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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.
1.
Introduction
2.
Overview of Operation
2.1.
Transports
2.2.
Allocations
2.3.
Exchanging Data with Peers
2.4.
Channels
2.5.
Permissions
2.6.
Preserving vs. Non-Preserving Allocations
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 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.
Send and Data Indications
9.1.
Sending a Send Indication
9.2.
Receiving a Send Indication
9.3.
Receiving a UDP Datagram
9.4.
Receiving a Data Indication
10.
Channels
10.1.
Sending a ChannelBind Request
10.2.
Receiving a ChannelBind Request
10.3.
Receiving a ChannelBind Response
10.4.
The ChannelData Message
10.5.
Sending a ChannelData Message
10.6.
Receiving a ChannelData Message
10.7.
Relaying
11.
IP and ICMP
11.1.
IP
11.2.
ICMP
12.
New STUN Methods
13.
New STUN Attributes
13.1.
CHANNEL-NUMBER
13.2.
LIFETIME
13.3.
PEER-ADDRESS
13.4.
DATA
13.5.
RELAY-ADDRESS
13.6.
REQUESTED-PROPS
13.7.
REQUESTED-TRANSPORT
13.8.
RESERVATION-TOKEN
14.
New STUN Error Response Codes
15.
Security Considerations
16.
IANA Considerations
17.
IAB Considerations
18.
Example
19.
Changes from Previous Versions
19.1.
Changed from -07 to -08
19.2.
Changes from -06 to -07
19.3.
Changes from -05 to -06
19.4.
Changes from -04 to -05
20.
Open Issues
21.
Acknowledgements
22.
References
22.1.
Normative References
22.2.
Informative References
§
Authors' Addresses
§
Intellectual Property and Copyright Statements
| TOC |
Session Traversal Utilities for NAT (STUN) [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.) provides a suite of tools for facilitating the traversal of NAT. Specifically, it defines the Binding method, which is used by a client to determine its reflexive transport address towards the STUN server. The reflexive transport address can be used by the client for receiving packets from peers, but only when the client is behind "good" NATs. In particular, if a client is behind a NAT whose mapping behavior [RFC4787] (Audet, F. and C. Jennings, “Network Address Translation (NAT) Behavioral Requirements for Unicast UDP,” January 2007.) is address or address and port dependent (sometimes called "bad" NATs), the reflexive transport address will not be usable for communicating with a peer.
The only reliable way to obtain a UDP transport address that can be used for corresponding with a peer through such a NAT is to make use of a relay. The relay sits on the public side of the NAT, and allocates transport addresses to clients reaching it from behind the private side of the NAT. These allocated transport addresses, called relayed transport address, are IP addresses and ports on the relay. When the relay receives a packet on one of these allocated addresses, the relay forwards it toward the client.
This specification defines an extension to STUN, called TURN, that allows a client to request a relayed transport address on a TURN server.
Though a relayed transport address is highly likely to work when corresponding with a peer, it comes at high cost to the provider of the relay service. As a consequence, relayed transport addresses should only be used as a last resort. Protocols using relayed transport addresses should make use of mechanisms to dynamically determine whether such an address is actually needed. One such mechanism, defined for multimedia session establishment protocols based on the offer/answer protocol in RFC 3264 (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.) [RFC3264], is Interactive Connectivity Establishment (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.).
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 in the later stages of its development to make sure that TURN is suitable for other types of applications.
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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.
+---------+
| |
| |
/ | Peer A |
Client's TURN // | |
Host Transport Server / | |
Address Address +-+ // +---------+
10.1.1.2:17240 192.0.2.15:3478 |N|/ 192.168.100.2:16400
| | |A|
| +-+ | /|T|
| | | | / +-+
v | | | / 192.0.2.210:18200
+---------+ | | |+---------+ / +---------+
| | |N| || | // | |
| TURN | | | 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 |
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 has allocated a local port on one of its addresses for use in communicating with the server. The combination of an IP address and a port is called a TRANSPORT ADDRESS and since this (IP address, port) combination is located on the client and not on the NAT, it is called the client’s HOST 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 ADDRESS. The client learns the server’s address through some unspecified means (e.g., configuration), and this address is typically used by many clients simultaneously. The TURN server address is used by the client to send both commands and data to the server; the commands are processed by the TURN server, while the data is relayed on to the peers.
Since the client is behind a NAT, the server sees these packets 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 allocate a RELAYED TRANSPORT ADDRESS, which is an transport address located on the TURN server. The server ensures that there is a one-to-one relationship between the client’s server-reflexive transport address and the relayed transport address; thus a packet received at the relayed transport address can be unambiguously relayed by the server to the client.
The client will typically communicate this relayed transport address to one or more peers through some mechanism not specified here (e.g., an ICE offer or answer [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.)). Once this is done, the client can send data to the server to relay towards its peers. In the reverse direction, peers can send data to the the relayed transport address of the client. The server will relay this data to the client as long as the client explicitly created a permission (see Section 2.5 (Permissions)) for the IP address of the peer.
| TOC |
TURN as defined in this specification only allows the use of 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.
| TURN client to TURN server | TURN server to peer |
|---|---|
| UDP | UDP |
| TCP | UDP |
| TLS over TCP | UDP |
If TCP or TLS over TCP is used between the client and the server, then the server will convert between stream transport and UDP transport when relaying data. TURN allows both TCP and TLS over TCP as transports in part because many firewalls are configured to not pass any UDP traffic.
For TURN clients, using TLS over TCP to communicate with the TURN server provides two benefits. First, the client can be assured that the addresses of its peers are not visible to any attackers between it and the server. Second, the client may be able to communicate with TURN servers using TLS when it would not be able to communicate with the same server using TCP or UDP, due to the policy of a firewall between the TURN client and its server. In this second case, TLS between the client and TURN server facilitates traversal.
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.
| TOC |
To allocate a relayed transport address, the client uses an Allocate transaction. The client sends a Allocate Request to the server, and the server replies with an Allocate 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). And 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 with the allocated related transport address. 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. If a client no longer wishes to use an Allocation, it should do a Refresh transaction with a requested lifetime of 0.
Note that sending or receiving data from a peer DOES NOT refresh the allocation.
The server keeps track of the client reflexive transport address and port, the server transport address and port, and the protocol used by the client to communicate with the server. (Together known as a 5-tuple. The server remembers the 5-tuple used in the Allocate Request. Subsequent transactions between the client and the server use this same 5-tuple. In this way, the server knows which client owns the allocated relayed transport address. If the client wishes to allocate a second relayed transport address, it must use a different 5-tuple for this allocation (e.g., by using a different client host address).,
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, many implementations use a file-descriptor in place of a 5-tuple to represent a TCP connection.
| TOC |
There are two ways for the client and peers to exchange data using the TURN server. The first way uses Send and Data indications, 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.
When using the first way, the client sends a Send indication to the TURN server containing, in attributes inside the indication, the transport address of the peer and the data to be sent to that peer. When the TURN server receives the Send Indication, it extracts the data from the Send Indication and sends it in a UDP datagram to the peer, using the allocated relay address as the source address. In the reverse direction, UDP datagrams arriving at the relay 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 attribute in the Data Indication.
TURN TURN Peer Peer client server A B |--- Allocate Req -->| | | |<-- Allocate Resp ---| | | | | | | |--- Send (Peer A)--->| | | | |=== data ===>| | | | | | | |<== data ====| | |<-- Data (Peer A)----| | | | | | | |--- Send (Peer B)--->| | | | |=== data =================>| | | | | | |<== data ==================| |<-- Data (Peer B)----| | |
| Figure 2 |
In the figure above, the client first allocates a relayed transport address. It then sends data to Peer A using a Send Indication; at the server, the 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. A similar exchange happens with Peer B.
| TOC |
For some applications (e.g. Voice over IP), the 36 bytes of overhead that a Send 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 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 ChannelData messages from the client to the server, or by rebinding the channel to the peer.
TURN TURN Peer Peer client server A B |--- Allocate Req -->| | | |<-- Allocate Resp ---| | | | | | | |--- Send (Peer A)--->| | | | |=== data ===>| | | | | | | |<== data ====| | |<-- Data (Peer A)----| | | | | | | |- ChannelBind Req -->| | | | (Peer A to 0x4001) | | | | | | | |<- ChannelBind Resp -| | | | | | | |-- [0x4001] data --->| | | | |=== data ===>| | | | | | | |<== data ====| | |<- [0x4001] data --->| | | | | | | |--- Send (Peer B)--->| | | | |=== data =================>| | | | | | |<== data ==================| |<-- Data (Peer B)----| | |
| Figure 3 |
The figure above shows the channel mechanism in use. The client begins by allocating a relayed transport address, and then uses that address to exchange data with Peer A. After a bit, the client decides to bind a channel to Peer A. To do this, it 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.
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 Send and Data indications.
Channel bindings are always initiated by the client.
| TOC |
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.).
The client can install a permission by sending data to a peer (or by doing certain other things). Once a permission is installed, any peer with the same IP address (the ports numbers can differ) is permitted to send data to the client. After 5 minutes, the permission times out and the server drops any UDP datagrams arriving at the relayed transport from that IP address. Note that permissions are within the context of an allocation, so adding or expiring a permission in one allocation does not affect other allocations.
Data received from the peer DOES NOT refresh the permission.
| TOC |
Some applications that use TURN are quite tolerant of the different possible ways a TURN server could set the Diff-Serv, ECN, TTL / Hop Limit, and Flow Label fields in the IP header of the outgoing packet. Other applications require that the TURN server set these fields in a specific way, and also require that the TURN server relay ICMP error packets. Applications in the second class typically wish to do Path MTU Discovery or end-to-end QOS.
Unfortunately, reading and manipulating fields in the IP header and relaying ICMP messages usually requires the server to have special permissions (e.g., access to RAW sockets or be loaded into the kernel), something that the person setting up the server may be unwilling or unable to grant. This is especially true when the server is part of a larger application, for example a peer-to-peer application. It is also significantly more difficult to implement this type of server than just relaying at the UDP layer.
To allow TURN to cater to both usage scenarios, TURN defines the concept of Preserving vs. Non-Preserving allocations. A Preserving allocation sets the fields in outgoing IP header correctly, and also relays ICMP messages, while a Non-Preserving allocation may not relay correctly in every case. The relaying rules for a Preserving are designed to guarantee the following:
If the client knows its application or usage scenario requires a Preserving allocation, then it can request one in its Allocate request. If the server is unable to grant this request, then it rejects the Allocate request.
Note that a Preserving allocation only makes sense when the transport protocol to the client is UDP; when the transport is TCP or TLS, the allocation is always Non-Preserving.
| TOC |
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 familar 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:
- A 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.
- Host Transport Address:
- A transport address allocated on a host.
- 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 that exists on a TURN server. If a permission exists, packets that arrive at this address are relayed towards the TURN client.
- 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.
- 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.
- Preserving Allocation
- An allocation that sets the the fields in the IP header in a specific manner when relaying application data, and which also relays ICMP messages. An allocation that may not do this in some cases is called a Non-Preserving allocation.
| TOC |
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 require that all TURN requests use 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. 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).
The client and/or the server MAY include the FINGERPRINT attribute in any of the methods defined in this document. However, 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.
| TOC |
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 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. Both of these MUST be unique across all allocations, so either one can be used to uniquely identify the allocation.
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.
The username and password of the allocation is the username and password of the authenticated Allocate request that creates the allocation. Subsequent requests on an allocation use the same username and password as those 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 438 (Wrong Credentials) error.
The transaction ID of the allocation is the transaction ID used in the Allocate request. This is used to detect retransmissions of the Allocate request over UDP (see Section 6.2 (Receiving an Allocate Request) for details).
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 is freed. However the server MUST ensure that neither the relayed transport address nor the client reflexive transport address from the 5-tuple are re-used in other allocations until 2 minutes after the allocation expires; this ensures that any messages that are in transit when the allocation expires are gone before either of these transport addresses are re-used.
The list of permissions is described in Section 8 (Permissions) and the list of channels is described in Section 10 (Channels).
The differences between a Preserving and a Non-Preserving allocation are described in Section 11 (IP and ICMP).
| TOC |
An allocation on the server is created using an Allocate transaction.
| TOC |
The client forms an Allocate request as follows.
The client first needs to pick a host transport address that the server does not think is currently in use, or was recently in use. The client SHOULD pick a currently-unused transport address on the client's host (typically by allowing its OS to pick a currently-unused port for a new socket).
The client needs to pick 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 (e.g., [I‑D.ietf‑behave‑turn‑tcp] (Perreault, S. and J. Rosenberg, “Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations,” March 2010.)).
If the client wishes the server to initialize the time-to-expire 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 tha the default value.
If the client required the allocation to satisfy certain properties, then the client includes the REQUESTED-PROPS attribute. This attribute is optional, and can be omitted if no special properties are required.
Using the E and R bits in the REQUESTED-PROPS attribute, the client can request:
Note that the client cannot request a pair of adjacent ports unless it also requests that the lower numbered port be even. Thus the combination (E=0, R=1) is not allowed.
Similarly, by setting the P bit to 1 in the REQUESTED-PROPS attribute, the client can request that the server allocate a Preserving allocation.
For all the various REQUESTED-PROPS flags, if the server cannot satisfy the request, the Allocate request is rejected.
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 either omit the REQUESTED-PROPS attribute or set E=0 and R=0, since doing otherwise would make no sense.
Once constructed, the client sends the Allocate request on the 5-tuple.
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When the server receives an Allocate request, it performs the following checks:
If the server rejects the request with one of the error codes 422 (Unsupported Transport Protocol), 486 (Allocation Quota Reached) or 508 (Insufficient Port Capacity), it MAY include an ALTERNATE-SERVER attribute in the error response redirecting the client to another server that it believes will accept the request. If the attribute is included, the address MUST be from the same address family as the server's transport address. Note that, if the attribute is included, the client will try this alternate server before trying the other servers given by the SRV procedures.
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.
When allocating a relayed transport address for the allocation, the server MUST allocate an IP address from the same family (e.g, IPv4 vs. IPv6) as that on which the request was received (i.e., the server's IP address in the 5-tuple for the allocation).
NOTE: An extension to TURN to allow an address from a different address family is currently in progress [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 addition, 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.
If the request contains a REQUESTED-PROPS attribute with the R flag set, 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).
The server determines the initial value of the time-to-expire 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 value. It is RECOMMENDED that the server impose a maximum lifetime of no more than 3600 seconds (1 hour).
NOTE: The time-to-expire is recomputed with each successful Refresh request. 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:
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.
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If the client receives a success response, then it MUST check that the relayed transport address is in an address family that the client understands and is prepared to deal with. This specification only covers the case where the relayed transport address is of the same address family as the client's transport address. If the relayed transport address is 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 IPv4, but the request would arrive at the server over IPv6, 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.
If the client receives an error response, then the processing depends on the actual error code returned:
If the error response contains an ALTERNATE-SERVER attribute, and the client elects to try a different server, the the client SHOULD try the alternate server specified in that attribute (while obeying the rules in [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.) for avoiding redirection loops) before trying any other servers found using the SRV procedures of [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.).
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A Refresh transaction can be used to either (a) refresh an existing allocation and update its time-to-expire, 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 change the time-to-expire of an allocation at any time for other reasons.
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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-expire timer of an allocation. If the client wishes the server to set the time-to-expire 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-expire 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.
The Refresh transaction is sent on the 5-tuple for the allocation.
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When the server receives a Refresh request, it processes it as follows. If, during processing, an error in the request is detected (for example, a syntax error in the request which causes a 400 error), then the request is rejected with an error response but the allocation is NOT deleted (but note that a 437 error will indicate that the allocation was not found).
The server determines the new value for the time-to-expire field as follows. If the request contains a LIFETIME attribute, and the attribute value is 0, then the server uses a value of 0, which causes the allocation to expire. Otherwise, if the request contains a LIFETIME attribute and the attribute value is greater than the default lifetime, and the attribute value is otherwise acceptable to the server, then the server uses the attribute value. Otherwise, the server uses the default value. It is RECOMMENDED that the server impose a maximum lifetime of no more than 3600 seconds (1 hour).
The server then constructs a success response containing:
The response is then sent on the 5-tuple.
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If the client receives a success response to its Refresh request, it updates its copy of the allocation data structure with the time-to-expire value contained in the response.
If the client receives an 437 (Allocation Mismatch) error response to its Refresh request, then it must consider the allocation as having expired, as described in Section 4 (General Behavior). All other errors indicate a software error on the part of either the client or the server.
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For each allocation, the server keeps a list of zero or more permissions. Each permission consists an IP address which uniquely identifies the permission, and an associated time-to-expiry. The IP address describes a peer that is allowed to send data to the client, and the time-to-expiry is the number of seconds until the permission expires.
Various events, as described in subsequent sections, can cause a permission for a given IP address to be installed or refreshed. This causes one of two things to happen:
The default permission lifetime MUST be 300 seconds (= 5 minutes).
Each permission’s time-to-expire decreases down once per second until it reaches 0, at which point the permission expires and is deleted.
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.
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TURN supports two ways to send and receive data from peers. This section describes the use of Send and Data indications, while Section 10 (Channels) describes the use of the Channel Mechanism.
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A client can use a Send Indication to pass data to the server for relaying to a peer. A client can also use a Send Indication without a DATA attribute to install or refresh a permission for the specified IP address. A client may use a Send indication to send data to a peer even if a channel is bound to that peer.
When forming a Send Indication, the client MUST include a PEER-ADDRESS attribute and MAY include a DATA attribute. If the DATA attribute is included, then the DATA attribute contains the actual application data to be sent to the peer, and the PEER-ADDRESS attribute contains the transport address of the peer to which the data is to be sent. If the DATA attribute is not present, then the PEER-ADDRESS attribute contains the IP address for which a permission is to be installed or refreshed; in this case the port specified in the attribute is ignored.
Note that no authentication attributes are included, since indications cannot be authenticated using the Long-Term Credential mechanism.
The Send Indication MUST be sent using the same 5-tuple used for the original allocation.
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When the server receives a Send indication, it processes it as follows.
If the received Send indication contains a DATA attribute, then it forms a UDP datagram as follows:
The resulting UDP datagram is then sent to the peer. If any errors are detected during this process (e.g., the Send indication does not contain a PEER-ADDRESS attribute), the received indication is silently discarded and no UDP datagram is sent.
When the server receives a valid Send Indication, either with or without a DATA attribute, it also installs or refreshes a permission for the IP address contained in the PEER-ADDRESS attribute (see Section 8 (Permissions)).
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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 Section 8 (Permissions)).
If relaying is permitted, and there is no channel bound to the peer that sent the UDP datagram (see ISection 10 (Channels)), then the server forms and sends a Data indication. The Data indication MUST contain both a PEER-ADDRESS and a DATA attribute. The DATA attribute is set to the value of the ‘data octets’ field from the datagram, and the 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.
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When the client receives a Data indication, it checks that the Data indication contains both a PEER-ADDRESS and a DATA attribute. It then 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 PEER-ADDRESS attribute.
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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:
Within the context of an allocation, a channel binding is uniquely identified either by the channel number or by the 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 last for 10 minutes unless refreshed. Refreshing the binding (by the server receiving either a ChannelBind request rebinding the channel to the same peer, or by the server receiving a ChannelData message on that channel) resets the time-to-expire timer back to 10 minutes. When the channel binding expires, the channel becomes unbound and available for binding to a different transport address.
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.
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A channel binding is created using a ChannelBind transaction. A channel binding can also be refreshed using a ChannelBind transaction.
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 PEER-ADDRESS attribute. Section 10.2 (Receiving a ChannelBind Request) describes the restrictions on these attributes.
Note that rebinding a channel to the same transport address that it is already bound to provides a way to refresh a channel binding without sending data to the peer.
Once formed, the ChannelBind Request is sent using the 5-tuple for the allocation.
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When the server receives a ChannelBind request, it checks the following:
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.
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 PEER-ADDRESS attribute. The server also installs or refreshes a permission for the IP address in the PEER-ADDRESS attribute.
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When the client receives a successful ChannelBind response, it updates its data structures to record that the channel binding is now active.
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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.
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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 10.4 (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.
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The receiver of the ChannelData message uses bits 0 and 1 to distinguish it from STUN-formatted messages, as described in Section 10.4 (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 10.5 (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.
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When a server receives a ChannelData message, it first processes it as described in the previous section. If no errors are detected, it relays the application data to the peer by forming a UDP datagram as follows:
The resulting UDP datagram is then sent to the peer.
If the ChannelData message is valid, then the server refreshes the channel binding, and also installs or refreshes a permission for the IP address part of the transport address to which the UDP datagram is sent (see Section 8 (Permissions)).
In the other direction, when the server receives a UDP datagram on the relayed transport address associated with an allocation, then it first checks to see if it is permitted to relay the datagram. This check is done as described in Section 8 (Permissions). If relaying is permitted, then the server checks to see if there is a channel bound to the peer that sent the UDP datagram. If there is, then it SHOULD form and send a ChannelData message as described in Section 10.5 (Sending a ChannelData Message). If no channel is bound to the peer, then it MUST form and send a Data indication as described in Section 9.3 (Receiving a UDP Datagram).
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This section describes how the server sets various fields in the IP header when relaying between the client and the peer or vica-versa. It also describes how the server relays ICMP messages. The descriptions in this section apply: (a) when the server receives a Send indication or ChannelData message from the client and sends a UDP datagram to the peer, (b) when the server receives a UDP datagram on the relayed-transport address and sends a Data indication or ChannelData message to the client, or (c) when the server receives an ICMP message. This section does not apply when the server sends TURN control messages.
The descriptions below have two parts: a preferred behavior and an alternate behavior. A Preserving allocation MUST implement the preferred behavior. A non-preserving allocation with UDP transport to the client SHOULD implement the preferred behavior, but if that is not possible for a particular field, then it SHOULD implement the alternative behavior. A non-preserving allocation with TCP or TLS transport to client SHOULD implement the alternate behavior, except where this conflicts with standard TCP or TLS behavior.
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This section describes the preferred and alternate behavior for various fields in the IP header.
Time to Live (IPv4) or Hop Count (IPv6)
Preferred Behavior: If the incoming value is 0, then send an ICMP Time Exceeded message back to the sender. Otherwise set the outgoing Time to Live/Hop Count to one less than the incoming value.
Alternate Behavior: Set the outgoing value to the default for outgoing packets.
Diff-Serv Code Point
Preferred Behavior: Set the outgoing value to the incoming value, unless the server, though configuration or other means, believes that a different setting is more appropriate.
Alternate Behavior: Set the outgoing value to Best Effort, unless the server, through configuration or other means, believes a different setting is more appropriate.
ECN
Preferred Behavior: Set the outgoing value to the incoming value, UNLESS the server is doing Active Queue Management, the incoming ECN field is 01 or 10, and the server wishes to indicate that congestion has been experienced, in which case set the outgoing value to 11.
Alternate Behavior: Set the outgoing value to 00 (ECN not supported)
Flow Label
Preferred Behavior: Set the outgoing flow label to 0.
Alternate Behavior: Same as the Preferred behavior.
IPv4 Fragmentation
Preferred Behavior:
If the outgoing packet size does not exceed the outgoing link's MTU, then send the outgoing packet unfragmented. Set the DF bit in the outgoing packet to the value of the DF bit in the incoming packet, and set the other fragmentation fields (Identification, MF, Fragment Offset) as appropriate for a packet originating from the server.
Otherwise, if the outgoing link's MTU is exceeded and the incoming DF bit is 0, then fragment the packet before sending. Set the outgoing DF to 0, and set the other fragmentation fields as appropriate for fragments originated from the server.
Otherwise [link MTU exceeded and incoming DF set], drop the outgoing packet and send an ICMP message of type 3 code 4 ("fragmentation needed and DF set") to the sender of the incoming packet.
Alternate Behavior: As described in the Preferred Behavior, except always assume the incoming DF bit is 0.
IPv6 Fragmentation
Preferred Behavior:
If the incoming packet did not include a Fragmentation header and the outgoing packet size does not exceed the outgoing link's MTU, then send the outgoing packet without a Fragmentation header.
If the incoming packet included a Fragment header and if the outgoing packet size (with a Fragmentation header included) does not exceed the outgoing link's MTU, then send the outgoing packet with a Fragmentation header. Set the fields of the Fragmentation header as appropriate for a packet originating from the server.
If the incoming packet did not include a Fragmentation header and the outgoing packet size exceeds the outgoing link's MTU , then drop the outgoing packet and send an ICMP message of type 2 code 0 ("Packet too big") to the sender of the incoming packet. If the packet is being sent to the peer, then reduce the MTU reported in the ICMP message by 48 bytes to allow room for the overhead of a Data indication.
Otherwise, if the link's MTU is exceeded and the incoming packet contained a Fragmentation header, then fragment the outgoing packet into fragments of no more than 1280 bytes. Set the fields of the Fragmentation header as appropriate for a packet originating from the server.
Alternate Behavior: As described in the Preferred Behavior, except always assume incoming packet has a Fragmentation header.
IPv4 Options
Preferred Behavior: The outgoing packet is sent without any IPv4 options.
Alternate Behavior: Same as preferred.
IPv6 Extention Headers
Preferred Behavior: The outgoing packet is sent without any IPv6 extension headers, with the exception of the Fragmentation header as described above
Alternate Behavior: Same as preferred.
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This sub-section describes the preferred behavior of ICMP relaying. The corresponding alternate behavior is to not relay ICMP messages.
When an ICMP message arrives at the server, the copy of the original IP packet present inside the ICMP message is examined. The server first checks that the original IP packet header is immediately followed by a UDP protocol header, such that the original source transport address was X and the original destination transport address was Y. The server also checks that the type and code values in the ICMP header are one of those relayed (see below). Other ICMP messages are either ignored, or used by the server internally in an unspecified manner.
The server then checks if one of the following two cases applies:
Case 1: X is a relayed-transport-address currently assigned to an active allocation on the server, and there exists a permission for the IP address of Y in the allocation.
In this case, the original IP packet was traveling from the server to a peer, so the the server relays the ICMP message back to the client. The server creates a Data indication where the PEER-ADDRESS attribute contains Y, and the ICMP attribute contains the type and code from the incoming ICMP message, and the DATA attribute contains application data from the original IP packet starting AFTER the UDP header. The server SHOULD include as much application data as possible consistent with not exceeding a total IP packet size of either 576 bytes (for IPv4) or 1280 bytes (for IPv6).
Note that there is no point in including the original IP or UDP header in the DATA attribute because those headers were generated by the server, not the client.
Case 2: There is an active allocation where X is the server transport address, Y is the client transport address, and UDP is used as transport between the client and the server. Furthermore, the packet after the UDP header is either (a) a ChannelData header which contains an active channel number in the allocation, or (b) a Data indication whose PEER-ADDRESS attribute contains an IP address for which there exists a permission in the allocation.
In this case, the original IP packet was traveling from the server to the client, so the server creates and sends an ICMP message to the peer. The outgoing ICMP message contains the type and code fields from the incoming ICMP message and then contains an approximation to the original IP packet sent from the peer to the server (the one the server was trying to relay to the client inside the ChannelData or Data indication). This approximation contains a synthesized IP header, a synthesized UDP header, and some application data. The synthesis is done as follows:
The remaining fields in the IP and UDP headers are simply set to sensible values, since for most of them there is no way to reconstruct the original values.
The server SHOULD relay as all ICMP type/code combinations and MUST relay at at least the following combinations. For IPv4:
Type 3, code 4: Fragmentation needed and DF set
For IPv6:
Type 2, code <any>: Packet too big
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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.
Request/Response Transactions
0x003 : Allocate
0x004 : Refresh
0x009 : ChannelBind
Indications
0x006 : Send
0x007 : Data
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This STUN extension defines the following new attributes:
0x000C: CHANNEL-NUMBER 0x000D: LIFETIME 0x0010: Reserved (was BANDWIDTH) 0x0012: PEER-ADDRESS 0x0013: DATA 0x0016: RELAY-ADDRESS 0x0018: REQUESTED-PROPS 0x0019: REQUESTED-TRANSPORT 0x0022: RESERVATION-TOKEN
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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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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.
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The 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.
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The DATA attribute is present in all Data Indications and most Send Indications. It contains raw payload data that is to be sent (in the case of a Send Request) or was received (in the case of a Data Indication).
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The RELAY-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.
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This attribute allows the client to request that the allocation have certain properties. The attribute is 32 bits long. Its format is:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E|R|P| MUST be 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first part of the attribute value contains a number of one-bit flags. These are:
- E:
- If 1, the port number for the relayed-transport-address must be even. If 0, the port number can be even or odd.
- R:
- If 1, the server must reserve the next highest port for a subsequent allocation. If 0, no such reservation is requested. If the client sets the R bit to 1, it MUST also set the E bit to 1 (however, the E bit may be 1 when the R bit is 0).
- P:
- If 1, the allocation must be a Preserving allocation. If 0, the allocation can be either Preserving or Non-Preserving.
All these flags have the property that if the bit is 1, and the server cannot create an allocation that satisfies the request, then the Allocate request is rejected. To allow future TURN extensions to define new flags that also have this property, the client MUST set the rest of the attribute to zero, and the server MUST fail the Allocate request if any bits which the server does not support are set to 1. By doing this, any new flags that are not recognized by the server will cause the Allocate request to fail.
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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 receiption. It is reserved for future uses.
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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.
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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.
- 438
- (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.
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TURN servers allocate resources to clients, in contrast to the Binding method defined in [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.). Therefore, a TURN server may require the authentication and authorization of STUN requests. This authentication is provided by mechanisms defined in the STUN specification itself, in particular digest authentication.
Because TURN servers allocate resources, they can be susceptible to denial-of-service attacks. All Allocate transactions are authenticated, so that an unknown attacker cannot launch an attack. An authenticated attacker can generate multiple Allocate Requests, however. To prevent a single malicious user from allocating all of the resources on the server, it is RECOMMENDED that a server implement a per user limit on the number of allocations that can active at one time. Such a mechanism does not prevent a large number of malicious users from each requesting a small number of allocations. Attacks such as these are possible using botnets, and are difficult to detect and prevent. Implementors of TURN should keep up with best practices around detection of anomalous botnet attacks.
A client will use the transport address learned from the RELAY-ADDRESS attribute of the Allocate Response to tell other users how to reach them. Therefore, a client needs to be certain that this address is valid, and will actually route to them. Such validation occurs through the message integrity checks provided in the Allocate response. They can guarantee the authenticity and integrity of the allocated addresses. Note that TURN is not susceptible to the attacks described in Section 12.2.3, 12.2.4, 12.2.5 or 12.2.6 of [I‑D.ietf‑behave‑rfc3489bis] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” July 2008.) [[TODO: Update section number references to 3489bis]]. These attacks are based on the fact that a STUN server mirrors the source IP address, which cannot be authenticated. STUN does not use the source address of the Allocate Request in providing the RELAY-ADDRESS, and therefore, those attacks do not apply.
TURN attempts to adhere as closely as possible to common firewall policies, consistent with allowing data to flow. TURN has fairly limited applicability, requiring a user to explicitly authorize permission to receive data from a peer, one IP address at a time. Thus, it does not provide a general technique for externalizing sockets. Rather, it has similar security properties to the placement of an address-restricted NAT in the network, allowing messaging in from a peer only if the internal client has sent a packet out towards the IP address of that peer. This limitation means that TURN cannot be used to run, for example, SIP servers, NTP servers, FTP servers or other network servers that service a large number of clients. Rather, it facilitates rendezvous of NATted clients that use some other protocol, such as SIP, to communicate IP addresses and ports for communications.
Confidentiality of the transport addresses learned through Allocate transactions does not appear to be that important. If required, it can be provided by running TURN over TLS.
TURN does not and cannot guarantee that UDP data is delivered in sequence or to the correct address. As most TURN clients will only communicate with a single peer, the use of a single channel number will be very common. Consider an enterprise where Alice and Bob are involved in separate calls through the enterprise NAT to their corporate TURN server. If the corporate NAT reboots, it is possible that Bob will obtain the exact NAT binding originally used by Alice. If Alice and Bob were using identical channel numbers, Bob will receive unencapsulated data intended for Alice and will send data accidentally to Alice's peer. This is not a problem with TURN. This is precisely what would happen if there was no TURN server and Bob and Alice instead provided a (STUN) reflexive transport address to their peers. If detecting this misdelivery is a problem, the client and its peer need to use message integrity on their data.
Relay servers are useful even for users not behind a NAT. They can provide a way for truly anonymous communications. A user can cause a call to have its media routed through a TURN server, so that the user's IP addresses are never revealed.
Any relay addresses learned through an Allocate request will not operate properly with IPSec Authentication Header (AH) (Kent, S., “IP Authentication Header,” December 2005.) [RFC4302] in transport or tunnel mode. However, tunnel-mode IPSec ESP (Kent, S., “IP Encapsulating Security Payload (ESP),” December 2005.) [RFC4303] should still operate.
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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 method, 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 12 (New STUN Methods).
The codepoints for the new STUN attributes defined in this specification are listed in Section 13 (New STUN Attributes).
The codepoints for the new STUN error codes defined in this specification are listed in Section 14 (New STUN Error Response Codes).
Extensions to TURN can be made through IETF consensus.
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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].
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TBD
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Note to RFC Editor: Please remove this section prior to publication of this document as an RFC.
This section lists the changes between the various versions of this specification.
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NOTE to RFC Editor: Please remove this section prior to publication of this document as an RFC.
Bandwidth: How should bandwidth be specified? What are the right rules around bandwidth?
Alternate Server: Do we still want this mechanism? Is the current proposal acceptable? Note that the usage of the ALTERNATE-SERVER attribute in this document is inconsistent with its usage in STUN. In STUN, if the ALTERNATE-SERVER attribute is used, then the error that the server would otherwise generate is replaced by a 300 (Try Alternate) code. In this document, the 300 error code is not used, and the server returns an appropriate error code and then includes the ALTERNATE-SERVER attribute in the response. In this way, the client can see the actual error code, rather than always seeing error code 300, and can thus make a more intelligent decision on whether it wishes to try the alternate server.
Public TURN servers: The text currently says that a server "SHOULD" use the Long-Term Credential mechanism, with the unstated idea that a public TURN server would not use it. But this really weakens the security of TURN. Is there a better way to allow public servers? Or should we just drop the notion of a public server entirely?
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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, Derek MacDonald, Cullen Jennings, Lars Eggert, Magnus Westerlund, and Eric Rescorla have been particularly helpful, with Eric also suggesting the channel allocation mechanism, and Cullen suggesting the REQUESTED-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.
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| [I-D.ietf-behave-rfc3489bis] | Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for (NAT) (STUN),” draft-ietf-behave-rfc3489bis-18 (work in progress), July 2008 (TXT). |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [RFC3697] | Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, “IPv6 Flow Label Specification,” RFC 3697, March 2004 (TXT). |
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| Jonathan Rosenberg | |
| Cisco Systems, Inc. | |
| Edison, NJ | |
| USA | |
| Email: | jdrosen@cisco.com |
| URI: | http://www.jdrosen.net |
| Rohan Mahy | |
| Plantronics, Inc. | |
| Email: | rohan@ekabal.com |
| Philip Matthews | |
| (Unaffiliated) | |
| Fax: | |
| Email: | philip_matthews@magma.ca |
| URI: |
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