[Networking] Foundation of Networking

What is the internet?

connected computing devices are called hosts = end systems
running network apps
End systems are connected by a network of communication links and packet switches
Different links can transmit data at different transmission rate: bandwidth
Packet switches: takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links
types of packet switches: routers and link-layer switches
route / path:the sequence of communication links and packet switches traversed by a packet from the sending end system to the receiving end system
Internet service Providers (ISPs): end system access the internet through ISPs.
Protocols: control sending, receiving of messages
Transmission Control Protocol (TCP) and the Internet Protocol (IP)
Internet standards
RFC: request For Comments
IETF: Internet Engineering Task Force

A network is an infrastructure that provides services to applications.

Application Programming Interface (API): specifies how a program running on one end system.

Protocol defines format, order of messages exchanged among network entities, and actions taken on message transmission, receipt.

Hosts/ end systems can be divided into client and servers
host sending function
- breaks into smaller chunks, known as packets of length \(L\) bits
- transmits packet into access network at transmission rate \(R\)

\[ \text{packet transmission delay} = \frac{L}{R} \]

Two most prevalent types of the residential access are Digital subscriber line (DSL) and cable.

DSL: use existing telephone line to central office DSLAM
data over DSL phone line goes to Internet
voice over DSL phone line goes to telephone net
Cable Network: makes use of the cable television company's existing cable television infrastructure
Hybrid Fiber Coax (HFC): both fiber and coaxial cable are employed in this system.

Local area network (LAN) is used to connect an end system to the edge router.
End systems connect into Ethernet switch.
Wireless LANs: Wi-Fi
Wide-area wireless access: 3G, 4G

bit: propagates between transmitter/receiver pairs
physical link: what lies between transmitter and receiver
guided media: e.g. copper, fiber, coax
coaxial cable: two concentric copper conductors
fiber optic cable: glass fiber carrying light pulses
unguided media: radio
terrestrial microwave (e.g. up to 45 Mbps channels); LAN (e.g. Wi-Fi); wide-area (e.g. 3G cellular); satellite
twisted pair (TP): two insulated copper wires

There are two fundamental approaches to move data through a network of links and switches: packet switching and circuit switching.

packet-switching: hosts/end system break application-layer messages into packets
each packet travels through communication links and packet switches (routers and link-layer switches)
Store-and-Forward Transmission: the packet switch must receive the entire packet before it can begin to transmit on next link.
\(d_{\text{end-to-end}}=2\frac{L}{R}\) (assuming zero propagation delay)
Queueing Delays and Packet Loss
Output buffer (output queue): stores packets that the router is about to send into that link
staying in output buffer --> queuing delays
packet loss: occurs when the buffer is completely full with other packets waiting for transmission.
Two key network-core functions: routing and forwarding
routing: determines source-destination route taken by packets
forwarding: move packets from router's input to appropriate router output

Frequency-division multiplexing (FDM): the frequency spectrum of a link is divided up among the connections established across the link
With FDM, each circuit continuously gets a fraction of the bandwidth.

time-division multiplexing (TDM): time is divided into frames of fixed duration, and each frame is divided into a fixed number of time slots
With TDM, each circuit gets all of the bandwidth periodically during brief intervals of time.

Packet Switching vs. Circuit Switching
Generally, packet switching allows more users to use network.
Packet switching offers better sharing of transmission capacity; it is simpler, more efficient, and less costly to implement.
Circuit switching is more suitable for real-time services

End systems connect to Internet via accessing ISPs (Internet Service Providers)
“tier-1” commercial ISPs (Level 3 Communications, AT&T, Sprint, and NTT )
Content provider network (e.g, Google): run their own network, to bring services, content close to end users
Internet Exchange Point (IXP): a meeting point where multiple ISPs can peer together.
regional networks may arise to connect access nets to ISPS

Delays: nodal processing delay, queuing delay, transmission delay, propagation delay
Processing Delay: examine the packet’s header and determine output link (where to direct the packet)
Queuing Delay: time waiting at output link for transmission
- depends on congestion level of router
- Traffic intensity: \(La/R\) (\(R\): link bandwidth; \(L\): packet length; \(a\): average packet arrival rate)
- \(La/R>1\): more "work" arriving than that can be serviced
Transmission Delay: \(L/R\) (\(L\): packet length; \(R\): link bandwidth)
Propagation Delay: \(d/s\) (\(d\): length of physical link; \(s\): propagation speed in medium)

\[ d_{\text{nodal}} = d_{\text{proc}} +d_{\text{queue}}+d_{\text{trans}}+d_{\text{prop}} \]

End-to-End Delay: \(d_{\text{end-end}}=N(d_{\text{proc}}+d_{\text{trans}}+d_{\text{prop}})\)
Traceroute program: provides {% label warning@delay measurement%} delay measurement from source to router along end-end Internet path towards destination
Loss: a packet can arrive to find a full queue with no place to store such packet, then a router will drop that packet (aka lost)

Throughput: rate (bits/time unit) at which bits transferred between sender/receiver
Instantaneous throughput: rate at given point in time
Average throughput: rate over longer period of time

Layer: each layer implements a service via its own internal-layer actions and relying on services provided by layer below
Layering modularization can ease maintenance, updating of system
Internet protocol stack
application: supporting network applications (FTP, SMTP, HTTP)
transport: process-process data transfer (TCP, UDP)
network: routing of datagrams from source to destination (IP, routing protocols)
link: data transfer between neighboring network elements (Ethernet, Wi-Fi, PPP)
physical: bits "on the wire" (fiber)

Malware: get in host and infect good stuff into malicious stuffhexo
virus: §self-replicating infection by receiving/executing object (e.g., e-mail attachment)
worm: self-replicating infection by passively receiving object that gets itself executed
Denial-of-Service (DoS): attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource
Packet Sniffer: a passive receiver that records a copy of every packet that flies
IP spoofing : send packet with false source address