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RFC 5389


Session Traversal Utilities for NAT (STUN)

Part 3 of 3, p. 31 to 51
Prev RFC Part


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15.  STUN Attributes

   After the STUN header are zero or more attributes.  Each attribute
   MUST be TLV encoded, with a 16-bit type, 16-bit length, and value.
   Each STUN attribute MUST end on a 32-bit boundary.  As mentioned
   above, all fields in an attribute are transmitted most significant
   bit first.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |         Type                  |            Length             |
      |                         Value (variable)                ....

                    Figure 4: Format of STUN Attributes

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   The value in the length field MUST contain the length of the Value
   part of the attribute, prior to padding, measured in bytes.  Since
   STUN aligns attributes on 32-bit boundaries, attributes whose content
   is not a multiple of 4 bytes are padded with 1, 2, or 3 bytes of
   padding so that its value contains a multiple of 4 bytes.  The
   padding bits are ignored, and may be any value.

   Any attribute type MAY appear more than once in a STUN message.
   Unless specified otherwise, the order of appearance is significant:
   only the first occurrence needs to be processed by a receiver, and
   any duplicates MAY be ignored by a receiver.

   To allow future revisions of this specification to add new attributes
   if needed, the attribute space is divided into two ranges.
   Attributes with type values between 0x0000 and 0x7FFF are
   comprehension-required attributes, which means that the STUN agent
   cannot successfully process the message unless it understands the
   attribute.  Attributes with type values between 0x8000 and 0xFFFF are
   comprehension-optional attributes, which means that those attributes
   can be ignored by the STUN agent if it does not understand them.

   The set of STUN attribute types is maintained by IANA.  The initial
   set defined by this specification is found in Section 18.2.

   The rest of this section describes the format of the various
   attributes defined in this specification.


   The MAPPED-ADDRESS attribute indicates a reflexive transport address
   of the client.  It consists of an 8-bit address family and a 16-bit
   port, followed by a fixed-length value representing the IP address.
   If the address family is IPv4, the address MUST be 32 bits.  If the
   address family is IPv6, the address MUST be 128 bits.  All fields
   must be in network byte order.

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   The format of the MAPPED-ADDRESS attribute is:

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

               Figure 5: Format of MAPPED-ADDRESS Attribute

   The address family can take on the following values:


   The first 8 bits of the MAPPED-ADDRESS MUST be set to 0 and MUST be
   ignored by receivers.  These bits are present for aligning parameters
   on natural 32-bit boundaries.

   This attribute is used only by servers for achieving backwards
   compatibility with RFC 3489 [RFC3489] clients.


   The XOR-MAPPED-ADDRESS attribute is identical to the MAPPED-ADDRESS
   attribute, except that the reflexive transport address is obfuscated
   through the XOR function.

   The format of the XOR-MAPPED-ADDRESS is:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |x x x x x x x x|    Family     |         X-Port                |
     |                X-Address (Variable)

             Figure 6: Format of XOR-MAPPED-ADDRESS Attribute

   The Family represents the IP address family, and is encoded
   identically to the Family in MAPPED-ADDRESS.

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   X-Port is computed by taking the mapped port in host byte order,
   XOR'ing it with the most significant 16 bits of the magic cookie, and
   then the converting the result to network byte order.  If the IP
   address family is IPv4, X-Address is computed by taking the mapped IP
   address in host byte order, XOR'ing it with the magic cookie, and
   converting the result to network byte order.  If the IP address
   family is IPv6, X-Address is computed by taking the mapped IP address
   in host byte order, XOR'ing it with the concatenation of the magic
   cookie and the 96-bit transaction ID, and converting the result to
   network byte order.

   The rules for encoding and processing the first 8 bits of the
   attribute's value, the rules for handling multiple occurrences of the
   attribute, and the rules for processing address families are the same

   Note: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their
   encoding of the transport address.  The former encodes the transport
   address by exclusive-or'ing it with the magic cookie.  The latter
   encodes it directly in binary.  RFC 3489 originally specified only
   MAPPED-ADDRESS.  However, deployment experience found that some NATs
   rewrite the 32-bit binary payloads containing the NAT's public IP
   address, such as STUN's MAPPED-ADDRESS attribute, in the well-meaning
   but misguided attempt at providing a generic ALG function.  Such
   behavior interferes with the operation of STUN and also causes
   failure of STUN's message-integrity checking.


   The USERNAME attribute is used for message integrity.  It identifies
   the username and password combination used in the message-integrity

   The value of USERNAME is a variable-length value.  It MUST contain a
   UTF-8 [RFC3629] encoded sequence of less than 513 bytes, and MUST
   have been processed using SASLprep [RFC4013].


   The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] of
   the STUN message.  The MESSAGE-INTEGRITY attribute can be present in
   any STUN message type.  Since it uses the SHA1 hash, the HMAC will be
   20 bytes.  The text used as input to HMAC is the STUN message,
   including the header, up to and including the attribute preceding the
   MESSAGE-INTEGRITY attribute.  With the exception of the FINGERPRINT
   attribute, which appears after MESSAGE-INTEGRITY, agents MUST ignore
   all other attributes that follow MESSAGE-INTEGRITY.

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   The key for the HMAC depends on whether long-term or short-term
   credentials are in use.  For long-term credentials, the key is 16

            key = MD5(username ":" realm ":" SASLprep(password))

   That is, the 16-byte key is formed by taking the MD5 hash of the
   result of concatenating the following five fields: (1) the username,
   with any quotes and trailing nulls removed, as taken from the
   USERNAME attribute (in which case SASLprep has already been applied);
   (2) a single colon; (3) the realm, with any quotes and trailing nulls
   removed; (4) a single colon; and (5) the password, with any trailing
   nulls removed and after processing using SASLprep.  For example, if
   the username was 'user', the realm was 'realm', and the password was
   'pass', then the 16-byte HMAC key would be the result of performing
   an MD5 hash on the string 'user:realm:pass', the resulting hash being

   For short-term credentials:

                          key = SASLprep(password)

   where MD5 is defined in RFC 1321 [RFC1321] and SASLprep() is defined
   in RFC 4013 [RFC4013].

   The structure of the key when used with long-term credentials
   facilitates deployment in systems that also utilize SIP.  Typically,
   SIP systems utilizing SIP's digest authentication mechanism do not
   actually store the password in the database.  Rather, they store a
   value called H(A1), which is equal to the key defined above.

   Based on the rules above, the hash used to construct MESSAGE-
   INTEGRITY includes the length field from the STUN message header.
   Prior to performing the hash, the MESSAGE-INTEGRITY attribute MUST be
   inserted into the message (with dummy content).  The length MUST then
   be set to point to the length of the message up to, and including,
   the MESSAGE-INTEGRITY attribute itself, but excluding any attributes
   after it.  Once the computation is performed, the value of the
   MESSAGE-INTEGRITY attribute can be filled in, and the value of the
   length in the STUN header can be set to its correct value -- the
   length of the entire message.  Similarly, when validating the
   MESSAGE-INTEGRITY, the length field should be adjusted to point to
   the end of the MESSAGE-INTEGRITY attribute prior to calculating the
   HMAC.  Such adjustment is necessary when attributes, such as

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   The FINGERPRINT attribute MAY be present in all STUN messages.  The
   value of the attribute is computed as the CRC-32 of the STUN message
   up to (but excluding) the FINGERPRINT attribute itself, XOR'ed with
   the 32-bit value 0x5354554e (the XOR helps in cases where an
   application packet is also using CRC-32 in it).  The 32-bit CRC is
   the one defined in ITU V.42 [ITU.V42.2002], which has a generator
   polynomial of x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1.
   When present, the FINGERPRINT attribute MUST be the last attribute in
   the message, and thus will appear after MESSAGE-INTEGRITY.

   The FINGERPRINT attribute can aid in distinguishing STUN packets from
   packets of other protocols.  See Section 8.

   As with MESSAGE-INTEGRITY, the CRC used in the FINGERPRINT attribute
   covers the length field from the STUN message header.  Therefore,
   this value must be correct and include the CRC attribute as part of
   the message length, prior to computation of the CRC.  When using the
   FINGERPRINT attribute in a message, the attribute is first placed
   into the message with a dummy value, then the CRC is computed, and
   then the value of the attribute is updated.  If the MESSAGE-INTEGRITY
   attribute is also present, then it must be present with the correct
   message-integrity value before the CRC is computed, since the CRC is
   done over the value of the MESSAGE-INTEGRITY attribute as well.


   The ERROR-CODE attribute is used in error response messages.  It
   contains a numeric error code value in the range of 300 to 699 plus a
   textual reason phrase encoded in UTF-8 [RFC3629], and is consistent
   in its code assignments and semantics with SIP [RFC3261] and HTTP
   [RFC2616].  The reason phrase is meant for user consumption, and can
   be anything appropriate for the error code.  Recommended reason
   phrases for the defined error codes are included in the IANA registry
   for error codes.  The reason phrase MUST be a UTF-8 [RFC3629] encoded
   sequence of less than 128 characters (which can be as long as 763

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |           Reserved, should be 0         |Class|     Number    |
      |      Reason Phrase (variable)                                ..

                      Figure 7: ERROR-CODE Attribute

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   To facilitate processing, the class of the error code (the hundreds
   digit) is encoded separately from the rest of the code, as shown in
   Figure 7.

   The Reserved bits SHOULD be 0, and are for alignment on 32-bit
   boundaries.  Receivers MUST ignore these bits.  The Class represents
   the hundreds digit of the error code.  The value MUST be between 3
   and 6.  The Number represents the error code modulo 100, and its
   value MUST be between 0 and 99.

   The following error codes, along with their recommended reason
   phrases, are defined:

   300  Try Alternate: The client should contact an alternate server for
        this request.  This error response MUST only be sent if the
        request included a USERNAME attribute and a valid MESSAGE-
        INTEGRITY attribute; otherwise, it MUST NOT be sent and error
        code 400 (Bad Request) is suggested.  This error response MUST
        be protected with the MESSAGE-INTEGRITY attribute, and receivers
        MUST validate the MESSAGE-INTEGRITY of this response before
        redirecting themselves to an alternate server.

             Note: Failure to generate and validate message integrity
             for a 300 response allows an on-path attacker to falsify a
             300 response thus causing subsequent STUN messages to be
             sent to a victim.

   400  Bad Request: The request was malformed.  The client SHOULD NOT
        retry the request without modification from the previous
        attempt.  The server may not be able to generate a valid
        MESSAGE-INTEGRITY for this error, so the client MUST NOT expect
        a valid MESSAGE-INTEGRITY attribute on this response.

   401  Unauthorized: The request did not contain the correct
        credentials to proceed.  The client should retry the request
        with proper credentials.

   420  Unknown Attribute: The server received a STUN packet containing
        a comprehension-required attribute that it did not understand.
        The server MUST put this unknown attribute in the UNKNOWN-
        ATTRIBUTE attribute of its error response.

   438  Stale Nonce: The NONCE used by the client was no longer valid.
        The client should retry, using the NONCE provided in the

   500  Server Error: The server has suffered a temporary error.  The
        client should try again.

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15.7.  REALM

   The REALM attribute may be present in requests and responses.  It
   contains text that meets the grammar for "realm-value" as described
   in RFC 3261 [RFC3261] but without the double quotes and their
   surrounding whitespace.  That is, it is an unquoted realm-value (and
   is therefore a sequence of qdtext or quoted-pair).  It MUST be a
   UTF-8 [RFC3629] encoded sequence of less than 128 characters (which
   can be as long as 763 bytes), and MUST have been processed using
   SASLprep [RFC4013].

   Presence of the REALM attribute in a request indicates that long-term
   credentials are being used for authentication.  Presence in certain
   error responses indicates that the server wishes the client to use a
   long-term credential for authentication.

15.8.  NONCE

   The NONCE attribute may be present in requests and responses.  It
   contains a sequence of qdtext or quoted-pair, which are defined in
   RFC 3261 [RFC3261].  Note that this means that the NONCE attribute
   will not contain actual quote characters.  See RFC 2617 [RFC2617],
   Section 4.3, for guidance on selection of nonce values in a server.

   It MUST be less than 128 characters (which can be as long as 763


   The UNKNOWN-ATTRIBUTES attribute is present only in an error response
   when the response code in the ERROR-CODE attribute is 420.

   The attribute contains a list of 16-bit values, each of which
   represents an attribute type that was not understood by the server.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |      Attribute 1 Type           |     Attribute 2 Type        |
      |      Attribute 3 Type           |     Attribute 4 Type    ...

             Figure 8: Format of UNKNOWN-ATTRIBUTES Attribute

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      Note: In [RFC3489], this field was padded to 32 by duplicating the
      last attribute.  In this version of the specification, the normal
      padding rules for attributes are used instead.

15.10.  SOFTWARE

   The SOFTWARE attribute contains a textual description of the software
   being used by the agent sending the message.  It is used by clients
   and servers.  Its value SHOULD include manufacturer and version
   number.  The attribute has no impact on operation of the protocol,
   and serves only as a tool for diagnostic and debugging purposes.  The
   value of SOFTWARE is variable length.  It MUST be a UTF-8 [RFC3629]
   encoded sequence of less than 128 characters (which can be as long as
   763 bytes).


   The alternate server represents an alternate transport address
   identifying a different STUN server that the STUN client should try.

   It is encoded in the same way as MAPPED-ADDRESS, and thus refers to a
   single server by IP address.  The IP address family MUST be identical
   to that of the source IP address of the request.

16.  Security Considerations

16.1.  Attacks against the Protocol

16.1.1.  Outside Attacks

   An attacker can try to modify STUN messages in transit, in order to
   cause a failure in STUN operation.  These attacks are detected for
   both requests and responses through the message-integrity mechanism,
   using either a short-term or long-term credential.  Of course, once
   detected, the manipulated packets will be dropped, causing the STUN
   transaction to effectively fail.  This attack is possible only by an
   on-path attacker.

   An attacker that can observe, but not modify, STUN messages in-
   transit (for example, an attacker present on a shared access medium,
   such as Wi-Fi), can see a STUN request, and then immediately send a
   STUN response, typically an error response, in order to disrupt STUN
   processing.  This attack is also prevented for messages that utilize
   MESSAGE-INTEGRITY.  However, some error responses, those related to
   authentication in particular, cannot be protected by MESSAGE-
   INTEGRITY.  When STUN itself is run over a secure transport protocol
   (e.g., TLS), these attacks are completely mitigated.

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   Depending on the STUN usage, these attacks may be of minimal
   consequence and thus do not require message integrity to mitigate.
   For example, when STUN is used to a basic STUN server to discover a
   server reflexive candidate for usage with ICE, authentication and
   message integrity are not required since these attacks are detected
   during the connectivity check phase.  The connectivity checks
   themselves, however, require protection for proper operation of ICE
   overall.  As described in Section 14, STUN usages describe when
   authentication and message integrity are needed.

   Since STUN uses the HMAC of a shared secret for authentication and
   integrity protection, it is subject to offline dictionary attacks.
   When authentication is utilized, it SHOULD be with a strong password
   that is not readily subject to offline dictionary attacks.
   Protection of the channel itself, using TLS, mitigates these attacks.
   However, STUN is most often run over UDP and in those cases, strong
   passwords are the only way to protect against these attacks.

16.1.2.  Inside Attacks

   A rogue client may try to launch a DoS attack against a server by
   sending it a large number of STUN requests.  Fortunately, STUN
   requests can be processed statelessly by a server, making such
   attacks hard to launch.

   A rogue client may use a STUN server as a reflector, sending it
   requests with a falsified source IP address and port.  In such a
   case, the response would be delivered to that source IP and port.
   There is no amplification of the number of packets with this attack
   (the STUN server sends one packet for each packet sent by the
   client), though there is a small increase in the amount of data,
   since STUN responses are typically larger than requests.  This attack
   is mitigated by ingress source address filtering.

   Revealing the specific software version of the agent through the
   SOFTWARE attribute might allow them to become more vulnerable to
   attacks against software that is known to contain security holes.
   Implementers SHOULD make usage of the SOFTWARE attribute a
   configurable option.

16.2.  Attacks Affecting the Usage

   This section lists attacks that might be launched against a usage of
   STUN.  Each STUN usage must consider whether these attacks are
   applicable to it, and if so, discuss counter-measures.

   Most of the attacks in this section revolve around an attacker
   modifying the reflexive address learned by a STUN client through a

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   Binding request/response transaction.  Since the usage of the
   reflexive address is a function of the usage, the applicability and
   remediation of these attacks are usage-specific.  In common
   situations, modification of the reflexive address by an on-path
   attacker is easy to do.  Consider, for example, the common situation
   where STUN is run directly over UDP.  In this case, an on-path
   attacker can modify the source IP address of the Binding request
   before it arrives at the STUN server.  The STUN server will then
   return this IP address in the XOR-MAPPED-ADDRESS attribute to the
   client, and send the response back to that (falsified) IP address and
   port.  If the attacker can also intercept this response, it can
   direct it back towards the client.  Protecting against this attack by
   using a message-integrity check is impossible, since a message-
   integrity value cannot cover the source IP address, since the
   intervening NAT must be able to modify this value.  Instead, one
   solution to preventing the attacks listed below is for the client to
   verify the reflexive address learned, as is done in ICE [MMUSIC-ICE].
   Other usages may use other means to prevent these attacks.

16.2.1.  Attack I: Distributed DoS (DDoS) against a Target

   In this attack, the attacker provides one or more clients with the
   same faked reflexive address that points to the intended target.
   This will trick the STUN clients into thinking that their reflexive
   addresses are equal to that of the target.  If the clients hand out
   that reflexive address in order to receive traffic on it (for
   example, in SIP messages), the traffic will instead be sent to the
   target.  This attack can provide substantial amplification,
   especially when used with clients that are using STUN to enable
   multimedia applications.  However, it can only be launched against
   targets for which packets from the STUN server to the target pass
   through the attacker, limiting the cases in which it is possible.

16.2.2.  Attack II: Silencing a Client

   In this attack, the attacker provides a STUN client with a faked
   reflexive address.  The reflexive address it provides is a transport
   address that routes to nowhere.  As a result, the client won't
   receive any of the packets it expects to receive when it hands out
   the reflexive address.  This exploitation is not very interesting for
   the attacker.  It impacts a single client, which is frequently not
   the desired target.  Moreover, any attacker that can mount the attack
   could also deny service to the client by other means, such as
   preventing the client from receiving any response from the STUN
   server, or even a DHCP server.  As with the attack in Section 16.2.1,
   this attack is only possible when the attacker is on path for packets
   sent from the STUN server towards this unused IP address.

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16.2.3.  Attack III: Assuming the Identity of a Client

   This attack is similar to attack II.  However, the faked reflexive
   address points to the attacker itself.  This allows the attacker to
   receive traffic that was destined for the client.

16.2.4.  Attack IV: Eavesdropping

   In this attack, the attacker forces the client to use a reflexive
   address that routes to itself.  It then forwards any packets it
   receives to the client.  This attack would allow the attacker to
   observe all packets sent to the client.  However, in order to launch
   the attack, the attacker must have already been able to observe
   packets from the client to the STUN server.  In most cases (such as
   when the attack is launched from an access network), this means that
   the attacker could already observe packets sent to the client.  This
   attack is, as a result, only useful for observing traffic by
   attackers on the path from the client to the STUN server, but not
   generally on the path of packets being routed towards the client.

16.3.  Hash Agility Plan

   This specification uses HMAC-SHA-1 for computation of the message
   integrity.  If, at a later time, HMAC-SHA-1 is found to be
   compromised, the following is the remedy that will be applied.

   We will define a STUN extension that introduces a new message-
   integrity attribute, computed using a new hash.  Clients would be
   required to include both the new and old message-integrity attributes
   in their requests or indications.  A new server will utilize the new
   message-integrity attribute, and an old one, the old.  After a
   transition period where mixed implementations are in deployment, the
   old message-integrity attribute will be deprecated by another
   specification, and clients will cease including it in requests.

   It is also important to note that the HMAC is done using a key that
   is itself computed using an MD5 of the user's password.  The choice
   of the MD5 hash was made because of the existence of legacy databases
   that store passwords in that form.  If future work finds that an HMAC
   of an MD5 input is not secure, and a different hash is needed, it can
   also be changed using this plan.  However, this would require
   administrators to repopulate their databases.

17.  IAB Considerations

   The IAB has studied the problem of Unilateral Self-Address Fixing
   (UNSAF), which is the general process by which a client attempts to
   determine its address in another realm on the other side of a NAT

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   through a collaborative protocol reflection mechanism (RFC3424
   [RFC3424]).  STUN can be used to perform this function using a
   Binding request/response transaction if one agent is behind a NAT and
   the other is on the public side of the NAT.

   The IAB has mandated that protocols developed for this purpose
   document a specific set of considerations.  Because some STUN usages
   provide UNSAF functions (such as ICE [MMUSIC-ICE] ), and others do
   not (such as SIP Outbound [SIP-OUTBOUND]), answers to these
   considerations need to be addressed by the usages themselves.

18.  IANA Considerations

   IANA has created three new registries: a "STUN Methods Registry", a
   "STUN Attributes Registry", and a "STUN Error Codes Registry".  IANA
   has also changed the name of the assigned IANA port for STUN from
   "nat-stun-port" to "stun".

18.1.  STUN Methods Registry

   A STUN method is a hex number in the range 0x000 - 0xFFF.  The
   encoding of STUN method into a STUN message is described in
   Section 6.

   The initial STUN methods are:

   0x000: (Reserved)
   0x001: Binding
   0x002: (Reserved; was SharedSecret)

   STUN methods in the range 0x000 - 0x7FF are assigned by IETF Review
   [RFC5226].  STUN methods in the range 0x800 - 0xFFF are assigned by
   Designated Expert [RFC5226].  The responsibility of the expert is to
   verify that the selected codepoint(s) are not in use and that the
   request is not for an abnormally large number of codepoints.
   Technical review of the extension itself is outside the scope of the
   designated expert responsibility.

18.2.  STUN Attribute Registry

   A STUN Attribute type is a hex number in the range 0x0000 - 0xFFFF.
   STUN attribute types in the range 0x0000 - 0x7FFF are considered
   comprehension-required; STUN attribute types in the range 0x8000 -
   0xFFFF are considered comprehension-optional.  A STUN agent handles
   unknown comprehension-required and comprehension-optional attributes

   The initial STUN Attributes types are:

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   Comprehension-required range (0x0000-0x7FFF):
     0x0000: (Reserved)
     0x0001: MAPPED-ADDRESS
     0x0002: (Reserved; was RESPONSE-ADDRESS)
     0x0003: (Reserved; was CHANGE-ADDRESS)
     0x0004: (Reserved; was SOURCE-ADDRESS)
     0x0005: (Reserved; was CHANGED-ADDRESS)
     0x0006: USERNAME
     0x0007: (Reserved; was PASSWORD)
     0x0009: ERROR-CODE
     0x000B: (Reserved; was REFLECTED-FROM)
     0x0014: REALM
     0x0015: NONCE

   Comprehension-optional range (0x8000-0xFFFF)
     0x8022: SOFTWARE
     0x8028: FINGERPRINT

   STUN Attribute types in the first half of the comprehension-required
   range (0x0000 - 0x3FFF) and in the first half of the comprehension-
   optional range (0x8000 - 0xBFFF) are assigned by IETF Review
   [RFC5226].  STUN Attribute types in the second half of the
   comprehension-required range (0x4000 - 0x7FFF) and in the second half
   of the comprehension-optional range (0xC000 - 0xFFFF) are assigned by
   Designated Expert [RFC5226].  The responsibility of the expert is to
   verify that the selected codepoint(s) are not in use, and that the
   request is not for an abnormally large number of codepoints.
   Technical review of the extension itself is outside the scope of the
   designated expert responsibility.

18.3.  STUN Error Code Registry

   A STUN error code is a number in the range 0 - 699.  STUN error codes
   are accompanied by a textual reason phrase in UTF-8 [RFC3629] that is
   intended only for human consumption and can be anything appropriate;
   this document proposes only suggested values.

   STUN error codes are consistent in codepoint assignments and
   semantics with SIP [RFC3261] and HTTP [RFC2616].

   The initial values in this registry are given in Section 15.6.

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   New STUN error codes are assigned based on IETF Review [RFC5226].
   The specification must carefully consider how clients that do not
   understand this error code will process it before granting the
   request.  See the rules in Section 7.3.4.

18.4.  STUN UDP and TCP Port Numbers

   IANA has previously assigned port 3478 for STUN.  This port appears
   in the IANA registry under the moniker "nat-stun-port".  In order to
   align the DNS SRV procedures with the registered protocol service,
   IANA is requested to change the name of protocol assigned to port
   3478 from "nat-stun-port" to "stun", and the textual name from
   "Simple Traversal of UDP Through NAT (STUN)" to "Session Traversal
   Utilities for NAT", so that the IANA port registry would read:

   stun   3478/tcp   Session Traversal Utilities for NAT (STUN) port
   stun   3478/udp   Session Traversal Utilities for NAT (STUN) port

   In addition, IANA has assigned port number 5349 for the "stuns"
   service, defined over TCP and UDP.  The UDP port is not currently
   defined; however, it is reserved for future use.

19.  Changes since RFC 3489

   This specification obsoletes RFC 3489 [RFC3489].  This specification
   differs from RFC 3489 in the following ways:

   o  Removed the notion that STUN is a complete NAT traversal solution.
      STUN is now a tool that can be used to produce a NAT traversal
      solution.  As a consequence, changed the name of the protocol to
      Session Traversal Utilities for NAT.

   o  Introduced the concept of STUN usages, and described what a usage
      of STUN must document.

   o  Removed the usage of STUN for NAT type detection and binding
      lifetime discovery.  These techniques have proven overly brittle
      due to wider variations in the types of NAT devices than described
      in this document.  Removed the RESPONSE-ADDRESS, CHANGED-ADDRESS,

   o  Added a fixed 32-bit magic cookie and reduced length of
      transaction ID by 32 bits.  The magic cookie begins at the same
      offset as the original transaction ID.

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   o  Added the XOR-MAPPED-ADDRESS attribute, which is included in
      Binding responses if the magic cookie is present in the request.
      Otherwise, the RFC 3489 behavior is retained (that is, Binding
      response includes MAPPED-ADDRESS).  See discussion in XOR-MAPPED-
      ADDRESS regarding this change.

   o  Introduced formal structure into the message type header field,
      with an explicit pair of bits for indication of request, response,
      error response, or indication.  Consequently, the message type
      field is split into the class (one of the previous four) and

   o  Explicitly point out that the most significant 2 bits of STUN are
      0b00, allowing easy differentiation with RTP packets when used
      with ICE.

   o  Added the FINGERPRINT attribute to provide a method of definitely
      detecting the difference between STUN and another protocol when
      the two protocols are multiplexed together.

   o  Added support for IPv6.  Made it clear that an IPv4 client could
      get a v6 mapped address, and vice versa.

   o  Added long-term-credential-based authentication.

   o  Added the SOFTWARE, REALM, NONCE, and ALTERNATE-SERVER attributes.

   o  Removed the SharedSecret method, and thus the PASSWORD attribute.
      This method was almost never implemented and is not needed with
      current usages.

   o  Removed recommendation to continue listening for STUN responses
      for 10 seconds in an attempt to recognize an attack.

   o  Changed transaction timers to be more TCP friendly.

   o  Removed the STUN example that centered around the separation of
      the control and media planes.  Instead, provided more information
      on using STUN with protocols.

   o  Defined a generic padding mechanism that changes the
      interpretation of the length attribute.  This would, in theory,
      break backwards compatibility.  However, the mechanism in RFC 3489
      never worked for the few attributes that weren't aligned naturally
      on 32-bit boundaries.

   o  REALM, SERVER, reason phrases, and NONCE limited to 127
      characters.  USERNAME to 513 bytes.

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   o  Changed the DNS SRV procedures for TCP and TLS.  UDP remains the
      same as before.

20.  Contributors

   Christian Huitema and Joel Weinberger were original co-authors of RFC

21.  Acknowledgements

   The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
   Jennings, Bob Penfield, Xavier Marjou, Magnus Westerlund, Miguel
   Garcia, Bruce Lowekamp, and Chris Sullivan for their comments, and
   Baruch Sterman and Alan Hawrylyshen for initial implementations.
   Thanks for Leslie Daigle, Allison Mankin, Eric Rescorla, and Henning
   Schulzrinne for IESG and IAB input on this work.

22.  References

22.1.  Normative References

   [ITU.V42.2002]    International Telecommunications Union, "Error-
                     correcting Procedures for DCEs Using Asynchronous-
                     to-Synchronous Conversion", ITU-T Recommendation
                     V.42, March 2002.

   [RFC0791]         Postel, J., "Internet Protocol", STD 5, RFC 791,
                     September 1981.

   [RFC1122]         Braden, R., "Requirements for Internet Hosts -
                     Communication Layers", STD 3, RFC 1122,
                     October 1989.

   [RFC1321]         Rivest, R., "The MD5 Message-Digest Algorithm",
                     RFC 1321, April 1992.

   [RFC2104]         Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
                     Keyed-Hashing for Message Authentication",
                     RFC 2104, February 1997.

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

   [RFC2460]         Deering, S. and R. Hinden, "Internet Protocol,
                     Version 6 (IPv6) Specification", RFC 2460,
                     December 1998.

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   [RFC2617]         Franks, J., Hallam-Baker, P., Hostetler, J.,
                     Lawrence, S., Leach, P., Luotonen, A., and L.
                     Stewart, "HTTP Authentication: Basic and Digest
                     Access Authentication", RFC 2617, June 1999.

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

   [RFC2818]         Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC2988]         Paxson, V. and M. Allman, "Computing TCP's
                     Retransmission Timer", RFC 2988, November 2000.

   [RFC3629]         Yergeau, F., "UTF-8, a transformation format of ISO
                     10646", STD 63, RFC 3629, November 2003.

   [RFC4013]         Zeilenga, K., "SASLprep: Stringprep Profile for
                     User Names and Passwords", RFC 4013, February 2005.

22.2.  Informative References

   [BEHAVE-NAT]      MacDonald, D. and B. Lowekamp, "NAT Behavior
                     Discovery Using STUN", Work in Progress, July 2008.

   [BEHAVE-TURN]     Rosenberg, J., Mahy, R., and P. Matthews,
                     "Traversal Using Relays around NAT (TURN): Relay
                     Extensions to Session  Traversal Utilities for NAT
                     (STUN)", Work in Progress, July 2008.

   [KARN87]          Karn, P. and C. Partridge, "Improving Round-Trip
                     Time Estimates in Reliable Transport Protocols",
                     SIGCOMM 1987, August 1987.

   [MMUSIC-ICE]      Rosenberg, J., "Interactive Connectivity
                     Establishment (ICE): A Protocol for Network Address
                     Translator (NAT) Traversal for Offer/Answer
                     Protocols", Work in Progress, October 2007.

   [MMUSIC-ICE-TCP]  Rosenberg, J., "TCP Candidates with Interactive
                     Connectivity Establishment (ICE)", Work
                     in Progress, July 2008.

   [RFC2616]         Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                     Masinter, L., Leach, P., and T. Berners-Lee,
                     "Hypertext Transfer Protocol -- HTTP/1.1",
                     RFC 2616, June 1999.

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   [RFC3261]         Rosenberg, J., Schulzrinne, H., Camarillo, G.,
                     Johnston, A., Peterson, J., Sparks, R., Handley,
                     M., and E. Schooler, "SIP: Session Initiation
                     Protocol", RFC 3261, June 2002.

   [RFC3264]         Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
                     Model with Session Description Protocol (SDP)",
                     RFC 3264, June 2002.

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

   [RFC3489]         Rosenberg, J., Weinberger, J., Huitema, C., and R.
                     Mahy, "STUN - Simple Traversal of User Datagram
                     Protocol (UDP) Through Network Address Translators
                     (NATs)", RFC 3489, March 2003.

   [RFC4107]         Bellovin, S. and R. Housley, "Guidelines for
                     Cryptographic Key Management", BCP 107, RFC 4107,
                     June 2005.

   [RFC5226]         Narten, T. and H. Alvestrand, "Guidelines for
                     Writing an IANA Considerations Section in RFCs",
                     BCP 26, RFC 5226, May 2008.

   [SIP-OUTBOUND]    Jennings, C. and R. Mahy, "Managing Client
                     Initiated Connections in the Session Initiation
                     Protocol  (SIP)", Work in Progress, June 2008.

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Appendix A.  C Snippet to Determine STUN Message Types

   Given a 16-bit STUN message type value in host byte order in msg_type
   parameter, below are C macros to determine the STUN message types:

   #define IS_REQUEST(msg_type)       (((msg_type) & 0x0110) == 0x0000)
   #define IS_INDICATION(msg_type)    (((msg_type) & 0x0110) == 0x0010)
   #define IS_SUCCESS_RESP(msg_type)  (((msg_type) & 0x0110) == 0x0100)
   #define IS_ERR_RESP(msg_type)      (((msg_type) & 0x0110) == 0x0110)

Authors' Addresses

   Jonathan Rosenberg
   Edison, NJ


   Rohan Mahy


   Philip Matthews


   Dan Wing
   771 Alder Drive
   San Jose, CA  95035


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