[BCP63]    Best Current Practice 63,
              <https://www.rfc-editor.org/info/bcp63>.
              At the time of writing, this BCP comprises the following:

              Vemuri, A. and J. Peterson, "Session Initiation Protocol
              for Telephones (SIP-T): Context and Architectures",
              BCP 63, RFC 3372, DOI 10.17487/RFC3372, September 2002,
              <https://www.rfc-editor.org/info/rfc3372>.