IP Fragmentation

Glen Kent glen.kent at gmail.com
Thu Aug 28 18:44:44 CDT 2008

> >
> I'm not sure how to address the above points since there appear to be some
> incorrect assumptions at play. It all depends on whether the Don't Fragment
> (DF) bit is set in IPv4 and how the source application responds to any
> resulting ICMP error responses (if the DF is set and one of the routes
> requires fragmentation).

OK, so what happens if a transit router does not support IP
fragmentation and it receives a packet which is bigger than the
outgoing link's MTU. Should it simply drop the packet or proactively
send an ICMP Dest Unreachable error (Frag required) to the peer?

I understand that routers usually must send this error only when a
fragmentation is required and they recieve a packet with DF bit set.
However, in this case this router would drop the packet (for it doesnt
support fragmentation) and sending an ICMP error back to the host,
warning it that its packets will get dropped seems to be a better

OTOH, what do most of the implementations do if they send a regular IP
packet and receive an ICMP dest unreachable - Fragmentation reqd
message back? Do they fragment this packet and then send it out, or
this message is silently ignored?


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