3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Readers are expected to be familiar with [RFC5389] and the terms
   defined there.

   The following terms are used in this document:

   TURN:  The protocol spoken between a TURN client and a TURN server.
      It is an extension to the STUN protocol [RFC5389].  The protocol
      allows a client to allocate and use a relayed transport address.

   TURN client:  A STUN client that implements this specification.

   TURN server:  A STUN server that implements this specification.  It
      relays data between a TURN client and its peer(s).

   Peer:  A host with which the TURN client wishes to communicate.  The
      TURN server relays traffic between the TURN client and its
      peer(s).  The peer does not interact with the TURN server using
      the protocol defined in this document; rather, the peer receives
      data sent by the TURN server and the peer sends data towards the
      TURN server.

   Transport Address:  The combination of an IP address and a port.

   Host Transport Address:  A transport address on a client or a peer.

   Server-Reflexive Transport Address:  A transport address on the
      "public side" of a NAT.  This address is allocated by the NAT to
      correspond to a specific host transport address.

   Relayed Transport Address:  A transport address on the TURN server
      that is used for relaying packets between the client and a peer.
      A peer sends to this address on the TURN server, and the packet is
      then relayed to the client.

   TURN Server Transport Address:  A transport address on the TURN
      server that is used for sending TURN messages to the server.  This
      is the transport address that the client uses to communicate with
      the server.

   Peer Transport Address:  The transport address of the peer as seen by
      the server.  When the peer is behind a NAT, this is the peer's
      server-reflexive transport address.

   Allocation:  The relayed transport address granted to a client
      through an Allocate request, along with related state, such as
      permissions and expiration timers.

   5-tuple:  The combination (client IP address and port, server IP
      address and port, and transport protocol (currently one of UDP,
      TCP, or TLS)) used to communicate between the client and the
      server.  The 5-tuple uniquely identifies this communication
      stream.  The 5-tuple also uniquely identifies the Allocation on
      the server.

   Channel:  A channel number and associated peer transport address.
      Once a channel number is bound to a peer's transport address, the
      client and server can use the more bandwidth-efficient ChannelData
      message to exchange data.

   Permission:  The IP address and transport protocol (but not the port)
      of a peer that is permitted to send traffic to the TURN server and
      have that traffic relayed to the TURN client.  The TURN server
      will only forward traffic to its client from peers that match an
      existing permission.

   Realm:  A string used to describe the server or a context within the
      server.  The realm tells the client which username and password
      combination to use to authenticate requests.

   Nonce:  A string chosen at random by the server and included in the
      message-digest.  To prevent reply attacks, the server should
      change the nonce regularly.

4.  General Behavior

   This section contains general TURN processing rules that apply to all
   TURN messages.

   TURN is an extension to STUN.  All TURN messages, with the exception
   of the ChannelData message, are STUN-formatted messages.  All the
   base processing rules described in [RFC5389] apply to STUN-formatted
   messages.  This means that all the message-forming and message-
   processing descriptions in this document are implicitly prefixed with
   the rules of [RFC5389].

   [RFC5389] specifies an authentication mechanism called the long-term
   credential mechanism.  TURN servers and clients MUST implement this
   mechanism.  The server MUST demand that all requests from the client
   be authenticated using this mechanism, or that a equally strong or
   stronger mechanism for client authentication is used.

   Note that the long-term credential mechanism applies only to requests
   and cannot be used to authenticate indications; thus, indications in
   TURN are never authenticated.  If the server requires requests to be
   authenticated, then the server's administrator MUST choose a realm
   value that will uniquely identify the username and password
   combination that the client must use, even if the client uses
   multiple servers under different administrations.  The server's
   administrator MAY choose to allocate a unique username to each
   client, or MAY choose to allocate the same username to more than one
   client (for example, to all clients from the same department or
   company).  For each allocation, the server SHOULD generate a new
   random nonce when the allocation is first attempted following the
   randomness recommendations in [RFC4086] and SHOULD expire the nonce
   at least once every hour during the lifetime of the allocation.

   All requests after the initial Allocate must use the same username as
   that used to create the allocation, to prevent attackers from
   hijacking the client's allocation.  Specifically, if the server
   requires the use of the long-term credential mechanism, and if a non-
   Allocate request passes authentication under this mechanism, and if
   the 5-tuple identifies an existing allocation, but the request does
   not use the same username as used to create the allocation, then the
   request MUST be rejected with a 441 (Wrong Credentials) error.

   When a TURN message arrives at the server from the client, the server
   uses the 5-tuple in the message to identify the associated
   allocation.  For all TURN messages (including ChannelData) EXCEPT an
   Allocate request, if the 5-tuple does not identify an existing
   allocation, then the message MUST either be rejected with a 437
   Allocation Mismatch error (if it is a request) or silently ignored
   (if it is an indication or a ChannelData message).  A client
   receiving a 437 error response to a request other than Allocate MUST
   assume the allocation no longer exists.

   [RFC5389] defines a number of attributes, including the SOFTWARE and
   FINGERPRINT attributes.  The client SHOULD include the SOFTWARE
   attribute in all Allocate and Refresh requests and MAY include it in
   any other requests or indications.  The server SHOULD include the
   SOFTWARE attribute in all Allocate and Refresh responses (either
   success or failure) and MAY include it in other responses or
   indications.  The client and the server MAY include the FINGERPRINT
   attribute in any STUN-formatted messages defined in this document.

   TURN does not use the backwards-compatibility mechanism described in
   [RFC5389].

   TURN, as defined in this specification, only supports IPv4.  The
   client's IP address, the server's IP address, and all IP addresses
   appearing in a relayed transport address MUST be IPv4 addresses.

   By default, TURN runs on the same ports as STUN: 3478 for TURN over
   UDP and TCP, and 5349 for TURN over TLS.  However, TURN has its own
   set of Service Record (SRV) names: "turn" for UDP and TCP, and
   "turns" for TLS.  Either the SRV procedures or the ALTERNATE-SERVER
   procedures, both described in Section 6, can be used to run TURN on a
   different port.

   To ensure interoperability, a TURN server MUST support the use of UDP
   transport between the client and the server, and SHOULD support the
   use of TCP and TLS transport.

   When UDP transport is used between the client and the server, the
   client will retransmit a request if it does not receive a response
   within a certain timeout period.  Because of this, the server may
   receive two (or more) requests with the same 5-tuple and same
   transaction id.  STUN requires that the server recognize this case
   and treat the request as idempotent (see [RFC5389]).  Some
   implementations may choose to meet this requirement by remembering
   all received requests and the corresponding responses for 40 seconds.
   Other implementations may choose to reprocess the request and arrange
   that such reprocessing returns essentially the same response.  To aid
   implementors who choose the latter approach (the so-called "stateless
   stack approach"), this specification includes some implementation
   notes on how this might be done.  Implementations are free to choose
   either approach or choose some other approach that gives the same
   results.

   When TCP transport is used between the client and the server, it is
   possible that a bit error will cause a length field in a TURN packet
   to become corrupted, causing the receiver to lose synchronization
   with the incoming stream of TURN messages.  A client or server that
   detects a long sequence of invalid TURN messages over TCP transport
   SHOULD close the corresponding TCP connection to help the other end
   detect this situation more rapidly.

   To mitigate either intentional or unintentional denial-of-service
   attacks against the server by clients with valid usernames and
   passwords, it is RECOMMENDED that the server impose limits on both
   the number of allocations active at one time for a given username and
   on the amount of bandwidth those allocations can use.  The server
   should reject new allocations that would exceed the limit on the
   allowed number of allocations active at one time with a 486
   (Allocation Quota Exceeded) (see Section 6.2), and should discard
   application data traffic that exceeds the bandwidth quota.

5.  Allocations

   All TURN operations revolve around allocations, and all TURN messages
   are associated with an allocation.  An allocation conceptually
   consists of the following state data:

   o  the relayed transport address;

   o  the 5-tuple: (client's IP address, client's port, server IP
      address, server port, transport protocol);

   o  the authentication information;

   o  the time-to-expiry;

   o  a list of permissions;

   o  a list of channel to peer bindings.

   The relayed transport address is the transport address allocated by
   the server for communicating with peers, while the 5-tuple describes
   the communication path between the client and the server.  On the
   client, the 5-tuple uses the client's host transport address; on the
   server, the 5-tuple uses the client's server-reflexive transport
   address.

   Both the relayed transport address and the 5-tuple MUST be unique
   across all allocations, so either one can be used to uniquely
   identify the allocation.

   The authentication information (e.g., username, password, realm, and
   nonce) is used to both verify subsequent requests and to compute the
   message integrity of responses.  The username, realm, and nonce
   values are initially those used in the authenticated Allocate request
   that creates the allocation, though the server can change the nonce
   value during the lifetime of the allocation using a 438 (Stale Nonce)
   reply.  Note that, rather than storing the password explicitly, for
   security reasons, it may be desirable for the server to store the key
   value, which is an MD5 hash over the username, realm, and password
   (see [RFC5389]).

   The time-to-expiry is the time in seconds left until the allocation
   expires.  Each Allocate or Refresh transaction sets this timer, which
   then ticks down towards 0.  By default, each Allocate or Refresh
   transaction resets this timer to the default lifetime value of 600
   seconds (10 minutes), but the client can request a different value in
   the Allocate and Refresh request.  Allocations can only be refreshed
   using the Refresh request; sending data to a peer does not refresh an

   allocation.  When an allocation expires, the state data associated
   with the allocation can be freed.

   The list of permissions is described in Section 8 and the list of
   channels is described in Section 11.

6.  Creating an Allocation

   An allocation on the server is created using an Allocate transaction.

6.1.  Sending an Allocate Request

   The client forms an Allocate request as follows.

   The client first picks a host transport address.  It is RECOMMENDED
   that the client pick a currently unused transport address, typically
   by allowing the underlying OS to pick a currently unused port for a
   new socket.

   The client then picks a transport protocol to use between the client
   and the server.  The transport protocol MUST be one of UDP, TCP, or
   TLS-over-TCP.  Since this specification only allows UDP between the
   server and the peers, it is RECOMMENDED that the client pick UDP
   unless it has a reason to use a different transport.  One reason to
   pick a different transport would be that the client believes, either
   through configuration or by experiment, that it is unable to contact
   any TURN server using UDP.  See Section 2.1 for more discussion.

   The client also picks a server transport address, which SHOULD be
   done as follows.  The client receives (perhaps through configuration)
   a domain name for a TURN server.  The client then uses the DNS
   procedures described in [RFC5389], but using an SRV service name of
   "turn" (or "turns" for TURN over TLS) instead of "stun" (or "stuns").
   For example, to find servers in the example.com domain, the client
   performs a lookup for '_turn._udp.example.com',
   '_turn._tcp.example.com', and '_turns._tcp.example.com' if the client
   wants to communicate with the server using UDP, TCP, or TLS-over-TCP,
   respectively.

   The client MUST include a REQUESTED-TRANSPORT attribute in the
   request.  This attribute specifies the transport protocol between the
   server and the peers (note that this is NOT the transport protocol
   that appears in the 5-tuple).  In this specification, the REQUESTED-
   TRANSPORT type is always UDP.  This attribute is included to allow
   future extensions to specify other protocols.

   If the client wishes the server to initialize the time-to-expiry
   field of the allocation to some value other than the default

   lifetime, then it MAY include a LIFETIME attribute specifying its
   desired value.  This is just a request, and the server may elect to
   use a different value.  Note that the server will ignore requests to
   initialize the field to less than the default value.

   If the client wishes to later use the DONT-FRAGMENT attribute in one
   or more Send indications on this allocation, then the client SHOULD
   include the DONT-FRAGMENT attribute in the Allocate request.  This
   allows the client to test whether this attribute is supported by the
   server.

   If the client requires the port number of the relayed transport
   address be even, the client includes the EVEN-PORT attribute.  If
   this attribute is not included, then the port can be even or odd.  By
   setting the R bit in the EVEN-PORT attribute to 1, the client can
   request that the server reserve the next highest port number (on the
   same IP address) for a subsequent allocation.  If the R bit is 0, no
   such request is made.

   The client MAY also include a RESERVATION-TOKEN attribute in the
   request to ask the server to use a previously reserved port for the
   allocation.  If the RESERVATION-TOKEN attribute is included, then the
   client MUST omit the EVEN-PORT attribute.

   Once constructed, the client sends the Allocate request on the
   5-tuple.

6.2.  Receiving an Allocate Request

   When the server receives an Allocate request, it performs the
   following checks:

   1.  The server MUST require that the request be authenticated.  This
       authentication MUST be done using the long-term credential
       mechanism of [RFC5389] unless the client and server agree to use
       another mechanism through some procedure outside the scope of
       this document.

   2.  The server checks if the 5-tuple is currently in use by an
       existing allocation.  If yes, the server rejects the request with
       a 437 (Allocation Mismatch) error.

   3.  The server checks if the request contains a REQUESTED-TRANSPORT
       attribute.  If the REQUESTED-TRANSPORT attribute is not included
       or is malformed, the server rejects the request with a 400 (Bad
       Request) error.  Otherwise, if the attribute is included but
       specifies a protocol other that UDP, the server rejects the
       request with a 442 (Unsupported Transport Protocol) error.

   4.  The request may contain a DONT-FRAGMENT attribute.  If it does,
       but the server does not support sending UDP datagrams with the DF
       bit set to 1 (see Section 12), then the server treats the DONT-
       FRAGMENT attribute in the Allocate request as an unknown
       comprehension-required attribute.

   5.  The server checks if the request contains a RESERVATION-TOKEN
       attribute.  If yes, and the request also contains an EVEN-PORT
       attribute, then the server rejects the request with a 400 (Bad
       Request) error.  Otherwise, it checks to see if the token is
       valid (i.e., the token is in range and has not expired and the
       corresponding relayed transport address is still available).  If
       the token is not valid for some reason, the server rejects the
       request with a 508 (Insufficient Capacity) error.

   6.  The server checks if the request contains an EVEN-PORT attribute.
       If yes, then the server checks that it can satisfy the request
       (i.e., can allocate a relayed transport address as described
       below).  If the server cannot satisfy the request, then the
       server rejects the request with a 508 (Insufficient Capacity)
       error.

   7.  At any point, the server MAY choose to reject the request with a
       486 (Allocation Quota Reached) error if it feels the client is
       trying to exceed some locally defined allocation quota.  The
       server is free to define this allocation quota any way it wishes,
       but SHOULD define it based on the username used to authenticate
       the request, and not on the client's transport address.

   8.  Also at any point, the server MAY choose to reject the request
       with a 300 (Try Alternate) error if it wishes to redirect the
       client to a different server.  The use of this error code and
       attribute follow the specification in [RFC5389].

   If all the checks pass, the server creates the allocation.  The
   5-tuple is set to the 5-tuple from the Allocate request, while the
   list of permissions and the list of channels are initially empty.

   The server chooses a relayed transport address for the allocation as
   follows:

   o  If the request contains a RESERVATION-TOKEN, the server uses the
      previously reserved transport address corresponding to the
      included token (if it is still available).  Note that the
      reservation is a server-wide reservation and is not specific to a
      particular allocation, since the Allocate request containing the
      RESERVATION-TOKEN uses a different 5-tuple than the Allocate
      request that made the reservation.  The 5-tuple for the Allocate

      request containing the RESERVATION-TOKEN attribute can be any
      allowed 5-tuple; it can use a different client IP address and
      port, a different transport protocol, and even different server IP
      address and port (provided, of course, that the server IP address
      and port are ones on which the server is listening for TURN
      requests).

   o  If the request contains an EVEN-PORT attribute with the R bit set
      to 0, then the server allocates a relayed transport address with
      an even port number.

   o  If the request contains an EVEN-PORT attribute with the R bit set
      to 1, then the server looks for a pair of port numbers N and N+1
      on the same IP address, where N is even.  Port N is used in the
      current allocation, while the relayed transport address with port
      N+1 is assigned a token and reserved for a future allocation.  The
      server MUST hold this reservation for at least 30 seconds, and MAY
      choose to hold longer (e.g., until the allocation with port N
      expires).  The server then includes the token in a RESERVATION-
      TOKEN attribute in the success response.

   o  Otherwise, the server allocates any available relayed transport
      address.

   In all cases, the server SHOULD only allocate ports from the range
   49152 - 65535 (the Dynamic and/or Private Port range [Port-Numbers]),
   unless the TURN server application knows, through some means not
   specified here, that other applications running on the same host as
   the TURN server application will not be impacted by allocating ports
   outside this range.  This condition can often be satisfied by running
   the TURN server application on a dedicated machine and/or by
   arranging that any other applications on the machine allocate ports
   before the TURN server application starts.  In any case, the TURN
   server SHOULD NOT allocate ports in the range 0 - 1023 (the Well-
   Known Port range) to discourage clients from using TURN to run
   standard services.

      NOTE: The IETF is currently investigating the topic of randomized
      port assignments to avoid certain types of attacks (see
      [TSVWG-PORT]).  It is strongly recommended that a TURN implementor
      keep abreast of this topic and, if appropriate, implement a
      randomized port assignment algorithm.  This is especially
      applicable to servers that choose to pre-allocate a number of
      ports from the underlying OS and then later assign them to
      allocations; for example, a server may choose this technique to
      implement the EVEN-PORT attribute.

   The server determines the initial value of the time-to-expiry field
   as follows.  If the request contains a LIFETIME attribute, then the
   server computes the minimum of the client's proposed lifetime and the
   server's maximum allowed lifetime.  If this computed value is greater
   than the default lifetime, then the server uses the computed lifetime
   as the initial value of the time-to-expiry field.  Otherwise, the
   server uses the default lifetime.  It is RECOMMENDED that the server
   use a maximum allowed lifetime value of no more than 3600 seconds (1
   hour).  Servers that implement allocation quotas or charge users for
   allocations in some way may wish to use a smaller maximum allowed
   lifetime (perhaps as small as the default lifetime) to more quickly
   remove orphaned allocations (that is, allocations where the
   corresponding client has crashed or terminated or the client
   connection has been lost for some reason).  Also, note that the time-
   to-expiry is recomputed with each successful Refresh request, and
   thus the value computed here applies only until the first refresh.

   Once the allocation is created, the server replies with a success
   response.  The success response contains:

   o  An XOR-RELAYED-ADDRESS attribute containing the relayed transport
      address.

   o  A LIFETIME attribute containing the current value of the time-to-
      expiry timer.

   o  A RESERVATION-TOKEN attribute (if a second relayed transport
      address was reserved).

   o  An XOR-MAPPED-ADDRESS attribute containing the client's IP address
      and port (from the 5-tuple).

      NOTE: The XOR-MAPPED-ADDRESS attribute is included in the response
      as a convenience to the client.  TURN itself does not make use of
      this value, but clients running ICE can often need this value and
      can thus avoid having to do an extra Binding transaction with some
      STUN server to learn it.

   The response (either success or error) is sent back to the client on
   the 5-tuple.

      NOTE: When the Allocate request is sent over UDP, section 7.3.1 of
      [RFC5389] requires that the server handle the possible
      retransmissions of the request so that retransmissions do not
      cause multiple allocations to be created.  Implementations may
      achieve this using the so-called "stateless stack approach" as
      follows.  To detect retransmissions when the original request was
      successful in creating an allocation, the server can store the

      transaction id that created the request with the allocation data
      and compare it with incoming Allocate requests on the same
      5-tuple.  Once such a request is detected, the server can stop
      parsing the request and immediately generate a success response.
      When building this response, the value of the LIFETIME attribute
      can be taken from the time-to-expiry field in the allocate state
      data, even though this value may differ slightly from the LIFETIME
      value originally returned.  In addition, the server may need to
      store an indication of any reservation token returned in the
      original response, so that this may be returned in any
      retransmitted responses.

      For the case where the original request was unsuccessful in
      creating an allocation, the server may choose to do nothing
      special.  Note, however, that there is a rare case where the
      server rejects the original request but accepts the retransmitted
      request (because conditions have changed in the brief intervening
      time period).  If the client receives the first failure response,
      it will ignore the second (success) response and believe that an
      allocation was not created.  An allocation created in this matter
      will eventually timeout, since the client will not refresh it.
      Furthermore, if the client later retries with the same 5-tuple but
      different transaction id, it will receive a 437 (Allocation
      Mismatch), which will cause it to retry with a different 5-tuple.
      The server may use a smaller maximum lifetime value to minimize
      the lifetime of allocations "orphaned" in this manner.

6.3.  Receiving an Allocate Success Response

   If the client receives an Allocate success response, then it MUST
   check that the mapped address and the relayed transport address are
   in an address family that the client understands and is prepared to
   handle.  This specification only covers the case where these two
   addresses are IPv4 addresses.  If these two addresses are not in an
   address family which the client is prepared to handle, then the
   client MUST delete the allocation (Section 7) and MUST NOT attempt to
   create another allocation on that server until it believes the
   mismatch has been fixed.

      The IETF is currently considering mechanisms for transitioning
      between IPv4 and IPv6 that could result in a client originating an
      Allocate request over IPv6, but the request would arrive at the
      server over IPv4, or vice versa.

   Otherwise, the client creates its own copy of the allocation data
   structure to track what is happening on the server.  In particular,
   the client needs to remember the actual lifetime received back from
   the server, rather than the value sent to the server in the request.

   The client must also remember the 5-tuple used for the request and
   the username and password it used to authenticate the request to
   ensure that it reuses them for subsequent messages.  The client also
   needs to track the channels and permissions it establishes on the
   server.

   The client will probably wish to send the relayed transport address
   to peers (using some method not specified here) so the peers can
   communicate with it.  The client may also wish to use the server-
   reflexive address it receives in the XOR-MAPPED-ADDRESS attribute in
   its ICE processing.

6.4.  Receiving an Allocate Error Response

   If the client receives an Allocate error response, then the
   processing depends on the actual error code returned:

   o  (Request timed out): There is either a problem with the server, or
      a problem reaching the server with the chosen transport.  The
      client considers the current transaction as having failed but MAY
      choose to retry the Allocate request using a different transport
      (e.g., TCP instead of UDP).

   o  300 (Try Alternate): The server would like the client to use the
      server specified in the ALTERNATE-SERVER attribute instead.  The
      client considers the current transaction as having failed, but
      SHOULD try the Allocate request with the alternate server before
      trying any other servers (e.g., other servers discovered using the
      SRV procedures).  When trying the Allocate request with the
      alternate server, the client follows the ALTERNATE-SERVER
      procedures specified in [RFC5389].

   o  400 (Bad Request): The server believes the client's request is
      malformed for some reason.  The client considers the current
      transaction as having failed.  The client MAY notify the user or
      operator and SHOULD NOT retry the request with this server until
      it believes the problem has been fixed.

   o  401 (Unauthorized): If the client has followed the procedures of
      the long-term credential mechanism and still gets this error, then
      the server is not accepting the client's credentials.  In this
      case, the client considers the current transaction as having
      failed and SHOULD notify the user or operator.  The client SHOULD
      NOT send any further requests to this server until it believes the
      problem has been fixed.

   o  403 (Forbidden): The request is valid, but the server is refusing
      to perform it, likely due to administrative restrictions.  The
      client considers the current transaction as having failed.  The
      client MAY notify the user or operator and SHOULD NOT retry the
      same request with this server until it believes the problem has
      been fixed.

   o  420 (Unknown Attribute): If the client included a DONT-FRAGMENT
      attribute in the request and the server rejected the request with
      a 420 error code and listed the DONT-FRAGMENT attribute in the
      UNKNOWN-ATTRIBUTES attribute in the error response, then the
      client now knows that the server does not support the DONT-
      FRAGMENT attribute.  The client considers the current transaction
      as having failed but MAY choose to retry the Allocate request
      without the DONT-FRAGMENT attribute.

   o  437 (Allocation Mismatch): This indicates that the client has
      picked a 5-tuple that the server sees as already in use.  One way
      this could happen is if an intervening NAT assigned a mapped
      transport address that was used by another client that recently
      crashed.  The client considers the current transaction as having
      failed.  The client SHOULD pick another client transport address
      and retry the Allocate request (using a different transaction id).
      The client SHOULD try three different client transport addresses
      before giving up on this server.  Once the client gives up on the
      server, it SHOULD NOT try to create another allocation on the
      server for 2 minutes.

   o  438 (Stale Nonce): See the procedures for the long-term credential
      mechanism [RFC5389].

   o  441 (Wrong Credentials): The client should not receive this error
      in response to a Allocate request.  The client MAY notify the user
      or operator and SHOULD NOT retry the same request with this server
      until it believes the problem has been fixed.

   o  442 (Unsupported Transport Address): The client should not receive
      this error in response to a request for a UDP allocation.  The
      client MAY notify the user or operator and SHOULD NOT reattempt
      the request with this server until it believes the problem has
      been fixed.

   o  486 (Allocation Quota Reached): The server is currently unable to
      create any more allocations with this username.  The client
      considers the current transaction as having failed.  The client
      SHOULD wait at least 1 minute before trying to create any more
      allocations on the server.

   o  508 (Insufficient Capacity): The server has no more relayed
      transport addresses available, or has none with the requested
      properties, or the one that was reserved is no longer available.
      The client considers the current operation as having failed.  If
      the client is using either the EVEN-PORT or the RESERVATION-TOKEN
      attribute, then the client MAY choose to remove or modify this
      attribute and try again immediately.  Otherwise, the client SHOULD
      wait at least 1 minute before trying to create any more
      allocations on this server.

   An unknown error response MUST be handled as described in [RFC5389].

7.  Refreshing an Allocation

   A Refresh transaction can be used to either (a) refresh an existing
   allocation and update its time-to-expiry or (b) delete an existing
   allocation.

   If a client wishes to continue using an allocation, then the client
   MUST refresh it before it expires.  It is suggested that the client
   refresh the allocation roughly 1 minute before it expires.  If a
   client no longer wishes to use an allocation, then it SHOULD
   explicitly delete the allocation.  A client MAY refresh an allocation
   at any time for other reasons.

7.1.  Sending a Refresh Request

   If the client wishes to immediately delete an existing allocation, it
   includes a LIFETIME attribute with a value of 0.  All other forms of
   the request refresh the allocation.

   The Refresh transaction updates the time-to-expiry timer of an
   allocation.  If the client wishes the server to set the time-to-
   expiry timer to something other than the default lifetime, it
   includes a LIFETIME attribute with the requested value.  The server
   then computes a new time-to-expiry value in the same way as it does
   for an Allocate transaction, with the exception that a requested
   lifetime of 0 causes the server to immediately delete the allocation.

7.2.  Receiving a Refresh Request

   When the server receives a Refresh request, it processes as per
   Section 4 plus the specific rules mentioned here.

   The server computes a value called the "desired lifetime" as follows:
   if the request contains a LIFETIME attribute and the attribute value
   is 0, then the "desired lifetime" is 0.  Otherwise, if the request
   contains a LIFETIME attribute, then the server computes the minimum

   of the client's requested lifetime and the server's maximum allowed
   lifetime.  If this computed value is greater than the default
   lifetime, then the "desired lifetime" is the computed value.
   Otherwise, the "desired lifetime" is the default lifetime.

   Subsequent processing depends on the "desired lifetime" value:

   o  If the "desired lifetime" is 0, then the request succeeds and the
      allocation is deleted.

   o  If the "desired lifetime" is non-zero, then the request succeeds
      and the allocation's time-to-expiry is set to the "desired
      lifetime".

   If the request succeeds, then the server sends a success response
   containing:

   o  A LIFETIME attribute containing the current value of the time-to-
      expiry timer.

      NOTE: A server need not do anything special to implement
      idempotency of Refresh requests over UDP using the "stateless
      stack approach".  Retransmitted Refresh requests with a non-zero
      "desired lifetime" will simply refresh the allocation.  A
      retransmitted Refresh request with a zero "desired lifetime" will
      cause a 437 (Allocation Mismatch) response if the allocation has
      already been deleted, but the client will treat this as equivalent
      to a success response (see below).

7.3.  Receiving a Refresh Response

   If the client receives a success response to its Refresh request with
   a non-zero lifetime, it updates its copy of the allocation data
   structure with the time-to-expiry value contained in the response.

   If the client receives a 437 (Allocation Mismatch) error response to
   a request to delete the allocation, then the allocation no longer
   exists and it should consider its request as having effectively
   succeeded.

8.  Permissions

   For each allocation, the server keeps a list of zero or more
   permissions.  Each permission consists of an IP address and an
   associated time-to-expiry.  While a permission exists, all peers
   using the IP address in the permission are allowed to send data to

   the client.  The time-to-expiry is the number of seconds until the
   permission expires.  Within the context of an allocation, a
   permission is uniquely identified by its associated IP address.

   By sending either CreatePermission requests or ChannelBind requests,
   the client can cause the server to install or refresh a permission
   for a given IP address.  This causes one of two things to happen:

   o  If no permission for that IP address exists, then a permission is
      created with the given IP address and a time-to-expiry equal to
      Permission Lifetime.

   o  If a permission for that IP address already exists, then the time-
      to-expiry for that permission is reset to Permission Lifetime.

   The Permission Lifetime MUST be 300 seconds (= 5 minutes).

   Each permission's time-to-expiry decreases down once per second until
   it reaches 0; at which point, the permission expires and is deleted.

   CreatePermission and ChannelBind requests may be freely intermixed on
   a permission.  A given permission may be initially installed and/or
   refreshed with a CreatePermission request, and then later refreshed
   with a ChannelBind request, or vice versa.

   When a UDP datagram arrives at the relayed transport address for the
   allocation, the server extracts the source IP address from the IP
   header.  The server then compares this address with the IP address
   associated with each permission in the list of permissions for the
   allocation.  If no match is found, relaying is not permitted, and the
   server silently discards the UDP datagram.  If an exact match is
   found, then the permission check is considered to have succeeded and
   the server continues to process the UDP datagram as specified
   elsewhere (Section 10.3).  Note that only addresses are compared and
   port numbers are not considered.

   The permissions for one allocation are totally unrelated to the
   permissions for a different allocation.  If an allocation expires,
   all its permissions expire with it.

      NOTE: Though TURN permissions expire after 5 minutes, many NATs
      deployed at the time of publication expire their UDP bindings
      considerably faster.  Thus, an application using TURN will
      probably wish to send some sort of keep-alive traffic at a much
      faster rate.  Applications using ICE should follow the keep-alive
      guidelines of ICE [RFC5245], and applications not using ICE are
      advised to do something similar.

9.  CreatePermission

   TURN supports two ways for the client to install or refresh
   permissions on the server.  This section describes one way: the
   CreatePermission request.

   A CreatePermission request may be used in conjunction with either the
   Send mechanism in Section 10 or the Channel mechanism in Section 11.

9.1.  Forming a CreatePermission Request

   The client who wishes to install or refresh one or more permissions
   can send a CreatePermission request to the server.

   When forming a CreatePermission request, the client MUST include at
   least one XOR-PEER-ADDRESS attribute, and MAY include more than one
   such attribute.  The IP address portion of each XOR-PEER-ADDRESS
   attribute contains the IP address for which a permission should be
   installed or refreshed.  The port portion of each XOR-PEER-ADDRESS
   attribute will be ignored and can be any arbitrary value.  The
   various XOR-PEER-ADDRESS attributes can appear in any order.

9.2.  Receiving a CreatePermission Request

   When the server receives the CreatePermission request, it processes
   as per Section 4 plus the specific rules mentioned here.

   The message is checked for validity.  The CreatePermission request
   MUST contain at least one XOR-PEER-ADDRESS attribute and MAY contain
   multiple such attributes.  If no such attribute exists, or if any of
   these attributes are invalid, then a 400 (Bad Request) error is
   returned.  If the request is valid, but the server is unable to
   satisfy the request due to some capacity limit or similar, then a 508
   (Insufficient Capacity) error is returned.

   The server MAY impose restrictions on the IP address allowed in the
   XOR-PEER-ADDRESS attribute -- if a value is not allowed, the server
   rejects the request with a 403 (Forbidden) error.

   If the message is valid and the server is capable of carrying out the
   request, then the server installs or refreshes a permission for the
   IP address contained in each XOR-PEER-ADDRESS attribute as described
   in Section 8.  The port portion of each attribute is ignored and may
   be any arbitrary value.

   The server then responds with a CreatePermission success response.
   There are no mandatory attributes in the success response.

      NOTE: A server need not do anything special to implement
      idempotency of CreatePermission requests over UDP using the
      "stateless stack approach".  Retransmitted CreatePermission
      requests will simply refresh the permissions.

9.3.  Receiving a CreatePermission Response

   If the client receives a valid CreatePermission success response,
   then the client updates its data structures to indicate that the
   permissions have been installed or refreshed.