The internet network layer consists of:

• Path selection algorithms
  • Traditionally implemented in routing protocols (OPSF, BGP)
  • Can also be implemented in SDN controller
  • Responsible for forwarding table, which dictates what routers should do given an IP packet
• IP protocol
  • Dictates format of datagram
  • Controls how hosts/routers/etc. are addressed, how to handle packets
• ICMP protocol
  • Control protocol
  • Responsible for how errors are reported, how routers can signals themselves

Datagram Format (IPv4)

• Typically has TCP/UDP segment, could find an ICMP message
• Start - 4-bit version nnumber
• Header length is necessary - size of IPv4 header is not static, changes depending on optional headers
• Length field is 16-bit field that gives total size of data
  • In practice, packets are 1500 bytes or less
  • Max size of IPv4 is dictated by MTU data link technology
• Service type marks packet according to level of service required by IP packet
  • In practice, only 6 of 8 bits are used to differentiate between IP packets (diffserv protocol)
  • Remaining 2 bits are used for ECN mechanism (marking packet for congestion)
• TTL is time to live, designates remaining max hops
• Upper-layer ID identifies protocol in payload of IP datagram
  • TCP - value 6
  • UDP - value 17
  • ICMP - value 1
• 16-bit identifier for IP packet
• 1-bit flags for fragmentation/reassembly
• Header checksum
  • Applied for header of datagram
  • Tested by routers to check for failure (dropped if checksum fails)
  • Errors in payload of datagram will not be detected by IP layer (IP layer only tests for errors in IP datagram header)
• 32-bit source/destination IP address
• Optional fields include timestamp, series of routers taken, etc.
• Minimum size of IP datagram:
  • 20 Bytes for TCP
  • 20 Bytes for IP
  • Total: 40 bytes + app layer overhead for TCP + IP