1. Relaying（转发）
The most reliable, but least efficient, method of implementing peer-
to-peer communication in the presence of a middlebox is to make the
peer-to-peer communication look to the network like client/server
communication through relaying. For example, suppose two client
hosts, A and B, have each initiated TCP or UDP connections with a
well-known server S having a permanent IP address. The clients
reside on separate private networks, however, and their respective
middleboxes prevent either client from directly initiating a
connection to the other.
Server S
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|
+----------------------+----------------------+
| |
NAT A NAT B
| |
| |
Client A Client B
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. Its obvious
disadvantages are that it consumes the server's processing power and
network bandwidth unnecessarily, and communication latency between
the two 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.
2. Connection reversal（反向连接）
The second technique works if only one of the clients is behind a
middlebox. For example, suppose client A is behind a NAT but client
B has a globally routable IP address, as in the following diagram:
Server S
18.181.0.31:1235
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|
+----------------------+----------------------+
| |
NAT A |
155.99.25.11:62000 |
| |
| |
Client A Client B
10.0.0.1:1234 138.76.29.7:1234
Client A has private IP address 10.0.0.1, and the application is
using TCP port 1234. This client has established a connection with
server S at public IP address 18.181.0.31 and port 1235. NAT A has
assigned TCP port 62000, at its own public IP address 155.99.25.11,
to serve as the temporary public endpoint address for A's session
with S: therefore, server S believes that client A is at IP address
155.99.25.11 using port 62000. Client B, however, has its own
permanent IP address, 138.76.29.7, and the peer-to-peer application
on B is accepting TCP connections at port 1234.
Now suppose client B would like to initiate a peer-to-peer
communication session with client A. B might first attempt to
contact client A either at the address client A believes itself to
have, namely 10.0.0.1:1234, or at the address of A as observed by
server S, namely 155.99.25.11:62000. In either case, however, the
connection will fail. In the first case, traffic directed to IP
address 10.0.0.1 will simply be dropped by the network because
10.0.0.1 is not a publicly routable IP address. In the second case,
the TCP SYN request from B will arrive at NAT A directed to port
62000, but NAT A will 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 middlebox.
A variety of current peer-to-peer systems implement this technique.
Its main limitation, of course, is that it only works as long as only
one of the communicating peers is behind a NAT: in the increasingly
common case where both peers are behind NATs, the method fails.
Because connection reversal is not a general solution to the problem,
it is NOT recommended as a primary strategy. Applications may choose
to attempt connection reversal, but should be able to fall back
automatically on another mechanism such as relaying if neither a
"forward" nor a "reverse" connection can be established.
3.UDP hole punching
The third technique, and the one of primary interest in this
document, is widely known as "UDP Hole Punching." UDP hole punching
relies on the properties of common firewalls and cone NATs to allow
appropriately designed peer-to-peer applications to "punch holes"
through the middlebox and establish direct connectivity with each
other, even when both communicating hosts may lie behind middleboxes.
This technique was mentioned briefly in section 5.1 of RFC 3027 [NAT-
PROT], and has been informally described elsewhere on the Internet
[KEGEL] and used in some recent protocols [TEREDO, ICE]. As the name
implies, unfortunately, this technique works reliably only with UDP.
We will consider two specific scenarios, and how applications can be
designed to handle both of them gracefully. In the first situation,
representing the common case, two clients desiring direct peer-to-
peer communication reside behind two different NATs. In the second,
the two clients actually reside behind the same NAT, but do not
necessarily know that they do.
3.1. Peers behind different NATs
Suppose clients A and B both have private IP addresses and lie behind
different network address translators. The peer-to-peer application
running on clients A and B and on server S each use UDP port 1234. A
and B have each initiated UDP communication sessions with server S,
causing NAT A to assign its own public UDP port 62000 for A's session
with S, and causing NAT B to assign its port 31000 to B's session
with S, respectively.
Server S
18.181.0.31:1234
|
|
+----------------------+----------------------+
| |
NAT A NAT B
155.99.25.11:62000 138.76.29.7:31000
| |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Now suppose that client A wants to establish a UDP communication
session directly with client B. If A simply starts sending UDP
messages to B's public address, 138.76.29.7:31000, then NAT B will
typically discard these incoming messages (unless it is a full cone
NAT), 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, 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
A to open up a new communication session between A's private address
and B's public address. At the same time, B's messages to A's public
address (155.99.25.11:62000) 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 "introduction" server S.
The UDP hole punching technique has several useful properties. Once
a direct peer-to-peer UDP connection has been established between two
clients behind middleboxes, either party on that connection can in
turn take over the role of "introducer" and help the other party
establish peer-to-peer connections with additional peers, minimizing
the load on the initial introduction server S. The application does
not need to attempt to detect explicitly what kind of middlebox it is
behind, if any [STUN], since the procedure above will establish peer-
to-peer communication channels equally well if either or both clients
do not happen to be behind a middlebox. The 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.
3.2. 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. Client A has
established a UDP session with server S, to which the common NAT has
assigned public port number 62000. Client B has similarly
established a session with S, to which the NAT has assigned public
port number 62001.
Server S
18.181.0.31:1234
|
|
NAT
A-S 155.99.25.11:62000
B-S 155.99.25.11:62001
|
+----------------------+----------------------+
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Suppose that A and B use the UDP hole punching technique as outlined
above to establish a communication channel using server S as an
introducer. Then A and B will learn each other's public IP addresses
and port numbers as observed by server S, and start sending each
other messages at those public addresses. The two clients will be
able to communicate with each other this way as long as the NAT
allows hosts on the internal network to open translated UDP sessions
with other internal hosts and not just with external hosts. We refer
to this situation as "loopback 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 155.99.25.11:62001. The NAT
receives this packet, translates it to have a source of
155.99.25.11:62000 (A's public address) and a destination of
10.1.1.3:1234, and then forwards it on to B. Even if loopback
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. 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 following situation.
Server S
18.181.0.31:1234
|
|
NAT X
A-S 155.99.25.11:62000
B-S 155.99.25.11:62001
|
|
+----------------------+----------------------+
| |
NAT A NAT B
192.168.1.1:30000 192.168.1.2:31000
| |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Suppose NAT X is a large industrial NAT deployed by an 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. Each client initiates an outgoing connection to
server S as before, causing NATs A and B each to create a single
public/private translation, and causing NAT X to establish a
public/private translation for each session.
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, 155.99.25.11:62000 and 155.99.25.11:62001. 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
loopback translation.
3.4. Consistent port bindings
The hole punching technique has one main caveat: it works only if
both NATs are cone NATs (or non-NAT firewalls), which maintain a
consistent port binding between a given (private IP, private UDP)
pair and a (public IP, public UDP) pair for as long as that UDP port
is in use. Assigning a new public port for each new session, as a
symmetric NAT does, makes it impossible for a UDP application to
reuse an already-established translation for communication with
different external destinations. Since cone NATs are the most
widespread, the UDP hole punching technique is fairly broadly
applicable; nevertheless a substantial fraction of deployed NATs are
symmetric and do not support the technique.
4. UDP port number prediction
A variant of the UDP hole punching technique discussed above exists
that allows peer-to-peer UDP sessions to be created in the presence
of some symmetric NATs. 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 again the situation in which two clients, A and B, each
behind a separate NAT, have each established UDP connections with a
permanently addressable server S:
Server S
18.181.0.31:1234
|
|
+----------------------+----------------------+
| |
Symmetric NAT A Symmetric NAT B
A-S 155.99.25.11:62000 B-S 138.76.29.7:31000
| |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
NAT A has assigned its own UDP port 62000 to the communication
session between A and S, and NAT B has assigned its port 31000 to the
session between B and S. By communicating through server S, A and B
learn each other's public IP addresses and port numbers as observed
by S. Client A now starts sending UDP messages to port 31001 at
address 138.76.29.7 (note the port number increment), and client B
simultaneously starts sending messages to port 62001 at address
155.99.25.11. If NATs A and B assign port numbers to new sessions
sequentially, and if not much time has passed since the A-S and B-S
sessions were initiated, then a working 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 below.
Server S
18.181.0.31:1234
|
|
+----------------------+----------------------+
| |
NAT A NAT B
A-S 155.99.25.11:62000 B-S 138.76.29.7:31000
A-B 155.99.25.11:62001 B-A 138.76.29.7:31001
| |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234
Obviously there are many things that can cause this trick to fail.
If the predicted port number at either NAT already happens to be in
use by an unrelated session, then the NAT will skip over that port
number and the connection attempt will fail. If either NAT sometimes
or always chooses port numbers non-sequentially, then the trick will
fail. If a different client behind NAT A (or B respectively) opens
up a new outgoing UDP connection to any external destination after A
(B) establishes its connection with S but before sending its first
message to B (A), then the unrelated client will inadvertently
"steal" the desired port number. This trick is therefore much less
likely to work when either NAT involved is under load.
Since in practice a P2P application implementing this trick would
still need to work if the NATs are cone NATs, or if one is a cone NAT
and the other is a symmetric NAT, the application would need to
detect beforehand what kind of NAT is involved on either end [STUN]
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 are
symmetric. For all of these reasons, it is NOT recommended that new
applications implement this trick; it is mentioned here for
historical and informational purposes.
5. Simultaneous TCP open
There is a method that can be used in some cases to establish direct
peer-to-peer TCP connections between a pair of nodes that are both
behind existing middleboxes. Most TCP sessions start with one
endpoint sending a SYN packet, to which the other party responds with
a SYN-ACK packet. It is possible and legal, however, for two
endpoints to start a TCP session by simultaneously sending each other
SYN packets, to which each party subsequently responds with a
separate ACK. This procedure is known as a "simultaneous open."
If a middlebox receives a TCP SYN packet from outside the private
network attempting to initiate an incoming TCP connection, the
middlebox will normally reject the connection attempt by either
dropping the SYN packet or sending back a TCP RST (connection reset)
packet. If, however, the SYN packet arrives with source and
destination addresses and port numbers that correspond to a TCP
session that the middlebox believes is already active, then the
middlebox will allow the packet to pass through. In particular, if
the middlebox 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 correctly predict the public port number
that its respective middlebox will assign the next outgoing TCP
connection, and if each client initiates an outgoing TCP connection
with the other client timed so that each client's outgoing SYN passes
through its local middlebox before either SYN reaches the opposite
middlebox, then a working peer-to-peer TCP connection will result.
Unfortunately, this trick may be even more fragile and timing-
sensitive than the UDP port number prediction trick described above.
First, unless both middleboxes are simple firewalls or implement cone
NAT behavior on their 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. In addition, if
either client's SYN arrives at the opposite middlebox too quickly,
then the remote middlebox may reject the SYN with a RST packet,
causing the local middlebox in turn to close the new session and make
future SYN retransmission attempts using the same port numbers
futile. Finally, even though support for simultaneous open is
technically a mandatory part of the TCP specification [TCP], it is
not implemented correctly in some common operating systems. For this
reason, this trick is likewise mentioned here only for historical
reasons; it is NOT recommended for use by applications. Applications
that require efficient, direct peer-to-peer communication over
existing NATs should use UDP.
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