Common Requirements for Carrier-Grade NATs (CGNs)
RFC 6888

• Status
• IESG evaluation record
• IESG writeups
• Email expansions
• History

• Versions
• 05
• 06
• 07
• 08
• 09
• 10

Internet Engineering Task Force (IETF)                 S. Perreault, Ed.
Request for Comments: 6888                                      Viagenie
BCP: 127                                                     I. Yamagata
Updates: 4787                                                S. Miyakawa
Category: Best Current Practice                       NTT Communications
ISSN: 2070-1721                                              A. Nakagawa
                                          Japan Internet Exchange (JPIX)
                                                               H. Ashida
                                                           Cisco Systems
                                                              April 2013

           Common Requirements for Carrier-Grade NATs (CGNs)

Abstract

   This document defines common requirements for Carrier-Grade NATs
   (CGNs).  It updates RFC 4787.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in 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
   http://www.rfc-editor.org/info/rfc6888.

Copyright Notice

   Copyright (c) 2013 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Perreault, et al.         Best Current Practice                 [Page 1]
RFC 6888                    CGN Requirements                  April 2013

Table of Contents
   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . .  2
   2. Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  3
   3. Requirements for CGNs  . . . . . . . . . . . . . . . . . . .  4
   4. Logging  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5. Port Allocation Scheme . . . . . . . . . . . . . . . . . . . 11
   6. Deployment Considerations  . . . . . . . . . . . . . . . . . 11
   7. Security Considerations  . . . . . . . . . . . . . . . . . . 12
   8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
   9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
      9.1. Normative References  . . . . . . . . . . . . . . . . . 12
      9.2. Informative Reference . . . . . . . . . . . . . . . . . 13

1.  Introduction

   With the shortage of IPv4 addresses, it is expected that more
   Internet Service Providers (ISPs) may want to provide a service where
   a public IPv4 address would be shared by many subscribers.  Each
   subscriber is assigned a private address, and a Network Address
   Translator (NAT) [RFC2663] situated in the ISP's network translates
   the traffic between private and public addresses.  When a second IPv4
   NAT is located at the customer edge, this results in two layers of
   NAT.

   This service can conceivably be offered alongside others, such as
   IPv6 services or regular IPv4 service assigning public addresses to
   subscribers.  Some ISPs started offering such a service long before
   there was a shortage of IPv4 addresses, showing that there are
   driving forces other than the shortage of IPv4 addresses.  One
   approach to CGN deployment is described in [RFC6264].

   This document describes behavior that is required of those multi-
   subscriber NATs for interoperability.  It is not an IETF endorsement
   of CGNs or a real specification for CGNs; rather, it is just a
   minimal set of requirements that will increase the likelihood of
   applications working across CGNs.

   Because subscribers do not receive unique IPv4 addresses, Carrier-
   Grade NATs introduce substantial limitations in communications
   between subscribers and with the rest of the Internet.  In
   particular, it is considerably more involved to establish proxy
   functionality at the border between internal and external realms.
   Some applications may require substantial enhancements, while some
   others may not function at all in such an environment.  Please see
   "Issues with IP Address Sharing" [RFC6269] for details.

Perreault, et al.         Best Current Practice                 [Page 2]
RFC 6888                    CGN Requirements                  April 2013