Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)
draft-ietf-behave-turn-10

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Abstract

If a host is located behind a NAT, then in certain situations it can be impossible for that host to communicate directly with other hosts (peers) located behind other NATs. In these situations, it is necessary for the host to use the services of an intermediate node that acts as a communication relay. This specification defines a protocol, called TURN (Traversal Using Relays around NAT), that allows the host to control the operation of the relay and to exchange packets with its peers using the relay.

The TURN protocol can be used in isolation, but is more properly used as part of the ICE (Interactive Connectivity Establishment) approach to NAT traversal.

Table of Contents

1.  Introduction
2.  Overview of Operation
    2.1.  Transports
    2.2.  Allocations
    2.3.  Permissions
    2.4.  Send Mechanism
    2.5.  Channels
    2.6.  Other Features
3.  Terminology
4.  General Behavior
5.  Allocations
6.  Creating an Allocation
    6.1.  Sending an Allocate Request
    6.2.  Receiving an Allocate Request
    6.3.  Receiving an Allocate Success Response
    6.4.  Receiving an Allocate Error Response
7.  Refreshing an Allocation
    7.1.  Sending a Refresh Request
    7.2.  Receiving a Refresh Request
    7.3.  Receiving a Refresh Response
8.  Permissions
9.  CreatePermission
    9.1.  Forming a CreatePermission request
    9.2.  Receiving a CreatePermission request
    9.3.  Receiving a CreatePermission response
10.  Send and Data Methods
    10.1.  Forming a Send Indication
    10.2.  Receiving a Send Indication
    10.3.  Receiving a UDP Datagram
    10.4.  Receiving a Data Indication
11.  Channels
    11.1.  Sending a ChannelBind Request
    11.2.  Receiving a ChannelBind Request
    11.3.  Receiving a ChannelBind Response
    11.4.  The ChannelData Message
    11.5.  Sending a ChannelData Message
    11.6.  Receiving a ChannelData Message
    11.7.  Relaying Data from the Peer
12.  IP Header Fields
13.  New STUN Methods
14.  New STUN Attributes
    14.1.  CHANNEL-NUMBER
    14.2.  LIFETIME
    14.3.  XOR-PEER-ADDRESS
    14.4.  DATA
    14.5.  XOR-RELAYED-ADDRESS
    14.6.  EVEN-PORT
    14.7.  REQUESTED-TRANSPORT
    14.8.  DONT-FRAGMENT
    14.9.  RESERVATION-TOKEN
15.  New STUN Error Response Codes
16.  Security Considerations
    16.1.  Outsider Attacks
        16.1.1.  Obtaining Unauthorized Allocations
        16.1.2.  Offline Dictionary Attacks
        16.1.3.  Faked Refreshes and Permissions
        16.1.4.  Fake Data
        16.1.5.  Impersonating a Server
        16.1.6.  Eavesdropping Traffic
    16.2.  Firewall Considerations
        16.2.1.  Faked Permissions
        16.2.2.  Blacklisted IP Addresses
        16.2.3.  Running Servers on Well-Known Ports
    16.3.  Insider Attacks
        16.3.1.  DoS Against TURN Server
        16.3.2.  Anonymous Relaying of Malicious Traffic
        16.3.3.  Manipulating other Allocations
    16.4.  Other Considerations
17.  IANA Considerations
18.  IAB Considerations
19.  Open Issues
20.  Changes from Previous Versions
    20.1.  Changes from -09 to -10
    20.2.  Changes from -08 to -09
    20.3.  Changes from -07 to -08
    20.4.  Changes from -06 to -07
    20.5.  Changes from -05 to -06
    20.6.  Changes from -04 to -05
21.  Acknowledgements
22.  References
    22.1.  Normative References
    22.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

A host behind a NAT may wish to exchange packets with other hosts, some of which may also be behind NATs. To do this, the hosts involved can use the ICE (Interactive Connectivity Exchange [I‑D.ietf‑mmusic‑ice] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” October 2007.)) protocol to discover a working communication path. In many cases, ICE will find a direct communication path between two hosts. However, if the NATs along the path have a mapping behavior [RFC4787] (Audet, F. and C. Jennings, “Network Address Translation (NAT) Behavioral Requirements for Unicast UDP,” January 2007.) of address or address and port dependent mapping, then it can be extremely difficult or impossible to find a direct path.

In these cases, it is necessary to use the services of an intermediate host that acts as a relay for the packets. This relay typically sits in the public Internet and relays packets between two hosts that both sit behind NATs.

This specification defines a protocol, called TURN, that allows a host behind a NAT (called the TURN client) to request that another host (called the TURN server) act as a relay. The client can arrange for the server to relay packets to certain other hosts (called peers) and can control aspects of how the relaying is done.

Though the use of a relay to enable communication between two hosts behind NATs is highly likely to work, it comes at a high cost to the provider of the relay, since the relay typically needs a high bandwidth connection to the Internet . As a consequence, it is best to use a relay only when a direct communication path cannot be found. When the client and a peer use the ICE protocol to determine a communication path, ICE will search for a direct path first and only use the relay (i.e. TURN server) when a direct path cannot be found.

TURN was originally invented to support multimedia sessions signaled using SIP. Since SIP supports forking, TURN supports multiple peers per client; a feature not supported by other approaches (e.g., SOCKS [RFC1928] (Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and L. Jones, “SOCKS Protocol Version 5,” March 1996.)). However, care has been taken to make sure that TURN is suitable for other types of applications.

TURN is an extension to the STUN (Session Traversal Utilities for NAT [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.)) protocol. Most, though not all, TURN messages are STUN-formatted messages. A reader of this document should be familiar with STUN.

2.  Overview of Operation

This section gives an overview of the operation of TURN. It is non-normative.

In a typical configuration, a TURN client is connected to a private network (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.) [RFC1918] and through one or more NATs to the public Internet. On the public Internet is a TURN server. Elsewhere in the Internet are one or more peers that the TURN client wishes to communicate with. These peers may or may not be behind one or more NATs. The client uses the server as a relay to send packets to these peers and to receive packets from these peers.

                                                          +---------+
                                                          |         |
                                                          |         |
                          TURN                          / |  Peer A |
    Client's              Server                      //  |         |
    Host Transport        Transport                  /    |         |
    Address               Address              +-+ //     +---------+
   10.1.1.2:17240       192.0.2.15:3478        |N|/  192.168.100.2:16400
            |               |                  |A|
            |   +-+         |                 /|T|
            |   | |         |                / +-+
            |   | |         |               /       192.0.2.210:18200
 +---------+|   | |         |+---------+   /              +---------+
 |         ||   |N|         ||         | //               |         |
 | TURN    |v   | |         v| TURN    |/                 |         |
 | Client  |----|A|----------| Server  |------------------|  Peer B |
 |         |    | |^         |         |^                ^|         |
 |         |    |T||         |         ||                ||         |
 +---------+    | ||         +---------+|                |+---------+
                | ||                    |                |
                | ||                    |                |
                +-+|                    |                |
                   |                    |                |
                   |                    |                |
             Client's                   |            Peer B
             Server-Reflexive    Relayed             Transport
             Transport Address   Transport Address   Address
             192.0.2.1:7000      192.0.2.15:9000     192.0.2.210:18200

Figure 1 shows a typical deployment. In this figure, the TURN client and the TURN server are separated by a NAT, with the client on the private side and the server on the public side of the NAT. This NAT is assumed to be a “bad” NAT; for example, it might have a mapping property of address-and-port-dependent mapping (see [RFC4787] (Audet, F. and C. Jennings, “Network Address Translation (NAT) Behavioral Requirements for Unicast UDP,” January 2007.) for a description of what this means).

The client talks to the server from a (IP address, port) combination called the client's HOST TRANSPORT ADDRESS. (The combination of an IP address and port is called a TRANSPORT ADDRESS).

The client sends TURN messages from its host transport address to a transport address on the TURN server which is known as the TURN SERVER TRANSPORT ADDRESS. The client learns the server’s transport address through some unspecified means (e.g., configuration), and this address is typically used by many clients simultaneously.

Since the client is behind a NAT, the server sees packets from the client as coming from a transport address on the NAT itself. This address is known as the client’s SERVER-REFLEXIVE transport address; packets sent by the server to the client’s server-reflexive transport address will be forwarded by the NAT to the client’s host transport address.

The client uses TURN commands to create and manipulate an ALLOCATION on the server. An allocation is a data structure on the server, an important component of which is a RELAYED TRANSPORT ADDRESS. The relayed transport address for the allocation is a transport address on the server which is used to send and receive packets to the peers.

Once an allocation is created, the client can send application data to the server along with an indication of which peer the data is to be sent to, and the server will relay this data to the appropriate peer. The client sends the application data to the server inside a TURN message; at the server, the data is extracted from the TURN message and sent to the peer in a UDP datagram. In the reverse direction, a peer can send application data in a UDP datagram to the relayed transport address for the allocation; the server will then encapsulate this data inside a TURN message and send it to the client along with an indication of which peer sent the data. Since the TURN message always contains an indication of which peer the client is communicating with, the client can use a single allocation to communicate with multiple peers.

Each allocation on the server belongs to a single client and has exactly one relayed transport address which is used only by that allocation. Thus when a packet arrives at a relayed transport address on the server, the server knows which client the data is intended for. However, the client may have multiple allocations on a server at the same time.

2.1.  Transports

TURN as defined in this specification always uses UDP between the server and the peer. However, this specification allows the use of any one of UDP, TCP, or TLS over TCP to carry the TURN messages between the client and the server.

If TCP or TLS over TCP is used between the client and the server, then the server will convert between these transports and UDP transport when relaying data to/from the peer.

TURN supports TCP transport between the client and the server because some firewalls are configured to block UDP entirely. These firewalls block UDP but not TCP in part because TCP has properties that make the intention of the nodes being protected by the firewall more obvious to the firewall. For example, TCP has a three-way handshake that makes in clearer that the protected node really wishes to have that particular connection established, while for UDP the best the firewall can do is guess which flows are desired by using filtering rules. Also, TCP has explicit connection teardown, while for UDP the firewall has to use timers to guess when the flow is finished.

TURN supports TLS over TCP transport between the client and the server because TLS provides additional security properties not provided by TURN's default digest authentication; properties which some clients may wish to take advantage of. In particular, TLS provides a way for the client to ascertain that it is talking to the server that it intended to, and also provides for confidentiality of TURN control messages. TURN does not require TLS because the overhead of using TLS is higher than that of digest authentication; for example, using TLS likely means that most application data will be doubly encrypted (once by TLS and once to ensure it is still encrypted in the UDP datagram).

There is a planned extension to TURN to add support for TCP between the server and the peers [I‑D.ietf‑behave‑turn‑tcp] (Perreault, S. and J. Rosenberg, “Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations,” March 2010.). For this reason, allocations that use UDP between the server and the peers are known as UDP allocations, while allocations that use TCP between the server and the peers are known as TCP allocations. This specification describes only UDP allocations.

TURN as defined in this specification only supports IPv4. All IP addresses in this specification must be IPv4 addresses. However, there is a planned extension to TURN to add support for IPv6 and for relaying between IPv4 and IPv6 [I‑D.ietf‑behave‑turn‑ipv6] (Camarillo, G., Novo, O., and S. Perreault, “Traversal Using Relays around NAT (TURN) Extension for IPv6,” March 2010.).

In some applications for TURN, the client may send and received packets other than TURN packets on the host transport address it uses to communicate with the server. This can happen, for example, when using TURN with ICE. In these cases, the client can distinguish TURN packets from other packets by examining the source address of the arriving packet: those arriving from the TURN server will be TURN packets.

2.2.  Allocations

To create an allocation on the server, the client uses an Allocate transaction. The client sends a Allocate request to the server, and the server replies with an Allocate success response containing the allocated relayed transport address. The client can include attributes in the Allocate request that describe the type of allocation it desires (e.g., the lifetime of the allocation). Since relaying data may require lots of bandwidth, the server typically requires that the client authenticate itself using STUN’s long-term credential mechanism, to show that it is authorized to use the server.

Once a relayed transport address is allocated, a client must keep the allocation alive. To do this, the client periodically sends a Refresh request to the server. TURN deliberately uses a different method (Refresh rather than Allocate) for refreshes to ensure that the client is informed if the allocation vanishes for some reason.

The frequency of the Refresh transaction is determined by the lifetime of the allocation. The client can request a lifetime in the Allocate request and may modify its request in a Refresh request, and the server always indicates the actual lifetime in the response. The client must issue a new Refresh transaction within 'lifetime' seconds of the previous Allocate or Refresh transaction. Once a client no longer wishes to use an Allocation, it should delete the allocation using a Refresh request with a requested lifetime of 0.

Both the server and client keep track of a value known as the 5-TUPLE. At the client, the 5-tuple consists of the client's host transport address, the server transport address, and the transport protocol used by the client to communicate with the server. At the server, the 5-tuple value is the same except that the client's host transport address is replaced by the client's server-reflexive address, since that is the client's address as seen by the server.

Both the client and the server remember the 5-tuple used in the Allocate request. Subsequent messages between the client and the server uses the same 5-tuple. In this way, the client and server know which allocation is being referred to. If the client wishes to allocate a second relayed transport address, it must create a second allocation using a different 5-tuple (e.g., by using a different client host address or port).

NOTE: While the terminology used in this document refers to 5-tuples, the TURN server can store whatever identifier it likes that yields identical results. Specifically, an implementation may use a file-descriptor in place of a 5-tuple to represent a TCP connection

TURN                                 TURN           Peer          Peer
client                               server          A             B
  |-- Allocate request --------------->|             |             |
  |                                    |             |             |
  |<--------------- Allocate failure --|             |             |
  |                 (401 Unauthorized) |             |             |
  |                                    |             |             |
  |-- Allocate request --------------->|             |             |
  |                                    |             |             |
  |<---------- Allocate success resp --|             |             |
  |            (192.0.2.15:9000)       |             |             |
  //                                   //            //            //
  |                                    |             |             |
  |-- Refresh request ---------------->|             |             |
  |                                    |             |             |
  |<----------- Refresh success resp --|             |             |
  |                                    |             |             |

In Figure 2, the client sends an Allocate request to the server without credentials. Since the server requires that all requests be authenticated using STUN's long-term credential mechanism, the server rejects the request with a 401 (Unauthorized) error code. The client then tries again, this time including credentials (not shown). This time, the server accepts the Allocate request and returns an Allocate success response containing (amongst other things) the relayed transport address assigned to the allocation. Sometime later the client decides to refresh the allocation and thus sends a Refresh request to the server. The refresh is accepted and the server replies with a Refresh success response.

2.3.  Permissions

To ease concerns amongst enterprise IT administrators that TURN could be used to bypass corporate firewall security, TURN includes the notion of permissions. TURN permissions mimic the address-restricted filtering mechanism of NATs that comply with [RFC4787] (Audet, F. and C. Jennings, “Network Address Translation (NAT) Behavioral Requirements for Unicast UDP,” January 2007.).

An allocation can have zero or more permissions. Each permission consists of an IP address and a lifetime. When the server receives a UDP datagram on the allocation's relayed transport address, it first checks the list of permissions. If the source IP address of the datagram matches a permission, the application data is relayed to the client, otherwise the UDP datagram is silently discarded.

A permission expires after 5 minutes if it is not refreshed. There is no way to explicitly delete a permission.

The client can install or refresh a permission using either a CreatePermission request or a ChannelBind request. For security reasons, permissions can only be installed or refreshed by transactions that can be authenticated; thus Send indications and ChannelData messages (which are used to send data to peers) do not install or refresh any permissions.

Note that permissions are within the context of an allocation, so adding or expiring a permission in one allocation does not affect other allocations.

2.4.  Send Mechanism

There are two mechanisms for the client and peers to exchange application data using the TURN server. The first mechanism uses the Send and Data methods, the second way uses channels. Common to both ways is the ability of the client to communicate with multiple peers using a single allocated relayed transport address; thus both ways include a means for the client to indicate to the server which peer to forward the data to, and for the server to indicate which peer sent the data.

The Send mechanism uses Send and Data indications. Send indications are used to send application data from the client to the server, while Data indications are used to send application data from the server to the client.

When using the Send mechanism, the client sends a Send indication to the TURN server containing (a) an XOR-PEER-ADDRESS attribute specify the transport address of the peer and (b) a DATA attribute holding the application data. When the TURN server receives the Send indication, it extracts the application data from the DATA attribute and sends it in a UDP datagram to the peer, using the allocated relay address as the source address. Note that there is no need to specify the relayed transport address, since it is implied by the 5-tuple used for the Send indication.

In the reverse direction, UDP datagrams arriving at the relayed transport address on the TURN server are converted into Data indications and sent to the client, with the transport address of the peer included in an XOR-PEER-ADDRESS attribute and the data itself in a DATA attribute. Since the relayed transport address uniquely identified the allocation, the server knows which client to relay the data to.

TURN                                 TURN           Peer          Peer
client                               server          A             B
  |                                    |             |             |
  |-- CreatePermission req (Peer A) -->|             |             |
  |<-- CreatePermission success resp --|             |             |
  |                                    |             |             |
  |--- Send ind (Peer A)-------------->|             |             |
  |                                    |=== data ===>|             |
  |                                    |             |             |
  |                                    |<== data ====|             |
  |<-------------- Data ind (Peer A) --|             |             |
  |                                    |             |             |
  |                                    |             |             |
  |--- Send ind (Peer B)-------------->|             |             |
  |                                    |=== data =================>|
  |                                    |             |             |
  |                                    |<== data ==================|
  |                            dropped |             |             |
  |                                    |             |             |

In Figure 3, the client has already created an allocation and now wishes to send data to its peers. The client first creates a permission by sending the server a CreatePermission request specifying peer A's IP address in the XOR-PEER-ADDRESS attribute; this allows peer A to send data to the client. The client then sends data to Peer A using a Send indication; at the server, the application data is extracted and forwarded in a UDP datagram to Peer A, using the relayed transport address as the source transport address. When a UDP datagram from Peer A is received at the relayed transport address, the contents are placed into a Data indication and forwarded to the client. Finally, the client then uses a Send indication to send a second data packet to Peer B; here the client has not installed a permission for peer B, so the UDP datagram from peer B is silently discarded by the server.

2.5.  Channels

For some applications (e.g. Voice over IP), the 36 bytes of overhead that a Send indication or Data indication adds to the application data can substantially increase the bandwidth required between the client and the server. To remedy this, TURN offers a second way for the client and server to associate data with a specific peer.

This second way uses an alternate packet format known as the ChannelData message. The ChannelData message does not use the STUN header used by other TURN messages, but instead has a 4-byte header that includes a number known as a channel number. Each channel number in use is bound to a specific peer and thus serves as a shorthand for the peer's host transport address.

To bind a channel to a peer, the client sends a ChannelBind request to the server, and includes an unbound channel number and the transport address of the peer. Once the channel is bound, the client can use a ChannelData message to send the server data destined for the peer. Similarly, the server can relay data from that peer towards the client using a ChannelData message.

Channel bindings last for 10 minutes unless refreshed. Channel bindings are refreshed by sending another ChannelBind request rebinding the channel to the peer. Like permissions (but unlike allocations), there is no way to explicitly delete a channel binding; the client must simply wait for it to time out.

TURN                                 TURN           Peer          Peer
client                               server          A             B
  |                                    |             |             |
  |-- ChannelBind req ---------------->|             |             |
  | (Peer A to 0x4001)                 |             |             |
  |                                    |             |             |
  |<---------- ChannelBind succ resp --|             |             |
  |                                    |             |             |
  |-- [0x4001] data ------------------>|             |             |
  |                                    |=== data ===>|             |
  |                                    |             |             |
  |                                    |<== data ====|             |
  |<------------------ [0x4001] data --|             |             |
  |                                    |             |             |
  |--- Send ind (Peer A)-------------->|             |             |
  |                                    |=== data ===>|             |
  |                                    |             |             |
  |                                    |<== data ====|             |
  |<------------------ [0x4001] data --|             |             |
  |                                    |             |             |
  |-- Send ind (Peer B)--------------->|             |             |
  |                                    |=== data =================>|
  |                                    |             |             |

Figure 4 shows the channel mechanism in use. The client has already created an allocation and now wishes to bind a channel to peer A. To do this, the client sends a ChannelBind request to the server, specifying the transport address of Peer A and a channel number (0x4001). After that, the client can send application data encapsulated inside ChannelData messages to Peer A: this is shown as "[0x4001] data" where 0x4001 is the channel number. When the ChannelData message arrives at the server, the server transfers the data to a UDP datagram and sends it to the peer A, as indicated by the channel number. When peer A sends a UDP datagram to the relayed transport address, the data is placed inside a ChannelData message and sent to the client.

Once a channel has been bound, the client is free to intermix ChannelData messages and Send indications. In the figure, the client later decides to use a Send indication rather than a ChannelData message to send additional data to peer A. The client might decide to do this, for example, so it can use the DONT-FRAGMENT attribute (see the next section). However, once a channel is bound, the server will always use a ChannelData message, as shown in the call flow.

Note that ChannelData messages can only be used for peers to which the client has bound a channel. In the example above, Peer A has been bound to a channel, but Peer B has not, so application data to and from Peer B uses the Send mechanism.

2.6.  Other Features

This section describes a few other features of TURN.

Old versions of RTP [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) required that the RTP stream be on an even port number and the associated RTCP stream, if present, be on the next highest port. To allow clients to work with nodes that still require this,TURN allows the client to request that the server allocate a relayed-transport-address with an even port number, and to optionally request the server reserve the next-highest port number for a subsequent allocation.

If appropriate, a TURN server can reject an Allocate request with the suggestion that the client try an alternative server.

TURN is designed so that the server can be implemented as an application that runs in userland under commonly available operating systems and which does not requiring special privileges. This design decision has the following implications:

• There is no end-to-end QOS, since the value of the Diff-Serv field is not preserved across the server;
• There is no end-to-end TTL decrementing, since the value of the TTL field is reset by the server;
• There is no end-to-end Explicit Congestion Notification, since the ECN field is reset by the server;
• ICMP messages are not relayed by the server;
• Path MTU Discovery does not work, except in the limited way available using the DONT-FRAGMENT attribute (see below); and
• There is no end-to-end fragmentation, since the packet is re-assembled at the server.

Future work may specify alternate TURN semantics that address these limitations.

To provide a limited form of Path MTU discovery, TURN has a DONT-FRAGMENT attribute. The client may include this attribute in a Send indication to specify that the server set the DF (Don't Fragment) bit in the UDP datagram that it sends to the peer. Since some servers may be unable to set the DF bit, the client should also include this attribute in the Allocate request; servers that do not support this feature will reject the Allocate request.

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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

Readers are expected to be familiar with [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.) 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 [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.). 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.

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 (UDP or TCP)) 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.

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 [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.) apply to STUN-formatted messages. This means that all the message-forming and -processing descriptions in this document are implicitly prefixed with the rules of [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.).

In addition, the server SHOULD demand that all requests from the client be authenticated, using the Long-Term Credential mechanism described in [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.), and the client MUST be prepared to authenticate requests if required. Note that this authentication 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).

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.

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.

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 [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.).

By default, TURN runs on the same port as STUN. However, either the SRV procedures or the ALTERNATE-SERVER procedures described in Section 6 (Creating an Allocation) may be used to run TURN on a different port.

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.

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 [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.)). 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.

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 (Receiving an Allocate Request)), 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:

• the relayed transport address
• The 5-tuple: (client's IP address, client's port, server IP address, server port, transport protocol)
• the username and password
• the time-to-expiry
• A list of permissions
• 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, while 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 username and password of the allocation is the username and password of the authenticated Allocate request that creates the allocation. These are used both to verify subsequent requests and to compute the message integrity of responses.

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 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 (Permissions) and the list of channels is described in Section 11 (Channels).

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 (Transports) for more discussion.

The client must also pick a server transport address. Typically, this is done by the client learning (perhaps through configuration) one or more domain names for TURN servers. In this case, the client uses the DNS procedures described in [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.), 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 specify other protocols.

If the client wishes the server to initialize the time-to-expiry field of the allocation to some value other 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 SHOULD require that the request be authenticated using the Long-Term Credential mechanism of [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.).
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 contain a REQUESTED-TRANPORT 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 (IP Header Fields)), 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 EVEN-PORT attribute. If yes, then the server checks that it satisfy the request. If the server cannot satisfy the request, then the server rejects the request with a 508 (Insufficient Port Capacity) error.
6. The server checks if the request contains a RESERVATION-TOKEN attribute. If yes, and the request also contains a 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 Port 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 [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.), with the modification that a TURN server MAY return this error code and attribute in unauthenticated error responses as well as in authenticated error responses..

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:

• 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.
• 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.
• 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 is one that the server is listening for TURN requests on).
• 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] (, “IANA Port Numbers Registry,” .)), 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 [I‑D.ietf‑tsvwg‑port‑randomization] (Larsen, M. and F. Gont, “Transport Protocol Port Randomization Recommendations,” April 2010.)). It is 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, and the proposed lifetime value is greater than the default lifetime, and the proposed lifetime value is otherwise acceptable to the server, then the server uses that value. Otherwise, the server uses the default lifetime. It is RECOMMENDED that the server impose a maximum lifetime 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 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:

• A XOR-RELAYED-ADDRESS attribute containing the relayed transport address;
• A LIFETIME attribute containing the current value of the time-to-expiry timer;
• A RESERVATION-TOKEN attribute (if a second relayed transport address was reserved).
• 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: Implementations may implement the idempotency of the Allocate request over UDP 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 deal with. This specification only covers the case where these two addresses are IPv4 addresses. If these two addresses are not in an address family that the client is prepared to deal with, then the client MUST delete the allocation (Section 7 (Refreshing an Allocation)) 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 vica-versa. Hence the importance of this check.

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:

• (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).
• 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 [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.) with the following changes: the client SHOULD accept unauthenticated error responses containing the 300 (Try Alternate) error code, the client MUST ensure that the realm value received from the alternate server is as expected, and the client MUST use the same username and password as it would have with the original server. The latter checks protect against an attacker sending the client an unauthenticated Allocate error response that redirects the client to some totally different and unexpected server.
• 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.
• 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.
• 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.
• 437 (Allocation Mismatch): This indicates that the client has picked a 5-tuple which 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 which 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.
• 438 (Stale Nonce): See the procedures for the Long-Term Credential mechanism [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.).
• 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.
• 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.
• 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.
• 508 (Insufficient Port 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.

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 also 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 it as follows:

1. The server checks the credentials of the request as per the Long-Term Credential mechanism, checks that the allocation exists, and does the additional username check of Section 4 (General Behavior).
2. 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 and the attribute value is greater than the default lifetime, and if the attribute value is otherwise acceptable to the server, then the desired lifetime is the attribute value. Otherwise the desired lifetime is the default value.
3. Subsequent processing depends on the desired lifetime value:
   • If desired lifetime is 0, then the request succeeds and the allocation is deleted.
   • 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 server sends a success response containing:
   • 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 which uniquely identifies the permission, and an associated time-to-expiry. The IP address describes a set of peers that are allowed to send data to the client, and the time-to-expiry is the number of seconds until the permission expires.

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:

• 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 the default permission lifetime.
• If a permission for that IP address already exists, then the lifetime for that permission is reset to the default permission lifetime.

The default 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 installed or refreshed at one point in time with a CreatePermission request, and then refreshed with a ChannelBind request at a different point in time, or vica-versa.

When a UDP datagram arrives at the relayed transport address for the allocation, the server checks the list of permissions for that allocation. If there is a permission with an IP address that is equal to the source IP address of the UDP datagram, then the UDP datagram can be relayed to the client. Otherwise, the UDP datagram is silently discarded. Note that only IP addresses are compared; port numbers are irrelevant.

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 [I‑D.ietf‑mmusic‑ice] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” October 2007.), 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 mechanismSection 10 (Send and Data Methods) or the Channel mechanismSection 11 (Channels).

9.1.  Forming a CreatePermission request

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

When forming a CreatePermission request, the client MUST include a XOR-PEER-ADDRESS attribute. The IP address portion of the XOR-PEER-ADDRESS attribute contains the IP address for which a permission should be installed or refreshed. The port portion of the XOR-PEER-ADDRESS attribute will be ignored and can be any arbitrary value.

9.2.  Receiving a CreatePermission request

When the server receives the CreatePermission request, it processes it as follows.

The message is first checked for validity. The CreatePermission request MUST contain a XOR-PEER-ADDRESS attribute. If this attribute is missing or invalid, then the message is discarded and a 400 (Bad Request) error is returned.

If the message is valid, then the server installs or refreshes a permission for the IP address contained in the XOR-PEER-ADDRESS attribute as described in Section 8 (Permissions). The port portion of the 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 allocation.

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 permission has been installed or refreshed.

10.  Send and Data Methods

TURN supports two mechanisms for sending and receive data from peers. This section describes the use of the Send and Data mechanism, while Section 11 (Channels) describes the use of the Channel mechanism.

10.1.  Forming a Send Indication

The client can use a Send indication to pass data to the server for relaying to a peer. A client may use a Send indication even if a channel is bound to that peer.

When forming a Send indication, the client MUST include a XOR-PEER-ADDRESS attribute and a DATA attribute. The XOR-PEER-ADDRESS attribute contains the transport address of the peer to which the data is to be sent, and the DATA attribute contains the actual application data to be sent to the peer.

The client MAY include a DONT-FRAGMENT attribute in the Send indication if it wishes the server to set the DF bit on the UDP datagram sent to the peer.

10.2.  Receiving a Send Indication

When the server receives a Send indication, it processes it as follows.

The message is first checked for validity. The Send indication MUST contain both a XOR-PEER-ADDRESS attribute and a DATA attribute. If one of these attributes is missing or invalid, then the message is discarded.

The Send indication may also contain the DONT-FRAGMENT attribute. If the server is unable to set the DF bit on outgoing UDP datagrams when this attribute is present, then the server acts as if the DONT-FRAGMENT attribute is an unknown comprehension-required attribute (and thus the Send indication is silently ignored).

If the message is valid, then the server forms a UDP datagram as follows:

• the source transport address is the relayed transport address of the allocation, where the allocation is determined by the 5-tuple on which the Send indication arrived;
• the destination transport address is taken from the XOR-PEER-ADDRESS attribute;
• the data following the UDP header is the contents of the value field of the DATA attribute.

The handling of the DONT-FRAGMENT attribute (if present), is described in Section 12 (IP Header Fields).

The resulting UDP datagram is then sent to the peer.

Send indications do not cause a permission to be installed or refresh; thus it is possible for the client to send data to a peer without installing a permission.

10.3.  Receiving a UDP Datagram

When the server receives a UDP datagram at a currently allocated relayed transport address, the server looks up the allocation associated with the relayed transport address. It then checks to see if relaying is permitted, as described in Section 8 (Permissions).

If relaying is permitted, then the server checks if there is a channel bound to the peer that sent the UDP datagram (see Section 11 (Channels)). If a channel is bound, then processing proceeds as described in Section 11.7 (Relaying Data from the Peer).

If relaying is permitted but no channel is bound to the peer, then the server forms and sends a Data indication. The Data indication MUST contain both a XOR-PEER-ADDRESS and a DATA attribute. The DATA attribute is set to the value of the ‘data octets’ field from the datagram, and the XOR-PEER-ADDRESS attribute is set to the source transport address of the received UDP datagram. The Data indication is then sent on the 5-tuple associated with the allocation.

10.4.  Receiving a Data Indication

When the client receives a Data indication, it checks that the Data indication contains both a XOR-PEER-ADDRESS and a DATA attribute, and discards the indication if it does not. The client SHOULD also check that the XOR-PEER-ADDRESS attribute value contains an IP address with which the client believes there is an active permission, and discard the Data indication otherwise.

NOTE: The latter check protects the client against an attacker who somehow manages to trick the server into installing permissions not desired by the client.

If the Data indication passes the above checks, the client delivers the data octets inside the DATA attribute to the application, along with an indication that they were received from the peer whose transport address is given by the XOR-PEER-ADDRESS attribute.

11.  Channels

Channels provide a way for the client and server to send application data using ChannelData messages, which have less overhead than Send and Data indications.

Channel bindings are always initiated by the client. The client can bind a channel to a peer at any time during the lifetime of the allocation. The client may bind a channel to a peer before exchanging data with it, or after exchanging data with it (using Send and Data indications) for some time, or may choose never to bind a channel it. The client can also bind channels to some peers while not binding channels to other peers.

Channel bindings are specific to an allocation, so that a binding in one allocation has no relationship to a binding in any other allocation. If an allocation expires, all its channel bindings expire with it.

A channel binding consists of:

• A channel number;
• A transport address (of the peer);
• A time-to-expiry timer.

Within the context of an allocation, a channel binding is uniquely identified either by the channel number or by the peer's transport address. Thus the same channel cannot be bound to two different transport addresses, nor can the same transport address be bound to two different channels.

A channel binding lasts for 10 minutes unless refreshed. Refreshing the binding (by the server receiving a ChannelBind request rebinding the channel to the same peer) resets the time-to-expiry timer back to 10 minutes.

When the channel binding expires, the channel becomes unbound. Once unbound, the channel number can be bound to a different transport address, and the transport address can be bound to a different channel number. To prevent race conditions, the client MUST wait 5 minutes after the channel binding expires before attempting to bind the channel number to a different transport address or the transport address to a different channel number.

When binding a channel to a peer, the client SHOULD be prepared to receive ChannelData messages on the channel from the server as soon as it has sent the ChannelBind request. Over UDP, it is possible for the client to receive ChannelData messages from the server before it receives a ChannelBind success response.

In the other direction, the client MAY elect to send ChannelData messages before receiving the ChannelBind success response. Doing so, however, runs the risk of having the ChannelData messages dropped by the server if the ChannelBind request does not succeed for some reason (e.g., packet lost if the request is sent over UDP, or the server being unable to fulfill the request). A client that wishes to be safe should either queue the data, or use Send indications until the channel binding is confirmed.

11.1.  Sending a ChannelBind Request

A channel binding is created or refreshed using a ChannelBind transaction. A ChannelBind transaction also creates or refreshes a permission towards the peer.

To initiate the ChannelBind transaction, the client forms a ChannelBind request. The channel to be bound is specified in a CHANNEL-NUMBER attribute, and the peer's transport address is specified in a XOR-PEER-ADDRESS attribute. Section 11.2 (Receiving a ChannelBind Request) describes the restrictions on these attributes.

Rebinding a channel to the same transport address that it is already bound to provides a way to refresh a channel binding and the corresponding permission without sending data to the peer. Note however, that permissions need to be refreshed more frequently than channels.

11.2.  Receiving a ChannelBind Request

When the server receives a ChannelBind request, it checks the following:

• The request contains both a CHANNEL-NUMBER and a XOR-PEER-ADDRESS attribute;
• The channel number is in the range 0x4000 to 0xFFFE (inclusive);
• The channel number is not currently bound to a different transport address (same transport address is OK);
• The transport address is not currently bound to a different channel number.

If any of these tests fail, the server replies with an error response with error code 400 "Bad Request". Otherwise, the ChannelBind request is valid and the server replies with a ChannelBind success response. There are no required attributes in a successful ChannelBind response.

If ChannelBind request is valid, then the server creates or refreshes the channel binding using the channel number in the CHANNEL-ADDRESS attribute and the transport address in the XOR-PEER-ADDRESS attribute. The server also installs or refreshes a permission for the IP address in the XOR-PEER-ADDRESS attribute as described in Section 8 (Permissions).

NOTE: A server need not do anything special to implement idempotency of ChannelBind requests over UDP using the "stateless stack approach". Retransmitted ChannelBind requests will simply refresh the channel binding and the corresponding permission. Furthermore, the client must wait 5 minutes before binding a previously bound channel number or peer address to a different channel, eliminating the possibility that the transaction would initially fail but succeed on a retransmission.

11.3.  Receiving a ChannelBind Response

When the client receives a ChannelBind success response, it updates its data structures to record that the channel binding is now active. It also updates its data structures to record that the corresponding permission has been installed or refreshed.

If the client receives a ChannelBind failure response that indicates that the channel information is out-of-sync between the client and the server (e.g., an unexpected 400 "Bad Request" response), then it is RECOMMENDED that the client immediately delete the allocation and start afresh with a new allocation.

11.4.  The ChannelData Message

The ChannelData message is used to carry application data between the client and the server. It has the following format:

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Channel Number        |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
/                       Application Data                        /
/                                                               /
|                                                               |
|                               +-------------------------------+
|                               |
+-------------------------------+

The Channel Number field specifies the number of the channel on which the data is traveling, and thus the address of the peer that is sending or is to receive the data. The channel number MUST be in the range 0x4000 – 0xFFFF, with channel number 0xFFFF being reserved for possible future extensions.

Channel numbers 0x0000 – 0x3FFF cannot be used because bits 0 and 1 are used to distinguish ChannelData messages from STUN-formatted messages (i.e., Allocate, Send, Data, ChannelBind, etc). STUN-formatted messages always have bits 0 and 1 as “00”, while ChannelData messages use combinations “01”, “10”, and “11”.

The Length field specifies the length in bytes of the application data field (i.e., it does not include the size of the ChannelData header). Note that 0 is a valid length.

The Application Data field carries the data the client is trying to send to the peer, or that the peer is sending to the client.

11.5.  Sending a ChannelData Message

Once a client has bound a channel to a peer, then when the client has data to send to that peer it may use either a ChannelData message or a Send indication; that is, the client is not obligated to use the channel when it exists and may freely intermix the two message types when sending data to the peer. The server, on the other hand, MUST use the ChannelData message if a channel has been bound to the peer.

The fields of the ChannelData message are filled in as described in Section 11.4 (The ChannelData Message).

Over stream transports, the ChannelData message MUST be padded to a multiple of four bytes in order to ensure the alignment of subsequent messages. The padding is not reflected in the length field of the ChannelData message, so the actual size of a ChannelData message (including padding) is (4 + Length) rounded up to the nearest multiple of 4. Over UDP, the padding is not required but MAY be included.

The ChannelData message is then sent on the 5-tuple associated with the allocation.

11.6.  Receiving a ChannelData Message

The receiver of the ChannelData message uses bits 0 and 1 to distinguish it from STUN-formatted messages, as described in Section 11.4 (The ChannelData Message).

If the ChannelData message is received in a UDP datagram, and if the UDP datagram is too short to contain the claimed length of the ChannelData message (i.e., the UDP header length field value is less than the ChannelData header length field value + 4 + 8), then the message is silently discarded.

If the ChannelData message is received over TCP or over TLS over TCP, then the actual length of the ChannelData message is as described in Section 11.5 (Sending a ChannelData Message).

If the ChannelData message is received on a channel which is not bound to any peer, then the message is silently discarded.

On the client, it is RECOMMENDED that the client discard the ChannelData message if the client believes there is no active permission towards the peer.

On the server, if no errors are detected, the server relays the application data to the peer by forming a UDP datagram as follows:

• the source transport address is the relayed transport address of the allocation, where the allocation is determined by the 5-tuple on which the ChannelData message arrived;
• the destination transport address is the transport address to which the channel is bound;
• the data following the UDP header is the contents of the data field of the ChannelData message.

The resulting UDP datagram is then sent to the peer. Note that if the Length field in the ChannelData message is 0, then there will be no data in the UDP datagram, but the UDP datagram is still formed and sent.

11.7.  Relaying Data from the Peer

When the server receives a UDP datagram on the relayed transport address associated with an allocation, the server processes it as described in Section 10.3 (Receiving a UDP Datagram). If that section indicates that a ChannelData message should be sent (because there is a channel bound to the peer that sent to UDP datagram), then the server forms and sends a ChannelData message as described in Section 11.5 (Sending a ChannelData Message).

12.  IP Header Fields

This section describes how the server sets various fields in the IP header when relaying between the client and the peer or vica-versa. The descriptions in this section apply: (a) when the server sends a UDP datagram to the peer, or (b) when the server sends a Data indication or ChannelData message to the client over UDP transport. The descriptions in this section do not apply to TURN messages sent over TCP or TLS transport from the server to the client.

Time to Live (TTL) field

Set the outgoing value to the default for outgoing packets.

Diff-Serv Code Point (DSCP) field ] (Nichols, K., Blake, S., Baker, F., and D. Black, “Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers,” December 1998.) [RFC2474]

Set the outgoing value to a fixed value, which by default is Best Effort unless configured otherwise.

Explicit Congestion Notification (ECN) field [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.)

Set the outgoing value to 00 (ECN not supported).

IPv4 Fragmentation fields

When the server sends a packet to a peer in response to a Send indication containing the DONT-FRAGMENT attribute, then the server MUST set the DF bit in the outgoing IP header to 1. In all other cases (e.g., Data indication, or DONT-FRAGMENT attribute not included in the Send indication), the server SHOULD set the DF bit in the outgoing packet to 0.

The server sets the other fragmentation fields (Identification, MF, Fragment Offset) as appropriate for a packet originating from the server.

IPv4 Options

The outgoing packet is sent without any IPv4 options.

13.  New STUN Methods

This section lists the codepoints for the new STUN methods defined in this specification. See elsewhere in this document for the semantics of these new methods.

  0x003  :  Allocate          (only request/response semantics defined)
  0x004  :  Refresh           (only request/response semantics defined)
  0x006  :  Send              (only indication semantics defined)
  0x007  :  Data              (only indication semantics defined)
  0x008  :  CreatePermission  (only request/response semantics defined
  0x009  :  ChannelBind       (only request/response semantics defined)

14.  New STUN Attributes

This STUN extension defines the following new attributes:

  0x000C: CHANNEL-NUMBER
  0x000D: LIFETIME
  0x0010: Reserved (was BANDWIDTH)
  0x0012: XOR-PEER-ADDRESS
  0x0013: DATA
  0x0016: XOR-RELAYED-ADDRESS
  0x0018: EVEN-PORT
  0x0019: REQUESTED-TRANSPORT
  0x001a: DONT-FRAGMENT
  0x0021: Reserved (was TIMER-VAL)
  0x0022: RESERVATION-TOKEN

14.1.  CHANNEL-NUMBER

The CHANNEL-NUMBER attribute contains the number of the channel. It is a 16-bit unsigned integer, followed by a two-octet RFFU (Reserved For Future Use) field which MUST be set to 0 on transmission and MUST be ignored on reception.

   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |        Channel Number         |         RFFU = 0              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

14.2.  LIFETIME

The LIFETIME attribute represents the duration for which the server will maintain an allocation in the absence of a refresh. It is a 32-bit unsigned integral value representing the number of seconds remaining until expiration.

14.3.  XOR-PEER-ADDRESS

The XOR-PEER-ADDRESS specifies the address and port of the peer as seen from the TURN server. It is encoded in the same way as XOR-MAPPED-ADDRESS.

14.4.  DATA

The DATA attribute is present in all Send and Data indications. The contents of DATA attribute is the application data (that is, the data that would immediately follow the UDP header if the data was been sent directly between the client and the peer).

14.5.  XOR-RELAYED-ADDRESS

The XOR-RELAYED-ADDRESS is present in Allocate responses. It specifies the address and port that the server allocated to the client. It is encoded in the same way as XOR-MAPPED-ADDRESS.

14.6.  EVEN-PORT

This attribute allows the client to request that the port in the relayed-transport-address be even, and (optionally) that the server reserve the next-highest port number. The attribute is 8 bits long. Its format is:

   0
   0 1 2 3 4 5 6 7
  +-+-+-+-+-+-+-+-+
  |R|             |
  +-+-+-+-+-+-+-+-+

The attribute contains a single 1-bit flag:

R:

If 1, the server is requested to reserve the next highest port number (on the same IP address) for a subsequent allocation. If 0, no such reservation is requested.

The other 7 bits of the attribute are ignored.

14.7.  REQUESTED-TRANSPORT

This attribute is used by the client to request a specific transport protocol for the allocated transport address. It has the following format:

   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |    Protocol   |                    RFFU                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Protocol field specifies the desired protocol. The codepoints used in this field are taken from those allowed in the Protocol field in the IPv4 header and the NextHeader field in the IPv6 header [Protocol‑Numbers] (, “IANA Protocol Numbers Registry,” 2005.). This specification only allows the use of codepoint 17 (User Datagram Protocol).

The RFFU field MUST be set to zero on transmission and MUST be ignored on reception. It is reserved for future uses.

14.8.  DONT-FRAGMENT

This attribute is used by the client to request that the server set the DF (Don't Fragment) bit in the IP header when relaying the application data onward to the peer. This attribute has no value part and thus the attribute length field is 0.

14.9.  RESERVATION-TOKEN

The RESERVATION-TOKEN attribute contains a token that uniquely identifies a relayed transport address being held in reserve by the server. The server includes this attribute in a success response to tell the client about the token, and the client includes this attribute in a subsequent Allocate request to request the server use that relayed transport address for the allocation.

The attribute value is a 64-bit-long field containing the token value.

15.  New STUN Error Response Codes

This document defines the following new error response codes:

437

(Allocation Mismatch): A request was received by the server that requires an allocation to be in place, but there is none, or a request was received which requires no allocation, but there is one.

441

(Wrong Credentials): The credentials in the (non-Allocate) request, though otherwise acceptable to the server, do not match those used to create the allocation.

442

(Unsupported Transport Protocol): The Allocate request asked the server to use a transport protocol between the server and the peer that the server does not support. NOTE: This does NOT refer to the transport protocol used in the 5-tuple.

486

(Allocation Quota Reached): No more allocations using this username can be created at the present time.

508

(Insufficient Port Capacity): The server has no more relayed transport addresses available right now, or has none with the requested properties, or the one that corresponds to the specified token is not available.

16.  Security Considerations

This section considers attacks that are possible in a TURN deployment, and discusses how they are mitigated by mechanisms in the protocol or recommended practices in the implementation.

16.1.  Outsider Attacks

Outsider attacks are ones where the attacker has no credentials in the system, and is attempting to disrupt the service seen by the client or the server.

16.1.1.  Obtaining Unauthorized Allocations

An attacker might wish to obtain allocations on a TURN server for any number of nefarious purposes. A TURN server provides a mechanism for sending and receiving packets while cloaking the actual IP address of the client. This makes TURN servers an attractive target for attackers who wish to use it to mask their true identity.

An attacker might also wish to simply utilize the services of a TURN server without paying for them. Since TURN services require resources from the provider, it is anticipated that their usage will come with a cost.

These attacks are prevented using the digest authentication mechanism which allows the TURN server to determine the identity of the requestor and whether the requestor is allowed to obtain the allocation.

16.1.2.  Offline Dictionary Attacks

The digest authentication mechanism used by TURN is subject to offline dictionary attacks. An attacker that is capable of eavesdropping on a message exchange between a client and server can determine the password by trying a number of candidate passwords and seeing if one of them is correct. This attack works when the passwords are low entropy, such as a word from the dictionary. This attack can be mitigated by using strong passwords with large entropy. In situations where even stronger mitigation is required, TLS transport between the client and the server can be used.

16.1.3.  Faked Refreshes and Permissions

An attacker might wish to attack an active allocation by sending it a Refresh with an immediate expiration, in order to delete it and disrupt service to the client. This is prevented by authentication of refreshes. Similarly, an attacker wishing to send CreatePermission requests to create permissions to undesirable destinations is prevented from doing so through authentication. The motivations for such an attack are described in Section 16.2 (Firewall Considerations).

16.1.4.  Fake Data

An attacker might wish to send data to the client or the peer, as if they came from the peer or client respectively. To do that, the attacker can send the client a faked Data Indication or ChannelData message, or send the TURN server a faked Send Indication or ChannelData message.

Indeed, since indications and ChannelData messages are not authenticated, this attack is not prevented by TURN. However, this attack is generally present in IP-based communications and is not substantially worsened by TURN. Consider an normal, non-TURN IP session between hosts A and B. An attacker can send packets to B as if they came from A by sending packets towards A with a spoofed IP address of B. This attack requires the attacker to know the IP addresses of A and B. With TURN, an attacker wishing to send packets towards a client using a Data indication needs to know its IP address (and port), the IP address and port of the TURN server, and the IP address and port of the peer (for inclusion in the XOR-PEER-ADDRESS attribute). To send a fake ChannelData message to a client, an attacker needs to know the IP address and port of the client, the IP address and port of the TURN server, and the channel number. This particular combination is mildly more guessable than in the non-TURN case.

These attacks are more properly mitigated by application layer authentication techniques. In the case of real time traffic, usage of SRTP [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) prevents these attacks completely.

16.1.5.  Impersonating a Server

When a client learns a relayed address from a TURN server, it uses that relayed address in application protocols to receive traffic. Therefore, an attacker wishing to intercept or redirect that traffic might try to impersonate a TURN server and provide the client with a faked relayed address.

This attack is prevented through the digest authentication mechanism, which provides message integrity for responses in addition to verifying that they came from the server. Furthermore, an attacker cannot replay old server responses; the transaction ID in the STUN header prevents this.

16.1.6.  Eavesdropping Traffic

TURN concerns itself primarily with authentication and message integrity. Confidentiality is only a secondary concern, as TURN control messages do not include information that is particularly sensitive. The primary protocol content of the messages is the IP address of the peer. If it is important to prevent an eavesdropper on a TURN connection from learning this, TURN can be run over TLS.

Confidentiality for the application data relayed by TURN is best provided by the application protocol itself, since running TURN over TLS does not protect application data between the server and the peer. If confidentiality of application data is important, then the application should encrypt or otherwise protect its data. For example, for real time media, confidentiality can be provided by using SRTP.

16.2.  Firewall Considerations

A key aspect of TURN's security considerations is that it should not weaken the protections afforded by firewalls deployed between a client and a TURN server. It is anticipated that TURN servers will often be present on the public Internet, and clients may often be inside enterprise networks with corporate firewalls. If TURN servers provide a 'backdoor' for reaching into the enterprise, TURN will be blocked by these firewalls.

TURN servers therefore emulate the behavior of NAT devices which implement address-dependent filtering [RFC4787] (Audet, F. and C. Jennings, “Network Address Translation (NAT) Behavioral Requirements for Unicast UDP,” January 2007.), a property common in many firewalls as well. When a NAT or firewall implements this behavior, packets from an outside IP address are only allowed to be sent to an internal IP address and port if the internal IP address and port had recently sent a packet to that outside IP address. TURN servers introduce the concept of permissions, which provide exactly this same behavior on the TURN server. An attacker cannot send a packet to a TURN server and expect it to be relayed towards the client, unless the client has tried to contact the attacker first.

It is important to note that some firewalls have policies which are even more restrictive than address-dependent filtering. Firewalls can also be configured with address and port dependent filtering, or can be configured to disallow inbound traffic entirely. In these cases, if a client is allowed to connect the TURN server, communications to the client will be less restrictive than what the firewall would normally allow.

16.2.1.  Faked Permissions

In firewalls and NAT devices, permissions are granted implicitly through the traversal of a packet from the inside of the network towards the outside peer. Thus, a permission cannot, by definition, be created by any entity except one inside the firewall or NAT. With TURN, this restriction no longer holds. Since the TURN server sits outside the firewall, at attacker outside the firewall can now send a message to the TURN server and try to create a permission for itself.

This attack is prevented because all messages which create permissions (i.e., ChannelBind and CreatePermission) are authenticated.

16.2.2.  Blacklisted IP Addresses

Many firewalls can be configured with blacklists which prevent a client behind the firewall from sending packets to, or receiving packets from, ranges of blacklisted IP addresses. This is accomplished by inspecting the source and destination addresses of packets entering and exiting the firewall, respectively.

If a client connects to a TURN server, it will be able to bypass such blacklisting policies and communicate with IP addresses which the firewall would otherwise restrict. This is a problem for other protocols that provide tunneling functions, such as VPNs. It is possible to build TURN-aware firewalls which inspect TURN messages, and check the IP address of the correspondent. TURN messages to offending destinations can then be rejected. TURN is designed so that this inspection can be done statelessly.

16.2.3.  Running Servers on Well-Known Ports

A malicious client behind a firewall might try to connect to a TURN server and obtain an allocation which it then uses to run a server. For example, a client might try to run a DNS server or FTP server.

This is not possible in TURN. A TURN server will never accept traffic from a peer which the client itself has not contacted. Thus, peers cannot just connect to the allocated port in order to obtain the service.

16.3.  Insider Attacks

In insider attacks, a client has legitimate credentials but defies the trust relationship that goes with those credentials. These attacks cannot be prevented by cryptographic means but need to be considered in the design of the protocol.

16.3.1.  DoS Against TURN Server

A client wishing to disrupt service to other clients might obtain an allocation and then flood it with traffic, in an attempt to swamp the server and prevent it from servicing other legitimate clients. This is mitigated by the recommendation that the server limit the amount of bandwidth it will relay for a given username. This won't prevent a client from sending a large amount of traffic, but it allows the server to immediately discard traffic in excess.

Since each allocation uses a port number on the IP address of the TURN server, the number of allocations on a server is finite. An attacker might attempt to consume all of them by requesting a large number of allocations. This is prevented by the recommendation that the server impose a limit of the number of allocations active at a time for a given username.

16.3.2.  Anonymous Relaying of Malicious Traffic

TURN servers provide a degree of anonymization. A client can send data to correspondent peers without revealing their own IP addresses. TURN servers may therefore become attractive vehicles for attackers to launch attacks against targets without fear of detection. Indeed, it is possible for a client to chain together multiple TURN servers, such that any number of relays can be used before a target receives a packet.

Administrators who are worried about this attack can maintain logs which capture the actual source IP and port of the client. This will allow for forensic tracing to determine the original source, should it be discovered that an attack is being relayed through a TURN server.

16.3.3.  Manipulating other Allocations

An attacker might attempt to disrupt service to other users of the TURN server by sending Refresh requests or CreatePermission requests which (through source address spoofing) appear to be coming from another user of the TURN server. TURN prevents this by requiring that the credentials used in CreatePermission, Refresh, and ChannelBind messages match those used to create the initial allocation. Thus, the fake requests from the attacker will be rejected.

16.4.  Other Considerations

Any relay addresses learned through an Allocate request will not operate properly with IPSec Authentication Header (AH) [RFC4302] (Kent, S., “IP Authentication Header,” December 2005.) in transport or tunnel mode. However, tunnel-mode IPSec ESP [RFC4303] (Kent, S., “IP Encapsulating Security Payload (ESP),” December 2005.) should still operate.

17.  IANA Considerations

Since TURN is an extension to STUN [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.), the methods, attributes and error codes defined in this specification are new methods, attributes, and error codes for STUN. This section directs IANA to add these new protocol elements to the IANA registry of STUN protocol elements.

The codepoints for the new STUN methods defined in this specification are listed in Section 13 (New STUN Methods).

The codepoints for the new STUN attributes defined in this specification are listed in Section 14 (New STUN Attributes).

The codepoints for the new STUN error codes defined in this specification are listed in Section 15 (New STUN Error Response Codes).

Extensions to TURN can be made through IETF consensus.

18.  IAB Considerations

The IAB has studied the problem of "Unilateral Self Address Fixing", which is the general process by which a client attempts to determine its address in another realm on the other side of a NAT through a collaborative protocol reflection mechanism [RFC3424] (Daigle, L. and IAB, “IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Translation,” November 2002.). The TURN extension is an example of a protocol that performs this type of function. The IAB has mandated that any protocols developed for this purpose document a specific set of considerations.

TURN is an extension of the STUN protocol. As such, the specific usages of STUN that use the TURN extensions need to specifically address these considerations. Currently the only STUN usage that uses TURN is ICE (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” October 2007.) [I‑D.ietf‑mmusic‑ice], and the IAB considerations for the ICE usage of TURN are discussed in the base ICE document.

19.  Open Issues

Note to RFC Editor: Please remove this section prior to publication of this document as an RFC.

This section lists the known issues in this version of the specification.

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20.  Changes from Previous Versions

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This section lists the changes between the various versions of this specification.

20.1.  Changes from -09 to -10

• Changed the recommendation for using the SOFTWARE attribute. Previously its use was recommended in all requests and responses; now it is only recommended in Allocate and Refresh requests and responses, though it may appear elsewhere. Also, version -09 incorrectly referred to this attribute as "SOFTWARE-TYPE".
• Changed the name of the PEER-ADDRESS and RELAYED-ADDRESS attributes to XOR-PEER-ADDRESS and XOR-RELAYED-ADDRESS respectively for consistency with other specifications.
• Removed the concept of a "preserving" allocation. All allocations are now non-preserving. This simplifies the base specification and allows it to advance more rapidly; see the discussion in the BEHAVE meeting of 29 July 2008. The concept of a preserving allocation will be advanced as an extension to TURN. As part of this change, the P bit in the REQUESTED-PROPS attribute, the ICMP attribute, and ICMP message relaying was removed. Further, in Section 12 (IP Header Fields), the preferred behaviors were removed, leaving the alternate behaviors as the specified behaviors.
• Replaced the REQUESTED-PROPS attribute with the EVEN-PORT attribute. The new attribute lacks the feature of the old attribute of being an alternate way to specify new allocation properties. As a consequence, the only way to specify a new allocation property is to define a new attribute.
• Added text recommending that the client check that the IP address in XOR-PEER-ADDRESS attribute in a received Data indication is one with which the client believes there is an active permission. Similarly, it is recommended that the client check that a permission exist when receiving a ChannelData message.
• Added text recommending that the client delete the allocation if it receives a ChannelBind failure response on an unbound channel.
• Added the CreatePermission request/response transaction which adds or updates permissions, and removed the ability for Send indications and ChannelBind messages to install or update permissions. The net effect is that only authenticate-able messages (i.e., CreatePermission requests and ChannelBind requests) can install or refresh permissions; unauthenticate-able Send indications and ChannelData messages do not.
• Removed all support for IPv6. All IPv6 support, including ways of relaying between IPv4 and IPv6, will now be covered in [I‑D.ietf‑behave‑turn‑ipv6] (Camarillo, G., Novo, O., and S. Perreault, “Traversal Using Relays around NAT (TURN) Extension for IPv6,” March 2010.).
• Reserved attribute code point 0x0021. This was previously used for the TIMER-VAL attribute, which was removed when the SetActiveDestination feature was removed.
• Added the DONT-FRAGMENT attribute which allows the client to request that the server set the DF bit when sending the UDP datagram to the peer. This attribute may appear in both Allocate requests and Send indications.
• Changed how the ALTERNATE-SERVER attribute is used. The attribute can no longer be used with any error code, but must be used with 300 (Try Alternative). It can now appear in unauthenticated responses, however there are restrictions around how the subsequent Allocate request is authenticated.
• Reworked the details of how idempotency of requests is handled, making it clear that the stack can either remember all transactions for 40 seconds, or can handle this using the so-called "stateless stack approach". Made some changes to the semantics of the Allocate, Refresh, and ChannelBind requests as a consequence.
• Added the requirement that a client cannot re-use previously bound channel number or transport address until 5 minutes after the channel binding expires. This avoids various race conditions.
• Removed the requirement that an allocation cannot be re-used within 2 minutes of having been deleted. This requirement was put in place to prevent mis-delivered packets but is no longer seen as having any real value.
• Added a recommendation that the server impose quotas on both the number of allocations and the amount of bandwidth a given username can use at one time. These quotas help protect against denial-of-service attacks.
• Completely rewrote the security considerations section.
• Made quite a few changes to the descriptive text in both the Overview and the normative text to try to further clarify concepts.

20.2.  Changes from -08 to -09

• Added text to properly define the ICMP attribute. This attribute was introduced in TURN-08, but not fully defined due to an oversight. Clarified that the attribute can appear in a Data indication, but not a Send indication. Added text to the section on receiving a Data indication that points out that this attribute may be present.
• Changed the wording around the handling of the DSCP field to allow the server to set the DSCP to an arbitrary value if the next hop is a Diff-Serv classifier and marker.
• When the server generates a 508 response due to an unsupported flag in the REQUESTED-PROPS attribute, the server now includes the REQUESTED-PROPS attribute in the response with all the flags it supports set to 1. This allows the client to see if the server does not understand one of its flags. Similarly, the client is now allowed to immediately retry the request if it modifies the included REQUESTED-PROPS attribute.
• Clarified that the REQUESTED-PROPS attribute can be used in conjunction with the RESERVATION-TOKEN attribute as long as both the E and R bits are 0. The spec previously contradicted itself on this point.
• Clarified that when the server receives a ChannelData message with a length field of 0, it sends a UDP Datagram to the peer that contains no application data.
• Rewrote some text around relaying incoming UDP Datagrams to avoid duplication of text in the Data indication and Channel sections.
• Added a note that points out that the on-going work on randomizing port allocations [I‑D.ietf‑tsvwg‑port‑randomization] (Larsen, M. and F. Gont, “Transport Protocol Port Randomization Recommendations,” April 2010.) may be applicable to TURN.
• Clarified that the Allocate request containing a RESERVATION-TOKEN attribute can use any 5-tuple, and that 5-tuple need not have any specific relationship to the 5-tuple of the Allocate request that created the reservation.
• Added a note that discusses retransmitted Allocate requests over UDP where the first request receives a failure response, but the second receives a success response. The server may elect to remember transmitted failure responses to avoid this situation.
• Added text about the usage of the SOFTWARE-TYPE attribute (formerly known as the SERVER attribute) in TURN messages.
• Rewrote the text in the Overview that motivates why TURN supports TCP and TLS between the client and the server. The previous text had been identified by various readers as inadequate and misleading.
• Rewrote the section how a server handles a Refresh request to clarify processing in various error conditions. The new text makes it clear that it is OK to delete a non-existent allocation. It also clarifies how to handle retransmissions of Refresh requests over UDP.
• Renamed the "RELAY-ADDRESS" attribute to "RELAYED-ADDRESS", since the text consistently uses the term "relayed transport address" for the concept and ICE uses the term "relayed candidate".
• Changed the codepoint assigned to the error code "Wrong Credentials" from 438 to 441 to avoid a conflict with the "Stale Nonce" error code of STUN.
• Changed the text to consistently use non-capitalized "request", "response" and "indication", except in headings, error code names, etc.
• Added a note mentioning that TURN packets can be demuxed from other packets arriving on the same socket by looking at the 5-tuple of the arriving packet.
• Clarified that there are no required attributes is a ChannelBind success response.

20.3.  Changes from -07 to -08

• Removed the BANDWIDTH attribute and all associated text (including error code 507 "Insufficient Bandwidth Capacity"), as the requirements for this feature were not clear and it was felt the feature could be easily added later.
• Changed the format of the REQUESTED-PROPS attribute from a one-byte field to a set of bit flags. Changed the semantics of the unused portion of the value from RFFU to "MUST be 0" to give a more desirable behavior when new flags are defined.
• Introduced the concept of Preserving vs. Non-Preserving allocations. As a result, completely revamped the rules for how to set the fields in the IP header, and added rules for relaying ICMP messages when the allocation is Preserving.

20.4.  Changes from -06 to -07

• Rewrote the General Behavior section, making various changes in the process.
• Changed the usage of authentication from MUST to SHOULD.
• Changed the requirement that subsequent requests use the same username and password from MUST to SHOULD to allow for the possibility of changing the credentials using some unspecified mechanism.
• Introduced a 438 (Wrong Credentials) error which is used when a non-Allocate request authenticates but does not use the same username and password as the Allocate request. Having a separate error code for this case avoids the client being confused over what the error actually is.
• The server must now prevent the relayed transport address and the 5-tuple from being reused in different allocations for 2 minutes after the allocation expires.
• Changed the usage of FINGERPRINT from MUST NOT to MAY, to allow for the possible multiplexing of TURN with some other protocol.
• Rewrote much of the section on Allocations, splitting it into three new sections (one on allocations in general, one on creating an allocation, and one on refreshing an allocation).
• Replaced the mechanism for requesting relayed transport addresses with specific properties. The new mechanism is less powerful: a client can request an even port, or a pair of ports, but cannot request a single odd port or a specific port as was possible under the old mechanism. Nor can the client request a specific IP address.
• Changed the rules for handling ALTERNATE-SERVER, removing the requirement that the referring server have "positive knowledge" about the state of the alternate server. The new rules instead rely on text in STUN to prevent referral loops.
• Changed the rules for allocation lifetimes. Allocations lifetimes are now a minimum of 10 minutes; the client can ask for longer values, but requests for shorter values are ignored. The text now recommends that the client refresh an allocation one minute before it expires.
• Put in temporary procedures for handling the BANDWIDTH attribute, modelled on the LIFETIME attribute. These procedures are mostly placeholders and likely to change in the next revision.
• Added a detailed description of how a client reacts to the various errors it can receive in reply to an Allocate request. This replaces the various descriptions that were previously scattered throughout the document, which were inconsistent and sometimes contradictory.
• Added a new section that gives the normative rules for permissions.
• Changed the rules around permission lifetimes. The text used to recommend a value of one minute; it MUST now be 5 minutes.
• Removed the errors "Channel Missing or Invalid", "Peer Address Missing or Invalid" and "Lifetime Malformed or Invalid" and used 400 "Bad Request" instead.
• Rewrote portions of the section on Send and Data indications and the section on Channels to try to make the client vs. server behavior clearer.
• Channel bindings now expire after 10 minutes, and must be refreshed to keep them alive.
• Binding a channel now installs or refreshes a permission for the IP address of corresponding peer.
• Changed the wording describing the situation when the client sends a ChannelData message before receiving the ChannelBind success response. -06 said that client SHOULD NOT do this; -07 now says that a client MAY, but describes the consequences of doing it.
• Added a section discussing the setting of fields in the IP header.
• Replaced the REQUESTED-PORT-PROPS attribute with the REQUESTED-PROPS attribute that has a different format and semantics, but reuses the same code point.
• Replaced the REQUESTED-IP attribute with the RESERVATION-TOKEN attribute, which has a different format and semantics, but reuses the same code point.
• Removed error codes 443 and 444, and replaced them with 508 (Insufficient Port Capacity). Also changed the error text for code 507 from "Insufficient Capacity" to "Insufficient Bandwidth Capacity".

20.5.  Changes from -05 to -06

• Changed the mechanism for allocating channels to the one proposed by Eric Rescorla at the Dec 2007 IETF meeting.
• Removed the framing mechanism (which was used to frame all messages) and replaced it with the ChannelData message. As part of this change, noted that the demux of ChannelData messages from TURN messages can be done using the first two bits of the message.
• Rewrote the sections on transmitted and receiving data as a result of the above to changes, splitting it into a section on Send and Data indications and a separate section on channels.
• Clarified the handling of Allocate request messages. In particular, subsequent Allocate request messages over UDP with the same transaction id are not an error but a retransmission.
• Restricted the range of ports available for allocation to the Dynamic and/or Private Port range, and noted when ports outside this range can be used.
• Changed the format of the REQUESTED-TRANSPORT attribute. The previous version used 00 for UDP and 01 for TCP; the new version uses protocol numbers from the IANA protocol number registry. The format of the attribute also changed.
• Made a large number of changes to the non-normative portion of the document to reflect technical changes and improve the presentation.
• Added the Issues section.

20.6.  Changes from -04 to -05

• Removed the ability to allocate addresses for TCP relaying. This is now covered in a separate document. However, communication between the client and the server can still run over TCP or TLS/TCP. This resulted in the removal of the Connect method and the TIMER-VAL and CONNECT-STAT attributes.
• Added the concept of channels. All communication between the client and the server flows on a channel. Channels are numbered 0..65535. Channel 0 is used for TURN messages, while the remaining channels are used for sending unencapsulated data to/from a remote peer. This concept adds a new Channel Confirmation method and a new CHANNEL-NUMBER attribute. The new attribute is also used in the Send and Data methods.
• The framing mechanism formally used just for stream-oriented transports is now also used for UDP, and the former Type and Reserved fields in the header have been replaced by a Channel Number field. The length field is zero when running over UDP.
• TURN now runs on its own port, rather than using the STUN port. The use of channels requires this.
• Removed the SetActiveDestination concept. This has been replaced by the concept of channels.
• Changed the allocation refresh mechanism. The new mechanism uses a new Refresh method, rather than repeating the Allocation transaction.
• Changed the syntax of SRV requests for secure transport. The new syntax is "_turns._tcp" rather than the old "_turn._tls". This change mirrors the corresponding change in STUN SRV syntax.
• Renamed the old REMOTE-ADDRESS attribute to PEER-ADDRESS, and changed it to use the XOR-MAPPED-ADDRESS format.
• Changed the RELAY-ADDRESS attribute to use the XOR-MAPPED-ADDRESS format (instead of the MAPPED-ADDRESS format)).
• Renamed the 437 error code from "No Binding" to "Allocation Mismatch".
• Added a discussion of what happens if a client's public binding on its outermost NAT changes.
• The document now consistently uses the term "peer" as the name of a remote endpoint with which the client wishes to communicate.
• Rewrote much of the document to describe the new concepts. At the same time, tried to make the presentation clearer and less repetitive.

21.  Acknowledgements

The authors would like to thank the various participants in the BEHAVE working group for their many comments on this draft. Marc Petit-Huguenin, Remi Denis-Courmont, Jason Fischl, Derek MacDonald, Scott Godin, Cullen Jennings, Lars Eggert, Magnus Westerlund, Benny Prijono, and Eric Rescorla have been particularly helpful, with Eric also suggesting the channel allocation mechanism, and Cullen suggesting the REQUESTED-PORT-PROPS mechanism. Christian Huitema was an early contributor to this document and was a co-author on the first few drafts. Finally, the authors would like to thank Dan Wing for both his contributions to the text and his huge help in restarting progress on this draft after work had stalled.

22.  References

22.1. Normative References

22.2. Informative References

Authors' Addresses

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