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

BEHAVE                                                           D. Wing
Internet-Draft                                              J. Rosenberg
Intended status:  Standards Track                          Cisco Systems
Expires:  January 9, 2008                                  H. Tschofenig
                                                  Nokia Siemens Networks
                                                            July 8, 2007


  Discovering, Querying, and Controlling Firewalls and NATs using STUN
              draft-wing-behave-nat-control-stun-usage-03

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   This Internet-Draft will expire on January 9, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Simple Traversal Underneath NAT (STUN) is a mechanism for traversing
   NATs.  STUN requests are transmitted through a NAT to external STUN
   servers.  While this works very well, its two primary drawbacks are
   the inability to modify the properties of a NAT binding and the need
   to query a public STUN server for every new NAT binding (e.g., every
   phone call).  These drawbacks require frequent messages which present



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   a load on servers (like SIP servers and STUN servers) and are bad for
   low speed access networks, such as cellular access.

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

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

Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].
































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
   4.  Discovery of Middleboxes (NATs and Firewalls)  . . . . . . . .  6
     4.1.  Outside-In . . . . . . . . . . . . . . . . . . . . . . . .  6
       4.1.1.  Nested NATs  . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  Tagging  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  Query and Control  . . . . . . . . . . . . . . . . . . . . . . 11
     5.1.  Client Procedures  . . . . . . . . . . . . . . . . . . . . 11
     5.2.  Server Procedures  . . . . . . . . . . . . . . . . . . . . 11
   6.  New Attributes . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  REFRESH-INTERVAL Attribute . . . . . . . . . . . . . . . . 12
     6.2.  XOR-INTERNAL-ADDRESS Attribute . . . . . . . . . . . . . . 12
     6.3.  PLEASE-TAG Attribute . . . . . . . . . . . . . . . . . . . 13
     6.4.  TAG Attribute  . . . . . . . . . . . . . . . . . . . . . . 13
     6.5.  BOOTNONCE Attribute  . . . . . . . . . . . . . . . . . . . 14
   7.  Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     7.1.  Simple Security Model  . . . . . . . . . . . . . . . . . . 15
     7.2.  Incremental Deployment . . . . . . . . . . . . . . . . . . 15
     7.3.  Optimize SIP-Outbound  . . . . . . . . . . . . . . . . . . 15
     7.4.  Optimize ICE . . . . . . . . . . . . . . . . . . . . . . . 16
       7.4.1.  Candidate Gathering  . . . . . . . . . . . . . . . . . 16
       7.4.2.  Keepalive  . . . . . . . . . . . . . . . . . . . . . . 16
       7.4.3.  Learning STUN Servers without Configuration  . . . . . 16
   8.  Limitations  . . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.1.  Overlapping IP Addresses with Nested NATs  . . . . . . . . 17
     8.2.  Address Dependent NAT on Path  . . . . . . . . . . . . . . 17
     8.3.  Address Dependent Filtering  . . . . . . . . . . . . . . . 18
     8.4.  Interacting with Legacy NATs . . . . . . . . . . . . . . . 18
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
     9.1.  Authorization  . . . . . . . . . . . . . . . . . . . . . . 19
     9.2.  Resource Exhaustion  . . . . . . . . . . . . . . . . . . . 19
     9.3.  Comparison to Other NAT Control Techniques . . . . . . . . 19
     9.4.  Rogue STUN Server  . . . . . . . . . . . . . . . . . . . . 20
   10. Open Issues and Discussion Points  . . . . . . . . . . . . . . 20
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     13.2. Informational References . . . . . . . . . . . . . . . . . 23
   Appendix A.  Changes . . . . . . . . . . . . . . . . . . . . . . . 24
     A.1.  Changes between -03 and 02 . . . . . . . . . . . . . . . . 24
   Appendix B.  Block Diagram of Internal NAT Operation . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
   Intellectual Property and Copyright Statements . . . . . . . . . . 27




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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 [UPnP], MIDCOM [RFC3303], Bonjour [Bonjour]), 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.  Motivation

   There are a number of problems with existing NAT traversal
   techniques, such as STUN [RFC3489], [UPnP], and [Bonjour]):

   nested NATs:
      Today, many ISPs provide their subscribers with modems that have
      embedded NATs, often with only one physical Ethernet port.  These
      subscribers then install NATs behind those devices to provide
      additional features, such as wireless access.  Another example is
      a NAT in the basement of an apartment building or a campus
      dormitory, which combined with a NAT within the home or dormitory
      room also result in nested NATs.  In both of these situations,
      UPnP and Bonjour no longer function at all, 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



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      within the same apartment building).

      One technique to avoid nested NATs is to disable one of the NATs,
      if it obtains an RFC 1918 address on its WAN interface.  This
      merely sidesteps the problem.  This technique is also ineffective
      if the ISP is NATting its subscribers and the ISP restricts each
      subscriber to one IP address.

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

   chattiness:
      To perform its binding discovery, a STUN client communicates 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).  STUN's chattiness is often cited as
      a reason to use other NAT traversal techniques such as UPnP or
      Bonjour.

      The technique described in this document provides a significant
      reduction in STUN's chattiness, to the point that the only time a
      STUN client needs to communicate with the STUN server on the
      public Internet is when the STUN client is initialized.

   incremental deployment:
      Many other NAT traversal techniques require the endpoint and its
      NAT to both support the new feature or else NAT traversal is not
      possible at all.

      The technique described in this document allows incremental
      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]) and ICE [I-D.ietf-mmusic-ice]
      procedures are used to traverse the NATs without the optimizations
      described in this document.


3.  Overview of Operation

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








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   Discovery of Middleboxes (NATs and Firewalls):
      This document describes two techniques for finding NATs or
      firewalls (see Section 4).  These two approaches are:

      Outside-In:
           Uses STUN to find the outer-most NAT and works itself towards
           the host.

      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.

   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 5.

   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 5.


4.  Discovery of Middleboxes (NATs and Firewalls)

   This document investigates 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.

4.1.  Outside-In

   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



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   server, and knows the 'public' IP address (and port) allocated by the
   outermost NAT.  For example, in the following diagram, the STUN
   client learns the public IP address of its NAT is 192.0.2.1:



     +--------+             +---------------+
     |  STUN  |             |          192.0.2.1             +--------+
     | Client +-------------+               +---<Internet>---+  STUN  |
     |   10.1.1.2/4193     10.1.1.1         |                | Server |
     +--------+             |               |                +--------+
                            |   NAT with    |
                            | Embedded STUN |
                            |    Server     |
                            +---------------+

                Figure 1: One NAT with embedded STUN server

   After learning the public IP address of its outer-most NAT, the STUN
   client attempts to communicate with the STUN server embedded in that
   outer-most NAT.  The STUN client does this by sending a STUN Binding
   Request to the NAT's public IP address.  The NAT will return a STUN
   Binding Response message including the XOR-INTERNAL-ADDRESS
   attribute, which will indicate the IP address and UDP port seen on
   the *internal* side of the NAT for that translation (see Figure 14 in
   Appendix B).  In the example above, the IP address and UDP port
   indicated in XOR-INTERNAL-ADDRESS are the same as that used by the
   STUN client (10.1.1.2/4193), which indicates there are no other NATs
   between the STUN client and that outer-most NAT.



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






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   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.

   The next steps are the additional steps performed by a STUN client
   that has implemented the STUN Control functionality:

   3:  The STUN client sends a UDP Binding Request to the IP address
       learned from the STUN server on the Internet.  This will be
       received by the STUN server embedded in the outer-most NAT.

   4:  The STUN server (embedded in the NAT) responds with a UDP 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).  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 4.1.1.  This functionality meets the need 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, as described in Section 6.1.  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.

4.1.1.  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.



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   The example in Figure 3 shows how a STUN client iterates over the
   NATs to communicate with all of the NATs in the path.

   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 STUN client would learn the outer-most NAT's IP address by
   performing the steps shown in Figure 2.  In the case below, 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 *outer* side 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
   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

   Once a shared secret has been obtained with each of the on-path NATs,
   the STUN client no longer needs the TLS/TCP connection -- all
   subsequent bindings for individual UDP streams (that is, for each



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   subsequent call) are obtained by just sending a Binding Request to
   each of the NATs.  By sending a Binding Request to both NAT-A and
   NAT-B, the STUN client has the opportunity to optimize the packet
   flow if their UDP peer is also behind the same NAT.

4.2.  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 4.1) 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
      [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.



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       If a previous STUN transaction id was found then the firewall
       inserts the TAG attribute, which contains the firewall's
       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.


5.  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 4.

5.1.  Client Procedures

   After discovering on-path NATs and firewalls with the procedure
   described in Section 4, 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 (at STUN UDP port (3478)).

   Client produces for handling the BOOTNONCE attribute can be found in
   Section 6.5.

5.2.  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).

      For example, looking at Figure 1, the XOR-INTERNAL-ADDRESS is the
      IP address and UDP port prior to the NAPT operation.  If there is
      only one NAT, as shown in Figure 1, XOR-INTERNAL-ADDRESS would
      contain the STUN client's own IP address and UDP port.  If there
      are multiple NATs, XOR-INTERNAL-ADDRESS would indicate the IP
      address of the next NAT (that is, the next NAT closer to the STUN
      client).




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   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 STUN Binding
   Request packet was received, i.e., the internal interface.

   Server produces for handling the BOOTNONCE attribute can be found in
   Section 6.5.

   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.


6.  New Attributes

6.1.  REFRESH-INTERVAL Attribute

   In a STUN request per [I-D.ietf-behave-rfc3489bis], the REFRESH-
   INTERVAL attribute indicates the number of milliseconds that the
   client wants the NAT binding (or firewall pinhole) to be opened.
   When the NAT Keepalive usage is being used, the server may become
   overloaded with keepalive messages (Binding Requests or other
   application-level keepalive messages).  The REFRESH-INTERVAL provies
   a mechanism for the client to learn and adjust the NAT's binding
   lifetime and thus reduce the frequency of client-initiated keepalive
   messages.

   In a STUN response, the same attribute indicates how long the STUN
   controlled NAT (or a STUN controlled firewall) is willing to allocate
   the binding (or to create the pinhole).

   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.

6.2.  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.







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   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).

6.3.  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, Bonjour, MIDCOM).

6.4.  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.



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   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.

   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,
   Bonjour, 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).

6.5.  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 NAT
   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 NAT 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



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   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.

   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


7.  Benefits

7.1.  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's simple because only the flow being
   used can be controlled (e.g., have its NAT timeout queried or
   extended).  Other flows cannot be created, queried, or controlled via
   STUN Control.

7.2.  Incremental Deployment

   NAT Control can be incrementally deployed.  If the outer-most NAT
   does not support it, the STUN client behaves as normal.  In this
   case, the tagging procedure described in Section 4.2, will still
   allow to gain some optimizations.  Where the outer-most STUN NAT does
   support it, 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 NAT 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.

7.3.  Optimize SIP-Outbound

   In SIP outbound [I-D.ietf-sip-outbound], the SIP proxy is also the
   STUN server.  STUN Control as described in this document enables two
   optimizations of SIP-Outbound's keepalive mechanism:




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   1.  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, and;

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

7.4.  Optimize ICE

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

7.4.1.  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.

7.4.2.  Keepalive

   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.

7.4.3.  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


8.  Limitations

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 11: 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



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   an on-path NAT.  If there is an on-path NAT, between the STUN client
   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 12: 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 the outside-in usage.
   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, Bonjour, 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
   that mapping.

   However, unlike those technologies, the NAT Control usage 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.  Rogue STUN Server

   Using the mechanisms described in this document, a STUN client learns
   the public IP addresses of its NATs which support the mechanisms
   described in this document.  However, without the STUN client's
   knowledge, those NATs may acquire a new IP address (e.g., DHCP lease
   expiration, a moving network).  When any of those NAT acquire a new
   IP address without the STUN client's knowledge, 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 ensure access to whatever media stream is being
   established by the STUN client (e.g., RTP traffic).  When such an
   attack occurs, the STUN client is unable to distinguish the
   attacker's replies from legitimate replies from the STUN server
   embedded in the STUN client's 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.

   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 the attacker must
   acquire 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.


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
      versions of SIMCO, UPnP, MIDCOM, or Bonjour).  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



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      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
      needed.









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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, Bajko Gabor, Markus Isomaki, Cullen
   Jennings, and Philip Matthews for their suggestions which have
   improved this document.


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., "Session Traversal Utilities for (NAT)
              (STUN)", draft-ietf-behave-rfc3489bis-06 (work in
              progress), March 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)
              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.





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13.2.  Informational References

   [I-D.ietf-behave-turn]
              Rosenberg, J., "Obtaining Relay Addresses from Simple
              Traversal Underneath NAT (STUN)",
              draft-ietf-behave-turn-03 (work in progress), March 2007.

   [UPnP]     UPnP Forum, "Universal Plug and Play", 2000,
              <http://www.upnp.org>.

   [Bonjour]  Apple Computer, "Bonjour", 2005,
              <http://www.apple.com/macosx/features/bonjour/>.

   [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-16 (work in progress), June 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-09 (work in progress), June 2007.

   [I-D.ietf-nsis-nslp-natfw]
              Stiemerling, M., "NAT/Firewall NSIS Signaling Layer
              Protocol (NSLP)", draft-ietf-nsis-nslp-natfw-14 (work in
              progress), March 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



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              Translation", RFC 3424, November 2002.

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


Appendix A.  Changes

A.1.  Changes between -03 and 02

   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.  Block Diagram of 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 | |
                       |         |    +--------+ |
                       |         |               |
                       | +-------+-------------+ |
                       | |   NAT Function      | |
                       | +-------+-------------+ |
                       |         |               |
                       +---------+---------------+
                                 |
                                 | inside interface
                                 |
                                 |

            Figure 14: Block Diagram of Internal NAT Operation


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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Full Copyright Statement

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