Network Working Group S. Guha, Ed. Internet-Draft Cornell U. Expires: December 20, 2006 K. Biswas Cisco Systems B. Ford M.I.T. P. Francis Cornell U. S. Sivakumar Cisco Systems P. Srisuresh Consultant June 18, 2006 NAT Behavioral Requirements for TCP draft-ietf-behave-tcp-01.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 20, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract Guha, et al. Expires December 20, 2006 [Page 1] Internet-Draft NAT TCP Requirements June 2006 This document defines a set of requirements for NATs that handle TCP that would allow many applications, such as peer-to-peer applications and on-line games, to work consistently. Developing NATs that meet this set of requirements will greatly increase the likelihood that these applications will function properly. Table of Contents 1. Applicability Statement . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4 4. TCP Connection Initiation . . . . . . . . . . . . . . . . . 4 4.1 Address and Port Mapping Behavior . . . . . . . . . . . . 4 4.2 Internally Initiated Connections . . . . . . . . . . . . . 5 4.3 Externally Initiated Connections . . . . . . . . . . . . . 6 5. NAT Session Refresh . . . . . . . . . . . . . . . . . . . . 9 6. Application Level Gateways . . . . . . . . . . . . . . . . . 11 7. UDP Specific Requirements Also Applicable to TCP . . . . . . 11 7.1 Port Assignment . . . . . . . . . . . . . . . . . . . . . 11 7.2 Hairpinning Behavior . . . . . . . . . . . . . . . . . . . 12 7.3 ICMP Responses to TCP Packets . . . . . . . . . . . . . . 12 8. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 13 9. Security considerations . . . . . . . . . . . . . . . . . . 14 10. IANA considerations . . . . . . . . . . . . . . . . . . . . 15 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 15 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 12.1 Normative References . . . . . . . . . . . . . . . . . . 15 12.2 Informational References . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17 Intellectual Property and Copyright Statements . . . . . . . 19 Guha, et al. Expires December 20, 2006 [Page 2] Internet-Draft NAT TCP Requirements June 2006 1. Applicability Statement This document is adjunct to [BEHAVE-UDP], which defines many terms relating to NATs, lays out general requirements for all NATs, and sets requirements for NATs that handle IP and unicast UDP traffic. The purpose of this document is to set requirements for NATs that handle TCP traffic. The requirements of this specification apply to Traditional NATs as described in [RFC2663]. This document only covers the TCP aspects of NAT traversal. Middle- box behavior that is not necessary for network address translation of TCP is out-of-scope. Firewalls, and packet inspection above the TCP layer are out-of-scope except for Application Level Gateways (ALG) behavior that may interfere with NAT traversal. Application and OS aspects of TCP NAT traversal are out-of-scope. Signaling based approaches to NAT traversal such as Midcom and UPnP that directly control the NAT are out-of-scope. 2. Introduction Network Address Translators (NATs) hinder connectivity in applications where sessions may be initiated to internal hosts. [BEHAVE-UDP] lays out the terminology and requirements for NATs in the context of IP and UDP. This document supplements these by setting requirements for NATs that handle TCP traffic. All definitions and requirements in [BEHAVE-UDP] are inherited here. [TCP-ROADMAP] chronicles the evolution of TCP from the original definition [RFC0793] to present day implementations. While much has changed in TCP with regards to congestion control and flow control, security, and support for high-bandwidth networks, the process of initiating a connection (i.e. the 3-way handshake or simultaneous- open) has changed little. It is the process of connection initiation that NATs affect the most. Experimental approaches such as T/TCP [RFC1644] have proposed alternate connection initiation approaches, but, have been found to be complex and susceptible to denial-of- service attacks. Modern operating systems and NATs consequently primarily support the 3-way handshake and simultaneous-open modes of connection initiation as described in [RFC0793]. Recently, many techniques have been devised to make peer-to-peer TCP applications work across NATs. [STUNT], [NATBLASTER], and [P2PNAT] describe Unilateral Self-Address Translation (UNSAF) mechanisms that allow peer-to-peer applications to establish TCP through NATs. These approaches require only endpoint applications to be modified and work with standards compliant OS stacks. The approaches, however, depend Guha, et al. Expires December 20, 2006 [Page 3] Internet-Draft NAT TCP Requirements June 2006 on specific NAT behavior that is usually, but not always, supported by NATs (see [TCPTRAV] and [P2PNAT] for details). Consequently a complete TCP NAT traversal solution is sometimes forced to rely on public TCP relays to traverse NATs that do not cooperate. This document defines requirements that ensure that TCP NAT traversal approaches are not forced to use data relays, while preserving the functionality and security provided by NATs. 3. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. This document uses the term "NAT session" as defined in [RFC2663]. "NAT" in this specification includes both "Basic NAT" and "Network Address/Port Translator (NAPT)" [RFC2663]. This document uses the term "TCP connection" (or just "connection") to refer to individual TCP flows identified by the 4-tuple (source and destination IP address and TCP port) and the initial sequence numbers (ISN). This document uses the term "address and port mapping" (or just "mapping") as defined in [BEHAVE-UDP] to refer to state at the NAT necessary for network address and port translation of TCP connections. This document also uses (and summarizes the definition of) the terms "endpoint independent mapping", "address dependent mapping", "address and port dependent mapping", "filtering behavior", "endpoint independent filtering", "address dependent filtering", "address and port dependent filtering", "port overloading", and "hairpinning" as defined in [BEHAVE-UDP]. 4. TCP Connection Initiation This section describes various NAT behaviors applicable to TCP connection initiation. 4.1 Address and Port Mapping Behavior A NAT uses a mapping to translate packets for each TCP connection. A mapping is dynamically allocated for connections initiated from the internal side, and potentially reused for certain subsequent connections. NAT behavior regarding when a mapping can be reused differs for different NATs as described here. Consider an internal IP address and TCP port (X:x) that initiates a TCP connection to an external (Y1:y1) tuple. Let the mapping Guha, et al. Expires December 20, 2006 [Page 4] Internet-Draft NAT TCP Requirements June 2006 allocated by the NAT for this connection be (X1':x1'). Shortly thereafter, the endpoint initiates a connection from (X:x) to an external address (Y2:y2) and gets the mapping (X2':x2') on the NAT. If (X1':x1') equals (X2':x2') for all values of (Y2:y2) then the NAT is defined to have "Endpoint Independent Mapping" behavior. If (X1':x1') equals (X2':x2') only when Y2 equals Y1 then the NAT is defined to have "Address Dependent Mapping" behavior. If (X1':x1') equals (X2':x2') only when (Y2:y2) equals (Y1:y1), that is for consecutive connections to the same external address (the second connection must be made shortly after the first is terminated), then the NAT is defined to have "Address and Port Dependent Mapping" behavior. If (X1':x1') never equals (X2':x2'), that is for each connection a new mapping is allocated, then the NAT is defined to have "Connection Dependent Mapping" behavior. REQ-1: A NAT MUST have an "Endpoint Independent Mapping" behavior for TCP. Justification: REQ-1 is necessary for UNSAF methods to work. Endpoint independent mapping behavior allows peer-to-peer applications to learn and advertise the external IP address and port allocated to an internal endpoint such that external peers can contact it (subject to the NAT's security policy). The security policy of a NAT is independent of its mapping behavior and is discussed later in Section 4.3. Having endpoint independent mapping behavior allows peer-to-peer applications to work consistently without compromising the security benefits of the NAT. 4.2 Internally Initiated Connections An internal endpoint initiates a TCP connection through a NAT by sending a SYN packet. The NAT allocates (or reuses) a mapping for the connection, as described in the previous section. The mapping defines the external IP address and port used for translation of all packets for that connection. In particular, for client-server applications where an internal client initiates the connection to an external server, the mapping is used to translate the outbound SYN, the resulting inbound SYNACK response, the subsequent outbound ACK, and other packets for the connection. This method of connection initiation corresponds to the 3-way handshake (defined in [RFC0793]) and is supported by all NATs. Peer-to-peer applications use an alternate method of connection initiation termed simultaneous-open (Fig. 8, [RFC0793]) to traverse NATs. In the Simultaneous-Open mode of operation, both peers send SYN packets for the same TCP connection. The SYN packets cross in the network. Upon receiving the other end's SYN packet each end Guha, et al. Expires December 20, 2006 [Page 5] Internet-Draft NAT TCP Requirements June 2006 responds with a SYNACK packet, which also cross in the network. The connection is considered established once the SYNACKs are received. From the perspective of the NAT, the internal host's SYN packet is met by an inbound SYN packet for the same connection (as opposed to a SYNACK packet during a 3-way handshake). Subsequent to this exchange, both an outbound and an inbound SYNACK are seen for the connection. Some NATs erroneously block the inbound SYN for the connection in progress. Some NATs block or incorrectly translate the outbound SYNACK. Such behavior breaks TCP simultaneous-open and prevents peer-to-peer applications from functioning correctly behind a NAT. In order to provide network address translation service for TCP, it is necessary for a NAT to correctly receive, translate, and forward all packets for a connection that conform to valid transitions of the TCP State-Machine (Fig. 6, [RFC0793]). REQ-2: For a TCP connection, a NAT MUST support all valid sequences of TCP packets (defined in [RFC0793]). In particular: a) A NAT MUST accept inbound SYN and subsequent outbound SYNACK packets when a connection initiation is in progress. Justification: The intent of this requirement is to allow standards compliant TCP stacks to traverse NATs no matter what path the stacks take through the TCP state-machine. a) Simultaneous-Open requires the NAT to accept an inbound SYN and an outbound SYNACK for an existing connection that has been initiated by an internal endpoint. 4.3 Externally Initiated Connections The NAT allocates a mapping for the first connection initiated by an internal endpoint to an external endpoint. In some scenarios, the NAT's policy may allow this mapping to be reused for connections initiated from the external side to the internal endpoint. Consider as before an internal IP address and port (X:x) that is assigned (or reuses) a mapping (X1':x1') when it initiates a connection to an external (Y1:y1). An external endpoint (Y2:y2) attempts to initiate a connection with the internal endpoint by sending a SYN to (X1':x1'). A NAT can choose to either allow the connection to be established, or to disallow the connection. If the NAT chooses to allow the connection, it translates the inbound SYN and routes it to (X:x) as per the existing mapping. It also translates the SYNACK generated by (X:x) in response and routes it to (Y2:y2) and so on. Alternately, the NAT can disallow the connection by filtering the inbound SYN. A NAT may allow an existing mapping to be reused by an externally Guha, et al. Expires December 20, 2006 [Page 6] Internet-Draft NAT TCP Requirements June 2006 initiated connection if its security policy permits. Several different policies are possible as described here. If a NAT allows the connection initiation from all (Y2:y2) then it is defined to have "Endpoint Independent Filtering" behavior. If the NAT allows connection initiations only when Y2 equals Y1 then the NAT is defined to have "Address Dependent Filtering" behavior. If the NAT allows connection initiations only when (Y2:y2) equals (Y1:y1), then the NAT is defined to have "Address and Port Dependent Filtering" behavior (possible only after the first connection has been terminated but the mapping is still active). If the NAT does not allow connection initiations from the external side, then the NAT is defined to have "Connection Dependent Filtering" behavior. REQ-3: If application transparency is most important, it is RECOMMENDED that a NAT have an "Endpoint independent filtering" behavior for TCP. If a more stringent filtering behavior is most important, it is RECOMMENDED that a NAT have an "Address dependent filtering" behavior. a) The filtering behavior MAY be an option configurable by the administrator of the NAT. b) The filtering behavior for TCP MAY be independent of the filtering behavior for UDP. Justification: The intent of this requirement is to allow peer-to- peer applications that do not always initiate connections from the internal side of the NAT to continue to work in the presence of NATs. This behavior also allows applications behind a BEHAVE compliant NAT to inter-operate with remote endpoints that are behind non-BEHAVE complaint (legacy) NATs. If the remote endpoint's NAT does not have endpoint independent mapping behavior but has only one external IP address, then an application can still traverse the combination of the two NATs if the local NAT has address dependent filtering. Section 9 contains a detailed discussion on the security implications of this requirement. If the inbound SYN packet is filtered, either because a corresponding mapping does not exist or because of the NAT's filtering behavior, a NAT has two basic choices: to ignore the packet silently, or to signal an error to the sender. Signaling an error through ICMP messages allows the sender to quickly detect that the SYN did not reach the intended destination. Silently dropping the packet, on the other hand, allows applications to perform Simultaneous-Open more reliably. Silently dropping the SYN aids Simultaneous-Open as follows. Consider that the application is attempting a Simultaneous-Open and the outbound SYN from the internal endpoint has not yet crossed the NAT (due to network congestion or clock skew between the two Guha, et al. Expires December 20, 2006 [Page 7] Internet-Draft NAT TCP Requirements June 2006 endpoints); this outbound SYN would otherwise have created the necessary mapping at the NAT to allow translation of the inbound SYN. Since the outbound SYN did not reach the NAT in time, the inbound SYN cannot be processed. If a NAT responds to the premature inbound SYN with an error message that forces the external endpoint to abandon the connection attempt, it hinders applications performing a TCP simultaneous-open. If instead the NAT silently ignores the inbound SYN, the external endpoint retransmits the SYN after a TCP timeout. In the meantime, the NAT creates the mapping in response to the (delayed) outbound SYN such that the retransmitted inbound SYN can be routed and simultaneous-open can succeed. NAT support for simultaneous-open as well as quickly signaling errors are both important for applications. Unfortunately, there is no way for a NAT to signal an error without forcing the endpoint to abort a potential simultaneous-open: TCP RST and ICMP Port Unreachable packets require the endpoint to abort the attempt while ICMP Host and Network Unreachable errors may adversely affect other connections to the same host or network [RFC1122]. In addition, when an unsolicited SYN is received by the NAT, the NAT may not know whether the application is attempting a simultaneous- open (and that it should therefore silently drop the SYN) or whether the SYN is in error (and that it should notify the sender). REQ-4: It is RECOMMENDED that a NAT respond to unsolicited SYN packets with an ICMP Port Unreachable error (Type 3, Code 3). If a NAT does so, it MUST delay the ICMP error by at least 6 seconds. Furthermore, it MUST cancel this delayed ICMP if in that time it receives and translates an outbound SYN for the connection. If a NAT does not have resources to delay the ICMP error or chooses not to send it, the NAT MUST silently drop the unsolicited SYN. Justification: The intent of this requirement is to allow simultaneous-open to work reliably in the presence of NATs as well as to quickly signal an error in case the unsolicited SYN is in error. As of writing this memo, it is not possible to achieve both; the requirement therefore represents a compromise. The NAT should tolerate some delay in the outbound SYN for a TCP simultaneous-open, which may be due to network congestion or loose synchronization between the endpoints. If the unsolicited SYN is not part of a simultaneous-open attempt and is in error, the NAT should endeavor to signal the error in accordance with [RFC1122]. There may, however, be reasons for the NAT to rate-limit such error notifications, for example in the case of an attack. Section 9 mentions the security considerations for this requirement. Guha, et al. Expires December 20, 2006 [Page 8] Internet-Draft NAT TCP Requirements June 2006 OPEN ISSUE: Alternate requirements for REQ-4 Option 1 - REQ-4: The NAT MUST silently drop inbound SYNs Option 2 - REQ-4: A NAT MUST NOT force an end-host to abort a TCP connection that could be a S-O. It SHOULD return an error if the error doesn't force the end to abort the attempt. Otherwise, it MUST silently drop the SYN Option 3 - REQ-4: If enabling P2P TCP apps is most important, a NAT MUST silently drop the SYN. If enabling quick diagnosis of network errors is most important, a NAT SHOULD signal an ICMP port unreachable. The behavior MAY be configurable by the administrator. Option 4 - REQ-4: It is RECOMMENDED that a NAT respond to unsolicited SYN packets with an ICMP Port Unreachable error (Type 3, Code 3). If a NAT does so, it MUST delay the ICMP error by at least 6 seconds unless REQ-4a) applies. Furthermore, it MUST cancel this delayed ICMP if in that time it receives and translates an outbound SYN for the connection. If a NAT does not have resources to delay the ICMP error or chooses not to send it, the NAT MUST silently drop the unsolicited SYN. a) If there is no active mapping that matches the unsolicited SYN, then the NAT SHOULD send the ICMP immediately. 5. NAT Session Refresh A NAT maintains state associated with in-progress and established connections. Because of this, a NAT is susceptible to a resource- exhaustion attack whereby an attacker (or virus) on the internal side attempts to cause the NAT to create more state than it has resources for. To prevent such an attack, a NAT needs to abandon sessions in order to free the state resources. A common method that is applicable only to TCP is to preferentially abandon sessions for crashed endpoints, followed by closed TCP connections and partially-open connections. A NAT can check if an endpoint for a session has crashed by sending a TCP keep-alive packet and receiving a TCP RST packet in response. If the NAT cannot determine whether the endpoint is active, it should not abandon the session until the TCP connection has been idle for some time. Noting that an established TCP connection can stay idle (but live) indefinitely, there is no fixed value for an idle-timeout that accommodates all applications. However, a large idle-timeout motivated by recommendations in [RFC1122] can reduce the chances of abandoning a live session. Guha, et al. Expires December 20, 2006 [Page 9] Internet-Draft NAT TCP Requirements June 2006 A TCP connection passes through three phases: partially-open, established, and closing. During the partially-open phase, endpoints synchronize initial sequence numbers. The phase is initiated by the first SYN for the connection and extends until both endpoints have sent a packet with the ACK flag set (TCP states: SYN_SENT and SYN_RCVD). ACKs in both directions mark the beginning of the established phase where application data can be exchanged indefinitely (TCP states: ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, and CLOSE_WAIT). The closing phase begins when both endpoint have terminated their half of the connection by sending a FIN packet. Once FIN packets are seen in both directions, application data can no longer be exchanged but the stacks still need to ensure that the FIN packets are received (TCP states: CLOSING and LAST_ACK). TCP connections can stay in established phase indefinitely without exchanging any packets. Some end-hosts can be configured to send keep-alive packets on such idle connections; by default, such keep- alive packets are sent every 2 hours if enabled [RFC1122]. Consequently, a NAT that waits for slightly over 2 hours can detect idle connections with keep-alive packets being sent at the default rate. TCP connections in the partially-open or closing phases, on the other hand, can stay idle for at most 4 minutes while waiting for in-flight packets to be delivered [RFC1122]. The "established connection idle-timeout" for a NAT is defined as the minimum time a connection in the established phase must remain idle before the NAT considers the associated session a candidate for removal. The "transitory connection idle-timeout" for a NAT is defined as the minimum time a connection in the partially-open or closing phases must remain idle before the NAT considers the associated session a candidate for removal. REQ-5: If a NAT cannot determine whether the endpoints of a TCP connection are active, it MAY abandon the session if it has been idle for some time. In such cases, the value of the "established connection idle-timeout" MUST NOT be less than 2 hours and 4 minutes (124 minutes). The value of the "transitory connection idle-timeout" MUST NOT be less than 4 minutes. a) The value of the NAT idle-timeouts MAY be configurable. Justification: The intent of this requirement is to minimize the cases where a NAT abandons session state for a live connection. While some NATs may choose to abandon sessions reactively in response to new connection initiations (allowing idle connections to stay up indefinitely in the absence of new initiations), other NATs may choose to proactively reap idle sessions. In cases where the NAT cannot actively determine if the connection is alive, this requirement ensures that applications can send keep-alive packets Guha, et al. Expires December 20, 2006 [Page 10] Internet-Draft NAT TCP Requirements June 2006 at a predefined rate (every 2 hours) such that the NAT can passively determine that the connection is alive. The additional 4 minutes allows time for in-flight packets to cross the NAT. NAT behavior for handling RST packets, or connections in TIME_WAIT state is left unspecified. The handling of non-SYN packets for connections for which there is no active mapping is left unspecified. Such packets may be received if the NAT silently abandons a live connection, or abandons a connection in TIME_WAIT state before the 4 minute TIME_WAIT period expires (Section 2.4, [RFC1644]). The decission to either silently drop such packets or to respond with a TCP RST packet is left up to the implementation. 6. Application Level Gateways Application Level Gateways (ALGs) in certain NATs modify IP addresses and TCP ports embedded inside application protocols. Such ALGs may interfere with UNSAF methods or protocols that try to be NAT-aware and must therefore be used with extreme caution. REQ-6: If a NAT includes ALGs that affect TCP, it is RECOMMENDED that all of those ALGs (except for FTP [RFC0959]) be disabled by default. Justification: The intent of this requirement is to prevents ALGs from interfering with UNSAF methods. The default state of a FTP ALG is left unspecified because of legacy concerns: as of writing this memo, a large fraction of legacy FTP clients do not enable PASV mode by default and require an ALG to traverse NATs. 7. UDP Specific Requirements Also Applicable to TCP A list of general and UDP specific NAT behavioral requirements are described in [BEHAVE-UDP]. The following requirements summarized here are TCP analogues to the corresponding UDP requirements. 7.1 Port Assignment NATs that allow different internal endpoints to simultaneously use the same mapping are defined to have a "Port assignment" behavior of "Port overloading". Such behavior prevents two internal endpoints sharing the same mapping from establishing simultaneous connections to a common external endpoint. Some applications expect the source TCP port to be in the well-known range (TCP ports from 0 to 1023). The "r" series of commands (rsh, Guha, et al. Expires December 20, 2006 [Page 11] Internet-Draft NAT TCP Requirements June 2006 rcp, rlogin, etc.) are an example. [FIXME: Need Citation] REQ-7 A NAT MUST NOT have a "Port assignment" behavior of "Port overloading" for TCP. a) If the host's source port was in the range 1-1023, it is RECOMMENDED the NAT's source port be in the same range. If the host's source port was in the range 1024-65535, it is RECOMMENDED that the NAT's source port be in that range. Justification: This requirement allows two applications on the internal side of the NAT to consistently communicate with the same destination. a) Certain applications expect the source TCP port to be in the well-known range. 7.2 Hairpinning Behavior NATs that forward packets originating from an internal address, destined for an external address that matches the active mapping for an internal address, back to that internal address are defined to support "hairpinning". If the NAT presents the hairpinned packet with an external source IP address and port (i.e. the mapped source address and port of the originating internal endpoint), then it is defined to have "External source IP address and port" for hairpinning. Hairpinning is necessary to allow two internal endpoints (known to each other only by their external mapped addresses) to communicate with each other. "External source IP address and port" behavior for hairpining avoids confusing implementations that expect the external source address and port. REQ-8: A NAT MUST support "Hairpinning" for TCP. a) A NAT Hairpinning behavior MUST be "External source IP address and port". Justification: This requirement allows two applications behind the same NAT that are trying to communicate with each other using their external addresses. a) Using the external source address and port for the hairpinned packet is necessary for applications that do not expect to receive a packet from a different address than the external address they are trying to communicate with. 7.3 ICMP Responses to TCP Packets ICMP responses are used by end-host TCP stacks for Path MTU Discovery and for quick error detection. ICMP messages SHOULD be rewritten by the NAT (specifically the IP headers and the ICMP payload) and forwarded to the appropriate internal or external host. Further, Guha, et al. Expires December 20, 2006 [Page 12] Internet-Draft NAT TCP Requirements June 2006 blocking any ICMP message is discouraged. REQ-9: Receipt of any sort of ICMP message MUST NOT terminate the NAT mapping or TCP connection for which the ICMP was generated. a) The NAT's default configuration SHOULD NOT filter ICMP messages based on their source IP address. Justification: This is necessary for reliably performing TCP simultaneous-open where a remote NAT may temporarily signal an ICMP error. It is also useful for MTU discovery. 8. Requirements A NAT that supports all of the mandatory requirements of this specification (i.e., the "MUST") and is compliant with [BEHAVE-UDP], is "compliant with this specification." A NAT that supports all of the requirements of this specification (i.e., included the "RECOMMENDED") and is fully compliant with [BEHAVE-UDP] is "fully compliant with all the mandatory and recommended requirements of this specification." REQ-1: A NAT MUST have an "Endpoint Independent Mapping" behavior for TCP. REQ-2: For a TCP connection, a NAT MUST support all valid sequences of TCP packets (defined in [RFC0793]). In particular: a) A NAT MUST accept inbound SYN and subsequent outbound SYNACK packets when a connection initiation is in progress. REQ-3: If application transparency is most important, it is RECOMMENDED that a NAT have an "Endpoint independent filtering" behavior for TCP. If a more stringent filtering behavior is most important, it is RECOMMENDED that a NAT have an "Address dependent filtering" behavior. a) The filtering behavior MAY be an option configurable by the administrator of the NAT. b) The filtering behavior for TCP MAY be independent of the filtering behavior for UDP. REQ-4: It is RECOMMENDED that a NAT respond to unsolicited SYN packets with an ICMP Port Unreachable error (Type 3, Code 3). If a NAT does so, it MUST delay the ICMP error by at least 6 seconds. Furthermore, it MUST cancel this delayed ICMP if in that time it receives and translates an outbound SYN for the connection. If a NAT does not have resources to delay the ICMP error or chooses not to send it, the NAT MUST silently drop the unsolicited SYN. REQ-5: If a NAT cannot determine whether the endpoints of a TCP connection are active, it MAY abandon the session if it has been idle for some time. In such cases, the value of the "established connection idle-timeout" MUST NOT be less than 2 hours and 4 minutes (124 minutes). The value of the "transitory connection Guha, et al. Expires December 20, 2006 [Page 13] Internet-Draft NAT TCP Requirements June 2006 idle-timeout" MUST NOT be less than 4 minutes. a) The value of the NAT idle-timeouts MAY be configurable. REQ-6: If a NAT includes ALGs that affect TCP, it is RECOMMENDED that all of those ALGs (except for FTP [RFC0959]) be disabled by default. REQ-7 A NAT MUST NOT have a "Port assignment" behavior of "Port overloading" for TCP. a) If the host's source port was in the range 1-1023, it is RECOMMENDED the NAT's source port be in the same range. If the host's source port was in the range 1024-65535, it is RECOMMENDED that the NAT's source port be in that range. REQ-8: A NAT MUST support "Hairpinning" for TCP. a) A NAT Hairpinning behavior MUST be "External source IP address and port". REQ-9: Receipt of any sort of ICMP message MUST NOT terminate the NAT mapping or TCP connection for which the ICMP was generated. a) The NAT's default configuration SHOULD NOT filter ICMP messages based on their source IP address. 9. Security considerations Concerns specific to handling TCP packets are discussed in this section. Security considerations for REQ-1: This requirement does not introduce any TCP-specific concerns. Security considerations for REQ-2: This requirement does not introduce any security concerns. Simultaneous-open and other transitions in the TCP state machine are by-design and necessary for TCP to work correctly in all scenarios. Further, this requirement only affects connections already in progress as authorized by the NAT in accordance with its policy. Security considerations for REQ-3: The security provided by the NAT is governed by its filtering behavior as addressed in [BEHAVE- UDP]. Connection dependent filtering behavior is most secure from a firewall perspective, but severely restricts connection initiations through a NAT. Endpoint independent filtering behavior, which is most transparent to applications, requires an attacker to guess the IP address and port of an active mapping in order to get his packet to an internal host. Address dependent filtering, on the other hand, is less transparent than endpoint independent filtering but more transparent than connection dependent filtering; it is more secure than endpoint independent filtering as it requires an attacker to additionally guess the address of the external endpoint for a NAT session associated with the mapping and be able to receive packets addressed to the same. While this protects against most attackers on the Internet, it does not necessarily protect against attacks that originate from Guha, et al. Expires December 20, 2006 [Page 14] Internet-Draft NAT TCP Requirements June 2006 behind a remote NAT with a single IP address that is also translating a legitimate connection to the victim. Security considerations for REQ-4: This document recommends a NAT to respond to unsolicited inbound SYN packets with an ICMP error delayed by a few seconds. Doing so may reveal the presence of a NAT to an external attacker. Silently dropping the SYN makes it harder to diagnose network problems and forces applications to wait for the TCP stack to finish several retransmissions before reporting an error. An implementer must therefore understand and carefully weigh the effects of not sending an ICMP error or rate- limiting such ICMP errors to a very small number. Security considerations for REQ-5: This document recommends that a NAT that passively monitors TCP state keep idle sessions alive for at least 2 hours and 4 minutes or 4 minutes depending on the state of the connection. If a NAT is under attack and cannot actively determine the liveness of a TCP connection, the NAT administrator may configure more conservative timeouts. Security considerations for REQ-6: This requirement does not introduce any TCP-specific concerns. Security considerations for REQ-7: This requirement does not introduce any TCP-specific concerns. Security considerations for REQ-8: This requirement does not introduce any TCP-specific concerns. Security considerations for REQ-9: This requirement does not introduce any TCP-specific concerns. NAT implementations that change local state based on TCP flags in packets must ensure that out-of-window TCP packets are properly handled. [TCP-ANTISPOOF] summarizes and discusses a variety of solutions designed to prevent attackers from affecting TCP connections. 10. IANA considerations This document does not change or create any IANA-registered values. 11. Acknowledgments Thanks to Mark Allman, Francois Audet, Paul Hoffman, Dave Hudson, Cullen Jennings, Philip Matthews, Tom Petch, Joe Touch, and Dan Wing for their contributions. 12. References 12.1 Normative References [BEHAVE-UDP] Audet, F. and C. Jennings, "NAT Behavioral Requirements Guha, et al. Expires December 20, 2006 [Page 15] Internet-Draft NAT TCP Requirements June 2006 for Unicast UDP", draft-ietf-behave-nat-udp (work in progress). [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC 959, October 1985. [RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. 12.2 Informational References [NATBLASTER] Biggadike, A., Ferullo, D., Wilson, G., and A. Perrig, "NATBLASTER: Establishing TCP connections between hosts behind NATs", Proceedings of the ACM SIGCOMM Asia Workshop (Beijing, China), April 2005. [P2PNAT] Ford, B., Srisuresh, P., and D. Kegel, "Peer-to-peer communication across network address translators", Proceedings of the USENIX Annual Technical Conference (Anaheim, CA), April 2005. [RFC1644] Braden, B., "T/TCP -- TCP Extensions for Transactions Functional Specification", RFC 1644, July 1994. [STUNT] Guha, S. and P. Francis, "NUTSS: A SIP based approach to UDP and TCP connectivity", Proceedings of the ACM SIGCOMM Workshop on Future Directions in Network Architecture (Portland, OR), August 2004. [TCP-ANTISPOOF] Touch, J., "Defending TCP Against Spoofing Attacks", draft-ietf-tcpm-tcp-antispoof (work in progress). [TCP-ROADMAP] Duke, M., Braden, R., Eddy, W., and E. Blanton, "A Roadmap for TCP Specification Documents", draft-ietf-tcpm-tcp-roadmap (work in progress). Guha, et al. Expires December 20, 2006 [Page 16] Internet-Draft NAT TCP Requirements June 2006 [TCPTRAV] Guha, S. and P. Francis, "Characterization and Measurement of TCP Traversal through NATs and Firewalls", Proceedings of the Internet Measurement Conference (Berkeley, CA), October 2005. Authors' Addresses Saikat Guha (editor) Cornell University 331 Upson Hall Ithaca, NY 14853 US Phone: +1 607 255 1008 Email: saikat@cs.cornell.edu Kaushik Biswas Cisco Systems, Inc. 170 West Tasman Dr. San Jose, CA 95134 US Phone: +1 408 525 5134 Email: kbiswas@cisco.com Bryan Ford M.I.T. Laboratory for Computer Science 77 Massachusetts Ave. Cambridge, MA 02139 US Phone: +1 617 253 5261 Email: baford@mit.edu Paul Francis Cornell University 4108 Upson Hall Ithaca, NY 14853 US Phone: +1 607 255 9223 Email: francis@cs.cornell.edu Guha, et al. Expires December 20, 2006 [Page 17] Internet-Draft NAT TCP Requirements June 2006 Senthil Sivakumar Cisco Systems, Inc. 7100-8 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709-4987 US Phone: +1 919 392 5158 Email: ssenthil@cisco.com Pyda Srisuresh Consultant 20072 Pacifica Dr. Cupertino, CA 95014 US Phone: +1 408 836 4773 Email: srisuresh@yahoo.com Guha, et al. Expires December 20, 2006 [Page 18] Internet-Draft NAT TCP Requirements June 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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Disclaimer of Validity 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. Copyright Statement Copyright (C) The Internet Society (2006). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Guha, et al. Expires December 20, 2006 [Page 19]