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rfc:rfc7112

Internet Engineering Task Force (IETF) F. Gont Request for Comments: 7112 Huawei Technologies Updates: 2460 V. Manral Category: Standards Track Ionos Networks ISSN: 2070-1721 R. Bonica

                                                      Juniper Networks
                                                          January 2014
            Implications of Oversized IPv6 Header Chains

Abstract

 The IPv6 specification allows IPv6 Header Chains of an arbitrary
 size.  The specification also allows options that can, in turn,
 extend each of the headers.  In those scenarios in which the IPv6
 Header Chain or options are unusually long and packets are
 fragmented, or scenarios in which the fragment size is very small,
 the First Fragment of a packet may fail to include the entire IPv6
 Header Chain.  This document discusses the interoperability and
 security problems of such traffic, and updates RFC 2460 such that the
 First Fragment of a packet is required to contain the entire IPv6
 Header Chain.

Status of This Memo

 This is an Internet Standards Track document.
 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
 Internet Standards 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/rfc7112.

Gont, et al. Standards Track [Page 1] RFC 7112 Implications of Oversized Header Chains January 2014

Copyright Notice

 Copyright (c) 2014 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 ....................................................2
 2. Requirements Language ...........................................3
 3. Terminology .....................................................3
 4. Motivation ......................................................4
 5. Updates to RFC 2460 .............................................5
 6. IANA Considerations .............................................5
 7. Security Considerations .........................................6
 8. Acknowledgements ................................................6
 9. References ......................................................7
    9.1. Normative References .......................................7
    9.2. Informative References .....................................7

1. Introduction

 With IPv6, optional internet-layer information is carried in one or
 more IPv6 Extension Headers [RFC2460].  Extension Headers are placed
 between the IPv6 header and the Upper-Layer Header in a packet.  The
 term "Header Chain" refers collectively to the IPv6 header, Extension
 Headers, and Upper-Layer Header occurring in a packet.  In those
 scenarios in which the IPv6 Header Chain is unusually long and
 packets are fragmented, or scenarios in which the fragment size is
 very small, the Header Chain may span multiple fragments.
 While IPv4 had a fixed maximum length for the set of all IPv4 options
 present in a single IPv4 packet, IPv6 does not have any equivalent
 maximum limit at present.  This document updates the set of IPv6
 specifications to create an overall limit on the size of the
 combination of IPv6 options and IPv6 Extension Headers that is
 allowed in a single IPv6 packet.  Namely, it updates RFC 2460 such
 that the First Fragment of a fragmented datagram is required to
 contain the entire IPv6 Header Chain.

Gont, et al. Standards Track [Page 2] RFC 7112 Implications of Oversized Header Chains January 2014

 It should be noted that this requirement does not preclude the use of
 large payloads but, instead, merely requires that all headers,
 starting from the IPv6 base header and continuing up to the Upper-
 Layer Header (e.g., TCP or the like) be present in the First
 Fragment.

2. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

3. Terminology

 For the purposes of this document, the terms Extension Header, IPv6
 Header Chain, First Fragment, and Upper-Layer Header are used as
 follows:
 Extension Header:
    Extension Headers are defined in Section 4 of [RFC2460].  As a
    result of [RFC7045], [IANA-PROTO] provides a list of assigned
    Internet Protocol Numbers and designates which of those protocol
    numbers also represent Extension Headers.
 First Fragment:
    An IPv6 fragment with Fragment Offset equal to 0.
 IPv6 Header Chain:
    The IPv6 Header Chain contains an initial IPv6 header, zero or
    more IPv6 Extension Headers, and optionally, a single Upper-Layer
    Header.  If an Upper-Layer Header is present, it terminates the
    header chain; otherwise, the "No Next Header" value (Next Header =
    59) terminates it.
    The first member of the IPv6 Header Chain is always an IPv6
    header.  For a subsequent header to qualify as a member of the
    header chain, it must be referenced by the "Next Header" field of
    the previous member of the header chain.  However, if a second
    IPv6 header appears in the header chain, as is the case when IPv6
    is tunneled over IPv6, the second IPv6 header is considered to be
    an Upper-Layer Header and terminates the header chain.  Likewise,
    if an Encapsulating Security Payload (ESP) header appears in the
    header chain, it is considered to be an Upper-Layer Header, and it
    terminates the header chain.

Gont, et al. Standards Track [Page 3] RFC 7112 Implications of Oversized Header Chains January 2014

 Upper-Layer Header:
    In the general case, the Upper-Layer Header is the first member of
    the header chain that is neither an IPv6 header nor an IPv6
    Extension Header.  However, if either an ESP header, or a second
    IPv6 header occur in the header chain, they are considered to be
    Upper-Layer Headers, and they terminate the header chain.
    Neither the upper-layer payload, nor any protocol data following
    the upper-layer payload, is considered to be part of the IPv6
    Header Chain.  In a simple example, if the Upper-Layer Header is a
    TCP header, the TCP payload is not part of the IPv6 Header Chain.
    In a more complex example, if the Upper-Layer Header is an ESP
    header, neither the payload data, nor any of the fields that
    follow the payload data in the ESP header are part of the IPv6
    Header Chain.

4. Motivation

 Many forwarding devices implement stateless firewalls.  A stateless
 firewall enforces a forwarding policy on a packet-by-packet basis.
 In order to enforce its forwarding policy, the stateless firewall may
 need to glean information from both the IPv6 and upper-layer headers.
 For example, assume that a stateless firewall discards all traffic
 received from an interface unless it is destined for a particular TCP
 port on a particular IPv6 address.  When this firewall is presented
 with a fragmented packet that is destined for a different TCP port,
 and the entire header chain is contained within the First Fragment,
 the firewall discards the First Fragment and allows subsequent
 fragments to pass.  Because the First Fragment was discarded, the
 packet cannot be reassembled at the destination.  Insomuch as the
 packet cannot be reassembled, the forwarding policy is enforced.
 However, when the firewall is presented with a fragmented packet and
 the header chain spans multiple fragments, the First Fragment does
 not contain enough information for the firewall to enforce its
 forwarding policy.  Lacking sufficient information, the stateless
 firewall either forwards or discards that fragment.  Regardless of
 the action that it takes, it may fail to enforce its forwarding
 policy.

Gont, et al. Standards Track [Page 4] RFC 7112 Implications of Oversized Header Chains January 2014

5. Updates to RFC 2460

 When a host fragments an IPv6 datagram, it MUST include the entire
 IPv6 Header Chain in the First Fragment.
 A host that receives a First Fragment that does not satisfy the
 above-stated requirement SHOULD discard the packet and SHOULD send an
 ICMPv6 error message to the source address of the offending packet
 (subject to the rules for ICMPv6 errors specified in [RFC4443]).
 However, for backwards compatibility, implementations MAY include a
 configuration option that allows such fragments to be accepted.
 Likewise, an intermediate system (e.g., router or firewall) that
 receives an IPv6 First Fragment that does not satisfy the above-
 stated requirement MAY discard that packet, and it MAY send an ICMPv6
 error message to the source address of the offending packet (subject
 to the rules for ICMPv6 error messages specified in [RFC4443]).
 Intermediate systems having this capability SHOULD support
 configuration (e.g., enable/disable) of whether or not such packets
 are dropped by the intermediate system.
 If a host or intermediate system discards a First Fragment because it
 does not satisfy the above-stated requirement and sends an ICMPv6
 error message due to the discard, then the ICMPv6 error message MUST
 be Type 4 ("Parameter Problem") and MUST use Code 3 ("First Fragment
 has incomplete IPv6 Header Chain").  The Pointer field contained by
 the ICMPv6 Parameter Problem message MUST be set to zero.  The format
 for the ICMPv6 error message is the same regardless of whether a host
 or intermediate system originates it.
 As a result of the above-mentioned requirement, a packet's header
 chain length cannot exceed the Path MTU associated with its
 destination.  Hosts discover the Path MTU using procedures such as
 those defined in [RFC1981] and [RFC4821].  Hosts that do not discover
 the Path MTU MUST limit the IPv6 Header Chain length to 1280 bytes.
 Limiting the IPv6 Header Chain length to 1280 bytes ensures that the
 header chain length does not exceed the IPv6 minimum MTU [RFC2460].

6. IANA Considerations

 IANA has added the following "Type 4 - Parameter Problem" message to
 the "Internet Control Message Protocol version 6 (ICMPv6) Parameters"
 registry:
    CODE     NAME/DESCRIPTION
     3       IPv6 First Fragment has incomplete IPv6 Header Chain

Gont, et al. Standards Track [Page 5] RFC 7112 Implications of Oversized Header Chains January 2014

7. Security Considerations

 No new security exposures or issues are raised by this document.
 This document describes how undesirably fragmented packets can be
 leveraged to evade stateless packet filtering.  Having made that
 observation, this document updates [RFC2460] so that undesirably
 fragmented packets are forbidden.  Therefore, a security
 vulnerability is removed.
 This specification allows nodes that drop the aforementioned packets
 to signal such packet drops with ICMPv6 "Parameter Problem, IPv6
 First Fragment has incomplete IPv6 header chain" (Type 4, Code 3)
 error messages.
 As with all ICMPv6 error/diagnostic messages, deploying Source
 Address Forgery Prevention filters helps reduce the chances of an
 attacker successfully performing a reflection attack by sending
 forged illegal packets with the victim's/target's IPv6 address as the
 IPv6 source address of the illegal packet [RFC2827] [RFC3704].
 A firewall that performs stateless deep packet inspection (i.e.,
 examines application payload content) might still be unable to
 correctly process fragmented packets, even if the IPv6 Header Chain
 is not fragmented.

8. Acknowledgements

 The authors would like to thank Ran Atkinson for contributing text
 and ideas that were incorporated into this document.
 The authors would like to thank (in alphabetical order) Ran Atkinson,
 Fred Baker, Stewart Bryant, Brian Carpenter, Benoit Claise, Dominik
 Elsbroek, Wes George, Mike Heard, Bill Jouris, Suresh Krishnan, Dave
 Thaler, Ole Troan, Eric Vyncke, and Peter Yee, for providing valuable
 comments on earlier versions of this document.

Gont, et al. Standards Track [Page 6] RFC 7112 Implications of Oversized Header Chains January 2014

9. References

9.1. Normative References

 [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
            for IP version 6", RFC 1981, August 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 2460, December 1998.
 [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
            Message Protocol (ICMPv6) for the Internet Protocol
            Version 6 (IPv6) Specification", RFC 4443, March 2006.
 [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
            Discovery", RFC 4821, March 2007.
 [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
            of IPv6 Extension Headers", RFC 7045, December 2013.

9.2. Informative References

 [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
            Defeating Denial of Service Attacks which employ IP Source
            Address Spoofing", BCP 38, RFC 2827, May 2000.
 [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
            Networks", BCP 84, RFC 3704, March 2004.
 [IANA-PROTO]
            Internet Assigned Numbers Authority, "Protocol Numbers",
            <http://www.iana.org/assignments/protocol-numbers>.

Gont, et al. Standards Track [Page 7] RFC 7112 Implications of Oversized Header Chains January 2014

Authors' Addresses

 Fernando Gont
 Huawei Technologies
 Evaristo Carriego 2644
 Haedo, Provincia de Buenos Aires  1706
 Argentina
 Phone: +54 11 4650 8472
 EMail: fgont@si6networks.com
 Vishwas Manral
 Ionos Networks
 Sunnyvale, CA  94089
 US
 Phone: 408-447-1497
 EMail: vishwas@ionosnetworks.com
 Ronald P. Bonica
 Juniper Networks
 2251 Corporate Park Drive
 Herndon, VA  20171
 US
 Phone: 571 250 5819
 EMail: rbonica@juniper.net

Gont, et al. Standards Track [Page 8]

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