





Internet Engineering Task Force (IETF)                      D. MacDonald
Request for Comments: 5780                                   B. Lowekamp
Category: Experimental                                             Skype
ISSN: 2070-1721                                                 May 2010


NAT Behavior Discovery Using Session Traversal Utilities for NAT (STUN)

Abstract

   This specification defines an experimental usage of the Session
   Traversal Utilities for NAT (STUN) Protocol that discovers the
   presence and current behavior of NATs and firewalls between the STUN
   client and the STUN server.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5780.



















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Copyright Notice

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   Without obtaining an adequate license from the person(s) controlling
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   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1.  Applicability  . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  5
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Example Diagnostic Use . . . . . . . . . . . . . . . . . .  6
     2.2.  Example Use with P2P Overlays  . . . . . . . . . . . . . .  7
     2.3.  Experimental Goals . . . . . . . . . . . . . . . . . . . .  8
   3.  Overview of Operations . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Determining NAT Mapping  . . . . . . . . . . . . . . . . . 10
     3.2.  Determining NAT Filtering  . . . . . . . . . . . . . . . . 10
     3.3.  Binding Lifetime Discovery . . . . . . . . . . . . . . . . 10
     3.4.  Diagnosing NAT Hairpinning . . . . . . . . . . . . . . . . 11
     3.5.  Determining Fragment Handling  . . . . . . . . . . . . . . 11
     3.6.  Detecting a Generic Application Level Gateway (ALG)  . . . 11
   4.  Discovery Process  . . . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Source Port Selection  . . . . . . . . . . . . . . . . . . 12
     4.2.  Checking for UDP Connectivity with the STUN Server . . . . 13
     4.3.  Determining NAT Mapping Behavior . . . . . . . . . . . . . 14
     4.4.  Determining NAT Filtering Behavior . . . . . . . . . . . . 14
     4.5.  Combining and Ordering Tests . . . . . . . . . . . . . . . 15
     4.6.  Binding Lifetime Discovery . . . . . . . . . . . . . . . . 15
   5.  Client Behavior  . . . . . . . . . . . . . . . . . . . . . . . 17
     5.1.  Discovery  . . . . . . . . . . . . . . . . . . . . . . . . 17
     5.2.  Security . . . . . . . . . . . . . . . . . . . . . . . . . 18
   6.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . . . 18
     6.1.  Preparing the Response . . . . . . . . . . . . . . . . . . 18
   7.  New Attributes . . . . . . . . . . . . . . . . . . . . . . . . 20
     7.1.  Representing Transport Addresses . . . . . . . . . . . . . 21
     7.2.  CHANGE-REQUEST . . . . . . . . . . . . . . . . . . . . . . 21
     7.3.  RESPONSE-ORIGIN  . . . . . . . . . . . . . . . . . . . . . 21
     7.4.  OTHER-ADDRESS  . . . . . . . . . . . . . . . . . . . . . . 22
     7.5.  RESPONSE-PORT  . . . . . . . . . . . . . . . . . . . . . . 22
     7.6.  PADDING  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   8.  IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 23
     8.1.  Problem Definition . . . . . . . . . . . . . . . . . . . . 23
     8.2.  Exit Strategy  . . . . . . . . . . . . . . . . . . . . . . 23
     8.3.  Brittleness Introduced by STUN NAT Behavior Discovery  . . 24
     8.4.  Requirements for a Long-Term Solution  . . . . . . . . . . 24
     8.5.  Issues with Existing NAPT Boxes  . . . . . . . . . . . . . 24
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
     9.1.  STUN Attribute Registry  . . . . . . . . . . . . . . . . . 25
     9.2.  Port Numbers and SRV Registry  . . . . . . . . . . . . . . 25
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     12.2. Informative References . . . . . . . . . . . . . . . . . . 27



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1.  Applicability

   This experimental NAT Behavior Discovery STUN usage provides
   information about a NAT device's observable transient behavior; it
   determines a NAT's behavior with regard to the STUN server used and
   the particular client ports used at the instant the test is run.
   This STUN usage does not allow an application behind a NAT to make an
   absolute determination of the NAT's characteristics.  NAT devices do
   not behave consistently enough to predict future behavior with any
   guarantee.  Applications requiring reliable reach between two
   particular endpoints must establish a communication channel through
   NAT using another technique.  IETF has proposed standards including
   [RFC5245] and [RFC5626] for establishing communication channels when
   a publicly accessible rendezvous service is available.

   The uses envisioned for the STUN attributes included in this document
   are diagnostics and real-time tuning of applications.  For example,
   determining what may work and should be tried first compared to more
   expensive methods.  The attributes can also be used to observe
   behaviors that causes an application's communication to fail, thus
   enabling better selection of methods of recovery.  The STUN
   attributes could also be a basis for a network technician's
   diagnostics tool to observe NAT behavior.

   This document proposes experimental usage of these attributes for
   real-time optimization of parameters for protocols in situations
   where a publicly accessible rendezvous service is not available.
   Such a use of these techniques is only possible when the results are
   applied as an optimization and a reliable fallback is available in
   case the NAT's behavior becomes more restrictive than determined by
   the Behavior Discovery tests.  One possible application is role
   selection in peer-to-peer (P2P) networks based on statistical
   experience with establishing direct connections and diagnosing NAT
   behavior with a variety of peers.  The experimental question is
   whether such a test is useful.  Consider a node that tries to join an
   overlay as a full peer when its NAT prevents sufficient connectivity;
   joining and withdrawing from the overlay might be expensive and/or
   lead to unreliable or poorly performing operations.  Even if the
   behavior discovery check is only "correct" 75% of the time, its
   relative cheapness may make it very useful for optimizing the
   behavior of the overlay network.  Section 2.2 describes this
   experimental application in more detail and discusses how to evaluate
   its success or failure.

   The applications of this STUN usage differ from the original use of
   STUN (originally RFC 3489 [RFC3489], now RFC 5389 [RFC5389]).  This
   specification acknowledges that the information gathered in this




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   usage is not, and cannot be, correct 100% of the time, whereas STUN
   focused only on getting information that could be known to be correct
   and static.

   This specification can also be compared to ICE.  ICE requires a
   fallback to TURN be available whereas RFC 3489 based applications
   tried to determine in advance whether they would need a relay and
   what their peer reflexive address will be, which is not generally
   achievable.

   This STUN usage requires an application using it to have a fallback.
   However, unlike ICE's focus on the problems inherent in VoIP
   sessions, this STUN usage doesn't assume that it will be used to
   establish a connection between a single pair of machines, so
   alternative fallback mechanisms may be available.

   For example, in a P2P application it may be possible to simply switch
   out of the role where such connections need to be established or to
   select an alternative indirect route if the peer discovers that, in
   practice, 10% of its connection attempts fail.

   It is submitted to the Internet community as an experimental protocol
   that, when applied with appropriate statistical underpinnings and
   application behavior that is ultimately based on experienced
   connectivity patterns, can lead to more stability and increased
   performance than is available without the knowledge it provides.

   If a Standards Track document specifies the use of any portion of
   this STUN usage, that document MUST describe how incorrect
   information derived using these methods will be managed, either
   through identifying when a NAT's behavior changed or because the
   protocol uses such knowledge as an optimization but remains
   functional when the NAT's behavior changes.  The referencing document
   MUST also define when the fallback mechanism will be invoked.
   Applications in different domains may vary greatly in how
   aggressively the fallback mechanism is utilized, so there must be a
   clear definition of when the fallback mechanism is invoked.

1.1.  Requirements Language

   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 RFC 2119 [RFC2119].








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2.  Introduction

   "Session Traversal Utilities for NAT (STUN)" [RFC5389] provides a
   mechanism to discover the reflexive transport address toward the STUN
   server, using the Binding Request.  This specification defines the
   NAT Behavior Discovery STUN usage, which allows a STUN client to
   probe the current behavior of the NAT/firewall (NAT/FW) devices
   between the client and the STUN server.  This usage defines new STUN
   attributes for the Binding Request and Binding Response.

   Many NAT/FW devices do not behave consistently and will change their
   behavior under load and over time.  Applications requiring high
   reliability must be prepared for the NAT's behavior to become more
   restrictive.  Specifically, it has been found that under load NATs
   may transition to the most restrictive filtering and mapping behavior
   and shorten the lifetime of new and existing bindings.  In short,
   applications can discover how bad things currently are, but not how
   bad things will get.

   Despite this limitation, instantaneous observations are often quite
   useful in troubleshooting network problems, and repeated tests over
   time, or in known load situations, may be used to characterize a
   NAT's behavior.  In particular, in the hands of a person
   knowledgeable about the needs of an application and the nodes an
   application needs to communicate with, it can be a powerful tool.

2.1.  Example Diagnostic Use

   Applications that work well in the lab, but fail in a deployment, are
   notoriously common within distributed systems.  There are few systems
   developers who have not had the experience of searching to determine
   the difference in the environments for insight as to what real-
   network behavior was missed in the testing lab.  The Behavior
   Discovery usage offers a powerful tool that can be used to check NAT
   and firewall behavior as the application is running.  For example, an
   application could be designed to perform Behavior Discovery tests
   whenever it experiences significant communications problems when
   running.  Such analysis might be included as part of the diagnostic
   information logged by the application.

   As they are being used to detect instantaneous behavior for analysis
   by an experienced developer or administrator, there are relatively
   few concerns about this application of the NAT Behavior Discovery
   STUN usage.  However, the user should be aware that

   o  adding new traffic to new destinations (STUN servers) has the
      potential to itself change the behavior of a NAT and




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   o  the user must be careful to select a STUN server that is
      appropriately located, ideally collocated (or even integrated)
      with the communication partners of the application in question,
      for the results to be applicable to the network conditions
      experienced by the application.

2.2.  Example Use with P2P Overlays

   An application could use Behavior Discovery in a P2P protocol to
   determine if a particular endpoint is a reasonable candidate to
   participate as a peer or supernode (defined here as a peer in the
   overlay that offers services, including message routing, to other
   members or clients of the overlay network).  This P2P network
   application is willing to select supernodes that might be located
   behind NATs to avoid the cost of dedicated servers.  A supernode
   candidate requires that its NAT or NATs offer Endpoint-Independent
   Filtering.  It might periodically re-run tests and would remove
   itself as a supernode if its NAT/FW chain lost this characteristic.
   These tests could be run with other supernodes acting as STUN servers
   as well as with dedicated STUN servers.  As many P2P algorithms
   tolerate non-transitive connectivity between a portion of their
   peers, guaranteed pair-wise reliable reach might be sacrificed in
   order to distribute the P2P overlay's load across peers that can be
   directly contacted by the majority of users.

   Consider an example from a hypothetical P2P protocol in more detail:
   when P2P node A starts up, it tests its NAT(s) relative to other
   peers already in the overlay.  If the results of its testing indicate
   A is behind a "good" NAT (with Endpoint-Independent Mapping and
   Filtering), A will join the overlay and establish connections with
   appropriate peers in the overlay to join the overlay's topology.
   Although A is reachable by routing messages across the overlay
   topology, A will also include in its communication with other nodes
   that they may reach it directly using its reflexive IP address (or
   addresses) that A discovered in its initial testing.  Suppose that
   later, node B wants to send a message to A, and B is not a neighbor
   of A in the overlay topology.  B may send the message directly to A's
   IP address and start a timer.  If B doesn't receive a response within
   a certain amount of time, then it routes the message to A across the
   overlay instead and includes a flag that indicates a direct
   connection was attempted but failed.  (Alternatively, B could
   simultaneously send the message to A's IP address across the overlay,
   which guarantees minimum response latency, but can waste bandwidth.)
   Over time, A observes the percentage of successful direct messages it
   receives out of those attempted.  If the percentage of successful
   direct connections is below some threshold (perhaps 75%), then A may
   stop advertising for direct connections because it has determined in
   practice that its NATs are not providing sufficiently reliable



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   connectivity to justify the cost of attempting the direct message.
   But if the percentage is high enough, A continues to advertise
   because the successful direct connections are improving the overlay's
   performance by reducing the routing load imposed on the overlay.  If
   at some point, A's NAT or NATs change behavior, A will notice a
   change in its percentage of successful direct connections and may re-
   evaluate its decision to advertise a public address.  In this
   hypothetical example, behavior discovery is used for A's initial
   operating mode selection, but the actual decision for whether to
   continue advertising that public IP/port pair is made based on actual
   operating data.  The results of the Behavior Discovery usage are also
   used as a performance optimization, as A is at all times able to
   establish connectivity through the overlay if the attempted direct
   connection fails.

   Use of behavior discovery for such an application requires:

   o  Use of a protocol capable of offering reliable end-user
      performance while using unreliable links between pairs of nodes.

   o  A protocol offering a reliable fallback to connections attempted
      based on the results of Behavior Discovery probing.

   o  The application is deployed behind NATs that provide Endpoint-
      Independent Filtering and that remain in this mode for an amount
      of time sufficient for the application to identify their behavior,
      distribute this information to the rest of the overlay, and
      provide useful work for the application.

   This document is experimental as applications implementing open
   protocols have yet to be deployed in such environments to demonstrate
   that these three requirements have been met.  However, anecdotal
   evidence suggests that NATs targeted at households and small
   businesses have stable behavior, especially when there are few
   clients behind them.  Numerous P2P applications have been deployed
   that appear to have these properties, although their protocols have
   not yet been subjected to rigorous evaluation by standards bodies.

2.3.  Experimental Goals

   The criteria for an application to successfully demonstrate use of
   the NAT Behavior Discovery STUN usage would include:

   o  An implementation that relies on this usage to determine its run-
      time behavior, most likely using it to determine an initial choice
      of options that are then adjusted based on experience with its
      network connections.




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   o  The implementation must either demonstrate its applicability in
      environments where it is realistic to expect a provider to deploy
      dedicated STUN servers with multiple IP addresses, or it must
      demonstrate duplicating the behavior of such a dedicated STUN
      server with two nodes that share the role of providing the
      address-changing operations required by this usage.

   o  Experimental evidence that the application of this usage results
      in improved behavior of the application in real-world conditions.
      The exact metrics for this improvement may vary, some
      possibilities include: faster convergence to the proper
      parameters, less work to set up initial connections, fewer
      reconfigurations required after startup, etc.

   o  A protocol specification that defines how the implementation
      applies this usage.

   The P2P scenario described above is a likely experimental test case
   for this usage, but others applications are possible as well.

3.  Overview of Operations

   In a typical configuration, a STUN client is connected to a private
   network and through one or more NATs to the public Internet.  The
   client is configured with the address of a STUN server on the public
   Internet.  The Behavior Discovery usage makes use of SRV records so
   that a server may use a different transport address for this usage
   than for other usages.  This usage does not provide backward
   compatibility with RFC 3489 [RFC3489] for either clients or servers.
   Implementors of clients that wish to be compliant with RFC 3489
   servers should see that specification.  Implementors of servers
   SHOULD NOT include support for RFC 3489 clients, as the original uses
   of that protocol have been deprecated.

   Because STUN forbids a server from creating a new TCP or TCP/TLS
   connection to the client, many tests apply only to UDP.  The
   applicability of the various tests is indicated below.

   The STUN NAT Behavior Discovery usage defines new attributes on the
   STUN Binding Request and STUN Binding Response that allow these
   messages to be used to diagnose the current behavior of the NAT(s)
   between the client and server.

   This section provides a descriptive overview of the typical use of
   these attributes.  Normative behavior is described in Sections 5, 6,
   and 7.





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3.1.  Determining NAT Mapping

   A client behind a NAT wishes to determine if that NAT is currently
   using Endpoint-Independent, Address-Dependent, or Address and Port-
   Dependent Mapping [RFC4787].  The client performs a series of tests
   that make use of the OTHER-ADDRESS attribute; these tests are
   described in detail in Section 4.  These tests send binding requests
   to the alternate address and port of the STUN server to determine
   mapping behavior.  These tests can be used for UDP, TCP, or TCP/TLS
   connections.

3.2.  Determining NAT Filtering

   A client behind a NAT wishes to determine if that NAT is currently
   using Endpoint-Independent, Address-Dependent, or Address and Port-
   Dependent Filtering [RFC4787].  The client performs a series of tests
   that make use of the OTHER-ADDRESS and CHANGE-REQUEST attributes;
   these tests are described in Section 4.  These tests request
   responses from the alternate address and port of the STUN server; a
   precondition to these tests is that no binding be established to the
   alternate address and port.  See below for more information.  Because
   the NAT does not know that the alternate address and port belong to
   the same server as the primary address and port, it treats these
   responses the same as it would those from any other host on the
   Internet.  Therefore, the success of the binding responses sent from
   the alternate address and port indicate whether the NAT is currently
   performing Endpoint-Independent Filtering, Address-Dependent
   Filtering, or Address and Port-Dependent Filtering.  This test
   applies only to UDP datagrams.

3.3.  Binding Lifetime Discovery

   Many systems, such as VoIP, rely on being able to keep a connection
   open between a client and server or between peers of a P2P system.
   Because NAT bindings expire over time, keepalive messages must be
   sent across the connection to preserve it.  Because keepalives impose
   some overhead on the network and servers, reducing the frequency of
   keepalives can be useful.

   A normal request-response protocol cannot be used to test binding
   lifetime because the initial request resets the binding timer.
   Behavior discovery defines the RESPONSE-PORT attribute to allow the
   client and server to set up a "control channel" using one port on the
   client that is used to test the binding lifetime of a different port
   allocated on the client.  More generally, RESPONSE-PORT allows the
   client to allocate two ports and request that responses to queries
   sent from one port be delivered to the other.  The client uses its
   second port and the STUN server's alternate address to check if an



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   existing binding that hasn't had traffic sent on it is still open
   after time T.  This approach is described in detail in Section 4.6.
   This test applies only to UDP datagrams.

3.4.  Diagnosing NAT Hairpinning

   STUN Binding Requests allow a client to determine whether it is
   behind a NAT that supports hairpinning of connections.  To perform
   this test, the client first sends a Binding Request to its STUN
   server to determine its mapped address.  The client then sends a STUN
   Binding Request to this mapped address from a different port.  If the
   client receives its own request, the NAT hairpins connections.  This
   test applies to UDP, TCP, or TCP/TLS connections.

3.5.  Determining Fragment Handling

   Some NATs exhibit different behavior when forwarding fragments than
   when forwarding a single-frame datagram.  In particular, some NATs do
   not hairpin fragments at all and some platforms discard fragments
   under load.  To diagnose this behavior, STUN messages may be sent
   with the PADDING attribute, which simply inserts additional space
   into the message.  By forcing the STUN message to be divided into
   multiple fragments, the NAT's behavior can be observed.

   All of the previous tests can be performed with PADDING if a NAT's
   fragment behavior is important for an application, or only those
   tests that are most interesting to the application can be retested.
   PADDING only applies to UDP datagrams.  PADDING can not be used with
   RESPONSE-PORT.

3.6.  Detecting a Generic Application Level Gateway (ALG)

   A number of NAT boxes are now being deployed into the market that try
   to provide "generic" ALG functionality.  These generic ALGs hunt for
   IP addresses, either in text or binary form within a packet, and
   rewrite them if they match a binding.  This behavior can be detected
   because the STUN server returns both the MAPPED-ADDRESS and XOR-
   MAPPED-ADDRESS in the same response.  If the result in the two does
   not match, there is a NAT with a generic ALG in the path.  This test
   apples to UDP and TCP, but not TLS over TCP connections.

4.  Discovery Process

   This section provides a descriptive overview of how the NAT Behavior
   Discovery usage primitives allow checks to be made to discover the
   current behavior of the NAT or NATs an application is behind.  These
   tests can only give the instantaneous behavior of a NAT; it has been
   found that NATs can change behavior under load and over time.  The



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   results of these tests therefore can be regarded as upper bounds --
   an application must assume that NAT behavior can become more
   restrictive at any time.  Results from tests performed using a
   particular port on the client may also not indicate the behavior
   experienced by a different port, as described in Section 4.1.

   Definitions for NAT filtering and mapping behavior are from
   [RFC4787].  The tests described here are for UDP connectivity, NAT
   mapping behavior, NAT filtering behavior, and NAT binding lifetime
   discovery; additional tests could be designed using this usage's
   mechanisms.  The tests described below include only tests that can be
   performed using a client with a single IP address.  A client with
   multiple IP addresses (or multiple clients collaborating) behind the
   same NAT can combine their probes to test additional aspects of NAT
   behavior, such as port overloading.  This section provides a
   descriptive overview of how the primitives provided by the STUN
   attributes in this specification may be used to perform behavior
   tests.

   Normative specifications for the attributes are defined in later
   sections.

4.1.  Source Port Selection

   Proper source port selection is important to ensuring the usefulness
   and accuracy of the Behavior Discovery tests.  There are two
   preconditions for tests:

   o  Because mapping behavior can vary on a port-by-port basis, an
      application should perform its tests using the source port
      intended for use by the application whenever possible.  If it
      intends to use multiple source ports, it should repeat these tests
      for each source port.  Such tests should be performed sequentially
      to reduce load on the NAT.

   o  Because the results of some diagnostic checks depend on previous
      state in the NAT created by prior traffic, the tests should be
      performed using a source port that has not generated recent
      traffic.  Therefore, the application should use a random source
      port or ensure that no traffic has previously occurred on the
      selected port prior to performing tests, generally by allocating a
      port and holding it unused for at least 15 minutes prior to the
      tests.

   Ensuring both of these preconditions can be challenging, particularly
   for a device or application wishing to perform Behavior Discovery
   tests at startup.  The following guidelines are suggested for
   reducing the likelihood of problems:



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   o  An application intended to operate behind a NAT should not attempt
      to allocate a specific or well-known port.  Because such software
      must be designed to interoperate using whatever port is mapped to
      it by the NAT, the specific port is unnecessary.  Instead, on
      startup, a random port should be selected (see below for
      recommended ranges).  An application, particularly on an embedded
      device, should not rely on the host operating system to select the
      next available port because that might result in the application
      receiving the same port on each restart.  An application using the
      same port between restarts may not receive accurate results from
      Behavior Discovery tests that are intended to test state-related
      behavior of NATs, such as filtering and binding lifetime.

   o  An application requiring multiple ports, such as separate ports
      for control and media, should allocate those ports on startup when
      possible.  Even if there is no immediate need for media flow, if
      Behavior Discovery tests will be run on those ports, allocating
      them early will allow them to be left idle, increasing the chance
      of obtaining accurate results from Behavior Discovery tests.

   o  Although the most reliable results are obtained when performing
      tests with the specific ports that the application will use, in
      many cases an application will need to allocate and use ports
      without being able to perform complete Behavior Discovery tests on
      those ports.  In those cases, an application should randomly
      select its ports from a range likely to receive the same treatment
      by the NAT.  This document recommends ranges of 32768-49151, which
      is the upper end of IANA's Registered Ports range, and 49152-
      65535, which is IANA's Dynamic and/or Private port range, for
      random selection.  To attempt to characterize a NAT's general
      treatment of ports in these ranges, a small number of ports within
      a range can be randomly selected and characterized.

   Those tests particularly sensitive to prior state on a NAT will be
   indicated below.

4.2.  Checking for UDP Connectivity with the STUN Server

   The client sends a STUN Binding Request to a server.  This causes the
   server to send the response back to the address and port that the
   request came from.  If this test yields no response, the client knows
   right away that it does not have UDP connectivity with the STUN
   server.  This test requires only STUN [RFC5389] functionality.








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4.3.  Determining NAT Mapping Behavior

   This will require at most three tests.  In test I, the client
   performs the UDP connectivity test.  The server will return its
   alternate address and port in OTHER-ADDRESS in the binding response.
   If OTHER-ADDRESS is not returned, the server does not support this
   usage and this test cannot be run.  The client examines the XOR-
   MAPPED-ADDRESS attribute.  If this address and port are the same as
   the local IP address and port of the socket used to send the request,
   the client knows that it is not NATed and the effective mapping will
   be Endpoint-Independent.

   In test II, the client sends a Binding Request to the alternate
   address, but primary port.  If the XOR-MAPPED-ADDRESS in the Binding
   Response is the same as test I the NAT currently has Endpoint-
   Independent Mapping.  If not, test III is performed: the client sends
   a Binding Request to the alternate address and port.  If the XOR-
   MAPPED-ADDRESS matches test II, the NAT currently has Address-
   Dependent Mapping; if it doesn't match it currently has Address and
   Port-Dependent Mapping.

4.4.  Determining NAT Filtering Behavior

   This will also require at most three tests.  These tests are
   sensitive to prior state on the NAT.

   In test I, the client performs the UDP connectivity test.  The server
   will return its alternate address and port in OTHER-ADDRESS in the
   binding response.  If OTHER-ADDRESS is not returned, the server does
   not support this usage and this test cannot be run.

   In test II, the client sends a binding request to the primary address
   of the server with the CHANGE-REQUEST attribute set to change-port
   and change-IP.  This will cause the server to send its response from
   its alternate IP address and alternate port.  If the client receives
   a response, the current behavior of the NAT is Endpoint-Independent
   Filtering.

   If no response is received, test III must be performed to distinguish
   between Address-Dependent Filtering and Address and Port-Dependent
   Filtering.  In test III, the client sends a binding request to the
   original server address with CHANGE-REQUEST set to change-port.  If
   the client receives a response, the current behavior is Address-
   Dependent Filtering; if no response is received, the current behavior
   is Address and Port-Dependent Filtering.






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4.5.  Combining and Ordering Tests

   Clients may wish to combine and parallelize these tests to reduce the
   number of packets sent and speed the discovery process.  For example,
   test I of the filtering and mapping tests also checks if UDP is
   blocked.  Furthermore, an application or user may not need as much
   detail as these sample tests provide.  For example, establishing
   connectivity between nodes becomes significantly more difficult if a
   NAT has any behavior other than Endpoint-Independent Mapping, which
   requires only test I and II of Section 4.3.  An application that
   determines its NAT does not always provide Endpoint-Independent
   Mapping might notify the user if no relay is configured, whereas an
   application behind a NAT that provides Endpoint-Independent Mapping
   might not notify the user until a subsequent connection actually
   fails or might provide a less urgent notification that no relay is
   configured.  Such a test does not alleviate the need for [RFC5245],
   but it does provide some information regarding whether ICE is likely
   to be successful establishing non-relayed connections.

   Care must be taken when combining and parallelizing tests, due to the
   sensitivity of certain tests to prior state on the NAT and because
   some NAT devices have an upper limit on how quickly bindings will be
   allocated.  Section 5 restricts the rate at which clients may begin
   new STUN transactions.

4.6.  Binding Lifetime Discovery

   STUN can also be used to probe the lifetimes of the bindings created
   by the NAT.  Such tests are sensitive to prior state on the NAT.  For
   many NAT devices, an absolute refresh interval cannot be determined;
   bindings might be closed more quickly under heavy load or might not
   behave as the tests suggest.  For this reason, applications that
   require reliable bindings must send keepalives as frequently as
   required by all NAT devices that will be encountered.  Suggested
   refresh intervals are outside the scope of this document.  [RFC5245]
   and OUTBOUND [RFC5626] have suggested refresh intervals.

   Determining the binding lifetime relies on two separate source ports
   being used to send STUN Binding Requests to the STUN server.  The
   general approach is that the client uses a source port X to send a
   single Binding Request.  After a period of time during which source
   port X is not used, the client uses a second source port Y to send a
   Binding Request to the STUN server that indicates the response should
   be sent to the binding established to port X.  If the binding for
   port X has timed out, that response will not be received.  By varying
   the time between the original Binding Request sent from X and the
   subsequent request sent from Y, the client can determine the binding
   lifetime.



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   To determine the binding lifetime, the client first sends a Binding
   Request to the server from a particular source port, X.  This creates
   a binding in the NAT.  The response from the server contains a
   MAPPED-ADDRESS attribute, providing the public address and port on
   the NAT.  Call this Pa and Pp, respectively.  The client then starts
   a timer with a value of T seconds.  When this timer fires, the client
   sends another Binding Request to the server, using the same
   destination address and port, but from a different source port, Y.
   This request contains an RESPONSE-PORT attribute, set to Pp, to
   request the response be delivered to (Pa, Pp).  This will create a
   new binding on the NAT, and cause the STUN server to send a Binding
   Response that would match the old binding, (Pa, Pp), if it still
   exists.  If the client receives the Binding Response on port X, it
   knows that the binding has not expired.  If the client receives the
   Binding Response on port Y (which is possible if the old binding
   expired, and the NAT allocated the same public address and port to
   the new binding), or receives no response at all, it knows that the
   binding has expired.

   Because some NATs only refresh bindings when outbound traffic is
   sent, the client must resend a binding request from the original
   source port before beginning a second test with a different value of
   T.  The client can find the value of the binding lifetime by doing a
   binary search through T, arriving eventually at the value where the
   response is not received for any timer greater than T, but is
   received for any timer less than T.  Note also that the binding
   refresh behavior (outbound only or all traffic) can be determined by
   sending multiple Binding Requests from port Y without refreshes from
   the original source port X.

   This discovery process takes quite a bit of time and is something
   that will typically be run in the background on a device once it
   boots.

   It is possible that the client can get inconsistent results each time
   this process is run.  For example, if the NAT should reboot, or be
   reset for some reason, the process may discover a lifetime that is
   shorter than the actual one.  Binding lifetime may also be dependent
   on the traffic load on the NAT.  For this reason, implementations are
   encouraged to run the test numerous times and be prepared to get
   inconsistent results.

   Like the other diagnostics, this test is inherently unstable.  In
   particular, an overloaded NAT might reduce binding lifetime to shed
   load.  A client might find this diagnostic useful at startup, for
   example, setting the initial keepalive interval on its connection to
   the server to 10 seconds while beginning this check.  After
   determining the current lifetime, the keepalive interval used by the



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   connection to the server can be set to this appropriate value.
   Subsequent checks of the binding lifetime can then be performed using
   the keepalives in the server connection.  The STUN Keepalive Usage
   [RFC5626] provides a response that confirms the connection is open
   and allows the client to check that its mapped address has not
   changed.  As that provides both the keepalive action and diagnostic
   that it is working, it should be preferred over any attempt to
   characterize the connection by a secondary technique.

5.  Client Behavior

   Unless otherwise specified here, all procedures for preparing,
   sending, and processing messages as described in the STUN Binding
   Usage [RFC5389] are followed.

   As support for RESPONSE-PORT is optional, a client MUST be prepared
   to receive a 420 (Unknown Attribute) error to requests that include
   RESPONSE-PORT.  Support for OTHER-ADDRESS and CHANGE-REQUEST is
   optional, but MUST be supported by servers advertised via SRV, as
   described below.  This is to allow the use of PADDING and RESPONSE-
   PORT in applications where servers do not have multiple IP addresses.
   Clients MUST be prepared to receive a 420 for requests that include
   CHANGE-REQUEST when OTHER-ADDRESS was not received in Binding
   Response messages from the server.

   If an application makes use of the NAT Behavior Discovery STUN usage
   by multiplexing it in a flow with application traffic, a FINGERPRINT
   attribute SHOULD be included unless it is always possible to
   distinguish a STUN message from an application message based on their
   header.

   When PADDING is used, it SHOULD be equal to the MTU of the outgoing
   interface.

   Clients SHOULD ignore an ALTERNATE-SERVER attribute in a response
   unless they are using authentication with a provider of STUN servers
   that is aware of the topology requirements of the tests being
   performed.

   A client SHOULD NOT generate more than ten new STUN transactions per
   second and SHOULD pace them such that the retransmission timeouts
   (RTOs) do not synchronize the retransmissions of each transaction.

5.1.  Discovery

   Unless the user or application is aware of the transport address of a
   STUN server supporting the NAT Behavior Discovery usage through other
   means, a client is configured with the domain name of the provider of



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   the STUN servers.  The domain is resolved to a transport address
   using SRV procedures [RFC2782].  The mechanism for configuring the
   client with the domain name of the STUN servers or of acquiring a
   specific transport address is out of scope for this document.

   For the Behavior Discovery usage, the service name is "stun-behavior"
   for UDP and TCP.  The service name is "stun-behaviors" for TLS over
   TCP.  Only "tcp" is defined as a protocol for "stun-behaviors".
   Other aspects of handling failures and default ports are followed as
   described in STUN [RFC5389].

5.2.  Security

   Servers MAY require authentication before allowing a client to make
   use of its services.  The method for obtaining these credentials,
   should the server require them, is outside the scope of this usage.
   Presumably, the administrator or application relying on this usage
   should have its own method for obtaining credentials.  If the client
   receives a 401 (Unauthorized) Response to a Request, then it must
   either acquire the appropriate credential from the application before
   retrying or report a permanent failure.  Procedures for encoding the
   MESSAGE-INTEGRITY attribute for a request are described in STUN
   [RFC5389].

6.  Server Behavior

   Unless otherwise specified here, all procedures for preparing,
   sending, and processing messages as described for the STUN Binding
   Usage of STUN [RFC5389] are followed.

   A server implementing the NAT Behavior Discovery usage SHOULD be
   configured with two separate IP addresses on the public Internet.  On
   startup, the server SHOULD allocate a pair of ports for each of the
   UDP, TCP, and TCP/TLS transport protocols, such that it can send and
   receive datagrams using the same ports on each IP address (normally a
   wildcard binding accomplishes this).  TCP and TCP/TLS MUST use
   different ports.  If a server cannot allocate the same ports on two
   different IP address, then it MUST NOT include an OTHER-ADDRESS
   attribute in any Response and MUST respond with a 420 (Unknown
   Attribute) to any Request with a CHANGE-REQUEST attribute.  A server
   with only one IP address MUST NOT be advertised using the SRV service
   name "stun-behavior" or "stun-behaviors".

6.1.  Preparing the Response

   After performing all authentication and verification steps, the
   server begins processing specific to this Usage if the Binding
   Request contains any request attributes defined in this document:



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   RESPONSE-PORT, CHANGE-REQUEST, or PADDING.  If the Binding Request
   does not contain any attributes from this document, OTHER-ADDRESS and
   RESPONSE-ORIGIN are still included in the Binding Response.

   The server MUST include both MAPPED-ADDRESS and XOR-MAPPED-ADDRESS in
   its Response.

   If the Request contains the CHANGE-REQUEST attribute and the server
   does not have an alternate address and port as described above, the
   server MUST generate an error response of type 420.

   The source address and port of the Binding Response depend on the
   value of the CHANGE-REQUEST attribute and on the address and port on
   which the Binding Request was received; this is summarized in
   Table 1.

   Let A1 and A2 be the two IP addresses used by the server, and P1 and
   P2 be the ports used by the server.  Let Da represent the destination
   IP address of the Binding Request (which will be either A1 or A2),
   and Dp represent the destination port of the Binding Request (which
   will be either P1 or P2).  Let Ca represent the other address, so
   that if Da is A1, Ca is A2.  If Da is A2, Ca is A1.  Similarly, let
   Cp represent the other port, so that if Dp is P1, Cp is P2.  If Dp is
   P2, Cp is P1.  If the "change port" flag was set in the CHANGE-
   REQUEST attribute of the Binding Request, and the "change IP" flag
   was not set, the source IP address of the Binding Response MUST be Da
   and the source port of the Binding Response MUST be Cp.  If the
   "change IP" flag was set in the Binding Request, and the "change
   port" flag was not set, the source IP address of the Binding Response
   MUST be Ca and the source port of the Binding Response MUST be Dp.
   When both flags are set, the source IP address of the Binding
   Response MUST be Ca and the source port of the Binding Response MUST
   be Cp.  If neither flag is set, or if the CHANGE-REQUEST attribute is
   absent entirely, the source IP address of the Binding Response MUST
   be Da and the source port of the Binding Response MUST be Dp.

   +--------------------+----------------+-------------+---------------+
   | Flags              | Source Address | Source Port | OTHER-ADDRESS |
   +--------------------+----------------+-------------+---------------+
   | none               | Da             | Dp          | Ca:Cp         |
   | Change IP          | Ca             | Dp          | Ca:Cp         |
   | Change port        | Da             | Cp          | Ca:Cp         |
   | Change IP and      | Ca             | Cp          | Ca:Cp         |
   | Change port        |                |             |               |
   +--------------------+----------------+-------------+---------------+

        Table 1: Impact of Flags on Packet Source and OTHER-ADDRESS




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   The server MUST add a RESPONSE-ORIGIN attribute to the Binding
   Response, containing the source address and port used to send the
   Binding Response.

   If the server supports an alternate address and port, the server MUST
   add an OTHER-ADDRESS attribute to the Binding Response.  This
   contains the source IP address and port that would be used if the
   client had set the "change IP" and "change port" flags in the Binding
   Request.  As summarized in Table 1, these are Ca and Cp,
   respectively, regardless of the value of the CHANGE-REQUEST flags.

   If the Request contained a PADDING attribute, PADDING MUST be
   included in the Binding Response.  The server SHOULD use a length of
   PADDING equal to the MTU on the outgoing interface, rounded up to an
   even multiple of four bytes.  If the Request also contains the
   RESPONSE-PORT attribute the server MUST return an error response of
   type 400.

   Following that, the server completes the remainder of the processing
   from STUN [RFC5389].  If authentication is being required, the server
   MUST include a MESSAGE-INTEGRITY and associated attributes as
   appropriate.  A FINGERPRINT attribute is only required if the STUN
   messages are being multiplexed with application traffic that requires
   use of a FINGERPRINT to distinguish STUN messages.

   An ALTERNATE-SERVER attribute MUST NOT be included with any other
   attribute defined in this specification.

   When the server sends the Response, it is sent from the source
   address as determined above and to the source address of the Request.
   If RESPONSE-PORT is present, the server sends the response to that
   port instead of the originating port.

7.  New Attributes

   This document defines several STUN attributes that are required for
   NAT Behavior Discovery.  These attributes are all used only with
   Binding Requests and Binding Responses.  CHANGE-REQUEST was
   originally defined in RFC 3489 [RFC3489] but is redefined here as
   that document is obsoleted by [RFC5389].

     Comprehension-required range (0x0000-0x7FFF):
       0x0003: CHANGE-REQUEST
       0x0026: PADDING
       0x0027: RESPONSE-PORT






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     Comprehension-optional range (0x8000-0xFFFF):
       0x802b: RESPONSE-ORIGIN
       0x802c: OTHER-ADDRESS

7.1.  Representing Transport Addresses

   Whenever an attribute contains a transport IP address and port, it
   has the same format as MAPPED-ADDRESS.  Similarly, the XOR-
   attributes have the same format as XOR-MAPPED-ADDRESS [RFC5389].

7.2.  CHANGE-REQUEST

   The CHANGE-REQUEST attribute contains two flags to control the IP
   address and port that the server uses to send the response.  These
   flags are called the "change IP" and "change port" flags.  The
   CHANGE-REQUEST attribute is allowed only in the Binding Request.  The
   "change IP" and "change port" flags are useful for determining the
   current filtering behavior of a NAT.  They instruct the server to
   send the Binding Responses from the alternate source IP address
   and/or alternate port.  The CHANGE-REQUEST attribute is optional in
   the Binding Request.

   The attribute is 32 bits long, although only two bits (A and B) are
   used:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A B 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The meanings of the flags are:

   A: This is the "change IP" flag.  If true, it requests the server to
      send the Binding Response with a different IP address than the one
      the Binding Request was received on.

   B: This is the "change port" flag.  If true, it requests the server
      to send the Binding Response with a different port than the one
      the Binding Request was received on.

7.3.  RESPONSE-ORIGIN

   The RESPONSE-ORIGIN attribute is inserted by the server and indicates
   the source IP address and port the response was sent from.  It is
   useful for detecting double NAT configurations.  It is only present
   in Binding Responses.




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7.4.  OTHER-ADDRESS

   The OTHER-ADDRESS attribute is used in Binding Responses.  It informs
   the client of the source IP address and port that would be used if
   the client requested the "change IP" and "change port" behavior.
   OTHER-ADDRESS MUST NOT be inserted into a Binding Response unless the
   server has a second IP address.

   OTHER-ADDRESS uses the same attribute number as CHANGED-ADDRESS from
   RFC 3489 [RFC3489] because it is simply a new name with the same
   semantics as CHANGED-ADDRESS.  It has been renamed to more clearly
   indicate its function.

7.5.  RESPONSE-PORT

   The RESPONSE-PORT attribute contains a port.  The RESPONSE-PORT
   attribute can be present in the Binding Request and indicates which
   port the Binding Response will be sent to.  For servers which support
   the RESPONSE-PORT attribute, the Binding Response MUST be transmitted
   to the source IP address of the Binding Request and the port
   contained in RESPONSE-PORT.  It is used in tests such as Section 4.6.
   When not present, the server sends the Binding Response to the source
   IP address and port of the Binding Request.  The server MUST NOT
   process a request containing a RESPONSE-PORT and a PADDING attribute.
   The RESPONSE-PORT attribute is optional in the Binding Request.
   Server support for RESPONSE-PORT is optional.

   RESPONSE-PORT is a 16-bit unsigned integer in network byte order
   followed by 2 bytes of padding.  Allowable values of RESPONSE-PORT
   are 0-65536.

7.6.  PADDING

   The PADDING attribute allows for the entire message to be padded to
   force the STUN message to be divided into IP fragments.  PADDING
   consists entirely of a free-form string, the value of which does not
   matter.  PADDING can be used in either Binding Requests or Binding
   Responses.

   PADDING MUST NOT be longer than the length that brings the total IP
   datagram size to 64K.  It SHOULD be equal in length to the MTU of the
   outgoing interface, rounded up to an even multiple of four bytes.
   Because STUN messages with PADDING are intended to test the behavior
   of UDP fragments, they are an exception to the usual rule that STUN
   messages be less than the MTU of the path.






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8.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self-Address Fixing"
   (UNSAF), which is the general process by which a client attempts to
   determine its address in another realm on the other side of a NAT
   through a collaborative protocol reflection mechanism [RFC3424].  The
   STUN NAT Behavior Discovery usage is an example of a protocol that
   performs this type of function.  The IAB has mandated that any
   protocols developed for this purpose document a specific set of
   considerations.  This section meets those requirements.

8.1.  Problem Definition

   From RFC 3424 [RFC3424], any UNSAF proposal must provide:

      Precise definition of a specific, limited-scope problem that is to
      be solved with the UNSAF proposal.  A short term fix should not be
      generalized to solve other problems.  Such generalizations lead to
      the prolonged dependence on and usage of the supposed short term
      fix -- meaning that it is no longer accurate to call it "short
      term".

   The specific problem being solved by the STUN NAT Behavior Discovery
   usage is for a client, which may be located behind a NAT of any type,
   to determine the instantaneous characteristics of that NAT.  This
   determination allows either the diagnosis of the cause of problems
   experienced by that or other applications or the modification of an
   application's behavior based on the current behavior of the NAT and
   an appropriate statistical model of the behavior required for the
   application to succeed.

8.2.  Exit Strategy

   From [RFC3424], any UNSAF proposal must provide:

      Description of an exit strategy/transition plan.  The better short
      term fixes are the ones that will naturally see less and less use
      as the appropriate technology is deployed.

   The STUN NAT Behavior Discovery usage does not itself provide an exit
   strategy for v4 NATs.  At the time of this writing, it appears some
   sort of NAT will be necessary between v6 clients and v4 servers, but
   this specification will not be necessary with those v6-to-v4 NATs
   because the IETF is planning to adequately describe their operation.
   This specification will be of no interest for v6-to-v6 connectivity.






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8.3.  Brittleness Introduced by STUN NAT Behavior Discovery

   From [RFC3424], any UNSAF proposal must provide:

      Discussion of specific issues that may render systems more
      "brittle".  For example, approaches that involve using data at
      multiple network layers create more dependencies, increase
      debugging challenges, and make it harder to transition.

   The STUN NAT Behavior Discovery usage allows a client to determine
   the current behavior of a NAT.  This information can be quite useful
   to a developer or network administrator outside of an application,
   and as such can be used to diagnose the brittleness induced in
   another application.  When used within an application itself, STUN
   NAT Behavior Discovery allows the application to adjust its behavior
   according to the current behavior of the NAT.  This document is
   experimental because the extent to which brittleness is introduced to
   an application relying on the Behavior Discovery usage is unclear and
   must be carefully evaluated by the designers of the protocol making
   use of it.  The experimental test for this protocol is essentially
   determining whether an application can be made less brittle through
   the use of behavior-discovery information than it would be if
   attempted to make use of the network without any awareness of the
   NATs its traffic must pass through.

8.4.  Requirements for a Long-Term Solution

   From [RFC3424], any UNSAF proposal must provide:

      Identify requirements for longer-term, sound technical solutions
      -- contribute to the process of finding the right longer-term
      solution.

   As long as v4 NATs are present, means of adapting to their presence
   will be required.  As described above, well-behaved v6 to v4 NATs and
   direct v6 to v6 connections will not require behavior
   characterization.

8.5.  Issues with Existing NAPT Boxes

   From [RFC3424], any UNSAF proposal must provide:

      Discussion of the impact of the noted practical issues with
      existing deployed NATs and experience reports.

   This usage provides a set of generic attributes that can be assembled
   to test many types of NAT behavior.  While tests for the most
   commonly known NAT box behaviors are described, the BEHAVE mailing



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   list regularly has descriptions of new behaviors, some of which may
   not be readily detected using the tests described herein.  However,
   the techniques described in this usage can be assembled in different
   combinations to test NAT behaviors not now known or envisioned.

9.  IANA Considerations

9.1.  STUN Attribute Registry

   This specification defines several new STUN attributes.  IANA has
   added these new protocol elements to the "STUN Attributes" registry.

   0x0003: CHANGE-REQUEST
   0x0027: RESPONSE-PORT
   0x0026: PADDING
   0x8027: CACHE-TIMEOUT
   0x802b: RESPONSE-ORIGIN
   0x802c: OTHER-ADDRESS

9.2.  Port Numbers and SRV Registry

   By default, the STUN NAT Behavior Discovery usage runs on the same
   ports as STUN: 3478 over UDP and TCP, and 5349 for TCP over TLS.
   However, the Behavior Discovery usage has its own set of Service
   Record (SRV) names: "stun-behavior" for UDP and TCP, and "stun-
   behaviors" for TLS.  Either the SRV procedures or the ALTERNATE-
   SERVER procedures, subject to the recommendations of Section 5, can
   be used to run Behavior Discovery on a different port.

   This specification defines the "stun-behavior" and "stun-behaviors"
   SRV service names. "stun-behavior" may be used with SRV protocol
   specifiers "udp" and "tcp". "stun-behaviors" may only be specified
   with "tcp".  Thus, the allowable SRV queries are:

   _stun-behavior._udp            UDP
   _stun-behavior._tcp            TCP
   _stun-behaviors._tcp           TLS over TCP

10.  Security Considerations

   This usage inherits the security considerations of STUN [RFC5389].
   This usage adds several new attributes; security considerations for
   those are detailed here.








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   OTHER-ADDRESS does not permit any new attacks; it provides another
   place where an attacker can impersonate a STUN server but it is not
   an interesting attack.  An attacker positioned where it can
   compromise the Binding Response can completely hide the STUN server
   from the client.

   o  Requests containing both RESPONSE-PORT and PADDING are rejected by
      the server.  This prevents an amplification attack that is
      targeted at the originating address.

   The only attack possible with the PADDING attribute is to have a
   large padding length that could cause a server to allocate a large
   amount of memory.  As servers will ignore any padding length greater
   than 64K so the scope of this attack is limited.  In general, servers
   should not allocate more memory than the size of the received
   datagram.  This attack would only affect non-compliant
   implementations.

   RESPONSE-ORIGIN and RESPONSE-PORT do not provide any additional
   attacks.

11.  Acknowledgements

   The authors would like to thank the authors of the original STUN
   specification [RFC3489] from which many of the ideas, attributes, and
   description in this document originated.  Thanks to Dan Wing, Cullen
   Jennings, and Magnus Westerlund for detailed comments.

12.  References

12.1.  Normative References

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

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

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

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.





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12.2.  Informative References

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

   [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.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

   [RFC5626]  Jennings, C., Mahy, R., and F. Audet, "Managing Client-
              Initiated Connections in the Session Initiation Protocol
              (SIP)", RFC 5626, October 2009.

Authors' Addresses

   Derek C. MacDonald
   Skype
   Palo Alto, CA
   USA

   EMail: derek.macdonald@gmail.com


   Bruce B. Lowekamp
   Skype
   Palo Alto, CA
   USA

   EMail: bbl@lowekamp.net















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