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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
Intended status: Proposed Standard                   Cisco Systems, Inc.
Expiration Date: June 2009
                                                          William Jaeger
                                                                    AT&T

                                                              Tom Scholl
                                                               AT&T Labs

                                                        December 7, 2008


  Requirements for Label Edge Router Forwarding of IPv4 Option Packets

                   draft-ietf-mpls-ip-options-00.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Abstract

   This document imposes a new requirement on Label Edge Routers (LER)
   specifying that when determining whether to MPLS encapsulate an IP
   packet, the determination is made independent of any IP options that
   may be carried in the IP packet header.  Lack of a formal standard
   has resulted in different LER forwarding behaviors for IP packets



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   with header options despite being associated with a prefix-based
   Forwarding Equivalence Class (FEC).  IP option packets that belong to
   a prefix-based FEC but fail to be MPLS encapsulated simply due to
   their header options 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. 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 carried by LSRs.



Table of Contents

    1          Specification of Requirements  ......................   2
    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          Conclusion  .........................................   7
    8          Acknowledgements  ...................................   8
    9          Normative References  ...............................   8
   10          Informational References  ...........................   8
   11          Authors' Addresses  .................................   9
   12          Full Copyright Statement  ...........................  10
   13          Intellectual Property  ..............................  10






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].










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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
   & applications include but are not limited to:
     o Strict & 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.  Many router
   implementations, for example, punt such IP option packets from the
   hardware forwarding (fast) path into the software forwarding (slow)
   path.

   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



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   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 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 below
   to a prefix-based FEC.  Lack of a formal standard has resulted in
   inconsistent forwarding behaviors by ingress LERs.  When MPLS
   encapsulated by an ingress LER, for example, the IP header including
   option fields of transit packets are transparent to downstream LSRs
   given MPLS forwarding.  Conversely, when IP routed by an ingress LER,
   downstream LSRs must apply IP forwarding rules which may expose the
   LSRs to IP security attacks and for which they (the LSRs) may have
   insufficient IP routing information.

   IP option packets that belong to a prefix-based FEC but fail to be
   MPLS encapsulated simply due to their header options 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.

   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] 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 the IP packets are received as
   they enter the MPLS domain.




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   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), for example, 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 below to a prefix-based FEC yet bypass MPLS
   encapsulation at a 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
   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 below to a prefix-based FEC yet
       bypass MPLS encapsulation at a 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



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       LSRs lack protection mechanisms.
     o Crafted IP option packets that below to a prefix-based FEC yet
       bypass MPLS encapsulation at a 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 below to a prefix-based FEC yet
       bypass MPLS encapsulation at a 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 below to a prefix-based FEC yet
       bypass MPLS encapsulation at a ingress LER may allow an attacker
       to bypass LSP Diff-Serv tunnels [RFC3270] and any associated MPLS
       CoS field [MPLSCOS] 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 IP strict and loose source route option packets that
       below to a prefix-based FEC yet bypass MPLS encapsulation at a
       ingress LER may allow an attacker to specify explicit IP
       forwarding path(s) across an MPLS network and, thereby, target
       specific LSRs with any of the DoS attacks outlined above.  This
       assumes, of course, that the MPLS network is enabled to process
       IP strict and loose source route options.
     o Crafted RSVP packets that below to a prefix-based FEC yet bypass
       MPLS encapsulation at a ingress LER may allow an attacker to
       build RSVP soft-states [RFC2205] 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 IP option
   packets that below 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



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   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 below 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.


7. Conclusion

   This document imposes a new requirement on ingress LERs in order to
   reduce the risk of IP options-based security attacks against LSRs as
   well as to assist LER operation across MPLS networks which minimize
   the IP routing information carried by LSRs.








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8. 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.


9. 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.


10. 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>.

   [MPLSCOS] Andersson, L., "EXP Field renamed to Traffic Class Field,"
   IETF draft-ietf-mpls-cosfield-def-07.txt, November 17, 2008.


11. Authors' Addresses


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



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



      Tom Scholl
      AT&T Labs
      200 S. Laurel Avenue
      Middletown, NJ  07748
      Email: ts3127@att.com



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



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12. Full Copyright Statement

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