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Unformatted text preview: es IP datagrams. file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm (7 of 11) [12/09/2001 14.46.33] file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm Figure 2.4 Processing of IP datagrams by loopback interface.
The key points to note in this figure are as follows:
1. Everything sent to the loopback address (normally 127.0.0.1) appears as IP input.
2. Datagrams sent to a broadcast address or a multicast address are copied to the loopback
interface and sent out on the Ethernet. This is because the definition of broadcasting and
multicasting (Chapter 12) includes the sending host.
3. Anything sent to one of the host's own IP addresses is sent to the loopback interface.
While it may seem inefficient to perform all the transport layer and IP layer processing of the
loopback data, it simplifies the design because the loopback interface appears as just another link
layer to the network layer. The network layer passes a datagram to the loopback interface like any
other link layer, and it happens that the loopback interface then puts the datagram back onto IP's
Another implication of Figure 2.4 is that IP datagrams sent to the one of the host's own IP
addresses normally do not appear on the corresponding network. For example, on an Ethernet,
normally the packet is not transmitted and then read back. Comments in some of the BSD Ethernet
device drivers indicate that many Ethernet interface cards are not capable of reading their own
transmissions. Since a host must process IP datagrams that it sends to itself, handling these file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm (8 of 11) [12/09/2001 14.46.33] file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm packets as shown in Figure 2.4 is the simplest way to accomplish this.
The 4.4BSD implementation defines the va...
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- Spring '12