1)
Doesn't take much thought if you've manually calculated a checksum and have a small math aptitude.
Parity may fail with a simple two-bit corruption:
transmitted: data = 00001111, parity = 0
corrupted: data = 10001111, parity = 1
corrupted again: data = 11001111, parity = 0
Checksums can fail by inserting errors where the errors sum to zero:
transmitted: data = 1 2 3, checksum = 6
corrupted: data = 0 3 3, checksum = 6
CRCs can fail by inserting errors where the polynomially-weighted errors sum to zero. I've used a trivial CRC, real CRC's don't individually weight every bit as this leads to too large a CRC.
transmitted data: 1 2 3, trivial crc = 1^3 + 2^2 + 3^1 = 1 + 4 + 3 = 8
corrupted: data = 0 1 2 3, trivial crc = 0^4 + 1^3 + 2^2 + 3^1 = 0 + 1 + 4 + 3 = 8
2)
What they are looking for here is the words "virtual private network".
A VPN is a set of tunnels (IP packet containing IP packets) which allow sites to be interconnected across the Internet without getting direct access to the Internet. The firewall on each site is configured to allow traffic which crosses the VPN tunnels but to prevent other Internet traffic getting direct access to the interior of the sites.
The effect is as if the sites have arranged to interconnect their routers using leased lines rather than using an Internet service provider.
VPNs commonly encrypt traffic so that the privacy of the inter-site traffic can be maintained. These encrypted tunnels remain vulnerable to traffic analysis; that is, undermining privacy by looking at the quantity and timing of encrypted data.
Without using a QoS class from the ISP which interconnects the sites, VPN-connected sites may lose connectivity if the ISP has a denial of service attack.
