Internet Draft P. Srisuresh Document: draft-srisuresh-behave-p2p-state-00.txt Caymas Systems Expires: June 30, 2005 B. Ford M.I.T. D. Kegel kegel.com December 2004 State of Peer-to-Peer(P2P) communication across Network Address Translators(NATs) Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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 methods known to be in use by the TCP/UDP based peer-to-peer (P2P) applications for communication in the presence of network address translators (NATs) at the current time. This memo is not an endorsement of the methods in use, but merely an attempt to undertsand the techniques used. Table of Contents 1. Introduction ................................................. 2. Terminology .................................................. Srisuresh, Ford & Kegel [Page 1]
Internet-Draft State of P2P communication actoss NATs December 2004 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. Assumption of P2P-friendly NAT devices enroute ... 3.4. Simultaneous TCP Open ................................... 3.5. UDP port number prediction .............................. 3.6. TCP port number prediction .............................. 4. Summary of observations ...................................... 4.1. TCP/UDP hole punching ................................... 4.2. Symmetric NATs are not P2P friendly ..................... 4.3. Peer discovery .......................................... 4.4. Hairpin translation ..................................... 5. Security considerations ...................................... 5.1. IP address aliasing ..................................... 5.2. Denial-of-service attacks ............................... 5.3. Man-in-the-middle attacks ............................... 5.4. Impact on NAT device security ........................... 6. Acknowledgments .............................................. 7. Informative References ....................................... 8. Authors' addresses ........................................... 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 the NAT devices in the deployments. The asymmetric addressing and connectivity regimes established by the 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 is 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 percieved as a privacy benefit. Srisuresh, Ford & Kegel [Page 2]
Internet-Draft State of P2P communication actoss NATs December 2004 In the peer-to-peer paradigm, however, Internet hosts that would normally be considered "clients" need to establish communication sessions directly with each other. 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, is for the participating application hosts to contact a well-known server for initialization and administration purposes. Subsequent to this, 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 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. In this document, we summarize the currently known methods by which P2P applications work around the presence of NAT devices. 2. Terminology Readers are urged to refer [NAT-TERM] for information on NAT taxonomy and terminology. Traditional NAT is the most common type of NAT device deployed. Readers may refer [NAT-TRAD] for detailed information on traditional NAT. Traditional NAT has two main varieties - Basic NAT and Network Address/Port Translator (NAPT). 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. The effect is that the NAPT establishes a NAT session to translate the (private IP address, private port number) tuple to (public IP address, public port number) tuple and vice versa for the duration of the session. An issue of relevance to P2P applications is how the NAT behaves when an internal host initiates multiple simultaneous sessions from a single (private IP, private port) endpoint to multiple distinct endpoints on the external network. Additional terms that further classify NAPT implementation are defined in more recent work [STUN] and are summarized below. Srisuresh, Ford & Kegel [Page 3]
Internet-Draft State of P2P communication actoss NATs December 2004 Cone NAT The fundamental property of Cone NAT is that it reuses port binding assigned to a private host endpoint (identified by the combination of private IP address and protocol specific port number) for all sessions initiated by the private host from the same endpoint, while the port binding is alive. Cone NAT creates port binding between a (private IP, private port) tuple and a (public IP, public port) tuple for translation purposes. For example, suppose Client A in figure 1 initiates two simultaneous outgoing sessions through a cone NAT, from the same internal endpoint (10.0.0.1:1234) to two different external servers, S1 and S2. The cone NAT assigns just one public endpoint 155.99.25.11:62000 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 Cone NAT. Srisuresh, Ford & Kegel [Page 4]
Internet-Draft State of P2P communication actoss NATs December 2004 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 | | 155.99.25.11:62000 | | | 155.99.25.11:62000 | | +--------------+ | 155.99.25.11 | | | | Any type of | | Cone NAT | +--------------+ | ^ 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: Cone NAT - Reuse of port binding for multiple sessions Symmetric NAT A symmetric NAT, in contrast, does not use port bindings. A Symmetric NAT assigns a new public port to each new session traversing the NAT device. For example, 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 symmetric NAT, the symmetric NAT assigns port 62000 to translate the client end-point. When the second session from the same client end-point to server S2 traverses the symmetric NAT, the symmetric NAT will assign a different port 62001 to translate the same client end-point. As a result, server S1 sees the client endpoint as 155.99.25.11:62000, whereas server S2 sees the same client endpoint differently as 155.99.25.11:62001. The symmetric NAT, 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. Srisuresh, Ford & Kegel [Page 5]
Internet-Draft State of P2P communication actoss NATs December 2004 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 | | 155.99.25.11:62000 | | | 155.99.25.11:62001 | | +---------------+ | 155.99.25.11 | | | | Symmetric | | NAT | +---------------+ | ^ 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: Symmetric NAT - Port binding not in use for sessions Cone NAT is further classified according to how liberally the NAT accepts incoming traffic directed to an already-established (public IP, public port) tuple. The following Cone NAT variations are defined in [STUN], but restated here for additional explanation. This classification generally applies only to UDP traffic, since NATs reject incoming TCP connection attempts unconditionally unless specifically configured to do otherwise. Full Cone NAT Subsequent to establishing port binding at the start of an outgoing session, a full cone NAT will accept incoming traffic to the corresponding public port from ANY external endpoint on the public network. Full cone NAT is also sometimes referred as "promiscuous" NAT. Address-restricted Cone NAT Subsequent to establishing port binding at the start of an outgoing session, Address-restricted Cone NAT will accept incoming traffic to the corresponding public port from only those external endpoints whose IP address match the address of a node to which the internal host has previously sent one Srisuresh, Ford & Kegel [Page 6]
Internet-Draft State of P2P communication actoss NATs December 2004 or more outgoing packets. Port-restricted Cone NAT Subsequent to establishing port binding at the start of an outgoing session, Port-restricted Cone NAT will accept incoming traffic to the corresponding public port from only those external endpoints to which the internal host has previously sent one or more outgoing packets. Port-restricted Cone NAT is the true-to-spirit implementation of NAPT, as defined. Port-restricted Cone NAT provides internal nodes the same level of protection against unsolicited incoming UDP traffic as does a symmetric NAT. This is because Port-restricted Cone NAT and Symmetric NAT have one thing in common. They both maintain granular NAT-sessions. I.e., every single 5-tuple UDP session permitted for traversal by the NAT is maintained within the NAT as a NAT-session. As a result, incoming packet traffic is limited to only those sessions for which the NAT is aware of an outgoing NAT-session. This is not the case with Address-restricted Cone NAT and Full Cone NAT. NAT sessions maintained by Address-restricted Cone NAT and Full Cone NAT are less granular. The NAT-sessions maintained by an Address-restricted Cone NAT, for example, use wildcard match on the external UDP port. The NAT-sessions maintained by a Full Cone NAT, for example, use wildcard match on the external address as well as the external UDP port. As a result, the NAT will permit new UDP sessions initiated from an external endpoint to the public port bound to the private endpoint, even as the private endpoint did not originate an outgoing session to the external endpoint. Address-restricted Cone NAT as well as Full Cone NAT will permit traversal of the new incoming session traffic. Finally, we define the following new terms for classifying P2P-relevant behavior across NAT devices. P2P-Application P2P-application as used in this document is an application in which each P2P participant registers with a public registration server, and subsequently uses either its private endpoint, or public endpoint, or both, to establish peering sessions. NAT-friendly P2P application NAT-friendly P2P application is a P2P application that is designed to work effectively even as peering nodes are Srisuresh, Ford & Kegel [Page 7]
Internet-Draft State of P2P communication actoss NATs December 2004 located in multiple distinct IP address realms, connected by one or more NATs. P2P-friendly NAT P2P-friendly NAT is a NAT device that permits the traversal of P2P application traffic across itself. A key requirement for a P2P-friendly NAT is the ability to maintain endpoint identity of a P2P application host when the P2P application is initiated. All variations of Cone NAT are good examples of P2P-friendly NAT devices. Symmetric NAT is a good example of a NAT device that is not P2P friendly. Loopback translation / 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, the NAT device performs the equivalent of a "Twice-nat" 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 performed by a NAT device as "Loopback translation". This is also referred sometimes as "Hairpin translation". 3. Techniques used by P2P applications to work with 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. The readers will note that the applications assume an Address/Port-restricted Cone NAT 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. For example, suppose two client hosts A and B, in figure 3, have each initiated TCP or UDP connections to a well-known server S, which has a permanent IP address. The clients reside on separate private networks, and their respective NAT devices prevent either client from directly initiating a connection to the other. Srisuresh, Ford & Kegel [Page 8]
Internet-Draft State of P2P communication actoss NATs December 2004 Server S 18.181.0.31:1234 | +----------------------------+----------------------------+ | | | ^ Relay-Req Session(A-S) ^ ^ Relay-Req Session(B-S) ^ | | | 18.181.0.31:1234 | | 18.181.0.31:1234 | | | | 155.99.25.11:62000 | | 138.76.29.7:31000 | | | | +--------------+ +--------------+ | 155.99.25.11 | | 138.76.29.7 | | | | | | Symmetric or | | Symmetric or | | Cone NAT A | | Cone NAT B | +--------------+ +--------------+ | | | ^ 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 Client-Server sessions & relay server to emulate 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. Its obvious disadvantages 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. 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. For example, suppose client A is behind a NAT but client B has a globally routable IP address, as in figure 4. Srisuresh, Ford & Kegel [Page 9]
Internet-Draft State of P2P communication actoss NATs December 2004 Server S 18.181.0.31:1234 | +----------------------------+----------------------------+ | | | ^ Relay-Req Session(A-S) ^ ^ Relay-Req Session(B-S) ^ | | | 18.181.0.31:1234 | | 18.181.0.31:1234 | | | | 155.99.25.11:62000 | | 138.76.29.7:1234 | | | | | ^ P2P Session (A-B) ^ | P2P Session (B-A) | | | | 138.76.29.7:1234 | | 155.99.25.11:62000 | | | | 155.99.25.11:62000 | v 138.76.29.7:31000 v | | | +--------------+ | | 155.99.25.11 | | | | | | Address/Port | | | Restricted | | | Cone NAT A | | +--------------+ | | | | ^ Relay-Req 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: Force private client to initiate session for 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 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 Srisuresh, Ford & Kegel [Page 10]
Internet-Draft State of P2P communication actoss NATs December 2004 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 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 and the NAT is P2P-friendly, such as a Cone NAT. In the increasingly common case where both peers can be behind NATs, the method fails. Because connection reversal is not a general solution to the problem, it is NOT recommended as a primary strategy. NAT-friendly P2P applications may choose to attempt connection reversal, but 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 cone 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 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 Srisuresh, Ford & Kegel [Page 11]
Internet-Draft State of P2P communication actoss NATs December 2004 necessarily know that they do. 3.3.1. Peers behind different NATs Suppose clients A and B both have private IP addresses and lie behind different network address translators as in figure 5. 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 | +----------------------------+----------------------------+ | | | ^ Relay-Req Session(A-S) ^ ^ Relay-Req Session(B-S) ^ | | | 18.181.0.31:1234 | | 18.181.0.31:1234 | | | | 155.99.25.11:62000 | | 138.76.29.7:31000 | | | | | ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ | | | 138.76.29.7:31000 | | 155.99.25.11:62000 | | | | 155.99.25.11:62000 | | 138.76.29.7:31000 | | | | +--------------+ +--------------+ | 155.99.25.11 | | 138.76.29.7 | | | | | | Address/Port | | Address/port | | Restricted | | Restricted | | Cone NAT A | | Cone NAT B | +--------------+ +--------------+ | | | ^ 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 | | | | | ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ | | | 138.76.29.7:31000 | | 155.99.25.11:62000 | | | | 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: Coordinate simultaneous outgoing sessions for Direct-P2P Now suppose that client A wants to establish a UDP communication session directly with client B. If A simply starts sending UDP Srisuresh, Ford & Kegel [Page 12]
Internet-Draft State of P2P communication actoss NATs December 2004 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 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 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 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, if any [STUN], since the procedure above will establish peer-to-peer communication channels equally well if either or both clients do not happen to be behind a NAT 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. 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 13]
Internet-Draft State of P2P communication actoss NATs December 2004 Server S 18.181.0.31:1234 | ^ Relay-Req Session(A-S) ^ | ^ Relay-Req Session(B-S) ^ | 18.181.0.31:1234 | | | 18.181.0.31:1234 | | 155.99.25.11:62000 | | | 155.99.25.11:62001 | | +--------------+ | 155.99.25.11 | | | | Address/Port | | Restricted | | Cone NAT | +--------------+ | +-----------------------------+----------------------------+ | | | | | ^ 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 | | | | | ^ 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)^ | | | 155.99.25.11:62001 | | 155.99.25.11:62000 | | | | 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: Register private identity & NAT identity with Relay server. 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 Srisuresh, Ford & Kegel [Page 14]
Internet-Draft State of P2P communication actoss NATs December 2004 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. 3.3.3. 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 situation, depicted in figure 7. Srisuresh, Ford & Kegel [Page 15]
Internet-Draft State of P2P communication actoss NATs December 2004 Server S 18.181.0.31:1234 | ^ Relay-Req Session(A-S) ^ | ^ Relay-Req Session(B-S) ^ | 18.181.0.31:1234 | | | 18.181.0.31:1234 | | 155.99.25.11:62000 | | | 155.99.25.11:62001 | | +--------------+ | 155.99.25.11 | | | | Address/Port | | Restricted | | Cone NAT X | | (Supporting | | Loopback | | Translation) | +--------------+ | +----------------------------+----------------------------+ | | | ^ Relay-Req Session(A-S) ^ ^ Relay-Req 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) ^ | | | 155.99.25.11:62001 | | 155.99.25.11:62000 | | | | 192.168.1.1:30000 | | 192.168.1.2:31000 | | | | +--------------+ +--------------+ | 192.168.1.1 | | 192.168.1.2 | | | | | | Address/Port | | Address/Port | | Restricted | | Restricted | | Cone-NAT A | | Cone-NAT B | +--------------+ +--------------+ | | | ^ 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 | | | | | ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ | | | 155.99.25.11:62001 | | 155.99.25.11:62000 | | | | 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: Use of Loopback translation to facilitate Direct-P2P Srisuresh, Ford & Kegel [Page 16]
Internet-Draft State of P2P communication actoss NATs December 2004 Suppose NAT X is a large industrial Cone 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. 3.3.4. Assumption of P2P-friendly NAT devices enroute The UDP hole punching technique has a caveat in that it works only if the traversing NAT is a P2P-friendly NAT, such as a Cone NAT. When a symmetric NAT is encountered enroute, P2P application is unable to reuse an already-established translation endpoint for communication with different external destinations and the technique would fail. However, Cone NATs are widely deployed in the Internet. That makes the UDP hole punching technique broadly applicable; nevertheless a substantial fraction of deployed NATs are symmetric NATs and do not support the UDP hole punching technique. 3.4. Simultaneous TCP Open Simultaneous TCP open (also known sometimes as TCP hole punching) technique is used in some cases to establish direct peer-to-peer Srisuresh, Ford & Kegel [Page 17]
Internet-Draft State of P2P communication actoss NATs December 2004 TCP connections between a pair of nodes that are both behind P2P-friendly NAT devices that implement Cone NAT behavior on 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 referred as "simultaneous TCP Open" technique. 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 did 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 cause the TCP connection to be not established. 3.5. UDP port number prediction Srisuresh, Ford & Kegel [Page 18]
Internet-Draft State of P2P communication actoss NATs December 2004 A variant of the UDP hole punching technique 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, as depicted in figure 8. Server S 18.181.0.31:1234 | +----------------------------+----------------------------+ | | | ^ Relay-Req Session(A-S) ^ ^ Relay-Req Session(B-S) ^ | | | 18.181.0.31:1234 | | 18.181.0.31:1234 | | | | 155.99.25.11:62000 | | 138.76.29.7:31000 | | | | | | +--------------+ +-------------+ | 155.99.25.11 | | 138.76.29.7 | | | | | | Symmetric | | Symmetric | | NAT A | | NAT B | +--------------+ +-------------+ | | | ^ 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 8: Use Peer's Symmetric-NAT Identity to predict P2P port 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 Srisuresh, Ford & Kegel [Page 19]
Internet-Draft State of P2P communication actoss NATs December 2004 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 20]
Internet-Draft State of P2P communication actoss NATs December 2004 Server S 18.181.0.31:1234 | | +----------------------------+----------------------------+ | | | ^ Relay-Req Session(A-S) ^ ^ Relay-Req Session(B-S) ^ | | | 18.181.0.31:1234 | | 18.181.0.31:1234 | | | | 155.99.25.11:62000 | | 138.76.29.7:31000 | | | | | ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ | | | 138.76.29.7:31001 | | 155.99.25.11:62001 | | | | 155.99.25.11:62001 | | 138.76.29.7:31001 | | | | +--------------+ +-------------+ | 155.99.25.11 | | 138.76.29.7 | | | | | | Symmetric | | Symmetric | | NAT A | | NAT B | +--------------+ +-------------+ | | | ^ 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 | | | | | ^ P2P Session (A-B) ^ ^ P2P Session (B-A) ^ | | | 138.76.29.7:31001 | | 155.99.25.11:62001 | | | | 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 Port Prediction on Symmetric NATs to setup Direct-p2p 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 21]
Internet-Draft State of P2P communication actoss NATs December 2004 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. This technique is mentioned here only for historical and informational purposes. 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 symmetric NATs. 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 Cone NAT behavior 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. For this reason, this trick is mentioned here only for historical reasons. It is NOT recommended for use by applications. 4. Summary of observations 4.1. TCP/UDP hole punching TCP/UDP hole punching is apparently the most efficient existing method of establishing direct TCP/UDP peer-to-peer communication between two nodes that are both behind NATs. These techniques have been used with a wide variety of existing NATs. However, applications should be prepared to fall back on simple relaying when direct communication cannot be established. 4.2. Symmetric NATs are not P2P friendly Symmetric NATs gained popularity with client-server applications such as web browsers, which only need to initiate outgoing Srisuresh, Ford & Kegel [Page 22]
Internet-Draft State of P2P communication actoss NATs December 2004 connections. However, in the recent times, P2P applications such as Instant messaging and audio conferencing have been in wide use. Symmetric NATs do not support TCP/UDP port binding and are not suitable for P2P applications. P2P-friendly NAT devices implement Cone NAT behavior, allowing applications to establish robust P2P connectivity using the TCP/UDP hole punching techniques. A Cone NAT maintains port bindings for TCP and UDP endpoints. 4.3. Peer discovery Applications should not assume all its peers to be outside its NAT boundary. As such, an application should register all its private IP addresses with the external server, so it can connect to some of its peers within the NAT boundary without having to traverse the NAT device. 4.4. Hairpin translation Hairpin translation support is highly benficial 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 technique, unless the first-level NAT also supports loopback translation. This would be the case even when all NAT devices in the deployment preserve endpoint identities, 5. 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. 5.1. IP address aliasing NAT-friendly P2P applications must use appropriate authentication mechanisms to protect their P2P connections from accidental Srisuresh, Ford & Kegel [Page 23]
Internet-Draft State of P2P communication actoss NATs December 2004 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, 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 server S, 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. 5.2. 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 Srisuresh, Ford & Kegel [Page 24]
Internet-Draft State of P2P communication actoss NATs December 2004 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 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. 5.3. Man-in-the-middle attacks Any network device on the path between a P2P client and a 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 Srisuresh, Ford & Kegel [Page 25]
Internet-Draft State of P2P communication actoss NATs December 2004 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. 5.4. Impact on NAT device security Designing NAT devices to preserve endpoint identities does not weaken the security provided by the NAT device. For example, a Port-restricted Cone NAT is inherently no more "promiscuous" than a Symmetric NAT in its policies for allowing either incoming or outgoing traffic to pass through the NAT device. As long as outgoing TCP/UDP sessions are enabled and the NAT device maintains consistent binding between internal and external TCP/UDP ports, the NAT device will filter out any incoming TCP/UDP packets that do not match the active sessions initiated from within the enclave. Filtering incoming traffic aggressively while maintaining consistent port bindings thus allows a NAT device to be P2P friendly without compromising the principle of rejecting unsolicited incoming traffic. Maintaining consistent port binding could arguably increase the predictability of traffic emerging from the NAT device, by revealing the relationships between different 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. 6. Acknowledgments The authors wish to thank Henrik, Dave, and Christian Huitema for their valuable feedback. Srisuresh, Ford & Kegel [Page 26]
Internet-Draft State of P2P communication actoss NATs December 2004 7. Informative References [NAT-TERM] P. Srisuresh and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [NAT-TRAD] P. Srisuresh and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [STUN] J. Rosenberg, J. Weinberger, C. Huitema, and R. Mahy, "STUN - Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs)", RFC 3489, March 2003. [NAT-APPL] D. Senie, "Network Address Translator (NAT)-Friendly Application Design Guidelines", RFC 3235, January 2002. [NAT-PROT] M. Holdrege and P. Srisuresh, "Protocol Complications with the IP Network Address Translator", RFC 3027, January 2001. [NAT-PT] G. Tsirtsis and P. Srisuresh, "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, February 2000. [BIDIR] Peer-to-Peer Working Group, NAT/Firewall Working Committee, "Bidirectional Peer-to-Peer Communication with Interposing Firewalls and NATs", August 2001. http://www.peer-to-peerwg.org/tech/nat/ [KEGEL] Dan Kegel, "NAT and Peer-to-Peer Networking", July 1999. http://www.alumni.caltech.edu/~dank/peer-nat.html [TCP] "Transmission Control Protocol", RFC 793, September 1981. [TURN] J. Rosenberg, J. Weinberger, R. Mahy, and C. Huitema, "Traversal Using Relay NAT (TURN)", draft-rosenberg-midcom-turn-01 (Work In Progress), March 2003. 8. Authors' Addresses Pyda Srisuresh Caymas Systems, Inc. 1179-A North McDowell Blvd. Srisuresh, Ford & Kegel [Page 27]
Internet-Draft State of P2P communication actoss NATs December 2004 Petaluma, CA 94954 Phone: (707) 283-5063 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/ 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/ Full Copyright Statement Copyright (C) The Internet Society (2004). 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 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 28]
