%% You should probably cite draft-holmberg-sipcore-keep instead of this I-D.
@techreport{holmberg-sip-keep-04,
    number =    {draft-holmberg-sip-keep-04},
    type =      {Internet-Draft},
    institution =   {Internet Engineering Task Force},
    publisher = {Internet Engineering Task Force},
    note =      {Work in Progress},
    url =       {https://datatracker.ietf.org/doc/draft-holmberg-sip-keep/04/},
    author =    {Christer Holmberg},
    title =     {{Indication of support for keep-alive}},
    pagetotal = 10,
    year =      2009,
    month =     mar,
    day =       26,
    abstract =  {This specification defines a new SIP Via header parameter, "keep" which SIP entities can use to indicate support of the NAT keep-alive techniques defined in SIP Outbound, in cases where the Outbound procedures are not supported or cannot be applied.},
}