One of the best articles describing udp hole punching
http://pdos.csail.mit.edu/~baford/nat/draft-ford-natp2p-00.txt
There is another article that goes into details on how to predict port on symmetrical NAT.
Here is the article/IETF DRAFT
Internet Draft B. Ford
Document: draft-ford-natp2p-00.txt M.I.T.
Expires: October 2003 April 2003
Network Address Translation and Peer-to-Peer Applications (NATP2P)
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026. Internet-Drafts are working documents of
the Internet Engineering Task Force (IETF), its areas, and its
working groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
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Distribution of this document is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes and recommends methods by which peer-to-peer
(P2P) applications can operate efficiently in the presence of Network
Address Translation (NAT). This document also provides
recommendations for the design of network address translators, in
order for them to support P2P applications effectively without
compromising security or performance. This memo focuses on the
interaction of P2P with NAT in the absence of any special proxy,
gateway, or relaying protocols. While not intending to preclude the
use of such protocols, the goal of this memo is to enable P2P
applications to function automatically without specific knowledge of
the type, location, or configuration of the NAT.
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1. Introduction
The ubiquitous deployment of Network Address Translation (NAT) on the
Internet has provided an effective if hopefully temporary means of
working around the ongoing depletion of the IP address space. At the
same time, however, the asymmetric addressing and connectivity
regimes established by NAT and firewall systems have created unique
problems for peer-to-peer (P2P) applications and protocols, such as
teleconferencing and multiplayer on-line gaming. This document
discusses these issues and how they can be addressed. Familiarity is
assumed with NAT terminology and conventions, as specified in [NAT-
TRAD] and [NAT-TERM]. As used throughout this document, the term
"NAT" refers to "Traditional NAT" in both of its standard variants:
namely Basic NAT, in which only IP addresses are translated, as well
as Network Address/Port Translation (NAPT), where both IP addresses
and transport-level port numbers are translated. In general, this
document always assumes NAPT as the standard "worst-case" scenario.
In the traditional client/server paradigm, for which current NAT and
firewall mechanisms are primarily designed, network communication
typically operates in terms of sessions that originate in a privately
addressed network behind a network address translator, and are
directed to a well-connected public server with a stable IP address
and DNS mapping. The client, or originator, of these connections
often does not have its own routable IP address on the public
Internet, but instead must share a single public IP address with a
number of other hosts on the same private network using the NAT as a
multiplexor. The lack of a stable, dedicated public IP address is
not a problem for most client software such as web browsers, because
the client only needs to be addressable for the duration of a
particular session.
In the peer-to-peer paradigm, however, Internet hosts that would
normally be considered to be "clients" need to establish
communication sessions directly with each other. In this case, both
the initiator and responder in a communication session might lie
behind a NAT/Firewall, without any permanent IP address or other form
of public network presence. A common on-line gaming architecture,
for example, is for the participating application hosts to contact a
well-known server for initialization and administration purposes, but
then to establish direct connections with each other for fast and
efficient propagation of updates during gameplay. Similarly, a file
sharing application might contact a well-known server for resource
discovery or searching purposes, but establish direct connections
with peer hosts for data transfer. NAT creates a problem for peer-
to-peer connections because unless the NAT is specially configured,
hosts behind the NAT have no consistent, permanently usable ports to
which incoming TCP or UDP connections from "the Internet at large"
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can be directed. Firewall functionality, which is typically but not
always bundled with NAT, creates a similar problem because firewalls
are generally designed as one-way filters: sessions initiated inside
the protected network are allowed, but attempts by external hosts on
the Internet at large to initiate communication sessions with hosts
inside the firewall are blocked. RFC 3235 [NAT-APPL] briefly
addresses this issue, but does not offer any general solutions that
do not compromise security; filling that gap is the purpose of this
document.
2. Techniques for P2P Communication with NAT
This section describes in detail the currently known techniques for
implementing peer-to-peer communication in the presence of NAT, from
the perspective of the application or protocol designer.
2.1. Relaying
The most reliable, but least efficient, method of implementing peer-
to-peer communication in the presence of NAT is to make the peer-to-
peer communication look to the network like client/server
communication. For example, suppose two client hosts, A and B, have
each initiated TCP or UDP connections with a well-known server S
having a permanent IP address. Clients A and B both reside on
privately addressed networks behind network address translators,
however, and neither of them have control over a public IP address or
permanently stable TCP or UDP port to which incoming connections can
be directed.
Server S
|
|
+----------------------+----------------------+
| |
NAT A NAT B
| |
| |
Client A Client B
Instead of attempting to establish a direct TCP or UDP connection
between clients A and B, the two clients can simply use the server S
to relay messages between them. For example, to send a message to
client B, client A simply sends the message to server S along its
already-established client/server connection, and server S then sends
the message on to client B using its existing client/server
connection with B. This method has the advantage that it will always
work as long as both clients have connectivity to the server. Its
obvious disadvantages are that it consumes the server's processing
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power and network bandwidth unnecessarily, and communication latency
between the two clients is likely to be increased even if the server
is well-connected.
2.2. Connection Reversal
The second technique works if only one of the clients is behind a
NAT. For example, suppose client A is behind a NAT but client B is
not, as in the following diagram:
Server S
18.181.0.31:1235
|
|
+----------------------+----------------------+
| |
NAT A |
155.99.25.11:62000 |
| |
| |
Client A Client B
10.0.0.1:1234 138.76.29.7:1234
Client A has private IP address 10.0.0.1, and the application is
using TCP port 1234. This client has established a connection with
server S at public IP address 18.181.0.31 and port 1235. NAT A has
assigned TCP port 62000, at its own public IP address 155.99.25.11,
to serve as the temporary public endpoint address for A's session
with S: therefore, server S believes that client A is at IP address
155.99.25.11 using port 62000. Client B, however, has its own
permanent IP address, 138.76.29.7, and the peer-to-peer application
on B is accepting TCP connections at port 1234.
Now suppose client B would like to initiate a peer-to-peer
communication session with client A. B might first attempt to
contact client A either at the address client A believes itself to
have, namely 10.0.0.1:1234, or at the address of A as observed by
server S, namely 155.99.25.11:62000. In either case, however, the
connection will fail. In the first case, traffic directed to IP
address 10.0.0.1 will simply be dropped by the network because
10.0.0.1 is not a publicly routable IP address. In the second case,
the TCP SYN request from B will arrive at NAT A directed to port
62000, but NAT A will typically reject the connection request with a
RST packet because only outgoing connections are allowed.
After attempting and failing to establish a direct connection to A,
client B can use server S to relay a request to client A to initiate
a "reversed" connection to client B. Client A, upon receiving this
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relayed request through S, opens a TCP connection to client B at B's
public IP address and port number. NAT A allows the connection to
proceed because it is originating inside the firewall, and client B
can receive the connection because it is not behind a NAT.
A variety of current peer-to-peer systems implement this technique.
Its main limitation, of course, is that it only works as long as only
one of the communicating peers is behind a NAT: if both peers are
behind NATs, then the method fails. Because connection reversal is
not a general solution to the problem, it is NOT recommended as a
primary strategy. Applications may choose to attempt connection
reversal, but should be table to fall back automatically on another
mechanism such as relaying if neither a "forward" nor a "reversed"
connection can be established.
2.3. UDP Hole Punching
The third technique, and the one of primary interest in this memo, is
sometimes known as "UDP Hole Punching." UDP hole punching relies on
well-established NAT conventions to allow appropriately designed
peer-to-peer applications to "punch holes" through NATs and firewalls
and establish direct connectivity with each other, even when both
communicating hosts may lie behind a NAT. This technique was
mentioned briefly in section 5.1 of RFC 3027 [NAT-PROT] and has been
informally described elsewhere on the Internet [KEGEL]. As the name
implies, unfortunately, this technique works reliably only with UDP.
We will consider two specific scenarios, and how applications can be
designed to handle both of them gracefully. In the first situation,
representing the common case, two clients desiring direct peer-to-
peer communication reside behind different NATs. In the second, the
two clients actually reside behind the same NAT, but do not
necessarily know that they do.
2.3.1. Clients Behind Different NATs
Suppose clients A and B both have private IP addresses and lie behind
different network address translators. The peer-to-peer application
running on clients A and B and on server S each use UDP port 1234. A
and B have each initiated UDP communication sessions with server S,
causing NAT A to assign its own public UDP port 62000 for A's session
with S, and causing NAT B to assign its port 31000 to B's session
with S, respectively.
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Server S
18.181.0.31:1234
|
|
+----------------------+----------------------+
| |
NAT A NAT B
155.99.25.11:62000 138.76.29.7:31000
| |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Now suppose that client A wants to establish a UDP communication
session directly with client B. If A simply starts sending UDP
requests to B's public address, 138.76.29.7:31000, then NAT B will
typically discard these incoming messages because the source address
and port number does not match those of S, with which the original
outgoing session was established. Similarly, if B simply starts
sending UDP requests to A's public address, then NAT A will discard
these messages.
Suppose A starts sending UDP requests to B's public address, however,
and simultaneously relays a request through server S to B, asking B
to start sending UDP requests to A's public address. A's outgoing
messages directed to B's public address (138.76.29.7:31000) will
cause NAT A to open up a new communication session between A's
private address and B's public address. At the same time, B's
messages to A's public address (155.99.25.11:62000) will cause NAT B
to open up a new communication session between B's private address
and A's public address. Once the new UDP sessions have been opened
up in each direction, client A and B can communicate with each other
directly without further reference to or burden on the "introduction"
server S.
The UDP hole punching technique has several useful properties. Once
a direct peer-to-peer UDP connection has been established between two
NATted clients, either party on that connection can in turn take over
the role of "introducer" and help the other party establish peer-to-
peer connections with additional peers, minimizing the load on the
initial introduction server S. The application does not need to
attempt to detect explicitly what kind of NAT it is behind, if any
[STUN], since the procedure above will establish peer-to-peer
communication channels equally well if either or both clients do not
happen to be behind a NAT. The hole punching technique even works
automatically under "Twice NAT", where one or both clients are
removed from the public Internet via two or more levels of address
translation.
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2.3.2. Clients Behind the Same NAT
Now consider the scenario in which the two clients (probably
unknowingly) happen to reside behind the same NAT, and are therefore
located in the same private IP address space. Client A has
established a UDP session with server S, to which the common NAT has
assigned public port number 62000. Client B has similarly
established a session with S, to which the NAT has assigned public
port number 62001.
Server S
18.181.0.31:1234
|
|
NAT
A-S 155.99.25.11:62000
B-S 155.99.25.11:62001
|
+----------------------+----------------------+
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Suppose that A and B use the UDP hole punching technique as outlined
above to establish a communication channel using server S as an
introducer. Then A and B will learn each other's public IP addresses
and port numbers as observed by server S, and start sending each
other messages at those public addresses. The two clients will be
able to communicate with each other this way as long as the NAT
allows hosts on the internal network to open translated UDP sessions
with other internal hosts and not just with external hosts. For
example, when A sends a UDP packet to B's public address, the packet
initially has a source IP address and port number of 10.0.0.1:124 and
a destination of 155.99.25.11:62001. The NAT receives this packet,
translates it to have a source of 155.99.25.11:62000 (A's public
address) and a destination of 10.1.1.3:1234, and then forwards it on
to B. Even if supported by the NAT, this translation and forwarding
step is obviously unnecessary in this situation, and is likely to add
latency to the dialog between A and B as well as burdening the NAT.
The solution to this problem is straightforward, however. When A and
B initially exchange address information through server S, they
should include their own IP addresses and port numbers as "observed"
by themselves, as well as their addresses as observed by S. The
clients then simultaneously start sending packets to each other at
each of the alternative addresses they know about, and use the first
address that leads to successful communication. If the two clients
are behind the same NAT, then the packets directed to their private
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addresses are likely to arrive first, resulting in a direct
communication channel not involving the NAT. If the two clients are
behind different NATs, then the packets directed to their private
addresses will fail to reach each other at all, but the clients will
hopefully establish connectivity using their respective public
addresses. It is important that these packets be authenticated in
some way, however, since in the case of different NATs it is entirely
possible for A's messages to B's private address to reach some other,
unrelated node on A's private network, or vice versa.
2.3.3. Consistent Port Mappings
The hole punching technique has one main caveat: it works only if
each of the NATs maintains a single, consistent mapping from a given
(private IP address, private UDP port) pair to a translated (public
IP address, public UDP port) pair, for as long as that UDP port is in
use. For a given UDP port on the private network, the NAT must
ensure that the internal port is always mapped to the same public UDP
port on the globally addressable Internet, even if communication
occurs between that internal UDP port and multiple distinct external
destinations on the Internet. In particular, the NAT must not just
naively allocate and assign a new public UDP port for each new
session initiated from within the firewall boundary, where a
"session" is defined by the addresses and port numbers of both
communicating endpoints. Assigning a new public port for each new
session makes it impossible for a UDP application to reuse an
already-established (public IP address, public UDP port) mapping for
communication with different external destinations.
While RFC 3022 [NAT-TRAD] suggests and explicitly allows NATs to
maintain a single mapping from a (private IP address, private port)
pair to a (public IP address, public port) pair, it unfortunately
does not mandate this behavior. Therefore, while many NATs implement
the desirable behavior and therefore allow direct UDP-based P2P
connections using the hole punching technique, other NATs currently
do not support the technique. Since this is the most efficient known
method of establishing direct peer-to-peer communication between two
nodes that are both behind NATs, and it works with a wide variety of
existing NATs, it is recommended that applications use this technique
if efficient peer-to-peer communication is required, but be prepared
to fall back on simple relaying when direct communication cannot be
established.
2.4. UDP Port Number Prediction
A variant of the UDP hole punching technique discussed above exists
that allows peer-to-peer UDP sessions to be created in the presence
of some NATs that do not maintain a consistent mapping between
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private and public UDP ports. This method, sometimes called the
"N+1" technique [BIDIR], works if public port numbers selected by the
NAT are not held constant across all sessions originating at a given
private IP address and port, but are nevertheless predictable.
Consider again the situation in which two clients, A and B, each
behind a separate NAT, have each established UDP connections with a
permanently addressable server S:
Server S
18.181.0.31:1234
|
|
+----------------------+----------------------+
| |
NAT A NAT B
A-S 155.99.25.11:62000 B-S 138.76.29.7:31000
| |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
NAT A has assigned its own UDP port 62000 to the communication
session between A and S, and NAT B has assigned its port 31000 to the
session between B and S. By communicating through server S, A and B
learn each other's public IP addresses and port numbers as observed
by S. Client A now starts sending UDP messages to port 31001 at
address 138.76.29.7 (note the port number increment), and client B
simultaneously starts sending messages to port 62001 at address
155.99.25.11. If NATs A and B assign port numbers to new sessions
sequentially, and if not much time has passed since the A-S and B-S
sessions were initiated, then a working bidirectional communication
channel between A and B should result. A's messages to B cause NAT A
to open up a new session, to which NAT A will (hopefully) assign
public port number 62001, because 62001 is next in sequence after the
port number 62000 it previously assigned to the session between A and
S. Similarly, B's messages to A will cause NAT B to open a new
session, to which it will (hopefully) assign port number 31001. If
both clients have correctly guessed the port numbers each NAT assigns
to the new sessions, then a bidirectional UDP communication channel
will have been established as shown below.
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Server S
18.181.0.31:1234
|
|
+----------------------+----------------------+
| |
NAT A NAT B
A-S 155.99.25.11:62000 B-S 138.76.29.7:31000
A-B 155.99.25.11:62001 B-A 138.76.29.7:31001
| |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Obviously there are many things that can cause this trick to fail.
If the predicted port number at either NAT already happens to be in
use by an unrelated session, then the NAT will skip over that port
number and the connection attempt will fail. If either NAT sometimes
or always chooses port numbers non-sequentially, then the trick will
fail. If a different client behind NAT A (or B respectively) opens
up a new outgoing UDP connection to any external destination after A
(B) establishes its connection with S but before sending its first
message to B (A), then the unrelated client will inadvertently
"steal" the desired port number. This trick is therefore much less
likely to work when either NAT involved is under load. For all of
these reasons, it is NOT recommended that new applications implement
this trick; it is described here for purely for historical and
informational purposes.
2.5. Simultaneous TCP Connection Initiation
There is a method that can be used in some cases to establish direct
peer-to-peer TCP connections between a pair of nodes that are both
behind NATs. Most TCP sessions start with one endpoint sending a SYN
packet, to which the other party responds with a SYN-ACK packet. It
is possible and legal, however, for two endpoints to start a TCP
session by simultaneously sending each other SYN packets, to which
each party subsequently responds with a separate ACK. This procedure
is known as a "simultaneous open."
If a NAT receives a TCP SYN packet from outside the private network
attempting to initiate an incoming TCP connection, the NAT will
normally reject the connection attempt by sending back a TCP RST
(connection reset) packet. If, however, the SYN packet arrives with
source and destination addresses and port numbers that correspond to
a TCP session that the NAT believes is already active, then the NAT
will allow the packet to pass through. In particular, if the NAT has
just recently seen and transmitted an outgoing SYN packet with the
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same addresses and port numbers, then it will consider the session
active and allow the incoming SYN through. If clients A and B can
each correctly predict the public port number that its respective NAT
will assign the next outgoing TCP connection, and if each client
initiates an outgoing TCP connection with the other client timed so
that each client's outgoing SYN passes through its local NAT before
either SYN reaches the opposite NAT, then a working peer-to-peer TCP
connection will result.
Unfortunately, this trick is even more fragile and timing-sensitive
than the UDP port number prediction trick described above. First,
all the same things can go wrong with each side's attempt to predict
the public port numbers the NATs will assign to the new sessions. In
addition, if either client's SYN arrives at the opposite NAT too
quickly, then the NAT will reject the SYN with a RST packet, causing
the local NAT in turn to close the new session. Finally, even though
support for simultaneous open is technically a mandatory part of the
TCP specification [TCP], it is not implemented correctly or at all in
many common operating systems. For this reason, this trick is
likewise mentioned here only for historical interest; it is NOT
recommended for use by applications. Applications that require
efficient, direct peer-to-peer communication should use UDP.
3. NAT Design Guidelines
This section discusses considerations for the design of network
address translators, as they affect peer-to-peer applications.
3.1. Maintaining Consistent Public/Private Mappings for UDP Ports
The primary and most important recommendation of this document for
NAT designers is that the NAT maintain a consistent and stable
mapping between a given (internal IP address, internal UDP port) pair
and a corresponding (public IP address, public UDP port) pair for as
long as any active sessions exist using that mapping. The NAT may
filter incoming traffic on a per-session basis, by examining both the
source and destination IP addresses and port numbers in each packet.
When a node on the private network starts sending outgoing packets to
a new external destination, however, while using the same source IP
address and UDP port as an existing translated UDP session, the NAT
should ensure that the new UDP session is given the same public IP
address and UDP port numbers as the existing session.
One method of implementing this behavior, meant to be only
illustrative and not prescriptive, is outlined here. The "critical-
path" processing performed by a NAT on a packet flowing in either
direction typically involves extracting the source and destination IP
addresses from the IP header, and the source and destination TCP/UDP
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port numbers from the TCP/UDP header, and using these four values to
index into a table of active sessions. When a packet is received for
which no entry is found in the session table, the NAT must decide
whether or not and how to establish a new session, and this is where
the typical "outgoing sessions only" firewall policy comes into
effect.
If the unknown packet was received from the public network, then it
is usually dropped (or perhaps rejected with a RST in the case of
TCP). If a UDP packet for an unknown session arrives from the
private network, however, and the NAT is configured in the typical
way to allow outgoing UDP sessions, the then NAT looks up the
packet's (source IP address, source UDP port) in a second internal
table: a "mapping table." For a given (private IP address, private
UDP port) on the internal network, the NAT stores in the mapping
table the corresponding (public IP address, public UDP port) it has
assigned to represent that private endpoint. If a mapping already
exists, the NAT simply uses the existing mapping when constructing
the new session. If no such entry is present in the mapping table,
then the NAT allocates a new public UDP port from its pool and
creates a new mapping table entry along with the new session. The
NAT also maintains with each entry in the mapping table a list or
count of the active sessions using that mapping, so that it can
reassign the public UDP port to other purposes once all of the
outstanding sessions for the mapping are deemed inactive.
3.1.1. Preserving Port Numbers
Some NATs, when establishing a new UDP session, attempt to assign the
same public port number as the corresponding private port number, if
that port number happens to be available. For example, if client A
at address 10.0.0.1 initiates an outgoing UDP session with a datagram
from port number 1234, and the NAT's public port number 1234 happens
to be available, then the NAT uses port number 1234 at the NAT's
public IP address as the translated endpoint address for the session.
This behavior might be beneficial to some legacy UDP applications
that expect to communicate only using specific UDP port numbers, but
it is not recommended that applications depend on this behavior since
it is only possible for a NAT to preserve the port number if at most
one node on the internal network is using that port number.
In addition, a NAT should NOT try to preserve the port number in a
new session if doing so would conflict with the goal of maintaining a
consistent mapping between public and private endpoint addresses.
For example, suppose client A at internal port 1234 has established a
session with external server S, and NAT A has assigned public port
62000 to this session because port number 1234 on the NAT was not
available at the time. Now suppose port number 1234 on the NAT
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subsequently becomes available, and while the session between A and S
is still active, client A initiates a new session from its same
internal port (1234) to a different external node B. In this case,
because an active mapping has already been established between client
A's port 1234 and the NAT's public port 62000, this mapping should be
maintained and the new session should also use port 62000 as the
public port corresponding to client A's port 1234. The NAT should
NOT assign public port 1234 to this new session just because port
1234 has become available: that behavior would not be likely to
benefit the application in any way since the application has already
been operating with a translated port number, and it would break any
attempts the application might make to establish peer-to-peer
connections using the UDP hole punching technique.
3.2. Maintaining Consistent Public/Private Mappings for TCP Ports
For consistency with the behavior of UDP translation, it is suggested
that NATs also maintain a consistent mapping between private and
public (IP address, TCP port number) pairs for TCP connections, in
the same way as described above for UDP. Maintaining consistent
mappings for TCP ports facilitates the operation of the simultaneous
TCP open technique, which although not recommended in general for
peer-to-peer applications, may be useful in controlled situations
when the two endpoints are sufficiently well synchronized.
Maintaining TCP endpoint mappings consistently may also increase the
NAT's compatibility with other applications that initiate multiple
TCP connections from the same source port.
3.3. Proxy Protocols
Besides adopting the above recommendations to make a NAT's basic
"transparent-mode" operation as peer-to-peer friendly as possible, it
is helpful for NATs also to support proxy protocols that allow
applications to request an explicitly managed presence on the public
side of the NAT. Unfortunately, several alternative protocols have
been proposed with varying characteristics [SOCKS, RSIP, MIDCOM,
UPNP], and as of this writing none of them have achieved clear
acceptance or dominance in the Internet community. Furthermore, it
is not clear yet how well these protocols will work in the
increasingly common "Twice NAT" situation where clients are located
behind multiple levels of NAT, especially if the NATs are from
different vendors, support different features and policies, and are
under different administrative domains. (In the common case, one is
owned and managed by the ISP and the other by the end user.) For
these reasons, this document makes no attempt to explore this issue
in detail or to recommend specific proxy protocols for NATs to
implement.
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4. Security Considerations
Using the UDP hole punching technique in peer-to-peer applications
and supporting it in NATs should not create any new security issues.
In particular, the technique does not require a NAT firewall to be
"promiscuous" in any way about acceping incoming UDP traffic. As
long as outgoing UDP sessions are enabled and the firewall maintains
consistent mappings between internal and external UDP ports, the
firewall can still filter out all incoming UDP packets except those
with (source IP, source port, destination IP, destination port)
tuples that exactly match those of active sessions initiated from
within the enclave. Filtering incoming traffic aggressively while
maintaining consistent mappings thus allows a firewall to be "peer-
to-peer friendly" without compromising the standard firewall security
principle of rejecting all unsolicited incoming traffic.
It might be argued that maintaining a consistent internal/external
port mapping can "leak" some information to the outside about the
applications on the internal network, particularly about the
relationships between different UDP sessions that cross the firewall
boundary. If the security requirements are so critical that such a
subtle information channel is of concern, however, then the firewall
almost certainly should not be configured to allow unrestricted
outgoing UDP traffic in the first place. Controlling information
flow to this degree generally requires that the firewall only allow
communication only via tightly-controllable application-level
gateways, in which case the firewall can either implement the proper
peer-to-peer communication behavior itself or disallow it entirely.
References
[BIDIR] Peer-to-Peer Working Group, NAT/Firewall Working Committee,
"Bidirectional Peer-to-Peer Communication with Interposing
Firewalls and NATs", August 2001.
http://www.peer-to-peerwg.org/tech/nat/
[KEGEL] Dan Kegel, "NAT and Peer-to-Peer Networking", July 1999.
http://www.alumni.caltech.edu/~dank/peer-nat.html
[MIDCOM] P. Srisuresh, J. Kuthan, J. Rosenberg, A. Molitor, and
A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[NAT-APPL] D. Senie, "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[NAT-PROT] M. Holdrege and P. Srisuresh, "Protocol Complications
with the IP Network Address Translator", RFC 3027,
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draft-ford-natp2p-00.txt April 2003
January 2001.
[NAT-TERM] P. Srisuresh and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
[NAT-TRAD] P. Srisuresh and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RSIP] M. Borella, J. Lo, D. Grabelsky, and G. Montenegro,
"Realm Specific IP: Framework", RFC 3102, October 2001.
[SOCKS] M. Leech, M. Ganis, Y. Lee, R. Kuris, D. Koblas, and
L. Jones, "OCKS Protocol Version 5", RFC 1928, March 1996.
[STUN] J. Rosenberg, J. Weinberger, C. Huitema, and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
March 2003.
[TCP] "Transmission Control Protocol", RFC 793, September 1981.
[UPNP] UPnP Forum, "Internet Gateway Device (IGD) Standardized
Device Control Protocol V 1.0", November 2001.
http://www.upnp.org/standardizeddcps/igd.asp
Author's Address
Bryan Ford
Laboratory for Computer Science
Massachusetts Institute of Technology
77 Massachusetts Ave.
Cambridge, MA 02139
Phone: (617) 253-5261
E-mail: baford@mit.edu
Web: http://www.brynosaurus.com/
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kind, provided that the above copyright notice and this paragraph are
Ford [Page 15]
draft-ford-natp2p-00.txt April 2003
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