One document matched: draft-ietf-behave-nat-behavior-discovery-07.xml
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<rfc category="exp" docName="draft-ietf-behave-nat-behavior-discovery-07"
ipr="pre5378Trust200902">
<front>
<title abbrev="NAT Behavior Discovery">NAT Behavior Discovery Using
STUN</title>
<author fullname="Derek C. MacDonald" initials="D.C." surname="MacDonald">
<organization>CounterPath Solutions, Inc.</organization>
<address>
<postal>
<street>Suite 300, One Bentall Centre, 505 Burrard St</street>
<city>Vancouver</city>
<code>V7X1M3</code>
<region>BC</region>
<country>Canada</country>
</postal>
<phone>+1-604-320-3344</phone>
<email>derek@counterpath.com</email>
</address>
</author>
<author fullname="Bruce B. Lowekamp" initials="B. B." surname="Lowekamp">
<organization>MYMIC LLC</organization>
<address>
<postal>
<street>1040 University Blvd., Suite 100</street>
<city>Portsmouth</city>
<region>VA</region>
<code>23703</code>
<country>USA</country>
</postal>
<email>bbl@lowekamp.net</email>
</address>
</author>
<date day="7" month="July" year="2009" />
<area>Transport Area</area>
<workgroup>BEHAVE</workgroup>
<keyword>nat type diagnostics</keyword>
<keyword>Draft</keyword>
<abstract>
<t>This specification defines an experimental usage of the Simple
Traversal Underneath Network Address Translators (NAT) (STUN) Protocol
that discovers the presence and current behaviour of NATs and firewalls
between the STUN client and the STUN server.</t>
</abstract>
<note title="Requirements Language">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119">RFC 2119</xref>.</t>
</note>
</front>
<middle>
<section anchor="Intro" title="Applicability" toc="default">
<t>This experimental NAT Behavior Discovery STUN usage provides
information about a NAT device's observable transient behavior; it
determines a NAT's behavior with regard to the STUN server used and the
particular client ports used at the instant the test is run. This STUN
usage does not allow an application behind a NAT to make an absolute
determination of the NAT’s characteristics. NAT devices do not
behave consistently enough to predict future behaviour with any
guarantee. Applications requiring reliable reach between two particular
endpoints must establish a communication channel through NAT using
another technique. IETF has proposed standards including <xref
target="I-D.ietf-mmusic-ice">ICE</xref> and <xref
target="I-D.ietf-sip-outbound">OUTBOUND</xref> for establishing
communication channels when a publicly accessible rendezvous service is
available.</t>
<t>The primary uses envisioned for the STUN attributes included in this
draft are diagnostics and real-time tuning of applications. The
techniques possible with this usage are powerful diagnostic tools in the
hands of a network administrator or system programmer trying to
determine the causes of network failure; particularly when behavior
varies by load, destination, or other factors that may be related to NAT
behavior.</t>
<t>This draft also proposes experimental usage of these attributes for
real-time optimization of parameters for protocols in situations where a
publicly accessible rendezvous service is not available. Such a use of
these techniques is only possible when the results are applied as an
optimization and a reliable fallback is available in case the NAT's
behavior becomes more restrictive than determined by the Behavior
Discovery tests. One possible application is role selection in P2P
networks based on statistical experience with establishing direct
connections and diagnosing NAT behavior with a variety of peers. The
experimental question is whether such a test is useful. If a node trying
to join an overlay as a full peer when its NAT prevents sufficient
connectivity and then withdrawing is expensive or leads to unreliable or
poorly performing operation, then even if the behavior discovery check
is only "correct" 75% of the time, its relative cheapness may make it
very useful for optimizing the behavior of the overlay network. Section
<xref format="counter" target="secP2PExperiment"></xref> describes this
experimental application in more detail and discusses how to evaluate
its success or failure.</t>
<t>The applications of this STUN usage are very different than the
original use of <xref target="RFC3489">RFC3489</xref>, which was
intended for static determination of device behavior. The NAT Behavior
Discovery STUN usage makes an explicit statement that it is not, and
cannot be, correct 100% of the time, but is still very useful. More
generally, one of the important differences between 3489 and ICE is that
ICE ensures there is always a fallback to TURN, and thus avoids the
problem experienced by 3489-based applications that tried to determine
in advance whether they would need a relay and what their peer reflexive
address will be, which are both impossible. This STUN usage requires an
application using it to have a fallback, but unlike ICE's focus on the
problems inherent in VoIP sessions, doesn't assume that it will only be
used to establish a connection between a single pair of machines, and so
alternative fallback mechanisms may make sense. For example, in a P2P
application it may be possible to simply switch out of the role where
such connections need to be established or to select an alternative
indirect route if the peer discovers that, in practice, 10% of its
connection attempts fail.</t>
<t>It is submitted to the Internet community as an experimental protocol
that, when applied with appropriate statistical underpinnings and
application behavior that is ultimately based on experienced
connectivity patterns, can lead to more stability and increased
performance than is available without the knowledge it provides.</t>
<t>If a draft specifies the use of any portion of this STUN usage, that
draft MUST document how incorrect information derived using these
methods will be managed, either through identifying when a NAT's
behavior changed or because the protocol uses such knowledge as an
optimization but remains functional when the NAT's behavior changes.</t>
</section>
<section title="Introduction">
<t>The <xref target="RFC5389">Simple Traversal Underneath Network
Address Translators (STUN)</xref> provides a mechanism to discover the
reflexive transport address toward the STUN server, using the Binding
Request. This specification defines the NAT Behavior Discovery STUN
usage, which allows a STUN client to probe the current behaviour of the
NAT/FW devices between the client and the STUN server. This usage
defines new STUN attributes for the Binding Request and Binding
Response.</t>
<t>Many NAT/FW devices do not behave consistently and will change their
behaviour under load and over time. Applications requiring high
reliability must be prepared for the NAT's behaviour to become more
restrictive. Specifically, it has been found that under load NATs may
transition to the most restrictive filtering and mapping behaviour and
shorten the lifetime of new and existing bindings. In short,
applications can discover how bad things currently are, but not how bad
things will get.</t>
<t>Despite this limitation, instantaneous observations are often quite
useful in troubleshooting network problems, and repeated tests over
time, or in known load situations, may be used to characterize a NAT's
behavior. In particular, in the hands of a person knowledgeable about
the needs of an application and the nodes an application needs to
communicate with, it can be a powerful tool.</t>
<section title="Example Diagnostic Use">
<t>Applications that work well in the lab, but fail in a deployment,
are notoriously common within distributed systems. There are few
systems developers who have not had the experience of searching to
determine the difference in the environments for insight as to what
real-network behavior was missed in the testing lab. The behavior
discovery usage offers a powerful tool that can be used to check NAT
and firewall behavior as the application is running. For example, an
application could be designed to perform Behavior Discovery tests
whenever it experiences significant communications problems when
running. Such analysis might be included as part of the diagnostic
information logged by the application.</t>
<t>As they are being used to detect instantaneous behavior for
analysis by an experienced developer or administrator, there are
relatively few concerns about this application of the NAT Behavior
Discovery STUN usage. However, the user should be aware that</t>
<t><list style="symbols">
<t>adding new traffic to new destinations (STUN servers) has the
potential to itself change the behavior of a NAT and</t>
<t>the user must be careful to select a STUN server that is
appropriately located, ideally collocated (or even integrated)
with the communication partners of the application in question,
for the results to be applicable to the network conditions
experienced by the application.</t>
</list></t>
</section>
<section anchor="secP2PExperiment" title="Example Use with P2P Overlays">
<t>An application could use Behavior Discovery in a Peer-to-Peer (P2P)
protocol to determine if a particular endpoint is a reasonable
candidate to participate as a peer or supernode (defined here as a
peer in the overlay that offers services, including message routing,
to other members or clients of the overlay network). This P2P network
application is willing to select supernodes that might be located
behind NATs to avoid the cost of dedicated servers. A supernode
candidate requires that its NAT(s) offer(s) Endpoint-Independent
Filtering. It might periodically re-run tests and would remove itself
as a supernode if its NAT/FW chain lost this characteristic. These
tests could be run with other supernodes acting as STUN servers as
well as with dedicated STUN servers. As many P2P algorithms tolerate
non-transitive connectivity between a portion of their peers,
guaranteed pair-wise reliable reach might be sacrificed in order to
distribute the P2P overlay's load across peers that can be directly
contacted by the majority of users.</t>
<t>Consider an example from a hypothetical P2P protocol in more
detail: When P2P node A starts up, it tests its NAT(s) relative to
other peers already in the overlay. If the results of its testing
indicate A is behind a "good" NAT (with endpoint-independent mapping
and filtering), A will join the overlay and establish connections with
appropriate peers in the overlay to join the overlay's topology.
Although A is reachable by routing messages across the overlay
topology, A will also include in its communication with other nodes
that they may reach it directly using its reflexive IP address (or
addresses) that A discovered in its initial testing. Suppose that
later, node B wants to send a message to A, and B is not a neighbor of
A in the overlay topology. B may send the message directly to A's IP
address and start a timer. If B doesn't receive a response within a
certain amount of time, then it routes the message to A across the
overlay instead and includes a flag that indicates a direct connection
was attempted but failed. (Alternatively, B could simultaneously send
the message to A's IP address and across the overlay, which guarantees
minimum response latency, but can waste bandwidth.) A over time
observes the percentage of successful direct messages it receives out
of those attempted. If the percentage of successful direct connections
is below some threshold (perhaps 75%), then A may stop advertising for
direct connections because it has determined in practice that its NATs
are not providing sufficiently reliable connectivity to justify the
cost of attempting the direct message. But if the percentage is high
enough, A continues to advertise because the successful direct
connections are improving the overlay's performance by reducing the
routing load imposed on the overlay. If at some point, A's NAT(s)
changes behavior, A will notice a change in its percentage of
successful direct connections and may re-evaluate its decision to
advertise a public address. In this hypothetical example, Behavior
Discovery is used for A's initial operating mode selection, but the
actual decision for whether to continue advertising that public
IP/port pair is made based on actual operating data. The results of
the behavior-discovery usage are also used as a performance
optimization, as A is at all times able to establish connectivity
through the overlay if the attempted direct connection fails.</t>
<t><!-- This is based on the assumption that NAT behaviours are not completely
chaotic. NAT(s) must also remain in the Endpoint-Independent Filtering mode
for a reasonable amount of time; long enough to advertise the supernode and
establish peer connections. -->Use of Behavior Discovery for such an
application requires:</t>
<t><list style="symbols">
<t>Use of a protocol capable of offering reliable end-user
performance using unreliable links between peers.</t>
<t>A protocol offering a reliable fallback to connections
attempted based on the results of Behavior Discovery probing.</t>
<t>The application is deployed behind NATs that provide
Endpoint-Independent Filtering and that remain in this mode for an
amount of time sufficient for the application to identify their
behavior, distribute this information to the rest of the overlay,
and provide useful work for the application.</t>
</list></t>
<t>This draft is experimental as applications implementing open
protocols have yet to be deployed in such environments to demonstrate
that these two requirements have been met. However, anecdotal evidence
suggests that NATs targeted at households and small businesses have
stable behaviour, especially when there are few clients behind them.
Numerous P2P applications have been deployed that appear to have these
properties, although their protocols have not yet been subjected to
rigorous evaluation by standards bodies.</t>
</section>
<section title="Experimental Success">
<t>The criteria for an application to successfully demonstrate use of
the NAT Behavior Discovery STUN usage would include:</t>
<t><list style="symbols">
<t>An implementation that relies on this usage to determine its
run-time behavior, most likely using it to determine an initial
choice of options that are then adjusted based on experience with
its network connections.</t>
<t>The implementation must either demonstrate its applicability in
environments where it is realistic to expect a provider to deploy
dedicated STUN servers with multiple IP addresses, or it must
demonstrate duplicating the behavior of such a dedicated STUN
server with two nodes that share the role of providing the
address-changing operations required by this usage.</t>
<t>Experimental evidence that the application of this usage
results in improved behavior of the application in real-world
conditions. The exact metrics for this improvement may vary, some
possibilities include: faster convergence to the proper
parameters, less work to set up initial connections, fewer
reconfigurations required after startup, etc.</t>
<t>A protocol specification that defines how the implementation
applies this usage.</t>
</list>The P2P scenario described above is a likely experimental
test case for this usage, but others applications are possible as
well.</t>
</section>
</section>
<section title="Overview of Operations">
<t>In a typical configuration, a STUN client is connected to a private
network and through one or more NATs to the public Internet. The client
is configured with the address of a STUN server on the public Internet.
The Behavior Discovery usage makes use of SRV records so that a server
may use a different transport address for this usage than for other
usages. This usage does not provide backward compatibility with <xref
target="RFC3489">RFC3489</xref> for either clients or servers.
Implementors of clients that wish to be compliant with RFC3489 servers
should see that specification. Implementors of servers SHOULD NOT
include support for RFC3489 clients as the original uses of that
protocol have been deprecated.</t>
<t>Because the STUN forbids a server from creating a new TCP or TCP/TLS
connection to the client, many tests apply only to UDP. The
applicability of the various tests is indicated below.</t>
<t>The STUN NAT Behavior Discovery usage defines new attributes on the
STUN Binding Request and STUN Binding Response that allow these messages
to be used to diagnose the current behavior of the NAT(s) between the
client and server.</t>
<t>This section provides a descriptive overview of the typical use of
these attributes. Normative behavior is described in Sections <xref
format="counter" target="secClient"></xref>, <xref format="counter"
target="secServer"></xref>, <xref format="counter"
target="secAttrib"></xref>, and <xref format="counter"
target="secResponse"></xref> .</t>
<section title="Determining NAT Mapping">
<t>A client behind a NAT wishes to determine if the NAT it is behind
is currently using endpoint-independent, address-dependent, or address
and port-dependent mapping<xref target="RFC4787"></xref>. The client
performs a series of tests that make use of the OTHER-ADDRESS
attribute; these tests are described in detail in <xref
format="default" target="sec-disc-tests"></xref>. These tests send
binding requests to the alternate address and port of the STUN server
to determine mapping behaviour. These tests can be used for UDP, TCP,
or TCP/TLS connections.</t>
</section>
<section title="Determining NAT Filtering">
<t>A client behind a NAT wishes to determine if the NAT it is behind
is currently using endpoint-independent, address-dependent, or address
and port-dependent filtering<xref target="RFC4787"></xref>. The client
performs a series of tests that make use of the OTHER-ADDRESS and
CHANGE-REQUEST attributes; these tests are described in <xref
format="default" target="sec-disc-tests"></xref>. These tests request
responses from the alternate address and port of the STUN server; a
precondition to these tests is that no binding be established to the
alternate address and port. See below for more information. Because
the NAT does not know that the alternate address and port belong to
the same server as the primary address and port, it treats these
responses the same as it would those from any other host on the
Internet. Therefore, the success of the binding responses sent from
the alternate address and port indicate whether the NAT is currently
performing endpoint-independent filtering, address-dependent
filtering, or address and port-dependent filtering. This test applies
only to UDP datagrams.</t>
</section>
<section title="Binding Lifetime Discovery">
<t>Many systems, such as VoIP, rely on being able to keep a connection
open between a client and server or between peers of a P2P system.
Because NAT bindings expire over time, keepalive messages must be sent
across the connection to preserve it. Because keepalives impose some
overhead on the network and servers, reducing the frequency of
keepalives can be useful.</t>
<t>A normal request-response protocol cannot be used to test binding
lifetime because the initial request resets the binding timer.
Behavior Discover defines the XOR-RESPONSE-TARGET attribute to allow
the client and server to set up a "control channel" using one port on
the client that is used to test the binding lifetime of a different
port allocated on the client. More generally, XOR-RESPONSE-TARGET
allows the client to allocate two ports and request that responses to
queries sent from one port be delivered to the other. The client uses
its second port and the STUN server's alternate address to check if an
existing binding that hasn't had traffic sent on it is still open
after time T. This approach is described in detail in <xref
target="sec-binding-desc"></xref>. This test applies only to UDP
datagrams.</t>
</section>
<section title="Diagnosing NAT Hairpinning">
<t>STUN Binding Requests allow a client to determine whether it is
behind a NAT that supports hairpinning of connections. To perform this
test, the client first sends a Binding Request to its STUN server to
determine its mapped address. The client then sends a STUN Binding
Request to this mapped address from a different port. If the client
receives its own request, the NAT hairpins connections. This test
applies to UDP, TCP, or TCP/TLS connections.</t>
</section>
<section title="Determining Fragment Handling">
<t>Some NATs exhibit different behavior when forwarding fragments than
when forwarding a single-frame datagram. In particular, some NATs do
not hairpin fragments at all and some platforms discard fragments
under load. To diagnose this behavior, STUN messages may be sent with
the PADDING attribute, which simply inserts additional space into the
message. By forcing the STUN message to be divided into multiple
fragments, the NAT's behavior can be observed.</t>
<t>All of the previous tests can be performed with PADDING if a NAT's
fragment behavior is important for an application, or only those tests
which are most interesting to the application can be retested. PADDING
only applies to UDP datagrams. PADDING can not be used with
XOR-RESPONSE-TARGET.</t>
</section>
<section title="Detecting Generic ALGs">
<t>A number of NAT boxes are now being deployed into the market which
try to 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 behavior can be detected
because the STUN server returns both the MAPPED-ADDRESS and
XOR-MAPPED-ADDRESS in the same response. If the result in the two does
not match, there is a NAT with a generic ALG in the path. This test
apples to UDP and TCP, but not TLS over TCP connections.</t>
</section>
</section>
<section anchor="sec-disc-tests" title="Discovery Process">
<t>This section provides a descriptive overview of how the NAT Behavior
Discovery usage primitives allow checks to be made to discover the
current behaviour of the NAT or NATs an application is behind. These
tests can only give the instantaneous behaviour of a NAT; it has been
found that NATs can change behaviour under load and over time. The
results of these tests therefore can be regarded as upper bounds---an
application must assume that NAT behaviour can become more restrictive
at any time. Results from tests performed using a particular port on the
client may also not indicate the behavior experienced by a different
port, as described in<xref format="default"
target="sec-source-port"></xref>.</t>
<t>Definitions for NAT filtering and mapping behaviour are from <xref
target="RFC4787"></xref>. The tests described here are for UDP
connectivity, NAT mapping behaviour, NAT filtering behaviour, and NAT
binding lifetime discovery; additional tests could be designed using
this usage's mechanisms. The tests described below include only tests
that can be performed using a client with a single IP address. A client
with multiple IP addresses (or multiple clients collaborating) behind
the same NAT can combine their probes to test additional aspects of NAT
behavior, such as port overloading. This section provides a descriptive
overview of how the primitives provided by the STUN attributes in this
specification may be used to perform behavior tests.</t>
<t>Normative specifications for the attributes are defined in later
sections.</t>
<section anchor="sec-source-port" title="Source Port Selection">
<t>Proper source port selection is important to ensuring the
usefulness and accuracy of the Behavior Discovery tests. There are two
preconditions for tests:</t>
<t><list style="symbols">
<t>Because mapping behavior can vary on a port-by-port basis, an
application should perform its tests using the source port
intended for use by the application whenever possible. If it
intends to use multiple source ports, it should repeat these tests
for each source port. Such tests should be performed sequentially
to reduce load on the NAT.</t>
<t>Because the results of some diagnostic checks depend on
previous state in the NAT created by prior traffic, the tests
should be performed using a source port that has not generated
recent traffic. Therefore the application should use a random
source port or ensure that no traffic has previously occurred on
the selected port prior to performing tests, generally by
allocating a port and holding it unused for at least 15 minutes
prior to the tests.</t>
</list>Ensuring both of these preconditions can be challenging,
particularly for a device or application wishing to perform Behavior
Discovery tests at startup. The following guidelines are suggested for
reducing the likelihood of problems:</t>
<t><list style="symbols">
<t>An application intended to operate behind a NAT should not
attempt to allocate a specific or well-known port. Because such
software must be designed to interoperate using whatever port is
mapped to it by the NAT, the specific port is unnecessary.
Instead, on startup a random port should be selected (see below
for recommended ranges). An application, particularly on an
embedded device, should not rely on the host operating system to
select the next available port, because that might result in the
application receiving the same port on each restart. An
application using the same port between restarts may not receive
accurate results from Behavior Discovery tests that are intended
to test state-related behavior of NATs, such as filtering and
binding lifetime.</t>
<t>An application requiring multiple ports, such as separate ports
for control and media, should allocate those ports on startup when
possible. Even if there is no immediate need for media flow, if
Behavior Discovery tests will be run on those ports, allocating
them early will allow them to be left idle, increasing the chance
of obtaining accurate results from Behavior Discovery tests.</t>
<t>Although the most reliable results are obtained when performing
tests with the specific ports that the application will use, in
many cases an application will need to allocate and use ports
without being able to perform complete Behavior Discovery tests on
those ports. In those cases, an application should randomly select
its ports from a range likely to receive the same treatment by the
NAT. This document recommends ranges of 32768-49151, which is the
upper end of IANA's Registered Ports range, and 49152-65535, which
is IANA's Dynamic and/or Private port range, for random selection.
To attempt to characterize a NAT's general treatment of ports in
these ranges, a small number of ports within a range can be
randomly selected and characterized.</t>
</list>Those tests particularly sensistive to prior state on a NAT
will be indicated below.</t>
</section>
<section title="Checking for UDP Connectivity with the STUN Server">
<t>The client sends a STUN Binding Request to a server. This causes
the server to send the response back to the address and port that the
request came from. If this test yields no response, the client knows
right away that it does not have UDP connectivity with the STUN
server. This test requires only <xref target="RFC5389">STUN</xref>
functionality.</t>
</section>
<section anchor="secNatMapping" title="Determining NAT Mapping Behavior">
<t>This will require at most three tests. In test I, the client
performs the UDP connectivity test. The server will return its
alternate address and port in OTHER-ADDRESS in the binding response.
If OTHER-ADDRESS is not returned, the server does not support this
usage and this test cannot be run. The client examines the
XOR-MAPPED-ADDRESS attribute. If this address and port are the same as
the local IP address and port of the socket used to send the request,
the client knows that it is not NATed and the effective mapping will
be Endpoint-Independent.</t>
<t>In test II, the client sends a Binding Request to the alternate
address, but primary port. If the XOR-MAPPED-ADDRESS in the Binding
Response is the same as test I the NAT currently has
Endpoint-Independent Mapping. If not, test III is performed: the
client sends a Binding Request to the alternate address and port. If
the XOR-MAPPED-ADDRESS matches test II, the NAT currently has
Address-Dependent Mapping; if it doesn't match it currently has
Address and Port-Dependent Mapping.</t>
</section>
<section title="Determining NAT Filtering Behavior">
<t>This will also require at most three tests. These tests are
sensitive to prior state on the NAT.</t>
<t>In test I, the client performs the UDP connectivity test. The
server will return its alternate address and port in OTHER-ADDRESS in
the binding response. If OTHER-ADDRESS is not returned, the server
does not support this usage and this test cannot be run.</t>
<t>In test II, the client sends a binding request to the primary
address of the server with the CHANGE-REQUEST attribute set to
change-port and change-IP. This will cause the server to send its
response from its alternate IP address and alternate port. If the
client receives a response the current behaviour of the NAT is
Endpoint-Independent Filtering.</t>
<t>If no response is received, test III must be performed to
distinguish between Address-Dependent Filtering and Address and
Port-Dependent Filtering. In test III, the client sends a binding
request to the original server address with CHANGE-REQUEST set to
change-port. If the client receives a response the current behaviour
is Address-Dependent Filtering; if no response is received the current
behaviour is Address and Port-Dependent Filtering.</t>
</section>
<section title="Combining and Ordering Tests">
<t>Clients may wish to combine and parallelize these tests to reduce
the number of packets sent and speed the discovery process. For
example, test I of the filtering and mapping tests also checks if UDP
is blocked. Furthermore, an application or user may not need as much
detail as these sample tests provide. For example, establishing
connectivity between nodes becomes significantly more difficult if a
NAT has any behavior other than endpoint-independent mapping, which
requires only test I and II of <xref target="secNatMapping"></xref>.
An application determining its NAT does not always provide independent
mapping might notify the user if no relay is configured, whereas an
application behind a NAT that provides endpoint-independent mapping
might not notify the user until a subsequent connection actually fails
or might provide a less urgent notification that no relay is
configured. Such a test does not alleviate the need for <xref
target="I-D.ietf-mmusic-ice">ICE</xref>, but it does provide some
information regarding whether ICE is likely to be successful
establishing non-relayed connections.</t>
<t>Care must be taken when combining and parallelizing tests, due to
the sensitivity of certain tests to prior state on the NAT and because
some NAT devices have an upper limit on how quickly bindings will be
allocated. Section <xref target="secClient"></xref> restricts the rate
at which clients may begin new STUN transactions.</t>
</section>
<section anchor="sec-binding-desc" title="Binding Lifetime Discovery">
<t>STUN can also be used to probe the lifetimes of the bindings
created by the NAT. Such tests are sensitive to prior state on the
NAT. For many NAT devices, an absolute refresh interval cannot be
determined; bindings might be closed quicker under heavy load or might
not behave as the tests suggest. For this reason applications that
require reliable bindings must send keep-alives as frequently as
required by all NAT devices that will be encountered. Suggested
refresh intervals are outside the scope of this document. <xref
target="I-D.ietf-mmusic-ice">ICE</xref> and <xref
target="I-D.ietf-sip-outbound">OUTBOUND</xref> have suggested refresh
intervals.</t>
<t>Determining the binding lifetime relies on two separate source
ports being used to send STUN Binding Requests to the STUN server. The
general approach is that the client uses a source port X to send a
single Binding Request. After a period of time during which source
port X is not used, the client uses a second source port Y to send a
Binding Request to the STUN server that indicates the response should
be sent to the binding established to port X. If the binding for port
X has timed out, that response will not be received. By varying the
time between the original Binding Request sent from X and the
subsequent request sent from Y, the client can determine the binding
lifetime.</t>
<t>To determine the binding lifetime, the client first sends a Binding
Request to the server from a particular source port, X. This creates a
binding in the NAT. The response from the server contains a
MAPPED-ADDRESS attribute, providing the public address and port on the
NAT. Call this Pa and Pp, respectively. The client then starts a timer
with a value of T seconds. When this timer fires, the client sends
another Binding Request to the server, using the same destination
address and port, but from a different source port, Y. This request
contains an XOR-RESPONSE-TARGET address attribute, set to (Pa,Pp), to
request the response be delivered to (Pa,Pp). This will create a new
binding on the NAT, and cause the STUN server to send a Binding
Response that would match the old binding, (Pa,Pp), if it still
exists. If the client receives the Binding Response on port X, it
knows that the binding has not expired. If the client receives the
Binding Response on port Y (which is possible if the old binding
expired, and the NAT allocated the same public address and port to the
new binding), or receives no response at all, it knows that the
binding has expired.</t>
<t>Because some NATs only refresh bindings when outbound traffic is
sent, the client must resend a binding request from the original
source port before beginning a second test with a different value of
T. The client can find the value of the binding lifetime by doing a
binary search through T, arriving eventually at the value where the
response is not received for any timer greater than T, but is received
for any timer less than T. Note also that the binding refresh behavior
(outbound only or all traffic) can be determined by sending multiple
Binding Requests from port Y without refreshes from the original
source port X.</t>
<t>This discovery process takes quite a bit of time and is something
that will typically be run in the background on a device once it
boots.</t>
<t>It is possible that the client can get inconsistent results each
time this process is run. For example, if the NAT should reboot, or be
reset for some reason, the process may discover a lifetime than is
shorter than the actual one. Binding lifetime may also be dependent on
the traffic load on the NAT. For this reason, implementations are
encouraged to run the test numerous times and be prepared to get
inconsistent results.</t>
<t>Like the other diagnostics, this test is inherently unstable. In
particular, an overloaded NAT might reduce binding lifetime to shed
load. A client might find this diagnostic useful at startup, for
example setting the initial keepalive interval on its connection to
the server to 10 seconds while beginning this check. After determining
the current lifetime, the keepalive interval used by the connection to
the server can be set to this appropriate value. Subsequent checks of
the binding lifetime can then be performed using the keepalives in the
server connection. The STUN Keepalive Usage <xref
target="I-D.ietf-sip-outbound"></xref> provides a response that
confirms the connection is open and allows the client to check that
its mapped address has not changed. As that provides both the
keepalive action and diagnostic that it is working, it should be
preferred over any attempt to characterize the connection by a
secondary technique.</t>
</section>
</section>
<section anchor="secClient" title="Client Behavior">
<t>Unless otherwise specified here, all procedures for preparing,
sending, and processing messages as described in the STUN Binding Usage
<xref target="RFC5389"></xref> are followed.</t>
<t>If a client intends to utilize an XOR-RESPONSE-TARGET attribute in
future transactions, as described in <xref
target="sec-binding-desc"></xref>, then it MUST include a CACHE-TIMEOUT
attribute in the Request with the value set greater than the longest
time duration it intends to test. The server will also include this
attribute in its Response, modified with its estimate of how long it
will be able to cache this connection. Because the returned value is
only an estimate, the client must be prepared for the value to be wrong,
and therefore to receive a 481 response to its subsequent Requests with
XOR-RESPONSE-TARGET.</t>
<t>Support for XOR-RESPONSE-TARGET is optional due to the state cost on
the server. Therefore, a client MUST be prepared to receive a 420
(Unknown Attribute) error to requests that include XOR-RESPONSE-TARGET
or CACHE-TIMEOUT. Support for OTHER-ADDRESS and CHANGE-REQUEST is
optional, but MUST be supported by servers advertised via SRV, as
described below. This is to allow the use of PADDING and
XOR-RESPONSE-TARGET in applications where servers do not have multiple
IP addresses. Clients MUST be prepared to receive a 420 for requests
that include CHANGE-REQUEST when OTHER-ADDRESS was not received in
Binding Response messages from the server.</t>
<t>If an application makes use of the NAT Behavior Discovery STUN usage
by multiplexing it in a flow with application traffic, a FINGERPRINT
attribute SHOULD be included unless it is always possible to distinguish
a STUN message from an application message based on their header.</t>
<t>When PADDING is used, it SHOULD be equal to the MTU of the outgoing
interface.</t>
<t>Clients SHOULD ignore an ALTERNATE-SERVER attribute in a response
unless they are using authentication with a provider of STUN servers
that is aware of the topology requirements of the tests being
performed.</t>
<t>A client SHOULD NOT generate more than ten new STUN transactions per
second and SHOULD pace them such that the RTOs do not synchronize the
retransmissions of each transaction.</t>
<section title="Discovery">
<t>Unless the user or application is aware of the transport address of
a STUN server supporting the NAT Behavior Discovery usage through
other means, a client is configured with the domain name of the
provider of the STUN servers. The domain is resolved to a transport
address using SRV procedures <xref target="RFC2782"></xref>. The
mechanism for configuring the client with the domain name of the STUN
servers or of acquiring a specific transport address is out of scope
for this document.</t>
<t>For the Behavior Discovery Usage the service name is
"stun-behavior" for UDP and TCP. The service name is "stun-behaviors"
for TLS over TCP. Only "tcp" is defined as a protocol for
"stun-behaviors". Other aspects of handling failures and default ports
are followed as described in <xref target="RFC5389">STUN</xref>.</t>
</section>
<section title="Security" toc="default">
<t>Servers MAY require authentication before allowing a client to make
use of its services. This is particularly important to requests used
to perform a Binding Lifetime Discovery test or other test requiring
use of the XOR-RESPONSE-TARGET attribute. The method for obtaining
these credentials, should the server require them, is outside the
scope of this usage. Presumably, the administrator or application
relying on this usage should have its own method for obtaining
credentials. If the client receives a 401 (Unauthorized) Response to a
Request, then it must either acquire the appropriate credential from
the application before retrying or report a permanent failure.
Procedures for encoding the MESSAGE-INTEGRITY attribute for a request
are described in <xref target="RFC5389">STUN</xref>.</t>
</section>
</section>
<section anchor="secServer" title="Server Behavior">
<t>Unless otherwise specified here, all procedures for preparing,
sending, and processing messages as described for the STUN Binding Usage
of <xref target="RFC5389">STUN</xref> are followed.</t>
<t>A server implementing the NAT Behavior Discovery usage SHOULD be
configured with two separate IP addresses on the public Internet. On
startup, the server SHOULD allocate a pair of ports for each of the UDP,
TCP, and TCP/TLS transport protocols, such that it can send and receive
datagrams using the same ports on each IP address (normally a wildcard
binding accomplishes this). TCP and TCP/TLS MUST use different ports. If
a server cannot allocate the same ports on two different IP address,
then it MUST NOT include an OTHER-ADDRESS attribute in any Response and
MUST respond with a 420 (Unknown Attribute) to any Request with a
CHANGE-REQUEST attribute. A server with only one IP address MUST NOT be
advertised using the SRV service name "stun-behavior" or
"stun-behaviors".</t>
<section anchor="secPrepResponse" title="Preparing the Response">
<t>After performing all authentication and verification steps the
server begins processing specific to this Usage if the Request
contains any request attributes defined in this document:
XOR-RESPONSE-TARGET, CHANGE-REQUEST, CACHE-TIMEOUT, or PADDING. If the
Request does not contain any attributes from this document,
OTHER-ADDRESS and RESPONSE-ORIGIN are still included in the
response.</t>
<t>The server MUST include both MAPPED-ADDRESS and XOR-MAPPED-ADDRESS
in its Response.</t>
<t>If the Request contains CHANGE-REQUEST attribute and the server
does not have an alternate address and port as described above, the
server MUST generate an error response of type 420.</t>
<t>If the Request contains a CACHE-TIMEOUT attribute, then the server
SHOULD include a CACHE-TIMEOUT attribute in its response indicating
the duration (in seconds) it anticipates being able to cache this
binding request in anticipation of a future Request using the
XOR-RESPONSE-TARGET attribute. The CACHE-TIMEOUT response value can be
greater or less than the value in the request. If the server is not
prepared to provide such an estimate, it SHOULD NOT include the
CACHE-TIMEOUT attribute in its Response. The server SHOULD NOT provide
a CACHE-TIMEOUT length longer than the amount of time it has been able
to cache recent requests.</t>
<t>If XOR-RESPONSE-TARGET is included in a Request, then the server
MUST verify that it has previously received a binding request with
CACHE-TIMEOUT from the same address as is specified in
XOR-RESPONSE-TARGET. If it has not, or if sufficient time has passed
that it no longer has a record of having received such a request due
to limited state, it MUST respond with an error response of type
481.</t>
<t>If the Request contains a XOR-RESPONSE-TARGET attribute and the
server is authenticating such requests, then the server checks the
message for a MESSAGE-INTEGRITY attribute and a USERNAME. If they are
not present the server MUST generate an error response of type
401.</t>
<t>Because XOR-RESPONSE-TARGET offers the potential for minor
indirection attacks, a server MUST either authenticate the users
requesting its use or rate-limit its response to those requests, in
addition to verifying that it previously received a binding request
from that address. Rate-limiting is RECOMMENDED even for responses to
authenticated requests unless the server is deployed for an
application that requires more frequent responses. If the Request
contains a XOR-RESPONSE-TARGET attribute and the server is
rate-limiting such requests, it MUST ensure that it does not generate
a Response on a particular address more often than one per second. If
the server receives requests whose responses are being rate-limited
more often than one per second, it MUST generate a 503 (Service
unavailable) Response to the Request.</t>
<t>The source address and port of the Binding Response depend on the
value of the CHANGE-REQUEST attribute and on the address and port the
Binding Request was received on, and are summarized in <xref
target="oatable"></xref>.</t>
<t>Let A1 and A2 be the two IP addresses used by the server and P1 and
P2 be the ports used by the server. Let Da represent the destination
IP address of the Binding Request (which will be either A1 or A2), and
Dp represent the destination port of the Binding Request (which will
be either P1 or P2). Let Ca represent the other address, so that if Da
is A1, Ca is A2. If Da is A2, Ca is A1. Similarly, let Cp represent
the other port, so that if Dp is P1, Cp is P2. If Dp is P2, Cp is P1.
If the "change port" flag was set in CHANGE-REQUEST attribute of the
Binding Request, and the "change IP" flag was not set, the source IP
address of the Binding Response MUST be Da and the source port of the
Binding Response MUST be Cp. If the "change IP" flag was set in the
Binding Request, and the "change port" flag was not set, the source IP
address of the Binding Response MUST be Ca and the source port of the
Binding Response MUST be Dp. When both flags are set, the source IP
address of the Binding Response MUST be Ca and the source port of the
Binding Response MUST be Cp. If neither flag is set, or if the
CHANGE-REQUEST attribute is absent entirely, the source IP address of
the Binding Response MUST be Da and the source port of the Binding
Response MUST be Dp.</t>
<texttable anchor="oatable"
title="Impact of Flags on Packet Source and OTHER-ADDRESS">
<ttcol width="25%">Flags</ttcol>
<ttcol>Source Address</ttcol>
<ttcol>Source Port</ttcol>
<ttcol>OTHER-ADDRESS</ttcol>
<c>none</c>
<c>Da</c>
<c>Dp</c>
<c>Ca:Cp</c>
<c>Change IP</c>
<c>Ca</c>
<c>Dp</c>
<c>Ca:Cp</c>
<c>Change port</c>
<c>Da</c>
<c>Cp</c>
<c>Ca:Cp</c>
<c>Change IP and Change port</c>
<c>Ca</c>
<c>Cp</c>
<c>Ca:Cp</c>
</texttable>
<t>The server MUST add a RESPONSE-ORIGIN attribute to the Binding
Response, containing the source address and port used to send the
Binding Response.</t>
<t>If the server supports an alternate address and port the server
MUST add an OTHER-ADDRESS attribute to the Binding Response. This
contains the source IP address and port that would be used if the
client had set the "change IP" and "change port" flags in the Binding
Request. As summarized in <xref target="oatable"></xref>, these are Ca
and Cp, respectively, regardless of the value of the CHANGE-REQUEST
flags.</t>
<t>Next the server inspects the Request for a XOR-RESPONSE-TARGET
attribute. If the XOR-RESPONSE-TARGET attribute is included, then it
includes an XOR-REFLECTED-FROM attribute with the source address the
Request was received from.</t>
<t>If the Request contained a PADDING attribute, PADDING MUST be
included in the Binding Response. The server SHOULD use a length of
PADDING equal to the MTU on the outgoing interface, rounded up to an
even multiple of four bytes. If the Request also contains the
XOR-RESPONSE-TARGET attribute the server MUST return an error response
of type 400.</t>
<t>Following that, the server completes the remainder of the
processing from <xref target="RFC5389">STUN</xref>. If authentication
is being required, the server MUST include a MESSAGE-INTEGRITY and
associated attributes as appropriate. A FINGERPRINT attribute is only
required if the STUN messages are being multiplexed with application
traffic that requires use of a FINGERPRINT to distinguish STUN
messages.</t>
<t>An ALTERNATE-SERVER attribute MUST NOT be included with any other
attribute defined in this specification.</t>
<t>When the server sends the Response, it is sent from the source
address as determined above and to the destination address determined
from the XOR-RESPONSE-TARGET, or to the source address of the Request
otherwise.</t>
</section>
</section>
<section anchor="secAttrib" title="New Attributes">
<t>This document defines several STUN attributes that are required for
NAT Behavior Discovery. These attributes are all used only with Binding
Requests and Binding Responses. CHANGE-REQUEST was originally defined in
<xref target="RFC3489">RFC3489</xref> but is redefined here as that
document is obsoleted by <xref target="RFC5389"></xref>.</t>
<figure>
<artwork><![CDATA[
Comprehension-required range (0x0000-0x7FFF):
0x0003: CHANGE-REQUEST
0x0026: PADDING
0x0027: XOR-RESPONSE-TARGET
0x0028: XOR-REFLECTED-FROM
Comprehension-optional range (0x8000-0xFFFF)
0x8027: CACHE-TIMEOUT
0x802b: RESPONSE-ORIGIN
0x802c: OTHER-ADDRESS
]]></artwork>
</figure>
<section title="Representing Transport Addresses">
<t>Whenever an attribute contains a transport IP address and port, it
has the same format as MAPPED-ADDRESS. Similarly, the XOR- attributes
have the same format as XOR-MAPPED-ADDRESS<xref
target="RFC5389"></xref>.</t>
</section>
<section title="CHANGE-REQUEST">
<t>The CHANGE-REQUEST attribute contains two flags to control the IP
address and port the server uses to send the response. These flags are
called the "change IP" and "change port" flags. The CHANGE-REQUEST
attribute is allowed only in the Binding Request. The "change IP" and
"change port" flags are useful for determining the current filtering
behavior of a NAT. They instruct the server to send the Binding
Responses from the alternate source IP address and/or alternate port.
The CHANGE-REQUEST attribute is optional in the Binding Request.</t>
<t>The attribute is 32 bits long, although only two bits (A and B) are
used:</t>
<figure>
<artwork xml:space="preserve"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A B 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The meanings of the flags are:<list hangIndent="3" style="hanging">
<t hangText="A:">This is the "change IP" flag. If true, it
requests the server to send the Binding Response with a different
IP address than the one the Binding Request was received on.</t>
<t hangText="B:">This is the "change port" flag. If true, it
requests the server to send the Binding Response with a different
port than the one the Binding Request was received on.</t>
</list></t>
</section>
<section title="RESPONSE-ORIGIN">
<t>The RESPONSE-ORIGIN attribute is inserted by the server and
indicates the source IP address and port the response was sent from.
It is useful for detecting twice NAT configurations. It is only
present in Binding Responses.</t>
</section>
<section title="OTHER-ADDRESS">
<t>The OTHER-ADDRESS attribute is used in Binding Responses. It
informs the client of the source IP address and port that would be
used if the client requested the "change IP" and "change port"
behavior. OTHER-ADDRESS MUST NOT be inserted into a Binding Response
unless the server has a second IP address.</t>
<t>OTHER-ADDRESS uses the same attribute number as CHANGED-ADDRESS
from RFC3489 <xref target="RFC3489"></xref>because it is simply a new
name with the same semantics as CHANGED-ADDRESS. It has been renamed
to more clearly indicate its function.</t>
</section>
<section title="XOR-REFLECTED-FROM">
<t>The XOR-REFLECTED-FROM attribute is present only in Binding
Responses when the Binding Request contained a XOR-RESPONSE-TARGET
attribute. The attribute contains the transport address of the source
where the request came from. Its purpose is to provide traceability,
so that a STUN server cannot be used as a reflector for anonymous
denial-of-service attacks.</t>
<t>The XOR-REFLECTED-FROM attribute is used in place of RFC3489's
REFLECTED-FROM attribute. It provides the same information, but
because the NAT's public address is obfuscated through the XOR
function, It can pass through a NAT that would otherwise attempt to
translate it to the private network address.</t>
</section>
<section title="XOR-RESPONSE-TARGET">
<t>The XOR-RESPONSE-TARGET attribute contains an IP address and port.
The XOR-RESPONSE-TARGET attribute can be present in the Binding
Request and indicates where the Binding Response is to be sent. It is
used in tests such as <xref target="sec-binding-desc"></xref>. When
not present, the server sends the Binding Response to the source IP
address and port of the Binding Request. The server MUST NOT process a
request containing a XOR-RESPONSE-TARGET and a PADDING attribute. The
XOR-RESPONSE-TARGET attribute is optional in the Binding Request.
Server support for XOR-RESPONSE-TARGET is optional.</t>
<t>XOR-RESPONSE-TARGET is used in place of RFC3489's RESPONSE-ADDRESS.
It provides the same information, but because the NAT's public address
is obfuscated through the XOR function, It can pass through a NAT that
would otherwise attempt to translate it to the private network
address.</t>
</section>
<section title="PADDING">
<t>The PADDING attribute allows for the entire message to be padded to
force the STUN message to be divided into IP fragments. PADDING
consists entirely of a freeform string, the value of which does not
matter. PADDING can be used in either Binding Requests or Binding
Responses.</t>
<t>PADDING MUST NOT be longer than the length that brings the total IP
datagram size to 64K. It SHOULD be equal in length to the MTU of the
outgoing interface, rounded up to an even multiple of four bytes.
Because STUN messages with PADDING are intended to test the behavior
of UDP fragments, they are an exception to the usual rule that STUN
messages be less than the MTU of the path.</t>
</section>
<section title="CACHE-TIMEOUT">
<t>The CACHE-TIMEOUT is used in Binding Requests and Responses. It
indicates the time duration (in seconds) that the server will cache
the source address and USERNAME of an original Binding Request that
will later by followed by a request from a different source address
with a XOR-RESPONSE-TARGET asking that a response be reflected to the
source address of the original Binding Request. A server MUST NOT send
a response to a target address requested with XOR-RESPONSE-TARGET
unless it has cached that the same a previous binding was received
from that target address. The client inserts a value in CACHE-TIMEOUT
into the Binding Request indicating the amount of time it would like
the server to cache that information. The server responds with a
CACHE-TIMEOUT in its Binding Response providing a prediction of how
long it will cache that information. The response value can be greater
than, equal to, or less than the requested value. If the server is not
able to provide such an estimate or the information in the response
would be meaningless, the server SHOULD NOT include a CACHE-TIMEOUT
attribute in its response. Server support for CACHE-TIMEOUT is
optional.</t>
</section>
</section>
<section anchor="secResponse" title="New Response Codes">
<t>This draft defines two new STUN response codes.</t>
<section title="481 Connection does not exist">
<t>This code is generated when a server has received an
XOR-RESPONSE-TARGET, but the server has no record of having received a
prior Binding Request from the address specified in
XOR-RESPONSE-TARGET. The client SHOULD send a new Binding Request from
the address it intends to specify in an XOR-RESPONSE-TARGET. This new
Binding Request SHOULD include a CACHE-TIMEOUT attribute with the
value set to the desired duration. If the server's response includes a
CACHE-TIMEOUT duration that is shorter than the client's requested
duration, the server is unable to satisfy the caching time requested
by the client and the client SHOULD NOT continue to retry the
request.</t>
</section>
<section title="503 Service Unavailable">
<t>This response is generated when a server receives Requests
specifying a particular address in their XOR-RESPONSE-TARGET attribute
more often than one per second.</t>
</section>
</section>
<section title="IAB Considerations">
<t>The IAB has studied the problem of ``Unilateral Self Address
Fixing'', which is the general process by which a client attempts to
determine its address in another realm on the other side of a NAT
through a collaborative protocol reflection mechanism <xref
target="RFC3424">RFC 3424</xref>. The STUN NAT Behavior Discovery usage
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.</t>
<section title="Problem Definition">
<t>From <xref target="RFC3424">RFC 3424</xref>, any UNSAF proposal
must provide: <list style="hanging">
<t>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".</t>
</list></t>
<t>The specific problem being solved by the STUN NAT Behavior
Discovery usage is for a client, which may be located behind a NAT of
any type, to determine the instantaneous characteristics of that NAT
in order to either diagnose the cause of problems experienced by that
or other applications or for an application to modify its behavior
based on the current behavior of the NAT and an appropriate
statistical model of the behavior required for the application to
succeed.</t>
</section>
<section title="Exit Strategy">
<t>From <xref target="RFC3424"></xref>, any UNSAF proposal must
provide: <list style="hanging">
<t>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.</t>
</list></t>
<t>The STUN NAT Behavior Discovery usage does not itself provide an
exit strategy for v4 NATs. At the time of this writing, it appears
some sort of NAT will be necessary between v6 clients and v4 servers,
but this specification will not be necessary with those v6 to v4 NATs,
because the IETF is planning to adequately describe their operation.
This specification will be of no interest for v6 to v6
connectivity.</t>
</section>
<section title="Brittleness Introduced by STUN NAT Behavior Discovery">
<t>From <xref target="RFC3424"></xref>, any UNSAF proposal must
provide: <list style="hanging">
<t>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.</t>
</list></t>
<t>The STUN NAT Behavior Discovery usage allows a client to determine
the current behavior of a NAT. This information can be quite useful to
a developer or network administrator outside of an application, and as
such can be used to diagnose the brittleness induced in another
application. When used within an application itself, STUN NAT Behavior
Discovery allows the application to adjust its behavior according to
the current behavior of the NAT. This draft is experimental because
the extent to which brittleness is introduced to an application
relying on the Behavior Discovery usage is unclear and must be
carefully evaluated by the designers of the protocol making use of it.
The experimental test for this protocol is essentially determining
whether an application can be made less brittle through the use of
behavior-discovery information than it would be if attempted to make
use of the network without any awareness of the NATs its traffic must
pass through.<!--While this can be helpful in
improving the performance of an application, an improperly written
application could use information from this usage and assume that the
NAT will always behave in the same manner, and thus failing to work
properly when the NAT changes its behavior. Regardless of whether an
application makes use of NAT Behavior Discovery or not, if it does not
use techniques such as <xref target="I-D.ietf-mmusic-ice">ICE</xref>
or <xref target="I-D.ietf-sip-outbound">OUTBOUND</xref> it exposes
itself to the inherent instability of NAT.--></t>
</section>
<section title="Requirements for a Long Term Solution">
<t>From <xref target="RFC3424"></xref>, any UNSAF proposal must
provide: <list style="hanging">
<t>Identify requirements for longer term, sound technical
solutions -- contribute to the process of finding the right longer
term solution.</t>
</list></t>
<t>As long as v4 NATs are present, means of adapting to their presence
will be required. As described above, well-behaved v6 to v4 NATs and
direct v6 to v6 connections will not require behavior
characterization.</t>
</section>
<section title="Issues with Existing NAPT Boxes">
<t>From <xref target="RFC3424"></xref>, any UNSAF proposal must
provide: <list style="hanging">
<t>Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.</t>
</list></t>
<t>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 usage avoids that problem
by using the XOR-REFLECTED-FROM and XOR-RESPONSE-TARGET attributes
instead of the older REFLECTED-FROM and RESPONSE-ADDRESS
attributes.</t>
<t>This usage provides a set of generic attributes that can be
assembled to test many types of NAT behavior. While tests for the most
commonly known NAT box behaviors are described, the BEHAVE mailing
list regularly has descriptions of new behaviors, some of which may
not be readily detected using the tests described herein. However, the
techniques described in this usage can be assembled in different
combinations to test NAT behaviors not now known or envisioned.</t>
</section>
</section>
<!-- IAB Considerations -->
<section anchor="IANA" title="IANA Considerations">
<t>This specification requests that IANA make additions to the "STUN
Attributes Registry" and "STUN Error Code Registry".</t>
<section title="STUN Attribute Registry">
<t>This specification defines several new STUN attributes. This
section directs IANA to add these new protocol elements to the IANA
registry of STUN protocol elements.</t>
<t>0x0003: CHANGE-REQUEST<vspace blankLines="0" />0x0027:
XOR-RESPONSE-TARGET<vspace blankLines="0" />0x0028:
XOR-REFLECTED-FROM<vspace blankLines="0" />0x0026: PADDING<vspace
blankLines="0" />0x8027: CACHE-TIMEOUT<vspace blankLines="0" />0x802b:
RESPONSE-ORIGIN<vspace blankLines="0" />0x802c: OTHER-ADDRESS</t>
</section>
<section title="STUN Error Code Registry">
<t>This specification defines two new STUN error response codes.</t>
<t>481: Connection does not exist<vspace blankLines="0" />503: Service
Unavailable</t>
</section>
<section title="SRV Registry">
<t>This specification defines the "stun-behavior" and "stun-behaviors"
SRV service names. "stun-behavior" may be used with SRV protocol
specifiers "udp" and "tcp". "stun-behaviors" may only be specified
with "tcp". Thus, the allowable SRV queries are:</t>
<figure>
<artwork><![CDATA[_stun-behavior._udp UDP
_stun-behavior._tcp TCP
_stun-behaviors._tcp TLS over TCP]]></artwork>
</figure>
</section>
</section>
<section anchor="Security" title="Security Considerations">
<t>This usage inherits the security considerations of <xref
target="RFC5389">STUN</xref>. This usage adds several new attributes;
security considerations for those are detailed here.</t>
<t>OTHER-ADDRESS does not permit any new attacks; it provides another
place where an attacker can impersonate a STUN server but it is not an
interesting attack. An attacker positioned where it can compromise the
Binding Response can completely hide the STUN server from the
client.</t>
<t>XOR-RESPONSE-TARGET allows a STUN server to be used as a reflector
for denial-of-service attacks. It does not provide any amplification of
the attack. The XOR-REFLECTED-FROM mitigates this by providing the
identity (in terms of IP address) of the source where the request came
from. Its purpose is to provide traceability, so that a STUN server
cannot be used as an anonymous reflector for denial-of-service attacks.
To mitigate the security threats of XOR-RESPONSE-TARGET:</t>
<t><list style="symbols">
<t>The server MUST NOT respond to requests with XOR-RESPONSE-TARGET
unless they have cached state that a binding request with
CACHE-TIMEOUT has previously been received from the target
address.</t>
<t>The server MUST either authenticate all requests using
XOR-RESPONSE-TARGET or rate-limit its responses to such requests.
Rate-limiting is RECOMMENDED even if authenticating requests, unless
the server is deployed for an application requiring more frequent
responses.</t>
<t>Requests containing both XOR-RESPONSE-TARGET and PADDING are
rejected by the server.</t>
<t>Implementing XOR-RESPONSE-TARGET is optional, allowing servers
that cannot store the required state and/or deployed for
applications that don't require its use to automatically reject any
requests containing it.</t>
</list></t>
<t>The only attack possible with the PADDING attribute is to have a
large padding length which could cause a server to allocate a large
amount of memory. As servers will ignore any padding length greater than
64k so the scope of this attack is limited. In general, servers should
not allocate more memory than the size of the received datagram. This
attack would only affect non-compliant implementations.</t>
<t>CHANGE-REQUEST provides no attacks, but adds three more reflection
sources for the XOR-RESPONSE-TARGET reflection attacks. It provides no
additional amplification, however, and the security mechanisms for
XOR-RESPONSE-TARGET protect all addresses on the server.</t>
<t>RESPONSE-ORIGIN, CACHE-TIMEOUT and XOR-REFLECTED-FROM do not provide
any additional attacks.</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>The authors would like to thank the authors of <xref
target="RFC3489">the original STUN specification</xref> from which many
of the ideas, attributes, and description in this document originated.
Thanks to Dan Wing, Cullen Jennings, and Magnus Westerlund for detailed
comments.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.4787"?>
<?rfc include="reference.RFC.5389"?>
<?rfc include='reference.RFC.2782'?>
</references>
<references title="Informative References">
<?rfc include="reference.I-D.ietf-sip-outbound"?>
<?rfc include='reference.RFC.3424'?>
<?rfc include='reference.RFC.3489'?>
<?rfc include="reference.I-D.ietf-mmusic-ice"?>
</references>
<section title="Change Log">
<t>RFC-EDITOR: Please remove this entire Change Log section while
formatting this document for publication.</t>
<section title="from draft-macdonald-behave-nat-behavior-diagnostics-00">
<t><list style="symbols">
<t>Only OTHER-ADDRESS, CHANGE-ADDRESS, and XOR-RESPONSE-TARGET
support is optional; support for PADDING and SOURCE-ADDRESS is now
mandatory</t>
<t>PADDING is now a mandatory attribute</t>
<t>OTHER-ADDRESS is returned in all binding responses if the
server has a second IP address</t>
</list></t>
</section>
<section title="from draft-ietf-behave-nat-behavior-discovery-00">
<t><list style="symbols">
<t>Clarified that only servers with two IP addresses should have
an SRV entry</t>
<t>Removed support for backward compatibility with 3489 clients by
removing non-XOR forms of attributes. Language states that
backward compatibility with 3489 clients is SHOULD NOT.
Compatibility with 3489 servers is left unspecified.</t>
<t>PADDING is mandatory and language has been changed to indicate
that if a server supports PADDING it must either actually provide
the padding or return an error (can't support it but refuse to do
it)</t>
<t>Require both MAPPED-ADDRESS and XOR-MAPPED-ADDRESS to be
returned to support detection of generic ALGs</t>
</list></t>
</section>
<section title="from draft-ietf-behave-nat-behavior-discovery-01">
<t><list style="symbols">
<t>Changed proposed status to experimental</t>
<t>Made significant changes to the introduction and applicability
statements to reflect the experimental status</t>
<t>Fixed the New Attributes and IANA considerations not listing
the same attribute numbers.</t>
<t>Removed mandatory shared secret credentials in favor of the
option of rate limiting or credentials. Specified that credentials
must be obtained from the user or parent application.</t>
<t>Made OTHER-ADDRESS and SOURCE-ADDRESS optional to address
compatibility with 3489bis clients. Renamed SOURCE-ADDRESS as
RESPONSE-ORIGIN to avoid conflicts with 3489.</t>
<t>Renamed XOR-RESPONSE-ADDRESS to XOR-RESPONSE-TARGET</t>
<t>Added discussion of FINGERPRINT and ALTERNATE-SERVER for
compliance with 3489bis stun usage definition requirements.</t>
</list></t>
</section>
<section title="from draft-ietf-behave-nat-behavior-discovery-02">
<t><list style="symbols">
<t>fix terminology for endpoint-independent, address-dependent,
and address and port-dependent from rfc4787</t>
<t>define the ALG detection to apply to UDP and TCP</t>
<t>fix >From typo in 9.5</t>
<t>added exception to single MTU size restriction for PADDING</t>
<t>removed OPEN ISSUE about CHANGE-REQUEST IANA registry based on
the belief that we need to list that definition here now that
3489bis is dropping it.</t>
</list></t>
</section>
<section title="from draft-ietf-behave-nat-behavior-discovery-03">
<t><list style="symbols">
<t>moved semantics of PADDING usage into behavior sections rather
than attributes section</t>
<t>removed reference to SERVER attribute</t>
<t>removed Open Issues section</t>
<t>Updated IAB considerations</t>
</list></t>
</section>
<section title="from draft-ietf-behave-nat-behavior-discovery-04">
<t><list style="symbols">
<t>Clarified that behavior may vary by port used as well as by
destination IP/particular STUN server, and therefore specified
that all tests should be performed using the port the application
will use</t>
<t>Added additional text on selecting random port/ensuring port
has been unused prior to starting filtering tests</t>
<t>specified limit to start rate of tests and that tests
retransmissions should not synchronize</t>
<t>additional explanatory text for XOR-RESPONSE-TARGET</t>
<t>added SRV entry to IANA section and subdivided to match STUN
registries from 5389</t>
<t>clarified that test combinations are non-normative</t>
<t>Numerous clarifications</t>
<t>Changed PADDING to default to interface MTU, and changed
maximum length to not make IP datagram exceed 64K</t>
<t>Added text that server should allocate TCP and TCP/TLS</t>
</list></t>
</section>
<section title="from draft-ietf-behave-nat-behavior-discovery-05">
<t><list style="symbols">
<t>In applicability section, add explicit requirement that citing
drafts must specify how they handle failure</t>
<t>Add new subsection on selecting the source port used for tests,
covering both requirements that the same port be used for tests as
for the application and dependencies on previous bindings.
Reference that section explicitly for tests that are sensitive to
previous binding state.</t>
<t>Change SRV discovery to stun-behavior for UDP and TCP and
stun-behaviors for TLS/TCP.</t>
<t>Add SRV registry subsection to IANA section.</t>
</list></t>
</section>
<section title="from draft-ietf-behave-nat-behavior-discovery-06">
<t><list style="symbols">
<t>Reworked the applicability section and provided more detail in
the P2P application example</t>
<t>Additional clarification on the limits of the tests, and that
the described tests include only those utilizing a single source
IP, so can't test overloading.</t>
<t>Several changes to clarify and make consistent security
requirements for XOR-RESPONSE-TARGET</t>
<t>Numerous other clarifications in response to comments.</t>
</list></t>
</section>
</section>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-24 05:56:24 |