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Versions: (draft-boucadair-behave-ipv6-portrange) 00 01 02 03 04 05 06 07 08 09 10 RFC 6346

Network Working Group                                            R. Bush
Internet-Draft                                 Internet Initiative Japan
Intended status: Standards Track                              O. Maennel
Expires: September 10, 2009                Deutsche Telekom Laboratories
                                                                 J. Zorz
                                                             S. Bellovin
                                                     Columbia University
                                                            L. Cittadini
                                                    Universita' Roma Tre
                                                           March 9, 2009

             The A+P Approach to the IPv4 Address Shortage

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   Provisions Relating to IETF Documents in effect on the date of

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   publication of this document (http://trustee.ietf.org/license-info).
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   We are facing the exhaustion of the IANA IPv4 free IP address pool.
   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 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
   IP to different clients (e.g., CPE's, mobile phones), each with its
   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 Large-Scale-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 . . . . . . . . 13
   4.  Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 15
     4.1.  A+P for Broadband Providers  . . . . . . . . . . . . . . . 15
     4.2.  A+P for Mobile Providers . . . . . . . . . . . . . . . . . 15
     4.3.  A+P from provider networks perspective . . . . . . . . . . 16
     4.4.  Dynamic allocation of port ranges  . . . . . . . . . . . . 18
     4.5.  Example of A+P-forwarded packets . . . . . . . . . . . . . 20
     4.6.  Forwarding of standard packets . . . . . . . . . . . . . . 24
     4.7.  Handling ICMP  . . . . . . . . . . . . . . . . . . . . . . 24
     4.8.  Limitations of the A+P approach  . . . . . . . . . . . . . 25
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 26
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 26
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27

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1.  Introduction

   This document addresses the imminent IPv4 address space exhaustion.
   Very soon there will be not enough IPv4 space allocatable to
   customers of broadband or mobile providers, while IPv6 is not widely
   enough deployed to migrate to an IPv6-only world.  Many large
   Internet Service Providers (ISPs) face the problem that their
   networks' customer edges are so large that it will soon not be
   possible anymore to provide each customer with a single IPv4 address.
   Therefore ISPs have to devise something more ingenious.  Although
   undesirable, address sharing is inevitable.

   To allow end-to-end connectivity between IPv4 speaking applications
   we propose to "steal" some bits from the UDP/TCP header and use them
   for addressing 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 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 anymore in face of the
   looming IPv4 address exhaustion, where so called Large-Scale-NATs
   (LSNs) might be deployed within the providers network - outside the
   control of the customer.

1.1.  Why Large-Scale-NATs are Harmful

   Various forms of NATs will be installed at various levels and places
   in the IPv4-Internet to achieve the necessary address compression.
   This document argues for mechanisms that end-customers will not be
   locked behind a walled-garden shrine without any control over the
   translation and that it is therefore essential to create mechanisms
   to "bypass" a NAT, and keep the control at the end-user:

   "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
   and policies.  This is the opposite of the "end-to-end" model of the

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   With the smarts at the edges, one can easily field new protocols
   between consenting end-points by "just" tweaking the NATs at the
   corresponding Customer Premises Equipment (CPE), even adding
   application layer gateways (ALGs) if they are needed.  However, LSNs
   do not build an Internet walled garden at the edges, they build it by
   restricting the core.

   With LSNs in the core, customers wanting new application protocols
   which require cooperation from the NAT, have to beg help from the
   broadband providers' engineers and lawyers, and all other users of
   the large-scale-NATs.  It is feared that all new application
   protocols have to go through the carrier-loving lawyers to be allowed
   to be handled by the NATs in their core.  Today's NATs are typically
   mitigated by ALGs over which the customer has some degree of control,
   e.g. port forwarding or UPnP.  However, this is not expected to work
   anymore with LSNs.  LSN proposals admit that it is not expected that
   applications that require specific port assignment or port mapping
   from the NAT box will keep working
   [I-D.durand-softwire-dual-stack-lite].  This is the ultimate horror
   the NAT-haters fear, and, in this case, they are not all that wrong.

   We believe this is not an option and that the end-user must have the
   ability to control its own ALGs.  So, if someone wants to deploy a
   new application, they can talk to the broadband 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 back 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.

   And remember that as IPv6 deploys, if we want to have one Internet,
   i.e.  IPv4 nodes talking freely with IPv6 nodes, then translation
   must be done somewhere.  The challenge is whether someone can figure
   out a scheme where it is done for these large networks?  We believe
   it should be at the customer edge, not in the core.

   Another issue with LSN is scalability.  ISPs face a tension between
   the placement of LSNs within their network to aggregate as much as
   possible, when too much aggregation creates a massive state problem.
   To reduce the state, the placement ends up somewhere closer to the
   edge, where the benefits are somewhat limited.  It is not clear how a
   LSN should maintain per-session state in a scalable manner.  State
   for improperly terminated sessions could remain stale for some time.
   The LSN hence trades scalability for the amount of state that needs
   to be kept, which makes optimally placing a LSN a hard engineering

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   In addition, NATs frequently need to initiate translation for
   secondary port numbers.  This may be a decision based on packet
   inspection (i.e., looking for PORT commands in FTP [RFC0959]
   sessions), or it may rely on explicit signaling from the end host via
   protocols such as UPnP.  Either way, LSNs pose a security threat
   and/or an administrative nightmare.

   The issue is proper authentication of such requests.  Most UPnP
   devices do not implement appropriate security features.  Even if they
   did, there would be no way to administer the security mechanism.
   Every end-user device would have to have a secret corresponding to
   some authentication field in the LSN.  End users will not set these
   up properly; providers do not want to maintain such a database.

   Decisions made based on packet inspection are just as problematic.  A
   request from one customer could easily request opening a port for an
   other customer's addresses, similar to the Java-based attack
   described by Martin et al in [Martin-Java].

   Furthermore, with LSNs, tracing hackers, spammers and other criminals
   will be impossible, unless all the connection based mapping
   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 possibility to send/receive packets unmodified and deploy
           new application protocols at will.  IPv4 address exhaustion
           is no clearance 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

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   3)      Incremental deployability and backward compatibility.  The
           approaches shall be transparent to unaware users.  Devices or
           existing applications shall be able to work without
           modification.  Emergence of new applications shall not be

   4)      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 are NOT supposed to lookup assigned ports
           manually on websites and then configure them on devices or

   5)      "Double-NAT" shall 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

   6)      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 the lowest impact
           possible on the storage and the ISP.  We assume that NATs on
           customer premises do not pose much of a problem, while
           provider NATs need to keep additional logs.

   7)      IPv6 deployment should be encouraged.

   While we acknowledge that A+P works in an IPv4-only environment
   (e.g., [I-D.boucadair-port-range]) we strongly believe that IPv6 is
   the long-term solution to the problem, and that A+P should be
   considered only as an intermediate hack towards an IPv6-only world.
   We therefore assume in constraint 7 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 an hindrance to the
   introduction of IPv6.

   Constraints 2 and 5 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 idea can be split into three distinct functionalities:
   encaps/decaps, NAT, and signaling functionalities.

   Encaps/decaps functionality: is used to forward port-restricted A+P-
   packets over intermediate legacy devices.  The encapsulation

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   functionality 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 result of such a check, the packet MAY be
   forwarded untranslated, it MAY be discarded or MAY be NATed.

   Network Address Translation (NAT) functionality: 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 functionality is performing any address or port translation,
   including application-level-gateways (ALGs).  The state that has to
   be kept to implement this functionality is the mapping for which
   external addresses and ports have been mapped to which internal
   addresses and ports.

   Signaling functionality: 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).  In addition, the
   signaling functionality is 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.
   Ideally they should be taken from [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 not.

3.  Overview of the A+P Solution

   The core architectural elements of the A+P solution are three
   separated and independent functionalities: the NAT functionality, the
   encaps/decaps functionality, and the signaling functionality.  The
   NAT functionality 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-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

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   restricted range of ports when mapping multiple internal addresses
   onto an external one, e.g., the external address realm might be port-

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

                              Traditional NAT

                                 Figure 1

   The encaps/decaps functionality, on the other hand, consists of the
   capability of establishing a tunnel with another endpoint providing
   the same functionality.  This implies some form of signaling to
   establish a tunnel This can be viewed as integrated with DHCP or a
   separate service.  Section 3.1 discusses the constraints of this
   signaling function.  The established tunnel can be encapsulation in
   IPv6, a layer-2 tunnel, or some other form of softwire.  Note that
   the presence of a tunnel allows for intermediate legacy devices
   between the two endpoints.

   Two or more devices which provide the encaps/decaps functionality and
   are linked by tunnels 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 functionality.

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

                          A simple A+P subsystem

                                 Figure 2

   Within an A+P subsystem, the external address realm is extended by
   "stealing" bits from the port number.  Each device is assigned one

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   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.  On the other hand, packets that are originated from
   outside the A+P subsystem need to be translated, since they belong to
   different realms.  For this reason, one of the two edges of the A+P
   subsystem MUST provide the NAT functionality (or both).  It is up to
   the provider to trade-off the placement of the NAT functionality.
   Hence, the design of A+P is deliberately agnostic to where packets in
   transit will be translated, provided that the translation happens
   exactly once (Constraint 5).

3.1.  Signaling

   The following information needs to be available on all the gateways
   in the A+P subsystem.  We propose to deploy a signaling protocol such
   as [I-D.boucadair-dhc-port-range],
   [I-D.bajko-v6ops-port-restricted-ipaddr-assign].  The information
   that needs to be shared are the following:

   o  a set of public IPv4 addresses,

   o  for each IPv4 address a set of allocated port-ranges (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 functionality is provided by the gateway

   o  a device identification number and some authentification

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

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

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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
   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:

   o  Allocate 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 (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.

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

   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 functionality 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 DHCP-similar solution.
   In general the following rule applys: 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 hosts.

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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 could
   actually consits 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
   functionality (e.g., LSN).  Packet forwarding is assumed in the
   following way: 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 any 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 options:

   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 allocated information.  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 functionality.

   "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 can now NAT the
       packet using its stored mapping and forward the translated packet
       in 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
       the 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 does neither fall 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 the A+P subsystem provides the encaps/decaps
   functionality, new devices can establish tunnels and become in turn
   part of the A+P subsystem.  As noted above, being part of the 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 the 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 the A+P subsystem.  This allows A+P-aware
   applications (or OSes) running on end hosts to enter the A+P
   subsystem and exploit untranslated connectivity.

   There are two modes for end-hosts to gain end-to-end connectivity.
   The first one is having end-hosts perform the NAT function (along
   with 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 who 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

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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 build on a paradigm
   that end-to-end connectivity is possible.  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 (a large-scale-NAT provided
   by the provider).  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 connectivity.
   One limitation applies, if "delayed translation" is used (e.g.,
   translation at the LSN instead of the CPE).  If devices using

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   "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 have not 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 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 same functionality, and
   hence is able to establish an A+P subsystem with the CPE.  This
   device is referred to as A+P border router or port-range router
   (PRR), and could be located close to the PE router.  The core of the
   network MUST support the tunneling protocol (which SHOULD be IPv6, as
   per Constraint 7).  In addition, we do not want to restrict any
   initiative of customers, who might want to run an A+P-capable network
   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.

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

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   is not within the provider network, but on the edge between the
   mobile phone devices and the base-station.  While desirable, IPv6
   connectivity may or may not be providable.

   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 many IPv4 addresses and is always in
   the data-path of the packets.  Transportation between 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
   necessarily 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 provider networks 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 between the CPE and a port-range router (PRR) within the
   ISP core network.  The PRR is placed somewhere within the ISPs
   network, preferably close to the customer edge and forms the border
   from where on packets are forwarded based on address and port.  The
   provider MAY deploy a LSN co-located with the PRR: in this case
   packets that have not been translated by the CPE will be handed to
   the LSN and NATted.  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 with 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 from 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.
   Hence, 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-

4.4.  Dynamic allocation of port ranges

   Allocating the same sized fixed range of ports to all CPE may lead to
   exhaustion of ports that are needed for NAT in a CPE to operate,
   because that customer has several hosts behind CPE and uses NAT to
   communicate with the Internet, any given restricted range of
   allocated ports might become exhausted.  This is a perfect recipe for
   upsetting the more demanding customer.  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

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   address and, on the other hand, not limiting the more demanding
   customers on their communication to/from Internet.

   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 is 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 allocation mechanism.  On the other hand,
   if customer configures the NAT function to access Internet from
   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
   signals 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 and additional mechanism that would return unused port ranges
   back to the PRR's pool.  This may be fixed by forcing the NAT to
   sequentially allocate ports for translation and reallocate to new
   requesta and released ports.  So the use of ports is controlled and
   unfragmented ranges can 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 very short time, leaving the
   customer unlikely to notice the event.

   The mechanism would prefer allocations of port ranges from the same
   IP for an initial allocation.  If it is not possible to allocate an
   additional port range from the same IP, than mechanism can allocate a
   port range from another IP within the same subnet.  With every
   additional port range allocation, the PRR updates its routing table
   and sends packets coming to allocated ports on that IP to the
   appropriate tunnel that ends on the CPE which requested and allocated
   that additional port range.  The mechanism for allocating additional
   port ranges may be part of normal signaling that is used to
   authenticate CPE to ISP.

   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 would be 512 ports initially and
   then dynamic allocations of ranges of 512 ports up to 6 more
   allocations maximum.  The maximum number of allocations should
   prevent from one customer acting in distructive manner, in case they

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   become infected.  The maximum number of allocations can also be fine
   grained with parameter of how many allocations a user can request per
   time frame.  If this is used, evasive applications are limited in bad
   behavior, for example one additional allocation per minute would
   considerably slow the port requesting storm.

   Note that there is 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.

4.5.  Example of A+P-forwarded packets

   This section provides a detailed example of A+P setup, configuration,
   and how packets 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 the situation of an A+P-
   unaware end-host, the NATing is done at the CPE.  Figure 8
   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, in 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, tunnel-interface).

   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 routes the encapsulated packet.  This is always the case for
   A+P-forwarded packets.  Otherwise, the PRR assumes that the packet is
   not A+P-forwarded, and then passes it to the LSN function, which in-
   turn NATs the packet and then releases it into the Internet.
   Figure 8 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 8: 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 tunnels the
   packets unmodified.  If the address space was not-delegated the
   packet will be handed to the LSN to check if a mapping is available.

   Figure 9 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 9: 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 he/she has an home gateway with customizable port-forwarding.

4.6.  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.durand-softwire-dual-stack-lite] for network
   diagrams which illustrate the packet flow in this case.

4.7.  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 BR 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.)

4.8.  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
   virtual hosting), 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 kind of NATs is the
   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.

   Port randomization is also a bit compromised in A+P solution.  As CPE
   can randomize ports only within port range that is allocated to it,
   randomness is more limited than in the the scenario with full range
   of ports, allowed for randomization.  We can assume, that CPE either
   gets port range from ephemeral range (49152-65535) or from
   "registered ports" range (1024-49151).  Both ranges can be used for
   randomization, see [I-D.ietf-tsvwg-port-randomization] for more

5.  IANA Considerations

   This document makes no request of IANA.

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

6.  Security Considerations

7.  Acknowledgments

   The authors wish to thank especially (in alphabetical order) Gabor
   Bajko, Remi Despres, Alain Durand, Pierre Levis, and Teemu Savolainen
   for their close collaboration on the development of the A+P approach.
   David Ward for review, constructive criticism, and interminable
   questions.  Cullen Jennings for discussion and review of
   fragmentation, 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: Bernhard Ager, Rob
   Austein, Gert Doering, Dino Farinacci, Russ Housley, and Ruediger

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2.  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. and T. Savolainen, "Port Restricted IP Address
              draft-bajko-v6ops-port-restricted-ipaddr-assign-02 (work
              in progress), November 2008.

              Boucadair, M., Grimault, J., Levis, P., and A.
              Villefranque, "DHCP Options for Conveying Port Mask and
              Port Range Router IP Address",
              draft-boucadair-dhc-port-range-01 (work in progress),
              October 2008.


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              Boucadair, M., Levis, P., Bajko, G., and T. Savolainen,
              "IPv4 Connectivity Access in the Context of IPv4 Address
              Exhaustion", draft-boucadair-port-range-01 (work in
              progress), January 2009.

              Durand, A., Droms, R., Haberman, B., and J. Woodyatt,
              "Dual-stack lite broadband deployments post IPv4
              exhaustion", draft-durand-softwire-dual-stack-lite-01
              (work in progress), November 2008.

              Larsen, M. and F. Gont, "Port Randomization",
              draft-ietf-tsvwg-port-randomization-02 (work in progress),
              August 2008.

              Martin, D., Rajagopalan, S., and A. Rubin, "Blocking Java
              Applets at the Firewall", Proceedings of the Internet
              Society Symposium on Network and Distributed System
              Security, pp. 16-26, 1997.

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, October 1985.

   [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.

Authors' Addresses

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

   Phone: +1 206 780 0431 x1
   Email: randy@psg.com

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   Olaf Maennel
   Deutsche Telekom Laboratories
   Ernst-Reuter-Platz 7
   Berlin  10587

   Phone: +3727120686
   Email: o@maennel.net

   Jan Zorz
   Frankovo naselje 165
   Skofja Loka  4220

   Phone: +38659042000
   Email: jan@go6.si

   Steven M. Bellovin
   Columbia University
   1214 Amsterdam Avenue
   MC 0401
   New York, NY  10027

   Phone: +1 212 939 7149
   Email: bellovin@acm.org

   Luca Cittadini
   Universita' Roma Tre
   via della Vasca Navale, 79
   Rome,   00146

   Phone: +39 06 5733 3215
   Email: luca.cittadini@gmail.com

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