Computer Networks Chapter 1 Flashcards
Access Networks
The access network is 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.
-Cable-based access
-DSL
-Home networks
-Enterprise networks
-Data center networks
Distributed Applications
Applications that involve multiple end systems that exchange data with each other.
RFC
Request For Comments. IETF standards documents. They define protocols such as TCP, IP, HTTP, and SMTP.
Internet Standards
-RFC
-IETF
Packet Switch
Takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links. The two most prominent types are routers and link-layer switches.
Host Sending Function
Host sends packets of data.
-takes application message
-breaks it into smaller chunks (packets) of length L bits
-transmits packet into access network at transmission rate R.
L bits/R bits per second = transmission time in seconds.
Bit
Short for “binary digit.” Propagates between transmitter/receiver pairs.
Wireless Radio
Signal carried in various “bands” in electromagnetic spectrum. No physical wire, but broadcast (half-duplex). Propagation environment effects:
-reflection
-obstruction by objects
-interference/noise
Fiber Optic Cable
Glass fiber carrying light pulses. Each pulse is a bit. High-speed point-to-point transmission (10s-100s Gbps). Low error rate: repeaters spaced far apart, immune to electromagnetic noise.
Coaxial Cable
Two concentric copper conductors. Bidirectional. Broadband: multiple frequency channels on cable, 100s Mbps per channel. May need repeaters to propagate with long distances.
Twisted Pair
“TP.” Two insulated copper wires twisted together to reduce interference from other signals. Parallel, not concentric.
-Category 5: 100 Mbps, 1 Gbps Ethernet.
-Category 6: 10 Gbps Ethernet.
Guided Media
Signals propagate in solid media.
ex. Copper, fiber, coax.
Unguided Media
Signals propagate freely.
ex. Radio, WiFi.
Physical Link
What lies between transmitter and receiver.
Half-duplex
Allowing the transmission of signals in both directions but not simultaneously.
Links: Physical Media
-Bit
-Physical link
-Guided media
-Unguided media
-Twisted pair
-Coaxial cable
-Fiber optic cable
-Wireless radio
Radio Link Types
-WLAN (WiFi)
-Wide-area
-Bluetooth
-Terrestrial microwave
-Satellite
Bluetooth
Type of radio link.
-Cable replacement
-Short distances, limited rates
Terrestrial Microwave
Type of radio link.
-Point-to-point; 45 Mbps channels
Satellite
Type of radio link.
-Up to more than 100 Mbps (Starlink) downlink
-270 msec end-end delay (geostationary)
Packet-switching
When hosts break application-layer messages into packets. Network forwards packets from one router to the next, across links on path from source to destination.
Two Key Network-core Functions
-Forwarding (switching)
-Routing
Forwarding
aka “switching.” local action: move arriving packets from router’s input link to appropriate router output link. One of two key network-core functions.
Store-and-forward
Packet-switching function in which entire oacket must arrive at router before it can be transmitted on next link.
Routing
One of two key network-core functions. Global action: determine source-destination paths taken by packets.
Queuing
Occurs when work (packets) arrives faster than it can be serviced. Happens when arrival rate (in bps) to link exceeds transmission rate (bps) of link for some period of time.
Circuit-Switching
End-end resources allocated to, reserved for “call” between source and destination. Dedicated resources. Circuit-like (guaranteed) performance. Circuit segment idle if not used by call (no sharing). Commonly used in traditional telephone networks. Alternative to packet-switching.
FDM
Frequency Division Multiplexing.
-optical, electromagnetic frequencies divided into narrow frequency bands.
-each call allocated its own band. Can transmit at max rate of that narrow band.
TDM
Time Division Multiplexing.
-time divided into solts.
-Each call allocated periodic slots. Can transmit at max rate of frequency band only during its time slot(s).
IXP
Internet Exchange Point. A meeting point where multiple ISPs can peer together.
“Center” of Internet Structure
-Tier-1 ISPs
-Content provider networks
Tier-1 ISPs
National and international coverage. At the “center” of the Internet structure.
ex. Sprint, AT&T.
Content-Provider Networks
Private networks that connect their data centers to the Internet, often bypassing tier-1 and regional ISPs. At the “center” of the Internet structure.
ex. Google, Facebook.
Packet Loss
Occurs when memory buffer to hold queued packets fills up. Packets arriving to a full queue are dropped (lost). Lost packet may be retransmitted by previous node, by source end system, or not at all.
Sources of Packet Delay
-d_proc
-d_queue
-d_trans
-d_prop
Total nodal delay: d_nodal
d_nodal = d_proc + d_queue + d_trans + d_prop
d_queue
Queuing delay. Time waiting at the output link for transmission.
-Depends on congestion level of the router.
-If no other packet is currently being transmitted, queuing delay = 0.
-Can be on the order of microseconds to milliseconds.
d_proc
Processing delay. The time it takes to examine a packet’s header and determine where to direct the packet.
-Can also include the time needed to check for bit-level errors.
-Typically < microseconds.
d_trans
Transmission delay. The time it takes to push (transmit) all of the packet’s bits into the link.
-Typically on the order of microseconds to milliseconds.
-L: packet length (bits)
-R: link transmission rate (bps)
d_trans = L/R
d_prop
Propagation delay. The time it takes to propagate from the beginning of the link to router B.
-d: distance between router A and router B.
-s: propagation speed of the link (~ 210^8 meters/sec to 310^8 meters/sec (a little less than the speed of light)).
-In wide-area networks, propagation delays are on the order of milliseconds.
d_prop = d/s
Traffic Intensity
Plays an important role in estimating the extent of the queuing delay.
-a: average packet arrival rate
-L: packet length (bits)
-R: link bandwidth (bit transmission rate)
(L*a)/R
==> (arrival of bits)/(service rate of bits)
==> “traffic intensity”
** (La)/R ~ 0: average queuing delay small (what we want)
** (La)/R ~ 1: average queuing delay large
** (L*a)/R > 1: average queuing delay infinite (packets get dropped)
Traceroute
Unix program that provides delay measurement from source to router along end-end Internet path towards destination.
*Windows variant is called “tracer.”
Throughput
Rate (bits/time unit) at which bits are being sent from sender to receiver.
-Instantaneous
-Average
Instantaneous Throughput
Rate at given point in time in bits/sec at which Host B is receiving the file.
Average Throughput
Rate over longer period of time in bits.sec.
Bottleneck Link
Link on end-end path that constrains end-end throughput.
min{R_c, R_s}
Packet
A segment of size L bits of a larger message. Data sent over computer networks, such as the Internet, is divided into packets. These packets are then recombined by the computer or device that receives them.
Hosts
Another name for end systems. Typically includes clients and servers.
Ethernet
Wired access at 100 Mbps, 1 Gbps, 10 Gbps.
Enterprise Networks
Used by companies, universities, etc. Mix of wired and wireless link technologies connecting a mix of switches and routers: Ethernet and WiFi.
WiFi
Wireless access points at 11, 54, 450 Mbps.
Wide-area Cellular Access Networks
Type of radio link. Provided by mobile, cellular networks.
-10s Mbps (4G) over ~10 km.
ex. 4G/5G.
WLANs
Type of radio link.
-10-100s Mbps, 10s of meters
-Typically within or around a building (~100 ft)
-802.11 b/g/n
*b/g/n are versions of WiFi.
Wireless Access Networks
Shared wireless access network connects end system to router via base station, aka “access point.”
-WLANs
-Wide-area cellular access networks
Home Networks
-Wireless and wired devices
-WiFi access point; router, firewall, NAT; and cable or DSL modem; often combined in a single box
DSLAM
Digital Subscriber Line Access Multiplexer (“DEE-slam”). A network device, often located in telephone exchanges, that connects multiple (sometimes thousands) customer DSL interfaces to a high-speed digital communications channel using multiplexing techniques.
DSL
Digital Subscriber Line. Use exisiting telephone line to central office DSLAM.
-Data over DSL phone line goes to Internet.
-Voice over DSL phone line goes to telephone net.
-24-52 Mbps dedicated downstream transmission rate.
-3.5-16 Mbps dedicated upstream transmission rate.
Cable-based Access
Network of cable and fiber attaches homes to ISP router. Homes share the access network to cable headend.
-Hybrid fiber coax.
-asymmetric:up to 40 Mbps-1.2 Gbps downstream transmission rate, 30-100 Mbps upstream transmission rate.
Internet Structure
-Network edge
-Network core
-Access networks and physical media
Protocols
Define the format and order of messages sent and received among network entities, and actions taken on message transmission and receipt.
Network Edge
Made up of:
-hosts
-servers (often in data centers)
Applications
A computer software package that performs a specific function for an end user or another application based on carefully designed features.
IETF
Internet Engineering Task Force. Responsible for ensuring RFCs are developed, published, and maintained.
Data Center Networks
High-bandwidth links (10s to 100s Gbps) connect hundreds to thousands of servers together, and to the Internet.
IP
Internet Protocol. Specifies the format of the packets that are sent and received among routers and end systems.
TCP
Transmission Control Protocol. TCP/IP are some of the most important protocols in existence.
ISP
Internet Service Provider. Each ISP is in itself a network of packet switches and communication links. ISPs provide a variety of types of network access to the end systems, including residential broadband access such as cable modem or DSL, high-speed LAN access, and mobile wireless access. ISPs also provide Internet access to content providers, connecting servers directly to the Internet.
Link-Layer Switch
Typically used in access networks. A type of packet swithc.
Overarching Goal of the Internet
To interconnect the access ISPs so that all end systems can send packets to each other.
Network Structure 1
Interconnects all of the access ISPs with a single global transit ISP.
Network Structure 2
Consists of hundreds of thousands of access ISPs and multiple global transit ISPs. Two-tier hierarchy.
-Introduces competition between global transit providers.
-Global transit ISPs must interconnect.
-May introduce a regional ISP.
Network Structure 3
Multiple competing regional ISPs.
-Multi-tier hierarchy.
-Customer-provider relationship at each level of the hierarchy.
PoP
Point of Presence. A PoP is a group of one or more routers (at the same location) in the provider’s network where customer ISPs can connect into the provider ISP. PoPs exist in all levels of the Network Structure hierarchy except for the access ISP level.
Multi-home
To connect to two or more provider ISPs. When an ISP multi-homes, it can continue to send and receive packets into the Internet even if one of its providers has a failure.
ex. An access ISP may multi-home with two regional ISPs and also with a tier-1 ISP.
ex. A regional ISP can multi-home with multiple tier-1 ISPs.
Peer
When a pair of nearby ISPs at the same level of the hierarchy directly connect their networks together so that all the traffic between them passes over the direct connection rather than through upstream intermediaries.
-This helps both ISPs save money by reducing the traffic they exchange with their providers.
-When two ISPs peer, it is typically settlement-free (they don’t pay each other).
Network Structure 4
The ecosystem consisting of:
-Access ISPs
-Regional ISPs
-Tier-1 ISPs
-PoPs
-Multi-homing
-Peering
-IXPs
Network Structure 5
Describes today’s Internet. Builds onto Network Structure 4 by adding content-provider networks.
Node
Host or router.
Nodal Delay
The delay at a single router. Denoted d_nodal.
End-to-End Delay
The total delay from source to destination. Denoted d_end-end.
Internet Protocol Stack
Includes the following layers:
-Application
-Transport
-Network
-Link
-Physical
Application Layer (Internet Protocol Stack)
The application layer is where network applications and their application-layer protocols reside. Exchanges messages, M, to implement some application service using services of the transport layer.
HTTP
Provides for Web document request and transfer.
SMTP
Provides for the transfer of e-mail messages.
FTP
Provides for the transfer of files between two end systems.
Transport Layer (Internet Protocol Stack)
Transfers message M from one process to another using services of the network layer. Transport-layer protocol encapsulates M with transport-layer header H_t to create a transport-layer segment.
Network Layer (Internet Protocol Stack)
Responsible for moving network-layer packets known as datagrams from one host to another using link layer services. Network-layer encapsulates transport-layer segment [H_t | M] with network-layer header H_n to create datagram [H_n | [H_t | M].
Link Layer (Internet Protocol Stack)
Transfers datagram [H_n | [H_t | M] from host to neighboring host using network-layer services. At each node, the network layer passes the datagram down to the link layer, which delivers the datagram to the next node along the route. At this next node, the link layer passes the datagram up to the network layer. Link layer packets are known as frames.
Physical Layer (Internet Protocol Stack)
The job of the physical layer is to move the individual bits within a frame from one node to the next.
Payload Field
A packet from the layer above.
ex. At the Network Layer, the header field is H_n and the payload field is [H_t | M].
Packet Sniffer
A passive receiver that records a copy of every packet that flies by.
IP Spoofing
The ability to inject packets into the Internet with a false source address.
Layer
Each layer implements a service via its own internal-layer actions, relying on services provided by the layer below.
OSI Reference Model
Developed by the Internet Standards Organization (ISO), the Open System Interconnection (OSI) Reference Model has 7 layers:
-Application
-Presentation
-Session
-Transport
-Network
-Link
-Physical
Presentation Layer (OSI Model)
Allow applications to interpret the meaning of data, e.g., encryption, compression, and machine-specified conventions.
Session Layer (OSI Model)
Synchronization, checkpointing, and recovery of data exchange.
Cerf and Kahn’s Internetworking Principles
-Minimalism, autonomy - no internal changes required to interconnect networks.
-Best-effort service model.
-Stateless routing.
-Decentralized control.
*These principles define today’s Internet architecture.
