BEHAVE D. Wing
Internet-Draft J. Rosenberg
Intended status: Standards Track Cisco Systems
Expires: August 17, 2007 February 13, 2007
Controlling NAT Bindings using STUN
draft-wing-behave-nat-control-stun-usage-01
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Copyright (C) The IETF Trust (2007).
Abstract
Simple Traversal Underneath NAT (STUN) is a mechanism for traversing
NATs. STUN requests are transmitted through a NAT to external STUN
servers. While this works very well, its two primary drawbacks are
the inability to modify the properties of a NAT binding and the need
to query a public STUN server for every NAT binding. These drawbacks
require frequent messages which present a load on servers (like SIP
servers and STUN servers) and are bad for low speed access networks,
such as cellular. This document proposes that the STUN server be
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embedded in the NAT itself, and describes how these STUN servers can
be readily discovered and utilized to reduce queries to public STUN
servers and to reduce NAT keepalive traffic.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Conventions Used in this Document . . . . . . . . . . . . . . 4
4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 4
4.1. Nested NATs . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Interacting with Legacy NATs . . . . . . . . . . . . . . . 9
5. NAT Control Usage . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Client Discovery of Server . . . . . . . . . . . . . . . . 10
5.3. Server Determination of Usage . . . . . . . . . . . . . . 10
5.4. New Requests or Indications . . . . . . . . . . . . . . . 10
5.5. New Attributes . . . . . . . . . . . . . . . . . . . . . . 10
5.5.1. XOR-INTERNAL-ADDRESS . . . . . . . . . . . . . . . . . 11
5.5.2. REFRESH-INTERVAL . . . . . . . . . . . . . . . . . . . 11
5.6. Client Procedures . . . . . . . . . . . . . . . . . . . . 11
5.7. Server Procedures . . . . . . . . . . . . . . . . . . . . 12
6. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Incremental Deployment . . . . . . . . . . . . . . . . . . 13
6.2. Optimize SIP-Outbound . . . . . . . . . . . . . . . . . . 14
6.3. Optimize ICE . . . . . . . . . . . . . . . . . . . . . . . 14
6.3.1. Candidate Gathering . . . . . . . . . . . . . . . . . 14
6.3.2. Keepalive . . . . . . . . . . . . . . . . . . . . . . 14
6.3.3. Learning STUN Servers without Configuration . . . . . 15
7. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Overlapping IP Addresses with Nested NATs . . . . . . . . 15
7.2. Address Dependent NAT on Path . . . . . . . . . . . . . . 16
7.3. Address Dependent Filtering . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8.1. Authorization and Resource Exhaustion . . . . . . . . . . 17
8.2. Comparison to Other NAT Control Techniques . . . . . . . . 17
8.3. Rogue STUN Server . . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
10.2. Informational References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 21
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1. Introduction
Two common usages of STUN [I-D.ietf-behave-rfc3489bis] are Binding
Discovery and NAT Keepalive. The Binding Discovery usage allows a
STUN client to learn its public IP address (from the perspective of
the STUN server it contacted) and the NAT keepalive usage allows a
STUN client to keep an active NAT binding alive. Unlike some other
techniques (e.g., UPnP [UPnP], MIDCOM [RFC3303], Bonjour [Bonjour]),
STUN does not interact directly with the NAT. Because STUN doesn't
interact directly with the NAT, STUN cannot request additional
services from the NAT such as longer lifetimes (which would reduce
keepalive messages).
This paper describes a mechanism for the STUN client to interact
directly with the NAT and request additional services, by using the
STUN protocol itself. Thereafter, the STUN client need only ask that
NAT's embedded STUN server for public IP addresses and UDP ports --
as it will return the same information as the public STUN server.
Additionally, the STUN client can ask the NAT's embedded STUN server
to extend the NAT binding for the flow, and the STUN client can learn
the IP address of the next-outermost NAT. By repeating this
procedure with the next-outermost NAT, all of the NATs along that
path can have their bindings extended. By learning all of the STUN
servers on the path between the public Internet and itself, an
endpoint can optimize the path of peer-to-peer communications.
2. Motivation
There are a number of problems with existing NAT traversal techniques
such as STUN [RFC3489], [UPnP], and [Bonjour]):
nested NATs
Today, many ISPs provide their subscribers with modems that have
embedded NATs, often with only one physical Ethernet port. These
subscribers then install NATs behind those devices to provide
additional features, such as wireless access.
Nested NATs are, unfortunately, becoming quite common and often
occur without the knowledge of users. For example, some ISPs
provide their subscribers with modems that include integrated NAT
functionality. When the subscriber installs another NAT, perhaps
to provide himself with wireless network access, the endpoints are
now behind nested NATs. Another example is a NAT in the basement
of an apartment building or a campus dormitory, which combined
with a NAT within the home or dormitory room also result in nested
NATs. In both of these situations, UPnP and Bonjour no longer
function at all, as those protocols can only control the first
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NAT. STUN continues to function, but is unable to optimize
network traffic behind those nested NATs (e.g., traffic that stays
within the same house or within the same apartment building). The
technique described in this paper allows optimization of the
traffic behind those NATs so that the traffic can traverse the
fewest NATs possible.
chattiness
To perform its binding discovery, a STUN client communicates to a
server on the Internet. This consumes bandwidth across the user's
access network which in some cases is bandwidth constrained (e.g.,
wireless, satellite).
STUN's chattiness is often cited as a reason to use other NAT
traversal techniques such as UPnP or Bonjour. However, those NAT
traversal techniques bring restrictions (they both require a UPnP-
aware or Bonjour-aware NAT, they do not work with nested NATs, and
they only work within one broadcast domain). The technique
described in this paper provides a significant reduction in STUN's
chattiness, to the point that the only time a STUN client needs to
communicate with the STUN server on the public Internet is when
the STUN client is initialized.
incremental deployment
Many NAT traversal techniques require the endpoint and the NAT to
both support the new feature or else NAT traversal isn't possible
at all.
However, the technique described in this paper allows incremental
deployment of local endpoints, local NATs, and remote endpoints
and their remote NATs which support the features described in this
paper. Only the local endpoints and the NATs on the path to their
STUN server need to implement the technique in this paper to
optimize their functionality.
3. Conventions Used in this Document
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 [RFC2119].
4. Overview of Operation
When a STUN client sends a STUN Request to a STUN server, it receives
a STUN Response which indicates the IP address and UDP port seen by
the STUN server. If the IP address and UDP port differs from the IP
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address and UDP port of the socket used to send the request, the STUN
client knows there is at least one NAT between itself and the STUN
server, and knows the 'public' IP address of that NAT. For example,
in the following diagram, the STUN client learns the public IP
address of its NAT is 192.0.2.1:
+--------+ +---------------+
| STUN | | 192.0.2.1 +--------+
| Client +-------------+ +---<Internet>---+ STUN |
| 10.1.1.2/4193 10.1.1.1 | | Server |
+--------+ | | +--------+
| NAT with |
| Embedded STUN |
| Server |
+---------------+
Figure 1: One NAT with embedded STUN server
After learning the public IP address of its outer-most NAT, the STUN
client attempts to communicate with the STUN server embedded in that
outer-most NAT. The STUN client does this by first obtaining a
shared secret, over a TLS connection, to the NAT's public IP address
(192.0.2.1 in the figure above). After obtaining a shared secret, it
sends a STUN Binding Request to the NAT's public IP address. The NAT
will return a STUN Binding Response message including the XOR-
INTERNAL-ADDRESS attribute, which will indicate the IP address and
UDP port seen on the *internal* side of the NAT for that translation.
In the example above, the IP address and UDP port indicated in XOR-
INTERNAL-ADDRESS are the same as that used by the STUN client
(10.1.1.2/4193), which indicates there are no other NATs between the
STUN client and that outer-most NAT.
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STUN Client NAT STUN Server
| | |
1. |-----TLS/TCP----------------------------->| }
2. |-----Shared Secret Request (TLS)--------->| }
3. |<----Shared Secret Response (TLS)---------| } normal STUN
4. |-----TCP connection closed--------------->| } behavior
5. |-----Binding Request (UDP)--------------->| }
6. |<----Binding Response (UDP)---------------| }
| | |
7. |-----TLS/TCP------------------>| | }
8. |--Shared Secret Request (TLS)->| | }
9. |<-Shared Secret Response (TLS)-| | } NAT Control
10. |--TCP connection closed------->| | } STUN Usage
11. |--Binding Request (UDP)------->| | }
12. |<-Binding Response (UDP)-------| | }
| | |
Figure 2: Communication Flow
In the call flow above, steps 1-6 are normal STUN behavior
[I-D.ietf-behave-rfc3489bis]:
1: STUN client initiates a TLS-over-TCP connection to its STUN
server on the Internet.
2: Using that connection, the STUN client sends Shared Secret
Request to that STUN server.
3: Using that connection, the STUN server sends Shared Secret
Response. This contains the STUN username the client should use
for subsequent queries to this STUN server, and the STUN password
that will be used to integrity-protect subsequent STUN messages
with this STUN server.
4: TCP connection is closed.
5: STUN client sends UDP Binding Request to its STUN server on the
Internet, using the STUN username obtained from that STUN server
(from step 3).
6: STUN server responds with UDP Binding Response, integrity
protected with the STUN password (from step 3). The STUN client
now knows the public IP address of its outer-most NAT. This is
used in the next step.
The next steps are the additional steps performed by a STUN client
that has implemented the NAT Control STUN Usage:
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7: STUN client initiates a TLS-over-TCP connection to the STUN
server embedded in its outer-most NAT.
8: Using that connection, the STUN client sends Shared Secret
Request to that STUN server.
9: Using that connection, the STUN server sends Shared Secret
Response. This contains the STUN username the client should use
for subsequent queries to this STUN server, and the STUN
password that will be used to integrity-protect subsequent STUN
messages with this STUN server.
10: TCP connection is closed.
11: STUN client sends UDP Binding Request to the STUN server
embedded in its outer-most NAT, using the STUN username obtained
from from that STUN server (from step 10).
12: STUN server responds with UDP Binding Response, integrity
protected with the STUN password (from step 10).
The response obtained in the message 12 contains the XOR-MAPPED-
ADDRESS attribute which will have the same value as when the STUN
server on the Internet responded (in step 6). The STUN client can
perform steps 11-12 for any new UDP communication (e.g., for every
new phone call), without needing to repeat steps 1-10. This meets
the desire to reduce chattiness.
The response obtained in message 12 will also contain the XOR-
INTERNAL-ADDRESS, which allows the STUN client to repeat steps 7-12
in order to communicate with all the on-path NATs between itself and
its STUN server on the Internet. This is described in detail in
section Section 4.1. This meets the desire to optimize traffic
between nested NATs.
The STUN client can request each NAT to increase the binding
lifetime, as described in Section 5.5. The STUN client receives
positive confirmation that the binding lifetime has been extended,
allowing the STUN client to significantly reduces its NAT keepalive
traffic. Additionally, as long as the NAT complies with [RFC4787],
the STUN client's keepalive traffic need only be sent to the outer-
most NAT's IP address. This further meets the desire to reduce
chattiness.
4.1. Nested NATs
Nested NATs are controlled individually. The nested NATs are
discovered, from outer-most NAT to the inner-most NAT, using the XOR-
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INTERNAL-ADDRESS attribute.
The following diagram shows how a STUN client iterates over the NATs
to communicate with all of the NATs in the path. First, the STUN
client would learn the outer-most NAT's IP address by performing the
steps above. In the case below, however, the IP address and UDP port
indicated by the XOR-INTERNAL-ADDRESS will not be the STUN client's
own IP address and UDP port -- rather, it's the IP address and UDP
port on the *outer* side of the NAT-B -- 10.1.1.2.
Because of this, the STUN client repeats the procedure and sends
another STUN Binding Request to that newly-learned address (the
*outer* side of NAT-B). NAT-B will respond with a STUN Binding
Response containing the XOR-INTERNAL-ADDRESS attribute, which will
match the STUN client's IP address and UDP port. The STUN client
knows there are no other NATs between itself and NAT-B, and finishes.
+------+ +--------+ +--------+
| 192.168.1.2 | 10.1.1.2 | 192.0.2.1 +-----------+
| STUN +------+ NAT-B +-----+ NAT-A +---<Internet>---+STUN Server|
|Client| 192.168.1.1 | 10.1.1.1 | +-----------+
+------+ +--------+ +--------+
Figure 3: Two NATs with embedded STUN servers
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Internally, the NAT can be diagrammed to function like this, where
the NAT operation occurs before the STUN server.
|
| outside interface
|
+---------+---------------+
| | |
| | +--------+ |
| |----+ STUN | |
| | | Server | |
| | +--------+ |
| | |
| +-------+-------------+ |
| | NAT Function | |
| +-------+-------------+ |
| | |
+---------+---------------+
|
| inside interface
|
|
Figure 4: Block Diagram of Internal NAT Operation
4.2. Interacting with Legacy NATs
There will be cases where the STUN client attempts to communicate
with an on-path NAT which does not support the usage described in
this document. There are two cases:
o the NAT does not run a STUN server on its public interface (this
will be the most common)
o the NAT does run a STUN server on its public interface, but
doesn't return the XOR-INTERNAL-ADDRESS attribute defined in this
document
In both cases the optimizations described in this document won't be
available to the STUN client and the STUN client. This is no worse
than the condition today. This allows incremental upgrades of
applications and NATs that implement the technique described in this
document.
5. NAT Control Usage
This section describes a new STUN usage, following the recommendation
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for defining a new usage in [I-D.ietf-behave-rfc3489bis].
5.1. Applicability
This STUN usage MAY be used by a STUN client that discovers there is
a NAT between itself and its STUN server. Such discovery would most
likely occur with a STUN Binding Request / Binding Response exchange
to a STUN server on the Internet, and by noticing the IP address and
UDP port indicated by the XOR-MAPPED-ADDRESS attribute of the STUN
Binding Response differs from the local socket's IP address and UDP
port. Such a difference indicates a NAT is present between the STUN
client and its STUN server.
5.2. Client Discovery of Server
As this usage involves communicating with on-path NATs directly, the
client needs to find those NATs. The outer-most NAT is found by the
normal XOR-MAPPED-ADDRESS attribute in a STUN Response. To iterate
through the inner NATs, each NAT needs to support the usage described
in this document, and the STUN client discovers each of those NATs by
iterating through the XOR-INTERNAL-ADDRESS attribute returned by
those NATs. This is described in diagrams and examples in Section 4.
5.3. Server Determination of Usage
If a STUN Binding Request is received from a NAT's private interface
and sent to the IP address of its public interface, the STUN server
can assume the NAT Control Usage.
5.4. New Requests or Indications
This usage does not define any new message types.
5.5. New Attributes
The following figure indicates which attributes are present in which
messages for this usage. An M indicates that inclusion of the
attribute in the message is mandatory, O means its optional, C means
it's conditional based on some other aspect of the message, and -
means that the attribute is not applicable to that message type.
Error
Attribute Request Response Response Indication
_________________________________________________________________
XOR-INTERNAL-ADDRESS - M - -
REFRESH-INTERVAL O C - -
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5.5.1. XOR-INTERNAL-ADDRESS
This attribute MUST be present in a Binding Response and may be used
in other responses as well. This attribute is necessary to allow a
STUN client to 'walk backwards' and communicate directly with all of
the STUN-aware NATs along the path.
The format of the XOR-INTERNAL-ADDRESS attribute is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x x x x x x x x| Family | X-Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| X-Address (Variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: XOR-INTERNAL-ADDRESS Attribute
The meaning of Family, X-Port, and X-Address are exactly as in
[I-D.ietf-behave-rfc3489bis]. The length of X-Address depends on the
address family (IPv4 or IPv6).
5.5.2. REFRESH-INTERVAL
The REFRESH-INTERVAL attribute is defined in
[I-D.ietf-behave-rfc3489bis] where it can only appear in a response.
In the NAT Control usage defined in this document, the REFRESH-
INTERVAL may also appear in a request.
In a Binding Request, the REFRESH-INTERVAL indicates the desired
mapping timeout. In a Binding Response, the REFRESH-INTERVAL
indicates the NAT's mapping timeout.
5.6. Client Procedures
The STUN client sends a STUN Binding Request to its STUN server on
the Internet and receives a STUN Binding Response, as normal. The
STUN Binding Response contains the XOR-MAPPED-ADDRESS attribute which
indicates the IP address and UDP port of the STUN Binding Request, as
seen by the STUN server. If this IP address differs from the STUN
client's IP address, the STUN client knows there is at least one NAT
between itself and the STUN server, and it continues with the
procedure; otherwise, it stops.
The STUN client now knows the public IP address of its outer-most NAT
-- it was returned in the XOR-MAPPED-ADDRESS attribute. The STUN
client performs the Shared Secret Usage (as described in
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[I-D.ietf-behave-rfc3489bis]) with the public IP address of its
outer-most NAT. After performing that usage, the STUN client now has
a STUN USERNAME and PASSWORD which authenticate subsequent messages
between the STUN client and this NAT's STUN server.
If subsequent messages from the STUN server fail authentication, the
STUN client MUST re-obtain its IP address from a public STUN server,
not from its outer-most NAT (see section Section 8.3).
To modify an existing NAT mapping's attributes, or to request a new
NAT mapping for a new UDP port, the STUN client can now send a STUN
Binding Request to the IP address of address in its outer-most NAT's
STUN UDP port (3478). The NAT's STUN server will respond with a STUN
Binding Response containing an XOR-MAPPED-ADDRESS attribute (which
points at the NAT's public IP address and port -- just as if the STUN
Binding Request had been sent to a STUN server on the public
Internet) and an XOR-INTERNAL-ADDRESS attribute (which points to the
source IP address and UDP port the packet STUN Binding Request packet
had prior to being NATted).
If the XOR-INTERNAL-ADDRESS attribute indicates an IP address and UDP
port different from the STUN client's own IP address and port, the
STUN client knows there is at least one NAT between itself and the
STUN server it last contacted. If the STUN client wants to use
multiple STUN servers, or wants to control the properties of the NAT
bindings in each of those NATs, the STUN client can iteratively
perform the Short-Term Password Usage followed by the Binding
Discovery Usage with each NAT learned via the XOR-INTERNAL-ADDRESS
attribute from the previous NAT.
In each case where the STUN client is sending STUN Binding Requests
to the NATs, the STUN client can also include other STUN attributes
to request certain properties for the flow. Requesting certain
properties may require the STUN client to obtain short-term
credentials. Defined in this document is a requested lifetime for
the NAT binding in order to reduce keepalive traffic (REFRESH-
INTERVAL).
5.7. Server Procedures
The server should listen for STUN Shared Secret Requests and STUN
Binding Requests on the STUN UDP and TCP ports (UDP/3478, TCP/3478)
on its public IP address, from hosts connected to its private
interface(s). The NAT SHOULD only respond to such message which
arrive from its 'internal' interface. STUN Binding Requests sent to
the NAT's public IP address which arrived from its public interface
SHOULD be handled as if the NAT isn't listening on that port (e.g.,
return an ICMP error).
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After receiving a STUN Shared Secret Request, the NAT follows the
procedures described in the Short-Term Usage section of
[I-D.ietf-behave-rfc3489bis]. The embedded STUN server MUST store
that username and password so long as any NAT bindings, created or
adjusted with that same STUN username, have active mappings on the
NAT.
After receiving a STUN Binding Request containing the REFRESH-
INTERVAL attribute, the server SHOULD authenticate the request using
the USERNAME attribute and the previously-shared STUN password (this
is to defend against resource starvation attacks, see Section 8.1).
If authenticated, the new binding's lifetime can be maximized against
the NAT's configured sanity check and the lifetime indicated in the
REFRESH-INTERVAL attribute of the STUN Binding Response.
In addition to its other attributes, the Binding Response always
contains the XOR-MAPPED-ADDRESS and XOR-INTERNAL-ADDRESS attributes.
The XOR-MAPPED-ADDRESS contains the public IP address and UDP port
for this binding. The XOR-INTERNAL-ADDRESS contains the IP address
and UDP port of the STUN Binding Request prior to the NAT
translation. The XOR-INTERNAL-ADDRESS is used by the STUN client to
walk backwards through nested NATs.
For example, looking at Figure 1, the XOR-INTERNAL-ADDRESS is the
IP address and UDP port _prior to_ the NAPT operation. If there
is only one NAT, as shown in Figure 1, XOR-INTERNAL-ADDRESS would
contain the STUN client's own IP address and UDP port. If there
are multiple NATs, XOR-INTERNAL-ADDRESS would indicate the IP
address of the next NAT (that is, the next NAT closer to the STUN
client). Iterating over this procedure allows the STUN client to
find all of the NATs along the path.
6. Benefits
6.1. Incremental Deployment
NAT Control can be incrementally deployed. If the outer-most NAT
does not support it, the STUN client behaves as normal. Where the
outer-most STUN NAT does support it, the STUN client can gain some
significant optimizations as described in the following sections.
Likewise, there is no change to applications if NATs are deployed
which support NAT Control.
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6.2. Optimize SIP-Outbound
In sip-outbound [I-D.ietf-sip-outbound], the SIP proxy is also the
STUN server. This document enables two optimizations of SIP-
Outbound's keepalive mechanism:
1. STUN keepalive messages need only be sent to the outer-most NAT,
rather than across the access link to the SIP proxy, which vastly
reduces the traffic to the SIP proxy, and;
2. all of the on-path NATs can explicitly indicate their timeouts,
reducing the frequency of keepalive messages.
6.3. Optimize ICE
The NAT Control usage provides several opportunities to optimize ICE
[I-D.ietf-mmusic-ice].
6.3.1. Candidate Gathering
During its candidate gathering phase, an ICE endpoint normally
contacts a STUN server on the Internet. If an ICE endpoint discovers
that its outer-most NAT runs a STUN server, the ICE endpoint can use
the outer-most NAT's STUN server rather than using the STUN server on
the Internet. This saves access bandwidth and reduces the reliance
on the STUN server on the Internet -- the STUN server on the Internet
need only be contacted once.
6.3.2. Keepalive
[Note: In ICE-13, the keepalives were changed to STUN
Indications. If this change to ICE becomes working group
consensus for ICE keepalives, this section in this document should
be deleted.]
ICE uses STUN as its primary media stream keepalive mechanism. This
document enables two optimizations of ICE's keepalive techniques:
1. STUN keepalive messages need only be sent to the outer-most NAT,
rather than across the access link to the remote peer, and;
2. all of the on-path NATs can explicitly indicate their timeouts,
reducing the frequency of keepalive messages.
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6.3.3. Learning STUN Servers without Configuration
ICE allows endpoints to have multiple STUN servers, but it is
difficult to configure all of the STUN servers in the ICE endpoint --
it requires some awareness of network topology. By using the 'walk
backward' technique described in this document, all the on-path NATs
and their embedded STUN servers can be learned without additional
configuration. By knowing the STUN servers at each address domain,
ICE endpoints can optimize the network path between two peers.
For example, if endpoint-1 is only configured with the IP address of
the STUN server on the left, endpoint-1 can learn about NAT-B and
NAT-A. Utilizing the STUN server in NAT-A, endpoint-1 and endpoint-2
can optimize their media path so they make the optimal path from
endpoint-1 to NAT-A to endpoint-2:
+-------+ +-------+ +-------------+
endpoint-1---| NAT-A +--+--+ NAT-B +-------| STUN Server |
+-------+ | +-------+ +-------------+
|
endpoint-2
7. Limitations
7.1. Overlapping IP Addresses with Nested NATs
If nested NATs have overlapping IP address space, there will be
undetected NATs on the path. When this occurs, the STUN client will
be unable to detect the presence of NAT-A if NAT-A assigns the same
UDP port. For example, in the following figure, NAT-A and NAT-B are
both using 10.1.1.x as their 'private' network.
+------+ +--------+ +--------+
| 10.1.1.2 | 10.1.1.2 | 192.0.2.1
| STUN +-------+ NAT-A +-----+ NAT-B +------<Internet>
|client| 10.1.1.1 | 10.1.1.1 |
+------+ +--------+ +--------+
Figure 8: Overlapping Addresses with Nested NATs
When this situation occurs, the STUN client can only learn the outer-
most address. This isn't a problem -- the STUN client is still able
to communicate with the outer-most NAT and is still able to avoid
consuming access network bandwidth and avoid communicating with the
public STUN server. All that is lost is the ability to optimize
paths within the private network that has overlapped addresses.
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7.2. Address Dependent NAT on Path
In order to utilize the mechanisms described in this document, a STUN
Request is sent from the same source IP address and source port as
the original STUN Binding Discovery message, but is sent to a
different destination IP address -- it is sent to the IP address of
an on-path NAT. If there is an on-path NAT, between the STUN client
and the STUN server, with 'address dependent' or 'address and port-
dependent' mapping behavior (as described in section 4.1 of
[RFC4787]), that NAT will prevent a STUN client from taking advantage
of the technique described in this document. When this occurs, the
ports indicated by XOR-MAPPED-ADDRESS from the public STUN server and
the NAT's embedded STUN server will differ.
An example of such a topology is shown in the following figure:
+------+ +--------+ +--------+
| STUN | | 10.1.1.2 | 192.0.2.1
|client+-----+ NAT-A +---+ NAT-B +------<Internet>
| | 10.1.1.1 | 10.1.1.1 |
+------+ +--------+ +--------+
In this figure, NAT-A is a NAT that has address dependent mapping.
Thus, when the STUN client sends a STUN Binding Request to 192.0.2.1
on UDP/3478, NAT-A will choose a new public UDP port for that
communication. NAT-B will function normally, returning a different
port in its XOR-MAPPED-ADDRESS, which indicates to the STUN client
that a symmetric NAT exists between the STUN client and the STUN
server it just queried (NAT-B, in this example).
Figure 9: Address Dependant NAT on Path
Open issue: We could resolve this problem by introducing a new
STUN attribute which indicates the UDP port the STUN client wants
to control. However, this changes the security properties of NAT
Control, so this seems undesirable.
Open issue: When the STUN client detects this situation, should
we recommend it abandon the NAT Control usage, and revert to
operation as if it doesn't support the NAT Control usage?
7.3. Address Dependent Filtering
If there is an NAT along the path that has address dependent
filtering (as described in section 5 of [RFC4787]), and the STUN
client sends a STUN packet directly to any of the on-path NATs public
addresses, the address-dependent filtering NAT will filter packets
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from the remote peer. Thus, after communicating with all of the on-
path NATs the STUN client MUST send a UDP packet to the remote peer,
if the remote peer is known.
Discussion: How many filter entries are in address dependent
filtering NATs? If only one, this does become a real limitation
if NATs are nested; if they're not nested, the outer-most NAT can
avoid overwriting its own address in its address dependent filter.
8. Security Considerations
This security considerations section will be expanded in a subsequent
version of this document. So far, the authors have identified the
following considerations:
8.1. Authorization and Resource Exhaustion
Only hosts that are 'inside' a NAT, which a NAT is already providing
services for, can query or adjust the timeout of a NAT mapping.
A malicious STUN client could ask for absurdly long NAT bindings
(days) for many UDP sessions, which would exhaust the resources in
the NAT. To ensure the STUN client is not spoofing its IP address
when launching such an attack, the STUN server can challenge requests
to extend the timeout by sending a NONCE to the STUN client. The
STUN server can authorize an extension to the refresh timeout if a
new request is sent with that same NONCE value.
Without considering this document and without considering STUN or
other UNSAF NAT traversal techniques, a malicious TCP client can open
many TCP connections, and keep them open, causing resource exhaustion
in the NAT. A NAT which provide protection against such a TCP attack
can provide a similar level of protection, via the NONCE challange/
response, as they can for TCP sessions.
8.2. Comparison to Other NAT Control Techniques
Like UPnP, Bonjour, and host-initiated MIDCOM, the STUN usage
described in this document allows a host to learn its public IP
address and UDP port mapping, and to request a specific lifetime for
that mapping.
However, unlike those technologies, the NAT Control usage described
in this document only allows each UDP port on the host to create and
adjust the mapping timeout of its own NAT mappings. Specifically, an
application on a host can only adjust the duration of a NAT bindings
for itself, and not for another application on that same host, and
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not for other hosts. This provides security advantages over other
NAT control mechanisms where malicious software on a host can
surreptitiously create NAT mappings to another application or to
another host.
8.3. Rogue STUN Server
As described in Section 6, a STUN client can learn its outer-most NAT
runs an embedded STUN server. However, without the STUN client's
knowledge, the outer-most NAT may acquire a new IP address. This
could occur when the NAT moves to a new mobile network or its DHCP
lease expires. When the NAT acquires a new IP address, the STUN
client will send a STUN Binding Request to the NAT's prior public IP
address, which will be routed to the NAT's previous address.
If an attacker runs a rogue STUN server on that address, the attacker
has effectively compromised the STUN server (the attacked described
in section 12.2.1 of [RFC3489]). The attacker will send STUN Binding
Responses indicating his IP address, which will be indistinguishable,
to the STUN client, from the behavior of the legitimate STUN server.
To defend against this attack, the STUN client and STUN server obtain
a short-term password as described in section Section 5.6.
9. IANA Considerations
This section registers one new STUN attribute per the procedures in
[I-D.ietf-behave-rfc3489bis]:
0x0026 XOR-INTERNAL-ADDRESS
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[I-D.ietf-behave-rfc3489bis]
Rosenberg, J., "Simple Traversal Underneath Network
Address Translators (NAT) (STUN)",
draft-ietf-behave-rfc3489bis-05 (work in progress),
October 2006.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
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RFC 4787, January 2007.
[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.
10.2. Informational References
[UPnP] UPnP Forum, "Universal Plug and Play", 2000,
<http://www.upnp.org>.
[Bonjour] Apple Computer, "Bonjour", 2005,
<http://www.apple.com/macosx/features/bonjour/>.
[RFC3303] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and
A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Methodology for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-13 (work in progress), January 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-07 (work in progress),
January 2007.
Authors' Addresses
Dan Wing
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Email: dwing@cisco.com
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Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
USA
Email: jdrosen@cisco.com
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