Tài liệu a professionals guide to data communication in a tcpip world

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:
ã Accepts the APDU from the application layer.
ã Records both source and destination ports.
ã Calculates a checksum and transmits the ones complement.
ã Encapsulates the APDU with a header (TH) containing this information.
ã Passes the TPDU to the Internet layer.
When receiving in the connectionless mode, the transport layer:
ã Accepts the TPDU from the network interface layer.
ã Checks the length and confirms it matches the value contained in TH. If it
does not agree, it discards the TPDU.
ã 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.
ã Passes the APDU to the receiving port identified in the TPDU.
When sending in the connection-oriented mode, the transport layer:
ã Establishes a duplex connection (real or virtual).
ã Accepts the APDU from the application layer.
ã Records source and destination ports.
ã Provides the number of the first byte to be sent.
ã Acknowledges receipt of previous frame (if any).
2.4 Internet Model 39
ã Identifies size of storage allocated to this segment.
ã Calculates a checksum and transmits the ones complement.
ã Requests options such as selective acknowledgement, larger window size, and
so forth from the destination.
ã Encapsulates APDU with a header (TH) containing this information to form
TPDU.