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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 Messaging and Message, Resource Record and Master File Formats

DNS Message Resource Record Field Formats 1 2 3 DNS Master File Format

Using Message Compression to Avoid Duplication of Part of a Name

The example above shows how pointers can be used to eliminate duplication of a whole name: the name “mail.xyzindustries.com” was used in two places and a pointer was used instead of the second. Pointers are even more powerful than this, however. They can also be used to point to only part of a real name, or can be combined with additional labels to provide a compressed representation of a name related to another name in a resource record. This provides even greater space savings.

In the example above, this means that even the first instance of “mail.xyzindustries.com” can be compressed. Recall that the MX record will have “xyzindustries.com” in the Name field and “mail.xyzindustries.com” in the RData field. If the Name field of that record starts at byte 19, then we can encode the RData field as:

“[4] m a i l [pointer-to-byte-19]”.

The device reading the record will get “mail” for the first label and then read “xyzindustries.com” from the Name field to get the complete name, “mail.xyzindustries.com”.

Similarly, suppose we had a record in this same message that contained a reference to the parent domain for “xyzindustries.com”, which is of course “com”. This could simply be encoded as:

“[pointer-to-byte-33]”

The reason is that byte 33 is where we find the “[3] c o m [0]” part of the Name field containing “[13] x y z i n d u s t r i e s [3] c o m [0]”.

DNS Message Resource Record Field Formats 1 2 3 DNS Master File Format