Typos:
* "From the application's viewpoint, the main **limitations** of the network layer service that its service is unreliable:" => "From the application's viewpoint, the main **limitation** of the network layer service **is** that its service is unreliable:"
* "We have already described in the datalink **layers** mechanisms to deal with data losses and transmission errors." => "We have already described in the datalink **layer** mechanisms to deal with data losses and transmission errors."
* "Transport **entity** are usually implemented inside the operating system kernel and share**s** memory with other parts of the system." => "Transport **entities** are usually implemented inside the operating system kernel and **share** memory with other parts of the system.""