Bill, To be brief, but hopefully not too fleeting, the majority of the standards orgs - ITU, MEF - use packet loss to derive availability. Loss% = the % of packets which were transmitted but not received by the destination host. As for availability, loss is measured across some time period. If during that period X% of the transmitted packets were NOT lost, then the network is said to be available. Typically a 20% figure is used, e.g. if 20% of the packets transmitted during a 5-minute period were received then the network is said to be 100% Available for that 5-minute time period. Some Carriers have taken this to the extreme to say that if at least 1 packet was successfully transmitted then the network was 100% Available for the time period. Loss is a measure of the networks usability, Availability is .......?? (Meaningless??) What utility does a network have that is "Available" yet sustaining a loss rate which renders it inoperable? Rich -----Original Message----- From: Bill Woodcock [mailto:woody@pch.net] Sent: Wednesday, July 29, 2009 12:34 AM To: nanog Subject: Ahoy, SLA boffins! 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