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Versions: 00 01

Internet Engineering Task Force                                MIDCOM WG
Internet Draft                         Rosenberg,Weinberger,Huitema,Mahy
draft-rosenberg-midcom-stun-00.txt           dynamicsoft,Microsoft,Cisco
October 1, 2001
Expires: March 2002


              STUN - Simple Traversal of UDP Through NATs

STATUS OF THIS MEMO

   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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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   http://www.ietf.org/ietf/1id-abstracts.txt

   To view the list Internet-Draft Shadow Directories, see
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Abstract

   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
   protocol is very simple, being almost identical to echo.


1 Introduction

   Network Address Translators (NATs), while providing many benefits,
   also come with many drawbacks. The most troublesome of those



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   drawbacks is the fact that they break many existing IP applications,
   and make it difficult to deploy new ones. Guidlines have been
   developed [1] 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
   games.

   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 [2].
   MIDCOM allows an application entity, such as an end client or network
   server of some sort (like a SIP proxy [3]) 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
   MIDCOM.

   Many existing proprietary protocols, such as those for online games
   (such as the games described in RFC 3027 [4]) 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.



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   STUN requires no changes to NATs, and works with an arbitrary number
   of NATs in tandem between the application entity and the public
   Internet.

2 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALLNOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [5] 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 types defined in [6] 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.

        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.



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        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 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.
   Thats it.

   The trick is using this simple protocol to discover the presence of
   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 [7] 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 [8], 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
   application through NAT).

   The STUN request is used to discover the presence of a NAT, and to



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                           /-----\          ............
                         // STUN  \\        .  STUN    .
                        |   Server  |       .Translator.
                         \\       //        .          .
                           \-----/          ............




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



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




                          /-----\
                        //  STUN \\
                       |    Client |
                        \\       //               Private NET 2
                          \-----/


   Figure 1: STUN Configuration


   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



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   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.

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, although that is not
   currently specified.

   Several STUN attributes are defined. The first is a MAPPED-ADDRESS
   attribute, which is an IP address and port. It is 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



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   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 FLAG 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 fourth attribute is the CHANGED-ADDRESS attribute. It is present
   in 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 final 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.

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
   "Response".

   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 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 present in the request, the source port
   of the response MUST NOT be the same as the destination port of the



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   query request. If the "change IP" FLAG was present in the request,
   the source IP address of the response MUST NOT be the same as the
   destination IP address of the query request. 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 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, and handle request
   reliability.

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 [8].

   Specifically, the service name is "stun". The protocol is "udp". The
   procedures of RFC 2782 are followed to determine the server to



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                                           +---------+
                             +-+           |  Query  |
                             | |           |  Server |
                             | |   ------->|    1    |
                             | |---        +---------+
              Query          | |                |
              S:10.0.1.1  ---| | Query          |
                        --   | | S:14.1.2.2     |    Query
                     --- (1) |N|                |    RESPONSE-ADDRESS=
                  ---        | |                |(2)   14.1.2.2
       +------+ --           |A|                |
       |      |              | |                |
       |Client|              |T|                |
       |      |<---          | |                |
       +------+    ------    | |                \/
                         ----| |   (3)     +---------+
               Query         | |-----      |  Query  |
               Response      | |     ------|  Server |
               D: 10.0.1.1   | | Query     |    2    |
                             +-+ Response  +---------+
                                 D:14.1.2.2


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


   contact, with the following additions. If an attempt is made to
   contact a server, and that attempt results in an ICMP error, or no
   response within 30 seconds, the client SHOULD attempt to contact the
   next server.

   There are some cases where the client needs to discover N servers.
   This is done by following the same process as above, but once a
   server is found, SRV processing continues until N-1 more are found.

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


        This would allow a firewall admin to open the STUN port, so



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        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
   attributes.

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




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        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
   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 II, 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
   port restricted 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.



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   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
   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 [7]. 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



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                       +--------+
                       |  Test  |
                       |   I    |
                       +--------+
                            |
                            |
                            V
                           /\               /\
                        N /  \ Y           /  \ Y            +--------+
         UDP     <-------/Resp\---------->/ IP \------------>|  Test  |
         Blocked         \ ?  /           \Same/             |   II   |
                          \  /             \? /              +--------+
                           \/               \/                    |
                                            | N                   |
                                            |                     V
                                            V                    /\
                                        +--------+  Sym.      N /  \
                                        |  Test  |  UDP    <---/Resp\
                                        |   II   |  Firewall   \ ?  /
                                        +--------+              \  /
                                            |                    \/
                                            V                     |Y
                 /\                         /\                    |
  Symmetric  N  /  \       +--------+   N  /  \                   V
     NAT  <--- / IP \<-----|  Test  |<--- /Resp\               Open
               \Same/      |   I    |     \ ?  /               Internet
                \? /       +--------+      \  /
                 \/                         \/
                 |                           |Y
                 |                           |
                 |                           V
                 |                           Full
                 |                           Cone
                 V              /\
             +--------+        /  \ Y
             |  Test  |------>/Resp\---->Restricted
             |   III  |       \ ?  /
             +--------+        \  /
                                \/
                                 |N
                                 |       Port
                                 +------>Restricted


   Figure 3: Flow for type discovery process


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   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
   address.

   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
   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 byes, of the size of the message,



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   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:


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Type                  |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Value                             ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   The following types are defined:


   0x0001: MAPPED-ADDRESS
   0x0002: RESPONSE-ADDRESS
   0x0003: FLAGS
   0x0004: SOURCE-ADDRESS



   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.

10.2.1 MAPPED-ADDRESS

   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     |   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



   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.

10.2.2 RESPONSE-ADDRESS

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

10.2.3 CHANGED-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
   MAPPED-ADDRESS.

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
   ignored.

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




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        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 dicard flag. If true, the message is discarded.

10.2.5 SOURCE-ADDRESS

   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.

11 Security Considerations

   Because query servers do not create state or perform any intensive
   functions, there is little need for them to even authenticate
   clients. In fact, the complexity of authenticating the request is far
   greater than just generating the response. Therefore, no
   authentication is provided.

   The stateless nature of query servers makes them immune to DoS
   attacks as well.

   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. Usage of a set of well known ports could
   also be useful to enable filtering to prevent the usage of STUN for
   reflector attacks [9]. This requires more consideration.

   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.
   More consideration is required to prevent such attacks.



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   STUN has the important property that compromise of the STUN servers
   cannot cause security breaches when the client is within an
   enterprise. The only thing that a compromised server can do is return
   false addresses, resulting in the inability of the client to receive
   any data at all. The protocol is therefore fail safe.

12 Authors Addresses



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

   Joel Weinberger
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   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





13 Bibliography

   [1] D. Senie, "NAT friendly application design guidelines," Internet
   Draft, Internet Engineering Task Force, Mar. 2001.  Work in progress.

   [2] P. Srisuresh, J. Kuthan, and J. Rosenberg, "Middlebox
   communication architecture and framework," Internet Draft, Internet
   Engineering Task Force, Feb. 2001.  Work in progress.



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

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

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

   [6] C. Huitema, "Short term NAT requirements for UDP based peer-to-
   peer applications," Internet Draft, Internet Engineering Task Force,
   Feb. 2001.  Work in progress.

   [7] 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.

   [8] 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.

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





                           Table of Contents



   1          Introduction ........................................    1
   2          Terminology .........................................    3
   3          Definitions .........................................    3
   4          NAT Variations ......................................    3
   5          Overview of Operation ...............................    4
   6          Message Overview ....................................    6
   7          Server Behavior .....................................    7
   8          Client Behavior .....................................    8
   8.1        Discovery ...........................................    8
   8.2        Formulating the Request .............................   10
   9          Use Cases ...........................................   10
   9.1        Discovery Process ...................................   10



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   9.2        Binding Lifetime Discovery ..........................   12
   9.3        Binding Acquisition .................................   12
   10         Protocol Details ....................................   14
   10.1       Message Header ......................................   14
   10.2       Message Attributes ..................................   15
   10.2.1     MAPPED-ADDRESS ......................................   15
   10.2.2     RESPONSE-ADDRESS ....................................   16
   10.2.3     CHANGED-ADDRESS .....................................   16
   10.2.4     FLAGS ...............................................   16
   10.2.5     SOURCE-ADDRESS ......................................   17
   11         Security Considerations .............................   17
   12         Authors Addresses ...................................   18
   13         Bibliography ........................................   18






































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