? how cisco router handle the out-of-order ICMP echo-reply packets

Scott Morris swm at emanon.com
Tue Jan 6 07:58:33 CST 2009


Guess I'll have to go back and look at wireshark output again...  I didn't
recall seeing sequence number used in pings between Cisco devices, although
that may just be the implementation ('may be used') part.

I'll stand corrected.  ;)

Scott

-----Original Message-----
From: Steve Bertrand [mailto:steve at ibctech.ca] 
Sent: Tuesday, January 06, 2009 8:52 AM
To: swm at emanon.com
Cc: 'Zhao Ping'; nanog at merit.edu
Subject: Re: ? how cisco router handle the out-of-order ICMP echo-reply
packets

Scott Morris wrote:
> There aren't sequence numbers with ICMP.  And the timeout value is
> watched/triggered before the next ICMP is sent, so there shouldn't really
be
> any ordering problem/interpretation anyway.

FYI, from RFC 792:

  Sequence Number

   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.

Steve





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