TSVWG                                                           R. Penno
Internet-Draft                                                     Cisco
Intended status: Best Current Practice                      S. Perreault
Expires: August 22, 2015                                        Viagenie February 13, 2016                           Jive Communications
                                                              S. Kamiset
                                                        Insieme Networks
                                                            M. Boucadair
                                                          France Telecom
                                                                K. Naito
                                                       February 18,
                                                         August 12, 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 NAPT44.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119]. NAT44.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 22, 2015. February 13, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3   2
     1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   3   2
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  TCP Session Tracking  . . . . . . . . . . . . . . . . . . . .   3
     2.1.  TCP Transitory Connection Idle-Timeout  . . . . . . . . .   4
     2.2.  TIME_WAIT State . . . . . . . . . . . . . . . . . . . . .   5
       2.2.1.   Proposal: Apply RFC6191 and PAWS to NAT  . . . . . .   5
     2.3.  TCP RST . . . . . . . . . . . . . . . . . . . . . . . .   8 .   5
   3.  Port Overlapping behavior Behavior . . . . . . . . . . . . . . . . . .   8   5
   4.  Address Pooling Paired (APP)  . . . . . . . . . . . . . . . .   8   6
   5.  EIF Security  . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  EIF Protocol Independence . . . . . . . . . . . . . . . . . .   9
   7.   6
   6.  EIF Mapping Refresh . . . . . . . . . . . . . . . . . . . . .   9
     7.1.   7
     6.1.  Outbound Mapping Refresh and Error Packets  . . . . . . .  10
   8.   7
   7.  EIM Protocol Independence . . . . . . . . . . . . . . . . . .  10
   9.   7
   8.  Port Parity . . . . . . . . . . . . . . . . . . . . . . . . .  10
   10.   7
   9.  Port Randomization  . . . . . . . . . . . . . . . . . . . . .  10
   11.   8
   10. IP Identification (IP ID) . . . . . . . . . . . . . . . . . .  11
   12.   8
   11. ICMP Query Mappings Timeout . . . . . . . . . . . . . . . . .  11
   13.   8
   12. Hairpinning Support for ICMP Packets  . . . . . . . . . . . .  11
   14.   9
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   15.   9
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   16.   9
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   17.   9
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     17.1.   9
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     17.2.  10
     16.2.  Informative References . . . . . . . . . . . . . . . . .  13  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13  11

1.  Introduction

   [RFC4787], [RFC5382] and [RFC5508] greatly advanced NAT
   interoperability and conformance.  But with widespread deployment and
   evolution of NAT Network Address Translation (NAT) more development and
   operational experience was acquired some areas of the original
   documents need further clarification or updates.  This document
   provides such clarifications and updates.

1.1.  Scope

   This document focuses solely on NAPT44 and its

   The goal of this document is to clarify,
   fill gaps or clarify and update requirements the set of
   requirements listed in [RFC4787], [RFC5382] and [RFC5508].

   It is out of the  The
   document focuses exclusively on NAT44.

   The scope of this document the creation of completely has been set so that it does not create
   new requirements not associated with beyond those specified in the documents cited above.  New
   Carrier-Grade NAT (CGN) related requirements would be better served elsewhere and if they are CGN
   specific defined in an update to

1.2.  Terminology

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

   The reader should is assumed to be familiar with the terms withe terminology defined in
   [RFC2663],[RFC4787],[RFC5382],and [RFC5508]

2.  TCP Session Tracking

   [RFC5382] specifies TCP timers associated with various connection 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 NAPT44 NAT44 should use
   follow as a basis to apply such timers.

   Update:  The TCP state machine depicted in Figure 1, adapted from
      [RFC6146], provides guidance on how SHOULD be implemented by a NAT for TCP session tracking could be implemented - it is non-normative.


                    |                            |
                    V                            |
                 +------+     CV4   Client               |
                 |CLOSED|-----SYN------+         |
                 +------+              |         |
                     ^                 |         |
                     |TCP_TRANS T.O.   |         |
                     |                 V         |
                 +-------+          +-------+    |
                 | TRANS |          |V4 INIT|          |  INIT |    |
                 +-------+          +-------+    |
                   |    ^               |        |
             data pkt   |               |        |
                   |  V4 or V4 Server/Client RST  |        |
                   |  TCP_EST T.O.      |        |
                   V    |              SV4           Server SYN   |
              +--------------+          |        |
              | ESTABLISHED  |<---------+        |
              +--------------+                   |
               |           |                     |
         Client FIN      SV4    Server FIN                |
               |           |                     |
               V           V                     |
        +---------+   +----------+               |
        |  C FIN  |   | SV4  S FIN   |               |
        |   RCV   |   |    RCV   |               |
        +---------+   +----------+               |
            |             |                      |
        Server FIN      CV4      Client FIN            TCP_TRANS
            |             |                    T.O.
            V             V                      |
        +----------------------+                 |
        | CV4   C FIN + SV4 S FIN RCV|--------------------+ RCV  |-----------------+

      * Messages sent to (resp. received from) the server
        are prefixed with "Server".
      * Messages sent to (resp. 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 1: 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.
   removal (REQ-5 of [RFC5382]).  But the document [RFC5382] does not clearly
   states if state
   whether these can be configured separately.

   Clarification:  This document clarifies that a NAT device SHOULD provide
   knobs configurable parameters for configuring the open and
      closing idle timeouts.  This
   document further acknowledges

      To accommodate deployments that most TCP flows are very short
   (less than 10 seconds) [FLOWRATE][TCPWILD] and therefore consider a partially open timeout
      of 4 minutes might be as being excessive if from a security is standpoint, a
   concern.  Therefore, it NAT
      MAY be configured allow to configure the timeout to be less than 4 minutes
   in such cases.  There also may minutes.
      Still, this specification recommends the default "transitory
      connection idle-timeout" minimum value to be cases that a timeout of set to 4 minutes
   might be excessive.  The case and the solution are written below. minutes.

2.2.  TIME_WAIT State

   The TCP TIME_WAIT state is described in [RFC0793].  The  TCP TIME_WAIT
   state needs to be kept for 2MSL before a connection is CLOSED, for
   the reasons listed below:

   1: In RST

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

   Update:  This document adopts a session are delayed similar default behavior as in
      [RFC6146].  Concretely, when the in-
      between network, and delivered to the end relatively later, we
      should prevent the packets from being transferred and interpreted
      as a packet that belongs to NAT receives a new session.

   2: If the remote TCP has not received the acknowledgment of its
      connection termination request, RST matching
      an existing mapping, it will re-send MUST translate the FIN packet
      several times.

   These points are important for the TCP to work without problems.

   [RFC5382] leaves the handling of TCP connections in TIME_WAIT state
   unspecified and mentions that TIME_WAIT state is not part of the
   transitory connection idle-timeout.  If the NAT device honors the
   TIME_WAIT state, each TCP connection and its associated resources is
   kept for a certain period, typically for four minutes, which consumes
   port resources.

   [RFC6191] explains that in certain situation it is necessary to
   reduce the TIME_WAIT state and defines such a mechanism using TCP
   timestamps and sequence numbers.  When a connection request is
   received with a four-tuple that is in the TIME-WAIT state, the
   connection request may be accepted if the sequence number or the
   timestamp of the incoming SYN segment is greater than the last
   sequence number seen on the previous incarnation of the connection.

   This document specifies that a NAT device should keep TCP connections
   in TIME_WAIT state unless it implements the proposal described in the
   following sub-section.

2.2.1.  Proposal: Apply RFC6191 and PAWS to NAT

   This section proposes to apply [RFC6191] mechanism at NAT.  This
   mechanism MAY be adopted for both clients' and remote hosts' TCP
   active close.

            client                     NAT                  remote host
              |                         |                         |
              |          FIN            |          FIN            |
              |                         |                         |
              |          ACK            |          ACK            |
              |          FIN            |          FIN            |
              |                         |                         |
              |        ACK(TSval=A)     |          ACK            |
              |                         |  -                      |
              |                         |  |                      |
              |                         |  |                      |
              |                         |  |                      |
              |                         |  | TIME_WAIT            |
              |                         |  |  ->assassinated at x |
              |                         |  |                      |
              |                         |  |                      |
              |                         |  |                      |
              |        SYN(TSval>A)     |  x      SYN             |
              |                         |  -                      |
              |                         |  |                      |
              |                         |  | SYN_SENT             |
              |                         |  |                      |
              |                         |  |                      |

   Also, PAWS works to discard old duplicate packets at NAT.  A packet
   can be discarded as an old duplicate if it is received with a
   timestamp or sequence number value less than a value recently
   received on the connection.

   To make these mechanisms work, we should concern the case that there
   are several clients with nonsuccessive timestamp or sequence number
   values are connected to a NAT device (i.e., not monotonically
   increasing among clients).  Two mechanisms to solve this mechanism
   and applying [RFC6191] and PAWS to NAT are described below.  These
   mechanisms are optional.  Rewrite timestamp and sequence number values at NAT

   Rewrite timestamp and sequence number values of outgoings packets at
   NAT to be monotonically increasing.  This can be done by adopting
   following mechanisms at NAT.

   A: Store the newest rewritten value of timestamp and sequence number
      as the "max value at the time".

   B: NAT rewrite timestamp and sequence number values of incoming
      packets to be monotonically increasing.

   When packets come back as replies from remote hosts, NAT rewrite
   again the timestamp and sequence number values to be the original
   values.  This can be done by adopting following mechanisms at NAT.

   C: Store the values of original timestamp and sequence number of
      packets, and rewritten values of those.  Split an assignable number of port space to each client

   Adopt following mechanisms at NAT.

   A: Choose clients that can be assigned ports.

   B: Split assignable port numbers between clients.

   Packets from other clients which are not chosen by these mechanisms
   are rejected at NAT, unless there is unassigned port left.  Resend the last ACK to the retransmisstted FIN

   We need to solve another scenario to make [RFC6191] work with NAT.
   In the case the remote TCP could not receive the acknowledgment of
   its connection termination request, the NAT device, on behalf of
   clients, resends the last ACK packet when it receives a FIN packet of
   the previous connection, and when the state of the previous
   connection has been deleted from the NAT.  This mechanism MAY be used
   when clients starts closing process, and the remote host could not
   receive the last ACK.  Remote host behavior of several implementations

   To solve the port shortage problem on the client side, the behavior
   of remote host should be compliant to [RFC6191] or according the mechanism
   written in Section of [RFC1122], since
      NAT may reuse mapping entry.  Moreover, the
   same 5 tuple NAT SHOULD wait for a new connection.  We have investigated behaviors of
   OSes (e.g., Linux, FreeBSD, Windows, MacOS), 4 minutes
      before deleting the session and found removing any state associate with
      it if no packets are received during that they
   implemented the server side behavior of the above two.

2.3.  TCP RST

   [RFC5382] leaves 4 minutes timeout.

      Admittedly, the handling of NAT has to verify whether received TCP RST packets unspecified.  This
   document does not try standardize such behavior but clarifies based
   on operational experience that
      belong to a connection.  These verification checks are required to
      avoid off-path attacks.

      If the NAT that receives a TCP RST for an
   active mapping and performs session tracking MAY removes immediately delete
   the sessions and remove any state associated with it.  If the NAT
   device that performs TCP session tracking receives mapping upon receipt of a
      TCP RST for message, stale connections may be maintained by endpoints
      if the first session that created a mapping, it MAY remove RST message is lost between the session NAT and the mapping immediately.

3.  Port Overlapping behavior Behavior

   REQ-1 from [RFC4787] [RFC5382]: and REQ-1 Current RFCs specifiy from [RFC5382] specify a specific port
   overlapping behavior, i.e., behavior; that is the external IP:port IP address and port can be
   reused for connections originating from the same internal source IP:port IP
   address and port irrespective of the destination.  This is known as endpoint-
   independent mapping.
   endpoint-independent mapping (EIM).

   Update:  This document clarifies that this port overlapping behavior can
      may be extended to connections originating from different internal
      source IP:ports IP addresses and ports as long as their destinations are
      different.  This known as EDM (Endpoint Dependent Mapping).

      The following mechanism below MAY be one optional implement to NAT. 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) . port).

      This enables concurrent use of a single NAT external port for
      multiple transport sessions, which enables allows a NAT to work correctly successfully
      process packets in an IP address resource limited network.


   [RFC4787] and [RFC5382] requires "endpoint-independent mapping" at
   NAT, and port overlapping NAT cannot meet the requirement.  This
   mechanism can degrade the transparency of NAT in that its mapping
   mechanism is endpoint-dependent and makes NAT traversal harder.
   However, if a NAT adopts endpoint-independent mapping together network (e.g.,
      deployment with
   endpoint-dependent filtering, then the actual behavior high address space multiplicative factor (refer to
      Appendix B.  of the NAT
   will be the same as port overlapping NAT. [RFC6269])).

4.  Address Pooling Paired (APP)

   [RFC4787]: REQ-2 [RFC5382]:ND

   The Address Pooling Paired (APP) behavior for a NAT
   is was recommended
   in previous documents REQ-2 from [RFC4787], but the behavior when a public IPv4
   run runs out
   of ports is was left undefined.

   Clarification:  This document clarifies that if APP is enabled enabled, new
      sessions from a subscriber host that already has a mapping associated with a public an
      external IP that ran out of ports SHOULD be dropped.

      The administrator MAY provide a knob configurable parameter that allows
      a NAT
   device to starting using ports from another public external IP address
      when the one that anchored the APP mapping ran out of ports.  This
      is a trade-off between subscriber service continuity and APP strict
      enforcement.  (Note, it this behavior is sometimes referred as 'soft-APP')

5.  EIF Security

   [RFC4787]:REQ-8 and [RFC5382]:REQ-3 End-point independent filtering
   could potentially result in security attacks from the public realm.
   In order to handle this, when possible there MUST be strict filtering
   checks in the inbound direction.  A knob SHOULD be provided to limit
   the number of inbound sessions and a knob SHOULD be provided to
   enable or disable EIF on a per application basis. 'soft-

   Update:  This is specially
   important in the case of Mobile networks where such attacks can
   consume radio resources and count against the user quota.

6. behavior SHOULD apply also for TCP.

5.  EIF Protocol Independence

   [RFC4787]:REQ-8 and[RFC5382]:

   REQ-8 from [RFC4787] and REQ-3 Current RFCs from [RFC5382] do not specify whether
   EIF mappings are protocol independent. protocol-independent.  In other words, if an
   outbound TCP SYN creates a mapping, it is left undefined whether
   inbound UDP packets destined to that mapping should be forwarded.

   Update:  This document specifies that EIF mappings SHOULD be protocol
      protocol-independent in order allow inbound packets for protocols
      that multiplex TCP and UDP over the same IP: IP address and port
      through the NAT and also maintain compatibility with stateful
      NAT64 RFC6146 [RFC6146].  But,
   the . The administrator MAY provide a configuration knob parameter to
      make it protocol-dependent.  The default value of this
      configuration parameter is to allow for protocol-independent EIF.

      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 dependent.

7. or the filtering mode enabled at the NAT.

6.  EIF Mapping Refresh

   [RFC4787]: REQ-6 [RFC5382]: ND

   The NAT mapping Refresh direction MAY may have a "NAT Inbound refresh
   behavior" of "True" according to REQ-6 from [RFC4787], but it [RFC4787]
   does not
   clarifies 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 device has a an
      inbound refresh behavior of TRUE, 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.

6.1.  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] [RFC5382]: and REQ-1 Current RFCs from [RFC5382] do not specify whether
   EIM are protocol independent. 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 stateful NAT64
   [RFC6146] clearly specifies three binding information bases (TCP,
   UDP, ICMP).  This document clarifies that

   Update:  EIM mappings SHOULD be
   protocol dependent . protocol-dependent.  A knob 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 use the client using them.

      2.  Explicit dynamic mappings refer to mappings that are created
          as a
   single mapping.

9.  Port Parity

   A NAT devices MAY disable port parity preservation result, for example, of explicit PCP MAP and PEER
          requests.  Explicit dynamic
   mappings.  Nevertheless, A NAT SHOULD support means to explicitly
   request mappings have a finite lifetime,
          and this lifetime is communicated to preserve port parity (e.g., [I-D.ietf-pcp-port-set]).

10. the client.

9.  Port Randomization

   Update:  A NAT SHOULD follow the recommendations specified in
      Section 4 of
   [RFC6056] [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 9 5.3.1 of [RFC6864].


   Discussion:  This recommendation may have undesired effects on the
      performance of the NAT in environments in which fragmentation is
      massively experienced.  Such issue can be used as an attack vector
      against NATs.

11.  ICMP Query Mappings Timeout

   Section 3.1 of [RFC5508] says precises that ICMP Query Mappings are to be
   maintained by NAT device. a NAT.  However, RFC the specification doesn't discuss about the
   Query Mapping timeout values.  Section 3.2 of that RFC [RFC5508] only
   about 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

   [RFC5508]:REQ-7 This requirement

   REQ-7 from [RFC5508] specifies that a NAT devices enforcing
   Basic NAT MUST '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 must be maintained to allow inbound ICMP
      Query sessions.  If an ICMP Query is received on an external
      address, a NAT device can then translate to an internal IP.  [RFC5508]:REQ-7 This requirement

   REQ-7 from [RFC5508] specifies that all NAT
   devices (i.e., Basic NAT as well as NAPT devices) MUST NATs must support the
   traversal of hairpinned ICMP Error messages.

   Clarification:  This behavior requires a NAT
   devices to maintain address
      mappings from external IP address to internal IP address in
      addition to the ICMP Query Mappings described in section Section 3.1 of that RFC.


13.  IANA Considerations

   This document does not require any IANA action.


14.  Security Considerations

   NAT behavioral considerations are discussed in [RFC4787].

   Security considerations discussed in Section 5 of [RFC6146] apply
   also fro NAT44.

   In the case of EIF mappings due to high risk of resource crunch, a
   NAT device MAY provide a knob configurable parameter to limit the number of
   inbound sessions spawned from a EIF mapping.

   [I-D.ietf-tcpm-tcp-security] contains a detailed discussion of the
   security implications of TCP Timestamps and of different timestamp
   generation algorithms.


15.  Acknowledgements

   Thanks to Dan Wing, Suresh Kumar, Mayuresh Bakshi, Rajesh Mohan and Mohan,
   Senthil Sivamular Sivamular, Lars Eggert, and Gorry Fairhurst for review and discussions


16.  References

16.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

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

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations", RFC
              2663, August 1999. 1997,

   [RFC4787]  Audet, F. F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January 2007.
              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. 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. 2009,

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

   [RFC6191]  Gont, F., "Reducing the TIME-WAIT State Using TCP
              Timestamps", BCP 159, RFC 6191, April 2011. 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. 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.

17.2. 2013, <http://www.rfc-editor.org/info/rfc6888>.

16.2.  Informative References

              Zhang, Y., Breslau, L., Paxson, V., and S. Shenker, "On
              the Characteristics and Origins of Internet Flow Rates", .

              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-09 (work in progress), November 2014.

              Gont, F., "Survey of Security Hardening Methods for
              Transmission May 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 (TCP) Implementations",
              draft-ietf-tcpm-tcp-security-03 (work in progress), March

   [TCPWILD]  Qian, F., Subhabrata, S., Spatscheck, O., Morley Mao, Z.,
              and W. Willinger, "TCP Revisited: A Fresh Look at TCP in
              the Wild", . (PCP)", RFC 6887,
              DOI 10.17487/RFC6887, April 2013,

Authors' Addresses

   Reinaldo Penno
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134

   Email: repenno@cisco.com

   Simon Perreault
   2875 boul. Laurier, suite D2-630
   Quebec, QC  G1V 2M2
   Jive Communications

   Email: simon.perreault@viagenie.ca sperreault@jive.com

   Sarat Kamiset
   Insieme Networks
   Mohamed Boucadair
   France Telecom
   Rennes  35000

   Email: mohamed.boucadair@orange.com

   Kengo Naito

   Email: kengo@lab.ntt.co.jp