Network Working Group                                          C. Donley
Internet-Draft                                             C. Grundemann
Intended status: Informational                                V. Sarawat
Expires: July 16, 2013                                     K. Sundaresan
                                                              O. Vautrin
                                                        Juniper Networks
                                                        January 12, 2013

  Deterministic Address Mapping to Reduce Logging in Carrier Grade NAT


   In some instances, Service Providers have a legal logging requirement
   to be able to map a subscriber's inside address with the address used
   on the public Internet (e.g. for abuse response).  Unfortunately,
   many Carrier Grade NAT logging solutions require active logging of
   dynamic translations.  Carrier Grade NAT port assignments are often
   per-connection, but could optionally use port ranges.  Research
   indicates that per-connection logging is not scalable in many
   residential broadband services.  This document suggests a way to
   manage Carrier Grade NAT translations in such a way as to
   significantly reduce the amount of logging required while providing
   traceability for abuse response.  While the authors acknowledge that
   IPv6 is a preferred solution, Carrier Grade NAT is a reality in many
   networks, and is needed in situations where either customer equipment
   or Internet content only supports IPv4; this approach should in no
   way slow the deployment of IPv6.

Requirements Language

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

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

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   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 July 16, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Deterministic Port Ranges  . . . . . . . . . . . . . . . . . .  5
     2.1.  IPv4 Port Utilization Efficiency . . . . . . . . . . . . .  9
     2.2.  Planning & Dimensioning  . . . . . . . . . . . . . . . . .  9
     2.3.  Deterministic CGN Example  . . . . . . . . . . . . . . . .  9
   3.  Additional Logging Considerations  . . . . . . . . . . . . . . 11
   4.  Impact on the IPv6 Transition  . . . . . . . . . . . . . . . . 11
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

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1.  Introduction

   It is becoming increasingly difficult to obtain new IPv4 address
   assignments from Regional/Local Internet Registries due to depleting
   supplies of unallocated IPv4 address space.  To meet the growing
   demand for Internet connectivity from new subscribers, devices, and
   service types, some operators will be forced to share a single public
   IPv4 address among multiple subscribers using techniques such as
   Carrier Grade Network Address Translation (CGN) [RFC6264] (e.g.,
   NAT444 [I-D.shirasaki-nat444], DS-Lite [RFC6333], NAT64 [RFC6146]
   etc.).  However, address sharing poses additional challenges to
   operators when considering how they manage service entitlement,
   public safety requests, or attack/abuse reports [RFC6269].  In order
   to identify a specific user associated with an IP address in response
   to such a request or for service entitlement, an operator will need
   to map a subscriber's internal source IP address and source port with
   the global public IP address and source port provided by the CGN for
   every connection initiated by the user.

   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, we have
   observed CGN log messages to be approximately 150 bytes long for
   NAT444 [I-D.shirasaki-nat444], and 175 bytes for DS-Lite [RFC6333]
   (individual log messages vary somewhat in size).  Although we are not
   aware of definitive studies of connection rates per subscriber,
   reports from several operators in the US sets 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 the authors to identify a way to reduce this impact while
   improving abuse response.

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   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
   [I-D.sivakumar-behave-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
   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 document 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.

2.  Deterministic Port Ranges

   While a subscriber uses thousands of connections per day, most
   subscribers use far fewer resources at any given time.  When the
   compression ratio (see Appendix B of RFC6269 [RFC6269]) 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 expect to have access to thousands of TCP/UDP ports
   at any given time.  Thus, as an alternative to logging each
   connection, CGNs could deterministically map customer private
   addresses (received on the customer-facing interface of the CGN,

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   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).  This 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.  This prevents an
   operator from having to transport and store massive amounts of
   session data from the CGN and then process it to identify a

   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)

   The CGN SHOULD provide a method for administrators to test both
   mapping functions (e.g., enter an External IP Address + Port Number
   and receive the corresponding Internal IP Address).

   Deterministic Port Range allocation requires configuration of the
   following variables:

   o  Inside IPv4/IPv6 address range (I);

   o  Outside IPv4 address range (O);

   o  Compression ratio (e.g. inside IP addresses I/outside IP addresses
      O) (C);

   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  Reserved TCP/UDP port list (R)

   Note: The inside address range (I) will be an IPv4 range in NAT444
   operation (NAT444 [I-D.shirasaki-nat444]) and an IPv6 range in DS-
   Lite operation (DS-Lite [RFC6333]).

   A subscriber is identified by an internal IPv4 address (e.g., NAT44)
   or an IPv6 prefix (e.g., DS-Lite or NAT64).

   The algorithm may be generalized to L2-aware NAT
   [I-D.miles-behave-l2nat] but this requires the configuration of the
   Internal interface identifiers (e.g., MAC addresses).

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   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.  Also, the CGN is not required to 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 as follow:

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

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

      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

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

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

      5: Vendor-specific.  Other vendor-specific algorithms may also be

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

   The CGN then reserves ports as follows:

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   1.  The CGN removes reserved ports (R) from the port candidate list
       (e.g., 0-1023 for TCP and UDP).  At a minimum, the CGN SHOULD
       remove system ports (RFC6335) [RFC6335] from the port candidate
       list reserved for deterministic assignment.

   2.  The CGN calculates the total compression ratio (C+D), and
       allocates 1/(C+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
       MUST use one of the ports allocated in step 2 for the translation
       as long as such ports are available.  The CGN SHOULD allocate
       ports randomly within the port range assigned by the
       deterministic algorithm.  This is to increase subscriber privacy.
       The CGN MUST use the preallocated port range from step 2 for Port
       Control Protocol (PCP, [I-D.ietf-pcp-base]) 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
       MUST log dynamically assigned port blocks to facilitate
       subscriber-to-address mapping.  The CGN SHOULD manage dynamic
       ports as described in [I-D.tsou-behave-natx4-log-reduction].

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

   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.

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2.1.  IPv4 Port Utilization Efficiency

   For Service Providers requiring an aggressive address sharing ratio,
   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.

2.2.  Planning & 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 sharing ratio, etc.).
   Dedicated alarms SHOULD be configured when some port utilization
   thresholds are fired 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.) would require external geolocation
   providers to recalibrate their mappings.

2.3.  Deterministic CGN Example

   To illustrate the use of deterministic NAT, let's consider a simple
   example.  The operator configures an inside address range (I) of and outside address (O) of  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 maps to
   ports 1024-5055, maps to ports 5056-9087, etc.
   The dynamic port range thus contains ports 57472-65535 (port
   allocation illustrated in the table below).  Finally, the maximum
   ports/subscriber is set to 5040.

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            | Inside Address / Pool | Outside Address & Port  |
            | Reserved              |      |
            |            |   |
            |            |   |
            |            |  |
            |            | |
            |            | |
            |            | |
            |            | |
            |            | |
            |            | |
            |           | |
            |           | |
            |           | |
            |           | |
            |           | |
            | Dynamic               | |

   When subscriber 1 using 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,
   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.

   If a public safety agency reports abuse from, 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, port 58204, the operator will
   determine that port 58204 is within the dynamic pool range, consult
   the log file, 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 RFC6292
   [RFC6292], accurate time-keeping (e.g., use of NTP or Simple NTP) is

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   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.

3.  Additional Logging Considerations

   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 CGN MUST
   generate a record any time such variables are changed.  The CGN
   SHOULD generate a log message any time such variables are changed.
   The CGN MAY keep such a record in the form of a router configuration
   file.  If the CGN does not generate a log message, it would be up to
   the operator to maintain version control of router config changes.
   Also, the CGN SHOULD generate such a log message once per day to
   facilitate quick identification of the relevant configuration in the
   event of an abuse notification.

   Such a log message MUST, at minimum, include the timestamp, inside
   prefix I, inside mask, outside prefix O, outside mask, D, M, A, and
   reserved port list R; for example:

   [Wed Oct 11 14:32:52 2000]:

4.  Impact on the IPv6 Transition

   The solution described in this document is applicable to Carrier
   Grade NAT transition technologies (e.g.  NAT444, DS-Lite, and NAT64).
   As discussed in [I-D.donley-nat444-impacts], the authors acknowledge
   that 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 device manufacturers to consider [RFC6540] 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.

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5.  IANA Considerations

   This document makes no request of IANA.

6.  Security Considerations

   The security considerations applicable to NAT operation for various
   protocols as documented in, for example, RFC 4787 [RFC4787] and RFC
   5382 [RFC5382] also apply to this document.

7.  Acknowledgements

   The authors 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

8.  References

8.1.  Normative References

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

   [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.

   [RFC6264]  Jiang, S., Guo, D., and B. Carpenter, "An Incremental
              Carrier-Grade NAT (CGN) for IPv6 Transition", RFC 6264,
              June 2011.

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

8.2.  Informative References

              Donley, C., Howard, L., Kuarsingh, V., Berg, J., and U.

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              Colorado, "Assessing the Impact of Carrier-Grade NAT on
              Network Applications", draft-donley-nat444-impacts-04
              (work in progress), May 2012.

              Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
              Selkirk, "Port Control Protocol (PCP)",
              draft-ietf-pcp-base-29 (work in progress), November 2012.

              Miles, D. and M. Townsley, "Layer2-Aware NAT",
              draft-miles-behave-l2nat-00 (work in progress),
              March 2009.

              Yamagata, I., Shirasaki, Y., Nakagawa, A., Yamaguchi, J.,
              and H. Ashida, "NAT444", draft-shirasaki-nat444-06 (work
              in progress), July 2012.

              Sivakumar, S. and R. Penno, "IPFIX Information Elements
              for logging NAT Events",
              draft-sivakumar-behave-nat-logging-05 (work in progress),
              July 2012.

              ZOU), T., Li, W., and T. Taylor, "Port Management To
              Reduce Logging In Large-Scale NATs",
              draft-tsou-behave-natx4-log-reduction-02 (work in
              progress), September 2010.

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

   [RFC6292]  Hoffman, P., "Requirements for a Working Group Charter
              Tool", RFC 6292, 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)

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              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.

Authors' Addresses

   Chris Donley
   858 Coal Creek Cir
   Louisville, CO  80027


   Chris Grundemann
   858 Coal Creek Cir
   Louisville, CO  80027


   Vikas Sarawat
   858 Coal Creek Cir
   Louisville, CO  80027


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   Karthik Sundaresan
   858 Coal Creek Cir
   Louisville, CO  80027


   Olivier Vautrin
   Juniper Networks
   1194 N Mathilda Avenue
   Sunnyvale, CA  94089


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