Ahoy, SLA boffins!

Bill Woodcock woody at pch.net
Tue Jul 28 23:34:04 CDT 2009

So I've embarked on the no-doubt-futile task of trying to interpret  
SLAs as empirically-verifiable technical specifications, rather than  
as marketing blather.  And there's something that I'm finding  
particularly puzzling:

In most SLAs, there seem to be two separate guarantees proffered: one  
concerning "network availability" and one concerning "packet loss."   
Now, if I were to put my engineer hat on, and try to _imagine_ what  
the difference might be, I might imagine "network availability" to  
have something to do with layer-2 link status being presented as "up,"  
while packet loss would be the percentage of packets dropped.  But  
when I actually read SLAs, "network availability" is generally defined  
as the portion of the month that the path from the customer's local  
loop to the transit or peering routers was "available" to transmit  
packets.  Packet loss, on the other hand, is generally defined as the  
portion of packets which are lost while crossing that exact same piece  
of network.

Now, what am I missing here?  Is this one of those Heisenberg things,  
where "network availability" is the time the network _could have_  
delivered a packet _when you weren't actually doing so_, while "packet  
loss" is the time the network _couldn't_ deliver a packet when you  
_were_ actually doing so?

Is "network availability" inherently unmeasurable on a network that's  
less than 100% utilized?

Am I over-thinking this?

Seriously, though, I know there are people who don't consider SLAs to  
be fantasy-fiction, and some of them must not be innumerate, and some  
subset of those must be on NANOG, and the intersection set might be  
equal to or greater than one, right?  Can anybody explain this to me  
in a way I can translate into code, while still taking myself seriously?


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