TRAM WG T. Reddy, Ed.
Internet-Draft McAfee
Obsoletes: 5766, 6156 (if approved) A. Johnston, Ed.
Intended status: Standards Track Villanova University
Expires: January 23, 2020 P. Matthews
Alcatel-Lucent
J. Rosenberg
jdrosen.net
July 22, 2019

Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)
draft-ietf-tram-turnbis-28

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). 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. TURN differs from other relay control protocols in that it allows a client to communicate with multiple peers using a single relay address.

The TURN protocol was designed to be used as part of the ICE (Interactive Connectivity Establishment) approach to NAT traversal, though it also can be used without ICE.

This document obsoletes RFC 5766 and RFC 6156.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on January 23, 2020.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

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


Table of Contents

1. Introduction

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 "hole punching" techniques (see [RFC5128]) in an attempt discover a direct communication path; that is, a communication path that goes from one host to another through intervening NATs and routers, but does not traverse any relays.

As described in [RFC5128] and [RFC4787], hole punching techniques will fail if both hosts are behind NATs that are not well behaved. For example, if both hosts are behind NATs that have a mapping behavior of "address-dependent mapping" or "address- and port- dependent mapping" (Section 4.1 in [RFC4787]), then hole punching techniques generally fail.

When a direct communication path cannot be found, 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.

In many enterprise networks, direct UDP transmissions are not permitted between clients on the internal networks and external IP addresses. To permit media sessions in such a situation to use UDP and to avoid forcing the media sessions through TCP, an Enterprise Firewall can be configured to allow UDP traffic relayed through an Enterprise relay server. This scenario is required to be supported by the WebRTC requirements (Section 2.3.5.1 in [RFC7478]). In addition, in a SIP or WebRTC call, if the user wants IP location privacy from the peer then the client can select a relay server offering IP location privacy and only convey the relayed candidates to the peer for ICE connectivity checks (see Section 4.2.4 in [I-D.ietf-rtcweb-security]).

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 and from certain other hosts (called peers) and can control aspects of how the relaying is done. The client does this by obtaining an IP address and port on the server, called the relayed transport address. When a peer sends a packet to the relayed transport address, the server relays the transport protocol data from the packet to the client. The client knows the peer from which the transport protocol data was relayed by the server. If the server receives an ICMP error packet, the server also relays certain layer 3/4 header fields from the ICMP header to the client. When the client sends a packet to the server, the server relays the transport protocol data from the packet to the intended peer using the relayed transport address as the source.

A client using TURN must have some way to communicate the relayed transport address to its peers, and to learn each peer's IP address and port (more precisely, each peer's server-reflexive transport address, see Section 3). How this is done is out of the scope of the TURN protocol. One way this might be done is for the client and peers to exchange email messages. Another way is for the client and its peers to use a special-purpose "introduction" or "rendezvous" protocol (see [RFC5128] for more details).

If TURN is used with ICE [RFC8445], then the relayed transport address and the IP addresses and ports of the peers are included in the ICE candidate information that the rendezvous protocol must carry. For example, if TURN and ICE are used as part of a multimedia solution using SIP [RFC3261], then SIP serves the role of the rendezvous protocol, carrying the ICE candidate information inside the body of SIP messages [I-D.ietf-mmusic-ice-sip-sdp]. If TURN and ICE are used with some other rendezvous protocol, then ICE provides guidance on the services the rendezvous protocol must perform.

Though the use of a TURN server to enable communication between two hosts behind NATs is very likely to work, it comes at a high cost to the provider of the TURN server, since the server typically needs a high-bandwidth connection to the Internet. As a consequence, it is best to use a TURN server only when a direct communication path cannot be found. When the client and a peer use ICE to determine the communication path, ICE will use hole punching techniques to search for a direct path first and only use a 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 relayed transport address; a feature not supported by other approaches (e.g., SOCKS [RFC1928]). However, care has been taken to make sure that TURN is suitable for other types of applications.

TURN was designed as one piece in the larger ICE approach to NAT traversal. Implementors of TURN are urged to investigate ICE and seriously consider using it for their application. However, it is possible to use TURN without ICE.

TURN is an extension to the STUN (Session Traversal Utilities for NAT) protocol [I-D.ietf-tram-stunbis]. Most, though not all, TURN messages are STUN-formatted messages. A reader of this document should be familiar with STUN.

The TURN specification was originally published as [RFC5766], which was updated by [RFC6156] to add IPv6 support. This document supersedes and obsoletes both [RFC5766] and [RFC6156].

2. Terminology

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

Readers are expected to be familiar with [I-D.ietf-tram-stunbis] 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-tram-stunbis]. 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 "external side" of a NAT. This address is allocated by the NAT to correspond to a specific host transport address.
Relayed Transport Address:
A transport address on the TURN server that is used for relaying packets between the client and a peer. A peer sends to this address on the TURN server, and the packet is then relayed to the client.
TURN Server Transport Address:
A transport address on the TURN server that is used for sending TURN messages to the server. This is the transport address that the client uses to communicate with the server.
Peer Transport Address:
The transport address of the peer as seen by the server. When the peer is behind a NAT, this is the peer's server-reflexive transport address.
Allocation:
The relayed transport address granted to a client through an Allocate request, along with related state, such as permissions and expiration timers.
5-tuple:
The combination (client IP address and port, server IP address and port, and transport protocol (currently one of UDP, TCP, DTLS/UDP or TLS/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.
Transport Protocol:
The protocols above IP that carries TURN Requests, Responses, and Indications as well as providing identifiable flows using a 5-tuple. In this specification, UDP and TCP are defined as transport protocols, as well as their combination with a security layer using DTLS and TLS respectively.
Channel:
A channel number and associated peer transport address. Once a channel number is bound to a peer's transport address, the client and server can use the more bandwidth-efficient ChannelData message to exchange data.
Permission:
The IP address and transport protocol (but not the port) of a peer that is permitted to send traffic to the TURN server and have that traffic relayed to the TURN client. The TURN server will only forward traffic to its client from peers that match an existing permission.
Realm:
A string used to describe the server or a context within the server. The realm tells the client which username and password combination to use to authenticate requests.
Nonce:
A string chosen at random by the server and included in the server response. To prevent replay attacks, the server should change the nonce regularly.
(D)TLS:
This term is used for statements that apply to both Transport Layer Security [RFC8446] and Datagram Transport Layer Security [RFC6347].

3. 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 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 with which the TURN client wishes to communicate. 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.

                                        Peer A
                                        Server-Reflexive    +---------+
                                        Transport Address   |         |
                                        192.0.2.150:32102   |         |
                                            |              /|         |
                          TURN              |            / ^|  Peer A |
    Client's              Server            |           /  ||         |
    Host Transport        Transport         |         //   ||         |
    Address               Address           |       //     |+---------+
 198.51.100.2:49721     192.0.2.15:3478     |+-+  //     Peer A
            |               |               ||N| /       Host Transport
            |   +-+         |               ||A|/        Address
            |   | |         |               v|T|     203.0.113.2:49582
            |   | |         |               /+-+       
 +---------+|   | |         |+---------+   /              +---------+
 |         ||   |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      192.0.2.15:50000     192.0.2.210:49191

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]).

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 that is known as the TURN SERVER TRANSPORT ADDRESS. The client learns the TURN server 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. This data structure contains, amongst other things, the RELAYED TRANSPORT ADDRESS for the allocation. The relayed transport address is the transport address on the server that peers can use to have the server relay data to the client. An allocation is uniquely identified by its relayed transport address.

Once an allocation is created, the client can send application data to the server along with an indication of to which peer the data is to be sent, and the server will relay this data to the intended 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.

When the peer is behind a NAT, then the client must identify the peer using its server-reflexive transport address rather than its host transport address. For example, to send application data to Peer A in the example above, the client must specify 192.0.2.150:32102 (Peer A's server-reflexive transport address) rather than 203.0.113.2:49582 (Peer A's host transport address).

Each allocation on the server belongs to a single client and has exactly one or two relayed transport addresses that is used only by that allocation. Thus, when a packet arrives at a relayed transport address on the server, the server knows for which client the data is intended.

The client may have multiple allocations on a server at the same time.

3.1. 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, Transport Layer Security (TLS) over TCP or Datagram Transport Layer Security (DTLS) over UDP 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
DTLS-over-UDP UDP

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.

Since this version of TURN only supports UDP between the server and the peer, it is expected that most clients will prefer to use UDP between the client and the server as well. That being the case, some readers may wonder: Why also support TCP and TLS-over-TCP?

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 and DTLS-over-UDP transport between the client and the server because (D)TLS provides additional security properties not provided by TURN's default digest authentication; properties that some clients may wish to take advantage of. In particular, (D)TLS provides a way for the client to ascertain that it is talking to the correct server, and provides for confidentiality of TURN control messages. If (D)TLS transport is used between the TURN client and the TURN server, the cipher suites, server certificate validation and authentication of TURN server are discussed in Section 6.2.3 of [I-D.ietf-tram-stunbis]. The guidance given in [RFC7525] MUST be followed to avoid attacks on (D)TLS. TURN does not require (D)TLS because the overhead of using (D)TLS is higher than that of digest authentication; for example, using (D)TLS likely means that most application data will be doubly encrypted (once by (D)TLS and once to ensure it is still encrypted in the UDP datagram).

There is an extension to TURN for TCP transport between the server and the peers [RFC6062]. 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.

In some applications for TURN, the client may send and receive 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. The algorithm of demultiplexing packets received from multiple protocols on the host transport address is discussed in [RFC7983].

3.2. Allocations

To create an allocation on the server, the client uses an Allocate transaction. The client sends an 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 has security implications, the server requires that the client authenticate itself, typically using STUN’s long-term credential mechanism or the STUN Extension for Third-Party Authorization [RFC7635], 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 default lifetime of an allocation is 10 minutes -- this value was chosen to be long enough so that refreshing is not typically a burden on the client, while expiring allocations where the client has unexpectedly quit in a timely manner. However, the client can request a longer 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 use 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).

TURN                                 TURN           Peer          Peer
client                               server          A             B
  |-- Allocate request --------------->|             |             |
  |   (invalid or missing credentials) |             |             |     
  |                                    |             |             |
  |<--------------- Allocate failure --|             |             |
  |              (401 Unauthenticated) |             |             |
  |                                    |             |             |
  |-- Allocate request --------------->|             |             |
  |               (valid credentials)  |             |             |
  |                                    |             |             |
  |<---------- Allocate success resp --|             |             |
  |            (192.0.2.15:50000)      |             |             |
  //                                   //            //            //
  |                                    |             |             |
  |-- Refresh request ---------------->|             |             |
  |                                    |             |             |
  |<----------- Refresh success resp --|             |             |
  |                                    |             |             |

Figure 2

In Figure 2, the client sends an Allocate request to the server with invalid or missing 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. 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.

3.3. 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].

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, and there is no way to explicitly delete a permission. This behavior was selected to match the behavior of a NAT that complies with [RFC4787].

The client can install or refresh a permission using either a CreatePermission request or a ChannelBind request. Using the CreatePermission request, multiple permissions can be installed or refreshed with a single request -- this is important for applications that use ICE. 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.

3.4. 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 mechanism uses channels. Common to both mechanisms is the ability of the client to communicate with multiple peers using a single allocated relayed transport address; thus, both mechanisms include a means for the client to indicate to the server which peer should receive the data, and for the server to indicate to the client 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 specifying the (server-reflexive) 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 server-reflexive 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 should receive the data.

Some ICMP (Internet Control Message Protocol) packets arriving at the relayed transport address on the TURN server may be 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 ICMP type and code in a ICMP attribute. ICMP attribute forwarding always uses Data indications containing the XOR-PEER-ADDRESS and ICMP attributes, even when using the channel mechanism to forward UDP data.

Send and Data indications cannot be authenticated, since the long-term credential mechanism of STUN does not support authenticating indications. This is not as big an issue as it might first appear, since the client-to-server leg is only half of the total path to the peer. Applications that want end-to-end security should encrypt the data sent between the client and a peer.

Because Send indications are not authenticated, it is possible for an attacker to send bogus Send indications to the server, which will then relay these to a peer. To partly mitigate this attack, TURN requires that the client install a permission towards a peer before sending data to it using a Send indication. The technique to fully mitigate the attack is discussed in Section 21.1.4.

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)-------------->|             |             |
  |                                    | dropped     |             |
  |                                    |             |             |
  |                                    |<== data ==================|
  |                            dropped |             |             |
  |                                    |             |             |

Figure 3

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 (server-reflexive) IP address in the XOR-PEER-ADDRESS attribute; if this was not done, the server would not relay data between the client and the server. 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. Later, the client attempts to exchange data with Peer B; however, no permission has been installed for Peer B, so the Send indication from the client and the UDP datagram from the peer are both dropped by the server.

3.5. 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 -- this lifetime was chosen to be longer than the permission lifetime. 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 --|             |             |
  |                                    |             |             |

Figure 4

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 Peer A (which is the peer bound to channel number 0x4001).

In the reverse direction, when Peer A sends a UDP datagram to the relayed transport address, this UDP datagram arrives at the server on the relayed transport address assigned to the allocation. Since the UDP datagram was received from Peer A, which has a channel number assigned to it, the server encapsulates the data into a ChannelData message when sending the data 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 would use the Send mechanism.

3.6. Unprivileged TURN Servers

This version of TURN is designed so that the server can be implemented as an application that runs in user space under commonly available operating systems without requiring special privileges. This design decision was made to make it easy to deploy a TURN server: for example, to allow a TURN server to be integrated into a peer-to-peer application so that one peer can offer NAT traversal services to another peer and to use (D)TLS to secure the TURN connection.

This design decision has the following implications for data relayed by a TURN server:

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

3.7. Avoiding IP Fragmentation

For reasons described in [Frag-Harmful], applications, especially those sending large volumes of data, should avoid having their packets fragmented. [I-D.ietf-intarea-frag-fragile] discusses issues associated with IP fragmentation and proposes alternatives to IP fragmentation. Applications using TCP can more or less ignore this issue because fragmentation avoidance is now a standard part of TCP, but applications using UDP (and thus any application using this version of TURN) need to avoid IP fragmentation by sending sufficiently small messages or use UDP fragmentation [I-D.ietf-tsvwg-udp-options]. Note that the UDP fragmentation option needs to be supported by both endpoints, and at the time of writing of this document, UDP fragmentation support is under discussion and is not deployed.

The application running on the client and the peer can take one of two approaches to avoid IP fragmentation until UDP fragmentation support is available. The first uses messages that are limited to a predetermined fixed maximum and the second relies on network feedback to adapt that maximum.

The first approach is to avoid sending large amounts of application data in the TURN messages/UDP datagrams exchanged between the client and the peer. This is the approach taken by most VoIP (Voice-over-IP) applications. In this approach, the application MUST assume a PMTU of 1280 bytes, as IPv6 requires that every link in the Internet have an MTU of 1280 octets or greater as specified in [RFC8200]. If IPv4 support on legacy or otherwise unusual networks is a consideration, the application MAY assume on an effective MTU of 576 bytes for IPv4 datagrams, as every IPv4 host must be capable of receiving a packet whose length is equal to 576 bytes as discussed in [RFC0791] and [RFC1122].

The exact amount of application data that can be included while avoiding fragmentation depends on the details of the TURN session between the client and the server: whether UDP, TCP, or (D)TLS transport is used, whether ChannelData messages or Send/Data indications are used, and whether any additional attributes (such as the DONT-FRAGMENT attribute) are included. Another factor, which is hard to determine, is whether the MTU is reduced somewhere along the path for other reasons, such as the use of IP-in-IP tunneling.

As a guideline, sending a maximum of 500 bytes of application data in a single TURN message (by the client on the client-to-server leg) or a UDP datagram (by the peer on the peer-to-server leg) will generally avoid IP fragmentation. To further reduce the chance of fragmentation, it is recommended that the client use ChannelData messages when transferring significant volumes of data, since the overhead of the ChannelData message is less than Send and Data indications.

The second approach the client and peer can take to avoid fragmentation is to use a path MTU discovery algorithm to determine the maximum amount of application data that can be sent without fragmentation. The classic path MTU discovery algorithm defined in [RFC1191] may not be able to discover the MTU of the transmission path between the client and the peer since:

As a result, the client and server need to use a path MTU discovery algorithm that does not require ICMP messages. The Packetized Path MTU Discovery algorithm defined in [RFC4821] is one such algorithm.

[I-D.ietf-tram-stun-pmtud] is an implementation of [RFC4821] that uses STUN to discover the path MTU, and so might be a suitable approach to be used in conjunction with a TURN server that supports the DONT-FRAGMENT attribute. When the client includes the DONT-FRAGMENT attribute in a Send indication, this tells the server to set the DF bit in the resulting 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 -- any server that does not support the DONT-FRAGMENT attribute will indicate this by rejecting the Allocate request. If the TURN server carrying out packet translation from IPv4-to-IPv6 is unable to access the state of Don't Fragment (DF) bit in the IPv4 header, it MUST reject the Allocate request with DONT-FRAGMENT attribute.

3.8. RTP Support

One of the envisioned uses of TURN is as a relay for clients and peers wishing to exchange real-time data (e.g., voice or video) using RTP. To facilitate the use of TURN for this purpose, TURN includes some special support for older versions of RTP.

Old versions of RTP [RFC3550] required that the RTP stream be on an even port number and the associated RTP Control Protocol (RTCP) stream, if present, be on the next highest port. To allow clients to work with peers 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.

3.9. Happy Eyeballs for TURN

If an IPv4 path to reach a TURN server is found, but the TURN server's IPv6 path is not working, a dual-stack TURN client can experience a significant connection delay compared to an IPv4-only TURN client. To overcome these connection setup problems, the TURN client needs to query both A and AAAA records for the TURN server specified using a domain name and try connecting to the TURN server using both IPv6 and IPv4 addresses in a fashion similar to the Happy Eyeballs mechanism defined in [RFC8305]. The TURN client performs the following steps based on the transport protocol being used to connect to the TURN server.

4. Discovery of TURN server

Methods of TURN server discovery, including using anycast, are described in [RFC8155]. If a host with multiple interfaces discovers a TURN server in each interface, the mechanism described in [RFC7982] can be used by the TURN client to influence the TURN server selection. The syntax of the "turn" and "turns" URIs are defined in Section 3.1 of [RFC7065]. DTLS as a transport protocol for TURN is defined in [RFC7350].

4.1. TURN URI Scheme Semantics

The "turn" and "turns" URI schemes are used to designate a TURN server (also known as a relay) on Internet hosts accessible using the TURN protocol. The TURN protocol supports sending messages over UDP, TCP, TLS-over-TCP or DTLS-over-UDP. The "turns" URI scheme MUST be used when TURN is run over TLS-over-TCP or in DTLS-over-UDP, and the "turn" scheme MUST be used otherwise. The required <host> part of the "turn" URI denotes the TURN server host. The <port> part, if present, denotes the port on which the TURN server is awaiting connection requests. If it is absent, the default port is 3478 for both UDP and TCP. The default port for TURN over TLS and TURN over DTLS is 5349.

5. 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-tram-stunbis] apply to STUN-formatted messages. This means that all the message-forming and message-processing descriptions in this document are implicitly prefixed with the rules of [I-D.ietf-tram-stunbis].

[I-D.ietf-tram-stunbis] specifies an authentication mechanism called the long-term credential mechanism. TURN servers and clients MUST implement this mechanism, and the authentication options are discussed in Section 7.2.

Note that the long-term credential mechanism applies only to requests and cannot be used to authenticate indications; thus, indications in TURN are never authenticated. If the server requires requests to be authenticated, then the server's administrator MUST choose a realm value that will uniquely identify the username and password combination that the client must use, even if the client uses multiple servers under different administrations. The server's administrator MAY choose to allocate a unique username to each client, or MAY choose to allocate the same username to more than one client (for example, to all clients from the same department or company). For each Allocate request, the server SHOULD generate a new random nonce when the allocation is first attempted following the randomness recommendations in [RFC4086] and SHOULD expire the nonce at least once every hour during the lifetime of the allocation. To indicate that the server supports [I-D.ietf-tram-stunbis], the server prepends the NONCE attribute value with the "nonce cookie" (Section 9.2 of [I-D.ietf-tram-stunbis]).

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

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

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

TURN does not use the backwards-compatibility mechanism described in [I-D.ietf-tram-stunbis].

TURN, as defined in this specification, supports both IPv4 and IPv6. IPv6 support in TURN includes IPv4-to-IPv6, IPv6-to-IPv6, and IPv6-to-IPv4 relaying. When only a single address type is desired, the REQUESTED-ADDRESS-FAMILY attribute is used to explicitly request the address type the TURN server will allocate (e.g., an IPv4-only node may request the TURN server to allocate an IPv6 address). If both IPv4 and IPv6 are desired, the single ADDITIONAL-ADDRESS-FAMILY attribute indicates a request to the server to allocate one IPv4 and one IPv6 relay address in a single Allocate request. This saves local ports on the client and reduces the number of messages sent between the client and the TURN server.

By default, TURN runs on the same ports as STUN: 3478 for TURN over UDP and TCP, and 5349 for TURN over (D)TLS. However, TURN has its own set of Service Record (SRV) names: "turn" for UDP and TCP, and "turns" for (D)TLS. Either the DNS resolution procedures or the ALTERNATE-SERVER procedures, both described in Section 7, can be used to run TURN on a different port.

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

When UDP or DTLS-over-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-tram-stunbis]). Some implementations may choose to meet this requirement by remembering all received requests and the corresponding responses for 40 seconds (Section 6.3.1 in [I-D.ietf-tram-stunbis]). 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 7.2), and since UDP does not include a congestion control mechanism, it should discard application data traffic that exceeds the bandwidth quota.

6. Allocations

All TURN operations revolve around allocations, and all TURN messages are associated with either a single or dual 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. On the client, the 5-tuple uses the client's host transport address; on the server, the 5-tuple uses the client's server-reflexive transport address. The relayed transport address MUST be unique across all allocations so it can be used to uniquely identify the allocation, and an allocation in this context can be either a single or dual allocation.

The authentication information (e.g., username, password, realm, and nonce) is used to both verify subsequent requests and to compute the message integrity of responses. The username, realm, and nonce values are initially those used in the authenticated Allocate request that creates the allocation, though the server can change the nonce value during the lifetime of the allocation using a 438 (Stale Nonce) reply. For security reasons, the server MUST NOT store the password explicitly and MUST store the key value, which is a cryptographic hash over the username, realm, and password (see Section 16.1.3 in [I-D.ietf-tram-stunbis]).

Note that if the response contains a PASSWORD-ALGORITHMS attribute and this attribute contains both MD5 and SHA-256 algorithms, and the client also supports both the algorithms, the request MUST contain a PASSWORD-ALGORITHM attribute with the SHA-256 algorithm.

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

The list of permissions is described in Section 9 and the list of channels is described in Section 12.

7. Creating an Allocation

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

7.1. 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 picks a currently unused transport address, typically by allowing the underlying OS to pick a currently unused port.

The client then picks a transport protocol that the client supports to use between the client and the server based on the transport protocols supported by the server. 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 discovery or by experiment, that it is unable to contact any TURN server using UDP. See Section 3.1 for more discussion.

The client also picks a server transport address, which SHOULD be done as follows. The client uses one or more procedures described in [RFC8155] to discover a TURN server and uses the TURN server resolution mechanism defined in [RFC5928] and [RFC7350] to get a list of server transport addresses that can be tried to create a TURN allocation.

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

If the client wishes to obtain a relayed transport address of a specific address type then it includes a REQUESTED-ADDRESS-FAMILY attribute in the request. This attribute indicates the specific address type the client wishes the TURN server to allocate. Clients MUST NOT include more than one REQUESTED-ADDRESS-FAMILY attribute in an Allocate request. Clients MUST NOT include a REQUESTED-ADDRESS-FAMILY attribute in an Allocate request that contains a RESERVATION-TOKEN attribute, for the reason that the server uses the previously reserved transport address corresponding to the included token and the client cannot obtain a relayed transport address of a specific address type.

If the client wishes to obtain one IPv6 and one IPv4 relayed transport address then it includes an ADDITIONAL-ADDRESS-FAMILY attribute in the request. This attribute specifies that the server must allocate both address types. The attribute value in the ADDITIONAL-ADDRESS-FAMILY MUST be set to 0x02 (IPv6 address family). Clients MUST NOT include REQUESTED-ADDRESS-FAMILY and ADDITIONAL-ADDRESS-FAMILY attributes in the same request. Clients MUST NOT include ADDITIONAL-ADDRESS-FAMILY attribute in a Allocate request that contains a RESERVATION-TOKEN attribute. Clients MUST NOT include ADDITIONAL-ADDRESS-FAMILY attribute in a Allocate request that contains an EVEN-PORT attribute with the R bit set to 1. The reason behind the restriction is if EVEN-PORT with R bit set to 1 is allowed with the ADDITIONAL-ADDRESS-FAMILY attribute, two tokens will have to be returned in success response and requires changes to the way RESERVATION-TOKEN is handled.

If the client wishes the server to initialize the time-to-expiry field of the allocation to some value other than the default lifetime, then it MAY include a LIFETIME attribute specifying its desired value. This is just a hint, 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.

7.2. Receiving an Allocate Request

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

  1. The TURN server provided by the local or access network MAY allow unauthenticated request in order to accept Allocation requests from new and/or guest users in the network who do not necessarily possess long term credentials for STUN authentication. Making STUN authentication optional and its security implications are discussed in [RFC8155]. Otherwise, the server MUST require that the request be authenticated. If the request is authenticated, the authentication MUST be done either using the long-term credential mechanism of [I-D.ietf-tram-stunbis] or the STUN Extension for Third-Party Authorization [RFC7635] unless the client and server agree to use another mechanism through some procedure outside the scope of this document.
  2. The server checks if the 5-tuple is currently in use by an existing allocation. If yes, the server rejects the request with a 437 (Allocation Mismatch) error.
  3. The server checks if the request contains a REQUESTED-TRANSPORT attribute. If the REQUESTED-TRANSPORT attribute is not included or is malformed, the server rejects the request with a 400 (Bad Request) error. Otherwise, if the attribute is included but specifies a protocol other than UDP that is not supported by the server, 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 14 and Section 15), then the server treats the DONT-FRAGMENT attribute in the Allocate request as an unknown comprehension-required attribute.
  5. The server checks if the request contains a RESERVATION-TOKEN attribute. If yes, and the request also contains an EVEN-PORT or REQUESTED-ADDRESS-FAMILY or ADDITIONAL-ADDRESS-FAMILY attribute, the server rejects the request with a 400 (Bad Request) error. Otherwise, it checks to see if the token is valid (i.e., the token is in range and has not expired and the corresponding relayed transport address is still available). If the token is not valid for some reason, the server rejects the request with a 508 (Insufficient Capacity) error.
  6. The server checks if the request contains both REQUESTED-ADDRESS-FAMILY and ADDITIONAL-ADDRESS-FAMILY attributes. If yes, then the server rejects the request with a 400 (Bad Request) error.
  7. If the server does not support the address family requested by the client in REQUESTED-ADDRESS-FAMILY or if the allocation of the requested address family is disabled by local policy, it MUST generate an Allocate error response, and it MUST include an ERROR-CODE attribute with the 440 (Address Family not Supported) response code. If the REQUESTED-ADDRESS-FAMILY attribute is absent and the server does not support IPv4 address family, the server MUST include an ERROR-CODE attribute with the 440 (Address Family not Supported) response code. If the REQUESTED-ADDRESS-FAMILY attribute is absent and the server supports IPv4 address family, the server MUST allocate an IPv4 relayed transport address for the TURN client.
  8. The server checks if the request contains an EVEN-PORT attribute with the R bit set to 1. If yes, and the request also contains an ADDITIONAL-ADDRESS-FAMILY attribute, the server rejects the request with a 400 (Bad Request) error. Otherwise, the server checks if it can satisfy the request (i.e., can allocate a relayed transport address as described below). If the server cannot satisfy the request, then the server rejects the request with a 508 (Insufficient Capacity) error.
  9. The server checks if the request contains an ADDITIONAL-ADDRESS-FAMILY attribute. If yes, and the attribute value is 0x01 (IPv4 address family), then the server rejects the request with a 400 (Bad Request) error. Otherwise, the server checks if it can allocate relayed transport addresses of both address types. If the server cannot satisfy the request, then the server rejects the request with a 508 (Insufficient Capacity) error. If the server can partially meet the request, i.e. if it can only allocate one relayed transport address of a specific address type, then it includes ADDRESS-ERROR-CODE attribute in the success response to inform the client the reason for partial failure of the request. The error code value signaled in the ADDRESS-ERROR-CODE attribute could be 440 (Address Family not Supported) or 508 (Insufficient Capacity). If the server can fully meet the request, then the server allocates one IPv4 and one IPv6 relay address, and returns an Allocate success response containing the relayed transport addresses assigned to the dual allocation in two XOR-RELAYED-ADDRESS attributes.
  10. 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.
  11. 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-tram-stunbis].

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:

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

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

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

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

7.3. 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 or addresses are part of an address family or families that the client understands and is prepared to handle. If these addresses are not part of an address family or families which the client is prepared to handle, then the client MUST delete the allocation (Section 8) and MUST NOT attempt to create another allocation on that server until it believes the mismatch has been fixed.

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.

If the client receives an Allocate success response but with ADDRESS-ERROR-CODE attribute in the response and the error code value signaled in the ADDRESS-ERROR-CODE attribute is 440 (Address Family not Supported), the client MUST NOT retry its request for the rejected address type. If the client receives an ADDRESS-ERROR-CODE attribute in the response and the error code value signaled in the ADDRESS-ERROR-CODE attribute is 508 (Insufficient Capacity), the client SHOULD wait at least 1 minute before trying to request any more allocations on this server for the rejected address type.

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.

7.4. Receiving an Allocate Error Response

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

Note that the error code values 486 and 508 indicate to a eavesdropper that several other users are using the server at this time, similar to that of the HTTP error response code 503, but does not reveal any information about the users using the TURN server.

An unknown error response MUST be handled as described in [I-D.ietf-tram-stunbis].

8. Refreshing an Allocation

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

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

8.1. 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.

When refreshing a dual allocation, the client includes REQUESTED-ADDRESS-FAMILY attribute indicating the address family type that should be refreshed. If no REQUESTED-ADDRESS-FAMILY is included then the request should be treated as applying to all current allocations. The client MUST only include the family type it previously allocated and has not yet deleted. This process can also be used to delete an allocation of a specific address type, by setting the lifetime of that refresh request to 0. Deleting a single allocation destroys any permissions or channels associated with that particular allocation; it MUST NOT affect any permissions or channels associated with allocations for the other address family.

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.

8.2. Receiving a Refresh Request

When the server receives a Refresh request, it processes the request as per Section 5 plus the specific rules mentioned here.

If the server receives a Refresh Request with a REQUESTED-ADDRESS-FAMILY attribute and the attribute value does not match the address family of the allocation, the server MUST reply with a 443 (Peer Address Family Mismatch) Refresh error response.

The server computes a value called the "desired lifetime" as follows: if the request contains a LIFETIME attribute and the attribute value is 0, then the "desired lifetime" is 0. Otherwise, if the request contains a LIFETIME attribute, then the server computes the minimum of the client's requested lifetime and the server's maximum allowed lifetime. If this computed value is greater than the default lifetime, then the "desired lifetime" is the computed value. Otherwise, the "desired lifetime" is the default lifetime.

Subsequent processing depends on the "desired lifetime" value:

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

8.3. 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 its request to refresh the allocation, it should consider the allocation no longer exists. If the client receives a 438 (Stale Nonce) error to its request to refresh the allocation, it should reattempt the request with the new nonce value.

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.

9. Permissions

For each allocation, the server keeps a list of zero or more permissions. Each permission consists of an IP address and an associated time-to-expiry. While a permission exists, all peers using the IP address in the permission are allowed to send data to the client. The time-to-expiry is the number of seconds until the permission expires. Within the context of an allocation, a permission is uniquely identified by its associated IP address.

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

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

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

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

When a UDP datagram arrives at the relayed transport address for the allocation, the server extracts the source IP address from the IP header. The server then compares this address with the IP address associated with each permission in the list of permissions for the allocation. Note that only addresses are compared and port numbers are not considered. If no match is found, relaying is not permitted, and the server silently discards the UDP datagram. If an exact match is found, the permission check is considered to have succeeded and the server continues to process the UDP datagram as specified elsewhere (Section 11.3).

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.

10. CreatePermission

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

A CreatePermission request may be used in conjunction with either the Send mechanism in Section 11 or the Channel mechanism in Section 12.

10.1. Forming a CreatePermission Request

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

When forming a CreatePermission request, the client MUST include at least one XOR-PEER-ADDRESS attribute, and MAY include more than one such attribute. The IP address portion of each XOR-PEER-ADDRESS attribute contains the IP address for which a permission should be installed or refreshed. The port portion of each XOR-PEER-ADDRESS attribute will be ignored and can be any arbitrary value. The various XOR-PEER-ADDRESS attributes MAY appear in any order. The client MUST only include XOR-PEER-ADDRESS attributes with addresses of the same address family as that of the relayed transport address for the allocation. For dual allocations obtained using the ADDITIONAL-ADDRESS-FAMILY attribute, the client MAY include XOR-PEER-ADDRESS attributes with addresses of IPv4 and IPv6 address families.

10.2. Receiving a CreatePermission Request

When the server receives the CreatePermission request, it processes as per Section 5 plus the specific rules mentioned here.

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

If an XOR-PEER-ADDRESS attribute contains an address of an address family that is not the same as that of a relayed transport address for the allocation, the server MUST generate an error response with the 443 (Peer Address Family Mismatch) response code.

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

If the message is valid and the server is capable of carrying out the request, then the server installs or refreshes a permission for the IP address contained in each XOR-PEER-ADDRESS attribute as described in Section 9. The port portion of each attribute is ignored and may be any arbitrary value.

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

10.3. Receiving a CreatePermission Response

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

11. Send and Data Methods

TURN supports two mechanisms for sending and receiving data from peers. This section describes the use of the Send and Data mechanisms, while Section 12 describes the use of the Channel mechanism.

11.1. 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. However, the client MUST ensure that there is a permission installed for the IP address of the peer to which the Send indication is being sent; this prevents a third party from using a TURN server to send data to arbitrary destinations.

When forming a Send indication, the client MUST include an 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.

11.2. Receiving a Send Indication

When the server receives a Send indication, it processes as per Section 5 plus the specific rules mentioned here.

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

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 discarded).

The server also checks that there is a permission installed for the IP address contained in the XOR-PEER-ADDRESS attribute. If no such permission exists, the message is discarded. Note that a Send indication never causes the server to refresh the permission.

The server MAY impose restrictions on the IP address and port values allowed in the XOR-PEER-ADDRESS attribute -- if a value is not allowed, the server silently discards the Send indication.

If everything is OK, then the server forms a UDP datagram as follows:

The handling of the DONT-FRAGMENT attribute (if present), is described in Section 14 and Section 15.

The resulting UDP datagram is then sent to the peer.

11.3. 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. The server then checks to see whether the set of permissions for the allocation allow the relaying of the UDP datagram as described in Section 9.

If relaying is permitted, then the server checks if there is a channel bound to the peer that sent the UDP datagram (see Section 12). If a channel is bound, then processing proceeds as described in Section 12.7.

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 an 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.

11.4. Receiving a Data Indication

When the client receives a Data indication, it checks that the Data indication contains an XOR-PEER-ADDRESS 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.

If the XOR-PEER-ADDRESS is present and valid, the client checks that the Data indication contains either a DATA attribute or an ICMP attribute and discards the indication if it does not. Note that a DATA attribute is allowed to contain zero bytes of data. Processing of Data indications with an ICMP attribute is described in Section 11.6.

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.

11.5. Receiving an ICMP Packet

When the server receives an ICMP packet, the server verifies that the type is either 3 or 11 for an ICMPv4 [RFC0792] packet or either 1, 2, or 3 for an ICMPv6 [RFC4443] packet. It also verifies that the IP packet in the ICMP packet payload contains a UDP header, and if a UDP header is present, the server extracts the UDP source and destination IP address and port information. If either of these conditions fail, then the ICMP packet is silently dropped.

The server looks up the allocation whose relayed transport address corresponds to the encapsulated packet's source IP address and UDP port. If no such allocation exists, the packet is silently dropped. The server then checks to see whether the set of permissions for the allocation allows the relaying of the ICMP packet. For ICMP packets, the source IP address MUST NOT be checked against the permissions list as it would be for UDP packets. Instead, the server extracts the destination IP address from the encapsulated IP header. The server then compares this address with the IP address associated with each permission in the list of permissions for the allocation. If no match is found, relaying is not permitted, and the server silently discards the ICMP packet. Note that only addresses are compared and port numbers are not considered.

If relaying is permitted then the server forms and sends a Data indication. The Data indication MUST contain both an XOR-PEER-ADDRESS and an ICMP attribute. The ICMP attribute is set to the value of the type and code fields from the ICMP packet. The IP address portion of XOR-PEER-ADDRESS attribute is set to the destination IP address in the encapsulated IP header. At the time of writing of this specification, Socket APIs on some operating systems do not deliver the destination port in the encapsulated UDP header to applications without superuser privileges. If destination port in the encapsulated UDP header is available to the server then the port portion of XOR-PEER-ADDRESS attribute is set to the destination port otherwise the port portion is set to 0. The Data indication is then sent on the 5-tuple associated with the allocation.

Implementation Note: New ICMP types or codes can be defined in future specifications. If the server receives an ICMP error packet, and the new type or code field can help the client to make use of the ICMP error notification and generate feedback to the application layer, the server sends the Data indication with an ICMP attribute conveying the new ICMP type or code.

11.6. Receiving a Data Indication with an ICMP attribute

When the client receives a Data indication with an ICMP attribute, it checks that the Data indication contains an XOR-PEER-ADDRESS 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 an active permission, and discard the Data indication otherwise.

If the Data indication passes the above checks, the client signals the application of the error condition, along with an indication that it was received from the peer whose transport address is given by the XOR-PEER-ADDRESS attribute. The application can make sense of the meaning of the type and code values in the ICMP attribute by using the family field in the XOR-PEER-ADDRESS attribute.

12. 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.

The ChannelData message (see Section 12.4) starts with a two-byte field that carries the channel number. The values of this field are allocated as follows:

Note that the channel number range is not backwards compatible with [RFC5766], and cannot be used in environments where such compatibility is required.

According to [RFC7983], ChannelData messages can be distinguished from other multiplexed protocols by examining the first byte of the message:

+------------+------------------------------+
| [0..3]     | STUN                         |
|            |                              | 
+-------------------------------------------+
| [16..19]   | ZRTP                         |
|            |                              | 
+-------------------------------------------+
| [20..63]   | DTLS                         |
|            |                              | 
+-------------------------------------------+
| [64..79]   | TURN Channel                 |
|            |                              | 
+-------------------------------------------+
| [128..191] | RTP/RTCP                     |
|            |                              | 
+-------------------------------------------+
| Others     | Reserved, MUST be dropped    |
|            | and an alert MAY be logged   |
+-------------------------------------------+

Figure 5

Reserved values may be used in the future by other protocols. When the client uses channel binding, it MUST comply with the demultiplexing scheme discussed above.

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 to 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 the use of a channel number or peer transport address in a channel binding in one allocation has no impact on their use in a different 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 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.

12.1. 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 (see Section 9).

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 an XOR-PEER-ADDRESS attribute. Section 12.2 describes the restrictions on these attributes. The client MUST only include an XOR-PEER-ADDRESS attribute with an address of the same address family as that of a relayed transport address for the allocation.

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.

12.2. Receiving a ChannelBind Request

When the server receives a ChannelBind request, it processes as per Section 5 plus the specific rules mentioned here.

The server checks the following:

If any of these tests fail, the server replies with a 400 (Bad Request) error. If the XOR-PEER-ADDRESS attribute contains an address of an address family that is not the same as that of a relayed transport address for the allocation, the server MUST generate an error response with the 443 (Peer Address Family Mismatch) response code.

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

If the request is valid, but the server is unable to fulfill the request due to some capacity limit or similar, the server replies with a 508 (Insufficient Capacity) error.

Otherwise, the server replies with a ChannelBind success response. There are no required attributes in a successful ChannelBind response.

If the server can satisfy the request, then the server creates or refreshes the channel binding using the channel number in the CHANNEL-NUMBER 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 9.

12.3. 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.

12.4. 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 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.

12.5. 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 server uses a Data indication to signal the XOR-PEER-ADDRESS and ICMP attributes to the client even if a channel has been bound to the peer.

The fields of the ChannelData message are filled in as described in Section 12.4.

Over TCP and TLS-over-TCP, 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 (See section 14 in [I-D.ietf-tram-stunbis]). 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.

12.6. Receiving a ChannelData Message

The receiver of the ChannelData message uses the first byte to distinguish it from other multiplexed protocols, as described in Figure 5. If the message uses a value in the reserved range (0x5000 through 0xFFFF), then the message is silently discarded.

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 12.5.

If the ChannelData message is received on a channel that 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, the receipt of a ChannelData message MUST NOT refresh either the channel binding or the 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:[RFC6263]).

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 (Section 4.1 in

12.7. 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 11.3. If that section indicates that a ChannelData message should be sent (because there is a channel bound to the peer that sent to the UDP datagram), then the server forms and sends a ChannelData message as described in Section 12.5.

When the server receives an ICMP packet, the server processes it as described in Section 11.5.

13. Packet Translations

This section addresses IPv4-to-IPv6, IPv6-to-IPv4, and IPv6-to-IPv6 translations. Requirements for translation of the IP addresses and port numbers of the packets are described above. The following sections specify how to translate other header fields.

As discussed in Section 3.6, translations in TURN are designed so that a TURN server can be implemented as an application that runs in user space under commonly available operating systems and that does not require special privileges. The translations specified in the following sections follow this principle.

The descriptions below have two parts: a preferred behavior and an alternate behavior. The server SHOULD implement the preferred behavior, but if that is not possible for a particular field, the server MUST implement the alternate behavior and MUST NOT do anything else for the reasons detailed in [RFC7915]. The TURN server solely relies on the DF bit in the IPv4 header and Fragmentation header in IPv6 header to handle fragmentation and does not rely on the DONT-FRAGMENT attribute.

13.1. IPv4-to-IPv6 Translations

Time to Live (TTL) field

Traffic Class

Flow Label

Hop Limit

Fragmentation

Extension Headers

13.2. IPv6-to-IPv6 Translations

Flow Label

The TURN server should consider that it is handling two different IPv6 flows. Therefore, the Flow label [RFC6437] SHOULD NOT be copied as part of the translation.

Hop Limit

Fragmentation

Extension Headers

13.3. IPv6-to-IPv4 Translations

Type of Service and Precedence

Time to Live

Fragmentation

14. UDP-to-UDP relay

This section describes how the server sets various fields in the IP header for UDP-to-UDP relay from the client to the peer or vice 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.

The descriptions below have two parts: a preferred behavior and an alternate behavior. The server SHOULD implement the preferred behavior, but if that is not possible for a particular field, then it SHOULD implement the alternative behavior.

Differentiated Services Code Point (DSCP) field [RFC2474]



Explicit Congestion Notification (ECN) field [RFC3168]



IPv4 Fragmentation fields



IPv4 Options

15. TCP-to-UDP relay

This section describes how the server sets various fields in the IP header for TCP-to-UDP relay from the client to the peer. The descriptions in this section apply when the server sends a UDP datagram to the peer. Note that the server does not perform per-packet translation for TCP-to-UDP relaying.

Multipath TCP [I-D.ietf-mptcp-rfc6824bis] is not supported by this version of TURN because TCP multi-path is not used by both SIP and WebRTC protocols [RFC7478] for media and non-media data. TCP connection between the TURN client and server can use TCP-AO [RFC5925] but UDP does not provide a similar type of authentication until UDP supports a similar authentication option [I-D.ietf-tsvwg-udp-options]. Even if both TCP-AO and UDP authentication would be used between TURN client and server, it would not change the end-to-end security properties of the application payload being relayed. Therefore applications using TURN will need to secure their application data end-to-end appropriately, e.g., SRTP for RTP applications. Note that TCP-AO option obsoletes TCP MD5 option. Unlike UDP, TCP without the TCP Fast Open extension [RFC7413] does not support 0-RTT session resumption. The TCP user timeout [RFC5482] equivalent for application data relayed by the TURN is the use of RTP control protocol (RTCP). As a reminder, RTCP is a fundamental and integral part of RTP.

The descriptions below have two parts: a preferred behavior and an alternate behavior. The server SHOULD implement the preferred behavior, but if that is not possible for a particular field, then it SHOULD implement the alternative behavior.

For the UDP datagram sent to the peer based on Send Indication or ChannelData message arriving at the TURN server over a TCP Transport, the server sets various fields in the IP header as follows:

Differentiated Services Code Point (DSCP) field [RFC2474]



Explicit Congestion Notification (ECN) field [RFC3168]



IPv4 Fragmentation fields

IPv6 Fragmentation fields



IPv4 Options

16. UDP-to-TCP relay

This section describes how the server sets various fields in the IP header for UDP-to-TCP relay from the peer to the client. The descriptions in this section apply when the server sends a Data indication or ChannelData message to the client over TCP or TLS transport. Note that the server does not perform per-packet translation for UDP-to-TCP relaying.

The descriptions below have two parts: a preferred behavior and an alternate behavior. The server SHOULD implement the preferred behavior, but if that is not possible for a particular field, then it SHOULD implement the alternative behavior.

The TURN server sets IP header fields in the TCP packets on a per-connection basis for the TCP connection as follows:

Differentiated Services Code Point (DSCP) field [RFC2474]



Explicit Congestion Notification (ECN) field [RFC3168]



Fragmentation



Extension Headers

IPv4 Options

17. STUN Methods

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

18. STUN Attributes

This STUN extension defines the following attributes:

  0x000C: CHANNEL-NUMBER
  0x000D: LIFETIME
  0x0010: Reserved (was BANDWIDTH)
  0x0012: XOR-PEER-ADDRESS
  0x0013: DATA
  0x0016: XOR-RELAYED-ADDRESS
  0x0017: REQUESTED-ADDRESS-FAMILY
  0x0018: EVEN-PORT
  0x0019: REQUESTED-TRANSPORT
  0x001A: DONT-FRAGMENT
  0x0021: Reserved (was TIMER-VAL)
  0x0022: RESERVATION-TOKEN  
  TBD-CA: ADDITIONAL-ADDRESS-FAMILY  
  TBD-CA: ADDRESS-ERROR-CODE
  TBD-CA: ICMP
 

Some of these attributes have lengths that are not multiples of 4. By the rules of STUN, any attribute whose length is not a multiple of 4 bytes MUST be immediately followed by 1 to 3 padding bytes to ensure the next attribute (if any) would start on a 4-byte boundary (see [I-D.ietf-tram-stunbis]).

18.1. CHANNEL-NUMBER

The CHANNEL-NUMBER attribute contains the number of the channel. The value portion of this attribute is 4 bytes long and consists of 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              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

18.2. LIFETIME

The LIFETIME attribute represents the duration for which the server will maintain an allocation in the absence of a refresh. The TURN client can include the LIFETIME attribute with the desired lifetime in Allocate and Refresh requests. The value portion of this attribute is 4-bytes long and consists of a 32-bit unsigned integral value representing the number of seconds remaining until expiration.

18.3. XOR-PEER-ADDRESS

The XOR-PEER-ADDRESS specifies the address and port of the peer as seen from the TURN server. (For example, the peer's server-reflexive transport address if the peer is behind a NAT.) It is encoded in the same way as XOR-MAPPED-ADDRESS [I-D.ietf-tram-stunbis].

18.4. DATA

The DATA attribute is present in all Send indications. If the ICMP attribute is not present in a Data indication, it contains a DATA attribute. The value portion of this attribute is variable length and consists of 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). The application data is equivalent to the "UDP user data" and does not include the "surplus area" defined in Section 4 of [I-D.ietf-tsvwg-udp-options]. If the length of this attribute is not a multiple of 4, then padding must be added after this attribute.

18.5. 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 [I-D.ietf-tram-stunbis].

18.6. REQUESTED-ADDRESS-FAMILY

This attribute is used in Allocate and Refresh requests to specify the address type requested by the client. The value of this attribute is 4 bytes with 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Family    |            Reserved                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Family:
there are two values defined for this field and specified in [I-D.ietf-tram-stunbis], Section 14.1: 0x01 for IPv4 addresses and 0x02 for IPv6 addresses.
Reserved:
at this point, the 24 bits in the Reserved field MUST be set to zero by the client and MUST be ignored by the server.

18.7. 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-higher port number. The value portion of this attribute is 1 byte long. Its format is:

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

The value contains a single 1-bit flag:

R:
If 1, the server is requested to reserve the next-higher port number (on the same IP address) for a subsequent allocation. If 0, no such reservation is requested.
RFFU:
Reserved For Future Use.

The RFFU field must be set to zero on transmission and ignored on reception.

Since the length of this attribute is not a multiple of 4, padding must immediately follow this attribute.

18.8. REQUESTED-TRANSPORT

This attribute is used by the client to request a specific transport protocol for the allocated transport address. The value of this attribute is 4 bytes with 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]. 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.

18.9. 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, and for determining the server capability in Allocate requests. This attribute has no value part and thus the attribute length field is 0.

18.10. 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 8 bytes and contains the token value.

18.11. ADDITIONAL-ADDRESS-FAMILY

This attribute is used by clients to request the allocation of a IPv4 and IPv6 address type from a server. It is encoded in the same way as REQUESTED-ADDRESS-FAMILY Section 18.6. The ADDITIONAL-ADDRESS-FAMILY attribute MAY be present in Allocate request. The attribute value of 0x02 (IPv6 address) is the only valid value in Allocate request.

18.12. ADDRESS-ERROR-CODE

This attribute is used by servers to signal the reason for not allocating the requested address family. The value portion of this attribute is variable length with 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Family       |    Reserved             |Class|     Number    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Reason Phrase (variable)                                ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Family:
there are two values defined for this field and specified in [I-D.ietf-tram-stunbis], Section 14.1: 0x01 for IPv4 addresses and 0x02 for IPv6 addresses.
Reserved:
at this point, the 13 bits in the Reserved field MUST be set to zero by the server and MUST be ignored by the client.
Class:
The Class represents the hundreds digit of the error code and is defined in section 14.8 of [I-D.ietf-tram-stunbis].
Number:
this 8-bit field contains the reason server cannot allocate one of the requested address types. The error code values could be either 440 (unsupported address family) or 508 (insufficient capacity). The number representation is defined in section 14.8 of [I-D.ietf-tram-stunbis].
Reason Phrase:
The recommended reason phrases for error codes 440 and 508 are explained in Section 19. The reason phrase MUST be a UTF-8 [RFC3629] encoded sequence of less than 128 characters (which can be as long as 509 bytes when encoding them or 763 bytes when decoding them).

18.13. ICMP

This attribute is used by servers to signal the reason an UDP packet was dropped. The following is the format of the ICMP attribute.

        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Reserved                     |  ICMP Type  |  ICMP Code      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Error Data                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved:
This field MUST be set to 0 when sent, and MUST be ignored when received.
ICMP Type:
The field contains the value in the ICMP type. Its interpretation depends whether the ICMP was received over IPv4 or IPv6.
ICMP Code:
The field contains the value in the ICMP code. Its interpretation depends whether the ICMP was received over IPv4 or IPv6.
Error Data:
This field size is 4 bytes long. If the ICMPv6 type is 2 (Packet Too Big Message) or ICMPv4 type is 3 ( Destination Unreachable) and Code is 4 (fragmentation needed and DF set), the Error Data field will be set to the Maximum Transmission Unit of the next-hop link (Section 3.2 of [RFC4443]) and Section 4 of [RFC1191]). For other ICMPv6 types and ICMPv4 types and codes, Error Data field MUST be set to zero.

19. STUN Error Response Codes

This document defines the following error response codes:

403
(Forbidden): The request was valid but cannot be performed due to administrative or similar restrictions.
437
(Allocation Mismatch): A request was received by the server that requires an allocation to be in place, but no allocation exists, or a request was received that requires no allocation, but an allocation exists.
440
(Address Family not Supported): The server does not support the address family requested by the client.
441
(Wrong Credentials): The credentials in the (non-Allocate) request 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.
443
(Peer Address Family Mismatch). A peer address is part of a different address family than that of the relayed transport address of the allocation.
486
(Allocation Quota Reached): No more allocations using this username can be created at the present time.
508
(Insufficient Capacity): The server is unable to carry out the request due to some capacity limit being reached. In an Allocate response, this could be due to the server having no more relayed transport addresses available at that time, having none with the requested properties, or the one that corresponds to the specified reservation token is not available.

20. Detailed Example

This section gives an example of the use of TURN, showing in detail the contents of the messages exchanged. The example uses the network diagram shown in the Overview (Figure 1).

For each message, the attributes included in the message and their values are shown. For convenience, values are shown in a human-readable format rather than showing the actual octets; for example, "XOR-RELAYED-ADDRESS=192.0.2.15:9000" shows that the XOR-RELAYED-ADDRESS attribute is included with an address of 192.0.2.15 and a port of 9000, here the address and port are shown before the xor-ing is done. For attributes with string-like values (e.g., SOFTWARE="Example client, version 1.03" and NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"), the value of the attribute is shown in quotes for readability, but these quotes do not appear in the actual value.

TURN                                 TURN           Peer          Peer
client                               server          A             B
  |                                    |             |             |
  |--- Allocate request -------------->|             |             |
  |    Transaction-Id=0xA56250D3F17ABE679422DE85     |             |
  |    SOFTWARE="Example client, version 1.03"       |             |
  |    LIFETIME=3600 (1 hour)          |             |             |
  |    REQUESTED-TRANSPORT=17 (UDP)    |             |             | 
  |    DONT-FRAGMENT                   |             |             |
  |                                    |             |             |
  |<-- Allocate error response --------|             |             |
  |    Transaction-Id=0xA56250D3F17ABE679422DE85     |             |
  |    SOFTWARE="Example server, version 1.17"       |             |
  |    ERROR-CODE=401 (Unauthorized)   |             |             |
  |    REALM="example.com"             |             |             |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"      |             |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256            |             |
  |                                    |             |             |
  |--- Allocate request -------------->|             |             |
  |    Transaction-Id=0xC271E932AD7446A32C234492     |             |
  |    SOFTWARE="Example client 1.03"  |             |             |
  |    LIFETIME=3600 (1 hour)          |             |             |
  |    REQUESTED-TRANSPORT=17 (UDP)    |             |             |
  |    DONT-FRAGMENT                   |             |             |
  |    USERNAME="George"               |             |             |
  |    REALM="example.com"             |             |             |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"      |             |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256            |             |
  |    PASSWORD-ALGORITHM=SHA256       |             |             |    
  |    MESSAGE-INTEGRITY=...           |             |             |
  |    MESSAGE-INTEGRITY-SHA256=...    |             |             |
  |                                    |             |             |
  |<-- Allocate success response ------|             |             |
  |    Transaction-Id=0xC271E932AD7446A32C234492     |             |
  |    SOFTWARE="Example server, version 1.17"       |             |
  |    LIFETIME=1200 (20 minutes)      |             |             |
  |    XOR-RELAYED-ADDRESS=192.0.2.15:50000          |             |
  |    XOR-MAPPED-ADDRESS=192.0.2.1:7000             |             |
  |    MESSAGE-INTEGRITY-SHA256=...    |             |             |

The client begins by selecting a host transport address to use for the TURN session; in this example, the client has selected 198.51.100.2:49721 as shown in Figure 1. The client then sends an Allocate request to the server at the server transport address. The client randomly selects a 96-bit transaction id of 0xA56250D3F17ABE679422DE85 for this transaction; this is encoded in the transaction id field in the fixed header. The client includes a SOFTWARE attribute that gives information about the client's software; here the value is "Example client, version 1.03" to indicate that this is version 1.03 of something called the Example client. The client includes the LIFETIME attribute because it wishes the allocation to have a longer lifetime than the default of 10 minutes; the value of this attribute is 3600 seconds, which corresponds to 1 hour. The client must always include a REQUESTED-TRANSPORT attribute in an Allocate request and the only value allowed by this specification is 17, which indicates UDP transport between the server and the peers. The client also includes the DONT-FRAGMENT attribute because it wishes to use the DONT-FRAGMENT attribute later in Send indications; this attribute consists of only an attribute header, there is no value part. We assume the client has not recently interacted with the server, thus the client does not include USERNAME, USERHASH, REALM, NONCE, PASSWORD-ALGORITHMS, PASSWORD-ALGORITHM, MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute. Finally, note that the order of attributes in a message is arbitrary (except for the MESSAGE-INTEGRITY, MESSAGE-INTEGRITY-SHA256 and FINGERPRINT attributes) and the client could have used a different order.

Servers require any request to be authenticated. Thus, when the server receives the initial Allocate request, it rejects the request because the request does not contain the authentication attributes. Following the procedures of the long-term credential mechanism of STUN [I-D.ietf-tram-stunbis], the server includes an ERROR-CODE attribute with a value of 401 (Unauthorized), a REALM attribute that specifies the authentication realm used by the server (in this case, the server's domain "example.com"), and a nonce value in a NONCE attribute. The NONCE attribute starts with the "nonce cookie" with the STUN Security Feature "Password algorithm" bit set to 1. The server includes a PASSWORD-ALGORITHMS attribute that specifies the list of algorithms that the server can use to derive the long-term password. If the server sets the STUN Security Feature "Username anonymity" bit to 1 then the client uses the USERHASH attribute instead of the USERNAME attribute in the Allocate request to anonymise the username. The server also includes a SOFTWARE attribute that gives information about the server's software.

The client, upon receipt of the 401 error, re-attempts the Allocate request, this time including the authentication attributes. The client selects a new transaction id, and then populates the new Allocate request with the same attributes as before. The client includes a USERNAME attribute and uses the realm value received from the server to help it determine which value to use; here the client is configured to use the username "George" for the realm "example.com". The client includes the PASSWORD-ALGORITHM attribute indicating the algorithm that the server must use to derive the long- term password. The client also includes the REALM, PASSWORD-ALGORITHMS and NONCE attributes, which are just copied from the 401 error response. Finally, the client includes MESSAGE-INTEGRITY-SHA256 attribute as the last attributes in the message, whose value is Hashed Message Authentication Code - Secure Hash Algorithm 2 (HMAC-SHA2) hash over the contents of the message (shown as just "..." above); this HMAC-SHA2 computation includes a password value. Thus, an attacker cannot compute the message integrity value without somehow knowing the secret password.

The server, upon receipt of the authenticated Allocate request, checks that everything is OK, then creates an allocation. The server replies with an Allocate success response. The server includes a LIFETIME attribute giving the lifetime of the allocation; here, the server has reduced the client's requested 1-hour lifetime to just 20 minutes, because this particular server doesn't allow lifetimes longer than 20 minutes. The server includes an XOR-RELAYED-ADDRESS attribute whose value is the relayed transport address of the allocation. The server includes an XOR-MAPPED-ADDRESS attribute whose value is the server-reflexive address of the client; this value is not used otherwise in TURN but is returned as a convenience to the client. The server includes a MESSAGE-INTEGRITY-SHA256 attribute to authenticate the response and to ensure its integrity; note that the response does not contain the USERNAME, REALM, and NONCE attributes. The server also includes a SOFTWARE attribute.

TURN                                 TURN           Peer          Peer
client                               server          A             B
  |--- CreatePermission request ------>|             |             |
  |    Transaction-Id=0xE5913A8F460956CA277D3319     |             |
  |    XOR-PEER-ADDRESS=192.0.2.150:0  |             |             |
  |    USERNAME="George"               |             |             |
  |    REALM="example.com"             |             |             |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"      |             |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256            |             |
  |    PASSWORD-ALGORITHM=SHA256       |             |             |  
  |    MESSAGE-INTEGRITY-SHA256=...    |             |             |
  |                                    |             |             |
  |<-- CreatePermission success resp.--|             |             |
  |    Transaction-Id=0xE5913A8F460956CA277D3319     |             |
  |    MESSAGE-INTEGRITY-SHA256=...    |             |             |

The client then creates a permission towards Peer A in preparation for sending it some application data. This is done through a CreatePermission request. The XOR-PEER-ADDRESS attribute contains the IP address for which a permission is established (the IP address of peer A); note that the port number in the attribute is ignored when used in a CreatePermission request, and here it has been set to 0; also, note how the client uses Peer A's server-reflexive IP address and not its (private) host address. The client uses the same username, realm, and nonce values as in the previous request on the allocation. Though it is allowed to do so, the client has chosen not to include a SOFTWARE attribute in this request.

The server receives the CreatePermission request, creates the corresponding permission, and then replies with a CreatePermission success response. Like the client, the server chooses not to include the SOFTWARE attribute in its reply. Again, note how success responses contains a MESSAGE-INTEGRITY-SHA256 attribute (assuming the server uses the long-term credential mechanism), but no USERNAME, REALM, and NONCE attributes.

TURN                                 TURN           Peer          Peer
client                               server          A             B
  |--- Send indication --------------->|             |             |
  |    Transaction-Id=0x1278E9ACA2711637EF7D3328     |             |
  |    XOR-PEER-ADDRESS=192.0.2.150:32102            |             |
  |    DONT-FRAGMENT                   |             |             |
  |    DATA=...                        |             |             |
  |                                    |-- UDP dgm ->|             |
  |                                    |  data=...   |             |
  |                                    |             |             |
  |                                    |<- UDP dgm --|             |
  |                                    |  data=...   |             |
  |<-- Data indication ----------------|             |             |
  |    Transaction-Id=0x8231AE8F9242DA9FF287FEFF     |             |
  |    XOR-PEER-ADDRESS=192.0.2.150:32102            |             |
  |    DATA=...                        |             |             |

The client now sends application data to Peer A using a Send indication. Peer A's server-reflexive transport address is specified in the XOR-PEER-ADDRESS attribute, and the application data (shown here as just "...") is specified in the DATA attribute. The client is doing a form of path MTU discovery at the application layer and thus specifies (by including the DONT-FRAGMENT attribute) that the server should set the DF bit in the UDP datagram to send to the peer. Indications cannot be authenticated using the long-term credential mechanism of STUN, so no MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute is included in the message. An application wishing to ensure that its data is not altered or forged must integrity-protect its data at the application level.

Upon receipt of the Send indication, the server extracts the application data and sends it in a UDP datagram to Peer A, with the relayed transport address as the source transport address of the datagram, and with the DF bit set as requested. Note that, had the client not previously established a permission for Peer A's server-reflexive IP address, then the server would have silently discarded the Send indication instead.

Peer A then replies with its own UDP datagram containing application data. The datagram is sent to the relayed transport address on the server. When this arrives, the server creates a Data indication containing the source of the UDP datagram in the XOR-PEER-ADDRESS attribute, and the data from the UDP datagram in the DATA attribute. The resulting Data indication is then sent to the client.

TURN                                 TURN           Peer          Peer
client                               server          A             B
  |--- ChannelBind request ----------->|             |             |
  |    Transaction-Id=0x6490D3BC175AFF3D84513212     |             |
  |    CHANNEL-NUMBER=0x4000           |             |             |
  |    XOR-PEER-ADDRESS=192.0.2.210:49191            |             |
  |    USERNAME="George"               |             |             |
  |    REALM="example.com"             |             |             |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"      |             |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256            |             |
  |    PASSWORD-ALGORITHM=SHA256       |             |             |  
  |    MESSAGE-INTEGRITY-SHA256=...    |             |             |
  |                                    |             |             |
  |<-- ChannelBind success response ---|             |             |
  |    Transaction-Id=0x6490D3BC175AFF3D84513212     |             |
  |    MESSAGE-INTEGRITY-SHA256=...    |             |             |

The client now binds a channel to Peer B, specifying a free channel number (0x4000) in the CHANNEL-NUMBER attribute, and Peer B's transport address in the XOR-PEER-ADDRESS attribute. As before, the client re-uses the username, realm, and nonce from its last request in the message.

Upon receipt of the request, the server binds the channel number to the peer, installs a permission for Peer B's IP address, and then replies with ChannelBind success response.

TURN                                 TURN           Peer          Peer
client                               server          A             B
  |--- ChannelData ------------------->|             |             |
  |    Channel-number=0x4000           |--- UDP datagram --------->|
  |    Data=...                        |    Data=...               |
  |                                    |             |             |
  |                                    |<-- UDP datagram ----------|
  |                                    |    Data=... |             |
  |<-- ChannelData --------------------|             |             |
  |    Channel-number=0x4000           |             |             |
  |    Data=...                        |             |             |

The client now sends a ChannelData message to the server with data destined for Peer B. The ChannelData message is not a STUN message, and thus has no transaction id. Instead, it has only three fields: a channel number, data, and data length; here the channel number field is 0x4000 (the channel the client just bound to Peer B). When the server receives the ChannelData message, it checks that the channel is currently bound (which it is) and then sends the data onward to Peer B in a UDP datagram, using the relayed transport address as the source transport address and 192.0.2.210:49191 (the value of the XOR-PEER-ADDRESS attribute in the ChannelBind request) as the destination transport address.

Later, Peer B sends a UDP datagram back to the relayed transport address. This causes the server to send a ChannelData message to the client containing the data from the UDP datagram. The server knows to which client to send the ChannelData message because of the relayed transport address at which the UDP datagram arrived, and knows to use channel 0x4000 because this is the channel bound to 192.0.2.210:49191. Note that if there had not been any channel number bound to that address, the server would have used a Data indication instead.

TURN                                 TURN           Peer          Peer
client                               server          A             B
  |--- ChannelBind request ----------->|             |             |
  |    Transaction-Id=0xE5913A8F46091637EF7D3328     |             |
  |    CHANNEL-NUMBER=0x4000           |             |             |
  |    XOR-PEER-ADDRESS=192.0.2.210:49191            |             |
  |    USERNAME="George"               |             |             |
  |    REALM="example.com"             |             |             |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"      |             |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256            |             |
  |    PASSWORD-ALGORITHM=SHA256       |             |             |  
  |    MESSAGE-INTEGRITY-SHA256=...    |             |             |
  |                                    |             |             |
  |<-- ChannelBind success response ---|             |             |
  |    Transaction-Id=0xE5913A8F46091637EF7D3328     |             |
  |    MESSAGE-INTEGRITY-SHA256=...    |             |             |

The channel binding lasts for 10 minutes unless refreshed. The TURN client refreshes the binding by sending ChannelBind request rebinding the channel to the same peer (Peer B's IP address). The server processes the ChannelBind request, rebinds the channel to the same peer and resets the time-to-expiry timer back to 10 minutes.

TURN                                 TURN           Peer          Peer
client                               server          A             B
  |--- Refresh request --------------->|             |             |
  |    Transaction-Id=0x0864B3C27ADE9354B4312414     |             |
  |    SOFTWARE="Example client 1.03"  |             |             |
  |    USERNAME="George"               |             |             |
  |    REALM="example.com"             |             |             |
  |    NONCE="oobMatJos2gAAAadl7W7PeDU4hKE72jda"     |             |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256            |             |
  |    PASSWORD-ALGORITHM=SHA256       |             |             |  
  |    MESSAGE-INTEGRITY-SHA256=...    |             |             |
  |                                    |             |             |
  |<-- Refresh error response ---------|             |             |
  |    Transaction-Id=0x0864B3C27ADE9354B4312414     |             |
  |    SOFTWARE="Example server, version 1.17"       |             |
  |    ERROR-CODE=438 (Stale Nonce)    |             |             |
  |    REALM="example.com"             |             |             |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"      |             |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256            |             |
  |                                    |             |             |
  |--- Refresh request --------------->|             |             |
  |    Transaction-Id=0x427BD3E625A85FC731DC4191     |             |
  |    SOFTWARE="Example client 1.03"  |             |             |
  |    USERNAME="George"               |             |             |
  |    REALM="example.com"             |             |             |
  |    NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"      |             |
  |    PASSWORD-ALGORITHMS=MD5 and SHA256            |             |
  |    PASSWORD-ALGORITHM=SHA256       |             |             | 
  |    MESSAGE-INTEGRITY-SHA256=...    |             |             |
  |                                    |             |             |
  |<-- Refresh success response -------|             |             |
  |    Transaction-Id=0x427BD3E625A85FC731DC4191     |             |
  |    SOFTWARE="Example server, version 1.17"       |             |
  |    LIFETIME=600 (10 minutes)       |             |             |

Sometime before the 20 minute lifetime is up, the client refreshes the allocation. This is done using a Refresh request. As before, the client includes the latest username, realm, and nonce values in the request. The client also includes the SOFTWARE attribute, following the recommended practice of always including this attribute in Allocate and Refresh messages. When the server receives the Refresh request, it notices that the nonce value has expired, and so replies with 438 (Stale Nonce) error given a new nonce value. The client then reattempts the request, this time with the new nonce value. This second attempt is accepted, and the server replies with a success response. Note that the client did not include a LIFETIME attribute in the request, so the server refreshes the allocation for the default lifetime of 10 minutes (as can be seen by the LIFETIME attribute in the success response).

21. 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.

Most of the attacks on TURN are mitigated by the server requiring requests be authenticated. Thus, this specification requires the use of authentication. The mandatory-to-implement mechanism is the long- term credential mechanism of STUN. Other authentication mechanisms of equal or stronger security properties may be used. However, it is important to ensure that they can be invoked in an inter-operable way.

21.1. 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.

21.1.1. 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 long-term credential mechanism, which allows the TURN server to determine the identity of the requestor and whether the requestor is allowed to obtain the allocation.

21.1.2. Offline Dictionary Attacks

The long-term credential 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, (D)TLS transport between the client and the server can be used.

21.1.3. Faked Refreshes and Permissions

An attacker might wish to attack an active allocation by sending it a Refresh request 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 21.2.

21.1.4. 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.

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 a 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 B with a spoofed IP address of A. 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] prevents these attacks.

In some situations, the TURN server may be situated in the network such that it is able to send to hosts to which the client cannot directly send. This can happen, for example, if the server is located behind a firewall that allows packets from outside the firewall to be delivered to the server, but not to other hosts behind the firewall. In these situations, an attacker could send the server a Send indication with an XOR-PEER-ADDRESS attribute containing the transport address of one of the other hosts behind the firewall. If the server was to allow relaying of traffic to arbitrary peers, then this would provide a way for the attacker to attack arbitrary hosts behind the firewall.

To mitigate this attack, TURN requires that the client establish a permission to a host before sending it data. Thus, an attacker can only attack hosts with which the client is already communicating, unless the attacker is able to create authenticated requests. Furthermore, the server administrator may configure the server to restrict the range of IP addresses and ports to which it will relay data. To provide even greater security, the server administrator can require that the client use (D)TLS for all communication between the client and the server.

21.1.5. 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 long-term credential 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 as the transaction id in the STUN header prevents this. Replay attacks are further thwarted through frequent changes to the nonce value.

21.1.6. Eavesdropping Traffic

If the TURN client and server use the STUN Extension for Third-Party Authorization [RFC7635] (for example it is used in WebRTC), the username does not reveal the real user's identity, the USERNAME attribute carries an ephemeral and unique key identifier. If the TURN client and server use the STUN long-term credential mechanism and the username reveals the real user's identity, the client MUST either use the USERHASH attribute instead of the USERNAME attribute to anonynmize the username or use (D)TLS transport between the client and the server.

If the TURN client and server use the STUN long-term credential mechanism and realm information is privacy sensitive, TURN can be run over (D)TLS. As a reminder, STUN Extension for Third-Party Authorization does not use realm.

The SOFTWARE attribute can reveal the specific software version of the TURN client and server to eavesdropper and it might possibly allow attacks against vulnerable software that is known to contain security vulnerabilities. If the software version is known to contain security vulnerabilities, TURN SHOULD be run over (D)TLS to prevent leaking the SOFTWARE attribute in clear text. If zero-day vulnerabilities are detected in the software version, the endpoint policy can be modified to mandate the use of (D)TLS till the patch is in place to fix the flaw.

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, with the exception of USERNAME, REALM and SOFTWARE. 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 (D)TLS.

Confidentiality for the application data relayed by TURN is best provided by the application protocol itself, since running TURN over (D)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.

21.1.7. TURN Loop Attack

An attacker might attempt to cause data packets to loop indefinitely between two TURN servers. The attack goes as follows. First, the attacker sends an Allocate request to server A, using the source address of server B. Server A will send its response to server B, and for the attack to succeed, the attacker must have the ability to either view or guess the contents of this response, so that the attacker can learn the allocated relayed transport address. The attacker then sends an Allocate request to server B, using the source address of server A. Again, the attacker must be able to view or guess the contents of the response, so it can send learn the allocated relayed transport address. Using the same spoofed source address technique, the attacker then binds a channel number on server A to the relayed transport address on server B, and similarly binds the same channel number on server B to the relayed transport address on server A. Finally, the attacker sends a ChannelData message to server A.

The result is a data packet that loops from the relayed transport address on server A to the relayed transport address on server B, then from server B's transport address to server A's transport address, and then around the loop again.

This attack is mitigated as follows. By requiring all requests to be authenticated and/or by randomizing the port number allocated for the relayed transport address, the server forces the attacker to either intercept or view responses sent to a third party (in this case, the other server) so that the attacker can authenticate the requests and learn the relayed transport address. Without one of these two measures, an attacker can guess the contents of the responses without needing to see them, which makes the attack much easier to perform. Furthermore, by requiring authenticated requests, the server forces the attacker to have credentials acceptable to the server, which turns this from an outsider attack into an insider attack and allows the attack to be traced back to the client initiating it.

The attack can be further mitigated by imposing a per-username limit on the bandwidth used to relay data by allocations owned by that username, to limit the impact of this attack on other allocations. More mitigation can be achieved by decrementing the TTL when relaying data packets (if the underlying OS allows this).

21.2. Firewall Considerations

A key security consideration of TURN is that TURN 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 that implement address-dependent filtering [RFC4787], 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 that 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.

21.2.1. 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 that create permissions (i.e., ChannelBind and CreatePermission) are authenticated.

21.2.2. Blacklisted IP Addresses

Many firewalls can be configured with blacklists that 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.

This feature is also present in TURN, since TURN servers are allowed to arbitrarily restrict the range of addresses of peers that they will relay to.

21.2.3. 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 for which the client has not installed a permission. Thus, peers cannot just connect to the allocated port in order to obtain the service.

21.3. 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.

21.3.1. 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.

21.3.2. Anonymous Relaying of Malicious Traffic

TURN servers provide a degree of anonymization. A client can send data to peers without revealing its own IP address. 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 that capture the actual source IP and port of the client, and perhaps even every permission that client installs. 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.

21.3.3. Manipulating Other Allocations

An attacker might attempt to disrupt service to other users of the TURN server by sending Refresh requests or CreatePermission requests that (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.

21.4. Tunnel Amplification Attack

An attacker might attempt to cause data packets to loop numerous times between a TURN server and a tunnel between IPv4 and IPv6. The attack goes as follows.

Suppose an attacker knows that a tunnel endpoint will forward encapsulated packets from a given IPv6 address (this doesn't necessarily need to be the tunnel endpoint's address). Suppose he then spoofs two packets from this address:

  1. An Allocate request asking for a v4 address, and
  2. A ChannelBind request establishing a channel to the IPv4 address of the tunnel endpoint

Then he has set up an amplification attack:

  IPv6: src=2001:DB8:1::1 dst=2001:DB8::2
  UDP:  <ports>
  TURN: <channel id>
  IPv6: src=2001:DB8:1::1 dst=2001:DB8::2
  UDP:  <ports>
  TURN: <channel id>
  IPv6: src=2001:DB8:1::1 dst=2001:DB8::2
  UDP:  <ports>
  TURN: <channel id>
  ...
      

So if the attacker sends a packet of the following form...

The amplification potential here is limited by the MTU, so it's not huge: IPv6+UDP+TURN takes 334 bytes, so a four-to-one amplification out of a 1500-byte packet is possible. But the attacker could still increase traffic volume by sending multiple packets or by establishing multiple channels spoofed from different addresses behind the same tunnel endpoint.

The attack is mitigated as follows. It is RECOMMENDED that TURN servers not accept allocation or channel binding requests from addresses known to be tunneled, and that they not forward data to such addresses. In particular, a TURN server MUST NOT accept Teredo or 6to4 addresses in these requests.

21.5. Other Considerations

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

22. IANA Considerations

[Paragraphs in braces should be removed by the RFC Editor upon publication]

The codepoints for the STUN methods defined in this specification are listed in Section 17. [IANA is requested to update the reference from [RFC5766] to RFC-to-be for the STUN methods listed in Section 17.]

The codepoints for the STUN attributes defined in this specification are listed in Section 18. [IANA is requested to update the reference from [RFC5766] to RFC-to-be for the STUN attributes CHANNEL-NUMBER, LIFETIME, Reserved (was BANDWIDTH), XOR-PEER-ADDRESS, DATA, XOR-RELAYED-ADDRESS, REQUESTED-ADDRESS-FAMILY, EVEN-PORT, REQUESTED-TRANSPORT, DONT-FRAGMENT, Reserved (was TIMER-VAL) and RESERVATION-TOKEN listed in Section 18.]

[The ADDITIONAL-ADDRESS-FAMILY, ADDRESS-ERROR-CODE and ICMP attributes requires that IANA allocate a value in the "STUN attributes Registry" from the comprehension-optional range (0x8000-0xFFFF), to be replaced for TBD-CA throughout this document]

The codepoints for the STUN error codes defined in this specification are listed in Section 19. [IANA is requested to update the reference from [RFC5766] and [RFC6156] to RFC-to-be for the STUN error codes listed in Section 19.]

IANA has allocated the SRV service name of "turn" for TURN over UDP or TCP, and the service name of "turns" for TURN over (D)TLS.

IANA has created a registry for TURN channel numbers, initially populated as follows:

Any change to this registry must be made through an IETF Standards Action.

23. IAB Considerations

The IAB has studied the problem of "Unilateral Self Address Fixing" (UNSAF), which is the general process by which a client attempts to determine its address in another realm on the other side of a NAT through a collaborative protocol-reflection mechanism [RFC3424]. 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. These considerations and the responses for TURN are documented in this section.

Consideration 1: Precise definition of a specific, limited-scope problem that is to be solved with the UNSAF proposal. A short-term fix should not be generalized to solve other problems. Such generalizations lead to the prolonged dependence on and usage of the supposed short-term fix -- meaning that it is no longer accurate to call it "short-term".

Response: TURN is a protocol for communication between a relay (= TURN server) and its client. The protocol allows a client that is behind a NAT to obtain and use a public IP address on the relay. As a convenience to the client, TURN also allows the client to determine its server-reflexive transport address.

Consideration 2: Description of an exit strategy/transition plan. The better short-term fixes are the ones that will naturally see less and less use as the appropriate technology is deployed.

Response: TURN will no longer be needed once there are no longer any NATs. Unfortunately, as of the date of publication of this document, it no longer seems very likely that NATs will go away any time soon. However, the need for TURN will also decrease as the number of NATs with the mapping property of Endpoint-Independent Mapping [RFC4787] increases.

Consideration 3: Discussion of specific issues that may render systems more "brittle". For example, approaches that involve using data at multiple network layers create more dependencies, increase debugging challenges, and make it harder to transition.

Response: TURN is "brittle" in that it requires the NAT bindings between the client and the server to be maintained unchanged for the lifetime of the allocation. This is typically done using keep-alives. If this is not done, then the client will lose its allocation and can no longer exchange data with its peers.

Consideration 4: Identify requirements for longer-term, sound technical solutions; contribute to the process of finding the right longer-term solution.

Response: The need for TURN will be reduced once NATs implement the recommendations for NAT UDP behavior documented in [RFC4787]. Applications are also strongly urged to use ICE [RFC8445] to communicate with peers; though ICE uses TURN, it does so only as a last resort, and uses it in a controlled manner.

Consideration 5: Discussion of the impact of the noted practical issues with existing deployed NATs and experience reports.

Response: Some NATs deployed today exhibit a mapping behavior other than Endpoint-Independent mapping. These NATs are difficult to work with, as they make it difficult or impossible for protocols like ICE to use server-reflexive transport addresses on those NATs. A client behind such a NAT is often forced to use a relay protocol like TURN because "UDP hole punching" techniques [RFC5128] do not work.

24. Changes since RFC 5766

This section lists the major changes in the TURN protocol from the original [RFC5766] specification.

25. Updates to RFC 6156

This section lists the major updates to [RFC6156] in this specification.

26. Acknowledgements

Most of the text in this note comes from the original TURN specification, [RFC5766]. The authors would like to thank Rohan Mahy co-author of original TURN specification and everyone who had contributed to that document. The authors would also like to acknowledge that this document inherits material from [RFC6156].

Thanks to Justin Uberti, Pal Martinsen, Oleg Moskalenko, Aijun Wang and Simon Perreault for their help on the ADDITIONAL-ADDRESS-FAMILY mechanism. Authors would like to thank Gonzalo Salgueiro, Simon Perreault, Jonathan Lennox, Brandon Williams, Karl Stahl, Noriyuki Torii, Nils Ohlmeier, Dan Wing, Vijay Gurbani, Joseph Touch, Justin Uberti, Christopher Wood, Roman Danyliw, Éric Vyncke, Adam Roach, Mirja Kühlewind, Benjamin Kaduk and Oleg Moskalenko for comments and review. The authors would like to thank Marc for his contributions to the text.

Special thanks to Magnus Westerlund for the detailed AD review.

27. References

27.1. Normative References

[I-D.ietf-tram-stunbis] Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing, D., Mahy, R. and P. Matthews, "Session Traversal Utilities for NAT (STUN)", Internet-Draft draft-ietf-tram-stunbis-21, March 2019.
[Protocol-Numbers] "IANA Protocol Numbers Registry", 2005.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, DOI 10.17487/RFC0792, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2474] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, DOI 10.17487/RFC2474, December 1998.
[RFC3168] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003.
[RFC4443] Conta, A., Deering, S. and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012.
[RFC6437] Amante, S., Carpenter, B., Jiang, S. and J. Rajahalme, "IPv6 Flow Label Specification", RFC 6437, DOI 10.17487/RFC6437, November 2011.
[RFC7065] Petit-Huguenin, M., Nandakumar, S., Salgueiro, G. and P. Jones, "Traversal Using Relays around NAT (TURN) Uniform Resource Identifiers", RFC 7065, DOI 10.17487/RFC7065, November 2013.
[RFC7350] Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport Layer Security (DTLS) as Transport for Session Traversal Utilities for NAT (STUN)", RFC 7350, DOI 10.17487/RFC7350, August 2014.
[RFC7525] Sheffer, Y., Holz, R. and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015.
[RFC7915] Bao, C., Li, X., Baker, F., Anderson, T. and F. Gont, "IP/ICMP Translation Algorithm", RFC 7915, DOI 10.17487/RFC7915, June 2016.
[RFC7982] Martinsen, P., Reddy, T., Wing, D. and V. Singh, "Measurement of Round-Trip Time and Fractional Loss Using Session Traversal Utilities for NAT (STUN)", RFC 7982, DOI 10.17487/RFC7982, September 2016.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: Better Connectivity Using Concurrency", RFC 8305, DOI 10.17487/RFC8305, December 2017.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018.

27.2. Informative References

[Frag-Harmful] Kent, C. and J. Mogul, ""Fragmentation Considered Harmful", In Proc. SIGCOMM '87 Workshop on Frontiers in Computer Communications Technology, DOI 10.1145/55483.55524", August 1987.
[I-D.ietf-intarea-frag-fragile] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O. and F. Gont, "IP Fragmentation Considered Fragile", Internet-Draft draft-ietf-intarea-frag-fragile-15, July 2019.
[I-D.ietf-mmusic-ice-sip-sdp] Petit-Huguenin, M., Nandakumar, S. and A. Keranen, "Session Description Protocol (SDP) Offer/Answer procedures for Interactive Connectivity Establishment (ICE)", Internet-Draft draft-ietf-mmusic-ice-sip-sdp-36, June 2019.
[I-D.ietf-mptcp-rfc6824bis] Ford, A., Raiciu, C., Handley, M., Bonaventure, O. and C. Paasch, "TCP Extensions for Multipath Operation with Multiple Addresses", Internet-Draft draft-ietf-mptcp-rfc6824bis-18, June 2019.
[I-D.ietf-rtcweb-security] Rescorla, E., "Security Considerations for WebRTC", Internet-Draft draft-ietf-rtcweb-security-12, July 2019.
[I-D.ietf-tram-stun-pmtud] Petit-Huguenin, M. and G. Salgueiro, "Path MTU Discovery Using Session Traversal Utilities for NAT (STUN)", Internet-Draft draft-ietf-tram-stun-pmtud-10, September 2018.
[I-D.ietf-tsvwg-udp-options] Touch, J., "Transport Options for UDP", Internet-Draft draft-ietf-tsvwg-udp-options-07, March 2019.
[Port-Numbers] "IANA Port Numbers Registry", 2005.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC0791, September 1981.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996.
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D. and L. Jones, "SOCKS Protocol Version 5", RFC 1928, DOI 10.17487/RFC1928, March 1996.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, DOI 10.17487/RFC3424, November 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, DOI 10.17487/RFC3711, March 2004.
[RFC4086] Eastlake 3rd, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, DOI 10.17487/RFC4302, December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January 2007.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007.
[RFC5128] Srisuresh, P., Ford, B. and D. Kegel, "State of Peer-to-Peer (P2P) Communication across Network Address Translators (NATs)", RFC 5128, DOI 10.17487/RFC5128, March 2008.
[RFC5482] Eggert, L. and F. Gont, "TCP User Timeout Option", RFC 5482, DOI 10.17487/RFC5482, March 2009.
[RFC5766] Mahy, R., Matthews, P. and J. Rosenberg, "Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)", RFC 5766, DOI 10.17487/RFC5766, April 2010.
[RFC5925] Touch, J., Mankin, A. and R. Bonica, "The TCP Authentication Option", RFC 5925, DOI 10.17487/RFC5925, June 2010.
[RFC5928] Petit-Huguenin, M., "Traversal Using Relays around NAT (TURN) Resolution Mechanism", RFC 5928, DOI 10.17487/RFC5928, August 2010.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-Protocol Port Randomization", BCP 156, RFC 6056, DOI 10.17487/RFC6056, January 2011.
[RFC6062] Perreault, S. and J. Rosenberg, "Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations", RFC 6062, DOI 10.17487/RFC6062, November 2010.
[RFC6156] Camarillo, G., Novo, O. and S. Perreault, "Traversal Using Relays around NAT (TURN) Extension for IPv6", RFC 6156, DOI 10.17487/RFC6156, April 2011.
[RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for Keeping Alive the NAT Mappings Associated with RTP / RTP Control Protocol (RTCP) Flows", RFC 6263, DOI 10.17487/RFC6263, June 2011.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S. and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014.
[RFC7478] Holmberg, C., Hakansson, S. and G. Eriksson, "Web Real-Time Communication Use Cases and Requirements", RFC 7478, DOI 10.17487/RFC7478, March 2015.
[RFC7635] Reddy, T., Patil, P., Ravindranath, R. and J. Uberti, "Session Traversal Utilities for NAT (STUN) Extension for Third-Party Authorization", RFC 7635, DOI 10.17487/RFC7635, August 2015.
[RFC7657] Black, D. and P. Jones, "Differentiated Services (Diffserv) and Real-Time Communication", RFC 7657, DOI 10.17487/RFC7657, November 2015.
[RFC7983] Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme Updates for Secure Real-time Transport Protocol (SRTP) Extension for Datagram Transport Layer Security (DTLS)", RFC 7983, DOI 10.17487/RFC7983, September 2016.
[RFC8155] Patil, P., Reddy, T. and D. Wing, "Traversal Using Relays around NAT (TURN) Server Auto Discovery", RFC 8155, DOI 10.17487/RFC8155, April 2017.
[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion Notification (ECN) Experimentation", RFC 8311, DOI 10.17487/RFC8311, January 2018.
[RFC8445] Keranen, A., Holmberg, C. and J. Rosenberg, "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal", RFC 8445, DOI 10.17487/RFC8445, July 2018.

Authors' Addresses

Tirumaleswar Reddy (editor) McAfee, Inc. Embassy Golf Link Business Park Bangalore, Karnataka 560071 India EMail: kondtir@gmail.com
Alan Johnston (editor) Villanova University Villanova, PA USA EMail: alan.b.johnston@gmail.com
Philip Matthews Alcatel-Lucent 600 March Road Ottawa, Ontario Canada EMail: philip_matthews@magma.ca
Jonathan Rosenberg jdrosen.net Edison, NJ USA EMail: jdrosen@jdrosen.net URI: http://www.jdrosen.net