Original text: /wind19/article/details/4669637
SDU (service Data Unit): Service data unit, also known as service data unit, is a data set of user services at the specified layer. When transmitted to the receiver, the data does not change when the same protocol layer is transmitted to the receiver, that is, the service part, and then sent to the lower layer, the lower layer encapsulates it in the PDU and sends it out. The service data unit is transmitted from the information unit from the high-level protocol to the low-level protocol. The N-layer service data unit SDU and the protocol data unit (PDU) of the previous layer are one by one. Depending on the data of the protocol data unit, it is sent to the designated layer at the receiving end.
PDU(N) = SDU(N - 1)
SDU(N) = PDU(N + 1)
PDU（Ptotocol data unit）: Protocol data unit:Unit information exchanged between peer entities at each level of a computer networkFor example, the PDU of the TCP layer is segment (partition), and the PDU exchanged between the application layers is application data (application data)
A simple understanding is:
The PDU of this layer is the SDU of the lower layer;
The SDU of this layer is the PDU of the upper layer
The SDU service data unit corresponds to data that is not processed in a certain sub-layer. For a certain sublayer, the SDU comes in.
The PDU protocol data unit corresponds to data that is processed by the sub-layer to form a specific format. For a certain sublayer, the one that goes out is the PDU.
NLayer user and NThe data passed between layer protocols is called the Service Data Unit (SDU)
NData passed between layer protocol entitiesIt is called a protocol data unit (PDU), and it is managed through data transmission/receptionUser-submitted SDUIn the form of a PDU, it is sent to the peer protocol entity through the lower channel. existThe receiver then redirects the PDURestore to SDU and give to the receiver.
PDUPackage/Decapsulation: On the sender, the SDU submitted by the user is added to the protocol control information PCI and packaged into a PDU; on the receiver, the received PDU is decapsulated, the PCI is removed, and the SDU is restored to the SDU and sent to the receiver user.
SDUSegmentation/Assembly: If the bandwidth of the lower channel cannot meet the needs of delivering SDUs, it is necessary to divide an SDU into multiple segments and package it into PDUs and send it out (segmented); the receiver then decapsulates these PDUs and reassembles them into SDUs.
SDUSplicing/Separation: Splicing refers to the transmission (n) layer protocol entity encapsulating multiple (n) SDUs with shorter lengths into one (n) PDU for transmission, and then decapsulating the received (n) PDUs and separating the multiple (n) SDUs. The purpose of using the splicing function is to improve the utilization rate of the channel.
PDUsegmentation/Combination: PDU segmentation refers to the division of a (n) PDU into multiple (n-1) SDUs at the (n) layer protocol entity at the sending end and sending it out from multiple (n-1) channels in parallel; the receiving end then combines the received multiple (n-1) SDUs into one (n) PDU. Since this is an N-layer function, the combination operation is performed in the N-layer, that is, the N-layer first obtains multiple split (n) PDUs, and then combines them into one (n) PDU.