A packet is the Layer PDU. The IP header wraps the segment.
IPv4 header contents:
- Version: for IPv4
- Total length: up to bytes
- TTL: decremented at each hop
- Protocol: TCP (), UDP ()
- Source/Destination IP: addresses
- Checksum: header validation
TTL prevents infinite loops. When TTL hits , the router discards the packet. This is how traceroute works.