Definition

• Physical Layer

  • The Physical Connection
  • Individual bits are modulated onto a wire or transmitted over radio
    • Physical layer specifies how bits are encoded at a signal level

• Data Link Layer

  • multiple computers on a LAN contend for the network medium

    • media access control (MAC) specifies how computers cooperate
    • Link layer also specifies how bits are “packetized” and network interface controllers are addressed

  • When there are a collision (two nics want to talk at the same time on the same wire)

    • both two stop talking
    • the first computer will receive a signal and start talking again
    • the second computer will wait

    

• Network Layer (IP)Z

  • Internet Protocol (IP) routes packets across multiple networks

    • Every computer has a unique IP address

    • Individual networks are connected by routers than span networks

    • A router

      • is a computer connecting to multiple networks
      • when it sees a package sending to another network, it will copy it to another network

      

  • There are protocols to:

    • Let a host map an IP to MAC address on the same network
    • Let a router learn about other routers to get IP packets one step closer to their destination

    

  • Packet encapsulation

    • An IP packet is encapsulated as the payload of an Ethernet frame
    • AS IP packets traverse networks, routers pull out the IP packet from an Ethernet frame and plunk it into a new one on the next network

    

• Transport Layer (TCP)

  • Provides applications with reliable, ordered, congestion-controlled byte streams

    • send stream data as multiple IP packets (differentiated by sequence numbers) and retransmits them as necessary
    • when receiving, puts packets back in order and detects missing packets

  • A single host (IP address) can have up to $2^{16} = 65535$ ports

    • kind of like an apartment number at a postal address

      • your applications are the residents who get mail sent an apartment #

      

  • TCP Level Encapsulation

  

  • Applications use OS services to establish TCP streams
    • The “Berkeley Sockets” API
      • Clients connect() to a server IP address + application port number
      • Servers listen() for and accept() client connections
      • Clients and servers read(0 and write(0 data to each other

• Transport Layer (UDP)

  • User Datagram Protocol
  • Provides application with unreliable packet delivery
  • UDP is a really thin, simple layer on top of IP
    • Datagrams still are fragmented into multiple IP packets

• The (Mostly missing) Layers 5 & 6

  • Session Layer
  • Presentation Layer

• Application Layer

  • Application protocols

  • The format and meaning of messages between application entities

  • Example: HTTP is an application-level protocol that dictates how web browsers and web servers communicate

    • HTTP is implemented on top of TCP streams

      

  • Popular application-level protocols

    • DNS: translates a domain name into one or more IP addresses
      • Domain Name System
      • An hierarchy of DNS servers cooperate to do this
    • HTTP: WEB PROTOCOLS
      • Hypertext Transfer Protocol
    • SMTP, IMAP, POP: mail delivery and access protocols
    • SSH
    • bittorrent