Polina Goltsman issue 13

"old:
   The general principle here is that the security mechanisms (TLS at
new:
   The general principle here is that the security mechanisms ((D)TLS
at"

- s/TLS/TLS or DTLS/
Merge pull request #220 from petithug/polina-goltsman-issue-13

Polina Goltsman issue 13