Intro

Internet

• Hardware
  • Billions of devices: hosts or end systems
  • Devices are connected with communication links (e.g. Fibre, with a transmission rate called bandwidth in bits per second)
  • Packet switches forward packets (chunks of data) with routers and switches
• Software
  • protocols: controls sending and receiving of messages (TCP, IP, HTTP, etc)
  • standards:
    • RFC (request for comments)
    • IETF (internet engineering task force)
• Infrastructure
  • Provides services to applications (web, VoIP, email, etc)
  • Provides programming interface to apps

Network edge:

• hosts: clients and servers
• servers are often in data centers

Access networks, physical media: wired, wireless communication links

Network core

• Interconnected routers
• network of networks

Frequency division multiplexing: different channels transmitted in different frequencies

HFC: hybrid fibre coax

• 30Mbps down, 2Mbps up
• Shared line for neighbourhood, not a direct connection like in DSL

Circuit switching

• Frequency division: each user gets a frequency band
  • Bit rate not evenly divided since not all frequencies have the same capacity
• Time division: rate per time slot is frames per second times the number of bits in one time slot
  • e.g. 1 frame per second, 24 slots, 1.536 Mbps wire
  • \(rate = \frac{1.536 Mbps}{24} = 64 Kbps\)
  • Time to transmit a 640 Kb file: \(\frac{640Kb}{64 Kbps} = 10s\)

Packet switching

• Allows more users to use the network than circuit switching
• Has queueing delay

e.g. 1Mbps link, each user has 100Kbps when active, but only active 10% of the time

• Circuit switching can only use 10 users
• Packet switching: with 35 users, probability > 10 active at the same time is less than 0.0004:

Probability that 11 or more users or more are sending is equal to 1 - the probability that 10 users are sending
\[\begin{align} 1 - P(\text{10 users sending}) &= 1 - \sum_{n = 0}^{10} {\binom{35}{n} 0.1^n (1-0.1)^{35 - n}}\\ &= 0.0004\\ \end{align}\]

• Packet switching is good for data that comes in bursts (resource sharing, no call setup)
• Packet delay and loss possible. Protocols needed for reliable data transfer, congestion control
• Bandwidth guarantees are needed for audio/video apps to provide circuit-like behaviour. Still an unsolved problem.

Internet structure

• Bottom: access ISPs
• Above that are by regional ISPs
• Above that are some Internet Exchange Points
• Above that are Tier 1 ISPs and content providers (e.g. Google)

Network loss and delays

• Packets queue in router buffers
• Loss and delays occur when packet arrival rate exceeds output link capacity
• Packets wait for their turn to be transmitted

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

• \(d_{proc}\): nodal processing
  • check for bit errors
  • determine output link
  • typically less than milliseconds
• \(d_{queue}\): queueing delay
  • Time waiting for output link
  • Depends on congestion
• \(d_{trans}\): transmission delay
  • Delay from receiving start of packet to receiving end of packet
  • Depends on packet length and bitrate
  • \(L\): packet length
  • \(R\): bandwidth
  • \(d_{trans} = \frac{L}{R}\)
• \(d_{prop}\): propagation delay
  • Delay from sending start of packet to receiving start of packet
  • \(d\): length of physical link
  • \(s\): propagation speed in medium
  • \(d_{prop} = \frac{d}{s}\)

Queueing delay

Given a packet length of \(L\) bits, an \(R\) bps link, and an average arrival rate \(a\), we define traffic utilization or traffic intensity:
\[\text{Traffic intensity} = \frac{\underbrace{La}_\text{rate of arriving bits}}{\underbrace{R}_\text{rate of outgoing bits}}\]

• \(La/R \approx 0\): avg queueing delay is small
• \(La/R \rightarrow 1\): avg queueing delay is large
• \(La/R \gt 1\): avg delay is infinite because more work arrives than can be serviced

traceroute provides delay measurement from source to router, for each router along the path to the destination. Can be used to find routers that are congested

Throughput is the rate (bits/time) at which bits are transferred between sender and receiver

• Instantaneous throughput is the rate at a given point in time
• Average is over a long period of time
• The link with the smallest rate will always be the bottleneck for throughput
• Throughput is always the minimum rate along the link

Layers

Each later represents a service

Internet protocol stack:

• application: supports network applications (e.g. FTP, SMTP, HTTP)
• transport: Process to process data transfer (TCP, UDP)
• network: routing from source to destination (IP, routing protocols)
• link: data transfer between network elements (Ethernet, 802.111 (wifi), PPP)
• physical: bits "on the wire"

OSI model:

• application
• presentation: allow applications to interpret meaning of data (e.g. encryption, compression)
• session: synchronization, checkpointing, recovery of data exchange
• transport
• network
• link
• physical

Encapsulation means taking something from another layer, adding a header, and passing it on (not directly modifying data from the previous layer, just extending it)

• The full, extended message is called a datagram
• Removing of headers is decapsulation

Security

• virus: self replication by receiving and executing
• worm: self replicating by passively receiving and getting itself executed
• spyware: records info and uploads
• botnet: a network of similarly infected hosts
• packet sniffing: promiscuous network interface reads/records all broadcasted packets
• IP spoofing: send a message claiming to be from another address