%% You should probably cite rfc4891 instead of this I-D.
@techreport{ietf-v6ops-ipsec-tunnels-05,
    number =    {draft-ietf-v6ops-ipsec-tunnels-05},
    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-v6ops-ipsec-tunnels/05/},
    author =    {Mohan Parthasarathy and Richard F. Graveman and Hannes Tschofenig and Pekka Savola},
    title =     {{Using IPsec to Secure IPv6-in-IPv4 Tunnels}},
    pagetotal = 23,
    year =      2007,
    month =     jan,
    day =       17,
    abstract =  {This document gives guidance on securing manually configured IPv6-in- IPv4 tunnels using IPsec in transport mode. No additional protocol extensions are described beyond those available with the IPsec framework. This memo provides information for the Internet community.},
}