MIDCOM J. Rosenberg
Internet-Draft J. Weinberger
Expires: September 1, 2003 dynamicsoft
R. Mahy
Cisco Systems
C. Huitema
Microsoft
March 3, 2003
Traversal Using Relay NAT (TURN)
draft-rosenberg-midcom-turn-01
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on September 1, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
Traversal Using Relay NAT (TURN) is a protocol that allows for an
element behind a NAT or firewall to receive incoming data over TCP or
UDP connections. It is most useful for elements behind symmetric NATs
or firewalls that wish to be on the receiving end of a connection to
a single peer. TURN does not allow for users to run servers on well
known ports if they are behind a nat; it supports the connection of a
user behind a nat to only a single peer. In that regard, its role is
to provide the same security functions provided by symmetric NATs and
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firewalls, but to ``turn'' the tables so that the element on the
inside can be on the receiving end, rather than the sending end, of a
connection that is requested by the client.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Applicability Statement . . . . . . . . . . . . . . . . . . 7
5. Overview of Operation . . . . . . . . . . . . . . . . . . . 8
6. Message Overview . . . . . . . . . . . . . . . . . . . . . . 10
7. Server Behavior . . . . . . . . . . . . . . . . . . . . . . 11
7.1 Shared Secret Request . . . . . . . . . . . . . . . . . . . 11
7.2 Allocate Request . . . . . . . . . . . . . . . . . . . . . . 13
7.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.2.2 Initial Requests . . . . . . . . . . . . . . . . . . . . . . 13
7.2.3 Requests for Pre-Allocated Ports . . . . . . . . . . . . . . 17
7.2.4 Subsequent Requests . . . . . . . . . . . . . . . . . . . . 18
7.3 Receiving Packets and Connections . . . . . . . . . . . . . 19
7.4 Lifetime Expiration . . . . . . . . . . . . . . . . . . . . 20
8. Client Behavior . . . . . . . . . . . . . . . . . . . . . . 22
8.1 Discovery . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.2 Obtaining a One Time Password . . . . . . . . . . . . . . . 22
8.3 Allocating a Binding . . . . . . . . . . . . . . . . . . . . 23
8.4 Processing Allocate Responses . . . . . . . . . . . . . . . 24
8.5 Allocating a Pre-Allocated Binding . . . . . . . . . . . . . 25
8.6 Refreshing a Binding . . . . . . . . . . . . . . . . . . . . 26
8.7 Tearing Down a Binding . . . . . . . . . . . . . . . . . . . 26
8.8 Receiving and Sending Data . . . . . . . . . . . . . . . . . 26
9. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 27
9.1 Message Types . . . . . . . . . . . . . . . . . . . . . . . 27
9.2 Message Attributes . . . . . . . . . . . . . . . . . . . . . 27
9.2.1 TRANSPORT-PREFERENCES . . . . . . . . . . . . . . . . . . . 27
9.2.2 LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.2.3 ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . . . . 28
9.2.4 MAGIC-COOKIE . . . . . . . . . . . . . . . . . . . . . . . . 28
9.2.5 BANDWIDTH . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.3 Response Codes . . . . . . . . . . . . . . . . . . . . . . . 29
10. Security Considerations . . . . . . . . . . . . . . . . . . 30
11. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 32
11.1 Problem Definition . . . . . . . . . . . . . . . . . . . . . 32
11.2 Exit Strategy . . . . . . . . . . . . . . . . . . . . . . . 32
11.3 Brittleness Introduced by TURN . . . . . . . . . . . . . . . 33
11.4 Requirements for a Long Term Solution . . . . . . . . . . . 34
11.5 Issues with Existing NAPT Boxes . . . . . . . . . . . . . . 34
12. Requirements Analysis . . . . . . . . . . . . . . . . . . . 35
13. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 36
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Normative References . . . . . . . . . . . . . . . . . . . . 37
Informative References . . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 38
Intellectual Property and Copyright Statements . . . . . . . 40
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1. Introduction
Network Address Translators (NATs), while providing many benefits,
also come with many drawbacks. The most troublesome of those
drawbacks is the fact that they break many existing IP applications,
and make it difficult to deploy new ones. Guidelines [9] have been
developed that describe how to build "NAT friendly" protocols, but
many protocols simply cannot be constructed according to those
guidelines. Examples of such protocols include multimedia
applications and file sharing.
Simple Traversal of UDP Through NAT (STUN) [1] provides one means for
an application to traverse a NAT. STUN allows a client to obtain a
transport address (and IP address and port) which may be useful for
receiving packets from a peer. However, addresses obtained by STUN
may not be usable by all peers. Those addresses work depending on the
topological conditions of the network. Therefore, STUN by itself
cannot provide a complete solution for NAT traversal.
A complete solution requires a means by which a client can obtain a
transport address from which it can receive media from any peer which
can send packets to the public Internet. This can only be
accomplished by relaying data though a server that resides on the
public Internet. This specification describes Traversal Using Relay
NAT (TURN), a protocol that allows a client to obtain IP addresses
and ports from such a relay.
Although TURN will almost always provide connectivity to a client, it
comes at high cost to the provider of the TURN server. It is
therefore desirable to use TURN as a last resort only, preferring
other mechanisms (such as STUN or direct connectivity) when possible.
To accomplish that, the Interactive Connectivity Establishment (ICE)
[13] methodology can be used to discover the optimal means of
connectivity.
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2. Terminology
In this document, the key words MUST, MUST NOT, REQUIRED, SHALL,
SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to
be interpreted as described in RFC 2119 [2] and indicate requirement
levels for compliant TURN implementations.
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3. Definitions
TURN Client: A TURN client (also just referred to as a client) is
an entity that generates TURN requests. A TURN client can be an
end system, such as a Session Initiation Protocol (SIP) [6] User
Agent, or can be a network element, such as a Back-to-Back User
Agent (B2BUA) SIP server. The TURN protocol will provide the STUN
client with IP addresses that route to it from the public
Internet.
TURN Server: A TURN Server (also just referred to as a server) is
an entity that receives TURN requests, and sends TURN responses.
The server is capable of acting as a data relay, receiving data on
the address it provides to clients, and forwarding them to the
clients.
Transport Address: An IP address and port.
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4. Applicability Statement
TURN is useful for applications that require a client to place a
transport address into a protocol message, with the expectation that
the client will be able to receive packets from a single host that
will send to this address. Examples of such protocols include SIP,
which makes use of the Session Description Protocol (SDP) [7]. SDP
carries and IP address on which the client will receive media packets
from its peer. Another example of a protocol meeting this criteria is
the Real Time Streaming Protocol (RTSP) [8].
When a client is behind a NAT, transport addresses obtained from the
local operating system will not be publically routable, and
therefore, not useful in these protocols. TURN allows a client to
obtain a transport address, from a server on the public Internet,
which can be used in protocols meeting the above criteria. However,
the transport addresses obtained from TURN servers are not generally
useful for receiving data from anywhere. They are only useful for
communicating with a single peer. Once a host sends packets to that
transport address, it is ``locked down'', meaning that packets from
other hosts will not be forwarded to the client. This is done
purposefully, so as to prevent TURN from being used to run servers
(such as a web server) on a client behind a NAT. In this way,
enterprises which deploy NATs and firewalls to prevent users from
running servers, can be confident that TURN will not cause any
violations in their enterprise security policies.
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5. Overview of Operation
The typical TURN configuration is shown in Figure 1. A TURN client is
connected to private network 1. This network connects to private
network 2 through NAT 1. Private network 2 connects to the public
Internet through NAT 2. On the public Internet is a TURN server.
/-----\
// TURN \\
| Server |
\\ //
\-----/
+--------------+ Public Internet
................| NAT 2 |.......................
+--------------+
+--------------+ Private NET 2
................| NAT 1 |.......................
+--------------+
/-----\
// TURN \\
| Client |
\\ // Private NET 1
\-----/
Figure 1
TURN is a simple client-server protocol. It is identical in syntax
and general operation to STUN, in order to facilitate a joint
implementation of both. TURN defines a request message, called
Allocate, which asks for a public IP address and port. TURN can run
over UDP and TCP, as it allows for a client to request address/port
pairs for receiving both UDP and TCP.
A TURN client first discovers the address of a TURN server. This can
be preconfigured, or it can be discovered using SRV records [3] This
will allow for different TURN servers for UDP and TCP. Once a TURN
server is discovered, the client sends a TURN Allocate request to the
TURN server. TURN provides a mechanism for mutual authentication and
integrity checks for both requests and responses, based on a shared
secret. Assuming the request is authenticated and has not been
tampered with, the TURN server remembers the source transport address
that the request came from (call this SA), and returns a public
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transport address, PA, in the TURN response. The TURN server is
responsible for guaranteeing that packets sent to PA route to the
TURN server. The TURN server then waits for data on PA. When data is
received (either a UDP packet or a TCP connection request), the TURN
server accepts the connection (in the case of TCP), and then stores
the remote address and port where the data came from (RA). The data
just received, if any, are then forwarded to SA. The TURN server then
acts as a relay. Any data received from SA are forwarded to RA. Any
data sent from RA to PA are sent to SA.
For TCP, the TURN server does not need to examine the data received;
it merely forwards all data between the socket pairs it has
associated together. In the case of UDP, the TURN server looks for a
magic cookie in the first 128 bytes of each UDP packet. If present,
it indicates that the packet is a TURN control packet, used for
keepalives and teardown of the binding. In the case of TCP, if either
side closes a connection, the TURN server closes the other
connection. For both UDP and TCP, the TURN server can also time out a
connection in the event data is not received after some configured
time out period. This period is sent to the client in the TURN
response to the Allocate request.
TURN also allows a client to request an odd or even port when one is
allocated, and for it to pre-allocate the next higher port. This is
useful for securing consecutive ports for usage with the Real Time
Transport Protocol (RTP) [5].
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6. Message Overview
TURN messages are identical to STUN messages in their syntax. TURN
defines three new messages - the Allocate Request, the Allocate
Response, and the Allocate Error Response. TURN also uses the Shared
Secret Request, Shared Secret Response, and Shared Secret Error
Response defined by STUN. TURN makes use of some of the STUN
attributes (MAPPED-ADDRESS, USERNAME, MESSAGE-INTEGRITY, ERROR-CODE,
and UNKNOWN-ATTRIBUTES) and also defines several of its own.
Specifically, TURN adds TRANSPORT-PREFERENCES attribute, which allows
a client to request an odd or even port, and to pre-allocate the next
higher port. It defines the LIFETIME attribute, which allows the TURN
server to tell the client when the binding will be released. It
defines the MAGIC-COOKIE attribute, which allows the TURN client to
find TURN messages in a stream of UDP packets. It defines the
BANDWIDTH attribute, which allows a client to inform the server of
the expected bandwidth usage on the connection. Finally, it defines
the ALTERNATE-SERVER attribute, which allows the server to redirect
the TURN client to connect to an alternate server.
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7. Server Behavior
The server behavior depends on whether the request is a Shared Secret
Request or an Allocate Request.
7.1 Shared Secret Request
Unlike a STUN server, a TURN server provides resources to clients
that connect to it. Therefore, only authorized clients can gain
access to a TURN server. This requires that TURN requests be
authenticated. TURN assumes the existence of a long-lived shared
secret between the client and the TURN server in order to achieve
this authentication. The client uses this long-lived shared secret to
authenticate itself in a Shared Secret Request, sent over TLS. The
Shared Secret Response provides the client with a one-time username
and password. This one-time credential is then used by the server to
authenticate an Allocate Request. The usage of a separate long lived
and one-time credentials prevents dictionary attacks, whereby an
observer of a message and its HMAC could guess the password by an
offline dictionary search.
When a TURN server receives a Shared Secret Request, it first
executes the processing described in the first three paragraphs of
Section 8.2 of STUN. This processing will ensure that the Shared
Secret Request is received over TLS.
Assuming it was, the server checks the Shared Secret Request for a
MESSAGE-INTEGRITY attribute. If not present, the server generates a
Shared Secret Error Response with an ERROR-CODE attribute with
response code 401. That response MUST include a NONCE attribute,
containing a nonce that the server wishes the client to reflect back
in a subsequent Shared Secret Request (and therefore include the
message integrity computation). The response MUST include a REALM
attribute, containing a realm from which the username and password
are scoped [4].
If the MESSAGE-INTEGRITY attribute was present, the server checks for
the existence of the REALM attribute. If the attribute is not
present, the server MUST generate a Shared Secret Error Response.
That response MUST include an ERROR-CODE attribute with response code
434. That response MUST include a NONCE and a REALM attribute.
If the REALM attribute was present, the server checks for the
existence of the NONCE attribute. If the NONCE attribute is not
present, the server MUST generate a Shared Secret Error Response.
That response MUST include an ERROR-CODE attribute with response code
435. That response MUST include a NONCE attribute and a REALM
attribute.
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If the NONCE attribute was present, the server checks for the
existence of the USERNAME attribute. If it was not present, the
server MUST generate a Shared Secret Error Response. The Shared
Secret Error Response MUST include an ERROR-CODE attribute with
response code 432. It MUST include a NONCE attribute and a REALM
attribute.
If the USERNAME is present, the server computes the HMAC over the
request as described in Section 11.2.8 of STUN. The key is computed
as MD5(unq(USERNAME-value) ":" unq(REALM-value) ":" passwd), where
the password is the password associated with the username and realm
provided in the request. If the server does not have a record for
that username within that realm, the server generates a Shared Secret
Error Response. That response MUST include an ERROR-CODE attribute
with response code 436. That response MUST include a NONCE attribute
and a REALM attribute.
This format for the key was chosen so as to enable a common
authentication database for SIP and for TURN, as it is expected
that credentials are usually stored in their hashed forms. [[OPEN
ISSUE: Is support of MD5-sess needed?]]
If the computed HMAC differs from the one from the MESSAGE-INTEGRITY
attribute in the request, the server MUST generate a Shared Secret
Error Response with an ERROR-CODE attribute with response code 431.
This response MUST include a NONCE attribute and a REALM attribute.
If the computed HMAC doesn't differ from the one in the request, but
the nonce is stale, the server MUST generate a Shared Secret Error
Response. That response MUST include an ERROR-CODE attribute with
response code 430. That response MUST include a NONCE attribute and a
REALM attribute.
In all cases, the Shared Secret Error Response is sent over the TLS
connection on which the Shared Secret Request was received.
The server proceeds to authorize the client. The means for
authorization are outside the scope of this specification. It is
anticipated that TURN servers will be run by providers that also
provide an application service, such as SIP or RTSP. In that case, a
user would be authorized to use TURN if they are authorized to use
the application service.
The server then generates a Shared Secret Response as in Section 8.2
of STUN. This response will contain a USERNAME and PASSWORD, which
are used by the client as a short-term shared secret in subsequent
Allocate requests. Note that STUN specifies that the server has to
invalidate this username and password after 30 minutes. This is not
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the case in TURN. In TURN, the server MUST store the allocated
username and password for a duration of at least 30 minutes. Once an
Allocate request has been authenticated using that username and
password, if the result was an Allocate Error Response, the username
and password are discarded. If the result was an Allocate Response,
resulting in the creation of a new binding, the username and password
become associated with that binding. They can only be used to
authenticate Allocate requests sent from the same source transport
address in order to refresh or de-allocate that binding. Once the
binding is deleted, the username and password are discarded.
This policy avoids replay attacks, whereby a recorded Allocate
request is replayed in order to obtain a binding without proper
authentication. It also ensures that existing bindings can be
refreshed without needed to continuously obtain one-time passwords
from the TURN server.
7.2 Allocate Request
7.2.1 Overview
Allocate requests are used to obtain an IP address and port that the
client can use to receive UDP and TCP packets from any host on the
network, even when the client is behind a symmetric NAT. To do this,
a TURN server allocates a local transport address, and passes it to
the client in an Allocate Response. When the server receives packets
on this allocated address, it acts as a relay, and forwards them
towards the source of the Allocate request. The server remembers the
source transport address where that packet came from, and "locks
down". This means that packets sent from the client to the TURN
server are forwarded to that address.
As a result, the server maintains a set of bindings. These bindings
are associations between the five-tuple of received Allocate requests
(source IP address and port, destination IP address and port, and
protocol), called the allocate five-tuple, and another five tuple,
called the remote five-tuple.
The behavior of the server when receiving an Allocate Request depends
on whether the request is an initial one, or a subsequent one. An
initial request is one received with a source transport address which
is not associated with any existing bindings. A subsequent request is
one received that is associated with an existing binding.
7.2.2 Initial Requests
A TURN server MUST be prepared to receive Binding Requests over TCP
and UDP. The port on which to listen is based on the DNS SRV entries
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provided by the server. Typically, this will be XXXX, the default
TURN port. [[OPEN ISSUE: Can we just use the STUN port?]].
The server MUST check the Allocate Request for a MESSAGE-INTEGRITY
attribute. If not present, the server generates a Allocate Error
Response with an ERROR-CODE attribute with response code 401.
If the MESSAGE-INTEGRITY attribute was present, the server checks for
the existence of the USERNAME attribute. If it was not present, the
server MUST generate a Allocate Error Response. The Allocate Error
Response MUST include an ERROR-CODE attribute with response code 432.
If the USERNAME is present, the server computes the HMAC over the
request as described in Section 11.2.8 of STUN. The key is equal to
the password associated with the username in the request, where that
username is a short term username allocated by the TURN server. The
username MUST be one which has been allocated by the server in a
Shared Secret Response, but has not yet been used to authenticate an
Allocate request. If that username is not known by the server, or has
already been used, the server generates an Allocate Error Response.
That response MUST include an ERROR-CODE attribute with response code
430.
If the computed HMAC differs from the one from the MESSAGE-INTEGRITY
attribute in the request, the server MUST generate a Allocate Error
Response with an ERROR-CODE attribute with response code 431.
Assuming the message integrity check passed, processing continues.
The server MUST check for any attributes in the request with values
less than or equal to 0x7fff which it does not understand. If it
encounters any, the server MUST generate an Allocate Error Response,
and it MUST include an ERROR-CODE attribute with a 420 response code.
That response MUST contain an UNKNOWN-ATTRIBUTES attribute listing
the attributes with values less than or equal to 0x7fff which were
not understood.
If the Allocate request arrived over TCP, the Allocate Error Response
is sent on the connection from which the request arrived. If the
Allocate request arrived over UDP, the Allocate Error Response is
sent to the transport address from which the request was received
(i.e., the source IP address and port), and sent from the transport
address on which the request was received (i.e., the destination IP
address and port).
Assuming the Allocate request was authenticated and was well-formed,
the server attempts to allocate transport addresses. It first looks
for the BANDWIDTH attribute for the request. If present, the server
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determines whether or not it has sufficient capacity to handle a
binding that will generate the requested bandwidth. If so, the server
looks for the presence of the TRANSPORT-PREFERENCES attribute in the
request. If the attribute indicates that an even port is requested,
the server attempts to allocate a transport address with an even
port. If the attribute indicates that an odd port is requested, the
server attempts to allocate a transport address with an odd port. If
the attribute indicates that there is no preference for port parity,
or if the TRANSPORT-PREFERENCES attribute was absent, the server
attempts to allocate a port with either parity. The server MUST NOT
allocate ports from the well-known port range (0-1023) and MUST NOT
allocate ports from the user registered port range (1024 through
49151).
This aspect of the protocol helps guarantee that users cannot run
servers (such as a web server, SIP server, or SMTP server) using
TURN.
The TRANSPORT-PREFERENCES attribute can also indicate a preference
for a specific port, pre-allocated previously by a prior Allocate
request. This case is described in Section 7.2.3.
If a port meeting the constraints (including bandwidth) cannot be
allocated, the server MUST generate a Allocate Error Response that
includes an ERROR-CODE attribute with a response code of 300. That
response MAY include an ALTERNATE-SERVER attribute pointing to an
alternate server which can be used by the client.
Assuming a port was allocated according to the preferences (call this
the base port), the server checks to see if the TRANSPORT-PREFERENCES
attribute is present, and indicates a desire to pre-allocate the next
higher port (called the pre-allocated port). If so, the server
attempts to allocate that port from its local operating system. If it
cannot be allocated, the server can do one of two things. First, it
MAY try to allocate a different base port, in the hopes that the next
higher port is available. If the server believes that there are no
adjacent ports meeting the parity constraints present in the request,
the server MAY generate an Allocate Error Response that includes an
ERROR-CODE attribute with a response code of 300. That response MAY
include an ALTERNATE-SERVER attribute pointing to an alternate server
which can be used by the client.
Once a base port is allocated, the server creates a binding for it.
This binding is a mapping between two five-tuples - the allocate
five-tuple and the remote five-tuple. The allocate five-tuple is set
to the five-tuple of the Allocate Request (that is, the protocol of
the allocate five-tuple is set to the protocol of the Allocate
Request (TCP or UDP), the source IP address and port of the allocate
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five-tuple are set to the source IP address and port in the Allocate
Request, and the destination IP address and port of the allocate
five-tuple are set to the destination IP address and port in the
Allocate Request). The protocol in the remote five-tuple is set to
the protocol from the Allocate Request. The source IP address of the
remote five-tuple is set to the interface from which the base port
was allocated. The source port of the remote five-tuple is set to the
base port. If the binding was allocated for TCP, the connection on
which the Allocate request was received is associated with the
allocate five-tuple in the binding.
The server MUST remember the one-time username and password used to
obtain the binding.
If a port was pre-allocated, a binding is also created for it. The
allocate five-tuple is left empty. The protocol in the remote
five-tuple is set to the protocol from the Allocate Request. The
source IP address of the remote five-tuple is set to the interface
from which the pre-allocated port was allocated. The source port of
the remote five-tuple is set to the pre-allocated port. The identity
of the user (defined as the username provided in the Shared Secret
Request used to obtain the one-time password used in the Allocate
Request) is associated with this pre-allocated tuple. Only that user
can perform an allocation for this tuple. Furthermore, a timer is
set. If no allocation is made against this pre-allocation within 5
minutes, the port is released and the binding is deleted.
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 binding, 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 default duration at its discretion. In either cae, the
resulting duration is added to the current time, and a timer is set
to fire at or after that time. Section 7.4 discusses behavior when
the timer fires.
Once the base port has been obtained from the operating system, the
pre-allocated port obtained, and the activity timer started for the
base port binding, the server generates an Allocate Response. The
Allocate Response MUST contain the same transaction ID contained in
the Allocate Request. The length in the message header MUST contain
the total length of the message in bytes, excluding the header. The
Allocate Response MUST have a message type of "Allocate Response".
The server MUST add a MAPPED-ADDRESS attribute to the Allocate
Response. The IP address component of this attribute MUST be set to
the interface from which the base port was allocated. The port
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component of this attribute MUST be set to the base port.
The server MUST add a LIFETIME attribute to the Allocate Response.
This attribute contains the duration, in seconds, of the activity
timer associated with this binding.
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-binding cap that the server is
placing on the bandwidth usage on each binding. Such caps are needed
to prevent against denial-of-service attacks (See Section 10.
The server MUST add, as the final attribute of the request, a
MESSAGE-INTEGRITY attribute. The key used in the HMAC MUST be the
same as that used to validate the request.
The TURN server then sends the response. If the Allocate request was
received over TCP, the response is sent over that TCP connection.
Once the response is sent, the TURN server begins acting as a relay
for that connection (see Section 7.3). If the Allocate request was
received over UDP, the response is sent to the transport address from
which the request was received (i.e., the source IP address and
port), and sent from the transport address on which the request was
received (i.e., the destination IP address and port).
Additionally, if the base port was for UDP, the server MUST be
prepared to receive UDP packets once the TURN response is sent. If
the base port was for TCP, the server MUST be prepared to receive a
TCP connection request on that port. Behavior when either occurs is
described in Section 7.3.
7.2.3 Requests for Pre-Allocated Ports
The TRANSPORT-PREFERENCES attribute of the Allocate Request can
indicate a desire to allocate a port that was previously
pre-allocated by a prior Allocate request. If such an indication is
present, the server checks that this port has been pre-allocated by a
previous Allocate Request. The only user authorized to allocate a
pre-allocated address is the same one that generated the
pre-allocation. Note that the one-time usernames for both requests
(the pre-allocation and the final allocation) will be different.
However, both MUST have been obtained through Shared Secret Requests
authenticated as being sent from the same user.
If the Allocate request arrives on a different protocol than was used
to make the pre-allocation, the server MUST send an Allocate Error
Response. That response MUST contain an ERROR-CODE attribute with a
response code of 400.
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Assuming the requested port has been pre-allocated by the same user,
the server completes the allocation by setting the allocate
five-tuple for the binding to be equal to that of the Allocate
request. The server sets the activity timer for this binding, and
generates an Allocate Response. This response MUST contain a
MAPPED-ADDRESS attribute which contains the interface from which the
pre-allocated port was obtained, along with the pre-allocated port.
The response MUST contain a LIFETIME attribute and a
MESSAGE-INTEGRITY attribute as well.
7.2.4 Subsequent Requests
Once a binding has been created, packets received from the client are
generally forwarded to the remote client. However, if the binding is
UDP, the client can send subsequent Allocate requests to the TURN
server. To determine which packets are for the TURN server, and which
need to be relayed, the server looks at the packet. If the packet is
shorter than 28 bytes, it is not a TURN request. If it is longer than
28 bytes, the server checks bytes 25-28. If these bytes are equal to
the MAGIC-COOKIE, the request is a TURN request. Otherwise, it is a
data packet, and is to be relayed.
The server first authenticates the request. This is done as in
Section 7.2.2. The request MUST be authenticated using the same
one-time username and password used to allocate that binding
previously. That is, the five-tuple from the Allocate request is
compared to the allocate five-tuples in existing bindings. The
matching binding is selected. The one-time username and password
associated with that binding MUST match the ones used in the request.
Any TRANSPORT-PREFERENCE attribute in the request is ignored. An
Allocate Request sent to an existing binding is always a refresh or
deallocation. The server looks for the LIFETIME attribute in the
Allocate Request. If not found, it determines the default refresh
duration, in seconds, for this binding. If the LIFETIME attribute was
present in the request, and the value is larger than the maximum
duration the server is willing to extend the lifetime of the binding,
the server MAY lower it to that maximum. However, the server MUST NOT
increase the duration requested in the LIFETIME attribute. The
resulting duration is added to the current time, and the activity
timer for this binding is reset to fire at or after that time.
Section 7.4 discusses behavior when the timer fires.
Once the timer is set, the server MUST generate an Allocate Response.
The Allocate Response MUST contain the same transaction ID contained
in the Allocate Request. The length in the message header MUST
contain the total length of the message in bytes, excluding the
header. The Allocate Response MUST have a message type of "Allocate
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Response". The response MUST contain a MAGIC-COOKIE as the first
attribute. It MUST contain a MAPPED-ADDRESS which contains the source
IP address and port from the remote five-tuple of the binding. It
MUST contain a LIFETIME attribute which contains the time from now
until the point at which the binding will be deleted. The final
attribute MUST be a MESSAGE-INTEGRITY attribute, which MUST use the
same one-time username and password used to authenticate the request.
The TURN server then sends the response. If the Allocate request was
received over TCP, the response is sent over that TCP connection. If
the Allocate request was received over UDP, the response is sent to
the transport address from which the request was received (i.e., the
source IP address and port), and sent from the transport address on
which the request was received (i.e., the destination IP address and
port).
7.3 Receiving Packets and Connections
If a TURN server receives a TCP connection request on a port it has
allocated, the server retrieves the binding whose remote five-tuple
has a source address and source port that match the IP address and
port to which the connection was made, and whose transport is TCP. If
the destination IP address and port of the remote five-tuple in the
binding are already filled in (which means that a connection was
already made to this tuple), the connection request is rejected.
Otherwise, it is accepted. If the connection is accepted, the server
MUST set the destination IP address and port of the remote five-tuple
to the source IP address and port in the SYN packet. It also
associates this connection with the remote five-tuple.
If a TURN server receives a UDP packet on a port it has allocated,
the server retrieves the binding whose remote five-tuple has a source
address and source port that match the IP address and port to which
the connection was made, and whose transport is UDP. If the
destination IP address and port of the remote five-tuple in the
binding are already filled in, and do not match the source IP address
and port of the UDP packet, the server drops the packet and MAY
generate a port unreachable ICMP error. If the packet was not
discareded, it is forwarded. To forward, the packet is sent with a
source IP address and port equal to the destination IP address and
port in the allocate five-tuple, and with a destination address and
port equal to the source IP address and port in the allocate
five-tuple. If the destination address and port of the remote
five-tuple were not filled in, they are populated at this time. The
server MUST set the destination IP address and port of the remote
five-tuple to the source IP address and port in the UDP packet.
The process of filling in the destination IP address and port of the
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remote five-tuple is called "locking down". Once done, the client can
only send and receive packets with the specific peer from which the
first packet or connection was received.
If a TURN server receives a UDP packet on a port it has allocated,
the server retrieves the binding whose remote five-tuple has a source
address and source port that match the IP address and port to which
the connection was made, and whose transport is UDP. If the
destination IP address and port of the remote five-tuple in the
binding are already filled in, and they match the source IP address
and port of the UDP packet, the server MUST forward the UDP packet as
above.
If a TURN server receives data on a TCP connection that was opened to
a port it had allocated, the server MUST forward this data onto the
connection associated with allocate-tuple in the binding.
If a TURN server receives data on a TCP connection that is associated
with an allocate five-tuple, the binding for that tuple is retrieved.
If the destination IP address and port of that tuple have not been
filled in yet, the data is discarded. If the destination address and
port have been filled in, the connection associated with the remote
five-tuple is obtained, and the data is forwarded on that connection.
Note that, because data is forwarded blindly across TCP bindings, TLS
will successfully operate over a TURN allocated TCP port.
Similarly, if a TURN server receives a UDP packet on one of its
public TURN ports, it checks to see if the source IP address and port
match those of the allocate five-tuples in an existing binding. If
there is a match, the the UDP packet is not a TURN request (see
Section 7.2.4 for details on how this determination is made), the
destination IP address and port in the remote five-tuple of the
binding are checked. If they are not filled in yet, the UDP packet is
discarded. If they are, the packet is forwarded. It is forwarded
using the source IP address and port from the remote five-tuple, and
a destination IP address and port from the remote five-tuple.
If a TCP connection associated with an allocate five-tuple is closed,
the connection associated with the corresponding remote five-tuple is
also closed. At that point, the binding is destroyed. Similarly, if
the TCP connection associated with a remote five-tuple is closed, the
connection associated with the corresponding allocate five-tuple is
closed, and the binding is destroyed.
7.4 Lifetime Expiration
When the activity timer for a binding fires, the server checks to see
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if there has been any activity on the binding since its creation, or
since the last firing of the timer, whichever is more recent.
Activity is defined as connection establishment, or packet
transmission in either direction. If there has been activity, the
timer is set to fire once again in M seconds, where M is the value of
the LIFETIME attribute returned in the most recent Allocate Response
for this binding.
If there has been no activity, the server MUST destroy the binding,
along with its associated one-time password. If the binding was over
TCP, the server MUST close any connections it is holding to the
client and to the remote client.
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8. Client Behavior
Client behavior is broken into several separate steps. First, the
client obtains a one-time username and password. Secondly, it
generates initial Allocate Requests, and processes the responses. It
manages those addresses (refreshing and tearing them down), and
processes data received on those addresses.
8.1 Discovery
Generally, the client will be configured with a domain name of the
provider of the TURN servers. This domain name is resolved to an IP
address and port of using the SRV procedures [3]. When sending a
Shared Secret request, the service name is "turn" and the protocol is
"tcp". RFC 2782 spells out the details of how a set of SRV records
are sorted and then tried. However, it only states that the client
should "try to connect to the (protocol, address, service)" without
giving any details on what happens in the event of failure. Those
details are described here for TURN.
For TURN requests, failure occurs if there is a transport failure of
some sort (generally, due to fatal ICMP errors in UDP or connection
failures in TCP). Failure also occurs if the the request does not
solicit a response after 9.5 seconds. If a failure occurs, the client
SHOULD create a new request, which is identical to the previous, but
has a different transaction ID and MESSAGE-INTEGRITY attribute. That
request is sent to the next element in the list as specified by
RFC~2782.
8.2 Obtaining a One Time Password
In order to allocate addresses, a client must obtain a one-time
username and password from the TURN server. A unique username and
password are required for each distinct address allocated from the
server.
To obtain a one-time username and password, the client generates and
sends a Shared Secret Request. This is done as described in Section
9.2 of STUN. This request will have no attributes, and therefore,
based on the processing in Section 7.1, the server will reject it
with a Shared Secret Error Response with a 401 response code. That
response will contain a NONCE and a REALM. The client SHOULD generate
a new Shared Secret Request (with a new transaction ID), which
contains the NONCE and REALM attributes copied from the 401 response.
The request MUST include the USERNAME attribute, which contains a
username supplied by the user for the specified realm. The request
MUST include a MESSAGE-INTEGRITY attribute as the last attribute. The
key for the HMAC is computed as described in Section 7.1.
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If the response (either to the initial request or to the second
attempt with the credentials) is a Shared Secret Error Response, the
processing depends on the the value of the response code in the
ERROR-CODE attribute. If the response code was a 430, the client
SHOULD generate a new Shared Secret Request, using the username and
password provided by the user, and the REALM and NONCE provided in
the 430 response. For a 431 or 436 response code, the client SHOULD
alert the user. For a 432, 434 and 435 response codes, if the client
had omitted the USERNAME, REALM or NONCE attributes, respectively,
from the previous request, it SHOULD retry, this time including the
USERNAME, NONCE, REALM, and MESSAGE-INTEGRITY attributes. For a 500
response code, the client MAY wait several seconds and then retry the
request. For a 600 response code, the client MUST NOT retry the
request, and SHOULD display the reason phrase to the user. Unknown
attributes between 400 and 499 are treated like a 400, unknown
attributes between 500 and 599 are treated like a 500, and unknown
attributes between 600 and 699 are treated like a 600. Any response
between 100 and 399 MUST result in the cessation of request
retransmissions, but otherwise is discarded.
If a client receives a Shared Secret Response with an attribute whose
type is greater than 0x7fff, the attribute MUST be ignored. If the
client receives a Shared Secret Response with an attribute whose type
is less than or equal to 0x7fff, the response is ignored.
If the response is a Shared Secret Response, it will contain the
USERNAME and PASSWORD attributes. The client can use these to
authenticate an Allocate Request, as described below.
A client MAY send multiple Shared Secret Requests over the same TLS
connection, and MAY do so without waiting for responses to previous
requests. The client SHOULD close its connection when it has
completed allocating usernames and passwords.
8.3 Allocating a Binding
When a client wishes to obtain a transport address, it sends an
Allocate Request to the TURN server. Requests for TCP transport
addresses MUST be sent over a TCP connection, and requests for UDP
transport addresses MUST be sent over UDP.
First, the client obtains a one-time username and password, using the
mechanisms described in Section 8.2. The client then formulates an
Allocate Request. The request MUST contain a transaction ID, unique
for each request, and uniformly and randomly distributed between 0
and 2**128 - 1. The message type of the request MUST be ``Allocate
Request''. The length is set as described in Section 11.1 of STUN.
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The Allocate request MUST contain the MAGIC-COOKIE attribute as the
first attribute. If the client wishes to allocate an odd or even
port, it can do so by including the TRANSPORT-PREFERENCES attribute
in the request. That attribute can also be used by the client if it
wishes to pre-allocate the port one higher.
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.
The client MAY request a particular lifetime for the binding by
including it in the LIFETIME attribute in the request. If the no data
is sent or received on the binding before expiration of the lifetime,
the binding will be deleted by the client.
The client MUST include a USERNAME attribute, containing a username
obtained from a previous Shared Secret Response. The request MUST
include a MESSAGE-INTEGRITY attribute as the last attribute. The key
is equal to the password obtained from the PASSWORD attribute of the
Shared Secret Response. The Allocate Request MUST be sent to the same
IP address and port as the Shared Secret Request. This is because one
time passwords are expected to be host-specific. Rules for
retransmissions for Allocate Requests sent over UDP are identical to
those for STUN Binding Requests. Allocate Requests sent over TCP are
not retransmitted. Transaction timeouts are identical to those for
STUN Binding Requests, independent of the transport protocol.
8.4 Processing Allocate Responses
If the response is an Allocate Error Response, the client checks the
response code from the ERROR-CODE attribute of the response. For a
400 response code, the client SHOULD display the reason phrase to the
user. For a 420 response code, the client SHOULD retry the request,
this time omitting any attributes listed in the UNKNOWN-ATTRIBUTES
attribute of the response. For a 430 response code, the client SHOULD
obtain a new one-time username and password, and retry the Allocate
Request with a new transaction. For 401 and 432 response codes, if
the client had omitted the USERNAME or MESSAGE-INTEGRITY attribute as
indicated by the error, it SHOULD try again with those attributes. A
new one-time username and password is needed in that case. For a 431
response code, the client SHOULD alert the user, and MAY try the
request again after obtaining a new username and password. For a 300
response code, the client SHOULD attempt a new TURN transaction to
the server indicated in the ALTERNATE-SERVER attribute. For a 500
response code, the client MAY wait several seconds and then retry the
request with a new username and password. For a 600 response code,
the client MUST NOT retry the request, and SHOULD display the reason
phrase to the user. Unknown attributes between 400 and 499 are
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treated like a 400, unknown attributes between 500 and 599 are
treated like a 500, and unknown attributes between 600 and 699 are
treated like a 600. Unknown attributes between 300 and 399 are
treated like 300. Any response between 100 and 299 MUST result in the
cessation of any request retransmissions, but otherwise is discarded.
If a client receives a response with an attribute whose type is
greater than 0x7fff, the attribute MUST be ignored. If the client
receives a response with an attribute whose type is less than or
equal to 0x7fff, any request retransmissions MUST cease, but the
entire response is otherwise ignored.
If the response is an Allocate Response, the client MUST check the
response for a MESSAGE-INTEGRITY attribute. If not present, the
client MUST discard the response. If present, the client computes the
HMAC over the response. The key MUST be same as used to compute the
MESSAGE-INTEGRITY attribute in the request. If the computed HMAC
differs from the one in the response, the client MUST discard the
response, and SHOULD alert the user about a possible attack. If the
computed HMAC matches the one from the response, processing
continues.
The MAPPED-ADDRESS in the Binding Response can be used by the client
for receiving packets. The server will expire the binding after
LIFETIME seconds have passed with no activity. The server will allow
the user to send and receive no more than the amount of data
indicated in the BANDWIDTH attribute.
8.5 Allocating a Pre-Allocated Binding
If the initial Allocate Request included TRANSPORT-PREFERENCES that
indicated a desire to pre-allocate the port one-higher, the client
MAY allocate that port at a later time. It MUST do so within 4
minutes of receiving the Allocate Response, or the pre-allocated port
will expire.
To allocate the port, the client generates an Allocate Request as
described in Section 8.3. A new username and password MUST be used
for this allocation. The request MUST contain a TRANSPORT-PREFERENCES
attribute. It MUST indicate an explicit interface and port, whose
value is one higher than the port number returned in the prior
Allocate Response.
Processing of the responses is identical to Section 8.4. However, the
client SHOULD explicitly check that received packets are TURN
responses, as opposed to data packets, using the techniques described
in Section 7.2.4.
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8.6 Refreshing a Binding
If there has been no activity on a UDP binding for a period of time
equalling 3/4 of the lifetime of the binding (as conveyed in the
LIFETIME attribute of the Allocate Response), the client SHOULD
refresh the binding with another Allocate Request if it wishes to
keep it. Note that only UDP bindings can be refreshed. For TCP,
application-specific keepalives are needed.
To perform a refresh, the client generates an Allocate Request as
described in Section 8.3. However, the one-time username and password
used MUST be the same as those used in the successful Allocate
Request for that binding. The client will need to look for the TURN
response amongst the data packets using the MAGIC-COOKIE, as
described in Section 7.2.4. Processing of that response is as defined
in Section 8.4. If the response was an Allocate Response, and the
MAPPED-ADDRESS contains the same transport address as previously
obtained, the binding has been refreshed. The LIFETIME attribute
indicates the amount of additional time the binding will live without
activity. If, however, the response was an Allocate Error Response
with an ERROR-CODE indicating a 430 response, it means that the
binding has expired at the server. The client MAY use the procedures
in Section 8.3 to obtain a new binding (this will require a new
one-time username and password. Other response codes do not imply
that the binding has been expired, just that the refresh has failed.
8.7 Tearing Down a Binding
If a client no longer needs a binding, it SHOULD tear it down. For
TCP, this is done by closing the connection. For UDP, this is done by
performing a refresh, as described in Section 8.6, but with a
LIFETIME attribute indicating a time of 0.
8.8 Receiving and Sending Data
Once a binding has been allocated by an Allocate Response, the client
MUST be prepared to receive data from the socket on which the
Allocate Request was sent. For UDP, the client MUST be prepared to
disambiguate TURN messages from data for a period of 32 seconds
following the first TURN response. This disambiguation is done using
the MAGIC-COOKIE, as described in Section 7.2.4.
Once data has been received, the client MAY send data to its peer by
sending data on that same socket. Sending data on the socket before
data is received will cause the data to be discarded by the server.
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9. Protocol Details
This section presents the detailed encoding of the message types,
attributes, and response codes which are new to TURN. The general
message structure of TURN is identical to STUN [1].
9.1 Message Types
TURN defines three new Message Types:
0x0003 : Allocate Request
0x0103 : Allocate Response
0x0113 : Allocate Error Response
9.2 Message Attributes
TURN defines the following message attributes:
0x000c: TRANSPORT-PREFERENCES
0x000d: LIFETIME
0x000e: ALTERNATE-SERVER
0x000f: MAGIC-COOKIE
0x0010: BANDWIDTH
9.2.1 TRANSPORT-PREFERENCES
The TRANSPORT-PREFERENCES attribute indicates preferences for the
ports allocated by the TURN server. It is either 32 or 96 bits long,
depending on the value of the Typ bits. These bits indicate the
preferences for the allocated port:
0b00: no preferences
0b01: odd port parity
0b10: even port parity
0b11: allocate a pre-allocated port
When the Typ bits are 0b11, the following 64 bits encode the
pre-allocated transport address. They are in the same format used for
MAPPED-ADDRESS.
The P bit indicates a desire for pre-allocating the port one-higher.
If 1, it means pre-allocation is desired. This bit MUST NOT be set to
1 if the Typ bits are 0b11. That is, pre-allocation cannot be done
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again when allocating a previously pre-allocated port.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |P|Typ|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x x x x x x x x| Family | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9.2.2 LIFETIME
The lifetime attribute represents the duration for which the server
will maintain a binding in the absence of data traffic either from or
to the client. It is a 32 bit value representing the number of
seconds remaining until expiration.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9.2.3 ALTERNATE-SERVER
The alternate server represents an alternate IP address and port for
a different TURN server to try. It is encoded in the same way as
MAPPED-ADDRESS.
9.2.4 MAGIC-COOKIE
The MAGIC-COOKIE is used by TURN clients and servers to disambiguate
TURN traffic from data traffic. Its value ix 0x72c64bc6.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1|1|1|0|0|1|0|1|1|0|0|0|1|1|0|0|1|0|0|1|0|1|1|1|1|0|0|0|1|1|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9.2.5 BANDWIDTH
The bandwidth attribute represents the peak bandwidth, measured in
kbits per second, that the client expects to use on the binding. The
value represents the sum in the receive and send directions.
[[Editors note: Need to define leaky bucket parameters for this.]]
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9.3 Response Codes
TURN defines the following new response codes:
300 (Try Alternate): The client should contact an alternate server
for this request.
434 (Missing Realm): The REALM attribute was not present in the
request.
435 (Missing Nonce): The NONCE attribute was not present in the
request.
436 (Unknown Username): The USERNAME supplied in the Shared Secret
Request is not known in the given REALM.
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10. Security Considerations
TURN servers allocate bandwidth and port resources to clients.
Therefore, a TURN server requires authentication and authorization of
TURN requests. This authentication is provided by a client digest
over TLS, which results in the generation of a one-time password that
is used in a single subsequent Allocate Request. This mechanism
protects against eavesdropping attacks and man-in-the-middle attacks.
The usage of one-time passwords ensures that the Allocate Requests,
which do not run over TLS, are not susceptible to offline dictionary
attacks that can be used to guess the long lived shared secret
between the client and the server.
Because TURN servers allocate resources, they can be susceptible to
denial-of-service attacks. All Allocate Requests are authenticated,
so that an unknown attacker cannot launch an attack. An authenticated
attacker can generate multiple Allocate Requests, but each requires a
new one-time username and password. It is RECOMMENDED that servers
implement a cap on the number of one-time passwords that are
allocated to any specific user at a time (around 5 or 10 should be
sufficient). This will prevent floods of Allocate requests from a
single user, in an attempt to use up the resources of the system. A
single malicious user could generate a single Allocate Request,
obtain a binding, and then flood the server with data over this
binding, in an attempt to deny others service. However, this attack
requires the attacker themselves to receive the data being sent at
the server. To ameliorate these kinds of attacks, servers SHOULD
implement a bandwidth cap on each binding (conveyed to the client in
the BANDWIDTH attribute of the Allocate Response), and discard
packets beyond the threshold.
A client will use the transport address learned from the
MAPPED-ADDRESS attribute of the Binding 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 TLS and HMAC-based authentication and integrity
checks provided in TURN. They can guarantee the authenticity and
integrity of the mapped addressses. 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 STUN. These attacks are based on the fact that a STUN server
mirrors the source IP address, which cannot be authenticated. TURN
does not use the source address of the Binding Request, and
therefore, those attacks do not apply.
Confidentiality of the transport addresses learned through TURN does
not appear to be that important, and therefore, this capability is
not provided.
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TURN 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.
TCP transport addresses allocated by TURN will properly work with TLS
and SSL. However, any addresses allocated by TURN will not operate
properly with IPSec Authentication Header (AH) [10] in transport
mode. IPSec ESP [11] and any tunnel-mode ESP or AH should still
operate.
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11. IAB Considerations
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 RFC 3424 [12].
TURN 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. This section meets those
requirements.
11.1 Problem Definition
From RFC 3424 [12], any UNSAF proposal must provide:
Precise definition of a specific, limited-scope problem that is to
be solved with the UNSAF proposal. A short term fix should not
be generalized to solve other problems; this is why "short term
fixes usually aren't".
The specific problem being solved by TURN is for a client, which may
be located behind a NAT of any type, to obtain an IP address and port
on the public Internet, useful for applications that require a client
to place a transport address into a protocol message, with the
expectation that the client will be able to receive packets from a
single host that will send to this address. Both UDP and TCP are
addressed. It is also possible to send packets so that the recipient
sees a source address equal to the allocated address. TURN, by
design, does not allow a client to run a server (such as a web or
SMTP server) using a TURN address. TURN is useful even when NAT is
not present, to provide anonymity services.
11.2 Exit Strategy
From [12], any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better short
term fixes are the ones that will naturally see less and less use
as the appropriate technology is deployed.
It is expected that TURN will be useful indefinitely, to provide
anonymity services. When used to facilitate NAT traversal, TURN does
not iself provide an exit strategy. That is provided by the
Interactive Connectivity Establishment (ICE) [13] mechanism. ICE
allows two cooperating clients to interactively determine the best
addresses to use when communicating. ICE uses TURN-allocated
addresses as a last resort, only when no other means of connectivity
exists. As a result, as NATs phase out, and as IPv6 is deployed, ICE
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will increasingly use other addresses (host local addresses).
Therefore, clients will allocate TURN addresses, but not use them,
and therefore, de-allocate them. Servers will see a decrease in
usage. Once a provider sees that its TURN servers are not being used
at all (that is, no media flows through them), they can simply remove
them. ICE will operate without TURN-allocated addresses.
11.3 Brittleness Introduced by TURN
From [12], any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
TURN introduces brittleness in a few ways. First, it adds another
server element to any system, which adds another point of failure.
TURN requires clients to demultiplex TURN packets and data based on
hunting for a MAGIC-COOKIE in the TURN messages. It is possible (with
extremely small probabilities) that this cookie could appear within a
data stream, resulting in mis-classification. That might introduce
errors into the data stream (they would appear as lost packets), and
also result in loss of a binding. TURN relies on any NAT bindings
existing for the duration of the bindings held by the TURN server.
Neither the client nor the TURN server have a way of reliably
determining this lifetime (STUN can provide a means, but it is
heuristic in nature and not reliable). Therefore, if there is no
activity on an address learned from TURN for some period, the address
might become useless spontaneously.
TURN will result in potentially significant increases in packet
latencies, and also increases in packet loss probabilities. That is
because it introduces an intermediary on the path of a packet from
point A to B, whose location is determined by application-layer
processing, not underlying routing topologies. Therefore, a packet
sent from one user on a LAN to another on the same LAN may do a trip
around the world before arriving. When combined with ICE, some of the
most problematic cases are avoided (such as this example) by avoiding
the usage of TURN addresses. However, when used, this problem will
exist.
Note that TURN does not suffer from many of the points of brittleness
introduced by STUN. TURN will work with all existing NAT types known
at the time of writing, and for the forseeable future. TURN does not
introduce any topological constraints. TURN does not rely on any
heuristics for NAT type classification.
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11.4 Requirements for a Long Term Solution
From [12]}, any UNSAF proposal must provide:
Identify requirements for longer term, sound technical solutions
-- contribute to the process of finding the right longer term
solution.
Our experience with TURN continues to validate our belief in the
requirements outlined in Section 14.4 of STUN.
11.5 Issues with Existing NAPT Boxes
From [12], any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.
A number of NAT boxes are now being deployed into the market which
try and provide "generic" ALG functionality. These generic ALGs hunt
for IP addresses, either in text or binary form within a packet, and
rewrite them if they match a binding. This will interfere with proper
operation of any UNSAF mechanism, including TURN. However, if a NAT
tries to modify a MAPPED-ADDRESS in a TURN Allocate Response, this
will be detected by the client as an attack.
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12. Requirements Analysis
TODO.
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13. Examples
TODO.
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Normative References
[1] Rosenberg, J., Huitema, C., Mahy, R. and J. Weinberger, "STUN -
Simple Traversal of UDP Through Network Address Translators",
draft-ietf-midcom-stun-05 (work in progress), December 2002.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[4] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:
Basic and Digest Access Authentication", RFC 2617, June 1999.
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Informative References
[5] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
[6] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[7] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[8] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[9] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[10] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[11] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[12] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[13] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Nettwork Address Translator (NAT) Traversal
for the Session Initiation Protocol (SIP)",
draft-rosenberg-sipping-ice-00 (work in progress), February
2003.
Authors' Addresses
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
East Hanover, NJ 07936
US
Phone: +1 973 952-5000
EMail: jdrosen@dynamicsoft.com
URI: http://www.jdrosen.net
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Joel Weinberger
dynamicsoft
72 Eagle Rock Avenue
East Hanover, NJ 07936
US
Phone: +1 973 952-5000
EMail: jweinberger@dynamicsoft.com
Rohan Mahy
Cisco Systems
101 Cooper St
Santa Cruz, CA 95060
US
EMail: rohan@cisco.com
Christian Huitema
Microsoft
One Microsoft Way
Redmond, WA 98052-6399
US
EMail: huitema@microsoft.com
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Rosenberg, et al. Expires September 1, 2003 [Page 41]
