Google LLC This memo defines data structures that can be appended to selected ICMP messages. The ICMP extensions defined herein can be used to identify any combination of the following: the IP interface upon which a datagram arrived, the sub-IP component of an IP interface upon which a datagram arrived, the IP interface through which the datagram would have been forwarded had it been forwardable, the sub- IP component of an IP interface through which the datagram would have been forwarded had it been forwardable, and the IP next hop to which the datagram would have been forwarded. Devices can use this ICMP extension to identify interfaces and their components by any combination of the following: ifIndex, IPv4 address, IPv6 address, name, and MTU. ICMP-aware devices can use these extensions to identify both numbered and unnumbered interfaces. This document obsoletes RFC 5837. To preserve strict backward compatibility with legacy implementations, it preserves the original Interface Information Object (Class-Num 2) as defined in RFC 5837, and introduces a new Extended Interface Information Object to accommodate new interface roles.