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draft-ietf-lsvr-bgp-spf-09.xml
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<rfc category="std" docName="draft-ietf-lsvr-bgp-spf-09"
ipr="trust200902">
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<!-- ***** FRONT MATTER ***** -->
<front>
<title abbrev="BGP Protocol SPF Extensions">
Shortest Path Routing Extensions for BGP Protocol </title>
<!-- add 'role="editor"' below for the editors if appropriate -->
<!-- Another author who claims to be an editor -->
<author fullname="Keyur Patel" initials="K"
surname="Patel">
<organization>Arrcus, Inc.</organization>
<address>
<email>[email protected]</email>
</address>
</author>
<author fullname="Acee Lindem" initials="A"
surname="Lindem">
<organization>Cisco Systems</organization>
<address>
<postal>
<street>301 Midenhall Way</street>
<city>Cary</city>
<region>NC</region>
<code>27513</code>
<country>USA</country>
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<email>[email protected]</email>
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surname="Zandi">
<organization>LinkedIn</organization>
<address>
<postal>
<street>222 2nd Street</street>
<city>San Francisco</city>
<region>CA</region>
<code>94105</code>
<country>USA</country>
</postal>
<email>[email protected]</email>
</address>
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<author fullname="Wim Henderickx" initials="W"
surname="Henderickx">
<organization>Nokia</organization>
<address>
<postal>
<street></street>
<city>Antwerp</city>
<region></region>
<code></code>
<country>Belgium</country>
</postal>
<email>[email protected]</email>
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<date/>
<!-- Meta-data Declarations -->
<area>General</area>
<workgroup>Network Working Group</workgroup>
<keyword>IDR</keyword>
<!-- Keywords will be incorporated into HTML output
files in a meta tag but they have no effect on text or nroff
output. If you submit your draft to the RFC Editor, the
keywords will be used for the search engine. -->
<abstract>
<t>
Many Massively Scaled Data Centers (MSDCs) have converged on simplified
layer 3 routing. Furthermore, requirements for operational simplicity
have led many of these MSDCs to converge on BGP as their single routing
protocol for both their fabric routing and their Data Center Interconnect
(DCI) routing. This document describes a solution which leverages BGP
Link-State distribution and the Shortest Path First (SPF) algorithm similar
to Internal Gateway Protocols (IGPs) such as OSPF.
</t>
</abstract>
</front>
<middle>
<section anchor="introduction" title="Introduction">
<t>
Many Massively Scaled Data Centers (MSDCs) have converged on simplified
layer 3 routing. Furthermore, requirements for operational simplicity
have led many of these MSDCs to converge on BGP <xref target="RFC4271"/>
as their single routing protocol for both their fabric routing and
their Data Center Interconnect (DCI) routing. Requirements and procedures
for using BGP are described in <xref target="RFC7938"/>.
This document describes an alternative solution which leverages
BGP-LS <xref target="RFC7752"/> and the Shortest Path First algorithm similar
to Internal Gateway Protocols (IGPs) such as OSPF <xref target="RFC2328"/>.
</t>
<t>
<xref target="RFC4271"/> defines the
Decision Process that is used to select routes for subsequent advertisement
by applying the policies in the local Policy Information Base (PIB) to the
routes stored in its Adj-RIBs-In. The output of the Decision Process is the
set of routes that are announced by a BGP speaker to its peers. These
selected routes are stored by a BGP speaker in the speaker's Adj-RIBs-Out
according to policy.
</t>
<t>
<xref target="RFC7752"/> describes a mechanism by which link-state and TE information can
be collected from networks and shared with external components using BGP.
This is achieved by defining NLRI advertised within the BGP-LS/BGP-LS-SPF
AFI/SAFI. The BGP-LS extensions defined in <xref target="RFC7752"/> makes use of the
Decision Process defined in <xref target="RFC4271"/>.
</t>
<t>
This document augments <xref target="RFC7752"/> by replacing its use of the existing
Decision Process. Rather than reusing the BGP-LS SAFI, the BGP-LS-SPF SAFI
is introduced to insure backward compatibility. The Phase 1 and 2 decision functions of the
Decision Process are replaced with the Shortest Path First (SPF) algorithm
also known as the Dijkstra algorithm. The Phase 3 decision function is also
simplified since it is no longer dependent on the previous phases.
This solution avails the benefits of both BGP and SPF-based IGPs.
These include TCP based flow-control, no periodic link-state refresh, and
completely incremental NLRI advertisement. These advantages can reduce the
overhead in MSDCs where there is a high degree of Equal Cost Multi-Path
(ECMPs) and the topology is very stable.
Additionally, using an SPF-based computation can support fast convergence and
the computation of Loop-Free Alternatives (LFAs) <xref target="RFC5286"/> in the
event of link failures.
Furthermore, a BGP based solution lends itself to multiple peering models
including those incorporating route-reflectors <xref target="RFC4456"/>
or controllers.
</t>
<t>
Support for Multiple Topology Routing (MTR) as described in
<xref target="RFC4915"/> is an area for further study dependent on deployment
requirements.
</t>
<section title="BGP Shortest Path First (SPF) Motivation">
<t>
Given that <xref target="RFC7938"/> already describes how BGP could be used
as the sole routing protocol in an MSDC, one might question the motivation for
defining an alternate BGP deployment model when a mature solution exists.
For both alternatives, BGP offers the operational benefits of a single
routing protocol. However, BGP SPF offers some unique advantages above
and beyond standard BGP distance-vector routing.
</t>
<t>
A primary advantage is that all BGP speakers in the BGP SPF routing domain
will have a complete view of the topology. This will allow support for ECMP,
IP fast-reroute (e.g., Loop-Free Alternatives), Shared Risk Link Groups
(SRLGs), and other routing enhancements without advertisement of addition
BGP paths or other extensions. In short, the advantages of an IGP such as
OSPF <xref target="RFC2328"/> are availed in BGP.
</t>
<t>
With the simplified BGP decision process as defined in <xref target="Phase-1"/>,
NLRI changes can be disseminated throughout the BGP routing domain much
more rapidly (equivalent to IGPs with the proper implementation).
</t>
<t>
Another primary advantage is a potential reduction in NLRI advertisement.
With standard BGP distance-vector routing, a single link failure may impact
100s or 1000s prefixes and result in the withdrawal or re-advertisement of
the attendant NLRI. With BGP SPF, only the BGP speakers corresponding to
the link NLRI need withdraw the corresponding BGP-LS Link NLRI. This
advantage will contribute to both faster convergence and better scaling.
</t>
<t>
With controller and route-reflector peering models, BGP SPF advertisement
and distributed computation require a minimal number of sessions and
copies of the NLRI since only the latest version of the NLRI from the
originator is required. Given that verification of the adjacencies is done
outside of BGP (see <xref target="peering-models"/>), each BGP speaker will
only need as many sessions and copies of the NLRI as required for
redundancy (e.g., one for the SPF computation and another for backup).
Functions such as Optimized Route Reflection (ORR)
are supported without extension by virtue of the primary advantages.
Additionally, a controller could inject topology that
is learned outside the BGP routing domain.
</t>
<t>
Given that controllers are already consuming BGP-LS NLRI
<xref target="RFC7752"/>, reusing for the BGP-LS SPF leverages the
existing controller implementations.
</t>
<t>
Another potential advantage of BGP SPF is that both IPv6 and IPv4 can be
supported in the same address family using the same topology. Although not
described in this version of the document, multi-topology extensions can
be used to support separate IPv4, IPv6, unicast, and multicast topologies
while sharing the same NLRI.
</t>
<t>
Finally, the BGP SPF topology can be used as an underlay for other BGP
address families (using the existing model) and realize all the above
advantages. A simplified peering model using IPv6 link-local addresses
as next-hops can be deployed similar to <xref target="RFC5549"/>.
</t>
</section>
<section title="Requirements Language">
<t>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 14 <xref target="RFC2119"/> <xref target="RFC8174"/>
when, and only when, they appear in all capitals, as shown here.</t>
</section>
</section> <!-- for Introductions section -->
<section anchor="peering-models" title="BGP Peering Models">
<t>
Depending on the requirements, scaling, and capabilities of the BGP speakers, various
peering models are supported. The only requirement is that all BGP speakers in the
BGP SPF routing domain receive link-state NLRI on a timely basis, run an SPF calculation,
and update their data plane appropriately. The content of the Link NLRI is described
in <xref target="Link-NLRI"/>.
</t>
<section title="BGP Single-Hop Peering on Network Node Connections">
<t>
The simplest peering model is the one described in section 5.2.1 of
<xref target="RFC7938"/>. In this model,
EBGP single-hop sessions are established over direct point-to-point links
interconnecting the SPF domain nodes. For the purposes of BGP SPF, Link NLRI is only
advertised if a single-hop BGP session has been established and the Link-State/SPF
address family capability has been exchanged <xref target="RFC4790"/> on the
corresponding session.
If the session goes down, the corresponding Link NLRI will be withdrawn. Topologically,
this would be equivalent to the peering model in <xref target="RFC7938"/> where there
is a BGP session on every link in the data center switch fabric.
</t>
</section>
<section title="BGP Peering Between Directly Connected Network Nodes">
<t>
In this model, BGP speakers peer with all directly connected
network nodes but the sessions may be multi-hop and the direct connection
discovery and liveliness detection for those connections are
independent of the BGP protocol. How this is accomplished is outside
the scope of this document.
Consequently, there will be a single session even if there are multiple
direct connections between BGP speakers.
For the purposes of BGP SPF, Link NLRI is advertised as long as
a BGP session has been established, the Link-State/SPF address family
capability has been exchanged <xref target="RFC4790"/> and
the corresponding link is considered is up and considered operational.
This is much like the previous peering model only peering is on a single
loopback address and the switch fabric links can be unnumbered. However,
there will be the same number of sessions as with the previous peering
model unless there are parallel links between switches in the fabric.
</t>
</section>
<section title="BGP Peering in Route-Reflector or Controller Topology">
<t>
In this model, BGP speakers peer solely with one or more Route Reflectors
<xref target="RFC4456"/> or controllers. As in the previous model, direct
connection discovery and liveliness detection for those connections are
done outside the BGP protocol. More specifically, the Liveliness detection is
done using BFD protocol described in <xref target="RFC5880"/>. For the
purposes of BGP SPF, Link NLRI is
advertised as long as the corresponding link is up and considered operational.
</t>
<t>This peering model, known as sparse peering, allows for many fewer BGP sessions
and, consequently, instances of the same NLRI received from multiple peers. It is
discussed in greater detail in <xref target="I-D.ietf-lsvr-applicability"/>.
</t>
</section>
</section>
<section title="BGP-LS Shortest Path Routing (SPF) SAFI">
<t>
In order to replace the Phase 1 and 2 decision functions of the
existing Decision Process with an SPF-based Decision Process and streamline
the Phase 3 decision functions in a backward compatible manner, this draft
introduces the BGP-LS-SFP SAFI for BGP-LS SPF operation.
The BGP-LS-SPF (AFI 16388 / SAFI TBD1) <xref target="RFC4790"/> is allocated by IANA
as specified in the <xref target="IANA"/>. A BGP speaker using the
BGP-LS SPF extensions described herein MUST exchange the AFI/SAFI using
Multiprotocol Extensions Capability Code <xref target="RFC4760"/> with
other BGP speakers in the SPF routing domain.
</t>
</section>
<section title="Extensions to BGP-LS">
<t>
<xref target="RFC7752"/> describes a mechanism by which link-state and TE
information can be collected from networks and shared with external components
using BGP protocol. It describes both the definition of BGP-LS NLRI
that describes links, nodes, and prefixes comprising IGP link-state
information and the definition of a BGP path attribute (BGP-LS
attribute) that carries link, node, and prefix properties and
attributes, such as the link and prefix metric or auxiliary
Router-IDs of nodes, etc.
</t>
<t>
The BGP protocol will be used in the Protocol-ID field specified in
table 1 of <xref target="I-D.ietf-idr-bgpls-segment-routing-epe"/>.
The local and remote node descriptors for all NLRI will be the BGP Router-ID (TLV 516)
and either the AS Number (TLV 512) <xref target="RFC7752"/> or the BGP Confederation
Member (TLV 517) <xref target="RFC8402"/>.
However, if the BGP Router-ID is known to be unique within the BGP Routing domain,
it can be used as the sole descriptor.
</t>
<section title="Node NLRI Usage">
<t>
The BGP Node NLRI will be advertised unconditionally by all routers in
the BGP SPF routing domain.
</t>
<section title="Node NLRI Attribute SPF Capability TLV">
<t>
The SPF capability is a new Node Attribute TLV that will be added
to those defined in table 7 of <xref target="RFC7752"/>. The
new attribute TLV will only be applicable when BGP is specified
in the Node NLRI Protocol ID field.
The TBD TLV type will be defined by IANA. The new Node
Attribute TLV will contain a single-octet SPF algorithm as defined
in <xref target="RFC8402"/>.
</t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Algorithm |
+-+-+-+-+-+-+-+-+
The SPF Algorithm may take the following values:
0 - Normal Shortest Path First (SPF) algorithm based on link
metric. This is the standard shortest path algorithm as
computed by the IGP protocol. Consistent with the deployed
practice for link-state protocols, Algorithm 0 permits any
node to overwrite the SPF path with a different path based on
its local policy.
1 - Strict Shortest Path First (SPF) algorithm based on link
metric. The algorithm is identical to Algorithm 0 but Algorithm
1 requires that all nodes along the path will honor the SPF
routing decision. Local policy at the node claiming support for
Algorithm 1 MUST NOT alter the SPF paths computed by Algorithm 1.
]]></artwork>
</figure>
</t>
<t>Note that usage of Strict Shortest Path First (SPF) algorithm is
defined in the IGP algorithm registry but usage is restricted to
<xref target="I-D.ietf-idr-bgpls-segment-routing-epe"/>. Hence, its
usage for BGP-LS SPF is out of scope.</t>
<t>
When computing the SPF for a given BGP routing domain, only BGP nodes
advertising the SPF capability attribute will be included the Shortest
Path Tree (SPT).
</t>
</section>
<section anchor="node-status-tlv" title="BGP-LS Node NLRI Attribute SPF Status TLV">
<t>
A BGP-LS Attribute TLV to BGP-LS Node NLRI is defined to indicate the status of
the node with respect to the BGP SPF calculation. This will be used to rapidly take a
node out of service or to indicate the node is not to be used for transit (i.e.,
non-local) traffic. If the
SPF Status TLV is not included with the Node NLRI, the node is considered to be up
and is available for transit traffic.
</t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
BGP Status Values: 0 - Reserved
1 - Node Unreachable with respect to BGP SPF
2 - Node does not support transit with respect
to BGP SPF
3-254 - Undefined
255 - Reserved
]]></artwork>
</figure>
</t>
</section>
</section>
<section anchor="Link-NLRI" title="Link NLRI Usage">
<t>
The criteria for advertisement of Link NLRI are discussed in
<xref target="peering-models"/>.
</t>
<t>
Link NLRI is advertised with local and remote node descriptors as described
above and unique link identifiers dependent on the addressing. For IPv4 links, the
links local IPv4 (TLV 259) and remote IPv4 (TLV 260) addresses will be used.
For IPv6 links, the local IPv6 (TLV 261) and remote IPv6 (TLV 262) addresses
will be used. For unnumbered links, the link local/remote identifiers (TLV 258)
will be used. For links supporting having both IPv4 and IPv6 addresses, both sets
of descriptors may be included in the same Link NLRI. The link identifiers are
described in table 5 of <xref target="RFC7752"/>.
</t>
<t>
The link IGP metric attribute TLV (TLV 1095) as well as any others required for non-SPF
purposes SHOULD be advertised. The metric value in this TLV is variable length dependent
on specific protocol usage (refer to section 3.3.2.4 in <xref target="RFC7752"/>).
For simplicity, the BGP-LS SPF metric length will be 4 octets.
Algorithms such as setting the metric inversely to the link speed as done in the
OSPF MIB <xref target="RFC4750"/> MAY be supported. However, this is beyond the scope
of this document.
</t>
<section title="BGP-LS Link NLRI Attribute Prefix-Length TLVs">
<t>
Two BGP-LS Attribute TLVs to BGP-LS Link NLRI are defined to advertise the prefix length
associated with the IPv4 and IPv6 link prefixes. The prefix length is used for the
optional installation of prefixes corresponding to Link NLRI as defined in
<xref target="BGP-SPF"/>.</t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD IPv4 or IPv6 Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix-Length |
+-+-+-+-+-+-+-+-+
Prefix-length - A one-octet length restricted to 1-32 for IPv4
Link NLRI endpoint prefixes and 1-128 for IPv6
Link NLRI endpoint prefixes.
]]></artwork>
</figure>
</t>
</section>
<section anchor="link-status-tlv" title="BGP-LS Link NLRI Attribute SPF Status TLV">
<t>
A BGP-LS Attribute TLV to BGP-LS Link NLRI is defined to indicate the status of
the link with respect to the BGP SPF calculation. This will be used to expedite
convergence for link failures as discussed in <xref target="failure-converge"/>. If the
SPF Status TLV is not included with the Link NLRI, the link is considered
up and available.
</t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
BGP Status Values: 0 - Reserved
1 - Link Unreachable with respect to BGP SPF
2-254 - Undefined
255 - Reserved
]]></artwork>
</figure>
</t>
</section>
</section>
<section title="Prefix NLRI Usage">
<t>
Prefix NLRI is advertised with a local node descriptor as described above and the prefix and
length used as the descriptors (TLV 265) as described in <xref target="RFC7752"/>.
The prefix metric attribute TLV (TLV 1155) as well as any others required for non-SPF
purposes SHOULD be advertised. For loopback prefixes, the metric should be 0. For non-loopback
prefixes, the setting of the metric is a local matter and beyond the scope of this document.
</t>
<section anchor="prefix-status-tlv" title="BGP-LS Prefix NLRI Attribute SPF Status TLV">
<t>
A BGP-LS Attribute TLV to BGP-LS Prefix NLRI is defined to indicate the status of
the prefix with respect to the BGP SPF calculation. This will be used to expedite
convergence for prefix unreachability as discussed in <xref target="failure-converge"/>.
If the SPF Status TLV is not included with the Prefix NLRI, the prefix is considered
reachable.
</t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
BGP Status Values: 0 - Reserved
1 - Prefix down with respect to SPF
2-254 - Undefined
255 - Reserved
]]></artwork>
</figure>
</t>
</section>
</section>
<section title="BGP-LS Attribute Sequence-Number TLV">
<t>
A new BGP-LS Attribute TLV to BGP-LS NLRI types is defined to assure the most
recent version of a given NLRI is used in the SPF computation.
The TBD TLV type will be defined by IANA. The new BGP-LS
Attribute TLV will contain an 8-octet sequence number. The usage of the Sequence Number TLV
is described in <xref target="Phase-1"/>.
</t>
<t>
<figure align="center">
<artwork align="left"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (High-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (Low-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</t>
<t>
Sequence Number <vspace blankLines="1" />
The 64-bit strictly increasing sequence number is incremented for every
version of BGP-LS NLRI originated. BGP speakers implementing this specification MUST use
available mechanisms to preserve the sequence number's strictly increasing property
for the deployed life of the BGP speaker (including cold restarts).
One mechanism for accomplishing this would be to use the high-order 32 bits of the
sequence number as a wrap/boot count that is incremented anytime the BGP router
loses its sequence number state or the low-order 32 bits wrap.
</t>
<t>
When incrementing the sequence number for each self-originated NLRI,
the sequence number should be treated as an unsigned 64-bit
value. If the lower-order 32-bit value wraps, the higher-order 32&nbhy;bit value should
be incremented and saved in non-volatile storage. If by some chance the BGP Speaker is
deployed long enough that there is a possibility that the 64-bit sequence number may wrap
or a BGP Speaker completely loses its sequence number state (e.g., the BGP speaker hardware
is replaced or experiences a cold-start), the phase 1 decision function
(see <xref target="Phase-1"/>) rules will insure convergence, albeit, not immediately.
</t>
</section>
</section>
<section title="Decision Process with SPF Algorithm">
<t>
The Decision Process described in <xref target="RFC4271"/> takes place in
three distinct phases. The Phase 1 decision function of the Decision Process is
responsible for calculating the degree
of preference for each route received from a BGP speaker's peer. The Phase 2 decision
function is invoked on completion of the Phase 1 decision function and is responsible
for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into the Loc-RIB.
The combination of the Phase 1 and 2 decision functions is characterized as
a Path Vector algorithm.
</t>
<t>
The SPF based Decision process replaces the BGP best-path Decision process described in
<xref target="RFC4271"/>. This process starts with selecting only those Node NLRI whose
SPF capability TLV matches with the local BGP speaker's SPF capability TLV value.
Since Link-State NLRI always contains the local descriptor
<xref target="RFC7752"/>, it will only
be originated by a single BGP speaker in the BGP routing domain.
These selected Node NLRI and their Link/Prefix NLRI are used to build a directed
graph during the SPF computation. The best paths for BGP prefixes
are installed as a result of the SPF process.
</t>
<t>
When BGP-LS-SPF NLRI is received, all that is required is to determine
whether it is the best-path by examining the Node-ID and sequence number as described
in <xref target="Phase-1"/>. If the received best-path NLRI had changed, it will be advertised
to other BGP-LS-SPF peers. If the attributes have changed (other than the sequence number),
a BGP SPF calculation will be scheduled. However, a changed NLRI MAY be
advertised to other peers almost immediately and propagation of changes can approach
IGP convergence times. To accomplish this, the MinRouteAdvertisementIntervalTimer and
MinASOriginationIntervalTimer <xref target="RFC4271"/> are not applicable
to the BGP-LS-SPF SAFI. Rather, SPF calculations SHOULD be triggered and dampened consistent
with the SPF back-off algorithm specified in <xref target="RFC8405"/>.
</t>
<t>
The Phase 3 decision function
of the Decision Process <xref target="RFC4271"/> is also simplified since under
normal SPF operation, a BGP speaker would advertise the NLRI
selected for the SPF to all BGP peers with the BGP-LS/BGP-LS-SPF AFI/SAFI.
Application of policy would not be prevented however its usage to best-path process
would be limited as the SPF relies solely on link metrics.
</t>
<section anchor="Phase-1" title="Phase-1 BGP NLRI Selection">
<t>
The rules for NLRI selection are greatly simplified from <xref target="RFC4271"/>.
<list style="numbers">
<t>
If the NLRI is received from the BGP speaker originating the NLRI (as determined by the
comparing BGP Router ID in the NLRI Node identifiers with the BGP speaker Router ID),
then it is preferred over the same NLRI from non-originators. This rule will assure that
stale NLRI is updated even if a BGP-LS router loses its sequence number state due to a
cold-start.
</t>
<t>
If the Sequence-Number TLV is present in the BGP-LS Attribute, then the NLRI with the
most recent, i.e., highest sequence number is selected. BGP-LS NLRI with a Sequence-Number
TLV will be considered more recent than NLRI without a BGP-LS Attribute or a
BGP-LS Attribute that doesn't include the Sequence-Number TLV.
</t>
<t>The final tie-breaker is the NLRI from the BGP Speaker with the numerically largest
BGP Router ID.
</t>
</list>
</t>
<t>
When a BGP speaker completely loses its sequence number state, i.e., due to a cold start, or
in the unlikely possibility that that sequence number wraps, the BGP routing domain will
still converge. This is due to the fact that BGP speakers adjacent to the router will
always accept self-originated NLRI from the associated speaker as more recent (rule # 1). When
BGP speaker reestablishes a connection with its peers, any existing session will be taken
down and stale NLRI will be replaced by the new NLRI and stale NLRI will be discarded
independent of whether or not BGP graceful restart is deployed, <xref target="RFC4724"/>. The adjacent
BGP speaker will update their NLRI advertisements in turn until the BGP routing domain has
converged.
</t>
<t>
The modified SPF Decision Process performs an SPF calculation rooted at the BGP
speaker using the metrics from
Link and Prefix NLRI Attribute TLVs <xref target="RFC7752"/>. As a result, any attributes that
would influence the Decision process defined in <xref target="RFC4271"/> like ORIGIN, MULTI_EXIT_DISC, and
LOCAL_PREF attributes are ignored by the SPF algorithm. Furthermore, the NEXT_HOP attribute
value is preserved but otherwise ignored during the SPF or best-path.
</t>
</section>
<section title="Dual Stack Support">
<t>
The SPF-based decision process operates on Node, Link, and Prefix NLRIs that support
both IPv4 and IPv6 addresses. Whether to run a single SPF instance or multiple
SPF instances for separate AFs is a matter of a local implementation. Normally, IPv4
next-hops are calculated for IPv4 prefixes and IPv6 next-hops are calculated for IPv6
prefixes. However, an interesting use-case is deployment of <xref target="RFC5549"/> where
IPv6 next-hops are calculated for both IPv4 and IPv6 prefixes. As stated in
<xref target="introduction"/>, support for Multiple Topology Routing (MTR) is an area
for future study.
</t>
</section>
<section anchor="BGP-SPF" title="SPF Calculation based on BGP-LS NLRI">
<t>This section details the BGP-LS SPF local routing information base (RIB) calculation.
The router will use BGP-LS Node, Link, and Prefix NLRI to populate the local RIB using the
following algorithm. This calculation yields the set of intra-area routes associated
with the BGP-LS domain. A router calculates the shortest-path tree using itself
as the root. Variations and optimizations of the algorithm are valid as long as it
yields the same set of routes. The algorithm below supports Equal Cost Multi-Path (ECMP)
routes. Weighted Unequal Cost Multi-Path are out of scope. The organization of this section
owes heavily to section 16 of <xref target="RFC2328"/>.</t>
<t>The following abstract data structures are defined in order to specify the algorithm.
<list style="symbols">
<t>Local Route Information Base (RIB) - This is abstract contains reachability information
(i.e., next hops) for all prefixes (both IPv4 and IPv6) as well as the Node NLRI
reachability. Implementations may choose to implement this as separate RIBs for each
address family and/or Node NLRI.</t>
<t>Link State NLRI Database (LSNDB) - Database of BGP-LS NLRI that facilitates access to
all Node, Link, and Prefix NLRI as well as all the Link and Prefix NLRI corresponding to
a given Node NLRI. Other optimization, such as, resolving bi-directional connectivity
associations between Link NLRI are possible but of scope of this document.</t>
<t>Candidate List - This is a list of candidate Node NLRI with the lowest cost Node NLRI
at the front of the list. It is typically implemented as a heap but other concrete
data structures have also been used.</t>
</list></t>
<t>The algorithm is comprised of the steps below:
<list style="numbers">
<t>The current local RIB is invalidated. The local RIB is
rebuilt during the course of the SPF computation. The existing routing entries
are preserved for comparison to
determine changes that need to be installed in the global RIB.</t>
<t>The computing router's Node NLRI is installed in the local RIB with a cost of 0 and as
the sole entry in the candidate list.</t>
<t>The Node NLRI with the lowest cost is removed from the candidate list for processing. If
the BGP-LS Node attribute includes an SPF Status TLV (<xref target="node-status-tlv"/>)
indicating the node is unreachable, the Node NLRI is ignored and the next lowest cost
Node NLRI is selected from candidate list. The
Node corresponding to this NLRI will be referred to as the Current Node. If the candidate
list is empty, the SPF calculation has completed and the algorithm proceeds to step 6.</t>
<t>All the Prefix NLRI with the same Node Identifiers as the Current Node will be considered
for installation. The cost for each prefix is the metric advertised in the Prefix NLRI
added to the cost to reach the Current Node.
<list style="symbols">
<t>If the BGP-LS Prefix attribute includes an SPF Status TLV indicating the prefix is
unreachable, the BGP-LS Prefix NLRI is considered unreachable and the next BGP-LS Prefix
NLRI is examined.</t>
<t>If the prefix is in the local RIB and the cost is greater than the Current route's metric,
the Prefix NLRI does not contribute to the route and is ignored.</t>
<t>If the prefix is in the local RIB and the cost is less than the current route's metric,
the Prefix is installed with the Current Node's next-hops replacing the local RIB route's
next-hops and the metric being updated.</t>
<t>If the prefix is in the local RIB and the cost is same as the current route's metric,
the Prefix is installed with the Current Node's next-hops being merged with local
RIB route's next-hops.</t>
</list> </t>
<t>All the Link NLRI with the same Node Identifiers as the Current Node will be considered
for installation. Each link will be examined and will be referred to in the following text
as the Current Link. The cost of the Current Link is the advertised metric in the Link NLRI
added to the cost to reach the Current Node.
<list style="symbols">
<t>Optionally, the prefix(es) associated with the Current Link are installed into the
local RIB using the same rules as were used for Prefix NLRI in the previous steps.</t>
<t>If the current Node NLRI attributes includes the SPF status TLV
(<xref target="node-status-tlv"/>) and the status
indicates that the Node doesn't support transit, the next link for the current node is
processed.</t>
<t>The Current Link's endpoint Node NLRI is accessed (i.e., the Node NLRI
with the same Node identifiers as the Link endpoint). If it exists, it will be referred to
as the Endpoint Node NLRI and the algorithm will proceed as follows:
<list style="symbols">
<t>If the BGP-LS Link NLRI attribute includes an SPF Status TLV indicating the link is
down, the BGP-LS Link NLRI is considered down and the next BGP-LS Link
NLRI is examined.</t>
<t>All the Link NLRI corresponding the Endpoint Node NLRI will be searched for a back-link
NLRI pointing to the current node. Both the Node identifiers and the
Link endpoint identifiers in the Endpoint Node's Link NLRI must match for a match. If there
is no corresponding Link NLRI corresponding to the Endpoint Node NLRI, the Endpoint Node
NLRI fails the bi-directional connectivity test and is not processed further.</t>
<t>If the Endpoint Node NLRI is not on the candidate list, it is inserted based on the
link cost and BGP Identifier (the latter being used as a tie-breaker).</t>
<t>If the Endpoint Node NLRI is already on the candidate list with a lower cost, it need
not be inserted again.</t>
<t>If the Endpoint Node NLRI is already on the candidate list with a higher cost, it must
be removed and reinserted with a lower cost.</t>
</list></t>
<t>Return to step 3 to process the next lowest cost Node NLRI on the candidate list.</t>
</list></t>
<t>The local RIB is examined and changes (adds, deletes, modifications) are installed into
the global RIB.</t>
</list>
</t>
</section>
<section title="NEXT_HOP Manipulation">
<t>
A BGP speaker that supports SPF extensions MAY interact with peers that don't support
SPF extensions. If the BGP-LS address family is advertised to a peer not
supporting the SPF extensions described herein, then the BGP speaker
MUST conform to the NEXT_HOP rules specified in <xref target="RFC4271"/> when announcing
the Link-State address family routes to those peers.
</t>
<t>
All BGP peers that support SPF extensions would locally compute the Loc-RIB next-hops
as a result of the SPF process. Consequently, the NEXT_HOP attribute is always ignored on
receipt. However, BGP speakers SHOULD set the NEXT_HOP address according to the
NEXT_HOP attribute rules specified in <xref target="RFC4271"/>.
</t>
</section>
<section title="IPv4/IPv6 Unicast Address Family Interaction">
<t>
While the BGP-LS SPF address family and the IPv4/IPv6 unicast address families install routes
into the same device routing tables, they will operate independently much the same as OSPF and
IS-IS would operate today (i.e., "Ships-in-the-Night" mode). There will be no implicit
route redistribution between the BGP address families. However, implementation specific
redistribution mechanisms SHOULD be made available with the restriction that redistribution
of BGP-LS SPF routes into the IPv4 address family applies only to IPv4 routes and redistribution
of BGP-LS SPF route into the IPv6 address family applies only to IPv6 routes.
</t>
<t>
Given the fact that SPF algorithms are based on the assumption that all routers in the
routing domain calculate the precisely the same SPF tree and install the same set of
routes, it is RECOMMENDED that BGP-LS SPF IPv4/IPv6 routes be given priority by default
when installed into their respective RIBs. In common implementations the prioritization
is governed by route preference or administrative distance with lower being more preferred.
</t>
</section>
<section anchor="NLRI-Advertise" title="NLRI Advertisement and Convergence">
<section anchor="failure-converge" title="Link/Prefix Failure Convergence">
<t>A local failure will prevent a link from being used in the SPF calculation
due to the IGP bi-directional connectivity requirement. Consequently, local link
failures should always be given priority over updates (e.g., withdrawing all
routes learned on a session) in order to ensure the highest priority propagation
and optimal convergence.
</t>
<t>
An IGP such as OSPF <xref target="RFC2328"/> will stop using the link as soon as
the Router-LSA for one side of the link is received. With normal BGP advertisement,
the link would continue to be used until the last copy of the BGP-LS Link NLRI
is withdrawn. In order to avoid this delay, the originator of the Link NLRI will
advertise a more recent version of the BGP-LS Link NLRI including the SPF Status TLV
<xref target="link-status-tlv"/> indicating the link
is down with respect to BGP SPF. After some configurable period of time, e.g., 2-3 seconds,
the BGP-LS Link NLRI can be withdrawn with no consequence.
If the link becomes available
in that period, the originator of the BGP-LS LINK NLRI will simply advertise a more recent
version of the BGP-LS Link NLRI without the SPF Status TLV in the BGP-LS Link Attributes.
</t>
<t>Similarly, when a prefix becomes unreachable, a more recent version of the BGP-LS
Prefix NLRI will be advertised with the SPF Status TLV <xref target="prefix-status-tlv"/>
indicating the prefix is unreachable in the BGP-LS Prefix Attributes and the prefix will be
considered unreachable with respect to BGP SPF. After some configurable period of time, e.g., 2-3 seconds,
the BGP-LS Prefix NLRI can be withdrawn with no consequence.
If the prefix becomes reachable in that period, the originator of the BGP-LS Prefix NLRI
will simply advertise a more recent version of the BGP-LS Prefix NLRI without the
SPF Status TLV in the BGP-LS Prefix Attributes.
</t>
</section>
<section anchor="node-failure" title="Node Failure Convergence">
<t>With BGP without graceful restart <xref target="RFC4724"/>, all the NLRI advertised
by node are implicitly withdrawn when a session failure is detected. If fast failure
detection such as BFD is utilized, and the node is on the fastest converging path, the most
recent versions of BGP-LS NLRI may be withdrawn while these versions are in-flight on longer
paths. This will result the older version of the NLRI being used until the new versions
arrive and, potentially, unnecessary route flaps. Therefore, BGP-LS SPF NLRI SHOULD always
be retained before being implicitly withdrawn for a brief configurable interval, e.g., 2-3
seconds. This will not delay convergence since the adjacent nodes will detect the
link failure and advertise a more recent NLRI indicating the link is down with respect to
BGP SPF <xref target="failure-converge"/> and the BGP-SPF calculation will failure the
bi-directional connectivity check.
</t>
</section>
</section>
<section title="Error Handling">
<t>
When a BGP speaker receives a BGP Update containing a malformed SPF Capability TLV
in the Node NLRI BGP-LS Attribute <xref target="RFC7752"/>,
it MUST ignore the received TLV and the Node NLRI and not pass it to other BGP peers as
specified in <xref target="RFC7606"/>.
When discarding a Node NLRI with malformed TLV, a BGP speaker SHOULD log an error for
further analysis.
</t>
</section>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>
This document defines an AFI/SAFI for BGP-LS SPF operation and
requests IANA to assign the BGP-LS/BGP-LS-SPF (AFI 16388 / SAFI TBD1)
as described in <xref target="RFC4760"/>.
</t>
<t>
This document also defines five attribute TLVs for BGP-LS NLRI.
We request IANA to assign types for the SPF capability TLV,
Sequence Number TLV, IPv4 Link Prefix-Length TLV, IPv6 Link Prefix-Length TLV,
and SPF Status TLV
from the "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor,
and Attribute TLVs" Registry.
</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>
This extension to BGP does not change the underlying security issues
inherent in the existing <xref target="RFC4271"/>, <xref target="RFC4724"/>,
and <xref target="RFC7752"/>.
</t>
</section>
<section anchor="Management" title="Management Considerations">
<t>
This section includes unique management considerations for the BGP-LS SPF address family.
</t>
<section anchor="Config" title="Configuration">
<t>
In addition to configuration of the BGP-LS SPF address family, implementations SHOULD