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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Where the fields before the TTL field have the same meanings as for a question<br />

entry <strong>and</strong>:<br />

TTL A 32-bit time-to-live value in seconds for the record. This defines<br />

how long it can be regarded as valid.<br />

RDlength A 16-bit length for the Rdata field.<br />

Rdata A variable length string whose interpretation depends on the<br />

Type field.<br />

Message compression<br />

In order to reduce the message size, a compression scheme is used to eliminate<br />

the repetition of domain names in the various RRs. Any duplicate domain name<br />

or list of labels is replaced with a pointer to the previous occurrence. The pointer<br />

has the form of a 2-byte field as shown in Figure 12-8.<br />

Figure 12-8 DNS message compression<br />

Where:<br />

► The first 2 bits distinguish the pointer from a normal label, which is restricted<br />

to a 63-byte length plus the length byte ahead of it (which has a value of

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