cloud services, MACsec supports close to the full line rate of the interface. On transmission, MACsec takes the existing Ethernet frame as input and turns it into a MACsec frame, adding a small amount of data in the MACsec security tag and the validation field. Figure 11-16 shows the formation of the MACsec frame, where SA and DA are the source and destination (MAC) address of the Ethernet frame and FCS is the frame check sequence. The MACsec tag is inserted before the Ethertype and an ICV is inserted before the FCS. MACsec supports both using just the integrity checking alone (against the ICV) or combining it with encryp- tion of the payload. Ethernet frame MACsec frame Encrypted data Ether- Ether- Payload Payload Integrity protected Figure 11-16 MACsec Frame Format On reception of a MACsec frame, the network element verifies the ICV field and decrypts the MACsec frame (if encrypted). When the NE determines the egress direction, if that path is con- figured to apply MACsec, it (optionally) encrypts the message, calculates the ICV, and forwards the message, with the MACsec tag, down the link to the next hop. The MACsec security tag contains a small amount of information, the most important of which is the packet number. This packet number is used to protect against replay attacks. It used to be a 32- bit value, but with very high-speed links, there is now a 64-bit extended packet number that can be used to reduce the frequency of rollover when using the smaller value. It also contains an Ethertype (88:E5) to indicate that this frame is now a MACsec frame. So, MSPs are looking at deploying MACsec in those sites that might be vulnerable to interception, and this raises a few challenges regarding the transport of PTP timing signals. When PTP messages are secured with MACsec, there are time error effects introduced by the implementation: Constant time error (cTE), due to the difference in time taken to encrypt and decrypt PTP mes- sages at a single node cTE, due to asymmetry based on the choice of the point at which 