rfc9717v1.txt		rfc9717.txt
	skipping to change at line 182 ¶		skipping to change at line 182 ¶
	LEO: Low Earth Orbit. A satellite in LEO has an altitude of 2,000		LEO: Low Earth Orbit. A satellite in LEO has an altitude of 2,000
	km or less.		km or less.
	Local gateway: Each user station is associated with a single gateway		Local gateway: Each user station is associated with a single gateway
	in its region.		in its region.
	LSP: Link State Protocol Data Unit. An IS-IS LSP is a set of		LSP: Link State Protocol Data Unit. An IS-IS LSP is a set of
	packets that describe a node's connectivity to other nodes.		packets that describe a node's connectivity to other nodes.
	MEO: Medium Earth Orbit. A satellite in MEO is between LEO and GEO		MEO: Medium Earth Orbit. A satellite in MEO is between LEO and GEO
	orbits and has an altitude between 2,000 km and 35,786 km.		and has an altitude between 2,000 km and 35,786 km.
	SID: Segment Identifier [RFC8402]		SID: Segment Identifier [RFC8402]
	Stripe: A set of satellites in a few adjacent orbits. These form an		Stripe: A set of satellites in a few adjacent orbits. These form an
	IS-IS L1 area.		IS-IS L1 area.
	SR: Segment Routing [RFC8402]		SR: Segment Routing [RFC8402]
	Uplink: The half of a link leading from an Earth station to a		Uplink: The half of a link leading from an Earth station to a
	satellite.		satellite.
	User station: An Earth station interconnected with a small end-user		User station: An Earth station interconnected with a small end-user
	network.		network.
	2. Overview		2. Overview
	2.1. Topological Considerations		2.1. Topological Considerations
	Satellites travel in specific orbits around their parent planet.		Satellites travel in specific orbits around their parent planets.
	Some of them have their orbital periods synchronized to planetary		Some of them have their orbital periods synchronized to planetary
	rotation, so they are effectively stationary over a single point.		rotation, so they are effectively stationary over a single point.
	Other satellites have orbits that cause them to travel across regions		Other satellites have orbits that cause them to travel across regions
	of the planet either gradually or quite rapidly. Respectively, these		of the planet either gradually or quite rapidly. Respectively, these
	are typically known as the Geostationary Earth Orbit (GEO), Medium		are typically known as the Geostationary Earth Orbit (GEO), Medium
	Earth Orbit (MEO), or Low Earth Orbit (LEO), depending on the		Earth Orbit (MEO), or Low Earth Orbit (LEO), depending on the
	altitude. This discussion is not Earth-specific; as we get to other		altitude. This discussion is not Earth-specific; as we get to other
	planets, we can test this approach's generality.		planets, we can test this approach's generality.
	Satellites may have data interconnections with one another through		Satellites may have data interconnections with one another through
	skipping to change at line 267 ¶		skipping to change at line 267 ¶
	hierarchical abstractions, so a key question of any routing		hierarchical abstractions, so a key question of any routing
	architecture will be about the abstractions that can be created to		architecture will be about the abstractions that can be created to
	contain topological information.		contain topological information.
	Normal routing protocols are architected to operate with a static but		Normal routing protocols are architected to operate with a static but
	somewhat unreliable topology. Satellite networks lack the static		somewhat unreliable topology. Satellite networks lack the static
	organization of terrestrial networks, so normal architectural		organization of terrestrial networks, so normal architectural
	practices for scalability may not apply, and alternative approaches		practices for scalability may not apply, and alternative approaches
	may need consideration.		may need consideration.
	In a traditional deployment of a link-state routing protocol, current		In a typical deployment of a link-state routing protocol, current
	implementations can be deployed with a single area that spans a few		implementations can be deployed with a single area that spans a few
	thousand routers. A single area would also provide no isolation for		thousand routers. A single area would also provide no isolation for
	topological changes, causing every link change to be propagated		topological changes, causing every link change to be propagated
	throughout the entire network. This would be insufficient for the		throughout the entire network. This would be insufficient for the
	needs of large satellite networks.		needs of large satellite networks.
	Multiple areas or multiple instances of an Interior Gateway Protocol		Multiple areas or multiple instances of an Interior Gateway Protocol
	(IGP) can be used to improve scalability, but there are limitations		(IGP) can be used to improve scalability, but there are limitations
	to traditional approaches.		to typical approaches.
	Currently, the IETF actively supports two link-state IGPs: OSPF		Currently, the IETF actively supports two link-state IGPs: OSPF
	[RFC2328] [RFC5340] and IS-IS.		[RFC2328] [RFC5340] and IS-IS.
	OSPF requires that the network operate around a backbone area, with		OSPF requires that the network operate around a backbone area, with
	subsidiary areas hanging off of the backbone. While this works well		subsidiary areas hanging off of the backbone. While this works well
	for traditional terrestrial networks, this does not seem appropriate		for typical terrestrial networks, this does not seem appropriate for
	for satellite networks, where there is no centralized portion of the		satellite networks, where there is no centralized portion of the
	topology.		topology.
	IS-IS has a different hierarchical structure, where Level 1 (L1)		IS-IS has a different hierarchical structure, where Level 1 (L1)
	areas are connected sets of nodes, and then Level 2 (L2) is a		areas are connected sets of nodes, and then Level 2 (L2) is a
	connected subset of the topology that intersects all of the L1 areas.		connected subset of the topology that intersects all of the L1 areas.
	Individual nodes can be L1, L2, or both (L1L2). Traditional IS-IS		Individual nodes can be L1, L2, or both (L1L2). Typical IS-IS
	designs require that any node or link that is to be used as transit		designs require that any node or link that is to be used as transit
	between L2 areas must appear as part of the L2 topology. In a		between L2 areas must appear as part of the L2 topology. In a
	satellite network, any satellite could end up being used for L2		satellite network, any satellite could end up being used for L2
	transit, and so every satellite and link would be part of L2,		transit, and so every satellite and link would be part of L2,
	negating any scalability benefits from IS-IS's hierarchical		negating any scalability benefits from IS-IS's hierarchical
	structure.		structure.
	We elaborate on considerations specific to IS-IS in Section 4.		We elaborate on considerations specific to IS-IS in Section 4.
	2.4. Assumptions		2.4. Assumptions
	skipping to change at line 402 ¶		skipping to change at line 402 ¶
	We assume that the satellite network is connected (in the graph		We assume that the satellite network is connected (in the graph
	theory sense) almost always, even if some links are down. This		theory sense) almost always, even if some links are down. This
	implies that there is almost always some path to the destination. In		implies that there is almost always some path to the destination. In
	the extreme case with no such path, we assume that it is acceptable		the extreme case with no such path, we assume that it is acceptable
	to drop the payload packets. We do not require buffering of traffic		to drop the payload packets. We do not require buffering of traffic
	when a link is down. Instead, traffic should be rerouted.		when a link is down. Instead, traffic should be rerouted.
	2.5. Problem Statement		2.5. Problem Statement
	The goal of the routing architecture is to provide an organizational		The goal of the routing architecture is to provide an organizational
	structure to protocols running on the satellite network such that		structure to protocols running on the satellite network. This
	topology information is conveyed through relevant portions of the		architecture must convey topology information to relevant portions of
	network, paths are computed across the network, and data can be		the network. This enables path computation that is used for data
	delivered along those paths so that the structure can scale without		forwarding. The architecture must also scale without global changes
	any changes to the organizational structure.		to the organizational structure.
	3. Forwarding Plane		3. Forwarding Plane
	The end goal of a network is to deliver traffic. In a satellite		The end goal of a network is to deliver traffic. In a satellite
	network where the topology is in a continual state of flux and the		network where the topology is in a continual state of flux and the
	user stations frequently change their association with the		user stations frequently change their association with the
	satellites, having a highly flexible and adaptive forwarding plane is		satellites, having a highly flexible and adaptive forwarding plane is
	essential. Toward this end, we propose using MPLS as the fundamental		essential. Toward this end, we propose using MPLS as the fundamental
	forwarding plane architecture [RFC3031]. Specifically, we propose		forwarding plane architecture [RFC3031]. Specifically, we propose
	using an approach based on Segment Routing (SR) [RFC8402] with an		using an approach based on Segment Routing (SR) [RFC8402] with an
	skipping to change at line 442 ¶		skipping to change at line 442 ¶
	on its TTL.		on its TTL.
	We assume that there is a link-layer mechanism for a user station to		We assume that there is a link-layer mechanism for a user station to
	associate with a satellite. User stations will have an IP address		associate with a satellite. User stations will have an IP address
	assigned from a prefix managed by its local gateway. The mechanisms		assigned from a prefix managed by its local gateway. The mechanisms
	for this assignment and its communication to the end station are not		for this assignment and its communication to the end station are not
	discussed herein but might be similar to DHCP [RFC2131]. User		discussed herein but might be similar to DHCP [RFC2131]. User
	station IP addresses change infrequently and do not reflect their		station IP addresses change infrequently and do not reflect their
	association with their first-hop satellite. Gateways and their		association with their first-hop satellite. Gateways and their
	supporting terrestrial networks advertise prefixes covering all its		supporting terrestrial networks advertise prefixes covering all its
	local user stations into the global Internet.		local user stations throughout the global Internet.
	User stations may be assigned a node SID, in which case MPLS		User stations may be assigned a node SID, in which case MPLS
	forwarding can be used for all hops to the user station.		forwarding can be used for all hops to the user station.
	Alternatively, if the user station does not have a node SID, then the		Alternatively, if the user station does not have a node SID, then the
	last hop from the satellite to the end station can be performed based		last hop from the satellite to the end station can be performed based
	on the destination IP address of the packet. This does not require a		on the destination IP address of the packet. This does not require a
	full longest-prefix-match lookup, as the IP address is merely a		full longest-prefix-match lookup, as the IP address is merely a
	unique identifier at this point.		unique identifier at this point.
	Similarly, gateways may be assigned a node SID. A possible		Similarly, gateways may be assigned a node SID. A possible
	skipping to change at line 548 ¶		skipping to change at line 548 ¶
	details about the satellites within the stripe. The resulting		details about the satellites within the stripe. The resulting
	architecture scales proportionately to the number of stripes		architecture scales proportionately to the number of stripes
	required, not the number of satellites.		required, not the number of satellites.
	Groups of MEO and GEO satellites with interconnecting ISLs can also		Groups of MEO and GEO satellites with interconnecting ISLs can also
	form an IS-IS L1L2 area. Satellites that lack intra-constellation		form an IS-IS L1L2 area. Satellites that lack intra-constellation
	ISLs are better as independent L2 nodes.		ISLs are better as independent L2 nodes.
	6. Traffic Forwarding and Traffic Engineering		6. Traffic Forwarding and Traffic Engineering
	Forwarding in this architecture is straightforward. A path from a		The forwarding architecture presented here is straightforward. A
	gateway to a user station on the same stripe only requires a single		path from a gateway to a user station on the same stripe only
	node SID for the satellite that provides the downlink to the user		requires a single node SID for the satellite that provides the
	station.		downlink to the user station.
	\		\
	Gateway --> X		Gateway --> X
	\		\
	\		\
	X		X
	\		\
	\		\
	X ---> x User Station		X ---> x User Station
	\		\
	skipping to change at line 850 ¶		skipping to change at line 850 ¶
	Routing", 2015 IEEE Global Communications Conference		Routing", 2015 IEEE Global Communications Conference
	(GLOBECOM), DOI 10.1109/GLOCOM.2015.7417097, December		(GLOBECOM), DOI 10.1109/GLOCOM.2015.7417097, December
	2015, <https://ieeexplore.ieee.org/document/7417097>.		2015, <https://ieeexplore.ieee.org/document/7417097>.
	[Handley] Handley, M., "Delay is Not an Option: Low Latency Routing		[Handley] Handley, M., "Delay is Not an Option: Low Latency Routing
	in Space", HotNets '18: Proceedings of the 17th ACM		in Space", HotNets '18: Proceedings of the 17th ACM
	Workshop on Hot Topics in Networks, pp. 85-91,		Workshop on Hot Topics in Networks, pp. 85-91,
	DOI 10.1145/3286062.3286075, November 2018,		DOI 10.1145/3286062.3286075, November 2018,
	<https://dl.acm.org/doi/10.1145/3286062.3286075#>.		<https://dl.acm.org/doi/10.1145/3286062.3286075#>.
	[ITU] ITU, "Radio Regulations - Articles", 2016,		[ITU] ITU, "Radio Regulations - Articles", 2024,
	<https://search.itu.int/history/		<https://search.itu.int/history/HistoryDigitalCollectionDo
	HistoryDigitalCollectionDocLibrary/1.43.48.en.101.pdf>.		cLibrary/1.49.48.en.101.pdf#search=radio%20regulation>.
	[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and		[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
	dual environments", RFC 1195, DOI 10.17487/RFC1195,		dual environments", RFC 1195, DOI 10.17487/RFC1195,
	December 1990, <https://www.rfc-editor.org/info/rfc1195>.		December 1990, <https://www.rfc-editor.org/info/rfc1195>.
	[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",		[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
	RFC 2131, DOI 10.17487/RFC2131, March 1997,		RFC 2131, DOI 10.17487/RFC2131, March 1997,
	<https://www.rfc-editor.org/info/rfc2131>.		<https://www.rfc-editor.org/info/rfc2131>.
	[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,		[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
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