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From: barmar@think.com (Barry Margolin)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: ICMP Time Exceeded sent on non-initial fragments?
Message-ID: <1990Dec1.075328.13627@Think.COM>
Date: 1 Dec 90 07:53:28 GMT
References: <17727@shlump.nac.dec.com>
Sender: news@Think.COM
Distribution: na
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 23

In article <17727@shlump.nac.dec.com> ciarfella@shug.enet.dec.com (Paul Ciarfella) writes:
>	RFC 1122 says that ICMP messages must not be generated as a
>	result of receiving non-initial fragments of a datagram.
>	But, what happens if reassembly of a datagram times out 
>	and the initial fragment was never received.  Does this
>	mean that I cannot send an ICMP Time Exceeded given that
>	I don't have the initial fragment?

Correct.  ICMP error messages are supposed to contain the first N data
octets from the IP datagram that caused the error.  This allows the
recipient of the ICMP error to determine which higher-level protocols got
the error, and report the error back to the appropriate entities.  If you
haven't received the first fragment then you can't have the first N data
octets, so you won't be able to construct a useful error datagram.

If you don't receive the initial fragment then the sender will simply
behave as if no fragments made it to you, presumably timing out and perhaps
retransmitting.
--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar


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