逍遥梦，梦逍遥

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最近思考一个问题，发送一个ping请求(ICMP ECHO)出去，操作系统收到回应(ICMP relay)后如何来区分属于哪个ICMP ECHO?

RFC 792对此有详细的说明:

Echo or Echo Reply Message

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Identifier          |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Data ...
   +-+-+-+-+-

.........

 Identifier

      If code = 0, an identifier to aid in matching echos and replies,
      may be zero.

 Sequence Number
      If code = 0, a sequence number to aid in matching echos and

  replies, may be zero.

   Description

      The data received in the echo message must be returned in the echo
      reply message.

      The identifier and sequence number may be used by the echo sender
      to aid in matching the replies with the echo requests.  For
      example, the identifier might be used like a port in TCP or UDP to
      identify a session, and the sequence number might be incremented
      on each echo request sent.  The echoer returns these same values
      in the echo reply.

      Code 0 may be received from a gateway or a host.

看来区分这些是通过 Identifier和 Sequence Number来区分的.

如果你想对ICMP了解更多，访问:http://www.faqs.org/rfcs/rfc792.html