Validate IPv4 router forwarding and ICMP behavior at the hop boundary

Frame this document as a source-led configuration and assurance check: Validate IPv4 router forwarding and ICMP behavior at the hop boundary. Only the official source and traced locations below supply facts. Confirm applicability before acting.

Source fact:

The official RFC 1812 — Requirements for IP Version 4 Routers from RFC Editor / Internet Engineering Task Force supports the following bounded statements:

• Before processing an IPv4 packet, a router must validate minimum lengths, checksum, version, header length, and total length; failures are discarded and should be logged. The research record locates this support at Section 5.2.2 (IP Header Validation).
• After deciding a packet is not for local delivery, a router determines whether the next hop is the destination itself or another adjacent router. The research record locates this support at Sections 5.2.3 (Local Delivery Decision) and 5.2.4 (Determining the Next Hop Address).
• A router must generate the appropriate Destination Unreachable when it cannot forward, except where the prohibitions on replying to errors, invalid packets, broadcasts, multicasts, or noninitial fragments apply. The research record locates this support at Sections 4.3.2.7 (When Not to Send ICMP Errors) and 4.3.3.1 (Destination Unreachable).

Do not import neighboring assumptions into the source record. The supported task is a scoped comparison involving address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains and the conditions the source actually describes.

What the source does not establish

This RFC evidence supports only the named network-protocol decision; it does not select vendor settings, topology, capacity, or an acceptable failure mode. A correct source interpretation can still be inapplicable to a particular design. Confirm DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring, ownership, and change authority instead of treating documented behavior as a deployment guarantee.

Applicability questions

• For source statement 1 at Section 5.2.2 (IP Header Validation), which observable configuration, record, or test can confirm applicability here?
• For source statement 2 at Sections 5.2.3 (Local Delivery Decision) and 5.2.4 (Determining the Next Hop Address), which observable configuration, record, or test can confirm applicability here?
• For source statement 3 at Sections 4.3.2.7 (When Not to Send ICMP Errors) and 4.3.3.1 (Destination Unreachable), which observable configuration, record, or test can confirm applicability here?
• Within address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains, which versions, roles, and configuration states define the review population?
• Could DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring invalidate the test, hide a failure, or change applicability?
• Who owns the decision, and which observation requires stopping, escalation, or rollback?

DSE recommendation:

DSE recommends using the cited source as the evidence anchor for this decision. Use a two-person review for the source interpretation and the resulting operational decision. Record the source location, examined part of address plans, interfaces, routes, peers, protocol roles, timers, middleboxes, and intended failure domains, observed and expected states, owner, and reason for deviation.

If the review warrants change, use a bounded implementation with prerequisites, test population, monitoring, abort criteria, and a rehearsed reversal. Sequence checks for DNS, Active Directory authentication, PKI, time, routing policy, transport reachability, and monitoring and sanitize protected material before retention.

Verification and evidence

Tie each conclusion back to Section 5.2.2 (IP Header Validation); Sections 5.2.3 (Local Delivery Decision) and 5.2.4 (Determining the Next Hop Address); Sections 4.3.2.7 (When Not to Send ICMP Errors) and 4.3.3.1 (Destination Unreachable) and to observable material such as configuration snapshots, route or neighbor state, packet captures, counters, topology records, and controlled failover results. Preserve provenance and stable identifiers without copying secrets into the evidence set.

Keep before-state evidence, approval, test or change result, exceptions, and after-state evidence together. Use an approved lab, window, or nonproduction path for risky tests. Set a recheck trigger for version, architecture, dependency, vendor, incident, or ownership change. A check proves only what was observed.

Official references

• RFC 1812 — Requirements for IP Version 4 Routers — RFC Editor / Internet Engineering Task Force