Stop a datagram from bouncing back on its link
03Some point-to-point media do not use Neighbor Discovery. A larger prefix can leave unused destinations that neither router owns, creating a ping-pong risk if the required forwarding check is missing.
A chosen response is ICMPv6 Destination Unreachable, Type 1/Code 3, toward the original source A. RFC 6164 says SHOULD, subject to general error rules; this error is a different packet, not the forbidden original forwarding.
Source for this drawing: RFC 6164 · Read RFC section 5.1 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
The ping-pong issue described here is for certain point-to-point links without Neighbor Discovery, not every Ethernet or IPv6 link. The separate risk example uses /126, with endpoints ::2/::3 and unused destination ::1.
- The ping-pong issue described here is for certain point-to-point links without Neighbor Discovery, not every Ethernet or IPv6 link. The separate risk example uses /126, with endpoints ::2/::3 and unused destination ::1.
- A router receiving a packet on a point-to-point link for a destination in a subnet assigned to that same link, other than one of its own addresses, MUST NOT forward it back on that link.
- RFC 6164 §5.1 says it SHOULD generate ICMPv6 Destination Unreachable code 3 instead. ICMPv6 error suppression and rate limits still apply (RFC 4443 §2.4); the drawing’s Type 1/Code 3 is not an unconditional reply promise.
- A /127 with both endpoints assigned has no third unused on-link address for this particular mechanism. It is not a claim that /127 prevents all routing loops or eliminates other forwarding checks.
Fictional documentation addresses within 2001:db8::/32. Bounded operator-chosen address examples, not a complete router, Neighbor Discovery, ICMPv6 or address-validation implementation.
Read the RFC section ↗