Database Reference
In-Depth Information
Network Layers
Any network is implemented as a set of layers, each layer assigned to a specific task. To understand performance
metrics of a network, you need to know about the flow of network packets through various network layers. Figure
9-1
shows an overview of packet transfer through five network layers in a RAC cluster. Clusterware processes (such as
CSSD, CRSD, etc.) use TCP/IP to send streams of data to other nodes. Database server processes predominantly use
User Datagram Protocol (UDP) to send data segments to other nodes.
Server
processes
Application
layer
Clusterware
TCP
UDP
Segments
IP
Packets
NIC
Frames
Switch
Frames
Figure 9-1.
Network layers
Each layer is implemented as a kernel function, and data flows through the layers in opposite directions when
sending and receiving packets. Both TCP and UDP layer kernel functions are implemented over IP layer functions.
An IP layer function calls a device driver function specific to the particular hardware in question to send packets
through physical interfaces.
Figure
9-2
shows how a stack of functions is called to send or receive a datagram. In Figure
9-2
, the application
layer populates the buffers to send and calls a network system function (such as udp_sendmsg, tcp_sendmsg, etc.) to
send a data stream. The system calls, in turn, invoke the next layer kernel function in the call hierarchy, which calls
the next layer function, and so on. Essentially, every layer performs a specific task and calls the next layer function in
the hierarchy to do additional processing. For example, a UDP protocol handler fills the UDP header for source and
destination port, and chunks the data stream into segments. UDP (or TCP) layer functions call IP layer functions to
perform IP layer sanity checks. IP layer functions break the segments into packets, and add IP header details. Then,
IP layer function invokes the device driver functions to send the packets. Device driver sends the frames through the
network interface, wire, and switch to the destination.
