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Unformatted text preview: what's happening with the third datagram that is sent (with 1473 bytes of
user data). It reiterates that any transport layer header appears only in the first fragment.
Also note the terminology: an IP datagram is the unit of end-to-end transmission at the IP layer
(before fragmentation and after reassembly), and a packet is the unit of data passed between the
IP layer and the link layer. A packet can be a complete IP datagram or a fragment of an IP
datagram. Figure 11.8 Example of UDP fragmentation. 11.6 ICMP Unreachable Error (Fragmentation Required)
Another variation of the ICMP unreachable error occurs when a router receives a datagram that
requires fragmentation, but the don't fragment (DF) flag is turned on in the IP header. This error
can be used by a program that needs to determine the smallest MTU in the path to a destinationcalled the path MTU discovery mechanism (Section 2.9).
Figure 11.9 shows the format of the ICMP unreachable error for this case. This differs from
Figure 6.10 because bits 16-31 of the second 32-bit word can provide the MTU of the next hop,
instead of being 0. Figure 11.9 ICMP unreachable error when fragmentation required but don't fragment bit set.
If a router doesn't provide this newer format ICMP error, the next-hop MTU is set to 0. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (9 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol The new Router Requirements RFC [Almquist 1993] states that a router must generate this
newer form when originating this ICMP unreachable error.
A problem encountered by the author involving fragmentation and this ICMP error is trying to
determine the MTU on the dialup SLIP link from the router netb to the host sun. We know
the MTU of this link from sun to netb: it's part of the SLIP configuration process when SLIP
was installed in the host sun, plus we saw it with the netstat command in Section 3.9. We
want to determine the MTU in the other dir...
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