ICMP control messaging: Which protocol is responsible for sending a “destination network unknown” message back to the originating host when routing fails?

Introduction / Context:
The Internet Control Message Protocol (ICMP) provides network-layer control and error messaging for IP. When a router cannot forward a packet due to an unreachable network or other conditions, ICMP communicates the error back to the sender, aiding troubleshooting and adaptive behavior.

Given Data / Assumptions:

• The scenario is a routing failure where a destination network cannot be reached.
• We want the protocol that carries the error notification.
• This is independent of the transport layer protocol of the original traffic.

Concept / Approach:

ICMP operates alongside IP and is used by routers and hosts to send error and informational messages (for example, Destination Unreachable, Time Exceeded). A “destination network unknown” is conveyed via an ICMP Destination Unreachable message with an appropriate code. It is not a TCP or ARP function, nor a BootP/DHCP bootstrapping mechanism.

Step-by-Step Solution:

Verification / Alternative check:

Packet captures during failed routes show ICMP Destination Unreachable originating from an intermediate router and addressed to the sender’s IP, confirming ICMP’s role.

Why Other Options Are Wrong:

TCP is a transport protocol for reliable streams; it does not carry network-layer errors.

ARP resolves IPv4 addresses to MAC addresses on a local LAN only.

BootP is for bootstrapping IP configuration, not error reporting.

Common Pitfalls:

Blocking ICMP entirely at firewalls, which breaks PMTUD and complicates diagnostics; confusing “ping” (ICMP Echo) with all ICMP functionality.

Final Answer:

ICMP