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Unformatted text preview: ditional information. First, the output
frag 26304 (lines 3 and 4) and frag 26313 (lines 5 and 6) specify the value of the
identification field in the IP header.
The next number in the fragmentation information, the 1480 between the colon and the at sign
in line 3, is the size, excluding the IP header. The first fragment of both datagrams contains
1480 bytes of data: 8 bytes for the UDP header and 1472 bytes of user data. (The 20-byte IP
header makes the packet exactly 1500 bytes.) The second fragment of the first datagram (line 4)
contains 1 byte of data-the remaining byte of user data. The second fragment of the second
datagram (line 6) contains the remaining 2 bytes of user data.
Fragmentation requires that the data portion of the generated fragments (that is, everything
excluding the IP header) be a multiple of 8 bytes for all fragments other than the final one. In
this example, 1480 is a multiple of 8.
The number following the at sign is the offset of the data in the fragment, from the start of the
datagram. The first fragment of both datagrams starts at 0 (lines 3 and 5) and the second
fragment of both datagrams starts at byte offset 1480 (lines 4 and 6). The plus sign following
this offset that is printed for the first fragment of both datagrams means there are more
fragments comprising this datagram. This plus sign corresponds to the "more fragments" bit in
the 3-bit flags in the IP header. The purpose of this bit is to let the receiver know when it has
completed the reassembly of all the fragments for a datagram. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (8 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol Finally, notice that lines 4 and 6 (fragments other than the first) omit the protocol (UDP) and
the source and destination ports. The protocol could be printed, since it's in the IP header that's
copied into the fragments. The port numbers, however, are in the UDP header, which only
occurs in the first fragment.
Figure 11.8 shows...
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