LER IPv4 Options: Label Edge Router Forwarding of IPv4 Option Packets

Internet Engineering Task Force (IETF)                          D. Smith
Request for Comments: 6178                                   J. Mullooly
Updates: 3031                                              Cisco Systems
Category: Standards Track                                      W. Jaeger
ISSN: 2070-1721                                                     AT&T
                                                               T. Scholl
                                                   nLayer Communications
                                                              March 2011


          Label Edge Router Forwarding of IPv4 Option Packets

Abstract

   This document specifies how Label Edge Routers (LERs) should behave
   when determining whether to MPLS encapsulate an IPv4 packet with
   header options.  Lack of a formal standard has resulted in different
   LER forwarding behaviors for IPv4 packets with header options despite
   being associated with a prefix-based Forwarding Equivalence Class
   (FEC).  IPv4 option packets that belong to a prefix-based FEC, yet
   are forwarded into an IPv4/MPLS network without being MPLS-
   encapsulated, present a security risk against the MPLS
   infrastructure.  Further, LERs that are unable to MPLS encapsulate
   IPv4 packets with header options cannot operate in certain MPLS
   environments.  While this newly defined LER behavior is mandatory to
   implement, it is optional to invoke.

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/rfc6178.

Copyright Notice

   Copyright (c) 2011 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. Motivation ......................................................2
   2. Introduction ....................................................2
   3. Specification of Requirements ...................................4
   4. Ingress Label Edge Router Requirement ...........................4
   5. Security Considerations .........................................5
      5.1. IPv4 Option Packets That Bypass MPLS Encapsulation .........5
      5.2. Router Alert Label Imposition ..............................7
   6. Acknowledgements ................................................7
   7. References ......................................................7
      7.1. Normative References .......................................7
      7.2. Informative References .....................................8

1.  Motivation

   This document is motivated by the need to formalize MPLS
   encapsulation processing of IPv4 packets with header options in order
   to mitigate the existing risks of IPv4 options-based security attacks
   against MPLS infrastructures.  We believe that this document adds
   details that have not been fully addressed in [RFC3031] and
   [RFC3032], and that the methods presented in this document update
   [RFC3031] as well as complement [RFC3270], [RFC3443], and [RFC4950].

2.  Introduction

   The IPv4 packet header provides for various IPv4 options as
   originally specified in [RFC791].  IPv4 header options are used to
   enable control functions within the IPv4 data forwarding plane that
   are required in some specific situations but not necessary for most
   common IPv4 communications.  Typical IPv4 header options include
   provisions for timestamps, security, and special routing.  Example
   IPv4 header options and applications include but are not limited to
   the following:

      o Strict and Loose Source Route Options: Used to IPv4 route the
        IPv4 packet based on information supplied by the source.

      o Record Route Option: Used to trace the route an IPv4 packet
        takes.

      o Router Alert Option: Indicates to downstream IPv4 routers to
        examine these IPv4 packets more closely.

   The list of current IPv4 header options can be accessed at [IANA].

   IPv4 packets may or may not use IPv4 header options (they are
   optional), but IPv4 header option handling mechanisms must be
   implemented by all IPv4 protocol stacks (hosts and routers).  Each
   IPv4 header option has distinct header fields and lengths.  IPv4
   options extend the IPv4 packet header length beyond the minimum of 20
   octets.  As a result, IPv4 packets received with header options are
   typically handled as exceptions and in a less efficient manner due to
   their variable length and complex processing requirements.  For
   example, many router implementations punt such IPv4 option packets
   from the hardware forwarding (fast) path into the software forwarding
   (slow) path causing high CPU utilization.  Even when the forwarding
   plane can parse a variable-length header, it may still need to punt
   to the control plane because the forwarding plane may not have the
   clock cycles or intelligence required to process the header option.

   Multi-Protocol Label Switching (MPLS) [RFC3031] is a technology in
   which packets associated with a prefix-based Forwarding Equivalence
   Class (FEC) are encapsulated with a label stack and then switched
   along a label switched path (LSP) by a sequence of label switch
   routers (LSRs).  These intermediate LSRs do not generally perform any
   processing of the IPv4 header as packets are forwarded. (There are
   some exceptions to this rule, such as ICMP processing and LSP ping,
   as described in [RFC3032] and [RFC4379], respectively.)  Many MPLS
   deployments rely on LSRs to provide layer 3 transparency much like
   ATM switches are transparent at layer 2.  Such deployments often
   minimize the IPv4 routing information (e.g., no BGP transit routes)
   carried by LSRs since it is not necessary for MPLS forwarding of
   transit packets.

   Even though MPLS encapsulation seems to offer a viable solution to
   provide layer 3 transparency, there is currently no formal standard
   for MPLS encapsulation of IPv4 packets with header options that
   belong to a prefix-based FEC.  Lack of a formal standard has resulted
   in inconsistent forwarding behaviors by ingress Label Edge Routers
   (LERs).  When IPv4 packets are MPLS encapsulated by an ingress LER,
   for example, the IPv4 header including option fields of transit
   packets are not acted upon by downstream LSRs that forward based on
   the MPLS label(s).  Conversely, when a packet is IPv4 forwarded by an
   ingress LER two undesirable behaviors can result.  First, a
   downstream LSR may not have sufficient IPv4 routing information to
   forward the packet resulting in packet loss.  Second, downstream LSRs
   must apply IPv4 forwarding rules that may expose them to IPv4
   security attacks.

   IPv4 option packets that belong to a prefix-based FEC, yet are
   forwarded into an IPv4/MPLS network without being MPLS-encapsulated,
   present a security risk against the MPLS infrastructure.  Further,
   LERs that are unable to MPLS encapsulate IPv4 packets with header
   options cannot operate as an LER in certain MPLS environments.  This
   new requirement will reduce the risk of IPv4 options-based security
   attacks against LSRs as well as assist LER operation across MPLS
   networks that minimize the IPv4 routing information (e.g., no BGP
   transit routes) carried by LSRs.

3.  Specification of Requirements

   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].

4.  Ingress Label Edge Router Requirement

   An ingress LER MUST implement the following policy:

      o When determining whether to push an MPLS label stack onto an
        IPv4 packet, the determination is made without considering any
        IPv4 options that may be carried in the IPv4 packet header.
        Further, the label values that appear in the label stack are
        determined without considering any such IPv4 options.

   This policy MAY be configurable on an ingress LER, however, it SHOULD
   be enabled by default.  When processing of signaling messages or data
   packets with more specific forwarding rules is enabled, this policy
   SHOULD NOT alter the specific processing rules.  This applies to, but
   is not limited to, Resource Reservation Protocol (RSVP) as per
   [RFC2205], source routing as per [RFC791], as well as other FEC
   elements defined by future specifications.  Further, how an ingress
   LER processes the IPv4 header options of packets before MPLS
   encapsulation is out of scope since these are processed before they
   enter the MPLS domain.

   Implementation of the above policy prevents IPv4 packets that belong
   to a prefix-based FEC from bypassing MPLS encapsulation due to header
   options.  The policy also prevents specific option types such as
   Router Alert (option value 148) from forcing MPLS imposition of the
   MPLS Router Alert Label (label value 1) at ingress LERs.  Without
   this policy, the MPLS infrastructure is exposed to security attacks
   using legitimate IPv4 packets crafted with header options.  Further,
   LERs that are unable to MPLS encapsulate IPv4 packets with header
   options cannot operate as an LER in certain MPLS environments as
   described in Section 2.

5.  Security Considerations

   There are two potential categories of attacks using crafted IPv4
   option packets that threaten existing MPLS infrastructures.  Both are
   described below.  To mitigate the risk of these specific attacks, the
   ingress LER policy specified above is required.

5.1.  IPv4 Option Packets That Bypass MPLS Encapsulation

   Given that a router's exception handling process (i.e., CPU,
   processor line-card bandwidth, etc.) used for IPv4 header option
   processing is often shared with IPv4 control and management protocol
   router resources, a flood of IPv4 packets with header options may
   adversely affect a router's control and management protocols,
   thereby, triggering a denial-of-service (DoS) condition.  Note, IPv4
   packets with header options may be valid transit IPv4 packets with
   legitimate sources and destinations.  Hence, a DoS-like condition may
   be triggered on downstream transit IPv4 routers that lack protection
   mechanisms even in the case of legitimate IPv4 option packets.

   IPv4 option packets that belong to a prefix-based FEC yet bypass MPLS
   encapsulation at an ingress LER may be inadvertently IPv4 routed
   downstream across the MPLS core network (not label switched).  This
   allows an external attacker the opportunity to maliciously craft
   seemingly legitimate IPv4 packets with specific IPv4 header options
   in order to intentionally bypass MPLS encapsulation at the MPLS edge
   (i.e., ingress LER) and trigger a DoS condition on downstream LSRs.
   Some of the specific types of IPv4 option-based security attacks that
   may be leveraged against MPLS networks include the following:

      o Crafted IPv4 option packets that belong to a prefix-based FEC
        yet bypass MPLS encapsulation at an ingress LER may allow an
        attacker to DoS downstream LSRs by saturating their software
        forwarding paths.  By targeting a LSR's exception path, control
        and management protocols may be adversely affected and, thereby,
        an LSR's availability.  This assumes, of course, that downstream
        LSRs lack protection mechanisms.

      o Crafted IPv4 option packets that belong to a prefix-based FEC
        yet bypass MPLS encapsulation at an ingress LER may allow for
        IPv4 Time to Live (TTL) expiry-based DoS attacks against
        downstream LSRs.  MPLS enables IPv4 core hiding whereby transit
        IPv4 traffic flows see the MPLS network as a single router hop
        [RFC3443].  However, MPLS core hiding does not apply to packets
        that bypass MPLS encapsulation and, therefore, IPv4 option
        packets may be crafted to expire on downstream LSRs which may
        trigger a DoS condition.  Bypassing MPLS core hiding is an
        additional security consideration since it exposes the network
        topology.

      o Crafted IPv4 option packets that belong to a prefix-based FEC
        yet bypass MPLS encapsulation at an ingress LER may allow for
        DoS attacks against downstream LSRs that do not carry the IPv4
        routing information required to forward transit IPv4 traffic.
        Lack of such IPv4 routing information may prevent legitimate
        IPv4 option packets from transiting the MPLS network and,
        further, may trigger generation of ICMP destination unreachable
        messages, which could lead to a DoS condition.  This assumes, of
        course, that downstream LSRs lack protection mechanisms and do
        not carry the IPv4 routing information required to forward
        transit traffic.

      o Crafted IPv4 option packets that belong to a prefix-based FEC
        yet bypass MPLS encapsulation at an ingress LER may allow an
        attacker to bypass LSP Diffserv tunnels [RFC3270] and any
        associated MPLS Class of Service (CoS) field [RFC5462] marking
        policies at ingress LERs and, thereby, adversely affect (i.e.,
        DoS) high-priority traffic classes within the MPLS core.
        Further, this could also lead to theft of high-priority services
        by unauthorized parties.  This assumes, of course, that the
        [RFC3270] Pipe model is deployed within the MPLS core.

      o Crafted RSVP packets that belong to a prefix-based FEC yet
        bypass MPLS encapsulation at an ingress LER may allow an
        attacker to build RSVP soft-states [RFC2205] [RFC3209] on
        downstream LSRs which could lead to theft of service by
        unauthorized parties or to a DoS condition caused by locking up
        LSR resources.  This assumes, of course, that the MPLS network
        is enabled to process RSVP packets.

   The security attacks outlined above specifically apply to IPv4 option
   packets that belong to a prefix-based FEC yet bypass ingress LER
   label stack imposition.  Additionally, these attacks only apply to
   IPv4 option packets forwarded using the global routing table (i.e.,
   IPv4 address family) of a ingress LER.  IPv4 option packets
   associated with a BGP/MPLS IPv4 VPN service are always MPLS
   encapsulated by the ingress LER per [RFC4364] given that packet
   forwarding uses a Virtual Forwarding/Routing (VRF) instance.
   Therefore, BGP/MPLS IPv4 VPN services are not subject to the threats
   outlined above [RFC4381].  Further, IPv6 [RFC2460] makes use of
   extension headers not header options and is therefore outside the
   scope of this document.  A separate security threat that does apply
   to both BGP/MPLS IPv4 VPNs and the IPv4 address family makes use of
   the Router Alert Label.  This is described directly below.

5.2.  Router Alert Label Imposition

   [RFC3032] defines a Router Alert Label (label value of 1), which is
   analogous to the Router Alert IPv4 header option (option value of
   148).  The MPLS Router Alert Label (when exposed and processed only)
   indicates to downstream LSRs to examine these MPLS packets more
   closely.  MPLS packets with the MPLS Router Alert Label are also
   handled as an exception by LSRs and, again, in a less efficient
   manner.  At the time of this writing, the only legitimate use of the
   Router Alert Label is for LSP ping/trace [RFC4379].  Since there is
   also no formal standard for Router Alert Label imposition at ingress
   LERs:

      o Crafted IPv4 packets with specific IPv4 header options (e.g.,
        Router Alert) and that belong to a prefix-based FEC may allow an
        attacker to force MPLS imposition of the Router Alert Label at
        ingress LERs and, thereby, trigger a DoS condition on downstream
        LSRs.  This assumes, of course, that downstream LSRs lack
        protection mechanisms.

6.  Acknowledgements

        The authors would like to thank Adrian Cepleanu, Bruce Davie,
        Rick Huber, Chris Metz, Pradosh Mohapatra, Ashok Narayanan,
        Carlos Pignataro, Eric Rosen, Mark Szczesniak, and Yung Yu for
        their valuable comments and suggestions.

7.  References

7.1.  Normative References

   [RFC791]    Postel, J., "Internet Protocol", STD 5, RFC 791,
               September 1981.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3031]   Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
               Label Switching Architecture", RFC 3031, January 2001.

   [RFC3032]   Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
               Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
               Encoding", RFC 3032, January 2001.

7.2.  Informative References

   [RFC2205]   Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and
               S. Jamin, "Resource ReSerVation Protocol (RSVP) --
               Version 1 Functional Specification", RFC 2205, September
               1997.

   [RFC2460]   Deering, S. and R. Hinden, "Internet Protocol, Version 6
               (IPv6) Specification", RFC 2460, December 1998.

   [RFC3209]   Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
               and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
               Tunnels", RFC 3209, December 2001.

   [RFC3270]   Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
               P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
               Protocol Label Switching (MPLS) Support of Differentiated
               Services", RFC 3270, May 2002.

   [RFC3443]   Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
               in Multi-Protocol Label Switching (MPLS) Networks", RFC
               3443, January 2003.

   [RFC4364]   Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
               Networks (VPNs)", RFC 4364, February 2006.

   [RFC4379]   Kompella, K. and G. Swallow, "Detecting Multi-Protocol
               Label Switched (MPLS) Data Plane Failures", RFC 4379,
               February 2006.

   [RFC4381]   Behringer, M., "Analysis of the Security of BGP/MPLS IP
               Virtual Private Networks (VPNs)", RFC 4381, February
               2006.

   [RFC4950]   Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "ICMP
               Extensions for Multiprotocol Label Switching", RFC 4950,
               August 2007.

   [IANA]      "IP Option Numbers," IANA, February 15, 2007,
               <www.iana.org>.

   [RFC5462]   Andersson, L. and R. Asati, "Multiprotocol Label
               Switching (MPLS) Label Stack Entry: "EXP" Field Renamed
               to "Traffic Class" Field", RFC 5462, February 2009.

Authors' Addresses

   David J. Smith
   Cisco Systems
   111 Wood Avenue South
   Iselin, NJ  08830
   EMail: djsmith@cisco.com

   John Mullooly
   Cisco Systems
   111 Wood Avenue South
   Iselin, NJ  08830
   EMail: jmullool@cisco.com

   William Jaeger
   AT&T
   200 S. Laurel Avenue
   Middletown, NJ  07748
   EMail: wjaeger@att.com

   Tom Scholl
   nLayer Communications
   209 West Jackson, Suite 700
   Chicago, IL  60606
   EMail: tscholl@nlayer.net