Internet Engineering Task Force                             M. Ford, Ed.
Internet-Draft                                          Internet Society
Intended status: Informational                              M. Boucadair
Expires: December 6, 2010                                 France Telecom
                                                               A. Durand
                                                                P. Levis
                                                          France Telecom
                                                              P. Roberts
                                                        Internet Society
                                                            June 4, 2010

                     Issues with IP Address Sharing


   The completion of IPv4 address allocations from IANA and the RIRs is
   causing service providers around the world to question how they will
   continue providing IPv4 connectivity service to their subscribers
   when there are no longer sufficient IPv4 addresses to allocate them
   one per subscriber.  Several possible solutions to this problem are
   now emerging based around the idea of shared IPv4 addressing.  These
   solutions give rise to a number of issues and this memo identifies
   those common to all such address sharing approaches.  Solution-
   specific discussions are out of scope.

Status of this Memo

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   This Internet-Draft will expire on December 6, 2010.

Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   ( in effect on the date of
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   described in the BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Shared Addressing Solutions  . . . . . . . . . . . . . . . . .  4
   3.  Analysis of Issues as they Relate to First and Third
       Parties  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Port Allocation  . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Outgoing Ports . . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  Incoming Ports . . . . . . . . . . . . . . . . . . . . . .  9
       4.2.1.  Port Negotiation . . . . . . . . . . . . . . . . . . . 10
       4.2.2.  Connection to a Well-Known Port Number . . . . . . . . 10
   5.  Impact on Applications . . . . . . . . . . . . . . . . . . . . 11
   6.  Geo-location and Geo-proximity . . . . . . . . . . . . . . . . 12
   7.  Tracking Service Usage . . . . . . . . . . . . . . . . . . . . 12
   8.  ICMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Fragmentation  . . . . . . . . . . . . . . . . . . . . . . . . 13
   10. Traceability . . . . . . . . . . . . . . . . . . . . . . . . . 13
   11. Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     11.1. Abuse Logging and Penalty Boxes  . . . . . . . . . . . . . 14
     11.2. Authentication . . . . . . . . . . . . . . . . . . . . . . 15
     11.3. Spam . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     11.4. Port Randomisation . . . . . . . . . . . . . . . . . . . . 15
     11.5. IPsec  . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.6. Policing Forwarding Behaviour  . . . . . . . . . . . . . . 16
   12. TCP Control Block Sharing  . . . . . . . . . . . . . . . . . . 16
   13. Reverse DNS  . . . . . . . . . . . . . . . . . . . . . . . . . 17
   14. Load Balancing . . . . . . . . . . . . . . . . . . . . . . . . 17
   15. IPv6 Transition Issues . . . . . . . . . . . . . . . . . . . . 17
   16. Introduction of Single Points of Failure . . . . . . . . . . . 18
   17. State Maintenance Reduces Battery Life . . . . . . . . . . . . 18
   18. Support of Multicast . . . . . . . . . . . . . . . . . . . . . 18
   19. Support of Mobile-IP . . . . . . . . . . . . . . . . . . . . . 18
   20. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   21. Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   22. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 19
   23. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   24. Annex  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     24.1. Classes of Address Sharing Solution  . . . . . . . . . . . 19
     24.2. Address Space Multiplicative Factor  . . . . . . . . . . . 20
   25. Informative References . . . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23

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1.  Introduction

   Allocations of IPv4 addresses from the Internet Assigned Numbers
   Authority (IANA) are currently forecast to be complete during 2011
   [IPv4_Report].  Allocations from some Regional Internet Registries
   (RIRs) are anticipated to be complete around a year later, although
   the exact date will vary from registry to registry.  This is causing
   service providers around the world to start to question how they will
   continue providing IPv4 connectivity service to their subscribers
   when there are no longer sufficient IPv4 addresses to allocate them
   one per subscriber.  Several possible solutions to this problem are
   now emerging based around the idea of shared IPv4 addressing.  These
   solutions give rise to a number of issues and this memo identifies
   those common to all such address sharing approaches and collects them
   in a single document.

   Over the long term, deploying IPv6 is the only way to ease pressure
   on the public IPv4 address pool and thereby mitigate the need for
   address sharing mechanisms that give rise to the issues identified
   herein.  In the short term, maintaining growth of IPv4 services in
   the presence of IPv4 address depletion will require address sharing.
   Address sharing will cause issues for end-users, service providers
   and third parties such as law enforcement agencies and content
   providers.  This memo is intended to highlight and briefly discuss
   these issues.

   Increased IPv6 deployment should reduce the burden being placed on an
   address sharing solution, and should reduce the costs of operating
   that solution.  Increasing IPv6 deployment should cause a reduction
   in the number of concurrent IPv4 sessions per subscriber.  If the
   percentage of end-to-end IPv6 traffic significantly increases, so
   that the volume of IPv4 traffic begins decreasing, then the number of
   IPv4 sessions will decrease.  The smaller the number of concurrent
   IPv4 sessions per subscriber, the higher the number of subscribers
   able to share the same IPv4 public address, and consequently, the
   lower the number of IPv4 public addresses required.  However, this
   effect will only occur for subscribers who have both an IPv6 access
   and a shared IPv4 access.  This motivates a strategy to
   systematically bind a shared IPv4 access to an IPv6 access.  It is
   difficult to foresee to what extent growing IPv6 traffic will reduce
   the number of concurrent IPv4 sessions, but in any event, IPv6
   deployment and use should reduce the pressure on the available public
   IPv4 address pool.

2.  Shared Addressing Solutions

   In many networks today a subscriber is provided with a single public

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   IPv4 address at their home or small business.  For instance, in fixed
   broadband access, an IPv4 public address is assigned to each CPE
   (Customer Premises Equipment).  CPEs embed a NAT function which is
   responsible for translating private IPv4 addresses ([RFC1918]
   addresses) assigned to hosts within the local network, to the public
   IPv4 address assigned by the service provider (and vice versa).
   Therefore, devices located with the LAN share the single public IPv4
   address and they are all associated with a single subscriber account
   and a single network operator.

   A number of proposals currently under consideration in the IETF rely
   upon the mechanism of multiplexing multiple subscribers' connections
   over a smaller number of shared IPv4 addresses.  This is a
   significant change.  These proposals include Carrier Grade NAT (CGN)
   [I-D.nishitani-cgn] , Dual-Stack-Lite
   [I-D.ietf-softwire-dual-stack-lite] , NAT64
   [I-D.ietf-behave-v6v4-xlate-stateful] , IVI
   [I-D.ietf-behave-v6v4-xlate] , Address+Port (A+P) proposals
   [I-D.ymbk-aplusp] , [I-D.boucadair-port-range] and SAM
   [I-D.despres-sam].  Section 24 provides a classification of these
   different types of solution and discusses some of the design
   considerations to be borne in mind when deploying large-scale address

   In these new proposals, a single public IPv4 address would be shared
   by multiple homes or small businesses (i.e. multiple subscribers) so
   the operational paradigm described above would no longer apply.  In
   this document we refer to this new paradigm as large-scale address
   sharing.  All these proposals extend the address space by adding port
   information, they differ in the way they manage the port value.

   Security issues associated with NAT have long been documented (see
   [RFC2663] and [RFC2993] ).  However, sharing IPv4 addresses across
   multiple subscribers by any means, either moving the NAT
   functionality from the home gateway to the core of the service
   provider network, or restricting the port choice in the subscriber's
   NAT, creates additional issues for subscribers, content providers and
   network operators.  Many of these issues are created today by public
   wi-fi hotspot deployments.  As such large-scale address sharing
   solutions become more widespread in the face of IPv4 address
   depletion, these issues will become both more severe and more
   commonly felt.  NAT issues in the past typically only applied to a
   single legal entity; as large-scale address sharing becomes more
   prevalent these issues will increasingly span across multiple legal
   entities simultaneously.

   All large-scale address sharing proposals share technical and
   operational issues and these are addressed in the sections that

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   follow.  These issues are common to any service-provider NAT,
   enterprise NAT, and also non-NAT solutions that share individual IPv4
   addresses across multiple subscribers.  This document is intended to
   bring all of these issues together in one place.

3.  Analysis of Issues as they Relate to First and Third Parties

   In this section we present an analysis of whether the issues
   identified in the remainder of this document are applicable to the
   organisation deploying the shared addressing mechanism (and by
   extension their subscribers) and/or whether these issues impact third
   parties (e.g. content providers, law enforcement agencies, etc.).  In
   this analysis, issues that affect end-users are deemed to affect 1st
   parties, as end-users can be expected to complain to their service
   provider when problems arise.  Where issues can expect to be foreseen
   and addressed by the party deploying the shared addressing solution,
   they are not attributed.

   |                   Issue                        |   1st  |   3rd   |
   |                                                |  party | parties |
   | Overly restrictive allocations of outgoing     |    x   |         |
   | ports will impact performance for end users    |        |         |
   |                                                |        |         |
   | Incoming port negotiation mechanisms may fail  |    x   |         |
   |                                                |        |         |
   | Incoming connections to Well-Known Ports will  |    x   |         |
   | not work                                       |        |         |
   |                                                |        |         |
   | Some applications will fail to operate         |    x   |    x    |
   |                                                |        |         |
   | TCP control block sharing will be affected     |    x   |    x    |
   |                                                |        |         |
   | Reverse DNS will be affected                   |    x   |    x    |
   |                                                |        |         |
   | Inbound ICMP will fail in many cases           |    x   |    x    |
   |                                                |        |         |
   | Amplification of security issues               |    x   |    x    |
   |                                                |        |         |
   | Fragmentation will require special handling    |    x   |         |
   |                                                |        |         |
   | Single points of failure and increased         |    x   |         |
   | network instability                            |        |         |
   |                                                |        |         |
   | Port randomisation will be affected            |    x   |         |
   |                                                |        |         |

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   |                   Issue                        |   1st  |   3rd   |
   |                                                |  party | parties |
   | Service usage monitoring and abuse logging     |    x   |    x    |
   | will be impacted for all elements in the chain |        |         |
   | between service provider and content provider  |        |         |
   |                                                |        |         |
   | Penalty boxes will no longer work              |    x   |    x    |
   |                                                |        |         |
   | Spam blacklisting will be affected             |    x   |    x    |
   |                                                |        |         |
   | Geo-location services will be impacted         |    x   |    x    |
   |                                                |        |         |
   | Geo-proximity mechanisms will be impacted      |    x   |    x    |
   |                                                |        |         |
   | Load balancing algorithms may be impacted      |        |    x    |
   |                                                |        |         |
   | Authentication mechanisms may be impacted      |        |    x    |
   |                                                |        |         |
   | Traceability of network usage and abusage will |        |    x    |
   | be affected                                    |        |         |
   |                                                |        |         |
   | IPv6 transition mechanisms will be affected    |    x   |         |
   |                                                |        |         |
   | Frequent keep-alives reduce battery life       |    x   |         |
   |                                                |        |         |

      Figure 1: Shared addressing issues for first and third parties

   As can be seen from Figure 1, deployment of large-scale address
   sharing will create almost as many problems for third parties as it
   does for the service provider deploying the address sharing
   mechanism.  All of these issues are discussed in further detail

4.  Port Allocation

   When we talk about port numbers we need to make a distinction between
   outgoing connections and incoming connections.  For outgoing
   connections, the actual source port number used is usually
   irrelevant.  (While this is true today, in a port-range solution it
   is necessary for the source port to be within the allocated range).
   But for incoming connections, the specific port numbers allocated to
   subscribers matter because they are part of external referrals (used
   by third parties to contact services run by the subscribers).

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   The total number of subscribers able to share a single IPv4 address
   will depend upon assumptions about the average number of ports
   required per active subscriber, and the average number of
   simultaneously active subscribers.  It is important to realise that
   the TCP design makes it undesirable for clients to re-use ports while
   they remain in the TIME-WAIT state (typically 4 minutes after the
   connection has concluded).  TIME-WAIT state removes the hazard of old
   duplicates for "fast" or "long" connections, in which clock-driven
   Initial Sequence Number selection is unable to prevent overlap of the
   old and new sequence spaces.  The TIME-WAIT delay allows all old
   duplicate segments time enough to die in the Internet before the
   connection is reopened [RFC1337].  Therefore ports in this state must
   be included in calculations concerning port usage per subscriber.

   Most of the time the source port selected by a client application
   will be translated (unless there is direct knowledge of a port-range
   restriction in the client's stack), either by a NAT in the
   subscriber's device, or by a CPE NAT, or by a CPE NAT and a CGN.

   IANA has classified the whole port space into three categories (as
   defined in

   o  The Well Known Ports are those from 0 through 1023.

   o  The Registered Ports are those from 1024 through 49151.

   o  The Dynamic and/or Private Ports are those from 49152 through

   [RFC4787] notes that current NATs have different policies with regard
   to this classification; some NATs restrict their translations to the
   use of dynamic ports, some also include registered ports, some
   preserve the port if it is in the well-known range.  [RFC4787] makes
   it clear that the use of port space (1024-65535) is safe: "mapping a
   source port to a source port that is already registered is unlikely
   to have any bad effects".  Therefore, for all address sharing
   solutions, there is no reason to only consider a subset of the port
   space (1024-65535) for outgoing source ports.

4.1.  Outgoing Ports

   According to measurements the average number of outgoing ports
   consumed per active subscriber is much, much smaller than the maximum
   number of ports a subscriber can use at any given time.  However, the
   distribution is heavy-tailed, so there are typically a small number
   of subscribers who use a very high number of ports [CGN_Viability].
   This means that an algorithm that dynamically allocates outgoing port
   numbers from a central pool will typically allow more subscribers to

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   share a single IPv4 address than algorithms that statically divide
   the resource by pre-allocating a fixed number of ports to each
   subscriber.  Similarly, such an algorithm should be more able to
   accommodate subscribers wishing to use a relatively high number of

   It is important to note here that the desire to dynamically allocate
   outgoing port numbers will make a service provider's job of
   maintaining records of subscriber port number allocations
   considerably more onerous (see Section 10).  The number of records
   per subscriber will increase from 1 in a scheme where ports are
   statically allocated, to a much larger number equivalent to the total
   number of outgoing ports consumed by that subscriber during the time
   period for which detailed logs must be kept.

   A potential problem with dynamic allocation occurs when one of the
   subscriber devices behind such a port-shared IPv4 address becomes
   infected with a worm, which then quickly sets about opening many
   outbound connections in order to propagate itself.  Such an infection
   could rapidly exhaust the shared resource of the single IPv4 address
   for all connected subscribers.  It is therefore necessary to impose
   limits on the total number of ports available to an individual
   subscriber to ensure that the shared resource (the IPv4 address)
   remains available in some capacity to all the subscribers using it.

4.2.  Incoming Ports

   It is desirable to ensure that incoming ports remain stable over
   time.  This is challenging as the network doesn't know anything in
   particular about the applications that it is supporting and therefore
   has no real notion of how long an application/service session is
   still ongoing and therefore requiring port stability.

   Early measurements [CGN_Viability] also seem to indicate that, on
   average, only very few ports are used by subscribers for incoming
   connections.  However, a majority of subscribers accept at least one
   inbound connection.

   This means that it is not necessary to pre-allocate a large number of
   incoming ports to each subscriber.  It is possible to either pre-
   allocate a small number of ports for incoming connections or do port
   allocation on demand when the application wishing to receive a
   connection is initiated.  The bulk of incoming ports can be reserved
   as a centralized resource shared by all subscribers using a given
   public IPv4 address.

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4.2.1.  Port Negotiation

   In current deployments, one important and widely used feature of many
   CPE devices is the ability to open incoming ports (port forwarding)
   either manually, or with a protocol such as UPnP IGD.  If a CGN is
   present, the port must also be open in the CGN.  The situation may be
   alleviated somewhat if the CGN architecture is composed of only one
   NAT level (no NAT in the CPE) as for DS-Lite, although a service
   provider operating this solution will still be required to offer some
   means for configuring incoming ports by their subscribers.  This may
   be either via a UPnP or NAT-PMP relay over a tunnelled direct
   connection between the CPE and CGN, or a web interface to configure
   the incoming port mapping on the CGN.  Note, that such an interface
   effectively makes public what was previously a private management
   interface and this raises security concerns that must be addressed.

   For port-range solutions, port forwarding capabilities may still be
   present at the CPE, with the limitation that the open incoming port
   must be within the allocated port-range (for instance it is not
   possible to open port 5002 for incoming connections if port 5002 is
   not within the allocated port-range).  Universal Plug and Play (UPnP)

   Using the UPnP semantic, an application asks "I want to use port
   number X, is that ok?" and the answer is yes or no.  If the answer is
   no, the application will typically try the next port in sequence,
   until it either finds one that works or gives up after a limited
   number of attempts.  UPnP has, currently, no way to redirect the
   application to use another port number.  UPnP IGD 2.0, currently
   being defined, should improve this and allow for allocation of any
   available port.  NAT Port Mapping Protocol (NAT-PMP)

   NAT-PMP already has a better semantic here, enabling the NAT to
   redirect the application to an available port number.

4.2.2.  Connection to a Well-Known Port Number

   Once an IPv4 address sharing mechanism is in place, connections to
   well-known port numbers will not work in the general case.  Any
   application that is not port-agile cannot be expected to work.  Some
   workaround (e.g. redirects to a port-specific URI) could be deployed
   given sufficient incentives.  There exist several proposals for
   'application service location' protocols which would provide a means
   of addressing this problem, but historically these proposals have not
   gained much deployment traction.

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   For example, the use of DNS SRV records [RFC2782] provides a
   potential solution for subscribers wishing to host services in the
   presence of a shared-addressing scheme.  SRV records make it possible
   to specify a port value related to a service, thereby making services
   accessible on ports other than the Well-Known ports.  It is worth
   noting that this mechanism is not applicable to HTTP.

5.  Impact on Applications

   Address sharing solutions will have an impact on the following types
   of applications:

   o  Applications that establish inbound communications - these
      applications will have to ensure that ports selected for inbound
      communications are either within the allocated range (for port-
      range solutions) or are forwarded appropriately by the CGN (for
      CGN-based solutions).  See Section 4.2 for more discussion of

   o  Applications that carry address and/or port information in their
      payload - where translation of port and/or address information is
      performed at the IP and transport layers by the address sharing
      solution, an ALG will also be required to ensure application layer
      data is appropriately modified;

   o  Applications that use fixed ports (e.g. well-known ports) - see
      Section 4.2.2 for more discussion of this;

   o  Applications that do not use any port (e.g.  ICMP) - where address
      sharing solutions map subscribers to (private) IP addresses on a
      one-to-one basis this will not be an issue, otherwise such
      applications will require special handling - see Section 8 for
      more discussion of this;

   o  Applications that assume the uniqueness of source addresses (e.g.
      IP address as identifier) - such applications will fail to operate
      correctly in the presence of multiple, discrete, simultaneous
      connections from the same source IP address;

   o  Applications that explicitly prohibit concurrent connections from
      the same address - such applications will fail when multiple
      subscribers sharing an IP address attempt to use them

   Applications already frequently implement mechanisms in order to
   circumvent the presence of NATs (typically CPE NATs):

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   o  Application Layer Gateways (ALGs): Many CPE devices today embed
      ALGs that allow applications to behave correctly despite the
      presence of NAT on the CPE.  When the NAT belongs to the
      subscriber, the subscriber has flexibility to tailor the device to
      his or her needs.  For CGNs, subscribers will be dependent on the
      set of ALGs that their service provider makes available.  A
      service provider-based NAT may, or may not, support NAT-traversal
      for IKE [RFC3947] for example.  For port-range solutions, ALGs
      will require modification to deal with the port-range restriction,
      but will otherwise have the same capabilities as today.

   o  NAT Traversal Techniques: ICE, STUN, TURN, etc.  See
      Section 4.2.1.

6.  Geo-location and Geo-proximity

   IP addresses are frequently used to indicate, with some level of
   granularity and some level of confidence, where a host is physically
   located.  Geo-location services are used by content providers to
   allow them to conform with regional content licensing restrictions,
   to target advertising at specific geographic areas, or to provide
   customised content.  Geo-location services are also necessary for
   emergency services provision.  In some deployment contexts (e.g.
   centralised CGN), shared addressing will reduce the level of
   confidence and level of location granularity that IP-based geo-
   location services can provide.  Other forms of geo-location will
   still work as usual.

   A slightly different use of an IP address is to calculate the
   proximity of a connecting host to a particular service delivery
   point.  This use of IP address information impacts the efficient
   delivery of content to an end-user.  If a CGN is introduced in
   communications and it is far from an end-user connected to it,
   application performance may be degraded insofar as IP-based geo-
   proximity is a factor.

7.  Tracking Service Usage

   As large-scale address sharing becomes more commonplace, monitoring
   the number of unique users of a service will become more complex than
   simply counting the number of connections from unique IP addresses.
   While this is a somewhat inexact methodology today due to the
   widespread use of CPE NAT, it will become a much less useful approach
   in the presence of widespread large-scale address sharing solutions.
   In general, all elements that monitor usage or abusage in the chain
   between a service provider that has deployed address sharing and a

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   content provider will need to be upgraded to take account of the port
   value in addition to IP addresses.

8.  ICMP

   ICMP does not carry any port information and is consequently
   problematic for address sharing mechanisms.  Sourcing ICMP from hosts
   behind an address sharing solution does not pose problems, although
   responses to outgoing ICMP will require special handling, such as
   making use of the ICMP identifier value to route the response

   For inbound ICMP there are two cases.  The first case is that of ICMP
   sourced from outside the network of the address sharing solution
   provider.  Several applications make use of this (e.g.  P2P
   applications) and measurements derived by such applications in the
   presence of an address sharing solution will be erroneous.  The
   second case is that of ICMP sourced from within the network of the
   address sharing solution provider (e.g. for network management and
   diagnostic purposes).  In this case ICMP can be routed normally for
   CGN-based solutions owing to the presence of locally unique private
   IP addresses for each CPE device.  For port-range solutions, ICMP
   will not be routable without special handling, e.g. placing a port
   number in the ICMP identifier field, and having port-range routers
   make routing decisions based upon that field.

   Another concern relating to ICMP in the presence of large-scale
   address sharing is that a malevolent user could send an ICMP "Packet
   Too Big" (Type 3, Code 4) message indicating a Next-Hop MTU of
   anything down to 68 octets.  This value will be cached by the off-net
   server for all subscribers sharing the address of the malevolent
   user.  This could lead to a DoS against both the remote server and
   the large-scale NAT device itself (as they will both have to handle
   many more packets per second).

9.  Fragmentation

   When a packet is fragmented, transport-layer port information (either
   UDP or TCP) is only present in the first fragment.  Subsequent
   fragments will not carry the port information and so will require
   special handling.

10.  Traceability

   Legal obligations require a service provider to provide the identity

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   of a subscriber upon request to the authorities.  Where one public
   IPv4 address is shared between several subscribers, the knowledge of
   the IP address alone is not enough to identify the appropriate
   subscriber.  The legal request should include the information: [IP
   address - Port - Protocol- Begin_Timestamp - End_Timestamp].
   Accurate time-keeping is clearly important.  A densely populated CGN
   could mean even very small amounts of clock skew between a content
   provider and the CGN operator will result in ambiguity about which
   customer was using a specific port at a given time.

   Considering that it is unrealistic to expect all servers to start
   logging port information of connection attempts, address sharing
   service providers may need to start logging destinations as well,
   giving rise to privacy concerns.

   Address sharing solutions must record and store all mappings
   (typically during 6 months to one year, depending on the
   jurisdiction) that they create.  If we consider one mapping per
   session, a service provider should record and retain traces of all
   sessions created by all subscribers during one year (if the legal
   storage duration is one year).  This may be challenging due to the
   volume of data requiring storage, the volume of data to repeatedly
   transfer to the storage location, and the volume of data to search in
   response to a query.

   Address sharing solutions may mitigate these issues to some extent by
   pre-allocating groups of ports.  Then only the allocation of the
   group needs to be recorded, and not the creation of every session
   binding within that group.  There are trade-offs to be made between
   the sizes of these groups, the ratio of public addresses to
   subscribers, whether or not these groups timeout, the impact on
   logging requirements and port randomisation security.

11.  Security

   Before noting some specific security-related issues caused by large-
   scale address sharing, it is perhaps worth noting that, in general,
   address sharing creates a vector for attack amplification in numerous
   ways.  See Section 8 for one example.

11.1.  Abuse Logging and Penalty Boxes

   When an abuse is reported today, it is usually done in the form: IPv4
   address X has done something bad at time T0.  This is not enough
   information to uniquely identify the subscriber responsible for the
   abuse when that IPv4 address is shared by more than one subscriber.
   Law enforcement authorities may be particularly impacted because of

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   this.  This particular issue can be fixed by logging port numbers,
   although this will increase logging data storage requirements.

   A number of application servers on the network today log IPv4
   addresses in connection attempts to protect themselves from certain
   attacks.  For example, if a server sees too many login attempts from
   the same IPv4 address, it may decide to put that address in a penalty
   box for a certain time.  If an IPv4 address is shared by multiple
   subscribers, this would have unintended consequences in a couple of
   ways.  First it may become the natural behavior to see many login
   attempts from the same address because it is now shared across a
   potentially large number of subscribers.  Second and more likely is
   that one user who fails a number of login attempts may block out
   other users who have not made any previous attempts but who will now
   fail on their first attempt.  In the presence of widespread large-
   scale address sharing, penalty box solutions to service abuse simply
   will not work.

11.2.  Authentication

   Simple address-based identification mechanisms that are used to
   populate access control lists will fail when an IP address is no
   longer sufficient to identify a particular subscriber.  Including
   port numbers in access control list definitions may be possible at
   the cost of extra complexity, and may also require the service
   provider to make static port assignments, which conflicts with the
   requirement for dynamic assignments discussed in Section 4.1 .

11.3.  Spam

   Another case of identifying abusers has to do with spam blacklisting.
   When a spammer is behind a CGN or using a port-shared address,
   blacklisting of their IP address will result in all other subscribers
   sharing that address having their ability to source SMTP packets
   restricted to some extent.

11.4.  Port Randomisation

   A blind attack that can be performed against TCP relies on the
   attacker's ability to guess the 5-tuple (Protocol, Source Address,
   Destination Address, Source Port, Destination Port) that identifies
   the transport protocol instance to be attacked.
   [I-D.ietf-tsvwg-port-randomization] describes a number of methods for
   the random selection of the source port number, such that the ability
   of an attacker to correctly guess the 5-tuple is reduced.  With
   shared IPv4 addresses, the port selection space is reduced.
   Preserving port randomisation is important and may be more or less
   difficult depending on the address sharing solution and the size of

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   the port space that is being manipulated.  Allocation of non-
   contiguous port ranges could help to mitigate this issue.

   It should be noted that guessing the port information may not be
   sufficient to carry out a successful blind attack.  The exact TCP
   Sequence Number (SN) should also be known.  A TCP segment is
   processed only if all previous segments have been received, except
   for some Reset Segment implementations which immediately process the
   Reset as long as it is within the Window.  If SN is randomly chosen
   it will be difficult to guess it (SN is 32 bits long); port
   randomisation is one protection among others against blind attacks.

11.5.  IPsec

   The impact of large-scale IP address sharing for IPsec operation
   should be evaluated and assessed.  [RFC3947] proposes a solution to
   solve issues documented in [RFC3715] .  The applicability of
   [RFC3947] in the context of shared IP address solutions should be
   evaluated.  In particular, service providers may wish to ensure that
   CGN deployments do not inadvertently block NAT traversal for security
   protocols such as IKE (refer to [I-D.gont-behave-nat-security] for
   more information).

11.6.  Policing Forwarding Behaviour

   [RFC2827] motivates and discusses a simple, effective, and
   straightforward method for using ingress traffic filtering to
   prohibit Denial-of-Service (DoS) attacks which use forged IP
   addresses.  Following this recommendation, service providers
   operating shared-addressing mechanisms should ensure that source
   addresses, or source ports in the case of port-range schemes, are set
   correctly in outgoing packets from their subscribers or they should
   drop the packets.

   If some form of IPv6 ingress filtering is deployed in the broadband
   network and DS-Lite service is restricted to those subscribers, then
   tunnels terminating at the CGN and coming from registered subscriber
   IPv6 addresses cannot be spoofed.  Thus a simple access control list
   on the tunnel transport source address is all that is required to
   accept traffic on the southbound interface of a CGN.

12.  TCP Control Block Sharing

   [RFC2140] defines a performance optimisation for TCP based on sharing
   state between TCP control blocks that pertain to connections to the
   same host, as opposed to maintaining state for each discrete
   connection.  This optimisation assumes that an address says something

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   about the properties of the path between two hosts, which is clearly
   not the case if the address in question is shared by multiple hosts
   at different physical network locations.  While CPE NAT today causes
   problems for sharing TCP control block state across multiple
   connections to a given IP address, large-scale address sharing will
   make these issues more severe and more widespread.

13.  Reverse DNS

   Many service providers populate forward and reverse DNS zones for the
   public IPv4 addresses that they allocate to their subscribers.  In
   the case where public addresses are shared across multiple
   subscribers, such strings are, by definition, no longer sufficient to
   identify an individual subscriber without additional information.

14.  Load Balancing

   Algorithms used to balance traffic load for popular destinations may
   be affected by the introduction of address sharing.  Where balancing
   is achieved by deterministically routing traffic from specific source
   IP addresses to specific servers, sudden imbalances in load may be
   experienced as address sharing is enabled for some of those source IP
   addresses.  This will require re-evaluation of the algorithms used in
   the load-balancing design.  In general, as the scale of address
   sharing grows, load-balancing designs will need to be re-evaluated
   and any assumptions about average load per source IP address

15.  IPv6 Transition Issues

   IPv4 address sharing solutions may interfere with existing IPv4 to
   IPv6 transition mechanisms, which were not designed with IPv4
   shortage considerations in mind.  With port-range solutions for
   instance, incoming 6to4 packets should be able to find their way from
   a 6to4 relay to the appropriate 6to4 CPE router, despite the lack of
   direct port range information (UDP/TCP initial source port did not
   pass through the CPE port range translation process).  One solution
   would be for a 6to4 IPv6 address to embed not only an IPv4 address
   but also a port range value.

   Subscribers allocated with private addresses will not be able to
   utilise 6to4 to access IPv6, but may be able to utilise Teredo.

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16.  Introduction of Single Points of Failure

   In common with all deployments of new network functionality, the
   introduction of new nodes or functions to handle the multiplexing of
   multiple subscribers across shared IPv4 addresses could create single
   points of failure in the network.  Any IP address sharing solution
   should consider the opportunity to add redundancy features in order
   to alleviate the impact on the robustness of the offered IP
   connectivity service.  The ability of the solution to allow hot
   swapping from one machine to another should be considered.  This is
   especially important where the address sharing solution in question
   requires the creation of per-flow state in the network.

17.  State Maintenance Reduces Battery Life

   In order for hosts to maintain network state in the presence of NAT,
   keep-alive messages have to be sent at frequent intervals.  For
   battery-powered devices, sending these keep-alive messages can result
   in significantly reduced battery performance than would otherwise be
   the case [Mobile_Energy_Consumption].

18.  Support of Multicast

   [RFC5135] specifies requirements for a NAT that supports Any Source
   IP Multicast or Source-Specific IP Multicast.  Port-range routers
   that form part of port-range solutions will need to support similar
   requirements if multicast support is required.

   [Placeholder for more details of impact of address sharing on
   multicast deployments.]

19.  Support of Mobile-IP

   IP address sharing within the context of Mobile-IP deployments (in
   the home network and/or in the visited network), will require Home
   Agents and/or Foreign Agents to be updated so as to take into account
   the relevant port information.  There may also be issues raised when
   an additional layer of encapsulation is required thereby causing, or
   increasing the need for, fragmentation and reassembly.

   Issues for Mobile-IP in the presence of NAT are discussed in

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20.  IANA Considerations

   This memo includes no request to IANA.

21.  Security Considerations

   This memo does not define any protocol and raises no security issues.
   Section 11 discusses some of the security and identity-related
   implications of address sharing.

22.  Contributors

   This document is based on sources co-authored by J.L. Grimault and A.
   Villefranque of France Telecom.

23.  Acknowledgements

   This memo was partly inspired by conversations that took place as
   part of Internet Society (ISOC) hosted roundtable events for
   operators and content providers deploying IPv6.  Participants in
   those discussions included John Brzozowski, Leslie Daigle, Tom
   Klieber, Yiu Lee, Kurtis Lindqvist, Wes George, Lorenzo Colliti, Erik
   Kline, Igor Gashinsky, Jason Fesler, Rick Reed, Adam Bechtel, Larry
   Campbell, Tom Coffeen, David Temkin, Pete Gelbman, Mark Winter, Will
   Charnock, Martin Levy, Greg Wood and Christian Jacquenet.  The
   authors are also grateful to Christian Jacquenet, Iain Calder, Joel
   Halpern, Brian Carpenter, Gregory Lebovitz, Bob Briscoe, Marcelo
   Bagnulo and Dan Wing for their helpful comments and suggestions for
   improving this document.

   This memo was created using the xml2rfc tool.

24.  Annex

24.1.  Classes of Address Sharing Solution

   IP address sharing solutions fall into two classes.  Either a
   service-provider operated NAT function is introduced and subscribers
   are allocated addresses from [RFC1918] space, or public IPv4
   addresses are shared across multiple subscribers by restricting the
   range of ports available to each subscriber.  These classes of
   solution are described in a bit more detail below.

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   o  CGN-based solutions: These solutions propose the introduction of a
      NAPT function in the service provider's network, denoted also as
      Carrier Grade NAT (CGN), or Large Scale NAT (LSN)
      [I-D.nishitani-cgn] , or Provider NAT.  The CGN is responsible for
      translating private addresses to publicly routable addresses.
      Private addresses are assigned to subscribers, a pool of public
      addresses is assigned to the CGN, and the number of public
      addresses is smaller than the number of subscribers.  A public
      IPv4 address in the CGN pool is shared by several subscribers at
      the same time.  Solutions making use of a service provider-based
      NAT include [I-D.shirasaki-nat444] (two layers of NAT) and
      [I-D.ietf-softwire-dual-stack-lite] (a single layer of NAT).

   o  Port-range solutions: These solutions avoid the presence of a CGN
      function.  A single public IPv4 address is assigned to several
      subscribers at the same time.  A restricted port range is also
      assigned to each subscriber so that two subscribers with the same
      IPv4 address have two different port ranges that do not overlap.
      These solutions are called A+P (Address+Port) [I-D.ymbk-aplusp] ,
      or Port Range [I-D.boucadair-port-range] , or SAM (Stateless
      Address Mapping) [I-D.despres-sam] .

24.2.  Address Space Multiplicative Factor

   The purpose of sharing public IPv4 addresses is to increase the
   addressing space.  A key parameter is the factor by which service
   providers want or need to multiply their IPv4 public address space;
   and the consequence is the number of subscribers sharing the same
   public IPv4 address.  We refer to this parameter as the address space
   multiplicative factor, the inverse is called the compression ratio.

   The multiplicative factor can only be applied to the subset of
   subscribers that are eligible for a shared address.  The reasons a
   subscriber cannot have a shared address can be:

   o  It would not be compatible with the service they are currently
      subscribed to (for example: business subscriber).

   o  Subscriber CPE is not compatible with the address sharing solution
      selected by the service provider (for example it does not handle
      port restriction for port-range solutions or it does not allow
      IPv4 in IPv6 encapsulation for the DS-Lite solution), and its
      replacement is not easy.

   Different service providers may have very different needs.  A long-
   lived service provider, whose number of subscribers is rather stable,
   may have an existing address pool that will only need a small
   extension to cope with the next few years, assuming that this address

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   pool can be re-purposed for an address sharing solution (small
   multiplicative factor, less than 10).  A new entrant or a new line of
   business will need a much bigger multiplicative factor (e.g. 1000).
   A mobile operator may see its addressing needs grow dramatically as
   the IP-enabled mobile handset market grows.

   When the multiplicative factor is large, the average number of ports
   per subscriber is small.  Given the large measured disparity between
   average and peak port consumption [CGN_Viability] , this will create
   service problems in the event that ports are allocated statically.
   In this case, it is essential for port allocation to map to need as
   closely as possible, and to avoid allocating ports for longer than
   necessary.  Therefore, the larger the multiplicative factor, the more
   dynamic the port assignment has to be.

25.  Informative References

              Alcock, S., "Research into the Viability of Service-
              Provider NAT", 2008, <

              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.

              Despres, R., "Scalable Multihoming across IPv6 Local-
              Address Routing Zones Global-Prefix/Local-Address
              Stateless Address Mapping (SAM)", draft-despres-sam-03
              (work in progress), July 2009.

              Gont, F. and P. Srisuresh, "Security implications of
              Network Address Translators (NATs)",
              draft-gont-behave-nat-security-03 (work in progress),
              October 2009.

              Haddad, W. and C. Perkins, "A Note on NAT64 Interaction
              with Mobile IPv6",
              draft-haddad-mext-nat64-mobility-harmful-01 (work in
              progress), April 2010.

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              Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", draft-ietf-behave-v6v4-xlate-20 (work in
              progress), May 2010.

              Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers",
              draft-ietf-behave-v6v4-xlate-stateful-11 (work in
              progress), March 2010.

              Durand, A., Droms, R., Haberman, B., Woodyatt, J., Lee,
              Y., and R. Bush, "Dual-Stack Lite Broadband Deployments
              Following IPv4 Exhaustion",
              draft-ietf-softwire-dual-stack-lite-04 (work in progress),
              March 2010.

              Larsen, M. and F. Gont, "Transport Protocol Port
              Randomization Recommendations",
              draft-ietf-tsvwg-port-randomization-08 (work in progress),
              May 2010.

              Yamagata, I., Nishitani, T., Miyakawa, S., Nakagawa, A.,
              and H. Ashida, "Common requirements for IP address sharing
              schemes", draft-nishitani-cgn-04 (work in progress),
              March 2010.

              Yamagata, I., Shirasaki, Y., Nakagawa, A., Yamaguchi, J.,
              and H. Ashida, "NAT444", draft-shirasaki-nat444-01 (work
              in progress), March 2010.

              Bush, R., "The A+P Approach to the IPv4 Address Shortage",
              draft-ymbk-aplusp-05 (work in progress), October 2009.

              Huston, G., "IPv4 Address Report", 2009,

              Haverinen, H., Siren, J., and P. Eronen, "Energy
              Consumption of Always-On Applications in WCDMA Networks",
              2007, <>.

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   [RFC1337]  Braden, B., "TIME-WAIT Assassination Hazards in TCP",
              RFC 1337, May 1992.

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

   [RFC2140]  Touch, J., "TCP Control Block Interdependence", RFC 2140,
              April 1997.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
              November 2000.

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

   [RFC3947]  Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
              "Negotiation of NAT-Traversal in the IKE", RFC 3947,
              January 2005.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC5135]  Wing, D. and T. Eckert, "IP Multicast Requirements for a
              Network Address Translator (NAT) and a Network Address
              Port Translator (NAPT)", BCP 135, RFC 5135, February 2008.

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Authors' Addresses

   Mat Ford (editor)
   Internet Society


   Mohamed Boucadair
   France Telecom


   Alain Durand


   Pierre Levis
   France Telecom
   42 rue des Coutures
   BP 6243
   Caen Cedex 4  14066


   Phil Roberts
   Internet Society
   Reston, VA


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