Network Working Group                                       R. Bush, Ed.
Internet-Draft                                 Internet Initiative Japan
Intended status: Standards Track                        October 27, 2009
Expires: April 30, 2010

             The A+P Approach to the IPv4 Address Shortage

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   We are facing the exhaustion of the IANA IPv4 free IP address pool.

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   Unfortunately, IPv6 is not yet deployed widely enough to fully
   replace IPv4, and it is unrealistic to expect that this is going to
   change before we run out of IPv4 addresses.  Letting hosts seamlessly
   communicate in an IPv4-world without assigning a unique globally
   routable IPv4 address to each of them is a challenging problem.

   This draft discusses the possibility of address sharing by treating
   some of the port number bits as part of an extended IPv4 address
   (Address plus Port, or A+P).  Instead of assigning a single IPv4
   address to a customer device, we propose to extended the address by
   "stealing" bits from the port number in the TCP/UDP header, leaving
   the applications a reduced range of ports.  This means assigning the
   same IPv4 address to multiple clients (e.g., CPE, mobile phones),
   each with its assigned port-range.  In the face of IPv4 address
   exhaustion, the need for addresses is stronger than the need to be
   able to address thousands of applications on a single host.  If
   address translation is needed, the end-user should be in control of
   the translation process - not some smart boxes in the core.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Why Carrier Grade NATs are Harmful . . . . . . . . . . . .  4
   2.  Design Constraints and Assumptions . . . . . . . . . . . . . .  6
     2.1.  Design constraints . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Overview of the A+P Solution . . . . . . . . . . . . . . . . .  8
     3.1.  Signaling  . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.2.  Address realm  . . . . . . . . . . . . . . . . . . . . . . 11
     3.3.  Reasons for allowing multiple A+P gateways . . . . . . . . 14
   4.  Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 16
     4.1.  A+P for Broadband Providers  . . . . . . . . . . . . . . . 16
     4.2.  A+P for Mobile Providers . . . . . . . . . . . . . . . . . 17
     4.3.  A+P from the provider network perspective  . . . . . . . . 17
     4.4.  Dynamic allocation of port ranges  . . . . . . . . . . . . 20
     4.5.  Overall A+P architecture . . . . . . . . . . . . . . . . . 22
     4.6.  Example of A+P-forwarded packets . . . . . . . . . . . . . 22
     4.7.  Forwarding of standard packets . . . . . . . . . . . . . . 27
     4.8.  Handling ICMP  . . . . . . . . . . . . . . . . . . . . . . 27
     4.9.  Limitations of the A+P approach  . . . . . . . . . . . . . 28
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   7.  Authors  . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 31
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 32
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 32
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 33

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1.  Introduction

   This document describes a technique to deal with the imminent IPv4
   address space exhaustion.  Many large Internet Service Providers
   (ISPs) face the problem that their networks' customer edges are so
   large that it will soon not be possible to provide each customer with
   a unique IPv4 address.  Therefore these ISPs have to devise something
   more ingenious.  Although undesirable, address sharing, a la NAT, is

   To allow end-to-end connectivity between IPv4 speaking applications
   we propose to "steal" some bits from the UDP/TCP header and use them
   to extend addressing of devices.  Assuming we could limit the
   applications' port addressing to 8 (or 4) bits, we can increase the
   effective size of an IPv4 address by 8 (or 12) additional bits.  In
   this scenario, 128 (or 4096) customers could be multiplexed on the
   same IPv4 address, while allowing them a fixed range of 512 (or 16)
   ports.  Customers that require larger port-ranges could dynamically
   request additional blocks, depending on their contract.  We call this
   "extended addressing" or "A+P" (Address plus Port) addressing.  The
   main advantage of A+P is that it preserves the Internet "end-to-end"
   paradigm by not translating (at least some ports of) an IP address.
   With NAT in the core of the network, this end-to-end connectivity is
   broken.  As long as the customer chooses to do this on his/her
   premises this is a choice that he/she takes, however this is not an
   option in face of the looming IPv4 address exhaustion, where so
   called Carrier Grade NATs (CGNs) might be deployed within the
   providers network - beyond control of the customer.  CGNs come with
   different names and in different flavors, such as NAT444, Large Scale
   NATs (LSNs) or Address Family Transition Routers (AFTR).

1.1.  Why Carrier Grade NATs are Harmful

   Various forms of NATs will be installed at various levels and places
   in the IPv4-Internet to achieve address compression.  This document
   argues for mechanisms where this happens as close to the edge as
   possible, thereby minimizing damage to the End to End Principle.
   End-customers will not be locked into a walled-garden without any
   control over the translation.  It is is essential to create
   mechanisms to "bypass" NATs in the core, and keep the control at the

   "Carrier grade" is a euphemism for centralized.  More semantics move
   to the core of the network.  This is bad in and of itself.  Net-heads
   call it "telco-think" because it is the telco model of smarts in the
   core as opposed to the Internet model of a simple, just-forward-
   packets core, with smart edges.  It also places the provider in the
   position, where the user is trapped behind unchangeable application

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   policies, and has the danger of invoking lawyers when users wish to
   deploy new applications needing Application Level Gateways (ALGs).
   This is the opposite of the "end-to-end" model of the Internet.

   With the smarts at the edges, one can easily field new protocols
   between consenting end-points by merely tweaking the NATs at the
   corresponding Customer Premises Equipment (CPE), even adding
   application layer gateways if they are needed.

   Today's NATs are typically mitigated by ALGs over which the customer
   has control, e.g. port forwarding or UPnP/NAT-PMP.  However, this is
   not expected to work with CGNs.  CGN proposals - other than DS-Lite
   [I-D.ietf-softwire-dual-stack-lite] with A+P - admit that it is not
   expected that applications that require specific port assignment or
   port mapping from the NAT box will keep working.  This is the
   ultimate horror the NAT-haters fear, and, in this case, they are not
   all that wrong.

   We believe this CGN approach is not an option and that the end-user
   must have the ability to control their own ALGs.  With CGN, if a user
   wishes to deploy a new application, they must talk to the providers'
   lawyers or run new disruptive technology over HTTP; we can pick our
   poison.  And if the NAT is not where the customer can directly
   control it, i.e., it is anywhere in the provider's network, then the
   provider controls what the user can control, i.e. it is not really
   under user control.  We do not wish to deal with the case where the
   provider has to decide whether to allow Skype v42 when they
   themselves provide a competing VoIP product.

   Another issue with CGN is scalability.  ISPs face a tension between
   the placement of CGNs within their network to aggregate as much as
   possible, when too much aggregation creates a massive state problem.
   CGNs also present a single point of failure.  And having a back-up
   CGN has the state transfer problem as well as exposure to network
   partition and dual-device failure.  When you start talking about
   'high reliability/availability, you have already lost the game.  The
   internet is about building a reliable network using unreliable

   To reduce the state, NAT placement ends up as CGNs somewhere closer
   to the edge.  It is not clear how a CGN should maintain per-session
   state in a scalable manner.  State for improperly terminated sessions
   could remain stale for some time.  The CGN hence trades scalability
   for the amount of state that needs to be kept, which makes optimally
   placing a CGN a hard engineering problem.

   Furthermore, with CGN, tracing hackers, spammers and other criminals
   will be impossible, unless all the connection based mapping

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   information is recorded and stored.  This would not only cause
   concern for law enforcement services, but also for privacy advocates.

2.  Design Constraints and Assumptions

   The problem of address space shortage is first felt by providers with
   a very large end-user customer base, such as broadband providers and
   mobile-service providers.  Though the cases and requirements are
   slightly different, they share many commonalities.  In the following
   we will develop a set of overall design constraints.

2.1.  Design constraints

   We regard several constraints as important for our design:

   1)      End-to-End is under customer control: Customers shall have
           the ability to deploy new application protocols at will.
           IPv4 address shortage should not be a license to break the
           Internet's end-to-end paradigm.

   2)      End-to-End transparency through multiple intermediate
           devices: Multiple gateways should be able to operate in
           sequence along one data path without interfering with each

   3)      Backward compatibility: Approaches should be transparent to
           unaware users.  Devices or existing applications should be
           able to work without modification.  Emergence of new
           applications should not be limited.

   4)      Incrementally deployable: The provider should not be forced
           to replace unaffected core devices or replace customer
           premises equipment (CPE).  In particular, the provider should
           be able to change only CPE where they wish to deploy A+P. And
           customers should be able to acquire A+P aware CPE at will.

   5)      Highly-scalable and minimal state core: Minimal state should
           be kept inside the ISP's network.  If the operator is rolling
           out A+P incrementally, it is understood there may be state in
           the core in the non-A+P part of such a roll-out.

   6)      Efficiency vs. complexity: Operators should have the
           flexibility to trade off port multiplexing efficiency and
           scalability and end-to-end transparency.

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   7)      Automatic configuration/administration: There should be no
           need for customers to call the ISP and tell them that they
           are operating their own A+P-gateway devices.  Customers/
           mobile phone users should not be expected to look-up assigned
           ports manually on websites and then configure them on devices
           or applications.

   8)      "Double-NAT" should be avoided: Based on Constraint 2
           multiple gateway devices might be present in a path, and once
           one has done some translation, those packets should not be

   9)      Legal traceability: ISPs must be able to provide the identity
           of a customer from the knowledge of the IPv4 public address
           and the port.  This should have as low an impact as is
           reasonable on storage by the ISP.  We assume that NATs on
           customer premises do not pose much of a problem, while
           provider NATs need to keep additional logs.

   10)     IPv6 deployment should be encouraged.  NAT444 strongly biases
           the users to the deployment of RFC 1918 addressing.  A+P
           should not.  While we acknowledge that A+P might be used in
           an IPv4-only environment (e.g., [I-D.boucadair-port-range])
           we strongly believe that IPv6 is the best long-term approach,
           and that A+P should be considered only as an intermediate
           hack towards an IPv6-only world.  We therefore prefer to
           assume in Constraint 10 that the ISP has migrated to a dual-
           stack core and A+P can use IPv6 as a transport inside the
           network.  This ensures that A+P will not be a hindrance to
           the introduction of IPv6.

   Constraints 2 and 8 are important: while many techniques have been
   deployed to allow applications to work through a NAT, traversing
   cascaded NATs is crucial if NATs are being deployed in the core of a
   provider network.

2.2.  Terminology

   The A+P architecture can be split into three distinct functions:
   encaps/decaps, NAT, and signaling.

   Encaps/decaps function: is used to forward port-restricted A+P-
   packets over intermediate legacy devices.  The encapsulation function
   takes an IPv4 packet, looks up the IP and TCP/UDP headers, and puts
   the packet into the appropriate tunnel.  The state needed to perform
   this action is comparable to a forwarding table.  The decapsulation
   device SHOULD check if the source address and port of packets coming
   out of the tunnel are legitimate (e.g., see [BCP38]).  Based on the

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   result of such a check, the packet MAY be forwarded untranslated, it
   MAY be discarded or MAY be NATed.  In this draft we refer to a device
   that provides this encaps/decaps functionality as Port-Range-Router

   Network Address Translation (NAT) function: is used to connect legacy
   end-hosts.  Unless upgraded, end-hosts or end-systems are not aware
   of A+P restrictions and therefore assume a full IP address.  The NAT
   function performs any address or port translation, including
   application-level-gateways (ALGs).  The state that has to be kept to
   implement this function is the mapping for which external addresses
   and ports have been mapped to which internal addresses and ports,
   just as in CPE NATs today.  A subtle, but very important, difference
   should be noted here: the customer has control over the NATing
   process or might choose to "bypass" the NAT.  If this is done, we
   call the NAT a large scale NAT (LSN).  However, if the NAT that does
   NOT allow the customer to control the translation process, we refer
   to as a CGN.

   Signaling function: is used in order to allow A+P-aware devices get
   to know which ports are assigned to be passed through untranslated
   and what will happen to packets outside the assigned port-range
   (e.g., could be NATed or discarded).  Signaling may also be used to
   learn the encapsulation method and any endpoint information needed.
   In addition, the signaling function may be used to dynamically
   increase/decrease the requested port-range.

   A+P address realm: a public routable IPv4 address that is port
   restricted (A+P).  Forwarding of packets is done based on the IPv4
   address and the TCP/UDP port numbers.  When this draft talks about
   "A+P packets" it is assumed that those packets pass untranslated.

   Private address realm: IPv4 addresses that are not globally routed.
   They may be taken from the [RFC1918] range.  However, this draft does
   not make such an assumption.  We regard as private address space any
   IPv4 address, which needs to be translated in order to gain global
   connectivity, irrespective of whether it falls in [RFC1918] space or

3.  Overview of the A+P Solution

   The core architectural elements of the A+P solution are three
   separated and independent functions: the NAT function, the encaps/
   decaps function, and the signaling function.  The NAT function is
   similar to a NAT as we know it today: it performs a translation
   between two different address realms.  When the external realm is
   public IPv4 address space, we assume that the translation is many-to-

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   one, in order to multiplex many customers on a single public IPv4
   address.  The only difference with a traditional NAT (Figure 1) is
   that the translator might only be able to use a restricted range of
   ports when mapping multiple internal addresses onto an external one,
   e.g., the external address realm might be port-restricted.

                    "internal-side"          "external-side"
                      internal     |  N  |     external
                      address  <---|  A  |---> address
                       realm       |  T  |      realm

                              Traditional NAT

                                 Figure 1

   The encaps/decaps function, on the other hand, is the ability to
   establish a tunnel with another end-point providing the same
   function.  This implies some form of signaling to establish a tunnel.
   Such signaling can be viewed as integrated with DHCP or as a separate
   service.  Section 3.1 discusses the constraints of this signaling
   function.  The tunnel can be an IPv6 or IPv4encapsulation, a layer-2
   tunnel, or some other form of softwire.  Note that the presence of a
   tunnel allows unmodified, naive, or even legacy devices between the
   two endpoints.

   Two or more devices which provide the encaps/decaps function and are
   linked by tunnels to form an A+P subsystem.  The function of each
   gateway is to encapsulate and decapsulate respectively.  Figure 2
   depicts the simplest possible A+P subsystem, that is, two devices
   providing the encaps/decaps function.

     port-restricted  | +----------+  tunnel  +----------+ |   external
      address realm --|-| gateway  |==========| gateway  |-|-- address
                      | +----------+          +----------+ |    realm
                                  A+P subsystem

                          A simple A+P subsystem

                                 Figure 2

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   Within an A+P subsystem, the external address realm is extended by
   "stealing" bits from the port number.  Each device is assigned one
   address from the external realm and a range of port numbers.  Hence,
   devices which are part of an A+P subsystem can communicate with the
   external address without the need for address translation (i.e.,
   preserving end-to-end packet integrity): an A+P packet originated
   from within the A+P subsystem can be simply forwarded over tunnels up
   to the endpoint, where it gets decapsulated and routed in the
   external realm.

3.1.  Signaling

   The following information needs to be available on all the gateways
   in the A+P subsystem.  It is expected that there will be a signaling
   protocol such as [I-D.bajko-pripaddrassign],
   [I-D.boucadair-dhcpv6-shared-address-option], or
   [I-D.boucadair-pppext-portrange-option].  The information that needs
   to be shared is the following:

   o  a set of public IPv4 addresses,

   o  for each IPv4 address a starting point for the allocated port-

   o  number of delegated ports,

   o  optional key that enables partial or full preservation of entropy
      in port randomization - see [I-D.bajko-pripaddrassign],

   o  lifetime for each IPv4 address and allocated port-set,

   o  the tunneling technology to be used (e.g., "IPv6-encapsulation")

   o  addresses of the tunnel endpoints (e.g., IPv6 address of tunnel

   o  whether or not NAT function is provided by the gateway

   o  a device identification number and some authentication mechanisms

   o  a version number and some reserved bits for future use.

   Note that the functions of encapsulation and decapsulation have been
   separated from the NAT function.  However, to accommodate legacy
   hosts, NATing is likely to be provided at some point in the path;
   therefore the availability or absence of NATing MUST be communicated
   in signaling, as A+P is agnostic about NAT placement.

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   The port-ranges can be allocated in two different ways:

   o  If applications or end-hosts behind the CPE are not UPnPv2/NAT-PMP
      aware, then the CPE SHOULD request ports via mechanisms, e.g. as
      described in [I-D.bajko-pripaddrassign] and
      [I-D.boucadair-pppext-portrange-option].  Note that different
      port-ranges can have different lifetimes, and the CPE is not
      entitled to use them after they expire - unless it refreshes those
      ranges.  It is up to the ISP to put mechanisms in place, that
      determine what percentage of already allocated port-ranges should
      be exhausted before a CPE may requests additional ranges, how
      often the CPE can request additional ranges, and so on.  (To
      prevent Denial of Service attacks.)

   o  If applications behind the CPE are UPnPv2/NAT-PMP aware additional
      ports MAY be requested through that mechanism.  In this case the
      CPE should forward those requests to the LSN and the LSN should
      reply reporting if the requested ports are available or not (and
      if they are not available some alternatives should be offered).
      Here again, to prevent potential denial of service attacks,
      mechanism should be in place to prevent UPnPv2/NAT-PMP packet
      storms and fast port allocation.

   Whatever signaling mechanism is used inside the tunnels, DHCP or IPCP
   based, synchronization between signaling server and PRR must be
   established in both directions.  For example, if we use DHCP as
   signaling mechanism, the PRR must communicate to DHCP server at least
   its IP range.  The DHCP server then starts to allocate IPs and port-
   ranges to CPEs and communicates back to the PRR which IP and port
   range have been allocated to which CPE, so the PRR knows to which
   tunnel redirect incoming traffic.  In addition, DHCP MUST also
   communicate lifetimes of port-ranges assigned to CPE via the PRR.

   If UPnPv2/NAT-PMP is used as dynamic port allocation mechanism, the
   PRR must also communicate to the DHCP (or IPCP) server to avoid those
   ports.  The PRR must somehow (DHCP or IPCP options) communicate back
   to CPE that allocation of ports was successful, so CPE adds those
   ports to existing port-ranges.

3.2.  Address realm

   Each gateway within the A+P subsystem manages a certain portion of
   A+P address space, that is, a portion of IPv4 space which is extended
   by borrowing bits from the port number.  This address space may be a
   single, port-restricted IPv4 address.  The gateway MAY use its
   managed A+P address space for several purposes:

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   o  Allocation of a sub-portion of the A+P address space to other
      authenticated A+P gateways in the A+P subsystem (referred to as
      delegation).  We call the allocated sub-portion delegated address

   o  Exchange of (untranslated) packets with the external address
      realm.  For this to work, such packets MUST use source address and
      port belonging to the non-delegated address space.

   If the gateway is also capable of performing the NAT function, it MAY
   translate packets arriving on an internal interface which are outside
   of its managed A+P address space into non-delegated address space.

   Hence, a provider may have 'islands' of A+P as they slowly deploy
   over time.  The provider does not have to replace CPE until they want
   to provide the A+P function to an island of users or even to one
   particular user in a sea of non-A+P users.

   An A+P gateway ("A"), accepts incoming connections from other A+P
   gateways ("B").  Upon connection establishment (provided appropriate
   authentication), B would "ask" A for delegation of an A+P address.
   In turn, A will inform B about its public IPv4 address, and will
   delegate a portion of its port-range to B. In addition, A will also
   negotiate the encaps/decaps function with B (e.g., let B know the
   address of the decaps device/other-end-point of the tunnel).

   This could be implemented for example via a NAT-PMP or DHCP-like
   solution.  In general the following rule applies: A sub-portion of
   the managed A+P address space is delegated as long as devices below
   ask for it, otherwise private IPv4 is provided to support legacy

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              private    +-----+          +-----+     public
              address ---|  B  |==========|  A  |---  Internet
               realm     +-----+          +-----+

                         Address space realm of A:
                         public IPv4 address =
                         port range = 0-65535

                         Address space realm of B:
                         public IPv4 address =
                         port range = 2560-3071

                                 Figure 3

   Figure 3 illustrates a sample configuration.  Note that A might
   actually consist of three different devices: one that handles
   signaling requests from B; one device that performs encapsulation and
   decapsulation; and, if provided, one device that performs NATing
   function (e.g., LSN).  Packet forwarding is assumed to be as follows:
   In the "out-bound" case, a packet arrives from the private address
   realm to B. As stated above, B has two options: it can either apply
   or not apply the NAT function.  The decision depends upon the
   specific configuration and/or the capabilities of A and B. Note that
   NAT functionality is required to support legacy hosts, however, this
   can be done at either of the two devices A or B. The term NAT refers
   to translating the packet into the managed A+P address (B has address and ports 2560-3071 in the example above).  We then have two

   1)  B NATs the packet.  The translated packet is then tunneled to A.
       A recognizes that the packet has already been translated, because
       the source address and port match the delegated space.  A
       decapsulates the packet and releases it in the public Internet.

   2)  B does not NAT the packet.  The untranslated packet is then
       tunneled to A. A recognizes that the packet has not been
       translated, so A forwards the packet to a co-located NATing
       device, which translates the packet and routes it in the public
       Internet.  This device, e.g., an LSN, has to store the mapping
       between the source port used to NAT and the tunnel where the
       packet came from, in order to correctly route the reply.  Note
       that A cannot use a port number from the range that has been
       delegated to B. As a consequence A has to assign a part of its
       non-delegated address space to the NATing function.

   "Inbound" packets are handled in the following way: a packet from the
   public realm arrives at A. A analyzes the destination port number to

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   understand whether the packet needs to be NATed or not.

   1)  If the destination port number belongs to the range that A
       delegated to B, then A tunnels the packet to B. B NATs the packet
       using its stored mapping and forwards the translated packet to
       the private domain.

   2)  If the destination port number is from the address space of the
       LSN, then A passes the packet on to the co-located LSN which uses
       its stored mapping to NAT the packet into the private address
       realm of B. The appropriate tunnel is stored as well in the
       mapping of the initial NAT.  The LSN then encapsulates the packet
       to B, which decapsulates it and normally routes it within its
       private realm.

   3)  Finally, if the destination port number neither falls in a
       delegated range, nor into the address range of the LSN, A
       discards the packet.  If the packet is passed to the LSN, but no
       mapping can be found, the LSN discards the packet.

3.3.  Reasons for allowing multiple A+P gateways

   Since each device in an A+P subsystem provides the encaps/decaps
   function, new devices can establish tunnels and become in turn part
   of an A+P subsystem.  As noted above, being part of an A+P subsystem
   implies the capability of talking to the external address realm
   without any translation.  In particular, as described in the previous
   section, a device X in an A+P subsystem can be reached from the
   external domain by simply using the public IPv4 address and a port
   which has been delegated to X. Figure 4 shows an example where three
   devices are connected in a chain.  In other words, A+P signaling can
   be used to extend end-to-end connectivity to the devices which are in
   an A+P subsystem.  This allows A+P-aware applications (or OSes)
   running on end hosts to enter an A+P subsystem and exploit
   untranslated connectivity.

   There are two modes for end-hosts to gain fine-grained control of
   end-to-end connectivity.  The first is where actual end-hosts perform
   the NAT function and the encaps/decaps function which is required to
   join the A+P subsystem.  This option works in a similar way to the
   NAT-in-the-host trick employed by virtualization software such as
   VMware, where the guest operating system is connected via a NAT to
   the host operating system.  The second mode is applications which
   autonomously ask for an A+P address and use it to join the A+P
   subsystem.  This capability is necessary for some applications that
   require end-to-end connectivity (e.g., applications that need to be
   contacted from outside).

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               +---------+      +---------+      +---------+
     internal  | gateway |      | gateway |      | gateway |  external
     realm   --|    1    |======|    2    |======|    3    |-- realm
               +---------+      +---------+      +---------+

                  An A+P subsystem with multiple devices

                                 Figure 4

   Whatever the reasons might be, the Internet was built on a paradigm
   that end-to-end connectivity is important.  A+P makes this still
   possible in a time where address shortage forces ISPs to use NATs at
   various levels.  In such sense, A+P can be regarded as a way to
   bypass NATs.

              +---+          (customer2)
              |A+P|-.         +---+
              +---+  \     NAT|A+P|-.
                      \       +---+ |
                       \            |       forward if in-range
              +---+     \+---+    +---+    /
              +---+     /+---+    +---+    \
                       /                    NAT if necessary
                      / (cust1)   (prov.    (e.g., provider NAT)
              +---+  /            router)

                          A complex A+P subsystem

                                 Figure 5

   Figure 5 depicts a complex scenario, where the A+P subsystem is
   composed by multiple devices organized in a hierarchy.  Each A+P
   gateway decapsulates the packet and then re-encapsulates it again to
   the next tunnel.

   A packet can either be NATed when it enters the A+P subsystem, or at
   intermediate devices, or when it exits the A+P subsystem.  This could
   be for example a gateway installed within the provider's network,
   together with a LSN.  Then each customer operates its own CPE.
   However, behind the CPE applications might also be A+P-aware and run
   their own A+P-gateways, which enables them to have end-to-end

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   One limitation applies, if "delayed translation" is used (e.g.,
   translation at the LSN instead of the CPE).  If devices using
   "delayed translation" want to talk to each other they SHOULD use A+P
   addresses or out-of-band addressing.

4.  Deployment Scenarios

4.1.  A+P for Broadband Providers

   Large broadband providers do not have enough IPv4 address space to
   provide every customer with a single IP.  The natural solution is
   sharing a single IP address among many customers.  Multiplexing
   customers is usually accomplished by allocating different port
   numbers to different customers somewhere within the network of the

   In this document we use the following terms and assumptions:

   1.  Customer Premises Equipment (CPE), i.e. cable/DSL modem.

   2.  Provider Edge Router (PE), AKA customer aggregation router

   3.  Port Range Router (PRR), edge behind which A+P addresses are

   4.  Provider Border Router (BR), providers edge to other providers

   5.  Network Core Routers (Core), provider routers which are not at
       the edge.

   It is expected that, when the provider wishes to enable A+P for a
   customer or a range of customers, the CPE can be upgraded or replaced
   to support A+P encaps/decaps functionality.  Ideally the CPE also
   provides NATing functionality.  Further, it is expected that at least
   another component in the ISP network provides the corresponding A+P
   functionality, and hence is able to establish an A+P subsystem with
   the CPE.  This device is referred to as A+P router or port-range
   router (PRR), and could be located close to PE routers.  The core of
   the network MUST support the tunneling protocol (which SHOULD be
   IPv6, as per Constraint 10) but MAY be another tunneling technology
   when necessary.  In addition, we do not wish to restrict any
   initiative of customers who might want to run an A+P-capable network
   on or behind their CPE.  To satisfy both Constraints 1 and 3
   unmodified legacy hosts should keep working seamlessly, while
   upgraded/new end-systems should be given the opportunity to exploit
   enhanced features.

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4.2.  A+P for Mobile Providers

   In the case of mobile service provider the situation is slightly
   different.  The A+P border is assumed to be the gateway (e.g., GGSN/
   PDN GW of 3GPP, or ASN GW of WiMAX).  The need to extend the address
   is not within the provider network, but on the edge between the
   mobile phone devices and the gateway.  While desirable, IPv6
   connectivity may or may not be provided.

   For mobile providers we use the following terms and assumptions:

   1.  Provider Network (PN)

   2.  Gateway (GW)

   3.  Mobile Phone device (phone)

   4.  Devices behind phone, e.g., laptop computer connecting via phone
       to Internet.

   We expect that the gateway has a pool of IPv4 addresses and is always
   in the data-path of the packets.  Transport between the gateway and
   phone devices is assumed to be an end-to-end layer-2 tunnel.  We
   assume that phone as well as gateway can be upgraded to support A+P.
   However, some applications running on the phone or devices behind the
   phone (such as laptop computers connecting via the phone), are not
   expected to be upgraded.  Again, while we do not expect that devices
   behind the phone will be A+P aware/upgraded we also do not want to
   hinder their evolution.  In this sense the mobile phone would be
   comparable to the CPE in the broadband provider case; the gateway to
   the PRR/LSN box in the network of the broadband provider.

4.3.  A+P from the provider network perspective

   ISPs suffering from IPv4 address space exhaustion are interested in
   achieving a high address space compression ratio.  In this respect,
   an A+P subsystem allows much more flexibility than traditional NATs:
   the NAT can be placed at the customer, and/or in the provider
   network.  In addition hosts or applications can request ports and
   thus have untranslated end-to-end connectivity.

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        private    | +------+  A+P-in  +-----+ |   dual-stacked
       (RFC1918) --|-| CPE  |==-IPv6-==| PRR |-|-- network
         space     | +------+  tunnel  +-----+ |   (public addresses)
                   |    ^              +-----+ |
                   |    |  IPv6-only   | LSN | |
                   |    |   network    +-----+ |
                   +----+----------------- ^ --+
                        |                  |
                   on customer        within provider
              premises and control      network

                      A simple A+P subsystem example

                                 Figure 6

   Consider the deployment scenario in Figure 6, where an A+P subsystem
   is formed by the CPE and a port-range router (PRR) within the ISP
   core network, preferably close to the customer edge, and represents
   the border from where on packets are forwarded based on address and
   port.  The provider MAY deploy a LSN co-located with the PRR to
   handle packets that have not been translated by the CPE.  In such a
   configuration, the ISP allows the customer to freely decide whether
   the translation is done at the CPE or at the LSN.  In order to
   establish the A+P subsystem, the CPE will be configured automatically
   (e.g. via a signaling protocol, that conforms to the requirements
   stated above).

   Note that the CPE in the example above is only provisioned with an
   IPv6 address on the external interface.

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    +------------ IPv6-only transport ------------+
    | +---------------+ |              |          |
    | |A+P-application| |  +--------+  |  +-----+ |   dual-stacked
    | | on end-host   |=|==| CPE w/ |==|==| PRR |-|-- network
    | +---------------+ |  +--------+  |  +-----+ |   (public addresses)
    +---------------+   |  +--------+  |  +-----+ |
      private IPv4 <-*--+->| NAT    |  |  | LSN | |
      address space   \ |  +--------+  |  +-----+ |
      for legacy       +|--------------|----------+
        hosts           |              |
                        |              |
      end-host with     |  CPE device  |  provider
        upgraded        |  on customer |  network
       application      |   premises   |

         An extended A+P subsystem with end-host running A+P-aware

                                 Figure 7

   Figure 7 shows an example of how an upgraded application running on a
   legacy end-host can connect.  The legacy host is provisioned with a
   private IPv4 address allocated by the CPE.  Any packet sent from the
   legacy host will be NATed either at the CPE (if configured to do so),
   or at the LSN (if available).

   An A+P-aware application running on the end-host MAY use the
   signaling described in Section 3.1 to connect to the A+P-subsystem.
   In this case, the application will be delegated some space in the A+P
   address realm, and will be able to contact the external realm (i.e.,
   the public Internet) without the need for translation.

   Note that part of A+P signaling is that the NATs are optional.
   However, if neither the CPE nor the PRR provides NATing
   functionality, then it will not be possible to connect legacy end-

   To enable packet forwarding with A+P, the ISP MUST install at its A+P
   border a PRR which encaps/decaps packets.  However, to achieve a
   higher address space compression ratio and/or to support CPEs without
   NATing functionality, the ISP MAY decide to provide an LSN as well.
   If no LSN is installed in some part of the ISP's topology, all CPE in
   that part of the topology MUST support NAT functionality.  For
   reasons of scalability, it is assumed that the PRR is located within
   the access-portion of the network.  The CPE would be configured
   automatically (e.g. via an extended DHCP or NAT-PMP, which has the
   signaling requirements stated above) with the address of the PRR, and

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   if a LSN is being provided or not.  Figure 6 illustrates a possible
   deployment scenario.

4.4.  Dynamic allocation of port ranges

   Allocating a fixed number of ports to all CPE may lead to exhaustion
   of ports for high usage customers.  This is a perfect recipe for
   upsetting more demanding customers.  On the other hand, allocating to
   all customers ports sufficient to match the needs of peak users will
   not be very efficient.  A mechanism for dynamic allocation of port
   ranges allows the ISP to achieve two goals; a more efficient
   compression ratio of number of customers on one IPv4 address and, on
   the other hand, not limiting the more demanding customers'

   Additional allocation of ports, or port ranges may be made after an
   initial static allocation of ports.

   The following mechanism applies to NAT functionality in CPE only: If
   a customer has an arrangement with the ISP for well-known ports, and
   the PRR allocates to this CPE WKP range, this range may be used for
   end-to-end communications to a server behind CPE with public IP
   address or if customer configures so for inbound NAT (1:1 or port
   forwarding).  This function has a fixed range of ports and is not
   considered in the dynamic pool allocation mechanism.  On the other
   hand, if customer configures the NAT function to access the Internet
   from a private address pool behind the CPE, this mechanism is
   automatically applied.  NAT keeps track of translation tables, so
   only a small "daemon" needs to be developed and implemented by the
   CPE manufacturer to keep track of allocated ranges of ports and how
   many are used.  In the case of 90% usage, the dynamic allocation
   daemon could signal to the PRR the need for additional ports.  A
   downside of this mechanism is that port allocation to a CPE might get
   quite large without an additional mechanism that would return unused
   port ranges back to the PRR's pool.  This may be dealt with by
   requiring the NAT to sequentially allocate ports for translation and
   reallocate to new requests and released ports.  So the use of ports
   is controlled and unfragmented ranges may be returned to pool.  An
   other, not so pretty, way is to reset the additional allocations to 0
   every 24 hours, and leave only the first allocation.  Additional
   allocations would be requested by mechanism in a very short time,
   leaving the customer unlikely to notice the event.

   The mechanism would prefer allocations of port ranges from the same
   IP address as the initial allocation.  If it is not possible to
   allocate an additional port range from the same IP, then mechanism
   can allocate a port range from another IP within the same subnet.
   With every additional port range allocation, the PRR updates its

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   routing table.  The mechanism for allocating additional port ranges
   may be part of normal signaling that is used to authenticate CPE to

   The ISP controls the dynamic allocation of port ranges by the PRR by
   setting the initial allocation size and maximum number of allocations
   per CPE, or the maximum allocations per subscription, depending on
   subscription level.  There is a general observation that the more
   demanding customer uses around 1024 ports when heavily communicating.
   So, for example, a first suggestion might be 128 ports initially and
   then dynamic allocations of ranges of 128 ports up to 511 more
   allocations maximum.  A configured maximum number of allocations
   could be used to prevent one customer acting in distructive manner
   should they become infected.  The maximum number of allocations might
   also be more finely grained, with parameters of how many allocations
   a user may request per some time frame.  If this is used, evasive
   applications may need to be limited in their bad behavior, for
   example one additional allocation per minute would considerably slow
   a port request storm.

   There is likely no minimum request size.  This is because A+P-aware
   applications running on end-hosts MAY request a single port (or a few
   ports) for the CPE to be contacted on (e.g., VoIP clients register a
   public IP and a single delegated port from the CPE, and accept
   incoming calls on that port).  The implementation on the CPE or PRR
   will dictate how to handle such requests for smaller blocks: For
   example, half of available blocks might be used for "block-
   allocations", 1/6 for single port requests, and the rest for NATing.

   Another possible mechanism to allocate additional ports is UPnP/
   NAT-PMP (as defined in Section 3.1), if applications behind CPE
   support it.  In case of the LSN implementation (DS-Lite), as
   described in the A+P overall architecture section, signaling packets
   are simply forwarded by the CPE to the LSN and back to the host
   running the application which requested the ports, and PRR allocates
   requested port to appropriate CPE.  The same behavior may be chosen
   with AFTR, if requested ports are outside of static initial port
   allocation.  If a full A+P implementation is selected, than UPnPv2/
   NAT-PMP packets are accepted by the CPE, processed, and the requested
   port number is communicated through normal signaling mechanism
   between CPE and PRR tunnel endpoints (DHCP or IPCP).

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4.5.  Overall A+P architecture

                           A+P architecture

         IPv4         Full-A+P          AFTR             CGN
          |              |               |                |
   <-- Full IPv4 ---- Port range ---- Port range  ---- Provider --->
       allocated      & dynamic         & LSN          NAT ONLY
                      allocation      (NAT on CPE      (No mechanism)
       (no NAT)      (NAT on CPE)     and on LSN)      for customer to
                                                       bypass CGN)

                    Figure 8: A+P overall architecture

   The A+P architecture defines various options to be deployed within an
   ISP.  Figure 8 shows the spectrum of deployment options.  On the far
   left today's status-quo, an IPv4 address unrestricted with full port-
   range.  Full-A+P, refers to a port-range allocation from the ISP.
   The customer must operate A+P-aware devices and no NATing
   functionality is provided by the ISP.  AFTR, such as DS-Lite
   [I-D.ietf-softwire-dual-stack-lite], is a hybrid.  There is NAT
   present in the core (in this draft referred to as LSN), but the user
   has the option to "bypass" that NAT in one form or an other, for
   example via A+P, NAT-PMP, etc...  Finally, a provider only CGN, will
   place a NAT in the providers core and does not allow the customer to
   "bypass" the translation process or modify ALGs on the NAT.  The
   customer is provider-locked.  Note as well that all options (besides
   full IPv4) require some form of tunneling mechanism (e.g., 4in6) and
   a signaling mechanism (see Section 3.1).

4.6.  Example of A+P-forwarded packets

   This section provides a detailed example of A+P setup, configuration,
   and packet flow from an end-host behind an A+P upgraded provider to
   any host in the IPv4 Internet, and how the return packets flow back.
   The following example discusses an A+P-unaware end-host, where the
   NATing is done at the CPE.  Figure 9 illustrates how the CPE receives
   an IPv4 packet from the end-user device.  We first describe the case
   where the CPE has been configured to provide the NAT functionality
   (e.g., by the customer via interaction via a website, or via
   automatic signaling).  In the following, we call a packet which is
   translated at the CPE an A+P-forwarded packet, an analogy with the
   port-forwarding function employed in today's CPEs.  Upon receiving a
   packet from the internal interface, the CPE NATs it and forwards it
   to the PRR.  The NAT on the CPE is assumed to store the 5-tuple
   (source_IPv4, source_port, destination_IPv4, destination_port,

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   When the PRR receives the A+P-forwarded packet, it de-capsulates the
   inner IPv4 packet and it checks the source address and port.  If the
   source address and port match the CPE's A+P address, then the PRR
   simply forwards the decapsulated packet onward.  This is always the
   case for A+P-forwarded packets.  Otherwise, the PRR assumes that the
   packet is not A+P-forwarded, sl passes it to the LSN function, which
   in-turn NATs the packet and then releases it into the Internet.
   Figure 9 shows the packet flow for an outgoing A+P-forwarded packet.

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                   |    Host   |
                      |  |
      IPv4 datagram 1 |  |
                      |  |
                      v  |
               |CPE      |         |
                      | |||     a::2
                      | ||| (100-200)
       IPv6 datagram 2| |||
                      | |||<-IPv4-in-IPv6
                      | |||
               /      | |||        \
              |  ISP access network |
               \      | |||        /
                      | |||
                      v |||     a::1
               |PRR     |||        |
                      |  |
      IPv4 datagram 3 |  |
               /      |  |         \
              |   ISP network /     |
               \      Internet     /
                      |  |
                      v  |
                   | IPv4 Host |

          Figure 9: Forwarding of Outgoing A+P-forwarded Packets

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     |        Datagram | Header field | Contents                    |
     | IPv4 datagram 1 |     IPv4 Dst |                   |
     |                 |     IPv4 Src |                    |
     |                 |      TCP Dst | 80                          |
     |                 |      TCP Src | 8000                        |
     | --------------- | ------------ | --------------------------- |
     | IPv6 Datagram 2 |     IPv6 Dst | a::1                        |
     |                 |     IPv6 Src | a::2                        |
     |                 |     IPv4 Dst |                   |
     |                 |     IPv4 Src |                    |
     |                 |      TCP Dst | 80                          |
     |                 |      TCP Src | 100                         |
     | --------------- | ------------ | --------------------------- |
     | IPv4 datagram 3 |     IPv4 Dst |                   |
     |                 |     IPv4 Src |                    |
     |                 |      TCP Dst | 80                          |
     |                 |      TCP Src | 100                         |

                         Datagram header contents

   An incoming packet undergoes the reverse process.  When the PRR
   receives an IPv4 packet on an external interface, it first checks
   whether the destination port number falls in a delegated range or
   not.  If the address space was delegated, then PRR encapsulates the
   incoming packet and forwards it through the appropriate tunnel for
   that IP/port range.  If the address space was not-delegated the
   packet would be handed to the LSN to check if a mapping is available.

   Figure 10 shows how an incoming packet is forwarded, under the
   assumption that the port number matches the port range which was
   delegated to the CPE.

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                   |    Host   |
                      ^  |
      IPv4 datagram 3 |  |
                      |  |
                      |  |
               |CPE      |         |
                      ^ |||     a::2
                      | ||| (100-200)
       IPv6 datagram 2| |||
                      | |||<-IPv4-in-IPv6
                      | |||
               /      | |||        \
              | ISP access network  |
               \      | |||        /
                      | |||
                      | |||     a::1
               |PRR     |||        |
                      ^  |
      IPv4 datagram 1 |  |
               /      |  |         \
              |  ISP network /      |
               \      Internet     /
                      |  |
                      |  |
                   | IPv4 Host |

          Figure 10: Forwarding of Incoming A+P-forwarded Packets

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     |        Datagram | Header field | Contents                    |
     | IPv4 datagram 1 |     IPv4 Dst |                    |
     |                 |     IPv4 Src |                   |
     |                 |      TCP Dst | 100                         |
     |                 |      TCP Src | 80                          |
     | --------------- | ------------ | --------------------------- |
     | IPv6 Datagram 2 |     IPv6 Dst | a::2                        |
     |                 |     IPv6 Src | a::1                        |
     |                 |     IPv4 Dst |                    |
     |                 |       IP Src |                   |
     |                 |      TCP Dst | 100                         |
     |                 |      TCP Src | 80                          |
     | --------------- | ------------ | --------------------------- |
     | IPv4 datagram 3 |     IPv4 Dst |                    |
     |                 |     IPv4 Src |                   |
     |                 |      TCP Dst | 8000                        |
     |                 |      TCP Src | 80                          |

                         Datagram header contents

   Note that datagram 1 travels untranslated up to the CPE, thus the
   customer has the same control over the translation as it has today
   where s/he has an home gateway with customizable port-forwarding.

4.7.  Forwarding of standard packets

   Packets for which the CPE does not have a corresponding port
   forwarding rule are tunneled to the PRR which provides the LSN
   function.  We underline that the LSN MUST NOT use the delegated space
   for NATting.  See [I-D.ietf-softwire-dual-stack-lite] for network
   diagrams which illustrate the packet flow in this case.

4.8.  Handling ICMP

   ICMP is problematic for all NATs, because it lacks port numbers.  A+P
   routing exacerbates the problem.

   Most ICMP messages fall into one of two categories: error reports, or
   ECHO/ECHO reply (commonly known as "ping").  For error reports, the
   offending packet header is embedded within the ICMP packet; NAT
   devices can then rewrite that portion and route the packet to the
   actual destination host.  This functionality will remain the same
   with A+P; however, the PRR will need to examine the embedded header
   to extract the port number, while the A+P gateway will do the
   necessary rewriting.

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   ECHO and ECHO reply are more problematic.  For ECHO, the A+P gateway
   device must rewrite the "Identifier" and perhaps "Sequence Number"
   fields in the ICMP request, treating them as if they were port
   numbers.  This way, the PRR can build the correct A+P address for the
   returning ECHO replies, so they can be correctly routed back to the
   appropriate host in the same way as TCP/UDP packets.  (Pings
   originated from an external domain/legacy Internet towards an A+P
   device are not supported.)

4.9.  Limitations of the A+P approach

   One limitation that A+P shares with any other IP address-sharing
   mechanism is the availability of well-known ports.  In fact, services
   run by customers that share the same IP address will be distinguished
   by the port number.  As a consequence, it will be impossible for two
   customers who share the same IP address to run services on the same
   port (e.g., port 80).  Unfortunately, working around this limitation
   usually implies application-specific hacks (e.g., HTTP and HTTPS
   redirection), discussion of which is out of the scope of this
   document.  Of course, a provider might charge more for giving a
   customer the well-known port range, 0..1024, thus allowing the
   customer to provide externally available services.  Many applications
   require the availability of well known ports.  However, those
   applications are not expected to work in A+P environment unless they
   can adapt to work with different ports.  However, such application do
   not work behind today's NATs either.

   Another problem which is common to all NATs is coexistence with
   IPsec.  In fact, a NAT which also translates port numbers prevents AH
   and ESP from functioning properly, both in tunnel and in transport
   mode.  In this respect, we stress that, since an A+P subsystem
   exhibits the same external behavior as a NAT, well-known workarounds
   (such as [RFC3715]) can be employed.

5.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

6.  Security Considerations

   The primary security issue any time a NAT is mentioned is the
   implicit firewall provided by a NAT.  Any proposal to eliminate NATs
   raises the spectre of insecure hosts lying naked before a hostile

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   Internet.  For a number of reasons, we do not think this is a serious
   issue here.  If nothing else, NATs are not really security devices;
   their protective value is limited.

   A NAT owned by a customer, whether a home consumer or a large
   enterprise, is under the control of that customer.  All machines on
   the customer's side of the NAT have unfettered access to other
   machines on the same side; generally, this is what is desired.  A+P
   NATs do not change this, as the customer has still controls what is
   being NATed.  LSN does not change the access property, either.
   However, with a CGN without A+P there are *many* machines on the
   inside of the translation, not all of which are in the customer's
   administrative domain.  Unless other firewall mechanisms are
   employed, LSNs create added risk of unauthorized access.

   By contrast, the protection scope of an A+P NAT is, by definition, at
   the boundary to the customer network.  The access properties are thus
   precisely what traditional NATs have provided.

   There is one notable exception to this point.  Inbound packets
   addressed to the assigned port number range are passed through
   unchanged, even if no outbound packets were sent to the originator.
   While this allows customers to run their own servers on certain
   ports, it also allows attackers to probe these servers without the
   protection provided today by provider-supplied NAT boxes.  The issue
   is not that internal machines are addressable -- that is an
   inevitable corollary to servers being run -- but that it may
   represent a change from today's behavior.  Furthermore, the effect on
   the customer varies greatly, depending on what port number range they
   are assigned; someone who is assigned 0-4K derives more benefit and
   runs more risk than someone who is assigned 48K-52K, since the latter
   is in the IANA-assigned dynamic port range.

   A useful middle ground would be provision of a customer-controllable
   switch in the CPE to control what happens to such packets.  If
   filtering is to be done, state must be kept, which might be costly.
   This suggests that perhaps it should only be done in the CPE if it is
   replacing current CPE that provides NAT functionality.  If
   applications on end-hosts installed A+P gateways, they might open up
   ports untranslated.

   Note that, regardless of the existence of such an option, the A+P
   gateway will need customer-controllable port number-mapping
   capability, as most customers will not be assigned a range which
   corresponds to the servers they wish to run.

   With CGN/LSNs, tracing hackers, spammers and other criminals will be
   extremely difficult, requiring logging, recording, and storing of all

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   connection based mapping information.  The need for storage implies a
   tradeoff.  On one hand, the LSNs can manage addresses and ports as
   dynamically as possible, in order to maximize aggregation.  On the
   other hand, the more quickly the mapping between private and public
   space changes, the more information needs to be recorded.  This would
   not only cause concern for law enforcement services, but also for
   privacy advocates.

   A+P offers a better set of tradeoffs.  All that needs to be logged is
   the allocation of a range of port numbers to a customer.  By design,
   this will be done rarely, improving scalability.  If the NAT
   functionality is moved further up the tree, the logging requirement
   will be as well, increasing the load on one node, but giving it more
   resources to allocate to a busy customer, perhaps decreasing the
   frequency of allocation requests.

   The other extreme is A+P NAT on the customer premises.  Such a node
   would be no different than today's NAT boxes, which do no such
   logging.  We thus conclude that A+P is no worse than today's
   situation, while being considerably better than CGNs.

7.  Authors

   This document has 8 primary authors, which is not allowed in the
   header of Internet-Drafts.  This is the list of actual authors of
   this document.

      Gabor Bajko
      Email: gabor(dot)bajko(at)nokia(dot)com

      Steven M. Bellovin
      Columbia University
      1214 Amsterdam Avenue
      MC 0401
      New York, NY  10027
      Phone: +1 212 939 7149

      Randy Bush
      Internet Initiative Japan
      5147 Crystal Springs
      Bainbridge Island, Washington  98110
      Phone: +1 206 780 0431 x1

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      Luca Cittadini
      Universita' Roma Tre
      via della Vasca Navale, 79
      Rome,   00146
      Phone: +39 06 5733 3215

      Alain Durand
      1 Comcast Center
      Philadelphia, PA

      Olaf Maennel
      Loughborough University
      Department of Computer Science - N.2.03
      United Kindom
      Phone: +44 115 714 0042

      Teemu Savolainen
      Hermiankatu 12 D
      TAMPERE, FI-33720

      Jan Zorz
      Frankovo naselje 165
      Skofja Loka  4220
      Phone: +38659042000

8.  Acknowledgments

   The authors wish to especially thank Remi Despres, and Pierre Levis
   for their help on the development of the A+P approach.  We also thank
   David Ward for review, constructive criticism, and interminable
   questions, and Dave Thaler for useful criticism on "stackable" A+P
   gateways.  We would also like to thank the following persons for
   their feedback on earlier versions of this work: Rob Austein, Gert

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   Doering, Dino Farinacci, Russ Housley, and Ruediger Volk.

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.

9.2.  Informative References

   [BCP38]    Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, May 2000.

              Bajko, G., Savolainen, T., Boucadair, M., and P. Levis,
              "Port Restricted IP Address Assignment",
              draft-bajko-pripaddrassign-01 (work in progress),
              March 2009.

              Boucadair, M., Levis, P., Grimault, J., Savolainen, T.,
              and G. Bajko, "Dynamic Host Configuration Protocol
              (DHCPv6) Options for Shared IP Addresses  Solutions",
              draft-boucadair-dhcpv6-shared-address-option-00 (work in
              progress), May 2009.

              Boucadair, M., Levis, P., Bajko, G., and T. Savolainen,
              "IPv4 Connectivity Access in the Context of IPv4 Address
              Exhaustion: Port  Range based IP Architecture",
              draft-boucadair-port-range-02 (work in progress),
              July 2009.

              Boucadair, M., Levis, P., Grimault, J., and A.
              Villefranque, "Port Range Configuration Options for PPP
              IPCP", draft-boucadair-pppext-portrange-option-01 (work in
              progress), July 2009.

              Durand, A., Droms, R., Haberman, B., Woodyatt, J., Lee,
              Y., and R. Bush, "Dual-stack lite broadband deployments
              post IPv4 exhaustion",
              draft-ietf-softwire-dual-stack-lite-01 (work in progress),
              July 2009.

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   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC3715]  Aboba, B. and W. Dixon, "IPsec-Network Address Translation
              (NAT) Compatibility Requirements", RFC 3715, March 2004.

Author's Address

   Randy Bush (editor)
   Internet Initiative Japan
   5147 Crystal Springs
   Bainbridge Island, Washington  98110

   Phone: +1 206 780 0431 x1

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