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Network Working Group                                     Camilo Cardona
Internet-Draft                                       IMDEA Networks/UC3M
Intended status: Informational                           Pierre Francois
Expires: August 27, 2015                                  IMDEA Networks
                                                           Paolo Lucente
                                                           Cisco Systems
                                                       February 23, 2015


        Impact of BGP filtering on Inter-Domain Routing Policies
                  draft-ietf-grow-filtering-threats-05

Abstract

   This document describes how unexpected traffic flows can emerge
   across an autonomous system, as the result of other autonomous
   systems filtering, or restricting the propagation of more specific
   prefixes.  We provide a review of the techniques to detect the
   occurrence of this issue and defend against it.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on August 27, 2015.

Copyright Notice

   Copyright (c) 2015 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



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Unexpected Traffic Flows  . . . . . . . . . . . . . . . . . .   4
     2.1.  Local filtering . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Unexpected traffic flows caused by local filtering of
               more specific prefixes  . . . . . . . . . . . . . . .   5
     2.2.  Remote filtering  . . . . . . . . . . . . . . . . . . . .   6
       2.2.1.  Unexpected traffic flows caused by remotely triggered
               filtering of more specific prefixes . . . . . . . . .   7
   3.  Techniques to detect unexpected traffic flows caused by
       filtering of more specific prefixes . . . . . . . . . . . . .   8
     3.1.  Existence of unexpected traffic flows within an AS  . . .   8
     3.2.  Contribution to the existence of unexpected traffic flows
           in another AS . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Techniques to counter unexpected traffic flows  . . . . . . .  10
     4.1.  Reactive counter-measures . . . . . . . . . . . . . . . .  11
     4.2.  Proactive counter-measures  . . . . . . . . . . . . . . .  12
       4.2.1.  Access lists  . . . . . . . . . . . . . . . . . . . .  12
       4.2.2.  Neighbor-specific forwarding  . . . . . . . . . . . .  13
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Informative References  . . . . . . . . . . . . . . . . .  14
     9.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   It is common practice for network operators to propagate a more
   specific prefix in the BGP routing system, along with the less
   specific prefix that they originate.  It is also possible for some
   Autonomous Systems (ASes) to apply different policies to the more
   specific and the less specific prefix.

   While BGP makes independent, policy driven decisions for the
   selection of the best path to be used for a given IP prefix, routers
   must forward packets using the longest-prefix-match rule, which
   "precedes" any BGP policy (RFC1812 [1]).  The existence of a prefix p
   that is more specific than a prefix p' in the Forwarding Information
   Base (FIB) will let packets whose destination matches p be forwarded



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   according to the next hop selected as best for p (the more specific
   prefix).  This process takes place by disregarding the policies
   applied in the control plane for the selection of the best next-hop
   for p'.  When an Autonomous System filters more specific prefixes and
   forwards packets according to the less specific prefix, the
   discrepancy in the routing policies applied to the less and the more
   specific prefixes can create unexpected traffic flows that infringe
   the policies of other ASes, still holding a path towards the more
   specific prefix.

   The objective of this draft is to shed light on possible side effects
   associated with more specific prefix filtering.  This document
   presents examples of such side effects and discusses approaches
   towards solutions to the problem.

   The rest of the document is organized as follows: In Section 2 we
   provide some scenarios in which the filtering of more specific
   prefixes leads to the creation of unexpected traffic flows.
   Section 3 and Section 4 discuss some techniques that ASes can use
   for, respectively, detect and react to unexpected traffic flows.  We
   conclude in Section 5.

1.1.  Terminology

   More specific prefix: A prefix in the routing table with an address
   range that is covered by a shorter prefix also present in the routing
   table.

   Less specific prefix: A prefix in the routing table with an address
   range partially covered by other prefixes.

   We re-use the definitions of customer-transit peering and settlement-
   free peering of RFC4384 [2].

   Selective advertisement: The behavior of only advertising a self
   originated BGP path for a prefix over a strict subset of the eBGP
   sessions of the AS.

   Selective propagation: The behavior of only propagating a BGP path
   for a prefix over a strict subset of the eBGP sessions of an AS.

   Local filtering: The behavior of explicitly ignoring a BGP path
   received over an eBGP session.

   Remote filtering: The behavior of triggering selective propagation of
   a BGP path at a distant AS.  Note that this is typically achieved by
   tagging a self-originated path with BGP communities defined by the
   distant AS.



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   Unexpected traffic flow: Traffic flowing between two neighboring ASes
   of an AS, although the transit policy of that AS is to not provide
   connectivity between these two neighbors.  A traffic flow across an
   AS, between two of its transit providers, or between a transit
   provider and one of its settlement-free peers, are classical examples
   of unexpected traffic flows.

2.  Unexpected Traffic Flows

   In this section, we describe how more specific prefix filtering can
   lead to unexpected traffic flows in other, remote, ASes.  We
   differentiate cases in which the filtering is performed locally from
   those where the filtering is triggered remotely.

2.1.  Local filtering

   Local filtering can be motivated by different reasons, such as: (1)
   Traffic engineering, where an AS wants to control its local outbound
   traffic distribution using only the policy applied to the less
   specific prefix.  Such a practice was notably documented in [3] (2)
   Enforcing contract compliance, where, for instance, an AS avoids a
   settlement-free peer to attract traffic to one link by using
   selective advertisement, when this is not allowed by their peering
   agreement. (3) The need for Forwarding Information Base memory
   preservation sometimes pushes ISP operators to filter more specific
   prefixes.

   Figure 1 illustrates a scenario in which one AS is motivated to
   perform local filtering due to outbound traffic engineering.  The
   figure depicts AS64504, and two of its neighboring ASes, AS64502 and
   AS64505.  AS64504 has a settlement-free peering with AS64502 and is a
   customer of AS64505.  AS64504 receives from AS64505 prefixes
   2001:DB8::/32 and 2001:DB8::/34, a less specific and a more specific
   prefix, respectively.  AS64504 receives only the less specific prefix
   2001:DB8::/32 from AS64502.
















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                 ,-----.
                /       \
               ( AS64505 )
                \       /
                 `--+--'
    2001:DB8::/32 | |
    2001:DB8::/34 v |
                    |
                 ,--+--.  2001:DB8::/32  ,-----.
                /       \           <-- /       \
               ( AS64504 )-------------( AS64502 )
                \       /               \       /
                 `-----'                 `-----'


                         Figure 1: Local Filtering

   Due to economic reasons, AS64504 might prefer to send traffic to
   AS64502 instead of AS64505.  However, even if paths received from
   AS64502 are given a large local preference, routers in AS64504 will
   still send traffic to prefix 2001:DB8::/34 via neighbor AS64505.
   This situation may push AS64504 to apply an inbound filter for the
   more specific prefix, 2001:DB8::/34, on the session with AS64505.
   After the filter is applied, traffic to the more specific prefix will
   be sent to AS64502

2.1.1.  Unexpected traffic flows caused by local filtering of more
        specific prefixes

   In this section, we show how the decision of AS64504 to perform local
   filtering creates unexpected traffic flows in AS64502.  Figure 2
   shows the whole picture of the scenario; where AS64501 is a customer
   of AS64503 and AS64502.  AS64503 is a settlement-free peer with
   AS64502.  AS64503 and AS64502 are customers of AS64505.  The AS
   originating the two prefixes, AS64501, performs selective
   advertisement with the more specific prefix and only announces it to
   AS64503.

   After AS64504 locally filters the more specific prefix, traffic from
   AS64504 to prefix 2001:DB8::/34 is forwarded towards AS64502.
   Because AS64502 receives the more specific prefix from AS64503,
   traffic from AS64504 to 2001:DB8::/34 follows the path
   AS64504-AS64502-AS64503-AS64501.  AS64502's BGP policies are
   implemented to avoid transporting traffic between AS64504 and
   AS64503.  However, due to the discrepancies of routes between the
   more and the less specific prefixes, unexpected traffic flows between
   AS64504 and AS64503 exist in AS64502's network.




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                          ____,,................______
                _,.---''''                            `''---..._
            ,-''   AS64505                                      `-.
            [                                                      /
             -.._                                             __.-'
              .  `'---....______                ______...---''
            + |/32              `'''''''''''''''         |
            | |/34               + |/32                  |
            v |                  v |/34                  |
              |                    |                   ^ |
              |                  ^ |/32                | |/32
              |                  + |                   + |/34
       _,,---.:_               _,,---.._              _,,---.._
     ,'         `.           ,'         `.          ,'         `.
    /  AS64504    \     <-+ /  AS64502    \        /  AS64503    \
    |             |_________|             |________|             |
    |             |     /32 |             |/32  /32|             |
    '.           ,'          .           ,'     /34 .           ,'
      `.       ,'             `.       ,'  +->  <-+  `.       ,'
        ``---''                 ``---''                ``---''
                                    |                  ^ |
                                  ^ |2001:DB8::/32     | |2001:DB8::/32
                                  | |                  + |2001:DB8::/34
                                  + | _....---------...._|
                                   ,-'AS64501            ``-.
                                 /'                          `.
                                 `.                         _,
                                   `-.._               _,,,'
                                        `''---------'''


         Figure 2: Unexpected traffic flows due to local filtering

2.2.  Remote filtering

   ISPs can tag the BGP paths that they propagate to neighboring ASes
   with communities, in order to tweak the propagation behavior of the
   ASes that handle these paths [1].  Some ISPs allow their customers to
   use such communities to let the receiving AS not export the path to
   some selected neighboring ASes.  By combining communities, the prefix
   could be advertised only to a given peer of the AS providing this
   feature.  Remote filtering can be leveraged by an AS to, for
   instance, limit the scope of prefixes and hence perform a more
   granular inbound traffic engineering.

   Figure 3 illustrates a scenario in which an AS uses BGP communities
   to command its provider to selectively propagate a more specific
   prefix.  Let AS64501 be a customer of AS64502 and AS64503.  AS64501



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   originates prefix 2001:DB8::/32, which it advertises to AS64502 and
   AS64503.  AS64502 and AS64503 are settlement-free peers.  Let AS64501
   do selective advertisement and only propagate 2001:DB8::/34 over
   AS64503.  AS64503 would normally propagate this prefix to its
   customers, providers, and peers, including AS64502.

   Let us consider that AS64501 decides to limit the scope of the more
   specific prefix.  AS64501 can make this decision based on its traffic
   engineering strategy.  To achieve this, AS64501 can tag the more
   specific prefix with a set of communities that leads AS64503 to only
   propagate the path to AS64502.

      ^     \         /     ^       ^    \         /    ^
      |  /32 \       / /32  |       | /32 \       / /32 |
               ,-----.                     ,-----.
             ,'       `.                 ,'       `.
            / AS64502   \               / AS64503   \
           (             )-------------(             )
            \           / /32       /32 \           /
             `.       ,'   ->       /34  `.       ,'
               '-----;              <-  /  '-----'
                      \                /
                    ^  \              /    ^
                    |   \            /     |
                    |    \          /      |
                    |     \ ,-----.'       |  2001:DB8::/32
                    |     ,'       `.      |  2001:DB8::/34
      2001:DB8::/32 +--  / AS64501   \   --+
                        (             )
                         \           /
                          `.       ,'
                            '-----'

                   Figure 3: Remote triggered filtering

2.2.1.  Unexpected traffic flows caused by remotely triggered filtering
        of more specific prefixes

   Figure 4 expands the scenario from Figure 3 and includes other AS
   peering with ASes 64502 and 64503.  Due to the limitation on the
   scope performed on the more specific prefix, ASes that are not
   customers of AS64502 will not receive a path for 2001:DB8::/34.
   These ASes will forward packets destined to 2001:DB8::/34 according
   to their routing state for 2001:DB8::/32.  Let us assume that AS64505
   is such an AS, and that its best path towards 2001:DB8::/32 is
   through AS64502.  Packets sent towards 2001:DB8::1 by AS64505 will
   reach AS64502.  However, in the data-plane of the nodes of AS64502,
   the longest prefix match for 2001:DB8::1 is 2001:DB8::/34, which is



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   reached through AS64503, a settlement-free peer of AS64502.  Since
   AS64505 is not in the customer branch of AS64502, we are in a
   situation in which traffic flows between non-customer ASes take place
   in AS64502.

                  ,-----.
                ,'       `. -------  Connections to other ASes
               / AS64505   \         /32
              (             )        <-+
               \           /
                `.       ,'
                  '-----'
                    ^    \         /     ^       ^    \         /    ^
                    | /32 \       / /32  |       | /32 \       / /32 |
                    +       ,-----.      +       +      ,-----.      +
                          ,'       `.                 ,'       `.
                         / AS64502   \               / AS64503   \
                        (             )-------------(             )
        ,-----.          \           / /32       /32 \           /
      ,'       `.---------`.       ,'  +->       /34  `.       ,'
     / AS64504   \    /32   '-----;              <-+ /  '-----'
    (             )   /34          \                /
     \           /    <-+        ^  \              /    ^
      `.       ,'                |   \            /     |
        '-----;                  |    \          /      |
                                 |     \ ,-----.'       |  2001:DB8::/32
                                 |     ,'       `.      |  2001:DB8::/34
                   2001:DB8::/32 +--+ / AS64501   \  +--+
                                     (             )
                                      \           /
                                       `.       ,'
                                         '-----'

   Figure 4: Unexpected traffic flows due to remote triggered filtering

3.  Techniques to detect unexpected traffic flows caused by filtering of
    more specific prefixes

3.1.  Existence of unexpected traffic flows within an AS

   To detect if unexpected traffic flows are taking place in its
   network, an ISP can monitor its traffic data to check if it is
   providing transit between two of its peers, although his policy is
   configured to not provide such transit.  IPFIX (RFC7011 [3]) is an
   example of a technology that can be used to export information
   regarding traffic flows across the network.  Traffic information must
   be analyzed under the perspective of the business relationships with




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   neighboring ASes.  Open source tools such as [4] can be used to this
   end.

   Note that the AS detecting the unexpected traffic flow may simply
   realize that his policy configuration is broken.  The first
   recommended action upon detection of an unexpected traffic flow is to
   verify the correctness of the BGP configuration.

   Once it has been assessed that the local configuration is correct,
   the operator should check if the problem detected in the data-plane
   arose due to filtering of BGP paths by neighboring ASes.  The
   operator should check if the destination address of the unexpected
   traffic flow is locally routed as per a more specific prefix only
   received from non-customer peers.  The operator should also checks if
   there are paths to a less specific prefix received from a customer,
   and hence propagated to peers.  If these two situations happen at the
   same time, the neighboring AS at the entry point of the unexpected
   flow is routing the traffic based on the less specific prefix,
   although the ISP is actually forwarding the traffic via non-customer
   links.

   To detect the origin of the problem, human interaction or looking
   glasses can be used in order to find out whether local filtering,
   remote filtering, or selective propagation was performed on the more
   specific prefix.  Due to the distributed nature and restricted
   visibility of the steering of BGP policies, such analysis is deemed
   to not identify the origin of the problem with guaranteed accuracy.
   We are not aware, at the time of this writing, of any openly
   available tool that can automatically perform this operation.

3.2.  Contribution to the existence of unexpected traffic flows in
      another AS

   It can be considered problematic to be causing unexpected traffic
   flows in other ASes.  It is thus advisable for an AS to assess the
   risks of filtering more specific prefixes before implementing them by
   obtaining as much data information as possible about its surrounding
   routing environment.

   There may be justifiable reasons for one ISP to perform filtering;
   either to enforce established policies or to provide prefix
   advertisement scoping features to its customers.  These can vary from
   trouble-shooting purposes to business relationship implementations.
   Restricting the use of these features for the sake of avoiding the
   creation of unexpected traffic flows is not a practical option.

   In order to assess the rist of filtering more specific prefixes, the
   AS would need information of the routing policies and the



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   relationships among external ASes to detect if its actions could
   trigger the appearance of unexpected traffic flows.  With this
   information, the operator could detect other ASes receiving the more
   specific prefix from non-customer ASes, while announcing the less
   specific prefix to other non-customer ASes.  If the filtering of the
   more specific prefix leads other ASes to send traffic for the more
   specific prefix to these ASes, an unexpected traffic flow can arise.
   However, the information required for this operation is difficult to
   obtain, due to the distributed nature of BGP policies.  We are not
   aware, at the time of this writing, of any openly available tool that
   can automatically perform this procedure.

4.  Techniques to counter unexpected traffic flows

   Network Operators can adopt different approaches with respect to
   unexpected traffic flows.  Note that due the complexity of inter-
   domain routing policies, there is not a single solution that can be
   applied to all situations.  We provide potential solutions that ISPs
   must evaluate against each particular case.  We classify these
   actions according to whether they are proactive or reactive.

   Reactive approaches are those in which the operator tries to detect
   the situations via monitoring and solve unexpected traffic flows,
   manually, on a case-by-case basis.

   Anticipant or preventive approaches are those in which the routing
   system will not let the unexpected traffic flows actually take place
   when the scenario arises.

   We use the scenario depicted in Figure 5 to describe these two kinds
   of approaches.  Based on our analysis, we observe that proactive
   approaches can be complex to implement and can lead to undesired
   effects.  Therefore, we conclude that the reactive approach is the
   more reasonable recommendation to deal with unexpected flows.

















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                         ____,,................______
               _,.---''''                            `''---..._
           ,-''   AS64505                                      `-.
           [                                                      /
            -.._                                             __.-'
             .  `'---....______                ______...---''
           + |/32              `'''''''''''''''         |
           | |/34               + |/32                  |
           v |                  v |/34                  |
             |                    |                   ^ |
             |                  ^ |/32                | |/32
             |                  + |                   + |/34
      _,,---.:_               _,,---.._              _,,---.._
    ,'         `.           ,'         `.          ,'         `.
   /  AS64504    \     <-+ /  AS64502    \        /  AS64503    \
   |             |_________|             |        |             |
   |             |     /32 |             |        |             |
   '.           ,'          .           ,'         .           ,'
     `.       ,'             `.       ,'            `.       ,'
       ``---''                 ``---''                ``---''
                                   |                  ^ |
                                 ^ |2001:DB8::/32     | |2001:DB8::/32
                                 | |                  + |2001:DB8::/34
                                 + | _....---------...._|
                                  ,-'AS64501            ``-.
                                /'                          `.
                                `.                         _,
                                  `-.._               _,,,'
                                       `''---------'''

      Figure 5: Counter-measures for unexpected traffic flows - Base
                                  example

4.1.  Reactive counter-measures

   An operator who detects unexpected traffic flows originated by any of
   the cases described in Section 2 can contact the ASes that are likely
   to have performed the propagation tweaks, inform them of the
   situation, and persuade them to change their behavior.

   If the situation remains, the operator can implement prefix filtering
   in order to stop the unexpected flows.  The operator can decide to
   perform this action over the session with the operator announcing the
   more specific prefix or over the session with the neighboring AS from
   which it is receiving the traffic.  Each of these options carry a
   different repercussion for the affected AS.  We describe briefly the
   two alternatives.




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   o  An operator can decide to stop announcing the less specific prefix
      at the peering session with the neighboring AS from which it is
      receiving traffic to the more specific prefix.  In the example of
      Figure 5, AS64502 would filter out the prefix 2001:DB8::/32 from
      the eBGP session with AS64504.  In this case, traffic heading to
      the prefix 2001:DB8::/32 from AS64501 would no longer traverse
      AS64502.  AS64502 should evaluate if solving the issues originated
      by the unexpected traffic flows are worth the loss of this traffic
      share.

   o  An operator can decide to filter out the more specific prefix at
      the peering session over which it was received.  In the example of
      Figure 5, AS64502 would filter out the incoming prefix
      2001:DB8::/34 from the eBGP session with AS64505.  As a result,
      the traffic destined to that /32 would be forwarded by AS64502
      along its link with AS64501, despite the actions performed by
      AS64501 to have this traffic coming in through its link with
      AS64503.  However, as AS64502 will no longer know a route to the
      more specific prefix, it risks losing the traffic share from
      customers different from AS64501 to that prefix.  Furthermore,
      this action can generate conflicts between AS64502 and AS64501,
      since AS64502 does not follow the routing information expressed by
      AS64501 in its BGP announcements.

   It is possible that the behavior of the neighboring AS causing the
   unexpected traffic flows opposes the peering agreement.  In this
   case, an operator could account the amount of traffic that has been
   subject to the unexpected flows, using traffic measurement protocols
   such as IPFIX, and charge the peer for that traffic.  That is, the
   operator can claim that it has been a provider of that peer for the
   traffic that transited between the two ASes.

4.2.  Proactive counter-measures

4.2.1.  Access lists

   An operator could install access-lists to prevent unexpected traffic
   flows from happening in the first place.  In the example of Figure 5,
   AS64502 would install an access-list denying packets matching
   2001:DB8::/34 associated with the interface connecting to AS64504.
   As a result, traffic destined to that prefix would be dropped,
   despite the existence of a valid route towards 2001:DB8::/32.

   The operational overhead of such a solution is considered high as the
   operator would have to constantly adapt these access-lists to
   accommodate inter-domain routing changes.  Moreover, this technique
   lets packets destined to a valid prefix be dropped while they are
   sent from a neighboring AS that may not know about the policy



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   conflict, and hence had no means to avoid the creation of unexpected
   traffic flows.  For this reason, this technique can be considered
   harmful and is thus not recommended for implementation.

4.2.2.  Neighbor-specific forwarding

   An operator can technically ensure that traffic destined to a given
   prefix will be forwarded from an entry point of the network based
   only on the set of paths that have been advertised over that entry
   point.

   As an example, let us analyze the scenario of Figure 5 from the point
   of view of AS64502.  The edge router connecting to the AS64504
   forwards packets destined to prefix 2001:DB8::/34 towards AS64505.
   Likewise, it forwards packets destined to prefix 2001:DB8::/32
   towards AS64501.  The router, however, only propagates the path of
   the less specific prefix (2001:DB8::/32) to AS64504.  An operator
   could implement the necessary techniques to force the edge router to
   forward packets coming from AS64504 based only on the paths
   propagated to AS64504.  Thus, the edge router would forward packets
   destined to 2001:DB8::/34 towards AS64501 in which case no unexpected
   traffic flow would occur.

   Different techniques could provide this functionality; however, their
   technical implementation can be complex to design and operate.  An
   operator could, for instance, employ Virtual Routing Forwarding (VRF)
   tables (RFC4364 [4]) to store the routes announced to a neighbor and
   forward traffic exclusively based on those routes. [2] describes the
   use of such an architecture for Internet routing, and provides a
   description of its limitations.

   In such an architecture, packets received from a peer would be
   forwarded solely based on the paths that fit the path propagation
   policy for that peer, and not based on the global routing table of
   the router.  As a result, a more specific path that would not be
   propagated to a peer will not be used to forward a packet from that
   peer, and the unexpected flow will not take place.  Packets will be
   forwarded based on the policy compliant less specific prefix.
   However, note that an operator must make sure that all their routers
   could support the potential performance impact of this approach.

   note that similarly to the solution described in Section 4.1, this
   approach could create conflicts between AS64502 and AS64501, since
   the traffic forwarding performed by AS64502 goes against the policy
   of AS64501.






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5.  Conclusions

   In this document, we described how the filtering of more specific
   prefixes can potentially create unexpected traffic flows in remote
   ASes.  We provided examples of scenarios in which unexpected traffic
   flows are caused by these practices and introduce some techniques for
   their detection and prevention.  Analyzing the different options for
   dealing with this kind of problems, we recommend ASes to implement
   monitoring systems that can detect them and react to them according
   to the specific situation.  Although we observe that there are
   reasonable situations in which ASes could filter more specific
   prefixes, we encourage network operators to implement this type of
   filters considering the cases described in this document.

6.  Security Considerations

   It is possible for an AS to use any of the methods described in this
   document to deliberately reroute traffic flowing through another AS.
   The objective of this document is to inform on this potential routing
   security issue, and analyse ways for operators to defend against
   them.

   It must be noted that, at the time of this document, there are no
   existing or proposed tools to automatically protect against such
   behavior.  Network monitoring allowing for the detection of
   unexpected traffic flows exist, but automated configuration changes
   to solve the problem do not.

7.  IANA Considerations

   This document has no IANA actions.

8.  Acknowledgments

   The authors would like to thank Wes George, Jon Mitchell, and Bruno
   Decraene for their useful suggestions and comments.

9.  References

9.1.  Informative References

   [1]        Donnet, B. and O. Bonaventure, "On BGP Communities", ACM
              SIGCOMM Computer Communication Review vol. 38, no. 2, pp.
              55-59, April 2008.







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   [2]        Vanbever, L., Francois, P., Bonaventure, O., and J.
              Rexford, "Customized BGP Route Selection Using BGP/MPLS
              VPNs", Cisco Systems, Routing Symposium
              http://www.cs.princeton.edu/~jrex/talks/cisconag09.pdf,
              October 2009.

   [3]        "INIT7-RIPE63", <http://ripe63.ripe.net/presentations/48-
              How-more-specifics-increase-your-transit-bill-v0.2.pdf>.

   [4]        "pmacct project: IP accounting iconoclasm",
              <http://www.pmacct.net>.

9.2.  URIs

   [1] http://www.ietf.org/rfc/rfc1812.txt

   [2] http://www.ietf.org/rfc/rfc4384.txt

   [3] http://www.ietf.org/rfc/rfc7011.txt

   [4] http://www.ietf.org/rfc/rfc4364.txt

Authors' Addresses

   Camilo Cardona
   IMDEA Networks/UC3M
   Avenida del Mar Mediterraneo, 22
   Leganes  28919
   Spain

   Email: juancamilo.cardona@imdea.org


   Pierre Francois
   IMDEA Networks
   Avenida del Mar Mediterraneo, 22
   Leganes  28919
   Spain

   Email: pierre.francois@imdea.org











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   Paolo Lucente
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Email: plucente@cisco.com












































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