IDR Working Group R. Raszuk, Ed.
Internet-Draft Bloomberg LP
Intended status: Standards Track C. Cassar
Expires: July 9, 2017 Cisco Systems
E. Aman
Telia Company
B. Decraene
S. Litkowski
Orange
K. Wang
Juniper Networks
January 5, 2017
BGP Optimal Route Reflection (BGP-ORR)
draft-ietf-idr-bgp-optimal-route-reflection-13
Abstract
This document proposes a solution for BGP route reflectors to allow
them to choose the best path their clients would have chosen under
the same conditions, without requiring further state or any new
features to be placed on the clients. This facilitates, for example,
best exit point policy (hot potato routing). This solution is
primarily applicable in deployments using centralized route
reflectors.
The solution relies upon all route reflectors learning all paths
which are eligible for consideration. Best path selection is
performed in each route reflector based on a configured virtual
location in the IGP. The location can be the same for all clients or
different per peer/update group or per peer. Best path selection can
also be performed based on user configured policies in each route
reflector.
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-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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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 July 9, 2017.
Copyright Notice
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document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Definitions of Terms Used in This Memo . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 3
2.2. Existing/Alternative Solutions . . . . . . . . . . . . . 4
3. Proposed Solutions . . . . . . . . . . . . . . . . . . . . . 5
3.1. Client's Perspective IGP Based Best Path Selection . . . 6
3.2. Client's Perspective Policy Based Best Path Selection . . 6
3.3. Solution Interactions . . . . . . . . . . . . . . . . . . 7
4. CPU and Memory Scalability . . . . . . . . . . . . . . . . . 8
5. Advantages and Deployment Considerations . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Definitions of Terms Used in This Memo
NLRI - Network Layer Reachability Information.
RIB - Routing Information Base.
AS - Autonomous System number.
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VRF - Virtual Routing and Forwarding instance.
PE - Provider Edge router
POP - Point Of Presence
L3VPN - Layer 3 Virtual Private Networks RFC4364
6PE - IPv6 Provider Edge Router
IGP - Interior Gateway Protocol
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]
2. Introduction
There are three types of BGP deployments within Autonomous Systems
today: full mesh, confederations and route reflection. BGP route
reflection [RFC4456] is the most popular way to distribute BGP routes
between BGP speakers belonging to the same Autonomous System. In
some situations, this method suffers from non-optimal path selection.
2.1. Problem Statement
[RFC4456] asserts that, because the Interior Gateway Protocol (IGP)
cost to a given point in the network will vary across routers, "the
route reflection approach may not yield the same route selection
result as that of the full IBGP mesh approach." One practical
implication of this assertion is that the deployment of route
reflection may thwart the ability to achieve hot potato routing. Hot
potato routing attempts to direct traffic to the best AS exit point
in cases where no higher priority policy dictates otherwise. As a
consequence of the route reflection method, the choice of exit point
for a route reflector and its clients will be the exit point best for
the route reflector - not necessarily the one best for the RR
clients.
Section 11 of [RFC4456] describes a deployment approach and a set of
constraints which, if satisfied, would result in the deployment of
route reflection yielding the same results as the iBGP full mesh
approach. This deployment approach makes route reflection compatible
with the application of hot potato routing policy. In accordance
with these design rules, route reflectors have traditionally often
been deployed in the forwarding path and carefully placed on the POP
to core boundaries.
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The evolving model of intra-domain network design has enabled
deployments of route reflectors outside of the forwarding path.
Initially this model was only employed for new address families, e.g.
L3VPNs and L2VPNs. This model has been gradually extended to other
BGP address families including IPv4 and IPv6 Internet using either
native routing or 6PE. In such environments, hot potato routing
policy remains desirable.
Route reflectors outside of the forwarding path can be placed on the
POP to core boundaries, but they are often placed in arbitrary
locations in the core of large networks.
Such deployments suffer from a critical drawback in the context of
best path selection: A route reflector with knowledge of multiple
paths for a given prefix will typically pick its best path and only
advertise that best path to its clients. If the best path for a
prefix is selected on the basis of an IGP tie break, the path
advertised will be the exit point closest to the route reflector.
But the clients will be in a different place in the network topology
than the route reflector. In networks where the route reflectors are
not in the forwarding path, this difference will be even more acute.
Beside this, there are also deployment scenarios where service
providers want to have more control of choosing the exit points for
clients based on other factors like traffic type, traffic load, etc.
This further complicated the issue and makes it less likely for the
route reflector to select the best path from the client's
perspective. It follows that the best path chosen by the route
reflector is not necessarily the same as the path which would have
been chosen by the client if the client had considered the same set
of candidate paths as the route reflector.
2.2. Existing/Alternative Solutions
One possible valid solution or workaround to the best path selection
problem requires sending all domain external paths from the RR to all
its clients. This approach suffers the significant drawback of
pushing a large amount of BGP state to all edge routers. Many
networks receive full Internet routing information in a large number
of locations. This could easily result in tens of paths for each
prefix that would need to be distributed to clients.
Notwithstanding this drawback, there are a number of reasons for
sending more than just the single best path to the clients. Improved
path diversity at the edge is a requirement for fast connectivity
restoration, and a requirement for effective BGP level load
balancing.
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In practical terms, add/diverse path deployments are expected to
result in the distribution of 2, 3 or n (where n is a small number)
good paths rather than all domain external paths. While the route
reflector chooses one set of n paths and distributes those same n
paths to all its route reflector clients, those n paths may not be
the right n paths for all clients. In the context of the problem
described above, those n paths will not necessarily include the best
exit point out of the network for each route reflector client. The
mechanisms proposed in this document are likely to be complementary
to mechanisms aimed at improving path diversity.
Another possibility to optimize exit points would be installing
physical hardware at various IGP locations or what is quite unlikely
to attach Route Reflectors over manually created tunnels.
The paradigm of control plane is shifting from traditional routers to
x86 virtual space or even cloud. As result without this proposal
operators have choice of their route reflectors distributing
suboptimal paths or distributing all paths and in turn allowing
clients to make independent best path selection.
Now while the latter could be even an option in router's world more
and more BGP is being observed on the compute servers where sending
there all present in an AS BGP paths would be for one undesired as
well would require to run also IGP there. Let's also note that
number of paths per BGP prefix varies a lot. Depending on the
network it can be anywhere from few to few hundreds.
3. Proposed Solutions
The goal of this document is to allow a route reflector to choose the
best path the client would have chosen had the client considered the
same set of candidate paths the reflector has available. For
purposes of route selection, the perspective of a client can differ
from that of a route reflector or another client in two distinct
ways: it can, and usually will, have a different position in the IGP
topology, and it can have a different routing policy. These
correspond to the issues described earlier. Accordingly, we propose
two distinct modifications to the best path algorithm, to address
these two distinct factors. A route reflector can implement either
or both of the modifications, as needed in order to allow it to
choose the best path the client would have chosen had the client
considered the same set of candidate paths.
Both modifications rely upon all route reflectors learning all paths
which are eligible for consideration. In order to satisfy this
requirement, path diversity enhancing mechanisms such as ADD-PATH/
diverse paths may need to be deployed between route reflectors.
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A significant advantage of these approaches is that the RR clients do
not need to run new software or hardware.
3.1. Client's Perspective IGP Based Best Path Selection
The core of this solution is the ability for an operator to specify
on a per route reflector basis or per peer/update group basis or per
peer basis the virtual IGP location placement of the route reflector.
This enables having a given group of clients receive routes with
optimal distance to the next hops from the position of the configured
virtual IGP location. This also provides for freedom of route
reflector location and allows transient or permanent migration of
such network control plane function to optimal location.
The choice of specific granularity is left to the implementation
decision. An implementation is considered compliant with the
document if it supports at least one listed grouping of virtual IGP
placement.
In this document we refer to optimal as the decision made during best
path selection at the IGP metric to BGP next hop comparison step.
This approach does not apply to path selection preference based other
policy steps and provisions.
The computation of the virtual IGP location with any of the above
described granularity is outside of the scope of this document. The
operator may configure it manually, implementation may automate it
based on specified heuristic or it can be computed centrally and
configured by an external system.
The solution does not require any BGP or IGP protocol changes as
required changes are contained within the RR implementation.
The solution applies to NLRIs of all address families which can be
route reflected.
3.2. Client's Perspective Policy Based Best Path Selection
Optimal route reflection based on virtual IGP location could reflect
the best path to the client from IGP cost perspective. However,
there are also cases where the client might want best path from
perspectives beyond IGP cost. Examples include, but not limited to:
o Select the best path for the clients from a traffic engineering
perspective.
o Dedicate certain exit points for certain ingress points.
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The solution proposed here is to allow the user to apply a general
policy to select a subset of exit points as the candidate exit points
for its clients. For a given client, the policy should also allow
service providers to select different candidate exit points for
different address families. Regular path selection, including
client's perspective IGP based best path selection stated above, will
be applied to the candidate paths to select the final paths to
advertise to the clients.
The policy is applied on the route reflector on behalf of its
clients. This way, the route reflector will be able to reflect only
the optimal paths to the clients. An additional advantage of this
approach is that configuration need only be done on a small number of
route reflectors rather than a significantly larger number of
clients.
3.3. Solution Interactions
Depending on the actual deployment scenarios, service providers may
configure IGP based optimal route reflection or policy based optimal
route reflection. It's also possible to configure both approaches
together. In that case, policy based optimal route reflection will
be applied first to select the candidate paths. Subsequently, IGP
based optimal route reflection will be applied on top of the
candidate paths to select the final path to advertise to the client.
The expected use case for optimal route reflection is to avoid
reflecting all paths to the client because the client either does not
support add-paths or does not have the capacity to process all of the
paths. Typically the route reflector would just reflect a single
optimal route to the client. However, the solutions MUST NOT prevent
reflecting more than one optimal path to the client; the client may
want path diversity for load balancing or fast restoration. In case
add-path and optimal route reflection are configured together, the
route reflector MUST reflect n optimal paths to a client, where n is
the add-path count.
The most complicated scenario is where add-path is configured
together with both IGP based and policy based optimal route
reflection. In this scenario, the policy based optimal route
reflection will be applied first to select the candidate paths.
Subsequently, IGP based optimal route reflection will be applied on
top of the candidate paths to select the best n paths to advertise to
the client.
In IGP based optimal route reflection, even though the virtual IGP
location could be specified on a per route reflector basis or per
peer group basis or per peer basis, in reality, it's most likely to
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be specified per peer group basis. All clients with the same or
similar IGP location can be grouped into the same peer group. A
virtual IGP location is then specified for the peer group. The
virtual location is usually specified as the location of one of the
clients from the peer group or an ABR to the area where clients are
located. Also, one or more backup virtual location SHOULD be allowed
to be specified for redundancy. Implementations may wish to take
advantage of peer group mechanisms in order to provide for better
scalability of optimal route reflector client groups with similar
properties.
4. CPU and Memory Scalability
For IGP based optimal route reflection, determining the shortest path
and associated cost between any two arbitrary points in a network
based on the IGP topology learned by a router is expected to add some
extra cost in terms of CPU resources. However SPF tree generation
code is now implemented efficiently in a number of implementations,
and therefore this is not expected to be a major drawback. The
number of SPTs computed is expected to be of the order of the number
of clients of an RR whenever a topology change is detected. Advanced
optimizations like partial and incremental SPF may also be exploited.
The number of SPTs computed is expected to be higher but comparable
to some existing deployed features such as (Remote) Loop Free
Alternate which computes a (r)SPT per IGP neighbor.
For policy based optimal route reflection, there will be some
overhead to apply the policy to select the candidate paths. This
overhead is comparable to existing BGP export policies therefore
should be manageable.
By the nature of route reflection, the number of clients can be split
arbitrarily by the deployment of more route reflectors for a given
number of clients. While this is not expected to be necessary in
existing networks with best in class route reflectors available
today, this avenue to scaling up the route reflection infrastructure
would be available.
If we consider the overall network wide cost/benefit factor, the only
alternative to achieve the same level of optimality would require
significantly increasing state on the edges of the network. This
will consume CPU and memory resources on all BGP speakers in the
network. Building this client perspective into the route reflectors
seems appropriate.
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5. Advantages and Deployment Considerations
The solutions described provide a model for integrating the client
perspective into the best path computation for RRs. More
specifically, the choice of BGP path factors in either the IGP cost
between the client and the nexthop (rather than the distance from the
RR to the nexthop) or user configured policies.
Implementation to be declared as compliant with this memo should
allow to configure per instance or per group of peers logical
location from which either for the entire instance or for set of
peers best path will be computed.
These solutions can be deployed in traditional hop-by-hop forwarding
networks as well as in end-to-end tunneled environments. In the
networks where there are multiple route reflectors and hop-by-hop
forwarding without encapsulation, such optimizations should be
enabled in a consistent way on all route reflectors. Otherwise
clients may receive an inconsistent view of the network and in turn
lead to intra-domain forwarding loops.
With this approach, an ISP can effect a hot potato routing policy
even if route reflection has been moved from the forwarding plane and
hop-by-hop switching has been replaced by end-to-end MPLS or IP
encapsulation.
As per above, these approaches reduce the amount of state which needs
to be pushed to the edge of the network in order to perform hot
potato routing. The memory and CPU resource required at the edge of
the network to provide hot potato routing using these approaches is
lower than what would be required in order to achieve the same level
of optimality by pushing and retaining all available paths
(potentially 10s) per each prefix at the edge.
The proposals allow for a fast and safe transition to a BGP control
plane with centralized route reflection without compromising an
operator's closest exit operational principle. This enables edge-to-
edge LSP/IP encapsulation for traffic to IPv4 and IPv6 prefixes.
Regarding the client's IGP best-path selection, it should be self
evident that this solution does not interfere with policies enforced
above IGP tie breaking in the BGP best path algorithm.
6. Security Considerations
No new security issues are introduced to the BGP protocol by this
specification.
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7. IANA Considerations
This document does not request any IANA allocations.
8. Acknowledgments
Authors would like to thank Keyur Patel, Eric Rosen, Clarence
Filsfils, Uli Bornhauser, Russ White, Jakob Heitz, Mike Shand, Jon
Mitchell, John Scudder, Jeff Haas, Martin Djernaes and Daniele
Ceccarelli for their valuable input.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <http://www.rfc-editor.org/info/rfc4360>.
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <http://www.rfc-editor.org/info/rfc5492>.
9.2. Informative References
[I-D.ietf-idr-add-paths]
Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", draft-ietf-idr-
add-paths-15 (work in progress), May 2016.
[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
<http://www.rfc-editor.org/info/rfc1997>.
[RFC1998] Chen, E. and T. Bates, "An Application of the BGP
Community Attribute in Multi-home Routing", RFC 1998,
DOI 10.17487/RFC1998, August 1996,
<http://www.rfc-editor.org/info/rfc1998>.
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[RFC4384] Meyer, D., "BGP Communities for Data Collection", BCP 114,
RFC 4384, DOI 10.17487/RFC4384, February 2006,
<http://www.rfc-editor.org/info/rfc4384>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<http://www.rfc-editor.org/info/rfc4456>.
[RFC4893] Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
Number Space", RFC 4893, DOI 10.17487/RFC4893, May 2007,
<http://www.rfc-editor.org/info/rfc4893>.
[RFC5283] Decraene, B., Le Roux, JL., and I. Minei, "LDP Extension
for Inter-Area Label Switched Paths (LSPs)", RFC 5283,
DOI 10.17487/RFC5283, July 2008,
<http://www.rfc-editor.org/info/rfc5283>.
[RFC5668] Rekhter, Y., Sangli, S., and D. Tappan, "4-Octet AS
Specific BGP Extended Community", RFC 5668,
DOI 10.17487/RFC5668, October 2009,
<http://www.rfc-editor.org/info/rfc5668>.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010,
<http://www.rfc-editor.org/info/rfc5714>.
[RFC6774] Raszuk, R., Ed., Fernando, R., Patel, K., McPherson, D.,
and K. Kumaki, "Distribution of Diverse BGP Paths",
RFC 6774, DOI 10.17487/RFC6774, November 2012,
<http://www.rfc-editor.org/info/rfc6774>.
Authors' Addresses
Robert Raszuk (editor)
Bloomberg LP
731 Lexington Ave
New York City, NY 10022
USA
Email: robert@raszuk.net
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Christian Cassar
Cisco Systems
10 New Square Park
Bedfont Lakes, FELTHAM TW14 8HA
UK
Email: ccassar@cisco.com
Erik Aman
Telia Company
Solna SE-169 94
Sweden
Email: erik.aman@teliacompany.com
Bruno Decraene
Orange
38-40 rue du General Leclerc
Issy les Moulineaux cedex 9 92794
France
Email: bruno.decraene@orange.com
Stephane Litkowski
Orange
9 rue du chene germain
Cesson Sevigne 35512
France
Email: stephane.litkowski@orange.com
Kevin Wang
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: kfwang@juniper.net
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