Network Working Group C.D. Donley
Internet-Draft C.G. Grundemann
Intended status: Experimental Protocol V.S. Sarawat
Expires: July 19, 2012 K.S. Sundaresan
January 18, 2012

Deterministic Address Mapping to Reduce Logging in Carrier Grade NAT Deployments


Many Carrier Grade NAT solutions require per-connection logging. Unfortunately, such logging is not scalable to 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.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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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This Internet-Draft will expire on July 19, 2012.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1. Introduction

The world is rapidly running out of unallocated IPv4 addresses. To ensure IPv4 service continuity under the growing demands from new subscribers, devices, and service types, ISPs 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], etc.). However, address sharing poses additional challenges to ISPs in responding to law enforcement requests or attack/abuse reports [RFC6269]. In order to respond to such requests to identify a specific user associated with an IP address, an ISP 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/law enforcement requests, but it imposes significant operational challenges to ISPs. 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 ISPs in the US sets the average number of connections per household per day 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 ISPs 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 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 RFC6292 [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) [RFC6056]."

For the greatest reduction in logging, 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 document describes a method for such CGN address mapping, combined with block port reservations, that significantly reduces the burden on ISPs 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 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, 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 subscriber.

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 users to test both mapping functions (e.g., enter an External IP Address + Port Number and recieve the corrosponding Internal IP Address).

Deterministic Port Range allocation requires configuration of the following variables:

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).

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).

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:

  1. The CGN removes reserved ports 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. Any remaining ports are allocated to the dynamic pool. Port allocation could be made sequentially (e.g. the first block goes to address 1, the second block to address 2, etc.), staggered (e.g. address 1 receives ports n*(C+D), address 2 receives ports 1+n*(C+D), etc.), or through some other deterministic algorithm left to CGN implementation. 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 range of ports can be contiguous or be randomly assigned (e.g., using the random function defined in Section 2.2 of RFC6431 [RFC6431].
  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 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. 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 uses a port from the dynamic assignment range for such a connection or for PCP reservations. The CGN MUST log dynamically assigned ports 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.

2.1. Stability and Load-Balancing Considerations

Using the procedure defined in this document assumes a deterministic distribution of customers among deployed CGN devices. Balancing the traffic among several CGNs based on their actual load may not be supported because of the potential conflict of enforced algorithmic mapping rule. When CGN redundancy group is used, the same mapping rule, including in particular the external IP address, MUST be used. Furthermore, traffic oscillation MUST be avoided (because, unless state synchronization is used, the actual NAT state may not be instantiated in the redundancy group).

2.2. IPv4 Port Utilization Efficiency

For Service Providers requiring an aggressive address sharing ration, the use of the algorithmic mapping may impact the efficiency of the address sharing. Using a dynamic port range scheme, dynamic port assignment or a mix of static mapping and dynamic port assignment is more suitable for those SPs.

2.3. 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.

2.4. 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 buffer factor 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. 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.

Inside Address / Pool Outside Address & Port

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 law enforcement 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, corrolate with connection records, and determine that subscriber 2 generated the traffic (assuming that the law enforcement timestamp matches the operator timestamp. As noted in RFC6292 [RFC6292], accurate time-keeping (e.g., use of NTP or Simple NTP) is vital).

In this example, there are no log entries for the majority of subscribers, who only use pre-allocated ports. Only minimal logging would be needed for those few subscribers who exceed their pre-allocated ports and obtain extra bulk port assignments from the dynamic pool. Logging data for those users will include inside address, outside address, outside port range, and timestamp.

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 log message any time such variables are changed. 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, and reserved port range; for example:

[Wed Oct 11 14:32:52 2000]:,5004,5060.

4. IANA Considerations

This document makes no request of IANA.

5. 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.

6. Acknowledgements


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

7.2. Informative References

[RFC6333] Durand, A., Droms, R., Woodyatt, J. and Y. Lee, "Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion", RFC 6333, August 2011.
[I-D.ietf-pcp-base] Wing, D, Cheshire, S, Boucadair, M, Penno, R and P Selkirk, "Port Control Protocol (PCP)", Internet-Draft draft-ietf-pcp-base-21, January 2012.
[RFC6292] Hoffman, P., "Requirements for a Working Group Charter Tool", RFC 6292, June 2011.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-Protocol Port Randomization", BCP 156, RFC 6056, January 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.
[I-D.miles-behave-l2nat] Miles, D and M Townsley, "Layer2-Aware NAT", Internet-Draft draft-miles-behave-l2nat-00, March 2009.
[I-D.shirasaki-nat444] Yamagata, I, Shirasaki, Y, Nakagawa, A, Yamaguchi, J and H Ashida, "NAT444", Internet-Draft draft-shirasaki-nat444-05, January 2012.
[I-D.tsou-behave-natx4-log-reduction] ZOU), T, Li, W and T Taylor, "Port Management To Reduce Logging In Large-Scale NATs", Internet-Draft draft-tsou-behave-natx4-log-reduction-02, September 2010.

Authors' Addresses

Chris Donley CableLabs 858 Coal Creek Cir Louisville, CO 80027 US EMail:
Chris Grundemann CableLabs 858 Coal Creek Cir Louisville, CO 80027 US EMail:
Vikas Sarawat CableLabs 858 Coal Creek Cir Louisville, CO 80027 US EMail:
Karthik Sundaresan CableLabs 858 Coal Creek Cir Louisville, CO 80027 US EMail: