week 14 virtual machines and networking

flashcards based on os slides from week 14

operating system

a layer of software between many applications and diverse hardware

what does the operating system do

• hardware abstraction so an application doesn’t have to know the details about the hardwarre

• arbitrates access to resources among multiple applications + sharing of resources

• provides protections

what kind of protection does an OS provide

• isolation protects app’s from each other

• isolation also protects OS from applications

• isolation limits resource consumption by any one app

OS prevents applications from writing into

privileged memory

OS prevents applications from invoking

privileged functions

privileged instruction examples

• memory address mapping

• flush or invalidate data cache

• invalidate TLB (translation lookaside buffer) entries

• load and read system registers

• change processor modes from K to U

• change the voltage and frequency of processors

• halt/reset processor

• perform I/O operations

unit of work for an OS

• application

• task

• job

• process

what does a TASK consist of

• code — placed into memory

• data — stored in memory

• OS data for task — task descriptors

mode bit

hardware that identifies whether the system is in user mode or supervisor/kernel mode

• requires extra support from the CPU hardware for this OS feature

supervisor or kernel mode (mode bit = 0)

• can execute all machine instructions, including privileged instructions

• can reference all memory locations

• kernel executes in this mode

user mode (mode bit = 1)

• can only execute a subset of non-privileged instructions

• can only reference a subset of memory locations

• all applications run in user mode

virtual machine

• a simulated computer running within a real computer

• virtual computer runs an OS that can be different than the host operating

in a virtual machine, all the requests to access real hardware are routed to the appropriate _________, then virtual os or applications don’t know they are _________

host hardware, virtual

a process is given the illusion that it has its

• own memory, via virtual memory

• own CPU, via time slicing

virtual extends this idea to give a process the illusion that it also has its own

hardware

• moreover, extend the concept from a process to an entire os being given the illusion that it has its own memory, CPU, and I/O devices

virtual machine benefits

• can run multiple os’s simultaneously on the same host

• fault isolation if an os fails — doesn’t crash another VM. this is also useful for debugging a new os

• easier to deploy applications — can deploy an app within a VM instance that is customized for the app, rather than directly deploying the app itself and worrying about compatibility with the target os — useful for cloud server deployments

a type 1 hypervisor provides a _____________ for guest OSs and resides just above the ___________

virtualization layer, hardware

virtual machine goal

create a virtual machine that executes at close to native speeds on a CPU, so emulation and interpreting instruction by instruction are not good options — too much software overhead

virtual machine solution

have the guest os execute normally, directly on the CPU, except that it is not in kernel mode

• therefore, any special privileged instructions invoked by the guest os will be trapped to the hypervisor, which is kernel mode

when the guest os trapped to the hypervisor, the hypervisor

then emulates only these privileged instructions and when done passes control back to the guest OS, also known as a “VM entry”

with the hypervisor trapping, most ordinary (non-privileged) instructions operate at _________, and only privileged instructions incur the overhead of a trap, also known as a ________, to the hypervisor/VMM

full speed, “VM exit”

trap-and-emulate

Executable code from the guest can execute directly on the host CPU by the hypervisor. The hypervisor configures the CPU in such a way that all potentially unsafe instructions will cause a “trap”. A trap is an exceptional condition that transfers control back to the hypervisor

it is very easy to provision and deploy VM instances on the ______

cloud

amazon’s elastic compute cloud (EC2) uses ____________

Xen virtualization

• different types of VMs or instances that can be deployed

  • standard, high-memory, high-cpu

• users can create and reboot their own VMs

• to store data peristently, need to supplement EC2 with an additional cloud service

  • amazon’s simple storage service (S3)

process VMs, e.g. Java VMs

differ from system VMs in that the goal is NOT to try to run multiple OSs on the same host, but to provide portable code execution of a single application across different hosts

java applications are compiled into __________ that can be run on any ________

Java byte code, Java VM

• java VM acts as an interpreter of byte code, translating each byte code instruction into a local action on the host OS

______________ compilation can be used to speed up execution of java code

just in time

• java byte code is compiled at run time into native machine code that is executed directly on the hardware, rather than being interpreted instruction by instruction

note Java VMs virtualize and abstract _______, not actual ________, unlike system VMs

machine, hardware

• i.e. the target machine that Java byte code being compiled for is a software specifiction

applications today leverage the internet to __________________ data

send and receive data

internet service examples

• web browser requests pages from a web server

• P2P systems

• streaming video

• social networks

• mobile apps

every networked application communicates to a remote process via a ___________

socket API

• send(message)

• recieve(message)

socket library talks via ___________ to OS kernel’s _____________

system call API, network stack

• send(message)

• receive(message)

the network stack’s architecture is organized into

multiple layers of protocols

• each protocol performs a specific set of duties

web (HTTP), Email (SMTP, IMAP) reside in

application layer 5

TCP, UDP reside in

transport layer 4

IP (v4 and v6) reside in

network layer 3

ethernet, wifi, bluetooth reside in

data link/MAC layer 2

CDMA, OFDM reside in

physical layer 1

to send a packet of data to a remote destination

each layer first passes a packet of data down the stack to the next lowest layer

to receive a packet of data

• each layer retrieves a packet of data from the layer below

• and after processing the packet sends the packet to the layer above

to send, an application calls _____________

socket API’s send()

• gives a pointer to the user space buffer containing the data to send

if file to be sent is large, the application

segments the file into smaller packets

application layer prepend a ________ header to the user data, forming a __________

layer 5, packet

• prepend the header AH1 to data1 forming packet 1

• header info is useful at the remote receiver to decode the packet

when sending a packet

• in general, at each layer N, a pack header NHx is prepended to data x and then set to a lower layer N-1

when sending a packet, packet grows as it __________ the network layered stack

descends

when receiving a packet

• at each layer N, strip off the layer N header

when receiving a packet, packet shrinks as it _________ the stack

moves up

how to recover from a lost packet

retransmit lost packets

• TCP, the transmission control protocol

transport layer’s job is _________________, if desired

end-to-end error recovery

how does the sender know if a packet was received correctly

receiver sends an Acknowledgement (ACK) packet back to sender

when does a sender know when to retransmit?

sets a timer

• if it times out before ACK received, then retransmit

TCP ensures ____________

in-order delivery

many apps require TCP’s reliable and in-order packet delivery service

• web, email, etc - can’t render a web page or read email if there are holes in the web page or email

• changing order of web/email text also makes it unreadable

apps that use TCP can view the network connection as a __________________

pipe abstraction

• any data sent into the pipe appears at the other end, hence it is reliable, i.e. pipes don’t lose data

• a pipe preserves the order of the data sent into it at the output of the pipe — no reordering is possible

reliability comes at the cost of ________ due to retransmissions

delay

not all apps need/want TCP’s

reliability

• interactive real-time apps like skype audio/video conferencing can’t wait for TCP’s retransmissions

• must get packet delivered in real time, e.g. within 30 ms

real-time voice-over-IP (VOIP) apps like Skype and Facetime can tolerate

packet loss

• may lose audio temporarily, but it’s OK

• such apps are built on top of unreliable UDP (user datagram protocal) at layer 4, not TCP

the internet consists of many _______ that connect together to form a ___________

routers, network graph

the internet protocal (IP) network layer must route the

ip packet to the correct destination

network layer tries to find the ______________, e.g. using _________________

“shortest path”, dijkstra’s algorithm

• the metric for shortest path may be minimum number of hops, shortest physical distance, lowest delay, minimum cost, etc

each router implements the

network layer

ip routing may _________

lose packets

• any router or link may fail at any time. also congested router buffers may overflow

• ok as long as TCP can retransmit them

each link between any two routers (endpoints) must be able to

transmit packets

• data link layer is responsible for transmitting packets between any 2 neighboring nodes in the network

data link layer must define

the beginning and end of packets (packet framing)

• packets may be lost, so this layer may also retransmit locally

along each link, the ___________ determines how 1’s and 0’s, i.e. digital bits, are transmitted

physical layer