This preview shows page 1. Sign up to view the full content.
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,
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
15 destination unreachable:
network unreachable (Section 9.3)
host unreachable (Section 9.3)
port unreachable (Section 6.5)
fragmentation needed but don't-fragment bit set
source route failed (Section 8.5)
destination network unknown
destination host unknown
source host isolated (obsolete)
destination network administratively prohibited
destination host administratively proh...
View Full Document
- Spring '12