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rfc:rfc5382

Network Working Group S. Guha, Ed. Request for Comments: 5382 Cornell U. BCP: 142 K. Biswas Category: Best Current Practice Cisco Systems

                                                               B. Ford
                                                               MPI-SWS
                                                          S. Sivakumar
                                                         Cisco Systems
                                                          P. Srisuresh
                                                        Kazeon Systems
                                                          October 2008
                NAT Behavioral Requirements for TCP

Status of This Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Abstract

 This document defines a set of requirements for NATs that handle TCP
 that would allow many applications, such as peer-to-peer applications
 and online games to work consistently.  Developing NATs that meet
 this set of requirements will greatly increase the likelihood that
 these applications will function properly.

Guha, et al. Best Current Practice [Page 1] RFC 5382 NAT TCP Requirements October 2008

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  . . . . . . . . . . . .  5
   4.2.  Internally Initiated Connections . . . . . . . . . . . . .  5
   4.3.  Externally Initiated Connections . . . . . . . . . . . . .  7
 5.  NAT Session Refresh  . . . . . . . . . . . . . . . . . . . . . 10
 6.  Application Level Gateways . . . . . . . . . . . . . . . . . . 12
 7.  Other Requirements Applicable to TCP . . . . . . . . . . . . . 12
   7.1.  Port Assignment  . . . . . . . . . . . . . . . . . . . . . 12
   7.2.  Hairpinning Behavior . . . . . . . . . . . . . . . . . . . 13
   7.3.  ICMP Responses to TCP Packets  . . . . . . . . . . . . . . 13
 8.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 14
 9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
 10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
   11.2. Informational References . . . . . . . . . . . . . . . . . 18

Guha, et al. Best Current Practice [Page 2] RFC 5382 NAT TCP Requirements October 2008

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.
 Middlebox behavior that is not necessary for network address
 translation of TCP is out of scope.  Packet inspection above the TCP
 layer and firewalls are out of scope except for Application Level
 Gateway (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 Middlebox
 Communication (MIDCOM) and Universal Plug and Play (UPnP), that
 directly control the NAT are out of scope.  Finally, TCP connections
 intended for the NAT (e.g., an HTTP or Secure Shell Protocol (SSH)
 management interface) and TCP connections initiated by the NAT (e.g.,
 reliable syslog client) are out of scope.

2. Introduction

 Network Address Translators (NATs) hinder connectivity in
 applications where sessions may be initiated to internal hosts.
 Readers may refer to [RFC3022] for detailed information on
 traditional NATs.  [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.
 [RFC4614] 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].

Guha, et al. Best Current Practice [Page 3] RFC 5382 NAT TCP Requirements October 2008

 Recently, many techniques have been devised to make peer-to-peer TCP
 applications work across NATs.  [STUNT], [NATBLASTER], and [P2PNAT]
 describe Unilateral Self-Address Fixing (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
 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.

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].
 "NAT" in this specification includes both "Basic NAT" and "Network
 Address/Port Translator (NAPT)" [RFC2663].  The term "NAT Session" is
 adapted from [NAT-MIB] and is defined as follows.
 NAT Session - A NAT session is an association between a TCP session
 as seen in the internal realm and a TCP session as seen in the
 external realm, by virtue of NAT translation.  The NAT session will
 provide the translation glue between the two session representations.
 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 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 assignment", "Port overloading", "hairpinning", and
 "External source IP address and port" as defined in [BEHAVE-UDP].

4. TCP Connection Initiation

 This section describes various NAT behaviors applicable to TCP
 connection initiation.

Guha, et al. Best Current Practice [Page 4] RFC 5382 NAT TCP Requirements October 2008

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 in [BEHAVE-UDP].
 Consider an internal IP address and TCP port (X:x) that initiates a
 TCP connection to an external (Y1:y1) tuple.  Let the mapping
 allocated by the NAT for this connection be (X1':x1').  Shortly
 thereafter, the endpoint initiates a connection from the same (X:x)
 to an external address (Y2:y2) and gets the mapping (X2':x2') on the
 NAT.  As per [BEHAVE-UDP], 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), possible only for consecutive connections to the same
 external address shortly after the first is terminated and if the NAT
 retains state for connections in TIME_WAIT state, then the NAT is
 defined to have "Address and Port-Dependent Mapping" behavior.  This
 document introduces one additional behavior where (X1':x1') never
 equals (X2':x2'), that is, for each connection a new mapping is
 allocated; in such a case, 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

Guha, et al. Best Current Practice [Page 5] RFC 5382 NAT TCP Requirements October 2008

 applications where an internal client initiates the connection to an
 external server, the mapping is used to translate the outbound SYN,
 the resulting inbound SYN-ACK 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
 responds with a SYN-ACK packet, which also cross in the network.  The
 connection is considered established once the SYN-ACKs 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
 SYN-ACK packet during a 3-way handshake).  Subsequent to this
 exchange, both an outbound and an inbound SYN-ACK 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 SYN-ACK.  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:  A NAT MUST support all valid sequences of TCP packets
    (defined in [RFC0793]) for connections initiated both internally
    as well as externally when the connection is permitted by the NAT.
    In particular:
    a) In addition to handling the TCP 3-way handshake mode of
       connection initiation, A NAT MUST handle the TCP simultaneous-
       open mode of connection initiation.
 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 and no matter which end
    initiates the connection as long as the connection is permitted by
    the filtering policy of the NAT (filtering policy is described in
    the following section).
    a) In addition to TCP packets for a 3-way handshake, A NAT must be
       prepared to accept an inbound SYN and an outbound SYN-ACK for
       an internally initiated connection in order to support
       simultaneous-open.

Guha, et al. Best Current Practice [Page 6] RFC 5382 NAT TCP Requirements October 2008

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 SYN-ACK
 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
 initiated connection if its security policy permits.  Several
 different policies are possible as described in [BEHAVE-UDP].  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 shortly after the first connection
 has been terminated but the mapping is still active).  One additional
 filtering behavior defined in this document is when the NAT does not
 allow any connection initiations from the external side; in such
 cases, the NAT is defined to have "Connection-Dependent Filtering"
 behavior.  The difference between "Address and Port-Dependent
 Filtering" and "Connection-Dependent Filtering" behavior is that the
 former permits an inbound SYN during the TIME_WAIT state of the first
 connection to initiate a new connection while the latter does not.
 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.

Guha, et al. Best Current Practice [Page 7] RFC 5382 NAT TCP Requirements October 2008

 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 compliant (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
 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.  The downside to this
 behavior is that in the event the inbound SYN is erroneous, the
 remote side does not learn of the error until after several TCP
 timeouts.
 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 the ICMP Host
 and Network Unreachable errors may adversely affect other connections
 to the same host or network [RFC1122].

Guha, et al. Best Current Practice [Page 8] RFC 5382 NAT TCP Requirements October 2008

 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:  A NAT MUST NOT respond to an unsolicited inbound SYN packet
    for at least 6 seconds after the packet is received.  If during
    this interval the NAT receives and translates an outbound SYN for
    the connection the NAT MUST silently drop the original unsolicited
    inbound SYN packet.  Otherwise, the NAT SHOULD send an ICMP Port
    Unreachable error (Type 3, Code 3) for the original SYN, unless
    REQ-4a applies.
    a) The NAT MUST silently drop the original SYN packet if sending a
       response violates the security policy of the NAT.
 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].
    a) There may, however, be reasons for the NAT to rate-limit or
       omit such error notifications, for example, in the case of an
       attack.  Silently dropping the SYN packet when under attack
       allows simultaneous-open to work without consuming any extra
       network bandwidth or revealing the presence of the NAT to
       attackers.  Section 9 mentions the security considerations for
       this requirement.
 For NATs that combine NAT functionality with end-host functionality
 (e.g., an end-host that also serves as a NAT for other hosts behind
 it), REQ-4 above applies only to SYNs intended for the NAT'ed hosts
 and not to SYNs intended for the NAT itself.  One way to determine
 whether the inbound SYN is intended for a NAT'ed host is to allocate
 NAT mappings from one port range, and allocate ports for local
 endpoints from a different non-overlapping port range.  More dynamic
 implementations can be imagined.

Guha, et al. Best Current Practice [Page 9] RFC 5382 NAT TCP Requirements October 2008

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 for which it has
 resources.  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.  Note
 that an established TCP connection can stay idle (but live)
 indefinitely; hence, 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.
 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 endpoints 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].

Guha, et al. Best Current Practice [Page 10] RFC 5382 NAT TCP Requirements October 2008

 The "established connection idle-timeout" for a NAT is defined as the
 minimum time a TCP 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 TCP connection in the partially open or
 closing phases must remain idle before the NAT considers the
 associated session a candidate for removal.  TCP connections in the
 TIME_WAIT state are not affected by the "transitory connection idle-
 timeout".
 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 4 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
    at the default 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.  A NAT MAY hold state for a connection in
 TIME_WAIT state to accommodate retransmissions of the last ACK.
 However, since the TIME_WAIT state is commonly encountered by
 internal endpoints properly closing the TCP connection, holding state
 for a closed connection may limit the throughput of connections
 through a NAT with limited resources.  [RFC1337] describes hazards
 associated with TIME_WAIT assassination.
 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.  The
 decision to either silently drop such packets or to respond with a
 TCP RST packet is left up to the implementation.

Guha, et al. Best Current Practice [Page 11] RFC 5382 NAT TCP Requirements October 2008

 NAT behavior for notifying endpoints when abandoning live connections
 is left unspecified.  When a NAT abandons a live connection, for
 example due to a timeout expiring, the NAT MAY either send TCP RST
 packets to the endpoints or MAY silently abandon the connection.
 Sending a RST notification allows endpoint applications to recover
 more quickly; however, notifying the endpoints may not always be
 possible if, for example, session state is lost due to a power
 failure.

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 prevent ALGs
    from interfering with UNSAF methods.  The default state of an FTP
    ALG is left unspecified because of legacy concerns: as of writing
    this memo, a large fraction of legacy FTP clients do not enable
    passive (PASV) mode by default and require an ALG to traverse
    NATs.

7. Other Requirements Applicable to TCP

 A list of general and UDP-specific NAT behavioral requirements are
 described in [BEHAVE-UDP].  A list of ICMP-specific NAT behavioral
 requirements are described in [BEHAVE-ICMP].  The requirements listed
 below reiterate the requirements from these two documents that
 directly affect TCP.  The following requirements do not relax any
 requirements in [BEHAVE-UDP] or [BEHAVE-ICMP].

7.1. Port Assignment

 NATs that allow different internal endpoints to simultaneously use
 the same mapping are defined in [BEHAVE-UDP] to have a "Port
 assignment" behavior of "Port overloading".  Such behavior is
 undesirable, as it prevents two internal endpoints sharing the same
 mapping from establishing simultaneous connections to a common
 external endpoint.
 REQ-7:  A NAT MUST NOT have a "Port assignment" behavior of "Port
    overloading" for TCP.

Guha, et al. Best Current Practice [Page 12] RFC 5382 NAT TCP Requirements October 2008

 Justification:  This requirement allows two applications on the
    internal side of the NAT to consistently communicate with the same
    destination.
 NAT behavior for preserving the source TCP port range for connections
 is left unspecified.  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, rcp, rlogin, etc.) are an example.  NATs
 that preserve the range from which the source port is picked allow
 such applications to function properly through the NAT; however, by
 doing so the NAT may compromise the security of the application in
 certain situations; applications that depend only on the IP address
 and source TCP port range for security (the "r" commands, for
 example) cannot distinguish between an attacker and a legitimate user
 behind the same NAT.

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 in
 [BEHAVE-UDP] as supporting "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 hairpinning avoids confusing
 implementations that expect the external source IP address and port.
 REQ-8:  A NAT MUST support "hairpinning" for TCP.
    a) A NAT's hairpinning behavior MUST be of type "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

 Several TCP mechanisms depend on the reception of ICMP error messages
 triggered by the transmission of TCP segments.  One such mechanism is
 path MTU discovery [RFC1191], which is required for the correct

Guha, et al. Best Current Practice [Page 13] RFC 5382 NAT TCP Requirements October 2008

 operation of TCP.  The current path MTU discovery mechanism requires
 the sender of TCP segments to be notified of ICMP "Datagram Too Big"
 responses.
 REQ-9:  If a NAT translates TCP, it SHOULD translate ICMP Destination
    Unreachable (Type 3) messages.
 Justification:  Translating ICMP Destination Unreachable messages,
    particularly the "Fragmentation Needed and Don't Fragment was Set"
    (Type 3, Code 4) message avoids communication failures ("black
    holes" [RFC2923]).  Furthermore, TCP's connection establishment
    and maintenance mechanisms also behave much more efficiently when
    ICMP Destination Unreachable messages arrive in response to
    outgoing TCP segments.
 REQ-10:  Receipt of any sort of ICMP message MUST NOT terminate the
    NAT mapping or TCP connection for which the ICMP was generated.
 Justification:  This is necessary for reliably performing TCP
    simultaneous-open where a remote NAT may temporarily signal an
    ICMP error.

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:  A NAT MUST support all valid sequences of TCP packets
    (defined in [RFC0793]) for connections initiated both internally
    as well as externally when the connection is permitted by the NAT.
    In particular:
    a) In addition to handling the TCP 3-way handshake mode of
       connection initiation, A NAT MUST handle the TCP simultaneous-
       open mode of connection initiation.
 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.

Guha, et al. Best Current Practice [Page 14] RFC 5382 NAT TCP Requirements October 2008

    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:  A NAT MUST NOT respond to an unsolicited inbound SYN packet
    for at least 6 seconds after the packet is received.  If during
    this interval the NAT receives and translates an outbound SYN for
    the connection the NAT MUST silently drop the original unsolicited
    inbound SYN packet.  Otherwise, the NAT SHOULD send an ICMP Port
    Unreachable error (Type 3, Code 3) for the original SYN, unless
    REQ-4a applies.
    a) The NAT MUST silently drop the original SYN packet if sending a
       response violates the security policy of the NAT.
 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 4 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.
 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.
 The following requirements reiterate requirements from [BEHAVE-UDP]
 or [BEHAVE-ICMP] that directly affect TCP.  This document does not
 relax any requirements in [BEHAVE-UDP] or [BEHAVE-ICMP].
 REQ-7:  A NAT MUST NOT have a "Port assignment" behavior of "Port
    overloading" for TCP.
 REQ-8:  A NAT MUST support "hairpinning" for TCP.
    a) A NAT's hairpinning behavior MUST be of type "External source
       IP address and port".
 REQ-9:  If a NAT translates TCP, it SHOULD translate ICMP Destination
    Unreachable (Type 3) messages.
 REQ-10:  Receipt of any sort of ICMP message MUST NOT terminate the
    NAT mapping or TCP connection for which the ICMP was generated.

Guha, et al. Best Current Practice [Page 15] RFC 5382 NAT TCP Requirements October 2008

9. Security Considerations

 [BEHAVE-UDP] discusses security considerations for NATs that handle
 IP and unicast UDP traffic.  Security concerns specific to handling
 TCP packets are discussed in this section.
 Security considerations for REQ-1:  This requirement does not
    introduce any TCP-specific security concerns.
 Security considerations for REQ-2:  This requirement does not
    introduce any TCP-specific 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 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 that a
    NAT 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.

Guha, et al. Best Current Practice [Page 16] RFC 5382 NAT TCP Requirements October 2008

 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 4 minutes or 4 minutes depending on the state of
    the connection.  If a NAT is under attack, it may attempt to
    actively determine the liveliness of a TCP connection or let the
    NAT administrator configure more conservative timeouts.
 Security considerations for REQ-6:  This requirement does not
    introduce any TCP-specific security concerns.
 Security considerations for REQ-7:  This requirement does not
    introduce any TCP-specific security concerns.
 Security considerations for REQ-8:  This requirement does not
    introduce any TCP-specific security concerns.
 Security considerations for REQ-9:  This requirement does not
    introduce any TCP-specific security concerns.
 Security considerations for REQ-10:  This requirement does not
    introduce any TCP-specific security concerns.
 NAT implementations that modify TCP sequence numbers (e.g., for
 privacy reasons or for ALG support) must ensure that TCP packets with
 Selective Acknowledgement (SACK) notifications [RFC2018] are properly
 handled.
 NAT implementations that modify local state based on TCP flags in
 packets must ensure that out-of-window TCP packets are properly
 handled.  [RFC4953] summarizes and discusses a variety of solutions
 designed to prevent attackers from affecting TCP connections.

10. Acknowledgments

 Joe Touch contributed the mechanism for handling unsolicited inbound
 SYNs.  Thanks to Mark Allman, Francois Audet, Lars Eggert, Paul
 Francis, Fernando Gont, Sam Hartman, Paul Hoffman, Dave Hudson,
 Cullen Jennings, Philip Matthews, Tom Petch, Magnus Westerlund, and
 Dan Wing for their many contributions, comments, and suggestions.

Guha, et al. Best Current Practice [Page 17] RFC 5382 NAT TCP Requirements October 2008

11. References

11.1. Normative References

 [BEHAVE-UDP]   Audet, F. and C. Jennings, "Network Address
                Translation (NAT) Behavioral Requirements for Unicast
                UDP", BCP 127, RFC 4787, January 2007.
 [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.
 [RFC1191]      Mogul, J. and S. Deering, "Path MTU discovery",
                RFC 1191, November 1990.
 [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

11.2. Informational References

 [BEHAVE-ICMP]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha,
                "NAT Behavioral Requirements for ICMP protocol", Work
                in Progress, June 2008.
 [NAT-MIB]      Rohit, R., Srisuresh, P., Raghunarayan, R., Pai, N.,
                and C. Wang, "Definitions of Managed Objects for
                Network Address Translators (NAT)", RFC 4008,
                March 2005.
 [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.
 [RFC1337]      Braden, B., "TIME-WAIT Assassination Hazards in TCP",
                RFC 1337, May 1992.

Guha, et al. Best Current Practice [Page 18] RFC 5382 NAT TCP Requirements October 2008

 [RFC1644]      Braden, B., "T/TCP -- TCP Extensions for Transactions
                Functional Specification", RFC 1644, July 1994.
 [RFC2018]      Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow,
                "TCP Selective Acknowledgment Options", RFC 2018,
                October 1996.
 [RFC2663]      Srisuresh, P. and M. Holdrege, "IP Network Address
                Translator (NAT) Terminology and Considerations",
                RFC 2663, August 1999.
 [RFC2923]      Lahey, K., "TCP Problems with Path MTU Discovery",
                RFC 2923, September 2000.
 [RFC3022]      Srisuresh, P. and K. Egevang, "Traditional IP Network
                Address Translator (Traditional NAT)", RFC 3022,
                January 2001.
 [RFC4614]      Duke, M., Braden, R., Eddy, W., and E. Blanton, "A
                Roadmap for Transmission Control Protocol (TCP)
                Specification Documents", RFC 4614, September 2006.
 [RFC4953]      Touch, J., "Defending TCP Against Spoofing Attacks",
                RFC 4953, July 2007.
 [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.
 [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.

Guha, et al. Best Current Practice [Page 19] RFC 5382 NAT TCP Requirements October 2008

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
 Max Planck Institute for Software Systems
 Campus Building E1 4
 D-66123 Saarbruecken
 Germany
 Phone: +49-681-9325657
 EMail: baford@mpi-sws.org
 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
 Kazeon Systems, Inc.
 1161 San Antonio Rd.
 Mountain View, CA  94043
 US
 Phone: +1 408 836 4773
 EMail: srisuresh@yahoo.com

Guha, et al. Best Current Practice [Page 20] RFC 5382 NAT TCP Requirements October 2008

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Guha, et al. Best Current Practice [Page 21]

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