%% You should probably cite draft-ietf-calext-subscription-upgrade instead of this I-D.
@techreport{douglass-subscription-upgrade-00,
    number =    {draft-douglass-subscription-upgrade-00},
    type =      {Internet-Draft},
    institution =   {Internet Engineering Task Force},
    publisher = {Internet Engineering Task Force},
    note =      {Work in Progress},
    url =       {https://datatracker.ietf.org/doc/draft-douglass-subscription-upgrade/00/},
    author =    {Michael Douglass},
    title =     {{Calendar subscription upgrades}},
    pagetotal = 6,
    year =      2017,
    month =     feb,
    day =       14,
    abstract =  {This specification introduces an approach to allow subscribers to calendar feeds to upgrade to a more performant protocol.},
}