Background: Know: Recognize:

echo request and echo reply

Echo request

The request is an ICMP message whose data is ecpected to be received back in an echo reply. The host must respond to all echo requests with an echo reply containing the exact data received in the request message.

  • The Identifier and Sequence Number can be used by the client to match the reply with the request that caused the reply. In practice, most Linux systems use a unique identifier for every ping process, and sequence number is an increasing number within that process. Windows uses a fixed identifier, which varies between Windows versions, and a sequence number that is only reset at boot time.
  • The data received by the Echo Request must be entirely included in the Echo Reply.

Echo reply

The echo reply is an ICMP message generated in response to an echo request, and is mandatory for all hosts and routers.

  • Type and code must be set to 0.
  • The identifier and sequence number can be used by the client to determine which echo requests are associated with the echo replies.
  • The data received in the echo request must be entirely included in the echo reply.