One document matched: draft-palet-v6ops-proto41-nat-03.txt
Differences from draft-palet-v6ops-proto41-nat-02.txt
Internet Engineering Task Force
Internet Draft Jordi Palet
Document: draft-palet-v6ops-proto41-nat-03.txt Cesar Olvera
Consulintel
Category: David Fernandez
UPM
Expires: April 2004 October 2003
Forwarding Protocol 41 in NAT Boxes
draft-palet-v6ops-proto41-nat-03.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
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http://www.ietf.org/shadow.html.
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Abstract
Some IPv4-only NAT boxes/routers allow the establishment of IPv6
tunnels from systems in the private LAN (using private IPv4
addresses) to routers or tunnel servers in the public Internet.
As far as we know [2] this is not a common way of using IPv6 tunnels;
the usual way is to finish the tunnel directly in a device with an
IPv4 public address.
This behavior provides a big opportunity to rapidly deploy a huge
number of IPv6 nodes and networks, without the need of new transition
mechanism. This option is very important to facilitate the IPv6
deployment when is not possible to offer native IPv6 or 6to4 [3].
From this point of view, this mechanism should be considered only as
a temporary solution until native IPv6 routers, or those that support
6to4, will become widely available.
Not all the IPv4-only NAT boxes/routers support this mechanism, but
this document describes this behavior and consequently provides hints
that should be applied in the IPv4-only NAT boxes and tunnel brokers
to facilitate it.
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Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [4].
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Table of Contents
1. Introduction...................................................4
2. Rationale of proto-41 forwarding...............................6
3. Behavior of different NAT types................................7
3.1 3.1 Traditional (or) Outbound NAT..........................7
3.2 Bi-directional (or) Two-Way NAT............................8
4. Applicability..................................................8
5. NAT design considerations and recommendations..................9
6. Tunnel broker design considerations...........................10
7. Security Considerations.......................................10
8. References....................................................11
Acknowledgments..................................................11
Authors' Addresses...............................................11
Intellectual Property Statement..................................12
Full Copyright Statement.........................................12
Acknowledgement..................................................13
1. Introduction
Most of the existing solutions for the transition to IPv6 rely in
tunnels assuming that the client end-point is an IPv6 capable router.
However, nowadays the installed base of IPv4-only NAT boxes/routers
is still quite big, while most of the client operating systems
already support IPv6.
The ability of some IPv4-only NAT boxes/routers to establish IPv6
tunnels from systems inside the private LAN (even using private IPv4
addresses) to routers or tunnel servers in the public Internet has
been used for some time. However, it has not been documented so far.
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The goal of this document is to describe in detail that functionality
and to show the rationale behind it, as well as to provide some
recommendations for IPv4-only NAT boxes and tunnel broker
implementers in order to facilitate its use and deployment.
The basic scenario of the mechanism presented is shown in the Figure
below. As can be seen, a Tunnel Client (a host or a router), which is
connected to Internet through an IPv4-only NAT box using a private
IPv4 address, establishes an IPv6 tunnel to a Tunnel Server with the
help of a Tunnel Broker. The mechanism can also be used without a
tunnel broker, ending the tunnel in an IPv6 router, which is
configured manually.
____
( )
( IPv6 )
(____)
|
+--------+ +--------+
| Tunnel |________| Tunnel |
| Broker | | Server |
+--------+ +--------+
\ / |
\____/ |
( ) |
( IPv4 ) |
(____) | IPv6 Tunnel
| |
Public IPv4 | |
+-----+ |
| NAT | |
| Box | |
+-----+ |
| |
Private IPv4 | |
+--------+ |
| Tunnel |------
| Client |------> (possible IPv6 or dual stack network)
+--------+
Typically, IPv6 routers on the Tunnel Server side support the
establishment of these tunnels without any additional configuration.
However, in the case of some clients under certain operating systems,
the tunnel configuration process or the tunnel broker scripts have to
be modified to reflect the private/public addressing conversion.
This fact should be taken into consideration by tunnel broker
implementations in future versions, in order to properly create the
script in case the client is located in a private network.
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This document describes the reasons why this scenario works as it is
using present NAT implementations. We consider that exploring this
option is very important to facilitate the IPv6 deployment, as it can
be used as a temporary fallback solution when neither native IPv6 nor
6to4 mechanisms are available.
The document does not discuss how the local private network is
organized, for example, in case the Tunnel Client is an IPv6 router
providing IPv6 connectivity to other systems. The behavior in this
case should be the same as any other IPv6 native network (that is
using stateless or stateful autoconfiguration, or any other typical
functionalities like Home Agent, etc).
Although this mechanism is not usable on all existing IPv4-only NAT
boxes/routers, the large number of them that already support it gives
an opportunity to rapidly deploy a huge number of IPv6 nodes and
networks (in case the node behind the NAT is an IPv6 router) without
the need of using or designing new transition mechanisms.
The scenario presented has been tested with several IPv4-only NAT
boxes that have successfully established IPv6 tunnels between tunnel
clients in a private network and tunnel servers in the public
Internet. In these test, we have used three well-known Tunnel Broker
implementations (BT, Freenet6 and TILAB) as well as manually
configured tunnels with routers from several manufacturers.
2. Rationale of proto-41 forwarding
As described in RFC 2663 [5]:
"Address translations performed by NAT are session based and would
include translation of incoming as well as outgoing packets belonging
to that session ... a session is defined as the set of traffic that
is managed as a unit for translation. TCP/UDP sessions are uniquely
identified by the tuple of (source IP address, source TCP/UDP port,
target IP address, target TCP/UDP port). ICMP query sessions are
identified by the tuple of (source IP address, ICMP query ID, target
IP address). All other sessions are characterized by the tuple of
(source IP address, target IP address, IP protocol)."
Basically, what the NAT router does in the scenario presented in this
document is a network address translation for protocol identifier 41
(the one used for IPv6 over IPv4 tunnels). The router considers each
tuple of the form [source IP address, target IP address, IP protocol
(41)] a different session, and typically creates a new proto 41 entry
in its table whenever an IPv6 over IPv4 packet flows from the private
network to the Internet (as it does, for example, for TCP
connections).
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3. Behavior of different NAT types
As mentioned before, some NAT routers do not support protocol 41
forwarding. They are usually limited to do network address
translation for common protocols like TCP, UDP and ICMP. This type of
NAT routers will not be considered in this document, although some
recommendations for them will be given in section 5.
RFC 2663 [5] distinguishes several types of NAT routers. This
document focuses on how proto 41 forwarding works over the two most
common types: Traditional NAT and NAPT.
3.1 3.1 Traditional (or) Outbound NAT
In traditional NAT routers sessions are unidirectional. This means
that, as IPv6 tunnels are treated as any other NAT dynamic session,
the tunnel entries are only added to the table whenever an IPv6
packet is sent from the private network to the public Internet, but
not with packets flowing in the opposite direction (i.e., coming from
the external tunnel endpoint).
Usually, an inactivity timer is started when the NAT entry is
created, so that the session (and consequently the tunnel) is deleted
if no packets are sent during the inactivity period (a few minutes
typically). In case the tunnel entry is deleted due to inactivity, it
will be created again whenever a new packet is sent from the private
network.
RFC 2663 distinguish between two types of traditional NAT routers:
Basic NAT and NAPT. Basic NATs do the address translation by means of
a one-to-one association between private and public addresses, so
entries on the NAT table have the form [private address, public
address].
In the case of NAPT, which is the most widely used at present, each
public address can be shared among several private systems, by using
different transport ports for each one. Entries in NAT table have the
form [source IP address, source TCP/UDP port, target IP address,
target TCP/UDP port]. Both types can be combined in the same NAT
router.
Support for protocol 41 forwarding in Traditional NAT routers
basically means that they should be prepared to manage sessions of
the form [source IP address, destination IP address, protocol ID] for
protocol 41.
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3.2 Bi-directional (or) Two-Way NAT
As stated in [5], “with a Bidirectional NAT, sessions can be
initiated from hosts in the public network as well as the private
network. Private network addresses are bound to globally unique
addresses, statically or dynamically as connections are established
in either direction”.
RFC 2663 mentions the use of a DNS-ALG algorithm in order to allow
public hosts to communicate with private ones. Basically, whenever a
DNS query is made for a private host name, the DNS-ALG in conjunction
with Bi-directional NAT answers the query using an available public
address and sets the corresponding NAT table entry.
However, this mechanism does not fit proto-41 forwarding
requirements, as no names are normally involved when setting up
tunnels.
What it is needed in our case is just the basic mechanism included in
bi-directional NAT to statically associate one of the private
addresses with a public address, only for protocol 41.
In this way, all ingoing IPv6 over IPv4 traffic will be forwarded to
the designated internal system and the tunnel will work in a complete
bidirectional way, even when no outgoing traffic is generated. IN
that case, the inactivity timer will probably not be needed, as the
entry on NAT table for outgoing traffic could also be statically
configured (by means of a configuration file, http interface, CLI,
etc.).
4. Applicability
In the case of Basic NAT and NAPT, IPv6 tunnels can only be initiated
by inside-to-outside sessions. So in this case, outside-to-inside
sessions only work whenever a previous inside-to-outside session has
created the proto-41 entry in the NAT table and the inactivity
timeout has not been reached.
This fact is only a problem when IPv6 servers or services inside the
private network are needed to be accessible from outside. If the
traffic is client initiated, the session will be created normally as
soon as the first packet is sent, allowing IPv6 communication.
The only way to maintain the session permanently is to constantly
send traffic, for example, with a periodic ping from the Tunnel
Client, a router solicitation message, or other means. Alternatively,
some simple keep-alive protocol could be integrated inside tunnel
broker implementations in order to maintain the tunnel.
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In the case of Bi-directional NATs, there are means to support also
incoming sessions, even when no outgoing traffic haa been generated.
However, they require some type of pre-configuration in the IPv4-only
NAT box.
To facilitate that, a default configuration could be defined. For
example, in the case of simple NAT routers used in most SOHO
accesses, the default configuration could include a pre-defined
private network address for the LAN interface and a pre-defined
private address for the host where all the proto-41 traffic is
forwarded.
In summary, the application of proto-41 forwarding procedure allows
in both cases the operation of private IPv6 networks connected by
means of non-IPv6 aware NAT boxes to tunnel brokers or manually
configured tunnels.
The most usual scope of application of the proto-41 forwarding
procedure described in this document seems to be SOHO and home
environments, but it is not only limited to those scenarios.
5. NAT design considerations and recommendations
This document has been written following a survey with users/vendors
of different IPv4-only NAT boxes, and the conclusion is that most of
the manufacturers support protocol-41 forwarding (78% in our survey).
Nevertheless not all support a bidirectional mode (over 22% of the
surveyed models do not support it).
NAT boxes should tend to support native IPv6. If this is not
feasible, 6to4 should be the second option, and as a last resort,
proto-41 forwarding.
6to4 and Proto-41 forwarding can coexist in the same NAT box. In that
case, an IPv6 over IPv4 packet received, will be forwarded to the
private LAN only if the IPv6 destination does not belong to the local
6to4 /48 prefix. Otherwise it will be decapsulated in the NAT box,
following 6to4 procedures. This fact avoids the problems created by
mobile users when they visit a network that uses 6to4, in the case
they have some automatic proto-41 setup.
New firmware/software versions of the NAT implementations should
ensure the support of protocol-41 forwarding, as a temporary
solution, while they are not supporting native IPv6 or 6to4.
Proto-41 make sense only in IPv4-only routers, but nevertheless, when
these routers are upgraded to support, for example, 6to4, for
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compatibility reasons (with existing network configurations), it
could be still considered to maintain the support proto-41.
In addition, considering that the code changes needed to support a
full bidirectional NAT will be minimum, this option should also be
considered, at least as a configurable option, in an easy way by the
user (very simple http interface).
Proto-41 adds an inexpensive feature to existing IPv4-only NAT boxes,
facilitating the gradual transition to IPv6, while preserving the
users investment in the existing IPv4 network.
6. Tunnel broker design considerations
New releases of tunnel brokers should provide means to cope with the
scenario defined in this document. They should automatically detect
it or, at least, they should allow the user to specify manually that
a NAT router is present between the tunnel client and server.
According to that the tunnel broker must properly create the script
or configuration file that will setup the client tunnel endpoint. In
that case they should have requested the public addresses (can be
automatically detected) and the local interface ID or name of the
tunnel client.
7. Security Considerations
It is important to note that IPv6 applications sending traffic over
the tunnels described here do not suffer the restrictions that apply
to NAT traversal scenarios, because NAT is made to IPv4 packets that
transport IPv6 ones, not to IPv6 packets.
Besides, the protection derived from the unidirectional nature of NAT
disappears for IPv6. Therefore, some security mechanism (network or
personal firewalls) could be necessary to protect IPv6 systems in the
private network.
A possible security problem is the one related to the DoS Attack than
can be created if a host in the local network, behind the NAT sends
IPv6 packets (using protocol 41) to the tunnel endpoint, simulating
to be the original "owner" of the tunnel. The behavior of the IPv4-
only NAT box will define the success or failure of this attack. In
any case, it seems not reasonable that this happens in small networks
(SOHO and home environments), where the attacker can be easily
identified.
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This considerations are generic to transition mechanisms, as
described in [6].
8. References
1 S. Bradner, "The Internet Standards Process -- Revision 3", BCP 9,
RFC 2026, October 1996.
2 J. Palet, C. Olvera, D. Fernandez, "IPv6 Tunnels through Routers
with NAT", Euro6IX Project,
http://www.euro6ix.org/documentation/euro6ix_co_upm-
consulintel_wp4_ipv6_tunnels_nat_v1_6.pdf, April 2003.
3 B. Carpenter, K. Moore, ”Connection of IPv6 Domains via IPv4
Clouds”, RFC 3056, February 2001.
4 S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
5 P. Srisuresh. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
6 P. Savola, ”IPv6 Transition/Co-existence Security Considerations”,
draft-savola-v6ops-security-overview-00, June 2003 (work in
progress).
Acknowledgments
The authors would also like to acknowledge the inputs from Tim Chown,
Miguel Angel Diaz, Alain Durand, Jun-ichiro "itojun" Hagino, Keith
Moore, Mariana Nikolova, Rute C. Sofia and the European Commission
support in the co-funding of the Euro6IX project, where this work is
being developed.
Authors' Addresses
Jordi Palet Martinez
Consulintel
San Jose Artesano, 1
28108 - Alcobendas (Madrid - Spain)
Phone: +34 91 151 81 99
Fax: +34 91 151 81 98
Email: jordi.palet@consulintel.es
Cesar Olvera Morales
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Consulintel
San Jose Artesano, 1
28108 - Alcobendas (Madrid - Spain)
Phone: +34 91 151 81 99
Fax: +34 91 151 81 98
Email: cesar.olvera@consulintel.es
David Fernandez
Technical University of Madrid (UPM)
Ciudad Universitaria s/n
28040 – Madrid (Spain)
Phone: +34 91 549 57 00
Fax: +34 91 336 73 33
Email: david@dit.upm.es
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