Để di chuyển một gói tin từ một nút (máy chủ lưu trữ hoặc chuyển đổi gói tin) đến nút tiếp theo trong lộ trình, các lớp mạng phải dựa vào các dịch vụ của Để chuyển các gói dữ liệu từ một máy chủ gửi đến các máy chủ đích, lớp mạng phải xác định đường đi hoặc lộ trình mà các gói tin phải tuân theo. | Principle of Reliable Data Transfer rdt_rcv rcvpkt notcorrupt rcvpkt hasseqO rcvpkt extract rcvpkt data del iver_data data compute chksum make_pkt sendpkt ACKO chksum udt_send sndpkt udt_send sndpkt rdt rcv rcvpkt corrupt rcvpkt I has seq 1 rcvpkt udt_send sndpkt rdt rcv rcvpkt corrupt rcvpkt I has_seqO rcvpkt rdtrcv rcvpkt notcorrupt rcvpkt has seq 1 rcvpkt extract rcvpktdata deliver_data data compute chksum make_pkt sendpkt ACKl chksum udtsend sndpkt Figure receiver Reliable Data Transfer over a Lossy Channel with Bit Errors Suppose now that in addition to corrupting bits the underlying channel can lose packets as well a not uncommon event in today s computer networks including the Internet . Two additional concerns must now be addressed by the protocol how to detect packet loss and what to do when this occurs. The use of checksumming sequence numbers ACK packets and retransmissions - the techniques already developed in rdt - will allow us to answer the latter concern. Handling the first concern will require adding a new protocol mechanism. There are many possible approaches towards dealing with packet loss several more of which are explored in the exercises at the end of the chapter . Here we ll put the burden of detecting and recovering from lost packets on the sender. Suppose that the sender transmits a data packet and either that packet or the receiver s ACK of that packet gets lost. In either case no reply is forthcoming at the sender from the receiver. If the sender is willing to wait long enough so that it is certain that a packet has been lost it can simply retransmit the data packet. You should convince yourself that this protocol does indeed work. But how long must the sender wait to be certain that something has been lost It must clearly wait at least as long as a round trip delay between the sender and receiver which may include buffering at intermediate routers or gateways plus whatever amount of time is needed to process a .
