Internet Protocol Security

Internet Protocol Security #

The Internet #

  • Global protocol that provides best-effort delivery of packet between connected hosts
  • Packet: structured sequence of bytes
    • Header: metadata used by network
    • Payload: user data to be transported
  • Every host has a unique identifier - IP address
  • Series of routers receive packets, look at header destination address, and send it one hop towards destination IP address

Network protocols #

  • Define how hosts communicate in published network protocols
    • Syntax: how communication is structured (e.g. format and order of messages)
    • Semantics: what communication means.
      • Actions taken on transmit or receipt of message, or when timer expiers.
      • What assumptions can be made.

Protocol layering #

  • Networks use stack of protocol layers
    • Each layer has different responsibility
    • Layers define abstraction boundaries
  • Lower layers provide services to layers above
    • Don’t care what higher layers do
  • Higher layers use services of layers below
    • Don’t worry about how it works

OSI 5 layer model #

  • Physical

    • How do bits get translated into electrical, optical, or radio signals.
  • Link (typically: ethernet)

    • How to get packet to the next hop. Transmission of data frames between two nodes connected by a physical link.
    • Assumes: local nodes are physically connected
    • Task: transfer bytes between two hosts on physically connected network
    • MAC address: 6-byte address given to physical hosts, assigned at manufacture time
    • Frame contains:
      • Destination MAC
      • Source MAC
      • Data type (usually “IP”)
      • Data
      • CRC checksum
    • Originally: transmission was broadcast, every local computer got every packet
    • Now: switched ethernet
      • Switch learns at which physical port each MAC address lives based on MAC source addresses
      • If switch knows MAC address M is at port P, it will only send a packet for M to port P
      • If not, sends packet everywhere
  • Network

    • Repsonsible for packet forwarding. How to get a packet to the final destination when there are many hops along the way.
    • Assumes: frames can be sent on a local network
    • Task: let any two computers exchange packets
    • IPv4 header:
      • Instruct routers and hosts what to do with a packet
      • All values are filled in by the sending host
    • ARP: Address Resolution Protocol
      • How do we map IP addr -> MAC addr?
      • How do routes learn where IPs are located?
  • Transport

    • Allows a client to establish a connection to specific services (e.g. web server on port 80): provides reliable communication
  • Application

    • Defines how individual applications communicate. For example, HTTP defines how browsers send requests to web servers.

Interaction of layers:

Layer interactions

Packet encapsulation #

  • Protocol N1 can use the services of lower layer protocol N2
    • A packet P1 of N1 is encapsulated into a packet P2 of N2
    • The payload of P2 is P1
    • The control information of P2 is derived from that of P1