[Docs] [txt|pdf]



[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


Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/