On the receiving side, the arrival of a network packet is signaled by an interrupt. The interrupt handler, which is part of the network interface device driver, encapsulates the packet in an mbuf, queues the packet in the IP queue, and posts a software interrupt. In the context of this software interrupt, the packet is processed by IP. After potential reassembly of multiple IP fragments, UDP's or TCP's input function is called, as appropriate. Finally--still in the context of the software interrupt--the packet is queued on the socket queue of the socket that is bound to the packet's destination port. The software interrupt has higher priority than any user process; therefore, whenever a user process is interrupted by a packet arrival, the protocol processing for that packet occurs before control returns to the user process. On the other hand, software interrupts have lower priority than hardware interrupts; thus, the reception of subsequent packets can interrupt the protocol processing of earlier packets.
When an application process performs a receive system call
on the socket, the packet's data is copied from
the mbufs into the application's address space. The mbufs are then
dequeued and deallocated. This final processing step occurs in the
context of the user process performing a system call.
On the sending side, data written to a socket by an application is copied into newly allocated mbufs. For datagram sockets (UDP), the mbufs are then handed to UDP and IP for transmission. After potential fragmentation, the resulting IP packets are then transmitted, or--if the interface is currently busy--placed in the driver's interface queue. All of these actions are executed in the context of the user process that performed the send system call on the socket. Packets queued in the interface queue are removed and transmitted in the context of the network interface's interrupt handler.
For stream sockets (TCP), the mbufs are queued in the socket's outgoing socket queue, and TCP's output function is called. Depending on the state of the TCP connection and the arguments to the send call, TCP makes a logical copy of all, some, or none of the queued mbufs, processes them for transmission, and calls IP's output function. The resulting IP packets are then transmitted or queued on the interface queue. Again, this processing occurs in the context of the application process performing a system call. As for UDP packets, data is removed from the interface queue and transmitted in the context of the network interface's interrupt handler.
Processing of any remaining data in the socket queue typically occurs in the context of a software interrupt. If TCP receives an acknowledgment, more data from the socket queue may be sent in the context of the software interrupt that was posted to process the incoming acknowledgment. Or, data may be sent in the context of a software interrupt that was scheduled by TCP to indicate a timeout. Data is not removed from the socket queue until its reception was acknowledged by the remote receiver.
CPU time consumed during the processing of network I/O is accounted for as follows. Any processing that occurs in the context of a user process performing a system call is charged to that process as system time. CPU time spent in software or hardware interrupt handlers is charged to the user process that was interrupted. Note that in general, the interrupted process may be unrelated to the network communication that caused the interrupt.