Physical Internet

Physically, the internet consists of billions of connected computing devices.
Packet switches are present to forward chunks of data (packets).
Communication links and networks exist.

Protocols

Protocols define the format, order of messages sent and recieved among network entities, and actions taken on message transmission, receipt

Access networks

HFC - Hybrid fiber coax

  • Asymmetric

Home networks
Wireless access networks
Enterprise networks

  • Twisted Pair Cable
  • Coaxial Cable
  • Fibre Optic Cable
  • Satellite
  • Microwave
  • Bluetooth
  • Cellular
  • Wireless LAN (WiFi)

Packet & Circuit Switching

  • Store-and-forward methods require the entire packet to arrive at a router before it can be transmitted to the next link
  • Queueing occurs when packets arrive faster than they can be processed
  • Circuit switches allocate end-to-end resources in stead
  • Circuit switching can use frequency division multiplexing or time division multiplexing

Packet switching can function with many more users at a time

Internet Structure

The internet is a network of networks, they are connected by ISPs which are connected by IXPs (Internet eXchange Points)

Internet Performance

Latency increases and packet loss can happen if router queues fill up.
Specificlly, packet delay can be caused by:

  • Nodal processing
  • Queueing delay
  • Transmission delay
  • Propagation delay

Throughput refers to the rate at which data is sent between two devices.

(De)Multiplexing

  • Recall that processes communicate over sockets
  • Multiplexing allows you to send data from multiple sockets over the internet
  • Demultiplexing lets you recieve data from multiple sockets
  • Multiplexing is possible thanks to fields such as:
  • Source and destination IP and port (for TCP)
  • Destination port number only (for UDP)

(De)Multiplexing happens at all layers.

Connectionless Transport: UDP

  • UDP is “connectionless” - no connection needs to be established before data is sent - reduce RTT delay
  • UDP is more bare-bones compared to TCP
  • It has a small header size
  • No congestion control
  • It is often used for streaming apps
  • It is also used for DNS*

Communication over UDP

  • Transport layer is passed an application-layer message
  • The UDP segment’s header field values are determined
  • The UDP segment is created and passed to the network (IP) layer … …
  • The transport layer recieves a segment from the IP layer
  • The UDP checksum header value is checked
  • The application-layer message is checked
  • The message is demultiplexed to the application via the socket

Checksums

  • UDP and IP have checksums
  • Internet checksum (used by IP) only covers the header
  • In some specific cases, it is unreliable

Recovering from errors

  • As the underlying channel may be unreliable, it must be possible to recover from errors
  • ACK and NACK allows you to do this
  • Sender waits some time for an ACK, if no ACK is recieved then it retransmits

TCP

  • TCP uses ACKs ofc
  • TCP flow control has the reciever controling how many bytes the sender transmits as to not overflow the buffer