draft-ietf-v6ops-natpt-to-exprmntl-01.txt   draft-ietf-v6ops-natpt-to-exprmntl-02.txt 
v6ops Working Group C. Aoun v6ops Working Group C. Aoun
Internet-Draft ENST Internet-Draft ZTE/ENST Paris
Updates: 2766 (if approved) E. Davies Updates: 2766 (if approved) E. Davies
Expires: January 6, 2006 Consultant Expires: October 3, 2005 Consultant
July 5, 2005 April 2005
Reasons to Move NAT-PT to Experimental Reasons to Move NAT-PT to Experimental
draft-ietf-v6ops-natpt-to-exprmntl-01 draft-ietf-v6ops-natpt-to-exprmntl-02
Status of this Memo Status of this Memo
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have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
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This Internet-Draft will expire on January 6, 2006. This Internet-Draft will expire on October 3, 2005.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2005).
Abstract Abstract
This document discusses general issues with all forms of IPv6-IPv4 This document discusses issues with the specific form of IPv6-IPv4
translation and specific issues related to the form of IPv6-IPv4
protocol translation mechanism implemented by the Network Address protocol translation mechanism implemented by the Network Address
Translator - Protocol Translator (NAT-PT) defined in RFC 2766. These Translator - Protocol Translator (NAT-PT) defined in RFC 2766. These
specific issues are sufficiently serious that recommending RFC 2766 issues are sufficiently serious that recommending RFC 2766 as a
as a general purpose transition mechanism is no longer desirable, and general purpose transition mechanism is no longer desirable, and this
this document recommends that the IETF should reclassify RFC 2766 document recommends that the IETF should reclassify RFC 2766 from
from Standards Track to Experimental status. Standards Track to Experimental status.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Issues Unrelated to DNS-ALG . . . . . . . . . . . . . . . . 6 2. Issues Unrelated to DNS-ALG . . . . . . . . . . . . . . . . . 6
2.1 Issues with Protocols Embedding IP Addresses . . . . . . . 6 2.1. Issues with Protocols Embedding IP Addresses . . . . . . . 6
2.2 NAPT-PT Redirection Issues . . . . . . . . . . . . . . . . 7 2.2. NAPT-PT Redirection Issues . . . . . . . . . . . . . . . . 7
2.3 NAT-PT Binding State Decay . . . . . . . . . . . . . . . . 8 2.3. NAT-PT Binding State Decay . . . . . . . . . . . . . . . . 7
2.4 Loss of Information through Incompatible Semantics . . . . 8 2.4. Loss of Information through Incompatible Semantics . . . . 8
2.5 NA(P)T-PT and Fragmentation . . . . . . . . . . . . . . . 9 2.5. NA(P)T-PT and Fragmentation . . . . . . . . . . . . . . . 9
2.6 NAT-PT Interaction with SCTP and Multihoming . . . . . . . 10 2.6. NAT-PT Interaction with SCTP and Multihoming . . . . . . . 10
2.7 NAT-PT as a Proxy Correspondent Node for MIPv6 . . . . . . 11 2.7. NAT-PT as a Proxy Correspondent Node for MIPv6 . . . . . . 10
2.8 NAT-PT and Multicast . . . . . . . . . . . . . . . . . . . 11 2.8. NAT-PT and Multicast . . . . . . . . . . . . . . . . . . . 11
3. Issues exacerbated by the Use of DNS-ALG . . . . . . . . . . 12 3. Issues exacerbated by the Use of DNS-ALG . . . . . . . . . . . 12
3.1 Network Topology Constraints Implied by NAT-PT . . . . . . 12 3.1. Network Topology Constraints Implied by NAT-PT . . . . . . 12
3.2 Scalability and Single Point of Failure Concerns . . . . . 13 3.2. Scalability and Single Point of Failure Concerns . . . . . 13
3.3 Issues with Lack of Address Persistence . . . . . . . . . 14 3.3. Issues with Lack of Address Persistence . . . . . . . . . 14
3.4 DOS Attacks on Memory and Address/Port Pools . . . . . . . 14 3.4. DOS Attacks on Memory and Address/Port Pools . . . . . . . 14
4. Issues Directly Related to use of DNS-ALG . . . . . . . . . 15 4. Issues Directly Related to use of DNS-ALG . . . . . . . . . . 15
4.1 Address Selection Issues when Communicating with 4.1. Address Selection Issues when Communicating with
Dual-Stack End-hosts . . . . . . . . . . . . . . . . . . . 15 Dual-Stack End-hosts . . . . . . . . . . . . . . . . . . . 15
4.2 Non-global Validity of Translated RR Records . . . . . . . 17 4.2. Non-global Validity of Translated RR Records . . . . . . . 17
4.3 Inappropriate Translation of Responses to A Queries . . . 17 4.3. Inappropriate Translation of Responses to A Queries . . . 17
4.4 DNS-ALG and Multi-addressed Nodes . . . . . . . . . . . . 17 4.4. DNS-ALG and Multi-addressed Nodes . . . . . . . . . . . . 17
4.5 Limitations on Deployment of DNS Security Capabilities . . 18 4.5. Limitations on Deployment of DNS Security Capabilities . . 18
5. Impact on IPv6 Application Development . . . . . . . . . . . 18 5. Impact on IPv6 Application Development . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . 19 6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 19 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 20 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 20 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1 Normative References . . . . . . . . . . . . . . . . . . 21 10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
10.2 Informative References . . . . . . . . . . . . . . . . . 22 10.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . 25 Intellectual Property and Copyright Statements . . . . . . . . . . 25
1. Introduction 1. Introduction
The Network Address Translator - Protocol Translator NAT-PT) document The Network Address Translator - Protocol Translator NAT-PT) document
[RFC2766] defines a set of network layer translation mechanisms. [RFC2766] defines a set of network layer translation mechanisms
These mechanisms are designed to allow nodes which only support IPv4 designed to allow nodes which only support IPv4 to communicate with
to communicate with nodes which only support IPv6 during the nodes which only support IPv6 during the transition to the use of
transition to the use of IPv6 in the Internet. IPv6 in the Internet.
[RFC2766] specifies the basic NAT-PT in which only addresses are [RFC2766] specifies the basic NAT-PT in which only addresses are
translated and NAPT-PT (Network Address Port Translator - Protocol translated and NAPT-PT (Network Address Port Translator - Protocol
Translator)which also translates transport identifiers, allowing for Translator)which also translates transport identifiers, allowing for
greater economy of scarce IPv4 addresses. Protocol translation is greater economy of scarce IPv4 addresses. Protocol translation is
performed using the Stateless IP/ICMP Translation Algorithm (SIIT) performed using the Stateless IP/ICMP Translation Algorithm (SIIT)
defined in [RFC2765]. defined in [RFC2765].
A number of previous documents have raised issues with NAT-PT. This A number of previous documents have raised issues with NAT-PT. This
document will summarize these issues, note several other issues document will summarize these issues, note several other issues
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intercommunication between IPv6 networks and IPv4 networks. intercommunication between IPv6 networks and IPv4 networks.
Although the [RFC2766] form of packet translation is not generally Although the [RFC2766] form of packet translation is not generally
applicable, it is likely that in some circumstances a node which can applicable, it is likely that in some circumstances a node which can
only support IPv4 will need to communicate with a node which can only only support IPv4 will need to communicate with a node which can only
support IPv6: this is bound to need a translation mechanism of some support IPv6: this is bound to need a translation mechanism of some
kind. Although this may be better carried out by an application kind. Although this may be better carried out by an application
level proxy or transport layer translator, there may still be level proxy or transport layer translator, there may still be
scenarios in which a (possibly restricted) version of NAT-PT can be a scenarios in which a (possibly restricted) version of NAT-PT can be a
suitable solution: accordingly this document recommends that the IETF suitable solution: accordingly this document recommends that the IETF
should reclassify RFC2766 from Standards Track to Experimental should reclassify RFC2766 from Standards Track to Experiemental
status. status.
The following documents relating directly to NAT-PT have been The following documents relating directly to NAT-PT have been
reviewed while drafting this document: reviewed while drafting this document:
o Network Address Translation - Protocol Translation (NAT-PT) o Network Address Translation - Protocol Translation (NAT-PT)
[RFC2766] [RFC2766]
o Stateless IP/ICMP Translation Algorithm (SIIT) [RFC2765] o Stateless IP/ICMP Translation Algorithm (SIIT) [RFC2765]
o NAT-PT applicability statement [I-D.satapati-v6ops-natpt- o NAT-PT applicability statement [I-D.satapati-v6ops-natpt-
applicability] applicability]
o Issues with NAT-PT DNS ALG in RFC2766 [I-D.durand-natpt-dns-alg- o Issues with NAT-PT DNS ALG in RFC2766 [I-D.durand-natpt-dns-alg-
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interactions between DNS and NAT-PT. However, detailed inspection of interactions between DNS and NAT-PT. However, detailed inspection of
[RFC2766] shows that the 'simple' case has not been worked out and it [RFC2766] shows that the 'simple' case has not been worked out and it
is unclear how information about the address translation could be is unclear how information about the address translation could be
passed to the hosts in the absence of the DNS-ALG. This document passed to the hosts in the absence of the DNS-ALG. This document
therefore assumes that the DNS-ALG is an integral part of NAT-PT: therefore assumes that the DNS-ALG is an integral part of NAT-PT:
accordingly issues with the DNS-ALG must be considered as issues for accordingly issues with the DNS-ALG must be considered as issues for
the whole specification. the whole specification.
Note that the issues which are not specifically related to the use of Note that the issues which are not specifically related to the use of
the DNS-ALG will apply to any network layer translation scheme, the DNS-ALG will apply to any network layer translation scheme,
including any based on the SIIT algorithm [RFC2765]. In the event including any based on the SIIT algorithm [RFC2765].
that new forms of translator are developed as alternatives to NAT-PT,
the generic issues relevant to all IPv6-IPv4 translators should be
borne in mind.
Issues raised with IPv6-IPv4 translators in general and NAT-PT in Issues raised with NAT-PT can be categorized as follows:
particular can be categorized as follows: o Issues which are independent of the use of a DNS-ALG:
o Issues which are independent of the use of a DNS-ALG and are,
therefore, applicable to any form of IPv6-IPv4 translator:
* Disruption of all protocols which embed IP addresses (and/or * Disruption of all protocols which embed IP addresses (and/or
ports) in packet payloads or which apply integrity mechanisms ports) in packet payloads or which apply integrity mechanisms
using IP addresses (and ports). using IP addresses (and ports).
* Inability to re-direct traffic for protocols without any * Inability to re-direct traffic for protocols without any
demultiplexing capabilities or not built on top of specific demultiplexing capabilities or not built on top of specific
transport layer protocols in situations where one NAPT-PT is transport layer protocols in situations where one NAPT-PT is
translating for multiple IPv6 hosts. translating for multiple IPv6 hosts.
* Requirement for applications to use keep alive mechanisms to * Requirement for applications to use keep alive mechanisms to
workaround connectivity issues caused by premature NAT-PT state workaround connectivity issues caused by premature NAT-PT state
timeout. timeout.
* Loss of information due to incompatible semantics between IPv4 * Loss of information due to incompatible semantics between IPv4
and IPv6 versions of headers and protocols. and IPv6 versions of headers and protocols.
* Need for additional state and/or packet reconstruction in * Need for additional state and/or packet reconstruction in
NAPT-PT translators dealing with packet fragmentation. NAPT-PT translators dealing with packet fragmentation.
* Interaction with SCTP and multihoming. * Interaction with SCTP and multihoming.
* Need for NAT-PT to act as proxy for correspondent node when * Need for NAT-PT to act as proxy for correspondent node when
IPv6 node is mobile, with consequent restrictions on mobility. IPv6 node is mobile, with consequent restrictions on mobility.
* NAT-PT not being able to handle multicast traffic. * NAT-PT not being able to handle multicast traffic.
o Issues which are exacerbated by the use of a DNS-ALG and are, o Issues which are exacerbated by the use of a DNS-ALG:
therefore also applicable to any form of IPv6-IPv4 translator:
* Constraints on network topology. * Constraints on network topology.
* Scalability concerns together with introduction of single point * Scalability concerns together with introduction of single point
of failure and security attack nexus. of failure and security attack nexus.
* Lack of address mapping persistence: Some applications require * Lack of address mapping persistence: Some applications require
address retention between sessions. The user traffic will be address retention between sessions. The user traffic will be
disrupted if a different mapping is used. The use of the DNS- disrupted if a different mapping is used. The use of the DNS-
ALG to create address mappings with limited lifetimes means ALG to create address mappings with limited lifetimes means
that applications must start using the address shortly after that applications must start using the address shortly after
the mapping is created, as well as keeping it alive once they the mapping is created, as well as keeping it alive once they
start using it. start using it.
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* Constraints on network topology. * Constraints on network topology.
* Scalability concerns together with introduction of single point * Scalability concerns together with introduction of single point
of failure and security attack nexus. of failure and security attack nexus.
* Lack of address mapping persistence: Some applications require * Lack of address mapping persistence: Some applications require
address retention between sessions. The user traffic will be address retention between sessions. The user traffic will be
disrupted if a different mapping is used. The use of the DNS- disrupted if a different mapping is used. The use of the DNS-
ALG to create address mappings with limited lifetimes means ALG to create address mappings with limited lifetimes means
that applications must start using the address shortly after that applications must start using the address shortly after
the mapping is created, as well as keeping it alive once they the mapping is created, as well as keeping it alive once they
start using it. start using it.
* Creation of a DOS threat relating to exhaustion of memory and * Creation of a DOS threat relating to exhaustion of memory and
address/port pool resources on the translator. address/port pool resources on the translator.
o Issues which result from the use of a DNS-ALG and are, therefore, o Issues which result from the use of a DNS-ALG:
specific to NAT-PT as defined in [RFC2766]:
* Address selection issues when either the internal or external * Address selection issues when either the internal or external
hosts implement both IPv4 and IPv6. hosts implement both IPv4 and IPv6.
* Restricted validity of translated DNS records: a translated * Restricted validity of translated DNS records: a translated
record may be forwarded to an application which cannot use it. record may be forwarded to an application which cannot use it.
* Inappropriate translation of responses to A queries from IPv6 * Inappropriate translation of responses to A queries from IPv6
nodes. nodes.
* Address selection issues and resource consumption in DNS-ALG * Address selection issues and resource consumption in DNS-ALG
with multi-addressed nodes. with multi-addressed nodes.
* Limitations on DNS security capabilities when using DNS-ALG. * Limitations on DNS security capabilities when using DNS-ALG.
Section 2, Section 3 and Section 4 discuss these groups of issues. Section 2, Section 3 and Section 4 discuss these groups of issues.
Section 5 examines the consequences of deploying NAT-PT for Section 5 examines the consequences of deploying NAT-PT for
application developers and the long term effects of NAT-PT or any application developers and the long term effects of NAT-PT on the
form of generally deployed IPv6-IPv4 translator on the further further development of IPv6.
development of IPv6.
The terminology used in this document is defined in [RFC2663], The terminology used in this document is defined in [RFC2663],
[RFC2766] and [RFC3314]. [RFC2766] and [RFC3314].
2. Issues Unrelated to DNS-ALG 2. Issues Unrelated to DNS-ALG
2.1 Issues with Protocols Embedding IP Addresses 2.1. Issues with Protocols Embedding IP Addresses
It is well known from work on IPv4 NATs (see Section 8 of [RFC2663] It is well known from work on IPv4 NATs (see Section 8 of [RFC2663]
and [RFC3027]) that the large class of protocols which embed numeric and [RFC3027]) that the large class of protocols which embed numeric
IP addresses in their payloads either cannot work through NATs or IP addresses in their payloads either cannot work through NATs or
require specific ALGs as helpers to translate the payloads in line require specific ALGs as helpers to translate the payloads in line
with the address and port translations. The same set of protocols with the address and port translations. The same set of protocols
cannot pass through NAT-PT. The problem is exacerbated because the cannot pass through NAT-PT. The problem is exacerbated because the
IPv6 and IPv4 addresses are of different lengths so that packet IPv6 and IPv4 addresses are of different lengths so that packet
lengths as well as contents are altered. [RFC2766] describes the lengths as well as contents are altered. [RFC2766] describes the
consequences as part of the description of the FTP ALG: similar consequences as part of the description of the FTP ALG: similar
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coordinated updates of the ALGs to keep them in step. coordinated updates of the ALGs to keep them in step.
Assuming that the NAT-PT contains a co-located ALG for one of the Assuming that the NAT-PT contains a co-located ALG for one of the
relevant protocols, the ALG could replace the embedded IP addresses relevant protocols, the ALG could replace the embedded IP addresses
and ports. However this replacement can only happen if no and ports. However this replacement can only happen if no
cryptographic integrity mechanism is used and the protocol messages cryptographic integrity mechanism is used and the protocol messages
are sent in clear (i.e. not encrypted). are sent in clear (i.e. not encrypted).
A possible workaround relies on the NAT-PT being party to the A possible workaround relies on the NAT-PT being party to the
security association used to provide authentication and/or security association used to provide authentication and/or
encryption. NAT-PT would then be aware of the cryptographic encryption. The NAT-PT should then be aware of the cryptographic
algorithms and keys used to secure the traffic. It could then modify algorithms and keys used to secure the traffic and could modify and
and re-secure the packets: this would certainly complicate network re-secure the packets: this would certainly complicate network
operations and provides additional points of security vulnerability. operations and provides additional points of security vulnerability.
Unless UDP encapsulation is used for IPsec [RFC3498], traffic using Unless UDP encapsulation is used for IPsec [RFC3498], traffic using
IPsec AH (in transport and tunnel mode) and IPsec ESP (in transport IPsec AH (in transport and tunnel mode) and IPsec ESP (in transport
mode) are unable to be carried through NAT-PT without terminating the mode) are unable to be carried through NAT-PT without terminating the
security associations on the NAT-PT, due to their usage of security associations on the NAT-PT, due to their usage of
cryptographic integrity protection. cryptographic integrity protection.
A related issue with DNS security is discussed in Section 4.5. A related issue with DNS security is discussed in Section 4.5.
2.2 NAPT-PT Redirection Issues 2.2. NAPT-PT Redirection Issues
Section 4.2 of [RFC3027] discusses problems specific to RSVP and Section 4.2 of [RFC3027] discusses problems specific to RSVP and
NATs, one of which is actually a more generic problem for all port NATs, one of which is actually a more generic problem for all port
translators. When several end-hosts are using a single NAPT-PT box, translators. When several end-hosts are using a single NAPT-PT box,
protocols that do not have a demultiplexing capability similar to protocols that do not have a demultiplexing capability similar to
transport layer port numbers may be unable to work through NAPT-PT transport layer port numbers may be unable to work through NAPT-PT
(and any other port translator) because there is nothing for NAPT-PT (and any other port translator) because there is nothing for NAPT-PT
to use to identify the correct binding. to use to identify the correct binding.
This type of issue affects IPsec encrypted packets where the This type of issue affects IPsec encrypted packets where the
transport port is not visible (although it might be possible to use transport port is not visible (although it might be possible to use
the SPI as an alternative demultiplexer) and protocols, such as RSVP, the SPI as an alternative demultiplexer) and protocols, such as RSVP,
which are carried directly in IP datagrams rather than using a which are carried directly in IP datagrams rather than using a
standard transport layer protocol such as TCP or UDP. In the case of standard transport layer protocol such as TCP or UDP. In the case of
RSVP, packets going from the IPv4 domain to the IPv6 domain do not RSVP, packets going from the IPv4 domain to the IPv6 domain do not
necessarily carry a suitable demultiplexing field, because the port necessarily carry a suitable demultiplexing field, because the port
fields in the flow identifier and traffic specifications are fields in the flow identifer and traffic specifications are optional.
optional.
Several ad hoc workarounds could be used to solve the demultiplexing Several ad hoc workarounds could be used to solve the demultiplexing
issues, however in most cases these solutions are not documented issues, however in most cases these solutions are not documented
anywhere which could lead to non-deterministic, undesirable behavior anywhere which could lead to non-deterministic, undesirable behavior
(for example, such workarounds often assume particular network (for example, such workarounds often assume particular network
topologies etc in order to function correctly; if the assumptions are topologies etc in order to function correctly; if the assumptions are
not met in a deployment the workaround may not work as expected). not met in a deployment the workaround may not work as expected).
This issue is closely related to the fragmentation issue described in This issue is closely related to the fragmentation issue described in
Section 2.5. Section 2.5.
2.3 NAT-PT Binding State Decay 2.3. NAT-PT Binding State Decay
NAT-PT will generally use dynamically created bindings to reduce the NAT-PT will generally use dynamically created bindings to reduce the
need for IPv4 addresses both for NAT-PT and NAPT-PT. NA(P)T-PT uses need for IPv4 addresses both for NAT-PT and NAPT-PT. NA(P)T-PT uses
soft state mechanisms to manage the address and port pools used for soft state mechanisms to manage the address and port pools used for
dynamically created address bindings. This allows the NA(P)T-PT to dynamically created address bindings. This allows the NA(P)T-PT to
operate autonomously without requiring clients to signal either operate autonomously without requiring clients to signal either
implicitly or explicitly that a binding is no longer required. In implicitly or explicitly that a binding is no longer required. In
any case, without soft state timeouts network and application any case, without soft state timeouts network and application
unreliability would inevitably lead to leaks, eventually causing unreliability would inevitably lead to leaks, eventually causing
address or port pool exhaustion. address or port pool exhaustion.
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NAT-PT binding will not be refreshed and the application connection NAT-PT binding will not be refreshed and the application connection
will be broken. Hence all applications need to maintain their NAT-PT will be broken. Hence all applications need to maintain their NAT-PT
bindings during long idle periods by incorporating a keep-alive bindings during long idle periods by incorporating a keep-alive
mechanism, which may not be possible for legacy systems. mechanism, which may not be possible for legacy systems.
Also [RFC2766] does not specify how to choose timeouts for bindings: Also [RFC2766] does not specify how to choose timeouts for bindings:
as is discussed in [RFC2663] for traditional NATs, selecting suitable as is discussed in [RFC2663] for traditional NATs, selecting suitable
values is a matter of heuristics and coordinating with application values is a matter of heuristics and coordinating with application
expectations may be impossible. expectations may be impossible.
2.4 Loss of Information through Incompatible Semantics 2.4. Loss of Information through Incompatible Semantics
NAT-PT reuses the SIIT header and protocol translations defined in NAT-PT reuses the SIIT header and protocol translations defined in
[RFC2765]. Mismatches in semantics between IPv4 and IPv6 versions [RFC2765]. Mismatches in semantics between IPv4 and IPv6 versions
can lead to loss of information when packets are translated. Three can lead to loss of information when packets are translated. Three
issues arising from this are: issues arising from this are:
o There is no equivalent in IPv4 for the flow label field of the o There is no equivalent in IPv4 for the flow label field of the
IPv6 header. Hence any special treatment of packets based on flow IPv6 header. Hence any special treatment of packets based on flow
label patterns cannot be propagated into the IPv4 domain. label patterns cannot be propagated into the IPv4 domain.
o IPv6 extension headers provide flexibility for improvements in the o IPv6 extension headers provide flexibility for improvements in the
IP protocol suite in future. New headers may be defined in future IP protocol suite in future. New headers may be defined in future
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Loss of information in any of these cases could be a constraint to Loss of information in any of these cases could be a constraint to
certain applications. certain applications.
A related matter concerns the propagation of the Differentiated A related matter concerns the propagation of the Differentiated
Services Code Point (DSCP). NAT-PT and SIIT simply copy the DSCP Services Code Point (DSCP). NAT-PT and SIIT simply copy the DSCP
field when translating packets. Accordingly the IPv4 and IPv6 field when translating packets. Accordingly the IPv4 and IPv6
domains must have equivalent Per-Hop Behaviors for the same code domains must have equivalent Per-Hop Behaviors for the same code
point or alternative means must be in place to translate the DSCP point or alternative means must be in place to translate the DSCP
between domains. between domains.
2.5 NA(P)T-PT and Fragmentation 2.5. NA(P)T-PT and Fragmentation
As mentioned in [RFC3027], simple port translators are unable to As mentioned in [RFC3027], simple port translators are unable to
translate packet fragments other than the first from a fragmented translate packet fragments other than the first from a fragmented
packet because subsequent fragments do not contain the port number packet because subsequent fragments do not contain the port number
information. information.
This means that generally fragmentation cannot be allowed for any This means that generally fragmentation cannot be allowed for any
traffic that traverses a NAPT-PT. One attempted workaround requires traffic that traverses a NAPT-PT. One attempted workaround requires
the NAPT-PT to maintain state about fragmented packets in transit. the NAPT-PT to maintain state about fragmented packets in transit.
This is not a complete solution because fragment misordering could This is not a complete solution because fragment misordering could
lead to the first fragment appearing at the NAPT-PT after later lead to the first fragment appearing at the NAPT-PT after later
fragments. The NAPT-PT would then not have the information needed to fragments. The NAPT-PT would then not have the information needed to
translate the fragments received before the first. translate the fragments received before the first.
Although it would not be expected in normal operation, NAPT-PT needs Although it would not be expected in normal operation, NAPT-PT needs
to be proofed against receiving short first fragments which don't to be proofed against receiving short first fragments which don't
contain the transport port numbers. Note that such packets are a contain the transport port numbers. Note that such packets are a
problem for IPv6 stateful packet inspection. The current problem for IPv6 stateful packet inspection. The current
specifications of IPv6 do not mandate any minimum packet size beyond specifications of IPv6 do not mandate any minimum packet size beyond
the need to carry the unfragmentable part (which doesn't include the the need to carry the unfragmentable part (which doesn't include the
transport port numbers) or reassembly rules to minimize the effects transport port numbers) or reassembly rules to minimise the effects
of overlapping fragments, leaving IPv6 open to the sort of attacks of overlapping fragments, leaving IPv6 open to the sort of attacks
described in [RFC1858] and [RFC3128]. described in [RFC1858] and [RFC3128].
An additional concern arises when a fragmented IPv4 UDP packet, which An additional concern arises when a fragmented IPv4 UDP packet, which
does not have a transport layer checksum, traverses a NAT-PT box. As does not have a transport layer checksum, traverses a NAT-PT box. As
described in [RFC2766], the NAT-PT has to reconstruct the whole described in [RFC2766], the NAT-PT has to reconstruct the whole
packet so that it can calculate the checksum needed for the packet so that it can calculate the checksum needed for the
translated IPv6 packet. This can result in significant delay to the translated IPv6 packet. This can result in significant delay to the
packet, especially if it has to be re-fragmented before transmission packet, especially if it has to be re-fragmented before transmission
on the IPv6 side. on the IPv6 side.
If NA(P)T-PT boxes reassembled all incoming fragmented packets (both If NA(P)T-PT boxes reassembled all incoming fragmented packets (both
from the IPv4 and IPv6 directions) in the same way as they have to do from the IPv4 and IPv6 directions) in the same way as they have to do
for unchecksummed IPv4 UDP packets, this would be a solution to the for unchecksummed IPv4 UDP packets, this would be a solution to the
first problem. The resource cost would be considerable apart from first problem. The cost would be considerable: apart from the
the potential delay problem if the outgoing packet has to be re- potential delay problem if the outgoing packet has to be fragmented,
fragmented. In any case fragmentation would mean that the NA(P)T-PT the NA(P)T-PT would consume extra memory and CPU resources, making
would consume extra memory and CPU resources, making the NAT-PT even the NAT-PT even less scaleable (see Section 3.2).
less scaleable (see Section 3.2).
Packet reassembly in NA(P)T-PT also opens up the possibility of Packet reassembly in NA(P)T-PT also opens up the possibility of
various fragment-related security attacks. Some of these are various fragment-related security attacks. Some of these are
analogous to attacks identified for IPv4. Of particular concern is a analagous to attacks identified for IPv4. Of particular concern is a
DOS attack based on sending large numbers of small fragments without DOS attack based on sending large numbers of small fragments without
a terminating last fragment which would potentially overload the a terminating last fragment which would potentially overload the
reconstruction buffers and consume large amounts of CPU resources. reconstruction buffers and consume large amounts of CPU resources.
2.6 NAT-PT Interaction with SCTP and Multihoming 2.6. NAT-PT Interaction with SCTP and Multihoming
The Stream Control Transmission Protocol (SCTP) [RFC2960] is a The Stream Control Transmission Protocol (SCTP) [RFC2960] is a
transport protocol which has been standardized since SIIT was transport protocol which has been standardized since SIIT was
specified. SIIT does not explicitly cover translation of SCTP, but specified. SIIT does not explicitly cover translation of SCTP, but
SCTP uses transport port numbers in the same way as UDP and TCP so SCTP uses transport port numbers in the same way as UDP and TCP so
that similar techniques could be used. that similar techniques could be used.
However, SCTP also supports multihoming. During connection setup However, SCTP also supports multihoming. During connection setup
SCTP control packets carry embedded addresses which would have to be SCTP control packets carry embedded addresses which would have to be
translated. This would also require that the types of the options translated. This would also require that the types of the options
fields in the SCTP control packets were changed with consequent fields in the SCTP control packets were changed with consequent
changes to packet length: the transport checksum would also have to changes to packet length: the transport checksum would also have to
be recalculated. The ramifications of multihoming as it might be recalculated. The ramifications of multihoming as it might
interact with NAT-PT have not been fully explored. Because of the interact with NAT-PT have not been fully explored. Because of the
'chunked' nature of data transfer it does not appear that state would 'chunked' nature of data transfer it does not appear that state would
have to be maintained to relate packets transmitted using the have to be maintained to relate packets transmitted using the
different IP addresses associated with the connection. different IP addresses associated with the connection. [Author's
Note: This needs to be considered by an SCTP expert].
Even if these technical issues can be overcome, using SCTP in a Even if these technical issues can be overcome, using SCTP in a
NAT-PT environment may effectively nullify the multihoming advantages NAT-PT environment may effectively nullify the multihoming advantages
of SCTP if all the connections run through the same NAT-PT. The of SCTP if all the connections run through the same NAT-PT. The
consequences of running a multihomed network with separate NAT-PT consequences of running a multihomed network with separate NAT-PT
boxes associated with each of the 'homes' have not been fully boxes associated with each of the 'homes' have not been fully
explored, but one issue that will arise is described in Section 4.4. explored, but one issue that will arise is described in Section 4.4.
SCTP will need an associated 'ALG' - actually a Transport Layer SCTP will need an associated 'ALG' - actually a Transport Layer
Gateway - to handle the packet payload modifications. If it turns Gateway - to handle the packet payload modifications. If it turns
out that state is required, the state would have to distributed and out that state is required, the state would have to distributed and
synchronized across several NAT-PT boxes in a multihomed environment. synchronized across several NAT-PT boxes in a multihomed environment.
SCTP running through NAT-PT in a multihomed environment is also SCTP running through NAT-PT in a multihomed environment is also
incompatible with IPsec as described in Section 2.1. incompatible with IPsec as described in Section 2.1.
2.7 NAT-PT as a Proxy Correspondent Node for MIPv6 2.7. NAT-PT as a Proxy Correspondent Node for MIPv6
As discussed in [I-D.lee-v6ops-natpt-mobility], it is not possible to As discussed in [I-D.lee-v6ops-natpt-mobility], it is not possible to
propagate Mobile IPv6 control messages into the IPv4 domain. propagate Mobile IPv6 control messages into the IPv4 domain.
According to the IPv6 Node Requirements [I-D.ietf-ipv6-node- According to the IPv6 Node Requirements [I-D.ietf-ipv6-node-
requirements], IPv6 nodes should normally be prepared to support the requirements], IPv6 nodes should normally be prepared to support the
route optimization mechanisms needed in a correspondent node. If route optimization mechanisms needed in a correspondent node. If
communications from an IPv6 mobile node are traversing a NAT-PT, the communications from an IPv6 mobile node is traversing a NAT-PT, the
destination IPv4 node will certainly not be able to support the destination IPv4 node is not going to support the correspondent node
correspondent node features needed for route optimization. features needed for route optimization.
This can be resolved in two ways: This can be resolved in two ways:
o The NAT-PT can discard messages and headers relating to changes of o The NAT-PT can discard messages and headers relating to changes of
care-of addresses including reverse routing checks. care-of addresses including reverse routing checks.
Communications with the mobile node will continue through the home Communications with the mobile node will continue through the home
agent without route optimization. This is clearly sub-optimal but agent without route optimization. This is clearly sub-optimal but
communication should remain possible. communication should remain possible.
o Additional functionality could be implemented in the NAT-PT to o Additional functionality could be implemented in the NAT-PT to
allow it to function as a proxy correspondent node for all IPv4 allow it to function as a proxy correspondent node for all IPv4
nodes for which it has bindings. This scheme adds considerably to nodes for which it has bindings. This scheme adds considerably to
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IPv6 PREFIX used for translated IPv4 addresses, it may also limit IPv6 PREFIX used for translated IPv4 addresses, it may also limit
the extent of mobility of the mobile node: all communications to the extent of mobility of the mobile node: all communications to
the IPv4 destination have to go through the same NAT-PT even if the IPv4 destination have to go through the same NAT-PT even if
the mobile node moves to a network which does not have direct IPv6 the mobile node moves to a network which does not have direct IPv6
connectivity with the NAT-PT. connectivity with the NAT-PT.
In both cases the existing NAT-PT specification would need to be In both cases the existing NAT-PT specification would need to be
extended to deal with IPv6 mobile nodes and neither is a fully extended to deal with IPv6 mobile nodes and neither is a fully
satisfactory solution. satisfactory solution.
2.8 NAT-PT and Multicast 2.8. NAT-PT and Multicast
SIIT [RFC2765] cannot handle translation of multicast packets and SIIT [RFC2765] cannot handle translation of multicast packets and
NAT-PT does not discuss a way to map multicast addresses between IPv4 NAT-PT does not discuss a way to map multicast addresses between IPv4
and IPv6. Some separate work has been done to provide an alternative and IPv6. Some separate work has been done to provide an alternative
mechanism to handle multicast. This uses a separate gateway which mechanism to handle multicast. This uses a separate gateway which
understands some or all of the relevant multicast control and routing understands some or all of the relevant multicast control and routing
protocols in each domain. This work has not been carried through protocols in each domain. This work has not been carried through
into standards as yet. into standards as yet.
A basic mechanism which involves only IGMP on the IPv4 side and MLD A basic mechanism which involves only IGMP on the IPv4 side and MLD
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comprehensive approach which includes proxying of the multicast comprehensive approach which includes proxying of the multicast
routing protocols is described in 'An IPv4 - IPv6 multicast gateway' routing protocols is described in 'An IPv4 - IPv6 multicast gateway'
[I-D.venaas-mboned-v4v6mcastgw]. Both approaches have several of the [I-D.venaas-mboned-v4v6mcastgw]. Both approaches have several of the
issues described in this section, notably issues with embedded issues described in this section, notably issues with embedded
addresses. addresses.
[I-D.okazaki-v6ops-natpt-security] identifies the possibility of a [I-D.okazaki-v6ops-natpt-security] identifies the possibility of a
multiplicative reflection attack if the NAT-PT can be spoofed into multiplicative reflection attack if the NAT-PT can be spoofed into
creating a binding for a multicast address. This attack would be creating a binding for a multicast address. This attack would be
very hard to mount because routers should not forward packets with very hard to mount because routers should not forward packets with
multicast addresses in the source address field. However, it muticast addresses in the source address field. However, it points
highlights the possibility that a naively implemented DNS-ALG could up that a naively implemented DNS-ALG could create such bindings from
create such bindings from spoofed DNS responses since [RFC2766] does spoofed DNS responses since [RFC2766] does not mention the need for
not mention the need for checks on the types of addresses in these checks on the types of addresses in these responses.
responses.
The issues for NAT-PT and multicast reflect the fact that NAT-PT is The issues for NAT-PT and multicast reflect the fact that NAT-PT is
at best a partial solution. Completing the translation solution to at best a partial solution. Completing the translation solution to
cater for multicast traffic is likely to carry a similar set of cater for multicast traffic is likely to carry a similar set of
issues to the current unicast NAT-PT and may open up significant issues to the current unicast NAT-PT and may open up significant
additional security risks. additional security risks.
3. Issues exacerbated by the Use of DNS-ALG 3. Issues exacerbated by the Use of DNS-ALG
3.1 Network Topology Constraints Implied by NAT-PT 3.1. Network Topology Constraints Implied by NAT-PT
Traffic flow initiators in a NAT-PT environment are dependent on the Traffic flow initiators in a NAT-PT environment are dependent on the
DNS-ALG in the NAT-PT to provide the mapped address needed to DNS-ALG in the NAT-PT to provide the mapped address needed to
communicate with the flow destination on the other side of the communicate with the flow destination on the other side of the
NAT-PT. Whether used for flows initiated in the IPv4 domain or the NAT-PT. Whether used for flows initiated in the IPv4 domain or the
IPv6 domain, the NAT-PT has to be on the path taken by the DNS query IPv6 domain, the NAT-PT has to be on the path taken by the DNS query
sent by the flow initiator to the relevant DNS server; otherwise the sent by the flow initiator to the relevant DNS server; otherwise the
DNS query will not be modified and the response type would not be DNS query will not be modified and the response type would not be
appropriate. appropriate.
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choose the correct DNS server for each query resulting in the same choose the correct DNS server for each query resulting in the same
issues as were raised for flows initiated from the IPv6 domain. issues as were raised for flows initiated from the IPv6 domain.
Although the engineering workaround, just described, provides a Although the engineering workaround, just described, provides a
partial solution to the topology constraints issue it mandates that partial solution to the topology constraints issue it mandates that
DNS queries and responses should still go through a NAT-PT even if DNS queries and responses should still go through a NAT-PT even if
there would normally be no reason to do so. This mandatory passage there would normally be no reason to do so. This mandatory passage
through the NAT-PT for all DNS requests will exacerbate the other DNS through the NAT-PT for all DNS requests will exacerbate the other DNS
related issues discussed in Section 3.4 and Section 4.1. related issues discussed in Section 3.4 and Section 4.1.
3.2 Scalability and Single Point of Failure Concerns 3.2. Scalability and Single Point of Failure Concerns
As with traditional NAT, NAT-PT is a bottleneck in the network with As with traditional NAT, NAT-PT is a bottleneck in the network with
significant scalability concerns and the anchoring of flows to a significant scalability concerns and the anchoring of flows to a
particular NAT-PT makes the NAT-PT a potential single point of particular NAT-PT makes the NAT-PT a potential single point of
failure in the network. The addition of the DNS-ALG in NAT-PT failure in the network. The addition of the DNS-ALG in NAT-PT
further increases the scalability concerns. further increases the scalability concerns.
Solutions to both problems have been envisaged using collections of Solutions to both problems have been envisaged using collections of
cooperating NAT-PT boxes, but such solutions require coordination and cooperating NAT-PT boxes, but such solutions require coordination and
state synchronization which has not yet been standardized and again state synchronization which has not yet been standardized and again
adds to the functional and operational complexity of NAT-PT. One adds to the functional and operational complexity of NAT-PT. One
such solution is described in [I-D.park-scalable-multi-natpt]. such solution is described in [I-D.park-scalable-multi-natpt].
As with traditional NAT, the concentration of flows through NAT-PT As with traditional NAT, the concentration of flows through NAT-PT
and the legitimate modification of packets in the NAT-PT make NAT-PTs and the legitimate modification of packets in the NAT-PT make NAT-PTs
enticing targets for security attacks. enticing targets for security attacks.
3.3 Issues with Lack of Address Persistence 3.3. Issues with Lack of Address Persistence
Using the DNS-ALG to create address bindings requires that the Using the DNS-ALG to create address bindings requires that the
application uses the translated address returned by the DNS query application uses the translated address returned by the DNS query
before the NAT-PT binding state is timed out (see Section 2.3). before the NAT-PT binding state is timed out (see Section 2.3).
Applications will not normally be aware of this constraint, which may Applications will not normally be aware of this constraint, which may
be different from the existing lifetime of DNS query responses. This be different from the existing lifetime of DNS query responses. This
could lead to 'difficult to diagnose' problems with applications. could lead to difficult diagnosis problems with applications.
Additionally, the DNS-ALG needs to determine the initial lifetime of Additionally, the DNS-ALG needs to determine the initial lifetime of
bindings which it creates. As noted in Section 2.3, this may need to bindings which it creates. As noted in Section 2.3, this may need to
be determined heuristically. The DNS-ALG does not know which be determined heuristically. The DNS-ALG does not know which
protocol the mapping is to be used for, and so needs another way to protocol the mapping is to be used for, and so needs another way to
determine the initial lifetime. This could be tied to the DNS determine the initial lifetime. This could be tied to the DNS
response lifetime but this might open up additional DOS attack response lifetime but this might open up additional DOS attack
possibilities if very long validities are allowed. Also the lifetime possibilities if very long validities are allowed. Also the lifetime
should be adjusted once the NAT-PT determines which protocol is being should be adjusted once the NAT-PT determines which protocol is being
used with the binding. used with the binding.
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the previously used mapped address to different hosts between the previously used mapped address to different hosts between
sessions. sessions.
Trying to keep NAT-PT from discarding an address mapping would Trying to keep NAT-PT from discarding an address mapping would
require either a NAT-PT extension protocol that would allow the require either a NAT-PT extension protocol that would allow the
application to request the NAT-PT device to retain the mappings, or application to request the NAT-PT device to retain the mappings, or
an extended ALG (which has all the issues discussed in Section 2.1) an extended ALG (which has all the issues discussed in Section 2.1)
that can interact with NAT-PT to keep the address mapping from being that can interact with NAT-PT to keep the address mapping from being
discarded after a session. discarded after a session.
3.4 DOS Attacks on Memory and Address/Port Pools 3.4. DOS Attacks on Memory and Address/Port Pools
As discussed in Section 2.3 a NAT-PT may create dynamic NAT bindings As discussed in Section 2.3 a NAT-PT may create dynamic NAT bindings
each of which consumes memory resources as well as an address (or each of which consumes memory resources as well as an address (or
port if NAPT-PT is used) from an address (or port) pool. A number of port if NAPT-PT is used) from an address (or port) pool. A number of
documents, including [RFC2766] and [I-D.okazaki-v6ops-natpt-security] documents, including [RFC2766] and [I-D.okazaki-v6ops-natpt-security]
discuss possible denial of service (DOS) attacks on NA(P)T-PT which discuss possible denial of service (DOS) attacks on NA(P)T-PT which
result in resource depletion associated with address and port pools. result in resource depletion associated with address and port pools.
NAT-PT does not specify any authentication mechanisms so that an NAT-PT does not specify any authentication mechanisms so that an
attacker may be able to create spurious bindings by spoofing attacker may be able to create spurious binds by spoofing addresses
addresses in packets sent through NAT-PT. The attack is more in packets sent through NAT-PT. The attack is more damaging if the
damaging if the attacker is able to spoof protocols with long binding attacker is able to spoof protocols with long binding timeouts
timeouts (typically used for TCP). (typically used for TCP).
The use of the DNS-ALG in NAT-PT introduces another vulnerability The use of the DNS-ALG in NAT-PT introduces another vulnerability
which can result in resource depletion. The attack identified in which can result in resource depletion. The attack identified in
[I-D.durand-natpt-dns-alg-issues] exploits the use of DNS queries [I-D.durand-natpt-dns-alg-issues] exploits the use of DNS queries
traversing NAT-PT to create dynamic bindings. Every time a DNS query traversing NAT-PT to create dynamic bindings. Every time a DNS query
is sent through the NAT-PT the NAT-PT may create a new NA(P)T-PT bind is sent through the NAT-PT the NAT-PT may create a new NA(P)T-PT bind
without any end-host authentication or authorization mechanisms. without any end-host authentication or authorization mechanisms.
This behavior could lead to a serious DOS attack on both memory and This behavior could lead to a serious DOS attack on both memory and
address or port pools. Address spoofing is not required for this address or port pools. Address spoofing is not required for this
attack to be successful. attack to be successful.
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The ideal mitigation solution would be to disallow dynamically The ideal mitigation solution would be to disallow dynamically
created binds until authentication and authorization of the end-host created binds until authentication and authorization of the end-host
needing the protocol translation has been carried out. This would needing the protocol translation has been carried out. This would
require that the proper security infrastructure be in place to require that the proper security infrastructure be in place to
support the authentication and authorization, which increases the support the authentication and authorization, which increases the
network operational complexity. network operational complexity.
4. Issues Directly Related to use of DNS-ALG 4. Issues Directly Related to use of DNS-ALG
4.1 Address Selection Issues when Communicating with Dual-Stack End- 4.1. Address Selection Issues when Communicating with Dual-Stack End-
hosts hosts
[I-D.durand-natpt-dns-alg-issues] discusses NAT-PT DNS-ALG issues [I-D.durand-natpt-dns-alg-issues] discusses NAT-PT DNS-ALG issues
with regard to address selection. As specified in [RFC2766], the with regard to address selection. As specified in [RFC2766], the
DNS-ALG returns AAAA RRs from two possible sources to the IPv6 host DNS-ALG returns AAAA RRs from two possible sources to the IPv6 host
which has made an AAAA DNS query. which has made an AAAA DNS query AAAA.
If the query relates to a dual-stack host, the query will return both If the query relates to a dual-stack host, the query will return both
the native IPv6 address(es) and the translated IPv4 address(es) in the native IPv6 address(es) and the translated IPv4 address(es) in
AAAA RRs. Without additional information, the IPv6 host address AAAA RRs. Without additional information, the IPv6 host address
selection may pick a translated IPv4 address instead of selecting the selection may pick a translated IPv4 address instead of selecting the
more appropriate native IPv6 address. Under some circumstances, the more appropriate native IPv6 address. Under some circumstances, the
address selection algorithms will always prefer the translated address selection algorithms [RFC3484] will always prefer the
address over the native IPv6 address which is obviously undesirable. translated address over the native IPv6 address which is obviously
undesirable.
[I-D.hallin-natpt-dns-alg-solutions] proposes a solution which [I-D.hallin-natpt-dns-alg-solutions] proposes a solution which
involves modification to the NAT-PT specification intended to return involves modification to the NAT-PT specification intended to return
only the most appropriate address(es) to an IPv6 capable host: only the most appropriate address(es) to an IPv6 capable host:
o When a DNS AAAA query traverses the NAT-PT DNS-ALG, the NAT-PT o When a DNS AAAA query traverses the NAT-PT DNS-ALG, the NAT-PT
will forward the query to the DNS server in the IPv4 domain will forward the query to the DNS server in the IPv4 domain
unchanged but using IPv4 transport: unchanged but using IPv4 transport:
* If the authoritative DNS server has one or more AAAA records, * If the authoritative DNS server has one or more AAAA records,
it returns them. The DNS-ALG then forwards this response to it returns them. The DNS-ALG then forwards this response to
the IPv6 host and does not send an A query as the standard the IPv6 host and does not send an A query as the standard
NAT-PT would do. NAT-PT would do.
* Otherwise, if the DNS server does not understand the AAAA query * Otherwise, if the DNS server does not understand the AAAA query
or has no AAAA entry for the host, it will return an error. or has no AAAA entry for the host, it will return an error.
The NAT-PT DNS-ALG will intercept the error or empty return and The NAT-PT DNS-ALG will intercept the error or empty return and
send an A query for the same host. If this query returns an send an A query for the same host. If this query returns an
IPv4 address, the ALG creates a binding and synthesizes a IPv4 address, the ALG creates a binding and synthesizes a
corresponding AAAA record which it sends back to the IPv6 host. corresponding AAAA record which it sends back to the IPv6 host.
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it returns them. The DNS-ALG then forwards this response to it returns them. The DNS-ALG then forwards this response to
the IPv6 host and does not send an A query as the standard the IPv6 host and does not send an A query as the standard
NAT-PT would do. NAT-PT would do.
* Otherwise, if the DNS server does not understand the AAAA query * Otherwise, if the DNS server does not understand the AAAA query
or has no AAAA entry for the host, it will return an error. or has no AAAA entry for the host, it will return an error.
The NAT-PT DNS-ALG will intercept the error or empty return and The NAT-PT DNS-ALG will intercept the error or empty return and
send an A query for the same host. If this query returns an send an A query for the same host. If this query returns an
IPv4 address, the ALG creates a binding and synthesizes a IPv4 address, the ALG creates a binding and synthesizes a
corresponding AAAA record which it sends back to the IPv6 host. corresponding AAAA record which it sends back to the IPv6 host.
o The NAT-PT thus forwards the result of the first successful DNS o The NAT-PT thus forwards the result of the first successful DNS
response back to the end-host or an error if neither succeeds. response back to the end-host or an error if neither succeeeds.
Consequently only AAAA RRs from one source will be provided Consequently only AAAA RRs from one source will be provided
instead of two as specified in [RFC2766] and it will contain the instead of two as specified in [RFC2766] and it will contain the
most appropriate address for a dual-stack or IPv6-only querier. most appropriate address for a dual-stack or IPv6-only querier.
There is, however, still an issue with the proposed solution: There is, however, still an issue with the proposed solution:
o The DNS client may timeout the query if it doesn't receive a o The DNS client may timeout the query if it doesn't receive a
response in time. This is more likely because the NAT-PT may have response in time. This is more likely because the NAT-PT may have
to make two separate queries sequentially which the client is not to make two separate queries sequentially which the client is not
aware of. It may be possible to reduce the response time by aware of. It may be possible to reduce the response time by
sending the two queries in parallel and ignoring the result of the sending the two queries in parallel and ignoring the result of the
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Neither of these solutions resolves a second issue related to address Neither of these solutions resolves a second issue related to address
selection identified in [I-D.durand-natpt-dns-alg-issues]. selection identified in [I-D.durand-natpt-dns-alg-issues].
Applications have no way of knowing that the IPv6 address returned Applications have no way of knowing that the IPv6 address returned
from the DNS-ALG is not a 'real' IPv6 address, but a translated IPv4 from the DNS-ALG is not a 'real' IPv6 address, but a translated IPv4
address. The application may therefore be led to believe that it has address. The application may therefore be led to believe that it has
end-to-end IPv6 connectivity with the destination. As a result, the end-to-end IPv6 connectivity with the destination. As a result, the
application may use IPv6 specific options that are not supported by application may use IPv6 specific options that are not supported by
NAT-PT. This issue is closely related to the issue described in NAT-PT. This issue is closely related to the issue described in
Section 4.2 and the discussion in Section 5. Section 4.2 and the discussion in Section 5.
4.2 Non-global Validity of Translated RR Records 4.2. Non-global Validity of Translated RR Records
Some applications propagate information records retrieved from DNS to Some applications propagate information records retrieved from DNS to
other applications. The published semantics of DNS imply that the other applications. The published semantics of DNS imply that the
results will be consistent to any user for the duration of the results will be consistent to any user for the duration of the
attached lifetime. RR records translated by NAT-PT violate these attached lifetime. RR records translated by NAT-PT violate these
semantics because the retrieved addresses are only usable for semantics because the retrieved addresses are only usable for
communications through the translating NAT-PT. communications through the translating NAT-PT.
Applications which pass on retrieved DNS records to other Applications which pass on retrieved DNS records to other
applications will generally assume that they can rely on the passed applications will generally assume that they can rely on the passed
on addresses to be usable by the receiving application. This may not on addresses to be usable by the receiving application. This may not
be the case if the receiving application is on another node be the case if the receiving application is on another node
especially if it is not in the domain served by the NAT-PT which especially if it is not in the domain served by the NAT-PT which
generated the translation. generated the translation.
4.3 Inappropriate Translation of Responses to A Queries 4.3. Inappropriate Translation of Responses to A Queries
Some applications running on dual-stack nodes may wish to query the Some applications running on dual-stack nodes may wish to query the
IPv4 address of a destination. If the resulting A query passes IPv4 address of a destination. If the resulting A query passes
through the NAT-PT DNS-ALG, the DNS-ALG will translate the response through the NAT-PT DNS-ALG, the DNS-ALG will translate the response
inappropriately into a AAAA record using a translated address. This inappropriately into a AAAA record using a translated address. This
happens because the DNS-ALG specified in [RFC2766] operates happens because the DNS-ALG specified in [RFC2766] operates
statelessly and hence has no memory of the IPv6 query which induced statelessly and hence has no memory of the IPv6 query which induced
the A request on IPv4 side. The default action is to translate the the A request on IPv4 side. The default action is to translate the
response. response.
The specification of NAT-PT could be modified to maintain minimal The specification of NAT-PT could be modified to maintain minimal
state about queries passed through the DNS-ALG, and hence to respond state about queries passed through the DNS-ALG, and hence to respond
correctly to A queries as well as AAAA queries. correctly to A queries as well as AAAA queries.
4.4 DNS-ALG and Multi-addressed Nodes 4.4. DNS-ALG and Multi-addressed Nodes
Many IPv6 nodes, especially in multihomed situations but also in Many IPv6 nodes, especially in multihomed situations but also in
single homed deployments, can expect to have multiple global single homed deployments, can expect to have multiple global
addresses. The same may be true for multihomed IPv4 nodes. addresses. The same may be true for multihomed IPv4 nodes.
Responses to DNS queries for these nodes will normally contain all Responses to DNS queries for these nodes will normally contain all
these addresses. Since the DNS-ALG in the NAT-PT has no knowledge these addresses. Since the DNS-ALG in the NAT-PT has no knowledge
which of the addresses can or will be used by the application issuing which of the addresses can or will be used by the application issuing
the query, it is obliged to translate all of them. the query, it is obliged to translate all of them.
This could be a significant drain on resources in both NAT-PT and This could be a significant drain on resources in both NAT-PT and
NAPT-PT, as bindings will have to be created for each address. NAPT-PT, as bindings will have to be created for each address.
When using SCTP in a multihomed network, the problem is exacerbated When using SCTP in a multihomed network, the problem is exacerbated
if multiple NAT-PTs translate multiple addresses: also it is not if multiple NAT-PTs translate mutiple addresses: also it is not clear
clear that SCTP will actually look up all the destination IP that SCTP will actually look up all the destination IP addresses via
addresses via DNS so that bindings may not be in place when packets DNS so that bindings may not be in place when packets arrive.
arrive.
4.5 Limitations on Deployment of DNS Security Capabilities 4.5. Limitations on Deployment of DNS Security Capabilities
Secure DNS (DNSSEC) [I-D.ietf-dnsext-dnssec-intro] uses public key Secure DNS (DNSSEC) [I-D.ietf-dnsext-dnssec-intro] uses public key
cryptographic signing to authenticate DNS responses. The DNS-ALG cryptographic signing to authenticate DNS responses. The DNS-ALG
modifies DNS query responses traversing the NAT-PT in both directions modifies DNS query responses traversing the NAT-PT in both directions
which would invalidate the signatures as (partially) described in which would invalidate the signatures as (partially) described in
Section 7.5 of [RFC2766]. Section 7.5 of [RFC2766].
Workarounds have been proposed, such as making the DNS-ALG behave Workarounds have been proposed, such as making the DNS-ALG behave
like a secure DNS server. This would need to be done separately for like a secure DNS server. This would need to be done separately for
both the IPv6 and IPv4 domains. This is operationally very complex both the IPv6 and IPv4 domains. This is operationally very complex
skipping to change at page 20, line 9 skipping to change at page 19, line 48
DNSSEC. DNSSEC.
7. IANA Considerations 7. IANA Considerations
There are no IANA considerations defined in this document. There are no IANA considerations defined in this document.
8. Conclusion 8. Conclusion
This document has discussed a number of significant issues with This document has discussed a number of significant issues with
NAT-PT as defined in [RFC2766]. From a deployment perspective 3GPP NAT-PT as defined in [RFC2766]. From a deployment perspective 3GPP
networks are currently the only 'standardized' scenario where an IPv6 networks are currently the only 'standardised' scenario where an IPv6
only host communicates with an IPv4 only host using NAT-PT as only host communicates with an IPv4 only host using NAT-PT as
described in the 3GPP IPv6 transition analysis [I-D.ietf-v6ops-3gpp- described in the 3GPP IPv6 transition analysis [I-D.ietf-v6ops-3gpp-
analysis] but NAT-PT has seen some limited usage for other purposes. analysis] but NAT-PT has seen some limited usage for other purposes.
Although some of issues identified with NAT-PT appear to have Although some of issues identified with NAT-PT appear to have
solutions, many of the solutions required significant alterations to solutions, many of the solutions required significant alterations to
the existing specification and would be likely to increase the existing specification and would be likely to increase
operational complexity. Even if these solutions were applied, we operational complexity. Even if these solutions were applied, we
have shown that NAT-PT still has significant irresolvable issues and have shown that NAT-PT still has significant irresolvable issues and
appears to have limited applicability. The potential constraints on appears to have limited applicability. The potential constraints on
skipping to change at page 21, line 5 skipping to change at page 20, line 44
9. Acknowledgments 9. Acknowledgments
This work builds on a large body of existing work examining the This work builds on a large body of existing work examining the
issues and applicability of NAT-PT: the work of the authors of the issues and applicability of NAT-PT: the work of the authors of the
documents referred in Section 1 has been extremely useful in creating documents referred in Section 1 has been extremely useful in creating
this document. Particular thanks are due to Pekka Savola for rapid this document. Particular thanks are due to Pekka Savola for rapid
and thorough review of the document. and thorough review of the document.
10. References 10. References
10.1 Normative References 10.1. Normative References
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm [RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000. (SIIT)", RFC 2765, February 2000.
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address [RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766, Translation - Protocol Translation (NAT-PT)", RFC 2766,
February 2000. February 2000.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions", [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999. RFC 2535, March 1999.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999. RFC 2663, August 1999.
[RFC3027] Holdrege, M. and P. Srisuresh, "Protocol Complications [RFC3027] Holdrege, M. and P. Srisuresh, "Protocol Complications
with the IP Network Address Translator", RFC 3027, with the IP Network Address Translator", RFC 3027,
January 2001. January 2001.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP, UDP,
ESP, and uncompressed", RFC 3095, July 2001.
[RFC3314] Wasserman, M., "Recommendations for IPv6 in Third [RFC3314] Wasserman, M., "Recommendations for IPv6 in Third
Generation Partnership Project (3GPP) Standards", Generation Partnership Project (3GPP) Standards",
RFC 3314, September 2002. RFC 3314, September 2002.
[RFC3484] Draves, R., "Default Address Selection for Internet [RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003. Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3574] Soininen, J., "Transition Scenarios for 3GPP Networks",
RFC 3574, August 2003.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[I-D.ietf-ipv6-node-requirements] [I-D.ietf-ipv6-node-requirements]
Loughney, J., "IPv6 Node Requirements", Loughney, J., "IPv6 Node Requirements",
draft-ietf-ipv6-node-requirements-11 (work in progress), draft-ietf-ipv6-node-requirements-11 (work in progress),
August 2004. August 2004.
[I-D.ietf-v6ops-3gpp-analysis] [I-D.ietf-v6ops-3gpp-analysis]
Wiljakka, J., "Analysis on IPv6 Transition in 3GPP Wiljakka, J., "Analysis on IPv6 Transition in 3GPP
Networks", draft-ietf-v6ops-3gpp-analysis-11 (work in Networks", draft-ietf-v6ops-3gpp-analysis-11 (work in
progress), October 2004. progress), October 2004.
[I-D.ietf-dnsext-dnssec-intro] [I-D.ietf-dnsext-dnssec-intro]
Arends, R., Austein, R., Massey, D., Larson, M., and S. Arends, R., Austein, R., Massey, D., Larson, M., and S.
Rose, "DNS Security Introduction and Requirements", Rose, "DNS Security Introduction and Requirements",
draft-ietf-dnsext-dnssec-intro-13 (work in progress), draft-ietf-dnsext-dnssec-intro-13 (work in progress),
October 2004. October 2004.
10.2 Informative References 10.2. Informative References
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security [RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858, Considerations for IP Fragment Filtering", RFC 1858,
October 1995. October 1995.
[RFC3128] Miller, I., "Protection Against a Variant of the Tiny [RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128, June 2001. Fragment Attack (RFC 1858)", RFC 3128, June 2001.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C., [RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
skipping to change at page 22, line 46 skipping to change at page 22, line 30
[I-D.durand-natpt-dns-alg-issues] [I-D.durand-natpt-dns-alg-issues]
Durand, A., "Issues with NAT-PT DNS ALG in RFC2766", Durand, A., "Issues with NAT-PT DNS ALG in RFC2766",
draft-durand-natpt-dns-alg-issues-00 (work in progress), draft-durand-natpt-dns-alg-issues-00 (work in progress),
February 2002. February 2002.
[I-D.hallin-natpt-dns-alg-solutions] [I-D.hallin-natpt-dns-alg-solutions]
Hallingby, P. and S. Satapati, "NAT-PT DNS ALG solutions", Hallingby, P. and S. Satapati, "NAT-PT DNS ALG solutions",
draft-hallin-natpt-dns-alg-solutions-01 (work in draft-hallin-natpt-dns-alg-solutions-01 (work in
progress), July 2002. progress), July 2002.
[I-D.durand-v6ops-dualstack-vs-natpt]
Durand, A., "Dual stack vs NAT-PT",
draft-durand-v6ops-dualstack-vs-natpt-00 (work in
progress), February 2003.
[I-D.lee-v6ops-natpt-mobility] [I-D.lee-v6ops-natpt-mobility]
Shin, M. and J. Lee, "Considerations for Mobility Support Shin, M. and J. Lee, "Considerations for Mobility Support
in NAT-PT", draft-lee-v6ops-natpt-mobility-00 (work in in NAT-PT", draft-lee-v6ops-natpt-mobility-01 (work in
progress), June 2003. progress), July 2005.
[I-D.okazaki-v6ops-natpt-security] [I-D.okazaki-v6ops-natpt-security]
Okazaki, S. and A. Desai, "NAT-PT Security Okazaki, S. and A. Desai, "NAT-PT Security
Considerations", draft-okazaki-v6ops-natpt-security-00 Considerations", draft-okazaki-v6ops-natpt-security-00
(work in progress), June 2003. (work in progress), June 2003.
[I-D.vanderpol-v6ops-translation-issues] [I-D.vanderpol-v6ops-translation-issues]
Pol, R., Satapati, S., and S. Sivakumar, "Issues when Pol, R., Satapati, S., and S. Sivakumar, "Issues when
translating between IPv4 and IPv6", translating between IPv4 and IPv6",
draft-vanderpol-v6ops-translation-issues-00 (work in draft-vanderpol-v6ops-translation-issues-00 (work in
skipping to change at page 23, line 44 skipping to change at page 24, line 8
February 2003. February 2003.
[I-D.park-scalable-multi-natpt] [I-D.park-scalable-multi-natpt]
Park, S., "Scalable mNAT-PT Solution", Park, S., "Scalable mNAT-PT Solution",
draft-park-scalable-multi-natpt-00 (work in progress), draft-park-scalable-multi-natpt-00 (work in progress),
May 2003. May 2003.
Authors' Addresses Authors' Addresses
Cedric Aoun Cedric Aoun
ENST ZTE Corporation/ENST Paris
Paris
France France
Email: cedric@caoun.net Email: aoun.cedric@zte.com.fr
Elwyn B. Davies Elwyn B. Davies
Consultant Consultant
Soham, Cambs Soham, Cambs
UK UK
Phone: +44 7889 488 335 Phone: +44 7889 488 335
Email: elwynd@dial.pipex.com Email: elwynd@dial.pipex.com
Intellectual Property Statement Intellectual Property Statement
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