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Unformatted text preview: examples that we've watched with tcpdump have used
UDP. When is each protocol used and why?
When the resolver issues a query and the response comes back with the TC bit set ("truncated") it means the size of the
response exceeded 512 bytes, so only the first 512 bytes were returned by the server. The resolver normally issues the
request again, using TCP. This allows more than 512 bytes to be returned. (Recall our discussion of the maximum UDP
datagram size in Section 11.10.) Since TCP breaks up a stream of user data into what it calls segments, it can transfer
any amount of user data, using multiple segments.
Also, when a secondary name server for a domain starts up it performs a zone transfer from the primary name server
for the domain. We also said that the secondary queries the primary on a regular basis (often every 3 hours) to see if the file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dns_the.htm (16 of 18) [12/09/2001 14.47.06] Chapter 14. DNS: The Domain Name System primary has had its tables updated, and if so, a zone transfer is performed. Zone transfers are done using TCP, since
there is much more data to transfer than a single query or response.
Since the DNS primarily uses UDP, both the resolver and the name server must perform their own timeout and
retransmission. Also, unlike many other Internet applications that used UDP (TFTP, BOOTP, and SNMP), which
operate mostly on local area networks, DNS queries and responses often traverse wide area networks. The packet loss
rate and variability in round-trip times are normally higher on a WAN than a LAN, increasing the importance of a good
retransmission and timeout algorithm for DNS clients. 14.9 Another Example
Let's look at another example that ties together many of the DNS features that we've described. We start an Rlogin
client, connecting to an Rlogin server in some other domain. Figure 14.16 shows the exchange of packets that takes
place. Figure 14.16 Summary of packets exchanged to start up Rlogin cli...
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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