[e2e] Re: Question on "identification" field of IP header

alok alok.dube at apara.com
Fri Dec 13 08:48:46 PST 2002

think about the usage of that fields in ATM and MPLS core scenarios....
TCP_MSS can get screwed up...

----- Original Message ----- 
From: der Mouse <mouse at Rodents.Montreal.QC.CA>
To: <end2end-interest at postel.org>; <tcp-impl at grc.nasa.gov>
Sent: Friday, December 13, 2002 9:11 PM
Subject: [e2e] Re: Question on "identification" field of IP header

> > If the "Don't fragment bit" is set in the IP header, what purpose
> > does the "identification" field serve?
> It says which datagram is affected if an ICMP is returned in resposne
> to it, if nothing else.
> > I remember coming across some e-mail chain in one of the mailing
> > lists (TCP-IMPL, e2e, TSVWG) about this issue and the interaction
> > with NAT.
> Heh.  My usual response to "NAT interacts badly with <foo>" is "don't
> use NAT, then".  It just breaks too many of the assumptions underlying
> IP-based networking.  But it wouldn't surprise me a bit if there were
> NAT implementations broken enough to assume that packets with identical
> IDs arriving within a small enough temporal window of one another were
> actually the same packet.
> Note that the portion of a packet returned in an ICMP does not include
> even the IP source and destination addresses; the identification value
> is almost the only value that can be relied upon to identify the
> original packet upon getting an ICMP....
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