Network Working Group K. Patel
Internet-Draft Arrcus, Inc.
Intended status: Standards Track A. Lindem
Expires: March 31, 2019 Cisco Systems
S. Zandi
Linkedin
W. Henderickx
Nokia
September 27, 2018
Shortest Path Routing Extensions for BGP Protocol
draft-ietf-lsvr-bgp-spf-03.txt
Abstract
Many Massively Scaled Data Centers (MSDCs) have converged on
simplified layer 3 routing. Furthermore, requirements for
operational simplicity have lead 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.
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
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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
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 March 31, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. BGP Shortest Path First (SPF) Motivation . . . . . . . . 4
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. BGP Peering Models . . . . . . . . . . . . . . . . . . . . . 5
2.1. BGP Single-Hop Peering on Network Node Connections . . . 5
2.2. BGP Peering Between Directly Connected Network Nodes . . 6
2.3. BGP Peering in Route-Reflector or Controller Topology . . 6
3. BGP-LS Shortest Path Routing (SPF) SAFI . . . . . . . . . . . 6
4. Extensions to BGP-LS . . . . . . . . . . . . . . . . . . . . 7
4.1. Node NLRI Usage and Modifications . . . . . . . . . . . . 7
4.2. Link NLRI Usage . . . . . . . . . . . . . . . . . . . . . 8
4.2.1. BGP-LS Link NLRI Attribute Prefix-Length TLVs . . . . 9
4.3. Prefix NLRI Usage . . . . . . . . . . . . . . . . . . . . 9
4.4. BGP-LS Attribute Sequence-Number TLV . . . . . . . . . . 9
5. Decision Process with SPF Algorithm . . . . . . . . . . . . . 10
5.1. Phase-1 BGP NLRI Selection . . . . . . . . . . . . . . . 11
5.2. Dual Stack Support . . . . . . . . . . . . . . . . . . . 12
5.3. SPF Calculation based on BGP-LS NLRI . . . . . . . . . . 12
5.4. NEXT_HOP Manipulation . . . . . . . . . . . . . . . . . . 15
5.5. IPv4/IPv6 Unicast Address Family Interaction . . . . . . 15
5.6. NLRI Advertisement and Convergence . . . . . . . . . . . 15
5.7. Error Handling . . . . . . . . . . . . . . . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
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8. Management Considerations . . . . . . . . . . . . . . . . . . 16
8.1. Configuration . . . . . . . . . . . . . . . . . . . . . . 16
8.2. Operational Data . . . . . . . . . . . . . . . . . . . . 16
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.1. Normative References . . . . . . . . . . . . . . . . . . 17
11.2. Information References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
Many Massively Scaled Data Centers (MSDCs) have converged on
simplified layer 3 routing. Furthermore, requirements for
operational simplicity have lead many of these MSDCs to converge on
BGP [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 [RFC7938].
This document describes an alternative solution which leverages BGP-
LS [RFC7752] and the Shortest Path First algorithm similar to
Internal Gateway Protocols (IGPs) such as OSPF [RFC2328].
[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.
[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 [RFC7752] makes use of the Decision Process defined in [RFC4271].
This document augments [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 a
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SPF-based computation can support fast convergence and the
computation of Loop-Free Alternatives (LFAs) [RFC5286] in the event
of link failures. Furthermore, a BGP based solution lends itself to
multiple peering models including those incorporating route-
reflectors [RFC4456] or controllers.
Support for Multiple Topology Routing (MTR) as described in [RFC4915]
is an area for further study dependent on deployment requirements.
1.1. BGP Shortest Path First (SPF) Motivation
Given that [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.
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 [RFC2328] are availed in
BGP.
With the simplified BGP decision process as defined in Section 5.1,
NLRI changes can be disseminated throughout the BGP routing domain
much more rapidly (equivalent to IGPs with the proper
implementation).
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.
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 Section 2), 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.
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Additionally, a controller could inject topology that is learned
outside the BGP routing domain.
Given that controllers are already consuming BGP-LS NLRI [RFC7752],
reusing for the BGP-LS SPF leverages the existing controller
implementations.
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.
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 [RFC5549].
1.2. 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
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. BGP Peering Models
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 Section 4.2.
2.1. BGP Single-Hop Peering on Network Node Connections
The simplest peering model is the one described in section 5.2.1 of
[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 [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 [RFC7938] where there is a BGP session on
every link in the data center switch fabric.
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2.2. BGP Peering Between Directly Connected Network Nodes
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 [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 unnumber of sessions as
with the previous peering model unless there are parrallel links
between switches in the fabric.
2.3. BGP Peering in Route-Reflector or Controller Topology
In this model, BGP speakers peer solely with one or more Route
Reflectors [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
[RFC5880]. For the purposes of BGP SPF, Link NLRI is advertised as
long as the corresponding link is up and considered operational.
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
[I-D.ietf-lsvr-applicability].
3. BGP-LS Shortest Path Routing (SPF) SAFI
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 (AF 16388 / SAFI TBD1) [RFC4790] is
allocated by IANA as specified in the Section 6. A BGP speaker using
the BGP-LS SPF extensions described herein MUST exchange the AFI/SAFI
using Multiprotocol Extensions Capability Code [RFC4760] with other
BGP speakers in the SPF routing domain.
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4. Extensions to BGP-LS
[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.
The BGP protocol will be used in the Protocol-ID field specified in
table 1 of [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) [RFC7752] or the BGP
Confederation Member (TLV 517) [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.
4.1. Node NLRI Usage and Modifications
The SPF capability is a new Node Attribute TLV that will be added to
those defined in table 7 of [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
[RFC8402].
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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.
Note that usage of Strict Shortest Path First (SPF) algorithm is
defined in the IGP algorithm registry but usage is restricted to
[I-D.ietf-idr-bgpls-segment-routing-epe]. Hence, its usage for BGP-
LS SPF is out of scope.
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).
4.2. Link NLRI Usage
The criteria for advertisement of Link NLRI are discussed in
Section 2.
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 [RFC7752].
The link IGP metric attribute TLV (TLV 1095) as well as any others
required for non-SPF purposes SHOULD be advertised. Algorithms such
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as setting the metric inversely to the link speed as done in the OSPF
MIB [RFC4750] MAY be supported. However, this is beyond the scope of
this document.
4.2.1. BGP-LS Link NLRI Attribute Prefix-Length TLVs
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 Section 5.3.
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 NLIR endpoint prefixes and 1-128 for IPv6
Link NLRI endpoint prefixes.
4.3. Prefix NLRI Usage
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 [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.
4.4. BGP-LS Attribute Sequence-Number TLV
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 Section 5.1.
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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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sequence Number
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.
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-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 Section 5.1) rules will insure convergence,
albeit, not immediately.
5. Decision Process with SPF Algorithm
The Decision Process described in [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.
The SPF based Decision process replaces the BGP best-path Decision
process described in [RFC4271]. This process starts with selecting
only those Node NLRI whose SPF capability TLV matches with the local
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BGP speaker's SPF capability TLV value. Since Link-State NLRI always
contains the local descriptor [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.
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 Section 5.1. 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 [RFC4271] are not applicable to the
BGP-LS-SPF SAFI. Rather, SPF calculations SHOULD be triggered and
dampened consistent with the SPF backoff algorithm specified in
[RFC8405].
The Phase 3 decision function of the Decision Process [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.
5.1. Phase-1 BGP NLRI Selection
The rules for NLRI selection are greatly simplified from [RFC4271].
1. 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.
2. 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.
3. The final tie-breaker is the NLRI from the BGP Speaker with the
numerically largest BGP Router ID.
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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, [RFC4724]. The adjacent BGP speaker will update their NLRI
advertisements in turn until the BGP routing domain has converged.
The modified SPF Decision Process performs an SPF calculation rooted
at the BGP speaker using the metrics from Link and Prefix NLRI
Attribute TLVs [RFC7752]. As a result, any attributes that would
influence the Decision process defined in [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.
5.2. Dual Stack Support
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
[RFC5549] where IPv6 next-hops are calculated for both IPv4 and IPv6
prefixes. As stated in Section 1, support for Multiple Topology
Routing (MTR) is an area for future study.
5.3. SPF Calculation based on BGP-LS NLRI
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 [RFC2328].
The following abstract data structures are defined in order to
specify the algorithm.
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o 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.
o 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.
o 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.
The algorithm is comprised of the steps below:
1. The current local RIB is invalidated. The local RIB is built
again from scratch. The existing routing entries are preserved
for comparision to determine changes that need to be installed in
the global RIB.
2. The computing router's Node NLRI is installed in the local RIB
with a cost of 0 and as as the sole entry in the candidate list.
3. The Node NLRI with the lowest cost is removed from the candidate
list for processing. 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.
4. 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.
* If the prefix is not in the local RIB, the prefix is installed
and will inherit the Current Node's next hops.
* 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.
* 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
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Current Node's next-hops replacing the local RIB route's next-
hops and the metric being updated.
* 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.
5. 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.
* 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.
* 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:
+ 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
NLIR fails the bi-directional connectivity test and is not
processed further.
+ 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).
+ If the Endpoint Node NLRI is already on the candidate list
with a lower cost, it need not be inserted again.
+ 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.
* Return to step 3 to process the next lowest cost Node NLRI on
the candidate list.
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6. The local RIB is examined and changes (adds, deletes,
modifications) are installed into the global RIB.
5.4. NEXT_HOP Manipulation
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 [RFC4271] when announcing the Link-State address family
routes to those peers.
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 [RFC4271].
5.5. IPv4/IPv6 Unicast Address Family Interaction
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.
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.
5.6. NLRI Advertisement and Convergence
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.
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Delaying the withdrawal of non-local routes is an area for further
study as more IGP-like mechanisms would be required to prevent usage
of stale NLRI.
5.7. Error Handling
When a BGP speaker receives a BGP Update containing a malformed SPF
Capability TLV in the Node NLRI BGP-LS Attribute [RFC7752], it MUST
ignore the received TLV and the Node NLRI and not pass it to other
BGP peers as specified in [RFC7606]. When discarding a Node NLRI
with malformed TLV, a BGP speaker SHOULD log an error for further
analysis.
6. IANA Considerations
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 [RFC4750].
This document also defines four attribute TLVs for BGP LS NLRI. We
request IANA to assign TLVs for the SPF capability, Sequence Number,
IPv4 Link Prefix-Length, and IPv6 Link Prefix-Length from the "BGP-LS
Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
TLVs" Registry.
7. Security Considerations
This extension to BGP does not change the underlying security issues
inherent in the existing [RFC4271], [RFC4724], and [RFC7752].
8. Management Considerations
This section includes unique management considerations for the BGP-LS
SPF address family.
8.1. Configuration
In addition to configuration of the BGP-LS SPF address family,
implementations SHOULD support the configuratio of the
INITIAL_SPF_DELAY, SHORT_SPF_DELAY, LONG_SPF_DELAY, TIME_TO_LEARN,
and HOLDDOWN_INTERVAL as documented in [RFC8405].
8.2. Operational Data
In order to troubleshoot SPF issues, implementations SHOULD support
an SPF log including entries for previous SPF computations, Each SPF
log entry would include the BGP-LS NLRI SPF triggering the SPF, SPF
scheduled time, SPF start time, SPF end time, and SPF type if
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different types of SPF are supported. Since the size of the log will
be finite, implementations SHOULD also maintain counters for the
total number of SPF computations of each type and the total number of
SPF triggering events. Additionally, to troubleshoot SPF scheduling
and backoff [RFC8405], the current SPF backoff state, remaining time-
to-learn, remaining holddown, last trigger event time, last SPF time,
and next SPF time should be available.
9. Acknowledgements
The authors would like to thank Sue Hares, Jorge Rabadan, Boris
Hassanov, Dan Frost, and Fred Baker for their review and comments.
10. Contributors
In addition to the authors listed on the front page, the following
co-authors have contributed to the document.
Derek Yeung
Arrcus, Inc.
derek@arrcus.com
Gunter Van De Velde
Nokia
gunter.van_de_velde@nokia.com
Abhay Roy
Cisco Systems
akr@cisco.com
Venu Venugopal
Cisco Systems
venuv@cisco.com
11. References
11.1. Normative References
[I-D.ietf-idr-bgpls-segment-routing-epe]
Previdi, S., Filsfils, C., Patel, K., Ray, S., and J.
Dong, "BGP-LS extensions for Segment Routing BGP Egress
Peer Engineering", draft-ietf-idr-bgpls-segment-routing-
epe-15 (work in progress), March 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>.
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Internet-Draft BGP Protocol SPF Extensions September 2018
[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>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
[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>.
[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>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8405] Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
Francois, P., and C. Bowers, "Shortest Path First (SPF)
Back-Off Delay Algorithm for Link-State IGPs", RFC 8405,
DOI 10.17487/RFC8405, June 2018, <https://www.rfc-
editor.org/info/rfc8405>.
11.2. Information References
[I-D.ietf-lsvr-applicability]
Patel, K., Lindem, A., Zandi, S., and G. Dawra, "Usage and
Applicability of Link State Vector Routing in Data
Centers", draft-ietf-lsvr-applicability-00 (work in
progress), July 2018.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998, <https://www.rfc-
editor.org/info/rfc2328>.
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[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,
<https://www.rfc-editor.org/info/rfc4456>.
[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
DOI 10.17487/RFC4724, January 2007, <https://www.rfc-
editor.org/info/rfc4724>.
[RFC4750] Joyal, D., Ed., Galecki, P., Ed., Giacalone, S., Ed.,
Coltun, R., and F. Baker, "OSPF Version 2 Management
Information Base", RFC 4750, DOI 10.17487/RFC4750,
December 2006, <https://www.rfc-editor.org/info/rfc4750>.
[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>.
[RFC4790] Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
Application Protocol Collation Registry", RFC 4790,
DOI 10.17487/RFC4790, March 2007, <https://www.rfc-
editor.org/info/rfc4790>.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC4915, June 2007,
<https://www.rfc-editor.org/info/rfc4915>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008, <https://www.rfc-
editor.org/info/rfc5286>.
[RFC5549] Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network
Layer Reachability Information with an IPv6 Next Hop",
RFC 5549, DOI 10.17487/RFC5549, May 2009,
<https://www.rfc-editor.org/info/rfc5549>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
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Authors' Addresses
Keyur Patel
Arrcus, Inc.
Email: keyur@arrcus.com
Acee Lindem
Cisco Systems
301 Midenhall Way
Cary, NC 27513
USA
Email: acee@cisco.com
Shawn Zandi
Linkedin
222 2nd Street
San Francisco, CA 94105
USA
Email: szandi@linkedin.com
Wim Henderickx
Nokia
Antwerp
Belgium
Email: wim.henderickx@nokia.com
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