Security Implications of Predictable Fragment Identification Values

The information below is for an old version of the document
Document Type Expired Internet-Draft (6man WG)
Author Fernando Gont 
Last updated 2014-11-10 (latest revision 2014-04-29)
Replaces draft-gont-6man-predictable-fragment-id
Stream Internet Engineering Task Force (IETF)
Expired & archived
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This Internet-Draft is no longer active. A copy of the expired Internet-Draft can be found at


IPv6 specifies the Fragment Header, which is employed for the fragmentation and reassembly mechanisms. The Fragment Header contains an "Identification" field which, 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 at the receiving host. The only requirement for setting the "Identification" value is that it must be different than that employed for any other fragmented packet sent recently with the same Source Address and Destination Address. Some implementations use simple a global counter for setting the Identification field, thus leading to predictable values. This document analyzes the security implications of predictable Identification values, and updates RFC 2460 specifying additional requirements for setting the Identification field of the Fragment Header, such that the aforementioned security implications are mitigated.


Fernando Gont (

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