Security Implications of Predictable Fragment Identification Values
RFC 7739

Document Type RFC - Informational (February 2016; No errata)
Last updated 2016-02-02
Replaces draft-gont-6man-predictable-fragment-id
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Internet Engineering Task Force (IETF)                           F. Gont
Request for Comments: 7739                           Huawei Technologies
Category: Informational                                    February 2016
ISSN: 2070-1721

  Security Implications of Predictable Fragment Identification Values

Abstract

   IPv6 specifies the Fragment Header, which is employed for the
   fragmentation and reassembly mechanisms.  The Fragment Header
   contains an "Identification" field that, together with the IPv6
   Source Address and the IPv6 Destination Address of a packet,
   identifies fragments that correspond to the same original datagram,
   such that they can be reassembled together by the receiving host.
   The only requirement for setting the Identification field is that the
   corresponding value must be different than that employed for any
   other fragmented datagram sent recently with the same Source Address
   and Destination Address.  Some implementations use a simple global
   counter for setting the Identification field, thus leading to
   predictable Identification values.  This document analyzes the
   security implications of predictable Identification values, and
   provides implementation guidance for setting the Identification field
   of the Fragment Header, such that the aforementioned security
   implications are mitigated.

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/rfc7739.

Gont                          Informational                     [Page 1]
RFC 7739        Implications of Predictable Fragment IDs   February 2016

Copyright Notice

   Copyright (c) 2016 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Implications of Predictable Fragment Identification
       Values  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Constraints for the Selection of Fragment Identification
       Values  . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Algorithms for Selecting Fragment Identification Values . . .   8
     5.1.  Per-Destination Counter (Initialized to a Random Value) .   8
     5.2.  Randomized Identification Values  . . . . . . . . . . . .   9
     5.3.  Hash-Based Fragment Identification Selection Algorithm  .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Information Leakage Produced by Vulnerable
                Implementations  . . . . . . . . . . . . . . . . . .  16
   Appendix B.  Survey of Fragment Identification Selection
                Algorithms Employed by Popular IPv6 Implementations   18
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  20
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

Gont                          Informational                     [Page 2]
RFC 7739        Implications of Predictable Fragment IDs   February 2016

1.  Introduction

   IPv6 specifies the Fragment Header, which is employed for the
   fragmentation and reassembly mechanisms.  The Fragment Header
   contains an "Identification" field that, together with the IPv6
   Source Address and the IPv6 Destination Address of a packet,
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