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Network Working Group                                           R. White
Internet-Draft                                                  Linkedin
Intended status: Informational                                 A. Retana
Expires: October 5, 2016                              Cisco Systems, Inc.
                                                                S. Hares
                                                                  Huawei
                                                           April 4, 2016


                    Filtering of Overlapping Routes
                 draft-white-grow-overlapping-routes-04

Abstract

   This document proposes an optional mechanism to remove a prefix when
   it overlaps with a functionally equivalent shorter prefix.  The
   proposed mechanism does not require any changes to the BGP protocol.

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
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   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 October 5, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   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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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Overlapping Route Filtering Mechanism . . . . . . . . . . . .   3
     3.1.  Marking Overlapping Routes  . . . . . . . . . . . . . . .   4
     3.2.  Preferring Marked Routes  . . . . . . . . . . . . . . . .   4
       3.2.1.  Using a Cost Community  . . . . . . . . . . . . . . .   4
       3.2.2.  Using the Local Preference  . . . . . . . . . . . . .   4
     3.3.  Handling Marked Routes Within the AS  . . . . . . . . . .   5
     3.4.  Handling Marked Routes at the Outbound Edge . . . . . . .   5
   4.  Examples of Filtering Overlapping Routes  . . . . . . . . . .   5
     4.1.  IPv4 Example  . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  IPv6 Example  . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   6
     5.1.  Advantages to the Service Provider  . . . . . . . . . . .   7
     5.2.  Implications for Router processing  . . . . . . . . . . .   7
     5.3.  Implications for Convergence Time . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .   8
     A.1.  Changes between the -00 and -01 versions. . . . . . . . .   8
     A.2.  Changes between the -01 and -02 versions  . . . . . . . .   9
     A.3.  Changes between the -02 and -03 versions  . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   One cause of the growth of the global Internet's default free zone
   table size is overlapping routes injected into the routing system to
   steer traffic among various entry points into a network.  Because
   padding AS Path lengths can only steer inbound traffic in a very
   small set of cases, and other mechanisms used to steer traffic to a
   particular inbound point are ineffective when multiple upstream
   providers are in use, advertising longer prefixes is often the only
   possible way for an AS to steer traffic into specific entry points
   along its edge.

   These longer prefix routes, called overlapping routes in this
   document, are often advertised along with a shorter prefix route,
   called a covering route, in order to ensure connectivity in the case



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   of link or device failures.  Overlapping routes not only add to the
   load on routers in the Internet core by simply expanding the table
   size; these routes may be less stable than the covering routes they
   are paired with.

   Given the importance of an autonomous system's ability to steer
   traffic into specific entry points, simply removing the longer
   prefixes in a longer prefix (overlapping)/shorter prefix (covering)
   pair of routes isn't a viable solution.

   This document proposes an optional mechanism to remove overlapping
   routes that are no longer useful for steering traffic towards a
   specific entry point in a particular AS.  Removing these routes would
   reduce the global table in size, and reduce its instability, while
   removing no capabilities, nor increasing the average path length.

   The mechanism proposed is simple to implement, requiring no changes
   to BGP [RFC4271] either in packet format or in the decision process.
   The removal described in this document is akin to filtering, not to
   route aggregation.

   The intent of the mechanism is for it to be used based on local
   decisions and policies, not on an Internet-wide fashion.  It is
   assumed that network operators using this mechanism have an incentive
   to do so.

2.  Requirements Language

   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].

3.  Overlapping Route Filtering Mechanism

   The handling of overlapping prefixes received from an external peer
   can be broken down into four parts: marking overlapping routes,
   preferring marked routes, handling marked routes within the AS, and
   handling marked routes at the AS exit point.

   The initial step in successfully filtering overlapping routes is to
   identify and mark them.  This document proposes the use of a BGP
   community called BOUNDED for that purpose.  Because the operation
   suggested takes place inside an Autonomous System (AS), then any
   locally assigned community can be used.

   The term BOUNDED is used to refer to a locally assigned community
   used to mark overlapping routes, and to these marked routes as well.




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3.1.  Marking Overlapping Routes

   As each prefix is received by a BGP speaker from an external peer, it
   is evaluated in the light of other prefixes already received.  If two
   prefixes overlap in space (such as 192.0.2.0/24 and 192.0.2.128/25,
   or 2001:DB8::/32 and 2001:DB8:1:/48), the longer prefix SHOULD be
   BOUNDED if it fully overlaps the covering prefix and it is the best
   path to the destination.

   An overlapping prefix is said to fully overlap the corresponding
   covering prefix if both have identical AS_PATH attributes (both in
   length and contents) and the same NEXT_HOP.

3.2.  Preferring Marked Routes

   Since the same overlapping route may be received at several peering
   points along the edge of the AS, and the covering route may not be
   present at each of these points, BOUNDED routes SHOULD be preferred
   over unmarked routes for overlapping routes to be properly handled.
   A router which marks an overlapping route should also use one of the
   two mechanisms described here to insure the marked route is preferred
   throughout the AS.

   Only one method described in this section SHOULD be deployed in any
   given AS.

3.2.1.  Using a Cost Community

   The recommended method for preferring BOUNDED routes is to use a Cost
   Community [I-D.ietf-idr-custom-decision] with the Point of Insertion
   set to ABSOLUTE_VALUE.  This mechanism leaves all existing local
   policy controls in place within the AS.

   If this method is used, only the BOUNDED routes need to be tagged
   using a lower than default Cost, as routes without a Cost Community
   are considered to have the default value.

3.2.2.  Using the Local Preference

   An alternate mechanism which may be used to prefer BOUNDED routes is
   to set their Local Preference to some number higher than the normal
   standard policy settings for a particular prefix.  It's not important
   that any particular BOUNDED route win over any other one; so simply
   adding a small amount to the normal Local Preference, as dictated by
   local policy, will ensure a BOUNDED route will always win over an
   unmarked route, so only these routes reach the outbound edge of the
   AS.




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3.3.  Handling Marked Routes Within the AS

   Routes marked with the BOUNDED community MAY not be installed in the
   local RIB of routers within the AS.  This optional step will reduce
   local RIB and forwarding table usage and volatility within the AS.

3.4.  Handling Marked Routes at the Outbound Edge

   If local policy dictates, routes marked with the BOUNDED community
   SHOULD NOT be advertised to external peers.  If they are advertised,
   they MAY then be marked with the NO_EXPORT community.

4.  Examples of Filtering Overlapping Routes

   Assume the following configuration of autonomous systems:

                    (   )
           /-------( AS2 )--------\
    (   ) /         (   )          \ (   )       (   )
   ( AS1 )                          ( AS4 )-----( AS5 )
    (   ) \         (   )          / (   )       (   )
           \-------( AS3 )--------/
                    (   )

   This network is used in both of the following examples.

4.1.  IPv4 Example

   o  AS1 is advertising 192.0.2.128/25 to both AS2 and AS3.

   o  AS2 is advertising both 192.0.2.128/25 and 192.0.2.0/24 into AS4.

   o  AS3 is advertising 192.0.2.128/25 into AS4

   o  Each BGP connection (session) is handled by a separate router
      within each AS (for instance, AS4 peers with AS2 and AS3 on
      separate routers).

   When the router in AS4 peering with AS2 receives both the
   192.0.2.128/25 and the 192.0.2.0/24 prefixes, it will mark
   192.0.2.128/25 as BOUNDED, and set a Cost Community (as described in
   Section 3.2.1) so the marked overlapping route is preferred over
   unmarked routes within AS4.

   The border router between AS4 and AS3 will receive the longer prefix
   from AS3, and the preferred BOUNDED overlapping route through iBGP.
   It will prefer the marked route, so the unmarked route towards
   192.0.2.128/25 will not be advertised throughout AS4.



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   If the link between AS1 and AS2 fails, the longer length prefix will
   be withdrawn from AS2, and thus the peering point between AS2 and AS4
   will no longer have an overlapping set of prefixes.  Within AS4, the
   border router which peers with AS2 will cease advertising the
   192.0.2.128/25 prefix, which allows the AS3/AS4 border router to
   begin advertising it into AS4, and through AS4 into AS5, restoring
   connectivity to AS1.

4.2.  IPv6 Example

   o  AS1 is advertising 2001:DB8:1:/48 to both AS2 and AS3.

   o  AS2 is advertising both 2001:DB8:1:/48 and 2001:DB8::/32 into AS4.

   o  AS3 is advertising 2001:DB8:1:/48 into AS4

   o  Each BGP connection (session) is handled by a separate router
      within each AS (for instance, AS4 peers with AS2 and AS3 on
      separate routers).

   When the router in AS4 peering with AS2 receives both the
   2001:DB8:1:/48 and 2001:DB8::/32 prefixes, it will mark
   2001:DB8:1:/48 as BOUNDED, and set a Cost Community (as described in
   Section 3.2.1) so the marked overlapping route is preferred over
   unmarked routes within AS4.

   The border router between AS4 and AS3 will receive the longer prefix
   from AS3, and the preferred BOUNDED overlapping route through iBGP.
   It will prefer the marked route, so the unmarked route towards
   2001:DB8:1:/48 will not be advertised throughout AS4.

   If the link between AS1 and AS2 fails, the longer length prefix will
   be withdrawn from AS2, and thus the peering point between AS2 and AS4
   will no longer have an overlapping set of prefixes.  Within AS4, the
   border router which peers with AS2 will cease advertising the
   2001:DB8:1:/48 prefix, which allows the AS3/AS4 border router to
   begin advertising it into AS4, and through AS4 into AS5, restoring
   connectivity to AS1.

5.  Operational Considerations

   The intent of the mechanism described in this document is for it to
   be used based on local policies, not on an Internet-wide fashion.  It
   is assumed that network operators using this mechanism have an
   incentive to do so.

   The practice of filtering exists today on the Internet.  While there
   may be local benefits to applying manual filters and/or the mechanism



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   specified in this document, the operator should be aware of the
   impact it may have on neighboring autonomous systems' policies
   [I-D.cardona-filtering-threats].

   The benefits and implications associated with this proposal are
   discussed in the sections below.  The text references the sample
   network in Section 4.

5.1.  Advantages to the Service Provider

   AS4, in each of the situations, reduces the number of prefixes
   advertised to transit peering autonomous systems by the number of
   longer prefixes that overlap with aggregates of those prefixes, so
   that AS5 receives fewer total routes, and a more stable routing
   table.  While one copy of the prefix continues to be carried through
   the autonomous system, this entry can be removed from the local
   forwarding table.

5.2.  Implications for Router processing

   This proposal requires a BGP speaker to perform an additional check
   on receiving a route, checking the route against existing routes for
   overlapping coverage of a set of reachable destinations.  This
   additional work, in terms of processing requirements, should be
   easily offset by the overall savings in processing through the
   reduction of the forwarding table size, and the additional stability
   in the routing table due to the removal of longer length prefixes.

5.3.  Implications for Convergence Time

   If the route to the AS providing the route to the covering route
   should be lost, the overlapping route must now propagate into the
   autonomous systems which had formerly received only the covering
   route.  This behavior increases convergence time and may create
   situations in which reachability is temporarily compromised.  Unlike
   the case where manual filters are used, normal BGP behavior should
   restore reachability without changes to the router configuration.

6.  Security Considerations

   This document presents a mechanism for an autonomous system to mark
   and filter overlapping prefixes.  Note that the result of this
   operation is akin to the implementation of local route filtering at
   an AS boundary.  As such, this document doesn't introduce any new
   security risks.






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7.  IANA Considerations

   This document has no IANA actions.

8.  Acknowledgements

   Cengiz Alaentinoglu, Daniel Walton, David Ball, Ted Hardie, Jeff
   Hass, Barry Greene, Bill Herrin and Robert Raszuk gave valuable
   comments on this document.

9.  References

9.1.  Normative References

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

9.2.  Informative References

   [I-D.cardona-filtering-threats]
              Cardona, C. and P. Francois, "Making BGP filtering a
              habit: Impact on policies", draft-cardona-filtering-
              threats-02 (work in progress), July 2013.

   [I-D.ietf-idr-custom-decision]
              Retana, A. and R. White, "BGP Custom Decision Process",
              draft-ietf-idr-custom-decision-04 (work in progress),
              November 2013.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

Appendix A.  Change Log

A.1.  Changes between the -00 and -01 versions.

   o  Updated authors' contact information.

   o  Changed intended status to Informational.

   o  General editorial changes.

   o  Clarified the intent of the draft in several places.

   o  Clarified when a route should be marked (3.1).

   o  Edited the operational considerations section.




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   o  Updated ACKs.

A.2.  Changes between the -01 and -02 versions

   o  Updated authors' contact information.

   o  General editorial changes.

   o  Refined the text about marking routes.

A.3.  Changes between the -02 and -03 versions

   o  Updated authors' contact information.

   o  Added IPv6 examples.

   o  Minor editorial changes.

A.4.  Changes between the -03 and -04 versions

   o  Updated authors' contact information.

Authors' Addresses

   Russ White
   Linkedin

   Email: russ@riw.us


   Alvaro Retana
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC  27709
   USA

   Email: aretana@cisco.com


   Susan Hares
   Huawei

   Email: shares@ndzh.com








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