIB entry, uniquely specified by
either the source IPv6 address and ICMPv6 Query Id (in the IPv6 -->
IPv4 direction) or the destination IPv4 address and the ICMPv4 Query
Id (in the IPv4 --> IPv6 direction). However, for all the BIBs, a
single BIB entry can have multiple corresponding sessions. When the
last corresponding session is deleted, if the BIB entry was
dynamically created, the BIB entry is deleted.
The NAT64 will receive packet through its interfaces. These packets
can be either IPv6 packets or IPv4 packets and they may carry TCP
traffic, UDP traffic or ICMP. The processing of the packets will be
described next. In the case that the processing is common to all the
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aforementioned types of packets, we will refer to the packet as the
incoming packet in general. In case that the processing is specific
to IPv6 packets, we will refer to the incoming IPv6 packet and
similarly to the IPv4 packets.
The processing of an incoming IP packet takes the following steps:
1. Determining the incoming tuple
2. Filtering and updating binding and session information
3. Computing the outgoing tuple
4. Translating the packet
5. Handling hairpinning
The details of these steps are specified in the following
subsections.
This breakdown of the 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.
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
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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.
If the incoming IP packet contains a fragment, then more processing
may be needed. This specification leaves open the exact details of
how a NAT64 handles incoming IP packets containing fragments, and
simply requires that the external behavior of the NAT64 is compliant
with the following conditions:
The NAT64 MUST handle fragments arriving out-of-order conditioned
to the following:
The NAT64 MUST limit the amount of resources devoted to the
storage of fragmented packets in order to protect from DoS
attack.
As long as the NAT64 has available resources, the NAT64 MUST
allow the fragments to arrive over a time interval. The time
interval MUST be configurable and the default value MUST be of
at least 10 seconds.
The NAT64 MAY require that the UDP, TCP, or ICMP header be
completely contained within the fragment that contains OFFSET
equal to zero.
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, subsequent fragments may arrive before
the first.
Implementers of NAT64 should be aware that there are a number of
well-known attacks against IP fragmentation; see [RFC1858] and
[RFC3128]. Implementers should also be aware of additional issues
with reassembling packets at high rates, described in [RFC4963].
Except from the retrieval of 5-tuple information from the incoming
packets as discussed above, all other fragmentation and PMTUD related
processing performed by the NAT64 device is performed as defined in
[I-D.ietf-behave-v6v4-xlate], including the translation of all
related fragmentation fields in the IP header, the determination of
the outgoing packet size, the fragmentation of outgoing packets and
the generation and processing of ICMP Packet Too Big errors.
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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.
Irrespectively of the transport protocol used, the NAT64 must
silently discard all incoming IPv6 packets containing a source
address that contains the Pref64::/n. This is required in order to
prevent hairpinning loops as described in the Security Considerations
section. In addition, the NAT64 function will only process incoming
IPv6 packets that contain a destination address that contains
Pref64::/n. Likewise, the NAT64 function will only process incoming
IPv4 packets that contain a destination address that belong to the
IPv4 pool assigned to the NAT64.
The details of this step depend on the protocol (UDP TCP or ICMP
Query).
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 with source transport address (X',x) and
destination transport address (Y',y) is processed as follows:
The NAT64 searches for a UDP BIB entry that contains an BIB IPv6
transport address that matches the IPv6 source transport address
(X',x). If such an entry does not exists, a new entry is created.
As BIB IPv6 transport address, the source IPv6 transport address
of the packet (X',x) is included and the BIB IPv4 transport
address is set to (T,t) which is allocated using the rules defined
in Section 3.2.4. The result is a BIB entry as follows: (X',x)
<--> (T,t).
The NAT64 searches for the session table entry corresponding to
the incoming 5-tuple. If no such entry is found, a new entry is
created. The information included in the session table is as
follows:
The STE source IPv6 transport address is set to (X',x) the
source IPv6 transport addresses contained in the received IPv6
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packet,
The STE destination IPv6 transport address is set to (Y',y) the
destination IPv6 transport addresses contained in the received
IPv6 packet,
The STE source IPv4 transport address is extracted from the
corresponding UDP BIB entry i.e. is set to (T,t),
The STE destination IPv4 transport is set to (Z(Y'),y), y being
the same port as the STE destination IPv6 transport address and
Z(Y') being algorithmically generated from the IPv6 destination
address (i.e. Y') using the reverse algorithm as specified in
Section 3.2.5 .
The result is a Session table entry as follows: (X',x),(Y',y) <-->
(T,t),(Z(Y'),y)
The NAT64 sets or resets the timer in the session table entry to
maximum session lifetime. By default, the maximum session
lifetime is 5 minutes. The packet is translated and forwarded as
described in the following sections.
An IPv4 incoming packet, with source IPv4 transport address (Y,y) and
destination IPv4 transport address (X,x) is processed as follows:
The NAT64 searches for a UDP BIB entry that contains an BIB IPv4
transport address matches (Y,y) i.e. the IPv4 destination
transport address in the incoming IPv4 packet. If such an 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 applies Address-Dependent filters on its IPv4
interface, then the NAT64 checks to see if the incoming packet is
allowed according to the address-dependent filtering rule. To do
this, it searches for a session table entry with a STE source IPv4
transport address equal to (X,x) (i.e. the destination IPv4
transport address in the incoming packet) and STE destination IPv4
address equal to Y (i.e. the source IPv4 address in the incoming
packet). If such an entry is found (there may be more than one),
packet processing continues. Otherwise, the packet is discarded.
If the packet is discarded, then an ICMP 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).
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In case the packet is not discarded in the previous processing
(either because the NAT64 is not filtering or because the packet
is compliant with the Address-dependent filtering rule), then the
NAT64 searches for the session table entry corresponding
containing the STE source IPv4 transport address equal to (X,x)
and the STE destination IPv4 transport address equal to (Y,y). If
no such entry is found, a new entry is created. In case a new UDP
session table entry is created, it contains the following
information:
The STE source IPv6 transport address is extracted from the
corresponding UDP BIB entry
The STE destination IPv6 transport address is set to (Z'(Y),y),
y being the same port y than the destination IPv4 transport
address and Z'(Y) being the IPv6 representation of Y, generated
using the algorithm described in Section 3.2.5
The STE source IPv4 transport address is set to (X,x) the
destination IPv4 transport addresses contained in the received
IPv4 packet,,
The STE destination IPv4 transport is set to (Y,y), the source
IPv4 transport addresses contained in the received IPv4 packet.
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.
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 TCP
session. When the timer expires, the TCP session is deleted. If
all the TCP sessions corresponding to a TCP BIB entry are deleted,
then the 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 at least 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, for each TCP session established
through it, it tracks the corresponding state machine as follows.
The states are the following ones:
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CLOSED: Analogous to [RFC0793], CLOSED is a fictional because it
represents the state when there is no state for this particular
5-tuple, and therefore, no connection.
V4 SYN RCV: An IPv4 packet containing a TCP SYN was received by
the NAT64, implying that a TCP connection is being initiated from
the IPv4 side. The NAT64 is now waiting for a matching IPv4
packet containing the TCP SYN in the opposite direction.
V6 SYN RCV: An IPv6 packet containing a TCP SYN was received by
the NAT64, implying that a TCP connection is being initiated from
the IPv6 side. The NAT64 is now waiting for a matching IPv4
packet containing the TCP SYN in the opposite direction.
ESTABLISHED: Represent an open connection, with data flowing in
both directions.
V4 FIN RCV: An IPv4 packet containing a TCP FIN was received by
the NAT64, data can still flow in the connection, the NAT64 is
waiting for a matching TCP FIN in the opposite direction.
V6 FIN RCV: An IPv6 packet containing a TCP FIN was received by
the NAT64, data can still flow in the connection, the NAT64 is
waiting for a matching TCP FIN in the opposite direction.
V6 FIN + V4 FIN RCV: Both an IPv4 packet containing a TCP FIN and
an IPv6 packet containing an TCP FIN for this connection were
received by the NAT64. The NAT64 keeps the connection state alive
and forwards packet in both directions for a short period of time
to allow remaining packets (in particular the ACKs) to be
delivered.
RST RCV: A packet containing a TCP RST was received by the NAT64
for this connection. The NAT64 will keep the state for the
connection for a short time and if no other data packets for that
connection are received, the assumption is that the node has
accepted the RST packet and the state for this connection is then
terminated.
The state machine used by the NAT64 for the TCP session processing is
depicted next. The described state machine handles all TCP segments
received through the IPv6 and IPv4 interface. There is one state
machine per TCP connection that is potentially established through
the NAT64. After bootstrapping of the NAT64 device, all TCP session
are in CLOSED state. As we mention above, the CLOSED state is a
fictional state that there is not state for that particular
connection in the NAT64.
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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.
+----------------------------+ +-----------------------------+
| | | |
| V V |
| V6 +------+ V4 |
| +----SYN------|CLOSED|-----SYN------+ |
| | +------+ | |
| | ^ | |
| | |4min T.O. | |
| V | V |
| +-------+ +-------+ +-------+ |
| |V6 SYN | |RST RCV| |V4 SYN | |
| | RCV | +-------+ | RCV | |
| +-------+ | ^ +-------+ |
| | data pkt | | |
| | | V4 or V6 RST | |
2:04Hrs V4 SYN V | V6 SYN |
T.O. | +--------------+ | |
| +--------->| ESTABLISHED |<---------+ |
+--------------------->| | |
+--------------+ |
| | |
V4 FIN V6 FIN |
| | |
V V |
+---------+ +----------+ |
| V4 FIN | | V6 FIN | |
+---------+ +----------+ |
| | |
V6 FIN V4 FIN 4 min
| | T.O.
V V |
+-------------------+ |
| V4 FIN + V6 FIN |----------------------+
+-------------------+
We next describe the state information and the transitions.
*** CLOSED ***
If a V6 SYN is received with source transport address (X',x) and
destination transport address (Y',y) (This is the case of a TCP
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connection initiated from the IPv6 side), the processing is as
follows:
The NAT64 searches for a TCP BIB entry that matches the IPv6
source transport address (X',x).
If such an entry does not exists, a new BIB entry is created.
The BIB IPv6 transport address is set to (X',x) (i.e. the
source IPv6 transport address of the packet) The BIB IPv4
transport address is the to an IPv4 transport address allocated
using the rules defined in Section 3.2.4 The processing of the
packet continues as described in the next paragraph.
If the entry already exists, then the processing continues as
described in the next paragraph.
Then a new TCP session entry is created in the TCP session table.
The information included in the session table is as follows:
The STE transport IPv6 source address is set to (X',x) (i.e.
the source transport address contained in the received V6 SYN
packet,
The STE transport IPv6 destination address is set to (Y',y)
(i.e. the destination transport address contained in the
received V6 SYN packet,
The STE transport IPv4 source address is set to the BIB IPv4
transport address of the corresponding TCP BIB entry.
The STE transport IPv4 destination address contains the port y
(i.e. the same port as the IPv6 destination transport address)
and the IPv4 address that is algorithmically generated from the
IPv6 destination address (i.e. Y') using the reverse algorithm
as specified in Section 3.2.5.
The lifetime of the TCP session table entry is set to at least
to 4 min (the transitory connection idle timeout as defined in
[RFC5382]).
The state of the session is moved to V6 SYN RCV.
The NAT64 translates and forwards the packet as described in the
following sections
If a V4 SYN packet is received with source IPv4 transport address
(Y,y) and destination IPv4 transport address (X,x) (This is the case
of a TCP connection initiated from the IPv4 side), the processing is
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as follows:
If the security policy requires silently dropping externally
initiated TCP connections, then the packet is silently discarded,
else,
If the destination transport address contained in the incoming V4
SYN (i.e. X,x) is not in use in the TCP BIB, then the 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 (i.e. X,x) is in use in the TCP BIB, then
A new session table entry is created in the TCP session table,
containing the following information:
The STE transport IPv4 source address is set to (X,x) (i.e.
the destination transport address contained in the V4 SYN)
The STE transport IPv4 destination address is set to (Y,y)
(i.e. the source transport address contained in the V4 SYN)
The STE source IPv6 transport address is set to the IPv6
transport address contained in the corresponding TCP BIB
entry.
The STE destination IPv6 transport address contains the port
y (i.e. the same port than the destination IPv4 transport
address) and the IPv6 representation of Y (i.e. 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 state is moved to V4 SYN RCV.
If the NAT64 is performing Address-Dependent filtering, the
packet is stored (The motivation for creating the session table
entry and storing the packet (instead of simply dropping the
packet based on the filtering) is to support simultaneous open
of TCP connections).
If the NAT64 is not performing Address-Dependent filtering, it
translates and forwards the packet as described in the
following sections.
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For any other packet belonging to this connection,
If there is a corresponding entry 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 there is no corresponding entry in the TCP BIB the packet is
silently discarded.
*** V4 SYN RCV ***
If a V6 SYN is received with source transport address (X',x) and
destination transport address (Y',y), then the lifetime of the
corresponding TCP session table entry is set to at least 2 hours 4
min (the established connection idle timeout as defined in
[RFC5382]). The packet is translated and forwarded. The state is
moved to ESTABLISHED.
If the lifetime expires, an ICMP Port Unreachable error (Type 3, Code
3) containing the IPv4 SYN packet stored is sent back to the source
of the v4 SYN, the session table entry is deleted and, the state is
moved to CLOSED.
For any other packet, 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), with
source IPv4 transport address (Y,y) and destination IPv4 transport
address (X,x), then the state is moved to ESTABLISHED. The timer is
set to at least 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.
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.
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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 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. (Since the NAT64 is uncertain whether the peer will accept
the RST packet, instead of moving the state to CLOSED, it moves to
the RST RCV, which has a shorter lifetime. If no other packets are
received for this connection during the short timer, the NAT64
assumes that the peer has accepted the RST packet and moves to
CLOSED. If packet keep flowing, the NAT64 assumes that the peer has
not accepted the RST packet and moves back to ESTABLISHED state.)
If any other packet is received, the packet is translated and
forwarded. The lifetime is set to at least 2 hours and 4 min. The
state remains unchanged as ESTABLISHED.
*** 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 any other packet is received, the packet is translated and
forwarded. The lifetime is set to at least 2 hours and 4 min. The
state remains unchanged as V4 FIN RCV.
If the lifetime expires, the session table entry is deleted and the
state is moved to CLOSED.
*** V6 FIN RCV ***
If a V4 FIN packet is received, the packet is translated and
forwarded. The lifetime is set to 4 min. The state is moved to V6
FIN + V4 FIN RCV.
If any other packet is received, the packet is translated and
forwarded. The lifetime is set to at least 2 hours and 4 min. The
state remains unchanged as V6 FIN RCV.
If the lifetime expires, the session table entry is deleted and the
state is moved to CLOSED.
*** V6 FIN + V4 FIN RCV ***
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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 other than a RST packet is received, the lifetime is set
to at least 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. 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. 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 with IPv6 source address X',
IPv6 destination address Y' and Identifier I1, 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 (X',I1) pair. If such entry does not exist, a new entry is
created with the following data:
The BIB IPv6 address is set to X' i.e. the source IPv6 address
of the IPv6 packet.
The BIB ICMPv6 Query Id is set to I1 i.e. the ICMPv6 Query
Identifier.
If there exists some other BIB entry containing the same IPv6
address X' and mapping it to some IPv4 address T, then use T as
the BIB IPv4 address for this new entry. Otherwise, use any
IPv4 address assigned to the IPv4 interface.
As the BIB 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.
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The NAT64 searches for an ICMP query session table entry
corresponding to the incoming 3-tuple (X',Y',I1). If no such
entry is found, a new entry is created. The information included
in the new session table entry is as follows:
The STE IPv6 source address is set to the X' i.e. the address
contained in the received IPv6 packet,
The STE IPv6 destination address is set to the Y' i.e. the
address contained in the received IPv6 packet,
The STE IPv6 identifier is set to the I1 I.e. the identifier
contained in the received IPv6 packet,
The STE IPv4 source address is set to the IPv4 address
contained in the corresponding BIB entry,
The STE IPv4 identifier is set to the IPv4 identifier contained
in the corresponding BIB entry,
The STE IPv4 destination address is algorithmically generated
from Y' using the reverse algorithm as specified in
Section 3.2.5.
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 with source IPv4 address Y,
destination IPv4 address X and Identifier I2 is processed as follows:
The NAT64 searches for a ICMP Query BIB entry that contains X as
IPv4 address matches and I2 as the IPv4 Identifier. If such an
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-
dependent filtering rule. To do this, it searches for a session
table entry with a STE source IPv4 address equal to X, an STE IPv4
Identifier equal to I2 and a STE destination IPv4 address equal to
Y. If such an entry is found (there may be more than one), packet
processing continues. Otherwise, the packet is discarded. If the
packet is discarded, then an ICMP message MAY be sent to the
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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).
In case the packet is not discarded in the previous processing
(either because the NAT64 is not filtering or because the packet
is compliant with the Address-dependent filtering rule), then the
NAT64 searches for a session table entry with a STE source IPv4
address equal to X, an STE IPv4 Identifier equal to I2 and a STE
destination IPv4 address equal to Y. If no such entry is found, a
new entry is created with the following information:
The STE source IPv4 address is set to X,
The STE IPv4 Identifier is set to I2,
The STE destination IPv4 address is set to Y,
The STE source IPv6 address is set to the IPv6 address of the
corresponding BIB entry,
The STE IPv6 Identifier is set to the IPv6 Identifier of the
corresponding BIB entry, and,
The STE destination IPv6 address is set to the IPv6
representation of the IPv4 address of Y, generated using the
algorithm described in Section 3.2.5.
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.
3.2.4. 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 of the IPv4 pool
assigned to the NAT64 to be used for translation.
If the port s is in the Well-Known port range 0..1023, then the
NAT64 SHOULD allocate a port t from this same range. Otherwise,
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if the port s is in the range 1024..65535, then the NAT64 SHOULD
allocate a port t from this range. Furthermore, if port s is
even, then t SHOULD be even, and if port s is odd, then t SHOULD
be odd. (this behavior is recommended in Section 7.1 of [RFC5382])
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 (referring to the UDP and TCP BIBs).
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. The NAT64 MAY send an ICMPv6 Destination Unreachable/
Address unreachable (Code 3) message.
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
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
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[I-D.ietf-behave-address-format]
NAT64 MUST support the algorithm for generating IPv6 representations
of IPv4 addresses defined in section 2 of
[I-D.ietf-behave-address-format]. The aforementioned algorithm
SHOULD be used as default algorithm.
3.3. 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
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.
3.3.1. 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.
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The BIB table is searched for an entry (X',x) <--> (D,d), and the
outgoing destination transport address is set to (X',x).
3.3.2. 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 BIB is searched for an
entry containing (X',I1) <--> (D,I2) and the outgoing destination
address is X' and the outgoing ICMPv6 Query Id is I1.
3.4. 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 as determined
in Section 3.3.1.
o When the protocol following the IP header is TCP or UDP, then the
source and destination ports are modified to the source and
destination ports from the outgoing 5-tuple. In addition, the TCP
or UDP checksum must also be updated to reflect the translated
addresses and ports; note that the TCP and UDP checksum covers the
pseudo-header 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 as determined in Section 3.3.2.
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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).
The size of outgoing packets as well and the potential need for
fragmentation is done according to the behavior defined in IP/ICMP
Translation Algorithm [I-D.ietf-behave-v6v4-xlate]
3.5. Handling Hairpinning
This step handles hairpinning if necessary. A NAT64 that forwards
packets originating from an IPv6 address, destined for an IPv4
address that matches the active mapping for another IPv6 address,
back to that IPv6 address are defined as as supporting "hairpinning".
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 Pref64::/n 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 Section 3.2
4. 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].
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),
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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
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
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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.
Avoiding hairpinning loops
If the IPv6-only client can guess the IPv4 binding address that will
be created, it can use the IPv6 representation of it as source
address for creating this binding. Then any packet sent to the
binding's IPv4 address will loop in the NAT64.
Consider the following example:
Suppose that the IPv4 pool is 192.0.2.0/24
Then the IPv6-only client sends this to NAT64:
Source: [Pref64::192.0.2.1]:500
Destination: whatever
The NAT64 allocates 192.0.2.1:500 as IPv4 binding address. Now
anything sent to 192.0.2.1:500, be it a hairpinned IPv6 packet or an
IPv4 packet, will loop.
It should be noted that it is not hard to guess the IPv4 address that
will be allocated. First the attacker creates a binding and use e.g.
STUN to know your external IPv4. New bindings will always have this
address. Then it uses a source port in the range 1-1023. This will
increase your chances to 1/512 (since range and parity must be
preserved).
In order to address this vulnerability, the NAT64 drops IPv6 packets
whose source address is in Pref64::/n.
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5. IANA Considerations
This document contains no IANA considerations.
6. Contributors
George Tsirtsis
Qualcomm
tsirtsis@googlemail.com
Greg Lebovitz
Juniper
gregory.ietf@gmail.com
Simon Parreault
Viagenie
simon.perreault@viagenie.ca
7. Acknowledgements
Dave Thaler, Dan Wing, Alberto Garcia-Martinez, Reinaldo Penno,
Ranjana Rao, Lars Eggert, Senthil Sivakumar 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.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC1035] Mockapetris, P., "Domain names - implementation and
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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,
"DNS64: DNS extensions for Network Address Translation
from IPv6 Clients to IPv4 Servers",
draft-ietf-behave-dns64-02 (work in progress),
October 2009.
[I-D.ietf-behave-v6v4-xlate]
Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", draft-ietf-behave-v6v4-xlate-04 (work in
progress), November 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-01 (work in progress),
October 2009.
8.2. 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,
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Internet-Draft NAT64 December 2009
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.
[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.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[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-03 (work in progress),
October 2009.
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Appendix A. Application scenarios
In this section, we describe how to apply NAT64/DNS64 to the suitable
scenarios described in [I-D.ietf-behave-v6v4-framework] .
A.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
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.
A.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.
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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 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.
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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