Arista Warrior - Cdn.oreilly.com

The kickin’ 10 Gbps data flow from Stream-Co has screamed across the country (or
even the world) until it gets right to your virtual doorstep, at which point the speed goes
from 10 Gbps to 50 Mbps. The difference in speed is not 10:1 like it is in a data center
switch, but rather 200:1!
Now let’s play a bit and assume that the cable distribution device has 24 MB buffers like
our Arista 7048T does. Remember, that 24 MB at 1 Gbps is 20 ms. Well, that same 24
MB at 50 Mbps is 4 seconds! Buffering for 20 ms is not a big deal, but buffering for 4
seconds will confuse the TCP windowing system, and your performance may be less
than optimal, to say the least. Additionally, although 24 MB is 4 seconds at 50 Mbps,
remember that it’s only 0.019 seconds at 10 Gbps. In other words, this buffer would take
less than 1/10th of a second to fill, but 4 seconds to empty.
Think about this, too: propagation delay (the time it takes for packets to travel over
distance) from New York to California might be 100 ms over multiple providers. Let’s
add that much on top for computational delay (the amount of time it takes for servers,
switches, and routers to process packets), which gives us 200 ms. That’s one-fifth of a
second, which is a pretty long time in our infinitely connected high-speed world. Imagine
that your service provider is getting packets in 200 ms, but is buffering multiple
seconds of your traffic. To quote some guy I met on the beach in California, that’s not
cool, man.
My point with this talk of buffer bloat is to consider all the information before coming
to rash conclusions. You may hear vendors pontificate about how big buffers are bad.
Big buffers within the data center make a lot more sense than big buffers for cable modem
distribution switches.
22 | Chapter 2: Buffers