A router MUST implement an ICMP Echo server function that receives Echo Requests sent to the router, and sends corresponding Echo Replies. A router MUST be prepared to receive, reassemble and echo an ICMP Echo Request datagram at least as the maximum of 576 and the MTUs of all the connected networks.
The Echo server function MAY choose not to respond to ICMP echo requests addressed to IP broadcast or IP multicast addresses. A router SHOULD have a configuration option that, if enabled, causes the router to silently ignore all ICMP echo requests; if provided, this option MUST default to allowing responses.
The neutral provision about responding to broadcast and multicast Echo Requests derives from [INTRO:2]'s "Echo Request/Reply" section.
As stated in Section [10.3.3], a router MUST also implement a user/application-layer interface for sending an Echo Request and receiving an Echo Reply, for diagnostic purposes. All ICMP Echo Reply messages MUST be passed to this interface.
The IP source address in an ICMP Echo Reply MUST be the same as the specific-destination address of the corresponding ICMP Echo Request message.
Data received in an ICMP Echo Request MUST be entirely included in the resulting Echo Reply.
If a Record Route and/or Timestamp option is received in an ICMP Echo Request, this option (these options) SHOULD be updated to include the current router and included in the IP header of the Echo Reply message, without truncation. Thus, the recorded route will be for the entire round trip.
If a Source Route option is received in an ICMP Echo Request, the return route MUST be reversed and used as a Source Route option for the Echo Reply message, unless the router is aware of policy that would prevent the delivery of the message.