wdiff draft-wing-behave-nat-control-stun-usage-01.txt draft-wing-behave-nat-control-stun-usage-02.txt

BEHAVE D. Wing
Internet-Draft J. Rosenberg
Intended status: Standards Track Cisco Systems
Expires: August 17, December 2, 2007 February 13, May 31, 2007

Discovering, Querying, and Controlling NAT Bindings Firewalls and NATs using STUN
draft-wing-behave-nat-control-stun-usage-01
draft-wing-behave-nat-control-stun-usage-02

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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 new NAT binding. binding (e.g., every
phone call). 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. cellular access.

This document proposes that the STUN server be
embedded in the NAT itself, and describes how several mechanisms to discover NATs and
firewalls and a method to query and control them. With these STUN servers
changes, binding discovery and keepalive traffic can be readily discovered and utilized reduced to reduce queries to public STUN
servers
involve only the necessary NATs or firewalls. At the same time,
backwards compatibility with NATs and to reduce NAT keepalive traffic. firewalls that do not support
this document is retrained.

This document is discussed on the BEHAVE mailing list,
<https://www1.ietf.org/mailman/listinfo/behave>, in anticipation of a
BoF at IETF69 in Chicago.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 4
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4
3. Conventions Used in this Document . . . . . . . . . . . . . . 4 5
4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 4
4.1. Nested NATs 5
5. Discovery of Middleboxes . . . . . . . . . . . . . . . . . . . 6
5.1. Outside-In . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Interacting with Legacy
5.1.1. Nested NATs . . . . . . . . . . . . . . . 9
5. NAT Control Usage . . . . . . 11
5.1.2. XOR-INTERNAL-ADDRESS Attribute . . . . . . . . . . . . 12
5.1.3. Interacting with Legacy NATs . . . . 9
5.1. Applicability . . . . . . . . . 13
5.2. Inside-Out . . . . . . . . . . . . . . . 10
5.2. Client Discovery of Server . . . . . . . . . 13
5.2.1. DEFAULT-ROUTE Attribute . . . . . . . . 10 . . . . . . . 14
5.3. Server Determination of Usage Tagging . . . . . . . . . . . . . . 10
5.4. New Requests or Indications . . . . . . . . . . . 14
5.3.1. PLEASE-TAG Attribute . . . . 10
5.5. New Attributes . . . . . . . . . . . . . 15
5.3.2. TAG Attribute . . . . . . . . . . . . 10
5.5.1. XOR-INTERNAL-ADDRESS . . . . . . . . 16
6. Query and Control . . . . . . . . . 11
5.5.2. REFRESH-INTERVAL . . . . . . . . . . . . . 17
6.1. REFRESH-INTERVAL Attribute . . . . . . 11
5.6. . . . . . . . . . . 17
6.2. Client Procedures . . . . . . . . . . . . . . . . . . . . 11
5.7. 17
6.3. Server Procedures . . . . . . . . . . . . . . . . . . . . 12
6. 18
7. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. 19
7.1. Simple Security Model . . . . . . . . . . . . . . . . . . 19
7.2. Incremental Deployment . . . . . . . . . . . . . . . . . . 13
6.2. 19
7.3. Optimize SIP-Outbound . . . . . . . . . . . . . . . . . . 14
6.3. 19
7.4. Optimize ICE . . . . . . . . . . . . . . . . . . . . . . . 14
6.3.1. 19
7.4.1. Candidate Gathering . . . . . . . . . . . . . . . . . 14
6.3.2. 20
7.4.2. Keepalive . . . . . . . . . . . . . . . . . . . . . . 14
6.3.3. 20
7.4.3. Learning STUN Servers without Configuration . . . . . 15
7. 20
8. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. 21
8.1. Overlapping IP Addresses with Nested NATs . . . . . . . . 15
7.2. 21
8.2. Address Dependent NAT on Path . . . . . . . . . . . . . . 16
7.3. 21
8.3. Address Dependent Filtering . . . . . . . . . . . . . . . 16
8. 22
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8.1. 22
9.1. Authorization and Resource Exhaustion . . . . . . . . . . 17
8.2. 23
9.2. Comparison to Other NAT Control Techniques . . . . . . . . 17
8.3. 23
9.3. Rogue STUN Server . . . . . . . . . . . . . . . . . . . . 18
9. 23
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. 24
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. 24
12.1. Normative References . . . . . . . . . . . . . . . . . . . 18
10.2. 24
12.2. Informational References . . . . . . . . . . . . . . . . . 19 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19 26
Intellectual Property and Copyright Statements . . . . . . . . . . 21 27

1. Introduction

Two common usages of STUN [I-D.ietf-behave-rfc3489bis] ([I-D.ietf-behave-rfc3489bis],[RFC3489])
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). messages), and each new NAT binding
(e.g., each phone call) requires communicating with the STUN server
on the Internet.

This paper describes a mechanism three mechanisms for the STUN client to interact
directly discover
NATs and firewalls that are on path with its STUN server. After
discovering the NAT NATs and request additional services, by using firewalls, the STUN protocol itself. Thereafter, the client can query and
control those devices using STUN. The STUN client need needs to only ask that
NAT's embedded
those STUN server servers (embedded in the NATs and firewalls) for public IP
addresses and UDP ports --
as it will return the same information as ports, thereby offloading that traffic from the public
STUN server. server on the Internet. 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 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 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
NAT. NAT closest to the host. 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).

One technique to avoid nested NATs is to disable one of the NATs
if it obtains an RFC1918 address on its WAN interface. This
merely sidesteps the problem. This technique is also ineffective
if the ISP is NATting its subscribers and the ISP restricts each
subscriber to one IP address.

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

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 deployment:
Many other NAT traversal techniques require the endpoint and the its
NAT to both support the new feature or else NAT traversal isn't
possible at all.

However, the

The technique described in this paper allows incremental
deployment of local endpoints, local NATs, and remote endpoints and their remote NATs which that support the features described in this
paper. Only STUN
Control. If the local endpoints and the NATs on endpoint, or its NATs, don't support the path to their
STUN server need Control functionality, normal STUN and ICE
[I-D.ietf-mmusic-ice] procedures are used to implement traverse the technique NATs
without the optimizations described in this paper to
optimize their functionality. paper.

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

This document describes three functions, which are all implemented
using the STUN protocol:

Discovery of Middleboxes (NATs and Firewalls):
This document describes three techniques to find NATs or
firewalls. The first technique uses STUN to find the outer-most
NAT and works itself towards the host. The second technique
requests on-path firewalls or NATs to append their IP address to a
STUN packet. The third technique sends a STUN packet to the
default router (which, in a small network, is often your NAT).
This is described in Section 5.

Querying Discovered Middleboxes:
After discovering a NAT or a firewall, it's useful to determine
characteristics of the NAT binding or the firewall pinhole. Two
of the most useful things to learn is the duration the NAT binding
or firewall pinhole will remain open if there is no traffic, and
the filtering applied to that binding or pinhole. This is
described in Section 6.

Discussion Point: After discovering NATs and firewalls,
querying those devices might also be done with a middlebox
control protocol (e.g., by using standard or slightly modified
versions of SIMCO [RFC4540], UPnP, MIDCOM, or Bonjour). This
is open for discussion as this document is scoped within the
IETF.

Controlling Discovered Middleboxes
A NAT or firewall might default to a more restrictive behavior
than desired by an application (e.g., aggressive timeout,
filtering). Requesting the NAT or firewall to change its default
behavior is useful for traffic optimization (e.g., reduce
keepalive traffic) and network optimization (e.g., adjust filters
to eliminate the need for a media relay
device[I-D.ietf-behave-turn]). This is described in Section 6.

Discussion Point: After discovering NATs and firewalls,
controlling those devices might also be done with a middlebox
control protocol (e.g., by using standard or slightly modified
versions of SIMCO, UPnP, MIDCOM, or Bonjour). This is open for
discussion as this document is scoped within the IETF.

5. Discovery of Middleboxes

There are three techniques to discover a middlebox (a NAT or a
firewall): outside-in, inside-out (useful when the outer-most NAT
device doesn't support STUN Control), or by tagging (useful for
firewalls).

These techniques can be combined in useful ways. For example, if
your inner-most NAT supports STUN Control, and your public STUN
server returns the same IP address and port as your inner-most NAT,
you know you don't have a NAT between your inner-most NAT and the
STUN server. Otherwise, you know there is a NAT between your inner-
most NAT and the STUN server (e.g., an ISP-supplied device or whoever
is providing your Internet service). Knowing this allows optimizing
NAT keepalives.

5.1. Outside-In

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
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

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 2: Block Diagram of Internal NAT Operation

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.

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: 3: 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:

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 query or control those on-path NATs between itself and
its STUN server on the Internet. This is described in detail in
section Section 4.1. 5.1.1. This functionality meets the desire need to optimize
traffic between nested NATs. NATs, without requiring configuration of
intermediate STUN servers.

The STUN client can request each NAT to increase the binding
lifetime, as described in Section 5.5. 6.1. 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 functionality meets the desire need to reduce
STUN's chattiness.

4.1.

5.1.1. Nested NATs

Nested NATs are controlled individually. The nested NATs are
discovered, from outer-most NAT to the inner-most NAT, using the XOR-
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.

The following figure shows two nested NATs:

+------+ +--------+ +--------+
| 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: 4: Two nested NATs with embedded STUN servers

Internally, the NAT can be diagrammed to function like this, where
the NAT operation occurs before the

The message flow with two nested NATs is shown below:

STUN server. Client NAT-B NAT-A STUN Server
| | outside interface |
+---------+---------------+ |
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)->| | |----+ STUN }
9. |<-Shared Secret Response (TLS)-| | }
10. |--TCP connection closed------->| | }
11. |--Binding Request (UDP)------->| | }
12. |<-Binding Response (UDP)-------| | } NAT Control
| Server | | | } STUN Usage
13. |-----TLS/TCP--------->| | +--------+ | }
14. |--Sh. Sec. Req (TLS)->| | | }
15. |<-Sh. Sec. Resp (TLS)-| | | +-------+-------------+ }
16. |-TCP conn. closed---->| | | }
17. |--Binding Req (UDP)-->| | NAT Function | }
18. |<-Binding Resp (UDP)--| | | +-------+-------------+ }
| | | |
+---------+---------------+

Figure 5: Message Flow for Outside-In with Two NATs

Once a shared secret has been obtained with each of the on-path NATs,
the STUN client no longer needs the TLS/TCP connection -- all
subsequent bindings for individual UDP streams (that is, for each
subsequent call) are obtained by just sending a Binding Request to
each of the NATs. By sending a Binding Request to both NAT-A and
NAT-B, the STUN client has the opportunity to optimize the packet
flow if their UDP peer is also behind the same NAT.

5.1.2. XOR-INTERNAL-ADDRESS Attribute

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 | inside interface
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| X-Address (32 bits or 128 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: Block Diagram 6: XOR-INTERNAL-ADDRESS Attribute

The meaning of Internal NAT Operation

4.2. 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).

5.1.3. 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 outside-in usage
described in
this document. Section 5.1. 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 section 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 In such a new
situation, the STUN usage, following client might implement the recommendation
for defining a new usage inside-out technique,
described in [I-D.ietf-behave-rfc3489bis].

5.1. Applicability Section 5.2.

5.2. Inside-Out

[[Discussion Point: This is being included as a discussion item.
Traditional traceroute provides similar functionality, and in many
cases traceroute survives traversing over a NAT that doesn't support
STUN usage MAY be used by Control. However, traceroute has significant disadvantages
(induces a load on intermediate devices to return ICMP error
messages, and those ICMP messages are routinely or inadvertently
filtered). Unlike the Inside-Out technique described below,
traceroute doesn't rely on the default route.]]

The STUN client that discovers sends a STUN request to UDP/3478 of the IP address of
its default router. If there is a NAT between itself STUN server listening there, it
will respond, and will indicate its default route via the new
DEFAULT-ROUTE attribute. With that information, the STUN server. Such client can
discover the next-outermost NAT by repeating the procedure.

5.2.1. DEFAULT-ROUTE Attribute

The DEFAULT-ROUTE attribute contains the XOR'd default route, which
is useful for finding the next-outer device.

The format of the DEFAULT-ROUTE 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-Address (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: DEFAULT-ROUTE Attribute

The meaning of X-Address is exactly as in
[I-D.ietf-behave-rfc3489bis].

5.3. Tagging

The Outside-In discovery would most
likely occur technique (Section 5.1) uses the public IP
address of the NAT to find the outer-most NAT that supports STUN
Control. Firewalls do not normally put their IP address into
packets, so a different technique is needed to identify firewalls.

To discover an on-path firewall, the PLEASE-TAG attribute is used
with a normal STUN Binding Request / usage. A firewall sees the normal
Binding Response exchange
to a Request usage (a STUN server on packet sent to UDP/3478) with the Internet, and by noticing
PLEASE-TAG attribute. When the IP address and
UDP port indicated by firewall sees the XOR-MAPPED-ADDRESS associated Binding
Response, the firewall appends a TAG attribute as the last attribute
of the STUN Binding Response differs from Response. This TAG attribute contains the local socket's firewall's
management IP address and UDP port. Such a difference indicates a NAT is present between the STUN
client and Each on-path firewall would be
able to insert its own TAG attribute. In this way, the STUN server.

5.2. Client Discovery Response
would contain pointer to each of Server

As this usage involves communicating with the on-path NATs directly, firewalls between the
client needs and that STUN server.

Note: Tagging is similar to find those NATs. The outer-most NAT how NSIS [I-D.ietf-nsis-nslp-natfw],
TIST [I-D.shore-tist-prot], and NLS [I-D.shore-nls-tl] function.

Discussion Point: Tagging would also be useful for the
Connectivity Check usage (which is found used by ICE), especially
considering that a different firewall may be traversed for media
than for the
normal XOR-MAPPED-ADDRESS attribute in initial Binding Discovery usage. In such a STUN Response. To iterate
through
situation, the inner NATs, each NAT needs new on-path firewall's policy might not allow a
binding request to support leave the usage described
in network or allow a binding response
to return. In this document, and case, the firewall would need to indicate its
presence to 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 another way. An ICMP error message
may be appropriate, and examples in Section 4.

5.3. Server Determination of Usage

If a an ICMP extension [RFC4884] could indicate
the firewall is controllable.

This figure shows how tagging functions.

STUN Client firewall STUN Server
| | |
1. |--Binding Request->|------------------>|
2. | |<-Binding Response-|
3. | [inserts tag] |
4. |<-Binding Response-| |
5. [firewall discovered] | |

Figure 8: Tagging Message Flow

1. Binding Request Request, containing PLEASE-TAG attribute, 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 server. This usage does not define any new message types.

5.5. New Attributes is seen by the firewall, and
the firewall remembers the STUN transaction id, and permits the
STUN Binding Request packet.

2. The following figure indicates STUN Binding Response packet is seen by the firewall.

3. The firewall inserts the TAG attribute, which attributes contains the
firewall's management address.

4. The firewall sends the (modified) STUN Binding Response towards
the STUN client.

5. The STUN client has now discovered the firewall, and can query it
or control it.

5.3.1. PLEASE-TAG Attribute

If a STUN client wants to discover on-path firewalls, it MUST include
this attribute in its Binding Response when performing the Binding
Discovery usage.

STUN servers are present not expected to understand this attribute; if they
return this attribute as an unknown attribute, it does not affect the
operation described in which
messages for this usage. An M indicates that inclusion document.

The format of the PLEASE-TAG attribute in 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Mech |x x x x x x x x x x x x x x x x x x x x x x x x x x x x x|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 9: PLEASE-TAG Attribute

The 3-bit Mechanism field indicates the message is mandatory, O means its optional, C means
it's conditional based on some other aspect of control mechanism desired.
Currently, the message, only defined mechanism is STUN Control, and -
means that the attribute is not applicable
indicated with all zeros. The intent of this field is to that message type.

Error allow
additional control mechanisms (e.g., UPnP, Bonjour, MIDCOM).

5.3.2. TAG Attribute Request Response Response Indication
_________________________________________________________________
XOR-INTERNAL-ADDRESS - M - -
REFRESH-INTERVAL O C - -

5.5.1. XOR-INTERNAL-ADDRESS

This

The TAG attribute MUST be present in contains the XOR'd management transport address of
the middlebox (typically a Binding Response and firewall, although a NAT may be used
in other responses find this
technique useful as well. This well).

A middlebox MUST append this attribute is necessary to allow as the last attribute of a
STUN client to 'walk backwards' response, and communicate directly with all of only if the STUN-aware NATs along associated STUN request (with the path. same
transaction-id) contained the PLEASE-TAG attribute.

The format of the XOR-INTERNAL-ADDRESS TAG attribute is:

Figure 6: XOR-INTERNAL-ADDRESS 10: TAG Attribute

The 3-bit Mechanism field indicates the control mechanism supported
on the described port. Currently, the only defined mechanism is STUN
Control, and is indicated with 0x0. The intent of this field is to
allow additional control mechanisms (e.g., UPnP, Bonjour, MIDCOM).

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

6. Query and Control

This section describes how to use STUN to query and control a response.
In the NAT Control usage defined in this document,
that was discovered using the REFRESH-
INTERVAL may also appear technique described in a request.

In a Binding Request, the Section 5.

6.1. REFRESH-INTERVAL indicates the desired
mapping timeout. Attribute

In a Binding Response, STUN request, the REFRESH-INTERVAL attribute indicates the NAT's mapping timeout.

5.6. Client Procedures

The STUN
number of milliseconds that the client sends a STUN Binding Request to its STUN server on wants the Internet and receives NAT binding (or
firewall pinhole) to be opened. In a STUN Binding Response, as normal. The
STUN Binding Response contains response, the XOR-MAPPED-ADDRESS same
attribute which indicates the IP address and UDP port length of the STUN Binding Request, time of that NAT binding (or
firewall pinhole).

REFRESH-INTERVAL is specified as
seen by the STUN server. If this IP address differs from the STUN
client's IP address, an unsigned 32 bit integer, and
represents an interval measured in milliseconds (thus the STUN client knows there maximum
value is at least one NAT
between itself approximately 50 days). This attribute can be present in
Binding Requests and in Binding Responses. In a request, the STUN server, value
0xFFFF means it's a query and it continues with the
procedure; otherwise, it stops.

The refresh interval isn't actually
changed.

6.2. Client Procedures

After discovering on-path NATs and firewalls, the STUN client now knows the public IP address of its outer-most NAT
-- it was returned in the XOR-MAPPED-ADDRESS attribute. begins
querying and controlling those devices. The STUN client first
performs the Shared Secret Usage (as described in
[I-D.ietf-behave-rfc3489bis]) with the public IP address of its
outer-most NAT. NAT or firewall it discovered.
After performing that usage, the STUN client now has a STUN USERNAME
and PASSWORD which authenticate PASSWORD. The username and password are used, thereafter, for
all subsequent messages between the STUN client and this NAT's STUN
server. This procedure might be done, for example, when the STUN
client first initializes such as upon bootup or initialization of the
application.

If subsequent messages from the that 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). 9.3).

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.

6.3. 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, address(es) and its private IP address(es), and
accept such STUN packets 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
interface(s) MAY be handled as if the NAT server isn't listening on that
port (e.g., return an ICMP error). error) -- this specification does not need
them.

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.
NAT, and for 60 seconds thereafter (to allow the STUN client to
obtain a new binding).

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). 9.1).
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 MUST
contain the XOR-MAPPED-ADDRESS and XOR-INTERNAL-ADDRESS attributes.
The XOR-MAPPED-ADDRESS contains the public IP address and UDP port
for this binding. binding, which is shared with the intended peer. The XOR-INTERNAL-ADDRESS XOR-
INTERNAL-ADDRESS contains the IP address and UDP port of the STUN
Binding Request prior to the NAT
translation. The XOR-INTERNAL-ADDRESS translation, which is used by the
STUN client to walk backwards through nested NATs. NATs (Section 5.1)

For example, looking at Figure 1, the XOR-INTERNAL-ADDRESS is the
IP address and UDP port _prior to_ 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.

7. Benefits

6.1.

7.1. Simple Security Model

Unlike other middlebox control techniques which have relatively
complex security models because a separate control channel is used,
STUN Control's is simple. It's simple because only the flow being
used can be controlled (e.g., have its NAT timeout queried or
extended). Other flows cannot be created, queried, or controlled via
STUN Control.

7.2. Incremental Deployment

NAT Control can be incrementally deployed. If the outer-most NAT
does not support it, the STUN client behaves as normal. In this
case, the STUN client might benefit from attempting inside-out
procedure described in Section 5.2, and still gain some
optimizations. 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 required to applications if NATs are
deployed which support NAT Control.

6.2. Control: such applications will be
unaware of the additional functionality in the NAT, and will not be
subject to any worse security risks due to the additional
functionality in the NAT.

7.3. Optimize SIP-Outbound

In sip-outbound [I-D.ietf-sip-outbound], the SIP proxy is also the
STUN server. This STUN Control as described in this document enables two
optimizations of SIP-
Outbound's 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.

7.4. Optimize ICE

The NAT Control usage provides several opportunities to optimize ICE
[I-D.ietf-mmusic-ice].

6.3.1.
[I-D.ietf-mmusic-ice], as described in this section.

7.4.1. 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, once -- when 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.] endpoint first
initializes.

7.4.2. Keepalive

ICE uses STUN Indications as its primary media stream keepalive
mechanism. This document enables two optimizations of ICE's
keepalive techniques: technique:

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,
which allows reducing the frequency of keepalive messages.

6.3.3. frequency.

7.4.3. 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

8. Limitations

7.1.

8.1. 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: 12: 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.

7.2.

Of course when such an overlap occurs the end host (STUN client)
cannot successfully establish bi-directional communication with hosts
in the overlapped network, anyway.

8.2. 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: 13: 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.

8.3. 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
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.

9. 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.

9.1. 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 attack is possible (without considering this
document and without considering STUN or other UNSAF [RFC3424] NAT
traversal techniques, 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 One way to thwart such a TCP an attack
can provide is to challenge the STUN
client with a similar level nonce, which is already part of protection, via the NONCE challange/
response, as they STUN specification.
By doing this, a NAT can provide DoS protection similar to what it
could do for TCP sessions.

8.2. today.

9.2. 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
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.

9.3. Rogue STUN Server

As described in Section 6, 7, 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. 6.2.

10. IANA Considerations

This section registers one new STUN attribute per the procedures in
[I-D.ietf-behave-rfc3489bis]:

0x0026

Mandatory range:
0x0028 XOR-INTERNAL-ADDRESS

10.

Optional range:
0x80.. PLEASE-TAG
0x80.. TAG

11. Acknowledgements

Thanks to Remi Denis-Courmont, Markus Isomaki, Cullen Jennings, and
Philip Matthews for their suggestions which have improved this
document.

12. References

10.1.

12.1. 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 "Session Traversal Underneath Network
Address Translators Utilities for (NAT)
(STUN)",
draft-ietf-behave-rfc3489bis-05 draft-ietf-behave-rfc3489bis-06 (work in
progress),
October 2006. March 2007.

[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
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.

12.2. Informational References

[I-D.ietf-behave-turn]
Rosenberg, J., "Obtaining Relay Addresses from Simple
Traversal Underneath NAT (STUN)",
draft-ietf-behave-turn-03 (work in progress), March 2007.

[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
draft-ietf-mmusic-ice-15 (work in progress), January March 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
draft-ietf-sip-outbound-08 (work in progress),
January March 2007.

[I-D.ietf-nsis-nslp-natfw]
Stiemerling, M., "NAT/Firewall NSIS Signaling Layer
Protocol (NSLP)", draft-ietf-nsis-nslp-natfw-14 (work in
progress), March 2007.

[RFC4884] Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
"Extended ICMP to Support Multi-Part Messages", RFC 4884,
April 2007.

[I-D.shore-tist-prot]
Shore, M., "The TIST (Topology-Insensitive Service
Traversal) Protocol", draft-shore-tist-prot-00 (work in
progress), May 2002.

[I-D.shore-nls-tl]
Shore, M., "Network-Layer Signaling: Transport Layer",
draft-shore-nls-tl-04 (work in progress), May 2007.

[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.

[RFC4540] Stiemerling, M., Quittek, J., and C. Cadar, "NEC's Simple
Middlebox Configuration (SIMCO) Protocol Version 3.0",
RFC 4540, May 2006.

Authors' Addresses

Dan Wing
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA

Email: dwing@cisco.com

Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
USA

Email: jdrosen@cisco.com

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