Chapter 1: Introduction Flashcards
Hosts
End systems
How are hosts connected?
By a network of communication links and packet switches
Packet switches
Take a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links
Route
The sequence of communication links and packet switches traversed by a packet from the sending end system to the receiving end system
Communication link examples
Fiber, copper, radio satellite
Transmission rate
Bandwidth (bits / second)
Networks
Collection of devices, routers, links that are managed by an organization
Internet
“Network of network”; consists of interconnected ISPs
What do protocols do?
Control sending and receiving of information within the Internet
Who developed the Internet standards?
The Internet Engineering Task Force
What are the internet standards called?
Request For Comments (RFS)
A “service” description of the Internet could be:
- Infrastructure that provides services to applications
- Provides programming interface to distributed applications
Distributed applications
Involve multiple end systems that exchange data with each other
What do “hooks” allow application to do?
Connect to / use the Internet “transport service”
How is the Internet analogous to the postal service?
Protocol
Defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event
What is the network edge?
Consists of hosts; clients and servers
Access network
The network that physically connects an end system to the first router on a path from the end system to any other distant end system
How can end systems be connected to the first router in an access network (also known as the edge router)?
Residential access networks, institutional access networks, and mobile access networks
How does the Digital Subscriber Line access network work?
- DSL modems at customer locations convert digital data from computers into high-frequency signals.
- These signals travel over existing telephone lines to the local telephone exchange.
- At the exchange, the DSLAM aggregates the signals from multiple customers and converts them into a format suitable for high-speed data transmission.
- The aggregated data is then sent over high-capacity fiber optic or other high-speed links to the internet backbone.
What sort of access does the DSL have?
Asymmetric
Is DSL a shared or dedicated access network?
Dedicated access network
How do cable-based access networks work?
Use a combination of fiber optic and coaxial cables to deliver high-speed internet, telephone and television services by transmitting signals from a hub to cable modems at the clients destination, that turn the analog signal into a digital format.
What sort of access do cable-based access networks (HFC) have?
Asymmetric
Are cable-based access networks shared or dedicated access networks?
Shared
How does Fiber To The Home (FTTH) work?
Delivers high speed internet from a Central Office (CO) directly to residences using fiber optic cables that carry a signal that travels through a network of fiber optic cables to local distribution points, here an optical splitter divides the signal to serve multiple homes. Then the optic signal is converted into an electrical signal by an ONT at each residence
Is FTTH a shared or dedicated access network?
Both
How do entreprise access networks work?
Mix of wired and wireless link technologies, connecting a mix of switches and routers
How do home access networks work?
Connect various devices within a household to the internet and to each other. Here’s a brief overview of how they work:
Internet Service Provider (ISP): The ISP provides internet access to the home via a modem or gateway.
Router: The router distributes the internet connection to multiple devices within the home, either wirelessly (Wi-Fi) or through wired connections (Ethernet).
Devices: Computers, smartphones, smart TVs, and other devices connect to the router to access the internet and communicate with each other.
How do wireless access networks work?
Access Point (AP): The central device, often a router, that broadcasts a wireless signal.
Wireless Signal: The AP uses radio waves to transmit data over the air.
Devices: Smartphones, laptops, tablets, and other wireless-enabled devices connect to the AP using Wi-Fi.
Internet Connection: The AP is connected to the internet via a modem, providing internet access to all connected devices.
Where is a physical link located?
Between transmitter and receiver
Guided media
Signals propagate in solid media
Unguided media
Signals propagate freely
Twisted Pair (TP)
Two insulated copper wires. Data rates depend on wire thickness and distance.
Used for residential Internet access.
Coaxial cable
Two concentric copper conductors. Bidirectional & can be used as a guided shared medium.
Fiber optic cable
Glass fiber carrying light pulses, each pulse a bit.
Used for long-haul guided transmission media - low error rate & high speed operation
Terrestrial Radio Channels
Wireless; signal carried with EM-waves over long distances
Packets
Segmented data with added header bytes to each segment
How does a host send an application message?
Breaks the message into packets, transmits the packets into access networks at transmission rate
Packet transmission delay
L / R
Packet-switching
Hosts break application layer messages into packets. Then forward packets from one router to the next, across links of path from source to destination.
How does the “store-and-forward” packet-switching technique work?
The packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link.
Output buffer
Stores packets that the router is about to send into a link
What happens if the arrival rate to link exceeds transmission rate of a link for a period of time when using packet-switching?
Packets will queue, waiting to be transmitted.
The arriving packet / one already queued can be dropped if the buffer in the router fills up
Forwarding tables
Maps destination addresses to a router’s outbound link
How does the network core use forwarding tables?
Move arriving packets from a router’s input link to the appropriate router output link based on a forwarding table
Routing protocol
Automatically sets the forwarding tables
How does the network core use routing protocols?
Determines the best source-destination paths that packets should take
Circuit switching
The resources needed along a path to provide for communication between the end systems are reserved for the duration of the communication session between the end systems. The resources are dedicated.
What are the two forms of multiplexing in circuit-switched networks?
Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM)
Frequency Division Multiplexing
A link dedicated a frequency band to each connection for the duration of the connection, where the connection can transmit at the max rate of the band
Time Division Multiplexing
Each connection is allocated periodic slots, and can transmit at the maximum rate of the frequency band, but only during its set time slots
What is the main benefit of packet switching in comparison to circuit switching?
Data is broken into packets and sent independently over the network. This allows for more efficient use of network resources, as multiple data streams can share the same network paths. More users can use the network!
Describe the internet structure from a “network of networks” POV
- Hosts connect to the internet through access ISPs that are interconnected.
- Regional ISPs that use PoPs, allowing customer ISPs to connect to provider ISPs.
- Internet Exchange Points (IXPs): IXPs are meeting points where multiple ISPs can peer together
- Core Networks: At the center are Tier-1 commercial ISPs and content provider networks
ISP
Internet Service Provider
PoP
A group of one or more routers in the provider’s network where customer ISPs can connect into the provider ISP
Internet Exchange Point (IXP)
A meeting point where multiple ISPs can peer together.
“Tier-1” commercial ISPs
National & international coverage networks
Content provider networks
Private networks that connect data centers to the Internet. Often bypass Tier-1 and regional ISPs.
How does packet loss and delay occur?
Packets queue in router buffers, and when the arrival rate to the link exceeds the output link capacity packet loss occurs
Equation for packet delay
d_proc + d_queue + d_trans + d_prop
What does it say about the queuing delay if the traffic intensity is 0?
The queueing delay is small
What does it say about the queuing delay if the traffic intensity is approaching 1?
The average queueing delay is large
What does it say about the queuing delay if the traffic intensity is bigger than one?
The average packet delay is infinite - arriving packets have to be dropped
Traffic intensity
La/R
When does packet loss occur?
When a packet arrived to a full queue.
d_end-end =
N(d_proc + d_trans + d_prop)
d_proc
Nodal processing delay
d_queue
Queueing delay
d_trans
Transmission delay
d_prop
Propagation delay
Throughput
Rate at which bits are being sent from sender to receiver
Instantaneous throughput
Rate at which receiver is receiving the bits at any instant point in time
Average throughput
Rate over longer periode of time
Bottleneck link
Link on end-end path that contrains end-end throughput
Virus
Self-replicating infection by receiving/executing object
Worm
Self-replicating infection by passively receiving object that gets itself executed
What are the categories of attacks on servers and network infrastructure?
Vulnerability attack, bandwidth flooding, and connection floowing
Denial of Service
Attackers make resources unavailable to legitimate traffic by overwhelming resource with bogus traffic
Packet sniffing
Promiscuous network interface that reads / records all packets passing by
IP spoofing
Sending packets with false source addresses
How is IP spoofing avoided?
End-point authentication
How do the protocol layers work?
Each protocol belongs to one of the layers, and each layer implements a service via its own internal layer actions and by relying on services provided by a layer below
Why do we use protocol layering?
Makes it easier to deal with complex systems - however layers may duplicated lower-layer functionality and need information that is only present in another layer
Describe the internet protocol stack
Application, transport, network, link, physical
Application layer
Supports network applications and their application layer protocols. Implemented in software in end-systems and distributed over multiple end systems
Transport layer
Transports application-layer messages between application endpoints
Network layer
Routing of datagrams (network-layer packets) from source to destination.
Link layer
Data transfer of network-layer packets between neighboring network elements
Frames
Link layer packets
Physical layer
Moves the individual bits within a frame from one node to the next. Dependent on the link and the transmission medium of the link
