Behavior Engineering for Hindrance J. Korhonen, Ed.
Avoidance (BEHAVE) Nokia Siemens Networks
Internet-Draft T. Savolainen, Ed.
Intended status: Informational Nokia
Expires: July 24, 2011 January 20, 2011
Analysis of solution proposals for hosts to learn NAT64 prefix
draft-korhonen-behave-nat64-learn-analysis-01.txt
Abstract
Hosts and applications may benefit from the knowledge if an IPv6
address is synthesized, which would mean a NAT64 is used to reach the
IPv4 network or Internet. This document analyses a number of
proposed solutions for communicating whether the synthesis is taking
place, used address format, and the IPv6 prefix used by the NAT64 and
DNS64. This enables both NAT64 avoidance and intentional utilization
by allowing local IPv6 address synthesis.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 24, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology and Assumptions . . . . . . . . . . . . . . . . . 5
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Proposed solutions to learn about synthesis and
Network-Specific Prefix . . . . . . . . . . . . . . . . . . . 8
4.1. EDNS0 option indicating AAAA Record synthesis and
format . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. Solution description . . . . . . . . . . . . . . . . . 8
4.1.2. Analysis and discussion . . . . . . . . . . . . . . . 8
4.1.3. Summary . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. EDNS0 flags indicating AAAA Record synthesis and format . 9
4.2.1. Solution description . . . . . . . . . . . . . . . . . 9
4.2.2. Analysis and discussion . . . . . . . . . . . . . . . 10
4.2.3. Summary . . . . . . . . . . . . . . . . . . . . . . . 11
4.3. DNS Query for a Well-Known Name . . . . . . . . . . . . . 11
4.3.1. Solution description . . . . . . . . . . . . . . . . . 11
4.3.2. Analysis and discussion . . . . . . . . . . . . . . . 11
4.3.3. Summary . . . . . . . . . . . . . . . . . . . . . . . 12
4.4. DNS Resource Record for IPv4-Embedded IPv6 address . . . . 12
4.4.1. Solution description . . . . . . . . . . . . . . . . . 12
4.4.2. Analysis and discussion . . . . . . . . . . . . . . . 12
4.4.3. Summary . . . . . . . . . . . . . . . . . . . . . . . 13
4.5. Learning the IPv6 Prefix of a Network's NAT64 using DNS . 13
4.5.1. Solution description . . . . . . . . . . . . . . . . . 14
4.5.2. Analysis and discussion . . . . . . . . . . . . . . . 14
4.5.3. Summary . . . . . . . . . . . . . . . . . . . . . . . 15
4.6. Learning the IPv6 Prefix of a Network's NAT64 using
DHCPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.6.1. Solution description . . . . . . . . . . . . . . . . . 15
4.6.2. Analysis and discussion . . . . . . . . . . . . . . . 15
4.6.3. Summary . . . . . . . . . . . . . . . . . . . . . . . 16
4.7. Learning the IPv6 Prefix of a Network's NAT64 using
Router Advertisements . . . . . . . . . . . . . . . . . . 16
4.7.1. Solution description . . . . . . . . . . . . . . . . . 17
4.7.2. Analysis and discussion . . . . . . . . . . . . . . . 17
4.7.3. Summary . . . . . . . . . . . . . . . . . . . . . . . 17
4.8. Using application layer protocols such as STUN . . . . . . 18
4.8.1. Solution description . . . . . . . . . . . . . . . . . 18
4.8.2. Analysis and discussion . . . . . . . . . . . . . . . 18
4.8.3. Summary . . . . . . . . . . . . . . . . . . . . . . . 20
5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 22
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
10. Informative References . . . . . . . . . . . . . . . . . . . . 22
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
Hosts and applications may benefit from the knowledge of whether an
IPv6 address is synthesized, which would mean a NAT64 is used to
reach the IPv4 network or Internet. There are two issues that can be
addressed with solutions that allow hosts and applications to learn
the Network Specific Prefix (NSP) [RFC6052] used by the NAT64
[I-D.ietf-behave-v6v4-xlate-stateful] and the DNS64
[I-D.ietf-behave-dns64] devices.
Firstly, finding out whether a particular address is synthetic and
therefore learning the presence of a NAT64. For example, a Dual-
Stack (DS) host with IPv4 connectivity could use this information to
bypass NAT64 and use native IPv4 transport for destinations that are
reachable through IPv4. We will refer this as 'Issue #1' throughout
the document.
Secondly, finding out how to construct from an IPv4 address an IPv6
address that will be routable to/by the NAT64. This is useful when
IPv4 literals can be found in the payload of some protocol or
applications do not use DNS to resolve names to addresses but know
the IPv4 address of the destination by some other means. We will
refer this as 'Issue #2' throughout the document.
Additionally three other issues have to be considered by a solution
addressing the first two issues: whether DNS is required 'Issue #3',
whether a solution supports changing NSP 'Issue #4', and whether
multiple NSPs are supported (either of the same or different length)
for load-balancing purposes 'Issue #5'.
This document analyses all known solution proposals known at the time
of writing for communicating if the synthesis is taking place, used
address format, and the IPv6 prefix used by the NAT64 and DNS64.
Based on the analysis we conclude whether the issue of learning the
Network-Specific Prefix is worth solving and what would be the
recommended solution(s) in that case.
2. Terminology and Assumptions
NSP
Network-Specific Prefix: A prefix chosen by network administrator
for NAT64/DNS64 to present IPv4 addresses in IPv6 namespace.
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WKP
Well-Known Prefix: A prefix (64:ff9b::/96) chosen by IETF and
configured by a network administrator for NAT64/DNS64 to present
IPv4 addresses in IPv6 namespace.
NAT64
Network Address and protocol Translation mechanism for translating
IPv6 packets to IPv4 packets and vice-versa: A network entity that
a host or an application may want to either avoid or utilize.
IPv6 packets hosts send to addresses in the NSP and/or WKP are
routed to NAT64.
DNS64
DNS extensions for network address translation from IPv6 clients
to IPv4 servers: A network entity that synthesizes IPv6 addresses
and AAAA records out of IPv4 addresses and A records, hence making
IPv4 namespaces visible into IPv6 namespace. DNS64 uses NSP
and/or WKP in the synthesis process.
Address Synthesis
A mechanism, in the context of this document, where an IPv4
address is represented as an IPv6 address understood by a NAT64
device. The synthesized IPv6 address is formed by embedding an
IPv4 address as-is into an IPv6 address prefixed with a NSP/WKP.
It is assumed that the 'unused' suffix bits of the synthesized
address are set to zero as described in Section 2.2 of [RFC6052].
Issue #1
The problem of distinguishing between a synthesized and a real
IPv6 addresses, which allows a host to learn the presence of a
NAT64 in the network.
Issue #2
The problem of learning the NSP used by the access network and
needed for local IPv6 address synthesis.
Issue #3
The problem of learning the NSP or WKP used by the access network
by a host not implementing DNS (hence applications are unable to
use DNS to learn prefix).
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Issue #4
The problem of supporting changing NSP.
Issue #5
The problem of supporting multiple NSPs.
3. Background
Certain applications, operating in protocol translation scenarios,
can benefit from knowing the IPv6 prefix used by a local NAT64 of the
attached access network. This applies to the Framework document
[I-D.ietf-behave-v6v4-framework] Scenario 1 ("IPv6 network to IPv4
Internet"), Scenario 5 ("An IPv6 network to an IPv4 network"), and
Scenario 7 ("The IPv6 Internet to the IPv4 Internet"). Scenario
3("The IPv6 Internet to an IPv4 network") is not considered
applicable herein as in that case a NAT64 is located at the front of
remote IPv4 network and host in IPv6 Internet can benefit very little
of learning NSP IPv6 prefix used by the remote NAT64. The NAT64
prefix can be either a Network Specific Prefix (NSP) or the Well-
known Prefix (WKP). Below is (an incomplete) list of various use
cases where it is beneficial for a host or an application to know the
presence of a NAT64 and the NSP/WKP:
o Host-based DNSSEC validation: as is documented in DNS64
[I-D.ietf-behave-dns64] section 5.5. point 3, synthetic AAAA
records cannot be successfully validated in a host. In order to
utilize NAT64 a security-aware and validating host has to perform
DNS64 function locally and hence it has to be able to learn WKP or
proper NSP.
o Protocols that use IPv4 literals: in IPv6-only access native IPv4
connections cannot be created. If a network has NAT64 it is
possible to synthesize IPv6 address by combining the IPv4 literal
and the IPv6 prefix used by NAT64. The synthesized IPv6 address
can then be used to create an IPv6 connection.
o Multicast translations
[I-D.venaas-behave-mcast46][I-D.venaas-behave-v4v6mc-framework].
o URI schemes with host IPv4 address literals rather than domain
names (e.g., http://192.0.2.1, ftp://192.0.2.1, imap://192.0.2.1,
ipp://192.0.2.1)): a host can synthesize IPv6 address out of the
literal in URI and use IPv6 to create connection through NAT64.
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o Updating host's [RFC3484] preference table to prefer native
prefixes over translated prefixes: this is useful as applications
are more likely able to traverse through NAT44 than NAT64.
DNS64 cannot serve applications that are not using DNS or that obtain
referral as an IPv4 literal address. One example application is the
Session Description Protocol (SDP) [RFC4566], as used by Real Time
Streaming Protocol (RTSP) [RFC2326] and Session Initiation Protocol
(SIP) [RFC3261]. Other example applications include web browsers, as
IPv4 address literals are still encountered in web pages and URLs.
Some of these applications could still work through NAT64, provided
they were able to create locally valid IPv6 presentations of peers'
IPv4 addresses.
4. Proposed solutions to learn about synthesis and Network-Specific
Prefix
4.1. EDNS0 option indicating AAAA Record synthesis and format
4.1.1. Solution description
Section 3 of [I-D.korhonen-edns0-synthesis-flag] defines a new EDNS0
option [RFC2671], which contain 3 flag bits (called SY-bits). The
EDNS0 option serves as an implicit indication of the presence of
DNS64 server and the NAT64 device. The EDNS0 option SY-bit values
other than '000' and '111' explicitly tell the NSP prefix length.
Only the DNS64 server can insert the EDNS0 option and the required
SY-bits combination into the synthesized AAAA Resource Record.
4.1.2. Analysis and discussion
The PROs of the proposal are listed below:
+ Can be used to solve Issue #1 and is designed to explicitly solve
Issue #2.
+ Solves issue #4 via DNS record lifetime.
+ Can partially solve issue #5 if multiple synthetic AAAA records
are included in the response and all use same format.
+ The solution is backward compatible from 'legacy' hosts and
servers point of view.
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+ Even if the solution is bundled with DNS queries and responses, a
standardization of a new DNS record type is not required, rather
just defining a new EDNS0 option.
+ EDNS0 option implementation requires changes only to DNS64
servers.
+ Does not require additional provisioning or management as the
EDNS0 option is added automatically by the DNS64 server to the
responses.
+ Does not involve additional queries towards the global DNS
infrastructure as EDNS0 logic can be handled within the DNS64
server.
The CONs of the proposal are listed below:
- Requires end hosts to support [RFC2671] EDSN0 extension mechanism.
- Requires host resolver changes and a mechanism/additions to the
host resolver API (or flags, hints etc) to deliver a note to the
querying application that the address is synthesized and what is
the NSP prefix length.
- Requires a modification to DNS64 servers to include the EDNS0
option to the synthesized responses.
- Does not provide solution for issue #3.
4.1.3. Summary
The EDNS0 option based solution works by extending the existing EDNS0
Resource Record. Although the solution has host resolver and DNS64
server impacts, the changes are limited to those entities (end host,
applications) that are interested in learning the presence of NAT64
and the used NAT64 prefix. The provisioning and management overhead
is minimal if not non-existent as the EDNS0 options are synthesized
in a DNS64 server in a same manner as the synthesized AAAA Resource
Records. Moreover, EDNS0 does not induce any load to DNS servers
because no new RRType query is defined.
4.2. EDNS0 flags indicating AAAA Record synthesis and format
4.2.1. Solution description
Section 3 of [EDNS0-Flag] defines 3 new flag bits (called SY-bits)
into EDNS0 OPT [RFC2671] header, which serve as an implicit
indication of the presence of DNS64 server and a NAT64 device. SY-
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bit values other than '000' or '111' explicitly tell the NSP prefix
length. Only the DNS64 server can insert the EDNS0 option and the
required SY-bits combination into the synthesized AAAA Resource
Record.
4.2.2. Analysis and discussion
The PROs of the proposal are listed below:
+ Can be used to solve Issue #1 and is designed to explicitly solve
Issue #2.
+ Solves issue #4 via DNS record lifetime.
+ Can partially solve issue #5 if multiple synthetic AAAA records
are included in the response and all use same format.
+ The solution is backward compatible from 'legacy' hosts and
servers point of view.
+ EDNS0 option implementation requires changes only to DNS64
servers.
+ Does not require additional provisioning or management as the
EDNS0 option is added automatically by the DNS64 server to the
responses.
+ Does not involve additional queries towards the global DNS
infrastructure as EDNS0 logic can be handled within the DNS64
server.
The CONs of the proposal are listed below:
- Requires end hosts to support [RFC2671] EDSN0 extension mechanism.
- Consumes scarce flag bits from EDNS0 OPT header.
- Requires a host resolver changes and a mechanism/additions to the
host resolver API (or flags, hints etc) to deliver a note to the
querying application that the address is synthesized and what is
the NSP prefix length.
- Requires a modification to DNS64 servers to include the EDNS0
option to the synthesized responses.
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- Does not provide solution for issue #3.
4.2.3. Summary
This option is included here for the sake of completeness. The
consumption of three bits of the limited EDNS0 OPT space can be
considered unfavorable and hence is unlikely to be accepted.
4.3. DNS Query for a Well-Known Name
4.3.1. Solution description
Section 3 of [I-D.savolainen-heuristic-nat64-discovery] describes a
host behavior for discovering the presence of a DNS64 server and a
NAT64 device, and heuristics for discovering the used NSP. A host
requiring information for local IPv6 address synthesis or for NAT64
avoidance sends a DNS query for an AAAA record of a Well-Known IPv4-
only Fully Qualified Domain Name (FQDN). If a host receives a
negative reply, it knows there are no DNS64 and NAT64 in the network.
If a host receives AAAA reply, it knows the network must be utilizing
IPv6 address synthesis. After receiving a synthesized AAAA Resource
Record, the host may examine the received IPv6 address and use
heuristics, such as "subtracting" the known IPv4 address out of
synthesized IPv6 address, to find out the NSP.
The Well-Known Name may be assigned by IANA or provided some third
party, including application or operating system vendor. The IPv4
address corresponding to the Well-Known Name may be resolved via A
query to Well-Known Name, assigned by IANA, or hard-coded.
4.3.2. Analysis and discussion
The PROs of the proposal are listed below:
+ Can be used to solve Issue #1 and Issue #2.
+ Solves issue #4 via DNS record lifetime.
+ Can partially solve issue #5 if multiple synthetic AAAA records
are included in the response. Can find multiple address formats.
+ Does not necessarily require any standards effort.
+ Does not require host stack or resolver changes. All required
logic and heuristics can be implemented in applications that are
interested in learning about address synthesis taking place.
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+ The solution is backward compatible from 'legacy' hosts and
servers point of view.
+ Hosts or applications interested in learning about synthesis and
the used NSP can do the "discovery" proactively at any time, for
example every time the host attaches to a new network.
+ Does not require explicit support from the network using NAT64
The CONs of the proposal are listed below:
- Requires hosting of a DNS resource record for the Well-Known Name.
- Does not provide solution for issue #3.
- This method is only able to find one NSP even if a network is
utilizing multiple NSPs (issue #5) (unless DNS64 includes multiple
synthetic AAAA records in response).
4.3.3. Summary
This is the only approach that can be deployed without explicit
support from the network or the host. This approach could also
complement explicit methods and be used as a fallback approach when
explicit methods are not supported by an access network.
4.4. DNS Resource Record for IPv4-Embedded IPv6 address
4.4.1. Solution description
Section 4 of [I-D.boucadair-behave-dns-a64] defines a new DNS
Resource Record (A64) that is a record specific to store a single
IPv4-Embedded IPv6 address [RFC6052]. Using a dedicated Resource
Record allows a host to distinguish between real IPv6 addresses and
synthesized IPv6 addresses. The solution requires host to send a
query for an A64 record. Positive answer with A64 record informs the
requesting host that the resolved address is not a native address but
an IPv4-Embedded IPv6 address. This would ease the local policies to
prefer direct communications (i.e., avoid using IPv4-Embedded IPv6
addresses when a native IPv6 address or a native IPv4 address is
available). Applications may be notified via new or modified API.
4.4.2. Analysis and discussion
The PROs of the proposal are listed below:
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+ Can be used to solve Issue #1 and #5.
+ Solves issue #4 via DNS record lifetime.
+ The solution is backward compatible from 'legacy' hosts and
servers point of view.
+ Synthesized addresses can be used in authoritative DNS servers.
+ Maintains the reliability of the DNS model (i.e., a synthesised
IPv6 address is presented as such and not as native IPv6 address).
+ When both IPv4-Converted and native IPv6 addresses are configured
for the same QNAME, native addresses are preferred.
The CONs of the proposal are listed below:
- Does not address Issue #2 or #3 in any way.
- Requires a host resolver changes and a mechanism/additions to the
host resolver API (or flags, hints etc) to deliver a note to the
querying application that the address is synthesized.
- Requires a standardization of a new DNS Resource Record type
(A64), and the implementation of it in both resolvers and servers.
- Requires a coordinated deployment between different flavors of DNS
servers within the provider to work deterministically.
- Additional load the DNS servers (3 Queries, A64, AAAA and A, may
be issued by a dual-stack host).
- Does not help to identify synthesized IPv6 addresses if the
session does not involve any DNS queries.
4.4.3. Summary
While the proposed solution delivers explicit information about
address synthesis taking place solving the Issue #1, a
standardization of a new DNS record type might turn out a too
overwhelming task for a solution for a temporary transition phase.
Defining a new record type increases load towards DNS server as the
host issues parallel A64, AAAA and A queries.
4.5. Learning the IPv6 Prefix of a Network's NAT64 using DNS
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4.5.1. Solution description
Section 4.1 of [I-D.wing-behave-learn-prefix] actually proposes two
DNS-based method for discovering the presence of a DNS64 server and a
NAT64 device, and then a mechanism for discovering the used NSP.
First, a host may learn the presence of a DNS64 server and a NAT64
device, by receiving a TXT Resource Record with a well-known (TBD
IANA registered?) string followed by the NAT64 unicast IPv6 address
and the prefix length. The DNS64 server would add the TXT Resource
Record into the DNS response.
Second, Section 4.1 of [I-D.wing-behave-learn-prefix] also proposes
specifying a new U-NAPTR [RFC4848] application to discover the
NAT64's IPv6 prefix and length. The input domain name is exactly the
same as would be used for a reverse DNS lookup, derived from the
host's IPv6 in the ".ip6.arpa." tree. The host doing the U-NAPTR
queries may need multiple queries until finds the provisioned domain
name with the correct prefix length. The response to a successful
U-NATPR query contains the unicast IPv6 address and the prefix length
of the NAT64 device.
4.5.2. Analysis and discussion
[Editor' note: can this be made to solve issue #5 by having multiple
NSPs in TXT record?]
The PROs of the proposal are listed below:
+ Can be used to solve Issue #1 and Issue #2.
+ Solves issue #4 via DNS record lifetime.
+ Does not require host stack or resolver changes if the required
logic and heuristics is implemented in applications that are
interested in learning about address synthesis taking place.
The CONs of the proposal are listed below:
- Requires standardization of a well-known names from IANA for TXT
Resource Record and/or a standardization of a new U-NAPTR
application.
- Requires a host resolver changes and a mechanism/additions to the
host resolver API (or flags, hints etc) to deliver a note to the
querying application that the address is synthesized and what is
the NSP prefix length. However, it is possible that the U-NAPTR
application logic is completely implemented by the application
itself as noted in PROs list.
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- U-NAPTR prefix learning method may entail multiple queries.
- U-NAPTR prefix learning method requires provisioning of NSPs in
".ip6.arpa." tree.
- RFC5507 [RFC5507] specifically recommends against reusing TXT
Resource Records to expand DNS.
- Requires configuration on the access network's DNS servers.
- Does not provide solution for issue #3.
4.5.3. Summary
The implementation of this solution requires some changes to the
applications and resolvers in a similar fashion as in solutions in
Section 4.1, Section 4.2 and Section 4.4. Unlike the other DNS-based
approaches, the U-NAPTR-based solution also requires provisioning
information into the '.ip6.arpa.' tree which is not any more entirely
internal to the provider hosting the NAT64/DNS64 service.
The iterative approach of learning the NAT64 prefix in U-NAPTR-based
solution may result in multiple DNS queries, which can be considered
more complex and inefficient compared to other DNS-based solutions.
4.6. Learning the IPv6 Prefix of a Network's NAT64 using DHCPv6
4.6.1. Solution description
Two individual drafts specify DHCPv6 based approaches.
Section 4.2 of [I-D.wing-behave-learn-prefix] describes a new DHCPv6
[RFC3315] option (OPTION_AFT_PREFIX_DHCP) that contains the IPv6
unicast prefix, IPv6 ASM prefix, and IPv6 SSM prefix (and their
lengths) for the NAT64.
Section 4 of [I-D.boucadair-dhcpv6-shared-address-option] defines a
DHCPv6 option that can be used to communicate to a requesting host
the prefix used for building IPv4-Converted IPv6 addresses together
with the format type. Provisioning the format type is required so as
to be correctly handled by the NAT64-enabled devices deployed in a
given domain.
4.6.2. Analysis and discussion
The PROs of the proposal are listed below:
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+ Can be used to solve Issue #1, Issue #2, Issue #3 and Issue #4 via
DHCPv6 information lifetime.
+ Does not involve DNS system. Therefore, applications that would
not normally initiate any DNS queries can still learn the NAT64
prefix.
+ DHCPv6 is designed to provide various kinds of configuration
information in a centrally managed fashion.
The CONs of the proposal are listed below:
- Change of NSP requires change to DHCPv6 configuration.
- Requires at least Stateless DHCPv6 client on hosts.
- Requires support on DHCPv6 clients, which is not trivial in all
operating systems.
- The DHCPv6-based solution involves changes and management on
network side nodes that are not really part of the NAT64/DNS64
deployment (or issues caused by their existence).
- A new DHCPv6 option is required and the corresponding changes to
both DHCPv6 clients and servers.
If DHCPv6 would include multiple NSPs issue #5 could be solved as
well, but only if nodes as a group would select different NSPs hence
supporting load-balancing. As this is not clear this item is not yet
listed under PRO nor CON.
4.6.3. Summary
The DHCPv6-based solution would be a good solution in a sense it
hooks into general IP configuration phase, allows easy updates when
configuration information changes and does not involve DNS in
general. Use of DHCPv6 requires configuration changes on DHCPv6
clients and servers and in some cases may also require implementation
changes. Furthermore, it is not obvious that all devices that need
translation services would implement stateless DHCPv6. For example,
cellular 3GPP networks do not mandate hosts or network to implement
or deploy DHCPv6.
4.7. Learning the IPv6 Prefix of a Network's NAT64 using Router
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4.7.1. Solution description
Section 3.3 of [RA-Learn-Prefix] describes a new Router Advertisement
(RA) [RFC4861] option (OPTION_AFT_PREFIX_RA) that contains the IPv6
unicast prefix, IPv6 ASM prefix, and IPv6 SSM prefix (and their
lengths) for the NAT64. The RA option is essentially the same as for
DHCPv6 discussed in Section 4.6.
4.7.2. Analysis and discussion
The PROs of the proposal are listed below:
+ Can be used to solve Issue #1, Issue #2, and Issue #3.
+ Can solve Issue #4 if lifetime information can be communicated.
The CONs of the proposal are listed below:
- Requires configuration and managements of all access routers to
emit correct information in RA. This could, for example, be
accomplished somehow by piggybacking on top of routing protocols
(which would then require enhancements to routing protocols)
- In some operating systems it may not be trivial to transfer
information obtained in RA to upper layers
- Requires changes to host operating system's IP stack
- NSP change requires changes to access router configuration
- Requires standardization of a new option to Router Advertisement
that is generally unfavored approach
- The RA-based solution involves changes and management on network
side nodes that are not really part of the NAT64/DNS64 deployment
(or issues caused by their existence).
If RA would include multiple NSPs issue #5 could be solved as well,
but only if nodes as a group would select different NSPs hence
supporting load-balancing. As this is not clear this item is not yet
listed under PRO nor CON.
4.7.3. Summary
The RA-based solution would be a good solution in a sense it hooks
into general IP configuration phase, allows easy updates when
configuration information changes and does not involve DNS in
general. However, generally introducing any changes to the Neighbor
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Discovery Protocol that are not absolutely necessary are unfavored
due the impact on both network node side and end host IP stack
implementations.
Compared to the DHCPv6 equivalent solution in Section 4.6 the
management overhead is greater with RA-based solution. In case of
DHCPv6-based solution the management can be centralized to few DHCPv6
servers compared to RA-based solution where each access router is
supposed to be configured with the same information.
4.8. Using application layer protocols such as STUN
4.8.1. Solution description
Application layer protocols, such as Session Traversal Utilities for
NAT (STUN) [RFC5389], which define methods for endpoints to learn
their external IP addresses could be used for NAT64 and NSP
discovery. This document focuses on STUN, but the protocol could be
something else as well.
A host must first use DNS to discover IPv6 representation(s) of STUN
server(s) IPv4 address(es), because the host has no way to directly
use IPv4 addresses to contact to STUN server(s).
After learning the IPv6 address of a STUN server the STUN client
sends a request to the STUN server containing new 'SENDING-TO'
attribute that tells to the server the IPv6 address the client sent
the request to. In a reply the server includes another new attribute
called 'RECEIVED-AS', which contains server's IP address the request
arrived on. After receiving the reply the client compares
'SENDING-TO' and 'RECEIVED-AS' attributes to find out an NSP
candidate.
4.8.2. Analysis and discussion
This solution is relatively similar as described in section 4.3, but
instead of using DNS uses STUN to get input for heuristic algorithms.
The PROs of the proposal are listed below:
+ Can be used to solve Issue #1 and Issue #2.
+ Does not require host changes or supportive protocols such as DNS
or DHCPv6. All required logic and heuristics can be implemented
in applications that are interested in learning about address
synthesis taking place.
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+ The solution is backward compatible from 'legacy' hosts and
servers point of view.
+ Hosts or applications interested in learning about synthesis and
the used NSP can do the "discovery" proactively at any time, for
example every time the host attaches to a new network.
+ Does not require explicit support from the network using NAT64.
+ Can possibly be bundled to existing STUN message exchanges as new
attributes and hence client can learn its external IPv4 address
and a NSP/WKP with the same exchange
+ Can be used to confirm the heuristics by synthesizing IPv6 address
of another STUN server or by synthesizing IPv6 address of first
STUN server after host has heuristically determined NSP using
method from section 4.3. I.e. the connectivity test could be done
with STUN.
+ True IPv4 destination address is used in NSP determination instead
of IPv4 address received from DNS. This may increase reliability.
+ The same STUN improvement could also be used to reveal NAT66 on
the data path, if the 'RECEIVED-AS' would contain different IPv6
address than 'SENDING-TO'.
The CONs of the proposal are listed below:
- Requires a server on the network to respond the queries.
- Requires standardization if done as extension to STUN.
- The solution involves changes and management on network side nodes
that are not really part of the NAT64/DNS64 deployment (or issues
caused by their existence).
- Does not solve issue #3 if STUN server's synthetic IPv6 address is
provisioned via DNS.
- Does not solve issue #4 as the STUN server would not be aware of
learned NSP's validity time.
- Does not solve issue #5 as the STUN server would not be aware of
multiple NSP prefixes.
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- Heavyweight solution especially if an application does not
otherwise support STUN.
4.8.3. Summary
The STUN, or similar, protocol based approach is a second way to
solve the problem without explicit access network support. The
heuristics for NSP discovery would still be in the client, however,
the result may be more reliable as actual IPv4 destination address is
compared to IPv6 address used in sending. The additional benefit of
STUN is that the client learns its public IPv4 address with the same
message exchange. STUN could also be used as the connectivity test
tool if the client would first heuristically determine NSP out of DNS
as described in section 4.3, synthesize IPv6 representation of STUN
server's IPv4 address, and then tests connectivity to the STUN
server.
As an additional benefit the STUN improvement could be used for NAT66
discovery.
5. Conclusion
As a general principle we prefer to have as minimal solution as
possible, avoid impacts to entities not otherwise involved in the
protocol translation scheme, minimize host impact, and that requires
minimal to no operational effort on the network side.
Of the different issues we give most weight for issues #1 and #2. We
are not giving much weight for the Issue #3 'DNS should not be
required', as cases where hosts need to synthesize IPv6 addresses but
do does not have DNS available seem rare for us. Even if application
does not otherwise utilize DNS, it ought to be able to trigger simple
DNS query to find out WKP/NSP. Issue #4 is handled by majority of
solutions and Issue #5 is considered to be mostly insignificant as
even if individual hosts would use only one NSP at a time, different
hosts would be using different NSPs, hence supporting load-balancing
targets. None of the discussed solutions support learning of
possible new or indicating support for multiple algorithms for
address synthesis other than the one described in [RFC6052].
The DNS64 entity has to be configured with WKP/NSP in order for it to
do synthetization and hence using DNS also for delivering the
synthetization information sounds logical. The fact that the
synthetization information fate-shares the information received in
the DNS response is a valuable attribute and reduces the possible
distribution of stale prefix information. On the contrary, use of
DHCPv6 would require additional trouble configuring DHCPv6 servers
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and ensuring DHCPv6 clients are in place, and furthermore that the
NAT64, DHCPv6 (and possible even some DNS64) servers are all in sync.
RA-based mechanisms are operationally expensive as configuration
would have to be placed and maintained in the access routers.
Furthermore, both DHCPv6 and RA based mechanisms involve entities
that do not otherwise need to be aware of protocol translation (only
need to know DNS server addresses).
Of the DNS-based mechanisms we favor EDNS0 option due to its
lightweight nature. The A64 and DNS SRV record approaches would
require significant amount of standardization and deployment effort
when compared to the size of the problem. The U-NAPTR-based approach
would require provisioning information into the '.ip6.arpa' tree
which would not be entirely internal for the provider.
The two heuristic-based approaches could be taken into use at once
and would provide benefits in networks utilizing protocol
translation, but on the long run their usefulness depends how well
networks will deploy explicit methods.
Our conclusion is to recommend publishing the Well-Known DNS Name
heuristic-based method as an informational IETF document for
applications and host implementors to implement as-is. If standards
track work is seen beneficial, then our recommendation is the
standardization of ENDS0 option.
6. Security Considerations
The security considerations are essentially similar to what is
described in DNS64 [I-D.ietf-behave-dns64]. Forgery of information
required for IPv6 address synthesis may allow an attacker to insert
itself as middle man or to perform denial-of-service attack. The
DHCPv6 and RA based approaches are vulnerable for the forgery as the
attacker may send forged RAs or act as a rogue DHCPv6 server (unless
DHCPv6 authentication or SEND are used). If the attacker is already
able to modify and forge DNS responses (flags, addresses of know
IPv4-only servers, records, etc), ability to influence local address
synthesis is likely of low additional value. Also, a DNS-based
mechanism is only as secure as the method used to configure the DNS
server's IP addresses on the host. Therefore, if the host cannot
trust e.g. DHCPv6 it cannot trust the DNS server learned via DHCPv6
either, unless the host has a way to authenticate all DNS responses.
7. IANA Considerations
This document is informative and has no actions to IANA.
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8. Contributors
The following people have contributed text to this document.
Mohamed Boucadair
9. Acknowledgements
Authors would like to thank Dan Wing and Christian Huitema especially
for the STUN idea for their valuable comments and discussions.
10. Informative References
[EDNS0-Flag]
Korhonen, J. and T. Savolainen, "EDNS0 Flags Indicating
AAAA Record Synthesis and Format",
draft-korhonen-edns0-synthesis-flag-00 (work in progress),
June 2010.
[I-D.boucadair-behave-dns-a64]
Boucadair, M. and E. Burgey, "A64: DNS Resource Record for
IPv4-Embedded IPv6 Address",
draft-boucadair-behave-dns-a64-02 (work in progress),
September 2010.
[I-D.boucadair-dhcpv6-shared-address-option]
Boucadair, M., Levis, P., Grimault, J., Savolainen, T.,
and G. Bajko, "Dynamic Host Configuration Protocol
(DHCPv6) Options for Shared IP Addresses Solutions",
draft-boucadair-dhcpv6-shared-address-option-01 (work in
progress), December 2009.
[I-D.ietf-behave-dns64]
Bagnulo, M., Sullivan, A., Matthews, P., and I. Beijnum,
"DNS64: DNS extensions for Network Address Translation
from IPv6 Clients to IPv4 Servers",
draft-ietf-behave-dns64-11 (work in progress),
October 2010.
[I-D.ietf-behave-v6v4-framework]
Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
IPv4/IPv6 Translation",
draft-ietf-behave-v6v4-framework-10 (work in progress),
August 2010.
[I-D.ietf-behave-v6v4-xlate-stateful]
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Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers",
draft-ietf-behave-v6v4-xlate-stateful-12 (work in
progress), July 2010.
[I-D.korhonen-edns0-synthesis-flag]
Korhonen, J. and T. Savolainen, "EDNS0 Option for
Indicating AAAA Record Synthesis and Format",
draft-korhonen-edns0-synthesis-flag-01 (work in progress),
September 2010.
[I-D.savolainen-heuristic-nat64-discovery]
Savolainen, T. and J. Korhonen, "Heuristic discovery of
NAT64 and Network-Specific Prefix",
draft-savolainen-heuristic-nat64-discovery-00 (work in
progress), September 2010.
[I-D.venaas-behave-mcast46]
Venaas, S., Asaeda, H., SUZUKI, S., and T. Fujisaki, "An
IPv4 - IPv6 multicast translator",
draft-venaas-behave-mcast46-02 (work in progress),
December 2010.
[I-D.venaas-behave-v4v6mc-framework]
Venaas, S., Li, X., and C. Bao, "Framework for IPv4/IPv6
Multicast Translation",
draft-venaas-behave-v4v6mc-framework-02 (work in
progress), December 2010.
[I-D.wing-behave-learn-prefix]
Wing, D., "Learning the IPv6 Prefix of a Network's IPv6/
IPv4 Translator", draft-wing-behave-learn-prefix-04 (work
in progress), October 2009.
[RA-Learn-Prefix]
Wing, D., Wang, X., and X. Xu, "Learning the IPv6 Prefixes
of an IPv6/IPv4 Translator",
draft-wing-behave-learn-prefix-03 (work in progress),
July 2009.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
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A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4848] Daigle, L., "Domain-Based Application Service Location
Using URIs and the Dynamic Delegation Discovery Service
(DDDS)", RFC 4848, April 2007.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5507] IAB, Faltstrom, P., Austein, R., and P. Koch, "Design
Choices When Expanding the DNS", RFC 5507, April 2009.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
October 2010.
Authors' Addresses
Jouni Korhonen (editor)
Nokia Siemens Networks
Linnoitustie 6
FI-02600 Espoo
Finland
Email: jouni.nospam@gmail.com
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Teemu Savolainen (editor)
Nokia
Hermiankatu 12 D
FI-33720 Tampere
Finland
Email: teemu.savolainen@nokia.com
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