Destination Unreachable

Description

ICMP Type 3 is the primary error reporting mechanism for undeliverable IP datagrams. Routers send codes 0-12 when they cannot forward a packet. Destination hosts send code 3 (Port Unreachable) when a UDP datagram arrives for a port with no listener.

Code 4 (Fragmentation Needed and DF Set) is critical for Path MTU Discovery (RFC 1191). When a router must fragment a packet but the Don't Fragment bit is set, it sends Type 3 Code 4 with the MTU of the next-hop link in the unused header field. TCP and QUIC use this to discover the path MTU and avoid fragmentation.

Blocking ICMP Type 3 Code 4 at firewalls breaks Path MTU Discovery, causing TCP black holes where connections establish but then stall when large packets are sent. This is one of the most common network misconfiguration patterns.

Common Causes

• –No route to destination network (code 0)
• –Host exists but is unreachable at layer 3 (code 1)
• –UDP datagram arrived but no process is listening on that port (code 3)
• –Packet too large and DF bit set – triggers PMTU Discovery (code 4)
• –Firewall or ACL blocking traffic to the destination (code 13)

Codes