TCP IP Illustrated

The final two columns in this figure specify whether

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Unformatted text preview: ally. For example, an ICMP error message is never generated in response to an ICMP error message. (If this were not the rule, we could end up with scenarios where an error generates an error, which generates an error, and so on, indefinitely) When an ICMP error message is sent, the message always contains the IP header and the first 8 bytes of the IP datagram that caused the ICMP error to be generated. This lets the receiving ICMP module associate the message with one particular protocol (TCP or UDP from the protocol field in the IP header) and one particular user process (from the TCP or UDP port numbers that are in the TCP or UDP header contained in the first 8 bytes of the IP datagram). We'll show an example of this in Section 6.5. An ICMP error message is never generated in response to 1. An ICMP error message. (An ICMP error message may, however, be generated in response to an ICMP query message.) 2. A datagram destined to an IP broadcast address (Figure 3.9) or an IP multicast address (a class D address, Figure 1.5). 3. A datagram sent as a link-layer broadcast. 4. A fragment other than the first. (We describe fragmentation in Section 11.5.) 5. A datagram whose source address does not define a single host. This means the file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/icmp_int.htm (2 of 16) [12/09/2001 14.46.42] Chapter 6. ICMP: Internet Control Message Protocol source address cannot be a zero address, a loopback address, a broadcast address, or a multicast address. type code 0 0 echo reply (Ping reply. Chapter 7) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 destination unreachable: network unreachable (Section 9.3) host unreachable (Section 9.3) protocol unreachable port unreachable (Section 6.5) fragmentation needed but don't-fragment bit set (Section 11.6) source route failed (Section 8.5) destination network unknown destination host unknown source host isolated (obsolete) destination network administratively prohibited destination host administratively proh...
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