IPv4 - Packet Header Format

• Version
  • specifies version of IP
  • current version is IPv4
  • future version is IPv6
• HLen
  • header length in size of 32-bit words
  • maximum header length = 2⁴ = 16 (i.e. 64 bytes)
  • most of the time Hlen = 5 32-bit words(without options and pad fields)
• TOS (Type of Service)
  • basic function is to allow packets to be treated differently based on application needs
• Length
  • length of datagram including header in bytes
  • Maximum Datagram Size (including header) = 2¹⁶- 1 = 65,535 bytes
  • Length = HLen + size(Data)
  • IP supports fragmentation and reassembly process for lower layer protocols
• fields used for fragmentation and reassembly
  • Ident (Identifier)
    • all fragments of same IP datagram carry the same identifier
  • Flags (3 bits)
    • bit 0: Reserved; must be zero
    • bit 1: Don’t Fragment (DF)
    • bit 2: More Fragments (MF)
  • Offset (13 bits)
    • counts 8-byte chunks, not bytes
• TTL (Time to Live)
  • TTL initial default value = 64
  • used to prevent an IP datagram from looping eternally within an internetwork
  • routers would decrement this field by 1 until it reached 0 in which case it would drop it
• Protocol
  • a demultiplexing key that identifies the transport layer protocols (e.g. TCP, UDP, etc)
• Checksum
  • Checksum = the result of checksum algorithm of IP header as a sequence of 16-bit words, adding them up using ones complement arithmetic, and taking the ones complement of the result
  • not the same as CRC but is much easier to calculate
• SourceAddr - example,172.217.6.142
• DestinationAddr - example, 208.67.222.222
• Options (optional)
• Pad (optional)
• Data
  • size of data is determined by header size
  • Maximum Data Size = 65,535 - header size
  • Data Size = Length - HLen