Packet Transfer

Network Stack

Application - Contructs message with address
Transport - Splits message into packets
Network - Handles routing through the Internet
Link - Handles actual transmission of packets

Common Ports

These are high level protocols built on the foundation of the TCP/IP protocol suite.

Port Service
20 File Transfer Protocol - FTP
21 FTP control
22 Secure Shell - SSH
23 Telnet
25 Simple Mail Transfer Protocol - SMTP
69 Trivial File Transfer Protocol - TFTP
80 Hypertext Transfer Protocol - HTTP
110 Post Office Protocol - POP3
115 Simple File Transfer Protocol - SFTP (not to be confused with SSH File Transfer Protocol, which operates on a different port)
119 Network News Transfer Protocol - NNTP
123 Network Time Protocol - NTP
143 Internet Message Access Protocol - IMAP
443 Secure Hypertext Transfer Protocol - HTTPS

Internet backbone - A set of high speed networks that carry Internet traffic. These are provided by large telecomms companies
Internet Service Provider - A company that provides other companies or individuals with access to the Internet

Packet Switching

To improve the efficiency of transferring information over a shared communication line, messages are divided into fixed-sized, numbered packets. Network devices called routers are used to direct packets between networks.

The message is split into packets. Packets are sent over the Internet by the most expedient route. Packets are reordered then reassembled into the original message.

Advantages of Packet Switching

  • No wasted bandwidth - Links are not reserved during idle periods
  • Multiplexing
  • Serivce - more connections of lesser quality, and no blocking of users
  • Adaptation - If there is a failure point, the network can recover and work around it

Disadvantages of Packet Switching

  • No guaranteed bandwidth - Harder to build applications requiring QoS
  • Per packet overhead - packet headers etc
  • Complex end-to-end control - Packets can be lost, corrupted during transit, or delivered in the incorrect order
  • Delay and congestion - due to no congestion control, arbitrary delays and packet drops can occur

Open Systems

Proprietary system - A system that uses technologies kept private by a particular commercial vendor
Interoperability - The ability for software and hardware on multiple machines and from multiple commercial vendors to communicate
Open systems - Systems based on a common model of network architecture and a suite of protocols used in its implementation

The ISO established the Open Systems Interconnection Reference Model - OSI. Each layer deals with a particular aspect of network communication.

Network Protocols

They are layered such that each one relies on the protocols that underlie it. Somtimes this results in it being referred to as a protocol stack.


TCP stands for Transmission Control Protocol

TCP software breaks messages into packets, hands them off to the IP software for delivery, and then orders and reassembles the packets at their destination.

IP stands for Internet Protocol

IP software deals with the routing of packets through the Internet to their final destination


UDP stands for User Datagram Protocol

It is a TCP alternative. The main difference being that TCP is highly reliable at the cost of descreased performance, while UDP is less reliable, but generally faster.

MIME Types


A method by which multiple signals are data streams are combined into one signal over a shared cable. The goal is to share an expensive resource.

Time Division Multiplexing


Frequency Division Multiplexing


Statistical Multiplexing

A smarter kind of multiplexing that can dedicate larger or smaller channels of bandwidth depending on the application. A large bandwidth application such as streaming TV, can be dedicated more bandwidth than an email application.