Internet DRAFT - draft-white-grow-overlapping-routes
draft-white-grow-overlapping-routes
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
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This Internet-Draft will expire on October 5, 2016.
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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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