4 ms·

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

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.

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.