Network Stack Functionality Link to heading

• Encapsulation

TCP State Link to heading

• TCP has state in contrast to UDP
• Need to store this state per TCP connection
• State processing often performed in kernel

• reusing packet id -> we wait 30 seconds to avoid conflicts

Data movement inside the network stack Link to heading

• Problem: while processing the data, we don’t want to copy it over all the time
• We need data to store it

Hardware accelerated networking Link to heading

Process Creation Link to heading

Spawn Link to heading

1. Constructs a running process from scratch
2. In windows, CreateProcess() creates a new process to run a program
3. Spawn is complex
   1. Has to specify everything about the new environments including the command line
   2. All rights a program needs must be granted in one go

Fork/Exec Link to heading

1. Child is almost exact copy of calling parent
2. Return value from fork indicated which process we are in (0: Child, > 0 Parent)
3. Can use full kernel API
   1. fork(): system call to create a copy of the current process
   2. Exec(): system call to change the program beng run by the current process

Remote Procedure Call = RPC

CPU Scheduling Link to heading

Scheduling Link to heading

• work is assigned to resources that complete the work

Dispatch Link to heading

• gives control of the CPU to the process selected by the scheduler

Preemptive Link to heading

• processes dispatched without warning

Non-preemptive Link to heading

• process explicitly has to give up the resource

Workloads Link to heading

Batch workload Link to heading

• Run this job to completion and tell me when you are done
• Main goals: throughput (jobs per hour), wait time, turnaround time, utilization

Interactive workloads Link to heading

• Wait for external events and react before the user gets annoyed
• Main goals: Response time, proportionality

Soft realtime workloads Link to heading

• This task must complete in less than 50ms
• Main goals: deadlines, guarantees, predictability

Hard realtime workloads Link to heading

• Ensure the plane’s control surface moves correctly in response to the pilot’s actions
• Difficult, not covered

Device Link to heading

• Piece of hardware visible from software
• Occupies some location on a bus
• Set of registers
  • Memory mapped or I/O space
• Source of interrupts
• MAy initiate Direct Memory Access transfers

Direct Memory Access Link to heading

• Avoid programmed I/O for lots of data
• Device needs to be able to do DMA
  • Generally built-in these days
• Bypasses CPU to transfer data directly between I/O device and memory
  • Doesn’t take up CPU time
  • Only one interrupt per transfer

Questions: Link to heading

Is the TCP protocol state processing done in userspace? Why or why not?

• The protocol state processing is done in the kernel.
• If this was done in userspace, an unprivileged application could potentially abuse the protocol.

The network stack in an OS only ever sends packets when explicitly requested by a user space application.

• False the TCP protocol requires sending packets that the user will never see. (Think of acks, or retransmissions)

What is the trouble this causes when implementing a TCP protocol in the top half?

• This would require the application to be scheduled at all kind of events not scheduled by the user program.

How is it solved in practice in Unix-like operating systems?

• We run TCP in the bottom half. Remember DPCs!! (Kernel call to handle the packet)

What is the difference between synonym and a homonym?

• synonym: same object different names
• homonym: same name different objects

Is an IP address a pure name?

• no it gives information

To what can a symbolic link point to?

• to path sections

• to files

• a name

• another link

What is the advantage of running a process in userspace?

• safer, you cannot break anything

1. 2. 3. 6. 7. 8. 9. 10. 11. 12. 13. 14. 4. 5.

What is the difference between a process and a program?

• A process consists of threads which execute a program.
• A process is a running instance of a program

How are processes identified?

• PID

In what state is a process after it exited?

• zombie state

How does having only one kernel thread simplify the kernel?

• no need for locks/synchronization

What is the point of a name server?

• To hold interface references for services