Topic: TCP/IP and the OSI Model

Protocols govern the interfaces Request For Comments(RFC) define calls / responses to be made in a certain case/device/process:

  • used at a certain time (waking up, sending data, done sending data)
  • particular way
  • limited meaning
  • limited response (fixed responses to choose from)
  • clear, clean, unambiguous

There are rules that govern the interface or interaction between layers. Layers can evolve independently, so long as they follow the expected rules of interaction with other layers above and below.

Example: [Message Source]->[Encoder]->[Transmitter]->[Transmission Medium]->[Receiver]->[Decoder]->[Message Destination]

History: Lots of different equipment that didn’t work together. Each producer made their own system to deal with networking. As companies evolved, things got really confusing. Standards evolved to determine the jobs.

Historical Evolution of Network Interfaces: IBM Network Architecture, ISO Suite, Apple Talk, DECnet, Novell Netware, TCP/IP Suite

1) DOD or TCP/IP Model

  • Application
  • Transport
  • Internet
  • Network

2) OSI Model

  • 7:Application
  • Define processes on either end of the communication
  • Define the types of messages
  • Define the syntax of messages
  • Define the meaning of any informational fields
  • Define how messages are sent and the expected response
  • Define interaction with the next lower layer
  • Examples: Application, Service, System operations, a program with multiprocessing

    Layer 6/7 Interface

  • 6:Presentation (Data format conversion, compression, encoding, and encrypting of data)
  • Formating
  • Can be done by app

    Layer 5/6 Interface

  • 5:Session (Establishes sessions between services. Synchronizes and performs translations for naming services)
  • Login to server
  • Timeout
  • Segmentation

    Layer 4/5 Interface

  • 4:Transport (Manages connections and provides reliable packet delivery. Operates in units of messages)
  • TCP Sessions
  • Three-way Handshakes: Syn, Syn-Ack, Ack

    Layer 3/4 Interface

  • 3:Network (Addresses and routes datagrams. Performs fragmentation and reassembly (IP). Operates in units of packets) Layer 2/3 Interface
  • 2:Data Link (Hardware addressing and error detection/correction. Operates in units of frames) Layer 1/2 Interface
  • 1:Physical (Defines connection, electrical, and wiring specifications. Operates in units of bits)

We can remember these layers by using mnemonics: Please Do Not Throw Stale Pizza Away All People Some Time Need Data Processing

As long as the rules are followed, everything should work as anticipated

Examples: Transmission Control Protocol(TCP), and User Datagram Protocol TCP is like registered mail: If it gets damaged in transit, then I get a notification, and I can resend that letter. If you need reliability, use this protocol. UDP is like a postcard: you put it out there but you don’t know if they received it. If information is lost it becomes useless or if time is of the essence, use this protocol.

Ethernet driver (frame type) -> (ARP/RARP) IP -> (ICMP/IGMP/OSPF) TCP/UDP -> Process The most common port numbers used are well known and can be found at

Two computers are typing to communicate using the TCP protocol: Gaceful Start: 1) A sends a SYN packet to B (SEQ=100 CTL=SYN), B receives the SYN packet. 2) B sends a SYN/ACK packet to A (SEQ=300 ACK=101 CTL=SYN, ACK), A receives the SYN/ACK packet. 3) A sends an ACK packet to B (SEQ=101 ACK=301 CTL=ACK), B receives the ACK packet. Connection between A and B is established

*When a protocol is finished sending information, but is not ready to disconnect, it sends what is called a keepalive packet. This is a packet used to maintain the channel.

Graceful End: 1) A sends a FIN packet to B, B receives packet. 2) B send an ACK packet to A, A receives packet. 3) B sends a FIN packet to A, A receives the packet. 4) A sends an ACK packet to B, B receives the packet. Connection between A and B is terminated

Protocol Data Unit (PDU): 1) Source -> Network address & Device address 2) Destination -> Network address and Device address 3) Data

RFCs HTTP: 2068, FTP: 0959, BOOTP: 1542, DHCP: 2132, SMTP: 0821, Telnet: 0854, NTP: 1305, TCP: 0793, UDP: 0768, DNS: 1034 & 1035, NetBIOS: 1001 & 1002, IP: 0791, BGP: 1771, EGP: 0904, OSFP: 2178, RIP: 1723, ICMP: 0792