Internet Engineering Task Force (IETF)
Request for Comments: 7857
BCP: 127
Updates: 4787, 5382, 5508
Category: Best Current Practice
ISSN: 2070-1721
R. Penno
S. Perreault
Jive Communications
M. Boucadair, Ed.
S. Sivakumar
K. Naito
April 2016

Updates to Network Address Translation (NAT) Behavioral Requirements


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

Copyright Notice

Copyright © 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 ( 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.

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

[RFC4787], [RFC5382], and [RFC5508] 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 [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.

Requirements related to Carrier-Grade NAT (CGN) are defined in [RFC6888].

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 [RFC2119].

   The reader is assumed to be familiar with the 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 addresses and transport identifiers (such as a 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 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 [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]). However, [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 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

[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 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.


  • 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 [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, 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 [RFC6269])).

4. Address Pooling Paired (APP)

The "IP address pooling" behavior of "Paired" (APP) was recommended in REQ-2 from [RFC4787], 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 [RFC6888].

   Update:  This behavior SHOULD apply also for TCP.

5. Endpoint-Independent Mapping (EIM) Protocol Independence

REQ-1 from [RFC4787] and REQ-1 from [RFC5382] 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) [RFC6146] that clearly specifies three binding information bases (TCP, UDP, and ICMP).

6. Endpoint-Independent Filtering (EIF) Protocol Independence

REQ-8 from [RFC4787] and REQ-3 from [RFC5382] 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 [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
      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 [RFC5389] or IKE INVALID_SYNTAX

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., [RFC7753]).

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.
  1. 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 [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] 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 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. 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 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 [RFC4787], [RFC5382], and [RFC5508].
  • 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
  • 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 [RFC4787], [RFC5382], and [RFC5508].

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

14. References

14.1. 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, <>.
   [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, <>.
   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, DOI 10.17487/RFC6864, February 2013,

14.2. Informative References

   [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,
   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
   [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, <>.
   [RFC7296]  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/RFC7296, October
              2014, <>.
   [RFC7753]  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/RFC7753,
              February 2016, <>.


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.


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


Simon Perreault
Jive Communications


   Mohamed Boucadair (editor)
   Rennes  35000

Senthil Sivakumar
Cisco Systems, Inc.
United States


Kengo Naito