BEHAVE D. MacDonald
Internet-Draft CounterPath Solutions, Inc.
Intended status: Experimental B. Lowekamp
Expires: August 28, 2008 SIPeerior Technologies and William
& Mary
February 25, 2008
NAT Behavior Discovery Using STUN
draft-ietf-behave-nat-behavior-discovery-03
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
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.
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Requirements Language
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 RFC 2119 [RFC2119].
Table of Contents
1. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Diagnostic Use . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Example Use with P2P Overlays . . . . . . . . . . . . . . 6
2.3. Experimental Success . . . . . . . . . . . . . . . . . . . 6
3. Overview of Operations . . . . . . . . . . . . . . . . . . . . 7
3.1. Determining NAT Mapping . . . . . . . . . . . . . . . . . 8
3.2. Determining NAT Filtering . . . . . . . . . . . . . . . . 8
3.3. Binding Lifetime Discovery . . . . . . . . . . . . . . . . 8
3.4. Diagnosing NAT Hairpinning . . . . . . . . . . . . . . . . 9
3.5. Determining Fragment Handling . . . . . . . . . . . . . . 9
3.6. Detecting Generic ALGs . . . . . . . . . . . . . . . . . . 9
4. Discovery Process . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Checking if UDP is Blocked . . . . . . . . . . . . . . . . 10
4.2. Determining NAT Mapping Behavior . . . . . . . . . . . . . 10
4.3. Determining NAT Filtering Behavior . . . . . . . . . . . . 10
4.4. Combining and Ordering Tests . . . . . . . . . . . . . . . 11
4.5. Binding Lifetime Discovery . . . . . . . . . . . . . . . . 11
5. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2. Security . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Preparing the Response . . . . . . . . . . . . . . . . . . 14
7. New Attributes . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1. Representing Transport Addresses . . . . . . . . . . . . . 17
7.2. CHANGE-REQUEST . . . . . . . . . . . . . . . . . . . . . . 17
7.3. RESPONSE-ORIGIN . . . . . . . . . . . . . . . . . . . . . 18
7.4. OTHER-ADDRESS . . . . . . . . . . . . . . . . . . . . . . 18
7.5. XOR-REFLECTED-FROM . . . . . . . . . . . . . . . . . . . . 18
7.6. XOR-RESPONSE-TARGET . . . . . . . . . . . . . . . . . . . 19
7.7. PADDING . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.8. CACHE-TIMEOUT . . . . . . . . . . . . . . . . . . . . . . 20
8. New Response Codes . . . . . . . . . . . . . . . . . . . . . . 20
8.1. 481 Connection does not exist . . . . . . . . . . . . . . 20
8.2. 503 Service Unavailable . . . . . . . . . . . . . . . . . 20
9. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 20
9.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 21
9.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 21
9.3. Brittleness Introduced by STUN NAT Behavior Discovery . . 21
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9.4. Requirements for a Long Term Solution . . . . . . . . . . 22
9.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 22
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
11. Security Considerations . . . . . . . . . . . . . . . . . . . 23
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 24
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
14.1. Normative References . . . . . . . . . . . . . . . . . . . 24
14.2. Informative References . . . . . . . . . . . . . . . . . . 25
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 25
A.1. from draft-macdonald-behave-nat-behavior-diagnostics-00 . 26
A.2. from draft-ietf-behave-nat-behavior-discovery-00 . . . . . 26
A.3. from draft-ietf-behave-nat-behavior-discovery-01 . . . . . 26
A.4. from draft-ietf-behave-nat-behavior-discovery-02 . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . . . 28
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1. Applicability
This experimental 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. This STUN usage provides information
about observable transient behavior; it only truly determines a NAT's
behavior with regard to the STUN server used at the instant the test
is run. Applications requiring reliable reach between two particular
endpoints must establish a communication channel through a NAT using
another technique. IETF has proposed standards including ICE
[I-D.ietf-mmusic-ice] and OUTBOUND [I-D.ietf-sip-outbound] for
establishing communication channels when a publicly accessible
rendezvous service is available.
This techniques available 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, in particular when
behavior varies by load, destination, or other factors that may be
related to NAT behavior.
This draft also proposes experimental applications of NAT Behavior
Discovery STUN for real-time selection of parameters for protocols in
situations where a publicly accessible rendezvous service is not
available. One such application is role selection in P2P networks
based on statistical experience with establishing 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 2.2 describes this experimental application in more
detail and discusses how to evaluate its success or failure.
The applications of this STUN usage are very different than the
original use of RFC3489 [RFC3489], 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. 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.
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2. Introduction
The Simple Traversal Underneath Network Address Translators (NAT)
(STUN) [I-D.ietf-behave-rfc3489bis] 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.
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.
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.
2.1. Diagnostic Use
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.
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
o adding new traffic to new destinations (STUN servers) has the
potential to itself change the behavior of a NAT and
o 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,
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for the results to be applicable to the network conditions
experienced by the application.
2.2. Example Use with P2P Overlays
An application could use Behavior Discovery in a 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 supernode candidates acting as
STUN servers as well as 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.
Use of Behavior Discovery for such an application requires:
o Specification of protocols capable of offering reliable end-user
performance using unreliable links between peers.
o 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.
This draft is experimental as deployed applications implementing open
protocols have yet to be deployed in such environments to demonstrate
that these two requirements have been met. However, apocryphal
evidence suggests that household- and small business-targeted NAT
devices 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.
2.3. Experimental Success
The criteria for an application to successfully demonstrate use of
the NAT Behavior Discovery STUN usage would include:
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o 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.
o 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.
o 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.
o A protocol specification that defines how the implementation
applies this usage.
The P2P scenario described above is a likely experimental test case
for this usage, but others applications are possible as well.
3. Overview of Operations
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 RFC3489 [RFC3489] 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.
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.
This section provides a descriptive overview of the typical use of
these attributes. Normative behavior is described in Sections 5, 6,
and 7.
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3.1. Determining NAT Mapping
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[RFC4787]. The client performs a series of
tests that make use of the OTHER-ADDRESS attribute; these tests are
described in detail in Section 4. 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.
3.2. Determining NAT Filtering
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[RFC4787]. The client performs a series
of tests that make use of the OTHER-ADDRESS and CHANGE-REQUEST
attributes; these tests are described in Section 4. 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. 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.
3.3. Binding Lifetime Discovery
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.
Binding lifetime can be discovered by performing timed tests that use
XOR-RESPONSE-TARGET. The client uses a 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 Section 4.5. This test applies
only to UDP datagrams.
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3.4. Diagnosing NAT Hairpinning
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.
3.5. Determining Fragment Handling
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.
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.
3.6. Detecting Generic ALGs
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.
4. Discovery Process
The NAT Behavior Discovery usage provides primitives that allow STUN
checks to be made to determine 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. An application must
assume that NAT behaviour can become more restrictive at any time.
The tests described here are for UDP connectivity, NAT mapping
behaviour, and NAT filtering behaviour; additional tests could be
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designed using this usage's mechanisms. Definitions for NAT
filtering and mapping behaviour are from [RFC4787].
4.1. Checking if UDP is Blocked
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 is not capable of UDP connectivity. This test
requires only RFC3489-bis [I-D.ietf-behave-rfc3489bis] functionality.
4.2. Determining NAT Mapping Behavior
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.
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.
4.3. Determining NAT Filtering Behavior
This will also require at most three tests. These tests should be
performed using a port that wasn't used for mapping or other tests as
packets sent during those tests may affect results. 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.
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.
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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.
4.4. Combining and Ordering Tests
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 Section 4.2. 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 ICE [I-D.ietf-mmusic-ice], but
it does provide some information regarding whether ICE is likely to
be successful establishing non-relayed connections.
Care must be taken when parallelizing tests, as some NAT devices have
an upper limit on how quickly bindings will be allocated.
4.5. Binding Lifetime Discovery
STUN can also be used to probe the lifetimes of the bindings created
by 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.
ICE [I-D.ietf-mmusic-ice] and OUTBOUND [I-D.ietf-sip-outbound] have
suggested refresh intervals.
To determine the binding lifetime, the client first sends a Binding
Request to the server from a particular socket, 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
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another Binding Request to the server, using the same destination
address and port, but from a different socket, Y. This request
contains an XOR-RESPONSE-TARGET address attribute, set 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, if it
still exists. If the client receives the Binding Response on socket
X, it knows that the binding has not expired. If the client receives
the Binding Response on socket 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.
Because some NATs only refresh bindings when outbound traffic is
sent, the client must resend a binding request on the original 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.
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.
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.
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
[I-D.ietf-sip-outbound]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.
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5. Client Behavior
Unless otherwise specified here, all procedures for preparing,
sending, and processing messages as described in the STUN Binding
Usage [I-D.ietf-behave-rfc3489bis] are followed.
If a client intends to utilize an XOR-RESPONSE-TARGET attribute in
future transactions, as described in Section 4.5, 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.
Support for XOR-RESPONSE-TARGET is optional due to the state cost on
the server. Therefore, a client MUST be prepared for receiving 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.
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.
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.
5.1. Discovery
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 [RFC2782]. 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.
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For the Behavior Discovery Usage the service name is "stun-behavior".
The protocol can be "udp", "tcp" or "tls". Other aspects of handling
failures and default ports are followed as described in STUN
[I-D.ietf-behave-rfc3489bis].
5.2. Security
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 STUN [I-D.ietf-behave-rfc3489bis].
6. Server Behavior
Unless otherwise specified here, all procedures for preparing,
sending, and processing messages as described for the STUN Binding
Usage of STUN [I-D.ietf-behave-rfc3489bis] are followed.
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 two UDP ports, such that it can
send and receive datagrams using the same ports on each IP address
(normally a wildcard binding accomplishes this). 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".
6.1. Preparing the Response
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, or PADDING. If the Request does not contain any
attributes from this document, OTHER-ADDRESS and RESPONSE-ORIGIN are
still included in the response.
The server MUST include both MAPPED-ADDRESS and XOR-MAPPED-ADDRESS in
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its Response.
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.
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.
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.
If XOR-RESPONSE-TARGET is included in a Request, then the server must
verify that it has previously received a binding request 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.
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.
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 it receives requests more often than one per second,
it MUST generate a 503 (Service unavailable) Response to the Request.
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 Table 1.
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
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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.
+--------------------+----------------+-------------+---------------+
| Flags | Source Address | Source Port | OTHER-ADDRESS |
+--------------------+----------------+-------------+---------------+
| none | Da | Dp | Ca:Cp |
| Change IP | Ca | Dp | Ca:Cp |
| Change port | Da | Cp | Ca:Cp |
| Change IP and | Ca | Cp | Ca:Cp |
| Change port | | | |
+--------------------+----------------+-------------+---------------+
Table 1: Impact of Flags on Packet Source and OTHER-ADDRESS
The server MUST add a RESPONSE-ORIGIN attribute to the Binding
Response, containing the source address and port used to send the
Binding Response.
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 Table 1, these are Ca and Cp,
respectively, regardless of the value of the CHANGE-REQUEST flags.
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.
If the Request contained a PADDING attribute, then the server SHOULD
insert a PADDING attribute of the same length into its response, but
no longer than 64K. If the Request also contains the XOR-RESPONSE-
TARGET attribute the server MUST return an error response of type
400.
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Following that, the server completes the remainder of the processing
from STUN [I-D.ietf-behave-rfc3489bis]. The server MAY include a
SERVER attribute. 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. An ALTERNATE-
SERVER attribute SHOULD NOT be included.
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.
7. New Attributes
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 RFC3489 [RFC3489] but is redefined here as that
document is obsoleted by RFC3489bis [I-D.ietf-behave-rfc3489bis].
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
7.1. Representing Transport Addresses
Whenever an attribute contains a transport address, it has the same
format as MAPPED-ADDRESS. Similarly, the XOR- attributes have the
same format as XOR-MAPPED-ADDRESS[I-D.ietf-behave-rfc3489bis].
7.2. CHANGE-REQUEST
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
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send the Binding Responses from the alternate source IP address
and/or alternate port. The CHANGE-REQUEST attribute is optional in
the Binding Request.
The attribute is 32 bits long, although only two bits (A and B) are
used:
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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The meanings of the flags are:
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.
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.
7.3. RESPONSE-ORIGIN
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.
7.4. OTHER-ADDRESS
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.
OTHER-ADDRESS uses the same attribute as CHANGED-ADDRESS from RFC3489
because it is simply a new name with the same semantics as CHANGED-
ADDRESS. It has been renamed to more clearly indicate its function.
7.5. XOR-REFLECTED-FROM
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-
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service attacks.
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.
7.6. XOR-RESPONSE-TARGET
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. 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 that does not contain
MESSAGE-INTEGRITY. The XOR-RESPONSE-TARGET attribute is optional in
the Binding Request.
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.
7.7. PADDING
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. When PADDING is used, it SHOULD be 1500 bytes long, unless a
more appropriate length is known based on the MTU of the path.
PADDING can be used in either Binding Requests or Binding Responses.
If PADDING is present in the Binding Request and the server supports
it, PADDING MUST be present in the Binding Response. The server
SHOULD use the same length PADDING as was used in the Binding
Request, but it MAY use another length if it knows what length is
required to cause fragmentation along the return path. If the server
supports PADDING (i.e. doesn't return a 420 in response to a Request
containing PADDING), then it MUST use either the requested length or
a length it knows is sufficient to cause fragmentation.
PADDING MUST be no longer than 64K and SHOULD be 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.
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7.8. CACHE-TIMEOUT
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 SHOULD NOT
send a response to a target address requested with XOR-RESPONSE-
TARGET unless it has cached that the same USERNAME made a previous
binding request 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.
8. New Response Codes
This draft defines new STUN response code.
8.1. 481 Connection does not exist
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 re-submit the original binding request with an
appropriate CACHE-TIMEOUT attribute. If the server's response
includes a CACHE-TIMEOUT that is shorter than the client's request,
the server is unable to satisfy the caching time requested by the
client and the client SHOULD NOT continue to retry the request.
8.2. 503 Service Unavailable
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.
9. 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
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through a collaborative protocol reflection mechanism RFC 3424
[RFC3424]. 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.
9.1. Problem Definition
From RFC 3424 [RFC3424], 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 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.
9.2. Exit Strategy
From [RFC3424], any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better short
term fixes are the ones that will naturally see less and less use
as the appropriate technology is deployed.
The STUN NAT Behavior Discovery usage does not itself provide an exit
strategy. Instead, that is provided by other initiatives. Work is
currently proceeding on proposals for protocols that allow clients to
determine the location of and control the behavior of NATs through
direct interaction with the NAT; Nat Control STUN Usage
[I-D.wing-behave-nat-control-stun-usage] STUN NAT Behavior Discovery
is no longer needed once NATs that can be communicated with directly
are in use. Finally, as NATs phase out and as IPv6 is deployed, STUN
NAT Behavior Discovery will no longer be of any interest.
9.3. Brittleness Introduced by STUN NAT Behavior Discovery
From [RFC3424], any UNSAF proposal must provide:
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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.
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.
9.4. Requirements for a Long Term Solution
From [RFC3424]}, any UNSAF proposal must provide:
Identify requirements for longer term, sound technical solutions
-- contribute to the process of finding the right longer term
solution.
As long as NATs are present, means of adapting to their presence will
be required. Direct control or discovery of NATs by applications,
such as proposed in Nat Control STUN Usage
[I-D.wing-behave-nat-control-stun-usage], will eliminate the need for
anonymous diagnostics of NAT behavior.
9.5. Issues with Existing NAPT Boxes
From [RFC3424], 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 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.
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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.
10. IANA Considerations
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.
0x0003: CHANGE-REQUEST
0x0027: XOR-RESPONSE-TARGET
0x0028: XOR-REFLECTED-FROM
0x0026: PADDING
0x8027: CACHE-TIMEOUT
0x802b: RESPONSE-ORIGIN
0x802c: OTHER-ADDRESS
This specification defines two new STUN error response codes.
481: Connection does not exist
503: Service Unavailable
11. Security Considerations
This usage inherits the security considerations of STUN
[I-D.ietf-behave-rfc3489bis]. This usage adds several new
attributes; security considerations for those are detailed here.
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 Request can completely hide the STUN server
from the client.
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. XOR-RESPONSE-TARGET is rate-limited or uses pre-existing
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credentials to alleviate this threat. Server caching previous
contacts before directing a response to a XOR-RESPONSE-TARGET further
eliminates the threat, although it introduces the complexity of state
into a STUN server. CACHE-TIMEOUT is used to reduce the amount of
additional state required.
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.
CHANGE-REQUEST provides no attacks, but adds three more reflection
sources for the XOR-RESPONSE-TARGET reflection attacks. It provides
no additional amplification and the security mechanisms for XOR-
RESPONSE-TARGET are deemed sufficient.
RESPONSE-ORIGIN, CACHE-TIMEOUT and XOR-REFLECTED-FROM do not provide
any additional attacks.
12. Open Issues
Does IANA consider attributes that were in 3489 but not in 3489bis to
have been removed from the registry and should be re-registered by
this document, or are there forever in the registry from 3489?
13. Acknowledgements
The authors would like to thank the authors of the original STUN
specification [RFC3489] from which many of the ideas, attributes, and
description in this document originated.
14. References
14.1. Normative References
[I-D.ietf-behave-rfc3489bis]
Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for (NAT) (STUN)",
draft-ietf-behave-rfc3489bis-15 (work in progress),
February 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
14.2. Informative References
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[I-D.ietf-sip-outbound]
Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-11 (work in progress),
November 2007.
[I-D.wing-behave-nat-control-stun-usage]
Wing, D., Rosenberg, J., and H. Tschofenig, "Discovering,
Querying, and Controlling Firewalls and NATs",
draft-wing-behave-nat-control-stun-usage-05 (work in
progress), October 2007.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
March 2003.
Appendix A. Change Log
RFC-EDITOR: Please remove this entire Change Log section while
formatting this document for publication.
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A.1. from draft-macdonald-behave-nat-behavior-diagnostics-00
o Only OTHER-ADDRESS, CHANGE-ADDRESS, and XOR-RESPONSE-TARGET
support is optional; support for PADDING and SOURCE-ADDRESS is now
mandatory
o PADDING is now a mandatory attribute
o OTHER-ADDRESS is returned in all binding responses if the server
has a second IP address
A.2. from draft-ietf-behave-nat-behavior-discovery-00
o Clarified that only servers with two IP addresses should have an
SRV entry
o 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.
o 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)
o Require both MAPPED-ADDRESS and XOR-MAPPED-ADDRESS to be returned
to support detection of generic ALGs
A.3. from draft-ietf-behave-nat-behavior-discovery-01
o Changed proposed status to experimental
o Made significant changes to the introduction and applicability
statements to reflect the experimental status
o Fixed the New Attributes and IANA considerations not listing the
same attribute numbers.
o 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.
o Made OTHER-ADDRESS and SOURCE-ADDRESS optional to address
compatibility with 3489bis clients. Renamed SOURCE-ADDRESS as
RESPONSE-ORIGIN to avoid conflicts with 3489.
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o Renamed XOR-RESPONSE-ADDRESS to XOR-RESPONSE-TARGET
o Added discussion of FINGERPRINT and ALTERNATE-SERVER for
compliance with 3489bis stun usage definition requirements.
A.4. from draft-ietf-behave-nat-behavior-discovery-02
o fix terminology for endpoint-independent, address-dependent, and
address and port-dependent from rfc4787
o define the ALG detection to apply to UDP and TCP
o fix >From typo in 9.5
o added exception to single MTU size restriction for PADDING
o 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.
Authors' Addresses
Derek C. MacDonald
CounterPath Solutions, Inc.
Suite 300, One Bentall Centre, 505 Burrard St
Vancouver, BC V7X1M3
Canada
Phone: +1-604-320-3344
Email: derek@counterpath.com
Bruce B. Lowekamp
SIPeerior Technologies and William & Mary
3000 Easter Circle
Williamsburg, Virginia 23188
USA
Phone: +1-757-565-0101
Email: lowekamp@sipeerior.com
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Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
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MacDonald & Lowekamp Expires August 28, 2008 [Page 28]
