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Versions: (draft-rosenberg-midcom-turn) 00 01
02 03 04 05 06 07 08 09 10 11 12 13
14 15 16 RFC 5766
Behave J. Rosenberg
Internet-Draft Cisco Systems
Expires: August 31, 2006 R. Mahy
Plantronics
C. Huitema
Microsoft
February 27, 2006
Obtaining Relay Addresses from Simple Traversal of UDP Through NAT
(STUN)
draft-ietf-behave-turn-00
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This specification defines a usage of the Simple Traversal of UDP
Through NAT (STUN) Protocol for asking the STUN server to relay
packets towards a client. This usage is useful for elements behind
NATs whose mapping behavior is address and port dependent. The
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extension purposefully restricts the ways in which the relayed
address can be used. In particular, it prevents users from running
well general purpose servers from ports obtained from the STUN
server.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Overview of Operation . . . . . . . . . . . . . . . . . . . 5
5. Applicability Statement . . . . . . . . . . . . . . . . . . 8
6. Client Discovery of Server . . . . . . . . . . . . . . . . . 9
7. Server Determination of Usage . . . . . . . . . . . . . . . 10
8. New Requests and Indications . . . . . . . . . . . . . . . . 10
8.1 Allocate Request . . . . . . . . . . . . . . . . . . . . . 11
8.1.1 Server Behavior . . . . . . . . . . . . . . . . . . . 11
8.1.2 Client Behavior . . . . . . . . . . . . . . . . . . . 15
8.2 Connect Request . . . . . . . . . . . . . . . . . . . . . 17
8.2.1 Server Behavior . . . . . . . . . . . . . . . . . . . 17
8.2.2 Client Behavior . . . . . . . . . . . . . . . . . . . 18
8.3 Set Active Destination Request . . . . . . . . . . . . . . 18
8.3.1 Server Behavior . . . . . . . . . . . . . . . . . . . 18
8.3.2 Client Behavior . . . . . . . . . . . . . . . . . . . 21
8.4 Send Indication . . . . . . . . . . . . . . . . . . . . . 24
8.4.1 Server Behavior . . . . . . . . . . . . . . . . . . . 24
8.4.2 Client Behavior . . . . . . . . . . . . . . . . . . . 25
8.5 Data Indication . . . . . . . . . . . . . . . . . . . . . 26
8.5.1 Server Behavior . . . . . . . . . . . . . . . . . . . 26
8.5.2 Client Behavior . . . . . . . . . . . . . . . . . . . 26
9. New Attributes . . . . . . . . . . . . . . . . . . . . . . . 27
9.1 LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.2 BANDWIDTH . . . . . . . . . . . . . . . . . . . . . . . . 27
9.3 DESTINATION-ADDRESS . . . . . . . . . . . . . . . . . . . 27
9.4 REMOTE-ADDRESS . . . . . . . . . . . . . . . . . . . . . . 28
9.5 DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.6 RELAY-ADDRESS . . . . . . . . . . . . . . . . . . . . . . 28
9.7 REQUESTED-PORT . . . . . . . . . . . . . . . . . . . . . . 28
9.8 REQUESTED-TRANSPORT . . . . . . . . . . . . . . . . . . . 29
9.9 REQUESTED-IP . . . . . . . . . . . . . . . . . . . . . . . 29
9.10 TIMER-VAL . . . . . . . . . . . . . . . . . . . . . . . 30
10. New Error Response Codes . . . . . . . . . . . . . . . . . . 30
11. Client Procedures . . . . . . . . . . . . . . . . . . . . . 31
11.1 Receiving and Sending Unencapsulated Data . . . . . . . 31
12. Server Procedures . . . . . . . . . . . . . . . . . . . . . 31
12.1 Receiving Data on Allocated Transport Addresses . . . . 31
12.1.1 TCP Processing . . . . . . . . . . . . . . . . . . . 31
12.1.2 UDP Processing . . . . . . . . . . . . . . . . . . . 32
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12.2 Receiving Data on Internal Local Transport Addresses . . 33
12.3 Lifetime Expiration . . . . . . . . . . . . . . . . . . 34
13. Security Considerations . . . . . . . . . . . . . . . . . . 34
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . 36
15. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 36
15.1 Problem Definition . . . . . . . . . . . . . . . . . . . 36
15.2 Exit Strategy . . . . . . . . . . . . . . . . . . . . . 36
15.3 Brittleness Introduced by TURN . . . . . . . . . . . . . 37
15.4 Requirements for a Long Term Solution . . . . . . . . . 38
15.5 Issues with Existing NAPT Boxes . . . . . . . . . . . . 38
16. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 38
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 44
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 44
18.1 Normative References . . . . . . . . . . . . . . . . . . 44
18.2 Informative References . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 45
Intellectual Property and Copyright Statements . . . . . . . 47
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1. Introduction
The Simple Traversal of UDP Through NAT (STUN) [1] provides a suite
of tools for facilitating the traversal of NAT. Specifically, it
defines the Binding Request, 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
[15] is address or address and port dependent (sometimes called "bad"
NATs), the reflexive transport address will not be usable for
corresponding with a peer.
The only way to obtain a 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 addresses are from interfaces on
the relay. When the relay receives a packet on one of these
allocated addresses, the relay forwards it towards the client.
This specification defines a usage of STUN, called the relay usage,
that allows a client to request an address on the STUN server itself,
so that the STUN server acts as a relay. To accomplish that, this
usage defines two new requests - the Allocate request and the Set
Active Destination request. It also defines two indications - Data
and Send. The Allocate request is the principal component of this
usage, and it is used to provide the client with a transport address
that is relayed through the STUN server. A transport address which
relays through an intermediary is called a relayed transport address.
Though a relayed address is highly likely to work when corresponding
with a peer, it comes at high cost to the provider of the STUN
server. 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 [7] is Interactive Connectivity Establishment (ICE) [14].
The mechanism defined here was previously a standalone protocol
called Traversal Using Relay NAT (TURN), and is now defined as a
usage of STUN.
2. Terminology
In this document, the key words MUST, MUST NOT, REQUIRED, SHALL,
SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to
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be interpreted as described in RFC 2119 [2] and indicate requirement
levels for compliant TURN implementations.
3. Definitions
Relayed Transport Address: A transport address that terminates on a
server, and is forwarded towards the client. The STUN Allocate
Request can be used to obtain a relayed transport address, for
example.
4. Overview of Operation
The typical configuration is shown in Figure 1. A 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 STUN server that
implements the relay usage.
/-----\
// STUN \\
| Server |
\\ //
\-----/
+--------------+ Public Internet
................| NAT 2 |.......................
+--------------+
+--------------+ Private NET 2
................| NAT 1 |.......................
+--------------+
/-----\
// STUN \\
| Client |
\\ // Private NET 1
\-----/
Figure 1
The STUN relay usage defines several new messages that add the
ability of the STUN server to act as a relay for packets. The client
sends an Allocate request to the server. This request is
authenticated by the server. The client can include requests for
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specific ports, transport protocols and IP addresses to be allocated
by the STUN server. The STUN server honors these if it can, and then
generates a response to the Allocate request. This response informs
the client of the address and port allocated to it, called the
allocated transport address. This address and port resides on the
STUN server itself.
The allocation will remain active as long as the client refreshes it
with subsequent Allocate requests. A basic negotiation mechanism is
defined which allows the client to request a specific lifetime, and
for the server to lower it and indicate the actual lifetime.
Once the client has obtained the allocated address from the STUN
server, it can use it to receive packets. However, when a packet
arrives at the allocated address, the STUN server does not forward
the packet. Instead, it will only forward a packet received from
some corresponent X if the client had previously sent a packet to X
through the relay. In that way, the relay is much like a NAT itself.
To send a packet through the relay towards some correspondent X, the
client issues a Send Indication to the STUN server. This indication
includes the destination address and port where the packet should be
sent to, and the data to send. The relay takes the data, and sends
it to X. It also adds a permission towards X, so that X can now send
packets to the allocated address, and the STUN server will relay
those towards the client. The clients are relayed towards the client
by encapsulating them in a Data Indication. This is a STUN
Indication which contains the data that was received by the STUN
server, along with the identity of the correspondent.
Since the primary usage of the STUN relay usage is in support of
multimedia communications, efficiency is a key design goal of this
STUN extension. The mechanism described so far will allow a client
behind the NAT to communicate with a correspondent. However, all
packets sent to and from the client will be encapsulated as STUN
Indications; a Send indication for data sent from the client to the
STUN server, and a Data indication for packets from the STUN server
to the client. This encapsulation adds 44 bytes to each packet.
With voice contents typically around 30 bytes (30 milliseconds of
G.729), this is a significant amount of overhead.
To optimize it, the relay usage provides a cut-through technique.
When the client has decided it would like to optimize the
transmission of packets with a particular correspondent, it issues a
Set Active Destination request to the server, and provides the IP
address and port of the correspondent. After a brief time during
which the client and server can determine they are synchronized on
the usage of the mechanism, the server enables an optimized path.
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Packets received from this correspondent are relayed to the client
without encapsulation in a STUN Data indication, and the client can
send unencapsulated packets to the server, which will be forwarded
towards the correspondent. This mechanism requires the STUN server
and client to disambiguate STUN from other packets when received on
the same IP address and port. That is provided by the magic cookie
field in the STUN message. This cookie reduces the likelihood of a
data packet from being confused with a STUN packet to 2.32x10^-10,
which is deemed sufficiently unlikely.
To do all of this, the STUN server will maintain a binding between an
internal 5-tuple and 1 or more external 5-tuples, as shown in
Figure 2. The internal 5-tuple represents the "connection" between
the STUN server and the STUN client. It is the actual connection in
the case of TCP, and in the case of UDP, it is the combination of the
IP address and port from which the STUN client sent its Allocate
Request, with the IP address and port to which that Allocate Request
was sent. The external local transport address is the IP address and
port allocated to the STUN client (the allocated transport address).
The external 5-tuple is the combination of the external local
transport address and the IP address and port of an external client
that the STUN client is communicating with through the STUN server.
Initially, there aren't any external 5-tuples, since the STUN client
hasn't communicated with any other hosts yet. As packets are
received on or sent from the allocated transport address, external
5-tuples are created.
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+---------+
| |
| External|
/ | Client |
// | |
/ | |
// +---------+
/
//
+-+ /
| | /
| | //
+---------+ | | +---------+ / +---------+
| | |N| | | // | |
| STUN | | | | |/ | External|
| Client |----|A|----------| STUN |------------------| Client |
| | | |^ ^| Server |^ ^| |
| | |T|| || || || |
+---------+ | || |+---------+| |+---------+
^ | || | | |
| | || | | |
| +-+| | | |
| | | | |
|
Internal Internal External External
Client Remote Local Local Remote
Performing Transport Transport Transport Transport
Allocations Address Address Address Address
| | | |
+-----+----+ +--------+-------+
| |
| |
Internal External
5-Tuple 5-tuple
Figure 2
5. Applicability Statement
STUN requires all usages to define the applicability of the usage
[1]. This section contains that information for the relay usage.
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The relayed transport address obtained from the Allocate request has
specific properties which limit its applicability. The transport
address will only be 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
small number of hosts (typically one), and only after sending packets
towards those hosts. Because of this limitation, relayed transport
addresses obtained from an Allocate request are only useful when
combined with rendezvous protocols of some sort, which allow the
client to discover the addresses of the hosts it will be
corresponding with. Examples of such protocols include the Session
Initiation Protocol (SIP) [6].
This limitation is purposeful. Because a client must send a packet
to a peer before it can receive packets from that peer, relayed
transport addresses obtained from the Allocate request can not be
used to run general purpose servers, such as a web or email server.
This means that the relay usage can be safely permitted to pass
through NATs and firewalls without fear of compromising the purpose
of having them there in the first place. Indeed, a relayed transport
address obtained from TURN has many of the properties of a transport
address obtained from a NAT whose filtering policies are address
dependent, but whose mapping properties are endpoint independent
[15], and thus "good" NATs. Indeed, to some degree, the relay turns
a bad NAT into a good NAT by, quite ironically, adding another NAT
function - the relay itself.
6. Client Discovery of Server
STUN requires all usages to define the mechanism by which a client
discovers the server [1]. This section contains that information for
the relay usage.
The relay usage differs from the other usages defined in [1] in that
it demands substantial resources from the STUN server. In addition,
it seems likely that administrators might want to block connections
from clients to the STUN server for relaying separated from
connections for the purposes of binding discovery. As a consequence,
the relay usage is defined to run on a separate port from other
usages. The client discovers the address and port of the STUN server
for the relay usage using the same DNS procedures defined in [1], but
using an SRV service name of "stun-relay" instead of just "stun".
[[TODO: Still need to sort out discovery for TLS vs. non-TLS, usage
of NAPTR, and so on.]]
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7. Server Determination of Usage
STUN requires all usages to define the mechanism by which the server
determines the usage [1]. This section contains that information for
the relay usage.
The relay usage is defined by a specific set of requests and
indications. As a consequence, the server knows that this usage in
being used because those request and indications were used.
8. New Requests and Indications
This usage defines two new requests (along with their success and
error responses) and two indications. It also defines processing
rules for the STUN server and client on receipt of non-STUN messages.
See Section 11 and Section 12
The new messages are:
0x0003 : Allocate Request
0x0103 : Allocate Response
0x0113 : Allocate Error Response
0x0004 : Send Indication
0x0115 : Data Indication
0x0006 : Set Active Destination Request
0x0106 : Set Active Destination Response
0x0116 : Set Active Destination Error Response
0x0007 : Connect Request
0x0107 : Connect Response
0x0117 : Connect Error Response
The server will receive the Allocate Request, Send Indiaction and Set
Active Destination Request on the transport address it has advertised
in DNS or that has been provided to clients through configuration.
However, the server will also receive non-STUN packets, meant for
relaying, on this port. STUN packets are disambiguated from data
packets through the MAGIC-COOKIE in the STUN header. Similarly, the
client will receive Allocate Responses, Allocate Error Responses,
Data Indications, Set Active Destination Responses, and Set Active
Destination Error Responses on the ephemeral port it uses to connect
to the STUN server. It will also receive non-STUN packets, relayed
to it by the STUN server, on this port. Like the server, it
disambiguates STUN and non-STUN packets through the presence of the
magic cookie.
[[OPEN ISSUE: The usage of a magic cookie in the STUN header provides
a nice, generic way to disambiguate stun from application packets for
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the turn usage, as well as sip-outbound, ice and other applications.
But, it introduces a problem as a consequence of this generalization.
When TURN is used with ICE, the agents will send p2p stun
connectivity checks through the turn relay. These being valid stun
packets, will also have the same magic cookie, and be processed by
the turn server, rather than the ice agent! The proposed remedy for
this is to use the DESTINATION-ADDRESS attribute in Allocate
requests, indicating the server to which the request is targeted. If
the turn server picks up a packet because of a magic cookie, but the
destination-address is not it or not there, it would forward the
packet as a regular datagram. ]]
8.1 Allocate Request
8.1.1 Server Behavior
The server first processes the request according to the general
request processing rules in [1]. This includes performing
authentication and checking for mandatory unknown attributes. Due to
the fact that the STUN server is allocating resources for processing
the request, Allocate requests MUST be authenticated, and
furthermore, MUST be authenticated using either a shared secret known
between the client and server, or a short term password derived from
it.
Note that Allocate requests, like all other STUN requests, can be
sent to the STUN server over UDP, TCP, or TCP/TLS.
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 whose source and destination transport address
matches the internal remote and local transport addresses of an
existing internal 5-tuple. A subsequent request is one whose source
and destination transport address do not match the internal remote
and local transport address of an existing internal 5-tuple.
8.1.1.1 Initial Requests
The server attempts to allocate transport addresses. It first looks
for the BANDWIDTH attribute for the request. If present, the server
determines whether or not it has sufficient capacity to handle a
binding that will generate the requested bandwidth.
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. First, the server checks for the
REQUESTED-TRANSPORT attribute. This indicates the transport protocol
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requested by the client. This specification defines values for UDP
and TCP. The server MUST allocate 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, the server MUST reject
the request and include a 442 (Unsupported Transport Protocol) in the
response, or else redirect the request. [[OPEN ISSUE: Should we
include a list of supported ones? Is this really an issue? If its
just ever TCP and UDP its not needed. Can always add it later, as
the hooks are here.]]. If the request did not contain a REQUESTED-
TRANSPORT attribute, the server MUST use the same transport protocol
as the request arrived on.
As a consequence of the REQUESTED-TRANSPORT attribute, it is possible
for a client to connect to the server over UDP and request a TCP
transport address, and for it to connect to the server over TCP (and
TLS, which uses TCP) and request a UDP transport address. In such a
case, the server will relay data between them.
Next, the server checks for the REQUESTED-IP attribute. If present,
it indicates a specific interface from which the client would like
its transport address allocated. If this interface 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 was not present,
the selection of an IP address is at the discretion of the server.
Finally, the server checks for the REQUESTED-PORT attribute. If
present, it indicates a specific port property desired by the client.
If the property is for a Specific Port, the server MUST attempt to
allocate that specific port for the client. If the port is not
available, the server MUST reject the request with a 444 (Invalid
Port) response or redirect to an alternate server. If the property
is for an even port, 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 property is for an odd port, 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 is similar to 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
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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 one 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 has to be even.
If any of the requested or desired constraints cannot be met, whether
it be bandwidth, transport protocol, IP address or port, instead of
rejecting the request, the server can alternately 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.
Furthermore, if the clients source port was in the range 1024-65535,
it is RECOMMENDED that the server allocate a port in that range. If
the clients source port was in the range of 1-1024, port selection is
at the discrtion of the administrator. It is RECOMMENDED that a port
in the range of 1024-65535 be allocated. This is one of several ways
to prohibit relayed transport addresses from being used to attempt to
run standard services. These guidelines are meant to be consistent
with [15], since the relay is effectively a NAT.
Once the port is allocated, the server associates it with the
internal 5-tuple and fills in that 5-tuple. The internal remote
transport address of the internal 5-tuple is set to the source
transport address of the Allocate Request. The internal local
transport address of the internal 5-tuple is set to the destination
transport address of the Allocate Request. For TCP, this amounts to
associating the TCP connection from the TURN client with the
allocated transport address.
If the Allocate request was authenticated using a shared secret
between the client and server, this credential MUST be associated
with the allocation. If the request was authenticated using a short
term password derived from a shared secret, that shared secret MUST
be associated with the allocation. This is used in subsequent
Allocate requests to ensure that only the same client can refresh or
modify the characteristics of the allocation it was given.
The allocation created by the Allocate request is also associated
with a transport address, called the active destination. This
transport address is used for forwarding data through the TURN
server, and is described in more detail later. It is initially set
to null when the allocation is created. In addition, the allocation
created by the server is associated with a set of permissions. Each
permission is a specific IP address identifying an external client.
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Initially, this list is null. Send Indications, Connect requests and
Set Active Destination requests add values to this list.
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 default duration at its
discretion. In either case, the resulting duration is added to the
current time, and a timer, called the allocation expiration timer, is
set to fire at or after that time. Section 12.3 discusses behavior
when the timer fires. Note that the LIFETIME attribute in the
request can be zero. This typically happens for subsequent
Allocations, and provides a mechanism to delete the allocation. It
will force the immediate firing of the allocation expiration timer.
Once the port has been obtained from the operating system and the
activity timer started for the port binding, the server generates an
Allocate Response using the general procedures defined in [1]. 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 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.
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.
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 13.
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.
If the allocated port was for TCP, the server MUST be prepared to
receive a TCP connection request on that port.
8.1.1.2 Subsequent Requests
A subsequent Allocate request is one received whose source and
destination IP address and ports match the internal 5-tuple of an
existing allocation. The request is processed used the general
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server procedures in [1] and is processed identically to
Section 8.1.1.1, with a few important exceptions.
First, the request MUST be authenticated using the same shared secret
as the one associated with the allocation, or be authenticated using
a short term password derived from that shared secret. If the
request was authenticated but not with such a matching credential,
the server MUST generate an Allocate Error Response with a 441
response code.
Secondly, if the allocated transport address given out previously to
the client still matches the constraints in the request (in terms of
request ports, IP addresses and transport protocols), the same
allocation granted previously MUST be returned. However, if one of
the constraints is not met any longer, because the client changed
some aspect of the request, the server MUST free the previous
allocation and allocate a new request to the client.
Finally, a subsequent Allocate request will set a new allocation
expiration timer for the allocation, effectively canceling the
previous timer that was running.
8.1.2 Client Behavior
Client behavior for Allocate requests depends on whether the request
is an initial one, for the purposes of obtaining a new relayed
transport address, or a subsequent one, used for refreshing an
existing allocation.
8.1.2.1 Initial Requests
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 [1]. The server will
challenge the request for credentials. The client MAY either provide
its credentials to the server directly, else obtain a short-term set
of credentials using the Shared Secret request, and then use those as
the credentials in the Allocate request.
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 allocation by
including it in the LIFETIME attribute in the request.
The client MAY include a REQUESTED-PORT, REQUESTED-TRANSPORT, or
REQUESTED-IP attribute in the request to obtain specific types of
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transport addresses. Whether these are needed depends on the
application using the relay usage. As an example, the Real Time
Transport Protocol (RTP) [5] requires that RTP and RTCP ports be even
and odd respectively, and contiguous. The REQUESTED-PORT attribute
allows the client to ask the relay for those properties.
Processing of the response follows the general procedures of [1]. A
successful response will include both a RELAY-ADDRESS and MAPPED-
ADDRESS attribute, providing both a relayed transport address and a
reflexive transport address, respectively, to the client. The server
will expire the allocation after LIFETIME seconds have passed if not
refreshed by another Allocate request. The server will allow the
user to send and receive no more than the amount of data indicated in
the BANDWIDTH attribute.
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.
8.1.2.2 Subsequent Requests
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
another Allocate Request if it wishes to keep the allocation.
To perform a refresh, the client generates an Allocate Request as
described in Section 8.1.2.1. If the initial request was
authenticated with a shared secret P that the client holds with the
server, or using a short term password derived from P through a
Shared Secret request, the client MUST use shared secret P, or a
short-term password derived from it, in the subsequent request.
In a successful response, the RELAY-ADDRESS contains the same
transport address as previously obtained, indicating that the binding
has been refreshed. The LIFETIME attribute indicates the amount of
additional time the binding will live without being refreshed. Note
that an error response do not imply that the binding has been
expired, just that the refresh has failed.
If the client wishes to explicitly remove the allocation because it
no longer needs it, it generates a subsequent Allocate request, but
sets the LIFETIME attribute to zero. This will cause the server to
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remove the allocation.
8.2 Connect Request
The Connect Request is used by a client when it has obtained an
allocated transport address that is TCP. The Connect request asks
the server to open a TCP connection to a specified destination
address, included in the request.
8.2.1 Server Behavior
Once the server has identified a request as a Connect request, the
server verifies that it has arrived with a source and destination
transport address that matches the internal remote and local
transport address of an internal 5-tuple associated with an existing
allocation. If there is no matching allocation, the server MUST
generate a 437 (No Binding) Send Error Response.
The request MUST be authenticated using the same shared secret as the
one associated with the allocation, or be authenticated using a short
term password derived from that shared secret. If the request was
authenticated but not with such a matching credential, the server
MUST generate an error response with a 441 response code.
If the allocation is not for TCP, the server MUST reject the request
with a 445 (Operation for TCP Only) response.
If the request does not contain a DESTINATION-ADDRESS attribute, the
server sends a Connect response, but otherwise does nothing.
If the request contains a DESTINATION-ADDRESS attribute, the IP
address contained within it is added to the permissions for this
allocation, if it was not already present. This happens regardless
of whether the subsequent TCP connection attempt succeeds or not.
The server then checks to see if it has any TCP connections in
existence from the allocated transport address to the IP address and
port in DESTINATION-ADDRESS. If it does, the server responds to the
request with a Connect response, indicating to the client that a
connection exists already.
Next, the server attempts to open a TCP connection from the allocated
transport address to the IP address and port in the DESTINATION-
ADDRESS attribute. If the connection succeeds, the server generates
a Connect Response. If the connection attempt fails or times out,
the server generates a Connect Error Response and includes an error
response of 446 (Connection Failure). If the connection attempt is
still pending prior to the the timeout of the STUN transaction, the
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server MUST send a 447 (Connection Timeout) error response. However,
the server continues to wait for the connection to get set up. If it
succeeds, the client holds on to the connection. The client can
retry the request at a later time, and if the connection has been
succesfully setup, it will result in a Success Response as described
above.
8.2.2 Client Behavior
If a client wishes to send data towards a peer on a TCP allocated
transport address, the client must first tell the server to open a
TCP connection towards the destination. To do that, the client sends
a Connect request to the server. The client MUST NOT send this
request for non-TCP allocated transport addresses. The request
SHOULD contain a DESTINATION-ADDRESS attribute indicating the desired
target for the connection attempt.
If the Connect request generates a successful response, it means that
a connection was opened, or was already opened, towards DESTINATION-
ADDRESS. If it generates a Connect Error response with a response
code of 446, it means that the servers attempt at the connection has
failed. If it generates a Connect Error response with a response
code of 447, it means that the server is still trying to connect, but
the attempt could not be completed before the STUN transaction needed
to end. Whether the client wishes to retry depends on the
application using the request. If the client wishes to determine the
disposition of the attempt, it MAY send a Connect request with the
same DESTINATION-ADDRESS at a later time.
[[OPEN ISSUE: yes, this is a hack. STUN transactions were designed
for immediate responses, and so the handshake is two-way, like SIP
non-INVITE. However, I am reluctant to include yet another new
transaction to SIP. The alternative to the above design is to have
the server send a request to the client when the connection
completes.]]
If the Connect request generates a 437, it means that the client's
allocation no longer exists, possibly due to server or network
failures. The client MAY obtain a new allocation if the application
so desires.
8.3 Set Active Destination Request
8.3.1 Server Behavior
The Set Active Destination Request is used by a client to set an
external 5-tuple that will be used as the forwarding destination of
all data that isn't to be processed by the STUN server itself. In
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addition, all data received from that external client will be
forwarded to the STUN client without encapsulation in a Data
Indication.
Once the server has identified a request as a Set Active Destination
request, the server verifies that it has arrived with a source and
destination transport address that matches the internal remote and
local transport address of an internal 5-tuple associated with an
existing allocation. If there is no matching allocation, the server
MUST generate a 437 (No Binding) Send Error Response.
The request MUST be authenticated using the same shared secret as the
one associated with the allocation, or be authenticated using a short
term password derived from that shared secret. If the request was
authenticated but not with such a matching credential, the server
MUST generate an error response with a 441 response code.
If the Set Active Destination request contains a DESTINATION-ADDRESS
attribute, the IP address contained within it is added to the
permissions for this allocation, if it was not already present.
Unfortunately, there is a race condition associated with the active
destination concept. Consider the case where the active destination
is set, and the server is relaying packets towards the client. The
client knows the IP address and port where the packets came from -
the current value of the active destination. The client issues a Set
Active Destination Request to change the active destination, and
receives a response. A moment later, a data packet is received, not
encapsulated in a STUN Data Indication. What is the source if this
packet? Is it the active destination that existed prior to the Set
Active Destination request, or the one after? If the transport
between the client and the STUN server is not reliable, there is no
way to know.
To deal with this problem, a small state machine is used to force a
"cooldown" period during which the server will not relay packets
towards the client without encapsulating them. This cooldown period
gives enough time for the client to be certain that any old data
packets have left the network. Once the cooldown period ends, the
server can begin relaying packets without encapsulation. There is an
instance of this state machine for each allocation.
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+-----+
| | Req Recvd, DA absent
| |
| |
| |
| V
+-----------+
| | timer fires
| | -----------
| None | active=null
| Set |<--------------------------------+
| | |
| | |
+-----------+ |
| | Req Recvd
| | ---------
| Req Recvd, DA present | 439
| ---------------------- | +----+
| active = DA | | |
| | | |
| | | |
V Req Recvd, | | V
+-----------+ DA!=active,absent +-----------+
| | ----------------- | |
| | Set timer | |
| Set |------------------------------>| Trans- |
| | | itioning |
| |<------------------------------| |
| | timer fires | |
+-----------+ ----------- +-----------+
| ^ active=DA
| |
| |
| |
+-----+
Req Recvd, DA=active
Figure 4
When the allocation is originally created, the active destination is
null, and the server sets the state to "None Set". In this state,
the server will relay all received packets in encapsulated form
towards the client. If the server receives a Set Active Destination
request, but the request contained no DESTINATION-ADDRESS attribute,
the state machine stays in the same state. The request is responded
to with a Set Active Destination Response. If, however, the Set
Active Destination request contained a DESTINATION-ADDRESS, the
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server sets the active destination to the transport address from the
DESTINATION-ADDRESS attribute, and enters the "Set" state. The
request is responded to with a Set Active Destination Response. In
this state, the server will relay packets from that transport address
towards the client in unencapsulated form.
If the server receives another Set Active Destination request while
in this state, and the DESTINATION-ADDRESS is present, but has a
value equal to the current active destination, the request causes no
change. The request is responded to with a Set Active Destination
Response. If, however, the request contained a DESTINATION-ADDRESS
which did not match the existing active destination, or omitted the
active destination, the server enters the "transitioning" state. The
request is responded to with a Set Active Destination Response. In
this state, the server will forward all packets to the client in
encapsulated form. In addition, when this state is entered, the
client sets a timer to fire in Ta seconds. If the connection between
the client and server is unreliable, this timer SHOULD be
configurable. It is RECOMMENDED that it be set to three seconds. If
the connection between the client and server is reliable, the timer
SHOULD be set to 0 seconds, causing it to fire immediately. This
makes the transitioning state transient for reliable transports. The
value of the timer used by the server, regardless of the transport
protocol, MUST be included in a TIMER-VAL attribute in the Set Active
Destination response.
If, while in the "transitioning" state, the server receives a Set
Active Destination Request, it generates a Set Active Destination
Error Response that includes a 439 (Transitioning) response code.
Once the timer fires, the server transitions to the "Set" state if
the Set Active Destination request that caused the server to enter
"transitioning" had contained the DESTINATION-ADDRESS. In this case,
the active destination is set to this transport address. If the Set
Active Destination request had not contained a DESTINATION-ADDRESS
attribute, the server enters the "Not Set" state and sets the active
destination to null.
8.3.2 Client Behavior
The Set Active Destination address allows the client to create an
optimized relay function between it and the server. When the server
receives packets from a particular preferred external client, the
server will forward those packets towards the client without
encapsulating them in a Data Indication. Similarly, the client can
send non-STUN packets to the server without encapsulation, and these
are forwarded to the external client. Sending and receiving data in
unencapsulated form is critical for efficiency purposes. One of the
primary use cases for the STUN relay usage is in support of Voice
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over IP (VoIP), which uses very small UDP packets to begin with. The
extra overhead of an additional layer of encapsulation is considered
unacceptable.
The Set Active Destination request is used by the client to provide
the identity of this preferred external client. The request also has
the side effect of adding a permission for the target of the
DESTINATION-ADDRESS.
The Set Active Destination address MAY contain a DESTINATION-ADDRESS
attribute. This attribute, when present, provides the address of the
preferred external client to the server. When absent, it clears the
value of the preferred external client.
In order for the client to know where incoming non-STUN packets were
sent from, and to be sure where non-STUN packets sent to the server
will go to, it is necessary to coordinate the value of the active
destination between the client and the server. As discussed above,
there is a race condition involved in this coordination which
requires a state machine to execute on both the client and the
server.
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+-----+
| | OK Recvd, DA absent
| |
| |
| |
| V
+-----------+
439 Recvd| | timer fires
+------| | -----------
| | None | active=null
| | Set |<--------------------------------+
+----->| | |
| | |
+-----------+ |
| |
| |
| OK Recvd, DA present |
| ---------------------- |
| active = DA |
| |
| |
V OK Recvd, |
+-----------+ DA!=active,absent +-----------+
| | ----------------- | |
| | Set timer | |
| Set |------------------------------>| Trans- |
| | | itioning |
| |<------------------------------| |
| | timer fires | |
+-----------+ ----------- +-----------+
| ^ active=DA
| |
| |
| |
+-----+
439 Recvd,
OK Recvd, DA=active
Figure 5
The state machine is shown in Figure 5. The client starts in the
"None Set" state. When the client is in either the "None Set" or
"Set" state, it can send Set Active Destination requests. The
transitions in the state machines are governed by responses to those
requests. Only success and 439 responses cause changes in state. A
437 response implies that the allocation has been removed, and thus
the state machine destroyed. A client MUST NOT send a new Set Active
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Destination request prior to the receipt of a response to the
previous. The state machine will further limit the transmission of
subsequent Set Active Destination requests.
If, while in the "None Set" state, the client sent a Set Active
Destination request without a DESTINATION-ADDRESS, and got a
successful response, there is no change in state. If a successful
response was received, but there was a DESTINATION-ADDRESS in the
request, the state machine transitions to the "Set" state, and the
client sets the active destination to the value of the DESTINATION-
ADDRESS attribute that was in the request.
If, while in the "Set" state, the client sends a Set Active
Destination request and received a 439 response, it means that there
was a temporal misalignment in the states between client and server.
The client thought that the active destination was updated on the
server, but the server was still in its transitioning state. When
this error is received, the client remains in the "Set" state. The
client SHOULD retry its Set Active Destination request, but no sooner
than 500ms after receipt of the 439 response. In addition, if, while
in the "Set" state, the client sends a Set Active Destination request
whose DESTINATION-ADDRESS attribute equals the current active
destination, and that request generates a success response, the
client remains in the "Set" state.
However, if, while in the "Set" state, the client sends a Set Active
Destination request whose DESTINATION-ADDRESS was either absent or
not equal to the current active destination, and receives a success
response, the client enters the "Transitioning" state. While in this
state, the client MUST NOT send a new Set Active Destination request.
The value of the active destination remains unchanged. In addition,
the client sets a timer. This timer MUST have a value equal to the
value of the TIMER-VAL attribute from the Set Active Destination
response. This is necessary for coordinating the state machines
between client and server.
Once the timer fires, if the DESTINATION-ADDRESS was not absent from
the Set Active Destination request which caused the client to start
the timer, the client moves back to the "Set" state, and then updates
the value of the active destination to the value of DESTINATION-
ADDRESS. If DESTINATION-ADDRESS was absent, the client sets the
active destination to null and enders the "None Set" state.
8.4 Send Indication
8.4.1 Server Behavior
A Send Indication is sent by a client after it has completed its
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Allocate transaction, in order to create permissions in the server
and send data to an external client.
Once the server has identified a message as a Send Indication, the
server verifies that it has arrived with a source and destination
transport address that matches the internal remote and local
transport address of an internal 5-tuple associated with an existing
allocation. If there is no matching allocation, the indication is
discarded. If there was no DESTINATION-ADDRESS, the indication is
discarded. If there was no DATA attribute, the indication is
discarded.
[[OPEN ISSUE: should message integrity checks be done for send? THey
cannot be challenged!]]
The server takes the contents of the DATA attribute present in the
indication. If the allocation was a UDP allocation, the server
creates a UDP packet whose payload equals that content. The server
sets the source IP address of the packet equal to the allocated
transport address. The destination transport address is set to the
contents of the DESTINATION-ADDRESS attribute. The server then sends
the UDP packet. Note that any retransmissions of this packet which
might be needed are not handled by the server. It is the clients
responsibility to generate another Send indication if needed. If the
TURN client hasn't previously sent to this destination IP address and
port, an external 5-tuple is instantiated in the TURN server. Its
local and remote transport addresses, respectively, are set to the
source and destination transport addresses of the UDP packet.
The server then adds the IP address of the DESTINATION-ADDRESS
attribute to the permission list for this allocation.
In the case of a TCP allocation, the server checks if it has an
existing TCP connection open from the allocated transport address to
the address in the DESTINATION-ADDRESS attribute. If so, the server
extracts the content of the DATA attribute and sends it on the
matching TCP connection. If the server doesn't have an existing TCP
connection to the destination, it discards the data and does nothing.
The client must first open a TCP connection with the Connect request
before it can send data.
8.4.2 Client Behavior
Before receiving any UDP or TCP data, a client has to send first.
Prior to the establishment of an active destination, or while the
client is in the transitioning state, transmission of data towards a
peer through the relay is done using the Send Indication. Indeed, if
the client is in the transitioning state, and it wishes to send data
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through the relay, it MUST use a Send indication.
For TCP allocated transport addresses, the client MUST first open a
connection towards an external client with a Connect request prior to
using the Send request. Data sent with a Send request prior to the
opening of a TCP connection is discarded silently by the server.
The Send Indication MUST contain a DESTINATION-ADDRESS attribute,
which contains the IP address and port that the data is being sent
to. The DATA attribute MAY be present, and contains the data that is
to be sent towards DESTINATION-ADDRESS. If absent, the server will
send an empty UDP packet in the case of UDP. In the case of TCP, the
server will do nothing.
Since Send is an Indication, it generates no response. The client
must relay on application layer mechanisms to determine if the data
was received by the peer.
8.5 Data Indication
8.5.1 Server Behavior
A server MUST send data packets towards the client using a Data
Indication under the conditions described in Section 12.1. Data
Indications MUST contain a DATA attribute containing the data to
send, and MUST contain a REMOTE-ADDRESS attribute indicating where
the data came from.
8.5.2 Client Behavior
Once a client has obtained an allocation and created permissions for
a particular external client, the server can begin to relay packets
from that external client towards the client. If the external client
is not the active destination, this data is relayed towards the
client in encapsulated form using the Data Indication.
The Data Indication contains two attributes - DATA and REMOTE-
ADDRESS. The REMOTE-ADDRESS attribute indicates the source transport
address that the request came from, and it will equal the external
remote transport address of the external client. When processing
this data, a client MUST treat the data as if it came from this
address, rather than the stun server itself. The DATA attribute
contains the data from the UDP packet or TCP segment that was
received. Note that the TURN server will not retransmit this
indication over UDP.
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9. New Attributes
The STUN relay usage defines the following new attributes:
0x000d: LIFETIME
0x0010: BANDWIDTH
0x0011: DESTINATION-ADDRESS
0x0012: REMOTE-ADDRESS
0x0013: DATA
0x0016: RELAY-ADDRESS
0x0018: REQUESTED-PORT
0x0019: REQUESTED-TRANSPORT
0x0020: REQUESTED-IP
0x0021: TIMER-VAL
9.1 LIFETIME
The lifetime attribute represents the duration for which the server
will maintain an allocation 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 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.]]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9.3 DESTINATION-ADDRESS
The DESTINATION-ADDRESS is present in Send Indications and Set Active
Destination Requests. It specifies the address and port where the
data is to be sent. It is encoded in the same way as MAPPED-ADDRESS.
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[[OPEN ISSUE: Should some of thes be xor-encoded? I don't see a need
really...]]
9.4 REMOTE-ADDRESS
The REMOTE-ADDRESS is present in Data Indications. It specifies the
address and port from which a packet was received. It is encoded in
the same way as MAPPED-ADDRESS.
9.5 DATA
The DATA attribute is present in 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).
9.6 RELAY-ADDRESS
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 MAPPED-ADDRESS.
9.7 REQUESTED-PORT
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:
x
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Property | Port Filter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The property is an unsigned integer from 0 to 65535 which identifies
the specific property that is desired. The meaning of the port
filter depends on the port property, and is not used for certain port
properties.
This specification defines the following port properties:
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0x0000: Even Port
0x0001: Odd Port
0x0002: Even Port, hold next higher port
0x0003: Specific Port
Even Port is a request to the server to allocate a port with even
parity. The port filter is not used with this property. Odd Port is
a request to the server to allocate a port with odd parity. The port
filter is not used with this property. Even port, with a hold on the
next higher port, is a request to the server to allocate an even
port. Furthermore, the client indicates that it will want the next
higher port as well. As such, the client requests that the server,
if it can, not allocate the next higher port to anyone unless that
port is explicitly requested, which the client will itself do. The
port filter is not used with this property. Finally, the Specific
Port property is a request for a specific port. The port that is
requested is contained in the Port filter.
Extensions to the relay usage can define additional port properties.
[[TODO: Add IANA registry]]
9.8 REQUESTED-TRANSPORT
This attribute is used by the client to request a specific transport
protocol for the allocated transport address. It is a 32 bit
unsigned integer. Its values are:
0x0000 0000: UDP
0x0000 0001: TCP
If an Allocate request is sent over TCP and requests a UDP
allocation, or an Allocate request is sent over UDP and requests a
TCP allocation, the server will relay data between the two
transports.
Extensions to the relay usage can define additional transport
protocols. [[TODO: Add IANA registry]]
9.9 REQUESTED-IP
The REQUESTED-IP attribute is used by the client to request that a
specific IP address be allocated to it. This attribute is needed
since it is anticipated that STUN relays 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 make that request.
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The format of this attribute is identical to MAPPED-ADDRESS.
However, the port component of the attribute is 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 attributes.
9.10 TIMER-VAL
The TIMER-VAL attribute is used only in conjunction with the Set
Active Destination response. It conveys from the server, to the
client, the value of the timer used in the server state machine.
Coordinated values are needed for proper operation of the mechanism.
The attribute is a 32 bit unsigned integer representing the number if
milliseconds used by the server for its timer.
10. New Error Response Codes
The STUN relay usage defines the following new Error response codes:
437 (No Binding): A request was received by the server that
requires an allocation to be in place. However, there is none yet
in place.
439 (Transitioning): A Set Active Destination request was received
by the server. However, a previous request was sent within the
last few seconds, and the server is still transitioning to that
active destination. Please repeat the request later.
441 (Wrong Username): A TURN request was received for an allocated
binding, but it did not use the same username and password that
were used in the allocation. The client must supply the proper
credentials, and if it cannot, it should teardown its binding,
allocate a new one time password, and try again.
442 (Unsupported Transport Protocol): The Allocate request asked
for a transport protocol to be allocated that is not supported by
the server.
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.
444 (Invalid Port): The Allocate request asked for a port to be
allocated that is not available on the server.
445 (Operation for TCP Only): The client tried to send a request
to perform a TCP-only operation on an allocation, and allocation
is UDP.
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446 (Connection Failure): The attempt by the server to open the
connection failed.
447 (Connection Timeout): The attempt by the server to open the
connection could not be completed, and is still in progress.
11. Client Procedures
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 8.1.2.2, but with a
LIFETIME attribute indicating a time of 0.
11.1 Receiving and Sending Unencapsulated Data
Once the active destination has been set, a client will receive both
STUN and non-STUN data on the socket on which the Allocate Request
was sent. If the client receives non-STUN data (disambiguated
through the magic cookie), it MUST be processed as if it had a source
IP address and port equal to the value of the active destination.
In addition, once the active destination has been set, if the client
is in the "Set" state, it MAY send data to the active destination by
sending data on that same socket. Unencapsulated data MUST NOT be
sent while in the "Not Set" or "Transitioning" states. However, it
is RECOMMENDED that the client not send unencapsulated data for
approximately 500 milliseconds after the client enters the "Set"
state. This eliminates any synchronization problems resulting from
network delays. Of course, even if the active destination is set,
the client can send data to that destination at any time by using the
Send Indication.
12. Server Procedures
Besides the processing of the request and indications described
above, this specification defines rules for processing of data
packets received by the STUN server. There are two cases - receipt
of any packets on an allocated address, and receipt of non-STUN data
on its internal local transport address.
12.1 Receiving Data on Allocated Transport Addresses
12.1.1 TCP Processing
If a server receives a TCP connection request on an allocated TCP
transport address, it checks the permissions associated with that
allocation. If the source IP address of the TCP SYN packet match one
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of the permissions, the TCP connection is accepted. Otherwise, it is
rejected. No information is passed to the client about the
acceptance of the connection; rather, data passed to the client with
a source transport address it has not seen before serves this
purpose.
If a server receives data on a TCP connection that terminates on the
allocated TCP transport address, the server checks the value of the
active destination. If it equals the source IP address and port of
the data packet (in other words, if the active destination identifies
the other side of the TCP connection), the server checks the state
machine of the allocation. If the state is "Set", the data is taken
from the TCP connection and sent towards the client in unencapsulated
form. Otherwise, the data is sent towards the client in a Data
Indication, also known as encapsulated form. In this form, the
server MUST add a REMOTE-ADDRESS which corresponds to the external
remote transport address of the TCP connection, and MUST add a DATA
attribute containing the data received on the TCP connection.
Sending of the data towards the client, whether in encapsulated or
unencapsulated form, depends on the linkage with the client. If the
linkage with the client is over UDP, the data is placed in a UDP
datagram and sent over the linkage. Note that the server will not
retransmit this data to ensure reliability. If the linkage with the
client is over TCP, the data is placed into the TCP connection
corresponding to the linkage. If the TCP connection generates an
error (because, for example, the incoming TCP packet rate exceeds the
throughput of the TCP connection to the client), the data is
discarded silently by the server.
Note that, because data is forwarded blindly across TCP bindings, TLS
will successfully operate over a TURN allocated TCP port if the
linkage to the client is also TCP.
12.1.2 UDP Processing
If a server receives a UDP packet on an allocated UDP transport
address, it checks the permissions associated with that allocation.
If the source IP address of the UDP packet matches one of the
permissions, the UDP packet is accepted. Otherwise, it is discarded.
Assuming the packet is accepted, it must be forwarded to the client.
It will be forwarded in either encapsulated or unencapsulated form.
To determine which, the server checks the value of the active
destination. If it equals the source IP address and port of the UDP
packet, the server checks the state machine of the allocation. If
the state is "Set", the data is taken from the UDP payload and sent
towards the client in unencapsulated form. Otherwise, the data is
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sent towards the client in a Data Indication, also known as
encapsulated form. In this form, the server MUST add a REMOTE-
ADDRESS which corresponds to the external remote transport address of
the UDP packet, and MUST add a DATA attribute containing the data
payload of the UDP packet.
Sending of the data towards the client, whether in encapsulated or
unencapsulated form, depends on the linkage with the client. If the
linkage with the client is over UDP, the data is placed in a UDP
datagram and sent over the linkage. Note that the server will not
retransmit this data to ensure reliability. If the linkage with the
client is over TCP, the data is placed into the TCP connection
corresponding to the linkage. If the TCP connection generates an
error (because, for example, the incoming UDP packet rate exceeds the
throughput of the TCP connection), the data is discarded silently by
the server.
12.2 Receiving Data on Internal Local Transport Addresses
If a server receives a UDP packet from the client on its internal
local transport address, and it is coming from an internal remote
transport address associated with an existing allocation, it
represents UDP data that the client wishes to forward. If the active
destination is not set, the server MUST discard the packet. If the
active destination is set, and the allocated transport protocol is
TCP, the server selects the TCP connection from the allocated
transport address to the active destination. The data is then sent
over that connection. If the transmission fails due to a TCP error,
the data is discarded silently by the server. If the active
destination is set, and the allocated transport protocol is UDP, the
server places the data from the client in a UDP payload, and sets the
destination address and port to the active destination. The UDP
packet is then sent with a source IP address and port equal to the
allocated transport address. Note that the server will not
retransmit the UDP datagram.
If a server receives data on a TCP connection to a client, the server
retrieves the allocation bound to that connection. If the active
destination for the allocation is not set, the server MUST discard
the data. If the active destination is set, and the allocated
transport protocol is TCP, the server selects the TCP connection from
the allocated transport address to the active destination. The data
is then sent over that connection. If the transmission fails due to
a TCP error, the data is discarded silently by the server. If the
active destination is set, and the allocated transport protocol is
UDP, the server places the data from the client in a UDP payload, and
sets the destination address and port to the active destination. The
UDP packet is then sent with a source IP address and port equal to
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the allocated transport address. Note that the server will not
retransmit the UDP datagram.
If a TCP connection from a client is closed, the associated
allocation is destroyed. This involves terminating any TCP
connections from the allocated transport address to external clients
(applicable only when the allocated transport address was TCP), and
then freeing the the allocated transport address (and all associated
state maintained by the server) for use by other clients.
Note that the state of the allocation, whether it is "Set", "Not
Set", or "Transitioning", has no bearing on the rules for forwarding
of packets received from clients. Only the value of the active
destination is relevant.
12.3 Lifetime Expiration
When the allocation expiration timer for a binding fires, the server
MUST destroy the allocation. This involves terminating any TCP
connections from the allocated transport address to external clients
(applicable only when the allocated transport address was TCP), and
then freeing the the allocated transport address (and all associated
state maintained by the server) for use by other clients.
[[OPEN ISSUE: This is a change from the previous version, which
allowed data traffic to keep allocations alive. This change was made
based on implementation considerations, as it allows an easier
separation of packet processing and signaling. Is this OK?]]
13. Security Considerations
TODO: Need to spend more time on this.
STUN servers implementing this relay usage allocate bandwidth and
port resources to clients, in constrast to the usages defined in [1].
Therefore, a STUN server providing the relay usage requires
authentication and authorization of STUN requests. This
authentication is provided by mechanisms defined in the STUN
specification itself. In particular, digest authentication and the
usage of short-term passwords, obtained through a digest exchange
over TLS, are available. The usage of short-tem passwords ensures
that the Allocate Requests, which often 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 STUN servers implementing the relay usage allocate resources,
they can be susceptible to denial-of-service attacks. All Allocate
Requests are authenticated, so that an unknown attacker cannot launch
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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 cap 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 as these are possible using botnets, and are
difficult to detect and prevent. Implementors of the STUN relay
usage should keep up with best practices around detection of
anomalous botnet attacks.
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 addresss. Note that the STUN relay usage is not
susceptible to the attacks described in Section 12.2.3, 12.2.4,
12.2.5 or 12.2.6 of RFC 3489 [[TODO: Update references once 3489bis
is more stable]]. 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.
The relay usage cannot be used by clients for subverting firewall
policies. The relay usage has fairly limited applicability,
requiring a user to send a packet to a peer before being able to
receive a packet from that peer. This applies to both TCP and UDP.
Thus, it does not provide a general technique for externalizing TCP
and UDP 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 the relay usage 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.
Confidentiality of the transport addresses learned through Allocate
requests does not appear to be that important, and therefore, this
capability is not provided.
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 STUN server, so that the
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user's IP addresses are never revealed.
TCP transport addresses allocated by Allocate requests will properly
work with TLS and SSL. However, any relay addresses learned through
an Allcoate will not operate properly with IPSec Authentication
Header (AH) [11] in transport mode. IPSec ESP [12] and any tunnel-
mode ESP or AH should still operate.
14. IANA Considerations
TODO.
15. 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 [13].
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.
15.1 Problem Definition
From RFC 3424 [13], 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.
15.2 Exit Strategy
From [13], any UNSAF proposal must provide:
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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) [14] 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
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.
15.3 Brittleness Introduced by TURN
From [13], 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
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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.
15.4 Requirements for a Long Term Solution
From [13]}, 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.
15.5 Issues with Existing NAPT Boxes
From [13], 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.
16. Example
In this example, a client is behind a NAT. The client has a private
address of 10.0.1.1. The STUN server is on the public side of the
NAT, and is listening for STUN relay requests on 192.0.2.3:8776. The
public side of the NAT has an IP address of 192.0.2.1. The client is
attempting to send a SIP INVITE to a peer, and wishes to allocate an
IP address and port for inclusion in the SDP of the INVITE.
Normally, TURN would be used in conjunction with ICE when applied to
SIP. For simplicities sake, TURN is showed without ICE.
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The client communicates with a SIP user agent on the public network.
This user agent uses a 192.0.2.17:12734 for receipt of its RTP
packets.
Client NAT STUN Srvr Called Pary
| | | |
|(1) Allocate | | |
|S=10.0.1.1:4334 | | |
|D=192.0.2.3:8776 | | |
|------------------>| | |
| | | |
| | | |
| | | |
| |(2) Allocate | |
| |S=192.0.2.1:63346 | |
| |D=192.0.2.3:8776 | |
| |------------------>| |
| | | |
| | | |
| | | |
| |(3) Error | |
| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
| |<------------------| |
| | | |
| | | |
| | | |
|(4) Error | | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
| | | |
| | | |
|(5) Allocate | | |
|S=10.0.1.1:4334 | | |
|D=192.0.2.3:8776 | | |
|------------------>| | |
| | | |
| | | |
| | | |
| |(6) Allocate | |
| |S=192.0.2.1:63346 | |
| |D=192.0.2.3:8776 | |
| |------------------>| |
| | | |
| |(7) Response | |
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| |RA=192.0.2.3:32766 | |
| |MA=192.0.2.1:63346 | |
| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
| |<------------------| |
| | | |
|(8) Response | | |
|RA=192.0.2.3:32766 | | |
|MA=192.0.2.1:63346 | | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
| | | |
| | | |
| | | |
|(9) INVITE | | |
|SDP=192.0.2.3:32766| | |
|---------------------------------------------------------->|
| | | |
| | | |
| | | |
| | | |
|(10) 200 OK | | |
|SDP=192.0.2.17:12734 | |
|<----------------------------------------------------------|
| | | |
| | | |
| | | |
| | | |
| | | |
|(11) ACK | | |
|---------------------------------------------------------->|
| | | |
|(12) Send | | |
|DATA=RTP | | |
|DA=192.0.2.17:12734| | |
|S=10.0.1.1:4334 | | |
|D=192.0.2.3:8776 | | |
|------------------>| | |
| | | |
| |(13) Send | |
| |DATA=RTP | |
| |DA=192.0.2.17:12734| |
| |S=192.0.2.1:63346 | |
| |D=192.0.2.3:8776 | |
| |------------------>| |
| | | |
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| | | |
| | | |
| | |(14) RTP |
| | |S=192.0.2.3:32766 |
| | |D=192.0.2.17:12734 |
| | |------------------>|
| | | |
| | | |
| | | |
| | |Permission |
| | |Created |
| | |192.0.2.17 |
| | | |
| | | |
| | | |
| | | |
| | |(15) RTP |
| | |S=192.0.2.17:12734 |
| | |D=192.0.2.3:32766 |
| | |<------------------|
| | | |
| |(16) DataInd | |
| |DATA=RTP | |
| |RA=192.0.2.17:12734| |
| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
| |<------------------| |
| | | |
|(17) DataInd | | |
|DATA=RTP | | |
|RA=192.0.2.17:12734| | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
| | | |
|(18) SetAct | | |
|DA=192.0.2.17:12734| | |
|S=10.0.1.1:4334 | | |
|D=192.0.2.3:8776 | | |
|------------------>| | |
| | | |
| | | |
| |(19) SetAct | |
| |DA=192.0.2.17:12734| |
| |S=192.0.2.1:63346 | |
| |D=192.0.2.3:8776 | |
| |------------------>| |
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| | | |
| | | |
| | | |
| |(20) Response | |
| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
| |<------------------| |
| | | |
| | | |
| | | |
|(21) Response | | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |after 3s
| | | |
| | | |
| | | |
| | | |
| | |(22) RTP |
| | |S=192.0.2.17:12734 |
| | |D=192.0.2.3:32766 |
| | |<------------------|
| | | |
| | | |
| | | |
| |(23) RTP | |
| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
| |<------------------| |
| | | |
| | | |
| | | |
|(24) RTP | | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
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| | | |
Figure 12
The call flow is shown in Figure 12. The client allocates a port
from the local operating system on its private interface, obtaining
4334. It then attempts to secure a port for RTP traffic. RTCP
processing is not shown. The client sends an Allocate request (1)
with a source address (denoted by S) of 10.0.1.1:4334 and a
destination (denoted by D) of 192.0.2.3:8776. This passes through
the NAT (2), which creates a mapping from the 192.0.2.1:63346 to the
source IP address and port of the request, 10.0.1.1:4334. This
request is received at the STUN server, which challenges it (3),
requesting credentials. This response is passed to the client (4).
The client retries the request, this time with credentials (5). This
arrives at the server (6). The request is now authenticated. The
server provides a UDP allocation, 192.0.2.3:32766, and places it into
the RELAY-ADDRESS (denoted by RA) in the response (7). It also
reflects the source IP address and port of the request into the
MAPPED-ADDRESS (denoted by MA) in the response. This passes through
the NAT to the client (8). The client now proceeds to perform a
basic SIP call setup. In message 9, it includes the relay address
into the SDP of its INVITE. The called party responds with a 200 OK,
and includes its IP address - 192.0.2.17:12734. The exchange
completes with an ACK (11).
Next, user A sends an RTP packet. Since the active destination has
not been set, the client decides to use the Send indication. It does
so, including the RTP packet as the contents of the DATA attribute.
The DESTINATION-ADDRESS attribute (denoted by DA) is set to
192.0.2.17:12734, learned from the 200 OK. This is sent through the
NAT (message 12) and arrives at the STUN server (message 13). The
server extracts the data contents, and sends the packet towards
DESTINATION-ADDRESS (message 14). Note how the source address and
port in this packet is 192.0.2.3:32766, the allocated transport
address given to the client. The act of sending the packet with Send
causes the STUN server to install a permission for 192.0.2.17.
Indeed, the called party now sends an RTP packet toward the client
(message 15). This arrives at the STUN server. Since a permission
has been set for the IP address in the source of this packet, it is
accepted. As no active destination is set, the STUN server
encapsulates the contents of the packet in a Data Indication (message
16), and sends it towards the client. The REMOTE-ADDRESS attribute
(denoted by RA) indicates the source of the packet - 192.0.2.17:
12734. This is forwarded through the NAT to the client (message 17).
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The client decides to optimize the path for packets to and from
192.0.2.17:12734. So, it issues a Set Active Destination request
(message 18) with a DESTINATION-ADDRESS of 192.0.2.17:12734. This
passes through the NAT and arrives at the STUN server (message 19).
This generates a successful response (message 20) which is passed to
the client (message 21). At this point, the server and client are in
the transitioning state. A little over 3 seconds later (by default),
the state machines transition back to "Set". Until this point,
packets from the called party would have been relayed back to the
client in Data Indications. Now, the next RTP packet shows up at the
STUN server (message 22). Since the source IP address and port match
the active destination, the RTP packet is relayed towards the client
without encapsulation (message 23 and 24).
17. Acknowledgements
The authors would like to thank Marc Petit-Huguenin for his comments
and suggestions.
18. References
18.1 Normative References
[1] Rosenberg, J., "Simple Traversal of UDP Through Network Address
Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-02 (work
in progress), July 2005.
[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.
18.2 Informative References
[5] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, July 2003.
[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.
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[7] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[8] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[9] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[10] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[11] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[12] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[13] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[14] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
Offer/Answer Protocols", draft-ietf-mmusic-ice-06 (work in
progress), October 2005.
[15] Audet, F. and C. Jennings, "NAT Behavioral Requirements for
Unicast UDP", draft-ietf-behave-nat-udp-04 (work in progress),
September 2005.
Authors' Addresses
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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Rohan Mahy
Plantronics
Email: rohan@ekabal.com
Christian Huitema
Microsoft
One Microsoft Way
Redmond, WA 98052-6399
US
Email: huitema@microsoft.com
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