Internet Engineering Task Force MIDCOM WG
Internet Draft J. Rosenberg
dynamicsoft
J. Weinberger
dynamicsoft
C. Huitema
Microsoft
R. Mahy
Cisco
draft-ietf-midcom-stun-00.txt
April 5, 2002
Expires: October 2002
STUN - Simple Traversal of UDP Through NATs
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
Simple Traversal of UDP Through NATs (STUN) is a lightweight protocol
that allows applications to discover the presence and types of
Network Address Translators (NATs) and firewalls between them and the
public Internet. It also provides the ability for applications to
determine the public IP addresses allocated to them by the NAT. STUN
works with many existing NATs, and does not require any special
behavior from them. As a result, it allows a wide variety of
applications to work through existing NAT infrastructure. The STUN
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protocol is simple, being almost identical to echo.
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Table of Contents
1 Applicability Statement ............................. 4
2 Introduction ........................................ 4
3 Terminology ......................................... 5
4 Definitions ......................................... 5
5 NAT Variations ...................................... 6
6 Overview of Operation ............................... 6
7 Message Overview .................................... 9
8 Server Behavior ..................................... 10
9 Client Behavior ..................................... 12
9.1 Discovery ........................................... 13
9.2 Formulating the Request ............................. 13
9.3 Authenticating the Response ......................... 14
10 Use Cases ........................................... 15
10.1 Discovery Process ................................... 15
10.2 Binding Lifetime Discovery .......................... 17
10.3 Binding Acquisition ................................. 19
11 Protocol Details .................................... 20
11.1 Message Header ...................................... 20
11.2 Message Attributes .................................. 20
11.2.1 MAPPED-ADDRESS ...................................... 21
11.2.2 RESPONSE-ADDRESS .................................... 22
11.2.3 CHANGED-ADDRESS ..................................... 22
11.2.4 FLAGS ............................................... 22
11.2.5 SOURCE-ADDRESS ...................................... 23
11.2.6 CMS-SIGNED-DATA ..................................... 23
11.2.7 COOKIE .............................................. 23
12 Security Considerations ............................. 24
13 IANA Considerations ................................. 25
14 IAB Considerations .................................. 25
14.1 Problem Definition .................................. 25
14.2 Exit Strategy ....................................... 26
14.3 Brittleness Introduced by STUN ...................... 27
14.4 Requirements for a Long Term Solution ............... 28
14.5 Issues with Existing NAPT Boxes ..................... 29
14.6 In Closing .......................................... 29
15 Changes since draft-rosenberg-midcom-stun-01 ........ 29
16 Acknowledgements .................................... 30
17 Authors Addresses ................................... 30
18 Normative References ................................ 31
19 Informative References .............................. 31
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1 Applicability Statement
This protocol is not a cure-all for the problems associated with NAT.
It does not enable incoming TCP connections through NAT. It allows
incoming UDP packets through NAT, but only through a subset of
existing NAT types. In particular, STUN does not enable incoming UDP
packets through symmetric NATs (defined below), which are common in
large enterprises. STUN's discovery procedures are based on
assumptions on NAT treatment of UDP; such assumptions may prove
invalid down the road as new NAT devices are deployed. For a more
complete discussion of the limitations of STUN, see Section 14.
2 Introduction
Network Address Translators (NATs), while providing many benefits,
also come with many drawbacks. The most troublesome of those
drawbacks is the fact that they break many existing IP applications,
and make it difficult to deploy new ones. Guidlines have been
developed [4] that describe how to build "NAT friendly" protocols,
but many protocols simply cannot be constructed according to those
guidelines. Examples of such protocols include almost all peer-to-
peer protocols, such as multimedia communications, file sharing and
games.
To combat that problem, Application Layer Gateways (ALGs) have been
embedded in NATs. ALGs perform the application layer functions
required for a particular protocol to traverse a NAT. Typically, this
involves rewriting messages to contain translated addresses, rather
than the ones inserted by the sender of the protocol message. ALGs
have serious limitations, including scalability, reliability, and
speed of deploying new applications. To resolve these problems, the
Middlebox Communciations (MIDCOM) protocol is being developed [5].
MIDCOM allows an application entity, such as an end client or network
server of some sort (like a SIP proxy [6]) to control a NAT (or
firewall), in order to obtain NAT bindings and open or close
pinholes. In this way, NATs and applications can be separated once
more, eliminating the need for embedding ALGs in NATs, and resolving
the limitations imposed by current architectures.
Unfortunately, MIDCOM requires upgrades to existing NAT and
firewalls, in addition to application components. Complete upgrades
of these NAT and firewall products will take a long time, potentially
years. This is due, in part, to the fact that the deployers of NAT
and firewalls are not the same people who are deploying and using
applications. As a result, the incentive to upgrade these devices
will be low in many cases. Consider, for example, an airport Internet
lounge that provides access with a NAT. A user connecting to the
natted network may wish to use a peer-to-peer service, but cannot,
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because the NAT doesn't support it. Since the administrators of the
lounge are not the ones providing the service, they are not motivated
to upgrade their NAT equipment to support it, using either an ALG, or
MIDCOM.
Another problem is that the MIDCOM protocol requires that the agent
controlling the middleboxes know the identity of those middleboxes,
and have a relationship with them which permits control. In many
configurations, this will not be possible. For example, many cable
access providers use NAT in front of their entire access network.
This NAT could be in addition to a residential NAT purchased and
operated by the end user. The end user will probably not have a
control relationship with the NAT in the cable access network, and
may not even know of its existence.
Many existing proprietary protocols, such as those for online games
(such as the games described in RFC 3027 [7]) and Voice over IP, have
developed tricks that allow them to operate through NATs without
changing those NATs. This draft is an attempt to take some of those
ideas, and codify them into an interoperable protocol that can meet
the needs of many applications.
The protocol described here, Simple Traversal of UDP Through NAT
(STUN), provides is an extremely simple protocol that allows entities
behind a NAT to first discover the presence of a NAT and the type of
NAT, and then to learn the addresses bindings allocated by the NAT.
STUN requires no changes to NATs, and works with an arbitrary number
of NATs in tandem between the application entity and the public
Internet.
3 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
indicate requirement levels for compliant STUN implementations.
4 Definitions
STUN Client: A STUN client (also just referred to as a client)
is an entity that generates STUN requests. A STUN client
can execute on an end system, such as a users PC, or can
run in a network element, such as a server.
STUN Server: A STUN Server (also just referred to as a server)
is an entity that receives STUN requests, and sends STUN
responses. STUN servers are generally attached to the
public Internet. STUN servers are stateless.
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5 NAT Variations
It is assumed that the reader is familiar with NATs. It has been
observed that NAT treatment of UDP is variable amongst
implementations. The four treatments observed in implementations are:
Full Cone: A full cone NAT is one where all requests from the
same internal IP address and port are mapped to the same
external IP address and port. Furthermore, any external
host can send a packet to the internal host, by sending a
packet to the mapped external address.
Restricted Cone: A restricted cone NAT is one where all requests
from the same internal IP address and port are mapped to
the same external IP address and port. Unlike a full cone
NAT, an external host (with IP address X) can send a packet
to the internal host only if the internal host had
previously sent a packet to IP address X.
Port Restricted Cone: A port restricted cone NAT is like a
restricted cone NAT, but the restriction includes port
numbers. Specifically, an external host can send a packet,
with source IP address X and source port P, to the internal
host only if the internal host had previously sent a packet
to IP address X and port P.
Symmetric: A symmetric NAT is one where all requests from the
same internal IP address and port, to a specific
destination IP address and port, are mapped to the same
external IP address and port. If the same host sends a
packet with the same source address and port, but to a
different destination, a different mapping is used.
Furthermore, only the external host that receives a packet
can send a UDP packet back to the internal host.
Determining the type of NAT is important in many cases. Depending on
what the application wants to do, the particular behavior may need to
be taken into account.
6 Overview of Operation
This section is descriptive only. Normative behavior is described in
Sections 8 and 9.
The typical STUN configuration is shown in Figure 1. A STUN client is
connected to private network 1. This network connects to private
network 2 through NAT 1. Private network 2 connects to the public
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/-----\
// STUN \\
| Server |
\\ //
\-----/
+--------------+ Public Internet
................| NAT 2 |.......................
+--------------+
+--------------+ Private NET 2
................| NAT 1 |.......................
+--------------+
/-----\
// STUN \\
| Client |
\\ // Private NET 1
\-----/
Figure 1: STUN Configuration
Internet through NAT 2. On the public Internet is a STUN server.
STUN is a simple client-server protocol. A client sends a request to
a server. The server examines the source IP address and port of the
request, and copies them into a response that is sent back to the
client. There are some parameters in the request that allow the
client to ask that the response be sent elsewhere, or that the server
send the response from a different address and port. There are also
security capabilities that allow the server to sign the response.
The trick is using this simple protocol to discover the presence of
nats, and to learn and use the bindings they allocate.
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The STUN client is typically embedded in an application which needs
to obtain a public IP address and port that can be used to receive
data. For example, it might need to obtain an IP address and port to
receive RTP [8] traffic. When the application starts, the STUN client
within the application sends a STUN request to its STUN server. STUN
servers can be discovered through DNS SRV records [2], and is
generally assumed that the client is configured with the domain to
use to find the STUN server. Generally, this will be the domain of
the provider of the service the application is using (such a provider
is incented to deploy STUN servers in order to allow its customers to
use its application through NAT). Of course, a client can determine
the address or domain name of a stun server through other means. A
STUN server can even be embedded within an end system.
The STUN request is used to discover the presence of a NAT, and to
discover the public IP address and port mappings generated by the
NAT. Requests are sent to the STUN server using UDP. When a request
arrives at the STUN server, it may have passed through one or more
NATs between the STUN client and the STUN server. As a result, the
source address of the request received by the server will be the
mapped address created by the nat closest to the server. The STUN
server copies that source IP address and port into a STUN response,
and sends it back to the source IP address and port of the STUN
request. For all of the NAT types above, this response will arrive at
the STUN client.
When the STUN client receives the STUN response, it compares the IP
address and port in the packet with the local IP address and port it
bound to when the request was sent. If these do not match, the STUN
client is behind one or more NATs. In the case of a full-cone NAT,
the IP address and port in the body of the STUN response are public,
and can be used by any host on the public Internet to send packets to
the application that sent the STUN request. An application need only
listen on the IP address and port from which the STUN request was
sent, and send the IP address and port learned in the STUN response
to hosts that wish to communicate with it.
Of course, the host may not be behind a full-cone NAT. Indeed, it
doesn't yet know what type of NAT it is behind. To determine that,
the client uses additional STUN requests. The exact procedure is
flexible, but would generally work as follows. The client would send
a second STUN request, this time to a different STUN server, but from
the same source IP address and port. If the IP address and port in
the response are different from those in the first response, the
client knows it is behind a symmetric NAT. To determine if its behind
a full-cone NAT, the client can send a STUN request with flags that
tell the STUN server to send a response from a different IP address
and port than the request was received on. In other words, if the
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client sent a request to IP address/port A/B using a source IP
address/port of X/Y, the STUN server would send the response to X/Y
using source IP address/port C/D. If the client receives this
response, it knows it is behind a full cone NAT.
STUN also allows the client to ask the server to send the response
from the same IP address the request was received on, but with a
different port. This can be used to detect whether the client is
behind a port restricted cone nat or just a restricted cone nat.
It should be noted that the configuration in Figure 1 is not the only
permissible configuration. The STUN server can be located anywhere,
including within another client. The only requirement is that the
STUN server is reachable by the client, and if the client is trying
to obtain a publically routable address, that the server reside on
the public Internet.
7 Message Overview
STUN messages are TLV (type-length-value) encoded using big endian
(network ordered) binary. All STUN messages start with a STUN header,
followed by a series of STUN attributes. The STUN header contains a
STUN message type, transaction ID, and length. The message type can
be request or response. The transaction ID is used to correlate
requests and responses. The length indicates the total length of the
STUN message. This allows STUN to run over TCP. Usage over TCP is
needed in order to fetch certificates from the server.
Several STUN attributes are defined. The first is a MAPPED-ADDRESS
attribute, which is an IP address and port. It is always placed in
the response, and it indicates the source IP address and port the
server saw in the request. There is also a RESPONSE-ADDRESS
attribute, which is also an IP address and port. The RESPONSE-ADDRESS
attribute can be present in the request, and indicates where the
response is to be sent. Its optional, and when not present, the
response is sent to the source IP address and port of the request.
The third attribute is the FLAGS attribute, and it contains boolean
flags to control behavior. Three flags are defined: "discard",
"change IP" and "change port". The FLAGS attribute is allowed only in
the request. The discard attribute tells the server to not send a
reply. The change IP and change port attributes are useful for
determining whether the client is behind a restricted cone nat or
restricted port cone nat. They instruct the server to send the
responses from a different source IP address and port. The FLAGS
attribute is optional in the request.
The fourth attribute is the CHANGED-ADDRESS attribute. It is present
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in all 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.
The fifth attribute is the SOURCE-ADDRESS attribute. It is only
present in responses. It indicates the source IP address and port
where the response was sent from. It is useful for detecting twice
NAT configurations.
The final two attributes provide security features. The SMS-SIGNED-
DATA attribute allows the server to provide a signature over its
response. This is useful for preventing several address-stealing
attacks that would otherwise be possible. See Section 12 for details
on the types of attacks possible. The COOKIE attribute enables a
four-way handshake between the client and the server for preventing
distributed denial-of-service attacks. When the server receives a
request without this cookie, or with an invalid one, it generates a
response that contains the cookie attribute. The client then retries
the request, including the cookie attribute. If the server receives a
request with a valid cookie attribute, it will add a signature to the
response.
8 Server Behavior
A STUN server MUST be prepared to receive requests on four
address/port combinations - (A1, P1), (A2, P1), (A1, P2), and (A2,
P2). (A1, P1) represent the primary address and port, and these are
the ones obtained through the client procedures below. Typically, P1
will be port 3478, the default STUN port. A2 and P2 are arbitrary. A2
and P2 are advertised by the server through the CHANGED-ADDRESS
attribute, as described below.
If the request contains the FLAGS attribute, and the discard flag is
true, the server MUST discard the request.
The server MUST generate a single response when a request is received
(assuming the request is not discarded). The response MUST contain
the same transaction ID contained in the request. The length in the
message header MUST contain the total length of the message in bytes,
excluding the header. The response MUST have a message type of
"Response".
The server MUST add a MAPPED-ADDRESS attribute to the response. The
IP address component of this attribute MUST be set to the source IP
address observed in the request. The port component of this attribute
MUST be set to the source port observed in the query request.
If the request arrived over TCP, the response MUST be sent on the
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same connection the request was received on. The server MAY close the
connection after sending the response, but SHOULD instead wait for
the client to close the connection.
If the request arrived over UDP, the procedures for sending the
response are as follows.
If the RESPONSE-ADDRESS attribute was absent from the Query request,
the destination address and port of the response MUST be the same as
the source address and port of the request. Otherwise, the
destination address and port of the response MUST be the value of the
IP address and port in the RESPONSE-ADDRESS attribute.
The source address and port of the response depend on the value of
the FLAGS attribute and on the address and port the request was
received on, and are summarized in Table 1.
Let Da represent the destination IP address of the request (which
will be either A1 or A2), and Dp represent the destination port of
the request (which will be either P1 or P2). Let Ca represent the
other address, so that if Da is A1, Ca is A2. If Da is A2, Ca is A1.
Similarly, let Cp represent the other port, so that if Dp is P1, Cp
is P2. If Dp is P2, Cp is P1. If the "change port" flag was set in
the request, and the "change IP" flag was not set, the source IP
address of the response MUST be Da and the source port of the
response MUST be Cp. If the "change IP" flag was set in the request,
and the "change port" flag was not set, the source IP address of the
response MUST be Ca and the source port of the response MUST be Dp.
When both flags are set, the source IP address of the response MUST
be Ca and the source port of the response MUST be Cp. If neither flag
is set, the source IP address of the response MUST be Da and the
source port of the response MUST be Dp.
Flags Source Address Source Port
none Da Dp
Change IP Ca Dp
Change port Da Cp
Change IP and
Change port Ca Cp
Table 1: Impact of Flags on Packet Source
The server MUST add a SOURCE-ADDRESS attribute to the response,
containing the address and port used to send the response.
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The server MUST add a CHANGED-ADDRESS attribute to the response. This
contains the source IP address and port that would be used if the
client had requested the "change IP" and "change port" capabilities
of the server in the request. These are Ca and Cp, respectively.
The server can optionally sign the response, in order to provide
additional security capabilities. The signature SHOULD NOT be present
if the request did not contain a valid COOKIE attribute. Valid, in
this case, means that the COOKIE was equal to a value previously
handed out by the server to the same client. See subsequent
paragraphs for more details. To sign the response, the server adds a
CMS-SIGNED-DATA attribute as the last attribute in the response. If
the response is sent over UDP (which it will, if the request came
over UDP), the SignedData object MUST NOT contain any certificates.
This is because the certificates would likely overflow the MTU,
causing IP level fragmentation. This will not function in the
presence of NAT. If the request arrived over TCP, the response MUST
contain certificates. The server SHOULD sign the response using a
site certificate whose domain matches the domain of the server, as
listed in DNS. However, an end user certificate MAY be used instead.
If there was no COOKIE in the request, or the COOKIE was invalid, the
response MAY contain a COOKIE attribute. This attribute is a 128 bit
opaque value, created by the server. It SHOULD be constructed in such
a way so that it can be reconstructed by the server based on a
subsequent request from the same client within a brief time interval.
It is RECOMMENDED that it be computed as a cryptographic hash of the
source IP address and port, in addition to a time stamp rounded to
the nearest 5 seconds. When the client receives the response, if it
wishes to authenticate the information it just received, it will
create a new request, identical to the previous, except with a
different transaction identifier, and this COOKIE reflected back. The
server then recomputes the COOKIE, using the same algorithm it used
to construct it previously, and verifies that this is the same value
in the request. If the values are the same, the COOKIE in the request
is valid. The purpose of this cookie exchange is to prevent
distributed denial of service attacks that would force a server to
perform expensive public key signatures. Instead, the server only
provides the signature in responses that come from valid clients;
valid, in this case, means the client meant to send the request,
instead of being manipulated into doing so through a dDoS attack.
The server SHOULD NOT retransmit the response. Reliability is
achieved by having the client periodically resend the request, each
of which triggers a response from the server.
9 Client Behavior
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The behavior of the client is very straightforward. Its task is to
discover the STUN server, formulate the request, handle request
reliability, and authenticate the response.
9.1 Discovery
Generally, the client will be configured with a domain name of the
provider of the STUN servers. This domain name is resolved to an IP
address and port of using the SRV procedures specified in RFC 2782
[2].
Specifically, the service name is "stun". The protocol is "udp" or
"tcp". The procedures of RFC 2782 are followed to determine the
server to contact. RFC 2782 spells out the details of how a set of
SRV records are sorted and then tried. However, it only states that
the client should "try to connect to the (protocol, address,
service)" without giving any details on what happens in the event of
failure. Those details are described here for STUN.
For STUN requests, failure occurs if there is a transport failure of
some sort (generally, due to fatal ICMP errors in UDP use or
connection failures in TCP). Failure also occurs if the the request
does not solicit a response after 30 seconds. If a failure occurs,
the client SHOULD create a new request, which is identical to the
previous, but has a different transaction ID. That request is sent to
the next element in the list as specified by RFC 2782.
The default port for STUN requests is 3478, for both TCP and UDP.
Administrators SHOULD use this port in their SRV records, but MAY use
others.
If no SRV records were found, the client performs an A or AAAA record
lookup of the domain name. The result will be a list of IP addresses,
each of which can be contacted at the default port.
This would allow a firewall admin to open the STUN port, so
hosts within the enterprise could access new applications.
Whether they will or won't do this is a good question.
9.2 Formulating the Request
A request formulated by the client follows the syntax rules defined
in Section 11. Any two requests that are not bit-wise identical, or
not sent to the same server from the same IP address and port, MUST
carry different transaction IDs. The transaction ID MUST be uniformly
and randomly chosen between 0 and 2**128 - 1. The large range is
needed because the transaction ID serves as a form of randomization,
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helping to prevent replays of previously signed responses from the
server. The message type of the request MUST be "Request".
The RESPONSE-ADDRESS attribute is optional in the request. It is used
if the client wishes the response to be sent to a different IP
address and port. This is useful for determining whether the client
is behind a firewall, and for applications that have separated
control and data components. See Section 10.3 for more details. The
FLAGS attribute is also optional. Whether it is present depends on
what the application is trying to accomplish. See Section 10 for some
example uses.
Once formulated, the client sends the request. Reliability is
accomplished through client retransmissions. Clients SHOULD
retransmit the request starting with an interval of 100ms, doubling
every retransmit until the interval reaches 1.6s. Retranmissions
continue with intervals of 1.6s until a total of 9 requests have been
sent, at which time the client SHOULD give up.
The response will contain the MAPPED-ADDRESS and SOURCE-ADDRESS
attributes.
9.3 Authenticating the Response
As discussed in Section 12, there are serious security
vulnerabilities introduced if the STUN response is not authenticated
and integrity protected. To combat that problem, STUN provides for
server signatures using CMS. The procedure for obtaining the
signature and certificates to validate it are as follows.
The initial STUN reply will arrive with a COOKIE attribute. The
client can choose to use the information in the STUN reply,
validating it in parallel, or can choose to authenticate the
information before using it. To authenticate the information, the
client formulates a new STUN request, identical to the initial one
except for two changes. First, is the usage of a new transaction ID.
Second, is the addition of the COOKIE attribute, which is copied from
the previous STUN response into the new request. The response to this
second STUN request will contain a CMS-SIGNED-DATA attribute. This
attribute is equal to the SignedData object defined in RFC 2630 [3].
If the other attributes in the response match the first response, and
the signature is valid, the client can trust that the information has
not been tampered with, and is authentic. If the certificate used for
the signature is a site certificate, the client SHOULD validate that
the domain it used to perform the STUN query is a sub-domain of the
domain in the site certificate. In other words, if the client queries
stun.example.com, the client SHOULD validate that the signature was
certified as coming from example.com.
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However, the CMS-SIGNED-DATA attribute in the second response will
not contain any certificates. The reason is that certificates can be
large. The STUN response may exceed the path MTU, causing IP layer
fragmentation. Many NAT implementations do not support fragmentation,
so that the response may not pass back through the NAT. The client
may have cached certificates from a previous exchange with this
server, in which case those certificates can be used. If the client
does not have a certificate chain for the server, the client creates
a third STUN request. This one is identical to the second, except it
contains a new transaction ID, and is sent over TCP. The response
will contain a CMS-SIGNED-DATA attribute, this time, with
certificates. The client SHOULD ignore all other elements of the
SignedData object with the exception of the certificates.
The result is that the exchange with the server could require up to
three requests in order to authenticate the response. This three pass
exchange is summarized in Figure 2.
10 Use Cases
The rules of Sections 8 and 9 describe exactly how a client and
server interact to send requests and get responses. However, they do
not dictate how the STUN protocol is used to accomplish useful tasks.
That is at the discretion of the client. Here, we provide some useful
scenarios for applying STUN.
10.1 Discovery Process
In this scenario, a user is running a multimedia application which
needs to determine which of the following scenarios applies to it:
o On the open Internet
o Firewall that blocks UDP
o Firewall that allows UDP out, and responses have to come back
to the source of the request (like a symmetric NAT, but no
translation. We call this symmetric UDP Firewall)
o Full-cone NAT
o Symmetric NAT
o Restricted cone or restricted port cone NAT
Which of the six scenarios applies can be determined through the flow
chart described in Figure 3.
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Client NAT Server
| | |
|(1) UDP Request 1 | |
|no cookie | |
|-------------------------------------->|
| | |
|(2) UDP Response 1 | |
|cookie | |
|<--------------------------------------|
| | |
|(3) UDP Request 2 | |
|cookie | |
|-------------------------------------->|
| | |
|(4) UDP Response 2 | |
|Signature | |
|<--------------------------------------|
| | |
| | |
|(5) TCP Request 3 | |
|-------------------------------------->|
| | |
|(6) TCP Response 3 | |
|Certificates | |
|<--------------------------------------|
| | |
| | |
| | |
| | |
Figure 2: Three pass exchange
The flow makes use of three tests. In test I, the client sends a STUN
request to a server, without any flags set, and without the
RESPONSE-ADDRESS attribute. This causes the server to send the
response back to the address and port that the request came from.
This response provides the IP address and port for the third party
address that would be used if the source IP and/or port were changed.
In test II, the client sends a request with both the "change IP" and
"change port" flags set. In test III, the client sends a request with
only the "change port" flag set.
The client begins by initiating test I. If this test yields no
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response, the client knows right away that it is not capable of UDP
connectivity. If the test produces a response, the client examines
the MAPPED-ADDRESS attribute. If this address is the same as the
local IP address and port of the socket used to send the request, the
client knows that it is not natted. It executes test II. If a
response is received, the client knows that it has open access to the
Internet (or, at least, its behind a firewall that behaves like a
full-cone NAT, but without the translation). If no response is
received, the client knows its behind a symmetric UDP firewall.
In the event that the IP address and port of the socket did not match
the MAPPED-ADDRESS attribute in the response to test I, the client
knows that it is behind a NAT. It performs test II. If a response is
received, the client knows that it is behind a full-cone NAT. If no
response is received, it performs test I again, but this time, does
so to the address from the CHANGED-ADDRESS attribute. If the IP
address returned in the MAPPED-ADDRESS attribute is not the same as
the one from the first test I, the client knows its behind a
symmetric NAT. If the address is the same, the client is either
behind a restricted or port restricted NAT. To make a determination
about which one it is behind, the client initiates test III. If a
response is received, its behind a restricted NAT, and if no response
is received, its behind a port restricted NAT.
This procedure yields substantial information about the operating
condition of the client application. In the event of multiple NATs
between the client and the Internet, the type that is discovered will
be the type of the most restrictive NAT between the client and the
Internet. The types of NAT, in order of restrictiveness, from most to
least, are symmetric, port restricted cone, restricted cone, and full
cone.
10.2 Binding Lifetime Discovery
STUN can also be used to discover the lifetimes of the bindings
created by the NAT. To do that, the client first sends a 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 another
request to the server, using the same destination address and port,
but from a different socket, Y. This request contains a RESPONSE-
ADDRESS address attribute, set to (Pa,Pp). This will create a new
binding on the NAT, and cause the stun server to send a response that
would match the old binding, if it still exists. If the client
receives the response on socket X, it knows that the binding has not
expired. If the client receives the response on socket Y (which is
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+--------+
| Test |
| I |
+--------+
|
|
V
/\ /\
N / \ Y / \ Y +--------+
UDP <-------/Resp\---------->/ IP \------------>| Test |
Blocked \ ? / \Same/ | II |
\ / \? / +--------+
\/ \/ |
| N |
| V
V /\
+--------+ Sym. N / \
| Test | UDP <---/Resp\
| II | Firewall \ ? /
+--------+ \ /
| \/
V |Y
/\ /\ |
Symmetric N / \ +--------+ N / \ V
NAT <--- / IP \<-----| Test |<--- /Resp\ Open
\Same/ | I | \ ? / Internet
\? / +--------+ \ /
\/ \/
| |Y
| |
| V
| Full
| Cone
V /\
+--------+ / \ Y
| Test |------>/Resp\---->Restricted
| III | \ ? /
+--------+ \ /
\/
|N
| Port
+------>Restricted
Figure 3: Flow for type discovery process
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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.
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.
10.3 Binding Acquisition
Consider once more the case of a VoIP phone. It used the discovery
process above when it started up, to discover its environment. Now,
it wants to make a call. As part of the discovery process, it
determined that it was behind a full-cone NAT.
Consider further that this phone consists of two logically separated
components - a control component that handles signaling, and a media
component that handles the audio, video, and RTP [8]. Both are behind
the same NAT. Because of this separation of control and media, we
wish to minimize the communication required between them. In fact,
they may not even run on the same host.
In order to make a voice call, the phone needs to obtain an IP
address and port that it can place in the call setup message as the
destination for receiving audio.
To obtain an address, the control component first sends a STUN
request to a server. No flags are present, and neither is the
RESPONSE-ADDRESS field. The response contains a mapped address. The
control component then formulates a second request. This request
contains a RESPONSE-ADDRESS, which is set to that mapped address.
This request is passed to the media component, along with the IP
address and port of the STUN server. The media component sends the
request. The request goes to the STUN server, which sends the
response back to the control component. The control component
receives this, and now has learned an IP address and port that will
be routed back to the media component that sent the request.
The client will be able to receive media from anywhere on this mapped
address.
In the case of silence suppression, there may be periods where the
client receives no media. In this case, the UDP bindings could
timeout (UDP bindings in nats are typically short). To deal with
this, the application can periodically retransmit the query in order
to keep the binding fresh.
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It is possible that both participants in the multimedia session are
behind the same NAT. In that case, both will repeat this procedure
above, and both will obtain public address bindings. When one sends
media to the other, the media is routed to the nat, and then turns
right back around to come back into the enterprise, where it is
translated to the private address of the recipient. This is not
particularly efficient, but it does work.
11 Protocol Details
This section presents the detailed encoding of a STUN message.
11.1 Message Header
All STUN messages consist of a 64 bit header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| STUN Message Type | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Transaction ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Types can take on the following values:
0x0001 : Request
0x0101 : Response
The message length is the count, in bytes, of the size of the
message, not including the 64 bit header.
The transaction ID is a 128 bit identifier. It also serves as salt to
randomize the response. All responses carry the same identifier as
the request they correspond to.
11.2 Message Attributes
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After the header are 0 or more attributes. Each attribute is TLV
encoded, with a 16 bit type, 16 bit length, and variable value:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following types are defined:
0x0001: MAPPED-ADDRESS
0x0002: RESPONSE-ADDRESS
0x0003: FLAGS
0x0004: SOURCE-ADDRESS
0x0005: CHANGED-ADDRESS
0x0006: CMS-SIGNED-DATA
0x0007: COOKIE
Future extensions MAY define new attributes. If a stun client or
server receives a message with an unknown attribute with a type lower
than or equal to 0x7fff, the message MUST be discarded. If the type
is greater than 0x7fff, the attribute MUST be ignored. The ordering
of attributes within a message is not important, and a client or
server MUST be prepared to receive them in any order. Any attributes
that are known, but are not supposed to be present in a message
(MAPPED-ADDRESS in a request, for example) MUST be ignored.
The length refers to the length of the value element.
11.2.1 MAPPED-ADDRESS
The MAPPED-ADDRESS attribute indicates the mapped IP address and
port. It consists of an eight bit address family, and a sixteen bit
port, followed by a fixed length value representing the IP address.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x x x x x x x x| Family | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The port is a network byte ordered representation of the mapped port.
The following families are defined:
0x01: IPv4
0x02: IPv6
The first 8 bits of the MAPPED-ADDRESS are ignored, for the purposes
of aligning parameters on natural boundaries.
For IPv4 addresses, the address is 32 bits. For IPV6, it is 128 bits.
New address families MAY be defined by extensions. A message with an
unknown address family is discarded.
11.2.2 RESPONSE-ADDRESS
The RESPONSE-ADDRESS attribute indicates where the response to a
request is sent. Its syntax is identical to MAPPED-ADDRESS.
11.2.3 CHANGED-ADDRESS
The CHANGED-ADDRESS attribute indicates the IP address and port of a
STUN server where responses will be sent from if the "change IP"
and/or "change port" flags were set. The attribute is always present
in the response, independent of the value of the flags. Its syntax is
identical to MAPPED-ADDRESS.
11.2.4 FLAGS
The FLAGS attribute is a series of boolean flags. It is 32 bits long:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |A|B|C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Only three flags, A,B,C, are currently defined. The other bits MAY be
used by extensions to define additional flags. Unknown flags are
ignored.
Each flag is a binary one if true, zero otherwise.
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The meaning of the flags is:
A: This is the "change IP" flag. If true, it requests the server
to send the response with a different IP address than the
one the request was received on.
B: This is the "change port" flag. If true, it requests the
server to send the response with a different port than the
one the request was received on.
C: This is the discard flag. If true, the message is discarded.
11.2.5 SOURCE-ADDRESS
The SOURCE-ADDRESS attribute is present in responses. It indicates
the source IP address and port that the server is sending the
response from. Its syntax is identical to that of MAPPED-ADDRESS.
11.2.6 CMS-SIGNED-DATA
STUN responses can be signed. The signatures are conveyed using the
Cryptographic Message Syntax (CMS), RFC 2630 [3]. Specifically. the
CMS-SIGNED-DATA is exactly equal to the CMS SignedData object, using
detached signatures. When present in a response, the CMS-SIGNED-DATA
attribute MUST be the last attribute in the response. The contentData
covered by the signature includes all of the bytes from the start of
the STUN message, up to, but not including, the CMS-SIGNED-DATA
attribute.
When the response is sent over UDP, the response won't contain
certificates (see Section 8. Responses over TCP will contain
certificates.
11.2.7 COOKIE
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie word 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie word 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie word 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie word 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The COOKIE attribute is used to prevent distributed denial of service
attacks on the STUN server. It is a 128 bit value, generated by the
server, and echoed by the client back to the server.
12 Security Considerations
There are several potential threats in a STUN system worthy of
consideration.
STUN can potentially introduce attacks which result in the theft of
addresses. When a client sends a request, an attacker can guess the
value of the mapped address used by the nat, and quickly generate its
own faked response, sending it to that address. This response would
contain a faked MAPPED-ADDRESS which actually routes to a different
host. This could enable DoS attacks, by using a victim's address, or
theft attacks, by using the address of the host run by the attacker.
This threat is potentially very serious. However, the attack is only
possible for attackers outside of the NAT when the NAT is full cone.
The attacker must also be on the message path, in order to observe
the transaction ID in the request, as it is needed to generate the
response. To combat the attack, STUN supports public key
authentication of responses. This allows a client to verify that the
response was indeed generated by the server to which the request was
sent.
Interestingly, there is little need for strong authentication of
requests. STUN servers are stateless. Their processing is not user
specific. The server is hardly more than an echo server. As a result,
client authentication provides no value. However, a request from a
client can result in expensive public key operations at the server,
for the purposes of signing the request. This makes a STUN server
potentially subject to denial-of-service attacks. To prevent against
such threats, STUN provides a simple cookie mechanism. The server
will not sign any responses until it has successfully passed a cookie
to the client and received it back in a subsequent request. This
cookie mechanism is similar to the techniques used to prevent the TCP
SYN attack.
Compromise of a STUN server can lead to discovery of open ports.
Knowledge of an open port creates an opportunity for DoS attacks on
those ports (or DDoS attacks if the traversed NAT is a full cone
NAT). Discovering open ports is already fairly trivial using port
probing, so this does not represent a major threat.
STUN servers constitute a reflector type of server, and can therefore
be used as launching grounds for distributed DoS attacks [9]. The
problem is amplified by the existence of the RESPONSE-ADDRESS
attribute, which can render ingress filtering useless in prevention
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of attacks. Interestingly, the MAPPED-ADDRESS in the response
provides a form of traceback in order to counter such attacks. An
attacker would need to spoof their source address in order to avoid
the traceback mechanism.
STUN has the important property that compromise of the STUN servers
cannot cause security breaches of a firewall when the client is
within an enterprise. The only thing that a compromised server can do
is return false addresses, resulting in the inability of the client
to receive any data at all. However, the attacker cannot send packets
to arbitrary servers within the enterprise, if the firewall prohibits
such communication.
13 IANA Considerations
There are no IANA considerations associated with this specification.
14 IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing",
which is the general process by which a client attempts to determine
its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism [10]. STUN 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.
14.1 Problem Definition
From [10], 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 problems being solved by STUN are:
o Provide a means for a client to detect the presence of one or
more NATs between it and a server run by a service provider on
the public Internet. The purpose of such detection is to
determine additional steps that might be necessary in order to
receive service from that particular provider.
o Provide a means for a client to detect the presence of one or
more NATs between it and another client, where the second
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client is reachable from the first, but it is not known
whether the second client resides on the public Internet.
o Provide a means for a client to obtain an address on the
public Internet from a non-symmetric NAT, for the express
purpose of receiving incoming UDP traffic from another host
targeted to that address.
STUN does not address TCP, either incoming or outgoing, and does not
address outgoing UDP communications.
14.2 Exit Strategy
From [10], 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.
STUN comes with its own built in exit strategy. This strategy is the
detection operation that is performed as a precursor to the actual
UNSAF address-fixing operation. This discovery operation, documented
in Section 10.1, attempts to discover the existence of, and type of,
any NATS between the client and the service provider network. Whilst
the detection of the specific type of NAT may be brittle, the
discovery of the existence of NAT is itself quite robust. As NATs are
phased out through the deployment of IPv6, the discovery operation
will return immediately with the result that there is no NAT, and no
further operations are required. Indeed, the discovery operation
itself can be used to help motivate deployment of IPv6; if a user
detects a NAT between themselves and the public Internet, they can
call up their access provider and complain about it.
STUN can also help facilitate the introduction of midcom. As midcom-
capable NATs are deployed, applications will, instead of using STUN
(which also resides at the application layer), first allocate an
address binding using midcom. However, it is a well-known limitation
of midcom that it only works when the agent knows the middleboxes
through which its traffic will flow. Once bindings have been
allocated from those middleboxes, a STUN detection procedure can
validate that there are no additional middleboxes on the path from
the public Internet to the client. If this is the case, the
application can continue operation using the address bindings
allocated from midcom. If it is not the case, STUN provides a
mechanism for self-address fixing through the remaining midcom-
unaware middlboxes. Thus, STUN provides a way to help transition to
full midcom-aware networks.
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14.3 Brittleness Introduced by STUN
From [10], any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data
at multiple network layers create more dependencies,
increase debugging challenges, and make it harder to
transition.
STUN introduces brittleness into the system in several ways:
o The discovery process assumes a certain classification of
devices based on their treatment of UDP. Their could be other
types of NATs that are deployed that would not fit into one of
these molds. Therefore, future NATs may not be properly
detected by STUN. STUN clients (but not servers) would need to
change to accomodate that.
o The binding acquisition usage of STUN does not work for all
NAT types. It will work for any application for full cone NATs
only. For restricted cone and port restricted cone NAT, it
will work for some applications dependening on the
application. Application specific processing will generally be
needed. For symmetric NATs, the binding acquisition will not
yield a usable address. The tight dependency on the specific
type of NAT makes the protocol brittle.
o STUN assumes that the server exists on the public Internet. If
the server is located in another private address realm, the
user may or may not be able to use its discovered address to
communicate with other users. There is no way to detect such a
condition.
o The bindings allocated from the NAT need to be continuously
refreshed. Since the timeouts for these bindings is very
implementation specific, the refresh interval cannot easily be
determined. When the binding is not being actively used to
receive traffic, but rather just wait for it, the binding
refresh will needlessly consume network bandwidth.
o The use of the STUN server as an additional network element
introduces another point of potential security attack. These
attacks are largely prevented by the security measures
provided by STUN, but not entirely.
o The use of the STUN server as an additional network element
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introduces another point of failure. If the client cannot
locate a STUN server, or if the server should be unavailable
to failure, the application cannot function.
o The use of STUN to discover address bindings will result in an
increase in latency for applications. For example, a Voice
over IP application will see an increase of call setup delays
equal to at least one RTT to the stun server.
14.4 Requirements for a Long Term Solution
From [10], any UNSAF proposal must provide:
Identify requirements for longer term, sound technical
solutions -- contribute to the process of finding the right
longer term solution.
Our experience with STUN has led to the following requirements for a
long term solution to the NAT problem:
Requests for bindings and control of other resources in a NAT
need to be explicit. Much of the brittleness in STUN
derives from its guessing at the parameters of the NAT,
rather than telling the NAT what parameters to use.
Control needs to be "in-band". There are far too many scenarios
in which the client will not know about the location of
middleboxes ahead of time. Instead, control of such boxes
needs to occur in band, traveling along the same path as
the data will itself travel. This guarantees that the right
set of middleboxes are controlled. This is only true for
first-party controls; third-party controls are best handled
using the midcom framework.
Control needs to be limited. Users will need to communicate
through NATs which are outside of their administrative
control. In order for providers to be willing to deploy
NATs which can be controlled by users in different domains,
the scope of such controls needs to be extremely limited -
typically, allocating a binding to reach the address where
the control packets are coming from.
Simplicity is Paramount. The control protocol will need to be
implement in very simple clients. The servers will need to
support extremely high loads. The protocol will need to be
extremely robust, being the precursor to a host of
application protocols. As such, simplicity is key.
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14.5 Issues with Existing NAPT Boxes
From [10], any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.
Several of the practical issues with STUN involve future proofing -
breaking the protocol when new NAT types get deployed. Fortunately,
this is not an issue at the current time, since most of the deployed
NATs are of the types assumed by STUN. The primary usage STUN has
found is in the area of VoIP, to facilitate allocation of addresses
for receiving RTP [8] traffic. In that application, the periodic
keepalives are provided by the RTP traffic itself. However, several
practical problems arise for RTP. First, RTP assumes that RTCP
traffic is on a port one higher than the RTP traffic. This pairing
property cannot be guaranteed through NATs that are not directly
controllable. As a result, RTCP traffic may not be properly received.
Protocol extensions to SDP have been proposed which mitigate this by
allowing the client to signal a different port for RTCP [11].
However, there will be interoperability problems for some time.
For VoIP, silence suppression can cause a gap in the transmission of
RTP packets. This could result in the loss of a binding in the middle
of a call, if that silence period exceeds the binding timeout. This
can be mitigated by sending occassional silence packets to keep the
binding alive. However, the result is additional brittleness; proper
operation depends on the the silence suppression algorithm in use,
the usage of a comfort noise codec, the duration of the silence
period, and the binding lifetime in the NAT.
14.6 In Closing
The problems with STUN are not design flaws in STUN. The problems in
STUN have to do with the lack of standardized behaviors and controls
in NATs. The result of this lack of standardization has been a
proliferation of devices whose behavior is highly unpredictable,
extremely variable, and uncontrollable. STUN does the best it can in
such a hostile environment. Ultimately, the solution is to make the
environment less hostile, and to introduce controls and standardized
behaviors into NAT. However, until such time as that happens, STUN
provides a good short term solution given the terrible conditions
under which it is forced to operate.
15 Changes since draft-rosenberg-midcom-stun-01
o Added IANA port 3478.
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o Removed bit about sending a request to a different server in
order to implement the changing of IP address and port.
o More rigorously specified the change address and port
behavior.
o Mandate that the stun server listens on all four address/port
combinations possible from change address/port.
o Extended the transaction ID to 128 bits, because it provides
randomization on the response.
o Reorganized the formatting of the attributes once again, to
support more convenient alignments.
o Changed the algorithm for detecting the binding lifetime
timers on the NAT. The previous algorithm only worked for
symmetric NAT.
o Added an applicability statement up front, summarizing some of
the issues with STUN.
o Noted that STUN servers introduce another point of failure in
the system.
o Mentioned that the adress theft attack is only possible in
certain situations.
16 Acknowledgements
The authors would like to thank Cedric Aoun, Pete Cordell and Cullen
Jennings for their comments, and Baruch Sterman and Alan Hawrylyshen
for initial implementations.
17 Authors Addresses
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
email: jdrosen@dynamicsoft.com
Joel Weinberger
dynamicsoft
72 Eagle Rock Avenue
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First Floor
East Hanover, NJ 07936
email: jweinberger@dynamicsoft.com
Christian Huitema
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
email: huitema@microsoft.com
Rohan Mahy
Cisco Systems
170 West Tasman Dr, MS: SJC-21/3
Phone: +1 408 526 8570
Email: rohan@cisco.com
18 Normative References
[1] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," Request for Comments 2119, Internet Engineering Task Force,
Mar. 1997.
[2] A. Gulbrandsen, P. Vixie, and L. Esibov, "A DNS RR for specifying
the location of services (DNS SRV)," Request for Comments 2782,
Internet Engineering Task Force, Feb. 2000.
[3] R. Housley, "Cryptographic message syntax," Request for Comments
2630, Internet Engineering Task Force, June 1999.
19 Informative References
[4] D. Senie, "Network address translator (nat)-friendly application
design guidelines," Request for Comments 3235, Internet Engineering
Task Force, Jan. 2002.
[5] P. Srisuresh, J. Kuthan, J. Rosenberg, A. Molitor, and A. Rayhan,
"Middlebox communication architecture and framework," Internet Draft,
Internet Engineering Task Force, Mar. 2002. Work in progress.
[6] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments 2543, Internet
Engineering Task Force, Mar. 1999.
[7] M. Holdrege and P. Srisuresh, "Protocol complications with the IP
network address translator," Request for Comments 3027, Internet
J. Rosenberg et. al. [Page 31]
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Engineering Task Force, Jan. 2001.
[8] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
transport protocol for real-time applications," Request for Comments
1889, Internet Engineering Task Force, Jan. 1996.
[9] V. Paxson, "An analysis of using reflectors for distributed
denial of service attacks," ACM Computer Communication Review, Vol.
31, July 2001.
[10] L. Daigle, "IAB considerations for UNilateral self-address
fixing (UNSAF)," Internet Draft, Internet Engineering Task Force,
Feb. 2002. Work in progress.
[11] C. Huitema, "RTCP attribute in SDP," Internet Draft, Internet
Engineering Task Force, Feb. 2002. Work in progress.
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