Network Working Group R. White Internet-Draft Verisign Intended status: Experimental A. Retana Expires: January 10, 2013 Hewlett-Packard Co. S. Hares Hauwei July 9, 2012 Filtering of Overlapping Routes draft-white-grow-overlapping-routes-00 Abstract This document proposes a 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 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 January 10, 2013. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as White, et al. Expires January 10, 2013 [Page 1] Internet-Draft Overlapping Route Filtering July 2012 described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 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 . . . . . . . . . . . . . . 5 3.3. Handling Marked Routes Within the AS . . . . . . . . . . . 5 3.4. Handling Marked Routes at the Outbound Edge . . . . . . . . 5 4. An Example of Automatic Filtering of TE Routes . . . . . . . . 5 5. Benefits and Implications . . . . . . . . . . . . . . . . . . . 6 5.1. Advantages to the Service Provider . . . . . . . . . . . . 6 5.2. Advantages to the Customer . . . . . . . . . . . . . . . . 6 5.3. Advantages to the Internet . . . . . . . . . . . . . . . . 6 5.4. Implications for Router processing . . . . . . . . . . . . 7 5.5. Implications for Traffic engineering . . . . . . . . . . . 7 5.6. Implications for Convergence Time . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 9.1. Normative References . . . . . . . . . . . . . . . . . . . 8 9.2. Informative References . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8 White, et al. Expires January 10, 2013 [Page 2] Internet-Draft Overlapping Route Filtering July 2012 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 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 are often less stable than the covering routes they are paired with. Overall, then, it is desirable to remove overlapping routes from the global routing table where possible. However, 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 a mechanism to detect routes that have been injected into the global default free zone, and to remove 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 the instability of the global table, while removing no capabilities, nor increasing the average path length. The mechanism proposed is simple to implement, requiring no changes to the BGP [RFC4271] protocol either in packet format or in the decision process. 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, White, et al. Expires January 10, 2013 [Page 3] Internet-Draft Overlapping Route Filtering July 2012 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. 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), the longer prefix SHOULD be BOUNDED. A BGP speaker MAY also choose to check the AS_PATH attribute length and contents before marking a prefix as BOUNDED. 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. White, et al. Expires January 10, 2013 [Page 4] Internet-Draft Overlapping Route Filtering July 2012 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. 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 Routes marked with the BOUNDED community SHOULD NOT be advertised to external peers. If they are advertised, they SHOULD then be marked with the NO_EXPORT community. 4. An Example of Automatic Filtering of TE Routes Assume the following configuration of autonomous systems: ( ) /-------( AS2 )--------\ ( ) / ( ) \ ( ) ( ) ( AS1 ) ( AS4 )-----( AS5 ) ( ) \ ( ) / ( ) ( ) \-------( AS3 )--------/ ( ) 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 seperate router within each AS (for instance, AS4 peers with AS2 and AS3 on a seperate 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 White, et al. Expires January 10, 2013 [Page 5] Internet-Draft Overlapping Route Filtering July 2012 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. 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. 5. Benefits and Implications 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. Advantages to the Customer In the example given here, the customer is represented as AS1. The customer will continue to receive some amount of traffic over both peering sessions, and dual homing through two Service Providers is still effective. If the customer's primary link fails, the alternate link through AS3 will take over receiving all inbound traffic automatically. 5.3. Advantages to the Internet Beyond the second AS hop, aggregation is preserved in all cases. While this would not reduce the backbone routing table by the dramatic amounts that other methods might, the advantages to the community are large, and the risk to individual autonomous systems White, et al. Expires January 10, 2013 [Page 6] Internet-Draft Overlapping Route Filtering July 2012 and providers is small. 5.4. Implications for Router processing This proposal requires all BGP speakers 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 global default free zone table size, and the additional stability in the routing table due to the removal of longer length prefixes. 5.5. Implications for Traffic engineering The implementation of filtering overlapping routes as described in this document risks magnifying or removing the effect of certain widely deployed traffic engineering methods. If, for example, an AS chose to prepend its own route to an announcement in order to alter the preference for that route, a BGP neighbor using automatic filtering of overlapping routes might now see that route as eligible for discard in favor of an aggregate. It should be fairly easy to define a local policy to work around that particular problem. 5.6. Implications for Convergence Time If the route to the AS providing the route to the aggregate should be lost, the more-specific must propagate into the ASes which had formerly heard only the aggregate. This increases convergence time and may create situations in which reachability is temporarily compromised. Unlike the filter case, however, 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 if 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. 7. IANA Considerations This document has no IANA actions. White, et al. Expires January 10, 2013 [Page 7] Internet-Draft Overlapping Route Filtering July 2012 8. Acknowledgements Cengiz Alaentinoglu, Daniel Walton, David Ball, Ted Hardie, Jeff Hass, and Barry Greene 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.ietf-idr-custom-decision] Retana, A. and R. White, "BGP Custom Decision Process", draft-ietf-idr-custom-decision-01 (work in progress), May 2012. [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006. Authors' Addresses Russ White Verisign 12061 Bluemont Way Reston, VA 20190 USA Email: riwhite@verisign.com Alvaro Retana Hewlett-Packard Co. 2610 Wycliff Road Raleigh, NC 27607 USA Email: alvaro.retana@hp.com White, et al. Expires January 10, 2013 [Page 8] Internet-Draft Overlapping Route Filtering July 2012 Susan Hares Hauwei USA Email: skh@ndzh.com White, et al. Expires January 10, 2013 [Page 9]