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Versions: (draft-rahman-rtg-router-alert-considerations) 00 01 02 03 04 05 06 07 08 09 10 RFC 6398

Network Working Group                                F. Le Faucheur, Ed.
Internet-Draft                                                     Cisco
Intended status: BCP                                    October 25, 2010
Expires: April 28, 2011


                IP Router Alert Considerations and Usage
           draft-ietf-intarea-router-alert-considerations-02

Abstract

   The IP Router Alert Option is an IP option that alerts transit
   routers to more closely examine the contents of an IP packet.  RSVP,
   PGM, IGMP/MLD, MRD and GIST are some of the protocols that make use
   of the IP Router Alert option.  This document discusses security
   aspects and usage guidelines around the use of the current IP Router
   Alert option.  Specifically, it provides recommendation against using
   the Router Alert in the end-to-end open Internet as well as identify
   controlled environments where protocols depending on Router Alert can
   be used safely.  It also provides recommendation about protection
   approaches for Service Providers.  Finally it provides brief
   guidelines for Router Alert implementation on routers.

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).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on April 28, 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



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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
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   This document may contain material from IETF Documents or IETF
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

































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Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Security Concerns of Router Alert  . . . . . . . . . . . . . .  7
   4.  Guidelines for use of Router Alert . . . . . . . . . . . . . . 10
     4.1.  Use of Router Alert End-to-End In the Internet (Router
           Alert in Peer Model) . . . . . . . . . . . . . . . . . . . 10
     4.2.  Use of Router Alert In Controlled Environments . . . . . . 11
       4.2.1.  Use of Router Alert Within an Administrative Domain  . 11
       4.2.2.  Use of Router Alert In Overlay Model . . . . . . . . . 13
     4.3.  Router Alert Protection Approaches for Service
           Providers  . . . . . . . . . . . . . . . . . . . . . . . . 16
   5.  Guidelines for Router Alert Implementation . . . . . . . . . . 18
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     10.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25




























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1.  Terminology

   For readability, this document uses the following loosely defined
   terms:

   o  Slow path : Software processing path for packets

   o  Fast path : ASIC/Hardware processing path for packets

   o  Next level protocol: the protocol transported in the IP datagram.
      In IPv4 [RFC0791], the next level protocol is identified by the
      IANA protocol number conveyed in the 8-bit "Protocol" field in the
      IPv4 header.  In IPv6 [RFC1883], the next level protocol is
      identified by the IANA protocol number conveyed in the 8-bit "Next
      Header" field in the IPv6 header.

1.1.  Conventions Used in This Document

   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.  Introduction

   [RFC2113] and [RFC2711] respectively define the IPv4 and IPv6 Router
   Alert Option (RAO).  In this document, we collectively refer to those
   as the IP Router Alert.  The IP Router Alert Option is an IP option
   that alerts transit routers to more closely examine the contents of
   an IP packet.

   RSVP ([RFC2205], [RFC3175], [RFC3209]), PGM ([RFC3208]), IGMP
   ([RFC3376]), MLD ([RFC2710], [RFC3810]), MRD ([RFC4286]) and NSIS
   General Internet Signalling Transport (GIST) ([I-D.ietf-nsis-ntlp])
   are some of the protocols that make use of the IP Router Alert.

   Section 3 describes the security concerns associated with the use of
   the Router Alert option.

   Section 4 provides guidelines for the use of Router Alert.  More
   specifically, Section 4.1 recommends that Router Alert not be used
   for end to end applications over the Internet, while Section 4.2
   presents controlled environments where applications/protocols relying
   on IP Router Alert can be deployed effectively and safely.
   Section 4.3 provides recommendations on protection approaches to be
   used by Service Providers in order to protect their network from
   Router Alert based attacks.

   Finally, Section 5 provides generic recommendations for router
   implementation of Router Alert aiming at increasing protection
   against attacks.

   The present document discusses considerations and practises based on
   the current specification of IP Router Alert ([RFC2113], [RFC2711]).
   Possible future enhancements to the specification of IP Router Alert
   (in view of reducing the security risks associated with the use of IP
   Router Alert) are outside the scope of this document.  A proposal for
   such enhancements can be found in
   [I-D.narayanan-rtg-router-alert-extension].

   The IPv6 base specification [RFC2460] defines the hop-by-hop option
   extension header.  The hop-by-hop option header is used to carry
   optional information that must be examined by every node along a
   packet's delivery path.  The IPv6 Router Alert Option is one
   particular hop by hop option.  Similar security concerns to those
   discussed in the present document for the IPv6 Router Alert apply
   more generically to the concept of IPv6 hop-by-hop option extension
   header.  However, addressing the broader concept of IPv6 hop-by-hop
   option thoroughly would require additional material so as to cover
   additional considerations associated with it (such as the attacks
   effectiveness depending on how many options are included and on the



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   range from to which the option-type value belongs, etc.), so this is
   kept outside the scope of the present document.  A detailed
   discussion about security risks and proposed remedies associated with
   IPv6 hop-by-hop option can be found in [I-D.krishnan-ipv6-hopbyhop].

   The IPv4 base specification [RFC0791] defines a general notion of
   IPv4 options that can be included in the IPv4 header (without
   distinguishing between hop-by-hop versus end-to-end option).  The
   IPv4 Router Alert Option is one particular IPv4 option.  Similar
   security concerns to those discussed in the present document for the
   IPv4 Router Alert apply more generically to the concept of IPv4
   option.  However, addressing the broader concept of IPv4 option
   thoroughly would require additional material so as to cover
   additional considerations associated with it (such as lack of option
   ordering, etc.), so this is kept outside the scope of the present
   document.



































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3.  Security Concerns of Router Alert

   The IP Router Alert option is defined ([RFC2113], [RFC2711]) as a
   mechanism that alerts transit routers to more closely examine the
   contents of an IP packet.  [RFC4081] and [RFC2711] mention the
   security risks associated with the use of the IP Router Alert:
   flooding a router with bogus (or simply undesired) IP datagrams which
   contain the IP Router Alert could impact operation of the router in
   undesirable ways.  For example, assuming the router punts the
   datagrams containing the IP Router Alert option to the slow path,
   such an attack could consume a significant share of the router's slow
   path and could also lead to packet drops in the slow path (thus,
   affecting operation of all other applications and protocols operating
   in the slow path).

   Furthermore, [RFC2113] specifies no (and [RFC2711] specifies very
   limited) mechanism for identifying different users of IP Router
   Alert.  As a result, many fast switching implementations of IP Router
   Alert punt most/all packets marked with IP Router Alert into the slow
   path (unless configured to systematically ignore or drop all Router
   Alert packets).

   Some IP Router Alert implementations may be able to take into account
   the next level protocol as a discriminator for the punting decision
   for different protocols using IP Router Alert.  However, this still
   only allows very coarse triage among various protocols using IP
   Router Alert for two reasons.  First, the next level protocol is the
   same when IP Router Alert is used for different applications of the
   same protocol (e.g., RSVP vs. RSVP-TE), or when IP Router Alert is
   used for different contexts of the same application (e.g., different
   levels of RSVP aggregation [RFC3175]).  Thus, it is not possible to
   achieve the necessary triage in the fast path across IP Router Alert
   packets from different applications or from different contexts of an
   application.  Secondly, some protocols requiring punting may be
   carried over a transport protocol (e.g., TCP or UDP) possibly because
   they require the services of that transport protocol or perhaps
   because the protocol does not justify allocation of a scarce next
   level protocol value.  Thus, considering the next level protocol does
   not allow triage in the fast path of IP Router Alert packets from
   different protocols sharing the same transport protocol.  Therefore,
   it is generally not possible to ensure that only the IP Router Alert
   packets of interest are punted to the slow path while other IP Router
   Alert packets are efficiently forwarded (i.e., in fast path).

   Some IP Router Alert implementations may be able to take into account
   the value field inside the router alert option.  However, only one
   value (zero) was defined in [RFC2113] and no IANA registry for IPv4
   Router Alert values was available until recently.  So this did not



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   allow most IPv4 Router Alert implementation to support useful
   classification based on the value field in the fast path.  Also,
   while [RFC2113] states that unknown values should be ignored (i.e.
   The packets should be forwarded as normal IP traffic), it has been
   reported that some existing implementations simply ignore the value
   field completely (i.e.  Process any packet with an IPv4 Router Alert
   regardless of its option value).  An IANA registry for further
   allocation of IPv4 Router Alert values has been introduced recently
   ([RFC5350]) but this would only allow coarse-grain classification,
   when, and if, supported by implementations.  For IPv6, [RFC2711]
   states that "the value field can be used by an implementation to
   speed processing of the datagram within the transit router" and
   defines an IANA registry for these values.  But again, this only
   allows coarse-grain classification.  Besides, some existing IPv6
   Router Alert implementations are reported to depart from that
   behavior.

   [RFC2711] mentions that limiting, by rate or some other means, the
   use of IP Router Alert option is a way of protecting against a
   potential attack.  However, if rate limiting is used as a protection
   mechanism but if the granularity of the rate limiting is not fine
   enough to distinguish among IP Router Alert packet of interest from
   unwanted IP Router Alert packet, a IP Router Alert attack could still
   severely degrade operation of protocols of interest that depend on
   the use of IP Router Alert.

   In a nutshell, the IP router alert option does not provide a
   convenient universal mechanism to accurately and reliably distinguish
   between IP Router Alert packets of interest and unwanted IP Router
   Alert packets.  This, in turn, creates a security concern when IP
   Router Alert option is used, because, short of appropriate router
   implementation specific mechanisms, the router slow path is at risk
   of being flooded by unwanted traffic.

   It can be observed that opening up a hole in the control plane of
   Service Provider routers is commonly done for other applications,
   such as BGP peering.  Depending on the actual environment and BGP
   security practises, the resulting DOS attack vector is similar, or
   somewhat less serious, with BGP peering than with Router Alert option
   for a number of reasons that include:

   o  With BGP, edge routers only exchange control plane information
      with pre-identified peers and can easily filter out any control
      plane traffic coming from other peers or non-authenticated peers,
      while the Router-Alert option can be received in a datagram with
      any source address and any destination source.  However, we note
      that effectiveness of such BGP filtering is dependent on proper
      security practises; poor BGP security practices (such as



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      infrequent or inexistent update of BGP peers authentication keys)
      create vulnerabilities through which the BGP authentication
      mechanisms can be compromised.

   o  with BGP Peering, the control plane hole is only open on the edge
      routers, and core routers are completely isolated from any direct
      control plane exchange with entities outside the administrative
      domain.  Thus, with BGP, a DOS attack would only affect the edge
      routers, while with Router Alert option, the attack could
      propagate to core routers.  However, in some BGP environments, the
      distinction between edge and core routers is not strict, and many/
      most/all routers act as both edge and core routers; in such BGP
      environments, a large part of the network is exposed to direct
      control plane exchanges with entities outside the administrative
      domain (as it would be with Router Alert).

   o  with BGP, the BGP policy control would typically prevent re-
      injection of undesirable information out of the attacked device,
      while with the Router-Alert option, the non-filtered attacking
      messages would typically be forwarded downstream.  However, we
      note that there has been real life occurences of situations where
      incorrect information was propagated through the BGP system,
      causing quite widespread problems.




























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4.  Guidelines for use of Router Alert

4.1.  Use of Router Alert End-to-End In the Internet (Router Alert in
      Peer Model)

   Because of the security concerns associated with Router Alert
   discussed in Section 3, network operators need to actively protect
   themselves against externally generated IP Router Alert packets.
   Because there is no convenient universal mechanisms to triage between
   desired and undesired router alert packets, network operators
   currently often protect themselves in ways that isolate them from
   externally generated IP Router Alert packets.  This may (ideally) be
   achieved by tunneling IP Router Alert packets
   [I-D.ietf-mpls-ip-options] so that the IP Router Alert option is
   hidden through that network, or it may be achieved via mechanisms
   resulting in occasional (e.g., rate limiting) or systematic drop of
   IP Router Alert packets.

   Thus, it is RECOMMENDED that applications and protocols not be
   deployed with a dependency on processing of the Router Alert option
   (as currently specified) across independent administrative domains in
   the Internet.  Figure 1 illustrates such a hypothetical use of Router
   Alert end-to-end in the Internet.  We refer to such a model of Router
   Alert option use as a "Peer Model" Router Alert option use, since
   core routers in different administrative domains would partake in
   processing of Router Alert option datagrams associated with the same
   signalling flow.


      --------         --------          --------          --------
     /   A    \       /   B    \        /   C    \        /   D    \
     | (*)    |       | (*)    |        | (*)    |        | (*)    |
     | | |<============>| |<=============>| |<=============>| |    |
     |  -     |       |  -     |        |  -     |        |  -     |
     \        /       \        /        \        /        \        /
      --------         --------          --------          --------

   (*) closer examination of Router Alert option datagrams

   <==>  flow of Router Alert option datagrams

   Figure 1: Use of Router Alert End-to-End in the Open Internet (Router
                           Alert in Peer Model)

   While this recommendation is framed here specifically in the context
   of router alert, the fundamental security risk that network operators
   want to preclude is to allow devices/protocols that are outside of
   their administrative domain (and therefore not controlled) to tap



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   into the control plane of their core routers.  Whether this control
   plane access is provided through router alert option or would be
   provided by any other mechanism (e.g.  Deep packet inspection)
   probably results in similar security concerns.  In other words, the
   fundamental security concern is associated with the notion of end to
   end signaling in a Peer Model across domains in the Internet.  As a
   result, it is expected that network operators would typically not
   want to have their core routers partake in end-to-end signalling with
   external uncontrolled devices through the open Internet, and
   therefore prevent deployment of end to end signalling in a Peer model
   through their network (regardless of whether that signalling uses
   Router Alert or not).

4.2.  Use of Router Alert In Controlled Environments

4.2.1.  Use of Router Alert Within an Administrative Domain

   In some controlled environments such as within a given Administrative
   Domain, the network administrator can determine that IP Router Alert
   packets will only be received from trusted well-behaved devices or
   can establish that specific protection mechanisms (e.g., RAO
   filtering and rate-limiting) against the plausible RAO-based DoS
   attacks are sufficient.  In that case, an application relying on
   exchange and handling of RAO packets (e.g., RSVP) MAY be safely
   deployed within the controlled network.  A private enterprise network
   firewalled from the Internet and using RSVP reservations for voice
   and video flows may be an example of such controlled environment.
   Such an environment is illustrated in Figure 2.


      -------------------------          --------          --------
     /            A            \        /   B    \        /   C    \
     | (*)              (*)    |   --   |        |        |        |
     | | |<============>| |    |--|FW|--|        |--------|        |
     |  -                -     |   --   |        |        |        |
     \                         /        \        /        \        /
      -------------------------          --------          --------

   (*) closer examination of Router Alert option datagrams

   <==>  flow of Router Alert option datagrams

   FW Firewall


       Figure 2: Use of Router Alert Within an Administrative Domain

   In some controlled environments, several Administrative Domains have



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   a special relationship whereby they cooperate very tightly and
   effectively operate as a single trust domain.  In that case, one
   domain is willing to trust another with respect to the traffic
   injected across the boundary.  In other words, a downstream domain is
   willing to trust that the traffic injected at the boundary has been
   properly validated/filtered by the upstream domain.  Where it has
   been established that such trust can be applied to router alert
   option packets, an application relying on exchange and handling of
   RAO packets (e.g., RSVP) MAY be safely deployed within such a
   controlled environment.  The entity within a company responsible for
   operating multimedia endpoints and the entity within the same company
   responsible for operating the network may be an example of such
   controlled environment.  For example, they may collaborate so that
   RSVP reservations can be used for video flows from endpoints to
   endpoints through the network.

   In some environments, the network administrator can reliably ensure
   that router alert packets from any untrusted device (e.g., from
   external routers) are prevented from entering a trusted area (e.g.,
   the internal routers).  For example, this may be achieved by ensuring
   that routers straddling the trust boundary (e.g., edge routers)
   always encapsulate those packets (without setting IP Router Alert -or
   equivalent- in the encapsulating header) through the trusted area (as
   discussed in [I-D.ietf-mpls-ip-options]).  In such environments, the
   risks of DOS attacks through the IP Router Alert vector is removed in
   the trusted area (or greatly reduced) even if IP Router Alert is used
   inside the trusted area (say for RSVP-TE).  Thus an application
   relying on IP Router Alert MAY be safely deployed within the trusted
   area.  A Service Provider running RSVP-TE within his network may be
   an example of such protected environment.  Such an environment is
   illustrated in Figure 3.




















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      --------         --------------------------          --------
     /   A    \       /             B            \        /   C    \
     |        |       |  (*)               (*)   |        |        |
     |        |-------TT | |<=============>| |  TT------- |        |
     |        |       |   -                 -    |        |        |
     \        /       \                          /        \        /
      --------         --------------------------          --------


   (*) closer examination of Router Alert option datagrams

   <==>  flow of Router Alert option datagrams

   TT   Tunneling of Router Alert option datagrams

       Figure 3: Use of Router Alert Within an Administrative Domain

4.2.2.  Use of Router Alert In Overlay Model

   In some controlled environment:

   o  the sites of a network A are interconnected through a service
      provider network B

   o  the service provider network B protects itself from IP Router
      Alert messages without dropping those when they transit over the
      transit network (for example using mechanisms discussed in
      [I-D.ietf-mpls-ip-options])

   In such controlled environment, an application relying on exchange
   and handling of RAO packets (e.g., RSVP) in the network A sites (but
   not inside network B) MAY be safely deployed.  We refer to such a
   deployment as a use of Router Alert in a Water-Tight Overlay.
   "Overlay" because Router Alert option datagrams are used in network A
   on top of, and completely transparently to, network B. "Water-Tight"
   because router alert option datagrams from A cannot leak inside
   network B. A private enterprise intranet, whose sites are
   interconnected through a Service Prover network, and using RSVP to
   perform reservations within the enterprise sites for voice and video
   flows may be an example of such controlled environment.  Such an
   environment is illustrated in Figure 4.










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      --------                                --------
     /   A    \                              /   A    \
     | (*)    |                              | (*)    |
     | | |<=================================>| | |    |
     |  -     |                              |  -     |
     \        /                              \        /
      --------                                --------
            \                                 /
             \   -------------------------   /
              \ /           B             \ /
               \|                         |/
                TT                       TT
                |                         |
                \                         /
                 -------------------------


   (*) closer examination of Router Alert option datagrams

   <==>  flow of Router Alert option datagrams

   TT   Tunneling of Router Alert option datagrams

           Figure 4: Use of Router Alert In Water-tight Overlay

   In the controlled environment described above, an application relying
   on exchange and handling of RAO packets (e.g.  RSVP-TE) in the
   service provider network B (but not in network A) MAY also be safely
   deployed simultaneously.  Such an environment with independent,
   isolated, deployment of router alert in overlay at two levels is
   illustrated in Figure 5.




















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      --------                                --------
     /   A    \                              /   A    \
     | (*)    |                              | (*)    |
     | | |<=================================>| | |    |
     |  -     |                              |  -     |
     \        /                              \        /
      --------                                --------
            \                                 /
             \   -------------------------   /
              \ /           B             \ /
               \|  (*)              (*)   |/
                TT | |<============>| | TT
                |   -                -    |
                \                         /
                 -------------------------


   (*) closer examination of Router Alert option datagrams

   <==>  flow of Router Alert option datagrams

   TT   Tunneling of Router Alert option datagrams

    Figure 5: Use of Router Alert In Water-tight Overlay at Two Levels

   In some controlled environment:

   o  the sites of a network A are interconnected through a service
      provider network B

   o  the service provider B processes router alert packets on the edge
      routers and protect these edge routers against RAO based attacks
      using mechanisms such as (possibly per port) RAO rate limiting and
      filtering

   o  the service provider network B protects its core routers from
      Router Alert messages without dropping those when they transit
      over the transit network (for example using mechanisms discussed
      in [I-D.ietf-mpls-ip-options])

   In such controlled environment, an application relying on exchange
   and handling of RAO packets (e.g., RSVP) in the network A sites and
   in network B Edges (but not in the core of network B) MAY be safely
   deployed.  We refer to such a deployment as a use of Router Alert in
   a Leak-Controlled Overlay.  "Overlay" because Router Alert option
   datagrams are used in network A on top of, and completely
   transparently to, network B core.  "Leak-Controlled" because router
   alert option datagrams from A leak inside network B's B edges but not



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   inside network B's core.  A private enterprise intranet, whose sites
   are interconnected through a Service Prover network, using RSVP for
   voice and video within network A sites as well as on Network B's edge
   to extend the reservation onto the attachment links between A and B
   (as specified in [RFC6016]) may be an example of such controlled
   environment.  Such an environment is illustrated in Figure 4.

      --------                                --------
     /   A    \                              /   A    \
     |        |                              |        |
     |        |   ------------------------   |        |
     | (*)    |  /(*)              (*)    \  | (*)    |
     | | |<======>| |<============>| |<=====>| | |    |
     |  -     |  | -                -     |  |  -     |
     \        /  |  \    -     -   /      |  \        /
      --------   |   TT-| |   | |-TT      |   --------
                 |       -     -          |
                 \                        /
                  ------------------------


   (*) closer examination of Router Alert option datagrams

   <==>  flow of Router Alert option datagrams

   TT   Tunneling of Router Alert option datagrams

         Figure 6: Use of Router Alert In Leak-Controlled Overlay

4.3.  Router Alert Protection Approaches for Service Providers

   Section 3 discusses the security risks associated with the use of the
   IP Router Alert and how it opens up a DOS vector in the router
   control plane.  Thus, it is RECOMMENDED that a Service Provider
   implements strong protection of his network against attacks based on
   IP Router Alert.

   As discussed in Section 4.2.2 some applications can benefit from the
   use of IP Router Alert packets in an Overlay model (i.e.  Where
   Router Alert packets are exchanged transparently on top of a Service
   Provider).  Thus, it is RECOMMENDED that a Service Provider protects
   his network from attacks based on IP Router Alert using mechanisms
   that avoid (or at least minimize) dropping of end to end IP Router
   Alert packets (other than those involved in an attack).

   For example, if the Service Provider does not run any protocol
   depending on IP Router Alert within his network, he may elect to
   simply turn-off punting/processing of IP Router Alert packet on his



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   routers; this will ensure that end-to-end IP Router Alert packet
   transit transparently and safely through his network.

   As another example, using protection mechanisms such selective
   filtering and rate-limiting (that Section 5 suggests be supported by
   IP Router Alert implementations) a Service Provider can protect the
   operation of a protocol depending on IP Router Alert within his
   network (e.g., RSVP-TE) while at the same time transporting IP Router
   Alert packets carrying another protocol that may be used end to end.
   Note that the Service Provider might additionally use protocol
   specific mechanisms that reduce the dependency on Router Alert for
   operation of this protocol inside the Service Provider environment;
   use of RSVP refresh reduction mechanisms ([RFC2961]) would be an
   example of such mechanisms in the case where the Service Provider is
   running RSVP-TE within his network since this allows refresh of
   existing Path and Resv states without use of the IP Router Alert
   option.

   As yet another example, using mechanisms such as those discussed in
   [I-D.ietf-mpls-ip-options] a Service Provider can safely protect the
   operation of a protocol depending on IP Router Alert within his
   network (e.g., RSVP-TE) while at the same time safely transporting IP
   Router Alert packets carrying another protocol that may be used end
   to end (e.g., IPv4/IPv6 RSVP).  We observe that while tunneling of
   Router Alert option datagrams over an MPLS backbone as discussed in
   [I-D.ietf-mpls-ip-options] is well understood, tunnelling Router
   Alert option datagrams over an non-MPLS IP backbone presents a number
   of issues (and in particular for determining where to forward the
   encapsulated datagram) and is not common practise at the time of
   writing this document.

   As a last resort, if the SP does not have any means to safely
   transport end to end IP Router Alert option packets over his network,
   the SP MAY drop those packets.  It must be noted that this has the
   undesirable consequence of preventing the use of the Router Alert
   option in the Overlay Model on top of this network, and therefore
   prevents users of that network from deploying a number of valid
   applications/protocols in their environment.













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5.  Guidelines for Router Alert Implementation

   It is RECOMMENDED that router implementations of IP Router Alert
   option include protection mechanisms against Router Alert based DOS
   attacks appropriate for their targeted deployment environments.  For
   example, this can include ability on an edge router to "tunnel" IP
   Router Alert option of received packets when forwarding those over
   the core as discussed in [I-D.ietf-mpls-ip-options].  As another
   example, although not always available from current implementations,
   new implementations MAY include protection mechanisms such as
   selective (possibly dynamic) filtering and rate-limiting of IP Router
   Alert option packets.

   If an IP packet contains the IP Router Alert option, but the next
   level protocol is not explicitly identified as a protocol of interest
   by the router examining the packet, the behavior is not explicitly
   defined by [RFC2113].  However, the behavior is implied and, for
   example, the definition of RSVP in [RFC2205] assumes that the packet
   will be forwarded using normal forwarding based on the destination IP
   address.  Thus, a router implementation SHOULD forward within the
   "fast path" (subject to all normal policies and forwarding rules) a
   packet carrying the IP Router Alert option containing a next level
   protocol that is not a protocol of interest to that router.  The "not
   punting" behavior protects the router from DOS attacks using IP
   Router Alert packets of a protocol unknown to the router.  The
   "forwarding" behavior contributes to transparent end to end transport
   of IP Router Alert packets (e.g., to facilitate their use by end to
   end application).

   Similarly, an implementation MAY support selective forwarding within
   "the fast path" (subject to all normal policies and forwarding rules)
   or punting of a packet with the IP Router Alert Option, based on the
   Value field of the Router Alert Option.  This would allow router
   protection against DOS attacks using IP Router Alert packets with
   value that is not relevant for that router (e.g. nesting levels of
   Aggregated RSVP Reservation [RFC5350]).















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6.  Security Considerations

   This document discusses security risks associated with current usage
   of the IP Router Alert Option and associated practices.















































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7.  IANA Considerations

   None.
















































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8.  Contributors

   The contributors to this document (in addition to the editors) are:

   o  Reshad Rahman:

      *  Cisco Systems

      *  rrahman@cisco.com

   o  David Ward:

      *  Juniper Networks

      *  dward@juniper.net

   o  Ashok Narayanan:

      *  Cisco Systems

      *  ashokn@cisco.com

   o  Adrian Farrell:

      *  OldDog Consulting

      *  adrian@olddog.co.uk

   o  Tony Li:

      *  tony.li@tony.li




















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9.  Acknowledgments

   We would like to thank Dave Oran, Magnus Westerlund, John Scudder,
   Ron Bonica, Ross Callon, Alfred Hines and Carlos Pignataro for their
   comments.  This document also benefited from discussions with Jukka
   Manner and Suresh Krishnan.  The discussion about use of the value
   field in the IPv4 Router Alert borrowed from a similar discussion in
   [I-D.ietf-nsis-ntlp].











































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10.  References

10.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC1883]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 1883, December 1995.

   [RFC2113]  Katz, D., "IP Router Alert Option", RFC 2113,
              February 1997.

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

   [RFC2711]  Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
              RFC 2711, October 1999.

   [RFC5350]  Manner, J. and A. McDonald, "IANA Considerations for the
              IPv4 and IPv6 Router Alert Options", RFC 5350,
              September 2008.

10.2.  Informative References

   [I-D.ietf-mpls-ip-options]
              Jaeger, W., Mullooly, J., Scholl, T., and D. Smith,
              "Requirements for Label Edge Router Forwarding of IPv4
              Option Packets", draft-ietf-mpls-ip-options-04 (work in
              progress), May 2010.

   [I-D.ietf-nsis-ntlp]
              Schulzrinne, H. and M. Stiemerling, "GIST: General
              Internet Signalling Transport", draft-ietf-nsis-ntlp-20
              (work in progress), June 2009.

   [I-D.krishnan-ipv6-hopbyhop]
              Krishnan, S., "The case against Hop-by-Hop options",
              draft-krishnan-ipv6-hopbyhop-05 (work in progress),
              October 2010.

   [I-D.narayanan-rtg-router-alert-extension]
              Narayanan, A., Faucheur, F., Ward, D., and R. Rahman, "IP
              Router Alert Option Extension",
              draft-narayanan-rtg-router-alert-extension-00 (work in
              progress), March 2009.

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



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              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,
              October 1999.

   [RFC2961]  Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.,
              and S. Molendini, "RSVP Refresh Overhead Reduction
              Extensions", RFC 2961, April 2001.

   [RFC3175]  Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie,
              "Aggregation of RSVP for IPv4 and IPv6 Reservations",
              RFC 3175, September 2001.

   [RFC3208]  Speakman, T., Crowcroft, J., Gemmell, J., Farinacci, D.,
              Lin, S., Leshchiner, D., Luby, M., Montgomery, T., Rizzo,
              L., Tweedly, A., Bhaskar, N., Edmonstone, R.,
              Sumanasekera, R., and L. Vicisano, "PGM Reliable Transport
              Protocol Specification", RFC 3208, December 2001.

   [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.

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, October 2002.

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC4081]  Tschofenig, H. and D. Kroeselberg, "Security Threats for
              Next Steps in Signaling (NSIS)", RFC 4081, June 2005.

   [RFC4286]  Haberman, B. and J. Martin, "Multicast Router Discovery",
              RFC 4286, December 2005.

   [RFC6016]  Davie, B., Le Faucheur, F., and A. Narayanan, "Support for
              the Resource Reservation Protocol (RSVP) in Layer 3 VPNs",
              RFC 6016, October 2010.







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Author's Address

   Francois Le Faucheur (editor)
   Cisco Systems
   Greenside, 400 Avenue de Roumanille
   Sophia Antipolis  06410
   France

   Phone: +33 4 97 23 26 19
   Email: flefauch@cisco.com









































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