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IETF RFC 7857
Last modified on Friday, April 29th, 2016
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Internet Engineering Task Force (IETF) R. Penno
Request for Comments: 7857 Cisco
BCP: 127 S. Perreault
Updates: 4787, 5382, 5508 Jive Communications
Category: Best Current Practice M. Boucadair, Ed.
ISSN: 2070-1721 Orange
S. Sivakumar
Cisco
K. Naito
NTT
April 2016
Updates to Network Address Translation (NAT) Behavioral Requirements
 Abstract
This document clarifies and updates several requirements of RFCs
4787, 5382, and 5508 based on operational and development experience.
The focus of this document is Network Address Translation from IPv4
to IPv4 (NAT44).
This document updates RFCs 4787, 5382, and 5508.
Status of This Memo
This memo documents an Internet Best Current Practice.
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
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7857.
Penno, et al. Best Current Practice PAGE 1 
RFC 7857 Updates to NAT Behavioral Requirements April 2016
Copyright Notice
Copyright (c) 2016 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) 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.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Penno, et al. Best Current Practice PAGE 2
RFC 7857 Updates to NAT Behavioral Requirements April 2016
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. TCP Session Tracking . . . . . . . . . . . . . . . . . . . . 4
2.1. TCP Transitory Connection Idle-Timeout . . . . . . . . . 6
2.2. TCP RST . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Port Overlapping Behavior . . . . . . . . . . . . . . . . . . 6
4. Address Pooling Paired (APP) . . . . . . . . . . . . . . . . 7
5. Endpoint-Independent Mapping (EIM) Protocol Independence . . 8
6. Endpoint-Independent Filtering (EIF) Protocol Independence . 8
7. Endpoint-Independent Filtering (EIF) Mapping Refresh . . . . 8
7.1. Outbound Mapping Refresh and Error Packets . . . . . . . 9
8. Port Parity . . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Port Randomization . . . . . . . . . . . . . . . . . . . . . 9
10. IP Identification (IP ID) . . . . . . . . . . . . . . . . . . 10
11. ICMP Query Mappings Timeout . . . . . . . . . . . . . . . . . 10
12. Hairpinning Support for ICMP Packets . . . . . . . . . . . . 10
13. Security Considerations . . . . . . . . . . . . . . . . . . . 11
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
14.1. Normative References . . . . . . . . . . . . . . . . . . 12
14.2. Informative References . . . . . . . . . . . . . . . . . 12
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
[RFC 4787], [RFC 5382], and [RFC 5508] contributed to enhance Network
Address Translation (NAT) interoperability and conformance.
Operational experience gained through widespread deployment and
evolution of NAT indicates that some areas of the original documents
need further clarification or updates. This document provides such
clarifications and updates.
1.1. Scope
The goal of this document is to clarify and update the set of
requirements listed in [RFC 4787], [RFC 5382], and [RFC 5508]. The
document focuses exclusively on NAT44.
The scope of this document has been set so that it does not create
new requirements beyond those specified in the documents cited above.
Requirements related to Carrier-Grade NAT (CGN) are defined in
[RFC 6888].
Penno, et al. Best Current Practice PAGE 3
RFC 7857 Updates to NAT Behavioral Requirements April 2016
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
The reader is assumed to be familiar with the terminology defined in
[RFC 2663], [RFC 4787], [RFC 5382], and [RFC 5508].
In this document, the term "NAT" refers to both "Basic NAT" and
"Network Address/Port Translator (NAPT)" (see Section 3 of
[RFC 4787]). As a reminder, Basic NAT and NAPT are two variations of
traditional NAT in that translation in Basic NAT is limited to IP
addresses alone, whereas translation in NAPT is extended to include
IP addresses and transport identifiers (such as a TCP/UDP port or
ICMP query ID); refer to Section 2 of [RFC 3022].
2. TCP Session Tracking
[RFC 5382] specifies TCP timers associated with various connection
states but does not specify the TCP state machine a NAT44 should
follow as a basis to apply such timers.
Update: The TCP state machine depicted in Figure 1, adapted from
[RFC 6146], SHOULD be implemented by a NAT for TCP session tracking
purposes.
Penno, et al. Best Current Practice PAGE 4
RFC 7857 Updates to NAT Behavioral Requirements April 2016
+----------------------------+
| |
V |
+------+ Client |
|CLOSED|-----SYN------+ |
+------+ | |
^ | |
|TCP_TRANS T.O. | |
| V |
+-------+ +-------+ |
| TRANS | | INIT | |
+-------+ +-------+ |
| ^ | |
data pkt | | |
| Server/Client RST | |
| TCP_EST T.O. | |
V | Server SYN |
+--------------+ | |
| ESTABLISHED |<---------+ |
+--------------+ |
| | |
Client FIN Server FIN |
| | |
V V |
+---------+ +----------+ |
| C FIN | | S FIN | |
| RCV | | RCV | |
+---------+ +----------+ |
| | |
Server FIN Client FIN TCP_TRANS
| | T.O.
V V |
+----------------------+ |
| C FIN + S FIN RCV |-----------------+
+----------------------+
Legend:
* Messages sent or received from the server are
prefixed with "Server".
* Messages sent or received from the client are
prefixed with "Client".
* "C" means "Client-side".
* "S" means "Server-side".
* TCP_EST T.O. refers to the established connection
idle-timeout as defined in [RFC 5382].
* TCP_TRANS T.O. refers to the transitory connection
idle-timeout as defined in [RFC 5382].
Figure 1: Simplified Version of the TCP State Machine
Penno, et al. Best Current Practice PAGE 5
RFC 7857 Updates to NAT Behavioral Requirements April 2016
2.1. TCP Transitory Connection Idle-Timeout
The transitory connection idle-timeout is defined as the minimum time
a TCP connection in the partially open or closing phases must remain
idle before the NAT considers the associated session a candidate for
removal (REQ-5 of [RFC 5382]). However, [RFC 5382] does not clearly
state whether these can be configured separately.
Clarification: This document clarifies that a NAT SHOULD provide
different configurable parameters for configuring the open and
closing idle timeouts.
To accommodate deployments that consider a partially open timeout
of 4 minutes as being excessive from a security standpoint, a NAT
MAY allow the configured timeout to be less than 4 minutes.
However, a minimum default transitory connection idle-timeout of 4
minutes is RECOMMENDED.
2.2. TCP RST
[RFC 5382] leaves the handling of TCP RST packets unspecified.
Update: This document adopts a similar default behavior as in
[RFC 6146]. Concretely, when the NAT receives a TCP RST matching
an existing mapping, it MUST translate the packet according to the
NAT mapping entry. Moreover, the NAT SHOULD wait for 4 minutes
before deleting the session and removing any state associated with
it if no packets are received during that 4-minute timeout.
Notes:
* Admittedly, the NAT has to verify whether received TCP RST
packets belong to a connection. This verification check is
required to avoid off-path attacks.
* If the NAT immediately removes the NAT mapping upon receipt of
a TCP RST message, stale connections may be maintained by
endpoints if the first RST message is lost between the NAT and
the recipient.
3. Port Overlapping Behavior
REQ-1 from [RFC 4787] and REQ-1 from [RFC 5382] specify a specific port
overlapping behavior; that is, the external IP address and port can
be reused for connections originating from the same internal source
IP address and port irrespective of the destination. This is known
as Endpoint-Independent Mapping (EIM).
Penno, et al. Best Current Practice PAGE 6
RFC 7857 Updates to NAT Behavioral Requirements April 2016
Update: This document clarifies that this port overlapping behavior
may be extended to connections originating from different internal
source IP addresses and ports as long as their destinations are
different.
The following mechanism MAY be implemented by a NAT:
If destination addresses and ports are different for outgoing
connections started by local clients, a NAT MAY assign the same
external port as the source ports for the connections. The
port overlapping mechanism manages mappings between external
packets and internal packets by looking at and storing their
5-tuple (protocol, source address, source port, destination
address, and destination port).
This enables concurrent use of a single NAT external port for
multiple transport sessions, which allows a NAT to successfully
process packets in a network that has a limited number of IP
addresses (e.g., deployment with a high address space
multiplicative factor (refer to Appendix B of [RFC 6269])).
4. Address Pooling Paired (APP)
The "IP address pooling" behavior of "Paired" (APP) was recommended
in REQ-2 from [RFC 4787], but the behavior when an external IPv4 runs
out of ports was left undefined.
Clarification: This document clarifies that if APP is enabled, new
sessions from a host that already has a mapping associated with an
external IP that ran out of ports SHOULD be dropped. A
configuration parameter MAY be provided to allow a NAT to start
using ports from another external IP address when the one that
anchored the APP mapping ran out of ports. Tweaking this
configuration parameter is a trade-off between service continuity
and APP strict enforcement. Note, this behavior is sometimes
referred to as "soft-APP".
As a reminder, the recommendation for the particular case of a CGN
is that an implementation must use the same external IP address
mapping for all sessions associated with the same internal IP
address, be they TCP, UDP, ICMP, something else, or a mix of
different protocols [RFC 6888].
Update: This behavior SHOULD apply also for TCP.
Penno, et al. Best Current Practice PAGE 7
RFC 7857 Updates to NAT Behavioral Requirements April 2016
5. Endpoint-Independent Mapping (EIM) Protocol Independence
REQ-1 from [RFC 4787] and REQ-1 from [RFC 5382] do not specify whether
EIM are protocol dependent or protocol independent. For example, if
an outbound TCP SYN creates a mapping, it is left undefined whether
outbound UDP packets can reuse such mapping.
Update: EIM mappings SHOULD be protocol dependent. A configuration
parameter MAY be provided to allow protocols that multiplex TCP
and UDP over the same source IP address and port number to use a
single mapping. The default value of this configuration parameter
MUST be protocol-dependent EIM.
This update is consistent with the stateful Network Address and
Protocol Translation from IPv6 Clients to IPv4 Servers (NAT64)
[RFC 6146] that clearly specifies three binding information bases
(TCP, UDP, and ICMP).
6. Endpoint-Independent Filtering (EIF) Protocol Independence
REQ-8 from [RFC 4787] and REQ-3 from [RFC 5382] do not specify whether
mappings with Endpoint-Independent Filtering (EIF) are protocol
independent or protocol dependent. For example, if an outbound TCP
SYN creates a mapping, it is left undefined whether inbound UDP
packets matching that mapping should be accepted or rejected.
Update: EIF filtering SHOULD be protocol dependent. A configuration
parameter MAY be provided to make it protocol independent. The
default value of this configuration parameter MUST be protocol-
dependent EIF.
This behavior is aligned with the update in Section 5.
Applications that can be transported over a variety of transport
protocols and/or support transport fallback schemes won't
experience connectivity failures if the NAT is configured with
protocol-independent EIM and protocol-independent EIF.
7. Endpoint-Independent Filtering (EIF) Mapping Refresh
The NAT mapping Refresh direction may have a "NAT Inbound refresh
behavior" of "True" according to REQ-6 from [RFC 4787], but [RFC 4787]
does not clarify how this behavior applies to EIF mappings. The
issue in question is whether inbound packets that match an EIF
mapping but do not create a new session due to a security policy
should refresh the mapping timer.
Penno, et al. Best Current Practice PAGE 8
RFC 7857 Updates to NAT Behavioral Requirements April 2016
Clarification: This document clarifies that even when a NAT has an
inbound refresh behavior set to "TRUE", such packets SHOULD NOT
refresh the mapping. Otherwise, a simple attack of a packet every
two minutes can keep the mapping indefinitely.
Update: This behavior SHOULD apply also for TCP.
7.1. Outbound Mapping Refresh and Error Packets
Update: In the case of NAT outbound refresh behavior, ICMP Errors or
TCP RST outbound packets sent as a response to inbound packets
SHOULD NOT refresh the mapping. Other packets that indicate the
host is not interested in receiving packets MAY be configurable to
also not refresh state, such as a Session Traversal Utilities for
NAT (STUN) error response [RFC 5389] or IKE INVALID_SYNTAX
[RFC 7296].
8. Port Parity
Update: A NAT MAY disable port parity preservation for all dynamic
mappings. Nevertheless, A NAT SHOULD support means to explicitly
request to preserve port parity (e.g., [RFC 7753]).
Note: According to [RFC 6887], dynamic mappings are said to be
dynamic in the sense that they are created on demand, either
implicitly or explicitly:
1. Implicit dynamic mappings refer to mappings that are created
as a side effect of traffic such as an outgoing TCP SYN or
outgoing UDP packet. Implicit dynamic mappings usually have a
finite lifetime, though this lifetime is generally not known
to the client using them.
2. Explicit dynamic mappings refer to mappings that are created
as a result, for example, of explicit Port Control Protocol
(PCP) MAP and PEER requests. Explicit dynamic mappings have a
finite lifetime, and this lifetime is communicated to the
client.
9. Port Randomization
Update: A NAT SHOULD follow the recommendations specified in
Section 4 of [RFC 6056], especially:
A NAPT that does not implement port preservation [RFC 4787]
[RFC 5382] SHOULD obfuscate selection of the ephemeral port of a
packet when it is changed during translation of that packet.
Penno, et al. Best Current Practice PAGE 9
RFC 7857 Updates to NAT Behavioral Requirements April 2016
A NAPT that does implement port preservation SHOULD obfuscate
the ephemeral port of a packet only if the port must be changed
as a result of the port being already in use for some other
session.
A NAPT that performs parity preservation and that must change
the ephemeral port during translation of a packet SHOULD
obfuscate the ephemeral ports. The algorithms described in
this document could be easily adapted such that the parity is
preserved (i.e., force the lowest order bit of the resulting
port number to 0 or 1 according to whether even or odd parity
is desired).
10. IP Identification (IP ID)
Update: A NAT SHOULD handle the Identification field of translated
IPv4 packets as specified in Section 5.3.1 of [RFC 6864].
11. ICMP Query Mappings Timeout
Section 3.1 of [RFC 5508] specifies that ICMP Query mappings are to be
maintained by a NAT. However, the specification doesn't discuss
Query mapping timeout values. Section 3.2 of [RFC 5508] only
discusses ICMP Query session timeouts.
Update: ICMP Query mappings MAY be deleted once the last session
using the mapping is deleted.
12. Hairpinning Support for ICMP Packets
REQ-7 from [RFC 5508] specifies that a NAT enforcing Basic NAT must
support traversal of hairpinned ICMP Query sessions.
Clarification: This implicitly means that address mappings from
external address to internal address (similar to Endpoint-
Independent Filters) must be maintained to allow inbound ICMP
Query sessions. If an ICMP Query is received on an external
address, a NAT can then translate to an internal IP.
REQ-7 from [RFC 5508] specifies that all NATs must support the
traversal of hairpinned ICMP Error messages.
Clarification: This behavior requires a NAT to maintain address
mappings from external IP address to internal IP address in
addition to the ICMP Query mappings described in Section 3.1 of
[RFC 5508].
Penno, et al. Best Current Practice PAGE 10
RFC 7857 Updates to NAT Behavioral Requirements April 2016
13. Security Considerations
NAT behavioral considerations are discussed in [RFC 4787], [RFC 5382],
and [RFC 5508].
Because some of the clarifications and updates (e.g., Section 2) are
inspired from NAT64, the security considerations discussed in
Section 5 of [RFC 6146] apply also for this specification.
The update in Section 3 allows for an optimized NAT resource usage.
In order to avoid service disruption, the NAT must not invoke this
functionality unless the packets are to be sent to distinct
destination addresses.
Some of the updates (e.g., Sections 7, 9, and 11) allow for increased
security compared to [RFC 4787], [RFC 5382], and [RFC 5508].
Particularly,
o the updates in Sections 7 and 11 prevent an illegitimate node to
maintain mappings activated in the NAT while these mappings should
be cleared, and
o port randomization (Section 9) complicates tracking hosts located
behind a NAT.
Sections 4 and 12 propose updates that increase the serviceability of
a host located behind a NAT. These updates do not introduce any
additional security concerns to [RFC 4787], [RFC 5382], and [RFC 5508].
The updates in Sections 5 and 6 allow for a better NAT transparency
from an application standpoint. Hosts that require a restricted
filtering behavior should enable specific policies (e.g., Access
Control List (ACL)) either locally or by soliciting a dedicated
security device (e.g., firewall). How a host updates its filtering
policies is out of scope of this document.
The update in Section 8 induces security concerns that are specific
to the protocol used to interact with the NAT. For example, if PCP
is used to explicitly request parity preservation for a given
mapping, the security considerations discussed in [RFC 6887] should be
taken into account.
The update in Section 10 may have undesired effects on the
performance of the NAT in environments in which fragmentation is
massively experienced. Such an issue may be used as an attack vector
against NATs.
Penno, et al. Best Current Practice PAGE 11
RFC 7857 Updates to NAT Behavioral Requirements April 2016
14. References
14.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 4787] Audet, F., Ed. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, DOI 10.17487/RFC 4787, January
2007, <http://www.rfc-editor.org/info/RFC 4787>.
[RFC 5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, DOI 10.17487/RFC 5382, October 2008,
<http://www.rfc-editor.org/info/RFC 5382>.
[RFC 5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP", BCP 148, RFC 5508,
DOI 10.17487/RFC 5508, April 2009,
<http://www.rfc-editor.org/info/RFC 5508>.
[RFC 6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC 6056, January 2011,
<http://www.rfc-editor.org/info/RFC 6056>.
[RFC 6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC 6146,
April 2011, <http://www.rfc-editor.org/info/RFC 6146>.
[RFC 6864] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, DOI 10.17487/RFC 6864, February 2013,
<http://www.rfc-editor.org/info/RFC 6864>.
14.2. Informative References
[RFC 2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, DOI 10.17487/RFC 2663, August 1999,
<http://www.rfc-editor.org/info/RFC 2663>.
Penno, et al. Best Current Practice PAGE 12
RFC 7857 Updates to NAT Behavioral Requirements April 2016
[RFC 3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
DOI 10.17487/RFC 3022, January 2001,
<http://www.rfc-editor.org/info/RFC 3022>.
[RFC 5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC 5389, October 2008,
<http://www.rfc-editor.org/info/RFC 5389>.
[RFC 6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269,
DOI 10.17487/RFC 6269, June 2011,
<http://www.rfc-editor.org/info/RFC 6269>.
[RFC 6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
DOI 10.17487/RFC 6887, April 2013,
<http://www.rfc-editor.org/info/RFC 6887>.
[RFC 6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
A., and H. Ashida, "Common Requirements for Carrier-Grade
NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC 6888,
April 2013, <http://www.rfc-editor.org/info/RFC 6888>.
[RFC 7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC 7296, October
2014, <http://www.rfc-editor.org/info/RFC 7296>.
[RFC 7753] Sun, Q., Boucadair, M., Sivakumar, S., Zhou, C., Tsou, T.,
and S. Perreault, "Port Control Protocol (PCP) Extension
for Port-Set Allocation", RFC 7753, DOI 10.17487/RFC 7753,
February 2016, <http://www.rfc-editor.org/info/RFC 7753>.
Acknowledgements
Thanks to Dan Wing, Suresh Kumar, Mayuresh Bakshi, Rajesh Mohan, Lars
Eggert, Gorry Fairhurst, Brandon Williams, and David Black for their
review and discussion.
Many thanks to Ben Laurie for the SecDir review and Dan Romascanu for
the Gen-ART review.
Dan Wing proposed some text for the configurable errors in
Section 7.1.
Penno, et al. Best Current Practice PAGE 13
RFC 7857 Updates to NAT Behavioral Requirements April 2016
Contributors
The following individual contributed text to the document:
Sarat Kamiset
Insieme Networks
United States
Authors' Addresses
Reinaldo Penno
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
United States
Email: repenno@cisco.com
Simon Perreault
Jive Communications
Canada
Email: sperreault@jive.com
Mohamed Boucadair (editor)
Orange
Rennes 35000
France
Email: mohamed.boucadair@orange.com
Senthil Sivakumar
Cisco Systems, Inc.
United States
Email: ssenthil@cisco.com
Kengo Naito
NTT
Tokyo
Japan
Email: k.naito@nttv6.jp
Penno, et al. Best Current Practice PAGE 14
RFC TOTAL SIZE: 29255 bytes
PUBLICATION DATE: Friday, April 29th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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