    Internet Engineering Task Force (IETF)                 M. Petit-Huguenin
    Request for Comments: 8489                            Impedance Mismatch
    Obsoletes: 5389                                             G. Salgueiro
    Category: Standards Track                                          Cisco
    ISSN: 2070-1721                                             J. Rosenberg
                                                                       Five9
                                                                     D. Wing
                                                                      Citrix
                                                                     R. Mahy
                                                                Unaffiliated
                                                                 P. Matthews
                                                                       Nokia
                                                               February 2020

                   Session Traversal Utilities for NAT (STUN)

    Abstract

       Session Traversal Utilities for NAT (STUN) is a protocol that serves
       as a tool for other protocols in dealing with NAT traversal.  It can
       be used by an endpoint to determine the IP address and port allocated
       to it by a NAT.  It can also be used to check connectivity between
       two endpoints and as a keep-alive protocol to maintain NAT bindings.
       STUN works with many existing NATs and does not require any special
       behavior from them.

       STUN is not a NAT traversal solution by itself.  Rather, it is a tool
       to be used in the context of a NAT traversal solution.

       This document obsoletes RFC 5389.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8489.

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    Copyright Notice

       Copyright (c) 2020 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Table of Contents

       1. Introduction ....................................................4
       2. Overview of Operation ...........................................5
       3. Terminology .....................................................7
       4. Definitions .....................................................7
       5. STUN Message Structure ..........................................9
       6. Base Protocol Procedures .......................................11
          6.1. Forming a Request or an Indication ........................11
          6.2. Sending the Request or Indication .........................12
               6.2.1. Sending over UDP or DTLS-over-UDP ..................13
               6.2.2. Sending over TCP or TLS-over-TCP ...................14
               6.2.3. Sending over TLS-over-TCP or DTLS-over-UDP .........15
          6.3. Receiving a STUN Message ..................................16
               6.3.1. Processing a Request ...............................17
                      6.3.1.1. Forming a Success or Error Response .......17
                      6.3.1.2. Sending the Success or Error Response .....18
               6.3.2. Processing an Indication ...........................18
               6.3.3. Processing a Success Response ......................19
               6.3.4. Processing an Error Response .......................19
       7. FINGERPRINT Mechanism ..........................................20
       8. DNS Discovery of a Server ......................................20
          8.1. STUN URI Scheme Semantics .................................21
       9. Authentication and Message-Integrity Mechanisms ................22
          9.1. Short-Term Credential Mechanism ...........................23
               9.1.1. HMAC Key ...........................................23
               9.1.2. Forming a Request or Indication ....................23
               9.1.3. Receiving a Request or Indication ..................23
               9.1.4. Receiving a Response ...............................25
               9.1.5. Sending Subsequent Requests ........................25
          9.2. Long-Term Credential Mechanism ............................26
               9.2.1. Bid-Down Attack Prevention .........................27
               9.2.2. HMAC Key ...........................................27

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               9.2.3. Forming a Request ..................................28
                      9.2.3.1. First Request .............................28
                      9.2.3.2. Subsequent Requests .......................29
               9.2.4. Receiving a Request ................................29
               9.2.5. Receiving a Response ...............................31
       10. ALTERNATE-SERVER Mechanism ....................................33
       11. Backwards Compatibility with RFC 3489 .........................34
       12. Basic Server Behavior .........................................34
       13. STUN Usages ...................................................35
       14. STUN Attributes ...............................................36
          14.1. MAPPED-ADDRESS ...........................................37
          14.2. XOR-MAPPED-ADDRESS .......................................38
          14.3. USERNAME .................................................39
          14.4. USERHASH .................................................40
          14.5. MESSAGE-INTEGRITY ........................................40
          14.6. MESSAGE-INTEGRITY-SHA256 .................................41
          14.7. FINGERPRINT ..............................................41
          14.8. ERROR-CODE ...............................................42
          14.9. REALM ....................................................44
          14.10. NONCE ...................................................44
          14.11. PASSWORD-ALGORITHMS .....................................44
          14.12. PASSWORD-ALGORITHM ......................................45
          14.13. UNKNOWN-ATTRIBUTES ......................................45
          14.14. SOFTWARE ................................................46
          14.15. ALTERNATE-SERVER ........................................46
          14.16. ALTERNATE-DOMAIN ........................................46
       15. Operational Considerations ....................................47
       16. Security Considerations .......................................47
          16.1. Attacks against the Protocol .............................47
               16.1.1. Outside Attacks ...................................47
               16.1.2. Inside Attacks ....................................48
               16.1.3. Bid-Down Attacks ..................................48
          16.2. Attacks Affecting the Usage ..............................50
               16.2.1. Attack I: Distributed DoS (DDoS) against a
                       Target ............................................51
               16.2.2. Attack II: Silencing a Client .....................51
               16.2.3. Attack III: Assuming the Identity of a Client .....52
               16.2.4. Attack IV: Eavesdropping ..........................52
          16.3. Hash Agility Plan ........................................52
       17. IAB Considerations ............................................53
       18. IANA Considerations ...........................................53
          18.1. STUN Security Features Registry ..........................53
          18.2. STUN Methods Registry ....................................54
          18.3. STUN Attributes Registry .................................54
               18.3.1. Updated Attributes ................................55
               18.3.2. New Attributes ....................................55
          18.4. STUN Error Codes Registry ................................56
          18.5. STUN Password Algorithms Registry ........................56

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               18.5.1. Password Algorithms ...............................57
                      18.5.1.1. MD5 ......................................57
                      18.5.1.2. SHA-256 ..................................57
          18.6. STUN UDP and TCP Port Numbers ............................57
       19. Changes since RFC 5389 ........................................57
       20. References ....................................................58
          20.1. Normative References .....................................58
          20.2. Informative References ...................................61
       Appendix A.  C Snippet to Determine STUN Message Types ............64
       Appendix B.  Test Vectors .........................................64
         B.1.  Sample Request with Long-Term Authentication with
               MESSAGE-INTEGRITY-SHA256 and USERHASH .....................65
       Acknowledgements ..................................................66
       Contributors ......................................................66
       Authors' Addresses ................................................67

    1.  Introduction

       The protocol defined in this specification, Session Traversal
       Utilities for NAT (STUN), provides a tool for dealing with Network
       Address Translators (NATs).  It provides a means for an endpoint to
       determine the IP address and port allocated by a NAT that corresponds
       to its private IP address and port.  It also provides a way for an
       endpoint to keep a NAT binding alive.  With some extensions, the
       protocol can be used to do connectivity checks between two endpoints
       [RFC8445] or to relay packets between two endpoints [RFC5766].

       In keeping with its tool nature, this specification defines an
       extensible packet format, defines operation over several transport
       protocols, and provides for two forms of authentication.

       STUN is intended to be used in the context of one or more NAT
       traversal solutions.  These solutions are known as "STUN Usages".
       Each usage describes how STUN is utilized to achieve the NAT
       traversal solution.  Typically, a usage indicates when STUN messages
       get sent, which optional attributes to include, what server is used,
       and what authentication mechanism is to be used.  Interactive
       Connectivity Establishment (ICE) [RFC8445] is one usage of STUN.  SIP
       Outbound [RFC5626] is another usage of STUN.  In some cases, a usage
       will require extensions to STUN.  A STUN extension can be in the form
       of new methods, attributes, or error response codes.  More
       information on STUN Usages can be found in Section 13.

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    2.  Overview of Operation

       This section is descriptive only.

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

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

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

                               /-----\
                             // STUN  \\
                            |   Client  |
                             \\       //               Private Network 1
                               \-----/

                     Figure 1: One Possible STUN Configuration

       One possible STUN configuration is shown in Figure 1.  In this
       configuration, there are two entities (called STUN agents) that
       implement the STUN protocol.  The lower agent in the figure is the
       client, which 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.  The upper agent in
       the figure is the server, which resides on the public Internet.

       STUN is a client-server protocol.  It supports two types of
       transactions.  One is a request/response transaction in which a
       client sends a request to a server, and the server returns a
       response.  The second is an indication transaction in which either
       agent -- client or server -- sends an indication that generates no
       response.  Both types of transactions include a transaction ID, which

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       is a randomly selected 96-bit number.  For request/response
       transactions, this transaction ID allows the client to associate the
       response with the request that generated it; for indications, the
       transaction ID serves as a debugging aid.

       All STUN messages start with a fixed header that includes a method, a
       class, and the transaction ID.  The method indicates which of the
       various requests or indications this is; this specification defines
       just one method, Binding, but other methods are expected to be
       defined in other documents.  The class indicates whether this is a
       request, a success response, an error response, or an indication.
       Following the fixed header comes zero or more attributes, which are
       Type-Length-Value extensions that convey additional information for
       the specific message.

       This document defines a single method called "Binding".  The Binding
       method can be used either in request/response transactions or in
       indication transactions.  When used in request/response transactions,
       the Binding method can be used to determine the particular binding a
       NAT has allocated to a STUN client.  When used in either request/
       response or in indication transactions, the Binding method can also
       be used to keep these bindings alive.

       In the Binding request/response transaction, a Binding request is
       sent from a STUN client to a STUN server.  When the Binding request
       arrives at the STUN server, it may have passed through one or more
       NATs between the STUN client and the STUN server (in Figure 1, there
       are two such NATs).  As the Binding request message passes through a
       NAT, the NAT will modify the source transport address (that is, the
       source IP address and the source port) of the packet.  As a result,
       the source transport address of the request received by the server
       will be the public IP address and port created by the NAT closest to
       the server.  This is called a "reflexive transport address".  The
       STUN server copies that source transport address into an XOR-MAPPED-
       ADDRESS attribute in the STUN Binding response and sends the Binding
       response back to the STUN client.  As this packet passes back through
       a NAT, the NAT will modify the destination transport address in the
       IP header, but the transport address in the XOR-MAPPED-ADDRESS
       attribute within the body of the STUN response will remain untouched.
       In this way, the client can learn its reflexive transport address
       allocated by the outermost NAT with respect to the STUN server.

       In some usages, STUN must be multiplexed with other protocols (e.g.,
       [RFC8445] and [RFC5626]).  In these usages, there must be a way to
       inspect a packet and determine if it is a STUN packet or not.  STUN
       provides three fields in the STUN header with fixed values that can

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       be used for this purpose.  If this is not sufficient, then STUN
       packets can also contain a FINGERPRINT value, which can further be
       used to distinguish the packets.

       STUN defines a set of optional procedures that a usage can decide to
       use, called "mechanisms".  These mechanisms include DNS discovery, a
       redirection technique to an alternate server, a fingerprint attribute
       for demultiplexing, and two authentication and message-integrity
       exchanges.  The authentication mechanisms revolve around the use of a
       username, password, and message-integrity value.  Two authentication
       mechanisms, the long-term credential mechanism and the short-term
       credential mechanism, are defined in this specification.  Each usage
       specifies the mechanisms allowed with that usage.

       In the long-term credential mechanism, the client and server share a
       pre-provisioned username and password and perform a digest challenge/
       response exchange inspired by the one defined for HTTP [RFC7616] but
       differing in details.  In the short-term credential mechanism, the
       client and the server exchange a username and password through some
       out-of-band method prior to the STUN exchange.  For example, in the
       ICE usage [RFC8445], the two endpoints use out-of-band signaling to
       exchange a username and password.  These are used to integrity
       protect and authenticate the request and response.  There is no
       challenge or nonce used.

    3.  Terminology

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

    4.  Definitions

       STUN Agent:  A STUN agent is an entity that implements the STUN
          protocol.  The entity can be either a STUN client or a STUN
          server.

       STUN Client:  A STUN client is an entity that sends STUN requests and
          receives STUN responses and STUN indications.  A STUN client can
          also send indications.  In this specification, the terms "STUN
          client" and "client" are synonymous.

       STUN Server:  A STUN server is an entity that receives STUN requests
          and STUN indications and that sends STUN responses.  A STUN server
          can also send indications.  In this specification, the terms "STUN
          server" and "server" are synonymous.

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       Transport Address:  The combination of an IP address and port number
          (such as a UDP or TCP port number).

       Reflexive Transport Address:  A transport address learned by a client
          that identifies that client as seen by another host on an IP
          network, typically a STUN server.  When there is an intervening
          NAT between the client and the other host, the reflexive transport
          address represents the mapped address allocated to the client on
          the public side of the NAT.  Reflexive transport addresses are
          learned from the mapped address attribute (MAPPED-ADDRESS or XOR-
          MAPPED-ADDRESS) in STUN responses.

       Mapped Address:  Same meaning as reflexive address.  This term is
          retained only for historic reasons and due to the naming of the
          MAPPED-ADDRESS and XOR-MAPPED-ADDRESS attributes.

       Long-Term Credential:  A username and associated password that
          represent a shared secret between client and server.  Long-term
          credentials are generally granted to the client when a subscriber
          enrolls in a service and persist until the subscriber leaves the
          service or explicitly changes the credential.

       Long-Term Password:  The password from a long-term credential.

       Short-Term Credential:  A temporary username and associated password
          that represent a shared secret between client and server.  Short-
          term credentials are obtained through some kind of protocol
          mechanism between the client and server, preceding the STUN
          exchange.  A short-term credential has an explicit temporal scope,
          which may be based on a specific amount of time (such as 5
          minutes) or on an event (such as termination of a Session
          Initiation Protocol (SIP) [RFC3261] dialog).  The specific scope
          of a short-term credential is defined by the application usage.

       Short-Term Password:  The password component of a short-term
          credential.

       STUN Indication:  A STUN message that does not receive a response.

       Attribute:  The STUN term for a Type-Length-Value (TLV) object that
          can be added to a STUN message.  Attributes are divided into two
          types: comprehension-required and comprehension-optional.  STUN
          agents can safely ignore comprehension-optional attributes they
          don't understand but cannot successfully process a message if it
          contains comprehension-required attributes that are not
          understood.

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       RTO:  Retransmission TimeOut, which defines the initial period of
          time between transmission of a request and the first retransmit of
          that request.

    5.  STUN Message Structure

       STUN messages are encoded in binary using network-oriented format
       (most significant byte or octet first, also commonly known as big-
       endian).  The transmission order is described in detail in Appendix B
       of [RFC0791].  Unless otherwise noted, numeric constants are in
       decimal (base 10).

       All STUN messages comprise a 20-byte header followed by zero or more
       attributes.  The STUN header contains a STUN message type, message
       length, magic cookie, and transaction ID.

          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|     STUN Message Type     |         Message Length        |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                         Magic Cookie                          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                                                               |
         |                     Transaction ID (96 bits)                  |
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2: Format of STUN Message Header

       The most significant 2 bits of every STUN message MUST be zeroes.
       This can be used to differentiate STUN packets from other protocols
       when STUN is multiplexed with other protocols on the same port.

       The message type defines the message class (request, success
       response, error response, or indication) and the message method (the
       primary function) of the STUN message.  Although there are four
       message classes, there are only two types of transactions in STUN:
       request/response transactions (which consist of a request message and
       a response message) and indication transactions (which consist of a
       single indication message).  Response classes are split into error
       and success responses to aid in quickly processing the STUN message.

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       The STUN Message Type field is decomposed further into the following
       structure:

                           0                 1
                           2  3  4 5 6 7 8 9 0 1 2 3 4 5
                          +--+--+-+-+-+-+-+-+-+-+-+-+-+-+
                          |M |M |M|M|M|C|M|M|M|C|M|M|M|M|
                          |11|10|9|8|7|1|6|5|4|0|3|2|1|0|
                          +--+--+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 3: Format of STUN Message Type Field

       Here the bits in the STUN Message Type field are shown as most
       significant (M11) through least significant (M0).  M11 through M0
       represent a 12-bit encoding of the method.  C1 and C0 represent a
       2-bit encoding of the class.  A class of 0b00 is a request, a class
       of 0b01 is an indication, a class of 0b10 is a success response, and
       a class of 0b11 is an error response.  This specification defines a
       single method, Binding.  The method and class are orthogonal, so that
       for each method, a request, success response, error response, and
       indication are possible for that method.  Extensions defining new
       methods MUST indicate which classes are permitted for that method.

       For example, a Binding request has class=0b00 (request) and
       method=0b000000000001 (Binding) and is encoded into the first 16 bits
       as 0x0001.  A Binding response has class=0b10 (success response) and
       method=0b000000000001 and is encoded into the first 16 bits as
       0x0101.

          Note: This unfortunate encoding is due to assignment of values in
          [RFC3489] that did not consider encoding indication messages,
          success responses, and errors responses using bit fields.

       The Magic Cookie field MUST contain the fixed value 0x2112A442 in
       network byte order.  In [RFC3489], the 32 bits comprising the Magic
       Cookie field were part of the transaction ID; placing the magic
       cookie in this location allows a server to detect if the client will
       understand certain attributes that were added to STUN by [RFC5389].
       In addition, it aids in distinguishing STUN packets from packets of
       other protocols when STUN is multiplexed with those other protocols
       on the same port.

       The transaction ID is a 96-bit identifier, used to uniquely identify
       STUN transactions.  For request/response transactions, the
       transaction ID is chosen by the STUN client for the request and
       echoed by the server in the response.  For indications, it is chosen
       by the agent sending the indication.  It primarily serves to
       correlate requests with responses, though it also plays a small role

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       in helping to prevent certain types of attacks.  The server also uses
       the transaction ID as a key to identify each transaction uniquely
       across all clients.  As such, the transaction ID MUST be uniformly
       and randomly chosen from the interval 0 .. 2**96-1 and MUST be
       cryptographically random.  Resends of the same request reuse the same
       transaction ID, but the client MUST choose a new transaction ID for
       new transactions unless the new request is bit-wise identical to the
       previous request and sent from the same transport address to the same
       IP address.  Success and error responses MUST carry the same
       transaction ID as their corresponding request.  When an agent is
       acting as a STUN server and STUN client on the same port, the
       transaction IDs in requests sent by the agent have no relationship to
       the transaction IDs in requests received by the agent.

       The message length MUST contain the size of the message in bytes, not
       including the 20-byte STUN header.  Since all STUN attributes are
       padded to a multiple of 4 bytes, the last 2 bits of this field are
       always zero.  This provides another way to distinguish STUN packets
       from packets of other protocols.

       Following the STUN fixed portion of the header are zero or more
       attributes.  Each attribute is TLV (Type-Length-Value) encoded.
       Details of the encoding and the attributes themselves are given in
       Section 14.

    6.  Base Protocol Procedures

       This section defines the base procedures of the STUN protocol.  It
       describes how messages are formed, how they are sent, and how they
       are processed when they are received.  It also defines the detailed
       processing of the Binding method.  Other sections in this document
       describe optional procedures that a usage may elect to use in certain
       situations.  Other documents may define other extensions to STUN, by
       adding new methods, new attributes, or new error response codes.

    6.1.  Forming a Request or an Indication

       When formulating a request or indication message, the agent MUST
       follow the rules in Section 5 when creating the header.  In addition,
       the message class MUST be either "Request" or "Indication" (as
       appropriate), and the method must be either Binding or some method
       defined in another document.

       The agent then adds any attributes specified by the method or the
       usage.  For example, some usages may specify that the agent use an
       authentication method (Section 9) or the FINGERPRINT attribute
       (Section 7).

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       If the agent is sending a request, it SHOULD add a SOFTWARE attribute
       to the request.  Agents MAY include a SOFTWARE attribute in
       indications, depending on the method.  Extensions to STUN should
       discuss whether SOFTWARE is useful in new indications.  Note that the
       inclusion of a SOFTWARE attribute may have security implications; see
       Section 16.1.2 for details.

       For the Binding method with no authentication, no attributes are
       required unless the usage specifies otherwise.

       All STUN messages sent over UDP or DTLS-over-UDP [RFC6347] SHOULD be
       less than the path MTU, if known.

       If the path MTU is unknown for UDP, messages SHOULD be the smaller of
       576 bytes and the first-hop MTU for IPv4 [RFC1122] and 1280 bytes for
       IPv6 [RFC8200].  This value corresponds to the overall size of the IP
       packet.  Consequently, for IPv4, the actual STUN message would need
       to be less than 548 bytes (576 minus 20-byte IP header, minus 8-byte
       UDP header, assuming no IP options are used).

       If the path MTU is unknown for DTLS-over-UDP, the rules described in
       the previous paragraph need to be adjusted to take into account the
       size of the (13-byte) DTLS Record header, the Message Authentication
       Code (MAC) size, and the padding size.

       STUN provides no ability to handle the case where the request is
       smaller than the MTU but the response is larger than the MTU.  It is
       not envisioned that this limitation will be an issue for STUN.  The
       MTU limitation is a SHOULD, not a MUST, to account for cases where
       STUN itself is being used to probe for MTU characteristics [RFC5780].
       See also [STUN-PMTUD] for a framework that uses STUN to add Path MTU
       Discovery to protocols that lack such a mechanism.  Outside of this
       or similar applications, the MTU constraint MUST be followed.

    6.2.  Sending the Request or Indication

       The agent then sends the request or indication.  This document
       specifies how to send STUN messages over UDP, TCP, TLS-over-TCP, or
       DTLS-over-UDP; other transport protocols may be added in the future.
       The STUN Usage must specify which transport protocol is used and how
       the agent determines the IP address and port of the recipient.
       Section 8 describes a DNS-based method of determining the IP address
       and port of a server that a usage may elect to use.

       At any time, a client MAY have multiple outstanding STUN requests
       with the same STUN server (that is, multiple transactions in
       progress, with different transaction IDs).  Absent other limits to

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       the rate of new transactions (such as those specified by ICE for
       connectivity checks or when STUN is run over TCP), a client SHOULD
       limit itself to ten outstanding transactions to the same server.

    6.2.1.  Sending over UDP or DTLS-over-UDP

       When running STUN over UDP or STUN over DTLS-over-UDP [RFC7350], it
       is possible that the STUN message might be dropped by the network.
       Reliability of STUN request/response transactions is accomplished
       through retransmissions of the request message by the client
       application itself.  STUN indications are not retransmitted; thus,
       indication transactions over UDP or DTLS-over-UDP are not reliable.

       A client SHOULD retransmit a STUN request message starting with an
       interval of RTO ("Retransmission TimeOut"), doubling after each
       retransmission.  The RTO is an estimate of the round-trip time (RTT)
       and is computed as described in [RFC6298], with two exceptions.
       First, the initial value for RTO SHOULD be greater than or equal to
       500 ms.  The exception cases for this "SHOULD" are when other
       mechanisms are used to derive congestion thresholds (such as the ones
       defined in ICE for fixed-rate streams) or when STUN is used in non-
       Internet environments with known network capacities.  In fixed-line
       access links, a value of 500 ms is RECOMMENDED.  Second, the value of
       RTO SHOULD NOT be rounded up to the nearest second.  Rather, a 1 ms
       accuracy SHOULD be maintained.  As with TCP, the usage of Karn's
       algorithm is RECOMMENDED [KARN87].  When applied to STUN, it means
       that RTT estimates SHOULD NOT be computed from STUN transactions that
       result in the retransmission of a request.

       The value for RTO SHOULD be cached by a client after the completion
       of the transaction and used as the starting value for RTO for the
       next transaction to the same server (based on equality of IP
       address).  The value SHOULD be considered stale and discarded if no
       transactions have occurred to the same server in the last 10 minutes.

       Retransmissions continue until a response is received or until a
       total of Rc requests have been sent.  Rc SHOULD be configurable and
       SHOULD have a default of 7.  If, after the last request, a duration
       equal to Rm times the RTO has passed without a response (providing
       ample time to get a response if only this final request actually
       succeeds), the client SHOULD consider the transaction to have failed.
       Rm SHOULD be configurable and SHOULD have a default of 16.  A STUN
       transaction over UDP or DTLS-over-UDP is also considered failed if
       there has been a hard ICMP error [RFC1122].  For example, assuming an
       RTO of 500 ms, requests would be sent at times 0 ms, 500 ms, 1500 ms,
       3500 ms, 7500 ms, 15500 ms, and 31500 ms.  If the client has not
       received a response after 39500 ms, the client will consider the
       transaction to have timed out.

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    6.2.2.  Sending over TCP or TLS-over-TCP

       For TCP and TLS-over-TCP [RFC8446], the client opens a TCP connection
       to the server.

       In some usages of STUN, STUN is the only protocol over the TCP
       connection.  In this case, it can be sent without the aid of any
       additional framing or demultiplexing.  In other usages, or with other
       extensions, it may be multiplexed with other data over a TCP
       connection.  In that case, STUN MUST be run on top of some kind of
       framing protocol, specified by the usage or extension, which allows
       for the agent to extract complete STUN messages and complete
       application-layer messages.  The STUN service running on the well-
       known port or ports discovered through the DNS procedures in
       Section 8 is for STUN alone, and not for STUN multiplexed with other
       data.  Consequently, no framing protocols are used in connections to
       those servers.  When additional framing is utilized, the usage will
       specify how the client knows to apply it and what port to connect to.
       For example, in the case of ICE connectivity checks, this information
       is learned through out-of-band negotiation between client and server.

       Reliability of STUN over TCP and TLS-over-TCP is handled by TCP
       itself, and there are no retransmissions at the STUN protocol level.
       However, for a request/response transaction, if the client has not
       received a response by Ti seconds after it sent the request message,
       it considers the transaction to have timed out.  Ti SHOULD be
       configurable and SHOULD have a default of 39.5 s.  This value has
       been chosen to equalize the TCP and UDP timeouts for the default
       initial RTO.

       In addition, if the client is unable to establish the TCP connection,
       or the TCP connection is reset or fails before a response is
       received, any request/response transaction in progress is considered
       to have failed.

       The client MAY send multiple transactions over a single TCP (or TLS-
       over-TCP) connection, and it MAY send another request before
       receiving a response to the previous request.  The client SHOULD keep
       the connection open until it:

       o  has no further STUN requests or indications to send over that
          connection,

       o  has no plans to use any resources (such as a mapped address
          (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address
          [RFC5766]) that were learned though STUN requests sent over that
          connection,

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       o  if multiplexing other application protocols over that port, has
          finished using those other protocols,

       o  if using that learned port with a remote peer, has established
          communications with that remote peer, as is required by some TCP
          NAT traversal techniques (e.g., [RFC6544]).

       The details of an eventual keep-alive mechanism are left to each STUN
       Usage.  In any case, if a transaction fails because an idle TCP
       connection doesn't work anymore, the client SHOULD send a RST and try
       to open a new TCP connection.

       At the server end, the server SHOULD keep the connection open and let
       the client close it, unless the server has determined that the
       connection has timed out (for example, due to the client
       disconnecting from the network).  Bindings learned by the client will
       remain valid in intervening NATs only while the connection remains
       open.  Only the client knows how long it needs the binding.  The
       server SHOULD NOT close a connection if a request was received over
       that connection for which a response was not sent.  A server MUST NOT
       ever open a connection back towards the client in order to send a
       response.  Servers SHOULD follow best practices regarding connection
       management in cases of overload.

    6.2.3.  Sending over TLS-over-TCP or DTLS-over-UDP

       When STUN is run by itself over TLS-over-TCP or DTLS-over-UDP, the
       TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and
       TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 ciphersuites MUST be
       implemented (for compatibility with older versions of this protocol),
       except if deprecated by rules of a specific STUN usage.  Other
       ciphersuites MAY be implemented.  Note that STUN clients and servers
       that implement TLS version 1.3 [RFC8446] or subsequent versions are
       also required to implement mandatory ciphersuites from those
       specifications and SHOULD disable usage of deprecated ciphersuites
       when they detect support for those specifications.  Perfect Forward
       Secrecy (PFS) ciphersuites MUST be preferred over non-PFS
       ciphersuites.  Ciphersuites with known weaknesses, such as those
       based on (single) DES and RC4, MUST NOT be used.  Implementations
       MUST disable TLS-level compression.

       These recommendations are just a part of the recommendations in
       [BCP195] that implementations and deployments of a STUN Usage using
       TLS or DTLS MUST follow.

       When it receives the TLS Certificate message, the client MUST verify
       the certificate and inspect the site identified by the certificate.
       If the certificate is invalid or revoked, or if it does not identify

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       the appropriate party, the client MUST NOT send the STUN message or
       otherwise proceed with the STUN transaction.  The client MUST verify
       the identity of the server.  To do that, it follows the
       identification procedures defined in [RFC6125], with a certificate
       containing an identifier of type DNS-ID or CN-ID, optionally with a
       wildcard character as the leftmost label, but not of type SRV-ID or
       URI-ID.

       When STUN is run multiplexed with other protocols over a TLS-over-TCP
       connection or a DTLS-over-UDP association, the mandatory ciphersuites
       and TLS handling procedures operate as defined by those protocols.

    6.3.  Receiving a STUN Message

       This section specifies the processing of a STUN message.  The
       processing specified here is for STUN messages as defined in this
       specification; additional rules for backwards compatibility are
       defined in Section 11.  Those additional procedures are optional, and
       usages can elect to utilize them.  First, a set of processing
       operations is applied that is independent of the class.  This is
       followed by class-specific processing, described in the subsections
       that follow.

       When a STUN agent receives a STUN message, it first checks that the
       message obeys the rules of Section 5.  It checks that the first two
       bits are 0, that the Magic Cookie field has the correct value, that
       the message length is sensible, and that the method value is a
       supported method.  It checks that the message class is allowed for
       the particular method.  If the message class is "Success Response" or
       "Error Response", the agent checks that the transaction ID matches a
       transaction that is still in progress.  If the FINGERPRINT extension
       is being used, the agent checks that the FINGERPRINT attribute is
       present and contains the correct value.  If any errors are detected,
       the message is silently discarded.  In the case when STUN is being
       multiplexed with another protocol, an error may indicate that this is
       not really a STUN message; in this case, the agent should try to
       parse the message as a different protocol.

       The STUN agent then does any checks that are required by a
       authentication mechanism that the usage has specified (see
       Section 9).

       Once the authentication checks are done, the STUN agent checks for
       unknown attributes and known-but-unexpected attributes in the
       message.  Unknown comprehension-optional attributes MUST be ignored
       by the agent.  Known-but-unexpected attributes SHOULD be ignored by
       the agent.  Unknown comprehension-required attributes cause
       processing that depends on the message class and is described below.

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       At this point, further processing depends on the message class of the
       request.

    6.3.1.  Processing a Request

       If the request contains one or more unknown comprehension-required
       attributes, the server replies with an error response with an error
       code of 420 (Unknown Attribute) and includes an UNKNOWN-ATTRIBUTES
       attribute in the response that lists the unknown comprehension-
       required attributes.

       Otherwise, the server then does any additional checking that the
       method or the specific usage requires.  If all the checks succeed,
       the server formulates a success response as described below.

       When run over UDP or DTLS-over-UDP, a request received by the server
       could be the first request of a transaction or could be a
       retransmission.  The server MUST respond to retransmissions such that
       the following property is preserved: if the client receives the
       response to the retransmission and not the response that was sent to
       the original request, the overall state on the client and server is
       identical to the case where only the response to the original
       retransmission is received or where both responses are received (in
       which case the client will use the first).  The easiest way to meet
       this requirement is for the server to remember all transaction IDs
       received over UDP or DTLS-over-UDP and their corresponding responses
       in the last 40 seconds.  However, this requires the server to hold
       state and is inappropriate for any requests that are not
       authenticated.  Another way is to reprocess the request and recompute
       the response.  The latter technique MUST only be applied to requests
       that are idempotent (a request is considered idempotent when the same
       request can be safely repeated without impacting the overall state of
       the system) and result in the same success response for the same
       request.  The Binding method is considered to be idempotent.  Note
       that there are certain rare network events that could cause the
       reflexive transport address value to change, resulting in a different
       mapped address in different success responses.  Extensions to STUN
       MUST discuss the implications of request retransmissions on servers
       that do not store transaction state.

    6.3.1.1.  Forming a Success or Error Response

       When forming the response (success or error), the server follows the
       rules of Section 6.  The method of the response is the same as that
       of the request, and the message class is either "Success Response" or
       "Error Response".

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       For an error response, the server MUST add an ERROR-CODE attribute
       containing the error code specified in the processing above.  The
       reason phrase is not fixed but SHOULD be something suitable for the
       error code.  For certain errors, additional attributes are added to
       the message.  These attributes are spelled out in the description
       where the error code is specified.  For example, for an error code of
       420 (Unknown Attribute), the server MUST include an UNKNOWN-
       ATTRIBUTES attribute.  Certain authentication errors also cause
       attributes to be added (see Section 9).  Extensions may define other
       errors and/or additional attributes to add in error cases.

       If the server authenticated the request using an authentication
       mechanism, then the server SHOULD add the appropriate authentication
       attributes to the response (see Section 9).

       The server also adds any attributes required by the specific method
       or usage.  In addition, the server SHOULD add a SOFTWARE attribute to
       the message.

       For the Binding method, no additional checking is required unless the
       usage specifies otherwise.  When forming the success response, the
       server adds an XOR-MAPPED-ADDRESS attribute to the response; this
       attribute contains the source transport address of the request
       message.  For UDP or DTLS-over-UDP, this is the source IP address and
       source UDP port of the request message.  For TCP and TLS-over-TCP,
       this is the source IP address and source TCP port of the TCP
       connection as seen by the server.

    6.3.1.2.  Sending the Success or Error Response

       The response (success or error) is sent over the same transport as
       the request was received on.  If the request was received over UDP or
       DTLS-over-UDP, the destination IP address and port of the response
       are the source IP address and port of the received request message,
       and the source IP address and port of the response are equal to the
       destination IP address and port of the received request message.  If
       the request was received over TCP or TLS-over-TCP, the response is
       sent back on the same TCP connection as the request was received on.

       The server is allowed to send responses in a different order than it
       received the requests.

    6.3.2.  Processing an Indication

       If the indication contains unknown comprehension-required attributes,
       the indication is discarded and processing ceases.

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       Otherwise, the agent then does any additional checking that the
       method or the specific usage requires.  If all the checks succeed,
       the agent then processes the indication.  No response is generated
       for an indication.

       For the Binding method, no additional checking or processing is
       required, unless the usage specifies otherwise.  The mere receipt of
       the message by the agent has refreshed the bindings in the
       intervening NATs.

       Since indications are not re-transmitted over UDP or DTLS-over-UDP
       (unlike requests), there is no need to handle re-transmissions of
       indications at the sending agent.

    6.3.3.  Processing a Success Response

       If the success response contains unknown comprehension-required
       attributes, the response is discarded and the transaction is
       considered to have failed.

       Otherwise, the client then does any additional checking that the
       method or the specific usage requires.  If all the checks succeed,
       the client then processes the success response.

       For the Binding method, the client checks that the XOR-MAPPED-ADDRESS
       attribute is present in the response.  The client checks the address
       family specified.  If it is an unsupported address family, the
       attribute SHOULD be ignored.  If it is an unexpected but supported
       address family (for example, the Binding transaction was sent over
       IPv4, but the address family specified is IPv6), then the client MAY
       accept and use the value.

    6.3.4.  Processing an Error Response

       If the error response contains unknown comprehension-required
       attributes, or if the error response does not contain an ERROR-CODE
       attribute, then the transaction is simply considered to have failed.

       Otherwise, the client then does any processing specified by the
       authentication mechanism (see Section 9).  This may result in a new
       transaction attempt.

       The processing at this point depends on the error code, the method,
       and the usage; the following are the default rules:

       o  If the error code is 300 through 399, the client SHOULD consider
          the transaction as failed unless the ALTERNATE-SERVER extension
          (Section 10) is being used.

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       o  If the error code is 400 through 499, the client declares the
          transaction failed; in the case of 420 (Unknown Attribute), the
          response should contain a UNKNOWN-ATTRIBUTES attribute that gives
          additional information.

       o  If the error code is 500 through 599, the client MAY resend the
          request; clients that do so MUST limit the number of times they do
          this.  Unless a specific error code specifies a different value,
          the number of retransmissions SHOULD be limited to 4.

       Any other error code causes the client to consider the transaction
       failed.

    7.  FINGERPRINT Mechanism

       This section describes an optional mechanism for STUN that aids in
       distinguishing STUN messages from packets of other protocols when the
       two are multiplexed on the same transport address.  This mechanism is
       optional, and a STUN Usage must describe if and when it is used.  The
       FINGERPRINT mechanism is not backwards compatible with RFC 3489 and
       cannot be used in environments where such compatibility is required.

       In some usages, STUN messages are multiplexed on the same transport
       address as other protocols, such as the Real-Time Transport Protocol
       (RTP).  In order to apply the processing described in Section 6, STUN
       messages must first be separated from the application packets.

       Section 5 describes three fixed fields in the STUN header that can be
       used for this purpose.  However, in some cases, these three fixed
       fields may not be sufficient.

       When the FINGERPRINT extension is used, an agent includes the
       FINGERPRINT attribute in messages it sends to another agent.
       Section 14.7 describes the placement and value of this attribute.

       When the agent receives what it believes is a STUN message, then, in
       addition to other basic checks, the agent also checks that the
       message contains a FINGERPRINT attribute and that the attribute
       contains the correct value.  Section 6.3 describes when in the
       overall processing of a STUN message the FINGERPRINT check is
       performed.  This additional check helps the agent detect messages of
       other protocols that might otherwise seem to be STUN messages.

    8.  DNS Discovery of a Server

       This section describes an optional procedure for STUN that allows a
       client to use DNS to determine the IP address and port of a server.
       A STUN Usage must describe if and when this extension is used.  To

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       use this procedure, the client must know a STUN URI [RFC7064]; the
       usage must also describe how the client obtains this URI.  Hard-
       coding a STUN URI into software is NOT RECOMMENDED in case the domain
       name is lost or needs to change for legal or other reasons.

       When a client wishes to locate a STUN server on the public Internet
       that accepts Binding request/response transactions, the STUN URI
       scheme is "stun".  When it wishes to locate a STUN server that
       accepts Binding request/response transactions over a TLS or DTLS
       session, the URI scheme is "stuns".

       The syntax of the "stun" and "stuns" URIs is defined in Section 3.1
       of [RFC7064].  STUN Usages MAY define additional URI schemes.

    8.1.  STUN URI Scheme Semantics

       If the <host> part of a "stun" URI contains an IP address, then this
       IP address is used directly to contact the server.  A "stuns" URI
       containing an IP address MUST be rejected.  A future STUN extension
       or usage may relax this requirement, provided it demonstrates how to
       authenticate the STUN server and prevent man-in-the-middle attacks.

       If the URI does not contain an IP address, the domain name contained
       in the <host> part is resolved to a transport address using the SRV
       procedures specified in [RFC2782].  The DNS SRV service name is the
       content of the <scheme> part.  The protocol in the SRV lookup is the
       transport protocol the client will run STUN over: "udp" for UDP and
       "tcp" for 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
       is sorted and then tried.  However, RFC 2782 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.
       When following these procedures, if the STUN transaction times out
       without receipt of a response, the client SHOULD retry the request to
       the next server in the order defined by RFC 2782.  Such a retry is
       only possible for request/response transmissions, since indication
       transactions generate no response or timeout.

       In addition, instead of querying either the A or the AAAA resource
       records for a domain name, a dual-stack IPv4/IPv6 client MUST query
       both and try the requests with all the IP addresses received, as
       specified in [RFC8305].

       The default port for STUN requests is 3478, for both TCP and UDP.
       The default port for STUN over TLS and STUN over DTLS requests is
       5349.  Servers can run STUN over DTLS on the same port as STUN over

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       UDP if the server software supports determining whether the initial
       message is a DTLS or STUN message.  Servers can run STUN over TLS on
       the same port as STUN over TCP if the server software supports
       determining whether the initial message is a TLS or STUN message.

       Administrators of STUN servers SHOULD use these ports in their SRV
       records for UDP and TCP.  In all cases, the port in DNS MUST reflect
       the one on which the server is listening.

       If no SRV records are found, the client performs both an A and AAAA
       record lookup of the domain name, as described in [RFC8305].  The
       result will be a list of IP addresses, each of which can be
       simultaneously contacted at the default port using UDP or TCP,
       independent of the STUN Usage.  For usages that require TLS, the
       client connects to the IP addresses using the default STUN over TLS
       port.  For usages that require DTLS, the client connects to the IP
       addresses using the default STUN over DTLS port.

    9.  Authentication and Message-Integrity Mechanisms

       This section defines two mechanisms for STUN that a client and server
       can use to provide authentication and message integrity; these two
       mechanisms are known as the short-term credential mechanism and the
       long-term credential mechanism.  These two mechanisms are optional,
       and each usage must specify if and when these mechanisms are used.
       Consequently, both clients and servers will know which mechanism (if
       any) to follow based on knowledge of which usage applies.  For
       example, a STUN server on the public Internet supporting ICE would
       have no authentication, whereas the STUN server functionality in an
       agent supporting connectivity checks would utilize short-term
       credentials.  An overview of these two mechanisms is given in
       Section 2.

       Each mechanism specifies the additional processing required to use
       that mechanism, extending the processing specified in Section 6.  The
       additional processing occurs in three different places: when forming
       a message, when receiving a message immediately after the basic
       checks have been performed, and when doing the detailed processing of
       error responses.

       Note that agents MUST ignore all attributes that follow MESSAGE-
       INTEGRITY, with the exception of the MESSAGE-INTEGRITY-SHA256 and
       FINGERPRINT attributes.  Similarly, agents MUST ignore all attributes
       that follow the MESSAGE-INTEGRITY-SHA256 attribute if the MESSAGE-
       INTEGRITY attribute is not present, with the exception of the
       FINGERPRINT attribute.

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    9.1.  Short-Term Credential Mechanism

       The short-term credential mechanism assumes that, prior to the STUN
       transaction, the client and server have used some other protocol to
       exchange a credential in the form of a username and password.  This
       credential is time-limited.  The time limit is defined by the usage.
       As an example, in the ICE usage [RFC8445], the two endpoints use out-
       of-band signaling to agree on a username and password, and this
       username and password are applicable for the duration of the media
       session.

       This credential is used to form a message-integrity check in each
       request and in many responses.  There is no challenge and response as
       in the long-term mechanism; consequently, replay is limited by virtue
       of the time-limited nature of the credential.

    9.1.1.  HMAC Key

       For short-term credentials, the Hash-Based Message Authentication
       Code (HMAC) key is defined as follow:

                           key = OpaqueString(password)

       where the OpaqueString profile is defined in [RFC8265].  The encoding
       used is UTF-8 [RFC3629].

    9.1.2.  Forming a Request or Indication

       For a request or indication message, the agent MUST include the
       USERNAME, MESSAGE-INTEGRITY-SHA256, and MESSAGE-INTEGRITY attributes
       in the message unless the agent knows from an external mechanism
       which message integrity algorithm is supported by both agents.  In
       this case, either MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 MUST
       be included in addition to USERNAME.  The HMAC for the MESSAGE-
       INTEGRITY attribute is computed as described in Section 14.5, and the
       HMAC for the MESSAGE-INTEGRITY-SHA256 attributes is computed as
       described in Section 14.6.  Note that the password is never included
       in the request or indication.

    9.1.3.  Receiving a Request or Indication

       After the agent has done the basic processing of a message, the agent
       performs the checks listed below in the order specified:

       o  If the message does not contain 1) a MESSAGE-INTEGRITY or a
          MESSAGE-INTEGRITY-SHA256 attribute and 2) a USERNAME attribute:

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          *  If the message is a request, the server MUST reject the request
             with an error response.  This response MUST use an error code
             of 400 (Bad Request).

          *  If the message is an indication, the agent MUST silently
             discard the indication.

       o  If the USERNAME does not contain a username value currently valid
          within the server:

          *  If the message is a request, the server MUST reject the request
             with an error response.  This response MUST use an error code
             of 401 (Unauthenticated).

          *  If the message is an indication, the agent MUST silently
             discard the indication.

       o  If the MESSAGE-INTEGRITY-SHA256 attribute is present, compute the
          value for the message integrity as described in Section 14.6,
          using the password associated with the username.  If the MESSAGE-
          INTEGRITY-SHA256 attribute is not present, then use the same
          password to compute the value for the message integrity as
          described in Section 14.5.  If the resulting value does not match
          the contents of the corresponding attribute (MESSAGE-INTEGRITY-
          SHA256 or MESSAGE-INTEGRITY):

          *  If the message is a request, the server MUST reject the request
             with an error response.  This response MUST use an error code
             of 401 (Unauthenticated).

          *  If the message is an indication, the agent MUST silently
             discard the indication.

       If these checks pass, the agent continues to process the request or
       indication.  Any response generated by a server to a request that
       contains a MESSAGE-INTEGRITY-SHA256 attribute MUST include the
       MESSAGE-INTEGRITY-SHA256 attribute, computed using the password
       utilized to authenticate the request.  Any response generated by a
       server to a request that contains only a MESSAGE-INTEGRITY attribute
       MUST include the MESSAGE-INTEGRITY attribute, computed using the
       password utilized to authenticate the request.  This means that only
       one of these attributes can appear in a response.  The response MUST
       NOT contain the USERNAME attribute.

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       If any of the checks fail, a server MUST NOT include a MESSAGE-
       INTEGRITY-SHA256, MESSAGE-INTEGRITY, or USERNAME attribute in the
       error response.  This is because, in these failure cases, the server
       cannot determine the shared secret necessary to compute the MESSAGE-
       INTEGRITY-SHA256 or MESSAGE-INTEGRITY attributes.

    9.1.4.  Receiving a Response

       The client looks for the MESSAGE-INTEGRITY or the MESSAGE-INTEGRITY-
       SHA256 attribute in the response.  If present and if the client only
       sent one of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
       attributes in the request (because of the external indication in
       Section 9.1.2 or because this is a subsequent request as defined in
       Section 9.1.5), the algorithm in the response has to match;
       otherwise, the response MUST be discarded.

       The client then computes the message integrity over the response as
       defined in Section 14.5 for the MESSAGE-INTEGRITY attribute or
       Section 14.6 for the MESSAGE-INTEGRITY-SHA256 attribute, using the
       same password it utilized for the request.  If the resulting value
       matches the contents of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
       SHA256 attribute, respectively, the response is considered
       authenticated.  If the value does not match, or if both MESSAGE-
       INTEGRITY and MESSAGE-INTEGRITY-SHA256 are absent, the processing
       depends on whether the request was sent over a reliable or an
       unreliable transport.

       If the request was sent over an unreliable transport, the response
       MUST be discarded, as if it had never been received.  This means that
       retransmits, if applicable, will continue.  If all the responses
       received are discarded, then instead of signaling a timeout after
       ending the transaction, the layer MUST signal that the integrity
       protection was violated.

       If the request was sent over a reliable transport, the response MUST
       be discarded, and the layer MUST immediately end the transaction and
       signal that the integrity protection was violated.

    9.1.5.  Sending Subsequent Requests

       A client sending subsequent requests to the same server MUST send
       only the MESSAGE-INTEGRITY-SHA256 or the MESSAGE-INTEGRITY attribute
       that matches the attribute that was received in the response to the
       initial request.  Here, "same server" means same IP address and port
       number, not just the same URI or SRV lookup result.

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    9.2.  Long-Term Credential Mechanism

       The long-term credential mechanism relies on a long-term credential,
       in the form of a username and password that are shared between client
       and server.  The credential is considered long-term since it is
       assumed that it is provisioned for a user and remains in effect until
       the user is no longer a subscriber of the system or until it is
       changed.  This is basically a traditional "log-in" username and
       password given to users.

       Because these usernames and passwords are expected to be valid for
       extended periods of time, replay prevention is provided in the form
       of a digest challenge.  In this mechanism, the client initially sends
       a request, without offering any credentials or any integrity checks.
       The server rejects this request, providing the user a realm (used to
       guide the user or agent in selection of a username and password) and
       a nonce.  The nonce provides a limited replay protection.  It is a
       cookie, selected by the server and encoded in such a way as to
       indicate a duration of validity or client identity from which it is
       valid.  Only the server needs to know about the internal structure of
       the cookie.  The client retries the request, this time including its
       username and the realm and echoing the nonce provided by the server.
       The client also includes one of the message-integrity attributes
       defined in this document, which provides an HMAC over the entire
       request, including the nonce.  The server validates the nonce and
       checks the message integrity.  If they match, the request is
       authenticated.  If the nonce is no longer valid, it is considered
       "stale", and the server rejects the request, providing a new nonce.

       In subsequent requests to the same server, the client reuses the
       nonce, username, realm, and password it used previously.  In this
       way, subsequent requests are not rejected until the nonce becomes
       invalid by the server, in which case the rejection provides a new
       nonce to the client.

       Note that the long-term credential mechanism cannot be used to
       protect indications, since indications cannot be challenged.  Usages
       utilizing indications must either use a short-term credential or omit
       authentication and message integrity for them.

       To indicate that it supports this specification, a server MUST
       prepend the NONCE attribute value with the character string composed
       of "obMatJos2" concatenated with the (4-character) base64 [RFC4648]
       encoding of the 24-bit STUN Security Features as defined in
       Section 18.1.  The 24-bit Security Feature set is encoded as 3 bytes,
       with bit 0 as the most significant bit of the first byte and bit 23
       as the least significant bit of the third byte.  If no security
       features are used, then a byte array with all 24 bits set to zero

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       MUST be encoded instead.  For the remainder of this document, the
       term "nonce cookie" will refer to the complete 13-character string
       prepended to the NONCE attribute value.

       Since the long-term credential mechanism is susceptible to offline
       dictionary attacks, deployments SHOULD utilize passwords that are
       difficult to guess.  In cases where the credentials are not entered
       by the user, but are rather placed on a client device during device
       provisioning, the password SHOULD have at least 128 bits of
       randomness.  In cases where the credentials are entered by the user,
       they should follow best current practices around password structure.

    9.2.1.  Bid-Down Attack Prevention

       This document introduces two new security features that provide the
       ability to choose the algorithm used for password protection as well
       as the ability to use an anonymous username.  Both of these
       capabilities are optional in order to remain backwards compatible
       with previous versions of the STUN protocol.

       These new capabilities are subject to bid-down attacks whereby an
       attacker in the message path can remove these capabilities and force
       weaker security properties.  To prevent these kinds of attacks from
       going undetected, the nonce is enhanced with additional information.

       The value of the "nonce cookie" will vary based on the specific STUN
       Security Feature bits selected.  When this document makes reference
       to the "nonce cookie" in a section discussing a specific STUN
       Security Feature it is understood that the corresponding STUN
       Security Feature bit in the "nonce cookie" is set to 1.

       For example, when the PASSWORD-ALGORITHMS security feature (defined
       in Section 9.2.4) is used, the corresponding "Password algorithms"
       bit (defined in Section 18.1) is set to 1 in the "nonce cookie".

    9.2.2.  HMAC Key

       For long-term credentials that do not use a different algorithm, as
       specified by the PASSWORD-ALGORITHM attribute, the key is 16 bytes:

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

       Where MD5 is defined in [RFC1321] and [RFC6151], and the OpaqueString
       profile is defined in [RFC8265].  The encoding used is UTF-8
       [RFC3629].

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       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 OpaqueString has already been applied); (2)
       a single colon; (3) the realm, with any quotes and trailing nulls
       removed and after processing using OpaqueString; (4) a single colon;
       and (5) the password, with any trailing nulls removed and after
       processing using OpaqueString.  For example, if the username is
       'user', the realm is 'realm', and the password is '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
       0x8493fbc53ba582fb4c044c456bdc40eb.

       The structure of the key when used with long-term credentials
       facilitates deployment in systems that also utilize SIP [RFC3261].
       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.  For example, this mechanism can be used with the
       authentication extensions defined in [RFC5090].

       When a PASSWORD-ALGORITHM is used, the key length and algorithm to
       use are described in Section 18.5.1.

    9.2.3.  Forming a Request

       The first request from the client to the server (as identified by
       hostname if the DNS procedures of Section 8 are used and by IP
       address if not) is handled according to the rules in Section 9.2.3.1.
       When the client initiates a subsequent request once a previous
       request/response transaction has completed successfully, it follows
       the rules in Section 9.2.3.2.  Forming a request as a consequence of
       a 401 (Unauthenticated) or 438 (Stale Nonce) error response is
       covered in Section 9.2.5 and is not considered a "subsequent request"
       and thus does not utilize the rules described in Section 9.2.3.2.
       Each of these types of requests have a different mandatory
       attributes.

    9.2.3.1.  First Request

       If the client has not completed a successful request/response
       transaction with the server, it MUST omit the USERNAME, USERHASH,
       MESSAGE-INTEGRITY, MESSAGE-INTEGRITY-SHA256, REALM, NONCE, PASSWORD-
       ALGORITHMS, and PASSWORD-ALGORITHM attributes.  In other words, the
       first request is sent as if there were no authentication or message
       integrity applied.

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    9.2.3.2.  Subsequent Requests

       Once a request/response transaction has completed, the client will
       have been presented a realm and nonce by the server and selected a
       username and password with which it authenticated.  The client SHOULD
       cache the username, password, realm, and nonce for subsequent
       communications with the server.  When the client sends a subsequent
       request, it MUST include either the USERNAME or USERHASH, REALM,
       NONCE, and PASSWORD-ALGORITHM attributes with these cached values.
       It MUST include a MESSAGE-INTEGRITY attribute or a MESSAGE-INTEGRITY-
       SHA256 attribute, computed as described in Sections 14.5 and 14.6
       using the cached password.  The choice between the two attributes
       depends on the attribute received in the response to the first
       request.

    9.2.4.  Receiving a Request

       After the server has done the basic processing of a request, it
       performs the checks listed below in the order specified.  Note that
       it is RECOMMENDED that the REALM value be the domain name of the
       provider of the STUN server:

       o  If the message does not contain a MESSAGE-INTEGRITY or MESSAGE-
          INTEGRITY-SHA256 attribute, the server MUST generate an error
          response with an error code of 401 (Unauthenticated).  This
          response MUST include a REALM value.  The response MUST include a
          NONCE, selected by the server.  The server MUST NOT choose the
          same NONCE for two requests unless they have the same source IP
          address and port.  The server MAY support alternate password
          algorithms, in which case it can list them in preferential order
          in a PASSWORD-ALGORITHMS attribute.  If the server adds a
          PASSWORD-ALGORITHMS attribute, it MUST set the STUN Security
          Feature "Password algorithms" bit to 1.  The server MAY support
          anonymous username, in which case it MUST set the STUN Security
          Feature "Username anonymity" bit set to 1.  The response SHOULD
          NOT contain a USERNAME, USERHASH, MESSAGE-INTEGRITY, or MESSAGE-
          INTEGRITY-SHA256 attribute.

          Note: Reusing a NONCE for different source IP addresses or ports
          was not explicitly forbidden in [RFC5389].

       o  If the message contains a MESSAGE-INTEGRITY or a MESSAGE-
          INTEGRITY-SHA256 attribute, but is missing either the USERNAME or
          USERHASH, REALM, or NONCE attribute, the server MUST generate an
          error response with an error code of 400 (Bad Request).  This
          response SHOULD NOT include a USERNAME, USERHASH, NONCE, or REALM

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          attribute.  The response cannot contain a MESSAGE-INTEGRITY or
          MESSAGE-INTEGRITY-SHA256 attribute, as the attributes required to
          generate them are missing.

       o  If the NONCE attribute starts with the "nonce cookie" with the
          STUN Security Feature "Password algorithms" bit set to 1, the
          server performs these checks in the order specified:

          *  If the request contains neither the PASSWORD-ALGORITHMS nor the
             PASSWORD-ALGORITHM algorithm, then the request is processed as
             though PASSWORD-ALGORITHM were MD5.

          *  Otherwise, unless (1) PASSWORD-ALGORITHM and PASSWORD-
             ALGORITHMS are both present, (2) PASSWORD-ALGORITHMS matches
             the value sent in the response that sent this NONCE, and (3)
             PASSWORD-ALGORITHM matches one of the entries in PASSWORD-
             ALGORITHMS, the server MUST generate an error response with an
             error code of 400 (Bad Request).

       o  If the value of the USERNAME or USERHASH attribute is not valid,
          the server MUST generate an error response with an error code of
          401 (Unauthenticated).  This response MUST include a REALM value.
          The response MUST include a NONCE, selected by the server.  The
          response MUST include a PASSWORD-ALGORITHMS attribute.  The
          response SHOULD NOT contain a USERNAME or USERHASH attribute.  The
          response MAY include a MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
          SHA256 attribute, using the previous key to calculate it.

       o  If the MESSAGE-INTEGRITY-SHA256 attribute is present, compute the
          value for the message integrity as described in Section 14.6,
          using the password associated with the username.  Otherwise, using
          the same password, compute the value for the MESSAGE-INTEGRITY
          attribute as described in Section 14.5.  If the resulting value
          does not match the contents of the MESSAGE-INTEGRITY attribute or
          the MESSAGE-INTEGRITY-SHA256 attribute, the server MUST reject the
          request with an error response.  This response MUST use an error
          code of 401 (Unauthenticated).  It MUST include the REALM and
          NONCE attributes and SHOULD NOT include the USERNAME, USERHASH,
          MESSAGE-INTEGRITY, or MESSAGE-INTEGRITY-SHA256 attribute.

       o  If the NONCE is no longer valid, the server MUST generate an error
          response with an error code of 438 (Stale Nonce).  This response
          MUST include NONCE, REALM, and PASSWORD-ALGORITHMS attributes and
          SHOULD NOT include the USERNAME and USERHASH attributes.  The
          NONCE attribute value MUST be valid.  The response MAY include a
          MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, using the

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          previous NONCE to calculate it.  Servers can revoke nonces in
          order to provide additional security.  See Section 5.4 of
          [RFC7616] for guidelines.

       If these checks pass, the server continues to process the request.
       Any response generated by the server MUST include the MESSAGE-
       INTEGRITY-SHA256 attribute, computed using the username and password
       utilized to authenticate the request, unless the request was
       processed as though PASSWORD-ALGORITHM was MD5 (because the request
       contained neither PASSWORD-ALGORITHMS nor PASSWORD-ALGORITHM).  In
       that case, the MESSAGE-INTEGRITY attribute MUST be used instead of
       the MESSAGE-INTEGRITY-SHA256 attribute, and the REALM, NONCE,
       USERNAME, and USERHASH attributes SHOULD NOT be included.

    9.2.5.  Receiving a Response

       If the response is an error response with an error code of 401
       (Unauthenticated) or 438 (Stale Nonce), the client MUST test if the
       NONCE attribute value starts with the "nonce cookie".  If so and the
       "nonce cookie" has the STUN Security Feature "Password algorithms"
       bit set to 1 but no PASSWORD-ALGORITHMS attribute is present, then
       the client MUST NOT retry the request with a new transaction.

       If the response is an error response with an error code of 401
       (Unauthenticated), the client SHOULD retry the request with a new
       transaction.  This request MUST contain a USERNAME or a USERHASH,
       determined by the client as the appropriate username for the REALM
       from the error response.  If the "nonce cookie" is present and has
       the STUN Security Feature "Username anonymity" bit set to 1, then the
       USERHASH attribute MUST be used; else, the USERNAME attribute MUST be
       used.  The request MUST contain the REALM, copied from the error
       response.  The request MUST contain the NONCE, copied from the error
       response.  If the response contains a PASSWORD-ALGORITHMS attribute,
       the request MUST contain the PASSWORD-ALGORITHMS attribute with the
       same content.  If the response contains a PASSWORD-ALGORITHMS
       attribute, and this attribute contains at least one algorithm that is
       supported by the client, then the request MUST contain a PASSWORD-
       ALGORITHM attribute with the first algorithm supported on the list.
       If the response contains a PASSWORD-ALGORITHMS attribute, and this
       attribute does not contain any algorithm that is supported by the
       client, then the client MUST NOT retry the request with a new
       transaction.  The client MUST NOT perform this retry if it is not
       changing the USERNAME, USERHASH, REALM, or its associated password
       from the previous attempt.

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       If the response is an error response with an error code of 438 (Stale
       Nonce), the client MUST retry the request, using the new NONCE
       attribute supplied in the 438 (Stale Nonce) response.  This retry
       MUST also include either the USERNAME or USERHASH, the REALM, and
       either the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute.

       For all other responses, if the NONCE attribute starts with the
       "nonce cookie" with the STUN Security Feature "Password algorithms"
       bit set to 1 but PASSWORD-ALGORITHMS is not present, the response
       MUST be ignored.

       If the response is an error response with an error code of 400 (Bad
       Request) and does not contain either the MESSAGE-INTEGRITY or
       MESSAGE-INTEGRITY-SHA256 attribute, then the response MUST be
       discarded, as if it were never received.  This means that
       retransmits, if applicable, will continue.

          Note: In this case, the 400 response will never reach the
          application, resulting in a timeout.

       The client looks for the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
       SHA256 attribute in the response (either success or failure).  If
       present, the client computes the message integrity over the response
       as defined in Sections 14.5 or 14.6, using the same password it
       utilized for the request.  If the resulting value matches the
       contents of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
       attribute, the response is considered authenticated.  If the value
       does not match, or if both MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-
       SHA256 are absent, the processing depends on the request being sent
       over a reliable or an unreliable transport.

       If the request was sent over an unreliable transport, the response
       MUST be discarded, as if it had never been received.  This means that
       retransmits, if applicable, will continue.  If all the responses
       received are discarded, then instead of signaling a timeout after
       ending the transaction, the layer MUST signal that the integrity
       protection was violated.

       If the request was sent over a reliable transport, the response MUST
       be discarded, and the layer MUST immediately end the transaction and
       signal that the integrity protection was violated.

       If the response contains a PASSWORD-ALGORITHMS attribute, all the
       subsequent requests MUST be authenticated using MESSAGE-INTEGRITY-
       SHA256 only.

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    10.  ALTERNATE-SERVER Mechanism

       This section describes a mechanism in STUN that allows a server to
       redirect a client to another server.  This extension is optional, and
       a usage must define if and when this extension is used.  The
       ALTERNATE-SERVER attribute carries an IP address.

       A server using this extension redirects a client to another server by
       replying to a request message with an error response message with an
       error code of 300 (Try Alternate).  The server MUST include at least
       one ALTERNATE-SERVER attribute in the error response, which MUST
       contain an IP address of the same address family as the source IP
       address of the request message.  The server SHOULD include an
       additional ALTERNATE-SERVER attribute, after the mandatory one, that
       contains an IP address of the address family other than the source IP
       address of the request message.  The error response message MAY be
       authenticated; however, there are use cases for ALTERNATE-SERVER
       where authentication of the response is not possible or practical.
       If the transaction uses TLS or DTLS, if the transaction is
       authenticated by a MESSAGE-INTEGRITY-SHA256 attribute, and if the
       server wants to redirect to a server that uses a different
       certificate, then it MUST include an ALTERNATE-DOMAIN attribute
       containing the name inside the subjectAltName of that certificate.
       This series of conditions on the MESSAGE-INTEGRITY-SHA256 attribute
       indicates that the transaction is authenticated and that the client
       implements this specification and therefore can process the
       ALTERNATE-DOMAIN attribute.

       A client using this extension handles a 300 (Try Alternate) error
       code as follows.  The client looks for an ALTERNATE-SERVER attribute
       in the error response.  If one is found, then the client considers
       the current transaction as failed and reattempts the request with the
       server specified in the attribute, using the same transport protocol
       used for the previous request.  That request, if authenticated, MUST
       utilize the same credentials that the client would have used in the
       request to the server that performed the redirection.  If the
       transport protocol uses TLS or DTLS, then the client looks for an
       ALTERNATE-DOMAIN attribute.  If the attribute is found, the domain
       MUST be used to validate the certificate using the recommendations in
       [RFC6125].  The certificate MUST contain an identifier of type DNS-ID
       or CN-ID (eventually with wildcards) but not of type SRV-ID or URI-
       ID.  If the attribute is not found, the same domain that was used for
       the original request MUST be used to validate the certificate.  If
       the client has been redirected to a server to which it has already
       sent this request within the last five minutes, it MUST ignore the
       redirection and consider the transaction to have failed.  This
       prevents infinite ping-ponging between servers in case of redirection
       loops.

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    11.  Backwards Compatibility with RFC 3489

       In addition to the backward compatibility already described in
       Section 12 of [RFC5389], DTLS MUST NOT be used with [RFC3489]
       (referred to as "classic STUN").  Any STUN request or indication
       without the magic cookie (see Section 6 of [RFC5389]) over DTLS MUST
       be considered invalid: all requests MUST generate a 500 (Server
       Error) error response, and indications MUST be ignored.

    12.  Basic Server Behavior

       This section defines the behavior of a basic, stand-alone STUN
       server.

       Historically, "classic STUN" [RFC3489] only defined the behavior of a
       server that was providing clients with server reflexive transport
       addresses by receiving and replying to STUN Binding requests.
       [RFC5389] redefined the protocol as an extensible framework, and the
       server functionality became the sole STUN Usage defined in that
       document.  This STUN Usage is also known as "Basic STUN Server".

       The STUN server MUST support the Binding method.  It SHOULD NOT
       utilize the short-term or long-term credential mechanism.  This is
       because the work involved in authenticating the request is more than
       the work in simply processing it.  It SHOULD NOT utilize the
       ALTERNATE-SERVER mechanism for the same reason.  It MUST support UDP
       and TCP.  It MAY support STUN over TCP/TLS or STUN over UDP/DTLS;
       however, DTLS and TLS provide minimal security benefits in this basic
       mode of operation.  It does not require a keep-alive mechanism
       because a TCP or TLS-over-TCP connection is closed after the end of
       the Binding transaction.  It MAY utilize the FINGERPRINT mechanism
       but MUST NOT require it.  Since the stand-alone server only runs
       STUN, FINGERPRINT provides no benefit.  Requiring it would break
       compatibility with RFC 3489, and such compatibility is desirable in a
       stand-alone server.  Stand-alone STUN servers SHOULD support
       backwards compatibility with clients using [RFC3489], as described in
       Section 11.

       It is RECOMMENDED that administrators of STUN servers provide DNS
       entries for those servers as described in Section 8.  If both A and
       AAAA resource records are returned, then the client can
       simultaneously send STUN Binding requests to the IPv4 and IPv6
       addresses (as specified in [RFC8305]), as the Binding request is
       idempotent.  Note that the MAPPED-ADDRESS or XOR-MAPPED-ADDRESS
       attributes that are returned will not necessarily match the address
       family of the server address used.

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       A basic STUN server is not a solution for NAT traversal by itself.
       However, it can be utilized as part of a solution through STUN
       Usages.  This is discussed further in Section 13.

    13.  STUN Usages

       STUN by itself is not a solution to the NAT traversal problem.
       Rather, STUN defines a tool that can be used inside a larger
       solution.  The term "STUN Usage" is used for any solution that uses
       STUN as a component.

       A STUN Usage defines how STUN is actually utilized -- when to send
       requests, what to do with the responses, and which optional
       procedures defined here (or in an extension to STUN) are to be used.
       A usage also defines:

       o  Which STUN methods are used.

       o  What transports are used.  If DTLS-over-UDP is used, then
          implementing the denial-of-service countermeasure described in
          Section 4.2.1 of [RFC6347] is mandatory.

       o  What authentication and message-integrity mechanisms are used.

       o  The considerations around manual vs. automatic key derivation for
          the integrity mechanism, as discussed in [RFC4107].

       o  What mechanisms are used to distinguish STUN messages from other
          messages.  When STUN is run over TCP or TLS-over-TCP, a framing
          mechanism may be required.

       o  How a STUN client determines the IP address and port of the STUN
          server.

       o  How simultaneous use of IPv4 and IPv6 addresses (Happy Eyeballs
          [RFC8305]) works with non-idempotent transactions when both
          address families are found for the STUN server.

       o  Whether backwards compatibility to RFC 3489 is required.

       o  What optional attributes defined here (such as FINGERPRINT and
          ALTERNATE-SERVER) or in other extensions are required.

       o  If MESSAGE-INTEGRITY-SHA256 truncation is permitted, and the
          limits permitted for truncation.

       o  The keep-alive mechanism if STUN is run over TCP or TLS-over-TCP.

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       o  If anycast addresses can be used for the server in case 1) TCP or
          TLS-over-TCP or 2) authentication is used.

       In addition, any STUN Usage must consider the security implications
       of using STUN in that usage.  A number of attacks against STUN are
       known (see the Security Considerations section in this document), and
       any usage must consider how these attacks can be thwarted or
       mitigated.

       Finally, a usage must consider whether its usage of STUN is an
       example of the Unilateral Self-Address Fixing approach to NAT
       traversal and, if so, address the questions raised in RFC 3424
       [RFC3424].

    14.  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

       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 MUST be set to zero on sending and MUST be ignored by
       the receiver.

       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

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       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 18.3.

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

    14.1.  MAPPED-ADDRESS

       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.

       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:

       0x01:IPv4
       0x02:IPv6

       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 [RFC3489] clients.

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    14.2.  XOR-MAPPED-ADDRESS

       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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |0 0 0 0 0 0 0 0|    Family     |         X-Port                |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                X-Address (Variable)
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 6: Format of XOR-MAPPED-ADDRESS Attribute

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

       X-Port is computed by XOR'ing the mapped port with the most
       significant 16 bits of the magic cookie.  If the IP address family is
       IPv4, X-Address is computed by XOR'ing the mapped IP address with the
       magic cookie.  If the IP address family is IPv6, X-Address is
       computed by XOR'ing the mapped IP address with the concatenation of
       the magic cookie and the 96-bit transaction ID.  In all cases, the
       XOR operation works on its inputs in network byte order (that is, the
       order they will be encoded in the message).

       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
       as for MAPPED-ADDRESS.

       Note: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their
       encoding of the transport address.  The former encodes the transport
       address by XOR'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 to provide a generic Application Layer Gateway
       (ALG) function.  Such behavior interferes with the operation of STUN
       and also causes failure of STUN's message-integrity checking.

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    14.3.  USERNAME

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

       The value of USERNAME is a variable-length value containing the
       authentication username.  It MUST contain a UTF-8-encoded [RFC3629]
       sequence of fewer than 509 bytes and MUST have been processed using
       the OpaqueString profile [RFC8265].  A compliant implementation MUST
       be able to parse a UTF-8-encoded sequence of 763 or fewer octets to
       be compatible with [RFC5389].

          Note: [RFC5389] mistakenly referenced the definition of UTF-8 in
          [RFC2279].  [RFC2279] assumed up to 6 octets per characters
          encoded.  [RFC2279] was replaced by [RFC3629], which allows only 4
          octets per character encoded, consistent with changes made in
          Unicode 2.0 and ISO/IEC 10646.

          Note: This specification uses the OpaqueString profile instead of
          the UsernameCasePreserved profile for username string processing
          in order to improve compatibility with deployed password stores.
          Many password databases used for HTTP and SIP Digest
          authentication store the MD5 hash of username:realm:password
          instead of storing a plain text password.  In [RFC3489], STUN
          authentication was designed to be compatible with these existing
          databases to the extent possible, which like SIP and HTTP
          performed no pre-processing of usernames and passwords other than
          prohibiting non-space ASCII control characters.  The next revision
          of the STUN specification, [RFC5389], used the SASLprep [RFC4013]
          stringprep [RFC3454] profile to pre-process usernames and
          passwords.  SASLprep uses Unicode Normalization Form KC
          (Compatibility Decomposition, followed by Canonical Composition)
          [UAX15] and prohibits various control, space, and non-text,
          deprecated, or inappropriate codepoints.  The PRECIS framework
          [RFC8264] obsoletes stringprep.  PRECIS handling of usernames and
          passwords [RFC8265] uses Unicode Normalization Form C (Canonical
          Decomposition, followed by Canonical Composition).  While there
          are specific cases where different username strings under HTTP
          Digest could be mapped to a single STUN username processed with
          OpaqueString, these cases are extremely unlikely and easy to
          detect and correct.  With a UsernameCasePreserved profile, it
          would be more likely that valid usernames under HTTP Digest would
          not match their processed forms (specifically usernames containing
          bidirectional text and compatibility forms).  Operators are free
          to further restrict the allowed codepoints in usernames to avoid
          problematic characters.

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    14.4.  USERHASH

       The USERHASH attribute is used as a replacement for the USERNAME
       attribute when username anonymity is supported.

       The value of USERHASH has a fixed length of 32 bytes.  The username
       MUST have been processed using the OpaqueString profile [RFC8265],
       and the realm MUST have been processed using the OpaqueString profile
       [RFC8265] before hashing.

       The following is the operation that the client will perform to hash
       the username:

       userhash = SHA-256(OpaqueString(username) ":" OpaqueString(realm))

    14.5.  MESSAGE-INTEGRITY

       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 SHA-1 hash, the HMAC will
       be 20 bytes.

       The key for the HMAC depends on which credential mechanism is in use.
       Section 9.1.1 defines the key for the short-term credential
       mechanism, and Section 9.2.2 defines the key for the long-term
       credential mechanism.  Other credential mechanisms MUST define the
       key that is used for the HMAC.

       The text used as input to HMAC is the STUN message, up to and
       including the attribute preceding the MESSAGE-INTEGRITY attribute.
       The Length field of the STUN message header is adjusted to point to
       the end of the MESSAGE-INTEGRITY attribute.  The value of the
       MESSAGE-INTEGRITY attribute is set to a dummy value.

       Once the computation is performed, the value of the MESSAGE-INTEGRITY
       attribute is filled in, and the value of the length in the STUN
       header is set to its correct value -- the length of the entire
       message.  Similarly, when validating the MESSAGE-INTEGRITY, the
       Length field in the STUN header must be adjusted to point to the end
       of the MESSAGE-INTEGRITY attribute prior to calculating the HMAC over
       the STUN message, up to and including the attribute preceding the
       MESSAGE-INTEGRITY attribute.  Such adjustment is necessary when
       attributes, such as FINGERPRINT and MESSAGE-INTEGRITY-SHA256, appear
       after MESSAGE-INTEGRITY.  See also [RFC5769] for examples of such
       calculations.

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    14.6.  MESSAGE-INTEGRITY-SHA256

       The MESSAGE-INTEGRITY-SHA256 attribute contains an HMAC-SHA256
       [RFC2104] of the STUN message.  The MESSAGE-INTEGRITY-SHA256
       attribute can be present in any STUN message type.  The MESSAGE-
       INTEGRITY-SHA256 attribute contains an initial portion of the HMAC-
       SHA-256 [RFC2104] of the STUN message.  The value will be at most 32
       bytes, but it MUST be at least 16 bytes and MUST be a multiple of 4
       bytes.  The value must be the full 32 bytes unless the STUN Usage
       explicitly specifies that truncation is allowed.  STUN Usages may
       specify a minimum length longer than 16 bytes.

       The key for the HMAC depends on which credential mechanism is in use.
       Section 9.1.1 defines the key for the short-term credential
       mechanism, and Section 9.2.2 defines the key for the long-term
       credential mechanism.  Other credential mechanism MUST define the key
       that is used for the HMAC.

       The text used as input to HMAC is the STUN message, up to and
       including the attribute preceding the MESSAGE-INTEGRITY-SHA256
       attribute.  The Length field of the STUN message header is adjusted
       to point to the end of the MESSAGE-INTEGRITY-SHA256 attribute.  The
       value of the MESSAGE-INTEGRITY-SHA256 attribute is set to a dummy
       value.

       Once the computation is performed, the value of the MESSAGE-
       INTEGRITY-SHA256 attribute is filled in, and the value of the length
       in the STUN header is set to its correct value -- the length of the
       entire message.  Similarly, when validating the MESSAGE-INTEGRITY-
       SHA256, the Length field in the STUN header must be adjusted to point
       to the end of the MESSAGE-INTEGRITY-SHA256 attribute prior to
       calculating the HMAC over the STUN message, up to and including the
       attribute preceding the MESSAGE-INTEGRITY-SHA256 attribute.  Such
       adjustment is necessary when attributes, such as FINGERPRINT, appear
       after MESSAGE-INTEGRITY-SHA256.  See also Appendix B.1 for examples
       of such calculations.

    14.7.  FINGERPRINT

       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 operation ensures
       that the FINGERPRINT test will not report a false positive on a
       packet containing a CRC-32 generated by an application protocol.)
       The 32-bit CRC is the one defined in ITU V.42 [ITU.V42.2002], which

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       has a generator polynomial of x^32 + x^26 + x^23 + x^22 + x^16 + x^12
       + x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1.  See the sample
       code for the CRC-32 in Section 8 of [RFC1952].

       When present, the FINGERPRINT attribute MUST be the last attribute in
       the message and thus will appear after MESSAGE-INTEGRITY and MESSAGE-
       INTEGRITY-SHA256.

       The FINGERPRINT attribute can aid in distinguishing STUN packets from
       packets of other protocols.  See Section 7.

       As with MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256, the CRC used
       in the FINGERPRINT attribute covers the Length field from the STUN
       message header.  Therefore, prior to computation of the CRC, this
       value must be correct and include the CRC attribute as part of the
       message length.  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 the value of the attribute is updated.
       If the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 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 and MESSAGE-INTEGRITY-SHA256
       attributes as well.

    14.8.  ERROR-CODE

       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]; it is also
       consistent in its code assignments and semantics with SIP [RFC3261]
       and HTTP [RFC7231].  The reason phrase is meant for diagnostic
       purposes 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-encoded [RFC3629] sequence of fewer than 128 characters (which
       can be as long as 509 bytes when encoding them or 763 bytes when
       decoding them).

          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: Format of 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 binary encoding of 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 either a USERNAME or USERHASH attribute and a
            valid MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 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 or MESSAGE-INTEGRITY-SHA256 attribute, and
            receivers MUST validate the MESSAGE-INTEGRITY or MESSAGE-
            INTEGRITY-SHA256 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 or MESSAGE-INTEGRITY-SHA256 for this error, so
            the client MUST NOT expect a valid MESSAGE-INTEGRITY or MESSAGE-
            INTEGRITY-SHA256 attribute on this response.

       401  Unauthenticated: 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
            response.

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

    14.9.  REALM

       The REALM attribute may be present in requests and responses.  It
       contains text that meets the grammar for "realm-value" as described
       in [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-encoded
       [RFC3629] sequence of fewer than 128 characters (which can be as long
       as 509 bytes when encoding them and as long as 763 bytes when
       decoding them) and MUST have been processed using the OpaqueString
       profile [RFC8265].

       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 in that realm for authentication.

    14.10.  NONCE

       The NONCE attribute may be present in requests and responses.  It
       contains a sequence of qdtext or quoted-pair, which are defined in
       [RFC3261].  Note that this means that the NONCE attribute will not
       contain the actual surrounding quote characters.  The NONCE attribute
       MUST be fewer than 128 characters (which can be as long as 509 bytes
       when encoding them and a long as 763 bytes when decoding them).  See
       Section 5.4 of [RFC7616] for guidance on selection of nonce values in
       a server.

    14.11.  PASSWORD-ALGORITHMS

       The PASSWORD-ALGORITHMS attribute may be present in requests and
       responses.  It contains the list of algorithms that the server can
       use to derive the long-term password.

       The set of known algorithms is maintained by IANA.  The initial set
       defined by this specification is found in Section 18.5.

       The attribute contains a list of algorithm numbers and variable
       length parameters.  The algorithm number is a 16-bit value as defined
       in Section 18.5.  The parameters start with the length (prior to
       padding) of the parameters as a 16-bit value, followed by the
       parameters that are specific to each algorithm.  The parameters are
       padded to a 32-bit boundary, in the same manner as an attribute.

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          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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         Algorithm 1           | Algorithm 1 Parameters Length |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                    Algorithm 1 Parameters (variable)
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         Algorithm 2           | Algorithm 2 Parameters Length |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                    Algorithm 2 Parameters (variable)
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                                                             ...

                 Figure 8: Format of PASSWORD-ALGORITHMS Attribute

    14.12.  PASSWORD-ALGORITHM

       The PASSWORD-ALGORITHM attribute is present only in requests.  It
       contains the algorithm that the server must use to derive a key from
       the long-term password.

       The set of known algorithms is maintained by IANA.  The initial set
       defined by this specification is found in Section 18.5.

       The attribute contains an algorithm number and variable length
       parameters.  The algorithm number is a 16-bit value as defined in
       Section 18.5.  The parameters starts with the length (prior to
       padding) of the parameters as a 16-bit value, followed by the
       parameters that are specific to the algorithm.  The parameters are
       padded to a 32-bit boundary, in the same manner as an attribute.
       Similarly, the padding bits MUST be set to zero on sending and MUST
       be ignored by the receiver.

          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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Algorithm           |  Algorithm Parameters Length   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                    Algorithm Parameters (variable)
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 9: Format of PASSWORD-ALGORITHM Attribute

    14.13.  UNKNOWN-ATTRIBUTES

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

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       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 10: Format of UNKNOWN-ATTRIBUTES Attribute

          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.

    14.14.  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-encoded
       [RFC3629] sequence of fewer than 128 characters (which can be as long
       as 509 when encoding them and as long as 763 bytes when decoding
       them).

    14.15.  ALTERNATE-SERVER

       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.

    14.16.  ALTERNATE-DOMAIN

       The alternate domain represents the domain name that is used to
       verify the IP address in the ALTERNATE-SERVER attribute when the
       transport protocol uses TLS or DTLS.

       The value of ALTERNATE-DOMAIN is variable length.  It MUST be a valid
       DNS name [RFC1123] (including A-labels [RFC5890]) of 255 or fewer
       ASCII characters.

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    15.  Operational Considerations

       STUN MAY be used with anycast addresses, but only with UDP and in
       STUN Usages where authentication is not used.

    16.  Security Considerations

       Implementations and deployments of a STUN Usage using TLS or DTLS
       MUST follow the recommendations in [BCP195].

       Implementations and deployments of a STUN Usage using the long-term
       credential mechanism (Section 9.2) MUST follow the recommendations in
       Section 5 of [RFC7616].

    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.

       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 13, STUN Usages describe when
       authentication and message integrity are needed.

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       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 or DTLS, mitigates these
       attacks.

       STUN supports both MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256,
       which makes STUN subject to bid-down attacks by an on-path attacker.
       An attacker could strip the MESSAGE-INTEGRITY-SHA256 attribute,
       leaving only the MESSAGE-INTEGRITY attribute and thus exploiting a
       potential vulnerability.  Protection of the channel itself, using TLS
       or DTLS, mitigates these attacks.  Timely removal of the support of
       MESSAGE-INTEGRITY in a future version of STUN is necessary.

       Note: The use of SHA-256 for password hashing does not meet modern
       standards, which are aimed at slowing down exhaustive password
       searches by providing a relatively slow minimum time to compute the
       hash.  Although better algorithms such as Argon2 [Argon2] are
       available, SHA-256 was chosen for consistency with [RFC7616].

    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 effectively.

       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.

    16.1.3.  Bid-Down Attacks

       This document adds the possibility of selecting different algorithms
       to protect the confidentiality of the passwords stored on the server
       side when using the long-term credential mechanism while still

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       ensuring compatibility with MD5, which was the algorithm used in
       [RFC5389].  This selection works by having the server send to the
       client the list of algorithms supported in a PASSWORD-ALGORITHMS
       attribute and having the client send back a PASSWORD-ALGORITHM
       attribute containing the algorithm selected.

       Because the PASSWORD-ALGORITHMS attribute has to be sent in an
       unauthenticated response, an on-path attacker wanting to exploit an
       eventual vulnerability in MD5 can just strip the PASSWORD-ALGORITHMS
       attribute from the unprotected response, thus making the server
       subsequently act as if the client was implementing the version of
       this protocol defined in [RFC5389].

       To protect against this attack and other similar bid-down attacks,
       the nonce is enriched with a set of security bits that indicates
       which security features are in use.  In the case of the selection of
       the password algorithm, the matching bit is set in the nonce returned
       by the server in the same response that contains the PASSWORD-
       ALGORITHMS attribute.  Because the nonce used in subsequent
       authenticated transactions is verified by the server to be identical
       to what was originally sent, it cannot be modified by an on-path
       attacker.  Additionally, the client is mandated to copy the received
       PASSWORD-ALGORITHMS attribute in the next authenticated transaction
       to that server.

       An on-path attack that removes the PASSWORD-ALGORITHMS will be
       detected because the client will not be able to send it back to the
       server in the next authenticated transaction.  The client will detect
       that attack because the security bit is set but the matching
       attribute is missing; this will end the session.  A client using an
       older version of this protocol will not send the PASSWORD-ALGORITHMS
       back but can only use MD5 anyway, so the attack is inconsequential.

       The on-path attack may also try to remove the security bit together
       with the PASSWORD-ALGORITHMS attribute, but the server will discover
       that when the next authenticated transaction contains an invalid
       nonce.

       An on-path attack that removes some algorithms from the PASSWORD-
       ALGORITHMS attribute will be equally defeated because that attribute
       will be different from the original one when the server verifies it
       in the subsequent authenticated transaction.

       Note that the bid-down protection mechanism introduced in this
       document is inherently limited by the fact that it is not possible to
       detect an attack until the server receives the second request after
       the 401 (Unauthenticated) response.

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       SHA-256 was chosen as the new default for password hashing for its
       compatibility with [RFC7616], but because SHA-256 (like MD5) is a
       comparatively fast algorithm, it does little to deter brute-force
       attacks.  Specifically, this means that if the user has a weak
       password, an attacker that captures a single exchange can use a
       brute-force attack to learn the user's password and then potentially
       impersonate the user to the server and to other servers where the
       same password was used.  Note that such an attacker can impersonate
       the user to the server itself without any brute-force attack.

       A stronger (which is to say, slower) algorithm, like Argon2 [Argon2],
       would help both of these cases; however, in the first case, it would
       only help after the database entry for this user is updated to
       exclusively use that stronger mechanism.

       The bid-down defenses in this protocol prevent an attacker from
       forcing the client and server to complete a handshake using weaker
       algorithms than they jointly support, but only if the weakest joint
       algorithm is strong enough that it cannot be compromised by a brute-
       force attack.  However, this does not defend against many attacks on
       those algorithms; specifically, an on-path attacker might perform a
       bid-down attack on a client that supports both Argon2 [Argon2] and
       SHA-256 for password hashing and use that to collect a MESSAGE-
       INTEGRITY-SHA256 value that it can then use for an offline brute-
       force attack.  This would be detected when the server receives the
       second request, but that does not prevent the attacker from obtaining
       the MESSAGE-INTEGRITY-SHA256 value.

       Similarly, an attack against the USERHASH mechanism will not succeed
       in establishing a session as the server will detect that the feature
       was discarded on path, but the client would still have been convinced
       to send its username in the clear in the USERNAME attribute, thus
       disclosing it to the attacker.

       Finally, when the bid-down protection mechanism is employed for a
       future upgrade of the HMAC algorithm used to protect messages, it
       will offer only a limited protection if the current HMAC algorithm is
       already compromised.

    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 countermeasures.

       Most of the attacks in this section revolve around an attacker
       modifying the reflexive address learned by a STUN client through a
       Binding request/response transaction.  Since the usage of the

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       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 and the
       intervening NAT must be able to modify this value.  Instead, one
       solution to prevent the attacks listed below is for the client to
       verify the reflexive address learned, as is done in ICE [RFC8445].

       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 described 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 allows 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.

       Note that this attack can be trivially launched by the STUN server
       itself, so users of STUN servers should have the same level of trust
       in the users of STUN servers as any other node that can insert itself
       into the communication flow.

    16.3.  Hash Agility Plan

       This specification uses HMAC-SHA256 for computation of the message
       integrity, sometimes in combination with HMAC-SHA1.  If, at a later
       time, HMAC-SHA256 is found to be compromised, the following remedy
       should be applied:

       o  Both a new message-integrity attribute and a new STUN Security
          Feature bit will be allocated in a Standards Track document.  The
          new message-integrity attribute will have its value computed using
          a new hash.  The STUN Security Feature bit will be used to
          simultaneously 1) signal to a STUN client using the long-term
          credential mechanism that this server supports this new hash
          algorithm and 2) prevent bid-down attacks on the new message-
          integrity attribute.

       o  STUN clients and servers using the short-term credential mechanism
          will need to update the external mechanism that they use to signal
          what message-integrity attributes are in use.

       The bid-down protection mechanism described in this document is new
       and thus cannot currently protect against a bid-down attack that
       lowers the strength of the hash algorithm to HMAC-SHA1.  This is why,

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       after a transition period, a new document updating this one will
       assign a new STUN Security Feature bit for deprecating HMAC-SHA1.
       When used, this bit will signal that HMAC-SHA1 is deprecated and
       should no longer be used.

       Similarly, if HMAC-SHA256 is found to be compromised, a new userhash
       attribute and a new STUN Security Feature bit will be allocated in a
       Standards Track document.  The new userhash attribute will have its
       value computed using a new hash.  The STUN Security Feature bit will
       be used to simultaneously 1) signal to a STUN client using the long-
       term credential mechanism that this server supports this new hash
       algorithm for the userhash attribute and 2) prevent bid-down attacks
       on the new userhash attribute.

    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
       through a collaborative protocol reflection mechanism [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 suggested that protocols developed for this purpose
       document a specific set of considerations.  Because some STUN Usages
       provide UNSAF functions (such as ICE [RFC8445]) and others do not
       (such as SIP Outbound [RFC5626]), answers to these considerations
       need to be addressed by the usages themselves.

    18.  IANA Considerations

    18.1.  STUN Security Features Registry

       A STUN Security Feature set defines 24 bits as flags.

       IANA has created a new registry containing the STUN Security Features
       that are protected by the bid-down attack prevention mechanism
       described in Section 9.2.1.

       The initial STUN Security Features are:

       Bit 0: Password algorithms
       Bit 1: Username anonymity
       Bit 2-23: Unassigned

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       Bits are assigned starting from the most significant side of the bit
       set, so Bit 0 is the leftmost bit and Bit 23 is the rightmost bit.

       New Security Features are assigned by Standards Action [RFC8126].

    18.2.  STUN Methods Registry

       A STUN method is a hex number in the range 0x000-0x0FF.  The encoding
       of a STUN method into a STUN message is described in Section 5.

       STUN methods in the range 0x000-0x07F are assigned by IETF Review
       [RFC8126].  STUN methods in the range 0x080-0x0FF are assigned by
       Expert Review [RFC8126].  The responsibility of the expert is to
       verify that the selected codepoint(s) is 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.

       IANA has updated the name for method 0x002 as described below as well
       as updated the reference from RFC 5389 to RFC 8489 for the following
       STUN methods:

       0x000: Reserved
       0x001: Binding
       0x002: Reserved; was SharedSecret prior to [RFC5389]

    18.3.  STUN Attributes 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 differently.

       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 [RFC8126].
       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 Expert Review
       [RFC8126].  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.

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    18.3.1.  Updated Attributes

       IANA has updated the names for attributes 0x0002, 0x0004, 0x0005,
       0x0007, and 0x000B as well as updated the reference from RFC 5389 to
       RFC 8489 for each the following STUN methods.

       In addition, [RFC5389] introduced a mistake in the name of attribute
       0x0003; [RFC5389] called it CHANGE-ADDRESS when it was actually
       previously called CHANGE-REQUEST.  Thus, IANA has updated the
       description for 0x0003 to read "Reserved; was CHANGE-REQUEST prior to
       [RFC5389]".

       Comprehension-required range (0x0000-0x7FFF):
       0x0000: Reserved
       0x0001: MAPPED-ADDRESS
       0x0002: Reserved; was RESPONSE-ADDRESS prior to [RFC5389]
       0x0003: Reserved; was CHANGE-REQUEST prior to [RFC5389]
       0x0004: Reserved; was SOURCE-ADDRESS prior to [RFC5389]
       0x0005: Reserved; was CHANGED-ADDRESS prior to [RFC5389]
       0x0006: USERNAME
       0x0007: Reserved; was PASSWORD prior to [RFC5389]
       0x0008: MESSAGE-INTEGRITY
       0x0009: ERROR-CODE
       0x000A: UNKNOWN-ATTRIBUTES
       0x000B: Reserved; was REFLECTED-FROM prior to [RFC5389]
       0x0014: REALM
       0x0015: NONCE
       0x0020: XOR-MAPPED-ADDRESS

       Comprehension-optional range (0x8000-0xFFFF)
       0x8022: SOFTWARE
       0x8023: ALTERNATE-SERVER
       0x8028: FINGERPRINT

    18.3.2.  New Attributes

       IANA has added the following attribute to the "STUN Attributes"
       registry:

       Comprehension-required range (0x0000-0x7FFF):
       0x001C: MESSAGE-INTEGRITY-SHA256
       0x001D: PASSWORD-ALGORITHM
       0x001E: USERHASH

       Comprehension-optional range (0x8000-0xFFFF)
       0x8002: PASSWORD-ALGORITHMS
       0x8003: ALTERNATE-DOMAIN

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    18.4.  STUN Error Codes 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 [RFC7231].

       New STUN error codes are assigned based on IETF Review [RFC8126].
       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 6.3.4.

       IANA has updated the reference from RFC 5389 to RFC 8489 for the
       error codes defined in Section 14.8.

       IANA has changed the name of the 401 error code from "Unauthorized"
       to "Unauthenticated".

    18.5.  STUN Password Algorithms Registry

       IANA has created a new registry titled "STUN Password Algorithms".

       A password algorithm is a hex number in the range 0x0000-0xFFFF.

       The initial contents of the "Password Algorithm" registry are as
       follows:

       0x0000: Reserved
       0x0001: MD5
       0x0002: SHA-256
       0x0003-0xFFFF: Unassigned

       Password algorithms in the first half of the range (0x0000-0x7FFF)
       are assigned by IETF Review [RFC8126].  Password algorithms in the
       second half of the range (0x8000-0xFFFF) are assigned by Expert
       Review [RFC8126].

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    18.5.1.  Password Algorithms

    18.5.1.1.  MD5

       This password algorithm is taken from [RFC1321].

       The key length is 16 bytes, and the parameters value is empty.

          Note: This algorithm MUST only be used for compatibility with
          legacy systems.

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

    18.5.1.2.  SHA-256

       This password algorithm is taken from [RFC7616].

       The key length is 32 bytes, and the parameters value is empty.

                  key = SHA-256(username ":" OpaqueString(realm)
                    ":" OpaqueString(password))

    18.6.  STUN UDP and TCP Port Numbers

       IANA has updated the reference from RFC 5389 to RFC 8489 for the
       following ports in the "Service Name and Transport Protocol Port
       Number Registry".

       stun   3478/tcp   Session Traversal Utilities for NAT (STUN) port
       stun   3478/udp   Session Traversal Utilities for NAT (STUN) port
       stuns  5349/tcp   Session Traversal Utilities for NAT (STUN) port

    19.  Changes since RFC 5389

       This specification obsoletes [RFC5389].  This specification differs
       from RFC 5389 in the following ways:

       o  Added support for DTLS-over-UDP [RFC6347].

       o  Made clear that the RTO is considered stale if there are no
          transactions with the server.

       o  Aligned the RTO calculation with [RFC6298].

       o  Updated the ciphersuites for TLS.

       o  Added support for STUN URI [RFC7064].

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       o  Added support for SHA256 message integrity.

       o  Updated the Preparation, Enforcement, and Comparison of
          Internationalized Strings (PRECIS) support to [RFC8265].

       o  Added protocol and registry to choose the password encryption
          algorithm.

       o  Added support for anonymous username.

       o  Added protocol and registry for preventing bid-down attacks.

       o  Specified that sharing a NONCE is no longer permitted.

       o  Added the possibility of using a domain name in the alternate
          server mechanism.

       o  Added more C snippets.

       o  Added test vector.

    20.  References

    20.1.  Normative References

       [ITU.V42.2002]
                  International Telecommunication Union, "Error-correcting
                  procedures for DCEs using asynchronous-to-synchronous
                  conversion", ITU-T Recommendation V.42, March 2002.

       [KARN87]   Karn, P. and C. Partridge, "Improving Round-Trip Time
                  Estimates in Reliable Transport Protocols", SIGCOMM '87,
                  Proceedings of the ACM workshop on Frontiers in computer
                  communications technology, Pages 2-7,
                  DOI 10.1145/55483.55484, August 1987.

       [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
                  DOI 10.17487/RFC0791, September 1981,
                  <https://www.rfc-editor.org/info/rfc791>.

       [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
                  Communication Layers", STD 3, RFC 1122,
                  DOI 10.17487/RFC1122, October 1989,
                  <https://www.rfc-editor.org/info/rfc1122>.

    Petit-Huguenin, et al.       Standards Track                   [Page 58]

    RFC 8489                          STUN                     February 2020

       [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
                  Application and Support", STD 3, RFC 1123,
                  DOI 10.17487/RFC1123, October 1989,
                  <https://www.rfc-editor.org/info/rfc1123>.

       [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
                  DOI 10.17487/RFC1321, April 1992,
                  <https://www.rfc-editor.org/info/rfc1321>.

       [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
                  Hashing for Message Authentication", RFC 2104,
                  DOI 10.17487/RFC2104, February 1997,
                  <https://www.rfc-editor.org/info/rfc2104>.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
                  specifying the location of services (DNS SRV)", RFC 2782,
                  DOI 10.17487/RFC2782, February 2000,
                  <https://www.rfc-editor.org/info/rfc2782>.

       [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
                  2003, <https://www.rfc-editor.org/info/rfc3629>.

       [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
                  Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
                  <https://www.rfc-editor.org/info/rfc4648>.

       [RFC5890]  Klensin, J., "Internationalized Domain Names for
                  Applications (IDNA): Definitions and Document Framework",
                  RFC 5890, DOI 10.17487/RFC5890, August 2010,
                  <https://www.rfc-editor.org/info/rfc5890>.

       [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
                  Verification of Domain-Based Application Service Identity
                  within Internet Public Key Infrastructure Using X.509
                  (PKIX) Certificates in the Context of Transport Layer
                  Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
                  2011, <https://www.rfc-editor.org/info/rfc6125>.

       [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
                  for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
                  RFC 6151, DOI 10.17487/RFC6151, March 2011,
                  <https://www.rfc-editor.org/info/rfc6151>.

    Petit-Huguenin, et al.       Standards Track                   [Page 59]

    RFC 8489                          STUN                     February 2020

       [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
                  "Computing TCP's Retransmission Timer", RFC 6298,
                  DOI 10.17487/RFC6298, June 2011,
                  <https://www.rfc-editor.org/info/rfc6298>.

       [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
                  Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
                  January 2012, <https://www.rfc-editor.org/info/rfc6347>.

       [RFC7064]  Nandakumar, S., Salgueiro, G., Jones, P., and M. Petit-
                  Huguenin, "URI Scheme for the Session Traversal Utilities
                  for NAT (STUN) Protocol", RFC 7064, DOI 10.17487/RFC7064,
                  November 2013, <https://www.rfc-editor.org/info/rfc7064>.

       [RFC7350]  Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
                  Layer Security (DTLS) as Transport for Session Traversal
                  Utilities for NAT (STUN)", RFC 7350, DOI 10.17487/RFC7350,
                  August 2014, <https://www.rfc-editor.org/info/rfc7350>.

       [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
                  Digest Access Authentication", RFC 7616,
                  DOI 10.17487/RFC7616, September 2015,
                  <https://www.rfc-editor.org/info/rfc7616>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

       [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
                  (IPv6) Specification", STD 86, RFC 8200,
                  DOI 10.17487/RFC8200, July 2017,
                  <https://www.rfc-editor.org/info/rfc8200>.

       [RFC8265]  Saint-Andre, P. and A. Melnikov, "Preparation,
                  Enforcement, and Comparison of Internationalized Strings
                  Representing Usernames and Passwords", RFC 8265,
                  DOI 10.17487/RFC8265, October 2017,
                  <https://www.rfc-editor.org/info/rfc8265>.

       [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
                  Better Connectivity Using Concurrency", RFC 8305,
                  DOI 10.17487/RFC8305, December 2017,
                  <https://www.rfc-editor.org/info/rfc8305>.

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    20.2.  Informative References

       [Argon2]   Biryukov, A., Dinu, D., Khovratovich, D., and S.
                  Josefsson, "The memory-hard Argon2 password hash and
                  proof-of-work function", Work in Progress, draft-irtf-
                  cfrg-argon2-09, November 2019.

       [BCP195]   Sheffer, Y., Holz, R., and P. Saint-Andre,
                  "Recommendations for Secure Use of Transport Layer
                  Security (TLS) and Datagram Transport Layer Security
                  (DTLS)", BCP 195, RFC 7525, May 2015,
                  <https://www.rfc-editor.org/info/bcp195>.

       [RFC1952]  Deutsch, P., "GZIP file format specification version 4.3",
                  RFC 1952, DOI 10.17487/RFC1952, May 1996,
                  <https://www.rfc-editor.org/info/rfc1952>.

       [RFC2279]  Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", RFC 2279, DOI 10.17487/RFC2279, January 1998,
                  <https://www.rfc-editor.org/info/rfc2279>.

       [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
                  A., Peterson, J., Sparks, R., Handley, M., and E.
                  Schooler, "SIP: Session Initiation Protocol", RFC 3261,
                  DOI 10.17487/RFC3261, June 2002,
                  <https://www.rfc-editor.org/info/rfc3261>.

       [RFC3424]  Daigle, L., Ed. and IAB, "IAB Considerations for
                  UNilateral Self-Address Fixing (UNSAF) Across Network
                  Address Translation", RFC 3424, DOI 10.17487/RFC3424,
                  November 2002, <https://www.rfc-editor.org/info/rfc3424>.

       [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
                  Internationalized Strings ("stringprep")", RFC 3454,
                  DOI 10.17487/RFC3454, December 2002,
                  <https://www.rfc-editor.org/info/rfc3454>.

       [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,
                  DOI 10.17487/RFC3489, March 2003,
                  <https://www.rfc-editor.org/info/rfc3489>.

       [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
                  and Passwords", RFC 4013, DOI 10.17487/RFC4013, February
                  2005, <https://www.rfc-editor.org/info/rfc4013>.

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       [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
                  Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
                  June 2005, <https://www.rfc-editor.org/info/rfc4107>.

       [RFC5090]  Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D.,
                  and W. Beck, "RADIUS Extension for Digest Authentication",
                  RFC 5090, DOI 10.17487/RFC5090, February 2008,
                  <https://www.rfc-editor.org/info/rfc5090>.

       [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
                  "Session Traversal Utilities for NAT (STUN)", RFC 5389,
                  DOI 10.17487/RFC5389, October 2008,
                  <https://www.rfc-editor.org/info/rfc5389>.

       [RFC5626]  Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed.,
                  "Managing Client-Initiated Connections in the Session
                  Initiation Protocol (SIP)", RFC 5626,
                  DOI 10.17487/RFC5626, October 2009,
                  <https://www.rfc-editor.org/info/rfc5626>.

       [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
                  Relays around NAT (TURN): Relay Extensions to Session
                  Traversal Utilities for NAT (STUN)", RFC 5766,
                  DOI 10.17487/RFC5766, April 2010,
                  <https://www.rfc-editor.org/info/rfc5766>.

       [RFC5769]  Denis-Courmont, R., "Test Vectors for Session Traversal
                  Utilities for NAT (STUN)", RFC 5769, DOI 10.17487/RFC5769,
                  April 2010, <https://www.rfc-editor.org/info/rfc5769>.

       [RFC5780]  MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
                  Using Session Traversal Utilities for NAT (STUN)",
                  RFC 5780, DOI 10.17487/RFC5780, May 2010,
                  <https://www.rfc-editor.org/info/rfc5780>.

       [RFC6544]  Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
                  "TCP Candidates with Interactive Connectivity
                  Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
                  March 2012, <https://www.rfc-editor.org/info/rfc6544>.

       [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
                  Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
                  DOI 10.17487/RFC7231, June 2014,
                  <https://www.rfc-editor.org/info/rfc7231>.

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       [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
                  Writing an IANA Considerations Section in RFCs", BCP 26,
                  RFC 8126, DOI 10.17487/RFC8126, June 2017,
                  <https://www.rfc-editor.org/info/rfc8126>.

       [RFC8264]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
                  Preparation, Enforcement, and Comparison of
                  Internationalized Strings in Application Protocols",
                  RFC 8264, DOI 10.17487/RFC8264, October 2017,
                  <https://www.rfc-editor.org/info/rfc8264>.

       [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
                  Connectivity Establishment (ICE): A Protocol for Network
                  Address Translator (NAT) Traversal", RFC 8445,
                  DOI 10.17487/RFC8445, July 2018,
                  <https://www.rfc-editor.org/info/rfc8445>.

       [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
                  Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
                  <https://www.rfc-editor.org/info/rfc8446>.

       [STUN-PMTUD]
                  Petit-Huguenin, M., Salgueiro, G., and F. Garrido,
                  "Packetization Layer Path MTU Discovery (PLMTUD) For UDP
                  Transports Using Session Traversal Utilities for NAT
                  (STUN)", Work in Progress, draft-ietf-tram-stun-pmtud-15,
                  December 2019.

       [UAX15]    Unicode Standard Annex #15, "Unicode Normalization Forms",
                  edited by Mark Davis and Ken Whistler.  An integral part
                  of The Unicode Standard,
                  <http://unicode.org/reports/tr15/>.

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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:

       <CODE BEGINS>
       #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)
       <CODE ENDS>

       A function to convert method and class into a message type:

       <CODE BEGINS>
       int type(int method, int cls) {
         return (method & 0x1F80) << 2 | (method & 0x0070) << 1
           | (method & 0x000F) | (cls & 0x0002) << 7
           | (cls & 0x0001) << 4;
         }
       <CODE ENDS>

       A function to extract the method from the message type:

       <CODE BEGINS>
       int method(int type) {
         return (type & 0x3E00) >> 2 | (type & 0x00E0) >> 1
           | (type & 0x000F);
         }
       <CODE ENDS>

       A function to extract the class from the message type:

       <CODE BEGINS>
       int cls(int type) {
         return (type & 0x0100) >> 7 | (type & 0x0010) >> 4;
         }
       <CODE ENDS>

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    Appendix B.  Test Vectors

       This section augments the list of test vectors defined in [RFC5769]
       with MESSAGE-INTEGRITY-SHA256.  All the formats and definitions
       listed in Section 2 of [RFC5769] apply here.

    B.1.  Sample Request with Long-Term Authentication with MESSAGE-
          INTEGRITY-SHA256 and USERHASH

       This request uses the following parameters:

       Username: "<U+30DE><U+30C8><U+30EA><U+30C3><U+30AF><U+30B9>" (without
       quotes) unaffected by OpaqueString [RFC8265] processing

       Password: "The<U+00AD>M<U+00AA>tr<U+2168>" and "TheMatrIX" (without
       quotes) respectively before and after OpaqueString [RFC8265]
       processing

       Nonce: "obMatJos2AAACf//499k954d6OL34oL9FSTvy64sA" (without quotes)

       Realm: "example.org" (without quotes)

            00 01 00 9c      Request type and message length
            21 12 a4 42      Magic cookie
            78 ad 34 33   }
            c6 ad 72 c0   }  Transaction ID
            29 da 41 2e   }
            00 1e 00 20      USERHASH attribute header
            4a 3c f3 8f   }
            ef 69 92 bd   }
            a9 52 c6 78   }
            04 17 da 0f   }  Userhash value (32 bytes)
            24 81 94 15   }
            56 9e 60 b2   }
            05 c4 6e 41   }
            40 7f 17 04   }
            00 15 00 29      NONCE attribute header
            6f 62 4d 61   }
            74 4a 6f 73   }
            32 41 41 41   }
            43 66 2f 2f   }
            34 39 39 6b   }  Nonce value and padding (3 bytes)
            39 35 34 64   }
            36 4f 4c 33   }
            34 6f 4c 39   }
            46 53 54 76   }
            79 36 34 73   }
            41 00 00 00   }

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            00 14 00 0b      REALM attribute header
            65 78 61 6d   }
            70 6c 65 2e   }  Realm value (11 bytes) and padding (1 byte)
            6f 72 67 00   }
            00 1c 00 20      MESSAGE-INTEGRITY-SHA256 attribute header
            e4 68 6c 8f   }
            0e de b5 90   }
            13 e0 70 90   }
            01 0a 93 ef   }  HMAC-SHA256 value
            cc bc cc 54   }
            4c 0a 45 d9   }
            f8 30 aa 6d   }
            6f 73 5a 01   }

    Acknowledgements

       Thanks to Michael Tuexen, Tirumaleswar Reddy, Oleg Moskalenko, Simon
       Perreault, Benjamin Schwartz, Rifaat Shekh-Yusef, Alan Johnston,
       Jonathan Lennox, Brandon Williams, Olle Johansson, Martin Thomson,
       Mihaly Meszaros, Tolga Asveren, Noriyuki Torii, Spencer Dawkins, Dale
       Worley, Matthew Miller, Peter Saint-Andre, Julien Elie, Mirja
       Kuehlewind, Eric Rescorla, Ben Campbell, Adam Roach, Alexey Melnikov,
       and Benjamin Kaduk for the comments, suggestions, and questions that
       helped improve this document.

       The Acknowledgements section of RFC 5389 appeared as follows:

       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.

    Contributors

       Christian Huitema and Joel Weinberger were original coauthors of
       RFC 3489.

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    Authors' Addresses

       Marc Petit-Huguenin
       Impedance Mismatch

       Email: marc@petit-huguenin.org

       Gonzalo Salgueiro
       Cisco
       7200-12 Kit Creek Road
       Research Triangle Park, NC  27709
       United States of America

       Email: gsalguei@cisco.com

       Jonathan Rosenberg
       Five9
       Edison, NJ
       United States of America

       Email: jdrosen@jdrosen.net
       URI:   http://www.jdrosen.net

       Dan Wing
       Citrix Systems, Inc.
       United States of America

       Email: dwing-ietf@fuggles.com

       Rohan Mahy
       Unaffiliated

       Email: rohan.ietf@gmail.com

       Philip Matthews
       Nokia
       600 March Road
       Ottawa, Ontario  K2K 2T6
       Canada

       Phone: 613-784-3139
       Email: philip_matthews@magma.ca

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