This preview shows page 1. Sign up to view the full content.
Unformatted text preview: 0
svr4 > bsdi: icmp: svr4 udp port
Figure 6.8 ICMP port unreachable generated by TFTP request. 8888 8888 8888 8888 An ICMP port unreachable is immediately returned (line 4). But the TFTP client appears to
ignore the ICMP message, sending another UDP datagram about 5 seconds later (line 5).
This continues three more times before the client gives up.
Notice that the ICMP messages are exchanged between hosts, without a port number
designation, while each 20-byte UDP datagram is from a specific port (2924) and to a
specific port (8888).
The number 20 at the end of each UDP line is the length of the data in the UDP datagram.
In this example 20 is the sum of the TFTP's 2-byte opcode, the 9-byte null terminated name
temp.foo, and the 9-byte null terminated string netascii. (See Figure 15.1 for the
details of the TFTP packet layout.)
If we run this same example using the -e option of tcpdump we see the exact length of
each ICMP port unreachable message that's returned to the sender. This length is 70 bytes,
and is allocated as shown in Figure 6.9. Figure 6.9 ICMP message returned for our "UDP port unreachable" example. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/icmp_int.htm (11 of 16) [12/09/2001 14.46.43] Chapter 6. ICMP: Internet Control Message Protocol One rule of ICMP is that the ICMP error messages (see the final column of Figure 6.3)
must include the IP header (including any options) of the datagram that generated the error
along with at least the first 8 bytes that followed this IP header. In our example, the first 8
bytes following the IP header contain the UDP header (Figure 11.2).
The important fact is that contained in the UDP header are the source and destination port
numbers. It is this destination port number (8888) that caused the ICMP port unreachable to
be generated. The source port number (2924) can be used by the system receiving the ICMP
error to associate the error with a particular user process (the TFTP client in this example).
One reason the IP head...
View Full Document
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