%% You should probably cite draft-ietf-sip-multiple-refer instead of this I-D.
@techreport{ietf-sipping-multiple-refer-06,
    number =    {draft-ietf-sipping-multiple-refer-06},
    type =      {Internet-Draft},
    institution =   {Internet Engineering Task Force},
    publisher = {Internet Engineering Task Force},
    note =      {Work in Progress},
    url =       {https://datatracker.ietf.org/doc/draft-ietf-sipping-multiple-refer/06/},
    author =    {Gonzalo Camarillo},
    title =     {{Refering to Multiple Resources in the Session Initiation Protocol (SIP)}},
    pagetotal = 14,
    year =      2006,
    month =     jun,
    day =       27,
    abstract =  {This document defines extensions to the SIP REFER method so that this method can be used to refer servers to multiple resources. These extensions include the use of pointers to Uniform Resource Identifier (URI)-lists in the Refer-To header field and the "multiple-refer" SIP option-tag.},
}