Lync's native protocol is SIP, a standard that Google could also have adopted years ago, but they went with the more modern and 'better' competing XMPP standard

Lync's native protocol is SIP, a standard that Google could also have adopted years ago, but they went with the more modern and 'better' competing XMPP standard. Microsoft went through a lot of problems implementing SIP it for Lync and its predecessors LCS and OCS, but they remained within the standard and worked on interoperability and improving the standard around security, even if it took more time than going it alone. The roles of Microsoft and Google have certainly switched around.