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The MAC address is meaningful only for a single link. It doesn't get copied through routers and get carried along to the destination. (At least in theory. Betwe
by Animats 1y ago
The MAC address is meaningful only for a single link. It doesn't get copied through routers and get carried along to the destination. (At least in theory. Between software defined network hardware and enterprise WiFi, there are cases where MAC addresses have wider scope. But they rarely leave an organization.)
The Xerox plan for Ethernet was to use XNS, with a 32-bit network ID and a 48-bit MAC. Routers only had to know how to find a network ID, and once the packet hit the correct destination network, the MAC address routed it to the destination.
Early on, there was a thing for "multi-protocol routers", that forwarded XNS, DECnet, IP, and a few other things such as Parc Universal Protocol, PUP. Cisco was once big on this, because Cisco came out of Stanford, which had a few of everything and tried to interconnect them.
In the 1990s, most of those branded protocols died out, although as late as Windows 2000, Microsoft still supported TP4, a bad idea from circuit-oriented telco people. There was a telco approach to networking, where you dialed up a reliable circuit to your destination, transmitted what you had to send, and later got a bill for the call. This, fortunately, lost out to IP.
(I had real doubts about pure datagram long distance networks. We didn't know how to deal with congestion in the middle of a pure datagram network. We still don't, but cheap fiber saved us from having to.)
- kstrauser 1y agoDon’t get me started on the ugly detour that was ATM, for when Bellheads wanted to build out the Internet like a phone switch, and when no one could agree on optimal cell sizes and they compromised on 53 bytes to piss off everyone equally. (Also, to have been a fly on the while when you worked out that whole congestion thing.)
- Animats 1y agoIt took over two decades for things to settle. The telco people had ATM and Frame Relay and DSL voice and a Bell Labs thing called Datakit. They were trying to get deterministic voice, so voice over packets wouldn't suck. What we ended up with is voice with lag, jitter, echo, dropout, and over-compression. So nobody talks on the phone much now.
- kstrauser 1y agoGive a fellow a little warning before conjuring those names! When it became clear that packet switching was going to win out, did the telcos ever try to emulate circuits with UDP flows or similar? It seems like IP over switched circuits would look a lot like virtual switched circuits over IP, if you squint just right.
- windexh8er 1y agoI remember working on a federal contract back in the early 2000s where there was a camp of ATM folks against us who were pushing Metro Ethernet and MPLS. The TL;DR of it was the ATM guys won, spent millions of dollars, only for us to come in and replace it all a year after it was supposed to have been done in the first place. Cisco sure made some good money on failed ATM projects back then!
- kstrauser 1y agoI witnessed the same. Yeah, charging twice to build out each link is a pretty good profit model.
- mannyv 1y agoMy understanding was that ATM was 53 bytes because it was a balance between latency and processing efficiency. If every packet is the same size your packet processing can be much faster.
- kstrauser 1y agoThat’s right. But… 53?! We were all gobsmacked when that was announced.
- gizmo686 1y ago> The MAC address is meaningful only for a single link This isn't entirely true. In ye-olden days, Ethernet was a proper bus architecture, where you would have many devices talking on a single line. As networks got larger, we wanted to have more devices than we could get on a single cable, so we introduced cheap booster hubs that would physically connect multiple cables. Since these hubs didn't have any fancy switching logic, it continued to look to devices like everyone was on a single bus (which they basically were). Eventually, compute became cheaper, and the limitations of having a massive bus more apparent, so we started replacing some of the Ethernet hubs with Ethernet switches to reduce contention. It still looks as if everyone is on a single bus; but now a node only sees traffic involving a node on its side of the nearest switch. Notably, these switches were drop in replacements for hubs, so they did not rewrite MAC addresses. Nowadays, actual Ethernet hubs are a dying breed (although I still keep one lying around to avoid buying an expensive managed switch with a monitor port). Almost every Ethernet cable you run into today is a genuine point-to-point connection connecting two specific nodes, and carrying only the data that needs to be sent between those nodes. Now that modern Ethernet is a full featured packet switched protocol, it has to deal with all the routing issues that come with that. This led to the introduction of the Spanning Tree Protocol, to allow Ethernet switches to construct Ethernet routing tables. Notably, nothing inside of these Ethernet networks replaces MAC addresses. When computer A sends to computer B on the same Ethernet network, the destination MAC address A uses is B's actual MAC address, regardless of how many switches are between them.
- nemothekid 1y ago>so we started replacing some of the Ethernet hubs with Ethernet switches to reduce contention. I just remembered having to answer questions about Ethernet hubs vs switches many years ago. To me, ethernet hubs are entirely theoretical, I don't think I've ever seen one in the wild.
- kstrauser 1y agoYou weren’t missing much. They sucked. Switches were an improvement in every way.
- 1y ago