[e2e] Switched Ethernet is Not an End-to-End System; was Protocols breaking the end-to-end argument

Richard Bennett richard at bennett.com
Sun Nov 1 13:26:02 PST 2009

There was a need for flow control as soon as full dupex was included in 
the 10BASET spec, but it became even more important with the addition of 
100BASETx to switches that were backward compatible with 10BASET. A 
collision is better than a silent drop, but neither is necessary. Full 
duplex Ethernet switches can transmit multiple frames at the same time, 
which is quite convenient in meet-me rooms at IXPs so I don't buy the 
routing vs. switching dichotomy; switching helps us do routing.

The point about the thinness of the IP layer doesn't have to do with 
routing as much as it has to do with what's in the IP header and what 
isn't. I would expect that a network layer protocol would have an 
unambiguous address for the host, like CYCLADES, DECNet, XNS, and ISO 
CLNP. But all IP has is an address that's a synonym for the LAN 
interface address, a point of attachment. So it's not fully separated 
from Layer 2. This is especially stark in IPv6 where they just throw in 
the whole MAC address into the IP header in order to bypass ARP.

In addition to IP lacking an actual host address, it doesn't do any 
protocol either - it's just a packet format and doesn't participate in 
any specific sequences of behavior, which is once again just like Blue 
Book Ethernet, AKA V2. It's perhaps worth noting that V2 is an odd MAC 
protocol  since most of its cousins have multiple frame types and state 
machines for each, even Switched Ethernet. Granted, there is a network 
address in the IP header, for what it's worth, but IP seems to be 
missing some function that would make networking a lot easier than it is 
in scenarios where a number of diverse applications contend for 
resources, and some of the function it's missing was also missing in V2. 
This is true in many universes, some of the alternates to this one.

Regarding Jon's comment on the rebirth of CSMA at the edge, there is 
some ironic truth to it, but Wi-Fi's not the same style as CSMA/CD 
because with 802.11n we have a selectively acknowledged windowing 
protocol, much more efficient than TCP where you have to discard 
everything after a dropped packet and do it again. I suspect that LTE is 
going to be a very large factor one day, and it uses a scheduled system 
that doesn't have collisions.


rick jones wrote:
> On Oct 31, 2009, at 2:46 PM, Richard Bennett wrote:
>> Dave Eckhardt wrote:
>>> So it's unclear
>>> that CSMA/CD was a structural limit of Ethernet--the reality
>>> is probably more like "It doesn't matter much how you contend
>>> among a few hosts, but you can't build large networks unless
>>> you limit contention domains to less than the size of the
>>> large network", which is almost a tautology.
>> That's part of the story, but the implications of the switched 
>> Ethernet killing off CSMA/CD Ethernet are much larger, and relate the 
>> end-to-end arguments principle. CSMA/CD Ethernet was an end-point 
>> managed system sharing a dump pipe, while switched Ethernet is a 
>> system that deploys intelligence - switching, flow control, 
>> buffering, QoS discrimination, VLANs - inside the network at multiple 
>> points. Switched Ethernet is scalable, manageable, diagnosable, and 
>> future-proof, while CSMA/CD Ethernet is none of these things. So the 
>> competition of CSMA/CD and Active Switching for markets demonstrates 
>> something about which approach to the design of layer 2 networks is 
>> superior.
> I think you left-out how Power over Ethernet will replace the global 
> power grid and that it also juliennes fries :)
> Color me a cynic, but I rather thought that today's switched 
> "Ethernet" needed flow control and buffering precisely because CSMA/CD 
> was removed from Ethernet when it went full-duplex?  I seem to recall 
> that flow-control was not initially present in full-duplex Ethernet.  
> I'm still not sure how much of the rest of the laundry list above has 
> been added to Ethernet has been in response to folks going "Routing is 
> hard, lets go shopping for switches" and the switch vendors being 
> quite happy to provide a solution to encourage people to buy new 
> switches.
> rick jones
> there is no rest for the wicked, yet the virtuous have no pillows

Richard Bennett
Research Fellow
Information Technology and Innovation Foundation
Washington, DC

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