ods defined in IEEE 1588-2019 depending on the pro- cessing model: GDOI: Group Domain of Interpretation, RFC 6407, is the scheme applied for group key dis- tribution to support immediate security processing for PTP ICV authentication. TESLA: Timed Efficient Stream Loss-tolerant Authentication, RFC 4082, is the scheme ap- plied when using delayed security processing. It is a very clever arrangement that allows imme- diate validation of the source of the message but delays the ability to validate the content until the key from the previous key interval has expired. Further discussion of these mechanisms and fields is beyond the scope of this book, not least because they have not yet been adopted by the main profiles. This is background information to give you an understanding of what might make its way into the popular profiles. For further in- formation on the ICV and AUTHENTICATION TLV, please see Section 16.14 of the IEEE 1588-2019 standard. 11.5.1 MACsec For reasons outlined previously, an increasing number of MSPs are looking to adopt transport-level security to protect the information within their networks, not least because elements of the network might be more accessible to nonauthorized personnel than previously. This is not just a problem with MSPs, but also cable operators, because increasingly they are adopting common packet-based transport standards such as Ethernet. Previously, only the very specialized hacker had access to a device to leavesdrop on cable protocols or Asynchronous Transfer Mode (ATM) cells, but now Eth- ernet devices are ubiquitous. The major backbone pipelines of the Internet, such as high-speed interconnection between data cen- ters and cloud services, are increasingly encrypted, and the technology most often deployed to pro- tect any data "in motion" is MACsec. The MACsec protocol was originally defined by IEEE 802.1 in 2006 and is now specified in the latest version, IEEE 802.1AE-2018. Because it is used to protect the very high-speed data rates needed to connect data centers and 