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(We'll see in Chapter 17 that TCP must establish a connection with the other end before the first
byte of data can be sent.) Also, there are no acknowledgments by the receiver when the data is
received. The sender, in this example, has no idea whether the other end receives the datagrams.
Finally note that the source UDP port number changes each time the program is run. First it is
1108 and then it is 1110. We mentioned in Section 1.9 that the ephemeral port numbers used by
clients are typically in the range 1024 through 5000, as we see here. 11.5 IP Fragmentation
As we described in Section 2.8, the physical network layer normally imposes an upper limit on
the size of the frame that can be transmitted. Whenever the IP layer receives an IP datagram to
send, it determines which local interface the datagram is being sent on (routing), and queries
that interface to obtain its MTU. IP compares the MTU with the datagram size and performs
fragmentation, if necessary. Fragmentation can take place either at the original sending host or
at an intermediate router.
When an IP datagram is fragmented, it is not reassembled until it reaches its final destination.
(This handling of reassembly differs from some other networking protocols that require
reassembly to take place at the next hop, not at the final destination.) The IP layer at the
destination performs the reassembly. The goal is to make fragmentation and reassembly
transparent to the transport layer (TCP and UDP), which it is, except for possible performance file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (6 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol degradation. It is also possible for the fragment of a datagram to again be fragmented (possibly
more than once). The information maintained in the IP header for fragmentation and reassembly
provides enough information to do this.
Recalling the IP header (Figure 3.1), the following fields are used in fragmentation. The
identification field contains a...
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