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A Professional’s Guide to Data Communication in a TCP/IP World phần 3

(Để đơn giản, tiền tố sẽ được bỏ qua bất kỳ đối tượng được mô tả trong phụ lục này, cũng giống như iso.org.dod.internet được bỏ qua từ các đối tượng SNMP)Hình 4.4 cho thấy các định dạng tiêu đề gói tin cho hai gói dữ liệu và một gói tin điều khiển. Tất cả bao gồm một số nhóm 4-bit và một số kênh 8-bit | 2.4 Internet Model 39 Combines user s data with generic function software to create a user s data block identified as information retrieval file transfer and mail. Encapsulates the user s data block with a header application header AH and identifies the source port from which it is sent and to which any reply must be addressed. Passes the application protocol data unit APDU to the transport layer. When receiving the application layer Removes the application header from the APDU to leave the user s data block. Provides any processing required to complete the transaction. Passes the user s data to the user s application. Confirms that the process is completed. 2.4.2 Transport Layer Two modes of operation are possible in the transport layer. The header may support a simple connectionless procedure called User Datagram Protocol UDP or may support a connection-oriented procedure called Transmission Control Protocol TCP . The transport layer PDU is called a segment or message. When sending in the connectionless mode the transport layer Accepts the APDU from the application layer. Records both source and destination ports. Calculates a checksum and transmits the ones complement. Encapsulates the APDU with a header TH containing this information. Passes the TPDU to the Internet layer. When receiving in the connectionless mode the transport layer Accepts the TPDU from the network interface layer. Checks the length and confirms it matches the value contained in TH. If it does not agree it discards the TPDU. Calculates a checksum and confirms it is all ones when added to the ones complement transmitted in the checksum field. If it is not it discards the frame. Passes the APDU to the receiving port identified in the TPDU. When sending in the connection-oriented mode the transport layer Establishes a duplex connection real or virtual . Accepts the APDU from the application layer. Records source and destination ports. Provides the number of the first byte to be sent. .