4 ms·

ARP is also nice and abstract and well-defined; it can bridge from any multi-endpoint subnet's layer-2 address to an IP address. Not sure if anyone actually use

ARP is also nice and abstract and well-defined; it can bridge from any multi-endpoint subnet's layer-2 address to an IP address. Not sure if anyone actually uses it for non-802, but the generality has forced a clean design. To add to your praises of DHCP - it can also configure routers, and is in fact the standard solution for that in IPv6. Instead of giving you one or several addresses for NAT through DHCP, it gives the router an address for itself, and also a prefix to assign to clients on its internal network. Super neat stuff, and a boon to administrators. Also to nitpick your summary, because nitpicking is what I do - layering violations are a thing, but only in the same way that violating software abstraction barriers are a thing. Not a hard-and-fast rule, and sometimes if you're doing weird enough stuff you just have to do it.

mcguire 7y ago
To go further off the rails, layering violations are not a thing, because "layers" are a remarkably poor abstraction for a network protocol "stack".
- azernik 7y ago
  Got to disagree. The level of abstraction is very useful as a means of swapping out one layer without changing the other technologies - e.g. running IP over point-to-point fiber, or AlohaNet, or 802, or carrier pigeon. Or running running ethernet over a phy with whatever ridiculous number of Mbps is the latest thing. (802.11, of course, has effectively zero phy/link distinction, but anything that has to deal with such high packet drop rates and negotiation of physical layer between endpoints is going to be a mess.) There's an issue with the specific OSI layering, but that's higher in the stack: it has waaay too many layers at the top. Everything up to maybe the transport layer (TCP/UDP/SCTP) is very well delinked in most implementations, but the session/presentation/application layer distinctions are total BS.
  - tptacek 7y ago
    They're a useful tool for understanding the mindset of the original developers, but as you go "up" in the layers, the division of responsibilities becomes more and more arbitrary, with a very sharp uptick after "layer 3". But more importantly, the notion that routing and forwarding "belongs" in IP, because that's the layer 3 protocol --- that's just false. There's no validity to it, and lots of systems have built overlays with layer 3 function on top of UDP (which in the "layering" model is a "layer 4" protocol, but is really best thought of as an escape hatch with which to build any new system you want on top of IP).
    - azernik 7y ago
      Who makes systems that do routing on UDP?
      - dnautics 7y ago
        Vxlan, for starters.
        azernik 7y ago
        That's more of a recursive version of the lower layers; using layers 1-4 of one instance of the OSI model as layer 2 of another instance. If anything, this demonstrates just how useful the clear abstraction barrier between layer 2 and layer 3 is; you can have a very complicated software package (like a VPN) as a layer 2 instead of a physical network and all the code from layer 3 up doesn't even need to know.
      - tptacek 7y ago
        What does "on UDP" mean? UDP is just a means of running an arbitrary datagram protocol that rides on top of IP; it's how you'd build a system that treats IP the way IP treats Ethernet.
        azernik 7y ago
        Sure, but you mentioned protocols that have "built overlays with layer 3 function on top of UDP". What are the examples you're referring to? EDIT: My comment in reply to the sibling comment, which mentioned vxlan: That's more of a recursive version of the lower layers; using layers 1-4 of one instance of the OSI model as layer 2 of another instance. If anything, this demonstrates just how useful the clear abstraction barrier between layer 2 and layer 3 is; you can have a very complicated software package (like a VPN) as a layer 2 instead of a physical network and all the code from layer 3 up doesn't even need to know.
  - mcguire 7y ago
    There are other models of modularity that make it easy to separate transport, routing, link, and physical protocols without starting from the assumption that "layer X can only interact with the minimum common denominator interface for layers X-1 and X+1". That assumption leads to everything from the PMTU discovery silliness to the pain of getting TCP to work correctly over links like wireless where packet loss does not imply congestion.
  - bdamm 7y ago
    I've heard some folks talk about TLS as a "session" layer, and it is fortunate that we no longer have to translate between ASCII and EBCDIC underneath the application, so the "presentation" layer now seems like it is mis-named. Ah how times change.

mcguire 7y ago

To go further off the rails, layering violations are not a thing, because "layers" are a remarkably poor abstraction for a network protocol "stack".

azernik 7y ago
Got to disagree. The level of abstraction is very useful as a means of swapping out one layer without changing the other technologies - e.g. running IP over point-to-point fiber, or AlohaNet, or 802, or carrier pigeon. Or running running ethernet over a phy with whatever ridiculous number of Mbps is the latest thing. (802.11, of course, has effectively zero phy/link distinction, but anything that has to deal with such high packet drop rates and negotiation of physical layer between endpoints is going to be a mess.) There's an issue with the specific OSI layering, but that's higher in the stack: it has waaay too many layers at the top. Everything up to maybe the transport layer (TCP/UDP/SCTP) is very well delinked in most implementations, but the session/presentation/application layer distinctions are total BS.
- tptacek 7y ago
  They're a useful tool for understanding the mindset of the original developers, but as you go "up" in the layers, the division of responsibilities becomes more and more arbitrary, with a very sharp uptick after "layer 3". But more importantly, the notion that routing and forwarding "belongs" in IP, because that's the layer 3 protocol --- that's just false. There's no validity to it, and lots of systems have built overlays with layer 3 function on top of UDP (which in the "layering" model is a "layer 4" protocol, but is really best thought of as an escape hatch with which to build any new system you want on top of IP).
  - azernik 7y ago
    Who makes systems that do routing on UDP?
    - dnautics 7y ago
      Vxlan, for starters.
      - azernik 7y ago
        That's more of a recursive version of the lower layers; using layers 1-4 of one instance of the OSI model as layer 2 of another instance. If anything, this demonstrates just how useful the clear abstraction barrier between layer 2 and layer 3 is; you can have a very complicated software package (like a VPN) as a layer 2 instead of a physical network and all the code from layer 3 up doesn't even need to know.
    - tptacek 7y ago
      What does "on UDP" mean? UDP is just a means of running an arbitrary datagram protocol that rides on top of IP; it's how you'd build a system that treats IP the way IP treats Ethernet.
      - azernik 7y ago
        Sure, but you mentioned protocols that have "built overlays with layer 3 function on top of UDP". What are the examples you're referring to? EDIT: My comment in reply to the sibling comment, which mentioned vxlan: That's more of a recursive version of the lower layers; using layers 1-4 of one instance of the OSI model as layer 2 of another instance. If anything, this demonstrates just how useful the clear abstraction barrier between layer 2 and layer 3 is; you can have a very complicated software package (like a VPN) as a layer 2 instead of a physical network and all the code from layer 3 up doesn't even need to know.
- mcguire 7y ago
  There are other models of modularity that make it easy to separate transport, routing, link, and physical protocols without starting from the assumption that "layer X can only interact with the minimum common denominator interface for layers X-1 and X+1". That assumption leads to everything from the PMTU discovery silliness to the pain of getting TCP to work correctly over links like wireless where packet loss does not imply congestion.
- bdamm 7y ago
  I've heard some folks talk about TLS as a "session" layer, and it is fortunate that we no longer have to translate between ASCII and EBCDIC underneath the application, so the "presentation" layer now seems like it is mis-named. Ah how times change.