LSVR K. Patel
Internet-Draft Arrcus, Inc.
Intended status: Informational A. Lindem
Expires: September 6, 2018 Cisco Systems
S. Zandi
G. Dawra
Linkedin
March 5, 2018
Usage and Applicability of Link State Vector Routing in Data Centers
draft-keyupate-lsvr-applicability-00.txt
Abstract
This document discusses the usage and applicability of Link State
Vector Routing (LSVR) extensions in the CLOS architecture of Data
Center Networks. The document is intended to provide a simplified
guide for the deployment of LSVR extensions.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
3. Recommended Reading . . . . . . . . . . . . . . . . . . . . . 2
4. Common Deployment Scenario . . . . . . . . . . . . . . . . . 3
5. Justification for BGP modifications . . . . . . . . . . . . . 3
6. LSVR Applicability to CLOS Networks . . . . . . . . . . . . . 4
6.1. Usage of LSVR SAFI . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
10.1. Normative References . . . . . . . . . . . . . . . . . . 6
10.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
This document complements [I-D.keyupate-lsvr-bgp-spf] by discussing
the applicability of the technology in a simple and fairly common
deployment scenario, which is described in Section 4.
After describing the deployment scenario, Section 5 will describe the
reasons for BGP modifications for such deployments.
Once the control plane routing protocol requirements are described,
Section 6 will cover the LSVR protocol enhancements to BGP to meet
these requirements and their applicability to Data Center CLOS
networks.
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] only when
they appear in all upper case. They may also appear in lower or
mixed case as English words, without any normative meaning.
3. Recommended Reading
This document assumes knowledge of existing data center networks and
data center network topologies [CLOS]. This document also assumes
knowledge of data center routing protocols like BGP [RFC4271], BGP-
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SPF [I-D.keyupate-lsvr-bgp-spf], OSPF [RFC2328], as well as, data
center OAM protocols like LLDP [RFC4957] and BFD [RFC5580].
4. Common Deployment Scenario
Within a Data Center, a common network design to interconnect servers
is done using the CLOS topology [CLOS]. The CLOS topology is fully
non-blocking and the topology is realized using Equal Cost Multipath
(ECMP). In a CLOS topology, the minimum number of parallel paths
between two servers is determined by the width of a tier-1 stage as
shown in the figure 1.
The following example illustrates multistage CLOS topology.
Tier-1
+-----+
|NODE |
+->| 12 |--+
| +-----+ |
Tier-2 | | Tier-2
+-----+ | +-----+ | +-----+
+------------>|NODE |--+->|NODE |--+--|NODE |-------------+
| +-----| 9 |--+ | 10 | +--| 11 |-----+ |
| | +-----+ +-----+ +-----+ | |
| | | |
| | +-----+ +-----+ +-----+ | |
| +-----+---->|NODE |--+ |NODE | +--|NODE |-----+-----+ |
| | | +---| 6 |--+->| 7 |--+--| 8 |---+ | | |
| | | | +-----+ | +-----+ | +-----+ | | | |
| | | | | | | | | |
+-----+ +-----+ | +-----+ | +-----+ +-----+
|NODE | |NODE | Tier-3 +->|NODE |--+ Tier-3 |NODE | |NODE |
| 1 | | 2 | | 3 | | 4 | | 5 |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | | |
A O B O <- Servers -> Z O O O
Figure 1: Illustration of the basic CLOS
5. Justification for BGP modifications
Many data centers use BGP as a routing protocol to create an overlay
as well as an underlay network for their CLOS Topologies to simplify
layer-3 routing and operations [RFC7938]. However, BGP is a path-
vector routing protocol. Since it does not have a way for creating a
topology, it uses hop-by-hop EBGP peering to facilitate hop-by-hop
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routing for creating underlay network and for resolving any overlay
next hops. The hop-by-hop BGP peering paradigm imposes several
restrictions within a CLOS. It severely prohibits a deployment of
Route Reflectors/Route Controllers as the EBGP peerings are inline
with the data path. The BGP best path algorithm is prefix based and
it prevents announcements of prefixes to other BGP speakers until the
best path decision process is performed for the prefix at each hop.
These restrictions significantly delay the overall convergence of the
underlay network within a CLOS.
The LSVR SPF modifications allow BGP to overcome these limitations.
Furthermore, using the BGP-LS NLRI format [RFC7752] allows the LSVR
data to be advertised for nodes, links, and prefixes in the BGP
routing domain and used for SPF computations.
6. LSVR Applicability to CLOS Networks
With the BGP SPF extensions [I-D.keyupate-lsvr-bgp-spf], the BGP best
path computation and route computation are replaced with OSPF-like
algorithms [RFC2328] both to determine whether an BGP-LS NLRI has
changed and needs to be re-advertised and to compute the routing
table. These modifications will significantly improve convergence of
the underlay while affording the operational benefits of a single
routing protocol [RFC7938].
Since every router in the BGP SPF domain will have a complete view of
the topology, BGP sessions are not required on every link in the data
center fabric as with the hop-by-hop peering model described in
[RFC7938]. Rather, protocols such as BFD [RFC5580] can be used to
determine the availability links and switches as opposed to requiring
a single-hop BGP session on every link in the data centric fabric.
Consequently, the BGP session topology can be much sparser than the
data center fabric topology itself and can utilize a BGP route
reflector hierarchy with the desired level of redundancy.
Data center controllers typically require visibility to the BGP
topology to compute traffic-engineered paths. These controllers
learn the topology and other relevant information via the BGP-LS
address family [RFC7752] which is totally independent of the underlay
address families (usually IPv4/IPv6 unicast). Furthermore, in
traditional BGP underlays, all the BGP routers will need to advertise
their BGP-LS information independently. With the BGP SPF extensions,
controllers can learn the topology using the same BGP advertisements
used to compute the underlay routes. Furthermore, these data center
controllers can avail the convergence advantages of the BGP SPF
extensions. The placement of controllers can be outside of the
forwarding path or within the forwarding path.
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Alternatively, as each and every router in the BGP SPF domain will
have a complete view of the topology, the operator can also choose to
configure BGP sessions in hop-by-hop peering model described in
[RFC7938] along with BFD [RFC5580]. In doing so, while the hop-by-
hop peering model lacks inherent benefits of the controller-based
model, BGP updates need not be serialized by BGP best path algorithm
in either of these models. This helps overall network convergence.
6.1. Usage of LSVR SAFI
The BGP SPF extensions [I-D.keyupate-lsvr-bgp-spf] define a new BGP-
LS SAFI for announcement of BGP SPF link-state. The NLRI format and
its associated attributes follow the format of BGP-LS for node, link,
and prefix announcements. Whether the peering model within a CLOS
follows hop-by-hop peering described in [RFC7938] or any controller-
based or route-reflector peering, an operator can exchange BGP SPF
SAFI routes over the BGP peering by simply configuring BGP SPF SAFI
between the necessary BGP speakers.
The BGP-LS SPF SAFI can also co-exist with BGP IP Unicast SAFI which
could exchange overlapping IP routes. The routes received by these
SAFIs are evaluated, stored, and announced separately according to
the rules of [RFC4760]. The tie-breaking of route installation is a
matter of the local policies and preferences of the network operator.
Finally, as the BGP SPF peering is done following the procedures
described in [RFC4271], all the existing transport security
mechanisms including [RFC5925] are available for the BGP-LS SPF SAFI.
7. IANA Considerations
No IANA updates are requested by this document.
8. Security Considerations
This document introduces no new security considerations above and
beyond those already specified in the [RFC4271] and
[I-D.keyupate-lsvr-bgp-spf].
9. Acknowledgements
The authors would like to thank Alvaro Retana and Yan Filyurin for
the review and comments.
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10. References
10.1. 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, <https://www.rfc-
editor.org/info/rfc2119>.
10.2. Informative References
[CLOS] "A Study of Non-Blocking Switching Networks", The Bell
System Technical Journal, Vol. 32(2), DOI
10.1002/j.1538-7305.1953.tb01433.x, March 1953.
[I-D.keyupate-lsvr-bgp-spf]
Patel, K., Lindem, A., Zandi, S., and W. Henderickx,
"Shortest Path Routing Extensions for BGP Protocol",
draft-keyupate-lsvr-bgp-spf-00 (work in progress), March
2018.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998, <https://www.rfc-
editor.org/info/rfc2328>.
[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, <https://www.rfc-
editor.org/info/rfc4271>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007, <https://www.rfc-
editor.org/info/rfc4760>.
[RFC4957] Krishnan, S., Ed., Montavont, N., Njedjou, E., Veerepalli,
S., and A. Yegin, Ed., "Link-Layer Event Notifications for
Detecting Network Attachments", RFC 4957,
DOI 10.17487/RFC4957, August 2007, <https://www.rfc-
editor.org/info/rfc4957>.
[RFC5580] Tschofenig, H., Ed., Adrangi, F., Jones, M., Lior, A., and
B. Aboba, "Carrying Location Objects in RADIUS and
Diameter", RFC 5580, DOI 10.17487/RFC5580, August 2009,
<https://www.rfc-editor.org/info/rfc5580>.
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[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016, <https://www.rfc-
editor.org/info/rfc7752>.
[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016, <https://www.rfc-
editor.org/info/rfc7938>.
Authors' Addresses
Keyur Patel
Arrcus, Inc.
2077 Gateway Pl
San Jose, CA 95110
USA
Email: keyur@arrcus.com
Acee Lindem
Cisco Systems
301 Midenhall Way
Cary, NC 95110
USA
Email: acee@cisco.com
Shawn Zandi
Linkedin
222 2nd Street
San Francisco, CA 94105
USA
Email: szandi@linkedin.com
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Gaurav Dawra
Linkedin
222 2nd Street
San Francisco, CA 94105
USA
Email: gdawra@linkedin.com
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