Intended status: INFORMATIONAL
Internet Draft P. Srisuresh
Expires: August 09, 2007 Kazeon Systems
B. Ford
M.I.T.
D. Kegel
kegel.com
February 09, 2007
State of Peer-to-Peer(P2P) Communication
Across Network Address Translators(NATs)
<draft-ietf-behave-p2p-state-01.txt>
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
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
http://www.ietf.org/shadow.html
Abstract
This memo documents the various methods known to be in use by
peer-to-peer (P2P) applications for communication in the presence
of Network Address Translators (NATs) at the current time. This
memo covers NAT traversal approaches used by both TCP and UDP
based applications. This memo is not an endorsement of the methods
described, but merely an attempt to capture them in a document.
Srisuresh, Ford & Kegel [Page 1]
Internet-Draft State of P2P Communication Across NATs February 2007
Table of Contents
1. Introduction .................................................
2. Terminology ..................................................
2.1. Endpoint ................................................
2.2. Endpoint Mapping ........................................
2.3. Endpoint-Independent Mapping ............................
2.4. Endpoint-Dependent Mapping ..............................
2.5. Endpoint-Independent Filtering ..........................
2.6. Endpoint-Dependent Filtering ............................
2.7. P2P Application .........................................
2.8. NAT-friendly P2P application ............................
2.9. P2P-friendly NAT ........................................
2.10. Hairpin translation ....................................
3. Techniques used by NAT-friendly P2P applications .............
3.1. Relaying ................................................
3.2. Connection reversal .....................................
3.3. UDP Hole Punching .......................................
3.3.1. Peers behind different NATs ......................
3.3.2. Peers behind the same NAT ........................
3.3.3. Peers separated by multiple NATs .................
3.3.4. Where UDP hole punching fails ....................
3.4. Simultaneous TCP Open ...................................
3.5. UDP port number prediction ..............................
3.6. TCP port number prediction ..............................
4. Recent Work on P2P NAT Traversal ..............................
5. Summary of observations ......................................
5.1. TCP/UDP hole punching ...................................
5.2. Symmetric NATs are not P2P friendly .....................
5.3. Peer discovery ..........................................
5.4. Hairpin translation .....................................
6. Security considerations ......................................
6.1. Lack of Authentication can cause connection hijacking ...
6.2. Denial-of-service attacks ...............................
6.3. Man-in-the-middle attacks ...............................
6.4. Security impact from a P2P-friendly NAT device ..........
7. IANA Considerations ..........................................
8. Acknowledgments ..............................................
9. Normative References .........................................
10. Informative References .......................................
1. Introduction
Present day Internet has seen ubiquitous deployment of network
address translators (NATs). There are a variety of NAT devices and
a variety of network topologies utilizing NAT devices in
deployments. The asymmetric addressing and connectivity regimes
Srisuresh, Ford & Kegel [Page 2]
Internet-Draft State of P2P Communication Across NATs February 2007
established by these NAT devices has created unique problems for
peer-to-peer (P2P) applications and protocols, such as
teleconferencing and multiplayer on-line gaming. These issues are
likely to persist even into the IPv6 world, where a NAT may be used
as an IPv4 compatibility mechanism [NAT-PT].
Currently deployed NAT devices are designed primarily around the
client/server paradigm, in which relatively anonymous client machines
inside a private network initiate connections to public servers with
stable IP addresses and DNS names. NAT devices encountered enroute
provide dynamic address assignment for the client machines. The
anonymity and inaccessibility of the internal hosts behind a NAT
device is not a problem for applications such as web browsers, which
only need to initiate outgoing connections. This inaccessibility is
sometimes perceived as a privacy benefit.
In the peer-to-peer paradigm, Internet hosts that would normally be
considered "clients" not only initiate sessions to peer nodes, but
also accept sessions initiated by peer nodes. The initiator and the
responder might lie behind different NAT devices with neither
endpoint having a permanent IP address or other form of public
network presence. A common on-line gaming architecture, for example,
involves all participating application hosts contacting a
well-known rendezvous server for registering themselves and
discovering peer hosts. Subsequent to the communication with
rendezvous server, the hosts establish direct connections with each
other for fast and efficient propagation of updates during game play.
Similarly, a file sharing application might contact a well-known
rendezvous server for resource discovery or searching, but establish
direct connections with peer hosts for data transfer. NAT devices
create problems for peer-to-peer connections because hosts behind a
NAT device normally have no permanently visible public ports on the
Internet to which incoming TCP or UDP connections from other peers
can be directed. RFC 3235 [NAT-APPL] briefly addresses this issue.
Unless prefixed with a NAT type or explicitly stated otherwise, the
term NAT, used throughout the document, refers to Traditional
NAT as described in [NAT-TRAD]. Traditional NATs include the
popular NAPT devices which use a single public IP address to
translate multiple private IP addresses.
NAT traversal strategies that involve explicit signaling between
applications and NAT devices, namely [NAT-PMP], [NSIS-NSLP],
[SOCKS], [RSIP], [MIDCOM], and [UPNP] are out of the scope of this
document. [UNSAF] is in scope.
In this document, we summarize the currently known methods by which
P2P applications work around the presence of NAT devices, without
Srisuresh, Ford & Kegel [Page 3]
Internet-Draft State of P2P Communication Across NATs February 2007
directly altering the NAT devices. IP addresses from [RFC3330] are
used, where possible, for illustration purposes. The traversal
techniques discussed are limited to TCP and UDP based applications.
It is not the objective of this document to provide solutions to NAT
traversal problem for P2P applications in general [BEH-APP] or to a
specific class of applications [ICE].
2. Terminology
Readers are urged to refer [NAT-TERM] for information on NAT
taxonomy and terminology. Traditional NAT [NAT-TRAD] is the most
common type of NAT device deployed. Traditional NAT has two main
variations - Basic NAT and Network Address Port Translator (NAPT).
Of these, NAPT is by far the most commonly deployed NAT device. NAPT
allows multiple internal hosts to share a single public IP address
simultaneously. When an internal host opens an outgoing TCP or UDP
session through a NAPT, the NAPT assigns the session a public IP
address and port number so that subsequent response packets from
the external endpoint can be received by the NAPT, translated, and
forwarded to the internal host. Unless specified otherwise, the
term NAT in this document simply refers to Traditional NAT.
An issue of relevance to P2P applications is how the NAT behaves
when an internal host initiates multiple simultaneous sessions from
a single endpoint (private IP, private port) to multiple distinct
endpoints on the external network.
[STUN] further classifies NAT implementations using the terms
"Full Cone", "Restricted Cone", "Port Restricted Cone" and
"Symmetric". Unfortunately, this terminology has been the source of
much confusion. For this reason, this draft adapts terminology from
[BEH-UDP] to distinguish between NAT implementations.
Listed below are terms used throughout the document.
2.1. Endpoint
An endpoint is a session specific tuple on an end host. An endpoint
may be represented differently for each IP protocol. For example,
a TCP session endpoint is represented as a tuple of (IP Address,
TCP port), and a UDP session endpoint is represented as a tuple
of (IP Address, UDP Port).
2.2. Endpoint Mapping
When a host in a private realm initiates an outgoing session to a
host in the public realm through a NAT device, the NAT device
assigns an external endpoint to translate the private endpoint so
Srisuresh, Ford & Kegel [Page 4]
Internet-Draft State of P2P Communication Across NATs February 2007
that subsequent response packets from the external host can be
received by the NAT, translated, and forwarded to the private
endpoint. The assignment by the NAT device to translate a private
endpoint to an external endpoint and vice versa is called the
Endpoint Mapping. NAT uses the Endpoint Mapping to perform
translation for the duration of the session.
2.3. Endpoint-Independent Mapping
"Endpoint-Independent Mapping" is defined in [BEH-UDP] as follows.
The NAT reuses the port mapping for subsequent packets sent
from the same internal IP address and port (X:x) to any
external IP address and port.
Endpoint-Independent Mapping is shared by all variations of Cone
NAT devices ([STUN]). The following text provides an example of
this. Suppose Client A in figure 1 initiates two simultaneous
outgoing sessions through a NAT device employing
Endpoint-Independent Mapping, from the same internal endpoint
(10.0.0.1:1234) to two different external servers, S1 and S2. The
NAT device assigns just one public endpoint 192.0.2.1:62000 to
both these sessions, ensuring that the "identity" of the client's
endpoint is maintained across address translation. Since Basic NAT
devices do not modify port numbers as packets traverse the device,
Basic NAT device can be viewed as a degenerate form of a NAT device
performing Endpoint-Independent Mapping.
Srisuresh, Ford & Kegel [Page 5]
Internet-Draft State of P2P Communication Across NATs February 2007
Server S1 Server S2
18.181.0.31:1235 138.76.29.7:1235
| |
| |
+----------------------+----------------------+
|
^ Session 1 (A-S1) ^ | ^ Session 2 (A-S2) ^
| 18.181.0.31:1235 | | | 138.76.29.7:1235 |
| 192.0.2.1:62000 | | | 192.0.2.1:62000 |
|
+----------------------+
| 192.0.2.1 |
| |
| NAT Device employing |
| Endpoint-Independent |
| Mapping |
+----------------------+
|
^ Session 1 (A-S1) ^ | ^ Session 2 (A-S2) ^
| 18.181.0.31:1235 | | | 138.76.29.7:1235 |
| 10.0.0.1:1234 | | | 10.0.0.1:1234 |
|
Client A
10.0.0.1:1234
Figure 1: Translation performed by Endpoint-Independent Mapping NAT
2.4. Endpoint-Dependent Mapping
"Endpoint-Dependent Mapping" refers to the combination of
"Address-Dependent Mapping" and "Address and Port-Dependent Mapping"
as defined in [BEH-UDP].
"Address-Dependent Mapping" is defined in [BEH-UDP] as follows.
The NAT reuses the port mapping for subsequent packets sent
from the same internal IP address and port (X:x) to the same
external IP address, regardless of the external port.
"Address and Port-Dependent Mapping" is defined in [BEH-UDP] as
follows.
The NAT reuses the port mapping for subsequent packets sent
from the same internal IP address and port (X:x) to the same
external IP address and port while the mapping is still active.
Symmetric NAT devices ([STUN]) are a good example of NAT devices
performing Endpoint-Dependent Mapping. The following text provides
an example of this.
Srisuresh, Ford & Kegel [Page 6]
Internet-Draft State of P2P Communication Across NATs February 2007
Suppose Client A in figure 2 initiates two outgoing sessions from
the same endpoint, one with S1 and another with S2. The same client
endpoint is connecting to the two external servers S1 and S2. When
the first session to server S1 traverses the NAT device employing
Endpoint-Dependent Mapping, the NAT device assigns port 62000 to
translate the client endpoint. When the second session from the
same client endpoint to server S2 traverses the NAT device, the NAT
device assigns a different port 62001 to translate the same client
endpoint. As a result, server S1 sees the client endpoint as
192.0.2.1:62000, whereas server S2 sees the same client endpoint
differently as 192.0.2.1:62001. The NAT device, however, is able
to differentiate between the two sessions for translation purposes
because the external endpoints involved in the two sessions (those
of S1 and S2) differ, even as the endpoint identity of the client
application is lost across the address translation boundary.
Server S1 Server S2
18.181.0.31:1235 138.76.29.7:1235
| |
| |
+----------------------+----------------------+
|
^ Session 1 (A-S1) ^ | ^ Session 2 (A-S2) ^
| 18.181.0.31:1235 | | | 138.76.29.7:1235 |
| 192.0.2.1:62000 | | | 192.0.2.1:62001 |
|
+----------------------+
| 192.0.2.1 |
| |
| NAT Device employing |
| Endpoint-Dependent |
| Mapping |
+----------------------+
|
^ Session 1 (A-S1) ^ | ^ Session 2 (A-S2) ^
| 18.181.0.31:1235 | | | 138.76.29.7:1235 |
| 10.0.0.1:1234 | | | 10.0.0.1:1234 |
|
Client A
10.0.0.1:1234
Figure 2: Endpoint-Dependent Mapping - NAT Translation
2.5. Endpoint-Independent Filtering
Srisuresh, Ford & Kegel [Page 7]
Internet-Draft State of P2P Communication Across NATs February 2007
"Endpoint-Independent Filtering" is defined in [BEH-UDP] as follows.
The NAT filters out only packets not destined to the internal
address and port X:x, regardless of the external IP address and
port source (Z:z). The NAT forwards any packets destined to
X:x. In other words, sending packets from the internal side of
the NAT to any external IP address is sufficient to allow any
packets back to the internal endpoint.
A NAT device employing the combination of "Endpoint-Independent
Mapping" and "Endpoint-Independent Filtering" will accept incoming
traffic to a mapped public port from ANY external endpoint on the
public network. Such a NAT device is also sometimes referred to as
"Promiscuous NAT" or "Full Cone NAT" [STUN].
2.6. Endpoint-Dependent Filtering
"Endpoint-Dependent Filtering" is same as "Address and Port-Dependent
Filtering" defined in [BEH-UDP]. "Address and Port-Dependent
Filtering" is defined in [BEH-UDP] as follows.
The NAT filters out packets not destined for the internal address
X:x. Additionally, the NAT will filter out packets from Y:y
destined for the internal endpoint X:x if X:x has not sent
packets to Y:y previously. In other words, for receiving packets
from a specific external endpoint, it is necessary for the
internal endpoint to send packets first to that external
endpoint's IP address and port.
Endpoint-Dependent Filtering is the least liberal form of filtering
incoming traffic on a NAT device.
2.7. P2P Application
A P2P application is an application that uses the same endpoint to
initiate outgoing sessions to peering hosts as well as accept
incoming sessions from peering hosts.
2.8. NAT-friendly P2P application
NAT-friendly P2P application is a P2P application that is designed
to work effectively even as peering nodes are located in distinct
IP address realms, connected by one or more NATs.
A NAT-friendly P2P application registers with a well-known
rendezvous server, used for node registration and peer node
discovery purposes. Pursuant to registering with rendezvous
server, a P2P-friendly application uses its private endpoint,
public endpoint, or a combination thereof to establish peering
Srisuresh, Ford & Kegel [Page 8]
Internet-Draft State of P2P Communication Across NATs February 2007
sessions.
2.9. P2P-friendly NAT
A P2P-friendly NAT is a NAT device that maintains the endpoint
identity of a P2P host application when the P2P application
initiates a session. P2P-friendly NAT devices permit traversal of
P2P applications traffic across themselves. NAT devices employing
Address-Independent Mapping are examples of P2P-friendly NAT
devices. A NAT device employing Address-Dependent Mapping is an
example of a NAT device that is not P2P friendly.
2.10. Hairpin translation
When a host in the private domain of a NAT device attempts to
connect with another host behind the same NAT device using the
public address of the host, a NAT device supporting hairpin
translation performs the equivalent of Twice NAT ([NAT-TERM])
translation on the packet as follows. The originating host's private
endpoint is translated into its assigned public endpoint, and the
target host's public endpoint is translated into its private
endpoint, before the packet is forwarded to the target host. We
refer the above translation as "Hairpin translation". Not all
currently deployed NATs support hairpin translation, although it is
mandated by [BEH-UDP].
3. Techniques used by P2P applications to traverse NATs
This section reviews in detail the currently known techniques for
implementing peer-to-peer communication over existing NAT devices,
from the perspective of the application or protocol designer.
Readers will note that the applications assume a NAT device
employing Endpoint-Independent Mapping and Endpoint-Dependent
Filtering in majority of the cases below.
3.1. Relaying
The most reliable, but least efficient method of implementing peer-
to-peer communication in the presence of a NAT device is to make the
peer-to-peer communication look to the network like client/server
communication through relaying. Consider the scenario in figure 3.
Two client hosts A and B, have each initiated TCP or UDP
connections to a well-known rendezvous server S. The Rendezvous
Server S has a permanent IP address and is used for the purposes of
registration, discovery and relay. Hosts behind NAT register with
the server. Peer hosts can discover hosts behind NATs and relay all
end-to-end messages using the server. The clients reside on separate
Srisuresh, Ford & Kegel [Page 9]
Internet-Draft State of P2P Communication Across NATs February 2007
private networks, and their respective NAT devices prevent either
client from directly initiating a connection to the other.
Registry, Discovery
combined with Relay
Server S
18.181.0.31:1234
|
+----------------------------+----------------------------+
| ^ Registry/ ^ ^ Registry/ ^ |
| | Relay-Req Session(A-S) | | Relay-Req Session(B-S) | |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 192.0.2.1:62000 | | 138.76.29.7:31000 | |
| |
+--------------+ +--------------+
| 192.0.2.1 | | 138.76.29.7 |
| | | |
| NAT A | | NAT B |
+--------------+ +--------------+
| |
| ^ Registry/ ^ ^ Registry/ ^ |
| | Relay-Req Session(A-S) | | Relay-Req Session(B-S) | |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Figure 3: Use of Registry and Relay Server to emulate direct-P2P
Instead of attempting a direct connection, 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. The enroute NAT device
is not assumed to be P2P friendly. The obvious disadvantages of
relaying are that it consumes the server's processing power and
network bandwidth, and communication latency between the peering
clients is likely to be increased even if the server is
well-connected. The TURN protocol [TURN] defines a method of
implementing relaying in a relatively secure fashion.
Srisuresh, Ford & Kegel [Page 10]
Internet-Draft State of P2P Communication Across NATs February 2007
3.2. Connection reversal
The following connection reversal technique for a direct P2P
communication works only when one of the clients (i.e., peers) is
behind a NAT device and the other is not. For example, consider
the scenario in figure 4. Client A is behind a NAT, but client B
has a permanent, globally routable IP address. A well-known
Rendezvous Server S has a permanent, globally routable IP address
and is used for the purposes of node registration and discovery.
Hosts behind NAT register with the server. Peer hosts discover
hosts behind NAT using the server.
Registry, Discovery
Server S
18.181.0.31:1234
|
+----------------------------+----------------------------+
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 192.0.2.1:62000 | | 138.76.29.7:1234 | |
| |
| ^ P2P Session (A-B) ^ | P2P Session (B-A) | |
| | 138.76.29.7:1234 | | 192.0.2.1:62000 | |
| | 192.0.2.1:62000 | v 138.76.29.7:31000 v |
| |
+--------------+ |
| 192.0.2.1 | |
| | |
| NAT A | |
+--------------+ |
| |
| ^ Registry Session(A-S) ^ |
| | 18.181.0.31:1234 | |
| | 10.0.0.1:1234 | |
| |
| ^ P2P Session (A-B) ^ |
| | 138.76.29.7:1234 | |
| | 10.0.0.1:1234 | |
| |
Private Client A Public Client B
10.0.0.1:1234 138.76.29.7:1234
Figure 4: Connection reversal to accomplish Direct-P2P
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
Srisuresh, Ford & Kegel [Page 11]
Internet-Draft State of P2P Communication Across NATs February 2007
assigned TCP port 62000, at its own public IP address 192.0.2.1,
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
192.0.2.1 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 192.0.2.1:62000. 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 reject the connection request 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
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 device.
A variety of current peer-to-peer applications implement this
technique. Its main limitation, of course, is that it only works so
long as only one of the communicating peers is behind a NAT device.
If the NAT device employs Endpoint-Independent Mapping, the public
Client can contact external server S to determine the specific
public endpoint from which to expect Client-A originated
connection. That would be P2P-friendly. However, if the NAT device
employs Endpoint-Dependent Mapping, the public Client cannot know
the specific public endpoint from which to expect Client-A
originated connection. In the increasingly common case where both
peers can be behind NATs, the Connection Reversal method fails. As
such, Connection Reversal is not a general solution to the
Peer-to-peer connection problem. Even if an application attempts
connection reversal method, the application should be able to fall
back automatically to another mechanism such as relaying if neither
a "forward" nor a "reverse" connection can be established.
3.3. UDP hole punching
UDP hole punching relies on the properties of common firewalls and
NATs employing Address-Independent Mapping to allow appropriately
Srisuresh, Ford & Kegel [Page 12]
Internet-Draft State of P2P Communication Across NATs February 2007
designed peer-to-peer applications to "punch holes" through the NAT
device and establish direct connectivity with each other, even when
both communicating hosts lie behind NAT devices. This technique was
mentioned briefly in section 5.1 of RFC 3027 [NAT-PROT], described
in [KEGEL], and used in some recent protocols [TEREDO, ICE]. This
technique has been used primarily with UDP applications, but not as
reliably with TCP applications. Readers may refer Section 3.4 for
details on "Simultaneous TCP open", also known sometimes as "TCP
hole punching".
We will consider two specific scenarios, and how applications are
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 two different NATs. In the second,
the two clients actually reside behind the same NAT, but do not
necessarily know that they do.
3.3.1. Peers behind different NATs
Consider the scenario in figure 5. Clients A and B both have private
IP addresses and lie behind different network address translators. A
well-known Rendezvous Server S has a permanent, globally routable IP
address and is used for the purposes of registration, discovery, and
limited relay. Hosts behind NAT register with the server. Peer hosts
discover hosts behind NAT using the server. Unlike in section 3.1,
peer hosts use the server to relay just connection initiation control
messages, instead of all end-to-end messages.
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.
Srisuresh, Ford & Kegel [Page 13]
Internet-Draft State of P2P Communication Across NATs February 2007
Registry, Discovery, combined
with limited Relay
Server S
18.181.0.31:1234
|
+----------------------------+----------------------------+
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 192.0.2.1:62000 | | 138.76.29.7:31000 | |
| |
| ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ |
| | 138.76.29.7:31000 | | 192.0.2.1:62000 | |
| | 192.0.2.1:62000 | | 138.76.29.7:31000 | |
| |
+--------------+ +--------------+
| 192.0.2.1 | | 138.76.29.7 |
| | | |
| P2P-friendly | | P2P-friendly |
| NAT A | | NAT B |
+--------------+ +--------------+
| |
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
| ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ |
| | 138.76.29.7:31000 | | 192.0.2.1:62000 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Figure 5: Simultaneous Hole Punching to accomplish Direct-P2P
Now suppose that client A wants to establish a UDP communication
session directly with client B. If A simply starts sending UDP
messages to B's public address, 138.76.29.7:31000, then NAT B will
typically discard these incoming messages (unless it employs
Endpoint-Independent Filtering), 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
messages to A's public address and port number, then NAT A will
typically discard these messages.
Suppose A starts sending UDP messages to B's public address, however,
and simultaneously relays a request through server S to B, asking B
to start sending UDP messages to A's public address. A's outgoing
messages directed to B's public address (138.76.29.7:31000) cause NAT
Srisuresh, Ford & Kegel [Page 14]
Internet-Draft State of P2P Communication Across NATs February 2007
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 (192.0.2.1:62000) 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 burden on the server S. Server S, which helps with
relaying connection initiation requests to peer nodes behind NAT ends
up like an "introduction" server to peer hosts.
The UDP hole punching technique has several useful properties. Once
a direct peer-to-peer UDP connection has been established between two
clients behind NAT devices, 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 the kind of NAT device it is behind,
as in [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 device. The UDP hole punching technique
even works automatically with multiple NATs, where one or both
clients are removed from the public Internet via two or more levels
of address translation.
3.3.2. Peers 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, as in figure 6. A
well-known Rendezvous Server S has a permanent, globally routable IP
address and is used for the purposes of registration, discovery, and
limited relay. Hosts behind NAT register with the server. Peer hosts
discover hosts behind NAT using the server and relay messages using
the server. Unlike in section 3.1, peer hosts use the server to relay
just control messages, instead of all end-to-end messages.
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.
Srisuresh, Ford & Kegel [Page 15]
Internet-Draft State of P2P Communication Across NATs February 2007
Registry, Discovery, combined
with limited Relay
Server S
18.181.0.31:1234
|
^ Registry Session(A-S) ^ | ^ Registry Session(B-S) ^
| 18.181.0.31:1234 | | | 18.181.0.31:1234 |
| 192.0.2.1:62000 | | | 192.0.2.1:62001 |
|
+--------------+
| 192.0.2.1 |
| |
| P2P-friendly |
| NAT |
+--------------+
|
+-----------------------------+----------------------------+
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
| ^ P2P Session-try1(A-B) ^ ^ P2P Session-try1(B-A) ^ |
| | 10.1.1.3:1234 | | 10.0.0.1:1234 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
| ^ P2P Session-try2(A-B) ^ ^ P2P Session-try2(B-A) ^ |
| | 192.0.2.1:62001 | | 192.0.2.1:62000 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Figure 6: Use local & public identities to communicate with peers.
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. We refer
to this situation as "Hairpin translation," because packets arriving
at the NAT from the private network are translated and then looped
back to the private network rather than being passed through to the
public network. 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 192.0.2.1:62001. The NAT
Srisuresh, Ford & Kegel [Page 16]
Internet-Draft State of P2P Communication Across NATs February 2007
receives this packet, translates it to have a source of
192.0.2.1:62000 (A's public address) and a destination of
10.1.1.3:1234, and then forwards it on to B. Even if hairpin
translation is 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
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 directed at B's private address to reach
some other, unrelated node on A's private network, or vice versa.
3.3.3. Peers separated by multiple NATs
In some topologies involving multiple NAT devices, it is not
possible for two clients to establish an "optimal" P2P route between
them without specific knowledge of the topology. Consider for
example the scenario in figure 7.
Srisuresh, Ford & Kegel [Page 17]
Internet-Draft State of P2P Communication Across NATs February 2007
Registry, Discovery, combined
with limited Relay
Server S
18.181.0.31:1234
|
^ Registry Session(A-S) ^ | ^ Registry Session(B-S) ^
| 18.181.0.31:1234 | | | 18.181.0.31:1234 |
| 192.0.2.1:62000 | | | 192.0.2.1:62001 |
|
+--------------+
| 192.0.2.1 |
| |
| P2P-friendly |
| NAT X |
| (Supporting |
| Hairpin |
| Translation) |
+--------------+
|
+----------------------------+----------------------------+
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 192.168.1.1:30000 | | 192.168.1.2:31000 | |
| |
| ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ |
| | 192.0.2.1:62001 | | 192.0.2.1:62000 | |
| | 192.168.1.1:30000 | | 192.168.1.2:31000 | |
| |
+--------------+ +--------------+
| 192.168.1.1 | | 192.168.1.2 |
| | | |
| P2P-friendly | | P2P-friendly |
| NAT A | | NAT B |
+--------------+ +--------------+
| |
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
| ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ |
| | 192.0.2.1:62001 | | 192.0.2.1:62000 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Figure 7: Hairpin translation support in NAT to facilitate Direct-P2P
Srisuresh, Ford & Kegel [Page 18]
Internet-Draft State of P2P Communication Across NATs February 2007
Suppose NAT X is a NAT device performing Address-Independent Mapping
and deployed by a large internet service provider (ISP) to multiplex
many customers onto a few public IP addresses, and NATs A and B are
small consumer NAT gateways deployed independently by two of the
ISP's customers to multiplex their private home networks onto their
respective ISP-provided IP addresses. Only server S and NAT X have
globally routable IP addresses; the "public" IP addresses used by
NAT A and NAT B are actually private to the ISP's addressing realm,
while client A's and B's addresses in turn are private to the
addressing realms of NAT A and B, respectively. Just as in previous
section, Server S is used for the purposes of registration,
discovery and limited relay. Peer hosts use the server to relay
connection initiation control messages, instead of all end-to-end
messages.
Now suppose clients A and B attempt to establish a direct peer-to-
peer UDP connection. The optimal method would be for client A to
send messages to client B's public address at NAT B,
192.168.1.2:31000 in the ISP's addressing realm, and for client B to
send messages to A's public address at NAT B, namely
192.168.1.1:30000. Unfortunately, A and B have no way to learn these
addresses, because server S only sees the "global" public addresses
of the clients, 192.0.2.1:62000 and 192.0.2.1:62001. Even if A
and B had some way to learn these addresses, there is still no
guarantee that they would be usable because the address assignments
in the ISP's private addressing realm might conflict with unrelated
address assignments in the clients' private realms. The clients
therefore have no choice but to use their global public addresses as
seen by S for their P2P communication, and rely on NAT X to provide
hairpin translation.
3.3.4. Where UDP hole punching fails
The UDP hole punching technique has a caveat in that it works only
if the traversing NAT is a P2P-friendly NAT. When a NAT device
employing Endpoint-Dependent Mapping is enroute, P2P application is
unable to reuse an already established endpoint mapping
for communication with different external destinations and the
technique would fail. However, many of the NAT devices deployed in
the Internet do employ Address-Independent Mapping. That makes the
UDP hole punching technique broadly applicable [P2P-NAT].
Nevertheless a substantial fraction of deployed NATs do employ
Endpoint-Dependent Mapping and do not support the UDP hole punching
technique.
3.4. Simultaneous TCP Open
Simultaneous TCP open (also known sometimes as TCP hole punching)
Srisuresh, Ford & Kegel [Page 19]
Internet-Draft State of P2P Communication Across NATs February 2007
is a technique used in some cases to establish direct peer-to-peer
TCP connections between a pair of nodes that are both behind
P2P-friendly NAT devices that implement Endpoint-Independent Mapping
for their TCP traffic. Most TCP sessions start with one endpoint
sending a SYN packet, to which the other party responds with a
SYN-ACK packet. It is permissible, 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 "Simultaneous TCP Open" technique
and may be found in figure 8 of the original TCP specification
([TCP]). However, "Simultaneous TCP Open" is not implemented
correctly on many systems, including NAT devices.
If a NAT device receives a TCP SYN packet from outside the private
network attempting to initiate an incoming TCP connection, the
NAT device will normally reject the connection attempt by either
dropping the SYN packet or sending back a TCP RST (connection reset)
packet. In the case of SYN timeout or connection reset, the P2P
endpoint will continue to resend a SYN packet, until the peer does
the same from its end.
When a SYN packet arrives with source and destination addresses and
port numbers that correspond to a TCP session that the NAT device
believes is already active, then the NAT device will allow the
packet to pass through. In particular, if the NAT device has just
recently seen and transmitted an outgoing SYN packet with the 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 initiate an outgoing TCP connection with the other client
timed so that each client's outgoing SYN passes through its local
NAT device before either SYN reaches the opposite NAT device, then
a working peer-to-peer TCP connection will result.
This technique may not always work reliably for the following
reason(s). If either node's SYN packet arrives at the remote NAT
device too quickly (before the peering node had a chance to send the
SYN packet), then the remote NAT device may either drop the SYN
packet or reject the SYN with a RST packet. This could cause the
local NAT device in turn to close the new NAT-session immediately or
initiate end-of-session timeout (refer section 2.6 of [NAT-TERM]) so
as to close the NAT-session at the end of the timeout. Even as both
peering nodes simultaneously initiate continued SYN retransmission
attempts, some remote NAT devices might not let the incoming SYNs
through if the NAT session is in end-of-session timeout state. This
in turn would prevent the TCP connection from being established.
In reality, the majority of the NAT devices (more than 50%) do
support Endpoint-Independent Mapping and do not send ICMP errors or
Srisuresh, Ford & Kegel [Page 20]
Internet-Draft State of P2P Communication Across NATs February 2007
RSTs in response to unsolicited incoming SYNs. As a result,
Simultaneous TCP Open technique does work across NAT devices in
the majority of TCP connection attempts ([P2P-NAT], [TCP-CHARACT]).
3.5. UDP port number prediction
A variant of the UDP hole punching technique exists that allows
peer-to-peer UDP sessions to be created in the presence of some
NATs implementing Endpoint-Dependent Mapping. This method is
sometimes called the "N+1" technique [BIDIR] and is explored in
detail by Takeda [SYM-STUN]. The method works by analyzing the
behavior of the NAT and attempting to predict the public port
numbers it will assign to future sessions. Consider the scenario
in figure 8. Two clients, A and B, each behind a separate NAT,
have established separate UDP connections with a rendezvous server
S. Rendezvous server S has a permanent, globally routable IP
address and is used for the purposes of registration and
discovery. Hosts behind NAT register with the server. Peer hosts
discover hosts behind NAT using the server.
Registry and Discovery
Server S
18.181.0.31:1234
|
+----------------------------+----------------------------+
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 192.0.2.1:62000 | | 138.76.29.7:31000 | |
| |
| |
+---------------------+ +--------------------+
| 192.0.2.1 | | 138.76.29.7 |
| | | |
| NAT A | | NAT B |
| (Endpoint-Dependent | | (Endpoint-Dependent|
| Mapping) | | Mapping) |
+---------------------+ +--------------------+
| |
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Figure 8: Port Prediction assuming Endpoint-Dependent Mapping NATs
Srisuresh, Ford & Kegel [Page 21]
Internet-Draft State of P2P Communication Across NATs February 2007
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 with 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 192.0.2.1. 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 bi-directional
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
bi-directional UDP communication channel will have been
established as shown in figure 9.
Srisuresh, Ford & Kegel [Page 22]
Internet-Draft State of P2P Communication Across NATs February 2007
Registry and Discovery
Server S
18.181.0.31:1234
|
|
+----------------------------+----------------------------+
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 192.0.2.1:62000 | | 138.76.29.7:31000 | |
| |
| ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ |
| | 138.76.29.7:31001 | | 192.0.2.1:62001 | |
| | 192.0.2.1:62001 | | 138.76.29.7:31001 | |
| |
+---------------------+ +--------------------+
| 192.0.2.1 | | 138.76.29.7 |
| | | |
| NAT A | | NAT B |
| (Endpoint-Dependent | | (Endpoint-Dependent|
| Mapping) | | Mapping) |
+---------------------+ +--------------------+
| |
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 18.181.0.31:1234 | | 18.181.0.31:1234 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
| ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ |
| | 138.76.29.7:31001 | | 192.0.2.1:62001 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Figure 9: Use of Port Prediction for Direct-P2P communication
Clearly, 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.
Since in practice a P2P application implementing this trick would
Srisuresh, Ford & Kegel [Page 23]
Internet-Draft State of P2P Communication Across NATs February 2007
still need to work even when one of the NATs employ
Endpoint-Independent Mapping, the application would need to detect
beforehand what kind of NAT is involved on either end and modify its
behavior accordingly, increasing the complexity of the algorithm and
the general brittleness of the network. Finally, port number
prediction has no chance of working if either client is behind two
or more levels of NAT and the NAT(s) closest to the client employ
Endpoint-Dependent Mapping.
3.6. TCP port number prediction
This is a variant of the "Simultaneous TCP open" technique that
allows peer-to-peer TCP sessions to be created in the presence of
some NATs employing Address-Dependent Mapping.
Unfortunately, this trick may be even more fragile and timing-
sensitive than the UDP port number prediction trick described
earlier. First, even as both NAT devices implement
Endpoint-Independent Mapping on the TCP traffic, all the same
things can go wrong with each side's attempt to predict the public
port numbers that the respective NATs will assign to the new
sessions can happen with TCP port prediction as well. In addition,
if either client's SYN arrives at the opposite NAT device too
quickly, then the remote NAT device may reject the SYN with a RST
packet, causing the local NAT device in turn to close the new
session and make future SYN retransmission attempts using the
same port numbers futile. This trick is mentioned here only for
historical reasons.
4. Recent Work on P2P NAT Traversal
[P2P-NAT] has a detailed discussion on the UDP and TCP hole punching
techniques for NAT traversal. [P2P-NAT] also lists empirical results
from running [NAT-CHECK] test program across a number of commercial
NAT devices. The results indicate that UDP hole punching is widely
supported on more than 80% of the NAT devices, whereas TCP hole
punching is supported on just over 60% of the NAT devices tested.
The results also indicate that TCP or UDP hairpinning is not yet
widely available on the commercial NAT devices, as less than 25% of
the devices passed the tests ([NAT-CHECK]) for Hairpinning.
[TCP-CHARACT] and [NAT-BLASTER] focus on TCP hole punching, exploring
and comparing several alternative approaches. [NAT-BLASTER] takes an
analytical approach, analyzing different cases of observed NAT
behavior and ways applications might address them. [TCP-CHARACT]
adopts a more empirical approach, measuring the commonality of
different types of NAT behavior relevant to TCP hole punching. This
Srisuresh, Ford & Kegel [Page 24]
Internet-Draft State of P2P Communication Across NATs February 2007
work finds that using more sophisticated techniques than those used
in [P2P-NAT], up to 88% of currently deployed NATs can support TCP
hole punching.
[TEREDO] is a NAT traversal service that uses relay technology to
connect IPv4 nodes behind NAT devices to IPv6 nodes, external to
the NAT devices. [TEREDO] provides for peer communication across
NAT devices by tunneling packets over UDP, across the NAT device(s)
to a relay node. Teredo relays act as Rendezvous servers to relay
traffic from private IPv4 nodes to the nodes in the external realm
and vice versa.
[ICE] is a NAT traversal protocol for setting up media sessions
between peer nodes for a class of multi-media applications. [ICE]
requires peering nodes to run STUN protocol ([STUN]) on the same
port number used to terminate media session(s). Applications that
use signaling protocols such as SIP ([SIP]) may embed the NAT
traversal attributes for the media session within the signaling
sessions and use the offer/answer type of exchange between peer
nodes to set up end-to-end media session(s)b across NAT devices.
[ICE-TCP] is an extension of ICE for TCP based media sessions.
A number of online gaming and media-over-IP applications, including
Instant Messaging application use the techniques described in the
document for peer-to-peer connection establishment. Commercially
deployed applications may use multiple distinct rendezvous servers
for registration, discovery and relay functions for load balancing,
among other reasons. For example, a well-known media over IP
application "Skype" uses a central public server for registration
and different public servers for end-to-end relay functon.
5. Summary of observations
5.1. TCP/UDP hole punching
TCP/UDP hole punching is the most efficient existing method of
establishing direct TCP/UDP peer-to-peer communication between two
nodes that are both behind NATs. This technique has been used with a
wide variety of existing NATs. However, applications should be
prepared to fall back to simple relaying when direct communication
cannot be established.
5.2. NATs Employing Endpoint-Dependent Mapping are not P2P friendly
NATs Employing Endpoint-Dependent Mapping gained popularity with
client-server applications such as web browsers, which only need to
initiate outgoing connections. However, in the recent times, P2P
applications such as Instant messaging and audio conferencing have
Srisuresh, Ford & Kegel [Page 25]
Internet-Draft State of P2P Communication Across NATs February 2007
been in wide use. NATs Employing Endpoint-Dependent Mapping are
not suitable for P2P applications.
5.3. Peer discovery
An application should not assume all its peers to be outside NAT
boundary. As such, an application should register all its private IP
addresses with rendezvous server, so it can connect to some of its
peers within the same NAT boundary without having to traverse the
NAT device. The rendezvous server must be on node with permanent,
globally routable IP address, and be able to provide registration,
discovery, and limited relay services, so an application is able
to discover peer hosts even as they are behind a NAT device.
5.4. Hairpin translation
Hairpin translation support is highly beneficial to allow hosts
behind a p2p-friendly NAT to communicate with other hosts behind
the same NAT device through their public, possibly translated
endpoints. Support for hairpin translation is particularly useful in
the case of large-capacity NATs deployed as the first level of a
multi-level NAT scenario. As described in section 3.3.3, hosts
behind the same first-level NAT but different second-level NATs do
not have a way to communicate with each other using TCP/UDP hole
punching techniques, unless the first-level NAT also supports
hairpin translation. This would be the case even when all NAT
devices in the deployment preserve endpoint identities,
6. Security considerations
This document does not inherently create new security issues.
Nevertheless, security risks may be present in the techniques
described. This section describes security risks the applications
could inadvertently create in attempting to support P2P
communication across NAT devices. Also described are implications
for the security policies of P2P-friendly NAT devices.
6.1. Lack of Authentication can cause connection hijacking
NAT-friendly P2P applications must use appropriate authentication
mechanisms to protect their P2P connections from accidental
confusion with other P2P connections as well as from malicious
connection hijacking or denial-of-service attacks. NAT-friendly
P2P applications effectively must interact with multiple distinct
IP address domains, but are not generally aware of the exact topology
or administrative policies defining these address domains. While
attempting to establish P2P connections via TCP/UDP hole punching,
Srisuresh, Ford & Kegel [Page 26]
Internet-Draft State of P2P Communication Across NATs February 2007
applications send packets that may frequently arrive at an entirely
different host than the intended one.
For example, many consumer-level NAT devices provide DHCP
services that are configured by default to hand out site-local
IP addresses in a particular address range. Say, a particular
consumer NAT device, by default, hands out IP addresses starting
with 192.168.1.100. Most private home networks using that NAT
device will have a host with that IP address, and many of these
networks will probably have a host at address 192.168.1.101 as
well. If host A at address 192.168.1.101 on one private network
attempts to establish a connection by UDP hole punching with
host B at 192.168.1.100 on a different private network, then as
part of this process host A will send discovery packets to
address 192.168.1.100 on its local network, and host B will send
discovery packets to address 192.168.1.101 on its network. Clearly,
these discovery packets will not reach the intended machine since
the two hosts are on different private networks, but they are very
likely to reach SOME machine on these respective networks at the
standard UDP port numbers used by this application, potentially
causing confusion, especially if the application is also running
on those other machines and does not properly authenticate its
messages.
This risk due to aliasing is therefore present even without a
malicious attacker. If one endpoint, say host A, is actually
malicious, then without proper authentication the attacker could
cause host B to connect and interact in unintended ways with
another host on its private network having the same IP address
as the attacker's (purported) private address. Since the two
endpoint hosts A and B presumably discovered each other through
a public rendezvous server S, providing registration, discovery
and limited relay services; and neither S nor B has any means to
verify A's reported private address, all P2P applications must
assume that any IP address they find to be suspect until they
successfully establish authenticated two-way communication.
6.2. Denial-of-service attacks
P2P applications and the public servers that support them must
protect themselves against denial-of-service attacks, and ensure
that they cannot be used by an attacker to mount denial-of-service
attacks against other targets. To protect themselves, P2P
applications and servers must avoid taking any action requiring
significant local processing or storage resources until
authenticated two-way communication is established. To avoid being
used as a tool for denial-of-service attacks, P2P applications and
servers must minimize the amount and rate of traffic they send to
Srisuresh, Ford & Kegel [Page 27]
Internet-Draft State of P2P Communication Across NATs February 2007
any newly-discovered IP address until after authenticated two-way
communication is established with the intended target.
For example, P2P applications that register with a public rendezvous
server can claim to have any private IP address, or perhaps multiple
IP addresses. A well-connected host or group of hosts that can
collectively attract a substantial volume of P2P connection attempts
(e.g., by offering to serve popular content) could mount a
denial-of-service attack on a target host C simply by including C's
IP address in their own list of IP addresses they register with the
rendezvous server. There is no way the rendezvous server can verify
the IP addresses, since they could well be legitimate private
network addresses useful to other hosts for establishing
network-local communication. The P2P application protocol must
therefore be designed to size- and rate-limit traffic to unverified
IP addresses in order to avoid the potential damage such a
concentration effect could cause.
6.3. Man-in-the-middle attacks
Any network device on the path between a P2P client and a public
rendezvous server can mount a variety of man-in-the-middle
attacks by pretending to be a NAT. For example, suppose
host A attempts to register with rendezvous server S, but a
network-snooping attacker is able to observe this registration
request. The attacker could then flood server S with requests
that are identical to the client's original request except with
a modified source IP address, such as the IP address of the
attacker itself. If the attacker can convince the server to
register the client using the attacker's IP address, then the
attacker can make itself an active component on the path of all
future traffic from the server AND other P2P hosts to the
original client, even if the attacker was originally only able
to snoop the path from the client to the server.
The client cannot protect itself from this attack by
authenticating its source IP address to the rendezvous server,
because in order to be NAT-friendly the application must allow
intervening NATs to change the source address silently. This
appears to be an inherent security weakness of the NAT paradigm.
The only defense against such an attack is for the client to
authenticate and potentially encrypt the actual content of its
communication using appropriate higher-level identities, so that
the interposed attacker is not able to take advantage of its
position. Even if all application-level communication is
authenticated and encrypted, however, this attack could still be
used as a traffic analysis tool for observing who the client is
communicating with.
Srisuresh, Ford & Kegel [Page 28]
Internet-Draft State of P2P Communication Across NATs February 2007
6.4. Security impact from a P2P-friendly NAT device
Designing NAT devices to preserve endpoint identities does not
weaken the security provided by the NAT device. For example, a
NAT device employing Endpoint-Independent Mapping and
Endpoint-Dependent Filtering is no more "promiscuous" than a NAT
device employing Endpoint-Dependent Mapping and Endpoint-Dependent
Filtering. Filtering incoming traffic aggressively using
Endpoint-Dependent Filtering, while employing Endpoint-Independent
Mapping allows a NAT device to be P2P friendly without
compromising the principle of rejecting unsolicited incoming
traffic.
Endpoint-Independent Mapping could arguably increase the
predictability of traffic emerging from the NAT device, by revealing
the relationships between different TCP/UDP sessions and hence about
the behavior of applications running within the enclave. This
predictability could conceivably be useful to an attacker in
exploiting other network or application level vulnerabilities.
If the security requirements of a particular deployment scenario
are so critical that such subtle information channels are of
concern, however, then the NAT device almost certainly should not be
configured to allow unrestricted outgoing TCP/UDP traffic in the
first place. Such a NAT device should only allow communication
originating from specific applications at specific ports, or
via tightly-controlled application-level gateways. In this
situation there is no hope of generic, transparent peer-to-peer
connectivity across the NAT device (or transparent client/server
connectivity for that matter); the NAT device must either
implement appropriate application-specific behavior or disallow
communication entirely.
7. IANA Considerations
There are no IANA considerations.
8. Acknowledgments
The authors wish to thank Henrik, Dave, Christian Huitema and Dan
Wing for their valuable feedback.
9. Normative References
[NAT-TERM] Srisuresh, P., and Holdrege, M., "IP Network Address
Srisuresh, Ford & Kegel [Page 29]
Internet-Draft State of P2P Communication Across NATs February 2007
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[NAT-TRAD] Srisuresh, P., and Egevang, K., "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[BEH-UDP] F. Audet and C. Jennings, "NAT Behavioral Requirements
for Unicast UDP", RFC 4787, January 2007.
10. Informative References
[BEH-APP] Ford, B., Srisuresh, P., and Kegel, D., "Application
Design Guidelines for Traversal through Network Address
Translators", draft-ford-behave-app-04.txt (Work In
Progress), October 2006.
[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/
[ICE] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Methodology for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-13.txt (work in Progress),
January 2007.
[ICE-TCP] Rosenberg, J., "TCP Candidates with Interactive
Connectivity Establishment (ICE)",
draft-ietf-mmusic-ice-tcp-02.txt (work in Progress),
October 2006.
[KEGEL] Kegel, D., "NAT and Peer-to-Peer Networking", July 1999.
http://www.alumni.caltech.edu/~dank/peer-nat.html
[MIDCOM] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A. and
Rayhan, A., "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[NAT-APPL] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[NAT-BLASTER] Biggadike, A., Ferullo, D., Wilson, G., and Perrig, A.,
"Establishing TCP Connections Between Hosts Behind
NATs", ACM SIGCOMM ASIA Workshop, April 2005.
Srisuresh, Ford & Kegel [Page 30]
Internet-Draft State of P2P Communication Across NATs February 2007
[NAT-CHECK] Ford, B., "NAT check Program" available online as
http://midcom-p2p.sourceforge.net, February 2005.
[NAT-PMP] Cheshire, S., Krochmal, M., and Sekar, K., "NAT Port
Mapping Protocol (NAT-PMP)",
draft-cheshire-nat-pmp-00.txt (Work In Progress),
June 2005.
[NAT-PROT] Holdrege, M., and Srisuresh, P., "Protocol Complications
with the IP Network Address Translator", RFC 3027,
January 2001.
[NAT-PT] Tsirtsis, G. and Srisuresh, P., "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766,
February 2000.
[NSIS-NSLP] Stiemerling, M., Tschofenig, H., Aoun, C., and Davies,
E., "NAT/Firewall NSIS Signaling Layer Protocol (NSLP)",
draft-ietf-nsis-nslp-natfw-13.txt (Work In Progress),
October 2006.
[P2P-NAT] Ford, B., Srisuresh, P., and Kegel, D., "Peer-to-Peer
Communication Across Network Address Translators",
Proceedings of the USENIX Annual Technical Conference
(Anaheim, CA), April 2005.
[RFC3330] IANA, "Special-Use IPv4 Addresses", RFC 3330,
September 2002.
[RSIP] Borella, M., Lo, J., Grabelsky, D., and Montenegro, G.,
"Realm Specific IP: Framework", RFC 3102, October 2001.
[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[SOCKS] Leech, M., Ganis, M., Lee, Y., Kuris, R.,Koblas, D., and
Jones, L., "SOCKS Protocol Version 5", RFC 1928,
March 1996.
[STUN] Rosenberg, J., Weinberger, J., Huitema, C., and Mahy, R.,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
March 2003.
[SYM-STUN] Takeda, Y., "Symmetric NAT Traversal using STUN",
Srisuresh, Ford & Kegel [Page 31]
Internet-Draft State of P2P Communication Across NATs February 2007
draft-takeda-symmetric-nat-traversal-00 (Work In
Progress), June 2003.
[TCP] Postel, J., "Transmission Control Protocol (TCP)
Specification", STD 7, RFC 793, September 1981.
[TCP-CHARACT] Guha, S., and Francis, P., "Characterization and
Measurement of TCP Traversal through NATs and Firewalls",
Proceedings of Internet Measurement Conference (IMC),
Berkeley, CA, Oct 2005, pp. 199-211.
[TEREDO] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[TURN] Rosenberg, J., Mahy, R., and Huitema, C.,
"Traversal Using Relay NAT (TURN)",
draft-ietf-behave-turn-02.txt (Work In Progress),
October 2006.
[UNSAF] Daigle, L., and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[UPNP] UPnP Forum, "Internet Gateway Device (IGD) Standardized
Device Control Protocol V 1.0", November 2001.
http://www.upnp.org/standardizeddcps/igd.asp
Authors' Addresses
Pyda Srisuresh
Kazeon Systems, Inc.
1161 San Antonio Rd.
Mountain View, CA 94043
U.S.A.
Phone: (408)836-4773
E-mail: srisuresh@yahoo.com
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/
Srisuresh, Ford & Kegel [Page 32]
Internet-Draft State of P2P Communication Across NATs February 2007
Dan Kegel
Kegel.com
901 S. Sycamore Ave.
Los Angeles, CA 90036
Phone: 323 931-6717
Email: dank@kegel.com
Web: http://www.kegel.com/
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Srisuresh, Ford & Kegel [Page 33]
Internet-Draft State of P2P Communication Across NATs February 2007
Acknowledgment
Funding for the RFC Editor function is currently provided by the
IETF Trust.
Srisuresh, Ford & Kegel [Page 34]
