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<pre>Internet Engineering Task Force (IETF) M. Boucadair
Request for Comments: 7225 France Telecom
Category: Standards Track May 2014
ISSN: 2070-1721
<span class="h1">Discovering NAT64 IPv6 Prefixes Using the Port Control Protocol (PCP)</span>
Abstract
This document defines a new Port Control Protocol (PCP) option to
learn the IPv6 prefix(es) used by a PCP-controlled NAT64 device to
build IPv4-converted IPv6 addresses. This option is needed for
successful communications when IPv4 addresses are used in referrals.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="http://www.rfc-editor.org/info/rfc7225">http://www.rfc-editor.org/info/rfc7225</a>.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
<span class="grey">Boucadair Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
Table of Contents
<a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-2">2</a>
<a href="#section-2">2</a>. Requirements Language . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-3">3</a>. Problem Statement . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-3.1">3.1</a>. Issues . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-3.2">3.2</a>. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-3.2.1">3.2.1</a>. AAAA Synthesis by the DNS Stub-resolver . . . . . . . <a href="#page-4">4</a>
<a href="#section-3.2.2">3.2.2</a>. Application Referrals . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-4">4</a>. PREFIX64 Option . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-4.1">4.1</a>. Format . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-4.2">4.2</a>. Server's Behavior . . . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-4.3">4.3</a>. Client's Behavior . . . . . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-5">5</a>. Flow Examples . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.1">5.1</a>. TCP Session Initiated from an IPv6-only Host . . . . . . <a href="#page-10">10</a>
<a href="#section-5.2">5.2</a>. SIP Flow Example . . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
<a href="#section-5.3">5.3</a>. Mapping of IPv4 Address Ranges to IPv6 Prefixes . . . . . <a href="#page-13">13</a>
<a href="#section-6">6</a>. IANA Considerations . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-7">7</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-8">8</a>. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-9">9</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-9.1">9.1</a>. Normative References . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-9.2">9.2</a>. Informative References . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
According to [<a href="./rfc6146" title=""Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers"">RFC6146</a>], NAT64 uses Pref64::/n to construct
IPv4-converted IPv6 addresses as defined in [<a href="./rfc6052" title=""IPv6 Addressing of IPv4/IPv6 Translators"">RFC6052</a>].
This document defines a new Port Control Protocol (PCP) option
[<a href="./rfc6887" title=""Port Control Protocol (PCP)"">RFC6887</a>] to inform PCP clients about the Pref64::/n and suffix
[<a href="./rfc6052" title=""IPv6 Addressing of IPv4/IPv6 Translators"">RFC6052</a>] used by a PCP-controlled NAT64 device [<a href="./rfc6146" title=""Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers"">RFC6146</a>]. It does
so by defining a new PREFIX64 option.
This PCP option is a deterministic solution to help establish
communications between IPv6-only hosts and remote IPv4-only hosts.
Unlike [<a href="./rfc7050" title=""Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis"">RFC7050</a>], this option solves all the issues identified in
[<a href="./rfc7051" title=""Analysis of Solution Proposals for Hosts to Learn NAT64 Prefix"">RFC7051</a>].
Some illustrative examples are provided in <a href="#section-5">Section 5</a>. Detailed
experiments conducted to assess the applicability of the PREFIX64
option for services (e.g., accessing a video server, establishing
SIP-based sessions, etc.) in NAT64 environments are available in
[<a href="#ref-EXPERIMENTS">EXPERIMENTS</a>].
The use of this PCP option for NAT64 load-balancing purposes is out
of scope.
<span class="grey">Boucadair Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Requirements Language</span>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Problem Statement</span>
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. Issues</span>
This document proposes a deterministic solution to solve the
following issues:
o Learn the Pref64::/n used by an upstream NAT64 function. This is
needed to help:
* distinguish between IPv4-converted IPv6 addresses [<a href="./rfc6052" title=""IPv6 Addressing of IPv4/IPv6 Translators"">RFC6052</a>] and
native IPv6 addresses.
* implement IPv6 address synthesis for applications not relying
on DNS (where DNS64 [<a href="./rfc6147" title=""DNS64: DNS Extensions for Network Address Translation from IPv6 Clients to IPv4 Servers"">RFC6147</a>] would provide the synthesis).
o Avoid stale Pref64::/n values.
o Discover multiple Pref64::/n values when multiple prefixes exist
in a network.
o Use DNSSEC ([<a href="./rfc4033" title=""DNS Security Introduction and Requirements"">RFC4033</a>], [<a href="./rfc4034" title=""Resource Records for the DNS Security Extensions"">RFC4034</a>], [<a href="./rfc4035" title=""Protocol Modifications for the DNS Security Extensions"">RFC4035</a>]) in the presence of
NAT64.
o Discover the suffix used by a NAT64 function when non-null
suffixes are in use (e.g., checksum neutral suffix).
o Support destination-based Pref64::/n (e.g., <a href="./rfc7050#section-5.1">Section 5.1 of
[RFC7050]</a>).
o Associate a Pref64::/n with a given NAT64 when distinct prefixes
are configured for each NAT64 enabled in a network.
A more extensive discussion can be found at [<a href="./rfc7051" title=""Analysis of Solution Proposals for Hosts to Learn NAT64 Prefix"">RFC7051</a>].
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. Use Cases</span>
This section provides some use cases to illustrate the problem space.
More details can be found at <a href="./rfc7051#section-4">Section 4 of [RFC7051]</a>.
<span class="grey">Boucadair Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
<span class="h4"><a class="selflink" id="section-3.2.1" href="#section-3.2.1">3.2.1</a>. AAAA Synthesis by the DNS Stub-Resolver</span>
The option defined in this document can be used for hosts with DNS64
capability [<a href="./rfc6147" title=""DNS64: DNS Extensions for Network Address Translation from IPv6 Clients to IPv4 Servers"">RFC6147</a>] added to the host's stub-resolver.
The stub resolver on the host will try to obtain (native) AAAA
records, and if they are not found, the DNS64 function on the host
will query for A records and then synthesize AAAA records. Using the
PREFIX64 PCP extension, the host's stub-resolver can learn the prefix
used for IPv6/IPv4 translation and synthesize AAAA records
accordingly.
Because synthetic AAAA records cannot be successfully validated in a
host, learning the Pref64::/n used to construct IPv4-converted IPv6
addresses allows the use of DNSSEC. As discussed in <a href="./rfc6147#section-5.5">Section 5.5 of
[RFC6147]</a>, a security-aware and validating host has to perform the
DNS64 function locally.
<span class="h4"><a class="selflink" id="section-3.2.2" href="#section-3.2.2">3.2.2</a>. Application Referrals</span>
As discussed in [<a href="#ref-REF-OBJECT">REF-OBJECT</a>], a frequently occurring situation is
that one entity A connected to a network needs to inform another
entity B how to reach either A itself or some third-party entity C.
This is known as address referral.
In the particular context of NAT64 [<a href="./rfc6146" title=""Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers"">RFC6146</a>], applications relying on
address referral will fail because an IPv6-only client won't be able
to make use of an IPv4 address received in a referral. A non-
exhaustive list of such applications is provided below:
o In SIP environments [<a href="./rfc3261" title=""SIP: Session Initiation Protocol"">RFC3261</a>], the SDP part ([<a href="./rfc4566" title=""SDP: Session Description Protocol"">RFC4566</a>]) of
exchanged SIP messages includes information required for
establishing RTP sessions (namely, IP address and port number).
When a NAT64 is involved in the path, an IPv6-only SIP User Agent
(UA) that receives an SDP offer/answer containing an IPv4 address
cannot send media streams to the remote endpoint.
o An IPv6-only WebRTC (Web Real-Time Communication [<a href="#ref-WebRTC" title=""Overview: Real Time Protocols for Brower- based Applications"">WebRTC</a>]) agent
cannot make use of an IPv4 address received in referrals to
establish a successful session with a remote IPv4-only WebRTC
agent.
o BitTorrent is a distributed file-sharing infrastructure that is
based on peer-to-peer (P2P) techniques for exchanging files
between connected users. To download a given file, a BitTorrent
client needs to obtain the corresponding torrent file. Then, it
connects to a tracker to retrieve a list of "leechers" (clients
that are currently downloading the file but do not yet possess all
<span class="grey">Boucadair Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
portions of the file) and "seeders" (clients that possess all
portions of the file and are uploading them to other requesting
clients). The client connects to those machines and downloads the
available portions of the requested file. In the presence of an
address-sharing function (see <a href="./rfc6269#appendix-A">Appendix A of [RFC6269]</a>), some
encountered issues are solved if PCP is enabled (see
[<a href="#ref-PCP-BITTORRENT">PCP-BITTORRENT</a>]). Nevertheless, an IPv6-only client cannot
connect to a remote IPv4-only machine even if the base PCP
protocol is used.
Learning the Pref64::/n solves the issues listed above.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. PREFIX64 Option</span>
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. Format</span>
The format of the PREFIX64 option is depicted in Figure 1. This
option follows the guidelines specified in <a href="./rfc6887#section-7.3">Section 7.3 of
[RFC6887]</a>.
This option allows the mapping of specific IPv4 address ranges
(contained in the IPv4 Prefix List) to separate Pref64::/n
prefixes as discussed in [<a href="./rfc6147" title=""DNS64: DNS Extensions for Network Address Translation from IPv6 Clients to IPv4 Servers"">RFC6147</a>].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Option Code=129| Reserved | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix64 Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
: Prefix64 (Variable) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Suffix (Variable) :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (optional) |
: IPv4 Prefix List (Variable) :
| (See Figure 2) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Prefix64 PCP Option
<span class="grey">Boucadair Standards Track [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
The description of the fields is as follows:
o Option Code: 129
o Reserved: This field is initialized as specified in <a href="./rfc6887#section-7.3">Section 7.3 of
[RFC6887]</a>.
o Option Length: Indicates in octets the length of the enclosed
data.
o Prefix64 Length: Indicates in octets the length of the Pref64::/n.
The allowed values are specified in [<a href="./rfc6052" title=""IPv6 Addressing of IPv4/IPv6 Translators"">RFC6052</a>] (i.e., 4, 5, 6, 7,
8, or 12).
o Prefix64: This field identifies the IPv6 unicast prefix to be used
for constructing an IPv4-converted IPv6 address from an IPv4
address as specified in <a href="./rfc6052#section-2.2">Section 2.2 of [RFC6052]</a>. This prefix can
be the Well-Known Prefix (i.e., 64:ff9b::/96) or a Network-
Specific Prefix. The address synthesis MUST follow the guidelines
documented in [<a href="./rfc6052" title=""IPv6 Addressing of IPv4/IPv6 Translators"">RFC6052</a>].
o Suffix: The length of this field is (12 - Prefix64 Length) octets.
This field identifies the suffix to be used for constructing an
IPv4-converted IPv6 address from an IPv4 address as specified in
<a href="./rfc6052#section-2.2">Section 2.2 of [RFC6052]</a>. No suffix is included if a /96 Prefix64
is conveyed in the option.
o IPv4 Prefix List: This is an optional field. The format of the
IPv4 Prefix List field is shown in Figure 2. This field may be
included by a PCP server to solve the destination-dependent
Pref64::/n discovery problem discussed in <a href="./rfc7050#section-5.1">Section 5.1 of
[RFC7050]</a>.
* IPv4 Prefix Count: indicates the number of IPv4 prefixes
included in the option. "IPv4 Prefix Count" field MUST be set
to 0 in a request and MUST be set to the number of included
IPv4 subnets in a response.
* An IPv4 prefix is represented as "IPv4 Address/IPv4 Prefix
Length" [<a href="./rfc4632" title=""Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan"">RFC4632</a>]. For example, to encode 192.0.2.0/24, "IPv4
Prefix Length" field is set to 24 and "IPv4 Address" field is
set to 192.0.2.0. If a Pref64::/n is configured for all IPv4
addresses, a wildcard IPv4 prefix (i.e., 0.0.0.0/0) may be
returned in the response together with the configured
Pref64::/n. If no IPv4 Prefix List is returned in a PREFIX64
option, the PCP client assumes the prefix is valid for any
destination IPv4 address. Valid IPv4 prefixes are listed in
<a href="./rfc4632#section-3.1">Section 3.1 of [RFC4632]</a>.
<span class="grey">Boucadair Standards Track [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Prefix Count | IPv4 Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Address (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Prefix Length | IPv4 Address (32 bits)... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... IPv4 Address (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format of IPv4 Prefix List field
Option Name: PREFIX64
Value: 129
Purpose: Learn the prefix used by the NAT64 to build
IPv4-converted IPv6 addresses. This is used by a host for
local address synthesis (e.g., when an IPv4 address is present
in referrals).
Valid for Opcodes: MAP, ANNOUNCE
Length: Variable
May appear in: request, response.
Maximum occurrences: 1 for a request. As many as fit within the
maximum PCP message size for a response.
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. Server's Behavior</span>
The PCP server controlling a NAT64 SHOULD be configured to return to
requesting PCP clients the value of the Pref64::/n and suffix used to
build IPv4-converted IPv6 addresses. When enabled, the PREFIX64
option conveys the value of the Pref64::/n and configured suffix. If
no suffix is explicitly configured to the PCP server, the null suffix
is used as the default value (see <a href="./rfc6052#section-2.2">Section 2.2 of [RFC6052]</a>).
If the PCP server is configured to honor the PREFIX64 option but no
Pref64::/n is explicitly configured, the PCP server MUST NOT include
any PREFIX64 option in its PCP messages.
<span class="grey">Boucadair Standards Track [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
The PCP server controlling a NAT64 MAY be configured to include a
PREFIX64 option in all MAP responses even if the PREFIX64 option is
not listed in the associated request. The PCP server controlling a
NAT64 MAY be configured to include a PREFIX64 option in its ANNOUNCE
messages.
The PCP server MAY be configured with a list of destination IPv4
prefixes associated with a Pref64::/n. This list is then included by
the PCP server in a PREFIX64 option sent to PCP clients.
The PCP server MAY be configured to return multiple PREFIX64 options
in the same message to the PCP client. In such case, the server does
the following:
o If no destination IPv4 prefix list is configured, the PCP server
includes in the first PREFIX64 option, which appears in the PCP
message it sends to the PCP client, the prefix and suffix to
perform local IPv6 address synthesis [<a href="./rfc6052" title=""IPv6 Addressing of IPv4/IPv6 Translators"">RFC6052</a>]. Additional
PREFIX64 options convey any other Pref64::/n values configured.
Returning these prefixes allows an end host to identify all
synthesized IPv6 addresses in a network; the host can prefer IPv4
or another network interface instead in order to avoid any NAT64
deployed in the network. The PCP server is required to
disambiguate prefixes used for IPv6 address synthesis and other
prefixes used to avoid any NAT64 deployed in the network. The PCP
server can be configured with a customized IPv6 prefix list (i.e.,
specific to a PCP client or a group of PCP clients) or system-wide
IPv6 prefix list (i.e., the same list is returned for any PCP
client). Note, it is NOT RECOMMENDED to include PREFIX64 options
in ANNOUNCE messages if a customized IPv6 prefix list is
configured to the PCP server.
o If IPv4 prefix lists are configured, the PCP server includes in
the first PREFIX64 options the Pref64::/n and suffix that are
associated with an IPv4 prefix list (i.e., each of these PREFIX64
options conveys a distinct Pref64::/n together with an IPv4 prefix
list). Additional PREFIX64 options convey any other Pref64::/n
values configured (i.e., the remaining Pref64::/n values not
mapped to any IPv4 prefix list).
If a distinct Pref64::/n or suffix is configured to the PCP-
controlled NAT64 device, the PCP server SHOULD issue an unsolicited
PCP ANNOUNCE message to inform the PCP client about the new
Pref64::/n and/or suffix.
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<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>. Client's Behavior</span>
The PCP client includes a PREFIX64 option in a MAP or ANNOUNCE
request to learn the IPv6 prefix and suffix used by an upstream PCP-
controlled NAT64 device. When enclosed in a PCP request, the
Prefix64 MUST be set to ::/96. The PREFIX64 option can be inserted
in a MAP request used to learn the external IP address as detailed in
<a href="./rfc6887#section-11.6">Section 11.6 of [RFC6887]</a>.
The PCP client MUST be prepared to receive multiple prefixes (e.g.,
if several PCP servers are deployed and each of them is configured
with a distinct Pref64::/n). The PCP client MUST associate each
received Pref64::/n and suffix with the PCP server from which the
Pref64::/n and suffix information was retrieved.
If the PCP client fails to contact a given PCP server, the PCP client
SHOULD clear the prefix(es) and suffix(es) it learned from that PCP
server. For example, a PCP client may fail to contact a PCP server
if the host embedding the PCP client moves to a new network or if
that PCP server is out of service. The use of these stale prefixes
is not recommended to build an IPv4-converted IPv6 address because
failures are likely to be encountered (see <a href="./rfc7051#section-3">[RFC7051], Section 3</a>,
Issue #4).
If the PCP client receives a PREFIX64 option that includes an invalid
IPv4 prefix, the PCP client ignores that IPv4 prefix. If one or more
valid IPv4 prefixes and/or IPv6 prefixes and suffixes are present,
the PCP client uses them.
Upon receipt of the message from the PCP server, the PCP client
replaces any old prefix(es)/suffix(es) received from the same PCP
server with the new one(s) included in the PREFIX64 option(s). If no
PREFIX64 option includes a destination IPv4 prefix list, the host
embedding the PCP client uses the prefix/suffix included in the first
PREFIX64 option for local address synthesis. Other prefixes learned
can be used by the host to avoid any NAT64 deployed in the network.
If one or multiple received PREFIX64 options contain a destination
IPv4 prefix list, the PCP client MUST associate the included IPv4
prefixes with the Pref64::/n and the suffix indicated in the same
PREFIX64 option. In such case, the host embedding the PCP client
MUST enforce a destination-based prefix Pref64::/n selection for
local address synthesis purposes. How the content of the PREFIX64
option(s) is passed to the OS is implementation specific.
Upon receipt of an unsolicited PCP ANNOUNCE message, the PCP client
replaces the old prefix/suffix received from the same PCP server with
the new Pref64::/n and suffix included in the PREFIX64 option.
<span class="grey">Boucadair Standards Track [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Flow Examples</span>
This section provides a non-normative description of use cases
relying on the PREFIX64 option.
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. TCP Session Initiated from an IPv6-Only Host</span>
The usage shown in Figure 3 depicts a typical usage of the PREFIX64
option when a DNS64 capability is embedded in the host.
In the example shown in Figure 3, once the IPv6-only client discovers
the IPv4 address of the remote IPv4-only server (e.g., using DNS), it
retrieves the Pref64::/n (i.e., 2001:db8:122:300::/56) to be used to
build an IPv4-converted IPv6 address for that server. This retrieval
is achieved using the PREFIX64 option (Steps (a) and (b)). The
client then uses 2001:db8:122:300::/56 to construct an IPv6 address
and then initiates a TCP connection (Steps (1) to (4)).
+---------+ +-----+ +---------+
|IPv6-only| |NAT64| |IPv4-only|
| Client | | | | Server |
+---------+ +-----+ +---------+
| | |
| (a) PCP MAP Request | |
| PREFIX64 | |
|======================>| |
| (b) PCP MAP Response | |
| PREFIX64 = | |
| 2001:db8:122:300::/56 | |
|<======================| |
| (1) TCP SYN | (2) TCP SYN |
|======================>|====================>|
| (4) TCP SYN/ACK | (3) TCP SYN/ACK |
|<======================|<====================|
| (5) TCP ACK | (6) TCP ACK |
|======================>|====================>|
| | |
Note: The DNS exchange to retrieve the IPv4 address of
the IPv4-only Server is not shown in the figure.
Figure 3: Example of a TCP Session Initiated from an IPv6-Only Host
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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. SIP Flow Example</span>
Figure 4 shows an example of the use of the option defined in <a href="#section-4">Section</a>
<a href="#section-4">4</a> in a SIP context. In order for RTP/RTCP flows to be exchanged
between an IPv6-only SIP UA and an IPv4-only UA without requiring any
ALG (Application Level Gateway) at the NAT64 or any particular
function at the IPv4-only SIP Proxy Server (e.g., hosted NAT
traversal [<a href="#ref-LATCHING" title=""Latching: Hosted NAT Traversal (HNT) for Media in Real-Time Communication"">LATCHING</a>]), the PORT_SET option [<a href="#ref-PORT-SET" title=""Port Control Protocol (PCP) Extension for Port Set Allocation"">PORT-SET</a>] is used in
addition to the PREFIX64 option.
In steps (a) and (b), the IPv6-only SIP UA retrieves a pair of ports
to be used for RTP/RTCP sessions, the external IPv4 address and the
Pref64::/n to build IPv4-embedded IPv6 addresses. This is achieved
by issuing a MAP request that includes a PREFIX64 option and a
PORT_SET option. A pair of ports (i.e., port_X/port_X+1) and an
external IPv4 address (together with a Pref64::/n, i.e.,
2001:db8:122::/48) are then returned by the PCP server to the
requesting PCP client.
The returned external IPv4 address and external port numbers are used
by the IPv6-only SIP UA to build its SDP offer, which contains
exclusively IPv4 addresses. (Especially in the "c=" line, the port
indicated for the media port is the external port assigned by the PCP
server.) The INVITE request including the SDP offer is then
forwarded by the NAT64 to the Proxy Server, which will relay it to
the called party, i.e., the IPv4-only SIP UA (Steps (1) to (3)).
The remote IPv4-only SIP UA accepts the offer and sends back its SDP
answer in a "200 OK" message that is relayed by the SIP Proxy Server
and NAT64 until being delivered to the IPv6-only SIP UA (Steps (4) to
(6)).
The Pref64::/n (2001:db8:122::/48) is used by the IPv6-only SIP UA to
construct a corresponding IPv6 address of the IPv4 address enclosed
in the SDP answer made by the IPv4-only SIP UA (Step (6)).
The IPv6-only SIP UA and IPv4-only SIP UA are then able to exchange
RTP/RTCP flows without requiring any ALG at the NAT64 or any special
function at the IPv4-only SIP Proxy Server.
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+---------+ +-----+ +------------+ +---------+
|IPv6-only| |NAT64| | IPv4 SIP | |IPv4-only|
| SIP UA | | | |Proxy Server| | SIP UA |
+---------+ +-----+ +------------+ +---------+
| (a) PCP MAP Request | | |
| PORT_SET | | |
| PREFIX64 | | |
|======================>| | |
| (b) PCP MAP Response | | |
| PORT_SET | | |
| PREFIX64: | | |
| 2001:db8:122::/48 | | |
|<======================| | |
| (1) SIP INVITE | (2) SIP INVITE | (3) SIP INVITE |
|======================>|===============>|================>|
| (6) SIP 200 OK | (5) SIP 200 OK | (4) SIP 200 OK |
|<======================|<===============|<================|
| (7) SIP ACK | (8) SIP ACK | (9) SIP ACK |
|======================>|===============>|================>|
| | | |
|src port: dst port:|src port: dst port:|
|port_A port_B|port_X port_B|
|<======IPv6 RTP=======>|<============IPv4 RTP============>|
|<===== IPv6 RTCP======>|<============IPv4 RTCP===========>|
|src port: dst port:|src port: dst port:|
|port_A+1 port_B+1|port_X+1 port_B+1|
| | |
Figure 4: Example of IPv6 to IPv4 SIP-Initiated Session
When the session is initiated from the IPv4-only SIP UA (see Figure
5), the IPv6-only SIP UA retrieves a pair of ports to be used for the
RTP/RTCP session, the external IPv4 address and the Pref64::/n to
build IPv4-converted IPv6 addresses (Steps (a) and (b)). These two
steps could instead be delayed until the INVITE message is received
(Step (3)).
The retrieved IPv4 address and port numbers are used to build the SDP
answer in Step (4), while the Pref64::/n is used to construct an IPv6
address corresponding to the IPv4 address enclosed in the SDP offer
made by the IPv4-only SIP UA (Step (3)). RTP/RTCP flows are then
exchanged between the IPv6-only SIP UA and the IPv4-only UA without
requiring any ALG at the NAT64 or any special function at the
IPv4-only SIP Proxy Server.
<span class="grey">Boucadair Standards Track [Page 12]</span></pre>
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+---------+ +-----+ +------------+ +---------+
|IPv6-only| |NAT64| | IPv4 SIP | |IPv4-only|
| SIP UA | | | |Proxy Server| | SIP UA |
+---------+ +-----+ +------------+ +---------+
| (a) PCP MAP Request | | |
| PORT_SET | | |
| PREFIX64 | | |
|======================>| | |
| (b) PCP MAP Response | | |
| PORT_SET | | |
| PREFIX64: | | |
| 2001:db8:122::/48 | | |
|<======================| | |
| (3) SIP INVITE | (2) SIP INVITE | (1) SIP INVITE |
|<======================|<===============|<================|
| (4) SIP 200 OK | (5) SIP 200 OK | (6) SIP 200 OK |
|======================>|===============>|================>|
| (9) SIP ACK | (8) SIP ACK | (7) SIP ACK |
|<======================|<===============|<================|
| | | |
|src port: dst port:|src port: dst port:|
|port_a port_b|port_Y port_b|
|<======IPv6 RTP=======>|<============IPv4 RTP============>|
|<===== IPv6 RTCP======>|<============IPv4 RTCP===========>|
|src port: dst port:|src port: dst port:|
|port_a+1 port_b+1|port_Y+1 port_b+1|
| | |
Figure 5: Example of IPv4 to IPv6 SIP-Initiated Session
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. Mapping of IPv4 Address Ranges to IPv6 Prefixes</span>
Figure 6 shows an example of a NAT64 configured with two Pref64::/n
values; each of these Pref64::/n values is associated with a distinct
IPv4 address range:
o 192.0.2.0/24 is mapped to 2001:db8:122:300::/56.
o 198.51.100.0/24 is mapped to 2001:db8:122::/48.
Once the IPv6-only client discovers the IPv4 address of the remote
IPv4-only server (i.e., 198.51.100.1), it retrieves two IPv6 prefixes
to be used to build an IPv4-converted IPv6 addresses. This retrieval
is achieved using two PREFIX64 options (Step (b)).
<span class="grey">Boucadair Standards Track [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
Because 198.51.100.1 matches the destination prefix 198.51.100.0/24,
the client uses the associated Pref64::/n (i.e., 2001:db8:122::/48)
to construct an IPv6 address for that IPv4-only server, and then it
initiates a TCP connection (Steps (1) to (6)).
+---------+ +-----+ +---------+
|IPv6-only| |NAT64| |IPv4-only|
| Client | | | | Server |
+---------+ +-----+ +---------+
| | 198.51.100.1
| (a) PCP MAP Request | |
| PREFIX64 | |
|=================================>| |
| (b) PCP MAP Response | |
|PREFIX64{ | |
| Pref64::/n =2001:db8:122:300::/56| |
| IPv4 Prefix=192.0.2.0/24} | |
|PREFIX64{ | |
| Pref64::/n =2001:db8:122::/48 | |
| IPv4 Prefix=198.51.100.0/24} | |
|<=================================| |
| (1) TCP SYN | (2) TCP SYN |
|=================================>|====================>|
| (4) TCP SYN/ACK | (3) TCP SYN/ACK |
|<=================================|<====================|
| (5) TCP ACK | (6) TCP ACK |
|=================================>|====================>|
| | |
Note: The DNS exchange to retrieve the IPv4 address of
the IPv4-only Server is not shown in the figure.
Figure 6: Mapping of IPv4 Address Ranges to IPv6 Prefixes
A similar behavior is to be experienced if these Pref64::/n values
and associated IPv4 prefix lists are configured to distinct NAT64
devices.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. IANA Considerations</span>
The following PCP Option Code has been allocated in the optional-to-
process range (the registry is maintained in
<a href="http://www.iana.org/assignments/pcp-parameters">http://www.iana.org/assignments/pcp-parameters</a>):
PREFIX64 set to 129 (see <a href="#section-4.1">Section 4.1</a>)
<span class="grey">Boucadair Standards Track [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Security Considerations</span>
PCP-related security considerations are discussed in [<a href="./rfc6887" title=""Port Control Protocol (PCP)"">RFC6887</a>].
As discussed in [<a href="./rfc6147" title=""DNS64: DNS Extensions for Network Address Translation from IPv6 Clients to IPv4 Servers"">RFC6147</a>], if an attacker can manage to change the
Pref64::/n used by the DNS64 function, the traffic generated by the
host that receives the synthetic reply will be delivered to the
altered Pref64. This can result in either a denial-of-service (DoS)
attack, a flooding attack, or a man-in-the-middle (MITM) attack.
This attack could be achieved either by altering PCP messages issued
by a legitimate PCP server or by using a fake PCP server.
Means to defend against attackers who can modify packets between the
PCP server and the PCP client, or who can inject spoofed packets that
appear to come from a legitimate PCP server, SHOULD be enabled. In
some deployments, access control lists (ACLs) can be installed on the
PCP client, PCP server, and the network between them, so those ACLs
allow only communications from a trusted PCP server to the PCP
client.
PCP server discovery is out of scope for this document. It is the
responsibility of documents about PCP server discovery to elaborate
on the security considerations to discover a legitimate PCP server.
Learning a Pref64::/n via PCP allows using DNSSEC in the presence of
NAT64. As such, NAT64 with DNSSEC and PCP is better than no DNSSEC
at all, but it is less safe than DNSSEC without DNS64/NAT64 and PCP.
The best mitigation action against Pref64::/n discovery attacks is
thus to add IPv6 support in all endpoints and hence reduce the need
to perform IPv6 address synthesis.
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Acknowledgements</span>
Many thanks to S. Perreault, R. Tirumaleswar, T. Tsou, D. Wing, J.
Zhao, R. Penno, I. van Beijnum, T. Savolainen, S. Savikumar, D.
Thaler, T. Lemon, S. Hanna, P. Resnick, R. Sparks, S. Farrell, and W.
Cui for their comments and suggestions.
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. References</span>
<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>. Normative References</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC4632">RFC4632</a>] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", <a href="https://www.rfc-editor.org/bcp/bcp122">BCP 122</a>, <a href="./rfc4632">RFC 4632</a>, August 2006.
<span class="grey">Boucadair Standards Track [Page 15]</span></pre>
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<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
[<a id="ref-RFC6052">RFC6052</a>] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", <a href="./rfc6052">RFC 6052</a>,
October 2010.
[<a id="ref-RFC6146">RFC6146</a>] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", <a href="./rfc6146">RFC 6146</a>, April 2011.
[<a id="ref-RFC6147">RFC6147</a>] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", <a href="./rfc6147">RFC 6147</a>,
April 2011.
[<a id="ref-RFC6887">RFC6887</a>] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", <a href="./rfc6887">RFC 6887</a>, April
2013.
<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>. Informative References</span>
[<a id="ref-PCP-BITTORRENT">PCP-BITTORRENT</a>]
Boucadair, M., Zheng, T., Deng, X., and J. Queiroz,
"Behavior of BitTorrent service in PCP-enabled networks
with Address Sharing", Work in Progress, May 2012.
[<a id="ref-EXPERIMENTS">EXPERIMENTS</a>]
Abdesselam, M., Boucadair, M., Hasnaoui, A., and J.
Queiroz, "PCP NAT64 Experiments", Work in Progress,
September 2012.
[<a id="ref-REF-OBJECT">REF-OBJECT</a>]
Carpenter, B., Jiang, S., and Z. Cao, "Problem Statement
for Referral", Work in Progress, February 2011.
[<a id="ref-LATCHING">LATCHING</a>] Ivov, E., Kaplan, H., and D. Wing, "Latching: Hosted NAT
Traversal (HNT) for Media in Real-Time Communication",
Work in Progress, May 2014.
[<a id="ref-PORT-SET">PORT-SET</a>] Qiong, Q., Boucadair, M., Sivakumar, S., Zhou, C., Tsou,
T., and S. Perreault, "Port Control Protocol (PCP)
Extension for Port Set Allocation", Work in Progress,
November 2013.
[<a id="ref-WebRTC">WebRTC</a>] Alvestrand, H., "Overview: Real Time Protocols for Brower-
based Applications", Work in Progress, February 2014.
<span class="grey">Boucadair Standards Track [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc7225">RFC 7225</a> PCP & NAT64 May 2014</span>
[<a id="ref-RFC3261">RFC3261</a>] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", <a href="./rfc3261">RFC 3261</a>,
June 2002.
[<a id="ref-RFC4033">RFC4033</a>] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", <a href="./rfc4033">RFC</a>
<a href="./rfc4033">4033</a>, March 2005.
[<a id="ref-RFC4034">RFC4034</a>] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
<a href="./rfc4034">RFC 4034</a>, March 2005.
[<a id="ref-RFC4035">RFC4035</a>] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", <a href="./rfc4035">RFC 4035</a>, March 2005.
[<a id="ref-RFC4566">RFC4566</a>] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", <a href="./rfc4566">RFC 4566</a>, July 2006.
[<a id="ref-RFC6269">RFC6269</a>] Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
Roberts, "Issues with IP Address Sharing", <a href="./rfc6269">RFC 6269</a>, June
2011.
[<a id="ref-RFC7050">RFC7050</a>] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
the IPv6 Prefix Used for IPv6 Address Synthesis", <a href="./rfc7050">RFC</a>
<a href="./rfc7050">7050</a>, November 2013.
[<a id="ref-RFC7051">RFC7051</a>] Korhonen, J. and T. Savolainen, "Analysis of Solution
Proposals for Hosts to Learn NAT64 Prefix", <a href="./rfc7051">RFC 7051</a>,
November 2013.
Author's Address
Mohamed Boucadair
France Telecom
Rennes 35000
France
EMail: mohamed.boucadair@orange.com
Boucadair Standards Track [Page 17]
</pre>
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