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RFC 8489

Session Traversal Utilities for NAT (STUN)

Pages: 67
Proposed Standard
Errata
Obsoletes:  5389
Part 1 of 3 – Pages 1 to 22
None   None   Next

Top   ToC   RFC8489 - Page 1
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.



(page 22 continued on part 2)

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