whole 231

What is Virtualisation?

gives each process the illusion of exclusive resource access (time + space sharing)

What is CPU Virtualisation?

OS runs multiple processes by time-sharing the CPU via a scheduler

What are the Components of the Cloud?

Virtual Machines, containers ans server clusters

What is the Goal of the Cloud?

on-demand, scalable resources with hidden complexity and isolation

What is UDP?

fast, connectionless, low-overhead transport protocol

How is UDP Connectionless?

• no handshaking between UDP sender, receiver

• each UDP segment handled independently of others

Why use UDP?

• no setup

• low delay

• small headers

What are the Sender Actions?

• is passed an application-layer message

• determines UDP segment header fields values

• creates UDP segment

• passes segment to IP

What are the Receiver Actions?

• receives segment from IP

• checks UDP checksum header value

• extracts application-layer message

• demultiplexes message up to application via socket

What does the Sender do at the Transport Layer?

• is passed an application-layer message

• determines segment header fields values

• creates segment

• passes segment to IP

What does the Receiver do at the Transport Layer?

• receives segment from IP

• checks header values

• extracts application-layer message

• demultiplexes message up to application via socket

What are the 2 Transport Protocols?

TCP and UDP

What are the Differences between the Transport and Network Layers?

transport layer:

• communication between processes

• relies on, enhances, network layer services

network layer:

• communication between hosts

What is TCP - Transmission Control Protocol?

• reliable

• ordered

• connection oriented transport protocol

What is Throughput?

the rate (bits/second) at which bits are transferred from a sender to a receiver

What is Instantaneous Throughput?

rate at given point in time

What is Average Throughput?

rate over a period of time

What is Peak Throughput?

highest instantaneous throughput rate seen so far

How do we Measure Throughput?

using iperf - send known data between client and server and calculate rate

How do we Calculate Rate in Iperf?

data / time

What are Threads?

lightweight process sharing memory with others

What does each Thread have?

own PC, registers, stack

What is a Process?

program in execution with its own memory and state

What does a Process include?

memory, registers, PCB, threads

What do Condition Variables do?

coordinate thread execution

What does the Condition Variable cond.wait() do?

release the lock and suspends

What does the Condition Variable cond.notify() do?

wakes one waiting thread

What is a Sentinel Value?

a special marker placed in the shared queue to signal consumers to stop when production is finished

What is Congestion?

too much data for network capacity

What does Congestion lead to?

delay or loss

What are the goals of TCP Congestion Control?

• efficiency

• distributedness

• fairness

What are the different Approaches to Congestion Control?

• end-end congestion control

• network-assisted congestion control

What is End-End Congestion Control?

• no explicit feedback from network

• congestion inferred from observed loss, delay

Which Congestion Control Approach does TCP take?

end-end

What is Network-Assisted Congestion Control?

• routers provide direct feedback to sending/receiving hosts with flows passing through congested router

• may indicate congestion level or explicitly set sending rate

What is AIMD?

senders can increase sending rate until packet loss (congestion) occurs, then decrease sending rate on loss event

What is Additive Increase in AIMD?

increase sending rate by 1 maximum segment size every RTT until loss detected

What is Multiplicative Decrease in AIMD?

cut sending rate in half at each loss event

What does it mean when we say AIMD has Sawtooth Behaviour?

probing for bandwidth

Why AIMD?

has been shown to

• optimise congested flow rates network wide

• have desirable stability properties

What is System Design?

combining resources into an efficient system

What is System Optimisation?

maximise performance metrics under resource constraints

What are Common Optimisation Resources?

• time

• space

• computation

• cost

• labour

What are the Constraints to System Optimisation?

• performance

• scalability

• resilience

• cost

• maintainability

• sustainability

What is a System Bottleneck?

most constrained element in a system

What Improves when you Remove a Bottleneck?

performance metrics

What is Multiplexing?

use the same resource to service multiple requests without interference

What are the Types of Multiplexing?

• time - CPU, I/O

• space - memory

• frequency - radio comms

What is Pipelining?

overlapping sequential tasks

What is Batching?

processing tasks together to reduce overhead

When does Batching work?

when 'overhead for N tasks' < 'N time overhead for one task'

What is Binding?

translate an abstraction into an instance

What is Indirection?

use a reference to access something, instead of directly

What does Indirection provide and why?

flexibility because the reference can change without affecting the higher-level system

What is Packet Switching?

data split into packets and forwarded independently

What are the Key Characteristics of Packet Switching?

store and forward, delays and possible loss

What is Store and Forward?

entire packet must arrive at router before it cam be transmitted on next link

What is Packet Processing Delay?

time a network takes to examine, process and transmit a packets header

What is Packet Transmission Delay?

takes L/R seconds to transmit L-bit packet into link at R bps

What is Packet Queuing and Loss?

if arrival rate to link exceeds transmission rate of link for some period of time

• packets will queue, waiting to be transmitted on output link

• packets can be dropped (lost) if memory (buffer) in router fills up

What does a Router do?

forwards packets between networks

What is Forwarding?

move packet locally

What is Routing?

determine path globally

What is Circuit Switching?

dedicated resources reserved for communication

What is Frequency Division Multiplexing (FDM)?

• optical electromagnetic frequencies divided into narrow frequency bands

• each call allocated its own band, can transmit at max rate of that narrow band

What is Time Division Multiplexing (TDM)?

• time divided into slots

• each call allocated periodic slot, can transmit at max rate of wider frequency band only during its time slot

What is Packet Switching at Risk of?

excessive congestion

What is Packet Switching good for?

‘bursty’ data - sometimes has data to send, but at other times not

What is a Socket?

interface between application and transport layer

How do we Identify a Connection?

5 tuple

What is a 5 tuple?

2 IP addresses, 2 port numbers, and the underlying transport protocol being used

What is the Difference between Well-Known and Ephemeral Ports?

• typically servers have a well-known

• client picks an unused ephemeral port

How is Process-to-Process Communication Enabled?

through socket-to-socket communication

What Calls can be used on a Socket?

• standard API calls (system calls) are used - link open(), read(), write(), close()

• plus some other socket-specific calls - send(), recv(), bind(), etc

What does the Router look like?

• routing, management control plane (software) operates in millisecond time frame

• forwarding data plane (hardware) operates in nanosecond timeframe

What are the Input Port functions?

• decentralised switching

• longest prefix matching

What is Decentralised Switching?

using header field values, lookup output port using forwarding table in input port memory (“match plus action”)

• input port queuing

• destination-based forwarding

What is the Goal of Decentralised Switching?

complete input port processing at ‘line speed’

What is Input Port Queuing?

if datagrams arrive faster than forwarding rate into switch fabric

What is Destination-Based Forwarding?

forward based only on destination IP addresses

What is Longest Prefix Matching?

choose routing entry with most specific match

What is Switching Fabrics?

transfer packet from input link to appropriate output link

What is Switching Rate?

rate at which packets can be transfer from inputs to outputs

What are the 3 types of Switching Fabrics?

• memory

• bus

• interconnection network

What is Head-of-the-Line (HOL) Blocking?

queued datagram at front of queue prevents others in queue from moving forward

What is Output Port Queuing?

buffering required when datagrams arrive from fabric faster than link transmission rate

What are the Principles of RDT?

sender, receiver do not know the “state” of each other, e.g., was a message received?

• unless communicated via a message

What are Interfaces?

What are the Functions for Interfaces?

• rdt_send()

• udt_send()

• rdt_rev()

• deliver_data()

How do we get started with RDT?

• incrementally develop sender, receiver sides of RDT protocol

• consider only unidirectional data transfer

  • but control info will flow in both directions

• use finite state machines (FSM) to specify sender, receiver

What is rdt1.0?

reliable transfer over a reliable channel

How do we Detect Errors?

checksum

What happens in a Channel with Bit Errors?

• underlying channel may flip bits in packet

• checksum to detect bit errors (i.e., corrupt packets)

• stop and wait - sender sends one packet, then waits for receiver response

How do we Recover from Bit Errors?

sender re-transmits the packet on receipt of NAK

What is a Negative Acknowledgement (NAK)?

receiver explicitly tells sender that the packet had bit errors (errors detected based on the checksum)

What is an Acknowledgement (ACK)?

receiver explicitly tells sender that the packet was received OK (no bit errors)

What happens if ACK/NAK corrupted?

• sender doesn’t know what happened at receiver

• can’t just re-transmit - possible duplicate

How do we Handle Duplicates?

• sender retransmits current packet if ACK/NAK corrupted

• sender adds sequence number to each packet

• receiver discards (doesn’t deliver up) duplicate packet

How does the Sender Handle Corrupted ACK/NAKs?

How does the Receiver Handle Corrupted ACK/NAKs?