Heuristics for Detecting ESP-NULL Packets
RFC 5879

Internet Engineering Task Force (IETF)                        T. Kivinen
Request for Comments: 5879                               AuthenTec, Inc.
Category: Informational                                      D. McDonald
ISSN: 2070-1721                                       Oracle Corporation
                                                                May 2010

               Heuristics for Detecting ESP-NULL Packets

Abstract

   This document describes a set of heuristics for distinguishing IPsec
   ESP-NULL (Encapsulating Security Payload without encryption) packets
   from encrypted ESP packets.  These heuristics can be used on
   intermediate devices, like traffic analyzers, and deep-inspection
   engines, to quickly decide whether or not a given packet flow is
   encrypted, i.e., whether or not it can be inspected.  Use of these
   heuristics does not require any changes made on existing IPsec hosts
   that are compliant with RFC 4303.

Status of This Memo

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

   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).  Not all documents
   approved by the IESG are 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
   http://www.rfc-editor.org/info/rfc5879.

Kivinen & McDonald            Informational                     [Page 1]
RFC 5879            Heuristics for Detecting ESP-NULL           May 2010

Copyright Notice

   Copyright (c) 2010 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
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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Applicability: Heuristic Traffic Inspection and
           Wrapped ESP ................................................4
      1.2. Terminology ................................................4
   2. Other Options ...................................................5
      2.1. AH .........................................................5
      2.2. Mandating by Policy ........................................6
      2.3. Modifying ESP ..............................................6
   3. Description of Heuristics .......................................6
   4. IPsec Flows .....................................................7
   5. Deep-Inspection Engine ..........................................9
   6. Special and Error Cases .........................................9
   7. UDP Encapsulation ..............................................10
   8. Heuristic Checks ...............................................10
      8.1. ESP-NULL Format ...........................................11
      8.2. Self Describing Padding Check .............................12
      8.3. Protocol Checks ...........................................14
           8.3.1. TCP Checks .........................................15
           8.3.2. UDP Checks .........................................16
           8.3.3. ICMP Checks ........................................16
           8.3.4. SCTP Checks ........................................17
           8.3.5. IPv4 and IPv6 Tunnel Checks ........................17
   9. Security Considerations ........................................17
   10. References ....................................................18
      10.1. Normative References .....................................18
      10.2. Informative References ...................................18
   Appendix A.  Example Pseudocode ...................................20
     A.1.  Fastpath ..................................................20
     A.2.  Slowpath ..................................................23

Kivinen & McDonald            Informational                     [Page 2]
RFC 5879            Heuristics for Detecting ESP-NULL           May 2010

1.  Introduction