draft-li-arch-sat-06.txt   draft-li-arch-sat-07.txt 
Network Working Group T. Li Network Working Group T. Li
Internet-Draft Juniper Networks Internet-Draft Juniper Networks
Intended status: Informational 21 February 2024 Intended status: Informational 3 July 2024
Expires: 24 August 2024 Expires: 4 January 2025
A Routing Architecture for Satellite Networks A Routing Architecture for Satellite Networks
draft-li-arch-sat-06 draft-li-arch-sat-07
Abstract Abstract
Satellite networks present some interesting challenges for packet Satellite networks present some interesting challenges for packet
networking. The entire topology is continually in motion, with links networking. The entire topology is continually in motion, with links
that are far less reliable than what is common in terrestrial far less reliable than what is common in terrestrial networks. Some
networks. Some changes to link connectivity can be anticipated due changes to link connectivity can be anticipated due to orbital
to orbital mechanics. mechanics.
This document proposes a scalable routing architecture for satellite This document proposes a scalable routing architecture for satellite
networks based on existing routing protocols and mechanisms, enhanced networks based on existing routing protocols and mechanisms, enhanced
with scheduled link connectivity change information. This document with scheduled link connectivity change information. This document
proposes no protocol changes. proposes no protocol changes.
This document presents the author's view and is neither the product This document presents the author's view and is neither the product
of the IETF nor a consensus view of the community. of the IETF nor a consensus view of the community.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering 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 Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 24 August 2024. This Internet-Draft will expire on 4 January 2025.
Copyright Notice Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Related Work . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Related Work . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terms and Abbreviations . . . . . . . . . . . . . . . . . 3 1.2. Terms and Abbreviations . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Topological Considerations . . . . . . . . . . . . . . . 5 2.1. Topological Considerations . . . . . . . . . . . . . . . 5
2.2. Link Changes . . . . . . . . . . . . . . . . . . . . . . 6 2.2. Link Changes . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Scalability . . . . . . . . . . . . . . . . . . . . . . . 6 2.3. Scalability . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 7 2.4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 7
2.4.1. Traffic Patterns . . . . . . . . . . . . . . . . . . 7 2.4.1. Traffic Patterns . . . . . . . . . . . . . . . . . . 7
2.4.2. User Station Contraints . . . . . . . . . . . . . . . 8 2.4.2. User Station Contraints . . . . . . . . . . . . . . . 8
2.4.3. Stochastic Connectivity . . . . . . . . . . . . . . . 8 2.4.3. Stochastic Connectivity . . . . . . . . . . . . . . . 9
2.5. Problem Statement . . . . . . . . . . . . . . . . . . . . 9 2.5. Problem Statement . . . . . . . . . . . . . . . . . . . . 9
3. Forwarding Plane . . . . . . . . . . . . . . . . . . . . . . 9 3. Forwarding Plane . . . . . . . . . . . . . . . . . . . . . . 9
4. IGP Suitability and Scalability . . . . . . . . . . . . . . . 10 4. IGP Suitability and Scalability . . . . . . . . . . . . . . . 11
5. Stripes and Areas . . . . . . . . . . . . . . . . . . . . . . 11 5. Stripes and Areas . . . . . . . . . . . . . . . . . . . . . . 12
6. Traffic Forwarding and Traffic Engineering . . . . . . . . . 12 6. Traffic Forwarding and Traffic Engineering . . . . . . . . . 12
7. Scheduling . . . . . . . . . . . . . . . . . . . . . . . . . 14 7. Scheduling . . . . . . . . . . . . . . . . . . . . . . . . . 15
8. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 16 8. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 16
9. Deployment Considerations . . . . . . . . . . . . . . . . . . 16 9. Deployment Considerations . . . . . . . . . . . . . . . . . . 17
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16 10. Security Considerations . . . . . . . . . . . . . . . . . . . 17
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
13.1. Normative References . . . . . . . . . . . . . . . . . . 17 13.1. Normative References . . . . . . . . . . . . . . . . . . 18
13.2. Informative References . . . . . . . . . . . . . . . . . 18 13.2. Informative References . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
Satellite networks present some interesting challenges for packet Satellite networks present some interesting challenges for packet
networking. The entire topology is continually in motion, with links networking. The entire topology is continually in motion, with links
that are far less reliable than what is common in terrestrial far less reliable than what is common in terrestrial networks. Some
networks. Some changes to link connectivity can be anticipated due changes to link connectivity can be anticipated due to orbital
to orbital mechanics. mechanics.
This document proposes a scalable routing architecture for satellite This document proposes a scalable routing architecture for satellite
networks based on existing routing protocols and mechanisms, enhanced networks based on existing routing protocols and mechanisms, enhanced
with scheduled link connectivity change information. This document with scheduled link connectivity change information. This document
proposes no protocol changes. proposes no protocol changes.
Large-scale satellite networks are being deployed, presenting an Large-scale satellite networks are being deployed, presenting an
unforeseen application for conventional routing protocols. The high unforeseen application for conventional routing protocols. The high
rate of intentional topological change coupled with the extreme scale rate of intentional topological change and the extreme scale are
are unprecedented in terrestrial networking. Links between unprecedented in terrestrial networking. Links between satellites
satellites can utilize free-space optics technology that allows can utilize free-space optics technology that allows liberal
liberal connectivity, but there are limitations due to the range of connectivity. Still, there are limitations due to the range of the
the links and conjunction with the sun, resulting in links that are links and conjunction with the sun, resulting in links that are far
far less reliable than network designers are used to. In addition, less reliable than network designers are used to. In addition, links
links can change their endpoints dynamically, resulting in structural can change their endpoints dynamically, resulting in structural
changes to the topology. changes to the topology.
Current satellite networks are proprietary and little information is
generally available for analysis and discussion. This document is
based on what is currently accessible.
This document proposes one approach to provide a routing architecture This document proposes one approach to provide a routing architecture
for such networks utilizing current routing technology and providing for such networks utilizing current standards-based routing
a solution for the scalability of the network while incorporating the technology and providing a solution for the scalability of the
rapid rate of topological change. network while incorporating the rapid rate of topological change.
This document intends to provide some initial guidance for satellite
network operators, but without specific details, this document can
only provide the basis for a more complete analysis and design.
This document presents the author's view and is neither the product This document presents the author's view and is neither the product
of the IETF nor a consensus view of the community. of the IETF nor a consensus view of the community.
1.1. Related Work 1.1. Related Work
A survey of related work can be found in [Westphal]. Link state A survey of related work can be found in [Westphal]. Link state
routing for satellite networks has been considered in [Cao] and routing for satellite networks has been considered in [Cao] and
[Zhang]. [Zhang].
1.2. Terms and Abbreviations 1.2. Terms and Abbreviations
* Constellation: A set of satellites. * Constellation: A set of satellites.
* Downlink: The half of a ground link leading from a satellite to an * Downlink: The half of a ground link leading from a satellite to an
Earth station. Earth station.
* Gateway: An Earth station that participates as part of the network * Gateway: An Earth station that participates in the network and
and acts as the interconnect between satellite constellations and acts as the interconnect between satellite constellations and the
the planetary network. Gateways have a much higher bandwidth than planetary network. Gateways have a much higher bandwidth than
user stations, have ample computing capabilities, and perform user stations, have ample computing capabilities, and perform
traffic engineering duties, subsuming the functionality of a traffic engineering duties, subsuming the functionality of a
network controller or Path Computation Element (PCE). [RFC4655] network controller or Path Computation Element (PCE). [RFC4655]
Multiple gateways are assumed to exist, with each serving a Multiple gateways are assumed to exist, each serving a portion of
portion of the network. the network.
* GEO: Geostationary Earth Orbit. A satellite in GEO has an orbit * GEO: Geostationary Earth Orbit. A satellite in GEO has an orbit
that is synchronized to planetary rotation, so it effectively sits that is synchronized to planetary rotation, so it effectively sits
over one spot on the planet. over one spot on the planet.
* Ground link: A link between a satellite and an Earth station. * Ground link: A link between a satellite and an Earth station.
* Earth station: A node in the network that is on or close to the * Earth station: A node in the network that is on or close to the
surface of the planet and has a link to a satellite. This planetary surface and has a link to a satellite. This includes
includes ships, aircraft, and other vehicles below LEO. [ITU] ships, aircraft, and other vehicles below LEO. [ITU]
* IGP: Interior Gateway Protocol. A routing protocol that is used * IGP: Interior Gateway Protocol. A routing protocol that is used
within a single administrative domain. Note that 'gateway' in within a single administrative domain. Note that 'gateway' in
this context is semantically equivalent to 'router' and has no this context is semantically equivalent to 'router' and has no
relationship to the 'gateway' used in the rest of this document. relationship to the 'gateway' used in the rest of this document.
* IS-IS: Intermediate System to Intermediate System routing * IS-IS: Intermediate System to Intermediate System routing
protocol. An IGP that is commonly used by service providers. protocol. An IGP that is commonly used by service providers.
[ISO10589] [RFC1195] [ISO10589] [RFC1195]
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* Local gateway: Each user station is associated with a single * Local gateway: Each user station is associated with a single
gateway in its region. gateway in its region.
* LSP: IS-IS Link State Protocol Data Unit. An LSP is a set of * LSP: IS-IS Link State Protocol Data Unit. An 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 * MEO: Medium Earth Orbit. A satellite in MEO is between LEO and
GEO orbits and has an altitude between 2,000km and 35,786km. GEO orbits and has an altitude between 2,000km and 35,786km.
* SID: Segment Identifier [RFC8402] * SID: Segment Identifier [RFC8402]
* Stripe: A set of satellites in a few adjacent orbits. These form * Stripe: A set of satellites in a few adjacent orbits. These form
an IS-IS L1 area. an 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 station: An Earth station interconnected with a small end-
user network. user 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 planet.
Some of them have their orbital periods synchronized to the rotation Some of them have their orbital periods synchronized to planetary
of the planet, so they are effectively stationary over a single rotation, so they are effectively stationary over a single point.
point. Other satellites have orbits that cause them to travel across Other satellites have orbits that cause them to travel across regions
regions of the planet gradually or quite rapidly. Respectively, of the planet gradually or quite rapidly. Respectively, these are
these are typically known as Geostationary Earth Orbits (GEO), Medium typically known as Geostationary Earth Orbits (GEO), Medium Earth
Earth Orbit (MEO), or Low Earth Orbit (LEO), depending on altitude. Orbit (MEO), or Low Earth Orbit (LEO), depending on altitude. This
This discussion is not Earth-specific; as we get to other planets, we discussion is not Earth-specific; as we get to other planets, we can
can test this approach's generality. test this approach's generality.
Satellites may have data interconnections with one another through Satellites may have data interconnections with one another through
Inter-Satellite Links (ISLs). Due to differences in orbits, ISLs may Inter-Satellite Links (ISLs). Due to differences in orbits, ISLs may
be connected temporarily, with periods of potential connectivity be connected temporarily, with periods of potential connectivity
computed through orbital mechanics. Multiple satellites may be in computed through orbital mechanics. Multiple satellites may be in
the same orbit but separated in space, with a roughly constant the same orbit but separated in space, with a roughly constant
separation. Satellites in the same orbit may have ISLs that have a separation. Satellites in the same orbit may have ISLs that have a
higher duty cycle than ISLs between different orbits but are still higher duty cycle than ISLs between different orbits but are still
not guaranteed to always be connected. not guaranteed to be always connected.
User +-----------------+ Local User +-----------------+ Local
Stations --- | Satellites |--- Gateway --- Internet Stations --- | Satellites |--- Gateway --- Internet
+-----------------+ +-----------------+
Figure 1: Overall network architecture Figure 1: Overall network architecture
Earth stations can communicate with one or more satellites that are Earth stations can communicate with one or more satellites in their
in their region. User stations are Earth stations that have a region. User stations are Earth stations with a limited capacity and
limited capacity and communicate with only a single satellite at a communicate with only a single satellite at a time. Other Earth
time. Other Earth stations that may have richer connectivity and stations that may have richer connectivity and higher bandwidth are
higher bandwidth are commonly called gateways and provide commonly called gateways and provide connectivity between the
connectivity between the satellite network and conventional wired satellite network and conventional wired networks. Gateways serve
networks. Gateways serve user stations that are in their geographic user stations in their geographic proximity and are replicated
proximity and are replicated across the globe as necessary to provide globally as necessary to provide coverage and meet service density
coverage and meet service density goals. User stations are goals. User stations are associated with a single local gateway.
associated with a single local gateway. Traffic from one Earth Traffic from one Earth station to another may need to traverse a path
station to another may need to traverse a path across multiple across multiple satellites via ISLs.
satellites via ISLs.
2.2. Link Changes 2.2. Link Changes
Like conventional network links, ISLs and ground links can fail at Like conventional network links, ISLs and ground links can fail
any time. However, unlike conventional links, there are predictable without warning. However, unlike terrestrial links, there are
times when ISLs and ground links can potentially connect and predictable times when ISLs and ground links can potentially connect
disconnect. These predictions can be computed and cataloged in a and disconnect. These predictions can be computed and cataloged in a
schedule that can be distributed to relevant network elements. schedule that can be distributed to relevant network elements.
Predictions of a link connecting are not a guarantee: a link may not Predictions of a link connecting are not guaranteed: a link may not
connect for a variety of reasons. Predictions of a link connect for many reasons. Link disconnection predictions due to
disconnecting due to orbital mechanics are effectively guaranteed, as orbital mechanics are effectively guaranteed, as the underlying
the underlying physics is extremely unlikely to improve unexpectedly. physics will not improve unexpectedly.
2.3. Scalability 2.3. Scalability
Some proposed satellite networks are fairly large, with tens of Some proposed satellite networks are fairly large, with tens of
thousands of proposed satellites. [CNN] A key concern is the ability thousands of proposed satellites. [CNN] A key concern is the ability
to reach this scale and larger, as useful networks tend to grow. to reach this scale and larger, as useful networks tend to grow.
As we know, the key to scalability is the ability to create As we know, the key to scalability is the ability to create
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
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[RFC4271] deployment and is not discussed further. [RFC4271] deployment and is not discussed further.
The satellite network interconnects user stations and gateways. The satellite network interconnects user stations and gateways.
Interconnection between the satellite network and the satellite Interconnection between the satellite network and the satellite
networks of other network operators is outside of the scope of this networks of other network operators is outside of the scope of this
document. document.
2.4.1. Traffic Patterns 2.4.1. Traffic Patterns
We assume that the primary use of the satellite network is to provide We assume that the primary use of the satellite network is to provide
access from a wide range of geographic locations. We assume that access from a wide range of geographic locations. We also assume
providing high-bandwidth bulk transit between peer networks is not a that providing high-bandwidth bulk transit between peer networks is
goal. It has been noted that satellite networks can provide lower not a goal. It has been noted that satellite networks can provide
latencies than terrestrial fiber networks [Handley]. This proposal lower latencies than terrestrial fiber networks [Handley]. This
does not preclude such applications but also does not articulate the proposal does not preclude such applications but does not articulate
mechanisms necessary for user stations to perform the necessary the mechanisms necessary for user stations to perform the appropriate
traffic engineering computations. Low-latency, multicast, and traffic engineering computations. Low-latency, multicast, and
anycast applications are not discussed further. anycast applications are not discussed further.
As with most access networks, we assume that there will be As with most access networks, we assume that there will be
bidirectional traffic between the user station and the gateway, but bidirectional traffic between the user station and the gateway, but
that the bulk of the traffic will be from the gateway to the user that the bulk of the traffic will be from the gateway to the user
station. We expect that the uplink from the gateway to the satellite station. We expect that the uplink from the gateway to the satellite
network to be the bandwidth bottleneck, and that gateways will need network to be the bandwidth bottleneck, and that gateways will need
to be replicated to scale the uplink bandwidth, as the satellite to be replicated to scale the uplink bandwidth, as the satellite
capacity reachable from a gateway will be limited. capacity reachable from a gateway will be limited.
We assume that it is not essential to provide optimal routing for We assume that it is not essential to provide optimal routing for
traffic from user station to user station. If this traffic is sent traffic from user station to user station. If this traffic is sent
first to a gateway and then back into the satellite network, this to a gateway first and then back into the satellite network, this
might be acceptable to some operators as long as the traffic volume might be acceptable to some operators as long as the traffic volume
remains very low. This type of routing is not discussed further. remains very low. This type of routing is not discussed further.
We assume that traffic for a user station should enter the satellite We assume that traffic for a user station should enter the satellite
network through a gateway that is in some close geographic proximity network through a gateway that is in some close geographic proximity
to the user station. This is to reduce the number of ISLs used by to the user station. This is to reduce the number of ISLs used by
the path to the user station. Similarly, we assume that user station the path to the user station. Similarly, we assume that user station
traffic should exit the satellite network through the gateway that is traffic should exit the satellite network through the gateway that is
in the closest geographic proximity to the user station. in the closest geographic proximity to the user station.
Jurisdictional requirements for landing traffic in certain regions Jurisdictional requirements for landing traffic in certain regions
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User stations that can concurrently access multiple satellites are User stations that can concurrently access multiple satellites are
not precluded by this proposal, but are not discussed in detail. not precluded by this proposal, but are not discussed in detail.
2.4.3. Stochastic Connectivity 2.4.3. Stochastic Connectivity
We assume that links in general will be available when scheduled. As We assume that links in general will be available when scheduled. As
with any network, there will be failures, and the schedule is not a with any network, there will be failures, and the schedule is not a
guarantee, but we also expect that the schedule is mostly accurate. guarantee, but we also expect that the schedule is mostly accurate.
We assume that at any given instant, there are enough working links We assume that at any given instant, there are enough working links
and aggregate bandwidth to run the network and support the traffic and aggregate bandwidth to run the network and support the traffic
demand. If this assumption does not hold, then no routing demand. If this assumption does not hold, no routing architecture
architecture can magically make the network more capable. can magically make the network more capable.
Satellites that are in the same orbit may be connected by ISLs. Satellites that are in the same orbit may be connected by ISLs.
These are called intra-orbit ISLs. Satellites that are in different These are called intra-orbit ISLs. Satellites that are in different
orbits may also be connected by ISLs. These are called inter-orbit orbits may also be connected by ISLs. These are called inter-orbit
ISLs. We assume that, in general, intra-orbit ISLs have higher ISLs. We assume that, in general, intra-orbit ISLs have higher
reliability and persistence than inter-orbit ISLs. reliability and persistence than inter-orbit ISLs.
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 all the time, 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 where there is no such path, we assume that it is the extreme case with no such path, we assume that it is acceptable
acceptable to drop the payload packets. We do not require buffering to drop the payload packets. We do not require buffering of traffic
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 such that
topology information is conveyed through relevant portions of the topology information is conveyed through relevant portions of the
network, that paths are computed across the network, and that data network, that paths are computed across the network, and that data
can be delivered along those paths, and the structure can scale to a can be delivered along those paths, and the structure can scale
very large network without any changes to the organizational without any changes to the organizational structure.
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 are frequently changing 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 to use MPLS as the essential. Toward this end, we propose to use MPLS as the
fundamental forwarding plane architecture [RFC3031]. Specifically, fundamental forwarding plane architecture [RFC3031]. Specifically,
we propose to use a Segment Routing (SR) [RFC8402] based approach we propose to use a Segment Routing (SR) [RFC8402] based approach
with an MPLS data plane [RFC8660], where each satellite is assigned a with an MPLS data plane [RFC8660], where each satellite is assigned a
node Segment Identifier (SID). This allows the architecture to node Segment Identifier (SID). This allows the architecture to
support both IPv4 and IPv6 concurrently. A path through the network support both IPv4 and IPv6 concurrently. A path through the network
can be then expressed as a label stack of node SIDs. IP forwarding can then be expressed as a label stack of node SIDs. IP forwarding
is not used within the internals of the satellite network, although is not used within the internals of the satellite network, although
each satellite may be assigned an IP address for management purposes. each satellite may be assigned an IP address for management purposes.
Existing techniques may be used to limit the size of the SR label Existing techniques may be used to limit the size of the SR label
stack so that it only contains the significant waypoints along the stack so that it only contains the significant waypoints along the
path.[Giorgetti] This implies that the label stack operates as a form path [Giorgetti]. The label stack operates as a loose source route
of loose-source routing through the network. If there is an through the network. If there is an unexpected topology change in
unexpected topology change in the network, then the IGP will compute the network, the IGP will compute a new path to the next waypoint,
a new path to the next waypoint, allowing packet delivery despite ISL allowing packet delivery despite ISL failures. While the IGP is
failures. While the IGP is converging, there may be micro-loops in converging, there may be micro-loops in the topology. These can be
the topology. These can be avoided through the use of TI-LFA avoided by using TI-LFA alternate paths
alternate paths [I-D.ietf-rtgwg-segment-routing-ti-lfa], or traffic [I-D.ietf-rtgwg-segment-routing-ti-lfa], or traffic will loop until
will loop until it is discarded based on its TTL. discarded based 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
that is assigned from a prefix managed by its local gateway. The assigned from a prefix managed by its local gateway. The mechanisms
mechanisms for this assignment and its communication to the end for this assignment and its communication to the end station are not
station are not discussed herein but might be similar to DHCP discussed herein but might be similar to DHCP [RFC2131]. User
[RFC2131]. User station IP addresses change infrequently and do not station IP addresses change infrequently and do not reflect their
reflect their association with their first-hop satellite. Gateways association with their first-hop satellite. Gateways and their
and their supporting terrestrial networks advertise prefixes covering supporting terrestrial networks advertise prefixes covering all its
all of its local user stations into the global Internet. local user stations into 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 unique full longest-prefix-match lookup as the IP address is merely a unique
identifier at this point. identifier at this point.
Similarly, gateways may be assigned a node SID. A possible Similarly, gateways may be assigned a node SID. A possible
optimization is that a single SID value be assigned as a global optimization is that a single SID value be assigned as a global
constant to always direct traffic to the topologically closest constant to always direct traffic to the topologically closest
gateway. If traffic engineering is required for traffic that is gateway. If traffic engineering is required for traffic that is
flowing to a gateway, a specific path may be encoded in a label stack flowing to a gateway, a specific path may be encoded in a label stack
that is attached to the packet by the user station or by the first- that is attached to the packet by the user station or by the first-
hop satellite. hop satellite.
Gateways can also perform traffic engineering by using different Gateways can also perform traffic engineering using different paths
paths and label stacks for different traffic flows. Routing a single and label stacks for separate traffic flows. Routing a single
traffic flow across multiple paths has proven to cause performance traffic flow across multiple paths has proven to cause performance
issues with transport protocols, so that approach is not recommended. issues with transport protocols, so that approach is not recommended.
Traffic engineering is discussed further in Section 6. Traffic engineering is discussed further in Section 6.
4. IGP Suitability and Scalability 4. IGP Suitability and Scalability
As discussed in Section 2.3, IS-IS is architecturally the best fit As discussed in Section 2.3, IS-IS is architecturally the best fit
for satellite networks, but does require some novel approaches to for satellite networks, but does require some novel approaches to
achieve the scalability goals for a satellite network. In achieve the scalability goals for a satellite network. In
particular, we propose that all nodes in the network be L1L2 so that particular, we propose that all nodes in the network be L1L2 so that
skipping to change at page 11, line 8 skipping to change at page 11, line 22
is done based on L2 information. is done based on L2 information.
IS-IS has the interesting property that it does not require interface IS-IS has the interesting property that it does not require interface
addresses. This feature is commonly known as 'unnumbered addresses. This feature is commonly known as 'unnumbered
interfaces'. This is particularly helpful in satellite topologies interfaces'. This is particularly helpful in satellite topologies
because it implies that ISLs may be used flexibly. Sometimes an because it implies that ISLs may be used flexibly. Sometimes an
interface might be used as an L1 link to another satellite and a few interface might be used as an L1 link to another satellite and a few
orbits later it might be used as an L1L2 link to a completely orbits later it might be used as an L1L2 link to a completely
different satellite without any reconfiguration or renumbering. different satellite without any reconfiguration or renumbering.
Scalability for IS-IS can be achieved through the use of a proposal Scalability for IS-IS can be achieved through a proposal known as
known as Area Proxy [I-D.ietf-lsr-isis-area-proxy]. With this Area Proxy [I-D.ietf-lsr-isis-area-proxy]. With this proposal, all
proposal, all of the nodes in an L1 area combine their information nodes in an L1 area combine their information into a single L2 Link
into a single L2 Link State Protocol Data Unit (LSP). This implies State Protocol Data Unit (LSP). This implies that the size of the L1
that the size of the L1 Link State Database (LSDB) scales as the Link State Database (LSDB) scales as the number of nodes in the L1
number of nodes in the L1 area and the size of the L2 LSDB scales area and the size of the L2 LSDB scales with the number of L1 areas.
with the number of L1 areas.
With Area Proxy, topological changes within an L1 area will not be With Area Proxy, topological changes within an L1 area will not be
visible to other areas, so the overhead of link state changes will be visible to other areas, so the overhead of link state changes will be
greatly reduced. greatly reduced.
The Area Proxy proposal also includes the concept of an Area SID. The Area Proxy proposal also includes the concept of an Area SID.
This is useful because it allows traffic engineering to construct a This is useful because it allows traffic engineering to construct a
path that traverses areas with a minimal number of label stack path that traverses areas with a minimal number of label stack
entries. entries.
Suppose, for example, that a network has 1,000 L1 areas, each with Suppose, for example, that a network has 1,000 L1 areas, each with
1,000 satellites. This would then mean that the network supports 1,000 satellites. This would then mean that the network supports
1,000,000 satellites, but only requires 1,000 entries in its L1 LSDB 1,000,000 satellites, but only requires 1,000 entries in its L1 LSDB
and 1,000 entries in its L2 LSDB; numbers that are easily achievable and 1,000 entries in its L2 LSDB; numbers that are easily achievable
today. The resulting MPLS label table would contain 1,000 node SIDs today. The resulting MPLS label table would contain 1,000 node SIDs
from the L1 (and L2) LSDB and 1,000 area SIDs from the L2 LSDB. If from the L1 (and L2) LSDB and 1,000 area SIDs from the L2 LSDB. If
each satellite advertises an IP address for management purposes, then each satellite advertises an IP address for management purposes, then
the IP routing table would have 1,000 entries for the L1 management the IP routing table would have 1,000 entries for the L1 management
addresses and 1,000 area proxy addresses from L2. addresses and 1,000 area proxy addresses from L2.
In this proposal, IS-IS does not carry any IP routes other than those In this proposal, IS-IS does not carry IP routes other than those in
that are in the satellite topology. In particular, there are no IP the satellite topology. In particular, there are no IP routes for
routes for user stations or the remainder of the Internet. user stations or the remainder of the Internet.
5. Stripes and Areas 5. Stripes and Areas
A significant problem with any link state routing protocol is that of A significant problem with any link state routing protocol is that of
area partition. While there have been many proposals for automatic area partition. While there have been many proposals for automatic
partition repair, none has seen significant production deployment. partition repair, none has seen notable production deployment. It
It seems best to simply avoid this issue altogether and ensure that seems best to avoid this issue and ensure areas have an extremely low
areas have an extremely low probability of partitioning. probability of partitioning.
As discussed above, intra-orbit ISLs are assumed to have higher As discussed above, intra-orbit ISLs are assumed to have higher
reliability and persistence than inter-orbit ISLs. However, even reliability and persistence than inter-orbit ISLs. However, even
intra-orbit ISLs are not sufficiently reliable to avoid partition intra-orbit ISLs are not sufficiently reliable to avoid partition
issues. Therefore, we propose to group a small number of adjacent issues. Therefore, we propose to group a small number of adjacent
orbits as an IS-IS L1 area, called a stripe. We assume that for any orbits as an IS-IS L1 area, called a stripe. We assume that for any
given reliability requirement, there is a small number of orbits that given reliability requirement, there is a small number of orbits that
can be used to form a stripe that satisfies the reliability can be used to form a stripe that satisfies the reliability
requirement. requirement.
Stripes are connected to other adjacent stripes using the same ISL Stripes are connected to other adjacent stripes using the same ISL
mechanism, forming the L2 topology of the network. Each stripe mechanism, forming the L2 topology of the network. Each stripe
should have multiple L2 connections and should never become should have multiple L2 connections and never become partitioned from
partitioned from the remainder of the network. the remainder of the network.
By using a stripe as an L1 area, in conjunction with Area Proxy, the By using a stripe as an L1 area, in conjunction with Area Proxy, the
overhead of the architecture is greatly reduced. Each stripe overhead of the architecture is greatly reduced. Each stripe
contributes a single LSP to the L2 LSDB, completely hiding all of the contributes a single LSP to the L2 LSDB, completely hiding all the
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
skipping to change at page 13, line 33 skipping to change at page 14, line 6
\ \
Figure 3: Off-stripe forwarding Figure 3: Off-stripe forwarding
As an example, consider a packet from an Internet source S to a user As an example, consider a packet from an Internet source S to a user
station D. A local gateway L has injected a prefix covering D into station D. A local gateway L has injected a prefix covering D into
BGP and advertised it globally. The packet is forwarded to L using BGP and advertised it globally. The packet is forwarded to L using
IP forwarding. When L receives the packet, it performs a lookup in a IP forwarding. When L receives the packet, it performs a lookup in a
pre-computed forwarding table. This contains a SID list for the user pre-computed forwarding table. This contains a SID list for the user
station that has already been converted into a label stack. Suppose station that has already been converted into a label stack. Suppose
that the user station is currently associated with a different stripe the user station is currently associated with a different stripe so
so that the label stack will contain an area label A and a label U that the label stack will contain an area label A and a label U for
for the satellite associated with the user station, resulting in a the satellite associated with the user station, resulting in a label
label stack (A, U). stack (A, U).
The local gateway forwards this into the satellite network. The The local gateway forwards this into the satellite network. The
first-hop satellite now forwards based on the area label A at the top first-hop satellite now forwards based on the area label A at the top
of the stack. All area labels are propagated as part of the L2 of the stack. All area labels are propagated as part of the L2
topology. This forwarding continues until the packet reaches a topology. This forwarding continues until the packet reaches a
satellite that is adjacent to the destination area. That satellite satellite adjacent to the destination area. That satellite pops
pops label A, removing that label and forwarding the packet into the label A, removing that label and forwarding the packet into the
destination area. destination area.
The packet is now forwarded based on the remaining label U, which was The packet is now forwarded based on the remaining label U, which was
propagated as part of the L1 topology. The last satellite forwards propagated as part of the L1 topology. The last satellite forwards
the packet based on the destination address D and forwards the packet the packet based on the destination address D and forwards the packet
to the user station. to the user station.
The return case is similar. The label stack, in this case, consists The return case is similar. The label stack, in this case, consists
of a label for the local gateway's stripe/area, A', and the label for of a label for the local gateway's stripe/area, A', and the label for
the local gateway, L, resulting in the stack (A', L). The forwarding the local gateway, L, resulting in the stack (A', L). The forwarding
mechanisms are similar to the previous case. mechanisms are similar to the previous case.
Very frequently, access networks congest due to oversubscription and Very frequently, access networks congest due to oversubscription and
the economics of access. Network operators can use traffic the economics of access. Network operators can use traffic
engineering to ensure that they are getting higher efficiency out of engineering to ensure that they get higher efficiency out of their
their networks by utilizing all available paths and capacity near any networks by utilizing all available paths and capacity near any
congestion points. In this particular case, the gateway will have congestion points. In this particular case, the gateway will have
information about all of the traffic that it is generating and can information about all of the traffic it is generating and can use all
use all of the possible paths through the network in its topological of the possible paths through the network in its topological
neighborhood. Since we're already using SR, this is easily done just neighborhood. Since we're already using SR, this is easily done just
by adding more explicit SIDs to the label stack. These can be by adding more explicit SIDs to the label stack. These can be
additional area SIDs, node SIDs, or adjacency SIDs. Path computation additional area SIDs, node SIDs, or adjacency SIDs. Path computation
can be performed by Path Computation Elements (PCE). [RFC4655] can be performed by Path Computation Elements (PCE). [RFC4655]
Each gateway or its PCE will need topological information from all of Each gateway or its PCE will need topological information from the
the areas that it will route through. It can do this by being a areas it will route through. It can do this by participating in the
participant in the IGP either directly, via a tunnel, or another IGP directly, via a tunnel, or another delivery mechanism such as
delivery mechanism such as BGP-LS [RFC9552]. User stations do not BGP-LS [RFC9552]. User stations do not participate in the IGP.
participate in the IGP.
Traffic engineering for traffic into a gateway can also be provided Traffic engineering for packets flowing into a gateway can also be
by an explicit SR path for the traffic. This can help ensure that provided by an explicit SR path. This can help ensure that ISLs near
ISLs near the gateway do not congest with traffic for the gateway. the gateway do not congest with traffic for the gateway. These paths
These paths can be computed by the gateway or PCE and then can be computed by the gateway or PCE and then distributed to the
distributed to the first-hop satellite or user station, which would first-hop satellite or user station, which would apply them to
then apply them to traffic. The delivery mechanism is outside of the traffic. The delivery mechanism is outside of the scope of this
scope of this document. document.
7. Scheduling 7. Scheduling
The most significant difference between terrestrial and satellite The most significant difference between terrestrial and satellite
networks from a routing perspective is that some of the topological networks from a routing perspective is that some of the topological
changes that will happen to the network can be anticipated and changes that will happen to the network can be anticipated and
computed. Both link and node changes will affect the topology and computed. Both link and node changes will affect the topology and
the network should react smoothly and predictably. the network should react smoothly and predictably.
The management plane is responsible for providing information about The management plane is responsible for providing information about
scheduled topological changes. The exact details of how the scheduled topological changes. The exact details of how the
information is disseminated are outside of the scope of this document information is disseminated are outside of the scope of this document
but could be done through a YANG model but could be done through a YANG model [I-D.ietf-tvr-schedule-yang].
[I-D.united-tvr-schedule-yang]. Scheduling information needs to be Scheduling information needs to be accessible to all of the nodes
accessible to all of the nodes that will make routing decisions based that will make routing decisions based on the topological changes in
on the topological changes in the schedule, so information about an the schedule, so data about an L1 topological change will need to be
L1 topological change will need to be circulated to all nodes in the circulated to all nodes in the L1 area and information about L2
L1 area and information about L2 changes will need to propagate to changes will need to propagate to all L2 nodes, plus the gateways and
all L2 nodes, plus the gateways and PCEs that carry the related PCEs that carry the related topological information.
topological information.
There is very little reaction that the network should do in response There is very little that the network should do in response to a
to a topological addition. A link coming up or a node joining the topological addition. A link coming up or a node joining the
topology should not have any functional change until the change is topology should not have any functional change until the change is
proven to be fully operational based on the usual liveness mechanisms proven to be fully operational based on the usual IS-IS liveness
found within IS-IS. Nodes may pre-compute their routing table mechanisms. Nodes may pre-compute their routing table changes but
changes but should not install them before all of the relevant should not install them before all relevant adjacencies are received.
adjacencies are received. The benefits of this pre-computation The benefits of this pre-computation appear to be very small.
appear to be very small. Gateways and PCEs may also choose to pre- Gateways and PCEs may also choose to pre-compute paths based on these
compute paths based on these changes, but should be careful to not changes, but should not install paths using the new parts of the
install paths using the new parts of the topology until they are topology until they are confirmed to be operational. If some path
confirmed to be operational. If some path pre-installation is pre-installation is performed, gateways and PCEs must be prepared for
performed, gateways and PCEs must be prepared for the situation where the situation where the topology fails to become operational and may
the topology does not become operational and may need to take need to take alternate steps instead, such as reverting any related
alternate steps instead, such as reverting any related pre-installed pre-installed paths.
paths.
The network may choose to not do any pre-installation or pre- The network may choose not to pre-install or pre-compute routes in
computation in reaction to topological additions, at a small cost of reaction to topological additions, at a small cost of some
some operational efficiency. operational efficiency.
Topological deletions are an entirely different matter. If a link or Topological deletions are an entirely different matter. If a link or
node is to be removed from the topology, then the network should act node is to be removed from the topology, the network should act
before the anticipated change to route traffic around the expected before the anticipated change to route traffic around the expected
topological loss. Specifically, at some point before the topology topological loss. Specifically, at some point before the topology
change, the affected links should be set to a high metric to direct change, the affected links should be set to a high metric to direct
traffic to alternate paths. This is a common operational procedure traffic to alternate paths. This is a common operational procedure
in existing networks when links are taken out of service, such as in existing networks when links are taken out of service, such as
when proactive maintenance needs to be performed. This type of when proactive maintenance needs to be performed. This type of
change does require some time to propagate through the network, so change does require some time to propagate through the network, so
the metric change should be initiated far enough in advance that the the metric change should be initiated far enough in advance that the
network converges before the actual topological change. Gateways and network converges before the actual topological change. Gateways and
PCEs should also update paths around the topology change and install PCEs should also update paths around the topology change and install
these changes before the topology change takes place. The time these changes before the topology change occurs. The time necessary
necessary for both IGP and path changes will vary depending on the for both IGP and path changes will vary depending on the exact
exact network and configuration. network and configuration.
Strictly speaking, changing to a high metric should not be necessary. Strictly speaking, changing to a high metric should not be necessary.
It should be possible for each router to simply exclude the link and It should be possible for each router to exclude the link and
recompute paths. However, it seems safer to also change the metric recompute paths. However, it seems safer to change the metric and
and use the IGP methods for indicating a topology change, as this can use the IGP methods for indicating a topology change, as this can
help avoid issues with incomplete information dissemination and help avoid issues with incomplete information dissemination and
synchronization. synchronization.
8. Future Work 8. Future Work
This architecture needs to be validated by satellite operators, both This architecture needs to be validated by satellite operators, both
via simulation and operational deployment. Meaningful simulation via simulation and operational deployment. Meaningful simulation
hinges on the exact statistics of ISL connectivity, and that hinges on the exact statistics of ISL connectivity, and that
information is not publicly available at present. information is not publicly available currently.
Current available information about ISLs indicates that links are
mechanically steered and will need to track the opposite end of the
link continually. The angles and distances that can be practically
supported are unknown, as are any limitations about the rate of
change.
It is expected that intra-orbit and inter-orbit ISL links will have
very different properties. Intra-orbit links should be much more
stable, but still far less stable than terrestrial links. Inter-
orbit links will be less stable. Links between satellites that are
roughly parallel should be possible, but will likely have a limited
duration. Two orbits may be roughly orthogonal, resulting in a
limited potential for connectivity. Finally, in some topologies
there may be parallel orbits where the satellites move in opposite
directions, giving a relative speed between satellites around
34,000mph. Links in this situation may not be possible or may be so
short-lived as to be impractical.
The key question to address is whether the parameters of a given
network can yield a stripe assignment that produces stable, connected
areas that work within the scaling bounds of the IGP. If links are
very stable, a stripe could be just a few parallel orbits, with only
a few hundred satellites. However, if links are unstable, a stripe
might have to encompass dozens of orbits and thousands of satellites,
which might be beyond the scaling limitations of a given IGP's
implementation.
9. Deployment Considerations 9. Deployment Considerations
The network behind a gateway is expected to be a normal terrestrial The network behind a gateway is expected to be a normal terrestrial
network. Conventional routing architectural principles apply. An network. Conventional routing architectural principles apply. An
obvious approach would be to extend IS-IS to the terrestrial network, obvious approach would be to extend IS-IS to the terrestrial network,
applying L1 areas as necessary for scalability. applying L1 areas as necessary for scalability.
The terrestrial network may have one or more BGP connections to the The terrestrial network may have one or more BGP connections to the
broader Internet. Prefixes for user stations should be advertised broader Internet. Prefixes for user stations should be advertised to
into the Internet near the associated gateway. If gateways are not the Internet near the associated gateway. If gateways are not
interconnected by the terrestrial network, then it may be advisable interconnected by the terrestrial network, then it may be advisable
to use one autonomous system per gateway as it might simplify the to use one autonomous system per gateway as it might simplify the
external perception of the network and subsequent policy external perception of the network and subsequent policy
considerations. Otherwise, one autonomous system may be used for the considerations. Otherwise, one autonomous system may be used for the
entire terrestrial network. entire terrestrial network.
10. Security Considerations 10. Security Considerations
This document discusses one possible routing architecture for This document discusses one possible routing architecture for
satellite networks. It proposes no new protocols or mechanisms and satellite networks. It proposes no new protocols or mechanisms and
skipping to change at page 18, line 40 skipping to change at page 19, line 38
[Handley] Handley, M., "Delay is Not an Option: Low Latency Routing [Handley] Handley, M., "Delay is Not an Option: Low Latency Routing
in Space", ACM Proceedings of the 17th ACM Workshop on Hot in Space", ACM Proceedings of the 17th ACM Workshop on Hot
Topics in Networks, DOI 10.1145/3286062.3286075, November Topics in Networks, DOI 10.1145/3286062.3286075, November
2018, <https://doi.org/10.1145/3286062.3286075>. 2018, <https://doi.org/10.1145/3286062.3286075>.
[I-D.ietf-rtgwg-segment-routing-ti-lfa] [I-D.ietf-rtgwg-segment-routing-ti-lfa]
Bashandy, A., Litkowski, S., Filsfils, C., Francois, P., Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute using Segment Routing", Work in Progress, Reroute using Segment Routing", Work in Progress,
Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa- Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
13, 16 January 2024, 16, 29 June 2024, <https://datatracker.ietf.org/doc/html/
<https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg- draft-ietf-rtgwg-segment-routing-ti-lfa-16>.
segment-routing-ti-lfa-13>.
[I-D.united-tvr-schedule-yang] [I-D.ietf-tvr-schedule-yang]
Qu, Y., Lindem, A., Kinzie, E., Fedyk, D., and M. Qu, Y., Lindem, A., Kinzie, E., Fedyk, D., and M.
Blanchet, "YANG Data Model for Scheduled Attributes", Work Blanchet, "YANG Data Model for Scheduled Attributes", Work
in Progress, Internet-Draft, draft-united-tvr-schedule- in Progress, Internet-Draft, draft-ietf-tvr-schedule-yang-
yang-00, 11 October 2023, 00, 16 April 2024, <https://datatracker.ietf.org/doc/html/
<https://datatracker.ietf.org/doc/html/draft-united-tvr- draft-ietf-tvr-schedule-yang-00>.
schedule-yang-00>.
[ITU] "ITU Radio Regulations, Article 1", 2016, [ITU] "ITU Radio Regulations, Article 1", 2016,
<https://search.itu.int/history/ <https://search.itu.int/history/
HistoryDigitalCollectionDocLibrary/1.43.48.en.101.pdf>. HistoryDigitalCollectionDocLibrary/1.43.48.en.101.pdf>.
[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",
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