One document matched: draft-ietf-behave-turn-08.xml
<?xml version="1.0" encoding="US-ASCII"?>
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<?rfc toc="yes"?>
<?rfc compact='yes'?>
<?rfc subcompact='no'?>
<?rfc symrefs="yes"?>
<rfc category="std" docName="draft-ietf-behave-turn-08" ipr="full3978">
<front>
<title abbrev="TURN">Traversal Using Relays around NAT (TURN): Relay
Extensions to Session Traversal Utilities for NAT (STUN)</title>
<author fullname="Jonathan Rosenberg" initials="J." surname="Rosenberg">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street></street>
<city>Edison</city>
<region>NJ</region>
<country>USA</country>
</postal>
<email>jdrosen@cisco.com</email>
<uri>http://www.jdrosen.net</uri>
</address>
</author>
<author fullname="Rohan Mahy" initials="R." surname="Mahy">
<organization abbrev="Plantronics">Plantronics, Inc.</organization>
<address>
<email>rohan@ekabal.com</email>
</address>
</author>
<author fullname="Philip Matthews" initials="P." surname="Matthews">
<organization>(Unaffiliated)</organization>
<address>
<facsimile></facsimile>
<email>philip_matthews@magma.ca</email>
<uri></uri>
</address>
</author>
<date year="2008" />
<area>Transport</area>
<workgroup>BEHAVE WG</workgroup>
<keyword>NAT</keyword>
<keyword>TURN</keyword>
<keyword>STUN</keyword>
<keyword>ICE</keyword>
<abstract>
<t>If a host is located behind a NAT, then in certain situations it can
be impossible for that host to communicate directly with other hosts
(peers) located behind other NATs. In these situations, it is necessary
for the host to use the services of an intermediate node that acts as a
communication relay. This specification defines a protocol, called TURN
(Traversal Using Relays around NAT), that allows the host to control the
operation of the relay and to exchange packets with its peers using the
relay.</t>
<t>The TURN protocol can be used in isolation, but is more properly used
as part of the ICE (Interactive Connectivity Establishment) approach to
NAT traversal.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>Session Traversal Utilities for NAT (STUN) <xref
target="I-D.ietf-behave-rfc3489bis"></xref> provides a suite of tools
for facilitating the traversal of NAT. Specifically, it defines the
Binding method, which is used by a client to determine its reflexive
transport address towards the STUN server. The reflexive transport
address can be used by the client for receiving packets from peers, but
only when the client is behind "good" NATs. In particular, if a client
is behind a NAT whose mapping behavior <xref target="RFC4787"></xref> is
address or address and port dependent (sometimes called "bad" NATs), the
reflexive transport address will not be usable for communicating with a
peer.</t>
<t>The only reliable way to obtain a UDP transport address that can be
used for corresponding with a peer through such a NAT is to make use of
a relay. The relay sits on the public side of the NAT, and allocates
transport addresses to clients reaching it from behind the private side
of the NAT. These allocated transport addresses, called relayed
transport address, are IP addresses and ports on the relay. When the
relay receives a packet on one of these allocated addresses, the relay
forwards it toward the client.</t>
<t>This specification defines an extension to STUN, called TURN, that
allows a client to request a relayed transport address on a TURN
server.</t>
<t>Though a relayed transport address is highly likely to work when
corresponding with a peer, it comes at high cost to the provider of the
relay service. As a consequence, relayed transport addresses should only
be used as a last resort. Protocols using relayed transport addresses
should make use of mechanisms to dynamically determine whether such an
address is actually needed. One such mechanism, defined for multimedia
session establishment protocols based on the offer/answer protocol in
<xref target="RFC3264">RFC 3264</xref>, is Interactive Connectivity
Establishment (ICE) <xref target="I-D.ietf-mmusic-ice"></xref>.</t>
<t>TURN was originally invented to support multimedia sessions signaled
using SIP. Since SIP supports forking, TURN supports multiple peers per
client; a feature not supported by other approaches (e.g., SOCKS <xref
target="RFC1928"></xref>). However, care has been taken in the later
stages of its development to make sure that TURN is suitable for other
types of applications.</t>
</section>
<section title="Overview of Operation">
<t>This section gives an overview of the operation of TURN. It is
non-normative.</t>
<t>In a typical configuration, a TURN client is connected to a <xref
target="RFC1918">private network</xref> and through one or more NATs to
the public Internet. On the public Internet is a TURN server. Elsewhere
in the Internet are one or more peers that the TURN client wishes to
communicate with. These peers may or may not be behind one or more
NATs.</t>
<figure anchor="fig-turn-model">
<artwork><![CDATA[
+---------+
| |
| |
/ | Peer A |
Client's TURN // | |
Host Transport Server / | |
Address Address +-+ // +---------+
10.1.1.2:17240 192.0.2.15:3478 |N|/ 192.168.100.2:16400
| | |A|
| +-+ | /|T|
| | | | / +-+
v | | | / 192.0.2.210:18200
+---------+ | | |+---------+ / +---------+
| | |N| || | // | |
| TURN | | | v| TURN |/ | |
| Client |----|A|----------| Server |------------------| Peer B |
| | | |^ | |^ ^| |
| | |T|| | || || |
+---------+ | || +---------+| |+---------+
| || | |
| || | |
+-+| | |
| | |
| | |
Client's | Peer B
Server-Reflexive Relayed Transport
Transport Address Transport Address Address
192.0.2.1:7000 192.0.2.15:9000 192.0.2.210:18200
]]></artwork>
</figure>
<t></t>
<t><xref target="fig-turn-model"></xref> 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 <xref target="RFC4787"></xref>) for a description of what this
means).</t>
<t>The client has allocated a local port on one of its addresses for use
in communicating with the server. The combination of an IP address and a
port is called a TRANSPORT ADDRESS and since this (IP address, port)
combination is located on the client and not on the NAT, it is called
the client’s HOST transport address.</t>
<t>The client sends TURN messages from its host transport address to a
transport address on the TURN server which is known as the TURN SERVER
ADDRESS. The client learns the server’s address through some
unspecified means (e.g., configuration), and this address is typically
used by many clients simultaneously. The TURN server address is used by
the client to send both commands and data to the server; the commands
are processed by the TURN server, while the data is relayed on to the
peers.</t>
<t>Since the client is behind a NAT, the server sees these packets as
coming from a transport address on the NAT itself. This address is known
as the client’s SERVER-REFLEXIVE transport address; packets sent
by the server to the client’s server-reflexive transport address
will be forwarded by the NAT to the client’s host transport
address.</t>
<t>The client uses TURN commands to allocate a RELAYED TRANSPORT
ADDRESS, which is an transport address located on the TURN server. The
server ensures that there is a one-to-one relationship between the
client’s server-reflexive transport address and the relayed
transport address; thus a packet received at the relayed transport
address can be unambiguously relayed by the server to the client.</t>
<t>The client will typically communicate this relayed transport address
to one or more peers through some mechanism not specified here (e.g., an
ICE offer or answer <xref target="I-D.ietf-mmusic-ice"></xref>). Once
this is done, the client can send data to the server to relay towards
its peers. In the reverse direction, peers can send data to the the
relayed transport address of the client. The server will relay this data
to the client as long as the client explicitly created a permission (see
<xref target="perms"></xref>) for the IP address of the peer.</t>
<section anchor="sec-transports" title="Transports">
<t>TURN as defined in this specification only allows the use of UDP
between the server and the peer. However, this specification allows
the use of any one of UDP, TCP, or TLS over TCP to carry the TURN
messages between the client and the server.</t>
<texttable>
<ttcol align="center">TURN client to TURN server</ttcol>
<ttcol align="center">TURN server to peer</ttcol>
<c>UDP</c>
<c>UDP</c>
<c>TCP</c>
<c>UDP</c>
<c>TLS over TCP</c>
<c>UDP</c>
</texttable>
<t>If TCP or TLS over TCP is used between the client and the server,
then the server will convert between stream transport and UDP
transport when relaying data. TURN allows both TCP and TLS over TCP as
transports in part because many firewalls are configured to not pass
any UDP traffic.</t>
<t>For TURN clients, using TLS over TCP to communicate with the TURN
server provides two benefits. First, the client can be assured that
the addresses of its peers are not visible to any attackers between it
and the server. Second, the client may be able to communicate with
TURN servers using TLS when it would not be able to communicate with
the same server using TCP or UDP, due to the policy of a firewall
between the TURN client and its server. In this second case, TLS
between the client and TURN server facilitates traversal.</t>
<t>There is a planned extension to TURN to add support for TCP between
the server and the peers <xref
target="I-D.ietf-behave-turn-tcp"></xref>. 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.</t>
</section>
<section title="Allocations">
<t>To allocate a relayed transport address, the client uses an
Allocate transaction. The client sends a Allocate Request to the
server, and the server replies with an Allocate Response containing
the allocated relayed transport address. The client can include
attributes in the Allocate Request that describe the type of
allocation it desires (e.g., the lifetime of the allocation). And
since relaying data may require lots of bandwidth, the server
typically requires that the client authenticate itself using
STUN’s long-term credential mechanism, to show that it is
authorized to use the server.</t>
<t>Once a relayed transport address is allocated, a client must keep
the allocation alive. To do this, the client periodically sends a
Refresh Request to the server with the allocated related transport
address. TURN deliberately uses a different method (Refresh rather
than Allocate) for refreshes to ensure that the client is informed if
the allocation vanishes for some reason.</t>
<t>The frequency of the Refresh transaction is determined by the
lifetime of the allocation. The client can request a lifetime in the
Allocate Request and may modify its request in a Refresh Request, and
the server always indicates the actual lifetime in the response. The
client must issue a new Refresh transaction within 'lifetime' seconds
of the previous Allocate or Refresh transaction. If a client no longer
wishes to use an Allocation, it should do a Refresh transaction with a
requested lifetime of 0.</t>
<t>Note that sending or receiving data from a peer DOES NOT refresh
the allocation.</t>
<t>The server keeps track of the client reflexive transport address
and port, the server transport address and port, and the protocol used
by the client to communicate with the server. (Together known as a
5-tuple. The server remembers the 5-tuple used in the Allocate
Request. Subsequent transactions between the client and the server use
this same 5-tuple. In this way, the server knows which client owns the
allocated relayed transport address. If the client wishes to allocate
a second relayed transport address, it must use a different 5-tuple
for this allocation (e.g., by using a different client host
address).,</t>
<t><list>
<t>While the terminology used in this document refers to 5-tuples,
the TURN server can store whatever identifier it likes that yields
identical results. Specifically, many implementations use a
file-descriptor in place of a 5-tuple to represent a TCP
connection.</t>
</list></t>
</section>
<section title="Exchanging Data with Peers">
<t>There are two ways for the client and peers to exchange data using
the TURN server. The first way uses Send and Data indications, the
second way uses channels. Common to both ways is the ability of the
client to communicate with multiple peers using a single allocated
relayed transport address; thus both ways include a means for the
client to indicate to the server which peer to forward the data to,
and for the server to indicate which peer sent the data.</t>
<t>When using the first way, the client sends a Send indication to the
TURN server containing, in attributes inside the indication, the
transport address of the peer and the data to be sent to that peer.
When the TURN server receives the Send Indication, it extracts the
data from the Send Indication and sends it in a UDP datagram to the
peer, using the allocated relay address as the source address. In the
reverse direction, UDP datagrams arriving at the relay address on the
TURN server are converted into Data Indications and sent to the
client, with the transport address of the peer included in an
attribute in the Data Indication.</t>
<figure anchor="fig-send-data">
<artwork><![CDATA[TURN TURN Peer Peer
client server A B
|--- Allocate Req -->| | |
|<-- Allocate Resp ---| | |
| | | |
|--- Send (Peer A)--->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<-- Data (Peer A)----| | |
| | | |
|--- Send (Peer B)--->| | |
| |=== data =================>|
| | | |
| |<== data ==================|
|<-- Data (Peer B)----| | |
]]></artwork>
<postamble></postamble>
</figure>
<t>In the figure above, the client first allocates a relayed transport
address. It then sends data to Peer A using a Send Indication; at the
server, the data is extracted and forwarded in a UDP datagram to Peer
A, using the relayed transport address as the source transport
address. When a UDP datagram from Peer A is received at the relayed
transport address, the contents are placed into a Data Indication and
forwarded to the client. A similar exchange happens with Peer B.</t>
</section>
<section title="Channels">
<t>For some applications (e.g. Voice over IP), the 36 bytes of
overhead that a Send or Data indication adds to the application data
can substantially increase the bandwidth required between the client
and the server. To remedy this, TURN offers a second way for the
client and server to associate data with a specific peer.</t>
<t>This second way uses an alternate packet format known as the
ChannelData message. The ChannelData message does not use the STUN
header used by other TURN messages, but instead has a 4-byte header
that includes a number known as a channel number. Each channel number
in use is bound to a specific peer and thus serves as a shorthand for
the peer's address.</t>
<t>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.</t>
<t>Channel bindings last for 10 minutes unless refreshed. Channel
bindings are refreshed by sending ChannelData messages from the client
to the server, or by rebinding the channel to the peer.</t>
<t></t>
<figure anchor="fig-channels">
<artwork><![CDATA[TURN TURN Peer Peer
client server A B
|--- Allocate Req -->| | |
|<-- Allocate Resp ---| | |
| | | |
|--- Send (Peer A)--->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<-- Data (Peer A)----| | |
| | | |
|- ChannelBind Req -->| | |
| (Peer A to 0x4001) | | |
| | | |
|<- ChannelBind Resp -| | |
| | | |
|-- [0x4001] data --->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<- [0x4001] data --->| | |
| | | |
|--- Send (Peer B)--->| | |
| |=== data =================>|
| | | |
| |<== data ==================|
|<-- Data (Peer B)----| | |
]]></artwork>
</figure>
<t>The figure above shows the channel mechanism in use. The client
begins by allocating a relayed transport address, and then uses that
address to exchange data with Peer A. After a bit, the client decides
to bind a channel to Peer A. To do this, it sends a ChannelBind
request to the server, specifying the transport address of Peer A and
a channel number (0x4001). After that, the client can send application
data encapsulated inside ChannelData messages to Peer A: this is shown
as "[0x4001] data" where 0x4001 is the channel number.</t>
<t>Note that ChannelData messages can only be used for peers to which
the client has bound a channel. In the example above, Peer A has been
bound to a channel, but Peer B has not, so application data to and
from Peer B uses Send and Data indications.</t>
<t>Channel bindings are always initiated by the client.</t>
</section>
<section anchor="perms" title="Permissions">
<t>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 <xref
target="RFC4787"></xref>.</t>
<t>The client can install a permission by sending data to a peer (or
by doing certain other things). Once a permission is installed, any
peer with the same IP address (the ports numbers can differ) is
permitted to send data to the client. After 5 minutes, the permission
times out and the server drops any UDP datagrams arriving at the
relayed transport from that IP address. Note that permissions are
within the context of an allocation, so adding or expiring a
permission in one allocation does not affect other allocations.</t>
<t>Data received from the peer DOES NOT refresh the permission.</t>
</section>
<section title="Preserving vs. Non-Preserving Allocations">
<t>Some applications that use TURN are quite tolerant of the different
possible ways a TURN server could set the Diff-Serv, ECN, TTL / Hop
Limit, and Flow Label fields in the IP header of the outgoing packet.
Other applications require that the TURN server set these fields in a
specific way, and also require that the TURN server relay ICMP error
packets. Applications in the second class typically wish to do Path
MTU Discovery or end-to-end QOS.</t>
<t>Unfortunately, reading and manipulating fields in the IP header and
relaying ICMP messages usually requires the server to have special
permissions (e.g., access to RAW sockets or be loaded into the
kernel), something that the person setting up the server may be
unwilling or unable to grant. This is especially true when the server
is part of a larger application, for example a peer-to-peer
application. It is also significantly more difficult to implement this
type of server than just relaying at the UDP layer.</t>
<t>To allow TURN to cater to both usage scenarios, TURN defines the
concept of Preserving vs. Non-Preserving allocations. A Preserving
allocation sets the fields in outgoing IP header correctly, and also
relays ICMP messages, while a Non-Preserving allocation may not relay
correctly in every case. The relaying rules for a Preserving are
designed to guarantee the following:<list style="symbols">
<t>Path MTU Discovery works end-to-end (i.e. client-to-peer),
using either the old algorithm (<xref target="RFC1191"></xref> and
<xref target="RFC1981"></xref>) or the new one (<xref
target="RFC4821"></xref>);</t>
<t>ECN and Diff-Serv works end-to-end;</t>
<t>Loops are prevented by copying and decrementing the TTL/Hop
Count field.</t>
</list></t>
<t>If the client knows its application or usage scenario requires a
Preserving allocation, then it can request one in its Allocate
request. If the server is unable to grant this request, then it
rejects the Allocate request.</t>
<t>Note that a Preserving allocation only makes sense when the
transport protocol to the client is UDP; when the transport is TCP or
TLS, the allocation is always Non-Preserving.</t>
</section>
</section>
<section title="Terminology">
<t>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 <xref
target="RFC2119">RFC 2119</xref>.</t>
<t>Readers are expected to be familar with <xref
target="I-D.ietf-behave-rfc3489bis"></xref> and the terms defined
there.</t>
<t>The following terms are used in this document:</t>
<t><list style="hanging">
<t hangText="TURN:">A protocol spoken between a TURN client and a
TURN server. It is an extension to the STUN protocol <xref
target="I-D.ietf-behave-rfc3489bis"></xref>. The protocol allows a
client to allocate and use a relayed transport address.</t>
<t hangText="TURN client:">A STUN client that implements this
specification.</t>
<t hangText="TURN server:">A STUN server that implements this
specification. It relays data between a TURN client and its
peer(s).</t>
<t hangText="Peer:">A host with which the TURN client wishes to
communicate. The TURN server relays traffic between the TURN client
and its peer(s). The peer does not interact with the TURN server
using the protocol defined in this document; rather, the peer
receives data sent by the TURN server and the peer sends data
towards the TURN server.</t>
<t hangText="Host Transport Address:">A transport address allocated
on a host.</t>
<t hangText="Server-Reflexive Transport Address:">A transport
address on the "public side" of a NAT. This address is allocated by
the NAT to correspond to a specific host transport address.</t>
<t hangText="Relayed Transport Address:">A transport address that
exists on a TURN server. If a permission exists, packets that arrive
at this address are relayed towards the TURN client.</t>
<t hangText="Allocation:">The relayed transport address granted to a
client through an Allocate request, along with related state, such
as permissions and expiration timers.</t>
<t hangText="5-tuple:">The combination (client IP address and port,
server IP address and port, and transport protocol (UDP or TCP))
used to communicate between the client and the server . The 5-tuple
uniquely identifies this communication stream. The 5-tuple also
uniquely identifies the Allocation on the server.</t>
<t hangText="Permission:">The IP address and transport protocol (but
not the port) of a peer that is permitted to send traffic to the
TURN server and have that traffic relayed to the TURN client. The
TURN server will only forward traffic to its client from peers that
match an existing permission.</t>
<t hangText="Preserving Allocation">An allocation that sets the the
fields in the IP header in a specific manner when relaying
application data, and which also relays ICMP messages. An allocation
that may not do this in some cases is called a Non-Preserving
allocation.</t>
</list></t>
</section>
<!-- Overview -->
<section anchor="sec-general-behavior" title="General Behavior">
<t>This section contains general TURN processing rules that apply to all
TURN messages.</t>
<t>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 <xref
target="I-D.ietf-behave-rfc3489bis"></xref> apply to STUN-formatted
messages. This means that all the message-forming and -processing
descriptions in this document are implicitly prefixed with the rules of
<xref target="I-D.ietf-behave-rfc3489bis"></xref>.</t>
<t>In addition, the server SHOULD require that all TURN requests use the
Long-Term Credential mechanism described in <xref
target="I-D.ietf-behave-rfc3489bis"></xref>, and the client MUST be
prepared to authenticate requests if required. The server's
administrator MUST choose a realm value that will uniquely identify the
username and password combination that the client must use, even if the
client uses multiple servers under different administrations. The
server's administrator MAY choose to allocate a unique username to each
client, or MAY choose to allocate the same username to more than one
client (for example, to all clients from the same department or
company).</t>
<t>The client and/or the server MAY include the FINGERPRINT attribute in
any of the methods defined in this document. However, TURN does not use
the backwards-compatibility mechanism described in <xref
target="I-D.ietf-behave-rfc3489bis"></xref>.</t>
<t>By default, TURN runs on the same port as STUN. However, either the
SRV procedures or the ALTERNATE-SERVER procedures described in <xref
target="sec-create-allocation"></xref> may be used to run TURN on a
different port.</t>
</section>
<section anchor="sec-allocations" title="Allocations">
<t>All TURN operations revolve around allocations, and all TURN messages
are associated with an allocation. An allocation conceptually consists
of the following state data:<list style="symbols">
<t>the relayed transport address</t>
<t>The 5-tuple: client IP address, client port, server IP address,
server port, transport protocol</t>
<t>the username</t>
<t>the transaction ID of the Allocate request</t>
<t>the time-to-expiry</t>
<t>A list of permissions</t>
<t>A list of channel to peer bindings</t>
<t>A flag indicating whether or not the allocation is Preserving</t>
</list>The relayed transport address is the transport address
allocated by the server for communicating with peers, while the 5-tuple
describes the communication path between the client and the server. Both
of these MUST be unique across all allocations, so either one can be
used to uniquely identify the allocation.</t>
<t>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.</t>
<t>The username and password of the allocation is the username and
password of the authenticated Allocate request that creates the
allocation. Subsequent requests on an allocation use the same username
and password as those used to create the allocation, to prevent
attackers from hijacking the client's allocation. Specifically, if the
server requires the use of the Long-Term Credential mechanism, and if a
non-Allocate request passes authentication under this mechanism, and if
the 5-tuple identifies an existing allocation, but the request does not
use the same username as used to create the allocation, then the request
MUST be rejected with a 438 (Wrong Credentials) error.</t>
<t>The transaction ID of the allocation is the transaction ID used in
the Allocate request. This is used to detect retransmissions of the
Allocate request over UDP (see <xref target="sec-rcv-allocate"></xref>
for details).</t>
<t>The time-to-expiry is the time in seconds left until the allocation
expires. Each Allocate or Refresh transaction sets this timer, which
then ticks down towards 0. By default, each Allocate or Refresh
transaction resets this timer to 600 seconds (10 minutes), but the
client can request a different value in the Allocate and Refresh
request. Allocations can only be refreshed using the Refresh request;
sending data to a peer does not refresh an allocation. When an
allocation expires, the state data associated with the allocation is
freed. However the server MUST ensure that neither the relayed transport
address nor the client reflexive transport address from the 5-tuple are
re-used in other allocations until 2 minutes after the allocation
expires; this ensures that any messages that are in transit when the
allocation expires are gone before either of these transport addresses
are re-used.</t>
<t>The list of permissions is described in <xref
target="sec-permissions"></xref> and the list of channels is described
in <xref target="sec-channels"></xref>.</t>
<t>The differences between a Preserving and a Non-Preserving allocation
are described in <xref target="sec-ip-and-icmp"></xref>.</t>
</section>
<section anchor="sec-create-allocation" title="Creating an Allocation">
<t>An allocation on the server is created using an Allocate
transaction.</t>
<section title="Sending an Allocate Request">
<t>The client forms an Allocate request as follows.</t>
<t>The client first needs to pick a host transport address that the
server does not think is currently in use, or was recently in use. The
client SHOULD pick a currently-unused transport address on the
client's host (typically by allowing its OS to pick a currently-unused
port for a new socket).</t>
<t>The client needs to pick a transport protocol to use between the
client and the server. The transport protocol MUST be one of UDP, TCP,
or TLS over TCP. Since this specification only allows UDP between the
server and the peers, it is RECOMMENDED that the client pick UDP
unless it has a reason to use a different transport. One reason to
pick a different transport would be that the client believes, either
through configuration or by experiment, that it is unable to contact
any TURN server using UDP. See <xref target="sec-transports"></xref>
for more discussion.</t>
<t>The client must also pick a server transport address. Typically,
this is done by the client learning (perhaps through configuration)
one or more domain names for TURN servers. In this case, the client
uses the DNS procedures described in <xref
target="I-D.ietf-behave-rfc3489bis"></xref>, but using an SRV service
name of "turn" (or "turns" for TURN over TLS) instead of "stun" (or
"stuns"). For example, to find servers in the example.com domain, the
client performs a lookup for '_turn._udp.example.com',
'_turn._tcp.example.com', and '_turns._tcp.example.com' if the client
wants to communicate with the server using UDP, TCP, or TLS over TCP,
respectively.</t>
<t>The client MUST include a REQUESTED-TRANSPORT attribute in the
request. This attribute specifies the transport protocol between the
server and the peers (note that this is NOT the transport protocol
that appears in the 5-tuple). In this specification, the
REQUESTED-TRANSPORT type is always UDP. This attribute is included to
allow future extensions specify other protocols (e.g., <xref
target="I-D.ietf-behave-turn-tcp"></xref>).</t>
<t>If the client wishes the server to initialize the time-to-expire
field of the allocation to some value other the default lifetime, then
it MAY include a LIFETIME attribute specifying its desired value. This
is just a request, and the server may elect to use a different value.
Note that the server will ignore requests to initialize the field to
less tha the default value.</t>
<t>If the client required the allocation to satisfy certain
properties, then the client includes the REQUESTED-PROPS attribute.
This attribute is optional, and can be omitted if no special
properties are required.</t>
<t>Using the E and R bits in the REQUESTED-PROPS attribute, the client
can request:<list style="symbols">
<t>(E=1, R=0 ) That the server allocate a relayed transport
address with an even port number; OR</t>
<t>(E=1, R=1) That the server reserve a pair of relayed transport
addresses with adjacent port numbers N and N+1, where N is even
and N+1 is odd, and then use port N for the current allocation. In
this case, the server returns a RESERVATION-TOKEN attribute in the
response which the client can then include in a subsequent
Allocate request to create an allocation with port number N+1.</t>
</list>Note that the client cannot request a pair of adjacent ports
unless it also requests that the lower numbered port be even. Thus the
combination (E=0, R=1) is not allowed.</t>
<t>Similarly, by setting the P bit to 1 in the REQUESTED-PROPS
attribute, the client can request that the server allocate a
Preserving allocation.</t>
<t>For all the various REQUESTED-PROPS flags, if the server cannot
satisfy the request, the Allocate request is rejected.</t>
<t>The client MAY also include a RESERVATION-TOKEN attribute in the
request to ask the server to use a previously reserved port for the
allocation. If the RESERVATION-TOKEN attribute is included, then the
client MUST either omit the REQUESTED-PROPS attribute or set E=0 and
R=0, since doing otherwise would make no sense.</t>
<t>Once constructed, the client sends the Allocate request on the
5-tuple.</t>
</section>
<section anchor="sec-rcv-allocate" title="Receiving an Allocate Request">
<t>When the server receives an Allocate request, it performs the
following checks:<list style="numbers">
<t>The server checks the credentials of the request, as per the
Long-Term Credential mechanism of <xref
target="I-D.ietf-behave-rfc3489bis"></xref>.</t>
<t>The server checks if the 5-tuple is currently in use by an
existing allocation, or was it in use by another allocation within
the last 2 minutes. If yes, then there are two sub-cases:<list
style="symbols">
<t>If the transport protocol in the 5-tuple is UDP, and if the
5-tuple is currently in use by an existing allocation, and if
the transaction id of the request matches the transaction id
stored with the allocation, then the request is a
retransmission of the original request. The server replies
either with a stored copy of the original response, or with a
response rebuilt from the stored state data. If the server
chooses to rebuild the response, then (a) it need not parse
the request further, but can immediately start building a
success response, (b) the value of the LIFETIME attribute can
be set to the current value of the time-to-expire timer, and
(c) the server may need to include an extra field in the
allocation to store the token returned in a RESERVATION-TOKEN
attribute.</t>
<t>Otherwise, the server rejects the request with a 437
(Allocation Mismatch) error.</t>
</list>NOTE: If the request includes credentials that are
acceptable to server, but the 5-tuple is already in use, then it
is important that the server reject the request with a 437
(Allocation Mismatch) error rather than a 401 (Unauthorized)
error. This ensures that the client knows that the problem is with
the 5-tuple, rather than (wrongly) believing that the problem lies
with its credentials.</t>
<t>The server checks if the request contain a REQUESTED-TRANPORT
attribute. If the REQUESTED-TRANSPORT attribute is not included or
is malformed, the server rejects the request with a 400 (Bad
Request) error. Otherwise, if the attribute is included but
specifies a protocol other that UDP, the server rejects the
request with a 422 (Unsupported Transport Protocol) error.</t>
<t>The server checks if the request contains a REQUESTED-PROPS
attribute. If yes, then the server checks that it understands and
can satisfy all the flags that are set to 1. If a flag is not
understood, or if the server cannot satisfy the request, then the
server rejects the request with a 508 (Insufficient Port Capacity)
error. Note that the combination (E=0, R=1) MUST be treated as
unsupported.</t>
<t>The server checks if the request contains a RESERVATION-TOKEN
attribute. If yes, and the request also contains a REQUESTED-PROPS
attribute, then the server rejectes the request with a 400 (Bad
Request) error. Otherwise it checks to see if the token is valid
(i.e., the token is in range and has not expired, and the
corresponding relayed transport address is still available). If
the token is not valid for some reason, the server rejects the
request with a 508 (Insufficient Port Capacity) error.</t>
<t>At any point, the server MAY also 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.</t>
</list></t>
<t>If the server rejects the request with one of the error codes 422
(Unsupported Transport Protocol), 486 (Allocation Quota Reached) or
508 (Insufficient Port Capacity), it MAY include an ALTERNATE-SERVER
attribute in the error response redirecting the client to another
server that it believes will accept the request. If the attribute is
included, the address MUST be from the same address family as the
server's transport address. Note that, if the attribute is included,
the client will try this alternate server before trying the other
servers given by the SRV procedures.</t>
<t>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.</t>
<t>When allocating a relayed transport address for the allocation, the
server MUST allocate an IP address from the same family (e.g, IPv4 vs.
IPv6) as that on which the request was received (i.e., the server's IP
address in the 5-tuple for the allocation). <list>
<t>NOTE: An extension to TURN to allow an address from a different
address family is currently in progress <xref
target="I-D.ietf-behave-turn-ipv6"></xref>.</t>
</list>In addition, the server SHOULD only allocate ports from the
range 49152 – 65535 (the Dynamic and/or Private Port range <xref
target="Port-Numbers"></xref>), 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.</t>
<t>If the request contains a REQUESTED-PROPS attribute with the R flag
set, then the server looks for a pair of port numbers N and N+1 on the
same IP address, where N is even. Port N is used in the current
allocation, while the relayed transport address with port N+1 is
assigned a token and reserved for a future allocation. The server MUST
hold this reservation for at least 30 seconds, and MAY choose to hold
longer (e.g. until the allocation with port N expires). The server
then includes the token in a RESERVATION-TOKEN attribute in the
success response.</t>
<t>If the request contains a RESERVATION-TOKEN, the server uses the
previously-reserved transport address corresponding to the included
token (if it is still available).</t>
<t>The server determines the initial value of the time-to-expire field
as follows. If the request contains a LIFETIME attribute, and the
proposed lifetime value is greater than the default lifetime, and the
proposed lifetime value is otherwise acceptable to the server, then
the server uses that value. Otherwise, the server uses the default
value. It is RECOMMENDED that the server impose a maximum lifetime of
no more than 3600 seconds (1 hour).</t>
<t><list>
<t>NOTE: The time-to-expire is recomputed with each successful
Refresh request. Thus the value computed here applies only until
the first refresh.</t>
</list></t>
<t>Once the allocation is created, the server replies with a success
response. The success response contains:<list style="symbols">
<t>A RELAYED-ADDRESS attribute containing the relayed transport
address;</t>
<t>A LIFETIME attribute containing the current value of the
time-to-expire timer;</t>
<t>A RESERVATION-TOKEN attribute (if a second relayed transport
address was reserved).</t>
<t>An XOR-MAPPED-ADDRESS attribute containing the client's IP
address and port (from the 5-tuple);</t>
</list></t>
<t><list>
<t>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.</t>
</list></t>
<t>The response (either success or error) is sent back to the client
on the 5-tuple.</t>
</section>
<section title="Receiving an Allocate Response">
<t>If the client receives a success response, then it MUST check that
the relayed transport address is in an address family that the client
understands and is prepared to deal with. This specification only
covers the case where the relayed transport address is of the same
address family as the client's transport address. If the relayed
transport address is not in an address family that the client is
prepared to deal with, then the client MUST delete the allocation
(<xref target="sec-refreshing-allocation"></xref>) and MUST NOT
attempt to create another allocation on that server until it believes
the mismatch has been fixed.</t>
<t><list>
<t>The IETF is currently considering mechanisms for transitioning
between IPv4 and IPv6 that could result in a client originating an
Allocate request over IPv4, but the request would arrive at the
server over IPv6, or vica-versa. Hence the importance of this
check.</t>
</list></t>
<t>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. </t>
<t>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.</t>
<t>If the client receives an error response, then the processing
depends on the actual error code returned:<list style="symbols">
<t>(Request timed out): There is either a problem with the server,
or a problem reaching the server with the chosen transport. The
client MAY choose to try again using a different transport (e.g.,
TCP instead of UDP), or the client MAY try a different server.</t>
<t>400 (Bad Request): The server believes the client's request is
malformed for some reason. 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. The client MAY try a
different server.</t>
<t>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. The
client SHOULD notify the user or operator and SHOULD NOT send any
further requests to this server until it believes the problem has
been fixed. The client MAY try a different server.</t>
<t>437 (Allocation Mismatch): This indicates that the client has
picked a 5-tuple which the server sees as already in use or which
was recently in use. One way this could happen is if an
intervening NAT assigned a mapped transport address that was
recently used by another allocation. 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.</t>
<t>438 (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. The client
MAY try a different server.</t>
<t>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 retry
the same request with this server until it believes the problem
has been fixed. The client MAY try a different server.</t>
<t>486 (Allocation Quota Reached): The server is currently unable
to create any more allocations with this username. The client
SHOULD wait at least 1 minute before trying to create any more
allocations on the server. The client MAY try a different
server.</t>
<t>508 (Insufficient Port Capacity): The server has no more
relayed transport addresses avaiable, or has none with the
requested properties, or the one that was reserved is no longer
available. If the client is using either the REQUESTED-PROPS or
the RESERVATION-TOKEN attribute, then the client MAY choose to
remove 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. The client MAY try a different
server.</t>
</list>If the error response contains an ALTERNATE-SERVER attribute,
and the client elects to try a different server, the the client SHOULD
try the alternate server specified in that attribute (while obeying
the rules in <xref target="I-D.ietf-behave-rfc3489bis"></xref> for
avoiding redirection loops) before trying any other servers found
using the SRV procedures of <xref
target="I-D.ietf-behave-rfc3489bis"></xref>.</t>
</section>
</section>
<section anchor="sec-refreshing-allocation"
title="Refreshing an Allocation">
<t>A Refresh transaction can be used to either (a) refresh an existing
allocation and update its time-to-expire, or (b) delete an existing
allocation.</t>
<t>If a client wishes to continue using an allocation, then the client
MUST refresh it before it expires. It is suggested that the client
refresh the allocation roughly 1 minute before it expires. If a client
no longer wishes to use an allocation, then it SHOULD explicitly delete
the allocation. A client MAY also change the time-to-expire of an
allocation at any time for other reasons.</t>
<section title="Sending a Refresh Request">
<t>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.</t>
<t>The Refresh transaction updates the time-to-expire timer of an
allocation. If the client wishes the server to set the time-to-expire
timer to something other than the default lifetime, it includes a
LIFETIME attribute with the requested value. The server then computes
a new time-to-expire value in the same way as it does for an Allocate
transaction, with the exception that a requested lifetime of 0 causes
the server to immediately delete the allocation.</t>
<t>The Refresh transaction is sent on the 5-tuple for the
allocation.</t>
</section>
<section title="Receiving a Refresh Request">
<t>When the server receives a Refresh request, it processes it as
follows. If, during processing, an error in the request is detected
(for example, a syntax error in the request which causes a 400 error),
then the request is rejected with an error response but the allocation
is NOT deleted (but note that a 437 error will indicate that the
allocation was not found).</t>
<t>The server determines the new value for the time-to-expire field as
follows. If the request contains a LIFETIME attribute, and the
attribute value is 0, then the server uses a value of 0, which causes
the allocation to expire. Otherwise, if the request contains a
LIFETIME attribute and the attribute value is greater than the default
lifetime, and the attribute value is otherwise acceptable to the
server, then the server uses the attribute value. Otherwise, the
server uses the default value. It is RECOMMENDED that the server
impose a maximum lifetime of no more than 3600 seconds (1 hour).</t>
<t>The server then constructs a success response containing:<list
style="symbols">
<t>A LIFETIME attribute containing the current value of the
time-to-expire timer.</t>
</list>The response is then sent on the 5-tuple.</t>
</section>
<section title="Receiving a Refresh Response">
<t>If the client receives a success response to its Refresh request,
it updates its copy of the allocation data structure with the
time-to-expire value contained in the response.</t>
<t>If the client receives an 437 (Allocation Mismatch) error response
to its Refresh request, then it must consider the allocation as having
expired, as described in <xref target="sec-general-behavior"></xref>.
All other errors indicate a software error on the part of either the
client or the server.</t>
</section>
</section>
<section anchor="sec-permissions" title="Permissions">
<t>For each allocation, the server keeps a list of zero or more
permissions. Each permission consists an IP address which uniquely
identifies the permission, and an associated time-to-expiry. The IP
address describes a peer that is allowed to send data to the client, and
the time-to-expiry is the number of seconds until the permission
expires.</t>
<t>Various events, as described in subsequent sections, can cause a
permission for a given IP address to be installed or refreshed. This
causes one of two things to happen:<list style="symbols">
<t>If no permission for that IP address exists, then a permission is
created with the given IP address and a time-to-expiry equal to the
default permission lifetime.</t>
<t>If a permission for that IP address already exists, then the
lifetime for that permission is reset to the default permission
lifetime.</t>
</list>The default permission lifetime MUST be 300 seconds (= 5
minutes).</t>
<t>Each permission’s time-to-expire decreases down once per second
until it reaches 0, at which point the permission expires and is
deleted.</t>
<t>When a UDP datagram arrives at the relayed transport address for the
allocation, the server checks the list of permissions for that
allocation. If there is a permission with an IP address that is equal to
the source IP address of the UDP datagram, then the UDP datagram can be
relayed to the client. Otherwise, the UDP datagram is silently
discarded. Note that only IP addresses are compared; port numbers are
irrelevant.</t>
<t>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.</t>
<t><list>
<t>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 <xref target="I-D.ietf-mmusic-ice"></xref>, and applications not
using ICE are advised to do something similar.</t>
</list></t>
</section>
<section anchor="sec-sendanddata" title="Send and Data Indications">
<t>TURN supports two ways to send and receive data from peers. This
section describes the use of Send and Data indications, while <xref
target="sec-channels"></xref> describes the use of the Channel
Mechanism.</t>
<section anchor="sec-forming-indication"
title="Sending a Send Indication">
<t>A client can use a Send Indication to pass data to the server for
relaying to a peer. A client can also use a Send Indication without a
DATA attribute to install or refresh a permission for the specified IP
address. A client may use a Send indication to send data to a peer
even if a channel is bound to that peer.</t>
<t>When forming a Send Indication, the client MUST include a
PEER-ADDRESS attribute and MAY include a DATA attribute. If the DATA
attribute is included, then the DATA attribute contains the actual
application data to be sent to the peer, and the PEER-ADDRESS
attribute contains the transport address of the peer to which the data
is to be sent. If the DATA attribute is not present, then the
PEER-ADDRESS attribute contains the IP address for which a permission
is to be installed or refreshed; in this case the port specified in
the attribute is ignored.</t>
<t>Note that no authentication attributes are included, since
indications cannot be authenticated using the Long-Term Credential
mechanism.</t>
<t>The Send Indication MUST be sent using the same 5-tuple used for
the original allocation.</t>
</section>
<section title="Receiving a Send Indication">
<t>When the server receives a Send indication, it processes it as
follows.</t>
<t>If the received Send indication contains a DATA attribute, then it
forms a UDP datagram as follows:<list style="symbols">
<t>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;</t>
<t>the destination transport address is taken from the
PEER-ADDRESS attribute;</t>
<t>the data following the UDP header is the contents of the value
field of the DATA attribute.</t>
</list>The resulting UDP datagram is then sent to the peer. If any
errors are detected during this process (e.g., the Send indication
does not contain a PEER-ADDRESS attribute), the received indication is
silently discarded and no UDP datagram is sent.</t>
<t>When the server receives a valid Send Indication, either with or
without a DATA attribute, it also installs or refreshes a permission
for the IP address contained in the PEER-ADDRESS attribute (see <xref
target="sec-permissions"></xref>).</t>
</section>
<section anchor="sec-sending-data-indication"
title="Receiving a UDP Datagram">
<t>When the server receives a UDP datagram at a currently allocated
relayed transport address, the server looks up the allocation
associated with the relayed transport address. It then checks to see
if relaying is permitted, as described in section <xref
target="sec-permissions"></xref>).</t>
<t>If relaying is permitted, and there is no channel bound to the peer
that sent the UDP datagram (see I<xref target="sec-channels"></xref>),
then the server forms and sends a Data indication. The Data indication
MUST contain both a PEER-ADDRESS and a DATA attribute. The DATA
attribute is set to the value of the ‘data octets’ field
from the datagram, and the PEER-ADDRESS attribute is set to the source
transport address of the received UDP datagram. The Data indication is
then sent on the 5-tuple associated with the allocation.</t>
</section>
<section title="Receiving a Data Indication">
<t>When the client receives a Data indication, it checks that the Data
indication contains both a PEER-ADDRESS and a DATA attribute. It then
delivers the data octets inside the DATA attribute to the application,
along with an indication that they were received from the peer whose
transport address is given by the PEER-ADDRESS attribute.</t>
</section>
</section>
<!-- Sending and Receiving Data -->
<section anchor="sec-channels" title="Channels">
<t>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.</t>
<t>Channel bindings are always initiated by the client. The client can
bind a channel to a peer at any time during the lifetime of the
allocation. The client may bind a channel to a peer before exchanging
data with it, or after exchanging data with it (using Send and Data
indications) for some time, or may choose never to bind a channel it.
The client can also bind channels to some peers while not binding
channels to other peers.</t>
<t>Channel bindings are specific to an allocation, so that a binding in
one allocation has no relationship to a binding in any other allocation.
If an allocation expires, all its channel bindings expire with it.</t>
<t>A channel binding consists of:<list style="symbols">
<t>A channel number;</t>
<t>A transport address (of the peer);</t>
<t>A time-to-expiry timer.</t>
</list>Within the context of an allocation, a channel binding is
uniquely identified either by the channel number or by the transport
address. Thus the same channel cannot be bound to two different
transport addresses, nor can the same transport address be bound to two
different channels.</t>
<t>A channel binding last for 10 minutes unless refreshed. Refreshing
the binding (by the server receiving either a ChannelBind request
rebinding the channel to the same peer, or by the server receiving a
ChannelData message on that channel) resets the time-to-expire timer
back to 10 minutes. When the channel binding expires, the channel
becomes unbound and available for binding to a different transport
address.</t>
<t>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.</t>
<t>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.</t>
<section title="Sending a ChannelBind Request">
<t>A channel binding is created using a ChannelBind transaction. A
channel binding can also be refreshed using a ChannelBind
transaction.</t>
<t>To initiate the ChannelBind transaction, the client forms a
ChannelBind request. The channel to be bound is specified in a
CHANNEL-NUMBER attribute, and the peer's transport address is
specified in a PEER-ADDRESS attribute. <xref
target="sec-receiving-ChannelBind"></xref> describes the restrictions
on these attributes.</t>
<t>Note that rebinding a channel to the same transport address that it
is already bound to provides a way to refresh a channel binding
without sending data to the peer.</t>
<t>Once formed, the ChannelBind Request is sent using the 5-tuple for
the allocation.</t>
</section>
<section anchor="sec-receiving-ChannelBind"
title="Receiving a ChannelBind Request">
<t>When the server receives a ChannelBind request, it checks the
following:<list style="symbols">
<t>The request contains both a CHANNEL-NUMBER and a PEER-ADDRESS
attribute;</t>
<t>The channel number is in the range 0x4000 to 0xFFFE
(inclusive);</t>
<t>The channel number is not currently bound to a different
transport address (same transport address is OK);</t>
<t>The transport address is not currently bound to a different
channel number.</t>
</list>If any of these tests fail, the server replies with an error
response with error code 400 "Bad Request". Otherwise, the ChannelBind
request is valid and the server replies with a ChannelBind success
response.</t>
<t>If ChannelBind request is valid, then the server creates or
refreshes the channel binding using the channel number in the
CHANNEL-ADDRESS attribute and the transport address in the
PEER-ADDRESS attribute. The server also installs or refreshes a
permission for the IP address in the PEER-ADDRESS attribute.</t>
</section>
<section title="Receiving a ChannelBind Response">
<t>When the client receives a successful ChannelBind response, it
updates its data structures to record that the channel binding is now
active.</t>
</section>
<section anchor="sec-channeldata-msg" title="The ChannelData Message">
<t>The ChannelData message is used to carry application data between
the client and the server. It has the following format:</t>
<figure>
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel Number | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Application Data /
/ /
| |
| +-------------------------------+
| |
+-------------------------------+]]></artwork>
</figure>
<t>The Channel Number field specifies the number of the channel on
which the data is traveling, and thus the address of the peer that is
sending or is to receive the data. The channel number MUST be in the
range 0x4000 – 0xFFFF, with channel number 0xFFFF being reserved
for possible future extensions.</t>
<t>Channel numbers 0x0000 – 0x3FFF cannot be used because bits 0
and 1 are used to distinguish ChannelData messages from STUN-formatted
messages (i.e., Allocate, Send, Data, ChannelBind, etc).
STUN-formatted messages always have bits 0 and 1 as “00”,
while ChannelData messages use combinations “01”,
“10”, and “11”.</t>
<t>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.</t>
<t>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.</t>
</section>
<section anchor="sec-sending-channeldata-msg"
title="Sending a ChannelData Message">
<t>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.</t>
<t>The fields of the ChannelData message are filled in as described in
<xref target="sec-channeldata-msg"></xref>.</t>
<t>Over stream transports, the ChannelData message MUST be padded to a
multiple of four bytes in order to ensure the alignment of subsequent
messages. The padding is not reflected in the length field of the
ChannelData message, so the actual size of a ChannelData message
(including padding) is (4 + Length) rounded up to the nearest multiple
of 4. Over UDP, the padding is not required but MAY be included.</t>
<t>The ChannelData message is then sent on the 5-tuple associated with
the allocation.</t>
</section>
<section title="Receiving a ChannelData Message">
<t>The receiver of the ChannelData message uses bits 0 and 1 to
distinguish it from STUN-formatted messages, as described in <xref
target="sec-channeldata-msg"></xref>.</t>
<t>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.</t>
<t>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 <xref target="sec-sending-channeldata-msg"></xref>.</t>
<t>If the ChannelData message is received on a channel which is not
bound to any peer, then the message is silently discarded.</t>
</section>
<section title="Relaying">
<t>When a server receives a ChannelData message, it first processes it
as described in the previous section. If no errors are detected, it
relays the application data to the peer by forming a UDP datagram as
follows:<list style="symbols">
<t>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;</t>
<t>the destination transport address is the transport address to
which the channel is bound;</t>
<t>the data following the UDP header is the contents of the data
field of the ChannelData message.</t>
</list>The resulting UDP datagram is then sent to the peer.</t>
<t>If the ChannelData message is valid, then the server refreshes the
channel binding, and also installs or refreshes a permission for the
IP address part of the transport address to which the UDP datagram is
sent (see <xref target="sec-permissions"></xref>).</t>
<t>In the other direction, when the server receives a UDP datagram on
the relayed transport address associated with an allocation, then it
first checks to see if it is permitted to relay the datagram. This
check is done as described in <xref target="sec-permissions"></xref>.
If relaying is permitted, then the server checks to see if there is a
channel bound to the peer that sent the UDP datagram. If there is,
then it SHOULD form and send a ChannelData message as described in
<xref target="sec-sending-channeldata-msg"></xref>. If no channel is
bound to the peer, then it MUST form and send a Data indication as
described in <xref target="sec-sending-data-indication"></xref>.</t>
</section>
</section>
<section anchor="sec-ip-and-icmp" title="IP and ICMP">
<t>This section describes how the server sets various fields in the IP
header when relaying between the client and the peer or vica-versa. It
also describes how the server relays ICMP messages. The descriptions in
this section apply: (a) when the server receives a Send indication or
ChannelData message from the client and sends a UDP datagram to the
peer, (b) when the server receives a UDP datagram on the
relayed-transport address and sends a Data indication or ChannelData
message to the client, or (c) when the server receives an ICMP message.
This section does not apply when the server sends TURN control
messages.</t>
<t>The descriptions below have two parts: a preferred behavior and an
alternate behavior. A Preserving allocation MUST implement the preferred
behavior. A non-preserving allocation with UDP transport to the client
SHOULD implement the preferred behavior, but if that is not possible for
a particular field, then it SHOULD implement the alternative behavior. A
non-preserving allocation with TCP or TLS transport to client SHOULD
implement the alternate behavior, except where this conflicts with
standard TCP or TLS behavior.</t>
<section title="IP">
<t>This section describes the preferred and alternate behavior for
various fields in the IP header.</t>
<t>Time to Live (IPv4) or Hop Count (IPv6)<list style="empty">
<t>Preferred Behavior: If the incoming value is 0, then send an
ICMP Time Exceeded message back to the sender. Otherwise set the
outgoing Time to Live/Hop Count to one less than the incoming
value.</t>
<t>Alternate Behavior: Set the outgoing value to the default for
outgoing packets.</t>
</list></t>
<t><vspace blankLines="1" /></t>
<t>Diff-Serv Code Point <list style="empty">
<t>Preferred Behavior: Set the outgoing value to the incoming
value, unless the server, though configuration or other means,
believes that a different setting is more appropriate.</t>
<t>Alternate Behavior: Set the outgoing value to Best Effort,
unless the server, through configuration or other means, believes
a different setting is more appropriate.</t>
</list></t>
<t><vspace blankLines="1" /></t>
<t>ECN<list style="empty">
<t>Preferred Behavior: Set the outgoing value to the incoming
value, UNLESS the server is doing Active Queue Management, the
incoming ECN field is 01 or 10, and the server wishes to indicate
that congestion has been experienced, in which case set the
outgoing value to 11.</t>
<t>Alternate Behavior: Set the outgoing value to 00 (ECN not
supported)</t>
</list></t>
<t><vspace blankLines="1" /></t>
<t>Flow Label <list style="empty">
<t>Preferred Behavior: Set the outgoing flow label to 0.</t>
<t>Alternate Behavior: Same as the Preferred behavior.</t>
</list></t>
<t><vspace blankLines="1" /></t>
<t>IPv4 Fragmentation<list>
<t>Preferred Behavior: <list>
<t>If the outgoing packet size does not exceed the outgoing
link's MTU, then send the outgoing packet unfragmented. Set
the DF bit in the outgoing packet to the value of the DF bit
in the incoming packet, and set the other fragmentation fields
(Identification, MF, Fragment Offset) as appropriate for a
packet originating from the server.</t>
<t>Otherwise, if the outgoing link's MTU is exceeded and the
incoming DF bit is 0, then fragment the packet before sending.
Set the outgoing DF to 0, and set the other fragmentation
fields as appropriate for fragments originated from the
server.</t>
<t>Otherwise [link MTU exceeded and incoming DF set], drop the
outgoing packet and send an ICMP message of type 3 code 4
("fragmentation needed and DF set") to the sender of the
incoming packet.</t>
</list></t>
<t>Alternate Behavior: As described in the Preferred Behavior,
except always assume the incoming DF bit is 0.</t>
</list></t>
<t><vspace blankLines="1" /></t>
<t>IPv6 Fragmentation<list>
<t>Preferred Behavior: <list>
<t>If the incoming packet did not include a Fragmentation
header and the outgoing packet size does not exceed the
outgoing link's MTU, then send the outgoing packet without a
Fragmentation header.</t>
<t>If the incoming packet included a Fragment header and if
the outgoing packet size (with a Fragmentation header
included) does not exceed the outgoing link's MTU, then send
the outgoing packet with a Fragmentation header. Set the
fields of the Fragmentation header as appropriate for a packet
originating from the server.</t>
<t>If the incoming packet did not include a Fragmentation
header and the outgoing packet size exceeds the outgoing
link's MTU , then drop the outgoing packet and send an ICMP
message of type 2 code 0 ("Packet too big") to the sender of
the incoming packet. If the packet is being sent to the peer,
then reduce the MTU reported in the ICMP message by 48 bytes
to allow room for the overhead of a Data indication.</t>
<t>Otherwise, if the link's MTU is exceeded and the incoming
packet contained a Fragmentation header, then fragment the
outgoing packet into fragments of no more than 1280 bytes. Set
the fields of the Fragmentation header as appropriate for a
packet originating from the server.</t>
</list></t>
<t>Alternate Behavior: As described in the Preferred Behavior,
except always assume incoming packet has a Fragmentation
header.</t>
</list></t>
<t><vspace blankLines="1" /></t>
<t>IPv4 Options<list>
<t>Preferred Behavior: The outgoing packet is sent without any
IPv4 options.</t>
<t>Alternate Behavior: Same as preferred.</t>
</list></t>
<t><vspace blankLines="1" /></t>
<t>IPv6 Extention Headers<list>
<t>Preferred Behavior: The outgoing packet is sent without any
IPv6 extension headers, with the exception of the Fragmentation
header as described above</t>
<t>Alternate Behavior: Same as preferred.</t>
</list></t>
</section>
<section title="ICMP">
<t>This sub-section describes the preferred behavior of ICMP relaying.
The corresponding alternate behavior is to not relay ICMP
messages.</t>
<t>When an ICMP message arrives at the server, the copy of the
original IP packet present inside the ICMP message is examined. The
server first checks that the original IP packet header is immediately
followed by a UDP protocol header, such that the original source
transport address was X and the original destination transport address
was Y. The server also checks that the type and code values in the
ICMP header are one of those relayed (see below). Other ICMP messages
are either ignored, or used by the server internally in an unspecified
manner.</t>
<t>The server then checks if one of the following two cases
applies:</t>
<t>Case 1: X is a relayed-transport-address currently assigned to an
active allocation on the server, and there exists a permission for the
IP address of Y in the allocation.</t>
<t>In this case, the original IP packet was traveling from the server
to a peer, so the the server relays the ICMP message back to the
client. The server creates a Data indication where the PEER-ADDRESS
attribute contains Y, and the ICMP attribute contains the type and
code from the incoming ICMP message, and the DATA attribute contains
application data from the original IP packet starting AFTER the UDP
header. The server SHOULD include as much application data as possible
consistent with not exceeding a total IP packet size of either 576
bytes (for IPv4) or 1280 bytes (for IPv6). <list>
<t>Note that there is no point in including the original IP or UDP
header in the DATA attribute because those headers were generated
by the server, not the client.</t>
</list></t>
<t>Case 2: There is an active allocation where X is the server
transport address, Y is the client transport address, and UDP is used
as transport between the client and the server. Furthermore, the
packet after the UDP header is either (a) a ChannelData header which
contains an active channel number in the allocation, or (b) a Data
indication whose PEER-ADDRESS attribute contains an IP address for
which there exists a permission in the allocation.</t>
<t>In this case, the original IP packet was traveling from the server
to the client, so the server creates and sends an ICMP message to the
peer. The outgoing ICMP message contains the type and code fields from
the incoming ICMP message and then contains an approximation to the
original IP packet sent from the peer to the server (the one the
server was trying to relay to the client inside the ChannelData or
Data indication). This approximation contains a synthesized IP header,
a synthesized UDP header, and some application data. The synthesis is
done as follows:<list style="symbols">
<t>The destination transport address is the
relayed-transport-address of the allocation;</t>
<t>The source transport address is the peer's transport address
determined from either (a) the channel number or (b) the
PEER-ADDRESS attribute;</t>
<t>The application data is taken from either (a) the ChannelData
message or (b) the DATA attribute. The server SHOULD include as
much application data as possible consistent with not exceeding
either 576 bytes (for IPv4) or 1280 bytes (for IPv6).</t>
</list>The remaining fields in the IP and UDP headers are simply set
to sensible values, since for most of them there is no way to
reconstruct the original values.</t>
<t>The server SHOULD relay as all ICMP type/code combinations and MUST
relay at at least the following combinations. For IPv4:<list>
<t>Type 3, code 4: Fragmentation needed and DF set</t>
</list>For IPv6:<list>
<t>Type 2, code <any>: Packet too big</t>
</list></t>
</section>
</section>
<section anchor="sec-stun-methods" title="New STUN Methods">
<t>This section lists the codepoints for the new STUN methods defined in
this specification. See elsewhere in this document for the semantics of
these new methods.</t>
<figure>
<preamble></preamble>
<artwork><![CDATA[ Request/Response Transactions
0x003 : Allocate
0x004 : Refresh
0x009 : ChannelBind
Indications
0x006 : Send
0x007 : Data
]]></artwork>
</figure>
</section>
<section anchor="sec-stun-attributes" title="New STUN Attributes">
<figure>
<preamble>This STUN extension defines the following new
attributes:</preamble>
<artwork><![CDATA[
0x000C: CHANNEL-NUMBER
0x000D: LIFETIME
0x0010: Reserved (was BANDWIDTH)
0x0012: PEER-ADDRESS
0x0013: DATA
0x0016: RELAY-ADDRESS
0x0018: REQUESTED-PROPS
0x0019: REQUESTED-TRANSPORT
0x0022: RESERVATION-TOKEN
]]></artwork>
</figure>
<section anchor="channelnums" title="CHANNEL-NUMBER">
<t>The CHANNEL-NUMBER attribute contains the number of the channel. It
is a 16-bit unsigned integer, followed by a two-octet RFFU (Reserved
For Future Use) field which MUST be set to 0 on transmission and MUST
be ignored on reception.</t>
<figure>
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel Number | RFFU = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</section>
<section title="LIFETIME">
<t>The lifetime attribute represents the duration for which the server
will maintain an allocation in the absence of a refresh. It is a
32-bit unsigned integral value representing the number of seconds
remaining until expiration.</t>
</section>
<section title="PEER-ADDRESS">
<t>The PEER-ADDRESS specifies the address and port of the peer as seen
from the TURN server. It is encoded in the same way as
XOR-MAPPED-ADDRESS.</t>
</section>
<section title="DATA">
<t>The DATA attribute is present in all Data Indications and most Send
Indications. It contains raw payload data that is to be sent (in the
case of a Send Request) or was received (in the case of a Data
Indication).</t>
</section>
<section title="RELAY-ADDRESS">
<t>The RELAY-ADDRESS is present in Allocate responses. It specifies
the address and port that the server allocated to the client. It is
encoded in the same way as XOR-MAPPED-ADDRESS.</t>
</section>
<section title="REQUESTED-PROPS">
<t>This attribute allows the client to request that the allocation
have certain properties. The attribute is 32 bits long. Its format
is:</t>
<figure>
<preamble></preamble>
<artwork><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|R|P| MUST be 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t></t>
<t>The first part of the attribute value contains a number of one-bit
flags. These are:<list style="hanging">
<t hangText="E:">If 1, the port number for the
relayed-transport-address must be even. If 0, the port number can
be even or odd.</t>
<t hangText="R:">If 1, the server must reserve the next highest
port for a subsequent allocation. If 0, no such reservation is
requested. If the client sets the R bit to 1, it MUST also set the
E bit to 1 (however, the E bit may be 1 when the R bit is 0).</t>
<t hangText="P:">If 1, the allocation must be a Preserving
allocation. If 0, the allocation can be either Preserving or
Non-Preserving.</t>
</list>All these flags have the property that if the bit is 1, and
the server cannot create an allocation that satisfies the request,
then the Allocate request is rejected. To allow future TURN extensions
to define new flags that also have this property, the client MUST set
the rest of the attribute to zero, and the server MUST fail the
Allocate request if any bits which the server does not support are set
to 1. By doing this, any new flags that are not recognized by the
server will cause the Allocate request to fail.</t>
</section>
<section anchor="sec-requested-transport" title="REQUESTED-TRANSPORT">
<t>This attribute is used by the client to request a specific
transport protocol for the allocated transport address. It has the
following format:</t>
<figure>
<artwork><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol | RFFU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>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 <xref
target="Protocol-Numbers"></xref>. This specification only allows the
use of codepoint 17 (User Datagram Protocol).</t>
<t>The RFFU field MUST be set to zero on transmission and MUST be
ignored on receiption. It is reserved for future uses.</t>
</section>
<section title="RESERVATION-TOKEN">
<t>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.</t>
<t>The attribute value is a 64-bit-long field containing the token
value.</t>
</section>
</section>
<!-- new attributes -->
<section anchor="sec-stun-errors" title="New STUN Error Response Codes">
<t>This document defines the following new error response codes:</t>
<t><list style="hanging">
<t hangText="437">(Allocation Mismatch): A request was received by
the server that requires an allocation to be in place, but there is
none, or a request was received which requires no allocation, but
there is one.</t>
<t hangText="438">(Wrong Credentials): The credentials in the
(non-Allocate) request, though otherwise acceptable to the server,
do not match those used to create the allocation.</t>
<t hangText="442">(Unsupported Transport Protocol): The Allocate
request asked the server to use a transport protocol between the
server and the peer that the server does not support. NOTE: This
does NOT refer to the transport protocol used in the 5-tuple.</t>
<t hangText="486">(Allocation Quota Reached): No more allocations
using this username can be created at the present time.</t>
<t hangText="508">(Insufficient Port Capacity): The server has no
more relayed transport addresses available right now, or has none
with the requested properties, or the one that corresponds to the
specified token is not available.</t>
</list></t>
</section>
<!-- error codes -->
<section anchor="sec-security" title="Security Considerations">
<t>TURN servers allocate resources to clients, in contrast to the
Binding method defined in <xref
target="I-D.ietf-behave-rfc3489bis"></xref>. Therefore, a TURN server
may require the authentication and authorization of STUN requests. This
authentication is provided by mechanisms defined in the STUN
specification itself, in particular digest authentication.</t>
<t>Because TURN servers allocate resources, they can be susceptible to
denial-of-service attacks. All Allocate transactions are authenticated,
so that an unknown attacker cannot launch an attack. An authenticated
attacker can generate multiple Allocate Requests, however. To prevent a
single malicious user from allocating all of the resources on the
server, it is RECOMMENDED that a server implement a per user limit on
the number of allocations that can active at one time. Such a mechanism
does not prevent a large number of malicious users from each requesting
a small number of allocations. Attacks such as these are possible using
botnets, and are difficult to detect and prevent. Implementors of TURN
should keep up with best practices around detection of anomalous botnet
attacks.</t>
<t>A client will use the transport address learned from the
RELAY-ADDRESS attribute of the Allocate Response to tell other users how
to reach them. Therefore, a client needs to be certain that this address
is valid, and will actually route to them. Such validation occurs
through the message integrity checks provided in the Allocate response.
They can guarantee the authenticity and integrity of the allocated
addresses. Note that TURN is not susceptible to the attacks described in
Section 12.2.3, 12.2.4, 12.2.5 or 12.2.6 of <xref
target="I-D.ietf-behave-rfc3489bis"></xref> [[TODO: Update section
number references to 3489bis]]. These attacks are based on the fact that
a STUN server mirrors the source IP address, which cannot be
authenticated. STUN does not use the source address of the Allocate
Request in providing the RELAY-ADDRESS, and therefore, those attacks do
not apply.</t>
<t>TURN attempts to adhere as closely as possible to common firewall
policies, consistent with allowing data to flow. TURN has fairly limited
applicability, requiring a user to explicitly authorize permission to
receive data from a peer, one IP address at a time. Thus, it does not
provide a general technique for externalizing sockets. Rather, it has
similar security properties to the placement of an address-restricted
NAT in the network, allowing messaging in from a peer only if the
internal client has sent a packet out towards the IP address of that
peer. This limitation means that TURN cannot be used to run, for
example, SIP servers, NTP servers, FTP servers or other network servers
that service a large number of clients. Rather, it facilitates
rendezvous of NATted clients that use some other protocol, such as SIP,
to communicate IP addresses and ports for communications.</t>
<t>Confidentiality of the transport addresses learned through Allocate
transactions does not appear to be that important. If required, it can
be provided by running TURN over TLS.</t>
<t>TURN does not and cannot guarantee that UDP data is delivered in
sequence or to the correct address. As most TURN clients will only
communicate with a single peer, the use of a single channel number will
be very common. Consider an enterprise where Alice and Bob are involved
in separate calls through the enterprise NAT to their corporate TURN
server. If the corporate NAT reboots, it is possible that Bob will
obtain the exact NAT binding originally used by Alice. If Alice and Bob
were using identical channel numbers, Bob will receive unencapsulated
data intended for Alice and will send data accidentally to Alice's peer.
This is not a problem with TURN. This is precisely what would happen if
there was no TURN server and Bob and Alice instead provided a (STUN)
reflexive transport address to their peers. If detecting this
misdelivery is a problem, the client and its peer need to use message
integrity on their data.</t>
<t>Relay servers are useful even for users not behind a NAT. They can
provide a way for truly anonymous communications. A user can cause a
call to have its media routed through a TURN server, so that the user's
IP addresses are never revealed.</t>
<t>Any relay addresses learned through an Allocate request will not
operate properly with <xref target="RFC4302">IPSec Authentication Header
(AH)</xref> in transport or tunnel mode. However, tunnel-mode <xref
target="RFC4303">IPSec ESP</xref> should still operate.</t>
</section>
<!-- Security -->
<section title="IANA Considerations">
<t>Since TURN is an extension to STUN <xref
target="I-D.ietf-behave-rfc3489bis"></xref>, the methods, attributes and
error codes defined in this specification are new method, attributes,
and error codes for STUN. This section directs IANA to add these new
protocol elements to the IANA registry of STUN protocol elements.</t>
<t>The codepoints for the new STUN methods defined in this specification
are listed in <xref target="sec-stun-methods"></xref>.</t>
<t>The codepoints for the new STUN attributes defined in this
specification are listed in <xref
target="sec-stun-attributes"></xref>.</t>
<t>The codepoints for the new STUN error codes defined in this
specification are listed in <xref target="sec-stun-errors"></xref>.</t>
<t>Extensions to TURN can be made through IETF consensus.</t>
</section>
<section title="IAB Considerations">
<t>The IAB has studied the problem of "Unilateral Self Address Fixing",
which is the general process by which a client attempts to determine its
address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism <xref
target="RFC3424"></xref>. 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.</t>
<t>TURN is an extension of the STUN protocol. As such, the specific
usages of STUN that use the TURN extensions need to specifically address
these considerations. Currently the only STUN usage that uses TURN is
<xref target="I-D.ietf-mmusic-ice">ICE</xref>.</t>
</section>
<!-- IAB Considerations -->
<section title="Example">
<t>TBD</t>
</section>
<!-- Example -->
<section title="Changes from Previous Versions">
<t>Note to RFC Editor: Please remove this section prior to publication
of this document as an RFC.</t>
<t>This section lists the changes between the various versions of this
specification.</t>
<section title="Changed from -07 to -08">
<t><list style="symbols">
<t>Removed the BANDWIDTH attribute and all associated text
(including error code 507 "Insufficient Bandwidth Capacity"), as
the requirements for this feature were not clear and it was felt
the feature could be easily added later.</t>
<t>Changed the format of the REQUESTED-PROPS attribute from a
one-byte field to a set of bit flags. Changed the semantics of the
unused portion of the value from RFFU to "MUST be 0" to give a
more desirable behavior when new flags are defined.</t>
<t>Introduced the concept of Preserving vs. Non-Preserving
allocations. As a result, completely revamped the rules for how to
set the fields in the IP header, and added rules for relaying ICMP
messages when the allocation is Preserving.</t>
</list></t>
</section>
<section title="Changes from -06 to -07">
<t><list style="symbols">
<t>Rewrote the General Behavior section, making various changes in
the process.</t>
<t>Changed the usage of authentication from MUST to SHOULD.</t>
<t>Changed the requirement that subsequent requests use the same
username and password from MUST to SHOULD to allow for the
possibility of changing the credentials using some unspecified
mechanism.</t>
<t>Introduced a 438 (Wrong Credentials) error which is used when a
non-Allocate request authenticates but does not use the same
username and password as the Allocate request. Having a separate
error code for this case avoids the client being confused over
what the error actually is.</t>
<t>The server must now prevent the relayed transport address and
the 5-tuple from being reused in different allocations for 2
minutes after the allocation expires.</t>
<t>Changed the usage of FINGERPRINT from MUST NOT to MAY, to allow
for the possible multiplexing of TURN with some other
protocol.</t>
<t>Rewrote much of the section on Allocations, splitting it into
three new sections (one on allocations in general, one on creating
an allocation, and one on refreshing an allocation).</t>
<t>Replaced the mechanism for requesting relayed transport
addresses with specific properties. The new mechanism is less
powerful: a client can request an even port, or a pair of ports,
but cannot request a single odd port or a specific port as was
possible under the old mechanism. Nor can the client request a
specific IP address.</t>
<t>Changed the rules for handling ALTERNATE-SERVER, removing the
requirement that the referring server have "positive knowledge"
about the state of the alternate server. The new rules instead
rely on text in STUN to prevent referral loops.</t>
<t>Changed the rules for allocation lifetimes. Allocations
lifetimes are now a minimum of 10 minutes; the client can ask for
longer values, but requests for shorter values are ignored. The
text now recommends that the client refresh an allocation one
minute before it expires.</t>
<t>Put in temporary procedures for handling the BANDWIDTH
attribute, modelled on the LIFETIME attribute. These procedures
are mostly placeholders and likely to change in the next
revision.</t>
<t>Added a detailed description of how a client reacts to the
various errors it can receive in reply to an Allocate request.
This replaces the various descriptions that were previously
scattered throughout the document, which were inconsistent and
sometimes contradictory.</t>
<t>Added a new section that gives the normative rules for
permissions.</t>
<t>Changed the rules around permission lifetimes. The text used to
recommend a value of one minute; it MUST now be 5 minutes.</t>
<t>Removed the errors "Channel Missing or Invalid", "Peer Address
Missing or Invalid" and "Lifetime Malformed or Invalid" and used
400 "Bad Request" instead.</t>
<t>Rewrote portions of the section on Send and Data indications
and the section on Channels to try to make the client vs. server
behavior clearer.</t>
<t>Channel bindings now expire after 10 minutes, and must be
refreshed to keep them alive.</t>
<t>Binding a channel now installs or refreshes a permission for
the IP address of corresponding peer.</t>
<t>Changed the wording describing the situation when the client
sends a ChannelData message before receiving the ChannelBind
success response. -06 said that client SHOULD NOT do this; -07 now
says that a client MAY, but describes the consequences of doing
it.</t>
<t>Added a section discussing the setting of fields in the IP
header.</t>
<t>Replaced the REQUESTED-PORT-PROPS attribute with the
REQUESTED-PROPS attribute that has a different format and
semantics, but reuses the same code point.</t>
<t>Replaced the REQUESTED-IP attribute with the RESERVATION-TOKEN
attribute, which has a different format and semantics, but reuses
the same code point.</t>
<t>Removed error codes 443 and 444, and replaced them with 508
(Insufficient Port Capacity). Also changed the error text for code
507 from "Insufficient Capacity" to "Insufficient Bandwidth
Capacity".</t>
</list></t>
</section>
<section title="Changes from -05 to -06">
<t><list style="symbols">
<t>Changed the mechanism for allocating channels to the one
proposed by Eric Rescorla at the Dec 2007 IETF meeting.</t>
<t>Removed the framing mechanism (which was used to frame all
messages) and replaced it with the ChannelData message. As part of
this change, noted that the demux of ChannelData messages from
TURN messages can be done using the first two bits of the
message.</t>
<t>Rewrote the sections on transmitted and receiving data as a
result of the above to changes, splitting it into a section on
Send and Data Indications and a separate section on channels.</t>
<t>Clarified the handling of Allocate Request messages. In
particular, subsequent Allocate Request messages over UDP with the
same transaction id are not an error but a retransmission.</t>
<t>Restricted the range of ports available for allocation to the
Dynamic and/or Private Port range, and noted when ports outside
this range can be used.</t>
<t>Changed the format of the REQUESTED-TRANSPORT attribute. The
previous version used 00 for UDP and 01 for TCP; the new version
uses protocol numbers from the IANA protocol number registry. The
format of the attribute also changed.</t>
<t>Made a large number of changes to the non-normative portion of
the document to reflect technical changes and improve the
presentation.</t>
<t>Added the Issues section.</t>
</list></t>
</section>
<section title="Changes from -04 to -05">
<t><list style="symbols">
<t>Removed the ability to allocate addresses for TCP relaying.
This is now covered in a separate document. However, communication
between the client and the server can still run over TCP or
TLS/TCP. This resulted in the removal of the Connect method and
the TIMER-VAL and CONNECT-STAT attributes.</t>
<t>Added the concept of channels. All communication between the
client and the server flows on a channel. Channels are numbered
0..65535. Channel 0 is used for TURN messages, while the remaining
channels are used for sending unencapsulated data to/from a remote
peer. This concept adds a new Channel Confirmation method and a
new CHANNEL-NUMBER attribute. The new attribute is also used in
the Send and Data methods.</t>
<t>The framing mechanism formally used just for stream-oriented
transports is now also used for UDP, and the former Type and
Reserved fields in the header have been replaced by a Channel
Number field. The length field is zero when running over UDP.</t>
<t>TURN now runs on its own port, rather than using the STUN port.
The use of channels requires this.</t>
<t>Removed the SetActiveDestination concept. This has been
replaced by the concept of channels.</t>
<t>Changed the allocation refresh mechanism. The new mechanism
uses a new Refresh method, rather than repeating the Allocation
transaction.</t>
<t>Changed the syntax of SRV requests for secure transport. The
new syntax is "_turns._tcp" rather than the old "_turn._tls". This
change mirrors the corresponding change in STUN SRV syntax.</t>
<t>Renamed the old REMOTE-ADDRESS attribute to PEER-ADDRESS, and
changed it to use the XOR-MAPPED-ADDRESS format.</t>
<t>Changed the RELAY-ADDRESS attribute to use the
XOR-MAPPED-ADDRESS format (instead of the MAPPED-ADDRESS
format)).</t>
<t>Renamed the 437 error code from "No Binding" to "Allocation
Mismatch".</t>
<t>Added a discussion of what happens if a client's public binding
on its outermost NAT changes.</t>
<t>The document now consistently uses the term "peer" as the name
of a remote endpoint with which the client wishes to
communicate.</t>
<t>Rewrote much of the document to describe the new concepts. At
the same time, tried to make the presentation clearer and less
repetitive.</t>
</list></t>
</section>
</section>
<section anchor="sec-open-issues" title="Open Issues">
<t>NOTE to RFC Editor: Please remove this section prior to publication
of this document as an RFC.</t>
<t>Bandwidth: How should bandwidth be specified? What are the right
rules around bandwidth?</t>
<t>Alternate Server: Do we still want this mechanism? Is the current
proposal acceptable? Note that the usage of the ALTERNATE-SERVER
attribute in this document is inconsistent with its usage in STUN. In
STUN, if the ALTERNATE-SERVER attribute is used, then the error that the
server would otherwise generate is replaced by a 300 (Try Alternate)
code. In this document, the 300 error code is not used, and the server
returns an appropriate error code and then includes the ALTERNATE-SERVER
attribute in the response. In this way, the client can see the actual
error code, rather than always seeing error code 300, and can thus make
a more intelligent decision on whether it wishes to try the alternate
server.</t>
<t>Public TURN servers: The text currently says that a server "SHOULD"
use the Long-Term Credential mechanism, with the unstated idea that a
public TURN server would not use it. But this really weakens the
security of TURN. Is there a better way to allow public servers? Or
should we just drop the notion of a public server entirely?</t>
</section>
<section title="Acknowledgements">
<t>The authors would like to thank the various participants in the
BEHAVE working group for their many comments on this draft. Marc
Petit-Huguenin, Remi Denis-Courmont, Derek MacDonald, Cullen Jennings,
Lars Eggert, Magnus Westerlund, and Eric Rescorla have been particularly
helpful, with Eric also suggesting the channel allocation mechanism, and
Cullen suggesting the REQUESTED-PROPS mechanism. Christian Huitema was
an early contributor to this document and was a co-author on the first
few drafts. Finally, the authors would like to thank Dan Wing for both
his contributions to the text and his huge help in restarting progress
on this draft after work had stalled.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.I-D.ietf-behave-rfc3489bis"?>
<?rfc include="reference.RFC.2119"?>
<?rfc include='reference.RFC.3697'?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.1918"?>
<?rfc include='reference.RFC.1981'?>
<?rfc include="reference.RFC.3264"?>
<?rfc include="reference.RFC.4302"?>
<?rfc include="reference.RFC.4303"?>
<?rfc include="reference.RFC.3424"?>
<?rfc include="reference.I-D.ietf-mmusic-ice"?>
<?rfc include="reference.RFC.4787"?>
<?rfc include="reference.I-D.ietf-behave-turn-tcp"?>
<?rfc include="reference.I-D.ietf-behave-turn-ipv6"?>
<?rfc include="reference.I-D.ietf-tsvwg-udp-guidelines"?>
<?rfc include="reference.RFC.1191"?>
<?rfc include="reference.RFC.4821"?>
<?rfc include='reference.RFC.1928'?>
<reference anchor="Port-Numbers"
target="http://www.iana.org/assignments/port-numbers">
<front>
<title>IANA Port Numbers Registry</title>
</front>
</reference>
<reference anchor="Protocol-Numbers"
target="http://www.iana.org/assignments/protocol-numbers">
<front>
<title>IANA Protocol Numbers Registry</title>
<author>
<organization></organization>
</author>
<date year="2005" />
</front>
</reference>
</references>
</back>
</rfc>
<!--
<?rfc include="reference.RFC.2782"?>
<?rfc include="reference.RFC.2617"?>
<?rfc include="reference.RFC.2327"?>
<?rfc include="reference.RFC.2326"?>
<?rfc include="reference.RFC.3235"?>
-->
<!-- Notes for Rohan from conversation with JDR
1. The Connect Request doesn't make sense. If the peer could receive a TCP connection request,
the client would just open a connection to the peer directly.
2. Still need the concept of a door if we want TCP to TCP case to work behind most NATs/FWs.
Took serveral steps to make sure door concept does not allow you to run a real server.
Can't ask for a door and specific port number. Can't get a door with a well known port.
Can't open a door twice.
(Also, the door can eliminate the use of two TURN servers if the caller and callee are both
behind bad NATs, as long as thier is no forking. With ICE, everything works fine.
You try one TURN server and if that doesn't work you try both.)
3. Introduce lightweight TCP framing. Send any data with magic cookie over UDP in a Data/Send
4. Disallow UDP (client to relay) to TCP (relay to peer) case.
5. Add connection status notification as an indication. This works well since the client to
server connection is always reliable now anytime you would get this indication.
6. Add "move my flow over here" feature.
-->
<!-- from my notes while doing turn-00-->
<!-- need to align the turn draft with the long term direction of stun
and turn for control of nat. In particular, this means ultimately
allowing multiple mapped address attributes returned from a stun query
(one from each nat processing the stun) and also having the binding
lifetime make its way into turn somehow -->
<!-- need to fix the big disambiguation problem, of how to know whether
this stun request is for me as a turn server, or for a downstream stun server
as in the ice case -->
<!-- terminology check - the draft uses the terms binding and allocation
too loosely. Allocation is the transport address allocated to the client,
and permissions needs to be used consistently. Also the term external
5-tuples is used a bunch and this probably needs to be removed. Other
terminology things: use correspondent instead of external client; it
sound sbetter. Also suggest allocated transport address instead of
external local, and well-known transport address instead of internal
local. Suggest reflexive for internal remote. -->
<!-- Draft needs to be a bit clearer on lifecycle management of
allocations. -->
<!-- Encoding of addresses - should these be using xor form or regular
form? in places like RELAY-ADDRESs and DESTINATION-ADDRESS -->
<!-- IANA registry needs to be added. THis includes registering the
new methods and attributes, and creating registries for the few things
in here, like port properties -->
<!-- Check to make sure all response codes mentioned in the back of
the document are used, and vice a versa -->
<!-- need to clean up terminology around shared secrets. There are two
types, long term and short term, with a relationship between them
(been using derived-from). That needs to be explained. Also, need to
be clearer on which credentials are reall required in a turn request -
needs to be equal to or cousin to one from original allocate -->
<!-- rewrite IAB consideratoins -->
<!-- major work on security considerations - what about data packets
and indications in particular? -->
<!-- scrub references, make sure all are used -->
<!-- spec says you need to include mapped address. But, more like, use
mapped address if magic cookie is not there, and xor-mapped if it is
-->
<!-- remove active 5-tuple terminology -->
<!-- I've used the term 'linkage' to try and include the traditional
tcp connection and udp connection. Need to decide if i like it and use
it or not -->
<!-- should i make turn into a mobile ip replacement? can be done, by
allowing internal address to be updated in an allocate. requires
changing keying structures around. today, incoming allocate uses
incoming 'linkage' or connection to refer to allocation. Would need to
be identified some other way. Would be cool though. Thats an OPEN
ISSUE -->
<!-- add table of mandatory/optional attributes here -->
<!-- maybe add a discussion about how alternate server can be used,
with a front end load balancer -->
<!-- Add text about using multiple virtual turn servers to deal with the
case where the turn server is effectively or virtually mutli-homed. -->
<!-- is message-integrity needed for the error responses? If so, it
belongs in rfc3489bis too -->
<!-- ISSUE: how to allow the server whether to know to start tls
procedures when it receives a tcp connection? Is this assuming
start-tls or are we using a separate port?? -->
<!-- ISSUE: once udp destination is set, server looks for TURN packets by magic cookie. But what
if packet being send is e2e stun connectivity check which
also has magic cookie! This is a real use case. Need to move to model where
control transitions, like for tcp.
-->
<!-- xtunnel 1: udp to tcp, specify qos parameters for leaky bucket -->
<!-- xteunnel 2: tcp one direction, udp the other?? -->
<!-- xtunnel 3: forking issue - requires creation of a new allocation. Alternative is to always
encapsulate with something more lightweight -->
<!-- xtunnel 4: some issue with requiring a third changed address to deal with running out of
local ports on the client. Rob says he'll send me some text
on the situation -->
<!-- add example -->
<!-- turn encapsulation issue raised by Justin Umberti - what if the data
that is sent unencapsulated is less than 7 bytes? Server has to buffer
7 (is it 7) bytes to look for the cookie. -->
<!-- from ietf65: add note that contiguous port requests is for legacy
interop -->
<!-- new framing proposal: use my 32 bit framing word for tcp. Note that,
you can put multiple tcp application frames within a data frame. Length
only indicates frequency of stun signaling. For UDP, don't use this.
However, packets inwards towards stun relay, stun relay looks for
magic cookie in the bytes 5-7 and uses data indication if they are
there. Similarly, client should look for cookie in those bytes before
sending and if present, use Send. This means you don't have the
problem of figuring out whether data is destined for this turn server
or not - those data are always done with Send. Nice. -->
<!-- agree to have refreshes come from different source/dest IP -->
<!-- from adam's note on behave 3/28, remove capability for udp from client
to turn server and then tcp outwards. No way this works -->| PAFTECH AB 2003-2026 | 2026-04-22 23:51:40 |