Diff: rfc9816v4.txt - rfc9816.txt

	rfc9816v4.txt	rfc9816.txt

	skipping to change at line 169 ¶	skipping to change at line 169 ¶
	sessions are congruent with the data path. The BGP best-path	sessions are congruent with the data path. The BGP best-path
	algorithm is prefix based, and it prevents announcements of prefixes	algorithm is prefix based, and it prevents announcements of prefixes
	to other BGP speakers until the best-path decision process has been	to other BGP speakers until the best-path decision process has been
	performed for the prefix at each intermediate hop. These	performed for the prefix at each intermediate hop. These
	restrictions significantly delay the overall convergence of the	restrictions significantly delay the overall convergence of the
	underlay network within a Clos network.	underlay network within a Clos network.

	The BGP SPF modifications allow BGP to overcome these limitations.	The BGP SPF modifications allow BGP to overcome these limitations.
	Furthermore, using the BGP-LS Network Layer Reachability Information	Furthermore, using the BGP-LS Network Layer Reachability Information
	(NLRI) format allows the BGP SPF data to be advertised for nodes,	(NLRI) format allows the BGP SPF data to be advertised for nodes,

	links, and prefixes in the BGP routing domain and used for SPF	links, and prefixes in the BGP routing domain [RFC9552] and used for
	computations [RFC9552].	SPF computations [RFC9815].

	Additional motivation for deploying BGP-SPF is included in [RFC9815].	Additional motivation for deploying BGP-SPF is included in [RFC9815].

	5. BGP-SPF Applicability to Clos Networks	5. BGP-SPF Applicability to Clos Networks

	With the BGP-SPF extensions [RFC9815], the BGP best-path computation	With the BGP-SPF extensions [RFC9815], the BGP best-path computation
	and route computation are replaced with link-state algorithms such as	and route computation are replaced with link-state algorithms such as
	those used by OSPF [RFC2328], both to determine whether a BGP-LS-SPF	those used by OSPF [RFC2328], both to determine whether a BGP-LS-SPF
	NLRI has changed and needs to be readvertised and to compute the BGP	NLRI has changed and needs to be readvertised and to compute the BGP
	routes. These modifications will significantly improve convergence	routes. These modifications will significantly improve convergence

	skipping to change at line 200 ¶	skipping to change at line 200 ¶
	their BGP-LS information independently. With the BGP-SPF extensions,	their BGP-LS information independently. With the BGP-SPF extensions,
	controllers can learn the topology using the same BGP advertisements	controllers can learn the topology using the same BGP advertisements
	used to compute the underlay routes. Furthermore, these data center	used to compute the underlay routes. Furthermore, these data center
	controllers can avail the convergence advantages of the BGP-SPF	controllers can avail the convergence advantages of the BGP-SPF
	extensions. The placement of controllers can be outside of the	extensions. The placement of controllers can be outside of the
	forwarding path or within the forwarding path.	forwarding path or within the forwarding path.

	Alternatively, as each and every router in the BGP-SPF domain will	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	have a complete view of the topology, the operator can also choose to
	configure BGP sessions in the hop-by-hop peering model described in	configure BGP sessions in the hop-by-hop peering model described in

	[RFC7938] along with BFD [RFC5580]. In doing so, while the hop-by-	[RFC7938] along with BFD [RFC5880]. In doing so, while the hop-by-
	hop peering model lacks the inherent benefits of the controller-based	hop peering model lacks the inherent benefits of the controller-based
	model, BGP updates need not be serialized by the BGP best-path	model, BGP updates need not be serialized by the BGP best-path
	algorithm in either of these models. This helps overall network	algorithm in either of these models. This helps overall network
	convergence.	convergence.

	5.1. Usage of BGP-LS-SPF SAFI	5.1. Usage of BGP-LS-SPF SAFI

	Section 5.1 of [RFC9815] defines a new BGP-LS-SPF SAFI for	Section 5.1 of [RFC9815] defines a new BGP-LS-SPF SAFI for
	announcement of the BGP-SPF link-state. The NLRI format and its	announcement of the BGP-SPF link-state. The NLRI format and its
	associated attributes follow the format of BGP-LS for node, link, and	associated attributes follow the format of BGP-LS for node, link, and
	prefix announcements. Whether the peering model within a Clos	prefix announcements. Whether the peering model within a Clos

	follows hop-by-hop peering described in [RFC7938] or any controller-	follows hop-by-hop peering as described in [RFC7938] or any
	based or route-reflector peering, an operator can exchange BGP-LS-SPF	controller-based or route-reflector peering, an operator can exchange
	SAFI routes over the BGP peering by simply configuring BGP-LS-SPF	BGP-LS-SPF SAFI routes over the BGP peering by simply configuring
	SAFI between the necessary BGP speakers.	BGP-LS-SPF SAFI between the necessary BGP speakers.

	The BGP-LS-SPF SAFI can also coexist with BGP IP Unicast SAFI	The BGP-LS-SPF SAFI can also coexist with BGP IP Unicast SAFI
	[RFC4760], which could exchange overlapping IP routes. One use case	[RFC4760], which could exchange overlapping IP routes. One use case
	for this is where BGP-LS-SPF routes are used for the underlay and BGP	for this is where BGP-LS-SPF routes are used for the underlay and BGP
	IP Unicast routes for VPNs are advertised in the overlay as described	IP Unicast routes for VPNs are advertised in the overlay as described
	in [RFC4364]. The routes received by these SAFIs are evaluated,	in [RFC4364]. The routes received by these SAFIs are evaluated,
	stored, and announced independently according to the rules of	stored, and announced independently according to the rules of
	[RFC4760]. The tiebreaking of route installation is a matter of the	[RFC4760]. The tiebreaking of route installation is a matter of the
	local policies and preferences of the network operator.	local policies and preferences of the network operator.


	skipping to change at line 250 ¶	skipping to change at line 250 ¶
	As previously stated, BGP-SPF can be deployed using the existing	As previously stated, BGP-SPF can be deployed using the existing
	peering model where there is a single-hop BGP session on each and	peering model where there is a single-hop BGP session on each and
	every link in the data center fabric [RFC7938]. This provides for	every link in the data center fabric [RFC7938]. This provides for
	both the advertisement of routes and the determination of link and	both the advertisement of routes and the determination of link and
	neighboring router availability. With BGP-SPF, the underlay will	neighboring router availability. With BGP-SPF, the underlay will
	converge faster due to changes to the decision process that will	converge faster due to changes to the decision process that will
	allow NLRI changes to be advertised faster after detecting a change.	allow NLRI changes to be advertised faster after detecting a change.

	5.2.1. Sparse Peering Model	5.2.1. Sparse Peering Model


	Alternately, BFD [RFC5580] can be used to swiftly determine the	Alternately, BFD [RFC5880] can be used to swiftly determine the
	availability of links, and the BGP peering model can be significantly	availability of links, and the BGP peering model can be significantly
	sparser than the data center fabric. BGP-SPF sessions only need to	sparser than the data center fabric. BGP-SPF sessions only need to
	be established with enough peers to provide a biconnected graph. If	be established with enough peers to provide a biconnected graph. If
	Internal BGP (IBGP) is used, then the BGP routers at tier N-1 will	Internal BGP (IBGP) is used, then the BGP routers at tier N-1 will
	act as route-reflectors for the routers at tier N.	act as route-reflectors for the routers at tier N.

	The obvious usage of sparse peering is to avoid parallel BGP sessions	The obvious usage of sparse peering is to avoid parallel BGP sessions
	on links between the same two routers in the data center fabric.	on links between the same two routers in the data center fabric.
	However, this use case is not very useful since parallel L3 links	However, this use case is not very useful since parallel L3 links
	between the same two BGP routers are rare in Clos or Fat Tree	between the same two BGP routers are rare in Clos or Fat Tree

	skipping to change at line 283 ¶	skipping to change at line 283 ¶
	of the spines dependent on the flooding redundancy required to be	of the spines dependent on the flooding redundancy required to be
	reasonably certain that every node within the BGP-SPF routing domain	reasonably certain that every node within the BGP-SPF routing domain
	has the complete topology.	has the complete topology.

	Alternately, controller-based data center topologies are envisioned	Alternately, controller-based data center topologies are envisioned
	where BGP speakers within the data center only establish BGP sessions	where BGP speakers within the data center only establish BGP sessions
	with two or more controllers. In these topologies, fabric nodes	with two or more controllers. In these topologies, fabric nodes
	below the first tier, as shown in Figure 1 of [RFC7938], will	below the first tier, as shown in Figure 1 of [RFC7938], will
	establish BGP multi-hop sessions with the controllers. For the	establish BGP multi-hop sessions with the controllers. For the
	multi-hop sessions, determining the route to the controllers without	multi-hop sessions, determining the route to the controllers without

	depending on BGP would need to be through some other means beyond the	depending on BGP would need to be through some other means, which is
	scope of this document. However, the BGP discovery mechanisms	beyond the scope of this document. However, the BGP discovery
	described in Section 5.5 would be one possibility.	mechanisms described in Section 5.5 would be one possibility.

	5.2.2. Biconnected Graph Heuristic	5.2.2. Biconnected Graph Heuristic

	With a biconnected graph heuristic, discovery of BGP SPF peers is	With a biconnected graph heuristic, discovery of BGP SPF peers is
	assumed, e.g., as described in Section 5.5. In this context,	assumed, e.g., as described in Section 5.5. In this context,
	"biconnected" refers to the fact that there must be an advertised	"biconnected" refers to the fact that there must be an advertised

	Link NLRI for both BGP and SPF peers associated with the link before	Link NLRI for both BGP SPF peers associated with the link before the
	the link can be used in the BGP SPF route calculation. Additionally,	link can be used in the BGP SPF route calculation. Additionally, it
	it is assumed that the direction of the peering can be ascertained.	is assumed that the direction of the peering can be ascertained. In
	In the context of a data center fabric, the direction is either	the context of a data center fabric, the direction is either
	northbound (toward the spine), southbound (toward the Top-of-Rack	northbound (toward the spine), southbound (toward the Top-of-Rack
	(ToR) routers), or east-west (same level in the hierarchy). The	(ToR) routers), or east-west (same level in the hierarchy). The
	determination of the direction is beyond the scope of this document.	determination of the direction is beyond the scope of this document.
	However, it would be reasonable to assume a technique where the ToR	However, it would be reasonable to assume a technique where the ToR
	routers can be identified and the number of hops to the ToR is used	routers can be identified and the number of hops to the ToR is used
	to determine the direction.	to determine the direction.

	In this heuristic, BGP speakers allow passive session establishment	In this heuristic, BGP speakers allow passive session establishment
	for southbound BGP sessions. For northbound sessions, BGP speakers	for southbound BGP sessions. For northbound sessions, BGP speakers
	will attempt to maintain two northbound BGP sessions with different	will attempt to maintain two northbound BGP sessions with different

	skipping to change at line 371 ¶	skipping to change at line 371 ¶
	session:	session:

	* The AS and BGP Identifier of a potential peer.	* The AS and BGP Identifier of a potential peer.

	* Supported security capabilities, and for cryptographic	* Supported security capabilities, and for cryptographic
	authentication, the security capabilities and possibly a key chain	authentication, the security capabilities and possibly a key chain
	[RFC8177] for use.	[RFC8177] for use.

	* A Session Policy Identifier, which is a group number or name used	* A Session Policy Identifier, which is a group number or name used
	to associate common session parameters with the peer. For	to associate common session parameters with the peer. For

	example, in a data center, BGP sessions with a ToR device could	example, in a data center, BGP sessions with a ToR router could
	have different parameters than BGP sessions between leaf and	have different parameters than BGP sessions between leaf and spine
	spine.	nodes.

	In a data center fabric, it is often useful to know whether a peer is	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	southbound (towards the servers) or northbound (towards the spine or
	super-spine), e.g., see Section 5.2.2. One mechanism, without	super-spine), e.g., see Section 5.2.2. One mechanism, without
	specifying all the details, might be for the ToR routers to be	specifying all the details, might be for the ToR routers to be
	identified when installed and for the other routers in the fabric to	identified when installed and for the other routers in the fabric to
	determine their level based on the distance from the closest ToR	determine their level based on the distance from the closest ToR
	router.	router.

	If there are multiple links between BGP speakers or the links between	If there are multiple links between BGP speakers or the links between

	skipping to change at line 436 ¶	skipping to change at line 436 ¶

	Since BGP-SPF is to be used for the routing underlay and Data Center	Since BGP-SPF is to be used for the routing underlay and Data Center
	Interconnect (DCI) gateway boxes typically have direct or very simple	Interconnect (DCI) gateway boxes typically have direct or very simple
	connectivity, BGP external sessions would typically not include the	connectivity, BGP external sessions would typically not include the
	BGP-LS-SPF SAFI.	BGP-LS-SPF SAFI.

	6. Non-Clos / Fat Tree Topology Applicability	6. Non-Clos / Fat Tree Topology Applicability

	The BGP-SPF extensions [RFC9815] can be used in other topologies and	The BGP-SPF extensions [RFC9815] can be used in other topologies and
	avail the inherent convergence improvements. Additionally, sparse	avail the inherent convergence improvements. Additionally, sparse

	peering techniques may be utilized Section 5.2. However, determining	peering techniques may be utilized as described in Section 5.2.
	whether to establish a BGP session is more complex, and the heuristic	However, determining whether to establish a BGP session is more
	described in Section 5.2.2 cannot be used. In such topologies, other	complex, and the heuristic described in Section 5.2.2 cannot be used.
	techniques such as those described in [RFC9667] may be employed. One	In such topologies, other techniques such as those described in
	potential deployment would be the underlay for a Service Provider	[RFC9667] may be employed. One potential deployment would be the
	(SP) backbone where usage of a single protocol, i.e., BGP, is	underlay for a Service Provider (SP) backbone where usage of a single
	desired.	protocol, i.e., BGP, is desired.

	7. Non-Transit Node Capability	7. Non-Transit Node Capability

	In certain scenarios, a BGP node wishes to participate in the BGP-SPF	In certain scenarios, a BGP node wishes to participate in the BGP-SPF
	topology but never be used for transit traffic. These include	topology but never be used for transit traffic. These include
	situations where a server wants to make application services	situations where a server wants to make application services
	available to clients homed at subnets throughout the BGP-SPF domain	available to clients homed at subnets throughout the BGP-SPF domain
	but does not ever want to be used as a router (i.e., carry transit	but does not ever want to be used as a router (i.e., carry transit
	traffic). Another specific instance is where a controller is	traffic). Another specific instance is where a controller is
	resident on a server and direct connectivity to the controller is	resident on a server and direct connectivity to the controller is

	skipping to change at line 534 ¶	skipping to change at line 534 ¶
	[RFC4957] Krishnan, S., Ed., Montavont, N., Njedjou, E., Veerepalli,	[RFC4957] Krishnan, S., Ed., Montavont, N., Njedjou, E., Veerepalli,
	S., and A. Yegin, Ed., "Link-Layer Event Notifications for	S., and A. Yegin, Ed., "Link-Layer Event Notifications for
	Detecting Network Attachments", RFC 4957,	Detecting Network Attachments", RFC 4957,
	DOI 10.17487/RFC4957, August 2007,	DOI 10.17487/RFC4957, August 2007,
	<https://www.rfc-editor.org/info/rfc4957>.	<https://www.rfc-editor.org/info/rfc4957>.

	[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF	[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
	for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,	for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
	<https://www.rfc-editor.org/info/rfc5340>.	<https://www.rfc-editor.org/info/rfc5340>.


	[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>.

	[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection	[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
	(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,	(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
	<https://www.rfc-editor.org/info/rfc5880>.	<https://www.rfc-editor.org/info/rfc5880>.

	[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP	[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
	Authentication Option", RFC 5925, DOI 10.17487/RFC5925,	Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
	June 2010, <https://www.rfc-editor.org/info/rfc5925>.	June 2010, <https://www.rfc-editor.org/info/rfc5925>.

	[RFC7404] Behringer, M. and E. Vyncke, "Using Only Link-Local	[RFC7404] Behringer, M. and E. Vyncke, "Using Only Link-Local
	Addressing inside an IPv6 Network", RFC 7404,	Addressing inside an IPv6 Network", RFC 7404,

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