Computer Networking
The Internet Control Message Protocol (ICMP)

provides communication between the Internet Protocol Software on one machine and the Internet Protocol Software on another
None of these
reports error conditions to the original source, the source must relate errors to individual application programs and take action to correct the problem
All of these
allows gateways to send error a control messages to other gateways or hosts

ANSWER DOWNLOAD EXAMIANS APP

Computer Networking
X.21 is

network access standard for connecting stations to a circuit-switched network
a method of determining which device has access to the transmission medium at any time
a method access control technique for multiple-access transmission media
None of these
a very common bit-oriented data link protocol issued by ISO

ANSWER DOWNLOAD EXAMIANS APP