IP identifier field for linux UDP/IP equals zero??!

Mark Newton newton at atdot.dotat.org
Thu May 15 09:50:20 CST 2003


On Thu, May 15, 2003 at 09:03:25AM +0930, Alan Kennington wrote:
 
 > > The "Identification" (aka Fragment ID) field is only used for
 > > fragmentation. Setting the DF bit turns off fragmentation, so the
 > > fragment ID isn't used.
 > 
 > Mark,
 > 
 > RFC 791, pages 8, doesn't seem to agree:

(BTW, RFC-791 has been obseleted by RFC-1349 and STD-5).

 > ===========================================
 > The originating protocol module of
 >     an internet datagram sets the identification field to a value that
 >     must be unique for that source-destination pair and protocol for the
 >     time the datagram will be active in the internet system.
 > ===========================================

... and where does it say that, Alan?  Ah, yes -- The section titled
"Fragmentation".  

The is absolutely no mention of the Identification field anywhere in 
RFC-791, RFC-1349 or STD-5 -except- in reference to fragment reassembly.

 > Some humans have also informed me in the last hour that in fact
 > the linux implementation does contradict RFC 791.
 > There is no indication that I can find in RFC 791 that an exception
 > to the rule is made for packets with the DF bit set.

... other than complete absence of the word "MUST" (which is how RFC's
denote mandatory behaviour).

 > > While I'm sure it'd be convenient for your application if the standard
 > > required more from the Identification field, it actually doesn't.  It's
 > > possible that some TCP/IP stacks set it on every IP packet, but they're
 > > going above and beyond the requirements of the TCP/IP spec in doing so.
 > 
 > Nope. It isn't "convenient" for my application. It's assumed by
 > another, more recent RFC.

Which one?

(note also that contradictory RFCs are by no means unusual :-)

  - mark

--------------------------------------------------------------------
I tried an internal modem,                    newton at atdot.dotat.org
     but it hurt when I walked.                          Mark Newton
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