Year Ended December 31,
Year Ended December 31,
Year Ended December 31,
2019
2018
2017
Subscription service revenue
$849,267
$681,138
$489,274
Subscription software revenue
25,739
23,262
17,081
Hardware and services
13,184
12,594
13,326
Total revenue
$888,190
$716,994
$519,681
proofpoint inc .
notes to consolidated financial statements -continued -dollars and share amounts in thousands except per share amounts .
of the transaction price to each performance obligation based on a relative standalone selling price or ssp basis .
• recognition of revenue when or as the company satisfies a performance obligation - the company recognizes revenue when control of the services or products are transferred to the customers in an amount that reflects the consideration the company expects to be entitled to in exchange for those services or products .
the company records its revenue net of any value added or sales tax .
the company generates sales directly through its sales team and to a growing extent through its channel partners .
sales to channel partners are made at a discount and revenues are recorded at this discounted the company generates sales directly through its sales team and to a growing extent through its channel partners .
sales to channel partners are made at a discount and revenues are recorded at this discounted price once all revenue recognition criteria are met .
channel partners generally receive an order from an end-customer prior to placing an order with the company and these partners do not carry any inventory of the company ’ s products or solutions .
payment from channel partners is not contingent on the partner ’ s success in sales to end-customers .
in the event that the company offers rebates joint marketing funds or other incentive programs to a partner recorded revenues are reduced by these amounts accordingly .
payment terms on invoiced amounts are typically 30 to 45 days .
disaggregation of revenue disaggregation of revenue .
the company derives its revenue primarily from : -1 subscription service revenue ; -2 subscription software revenue and -3 hardware and services which include professional service and training revenue provided to customers related to their use of the platform .
the following table presents the company ’ s revenue disaggregation : .
subscription service revenue .
subscription service revenue is derived from a subscription-based enterprise licensing model with contract terms typically ranging from one to three years and consists of -1 subscription fees from the licensing of the company ’ s security-as-a-service platform and it ’ s various components -2 subscription fees for software with support and related future updates where the software updates are critical to the customers ’ ability to derive benefit from the software due to the fast changing nature of the technology .
these function together as one performance obligation and -3 subscription fees for the right to access the company ’ s customer support services for software with significant standalone functionality and support services for hardware .
the hosted on-demand service arrangements do not provide customers with the right to take possession of the software supporting the hosted services .
support revenue is derived from ongoing security updates upgrades bug fixes and maintenance .
a time-elapsed method is used to measure progress because the company transfers control evenly over the contractual period .
accordingly the fixed consideration related to subscription service revenue is generally recognized on a straight-line basis over the contract term beginning on the date access is provided as long as other revenue recognition criteria have been met .
most of the company ’ s contracts are non-inate their contract for cause if the subscription service revenue is derived from a subscription-based enterprise licensing model with contract terms typically ranging from one to three years and consists of -1 subscription fees from the licensing of the company ’ s security-as-a-service platform and it ’ s various components -2 subscription fees for software with support and related future updates where the software updates are critical to the customers ’ ability to derive benefit from the software due to the fast changing nature of the technology .
these function together as one performance obligation and -3 subscription fees for the right to access the company ’ s customer support services for software with significant standalone functionality and support services for hardware .
the hosted on-demand service arrangements do not provide customers with the right to take possession of the software supporting the hosted services .
support revenue is derived from ongoing security updates upgrades bug fixes and maintenance .
a time-elapsed method is used to measure progress because the company transfers control evenly over the contractual period .
accordingly the fixed consideration related to subscription service revenue is generally recognized on a straight-line basis over the contract term beginning on the date access is provided as long as other revenue recognition criteria have been met .
most of the company ’ s contracts are non-cancelable over the contract term .
customers typically have the right to terminate their contract for cause if the company fails to perform in accordance with the contractual terms .
some of the company ’ s customers have the option to purchase additional subscription services at a stated price .
these options are evaluated on a case-by-case basis but generally do not provide a material right as they are priced at or above the company ’ s ssp and as such would not result in a separate performance obligation .

Question: What method is used to measure the progression of how the company transfer its control?
Answer: time-elapsed method