Security Implications of Predictable Fragment Identification Values
RFC 7739
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
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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
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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,
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 its
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