TCP/IP Overview and Vulnerabilities, continued

• the Internet layer protocols primarily consist of the Internet protocol (IP), and the Internet Control Message Protocol (ICMP) provide the actual routed delivery of messages between source and destination, and provide only a basic network management function by reporting any observed errors.
  IPv4, particularly, is vulnerable to attack and may be exploited to not deliver messages, deliver messages to the wrong destination, or 'confuse' a destination to the extent that it may stop providing any service.
  As examples, IP datagrams may be transmitted from one (attacking) host while claiming to be from another, and forged ICMP messages may make a destination network or host appear unreachable.

• physical layer protocols are not strictly part of the TCP/IP suite, but define how packets or frames are received via hardware, and provided to the IP (software) layer above. By its nature, interface hardware must see all packets destined for, or passing by, the interface, and most hardware may be configured by software (the operating system) to report all activity seen.
  Trivially, on a shared network, an operating system (and probably some of its programs) may capture all packets that are visible on a network.