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Versions: (draft-dasmith-mpls-ip-options) 00 01 02 03 04 05 06 07 RFC 6178

Network Working Group                                     David J. Smith
Internet Draft                                             John Mullooly
Updates: 3031                                              Cisco Systems
Intended Status: Proposed Standard
Expiration Date: June 2011                                William Jaeger
                                                                    AT&T

                                                              Tom Scholl
                                                   nLayer Communications

                                                       December 15, 2010


  Requirements for Label Edge Router Forwarding of IPv4 Option Packets

                   draft-ietf-mpls-ip-options-06.txt


Abstract

   This document specifies how Label Edge Routers (LER) should behave
   when determining whether to MPLS encapsulate an IP packet with header
   options.  Lack of a formal standard has resulted in different LER
   forwarding behaviors for IP packets with header options despite being
   associated with a prefix-based Forwarding Equivalence Class (FEC).
   IP option packets that belong to a prefix-based FEC, yet are
   forwarded into an IP/MPLS network without being MPLS-encapsulated,
   present a security risk against the MPLS infrastructure.  Further,
   LERs that are unable to MPLS encapsulate IP 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference



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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on June 1, 2011.


Copyright Notice

   Copyright (c) 2010 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          Specification of Requirements  ......................   3
    2          Motivation  .........................................   3
    3          Introduction  .......................................   3
    4          Ingress Label Edge Router Requirement  ..............   4
    5          Security Considerations  ............................   5
    5.1        IP Option Packets that Bypass MPLS Encapsulation  ...   5
    5.2        Router Alert Label Imposition  ......................   7
    6          IANA Considerations  ................................   7
    7          Acknowledgements  ...................................   8
    8          Normative References  ...............................   8
    9          Informational References  ...........................   8
   10          Authors' Addresses  .................................   9










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1. 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].


2. 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 IP 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].


3. Introduction

   The IP packet header provides for various IP options as originally
   specified in [RFC791].  IP header options are used to enable control
   functions within the IP data forwarding plane that are required in
   some specific situations but not necessary for most common IP
   communications.  Typical IP header options include provisions for
   timestamps, security, and special routing.  Example IP header options
   and applications include but are not limited to:
     o Strict and Loose Source Route Options: Used to IP route the IP
       packet based on information supplied by the source.
     o Record Route Option: Used to trace the route an IP packet takes.
     o Router Alert Option: Indicates to downstream IP routers to
       examine these IP packets more closely.
   The list of current IP header options can be accessed at [IANA].

   IP packets may or may not use IP header options (they are optional)
   but IP header option handling mechanisms must be implemented by all
   IP protocol stacks (hosts and routers).  Each IP header option has
   distinct header fields and lengths.  IP options extend the IP packet
   header length beyond the minimum of 20 octets.  As a result, IP
   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 IP 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.



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   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 IP 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 IP 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 IP 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 IP packets are MPLS encapsulated by an ingress LER, for
   example, the IP header including option fields of transit packets are
   not acted upon by downstream LSRs which forward based on the MPLS
   label(s).  Conversely, when a packet is IP forwarded by an ingress
   LER two undesirable behaviors can result. First, a downstream LSR may
   not have sufficient IP routing information to forward the packet
   resulting in packet loss.  Second, downstream LSRs must apply IP
   forwarding rules which may expose them to IP security attacks.

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


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 IP
       packet, the determination is made without considering any IP
       options that may be carried in the IP packet header.  Further,
       the label values that appear in the label stack are determined
       without considering any such IP options.



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   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, 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 IP 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 IP 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 IP packets crafted with header options.  Further,
   LERs that are unable to MPLS encapsulate IP packets with header
   options cannot operate as an LER in certain MPLS environments as
   described above in Section 3.


5. Security Considerations

   There are two potential categories of attacks using crafted IP 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. IP Option Packets that Bypass MPLS Encapsulation

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

   IP option packets that belong to a prefix-based FEC yet bypass MPLS
   encapsulation at an ingress LER may be inadvertently IP routed
   downstream across the MPLS core network (not label switched).  This
   allows an external attacker the opportunity to maliciously craft
   seemingly legitimate IP packets with specific IP header options in



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   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 IP option-based security attacks that
   may be leveraged against MPLS networks include:
     o Crafted IP 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, a
       LSR's availability.  This assumes, of course, that downstream
       LSRs lack protection mechanisms.
     o Crafted IP option packets that belong to a prefix-based FEC yet
       bypass MPLS encapsulation at an ingress LER may allow for IP TTL
       expiry-based DoS attacks against downstream LSRs.  MPLS enables
       IP core hiding whereby transit IP 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, IP 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 IP 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 IP routing
       information required to forward transit IP traffic. Lack of such
       IP routing information may prevent legitimate IP 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
       IP routing information required to forward transit traffic.
     o Crafted IP option packets that belong to a prefix-based FEC yet
       bypass MPLS encapsulation at an ingress LER may allow an attacker
       to bypass LSP Diff-Serv tunnels [RFC3270] and any associated MPLS
       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.




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   The security attacks outlined above specifically apply to IP option
   packets that belong to a prefix-based FEC yet bypass ingress LER
   label stack imposition.  Additionally, these attacks only apply to IP
   option packets forwarded using the global routing table (i.e., IPv4
   address family) of a ingress LER.  IP option packets associated with
   a BGP/MPLS IP 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 IP 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 IP 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" IP 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 IP packets with specific IP 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. IANA Considerations

   This document has no actions for IANA.












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7. 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.


8. Normative References

   [RFC791] Postel, J., "Internet Protocol Specification," RFC791,
   September 1981.

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

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

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


9. Informational References

   [RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S., Jamin, S.,
   "Resource ReSerVation Protocol -- Version 1 Functional
   Specification," RFC2205, September 1997.

   [RFC2460] Deering, S., Hinden, R. "Internet Protocol, Version 6
   Specification," RFC2460, December 1998.

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

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

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

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




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

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

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

   [IANA] "IP Option Numbers," IANA, February 15, 2007,
   <www.iana.org/assignments/ip-parameters>.

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


10. Authors' Addresses


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



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



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










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













































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