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BEHAVE                                                           D. Wing
Internet-Draft                                              J. Rosenberg
Intended status:  Standards Track                          Cisco Systems
Expires:  April 18, 2008                                   H. Tschofenig
                                                  Nokia Siemens Networks
                                                        October 16, 2007


       Discovering, Querying, and Controlling Firewalls and NATs
              draft-wing-behave-nat-control-stun-usage-05

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on April 18, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   A drawback with many NAT UDP hole punching techniques is the
   keepalive traffic necessary to keep the UDP binding open.  It it
   necessary to send keepalives frequently because it is not possible to
   determine or modify the NAT's binding lifetime.  This keepalive
   traffic causes server load and additional network traffic, which is
   especially problematic with battery-operated wireless devices.



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   This document describes two mechanisms to discover NATs and firewalls
   and a mechanism to query and control their binding lifetime.  With
   these mechanisms, UDP binding discovery and UDP keepalive traffic can
   be reduced to involve only the necessary NATs or firewalls.  This
   eliminates the keepalive traffic to servers, and vastly reduces
   keepalive traffic across the network.  At the same time, backwards
   compatibility with NATs and firewalls that do not support this
   specification is retained, which allows for incremental deployment of
   this mechanism.

   This document is discussed on the SAFE mailing list,
   <http://www1.ietf.org/mailman/listinfo/safe>.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4
   3.  Motivation and Benefits  . . . . . . . . . . . . . . . . . . .  4
     3.1.  Comparison with other NAT Traversal Techniques . . . . . .  6
       3.1.1.  Simple Security Model  . . . . . . . . . . . . . . . .  6
       3.1.2.  Incremental Deployment . . . . . . . . . . . . . . . .  6
     3.2.  Reduce Keepalive Messages  . . . . . . . . . . . . . . . .  7
       3.2.1.  SIP Outbound . . . . . . . . . . . . . . . . . . . . .  7
       3.2.2.  IKE/IPsec NAT Traversal  . . . . . . . . . . . . . . .  7
       3.2.3.  Teredo . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Optimize ICE . . . . . . . . . . . . . . . . . . . . . . .  8
       3.3.1.  Candidate Gathering  . . . . . . . . . . . . . . . . .  8
       3.3.2.  Learning STUN Servers without Configuration  . . . . .  8
       3.3.3.  Reduce Media Keepalive Messages  . . . . . . . . . . .  9
   4.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  9
   5.  Discovery of Middleboxes (NATs and Firewalls)  . . . . . . . . 10
     5.1.  Outside-In . . . . . . . . . . . . . . . . . . . . . . . . 10
       5.1.1.  Nested NATs  . . . . . . . . . . . . . . . . . . . . . 12
     5.2.  Tagging  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Query and Control  . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  Client Procedures  . . . . . . . . . . . . . . . . . . . . 15
     6.2.  Server Procedures  . . . . . . . . . . . . . . . . . . . . 15
   7.  New Attributes . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  REFRESH-INTERVAL Attribute . . . . . . . . . . . . . . . . 16
     7.2.  XOR-INTERNAL-ADDRESS Attribute . . . . . . . . . . . . . . 16
     7.3.  PLEASE-TAG Attribute . . . . . . . . . . . . . . . . . . . 17
     7.4.  TAG Attribute  . . . . . . . . . . . . . . . . . . . . . . 17
     7.5.  BOOTNONCE Attribute  . . . . . . . . . . . . . . . . . . . 18
   8.  Limitations of STUN Control  . . . . . . . . . . . . . . . . . 19
     8.1.  Overlapping IP Addresses with Nested NATs  . . . . . . . . 19
     8.2.  Address Dependent NAT on Path  . . . . . . . . . . . . . . 19
     8.3.  Address Dependent Filtering  . . . . . . . . . . . . . . . 20



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     8.4.  Interacting with Legacy NATs . . . . . . . . . . . . . . . 20
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
     9.1.  Authorization  . . . . . . . . . . . . . . . . . . . . . . 21
     9.2.  Resource Exhaustion  . . . . . . . . . . . . . . . . . . . 21
     9.3.  Comparison to Other NAT Control Techniques . . . . . . . . 21
     9.4.  BOOTNONCE Attribute  . . . . . . . . . . . . . . . . . . . 22
   10. Open Issues and Discussion Points  . . . . . . . . . . . . . . 22
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 24
     13.2. Informational References . . . . . . . . . . . . . . . . . 25
   Appendix A.  Changes . . . . . . . . . . . . . . . . . . . . . . . 26
     A.1.  Changes in -05 . . . . . . . . . . . . . . . . . . . . . . 26
     A.2.  Changes in -04 . . . . . . . . . . . . . . . . . . . . . . 27
     A.3.  Changes in -03 . . . . . . . . . . . . . . . . . . . . . . 27
   Appendix B.  Implementation Details  . . . . . . . . . . . . . . . 27
     B.1.  Internal NAT Operation . . . . . . . . . . . . . . . . . . 27
     B.2.  Linux specifics  . . . . . . . . . . . . . . . . . . . . . 28
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
   Intellectual Property and Copyright Statements . . . . . . . . . . 31






























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1.  Introduction

   Two common usages of Simple Traversal Underneath NAT (STUN)
   ([I-D.ietf-behave-rfc3489bis],[RFC3489]) are Binding Discovery and
   NAT Keepalive.  The Binding Discovery usage allows a STUN client to
   learn its public IP address (from the perspective of the STUN server
   it contacted) and the NAT keepalive usage allows a STUN client to
   keep an active NAT binding alive.  Unlike some other techniques
   (e.g., UPnP IGD [UPnP-IGD], MIDCOM [RFC3303], NAT-PMP
   [I-D.cheshire-nat-pmp]), NSIS-NSLP [I-D.ietf-nsis-nslp-natfw]), STUN
   does not interact directly with the NAT.  Thus, STUN cannot request
   additional services from the NAT, such as longer lifetimes which
   would reduce keepalive messages.  Furthermore, allocating new NAT
   bindings (e.g., each phone call) requires communication with a STUN
   server located somewhere on the Internet.

   This document describes three mechanisms for the STUN client to
   discover NATs and firewalls that are on path with its STUN server.
   After discovering the NATs and firewalls, the STUN client can query
   and control those devices using STUN.  The STUN client needs to only
   ask those STUN servers (embedded in the NATs and firewalls) for
   public IP addresses and UDP ports, thereby offloading that traffic
   from the STUN server on the Internet.  Additionally, the STUN client
   can ask the NAT's embedded STUN server to extend the NAT binding for
   the flow, and the STUN client can learn the IP address of the next-
   outermost NAT.  By repeating this procedure with the next-outermost
   NAT, all of the NATs along that path can have their bindings
   extended.  By learning all of the STUN servers on the path between
   the public Internet and itself, an endpoint can optimize the path of
   peer-to-peer communications.


2.  Notational Conventions

   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].


3.  Motivation and Benefits

   There are a number of problems with existing NAT traversal
   techniques, such as STUN, UPnP IGD, MIDCOM, NAT-PMP, and NSIS-NSLP:








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   nested NATs:
      Today, many ISPs provide their subscribers with modems that have
      embedded NATs or within the ISP's network.  These subscribers then
      install NATs behind those devices to provide additional features,
      such as wireless access.  In these situations, UPnP IGD and NAT-
      PMP no longer function, as those protocols can only control the
      first NAT closest to the host.  STUN continues to function, but is
      unable to optimize network traffic behind those nested NATs (e.g.,
      traffic that stays within the same house or within the same
      apartment building).

      One technique to avoid nested NATs is to disable one of the NATs,
      as recommended by [Vista-cert].  However, this merely sidesteps
      the problem of nested NATs, as some NATs are installed for a
      reason (e.g., reduce IP address consumption or provide a modicum
      of security).  Disabling the NAT is also ineffective if the ISP is
      NATting subscribers within the ISP's network, as ISP NATs do not
      typically support UPnP.

      The technique described in this document allows optimization of
      the traffic behind those NATs so that the traffic can traverse the
      fewest NATs possible.

   keepalive chatter:
      To keep NAT bindings from timing out and to perform its binding
      discovery, keepalive packets are sent to a server on the Internet.
      This consumes bandwidth across the user's access network, which in
      some cases is bandwidth constrained (e.g., wireless, satellite),
      creates a load on the server, and (for battery-powered devices)
      consumes battery power.  This chattiness can be avoided by using a
      NAT control mechanism such as UPnP IGD or NAT-PMP.  However,
      relying on such NAT control mechanisms exclusively for NAT
      traversal is problematic, as they are not universally deployed.
      Thus, many UDP NAT traversal techniques instead rely on UDP hole
      punching.

      The technique described in this document provides a significant
      reduction of keepalive traffic.  The keepalive traffic can be
      reduced in frequency and can even be sent to just the necessary
      NAT or firewall (rather than the server).

   lack of incremental deployment:
      Many other NAT traversal techniques require the endpoint and its
      NAT to both support the same NAT traversal technique or else NAT
      traversal is not possible at all.  Examples include NSIS-NSLP,
      NAT-PMP, UPnP IGD, and MIDCOM.

      The technique described in this document allows incremental



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      deployment of local endpoints and NATs that support STUN Control.
      If the local endpoint, or its NATs, does not support the STUN
      Control functionality, then STUN (see
      [I-D.ietf-behave-rfc3489bis]), [I-D.ietf-sip-outbound], and ICE
      [I-D.ietf-mmusic-ice] procedures are used to traverse the NATs
      without the optimizations described in this document.

   The protocol described in this document retains the positive features
   of STUN -- incremental deployment and support of nested NATs --
   without introducing drawbacks inherent in other NAT traversal
   techniques.  The protocol optimizes the operation of STUN clients
   when those STUN clients are behind a NAT that supports the protocol
   described in this document.  STUN clients that are behind a NAT that
   doesn't support the protocol described in this document continue to
   function as they do today, without those optimizations.

3.1.  Comparison with other NAT Traversal Techniques

   STUN Control offers the following benefits over other NAT traversal
   and NAT control techniques such as NSIS-NSLP, MIDCOM, NAT-PMP, and
   UPnP IGD.

3.1.1.  Simple Security Model

   Unlike other middlebox control techniques which have relatively
   complex security models because a separate control channel is used,
   STUN Control's is simple.  It is simple because only flows
   originating from the same source IP and UDP port can be controlled
   (i.e., have its NAT timeout queried or extended).  Other flows cannot
   be created, queried, or controlled via STUN Control.

3.1.2.  Incremental Deployment

   STUN Control can be incrementally deployed.  If the outer-most NAT
   does not support it, the STUN client behaves as normal -- it merely
   isn't able to optimize its keepalive (see also Section Section 8.4).
   If the outer-most NAT does support STUN Control, the STUN client can
   gain some significant optimizations as described in the following
   sections.

   Likewise, there is no change required to applications if NATs are
   deployed which support STUN Control:  such applications will be
   unaware of the additional functionality in the NAT, and will not be
   subject to any worse security risks due to the additional
   functionality in the NAT.






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3.2.  Reduce Keepalive Messages

   The primary value of the protocol and technique described in this
   document is the reduction of UDP keepalive messages.  This is helpful
   for several protocols.

   For each of the protocols below, STUN Control as described in this
   document enables two optimizations:

   1.  all of the on-path NATs can explicitly indicate their timeouts,
       reducing the frequency of keepalive messages, and;

   2.  STUN keepalive messages need only be sent to the outer-most NAT,
       rather than across the access link to the SIP proxy, which vastly
       reduces the traffic to the SIP proxy.

3.2.1.  SIP Outbound

   In SIP outbound [I-D.ietf-sip-outbound], the SIP proxy is also the
   STUN server.  Through the initial STUN request/response exchange with
   that server, the STUN client learns it is behind a NAT, and learns
   that NAT's public IP address.  Once it has learned the NAT's public
   IP address, it can query and control that NAT by following the
   procedures in Section 6.

3.2.2.  IKE/IPsec NAT Traversal

   In both the NAT traversal for IKEv1 [RFC3947] and IKEv2 (Section 2.23
   of [RFC4306]) the IKE endpoints can only learn that a NAT is present,
   but cannot learn the IP address of that NAT because the IP address is
   hashed.  Thus, IKE itself isn't usable to learn the IP address of the
   outer-most NAT.  STUN can be used to learn the IP address of the
   outer-most NAT, and STUN Control can then be used to extend the
   binding lifetime for the UDP port that is being used by IKE.  Once
   this is done, the IPsec NAT keepalive interval can be reduced
   (Section 4 of [RFC3948]).

   With IKE/IPsec NAT traversal, there are two ways to use STUN to learn
   the outer-most NAT:

   o  STUN packets can be sent between the IKE peers on the same port as
      IKE.  IKE, IPsec ESP, and STUN can be demultiplexed.  However,
      this does require changing software in both IKE peers.

   o  STUN packets can be sent to STUN port of the IKE peer's IP
      address.  This does not require changing software on the remote
      IKE peer, but requires a separate server process running on the
      remote peer.



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3.2.3.  Teredo

   Endpoints that implement Teredo [RFC4380] learn their outer-most NATs
   address as their Teredo Mapped Address.  Once learned, the Teredo
   client can utilize STUN Control to query and control that NAT's (and
   nested NAT's) UDP keepalive timeout, and thus reduce the refresh
   interval.

   In contrast, Teredo's existing s refresh interval determination
   procedure (Section 5.2.7 of [RFC4380]) allows the Teredo host to
   learn (but not adjust) the NAT's binding lifetime.  There is also a
   small risk that the NAT will use different refresh intervals for
   different ports (e.g., due to resource constraints), which
   contributes to some brittleness.

3.3.  Optimize ICE

   The STUN Control usage provides several opportunities to optimize ICE
   [I-D.ietf-mmusic-ice], as described in this section.

3.3.1.  Candidate Gathering

   During its candidate gathering phase, an ICE endpoint normally
   contacts a STUN server on the Internet.  If an ICE endpoint discovers
   that its outer-most NAT runs a STUN server, the ICE endpoint can use
   the outer-most NAT's STUN server rather than using the STUN server on
   the Internet.  This saves access bandwidth and reduces the reliance
   on the STUN server on the Internet -- the STUN server on the Internet
   need only be contacted once -- when the ICE endpoint first
   initializes.

3.3.2.  Learning STUN Servers without Configuration

   ICE allows endpoints to have multiple STUN servers, but it is
   difficult to configure all of the STUN servers in the ICE endpoint --
   it requires some awareness of network topology.  By using the 'walk
   backward' technique described in this document, all the on-path NATs
   and their embedded STUN servers can be learned without additional
   configuration.  By knowing the STUN servers at each address domain,
   ICE endpoints can optimize the network path between two peers.











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   For example, if endpoint-1 is only configured with the IP address of
   the STUN server on the left, endpoint-1 can learn about NAT-B and
   NAT-A.  Utilizing the STUN server in NAT-A, endpoint-1 and endpoint-2
   can optimize their media path so they make the optimal path from
   endpoint-1 to NAT-A to endpoint-2:

                      +-------+     +-------+       +-------------+
         endpoint-1---| NAT-A +--+--+ NAT-B +-------| STUN Server |
                      +-------+  |  +-------+       +-------------+
                                 |
                            endpoint-2

3.3.3.  Reduce Media Keepalive Messages

   While very minor, STUN Control makes it possible to optimize media
   keepalives.  This is useful if a video or audio stream is placed on
   'hold' or 'mute', but is expected to be resumed in the future.  ICE
   uses STUN Indications as its primary media stream keepalive
   mechanism.  This document enables two optimizations of ICE's
   keepalive technique:

   1.  STUN keepalive messages need only be sent to the outer-most NAT,
       rather than across the access link to the remote peer, and;

   2.  all of the on-path NATs can explicitly indicate their timeouts,
       which allows reducing the keepalive frequency.


4.  Overview of Operation

   This document describes three functions, which are all implemented
   using the STUN protocol:

   Discovery of Middleboxes (NATs and Firewalls):
      This document describes two techniques for finding NATs or
      firewalls (see Section 5).  These two approaches are:

      Outside-In:
           Uses STUN or Teredo to find the outer-most NAT.  Then STUN is
           used to communicate with that NAT and discover the other
           nested NATs (if any) along that path towards the host by
           repeated use of STUN with each of those NATs.

      Tagging:
           Send a STUN Request packet to your STUN server, and asks for
           compliant firewalls along the path to indicate their presence
           by adding an IP address to the STUN Response packet.




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   Querying Discovered Middleboxes:
      After discovering a NAT or a firewall, it is useful to determine
      characteristics of the NAT binding or the firewall pinhole.  Two
      of the most useful things to learn is the duration the NAT binding
      or firewall pinhole will remain open if there is no traffic, and
      the filtering applied to that binding or pinhole.  This is
      described in Section 6.

   Controlling Discovered Middleboxes:
      A NAT or a firewall might default to a more restrictive behavior
      than desired by an application (e.g., aggressive timeout,
      filtering).  Requesting the NAT or firewall to change its default
      behavior is useful for traffic optimization (e.g., reduce
      keepalive traffic) and network optimization (e.g., adjust filters
      to eliminate the need for a media relay device
      [I-D.ietf-behave-turn]).  A discussion of this functionality can
      be found in Section 6.


5.  Discovery of Middleboxes (NATs and Firewalls)

   This section describes two techniques to discover a NAT and a
   firewall:  outside-in and by tagging.

   Ideally, a single technique could be selected as an outcome of the
   standardization process.  However, it is possible to combine these
   two techniques.

5.1.  Outside-In

   The endpoint must first discover its outer-most NAT.  This can be
   accomplished using STUN or Teredo.

   STUN:  When a STUN client sends a STUN Request to a STUN server, it
      receives a STUN Response that indicates the IP address and UDP
      port seen by the STUN server.  If the IP address and UDP port
      differs from the IP address and UDP port of the socket used to
      send the request, the STUN client knows there is at least one NAT
      between itself and the STUN server.  The STUN client also learns
      the 'public' IP address (and port) allocated by the outermost NAT.

   Teredo:  As part of the Teredo qualification procedure, the Teredo
      client learns the IP address of its outer-most NAT.  With that
      information, the Teredo client can proceed to the next step.

   After learning the public IP address of its outer-most NAT, the
   endpoint sends a STUN packet to the STUN port (UDP/3478) of its
   outer-most NAT's public IP address.  The NAT will return a STUN



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   Binding Response message including two important STUN attributes:

      XOR-MAPPED-ADDRESS, which indicates the public IP address and UDP
      port for the mapping.  As the endpoint just learned this
      information via STUN or Teredo, this isn't terribly interesting to
      the endpoint at this time.  However, if the endpoint wants to
      create a new UDP mapping (e.g., for a new UDP flow), the endpoint
      need only send a STUN request to this outer-most NAT rather than
      to a host on the Internet.

      XOR-INTERNAL-ADDRESS, which indicates the IP address and UDP port
      seen on the *internal* side of the NAT for that translation (see
      Figure 13).  This allows the endpoint to discover, query, and
      control multiple NATs (nested NATs) along that path.



             Endpoint                           NAT     STUN Server
                 |                               |          |
            1.   |-----Binding Request (UDP)--------------->|
            2.   |<----Binding Response (UDP)---------------|
                 |                               |          |
            3.   |--Binding Request (UDP)------->|          |
            4.   |<-Binding Response (UDP)-------|          |
                 |                               |          |

                       Figure 2: Communication Flow

   In the message flow above, steps 1 and 2 correspond to the STUN
   behavior described in [I-D.ietf-behave-rfc3489bis]:

   1:  The STUN client sends a UDP Binding Request to its STUN server
       that is located on the Internet.

   2:  The STUN server on the Internet responds with a UDP Binding
       Response.

   After steps 1 and 2, the endpoint has learned the IP address of its
   outer-most NAT.  The endpoint could also have used Teredo to learn
   that IP address.

   The next steps are the additional steps performed by the endpoint
   implementing STUN Control:








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   3:  The endpoint sends a STUN Binding Request to the IP address of
       its outer-most NAT.  This will be received by the STUN server
       embedded in that outer-most NAT.

   4:  The STUN server (embedded in the NAT) responds with a STUN
       Binding Response.

   The response obtained in message (4) contains the XOR-MAPPED-ADDRESS
   attribute, which will have the same value as when the STUN server on
   the Internet responded (in step 2).  Thereafter, so long as the
   BOOTNONCE value doesn't change, the STUN client can perform steps (3)
   and (4) for any new UDP communication, without needing to repeat
   steps (1) and (2).  This meets the desire to reduce chattiness.  The
   STUN client also only needs to send keepalives towards the outer-most
   NAT's IP address, as well (reduces chatter for SIP outbound
   [I-D.ietf-sip-outbound]).

   The response obtained in message (4) will also contain the XOR-
   INTERNAL-ADDRESS, which allows the STUN client to repeat steps (3)
   and (4) in order to query or control those on-path NATs between
   itself and its STUN server on the Internet.  This is described in
   detail in Section 5.1.1.  This functionality allows ICE to learn more
   NAT bindings Section 3.3.2 and gives ICE the opportunity to optimize
   traffic between nested NATs, without requiring configuration of
   intermediate STUN servers.

   The STUN client can request each NAT to increase the binding lifetime
   for that source IP address and source UDP port, as described in
   Section 7.1.  The STUN client receives positive confirmation that the
   binding lifetime has been extended, allowing the STUN client to
   significantly reduces its NAT keepalive traffic.  Additionally, as
   long as the NAT complies with [RFC4787] (which is indicated by its
   support of this document), the STUN client's keepalive traffic need
   only be sent to the outer-most NAT's IP address.  This functionality
   meets the need to reduce STUN's chattiness.

5.1.1.  Nested NATs

   Nested NATs are controlled individually.  The nested NATs are
   discovered, from outer-most NAT to the inner-most NAT, using the XOR-
   INTERNAL-ADDRESS attribute.

   If there is only one NAT between an endpoint and the Internet, XOR-
   INTERNAL-ADDRESS will return the same IP address and UDP port the
   endpoint is using.  If there are multiple NATs between an endpoint
   and the Internet, XOR-INTERNAL-ADDRESS will return a different IP
   address than the endpoint is using, which points towards the NAT
   closer to the endpoint.  By repeating this procedure, the endpoint



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   can discover all of the NATs.  Note, however, the limitation
   described in Section 8.1.

   The following figure shows two nested NATs:


    +------+      +--------+     +--------+
    | 192.168.1.2 |    10.1.1.2  |  192.0.2.1              +-----------+
    | STUN +------+ NAT-B  +-----+ NAT-A  +---<Internet>---+STUN Server|
    |Client|   192.168.1.1 |   10.1.1.1   |                +-----------+
    +------+      +--------+     +--------+

           Figure 3: Two nested NATs with embedded STUN servers

   First, the endpoint would learn the outer-most NAT's IP address via
   STUN or Teredo.  The endpoint will then send a STUN binding request
   to that outer-most NAT.  With nested NATs, however, the IP address
   and UDP port indicated by the XOR-INTERNAL-ADDRESS will not be the
   STUN client's own IP address and UDP port -- rather, it is the IP
   address and UDP port on the inside of NAT-A, which are the same as
   the IP address and UDP port on the outside of the NAT-B -- 10.1.1.2.

   Because of this, the STUN client repeats the procedure and sends
   another STUN Binding Request to that newly-learned address (the
   *outer* side of NAT-B).  NAT-B will respond with a STUN Binding
   Response containing the XOR-INTERNAL-ADDRESS attribute, which will
   match the STUN client's IP address and UDP port.  The STUN client
   then knows there are no other NATs between itself and NAT-B, and
   finishes.

   The message flow with two nested NATs is shown below:


      STUN Client             NAT-B     NAT-A     STUN Server
          |                      |        |          |
     1.   |-----Binding Request (UDP)--------------->|
     2.   |<----Binding Response (UDP)---------------|
          |                      |        |          |
     3.   |--Binding Request (UDP)------->|          |     }
     4.   |<-Binding Response (UDP)-------|          |     } NAT Control
          |                      |        |          |     } STUN Usage
     5.   |--Binding Req (UDP)-->|        |          |     }
     6.   |<-Binding Resp (UDP)--|        |          |     }
          |                      |        |          |

            Figure 4: Message Flow for Outside-In with Two NATs

   A BOOTNONCE value is obtained from each of these NATs, and is



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   validated whenever a subsequent STUN Binding Request is sent to any
   of those learned NATs.

5.2.  Tagging

   To discover an on-path firewall, the PLEASE-TAG attribute is used
   with a STUN Binding Request (a STUN packet sent to UDP/3478) message.
   A firewall would inspect bypassing Binding Request messages and
   determine whether there is a PLEASE-TAG attribute.  When the firewall
   sees the associated Binding Response, the firewall appends a TAG
   attribute as the last attribute of the Binding Response.  This TAG
   attribute contains the firewall's management IP address and UDP port.
   Each on-path firewall would be able to insert its own TAG attribute.
   In this way, the STUN Response would contain a pointer to each of the
   on-path firewalls between the client and that STUN server.

      Motivation for developing the Tagging mechanism:  The Outside-In
      discovery technique (Section 5.1) uses the public IP address of
      the NAT to find the outer-most NAT that supports STUN Control.
      Firewalls do not translate packets and hence a different technique
      is needed to identify firewalls.

      Note that tagging is similar to how NSIS-NSLP
      [I-D.ietf-nsis-nslp-natfw], TIST [I-D.shore-tist-prot], and NLS
      [I-D.shore-nls-tl] function.

   This figure shows how tagging functions.

               STUN Client          firewall           STUN Server
                   |                   |                   |
            1.     |--Binding Request->|------------------>|
            2.     |                   |<-Binding Response-|
            3.     |             [inserts tag]             |
            4.     |<-Binding Response-|                   |
            5. [firewall discovered]   |                   |

                      Figure 5: Tagging Message Flow

   1.  A Binding Request, containing the PLEASE-TAG attribute, is sent
       to the IP address of the STUN server that is located somewhere on
       the Internet.  This is seen by the firewall, and the firewall
       remembers the STUN transaction id, and permits the STUN Binding
       Request packet.

   2.  When the firewall observes a STUN Binding Response packet it
       checks its cache for the previously stored STUN transaction id.
       If a previous STUN transaction id was found then the firewall
       inserts the TAG attribute, which contains the firewall's



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       management address.

   3.  The firewall sends the (modified) STUN Binding Response towards
       the STUN client.

   4.  The STUN client has now discovered the firewall, and can query it
       or control it.


6.  Query and Control

   This section describes how to use STUN to query and control a NAT
   that was discovered using the technique described in Section 5.

6.1.  Client Procedures

   After discovering on-path NATs and firewalls with the procedure
   described in Section 5, the STUN client begins querying and
   controlling those devices.

   To modify an existing NAT mapping's attributes, or to request a new
   NAT mapping for a new UDP port, the STUN client can now send a STUN
   Binding Request to the IP address of address of the respective NAT or
   firewall (using the STUN UDP port, 3478).

   Client produces for handling the BOOTNONCE attribute can be found in
   Section 7.5.

6.2.  Server Procedures

   When receiving a STUN Binding Request the STUN controlled NAT will
   respond with a STUN Binding Response containing an XOR-MAPPED-ADDRESS
   attribute (which points at the NAT's public IP address and port --
   just as if the STUN Binding Request had been sent to a STUN server on
   the public Internet) and an XOR-INTERNAL-ADDRESS attribute (which
   points to the source IP address and UDP port the packet STUN Binding
   Request packet had prior to being NATted).  See Figure 13 which
   depicts how this might be implemented in a NAT.

   When receiving a STUN Binding Request the STUN controlled firewall
   will respond with a STUN Binding Response containing an XOR-MAPPED-
   ADDRESS attribute (which points at the public IP address and port)
   and an XOR-INTERNAL-ADDRESS attribute (which points to the source IP
   address of the interface and UDP port where the packet was received,
   i.e., the internal interface).

   Server procedures for handling the BOOTNONCE and REFRESH-INTERVAL
   attributes can be found in Section 7.5 and Section 7.1.



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   STUN Binding Requests, which arrived from its public interface(s),
   MAY be handled as if the server is not listening on that port (e.g.,
   return an ICMP error).  This specification does not need them.


7.  New Attributes

7.1.  REFRESH-INTERVAL Attribute

   In a STUN request, the REFRESH-INTERVAL attribute indicates the
   number of milliseconds that the client wants the NAT binding (or
   firewall pinhole) to be opened.  This applies to all bindings that
   exist in that NAT from that same source IP address and same source
   UDP port (see also Appendix B.2).  In a STUN response, the REFRESH-
   INTERVAL attribute indicates the number of milliseconds the STUN
   server (embedded in the NAT or firewall) will keep the bindings open.

   REFRESH-INTERVAL is specified as an unsigned 32 bit integer, and
   represents an interval measured in milliseconds (thus the maximum
   value is approximately 50 days).  This attribute can be present in
   Binding Requests and in Binding Responses.

7.2.  XOR-INTERNAL-ADDRESS Attribute

   This attribute MUST be present in a Binding Response and is necessary
   to allow a STUN client to perform the outside-in discovery technique,
   in order to discover all of the STUN Control-aware NATs along the
   path.

   The format of the XOR-INTERNAL-ADDRESS attribute is:


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |x x x x x x x x|    Family     |         X-Port                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                X-Address (32 bits or 128 bits)                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 6: XOR-INTERNAL-ADDRESS Attribute

   The meaning of Family, X-Port, and X-Address are exactly as in
   [I-D.ietf-behave-rfc3489bis].  The length of X-Address depends on the
   address family (IPv4 or IPv6).






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7.3.  PLEASE-TAG Attribute

   If a STUN client wants to discover on-path firewalls, it MUST include
   this attribute in its Binding Response when performing the Binding
   Discovery usage.

   STUN servers are not expected to understand this attribute; if they
   return this attribute as an unknown attribute, it does not affect the
   operation described in this document.

   The format of the PLEASE-TAG attribute is:


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Mech.|x x x x x x x x x x x x x x x x x x x x x x x x x x x x x|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 7: PLEASE-TAG Attribute

   The 3-bit Mechanism field indicates the control mechanism desired.
   Currently, the only defined mechanism is STUN Control, and is
   indicated with all zeros.  The intent of this field is to allow
   additional control mechanisms (e.g., UPnP IGD, NAT-PMP, MIDCOM).

7.4.  TAG Attribute

   The TAG attribute contains the XOR'd management transport address of
   the middlebox.  Typically, a firewall as well as a NAT may find this
   technique useful as well.

   If the associated STUN Request contained the PLEASE-TAG attribute, a
   middlebox MUST append this attribute as the last attribute of the
   STUN Response (with that same transaction-id).  After appending this
   attribute, the STUN length field MUST be also be adjusted.















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   The format of the TAG attribute is:


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Mech.|M|x x x x|    Family     |         X-Port                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                X-Address (32 bits or 128 bit)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 8: TAG Attribute

   Mech:  The 3-bit Mechanism field indicates the control mechanism
   supported on the described port.  Currently, the only defined
   mechanism is STUN Control, and is indicated with 0x0.  The intent of
   this field is to allow additional control mechanisms (e.g., UPnP IGD,
   NAT-PMP, MIDCOM).

   The one-bit M field indicates if this firewall permits Mobility
   Header packets to flow through it ([RFC3775]).

   The meaning of Family, X-Port, and X-Address are exactly as in
   [I-D.ietf-behave-rfc3489bis].  The length of X-Address depends on the
   address family (IPv4 or IPv6).

7.5.  BOOTNONCE Attribute

   The BOOTNONCE attribute protects against the attack described in
   Section 9.4.

   Client procedures:  The STUN client expects each NAT to return the
   same BOOTNONCE value each time that NAT is contacted.  If a NAT
   returns a different value, the STUN client MUST NOT use any
   information returned in the Binding Response and MUST re-run the STUN
   Control procedures from the beginning (i.e., obtain its public IP
   address from the STUN server on the Internet).  This would only occur
   if an attack is in progress or if the NAT rebooted.  If the NAT
   rebooted, it is good practice to re-run the STUN Control procedures
   anyway, as the network topology could be different as well.

   Server procedures:  This attribute's value is a hash of the STUN
   client's IP address and a value that is randomly-generated each time
   the NAT is initialized.  The STUN client's IP address is included in
   this hash to thwart an attacker attaching to the NAT's internal
   network and learning the BOOTNONCE value.





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   The format of the BOOTNONCE attribute is:


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Boot Nonce value (32 bits)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 9: BOOTNONCE Attribute


8.  Limitations of STUN Control

8.1.  Overlapping IP Addresses with Nested NATs

   If nested NATs have overlapping IP address space, there will be
   undetected NATs on the path.  When this occurs, the STUN client will
   be unable to detect the presence of NAT-A if NAT-A assigns the same
   UDP port.  For example, in the following figure, NAT-A and NAT-B are
   both using 10.1.1.x as their 'private' network.

          +------+       +--------+     +--------+
          |  10.1.1.2    |  10.1.1.2    |  192.0.2.1
          | STUN +-------+  NAT-A +-----+  NAT-B +------<Internet>
          |client|    10.1.1.1    |    10.1.1.1  |
          +------+       +--------+     +--------+

             Figure 10: Overlapping Addresses with Nested NATs

   When this situation occurs, the STUN client can only learn the outer-
   most address.  This is not a problem -- the STUN client is still able
   to communicate with the outer-most NAT and is still able to avoid
   consuming access network bandwidth and avoid communicating with the
   public STUN server.  All that is lost is the ability to optimize
   paths within the private network that has overlapped addresses.

   Of course when such an overlap occurs the end host (STUN client)
   cannot successfully establish bi-directional communication with hosts
   in the overlapped network, anyway.

8.2.  Address Dependent NAT on Path

   In order to utilize the mechanisms described in this document, a STUN
   Request is sent from the same source IP address and source port as
   the original STUN Binding Discovery message, but is sent to a
   different destination IP address -- it is sent to the IP address of
   an on-path NAT.  If there is an on-path NAT, between the STUN client



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   and the STUN server, with 'address dependent' or 'address and port-
   dependent' mapping behavior (as described in Section 4.1 of
   [RFC4787]), that NAT will prevent a STUN client from taking advantage
   of the technique described in this document.  When this occurs, the
   ports indicated by XOR-MAPPED-ADDRESS from the public STUN server and
   the NAT's embedded STUN server will differ.

   An example of such a topology is shown in the following figure:

            +------+     +--------+   +--------+
            | STUN |     |  10.1.1.2  |  192.0.2.1
            |client+-----+  NAT-A +---+  NAT-B +------<Internet>
            |      |  10.1.1.1    |  10.1.1.1  |
            +------+     +--------+   +--------+

   In this figure, NAT-A is a NAT that has address dependent mapping.
   Thus, when the STUN client sends a STUN Binding Request to 192.0.2.1
   on UDP/3478, NAT-A will choose a new public UDP port for that
   communication.  NAT-B will function normally, returning a different
   port in its XOR-MAPPED-ADDRESS, which indicates to the STUN client
   that a symmetric NAT exists between the STUN client and the STUN
   server it just queried (NAT-B, in this example).

                 Figure 11: Address Dependant NAT on Path

8.3.  Address Dependent Filtering

   If there is an NAT along the path that has address dependent
   filtering (as described in section 5 of [RFC4787]), and the STUN
   client sends a STUN packet directly to any of the on-path NATs public
   addresses, the address-dependent filtering NAT will filter packets
   from the remote peer.  Thus, after communicating with all of the on-
   path NATs the STUN client MUST send a UDP packet to the remote peer,
   if the remote peer is known.

8.4.  Interacting with Legacy NATs

   There will be cases where the STUN client attempts to communicate
   with an on-path NAT, which does not support STUN Control.  There are
   two cases:

   o  the NAT does not run a STUN server on its public interface (this
      will be the most common)

   o  the NAT does run a STUN server on its public interface, but does
      not return the XOR-INTERNAL-ADDRESS attribute defined in this
      document




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   In both cases the optimizations described in this section will not be
   available to the STUN client.  This is no worse than the condition
   today.  This allows incremental upgrades of applications and NATs
   that implement the technique described in this document.


9.  Security Considerations

   This security considerations section will be expanded in a subsequent
   version of this document.  So far, the authors have identified the
   following considerations:

9.1.  Authorization

   Only hosts that are 'inside' a NAT, which a NAT is already providing
   services for, can query or adjust the timeout of a NAT mapping.

   A discussion of additional authorization mechanisms that might be
   needed for firewall traversal can be found at
   [I-D.wing-session-auth].

9.2.  Resource Exhaustion

   A malicious STUN client could ask for absurdly long NAT bindings
   (days) for many UDP sessions, which would exhaust the resources in
   the NAT.  The same attack is possible (without considering this
   document and without considering STUN or other UNSAF [RFC3424] NAT
   traversal techniques) -- a malicious TCP (or UDP) client can open
   many TCP (or UDP) connections, and keep them open, causing resource
   exhaustion in the NAT.

9.3.  Comparison to Other NAT Control Techniques

   Like UPnP IGD, NAT-PMP, and host-initiated MIDCOM, the STUN usage
   described in this document allows a host to learn its public IP
   address and UDP port mapping, and to request a specific lifetime for
   mappings from that same source IP address and same source UDP port.

   However, unlike other NAT traversal technologies, STUN Control
   described in this document only allows each UDP port on the host to
   create and adjust the mapping timeout of its own NAT mappings.
   Specifically, an application on a host can only adjust the duration
   of a NAT bindings for itself, and not for another application on that
   same host, and not for other hosts.  This provides security
   advantages over other NAT control mechanisms where malicious software
   on a host can surreptitiously create NAT mappings to another
   application or to another host.




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9.4.  BOOTNONCE Attribute

   Using the mechanisms described in this document, a STUN client learns
   the public IP addresses of its NAT which supports the mechanisms
   described in this document.  However, without the STUN client's
   knowledge, that NAT may acquire a new IP address (e.g., due to DHCP
   lease expiration or network renumbering).  When this occurs, the STUN
   client will send a STUN Binding Request to the NAT's previous public
   IP address.  If an attacker were to run a rogue STUN server on that
   address, the attacker will have effectively compromised the STUN
   server, as described in Section 12.2.1 of [RFC3489].  The attacker,
   upon receiving STUN Binding Requests, will reply with STUN Binding
   Responses indicating an IP address the attacker controls.  The
   attacker will thus have access to the subsequent flow established by
   the STUN client (e.g., RTP traffic).  This attack is possible because
   the STUN client is unable to distinguish the attacker's replies from
   replies from the legitimate NAT.

   To defend against this attack, the STUN server embedded in the NAT
   returns a BOOTNONCE value.  The STUN client validates that it
   receives the same BOOTNONCE value in each STUN Binding Response from
   that NAT.  If the STUN client receives a new BOOTNONCE value, the
   STUN client discards information about NATs it has learned through
   the procedures in this document, and restarts the procedure described
   in this document.

   A weakness of this approach is that an attacker can learn the
   BOOTNONCE value if the attacker is able to connect to the NAT's
   internal network prior to initiating the attack.  This is plausible
   if the internal network has no security (e.g., public WiFi network).
   For this reason, it is RECOMMENDED that the BOOTNONCE value is hashed
   with the STUN client's IP address.  Doing so means that a successful
   attacker must acquire both the same IP address as the victim from
   behind the NAT (to learn the BOOTNONCE), and must also acquire the
   NAT's previous public IP address, or needs to be on-path between the
   victim and its NAT (in which case the attacker has no incentive to
   redirect traffic elsewhere to observe such traffic; however, the
   attacker might be interested in redirecting traffic towards another
   endpoint on the Internet.  To thwart that attack, the STUN client
   MUST only honor STUN responses that have an X-MAPPED-ADDRESS that
   matches the public IP address of the NAT-embedded STUN server.


10.  Open Issues and Discussion Points

   o  Discussion Point:  After discovering NATs and firewalls,
      controlling those devices might also be done with a middlebox
      control protocol (e.g., by using standard or slightly modified



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      versions of SIMCO, UPnP IGD, MIDCOM, or NAT-PMP).  This is open
      for discussion as this document is scoped within the IETF.

   o  Discussion Point:  Tagging would also be useful for the
      Connectivity Check usage (which is used by ICE), especially
      considering that a different firewall may be traversed for media
      than for the initial Binding Discovery usage.  In such a
      situation, the new on-path firewall's policy might not allow a
      binding request to leave the network or allow a binding response
      to return.  In this case, the firewall would need to indicate its
      presence to the STUN client in another way.  An ICMP error message
      may be appropriate, and an ICMP extension [RFC4884] could indicate
      the firewall is controllable.

   o  Open issue:  We could resolve the problem of address dependant
      NATs along the path by introducing a new STUN attribute which
      indicates the UDP port the STUN client wants to control.  However,
      this changes the security properties of STUN Control, so this
      seems undesirable.

      Open issue:  When the STUN client detects an address dependant
      NAT, should we recommend it abandon the STUN Control usage, and
      revert to operation as if it doesn't support the STUN Control
      usage?

   o  Open issue:  How many filter entries are in address dependent
      filtering NATs?  If only one, this does become a real limitation
      if NATs are nested; if they're not nested, the outer-most NAT can
      avoid overwriting its own address in its address dependent filter.

   o  Discussion:  One way to thwart a resource consumption attack is to
      challenge the STUN client.  This would allow the STUN server to
      delay the establishment of resources before a return-routability
      test is performed.  This functionality is currently not provided
      by this specification.  The NONCE attribute
      [I-D.ietf-behave-rfc3489bis] could be useful to provide this
      function.  However, the mere sending of a UDP packet across a NAT
      creates a binding (for ~2 minutes), and there isn't a return-
      routability check for that.

   o  The inside-out discovery technique was removed with version -03 of
      this document.  The procedure worked as follows:  The STUN client
      sends a STUN request to UDP/3478 of the IP address of its default
      router.  If there is a STUN server listening there, it will
      respond, and will indicate its default route via the new DEFAULT-
      ROUTE attribute.  With that information, the STUN client can
      discover the next-outermost NAT by repeating the procedure.  More
      feedback is needed to determine whether the functionality is



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      needed.


11.  IANA Considerations

   This section registers new STUN attributes per the procedures in
   [I-D.ietf-behave-rfc3489bis]:

   Mandatory range:
     0x0029   XOR-INTERNAL-ADDRESS
     0x00..   BOOTNONCE

   Optional range:
     0x8024   REFRESH-INTERVAL
     0x80..   PLEASE-TAG
     0x80..   TAG



12.  Acknowledgements

   Thanks to Remi Denis-Courmont, Christian Dickmann, Bajko Gabor,
   Markus Isomaki, Cullen Jennings, and Philip Matthews for their
   suggestions which have improved this document.

   Thanks to Christian Dickmann and Yan Sun for their initial
   implementations of STUN Control.


13.  References

13.1.  Normative References

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

   [I-D.ietf-behave-rfc3489bis]
              Rosenberg, J., Huitema, C., Mahy, R., Matthews, P., and D.
              Wing, "Session Traversal Utilities for (NAT) (STUN)",
              draft-ietf-behave-rfc3489bis-11 (work in progress),
              October 2007.

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

   [RFC3489]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
              "STUN - Simple Traversal of User Datagram Protocol (UDP)



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              Through Network Address Translators (NATs)", RFC 3489,
              March 2003.

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

13.2.  Informational References

   [I-D.ietf-behave-turn]
              Rosenberg, J., "Traversal Using Relays around NAT (TURN):
              Relay Extensions to Session  Traversal Utilities for NAT
              (STUN)", draft-ietf-behave-turn-04 (work in progress),
              July 2007.

   [UPnP-IGD]
              UPnP Forum, "Universal Plug and Play (UPnP) Internet
              Gateway Device (IGD)", November 2001,
              <http://www.upnp.org/standardizeddcps/igd.asp>.

   [Vista-cert]
              Microsoft, "Windows Logo Program Device Requirements",
              2006, <http://download.microsoft.com/download/d/e/1/
              de1e0c8f-a222-47bc-b78b-1656d4cf3cf7/
              WLP-DeviceReqs_309.pdf>.

   [I-D.cheshire-nat-pmp]
              Cheshire, S., "NAT Port Mapping Protocol (NAT-PMP)",
              draft-cheshire-nat-pmp-02 (work in progress),
              October 2006.

   [RFC3303]  Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and
              A. Rayhan, "Middlebox communication architecture and
              framework", RFC 3303, August 2002.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-18 (work in progress),
              September 2007.

   [I-D.ietf-sip-outbound]
              Jennings, C. and R. Mahy, "Managing Client Initiated
              Connections in the Session Initiation Protocol  (SIP)",
              draft-ietf-sip-outbound-10 (work in progress), July 2007.

   [I-D.ietf-nsis-nslp-natfw]
              Stiemerling, M., "NAT/Firewall NSIS Signaling Layer



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              Protocol (NSLP)", draft-ietf-nsis-nslp-natfw-15 (work in
              progress), July 2007.

   [RFC4884]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
              "Extended ICMP to Support Multi-Part Messages", RFC 4884,
              April 2007.

   [I-D.shore-tist-prot]
              Shore, M., "The TIST (Topology-Insensitive Service
              Traversal) Protocol", draft-shore-tist-prot-00 (work in
              progress), May 2002.

   [I-D.shore-nls-tl]
              Shore, M., "Network-Layer Signaling: Transport Layer",
              draft-shore-nls-tl-05 (work in progress), June 2007.

   [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", RFC 3424, November 2002.

   [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
              Stenberg, "UDP Encapsulation of IPsec ESP Packets",
              RFC 3948, January 2005.

   [I-D.wing-session-auth]
              Wing, D., "Media Session Authorization",
              draft-wing-session-auth-00 (work in progress),
              February 2006.

   [RFC3947]  Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
              "Negotiation of NAT-Traversal in the IKE", RFC 3947,
              January 2005.

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              February 2006.


Appendix A.  Changes

A.1.  Changes in -05

   o  Teredo is another mechanism to learn outer-most NAT, and Teredo
      also benefits from STUN Control with reduced frequency of
      keepalives.



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   o  Provided more detail in how IKE/IPsec-over-UDP would operate with
      STUN Control.

A.2.  Changes in -04

   o  Clarified that all existing bindings, for that source IP address
      and UDP port, are controlled with STUN Control.

   o  Introduction now concentrates on the primary purpose of STUN
      Control, namely reducing keepalive traffic for SIP-Outbound.

A.3.  Changes in -03

   o  Removed TLS from normal STUN operation (as few use it, and ICE
      makes it unnecessary anyway)

   o  BOOTNONCE attribute replaces STUN Control's previous use of TLS.

   o  Added "MIP-capable" bit to TAG attribute

   o  Removed "inside-out" discovery technique.


Appendix B.  Implementation Details

B.1.  Internal NAT Operation

























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   Internally, the NAT can be diagrammed to function like this, where
   the NAT operation occurs before the STUN server:

                                 |
                                 | outside interface
                                 |
                       +---------+---------------+
                       |         |               |
                       |         |    +--------+ |
                       |         |----+ STUN   | |
                       |         |    | Server | |
                       |         |    +---^----+ |
                       |         |        |      |
                       |         |       API     |
                       |         |        |      |
                       | +-------+--------V----+ |
                       | |   NAT Function      | |
                       | +-------+-------------+ |
                       |         |               |
                       +---------+---------------+
                                 |
                                 | inside interface
                                 |
                                 |
   The host on the 'inside' interface of the NAT sends packets to the
   NAT's public interface, where the STUN server is listening.  This
   STUN server returns the same public IP address (XOR-MAPPED-ADDRESS)
   as a STUN server that resides on a separate server on the 'outside'
   interface.  In order to query and to control the NAT binding
   lifetimes, the STUN server uses an API with the NAT function.

            Figure 13: Block Diagram of Internal NAT Operation

B.2.  Linux specifics

   The Linux NAT implementation maintains a separate connection table
   entry for every binding.  When STUN Control is used to control the
   binding lifetime (e.g., extend the lifetime), the binding lifetime
   for each of those connection table entries is modified to the new
   value.











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   For example, with the following message flow:
                STUN Client                        NAT     STUN Server
                    |                               |          |
               1.   |-----Binding Request (UDP)--------------->|
               2.   |<----Binding Response (UDP)---------------|
                    |                               |          |
               3.   |--Binding Request (UDP)------->|          |
               4.   |<-Binding Response (UDP)-------|          |
                    |                               |          |

   the following two connection table entries are created:
     udp      17 24 src=10.7.2.4 dst=10.7.1.2 sport=1024
              dport=3478 packets=1 bytes=64 src=10.7.1.2
              dst=10.7.1.3 sport=3478 dport=1024 packets=1
              bytes=84 mark=0 use=1
     udp      17 25 src=10.7.2.4 dst=10.7.1.3 sport=1024
              dport=3478 packets=2 bytes=64 src=10.7.1.3
              dst=10.7.2.4 sport=3478 dport=1024 packets=2
              bytes=208 mark=0 use=1
   the first src/dst/sport/dport combination is the internal and the
   second one is the external version.  Both are equal in the second
   connection, as the NAT function wasn't active for the "internal"
   message.

   s


Authors' Addresses

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email:  dwing@cisco.com


   Jonathan Rosenberg
   Cisco Systems, Inc.
   Edison, NJ  07054
   USA

   Email:  jdrosen@cisco.com







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   Hannes Tschofenig
   Nokia Siemens Networks
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email:  Hannes.Tschofenig@nsn.com
   URI:    http://www.tschofenig.com











































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