[e2e] Historical question: Link layer flow control / silent discard
mattmathis at google.com
Sun May 26 09:50:00 PDT 2013
On Sat, May 25, 2013 at 4:34 PM, Lachlan Andrew <lachlan.andrew at gmail.com>wrote:
> I've always wondered why
> switched ethernet (which does the ISO layer-3 tasks of addressing,
> routing over multiple point-to-point links and buffering) is called a
> "link layer" by the internet community...
It's called "feature creep". Ethernet 2.0 (predates 802.*) was clearly a
link layer. IEEE keeps adding stuff. Many Internet purists complained.
We now are paying for doing nearly everything twice: once in silicon by
way of the IEEE and once in the Internet proper. As far as the internet
is concerned, the extra complexity in the lower layer is mostly a waste.
However there are a couple of really important exceptions: there is a
multiplicative scale increase from doing both routing and switching. Today
a single router with dozens of interfaces connected to switching fabrics
can have hundreds or thousands of peers. This is in part how the Internet
itself beats Moore's law. (The switches typically use MPLS)
Otherwise there would be some key router components that need to beat
Moore's law squared: router buffer memory size needs to scale with the data
rate, router FIB memory needs to scale with routing table size,
and access time for both also needs to scale with data rate. We at least
partially ease the scale pressure on these components by increasing the
Internet branchiness rather than path lengths.
As for link layer flow control, it doesn't work. You can buy it today in
many technologies, but if you turn it on, you will rediscover head-of-line
blocking and global resource contention. e.g. If you have one overloaded
exit, the cascaded backlogs at each stage through the fabric guarantee
congestion and/or resource starvation everywhere, even if the rest of the
fabric is otherwise lightly loaded.
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