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rfc:bcp:bcp127

[Note that this file is a concatenation of more than one RFC.]

Network Working Group F. Audet, Ed. Request for Comments: 4787 Nortel Networks BCP: 127 C. Jennings Category: Best Current Practice Cisco Systems

                                                          January 2007
     Network Address Translation (NAT) Behavioral Requirements
                          for Unicast UDP

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.

Copyright Notice

 Copyright (C) The IETF Trust (2007).

Abstract

 This document defines basic terminology for describing different
 types of Network Address Translation (NAT) behavior when handling
 Unicast UDP and also defines a set of requirements that would allow
 many applications, such as multimedia communications or online
 gaming, to work consistently.  Developing NATs that meet this set of
 requirements will greatly increase the likelihood that these
 applications will function properly.

Audet & Jennings Best Current Practice [Page 1] RFC 4787 NAT UDP Unicast Requirements January 2007

Table of Contents

 1.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  3
 2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
 3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
 4.  Network Address and Port Translation Behavior  . . . . . . . .  5
   4.1.  Address and Port Mapping . . . . . . . . . . . . . . . . .  5
   4.2.  Port Assignment  . . . . . . . . . . . . . . . . . . . . .  9
     4.2.1.  Port Assignment Behavior . . . . . . . . . . . . . . .  9
     4.2.2.  Port Parity  . . . . . . . . . . . . . . . . . . . . . 11
     4.2.3.  Port Contiguity  . . . . . . . . . . . . . . . . . . . 11
   4.3.  Mapping Refresh  . . . . . . . . . . . . . . . . . . . . . 12
   4.4.  Conflicting Internal and External IP Address Spaces  . . . 13
 5.  Filtering Behavior . . . . . . . . . . . . . . . . . . . . . . 15
 6.  Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . . 16
 7.  Application Level Gateways . . . . . . . . . . . . . . . . . . 17
 8.  Deterministic Properties . . . . . . . . . . . . . . . . . . . 18
 9.  ICMP Destination Unreachable Behavior  . . . . . . . . . . . . 19
 10. Fragmentation of Outgoing Packets  . . . . . . . . . . . . . . 20
 11. Receiving Fragmented Packets . . . . . . . . . . . . . . . . . 20
 12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 21
 13. Security Considerations  . . . . . . . . . . . . . . . . . . . 24
 14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 25
 15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 26
 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   16.1. Normative References . . . . . . . . . . . . . . . . . . . 26
   16.2. Informative References . . . . . . . . . . . . . . . . . . 26

Audet & Jennings Best Current Practice [Page 2] RFC 4787 NAT UDP Unicast Requirements January 2007

1. Applicability Statement

 The purpose of this specification is to define a set of requirements
 for NATs that would allow many applications, such as multimedia
 communications or online gaming, to work consistently.  Developing
 NATs that meet this set of requirements will greatly increase the
 likelihood that these applications will function properly.
 The requirements of this specification apply to Traditional NATs as
 described in [RFC2663].
 This document is meant to cover NATs of any size, from small
 residential NATs to large Enterprise NATs.  However, it should be
 understood that Enterprise NATs normally provide much more than just
 NAT capabilities; for example, they typically provide firewall
 functionalities.  A comprehensive description of firewall behaviors
 and associated requirements is specifically out-of-scope for this
 specification.  However, this specification does cover basic firewall
 aspects present in NATs (see Section 5).
 Approaches using directly signaled control of middle boxes are out of
 scope.
 UDP Relays (e.g., Traversal Using Relay NAT [TURN]) are out of scope.
 Application aspects are out of scope, as the focus here is strictly
 on the NAT itself.
 This document only covers aspects of NAT traversal related to Unicast
 UDP [RFC0768] over IP [RFC0791] and their dependencies on other
 protocols.

2. Introduction

 Network Address Translators (NATs) are well known to cause very
 significant problems with applications that carry IP addresses in the
 payload (see [RFC3027]).  Applications that suffer from this problem
 include Voice Over IP and Multimedia Over IP (e.g., SIP [RFC3261] and
 H.323 [ITU.H323]), as well as online gaming.
 Many techniques are used to attempt to make realtime multimedia
 applications, online games, and other applications work across NATs.
 Application Level Gateways [RFC2663] are one such mechanism.  STUN
 [RFC3489bis] describes a UNilateral Self-Address Fixing (UNSAF)
 mechanism [RFC3424].  Teredo [RFC4380] describes an UNSAF mechanism
 consisting of tunnelling IPv6 [RFC2460] over UDP/IPv4.  UDP Relays
 have also been used to enable applications across NATs, but these are
 generally seen as a solution of last resort.  Interactive

Audet & Jennings Best Current Practice [Page 3] RFC 4787 NAT UDP Unicast Requirements January 2007

 Connectivity Establishment [ICE] describes a methodology for using
 many of these techniques and avoiding a UDP relay, unless the type of
 NAT is such that it forces the use of such a UDP relay.  This
 specification defines requirements for improving NATs.  Meeting these
 requirements ensures that applications will not be forced to use UDP
 relay.
 As pointed out in UNSAF [RFC3424], "From observations of deployed
 networks, it is clear that different NAT box implementations vary
 widely in terms of how they handle different traffic and addressing
 cases".  This wide degree of variability is one factor in the overall
 brittleness introduced by NATs and makes it extremely difficult to
 predict how any given protocol will behave on a network traversing
 NAT.  Discussions with many of the major NAT vendors have made it
 clear that they would prefer to deploy NATs that were deterministic
 and caused the least harm to applications while still meeting the
 requirements that caused their customers to deploy NATs in the first
 place.  The problem NAT vendors face is that they are not sure how
 best to do that or how to document their NATs' behavior.
 The goals of this document are to define a set of common terminology
 for describing the behavior of NATs and to produce a set of
 requirements on a specific set of behaviors for NATs.
 This document forms a common set of requirements that are simple and
 useful for voice, video, and games, which can be implemented by NAT
 vendors.  This document will simplify the analysis of protocols for
 deciding whether or not they work in this environment and will allow
 providers of services that have NAT traversal issues to make
 statements about where their applications will work and where they
 will not, as well as to specify their own NAT requirements.

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].
 Readers are urged to refer to [RFC2663] for information on NAT
 taxonomy and terminology.  Traditional NAT is the most common type of
 NAT device deployed.  Readers may refer to [RFC3022] 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

Audet & Jennings Best Current Practice [Page 4] RFC 4787 NAT UDP Unicast Requirements January 2007

 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 a (public IP address, public port
 number) tuple, and vice versa, for the duration of the session.  An
 issue of relevance to peer-to-peer 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.  In this
 specification, the term "NAT" refers to both "Basic NAT" and "Network
 Address/Port Translator (NAPT)".
 This document uses the term "session" as defined in RFC 2663: "TCP/
 UDP sessions are uniquely identified by the tuple of (source IP
 address, source TCP/UDP ports, target IP address, target TCP/UDP
 Port)".
 This document uses the term "address and port mapping" as the
 translation between an external address and port and an internal
 address and port.  Note that this is not the same as an "address
 binding" as defined in RFC 2663.
 This document uses IANA terminology for port ranges, i.e., "Well
 Known Ports" is 0-1023, "Registered" is 1024-49151, and "Dynamic
 and/or Private" is 49152-65535, as defined in
 http://www.iana.org/assignments/port-numbers.
 STUN [RFC3489] used the terms "Full Cone", "Restricted Cone", "Port
 Restricted Cone", and "Symmetric" to refer to different variations of
 NATs applicable to UDP only.  Unfortunately, this terminology has
 been the source of much confusion, as it has proven inadequate at
 describing real-life NAT behavior.  This specification therefore
 refers to specific individual NAT behaviors instead of using the
 Cone/Symmetric terminology.

4. Network Address and Port Translation Behavior

 This section describes the various NAT behaviors applicable to NATs.

4.1. Address and Port Mapping

 When an internal endpoint opens an outgoing session through a NAT,
 the NAT assigns the session an external IP address and port number so
 that subsequent response packets from the external endpoint can be
 received by the NAT, translated, and forwarded to the internal
 endpoint.  This is a mapping between an internal IP address and port
 IP:port and external IP:port tuple.  It establishes the translation

Audet & Jennings Best Current Practice [Page 5] RFC 4787 NAT UDP Unicast Requirements January 2007

 that will be performed by the NAT for the duration of the session.
 For many applications, it is important to distinguish the behavior of
 the NAT when there are multiple simultaneous sessions established to
 different external endpoints.
 The key behavior to describe is the criteria for reuse of a mapping
 for new sessions to external endpoints, after establishing a first
 mapping between an internal X:x address and port and an external
 Y1:y1 address tuple.  Let's assume that the internal IP address and
 port X:x are mapped to X1':x1' for this first session.  The endpoint
 then sends from X:x to an external address Y2:y2 and gets a mapping
 of X2':x2' on the NAT.  The relationship between X1':x1' and X2':x2'
 for various combinations of the relationship between Y1:y1 and Y2:y2
 is critical for describing the NAT behavior.  This arrangement is
 illustrated in the following diagram:
                                    E
 +------+                 +------+  x
 |  Y1  |                 |  Y2  |  t
 +--+---+                 +---+--+  e
    | Y1:y1            Y2:y2  |     r
    +----------+   +----------+     n
               |   |                a
       X1':x1' |   | X2':x2'        l
            +--+---+-+
 ...........|   NAT  |...............
            +--+---+-+              I
               |   |                n
           X:x |   | X:x            t
              ++---++               e
              |  X  |               r
              +-----+               n
                                    a
                                    l
                       Address and Port Mapping
 The following address and port mapping behavior are defined:
    Endpoint-Independent Mapping:
       The NAT reuses the port mapping for subsequent packets sent
       from the same internal IP address and port (X:x) to any
       external IP address and port.  Specifically, X1':x1' equals
       X2':x2' for all values of Y2:y2.

Audet & Jennings Best Current Practice [Page 6] RFC 4787 NAT UDP Unicast Requirements January 2007

    Address-Dependent Mapping:
       The NAT reuses the port mapping for subsequent packets sent
       from the same internal IP address and port (X:x) to the same
       external IP address, regardless of the external port.
       Specifically, X1':x1' equals X2':x2' if and only if, Y2 equals
       Y1.
    Address and Port-Dependent Mapping:
       The NAT reuses the port mapping for subsequent packets sent
       from the same internal IP address and port (X:x) to the same
       external IP address and port while the mapping is still active.
       Specifically, X1':x1' equals X2':x2' if and only if, Y2:y2
       equals Y1:y1.
 It is important to note that these three possible choices make no
 difference to the security properties of the NAT.  The security
 properties are fully determined by which packets the NAT allows in
 and which it does not.  This is determined by the filtering behavior
 in the filtering portions of the NAT.
 REQ-1:  A NAT MUST have an "Endpoint-Independent Mapping" behavior.
 Justification:  In order for UNSAF methods to work, REQ-1 needs to be
    met.  Failure to meet REQ-1 will force the use of a UDP relay,
    which is very often impractical.
 Some NATs are capable of assigning IP addresses from a pool of IP
 addresses on the external side of the NAT, as opposed to just a
 single IP address.  This is especially common with larger NATs.  Some
 NATs use the external IP address mapping in an arbitrary fashion
 (i.e., randomly): one internal IP address could have multiple
 external IP address mappings active at the same time for different
 sessions.  These NATs have an "IP address pooling" behavior of
 "Arbitrary".  Some large Enterprise NATs use an IP address pooling
 behavior of "Arbitrary" as a means of hiding the IP address assigned
 to specific endpoints by making their assignment less predictable.
 Other NATs use the same external IP address mapping for all sessions
 associated with the same internal IP address.  These NATs have an "IP
 address pooling" behavior of "Paired".  NATs that use an "IP address
 pooling" behavior of "Arbitrary" can cause issues for applications
 that use multiple ports from the same endpoint, but that do not
 negotiate IP addresses individually (e.g., some applications using
 RTP and RTCP).

Audet & Jennings Best Current Practice [Page 7] RFC 4787 NAT UDP Unicast Requirements January 2007

 REQ-2:  It is RECOMMENDED that a NAT have an "IP address pooling"
    behavior of "Paired".  Note that this requirement is not
    applicable to NATs that do not support IP address pooling.
 Justification:  This will allow applications that use multiple ports
    originating from the same internal IP address to also have the
    same external IP address.  This is to avoid breaking peer-to-peer
    applications that are not capable of negotiating the IP address
    for RTP and the IP address for RTCP separately.  As such it is
    envisioned that this requirement will become less important as
    applications become NAT-friendlier with time.  The main reason why
    this requirement is here is that in a peer-to-peer application,
    you are subject to the other peer's mistake.  In particular, in
    the context of SIP, if my application supports the extensions
    defined in [RFC3605] for indicating RTP and RTCP addresses and
    ports separately, but the other peer does not, there may still be
    breakage in the form of the stream losing RTCP packets.  This
    requirement will avoid the loss of RTP in this context, although
    the loss of RTCP may be inevitable in this particular example.  It
    is also worth noting that RFC 3605 is unfortunately not a
    mandatory part of SIP [RFC3261].  Therefore, this requirement will
    address a particularly nasty problem that will prevail for a
    significant period of time.

Audet & Jennings Best Current Practice [Page 8] RFC 4787 NAT UDP Unicast Requirements January 2007

4.2. Port Assignment

4.2.1. Port Assignment Behavior

 This section uses the following diagram for reference.
                                    E
 +-------+               +-------+  x
 |  Y1   |               |  Y2   |  t
 +---+---+               +---+---+  e
     | Y1:y1          Y2:y2  |      r
     +---------+   +---------+      n
               |   |                a
       X1':x1' |   | X2':x2'        l
            +--+---+--+
 ...........|   NAT   |...............
            +--+---+--+             I
               |   |                n
     +---------+   +---------+      t
     | X1:x1           X2:x2 |      e
 +---+---+               +---+---+  r
 |  X1   |               |  X2   |  n
 +-------+               +-------+  a
                                    l
                            Port Assignment
 Some NATs attempt to preserve the port number used internally when
 assigning a mapping to an external IP address and port (e.g., x1=x1',
 x2=x2').  This port assignment behavior is referred to as "port
 preservation".  In case of port collision, these NATs attempt a
 variety of techniques for coping.  For example, some NATs will
 overridden the previous mapping to preserve the same port.  Other
 NATs will assign a different IP address from a pool of external IP
 addresses; this is only possible as long as the NAT has enough
 external IP addresses; if the port is already in use on all available
 external IP addresses, then these NATs will pick a different port
 (i.e., they don't do port preservation anymore).
 Some NATs use "Port overloading", i.e., they always use port
 preservation even in the case of collision (i.e., X1'=X2' and
 x1=x2=x1'=x2').  Most applications will fail if the NAT uses "Port
 overloading".
 A NAT that does not attempt to make the external port numbers match
 the internal port numbers in any case is referred to as "no port
 preservation".

Audet & Jennings Best Current Practice [Page 9] RFC 4787 NAT UDP Unicast Requirements January 2007

 When NATs do allocate a new source port, there is the issue of which
 IANA-defined range of port to choose.  The ranges are "well-known"
 from 0 to 1023, "registered" from 1024 to 49151, and "dynamic/
 private" from 49152 through 65535.  For most protocols, these are
 destination ports and not source ports, so mapping a source port to a
 source port that is already registered is unlikely to have any bad
 effects.  Some NATs may choose to use only the ports in the dynamic
 range; the only downside of this practice is that it limits the
 number of ports available.  Other NAT devices may use everything but
 the well-known range and may prefer to use the dynamic range first,
 or possibly avoid the actual registered ports in the registered
 range.  Other NATs preserve the port range if it is in the well-known
 range.  [RFC0768] specifies that the source port is set to zero if no
 reply packets are expected.  In this case, it does not matter what
 the NAT maps it to, as the source port will not be used.  However,
 many common OS APIs do not allow a user to send from port zero,
 applications do not use port zero, and the behavior of various
 existing NATs with regards to a packet with a source of port zero is
 unknown.  This document does not specify any normative behavior for a
 NAT when handling a packet with a source port of zero which means
 that applications cannot count on any sort of deterministic behavior
 for these packets.
 REQ-3:  A NAT MUST NOT have a "Port assignment" behavior of "Port
    overloading".
    a) If the host's source port was in the range 0-1023, 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 must be met in order to enable two
    applications on the internal side of the NAT both to use the same
    port to try to communicate with the same destination.  NATs that
    implement port preservation have to deal with conflicts on ports,
    and the multiple code paths this introduces often result in
    nondeterministic behavior.  However, it should be understood that
    when a port is randomly assigned, it may just randomly happen to
    be assigned the same port.  Applications must, therefore, be able
    to deal with both port preservation and no port preservation.
    a) Certain applications expect the source UDP port to be in the
       well-known range.  See the discussion of Network File System
       port expectations in [RFC2623] for an example.

Audet & Jennings Best Current Practice [Page 10] RFC 4787 NAT UDP Unicast Requirements January 2007

4.2.2. Port Parity

 Some NATs preserve the parity of the UDP port, i.e., an even port
 will be mapped to an even port, and an odd port will be mapped to an
 odd port.  This behavior respects the [RFC3550] rule that RTP use
 even ports, and RTCP use odd ports.  RFC 3550 allows any port numbers
 to be used for RTP and RTCP if the two numbers are specified
 separately; for example, using [RFC3605].  However, some
 implementations do not include RFC 3605, and do not recognize when
 the peer has specified the RTCP port separately using RFC 3605.  If
 such an implementation receives an odd RTP port number from the peer
 (perhaps after having been translated by a NAT), and then follows the
 RFC 3550 rule to change the RTP port to the next lower even number,
 this would obviously result in the loss of RTP.  NAT-friendly
 application aspects are outside the scope of this document.  It is
 expected that this issue will fade away with time, as implementations
 improve.  Preserving the port parity allows for supporting
 communication with peers that do not support explicit specification
 of both RTP and RTCP port numbers.
 REQ-4:  It is RECOMMENDED that a NAT have a "Port parity
    preservation" behavior of "Yes".
 Justification:  This is to avoid breaking peer-to-peer applications
    that do not explicitly and separately specify RTP and RTCP port
    numbers and that follow the RFC 3550 rule to decrement an odd RTP
    port to make it even.  The same considerations apply, as per the
    IP address pooling requirement.

4.2.3. Port Contiguity

 Some NATs attempt to preserve the port contiguity rule of RTCP=RTP+1.
 These NATs do things like sequential assignment or port reservation.
 Sequential port assignment assumes that the application will open a
 mapping for RTP first and then open a mapping for RTCP.  It is not
 practical to enforce this requirement on all applications.
 Furthermore, there is a problem with glare if many applications (or
 endpoints) are trying to open mappings simultaneously.  Port
 preservation is also problematic since it is wasteful, especially
 considering that a NAT cannot reliably distinguish between RTP over
 UDP and other UDP packets where there is no contiguity rule.  For
 those reasons, it would be too complex to attempt to preserve the
 contiguity rule by suggesting specific NAT behavior, and it would
 certainly break the deterministic behavior rule.
 In order to support both RTP and RTCP, it will therefore be necessary
 that applications follow rules to negotiate RTP and RTCP separately,
 and account for the very real possibility that the RTCP=RTP+1 rule

Audet & Jennings Best Current Practice [Page 11] RFC 4787 NAT UDP Unicast Requirements January 2007

 will be broken.  As this is an application requirement, it is outside
 the scope of this document.

4.3. Mapping Refresh

 NAT mapping timeout implementations vary, but include the timer's
 value and the way the mapping timer is refreshed to keep the mapping
 alive.
 The mapping timer is defined as the time a mapping will stay active
 without packets traversing the NAT.  There is great variation in the
 values used by different NATs.
 REQ-5:  A NAT UDP mapping timer MUST NOT expire in less than two
    minutes, unless REQ-5a applies.
    a) For specific destination ports in the well-known port range
       (ports 0-1023), a NAT MAY have shorter UDP mapping timers that
       are specific to the IANA-registered application running over
       that specific destination port.
    b) The value of the NAT UDP mapping timer MAY be configurable.
    c) A default value of five minutes or more for the NAT UDP mapping
       timer is RECOMMENDED.
 Justification:  This requirement is to ensure that the timeout is
    long enough to avoid too-frequent timer refresh packets.
    a) Some UDP protocols using UDP use very short-lived connections.
       There can be very many such connections; keeping them all in a
       connections table could cause considerable load on the NAT.
       Having shorter timers for these specific applications is,
       therefore, an optimization technique.  It is important that the
       shorter timers applied to specific protocols be used sparingly,
       and only for protocols using well-known destination ports that
       are known to have a shorter timer, and that are known not to be
       used by any applications for other purposes.
    b) Configuration is desirable for adapting to specific networks
       and troubleshooting.
    c) This default is to avoid too-frequent timer refresh packets.
 Some NATs keep the mapping active (i.e., refresh the timer value)
 when a packet goes from the internal side of the NAT to the external
 side of the NAT.  This is referred to as having a NAT Outbound
 refresh behavior of "True".

Audet & Jennings Best Current Practice [Page 12] RFC 4787 NAT UDP Unicast Requirements January 2007

 Some NATs keep the mapping active when a packet goes from the
 external side of the NAT to the internal side of the NAT.  This is
 referred to as having a NAT Inbound Refresh Behavior of "True".
 Some NATs keep the mapping active on both, in which case, both
 properties are "True".
 REQ-6:  The NAT mapping Refresh Direction MUST have a "NAT Outbound
    refresh behavior" of "True".
    a) The NAT mapping Refresh Direction MAY have a "NAT Inbound
       refresh behavior" of "True".
 Justification:  Outbound refresh is necessary for allowing the client
    to keep the mapping alive.
    a) Inbound refresh may be useful for applications with no outgoing
       UDP traffic.  However, allowing inbound refresh may allow an
       external attacker or misbehaving application to keep a mapping
       alive indefinitely.  This may be a security risk.  Also, if the
       process is repeated with different ports, over time, it could
       use up all the ports on the NAT.

4.4. Conflicting Internal and External IP Address Spaces

 Many NATs, particularly consumer-level devices designed to be
 deployed by nontechnical users, routinely obtain their external IP
 address, default router, and other IP configuration information for
 their external interface dynamically from an external network, such
 as an upstream ISP.  The NAT, in turn, automatically sets up its own
 internal subnet in one of the private IP address spaces assigned to
 this purpose in [RFC1918], typically providing dynamic IP
 configuration services for hosts on this internal network.
 Auto-configuration of NATs and private networks can be problematic,
 however, if the NAT's external network is also in RFC 1918 private
 address space.  In a common scenario, an ISP places its customers
 behind a NAT and hands out private RFC 1918 addresses to them.  Some
 of these customers, in turn, deploy consumer-level NATs, which, in
 effect, act as "second-level" NATs, multiplexing their own private
 RFC 1918 IP subnets onto the single RFC 1918 IP address provided by
 the ISP.  There is no inherent guarantee, in this case, that the
 ISP's "intermediate" privately-addressed network and the customer's
 internal privately-addressed network will not use numerically
 identical or overlapping RFC 1918 IP subnets.  Furthermore, customers
 of consumer-level NATs cannot be expected to have the technical

Audet & Jennings Best Current Practice [Page 13] RFC 4787 NAT UDP Unicast Requirements January 2007

 knowledge to prevent this scenario from occurring by manually
 configuring their internal network with non-conflicting RFC 1918
 subnets.
 NAT vendors need to design their NATs to ensure that they function
 correctly and robustly even in such problematic scenarios.  One
 possible solution is for the NAT to ensure that whenever its external
 link is configured with an RFC 1918 private IP address, the NAT
 automatically selects a different, non-conflicting RFC 1918 IP subnet
 for its internal network.  A disadvantage of this solution is that,
 if the NAT's external interface is dynamically configured or re-
 configured after its internal network is already in use, then the NAT
 may have to renumber its entire internal network dynamically if it
 detects a conflict.
 An alternative solution is for the NAT to be designed so that it can
 translate and forward traffic correctly, even when its external and
 internal interfaces are configured with numerically overlapping IP
 subnets.  In this scenario, for example, if the NAT's external
 interface has been assigned an IP address P in RFC 1918 space, then
 there might also be an internal node I having the same RFC 1918
 private IP address P.  An IP packet with destination address P on the
 external network is directed at the NAT, whereas an IP packet with
 the same destination address P on the internal network is directed at
 node I.  The NAT therefore needs to maintain a clear operational
 distinction between "external IP addresses" and "internal IP
 addresses" to avoid confusing internal node I with its own external
 interface.  In general, the NAT needs to allow all internal nodes
 (including I) to communicate with all external nodes having public
 (non-RFC 1918) IP addresses, or having private IP addresses that do
 not conflict with the addresses used by its internal network.
 REQ-7:  A NAT device whose external IP interface can be configured
    dynamically MUST either (1) automatically ensure that its internal
    network uses IP addresses that do not conflict with its external
    network, or (2) be able to translate and forward traffic between
    all internal nodes and all external nodes whose IP addresses
    numerically conflict with the internal network.
 Justification:  If a NAT's external and internal interfaces are
    configured with overlapping IP subnets, then there is, of course,
    no way for an internal host with RFC 1918 IP address Q to initiate
    a direct communication session to an external node having the same
    RFC 1918 address Q, or to other external nodes with IP addresses
    that numerically conflict with the internal subnet.  Such nodes
    can still open communication sessions indirectly via NAT traversal
    techniques, however, with the help of a third-party server, such
    as a STUN server having a public, non-RFC 1918 IP address.  In

Audet & Jennings Best Current Practice [Page 14] RFC 4787 NAT UDP Unicast Requirements January 2007

    this case, nodes with conflicting private RFC 1918 addresses on
    opposite sides of the second-level NAT can communicate with each
    other via their respective temporary public endpoints on the main
    Internet, as long as their common, first-level NAT (e.g., the
    upstream ISP's NAT) supports hairpinning behavior, as described in
    Section 6.

5. Filtering Behavior

 This section describes various filtering behaviors observed in NATs.
 When an internal endpoint opens an outgoing session through a NAT,
 the NAT assigns a filtering rule for the mapping between an internal
 IP:port (X:x) and external IP:port (Y:y) tuple.
 The key behavior to describe is what criteria are used by the NAT to
 filter packets originating from specific external endpoints.
    Endpoint-Independent Filtering:
       The NAT filters out only packets not destined to the internal
       address and port X:x, regardless of the external IP address and
       port source (Z:z).  The NAT forwards any packets destined to
       X:x.  In other words, sending packets from the internal side of
       the NAT to any external IP address is sufficient to allow any
       packets back to the internal endpoint.
    Address-Dependent Filtering:
       The NAT filters out packets not destined to the internal
       address X:x.  Additionally, the NAT will filter out packets
       from Y:y destined for the internal endpoint X:x if X:x has not
       sent packets to Y:any previously (independently of the port
       used by Y).  In other words, for receiving packets from a
       specific external endpoint, it is necessary for the internal
       endpoint to send packets first to that specific external
       endpoint's IP address.
    Address and Port-Dependent Filtering:
       This is similar to the previous behavior, except that the
       external port is also relevant.  The NAT filters out packets
       not destined for the internal address X:x.  Additionally, the
       NAT will filter out packets from Y:y destined for the internal
       endpoint X:x if X:x has not sent packets to Y:y previously.  In
       other words, for receiving packets from a specific external
       endpoint, it is necessary for the internal endpoint to send
       packets first to that external endpoint's IP address and port.

Audet & Jennings Best Current Practice [Page 15] RFC 4787 NAT UDP Unicast Requirements January 2007

 REQ-8:  If application transparency is most important, it is
    RECOMMENDED that a NAT have an "Endpoint-Independent Filtering"
    behavior.  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.
 Justification:  The recommendation to use Endpoint-Independent
    Filtering is aimed at maximizing application transparency; in
    particular, for applications that receive media simultaneously
    from multiple locations (e.g., gaming), or applications that use
    rendezvous techniques.  However, it is also possible that, in some
    circumstances, it may be preferable to have a more stringent
    filtering behavior.  Filtering independently of the external
    endpoint is not as secure: An unauthorized packet could get
    through a specific port while the port was kept open if it was
    lucky enough to find the port open.  In theory, filtering based on
    both IP address and port is more secure than filtering based only
    on the IP address (because the external endpoint could, in
    reality, be two endpoints behind another NAT, where one of the two
    endpoints is an attacker).  However, such a policy could interfere
    with applications that expect to receive UDP packets on more than
    one UDP port.  Using Endpoint-Independent Filtering or Address-
    Dependent Filtering instead of Address and Port-Dependent
    Filtering on a NAT (say, NAT-A) also has benefits when the other
    endpoint is behind a non-BEHAVE compliant NAT (say, NAT-B) that
    does not support REQ-1.  When the endpoints use ICE, if NAT-A uses
    Address and Port-Dependent Filtering, connectivity will require a
    UDP relay.  However, if NAT-A uses Endpoint-Independent Filtering
    or Address-Dependent Filtering, ICE will ultimately find
    connectivity without requiring a UDP relay.  Having the filtering
    behavior being an option configurable by the administrator of the
    NAT ensures that a NAT can be used in the widest variety of
    deployment scenarios.

6. Hairpinning Behavior

 If two hosts (called X1 and X2) are behind the same NAT and
 exchanging traffic, the NAT may allocate an address on the outside of
 the NAT for X2, called X2':x2'.  If X1 sends traffic to X2':x2', it
 goes to the NAT, which must relay the traffic from X1 to X2.  This is
 referred to as hairpinning and is illustrated below.

Audet & Jennings Best Current Practice [Page 16] RFC 4787 NAT UDP Unicast Requirements January 2007

   NAT
 +----+ from X1:x1 to X2':x2'   +-----+ X1':x1'
 | X1 |>>>>>>>>>>>>>>>>>>>>>>>>>>>>>--+---
 +----+                         |  v  |
                                |  v  |
                                |  v  |
                                |  v  |
 +----+ from X1':x1' to X2:x2   |  v  | X2':x2'
 | X2 |<<<<<<<<<<<<<<<<<<<<<<<<<<<<<--+---
 +----+                         +-----+
                         Hairpinning Behavior
 Hairpinning allows two endpoints on the internal side of the NAT to
 communicate even if they only use each other's external IP addresses
 and ports.
 More formally, a NAT that supports hairpinning forwards packets
 originating from an internal address, X1:x1, destined for an external
 address X2':x2' that has an active mapping to an internal address
 X2:x2, back to that internal address, X2:x2.  Note that typically X1'
 is the same as X2'.
 Furthermore, the NAT may present the hairpinned packet with either an
 internal (X1:x1) or an external (X1':x1') source IP address and port.
 Therefore, the hairpinning NAT behavior can be either "External
 source IP address and port" or "Internal source IP address and port".
 "Internal source IP address and port" may cause problems by confusing
 implementations that expect an external IP address and port.
 REQ-9:  A NAT MUST support "Hairpinning".
    a) A NAT Hairpinning behavior MUST be "External source IP address
       and port".
 Justification:  This requirement is to allow communications between
    two endpoints behind the same NAT when they are trying each
    other's external IP addresses.
    a) Using the external source IP address is necessary for
       applications with a restrictive policy of not accepting packets
       from IP addresses that differ from what is expected.

7. Application Level Gateways

 Certain NATs have implemented Application Level Gateways (ALGs) for
 various protocols, including protocols for negotiating peer-to-peer
 sessions, such as SIP.

Audet & Jennings Best Current Practice [Page 17] RFC 4787 NAT UDP Unicast Requirements January 2007

 Certain NATs have these ALGs turned on permanently, others have them
 turned on by default but allow them to be turned off, and others have
 them turned off by default but allow them be turned on.
 NAT ALGs may interfere with UNSAF methods or protocols that try to be
 NAT-aware and therefore must be used with extreme caution.
 REQ-10:  To eliminate interference with UNSAF NAT traversal
    mechanisms and allow integrity protection of UDP communications,
    NAT ALGs for UDP-based protocols SHOULD be turned off.  Future
    standards track specifications that define ALGs can update this to
    recommend the defaults for the ALGs that they define.
    a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow
       the NAT administrator to enable or disable each ALG separately.
 Justification:  NAT ALGs may interfere with UNSAF methods.
    a) This requirement allows the user to enable those ALGs that are
       necessary to aid in the operation of some applications without
       enabling ALGs, which interfere with the operation of other
       applications.

8. Deterministic Properties

 The classification of NATs is further complicated by the fact that,
 under some conditions, the same NAT will exhibit different behaviors.
 This has been seen on NATs that preserve ports or have specific
 algorithms for selecting a port other than a free one.  If the
 external port that the NAT wishes to use is already in use by another
 session, the NAT must select a different port.  This results in
 different code paths for this conflict case, which results in
 different behavior.
 For example, if three hosts X1, X2, and X3 all send from the same
 port x, through a port preserving NAT with only one external IP
 address, called X1', the first one to send (i.e., X1) will get an
 external port of x, but the next two will get x2' and x3' (where
 these are not equal to x).  There are NATs where the External NAT
 mapping characteristics and the External Filter characteristics
 change between the X1:x and the X2:x mapping.  To make matters worse,
 there are NATs where the behavior may be the same on the X1:x and
 X2:x mappings, but different on the third X3:x mapping.
 Another example is that some NATs have an "Endpoint-Independent
 Mapping", combined with "Port Overloading", as long as two endpoints
 are not establishing sessions to the same external direction, but
 then switch their behavior to "Address and Port-Dependent Mapping"

Audet & Jennings Best Current Practice [Page 18] RFC 4787 NAT UDP Unicast Requirements January 2007

 without "Port Preservation" upon detection of these conflicting
 sessions establishments.
 Any NAT that changes the NAT Mapping or the Filtering behavior
 without configuration changes, at any point in time, under any
 particular conditions, is referred to as a "non-deterministic" NAT.
 NATs that don't are called "deterministic".
 Non-deterministic NATs generally change behavior when a conflict of
 some sort happens, i.e., when the port that would normally be used is
 already in use by another mapping.  The NAT mapping and External
 Filtering in the absence of conflict is referred to as the Primary
 behavior.  The behavior after the first conflict is referred to as
 Secondary and after the second conflict is referred to as Tertiary.
 No NATs have been observed that change on further conflicts, but it
 is certainly possible that they exist.
 REQ-11:  A NAT MUST have deterministic behavior, i.e., it MUST NOT
    change the NAT translation (Section 4) or the Filtering
    (Section 5) Behavior at any point in time, or under any particular
    conditions.
 Justification:  Non-deterministic NATs are very difficult to
    troubleshoot because they require more intensive testing.  This
    non-deterministic behavior is the root cause of much of the
    uncertainty that NATs introduce about whether or not applications
    will work.

9. ICMP Destination Unreachable Behavior

 When a NAT sends a packet toward a host on the other side of the NAT,
 an ICMP message may be sent in response to that packet.  That ICMP
 message may be sent by the destination host or by any router along
 the network path.  The NAT's default configuration SHOULD NOT filter
 ICMP messages based on their source IP address.  Such 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.  The NAT needs to perform this function for as long as the UDP
 mapping is active.  Receipt of any sort of ICMP message MUST NOT
 destroy the NAT mapping.  A NAT that performs the functions described
 in the paragraph above is referred to as "support ICMP Processing".
 There is no significant security advantage to blocking ICMP
 Destination Unreachable packets.  Additionally, blocking ICMP
 Destination Unreachable packets can interfere with application
 failover, UDP Path MTU Discovery (see [RFC1191] and [RFC1435]), and
 traceroute.  Blocking any ICMP message is discouraged, and blocking
 ICMP Destination Unreachable is strongly discouraged.

Audet & Jennings Best Current Practice [Page 19] RFC 4787 NAT UDP Unicast Requirements January 2007

 REQ-12:  Receipt of any sort of ICMP message MUST NOT terminate the
    NAT mapping.
    a) The NAT's default configuration SHOULD NOT filter ICMP messages
       based on their source IP address.
    b) It is RECOMMENDED that a NAT support ICMP Destination
       Unreachable messages.
 Justification:  This is easy to do and is used for many things
    including MTU discovery and rapid detection of error conditions,
    and has no negative consequences.

10. Fragmentation of Outgoing Packets

 When the MTU of the adjacent link is too small, fragmentation of
 packets going from the internal side to the external side of the NAT
 may occur.  This can occur if the NAT is doing Point-to-Point over
 Ethernet (PPPoE), or if the NAT has been configured with a small MTU
 to reduce serialization delay when sending large packets and small
 higher-priority packets, or for other reasons.
 It is worth noting that many IP stacks do not use Path MTU Discovery
 with UDP packets.
 The packet could have its Don't Fragment bit set to 1 (DF=1) or 0
 (DF=0).
 REQ-13:  If the packet received on an internal IP address has DF=1,
    the NAT MUST send back an ICMP message "Fragmentation needed and
    DF set" to the host, as described in [RFC0792].
    a) If the packet has DF=0, the NAT MUST fragment the packet and
       SHOULD send the fragments in order.
 Justification:  This is as per RFC 792.
    a) This is the same function a router performs in a similar
       situation [RFC1812].

11. Receiving Fragmented Packets

 For a variety of reasons, a NAT may receive a fragmented packet.  The
 IP packet containing the header could arrive in any fragment,
 depending on network conditions, packet ordering, and the
 implementation of the IP stack that generated the fragments.

Audet & Jennings Best Current Practice [Page 20] RFC 4787 NAT UDP Unicast Requirements January 2007

 A NAT that is capable only of receiving fragments in order (that is,
 with the header in the first packet) and forwarding each of the
 fragments to the internal host is described as "Received Fragments
 Ordered".
 A NAT that is capable of receiving fragments in or out of order and
 forwarding the individual fragments (or a reassembled packet) to the
 internal host is referred to as "Receive Fragments Out of Order".
 See the Security Considerations section of this document for a
 discussion of this behavior.
 A NAT that is neither of these is referred to as "Receive Fragments
 None".
 REQ-14:  A NAT MUST support receiving in-order and out-of-order
    fragments, so it MUST have "Received Fragment Out of Order"
    behavior.
    a) A NAT's out-of-order fragment processing mechanism MUST be
       designed so that fragmentation-based DoS attacks do not
       compromise the NAT's ability to process in-order and
       unfragmented IP packets.
 Justification:  See Security Considerations.

12. Requirements

 The requirements in this section are aimed at minimizing the
 complications caused by NATs to applications, such as realtime
 communications and online gaming.  The requirements listed earlier in
 the document are consolidated here into a single section.
 It should be understood, however, that applications normally do not
 know in advance if the NAT conforms to the recommendations defined in
 this section.  Peer-to-peer media applications still need to use
 normal procedures, such as ICE [ICE].
 A NAT that supports all the mandatory requirements of this
 specification (i.e., the "MUST"), is "compliant with this
 specification".  A NAT that supports all the requirements of this
 specification (i.e., including the "RECOMMENDED") is "fully compliant
 with all the mandatory and recommended requirements of this
 specification".

Audet & Jennings Best Current Practice [Page 21] RFC 4787 NAT UDP Unicast Requirements January 2007

 REQ-1:  A NAT MUST have an "Endpoint-Independent Mapping" behavior.
 REQ-2:  It is RECOMMENDED that a NAT have an "IP address pooling"
    behavior of "Paired".  Note that this requirement is not
    applicable to NATs that do not support IP address pooling.
 REQ-3:  A NAT MUST NOT have a "Port assignment" behavior of "Port
    overloading".
    a) If the host's source port was in the range 0-1023, 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-4:  It is RECOMMENDED that a NAT have a "Port parity
    preservation" behavior of "Yes".
 REQ-5:  A NAT UDP mapping timer MUST NOT expire in less than two
    minutes, unless REQ-5a applies.
    a) For specific destination ports in the well-known port range
       (ports 0-1023), a NAT MAY have shorter UDP mapping timers that
       are specific to the IANA-registered application running over
       that specific destination port.
    b) The value of the NAT UDP mapping timer MAY be configurable.
    c) A default value of five minutes or more for the NAT UDP mapping
       timer is RECOMMENDED.
 REQ-6:  The NAT mapping Refresh Direction MUST have a "NAT Outbound
    refresh behavior" of "True".
    a) The NAT mapping Refresh Direction MAY have a "NAT Inbound
       refresh behavior" of "True".
 REQ-7  A NAT device whose external IP interface can be configured
    dynamically MUST either (1) Automatically ensure that its internal
    network uses IP addresses that do not conflict with its external
    network, or (2) Be able to translate and forward traffic between
    all internal nodes and all external nodes whose IP addresses
    numerically conflict with the internal network.
 REQ-8:  If application transparency is most important, it is
    RECOMMENDED that a NAT have "Endpoint-Independent Filtering"
    behavior.  If a more stringent filtering behavior is most
    important, it is RECOMMENDED that a NAT have "Address-Dependent
    Filtering" behavior.

Audet & Jennings Best Current Practice [Page 22] RFC 4787 NAT UDP Unicast Requirements January 2007

    a) The filtering behavior MAY be an option configurable by the
       administrator of the NAT.
 REQ-9:  A NAT MUST support "Hairpinning".
    a) A NAT Hairpinning behavior MUST be "External source IP address
       and port".
 REQ-10:  To eliminate interference with UNSAF NAT traversal
    mechanisms and allow integrity protection of UDP communications,
    NAT ALGs for UDP-based protocols SHOULD be turned off.  Future
    standards track specifications that define an ALG can update this
    to recommend the ALGs on which they define default.
    a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow
       the NAT administrator to enable or disable each ALG separately.
 REQ-11:  A NAT MUST have deterministic behavior, i.e., it MUST NOT
    change the NAT translation (Section 4) or the Filtering
    (Section 5) Behavior at any point in time, or under any particular
    conditions.
 REQ-12:  Receipt of any sort of ICMP message MUST NOT terminate the
    NAT mapping.
    a) The NAT's default configuration SHOULD NOT filter ICMP messages
       based on their source IP address.
    b) It is RECOMMENDED that a NAT support ICMP Destination
       Unreachable messages.
 REQ-13  If the packet received on an internal IP address has DF=1,
    the NAT MUST send back an ICMP message "Fragmentation needed and
    DF set" to the host, as described in [RFC0792].
    a) If the packet has DF=0, the NAT MUST fragment the packet and
       SHOULD send the fragments in order.
 REQ-14:  A NAT MUST support receiving in-order and out-of-order
    fragments, so it MUST have "Received Fragment Out of Order"
    behavior.
    a) A NAT's out-of-order fragment processing mechanism MUST be
       designed so that fragmentation-based DoS attacks do not
       compromise the NAT's ability to process in-order and
       unfragmented IP packets.

Audet & Jennings Best Current Practice [Page 23] RFC 4787 NAT UDP Unicast Requirements January 2007

13. Security Considerations

 NATs are often deployed to achieve security goals.  Most of the
 recommendations and requirements in this document do not affect the
 security properties of these devices, but a few of them do have
 security implications and are discussed in this section.
 This document recommends that the timers for mapping be refreshed on
 outgoing packets (see REQ-6) and does not make recommendations about
 whether or not inbound packets should update the timers.  If inbound
 packets update the timers, an external attacker can keep the mapping
 alive forever and attack future devices that may end up with the same
 internal address.  A device that was also the DHCP server for the
 private address space could mitigate this by cleaning any mappings
 when a DHCP lease expired.  For unicast UDP traffic (the scope of
 this document), it may not seem relevant to support inbound timer
 refresh; however, for multicast UDP, the question is harder.  It is
 expected that future documents discussing NAT behavior with multicast
 traffic will refine the requirements around handling of the inbound
 refresh timer.  Some devices today do update the timers on inbound
 packets.
 This document recommends that the NAT filters be specific to the
 external IP address only (see REQ-8) and not to the external IP
 address and UDP port.  It can be argued that this is less secure than
 using the IP and port.  Devices that wish to filter on IP and port do
 still comply with these requirements.
 Non-deterministic NATs are risky from a security point of view.  They
 are very difficult to test because they are, well, non-deterministic.
 Testing by a person configuring one may result in the person thinking
 it is behaving as desired, yet under different conditions, which an
 attacker can create, the NAT may behave differently.  These
 requirements recommend that devices be deterministic.
 This document requires that NATs have an "external NAT mapping is
 endpoint independent" behavior.  This does not reduce the security of
 devices.  Which packets are allowed to flow across the device is
 determined by the external filtering behavior, which is independent
 of the mapping behavior.
 When a fragmented packet is received from the external side, and the
 packets are out of order so that the initial fragment does not arrive
 first, many systems simply discard the out-of-order packets.
 Moreover, since some networks deliver small packets ahead of large
 ones, there can be many out-of-order fragments.  NATs that are
 capable of delivering these out-of-order packets are possible, but
 they need to store the out-of-order fragments, which can open up a

Audet & Jennings Best Current Practice [Page 24] RFC 4787 NAT UDP Unicast Requirements January 2007

 Denial-of-Service (DoS) opportunity, if done incorrectly.
 Fragmentation has been a tool used in many attacks, some involving
 passing fragmented packets through NATs, and others involving DoS
 attacks based on the state needed to reassemble the fragments.  NAT
 implementers should be aware of [RFC3128] and [RFC1858].

14. IAB Considerations

 The IAB has studied the problem of "Unilateral Self Address Fixing",
 which is the general process by which a client attempts to determine
 its address in another realm on the other side of a NAT through a
 collaborative protocol reflection mechanism [RFC3424].
 This specification does not, in itself, constitute an UNSAF
 application.  It consists of a series of requirements for NATs aimed
 at minimizing the negative impact that those devices have on peer-to-
 peer media applications, especially when those applications are using
 UNSAF methods.
 Section 3 of UNSAF lists several practical issues with solutions to
 NAT problems.  This document makes recommendations to reduce the
 uncertainty and problems introduced by these practical issues with
 NATs.  In addition, UNSAF lists five architectural considerations.
 Although this is not an UNSAF proposal, it is interesting to consider
 the impact of this work on these architectural considerations.
 Arch-1:  The scope of this is limited to UDP packets in NATs like the
          ones widely deployed today.  The "fix" helps constrain the
          variability of NATs for true UNSAF solutions such as STUN.
 Arch-2:  This will exit at the same rate that NATs exit.  It does not
          imply any protocol machinery that would continue to live
          after NATs were gone, or make it more difficult to remove
          them.
 Arch-3:  This does not reduce the overall brittleness of NATs, but
          will hopefully reduce some of the more outrageous NAT
          behaviors and make it easer to discuss and predict NAT
          behavior in given situations.
 Arch-4:  This work and the results [RESULTS] of various NATs
          represent the most comprehensive work at IETF on what the
          real issues are with NATs for applications like VoIP.  This
          work and STUN have pointed out, more than anything else, the
          brittleness NATs introduce and the difficulty of addressing
          these issues.

Audet & Jennings Best Current Practice [Page 25] RFC 4787 NAT UDP Unicast Requirements January 2007

 Arch-5:  This work and the test results [RESULTS] provide a reference
          model for what any UNSAF proposal might encounter in
          deployed NATs.

15. Acknowledgments

 The editor would like to acknowledge Bryan Ford, Pyda Srisuresh, and
 Dan Kegel for their multiple contributions on peer-to-peer
 communications across a NAT.  Dan Wing contributed substantial text
 on IP fragmentation and ICMP behavior.  Thanks to Rohan Mahy,
 Jonathan Rosenberg, Mary Barnes, Melinda Shore, Lyndsay Campbell,
 Geoff Huston, Jiri Kuthan, Harald Welte, Steve Casner, Robert
 Sanders, Spencer Dawkins, Saikat Guha, Christian Huitema, Yutaka
 Takeda, Paul Hoffman, Lisa Dusseault, Pekka Savola, Peter Koch, Jari
 Arkko, and Alfred Hoenes for their contributions.

16. References

16.1. Normative References

 [RFC0768]     Postel, J., "User Datagram Protocol", STD 6, RFC 768,
               August 1980.
 [RFC0791]     Postel, J., "Internet Protocol", STD 5, RFC 791,
               September 1981.
 [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

16.2. Informative References

 [RFC0792]     Postel, J., "Internet Control Message Protocol", STD 5,
               RFC 792, September 1981.
 [RFC1191]     Mogul, J. and S. Deering, "Path MTU discovery",
               RFC 1191, November 1990.
 [RFC1435]     Knowles, S., "IESG Advice from Experience with Path MTU
               Discovery", RFC 1435, March 1993.
 [RFC1812]     Baker, F., "Requirements for IP Version 4 Routers",
               RFC 1812, June 1995.
 [RFC1858]     Ziemba, G., Reed, D., and P. Traina, "Security
               Considerations for IP Fragment Filtering", RFC 1858,
               October 1995.

Audet & Jennings Best Current Practice [Page 26] RFC 4787 NAT UDP Unicast Requirements January 2007

 [RFC1918]     Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G.,
               and E. Lear, "Address Allocation for Private
               Internets", BCP 5, RFC 1918, February 1996.
 [RFC2460]     Deering, S. and R. Hinden, "Internet Protocol, Version
               6 (IPv6) Specification", RFC 2460, December 1998.
 [RFC2623]     Eisler, M., "NFS Version 2 and Version 3 Security
               Issues and the NFS Protocol's Use of RPCSEC_GSS and
               Kerberos V5", RFC 2623, June 1999.
 [RFC2663]     Srisuresh, P. and M. Holdrege, "IP Network Address
               Translator (NAT) Terminology and Considerations",
               RFC 2663, August 1999.
 [RFC3022]     Srisuresh, P. and K. Egevang, "Traditional IP Network
               Address Translator (Traditional NAT)", RFC 3022,
               January 2001.
 [RFC3027]     Holdrege, M. and P. Srisuresh, "Protocol Complications
               with the IP Network Address Translator", RFC 3027,
               January 2001.
 [RFC3128]     Miller, I., "Protection Against a Variant of the Tiny
               Fragment Attack (RFC 1858)", RFC 3128, June 2001.
 [RFC3261]     Rosenberg, J., Schulzrinne, H., Camarillo, G.,
               Johnston, A., Peterson, J., Sparks, R., Handley, M.,
               and E. Schooler, "SIP: Session Initiation Protocol",
               RFC 3261, June 2002.
 [RFC3424]     Daigle, L. and IAB, "IAB Considerations for UNilateral
               Self-Address Fixing (UNSAF) Across Network Address
               Translation", RFC 3424, November 2002.
 [RFC3489]     Rosenberg, J., Weinberger, J., Huitema, C., and R.
               Mahy, "STUN - Simple Traversal of User Datagram
               Protocol (UDP) Through Network Address Translators
               (NATs)", RFC 3489, March 2003.
 [RFC3550]     Schulzrinne, H., Casner, S., Frederick, R., and V.
               Jacobson, "RTP: A Transport Protocol for Real-Time
               Applications", STD 64, RFC 3550, July 2003.
 [RFC3605]     Huitema, C., "Real Time Control Protocol (RTCP)
               attribute in Session Description Protocol (SDP)",
               RFC 3605, October 2003.

Audet & Jennings Best Current Practice [Page 27] RFC 4787 NAT UDP Unicast Requirements January 2007

 [RFC4380]     Huitema, C., "Teredo: Tunneling IPv6 over UDP through
               Network Address Translations (NATs)", RFC 4380,
               February 2006.
 [RFC3489bis]  Rosenberg, J., "Simple Traversal Underneath Network
               Address Translators (NAT) (STUN)", Work in Progress,
               October 2006.
 [ICE]         Rosenberg, J., "Interactive Connectivity Establishment
               (ICE): A Methodology for Network Address Translator
               (NAT) Traversal for Offer/Answer Protocols", Work
               in Progress, October 2006.
 [RESULTS]     Jennings, C., "NAT Classification Test Results", Work
               in Progress, October 2006.
 [TURN]        Rosenberg, J., "Obtaining Relay Addresses from Simple
               Traversal Underneath NAT (STUN)", Work in Progress,
               October 2006.
 [ITU.H323]    "Packet-based Multimedia Communications Systems", ITU-
               T Recommendation H.323, July 2003.

Authors' Addresses

 Francois Audet (editor)
 Nortel Networks
 4655 Great America Parkway
 Santa Clara, CA  95054
 US
 Phone: +1 408 495 2456
 EMail: audet@nortel.com
 Cullen Jennings
 Cisco Systems
 170 West Tasman Drive
 MS: SJC-21/2
 San Jose, CA  95134
 US
 Phone: +1 408 902 3341
 EMail: fluffy@cisco.com

Audet & Jennings Best Current Practice [Page 28] RFC 4787 NAT UDP Unicast Requirements January 2007

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Audet & Jennings Best Current Practice [Page 29]

Internet Engineering Task Force (IETF) S. Perreault, Ed. Request for Comments: 6888 Viagenie BCP: 127 I. Yamagata Updates: 4787 S. Miyakawa Category: Best Current Practice NTT Communications ISSN: 2070-1721 A. Nakagawa

                                        Japan Internet Exchange (JPIX)
                                                             H. Ashida
                                                         Cisco Systems
                                                            April 2013
         Common Requirements for Carrier-Grade NATs (CGNs)

Abstract

 This document defines common requirements for Carrier-Grade NATs
 (CGNs).  It updates RFC 4787.

Status of This Memo

 This memo documents an Internet Best Current Practice.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 BCPs is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6888.

Copyright Notice

 Copyright (c) 2013 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Perreault, et al. Best Current Practice [Page 1] RFC 6888 CGN Requirements April 2013

Table of Contents

 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . .  2
 2. Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  3
 3. Requirements for CGNs  . . . . . . . . . . . . . . . . . . .  4
 4. Logging  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
 5. Port Allocation Scheme . . . . . . . . . . . . . . . . . . . 11
 6. Deployment Considerations  . . . . . . . . . . . . . . . . . 11
 7. Security Considerations  . . . . . . . . . . . . . . . . . . 12
 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
    9.1. Normative References  . . . . . . . . . . . . . . . . . 12
    9.2. Informative Reference . . . . . . . . . . . . . . . . . 13

1. Introduction

 With the shortage of IPv4 addresses, it is expected that more
 Internet Service Providers (ISPs) may want to provide a service where
 a public IPv4 address would be shared by many subscribers.  Each
 subscriber is assigned a private address, and a Network Address
 Translator (NAT) [RFC2663] situated in the ISP's network translates
 the traffic between private and public addresses.  When a second IPv4
 NAT is located at the customer edge, this results in two layers of
 NAT.
 This service can conceivably be offered alongside others, such as
 IPv6 services or regular IPv4 service assigning public addresses to
 subscribers.  Some ISPs started offering such a service long before
 there was a shortage of IPv4 addresses, showing that there are
 driving forces other than the shortage of IPv4 addresses.  One
 approach to CGN deployment is described in [RFC6264].
 This document describes behavior that is required of those multi-
 subscriber NATs for interoperability.  It is not an IETF endorsement
 of CGNs or a real specification for CGNs; rather, it is just a
 minimal set of requirements that will increase the likelihood of
 applications working across CGNs.
 Because subscribers do not receive unique IPv4 addresses, Carrier-
 Grade NATs introduce substantial limitations in communications
 between subscribers and with the rest of the Internet.  In
 particular, it is considerably more involved to establish proxy
 functionality at the border between internal and external realms.
 Some applications may require substantial enhancements, while some
 others may not function at all in such an environment.  Please see
 "Issues with IP Address Sharing" [RFC6269] for details.

Perreault, et al. Best Current Practice [Page 2] RFC 6888 CGN Requirements April 2013

 This document builds upon previous works describing requirements for
 generic NATs [RFC4787][RFC5382][RFC5508].  These documents, and their
 updates if any, still apply in this context.  What follows are
 additional requirements, to be satisfied on top of previous ones.

2. 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].
 Readers are expected to be familiar with "Network Address Translation
 (NAT) Behavioral Requirements for Unicast UDP" [RFC4787] and the
 terms defined there.  The following additional term is used in this
 document:
 Carrier-Grade NAT (CGN):  A NAT-based [RFC2663] logical function used
    to share the same IPv4 address among several subscribers.  A CGN
    is not managed by the subscribers.
       Note that the term "carrier-grade" has nothing to do with the
       quality of the NAT; that is left to discretion of implementers.
       Rather, it is to be understood as a topological qualifier: the
       NAT is placed in an ISP's network and translates the traffic of
       potentially many subscribers.  Subscribers have limited or no
       control over the CGN, whereas they typically have full control
       over a NAT placed on their premises.
       Note also that the CGN described in this document is IPv4-only.
       IPv6 address translation is not considered.
       However, the scenario in which the IPv4-only CGN logical
       function is used may include IPv6 elements.  For example, Dual-
       Stack Lite (DS-Lite) [RFC6333] uses an IPv4-only CGN logical
       function in a scenario making use of IPv6 encapsulation.
       Therefore, this document would also apply to the CGN part of
       DS-Lite.

Perreault, et al. Best Current Practice [Page 3] RFC 6888 CGN Requirements April 2013

 Figure 1 summarizes a common network topology in which a CGN
 operates.
                                 .
                                 :
                                 |       Internet
                 ............... | ...................
                                 |       ISP network
                 External pool:  |
                 192.0.2.1/26    |
                             ++------++  External realm
                 ........... |  CGN   |...............
                             ++------++  Internal realm
                      10.0.0.1 |    |
                               |    |
                               |    |    ISP network
                 ............. | .. | ................
                               |    |  Customer premises
                    10.0.0.100 |    | 10.0.0.101
                       ++------++  ++------++
                       |  CPE1  |  |  CPE2  |  etc.
                       ++------++  ++------++
             (IP addresses are only for example purposes)
                    Figure 1: CGN Network Topology
 Another possible topology is one for hotspots, where there is no
 customer premise or customer premises equipment (CPE), but where a
 CGN serves a bunch of customers who don't trust each other; hence,
 fairness is an issue.  One important difference with the previous
 topology is the absence of a second layer of NAT.  This, however, has
 no impact on CGN requirements since they are driven by fairness and
 robustness in the service provided to customers, which applies in
 both cases.

3. Requirements for CGNs

 What follows is a list of requirements for CGNs.  They are in
 addition to those found in other documents such as [RFC4787],
 [RFC5382], and [RFC5508].
 REQ-1:  If a CGN forwards packets containing a given transport
    protocol, then it MUST fulfill that transport protocol's
    behavioral requirements.  Current applicable documents are as
    follows:
    a.  "NAT Behavioral Requirements for Unicast UDP" [RFC4787]

Perreault, et al. Best Current Practice [Page 4] RFC 6888 CGN Requirements April 2013

    b.  "Network Address Translation (NAT) Behavioral Requirements for
        TCP" [RFC5382]
    c.  "NAT Behavioral Requirements for ICMP" [RFC5508]
    d.  "Network Address Translation (NAT) Behavioral Requirements for
        the Datagram Congestion Control Protocol (DCCP)" [RFC5597]
    Any future NAT behavioral requirements documents for IPv4
    transport protocols will impose additional requirements for CGNs
    on top of those stated here.
 Justification:  It is crucial for CGNs to maximize the set of
    applications that can function properly across them.  The IETF has
    documented the best current practices for UDP, TCP, ICMP, and
    DCCP.
 REQ-2:  A CGN MUST have a default "IP address pooling" behavior of
    "Paired" (as defined in Section 4.1 of [RFC4787]).  A CGN MAY
    provide a mechanism for administrators to change this behavior on
    an application protocol basis.
  • When multiple overlapping internal IP address ranges share the

same external IP address pool (e.g., DS-Lite [RFC6333]), the

       "IP address pooling" behavior applies to mappings between
       external IP addresses and internal subscribers rather than
       between external and internal IP addresses.
 Justification:  This stronger form of REQ-2 from [RFC4787] is
    justified by the stronger need for not breaking applications that
    depend on the external address remaining constant.
    Note that this requirement applies regardless of the transport
    protocol.  In other words, a CGN must use the same external IP
    address mapping for all sessions associated with the same internal
    IP address, be they TCP, UDP, ICMP, something else, or a mix of
    different protocols.
    The justification for allowing other behaviors is to allow the
    administrator to save external addresses and ports for application
    protocols that are known to work fine with other behaviors in
    practice.  However, the default behavior MUST be "Paired".
 REQ-3:  The CGN function SHOULD NOT have any limitations on the size
    or the contiguity of the external address pool.  In particular,
    the CGN function MUST be configurable with contiguous or non-
    contiguous external IPv4 address ranges.

Perreault, et al. Best Current Practice [Page 5] RFC 6888 CGN Requirements April 2013

 Justification:  Given the increasing rarity of IPv4 addresses, it is
    becoming harder for an operator to provide large contiguous
    address pools to CGNs.  Additionally, operational flexibility may
    require non-contiguous address pools for reasons such as
    differentiated services, routing management, etc.
    The reason for having SHOULD instead of MUST is to account for
    limitations imposed by available resources as well as constraints
    imposed for security reasons.
 REQ-4:  A CGN MUST support limiting the number of external ports (or,
    equivalently, "identifiers" for ICMP) that are assigned per
    subscriber.
    a.  Per-subscriber limits MUST be configurable by the CGN
        administrator.
    b.  Per-subscriber limits MAY be configurable independently per
        transport protocol.
    c.  Additionally, it is RECOMMENDED that the CGN include
        administrator-adjustable thresholds to prevent a single
        subscriber from consuming excessive CPU resources from the CGN
        (e.g., rate-limit the subscriber's creation of new mappings).
 Justification:  A CGN can be considered a network resource that is
    shared by competing subscribers.  Limiting the number of external
    ports assigned to each subscriber mitigates the denial-of-service
    (DoS) attack that a subscriber could launch against other
    subscribers through the CGN in order to get a larger share of the
    resource.  It ensures fairness among subscribers.  Limiting the
    rate of allocation mitigates a similar attack where the CPU is the
    resource being targeted instead of port numbers.  However, this
    requirement is not a MUST because it is very hard to explicitly
    call out all CPU-consuming events.
 REQ-5:  A CGN SHOULD support limiting the amount of state memory
    allocated per mapping and per subscriber.  This may include
    limiting the number of sessions, the number of filters, etc.,
    depending on the NAT implementation.
    a.  Limits SHOULD be configurable by the CGN administrator.
    b.  Additionally, it SHOULD be possible to limit the rate at which
        memory-consuming state elements are allocated.

Perreault, et al. Best Current Practice [Page 6] RFC 6888 CGN Requirements April 2013

 Justification:  A NAT needs to keep track of TCP sessions associated
    with each mapping.  This state consumes resources for which, in
    the case of a CGN, subscribers may compete.  It is necessary to
    ensure that each subscriber has access to a fair share of the
    CGN's resources.  Limiting the rate of allocation is intended to
    prevent CPU resource exhaustion.  Item "B" is at the SHOULD level
    to account for the fact that means other than rate limiting may be
    used to attain the same goal.
 REQ-6:  It MUST be possible to administratively turn off translation
    for specific destination addresses and/or ports.
 Justification:  It is common for a CGN administrator to provide
    access for subscribers to servers installed in the ISP's network
    in the external realm.  When such a server is able to reach the
    internal realm via normal routing (which is entirely controlled by
    the ISP), translation is unneeded.  In that case, the CGN may
    forward packets without modification, thus acting like a plain
    router.  This may represent an important efficiency gain.
    Figure 2 illustrates this use-case.
                X1:x1            X1':x1'            X2:x2
                +---+from X1:x1  +---+from X1:x1    +---+
                | C |  to X2:x2  |   |  to X2:x2    | S |
                | l |>>>>>>>>>>>>| C |>>>>>>>>>>>>>>| e |
                | i |            | G |              | r |
                | e |<<<<<<<<<<<<| N |<<<<<<<<<<<<<<| v |
                | n |from X2:x2  |   |from X2:x2    | e |
                | t |  to X1:x1  |   |  to X1:x1    | r |
                +---+            +---+              +---+
                      Figure 2: CGN Pass-Through
 REQ-7:  It is RECOMMENDED that a CGN use an "endpoint-independent
    filtering" behavior (as defined in Section 5 of [RFC4787]).  If it
    is known that "Address-Dependent Filtering" does not cause the
    application-layer protocol to break (how to determine this is out
    of scope for this document), then it MAY be used instead.
 Justification:  This is a stronger form of REQ-8 from [RFC4787].
    This is based on the observation that some games and peer-to-peer
    applications require EIF for the NAT traversal to work.  In the
    context of a CGN, it is important to minimize application
    breakage.

Perreault, et al. Best Current Practice [Page 7] RFC 6888 CGN Requirements April 2013

 REQ-8:  Once an external port is deallocated, it SHOULD NOT be
    reallocated to a new mapping until at least 120 seconds have
    passed, with the exceptions being:
    a.  If the CGN tracks TCP sessions (e.g., with a state machine, as
        in Section 3.5.2.2 of [RFC6146]), TCP ports MAY be reused
        immediately.
    b.  If external ports are statically assigned to internal
        addresses (e.g., address X with port range 1000-1999 is
        assigned to subscriber A, 2000-2999 to subscriber B, etc.),
        and the assignment remains constant across state loss, then
        ports MAY be reused immediately.
    c.  If the allocated external ports used address-dependent or
        address-and-port-dependent filtering before state loss, they
        MAY be reused immediately.
    The length of time and the maximum number of ports in this state
    MUST be configurable by the CGN administrator.
 Justification:  This is necessary in order to prevent collisions
    between old and new mappings and sessions.  It ensures that all
    established sessions are broken instead of redirected to a
    different peer.
    The exceptions are for cases where reusing a port immediately does
    not create a possibility that packets would be redirected to the
    wrong peer.  One can imagine other exceptions where mapping
    collisions are avoided, thus justifying the SHOULD level for this
    requirement.
    The 120 seconds value corresponds to the Maximum Segment Lifetime
    (MSL) from [RFC0793].
    Note that this requirement also applies to the case when a CGN
    loses state (due to a crash, reboot, failover to a cold standby,
    etc.).  In that case, ports that were in use at the time of state
    loss SHOULD NOT be reallocated until at least 120 seconds have
    passed.
 REQ-9:  A CGN MUST implement a protocol giving subscribers explicit
    control over NAT mappings.  That protocol SHOULD be the Port
    Control Protocol [RFC6887].
 Justification:  Allowing subscribers to manipulate the NAT state
    table with PCP greatly increases the likelihood that applications
    will function properly.

Perreault, et al. Best Current Practice [Page 8] RFC 6888 CGN Requirements April 2013

    A study of PCP-less CGN impacts can be found in [NAT444].  Another
    study considering the effects of PCP on a peer-to-peer file
    sharing protocol can be found in [BITTORRENT].
 REQ-10: CGN implementers SHOULD make their equipment manageable.
    Standards-based management using standards such as "Definitions of
    Managed Objects for NAT" [RFC4008] is RECOMMENDED.
 Justification:  It is anticipated that CGNs will be primarily
    deployed in ISP networks where the need for management is
    critical.  This requirement is at the SHOULD level to account for
    the fact that some CGN operators may not need management
    functionality.
    Note also that there are efforts within the IETF toward creating a
    MIB tailored for CGNs (e.g., [NAT-MIB]).
 REQ-11: When a CGN is unable to create a dynamic mapping due to
    resource constraints or administrative restrictions (i.e.,
    quotas):
    a.  it MUST drop the original packet;
    b.  it SHOULD send an ICMP Destination Unreachable message with
        code 1 (Host Unreachable) to the sender;
    c.  it SHOULD send a notification (e.g., SNMP trap) towards a
        management system (if configured to do so); and
    d.  it MUST NOT delete existing mappings in order to "make room"
        for the new one.  (This only applies to normal CGN behavior,
        not to manual operator intervention.)
 Justification:  This is a slightly different form of REQ-8 from
    [RFC5508].  Code 1 is preferred to code 13 because it is listed as
    a "soft error" in [RFC1122], which is important because we don't
    want TCP stacks to abort the connection attempt in this case.  See
    [RFC5461] for details on TCP's reaction to soft errors.
    Sending ICMP errors and SNMP traps may be rate-limited for
    security reasons, which is why requirements B and C are SHOULDs,
    not MUSTs.
    Applications generally handle connection establishment failure
    better than established connection failure.  This is why dropping
    the packet initiating the new connection is preferred over
    deleting existing mappings.  See also the rationale in Section 6
    of [RFC5508].

Perreault, et al. Best Current Practice [Page 9] RFC 6888 CGN Requirements April 2013

4. Logging

 It may be necessary for CGN administrators to be able to identify a
 subscriber based on external IPv4 address, port, and timestamp in
 order to deal with abuse.  When multiple subscribers share a single
 external address, the source address and port that are visible at the
 destination host have been translated from the ones originated by the
 subscriber.
 In order to be able to do this, the CGN would need to log the
 following information for each mapping created (this list is for
 informational purposes only and does not constitute a requirement):
 o  transport protocol
 o  subscriber identifier (e.g., internal source address or tunnel
    endpoint identifier)
 o  external source address
 o  external source port
 o  timestamp
 By "subscriber identifier" we mean information that uniquely
 identifies a subscriber.  For example, in a traditional NAT scenario,
 the internal source address would be sufficient.  In the case of DS-
 Lite, many subscribers share the same internal address and the
 subscriber identifier is the tunnel endpoint identifier (i.e., the
 B4's IPv6 address).
 A disadvantage of logging mappings is that CGNs under heavy usage may
 produce large amounts of logs, which may require large storage
 volume.
 REQ-12: A CGN SHOULD NOT log destination addresses or ports unless
    required to do so for administrative reasons.
 Justification:  Destination logging at the CGN creates privacy
    issues.  Furthermore, readers should be aware of logging
    recommendations for Internet-facing servers [RFC6302].  With
    compliant servers, the destination address and port do not need to
    be logged by the CGN.  This can help reduce the amount of logging.
    This requirement is at the SHOULD level to account for the fact
    that there may be other reasons for logging destination addresses
    or ports.  One such reason might be that the remote server is not
    following [RFC6302].

Perreault, et al. Best Current Practice [Page 10] RFC 6888 CGN Requirements April 2013

5. Port Allocation Scheme

 A CGN's port allocation scheme is subject to three competing
 requirements:
 REQ-13: A CGN's port allocation scheme SHOULD maximize port
    utilization.
 Justification:  External ports are one of the resources being shared
    by a CGN.  Efficient management of that resource directly impacts
    the quality of a subscriber's Internet connection.
    Some schemes are very efficient in their port utilization.  In
    that sense, they have good scaling properties (nothing is wasted).
    Others will systematically waste ports.
 REQ-14: A CGN's port allocation scheme SHOULD minimize log volume.
 Justification:  Huge log volumes can be problematic to CGN operators.
    Some schemes create one log entry per mapping.  Others allow
    multiple mappings to generate a single log entry, which sometimes
    can be expressed very compactly.  With some schemes, the logging
    frequency can approach that of DHCP servers.
 REQ-15: A CGN's port allocation scheme SHOULD make it hard for
    attackers to guess port numbers.
 Justification:  Easily guessed port numbers put subscribers at risk
    of the attacks described in [RFC6056].
    Some schemes provide very good security in that ports numbers are
    not easily guessed.  Others provide poor security to subscribers.
 A CGN implementation's choice of port allocation scheme optimizes to
 satisfy one requirement at the expense of another.  Therefore, these
 are soft requirements (SHOULD as opposed to MUST).

6. Deployment Considerations

 Several issues are encountered when CGNs are used [RFC6269].  There
 is current work in the IETF toward alleviating some of these issues.
 For example, see [NAT-REVEAL].

Perreault, et al. Best Current Practice [Page 11] RFC 6888 CGN Requirements April 2013

7. Security Considerations

 If a malicious subscriber can spoof another subscriber's CPE, it may
 cause a DoS to that subscriber by creating mappings up to the allowed
 limit.  An ISP can prevent this with ingress filtering, as described
 in [RFC2827].
 This document recommends endpoint-independent filtering (EIF) as the
 default filtering behavior for CGNs.  EIF has security considerations
 that are discussed in [RFC4787].
 NATs sometimes perform fragment reassembly.  CGNs would do so at
 presumably high data rates.  Therefore, the reader should be familiar
 with the potential security issues described in [RFC4963].

8. Acknowledgements

 Thanks for the input and review by Alexey Melnikov, Arifumi
 Matsumoto, Barry Leiba, Benson Schliesser, Dai Kuwabara, Dan Wing,
 Dave Thaler, David Harrington, Francis Dupont, Jean-Francois
 Tremblay, Joe Touch, Lars Eggert, Kousuke Shishikura, Mohamed
 Boucadair, Martin Stiemerling, Meng Wei, Nejc Skoberne, Pete Resnick,
 Reinaldo Penno, Ron Bonica, Sam Hartman, Sean Turner, Senthil
 Sivakumar, Stephen Farrell, Stewart Bryant, Takanori Mizuguchi,
 Takeshi Tomochika, Tina Tsou, Tomohiro Fujisaki, Tomohiro Nishitani,
 Tomoya Yoshida, Wes George, Wesley Eddy, and Yasuhiro Shirasaki.

9. References

9.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4008]  Rohit, R., Srisuresh, P., Raghunarayan, R., Pai, N., and
            C. Wang, "Definitions of Managed Objects for Network
            Address Translators (NAT)", RFC 4008, March 2005.
 [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
            (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
            RFC 4787, January 2007.
 [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
            Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
            RFC 5382, October 2008.

Perreault, et al. Best Current Practice [Page 12] RFC 6888 CGN Requirements April 2013

 [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
            Behavioral Requirements for ICMP", BCP 148, RFC 5508,
            April 2009.
 [RFC5597]  Denis-Courmont, R., "Network Address Translation (NAT)
            Behavioral Requirements for the Datagram Congestion
            Control Protocol", BCP 150, RFC 5597, September 2009.
 [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
            P.  Selkirk, "Port Control Protocol (PCP)", RFC 6887,
            April 2013.

9.2. Informative Reference

 [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
            793, September 1981.
 [RFC1122]  Braden, R., "Requirements for Internet Hosts -
            Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
            Translator (NAT) Terminology and Considerations", RFC
            2663, August 1999.
 [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
            Defeating Denial of Service Attacks which employ IP Source
            Address Spoofing", BCP 38, RFC 2827, May 2000.
 [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
            Errors at High Data Rates", RFC 4963, July 2007.
 [RFC5461]  Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
            February 2009.
 [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
            Protocol Port Randomization", BCP 156, RFC 6056, January
            2011.
 [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
            NAT64: Network Address and Protocol Translation from IPv6
            Clients to IPv4 Servers", RFC 6146, April 2011.
 [RFC6264]  Jiang, S., Guo, D., and B. Carpenter, "An Incremental
            Carrier-Grade NAT (CGN) for IPv6 Transition", RFC 6264,
            June 2011.

Perreault, et al. Best Current Practice [Page 13] RFC 6888 CGN Requirements April 2013

 [RFC6269]  Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
            Roberts, "Issues with IP Address Sharing", RFC 6269, June
            2011.
 [RFC6302]  Durand, A., Gashinsky, I., Lee, D., and S. Sheppard,
            "Logging Recommendations for Internet-Facing Servers", BCP
            162, RFC 6302, June 2011.
 [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
            Stack Lite Broadband Deployments Following IPv4
            Exhaustion", RFC 6333, August 2011.
 [NAT-MIB]  Perreault, S., Tsou, T., and S. Sivakumar, "Additional
            Managed Objects for Network Address Translators (NAT)",
            Work in Progress, February 2013.
 [NAT-REVEAL]
            Boucadair, M., Touch, J., Levis, P., and R. Penno,
            "Analysis of Solution Candidates to Reveal a Host
            Identifier (HOST_ID) in Shared Address Deployments", Work
            in Progress, April 2013.
 [NAT444]   Donley, C., Ed., Howard, L., Kuarsingh, V., Berg, J., and
            J. Doshi, "Assessing the Impact of Carrier-Grade NAT on
            Network Applications", Work in Progress, April 2013.
 [BITTORRENT]
            Boucadair, M., Zheng, T., Deng, X., and J. Queiroz,
            "Behavior of BitTorrent service in PCP-enabled networks
            with Address Sharing", Work in Progress, May 2012.

Authors' Addresses

 Simon Perreault (editor)
 Viagenie
 246 Aberdeen
 Quebec, QC  G1R 2E1
 Canada
 Phone: +1 418 656 9254
 EMail: simon.perreault@viagenie.ca
 URI:   http://www.viagenie.ca

Perreault, et al. Best Current Practice [Page 14] RFC 6888 CGN Requirements April 2013

 Ikuhei Yamagata
 NTT Communications Corporation
 Gran Park Tower 17F, 3-4-1 Shibaura, Minato-ku
 Tokyo  108-8118
 Japan
 Phone: +81 50 3812 4704
 EMail: ikuhei@nttv6.jp
 Shin Miyakawa
 NTT Communications Corporation
 Gran Park Tower 17F, 3-4-1 Shibaura, Minato-ku
 Tokyo  108-8118
 Japan
 Phone: +81 50 3812 4695
 EMail: miyakawa@nttv6.jp
 Akira Nakagawa
 Japan Internet Exchange Co., Ltd. (JPIX)
 Otemachi Building 21F, 1-8-1 Otemachi, Chiyoda-ku
 Tokyo  100-0004
 Japan
 Phone: +81 90 9242 2717
 EMail: a-nakagawa@jpix.ad.jp
 Hiroyuki Ashida
 Cisco Systems
 Midtown Tower, 9-7-1, Akasaka
 Minato-Ku, Tokyo  107-6227
 Japan
 EMail: hiashida@cisco.com

Perreault, et al. Best Current Practice [Page 15]

Internet Engineering Task Force (IETF) R. Penno Request for Comments: 7857 Cisco BCP: 127 S. Perreault Updates: 4787, 5382, 5508 Jive Communications Category: Best Current Practice M. Boucadair, Ed. ISSN: 2070-1721 Orange

                                                          S. Sivakumar
                                                                 Cisco
                                                              K. Naito
                                                                   NTT
                                                            April 2016
Updates to Network Address Translation (NAT) Behavioral Requirements

Abstract

 This document clarifies and updates several requirements of RFCs
 4787, 5382, and 5508 based on operational and development experience.
 The focus of this document is Network Address Translation from IPv4
 to IPv4 (NAT44).
 This document updates RFCs 4787, 5382, and 5508.

Status of This Memo

 This memo documents an Internet Best Current Practice.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 BCPs is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7857.

Penno, et al. Best Current Practice [Page 1] RFC 7857 Updates to NAT Behavioral Requirements April 2016

Copyright Notice

 Copyright (c) 2016 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Penno, et al. Best Current Practice [Page 2] RFC 7857 Updates to NAT Behavioral Requirements April 2016

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
 2.  TCP Session Tracking  . . . . . . . . . . . . . . . . . . . .   4
   2.1.  TCP Transitory Connection Idle-Timeout  . . . . . . . . .   6
   2.2.  TCP RST . . . . . . . . . . . . . . . . . . . . . . . . .   6
 3.  Port Overlapping Behavior . . . . . . . . . . . . . . . . . .   6
 4.  Address Pooling Paired (APP)  . . . . . . . . . . . . . . . .   7
 5.  Endpoint-Independent Mapping (EIM) Protocol Independence  . .   8
 6.  Endpoint-Independent Filtering (EIF) Protocol Independence  .   8
 7.  Endpoint-Independent Filtering (EIF) Mapping Refresh  . . . .   8
   7.1.  Outbound Mapping Refresh and Error Packets  . . . . . . .   9
 8.  Port Parity . . . . . . . . . . . . . . . . . . . . . . . . .   9
 9.  Port Randomization  . . . . . . . . . . . . . . . . . . . . .   9
 10. IP Identification (IP ID) . . . . . . . . . . . . . . . . . .  10
 11. ICMP Query Mappings Timeout . . . . . . . . . . . . . . . . .  10
 12. Hairpinning Support for ICMP Packets  . . . . . . . . . . . .  10
 13. Security Considerations . . . . . . . . . . . . . . . . . . .  11
 14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   14.1.  Normative References . . . . . . . . . . . . . . . . . .  12
   14.2.  Informative References . . . . . . . . . . . . . . . . .  12
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
 Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1. Introduction

 [RFC4787], [RFC5382], and [RFC5508] contributed to enhance Network
 Address Translation (NAT) interoperability and conformance.
 Operational experience gained through widespread deployment and
 evolution of NAT indicates that some areas of the original documents
 need further clarification or updates.  This document provides such
 clarifications and updates.

1.1. Scope

 The goal of this document is to clarify and update the set of
 requirements listed in [RFC4787], [RFC5382], and [RFC5508].  The
 document focuses exclusively on NAT44.
 The scope of this document has been set so that it does not create
 new requirements beyond those specified in the documents cited above.
 Requirements related to Carrier-Grade NAT (CGN) are defined in
 [RFC6888].

Penno, et al. Best Current Practice [Page 3] RFC 7857 Updates to NAT Behavioral Requirements April 2016

1.2. 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].
 The reader is assumed to be familiar with the terminology defined in
 [RFC2663], [RFC4787], [RFC5382], and [RFC5508].
 In this document, the term "NAT" refers to both "Basic NAT" and
 "Network Address/Port Translator (NAPT)" (see Section 3 of
 [RFC4787]).  As a reminder, Basic NAT and NAPT are two variations of
 traditional NAT in that translation in Basic NAT is limited to IP
 addresses alone, whereas translation in NAPT is extended to include
 IP addresses and transport identifiers (such as a TCP/UDP port or
 ICMP query ID); refer to Section 2 of [RFC3022].

2. TCP Session Tracking

 [RFC5382] specifies TCP timers associated with various connection
 states but does not specify the TCP state machine a NAT44 should
 follow as a basis to apply such timers.
 Update:  The TCP state machine depicted in Figure 1, adapted from
    [RFC6146], SHOULD be implemented by a NAT for TCP session tracking
    purposes.

Penno, et al. Best Current Practice [Page 4] RFC 7857 Updates to NAT Behavioral Requirements April 2016

                  +----------------------------+
                  |                            |
                  V                            |
               +------+   Client               |
               |CLOSED|-----SYN------+         |
               +------+              |         |
                   ^                 |         |
                   |TCP_TRANS T.O.   |         |
                   |                 V         |
               +-------+          +-------+    |
               | TRANS |          |  INIT |    |
               +-------+          +-------+    |
                 |    ^               |        |
           data pkt   |               |        |
                 | Server/Client RST  |        |
                 |  TCP_EST T.O.      |        |
                 V    |           Server SYN   |
            +--------------+          |        |
            | ESTABLISHED  |<---------+        |
            +--------------+                   |
             |           |                     |
       Client FIN    Server FIN                |
             |           |                     |
             V           V                     |
      +---------+   +----------+               |
      |  C FIN  |   |  S FIN   |               |
      |   RCV   |   |    RCV   |               |
      +---------+   +----------+               |
          |             |                      |
      Server FIN      Client FIN            TCP_TRANS
          |             |                    T.O.
          V             V                      |
      +----------------------+                 |
      |   C FIN + S FIN RCV  |-----------------+
      +----------------------+
  Legend:
    * Messages sent or received from the server are
      prefixed with "Server".
    * Messages sent or received from the client are
      prefixed with "Client".
    * "C" means "Client-side".
    * "S" means "Server-side".
    * TCP_EST T.O. refers to the established connection
      idle-timeout as defined in [RFC5382].
    * TCP_TRANS T.O. refers to the transitory connection
      idle-timeout as defined in [RFC5382].
         Figure 1: Simplified Version of the TCP State Machine

Penno, et al. Best Current Practice [Page 5] RFC 7857 Updates to NAT Behavioral Requirements April 2016

2.1. TCP Transitory Connection Idle-Timeout

 The transitory connection idle-timeout 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 (REQ-5 of [RFC5382]).  However, [RFC5382] does not clearly
 state whether these can be configured separately.
 Clarification:  This document clarifies that a NAT SHOULD provide
    different configurable parameters for configuring the open and
    closing idle timeouts.
    To accommodate deployments that consider a partially open timeout
    of 4 minutes as being excessive from a security standpoint, a NAT
    MAY allow the configured timeout to be less than 4 minutes.
    However, a minimum default transitory connection idle-timeout of 4
    minutes is RECOMMENDED.

2.2. TCP RST

 [RFC5382] leaves the handling of TCP RST packets unspecified.
 Update:  This document adopts a similar default behavior as in
    [RFC6146].  Concretely, when the NAT receives a TCP RST matching
    an existing mapping, it MUST translate the packet according to the
    NAT mapping entry.  Moreover, the NAT SHOULD wait for 4 minutes
    before deleting the session and removing any state associated with
    it if no packets are received during that 4-minute timeout.
    Notes:
  • Admittedly, the NAT has to verify whether received TCP RST

packets belong to a connection. This verification check is

       required to avoid off-path attacks.
  • If the NAT immediately removes the NAT mapping upon receipt of

a TCP RST message, stale connections may be maintained by

       endpoints if the first RST message is lost between the NAT and
       the recipient.

3. Port Overlapping Behavior

 REQ-1 from [RFC4787] and REQ-1 from [RFC5382] specify a specific port
 overlapping behavior; that is, the external IP address and port can
 be reused for connections originating from the same internal source
 IP address and port irrespective of the destination.  This is known
 as Endpoint-Independent Mapping (EIM).

Penno, et al. Best Current Practice [Page 6] RFC 7857 Updates to NAT Behavioral Requirements April 2016

 Update:  This document clarifies that this port overlapping behavior
    may be extended to connections originating from different internal
    source IP addresses and ports as long as their destinations are
    different.
    The following mechanism MAY be implemented by a NAT:
       If destination addresses and ports are different for outgoing
       connections started by local clients, a NAT MAY assign the same
       external port as the source ports for the connections.  The
       port overlapping mechanism manages mappings between external
       packets and internal packets by looking at and storing their
       5-tuple (protocol, source address, source port, destination
       address, and destination port).
    This enables concurrent use of a single NAT external port for
    multiple transport sessions, which allows a NAT to successfully
    process packets in a network that has a limited number of IP
    addresses (e.g., deployment with a high address space
    multiplicative factor (refer to Appendix B of [RFC6269])).

4. Address Pooling Paired (APP)

 The "IP address pooling" behavior of "Paired" (APP) was recommended
 in REQ-2 from [RFC4787], but the behavior when an external IPv4 runs
 out of ports was left undefined.
 Clarification:  This document clarifies that if APP is enabled, new
    sessions from a host that already has a mapping associated with an
    external IP that ran out of ports SHOULD be dropped.  A
    configuration parameter MAY be provided to allow a NAT to start
    using ports from another external IP address when the one that
    anchored the APP mapping ran out of ports.  Tweaking this
    configuration parameter is a trade-off between service continuity
    and APP strict enforcement.  Note, this behavior is sometimes
    referred to as "soft-APP".
    As a reminder, the recommendation for the particular case of a CGN
    is that an implementation must use the same external IP address
    mapping for all sessions associated with the same internal IP
    address, be they TCP, UDP, ICMP, something else, or a mix of
    different protocols [RFC6888].
 Update:  This behavior SHOULD apply also for TCP.

Penno, et al. Best Current Practice [Page 7] RFC 7857 Updates to NAT Behavioral Requirements April 2016

5. Endpoint-Independent Mapping (EIM) Protocol Independence

 REQ-1 from [RFC4787] and REQ-1 from [RFC5382] do not specify whether
 EIM are protocol dependent or protocol independent.  For example, if
 an outbound TCP SYN creates a mapping, it is left undefined whether
 outbound UDP packets can reuse such mapping.
 Update:  EIM mappings SHOULD be protocol dependent.  A configuration
    parameter MAY be provided to allow protocols that multiplex TCP
    and UDP over the same source IP address and port number to use a
    single mapping.  The default value of this configuration parameter
    MUST be protocol-dependent EIM.
    This update is consistent with the stateful Network Address and
    Protocol Translation from IPv6 Clients to IPv4 Servers (NAT64)
    [RFC6146] that clearly specifies three binding information bases
    (TCP, UDP, and ICMP).

6. Endpoint-Independent Filtering (EIF) Protocol Independence

 REQ-8 from [RFC4787] and REQ-3 from [RFC5382] do not specify whether
 mappings with Endpoint-Independent Filtering (EIF) are protocol
 independent or protocol dependent.  For example, if an outbound TCP
 SYN creates a mapping, it is left undefined whether inbound UDP
 packets matching that mapping should be accepted or rejected.
 Update:  EIF filtering SHOULD be protocol dependent.  A configuration
    parameter MAY be provided to make it protocol independent.  The
    default value of this configuration parameter MUST be protocol-
    dependent EIF.
    This behavior is aligned with the update in Section 5.
    Applications that can be transported over a variety of transport
    protocols and/or support transport fallback schemes won't
    experience connectivity failures if the NAT is configured with
    protocol-independent EIM and protocol-independent EIF.

7. Endpoint-Independent Filtering (EIF) Mapping Refresh

 The NAT mapping Refresh direction may have a "NAT Inbound refresh
 behavior" of "True" according to REQ-6 from [RFC4787], but [RFC4787]
 does not clarify how this behavior applies to EIF mappings.  The
 issue in question is whether inbound packets that match an EIF
 mapping but do not create a new session due to a security policy
 should refresh the mapping timer.

Penno, et al. Best Current Practice [Page 8] RFC 7857 Updates to NAT Behavioral Requirements April 2016

 Clarification:  This document clarifies that even when a NAT has an
    inbound refresh behavior set to "TRUE", such packets SHOULD NOT
    refresh the mapping.  Otherwise, a simple attack of a packet every
    two minutes can keep the mapping indefinitely.
 Update:  This behavior SHOULD apply also for TCP.

7.1. Outbound Mapping Refresh and Error Packets

 Update:  In the case of NAT outbound refresh behavior, ICMP Errors or
    TCP RST outbound packets sent as a response to inbound packets
    SHOULD NOT refresh the mapping.  Other packets that indicate the
    host is not interested in receiving packets MAY be configurable to
    also not refresh state, such as a Session Traversal Utilities for
    NAT (STUN) error response [RFC5389] or IKE INVALID_SYNTAX
    [RFC7296].

8. Port Parity

 Update:  A NAT MAY disable port parity preservation for all dynamic
    mappings.  Nevertheless, A NAT SHOULD support means to explicitly
    request to preserve port parity (e.g., [RFC7753]).
    Note: According to [RFC6887], dynamic mappings are said to be
    dynamic in the sense that they are created on demand, either
    implicitly or explicitly:
    1.  Implicit dynamic mappings refer to mappings that are created
        as a side effect of traffic such as an outgoing TCP SYN or
        outgoing UDP packet.  Implicit dynamic mappings usually have a
        finite lifetime, though this lifetime is generally not known
        to the client using them.
    2.  Explicit dynamic mappings refer to mappings that are created
        as a result, for example, of explicit Port Control Protocol
        (PCP) MAP and PEER requests.  Explicit dynamic mappings have a
        finite lifetime, and this lifetime is communicated to the
        client.

9. Port Randomization

 Update:  A NAT SHOULD follow the recommendations specified in
    Section 4 of [RFC6056], especially:
       A NAPT that does not implement port preservation [RFC4787]
       [RFC5382] SHOULD obfuscate selection of the ephemeral port of a
       packet when it is changed during translation of that packet.

Penno, et al. Best Current Practice [Page 9] RFC 7857 Updates to NAT Behavioral Requirements April 2016

       A NAPT that does implement port preservation SHOULD obfuscate
       the ephemeral port of a packet only if the port must be changed
       as a result of the port being already in use for some other
       session.
       A NAPT that performs parity preservation and that must change
       the ephemeral port during translation of a packet SHOULD
       obfuscate the ephemeral ports.  The algorithms described in
       this document could be easily adapted such that the parity is
       preserved (i.e., force the lowest order bit of the resulting
       port number to 0 or 1 according to whether even or odd parity
       is desired).

10. IP Identification (IP ID)

 Update:  A NAT SHOULD handle the Identification field of translated
    IPv4 packets as specified in Section 5.3.1 of [RFC6864].

11. ICMP Query Mappings Timeout

 Section 3.1 of [RFC5508] specifies that ICMP Query mappings are to be
 maintained by a NAT.  However, the specification doesn't discuss
 Query mapping timeout values.  Section 3.2 of [RFC5508] only
 discusses ICMP Query session timeouts.
 Update:  ICMP Query mappings MAY be deleted once the last session
    using the mapping is deleted.

12. Hairpinning Support for ICMP Packets

 REQ-7 from [RFC5508] specifies that a NAT enforcing Basic NAT must
 support traversal of hairpinned ICMP Query sessions.
 Clarification:  This implicitly means that address mappings from
    external address to internal address (similar to Endpoint-
    Independent Filters) must be maintained to allow inbound ICMP
    Query sessions.  If an ICMP Query is received on an external
    address, a NAT can then translate to an internal IP.
 REQ-7 from [RFC5508] specifies that all NATs must support the
 traversal of hairpinned ICMP Error messages.
 Clarification:  This behavior requires a NAT to maintain address
    mappings from external IP address to internal IP address in
    addition to the ICMP Query mappings described in Section 3.1 of
    [RFC5508].

Penno, et al. Best Current Practice [Page 10] RFC 7857 Updates to NAT Behavioral Requirements April 2016

13. Security Considerations

 NAT behavioral considerations are discussed in [RFC4787], [RFC5382],
 and [RFC5508].
 Because some of the clarifications and updates (e.g., Section 2) are
 inspired from NAT64, the security considerations discussed in
 Section 5 of [RFC6146] apply also for this specification.
 The update in Section 3 allows for an optimized NAT resource usage.
 In order to avoid service disruption, the NAT must not invoke this
 functionality unless the packets are to be sent to distinct
 destination addresses.
 Some of the updates (e.g., Sections 7, 9, and 11) allow for increased
 security compared to [RFC4787], [RFC5382], and [RFC5508].
 Particularly,
 o  the updates in Sections 7 and 11 prevent an illegitimate node to
    maintain mappings activated in the NAT while these mappings should
    be cleared, and
 o  port randomization (Section 9) complicates tracking hosts located
    behind a NAT.
 Sections 4 and 12 propose updates that increase the serviceability of
 a host located behind a NAT.  These updates do not introduce any
 additional security concerns to [RFC4787], [RFC5382], and [RFC5508].
 The updates in Sections 5 and 6 allow for a better NAT transparency
 from an application standpoint.  Hosts that require a restricted
 filtering behavior should enable specific policies (e.g., Access
 Control List (ACL)) either locally or by soliciting a dedicated
 security device (e.g., firewall).  How a host updates its filtering
 policies is out of scope of this document.
 The update in Section 8 induces security concerns that are specific
 to the protocol used to interact with the NAT.  For example, if PCP
 is used to explicitly request parity preservation for a given
 mapping, the security considerations discussed in [RFC6887] should be
 taken into account.
 The update in Section 10 may have undesired effects on the
 performance of the NAT in environments in which fragmentation is
 massively experienced.  Such an issue may be used as an attack vector
 against NATs.

Penno, et al. Best Current Practice [Page 11] RFC 7857 Updates to NAT Behavioral Requirements April 2016

14. References

14.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
            Translation (NAT) Behavioral Requirements for Unicast
            UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
            2007, <http://www.rfc-editor.org/info/rfc4787>.
 [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
            Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
            RFC 5382, DOI 10.17487/RFC5382, October 2008,
            <http://www.rfc-editor.org/info/rfc5382>.
 [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
            Behavioral Requirements for ICMP", BCP 148, RFC 5508,
            DOI 10.17487/RFC5508, April 2009,
            <http://www.rfc-editor.org/info/rfc5508>.
 [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
            Protocol Port Randomization", BCP 156, RFC 6056,
            DOI 10.17487/RFC6056, January 2011,
            <http://www.rfc-editor.org/info/rfc6056>.
 [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
            NAT64: Network Address and Protocol Translation from IPv6
            Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
            April 2011, <http://www.rfc-editor.org/info/rfc6146>.
 [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
            RFC 6864, DOI 10.17487/RFC6864, February 2013,
            <http://www.rfc-editor.org/info/rfc6864>.

14.2. Informative References

 [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
            Translator (NAT) Terminology and Considerations",
            RFC 2663, DOI 10.17487/RFC2663, August 1999,
            <http://www.rfc-editor.org/info/rfc2663>.

Penno, et al. Best Current Practice [Page 12] RFC 7857 Updates to NAT Behavioral Requirements April 2016

 [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
            Address Translator (Traditional NAT)", RFC 3022,
            DOI 10.17487/RFC3022, January 2001,
            <http://www.rfc-editor.org/info/rfc3022>.
 [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
            "Session Traversal Utilities for NAT (STUN)", RFC 5389,
            DOI 10.17487/RFC5389, October 2008,
            <http://www.rfc-editor.org/info/rfc5389>.
 [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
            P. Roberts, "Issues with IP Address Sharing", RFC 6269,
            DOI 10.17487/RFC6269, June 2011,
            <http://www.rfc-editor.org/info/rfc6269>.
 [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
            P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
            DOI 10.17487/RFC6887, April 2013,
            <http://www.rfc-editor.org/info/rfc6887>.
 [RFC6888]  Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
            A., and H. Ashida, "Common Requirements for Carrier-Grade
            NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,
            April 2013, <http://www.rfc-editor.org/info/rfc6888>.
 [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
            Kivinen, "Internet Key Exchange Protocol Version 2
            (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
            2014, <http://www.rfc-editor.org/info/rfc7296>.
 [RFC7753]  Sun, Q., Boucadair, M., Sivakumar, S., Zhou, C., Tsou, T.,
            and S. Perreault, "Port Control Protocol (PCP) Extension
            for Port-Set Allocation", RFC 7753, DOI 10.17487/RFC7753,
            February 2016, <http://www.rfc-editor.org/info/rfc7753>.

Acknowledgements

 Thanks to Dan Wing, Suresh Kumar, Mayuresh Bakshi, Rajesh Mohan, Lars
 Eggert, Gorry Fairhurst, Brandon Williams, and David Black for their
 review and discussion.
 Many thanks to Ben Laurie for the SecDir review and Dan Romascanu for
 the Gen-ART review.
 Dan Wing proposed some text for the configurable errors in
 Section 7.1.

Penno, et al. Best Current Practice [Page 13] RFC 7857 Updates to NAT Behavioral Requirements April 2016

Contributors

 The following individual contributed text to the document:
    Sarat Kamiset
    Insieme Networks
    United States

Authors' Addresses

 Reinaldo Penno
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, California  95134
 United States
 Email: repenno@cisco.com
 Simon Perreault
 Jive Communications
 Canada
 Email: sperreault@jive.com
 Mohamed Boucadair (editor)
 Orange
 Rennes  35000
 France
 Email: mohamed.boucadair@orange.com
 Senthil Sivakumar
 Cisco Systems, Inc.
 United States
 Email: ssenthil@cisco.com
 Kengo Naito
 NTT
 Tokyo
 Japan
 Email: k.naito@nttv6.jp

Penno, et al. Best Current Practice [Page 14]

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