Internet DRAFT - draft-keyupate-lsvr-applicability
draft-keyupate-lsvr-applicability
LSVR K. Patel
Internet-Draft Arrcus, Inc.
Intended status: Informational A. Lindem
Expires: January 3, 2019 Cisco Systems
S. Zandi
G. Dawra
Linkedin
July 2, 2018
Usage and Applicability of Link State Vector Routing in Data Centers
draft-keyupate-lsvr-applicability-02.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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 3, 2019.
Copyright Notice
Copyright (c) 2018 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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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
3. Recommended Reading . . . . . . . . . . . . . . . . . . . . . 3
4. Common Deployment Scenario . . . . . . . . . . . . . . . . . 3
5. Justification for BGP SPF Extension . . . . . . . . . . . . . 4
6. LSVR Applicability to CLOS Networks . . . . . . . . . . . . . 4
6.1. Usage of BGP-LS SAFI . . . . . . . . . . . . . . . . . . 5
6.1.1. Relationship to Other BGP AFI/SAFI Tuples . . . . . . 5
6.2. Peering Models . . . . . . . . . . . . . . . . . . . . . 5
6.2.1. Bi-Connected Graph Heuristic . . . . . . . . . . . . 6
6.3. BGP Peer Discovery . . . . . . . . . . . . . . . . . . . 6
6.3.1. BGP Peer Discovery Requirements . . . . . . . . . . . 6
6.3.2. BGP Peer Discovery Alternatives . . . . . . . . . . . 7
6.3.3. Data Center Interconnect (DCI) Applicability . . . . 7
6.4. Non-CLOS/FAT Tree Topology Applicability . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
This document complements [I-D.ietf-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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
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14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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-
SPF [I-D.ietf-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
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5. Justification for BGP SPF Extension
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 create a fabric topology,
it uses hop-by-hop EBGP peering to facilitate hop-by-hop routing to
create the underlay network and to resolve 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 sessions 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
intermediate 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.ietf-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].
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.
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
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[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 BGP-LS SAFI
The BGP SPF extensions [I-D.ietf-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.
6.1.1. Relationship to Other BGP AFI/SAFI Tuples
Normally, the BGP-LS AFI/SAFI is used solely to compute the underlay
and is given preference over other AFI/SAFIs. Other BGP SAFIs, e.g.,
IPv6/IPv6 Unicast VPN would use the BGP-SPF computed routes for next
hop resolution. However, if BGP-LS NLRI is also being advertised for
controller consumption, there is no need to replicate the Node, Link,
and Prefix NLRI in BGP-NLRI. Rather, additional NLRI attributes can
be advertised in the BGP-LS SPF AFI/SAFI as required.
6.2. Peering Models
As previously stated, BGP SPF can be deployed using the existing
peering model where there is a single hop BGP session on each and
every link in the data center fabric [RFC7938]. This provides for
both the advertisement of routes and the determination of link and
neighboring switch availability. With BGP SPF, the underlay will
converge faster due to changes in the decision process which will
allow NLRI changes to be advertised faster after detecting a change.
Alternately, BFD [RFC5580] can be used to swiftly determine the
availability of links and the BGP peering model can be significantly
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sparser than the data center fabric. BGP SPF sessions then only be
established with enough peers to provide a bi-connected graph. If
IEBGP is used, then the BGP routers at tier N-1 will act as route-
reflectors for the routers at tier N.
6.2.1. Bi-Connected Graph Heuristic
With this heuristic, discovery of BGP peers is assumed Section 6.3.
Additionally, it assumed that the direction of the peering can be
ascertained. In the context of a data center fabric, direction is
either northbound (toward the spine), southbound (toward the Top-Of-
Rack (TOR) switches) or east-west (same level in hierarchy. The
determination of the direction is beyond the scope of this document.
However, it would be reasonable to assume a technique where the TOR
switches can be identified and the number of hops to the TOR is used
to determine the direction.
In this heuristic, BGP speakers allow passive session establishment
for southbound BGP sessions. For northbound sessions, BGP speakers
will attempt to maintain two northbound BGP sessions with different
switches (in data center fabrics there is normally a single layer-3
connection anyway). For east-west sessions, passive BGP session
establishment is allowed. However, BGP speaker will never actively
establish an east-west BGP session unless it can't establish two
northbound BGP sessions.
6.3. BGP Peer Discovery
6.3.1. BGP Peer Discovery Requirements
The most basic requirement is to be able to discover the address of a
single-hop peer without pre-configuration. This is being
accomplished today with using IPv6 Router Advertisements (RA)
[RFC4861] and assuming that a BGP sessions is desired with any
discovered peer. Beyond the basic requirement, it is useful to have
to following information relating to the BGP session:
o Autonomous System (AS) and BGP Identifier of a potential peer.
The latter can be used for debugging and to decrease the
likelihood of BGP session establishment collisions.
o Security capabilities supported and for cryptographic
authentication, the security capabilities and possibly a key-chain
[RFC8177] to be used.
o Session Policy Identifier - A group number or name used to
associate common session parameters with the peer. For example,
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in a data center, BGP sessions with a Top of Rack (ToR) device
could have parameters than BGP sessions between leaf and spine.
In a data center fabric, it is often useful to know whether a peer is
southbound (towards the servers) or northbound (towards the spine or
super-spine) Section 6.2.1. A potential requirement would also be to
determine this dynamically. One mechanism, without specifying all
the details, might be for the ToRs to be identified when installed
and for the others switches in the fabric to determine their level
based on the distance from the closest ToR.
If there are multiple links between BGP speakers or the links between
BGP speakers are unnumbered, it is also useful to be able to
establish multi-hop sessions using the loopback addresses. This will
often require the discovery protocol to install route(s) toward the
potential peer loopback addresses prior to BGP session establishment.
Finally, a simple BGP discovery protocol could also be used to
establish a multi-hop session with one or more controllers by
advertising connectivity to one or more controllers. However, once
the multi-hop session actually traverses multiple nodes, it is
bordering a distance-vector routing protocol and possibly this is not
a good requirement for the discovery protocol.
6.3.2. BGP Peer Discovery Alternatives
While BGP peer discovery is not part of [I-D.ietf-lsvr-bgp-spf],
there are, at least, three proposals for BGP peer discovery. At
least one of these mechanisms will be adopted and will be applicable
to deployments other than the data center. It is strongly
RECOMMENDED that the accepted mechanism be used in conjunction with
BGP SPF in data centers. The BGP discovery mechanism should
discovery both peer addresses and endpoints for BFD discovery.
Additionally, it would be great if there were a heuristic for
determining whether the peer is at a tier above or below the
discovering BGP speaker (refer to Section 6.2.1).
The BGP discovery mechanisms under consideration are
[I-D.acee-idr-lldp-peer-discovery],
[I-D.xu-idr-neighbor-autodiscovery], and [I-D.ymbk-lsvr-lsoe].
6.3.3. Data Center Interconnect (DCI) Applicability
Since BGP SPF is to be used for the routing underlay and DCI gateway
boxes typically have direct or very simple connectivity, BGP external
sessions would typically not include the BGP SPF SAFI.
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6.4. Non-CLOS/FAT Tree Topology Applicability
The BGP SPF extensions [I-D.ietf-lsvr-bgp-spf] can be used in other
topologies and avail the inherent convergence improvements.
Additionally, sparse peering techniques may be utilized Section 6.2.
However, determining whether or to establish a BGP session is more
complex and the heuristic described in Section 6.2.1 cannot be used.
In such topologies, other techniques such as those described in
[I-D.li-dynamic-flooding] may be employed. One potential deployment
would be the underlay for a Service Provider (SP) backbone where
usage of a single protocol, i.e., BGP, is desired.
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.ietf-lsvr-bgp-spf].
9. Acknowledgements
The authors would like to thank Alvaro Retana and Yan Filyurin for
the review and comments.
10. References
10.1. Normative References
[I-D.ietf-lsvr-bgp-spf]
Patel, K., Lindem, A., Zandi, S., and W. Henderickx,
"Shortest Path Routing Extensions for BGP Protocol",
draft-ietf-lsvr-bgp-spf-01 (work in progress), May 2018.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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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.acee-idr-lldp-peer-discovery]
Lindem, A., Patel, K., Zandi, S., Haas, J., and X. Xu,
"BGP Logical Link Discovery Protocol (LLDP) Peer
Discovery", draft-acee-idr-lldp-peer-discovery-03 (work in
progress), June 2018.
[I-D.li-dynamic-flooding]
Li, T. and P. Psenak, "Dynamic Flooding on Dense Graphs",
draft-li-dynamic-flooding-05 (work in progress), June
2018.
[I-D.xu-idr-neighbor-autodiscovery]
Xu, X., Bi, K., Tantsura, J., Triantafillis, N., and K.
Talaulikar, "BGP Neighbor Autodiscovery", draft-xu-idr-
neighbor-autodiscovery-08 (work in progress), May 2018.
[I-D.ymbk-lsvr-lsoe]
Bush, R. and K. Patel, "Link State Over Ethernet", draft-
ymbk-lsvr-lsoe-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>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007, <https://www.rfc-
editor.org/info/rfc4861>.
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[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>.
[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>.
[RFC8177] Lindem, A., Ed., Qu, Y., Yeung, D., Chen, I., and J.
Zhang, "YANG Data Model for Key Chains", RFC 8177,
DOI 10.17487/RFC8177, June 2017, <https://www.rfc-
editor.org/info/rfc8177>.
Authors' Addresses
Keyur Patel
Arrcus, Inc.
2077 Gateway Pl
San Jose, CA 95110
USA
Email: keyur@arrcus.com
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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
Gaurav Dawra
Linkedin
222 2nd Street
San Francisco, CA 94105
USA
Email: gdawra@linkedin.com
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