TSVWG                                                           R. Penno
Internet-Draft                                                     Cisco
Updates: 4787, 5382, 5508  (if approved)                    S. Perreault
Intended status: Best Current Practice                      S. Perreault
Expires: February 15, 2016               Jive Communications
Expires: May 7, 2016                                        M. Boucadair
                                                          France Telecom
                                                            S. Sivakumar
                                                                K. Naito
                                                         August 14,
                                                        November 4, 2015

   Network Address Translation (NAT) Behavioral Requirements Updates


   This document clarifies and updates several requirements of RFC4787,
   RFC5382 and RFC5508 based on operational and development experience.
   The focus of this document is NAT44.

   This document updates RFC4787, RFC5382 and RFC5508.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   3
     1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   2   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  TCP Session Tracking  . . . . . . . . . . . . . . . . . . . .   3
     2.1.  TCP Transitory Connection Idle-Timeout  . . . . . . . . .   4   5
     2.2.  TCP RST . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Port Overlapping Behavior . . . . . . . . . . . . . . . . . .   5
   4.  Address Pooling Paired (APP)  . . . . . . . . . . . . . . . .   6
   5.  EIF  EIM Protocol Independence . . . . . . . . . . . . . . . . . .   6
   6.  EIF Mapping Refresh . . . Protocol Independence . . . . . . . . . . . . . . . . . .   7
     6.1.  Outbound
   7.  EIF Mapping Refresh and Error Packets . . . . . . .   7
   7.  EIM Protocol Independence . . . . . . . . . . . . . .   7
     7.1.  Outbound Mapping Refresh and Error Packets  . . . . . . .   7   8
   8.  Port Parity . . . . . . . . . . . . . . . . . . . . . . . . .   7   8
   9.  Port Randomization  . . . . . . . . . . . . . . . . . . . . .   8
   10. IP Identification (IP ID) . . . . . . . . . . . . . . . . . .   8   9
   11. ICMP Query Mappings Timeout . . . . . . . . . . . . . . . . .   8   9
   12. Hairpinning Support for ICMP Packets  . . . . . . . . . . . .   9
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   14. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     15.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   [RFC4787], [RFC5382] and [RFC5508] greatly advanced NAT Network Address
   Translation (NAT) interoperability and conformance.  But with  Operational
   experience gained through widespread deployment and evolution of Network Address Translation (NAT) more development and
   operational experience was acquired 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 [RFC4787], [RFC5382] and [RFC5508].  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.

   Carrier-Grade NAT (CGN) related requirements are defined in

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The reader is assumed to be familiar withe terminology defined in:
   [RFC2663],[RFC4787],[RFC5382], and [RFC5508].

   In this document, the term "NAT" refers to both "Basic NAT" and
   "Network Address/Port Translator (NAPT)" (see Section 3 of
   [RFC4787]).  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 address and Transport identifier (such as TCP/UDP port or ICMP
   query ID) (refer to Section 2 of [RFC3022]).

2.  TCP Session Tracking

   [RFC5382] 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
      [RFC6146], SHOULD be implemented by a NAT for TCP session tracking

                    |                            |
                    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  |-----------------+
      * Messages sent to (resp. or received from) from the server are
        prefixed with "Server".
      * Messages sent to (resp. or received from) 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 [RFC5382].
      * TCP_TRANS T.O: refers to the transitory connection
        idle timeout as defined in [RFC5382].

           Figure 1: Simplified version of the TCP State Machine

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 [RFC5382]).  But [RFC5382] 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 to configure the configured timeout to be less than 4 minutes.
      Still, this specification recommends the
      However, a minimum default "transitory transitory connection idle-timeout" minimum value to be set to idle-timeout of 4 minutes.
      minutes is RECOMMENDED.

2.2.  TCP RST

   [RFC5382] leaves the handling of TCP RST packets unspecified.

   Update:  This document adopts a similar default behavior as in
      [RFC6146].  Concretely, when the NAT receives a TCP RST matching
      an existing mapping, it MUST translate the packet according the
      NAT mapping entry.  Moreover, the NAT SHOULD wait for 4 minutes
      before deleting the session and removing any state associate associated with
      it if no packets are received during that 4 minutes timeout.

      Admittedly, the NAT has to verify whether received TCP RST packets
      belong to a connection.  These  This verification checks are check is required to
      avoid off-path attacks.

      If the NAT removes immediately 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

3.  Port Overlapping Behavior

   REQ-1 from [RFC4787] and REQ-1 from [RFC5382] 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).

   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

      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, 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 an IP address resource limited network (e.g.,
      deployment with high address space multiplicative factor (refer to
      Appendix B.  of [RFC6269])).

4.  Address Pooling Paired (APP)

   The Address Pooling Paired (APP) behavior for a NAT was recommended
   in REQ-2 from [RFC4787], but the behavior when a public 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.

      The administrator MAY provide a configurable  A
      configuration parameter that allows MAY be provided to allow a NAT to starting
      using ports from another external IP address when the one that
      anchored the APP mapping ran out of ports.  This  Tweaking this
      configuration parameter is a trade-off between service continuity
      and APP strict enforcement.  (Note,  Note, this behavior is sometimes
      referred as 'soft-
      APP'.) '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 [RFC6888].

   Update:  This behavior SHOULD apply also for TCP.

5.  EIF  EIM Protocol Independence


   REQ-1 from [RFC4787] and REQ-3 REQ-1 from [RFC5382] do not specify whether
   EIF mappings
   EIM are protocol-dependent or protocol-independent.  In other words,  For example, if
   an outbound TCP SYN creates a mapping, it is left undefined whether
   outbound UDP packets destined to that mapping should be forwarded. can reuse such mapping.

   Update:  This document specifies that EIF  EIM mappings SHOULD be
      protocol-independent in order protocol-dependent.  A configuration
      parameter MAY be provided to allow inbound packets for protocols that multiplex TCP
      and UDP over the same source IP address and port
      through the NAT and also maintain compatibility number to use a
      single mapping.  The default value of this configuration parameter
      MUST be protocol-dependent EIM.

      This update is compliant with the stateful NAT64 [RFC6146] that
      clearly specifies three binding information bases (TCP, UDP,

6.  EIF Protocol Independence

   REQ-8 from [RFC4787] and REQ-3 from [RFC5382] do not specify whether
   EIF mappings are protocol-independent or protocol-dependent . The administrator MAY provide 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-dependent. protocol-independent.  The
      default value of this configuration parameter is to allow for protocol-independent 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 fall back schemes won't
      experience connectivity failures as a function of the underlying
      transport protocol or the filtering mode enabled at if the NAT.

6. NAT is configured with
      protocol-independent EIM and protocol-independent EIF.

7.  EIF Mapping Refresh

   The NAT mapping Refresh direction may have a "NAT Inbound refresh
   behavior" of "True" according to REQ-6 from [RFC4787], but [RFC4787]
   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.

   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
      2 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 there are
      certain types of packets that should not refresh the mapping even
      if their direction is outbound.  For example, if the mapping is
      kept alive by ICMP Errors or TCP RST outbound packets sent as
      response to inbound packets, these SHOULD NOT refresh the mapping.

7.  EIM Protocol Independence

   REQ-1 from [RFC4787] and REQ-1 from [RFC5382] do not specify whether
   EIM are protocol-independent.  In other words, if a outbound TCP SYN
   creates a mapping it is left undefined whether outbound UDP can reuse
   such mapping and create session.  On the other hand, stateful NAT64
   [RFC6146] clearly specifies three binding information bases (TCP,
   UDP, ICMP).

   Update:  EIM mappings SHOULD be protocol-dependent.  A configuration
      parameter MAY be provided in order allow protocols that multiplex
      TCP and UDP over the same source IP address and port number to use
      a single mapping.

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., [I-D.ietf-pcp-port-set]).

      Note: According to [RFC6887], 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 PCP Port Control Protocol
          (PCP) MAP and PEER requests.  Explicit dynamic mappings have a
          finite lifetime, and this lifetime is communicated to the

9.  Port Randomization

   Update:  A NAT SHOULD follow the recommendations specified in
      Section 4 of [RFC6056], especially:

         "A NAPT that does not implement port preservation [RFC4787]
         [RFC5382] SHOULD obfuscate selection of the ephemeral port of a
         packet when it is changed during translation of that packet.  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 [RFC6864].

11.  ICMP Query Mappings Timeout

   Section 3.1 of [RFC5508] precises 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 [RFC5508] only
   discusses ICMP Query Session Timeouts.

   Update:  ICMP Query Mappings MAY be deleted once the last the session
      using the mapping is deleted.

12.  Hairpinning Support for ICMP Packets

   REQ-7 from [RFC5508] 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 [RFC5508] 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

13.  IANA Considerations

   This document does not require any IANA action.

14.  Security Considerations

   NAT behavioral considerations are discussed in [RFC4787], [RFC5382],
   and [RFC5508].

   Because some of the clarifications and updates (e.g., Section 2) are
   inspired from NAT64, the security considerations discussed in
   Section 5 of [RFC6146] 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 invoke this
   functionality only if packets are to be sen to distinct destination

   Some of the updates (e.g., Section 6, 7, Section 9, and Section 11)
   allow for an increased security compared to [RFC4787], [RFC5382], and
   [RFC5508].  Particularly:

   o  The updates in Section 6 7 and Section 11 prevent an illegitimate
      node to maintain mappings activated in the NAT while these
      mappings should be cleared.

   o  Port randomization (Section 9) complicates tracking hosts located
      behind a NAT.

   Section 4 and Section 12 propose updates that increase the
   serviceability of a host located behind a NAT.  These updates do not
   introduce any additional security concerns to [RFC4787], [RFC5382],
   and [RFC5508].

   The updates in Section 5 and Section 7 6 allow for a better NAT
   transparency from an application standpoint.  Hosts which require a
   restricted filtering behavior should enable security-dedicated
   features (e.g., ACL) access control list (ACL)) either locally or by
   soliciting a dedicated security device (e.g., firewall).

   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 [RFC6887] 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 issue may be used as an attack vector
   against NATs.

15.  References

15.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <http://www.rfc-editor.org/info/rfc4787>.

   [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, DOI 10.17487/RFC5382, October 2008,

   [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
              Behavioral Requirements for ICMP", BCP 148, RFC 5508,
              DOI 10.17487/RFC5508, April 2009,

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              DOI 10.17487/RFC6056, January 2011,

   [RFC6146]  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/RFC6146,
              April 2011, <http://www.rfc-editor.org/info/rfc6146>.

   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, DOI 10.17487/RFC6864, February 2013,

   [RFC6888]  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/RFC6888,
              April 2013, <http://www.rfc-editor.org/info/rfc6888>.

15.2.  Informative References

              Qiong, Q., Boucadair, M., Sivakumar, S., Zhou, C., Tsou,
              T., and S. Perreault, "Port Control Protocol (PCP)
              Extension for Port Set Allocation", draft-ietf-pcp-port-
              set-13 (work in progress), May October 2015.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, DOI 10.17487/RFC2663, August 1999,

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              DOI 10.17487/RFC3022, January 2001,

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              DOI 10.17487/RFC6269, June 2011,

   [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
              P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
              DOI 10.17487/RFC6887, April 2013,

   [RFC6888]  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/RFC6888,
              April 2013, <http://www.rfc-editor.org/info/rfc6888>.


   Thanks to Dan Wing, Suresh Kumar, Mayuresh Bakshi, Rajesh Mohan, Lars
   Eggert, and Gorry Fairhurst Fairhurst, and Brandon Williams for their review and


   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

   Email: repenno@cisco.com

   Simon Perreault
   Jive Communications

   Email: sperreault@jive.com
   Mohamed Boucadair
   France Telecom
   Rennes  35000

   Email: mohamed.boucadair@orange.com

   Senthil Sivakumar
   Cisco Systems, Inc.
   United States

   Email: ssenthil@cisco.com

   Kengo Naito

   Email: k.naito@nttv6.jp