Network Working Group P. Srisuresh
Request for Comments: 5128 Kazeon Systems
Category: Informational B. Ford
M.I.T.
D. Kegel
kegel.com
March 2008
State of Peer-to-Peer (P2P) Communication across
Network Address Translators (NATs)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
This memo documents the various methods known to be in use by
applications to establish direct communication in the presence of
Network Address Translators (NATs) at the current time. Although
this memo is intended to be mainly descriptive, the Security
Considerations section makes some purely advisory recommendations
about how to deal with security vulnerabilities the applications
could inadvertently create when using the methods described. 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, et al. Informational [Page 1]RFC 5128 State of P2P Communication across NATs March 2008
Table of Contents
1. Introduction and Scope ..........................................3
2. Terminology and Conventions Used ................................4
2.1. Endpoint ...................................................5
2.2. Endpoint Mapping ...........................................5
2.3. Endpoint-Independent Mapping ...............................5
2.4. Endpoint-Dependent Mapping .................................5
2.5. Endpoint-Independent Filtering .............................6
2.6. Endpoint-Dependent Filtering ...............................6
2.7. P2P Application ............................................7
2.8. NAT-Friendly P2P Application ...............................7
2.9. Endpoint-Independent Mapping NAT (EIM-NAT) .................7
2.10. Hairpinning ...............................................7
3. Techniques Used by P2P Applications to Traverse NATs ............7
3.1. Relaying ...................................................8
3.2. Connection Reversal ........................................9
3.3. UDP Hole Punching .........................................11
3.3.1. Peers behind Different NATs ........................12
3.3.2. Peers behind the Same NAT ..........................14
3.3.3. Peers Separated by Multiple NATs ...................16
3.4. TCP Hole Punching .........................................18
3.5. UDP Port Number Prediction ................................19
3.6. TCP Port Number Prediction ................................21
4. Recent Work on NAT Traversal ...................................22
5. Summary of Observations ........................................23
5.1. TCP/UDP Hole Punching .....................................23
5.2. NATs Employing Endpoint-Dependent Mapping .................23
5.3. Peer Discovery ............................................24
5.4. Hairpinning ...............................................24
6. Security Considerations ........................................24
6.1. Lack of Authentication Can Cause Connection Hijacking .....24
6.2. Denial-of-Service Attacks .................................25
6.3. Man-in-the-Middle Attacks .................................26
6.4. Security Impact from EIM-NAT Devices ......................26
7. Acknowledgments ................................................27
8. References .....................................................27
8.1. Normative References ......................................27
8.2. Informative References ....................................27
Srisuresh, et al. Informational [Page 2]RFC 5128 State of P2P Communication across NATs March 2008
1. Introduction and Scope
The 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 established by
these NAT devices have created unique problems for peer-to-peer (P2P)
applications and protocols, such as teleconferencing and multiplayer
online gaming. These issues are likely to persist even into the IPv6
world. During the transition to IPv6, some form of NAT may be
required to enable IPv4-only nodes to communicate with IPv6-only
nodes [NAT-PT], although the appropriate protocols and guidelines for
this use of NAT are still unresolved [NAT-PT-HIST]. Even a future
"pure IPv6 world" may still include firewalls, which employ similar
filtering behavior of NATs but without the address translation
[V6-CPE-SEC]. The filtering behavior interferes with the functioning
of P2P applications. For this reason, IPv6 applications that use the
techniques described in this document for NAT traversal may also work
with some firewalls that have filtering behavior similar to NATs.
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 en route
provide dynamic address assignment for the client machines. The
illusion of anonymity (private IP addresses) 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 illusion of anonymity and inaccessibility
is sometimes perceived as a privacy benefit. As noted in Section 2.2
of [RFC4941], this perceived privacy may be illusory in a majority of
cases utilizing Small-Office-Home-Office (SOHO) NATs.
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 online gaming architecture, for example,
involves all participating application hosts contacting a publicly
addressable rendezvous server for registering themselves and
discovering peer hosts. Subsequent to the communication with the
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
Srisuresh, et al. Informational [Page 3]
RFC 5128 State of P2P Communication across NATs March 2008
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.
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.
These techniques, if available, are a complement to the techniques
described in the document. [UNSAF] is in scope.
In this document, we summarize the currently known methods by which
applications work around the presence of NAT devices, without
directly altering the NAT devices. The techniques described predate
BEHAVE documents ([BEH-UDP], [BEH-TCP], and [BEH-ICMP]). The scope
of the document is restricted to describing currently known
techniques used to establish 2-way communication between endpoints of
an application. Discussion of timeouts, RST processing, keepalives,
and so forth that concern a running session are outside the scope of
this document. The scope is also restricted to describing techniques
for TCP- and UDP-based applications. It is not the objective of this
document to provide solutions to NAT traversal problems for
applications in general [BEH-APP] or to a specific class of
applications [ICE].
2. Terminology and Conventions Used
In this document, the IP addresses 192.0.2.1, 192.0.2.128, and
192.0.2.254 are used as example public IP addresses [RFC3330].
Although these addresses are all from the same /24 network, this is a
limitation of the example addresses available in [RFC3330]. In
practice, these addresses would be on different networks. As for the
notation for ports usage, all clients use ports in the range of
1-2000 and servers use ports in the range of 20000-21000. NAT
devices use ports 30000 and above for endpoint mapping.
Readers are urged to refer to [NAT-TERM] for information on NAT
taxonomy and terminology. Unless prefixed with a NAT type or
explicitly stated otherwise, the term NAT, used throughout this
document, refers to Traditional NAT [NAT-TRAD]. Traditional NAT has
two variations, namely, Basic NAT and Network Address Port Translator
(NAPT). Of these, NAPT is by far the most commonly deployed NAT
device. NAPT allows multiple private hosts to share a single public
IP address simultaneously.
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.
Srisuresh, et al. Informational [Page 4]
RFC 5128 State of P2P Communication across NATs March 2008
[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 document adapts terminology from
[BEH-UDP] to distinguish between NAT implementations.
Listed below are terms used throughout this 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
UDP or TCP session endpoint is represented as a tuple of (IP address,
UDP/TCP 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
a public endpoint to translate the private endpoint so 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 a
public endpoint and vice versa is called Endpoint Mapping. NAT uses
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.
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
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.
Srisuresh, et al. Informational [Page 5]RFC 5128 State of P2P Communication across NATs March 2008
Address and Port-Dependent Mapping
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.
2.5. Endpoint-Independent Filtering
"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.
2.6. Endpoint-Dependent Filtering
"Endpoint-Dependent Filtering" refers to the combination of "Address-
Dependent Filtering" and "Address and Port-Dependent Filtering" as
defined in [BEH-UDP].
Address-Dependent Filtering
The NAT filters out packets not destined to 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:any previously (independently of the port used by
Y). In other words, for receiving packets from a specific
external endpoint, it is necessary for the internal endpoint to
send packets first to that specific external endpoint's IP
address.
Address and Port-Dependent Filtering
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.
Srisuresh, et al. Informational [Page 6]RFC 5128 State of P2P Communication across NATs March 2008
A NAT device employing "Endpoint-Dependent Filtering" will accept
incoming traffic to a mapped public port from only a restricted set
of external endpoints on the public network.
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. A P2P application may use
multiple endpoints for peer-to-peer communication.
2.8. NAT-Friendly P2P Application
A 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.
One common way P2P applications establish peering sessions and remain
NAT-friendly is by using a publicly addressable rendezvous server for
registration and peer discovery purposes.
2.9. Endpoint-Independent Mapping NAT (EIM-NAT)
An Endpoint-Independent Mapping NAT (EIM-NAT, for short) is a NAT
device employing Endpoint-Independent Mapping. An EIM-NAT can have
any type of filtering behavior. BEHAVE-compliant NAT devices are
good examples of EIM-NAT devices. A NAT device employing Address-
Dependent Mapping is an example of a NAT device that is not EIM-NAT.
2.10. Hairpinning
Hairpinning is defined in [BEH-UDP] as follows:
If two hosts (called X1 and X2) are behind the same NAT and
exchanging traffic, the NAT may allocate an address on the
outside of the NAT for X2, called X2':x2'. If X1 sends traffic
to X2':x2', it goes to the NAT, which must relay the traffic
from X1 to X2. This is referred to as hairpinning.
Not all currently deployed NATs support hairpinning.
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.
Srisuresh, et al. Informational [Page 7]RFC 5128 State of P2P Communication across NATs March 2008
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 1.
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
publicly addressable 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
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
192.0.2.128:20001
|
+----------------------------+----------------------------+
| ^ Registry/ ^ ^ Registry/ ^ |
| | Relay-Req Session(A-S) | | Relay-Req Session(B-S) | |
| | 192.0.2.128:20001 | | 192.0.2.128:20001 | |
| | 192.0.2.1:62000 | | 192.0.2.254:31000 | |
| |
+--------------+ +--------------+
| 192.0.2.1 | | 192.0.2.254 |
| | | |
| NAT A | | NAT B |
+--------------+ +--------------+
| |
| ^ Registry/ ^ ^ Registry/ ^ |
| | Relay-Req Session(A-S) | | Relay-Req Session(B-S) | |
| | 192.0.2.128:20001 | | 192.0.2.128:20001 | |
| | 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 1: Use of a Relay Server to communicate with peers
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.
Srisuresh, et al. Informational [Page 8]RFC 5128 State of P2P Communication across NATs March 2008
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 required to be EIM-NAT. 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 has sufficient I/O
bandwidth and is located correctly topology-wise. The TURN protocol
[TURN] defines a method of implementing application agnostic,
session-oriented, packet relay in a relatively secure fashion.
3.2. Connection Reversal
The following connection reversal technique for a direct
communication works only when one of the peers is behind a NAT device
and the other is not. For example, consider the scenario in figure
2. Client A is behind a NAT, but client B has a publicly addressable
IP address. Rendezvous Server S has a publicly addressable IP
address and is used for the purposes of registration and discovery.
Hosts behind a NAT register their endpoints with the server. Peer
hosts discover endpoints of hosts behind a NAT using the server.
Srisuresh, et al. Informational [Page 9]
RFC 5128 State of P2P Communication across NATs March 2008
Registry and Discovery
Server S
192.0.2.128:20001
|
+----------------------------+----------------------------+
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 192.0.2.128:20001 | | 192.0.2.128:20001 | |
| | 192.0.2.1:62000 | | 192.0.2.254:1234 | |
| |
| ^ P2P Session (A-B) ^ | P2P Session (B-A) | |
| | 192.0.2.254:1234 | | 192.0.2.1:62000 | |
| | 192.0.2.1:62000 | v 192.0.2.254:1234 v |
| |
+--------------+ |
| 192.0.2.1 | |
| | |
| NAT A | |
+--------------+ |
| |
| ^ Registry Session(A-S) ^ |
| | 192.0.2.128:20001 | |
| | 10.0.0.1:1234 | |
| |
| ^ P2P Session (A-B) ^ |
| | 192.0.2.254:1234 | |
| | 10.0.0.1:1234 | |
| |
Private Client A Public Client B
10.0.0.1:1234 192.0.2.254:1234
Figure 2: Connection reversal using Rendezvous server
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 192.0.2.128 and port 20001. NAT A has
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, 192.0.2.254, and the application on B is accepting TCP
connections at port 1234.
Now suppose client B wishes to establish a direct 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, the connection will fail.
In the first case, traffic directed to IP address 10.0.0.1 will
Srisuresh, et al. Informational [Page 10]RFC 5128 State of P2P Communication across NATs March 2008
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 is EIM-NAT, the public client can contact external
server S to determine the specific public endpoint from which to
expect Client-A-originated connection and allow connections from just
those endpoints. If the NAT device is EIM-NAT, the public client can
contact the external server S to determine the specific public
endpoint from which to expect connections originated by client A, and
allow connections from just that endpoint. If the NAT device is not
EIM-NAT, the public client cannot know the specific public endpoint
from which to expect connections originated by client A. In the
increasingly common case where both peers can be behind NATs, the
Connection Reversal method fails. Connection Reversal is not a
general solution to the peer-to-peer connection problem. If neither
a "forward" nor a "reverse" connection can be established,
applications often fall back to another mechanism such as relaying.
3.3. UDP Hole Punching
UDP hole punching relies on the properties of EIM-NATs to allow
appropriately designed peer-to-peer applications to "punch holes"
through the NAT device(s) enroute and establish direct connectivity
with each other, even when both communicating hosts lie behind NAT
devices. When one of the hosts is behind a NAT that is not EIM-NAT,
the peering host cannot predictably know the mapped endpoint to which
to initiate a connection. Further, the application on the host
behind non-EIM-NAT would be unable to reuse an already established
endpoint mapping for communication with different external
destinations, and the hole punching technique would fail.
This technique was mentioned briefly in Section 5.1 of RFC 3027
[NAT-PROT], first described in [KEGEL], and used in some recent
protocols [TEREDO, ICE]. Readers may refer to Section 3.4 for
details on "TCP hole punching".
Srisuresh, et al. Informational [Page 11]
RFC 5128 State of P2P Communication across NATs March 2008
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 3. Clients A and B both have private
IP addresses and lie behind different NAT devices. Rendezvous Server
S has a publicly addressable IP address and is used for the purposes
of registration, discovery, and limited relay. Hosts behind a NAT
register their public endpoints with the server. Peer hosts discover
the public endpoints of hosts behind a NAT using the server. Unlike
in Section 3.1, peer hosts use the server to relay just connection
initiation control messages, instead of end-to-end messages.
The peer-to-peer application running on clients A and B use UDP port
1234. The rendezvous server S uses UDP port 20001. 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, et al. Informational [Page 12]
RFC 5128 State of P2P Communication across NATs March 2008
Registry and Discovery Combined
with Limited Relay
Server S
192.0.2.128:20001
|
+----------------------------+----------------------------+
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 192.0.2.128:20001 | | 192.0.2.128:20001 | |
| | 192.0.2.1:62000 | | 192.0.2.254:31000 | |
| |
| ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ |
| | 192.0.2.254:31000 | | 192.0.2.1:62000 | |
| | 192.0.2.1:62000 | | 192.0.2.254:31000 | |
| |
+--------------+ +--------------+
| 192.0.2.1 | | 192.0.2.254 |
| | | |
| EIM-NAT A | | EIM-NAT B |
+--------------+ +--------------+
| |
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 192.0.2.128:20001 | | 192.0.2.128:20001 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
| ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ |
| | 192.0.2.254:31000 | | 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 3: UDP Hole Punching to set up direct connectivity
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 endpoint 192.0.2.254:31000, then NAT B will
typically discard these incoming messages (unless it employs
Endpoint-Independent Filtering), because the source address and port
number do 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 endpoint, then NAT A will typically discard
these messages.
Suppose A starts sending UDP messages to B's public endpoint, and
simultaneously relays a request through server S to B, asking B to
start sending UDP messages to A's public endpoint. A's outgoing
messages directed to B's public endpoint (192.0.2.254:31000) cause
EIM-NAT A to open up a new communication session between A's private
Srisuresh, et al. Informational [Page 13]RFC 5128 State of P2P Communication across NATs March 2008
endpoint and B's public endpoint. At the same time, B's messages to
A's public endpoint (192.0.2.1:62000) cause EIM-NAT B to open up a
new communication session between B's private endpoint and A's public
endpoint. Once the new UDP sessions have been opened up in each
direction, clients 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
devices, 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,
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
distant 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 EIM-NAT, and are
therefore located in the same private IP address space, as in figure
4. A well-known Rendezvous Server S has a publicly addressable IP
address and is used for the purposes of registration, discovery, and
limited relay. Hosts behind the NAT register with the server. Peer
hosts discover hosts behind the 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 EIM-NAT has assigned public port number 62000. Client B has
similarly established a session with S, to which the EIM-NAT has
assigned public port number 62001.
Srisuresh, et al. Informational [Page 14]
RFC 5128 State of P2P Communication across NATs March 2008
Registry and Discovery Combined
with Limited Relay
Server S
192.0.2.128:20001
|
^ Registry Session(A-S) ^ | ^ Registry Session(B-S) ^
| 192.0.2.128:20001 | | | 192.0.2.128:20001 |
| 192.0.2.1:62000 | | | 192.0.2.1:62001 |
|
+--------------+
| 192.0.2.1 |
| |
| EIM-NAT |
+--------------+
|
+-----------------------------+----------------------------+
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 192.0.2.128:20001 | | 192.0.2.128:20001 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
| ^ P2P Session-try1(A-B) ^ ^ P2P Session-try1(B-A) ^ |
| | 192.0.2.1:62001 | | 192.0.2.1:62000 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
| ^ P2P Session-try2(A-B) ^ ^ P2P Session-try2(B-A) ^ |
| | 10.1.1.3:1234 | | 10.0.0.1:1234 | |
| | 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 4: Use of local and public endpoints 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 endpoints as
observed by server S, and start sending each other messages at those
public endpoints. 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. This situation is referred to as
"Hairpinning", 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, consider P2P session-try1 above. When A sends a UDP
packet to B's public endpoint, the packet initially has a source
endpoint of 10.0.0.1:1234 and a destination endpoint of
Srisuresh, et al. Informational [Page 15]RFC 5128 State of P2P Communication across NATs March 2008
192.0.2.1:62001. The NAT receives this packet, translates it to have
a source endpoint of 192.0.2.1:62000 and a destination endpoint of
10.1.1.3:1234, and then forwards it on to B.
Even if the NAT device supports hairpinning, this translation and
forwarding step is clearly unnecessary in this situation, and adds
latency to the dialog between A and B, besides burdening the NAT.
The solution to this problem is straightforward and is described as
follows.
When A and B initially exchange address information through the
Rendezvous server S, they include their own IP addresses and port
numbers as "observed" by themselves, as well as their public
endpoints 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, as is the
case in figure 4 above, then the packets directed to their private
endpoints (as attempted using P2P session-try2) 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 endpoints will fail to reach each other at
all, but the clients will hopefully establish connectivity using
their respective public endpoints. 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 endpoint to reach some other, unrelated node on A's
private network, or vice versa.
The [ICE] protocol employs this technique effectively, in that
multiple candidate endpoints (both private and public) are
communicated between peering end hosts during an offer/answer
exchange. Endpoints that offer the most efficient end-to-end
connection(s) are selected eventually for end-to-end data transfer.
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 5.
Srisuresh, et al. Informational [Page 16]
RFC 5128 State of P2P Communication across NATs March 2008
Registry and Discovery Combined
with Limited Relay
Server S
192.0.2.128:20001
|
^ Registry Session(A-S) ^ | ^ Registry Session(B-S) ^
| 192.0.2.128:20001 | | | 192.0.2.128:20001 |
| 192.0.2.1:62000 | | | 192.0.2.1:62001 |
|
+--------------+
| 192.0.2.1 |
| |
| EIM-NAT X |
| (Supporting |
| Hairpinning) |
+--------------+
|
+----------------------------+----------------------------+
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 192.0.2.128:20001 | | 192.0.2.128:20001 | |
| | 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 |
| | | |
| EIM-NAT A | | EIM-NAT B |
+--------------+ +--------------+
| |
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 192.0.2.128:20001 | | 192.0.2.128:20001 | |
| | 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 5: Use of Hairpinning in setting up direct communication
Suppose NAT X is an EIM-NAT 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
Srisuresh, et al. Informational [Page 17]RFC 5128 State of P2P Communication across NATs March 2008
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 NATs A
and B, respectively. Just as in the 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 endpoints
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 endpoints as seen by S
for their P2P communication, and rely on NAT X to provide
hairpinning.
3.4. TCP Hole Punching
In this section, we will discuss the "TCP hole punching" technique
used for establishing direct TCP connection between a pair of nodes
that are both behind EIM-NAT devices. Just as with UDP hole
punching, TCP hole punching relies on the properties of EIM-NATs to
allow appropriately 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 is also known sometimes as "Simultaneous TCP
Open".
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 6 of
the original TCP specification ([TCP]). However, "Simultaneous TCP
Open" is not implemented correctly on many systems, including NAT
devices.
Srisuresh, et al. Informational [Page 18]
RFC 5128 State of P2P Communication across NATs March 2008
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 application
endpoint will continue to resend a SYN packet, until the peer does
the same from its end.
Let us consider the case where a NAT device supports "Simultaneous
TCP Open" sessions. When a SYN packet arrives with source and
destination endpoints that correspond to a TCP session that the NAT
device believes is already active, then the NAT device would 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 address 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 to 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 an end-of-session
timeout state. This in turn would prevent the TCP connection from
being established.
In reality, the majority of NAT devices (more than 50%) support
Endpoint-Independent Mapping and do not send ICMP errors or RSTs in
response to unsolicited incoming SYNs. As a result, the 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
Srisuresh, et al. Informational [Page 19]
RFC 5128 State of P2P Communication across NATs March 2008
NAT and attempting to predict the public port numbers it will assign
to future sessions. The public ports assigned are often predictable
because most NATs assign mapping ports in sequence.
Consider the scenario in figure 6. Two clients, A and B, each behind
a separate NAT, have established separate UDP connections with
rendezvous server S. Rendezvous server S has a publicly addressable
IP address and is used for the purposes of registration and
discovery. Hosts behind a NAT register their endpoints with the
server. Peer hosts discover endpoints of the hosts behind NAT using
the server.
Registry and Discovery
Server S
192.0.2.128:20001
|
|
+----------------------------+----------------------------+
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 192.0.2.128:20001 | | 192.0.2.128:20001 | |
| | 192.0.2.1:62000 | | 192.0.2.254:31000 | |
| |
| ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ |
| | 192.0.2.254:31001 | | 192.0.2.1:62001 | |
| | 192.0.2.1:62001 | | 192.0.2.254:31001 | |
| |
+---------------------+ +--------------------+
| 192.0.2.1 | | 192.0.2.254 |
| | | |
| NAT A | | NAT B |
| (Endpoint-Dependent | | (Endpoint-Dependent|
| Mapping) | | Mapping) |
+---------------------+ +--------------------+
| |
| ^ Registry Session(A-S) ^ ^ Registry Session(B-S) ^ |
| | 192.0.2.128:20001 | | 192.0.2.128:20001 | |
| | 10.0.0.1:1234 | | 10.1.1.3:1234 | |
| |
| ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ |
| | 192.0.2.254:31001 | | 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 6: UDP Port Prediction to set up direct connectivity
Srisuresh, et al. Informational [Page 20]RFC 5128 State of P2P Communication across NATs March 2008
NAT A has assigned its 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 endpoints as observed by S. Client A now starts
sending UDP messages to port 31001 at address 192.0.2.254 (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
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.
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 an application implementing this trick would still
need to work even when one of the NATs employs 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
little chance of working if either client is behind two or more
levels of NAT and the NAT(s) closest to the client employs Endpoint-
Dependent Mapping.
3.6. TCP Port Number Prediction
This is a variant of the "TCP Hole Punching" technique to set up
direct peer-to-peer TCP sessions across NATs employing Address-
Dependent Mapping.
Srisuresh, et al. Informational [Page 21]
RFC 5128 State of P2P Communication across NATs March 2008
Unfortunately, this trick may be even more fragile and timing-
sensitive than the UDP port number prediction trick described
earlier. First, predicting the public port a NAT would assign could
be wrong. 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.
4. Recent Work on 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 a test program [NAT-CHECK] across a number of commercial
NAT devices. The results indicate that UDP hole punching works
widely on more than 80% of the NAT devices, whereas TCP hole punching
works on just over 60% of the NAT devices tested. The results also
indicate that TCP or UDP hairpinning is not yet widely available on
commercial NAT devices, as less than 25% of the devices passed the
tests ([NAT-CHECK]) for Hairpinning. Readers may also refer to
[JENN-RESULT] and [SAIK-RESULT] for empirical test results in
classifying publicly available NAT devices. [JENN-RESULT] provides
results of NAT classification using tests spanning across different
IP protocols. [SAIK-RESULT] focuses exclusively on classifying NAT
devices by the TCP behavioral characteristics.
[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
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 the Simple Traversal of the UDP
Protocol through NAT (STUN) protocol [STUN] on the same port number
Srisuresh, et al. Informational [Page 22]
RFC 5128 State of P2P Communication across NATs March 2008
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) 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 applications, use the techniques described in the
document for peer-to-peer connection establishment. Some
applications may use multiple distinct rendezvous servers for
registration, discovery, and relay functions for load balancing,
among other reasons. For example, the well-known media-over-IP
application "Skype" uses a central public server for login and
different public servers for end-to-end relay function.
5. Summary of Observations
5.1. TCP/UDP Hole Punching
TCP/UDP hole punching appears to be 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 may
need to prepare to fall back to simple relaying when direct
communication cannot be established.
The TCP/UDP hole punching technique has a caveat in that it works
only when the traversing NAT is EIM-NAT. When the NAT device enroute
is not EIM-NAT, the 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 are EIM-NAT devices. That
makes the TCP/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 TCP/UDP hole
punching technique.
5.2. NATs Employing Endpoint-Dependent Mapping
NATs Employing Endpoint-Dependent Mapping weren't a problem with
client-server applications such as Web browsers, which only need to
initiate outgoing connections. However, in recent times, P2P
applications such as Instant Messaging and Voice-over-IP have been in
wide use. NATs employing Endpoint-Dependent Mapping are not suitable
for P2P applications as techniques such as TCP/UDP hole punching will
not work across these NAT devices.
Srisuresh, et al. Informational [Page 23]
RFC 5128 State of P2P Communication across NATs March 2008
5.3. Peer Discovery
Application peers may be present within the same NAT domain or
external to the NAT domain. In order for all peers (those within or
external to the NAT domain) to discover the application endpoint, an
application may choose to register its private endpoints in addition
to public endpoints with the rendezvous server.
5.4. Hairpinning
Support for hairpinning is highly beneficial to allow hosts behind
EIM-NAT to communicate with other hosts behind the same NAT device
through their public, possibly translated, endpoints. Support for
hairpinning 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 hairpinning. This would be the case even
when all NAT devices in a 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 direct
communication across NAT devices.
6.1. Lack of Authentication Can Cause Connection Hijacking
Applications must use appropriate authentication mechanisms to
protect their connections from accidental confusion with other
connections as well as from malicious connection hijacking or
denial-of-service attacks. 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 connections via
TCP/UDP hole punching, applications send packets that may frequently
arrive at an entirely different host than the intended one.
For example, many consumer-level NAT devices provide Dynamic Host
Configuration Protocol (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
Srisuresh, et al. Informational [Page 24]
RFC 5128 State of P2P Communication across NATs March 2008
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, applications may be advised to assume that any IP
address they find to be suspect until they successfully establish
authenticated two-way communication.
6.2. Denial-of-Service Attacks
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, 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, applications and servers must minimize the
amount and rate of traffic they send to any newly discovered IP
address until after authenticated two-way communication is
established with the intended target.
For example, 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 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 its own list of IP addresses it registers with the
rendezvous server. There is no way the rendezvous server can verify
Srisuresh, et al. Informational [Page 25]
RFC 5128 State of P2P Communication across NATs March 2008
the IP addresses, since they could well be legitimate private network
addresses useful to other hosts for establishing network-local
communication. The 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 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 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.
6.4. Security Impact from EIM-NAT Devices
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 friendly to applications without compromising the
principle of rejecting unsolicited incoming traffic.
Srisuresh, et al. Informational [Page 26]
RFC 5128 State of P2P Communication across NATs March 2008
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, then
perhaps the NAT device was not to have been configured to allow
unrestricted outgoing TCP/UDP traffic in the first place. A NAT
device configured to allow communication originating from specific
applications at specific ports, or via tightly controlled
application-level gateways, may accomplish the security requirements
of such deployment scenarios.
7. Acknowledgments
The authors wish to thank Henrik Bergstrom, David Anderson, Christian
Huitema, Dan Wing, Eric Rescorla, and other BEHAVE work group members
for their valuable feedback on early versions of this document. The
authors also wish to thank Francois Audet, Kaushik Biswas, Spencer
Dawkins, Bruce Lowekamp, and Brian Stucker for agreeing to be
technical reviewers for this document.
8. References
8.1. Normative References
[NAT-TERM] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
[NAT-TRAD] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[BEH-UDP] Audet, F., Ed., and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, January 2007.
8.2. Informative References
[BEH-APP] Ford, B., Srisuresh, P., and D. Kegel, "Application
Design Guidelines for Traversal through Network Address
Translators", Work in Progress, March 2007.
Srisuresh, et al. Informational [Page 27]RFC 5128 State of P2P Communication across NATs March 2008
[BEH-ICMP] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha,
"NAT Behavioral Requirements for ICMP protocol", Work
in Progress, February 2008.
[BEH-TCP] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", Work
in Progress, April 2007.
[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", Work in
Progress, October 2007.
[ICE-TCP] Rosenberg, J., "TCP Candidates with Interactive
Connectivity Establishment (ICE)", Work in Progress,
July 2007.
[JENN-RESULT] Jennings, C., "NAT Classification Test Results", Work
in Progress, July 2007.
[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 A. Rayhan, "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.
[NAT-CHECK] Ford, B., "NAT check Program" available online as
http://midcom-p2p.sourceforge.net, February 2005.
[NAT-PMP] Cheshire, S., Krochmal, M., and K. Sekar, "NAT Port
Mapping Protocol (NAT-PMP)", Work in Progress, October
2006.
Srisuresh, et al. Informational [Page 28]RFC 5128 State of P2P Communication across NATs March 2008
[NAT-PROT] Holdrege, M. and P. Srisuresh, "Protocol Complications
with the IP Network Address Translator", RFC 3027,
January 2001.
[NAT-PT] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766,
February 2000.
[NAT-PT-HIST] Aoun, C. and E. Davies, "Reasons to Move the Network
Address Translator - Protocol Translator (NAT-PT) to
Historic Status", RFC 4966, July 2007.
[NSIS-NSLP] Stiemerling, M., Tschofenig, H., Aoun, C., and E.
Davies, "NAT/Firewall NSIS Signaling Layer Protocol
(NSLP)", Work in Progress, July 2007.
[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.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RSIP] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro,
"Realm Specific IP: Framework", RFC 3102, October 2001.
[SAIK-RESULT] Guha, Saikat, "NAT STUNT Results" available online as
https://www.guha.cc/saikat/stunt-results.php.
[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 L. Jones, "SOCKS Protocol Version 5", RFC 1928,
March 1996.
[STUN] Rosenberg, J., Weinberger, J., Huitema, C., and R.
Mahy, "STUN - Simple Traversal of User Datagram
Protocol (UDP) Through Network Address Translators
(NATs)", RFC 3489, March 2003.
Srisuresh, et al. Informational [Page 29]RFC 5128 State of P2P Communication across NATs March 2008
[SYM-STUN] Takeda, Y., "Symmetric NAT Traversal using STUN", Work
in Progress, June 2003.
[TCP] Postel, J., "Transmission Control Protocol", 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, October 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 P. Matthews, "Traversal
Using Relays around NAT (TURN): Relay Extensions to
Session Traversal Utilities for NAT (STUN)", Work in
Progress, January 2008.
[UNSAF] Daigle, L., Ed., 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
[V6-CPE-SEC] Woodyatt, J., "Recommended Simple Security Capabilities
in Customer Premises Equipment for Providing
Residential IPv6 Internet Service", Work in Progress,
June 2007.
Srisuresh, et al. Informational [Page 30]RFC 5128 State of P2P Communication across NATs March 2008
Authors' Addresses
Pyda Srisuresh
Kazeon Systems, Inc.
1161 San Antonio Rd.
Mountain View, CA 94043
USA
Phone: (408)836-4773
EMail: srisuresh@yahoo.com
Bryan Ford
Laboratory for Computer Science
Massachusetts Institute of Technology
77 Massachusetts Ave.
Cambridge, MA 02139
USA
Phone: (617) 253-5261
EMail: baford@mit.edu
Web: http://www.brynosaurus.com/
Dan Kegel
Kegel.com
901 S. Sycamore Ave.
Los Angeles, CA 90036
USA
Phone: 323 931-6717
EMail: dank@kegel.com
Web: http://www.kegel.com/
Srisuresh, et al. Informational [Page 31]
RFC 5128 State of P2P Communication across NATs March 2008
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
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.
Intellectual Property
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.
Srisuresh, et al. Informational [Page 32]
