IPv4

• IPv4 Datagrams:
  • Version number, header ln, service type, datagram ln
  • Fragmentation offset, flags, 16-bit identifier
    • ID flag: 1 if there are more fragments after, 0 if there aren't
    • Offset: How many units have already been sent (byte size of all previous, not including header, divided by 8)
  • TTL, upper layer (send to TCP or UDP?), header checksum
  • Source IP
  • Destination IP
  • Options
  • Payload
• Fragmentation & Reassembly
  • Links have MTUs---therefore, datagrams must be fragmented if IP hdr + Transport Hdr + MSS are greater than MTU
  • Given a 4000 Byte datagram, and an MTU of 1500 Bytes:
    • 4000-20=3980 (Remove IP Hdr)
    • 1500-20=1480 (For IP Hdr)
    • 3980/1480 = 2.68 (therefore, 3 fragments)
  • Fragmentation is discouraged