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0 timestamp request
timestamp reply kernel generates reply
user process 15
0 information request (obsolete)
information reply (obsolete) (ignored)
user process 17
0 address mask request
address mask reply kernel generates reply
user process 4 Figure 6.12 Handling of the ICMP message types by 4.4BSD. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/icmp_int.htm (15 of 16) [12/09/2001 14.46.43] Chapter 6. ICMP: Internet Control Message Protocol 6.7 Summary
This chapter has been a look at the Internet Control Message Protocol, a required part of
every implementation. Figure 6.3 lists all the ICMP message types, most of which we'll
discuss later in the text.
We looked at the ICMP address mask request and reply and the timestamp request and
reply in detail. These are typical of the request-reply messages. Both have an identifier and
sequence number in the ICMP message. The sending application stores a unique value in
the identifier field, to distinguish between replies for itself and replies for other processes.
The sequence number field lets the client match replies with requests.
We also saw the ICMP port unreachable error, a common ICMP error. This let us examine
the information returned in an ICMP error: the IP header and the next 8 bytes of the IP
datagram that caused the error. This information is required by the receiver of the ICMP
error, to know more about the cause of the error. Both TCP and UDP store the source and
destination port numbers in the first 8 bytes of their headers for this reason.
Finally, we presented our first time line of tcpdump output, a presentation format we'll use
in later chapters.
6.1 At the end of Section 6.2 we listed five special conditions under which an ICMP error
message is not sent. What would happen if these five conditions weren't followed and we
sent a broadcast UDP datagram to an unlikely port on the local cable?
6.2 Read the Host Requirements RFC [Braden 1989a] to see if the generation of an ICM...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12