Network Working Group                                            G. Chen
Internet-Draft                                              China Mobile
Intended status: Informational                                     W. Li
Expires: April 2, 2015                                     China Telecom
                                                                 T. Tsou
                                                                J. Huang
                                                     Huawei Technologies
                                                               T. Taylor
                                                    PT Taylor Consulting
                                                      September 29, 2014

  Analysis of NAT64 Port Allocation Methods for Shared IPv4 Addresses


   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
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   Drafts is at

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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 2, 2015.

Copyright Notice

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

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Considerations For the Choice of Port Allocation Methods  . .   3
     2.1.  Port Consumption on NAT64 . . . . . . . . . . . . . . . .   3
     2.2.  Classification of Port Allocation Models  . . . . . . . .   4
       2.2.1.  Stateful vs. Stateless  . . . . . . . . . . . . . . .   4
       2.2.2.  Dynamic vs. Static  . . . . . . . . . . . . . . . . .   5
       2.2.3.  Centralized vs. Distributed . . . . . . . . . . . . .   6
     2.3.  Port Allocation Solutions . . . . . . . . . . . . . . . .   6
       2.3.1.  Other 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 . . . . . . . . . . .   9
       2.4.3.  Port Randomization  . . . . . . . . . . . . . . . . .  10
   3.  Considerations For the Dynamic Assignment of Port-Ranges  . .  11
     3.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Implementation Issues -- Port Randomization and Port-
           Range Deallocation  . . . . . . . . . . . . . . . . . . .  11
     3.3.  Issues Of Traceability  . . . . . . . . . . . . . . . . .  13
     3.4.  Other Considerations  . . . . . . . . . . . . . . . . . .  14
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   As a resdult of 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 connectivity to

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   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.  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.

   The considerations in this document do not apply where the CGN does
   only Network Address Translation (NAT) [RFC3022].  In this scenario,
   there is no concern about port assignment.  Similarly, this document
   does not apply where encapsulation rather than translation is used as
   the IPv6 transition method.

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;

   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

   China Mobile 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.

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   China Mobile 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.  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.

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

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-v6ops-siit-dc].  NAT usage in

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   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
      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.  The China Mobile study mentioned earlier
          observed that the average number of sessions consumed by one
          user's device was around 200 to 300 ports.  Several devices
          may appear behind a CPE.  Based on this observation, 1000
          ports per subscriber household will provide enough room for
          multiple active users.  Administrators should monitor usage to
          adjust this number if users are being limited by this number,
          or if usage is so low that fewer ports would be sufficient.

      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

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      Static assignment makes port reservations in bulk for each
      internal address before subscriber connection.  The assigned ports
      can be in either a contiguous port range or a non-contiguous port
      range for the sake of defense against port-guessing attacks (see
      Section 3.2).  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.
      [I-D.donley-behave-deterministic-cgn] 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).

   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 [RFC6346] 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

2.3.  Port Allocation Solutions

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2.3.1.  Other Transition Technologies

   In other 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).  Lookin beyond
   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

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 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
      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.

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   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
   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

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   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

   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.

   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

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   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

   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.
      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 allocation normally follows this

   See Section 4 for a fuller discussion of port randomization.

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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 the introduction to
   [I-D.donley-behave-deterministic-cgn] for U.S.  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.

   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

   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

   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

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   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 4.

   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
   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

   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.

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   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 12 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) [RFC6302].  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.

   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

   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.

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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.  Security Considerations

   The discussion which follows addresses an issue that is particularly
   relevant to the strategies described in Section 3 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

   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

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   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

5.  IANA Considerations

   This document makes no request of IANA.

6.  Acknowledgements

   This document is the result of a merger of the original draft-chen-
   sunset4-cgn-port-allocation and draft-tsou-behave-natx4-log-
   reduction.  Version -02 of draft-chen contains the following

      The author would like to thank Lee Howard and Simon Perreault for
      their helpful comments.

      Many thanks to Wesley George and Marc Blanchet encourage the
      author to continue this work.

   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.

   The present version of the document benefited from further comments
   by Lee Howard.

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7.  References

7.1.  Normative References

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

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

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

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

   [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.

7.2.  Informative References

              Anderson, T., "SIIT-DC: Stateless IP/ICMP Translation for
              IPv6 Data Centre Environments (Work in progress)",
              September 2014.

              Bajko, G., Savolainen, T., Boucadair, M., and P. Levis,
              "Port Restricted IP Address Assignment (expired Work in
              Progress)", April 2012.

              Donley, C., Grundemann, C., Sarawat, V., Sundaresan, K.,
              and O. Vautrin, "Deterministic Address Mapping to Reduce
              Logging in Carrier Grade NAT Deployments (Work in
              progress)", January 2014.

              Chen, Z., Zhou, C., Tsou, T., and T. Taylor, "Syslog
              Format for NAT Logging (Work in Progress)", January 2014.

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              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)", July 2014.

              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)", April 2014.

              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

              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)", July 2014.

              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.

              Boucadair, M., Matsushima, S., Lee, Y., Bonness, O.,
              Borges, I., and G. Chen, "Motivations for Carrier-side
              Stateless IPv4 over IPv6 Migration Solutions (Expired work
              in Progress)", November 2012.

              Penno, R., Perrault, S., Kamiset, S., Boucadair, M., and
              K. Naito, "Network Address Translation (NAT) Behavioral
              Requirements Updates (expired Work in Progress)", January

              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)", June 2014.

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

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

   [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.

   [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.

   [RFC6346]  Bush, R., "The Address plus Port (A+P) Approach to the
              IPv4 Address Shortage", RFC 6346, August 2011.

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

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

   [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

Authors' Addresses

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


   Weibo Li
   China Telecom
   109, Zhongshan Ave. West, Tianhe District
   Guangzhou  510630
   P.R. China


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


   James Huang
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China


   Tom Taylor
   PT Taylor Consulting
   Ottawa, Ontario


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