This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.

The following 'Verified' errata have been incorporated in this document: EID 4211
Independent Submission                                         C. Donley
Request for Comments: 7422                                     CableLabs
Category: Informational                                    C. Grundemann
ISSN: 2070-1721                                         Internet Society
                                                              V. Sarawat
                                                           K. Sundaresan
                                                              O. Vautrin
                                                        Juniper Networks
                                                           December 2014

           Deterministic Address Mapping to Reduce Logging in
                     Carrier-Grade NAT Deployments


   In some instances, Service Providers (SPs) 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 logging solutions for Carrier-Grade NATs (CGNs)
   require active logging of dynamic translations.  CGN port assignments
   are often per connection, but they 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 CGN translations in such a way as to significantly reduce the
   amount of logging required while providing traceability for abuse
   response.  IPv6 is, of course, the preferred solution.  While
   deployment is in progress, SPs are forced by business imperatives to
   maintain support for IPv4.  This note addresses the IPv4 part of the
   network when a CGN solution is in use.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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   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
   ( 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.

Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................4
   2. Deterministic Port Ranges .......................................4
      2.1. IPv4 Port Utilization Efficiency ...........................7
      2.2. Planning and Dimensioning ..................................7
      2.3. Deterministic CGN Example ..................................8
   3. Additional Logging Considerations ...............................9
      3.1. Failover Considerations ...................................10
   4. Impact on the IPv6 Transition ..................................10
   5. Privacy Considerations .........................................11
   6. Security Considerations ........................................11
   7. References .....................................................11
      7.1. Normative References ......................................11
      7.2. Informative References ....................................12
   Acknowledgements ..................................................13
   Authors' Addresses ................................................14

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 NAT (CGN) [RFC6264] (e.g., NAT444 [NAT444], Dual-Stack
   Lite (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/fraud 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 [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 SP will generate approximately 150
   terabytes of log data per month, or 1.8 petabytes per year.  Note
   that many SPs compress log data after collection; compression factors
   of 2:1 or 3:1 are common.

   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 it 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.

   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], which recommends the
   logging of source ports at the server and/or destination logging at
   the CGN, and [NAT-LOGGING], which describes information to be logged
   at a NAT.

   However, the existing solutions still pose 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
   Application-specific functions (APP).

1.1.  Requirements Language

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

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 RFC 6269 [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,
   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 [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 [L2NAT], but this
   requires the configuration of the Internal interface identifiers
   (e.g., Media Access Control (MAC) addresses).

   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
   of the 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 follows:

   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 SP needs to retain the
      keying material and specific cryptographic function to support

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

   The assigned range of ports MAY also be used when translating ICMP
   requests (when rewriting 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, the CGN SHOULD
       remove system ports [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 pre-allocated 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
       MUST log dynamically assigned port blocks to facilitate
       subscriber-to-address mapping.  The CGN SHOULD manage dynamic
       ports as described in [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.

2.1.  IPv4 Port Utilization Efficiency

   For SPs 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

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 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 [RFC6598] 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
   pre-allocate ((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.

            | 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 pre-allocated 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.

   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, 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 RFC 6269 [RFC6269], accurate timekeeping 
   (e.g., use of NTP or Simple NTP) is vital.
EID 4211 (Verified) is as follows:

Section: 2.3

Original Text:

As noted in RFC 6292 [RFC6292], accurate timekeeping
   (e.g., use of NTP or Simple NTP) is vital).

Corrected Text:

As noted in RFC 6269 [RFC6269], accurate timekeeping
   (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. Note that in a production environment, operators are encouraged to consider [RFC6598] for assigning inside addresses. 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 regard 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]:,5004,5060. 3.1. 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 port 3456 maps to port 1000 on CGN 1 and 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. Impact on the IPv6 Transition The solution described in this document is applicable to CGN transition technologies (e.g., NAT444, DS-Lite, and NAT64). As discussed in [RFC7021], the authors acknowledge that native IPv6 will offer subscribers a better experience than CGN. However, many Customer Premises Equipment (CPE) devices only support IPv4. Likewise, as of October 2014, only approximately 5.2% 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; however, until such time as IPv6 content and devices are widely available, 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. Privacy Considerations The algorithm described above makes it easier for SPs and public safety officials to identify the IP address of a subscriber through a CGN system. This is the equivalent level of privacy users could expect when they are assigned a public IP address and their traffic is not translated. However, this algorithm could be used by other actors on the Internet to map multiple transactions to a single subscriber, particularly if ports are distributed sequentially. While still preserving traceability, subscriber privacy can be increased by using one of the other values of the Address Assignment Algorithm (A), which would require interested parties to know more about the Service Provider's CGN configuration to be able to tie multiple connections to a particular subscriber. 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. 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. 7. References 7.1. 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, <>. 7.2. Informative References [L2NAT] Miles, D. and M. Townsley, "Layer2-Aware NAT", Work in Progress, draft-miles-behave-l2nat-00, March 2009. [LOG-REDUCTION] Tsou, T., Li, W., Taylor, T., and J. Huang, "Port Management To Reduce Logging In Large-Scale NATs", Work in Progress, draft-tsou-behave-natx4-log-reduction-04, July 2013. [NAT-LOGGING] Sivakumar, S. and R. Penno, "IPFIX Information Elements for logging NAT Events", Work in Progress, draft-ietf-behave-ipfix-nat-logging-04, July 2014. [NAT444] Yamagata, I., Shirasaki, Y., Nakagawa, A., Yamaguchi, J., and H. Ashida, "NAT444", Work in Progress, draft-shirasaki-nat444-06, July 2012. [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, <>. [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, <>. [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, <>. [RFC6887] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, April 2013, <>. [RFC7021] Donley, C., Howard, L., Kuarsingh, V., Berg, J., and J. Doshi, "Assessing the Impact of Carrier-Grade NAT on Network Applications", RFC 7021, September 2013, <>. 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, Fred Baker, Brian Carpenter, and Reinaldo Penno. Authors' Addresses Chris Donley CableLabs 858 Coal Creek Cir Louisville, CO 80027 United States EMail: Chris Grundemann Internet Society Denver, CO United States EMail: Vikas Sarawat CableLabs 858 Coal Creek Cir Louisville, CO 80027 United States EMail: Karthik Sundaresan CableLabs 858 Coal Creek Cir Louisville, CO 80027 United States EMail: Olivier Vautrin Juniper Networks 1194 N Mathilda Avenue Sunnyvale, CA 94089 United States EMail: