Unformatted text preview: the outgoing datagram has the same checksum as the
incoming datagram (lines 3 and 4, 5 and 6). Looking at Figure 11.3 we see that the two IP
addresses are swapped, as are the two port numbers. The other fields in the pseudo-header and
the UDP header are the same, as is the data being echoed. This reiterates that the UDP
checksums (indeed, all the checksums in the TCP/IP protocol suite) are simple 16-bit sums. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (4 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol They cannot detect an error that swaps two of the 16-bit values.
The author also directed a DNS query at each of the eight root name servers described in
Section 14.2. The DNS uses UDP primarily, and only two of the eight had UDP checksums
[Mogul 1992] provides counts of various checksum errors on a busy NFS (Network File
System) server that had been up for 40 days. Figure 11.5 summarizes these numbers. Layer Number of checksum
errors Approximate total number of packets Ethernet
30,000,000 Figure 11.5 Counts of corrupted packets detected by various checksums.
The final column is only the approximate total for each row, since other protocols are in use at
the Ethernet and IP layers. For example, not all the Ethernet frames are IP datagrams, since
minimally ARP is also used on an Ethernet. Not all IP datagrams are UDP or TCP, since ICMP
also uses IP.
Note the much higher percentage of TCP checksum errors compared to UDP checksum errors.
This is probably because the TCP connections on this system tended to be "long distance"
(traversing many routers, bridges, etc.) while the UDP traffic was local. The bottom line is not
to trust the data-link (e.g., Ethernet, token ring, etc.) CRC completely. You should enable the
end-to-end checksums all the time. Also, if your data is valuable, you might not want to trust
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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