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Versions: 00 01 draft-ietf-midcom-stun

Internet Engineering Task Force                                MIDCOM WG
Internet Draft                                              J. Rosenberg
                                                           J. Weinberger
                                                              C. Huitema
                                                                 R. Mahy
March 1, 2002
Expires: September 2002

              STUN - Simple Traversal of UDP Through NATs


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see


   Simple Traversal of UDP Through NATs (STUN) is a lightweight protocol
   that allows applications to discover the presence and types of
   Network Address Translators (NATs) and firewalls between them and the
   public Internet. It also provides the ability for applications to
   determine the public IP addresses allocated to them by the NAT. STUN
   works with nearly all existing NATs, and does not require any special
   behavior from them. As a result, it allows a wide variety of
   applications to work through existing NAT infrastructure. The STUN

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   protocol is very simple, being almost identical to echo.

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                           Table of Contents

   1          Introduction ........................................    4
   2          Terminology .........................................    5
   3          Definitions .........................................    5
   4          NAT Variations ......................................    5
   5          Overview of Operation ...............................    6
   6          Message Overview ....................................    9
   7          Server Behavior .....................................   10
   8          Client Behavior .....................................   12
   8.1        Discovery ...........................................   12
   8.2        Formulating the Request .............................   14
   8.3        Authenticating the Response .........................   14
   9          Use Cases ...........................................   15
   9.1        Discovery Process ...................................   15
   9.2        Binding Lifetime Discovery ..........................   16
   9.3        Binding Acquisition .................................   17
   10         Protocol Details ....................................   18
   10.1       Message Header ......................................   18
   10.2       Message Attributes ..................................   18
   10.2.1     MAPPED-ADDRESS ......................................   19
   10.2.2     RESPONSE-ADDRESS ....................................   20
   10.2.3     CHANGED-ADDRESS .....................................   20
   10.2.4     FLAGS ...............................................   20
   10.2.5     SOURCE-ADDRESS ......................................   21
   10.2.6     CMS-SIGNED-DATA .....................................   21
   10.2.7     COOKIE ..............................................   21
   11         Security Considerations .............................   22
   12         IANA Considerations .................................   23
   13         IAB Considerations ..................................   23
   13.1       Problem Definition ..................................   23
   13.2       Exit Strategy .......................................   23
   13.3       Brittleness Introduced by STUN ......................   24
   13.4       Requirements for a Long Term Solution ...............   25
   13.5       Issues with Existing NAPT Boxes .....................   26
   13.6       In Closing ..........................................   27
   14         Acknowledgements ....................................   27
   15         Authors Addresses ...................................   27
   16         Normative References ................................   28
   17         Informative References ..............................   28

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1 Introduction

   Network Address Translators (NATs), while providing many benefits,
   also come with many drawbacks. The most troublesome of those
   drawbacks is the fact that they break many existing IP applications,
   and make it difficult to deploy new ones. Guidlines have been
   developed [4] that describe how to build "NAT friendly" protocols,
   but many protocols simply cannot be constructed according to those
   guidelines. Examples of such protocols include almost all peer-to-
   peer protocols, such as multimedia communications, file sharing and

   To combat that problem, Application Layer Gateways (ALGs) have been
   embedded in NATs. ALGs perform the application layer functions
   required for a particular protocol to traverse a NAT. Typically, this
   involves rewriting messages to contain translated addresses, rather
   than the ones inserted by the sender of the protocol message. ALGs
   have serious limitations, including scalability, reliability, and
   speed of deploying new applications. To resolve these problems, the
   Middlebox Communciations (MIDCOM) protocol is being developed [5].
   MIDCOM allows an application entity, such as an end client or network
   server of some sort (like a SIP proxy [6]) to control a NAT (or
   firewall), in order to obtain NAT bindings and open or close
   pinholes. In this way, NATs and applications can be separated once
   more, eliminating the need for embedding ALGs in NATs, and resolving
   the limitations imposed by current architectures.

   Unfortunately, MIDCOM requires upgrades to existing NAT and
   firewalls, in addition to application components. Complete upgrades
   of these NAT and firewall products will take a long time, potentially
   years. This is due, in part, to the fact that the deployers of NAT
   and firewalls are not the same people who are deploying and using
   applications. As a result, the incentive to upgrade these devices
   will be low in many cases. Consider, for example, an airport Internet
   lounge that provides access with a NAT. A user connecting to the
   natted network may wish to use a peer-to-peer service, but cannot,
   because the NAT doesn't support it. Since the administrators of the
   lounge are not the ones providing the service, they are not motivated
   to upgrade their NAT equipment to support it, using either an ALG, or

   Another problem is that the MIDCOM protocol requires that the agent
   controlling the middleboxes know the identity of those middleboxes,
   and have a relationship with them which permits control. In many
   configurations, this will not be possible. For example, many cable
   access providers use NAT in front of their entire access network.
   This NAT could be in addition to a residential NAT purchased and
   operated by the end user. The end user will probably not have a

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   control relationship with the NAT in the cable access network, and
   may not even know of its existence.

   Many existing proprietary protocols, such as those for online games
   (such as the games described in RFC 3027 [7]) and Voice over IP, have
   developed tricks that allow them to operate through NATs without
   changing those NATs. This draft is an attempt to take some of those
   ideas, and codify them into an interoperable protocol that can meet
   the needs of many applications.

   The protocol described here, Simple Traversal of UDP Through NAT
   (STUN), provides is an extremely simple protocol that allows entities
   behind a NAT to first discover the presence of a NAT and the type of
   NAT, and then to learn the addresses bindings allocated by the NAT.
   STUN requires no changes to NATs, and works with an arbitrary number
   of NATs in tandem between the application entity and the public

2 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
   indicate requirement levels for compliant STUN implementations.

3 Definitions

        STUN Client: A STUN client (also just referred to as a client)
             is an entity that generates STUN requests. A STUN client
             can execute on an end system, such as a users PC, or can
             run in a network element, such as a server.

        STUN Server: A STUN Server (also just referred to as a server)
             is an entity that receives STUN requests, and sends STUN
             responses. STUN servers are generally attached to the
             public Internet. STUN servers are stateless.

4 NAT Variations

   It is assumed that the reader is familiar with NATs. It has been
   observed that NAT treatment of UDP is variable amongst
   implementations. The four treatments observed in implementations are:

        Full Cone: A full cone NAT is one where all requests from the
             same internal IP address and port are mapped to the same
             external IP address and port. Furthermore, any external
             host can send a packet to the internal host, by sending a
             packet to the mapped external address.

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        Restricted Cone: A restricted cone NAT is one where all requests
             from the same internal IP address and port are mapped to
             the same external IP address and port. Unlike a full cone
             NAT, an external host (with IP address X) can send a packet
             to the internal host only if the internal host had
             previously sent a packet to IP address X.

        Port Restricted Cone: A port restricted cone NAT is like a
             restricted cone NAT, but the restriction includes port
             numbers. Specifically, an external host can send a packet,
             with source IP address X and source port P, to the internal
             host only if the internal host had previously sent a packet
             to IP address X and port P.

        Symmetric: A symmetric NAT is one where all requests from the
             same internal IP address and port, to a specific
             destination IP address and port, are mapped to the same
             external IP address and port. If the same host sends a
             packet with the same source address and port, but to a
             different destination, a different mapping is used.
             Furthermore, only the external host that receives a packet
             can send a UDP packet back to the internal host.

   Determining the type of NAT is important in many cases. Depending on
   what the application wants to do, the particular behavior may need to
   be taken into account.

5 Overview of Operation

   This section is descriptive only. Normative behavior is described in
   Sections 7 and 8.

   The typical STUN configuration is shown in Figure 1. A STUN client is
   connected to private network 1. This network connects to private
   network 2 through NAT 1. Private network 2 connects to the public
   Internet through NAT 2. On the public Internet is a STUN server.

   STUN is a simple client-server protocol. Its operation is trivial. A
   client sends a request to a server. The server examines the source IP
   address and port of the request, and copies them into a response that
   is sent back to the client. There are some parameters in the request
   that allow the client to ask that the response be sent elsewhere, or
   that the server send the response from a different address and port.
   There are also security capabilities that allow the server to sign
   the response. Thats it.

   The trick is using this simple protocol to discover the presence of

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                         // STUN  \\
                        |   Server  |
                         \\       //

                      +--------------+             Public Internet
      ................|     NAT 2    |.......................

                      +--------------+             Private NET 2
      ................|     NAT 1    |.......................

                        //  STUN \\
                       |    Client |
                        \\       //               Private NET 1

   Figure 1: STUN Configuration

   nats, and to learn and use the bindings they allocate.

   The STUN client is typically embedded in an application which needs
   to obtain a public IP address and port that can be used to receive
   data. For example, it might need to obtain an IP address and port to
   receive RTP [8] traffic. When the application starts, the STUN client
   within the application sends a STUN request to its STUN server. STUN
   servers are discovered through DNS SRV records [2], and is generally
   assumed that the client is configured with the domain to use to find
   the STUN server. Generally, this will be the domain of the provider
   of the service the application is using (such a provider is incented
   to deploy STUN servers in order to allow its customers to use its

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   application through NAT).

   The STUN request is used to discover the presence of a NAT, and to
   discover the public IP address and port mappings generated by the
   NAT. Requests are sent to the STUN server using UDP. When a request
   arrives at the STUN server, it may have passed through one or more
   NATs between the STUN client and the STUN server. As a result, the
   source address of the request received by the server will be the
   mapped address created by the nat closest to the server. The STUN
   server copies that source IP address and port into a STUN response,
   and sends it back to the source IP address and port of the STUN
   request. For all of the NAT types above, this response will arrive at
   the STUN client.

   When the STUN client receives the STUN response, it compares the IP
   address and port in the packet with the local IP address and port it
   bound to when the request was sent. If these do not match, the STUN
   client is behind one or more NATs. In the case of a full-cone NAT,
   the IP address and port in the body of the STUN response are public,
   and can be used by any host on the public Internet to send packets to
   the application that sent the STUN request. An application need only
   listen on the IP address and port from which the STUN request was
   sent, and send the IP address and port learned in the STUN response
   to hosts that wish to communicate with it.

   Of course, the host may not be behind a full-cone NAT. Indeed, it
   doesn't yet know what type of NAT it is behind. To determine that,
   the client uses additional STUN requests. The exact procedure is
   flexible, but would generally work as follows. The client would send
   a second STUN request, this time to a different STUN server, but from
   the same source IP address and port. If the IP address and port in
   the response are different from those in the first response, the
   client knows it is behind a symmetric NAT. To determine if its behind
   a full-cone NAT, the client can send a STUN request with flags that
   tell the STUN server to send a response from a different IP address
   and port than the request was received on. In other words, if the
   client sent a request to IP address/port A/B using a source IP
   address/port of X/Y, the STUN server would send the response to X/Y
   using source IP address/port C/D. If the client receives this
   response, it knows it is behind a full cone NAT.

   STUN also allows the client to ask the server to send the response
   from the same IP address the request was received on, but with a
   different port. This can be used to detect whether the client is
   behind a port restricted cone nat or just a restricted cone nat.

   It should be noted that the configuration in Figure 1 is not the only
   permissible configuration. The STUN server can be located anywhere,

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   including within another client. The only requirement is that the
   STUN server is reachable at a public IP address.

6 Message Overview

   STUN messages are TLV (type-length-value) encoded using big endian
   (network ordered) binary. All STUN messages start with a STUN header,
   followed by a series of STUN attributes. The STUN header contains a
   STUN message type, transaction ID, and length. The message type can
   be request or response. The transaction ID is used to correlate
   requests and responses. The length indicates the total length of the
   STUN message. This allows STUN to run over TCP. Usage over TCP is
   needed in order to fetch certificates from the server.

   Several STUN attributes are defined. The first is a MAPPED-ADDRESS
   attribute, which is an IP address and port. It is always placed in
   the response, and it indicates the source IP address and port the
   server saw in the request. There is also a RESPONSE-ADDRESS
   attribute, which is also an IP address and port. The RESPONSE-ADDRESS
   attribute can be present in the request, and indicates where the
   response is to be sent. Its optional, and when not present, the
   response is sent to the source IP address and port of the request.

   The third attribute is the FLAGS attribute, and it contains boolean
   flags to control behavior. Three flags are defined: "discard",
   "change IP" and "change port". The FLAG attribute is allowed only in
   the request. The discard attribute tells the server to not send a
   reply. The change IP and change port attributes are useful for
   determining whether the client is behind a restricted cone nat or
   restricted port cone nat. They instruct the server to send the
   responses from a different source IP address and port. The FLAGS
   attribute is optional in the request.

   The fourth attribute is the CHANGED-ADDRESS attribute. It is present
   in all 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.

   The fifth attribute is the SOURCE-ADDRESS attribute. It is only
   present in responses. It indicates the source IP address and port
   where the response was sent from. It is useful for detecting twice
   NAT configurations.

   The final two attributes provide security features. The SMS-SIGNED-
   DATA attribute allows the server to provide a signature over its
   response. This is useful for preventing several address-stealing
   attacks that would otherwise be possible. See Section 11 for details
   on the types of attacks possible. The COOKIE attribute enables a

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   four-way handshake between the client and the server for preventing
   distributed denial-of-service attacks. When the server receives a
   request without this cookie, or with an invalid one, it generates a
   response that contains the cookie attribute. The client then retries
   the request, including the cookie attribute. If the server receives a
   request with a valid cookie attribute, it continues to process the

7 Server Behavior

   If the request contains the flag attribute, and the discard flag is
   true, the server MUST discard the request.

   The server MUST generate a single response when a request is received
   (assuming the request is not discarded). The response MUST contain
   the same transaction ID contained in the request. The length in the
   message header MUST contain the total length of the message in bytes,
   excluding the header. The response MUST have a message type of

   The server MUST add a MAPPED-ADDRESS attribute to the response. The
   IP address component of this attribute MUST be set to the source IP
   address observed in the request. The port component of this attribute
   MUST be set to the source port observed in the query request.

   If the request arrived over TCP, the response MUST be sent on the
   same connection the request was received on. The server MAY close the
   connection after sending the response, but SHOULD instead wait for
   the client to close the connection.

   If the request arrived over UDP, the procedures for sending the
   response are as follows.

   If the RESPONSE-ADDRESS attribute was absent from the Query request,
   the destination address and port of the response MUST be the same as
   the source address and port of the request. Otherwise, the
   destination address and port of the response MUST be the value of the
   IP address and port in the RESPONSE-ADDRESS attribute.

   The source address and port of the response are computed as follows.
   If the "change port" FLAG was set in the request, and the "change IP"
   flag was not set, the source port of the response MUST NOT be the
   same as the destination port of the query request, and the source
   address of the response MUST be the same as the destination address
   of the request. If the "change IP" FLAG was set in the request, and
   the "change port" flag was not set, the source IP address of the
   response MUST NOT be the same as the destination IP address of the
   query request, and the source port of the response MUST be the same

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   as the destination port of the request. When both flags are set, both
   the source port and source address MUST be different. Furthermore,
   the source port MUST be the same as if the the "change port" flag
   alone was present in the request, and the source address MUST be the
   same as if the "change IP" flag alone was present in the request. The
   result is that if the source port differs from the received port, it
   is the same independently of whether the source IP address is
   different from the received IP address. Exactly how this is
   implemented is a local decision.

   The server MUST add a SOURCE-ADDRESS attribute to the response,
   containing the address and port used to send the response.

   The server MUST add a CHANGED-ADDRESS attribute to the response. This
   contains the source IP address and port that would be used if the
   client requested the "change IP" and "change port" capabilities of
   the server. This address MUST be invariant across requests with the
   same source IP address and port for a duration of 10 minutes. In
   other words, if the client sends a request from a particular socket,
   and the response contains a specific CHANGED-ADDRESS, subsequent
   requests from the same socket should return the same CHANGED-ADDRESS.

   One potential way to implement the change-IP feature is for the
   server to generate its own request, and send it to another server,
   running on a different host. That request is the same as the request
   received by the first server, except that a RESPONSE-ADDRESS
   attribute has been added, containing the source address and port of
   the original request. If the server receives a request with a
   RESPONSE-ADDRESS attribute, it must send the response to the address
   and port in that attribute. The second server will therefore send the
   response back to the original client. Since the response is sent by a
   different server, the IP address and port are different. This is
   shown in Figure 2.

   The server can optionally sign the response, in order to provide
   additional security capabilities. The signature SHOULD NOT be present
   if the request did not contain a valid COOKIE attribute. Valid, in
   this case, means that the COOKIE was equal to a value previously
   handed out by the server to the same client. See subsequent
   paragraphs for more details. To sign the response, the server adds a
   CMS-SIGNED-DATA attribute as the last attribute in the response. If
   the response is sent over UDP (which it will, if the request came
   over UDP), the SignedData object MUST NOT contain any certificates.
   This is because the certificates would likely overflow the MTU,
   causing IP level fragmentation. This will not function in the
   presence of NAT. If the request arrived over TCP, the response MUST
   contain certificates. The server SHOULD sign the response using a
   site certificate whose domain matches the domain of the server, as

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   listed in DNS. However, an end user certificate MAY be used instead.

   If there was no COOKIE in the request, or the COOKIE was invalid, the
   response MAY contain a COOKIE attribute. This attribute is a 128 bit
   opaque value, created by the server. It SHOULD be constructed in such
   a way so that it can be reconstructed by the server based on a
   subsequent request from the same client within a brief time interval.
   It is RECOMMENDED that it be computed as a cryptographic hash of the
   source IP address and port, in addition to a time stamp rounded to
   the nearest 5 seconds. When the client receives the response, if it
   wishes to authenticate the information it just received, it will
   create a new request, identical to the previous, except with a
   different transaction identifier, and this COOKIE reflected back. The
   server then recomputes the COOKIE, using the same algorithm it used
   to construct it previously, and verifies that this is the same value
   in the request. If the values are the same, the COOKIE in the request
   is valid. The purpose of this cookie exchange is to prevent
   distributed denial of service attacks that would force a server to
   perform expensive public key signatures. Instead, the server only
   provides the signature in responses that come from valid clients;
   valid, in this case, means the client meant to send the request,
   instead of being manipulated into doing so through a dDoS attack.

   The server SHOULD NOT retransmit the response. Reliability is
   achieved by having the client periodically resend the request, each
   of which triggers a response from the server.

8 Client Behavior

   The behavior of the client is very simple. Its main task is to
   discover the STUN server, formulate the request, handle request
   reliability, and authenticate the response.

8.1 Discovery

   Generally, the client will be configured with a domain name of the
   provider of the STUN servers. This domain name is resolved to an IP
   address and port of using the SRV procedures specified in RFC 2782

   Specifically, the service name is "stun". The protocol is "udp" or
   "tcp". The procedures of RFC 2782 are followed to determine the
   server to contact. RFC 2782 spells out the details of how a set of
   SRV records are sorted and then tried. However, it only states that
   the client should "try to connect to the (protocol, address,
   service)" without giving any details on what happens in the event of
   failure. Those details are described here for STUN.

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                             +-+           |  Query  |
                             | |           |  Server |
                             | |   ------->|    1    |
                             | |---        +---------+
              Query          | |                |
              S:  ---| | Query          |
                        --   | | S:     |    Query
                     --- (1) |N|                |    RESPONSE-ADDRESS=
                  ---        | |                |(2)
       +------+ --           |A|                |
       |      |              | |                |
       |Client|              |T|                |
       |      |<---          | |                |
       +------+    ------    | |                \/
                         ----| |   (3)     +---------+
               Query         | |-----      |  Query  |
               Response      | |     ------|  Server |
               D:   | | Query     |    2    |
                             +-+ Response  +---------+

   Figure 2: Sending a response from a different address/port

   For STUN requests, failure occurs if there is a transport failure of
   some sort (generally, due to fatal ICMP errors in UDP use or
   connection failures in TCP). Failure also occurs if the the request
   does not solicit a response after 30 seconds. If a failure occurs,
   the client SHOULD create a new request, which is identical to the
   previous, but has a different transaction ID. That request is sent to
   the next element in the list as specified by RFC 2782.

   The default port for STUN requests is [to be assigned by IANA].
   Administrators SHOULD use this port in their SRV records, but MAY use

   If no SRV records were found, the client performs an A or AAAA record
   lookup of the domain name. The result will be a list of IP addresses,
   each of which can be contacted at the default port.

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        This would allow a firewall admin to open the STUN port, so
        hosts within the enterprise could access new applications.
        Whether they will or won't do this is a good question.

8.2 Formulating the Request

   A request formulated by the client follows the syntax rules defined
   in Section 10. Any two requests that are not bit-wise identical, or
   not sent to the same server from the same IP address and port, MUST
   carry different transaction IDs. The transaction ID MUST be uniformly
   and randomly chosen between 0 and 2^^32 - 1. The message type of the
   request MUST be "Request".

   The RESPONSE-ADDRESS attribute is optional in the request. It is used
   if the client wishes the response to be sent to a different IP
   address and port. This is useful for determining whether the client
   is behind a firewall, and for applications that have separated
   control and data components. See Section 9.3 for more details. The
   FLAGS attribute is also optional. Whether it is present depends on
   what the application is trying to accomplish. See Section 9 for some
   example uses.

   Once formulated, the client sends the request. Reliability is
   accomplished through client retransmissions. Clients SHOULD
   retransmit the request starting with an interval of 100ms, doubling
   every retransmit until the interval reaches 1.6s. Retranmissions
   continue with intervals of 1.6s until a total of 9 requests have been
   sent, at which time the client SHOULD give up.

   The response will contain the MAPPED-ADDRESS and SOURCE-ADDRESS

8.3 Authenticating the Response

   As discussed in Section 11, there are serious security
   vulnerabilities introduced if the STUN response is not authenticated
   and integrity protected. To combat that problem, STUN provides for
   server signatures using CMS. The procedure for obtaining the
   signature and certificates to validate it are as follows.

   The initial STUN reply will arrive with a COOKIE attribute. The
   client can choose to use the information in the STUN reply,
   validating it in parallel, or can choose to authenticate the
   information before using it. To authenticate the information, the
   client formulates a new STUN request, identical to the initial one
   except for two changes. First, is the usage of a new transaction ID.
   Second, is the addition of the COOKIE attribute, which is copied from
   the previous STUN response into the new request. The response to this

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   second STUN request will contain a CMS-SIGNED-DATA attribute. This
   attribute is equal to the SignedData object defined in RFC 2630 [3].
   If the other attributes in the response match the first response, and
   the signature is valid, the client can trust that the information has
   not been tampered with, and is authentic. If the certificate used for
   the signature is a site certificate, the client SHOULD validate that
   the domain it used to perform the STUN query is a sub-domain of the
   domain in the site certificate. In other words, if the client queries
   stun.example.com, the client SHOULD validate that the signature was
   certified as coming from example.com.

   However, the CMS-SIGNED-DATA attribute in the second response will
   not contain any certificates. The client may have cached certificates
   from a previous exchange with this server, in which case those
   certificates can be used. If the client does not have a certificate
   chain for the server, the client creates a third STUN request. This
   one is identical to the second, except it contains a new transaction
   ID, and is sent over TCP. The response will contain a CMS-SIGNED-DATA
   attribute, this time, with certificates.

9 Use Cases

   The rules of Sections 7 and 8 describe exactly how a client and
   server interact to send requests and get responses. However, they do
   not dictate how the STUN protocol is used to accomplish useful tasks.
   That is at the discretion of the client. Here, we provide some useful
   scenarios for applying STUN.

9.1 Discovery Process

   In this scenario, a user is running a multimedia application which
   needs to determine which of the following scenarios applies to it:

        o On the open Internet

        o Firewall that blocks UDP

        o Firewall that allows UDP out, and responses have to come back
          to the source of the request (like a symmetric NAT, but no
          translation. We call this symmetric UDP Firewall)

        o Full-cone NAT

        o Symmetric NAT

        o Restricted cone or restricted port cone NAT

   Which of the six scenarios applies can be determined through the flow

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   chart described in Figure 3.

   The flow makes use of three tests. In test I, the client sends a STUN
   request to a server, without any flags set, and without the
   RESPONSE-ADDRESS attribute. This causes the server to send the
   response back to the address and port that the request came from.
   This response provides the IP address and port for the third party
   address that would be used if the source IP and/or port were changed.
   In test II, the client sends a request with both the "change IP" and
   "change port" flags set. In test III, the client sends a request with
   only the "change port" flag set.

   The client begins by initiating test I. If this test yields no
   response, the client knows right away that it is not capable of UDP
   connectivity. If the test produces a response, the client examines
   the MAPPED-ADDRESS attribute. If this address is the same as the
   local IP address and port of the socket used to send the request, the
   client knows that it is not natted. It executes test II. If a
   response is received, the client knows that it has open access to the
   Internet (or, at least, its behind a firewall that behaves like a
   full-cone NAT, but without the translation). If no response is
   received, the client knows its behind a symmetric UDP firewall.

   In the event that the IP address and port of the socket did not match
   the MAPPED-ADDRESS attribute in the response to test I, the client
   knows that it is behind a NAT. It performs test II. If a response is
   received, the client knows that it is behind a full-cone NAT. If no
   response is received, it performs test I again, but this time, does
   so to the address from the CHANGED-ADDRESS attribute. If the IP
   address returned in the MAPPED-ADDRESS attribute is not the same as
   the one from the first test I, the client knows its behind a
   symmetric NAT. If the address is the same, the client is either
   behind a restricted or port restricted NAT. To make a determination
   about which one it is behind, the client initiates test III. If a
   response is received, its behind a restricted NAT, and if no response
   is received, its behind a port restricted NAT.

   This simple procedure yields substantial information about the
   operating condition of the client application. In the event of
   multiple NATs between the client and the Internet, the type that is
   discovered will be the type of the most restrictive NAT between the
   client and the Internet. The types of NAT, in order of
   restrictiveness, from most to least, are symmetric, port restricted
   cone, restricted cone, and full cone.

9.2 Binding Lifetime Discovery

   STUN can also be used to discover the lifetimes of the bindings

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   created by the NAT. To do that, the client first sends a simple
   request (no attributes) to server A. The response from A will contain
   the CHANGED-ADDRESS attribute. The client sends another simple
   request to that address (server B). It then starts a timer with a
   value of T seconds. When this timer fires, the client sends a request
   to server A, with the "change IP" and "change port" flags set. If the
   binding is still active, this response should be received through all
   nat types. 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.

9.3 Binding Acquisition

   Consider once more the case of a VoIP phone. It used the discovery
   process above when it started up, to discover its environment. Now,
   it wants to make a call. As part of the discovery process, it
   determined that it was behind a full-cone NAT.

   Consider further that this phone consists of two logically separated
   components - a control component that handles signaling, and a media
   component that handles the audio, video, and RTP [8]. Both are behind
   the same NAT. Because of this separation of control and media, we
   wish to minimize the communication required between them. In fact,
   they may not even run on the same host.

   In order to make a voice call, the phone needs to obtain an IP
   address and port that it can place in the call setup message as the
   destination for receiving audio.

   To obtain an address, the control component first sends a STUN
   request to a server. No flags are present, and neither is the
   RESPONSE-ADDRESS field. The response contains a mapped address. The
   control component then formulates a second request. This request
   contains a RESPONSE-ADDRESS, which is set to that mapped address.
   This request is passed to the media component, along with the IP
   address and port of the STUN server. The media component sends the
   request. The request goes to the STUN server, which sends the
   response back to the control component. The control component
   receives this, and now has learned an IP address and port that will
   be routed back to the media component that sent the request.

   The client will be able to receive media from anywhere on this mapped

   In the case of silence suppression, there may be periods where the
   client receives no media. In this case, the UDP bindings could
   timeout (UDP bindings in nats are typically short). To deal with

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   this, the application can periodically retransmit the query in order
   to keep the binding fresh.

   It is possible that both participants in the multimedia session are
   behind the same NAT. In that case, both will repeat this procedure
   above, and both will obtain public address bindings. When one sends
   media to the other, the media is routed to the nat, and then turns
   right back around to come back into the enterprise, where it is
   translated to the private address of the recipient. This is not
   particularly efficient, but it does work.

10 Protocol Details

   This section presents the detailed encoding of a STUN message.

10.1 Message Header

   All STUN messages consist of a 64 bit header:

   |      STUN Message Type        |         Message Length        |
   |                      Transaction ID                           |

   The Message Types can take on the following values:

   0x0001  :  Request
   0x0101  :  Response

   The message length is the count, in bytes, of the size of the
   message, not including the 64 bit header.

   The transaction ID is a 32 bit identifier. All responses carry the
   same identifier as the request they correspond to.

10.2 Message Attributes

   After the header are 0 or more attributes. Each attribute is TLV
   encoded, with a 16 bit type, 16 bit length, and variable value:

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   |         Type                  |            Length             |
   |                             Value                             ....

   The following types are defined:

   0x0003: FLAGS
   0x0006: CMS-SIGNED-DATA
   0x0007: COOKIE

   Future extensions MAY define new attributes. If a stun client or
   server receives a message with an unknown attribute with a type lower
   than or equal to 0x7fff, the message MUST be discarded. If the type
   is greater than 0x7fff, the attribute MUST be ignored. The ordering
   of attributes within a message is not important, and a client or
   server MUST be prepared to receive them in any order. Any attributes
   that are known, but are not supposed to be present in a message
   (MAPPED-ADDRESS in a request, for example) MUST be ignored.

   The length refers to the length of the value element.


   The MAPPED-ADDRESS attribute indicates the mapped IP address and
   port. It consists of a sixteen bit port, eight bit address family,
   followed by a fixed length value representing the IP address.

   |           Port                |    Family     |x x x x x x x x|
   |                             Address..

   The port is a network byte ordered representation of the mapped port.

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   The following families are defined:

   0x01:   IPv4
   0x02:   IPv6

   The family is followed by 8 bits which are ignored, for the purposes
   of aligning the address on 32 bit boundaries.

   For IPv4 addresses, the address is 32 bits. For IPV6, it is 128 bits.

   New address families MAY be defined by extensions. A message with an
   unknown address family is discarded.


   The RESPONSE-ADDRESS attribute indicates where the response to a
   request is sent. Its syntax is identical to MAPPED-ADDRESS.


   The CHANGED-ADDRESS attribute indicates the IP address and port of a
   STUN server where responses will be sent from if the "change IP"
   and/or "change port" flags are set. Its syntax is identical to

10.2.4 FLAGS

   The FLAGS attribute is a series of boolean flags. It is 32 bits long:

   |A|B|C|                                                         |

   Only three flags, A,B,C, are currently defined. The other bits MAY be
   used by extensions to define additional flags. Unknown flags are

   Each flag is a binary one if true, zero otherwise.

   The meaning of the flags is:

        A: This is the "change IP" flag. If true, it requests the server

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             to send the response with a different IP address than the
             one the request was received on.

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

        C: This is the discard flag. If true, the message is discarded.


   The SOURCE-ADDRESS attribute is present in responses. It indicates
   the source IP address and port that the server is sending the
   response from. Its syntax is identical to that of MAPPED-ADDRESS.


   STUN responses can be signed. The signatures are conveyed using the
   Cryptographic Message Syntax (CMS), RFC 2630 [3]. Specifically. the
   CMS-SIGNED-DATA is exactly equal to the CMS SignedData object, using
   detached signatures. When present in a response, the CMS-SIGNED-DATA
   attribute MUST be the last attribute in the response. The contentData
   covered by the signature includes all of the bytes from the start of
   the STUN message, up to, but not including, the CMS-SIGNED-DATA

   When the response is sent over UDP, the response won't contain
   certificates (see Section 7. Responses over TCP will contain

10.2.7 COOKIE

   |                   Cookie word 1                               |
   |                   Cookie word 2                               |
   |                   Cookie word 3                               |
   |                   Cookie word 4                               |

   The COOKIE attribute is used to prevent distributed denial of service
   attacks on the STUN server. It is a 128 bit value, generated by the
   server, and echoed by the client back to the server.

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11 Security Considerations

   There are several potential threats in a STUN system worthy of

   STUN can potentially introduce attacks which result in the theft of
   addresses. When a client sends a request, an attacker can guess the
   value of the mapped address used by the nat, and quickly generate its
   own faked response, sending it to that address. This response would
   contain a faked MAPPED-ADDRESS which actually routes to a different
   host. This could enable DoS attacks, by using a victim's address, or
   theft attacks, by using the address of the host run by the attacker.
   This threat is potentially very serious. To combat it, STUN supports
   public key authentication of responses. This allows a client to
   verify that the response was indeed generated by the server to which
   the request was sent.

   Interestingly, there is little need for strong authentication of
   requests. STUN servers are stateless. Their processing is not user
   specific. The server is hardly more than an echo server. As a result,
   client authentication provides no value. However, a request from a
   client can result in expensive public key operations at the server,
   for the purposes of signing the request. This makes a STUN server
   potentially subject to denial-of-service attacks. To prevent against
   such threats, STUN provides a simple cookie mechanism. The server
   will not sign any responses until it has successfully passed a cookie
   to the client and received it back in a subsequent request. This
   cookie mechanism is similar to the techniques used to prevent the TCP
   SYN attack.

   Compromise of a STUN server can lead to discovery of open ports.
   Knowledge of an open port creates an opportunity for DoS attacks on
   those ports (or DDoS attacks if the traversed NAT is a full cone
   NAT).  Discovering open ports is already fairly trivial using port
   probing, so this does not represent a major threat.

   STUN servers constitute a reflector type of server, and can therefore
   be used as launching grounds for distributed DoS attacks [9]. The
   problem is amplified by the existence of the RESPONSE-ADDRESS
   attribute, which can render ingress filtering useless in prevention
   of attacks. Interestingly, the MAPPED-ADDRESS in the response
   provides a form of traceback in order to counter such attacks. An
   attacker would need to spoof their source address in order to avoid
   the traceback mechanism.

   STUN has the important property that compromise of the STUN servers
   cannot cause security breaches of a firewall when the client is
   within an enterprise. The only thing that a compromised server can do

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   is return false addresses, resulting in the inability of the client
   to receive any data at all. However, the attacker cannot send packets
   to arbitrary servers within the enterprise, if the firewall prohibits
   such communication.

12 IANA Considerations

   There are no IANA considerations associated with this specification.

13 IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing",
   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 [10]. STUN 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

13.1 Problem Definition

   From [10], 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; this
        is why "short term fixes usually aren't".

   The specific problems being solved by STUN are:

        o Provide a means for a client to detect the presence of one or
          more NATs between it and a server run by a service provider on
          the public Internet. The purpose of such detection is to
          determine additional steps that might be necessary in order to
          receive service from that particular provider.

        o Provide a means for a client to obtain an address on the
          public Internet from a non-symmetric NAT, for the express
          purpose of receiving incoming UDP traffic from another host
          targeted to that address.

   STUN does not address TCP, either incoming or outgoing, and does not
   address outgoing UDP communications.

13.2 Exit Strategy

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   From [10], 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.

   STUN comes with its own built in exit strategy. This strategy is the
   detection operation that is performed as a precursor to the actual
   UNSAF address-fixing operation. This discovery operation, documented
   in Section 9.1, attempts to discover the existence of, and type of,
   any NATS between the client and the service provider network. Whilst
   the detection of the specific type of NAT may be brittle, the
   discovery of the existence of NAT is itself quite robust. As NATs are
   phased out through the deployment of IPv6, the discovery operation
   will return immediately with the result that there is no NAT, and no
   further operations are required. Indeed, the discovery operation
   itself can be used to help motivate deployment of IPv6; if a user
   detects a NAT between themselves and the public Internet, they can
   call up their access provider and complain about it.

   STUN can also help facilitate the introduction of midcom. As midcom-
   capable NATs are deployed, applications will, instead of using STUN
   (which also resides at the application layer), first allocate an
   address binding using midcom. However, it is a well-known limitation
   of midcom that it only works when the agent knows the middleboxes
   through which its traffic will flow. Once bindings have been
   allocated from those middleboxes, a STUN detection procedure can
   validate that there are no additional middleboxes on the path from
   the public Internet to the client. If this is the case, the
   application can continue operation using the address bindings
   allocated from midcom. If it is not the case, STUN provides a
   mechanism for self-address fixing through the remaining midcom-
   unaware middlboxes. Thus, STUN provides a way to help transition to
   full midcom-aware networks.

13.3 Brittleness Introduced by STUN

   From [10], 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

   STUN introduces brittleness into the system in several ways:

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        o The discovery process assumes a certain classification of
          devices based on their treatment of UDP. Their could be other
          types of NATs that are deployed that would not fit into one of
          these molds. Therefore, future NATs may not be properly
          detected by STUN. STUN clients (but not servers) would need to
          change to accomodate that.

        o The binding acquisition usage of STUN does not work for all
          NAT types. It will work for any application for full cone NATs
          only. For restricted cone and port restricted cone NAT, it
          will work for some applications dependening on the
          application. Application specific processing will generally be
          needed. For symmetric NATs, the binding acquisition will not
          yield a usable address. The tight dependency on the specific
          type of NAT makes the protocol brittle.

        o STUN assumes that the server exists on the public Internet. If
          the server is located in another private address realm, the
          user may or may not be able to use its discovered address to
          communicate with other users. There is no way to detect such a

        o The bindings allocated from the NAT need to be continuously
          refreshed. Since the timeouts for these bindings is very
          implementation specific, the refresh interval cannot easily be
          determined. When the binding is not being actively used to
          receive traffic, but rather just wait for it, the binding
          refresh will needlessly consume network bandwidth.

        o The use of the STUN server as an additional network element
          introduces another point of potential security attack. These
          attacks are largely prevented by the security measures
          provided by STUN, but not entirely.

        o The use of STUN to discover address bindings will result in an
          increase in latency for applications. For example, a Voice
          over IP application will see an increase of call setup delays
          equal to at least one RTT to the stun server.

13.4 Requirements for a Long Term Solution

   From [10], any UNSAF proposal must provide:

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

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   Our experience with STUN has led to the following requirements for a
   long term solution to the NAT problem:

        Requests for bindings and control of other resources in a NAT
             need to be explicit. Much of the brittleness in STUN
             derives from its guessing at the parameters of the NAT,
             rather than telling the NAT what parameters to use.

        Control needs to be "in-band". There are far too many scenarios
             in which the client will not know about the location of
             middleboxes ahead of time. Instead, control of such boxes
             needs to occur in band, traveling along the same path as
             the data will itself travel. This guarantees that the right
             set of middleboxes are controlled. This is only true for
             first-party controls; third-party controls are best handled
             using the midcom framework.

        Control needs to be limited. Users will need to communicate
             through NATs which are outside of their administrative
             control. In order for providers to be willing to deploy
             NATs which can be controlled by users in different domains,
             the scope of such controls needs to be extremely limited -
             typically, allocating a binding to reach the address where
             the control packets are coming from.

        Simplicity is Paramount. The control protocol will need to be
             implement in very simple clients. The servers will need to
             support extremely high loads. The protocol will need to be
             extremely robust, being the precursor to a host of
             application protocols. As such, simplicity is key.

13.5 Issues with Existing NAPT Boxes

   From [10], any UNSAF proposal must provide:

        Discussion of the impact of the noted practical issues with
        existing, deployed NA[P]Ts and experience reports.

   Several of the practical issues with STUN involve future proofing -
   breaking the protocol when new NAT types get deployed. Fortunately,
   this is not an issue at the current time, since most of the deployed
   NATs are of the types assumed by STUN. The primary usage STUN has
   found is in the area of VoIP, to facilitate allocation of addresses
   for receiving RTP [8] traffic. In that application, the periodic
   keepalives are provided by the RTP traffic itself. However, several
   practical problems arise for RTP. First, RTP assumes that RTCP
   traffic is on a port one higher than the RTP traffic. This pairing

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   property cannot be guaranteed through NATs that are not directly
   controllable. As a result, RTCP traffic may not be properly received.
   Protocol extensions to SDP have been proposed which mitigate this by
   allowing the client to signal a different port for RTCP [11].
   However, there will be interoperability problems for some time.

   For VoIP, silence suppression can cause a gap in the transmission of
   RTP packets. This could result in the loss of a binding in the middle
   of a call, if that silence period exceeds the binding timeout. This
   can be mitigated by sending occassional silence packets to keep the
   binding alive. However, the result is additional brittleness; proper
   operation depends on the the silence suppression algorithm in use,
   the usage of a comfort noise codec, the duration of the silence
   period, and the binding lifetime in the NAT.

13.6 In Closing

   The problems with STUN are not design flaws in STUN. The problems in
   STUN have to do with the lack of standardized behaviors and controls
   in NATs. The result of this lack of standardization has been a
   proliferation of devices whose behavior is highly predictable,
   extremely variable, and uncontrollable. STUN does the best it can in
   such a hostile environment. Ultimately, the solution is to make the
   environment less hostile, and to introduce controls and standardized
   behaviors into NAT. However, until such time as that happens, STUN
   provides a good short term solution given the terrible conditions
   under which it is forced to operate.

14 Acknowledgements

   The authors would like to thank Cullen Jennings for his comments, and
   Baruch Sterman and Alan Hawrylyshen for initial implementations.

15 Authors Addresses

   Jonathan Rosenberg
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: jdrosen@dynamicsoft.com

   Joel Weinberger
   72 Eagle Rock Avenue
   First Floor

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   East Hanover, NJ 07936
   email: jweinberger@dynamicsoft.com

   Christian Huitema
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052-6399
   email: huitema@microsoft.com

   Rohan Mahy
   Cisco Systems
   170 West Tasman Dr, MS: SJC-21/3
   Phone: +1 408 526 8570
   Email: rohan@cisco.com

16 Normative References

   [1] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," Request for Comments 2119, Internet Engineering Task Force,
   Mar. 1997.

   [2] A. Gulbrandsen, P. Vixie, and L. Esibov, "A DNS RR for specifying
   the location of services (DNS SRV)," Request for Comments 2782,
   Internet Engineering Task Force, Feb. 2000.

   [3] R. Housley, "Cryptographic message syntax," Request for Comments
   2630, Internet Engineering Task Force, June 1999.

17 Informative References

   [4] D. Senie, "Network address translator (nat)-friendly application
   design guidelines," Request for Comments 3235, Internet Engineering
   Task Force, Jan. 2002.

   [5] P. Srisuresh, J. Kuthan, J. Rosenberg, A. Molitor, and A. Rayhan,
   "Middlebox communication architecture and framework," Internet Draft,
   Internet Engineering Task Force, Nov. 2001.  Work in progress.

   [6] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
   session initiation protocol," Request for Comments 2543, Internet
   Engineering Task Force, Mar. 1999.

   [7] M. Holdrege and P. Srisuresh, "Protocol complications with the IP
   network address translator," Request for Comments 3027, Internet
   Engineering Task Force, Jan. 2001.

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   [8] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
   transport protocol for real-time applications," Request for Comments
   1889, Internet Engineering Task Force, Jan. 1996.

   [9] V. Paxson, "An analysis of using reflectors for distributed
   denial of service attacks," ACM Computer Communication Review , Vol.
   31, July 2001.

   [10] L. Daigle, "IAB considerations for UNilateral self-address
   fixing (UNSAF)," Internet Draft, Internet Engineering Task Force,
   Feb. 2002.  Work in progress.

   [11] C. Huitema, "RTCP attribute in SDP," Internet Draft, Internet
   Engineering Task Force, Feb. 2002.  Work in progress.

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