Ahoy, SLA boffins!

Net funkyfun at gmail.com
Wed Jul 29 17:49:42 UTC 2009


On 7/29/09, Bill Woodcock <woody at pch.net> wrote:
> 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?
>                                  -Bill

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