One document matched: draft-ietf-behave-turn-06.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-06" 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.R." 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.M." surname="Mahy">
<organization abbrev="Plantronics">Plantronics, Inc.</organization>
<address>
<email>rohan@ekabal.com</email>
</address>
</author>
<author fullname="Philip Matthews" initials="P.M." surname="Matthews">
<organization abbrev="Avaya">Avaya, Inc.</organization>
<address>
<postal>
<street>1135 Innovation Drive</street>
<city>Ottawa</city>
<region>Ontario</region>
<code>K2K 3G7</code>
<country>Canada</country>
</postal>
<phone>+1 613 592-4343 x223</phone>
<facsimile></facsimile>
<email>philip_matthews@magma.ca</email>
<uri></uri>
</address>
</author>
<date year="2008" />
<area>Transport</area>
<workgroup>Behave</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>NOTE TO THE READER: This document is a work-in-progress. Please see
the list of open and closed issues in <xref target="sec-issues"></xref>.
With only a few exceptions, if there is an open issue the text has NOT
been updated in this area pending resolution of this issue - keep this
in mind when reading the text. In addition, in the interest of getting
the document out quickly in order to make progress on open issues, the
authors have elected to release the document is a bit more "raw" state
than they would prefer, resulting in some rough spots in the
presentation.</t>
<t><vspace blankLines="3" /></t>
<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 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 are from IP addresses belonging
to 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 an address on the TURN server, so that the
TURN server acts as a relay. This extension defines a handful of new
STUN methods. The Allocate method is the most fundamental component of
this set of extensions. It is used to provide the client with a
transport address that is relayed through the TURN server. A transport
address which relays through an intermediary is called a relayed
transport address.</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>Though originally invented for Voice over IP applications, TURN is
designed to be a general-purpose relay mechanism for NAT traversal.</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 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, peers can send data packets to the relayed transport
address and the server will forward them to the client. In the reverse
direction, the client can send data packets to the server (at its TURN
server address) and these will be forwarded by the server to the
appropriate peer, and the peer will see them as coming from the relayed
transport address; in this direction, the client must specify the
appropriate peer.</t>
<section 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>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 can require lots of bandwidth, the server may
require 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. This is done by the client periodically doing a
Refresh transaction with the server, where the client includes the
allocated relayed transport address in the Refresh Request. 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 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>The client can use the relayed transport address to exchange data
with its peers by using Send and Data indications. A Send Indication
is sent from a client to the TURN server and contains, in attributes
inside the message, 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>
<t>Note that a client can use a single relayed transport address to
exchange data with multiple peers at the same time.</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="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>A TURN server will drop a UDP datagram arriving at a relayed
transport address from a peer unless the client has recently sent data
to a peer with the same IP address (the port numbers can differ). See
the normative description for the precise definition of
“recently”. </t>
<t>A permission will timeout if not refreshed periodically. The client
refreshes a permission by sending data to the corresponding peer. Data
received from the peer DOES NOT refresh the permission. </t>
</section>
<section title="Channels">
<t>In some applications, the overhead of using Send and Data
indications can be substantial. For example, for applications like
VoIP which utilize small packets, Send and Data Indications, with 36
bytes of overhead, can have a substantial impact on overall bandwidth
usage. To remedy this, TURN clients can assign a CHANNEL to a peer.
Data to and from such a peer can then be sent using an alternate
packet format that adds only 4 bytes per packet of overhead.</t>
<t>The alternate packet format is 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.</t>
<t>To create a channel, the client sends a ChannelBind request to the
server, and includes an unallocated channel number and the transport
address of the peer. Once the client receives the response to the
ChannelBind request, it can send data to that peer using a ChannelData
message. Similarly, once the server has received the request, it can
relay data from that peer towards the client using a ChannelData
message. There is no way to modify channel bindings, so once a channel
is bound to a peer, it remains bound for the lifetime of the
allocation.</t>
<t>When the server receives a ChannelData message from the client, it
uses the channel number to determine the destination peer and then
forwards the data inside a UDP datagram to the peer. In the reverse
direction, when a UDP datagram arives at the relayed transport address
from that peer, the server inserts it into a ChannelData message
containing the channel number bound to that peer; in this way the
client can determine the peer that send the UDP datagram.</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>
<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 transport address granted to a client
through an Allocate request, along with related state, such as
permissions and expiration timers. See also Relayed Transport
Address.</t>
<t hangText="5-tuple:">A combination of the source IP address and
port, destination IP address and port, and transport protocol (UDP
or TCP). A 5-tuple uniquely identifies a TCP connection or the
bi-directional flow of UDP datagrams.</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>
</list></t>
</section>
<!-- Overview -->
<section title="General Behavior">
<t>After the initial Allocate transaction, all subsequent TURN
transactions need to be sent in the context of a valid allocation. The
source and destination IP address and ports for these TURN messages MUST
match the those used in the initial Allocate Request. These are
processed using the general server procedures in <xref
target="I-D.ietf-behave-rfc3489bis"></xref> with a few important
additions. For requests, if there is no matching allocation, the server
MUST generate a 437 (Allocation Mismatch) error response. For
indications, if there is no matching allocation, the indication is
silently discarded. An Allocate request MUST NOT be sent by a client
within the context of an existing allocation. Such a request MUST be
rejected by the server with a 437 (Allocation Mismatch) error
response.</t>
<t>A subsequent request MUST be authenticated using the same username,
password and realm as the one used in the Allocate request that created
the allocation. If the request was authenticated but not with the
matching credential, the server MUST reject the request with a 401
(Unauthorized) error response.</t>
<t>When a server returns an error response, it MAY include an
ALTERNATE-SERVER attribute if it has positive knowledge that the problem
reported in the error response will not be a problem on the alternate
server. For example, a 443 response (Invalid IP Address) with an
ALTERNATE-SERVER means that the other server is responsible for that IP
address. A 442 (Unsupported Transport Protocol) with this attribute
means that the other server is known to support that transport protocol.
A 507 (Insufficient Capacity) means that the other server is known to
have sufficient capacity. Using the ALTERNATE-SERVER mechanism in the
507 (Insufficient Capacity) response can only be done if the rejecting
server has definitive knowledge of available capacity on the target.
This will require some kind of state sharing mechanism between TURN
servers, which is beyond the scope of this specification. If a TURN
server attempts to redirect to another server without knowledge of
available capacity, it is possible that all servers are in a congested
state, resulting in series of rejections that only serve to further
increase the load on the system. This can cause congestion collapse.</t>
<t>If a client sends a request to a server and gets a 500 class error
response without an ALTERNATE-SERVER, or the STUN transaction times out
without a response, and the client was utilizing the SRV procedures of
<xref target="I-D.ietf-behave-rfc3489bis"></xref> to contact the server,
the client SHOULD try another server based on those procedures. However,
the client SHOULD cache the fact that the request to this server failed,
and not retry that server again for a configurable period of time. Five
minutes is RECOMMENDED.</t>
<t>TURN clients and servers MUST NOT include the FINGERPRINT attribute
in any of the methods defined in this document.</t>
</section>
<section anchor="allocations" title="Managing Allocations">
<t>Communications between a TURN client and a TURN server begin with an
Allocate transaction. All subsequent transactions happen in the context
of that allocation, and happen on the same 5-tuple. The client refreshes
allocations and deallocates them using a Refresh transaction.</t>
<section title="Client Behavior">
<section anchor="sec-alloc" title="Initial Allocate Requests">
<t>When a client wishes to obtain a transport address, it sends an
Allocate request to the server. This request is constructed and sent
using the general procedures defined in <xref
target="I-D.ietf-behave-rfc3489bis"></xref>. Clients MUST implement
the long term credential mechanism defined in <xref
target="I-D.ietf-behave-rfc3489bis"></xref>, and be prepared for the
server to demand credentials for requests.</t>
<t>The client SHOULD include a BANDWIDTH attribute, which indicates
the maximum bandwidth that will be used with this binding. If the
maximum is unknown, the attribute is not included in the
request.</t>
<t>The client MAY request a particular lifetime for the allocation
by including it in the LIFETIME attribute in the request.</t>
<t>The client MUST include a REQUESTED-TRANSPORT attribute. In this
specification, the REQUESTED-TRANSPORT MUST always be UDP. This
attribute is included to allow for future extensions to TURN (e.g.,
<xref target="I-D.ietf-behave-turn-tcp"></xref>)</t>
<t>The client MAY include a REQUESTED-PORT-PROPS or REQUESTED-IP
attribute in the request to obtain specific types of transport
addresses, if desired.</t>
<t>Processing of the response follows the general procedures of
<xref target="I-D.ietf-behave-rfc3489bis"></xref>. A successful
response will include both a RELAY-ADDRESS and an XOR-MAPPED-ADDRESS
attribute, providing both a relayed transport address and a
reflexive transport address, respectively, to the client. The value
of the LIFETIME attribute in the response indicates the amount of
time after which the server will expire the allocation, if not
refreshed with a Refresh request. The server will allow the user to
send and receive at least the amount of data indicated in the
BANDWIDTH attribute per allocation. (At its discretion the server
can optionally discard UDP data above this threshold.)</t>
<t>If the response is an error response and contains a 442, 443 or
444 error code, the client knows that its requested properties could
not be met. The client MAY retry with different properties, with the
same properties (in a hope that something has changed on the
server), or give up, depending on the needs of the application.
However, if the client retries, it SHOULD wait 500ms, and if the
request fails again, wait 1 second, then 2 seconds, and so on,
exponentially backing off.</t>
</section>
<section anchor="sec-refresh" title="Refresh Requests">
<t>TURN permissions are kept alive by traffic flowing through them,
and persist for the lifetime of the allocation. However, The
allocations themselves have to be kept alive through Refresh
Requests.</t>
<t>Before 3/4 of the lifetime of the allocation has passed (the
lifetime of the allocation is conveyed in the LIFETIME attribute of
the Allocate Response), the client SHOULD refresh the allocation
with a Refresh transaction if it wishes to keep the allocation.</t>
<t>To perform a refresh, the client generates a Refresh Request. The
client MUST use the same username, realm and password for the
Refresh request as it used in its initial Allocate Request. The
Refresh request MAY contain a proposed LIFETIME attribute. The
client MAY include a BANDWIDTH attribute if it wishes to request
more or less bandwidth than in the original request (this might also
be the first time the TURN client indicates bandwidth to the TURN
server). If the BANDWIDTH attribute is absent, it indicates no
change in the requested bandwidth from the Allocate request. The
client MUST NOT include a REQUESTED-IP, REQUESTED-TRANSPORT, or
REQUESTED-PORT-PROPS attribute in the Refresh request.</t>
<t>In a successful response, the LIFETIME attribute indicates the
amount of additional time (the number of seconds after the response
is received) that the allocation will live without being refreshed.
A successful response will also contain a BANDWIDTH attribute,
indicating the bandwidth the server is allowing for this allocation.
Note that an error response does not imply that the allocation has
expired, just that the refresh has failed.</t>
<t>If a client no longer needs an allocation, it SHOULD perform an
explicit deallocation. If the client wishes to explicitly remove the
allocation because it no longer needs it, it sends a Refresh
request, but sets the LIFETIME attribute to zero. This will cause
the server to remove the allocation, and all associated permissions
and channel numbers. For connection-oriented transports such as TCP,
the client can also remove the allocation (and all associated
bindings) by closing the relevant connection with the TURN
server.</t>
</section>
<!-- close client -->
</section>
<section title="Server Behavior">
<section anchor="sec-initial" title="Receiving an Allocate Request">
<t>When the server receives an Allocate request, the server attempts
to allocate a relayed transport address.</t>
<t>When the server receives the Allocate Request, it begins by
processing it according to the base protocol procedures described in
<xref target="I-D.ietf-behave-rfc3489bis"></xref>, plus the
Long-Term Credential Mechanism procedures if the server is using
this mechanism.</t>
<t>It then checks if the 5-tuple used for the Allocate Request
matches the 5-tuple used for an existing allocation. If there is a
match, then:<list style="symbols">
<t>If the transport protocol is UDP, and the transaction id in
the request message matches the transaction id used for the
original allocation, then the server treats this as a
retransmission of the original request, and replies with the
same response as it did to the original request. The server may
do this by either storing its original response and resending
it, or by rebuilding its original response from other state
data.</t>
<t>If the transport protocol is not UDP, or if the transaction
id in the request message does not match the transaction id used
for the original allocation, then the server replies with an
error response containing the error code 437 Allocation
Mismatch.</t>
</list>If the 5-tuple does not match an existing allocation, then
processing continues as described below.</t>
<section title="BANDWIDTH">
<t>The server checks for the BANDWIDTH attribute in the request.
If present, the server determines whether or not it has sufficient
capacity to handle a binding that will generate the requested
bandwidth.</t>
<t>If it does, the server attempts to allocate a transport address
for the client. The Allocate Request can contain several
additional attributes that allow the client to request specific
characteristics of the transport address. If it doesn't, it sends
an error response.</t>
</section>
<section title="REQUESTED-TRANSPORT">
<t>The server checks for the REQUESTED-TRANSPORT attribute. This
indicates the transport protocol requested by the client. This
specification defines a value for UDP only, but support for TCP
allocations is planned in <xref
target="I-D.ietf-behave-turn-tcp"></xref>.</t>
<t><list>
<t>As a consequence of the REQUESTED-TRANSPORT attribute, it
is possible for a client to connect to the server over TCP or
TLS over TCP and request a UDP transport address. In this
case, the server will relay data between the transports.</t>
</list></t>
<t>If the requested transport is supported, the server allocates a
port using the requested transport protocol. If the
REQUESTED-TRANSPORT attribute contains a value of the transport
protocol unknown to the server, or known to the server but not
supported by the server in the context of this request, the server
MUST reject the request and include a 442 (Unsupported Transport
Protocol) in the response. If the request did not contain a
REQUESTED-TRANSPORT attribute, the server MUST use the same
transport protocol as the request arrived on.</t>
</section>
<section title="REQUESTED-IP">
<t>The server checks for the REQUESTED-IP attribute. If present,
it indicates a specific IP address from which the client would
like its transport address allocated. (The client could do this if
it requesting the second address in a specific port pair). If this
IP address is not a valid one for allocations on the server, the
server MUST reject the request and include a 443 (Invalid IP
Address) error code in the response, or else redirect the request
to a server that is known to support this IP address. If the IP
address is one that is valid for allocations (presumably, the
server is configured to know the set of IP addresses from which it
performs allocations), the server MUST provide an allocation from
that IP address. If the attribute is not present, the selection of
an IP address is at the discretion of the server.</t>
</section>
<section title="REQUESTED-PORT-PROPS">
<t>The server checks for the REQUESTED-PORT-PROPS attribute. If
present, it indicates specific port properties desired by the
client. This attribute is split into two portions: one portion for
port behavior and the other for requested port alignment (whether
the allocated port is odd, even, reserved as a pair, or at the
discretion of the server).</t>
<t>If the port behavior requested is for a Specific Port, the
server MUST attempt to allocate that specific port for the client.
If the specific port is not available (in use or reserved), the
server MUST reject the request with a 444 (Invalid Port) response.
For example, the STUN server could reject a request for a Specific
Port because the port is temporarily reserved as part of an
adjacent pair of ports, or because the requested port is a
well-known port (1-1023).</t>
<t>If the client requests "even" port alignment, the server MUST
attempt to allocate an even port for the client. If an even port
cannot be obtained, the server MUST reject the request with a 444
(Invalid Port) response or redirect to an alternate server. If the
client requests odd port alignment, the server MUST attempt to
allocate an odd port for the client. If an odd port cannot be
obtained, the server MUST reject the request with a 444 (Invalid
Port) response or redirect to an alternate server. Finally, the
"Even port with hold of the next higher port" alignment is similar
to requesting an even port. It is a request for an even port, and
MUST be rejected by the server if an even port cannot be provided,
or redirected to an alternate server. However, it is also a hint
from the client that the client will request the next higher port
with a separate Allocate request. As such, it is a request for the
server to allocate an even port whose next higher port is also
available, and furthermore, a request for the server to not
allocate that one higher port to any other request except for one
that asks for that port explicitly. The server can honor this
request for adjacency at its discretion. The only constraint is
that the allocated port number MUST be even.</t>
<t><list>
<t>Port alignment requests exist for compatibility with
implementations of RTP which predate <xref
target="RFC3550"></xref>. These implementations use the port
numbering conventions in (now obsolete) <xref
target="RFC1889"></xref>.</t>
</list></t>
</section>
<section title="Lifetime">
<t>The server checks for a LIFETIME attribute. If present, it
indicates the lifetime the client would like the server to assign
to the allocation.</t>
<t>If the LIFETIME attribute is malformed, or if the requested
lifetime value is less than 32 seconds, the server replies with an
error response with an error code of XXX Lifetime Malformed or
Invalid.</t>
</section>
<section title="Creating the Allocation">
<t>If any of the requested or desired constraints cannot be met,
whether it be bandwidth, transport protocol, IP address or port,
the server can redirect the client to a different server that may
be able to fulfill the request. This is accomplished using the 300
error response and ALTERNATE-SERVER attribute. If the server does
not redirect and cannot service the request because the server has
reached capacity, it sends a 507 (Insufficient Capacity) response.
The server can also reject the request with a 486 (Allocation
Quota Reached) if the user or client is not authorized to request
additional allocations.</t>
<t>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>Once a port is allocated, the server associates the allocation
with the 5-tuple used to communicate between the client and the
server. For TCP, this amounts to associating the TCP connection
from the TURN client with the allocated transport address.</t>
<t>The new allocation MUST also be associated with the username,
password and realm used to authenticate the request. These
credentials are used in all subsequent requests to ensure that
only the same client can use or modify the allocation it was
given.</t>
<t>In addition, the allocation created by the server is associated
with a set of permissions and a set of channel bindings. Each set
is initially empty.</t>
<t>If the LIFETIME attribute was present in the request, and the
value is larger than the maximum duration the server is willing to
use for the lifetime of the allocation, the server MAY lower it to
that maximum. However, the server MUST NOT increase the duration
requested in the LIFETIME attribute. If there was no LIFETIME
attribute, the server may choose a duration at its discretion. Ten
minutes is RECOMMENDED. In either case, the resulting duration is
added to the current time, and a timer, called the allocation
expiration timer, is set to expire at or after that time. Note
that the LIFETIME attribute in an Allocate request can be zero,
though this is effectively a no-op, since it will create and
destroy the allocation in one transaction.</t>
</section>
<section title="Sending the Allocate Response">
<t>Once the port has been obtained and the allocation expiration
timer has been started, the server generates an Allocate Response
using the general procedures defined in <xref
target="I-D.ietf-behave-rfc3489bis"></xref>, including the ones
for long term authentication. The transport address allocated to
the client MUST be included in the RELAY-ADDRESS attribute in the
response. In addition, this response MUST contain the
XOR-MAPPED-ADDRESS attribute. This allows the client to determine
its reflexive transport address in addition to a relayed transport
address, from the same Allocate request.</t>
<t>The server MUST add a LIFETIME attribute to the Allocate
Response. This attribute contains the duration, in seconds, of the
allocation expiration timer associated with this allocation.</t>
<t>The server MUST add a BANDWIDTH attribute to the Allocate
Response. This MUST be equal to the attribute from the request, if
one was present. Otherwise, it indicates a per-allocation limit
that the server is placing on the bandwidth usage on each binding.
Such limits are needed to prevent against denial-of-service
attacks (see <xref target="sec-security"></xref>).</t>
</section>
</section>
<section anchor="sec-subsequent" title="Refresh Requests">
<t>A Refresh request is processed using the general server and long
term authentication procedures in <xref
target="I-D.ietf-behave-rfc3489bis"></xref>. It is used to refresh
and extend an allocation, or to cause an immediate deallocation. It
is processed as follows.</t>
<t>First, the request MUST be authenticated using the same shared
secret as the one associated with the allocation. If the request was
authenticated but not with such a matching credential, the server
MUST generate a Refresh Error Response with a 401 response.</t>
<t>If the Refresh request contains a BANDWIDTH attribute, the server
checks that it can relay the requested volume of traffic.</t>
<t>Finally, a Refresh Request will set a new allocation expiration
timer for the allocation, effectively canceling the previous
allocation expiration timer. As with an Allocate request, the server
MAY utilize a shorter allocation lifetime, but MUST NOT utilize a
longer lifetime.</t>
<t>A success Refresh response MUST contain a LIFETIME attribute. If
its associated Allocate request contained the BANDWIDTH attribute,
or this Refresh request contained a new BANDWIDTH attribute, the
response MUST also contain the BANDWIDTH attribute.</t>
</section>
<!-- close server -->
</section>
<!-- close allocate behavior -->
</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="Forming and Sending an Indication">
<t>When the client has data to send to a peer, it uses a Send
Indication to pass the data to the server. When the server has data to
send to the client, it uses a Data Indication to pass the data to the
client. A client can also use a Send Indication without a DATA
attribute to install or refresh a permission for the specified IP
address. Both indications are formed following the general rules
described in [ref 3489bis] with the extra considerations described
below.</t>
<t>A Send Indication MUST contain a PEER-ADDRESS attribute and MAY
contain a DATA attribute, while a Data Indication MUST contain both
attributes. The PEER-ADDRESS attribute contains the transport address
of the peer to which the data is to be sent (in the case of a Send
Indication) or from which the data was received (in the case of a Data
Indication). This peer address is the transport address of the peer as
seen by the server, which may not be the same as the host transport
address of the peer. The DATA attribute contains the actual
application data. Note that the application data may need to be padded
to ensure the DATA attribute length is a multiple of 4.</t>
<t>No other attributes are included. For example, neither the
FINGERPRINT attribute nor any authentication attributes are included.
The latter holds even if the server is using the Long-Term Credential
Mechanism, since indications cannot be authenticated using this
mechanism.</t>
<t>Both the Send and Data indications MUST be sent using the 5-tuple
of the original allocation. Thus, in the case of the Send Indication,
the source transport address is the client’s host transport
address, the destination transport address is the TURN server address,
and the transport protocol is the same as was used for the Allocate
request. For the Data Indication, the source and destination transport
addresses are the reverse.</t>
</section>
<section title="Receiving an Indication">
<t>When a Send Indication is received at the server, or a Data
Indication is received at the client, the receiver first does the
basic indication processing described in [3489bis]. Once this is done,
it does the processing specific to the Send and Data methods described
below.</t>
<t>A Send Indication MUST contain a PEER-ADDRESS attribute and MAY
contain a DATA attribute, while a Data Indication MUST contain both
attributes. Any other attributes appearing in the message are treated
as unexpected.</t>
<t><list>
<t>TODO: Add check that Send or Data indication arrives with
appropriate 5-tuple. Since this check applies to all STUN
messages, not just Send and Data indications, perhaps this goes
under the general processing section.</t>
</list></t>
</section>
<section title="Relaying">
<t>When the server receives a valid Send Indication contains a DATA
attribute, 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>
<t>the Length field in the UDP header is set to the Length field
of the DATA attribute;</t>
<t>the Checksum field in the UDP header is computed as described
in [RFC 768].</t>
</list>The resulting UDP datagram is then sent to the peer.</t>
<t>When the server receives a valid Send Indication (with or without a
DATA attribute), it also updates the permission associated with the IP
address contained in the PEER-ADDRESS attribute. For a certain
interval after the permission is updated, UDP datagrams received from
peers with source IP address equal to the IP address contained in the
PEER-ADDRESS attribute can be forwarded to the client. Note that only
the IP addresses are considered and the port numbers are irrelevent.
This permission is specific to the allocation and has no affect on any
other allocation. The recommended length of time is 60 seconds from
when the Send Indication is received.</t>
<t>When the server receives a UDP datagram with a destination
transport address corresponding to an active (i.e., still alive)
allocation, then it first checks to see if it is permitted to relay
the datagram. If it is not permitted, the UDP datagram MUST be
discarded.</t>
<t>If relaying is permitted, the server forms and send a Data
Indication as described in <xref
target="sec-forming-indication"></xref>, using the data following the
UDP header as the application data.</t>
</section>
</section>
<!-- Sending and Receiving Data -->
<section anchor="sec-channels" title="Channel Mechanism">
<t>As described in the overview, channel mechanism provides a way for a
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>Once a channel is bound to a peer, the channel binding cannot be
changed. There is no way to unbind a channel or bind it to a different
peer.</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>
<section title="Forming and Sending a ChannelBind Request">
<t>When a client wishes to bind a channel to a peer in an allocation,
it forms a ChannelBind Request. The Request formed following the
general rules described in <xref
target="I-D.ietf-behave-rfc3489bis"></xref> with the extra
considerations described below.</t>
<t>A ChannelBind Request MUST contain both a CHANNEL-NUMBER attribute
and a PEER-ADDRESS attribute. The CHANNEL-NUMBER attribute specifies
the number of the channel that the client wishes to bind to the peer.
The channel number MUST be in the range 0x4000 to 0xFFFE (inclusive)
and the channel MUST NOT be already bound to a different peer. It is
acceptable to rebind a channel to the peer it is already bound to. The
PEER-ADDRESS attribute specifies the peer address to bind the channel
to.</t>
<t>Once formed, the ChannelBind Request is sent using the 5-tuple for
the allocation.</t>
<t>The client SHOULD be prepared to receive ChannelData messages on
the channel as soon as it has sent the ChannelBind Request. Over UDP,
it is possible for the client to receive these before it receives a
ChannelBind Success Response. </t>
<t>Over UDP, the client SHOULD NOT send ChannelData messages on the
channel until it has received a ChannelBind Success Response for the
binding attempt. Sending them before the success response is received
risks having them dropped by the server if he ChannelBind Request was
lost.</t>
</section>
<section title="Receiving a ChannelBind Request and Sending a Response">
<t>When the server receives a ChannelBind Request, it first does the
basic request processing described in <xref
target="I-D.ietf-behave-rfc3489bis"></xref>. Once this is done, it
does the processing specific to the ChannelBind method described
below.</t>
<t>The server checks that the ChannelBind Request contains both a
CHANNEL-NUMBER attribute and a PEER-ADDRESS attribute. If the
PEER-ADDRESS attribute is missing or malformed, then the server
rejects the request with an Error Response containing the error code
XXX “Peer address missing or invalid”. If the
CHANNEL-NUMBER attribute is missing or malformed, or the channel
number is not in the range 0x4000 to 0xFFFE (inclusive), or the
channel is already bound to another peer (already bound to the same
peer is OK) the server rejects the request with an Error Response
containing the error code XXX “Channel number missing or
invalid”. Otherwise, if no errors are detected, the server
replies with a ChannelBind Success Response.</t>
</section>
<section title="Receiving a ChannelBind Response">
<t>When the client receives a ChannelBind response (either success or
error), it processes it as specified in [3489bis]. Any additional
processing is implementation specific.</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="Forming and 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, SHOULD
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 the server receives a valid ChannelData message, it forms a
UDP datagram as follows: the source transport address is the relayed
transport address of the allocation, where the allocation is
determined by the 5-tuple on which the ChannelData message arrived;
the destination transport address is the peer address to which the
channel is bound; the data following the UDP header is the contents of
the data field of the ChannelData message; the Length field in the UDP
header is set to the Length field of the ChannelData message + 8; and
the Checksum field in the UDP header is computed as described in [RFC
768]. The resulting UDP datagram is then sent to the peer.</t>
<t>The server also updates the permission associated with the IP
address part of the peer address to which the UDP datagram is
sent.</t>
<t>When the server receives a UDP datagram with a destination
transport address corresponding to an active (i.e., still alive)
allocation, then it first checks to see if it is permitted to relay
the datagram. If the allocation contains an active permission for the
source IP address (from the IP header) of the received UDP datagram,
then the UDP datagram is permitted. Otherwise, the UDP datagram MUST
be discarded.</t>
<t>To relay the UDP datagram, the server forms and send a ChannelData
message as described in <xref
target="sec-sending-channeldata-msg"></xref></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
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: BANDWIDTH
0x0012: PEER-ADDRESS
0x0013: DATA
0x0016: RELAY-ADDRESS
0x0018: REQUESTED-PORT-PROPS
0x0019: REQUESTED-TRANSPORT
0x0022: REQUESTED-IP
]]></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 field which
MUST be set to 0 on transmission and 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></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="BANDWIDTH">
<t>The bandwidth attribute represents the peak bandwidth, measured in
kilobits per second, that the client expects to use on the allocation
in each direction.</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 most Send Indications and Data
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-PORT-PROPS">
<t>This attribute allows the client to request certain properties for
the port that is allocated by the server. The attribute can be used
with any transport protocol that has the notion of a 16 bit port space
(including TCP and UDP). 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved = 0 | A | Specific Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>The two bits labeled A in the diagram above are for requested port
alignment and have the following meaning:</t>
<figure>
<artwork><![CDATA[
00 no specific port alignment
01 odd port number
10 even port number
11 even port number; reserve next higher port
]]></artwork>
</figure>
<t>If the value of the A field is 00 (no specific port alignment),
then the Specific Port Number field can either be 0 or some non-zero
port number. If the Specific Port Number field is 0, then the client
is not putting any restrictions on the port number it would like
allocated. If the Specific Port Number is some non-zero port number,
then the client is requesting that the server allocate the specified
port and the server MUST provide that port.</t>
<t>If the value of the A field is 01 (odd port number), then the
Specific Port Number field MUST be zero, and the client is requesting
the server allocate an odd-numbered port. The server MUST provide an
odd port number.</t>
<t>If the value of the A field is 10 (even port number), then the
Specific Port number field MUST be zero, and the client is requesting
the server allocate an even-numbered port. The server MUST provide an
even port number.</t>
<t>If the value of the A field is 11 (even port number; reserve next
higher port), then the Specific Port Number field MUST be zero, and
the client is requesting the server allocate an even-numbered port.
The server MUST return an even port number. In addition, the client is
requesting the server reserve the next higher port (i.e., N+1 if the
server allocates port N). The server SHOULD only allocate the N+1 port
number if it is explicitly requested (with a subsequent request
specifying that exact port number by the same TURN client, over a
different alllocation).</t>
<t>In all cases, if a port with the requested properties cannot be
allocated, the server MUST respond with a error response with an error
code of 444 (Invalid Port).</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 is set to zero on transmission and ignored on
receiption. It is reserved for future uses.</t>
</section>
<section title="REQUESTED-IP">
<t>The REQUESTED-IP attribute is used by the client to request that a
specific IP address be allocated by the TURN server. This attribute is
needed since it is anticipated that TURN servers will be multi-homed
so as to be able to allocate more than 64k transport addresses. As a
consequence, a client needing a second transport address on the same
interface as a previous one can use this attribute to request a remote
address from the same TURN server interface as the TURN client's
previous remote address.</t>
<t>The format of this attribute is identical to XOR-MAPPED-ADDRESS.
However, the port component of the attribute MUST be ignored by the
server. If a client wishes to request a specific IP address and port,
it uses both the REQUESTED-IP and REQUESTED-PORT-PROPS attributes.</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="442">(Unsupported Transport Protocol): The Allocate
request asked for a transport protocol to be allocated that is not
supported by the server. If the server is aware of another server
that supports the requested protocol, it SHOULD include the other
server's address in an ALTERNATE-SERVER attribute in the error
response.</t>
<t hangText="443">(Invalid IP Address): The Allocate request asked
for a transport address to be allocated from a specific IP address
that is not valid on the server.</t>
<t hangText="444">(Invalid Port): The Allocate request asked for a
port to be allocated that is not available on the server.</t>
<t hangText="486">(Allocation Quota Reached): The user or client is
not authorized to request additional allocations.</t>
<t hangText="(tbd)">(Channel Number Missing or Invalid): The request
requires a channel number, but the CHANNEL-NUMBER attribute is
missing, or the specified channel number is invalid in some way.</t>
<t hangText="(tbd)">(Peer Address Missing or Invalid): The request
requires a peer transport address, but the PEER-ADDRESS attribute is
missing, or the specified peer transport address is invalid in some
way.</t>
<t hangText="(tbd)">(Lifetime Malformed or Invalid): The LIFETIME
attribute is malformed or the specified lifetime is invalid in some
way.</t>
<t hangText="507">(Insufficient Capacity): The server cannot
allocate a new port for this client as it has exhausted its relay
capacity.</t>
</list></t>
</section>
<!-- error codes -->
<section title="Client Discovery of TURN Servers">
<t>The STUN extensions introduced by TURN differ from the binding
requests defined in <xref target="I-D.ietf-behave-rfc3489bis"></xref> in
that they are sent with additional framing and demand substantial
resources from the TURN server. In addition, it seems likely that
administrators might want to block connections from clients to the TURN
server for relaying separately from connections for the purposes of
binding discovery. As a consequence, TURN runs on a separate port from
STUN. The client discovers the address and port of the TURN server using
the same DNS procedures defined 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 just
"stun".</t>
<t>For example, to find TURN servers in the example.com domain, the TURN
client performs a lookup for '_turn._udp.example.com',
'_turn._tcp.example.com', and '_turns._tcp.example.com' if the STUN
client wants to communicate with the TURN server using UDP, TCP, or TLS
over TCP, respectively.</t>
</section>
<!-- Client Discovery -->
<section anchor="sec-security" title="Security Considerations">
<t>TURN servers allocate bandwidth and port resources to clients, in
contrast to the Binding method defined in <xref
target="I-D.ietf-behave-rfc3489bis"></xref>. Therefore, a TURN server
requires 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 modest per user
limit on the amount of bandwidth that can be allocated. 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 cannot be used by clients for subverting firewall policies. 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 web servers, email servers, SIP 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>One TURN-specific DoS attack bears extra discussion. An attacker who
can corrupt, drop, or cause the loss of a Send or Data indication sent
over UDP, and then forge a Channel Confirmation indication for the
corresponding channel number, can cause a TURN client (server) to start
sending unencapsulated data that the server (client) will discard. Since
indications are not integrity protected, this attack is not prevented by
cryptographic means. However, any attacker who can generate this level
of network disruption could simply prevent a large fraction of the data
from arriving at its destination, and therefore protecting against this
attack does not seem important. The ChannelConfirmation forging attack
is not possible when the client to server communication is over TCP or
TLS over TCP.</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="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-issues" title="Issues">
<t>NOTE to RFC Editor: Please remove this section prior to publication
of this document as an RFC.</t>
<t>This section lists the open and now closed issues in this document.
The descriptions here are brief, and the reader should consult the
corresponding thread on the mailing list for a more in-depth description
of the issue and the resolutions being considered.</t>
<section title="Open Issues">
<t><list style="numbers">
<t>Bandwidth: What should we do with the BANDWIDTH attribute,
which is currently ill-specified? Should we remove it? Or should
we try to come up with a good specification, perhaps using ideas
from RSVP?</t>
<t>Permission Policy: What should the permission policy be?
Address-restricted, as is currently specified in the document? Or
address-and-port-restricted, as many firewalls implement today? Or
should we leave this open to the implementor, under the assumption
that the IT administrator will only allow clients to contact those
servers that implement whatever permission policy the IT
administrator can accept?</t>
<t>Port Adjacency: The spec currently allows a client to request
that the server allocate a port and also reserve the next higher
port number for a possible future allocation (on a different
5-tuple). However, the exact behavior of the server in this case
is ill-specified. For example, must the next-higher-port be
available for the allocation of the lower port number to succeed?
How long is the next-higher-port reserved? 30 seconds? For the
lifetime of the lower-numbered-port's allocation? Or should we
just ditch this feature, since it is difficult to implement, it is
at odds with port randomization, and paired port numbers
applications don't work well with NATs anyway?</t>
<t>Demuxing ChannelData messages: How does a client or server
demux STUN-formatted messages from ChannelData messages? Does it
use the first two bits (as currently specified) or just one bit?
And how many channels do we need anyway? Some people are
questioning the need for any more than 200 channels. If we don't
need many channels, then the demux algorithm might become
simpler.</t>
<t>Deallocating Channels: Do we need a mechanism for deallocating
channels? Some have argued for this feature, because a TURN server
administrator will want a way to recover resources for channels no
longer in active use. If yes, then what is the mechanism? For
example, should a channel binding expire when the corresponding
permission expires?</t>
<t>Permissions and Channel Allocations: Should allocating a
channel for a peer automatically install a permission for that
peer's IP address?</t>
<t>Permission and Allocation Lifetimes: What should the default
permission lifetime be? Should there be a minumum value? Should
there be a way for the client to modify the permission lifetime?
Should there be a way for the client to learn the current
permission lifetime? And what is the relationship of the
permission lifetime to the allocation lifetime? Does it make sense
for the allocation lifetime to be less than the permission
lifetime?</t>
<t>Preserving bits in the IP header: What bits (if any) should be
preserved in the IP header when a packet is relayed by the server?
The bits under consideration are currently the Don't Fragment (DF)
bit, the Explicit Congestion Notification (ECN) bits, and the
DiffServ (DS) bits.</t>
<t>Exceeding the Path MTU Size: TURN adds an overhead of 4 bytes
(ChannelData msg) or 36 bytes (Send or Data Indication), thus
potentially exceeding the path MTU between the client and server.
This could either cause IP fragmentation, or cause the packet to
be dropped if the DF bit is set. Who handles this problem? Does
TURN need to handle this, or is this left up to the application to
handle?</t>
<t>Allowed PEER-ADDRESS values: Should there be any restrictions
on the IP address the client can specify in the PEER-ADDRESS
attribute? Are multicast addresses allowed? What about 0.0.0.0?
Any other restrictions?</t>
<t>Discarding UDP datagrams: If the server discards a received UDP
datagram on the relayed transport address (because there is no
corresponding permission), then does the server send an ICMP
response? If so, what error code does it use? (What does RFC 4787
say about the corresponding situation in NATs? I believe many NATs
silently discard these packets by default, or have a "stealth
mode" that enables this behavior.)</t>
<t>Authentication: Is the use of STUN's Long-Term Authentication
Mechanism by a TURN server mandatory? The document currently
implicitly assumes "yes", but what about someone who wants to
operate a public TURN server?</t>
<t>Re-using the 5-tuple: If an allocation expires, is there any
reason a client should not be able to immediately create a new
allocation using the same 5-tuple?</t>
<t>Password change: Is it possible to change the password for the
Long-Term Authentication mechanism during the lifetime of an
allocation? If so, how is it done?</t>
<t>IPv6: TURN probably works fine in an all IPv6 environment, but
there are a number of mixed IPv4/IPv6 cases that are
ill-specified. As an example, the server needs to check that the
PEER-ADDRESS in a Send Indication is of the same address family as
the relayed transport address. Should we carefully work through
all these cases and make sure we have caught them all, or should
we just state that this document covers the IPv4 case only, and
punt the specification of IPv6 and mixed IPv4/IPv6 operation to
draft-ietf-behave-turn-ipv6? Does the current interest in
resurecting IPv4-to-IPv6 NATs have any impact on TURN?</t>
</list></t>
</section>
<section title="Closed Issues">
<t><list style="numbers">
<t>Channel Allocation: Should TURN use the mechanism proposed by
EKR to allocate channels? RESOLUTION: Yes. Document now reflects
this.</t>
<t>Stateful Allocations: Does a TURN server need to distinguish
between the case where the client retransmits the initial Allocate
Request because the Allocate Response was lost and the case where
the client sends an Allocate Request because it thinks the
allocation does not exist? RESOLUTION: Yes. Document now reflects
this.</t>
<t>Port Range: From what range of port numbers should a TURN
server allocate ports? RESOLUTION: The server SHOULD allocate from
the Dynamic and/or Private Port range unless it is sure it will
not interfere with other apps on the same machine. Document now
reflects this.</t>
<t>Framing Header for STUN-formatted messages: Should TURN use the
framing mechanism for STUN-formatted messages? RESOLUTION: NO.
Document now reflects this. However, see related issues.</t>
<t>Length field in ChannelData header: Over UDP, the length of the
application data field in the ChannelData message can be
determined from the length field in the UDP header. So should the
length field in the ChannelData header be set to zero in this
case? RESOLUTION: No, the ChannelData length field should have the
same semantics over both TCP and UDP. Document now reflects
this.</t>
</list></t>
</section>
</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, Cullen Jennings, Lars Eggert,
Magnus Westerlund, and Eric Rescorla have been particularly helpful,
with Eric also suggesting the channel allocation 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 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"?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.3550"?>
<?rfc include="reference.RFC.1889"?>
<?rfc include="reference.RFC.1918"?>
<?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"?>
<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 | 2026-04-23 04:46:57 |