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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  Name Systems and TCP/IP Name Registration and Name Resolution
           9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)
                9  TCP/IP Domain Name System (DNS)
                     9  DNS Name Servers and Name Resolution
                          9  DNS Name Server Concepts and Operation

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DNS Name Server Load Balancing
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DNS Resolution Concepts and Resolver Operations
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DNS Name Server Enhancements: DNS Notify, Incremental Zone Transfers, and DNS Update (Dynamic DNS)
(Page 3 of 4)

Improving Zone Transfer Efficiency: Incremental Transfers

The second issue with regular DNS is the need to transfer the entire zone whenever a change to any part of it is made. There are many zones on the Internet that have truly enormous master files that change constantly. Consider the master files for the “.COM” zone for example; having to copy the entire thing to slave name servers every time there is a change to even one record is beyond “inefficient”—it's downright insane!

RFC 1995, Incremental Zone Transfer in DNS, specifies a new type of zone transfer called an incremental zone transfer. When this feature is implemented on master and slave name servers in a zone, the master server keeps track of the most recent changes made to the database. Each time a slave server determines that a change has occurred and the secondary's database needs to be updated, it sends an IXFR (incremental transfer) query to the master, which contains the Serial number of the slave’s current copy of the database. The master then looks to see what resource records have changed since that Serial number was the current one, and sends only the updated resource records to the secondary server.

To conserve storage, the master server obviously doesn't keep all the changes made to its database forever. It will generally track the last few modifications to the database, with the Serial number associated with each. If the slave sends an IXFR request that contains a Serial number for which recent change information is still on the primary server, only the changes are sent in reply. If the request has a Serial number so old that the master server no longer has information about some of the changes since that version of the database, a complete zone transfer is performed instead of an incremental one.

Key Concept: The DNS incremental zone transfer enhancement uses a special message type that allows a slave name server to determine what changes have occurred since it last synchronized with the master server. By transferring only the changes, the amount of time and bandwidth used for zone transfers can be significantly reduced.



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DNS Name Server Load Balancing
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DNS Resolution Concepts and Resolver Operations
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Version 3.0 - Version Date: September 20, 2005

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