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Unformatted text preview: at ICMP error. Setting the MTU down
to 1006 did work.
The conclusion we can make from this experiment is that many, but not all, WANs today can
handle packets larger than 512 bytes. Using the path MTU discovery feature will allow
applications to take advantage of these larger MTUs. 11.8 Path MTU Discovery with UDP
Let's examine the interaction between an application using UDP and the path MTU discovery
mechanism. We want to see what happens when the application writes datagrams that are too
big for some intermediate link.
Since the only system that we've been using that supports the path MTU discovery mechanism
is Solaris 2.x, we'll use it as the source host to send 650-byte datagrams to slip. Since our host
slip sits behind a SLIP link with an MTU of 296, any UDP datagram greater than 268 bytes
(296 -20-8) with the "don't fragment" bit set should cause the router bsdi to generate the ICMP
"can't fragment" error. Figure 11.13 shows the topology and the MTUs. Figure 11.13 Systems used for path MTU discovery using UDP.
The following command generates ten 650-byte UDP datagrams, with a 5-second pause file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (13 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol between each datagram:
solaris % sock -u -i -n10 -w650 -p5 slip discard
Figure 11.14 shows the tcpdump output. When this example was run, the router bsdi was set
to not return the next-hop MTU as part of the ICMP "can't fragment" error.
The first datagram is sent with the DF bit set (line 1) and generates the expected error from the
router bsdi (line 2). What's puzzling is that the next datagram is also sent with the DF bit set
(line 3) and generates the same ICMP error (line 4). We would expect this datagram to be sent
with the DF bit off.
On line 5 it appears IP has finally learned that datagrams to this destination should not be sent
with the DF bit set, so IP goes ahead and fragm...
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- Spring '12