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A really interesting article covering this: “The world in which IPv6 was a good design” https://apenwarr.ca/log/20170810 https://apenwarr.ca/log/20170810 It

A really interesting article covering this: “The world in which IPv6 was a good design” https://apenwarr.ca/log/20170810 https://apenwarr.ca/log/20170810 It talks about how when IPv6 was being designed, they wanted to do exactly that: drop most of the layer 2 stuff, abandon the idea of a bus network, make everything point-to-point, all switches would be L3 routers, etc. Search for “What if instead the world worked like this?” for the relevant part. My question though, is how would IP assignment work for each of the intermediary devices between me and (say) my ISP’s gateway? My computer is plugged into a switch right now, which is plugged into another switch, which is plugged into my router, which has a point-to-point link to the ISP gateway. Would my router get a /64, then delegate a /68 to the next “router” (ie. The physical thing I currently call a switch), which would delegate a /72 to the next one, etc? How would it determine the optimal IP allocation? What if there’s a cycle? Aren’t we sorta reinventing spanning tree at this point? (I’m genuinely curious about this, because I don’t really grok all of the implications of an “everything is L3” world like this.)

p_l 2y ago
For v6-specific world, scoped addresses and scoped multicast are explicitly for that purpose. You do not need to hierarchically subnet each following router, you just need to be able to express "next hop" for the subnets you need to route towards. You use link-local autoconfiguration, and use appropriately-scoped multicast addresses to ask "all-nodes" or "all-routers", making autoconfiguration a breeze compared to v4 world. In v4 world a similar setup is also possible, though specific details of the setup differ, and you have to setup addresses manually for each p2p link.
- ninkendo 2y ago
  I mean sure, you’d definitely use a scoped address to talk to the next hop, but it still doesn’t solve how the router/switch knows which port to send the packet to for its next destination. Say I have a global unicast address on my desktop, 2 hops from my router, and I want to allow traffic to it. My router gets a packet sent to it over its link-local address, with a destination header of my desktop’s IP. Say it has 4 ports (each going to another router/switch, each with its own link-local address.) How would it know which port to use as the next hop? It would need a routing table, and that would need to be configured automatically if we want to work as well as switches do today. What would be the protocol for this auto configuration? BGP or something like it? How do the routers know the available address space? Or are we just stipulating that we’d invent a protocol for this, if it had ever happened? In ethernet we have the Spanning Tree Protocol for this, to discover the topology of an Ethernet network and know which links to use for which MAC addresses (including the ability to detect cycles.) I feel like something like spanning tree would still need to exist in an all-unicast, no-ethernet, L3-only world. Does such a thing exist already, or would we need to invent it in this counterfactual universe?
  - p_l 2y ago
    STP does not cover filling in MAC forwarding tables, it only tells the switches which ports to shut down so that no loops would be created. RSTP and later evolutions just made it faster as well as capable of creating multiple spanning trees, but the core logic is still based on the "oh fuck" realisation of a corporation that sold nonrouteable ethernet protocol and thus had to figure how to deal with larger L2 domains than designed (while keeping logic simple to not make switches too expensive). For both IP, OSI, and routed ethernet, you run a complete routing protocol (IS-IS actually will handle both OSI, IP, and routed ethernet) where routers tend to send "hello" packets to announce themselves on the links to establish communication. This is how IP routing works since some of the oldest routing setups. On IP-bound protocols like OSPFv2/V3 or BGP you have to configure connections between the routers then tell them to peer with router at given IP, with IS-IS it has its own independent low level mechanism so you connect to a link and tell it which router ids to trust/peer with.
    - ninkendo 2y ago
      Interesting. I guess since IPv6 solves the address allocation problem natively (with SLAAC), it’s not allocation you need to worry about so much as “what IP’s does this router see vs that router, recursively”, which it sounds like IS-IS can provide. This would mean each router box in my network would grab its own random (SLAAC) address within the /64 advertised by my “main” router (found via multicast all-routers), and would then each (through IS-IS or something similar) announce and forward individual /128 routes for each host plugged into them. The network would converge such that each individual box would have a complete routing table of what next-hop to use for each IP address in the tree. I can see this being a preferable setup if it allows you to completely eliminate layer 2 from the equation… alas this never happened so it’s all a thought exercise.
thaumasiotes 2y ago
> Would my router get a /64, then delegate a /68 to the next “router” (ie. The physical thing I currently call a switch) This is another weird thing about networking. As far as I've been able to learn, a "router" is a device with two ports that handles transmission of data between those ports, whereas a "switch" is a device with more than two ports that handles transmission of data between those ports. But nobody would ever care about that distinction.
- kjs3 2y ago
  This is wrong and networking people care very much. (Wildly oversimplifying here, there are always exception, YMMV, no warranty expressed or implied, may cause blurred vision or a rash) A switch is a device that handles things like "I need to get a packet from my desktop PC to the printer down the hall". It has lots of ports because, usually, there are a lot of things local to you that you might want to talk to, and you want that traffic as fast as possible without the expense and overhead of 'routing'. If those things aren't on the same L3 network, a switch won't be able to get there[1]. A router is something that maintains a table that maps non-local (not the same L2 network, collision domain, VLAN, whatever) destinations to a 'next hop' based on various metrics[2]. In the general case, routers are concerned with questions like "I'm in Atlanta and I need to get this packet to Tokyo so is the best way to send it via my connection to Comcast or Level-3?". The degenerate case for routers is a two-port box that does nothing but move packets not destined for something in your local network to your ISP or other upstream network for forwarding to a non-local destination. Since that's the use case most folks see, it's easy to misunderstand the bigger pix. Much is done via various kinds of virtual interfaces now, but I have in my career worked on routers with hundreds of physical ports. [1] Yes, some switches have a router in them. Stop overthinking. [2] Yes, it's more complicated than that. Stop overthinking.

p_l 2y ago

For v6-specific world, scoped addresses and scoped multicast are explicitly for that purpose. You do not need to hierarchically subnet each following router, you just need to be able to express "next hop" for the subnets you need to route towards. You use link-local autoconfiguration, and use appropriately-scoped multicast addresses to ask "all-nodes" or "all-routers", making autoconfiguration a breeze compared to v4 world. In v4 world a similar setup is also possible, though specific details of the setup differ, and you have to setup addresses manually for each p2p link.

ninkendo 2y ago
I mean sure, you’d definitely use a scoped address to talk to the next hop, but it still doesn’t solve how the router/switch knows which port to send the packet to for its next destination. Say I have a global unicast address on my desktop, 2 hops from my router, and I want to allow traffic to it. My router gets a packet sent to it over its link-local address, with a destination header of my desktop’s IP. Say it has 4 ports (each going to another router/switch, each with its own link-local address.) How would it know which port to use as the next hop? It would need a routing table, and that would need to be configured automatically if we want to work as well as switches do today. What would be the protocol for this auto configuration? BGP or something like it? How do the routers know the available address space? Or are we just stipulating that we’d invent a protocol for this, if it had ever happened? In ethernet we have the Spanning Tree Protocol for this, to discover the topology of an Ethernet network and know which links to use for which MAC addresses (including the ability to detect cycles.) I feel like something like spanning tree would still need to exist in an all-unicast, no-ethernet, L3-only world. Does such a thing exist already, or would we need to invent it in this counterfactual universe?
- p_l 2y ago
  STP does not cover filling in MAC forwarding tables, it only tells the switches which ports to shut down so that no loops would be created. RSTP and later evolutions just made it faster as well as capable of creating multiple spanning trees, but the core logic is still based on the "oh fuck" realisation of a corporation that sold nonrouteable ethernet protocol and thus had to figure how to deal with larger L2 domains than designed (while keeping logic simple to not make switches too expensive). For both IP, OSI, and routed ethernet, you run a complete routing protocol (IS-IS actually will handle both OSI, IP, and routed ethernet) where routers tend to send "hello" packets to announce themselves on the links to establish communication. This is how IP routing works since some of the oldest routing setups. On IP-bound protocols like OSPFv2/V3 or BGP you have to configure connections between the routers then tell them to peer with router at given IP, with IS-IS it has its own independent low level mechanism so you connect to a link and tell it which router ids to trust/peer with.
  - ninkendo 2y ago
    Interesting. I guess since IPv6 solves the address allocation problem natively (with SLAAC), it’s not allocation you need to worry about so much as “what IP’s does this router see vs that router, recursively”, which it sounds like IS-IS can provide. This would mean each router box in my network would grab its own random (SLAAC) address within the /64 advertised by my “main” router (found via multicast all-routers), and would then each (through IS-IS or something similar) announce and forward individual /128 routes for each host plugged into them. The network would converge such that each individual box would have a complete routing table of what next-hop to use for each IP address in the tree. I can see this being a preferable setup if it allows you to completely eliminate layer 2 from the equation… alas this never happened so it’s all a thought exercise.

thaumasiotes 2y ago

> Would my router get a /64, then delegate a /68 to the next “router” (ie. The physical thing I currently call a switch) This is another weird thing about networking. As far as I've been able to learn, a "router" is a device with two ports that handles transmission of data between those ports, whereas a "switch" is a device with more than two ports that handles transmission of data between those ports. But nobody would ever care about that distinction.

kjs3 2y ago
This is wrong and networking people care very much. (Wildly oversimplifying here, there are always exception, YMMV, no warranty expressed or implied, may cause blurred vision or a rash) A switch is a device that handles things like "I need to get a packet from my desktop PC to the printer down the hall". It has lots of ports because, usually, there are a lot of things local to you that you might want to talk to, and you want that traffic as fast as possible without the expense and overhead of 'routing'. If those things aren't on the same L3 network, a switch won't be able to get there[1]. A router is something that maintains a table that maps non-local (not the same L2 network, collision domain, VLAN, whatever) destinations to a 'next hop' based on various metrics[2]. In the general case, routers are concerned with questions like "I'm in Atlanta and I need to get this packet to Tokyo so is the best way to send it via my connection to Comcast or Level-3?". The degenerate case for routers is a two-port box that does nothing but move packets not destined for something in your local network to your ISP or other upstream network for forwarding to a non-local destination. Since that's the use case most folks see, it's easy to misunderstand the bigger pix. Much is done via various kinds of virtual interfaces now, but I have in my career worked on routers with hundreds of physical ports. [1] Yes, some switches have a router in them. Stop overthinking. [2] Yes, it's more complicated than that. Stop overthinking.