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Network Working Group                                            G. Chen
Internet-Draft                                              China Mobile
Intended status: Informational                                   T. Tsou
Expires: October 12, 2014                            Huawei Technologies
                                                               C. Donley
                                                               CableLabs
                                                               T. Taylor
                                                    PT Taylor Consulting
                                                          April 10, 2014


  Analysis of NAT64 Port Allocation Methods for Shared IPv4 Addresses
               draft-chen-sunset4-cgn-port-allocation-04

Abstract

   This document enumerates methods of port assignment in Carrier Grade
   NATs (CGNs), focused particularly on NAT64 environments.  A
   theoretical framework of different NAT port allocation methods is
   described.  The memo is intended to clarify and focus the port
   allocation discussion and propose an integrated view of the
   considerations for selection of the port allocation mechanism in a
   given deployment.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 12, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Considerations For the Choice of  Port Allocation Methods . .   3
     2.1.  Port Consumption on NAT64 . . . . . . . . . . . . . . . .   4
     2.2.  Classification of Port Allocation Models  . . . . . . . .   5
       2.2.1.  Stateful vs. Stateless  . . . . . . . . . . . . . . .   5
       2.2.2.  Dynamic vs. Static  . . . . . . . . . . . . . . . . .   5
       2.2.3.  Centralized vs. Distributed . . . . . . . . . . . . .   6
     2.3.  Port Allocation Solutions . . . . . . . . . . . . . . . .   7
       2.3.1.  Older Transition Technologies . . . . . . . . . . . .   7
       2.3.2.  Current Work On Stateless Transition Technologies . .   7
       2.3.3.  Port Control Protocol (PCP) . . . . . . . . . . . . .   8
     2.4.  Specific Considerations . . . . . . . . . . . . . . . . .   8
       2.4.1.  Log Volume Optimization . . . . . . . . . . . . . . .   8
       2.4.2.  Connectivity State Optimization . . . . . . . . . . .  10
       2.4.3.  Port Randomization  . . . . . . . . . . . . . . . . .  11
   3.  Considerations For the Dynamic Assignment of Port-Ranges  . .  11
     3.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Implementation Issues -- Port Randomization and Port-
           Range Deallocation  . . . . . . . . . . . . . . . . . . .  12
     3.3.  Issues Of Traceability  . . . . . . . . . . . . . . . . .  13
     3.4.  Other Considerations  . . . . . . . . . . . . . . . . . .  14
   4.  Deterministic Port Allocation . . . . . . . . . . . . . . . .  14
     4.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .  14
     4.2.  Deterministic Port Ranges . . . . . . . . . . . . . . . .  16
       4.2.1.  IPv4 Port Utilization Efficiency  . . . . . . . . . .  19
       4.2.2.  Planning and Dimensioning . . . . . . . . . . . . . .  19
       4.2.3.  Deterministic CGN Example . . . . . . . . . . . . . .  20
       4.2.4.  Additional Management Considerations  . . . . . . . .  21
     4.3.  Failover Considerations . . . . . . . . . . . . . . . . .  22
     4.4.  Impact On IPv6 Transition . . . . . . . . . . . . . . . .  22
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  24
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  24
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  24
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  25
   Appendix A.  Configuration of Server Software to Log Source



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                Port . . . . . . . . . . . . . . . . . . . . . . . .  28
     A.1.  Apache  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     A.2.  Postfix . . . . . . . . . . . . . . . . . . . . . . . . .  28
     A.3.  Sendmail  . . . . . . . . . . . . . . . . . . . . . . . .  28
     A.4.  sshd  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     A.5.  Cyrus IMAP and UW IMAP  . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   With the depletion of IPv4 addresses, Carrier Grade NAT (CGN) has
   been adopted by ISPs to expand IPv4 spaces.  CGN maps IP addresses
   from one address realm to another, relying upon the mechanism of
   multiplexing multiple subscribers' connections over a smaller number
   of shared IPv4 addresses to provide transparent routing to end hosts.
   [RFC6888] specifies a number of CGN requirements.  A network-based
   NAT is implied by several approaches to IPv6 transition including DS-
   Lite [RFC6333], NAT64 ([RFC6145] and [RFC6146]), and NAT444.  All of
   these would likely fall within the scope of the CGN requirements
   document [RFC6888].

   The first part of this memo (Section 2) focusses on the topic of IPv6
   migration.  The CGN may not do Network Address Port Translation
   (NAPT), but only Network Address Translation (NAT) [RFC3022].  In
   this scenario, there is no concern about port assignment.  When NAPT
   is involved, Section 2 elaborates on the considerations for address
   sharing and particularly port assignment in the NAT64 environment,
   where IPv6-only nodes are connected to external dual-stack or IPv4
   networks.

   Section 3 looks more closely at dynamic bulk assignment of ports to
   individual subscriber sites, particularly as a means of log volume
   reduction.  The proposals made in this section are applicable to the
   CGN environment in general, independently of the particular flavour
   of translation being used.

   Finally, Section 4 looks at a scheme for assignment of ports using a
   deterministic algorithm that has the potential to simplify
   operations.

2.  Considerations For the Choice of Port Allocation Methods

   For port allocations on NAT64, several aspects may have to be
   considered when selecting a suitable method.  Here is a list of the
   potential considerations, which are covered in more detail below.

   o  specific features of port usage in a NAT64 environment;




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   o  classification of different port allocation methods;

   o  port allocation to improve connectivity;

   o  port allocation to optimize log volume;

   o  port allocation to enhance security.

   Both analysis and relevant experimental results are presented in the
   sub-sections that follow.

2.1.  Port Consumption on NAT64

   Thanks to its simplicity and efficiency, NAT64 will likely be
   deployed widely.  In a typical scenario, NAT64 will enable internal
   IPv6-only hosts to connect to external dual-stack or IPv4 networks.
   Compared with NAT44, fewer ports per subscriber are consumed on
   NAT64, because only flows between different address families require
   ports to be assigned.  That is, a NAT44 will be deployed in an
   IPv4-only environment.  Since all traffic will have to traverse the
   NAT, all flows will need ports.  Conversely, NAT64 only requires a
   port when one end is IPv4-only.  Therefore, the more hosts support
   IPv6, the fewer ports are needed on the NAT64.

   One of the authors did a test comparison of port consumption on NAT64
   and NAT44.  Top100 websites (referring to Alexa statistics) were
   assessed to evaluate status of port usage on NAT44 and NAT64
   respectively.  It was observed that the port consumption per session
   on NAT64 is roughly only half that on NAT44. 43 percent of top100
   websites have AAAA records, therefore the NAT64 didn't have to assign
   ports to the traffic going to those websites.  The results may be
   different if more services (e.g. game, web-mail, etc) are considered.
   But it is apparent that the effects of port saving on NAT64 will be
   amplified by increasing native IPv6 support.

   Apart from the above observation, port allocation can be tuned
   according to the phase of IPv6 migration.  The use of NAT64 will
   advance IPv6 deployment, because it provides everyone with incentives
   to use IPv6, and eventually the result is an end-to-end IPv6-only
   network with no need for port allocations.  As more content providers
   and services become available over IPv6, the utilization of NAT64
   goes down since fewer destinations require translation progressing.
   Thus as IPv6 migration proceeds, it will be possible to relax the
   multiplexing ratio of IPv4 address sharing.







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2.2.  Classification of Port Allocation Models

   This section lists several models to allocate the port information in
   NAT64 equipment.  It also describes example cases for each allocation
   model.

2.2.1.  Stateful vs. Stateless

   o  Stateful

      The stateful NAT can be implemented either by static address
      translation or dynamic address translation.

      In the case of static address assignment, a one-to-one address
      mapping for hosts between a IPv6 network address and an IPv4
      network address is pre-configured on the NAT operation.  This case
      normally occurs when a server is deployed in a IPv6 domain.  The
      static configuration ensures stable inbound connectivity.

      Dynamic address assignment would periodically free the binding so
      that the global address could be recycled for later use.  This
      increases the efficiency of usage of IPv4 addresses.

   o  Stateless

      Stateless NAT is performed in compliance with [RFC6145].  The
      public IPv4 address is required to be embedded in the IPv6
      address.  Thus the NAT64 can directly extract the address and has
      no need to record mapping states.

   A promising usage of stateless NAT may appear in the data centre
   environment where IPv6 server pools receive inbound connections from
   IPv4 users externally [I-D.anderson-siit-dc].  NAT usage in other
   cases may be controversial.  First off, the static one-to-one mapping
   does not address the issue of IPv4 depletion.  Secondly, it
   introduces a dependency between IPv4 and IPv6 addressing.  That
   creates new limitations since a change of IPv4 address will cause
   renumbering of IPv6 addresses.

2.2.2.  Dynamic vs. Static

   Port assignment can be dynamic (ports allocated on demand) or static
   (ports allocated as part of the configuration process).

   o  Dynamic assignment

      NAT64 normally uses dynamic assignment, since this achieves higher
      port utilization.  Port allocations can be made with per-session



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      or per-customer granularity.  Per-session assignment is configured
      on the NAT64 by default since it maximizes port utilization.
      However, this can result in a heavy log volume that may have to be
      recorded for lawful interception systems.  To mitigate that
      concern, the NAT64 may dynamically allocate a port range for each
      connected subscriber.  This will significantly reduce log volume.

      A proper port-range configuration may have to take into account
      two considerations:

      A.  The number of session initiations for each subscriber.  A
          subscriber normally uses multiple applications simultaneously,
          e.g. map, online video or game.  The number of concurrent
          sessions is essential to determine the number of ports the
          subscriber needs.  It has been learned from subscribers'
          behaviors that the average number of sessions consumed by one
          user's device is around 200 to 300 ports.  Several devices may
          appear behind a CPE.  Administrators may configure a range
          with 1000 ports to each CPE in fixed networks.

      B.  Impacts on NAT64 capacity.  Preassigned port ranges occupy
          memory even when there are unused ports.  Therefore, the
          operator should be cautious about the impact of port-range
          reservation on the capacity for attempted concurrent sessions,
          especially in the case of a centralized NAT64 CGN serving
          numerous subscribers.

   o  Static assignment

      Static assignment makes port reservations in bulk for each
      internal address before subscriber connection.  The assigned ports
      can be in either a contiguous or non-contiguous port range for the
      sake of attack defense.  Log recording may not be necessary due to
      the stable mapping relations.  Considerations of the interaction
      between port-range allocation and capacity impact are also
      applicable in the case of static assignment.  Section 4 describes
      a deterministic algorithm to assign a port range for an internal
      IP address pool in a sequence.

2.2.3.  Centralized vs. Distributed

   There is an increasing need to connect NAT64 with downstream
   NAT46-capable devices to support IPv4 users/applications on an
   IPv6-only path.  Several solutions have been proposed in this area,
   e.g., 464xlat [RFC6877], MAP-T [I-D.ietf-softwire-map-t] and 4rd
   [I-D.ietf-softwire-4rd].  Port allocation can be categorized as a
   centralized assignment on NAT64 or as a port delegation distributed
   to downstream devices (e.g, Customer Edge connected with NAT64).



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   o  Centralized Assignment

      A centralized method makes port assignments once IP flows come to
      the NAT64.  The allocation policy is enforced on a centralized
      point.  Either a dynamic or static port assignment is made for
      received sessions.

   o  Distributed Assignment

      NAT64 can also delegate the pre-allocated port range to customer
      edge devices.  That can be achieved through additional out-of-band
      provisioning signals (e.g., [I-D.ietf-pcp-port-set],
      [I-D.ietf-softwire-map-dhcp]).  The distributed model normally is
      performed A+P style for static port assignment.  The NAT64 should
      also hold the corresponding mapping in order to validate port
      usage in the outgoing direction and route inbound packets.
      Delegated port ranges shift NAT64 port computations/states into
      downstream devices.  The detailed benefits of this approach are
      documented in [I-D.ietf-softwire-stateless-4v6-motivation].

2.3.  Port Allocation Solutions

2.3.1.  Older Transition Technologies

   In older work, stateful NAT64 [RFC6146] uses bindings between IPv4
   and IPv6 addresses that may be either static or dynamic.  [RFC6146]
   describes a process where the dynamic binding is created by an
   outgoing packet, but it may also be created by other means such as a
   Port Control Protocol request (see Section 2.3.3).  Stepping outside
   the boundaries of NAT64 for the moment, DS-Lite [RFC6333] refers to
   the cautions in [RFC6269] but does not specify any port allocation
   method.  Both technologies assume a centralized model.

   The specifications for both transition methods thus allow
   implementations to use the proposals made in Section 3 and Section 4.

2.3.2.  Current Work On Stateless Transition Technologies

   The port allocation solutions that are being specified at the time of
   writing of this document are all variations on the static distributed
   model, to minimize the amount of state that has to be held in the
   network.  The proposals made in Section 3 and Section 4 do not apply
   to the current work in progress because that work has gone in another
   direction.  That work includes:

   o  Light-weight 4over6 (LW4o6 [I_D.ietf-softwire-lw4over6]), which
      requires the CPE to be configured explicitly with the shared IPv4
      address and port set it will use on the WAN side of its NAT44



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      function.  The border router is configured with the same
      information, reducing the state it must hold from per-session to
      per-subscriber amounts.

   o  Mapping of Address and Port with Encapsulation (MAP-E
      [I-D.ietf-softwire-map]) and the experimental specifications
      Mapping of Address and Port with Translation (MAP-T
      [I-D.ietf-softwire-map-t]) and 4rd [I-D.ietf-softwire-4rd],
      already mentioned.  These rely on an algorithmic embedding of WAN-
      side IPv4 address and assigned port set within the IPv6 prefix
      assigned to each CPE.  Both the CPE and the border router must be
      configured with this information.  However, the algorithm is
      designed to aggregate routing information such that the amount of
      state carried by the border router is of a lower order of
      magnitude than even the per-subscriber level.

   MAP-E also supports a 1-1 mapping mode, where the IPv4 and IPv6
   addresses assigned to a CPE are independent.  This can be helpful in
   transition, but, as with LW4o6, raises the amount of state in the
   network back to the per-subscriber level.

   For a packet destined to a host outside the MAP domain from which the
   packet originated: MAP-E and 4rd treat the packet as an IPv4 over
   IPv6 tunnel via the border router.

   MAP-T uses stateless mapping in the sense of Section 2.2.1 by
   embedding the destination IPv4 address within the IPv6 address of the
   packet sent to the border router.

2.3.3.  Port Control Protocol (PCP)

   The Port Control Protocol (PCP, [RFC6887]) can be used to reserve a
   single port or a port set [I-D.ietf-pcp-port-set] for applications.
   It requires that the NAT be collocated with a PCP server function.
   PCP provides an out-of-band signalling mechanism for coordinating
   dynamic allocation of ports between hosts and the border router.

2.4.  Specific Considerations

2.4.1.  Log Volume Optimization

   [RFC6269] has provided a thoughtful analysis on the issues of IP
   sharing.  It points out that IP sharing may impact law enforcement
   since source address information will be lost during the translation.
   Network administrators have to log the mapping status for each
   connection in order to identify a specific user associated with an IP
   address in a particular time slot.  The storage of log information
   may post a challenge to operators, since it requires additional



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   resources and data inspection processes to identify users.  For
   concrete details of what should be logged, see Section 3.1 of
   [I-D.ietf-behave-syslog-nat-logging].  The actual logging may use
   either IPFIX [RFC7011] or Syslog [RFC5424] depending on the
   operator's requirements.

   It is desirable to reduce the volume of the logged information.
   Referring to the classification of port allocation methods given
   above, dynamic assignments can be managed on either a per-session or
   per-customer granularity.  The coarser granularity will lead to lower
   log volume storage.  A test was made by recording the log information
   from 200,000 subscribers in the Chinese network for 60 days.  The
   volume of recorded information reached up to 42.5 terabytes with per-
   session logging in the raw format.  The volume could be reduced to
   10.6 terabytes with gzip format.  Compared with that, it only
   occupied 40.6 gigabytes, three orders of magnitude smaller volume,
   with per-customer logging in the raw format.  With static allocation,
   of course, no logs at all are required.

   On the other hand, the lower logging volumes are associated with
   lower efficiency of port utilization.  A port allocation based on
   per-customer granularity has to retain vacant ports in order to avoid
   traffic overflow.  The efficiency can be evaluated by port
   utilization rate, and will be even lower if the static port
   allocation method is used.  Inactive users may also impact the
   efficiency.

   Table 1 summarizes the test results using Syslog.  The ports were
   pre-allocated to customers regardless of online or offline status.

   +--------------------+--------------+----------------+--------------+
   | Port Allocation    | Log          | Estimated Log  | Port         |
   | Method             | Granularity  | Volume         | Utilization  |
   +--------------------+--------------+----------------+--------------+
   | Dynamic NAPT       | Per-session  | 42.5 terabytes | 100%         |
   | Dynamic port-range | Per-customer | 40.6 Gigabytes | 75%          |
   | Deterministic NAT, | None         | None           | (60% * 75%)  |
   | MAP-T, 4rd         |              |                | = 45%        |
   +--------------------+--------------+----------------+--------------+

       Table 1: Estimated Log Volumes For 200,000 Users Over 60 Days

   Note: 75% is the estimated port utilization ratio per active
   subscriber. 60% is the estimated ratio of active subscribers to the
   total number of subscribers.






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   The data shown in Table 1 roughly demonstrates the tradeoff between
   port utilization and log volume reduction.  Administrators may
   consider the following factors to determine their own solution:

   o  average connectivity per customer per day;

   o  peak connectivity per day;

   o  the number of public IPv4 addresses available to the NAT64;

   o  application demands for specific ports;

   o  processing capabilities of the NAT64;

   o  tolerable log volume.

2.4.2.  Connectivity State Optimization

   It has been observed that port consumption is significantly increased
   once subscribers land on a web page for video on demand, an online
   game, or map services.  In those cases, multiple TCP connections may
   be initiated to optimize the performance of data transmissions for
   video download and message exchange.  Given the video traffic growth
   trend, this likely presents a challenge for network operators who
   need to optimize connectivity states and avoid port depletion.  Those
   optimizations may even affect the method of port-range allocation,
   because a subscriber is only allowed to use a pre-configured port
   resource.

   Two optimizations may be considered:

   o  Reducing the TIME-WAIT state.  The user's behavior normally
      correlates with system performance.  It is rather common that
      users change video channels often.  Investigations have shown that
      60% of videos are watched for less than 20% of their duration.
      The user's access patterns may leave a number of the TIME-WAIT
      states.  Therefore, acceleration of TIME-WAIT state transitions
      could increase the efficiency of port utilization.  [RFC6191]
      defines a mechanism for reducing TIME-WAIT state by proposing TCP
      timestamps and sequence numbers.

      [I-D.penno-behave-rfc4787-5382-5508-bis] recommended applying
      [RFC6191] and PAWS (Protect Against Wrapped Sequence numbers,
      described in [RFC1323]) to NAT.  This may also be a way to improve
      port utilization.

   o  Another possibility is to use Address-Dependent Mapping or Address
      and Port-Dependent Mapping [RFC4787] to increase port utilization.



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      This feature has already been implemented on a vendor-specific
      basis.  However, it should be noted that REQ-7 and REQ-12 in
      [RFC6888] may reduce the incentive to use anything but the
      Address-Independent Mapping behaviour recommended by [RFC4787].

2.4.3.  Port Randomization

   Port randomization is a feature to enhance the defense against
   hijacking of flows.  [RFC6056] specifies that:

      "A NAPT that does not implement port preservation ([RFC4787],
      [RFC5382]) should obfuscate selection of the ephemeral port of a
      packet when it is changed during translation of that packet."

   A NAPT based on per-session alllocation normally follows this
   recommendation.  However, a simple algorithm for port assignment is
   generally desirable for a deterministic NAT even if it increases
   hijack vulnerability.

   See Section 5 for a fuller discussion of port randomization.

3.  Considerations For the Dynamic Assignment of Port-Ranges

3.1.  Motivation

   During the IPv6 transition period, large-scale NAT devices may be
   introduced, e.g. DS-Lite AFTR, NAT64.  When a NAT device needs to set
   up a new connection for a given internal address behind the NAT, it
   needs to create a new mapping entry for the new connection, which
   will contain source IP address, source port or ICMP identifier,
   converted source IP address, converted source port, protocol (TCP/
   UDP), etc.

   For various reasons it is necessary to log these mappings.  Some high
   performance NAT devices may need to create a large amount of new
   sessions per second.  As seen in Section 2.4.1, if the logs are
   generated for each mapping entry, the log traffic could reach tens of
   megabytes per second or more, which would be a problem for log
   generation, transmission and storage.  (The per-session volumes in
   Table 1 amount to 42 bytes per served subscriber per second.  The
   volumes reported in Section 2.4.2 for US users are even higher,
   around 58 bytes per second per subscriber served.)

   [RFC6888], REQ-13, REQ-14, and REQ-15 deal explicitly with port
   allocation schemes and logging.  However, it is recognized that these
   are conflicting requirements, requiring a tradeoff between the
   efficiency with which ports are used and the rate of generation of
   log records.



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   Allocating a range of N ports at once reduces the log volume by a
   factor of N, while also reducing port utilization by a factor which
   varies with the address sharing ratio and other configuration
   parameters.  This provides a clear motivation to use dynamic
   allocation of port-ranges rather than individual ports when it is
   possible to do so while maintaining a satisfactory level of port
   utilization (and by implication, shared global IPv4 address
   utilization).

   Dynamic allocation of port ranges may be used either as the sole
   strategy for port allocation on the NAPT, or as a supplement to an
   initial static allocation.

3.2.  Implementation Issues -- Port Randomization and Port-Range
      Deallocation

   Here is how dynamic allocation of port-ranges would work in greater
   detail.  When the user sends out the first packet, a port resource
   pool is allocated for the user, e.g., assigning ports 2001~2300 of a
   public IP address to the user's resource pool.  Only one log should
   be generated for this port block.  When the NAT needs to set up a new
   mapping entry for the user, it can use a port in the user's resource
   pool and the corresponding public IP address.  If the user needs more
   port resources, the NAT can allocate another port block, e.g., ports
   3501~3800, to the user's resource pool.  Again, just one log needs to
   be generated for this port block.

   [I-D.bajko-pripaddrassign] takes this idea further by allocating non-
   contiguous sets of ports using a pseudorandom function.  Scattering
   the allocated ports in this way provides a modest barrier to port
   guessing attacks.  The use of randomization is discussed further in
   Section 5.

   Suppose now that a given internal address has been assigned more than
   one block of ports.  The individual sessions using ports within a
   port block will start and end at different times.  If no ports in
   some port block are used for some configurable time, the NAT can
   remove the port block from the resource pool allocated to a given
   internal address, and make it available for other users.  In theory,
   it is unnecessary to log deallocations of blocks of ports, because
   the ports in deallocated blocks will not be used again until the
   blocks are reallocated.  However, the deallocation may be logged when
   it occurs to add robustness to troubleshooting or other procedures.

   The deallocation procedure presents a number of difficulties in
   practice.  The first problem is the choice of timeout value for the
   block.  If idle timers are applied for the individual mappings
   (sessions) within the block, and these conform to the recommendations



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   for NAT behaviour for the protocol concerned, then the additional
   time that might be configured as a guard for the block as a whole
   need not be more than a few minutes.  The block timer in this case
   serves only as a slightly more conservative extension of the
   individual session idle timers.  If, instead, a single idle timer is
   used for the whole block, it must itself conform to the
   recommendations for the protocol with which that block of ports is
   associated.  For example, REQ-5 of [RFC5382] requires an idle timer
   expiry duration of at least 2 hours and 4 minutes for TCP.  The
   suggestions made in Section 2.4.2 may be considered for reducing this
   time.

   The next issue with port block deallocation is the conflict between
   the desire to randomize port allocation and the desire to make unused
   resources available to other internal addresses.  As mentioned above,
   ideally port selection will take place over the entire set of blocks
   allocated to the internal address.  However, taken to its fullest
   extent, such a policy will minimize the probability that all ports in
   any given block are idle long enough for it to be released.

   As an alternative, it is suggested that when choosing which block to
   select a port from, the NAT should omit from its range of choice the
   block that has been idle the longest, unless no ports are available
   in any of the other blocks.  The expression "block that has been idle
   the longest" designates the block in which the time since the last
   packet was observed in any of its sessions, in either direction, is
   earlier than the corresponding time in any of the other blocks
   assigned to that internal address.  As [RFC6269] points out, port
   randomization is just one security measure of several, and the loss
   of randomness incurred by the suggested procedure is justified by the
   increased utilization of port resources it allows.

3.3.  Issues Of Traceability

   Section 11 of [RFC6269] provides a good discussion of the
   traceability issue.  Complete traceability given the NAT logging
   practices proposed in this draft requires that the remote destination
   record the source port of a request along with the source address
   (and presumably protocol, if not implicit).  In addition, the logs at
   each end must be timestamped, and the clocks must be synchronized
   within a certain degree of accuracy.  Here is one reason for the
   guard timing on block release, to increase the tolerable level of
   clock skew between the two ends.

   The ability to configure various server applications to record source
   ports has been investigated, with the following results:





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   o  Source port recording can be configured in Apache, Postfix,
      sendmail and sshd.  Please refer to the appendix for a
      configuration guide.

   o  Source port recording is not supported by IIS, Cyrus IMAP and UW
      IMAP.  But it should not be too difficult to get Cyrus IMAP and UW
      IMAP to support it by modifying the source code.

   Where source port logging can be enabled, this memo strongly urges
   the operators to do so.  Similarly, intrusion detection systems
   should capture source port as well as source address of suspect
   packets.

   In some cases [RFC6269], a server may not record the source port of a
   connection.  To allow traceability, the NAT device needs to record
   the destination IP address of a connection.  As [RFC6269] points out,
   this will provide an incomplete solution to the issue of traceability
   because multiple users of the same shared public IP address may
   access the service at the same time.  From the point of view of this
   draft, in such situations the game is lost, so to speak, and port
   allocation at the NAT might as well be completely dynamic.

   The final possibility to consider is where the NAT does not do per-
   session logging even given the possibility that the remote end is
   failing to capture source ports.  In that case, the port allocation
   strategy proposed in this section can be used.  The impact on
   traceability is that analysis of the logs would yield only the list
   of all internal addresses mapped to a given public address during the
   period of time concerned.  This has an impact on privacy as well as
   traceability, depending on the follow-up actions taken.

3.4.  Other Considerations

   [RFC6269] notes several issues introduced by the use of dynamic as
   opposed to static port assignment.  For example, Section 12.2 of that
   document notes the effect on authentication procedures.  These issues
   must be resolved, but are not specific to the dynamic port-range
   allocation strategy.

4.  Deterministic Port Allocation

4.1.  Motivation

   CGN connection logging satisfies the need to identify attackers and
   respond to abuse/public safety requests, but it imposes significant
   operational challenges to operators.  In lab testing, CGN log
   messages were observed to be approximately 150 bytes long for NAT444
   [I-D.shirasaki-nat444], and 175 bytes for DS-Lite [RFC6333]



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   (individual log messages vary somewhat in size).  Although the
   authors are not aware of definitive studies of connection rates per
   subscriber, reports from several operators in the US set the average
   number of connections per household at approximately 33,000
   connections per day.  If each connection is individually logged, this
   translates to a data volume of approximately 5 MB per subscriber per
   day, or about 150 MB per subscriber per month; however, specific data
   volumes may vary across different operators based on myriad factors.
   Based on available data, a 1-million subscriber service provider will
   generate approximately 150 terabytes of log data per month, or 1.8
   petabytes per year.

   The volume of log data poses a problem for both operators and the
   public safety community.  On the operator side, it requires a
   significant infrastructure investment by operators implementing CGN.
   It also requires updated operational practices to maintain the
   logging infrastructure, and requires approximately 23 Mbps of
   bandwidth between the CGN devices and the logging infrastructure per
   50,000 users.  On the public safety side, it increases the time
   required for an operator to search the logs in response to an abuse
   report, and could delay investigations.  Accordingly, an
   international group of operators and public safety officials
   approached some of the authors and contributors to this document to
   identify a way to reduce this impact while improving abuse response.

   As noted in Section 3.1, the volume of CGN logging can be reduced by
   assigning port ranges instead of individual ports.  Using this
   method, only the assignment of a new port range is logged.  This may
   massively reduce logging volume.  The log reduction may vary
   depending on the length of the assigned port range, whether the port
   range is static or dynamic, etc.  This has been acknowledged in
   [RFC6269] and Section 5.6.10 of [I-D.ietf-behave-ipfix-nat-logging].
   Per [RFC6269]:

      "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 port groups, the ratio of
      public addresses to subscribers, whether or not these groups
      timeout, and the impact on logging requirements and port
      randomization security ([RFC6056])."

   However, the existing solution still poses an impact on operators and
   public safety officials for logging and searching.  Instead, CGNs
   could be designed and/or configured to deterministically map internal
   addresses to {external address + port range} in such a way as to be
   able to algorithmically calculate the mapping.  Only inputs and



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   configuration of the algorithm need to be logged.  This approach
   reduces both logging volume and subscriber identification times.  In
   some cases, when full deterministic allocation is used, this approach
   can eliminate the need for translation logging.

   This section describes a method for such CGN address mapping,
   combined with block port reservations, that significantly reduces the
   burden on operators while offering the ability to map a subscriber's
   inside IP address with an outside address and external port number
   observed on the Internet.

   The activation of the proposed port range allocation scheme is
   compliant with BEHAVE requirements such as the support of APP.  [What
   is APP?  Reference for the complied-with requirements?  Or can this
   para be removed?]

4.2.  Deterministic Port Ranges

   While a subscriber uses thousands of connections per day, most
   subscribers use far fewer resources at any given time.

   Appendix B of [RFC6269] introduces the term "address space
   multiplicative factor" to denote the number of subscribers sharing
   the same public IPv4 address, and goes on to qualify how this value
   should be calculated.  When the address space multiplicative factor
   is low (e.g., the ratio of the number of subscribers to the number of
   public IPv4 addresses allocated to a CGN is closer to 10:1 than
   1000:1), each subscriber could have access to thousands of TCP/UDP
   ports at any given time.  Thus, as an alternative to logging each
   connection, CGNs can deterministically map customer private addresses
   (received on the customer-facing interface of the CGN, a.k.a.,
   internal side) to public addresses extended with port ranges (used on
   the Internet-facing interface of the CGN, a.k.a., external side).

   The mapping algorithm allows an operator to identify a subscriber
   internal IP address when provided the public side IP and port number
   without having to examine the CGN translation logs, and avoids having
   to transport and store massive amounts of session data from the CGN
   and then process it to identify a subscriber.  It can be classified
   as a static centralized port allocation strategy.

   The algorithmic mapping can be expressed as:

      (External IP Address, Port Range) = function 1 (Internal IP
      Address)

      Internal IP Address = function 2 (External IP Address, Port
      Number)



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   Deterministic Port Range allocation requires configuration of the
   following variables:

   o  The set of inside IPv4/IPv6 addresses <I>;

   o  the set of outside IPv4 addresses <O>;

   o  the address space multiplicative factor (F), i.e., ratio of number
      of inside IP addresses to outside IP addresses;

   o  dynamic address pool factor (D), to be added to the compression
      ratio in order to create an overflow address pool;

   o  maximum ports per user (M);

   o  address assignment algorithm (A) (see below); and

   o  number of reserved TCP/UDP ports (R).

   Note: The inside address set <I> will consist of IPv4 addresses in
   NAT444 operation (NAT444 [I-D.shirasaki-nat444]) and of IPv6
   addresses in DS-Lite [RFC6333] operation.

   A subscriber may be identified by an internal IPv4 address (e.g.,
   NAT44) or an IPv6 prefix (e.g., DS-Lite or NAT64).  For a fuller
   discussion of subscriber identification, see Section 2.4 of
   [I-D.ietf-behave-syslog-nat-logging].

   The algorithm is not designed to retrieve an internal host among
   those sharing the same internal IP address (e.g., in a DS-Lite
   context, only an IPv6 address/prefix can be retrieved using the
   algorithm while the internal IPv4 address used for the encapsulated
   IPv4 datagram is lost).

   Several address assignment algorithms are possible.  Using predefined
   algorithms, such as those that follow, simplifies the process of
   reversing the algorithm when needed.  However, the CGN may support
   additional algorithms, and may not support all algorithms described
   below.  Subscribers could be restricted to ports from a single IPv4
   address, or could be allocated ports across all addresses in a pool,
   for example.  The following algorithms and corresponding values of A
   are suggested as a starting set:

   A = 0:  Sequential (e.g. the first block goes to address 1, the
      second block to address 2, etc.)






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   A = 1:  Staggered (e.g. for every n between 0 and ((65536-R)/(F+D))-1
      , address 1 receives ports n*F+R, address 2 receives ports
      (1+n)*F+R, etc.)

   A = 2:  Round robin (e.g. the subscriber receives the same port
      number across a pool of external IP addresses.  If the subscriber
      is to be assigned more ports than there are in the external IP
      pool, the subscriber receives the next highest port across the IP
      pool, and so on.  Thus, if there are 10 IP addresses in a pool and
      a subscriber is assigned 1000 ports, the subscriber would receive
      a range such as ports 2000-2099 across all 10 external IP
      addresses).

   A = 3:  Interlaced horizontally (e.g. each address receives every Cth
      port spread across a pool of external IP addresses).

   A = 4:  Cryptographically random port assignment (Section 2.2 of
      [RFC6431]).  If this algorithm is used, the Service Provider needs
      to retain the keying material and specific cryptographic function
      to support reversibility.

   The assigned range of ports can also be used when translating ICMP
   requests (when re-writing the Identifier field).

   The CGN then reserves ports as follows:

   1.  The CGN removes reserved ports (R) from the port candidate list
       (e.g., 0-1023 for TCP and UDP).  At a minimum, it is likely that
       the operator will prefer the CGN to remove system ports [RFC6335]
       from the port candidate list reserved for deterministic
       assignment.

   2.  The CGN calculates the total address space multiplicative factor
       (F+D), and allocates 1/(F+D) of the available ports to each
       internal IP address.  Specific port allocation is determined by
       the algorithm (A) configured on the CGN.  Any remaining ports are
       allocated to the dynamic pool.

       Note: Setting D to 0 disables the dynamic pool.  This option
       eliminates the need for per-subscriber logging at the expense of
       limiting the number of concurrent connections that 'power users'
       can initiate.

   3.  When a subscriber initiates a connection, the CGN creates a
       translation mapping between the subscriber's inside local IP
       address/port and the CGN outside global IP address/port.  The CGN
       uses one of the ports allocated in step 2 for the translation as
       long as such ports are available.  The CGN allocates ports



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       randomly within the port range assigned by the deterministic
       algorithm.  This is to increase subscriber privacy.  The CGN must
       also use the preallocated port range from step 2 for Port Control
       Protocol (PCP, [RFC6887]) reservations as long as such ports are
       available.  While the CGN maintains its mapping table, it need
       not generate a log entry for translation mappings created in this
       step.

   4.  If D>0, the CGN will have a pool of ports left for dynamic
       assignment.  If a subscriber uses more than the range of ports
       allocated in step 2 (but fewer than the configured maximum ports
       M), the CGN assigns a block of ports from the dynamic assignment
       range for such a connection or for PCP reservations.  The CGN
       logs dynamically assigned port blocks to facilitate subscriber-
       to-address mapping.  The CGN should manage dynamic ports as
       described in Section 3.

   5.  Configuration of reserved ports (e.g., system ports) is left to
       the operator.

   Thus, the CGN will maintain translation mapping information for all
   connections within its internal translation tables; however, it only
   needs to externally log translations for dynamically-assigned ports.

4.2.1.  IPv4 Port Utilization Efficiency

   For Service Providers requiring an aggressive address space
   multiplicative factor, the use of the algorithmic mapping may impact
   the efficiency of the address sharing.  A dynamic port range
   allocation assignment is more suitable in those cases.

4.2.2.  Planning and Dimensioning

   Unlike dynamic approaches, the use of the algorithmic mapping
   requires more effort from operational teams to tweak the algorithm
   (e.g., size of the port range, address space multiplicative factor,
   etc.).  Operators should configure dedicated alarms triggered by port
   utilization threshold crossings so that the configuration can be
   refined.

   The use of algorithmic mapping also affects geolocation.  Changes to
   the inside and outside address ranges (e.g. due to growth, address
   allocation planning, etc.) will require external geolocation
   providers to recalibrate their mappings.







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4.2.3.  Deterministic CGN Example

   To illustrate the use of deterministic NAT, let us consider a simple
   example.  The operator configures an inside address range (I) of
   100.64.0.0/28 [RFC6598] and outside address (O) of 203.0.113.1.  The
   dynamic address pool factor (D) is set to '2'.  Thus, the total
   compression ratio is 1:(14+2) = 1:16.  Only the system ports (e.g.
   ports < 1024) are reserved (R).  This configuration causes the CGN to
   preallocate (65536-1024)/16 = 4032 TCP and 4032 UDP ports per inside
   IPv4 address.  For the purposes of this example, let's assume that
   they are allocated sequentially, where 100.64.0.1 maps to 203.0.113.1
   ports 1024-5055, 100.64.0.2 maps to 203.0.113.1 ports 5056-9087, etc.
   The dynamic port range thus contains ports 57472-65535 (port
   allocation illustrated in Table 2).  Finally, the maximum ports/
   subscriber is set to 5040.

            +-----------------------+-------------------------+
            | Inside Address / Pool | Outside Address & Port  |
            +-----------------------+-------------------------+
            | Reserved              | 203.0.113.1:0-1023      |
            | 100.64.0.1            | 203.0.113.1:1024-5055   |
            | 100.64.0.2            | 203.0.113.1:5056-9087   |
            | 100.64.0.3            | 203.0.113.1:9088-13119  |
            | 100.64.0.4            | 203.0.113.1:13120-17151 |
            | 100.64.0.5            | 203.0.113.1:17152-21183 |
            | 100.64.0.6            | 203.0.113.1:21184-25215 |
            | 100.64.0.7            | 203.0.113.1:25216-29247 |
            | 100.64.0.8            | 203.0.113.1:29248-33279 |
            | 100.64.0.9            | 203.0.113.1:33280-37311 |
            | 100.64.0.10           | 203.0.113.1:37312-41343 |
            | 100.64.0.11           | 203.0.113.1:41344-45375 |
            | 100.64.0.12           | 203.0.113.1:45376-49407 |
            | 100.64.0.13           | 203.0.113.1:49408-53439 |
            | 100.64.0.14           | 203.0.113.1:53440-57471 |
            | Dynamic               | 203.0.113.1:57472-65535 |
            +-----------------------+-------------------------+

          Table 2: Port Allocation For Deterministic NAT Example

   When subscriber 1 using 100.64.0.1 initiates a low volume of
   connections (e.g. < 4032 concurrent connections), the CGN maps the
   outgoing source address/port to the preallocated range.  These
   translation mappings are not logged.

   Subscriber 2 concurrently uses more than the allocated 4032 ports
   (e.g. for peer-to-peer, mapping, video streaming, or other
   connection- intensive traffic types), the CGN allocates up to an
   additional 1008 ports using bulk port reservations.  In this example,



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   subscriber 2 uses outside ports 5056-9087, and then 100-port blocks
   between 58000- 58999.  Connections using ports 5056-9087 are not
   logged, while 10 log entries are created for ports 58000-58099,
   58100-58199, 58200-58299, ..., 58900-58999.

   In order to identify a subscriber behind a CGN (regardless of port
   allocation method), public safety agencies need to collect source
   address and port information from content provider log files.  Thus,
   content providers are advised to log source address, source port, and
   timestamp for all log entries, per [RFC6302].  If a public safety
   agency collects such information from a content provider and reports
   abuse from 203.0.113.1, port 2001, the operator can reverse the
   mapping algorithm to determine that the internal IP address
   subscriber 1 has been assigned generated the traffic without
   consulting CGN logs (by correlating the internal IP address with DHCP
   /PPP lease connection records).  If a second abuse report comes in
   for 203.0.113.1, port 58204, the operator will determine that port
   correlate with connection records, and determine that subscriber 2
   generated the traffic (assuming that the public safety timestamp
   matches the operator timestamp.  As noted in [RFC6269], accurate
   time-keeping (e.g., use of NTP or Simple NTP) is vital).

   In this example, there are no log entries for the majority of
   subscribers, who only use pre-allocated ports.  Only minimal logging
   would be needed for those few subscribers who exceed their pre-
   allocated ports and obtain extra bulk port assignments from the
   dynamic pool.  Logging data for those users will include inside
   address, outside address, outside port range, and timestamp.  See
   [I-D.ietf-behave-syslog-nat-logging] Section 3.1.4 for a detailed
   specification of the information required.

4.2.4.  Additional Management Considerations

   The CGN should provide a method for administrators to test the
   mapping function in both directions, i.e., enter an External IP
   Address + Port Number and receive the corresponding Internal IP
   Address and vice versa.

   In order to be able to identify a subscriber based on observed
   external IPv4 address, port, and timestamp, an operator needs to know
   how the CGN was configured with regards to internal and external IP
   addresses, dynamic address pool factor, maximum ports per user, and
   reserved port range at any given time.  Therefore, the operator needs
   to keep a record of the current configuration and changes to it.  The
   record itself may be generated by the CGN, or may be retrieved from a
   router configuration management system.  For auditing purposes, such
   records should be generated on a daily basis and checked for
   unauthorized or unintended changes.



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4.3.  Failover Considerations

   Due to the deterministic nature of algorithmically-assigned
   translations, no additional logging is required during failover
   conditions provided that inside address ranges are unique within a
   given failover domain.  Even when directed to a different CGN server,
   translations within the deterministic port range on either the
   primary or secondary server can be algorithmically reversed, provided
   the algorithm is known.  Thus, if 100.64.0.1 port 3456 maps to
   203.0.113.1 port 1000 on CGN 1 and 198.51.100.1 port 1000 on Failover
   CGN 2, an operator can identify the subscriber based on outside
   source address and port information.

   Similarly, assignments made from the dynamic overflow pool need to be
   logged as described above, whether translations are performed on the
   primary or failover CGN.

4.4.  Impact On IPv6 Transition

   The solution described in this section is applicable to Carrier Grade
   NAT transition technologies (e.g. NAT444, DS-Lite, and NAT64).
   Native IPv6 will offer subscribers a better experience than CGN.
   However, many CPE devices only support IPv4.  Likewise, as of July
   2012, only approximately 4% of the top 1 million websites were
   available using IPv6.  Accordingly, deterministic CGN should in no
   way be understood as making CGN a replacement for IPv6 service.  The
   authors encourage [RFC6540] device manufacturers to consider and
   include IPv6 support.  In the interim, however, CGN has already been
   deployed in some operator networks.  Deterministic CGN will provide
   operators with the ability to quickly respond to public safety
   requests without requiring excessive infrastructure, operations, and
   bandwidth to support per-connection logging.

5.  Security Considerations

   The discussion which follows addresses an issue that is particularly
   relevant to the strategies described in Section 3 and Section 4 of
   this document.  The security considerations applicable to NAT
   operation for various protocols as documented in, for example,
   [RFC4787] and [RFC5382] also apply to this proposal.

   [RFC6056] summarizes the TCP port-guessing attack, by means of which
   an attacker can hijack one end of a TCP connection.  One mitigating
   measure is to make the source port number used for a TCP connection
   less predictable.  [RFC6056] provides various algorithms for this
   purpose.





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   As Section 3.1 of that RFC notes: "...provided adequate algorithms
   are in use, the larger the range from which ephemeral ports are
   selected, the smaller the chances of an attacker are to guess the
   selected port number."  Conversely, the reduced range sizes proposed
   by the present document increase the attacker's chances of guessing
   correctly.  This result cannot be totally avoided.  However,
   mitigating measures to improve this situation can be taken both at
   port block assignment time and when selecting individual ports from
   the blocks that have been allocated to a given user.

   At assignment time, one possibility is to assign ports as non-
   contiguous sets of values as proposed in [I-D.bajko-pripaddrassign].
   However, this approach creates a lot of complexity for operations,
   and the pseudo randomization can create uncertainty when the accuracy
   of logs is important to protect someone's life or liberty.

   Alternatively, the NAT can assign blocks of contiguous ports.
   However, at assignment time the NAT could attempt to randomize its
   choice of which of the available idle blocks it would assign to a
   given user.  This strategy has to be traded off against the
   desirability of minimizing the chance of conflict between what
   [RFC6056] calls "transport protocol instances" by assigning the most-
   idle block, as suggested in Section 3.  A compromise policy might be
   to assign blocks only if they have been idle for a certain amount of
   time whenever possible, and select pseudorandomly between the blocks
   available according to this criterion.  In this case it is suggested
   that the time value used be greater than the guard timing mentioned
   in Section 3, and that no block should ever be reassigned until it
   has been idle at least for the duration given by the guard timer.

   Note that with the possible exception of cryptographically-based port
   allocations, attackers could reverse-engineer algorithmically-derived
   port allocations to either target a specific subscriber or to spoof
   traffic to make it appear to have been generated by a specific
   subscriber.  However, this is exactly the same level of security that
   the subscriber would experience in the absence of CGN.  CGN is not
   intended to provide additional security by obscurity.

   While the block assignment strategy can provide some mitigation of
   the port guessing attack, the largest contribution will come from
   pseudo-randomization at port selection time.  [RFC6056] provides a
   number of algoriths for achieving this pseudo-randomization.  When
   the available ports are contained in blocks which are not in general
   consecutive, the algorithms clearly need some adaptation.  The task
   is complicated by the fact that the number of blocks allocated to the
   user may vary over time.  Adaptation is left as an exercise for the
   implementor.




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6.  IANA Considerations

   This document makes no request of IANA.

7.  Contributors

   This document is the result of merging three separate Internet
   Drafts: the original Chen document (version -03), draft-tsou-behave-
   natx4-log-reduction-04, and draft-donley-behave-deterministic-cgn-07.
   Aside from the authors listed on the front of the present document,
   the following co-authors of the other two original drafts deserve
   credit for their contributions:

   o  Weibo Li (China Telecom) and James Huang (Huawei) for their work
      on draft-tsou-behave-natx4-log-reduction, and

   o  Chris Grundemann (Internet Society), Vikas Sarawat and Karthik
      Sundaresan (CableLabs), and Olivier Vautrin (Juniper) for their
      work on draft-donley-behave-deterministic-cgn.

8.  Acknowledgements

   The authors of draft-donley-behave-deterministic-cgn would like to
   thank the following people for their suggestions and feedback: Bobby
   Flaim, Lee Howard, Wes George, Jean-Francois Tremblay, Mohammed
   Boucadair, Alain Durand, David Miles, Andy Anchev, Victor Kuarsingh,
   Miguel Cros Cecilia, and Reinaldo Penno.

   The authors of draft-tsou-behave-natx4-log-reduction have their own
   thanks to give.  Mohamed Boucadair reviewed the initial document and
   provided useful comments to improve it.  Reinaldo Penno, Joel
   Jaeggli, and Dan Wing provided comments on the subsequent version
   that resulted in major revisions.  Serafim Petsis provided
   encouragement to publication after a hiatus of two years.

9.  References

9.1.  Normative References

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056, January
              2011.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.






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   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

   [RFC6269]  Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
              Roberts, "Issues with IP Address Sharing", RFC 6269, June
              2011.

   [RFC6888]  Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
              and H. Ashida, "Common Requirements for Carrier-Grade NATs
              (CGNs)", BCP 127, RFC 6888, April 2013.

9.2.  Informative References

   [APACHE_LOG_CONFIG]
              The Apache Software Foundation, "http://httpd.apache.org/
              docs/2.4/mod/mod_log_config.html", 2013.

   [I-D.anderson-siit-dc]
              Anderson, T., "Stateless IP/ICMP Translation in IPv6 Data
              Centre Environments (expired work in progress)", November
              2012.

   [I-D.bajko-pripaddrassign]
              Bajko, G., Savolainen, T., Boucadair, M., and P. Levis,
              "Port Restricted IP Address Assignment (expired Work in
              Progress)", March 2012.

   [I-D.ietf-behave-ipfix-nat-logging]
              Sivakumar, S. and R. Penno, "IPFIX Information Elements
              for Logging NAT Events (Work in Progress)", February 2014.

   [I-D.ietf-behave-syslog-nat-logging]
              Chen, Z., Zhou, C., Tsou, T., and T. Taylor, "Syslog
              Format for NAT Logging (Work in Progress)", January 2014.

   [I-D.ietf-pcp-port-set]
              Sun, Q., Boucadair, M., Sivakumar, S., Zhou, C., Tsou, T.,
              and S. Perrault, "Port Control Protocol (PCP) Extension
              for Port Set Allocation (Work in Progress)", November
              2013.

   [I-D.ietf-softwire-4rd]
              Despres, R., Jiang, S., Penno, R., Lee, Y., Chen, G., and
              M. Chen, "IPv4 Residual Deployment via IPv6 - a Stateless
              Solution (4rd) (Work in Progress)", October 2013.





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   [I-D.ietf-softwire-map-dhcp]
              Mrugalski, T., Troan, O., Dec, W., Farrer, I., Perrault,
              S., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
              configuration of Softwire Address and Port Mapped Clients
              (Work in Progress)", March 2014.

   [I-D.ietf-softwire-map-t]
              Li, X., Bao, C., Dec, W., Troan, O., Matsushima, S., and
              T. Murakami, "Mapping of Address and Port using
              Translation (MAP-T) (Work in progress)", February 2014.

   [I-D.ietf-softwire-map]
              Troan, O., Dec, W., Li, X., Bao, C., Matsushima, S.,
              Murakami, T., and T. Taylor, "Mapping of Address and Port
              with Encapsulation (MAP) (Work in Progress)", January
              2014.

   [I-D.ietf-softwire-stateless-4v6-motivation]
              Boucadair, M., Matsushima, S., Lee, Y., Bonness, O.,
              Borges, I., and G. Chen, "Motivations for Carrier-side
              Stateless IPv4 over IPv6 Migration Solutions (Work in
              Progress)", November 2012.

   [I-D.penno-behave-rfc4787-5382-5508-bis]
              Penno, R., Perrault, S., Kamiset, S., Boucadair, M., and
              K. Naito, "Network Address Translation (NAT) Behavioral
              Requirements Updates (expired Work in Progress)", January
              2013.

   [I-D.shirasaki-nat444]
              Yamagata, I., Shirasaki, Y., Nakagawa, A., Yamaguchi, J.,
              and H. Ashida, "NAT444 (expired Work in Progress)", July
              2012.

   [I_D.ietf-softwire-lw4over6]
              Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
              Farrer, "Lightweight 4over6: An Extension to the DS-Lite
              Architecture (Work in Progress)", March 2014.

   [POSTFIX_LOG_CONFIG]
              "http://www.postfix.org/postconf.5.html", 2013.

   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022, January
              2001.



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

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

   [RFC5424]  Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.

   [RFC6191]  Gont, F., "Reducing the TIME-WAIT State Using TCP
              Timestamps", BCP 159, RFC 6191, April 2011.

   [RFC6269.e46ua]
              Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
              Roberts, "Issues with IP Address Sharing", RFC 6269, June
              2011.

   [RFC6302]  Durand, A., Gashinsky, I., Lee, D., and S. Sheppard,
              "Logging Recommendations for Internet-Facing Servers", BCP
              162, RFC 6302, June 2011.

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, August 2011.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165, RFC
              6335, August 2011.

   [RFC6431]  Boucadair, M., Levis, P., Bajko, G., Savolainen, T., and
              T. Tsou, "Huawei Port Range Configuration Options for PPP
              IP Control Protocol (IPCP)", RFC 6431, November 2011.

   [RFC6540]  George, W., Donley, C., Liljenstolpe, C., and L. Howard,
              "IPv6 Support Required for All IP-Capable Nodes", BCP 177,
              RFC 6540, April 2012.

   [RFC6598]  Weil, J., Kuarsingh, V., Donley, C., Liljenstolpe, C., and
              M. Azinger, "IANA-Reserved IPv4 Prefix for Shared Address
              Space", BCP 153, RFC 6598, April 2012.

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation", RFC
              6877, April 2013.




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   [RFC6887]  Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
              Selkirk, "Port Control Protocol (PCP)", RFC 6887, April
              2013.

   [RFC7011]  Claise, B., Trammell, B., and P. Aitken, "Specification of
              the IP Flow Information Export (IPFIX) Protocol for the
              Exchange of Flow Information", STD 77, RFC 7011, September
              2013.

   [SENDMAIL_LOG_CONFIG]
              O'Reilly, "Sendmail, 3rd Edition, Page 798", December
              2002.

   [SSHD_LOG_CONFIG]
              "http://www.openbsd.org/cgi- bin/
              man.cgi?query=sshd_config&sektion=5", April 2013.

Appendix A.  Configuration of Server Software to Log Source Port

A.1.  Apache

   The user can use LogFormat command to define a customized log format
   and use CustomLog command to apply that log format. "%a" and
   "%{remote}p" can be used in the format string to require logging the
   client's IP address and source port respectively.  This feature is
   available since Apache version 2.1.

   A detailed configuration guide can be found at [APACHE_LOG_CONFIG].

A.2.  Postfix

   In order to log the client source port, macro
   smtpd_client_port_logging should be set to "yes" in the configuration
   file.  See [POSTFIX_LOG_CONFIG].

   This feature has been available since Postfix version 2.5.

A.3.  Sendmail

   Sendmail has a macro ${client_port} storing the client port.  To log
   the source port, the user can define some check rules.  Here is an
   example which should be in the .mc configuration macro
   [SENDMAIL_LOG_CONFIG]:








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   LOCAL_CONFIG
   Klog syslog

   LOCAL_RULESETS
   SLocal_check_mail
   R $* $@ $(log Port_Stat $&{client_addr} $&{client_port} $)

   This feature has been available since version 8.10.

A.4.  sshd

   SSHD_CONFIG(5) OpenBSD Programmer's Manual SSHD_CONFIG(5) NAME
   sshd_config - OpenSSH SSH daemon configuration file LogLevel Gives
   the verbosity level that is used when logging messages from sshd(8).
   The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG,
   DEBUG1, DEBUG2, and DEBUG3.  The default is INFO.  DEBUG and DEBUG1
   are equivalent.  DEBUG2 and DEBUG3 each specify higher levels of
   debugging output.  Logging with a DEBUG level violates the privacy of
   users and is not recommended.  SyslogFacility Gives the facility code
   that is used when logging messages from sshd(8).  The possible values
   are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4,
   LOCAL5, LOCAL6, LOCAL7.  The default is AUTH.

   sshd supports logging the client IP address and client port when a
   client starts connection since version 1.2.2, here is the source code
   in sshd.c:

   ...
   verbose("Connection from %.500s port %d", remote_ip, remote_port);
   ...

   sshd supports logging the client IP address when a client
   disconnects, from version 1.2.2 to version 5.0.  Since version 5.1
   sshd supports logging the client IP address and source port.  Here is
   the source code in sshd.c:

   ...
   /* from version 1.2.2 to 5.0*/
   verbose("Closing connection to %.100s", remote_ip);
   ...

   /* since version 5.1*/
   verbose("Closing connection to %.500s port %d",
   remote_ip, remote_port);

   In order to log the source port, the LogLevel should be set to
   VERBOSE [SSHD_LOG_CONFIG] in the configuration file:




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   LogLevel VERBOSE

A.5.  Cyrus IMAP and UW IMAP

   Cyrus IMAP and UW IMAP do not support logging the source port for the
   time being.  Both software packages use syslog to create logs; it
   should not be too difficult to get source port logging supported by
   adding some new code.

Authors' Addresses

   Gang Chen
   China Mobile
   53A,Xibianmennei Ave.,
   Xuanwu District,
   Beijing  100053
   China

   Email: phdgang@gmail.com


   Tina Tsou
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China

   Email: tina.tsou.zouting@huawei.com


   Chris Donley
   CableLabs
   858 Coal Creek Cir
   Louisville, CO  80027
   USA

   Email: c.donley@cablelabs.com


   Tom Taylor
   PT Taylor Consulting
   Ottawa, Ontario
   Canada

   Email: tom.taylor.stds@gmail.com






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