Traversal Using Relays around NAT (TURN): Relay
Extensions to Session Traversal Utilities for NAT (STUN)McAfee, Inc.Embassy Golf Link Business ParkBangaloreKarnataka560071Indiakondtir@gmail.comVillanova UniversityVillanovaPAUSAalan.b.johnston@gmail.comAlcatel-Lucent600 March RoadOttawaOntarioCanadaphilip_matthews@magma.cajdrosen.netEdisonNJUSAjdrosen@jdrosen.nethttp://www.jdrosen.net
Transport
TRAM WGNATTURNSTUNICEIf 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 some 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.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 )
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 and , 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", 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.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 packet to the client. When the
client sends a data packet to the server, the server relays it to the
appropriate 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 ). 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 for more details).If TURN is used with ICE , 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 , then SIP
serves the role of the rendezvous protocol, carrying the ICE candidate
information inside the body of SIP messages. If TURN and ICE are used
with some other rendezvous protocol, then 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 ). 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 . 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 , which was updated by to add IPv6 support. This document supersedes
and obsoletes both and .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. 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 ).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 appropriate peer.
The client sends the application data to the server inside a TURN
message; at the server, the data is extracted from the TURN message and
sent to the peer in a UDP datagram. In the reverse direction, a peer can
send application data in a UDP datagram to the relayed transport address
for the allocation; the server will then encapsulate this data inside a
TURN message and send it to the client along with an indication of which
peer sent the data. Since the TURN message always contains an indication
of which peer the client is communicating with, the client can use a
single allocation to communicate with multiple peers.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 relayed transport address 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.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 serverTURN server to peerUDPUDPTCPUDPTLS-over-TCPUDPDTLS-over-UDPUDPIf 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 guidance given in
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 . 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 .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 , 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).NOTE: While the terminology used in this document refers to
5-tuples, the TURN server can store whatever identifier it likes
that yields identical results. Specifically, an implementation may
use a file-descriptor in place of a 5-tuple to represent a TCP
connection.In , 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.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 .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 .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.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 proper 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.In , 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.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. 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.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.This design decision has the following implications for data
relayed by a TURN server:The value of the Diffserv field may not be preserved across the
server;The Time to Live (TTL) field may be reset, rather than
decremented, across the server;The Explicit Congestion Notification (ECN) field may be reset
by the server;There is no end-to-end fragmentation, since the packet is
re-assembled at the server.Future work may specify alternate TURN semantics that address
these limitations.For reasons described in ,
applications, especially those sending large volumes of data, should
try hard to avoid having their packets fragmented. 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) must handle fragmentation
avoidance themselves.The application running on the client and the peer can take one of
two approaches to avoid IP fragmentation.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 exploits the fact that
the IP specification specifies that IP
packets up to 576 bytes should never need to be fragmented.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
may not be able to discover the MTU of
the transmission path between the client and the peer since:- a probe packet with DF bit set to test a path for a larger
MTU can be dropped by routers, or- ICMP error messages can be dropped by middle boxes.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 is one
such algorithm. is an
implementation of 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.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 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.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 . The TURN
client performs the following steps based on the transport protocol
being used to connect to the TURN server.For TCP or TLS-over-TCP, initiate TCP connection to both IP
address families as discussed in ,
and use the first TCP connection that is established. If
connections are established on both IP address families then
terminate the TCP connection using the IP address family with
lower precedence .For clear text UDP, send TURN Allocate requests to both IP
address families as discussed in ,
without authentication information. If the TURN server requires
authentication, it will send back a 401 unauthenticated response
and the TURN client uses the first UDP connection on which a 401
error response is received. If a 401 error response is received
from both IP address families then the TURN client can silently
abandon the UDP connection on the IP address family with lower
precedence. If the TURN server does not require authentication (as
described in Section 9 of ), it is
possible for both Allocate requests to succeed. In this case, the
TURN client sends a Refresh with LIFETIME value of 0 on the
allocation using the IP address family with lower precedence to
delete the allocation.For DTLS over UDP, initiate DTLS handshake to both IP address
families as discussed in and use
the first DTLS session that is established. If the DTLS session is
established on both IP address families then the client sends DTLS
close_notify alert to terminate the DTLS session using the IP
address family with lower precedence. If TURN over DTLS server has
been configured to require a cookie exchange (Section 4.2 in ) and HelloVerifyRequest is received from
the TURN servers on both IP address families then the client can
silently abandon the connection on the IP address family with
lower precedence.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.Readers are expected to be familiar with and the terms defined there.The following terms are used in this document:The protocol spoken between a TURN client and a
TURN server. It is an extension to the STUN protocol . The protocol allows a client
to allocate and use a relayed transport address.A STUN client that implements this
specification.A STUN server that implements this
specification. It relays data between a TURN client and its
peer(s).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.The combination of an IP address
and a port.A transport address on a
client or a peer.A transport
address on the "public side" of a NAT. This address is allocated by
the NAT to correspond to a specific host transport address.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.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.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.The relayed transport address granted to a
client through an Allocate request, along with related state, such
as permissions and expiration timers.The combination (client IP address and port,
server IP address and port, and transport protocol (currently one of
UDP, TCP, or (D)TLS)) used to communicate between the client and the
server. The 5-tuple uniquely identifies this communication stream.
The 5-tuple also uniquely identifies the Allocation on the
server.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.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.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.A string chosen at random by the server and
included in the message-digest. To prevent replay attacks, the
server should change the nonce regularly.This term is used for statements that apply to
both Transport Layer Security and
Datagram Transport Layer Security .Methods of TURN server discovery, including using anycast, are
described in . The syntax of the "turn"
and "turns" URIs are defined in Section 3.1 of .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.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 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
. specifies an
authentication mechanism called the long-term credential mechanism. TURN
servers and clients MUST implement this mechanism. The server MUST
demand that all requests from the client be authenticated using this
mechanism, or that a equally strong or stronger mechanism for client
authentication is used.Note that the long-term credential mechanism applies only to requests
and cannot be used to authenticate indications; thus, indications in
TURN are never authenticated. If the server requires requests to be
authenticated, then the server's administrator MUST choose a realm value
that will uniquely identify the username and password combination that
the client must use, even if the client uses multiple servers under
different administrations. The server's administrator MAY choose to
allocate a unique username to each client, or MAY choose to allocate the
same username to more than one client (for example, to all clients from
the same department or company). For each Allocate request, the server
SHOULD generate a new random nonce when the allocation is first
attempted following the randomness recommendations in and SHOULD expire the nonce at least once every
hour during the lifetime of the allocation.All requests after the initial Allocate must use the same username as
that used to create the allocation, to prevent attackers from hijacking
the client's allocation. Specifically, if the server requires the use of
the long-term credential mechanism, and if a non-Allocate request passes
authentication under this mechanism, and if the 5-tuple identifies an
existing allocation, but the request does not use the same username as
used to create the allocation, then the request MUST be rejected with a
441 (Wrong Credentials) error.When a TURN message arrives at the server from the client, the server
uses the 5-tuple in the message to identify the associated allocation.
For all TURN messages (including ChannelData) EXCEPT an Allocate
request, if the 5-tuple does not identify an existing allocation, then
the message MUST either be rejected with a 437 Allocation Mismatch error
(if it is a request) or silently ignored (if it is an indication or a
ChannelData message). A client receiving a 437 error response to a
request other than Allocate MUST assume the allocation no longer
exists. 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
.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. The REQUESTED-ADDRESS-FAMILY attribute allows a
client 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). The ADDITIONAL-ADDRESS-FAMILY attribute
allows a client to request 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 , 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 ). Some implementations may choose
to meet this requirement by remembering all received requests and the
corresponding responses for 40 seconds. Other implementations may choose
to reprocess the request and arrange that such reprocessing returns
essentially the same response. To aid implementors who choose the latter
approach (the so-called "stateless stack approach"), this specification
includes some implementation notes on how this might be done.
Implementations are free to choose either approach or choose some other
approach that gives the same results.When TCP transport is used between the client and the server, it is
possible that a bit error will cause a length field in a TURN packet to
become corrupted, causing the receiver to lose synchronization with the
incoming stream of TURN messages. A client or server that detects a long
sequence of invalid TURN messages over TCP transport SHOULD close the
corresponding TCP connection to help the other end detect this situation
more rapidly.To mitigate either intentional or unintentional denial-of-service
attacks against the server by clients with valid usernames and
passwords, it is RECOMMENDED that the server impose limits on both the
number of allocations active at one time for a given username and on the
amount of bandwidth those allocations can use. The server should reject
new allocations that would exceed the limit on the allowed number of
allocations active at one time with a 486 (Allocation Quota Exceeded)
(see ), and should discard
application data traffic that exceeds the bandwidth quota.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 or addresses;the 5-tuple: (client's IP address, client's port, server IP
address, server port, transport protocol);the authentication information;the time-to-expiry for each relayed transport address;a list of permissions for each relayed transport address;a list of channel to peer bindings for each relayed transport
address.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.Both the relayed transport address and the 5-tuple MUST be unique
across all allocations, so either one can be used to uniquely identify
the allocation, 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.
Note that, rather than storing the password explicitly, for security
reasons, it may be desirable for the server to store the key value,
which is a secure hash over the username, realm, and password (see ).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 and the list of channels is described
in .An allocation on the server is created using an Allocate
transaction.The client forms an Allocate request as follows.The client first picks a host transport address. It is RECOMMENDED
that the client pick a currently unused transport address, typically
by allowing the underlying OS to pick a currently unused port for a
new socket.The client then picks a transport protocol to use between the
client and the server. The transport protocol MUST be one of UDP, TCP,
TLS-over-TCP or DTLS-over-UDP. Since this specification only allows
UDP between the server and the peers, it is RECOMMENDED that the
client pick UDP unless it has a reason to use a different transport.
One reason to pick a different transport would be that the client
believes, either through configuration or by experiment, that it is
unable to contact any TURN server using UDP. See 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
to discover a TURN server and uses the
TURN server resolution mechanism defined in 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 reasons outlined in
.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.When the server receives an Allocate request, it performs the
following checks:The server SHOULD require that the request be authenticated.
The authentication of the request is optional to allow TURN
servers provided by the local or access network to accept
Allocation requests from new and/or guest users in the network who
do not necessarily possess long term credentials for STUN
authentication and its security implications are discussed in
. If the request is authenticated,
the authentication MUST be done using the long-term credential
mechanism of unless
the client and server agree to use another mechanism through some
procedure outside the scope of this document.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.The server checks if the request contains a REQUESTED-TRANSPORT
attribute. If the REQUESTED-TRANSPORT attribute is not included or
is malformed, the server rejects the request with a 400 (Bad
Request) error. Otherwise, if the attribute is included but
specifies a protocol other that UDP, the server rejects the
request with a 442 (Unsupported Transport Protocol) error.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 ),
then the server treats the DONT-FRAGMENT attribute in the Allocate
request as an unknown comprehension-required attribute.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.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.If the server does not support the address family requested by
the client in REQUESTED-ADDRESS-FAMILY or 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.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.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
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.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.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 .If all the checks pass, the server creates the allocation. The
5-tuple is set to the 5-tuple from the Allocate request, while the
list of permissions and the list of channels are initially empty.The server chooses a relayed transport address for the allocation
as follows:If the request contains a RESERVATION-TOKEN attribute, the
server uses the previously reserved transport address
corresponding to the included token (if it is still available).
Note that the reservation is a server-wide reservation and is not
specific to a particular allocation, since the Allocate request
containing the RESERVATION-TOKEN uses a different 5-tuple than the
Allocate request that made the reservation. The 5-tuple for the
Allocate request containing the RESERVATION-TOKEN attribute can be
any allowed 5-tuple; it can use a different client IP address and
port, a different transport protocol, and even different server IP
address and port (provided, of course, that the server IP address
and port are ones on which the server is listening for TURN
requests).If the request contains an EVEN-PORT attribute with the R bit
set to 0, then the server allocates a relayed transport address
with an even port number.If the request contains an EVEN-PORT attribute with the R bit
set to 1, then the server looks for a pair of port numbers N and
N+1 on the same IP address, where N is even. Port N is used in the
current allocation, while the relayed transport address with port
N+1 is assigned a token and reserved for a future allocation. The
server MUST hold this reservation for at least 30 seconds, and MAY
choose to hold longer (e.g., until the allocation with port N
expires). The server then includes the token in a
RESERVATION-TOKEN attribute in the success response.Otherwise, the server allocates any available relayed transport
address.In all cases, the server SHOULD only allocate ports from the range
49152 – 65535 (the Dynamic and/or Private Port range ), unless the TURN server application
knows, through some means not specified here, that other applications
running on the same host as the TURN server application will not be
impacted by allocating ports outside this range. This condition can
often be satisfied by running the TURN server application on a
dedicated machine and/or by arranging that any other applications on
the machine allocate ports before the TURN server application starts.
In any case, the TURN server SHOULD NOT allocate ports in the range 0
- 1023 (the Well-Known Port range) to discourage clients from using
TURN to run standard services.NOTE: The use of randomized port assignments to avoid certain
types of attacks is described in .
It is RECOMMENDED that a TURN server implement a randomized port
assignment algorithm from . This is
especially applicable to servers that choose to pre-allocate a
number of ports from the underlying OS and then later assign them
to allocations; for example, a server may choose this technique to
implement the EVEN-PORT attribute.The server determines the initial value of the time-to-expiry field
as follows. If the request contains a LIFETIME attribute, then the
server computes the minimum of the client's proposed lifetime and the
server's maximum allowed lifetime. If this computed value is greater
than the default lifetime, then the server uses the computed lifetime
as the initial value of the time-to-expiry field. Otherwise, the
server uses the default lifetime. It is RECOMMENDED that the server
use a maximum allowed lifetime value of no more than 3600 seconds (1
hour). Servers that implement allocation quotas or charge users for
allocations in some way may wish to use a smaller maximum allowed
lifetime (perhaps as small as the default lifetime) to more quickly
remove orphaned allocations (that is, allocations where the
corresponding client has crashed or terminated or the client
connection has been lost for some reason). Also, note that the time-
to-expiry is recomputed with each successful Refresh request, and thus
the value computed here applies only until the first refresh.Once the allocation is created, the server replies with a success
response. The success response contains:An XOR-RELAYED-ADDRESS attribute containing the relayed
transport address.A LIFETIME attribute containing the current value of the
time-to-expiry timer.A RESERVATION-TOKEN attribute (if a second relayed transport
address was reserved).An XOR-MAPPED-ADDRESS attribute containing the client's IP
address and port (from the 5-tuple).NOTE: The XOR-MAPPED-ADDRESS attribute is included in the
response as a convenience to the client. TURN itself does not make
use of this value, but clients running ICE can often need this
value and can thus avoid having to do an extra Binding transaction
with some STUN server to learn it.The response (either success or error) is sent back to the client
on the 5-tuple.NOTE: When the Allocate request is sent over UDP, requires that the server
handle the possible retransmissions of the request so that
retransmissions do not cause multiple allocations to be created.
Implementations may achieve this using the so-called "stateless
stack approach" as follows. To detect retransmissions when the
original request was successful in creating an allocation, the
server can store the transaction id that created the request with
the allocation data and compare it with incoming Allocate requests
on the same 5-tuple. Once such a request is detected, the server
can stop parsing the request and immediately generate a success
response. When building this response, the value of the LIFETIME
attribute can be taken from the time-to-expiry field in the
allocate state data, even though this value may differ slightly
from the LIFETIME value originally returned. In addition, the
server may need to store an indication of any reservation token
returned in the original response, so that this may be returned in
any retransmitted responses.For the case where the original request was unsuccessful in
creating an allocation, the server may choose to do nothing
special. Note, however, that there is a rare case where the server
rejects the original request but accepts the retransmitted request
(because conditions have changed in the brief intervening time
period). If the client receives the first failure response, it
will ignore the second (success) response and believe that an
allocation was not created. An allocation created in this matter
will eventually timeout, since the client will not refresh it.
Furthermore, if the client later retries with the same 5-tuple but
different transaction id, it will receive a 437 (Allocation
Mismatch), which will cause it to retry with a different 5-tuple.
The server may use a smaller maximum lifetime value to minimize
the lifetime of allocations "orphaned" in this manner.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 () 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.If the client receives an Allocate error response, then the
processing depends on the actual error code returned:(Request timed out): There is either a problem with the server,
or a problem reaching the server with the chosen transport. The
client considers the current transaction as having failed but MAY
choose to retry the Allocate request using a different transport
(e.g., TCP instead of UDP).300 (Try Alternate): The server would like the client to use
the server specified in the ALTERNATE-SERVER attribute instead.
The client considers the current transaction as having failed, but
SHOULD try the Allocate request with the alternate server before
trying any other servers (e.g., other servers discovered using the
DNS resolution procedures). When trying the Allocate request with
the alternate server, the client follows the ALTERNATE-SERVER
procedures specified in .400 (Bad Request): The server believes the client's request is
malformed for some reason. The client considers the current
transaction as having failed. The client MAY notify the user or
operator and SHOULD NOT retry the request with this server until
it believes the problem has been fixed.401 (Unauthorized): If the client has followed the procedures
of the long-term credential mechanism and still gets this error,
then the server is not accepting the client's credentials. In this
case, the client considers the current transaction as having
failed and SHOULD notify the user or operator. The client SHOULD
NOT send any further requests to this server until it believes the
problem has been fixed.403 (Forbidden): The request is valid, but the server is
refusing to perform it, likely due to administrative restrictions.
The client considers the current transaction as having failed. The
client MAY notify the user or operator and SHOULD NOT retry the
same request with this server until it believes the problem has
been fixed.420 (Unknown Attribute): If the client included a DONT-FRAGMENT
attribute in the request and the server rejected the request with
a 420 error code and listed the DONT-FRAGMENT attribute in the
UNKNOWN-ATTRIBUTES attribute in the error response, then the
client now knows that the server does not support the
DONT-FRAGMENT attribute. The client considers the current
transaction as having failed but MAY choose to retry the Allocate
request without the DONT-FRAGMENT attribute.437 (Allocation Mismatch): This indicates that the client has
picked a 5-tuple that the server sees as already in use. One way
this could happen is if an intervening NAT assigned a mapped
transport address that was used by another client that recently
crashed. The client considers the current transaction as having
failed. The client SHOULD pick another client transport address
and retry the Allocate request (using a different transaction id).
The client SHOULD try three different client transport addresses
before giving up on this server. Once the client gives up on the
server, it SHOULD NOT try to create another allocation on the
server for 2 minutes.438 (Stale Nonce): See the procedures for the long-term
credential mechanism .440 (Address Family not Supported): The server does not support
the address family requested by the client. If the client receives
an Allocate error response with the 440 (Unsupported Address
Family) error code, the client MUST NOT retry the request.441 (Wrong Credentials): The client should not receive this
error in response to a Allocate request. The client MAY notify the
user or operator and SHOULD NOT retry the same request with this
server until it believes the problem has been fixed.442 (Unsupported Transport Address): The client should not
receive this error in response to a request for a UDP allocation.
The client MAY notify the user or operator and SHOULD NOT
reattempt the request with this server until it believes the
problem has been fixed.486 (Allocation Quota Reached): The server is currently unable
to create any more allocations with this username. The client
considers the current transaction as having failed. The client
SHOULD wait at least 1 minute before trying to create any more
allocations on the server.508 (Insufficient Capacity): The server has no more relayed
transport addresses available, or has none with the requested
properties, or the one that was reserved is no longer available.
The client considers the current operation as having failed. If
the client is using either the EVEN-PORT or the RESERVATION-TOKEN
attribute, then the client MAY choose to remove or modify this
attribute and try again immediately. Otherwise, the client SHOULD
wait at least 1 minute before trying to create any more
allocations on this server.An unknown error response MUST be handled as described in
.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.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 family types 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.When the server receives a Refresh request, it processes the
request as per 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 "desired lifetime" is 0, then the request succeeds and
the allocation is deleted.If the "desired lifetime" is non-zero, then the request
succeeds and the allocation's time-to-expiry is set to the
"desired lifetime".If the request succeeds, then the server sends a success
response containing:A LIFETIME attribute containing the current value of the
time-to-expiry timer.NOTE: A server need not do anything special to implement
idempotency of Refresh requests over UDP using the "stateless
stack approach". Retransmitted Refresh requests with a non-zero
"desired lifetime" will simply refresh the allocation. A
retransmitted Refresh request with a zero "desired lifetime" will
cause a 437 (Allocation Mismatch) response if the allocation has
already been deleted, but the client will treat this as equivalent
to a success response (see below).If the client receives a success response to its Refresh request
with a non-zero lifetime, it updates its copy of the allocation data
structure with the time-to-expiry value contained in the response.If the client receives a 437 (Allocation Mismatch) error response
to a request to delete the allocation, then the allocation no longer
exists and it should consider its request as having effectively
succeeded.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:If no permission for that IP address exists, then a permission is
created with the given IP address and a time-to-expiry equal to
Permission Lifetime.If a permission for that IP address already exists, then the
time-to-expiry for that permission is reset to Permission
Lifetime.The Permission Lifetime MUST be 300 seconds (= 5 minutes).Each permission’s time-to-expiry decreases down once per second
until it reaches 0; at which point, the permission expires and is
deleted.CreatePermission and ChannelBind requests may be freely intermixed on
a permission. A given permission may be initially installed and/or
refreshed with a CreatePermission request, and then later refreshed with
a ChannelBind request, or vice versa.When a UDP datagram arrives at the relayed transport address for the
allocation, the server extracts the source IP address from the IP
header. The server then compares this address with the IP address
associated with each permission in the list of permissions for the
allocation. 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 ().The permissions for one allocation are totally unrelated to the
permissions for a different allocation. If an allocation expires, all
its permissions expire with it.NOTE: Though TURN permissions expire after 5 minutes, many NATs
deployed at the time of publication expire their UDP bindings
considerably faster. Thus, an application using TURN will probably
wish to send some sort of keep-alive traffic at a much faster rate.
Applications using ICE should follow the keep-alive guidelines of
ICE , and applications not using ICE
are advised to do something similar.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 or the Channel
mechanism in .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.When the server receives the CreatePermission request, it processes
as per 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 . The port portion
of each attribute is ignored and may be any arbitrary value.The server then responds with a CreatePermission success response.
There are no mandatory attributes in the success response.NOTE: A server need not do anything special to implement
idempotency of CreatePermission requests over UDP using the
"stateless stack approach". Retransmitted CreatePermission
requests will simply refresh the permissions.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.TURN supports two mechanisms for sending and receiving data from
peers. This section describes the use of the Send and Data mechanisms,
while describes the use of the
Channel mechanism.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.When the server receives a Send indication, it processes as per
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 source transport address is the relayed transport address
of the allocation, where the allocation is determined by the
5-tuple on which the Send indication arrived;the destination transport address is taken from the
XOR-PEER-ADDRESS attribute;the data following the UDP header is the contents of the value
field of the DATA attribute.The handling of the DONT-FRAGMENT attribute (if present), is
described in .The resulting UDP datagram is then sent to the peer.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 .If relaying is permitted, then the server checks if there is a
channel bound to the peer that sent the UDP datagram (see ). If a channel is bound, then processing
proceeds as described in .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.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.NOTE: The latter check protects the client against an attacker
who somehow manages to trick the server into installing
permissions not desired by the client.If the 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 .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.When the server receives an ICMP packet, the server verifies that
the type is either 3, 11 or 12 for an ICMPv4 packet or either 1, 2, or 3 for an ICMPv6
packet. It also verifies that the IP
packet in the ICMP packet payload contains a UDP header. 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.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.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 ) starts with a two-byte field that
carries the channel number. The values of this field are allocated as
follows:0x0000 through 0x3FFF: These values can never be used for channel
numbers.0x4000 through 0x4FFF: These values are the allowed channel
numbers (4096 possible values).0x5000-0xFFFF: Reserved (For DTLS-SRTP multiplexing collision
avoidance, see .According to , ChannelData messages can
be distinguished from other multiplexed protocols by examining the first
byte of the message: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:a channel number;a transport address (of the peer); andA time-to-expiry timer.Within the context of an allocation, a channel binding is
uniquely identified either by the channel number or by the peer's
transport address. Thus, the same channel cannot be bound to two
different transport addresses, nor can the same transport address be
bound to two different channels.A channel binding lasts for 10 minutes unless refreshed. Refreshing
the binding (by the server receiving a ChannelBind request rebinding the
channel to the same peer) resets the time-to-expiry timer back to 10
minutes.When the channel binding expires, the channel becomes unbound. Once
unbound, the channel number can be bound to a different transport
address, and the transport address can be bound to a different channel
number. To prevent race conditions, the client MUST wait 5 minutes after
the channel binding expires before attempting to bind the channel number
to a different transport address or the transport address to a different
channel number.When binding a channel to a peer, the client SHOULD be prepared to
receive ChannelData messages on the channel from the server as soon as
it has sent the ChannelBind request. Over UDP, it is possible for the
client to receive ChannelData messages from the server before it
receives a ChannelBind success response.In the other direction, the client MAY elect to send ChannelData
messages before receiving the ChannelBind success response. Doing so,
however, runs the risk of having the ChannelData messages dropped by the
server if the ChannelBind request does not succeed for some reason
(e.g., packet lost if the request is sent over UDP, or the server being
unable to fulfill the request). A client that wishes to be safe should
either queue the data or use Send indications until the channel binding
is confirmed.A channel binding is created or refreshed using a ChannelBind
transaction. A ChannelBind transaction also creates or refreshes a
permission towards the peer (see ).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. 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.When the server receives a ChannelBind request, it processes as per
plus the specific rules
mentioned here.The server checks the following:The request contains both a CHANNEL-NUMBER and an
XOR-PEER-ADDRESS attribute;The channel number is in the range 0x4000 through 0x4FFF
(inclusive);The channel number is not currently bound to a different
transport address (same transport address is OK);The transport address is not currently bound to a different
channel number.If 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.If any of these tests fail, the server replies with a 400 (Bad
Request) error.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 .NOTE: A server need not do anything special to implement
idempotency of ChannelBind requests over UDP using the "stateless
stack approach". Retransmitted ChannelBind requests will simply
refresh the channel binding and the corresponding permission.
Furthermore, the client must wait 5 minutes before binding a
previously bound channel number or peer address to a different
channel, eliminating the possibility that the transaction would
initially fail but succeed on a retransmission.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.The ChannelData message is used to carry application data between
the client and the server. It has the following format: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.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
.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. 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.The receiver of the ChannelData message uses the first byte to
distinguish it from other multiplexed protocols, as described above.
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 .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:the source transport address is the relayed transport address
of the allocation, where the allocation is determined by the
5-tuple on which the ChannelData message arrived;the destination transport address is the transport address to
which the channel is bound;the data following the UDP header is the contents of the data
field of the ChannelData message.The resulting UDP datagram is then sent to the peer. Note
that if the Length field in the ChannelData message is 0, then there
will be no data in the UDP datagram, but the UDP datagram is still
formed and sent.When the server receives a UDP datagram on the relayed transport
address associated with an allocation, the server processes it as
described in . 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 .When the server receives an ICMP packet, the server processes it as
described in .
A Data indication MUST be sent regardless of whether there is a
channel bound to the peer that was the destination of the UDP datagram
that triggered the reception of the ICMP packet.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 , translations in TURN
are designed so that a TURN server can be implemented as an application
that runs in userland 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.
Otherwise, the server MUST implement the alternate behavior and MUST NOT
do anything else for the reasons detailed in .Traffic ClassPreferred behavior: As specified in Section 4 of .Alternate behavior: The relay sets the Traffic Class to the
default value for outgoing packets.Flow LabelPreferred behavior: The relay sets the Flow label to 0. The
relay can choose to set the Flow label to a different value if it
supports the IPv6 Flow Label field .Alternate behavior: The relay sets the Flow label to the
default value for outgoing packets.Hop LimitPreferred behavior: As specified in Section 4 of .Alternate behavior: The relay sets the Hop Limit to the default
value for outgoing packets.FragmentationPreferred behavior: As specified in Section 4 of .Alternate behavior: The relay assembles incoming fragments. The
relay follows its default behavior to send outgoing packets.For both preferred and alternate behavior, the DONT-FRAGMENT
attribute MUST be ignored by the server.Extension HeadersPreferred behavior: The relay sends outgoing packet without any
IPv6 extension headers, with the exception of the Fragmentation
header as described above.Alternate behavior: Same as preferred.Flow LabelThe relay should consider that it is handling two different IPv6
flows. Therefore, the Flow label SHOULD
NOT be copied as part of the translation.Preferred behavior: The relay sets the Flow label to 0. The
relay can choose to set the Flow label to a different value if it
supports the IPv6 Flow Label field .Alternate behavior: The relay sets the Flow label to the
default value for outgoing packets.Hop LimitPreferred behavior: The relay acts as a regular router with
respect to decrementing the Hop Limit and generating an ICMPv6
error if it reaches zero.Alternate behavior: The relay sets the Hop Limit to the default
value for outgoing packets.FragmentationPreferred behavior: If the incoming packet did not include a
Fragment header and the outgoing packet size does not exceed the
outgoing link's MTU, the relay sends the outgoing packet without a
Fragment header.If the incoming packet did not include a Fragment header and
the outgoing packet size exceeds the outgoing link's MTU, the
relay drops the outgoing packet and send an ICMP message of type 2
code 0 ("Packet too big") to the sender of the incoming packet.
If the packet is being sent to the peer, the relay reduces
the MTU reported in the ICMP message by 48 bytes to allow room for
the overhead of a Data indication.If the incoming packet included a Fragment header and the
outgoing packet size (with a Fragment header included) does not
exceed the outgoing link's MTU, the relay sends the outgoing
packet with a Fragment header. The relay sets the fields of the
Fragment header as appropriate for a packet originating from the
server.If the incoming packet included a Fragment header and the
outgoing packet size exceeds the outgoing link's MTU, the relay
MUST fragment the outgoing packet into fragments of no more than
1280 bytes. The relay sets the fields of the Fragment header as
appropriate for a packet originating from the server.Alternate behavior: The relay assembles incoming fragments. The
relay follows its default behavior to send outgoing packets.For both preferred and alternate behavior, the DONT-FRAGMENT
attribute MUST be ignored by the server.Extension HeadersPreferred behavior: The relay sends outgoing packet without any
IPv6 extension headers, with the exception of the Fragmentation
header as described above.Alternate behavior: Same as preferred.Type of Service and PrecedencePreferred behavior: As specified in Section 5 of .Alternate behavior: The relay sets the Type of Service and
Precedence to the default value for outgoing packets.Time to LivePreferred behavior: As specified in Section 5 of .Alternate behavior: The relay sets the Time to Live to the
default value for outgoing packets.FragmentationPreferred behavior: As specified in Section 5 of . Additionally, when the outgoing packet's
size exceeds the outgoing link's MTU, the relay needs to generate
an ICMP error (ICMPv6 Packet Too Big) reporting the MTU size. If
the packet is being sent to the peer, the relay SHOULD reduce the
MTU reported in the ICMP message by 48 bytes to allow room for the
overhead of a Data indication.Alternate behavior: The relay assembles incoming fragments. The
relay follows its default behavior to send outgoing packets.For both preferred and alternate behavior, the DONT-FRAGMENT
attribute MUST be ignored by the server.This section describes how the server sets various fields in the IP
header when relaying between the client and 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.Time to Live (TTL) fieldPreferred Behavior: If the incoming value is 0, then the drop the
incoming packet. Otherwise, set the outgoing Time to Live/Hop Count
to one less than the incoming value.Alternate Behavior: Set the outgoing value to the default for
outgoing packets.Differentiated Services Code Point (DSCP) field Preferred Behavior: Set the outgoing value to the incoming value,
unless the server includes a differentiated services classifier and
marker .Alternate Behavior: Set the outgoing value to a fixed value,
which by default is Best Effort unless configured otherwise.In both cases, if the server is immediately adjacent to a
differentiated services classifier and marker, then DSCP MAY be set
to any arbitrary value in the direction towards the classifier.Explicit Congestion Notification (ECN) field Preferred Behavior: Set the outgoing value to the incoming value,
UNLESS the server is doing Active Queue Management, the incoming ECN
field is ECT(1) (=0b01) or ECT(0) (=0b10), and the server wishes to
indicate that congestion has been experienced, in which case set the
outgoing value to CE (=0b11).Alternate Behavior: Set the outgoing value to Not-ECT
(=0b00).IPv4 Fragmentation fieldsPreferred Behavior: When the server sends a packet to a peer in
response to a Send indication containing the DONT-FRAGMENT
attribute, then set the DF bit in the outgoing IP header to 1. In
all other cases when sending an outgoing packet containing
application data (e.g., Data indication, ChannelData message, or
DONT-FRAGMENT attribute not included in the Send indication), copy
the DF bit from the DF bit of the incoming packet that contained the
application data.Set the other fragmentation fields (Identification, More
Fragments, Fragment Offset) as appropriate for a packet originating
from the server.Alternate Behavior: As described in the Preferred Behavior,
except always assume the incoming DF bit is 0.In both the Preferred and Alternate Behaviors, the resulting
packet may be too large for the outgoing link. If this is the case,
then the normal fragmentation rules apply .IPv4 OptionsPreferred Behavior: The outgoing packet is sent without any IPv4
options.Alternate Behavior: Same as preferred.This section lists the codepoints for the STUN methods defined in
this specification. See elsewhere in this document for the semantics of
these methods.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 ).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.The LIFETIME attribute represents the duration for which the server
will maintain an allocation in the absence of a refresh. 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.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 .The DATA attribute is present in all Send and Data indications. 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). If the length of this attribute is not a multiple of 4,
then padding must be added after this attribute.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 .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:there are two values defined for this field
and specified in ,
Section 14.1: 0x01 for IPv4 addresses and 0x02 for IPv6
addresses.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.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:The value contains a single 1-bit flag: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.The other 7 bits of the attribute's value 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.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: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 . 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.This attribute is used by the client to request that the server set
the DF (Don't Fragment) bit in the IP header when relaying the
application data onward to the peer. This attribute has no value part
and thus the attribute length field is 0.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.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 . 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.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:there are two values defined for this field
and specified in ,
Section 14.1: 0x01 for IPv4 addresses and 0x02 for IPv6
addresses.at this point, the 13 bits in the Reserved
field MUST be set to zero by the client and MUST be ignored by the
server.The Class represents the hundreds digit of
the error code and is defined in section 14.8 of .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 .The recommended reason phrases for
error codes 440 and 508 are explained in .This attribute is used by servers to signal the reason an UDP
packet was dropped. The following is the format of the ICMP
attribute.This field MUST be set to 0 when sent, and
MUST be ignored when received.The field contains the value in the ICMP
type. Its interpretation depends whether the ICMP was received
over IPv4 or IPv6.The field contains the value in the ICMP
code. Its interpretation depends whether the ICMP was received
over IPv4 or IPv6.This document defines the following error response codes:(Forbidden): The request was valid but cannot be
performed due to administrative or similar restrictions.(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.(Address Family not Supported): The server does
not support the address family requested by the client.(Wrong Credentials): The credentials in the
(non-Allocate) request do not match those used to create the
allocation.(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.(Peer Address Family Mismatch). A peer address is
part of a different address family than that of the relayed
transport address of the allocation.(Allocation Quota Reached): No more allocations
using this username can be created at the present time.(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.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 ().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="obMatJos2AAABadl7W7PeDU4hKE72jda"), the value
of the attribute is shown in quotes for readability, but these quotes do
not appear in the actual value.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 .
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
, 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 and NONCE attributes, which are just copied from the
401 error response. Finally, the client includes MESSAGE-INTEGRITY and
MESSAGE-INTEGRITY-SHA256 attributes as the last attributes in the
message, whose values are Hashed Message Authentication Code - Secure
Hash Algorithm 1 (HMAC-SHA1) hash and Hashed Message Authentication Code
- Secure Hash Algorithm 2 (HMAC-SHA2) hash over the contents of the
message (shown as just "..." above); this HMAC-SHA1 and 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 either a MESSAGE-INTEGRITY or 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.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
contain a MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute
(assuming the server uses the long-term credential mechanism), but no
USERNAME, REALM, and NONCE attributes.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.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.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.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).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.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.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.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.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 .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 A with a spoofed IP
address of B. This attack requires the attacker to know the IP
addresses of A and B. With TURN, an attacker wishing to send packets
towards a client using a Data indication needs to know its IP
address (and port), the IP address and port of the TURN server, and
the IP address and port of the peer (for inclusion in the
XOR-PEER-ADDRESS attribute). To send a fake ChannelData message to a
client, an attacker needs to know the IP address and port of the
client, the IP address and port of the TURN server, and the channel
number. This particular combination is mildly more guessable than in
the non-TURN case.These attacks are more properly mitigated by application-layer
authentication techniques. In the case of real-time traffic, usage
of SRTP 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.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.TURN concerns itself primarily with authentication and message
integrity. Confidentiality is only a secondary concern, as TURN
control messages do not include information that is particularly
sensitive. The primary protocol content of the messages is the IP
address of the peer. If it is important to prevent an eavesdropper
on a TURN connection from learning this, TURN can be run over
(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.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).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 ,
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.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.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.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.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.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.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.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.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: An Allocate request asking for a v4 address, andA ChannelBind request establishing a channel to the IPv4
address of the tunnel endpointThen he has set up an amplification attack: The TURN relay will re-encapsulate IPv6 UDP data in v4 and send
it to the tunnel endpointThe tunnel endpoint will de-encapsulate packets from the v4
interface and send them to v6So if the attacker sends a packet of the following form... Then the TURN relay and the tunnel endpoint will send it
back and forth until the last TURN header is consumed, at which point
the TURN relay will send an empty packet, which the tunnel endpoint
will drop.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
relays 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 relay MUST NOT accept Teredo or 6to4
addresses in these requests.Any relay addresses learned through an Allocate request will not
operate properly with IPsec Authentication Header (AH) in transport or tunnel mode. However,
tunnel-mode IPsec Encapsulating Security Payload (ESP) should still operate.[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 . [IANA is requested to
update the reference from to RFC-to-be
for the STUN methods listed in .]The codepoints for the STUN attributes defined in this specification
are listed in . [IANA is
requested to update the reference from 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 .][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 . [IANA is requested
to update the reference from to RFC-to-be
for the STUN error codes listed in .]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:0x0000 through 0x3FFF: Reserved and not available for use, since
they conflict with the STUN header.0x4000 through 0x4FFF: A TURN implementation is free to use
channel numbers in this range.0x5000 through 0xFFFF: Unassigned.Any change to this registry must be made through an IETF
Standards Action.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 . 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 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 . Applications are also strongly urged to use
ICE 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 do not
work.This section lists the major changes in the TURN protocol from the
original specification.IPv6 support.REQUESTED-ADDRESS-FAMILY, ADDITIONAL-ADDRESS-FAMILY, AND
ADDRESS-ERR-CODE attributes.440 (Address Family not Supported) and 443 (Peer Address Family
Mismatch) responses.Description of the tunnel amplification attack.DTLS support.More details on packet translations.Add support for receiving ICMP packets.Updates PMTUD.Most of the text in this note comes from the original TURN
specification, . 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 .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, Justin Uberti and Oleg Moskalenko for
comments and review. The authors would like to thank Marc for his
contributions to the text.IANA Port Numbers RegistryFragmentation Considered HarmfulIANA Protocol Numbers Registry